Mainak Ganguly
a,
Jayasmita Jana
b,
Anjali Pal
c and
Tarasankar Pal
*b
aDepartment of Chemistry, Furman University, Greenville, South Carolina-29613, USA
bDepartment of Chemistry, Indian Institute of Technology, Kharagpur-721302, India. E-mail: tpal@chem.iitkgp.ernet.in
cDepartment of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, India
First published on 26th January 2016
Synergism of gold and silver, causing enhanced fluorescence, has been reported in cluster science with higher photochemical stability and practical applications. The electronic factor, nuclearity, size effect etc., bring autofluorescence, doping/alloying, aggregation induced fluorescence, core–shell interaction, oxidation induced interaction and silver effect. The plausible interplaying mechanisms behind the synergism in the bimetallic clusters has been focused upon. Appropriate selection of the template is mandatory to achieving AuAg bimetallic clusters. Such templates are scarce and only a few are reported in the literature while templates to obtain individual Au and Ag clusters are numerous. Semi/complete reduced gold and non/complete reduced silver are one of the important features of such bimetallic clusters. Mingled Au and Ag has a profound effect on the stability, electronic structure and band energy of the bimetallic clusters. The arrangement of Au and Ag atoms in the bimetallic clusters is also a matter of interest. The bimetallic AuAg clusters are found to be superior to not only individual Au/Ag clusters but also carbon and semiconductor quantum dots, considering their emissive nature, toxicity, ease of synthesis, robustness etc. Water miscible as well as water immiscible solvents are equally efficient for the production of AuAg bimetallic clusters. Finally, such bimetallic clusters have proved to be unique candidates in the context of practical applications, namely sensing, catalysis, surface enhanced Raman spectroscopy (SERS), metal enhanced fluorescence (MEF), bio-imaging, synthesis of anti-bacterial cotton/papers etc. The ratio of Au and Ag not only tune the fluorescence behavior but also toxicity, as described.
Fig. 1 The gene for total baldness is epistatic to those for blond or red hair. The baldness phenotype supersedes genes for hair colour and so the effects are non-additive. |
Synergism is a well known in the field of catalysis. Metals such as Ag, Cu, Ni, Pd and Co bestow pronounced effects in promoting oxygen activation in gold-catalyzed oxidation reactions.2 Gold and silver, being present in the same group (11), show synergistic effects very frequently unlike the other elements in the periodic table. Again, Liu et al.3 reported that Au–Ag alloy nanoparticles exhibit exceptionally higher catalytic activity for low-temperature CO oxidation compared with monometallic gold. Interestingly, such alloys have a lower surface-to-volume ratio in comparison to monometallic gold, explaining the essence of the synergistic effect. Not only in catalysis, but also in other fields of research synergism between gold and silver is observable. Phytofabricated Ag–Au nanoparticles was found to have improved antibacterial and anticandidal potential compared with their monometallic counterparts with specific reference to some pathogenic bacteria and Candida sp.4
Luminescent noble metal (gold, silver) nanoclusters (NCs), with discrete electronic states, have become highly promising over the last few years, possessing notable optical properties for extensive use in sensing,5–8 imaging9,10 and biolabeling.11–13 As the sizes of Ag and Au NCs are analogous to the Fermi wavelength of an electron (i.e., the electron de Broglie wavelength at the Fermi level: ∼0.5 nm for Au and Ag), they bestow molecular-like behavior including size-dependent fluorescence.14 For defining the size-dependent electronic structure and relative electronic transitions of the small NCs, the Jellium EFermi/N1/3 energy scaling law can be employed as a model.15
Template-based synthetic approaches have been employed to form fluorescent Au and Ag NCs from their corresponding salts using reducing agents such as NaBH4 and templates namely poly(amidoamine) (PAMAM) dendrimers, polyglycerol-block-poly(acrylic acid) copolymers, proteins, DNA, thiols etc.14,16–30 However, the templates for the gold–silver bimetallic silver clusters are limited in the literature. Au NCs are easier to synthesize than Ag NCs. This is mainly due to the poor stability associated with Ag NCs as they are vulnerable to oxidation. A suitable choice of templates is found to be somewhat useful to prevent rapid aggregation and oxidation of the Ag clusters. In comparison to the Au NC, the Ag NC show intense fluorescence, augmenting its efficacy to different key applications.16 Recently, extensive research on AuAg NCs has been conducted to overcome the shortcomings of individual Au NCs and Ag NCs.
Gold–silver synergism in fluorescence is becoming a very promising phenomenon and many researchers have ventured into this area recently. Bimetallic particles, made of silver and gold, can easily be synthesized in different size regimes and compositions for their similar lattice constants. Thus, mixed crystals over the almost whole concentration range of Au and Ag can be produced.31–33 Bimetallic clusters of silver and gold have proven to be a promising route to bypass the usually weak emission (in comparison to other common fluorophores) of gold clusters as the bimetallic clusters exhibit a noticeably enhanced quantum yield.34,35 Such an intense emission of AuAg NCs is the key feature of various useful applications. Of late, luminescent Au/Ag NCs have been employed as novel luminescent probes for sensing toxic metals, bioimaging and biosensing.36–44 To achieve various improved applications, the research community has become interested in developing facile synthetic protocols of strongly luminescent and stable AuAg NCs.45,46 Among the diverse techniques, mass spectrometry has become a crucial analytical tool to comprehend the atomic composition of such NCs. Moreover, the theoretical prediction of the structure of the these bimetallic clusters is also in progress.47
Plausible mechanisms of synergism in AuAg clusters, suitable templates for the synthesis of such clusters, their oxidation states, electronic-band structure as well as applications have been summarized in this review.
Fig. 2 Excitation and emission spectra of nanoparticles with different molar Ag:Au compositions. The numbers in parentheses indicate the maxima of the absorption and emission bands, respectively, for the six samples. [Ref. 48] – ©The Royal Society of Chemistry. |
Ag NCs have proven to be more fluorescent than Au NCs.50–53 However, the main limitations to biological applications are due to the high reactivity and Ag related cytotoxicity with the discharge of Ag ions from Ag NCs. Thus, Ag NCs are less popular for practical applications. Moreover, oxidation of Ag NCs causes a rapid decrease in the emissive property. In order to develop brighter metal clusters, silver has been added by Guével et al.49 during the synthesis of Au NCs, protected by glutathione (GSH). Thus Ag doped AuGSH (AuAgGSH) has been produced ([Au]/[Ag] = 50.1). AuAgGSH are non-cytotoxic and possess a stronger emissive property. An enhancement of the emission intensity by a factor of 4 to 5 with silver compared to that of AuGSH (excitation at 400 nm and emission at 615 nm) has been obtained [Fig. 3].
