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A Grand Avenue to Au Nanocluster Electrochemiluminescence Mahdi Hesari and Zhifeng Ding* Department of Chemistry, The University of Western Ontario, London, ON N6A 5B7, Canada CONSPECTUS: In most cases of semiconductor quantum dot nanocrystals, the inherent optical and electrochemical properties of these interesting nanomaterials do not translate into expected efficient electrochemiluminescence or electrogenerated chemiluminescence (ECL) because of the surface-state induction effect. Thus, their low ECL efficiencies, while very interesting to explore, limit their applications. As their electrochemistry is not welldefined, insight into their ECL mechanistic details is also limited. Alternatively, gold nanoclusters possess monodispersed sizes with atomic precision, low and well defined HOMO−LUMO energy gaps, and stable optical and electrochemical properties that make them suitable for potential ECL applications. In this Account, we demonstrate strong and sustainable ECL of gold nanoclusters Au25z (i.e., Au25(SR)18z, z = 1−, 0, 1+), Au38(SR)24, and Au144(SR)60, where the ligand SR is 2-phenylethanethiol. By correlation of the optical and electrochemical features of Au25 nanoclusters, a Latimer-type diagram can be constructed to reveal thermodynamic relationships of five oxidation states (Au252+, Au25+, Au250, Au25−, and Au252−) and three excited states (Au25−*, Au250*, and Au25+*). We describe ECL mechanisms and reaction kinetics by means of conventional ECL−voltage curves and novel spooling ECL spectroscopy. Notably, their ECL in the presence of tri-n-propylamine (TPrA), as a coreactant, is attributed to emissions from Au25−* (950 nm, strong), Au250* (890 nm, very strong), and Au25+* (890 nm, very strong), as confirmed by the photoluminescence (PL) spectra of the three Au25 clusters electrogenerated in situ. The ECL emissions are controllable by adjustment of the concentrations of TPrA· and Au25−, Au250, and Au25+ species in the vicinity of the working electrode and ultimately the applied potential. It was determined that the Au25−/TPrA coreactant system should have an ECL efficiency of >50% relative to the Ru(bpy)32+/TPrA, while those of Au250/TPrA and Au25+/TPrA reach 103% and 116%, respectively. Au25−* is the main light emission source for Au25z in the presence of benzoyl peroxide (BPO) as a coreactant, with a relative efficiency of up to 30%. For Au38, BPO leads to the Au38−* excited state, which emits light at 930 nm. In the Au38/TPrA coreactant system, we find that highly efficient light emission at 930 nm is mainly from Au38+* (and also Au383+*), with an efficiency 3.5 times that of the Ru(bpy)32+/TPrA reference. We show that the ECL and PL of the various Au38 charge states, namely, Au382−, Au38−, Au380, Au38+, Au382+, and Au384+, have the same peak wavelength of 930 nm. Finally, we demonstrate ECL with a peak wavelength of 930 nm from the Au144/TPrA coreactant system, which is released from the electrogenerated excited states Au144+* and Au1443+*. In our opinion, these gold nanoclusters represent a new class of effective near-IR ECL emitters, from which applications such as bioimaging, biological testing, and medical diagnosis are anticipated once they are made water-dispersible with hydrophilic capping ligands.

1. INTRODUCTION

coreactant is employed to react with one of the above luminophore radical species to produce the radiative excited state.6 Noble-metal nanoclusters possess various inherent properties that have found applications in catalysis,7,8 electrocatalysis,9 and biology.10 Among these metallic nanoclusters, gold ones are the most interesting because of their catalytic,7 optical,11,12 and electrochemical13−17 properties. Thus, gold nanoclusters with the general formula Aun(SR)m have been shown to be suitable candidates as nanoemitters.15,18 For instance, rich luminescence features of Au25(SR)18z (z = 1−, 0, 1+), Au38(SR)24, and Au144(SR)60 are distinct along with their electrochemistry.19−28

Electrochemiluminescence or electrogenerated chemiluminescence (ECL) has a history of more than 50 years and is known as a radiative process based on interactions between electrochemically produced, highly reactive radical species.1−6 The two main pathways to generate reactive species in the vicinity of a working electrode are ion annihilation and “coreactant”. In the ion annihilation route, the applied potential is pulsed or scanned between the first oxidation and reduction potentials of a luminophore. The resulting radical cations and anions participate in an electron transfer (ET) process to generate an excited state, which relaxes to the ground state and emits light. If the lifetime of the radical species is too short or they are not chemically stable, a reducing or oxidizing intermediate electrogenerated from a © 2017 American Chemical Society

