Discovery, Mechanism, and Application of Antigalvanic Reaction

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Discovery, Mechanism, and Application of Antigalvanic Reaction Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Zibao Gan, Nan Xia, and Zhikun Wu*

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Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Institute of Physical Science and Information Technology, Anhui University, Hefei 230601, China CONSPECTUS: Among many outstanding findings associated with the quantum size effect, one of the most exciting is the discovery of the antigalvanic reaction (AGR), which is the opposite of the classic galvanic reaction (GR) that has a history of nearly 240 years. The GR, named after Italian scientist Luigi Galvani, involves the spontaneous reduction of a noble-metal cation by a less noble metal in solution driven by the difference in electrochemical potentials. Classic galvanic reduction has been widely applied and has recently received particular interest in nanoscience and nanotechnology. However, the opposite of GR, that is, reduction of metal ions by less reactive (or more noble) metals, has long been regarded as a virtual impossibility until the recent surprising findings regarding atomically precise ultrasmall metal nanoparticles (nanoclusters), which bridge the gap between metal atoms (complexes) and metal nanocrystals and provide opportunities for novel scientific findings due to their well-defined compositions and structures. The AGR is significant not only because it is the opposite of the classic galvanic theory but also because it opens extensive applications in a large range of fields, such as sensing and tuning the compositions, structures, and properties of nanostructures that are otherwise difficult to obtain. Starting with the proposal of the general AGR concept in 2012 by Wu, a new era began, in which AGR received widespread attention and was extensively studied. After years of effort, great advances have been achieved in the research on AGR, which will be reviewed below. In this Account, we first provide a short introduction to the AGR concept and then discuss the driving force of the AGR together with the effecting factors, including the ligand, particle size, solvent, metal ion precursor, and ion dose. Subsequently, the application of the AGR in engineering atomically precise alloy (bimetallic and trimetallic) and monometallic nanoclusters is described, and tuning the properties of the parent nanoclusters is also included. In particular, four alloying modes (namely, (i) addition, (ii) replacement, (iii) replacement and structural transformation, and (iv) nonreplacement and structural transformation) associated with the AGR are discussed. After that, the applications of the AGR in metal ion sensing and antioxidation are reviewed. Finally, future prospects are discussed, and some challenging issues are presented at the end of this Account. It is expected that this Account will stimulate more scientific and technological interests in the AGR, and exciting progress in the understanding and application of the AGR will be made in the coming years.

1. INTRODUCTION The galvanic reaction (GR), named after Italian scientist Luigi Galvani (1737−1798), who discovered animal electricity, involves the spontaneous reduction of a noble-metal cation by a less noble metal in solution driven by the difference in electrochemical potentials.1 The metal atoms are oxidized and enter the solution phase, while the less reactive metal ions are reduced and deposited on the surface of the metal template.2 For an illustration, see Scheme 1. Classic galvanic reduction has been widely applied and has recently received particular interest in nanoscience and nanotechnology due to its high tunability and feasibility in engineering the metal nanostructures.3 However, the opposite of the GR, that is, reduction of metal ions by less reactive (or more noble) metals (antigalvanic reaction, AGR), © XXXX American Chemical Society

has long been regarded as a virtual impossibility until recent surprising findings in metal nanoparticles (NPs).1 The early works have actually found the size dependent oxidation− reduction property of metal NPs.4,5 For instance, Plieth even provided an equation to predict the reduction potential of metal NPs,4 as shown below: μd = μ b − (2M /zFρ)γ /r

μd is the reduction potential of metal NPs, μb is the reduction potential of electrode, M is the molar mass, ρ is the specific mass, z is the number of charge transfer, F is Faraday’s constant, γ is the Received: July 30, 2018

A

DOI: 10.1021/acs.accounts.8b00374 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research Scheme 1. Schematic Illustration of GR and AGR Processes (0 ≤ X < Y ≤ 1, 0 ≤ N < M ≤ 1; X, Y, M, and N Represent the Oxidation States of the According Atoms); Note That Only Au and Ag Are Chosen for the Illustration

Figure 1. Cyclic voltammograms of 4.5 mM AgNO3 (A) and surfactantand ligand-free gold NPs (B) in 0.1 M Bu4NPF6/CH3CN-THF. Reference electrodes: Ag/Ag+ (0.1 M in CH3CN); scan rate: 10 mV/s. Adapted from ref 15 with permission from John Wiley and Sons. Cyclic voltammograms of (C) the bare bulk Au electrode before and after self-assembly of the 11-MUA monolayer and (D) the 11-MUA-Ag/Au NCs, 11-MUA-Au NCs, and 11-MUA-Au NPs. Adapted with permission from ref 17. Copyright 2014 Royal Society of Chemistry.