Fig. 3 (a) Excitation (dashed line)/emission (solid line) spectra of AuAgGSH. (b) Normalised excitation spectra of AuGSH (blue line) and AuAgGSH (red line) with λem = 615 nm. The picture in the inset shows the fluorescence of AuGSH and AuAgGSH diluted to the same optical absorption under UV light (λex = 366 nm). (c) The relative fluorescence intensity of diluted AuAgGSH solutions prepared with different amounts of silver salt during the synthesis. Results show an increase of the fluorescence of the clusters in the presence of silver independently of the dilution. Ref. 49 © 2012 Royal Society of Chemistry. Reproduced from ref. 49 with permission from The Royal Society of Chemistry. |
Several noble metal clusters exhibit a much longer fluorescence lifetime (one or two magnitudes higher than organic dyes and quantum dots).14,54 Multiple components of the decay profile for lifetime measurements are dependent on the excitation wavelength. AuGSH and AuAgGSH, synthesized by Guével et al.,49 showed a fluorescence lifetime having a biexponential decay: a short component s1 (15 ± 2 ns) and a long component s2 (250 ± 20 ns) [Fig. 4]. The absolute value of either lifetime component of AuGSH remained unaffected with the variation of emission wavelength, pH and the existence of silver. Interestingly, there was a distinguishable impact on the long lifetime component s2, when Ag was doped. Ag caused a notable increase of the amplitude of s2. Again, ∼95 nm red shifted emission was favored because of silver doping. Patel et al. connected the long component to the cluster–ligand electron transfer, associated with the redox process.55
Fig. 4 Relative amounts of the long lifetime component s2 (∼250 ns) for AuGSH and AuAgGSH at two different emission wavelengths with λex = 405 nm. The presence of silver and an emission wavelength in the red region of AuAgGSH at higher wavelength led to a “quasi monoexponential” long lifetime decay. Lifetime decays were fitted with global lifetimes. Ref. 49 © 2012 Royal Society of Chemistry. Reproduced from ref. 49 with permission from The Royal Society of Chemistry. |
Zhang et al.24 also revealed a rapid synthesis of GSH-capped AuAg NCs (GS-AuAg NCs) via microwave irradiation. The doped GS-AuAg NCs have a much higher quantum yield (7.8% for GS-AuAg and 2.2% for GS-Au NCs) in comparison to GS-Au NCs. Their work has demonstrated that Ag/Au 0.2 brings the maximum quantum yield [Table 1]. Au–Ag charge redistribution to compensate the electron-withdrawing properties of the thiolate has been attributed to the cluster stability. Absorption spectra of the samples contain peaks at 500 nm as well as shoulder peaks at ∼400 nm. Stepwise multiple bands and localized surface plasmon resonance (LSPR) bands, characteristic of plasmonic gold or silver nanocrystals,56 do not appear in the absorption spectra contradictory to nanocrystal formation.24
Ag/Au | Emission wavelength (nm) | Quantum yield (%) |
---|---|---|
0 | 610 | 2.2 |
0.1 | 614 | 4.6 |
0.2 | 618 | 7.8 |
0.5 | 614 | 4.3 |
1 | 614 | 4.3 |
Alloy formation between gold and silver produces a distinctly different metal core, causing a large fluorescence enhancement. Udayabhaskararao et al.57 synthesized Ag7Au6 (a 13-Atom Alloy Quantum Cluster, QC), protected with mercaptosuccinic acid from the Ag7,8 cluster. The excitation and emission maxima were at 670 and 770 nm for Ag7,8, while the excitation and emission maxima are at 390 and 650 nm, respectively for the alloy cluster. The Ag7,8 cluster shows weak fluorescence (quantum yield 8 × 10−3) at room temperature (300 K) and the quantum yield is 3.5 × 10−2 for Ag7Au8 clusters under similar conditions. Alteration in luminescence property between Ag7,8 and Ag7Au8 were attributed to the core-modifications owing to alloying. The lesser extent of electron donation from the Au core of the QCs to thiol may be a vital point in this issue. One of the possible structures of the alloy QC, as reported by Udayabhaskararao et al.,57 is a distorted icosahedral core of C2v symmetry and the thiolates are bonded in a bridged form (–Au/Ag–thiolate–Ag/Au) [Fig. 5].
Fig. 5 Changes observed during synthesis. (A and B) Solutions of Ag7,8 (A) and the alloy QC (B) after synthesis under visible light. Insets: the same samples under UV light. (C) UV/vis profile of (a) Ag7,8 and (b) alloy QC measured in water; arrows indicate the well-defined optical features of the cluster. (D) Luminescence spectra of (a) Ag7,8 and (b) alloy QC in water at 300 K. (E) Photographs of (a, a1) alloy QC in water and (b, b1) in the solid state under visible and UV light. (F) Comparison of the PAGE of (a, a1) Ag8 and (b, b1) alloy QC. Photographs of gel in visible (a, b) and UV (a1, b1) light. Band positions are marked with circles on the gels. (G) Simulated absorption spectrum using time-dependent density functional theory (TDDFT). The data are compared with the experimental spectrum (full line). The region in between 1.4 to 2.3 eV is expanded. Inset: one of the optimized structures of the model cluster, Ag7Au6(SCH3)10, with which the spectrum was simulated. Ag large light gray, Au black, S small dark gray, CH3 small light gray. Reprinted with permission from ref. 57. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. |
Doping and alloying are similar to some extent. Both of them indicate mixing. However, more specifically for doping, the dopant has a remarkably smaller concentration unlike alloying. The perturbation of the properties of individual substances is expected to be greater in the case of doping.
The luminescence lifetime value assisted in understanding the luminescence property of the as-synthesized Au@Ag NCs. Luminescence decay profile indicated the predominance of long lifetime (in the microsecond scale) components in the luminescent Au@Ag NCs: 2.21 ms (56%), 0.641 ms (31.4%), 0.121 ms (10.3%), and 11.8 ns (2.3%). The microsecond-scale lifetime values were also reported for aggregation induced luminescence of Au NCs.20 These data support emission of the so-produced Au@Ag NCs is due to AIE of Au(I)/Ag(I)–thiolate complexes on the surface of NC. It was actually phosphorescence involving the metal-centered triplet states. Clearly this luminescence property was unlike the nanosecond emission from the singlet excited states, previously stated for luminescent DNA-protected Ag NCs.36
AIE of Au@Ag NCs originated from the complexes on the NC surface and the size/structure of the Au(I)/Ag(I)–thiolate complexes became the main reason for their strong luminescence behavior in the visible to near-infrared region. Dou et al.44 assumed that the intense red emission of the Au@Ag NCs is caused by the linking of tiny Au(I)–thiolate motifs on the parental Au NCs via the Ag(I) linkers. As a result, large Au(I)/Ag(I)–thiolate motifs are formed exhibiting strong luminescence via AIE. Since the Ag(I) ions serve as linkers in forming the large Au(I)/Ag(I)–thiolate complexes on the Au NCs, the removal of Ag(I) linkers breaks those emission-active species [large Au(I)/Ag(I)–thiolate complexes] and scrubs off those Ag(I) ions from the NC surface. Thus, the Au@Ag NCs revert back to the parental Au NCs, annulling their strong luminescence in solution. The Ag(I) linker, present on the surface of luminescent Au@Ag NCs, is detached by the introduction of a specific thiolate ligand, cysteine (Cys), that is capable of interacting efficiently with Ag(I) ions to form Ag(I)–thiolate complexes.58 Thus, the large red emission of Au@Ag NCs is instantaneously destroyed with the introduction of Cys [Fig. 6].
