Nanodiamonds: Synthesis and Application in Sensing, Catalysis, and the Possible Connection with Some Processes Occurring in Space

applied sciences

Review Nanodiamonds: Synthesis and Application in Sensing, Catalysis, and the Possible Connection with Some Processes Occurring in Space

Luca Basso , Massimo Cazzanelli , Michele Orlandi and Antonio Miotello *

Department of Physics, University of Trento, Via Sommarive 14, 38123 Povo, Italy; [email protected] (L.B.); [email protected] (M.C.); [email protected] (M.O.) * Correspondence: [email protected]; Tel.: +39-0461-281637

Received: 22 May 2020; Accepted: 10 June 2020; Published: 14 June 2020

Abstract: The relationship between the unique characteristics of nanodiamonds (NDs) and the fluorescence properties of nitrogen-vacancy (NV) centers has lead to a tool with quantum sensing capabilities and nanometric spatial resolution; this tool is able to operate in a wide range of temperatures and pressures and in harsh chemical conditions. For the development of devices based on NDs, a great effort has been invested in researching cheap and easily scalable synthesis techniques for NDs and NV-NDs. In this review, we discuss the common fluorescent NDs synthesis techniques as well as the laser-assisted production methods. Then, we report recent results regarding the applications of fluorescent NDs, focusing in particular on sensing of the environmental parameters as well as in catalysis. Finally, we underline that the highly non-equilibrium processes occurring in the interactions of laser-materials in controlled laboratory conditions for NDs synthesis present unique opportunities for investigation of the phenomena occurring under extreme thermodynamic conditions in planetary cores or under warm dense matter conditions.

Keywords: nanodiamonds; nitrogen-vacancy centers; quantum sensing; catalysis; space messengers

1. Introduction Nanodiamonds are diamond nanoparticles; they have most of the unique properties of bulk diamond but at a nanoscale. For this reason, nanodiamonds (NDs) have attracted a great deal of interest and, consequently, active research. These properties [1] include high hardness, the most known characteristic of the diamond-phase; high thermal conductivity and electrical resistivity; chemical stability; biocompatibility; a tunable surface structure; and resistance to harsh environments. These physical properties make NDs suitable for many applications in different fields: tribology, catalysis [2], biomedical applications [3], and drug delivery [4]. In the last decade, fluorescent nanodiamonds (NDs) have received a lot of attention. The fluorescence of NDs is caused by point-defects in the diamond structure, known as color centers. The nitrogen-vacancy (NV) center is the most studied color center. This defect consists of a substitutional nitrogen atom in a diamond lattice site having a carbon vacancy as its nearest 0 neighbor. NV centers exist in two charge states: neutral, NV , and negatively charged, NV−. Of the two states, NV− is the most investigated and probably the most interesting in terms of quantum technology applications [5,6]. This is because of its optical properties. The NV− shows bright and stable photoluminescence (PL) under excitation by visible light (usually a 532 nm laser source is used). The NV PL emission has a zero phonon line at 637 nm, with a sideband due to NV–phonon coupling extending to ~800 nm. The NV emission is highly suitable for many applications given its photostability (no bleaching is observed at room temperature) and short relaxation time (~10 ns) [7].

Appl. Sci. 2020, 10, 4094; doi:10.3390/app10124094 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4094 2 of 28

However, the most important property of NV optical transitions is their dependence on electron spin-states [8]. This means that NV fluorescence intensity changes in the presence of perturbations acting on the spin-state, such as electromagnetic fields. This, combined with the possibility of accurately manipulating the population of spin levels in the ground state, has allowed NV centers to be used as quantum sensors of environmental physical parameters with nanoscale precision, such as for magnetic [9] or electric fields [10], temperature [11] (from cryogenic to well above 300 K), pressure, as well as in harsh chemical environments. These sensors can be used in biological material given NDs’ non-toxicity. Measurements of temperature [12] and magnetic/electric fields [13] with NV-enriched NDs have already been reported. Additionally, as NDs can be chemically attached to a large variety of species (from DNA molecules to proteins) [14], NV-fluorescence can be used to track the position of the macromolecules [15]. Moreover, NDs linked to cell membranes could provide a sensor to study cell activity [16]. Furthermore, spin-dependent fluorescence intensity can be used to read-out the NV spin-state [17]. This property, together with the possibility of optically pumping most of the electrons in specific spin-sublevels with very large spin-coherence lifetimes [18] (as long as milliseconds), makes NV centers an ideal solid-state qubit for quantum information science. Finally, it was shown that the NV spin polarization can be transferred to the surrounding environment [19], such as 13C nuclear spin, opening the potential for use in nuclear magnetic resonance (NMR) applications [20]. Given the wide application field of NV-enriched NDs, an efficient technique for their synthesis is critical for further development. Many ND production methods have been proposed, the main ones being detonation [1] and milling [21] of macroscopic diamond synthesized either by chemical vapor deposition (CVD) or high pressure, high temperature (HPHT). These standard techniques, however, present common drawbacks: The control of the size and shape of NDs is poor, and the synthesis process often introduces contaminants, thus requiring post-production processes to clean ND surfaces (usually strong chemical treatments are required) [22]. Moreover, the nitrogen doping of the NDs, which is the first step to obtain NV centers, is inefficient, since only the native nitrogen atoms in the carbon precursor are exploited, resulting in NDs with a small density of NVs. To increase the NV number, a multi-step, post-synthesis treatment of the NDs is required, consisting in nitrogen ions implantation and subsequent annealing at high temperatures to create the NV center [23]. An alternative means of NDs production is represented by pulsed laser ablation (PLA). PLA is a standard top-down technique for the synthesis of a wide variety of nanoparticles [24]. It consists on irradiating the surface of a bulk 2 target with short (ps, ns) and intense (GW cm− ) laser pulses; the nanoparticles are mainly formed in a well-known physical phenomenon called phase explosion [25] or collisional cooling. Indeed, PLA can be performed in different environments, both liquid or gaseous, to cause chemical reactions between the ablated species and the atoms of the confining medium. When PLA is performed on a graphite target, the extreme thermodynamic conditions reached during laser irradiation, namely high temperature and pressure, allow for the transition of the nanodroplets expelled from the surface from liquid carbon into diamond. As is reported in this review, when laser ablation occurs in a nitrogen containing medium, direct synthesis of NDs containing NV centers is achieved. Standard production techniques will also be reviewed together with the applications of these NDs-based systems. In detail, the first part of this paper describes NDs synthesis techniques, focusing on the ones used for commercial production of NDs. Moreover, standard synthesis of fluorescent NDs is reported, together with the latest updates, in particular, regarding laser-assisted synthesis methods. The second part details the application of fluorescent NDs as sensors for environmental parameters, such as electromagnetic field and temperature. The subsequent section reviews the results obtained in catalysis by using NDs, where the number of investigated reactions is still limited, but the potential interest for these materials is high. Finally, the last section discusses an open astrophysical problem, the so-called extended red emission, and its possible relation with fluorescent NDs. The production of fluorescent NDs with PLA techniques allows us to establish the pressure and temperature conditions necessary to obtain these nanosystems for study, in addition to the correct relative composition of carbon and nitrogen. The study of these specific conditions is more difficult when using traditional techniques. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 30

the correct relative composition of carbon and nitrogen. The study of these specific conditions is more Appl. Sci. 2020, 10, 4094 3 of 28 difficult when using traditional techniques.

2.2. NanodiamondsNanodiamonds SynthesisSynthesis TechniquesTechniques ItIt isis wellwell knownknown thatthat the stable allotrope ofof carbon at ambient pressure andand room temperaturetemperature isis graphite.graphite. As As can can be be seen seen in the in carbon the carbon phase phase diagram diagram of Figure of1 ,Figure diamond 1, requiresdiamond extreme requires conditions extreme ofconditions temperature of temperature and pressure and to bepressure formed. to For be instance,formed. For natural instance, diamond natural may diamond be formed may in thebe formed Earth’s mantle,in the Earth 140 ’s200 mantle, km below 140−200 the km surface below [26 the], wheresurface the [26] temperature, where the andtemperature pressure and are inpressure the range are ofin − 900the range1400 ofC 900 and−1400 4.5 6°C GPa, and respectively4.5−6 GPa, respectively [27]. Once the[27] diamond. Once the phase diamond is formed, phase is the formed, transition the − ◦ − backtransition to graphite back to at graphite ambient at conditions ambient condition is avoideds is by avoided the high by energy the high barrier energy for barrier phase transition.for phase Indeed,transition even. Indeed, if graphite even is if thermodynamically graphite is thermodynamically favored (energy favored difference (energy of 0.02difference eV per of atom 0.02 between eV per 2 diamondatom between and graphite),diamond and a 0.4 graphite), eV energy a barrier0.4 eV energy must be barrier overcome must to be move overcome from to sp 2moveto sp fro3 chemicalm sp to 3 bonds.sp chemical This makes bonds diamond. This makes a metastable diamond phase, a metastable as it is thermodynamically phase, as it is thermodynamically unstable, but the transitionunstable, kineticsbut the transition to graphite kinetics is prohibitive. to graphite Currently, is prohibitive many di. ffCurrentlyerent methods, many for different artificial methods synthesis for of artificial NDs are availablesynthesis[ 1of]. NDs In the are following available sections, [1]. In the the following three main sections techniques, the commerciallythree main techniques available commercially are described: detonation,available are chemical described: vapor detonation, deposition chemical (CVD), andvapor milling deposition of high-pressure, (CVD), and high-temperature milling of high-pressure (HPHT), micro-sizedhigh-temperature diamonds. (HPHT) micro-sized diamonds.