surface free energy, and r is the particle radius. Obviously, on the basis of the proposed equation, the reduction potential of metal NPs decreases with the size decrease. In other words, the reducibility of metal NPs increases with the size decrease. It is the research advancement of metal nanoclusters (NCs, note that NCs are also NPs, but they are ultrasmall NPs with size generally less than 3 nm) that plays a vital role in finding the unexpected AGR phenomena because the metal NCs can be precisely characterized with atomic precision.6−12 In 2010, Murray and co-workers first identified the species Au 24 Ag(PET) 18 , Au23Ag2(PET)18, and so on by mass spectrometry after mixing the negative Au25(PET)18 nanocluster with Ag+ (PET: phenylethanethiolate), 13 after which Wu revealed that not only negative Au25(SR)18 but also neutral Au25(SR)18 and other ultrasmall gold and silver NPs can react with silver or copper ions.14 On the basis of these facts, he proposed the general AGR concept. Further, Wu’s group extended the AGR to Pt and Pd NPs, and demonstrated that the AGR is not caused by the reducing thiolates but is intrinsic and is associated with the size effect of a metal,15 thus truly establishing the concept and beginning a new research field. Afterward, the same group found that metal NCs could react with the same metal ions,16 which was termed the quasi-antigalvanic reaction. After the Wu group’s ground-breaking work and continuous advances in the study of AGR, other groups have also been involved in this frontier area of research, as will be also discussed (vide infra).

potential decrease of ∼2 nm gold NCs protected by 11-mercaptoundecanoic acid (11-MUA) (Figure 1C, D).17 Although the driving force of AGR can be explained in terms of reduction potential differences, the product diversity is not well understood. By now, several factors have been found to influence the AGR products. The type of surface ligand distinctly influences the AGR products. For example, Wu et al. demonstrated that phenylethanethiolated Au25 can react with silver ions to produce Au− Ag alloy NCs, while dominant alloy NCs were not found in the reaction of glutathiolated Au25 with silver ions.18,19 The AGR is size dependent: when the size of the metal NPs decreases, the reactivity of metal NPs toward metal ions (including free and unfree ions) increases.15 Wu et al. also demonstrated that the AGR is ion precursor dependent: different silver ion precursors (including AgNO3, Ag-EDTA, Ag-PET, and Ag-DTZ; EDTA: ethylenediamine tetraacetic acid disodium salt; DTZ: dithiazone) in the reaction with Au25(PET)18 under similar conditions lead to various products, as illustrated by thin-layer chromatography (TLC) in combination with mass spectrometry (Figure 2A, C).18 In another AGR, when cadmium ions were replaced by Cd(CHT)2, the product changed from Au28(CHT)20 to Au20Cd4(SH)(CHT)19 (CHT = cyclohexanethiolate).20 Additionally, the AGR is ion dose dependent. For example, in the reaction of Au25(PET)18 and Ag-DTZ, as the Ag-DTZ dose increases, the content of Au24Ag(PET)18 in the product mixture increases, while the content of Au25Ag2(PET)18 decreases and is finally negligible when the Ag:Au atomic ratio reaches 2:1 (Figure 2B, C).18 The solvent effect also exists in the AGR. Taking the reaction of Au25(PET)18 with Ag+ as an example, polydisperse Au24−xAgx(PET)18 NCs were obtained in dichloromethane or toluene, whereas only Au25Ag2(PET)18 is produced in acetonitrile.21 In other words, the AGR products are diverse and sensitive to multiple factors.

2. DRIVING FORCE AND EFFECTING FACTORS AGR is unexpected, and its mechanism attracts intense interest. Although AGR opposes the classic galvanic theory, it does not violate the laws of thermodynamics, and the driving force can be explained in terms of redox potential differences. Due to the quantum confinement effect, the oxidation potential of metal NPs greatly decreases, becoming even lower than the reduction potential of some less noble metal ions. For example, Wu et al. demonstrated that the oxidation potential of surfactant- and ligand-free gold NPs (−0.76 V vs Ag+/Ag) is obviously lower than the reduction potential of Ag+ (−0.21 V vs Ag+/Ag), which leads to the oxidation of gold NPs and the reduction of Ag+ (Figure 1A, B).15 Jin et al. also revealed the similar oxidation B