Received: August 31, 2016 Published: January 12, 2017 218

DOI: 10.1021/acs.accounts.6b00441 Acc. Chem. Res. 2017, 50, 218−230

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Accounts of Chemical Research

which agree very well with those reported by Jin’s group.28 Notably, one can identify Au25−* on the basis of its longer wavelength, while Au25+* can be distinguished from Au25−* and Au250* by its higher emission intensity due to higher probability for electron relaxation. Further elucidation of the excited states was carried out using our improved in situ spectrophotoelectrochemistry technique (Figure 2).34 Figure 3 shows the in situ PL spectra of Au252+,

The combination of their promising luminescence and electrochemical characteristics is anticipated to lead to their analytical applications such as determinations of dopamine and Pb2+.29,30 The goal of this Account is to provide a snapshot of our recent advanced studies of the ECL of the gold nanoclusters Au25(SR)18z, Au38(SR)24, and Au144(SR)60, denoted as Au25−, Au250, Au25+, Au38, and Au144, and to gain insight into their emission mechanisms, enhancement, and control.20,31−35

2. THERMODYNAMICS REVEALED BY ELECTROCHEMISTRY AND PHOTOLUMINESCENCE Figure 1A shows differential pulse voltammograms (DPVs) of a 0.1 mM Au250 solution.34 Two peaks at formal potentials (E°′) of

Figure 2. In situ photoluminescence spectroscopy setup. The picture at the right shows a thin-layer spectroelectrochemical cell (SEC). Adapted with permission from ref 34. Copyright 2014 Wiley.

Figure 1. (A) Differential pulse voltammograms of 0.1 mM Au250 in 1:1 acetonitrile/benzene containing 0.1 M TBAP. (B) Photoluminescence spectra of Au25+, Au250, and Au25− in the same solvents. Adapted with permission from ref 34. Copyright 2014 Wiley.

−0.06 and −1.68 V were observed, corresponding to reduction reactions of Au250 to Au25− and Au252−.34 In the anodic region, Au250 underwent oxidation reactions to give Au25+ and Au252+ at 0.19 and 0.84 V, respectively. These results agree well with those of Murray for Au25−.21 The redox potentials in Figure 1A link the five oxidation states and provide the Latimer diagram shown at the bottom of Scheme 1. Figure 1B presents photoluminescence (PL) spectra of the three individually synthesized Au25−, Au250, and Au25+ nanoclusters excited by a 532 nm laser.34 The dominant peak wavelengths assigned for relaxation of the excited states across the HOMO−LUMO gaps are 860 nm (1.44 eV), 865 nm (1.43 eV), and 945 nm (1.31 eV) for Au25+, Au250, and Au25−, respectively,

Figure 3. In situ PL spectra of Au25z (z = 2+, 1+, 0, 1−, 2−) during electrolysis of a 0.1 mM Au250 solution to the destination oxidation states. The applied potentials were determined from the DPVs in Figure 1A. A 532 nm laser was employed along with a razor-edge filter to cut the laser light from PL. Adapted with permission from ref 34. Copyright 2014 Wiley. 219


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Accounts of Chemical Research Scheme 1. Latimer-Type Diagram Showing the Relationships among Five Au25 Oxidation States and Three Excited Statesa

a

The paramagnetic configurations such as in Au25+ have been ignored. Adapted with permission from ref 34. Copyright 2014 Wiley.

Figure 4. (A) ECL−voltage curve and CV of 0.1 mM Au25− with 12.5 mM TPrA at a scan rate of 100 mV/s. The inset shows a schematic of the apparatus. (B) Spooling ECL spectra in the potential scanning in (A). The inset shows the stacked spooling spectra. Adapted with permission from ref 33. Copyright 2014 Royal Society of Chemistry.

