A Review on Van Der Waals Boron Nitride Quantum Dots

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Journal of C Carbon Research

Review A Review on van der Waals Boron Nitride Quantum Dots

Amit Acharya 1,2, Sambhawana Sharma 1,2, Xiuling Liu 1,3, Dongyan Zhang 1,2,* and Yoke Khin Yap 1,2

1 Department of Physics, Michigan Technological University, Houghton, MI 49931, USA; [email protected] (A.A.); [email protected] (S.S.); [email protected] (X.L.); [email protected] (Y.K.Y.) 2 Elizabeth and Richard Henes Center for Quantum Phenomena, Michigan Technological University, Houghton, MI 49931, USA 3 StabiLux Biosciences, Inc., Ann Arbor, MI 48103, USA * Correspondence: [email protected]; Tel.: +1-906-487-2086

Abstract: Boron nitride quantum dots (BNQDs) have gained increasing attention for their versatile ﬂuorescent, optoelectronic, chemical, and biochemical properties. During the past few years, sig- niﬁcant progress has been demonstrated, started from theoretical modeling to actual application. Many interesting properties and applications have been reported, such as excitation-dependent emission (and, in some cases, non-excitation dependent), chemical functionalization, bioimaging, phototherapy, photocatalysis, chemical, and biological sensing. An overview of this early-stage research development of BNQDs is presented in this article. We have prepared un-bias assessments on various synthesis methods, property analysis, and applications of BNQDs here, and provided our perspective on the development of these emerging nanomaterials for years to come.

Keywords: quantum dots; boron nitride; van der Waals; ﬂuorescent; sensing; photocatalysis

Citation: Acharya, A.; Sharma, S.; 1. Introduction Liu, X.; Zhang, D.; Yap, Y.K. A Review Low dimensional boron nitride (BN) materials such as BN nanosheets and BN nan- on van der Waals Boron Nitride otubes (BNNTs) have gained substantial attention [1–9]. BN materials are isoelectric to car- Quantum Dots. C 2021, 7, 35. bon solids. For example, the three-dimensional (3D) cubic phase (c-BN), two-dimensional https://doi.org/10.3390/c7020035 (2D) hexagonal phase (h-BN), and one-dimensional (1D) BN nanotubes (BNNTs) are struc- turally like diamonds, graphene, and carbon nanotubes (CNTs), respectively. Despite Academic Editor: these structural similarities, BN materials offer different physical and chemical properties. Jean-François Morin For instance, all BN materials are electrically insulating with about 6eV bandgaps, while most carbon nanomaterials are metallic, except semiconducting single-walled CNTs [10]. Received: 26 February 2021 Like other 2D materials such as graphene [11], and transitional metal dichalcogenides Accepted: 23 March 2021 Published: 27 March 2021 (TMDC), 2D BN nanosheets have gained signiﬁcant research attention [12–14]. Boron nitride nanoribbons, the 1D version of BN nanosheets, are also attractive for their spintronics,

Publisher’s Note: MDPI stays neutral magnetic, and electronic properties [15–18]. More recently, zero-dimensional (0D) BN with regard to jurisdictional claims in quantum dots (BNQDs) or BN dots (BNDs) have emerged as the new BN nanomaterials, published maps and institutional affil- with extremely high surface/volume ratios. Many optical and electronic properties of iations. BNQDs are yet to be clarified and therefore are expected to stimulate a lot more theoretical and experimental investigation. BNQDs are a 0D version of h-BN nanosheets. Due to their small dimension, the ratio of dangling bonds at the edges of BNQDs to the saturated sp2 h-BN bonds on the bulk of the 0D structure is high. It is expected that these edges will alter the physical and chemical Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. properties of the h-BN nanosheets. Their high chemical stability and the non-toxic nature This article is an open access article of BN materials are expected to allow for many biomedical applications in BNQDs. The distributed under the terms and theoretical study of electronic structures and spin polarization in BNQDs was first con- conditions of the Creative Commons ducted in 2011 [19] based on first-principle calculations and an extended Hubbard model. Attribution (CC BY) license (https:// Later in 2013, similar studies [20,21] on the structural stability, electronic, optical, and mag- creativecommons.org/licenses/by/ netic properties of BNQDs were performed. Experimentally, BNQDs synthesis [22,23] was 4.0/). reported thereafter. As shown in Figure1, the number of peer-reviewed publications has

C 2021, 7, 35. https://doi.org/10.3390/c7020035 https://www.mdpi.com/journal/carbon C 2021, 7, x FOR PEER REVIEW 2 of 26

C 2021, 7, 35 2 of 26 was reported thereafter. As shown in Figure 1, the number of peer-reviewed publications has significantly increased during the past two years. This publication statistic has con- sideredsigniﬁcantly papers increased consisting during of the the keyword past two “boron years. nitride This publication quantum dots”, statistic and has did considered not in- cludepapers papers consisting with other of the 0D keyword boron nitride “boron nanoparticles. nitride quantum Two dots”, recent and review did not articles include briefly papers discussedwith other 0D 0D BN boron mate nitriderials along nanoparticles. with other Two BN recentnanoparticles review articles[24,25]. brieﬂy In this discussed article, we 0D willBN focus materials on reviewing along with the otherrecent BN development nanoparticles of BNQDs, [24,25]. Infrom this the article, synthesis, we will character- focus on ization,reviewing and various the recent applications. development of BNQDs, from the synthesis, characterization, and various applications.

. Figure 1. The number of papers published with title or topic containing boron nitride quantum dots Figure(BNQDs) 1. The based number on Web of papers of Science publishedTM analysis with title with or the topic keyword containing “boron boron nitride nitride quantum quantum dots”. dots (BNQDs) based on Web of Science TM analysis with the keyword “boron nitride quantum dots”.2. Crystal Structures BN is the lightest III-IV compound and is isoelectric to carbon structures by sharing 2.the Crystal same Structure total numbers of electrons between the neighboring atoms [26]. As shown in FigureBN2 isa, the BN lightest has different III-IV polymorphs,compound and including is isoelectric hexagonal to carbon BN (h-BN), structures cubic by BN sharing (c-BN), therhombohedral same total number BN (r-BN), of electrons and wurtzite between BN the (w-BN) neighboring [27–31]. atoms The h-BN [26]. andAs shown r-BN have in sp2 Figurehybridized 2a, BN B-N has bondsdifferent while polymorphs, the c-BN andincluding w-BN hexagonal are sp3 hybridized. BN (h-BN), The cubic h-BN BN phase (c- BN),is the rhombohedral most stable BN phase (r-BN), under and standard wurtzite conditions,BN (w-BN) consists[27–31]. The of strong h-BN and boron–nitrogen r-BN have spcovalent2 hybridized intralayer B-N bonds bonds while making the c hexagonal-BN and w planar-BN are geometry sp3 hybridized. and weak The van h-BN der phase Waals is interactionthe most stable between phase layers under in thestandard bulk state conditions, as shown consists in Figure of2 strongb. There boron is also–nitrogen electrostatic co- valentinteraction intralayer due bonds to the electronegativitymaking hexagonal difference planar geometry between and the boronweak andvan nitrogender Waals atoms in- teractionbetween between adjacent layers layers in [ 32the] inbulk the state interlayer as shown stacking. in Figure Unlike 2b. electronThere is distributionalso electrostatic in the interactiongraphene due C-C to bond, the electronegativity the electrons in thedifference B-N bond between move the towards boronthe and electronegative nitrogen atoms N betweenatoms vacating adjacent the layers boron [32] atoms in the and interlayer provide s atacking. partial Unlike ionic character electron through distribution polarization in the grapheneof the B-N C-C bond. bond, Thethe electrons basal planes in the in B h-BN-N bond are stackedmove towards in an alternatingthe electronegative AA’AA’ N... atomssequence vacating which the is boron different atoms when and compared provide a withpartial graphite ionic character in which through hexagonal polarization nanosheets ofare the stacked B-N bond. in the The ABAB basal sequence. planes in Theh-BN w-BN are stack has aed diamond-like in an alternating tetrahedral AA’AA structure’… se- quencewith similar which AA’AA’is different staking when to compared that of h-BN. with The graph r-BNitee has in which a three-fold hexagonal stacking nanosheets sequence areABCABC stacked ...in theasshown ABAB insequence Figure2.a. The Similarly, w-BN has the stackinga diamond sequence-like tetrahedral for c-BN isstructure like that withof r-BNsimilar but AA’AA’ has a tetrahedral staking to that structure. of h-BN. In The this r- review,BN has wea three will-fold discuss stacking only sequence the stable ABCABC…h-BN materials. as shown in Figure 2a. Similarly, the stacking sequence for c-BN is like that of r-BN but has a tetrahedral structure. In this review, we will discuss only the stable h-BN materials. CC2021 2021, ,7 7,, 35 x FOR PEER REVIEW 33 of of 26 26

FigureFigure 2.2. ((aa)) Crystal Crystal structure structuress of of h- h-BN,BN, w w-BN,-BN, r-BN, r-BN, and and c-BN. c-BN. Boron Boron atoms atoms are areshown shown in red in, red, whilewhile nitrogennitrogen atoms atoms are are shown shown in in blue blue [28 [28]]. Reproduced. Reproduced with with permission. permission. 2007 2007 IOP IOP Publishing Publishing Ltd. (Ltd.b) Structural (b) Structural basis basis for two-dimensional for two-dimensional h-BN h nanostructures,-BN nanostructures, where where B-N bondsB-N bonds have have the length the of 0.145length nm, of the0.145 distance nm, the between distance two between adjacent two borazine adjacent rings borazine is 0.250 rings nm, is and 0.250 the nm, interlayer and the distance inter- is 0.333layer nm.distance The edge is 0.333 of the nm. monolayer The edgeBN of the nanosheet monolayer can beBN either nanosheet zigzag can or armchair be either [13zigzag]. Reproduced or arm- withchair permission. [13]. Reproduced Copyright with2017 permission. WILEY-VCH Copyright Verlag 2017 GmbH WIL &EY Co.-VCH kGaA Verlag (Weinheim, GmbH & Germany). Co. kGaA (Weinheim, Germany). 3. Synthesis of BNQDs 3. SynthesisExperimental of BNQDs research on BNQDs is still at an early stage. BNQDs can be synthesized via top-downExperimental or bottom-up research on approaches. BNQDs is still For at the an top-down early stage. process, BNQDs bulk can be h-BN synthesized powder isvia exfoliated top-down into or bottom thinner-up nanosheets approaches. and For subsequently the top-down into process, QDs by bulk combining h-BN powder various is processes.exfoliated Forinto the thinner bottom-up nanosheets approach, and subsequently B and N precursors into QDs are by used combining to grow various the BNQDs. pro- Bothcesses. approaches For the bottom have their-up advantagesapproach, B and and shortcomings. N precursors BNQDsare used obtained to grow by the exfoliation BNQDs. haveBoth goodapproaches crystallinity have buttheir are advantages poly-dispersed and shortcomings. in size. The weak BNQDs van der obtained Waals interactionby exfolia- betweention have h-BN good nanosheetscrystallinity makes but are the poly exfoliation-dispersed possible; in size. The however, weak van the strongder Waal in-planes inter- covalentaction between bonds are h-BN preventing nanosheets the formationmakes the of exfoliation 0D structures. possible Breaking; however, the h-BN the sheetstrong into in- 0Dplane is time-consuming, covalent bonds andare preventing the yield is low.the formation In contrast, of the 0D bottom-up structures. approach Breaking allows the h- forBN bettersheet into control 0D is in time the shape-consuming, and size and of the BNQDs, yield butis low. the In crystallinity contrast, the is bottom relatively-up inferior approach to thoseallows produced for better by control the top-down in the shape approach. and size of BNQDs, but the crystallinity is relatively inferior to those produced by the top-down approach. 3.1. Top-Down Approaches 3.1. TopThe-D top-downown Approaches method is the most used method for the exfoliation of layered structure materialsThe top-down due to method its relative is the simplicity most used and method effectiveness. for the exfoliation Researchers of havelayered used struc- the micromechanicalture materials due cleavage to its relative method, simplicity also called and the effectiveness. scotch tape Researchers method, to obtainhave used the BNthe nanosheets by repeatedly peeling the adhesive tape from the bulk h-BN sample up to micromechanical cleavage method, also called the scotch tape method, to obtain the BN the monolayer [33,34]. This method could produce BN nanosheets with a lateral size of nanosheets by repeatedly peeling the adhesive tape from the bulk h-BN sample up to the 100 micrometers. Other methods such as liquid exfoliation, defect engineering, and chemi- monolayer [33,34]. This method could produce BN nanosheets with a lateral size of 100 cal exfoliation were used to obtain the BN nanosheets, which were further assisted with micrometers. Other methods such as liquid exfoliation, defect engineering, and chemical sonication and heating to obtain the BNQDs. These methods are discussed in detail here. exfoliation were used to obtain the BN nanosheets, which were further assisted with son- 3.1.1.ication Sonication-Assisted and heating to obtain Liquid the ExfoliationBNQDs. These Method methods are discussed in detail here. This process involves the use of liquids to disperse bulk h-BN powders, followed by 3.1.1. Sonication-Assisted Liquid Exfoliation Method high-power sonication and refluxing. The polarity of liquid is the important parameter in liquidThis exfoliation process and involves is usually the use directly of liquids proportional to disperse to the bulk surface h-BN energy powders, [35] tofollowed overcome by thehigh weak-power van sonication der Waals and force refluxing. between theThe h-BNpolarity layers. of liquid Liquid is withthe important higher polarity parameter would in stronglyliquid exfoliation interact with and h-BN is usually layers, directly facilitating proportional easier exfoliation to the surface to BN nanosheetsenergy [35] and to over- then tocome BNQDs. the weak Various van liquidsder Waals used force in the between production the h- ofBN BNQDs layers. areLiquid discussed with higher here. polarity wouldWater: strongly Stengl interact et al. [ 22with] were h-BN the layers first research, facilitating group easier to make exfoliation BNQDs to using BN nanosheets sonication- assistedand then liquid to BNQDs. exfoliation Various and liquids heating. used They in the first production prepared of h-BN BNQDs particles are discussed by annealing here. boric acid and urea under a nitrogen atmosphere. These h-BN particles were then exfoliated C 2021, 7, x FOR PEER REVIEW 4 of 27