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Accounts of Chemical Research

Figure 2. Monitoring the product by TLC: (A) AgNO3, Ag-EDTA, Ag-PET, and Ag-DTZ were employed as the precursors with a Au:Ag ratio of 1:2 and (B) different Au:Ag ratios, 2:1, 1:1, and 1:2, in the case of Ag-DTZ. (C) Mass spectra of the products S1−S5, S8, and S10−S13. The spectra of S1, S3, S5, S8, S10, and S12 were acquired in negative ionizaton mode, while the others were collected in positive ionizaton mode. Adapted with permission from ref 18. Copyright 2015 Royal Society of Chemistry.

Figure 3. Optical absorption evolution of Au25 NCs after the addition of Cu2+ and the proposed transformation process. Adapted with permission from ref 22. Copyright 2015 Royal Society of Chemistry.

3. APPLICATION OF THE AGR The AGR is a significant finding not only because it does not comply with the classic antigalvanic theory but also because it is of potential application in a large range of fields. To date, the reaction has been applied in sensing, antioxidation, and tuning of the compositions, structures, and properties of NPs, which will be discussed below.

monometallic, bimetallic and trimetallic NPs can be obtained by means of the AGR. 3.1.1. Synthesis of Monometallic NCs by Way of AGR. The first example of the synthesis of monometallic NCs by means of the AGR is the synthesis of Au44(PET)32. Wu et al. reported that Cu2+ can convert negative Au25 NCs into neutral Au25, cationic Au25, and finally a new product, Au44(PET)32.22 The whole reaction process can be broken into three steps: oxidation−reduction, decomposition, and recombination, as shown in Figure 3. The as-obtained Au44(PET)32 NCs show higher catalytic activity toward the reduction of 4-nitrophenol at both room temperature and freezing point compared with the parent NC, becoming even higher than those of some other common NPs (Au38(PET)24, Au 4 4 (TBBT) 28 (TBBT: 4-tert-butylbenzenethiolate),

3.1. Tuning of Nanoparticle Compositions, Structures, and Properties

As noted by Wu in his pioneering work, the most straightforward application of the AGR is the tuning of the compositions, structures and properties of NPs, which attracts widespread interest because the AGR provides a facile and irreplaceable method of engineering NPs precisely.14 And it is known that C


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Figure 4. Emission comparison of Au24(PET)20 (black), Au25(PET)18 (red), Au38(PET)24 (blue), and Au144(PET)60 (dark cyan) (left) and photos of the four NCs under UV and visiblelight radiation (right). Reproduced with permission from ref 16. Copyright 2015 Royal Society of Chemistry.

Figure 5. “Molecular surgery” by a combination method of AGR and quasi-AGR.

and to date a few such bimetallic NCs, including Au25Ag2(PET)18,21 [Au 1 3 Cd 2 (PPh 3 ) 6 (SC 2 H 4 Ph) 6 (NO 3 ) 2 ] 2 Cd(NO 3 ) 4 , 2 5 Au24Ag1(PPh3)10(PET)5Cl2,26 Au24Cu1(PPh3)10(PET)5Cl2,26 Au38Cu(PET)24,27 Au24Hg(PET)18,28 and Au24Cd(PET)18,29 have been obtained. Interestingly, four main alloying modes of the AGR (see Scheme 2) have been found thus far:20

Au144(PET)60, and Ni, Ag, Co, and Cu NPs). Further, the same group showed that the reaction between [Au23(CHT)16]− and Cd2+ produces monometallic Au28(CHT)20 NCs.20 In addition, Wu et al. found that gold NCs can react with gold complexes (or salts), and they named this kind of reaction pseudo-AGR.16 As an illustration, Au24(PET)20 was obtained by reacting Au25(PET)18 with the Au-PET complex. Note that, when mixing Au25(PET)18 with AuCl4−, only large gold NPs (>3 nm) were produced due to the stronger oxidation ability of AuCl4− compared with that of the Au-PET complex. In comparison with the parent NC Au25(PET)18 and some other NCs, the as-obtained Au24(PET)20 NCs exhibit remarkably stronger emission, as shown in Figure 4. Recently, Jin and co-workers reported an interesting molecular “surgery” via combination of an AGR and a quasi-AGR (Figure 5): “resection” of two surface Au atoms of Au23(CHT)16 by sequentially reacting with Ag-SR and Au2Cl2(P−C−P) (P−C−P: Ph2PCH2PPh2).23 The slight tailoring on the staples had little influence on optical absorption but a remarkable effect on fluorescence (∼10-fold enhancement). 3.1.2. Synthesis of Bimetallic NCs by Means of the AGR. Bimetallic NPs are currently a focus in nanoscience and nanotechnology research due to their intriguing properties and promising applications.22 In particular, atomically monodisperse bimetallic NPs (AMBNs) have been paid more attention owing to the insightful correlation between composition/structure and property and the precise tuning of their properties. Although synchro-synthesis (reducing the mixed metal−complexes) was introduced early to synthesize bimetallic NPs, limited AMBNs have been obtained because that the as-obtained products by the synchro-synthesis are sometimes uncontrolled and polydisperse. Overall, the synthesis of AMBNs is still challenging.24 Fortunately, the recent discovery of the AGR opens another door to the synthesis of AMBNs that are otherwise difficult to obtain,