FigureFigure 1.1. CarboCarbonn phase phase diagram diagram indicating indicating graphite graphite (grey) (grey),, diamond diamond (white), (white), and and liquid liquid (yellow) (yellow).. In Inaddition, addition, pressure pressure-temperature-temperature conditions conditions reached reached with with different different synthesis synthesis technique technique are are indicated. HPHT:HPHT: highhigh pressure,pressure, highhigh temperature.temperature. PECVD: PECVD: Plasma Plasma Enhanced Enhanced Chemical Chemical Vapor Vapor Deposition. Deposition. 2.1. Detonation Synthesis 2.1. Detonation Synthesis In detonation synthesis, the energy of an explosion is used to drive the diamond phase In detonation synthesis, the energy of an explosion is used to drive the diamond phase formation formation [28]. In a closed metallic chamber, explosives with a negative oxygen balance are [28]. In a closed metallic chamber, explosives with a negative oxygen balance are detonated; usually detonated; usually a mixture of 60% TNT (C6H2(NO2)CH3) and 40% hexogen (C3H6N6O6) is employed a mixture of 60% TNT (C6H2(NO2)CH3) and 40% hexogen (C3H6N6O6) is employed (Figure 2) [1]. The (Figure2)[ 1]. The carbon atoms that eventually form the NDs are provided either by the molecules of carbon atoms that eventually form the NDs are provided either by the molecules of the explosives or the explosives or by precursor graphite put inside the detonation chamber. The synthesis is called by precursor graphite put inside the detonation chamber. The synthesis is called “dry” or “wet” when “dry” or “wet” when the chamber is filled with a gas (N2, Ar, CO2) or with water (ice), respectively, the chamber is filled with a gas (N2, Ar, CO2) or with water (ice), respectively, that act as coolants. that act as coolants. The cooling media play an important role in the carbon yield, which is usually The cooling media play an important role in the carbon yield, which is usually around 10% of the around 10% of the explosive weight [29]. The process leading to NDs formation is schematized in explosive weight [29]. The process leading to NDs formation is schematized in Figure 2. After Figure2. After detonation, the carbon atoms released during the explosive molecule’s dissociation detonation, the carbon atoms released during the explosive molecule’s dissociation condense and condense and crystallize into nanoclusters [30]. The pressure/temperature increase (Figure1) reached crystallize into nanoclusters [30]. The pressure/temperature increase (Figure 1) reached in the in the chamber during the detonation leads to crystallization of the carbon nanoclusters into diamond chamber during the detonation leads to crystallization of the carbon nanoclusters into diamond phase. Finally, the formed NDs grow and agglomerate, resulting in NDs of size 4 5 nm. The main phase. Finally, the formed NDs grow and agglomerate, resulting in NDs of size 4−−5 nm. The main drawback of this technique is that the resulting sample, called detonation soot, must be purified drawback of this technique is that the resulting sample, called detonation soot, must be purified to to remove contaminants. Indeed, detonation soot is not only made of diamond phase, but also of remove contaminants. Indeed, detonation soot is not only made of diamond phase, but also of non- non-diamond carbon (25–85 wt %) and metals coming from the wall of the chamber (1 8 wt %) [31]. − To remove the impurities, cleaning of the detonation soot in a strong acid environment (usually a Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 30 diamondAppl. Sci. 2020 carbon, 10, 4094 (25–85 wt %) and metals coming from the wall of the chamber (1−8 wt %) [31]4. of To 28 remove the impurities, cleaning of the detonation soot in a strong acid environment (usually a mixture of HNO3/H2SO4/HClO4) is required [28], which is a dangerous and expensive process. mixture of HNO3/H2SO4/HClO4) is required [28], which is a dangerous and expensive process. Another Anotherproblem isproblem NDs agglomeration is NDs agglomeration occurring duringoccurring synthesis, during wheresynthesis, NDs where clusters NDs of several clusters hundreds of several of hundredsnanometers of arenanometers obtained. are To obtained. isolate the To single isolate 4 the5 nm single sized 4− NDs,5 nm millingsized NDs, with milling ceramic with microbeads ceramic − microbeads(that again introduces(that again contaminants)introduces contaminants) or ultrasonic or disintegration ultrasonic disintegration are the standard are the de-aggregation standard de- aggregationmethods [32 ].methods [32].

Figure 2. (a) Detonation process. A mixture of 60% TNT (C H (NO )CH ) and 40% hexogen Figure 2. (a) Detonation process. A mixture of 60% TNT (C66H2(NO22)CH33) and 40% hexogen (C H N O ) is detonated inside a metallic chamber filled either with water (liquid or ice) or gas (C3H66N66O6)6 is detonated inside a metallic chamber filled either with water (liquid or ice) or gas (an (an atmosphere of N , Ar, or CO ). Then, 4–5 nm sized nanodiamonds (NDs) agglomerated into atmosphere of N2, Ar2, or CO2). Then,2 4–5 nm sized nanodiamonds (NDs) agglomerated into sub- micrometersub-micrometer clusters clusters are obtained. are obtained. (b) Schematics (b) Schematics of NDs of NDssynthesis synthesis uponupon shock shock wave wavepropagation. propagation. The detonationThe detonation (I) leads (I) leads to chemical to chemical dissociation dissociation of ofthe the carbon carbon precursor precursor (II) (II),, which which can can be be the the explosive explosive molecules or graphite. The dasheddashed lineline (III)(III)represents represents the the region regionwhere where the pressurepressure required to form the diamond phase is reached. The detonation products expand (IV), and the carbon atoms condense the diamond phase is reached. The detonation products expand (IV), and the carbon atoms condense and crystallize (V) to form nanoclusters (VI). Finally, NDs crystallize, starting from carbon nanoclusters, and crystallize (V) to form nanoclusters (VI). Finally, NDs crystallize, starting from carbon grow, and agglomerate (VII). Reproduced with permission from [1], Springer, 2012. nanoclusters, grow, and agglomerate (VII). Reproduced with permission from [1], Springer, 2012. 2.2. Chemical Vapor Deposition (CVD) 2.2. Chemical Vapor Deposition (CVD) Chemical vapor deposition is one of the most popular methods for the deposition of thin film, and itChemical has been vapor used fordeposition the synthesis is one of of nanocrystalline the most popular diamond method films [for33]. the In deposition detail, the depositionof thin film, of andcarbon it has atoms been occurs used duringfor the decompositionsynthesis of nanocrystalline of a gas mixture diamond with a carbonfilm [33] containing. In detail, species, the deposition usually of carbon atoms occurs during decomposition of a gas mixture with a carbon containing species, methane CH4, in an excess of hydrogen. The gas phase is decomposed by using a hot filament or a usually methane CH4, in an excess of hydrogen. The gas phase is decomposed by using a hot filament microwave plasma to form radicals such as H• and CH3•, which are essential for diamond growth. • • orThe a ND microwave film forms plasma on a substrate, to form radicals typically such a silicon as H wafer and coated CH3 , withwhich a micrometerare essential sized for diamond growth.powder actingThe ND as film a seed form fors NDon a nucleation substrate, [typically33], and eventuallya silicon wafer forms coated a continuous with a microme film. Theter typical sized diamondset-up [34 ]powder for CVD acting synthesis as a isseed reported for ND in Figurenucleation3a. The [33] size, and of theeventually grains composing forms a continuous the film ranges film. The typical set-up [34] for CVD synthesis is reported in Figure 3a. The size of the grains composing to tens of micron down to a few nanometers, depending on CH4/H2 relative concentration [35]. the film ranges to tens of micron down to a few nanometers, depending on CH4/H2 relative concentration [35]. Appl. Sci. 2020, 10, 4094 5 of 28 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 30

FigureFigure 3.3. ((a)a) ExperimentalExperimental setup setup for for chemical chemical vapor vapor deposition deposition (CVD (CVD)) synthesis. synthesis. The The hot hot filament ﬁlament is is required to dissociate the gas phase (CH /H mixture) into radicals, whereas the electrode plate is required to dissociate the gas phase (CH4/H4 2 2mixture) into radicals, whereas the electrode plate is usedused to to attract attract thosethose radicalsradicals (mainly(mainly HH•• and CH •)) toward toward the the substrate. substrate. Reproduced Reproduced with with permission permission fromfrom [ 34[34]],, Elsevier, Elsevier,2016. 2016. ((bb)) SchematizationSchematization of the standard growth mechanism mechanism of of n nanocrystallineanocrystalline diamonddiamond ﬁlmfilmduring during CVDCVD synthesis.synthesis.

A weak concentration of CH produces microcrystalline diamonds, whereas a high value of the A weak concentration of CH44 produces microcrystalline diamonds, whereas a high value of the CH /H ratio decreases grain size to the order of tens of nanometers for 1 5% CH /H . The standard CH44/H22 ratio decreases grain size to the order of tens of nanometers for 1−−5% CH44/H22. The standard diamonddiamond growth growth mechanism mechanism [35 [35]] is schematizedis schematized in Figure in Figure3b. Carbon 3b. Carbon atoms at atoms the surface at the of surface diamond of seedsdiamond are leftseeds with are dangling left with bondsdangling after bonds hydrogen after hydrogen abstraction abstraction by H• radicals. by H• radicals. These bonds These are bonds then ﬁlledare then by CHfilled3• bymolecules. CH3• molecules When. thisWhen process this process takes placetakes place in two in adjacenttwo adjacent sites, sites, the newthe new carbons carbons can bondcan bond together, together, and ﬁnallyand finally be locked be locked into theinto diamondthe diamond lattice. lattice.

2.3.2.3. MillingMilling ofof High-Pressure,High-Pressure, High-TemperatureHigh-Temperature (HPHT)(HPHT) MicrodiamondsMicrodiamonds TheThe HPHTHPHT synthesissynthesis techniquetechnique resembles the natural process process of of diamond diamond formation, formation, where where a a carboncarbon precursor, precursor, usually usually graphite, graphite, isis broughtbrought toto a state of high pressure pressure and and high high temperature. temperature. Inside Inside aa chamber, chamber, thethe temperaturetemperature isis broughtbrought toto ~2000~2000 °◦C and a a set set of of anvils anvils increase increasess the the pressure pressure up up to to severalseveral GPaGPa [[36]36].. This This technique technique allows allows for for the the formation formation of ofbulk bulk or ormicrodiamonds microdiamonds,, which which must must be bemilled milled to toobtain obtain NDs NDs [21] [21. The]. The milling milling process process does does not allow not allow for good for good control control of nanoparticle of nanoparticle size sizeand and shape shape [37] [,37 so], soadditional additional work work-up-up is required. is required. For For instance, instance, an an acid acid treatment treatment to to remove remove the the contaminantscontaminants comingcoming fromfrom thethe millingmilling process or centrifugation and and filtration ﬁltration to to isolate isolate NDs NDs with with a a narrowernarrower size size distribution.distribution.

2.4.2.4. PulsedPulsed LaserLaser AblationAblation StartingStarting fromfrom thethe pioneeringpioneering work of Yang et al. [38] [38],, PLA has has become become a a viable viable method method for for the the synthesissynthesis of of NDs. NDs. AA standard standard PLA PLA experimental experimental setup setup for for NDs NDs is is given given in in Figure Figure4. 4 The. The laser laser beam beam is focusedis focused on on the the top top surface surface of of a a graphite graphite target target that that isis immersedimmersed in a liquid, which which is is typically typically water, water, butbut NDsNDs productionproduction has been been observed observed in in othe otherr liquid liquid environment environments,s, such such as cyclohehexane as cyclohehexane [39] [. 39A ]. Ap pulsedulsed laser laser is is used used in in order order to to release release an an extremely extremely high high power power on on the the target target;; usually usually,, laser laser pulses pulses 2 inin the the nsns regionregion with energ energiesies in in the the order order of of 100 100 mJ, mJ, for for a final a ﬁnal power power density density of up of to up GW to GWcm−2 cm, are− , areemployed. employed. Appl. Sci. 2020, 10, 4094 6 of 28 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 30

Figure 4. Experimental setup of p pulsedulsed laser ablation in liquid. Reproduced with permission from [[40]40],, Springer, 2018 2018..