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Water: Stengl et al. [22] were the first research group to make BNQDs using sonication-assisted liquid exfoliation and heating. They first prepared h-BN particles by an- nealingin water boric to acid obtain and BNurea nanosheets under a nitrogen using atmosphere. a high-intensity These h cavitation-BN particles field were in then a pressurized exfoliatedbatch reactor. in water The to obtain products BN nanosheets were further using refluxed a high- atintensity 198 ◦C cavitation in ethylene field glycol in a at atmo- pressurizedspheric pressure batch reactor. for 48 The h. products The resulting were further dispersion refluxed was at 198 filtered0C in ethylene off using glycol a membrane atfilter atmospheric to obtain pressure a clear for and 48 transparenth. The resulting solution dispersion that was contains filtered BNQDs. off using The a mem- heat treatment branebroke filter the to BNobtain nanosheets a clear and intransparent a boiling solution solution that to contains produce BNQDs. few-layered The heat BNQDstreat- whose mentsurface broke was the BN passivated nanosheets by in the a boiling ethylene solution glycol to produce chains. few The-layered as-prepared BNQDs BNQDswhose showed surface was passivated by the ethylene glycol chains. The as-prepared BNQDs showed excitation-dependent emission, i.e., the fluorescent/luminescent wavelength is tunable by excitation-dependent emission, i.e., the fluorescent/luminescent wavelength is tunable by usingusing different different excitation excitation wavelengths. wavelengths. Figure 3 Figure shows3 the shows excitation the excitation-dependent-dependent photolu- photo- minescenceluminescence (PL) spec (PL)tra spectra of h-BN of QDs h-BN, excited QDs, excitedat a wavelength at a wavelength from 290 to from 400 290nm to[36] 400. nm [36]. TransmissionTransmission electron electron microscopy microscopy (TEM) (TEM) revealed revealed non-uniform non-uniform crystallinity crystallinity comprising comprising r-BNr-BN and and some some amorphous amorphous BN. This BN. work This was work inspired was inspired by previous by previous works in worksthe making in the making ofof graphene graphene QDs QDs and andcarbon carbon QDs QDs[37,38] [37. Similar,38]. Similar experiments experiments to obtainto BNQDs obtain from BNQDs from chemicallychemically exfoliated exfoliated h-BN h-BN (using (using H2SO H4 and2SO KMnO4 and4 KMnO) did not4) yield did notany yieldQDs. any QDs.

FigureFigure 3. Photoluminescence 3. Photoluminescence (PL) of (PL) h-BN of quantum h-BN quantum dots showing dots showingexcitation- excitation-dependentdependent emission. emission. TheThe inset inset plot plot shows shows the corresponding the corresponding normalized normalized PL spectra PL [36] spectra. Reproduced [36]. Reproduced with permission. with permission. Copyright 2015 WILEY-VCH Verlag GmbH & Co. kGaA (Weinheim, Germany). Copyright 2015 WILEY-VCH Verlag GmbH & Co. kGaA (Weinheim, Germany). Organic solvents: Li et al. [36] reported on the fabrication of BNQDs with controllable size usingOrganic sonication solvents: in dimethylformamide Li et al. [36] reported (DMF) on and the the fabrication subsequent of solvothermal BNQDs with pro- controllable cesssize as usingillustrated sonication in Figure in 4 dimethylformamide. They reported on the size (DMF) control and of the BNQDs subsequent with an solvothermalaverage pro- sizecess of as2.62 illustrated nm. A fluorescent in Figure quantum4. They yield reported (QY) onof up the to size 19.5% control was obtained of BNQDs by a withdjust- an average ingsize the of solvothermal 2.62 nm. A fluorescentsynthesis parameters quantum, such yield as (QY) filling of upfactor, to 19.5%synthesis was temperature, obtained by adjusting andthe process solvothermal duration. synthesis Similar methods parameters, were suchlater adopted as filling by factor, various synthesis research temperature,groups and usingprocess different duration. organic Similar solvents methods, such as DMF, were laterdimethyl adopted sulfoxide by various(DMSO), research ethanol, iso- groups using propyldifferent alcohol organic (IPA), solvents,and N-methyl such-2 as-pyrrolidone DMF, dimethyl (NMP) sulfoxide[39–41]. Liu (DMSO), et al. reported ethanol, the isopropyl synthesisalcohol of (IPA), BNQDs and withN-methyl-2-pyrrolidone tunable blue/green PL using (NMP) three [39 –solvents41]. Liu (ethanol, et al. reported DMF, and the synthesis of BNQDs with tunable blue/green PL using three solvents (ethanol, DMF, and NMP) in their liquid exfoliation-solvothermal treatment. The authors observed blue fluorescence from samples prepared in ethanol and DMF, but green fluorescence from samples prepared in NMP under the same UV irradiation. These results suggest that the sizes, QY, and optical properties of these BNQDs were dependent on the polarity of different organic solvents. The PL for BNQDs prepared in ethanol was reported to show excitation-independent emission. In contrast, BNQDs prepared in DMF and NMP were demonstrating excitation- C 2021, 7, 35 5 of 26

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dependent emission. The quantum yields (QYs) and average lateral sizes of the BNQDs are reported as 12.6% and 4.1 ± 0.2 nm, 16.4% and 2.8 ± 0.3 nm, as well as 21.3% and 2.0the ±BNQDs0.2 nm are for reported samples preparedas 12.6% inand ethanol, 4.1 ± 0.2 DMF, nm, and 16.4% NMP, and respectively 2.8 ± 0.3 nm, [40]. as well as 21.3% and 2.0 ± 0.2 nm for samples prepared in ethanol, DMF, and NMP, respectively [40].

Figure 4. Schematic illustration of thethe BNQDBNQD synthesissynthesis procedure,procedure, includingincludingthe thesonication-assisted sonication-as- liquidsisted liquid exfoliation exfoliation of h-BN of bulkh-BN powder bulk powder to synthesize to synthesize BN nanosheets BN nanosheets and solvothermal and solvothermal treatment treat- to fabricatement to fabricate BNQDs [BNQDs36]. Reproduced [36]. Reproduced with permission. with permission. Copyright Copyright 2017 WILEY-VCH 2017 WILEY Verlag-VCH GmbH Verlag & Co.GmbH kGaA & Co. (Weinheim, kGaA (Weinheim Germany)., Germany).

Fan etet al. al. reported reported the the use use of a ofmicrowave a microwave irradiation irradiation technique technique to synthesize to synthesize BNQDs. TheseBNQDs. BNQDs These are BNQDs 1.98 to are 7.05 1.98 nm to in 7.05 size nm with in a size high with QY ofa high 23.44% QY and of 23.44% excellent and electromag- excellent neticelectromagnetic wave absorption wave performance.absorption performance.The author reported The author synthesizing reported BN synthesizing nanoplates (BNNPs) BN na- fromnoplates bulk (BN h-BNNPs) powder from using bulk tiph-BN ultrasonication powder using in DMF tip ultrasonication as illustrated in Figurein DMF4. as The illustrated BNNPs’ ◦ dispersionin Figure 4. was The irradiated BNNPs’ withdispersion microwaves was irradiated in a solvothermal with microwave process ats in 150 a solvothermalC for 10 min. Theprocess products at 150 were ⁰C for then 10 min centrifuged. The products and finally were filtered then centrifuged with microporous and finally membrane filtered filters with tomicrop obtainorous BNQDs. membrane The size filters and dispersityto obtain BNQDs. of the as-prepared The size and BNQDs dispersity were reportedof the as to-pre- be tunedpared withBNQDs reaction were temperaturereported to be and tuned duration with timereaction of microwave temperature irradiation and duration [42]. time of microwaveAcid: Kumar irradiation et al. [42] reported. a facile, scalable, and single-step method to make BNQDs usingAcid: high-intensity Kumar et al. sonication. reported a This facile, approach scalable, resulted and single in the-step exfoliation method to and make etching BNQDs of BNusing nanosheets high-intensity in Bronsted sonication. acid (H This3PO approach4 in this case). resulted The in majority the exfoliation of BNQDs and were etching formed of by the etching at the edge site of BN nanosheets. The average size of synthesized particles BN nanosheets in Bronsted acid (H3PO4 in this case). The majority of BNQDs were formed wasby the 3–6 etching nm and at thicknessthe edge site 3–12 of atomic BN nanosheets. BN layers The [43]. average size of synthesized particles was 3Base:–6 nm Xue and et thickness al. [44] fabricated 3–12 atomic fluorescent BN layers monolayer [43]. BNQDs with an average lateral size ofBase: around Xue 3.3et al. nm, [44] an fabricated average thickness fluorescent of 1.05 monolayer nm, and BNQDs a QY of with 1.8%. an The average authors lateral use asize hydrothermal of around 3.3 method nm, an by average breaking thickness the bulk of h-BN 1.05 powdersnm, and a into QY a of basic 1.8%. solution. The authors The bulk use h-BN powder was mixed and grounded with sodium hydroxide and potassium hydroxide a hydrothermal method by breaking the bulk h-BN powders into a basic solution. The (KOH), and then heated in an autoclave. The products were then centrifuged to obtain bulk h-BN powder was mixed and grounded with sodium hydroxide and potassium hy- BN nanosheets. These exfoliated BN nanosheets were again hydrothermally treated with droxide (KOH), and then heated in an autoclave. The products were then centrifuged to water and KOH to create defects in the BN nanosheets which were further tip sonicated obtain BN nanosheets. These exfoliated BN nanosheets were again hydrothermally and filtered to obtain BNQDs. A similar synthesis approach for BNQDs was reported treated with water and KOH to create defects in the BN nanosheets which were further by Dehghani et al. [45] using liquid exfoliation of hBN in KOH and ethanol, followed by tip sonicated and filtered to obtain BNQDs. A similar synthesis approach for BNQDs was hydrothermal treatments. The as-synthesized BNQDs emit blue fluorescence. After surface reported by Dehghani et al. [45] using liquid exfoliation of hBN in KOH and ethanol, fol- passivation with polyethylene glycol (PEG ), these BNQDs produce green emission lowed by hydrothermal treatments. The as-200synthesized BNQDs emit blue fluorescence. under the same UV irradiation. These BNQDs have an average size of 1.8 ± 0.3 nm, an After surface passivation with polyethylene glycol (PEG200), these BNQDs produce green average thickness of 2 ± 0.5 nm, and a QY of 32% for blue fluorescence, and 38% for emission under the same UV irradiation. These BNQDs have an average size of green fluorescence. These BNQDs also demonstrate excellent dispersion in water. The synthesis1.8 ± 0.3 nm, of highly an average water-soluble thickness BN of nanosheets 2 ± 0.5 nm, and and BNQDs a QY of was 32% simultaneously for blue fluorescence, reported byand Liu 38% et al.for [46green] using fluorescence. solvothermal These treatment BNQDs of also bulk demonstrate h-BN in ethanol excellent with dispersion the presence in ofwater. sodium The hydroxidesynthesis of (NaOH). highly Theirwater- methodsoluble reliesBN nanosheets on the insertion and BNQDs of Na+ wasand simultane- OH− ions intoously the reported BN layers by atLiu high et temperatureal. [46] using and solvothermal pressure, thus treatment exfoliating of bulk it layer h-BN by i layer,n ethanol and with the presence of sodium hydroxide (NaOH). Their method relies on the insertion of Na+ and OH-ions into the BN layers at high temperature and pressure, thus exfoliating it C 2021, 7, 35 6 of 26 C 2021, 7, x FOR PEER REVIEW 6 of 26