Scheme 2. Schematic Illustration of the Four Alloying Modes of the AGR

(i) addition: the heteroatom is introduced without changing the parent NC’s composition and structure; (ii) replacement: the heteroatom replaces the parent metal atom, but the previous structure framework remains; (iii) replacement and structural transformation: the heteroatom replaces the parent metal atom and leads to structural transformation of the parent NC; and D


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Figure 6. (A) Mass spectra of the as-obtained Au25Ag2 and the mother Au25 NCs. (B) Structural comparison of Au25Ag2 and Au25NCs and the experimental and calculated spectra of Au25Ag2. Color labels: yellow, S; green, Au; red, Ag. All H and C atoms are omitted for clarity. Reproduced with permission from ref 21. Copyright 2015 American Chemical Society.

in the catalysis and may serve as active sites. In addition, such a Ag adduct was found to show ∼3.5-fold enhanced fluorescence in comparison with that of Au25(PET)18, while Ag replacement to form Au25−xAgx(PET)18 (x = ∼ 3) results in essentially no change in the fluorescence of Au25(PET)18. Another example of addition is the synthesis of [Au13Cd2(PPh3)6(SC2H4Ph)6(NO3)2]2Cd(NO3)4 by reacting Au25(PET)18 with Cd(PPh3)2(NO3)2.25 The resulting bimetallic NC was mainly composed of two Au13Cd2(PPh3)6(SC2H4Ph)6(NO3)2 (Au13Cd2 for short) units, which were produced by a peeling and doping process (Figure 7). Single-crystal X-ray crystallography (SCXC) revealed two Cd atoms capping two contrapositioned Au3 faces on the surface of the Au13 icosahedron. The cooperation between Cd and the neighboring gold atoms might explain the high activity compared with Au25(PET)18 and some other comparable catalysts in catalyzing the A3-coupling reaction. For the detailed mechanism, see ref 25. Recently, Jin and co-workers successfully implemented a single metal atom shuttling into and out of one Au24(PPh3)10(PET)5Cl2 NC with a central hollow space.26 They found that the foreign Ag atom can squeeze a preexisting gold atom into the hollow site to produce a novel NC Au24Ag1(PPh3)10(PET)5Cl2 without changing the composition or structure of the parent NC, Au24(PPh3)10(PET)5Cl2. Au24Cu1(PPh3)10(PET)5Cl2 can also be obtained in a similar way when the oxidant AgCl is replaced by CuCl. The difference between Cu and Ag doping is that Cu can occupy either the apex or waist positions of the rodshaped nanocluster, while Ag is only found at the apex of the nanocluster (Figure 8). Bürgi and co-workers also reported the synthesis of a Au38Cu(PET)24 adduct by the reaction between Au38(PET)24 and the Cux(PET)y complex.27 (ii) Replacement. The “replacement mode” is popular in the earlier reported bimetallic NCs, such as Ag-, Pt-, or Pd-doped Au25(PET)18; Cu-, Ag-, or Pd-doped Au38(PET)24; Ag-doped Au36(TBBT)24; and Cu- or Ag-doped Au144(PET)60, most of which are not atomically monodisperse.24 And Cd- and Hg-substituted gold NCs were not reported before 2015, in which year Wu et al. obtained atomically monodisperse Au25Hg1(PET)18 NCs by means of the AGR.28 SCXC reveals that the structure of Au24Hg1(PET)18 remains the structural framework of Au25(PET)18. Matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and thermogravimetric analysis (TGA) together with theoretical simulations indicate that the invading Hg replaces one