Au25+, Au250, Au25−, and Au252−. Except for Au250, all of the oxidation states were obtained by electrolysis of a 0.1 mM Au250 electrolyte solution at the corresponding potential values in Figure 1A applied to the Pt mesh in a 1 mm thin-layer quartz cuvette (Figure 2). The electrogenerated Au25+ and Au25− (Figure 3) show PL spectra very similar to those of the two pure species in Figure 1B: the transition from Au250 to Au25+ revealed an increase in PL intensity at a similar peak wavelength, while the conversion from Au250 to Au25− displayed a red shift in the peak wavelength and lower intensity. The above PL data connect the three excited states vertically with their ground states, as shown in Scheme 1, where a comprehensive Latimer-type diagram was completed by thermodynamic calculations.34

3. ELECTROCHEMILUMINESCENCE OF Au25−, Au250, AND Au25+ NANOCLUSTERS 3.1. Electrochemiluminescence of Au25− Nanoclusters

Au25− undergoes electrochemical reactions21,33 as its counterpart, the neutral Au250, as illustrated in Figure 1A. Very weak ECL from the Au25− solution was detected33 upon alternating oxidations and reductions between the potentials of the Au250/Au25− and Au25−/ Au252− or Au25+/Au250 couples and between the Au25−/Au252− or Au252+/Au25+ and Au25−/Au252− couples defined by Figure 1A. This could be due to the short lifetime and/or low reactivity of either Au252− or Au252+ species, leading to low production of Au25−*. Figure 4A shows a cyclic voltammogram (CV) of 0.1 mM Au25− with 12.5 mM tri-n-propylamine (TPrA), which is 220


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Au25− and its reaction with TPrA· (Scheme 2B). This did not occur since a potential much lower than 0.976 V is required. (2) At the applied potential of 1.276 V (green solid circle on the CV in Figure 4A), Au25− was oxidized to Au252+, which was then reduced to Au25+ by TPrA· in excess, forming Au250* (Scheme 3).

dominated by the oxidation of the higher concentration TPrA at 0.81 V. TPrA· generated via its oxidation and fast deprotonation36 has a high reducing power of −1.7 eV.37 The ECL−voltage curve measured simultaneously with a photomultiplier tube (PMT) (Figure 4A inset) increases significantly when TPrA· reacts with the electrogenerated Au25z species and emissive Au25z* states form consequently. Spooling ECL spectroscopy was developed in our lab. In this method, ECL spectra are taken at certain time interval during ECL processes driven by potential scanning (Figure 5).33,35

Scheme 3. Proposed ECL Mechanism for Au25−/TPrA at Potentials between 0.976 and 1.174 V

(3) Upon reversal of the potential scanning direction to cathodic, there would be just enough electrogenerated TPrA· to react with Au252+ at a potential of 1.174 V (red solid circle on the CV in Figure 4A). The ET between these two species results in electron injection to the LUMO of Au252+, forming Au25+* (Scheme 4; see Scheme 1 for the electron configurations of the species).33

Figure 5. Spooling ECL spectroscopy setup.

Scheme 4. Proposed ECL Mechanism for Au25−/TPrA in the Reverse Potential Scan below 1.174 V

Figure 4B illustrates the spooling ECL spectra in the potential scanning as in Figure 4A. As highlighted in colors, there are three potentials at which the ECL peak wavelength changes from 950 nm at the ECL onset potential of 0.796 V to 900 nm with low intensity at a potential of 1.276 V to 900 nm with high intensity at a backward scan potential of 1.174 V. The ECL spectra are stacked in the inset of Figure 4B, where the clear peak shift can be observed. These three emissions match the PL emissions from Au25−*, Au250*, and Au25+* in Figure 1B. The slight red shift of an ECL spectrum relative to the PL spectrum is related to selfabsorption and inner-filter effects in ECL processes. It was discovered that the ECL emissions depended on the concentrations of TPrA· and Au25−, Au250, and Au25+ species in the vicinity of the working electrode and ultimately on the applied potential. (1) At the applied potential of 0.976 V (orange solid circle on the CV in Figure 4A), Au252+ was generated from Au25− via sequential oxidation reactions in the vicinity of the electrode (Figure 1A). At the same potential, the local TPrA· concentration was 125 times higher than that of Au252+. The Au252+ underwent several successive reduction reactions with TPrA· to Au252−. The electro- and chemically generated Au252+ and Au252− can react, generating Au25−*, which returns to the ground state and emits light at 950 nm (Scheme 2A). The Au25−* could be produced via generation of Au250 from the oxidation of

When [TPrA] was further increased to 200 mM, for instance, spooling ECL spectra of 0.1 mM Au25− showed a unique peak wavelength at 983 nm.33 The very high [TPrA·] profile with high reducing power kept the Au25 oxidation state as Au252− in the diffusion layer. Au252+ generated in the vicinity of the electrode met and reacted with the Au252− immediately, favoring the of generation Au25−* (Scheme 2A). This leads to ECL evolution

Scheme 2. Probable (A) and Unlikely (B) Mechanisms for Au25−/TPrA ECL at Potentials up to 0.976 V

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Accounts of Chemical Research Scheme 5. ECL Mechanisms for the (A) Au25−/BPO and (B) Au25+/BPO Coreactant Systems

Figure 6. (A) ECL−voltage curve and CV of 0.1 mM Au25− with 5 mM BPO at a scan rate of 100 mV/s. (B) Corresponding spooling ECL spectra. The inset displays stacked spooling spectra. Adapted with permission from ref 33. Copyright 2014 Royal Society of Chemistry.