ﬁnally breaking into BN nanosheets and BNQDs. They reported the yield of BN nanosheets andlayer BNQDs by layer, to beand as finally high as breaking 18.1% and into 2.9%, BN nanosheets respectively. and BNQDs. They reported the yield of BN nanosheets and BNQDs to be as high as 18.1% and 2.9%, respectively. 3.1.2. Defect Engineering Method 3.1.2. Defect Engineering Method In this method, defects are created on an atomic scale using different physical sources suchIn as this microwaves, method, defects lasers, are and created ion beams. on an Theseatomic defects scale using work different as active physical nucleation sources sites forsuch covalent as microwaves, bonding lasers, between and two ion heterobeams. nanostructures.These defects work Jung as etactive al. reported nucleation a defectsites engineeringfor covalent methodbonding tobetween synthesize two hetero the blue nanostructures. luminescent BNQDsJung et al. and reported edge-hydroxylated a defect en- BNQDsgineering (EH-BNQDs) method to synthesizefrom bulk h-BNthe blue powders luminescent using physical BNQDs energy and edge sources-hydroxylated including anBNQDs impinging (EH-BNQDs) process offrom heated bulk iron h-BN nanoparticles, powders using microwave physical irradiation,energy sources and including sonication asan illustrated impinging inprocess Figure of5. heated This process iron nanoparticles, helped disintegrate microwave bulk irradiation, BN not only and at sonication the edges butas illustrated also on the in surfaces. Figure 5. The This as-prepared process helped BNQDs disintegrate had poly-dispersed bulk BN not sizesonly rangingat the edges from 10but to also 50 nm on withthe surfaces. an average The thicknessas-prepared of 3.2BNQDs nm. A had relatively poly-dispersed low QY ofsizes 1.19 rang± 0.34%ing from was obtained10 to 50 nm from with such an BNQDs.average Thethickness QY for of EH-BNQDs 3.2 nm. A relatively was 0.7%. low The QY decrease of 1.19 ± in 0.34% QY for was the latterobtained was from due tosuch the BNQDs. reduction The of QY defect for EH sites-BNQDs by hydroxylate was 0.7%. groups The decrease [47]. in QY for the latter was due to the reduction of defect sites by hydroxylate groups [47].

FigureFigure 5.5. DefectDefect engineering route to boron nitride quantum dots dots (BNQDs) (BNQDs) and and edge edge-hydroxylated-hydrox- BNQDsylated BNQDs for bioimaging. for bioimaging. (a) Hexagonal (a) Hexagonal boron nitride,boron nitride, (b) impinging (b) impinging process process of heated of ironheated nanoparti- iron cles,nanoparticles, (c) cracks and(c) cracks physical and defectsphysical of defects h-BN, of (d )h BN-BN, QD (d) afterBN QD microwave after microwave radiation radiation and sonication and treatments,sonication treatments, (e) edge-hydroxylated (e) edge-hydroxylated BN QD (EH-BN BN QD QD), (EH and-BN (QD),f) EH-BN and (f QD) EH for-BN bioimaging. QD for bioim- Repro- aging. Reproduced from [47] with permission from The Royal Society of Chemistry. duced from [47] with permission from The Royal Society of Chemistry.

3.1.3.3.1.3. IntercalationIntercalation MethodMethod LinLin etet al.al. [[23]23] adoptedadopted the same approach to to prepare prepare BNQDs BNQDs as as they they did did previously previously forfor graphenegraphene QDs QDs [48] [48] by by exfoliating exfoliating and and disintegrating disintegrating h-BN h-BN flakes. ﬂakes. Their Their approach approach in- includedcluded potassium potassium (K) (K) intercalation intercalation between between noncovalently noncovalently bonded bonded h-BN h-BN nanosheets, nanosheets, de- de-intercalationintercalation by short by short exposure exposure of K- ofhBN K-hBN to air, to and air, disintegration and disintegration of BN ofedges BN by edges react- by reactinging K-hBN K-hBN and andethan ethanol-waterol-water under under ultrasonic ultrasonic treatment. treatment. Monolayered Monolayered BNQDs BNQDs with with an anaverage average lateral lateral size size of 10 of nm 10 were nm were obtained. obtained. These These QDs have QDs a have QY of a QY2.5% of after 2.5% the after intro- the − − introductionduction of oxidized of oxidized defect defect centers centers (e.g., BO (e.g.,2 , carbene BO2 , carbenezigzag edge, zigzag C replaced edge, C N replaced vacancy N vacancypoint) that point) caused that a blue caused-green a blue-greenluminescence. luminescence. Also, with the Also, decrease with in the the decrease thickness in and the thicknesssize of QDs, and strong size ofquantum QDs, strong confinement quantum was conﬁnement seen, which wasincreased seen, the which optical increased band gap the opticalto 6.51 bandeV in gapcomparison to 6.51 eV to in6 eV comparison of the bulk to h 6-BN eV powders. of the bulk h-BN powders. PengPeng et al al.. [49] [49] reported reported another another synthesis synthesis method method of ofBNQDs BNQDs by bythe the intercalation intercalation of ofhydrazine hydrazine hydrate hydrate (N2 (NH42) Hinto4) into the theBN BNinterlayer interlayer space space of h- ofBN h-BN powders powders using using the sol- the solvothermalvothermal technique. technique. This Thiswas followed was followed by ultrasonic by ultrasonic treatment treatment in water/ethanol in water/ethanol mixture C 2021, 7, 35 7 of 26

mixture solution to obtain BN nanosheets. After evaporating ethanol and adjusting the pH to 8 with KOH, the obtained BN nanosheets were hydrothermally treated to obtain BNQDs after filtration using a microporous membrane. These BNQDs have lateral sizes and thicknesses in the range of 1–2.6 and 0.86–1.18 nm, respectively. A fluorescence QY of 4.16% was obtained. Furthermore, the as-prepared BNQDs were reported to exhibit excitation- independent fluorescence emission. This result suggests that fluorescence was due to the same surface states rather than the particle size-dependent quantum confinement effect. Besides, as-prepared BNQDs were reported to show high thermostability, prominent anti- photobleaching capability, good salt tolerance stability, and pH-independence ability [49].

3.1.4. Other Methods Thangasamy et al. [50] reported a supercritical fluid (SCF) approach for rapid (within 40 min) and facile synthesis of blue luminescent BNQDs. These QDs exhibit an average lateral size of 5 nm and a fluorescence QY of 2.8% from bulk BN via in situ exfoliation followed by disintegration. SCF has the combination of vapor and liquid properties such as low interfacial tension, excellent wetting of surfaces, and high diffusion coefficients required for efficient exfoliation/functionalization [51–54] and was created by increasing the temperature and pressure of the solvent (DMF) above its critical levels, where distinct liquid and gas phases do not exist. Their results also showed the existence of oxygen doping and more important adsorption of DMF molecules on the BNQD surface, which was not reported previously. Angizi et al. [55] reported the mechanochemical exfoliation approach of the synthesis of hydroxylated (-OH) functionalized BNQDs in ethanol. These BNQDs exhibit an average thickness of 2 nm, an average lateral dimension of 4 nm, and a QY of 6.2%. This approach included high-energy ball milling (up to 16 h) in wet conditions (ethanol). Such a process grinds and exfoliates bulk h-BN powders to BN nanosheets because of shear and normal stress. Such a process was followed by solvothermal treatment in ethanol for 18 h at 250 ◦C to enhance -OH functionalized BNQDs. The reported spectroscopic analysis showed that the occurrence of interaction between oxygen groups of the solvent and boron sites of h-BN has led to efficient exfoliation of h-BN and -OH functionalized BNQDs, as aided by mechanical forces. The authors correlate the blue PL fluorescence to the oxygen-and carbon-based emission centers. This excitation-independent PL suggests uniform surface states on BNQDs. The authors continued to explore the possibility of other methods for the surface/edge functionalization of BNQDs [56]. They reported the use of various solvents— ethanol, ethylenediamine, and DMF for the hydroxyl, amine, and methyl functionalization of BNQDs, respectively—and studied their effects on the QD bandgap. They applied the same mechanochemical exfoliation approach to synthesize BNQDs in these solvents. The polarity of these solvents ranged from 4.3 (ethanol) to 6.4 (DMF). It was demonstrated that and the use of solvents with higher polarities yielded better exfoliation efficiencies. The authors reported the bandgap modification of BNQDs using chemical molecule interaction with dangling bonds. The splitting of the main transition bandgap of bulk BN from 5.9 eV to two absorption edges for hydroxyl (2.3–3.6 eV), methyl (3.2–4.2 eV), and amine (3.1–4 eV) was evident from UV-Vis absorption and PL plots. They correlate these absorptions to delocalizing of the Px and Pz orbitals due to edge/surface functional groups (hydroxyl, methyl, and amine) as explained earlier. The chemical motifs provided by the solvents at high temperature and pressure interacted with the boron sites on BNQDs, resulting in the decrease in optical bandgap due to different surface functionalization. Cai et al. [57] reported the turbulence-induced disintegration method by the use of a food blender to prepare BNQDs from bulk BN crystals in isopropanol (IPA) and de-ionized water. The high-shear turbulence flow generated by the blender would break down the interlayer van der Waals forces between h-BN layers, and disintegrate them into QDs with an average lateral size of 2.13 ± 0.25 nm and thicknesses of 0.6 to 4 nm while using IPA. These average particle sizes are about double when water was used in the blending process. They introduced the formation mechanism of BNQDs based on fluid dynamics. In the case C 2021, 7, 35 8 of 26

of IPA, the calculated Reynolds number was 1.37 × 106 and the mean turbulent shear rate was 2.34 × 104 s−1, both above the threshold value of 104 for both parameters required for the exfoliation of nanoflakes to QDs. The particle yield was reported to be 4.02%. Radhakrishnan et al. [58] reported the synthesis of fluorinated BNQDs (FBNQDs) by the hydrothermal fluorination of BN nanosheets using DMF and Nafion. Nafion acted as the source for highly reactive fluorine-free radicals which reacted with BN nanosheets to break it down to form FBNQDs. They reported the fluorination using different solvents, DMF and formamide, where the latter gave higher vapor pressure, which was considered an important parameter for fluorination. This heterostructure of FBNQDs was reported to decrease the optical band gap of h-BN and have broad-spectrum light absorption with luminescence in the blue-green region and a high QY of 19.5%. Duong et al. [59] reported the synthesis of BNQDs using the cryogenic-mediated exfoliation of h-BN powders, as assisted with ultra-sonication and filtration. As reported, the bulk powders were soaked in liquid nitrogen for an hour and thermally shocked by introducing a mixture of IPA and water (1:1 ratio). This process created the peeling of h-BN flakes and formed cracks and defects in all dimensions. Further, ultrasonication induced full liquid-phase exfoliation. After centrifuging and filtration, the obtained BNQDs had narrow size distribution with sizes as small as 3 nm. These BNQDs contained fluorescent point defects that acted as photostable single-photon emitters which can be used as fluorescent markers in bioimaging applications and are suitable for far-field super-resolution fluorescence nanoscopy. In summary, the top-down methods are based on reducing the bulk h-BN powders to nanometer sizes using different approaches. Poor material yield is problematic for mass production, but the crystallinity is excellent. The dangling bonds at the edges of BNQDs and surface defects will allow for the functionalization of various molecules, which makes BNQDs prospective for fluorescent and optoelectronic applications.