(iv) nonreplacement and structural transformation: the heteroatom does not replace the mother metal atom but instead leads to structural transformation of the parent NC. (i) Addition. Owing to the similarity of gold and silver in terms of physical/chemical properties, atom-monodisperse silver-doped gold NCs have not yet been obtained by synchro-synthesis to the best of our knowledge. For the first time, Wu et al. introduced the AGR to rapidly prepare (2 min.) atomically monodisperse Au25Ag2(PET)18 (Au25Ag2 for short) NCs in a yield of 89% by simply mixing Au25(PET)18 with AgNO3 in acetonitrile.21 The two introduced Ag atoms do not replace the Au atoms in Au25(PET)18 but simply deposit on Au25(PET)18, as determined by mass spectrometry, UV−vis/ NIR absorption, and theoretical calculations (Figure 6). Interestingly, the as-obtained Au25Ag2 NCs distinctly accelerated the hydrolysis of 1,3-diphenylprop-2-ynyl acetate compared with Au25(PET)18 and some other NPs or metal complexes (Table 1), which indicates that the two Ag atoms of Au25Ag2 play a key role Table 1. Catalytic Activity Comparison of Selected Catalysts for the Hydrolysis of 1,3-Diphenylprop-2-ynyl Acetateab

entry

catalyst

additive

yield (%)c

1 2 3 4 5 6 7 8 9 10 11 12 13 14

none Au25− Au25+ Au25−xAgx Au25Ag2 AuCl AgNO3 AuCl + AgNO3 Au(PPh3)Cl HAuCl4 Ag152 Au38 ∼3 nm Ag ∼3 nm Au

K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3

7 11 17 18 52 9 8 10 10 8 34 12 45 15

a

Reproduced with permission from ref 21. Copyright 2015 American Chemical Society. bReaction conditions: 0.2 mol % catalyst, 0.10 mmol 1,3-diphenylprop-2-ynyl acetate, 0.05 mmol K2CO3 (), 2.0 mL solvent (DMF/H2O = 10:1), temperature (25 °C). cIsolated yield. E


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Figure 7. Transformation from Au25 to Au26Cd5. Color labels: red, Au; black, Cd; yellow, S; green, P; purple, N; blue, O; C and H atoms are omitted for clarity. Reproduced with permission from ref 25. Copyright 2017 John Wiley and Sons.

Figure 8. Transporting one Ag or Cu atom into the hollow site of Au24(PPh3)10(PET)5Cl2 by AGR. Reproduced with permission from ref 26. Copyright 2017 Nature Publishing Group.

Figure 9. (A) Structure of Au24Hg with Hg occupying in different positions (a, outer-shell; b, inner-shell; c, core) and theoritical and experimental UV−vis/NIR absorption spectra (d). (B) UV−vis/NIR absorption of Au24Cd and Au24Hg. The inset shows the irreversible transformation between Au24Cd and Au24Hg. Adapted with permission from refs 28 and 29. Copyright 2015 American Chemical Society.

outer-shell Au rather than one inner-shell Au or the central Au in Au25(PET)18. In strong contrast, in a later work reported by the same group, the Cd occupies the core position of Au24Cd1(PET)18 synthesized by a similar AGR method.29 Au24Cd1(PET)18 can be transformed to Au24Hg1(PET)18 immediately after the addition of Hg2+, while the opposite process is not feasible under the investigated conditions (Figure 9B), indicating that the two structures have obviously different thermodynamic stabilities. Cd or Hg doping can subtly tune the optical properties of the parent NCs: Hg doping leads to a red-shift of the maximum absorption band of Au25(PET)18 in the long-wavelength range from ∼680 to ∼800 nm, while Cd doping leads to a blueshift of the same absorption band from ∼680 to ∼658 nm. Note that Zhu et al. also independently synthesized Au24Hg1(PET)18 and Au24Cd1(PET)18 by means of the AGR and identified the “replacement mode” in alloying.30

(iii) Replacement and structural transformation. This alloying mode has only been reported by Jin et al. In their work, foreign metal atoms not only replace the Au atoms in NC precursors but also induce the overall structural transformation of the parent NCs. Specifically, Au23(CHT)16 was transformed to Ag-doped Au25(CHT)18 by reacting Au23(CHT)16 with Ag(I)(CHT).23,31 The reaction involves two steps: Au23(CHT)16 is first converted to Au23−xAgx(CHT)16 (0 ≤ x ≤ 2), and then it grows to Au25−xAgx(CHT)18 (x ∼ 4), with the doped Ag occupying only in the 12-atom icosahedral inner shell. Nevertheless, the heavily doped Au25−xAgx(CHT)18 (x ∼ 19) with partial doped Ag located in the staple motif is obtained by further doping (Figure 10). (iv) Nonreplacement and structural transformation. The titled alloying mode was very recently reported by Wu et al., who obtained a novel bimetallic nanocluster, Au20Cd4(SH)(CHT)19, F