Figure 6A shows the ECL−voltage curve from the PMT and the corresponding CV of 0.1 mM Au25− with 5 mM benzoyl peroxide (BPO), which shows a dominant reduction wave due to the higher BPO concentration. The reduction of Au25− to Au252− (Figure 1A) and the reduction of BPO to produce oxidizing C6H5CO2· (via the pathway of the BPO radical anion that decomposes to the radical) occurred at very similar potentials, hopefully leading to more efficient generation of Au25−*. The ECL onset potential was found to be −0.658 V in Figure 6A, at which the peak wavelength was determined to be 950 nm. The ECL is attributed to the emission of Au25−* (Figure 6B), which remained the same during evolution and devolution driven by the potentiodynamic scan. [BPO] was varied from 2.5 to 25 and 50 mM. At 2.5 mM, BPO did not generate sufficient C6H5CO2· to observe ECL. In the presence of 25 and 50 mM, the ECL intensity was low because of the quenching effect of the coreactant.38 Thus, the best relative ECL efficiency with 5 mM BPO was estimated to be >30%.

and devolution at the same peak wavelength. All of the other ECL spectra in Figure 4 can be considered as emission transitions among the above three cases. It is remarkable that the excited state and therefore the ECL emission in the Au25−/TPrA system can be controlled via [TPrA] and the applied potential. The relative ECL efficiency was determined as the number of photons emitted per redox event relative to the Ru(bpy)32+/ TPrA system. Because of the depleted PMT sensitivity in the near-IR (NIR) region, the strong ECL might be underestimated. It is plausible that the ECL quantum yields measured by our Andor camera and Acton spectrograph set are more reliable.33 The Au25−/TPrA system should have a relative ECL efficiency of >50%. While the Au25−/TPrA system did produce strong ECL intensity, the Au250* and Au25+* generation mechanisms showed a zigzagged energy pathway: oxidation of TPrA to produce TPrA·, sequential oxidation of Au25− to Au252+ for generating active radicals, and then successive reduction of Au252+ to Au25+ by TPrA· in the vicinity of the electrode and reduction of Au25− to Au252− in the bulk for the final generation of the three excited states. Because of the presence of a reduction peak at −1.5 V for Au25−/Au252−, it is natural to take into consideration scanning of the applied potential in the cathodic region with an electrogenerated oxidative intermediate such as benzoate radical (C6H5CO2·) that can react with Au252− to form Au25−* (Scheme 5A).

3.2. Electrochemiluminescence of Au250

Similar to the annihilation ECL process in the Au25− solution,33 no appreciable ECL via annihilation from the 0.1 mM Au250 solution was detected.34 Thus, TPrA was added to the solution for enhanced ECL generation. The TPrA concentration was varied from 6.3 mM to 12.5, 25, 50, 100, and 200 mM in order to 222


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Figure 7. Spooling ECL spectra of 0.1 mM Au250 in the presence of (A) 12.5 mM, (B) 50 mM, and (C) 200 mM TPrA in 1:1 acetonitrile/benzene containing 0.1 M TBAP. The applied potentials are labeled for each spectrum. The insets show the ECL evolution (left) and devolution (right) in a better visualization. Adapted with permission from ref 34. Copyright 2014 Wiley.

Scheme 6. Mechanisms for Au250/TPrA To Generate (A) Au250* and (B) Au25+*

intense than that of Au250* at the same wavelength of 880 nm. The ECL emission decreased as the applied potential was increased further as a result of depletion of the TPrA· concentration. It was found that ECL is highly dependent on the initial [TPrA] and ultimately on [TPrA·]. The ECL intensity increases with increasing [TPrA] in the range of 6.3 to 25.0 mM, following the evolution and devolution pattern shown in Figure 7A. With [TPrA] of up to 50 mM, the ECL intensity increased continually until the reverse potential scanning to 0.954 V (Figure 7B). In the potential range between 0.737 V (ECL onset potential)

gain insight into the ECL mechanisms and related reaction kinetics as well as tuning of the excited states. Figure 7A displays the spooling ECL spectra acquired for 0.1 mM Au250 solution with 12.5 mM TPrA during potential scanning between 0.025 and 1.235 V. The ECL onset potential is at 0.725 V, at which the electrogenerated Au25+ and TPrA· interact to form Au250* (Scheme 6A), which emits light at a peak wavelength of 880 nm (left inset of Figure 7A). Upon further anodic scanning in the range of 0.825 to 1.125 V, the existing Au25+ undergoes further oxidation to Au252+, which reacts with TPrA· to generate Au25+* (Scheme 6B). The ECL became more 223