3.2. Bottom-Up Approaches The top-down approach used for the synthesis of BNQDs requires lengthy processing of bulk h-BN powders and involves some complicated post-process treatment such as centrifugation and ﬁltration. To tackle some of these problems, the bottom-up method could be utilized where QDs can be synthesized by using atomic, ionic, or molecular starting materials. Various groups have utilized such bottom-up approaches to synthesize BN nanostructures which included hydrothermal [60–62] and chemical vapor deposition (CVD) methods [63]. Some of these bottom-up approaches for the synthesis of BNQDs will be discussed in this section.

3.2.1. Hydrothermal Methods Liu et al. [64] were the first group to adopt the bottom-up approach to synthesize hydroxyl-and amino-functionalized BNQDs. The BNQDs exhibit a lattice constant of 0.22 nm, an average thickness of 0.91 nm, a particle size of 2.56 to 3.20 nm within reaction times of 9 to 15 h, and a high QY of 18.3%. The authors hydrothermally treated the mixture of boric acid and ammonia solution in one pot at 200 ◦C for 12 h. The mechanism for the hydrothermal reaction and fluorescence quenching was also modeled theoretically using density functional theory (DFT). The UV absorption at a wavelength of 229 nm was reported which matched the DFT. Two sharp excitation peaks at 230 and 295 nm were detected, which are different from those previously reported [23,39] and were attributed to the difference in the chemical environment. As shown in Figure6, excitation-independent fluorescence was reported suggesting that the PL properties are due to specific surface states rather than the particle sizes of the QDs. A similar method for BNQDs synthesis was reported later with slight or no modification [65–69]. C 2021, 7, 35 9 of 26 C 2021, 7, x FOR PEER REVIEW 9 of 26

Figure 6. (a) UV-Vis absorption; inset: photographs of the BNQDs dispersed in aqueous solution underFigure 6. visible (a) UV (left)-Vis absorption; and λ = 365inset: nm photographs UV (right) of the lights. BNQDs (b )dispersed Photoluminescence in aqueous solution excitation (PLE) under visible (left) and λ = 365 nm UV (right) lights. (b) Photoluminescence excitation (PLE) spec- spectrum of the prepared BNQDs and excitation-independent PL emission behavior of BNQDs trum of the prepared BNQDs and excitation-independent PL emission behavior of BNQDs excited excitedat different at different wavelengths wavelengths from λ = 230 from–370λ nm.= 230–370 Reproduced nm. from Reproduced [64] with frompermission [64] with from permission 2016 from 2016Wiley Wiley-VCH-VCH Verlag Verlag GmbH GmbH& Co. KGaA & Co. (Weinheim KGaA (Weinheim,, Germany). Germany).

Researchers are are finding finding alternative alternative sources sources of nitrogen of nitrogen for the for bottom the bottom-up-up synthesis synthesis ofof BNQDsBNQDs because because ammonia ammonia solution solutionss are arevolatile, volatile, pollution, pollution, irritative irritative,, and harmful and harmfulto to humanhuman tissues. tissues. Huo Huo et etal. al.[70] [70 reported] reported the use the of use melamine of melamine as the asnitrogen the nitrogen source and source and boricboric acid as as the the boron boron source source for forthe thesynthesis synthesis of BNQDs of BNQDs by the byeconomical the economical hydrothermal hydrothermal method.method. The The as- as-growngrown BNQDs BNQDs have an have average an average lateral size lateral of 3 nm, size and of an 3 average nm, and thick- an average thicknessness of 0.7 ofnm. 0.7 A nm.similar A aqueous similar aqueoussynthesis method synthesis of BNQDs method was of BNQDsreported by was others reported by [71–73]. Later, Liu et al. [74] used the microwave method to synthesize BNQDs with a QY others [71–73]. Later, Liu et al. [74] used the microwave method to synthesize BNQDs with 10.31% by dissolving boric acid and melamine in ultra-pure water and placing them in a a QY 10.31% by dissolving boric acid and melamine in ultra-pureo water and placing them microwave oven set at a frequency of 1000 Hz for 70 min at 120 C. The authors◦ also re- inported a microwave for the synthesis oven setof sulfur at a frequency S-regulated of BNQDs 1000 Hz using for the 70 minsame at microwave 120 C. The method authors also reportedand same for precursors the synthesis (boric ofacid sulfur and S-regulatedmelamine) [75] BNQDs. They usingreported the the same use microwaveof thiourea method andand L same-cysteine precursors as the sulfur (boric precursors, acid and which melamine) yielded [S75-regulated]. They reportedBNQDs with the different use of thiourea andoptoelectronic, L-cysteine electro as thechemiluminescence sulfur precursors, (ECL), which and yielded electro S-regulated-optical properties. BNQDs The with use different optoelectronic,of different sulfur electrochemiluminescence precursors introduced different (ECL), functional and electro-optical groups on the properties. surfaces of The use ofBNQDs, different which sulfur would precursors result in electron introduced transitions different, as verified functional by zeta groups potentials, on UV the-Vis surfaces of BNQDs,absorption, which and FTIR would spectroscopy result in electron measurements. transitions, Interestingly, as verified the QY bys zeta for S potentials,-regulated UV-Vis absorption,BNQDs prepared and FTIRwith spectroscopythiourea and L measurements.-cysteine were 8.9% Interestingly, and 2.0%, respectively, the QYs for both S-regulated BNQDslower than prepared for pristine with BNQDs thiourea (10.31%) and L-cysteine due to additional were 8.9% nonradiative and 2.0%, respectively,pathways intro- both lower duced by the sulfur dopants. Besides, the authors reported that the PL and ECL behaviors than for pristine BNQDs (10.31%) due to additional nonradiative pathways introduced by (emission peak) strongly depend on the surface states and functional group of the S-reg- the sulfur dopants. Besides, the authors reported that the PL and ECL behaviors (emission ulated BNQDs. Recently, Yasno et al. [76] reported the short-time microwave method in peak)the synt stronglyhesis of dependBN nanoparticles on the surface with a states disc-like and morphology functional groupusing boric of the acid S-regulated and mel- BNQDs. Recently,amine as raw Yasno materials et al. [ 76using] reported a 2.4 GHz the microwave short-time field microwave for 90 min method. in the synthesis of BN nanoparticlesBesides melamine, with a disc-like researchers morphology have attempted using to boric use urea acid dissolved and melamine in ammonia as raw so- materials usinglution aas 2.4 the GHz nitrogen microwave source. For field example, for 90 min.Jerome et al. have synthesized BNQDs by the hydrothermalBesides melamine,method by using researchers such a nitrogen have attempted source and to boric use acid urea dissolved dissolved in etha- in ammonia solutionnol/water as as thethe nitrogenboron source source. [77]. ForThe example,same boron Jerome and nitrogen et al. have source synthesized were used BNQDsby by theother hydrothermal research groups method [78] forby BNQDs using synthesis. such a nitrogen Another hydrothermal source and boricmethod acid was dissolved re- in ethanol/waterported by Yao et as al. the [79] boron for the source fabrication [77]. Theof BNQDs same boronwith a andhigh nitrogenQY of 32.3%. source These were used byBNQDs other have research an average groups size [ 78of ]4 fornm BNQDsand were synthesis. made by using Another boric hydrothermalacid and ammonium method was reportedhydroxide by as Yaostarting et al. materials. [79] for Besi thedes, fabrication Kong et al. of [80] BNQDs have reported with a high on the QY hydrother- of 32.3%. These mal synthesis of BNQDs using amino acids (tryptophan) as a nitrogen source and boric BNQDs have an average size of 4 nm and were made by using boric acid and ammonium acid as a boron source. As compared to other nitrogen precursors, amino acids are high hydroxidein nitrogen ascontent, starting good materials. in biocompatibility, Besides, Kong and etlow al. in [80 price.] have The reported as-prepared on the BNQDs hydrothermal synthesisexhibit spherical of BNQDs shapes using with aminoan average acids diameter (tryptophan) of 3.05 ± as 0.57 a nitrogen nm, and a source QY of and8.45%. boric acid as a boron source. As compared to other nitrogen precursors, amino acids are high in nitrogen content, good in biocompatibility, and low in price. The as-prepared BNQDs exhibit spherical shapes with an average diameter of 3.05 ± 0.57 nm, and a QY of 8.45%. These BNQDs initiated a strong absorption peak at 280 nm, and always emit the same PL peaks when excited with different excitation wavelengths ranging from 280 to 400 nm (excitation independent). The authors have explored the use of other amino acids for the C 2021, 7, 35 10 of 26

synthesis of BNQDs. All these BNQDs exhibit strong blue ﬂuorescence but their UV-Vis absorption peaks and PL emission spectra are varying. These results suggest the role of different surface states were formed on BNQDs, which are corresponding to varying optical and spectroscopic properties.

3.2.2. Carbothermal Reduction Reaction Zhao et al. [81] have reported a different bottom-up approach towards the mass production of BNQDs with a QY as high as 18.2%. This was based on a biomass-directed strategy from steam-exploded rice straw lignin (SERSL). This work was inspired by a similar approach used for the growth of BN nanosheets based on carbothermal reduction [82]. The BNQD formation mechanism can be represented by the following reaction equations:

∆ 2B2O3(S) → 2B2O2(g) + O2(g) (1)

∆ B2O2(g) + 2C(s) + N2(g) → 2BN(s) + 2CO(g) (2)

As shown in Figure7, the B 2O3 solid powders were converted into the B2O2 and oxygen vapors at temperatures above 1000 ◦C. The carbon atom at the defect sites of SERSL will then be oxidized by oxygen. The B2O2 vapors will then react with the activated carbon atoms near the dangling bonds and gradually replace the graphite’s honeycomb lattice with B and N to form BN nanosheets. To prevent the formation of BCN/BN complexes during the reaction, the ratio of B2O3 to SERSL was optimized in the range 5:1 to 7:1. The optimum growth time was over 4 h, at temperatures above 1200 ◦C. Unlike another C 2021, 7, x FOR PEER REVIEW bottom-up approach of BNQDs synthesis, this method yielded BNQDs with11 excitation- of 27 dependent emission.

Figure 7. (a) Schematic for the synthesisFigure 7. (a of) Schematic BN half-layer for the nanosheets synthesis of from BN half steam-exploded-layer nanosheets rice from straw steam lignin-exploded (SERSL). rice SERSL straw lignin (SERSL). SERSL covered the surface of B2O3 powder inside the crucible and was covered the surface of B O powder inside the crucible and was heated at 1300 ◦C in nitrogen atmosphere. (b) The optical 2 3 heated at 1300 0C in nitrogen atmosphere. (b) The optical images before (left) and after (right) reac- images before (left) and after (right)tion. The reaction. BN half The-layer BN nanosheets half-layer were nanosheets sonicated were in dimethylformamide sonicated in dimethylformamide (DMF) to obtain (DMF) to obtain BNQDs. (c) The SEM imageBNQDs. of BNQDs. (c) The SEM (d,e image) The TEMof BNQDs. images (d,e of) The SERSL. TEM The images low-resolution of SERSL. The (f low) and-resolution high-resolution (f) (g) TEM images of BNQDs. Reproducedand high-resolution from [ 81(g)] TEM with images permission of BNQDs. from Reproduced 2019 Wiley-VCH from [81] Verlag with permission GmbH & from Co. 2019 KGaA (Weinheim, Germany). Wiley-VCH Verlag GmbH & Co. KGaA (Weinheim, Germany).

C 2021, 7, 35 11 of 26

3.2.3. Other Methods Ahmad et al. [83] reported the fabrication of large BN particles with a beads shape and an average lateral size of 50–200 nm. This was performed in a 25 min growth duration at 1200 ◦C using carbon-free precursors. The reported precursors comprised of nanosized B, magnesium oxide, and Maghemite (γ-Fe2O3) mixed in the ratio 2:1:1. The precursors were loaded in an alumina boat and heated in a horizontal tube furnace in ammonia gas as the nitrogen source for the formation of BN particles.

3.3. Combined Method Li et al. [84] used the combined bottom-up and top-down approaches. Firstly, boron trioxide (B2O3) and urea (CO(NH2)2) were mixed in a molar ratio of 1:10 in methanol to form a transparent solution under stirring. The white crystalline precursor was obtained after 24 h, which was further grounded and heated at high temperatures under ammonia (NH3) to produce ultrathin BN nanosheets. Secondly, the obtained BN nanosheets were treated ultrasonically and solvo-thermally in DMF to obtain BNQDs in a top-down manner. These BNQDs exhibit blue-green PL with a QY of 10%. The as-prepared BNQDs were reported to have oxygenous functional groups on the surface that had a strong inﬂuence on their PL characteristic.