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Au25(PET)18 trimetallic NCs (Au16.8Ag7.2Hg1(PET)18) in high yield (90%) via the AGR method.32 For the first time, the trimetallic NPs were characterized by SCXC, together with theoretical calculations, which indicate that Hg and Ag replace outer-shell and inner-shell Au atom(s) of Au25(PET)18, respectively. Interestingly, counteractive effects were first found in the trimetallic NPs. For example, sole Hg doping decreases the electrochemical gap of Au25(PET)18 by 0.12 V, sole Ag doping increases the electrochemical gap of Au25(PET)18, and the increase in the electrochemical gap from Ag doping and the decrease from Hg doping are counterbalanced in the trimetallic NCs, leading to a similar electrochemical gap to that of Au25(PET)18 (1.64 vs 1.63 V, see Figure 12). Meanwhile, a synergistic effect was found to coexist with the counteractive effect: Au24−xAgxHg1 shows markedly superior catalytic activity to the 4-nitrobenzene reduction than does the Au24Hg1 or Au24−xAgx. Zhu et al. also synthesized trimetallic M1AgxAu24−x(PET)18 (x = 2−6, M = Cd/Hg) NCs by reacting Au25(PET)18 with Ag-PET and M(PET)2 successively and found that the electronic structure of the bimetallic NC (AgxAu25‑x) was largely changed by doping a third metal atom.33 Recently, Negishi and co-workers reported two trimetallic NCs: Au∼20Ag∼4Pd(PET)18 and Au∼20Ag∼4Pt(PET)18 prepared by the AGR between Au24Pd(PET)18 or Au24Pt(PET)18 and Ag-PET. The electronic structures of the trimetallic NCs become more discrete due to the increased orbital splitting after Ag doping.34

Figure 10. Third AGR alloying mode found in the transformation from Au23(CHT)16 to Au25−xAgx(CHT)18 (x ∼ 19).

by means of the AGR.20 The novel Au20Cd4(SH)(CHT)19 NC consists of a distorted central icosahedral Au11Cd2 kernel (similar to the 13-atom icosahedral kernel of Au25(PET)18), two nonequivalent trimeric staples, one dimeric staple, two monomeric staples, four plain bridging thiolates, and one CdSH unit (Figure 11). The H atom in the CdSH unit was confirmed by

3.2. Sensing

The optic properties (e.g., photoluminescence) of metal NPs have been widely utilized in sensing, including the detection of chemicals, metal ions, biomolecules, and so forth. However, the sensing mechanism is not well understood due to the undefined composition and structure of relatively large metal NPs. The emergence of NCs provides an opportunity for understanding the sensing mechanism in detail. Wu et al. first revealed an AGR mechanism in the sensing of silver ions by the fluorescent nanosensor Au25(SG)18.19 Ag+ oxidizes Au25(SG)18 and leads to an increase in the photoluminescence intensity, mainly on the basis of which the silver ion can be quantitatively detected, with a detection limit of approximately 20 ppb. Further experiments revealed that 19 other metal ions (Pb2+, Cd2+, Hg2+, Cu2+, Zn2+, Ni2+, Co3+, Tb3+, Eu3+, Pd2+, Fe2+, Fe3+, Mg2+, K+, Na+, Ca2+, Cr3+, Mn2+, and Au3+) partially quench the photoluminescence of Au25(SG)18 over the concentration range from ∼10−7 to ∼10−3 M, indicating excellent selectivity of Au25(SG)18 toward Ag+. Inspired by this, Wu et al. developed a potential colorimetric probe, Ag30(Capt)18 (CaptH: captopril), and detected an ∼6 ppb level of Hg2+ in the environmental samples (e.g., lake water).35 It is worth noting that Cu2+ interferes with the detection of Hg2+ because of the similar reaction between Ag30(Capt)18 and Cu2+. Fortunately, the interference from Cu2+ can be excluded by the introduced low-temperature masking method because the oxidation−reduction between Ag30(Capt)18 and Cu2+ is temperature dependent and proceeds slowly at low temperature. The AGR process was confirmed by XPS analyses (Figure 13). Zhu and co-workers reported [Ag62S13(S-tBu)32]4+ as a probe for detecting Cu2+ ions employing the same mechanism.36 Wu et al. also reported selective Ag+ sensing by small Au NPs protected by bovine serum albumin (BSA), and XPS analysis confirmed the reduction of Ag+ to Ag(0) by Au NPs. Here, the AGR-based metal ion sensing is summarized in Table 2.

