Recent Trends in Noble Metal Nanoparticles for Colorimetric Chemical Sensing and Micro-Electronic Packaging Applications

Review Recent Trends in Noble Metal Nanoparticles for Colorimetric Chemical Sensing and Micro-Electronic Packaging Applications

Anurag Gautam 1,*, Pragya Komal 2 , Prabhat Gautam 3 , Ashutosh Sharma 4 , Neeraj Kumar 5 and Jae Pil Jung 6,*

1 Department of Chemistry, Geethanjali College of Engineering and Technology, Cheeryal, Hyderabad, Telangana 501301, India 2 Department of Biology, Birla Institute of Technology and Science, Pilani-Hyderabad Campus, Jawaharnagar, Shamirpet Mandal, Hyderabad, Telangana 500078, India; [email protected] 3 Department of Chemistry, CMR Institute of Technology, Bengaluru, Karnataka 560037, India; [email protected] 4 Department of Materials Science and Engineering, Ajou University, 206 Worldcup-ro, Yeongtong-gu, Suwon 16499, Korea; [email protected] 5 Department of Metallurgical Engineering, SOE, O.P. Jindal University, Raigarh 496109, India; [email protected] 6 Department of Materials Science and Engineering, University of Seoul, Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, Korea * Correspondence: [email protected] (A.G.); [email protected] (J.P.J.)

Abstract: Noble metal NPs are highly attractive candidates because of their unique combination of physical, chemical, mechanical, and structural properties. A lot of developments in this area are still fascinating the materials research community, and are broadly categorized in various sectors such as chemical sensors, biosensors, Förster resonance energy transfer (FRET), and microelectronic applications. The related function and properties of the noble metals in these areas can be further Citation: Gautam, A.; Komal, P.; tailored by tuning their chemical, optical, and electronic properties that are inﬂuenced by their Gautam, P.; Sharma, A.; Kumar, N.; Jung, J.P. Recent Trends in Noble size, shape, and distribution. The most widely used Au and Ag NPs in dispersed phase below Metal Nanoparticles for Colorimetric 100 nm exhibit strong color change in the visible range which alters upon aggregation of the NPs. Chemical Sensing and The chemical sensing of the analyte is inﬂuenced by these NPs aggregates. In this article, we have Micro-Electronic Packaging summarized the uniqueness of noble metal NPs, their synthesis methods, nucleation and growth Applications. Metals 2021, 11, 329. process, and their important applications in chemical sensing, microelectronic packaging, and Förster https://doi.org/10.3390/met11020329 resonance energy transfer.

Academic Editor: Marco Martino Keywords: colloid; nanostructure; microelectronic systems; crystalline; hydrothermal; nucleation Received: 22 January 2021 and growth Accepted: 9 February 2021 Published: 14 February 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Nanomaterial is defined as a material in which the maximum value of one dimension −9 iations. can be 100 nm, which can be further defined as one billionth of meter or 10 m [1–6]. It is approximately 10 H or 5 Si atoms in a line. It is continuing to be the most rapidly growing R/D sector in last decades, which is evident from more than several billion dollars of an- nual investment in this particular field [7,8]. Due to its unique features, nanomaterials and NPs allow them to be used for a wide variety of applications in nanotechnology covering Copyright: © 2021 by the authors. medical science, chemical, bio-network, applied physics, materials, microelectronic and Licensee MDPI, Basel, Switzerland. This article is an open access article metallurgy science, and engineering. There are lots of investments in the area of medical distributed under the terms and science, in particular, theragnostics, which refers to two kinds of word therapeutics and conditions of the Creative Commons diagnostics [9]. It is an advanced technique in which cancer diagnosis and therapy is done Attribution (CC BY) license (https:// simultaneously, for early detection and cure of the cancer [10,11]. To achieve this, some creativecommons.org/licenses/by/ special metals in the periodic table include alkaline to alkaline metallics, rare metallics, and 4.0/). noble metallics used for theranostics application [12]. Compared to these metals, noble

Metals 2021, 11, 329. https://doi.org/10.3390/met11020329 https://www.mdpi.com/journal/metals Metals 2021, 11, 329 2 of 21

metals have exceptional resistance in corrosive environments (in acidic or basic medium) and they also resist to oxidation even at elevated temperature. Moreover, these metals, especially gold and silver at a nanometer scale, exhibit unique and tunable plasmonic properties that have fascinated the researcher greatly over the last decade and still receiving increased attention up to date. [13–15]. Their plasmonic features bring more advantages in the biomedical and biosensor fields [5]. The use of noble metals dates back to the ancient Roman and Greek era [5,16]. Precious metals such as gold and silver were used in fabrication for various purposes such as pots, liquids containers, and also in the manufacturing of coins. The silver colloids were extensively used in antimicrobial therapy until antibiotics were developed in the early 1940s [17]. In addition, in the ancient Chinese era, gold and its complexes were used for medicines. Moreover, Europeans, also used colloidal gold in medieval period particularly by chemist/alchemist in laboratories for the treatment of many infectious diseases and even recommended to provide long life [18]. The unique term nanotechnology was specified by Noble Laureate Dr. R. Feynman on his visionary lecture entitled “There is Plenty of Room at the Bottom” at the meeting of American Physical Society, at California Institute of Technology, USA in December 1959 [19]. Noble metals have always been dispersed in various supports or supporting materials for a range of catalytic and sensing applications [20,21]. However, particle growth at high temperatures is a serious issue faced by many researchers which causes a loss in sensitivity and catalytic activities [22]. Among methods of dispersion, Arnal et al. developed stabi- lization of Au/ZrO2 catalyst against sintering at high temperatures [23]. In very recent sudy, Yang et al. [24] studied the uniformly dispersed fluorescent Ag NPs in aqueous solution capped with poly-methacrylic acid for sensing the lead ions in water. Their study suggested the fluorescence intensity of Ag nanoclusters was increased in presence of Pb(II) ions in water upon excitation with 320 nm. The detection limit was estimated to be 60 nM. Additionally, hollow silica spheres encapsulated silver nanoparticles (Ag@SiO2) that can be useful for the sensing and drug delivery at the targeted site. A similar method has also been used to encapsulate Au NPs by hollow silica that can have a wider application, such as catalysis, detection, portable microelectronics, and antibacterial areas [23,24]. The future generation of metal NPs promises attractive chemical sensors that are expected to rely on various principles, such as plasmon sensors, nanobiosensors, coulo- metric and fluorometric principles. Therefore, in this review we have highlighted various aspects of Au NPs sensors for chemical sensing and flexible sensors for microelectronic packaging devices.

2. Surface Charge Determination on the NPS The stability of the NPs in the colloidal solution is due the presence of surface charge on the nanoparticles surface. These charges are measured by a technique that is called zeta-potential analyzer that predict their stability [25,26]. If a measured zeta-potential of a surface modified nanoparticles showed a large positive or large negative, then the nanocolloids would have good physical stability due to electrostatic repulsion between the individual nanoparticles. The Zeta-potential value of nanoparticles +30 mV or −30 mV suggests good nanocolloids stability. Zeta-potential observed in the all colloidal system that has solid-liquid and liquid-liquid dispersion [27]. It is basically an electrical potential developed at the interfacial double layer which is dispersed nanoparticles and the continuous phase that could be the dispersion medium. It is determined through velocity measurement of the charged nanoparticles moving toward the electrode across the sample solution in the presence of an external electric field [25–27]. Rasmussen et al. determined the surface charge on various nanoparticles using salt gradient which caused a charge reversal of NPs in opposite direction. The increase in the NPs concentration and their spatial distribution provides both size (69–73 nm) and charge (−30 to −48 mV) on the surface of NPs [28]. Various surfactants are available in literature to provide anionic, ionic, or non-ionic ones to control the surface charge on the surface of NPs in the field of colloids, nanofluids, and nanocoatings [29–31]. Metals 2021, 11, x FOR PEER REVIEW 3 of 21

crease in the NPs concentration and their spatial distribution provides both size (69–73 nm) and charge (−30 to −48 mV) on the surface of NPs [28]. Various surfactants are available in literature to provide anionic, ionic, or non-ionic ones to control the surface charge on the surface of NPs in the field of colloids, nanofluids, and nanocoatings [29–31]. Metals 2021, 11, 329 3 of 21 3. Estimation of Surface to Volume Ratio The reduction in particle range (nm) progressively scales the surface area of atoms and 3.the Estimation surface to of volume Surface ratio to Volume of nano Ratiorange particles. Consequently, particle size and surface atomsThe reduction are two in essential particle rangefactors (nm) in progressivelynanomaterial scalesresearch. the surfaceFor spherical area of atomsshape and particles,the surface the particle to volume radius ratio (r) of varies nano rangeinversely particles. with Consequently,the surface to particlevolume sizeratio and (S/V), surface i.e., atoms are two essential factors in nanomaterial research. For spherical shape particles, the particle radius (r) varies inversely with the surface to volume ratio (S/V), i.e., = = (1) / S 4πr2 3 = = (1) where r = particle radius. The surfaceV to volume4πr3/3 ratio isr consequently interrelated, increasing with a decrease in the size of NPs. In an investigation, Nutzenadel et al. [32] reportedwhere the r =variation particle in radius. the surface The surface atoms of to Pd-metal volume ratiocluster is consequently(clusters are aggregates interrelated, of in- atoms/molecules)creasing with over a decrease the range in the of size1 to of4 nm NPs.. They In an suggested investigation, that, with Nutzenadel a diameter et al. of[ 32] approximatelyreported the 4 variationnm, 30% inof theatoms surface are on atoms the ofsurface, Pd-metal and cluster after decreasing (clusters are the aggregates particle of size atoms/molecules)close to 1 nm, the surface over the becomes range of 75% 1 to as 4 depicted nm. They in suggestedFigure 1. that, with a diameter of approximately 4 nm, 30% of atoms are on the surface, and after decreasing the particle size close to 1 nm, the surface becomes 75% as depicted in Figure1.

100

80 •

60 •

40 •

Surface atoms [%] Surface atoms 20 0 • 10-1 100 101 102 103 104 105

dcluster [nm]

FigureFigure 1. Variation 1. Variation of the of surface the surface atoms atoms in Pd in nanoparticle Pd nanoparticle clusters. clusters. The substantial quantity of atoms in these materials lies on the surface boundary and The substantial quantity of atoms in these materials lies on the surface boundary and therefore, there is improved contact and bonding at the nanoscale. For example, noble metal therefore, there is improved contact and bonding at the nanoscale. For example, noble NPs have novel optical properties when exposed to electromagnetic radiations known metal NPs have novel optical properties when exposed to electromagnetic radiations as surface plasmon resonance (SPR). The SPR occurs because the conduction electron known as surface plasmon resonance (SPR). The SPR occurs because the conduction oscillates continuously close to the metallic surface. If the size of NPs is usually less than electron oscillates continuously close to the metallic surface. If the size of NPs is usually the wavelength of the incident light, they interact and obey Rayleigh’s theory rather than less than the wavelength of the incident light, they interact and obey Rayleigh’s theory the Mie theory. In this state, SPR is subjected to electromagnet radiations primarily by rather than the Mie theory. In this state, SPR is subjected to electromagnet radiations dipolar mode as clearly shown in Figure2. primarily by dipolar mode as clearly shown in Figure 2.

Metals 2021, 11, 329 4 of 21 Metals 2021, 11, x FOR PEER REVIEW 4 of 21

FigureFigure 2.2. SchematicSchematic diagram diagram of of surface surface plasmon plasmon resonance resonance (SPR) (SPR) incoherent incoherent interaction interactio ofn the of electronsthe electrons in conduction in conduction band ofband metal of metal NPs with NPs an with applied an applied electromagnetic electromagnetic ﬁeld. field.