4. Applications of BNQDs 4.1. Biological Application Metal-free quantum dots such as carbon and BN quantum dots (CQDs, BNQDs) have multiple advantages over conventional semiconductor quantum dots. It is believed that CQDs and BNQDs exhibit higher photophysical, chemical, and photochemical stability. Besides, CQDs and BNQDs are non-toxic and more biocompatible [39,85]. It was reported that BNQDs are more biocompatible than carbon QDs as the latter would undesirably interact with biomolecules (DNA, proteins, or enzymes) that would compromise the intrinsic properties of these biomolecules [70]. Besides, BNQDs are more tolerant of different pH environments in the range of 2–12, with no signiﬁcant effect on the ﬂuorescence of BNQDs [40]. The cytotoxicity and biocompatibility of BNQDs were studied; excellent biocompatibility was shown [23,36,39,40,44,45,47,57,81].

4.1.1. Bioimaging Organic dyes are popular for bioimaging applications for their relatively low cost and good biocompatibility. However, organic dyes are limited by photobleaching where their fluorescence brightness is degrading under the prolonged irradiation of excitation laser light. The use of nanoparticles such as BNQDs with strong fluorescence, photostable, nonblinking, and nonbleaching would be a good alternative for cellular imaging probes. Lin et al. have reported on the use of BNQDs for the imaging of MDCK-II (Madin- Darby canine kidney) cells by confocal microscopy in the FITC (fluorescein isothiocyanate) mode as shown in Figure8. The BNQDs were internalized into cells but did not pene- trate the cell nuclei [23]. A similar study was performed on different types of cells by others [39,40,44,46,57,81]. Jung et al. [47] studied the internalization of edge-hydroxylated BNQDs (EH-BNQDs) by MCF-7 breast cancer cells and PC-3 prostate cancer cells. Blue fluorescence in the perinuclear region was demonstrated. The locations of EH-BNQDs inside the cells were compared with the locations of intracellular endosomes that were labeled with Lysotracker (red dyes), which resulted in a purple color due to the overlapping of blue and red fluorescence under confocal microscope proving the successful endocytosis of QDs. A similar study was performed on HeLa cells [40]. C 2021, 7, 35 12 of 26 C 2021, 7, x FOR PEER REVIEW 12 of 26

FigureFigure 8. 8.Confocal Confocal microscopy microscopy images images of of mammalian mammalian cells. cells. ( a(a)) Agglomerated Agglomerated BNQDs BNQDs surrounding surrounding eacheach nucleus nucleus (cells (cells are are stained stained by by BNQDs BNQDs only). only) (b. ()b Individual) Individual nucleus nucleus stained stained blue blue with with DAPI DAP (4’,I (4’, 6- diamidino-2-phenylindole).6-diamidino-2-phenylindole) (c ). ( BNQDsc) BNQDs with with green green luminescence luminescence surrounding surrounding the the nuclei. nuclei (.d ()d The) overlayThe overlay image image of cells of stainedcells stained with DAPIwith DAPI and BNQDs. and BNQDs. Reproduced Reproduced from [from23] with [23] permission with permission from 2013from Wiley-VCH 2013 Wiley Verlag-VCH Verlag GmbH GmbH & Co. KGaA& Co. (Weinheim,KGaA (Weinheim Germany)., Germany).

ThangasamyThangasamy etet al. al. [ 50[50]] reported reported the the imaging imaging of of bacterial bacterial cells cells (Gram-negative (Gram-negative and and Gram-positive)Gram-positive) using using BNQDs. BNQDs. Both Both types types of of cells cells were were stained stained with with 100 100µ µgg/mL/mL of of BNQDs.BNQDs. AccordingAccording toto imagesimages from epi epi-fluorescence-fluorescence microscopy microscopy and and confocal confocal microscopy, microscopy, the the in- internalizationternalization of of BNQDs BNQDs occurred occurred for for Gram Gram-negative-negative cells cells but butnot notfor G forram Gram-positive-positive cells. cells.It is to It be is tonoted benoted that the that Gram the- Gram-negativenegative cells possess cells possess an extra an lipid extra layer lipid outside layer outsidethe pep- thetidoglycan peptidoglycan layer, which layer, whichGram-positive Gram-positive cells lack. cells This lack. interesting This interesting interaction interaction of BNQDs of BNQDswith the with outer the lipid outer layer lipid of layerGram of-negative Gram-negative cells can cells be used can befor usedGram for-negative Gram-negative cell iden- celltification identification and selective and selective staining staining from environmental from environmentallyly mixed bacterial mixed bacterial samples. samples. BNQDsBNQDs exhibit exhibit a a long long fluorescent fluorescent lifetime lifetime (FLT) (FLT) and and a a unique unique excitation-dependent excitation-dependent emissionemission property. property. Therefore, Therefore, BNQDs BNQDs can can be be a a good good candidate candidate for for multiplex multiplex fluorescence fluorescence imagingimaging when when used used in in conjunction conjunction with with organic organic dyes. dyes. The The combined combined fluorescent fluorescent signals signals cancan bebe resolvedresolved spectrally and and temporally temporally based based on on the the emission emission wavelength wavelength and and the FLT, the FLT,respectively. respectively. For multiple For multiple probes probes having having overlapping overlapping emission emission wavelengths wavelengths but different but differentfluorescent fluorescent lifetimes, lifetimes, both spectral both spectraland temporal and temporal imaging imaging can be combined. can be combined. Dehghani De- et hghanial. [45] et reported al. [45] reported the use theof usetwo of-photon two-photon excitation excitation features features of BNQD of BNQDss for formultiplex multiplex cell cellimaging. imaging. The The fluorescence fluorescence lifetime lifetime imaging imaging microscopy microscopy (FLIM) (FLIM) was usedused toto accuratelyaccurately distinguishdistinguish thethe emission emission signals signals ofof BNQDs BNQDs with with longer longer FLT FLT in in comparison comparison to to relatively relatively shortershorter cell cell autofluorescence autofluorescence and and those those from from organic organic dyes. dyes. They They reported reported the the use use of of two two nuclearnuclear stains:stains: DAPI (4’, 6 6-diamidino-2-phenylindole)-diamidino-2-phenylindole) with with blue blue and and Sytox Sytox with with green green flu- fluorescence,orescence, both both with with shorter shorter fluorescent fluorescent decay decay lifetimes, to achieveachieve thethe temporallytemporally and and spectrallyspectrally resolved resolved fluorescent fluorescent signals signals as as shown shown in in Figure Figur9e. 9. They They co-stained co-stained the the RL-14 RL-14 cells with b-BNQDs/DAPI (b-BNQDs: blue-emitting BNQDs) and g-BNQD/Sytox green cells with b-BNQDs/DAPI (b-BNQDs: blue-emitting BNQDs) and g-BNQD/Sytox green (g-BNQDs: green-emitting BNQDs) separately. The emission of BNQDs and dyes was (g-BNQDs: green-emitting BNQDs) separately. The emission of BNQDs and dyes was not not separated by spectral analysis of the image, but this issue was resolved by using the separated by spectral analysis of the image, but this issue was resolved by using the dif- difference in fluorescence lifetimes of BNQDs and dyes using the FLIM technique. ference in fluorescence lifetimes of BNQDs and dyes using the FLIM technique. C 2021, 7, 35 13 of 26 C 2021, 7, x FOR PEER REVIEW 13 of 26

FigureFigure 9. 9. SpectralSpectral imagingimaging (A–C,G–I) and fluorescence fluorescence lifetime lifetime imaging imaging (D (D––FF,J,–J–LL) )of of RL RL-14-14 cells cells separatelyseparately treated treated with with b-BNQDs/DAPI b-BNQDs/DAPI and g-BNQDs/Sytoxg-BNQDs/Sytox green. green. No No emission emission was was detected detected in in panelspanels B B and and G G co-stained co-stained with with QDs QDs and and stains. stains. Fluorescence Fluorescence signals signals from from b-BNQDs b-BNQDs and and g-BNQDs g- wereBNQDs resolved were usingresolved spectrally using spectrally matched detection matched rangesdetection as seen ranges in panelsas seen A in and panels H. The A and fluorescence H. The lifetimesfluorescence were lifetimes obtained were by bi-exponential obtained by b fittingi-exponential and mapped fitting to and a false-color mapped scaleto a false from-color 0 ns (blue) scale to >4from ns (red).0 ns (blue) The resulting to >4 ns false-color(red). The resulting shows the false difference-color shows between the signalsdifference originating between from signals BNQDs originating from BNQDs (panels D and J) and organic dyes (E,K). Reproduced from [45] with permis- (panels D and J) and organic dyes (E,K). Reproduced from [45] with permission from Copyright © sion from Copyright © 2018 American Chemical Society. 2018 American Chemical Society.

4.1.2.4.1.2. BiosensingBiosensing AngiziAngizi et et al. al. reported reported the the use use of of BNQDs BNQDs for for the the sensitive sensitive and and selective selective detection detection of vi- of taminvitamin C (VC) C (VC) [55]. [55] This. This was performedwas performed by amodified by a modified screen-printed screen-printed gold electrode gold electrode (GSPE) with(GSPE) functionalized with functionalized BNQDs.BNQDs BNQDs. are BNQDs superior are to superiorcarbon nanostructures to carbon nanostructures in terms of the in linearterms range of the oflinear sensing range and of thesensing detection and the limit. detection For example, limit. For a detection example, limit a detection of 0.45 limitµM wasof 0.45 recorded µM was in recorded Angizi et in al.’s Angizi work et while al.’s work using while the BNQDs/Au using the BNQDs/Au system, significantly system, sig- lowernificantly than lower 3 µMfor than the 3cellulose µM for the acetate/graphite cellulose acetate/graphite system. The highsystem. electrocatalytic The high electrocat- activity foralytic electro-oxidation activity for electro of VC-oxidation was reported of VC and was the reported oxidation and occurred the oxidation at the occurred surface of at the the BNsurface QD/Au of the electrode BN QD/Au at a lower electrode positive at a potential lower positive than the potential GSPE. Later, than Jeromethe GSPE. et al. Later, [77] reportedJerome et a al. rapid [77] response reported sensora rapid (1.8 response sec) to sensor detect (1.8 vitamin sec) Cto (ascorbicdetect vitamin acid, AA)C (ascorbic using BNQDsacid, AA) and using polyluminol BNQDs and (Plu) polyluminol coated glassy (Plu) carbon coated electrode glassy (GCE).carbon As-preparedelectrode (GCE). hybrid As- Plu/BNQDsprepared hybrid coated Plu/BNQDs GCE was reportedcoated GCE to showwas reported improved to electrocatalyticshow improved activity electrocatal for AAytic oxidationactivity for at AA 0.2 oxidation V using the at 0.2 amperometry V using the detectionamperometry method. detection Further, method. they Further, carried outthey thecarried interference out the effectsinterference in the presenceeffects in ofthe uric presence acid, dopamine, of uric acid, and dopamine, glucose which and didglucose not respondwhich did to not the respond Plu/BNQDs/GCE to the Plu/BNQDs/GCE sensors, indicating sensors, good indicating selectivity good of selec thetivity sensor of for the AA.sensor In anotherfor AA. study,In another Kong study, et al. Kong [80] reported et al. [80] a reported fluorescent a fluorescent “on-off-on” “on sensor-off-on” to detectsensor 3+ AAto detect with FeAA withas a medium.Fe3+ as a medium. The fluorescence The fluorescence of BNQDs of BNQDs was quenched was quenched due to thedue inner to the 3+ filterinner effect filter betweeneffect between Fe andFe3+ and BNQDs. BNQDs. Then, Then, the the fluorescence fluorescence of of BNQDs BNQDs was was restored restored 3+ withwith increasing increasing AA AA due due to to the the oxidation-reduction oxidation-reduction between between Fe Fe3+and and AA. AA. This This technique technique enabledenabled the the detection detection of of AA AA in in the the concentration concentration range range 1–100 1–100µ µMM with with a a detection detection limit limit of of 0.08330.0833µ µMM under under the the optimized optimized experimental experimental conditions conditions at at pH pH 6. 6. The The sensing sensing selectivity selectivity 3+ ofof BNQDs BNQDs to to Fe Fe3+ ionsions waswas confirmedconfirmed byby comparingcomparing datadata obtainedobtained byby using using other other metal metal ions.ions. Other Other metal metal ions ions had had a a negligible negligible effect effect on on quenching quenching BNQD BNQD fluorescence. fluorescence. The The selec- se- tivitylectivity of BNQDs of BNQDs on AAon AA sensing sensing was was also also verified verified by replacingby replacing AA AA with with other other interfering interfer- substances,ing substances such, such as uric as acid,uric acid, dopamine, dopamine, and glucose. and glucose. These These substances substances have shownhave shown little recoveringlittle recovering effect oneffect the fluorescenceon the fluorescence of BNQDs. of BNQDs. All these All data these support data thesupport fact that the BNQDsfact that 3+ exhibitBNQDs high exhibit selectivity high selectivity for Fe and for Fe AA.3+ and AA. DehghaniDehghani et et al. al. [45 [45]] reported reported the the use use of of g-BNQDs g-BNQDs passivated passivated with with polyethylene polyethylene glycol, gly- for the biosensing of intracellular pH variations and their distribution inside cells. This was col, for the biosensing of intracellular pH variations and their distribution inside cells. performed by confocal microscopy and FLIM techniques. It is to be noted that the increase This was performed by confocal microscopy and FLIM techniques. It is to be noted that in incubation time will cause the micro-environment of the cells to become more acidic due the increase in incubation time will cause the micro-environment of the cells to become to the formation of more endosomes. The acidic environment would result in a shorter FLT. C 2021, 7, 35 14 of 26