Fig. 6 (a) Schematic illustration of the light-up process for the synthesis of highly luminescent Au@Ag NCs by using Ag(I) ions as linkers connecting the small Au(I)–thiolate motifs on the parental Au NC surface. (b) UV-vis absorption (solid lines) and luminescence spectra (dashed lines, λex = 520 nm): spectra of the parental Au18(SG)14 NCs (black lines) and luminescent Au@Ag NCs (red lines). (Insets) Digital photos of the parental Au18(SG)14 NCs (item 1 and 2) and luminescent Au@Ag NCs (item 3 and 4), under visible (item 1 and 3) and UV (item 2 and 4) light. (c) Luminescence decay profiles (top panel) of the luminescent Au@Ag NCs. The red line is a tetra-exponential fit of the experimental data. The bottom panel shows the residuals of fitting. Ref. 44 © 2013 Royal Society of Chemistry. Reproduced from ref. 44 with permission from The Royal Society of Chemistry. |
Scheme 1 Synergistic evolution of highly fluorescent Ag2 and Ag3 clusters on the Au(I) surface by solar light irradiation of Ag(I)–glutathione and Au(I)–glutathione. Reprinted with permission from (ref. 60). © 2013 American Chemical Society. |
Metal enhanced fluorescence (MEF) demands large-aggregated particles with higher scattering cross-section so that there is no lossy surface wave.62–66 Concentrating the electric field around the metalized surface in the presence of a weakly fluorescing fluorophore to enhance the rate of excitation (lightening rod effect) and higher radioactive decay rates of such fluorophores in the presence of the metal causing the higher rate of emission are the key issues for MEF. On the contrary, fluorescent clusters are very tiny and often sub-nanometer in size containing discrete energy levels. Again, emissive giant clusters are, as a whole, micro particles. However, it does not fulfil the criterion of MEF. Aqueous glutathione is not at all fluorescent, at least under ∼400 nm excitation (the optimal excitation wavelength for giant clusters). Moreover, using glutathione (reducing and capping agent for the giant clusters), separate Ag NCs and Au NCs have been produced by many groups.67,68
It was revealed that the amounts of silver ions required to obtain the highest emission after the completion of Au25(SG)18 oxidation was 5 equivalents (with respect to Au25) suggesting that a certain structured complex was responsible for the most intense emission. Added Ag+ followed the chelation-enhanced fluorescence (CHEF) mechanism for the increase of fluorescence of Au25(SG)18 NCs.70–72 The red-shifted emission with the silver addition, after the completion of oxidation of Au25(SG)18, supported the association of Ag+ with Au25.
Wu et al.69 also studied the consequence of the reduced silver [neutral silver, Ag(0), from the stoichiometric redox reaction between Au25(SG)18 and Ag+] on the emission of Au25(SG)18. In both these circumstances, the post-oxidation processes became similar. But, an alteration was observed in the fluorescence increase during the oxidation course. In one case (oxidized directly by Ag+), 188% fluorescence enhancement took place. In the other case, 126% enhancement (oxidized by H2O2) was observed. As there was no other suspected species except for mainly Ag(0) during the redox process, the plausible cause was explained to be the interaction between oxidized Au25 and Ag(0). The phenomenon was described using the MEF effect. However, XPS analyses could not detect Ag(0) species due to facile oxidation of Ag(0) and capping agent could not inhibit such oxidation under the experimental conditions.
Two opposing opinions, however, were found regarding the “silver effect” in fluorescence. Such an effect could increase75 and decrease76 the luminescence intensity of bimetallic AuAg NCs with respect to Au NCs.
Glutathione is mostly used for the synthesis of strongly fluorescent AuAg bimetallic clusters. Ganguly et al.20,34,60,77 achieved success for Ag(I)@Ag(0) fabrication using glutathione via UV irradiation, sunlight exposure and a modified hydrothermal (MHT) protocol. They also used S-lactoylglutathione for synthetic purposes.60 They were able to prepare the bimetallic NCs by employing green chemical approaches.60 Dou et al.44 also reported glutathione as a template for AuAg NCs, exhibiting AIE. Zhang et al.24 presented the rapid microwave-assisted technique for the synthesis of strongly luminescent AuAg nanoclusters using glutathione. Le Guével et al. also used glutathione for the synthesis of AuAg bimetallic clusters.49 By adding Ag+ to the pre-synthesized AuGSH, they made such bimetallic clusters. Again, Wu et al.69 used glutathione to show oxidation-interaction induced fluorescence enhancement for bimetallic clusters.
An interesting Ag7Au6 alloy NC57 was made by employing interfacial etching of mercaptosuccinic acid (MSA)-stabilized Ag nanoparticles (NPs) and a galvanic exchange reaction. However, the synthetic procedure demands toxic toluene and multistep washings via centrifugation. Zhou et al.35 also prepared highly fluorescent AgAu alloy NCs, protected with MSA by core etching of Ag NPs and a galvanic exchange reaction without any organic solvent as well as multistep centrifuge washing. Again, to withstand hostile environments the MSA–AgAu NCs were derivatized by poly(ethylene glycol) (PEG) using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) chemistry.78,79 Such PEGylated MSA–AgAu NCs exhibited efficient fluorescence enhancement in the presence of Al3+ [Fig. 7].
Fig. 7 Schematic representation for the formation of AuAg NCs that show fluorescence enhancement in the presence of Al3+. Reprinted with permission from (ref. 35). © 2013 American Chemical Society. |
Ristig et al.48 used 11-mercaptoundecanoic acid (11-MUA) to synthesize fluorescent AuAg alloy by co-reduction of gold and silver with NaBH4 in aqueous medium and 11-MUA caps the particle simultaneously. Paramanik and Patra80 prepared highly efficient blue bimetallic AuAg alloy emitter clusters using an easy one-pot bottom-up approach with 11-MUA. They also presented an alternative top-down protocol for the formation of the same bimetallic clusters.
A bimetallic AuAg alloy nanocluster was synthesized on the macroscale via sequential reduction employing simple mortar grinding. In this report, chitosan biopolymer was used for both stabilization and reducing purposes.81
Kumara and Dass82 reported Au38−nAgn(SCH2CH2PH)24, synthesized involving two steps. The first step was the formation of a crude product containing polydisperse AuAg clusters, demonstrated by Negishi et al.83 The next step involved the thermo-chemical treatment of the crude product in the presence of excess thiol to form Au38−nAgn(SCH2CH2PH)24 alloy nanomolecules.