All the electrons and nuclei oscillate in thethe same phase at thethe samesame frequencyfrequency inin thethe applied electricelectric fieldfield [ 33[33,34].,34]. Furthermore, Furthermore, as as long long as allas NPsall NPs are uniformare uniform and smallerand smaller than thethan wavelength the wavelength of the of light, the SPRlight, does SPR not does reveal not reveal any noticeable any noticeable changes changes due to thedue similar to the cross-sectionsimilar cross-section of the NPs. of the NPs. 4. Nucleation and Growth Process 4. Nucleation and Growth Process Nucleation is a framework that begins at the atomic scale where atoms serve as seeds Nucleation is a framework that begins at the atomic scale where atoms serve as in the liquid phase. These seeds such as atoms are arranged in particular patterns that seeds in the liquid phase. These seeds such as atoms are arranged in particular patterns create the final crystal structure of the specific compound. These seeds or nuclei are very that create the final crystal structure of the specific compound. These seeds or nuclei are prone to the external particles or impurity of the system, so they are intentionally applied invery many prone industrial-scale to the external processes particles to or enhance impurity the of nucleation the system, rate. so Thethey rate are ofintentionally nucleation canapplied be further in many divided industrial-scale into two categories: processes homogeneousto enhance the and nucleation heterogeneous. rate. The There rate is of a greaternucleation chance can ofbe reactionfurther di invided solution into phase two categories: synthesis to ho proceedmogeneous with and nucleation heterogeneous. through heterogeneousThere is a greater since chance structural of reaction inhomogeneities in solution such phase as thesynthesis surface to of proceed the vessel, with stirrer nuclea- rod, andtion impuritiesthrough heterogeneous may act as a stabilizersince structural surface inhomogeneities for nucleating. Homogeneoussuch as the surface nucleation, of the however,vessel, stirrer is more rod, likely and toimpurities begin somewhere may act inas the a stabilizer bulk solution surface from for the nucleating. walls of the Homo- vessel. Thegeneous process nucleation, of homogeneous however, nucleationis more likely can beto viewed,begin somewhere according in to thermodynamics,the bulk solution asfrom the the change walls in of the the total vessel. energy The process of the particle, of homogeneous which isa nucleation sum of the can surface be viewed, and bulk ac- volumetriccording to thermodynamics, energy as shown in as Figure the change3. Furthermore, in the total for energy spherical of the particles particle, with which radius is ra andsum surfaceof the surface energy/area, and bulk the freevolumetric energy/volume energy as of shown crystals in is Figure Gv. The 3. total Furthermore, Gibbs energy for changespherical is givenparticles by Equationwith radius (2): r and surface energy/area, the free energy/volume of crystals is Gv. The total Gibbs energy change is given by Equation (2): 2 4 3 ∆G = 4πr σ + π4r ∆Gv (2) G=4𝑟 +3 𝑟 G (2) 3 Usually,the free energy of the crystal (∆Gv) can be written in correlation with temperature Usually, the free energy of the crystal (Gv) can be written in correlation with tem- T, Boltzmann constant (k and supersaturating solution (S) as below Equation (3): perature T, Boltzmann constantB), (kB), and supersaturating solution (S) as below Equation (3): ∆Gv = −kBT ln(S)/v (3)

G = 𝑘𝑇ln(S)/𝑣 (3) Further, the entire energy change of the crystal can be given as below Further, the entire energy change of the crystal can be given as below 4 ∆G = 4πr2σ − π4r3∆G (4) G=4𝑟 3 𝑟 vG (4) 3 Furthermore, inin orderorder to to obtain obtain an an equilibrium equilibrium condition, condition, we we can can obtain obtain the totalthe total free energyfree energy barrier barrier by the by differentiation the differentiation of Equation of Equation (4) (preserving (4) (preserving d(∆G)/dr d(G)/dr = 0). In= other0). In words,other words, the critical the critical radius radius (r*) of a(r*) nanoparticle of a nanoparticle can be can computed be computed from Equation from Equation (5) [35]. (5) [35]. d(∆G) = 8πrσ − 4πr2∆G (5) dd(rG) v =8𝑟 4𝑟 G (5) d𝑟

Metals 2021, 11, x FOR PEER REVIEW 5 of 21

Metals 2021, 11, 329 5 of 21

ΔG +Ve S G Δ ∗ ΔG ∗ r r Free Energy, Energy, Free

ΔG

-Ve ΔG V

FigureFigure 3. 3.Schematic Schematic representationrepresentation ofof GibbsGibbs freefree energyenergy changechange inin nucleationnucleation and growth process. Δ Δ Δ ∆GG representsrepresents totaltotal Gibb’sGibb’s energy, ∆Gv and ∆G*G* are are free free energy energy and and critical critical free free energy energy ( (G*)∆G*) of ofthe thecrystal. crystal.

NowNow from from (5) (5) 8πrσ = 4πr2∆G (6) v 8𝑟 =4𝑟 G (6) and 2σ and r∗ = (7) ∆Gv ∗ 2 Substitution of Equation (7) in (4) provides𝑟 = the value of total Gibbs free energy change,(7) 𝐺 for example, Substitution of Equation (7) in (4) prov16idesπσ3 the value of total Gibbs free energy ∗ = change, for example, ∆G 2 (8) 3∆Gv The critical threshold of free energy (∆G*) required∗ 16 for NPs to grow in the liquid phase 𝐺 = (8) is given by Equation (8). 3𝐺

5. SyntheticThe critical Methods threshold of Noble of free Metal energy NPs (G*) required for NPs to grow in the liquid phase is given by Equation (8). The most important groundbreaking ﬁelds of nanoscience are a vast variety of nanomaterials. The synthesis of NPs can essentially be accomplished via two major approaches: 5. Synthetic Methods of Noble Metal NPs (1) Top-down and (2) Bottom-up technique, as clearly shown in Figure4. The dimension of theThe bulk most crystal important reduces groundbreaking down to the range fields of nanometersof nanoscience in theare top-downa vast variety phase of byna- applyingnomaterials. different The chemicalsynthesis and of NPs physical can approaches.essentially be External accomplished variables, via e.g., two friction major and ap- pressure,proaches: constrain (1) Top-down the particles and (2) in Bottom-up a desired shapetechnique, and sizesas clearly to reduce shown in dimensionin Figure 4. [ 36The]. Fordimension example, of many the bulk approaches crystal reduces have been down used to the in the range processing of nanometers of nanostructured in the top-down ma- terialsphase inby recent applying years, different such as chemical the ball millingand physical process approaches. [37], the lithography External variables, process [ 38e.g.,], thefriction pyrolysis and pressure, process [ 39constrain], and the the thermolysis particles in processa desired [40 shape]. However, and sizes the to top-down reduce in techniquesdimension have[36]. fewFor drawbacks,example, many e.g., approaches large size variation, have been residual used in stresses, the processing and there- of forenanostructured more surface materials defects. in In addition,recent years, this su processch as the is less ball cost-effective milling process because [37], ofthe the li- high-pressurethography process environment, [38], the warm pyrolysis temperatures, process and[39], extremelyand the thermolysis advanced nanostructure process [40]. materialHowever, processing the top-down equipment. techniques have few drawbacks, e.g., large size variation, residual stresses, and therefore more surface defects. In addition, this process is less cost-effective because of the high-pressure environment, warm temperatures, and ex- tremely advanced nanostructure material processing equipment.

MetalsMetals 20212021,, 1111,, 329x FOR PEER REVIEW 66 of of 2121

Figure 4. Two majormajor approachapproach forfor synthesissynthesis ofof NPs.NPs.

Powder metallurgy isis aa method consistingconsisting ofof millingmilling ofof microparticlesmicroparticles downdown toto nanonano levellevel andand subsequentsubsequent densificationdensification toto achieveachieve bulkbulk product.product. The balls areare configuredconfigured toto rotate atat somesome rev/minrev/min (RPM) (RPM) in in this process, the rpm offers the balls withwith energyenergy thatthat isis eventually usedused toto breakdown breakdown the the particles particles from from the the micron micron size size to theto the nano-sized nano-sized particles. parti- Incles. addition, In addition, specific specific parameters parameters of milling of milling such as such atmosphere, as atmosphere, agent media,agent media, speed, ballspeed, to weightball to weight ratio, grinding ratio, grinding time, and time, temperature and temperature must be must further be further tuned totuned obtain to obtain optimum op- sizetimum of NPs.size of NPs. Lithography isis alsoalso another another top-down top-down process process by by which which it is it possible is possible to design to design com- plexcomplex two-dimensional two-dimensional and and three-dimensional three-dimensional nano-scale nano-scale patterns patterns with with better better control control of theof the structure. structure. By By introducing introducing photolithography, photolithography, soft-lithography,soft-lithography, imprintimprint lithography, and scanningscanning probe-lithography (SPL) to a substratesubstrate surface,surface, thethe desireddesired patternpattern cancan bebe developed in various ways. In the fieldfield ofof electronicelectronic circuitcircuit synthesissynthesis andand processingprocessing ofof electronics componentscomponents [[41–45],41–45], thisthis approachapproach cancan bebe used.used. Further, anotheranother top-downtop-down method used forfor thethe processingprocessing atat anan industrialindustrial scalescale isis pyrolysis.pyrolysis. In In this process, the precursor contentcontent is injected at highhigh pressurepressure throughthrough aa hole, followed by high-temperaturehigh-temperature annealingannealing providedprovided byby thethe NPs.NPs. Since thethe higherhigher pressurepressure andand temperaturetemperature werewere involvedinvolved inin thethe synthesissynthesis method,method, thethe NPsNPs obtainedobtained areare often aggregated, so theythey havehave aa largelarge distributiondistribution ofof particleparticle size.size. ThisThis methodmethod maymay notnot be very expensiveexpensive since it takestakes moremore pressurepressure andand temperatures,temperatures, but itit isis alsoalso usedused forfor processingprocessing onon aa commercialcommercial scalescale [[46].46]. The creation of the blocksblocks of atoms begins from thethe bottombottom inin thethe bottom-upbottom-up processprocess and itit goesgoes atomatom by by atom, atom, creating creating the the complete complete structure. structure. These These atoms, atoms, however, however, originate origi- fromnate from their their respective respective ions andions saltsand salts by reduction by reduction with with appropriate appropriate chemical chemical reagents reagents [47]. Bottom-up[47]. Bottom-up nanostructures nanostructures have have lower lower surface surface defects defects and and good good atomic atomic packaging packaging toto minimize the total free energyenergy ofof GibbGibb andand provideprovide greatergreater reliabilityreliability comparedcompared toto thethe top-down method. This improvement in reliability will further expand the applications top-down method. This improvement in reliability will further expand the applications of NPs. of NPs. In previous times that it was used widely, for instance, the chemical reduction process In previous times that it was used widely, for instance, the chemical reduction pro- is divided into green synthesis and the polyol synthesis methods [46,48]. There are a variety cess is divided into green synthesis and the polyol synthesis methods [46,48]. There are a of familiar approaches from bottom to top. In the chemical reduction process, an effective variety of familiar approaches from bottom to top. In the chemical reduction process, an reduction agent is typically used for removing metal ions from their respective salts. The effective reduction agent is typically used for removing metal ions from their respective polar or non-polar and universal solvents may be liquid such as water. The popular salts. The polar or non-polar and universal solvents may be liquid such as water. The reducing agents, such as hydrazine (N2H4), alcohol (C2H5OH), lithium aluminum hydride popular reducing agents, such as hydrazine (N2H4), alcohol (C2H5OH), lithium aluminum (LiAlH4), and sodium borohydride (NaBH4) were reported earlier [48–51]. However, hydride (LiAlH4), and sodium borohydride (NaBH4) were reported earlier [48–51]. chemical reagents used as a reductant in the green synthesis method are moderate in nature However, chemical reagents used as a reductant in the green synthesis method are as well as the most important sources are the reducing agents from green plants [52]. moderateIn the in polyol nature process, as well polyalcohols as the most such important as polyethylene sources are glycol the andreducing polyvinyl agents alcohols from reducegreen plants metal [52]. ions into metallic NPs [53]. The polymer may also serve as a capping layer