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Figure 8. Spooling ECL spectroscopy of 0.1 mM Au25+ in the presence of 50 mM TPrA in 1:1 acetonitrile/benzene at a scan rate of 100 mV/s between −0.557 and 1.142 V vs SCE. The insets present stacked ECL spectra in the course of the evolution and devolution processes. Adapted with permission from ref 32. Copyright 2014 Royal Society of Chemistry.

Figure 9. (A) Spooled ECL spectra of Au25+ nanoclusters with 5 mM BPO during one cycle of the applied potential from 0.04 to −1.86 V and then back to 0.04 V at a scan rate of 100 mV/s. (B) Accumulated ECL spectrum of the same solution. Adapted from ref 35. Copyright 2012 American Chemical Society.

and 0.837 V (before the oxidation of Au25+ to Au252+), excess [TPrA·] reacted with the bulk Au250 to generate Au25−*, giving off an ECL onset peak at a wavelength of 960 nm (similar to Scheme 2A). At more positive applied potentials, Au252+ dominated in the vicinity of the electrode, and Au25+* was generated (Scheme 6B). Upon the reverse potential scanning from 0.954 to 0.554 V, the ECL has a contribution from Au25−* again, as identified by the red-shifted peak wavelength. ECL wavelengths are facile to tune and control on the basis of the above observations. Further increases in [TPrA] to 100 and 200 mM (Figure 7C) caused the ECL evolution and devolution to display a unique peak wavelength at 960 nm, correlating to Au25−* emission. Remarkably, the ECL efficiency of the Au250/TPrA system reached up to 103% relative to that of the well-known Ru(bpy)32+/TPrA system at 25 mM TPrA.

(Figure 1A). The ECL process must follow the path of the electrogenerated TPrA· reacting with the bulk Au25+ to yield the emissive Au250*. At potentials greater than 0.847 V, Au252+ was generated in the vicinity of the working electrode. Au25+* was produced via the interaction of Au252+ with TPrA·, giving off light at the same peak wavelength with dramatically increased intensity (Figure 1A and Scheme 4). The ECL intensity was augmented as the potential was scanned from 0.643 to 1.043 V and dropped gradually upon potential scanning to a little more positive potential (1.141 V) and back to 0.541 V (insets in Figure 8). The Au25+ solutions with TPrA concentrations of 6.3, 12.5, and 25 mM displayed a gradual increase in ECL intensity, with the same peak wavelength of 875 nm in the ECL evolution and devolution processes as that with 50 mM TPrA, featuring emissions from Au250* and Au25+*.32 Further increases in [TPrA] promoted a very high local [TPrA·], which must act as an electron carrier to drive reduction reactions of Au25+ (in the bulk at the ECL onset potential) and Au252+ (generated at the electrode biased with more positive potentials) to Au250 and all the way to Au252−. Au25−* was formed either via electron transfer from TPrA· to the Au250 LUMO or from the Au252− HOMO to other Au25 HOMOs (Scheme 1), leading to a peak wavelength of 945 nm (e.g., Scheme 2A).32 Au25+ nanoclusters are the most suitable candidate to be used in the presence of TPrA because of the potential matching. The ECL quantum yield of Au25+ in the presence of 25 mM TPrA reached a relative efficiency as high as 116%.32

3.3. Electrochemiluminescence of Au25+

The ECL was discovered to be very weak in the annihilation path for a 0.67 mg/mL Au25+ cluster electrolyte solution, where the electrogenerated Au252+ and Au252− reacted and the Au25−* excited state was produced.35 It was found that Au252− is more stable than Au252+ since ECL can be observed only in the anodic potential region. Figure 8 displays the spooling ECL spectra acquired at a time interval of 1 s for a 0.1 mM Au25+ electrolyte solution with 50 mM TPrA during a cycle of potential scanning between −0.557 and 1.142 V at a scan rate of 100 mV/s.32 The ECL onset potential and peak wavelength are 0.654 V and 872 nm, respectively, whereas the formal potential for Au252+/Au25+ is 0.817 V 224