The authors reported the translocation and accumulation of g-BNQDs at endosomes over 4 h, which resulted in the change of cell morphology. The quenching of photoluminescence of BNQDs in an acidic medium can be utilized for detecting changes in metabolic activity inside human cells. Owing to the Warburg effect [86], lactate production in cancer cells during glycolysis is high. Therefore, the extracellular pH of tumor cells is often acidic, leading to a high extracellular acidification rate (ECAR) compared to benign cells. Rad- hakrishnan et al. [58] reported the use of the glycolytic inhibitor, 3-bromopyruvate(3-BP), to suppress ECAR. This was demonstrated by monitoring the increase in fluorescence from FBNQDs, as a method to detect the glycolytic activity in cancer cells. In this case, the green fluorescence channel of flow cytometry was used to detect the enhanced fluorescent signal after introducing 3-BP to cancer cells. There was no considerable change in fluorescence in benign cells due to a less active glycolytic pathway. In another study, Yola et al. [41] reported a stable, repeatable, reusable, and selective imprinted biosensor based on BNQDs for cardiac Troponin-I (cTnI) detection in plasma samples. The cTnI is widely used for the diagnosis of acute myocardial infarction (AMI) diseases. Later, the same group [87] reported the preparation of a novel voltammetry sensor for the detection of various organophosphate pesticides in water samples based on BNQDs on graphene oxide. Owing to the strong inner filter effect between 2,4,6-trinitrophenol (TNP) and BNQDs, the fluorescence intensity of BNQDs can be reduced upon quenching by TNP. By this mean, TNP can be selectively and sensitively detected in the concentration range of 0.25−200 µM, with a detection limit of 0.14 µM when BNQDs are used as fluorescence probes [49]. There- fore, BNQDs can be used as the turn-off sensors for the rapid detection of TNP from natural water sources without tedious pretreatment processes. Similarly, Peng et al. [67] reported a “switch on” nanosensor for sensitive assay of glutathione (GSH). GSH can regulate the inner filter effect of MnO2 nanosheets (NS) on BNQDs. Owing to the superior light absorption capability of redox-able MnO2NS centered at 380 nm, the fluorescence of BNQDs (with a maximum emission wavelength 380 nm) can be selectively quenched. For an optimal inner filter effect to take place between the two substances, the maximum absorption of one should overlap the maximum emission of the other. However, the introduction of GSH will help to recover the fluorescence of BNQDs, dependent on the concentration of GSH, by weakening the inner filter effect as initiated by the decomposition of MnO2 to 2+ Mn . This MnO2NS/BNQDs nanoprobe was reported to exhibit good selectivity on GSH detection in the range of 0.5–250 µM with a detection limit of 160 nm in human plasma samples. They reported the use of other interference substances such as histidine, glutamic acid, cysteine, vitamin C, sodium chloride, potassium chloride, etc., instead of GSH in the same concentration which did not show much change in fluorescence recovery, thus suggesting the need for the separation of interfering reductants from real samples. The sensing principle is shown in Figure 10. In addition to the inner filter effect, fluorescence resonance energy transfer (FRET) on BNQDs also offers interesting application. For example, FRET between BNQDs and gold nanoparticles (AuNPs) has enabled the fabrication of rapid, label-free, and highly sensitive fluorescence-based biosensors [88]. Such sensors were used to detect acetylcholinesterase (AChE), a key enzyme in the biological nerve conduction system, whose activities have been connected to several diseases such as muscular paralysis, convulsions, and bronchial constriction. As reported, AChE can hydrolyze acetylthiocholine (ATCh) to generate thiocholine (TCh) whose thiol (-SH) group can reduce chloroauric acid (HAuCl4) to AuNPs. The formation of TCh-BNQDs/AuNPs aggregates can quench the fluorescence of BNQDs via FRET. An inhibitor, paraoxon, was used to lower the activity of AChE and decreased the fluorescence quenching of BNQDs. In this way, a simple ‘one-pot’ FRET-based biosensor was reported to be constructed to assess AChE activity and its inhibitor by analyzing the fluorescence of BNQDs. C 2021, 7, 35 15 of 26 C 2021, 7, x FOR PEER REVIEW 15 of 26

FigureFigure 10.10.Schematic Schematic principle principle of of glutathione glutathione (GSH) (GSH regulated) regulated IFE IFE (inner (inner ﬁlter filter effect) effect) of of MnO MnO2NS2NS on BNQDson BNQDs for sensing.for sensing. The The observed observed ﬂuorescence fluorescence of BNQDs of BNQDs is quenched is quenched with with the MnOthe MnO2NS2NS presence pres- andence later and recoveredlater recovered with GSH with application.GSH application. Reproduced Reproduced from [ 67from] with [67] permission with permission from copyright from copy-© 2019right American © 2019 American Chemical Chemical Society. Society.

4.2.4.2. ChemicalChemical SensingSensing LiuLiu etet al.al. reported reported the the use use of of BNQDs BNQDs as as fluorescence fluorescence probes probes to todetermine determine the the concen- con- centrationtration of ferrous of ferrous (Fe (Fe2+) 2+and) and ferric ferric (Fe3+ (Fe) ions3+) ions with with high high selectivity, selectivity, sensitivity, sensitivity, and and a low a lowdetection detection limit limit [64]. [ 64For]. Forthis thisstudy, study, the change the change in fluorescence in fluorescence intensity intensity with with various various con- concentrationscentrations of ofions ions (10 (10–600–600 µM)µM) was was investigated investigated at at the the excitation excitation wavelength wavelength of 310 nm.nm. GradualGradual quenchingquenching with with increasing increasing concentration concentration resulting resulting in lowerin lower fluorescence fluorescence intensity inten- duesity todue electron to electron transfer transfer fromBNQDs from BNQDs to Fe2+ toand Fe Fe2+ 3+andions Fe3+ was ions reported was reported as seen inas Figureseen in 11Figure. The PL11. quenchingThe PL quenching efficiency efficiency at excitation at excitation wavelength wavelength 310 nm at various310 nm ionsat various concentra- ions tionconcentration showed two showed linear trendstwo linear (one trends for a concentration (one for a concentration range of 0–250 rangeµM, of and 0–250 another µM, oneand foranother the range one for of 300–650 the rangeµM) of for300 both–650 typesµM) for of ions.both Thoughtypes of theions. total Though concentration the total ofconcen- both ionstration can of be both determined ions can regardless be determined of the regardless oxidation of states, the oxidation detecting states, individual detecting ions from indi- thevidual mixture ions isfrom still the challenging mixture usingis still thischallenging procedure. using A similar this procedure. study for FeA 3+similardetection study was for reportedFe3+ detection by Kong was et reported al. [80]. by Kong et al. [80]. Nickel (Ni2+) ions were reported to be detected in natural water samples via quenching of the fluorescence of BNQDs [79]. Liu et al. reported that the BNQDs prepared in N- methyl-2-pyrrolidone (NMP) with green fluorescence were much more competitive than those prepared in ethanol and DMF with blue fluorescence as a competitive fluorescent sensing probe for label-free specific detection of Fe3+ or Cu2+ [40]. C 2021, 7, 35 16 of 26 C 2021, 7, x FOR PEER REVIEW 16 of 26

FigureFigure 11.11. BNQDsBNQDs-based-based sensing sensing of of(a) (Fea)3+ Fe and3+ (andb) Fe (b2+) by Fe measuring2+ by measuring the PL intensity the PL intensityat at λλexex = 310 nm, with ion ion concentrations concentrations of of 10, 10, 20, 20, 30, 30, 40, 40, 50, 50, 60, 60, 70, 70, 80, 80, 90, 90, 100, 100, 150, 150, 2 200,00, 250, 250, 300, 300, 400, 400, 500,500, andand 600600 µμm,m, and the linear relationship relationship between between the the PL PL-quenching-quenching efficiency efﬁciency (I/ (I0/) Iat0) at λex = 310λex nm= 310 and nm the and concentration the concentration of (c) of Fe 3+(c)and Fe3+ (andd) Fe (d2+) .Fe Reproduced2+. Reproduced from from [64] [64] withwith permission permission from copyrightfrom copyright © 2016 © Wiley-VCH 2016 Wiley Verlag-VCH Verlag GmbH GmbH & Co. KGaA& Co. KGaA (Weinheim, (Weinheim Germany)., Germany).

HuoNickel et al.(Ni [702+)] ions reported were the reported strongest to be fluorescence detected in quenching natural water phenomenon samples via for BNQDsquench- dispersioning of the fluorescence with Fe3+ ions of BNQDs due to [79] the. aggregationLiu et al. reported of BNQDs. that the There BNQDs was prepared no obvious in N- fluorescencemethyl-2-pyrrolidone emission quenching(NMP) with due green to other fluorescence metal ions were of themuch same more concentration. competitive Thisthan makesthose prepared BNQDs unique in ethanol as a and selective DMF fluorescencewith blue fluorescence sensing agent as a for competitive Fe3+ ions. fluorescent In addition tosensing Fe3+ ion probe detection for label in DI-free (deionized) specific detection water, Dehghani of Fe3+ or et Cu al.2+ [ 45[40]] explored. the intracellular Fe3+ ionsHuo detectionet al. [70] with reported the help the of strongest non-passivated fluorescence blue-emitting quenching BNQDs phenomenon (b-BNQDs). for TheBNQDs intercellular dispersion detection with Fe3+ limit ions for due Fe to3+ theions aggregation was reported of BN asQDs. 20.6 nm,There which was no is amongobvious thefluorescence highest sensitivity emission reportedquenching to date.due to Further, other metal Han ions et al. of [89 the] reported same concentration. the fluorescence This quenchingmakes BNQDs of BNQDs unique using as a Feselective3+ and Cufluorescence2+ ions. Interestingly, sensing agent the fluorescencefor Fe3+ ions. signalsIn addition can beto switchedFe3+ ion detection on and off in uponDI (deionized) the addition water, of pyrophosphate Dehghani et al. (PPi) [45]and explored alkaline the phosphatase intracellular (ALP),Fe3+ ions respectively. detection with The the specific help of detection non-passivated of PPi and blue ALP-emitting was achievable BNQDs (b- basedBNQDs). on theThe switch-onintercellular and detection switch-off limit states for Fe of3+ the ions fluorescence was reported of BNQDs.as 20.6 nm By, which comparison is among ofthe the percentagehighest sensitivity of fluorescence reported quenching to date. Further, and recovery Han et of al. BNQDs, [89] reported 300 µM the Fe 3+ fluorescence-mediated fluorescencequenching of quenching BNQDs using was Fe found3+ and to Cu be2+ i moreons. Interestingly, efficient for thethe sensitivefluorescence and signals selective can sensingbe switched of ALP. on Theand ALPoff upon activity the measurementaddition of pyrophosphate was achieved (PPi) in a linear and alkaline range between phospha- 2 −1 −1 andtase 200 (ALP), U L respectively.with a low limitThe specific of detection detection of 0.8 of U PPi L .and Recently, ALP was Yang achievable et al. [78] reportedbased on anthe efficient switch- methodon and toswitch determine-off states tetracyclines of the fluorescence (TCs) using of ratiometric BNQDs. By fluorescence comparison sensors of the 3+ basedpercentage on BNQDs of fluorescence and europium quenching ion (Eu and) recovery systems. of This BNQDs, system 300 worked M Fe as3+-mediated a probe that flu- detectedorescence the quenching quenching was of bluefound fluorescence to be more efficient of BNQDs for when the sensitive introduced and withselective TCs, sensing while 3+ enhancingof ALP. The the ALP red activity fluorescence measurement of Eu . was This achieved phenomenon in a linear can be range attributed between to the2 and inner 200 filterU L−1 effect, with photo-induceda low limit of detection electron transfer,of 0.8 U andL−1. Recently, antenna effect. Yang Theet al. authors [78] reported also reported an effi- thecient use method of this sensor to determine in the quantitative tetracyclines detection (TCs) using of TCs ratiometric in milk and fluorescence beef samples sensor withs acceptablebased on BNQDs recoveries. and Further, europium they ion developed (Eu3+) system a tests. paper This system for the visualworked detection as a probe of TCs that demonstratingdetected the quenching the sensor of practical blue fluorescence application. of BNQDs when introduced with TCs, while enhancing the red fluorescence of Eu3+. This phenomenon can be attributed to the inner C 2021, 7, 35 17 of 26