Metals 2021, 11, x FOR PEER REVIEW 7 of 21

In the polyol process, polyalcohols such as polyethylene glycol and polyvinyl alcohols reduce metal ions into metallic NPs [53]. The polymer may also serve as a capping Metals 2021, 11, 329 layer of metallic NPs that provides them longer longevity in a specific application. 7It of is 21 also an expense mechanism and can easily be customized to the demands. However, the NPs can be regulated by changing certain parameters including photovoltaic current, of metallic NPs that provides them longer longevity in a specific application. It is also voltage, and temperature. Furthermore, Schulman et al. suggested a microemulsion an expense mechanism and can easily be customized to the demands. However, the NPs process where at least one polar and non-polar phase is mixed with a stable surfactant can be regulated by changing certain parameters including photovoltaic current, voltage, [54]. The micro-emulsion is therefore characterized as a consistent, symmetric, and and temperature. Furthermore, Schulman et al. suggested a microemulsion process where steady dispersion of two or more components. The surfactant forms the interfacial film at least one polar and non-polar phase is mixed with a stable surfactant [54]. The micro- thatemulsion separates is therefore these two characterized layers between as a consistent,the polar and symmetric, the non-polar and steady layer, dispersion producing of micro-emulsionstwo or more components. at this interfacial The surfactant region. Mi formscro-emulsion the interfacial will usually film that occur separates in contin- these uoustwo layerswater betweenor a continuous the polar oil and process the non-polar as a consequence layer, producing of dispersing micro-emulsions the oil drops at this [55–58].interfacial region. Micro-emulsion will usually occur in continuous water or a continuous oil processLaser ablation as a consequence is also a bottom-up of dispersing method the oil where drops we [55 produce–58]. a coating by heating, vaporizing,Laser ablationor sublimating is also ait bottom-up using laser method source where energy we used produce for the a coating ablation. by heating,Conse- quently,vaporizing, the orsubstance sublimating is fully it using transformed laser source into energy the plasma used for phase. the ablation. This technique Consequently, has beenthe substance beneficial is over fully others transformed because into it does the plasma not need phase. to evaporate This technique the excess has been solvent beneficial that canover be others used becausefor aqueous it does and not non-aqueous need to evaporate synthesis the purposes. excess solvent In comparison that can be to used other for approaches,aqueous and this non-aqueous technique also synthesis provides purposes. some additional In comparison advantages, to other including approaches, a short this periodtechnique of time, also industrial provides production, some additional and easily advantages, controlled including sizes and a shortshapes. period of time, industrialMicrowave production, processing and easilyis also controlleda method of sizes irradiation and shapes. where the sample is irradiated with aMicrowave microwave processing source of energy is also a[59]. method When of irradiated irradiation in where the presence the sample of ais surfactant, irradiated thewith metallic a microwave salts would source decompose of energy and [59]. lead When to metal irradiated NPs. in the presence of a surfactant, the metallic salts would decompose and lead to metal NPs. 6. Applications of Noble Metal NPs 6. ApplicationsSensors are generally of Noble Metaldescribed NPs by functional molecules or systems for detecting or sensingSensors a given are analyte generally molecule described in the by surro functionalunding molecules environment. or systems The output for detecting signal is or providedsensing aby given a change analyte in moleculethe analyte in species the surrounding concentration environment. in the sample The output[60–64]. signal There- is fore,provided NPs’ bysensing a change application in the analyte depends species on concentrationthree factors (i) in theanalyte, sample (ii) [ 60a –binding64]. Therefore, sensi- tivityNPs’ detection sensing application part of an analyte, depends and on (iii) three transducers. factors (i) analyte,It is possible (ii) a to binding characterize sensitivity the transducerdetection partas a ofsystem an analyte, capable and of (iii)converting transducers. binding Itis events possible into to comparable characterize voltage the trans- or current.ducer as Therefore, a systemcapable the three of convertingaspects listed binding above events are important into comparable for any voltage sensor orto current.be ef- fectiveTherefore, to produce the three a small aspects error listed with above higher are signal-to-noise important for any(S/N) sensor ratio and to be a effectiveminimum to analyteproduce concentration. a small error However, with higher noble signal-to-noise metal NPs are (S/N) less ratiostable and in open a minimum environments analyte andconcentration. can have difficult However, surface noble chemicals metal NPs that are restrict less stable their in openapplications environments in the field and canof bio-sensing.have difficult surface chemicals that restrict their applications in the field of bio-sensing. TheThe functional functional NPs NPs of of the the noble metal maymay therefore largelylargely possesspossess excellentexcellent photo-pho- tophysicalphysical characteristics characteristics such such as as high high fluorescence fluorescence and and visible visible region region absorption, absorption, conduc- con- ductivity,tivity, and and redox redox properties properties to completelyto completely fulfill fulfill sensing sensing requirements. requirements. The The Ag Ag NPs, NPs, in indifferent different sizes, sizes, reveal reveal different different colors, colors, as asshown shown inin FigureFigure5 5.. TheThe AgAg NPsNPs areare also very stable in colloidal suspension and in the form of nanocomposite films. Figure5A,B displays stable in colloidal suspension and in the form of nanocomposite films. Figure 5A,B dis- the Ag colloidal NPs showing various colors due to particle size differences. plays the Ag colloidal NPs showing various colors due to particle size differences.

Figure 5. Color variation in (A) Ag nanocolloids dispersed in water (B) nanocomposite ﬁlms casted onto silicate glass. Samples 1–6 contain Ag in 0.0, 0.2, 0.5, 1.0, 2.0, and 5.0 wt%, respectively. [with permission reference [13]]. Metals 2021, 11, x FOR PEER REVIEW 8 of 21

Figure 5. Color variation in (A) Ag nanocolloids dispersed in water (B) nanocomposite films casted Metals 2021, 11, 329 8 of 21 onto silicate glass. Samples 1–6 contain Ag in 0.0, 0.2, 0.5, 1.0, 2.0, and 5.0 wt%, respectively. [with permission reference [13]].

WithWith thethe increasingincreasing AgAg content,content, thethe variousvarious sizessizes ofof NPsNPs werewere preparedprepared usingusing thethe polyol method. After that, a viscous solution was cast into glass petri dish in the form of polyol method. After that, a viscous solution was cast into glass petri dish in the form of ﬁlm, the excessive water solvent was evaporated. The color shift in the ﬁlm with the silver film, the excessive water solvent was evaporated. The color shift in the film with the sil- content increasing is seen in Figure5b. The gold particles also displayed exceptional color ver content increasing is seen in Figure 5b. The gold particles also displayed exceptional change based on size and the wavelength of the absorption was tunable in the infrared zone color change based on size and the wavelength of the absorption was tunable in the in- (Figure6). The change of colors in Au NPs was seen by El-Sayed and his coworkers [ 5,62] frared zone (Figure 6). The change of colors in Au NPs was seen by El-Sayed and his as their aspect ratio was different (as shown Figure6a). In addition, the gold shell SPR on coworkers [5,62] as their aspect ratio was different (as shown Figure 6a). In addition, the silica NPs was shown to be core and the silica NPs were tuned to around 600–11,000 nm gold shell SPR on silica NPs was shown to be core and the silica NPs were tuned to (Figure6b). around 600–11,000 nm (Figure 6b).

FigureFigure 6.6. Optical properties of of Au Au (a (a) )color color change change in in Au Au nanorods nanorods with with different different aspect aspect ratios ratios of gold of gold nanoparticles, nanoparticles, (b) (tunableb) tunable SPR SPR of ofgold gold nanoparticles nanoparticles with with differe differentnt aspect aspect ratio ratio of of gold gold nanoparticles, nanoparticles, ( (cc)) color color change noted inin AuAu nanoshellsnanoshells withwith differentdifferent shell-thicknessshell-thickness ((dd),), andand tunabilitytunability ofof theirtheir wavelengthwavelength atat differentdifferent shellshell thickness,thickness, ((ee)) physicalphysical appearance of Au nanocages with different HAuCl4 content in their synthesis, and (f) tunability of their wavelength (with appearance of Au nanocages with different HAuCl4 content in their synthesis, and (f) tunability of their wavelength (with permissionpermission fromfrom [[62]).62]).

InIn otherother reports,reports, XiaXia etet al.al. [[64],64], producedproduced AgAg nanocagesnanocages viavia thethe PolyolPolyol method.method. TheThe subsequentsubsequent transformationtransformation of of Ag Ag into into Au Au nanocages nanocages was was achieved achieved by by adding adding auric auric chloride chlo- solution. The nanocages of gold were created by the silver metal galvanizing as given by:

3Ag (s) + HAuCl4 → Au (s) + 3AgCl (s) + HCl (aq) (9)

Metals 2021, 11, 329 9 of 21

The SPR could be tuned between about 470 nm and 1000 nm (Figure6e). The change of color of gold particles found in the PVA solution was also seen by our group [15]. The PVA was a mild capping agent that stabilized the NPs [53]. These NPs were later reinforced it into PVA to produced Au-PVA composite ﬁlms by evaporating water surplus at 70 ◦C.

6.1. Colorimetric Sensing Colorimetric sensing involves color shift detection of the target solution which is caused by the NPs being aggregated. The aggregation of Ag or Au NPs is mainly correlated with the affinity to bind the targeted analytes of the surface ligands, so aggregation occurs primarily when the analyst binds the ligands on the gold NPs [13,65–68]. In general, however, the golden nanometers are red, their color after aggregation shifts blue, and their SPRs turn to red. Au NPs in the size of ~13 nm, for example, had maximum absorption at 523 nm when the analyte component is attached. In general, nanoparticle aggregation factors are affected by the changes in solution ion power, (ii) functioning of NPs that have DNA or carbohydrate, enzymes, or other threatening agents such as uranium compounds [69,70]. Furthermore, Au NPs were comfortably detected as harmful heavy metal ions (HMI) such as (Pb2+, Cd2+, Hg2+)[71]. Due to HMI chelation, the solution color has been switched from red to blue. On the region of MUA, the carboxylate group was HMI ionic receptors. The detection limit observed was above 400 µM for the entire HMI. The HMI was observed for the calorimetric sensing of aqueous Hg2+ ions, the Mirkin Group functionalized Gold NPs with DNA [72]. The sensing mechanism was related to the coordination compounds of thymine-Hg2+-thymine (T-T). The sensitivity was measured up to 100 nM in the designed sample. Furthermore, in colorimetric sensors for detecting Hg2+ ions, Liu group [73] used a very similar T-T mismatch method. The detection limit was 3 µM. Xie et al. [74] also published an ultra-sensitive Hg2+ detection sample based on fluorescent NPs from the gold. The quenchings are caused by interactions between Hg2+-Au+ and the surface of Au NPs with the small amount (~17%) of Au+ ions modified the surface of the Au metal NPs. Recently the Hg2+ ions, produced by the reaction of tetrachlorinated uric acid in the presence of golden nanoclusters acting as a catalyst, were colorimetrically detected in another study by Zhou et al. The detection limit of Hg2+ was 10 nM, far lower than the allowed level in USA [75]. Silver NPs are also frequently used as optical sensor materials [76–78]. For instance, for Farhadi et al. [76], the newly synthesized silver nanostructure with plant extract was used for detecting Hg2+ ions. It has a scattered, yellowish-brown color that becomes color-free with the Hg2+ sample solution. They also researched the susceptibility of silver metal NPs for alkaline metallic ions, metal ions, and earth metallic alkaline. In comparison with other metals with a detection limit of 2.20 × 10−6 M, however, the findings are optimum for Hg2+ Ions. Sebastian et al. [77] studied the sensing property of stabilized Ag NPs of Agaricus bisporus in a further study These noble metal NPs were also used to detect possible hazards in addition to detecting drinking water toxic ions [78]. 2+ For example, uranium (UO2 ), the calorimetric sensors that have excellent radioactive uranium selectivity, can be detected with gold NPs. The gold NPs detection limit for 2+ UO2 was much lower than the infection level according to Environmental Protection 2+ Agency (EPA-USA) criterion for UO2 in consumable water. To experiment, Liu et al. [79] identified the uranyl specific sensor DNAzyme with a detection limit of 45 pm with high selectivity. Similarly, Lee et al. [69] used both a labeled and label-free method for Uranil sensing and the findings were remarkable. The labeled method showed a higher detection limit of 50 nM and larger response time compared to the label-free method. The various developments in the chemical colorimetric sensing are given in Table1. Metals 2021, 11, 329 10 of 21

Table 1. Colorimetric sensing characteristics of the probes.

Analyte Size of Nps LOD NPs Detection Method Ref. MUA-functionalized Pb2+,Cd2+, Hg2+ 13 nm 400 µM Naked eyes/UVS [71] AuNPs Hg2+ 15 nm 100 nM DNA functionalized AuNPs Naked eyes/UVS [72] Hg2+ 25 Au-atoms 50 µM Au cluster stabilized by Au+ Naked eyes/UVS [74] Hg2+ 15 nm 10 nM BSA Stabilized AuNPs Naked eyes/UVS [75] Pb2+ 5–8 nm 100 nM GSH- functionalized AuNPs Naked eyes/UVS [80] Pb2+ 14 nm 45 nM 2-Mercaptoethanol/AuNPs Naked eyes/UVS [81] 2+ UO2 45 pM DNAzyme- AuNp DNAzyme- AuNPs UVS [79] 2+ UO2 - 1 nM Bare UVS [69] Ba2+ 12 3 nM Guanine-AgNPs Naked eyes [82] Hg2+ - 2.2 µM Soap plant AgNP Naked eyes [76] Hg2+ 14 nm 2.1 µM AgaricusbisporusAgNP Naked eyes [77] BSA = Bovine Serum Albumin, LOD = Limit of detection, CMC = carboxymethyl cellulose, UVS = UV-Visible spectrometry.