Mohanty et al.84 reported the preparation protocol of luminescent AuAg alloy quantum clusters (QCs) using protein template, bovine serum albumin (BSA). They mixed the as-synthesized protein-protected Au and Ag clusters to form AuAg alloy clusters within BSA. Mass-spectrometric analyses of the AuAg alloy quantum clusters from a 1:1 molar ratio reaction mixture of AuQC@BSA and AgQC@BSA indicated the alloy clusters to be Au38−xAgx@BSA. Zhang et al.73 also prepared bimetallic AuAg NCs, using BSA as the protein stabilization and reduction agent. They used HAuCl4, AgNO3 and NaOH for synthetic purposes (no preformed gold or silver clusters) and a one-pot biomineralization synthetic route.
Chen et al.21 reported a one-pot synthetic route of fluorescent DNA–AuAg NCs employing HAuCl4, AgNO3, and NaBH4 (reducing agent) in the presence of 5′-CCCTTAATCCCC-3′, used as a template. They described the relative molar ratios of Au3+ to Ag+ and [NaBH4] to make stable and emissive DNA–AuAg NCs.
Material | Oxidation state of Au | Oxidation state of Ag | Binding energy of Au 4f7/2 (eV) | Binding energy of Ag 3d5/2 (eV) | Reference |
---|---|---|---|---|---|
MSA–AgAu NCs | 0, +1 | 0 | 84.7 | 368.1 | Zhou et al.35 |
DNA–AgAu NCs | 0, +1 | 0, +1 | 83.6, 85.0 | 368.7 | Chen et al.21 |
MUA–AgAu NCs | 0, +1 | 0 | 85.03 | 367.54 | Paramanik and Patra80 |
GSH–AgAu NCs | +1 | +1 | 84.4 | 367.9 | Dou et al.44 |
BSA–AgAu NCs | 0 | 0 | 84.1 | 368.2 | Mohanty et al.84 |
GSH–AgAu NCs | 0, +1 | +1 | 83.8 | 367.9 | Zhang et al.24 |
MSA–AgAu NCs | 0, +1 | 0 | 84.7 | 368.1 | Udayabhaskararao et al.57 |
BSA–AgAu NCs | 0 | 0, +1 | 84.1 | 367.8 | Zhang et al.73 |
GSH–AgAu NCs | +1 | 0 | 85 | 368.5 | Ganguly et al.23 |
Fig. 8 (A) Optimized structures of the considered clusters 1–5 of composition Au144−xAgx(SH)60 (see the main text for the detailed descriptions) brown, gold; gray, silver; yellow, sulfur. The hydrogen atoms are not shown for clarity. (B) Formation energy per metal atom, Eform, of clusters 1–5. The smallest value indicates optimal formation of cluster 4. Adapted with permission from (ref. 85). © 2011 American Chemical Society. |
(1) Au144(SR)60 [written as Au114(RSAuSR)30] in the “Divide and Protect” scheme.
(2) Ag54Au60(RSAuSR)30 where the inner 54-atom Mackay icosahedron of the metal core are formed by the Ag atom;
(3) Au54Ag60(RSAuSR)30 contains 60 Ag atoms, randomly populated in both the Mackay icosahedron core and the anti-Mackay surface layer (30 each);
(4) Au54Ag60(RSAuSR)30 contains anti-Mackay surface sites of the metal core of 60 Ag; and
(5) Au114(RSAgSR)30, where in the thiolate layer, Ag substitutes Au.
However, it is the energetically optimal cluster that possesses the lowest (positive) formation energy. The minimal value of Eform comes from cluster 4 (∼0.07 eV/metal atom) and the energy value is ∼ half of the Eform of Au144(SR)60 [Fig. 8(B)]. Thus, Au and Ag like to be in a separate atom shell in the core. The theoretical investigations also indicate that the intensity of Au(5d)–Au(6sp) and Ag(4d)–Ag(5sp) interband transitions in the plasmonic region depends on the mixing ratio of Au to Ag. The lowest energy feature in the absorption spectra is due to gold-based transitions while the contribution of silver becomes dominant at higher energies of excitation. For the case of smaller nanoclusters, Ag13Au12(SH)18−1, where all silver atoms are present in the core, bestowed more low-energy “superatomic” P–D transitions than all-gold Au25(SR)18−1 as reported by Aikens.86
Again, Negishi et al.87 discovered the technique to form stable anionic Au25−xAgx(SR)18−1 clusters and the maximal x was found to be 12. The affinity of silver to be populated at the surface-site of the metal core in the well-investigated structure of Au25(SR)18−1 is also known.
A theoretical prediction of the structure of the Au6Ag7(SR)10 (R = CH3) cluster, protected by mercaptosuccinic acid, was performed by Tlahuice-Flores.47 Using a DFT approach, he followed an isoelectronic replacement of seven Au atoms by Ag atoms on the [Au13(SR)10]+ cluster. With a target in mind to point out the lowest energy isomers, he revealed the most plausible structure to be an octahedral Ag6 core that is covered by two monoatomic dimer motifs and one Au2Ag1(SR)4 staple-like motif. Ag atoms prefer to stay at the inner (core) portion and the surface has Au atoms attached by staple-like motifs.
Zhang et al.88 performed a first-principles investigation of an Ag-doped gold cluster. A stable Au20 cluster with a distinct electronic structure can be manipulated by including Ag. Consequently, the gap between the HOMO and LUMO of Au20−nAgnclusters is remarkably altered with the electronic states of Ag in the HOMO and LUMO. The imaginary part of the dielectric function implied that the Ag atom shifts the HOMO–LUMO transition of Au clusters towards the lower energy range and it is related to the concentration of Ag.
Ground state geometries of Au20−nAgnwere calculated by Zhang et al.88 Electronic and optical properties of Au NCs are greatly related to the stability of the NCs. The binding energy/atom [Eb(n)] of Au NCs was calculated to predict the cluster-stability using the following equation.
[Eb(n)] = [E(Ag) + nE(Au) − nE(AunAg)]/(n + 1) |
E(Ag), E(Au), and E(AunAg) stand for the total energies of the most stable Ag, Au, and AuAg clusters, respectively.
All of the clusters prefer the lowest spin state. The 2.40 eV binding energy of Au20 from the study of Zhang et al.88 is very close to the report of Idrobo et al.89 The increased size of gold clusters is related to the increase in binding energy.90 Enhanced binding energy denotes improved stability because of increased core electron configurations. Owing to the incorporation of Ag in Au NCs, the obtained binding energies of Au19Ag1, Au18Ag2, Au17Ag3, and Au16Ag4 became 2.68, 2.68, 2.68, and 2.69 eV, respectively. The increased Ag atom caused little perturbation in the binding energy. However, the binding energy of AuAg alloy was higher in comparison to Au20. Thus, the introduction of Ag atom improves the structural stability. Again, the Ag–Au bond is stronger in comparison to the Au–Au bond, contributing an additional stability for the σ-bonding interaction that occurs due to the overlap between the valence Au 6s (5d) orbital and vacant Ag 4p orbital.