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Figure 10. (A) Cyclic voltammogram of 0.1 mM Au380 in 1:1 acetonitrile/benzene containing 0.1 M TBAP at a scan rate of 100 mV/s. The insets represent electronic configurations of various redox species (the degeneracies of orbitals are neglected). (B) UV−vis−NIR absorption spectrum of 0.05 mM Au38 in the acetonitrile/benzene solution vs photon energy. The insets show peak energies and the most probable corresponding electronic excitations. Adapted from ref 20. Copyright 2014 American Chemical Society.

Figure 11. (A) ECL−voltage curve of 0.1 mM Au38 in the presence of 6.3 mM TPrA in 1:1 acetonitrile/benzene containing 0.1 M TPAP. The insets show schematic electronic reaction diagrams of Au382+ and TPrA· (left (blue)) and of Au384+ and TPrA· (right (brown)) to generate the excited species Au38+* and Au383+*, respectively. (B) Spooling ECL spectra of the same solution as in (A). The insets show stacked spectra demonstrating the ECL evolution and devolution during cyclic potential scanning between −0.51 and 1.30 V. Adapted from ref 20. Copyright 2014 American Chemical Society.

Au25−*, Scheme 5B. The ECL light generation kept the same emission wavelength during the evolution and devolution, and the intensity depended on the local C6H5CO2· concentration and ultimately on the potentiodynamic scanning (Figure 9).

In fact, the Au25+ solution containing with 5 mM BPO was first employed to enhance the ECL production.35 The Au25+ cluster was electrochemically reduced all the way to Au252−, which then reacted with C6H5CO2·. The radical removed an electron from the Au252− species, forming the emissive 225


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Accounts of Chemical Research Scheme 7. Mechanisms for Au380/TPrAa

a

Furthermore, ECL in the range of 0.900 and 1.200 V under the brown current segment in Figure 11A is attributed to the excited states Au383+* and Au382+* (most probably to Au383+* because of the indistinguishable two-electron reaction) and TPrA· (Scheme 7B,C). The ECL peak wavelength remained the same in ECL evolution and devolution that followed those of TPrA·, as illustrated by the color-coded current segments in Figure 11A. In situ spectrophotoelectrochemistry (Figure 12) revealed the same PL peak wavelength of 930 nm for various Au38z charge states (z = 0 (A), 1+ (B), 2+ (C), 3+/4+ (D), 1− (E), and 2− (F)). Single or spooling PL spectra were acquired from the in situ formation of the desired charge species Au383+/4+, Au382+, Au38+, Au38−, and Au382− at 1.0, 0.65, 0.45, −0.85, and −1.10 V vs SCE, respectively (Figure 12B−F). It is plausible that ECL might have the same transitions as those of the PL process and displays the same peak wavelength of 930 nm. With the increased [TPrA] in the range of 6.3 to 50 mM, the amount of TPrA· was augmented in the vicinity of the electrode at the same potential, and thus, an increase in ECL intensity was observed. With 50 mM TPrA, the ECL efficiency can reach 3.5 times that of the Ru(bpy)32+/TPrA system. The ECL intensity dropped when the TPrA concentration was higher than 50 mM, indicating possible quenching of the ECL by the TPrA at very high concentrations, an effect observed in other ECL systems using this coreactant.37 On the basis of the two consecutive electrochemical reduction reactions of Au38 in the cathodic region, it was also of interest to investigate ECL emission in the presence of BPO. Figure 13A presents the CV and ECL−voltage curve of 0.1 mM Au38 clusters in the presence of 5 mM BPO in the acetonitrile/benzene mixture. By comparing the onset potential (at −1.00 V) and the reduction potentials of Au380/Au38− (−0.762 V) and Au38−/ Au382− (−1.01 V), one can conclude that the observed ECL is due to the reaction between electrogenerated Au382− and C6H5CO2·, resulting in the formation of Au38−*. The accumulated ECL spectrum shows a peak wavelength of 922 nm, which matches that of the Au38− PL spectrum. In fact, C6H5CO2· accepts an electron from one of the two Au382− HOMOs (Figure 10A inset), producing the Au38−* excited state (Figure 13C). The ECL evolution and devolution in Figure 13A depend on the concentrations of both electrogenerated C6H5CO2· and Au382−. The ECL efficiency of Au38 with 5 mM BPO was determined to be 7.2%.