Figure 11. Structure anatomy of Au20Cd4(SH)(CHT)19 NC. Note: carbon and hydrogen atoms are not shown for clarity. Reproduced with permission from ref 20. Copyright 2018 John Wiley and Sons.

MS in combination with other techniques. The unprecedented CdSH motif, which is included in the new structure in a nonreplacement fashion, was first found in the NCs. The Au23(CHT)16-to-Au20Cd4(SH)(CHT)19 transformation is a thorough one without resembling the mother structure; thus, the alloying mode is a nonreplacement and structural transformation mode. The introduction of Cd enhances the stability of Au20Cd4(SH)(CHT)19 and improves its catalytic selectivity for the semihydrogenation of acetylene in an ethylene-rich stream, although the acetylene conversion rate was decreased. 3.1.3. Trimetallic NCs. Since the AGR can be employed for the synthesis of bimetallic NPs, it is natural for one to speculate the possibility of synthesizing trimetallic NPs by means of the AGR. Wu et al. successfully obtained Hg and Ag doped G


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Figure 12. Bimetal doping of Au25(PET)18 by AGR and the counteraction/synergistic effect revealed in the bimetal doping. Reproduced with permission from ref 32. Copyright 2016 American Chemical Society.

Figure 13. (A) Incubation temperature-dependent absorbance responses of Ag30 at 493 nm to Hg2+ and Cu2+. (B) Absorbance response of Ag30 at 493 nm to mixed ions (temperature: 10 °C). Mixed ions include K+, Na+, Ba2+, Ca2+, Mg2+, Fe3+, Fe2+, Cr3+, Ni2+, Co2+, Cd2+, Zn2+, and Pb2+. Enlarged Ag30 XPS scan after the addition of (C) Hg2+ and (D) Cu2+. The inset shows the spectrum of Cu LMM XANES. Adapted with permission from ref 35. Copyright 2015 Royal Society of Chemistry.

3.3. Antioxidation

4. PROSPECTS FOR THE FUTURE Although great advances have been achieved since the proposal of the general concept of AGR in 2012, AGR research is still in its infancy, and much work remains to be pursued in the future. The product diversity is not well understood, and details of the reaction mechanisms are ambiguous. The AGR provides a unique and irreplaceable method of engineering metal NPs; however, only a small portion of NPs have been tuned by the AGR method, and a large range of materials, including metal NPs (e.g., Rh, Cu NPs) and semiconductor NPs, have not been investigated. A comprehensive model for AGR needs to be developed in the future, and the applicability of AGR to relatively big NPs (>3 nm) even need to be verified by powerful analysis tools such as mass spectrometry and SCXC. Except for the above-mentioned applications in engineering NPs, sensing, and antioxidation, other applications in biomedicine, energy conversion, and so forth should be explored. A new era of AGR research has just begun, and there are still many challenging issues that must be addressed. In the coming years, we expect major

Preventing the oxidation of Cu(0) is very important for its applications in optics, electronics, sensing and catalysis, especially for catalytic applications because the oxidation of Cu NPs in air usually decreases the catalytic activity; thus, developing a method to maintain the Cu(0) species is in serious need. Astruc et al. prepared Au/Cu and Ag/Cu bimetallic NPs and investigated their antioxidation ability.43 They found that the Ag/Cu bimetallic NPs are not stable, as shown in Figure 14A, the color of solution (orange) fast changed to dark green, and an absorption band (345 nm) assigned to Cu2O appeared after 1 day, while the Au/Cu bimetallic NPs are still stable after 2 months, as monitored by UV−vis/NIR spectrometry (Figure 14B). XPS analysis confirmed that Cu(0) in Ag/Cu NPs is oxidized to Cu(I), whereas Cu(0) in Au/Cu NPs is not oxidized. The Cu in Ag/ Cu NPs is much more easily oxidized than that in Au/Cu NPs because Au is more stable than silver and the AGR protects the Cu(0) from oxidation, indicating another application of the AGR for antioxidation. H