In pulsed laser laser ablation, ablation, the the water water plays plays a adouble double role role;; it is it a is medium a medium,, where where nanoparticles nanoparticles are aresuspended suspended and and collected collected during during ablation ablation,, and and it itconfine confiness the the ablation ablation plume, plume, which which favors the diamond-phasediamond-phase formation. Indeed, Indeed, the the presence presence of of a a confining confining layer layer in in front front of of the target is a 2 3 fundamental aspect of thethe transitiontransition fromfrom spsp2 carbon atoms of graphitegraphite toto spsp3 hybridization of the diamond phase.phase. TheThe details details of of the the formation formation mechanism mechanism are are provided provided in [ 40in, 41[40,41]]. The. synthesisThe synthesis of NDs of uponNDs upon the PLA the ofPLA graphite of graphite in water in water occurs occurs in three in three steps: steps: 1. 1.The T absorptionhe absorption of the of the high high power power laser laser pulse pulse induces induces melting melting of the of graphite the graphite surface. surface As the. As laser the energylaser is energy deposited is in deposited a short timescale, in a short the target timescale, surface the rapidly target becomes surface arapidly superheated becomes liquid. a Whensuperheated the temperature liquid. ofWhen this liquidthe temperature reaches a valueof this of liquid about reaches 90% of thea value critical of about temperature, 90% of the TC, a processcritical known temperature as phase, TC explosion, a process occurs known [25 ], as leading phase to explosion the ejection occurs of nanodroplets [25], leading [42 to] that the eventuallyejection turnof nanodroplets into nanoparticles [42] that after eventually solidification. turn into nanoparticles after solidification. 2. 2.Strong Strong vaporization vaporization of the of graphitethe graphite target target and phase and p explosionhase explosion lead to lead the emissionto the emission of a plasma of a plumeplasma containing plume the containi ablatedng material; the ablated inside material the liquid,; inside a state the of liquid extreme, a state thermodynamic of extreme conditionsthermodynamic of temperature conditions and of pressure temperature is created, and pressure of 5000–6000 is created K [,43 of] 5000 and– 2–46000 GPa K [43] [41 ,and44], respectively.2–4 GPa [41,44], respectively. 3. 3.The The last last step step concerns concerns the cooling the cooling of the of ablation the ablation plume. plume. Due to Due the confinementto the confinement effect, the effect, ablation the plumeablation dissipates plume excess dissipates heat very excess effi cientlyheat very through efficiently collisional through cooling collisional with the cooling liquid molecules, with the resultingliquid in molecules, a short quenching resulting time. in a s Thehort fast quenching cooling rate time is. theThe most fast peculiarcooling rate characteristic is the most of 10 11 1 10 11 −1 laserpeculiar ablation characteristic in liquid, which of laser can ablation be in the in orderliquid of, which 10 –10 can beK sin− the[45 ].order This of strong 10 –10 and K fast s reduction[45]. This in the strong temperature and fast is reduction enoughto in produce the temperature the carbon is nanodroplets enough to produce in an undercooling the carbon regimenanodroplets in a few nanoseconds. in an undercooling In this condition regimeof in undercooling, a few nanoseconds. NDs form as In a this metastable condition phase, of startingundercooling, from the nanodropletsNDs form as expelled a metastabl by phasee phase explosion., starting The from transition the nanodroplets to the thermodynamic expelled by stablephase allotrope explosion. of carbon, The transition namely graphite, to the thermodynamic is prevented by the stable rapid allotrope quenching; of carbon, the metastable namely phasegraphite, is literally is prevented frozen under by the the rapid undercooling quenching action.; the metastable phase is literally frozen under the undercooling action. Figure5 shows the characterization of NDs produced through the PLA of graphite in water as reportedFigure in [540 shows]. the characterization of NDs produced through the PLA of graphite in water as reportedPLA in for [40] NDs. synthesis presents some advantages compared to the standard production processes, such as HPHT or detonation. For instance, the sample is not contaminated by metallic impurities that require harsh chemical purification. NDs produced by laser ablation only require removal of the graphitic shell covering the nanoparticles. Indeed, when the temperature and pressure conditions inside the ablation plume are no longer the ones required for diamond phase formation, a sp2 shell forms around the NDs. An efficient and simple way to remove the shell was found by Osawa Osswald [31]; it consists of air oxidation at ~400 ◦C. Furthermore, the main advantage of PLA for NDs production is the possibility of achieving diamond phase formation with a simple and cheap experimental apparatus, and performing the synthesis process at ambient pressure and temperature conditions. Appl. Sci. 2020, 10, 4094 7 of 28 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 30

Figure 5. Characterization of of the the NDs from graphite ablation in water water.. ( aa)) Typical Typical Scanning Electron Microscopy (SEM)(SEM) image, image, after after removal removal of of the the non-diamond non-diamond phase phase of of carbon, carbon, showingshowing clustered nanonanoparticlesparticles ofof about about 100 100 nm nm size. size (b). Energy-dispersive(b) Energy-dispersive X-rayX-ray SpectroscopySpectroscopy (EDXS (EDXS)) spectrum spectrum of of the nanoparticles. The carboncarbon peak is detected atat 0.27 keV. Moreover, thethe peakspeaks ofof siliconsilicon atat 1.741.74 keVkeV and of oxygen atat 0.520.52 keVkeV areare observed. observed. They They come come from from the the substrate substrate on on which which the the particles particles are depositedare deposited and andfrom from the oxidizedthe oxidized surface surface layers layers of of the the ablated ablated powders, powders, respectively. respectively. ((cc)) RamanRaman spectrumspectrum (obtained 1 under 532 nm nm excitation excitation wavelength) wavelength) of of purified purified particles. particles. The The main main peak peak at at 1580 cm−1 isis the the graphite graphite G G 1 peak, while the peak at 1335 cm−1 (shown(shown in in detail detail in in the the inset) inset) is is attributed attributed to to compressiv compressively-strainedely-strained NDsNDs.. Reproduced Reproduced with permission from [40] [40],, Springer, 2018.2018. 2.5. Nanodiamond Purification Methods PLA for NDs synthesis presents some advantages compared to the standard production processWork-upes, such and as post-processingHPHT or detonation. procedures For aimedinstance, at purifyingthe sample reaction is not productscontaminated are often by ametallic critical impuritiesstep in the that synthesis require of harsh NDs. chemical Several methodologiespurification. NDs have produced been developed by laser toablation cope with only therequire two removalmost common of the graphitic and abundant shell covering contaminants: the nanoparticles. other forms ofIndeed, carbon when (mostly the temperature graphitic) and and metals pressure and conditionstheir oxides. inside the ablation plume are no longer the ones required for diamond phase formation, a sp2For shell the forms carbon around contaminants, the NDs. the An common efficient principle and simple underlying way to purificationremove theis shell selective was oxidation,found by Osawaexploiting Osswald the higher [31]; it reactivity consists of of air sp 2oxidationcarbon with at ~400 respect °C. Furthermore, to the sp3 carbon the main of the advantage diamond of phase. PLA forExamples NDs production are the above is the mentioned possibility oxidation of achieving in air diamond reported byphase Osawa formation Osswald with [31 ],a whichsimple was and further cheap experimentalinvestigated in apparatus, a subsequent and paper performing [46] where the synthesis selectivity process towards at sp ambient2 carbon pressure was achieved and temper by keepingature conditions.the temperature in the range of 400–430 ◦C. This procedure provides a simple and effective solution for 2 the case of PLA NDs where, even though metallic impurities are avoided, the sp carbon fraction can be so high as to actually be the main reaction product. A similar method, but employing boric anhydride to enhance selectivity, was also reported [47]. Another route that has been explored is oxidation with ozone-enriched air [48–50], though these procedures require significantly more complex equipment. Appl. Sci. 2020, 10, 4094 8 of 28

Liquid-phase oxidation reactions are by far the most commonly employed procedures. These generally require strong acids, which provide the additional advantage of also removing some metal-based impurities. Examples include HClO4, concentrated HNO3,H2SO4 alone and in combination with HNO3, HCl in combination with HNO3, and HF [29,51–53]. These treatments require temperatures from 80 ◦C to more than 200 ◦C, thus, specialized equipment is needed that adds to the complexity and cost. Yet, this is necessary, especially in the case of detonation NDs, where metal-based impurities are present in signiﬁcant quantities, coming from the equipment involved in these processes, such as steel detonation chambers. For a comprehensive review, though it is focused on detonation NDs, the reader should consider Aleksenskii [54].

2.6. Synthesis of Nitrogen-Vacancy (NV)Centers-Enriched Nanodiamonds (NDs)

Recently, Reineck et al. [55] reported the observation of NV− emissions from unprocessed detonation NDs, with a brightness comparable to what is obtained with highly processed fluorescent 100 nm HPHT NDs. Moreover, diamond CVD synthesis of highly-oriented NV centers was reported [56], with the possibility of crushing fluorescent bulk diamond into NDs [57]. Despite that, the standard way to produce NV-centers-enriched NDs is a complex, multi-step, and expensive process. Synthetic HPHT diamonds typically contain 100 ppm of nitrogen atoms as an impurity in the diamond structure [58] and to form NV centers, vacancies are created in the lattice by high-energy particle irradiation (electrons, protons, He+) followed by a vacuum annealing at 700–1000 ◦C[59]. The role of the annealing is to increase the mobility of the nitrogen atoms inside the lattice, which are then trapped by the carbon vacancies to form NVs. One could directly irradiate NDs to increase the NV-fluorescence, but it has been proven by density functional theory [60] and Monte Carlo simulations [61] that NV concentration increases non-linearly with crystal size. In particular, the probability of forming NV centers in 5 nm sized NDs is 4.5 and 25 times lower compared to 20 and 55 nm NDs, respectively [61]. Thus, it is more efficient to increase NV concentration in micro-sized diamonds and then reduce their size to obtain fluorescent NDs. This was done by Boudou et al. [23], in what has become the standard fluorescent NDs synthesis technique. The procedure is shown in Figure6. The starting material is HPHT-synthesized microdiamonds that are then irradiated with a 10 MeV electron beam to form vacancies. The following step is annealing at 750 ◦C to form fluorescent microdiamonds. The next step is the reduction of diamond size, achieved by nitrogen jet milling and ball milling to obtain NDs with a crystal size smaller than 10 nm. For bioimaging, which is the main field of application for fluorescent NDs, the number of NV centers should be as high as possible. For this reason, only HPHT diamonds that contain a relevant amount of nitrogen impurities are used. In contrast, CVD diamond films contain a very small number of nitrogen atoms [62]. Consequently, highly fluorescent NDs are not produced by this mechanism; rather CVD is used to produce nanocrystalline diamond films with single NV centers, suitable for quantum information technologies.