6.2. Fluorimetric Sensing Fluorimetric detection is a fluorescence phenomenon regardless of the power losses typically in the solution between the excitation and the emission spectrum. Fluorescence is superior to absorption spectroscopy since the absorbent process is very fast (approximately 10–15 s), so that molecular motion does not occur during the absorbent process, as per the Frank-Condon theory. However, compared to the absorption process, fluorescence takes place over a longer time (environ 10 ns) imply that the fluorophore molecule stays in the exit for a longer period, thus, interacts with its neighboring molecule [61]. This relationship changes; the analyte may increase or decrease fluorescence intensity depending on the fluorophore alignment with the quencher, while fluorophore is linked to the analyte [70]. Furthermore, as stated in the literature in a few cases, static and dynamic quenching can take place together [70]. The major groups of the molecules are hydroxyl, amine, and halogens typically serve as quenchers [61]. The difference in fluorophore intensity occurs due to the resonance energy transfer Förster (FRET). Theoretical calculations by Theodor Förster were based on the induced dipole-dipole interactions between donor molecules and the receiver molecule in 1949. In this phase, fluorophore (donor) initially goes to the excited state through absorption of a photon, when the FRET Mechanism model is followed by the radiation-free energy transfer that transfers energy to another molecule which is a recipient. Then, the acceptor becomes aroused and the acceptor molecule returns to the ground through photon emission, as shown in Figure7. In this method, which is possible by the FRET method, the fluorescence intensity of the acceptor molecule will change. The interaction is studied at molecular levels and the distance from each other on a molecule or like the ligand-guided ion channel can be quantified [78]. Metals 2021, 11, x FOR PEER REVIEW 11 of 21

Metals 2021, 11, 329 11 of 21

S1 Excitation hν FRET Emission Emission

S0 S0 Donor Acceptor

FigureFigure 7. The 7.schematicThe schematic of resonance of resonance energy energy transfer transfer Förster Förster (FRET) (FRET) process. process.

BeyondBeyond the range the range of Förste of Försterr distance, distance, energy energy transfer transfer takes takes place place between between the the do- donor- nor-acceptoracceptor molecules. molecules. The The Förster Förster gap usuallyusually lieslies between between 10 10 Å Å and and 60 60 Å andÅ and many many proteins proteinsare are comparable comparable in size in size [61, 83[61,83].]. Another Another term term RET RET is a specialis a special state state of FRET of FRET when when a donor’s a donor’semission emission spectrum spectrum overlaps overlaps with thewith absorption the absorption spectrum spectrum of the recipient.of the recipient. FRET is also FRETcalled is also RET called in thisRET condition. in this condition. The energy The transferenergy transfer efficiency efficiency can be calculated can be calculated by the Förster by theEquation Förster Equation (10). (10). 1 E = 1 i (10) 𝐸= [1 + ( R )6 𝑅 R0 (10) [1 + ( )] 𝑅 The Förster range, R0, is at that distance where the donor to acceptor distance is Thesuch Förster that the range, energy R0, is transfer at that distance efficiency wh isere 50%. the donor The transfer to acceptor of energies distance betweenis such the that thedonor energy and transfer the accepter efficiency depends, is 50%. for Th example,e transfer on of the energies medium between refractive the indexdonor for and energy the acceptertransmission, depends, donor for example, and acceptor on the spectrum medium form, refractive and shape. index for Noble energy metal transmis- NPs such as sion, donorgold are and closely acceptor connected spectrum to biomoleculesform, and shape. such Noble as protein metal and NPs nucleic such as acids gold and are hence closelyare connected used in fluorescent to biomolecules sensing. such The as pr Auotein NPs’ and association nucleic acids with and the hence biomolecule are used however in fluorescentdepends sensing. on its The composition Au NPs’ andassociation structure. with For the example, biomolecule the spiral however form depends single-stranded on its composition(ss)-DNA andand double-strandedstructure. For example, (ds-DNA) the showspiral differentform single-stranded absorption patterns (ss)-DNA on theand noble double-strandedmetal surface. (ds-DNA) There was show a different different scale. absorption The ss-DNA patterns was absorbedon the noble irreversibly metal sur- and rigid face. ds-DNAThere was (dorsale a different phosphate) scale. wasThe reflectedss-DNA backwas dueabsorbed to repulsion irreversibly between and the rigid negative ds-DNAAuNPs (dorsale stabilized. phosphate) Au metal was NPs reflected can be ba usedck due as anto acceptorrepulsion in between the FRET the phase negative because of 8 −1 −1 AuNPsits stabilized. exceptionally Au highmetal extinction NPs can be coefficient used as an of ~10acceptorM incm the FRETfor 15 phase nm Au because NPs, whereas of 5 −1 −1 its exceptionallyit is ~10 M highcm extinctionfor traditional coefficient sensors. of ~108 Furthermore, M−1 cm−1 for 15 gold nm NPs Au NPs, also havewhereas additional it is ~10characteristics5 M−1cm−1 for traditional such ashigh sensors. conductivity, Furthermore, simple gold surface NPs also alteration, have additional high fluorescence char- acteristicsabnormality, such as andhigh a tunableconductivity, absorption simple spectrum surface varyingalteration, with high their fluorescence size and shape. ab- For normality,example, and Aua tunable nanorods absorption with different spectrum aspect varying ratios with may their be tuned size and to a shape. visible For infrared example,absorption Au nanorods spectrum with (Figure different6), thus aspect the spectrumratios may of be Au tuned NPs canto a alter visible donor infrared emissions absorptionacross spectrum the range (Figure [60,62,84 6),]. thus the spectrum of Au NPs can alter donor emissions across the Dubertretrange [60,62,84]. et al. [ 33 ] prepared the first study on gold-based FRET, which was useful for chemical fluorophores. The FRET was due to the interaction between the dipole and Dubertret et al. [33] prepared the first study on gold-based FRET, which was useful the dipole and the right orientation of the gold NPs. However, for gold spherical NPs for chemical fluorophores. The FRET was due to the interaction between the dipole and with no dipole moment the energy may be transferred into gold NPs in either direction as the dipole and the right orientation of the gold NPs. However, for gold spherical NPs opposed to dyes. This can gain from modern spherical-form metal NPs compared with with no dipole moment the energy may be transferred into gold NPs in either direction as the other types of NPs. Organic molecules that make the receiver excellent. Benzoic acid opposed to dyes. This can gain from modern spherical-form metal NPs compared with (DABCYL) is, for example, considered to be the most capable acceptor of organic quencher the other types of NPs. Organic molecules that make the receiver excellent. Benzoic acid 4-((4’-(dimethylamino)phenyl) azobenzoic acid. The fluorescence intensity of a given color is quenched to 99.0 percent. The efficiency for quenching is not standardized on the whole absorption spectrum, however, and the dyes’ efficiency decreases with long wavelengths. The DABCYL structure is given in Figure8.

Metals 2021, 11, x FOR PEER REVIEW 12 of 21

(DABCYL) is, for example, considered to be the most capable acceptor of organic quencher 4-((4’-(dimethylamino)phenyl) azobenzoic acid. Metals 2021, 11, 329 The fluorescence intensity of a given color is quenched to 99.0 percent. The efficiency12 of 21 for quenching is not standardized on the whole absorption spectrum, however, and the dyes’ efficiency decreases with long wavelengths. The DABCYL structure is given in TheFigure extended 8. The extended length is estimatedlength is estimated to be about to 1.2be about nm, which 1.2 nm, is compatiblewhich is compatible with ~1.4 with nm gold~1.4 nm NPs gold in size. NPs However, in size. However, the gold NPsthe gold are moreNPs are than more 100-fold than more100-fold effective more thaneffective the DABCYL.than the DABCYL. Furthermore, Furthermore, as opposed as toopposed coloring, to in coloring, the infrared in the zone infrared the performance zone the per- of goldformance NPs doesof gold not NPs decrease does [not33, 85decrease]. [33,85].

FigureFigure 8.8. 4-((4’-(dimethylamino)4-((4’-(dimethylamino) phenyl)phenyl) azobenzoicazobenzoic acid.acid.

AlthoughAlthough thethe goldgold NPs NPs are are successful successful quenchers, quenchers, few few reports reports also also indicate indicate a fluores- a fluo- cencerescence enhancement enhancement of goldof gold NPs NPs for for solid solid substrates substrates at at the the required required distances distances from from thethe fluorophorefluorophore metal.metal. However,However, thethe increasedincreased fluorescencefluorescence intensityintensity isis expectedexpected toto bebe duedue toto thethe far-reachingfar-reaching reflectionreflection ofof fluorophorefluorophore radiationradiation thatthat hashas recurredrecurred to fluorophorefluorophore [[86].86]. Recently,Recently, SchietingerSchietinger etet al.al. publishedpublished inin anotheranother paperpaper onon thethe plasmonplasmon intensitiesintensities of up-up- gradegrade luminescence of 30 30 nm nm in in diameter diameter UCNPs UCNPs attached attached to to 30–60 30–60 nm nm gold gold NPs NPs [87]. [87 In]. Inrecent recent years years several several reports reports on on gold gold NPs NPs for for various various fluorometric, fluorometric, quantumquantum dots,dots, andand UCNPUCNP areare summarizedsummarized inin TableTable2 [288 [88–93].–93]. Table 2. Au NPs as FRET acceptors. Table 2. Au NPs as FRET acceptors.

Biological Target Biological Donor/Acceptor Target Donor/Acceptor Limit of Detection Limit of Detection Detection Detection Range Range Ref. Ref. Quantum Glucose QuantumGlucose dot/AuNp 50 nM 50 nM 0.10–50 0.10–50 μM µM[[88]88 ] dot/AuNp Avidin Quantum dot/AuNp 10 nM 10 nm–2 μM [89] Quantum Avidin 10 nM 10 nm–2 µM[89] IgG Fluorophore/AuNp dot/AuNp 1 nM 1–50 nM [90] IgM Fluorophore/AuNpIgG Fluorophore/AuNp 42 pM 1 nM 0.35 to 5 1–50 nM nM [91] [90 ] MMP-7 Fluorophore/AuNpIgM Fluorophore/AuNp 10 ng mL−1 42 pM 10 to 1000 0.35 ng tomL 5− nM1 [92] [91 ] −1 −1 cTnT Fluorophore/AuNpMMP-7 Fluorophore/AuNp 0.02 nM10 ng mL 0.02 10to to0.15 1000 nM ng mL [93][92 ] cTnT Fluorophore/AuNp 0.02 nM 0.02 to 0.15 nM [93] IgG = Immunoglobulin G, IgM = Immunoglobulin M, MMP-7 = Metalloproteinase-7, cTnT = Cardiac troponin T. IgG = Immunoglobulin G, IgM = Immunoglobulin M, MMP-7 = Metalloproteinase-7, cTnT = Cardiac troponin T. Further, FRET performance improvers for the interaction with fluorophore silver Further, FRET performance improvers for the interaction with fluorophore silver NPs NPs were reported as FRET in recent times [94–96]. The studied growth factor-BB iden- were reported as FRET in recent times [94–96]. The studied growth factor-BB identification tification of the human platelet using the enhanced silver nanoparticular FRET sensor of of the human platelet using the enhanced silver nanoparticular FRET sensor of BHQ- BHQ-2 quencher strands. In this study, fluorophore aptamer’s emission spectrum has 2 quencher strands. In this study, fluorophore aptamer’s emission spectrum has been been enhanced near silver NPs. These sensors were more sensitive than bare FRET and enhanced near silver NPs. These sensors were more sensitive than bare FRET and gold gold NPs dependent FRET with regard to sensitivity and selectivity. Their findings NPs dependent FRET with regard to sensitivity and selectivity. Their findings showed that showed that silver NPs are used for improved FRET sensor selectivity and sensitivity of silver NPs are used for improved FRET sensor selectivity and sensitivity of 0.8 ng/mL. 0.8 ng/mL. Another research planned and developed improved FRET imagery by Zhao et Another research planned and developed improved FRET imagery by Zhao et al. [96]. al. [96]. 6.3. Metal NPs in Microelectronic Packaging 6.3. Metal NPs in Microelectronic Packaging Metal NPs have been consistently used in microjoining of several electronic compo- nentsMetal such asNPs electronic have been circuit consistently boards, microfluidic used in microjoining devices, and of nanodielectrics. several electronic Besides, com- enormousponents such research as electronic efforts havecircuit been boards, devoted microfluidic in this area devices, to develop and nanodielectrics. metal NP pastes Be- containingsides, enormous several research hundred efforts nm sizes have for advancedbeen devoted microfluidic in this area and stretchableto develop electronics metal NP inpastes medical containing applications. several Generally, hundred the nm metal sizes NPs for areadvanced prepared microfluidic by using capping and stretchable agents to controlelectronics their in growth medical and applications. particle size Generally, for a variety the of metal applications NPs are [prepared97,98]. Monodispersed by using cap- Auping NPs agents have to been control successfully their growth printed and particle for chemoreceptive size for a variety sensor of circuitapplications assembly [97,98]. on polyimide substrate as shown in Figure9[ 99]. The Ag interdigitated electrodes were printed on polyimide by a robotic dispenser. The printed Au NPs were of less than tens of nanometers. When heat-treated at high temperatures, these metal NPs decompose leaving behind the organic capping layer around the metal core surface which further forms a conducting thin layer over the circuit components. Metals 2021, 11, x FOR PEER REVIEW 13 of 21

Monodispersed Au NPs have been successfully printed for chemoreceptive sensor circuit assembly on polyimide substrate as shown in Figure 9 [99]. The Ag interdigitated electrodes were printed on polyimide by a robotic dispenser. The printed Au NPs were of less than tens of nanometers. When heat-treated at high temperatures, these metal NPs Metals 2021, 11, 329 decompose leaving behind the organic capping layer around the metal core surface13 of 21 which further forms a conducting thin layer over the circuit components.