The electronic structure of Au20−nAgn clusters can be predicted by the density of states (DOS) calculated by Zhang et al.88 The Au20 cluster possesses a large HOMO–LUMO gap.89,91 However, the theoretical band gap of Au20 was found to be 1.47 eV (0.31 eV fewer than the experimental result of 1.78 eV) for the DFT induced underestimation of electronic states.92 The Au d states (in the range of −6 and 0 eV) are dominated in HOMO, consisting of s and d states. The DOS alters with the value of n. The band gap of pure Au20 clusters decreases with the incorporation of Ag indicating the effect of Ag in the band gap. It was found that 1.42, 1.34, 1.40, and 1.68 eV were the band gaps of Au19Ag1, Au18Ag2, Au17Ag3 and Au16Ag4, respectively.88 The shift of LUMO (of Au19Ag1, Au18Ag2, and Au17Ag3) towards the lower energy region reduces the band gap owing to the change of electronic properties. Successive incorporation of Ag in the Au20 clusters causes significant p states in LUMO, resulting in the shift of LUMO and narrowing of the band gap. On the other hand, improvement of structural stability and enclosing the electronic configurations describes the increase of the band gap for the case of Au16Ag4.93 Meanwhile, the p and s states are obviously enhanced in the LUMO, and these electronic states can have effects on the optical properties.
So, Ag insertion greatly perturbs the HOMO–LUMO transition of pure gold clusters. Such a deviation in the energy level can be explained in two stages. Firstly, for Au19Ag1 and Au18Ag2, both HOMO and LUMO are contributed by Ag electronic states. Again, LUMO is slightly shifted towards the low energy region, responsible for the decrease of the transition level. Secondly, for Au17Ag3 and Au16Ag4, increasing the number of Ag atoms creates more dispersive d states lifting LUMO to the high energy region and the band gap alters. The broadening of the band gap for Au16Ag4 implies improvement in the structural stability and binding energy [Fig. 9].93
Fig. 9 (A) Calculated ground state geometries of Au20−nAgn, (B) the partial DOS of (a) Au20−nAgn clusters, (C) the outline of optical transition of Au20−nAgn clusters calculated by energy level. Ref. 88 is an open access article distributed under the Creative Commons Attribution License (CC BY) which permits unrestricted use, distribution, and reproduction in any medium. |
In contrast to the wide applicability of silver compounds in numerous industries (photography, electronics, optical instruments, mirrors, etc.), Ag+ is termed as a toxic heavy metal. Ag+ disables sulphydryl enzymes, explained by the HSAB theory, being amassed in the body. It has been found that ∼2500 tons of Ag+ is released annually into the environment through industrial wastes and emissions. Amongst these, 150 tons are released into the sludge of waste-water while 80 tons are released into surface waters together with other contaminants. So, selective detection of Ag+ is an important issue in modern research.95,96
Mercury, one of the most hazardous heavy metals, is expansively found in air, water, soil, and food.97 The accumulation of Hg in the human body causes several problems including damage to the digestive, excretory, and central nervous systems. As a result, a variety of serious diseases such as tremors, deafness, arthritis, loss of muscle coordination/sensation/memory, motor disorders etc. occur. Again, copper ions carry out pivotal roles in many essential biological activities including metabolism, growth, and immune system development.98 Copper deficiency is thus a cause of various illnesses. Besides, a high concentration of copper in tissues also creates lethal effects. Long-standing contact of high [Cu2+] causes cellular toxicity, damaged kidney and liver.99,100 Therefore, it is of great interest to develop a simple, rapid, and highly sensitive detection method for mercury and copper ions.
Aluminum is the most abundant metal in the earth’s crust and is used expansively in our daily life as well as industrially. However, excess Al3+ affects plant performance, the growth of fish, and causes a number of diseases (Parkinson’s disease, Alzheimer’s disease etc.) in humans. So, easy recognition of Al3+ is an important issue now-a-days.101–104
Pb2+ is one of the highly poisonous heavy metal cations. Ingestion of even very small quantities causes numerous health hazards namely anaemia, memory loss, and slow nerve conduction velocity especially in children. A report by the CDC (Centers of Disease Control) reveals that ≥100 mg L−1 (0.48 mM) [Pb] in the blood is deleterious to children.105,106 Thus, sensitive detection of Pb2+ is essential.
By employing the phenomenon that synergism of Au and Ag enhances fluorescence, Wu et al.69 showed intriguing fluorescence enhancement of Au25(SG)18 in the presence of Ag+ and designed a Ag+ sensor. They explained it using oxidation and interaction induced fluorescence enhancement (discussed earlier). A good linear relationship existed between [Ag+] and the emission intensity (20.2 nm to 11.1 μm). A detection limit of ∼200 nM was obtained by their protocol. It is satisfactorily lower than the maximum allowable level (MAL) of Ag+ (460 nm) in drinking water as required by the U.S. Environmental Protection Agency (EPA).
Zhang et al.73 designed a fluorimetric technique for the separate recognition of Hg2+ and Cu2+ ions using BSA protected AuAg NCs. The fluorescence quenching of AuAg NCs with Hg2+ ions at different concentrations is demonstrated in Fig. 10. A plot of logarithmic concentration of Hg2+ vs. fluorescence quenching efficiency indicates that Hg2+ ions can be sensed over a linear concentration range of 0.20 to 2500 nM (R2 = 0.9903) and LOD (limit of detection) of 0.10 nM (3σ rule). Likewise, a linear correlation for Cu2+ was reported from 0.50 nM to 2500 nM (R2 = 0.9960) and a LOD of 0.30 nM. Structural analysis from TEM imaging for such bimetallic AuAg NCs with/without Cu2+ or Hg2+ ions and in the presence of EDTA revealed insignificant changes in the size and morphology for the case of Hg2+. Fluorescence microscopy indicated that quenched fluorescence with Hg2+ ions does not alter with the introduction of EDTA. On the contrary, Cu2+ ions induce the aggregation of the protein template of Au–Ag NCs. Then, EDTA causes a deaggregation effect on the resulting mixture with the restoration of the fluorescence. The Hg2+ induced quenching fluorescence of AuAg NCs was explained by the interaction between Hg2+ ions and Au of AuAg NCs, resulting in metallophilic bonding of their 5d10 centers.75,107 On the contrary, histidyl and carboxyl groups of the protein scaffold of AuAg NCs react with Cu2+ ions, forming the protein–Cu2+ adduct.108 Cu2+stimulated protein–protein cross-linking is responsible for aggregation of the protein scaffold. Amino acid residues of the protein scaffolds can also reduce Cu2+ to Cu+ ions, originating metallophilic actions of 3d10(Cu+)–4d10(Ag+) with the fluorescence quenching. EDTA, being a strong chelating ligand, possesses a higher affinity for copper than the protein–Cu2+complex.109 Thus, the deaggregation of the protein–Cu2+complex takes place with the restoration of the vanished fluorescence of AuAg NCs. However, strong metallophilic Au–Hg2+ interaction wins over Hg2+–EDTA chelation with the permanent loss of fluorescence in the presence of Hg2+. This method was also employed to detect Hg2+ and Cu2+ selectively in blood samples down to 0.30 nM and 0.60 nM levels.
