Luminescent Au(I)âThiolate Complexes through Aggregation-Induced

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Luminescent Au(I)-thiolate Complexes through AggregationInduced Emission: The Effect of pH during and post Synthesis Meihua Wu, Jia Zhao, Daniel M. Chevrier, Peng Zhang, and Lijia Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11716 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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The Journal of Physical Chemistry

Luminescent Au(I)-thiolate Complexes through Aggregation-Induced Emission: The Effect of pH during and post Synthesis Meihua Wu,1 Jia Zhao,1 Daniel M. Chevrier,2 Peng Zhang,2 and Lijia Liu1* 1

Jiangsu Key Laboratory of Carbon-based Materials, Institute of Functional Nano and Soft

Materials (FUNSOM), Soochow University, 199 Ren’ai Road, Jiangsu, Suzhou, 215123 China 2

Department of Chemistry, Dalhousie University, 6274 Coburg Road, Halifax, NS B3H4J3

Canada *Email: [email protected], Telephone: +86-512-65884530

ACS Paragon Plus Environment

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ABSTRACT: Luminescent Au(I)-thiolate complexes (Au(I)-SR) are synthesized at room temperature using glutathione (GSH) as both a reducing agent and capping ligand. The photoluminescence (PL) intensity and the emission wavelength are found to be strongly affected by the pH environment during and post synthesis. Although there is no Au(0) core present in these complexes, Au(I)-SR exhibit PL which can be further enhanced through aggregation-induced emission (AIE) mechanism. The structures of these Au(I)-SR at each pH stage are carefully characterized. Using X-ray absorption find structure (XAFS), we further establish a relationship between the PL intensity and the degree of aurophilic interactions. We find that the luminescence of Au(I)-SR is tunable over a wide pH range with good reversibility.


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INTRODUCTION Thiolate-protected gold nanoclusters (AuNCs) typically contain a few tens to hundreds of Au atoms stabilized by thiolate ligands, and show molecular-like properties due to quantum confinement effects.1-3 In particular, smaller AuNCs can exhibit photoluminescence (PL) with good photostability, large Stokes shifts and reasonable tunability of emission energy.4-7 AuNCs are thus becoming increasingly promising as new fluorescent probes for biomedical, environmental applications, and sensing.8-13 In addition, AuNCs (and other metal NCs) hybrid materials also demonstrate good performance in catalysis and light harvesting.14,15 It has been demonstrated that the PL properties of AuNCs have a strong dependency on the number of Au in the metal core as well as the coordination environment of the S-Au(I)-S surface shell.5 From the past two decades of research, nanoclusters with specific compositions of Aun(SR)m can be synthesized with atomic precision,7,16 but for most AuNCs the PL is intrinsically too weak for potential biomedical imaging applications.17,18 A major advance in increasing the PL is the finding that only AuNCs with a large quantity of Au(I) on the surface can luminesce with high intensity through aurophilic interactions.19,20 In addition, AuNCs can undergo aggregationinduced emission (AIE) mechanism that further enhances the PL.21-24 AIE refers to a group of molecules which are non-emissive in diluted solutions but emit intense light in the concentrated solution when the intramolecular rotation is restricted.25 In the case of AuNCs, the ligand shell can be rigidified either by inducing a strong intermolecular force between the functional groups of the ligand26,27 or between ligand and surrounding environment.20,22,28,29 Even for AuNCs with a definite core-ligand ratio, AIE has been reported to be the dominant PL mechanism.30 Glutathione (GSH), a common tripeptide, acts as a reducing agent in biological systems. When synthesizing AuNCs, the thiol group reduces the Au(III) precursor to Au(I), and binds easily with


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the latter to form an Au(I)-thiolate complex (Au(I)-SG). GSH has two carboxylic acid and an amine group, and these zwitterionic groups allow it to undergo a reversible assembly-disassembly process by tuning the pH.22,31,32 This property can be used in tuning the PL of Au(I)-SG as well as GSH-capped AuNCs. For example, Yao et al. found that the aggregation-dissociation equilibrium between aggregated and free Au(I)-SG complexes can be easily controlled by the pH.33 Yu et al. also reported that the pH level is a key factor to adjust the reduction kinetics and therefore to tailor the size of AuNCs.34 Wang et al. used pH-controlled AIE to produce highly luminescent AuNCs.23 However, the understanding of the AIE mechanism in free Au(I)-SG and immobilized Au(I)-SG on AuNC surface is still far from complete. Most of the studies on pH response of GSH so far investigated a relatively narrow pH range.23,31-36 GSH has four ionizable functional groups with pKa 2.12, 3.53, 8.66, 9.12, respectively.37 Although the last pKa (9.12), which belongs to the thiol group, can be ignored due to its bonding with Au, it is still necessary to perform a study on the assembly-disassembly of Au(I)-SG that covers a wider pH range (i.e. from pH lower than 2 to pH above 9). Although having a large portion of Au(I)-thiolate surface moieties is believed to be the key factor for producing highly luminescent AuNCs, many AuNCs systems investigated so far have a Au(0)core-Au(I)-shell structure, which actually makes the situation more complicated.23,38 In addition, in order to synthesize such structure, a second reducing agent has to be introduced to reduce Au(I)thiolate species to Au(0).16,39-41 On the other hand, there are also reports of luminescent AuNCs made using GSH only.9,10,19 The reducing power of GSH alone is not sufficient to producing Au(0) species, but transmission electron microscopy (TEM) images confirm the presence of small particles. Although in almost all the studies, Au-based species that are visible under TEM as small clusters are referred to as AuNCs, the exact species formed is not fully understood, and neither is