FigureFigure 9.9. ((a)) SchematicSchematic ofof chemoreceptivechemoreceptive sensorsensor circuitcircuit fabricationfabrication using AuAu NPsNPs onon polyimide,polyimide, andand ((b)) AuAu NPsNPs sizesize distribution [99]. distribution [99].

However, compared to conventionalconventional microparticlesmicroparticles decompose quickly at lowerlower temperatures andand formform conductingconducting layer layer at at pretty pretty low low temperatures temperatures (300 (300◦C) °C) compared compared to theto the microparticles microparticles owing owing to to quantum quantum size size effect. effect. The The thermal thermal stability stability of of Au Au NPsNPs isis alsoalso important forfor micropatternedmicropatterned sensorssensors andand ﬂexibleflexible electronics.electronics. The most widely used Au NPsNPs havehave excellentexcellent corrosioncorrosion resistanceresistance andand thermalthermal andand electrical conductivity, which enables them to bond with a ﬂuxlessfluxless processprocess onon plasticplastic substrates. The melting points of Au NPs decreasedecrease comparedcompared toto bulkybulky AuAu (1064(1064 ◦°C)C) when the Au NPsNPs sizesize isis belowbelow 1010 nmnm asas shownshown inin FigureFigure 1010.. Gibbs–ThomsonGibbs–Thomson equationequation showsshows the relationshiprelationship ofof meltingmelting pointspoints of of nanoparticle nanoparticle and and bulk bulk particle particle having having a diametera diameter of of d asd as follows. follows. This This equation equation explains explains the the depression depression in melting in melting point point of the of nanostructured the nanostruc- materialstured materials with particle with particle size [100 size,101 [100,101]]. . 4𝜎 ! 4σsl 𝑇(𝑑)Tm=𝑇(d)= T1MB 1 − (11)(11) 𝐻𝜌H𝑑f ρsd

MB == bulk bulk melting melting temperature, temperature, H ff == heat heat of of fusion, fusion, σσslsl == solid-liquid solid-liquid interfacial interfacial energy, energy, and ρs == solid solid material material density. density. Further, Further, if the di diameterameter reaches around 5 nm, then the AuAu NPs can melt around room temperaturetemperature asas shownshown inin [[102,103].102,103]. Thus,Thus, nanosizednanosized particlesparticles are frequently used for lower melting point bonding or electronics packaging. Wang Wang et al. suggested meltingmelting point point of of Au Au NPs NPs entrapped entrapp ined double in double layered layered graphene graphene sheets sheets increased in- whencreased the when length the of length Au NPs of wasAu NPs decreased. was decreased. The melting The ofmelting the Au of NPs the beginsAu NPs from begins the innermostfrom the innermost layer [103 layer]. The [103]. NPs mixedThe NPs with mixe smallerd with and smaller larger and sizes larger showed sizes an showed increased an ﬁllingincreased ratio filling during ratio sintering during for sintering electronics for el packaging.ectronics packaging. German et German al. reported et al. the reported ﬁlling ratiothe filling of sintered ratio of particles sintered as particles following as equationfollowing [ 104equation]: [104]:

max L L s (12) fmaxf − −f Lf + + (1 (1− −fL) fs )f (12)

where fL and fS indicate the filling ratio of large and small particles, respectively. The where f and f indicate the filling ratio of large and small particles, respectively. The shape of shape ofL particlesS also affects the microjoining in noble metal NPs for die-attach materi- particles also affects the microjoining in noble metal NPs for die-attach materials. Figure 11a–c als. Figure 11a–c shows Ag micro powder having a submicron sized chestnut-burr-like shows Ag micro powder having a submicron sized chestnut-burr-like shape. In this shape, the powder particles have a higher surface/volume ratio than the spherical powder, the bonding mechanism is almost similar to the Ag NPs during sintering [105,106].

Metals 2021, 11, x FOR PEER REVIEW 14 of 21

shape. In this shape, the powder particles have a higher surface/volume ratio than the Metals 2021, 11, 329 shape. In thisspherical shape, the powder, powder the particles bonding have mechanism a higher is surface/volume almost similar ratioto the than Ag NPsthe during14 of sin- 21 spherical powder,tering the[105,106]. bonding mechanism is almost similar to the Ag NPs during sintering [105,106].

FigureFigure 10. 10.Effect Effect of of particle particle radius radius on on the the melting melting point point of of Au Au NPs NPs [ 100[100].]. Figure 10. Effect of particle radius on the melting point of Au NPs [100].

Figure 11. (a) A single chestnut burr like Ag particle, (b) mixed chestnut burr and spherical particles, and (c) Ag particles after sintering [105]. Figure 11. (a) AFigure single 11. chestnut (a) A single burr chestnut like Ag particle, burr like (b Ag) mixed particle, chestnut (b) mixed burr chestnut and spherical burr and particles, spherical and parti- (c) Ag particles cles, and (c) Ag particles after sintering [105]. after sintering [105]. If the pressure is applied during sintering, the bonding layer of powders can have a If the pressuredenserIf the isstructure. applied pressure duringThe is appliedsintering sinterin during drivingg, the sintering, bonding force with thelayer applied bonding of powders pressure layer can of can powders have be representeda can have aas denser structure.denserthe Equation The structure. sintering (13). The driving sintering force driving with applied force with pressure applied can pressure be represented can be representedas as the Equation (13). the Equation (13). DE = γΩK +gPa (13) DE = γΩK +gPa (13) DE = γΩK +gPa (13) where, γ = surface energy, Ω = molecular wt., K = curvature of voids, g = geometric con- where, γ = surface energy, Ω = molecular wt., K = curvature of voids, g = geometric constant, where, γ = surfacestant, energy,and Pa = Ωapplied = molecular pressure wt., [107]. K = curvature All these ofactivities voids, gon = Augeometric and Ag con- NPs at low tem- and Pa = applied pressure [107]. All these activities on Au and Ag NPs at low temperature stant, and Pa =perature applied joiningpressure materials [107]. All suggest these activities that the on metal Au andNPs Ag can NPs be atsuccessfully low tem- applied to joining materials suggest that the metal NPs can be successfully applied to flexible polymer perature joiningflexible materials polymer suggest substrates, that thglasse metal and ceramicsNPs can asbe wellsuccessfully [97,108–110]. applied Fine to electronic cir- substrates, glass and ceramics as well [97,108–110]. Fine electronic circuitries with line and flexible polymercuitries substrates, with line glass and and space ceramics < 50 mm as arewell usua [97,108–110].lly formed Fineby screen electronic printing cir- where tradi- space < 50 mm are usually formed by screen printing where traditional metal particles cuitries with tionalline and metal space particles < 50 mm cannot are usua be appliedlly formed due by to screentheir bulk printing size. whereFurther, tradi- the superior dis- cannot be applied due to their bulk size. Further, the superior dispersion of metal NPs tional metal particlespersion ofcannot metal be NPs applied makes due it possibleto their bulkto print size. fine Further, traces the in largesuperior scale dis- integration de- makes it possible to print fine traces in large scale integration devices in microjoining [97]. persion of metalvices NPs in microjoining makes it possible [97]. to print fine traces in large scale integration de- Various kinds of metal NPs have been used in screen printing of electronic circuit vices in microjoiningVarious [97]. kinds of metal NPs have been used in screen printing of electronic circuit patterns such as Ag, Au, Pd, Ag-Pd, etc. [97,108–110]. Dispersed Au and Ag NPs with Various patternskinds of suchmetal as NPs Ag, have Au, Pd,been Ag-Pd, used inetc. screen [97,108–110]. printing Dispersedof electronic Au circuit and Ag NPs with average size smaller than 10 nm [97] were widely prepared by evaporation routes. These patterns suchaverage as Ag, sizeAu, smallerPd, Ag-Pd, than etc. 10 nm[97,108–110]. [97] were widelyDispersed prepared Au and by Ag evaporation NPs with routes. These metal NPs were dispersed in an organic solvent which is further activated by heating average size smallermetal NPs than were 10 nm dispersed [97] were in widely an organic prepared solvent by evaporationwhich is further routes. activated These by heating around 200 ◦C to leave behind a conductive film (0.1 to 10 µm thick) with a resistivity around 200 °C to leave behind a conductive film (0.1 to 10 μm thick) with a resistivity of 3 metal NPs wereof 3 todispersed 50 µΩcm in [108 an, 109organic]. Figure solvent 12 shows which a is typical further design activated of a multilayer by heating circuit board to 50 μΩcm [108,109]. Figure 12 shows a typical design of a multilayer circuit board con- around 200 °Cconsisting to leave ofbehind copper a conductive layers, dielectric, film (0.1 and to mask10 μm layers thick) for with generating a resistivity patterns. of 3 to 50 μΩcm [108,109].sisting of Figurecopper 12 layers, shows dielectric, a typical and design mask of layersa multilayer for generating circuit board patterns. con- sisting of copper layers, dielectric, and mask layers for generating patterns.

Metals 2021, 11, x FOR PEER REVIEW 15 of 21 MetalsMetals 20212021,, 1111,, 329x FOR PEER REVIEW 1515 of of 2121

Figure 12. Schematic diagram of a multilayered circuit board [111]. Figure 12. Schematic diagram of a multilayered circuitcircuit boardboard [[111].111]. The copper layers are filled through the substrate for electrical continuity to the The copper layers are filledfilled throughthrough thethe substratesubstrate forfor electricalelectrical continuitycontinuity toto thethe patterned electric circuit over the top surface [111]. In some reports, Ag-Pd NPs have patterned electricelectric circuitcircuit over over the the top top surface surface [111 [111].]. In someIn some reports, reports, Ag-Pd Ag-Pd NPs haveNPs beenhave utilizedbeen utilized for the for printing the printing of electronic of electronic devices devices [110]. Ag85-Pd15 [110]. Ag85-Pd15 nanoparticle nanoparticle paste affords paste been utilized for the printing of electronic devices [110]. Ag85-Pd15 nanoparticle paste theaffords 4 µm the thick 4 μ filmsm thick with films resistivity with resistivity of 12 µΩ cm,of 12 exhibiting μΩcm, exhibiting anti electromigration anti electromigration behavior. affords the 4 μm thick films with resistivity of 12 μΩcm, exhibiting anti electromigration Somebehavior. researchers Some researchers have used hybridhave used Ag pastehybrid composed Ag paste of composed micron and of nanoparticlemicron and nano- sized behavior. Some researchers have used hybrid Ag paste composed of micron and nano- Agparticle prepared sized byAg an prepared evaporation by an methodevaporation (diameter method 3–5 (diameterµm) [97, 1123–5]. μ Highlym) [97,112]. viscous Highly Ag particle sized Ag prepared by an evaporation method (diameter 3–5 μm) [97,112]. Highly nanoparticleviscous Ag nanoparticle pastes are also pastes used are for also printing used up for to printing 18 µm width up to by 18 dispensing μm width method.by dis- viscous Ag nanoparticle pastes are also used for printing up to 18 μm width by dis- Thispensing dispensing method. method This dispensing can also repair method any can disconnected also repair part any throughdisconnected heat treatment part through and pensing method. This dispensing method can also repair any disconnected part through laserheat treatment irradiation and [112 laser]. irradiation [112]. heat treatment and laser irradiation [112]. Inkjet printingprinting and and dispersed dispersed metal metal NPs NP haves have shown shown promise promise for superfine for superfine patterning pat- Inkjet printing and dispersed metal NPs have shown promise for superfine pat- withterning several with micronsseveral microns line width line which width iswhich hardly is possiblehardly possible via screen via printing.screen printing. For inkjet For terning with several microns line width which is hardly possible via screen printing. For printing,inkjet printing, around around 2 pL 2 of pL liquid of liquid can can produce produce a 15–16 a 15–16µm μm size size dot dot and and form form a a lineline byby inkjet printing, around 2 pL of liquid can produce a 15–16 μm size dot and form a line by connecting each dot by inkjetinkjet printing.printing. As comparedcompared to pastepaste andand screenscreen printing,printing, metalmetal connecting each dot by inkjet printing. As compared to paste and screen printing, metal NPs are lessless viscousviscous andand wellwell disperseddispersed inin inkjetinkjet printing.printing. The finefine electronicelectronic patternpattern NPs are less viscous and well dispersed in inkjet printing. The fine electronic pattern fabricated on procured flexibleflexible polydimethylsiloxanepolydimethylsiloxane (PDMS)(PDMS) substratesubstrate withwith AgAg nanoinknanoink fabricated on procured flexible polydimethylsiloxane (PDMS) substrate with Ag nanoink is inkjet printing isis shownshown inin FigureFigure 1313.. is inkjet printing is shown in Figure 13.