Adapted from ref 20. Copyright 2014 American Chemical Society.

The relative ECL efficiency of the Au25+/BPO system was determined to be 2.6%, which is lower than that of the Au25−/ BPO system because the reduction of Au25+ to Au25− is not energetically favorable.35

4. ELECTROCHEMILUMINESCENCE OF Au38 CLUSTERS Au38 undergoes five successive redox reactions with formal potentials of −0.762, −1.010, 0.390, 0.598, and 0.994 V vs SCE, assigned as Red1 (Au380/Au38−), Red2 (Au38−/Au382−), Ox1 (Au380/Au38+), Ox2 (Au38+/Au382+), and Ox3 (Au382+/Au383+/4+), respectively, as reported by us (Figure 10A) and Quinn, Liljeroth, and co-workers.16,20 The HOMO−LUMO gap was determined to be 0.952 eV, which was calculated from the potential difference between the first oxidation and first reduction waves (EOx ° 1 − ERed ° 1= 1.158 eV) with an associated charge correction (Ox1 and Ox2 potential differences, ca. 0.206 eV). This value is close to that determined from the optical HOMO−LUMO gap (0.9 eV), as shown in Figure 10B. The ECL−voltage curve displays no significant light production in the course of scanning between the highest and lowest potential values, corresponding to the generation of Au383/4+ and Au382−, respectively (Figure 10A). This is most likely due to the short lifetimes of the electrogenerated species in the course of potential scanning or their low reactivity. Figure 11 displays the ECL−voltage curve of the above Au38 electrolyte solution with 6.3 mM TPrA at a scan rate of 100 mV/s, along with the corresponding spooling ECL spectra. The ECL onset was at 0.796 V vs SCE, at which the Au38 was already oxidized to Au382+ (E°′ = 0.598 V). At this potential, TPrA began to undergo an oxidation reaction36 to form TPrA·.37 The highly reducing TPrA· injected an electron into the Au382+ LUMO, producing the excited state Au38+* (left (blue) inset in Figure 11A), which emitted light upon relaxing to the ground state (Scheme 7A). The onset ECL spectrum showed a peak wavelength of 930 nm.

5. ELECTROCHEMILUMINESCENCE OF Au144 CLUSTERS Au144 clusters (originally called monolayer-protected clusters (MPCs)14) stand at the boundary of nanoclusters and quantum dotes (QDs).14 The DPVs of a Au144 cluster electrolyte solution show multistep electron transfers including 15 successive redox couples (Figure 14A).27,31 The open-circuit potential was measured as 0.004 V vs SCE, representing no charge on the Au144 clusters. Thus, the redox couples can be assigned from 0/1+ to 5+/6+ and also 0/1− to 7−/8− in the oxidative and reductive directions, respectively (Figure 14A). The loss of reversibility beyond the 6−/7− states is known to be due to reductive desorption of the thiolate ligands at these more negative potentials.31 In an annihilation experiment, the applied potential was scanned between 1.25 and −1.60 V vs SCE, at which Au1448− and Au1446+ formed but no appreciable ECL was detected. The emission of the expected excited 226


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Figure 12. PL spectra of Au38z (z = 2−, 1−, 0, 1+, 2+, 3+/4+) in 1:1 acetonitrile/benzene containing 0.1 M TBAP using a thin-layer spectrophotoelectrochemical cell. The applied potential was scanned over the range as indicated in each panel, corresponding to (A) Au380, (B) Au38+, (C) Au382+, (D) Au383+/4+, (E) Au38−, and (F) Au382−. The insets show the typical spooling PL spectra in the course of continuous potential scanning at a scan rate of 100 mV/s over the range indicated by ΔE. Each spectrum was acquired for 2 s, and the time interval was 50 s. The arrows indicate the electrolysis time and spooling directions. All samples were excited with a 532 nm laser. Adapted from ref 20. Copyright 2014 American Chemical Society.