Fig. 10 Fluorescence spectra of Au–AgNCs (0.417 mM) upon the addition of (A) Hg2+ ions (0, 0.20, 1.0, 5.0, 25, 125, 625, 1250, 2500 nM) and (C) Cu2+ ions (0, 0.5, 1.0, 5.0, 25, 125, 625, 1250, 2500 nM) at λex 370 nm, corresponding to fluorescence quenching efficiencies versus the logarithmic concentrations of (B) Hg2+ ions and (D) Cu2+ ions in water (insert: photographs under UV light). Reprinted with permission from (ref. 73). © 2014 American Chemical Society. |
A strongly fluorescent Au(I)core–(Ag2/Ag3)shell giant cluster, reported by Ganguly et al.,61 loses its emissive property in the presence of Hg2+. Along with fluorescence quenching, red shifting of the emission maxima is also noticed due to Hg2+ [Scheme 2]. d10(Hg2+)–4d10(Agδ+) metallophilic interaction and aggregation of the silver clusters were ascribed to the observed fluorescence quenching phenomenon and red shifting, respectively. Cu(II) and Fe(III) also quench the fluorescence of giant clusters. Na2–EDTA and ammonium bifluoride were employed to overcome Cu(II) and Fe(III) induced quenching. Thus, a highly selective Hg(II) sensor was designed. A linear correlation was found between (I0 − I)/I0 and [Hg(II)] over the range 0–10 μM (R2 = 0.98) while the LOD [at an S/N ratio of 3 for Hg(II)] was reported to be 6 nM. It is much lower than the permissible level of the United States Environmental Protection Agency (EPA).110 This method was also found to be useful for the estimation of Hg2+ in real environmental samples.
Scheme 2 Giant fluorescent Au(I)core–(Ag2/Ag3)shell clusters as a highly selective sensing platform for Hg(II). Ref. 34 © 2014 Royal Society of Chemistry. Reproduced from ref. 34 with permission from The Royal Society of Chemistry. |
The photoluminescence intensity of microwave-assisted glutathione capped AuAg bimetallic clusters was reported to be reduced successively with the gradual increase of the concentration of Cu2+ ion.24 No other metal ion bestowed any significant quenching, helping to design the Cu2+sensor. The linear correlation between I/I0 [I and I0 are the luminescence intensity with and without Cu2+ ion] and [Cu2+] exists in the range 0 to 100 nM (y = 1–0.00467x, R2 = 0.996) with a limit of detection (LOD) of 2 nM (S/N = 3).
According to the report of Zhou et al.,35 the PEGylated MSA–AgAu NCs can be employed extremely selectively as well as a sensitive sensing platform in an aqueous medium of Al3+ (detection limit of 0.8 μM that is much lower than the permitted level 7.4 μM of World Health Organization, WHO).111 Introduction of Al3+ to the MSA–AgAu NCs causes aggregation of the particle as revealed by DLS analysis. The hydrodynamic diameter changed from 15 to 72 nm with 50 μM Al3+. To offer more chelating sites with Al3+ and completion with the ligand shell of metal NCs, free succinic acid (SA) or MSA was added in the reaction mixture. However, no significant reduction in the fluorescence intensity and hydrodynamic diameters of PEGylated MSA–AgAu NCs took place calling into question the Al3+ mediated aggregation induced fluorescence enhancement. Zhou et al.35 explained the fluorescence enhancement due to the AlAu alloy formation. XPS, DLS, and ζ-potential were employed to support the formation of such an alloy. In other words, deposition of Al3+ on the surface of the bimetallic core is the main factor for fluorescence enhancement and selective Al3+ sensing.
Glutathione capped Au(I)core–(Ag2/Ag3)shell giant cluster112 loses its fluorescence selectively in the presence of DMSO that replaces glutathione. Pb(II) binds DMSO because of the strong affinity between Pb(II) and S. Pb(II) brings back the lost fluorescence by removing DMSO from the surface of the giant clusters and glutathione takes its original position. Thus, a highly selective and sensitive Pb2+ sensor [Scheme 3] was made by Ganguly et al.112 Like Pb(II), Ag(I) and Hg(II) possess a strong affinity towards the sulfur of DMSO.113 But, extra Ag(I) ion destabilizes the fluorescent silver clusters via aggregation and Hg(II) forms an amalgam with silver. Thus, Ag(I) and Hg(II) could not regenerate the lost fluorescence (due to DMSO). The fluorescence property is re-established only in the presence of Pb(II). A linear correlation between I0/I [I0 and I are the fluorescence intensity before and after the addition of Pb2+] and [Pb2+] exists in the range 0–20 μM and the LOD was found to be 2 × 10−7 M. However, EDTA again quenches the Pb2+ induced fluorescence by forming a strong chelating complex with Pb2+.
Scheme 3 Schematic representation of DMSO induced fluorescence quenching, Pb2+ induced regaining of fluorescence of Ag(I)@Au(0)–GSH. Na2–EDTA eliminates the Pb2+ induced enhanced fluorescence. Reprinted with permission from (ref. 112). © 2014 American Chemical Society. |
To detect the toxic and corrosive S2−, Ganguly et al.77 made highly fluorescent faint yellow colored Au(I)@Ag particles via a modified hydrothermal technique in glutathione (GSH) matrix. Here, tiny Ag2, Ag3 clusters embedded on Au(I) are the origin of fluorescence. Due to the smaller size associated with a high diffusing capability, S2− selectively covers the surface of Au(I)@Ag surface generating an orange coloration and removes GSH from the fluorescent particle. Due to a higher reducing property of S2− than GSH, Au(I) is reduced to Au(0) causing the aggregation of silver clusters and eventually destroys the fluorescence. Thus, naked eye and fluorimetric detection of S2− become possible. In fluorimetric sensing via selective quenching, I− and S2O32− interfere as they are also quenchers under the experimental conditions. Pb2+ removes interference by causing fluorescence enhancement in the case of I− and S2O32−. However, S2− induced quenching remains unaffected in the presence of Pb2+. A linear correlation was found between I0/I (I0 and I are the fluorescence intensities of the solutions containing Au(I)@Ag particles without and with S2−, respectively) and [S2−] through the range of 0–10 μM (R2 = 0.985) and the LOD was 10 nM (S/N ratio of 3). Not only in water but also in water miscible solvents, sulfide detection is possible by this regime. Ganguly et al. also investigated the effect of straight chain organothiols with different chain lengths in fluorescence quenching and found that with the increase of chain length of thiols, the quenching capability decreases. This phenomenon supports the strong quenching by sulfide to be related to its smaller size and higher diffusing power [Fig. 11].