2.7. Fluorescent NDs Synthesis by Pulsed Laser Ablation PLA has been demonstrated to be a promising process for the direct synthesis of NV-enriched NDs. Narayan et al. [63] achieved synthesis of NV-doped diamond in the form of single-crystal nanodiamonds, nanoneedles, microneedles, and thin films by pulsed laser irradiation of N-doped carbon film. Diamond phase formation is obtained through conversion of the sp2 carbon atoms through rapid melting in a super undercooled state and consequent quenching at ambient temperatures and pressures in air. Recently, we demonstrated direct synthesis of NV-centers-enriched NDs through the PLA of graphite in two different nitrogen-containing media: controlled nitrogen atmosphere [64] and liquid nitrogen [65]. In the first case, graphite is laser ablated inside a vacuum chamber filled with 1 Pa of nitrogen gas, and the ablated material deposits onto a silicon substrate. The resulting sample is a diamond-like carbon (DLC) film embedded with the fluorescent NDs (Figure7). Appl. Sci. 2020, 10, 4094 9 of 28 Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 30

Figure 6. Nitrogen-vacancy (NV)-enriched NDs preparation. Reproduced with permission from [23], Figure 6. Nitrogen-vacancy (NV)-enriched NDs preparation. Reproduced with permission from Elsevier, 2013. [23], Elsevier, 2013.

2.7 Fluorescent NDs Synthesis by Pulsed Laser Ablation PLA has been demonstrated to be a promising process for the direct synthesis of NV-enriched NDs. Narayan et al. [63] achieved synthesis of NV-doped diamond in the form of single-crystal nanodiamonds, nanoneedles, microneedles, and thin films by pulsed laser irradiation of N-doped carbon film. Diamond phase formation is obtained through conversion of the sp2 carbon atoms through rapid melting in a super undercooled state and consequent quenching at ambient temperatures and pressures in air. Recently, we demonstrated direct synthesis of NV-centers- Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 30

enriched NDs through the PLA of graphite in two different nitrogen-containing media: controlled nitrogen atmosphere [64] and liquid nitrogen [65]. In the first case, graphite is laser ablated inside a vacuum chamber filled with 1 Pa of nitrogen gas, and the ablated material deposits onto a silicon Appl.substrate. Sci. 2020 The, 10, 4094resulting sample is a diamond-like carbon (DLC) film embedded with the fluorescent10 of 28 NDs (Figure 7).

FigureFigure 7. 7. ((aa)) ExperimentalExperimental setup setup of of pulsed pulsed laser laser ablation ablation (PLA) (PLA).. (b) (SEMb) SEM image image of the of thesynthesized synthesized sample:sample: onon the Si Si substrate substrate a athin thin diamond diamond-like-like carbon carbon (DLC (DLC)) film filmis deposited is deposited over which over NDs which are NDs − aredispersed. dispersed. (c) NV (c) NVcenter− center energy energy levels. levels.When the When NV is the excited NV isby excited using a bygreen using laser a green(green laserarrow), (green arrow),it can relax it can either relax radiatively either radiatively (red arrow) (red or arrow) non-radiative or non-radiativelyly (yellow dotted (yellow arrow). dotted The arrow). effect of The the e ffect oflattice the lattice strain, strain, both in both its parallel in its parallel and perpendicular and perpendicular component componentss with respect with to the respect NV axis, to the on NV the axis, ondegenerate the degeneratedd ms = 1 spinms =states1 spin is represented states is represented (see text for (seedetails). text In for the details). red box: Inscheme the red of diamond box: scheme ofcrystal diamond structure crystal with structure an NV center, with consisting an NV center, of a nitrogen consisting atom of(blue a nitrogen sphere) and atom a nearest (blue sphere)-neighbor and a nearest-neighborcarbon-vacancy ( carbon-vacancyyellow circle) along (yellow the (111) circle) direction. along the ((111)d) Left direction.-hand side: (d )SEM Left-hand image of side: one SEM imagemicroparticle of one microparticle deposited on the deposited DLC film, onconsisting the DLC of clustered film, consisting C-nanoparticles of clustered with sizes C-nanoparticles below 50 nm. with sizesRight below-hand 50side: nm. corresponding Right-hand side: EDXS corresponding spectrum. The EDXS carbon spectrum. and nitrogen The carbonpeaks are and detected nitrogen at peaks, respectively, 0.27 and 0.38 KeV, indicating that the nitrogen is contained inside the nanoparticles’ cluster. are detected at, respectively, 0.27 and 0.38 KeV, indicating that the nitrogen is contained inside the In addition, the silicon peak from the Si substrate was detected at 1.74 KeV (not shown). Reproduced nanoparticles’ cluster. In addition, the silicon peak from the Si substrate was detected at 1.74 KeV with permission from [64], Royal Society of Chemistry, 2018. (not shown). Reproduced with permission from [64], Royal Society of Chemistry, 2018. In the case of ablation in liquid nitrogen, the NDs are directly dispersed into the liquid medium. In the case of ablation in liquid nitrogen, the NDs are directly dispersed into the liquid medium. In both cases, NDs show intense native photoluminescence (PL) without the need of post-synthesis In both cases, NDs show intense native photoluminescence (PL) without the need of post-synthesis thermal activation or additional procedures. Proof of NV centers formation is obtained through thermal activation or additional procedures. Proof of NV centers formation is obtained through optically detected magnetic resonance (ODMR), reported in Figure 8, for synthesis in liquid nitrogen. optically detected magnetic resonance (ODMR), reported in Figure8, for synthesis in liquid nitrogen. Appl. Sci. 2020, 10, 4094 11 of 28 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 30

FigureFigure 8. 8.( a(a)) Wide Wide ﬁeld field optical optical imageimage andand ((bb)) relativerelative photoluminescencephotoluminescence ( (PL)PL) image. ( (cc,d,d)) Typical Typical ODMRODMR spectra spectraobtained obtained fromfrom the bright spot spot of of (b (b),), confirming conﬁrming the the Nitrogen Nitrogen-Vacancy-Vacancy (NV (NV)) origin origin of the of thecollected collected PL. PL. The The intensity intensity reduction reduction at at2.87 2.87 GHz GHz is isevident evident in in (c (),c ),while while in in (d ()d a) adouble double resonance resonance is is observed due to strain of the NDs hosting the NV centers and removing the degeneracy of the ms = 1 observed due to strain of the NDs hosting the NV centers and removing the degeneracy of the ms = ±1± states.states. Reproduced Reproduced with with permissionpermission fromfrom [[65]65],, Elsevier,Elsevier, 2019.2019.

TheThe mechanism mechanism responsibleresponsible forfor NDND formationformation isis thethe samesame reportedreported in SectionSection 2.4. The The whole whole graphite–liquid–diamondgraphite–liquid–diamond transition transition is is represented represented by by the the purple purple arrow arrow inthe in the carbon carbon phase phase diagram diagram [41 ] in[41] Figure in Figure9a. The 9 tilteda. The red tilted lines red indicate lines the indicate region the where region undercooled where undercooled liquid carbon liquid and carbon metastable and diamondmetastable coexist, diamond referred coexist to as, referred a “diamond-like” to as a “diamond liquid [-66like].” The liquid transition [66]. The to t diamondransition phaseto diamond occurs becausephase occurs in this because thermodynamic in this thermodynamic condition, the energycondition barrier, the energy for diamond barrier formation for diamond is lower formation than that is forlower graphite than formationthat for graphite [67]. It formation is important [67] to. noteIt is important the presence to note of atomic the presence nitrogen, of which atomic is nitrogen, required + 4+ forwhich NV-centers is required formation. for NV-centers The energy formation. of the The atoms, energy ions of the (from atoms, C to ions C (from), and C+ electrons to C4+), and leaving electrons the laserleaving ablated the laser target ablated can be target higher can than be higher 100 eV than for the100 typicaleV for the laser typical fluence laser used fluence [68]. used This [68] energy. This is enoughenergy tois breakenough the toN break2 molecules, the N2 molecules, having a bondhaving dissociation a bond dissociation energy at energy room temperature at room temperature of 9.79 eV. Thus,of 9.79 the eV. ablated Thus, energeticthe ablated species energetic can easilyspecies break can easily N2 molecules, break N2 andmolecules single, Nand atoms single may N atoms be trapped may insidebe trapped the nanodroplets inside the nanodroplets expelled from expelled the graphite from the target. graphite Interestingly, target. Interestingly, we performed we aperformed comparison a betweencomparison these between two methods these two (ablation methods of graphite(ablation inof gasgraphite and liquid in gas nitrogen) and liquid in nitrogen) term of the in term total PLof emissionthe total ofPL the emission produced of the sample produced (Figure sample9b,c). (Figure 9b,c). The results, shown in Figure9d, prove that the LN 2-NDs present a PL emission intensity (7000 2000 a.u.) that is more than one order of magnitude higher with respect to DLC-NDs ± (400 100 a.u.). The explanation of this larger NV-enriched ND production efficiency can be related to ± the different conditions attained inside the ablation plume. In particular, a strong sp2 contamination of the NV-enriched NDs surface is observed for laser ablation in a gaseous environment [65] due to both lower pressure and smaller cooling rates achieved in the ablation plume. Appl. Sci. 2020, 10, 4094 12 of 28 Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 30