Figure 13. Flexible circuit pattern fabricated usingusing AgAg inkjetinkjet printingprinting [[113].113]. Figure 13. Flexible circuit pattern fabricated using Ag inkjet printing [113]. The AgAg precursorprecursor inkink was was printed printed on on the the PDMS PDMS substrate substrate and and cured. cured. Further Further reduc- re- The Ag precursor ink was printed on the PDMS substrate and cured. Further re- tionduction of printed of printed deposits deposits with with hydrazine hydrazine was was done done to produce to produce the semi-wrappedthe semi-wrapped patterns pat- duction of printed deposits with hydrazine was done to produce the semi-wrapped pat-

Metals 2021, 11, 329 16 of 21

directly. Recent inkjet printing uses femtoliter drops of dispersed Ag nano-ink to generate fine patterns enclosing fine lines in a given space which is a viable technique in developing flexible electronics [114,115]. In addition, multilayer inkjet printing is beneficial for em- bedded IC circuits for large scale integration for flexible electronics, medical and chemical sensing applications [116,117].

6.4. Impact of Nanoparticles of Human Health and Enviroments The rapid progress has been taking place in the nanotechnology leading to concerns about the potential risk associated with the use and application of nanoparticles on human health and the environment. Obviously, the nanoparticles are more active with their bulk counterpart that may need some more careful investigation before it really becomes applied to the human. In this regard, nanoparticles have been investigated in different models such as cell model, mouse model and human model. The available data suggest that the over exposure can lead to accumulation in the organs. For instance, Mats Hulander et al. [118] reported that noble metal can be used as a biomaterial because these metals have a special antimicrobial and anti-allergic capability. The use of noble metals such as silver colloidal NPs often increases the risk of night vision, bow pain, and respiratory and neurological disorders signiﬁcantly. Additionally, some skin allergies have also been noted with the use of colloidal noble metal nanoparticles [119]. In addition, Zhu et al. [120] have found that noble metals have been applied as a multifunctional cancer therapy because of their remarkable surface plasmon responses. Furthermore, a number of hazardous pollutants are also being generated as byproducts due to the rapid development of chemical industries. This is very difﬁcult and huge challenge to decompose these NPs with traditional biological treatment. Such issues can be easily solved with the help of noble metal nanomaterials- based catalysts because they have outstanding absorption and dissociating ability [121]. In this regard, the noble metals not only contribute to human life but also minimize environment pollution.

7. Summary and Conclusions We have overviewed the various routes utilized for the synthesis of metallic NPs, their nucleation and growth behavior. The properties of the bulk materials depend upon the surface atoms density that determines the chemical sensing and electronic behavior. Be- sides, we have summarized the various developments in this metal NPs technology related to Au and Ag NPs in various chemical analysis, biosensing, FRET, and microelectronic applications. Precious noble metal NPs have shown remarkable usage in sensing of water contamination at much lower detection limits as compared to traditional sensors. Au NPs have been shown to sense various radioactive uranyl compounds and saved the human life. We have also discussed the shape and size of NPs, such as how spherical NPs have more advantages over the other shapes of the NPs. Moreover, we discussed why the gold NPs have superiority in FRET process compare to the traditional FRET sensor such as DABCYL which also retain their efficiency in the infrared region. However, in contrast to Au, Ag NPs have been established as FRET efficiency enhancer when interacted with fluorophore. The technology of metal NPs is growing across microelectronic systems for screen printing and inkjet printing as a new way to fabricate fine pitch circuit and multilayer printing for systems in packaging all at once [3]. The future trends in chemical sensing are expected to depend on the development of SPR sensors, nanobiosensors, FRET, etc., for chemical, flexible portable diagnostic sensors in micro- and nanoelectronics. Nanobiosensors have yet to be fulfill their potential in chemical sensing with a high sensitivity and selectivity, as they still need comprehensive developments in future. The future generation of nanobiosensors based on AuNPs will definitely revolutionize the field of chemical sensing and monitoring at reduced costs and widespread usage of medicines and therapies at ease. Therefore, the employment of metal NPs technology has further advanced the field of electronics, medical, chemical, and bio-sensing to design smart engineering devices and applications. Metals 2021, 11, 329 17 of 21

Author Contributions: Conceptualization and methodology, A.G. and J.P.J.; formal analysis and validation, P.K. and P.G.; resources and supervision, J.P.J.; writing—original draft preparation, A.G. and J.P.J.; writing—review and editing, A.S. and N.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the 2020 Research Fund of the University of Seoul for Jae Pil Jung. Institutional Review Board Statement: Not applicable. Acknowledgments: This work was supported by the 2020 Research Fund of the University of Seoul for Jae Pil Jung. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Huo, D.; Kim, M.J.; Lyu, Z.; Shi, Y.; Wiley, B.J.; Xia, Y. One-Dimensional Metal Nanostructures: From Colloidal Syntheses to Applications. Chem. Rev. 2019, 119, 8972–9073. [CrossRef] 2. West, J.L.; Halas, N.J. Engineered Nanomaterials for Biophotonics Applications: Improving Sensing, Imaging, and Therapeutics. Annu. Rev. Biomed. Eng. 2003, 5, 285–292. [CrossRef] 3. Chen, N.; Zhang, Y.; Liu, H.; Wu, X.; Li, Y.; Miao, L.; Shen, Z.; Wu, A. High-Performance Colorimetric Detection of Hg2+ Based on Triangular Silver Nanoprisms. ACS Sens. 2016, 5, 521–527. [CrossRef] 4. Gautam, A.; Komal, P. Probable ideal size of Ln3+-based upconversion NPs for single and multimodal imaging. Coord. Chem. Rev. 2018, 376, 393–404. [CrossRef] 5. Dreaden, E.C.; Alkilany, A.M.; Huang, X.; Murphy, C.J.; El-Sayed, M.A. The golden age: Gold NPs for biomedicine. Chem. Soc. Rev. 2012, 41, 2740–2779. [CrossRef][PubMed] 6. Sztandera, K.; Gorzkiewicz, M.; Klajnert-Maculewicz, B. Gold NPs in Cancer Treatment. Mol. Pharm. 2019, 16, 1–23. [CrossRef] [PubMed] 7. Paull, R.; Wolfe, J.; Hébert, P.; Sinkula, M. Investing in nanotechnology. Nat. Biotechnol. 2003, 21, 1144–1147. [CrossRef][PubMed] 8. Jia, L. Global Governmental Investment in Nanotechnologies. Curr. Nanosci. 2005, 1, 263–266. [CrossRef] 9. Gharatape, A.; Salehi, R. Recent Progress in Theranostic Applications of Hybrid Gold NPs. Eur. J. Med. Chem. 2017, 138, 221–233. [CrossRef] 10. Xia, Y.; Li, W.; Cobley, C.M.; Chen, J.; Xia, X.; Zhang, Q.; Yang, M.; Cho, E.C.; Brown, P.K. Gold Nanocages: From Synthesis to Theranostic Applications. Acc. Chem. Res. 2011, 44, 914–924. [CrossRef] 11. Zhu, W.; Zhao, J.; Chen, Q.; Liu, Z. Nanoscale metal-organic frameworks and coordination polymers as theranostic platforms for cancer treatment. Coord. Chem. Rev. 2019, 398, 113009–113023. [CrossRef] 12. Liu, Y.; Yang, L.; Shen, Y. Hydrothermal Synthesis of Gold Nanoplates and their Structure-Dependent LSPR Properties. J. Mater. Res. 2018, 33, 2671–2679. [CrossRef] 13. Gautam, A.; Ram, S. Preparation and thermomechanical properties of Ag-PVA nanocomposite films. Mater. Chem. Phys. 2010, 119, 266–271. [CrossRef] 14. Ram, S.; Tripathy, P.; Fecht, H.J. Gold NPs reinforced poly (vinyl alcohol) of self-standing optical films. J. Nanosci. Nanotechnol. 2007, 7, 3200–3206. [CrossRef][PubMed] 15. Ram, S.; Gautam, A.; Fecht, H.J.; Cai, J.; Bansmann, J.; Behm, R.J. A new allotropic structure of silver nanocrystals nucleated and grown over planar polymer molecules. Philos. Mag. Lett. 2007, 87, 361–372. [CrossRef] 16. Cametti, M.; Džolić,Z. New frontiers in hybrid materials: Noble metal NPs Supramolecular gel systems. Chem. Commun. 2014, 50, 8273–8286. [CrossRef] 17. Alexander, J. History of the Medical Use of Silver. Surg. Infect. 2009, 10, 289–292. [CrossRef][PubMed] 18. Dykman, L.A.; Khlebtsov, N.G. Gold NPs in Biology and Medicine: Recent Advances and Prospects. Acta Nat. 2011, 3, 34–55. [CrossRef] 19. “Plenty of Room” Revisited. Nat. Nanotechnol. 2009, 4, 781. [CrossRef][PubMed] 20. Blanco, E.; Shen, H.; Ferrari, M. Principles of nanoparticle design for overcoming biological barriers to drug delivery. Nat. Biotechnol. 2015, 33, 941. [CrossRef] 21. Chang, C.-C.; Chen, C.-P.; Wu, T.-H.; Yang, C.-H.; Lin, C.-W.; Chen, C.-Y. Gold Nanoparticle-Based Colorimetric Strategies for Chemical and Biological Sensing Applications. Nanomaterials 2019, 9, 861. [CrossRef] 22. Argyle, M.D.; Bartholomew, C.H. Heterogeneous Catalyst Deactivation and Regeneration: A Review. Catalyst 2015, 5, 145–269. [CrossRef] 23. Arnal, P.M.; Comotti, M.; Schüth, F. High-Temperature-Stable Catalysts by Hollow Sphere Encapsulation. Angew. Chem. Int. Ed. 2006, 45, 8224–8227. [CrossRef] 24. Yang, Q.; Li, L.; Zhao, F.; Wang, Y.; Ye, Z.; Hua, C.; Liu, Z.; Bohinc, K.; Guo, X. Spherical Polyelectrolyte Brushes as Templates to Prepare Hollow Silica Spheres Encapsulating Metal Nanoparticles. Nanomaterials 2020, 10, 799. [CrossRef][PubMed] 25. Bohinc, K.; Bossa, G.V.; May, S. Incorporation of ion and solvent structure into mean-field modeling of the electric double layer. Adv. Colloid Interface Sci. 2017, 249, 220–233. [CrossRef][PubMed] Metals 2021, 11, 329 18 of 21