Fig. 11 (A) Fluorescence spectral profile and (B) relative fluorescence intensity (I = fluorescence intensity of alkanethiol-passivated HFL and I0 = fluorescence intensity of HFL) showing the effect of chain length of alkane thiols on the fluorescence quenching of HFL. Conditions: 0.5 mL HFL diluted to 3 mL with distilled water, [alkanethiol] = 2.5 × 10−4 M, λex = 390 nm. Ref. 77 © 2015 Royal Society of Chemistry. Reproduced from ref. 77 with permission from The Royal Society of Chemistry. |
Chen et al.21 synthesized DNA–AuAg NCs employing HAuCl4, AgNO3, NaBH4 and 5′-CCCTTAATCCCC-3′. S2− ions quenches the emission of the DNA–AuAg NCs due to the interaction between the Au/Ag atoms/ions and S2− ions. The solubility products (Ksp) of Au2S and Ag2S are reported to be 1.58 × 10−73 and 8.0 × 10−51 M2, respectively.119,120 Owing to the formation of Au2S and Ag2S, the DNA–AuAg NCs are broken down and the DNA scaffolds are converted to random coiled structures. The emission at 630 nm of such DNA–AuAg NCs is quenched with the successive introduction of S2− ions. Two linear regions 0–0.01 μM and 0.01–9 μM were found (I0F − IF)/I0F [I0F and IF are the fluorescence intensity before and after the addition of S2−] versus [S2−]. The LOD for sulfide in this process has been reported to be 0.83 nM.
Green chemically synthesized fluorescent Au(I)@(Ag2/Ag3)–thiolate core–shell particles112 (synthesized from HAuCl4, AgNO3 and GSH under the Sun) were used for selective detection of dimethyl sulfoxide (DMSO) via fluorescence quenching. No other water-miscible solvent causes quenching of fluorescence. DMSO removes capped GSH and GSSG from the particle surface resulting in fluorescence quenching. DMSO can be recognized even when DMSO molecules are 4.09 × 10−5 times the number of water molecules in a solution. With the increasing [DMSO], the fluorescence of the fluorescent particles is monotonously decreased until DMSO:H2O [molar ratio] = 0.01:1. DMSO sensing can be performed in water when the molar concentration of water is 2.4 × 104 to 1 × 102 (a wide range) times higher than [DMSO] and the LOD is 178 ppm. When [DMSO]/[H2O] (molar ratio) > 0.01, further increase of [DMSO] causes fluorescence enhancement along with a continuing red shift. [DMSO]/[H2O] (molar ratio) = 0.08 causes maximum fluorescence intensity with a red shift of ∼29 nm. Then, emission intensity is reduced with the further increase of [DMSO] in water.
Green chemically synthesized, GSH capped Au(I)@Ag2/Ag3 core–shell giant cluster was used as a highly selective and sensitive Cys sensor in the solution phase.128 Giant clusters can have robust hydrophilic moieties where the adsorbate monolayer is accumulated with the sterically demanding –GS group.129 When Cys is introduced, GSH (molecular weight 307 Da) is replaced via place exchange reaction by Cys (an amino acid of low molecular weight, ∼121 Da). Such a place exchange reaction is facilitated by the release of steric crowding, destroying the fluorescence property of the giant cluster. The place exchange reaction is also favored by the strong penetrating power of Cys. A report by Tehrani et al.130 demonstrated that GSH is prone to forming intramolecular hydrogen bonds with noble metals, contributing to the long-term stability of the fluorescent silver clusters. However, Cys is unable to produce such hydrogen bonding and a non-fluorescent solution is generated due to the removal of GSH by Cys from the cluster surface. A linear correlation was found between (I0 − I)/I0 (I0 and I is the fluorescence intensity of giant cluster without and with Cys, respectively) and [Cys] in the range of 50 nM to 10 mM and the LOD was 50 nM.
It is found that the enhanced dose of silver in alloyed AgAu nanoparticles does not cause a monotonous decrease in cell viability. Importantly, the nanoparticles with Ag:Au = 90:10 were reported to be more toxic in comparison to pure silver nanoparticles (Ag:Au = 100:0). 11-MUA capped AuAg clusters, synthesized by Ristig et al.,48 reported a similar observation. Again, Barcikowski et al.131 demonstrated a similar fact for alloyed silver–gold nanoparticles, obtained from laser ablation in mammalian gametes. Ristig et al.48 also incubated the HeLa cells in their 11-MUA capped AuAg clusters (Ag:Au = 30:70 and Ag:Au = 50:50) for 24 h in serum supplemented medium and found red emitters are located at the cytoplasm. It is an efficacious internalization of bimetallic particles inside the cells. The red emissive dots in the cells are not separate nanoparticles. They are due to the gathering of nanoparticles in endosomal vesicles [Fig. 12 and 13].
Fig. 12 MTT assays of HeLa cells after incubation with different concentrations of alloyed silver–gold nanoparticles for 24 h (A) and 72 h (B). [Ref. 48] – published by The Royal Society of Chemistry. |
Fig. 13 CLSM images of HeLa cells after incubation with Ag:Au 30:70 (A) and Ag:Au 50:50 (B) nanoparticles for 24 h in serum supplemented medium. The nanoparticles have a red colour and the cell cytoskeleton (actin) is stained green. [Ref. 48] – © The Royal Society of Chemistry. |
MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay of hydrothermally synthesized fluorescent Au(I)@Ag particles (Au:Ag = 10:17) showed that the particles are bio-compatible.77 The optical study on both primary [human fibroblast (foreskin origin) cells] and cancer cell (MG-63 cells) lines indicate that for primary cells, fluorescent particles have an inclination to accrue in the cytoplasm. On the other hand, the fluorescent particles are likely to be accumulated inside the nucleus for cancer cells. This observation was enlightened using the electric properties of a cancer cell. The cytoplasm of a cancer cell has a paucity of electrons in comparison to a primary/normal cell. Due to the positive zeta potential of the emissive particles (+11.9 mV), the cytoplasm repels the particles strongly in a cancer cell unlike the primary cell. This study increases the opportunity of bimetallic nanoclusters to be used as nanoprobes for the in vitro diagnostics of cancer cells employing a traditional biopsy strategy, related to fluorescence microscopy.
EF = (ISERS/Nads)/(Ibulk/Nbulk) [ISERS = the intensity of a vibrational mode in the surface enhanced spectrum, Ibulk = intensity of the Raman spectrum of same vibrational mode in the bulk material, Nads = number of adsorbed molecules on the SERS-active substrate, and Nbulk = the number of sampled molecules in the bulk material].