FigureFigure 9.9. ((aa)) Carbon Carbon phase phase diagram. diagram. Stability Stability regions regions of diamond of diamond (D), (D), graphite graphite (G), (G),vapor vapor (V), (V),and andliquid liquid carbon carbon (L) are (L) indicated, are indicated, respectively, respectively, in white, in white, grey, grey,blue, blue, and green. and green. The purple The purple arrow arrowshows showsthe graphite the graphite–liquid–diamond–liquid–diamond transition transitionused to explain used ND to explainformation ND under formation pulsed under laser irradiation pulsed laser of

irradiationgraphite in of LN graphite2. The in tilted LN2. Thered tilted lines redshow lines the show region the known region as known “diamond as “diamond-like-like liquid” liquid” [33], where [33], whereundercooled undercooled liquid carbon liquid carbon and metastable and metastable diamond diamond coexist. coexist. Typical TypicalPL wide PL-field wide-field images imagesof NDs ofsynthesized NDs synthesized by Pulsed by Laser Pulsed Deposition Laser Deposition (PLD) in (PLD) nitrogen in nitrogen atmosphere atmosphere (b) and ( bby) andPLA by in PLA liquid in liquidnitrogen nitrogen (c). (d ()c ). PL (d ) strength PL strength comparison comparison between between the the two two samples, samples, obtained obtained by by integrating integrating the the intensity of the whole PL image after background subtraction: LN -NDs show a more than 10 higher intensity of the whole PL image after background subtraction: LN2 2-NDs show a more than 10× higher emissionemission comparedcompared toto Diamond-likeDiamond-like CarbonCarbon (DLC)-embedded(DLC)-embedded NDs. The error bars of ((dd)) areare givengiven byby the standard deviation of the didifferentfferent integratedintegrated images.images. Reproduced with permission from [[65]65],, Elsevier,Elsevier, 2019.2019. 3. Optical Sensing with NDs The results, shown in Figure 9d, prove that the LN2-NDs present a PL emission intensity (7000 ± 2000The a.u.) reason that is behind more than the quest one order for fluorescent of magnitude NV-NDs higher resides with inrespect the peculiar to DLC properties-NDs (400that ± 100 such a.u.). a point-likeThe explanation color center of this possesses. larger NV In-enriched fact, the NV-relatedND production fluorescence efficiency is can strongly be related influenced, to the different even at roomcondition temperature,s attained byinside surrounding the ablation electrical plume. and In magneticparticular, fields a strong as well sp2 ascontamination by local environmental of the NV- parametersenriched NDs such surface as temperature. is observed The fordependence laser ablation of in the a gaseous electronic environment energy levels [65] on due the to surrounding both lower electromagneticpressure and smaller fields cooling or temperature rates achieved is responsible in the ablation for such plume. behavior. Panel (c) of Figure7 shows how the ground level population is responsible for the activation of the more or less efficient fluorescent recombination3. Optical Sensing processes with afterNDs green optical pumping of the electronics states. In this section, we review the main sensing capabilities of NV-NDs [69–79]. The reason behind the quest for fluorescent NV-NDs resides in the peculiar properties that such 3.1.a point Magnetic-like color Sensing center possesses. In fact, the NV-related fluorescence is strongly influenced, even at room temperature, by surrounding electrical and magnetic fields as well as by local environmental parametersMagnetic such field assensing temperature. with commercial The dependence NV-NDs of hasthe beenelectronic demonstrated energy levels to detect on the feeble surrounding magnetic fields with a limit of sensitivity down to nT/ √Hz [13,18]. The spatial scale involved within this electromagnetic fields or temperature is responsible≈ for such behavior. Panel (c) of Figure 7 shows detectionhow the schemeground is level potentially population a nanometric is responsible one, and for this the astonishing activation spatial of the resolution more or lessis inherently efficient fluorescent recombination processes after green optical pumping of the electronics states. In this section, we review the main sensing capabilities of NV-NDs [69–79]. Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 30

Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 30 3.1. Magnetic Sensing 3.1. MagneticMagnetic S ensing field sensing with commercial NV-NDs has been demonstrated to detect feeble magneticMagnetic fields fieldwith a sensing limit of with sensitivity commercial down NVto ≈- NDsnT/√ Hz has [13,18] been . demonstratedThe spatial scale to involved detect feeble within Appl.thismagnetic Sci. detection2020 field, 10, s 4094 scheme with a limit is potentially of sensitivity a nanometricdown to ≈ nT/ one,√Hz and[13,18] this. The astonishing spatial scale spatial involved resolution within13 of 28 is inherentlythis detection due schemeto the point is potentially-like nature a nanometric of the NV one,color and cent thiser and astonishing to its nm spatial-proximity resolution to the is ND surface.inherently The due interest to the in point measuring-like nature nanometrically of the NV -colorresolved cent ertiny and magnetic to its nm fields-proximity is strong, to theand ND a few due to the point-like nature of the NV color center and to its nm-proximity to the ND surface. The interest examplessurface. The include interest their in measuring use for measurement nanometrically of- resolved in-vivo neuronaltiny magnetic activity fields [11] is ,strong, blood and hemoglobin a few in measuring nanometrically-resolved tiny magnetic ﬁelds is strong, and a few examples include their oxygenationexamples include [80], or their more use general for measuremently non-invasive of in biological-vivo neuronal measurements activity [11] of, intra blood-cellular hemoglobin moving use for measurement of in-vivo neuronal activity [11], blood hemoglobin oxygenation [80], or more chargeoxygenation states. [80] The, or basic more idea general andly the non experiment-invasive biologicals behind measurementssuch detection of schemes intra-cellular are reported moving in generallyFigurecharge 10 states.. non-invasive The basic biological idea and measurements the experiment ofs intra-cellularbehind such detection moving charge schemes states. are reported The basic in idea andFigure the 10 experiments. behind such detection schemes are reported in Figure 10.

Figure 10. (a) Photoluminescence spectrum of an ensemble of NV centers under 532 nm laser FigureFigure 10. 10.( a()a Photoluminescence) Photoluminescence spectrum spectrum of anof ensemblean ensemble of NV of centers NV centers under under 532 nm 532 laser nm pumping. laser pumping.0 The NV0 (575 nm) and NV- (638 nm) zero phonon lines are indicated. The red phonon Thepumping. NV (575 The nm) NV and0 (575 NV nm)− (638 and nm) NV zero- (638 phonon nm) zero lines phonon are indicated. lines are The indicated. red phonon The sidebandred phonon (PSB) sideband (PSB) is evident. (b) Continuous wave optically-detected magnetic resonance (CW-ODMR) issideband evident. (PSB) (b) Continuous is evident. ( waveb) Continuous optically-detected wave optically magnetic-detected resonance magnetic (CW-ODMR) resonance (CW spectrum-ODMR) of NV centersspectrumspectrum at zeroof NV magnetic centers field atat zerozero (b) magnetic recordedmagnetic field using field ( b the()b )recorded opticallyrecorded using detected using the the magneticoptically optically detected resonance detected magnetic technique. magnetic resonance technique. (c) CW-ODMR with an external magnetic field B ≠ 0, where the degeneracy of (cresonance) CW-ODMR technique. with an (c) external CW-ODMR magnetic with an field external B , 0, magnetic where the field degeneracy B ≠ 0, where of thethe mdegeneracys = 1 states of is s ± removed,thethe mms == ±± thus1 states leading is removed,removed, to two di thusthusfferent leading leading optical to to transitionstwo two different different split optical optical by the transitions Zeemantransitions factor split split by 2γ B,bythe withthe Zeeman Zeemanγ being thefactorfactor electron 2γB,2γB, with gyromagnetic γ being tthehe ratio. electronelectron Panels gyromagnetic gyromagnetic (b,c) adapted ratio. ratio. from Panel Panel [81s]. (sb (,cb), cadapted) adapted from from [81]. [81].

NV-NDsNVNV--NDsNDs areare goodgood candidatescandidates for forfor biological biologicalbiological magnetic magneticmagnetic field fieldfield sensing sensingsensing due duedue to totothe thethe B- sensitivity B-sensitivityB-sensitivity they theythey oofferofferffer... AA continuouscontinuouscontinuous wave wave electronelectelectronron paramagnetic paramagnetic resonance resonance resonance (CW (CW-EPR)optically-detected(CW-EPR-EPR)optically)optically-detected-detected magnetic magnetic magnetic resonanceresonanceresonance (ODMR) ( (ODMRODMR B-detection) B-detection scheme scheme scheme capable capable capable of sensingof of sens sensing magneticing magnetic magnetic fields fields fields down down todown few to mT tofew few was mT mT recentlywas was demonstratedrecentlyrecently demonstrateddemonstrated (Figure 11 (Figure(Figure)[82]. 11)11) [8 [822].] .

FigureFigure 11.11. ODMRODMR spectra spectra under under external external magnetic magnetic field: field: zerozero-fieldfield (red curve), (red curve), 1 mT ( 1blue mT curve) (blue, curve),and andFigure3 mT 3 mT (11.green (greenODMR curve). curve). spectra TheThe circlesunder circles external are are the the experimentalmagnetic experimental field: data zero data while-field while the (red the lines curve), lines are are 1 multi mT multi-gaussian -(gaussianblue curve) fits., andfits. Adapted3Adapted mT (greenwith with curve). permissionpermission The from circles [8 [82 2 are]],, Royal Royal the experimentalSociety Society of of Chemistry, Chemistry, data while 2016 2016. . the lines are multi-gaussian fits. Adapted with permission from [82], Royal Society of Chemistry, 2016. Another interesting magneto-optical application of commercial NDs is related to the magnetic modulation of the red NV-fluorescent signal, allowing for background-free imaging of the fluorescence of complex biological systems containing NDs, where the sensing characteristics of NDs are exploited to filter out the target fluorescence signals. As evidenced by [83], the magnetic modulation achieved Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 30

Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 30 Another interesting magneto-optical application of commercial NDs is related to the magnetic modulation of the red NV-fluorescent signal, allowing for background-free imaging of the Another interesting magneto-optical application of commercial NDs is related to the magnetic fluorescence of complex biological systems containing NDs, where the sensing characteristics of NDs modulation of the red NV-fluorescent signal, allowing for background-free imaging of the Appl.are Sci.exploited2020, 10, 4094 to filter out the target fluorescence signals. As evidenced by [83], the magnetic14 of 28 fluorescence of complex biological systems containing NDs, where the sensing characteristics of NDs modulation achieved by [84] allowed for the creation of high-contrast imaging of NV-PL in high are exploited to filter out the target fluorescence signals. As evidenced by [83], the magnetic background environments (see Figure 12 for more details). bymodulation [84] allowed achieved for the by creation [84] allowed of high-contrast for the creation imaging of NV-PLhigh-contrast in high imaging background of NV environments-PL in high (backgroundsee Figure 12 env forironments more details). (see Figure 12 for more details).