4.3. Gas Sensing Gas sensor fabrication based on BNQDs’ decorated zinc oxide (ZnO) nanoplates was reported by Choudhury et al. [69]. These sensors were used for sensing benzene, toluene, ethyl benzene, and xylene (BTEX) vapors. Sensitive sensing of these vapors is important as they are highly toxic and carcinogenic. The sensing response of these ZnO-BNQDs was enhanced greatly as compared to pure ZnO. The X-ray Photoelectron Spectroscopy (XPS) study on this BNQDs-ZnO composite showed that BNQDs are electrically interacted with ZnO via the oxygen atoms and acted as the catalysts for generating charge carriers. The increase in the number of oxygen species decides the sensing capabilities of the composites. This resistance-based gas sensor is based on the variation of conductance resulting from the chemisorption of oxygen molecules on the surface of the sensor. When the gas molecules of BTEX are adsorbed on the sensor surface, they react with the ionized oxygen and lead to oxidation decomposition of BTEX molecules, resulting in electron transfer to the sensor. An optimal temperature of 370 ◦C was used for the concentration-dependent studies. The response and recovery time of the sensor were reported within 20 s and had good stability.

4.4. Temperature Sensing Li et al. utilize the variation of PL spectrum intensities of BNQDs with the temperature change as the working principle of temperature sensors [84]. They reported that PL intensities decrease linearly with the increase in temperature from 80 to 440 K when excited at 394 nm, thus making portable non-contact temperature sensors by using BNQDs. The decrease in PL intensity with the increase in temperature is due to the increase in the thermally active nonradiative relaxation process. The PL intensity is reported to be thermally activated with an activation energy of 83.9 meV and has good reversibility. The authors also reported the effect of the size of BNQDs on the sensitivity of the sensor. The relative sensitivity of the sensor with 17.7 nm BNQDs reached the maximum of 0.45 % K−1 at 440 K as compared to 0.73% K−1 at 440 K with 4.6 nm BNQDs. The relative sensitivity of the sensor based on 4.6 nm BNQDs is better than other materials such as Manganese (Mn), Zinc oxide (ZnO), Erbium (Er), and Ytterbium (Yb)-based sensors reported so far [90–92]. Besides, the sensors based on BNQDs exhibit the widest temperature detection range as compared with other noncontact temperature sensors based on silver nanoclusters, cadmium telluride (CdTe), Zinc sulﬁde (ZnS), and carbon QDs, as reported in [93–97].

4.5. Fluorescent Staining Cotton fibers stained with BNQDs were reported to show the excitation wavelength- dependent fluorescence. These fibers emit green and red fluorescence when exposed to blue (450−480 nm) and green (510−550 nm) excitation light, respectively, as shown in C 2021, 7, x FOR PEER REVIEW 18 of 26 Figure 12. This work demonstrated that BNQDs are potential fluorochromes for chemical staining [70]. A similar study was reported later on cotton and silk fibers [40].

FigureFigure 12.12. ((a)) Bright-fieldBright-field image image of of cotton cotton fibers fibers stained stained with with BNQDs. BNQDs. (b ()b Fluorescence) Fluorescence image image of of cottoncotton fibersfibers stained stained with with BNQDs BNQDs obtained obtained at at the the excitation excitation wavelengths wavelengths of of 510–550 510–550 nm. nm. (c ()c Fluores-) Fluo- cencerescence image image of cotton of cotton fibers fibers stained stained with with BNQDs BNQDs obtained obtained at the at excitationthe excitation wavelengths wavelengths of 450–480 of 450– nm.480 nm. Scale Scale bar: bar: 50 µ50m. μm. Reproduced Reproduced from from [70 [70]] with with permission permission from from copyright copyright© ©2017 2017 AmericanAmerican ChemicalChemical Society.Society.

4.6. Electrochemiluminescence (ECL) Responses ECL is luminescence produced by the electrochemical process where a species generated at the electrodes undergo electron transfer reactions to form excited states and emit light. This is a means to convert electrical energy into light energy without the use of excitation light. The ECL mechanism could enable an analytical tool for clinical detection. For example, the ELC of Ruthenium(II) Tris(2,2′-bipyridyl) (Ru(bpy)32+) and its derivatives-based was demonstrated with excellent efficiency [98]. ECL was also applied in commercial instruments for immunoassays and DNA analyses with high sensitivity and selectivity [99]. For enhanced ECL performance, a suitable co-reactant is needed. BNQDs, having excellent chemical inertness, exhibit low toxicity and can be used as the alternative ECL luminophores. The ECL property of BNQDs was first reported by Liu et al. [40] using L-cysteine (RSH) as a co-reactant. As shown in Figure 13A, BNQDs displayed ECL emission at an electrode potential of −1.91 V. Adding RSH as the co-reactant, the ECL intensity increased almost five times. ECL was not detected with RSH alone without BNQDs. The reported ECL mechanism is illustrated in Figure 13B. As shown, oxidation of RSH produces free radical intermediates including RS, RSO, RSO2, and RSO3, which reacted with the electron-rich BNQDs (electron transferred from Pt electrode to BNQDs). BNQDs were excited after such electron transfers and emitted measurable luminescent signals. The effect of buffer pH on the ECL behavior was reported. It was shown that the ECL intensity increase with the increase in the buffer pH from 3 to 7, and then decreases when the pH increased over 7, as summarized in Figure 13C. Figure 13D shows the ECL emission of BNQDs with RSH under continuous 14 cyclic Volta metric sweeping with good repeatedly [40]. Later, Xing et al. [65] reported an enhanced and stable anodic ECL from a suspension of BNQDs and Ru(bpy)32+, which had a 400-fold enhancement compared with individual Ru(bpy)32+. BNQDs functionalized with amino groups and the electrocatalytic effect made them efficient co-reactants of Ru(bpy)32+. The effect of atmosphere and electrode materials on ECL performance was reported. For example, ECL intensity under saturated nitrogen was reported to be higher than that obtained in the air and oxygen atmosphere. The use of glassy carbon electrode yielded better ECL intensity as compared with gold and platinum electrodes. Although tripropylamine (TPA) was shown to be the better co-reactant, BNQDs are the better choice for their better solubility, chemical stability, biocompatibility, and ease of synthesis. Further, ECL sensing for dopamine (DA) was reported based on the quenching effect between the excited state of Ru(bpy)32+ and the oxidation form of DA in the ECL system of Ru(bpy)32+/BNQDs [65]. In another study, BNQDs were used as the co- reactants in label-free ECL immune-sensors. Such sensors are based on antigen–antibody interaction for alpha-fetoprotein (AFP) detection in human serum as reported by Qin et al. [68]. The authors integrated Ru(bpy)32+-doped silica nanoparticles (Ru-SiO2) and AuNP–modified graphene oxide (rGO@Au) to form a rGO@Au@Ru-SiO2 composite C 2021, 7, 35 18 of 26

4.6. Electrochemiluminescence (ECL) Responses ECL is luminescence produced by the electrochemical process where a species generated at the electrodes undergo electron transfer reactions to form excited states and emit light. This is a means to convert electrical energy into light energy without the use of excitation light. The ECL mechanism could enable an analytical tool for clinical detection. 0 2+ For example, the ELC of Ruthenium(II) Tris(2,2 -bipyridyl) (Ru(bpy)3 ) and its derivatives- based was demonstrated with excellent efficiency [98]. ECL was also applied in commercial instruments for immunoassays and DNA analyses with high sensitivity and selectivity [99]. For enhanced ECL performance, a suitable co-reactant is needed. BNQDs, having excellent chemical inertness, exhibit low toxicity and can be used as the alternative ECL luminophores. The ECL property of BNQDs was first reported by Liu et al. [40] using L-cysteine (RSH) as a co-reactant. As shown in Figure 13A, BNQDs displayed ECL emission at an electrode potential of −1.91 V. Adding RSH as the co- reactant, the ECL intensity increased almost five times. ECL was not detected with RSH C 2021, 7, x FOR PEER REVIEW 19 of 26 alone without BNQDs. The reported ECL mechanism is illustrated in Figure 13B. As shown, oxidation of RSH produces free radical intermediates including RS, RSO, RSO2, and RSO3, which reacted with the electron-rich BNQDs (electron transferred from Pt electrode to BNQDs).which was BNQDs used as werethe ECL excited probe after of the such sensors. electron The transfersanode ECL and intensity emitted of measurable this compo- luminescentsite on GCE signals.was low, The but effect the addition of buffer of pH BNQDs on the enhanced ECL behavior the ECL was intensity reported. by It about was shown100 times. that The the ECL immuno intensity-sensor increase was with applied the increase for quantitative in the buffer detection pH from of AFP 3 to antigen 7, and thenas ECL decreases signal decreased when the pHwith increased the increase over in 7, AFP as summarized concentration in from Figure 0.0001 13C. to Figure 13D shows100 ng themL ECL−1. This emission immune of sensor BNQDs exhibited with RSH a low under detection continuous limit, high 14cyclic selectivity Volta and metric sen- sweepingsitivity, acceptable with good stability, repeatedly and [ 40a ].wide linear detection range.

FigureFigure 13. 13.( (AA)) Electrochemiluminescence Electrochemiluminescence (ECL) (ECL) intensity–potential intensity–potential curves curves of of 100 100 mg mg mL mL−−11 BNQDs preparedprepared in in ethanol ethanol in in 100 100× ×10 10−−3 mm PBS PBS (pH (pH 7.0) 7.0) with with varying condition condition.. ( B) Schematic illustration ofof thethe ECLECL mechanism of the the BNQDs/ BNQDs/L-cysteineL-cysteine (RSH (RSH)) system system.. (C) ( CInfluences) Inﬂuences of buffer of buffer pH pHon the on ECL intensity of 100 mg mL−1 BNQDs. (D) ECL intensity–time curve under continuous cyclic volt- the ECL intensity of 100 mg mL−1 BNQDs. (D) ECL intensity–time curve under continuous cyclic ammetry scanning between −2.0 and 0 V for 600 s for the BNQDs/RSH system. The scan rate is 100 voltammetry scanning between −2.0 and 0 V for 600 s for the BNQDs/RSH system. The scan rate mV s−1. Reproduced from [40] with permission from 2016 Wiley-VCH Verlag GmbH & Co. KGaA −1 is(Weinheim 100 mV s , Germany).. Reproduced from [40] with permission from 2016 Wiley-VCH Verlag GmbH & Co. KGaA (Weinheim, Germany). The emission spectrum of BNQDs is perfectly matched with the absorption spectrum of nitrogen-doped graphene QDs (NGQDs). Based on this property, Li et al. have demonstrated folic acid (FA) sensors with good selectivity and high recoveries based on ECL resonance energy transfer (ECL-RET) between BNQDs and NGQDs in the presence of co- reactant K2S2O8 [66]. The intermediate state of K2S2O8, SO42-, oxidizes FA and decreases the NGQDs/BNQDs/K2S2O8 ECL signal, resulting in the signal-off detection of FA. The ECL intensity of NGQDs by RET was improved by about 10-fold by the addition of BNQDs. This reported method has an advantage against other reported ECL-RET-based sensors [100–103] that use toxic heavy metal QDs that cause serious health and environmental concerns. Another study by Liu et al. demonstrated the use of BNQDs and gold nanoparticles (AuNPs) as biosensors to accurately quantify the Shiga toxin-producing Escherichia Coli gene from 1 pmol L−1 to 5 nmol L−1 with a limit of detection of 0.3 pmol L−1 [74]. This biosensor can be used for DNA detection and was designed based on the ECL-RET principle, and surface plasmon coupling (SPC) effects between AuNPs and BNQDs. In brief, a hairpin DNA probe was made by binding BNQDs and AuNPs to its end. The ECL signal of the BNQDs was quenched by the AuNPs due to ECL-RET between the AuNPs and BNQDs. When the hairpin DNA probe identified the target DNA, the distance between BNQDs and AuNPs increased due to the linear transformation of the hairpin structure. C 2021, 7, 35 19 of 26