The bimetallic AuAg alloy nanoclusters have also been employed for SERS experiments. As the surface area of NCs is significantly higher in comparison to the corresponding NPs/bulk materials, a greater degree of absorption on the cluster surface takes place, causing SERS. Paramanik and Patra80 showed the MUA protected AuAg nanoclusters in the context of SERS when the probe is methylene blue (MB).
In the Raman spectra of MB, strong peaks for C–C stretching of the ring carbon (1618 cm−1) and peaks for the symmetric, asymmetric stretching and bending of the C–N–C skeleton (1395 cm−1, 1450 cm−1 and 445 cm−1) were found133 When AuNCs are added to MB (non-fluorescent), an insignificant Raman signal is obtained at 1618 cm−1 or 445 cm−1. Interestingly, AuAg alloy clusters cause remarkable SERS enhancement with an EF (enhancement factor) of 1.44 × 106 for MB. MB dye adsorbs on the surface of AuAg clusters, concentrating the local electromagnetic field. A study by Paramanik and Patra80 revealed that AuAg alloy NCs have a higher potential for SERS activity in comparison to pure Au NCs.
Fluorescent giant clusters [Au(I)core–(Ag2/Ag3)shell], prepared by Ganguly et al.,60 were impregnated in cotton and paper to produce fluorescent cotton and paper with antibacterial activity. Paper substrate and cotton wool containing fluorescing giant clusters exhibited decent antibacterial activity against both Gram-positive (S. aureus) and Gram-negative (E. coli) bacterial strains. Clear zones were found around the fluorescent cotton wool and paper substrate when the disk diffusion assay method was performed. The fluorescent particle treated cotton wool/paper showed a decrease in the bacterial number for both Gram-positive and Gram-negative bacteria. It was reported that the fluorescent cotton wool caused 99.91% reduction in viability of the Gram-negative bacteria after 3 h of incubation and 98.6% was obtained for the Gram-positive bacteria after 3 h. It was concluded that fluorescent cotton wool, impregnated with giant clusters, was marginally less effective for Gram-positive bacteria than Gram-negative bacteria within the initial 3 h of incubation [Fig. 14].
Fig. 14 Antibacterial activity of the fluorescent cotton wool in agar plates for E. coli (a) and S. aureus (b). Antibacterial activity of the fluorescent paper substrate using agar plates for E. coli (a1) and S. aureus (b1). Reprinted with permission from (ref. 60). © 2013 American Chemical Society. |
Fig. 15 (a) Decay of the logarithmic absorbance of 4NP as a function of time for different NC catalysts. (b) Arrhenius plots for reactions catalyzed by Ag and Au–Ag NCs. Ref. 81 © 2012 Royal Society of Chemistry. [Ref. 81] – reproduced by permission of The Royal Society of Chemistry. |
Researchers are also involved in the synthesis of carbon dots.139,140 The synthesis of carbon dots is cheap and they are mostly cytocompatible as the precursor compounds are cytocompatible. However, the main problem is that in most of the synthetic strategies the particle size of the carbon dots cannot be controlled properly and the quantum yield is poor.141 Semiconductor quantum dots142,143 are known for their strong luminescent property. However, high toxicity associated with severe health hazards limits their in vivo applications.
Bimetallic AuAg NCs are superior to individual Au NCs or Ag NCs due to their high emissive behavior and photostabilty.49 The toxicity of AuAg NCs can be controlled by altering the ratio of Au and Ag in the bimetallic clusters.48 They are very robust. They can be vacuum dried to obtain fluorescent solids and can be reversibly transferred in solid and liquid phase.60 Usually, fluorescent Ag NCs/Au NCs change the emission maximum with variation of the solvent.144 However, Au(I)core–(Ag2,Ag3)shell giant clusters are virtually unchanged in terms of their emission maxima with solvent variation. Different extents of fluorescent enhancement were observed in different water-miscible solvents in comparison to water [Fig. 16]. The enhancement was probably due to release of self-quenching and hydrogen bonding. Not only in water miscible solvents, but also in water immiscible solvents, Au(I)core–(Ag2,Ag3)shell giant clusters can be prepared using the water pool of the AOT reverse micelle.60 Such giant clusters can also be impregnated in cotton wool and paper to obtain fluorescent cotton and paper with antibacterial activity. Bimetallic clusters are also superior to monometallic clusters in the context of different useful applications such as sensing, surface enhanced Raman scattering, catalytic activity etc.
Fig. 16 Fluorescence spectra of the fluorescing solution containing giant clusters in different water miscible organic solvents. Reprinted with permission from (ref. 60). © 2013 American Chemical Society. |
Zhou et al.145 examined the excited state dynamics of a highly luminescent cluster [Au6Ag2(C) (dppy)6](BF4)4 in three different solvents (CH2Cl2, CH3CN, CH3OH). They employed femtosecond transient absorption spectroscopy associated with quantum chemical calculations in this context. It is a Au(I)–Ag(I) cluster with hypercoordinated carbon. The cluster possessed an octahedral Au6 core and two Ag atoms. The two Ag atoms cap the two opposite Au3 triangles and six peripheral diphenylphoshpino-2-pyridine (dppy) ligands. The cluster was strongly emissive in the solid state as well as the solution phase. Solvents of altered polarity and hydrogen bond formation capability (CH2Cl2, CH3CN, CH3OH) were used to monitor the excited state dynamics and the electronic structure. The shift of the steady state absorption spectra in the visible range was observed with alteration of the solvent systems. Interestingly, the emission wavelength was unaltered [Fig. 17 and 18]. Actually, the carbon hypercoordinated Au(I)–Ag(I) cluster exhibits an ultrafast intersystem crossing rate (1–3 ps) and solvent dependent energy transfer. This is the reason for different emissive behaviors. Again, the population of various states and the phosphorescence behaviors of the carbon hypercoordinated clusters can be manipulated with the variation of solvent.
Fig. 17 Crystal Structure of [Au6Ag2(C) (dppy)6]4+ (A) and simplified optimized structure of ground state [Au6Ag2(C) (PH2py)6]4+ (B). Au atoms are colored yellow, Ag atoms green, N atoms blue, C atoms gray, P atoms orange, H atoms light gray. Reprinted with permission from (ref. 145). © 2015 American Chemical Society. |
Fig. 18 (A) Normalized steady state absorption spectra of [Au6Ag2(C)-(dppy)6](BF4)4 dissolved in three solvents, where the concentrations were adjusted to be the same (∼8 × 10−5 mol L−1) for all measurements. (B) Luminescence spectra of [Au6Ag2(C)-(dppy)6](BF4)4 with excitation of 350 nm in three solvents. Reprinted with permission from (ref. 145). © 2015 American Chemical Society. |
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