Figure 12. (a) Fluorescence spectrum of NDs in water under 532 nm Continuous-Wave (CW)-laser Figurepump 12.ing.( a ()b Fluorescence) Temporal evolution spectrum of of the NDs fluorescence in water under intensity 532 nm at Continuous-Wave 687 nm under 2 Hz (CW)-laser magnetic Figure 12. (a) Fluorescence spectrum of NDs in water under 532 nm Continuous-Wave (CW)-laser pumping.modulation (b.) The Temporal inset shows evolution Fourier of transf the fluorescenceormed frequency intensity spectrum. at 687 ( nmc) Norma underlized 2 Hz fluorescence magnetic pumping. (b) Temporal evolution of the fluorescence intensity at 687 nm under 2 Hz magnetic modulation.spectrum of The NDs inset in shows water Fourier (black) transformed and its Fast frequency Fourier spectrum. Transform (c ) (FFT Normalized) spectrum fluorescence (red) after modulation. The inset shows Fourier transformed frequency spectrum. (c) Normalized fluorescence spectrumdemodulation of NDs (NDs in water concentration (black) and itsin water Fast Fourier 0.1 μg TransformmL−1). The (FFT)asterisk spectrum indicates (red) Raman after demodulationpeaks of water. spectrum of NDs in water (black) and its Fast Fourier Transform (FFT) spectrum (red) after (NDsReproduced concentration with permission in water 0.1 fromµg mL [851],). Springer The asterisk, 2017 indicates. Raman peaks of water. Reproduced demodulation (NDs concentration in water− 0.1 μg mL−1). The asterisk indicates Raman peaks of water. with permission from [85], Springer, 2017. Reproduced with permission from [85], Springer, 2017. The cutting edge of research in the field of opto-magnetic imaging of NDs is represented in the workThe by cutting the authors edge ofof research[86], who in successfully the field of opto-magnetic monitored the imaging position of of NDs fluorescent is represented NDs by in using the The cutting edge of research in the field of opto-magnetic imaging of NDs is represented in the workthem by as the beacons authors for of the [86 early], who diagnosis/imaging successfully monitored of tumors the positionvia their of positioning fluorescent inside NDs bybiological using work by the authors of [86], who successfully monitored the position of fluorescent NDs by using themtissues. as beaconsThe NV− for center the earlyis suitable diagnosis for intra/imaging-tissue ofimaging tumors because via their it emits positioning fluorescence inside in biological the near‐ them as beacons for the early diagnosis/imaging of tumors via their positioning inside biological tissues.infrared. The Nearly NV− 70%center of its is light suitable emission for intra-tissue lies in the near‐infrared imaging because (NIR) itwindow emits fluorescence from 650 to 1350 in the nm tissues. The NV− center is suitable for intra-tissue imaging because it emits fluorescence in the near‐ near-infrared.(Figure 13) [87 Nearly], where 70% light of has its lightthe maximum emission liespenetration in the near-infrared depth in biological (NIR) windowtissue [88 from]. 650 to 1350infrared. nm (Figure Nearly 1370%)[87 of], its where light lightemission has the lies maximum in the near‐infrared penetration (NIR) depth window in biological from 650 tissue to 1350 [88]. nm (Figure 13) [87], where light has the maximum penetration depth in biological tissue [88].

Figure 13. Fluorescence spectrum (red curve) of NDs superimposed on the near-infrared window

ofFigure biological 13. F tissues.luorescence The spectrum black and (red gray curve) curves of are NDs the superimposed absorption spectra on theof near‐infrared water (H2O), window oxygen of boundbiological hemoglobin tissues. (HbOThe black), and and hemoglobin gray curves (Hb), are the respectively. absorption Reprinted spectra of withwater permission (H2O), oxygen from bound [87], Figure 13. Fluorescence spectrum2 (red curve) of NDs superimposed on the near‐infrared window of Elsevier,hemoglobin 2012. (HbO2), and hemoglobin (Hb), respectively. Reprinted with permission from [87], biological tissues. The black and gray curves are the absorption spectra of water (H2O), oxygen bound Elsevier, 2012. Fluorescenthemoglobin commercial (HbO2), and NDs hemoglobin where observed (Hb), respectively. with optically Reprinted detected with magnetic permission resonance from (ODMR)[87], imagingElsevier, through 2012 1.5. to 5 mm thick sections of breast tissue of chicken. The idea behind this imaging scheme is to detect the 0-magnetic-field fluorescence signal, slicing the biological tissue with B gradient, as in normal magnetic resonance imaging, but collecting the red NV B-dependent fluorescence, see Figure 14. It is clear that the main advantage of fluorescent NDs with respect to a normal MRI is that the first one pumps and collects optical waves (with a potential resolution close to the wavelength of Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 30

Fluorescent commercial NDs where observed with optically detected magnetic resonance (ODMR) imaging through 1.5 to 5 mm thick sections of breast tissue of chicken. The idea behind this imaging scheme is to detect the 0-magnetic-field fluorescence signal, slicing the biological tissue with B gradient, as in normal magnetic resonance imaging, but collecting the red NV B-dependent Appl.fluorescence, Sci. 2020, 10 ,s 4094ee Figure 14. It is clear that the main advantage of fluorescent NDs with respect15 ofto 28 a normal MRI is that the first one pumps and collects optical waves (with a potential resolution close to the wavelength of observation, i.e., sub-micrometric resolution) whereas in the usual MRI, the RF observation, i.e., sub-micrometric resolution) whereas in the usual MRI, the RF pumping and collection pumping and collection limits the resolution to millimeters. In the case of tumor diagnosis/imaging, limits the resolution to millimeters. In the case of tumor diagnosis/imaging, this can mean earlier this can mean earlier observation of the insurgence of the illness. observation of the insurgence of the illness. Other magnetic detection schemes with NDs have been demonstrated, and comprehensive lists Other magnetic detection schemes with NDs have been demonstrated, and comprehensive lists can be found in References [89–99]. can be found in References [89–99].

Figure 14. Panel (a) ND phantoms made of commercial NDs. (b) Chicken breast illuminated by an LED. TheFigure stripe 14. onPanel the chicken(a) ND phantoms breast marks made the of edge commercial of the phantom, NDs. (b which) Chicken is placed breast within illuminat theed chicken by an breast,LED. The 5 mm stripe behind on the the chicken front surface, breast facingmarks the the LED edge emitting of the phantom, at 621 nm. which Fluorescence is placed is within collected the outchicken to the breast, side. (5c )mm ND behind phantoms the imagedfront surface, outside facing of the the chicken LED emitting breast. Reprinted at 621 nm. with Fluorescence permission is fromcollected [86]. outReproduced to the side. with (c permission) ND phantoms of the imaged American outside Chemical of the Society, chicken 2013. breast. Reprinted with permission from [86]. Reproduced with permission of the American Chemical Society,2013. 3.2. Electric Field Sensing 3.2. ElectricThe electric Field fieldSensing sensing capability of the NV-NDs is related to electric-field-dependent shifts in energyThe electric levels [field10]. Thesensing Stark capability shift in theof the excited NV-ND triplets is related states ofto NVelectric centers-field was-dependent detected shifts at low in temperatureenergy levels [100 [10]], and. The the Stark ground shift state in Stark the excited shift of NV triplet ensembles states of was NV proven centers at low was temperature detected at [101 low]. Atemperature voltage [10] [ was100], applied and the to ground a gold state layer Stark deposited shift onof NV a bulk ensembles diamond was sample prove containingn at low t NVemperature centers, to[10 generate1]. A voltage the electric [10] was field applied (see Figureto a gold 15 layerb). To deposited generate on and a orientatebulk diamond the applied sample magneticcontaining field, NV twocenters Helmholtz, to generate coil pairsthe electric (x-y axes) field and(see aF singleigure 1 coil5b). (z-axis)To generate were and used. orientate A set of the coils, applied fabricated magnetic as partfield, of two the Helmholtz gold layer, generatedcoil pairs ( thex-y resonantaxes) and microwave a single coil field. (z-axis To) measure were used. the A shifts set of of coils, spin sub-levelsfabricated inducedas part of by the the gold electric layer, field, generated the electron the resonant spin resonance microwave transition field. To between measure the the m sshifts= 0 and of spin ms = sub1- ± sub-levelslevels induced in the by triplet the electric ground field state, th ofe theelectron NV center spin resonance was optically transition detected. between Upon applicationthe ms = 0 and of anms electric= ±1 sub field-levels of ca.in the 1000 tripl V/etcm, ground resonance state lineof the shifts NV ofcenter 28.4 kHzwas optically were observed. detected. The Upon final application estimated 1 0.5 electricof an electric field sensitivity field of ca.was 1000 202.6 V/cm V cm, −resonanceHz− . line shifts of 28.4 kHz were observed. The final estimated electric field sensitivity was 202.6 V cm−1 Hz−0.5. 3.3. Temperature Sensing ODMR is very useful for testing the temperature sensing properties of NV centers in diamond and ND. Both the spin resonance and coherence time of the NV− center are sensitive to the variations of temperature at the nanoscale [16]. In Reference [102], the ODMR spectra of single 100-nm NDs under a green laser pump (Figure 16a,b) were measured, and temperatures with a precision of 0.1 K were demonstrated (Figure 16c). The local nano-sources of heat were gold nanoparticles independently laser-heated and made to interact with the fluorescent NDs (Figure 16b). More complex magneto-optical detection schemes demonstrated that fluorescent NDs can achieve temperature sensitivities down to the 1 mK Hz–1/2 regime [102–104]. These results prove that the NDs are among the most sensitive temperature probes known. Appl.Appl. Sci. Sci.2020 2020, 10, 10, 4094, x FOR PEER REVIEW 1616 of of 28 30

Figure 15. (a) Schematic drawing of the NV center; (b) simulated electric field 6 µm below the microstructureFigure 15. (a) (where Schematic the NV drawing is located) of the for NV 1 Vcenter voltage; (b) applied; simulated (c) observedelectric field shift 6 of µm the below ODMR the resonance lines for different voltages applied; (d) schematic of the used confocal setup with Helmholtz Appl. Sci.microstructure 2020, 10, x FOR PEER(where REVIEW the NV is located) for 1 V voltage applied; (c) observed shift of the ODMR17 of 30 coilsresonance for magnetic lines fieldfor different alignment. voltages Reprinted applied; withpermission (d) schematic from of [10 ], the Springer, used confocal 2011. setup with Helmholtz coils for magnetic field alignment. Reprinted with permission from [10], Springer, 2011.

3.3. Temperature Sensing ODMR is very useful for testing the temperature sensing properties of NV centers in diamond and ND. Both the spin resonance and coherence time of the NV- center are sensitive to the variations of temperature at the nanoscale [16]. In Reference [102], the ODMR spectra of single 100‐nm NDs under a green laser pump (Figure 16a,b) were measured, and temperatures with a precision of 0.1 K were demonstrated (Figure 16c). The local nano-sources of heat were gold nanoparticles independently laser-heated and made to interact with the fluorescent NDs (Figure 16b). More complex magneto-optical detection schemes demonstrated that fluorescent NDs can achieve temperature sensitivities down to the 1 mK Hz−1/2 regime [102–104]. These results prove that the NDs are among the most sensitive temperature probes known. Some other temperature sensing schemes have been demonstrated, and selection of them can be found in [105–110].