state (due to electron transfer from the donor, Au144−n, to the acceptor, Au144+n) was not observed. Figure 14B shows the CV of the above Au144 solution along with the ECL−voltage curve in the presence of 50 mM TPrA. The onset of the ECL−voltage curve was found at ca. 0.8 V vs SCE, at which Au144 was oxidized to Au1445+ (see Figure 14A) and the reducing agent TPrA· was generated (i.e., TPrA oxidation had begun). The reaction would produce Au1444+*, which was likely responsible for the observed ECL at around 930 nm in the accumulation ECL spectrum in Figure 15A. When the potential was scanned toward more positive values, Au1446+ was generated and reacted with TPrA· to produce Au1445+*. The ECL peak wavelength was again found to be 930 nm in the accumulation ECL spectrum in Figure 15B. The PL spectra of various Au144 oxidation states electrogenerated in situ in the spectrophotoelectrochemistry

displayed a constant peak wavelength of 915 nm.31 The proposed mechanisms for the two ECL emissions are shown in Figure 15C,D. The Au144 clusters showed a very low ECL efficiency. This could be due to the two following reasons: (i) a very small HOMO−LUMO gap that makes the corresponding ECL emission very weak and (ii) less accessible reaction sites on the surface of the Au144 to react with TPrA· and generate the excited states.

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CONCLUSION AND PERSPECTIVE ECL is an adaptable and versatile technique that creates many possibilities for both fundamental and application perspectives.2,4 Despite the existence of many visible ECL emitters, there are few NIR ones. The very strong NIR ECL of the Au nanocluster/TPrA coreactant systems with efficiencies of up to 227


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Figure 13. (A) ECl−voltage curve of 0.1 mM Au380 in the presence of 5 mM BPO in 1:1 acetonitrile/benzene containing 0.1 M TBAP at a scan rate of 100 mV/s. (B) ECL accumulated spectrum (red curve) of the solution in (A). (C) Illustration of the proposed ECL mechanism for the Au38/BPO system. Adapted from ref 20. Copyright 2014 American Chemical Society.

Figure 14. (A) Differential pulse voltammograms of 0.05 mM Au144 in an acetonitrile/benzene electrolyte solution. (B) ECL−voltage curve of 0.05 mM Au144 in the presence of 50 mM TPrA at a scan rate of 100 mV/s. Adapted from ref 31. Copyright 2014 American Chemical Society.

Figure 15. ECL spectrum of the Au144/TPrA coreactant system recorded during potential scanning between (A) −0.089 and +0.9 and (B) −0.089 and +1.20 V vs SCE. (C, D) Proposed ECL mechanisms. Adapted from ref 31. Copyright 2014 American Chemical Society.

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Accounts of Chemical Research 3.5 times that of the Ru(bpy)32+/TPrA commercial standard can be controlled by adjusting the applied potential and TPrA concentration. Very importantly, by means of the comprehensive Latimer-type diagram and spooling ECL spectroscopy combining spectroscopic and electrochemical features during the ECL processes, insight into kinetic and thermodynamic details is facile to obtain. Interestingly, among the Au25 nanoclusters with different charge states, the ECL intensities follow the order Au25+/TPrA > Au250/TPrA > Au25−/TPrA. For the neutral forms of the nanoclusters with various core sizes, the obtained ECL efficacies of Au38, Au25, and Au144 in the TPrA coreactant systems demonstrate a descending tendency. These can be deeply understood by correlating their structures, reactive sites, and electron configurations. Unlike many semiconductor nanocrystal systems showing weak ECL,3,39 atomically precise Au nanoclusters represent a novel class of efficient ECL emitters in acetonitrile/benzene, as revealed by our investigations described above. Further studies of these and other noble-metal nanoclsuters should be devoted to the development of ECL applications in water solutions,29,30,40 leading to the grand avenue toward biological testing and medical diagnosis.2,4

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhifeng Ding: 0000-0001-9252-9320 Notes

The authors declare no competing financial interest. Biographies Mahdi Hesari studied chemistry at Bu-Ali Sina University for his B.Sc. and M.Sc. He received his Ph.D. at the University of Western Ontario. He is currently a postdoc in single-molecule photoelectrocatalysis at Cornell University. Zhifeng Ding obtained his B.Sc. at Southeast University, his M.Sc. at Nanjing University, and his Ph.D. at EPFL. After his postdoctoral research at UT Austin, he joined the University of Western Ontario as a faculty member in 2002. His research focuses on electrochemiluminescence, solar cells, scanning electrochemical microscopy, and ionic liquid electrochemistry.

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ACKNOWLEDGMENTS We acknowledge the research support by NSERC, CFI/OIT, PREA, and Western and the quality service from the Western Electronic Shop and ChemBio Store. We are grateful to Mark S. Workentin for his collaborations.

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REFERENCES

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on January 12, 2017, with an error in Figure 1B. The corrected version was reposted on January 19, 2017.

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