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the luminescence mechanism. Therefore, more precise structure-PL relationships are required. In addition, if Au(I)-SG species are the key contribution to the intense PL of AuNCs systems, producing nanoclusters without Au(0) would help investigate the AIE effect of Au(I)-SG directly. So far, very few studies have done on the correlation between Au(I)-SG species and their PL, and although it is known that pH can play a significant role in tuning the PL property, the detailed understanding is still lacking. In this study, Au(III) chloride salt is used as a precursor, and GSH is used as both the capping ligand and the reducing agent to produce Au(I)-thiolate complexes (Au(I)-SR) of various PL properties. The reaction is carried out for an extended period of time to reach a thermodynamically stable state. Such a method does not involve heating or the use of a second reducing agent, and has also been reported to be successful in synthesizing other luminescent metal NCs such as CuNCs.35 Herein, the role of pH on the Au(I)-SR and the corresponding PL during and post-synthesis is investigated over a wide range (pH 1.6 – 11.9). The local structure of these Au(I)-SR was studied using X-ray absorption fine structure (XAFS) at the Au L3-edge.

EXPERIMENTAL Materials. L-Glutathione (GSH) and HAuCl4·3H2O were purchased from Sigma-Aldrich. Sodium hydroxide (NaOH) was obtained from Sinopharm Chemical Reagent. Hydrochloric acid (HCl, 36%~38%) was from Chinasun Specialty Products Co., Ltd. Ultrapure water (resistivity 18.2 MΩ) was used throughout the work. Synthesis of Au(I)-SR at Various pHs. Au(I)-SR were synthesized by a chemical reduction method in aqueous media following a previously reported protocol.10 Briefly, freshly prepared 50 mM HAuCl4 (1 mL) and ultrapure water was mixed with 50 mM glutathione (1.25 mL). The


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mixture was vigorously stirred for 2 min at room temperature. 1 M NaOH was then used to tune the pH values of the solution. In this work, four pH values were studied: 1.6 (pristine), 3.1 (200 µL NaOH), 7.1 (290 µL NaOH) and 11.9 (500 µL NaOH). The total volume of the solution mixture is kept at 10 mL. The solution was left in a 37 °C water bath under continuous stirring for 24 h. The raw product was purified using an ultrafiltration centrifuge tube with a molecular weight cut-off (MWCO) of 3 kDa. Characterization. The photoluminescence (PL) spectra of Au(I)-SR was recorded using a ﬂuorescence spectrophotometer (Fluromax 4) and the UV-vis absorption spectra were obtained with UV-vis spectrophotometer (Lambda 750). The PL lifetime was measured by HORIB-FM2015 ( France JY Company). Particle size distribution was obtained on the Malvern Nano ZS90. The morphology was measured by transmission electron microscopy (TEM, Tecnai G2 F20, FEI). A pH meter (Mettler Toledo, FE20) was used to monitor the pH of the solution. The Au L3-edge XAFS measurements were conducted at the National Synchrotron Radiation Research Center (NSRRC), Taiwan, BL01C1 and BL14W1 at Shanghai Synchrotron Radiation Facility (SSRF), Shanghai. The samples were measured in solution form and the spectra were recorded in X-ray fluorescence mode. Background subtraction, scan averaging, energy calibration, normalization, and EXAFS fitting were all performed using the WinXAS 3.1 software package. The amplitude reduction factor (S02) was fixed at 0.9 for EXAFS fitting. Theoretical phase and scattering amplitudes for all scattering paths used in EXAFS fitting were simulated using the FEFF8.2 computational package.42 A krange of 3.0 – 12.0 Å-1 was selected for the conversion to FT-EXAFS, which was the range subsequently used for EXAFS fitting. A R-space fitting range of 1.5 – 2.5 Å or 1.5 – 3.5 Å was used to account for one or two scattering paths, respectively. Au-S and Au-Au scattering paths


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(scattering amplitudes and phase-shift functions calculated with FEFF8.2) from the Au25(SR)18 structural model43 were used fitting experimental data. Uncertainties in EXAFS fitting parameters were computed from off-diagonal elements of the correlation matrix and weighted by the square root of the reduced chi-squared value obtained for the simulated fit. The amount of experimental noise from 15 - 25 Å in R-space was also taken into consideration for each EXAFS spectrum.44