Figure 16. (a) ODMR spectrum around the NV center of a single ND. The four red points indicate Figurethe RF 16. frequencies (a) ODMR used spectru to extractm around the temperaturethe NV center (maximum of a single slopeND. The of the four dip). red (pointsb) Confocal indicate spatial the RFmap frequencies of ﬂuorescent used NDsto extract (circles) the andtemperature gold nanoparticles (maximum (cross).slope of ( cthe) Temperature dip). (b) Confocal of a single spatial ND map as a offunction fluorescent of laser NDs power (circles) with and (red) gold or without nanoparticles (blue) laser(cross). illumination (c) Temperature on a nearby of a gold single nanoparticle. ND as a function(d) Temperature of laser power changes with of (red) a single or without ND, as (blue) a function laser illumination of the distance on a fromnearby the gold illuminated nanoparticle. gold (dnanoparticle) Temperature indicated changes by of the a cross. single Reprinted ND, as a with function permission of the fromdistance [102 from], Springer, the illuminated 2013. gold nanoparticle indicated by the cross. Reprinted with permission from [102], Springer, 2013. Some other temperature sensing schemes have been demonstrated, and selection of them can be 3.4.found Other in I [mplementations105–110]. of NV-NDs Local, environmental, mechanical stress acts as a strain field on the fluorescent PLA NDs that can be detected via ODMR as an additional source of the ODMR fluorescence dip splitting. We demonstrated in [64] that such a field is well explained by taking into account the spin-Hamiltonian of the NV center with the added of a potential term, Vgs, which accounts for the interaction of the NV center with the local strain field e. Strain fields down to 10−5 were detected and good compatibility with Raman peak shifts was proven. Figure 17 reports the wide-field ODMR where micrometric- sized regions-of-interest (ROIs) can be extrapolated in ODMR curves [64]. Appl. Sci. 2020, 10, 4094 17 of 28

3.4. Other Implementations of NV-NDs Local, environmental, mechanical stress acts as a strain field on the fluorescent PLA NDs that can be detected via ODMR as an additional source of the ODMR fluorescence dip splitting. We demonstrated in [64] that such a field is well explained by taking into account the spin-Hamiltonian of the NV center with the added of a potential term, Vgs, which accounts for the interaction of the NV center with the 5 local strain field e. Strain fields down to 10− were detected and good compatibility with Raman peak shifts was proven. Figure 17 reports the wide-field ODMR where micrometric-sized regions-of-interest Appl. Sci. 2020, 10, x FOR PEER REVIEW 18 of 30 (ROIs) can be extrapolated in ODMR curves [64].

Figure 17. Spatially resolved OMDR wide-field imaging and strain-field dependence. Panel (a): Figure 17. Spatially resolved OMDR wide-field imaging and strain-field dependence. Panel (a): Schematics of the inverted wide-field ODMR microscope. The green laser is steered onto the sample and Schematics of the inverted wide-field ODMR microscope. The green laser is steered onto the sample focused through the 40 objective through a dichroic mirror transmitting the red photoluminescence. and focused through× the 40× objective through a dichroic mirror transmitting the red The RF irradiation is obtained with an Au-coated copper loop, here represented by the microwave photoluminescence. The RF irradiation is obtained with an Au-coated copper loop, here represented antenna wire located under the sample’s surface. Panel (b): Temporal diagram of the continuous wave by the microwave antenna wire located under the sample’s surface. Panel (b): Temporal diagram of electron spin resonance experiment. The camera detects the PL image synchronously with the RF the continuous wave electron spin resonance experiment. The camera detects the PL image irradiation, and the laser is kept ON all along the measurement time. Panel (c) Photoluminescence synchronously with the RF irradiation, and the laser is kept ON all along the measurement time. Panel images as the RF frequency sweeps in the 2.85–2.90 GHz range. An ROI (red square) containing a (c) Photoluminescence images as the RF frequency sweeps in the 2.85–2.90 GHz range. An ROI (red bright spot was selected and the average intensity inside that ROI was plotted as a function of the square) containing a bright spot was selected and the average intensity inside that ROI was plotted RF frequency to generate an ODMR spectrum. Panel (d) reports three typical examples of signals on as a function of the RF frequency to generate an ODMR spectrum. Panel (d) reports three typical diamond nano-agglomerates showing different strain effects (curves d1, d2, and d3). Reproduced with permissionexamples of from signals [64], on Royal diamo Societynd nano of Chemistry,-agglomerates 2018. showing different strain effects (curves d1, d2, and d3). Reproduced with permission from [64], Royal Society of Chemistry, 2018. NDs are able to determine the structure of single proteins like ferritin. Ermakova et al. [111] whereNDs able are to detectable to the determine presence ofthe ferritin structure molecules of single on proteins the NDs’ like surface ferritin. via theErmakova increased et magnetic al. [111] relaxationwhere able experienced to detect the in presence such a situation of ferritin compared molecules to on uncoated the NDs NDs.’ surface The via strongly the increased fluctuating magnetic spin bathrelaxation created experienced by the ferritin in such coating a situation at room compared temperature to uncoated made the NDs. spin relaxationThe strongly time fluctuating (T1) of the spin NV centerbath created relax 10 by times the ferritin faster thancoating that at of room the uncoatedtemperature NDs, made see Figurethe spin 18 relaxation. It is worth time mentioning (T1) of the here NV thatcenter single relax Gd 10 ionstimes have faster been than detected that of onthe the uncoated surface NDs, of NDs see with Figure this 1 technique8. It is worth [112 mentioning]. here that singleTo conclude Gd ion thiss have section been detected on sensing on bythe NV-containingsurface of NDs NDs,with this we technique want to underline [112]. that PLA NDsTo permit conclude “doping” this section of the NDson sensing with, forby example,NV-containing silicon, NDs germanium,, we want orto underline other dopants, that PLA by using NDs dopedpermit graphite“doping” targets. of the NDs This with will, for contribute example, to silicon, the exploration germanium of, or other other emission dopants, spectral by using regions doped (seegraphite Figure targets. 19), which This will could contribute open a wealth to the ofexplor newation sensing of other opportunities. emission spectral regions (see Figure 19), which could open a wealth of new sensing opportunities. Appl. Sci. 2020, 10, x FOR PEER REVIEW 19 of 30 Appl. Sci. 2020, 10, 4094 18 of 28 Appl. Sci. 2020, 10, x FOR PEER REVIEW 19 of 30

Figure 18. (a) High resolution Transmission Electron Microscopy (TEM) of ferritin molecules (right) FigureFigure 18. 18. (a()a )High High resolution resolution Transmission Transmission Electron Electron Microscopy Microscopy (TEM (TEM)) of offerritin ferritin molecules molecules (right) (right) showing the iron-containing core and a single ND covered with ferritin (left). (b) The spin relaxation showingshowing the the iron iron-containing-containing core core and and a asingle single ND ND covered covered with with ferritin ferritin (left). (left). (b) ( bThe) The spin spin relaxation relaxation time T1 of naked NV (left) and in ferritin-coated NDs (left, inset). Statistical distribution of the T1 for timetime T1 T1 of of naked naked NV NV (left) (left) and and in in ferritin ferritin-coated-coated NDs NDs (left, (left, inset). inset). Statistical Statistical distribution distribution of the of the T1 T1for for bare NDs (right, blue bars) and for ferritin-coated NDs (right, red bars). Reprinted with permission barebare NDs NDs (right, (right, blue blue bars) bars) and and for for ferritin ferritin-coated-coated NDs NDs (right, (right, red red bars). bars). Reprinted Reprinted with with permission permission from [111], American Chemical Society, 2013. fromfrom [111] [111,], American American Chemical Chemical Society, Society, 2013 2013..

Figure 19. Fluorescence spectra of color centers in diamonds under 532 nm laser excitation. Lead-related FigureFigure 19. 19. Fluorescence Fluorescence spectra spectra of ofcolor color centers centers in indiamonds diamonds under under 532 532 nm nm laser laser excitation. excitation. Lead Lead- - (Pb-rel), germanium-vacancy (GeV), tin-vacancy (SnV), and silicon-vacancy (SiV). Reproduced with relatedrelated (Pb (Pb-rel),-rel), germanium germanium-vacancy-vacancy (GeV) (GeV), , tin tin-vacancy-vacancy (SnV), (SnV), and and silicon silicon-vacancy-vacancy (SiV). (SiV). permission from [113], Springer, 2013. ReproducedReproduced with with permission permission from from [113] [113], Springer, Springer, 2013, 2013. . Appl. Sci. 2020, 10, 4094 19 of 28

4. Nanodiamonds as Catalysts NDs recently attracted attention in the catalysis field where they have been investigated in two main roles: as a support material to be functionalized with a catalyst or as a proper catalyst material. In the former case, NDs are attractive due to their chemical and mechanical stability, large surface area, and relatively easy surface functionalization, allowing them to load a variety of catalysts. The catalysis function is then only loosely influenced by the NDs’ properties and this body of literature will not be considered here. For a recent review we point the reader to Campisciano et al. [114]. The following section will instead be focused only on those investigations where NDs are reported as catalyst materials themselves. Catalysis by NDs has been reported for a few reactions of industrial interest, including oxidative dehydrogenations and dechlorination reactions. Recently, conductive NDs were also considered as electrocatalysts for CO2-reduction schemes with promising results. Dehydrogenation of ethylbenzene is the main route for the industrial synthesis of styrene, employed for more than 80% of its production worldwide [115]. Styrene is then used to obtain a variety of specialty polymers applied as resins, synthetic rubbers, and plastics. The process generally involves a Fe2O3 catalyst and requires superheated steam to limit an otherwise rapid catalyst poisoning by carbon-based species. [116]. When investigated as an alternative catalyst, NDs compared favorably with respect to the benchmark, providing longer duration without requiring superheated steam [117]. The activity was attributed to high surface area and to a partially-ordered geometry promoting reagent adsorption. However, the NDs employed were powders from detonation synthesis, which are of limited industrial interest due to incompatibility with fixed-bed configurations. Steps towards solving this issue by immobilizing NDs were taken by using a SiC foam [118] or few-layered graphene [119] as the supporting material. Dechlorination is a class of reactions where chlorine in an organochlorine compound is removed. Three main routes are commonly employed: the high temperature removal of HCl (dehydrochlorination), reductive replacement of chlorine with hydrogen from catalyst-activated hydrogen gas (hydro-dechlorination), and chlorine gas extraction by a metal-based catalyst (simple dechlorination). These reactions are used in industrial synthesis and are especially important for the disposal of chlorinated chemicals, such as chlorinated solvents. The high temperature and/or metal-based catalysis requirement generated interest towards investigating carbon-based catalysts as replacements. NDs in particular proved to be active for the dechlorination of 1,2-dichloroethane in relatively mild conditions (300 ◦C), promoting a combination of hydro-dechlorination and simple dechlorination. Interestingly, hydro-dechlorination took place from hydrogen-terminated surface sites on NDs [120]. Electrocatalytic reduction to fuels (CH4, CO, ethanol) or value-added chemicals (acetic acid, formic acid, methanol, ethylene, propanols) is currently one of the most promising routes for CO2 conversion, providing at the same time a storage method for intermittent renewable energy sources and CO2 extraction from the atmosphere [121,122]. Materials requirements for a CO2-reduction electrocatalyst are quite strict: In addition to long-term stability, a high activity is needed to overcome the difficult kinetics dictated by the multi-electron redox nature of the reaction. Another important point is selectivity, as the hydrogen evolution reaction often competes successfully with the CO2 reduction pathway [123,124]. As a result, the field is very active and rapidly evolving with novel materials being investigated at a quick pace. In this context, carbon-based materials are attractive because they can offer a cost-effective alternative with reasonably high conductivity, surface area, and the possibility of tuning their properties by doping. In fact, while carbon materials in general were found to be poor CO2-reduction catalysts with poor selectivity [125], doping with nitrogen [126] or boron [127] resulted in very different properties. In both cases, doping induced a lower onset potential and very high (77% for N and 74% for B) faradaic efficiency, associated to acetate and formate production, respectively, indicating a remarkable selectivity. Appl. Sci. 2020, 10, 4094 20 of 28