Later, Xing et al. [65] reported an enhanced and stable anodic ECL from a suspension 2+ of BNQDs and Ru(bpy)3 , which had a 400-fold enhancement compared with individual 2+ Ru(bpy)3 . BNQDs functionalized with amino groups and the electrocatalytic effect made 2+ them efficient co-reactants of Ru(bpy)3 . The effect of atmosphere and electrode materials on ECL performance was reported. For example, ECL intensity under saturated nitrogen was reported to be higher than that obtained in the air and oxygen atmosphere. The use of glassy carbon electrode yielded better ECL intensity as compared with gold and platinum electrodes. Although tripropylamine (TPA) was shown to be the better co-reactant, BNQDs are the better choice for their better solubility, chemical stability, biocompatibility, and ease of synthesis. Further, ECL sensing for dopamine (DA) was reported based on the 2+ quenching effect between the excited state of Ru(bpy)3 and the oxidation form of DA 2+ in the ECL system of Ru(bpy)3 /BNQDs [65]. In another study, BNQDs were used as the co-reactants in label-free ECL immune-sensors. Such sensors are based on antigen– antibody interaction for alpha-fetoprotein (AFP) detection in human serum as reported by 2+ Qin et al. [68]. The authors integrated Ru(bpy)3 -doped silica nanoparticles (Ru-SiO2) and AuNP–modified graphene oxide (rGO@Au) to form a rGO@Au@Ru-SiO2 composite which was used as the ECL probe of the sensors. The anode ECL intensity of this composite on GCE was low, but the addition of BNQDs enhanced the ECL intensity by about 100 times. The ECL immuno-sensor was applied for quantitative detection of AFP antigen as ECL signal decreased with the increase in AFP concentration from 0.0001 to 100 ng mL−1. This immune sensor exhibited a low detection limit, high selectivity and sensitivity, acceptable stability, and a wide linear detection range. The emission spectrum of BNQDs is perfectly matched with the absorption spectrum of nitrogen-doped graphene QDs (NGQDs). Based on this property, Li et al. have demonstrated folic acid (FA) sensors with good selectivity and high recoveries based on ECL resonance energy transfer (ECL-RET) between BNQDs and NGQDs in the presence 2- of co-reactant K2S2O8 [66]. The intermediate state of K2S2O8, SO4 , oxidizes FA and decreases the NGQDs/BNQDs/K2S2O8 ECL signal, resulting in the signal-off detection of FA. The ECL intensity of NGQDs by RET was improved by about 10-fold by the addition of BNQDs. This reported method has an advantage against other reported ECL-RET- based sensors [100–103] that use toxic heavy metal QDs that cause serious health and environmental concerns. Another study by Liu et al. demonstrated the use of BNQDs and gold nanoparticles (AuNPs) as biosensors to accurately quantify the Shiga toxin-producing Escherichia Coli gene from 1 pmol L−1 to 5 nmol L−1 with a limit of detection of 0.3 pmol L−1 [74]. This biosensor can be used for DNA detection and was designed based on the ECL-RET principle, and surface plasmon coupling (SPC) effects between AuNPs and BNQDs. In brief, a hairpin DNA probe was made by binding BNQDs and AuNPs to its end. The ECL signal of the BNQDs was quenched by the AuNPs due to ECL-RET between the AuNPs and BNQDs. When the hairpin DNA probe identified the target DNA, the distance between BNQDs and AuNPs increased due to the linear transformation of the hairpin structure. As a result, the SPC-ECL effect suppressed the ECL-RET effect, thus enhancing the ECL signal (due to coupling of light emission with surface plasmon resonance) by about six times higher than BNQDs-Au NPs- hairpin DNA conjugates. The authors further reported on the use of S-regulated BNQDs along with AuNPs with higher ECL performance for BRAF gene detection and quantification using the same SPC-ECL strategy as discussed above [75]. Further, Liang et al. [104] reported the first polarized-ECL biosensor developed using SPC-ECL strategy and fluorine-doped BNQDs to detect the K-ras gene. The fluorine-doped BNQDs ECL signal was reported to be enhanced at a specific polarization angle, thanks to polarization angle-dependent SPC-ECL. This polarized-biosensor was reported to quantify the K-ras gene from 0.1 fM to 10 nM, with the detection limit as 0.03 fM. Recently, Wang et al. [105] reported the construction of ECL biosensor using BNQDs as a co-reactant of luminol for the quantitative determination of concanavalin A (Con A). BNQDs are used for boosting the ECL intensity of luminol, almost producing a 10-fold C 2021, 7, 35 20 of 26

enhancement as compared with single poly (luminol/aniline) nanorods loaded on reduced graphene oxide using GCE. The detection limit of Con A was reported in the range of 1.0 pgmL−1 to 1.0 µgmL−1 with a low detection limit of 0.15 pgmL−1.

4.7. Photocatalyst

Radhakrishnan et al. reported the use of TiO2 nanotubes/FBNQDs heterojunctions as the broad-spectrum photocatalyst for efficient water splitting [58]. Hydrogen production enhancement was reported when FBNQDs were loaded on TiO2 nanotubes (NT). This was demonstrated by using photoelectrochemical cells consisting of the platinum cathode, and TiO2 nanotubes/FBNQDs photoanode, where visible light from solar radiation was used to generate hydrogen and oxygen from water. A hydrogen production conversion efficiency of up to 24% was obtained after adding FBNQDs onto TiO2 nanotubes, paving an alternative way in the design of efficient visible-light photocatalysts. Later, Yang et al. reported metal-free heterostructure photocatalyst using ultrathin porous graphitic carbon nitride (UPCN) decorated with BNQDs [71]. The use of BNQDs has promoted the dissociation of excitons and accelerated the transfer of charges. This BNQDs/UPCN (BU) photocatalyst used visible light to degrade oxytetracycline hydrochloride (OTC−HCl) for molecular − oxygen activation by the generation of superoxide radical (O2 ). Further, BU photocatalyst along with visible light irradiation was used to produce hydrogen peroxide (H2O2) from − the mixture of IPA and deionized water. The average O2 generation rate for the best- optimized sample was about 2.3 and 1.6 times better than that of bulk graphitic carbon nitride (g-C3N4) and UPCN, respectively. Moreover, the H2O2 production by the same sample was 3.2 and 2 times higher than that of bulk g-C3N4 and UPCN, respectively, for 60 min. The incorporation of BNQDs to tungsten trioxide (WO3) photoanode was reported for efficient photogenerated charge separation, and charge recombination for the photoelectrochemical (PEC) water splitting application [72]. It was thought that BNQDs help to extract the photogenerated holes, thus enhancing charge separation and water oxidation potency. A 2.4-fold enhancement in photocurrent density and 2.44-fold enhancement in charge carrier density were reported by using such BNQD-modified photoanode in comparison to the pristine WO3 anode. Moreover, the incident photon to current conversion efficiency (IPCE) using the modified photoanode has reached 32%, which is 2.1 times higher than that when pristine WO3 anode was used. On the other hand, Sahu et al. reported superior PEC performance in water splitting using BNQDs incorporated over fluorine-doped hematite (F-Fe2O3) nanorods [73]. 1-D hematite nanorod arrays quickly transferred electrons to the FTO substrate, while BNQDs acted as the efficient hole extractors that enhanced charge separation on the hematite surface by decreasing the hole trapping probability. In the optimal condition, a carrier density of 2.08 × 1020 cm−3 was obtained, which is two orders of magnitude higher than that when bare α-Fe2O3 photoanode was used. This resulted in a photocurrent density of 2.24 mA cm-2 at 1.23 V, which is six-fold higher than the case when bare hematite was used. The faradaic efficiency test of F-Fe2O3-BNQDs for oxygen evolution was reported to be 90% after 1 h of irradiation, confirming that most of the photo-generated charges were utilized for water oxidation.

4.8. Other Applications Because of the low shielding effect of the h-BN planes in proton transport [106], BN- QDs suspended in water were investigated for their roles in proton exchange membranes (PEMs) by Lei et al. [39]. The authors reported easy integration of BNQDs into polymer matrix resulting in enhanced thermal stability and proton conductivities of the composite PEMs with an enhanced water retention capability. With the electronic devices getting smaller and smaller with high integration and power density, dissipating heat effectively is highly desirable for ensuring their long lifetime and reliability. The good thermal conductivity and excellent electrical insulation in h-BN materials make them a good candidate for thermal management applications. BN C 2021, 7, 35 21 of 26

nanosheets were added into the polymer matrix as filler to form effective thermal conductive paths that significantly improve the thermal conductivity of the composites [107–114]. Theoretical studies on thermoelectric properties of BNQDs were conducted to inspire experimentalists to seek ways for real-world applications [3]. For example, Zhou et al. [115] reported the doping of BNQDs on a flexible polyamide-imide (PAI)/BN nanosheets composite film to improve the thermal conductivity (TC) of the polymer and track the thermal conductive paths utilizing the photoluminescence (PL) of BNQDs. In this case, BNQDs were filled into the gaps between the BN nanosheet fillers and the polymer matrix, forming the continuous thermal conductive paths. For low filler content (BN nanosheets 9% wt) and 1% wt BNQDs, the hybrid composite film resulted in an in-plane TC of 7.69 W m−1 K−1 and had good optical transparency, making it a possible candidate for use as a flexible display substrate. The use of BNQDs as readout labels for logic gate construction was first reported by Han et al. [89] based on the principle of signal-on and-off in the BNQDs-Fe3+ FL in the PPi and ALP assays. The existence of PPi and ALP was used as the inputs (“1” state for present and “0” state for absence). The normalized FL intensity of BNQDs under optimal excitation conditions was used as the output signal. The reported application for inhibiting “INH” logic gate construction could pave a path for the use of other 2D-QDs for a future logic gate design. Beytur [116] reported the use of BNQDs in preparation for Platinum nanoparticle (PtNPs)/BNQDs/6-methyl-2-(3-hydroxy-4-methoxybenzylidenamino)-benzothiazole(ILs) nanocomposite (PtNPs/BNQDs/ILs) for the electrocatalytic oxidation of methanol. The performance of this nanocomposite was better than while using only ILs and BNQDs/ILs.

5. Conclusions Since the successful synthesis in 2014, BNQDs have gained considerable attention for their photochemical properties, photostability, and biocompatibility. All the publications so far have shown that the synthetic route is simple and cost-efﬁcient; however, more creativity to scale-up the production efﬁciency is needed for the large-scale and real-world application of BNQDs. There is no doubt that BNQDs have already shown immense potential in sensors, bioimaging, phototherapy, photocatalysis, and biomedicine. We are expected to see creativity in the use of BNQDs for many other applications, for example, based on the edge functionalization of these van der Walls nanostructures, and solar energy harvesting based on the broad light absorption nature of the QDs. All these will rely on the high-yield synthesis of BNQDs with controllable sizes. Breakthrough in large-scale synthesis and real-world application will promote the future science, engineering, and application of BNQDs.

Author Contributions: Performed a literature study and discuss the findings, A.A., S.S., X.L., D.Z. and Y.K.Y.; wrote the manuscript, A.A. and S.S.; X.L., D.Z., discussed, reviewed, and edited the contents of the manuscript, Y.K.Y., A.A. and S.S. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the Elizabeth and Richard Henes Center for Quantum Phenomena. Institutional Review Board Statement: Not applicable. Data Availability Statement: All data presented here are adopted from the published work cited in the specific reference. Conflicts of Interest: The authors declare no conflict of interest.

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