26. Mojarad, N.; Krishnan, M. Measuring the size and charge of single nanoscale objects in solution using an electrostatic fluidic trap. Nat. Nanotechnol. 2012, 7, 448. [CrossRef] 27. Skaug, M.J.; Schwemmer, C.; Fringes, S.; Rawlings, C.D.; Knoll, A.W. Nanofluidic rocking brownian motors. Science 2018, 359, 1505–1508. [CrossRef][PubMed] 28. Rasmussen, M.K.; Pedersen, J.N.; Marie, R. Size and surface charge characterization of nanoparticles with a salt gradient. Nat. Commun. 2020, 11, 2337. [CrossRef] 29. Smith, G.N.; Grillo, I.; Rogers, S.E.; Eastoe, J. Surfactants with colloids: Adsorption or absorption? J. Colloid Interface Sci. 2015, 449, 205–214. [CrossRef] 30. Al-Waeli, A.H.A.; Chaichan, M.t.; Kazem, H.A.; Sopian, K. Evaluation and analysis of nanofluid and surfactant impact onphotovoltaic-thermal systems. Case Stud. Therm. Eng. 2019, 13, 100392. [CrossRef] 31. Sharma, A.; Das, S.; Das, K. Pulse Electroplating of Ultrafine Grained Tin Coating. In Electroplating of Nanostructures; IntechOpen: Rijeka, Croatia, 2015; pp. 105–129. 32. Nützenadel, C.; Züttel, A.; Chartouni, D.; Schmid, G.; Schlapbach, L. Critical Size and Surface Effect of the Hydrogen Interaction of Palladium Clusters. Eur. Phys. J. D 2000, 8, 245–250. [CrossRef] 33. Dubertret, B.; Calame, M.; Libchaber, A.J. Single-Mismatch Detection Using Gold-Quenched Fluorescent Oligonucleotides. Nat. Biotechnol. 2001, 19, 365–370. [CrossRef] 34. Link, S.; El-Sayed, M.A. Shape and Size Dependence of Radiative, Non-radiative and Photothermal Properties of Gold Nanocrys- tals. Int. Rev. Phys. Chem. 2000, 19, 409–453. [CrossRef] 35. Thanh, N.T.K.; Maclean, N.; Mahiddine, S. Mechanisms of Nucleation and Growth of NPs in Solution. Chem. Rev. 2014, 114, 7610–7630. [CrossRef][PubMed] 36. Chen, Y.; Fan, Z.; Zhang, Z.; Niu, W.; Li, C.; Yang, N.; Chen, B.; Zhang, H. Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications. Chem. Rev. 2018, 118, 6409–6455. [CrossRef][PubMed] 37. Wei, X.; Wang, X.; Gao, B.; Zou, W.; Dong, L. Facile Ball-Milling Synthesis of CuO/Biochar Nanocomposites for Efficient Removal of Reactive Red 120. ACS Omega 2020, 5, 5748–5755. [CrossRef][PubMed] 38. Korczagin, I.; Golze, S.; Hempenius, M.A.; Vancso, G.J. Surface Micropatterning and Lithography with Poly(Ferrocenylmethylphenylsilane). Chem. Mater. 2003, 15, 3663–3668. [CrossRef] 39. Kim, S.H.; Liu, B.Y.H.; Zachariah, M.R. Synthesis of Nanoporous Metal Oxide Particles by a New Inorganic Matrix Spray Pyrolysis Method. Chem. Mater. 2002, 14, 2889–2899. [CrossRef] 40. Susha, A.S.; Ringler, M.; Ohlinger, A.; Paderi, M.; LiPira, N.; Carotenuto, G.; Rogach, A.L.; Feldmann, J. Strongly Luminescent Films Fabricated by Thermolysis of Gold−Thiolate Complexes in a Polymer Matrix. Chem. Mater. 2008, 20, 6169–6175. [CrossRef] 41. Zhu, M.; Baffou, G.; Meyerbröker, N.; Polleux, J. Micropatterning Thermoplasmonic Gold Nanoarrays To Manipulate Cell Adhesion. ACS Nano. 2012, 6, 7227–7233. [CrossRef] 42. Marques-Hueso, J.; Morton, J.A.S.; Wang, X.; Bertran-Serra, E.; Desmulliez, M.P.Y. Photolithographic Nanoseeding Method for Selective Synthesis of Metal-Catalysed Nanostructures. Nanotechnology 2019, 30, 15302–15309. [CrossRef] 43. Qin, D.; Xia, Y.; Whitesides, G.M. Soft lithography for Micro- and Nanoscale Patterning. Nat. Protoc. 2010, 5, 491–502. [CrossRef] 44. Sreenivasan, S.V. Nanoimprint Lithography Steppers for Volume Fabrication of Leading-Edge Semiconductor Integrated Circuits. Microsyst. Nanoeng. 2017, 3, 17075–17093. [CrossRef][PubMed] 45. Garno, J.C.; Yang, Y.; Amro, N.A.; Cruchon-Dupeyrat, S.; Chen, S.; Liu, G.-Y. Precise Positioning of NPs on Surfaces Using Scanning Probe Lithography. Nano Lett. 2003, 3, 389–395. [CrossRef] 46. Heuer-Jungemann, A.; Feliu, N.; Bakaimi, I.; Hamaly, M.; Alkilany, A.; Chakraborty, I.; Masood, A.; Casula, M.F.; Kostopoulou, A.; Oh, E.; et al. The Role of Ligands in the Chemical Synthesis and Applications of Inorganic NPs. Chem. Rev. 2019, 119, 4819–4880. [CrossRef] 47. Gautam, A.; Mukherjee, S.; Ram, S. Controlled Novel Route to Synthesis and Characterization of Silver Nanorods. J. Nanosci. Nanotechnol. 2010, 10, 4329–4334. [CrossRef][PubMed] 48. Gautam, A.; Singh, G.P.; Ram, S. A Simple Polyol Synthesis of Silver Metal Nanopowder of Uniform Particles. Synth. Met. 2007, 157, 5–10. [CrossRef] 49. Evanoff, D.D.; Chumanov, G. Size-Controlled Synthesis of NPs. 1. “Silver-Only” Aqueous Suspensions via Hydrogen Reduction. J. Phys. Chem. B 2004, 108, 13948–13956. [CrossRef] 50. Tatarchuk, V.V.; Sergievskaya, A.P.; Korda, T.M.; Druzhinina, I.A.; Zaikovsky, V.I. Kinetic Factors in the Synthesis of Silver NPs by Reduction of Ag+ with Hydrazine in Reverse Micelles of Triton N-42. Chem. Mater. 2013, 25, 3570–3579. [CrossRef] 51. Fountoulaki, S.; Daikopoulou, V.; Gkizis, P.L.; Tamiolakis, I.; Armatas, G.S.; Lykakis, I.N. Mechanistic Studies of the Reduction of Nitroarenes by NaBH4 or Hydrosilanes Catalyzed by Supported Gold NPs. ACS Catal. 2014, 4, 3504–3511. [CrossRef] 52. Ahmad, S.; Munir, S.; Zeb, N.; Ullah, A.; Khan, B.; Ali, J.; Bilal, M.; Omer, M.; Alamzeb, M.; Salman, S.M.; et al. Green Nanotechnology: A Review on Green Synthesis of Silver NPs An Ecofriendly Approach. Int. J. Nanomed. 2019, 14, 5087–5107. [CrossRef][PubMed] 53. Gautam, A.; Tripathy, P.; Ram, S. Microstructure, topology and X-ray diffraction in Ag-Metal Reinforced Polymer of Polyvinyl Alcohol of Thin Laminates. J. Mater. Sci. 2006, 41, 3007–3016. [CrossRef] 54. Schulman, J.H.; Stoeckenius, W.; Prince, L.M. Mechanism of Formation and Structure of Micro Emulsions by Electron Microscopy. J. Phys. Chem. 1959, 63, 1677–1680. [CrossRef] Metals 2021, 11, 329 19 of 21

55. Tissandier, C.; Diop, N.; Martini, M.; Roux, S.; Tillement, O.; Hamaide, T. One-Pot Synthesis of Hybrid Multifunctional Silica NPs with Tunable Coating by Click Chemistry in Reverse W/O Microemulsion. Langmuir 2012, 28, 209–218. [CrossRef][PubMed] 56. Gautam, A.; van Veggel, F.C.J.M. Synthesis of InN@SiO2 Nanostructures and Fabrication of Blue LED Devices. ACS Appl. Mater. Interfaces 2012, 4, 3902–3909. [CrossRef] 57. Ganguli, A.K.; Ganguly, A.; Vaidya, S. Microemulsion-Based Synthesis of Nanocrystalline Materials. Chem. Soc. Rev. 2010, 39, 474–485. [CrossRef][PubMed] 58. Bueno-Alejo, C.J.; D’Alfonso, C.; Pacioni, N.L.; González-Béjar, M.; Grenier, M.; Lanzalunga, O.; Alarcon, E.I.; Scaiano, J.C. Ultraclean Derivatized Monodisperse Gold NPs through Laser Drop Ablation Customization of Polymorph Gold Nanostructures. Langmuir 2012, 28, 8183–8189. [CrossRef] 59. Dahal, N.; García, S.; Zhou, J.; Humphrey, S.M. Beneficial Effects of Microwave-Assisted Heating versus Conventional Heating in Noble Metal Nanoparticle Synthesis. ACS Nano. 2012, 6, 9433–9446. [CrossRef] 60. Saha, K.; Agasti, S.S.; Kim, C.; Li, X.; Rotello, V.M. Gold NPs in Chemical and Biological Sensing. Chem. Rev. 2012, 112, 2739–2779. [CrossRef] 61. Lakowicz, J.R. Principles of Fluorescence Spectroscopy; Springer: Berlin/Heidelberg, Germany, 2013. [CrossRef] 62. Huang, X.; El-Sayed, M.A. Gold NPs: Optical Properties and Implementations in Cancer Diagnosis and Photothermal Therapy. J. Adv. Res. 2010, 1, 13–28. [CrossRef] 63. Loo, C.; Lin, A.; Hirsch, L.; Lee, M.-H.; Barton, J.; Halas, N.; West, J.; Drezek, R. Nanoshell-Enabled Photonics-Based Imaging and Therapy of Cancer. Technol. Cancer Res. Treat. 2004, 3, 33–40. [CrossRef] 64. Skrabalak, S.E.; Au, L.; Li, X.; Xia, Y. Facile Synthesis of Ag Nanocubes and Au Nanocages. Nat. Protoc. 2007, 2, 2182–2190. [CrossRef] 65. Polavarapu, L.; Pérez-Juste, J.; Xu, Q.-H.; Liz-Marzán, L.M. Optical Sensing of Biological, Chemical and Ionic Species Through Aggregation of Plasmonic NPs. J. Mater. Chem. C 2014, 2, 7460–7476. [CrossRef] 66. Chen, Y.; Xianyu, Y.; Jiang, X. Surface Modification of Gold NPs with Small Molecules for Biochemical Analysis. Acc. Chem. Res. 2017, 50, 310–319. [CrossRef] 67. Deng, H.; Zhang, X.; Kumar, A.; Zou, G.; Zhang, X.; Liang, X.-J. Long Genomic DNA Amplicons Adsorption onto Unmodified Gold NPs for Colorimetric Detection of Bacillus Anthracis. Chem. Commun. 2013, 49, 51–53. [CrossRef][PubMed] 68. Vilela, D.; González, M.C.; Escarpa, A. Sensing Colorimetric Approaches Based on Gold and Silver NPs Aggregation: Chemical Creativity Behind the Assay. A Review. Anal. Chim. Acta 2012, 751, 24–43. [CrossRef] 69. Lee, J.H.; Wang, Z.; Liu, J.; Lu, Y. Highly Sensitive and Selective Colorimetric Sensors for Uranyl (UO22+): Development and Comparison of Labeled and Label-Free DNAzyme-Gold Nanoparticle Systems. J. Am. Chem. Soc. 2008, 130, 14217–14226. [CrossRef] 70. Upadhyayula, V.K.K. Functionalized Gold Nanoparticle Supported Sensory Mechanisms Applied in Detection of Chemical and Biological Threat agents: A review. Anal. Chim. Acta 2012, 715, 1–18. [CrossRef][PubMed] 71. Kim, Y.; Johnson, R.C.; Hupp, J.T. Gold Nanoparticle-Based Sensing of “Spectroscopically Silent” Heavy Metal Ions. Nano Lett. 2001, 1, 165–167. [CrossRef] 72. Lee, J.-S.; Han, M.S.; Mirkin, C.A. Colorimetric Detection of Mercuric ion (Hg2+) in Aqueous Media Using DNA-Functionalized Gold NPs. Angew. Chem. Int. Ed. Engl. 2007, 46, 4093–4096. [CrossRef] 73. Xue, X.; Wang, F.; Liu, X. One-Step, Room Temperature, Colorimetric Detection of Mercury (Hg2+) Using DNA/Nanoparticle Conjugates. J. Am. Chem. Soc. 2008, 130, 3244–3245. [CrossRef][PubMed] 74. Xie, J.; Zheng, Y.; Ying, J.Y. Highly Selective and Ultrasensitive Detection of Hg2+ Based on Fluorescence Quenching of Au Nanoclusters by Hg2+–Au+ Interactions. Chem. Commun. 2010, 46, 961–963. [CrossRef] 75. Zhou, Y.; Ma, Z. Colorimetric Detection of Hg2+ by Au NPs Formed by H2O2 Reduction of HAuCl4 Using Au Nanoclusters as the Catalyst. Sens. Actuat. B Chem. 2017, 241, 1063–1068. [CrossRef] 76. Farhadi, K.; Forough, M.; Molaei, R.; Hajizadeh, S.; Rafipour, A. Highly Selective Hg2+ Colorimetric Sensor Using Green Synthesized and Unmodified Silver NPs. Sens. Actuat. B Chem. 2012, 161, 880–885. [CrossRef] 77. Sebastian, M.; Aravind, A.; Mathew, B. Green Silver-Nanoparticle-Based Dual Sensor for Toxic Hg(II) Ions. Nanotechnology 2018, 29, 355502. [CrossRef][PubMed] 78. Alagan, J.; Dhesingh, R.S. Functionalized Silver Nanoparticle Probe for Visual Colorimetric Sensing of Mercury. Mater. Res. Bull. 2016, 83, 48–55. 79. Liu, J.; Brown, A.K.; Meng, X.; Cropek, D.M.; Istok, J.D.; Watson, D.B.; Lu, Y. A Catalytic Beacon Sensor for Uranium with Parts-Per-Trillion Sensitivity and Millionfold Selectivity. Proc. Natl. Acad. Sci. USA 2007, 104, 2056–2061. [CrossRef][PubMed] 80. Chai, F.; Wang, C.; Wang, T.; Li, L.; Su, Z. Colorimetric Detection of Pb2+ Using Glutathione Functionalized Gold NPs. ACS Appl. Mater. Interfaces 2010, 2, 1466–1470. [CrossRef] 81. Hung, Y.-L.; Hsiung, T.-M.; Chen, Y.-Y.; Huang, Y.-F.; Huang, C.-C. Colorimetric Detection of Heavy Metal Ions Using Label-Free Gold NPs and Alkanethiols. J. Phys. Chem. C 2010, 114, 16329–16334. [CrossRef] 82. Patel, K.; Bhamore, J.R.; Park, T.J.; Kailasa, S.K. Selective and Sensitive Colorimetric Recognition of Ba2+ Ion Using Guanine- Functionalized Silver NPs. Chem. Sel. 2018, 3, 10182–10187. 83. Dau, A.; Komal, P.; Truong, M.; Morris, G.; Evans, G.; Nashmi, R. RIC-3 Differentially Modulates α4β2 and α7 Nicotinic Receptor Assembly, Expression, and Nicotine-Induced Receptor Upregulation. BMC Neurosci. 2013, 14, 14–47. [CrossRef][PubMed] Metals 2021, 11, 329 20 of 21