While the number of reports investigating NDs as catalysts is still small, the potential of NDs in this field is apparent and makes the development of reliable, cheap, and tunable synthesis methods an Appl. Sci. 2020, 10, x FOR PEER REVIEW 21 of 30 important task. 5. Possible Connections with Some Processes Occurring in Space 5. Possible Connections with Some Processes Occurring in Space There has also been evidence that NV centers in NDs could be at the very origin of a long-lasting, There has also been evidence that NV centers in NDs could be at the very origin of a long-lasting, puzzling, visible (red) emission observed in particular classes of nebulae, in particular, carbon-rich puzzling, visible (red) emission observed in particular classes of nebulae, in particular, carbon-rich planetary nebulae. This is the so-called extended red emission (ERE) that has been a literature topic planetary nebulae. This is the so-called extended red emission (ERE) that has been a literature topic since the 1980s [128–130]. In Figure 20 we report the spectrum of the extended red emission (ERE) since the 1980s [128–130]. In Figure 20 we report the spectrum of the extended red emission (ERE) from the so called red rectangle (right panel), from [129], collected at different observation angles and from the so called red rectangle (right panel), from [129], collected at different observation angles and compared with a typical spectrum of crushed CVD NDs with an average tail ≈40 nm. As can be seen, compared with a typical spectrum of crushed CVD NDs with an average tail 40 nm. As can be seen, the ERE emission from the red rectangle (which is 25 times more intense than the≈ other ERE nebular the ERE emission from the red rectangle (which is 25 times more intense than the other ERE nebular emissions, probably explaining why in all other lower signal strength spectra, they have not been emissions, probably explaining why in all other lower signal strength spectra, they have not been resolved) is compatible with the photoluminescence spectrum of commercial uncoated Bikanta [131] resolved) is compatible with the photoluminescence spectrum of commercial uncoated Bikanta [131] 40 nm-tailed NDs, if the VIS (532 nm) pumping redshift with respect to far-UV pumping is taken into 40 nm-tailed NDs, if the VIS (532 nm) pumping redshift with respect to far-UV pumping is taken account, as was done in [131]. In particular, it is evident how the zero-phonon-line (ZPL) of the NV- into account, as was done in [131]. In particular, it is evident how the zero-phonon-line (ZPL) of the emission is present both in the ERE as well as in the crushed commercial NDs, thus suggesting a NV− emission is present both in the ERE as well as in the crushed commercial NDs, thus suggesting a significant VIS pumping versus the far-UV pumping of the NDs causing the ERE emission from the significant VIS pumping versus the far-UV pumping of the NDs causing the ERE emission from the red rectangle [128,132]. red rectangle [128,132].

Figure 20. Comparison between a typical photoluminescence spectrum of 40 nm crushed NDs obtained Figureunder 20. 532 Comparison nm optical pumpingbetween aand typical the extended photoluminescence red emission spec (ERE)trum emission of 40 from nm crushedthe redrectangle. NDs obtainedThe peak under at 532 637 /nm638 optical nm is stronglypumping reproducible and the extended and wasred emission ﬁrst observed (ERE) in emission [128]. Its from compatibility the red

rectanglewith the. The zero peak phonon at 637/638 line of nm the is NV strongly− center reproducible in NDs is evidenced and was firs witht observed a vertical in line. [128 A]. better, Its compatibilitylonger wavelength, with the zero PL-tail phonon compatibility line of the is expected NV- center under in NDs far-UV is evidenced irradiation, with as from a vertical Reference line. A [132 ]. betterThe, peaklonger at aroundwavelength 580 nm, PL could-tail compatibilitybe correlated with is expec the NVted0 zero under phonon far-UV line. irradiation Data of ERE, as (blue from and Refrederence curve) [132] are. The reproduced peak at around from [ 129580]. nm could be correlated with the NV0 zero phonon line. Data of ERE (blue and red curve) are reproduced from [129]. Nanometer-tailed ﬂuorescent NDs produced via PLA techniques and the proposed thermodynamic model for their formation can contribute to interpretation of ERE space emission because in the PLA technique, high pressure, high temperature, and fast cooling conditions are controllable quantities that permitNanomete us tor infer-tailed the circumstellar fluorescent NDs environmental produced conditions. via PLA Figure techniques 21 gives and an idea the of the proposed relevance thermodynamicof this type of model cross-analysis. for their formation can contribute to interpretation of ERE space emission because in the PLA technique, high pressure, high temperature, and fast cooling conditions are controllable quantities that permit us to infer the circumstellar environmental conditions. Figure 21 gives an idea of the relevance of this type of cross-analysis. Appl. Sci. 2020, 10, x FOR PEER REVIEW 22 of 30

Appl. Sci. 2020, 10, 4094 21 of 28

Figure 21. The black line is the PL spectrum under a 532 nm pump of PLA NDs containing NV centers, while the blue and red curves are typical ERE emissions, pertinent to NGC2023 and NGC7023 nebulae Figure 21. The black line is the PL spectrum under a 532 nm pump of PLA NDs containing NV centers, Data reproduced from [64] (black curve), [133] (red curve) and [129] (blue curve). while the blue and red curves are typical ERE emissions, pertinent to NGC2023 and NGC7023 nebulae DataMoreover, reproduced in from a recently [64] (black published curve), paper[133] (red regarding curve) and the [129] liquid (blue state curve). of carbon [134], it was pointed out that while the temperature–pressure conditions to have liquid carbon are hardly ever found on

earth, this is not the case in stellar and planetary interiors [135,136], where the existence of the liquid state is quite probable. Indeed, as pointed out by Ross [136], the thermodynamic conditions in the Finally, Moreover, in a recently published paper regarding the liquid state of carbon [134], it was liquid central layers of, for example, Uranus and Neptune planets, favor the pyrolysis of methane pointed out that while the temperature–pressure conditions to have liquid carbon are hardly ever gas into a mixture of elemental carbon and hydrogen. In the PLA synthesis of NDs [41,64,65] the found on earth, this is not the case in stellar and planetary interiors [135,136], where the existence of conditions to generate liquid carbon from which NDs are formed are investigated fairly well in the liquid state is quite probable. Indeed, as pointed out by Ross [136], the thermodynamic conditions controlled experimental conditions. These studies then allow us to generate laboratory conditions that in the liquid central layers of, for example, Uranus and Neptune planets, favor the pyrolysis of help to formulate plausible hypotheses in relation to some astrophysical processes. methane gas into a mixture of elemental carbon and hydrogen. In the PLA synthesis of NDs [41,64,65] Finally,a comprehensive list of papers on regarding the liquid state of carbon in stellar and planetary the conditions to generate liquid carbon from which NDs are formed are investigated fairly well in interiors, and possible nanodiamond-related origin of ERE can be found in References [135–145]. controlled experimental conditions. These studies then allow us to generate laboratory conditions that6. help Conclusions to formulate and plausible Perspectives hypotheses in relation to some astrophysical processes. Finally, a comprehensive list of papers on regarding the liquid state of carbon in stellar and planetaryThe interiors, interest inand NDs possible and in particularnanodiamond fluorescent-related NDs origin in a of large ERE variety can be of found science in fields References is pushing [135the–14 research5]. on this particular nanoparticle. This interest is due to the properties of fluorescent NDs, which in turn leads, as described in this review, to many different applications. Indeed, the significant 6. Conclusionsand growing and potential Perspective for commercials applications of nanodiamonds is testified to by the volume and timeline of published patents. A search using the “nanodiamonds” keyword on the Espacenet The interest in NDs and in particular fluorescent NDs in a large variety of science fields is database yields 2077 results [146], steadily rising in a time window starting in 1995 and peaking in 2017. pushing the research on this particular nanoparticle. This interest is due to the properties of The largest group concerns fabrication and modification/functionalization methods (570), but patents fluorescent NDs, which in turn leads, as described in this review, to many different applications. regarding specific applications are the second largest group (240) and are concentrated in the last Indeed, the significant and growing potential for commercial applications of nanodiamonds is five years. To date, several industrial-grade producers are active, and commercial nanodiamonds for testified to by the volume and timeline of published patents. A search using the “nanodiamonds” research purposes are available via both general chemical suppliers Sigma-Aldrich (St Louis, MO, USA) keyword on the Espacenet database yields 2077 results [146], steadily rising in a time window and specialized companies such as Ray Techniques Ltd. (Jerusalem, Israel), Adàmas Nanotechnologies starting in 1995 and peaking in 2017. The largest group concerns fabrication and (Raleigh, NC, USA), PlasmaChem (Berlin, Germany), Tong Li Chemicals (Shenzhen, China), and TCI modification/functionalization methods (570), but patents regarding specific applications are the Chemicals (Tokyo, Japan), among others. second largest group (240) and are concentrated in the last five years. To date, several industrial- However, many challenges remain, in particular concerning the synthesis process. To enable a grade producers are active, and commercial nanodiamonds for research purposes are available via wide use of nanodiamonds, large-volume cost-effective production is required, as well as a better Appl. Sci. 2020, 10, 4094 22 of 28 control of the surface chemistry, which is essential for certain applications. In addition, control of the size of NDs with limited dispersion and established content of NV centers are objectives to be pursued. Moreover, preventing aggregation during synthesis of ultrasmall fluorescent nanodiamonds (<4 nm) is a fundamental step towards improving sensing capabilities. In summary, synthesis techniques should be improved to obtain fluorescent NDs with a quality closer to bulk diamond. This research would lead to a better control of NDs’ properties and thus to novel applications.

Author Contributions: Conceptualization, L.B., M.C., M.O. and A.M; writing—original draft preparation, L.B., M.C., M.O. and A.M.; writing—review and editing, L.B., M.C., M.O. and A.M.; supervision, A.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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