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Accounts of Chemical Research Table 2. List of Recent Metal Ion Sensing Related to the AGR Mechanisma detection method fluorescence quenching fluorescence enhancement

colorimetric changes

resonance light scattering

analyte

NC/hybrid system used

Cu

Ag62S13(S-tBu)32

Ag

Au25(SG)18

Ag Hg Cu

AuNC@BSA AgNC@[HSC10mim]BrRh6G AuNC@calixarene

Hg

Ag30(Capt)18

Ag

AuNC@BSA

Ag Cu

AuNP AgNC@ssDNA

selectivity among other ions

LOD

K+, Na+, Cd2+, Zn2+, Pb2+, Ni2+, Mn2+, Mg2+, Hg2+, Cr3+, Ba2+, Fe3+ Pb2+, Cd2+, Hg2+, Cu2+, Zn2+, Ni2+, Co3+, Tb3+, Eu3+, Pd2+, Fe2+, Fe3+, Mg2+, K+, Na+, Ca2+, Cr3+, Mn2+, Au3+ Al3+, Ca2+, Cd2+, K+, Mg2+, Mn2+ K+, Na+, Pb2+, Mg2+, Ba2+, Zn2+, Co2+, Cu2+, Fe3+, Ca2+, Cd2+, Ni2+

not mentioned

MeCN/H2O 1/1

36

200 nM

aqueous solution

19

not mentioned 1 × 10−13 M

aqueous solution aqueous solution

37 38

Li+, Na+, K+, Ba2+, Mg2+, Fe3+, Cs+, Hg2+, Ca2+, Zn2+, Cd2+, Ni2+, Sr2+ Na+, K+, Ca2+, Mg2+, Ba2+, Cr3+, Fe3+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Pb2+, Cu2+ Co2+, Ni2+, Na+, Fe3+, Mn2+, Pb2+, Cd2+, Zn2+, Ca2+, Mg2+ Cu2+, Cd2+, Ce2+, Hg2+, Na+, K+ Na+, K+, Ag+, Ca2+, Mg2+, Zn2+, Ni2+, Mn2+, Co2+, Hg2+, Fe3+, Al3+

0.65 ppm

toluene/THF 1/4

39

6 ppb

aqueous solution, lake water, soil solution aqueous solution, lake water film aqueous solution

35

0.204 mM 100 nM 2 nM

media

ref

40 41 42

a Abbreviations: LOD, limit of detection; S-tBu, tert-butyl thiolate; BSA, bovine serum albumin; AgNC@[HS-C10mim]Br-Rh6G, the composite of 1-(10-mercaptodecyl)-3-methylimidazolium bromide protected AgNC and rhodamine 6G; Capt, captopril; ssDNA, C-rich ssDNA, 5′-(CCCTAA)3CCCTA-3′, 23 bp.

Figure 14. UV−vis/NIR spectra and photographs of bimetallic NPs: (a) Ag/Cu bimetallic NPs after staying for 5 min; (b) Ag/Cu bimetallic NPs after staying for 1 day in air; (c) Au/Cu bimetallic NPs after staying for 5 min; and (d) Au/Cu bimetallic NPs after staying for 2 months in air. Reproduced with permission from ref 43. Copyright 2017 Royal Society of Chemistry.

advances and exciting research on the understanding and application of the AGR.

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Zhikun Wu received his Ph.D. in Chemistry from Institute of Chemistry, Chinese Academy of Sciences in 2004. He is a full professor of chemistry/material with research interests in NCs.

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AUTHOR INFORMATION

Corresponding Author

ACKNOWLEDGMENTS We would like to thank National Natural Science Foundation of China (Nos. 21222301, 51502299, 21701179, 21771186, 21829501, 21171170, and 21528303), the Natural Science Foundation of Anhui Province (1708085QB36), Key Program of 13th five-year plan, CASHIPS (KP-2017-16), Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2017FXCX002), and the CAS/SAFEA International Partnership Program for Creative Research Teams for financial support.

*E-mail: [email protected]. ORCID

Zhikun Wu: 0000-0002-2711-3860 Notes

The authors declare no competing financial interest. Biographies Zibao Gan received his Ph.D. degree in chemistry from USTC in 2013. He is now an associated professor in the group of Prof. Zhikun Wu.

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Nan Xia received his Ph.D. degree in chemistry from Nankai University in 2011. He is now an associated professor in the group of Prof. Zhikun Wu.

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