84. Shi, J.; Tian, F.; Lyu, J.; Yang, M. Nanoparticle Based Fluorescence Resonance Energy Transfer (FRET) for Biosensing Applications. J. Mater. Chem. B 2015, 3, 6989–7005. [CrossRef][PubMed] 85. Gersten, J.; Nitzan, A. Spectroscopic Properties of Molecules Interacting with Small Dielectric Particles. J. Chem. Phys. 1981, 75, 1139–1152. [CrossRef] 86. Lakowicz, J.R. Radiative decay engineering 5: Metal-Enhanced Fluorescence and Plasmon Emission. Anal. Biochem. 2005, 337, 171–194. [CrossRef] 87. Mendez-Gonzalez, D.; Melle, S.; Calderón, O.G.; Laurenti, M.; Cabrera-Granado, E.; Egatz-Gómez, A.; López-Cabarcos, E.; Rubio-Retama, J.; Díaz, E. Control of Upconversion Luminescence by Gold Nanoparticle Size: From Quenching to Enhancement. Nanoscale 2019, 11, 13832–13844. [CrossRef][PubMed] 88. Tang, B.; Cao, L.; Xu, K.; Zhuo, L.; Ge, J.; Li, Q.; Yu, L. A New Nanobiosensor for Glucose with High Sensitivity and Selectivity in Serum Based on Fluorescence Resonance Energy Transfer (FRET) between CdTe Quantum Dots and Au NPs. Chem. A Eur. J. 2008, 14, 3637–3644. [CrossRef][PubMed] 89. Oh, E.; Hong, M.-Y.; Lee, D.; Nam, S.-H.; Yoon, H.C.; Kim, H.-S. Inhibition Assay of Biomolecules Based on Fluorescence Resonance Energy Transfer (FRET) Between Quantum Dots and Gold NPs. J. Am. Chem. Soc. 2005, 127, 3270–3271. [CrossRef] [PubMed] 90. Angelatos, A.S.; Radt, B.; Caruso, F. Light-Responsive Polyelectrolyte/Gold Nanoparticle Microcapsules. J. Phys. Chem. B 2005, 109, 3071–3076. [CrossRef][PubMed] 91. Chen, J.; Huang, Y.; Zhao, S.; Lu, X.; Tian, J. Gold NPs-based fluorescence resonance energy transfer for competitive immunoassay of biomolecules. Analyst 2012, 137, 5885–5890. [CrossRef] 92. Park, S.Y.; Lee, S.M.; Kim, G.B.; Kim, Y.-P. Gold Nanoparticle-Based Fluorescence Quenching Via Metal Coordination for Assaying Protease Activity. Gold Bull. 2012, 45, 213–219. [CrossRef] 93. Mayilo, S.; Kloster, M.A.; Wunderlich, M.; Lutich, A.; Klar, T.A.; Nichtl, A.; Kürzinger, K.; Stefani, F.D.; Feldmann, J. Long-Range Fluorescence Quenching by Gold NPs in a Sandwich Immunoassay for Cardiac Troponin T. Nano Lett. 2009, 9, 4558–4563. [CrossRef] 94. Amjadi, M.; Abolghasemi-Fakhri, Z.; Hallaj, T. Carbon Dots-Silver NPs Fluorescence Resonance Energy Transfer System as a Novel Turn-on Fluorescent Probe for Selective Determination of Cysteine. J. Photochem. Photobiol. A Chem. 2015, 309, 8–14. [CrossRef] 95. Li, H.; Wang, M.; Wang, C.; Li, W.; Qiang, W.; Xu, D. Silver Nanoparticle-Enhanced Fluorescence Resonance Energy Transfer Sensor for Human Platelet-Derived Growth Factor-BB Detection. Anal. Chem. 2013, 85, 4492–4499. [CrossRef][PubMed] 96. Zhao, T.; Li, T.; Liu, Y. Silver Nanoparticle Plasmonic Enhanced Förster Resonance Energy Transfer (FRET) Imaging of Protein- Specific Sialylation on the Cell Surface. Nanoscale 2017, 9, 9841–9847. [CrossRef] 97. Oda, J.M. Metal Nano-Particles. Jpn. Inst. Electron. Packag. 2002, 5, 523–528. [CrossRef] 98. Wu, B.-H.; Yang, H.-Y.; Huang, H.-Q.; Chen, G.X.; Zheng, N.-F. Solvent Effect on the Synthesis of Monodisperse Amine-Capped Au Nanoparticles. Chin. Chem. Lett. 2013, 24, 457–462. [CrossRef] 99. Ketelsen, B.; Tjarks, P.P.; Schlicke, H.; Liao, Y.-C.; Vossmeyer, T. Fully Printed Flexible Chemiresistors with Tunable Selectivity Based on Gold NPs. Chemosensors 2020, 8, 116. [CrossRef] 100. Schmid, G.; Corain, B. Nanoparticulated Gold: Syntheses, Structures, Electronics, and Reactivities. Eur. J. Inorg. Chem. 2003, 3081–3098. [CrossRef] 101. Castro, T.; Reifenberger, R.; Choi, E.; Andres, R.P. Size-dependent melting temperature of individual nanometer-sized metallic clusters. Phys. Rev. B 1990, 42, 8548–8556. [CrossRef] 102. Chin, H.; Cheong, K.; Ismail, A. A Review on Die Attach Materials for SiC-Based High-Temperature Power Device. Metall. Mater. Trans. B 2010, 41, 824. [CrossRef] 103. Wang, G.; Wu, N.; Wang, J.; Shao, J.; Zhu, X.; Lu, X.; Guo, L. Abnormal change of melting points of gold NPs confined between two-layer graphene nanosheets. RSC Adv. 2016, 6, 108343–108346. [CrossRef] 104. German, R.M. Prediction of Sintered Density for Bimodal Powder Mixtures. Metall. Trans. A 1992, 23, 1455–1465. [CrossRef] 105. Roh, M.H.; Nishikawa, H.; Jung, J.P. A Review of Ag Paste Bonding for Automotive Power Device Packaging. J. Microelectron. Packag. Soc. 2015, 22, 15–23. [CrossRef] 106. Jung, D.-H.; Roh, M.H.; Lee, J.H.; Kim, K.H.; Jung, J.P. Transient Liquid Phase (TLP) Bonding of Device for High Temperature Operation. J. Microelectron. Packag. Soc. 2017, 24, 17–25. [CrossRef] 107. Skandan, G.; Hahn, H.; Kear, B.H.; Roddy, M.; Cannon, W.R. The Effect of Applied Stress on Densification of Nano-structured Zirconia during Sinter-forging. Mater. Lett. 1994, 20, 302–309. [CrossRef] 108. Matsuba, Y. Direct Patterning Using Metal Nano-Particles. J. Jpn. Inst. Electron. Packag. 2003, 6, 130–135. [CrossRef] 109. Bishop, P.T.; Ashfield, L.J.; Berzins, A.; Boardman, A.; Buche, V.; Cookson, J.; Gordon, R.J.; Salcianu, C.; Sutton, P.A. Printed Gold for Electronic Applications. Gold Bull. 2010, 43, 181–185. [CrossRef] 110. Nakamoto, M.; Yamamoto, M.; Kashiwagi, Y.; Kakiuchi, H.; Tsujimoto, T.; Yoshida, Y. In Proceedings of Microelectronics Symposium, 2005; pp. 241–244. 111. Shamkhalichenar, H.; Bueche, C.J.; Choi, J.-W. Printed Circuit Board (PCB) Technology for Electrochemical Sensors and Sensing Platforms. Biosensors 2020, 10, 159. [CrossRef][PubMed] 112. Ohsako, Y. In: Abstract of Chemical Engineering, 34th Meeting, K122, 2001. Metals 2021, 11, 329 21 of 21

113. Sun, J.; Jiang, J.; Bao, B.; Wang, S.; He, M.; Zhang, X.; Song, Y. Fabrication of Bendable Circuits on a Polydimethylsiloxane (PDMS) Surface by Inkjet Printing Semi-Wrapped Structures. Materials 2016, 9, 253. [CrossRef] 114. Yin, Z.P.; Huang, Y.A.; Bu, N.B.; Wang, X.M.; Xiong, Y.L. Inkjet Printing for Flexible Electronics: Materials, Processes and Equipments. Chin. Sci. Bull. 2010, 55, 3383–3407. [CrossRef] 115. Jang, J. Displays Develop A New Flexibility. Mater. Today 2006, 9, 46–52. [CrossRef] 116. Sharma, A.; Lim, D.U.; Jung, J.P. Microstructure and brazeability of SiC nanoparticles reinforced Al–9Si–20Cu produced by induction melting. Mater. Sci. Technol. 2016, 32, 773–779. [CrossRef] 117. Sharma, A.; Das, S.; Das, K. Pulse Electrodeposition of Lead-Free Tin-Based Composites for Microelectronic Packaging. In Electrodeposition of Composite Materials; InTech: Rijeka, Croatia, 2016; pp. 253–274. 118. Hulander, M.; Hong, J.; Andersson, M.; Gervén, F.; Ohrlander, M.; Tengvall, P.; Elwing, H. Blood interactions with noble metals: Coagulation and immune complement activation. ACS Appl. Mater. Interfaces 2009, 1, 1053–1062. [CrossRef][PubMed] 119. Arvizo, R.R.; Bhattacharyya, S.; Kudgus, R.A.; Giri, K.; Bhattacharya, R.; Mukherjee, P. Intrinsic therapeutic applications of noble metal nanoparticles: Past, present and future. Chem. Soc. Rev. 2012, 41, 2943–2970. [CrossRef][PubMed] 120. Lv, Z.; He, S.; Wang, Y.; Zhu, X. Noble Metal Nanomaterials for NIR-Triggered Photothermal Therapy in Cancer. Adv. Healthc. Mater. 2021, 2001806. [CrossRef][PubMed] 121. Lin, Y.; Cao, Y.; Yao, Q.; Chai, O.J.H.; Xie, J. Engineering Noble Metal Nanomaterials for Pollutant Decomposition. Ind. Eng. Chem. Res. 2020, 59, 20561–20581. [CrossRef]