RESULTS AND DISCUSSION Figure 1 shows the UV-vis and PL of the Au(I)-SR prepared under various pH values. The UVvis spectrum of Au(I)-SR prepared under basic conditions is clearly different from the spectra made under acidic and neutral conditions. Unlike AuNCs made with the presence of a second reducing agent, no characteristic absorption features were found that belong to AuNCs with a specific Au(0) core size.17,30,45,46 We therefore refer all Au species synthesized in our study as Au(I)-SR (i.e. Au(I)-thiolate complexes. The presence of a Au(0) core, if there is one, is too small or too few on average to be detected by UV-Vis. A broad shoulder feature appears at ~400 nm in the pH=11.9 sample, and similar feature has been observed from oligomeric Au(I)-thiolate species.6,23,31 An enhanced absorption at this region could be due to a stronger ligand-metal charge transfer (LMCT) across the HOMO of S to LUMO of Au.47 At pH=11.9, the GSH ligands are completely deprotonated (see detailed molecular structures of GSH at various pHs in Supporting Information Scheme S1), and an increased charge density around S could lead to an increased intensity in the UV-vis spectrum.


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Figure 1. (a) UV−vis absorption spectra of Au(I)-SR. (b) PL spectra of Au(I)-SR with an excitation wavelength of 420 nm. (c) PL decay lifetime of Au(I)-SR prepared at pH=3.1 (λem=655 nm and λex=405 nm) and pH=11.9 (λem=670 nm and λex=420 nm).

Regardless of the preparation conditions, all Au(I)-SR are light-emitting. Shown in Figure 1b, Au(I)-SR prepared at pH = 3.1 have the strongest luminescence with a quantum yield (QY) of about 1.89% compared to Rhodamine 6G (QY≈99%). Samples prepared under strongly acidic


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(i.e., pH=1.6) and neutral conditions emit weaker luminescence. The PLQY values for all Au(I)SR are summarized in the Supporting Information Table S1. All samples exhibit Stokes shift larger than 8300 cm-1 (see Supporting Information Figure S1). It is interesting to see that the sample prepared under basic conditions has a symmetric PL peak centred at 670 nm, but others contain two PL components at 620 nm and 660 nm. Asymmetric PL has been observed from other thiol-protected AuNCs.6,22,48 An earlier study on bovine serum albumin (BSA)-protected Au25 nanoclusters suggested that the long wavelength PL (>700 nm) is from Au(0) and the short wavelength PL (610 nm) is from Au(I)-thiol and Au(I)-Au(I) interaction.49 However, this is still under debate, because recent research carried out by our authors suggests the absence of Au(0) core in luminescent Au-BSA nanoclusters.50 A similar study also points out that the longer wavelength PL (>800 nm) originates from Au with low degree of thiol bonding.6 However, PL from AuNCs reported to be mixed valence only exhibits a single component at 585 nm.19 In our case, all PL is centered around 600 ~ 700 nm; thus, we may rule out the possibility of Au(0), since our UV-Vis results also suggest so. This can be further confirmed by comparing the X-ray absorption near-edge structure (XANES) at the Au L3-edge of the Au(I)SR and metallic Au foil (Figure S2). When Au has a less-filled 5d orbital (i.e. higher valence), the spectrum will exhibit an increased absorption intensity at the edge. Here, all Au(I)-SR exhibit similar absorption profiles reflecting an oxidized Au chemical state (i.e., Au(I)). The origins of the two PL emission features should be both attributed to a LMCT mechanism. A recent study performed by our authors also noticed a minor PL peak red-shift when the degree of Au(I)-SR inter-cluster interaction increases.50 Our observation suggests that, under neutral and acidic conditions, there are two types of metal-thiol interactions/electronic transitions that induce photoemissive relaxation of Au(I)-SR. In acidic and neutral conditions, these two types have nearly


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equivalent contribution to the total PL, but in basic condition, the shorter wavelength PL component decreases significantly. The PL decay curves of Au(I)-SR prepared at pH=3.1 and pH=11.9 are shown in Figure 1c. A polyexponential fit reveals that the average decay lifetime is 254 ns for Au(I)-SR prepared at pH=3.1 and 311 ns for the one at pH=11.9 (Supporting Information Table S2). Since PL from singlet excited states of metallic Au(0) is usually at the order of a few nanoseconds,19,51 our results suggest that the observed PL of our Au(I)-SR samples, in both aggerated and dispersed form, are originated from triplet excited states (i.e. Au(I)-ligand interaction). In particular, only Au(I)-SR prepared at pH=3.1 has a microsecond-scale decay lifetime (1.01 µs), which is similar to previously reported Au-thiolate nanoclusters upon aggregation and rigidified Au-thiolate structure, and the origin of the enhanced luminescence has both been attributed to the restrain of intramolecular vibration and rotation.22,50 The relative intensity ratio between the two components also varies with pH. PL spectra of Au(I)-SR prepared at pH=1.6, 3.1, and 7.1 are fitted with Gaussian peaks (see Supporting Information Figure S3). The shorter wavelength band, which is centered at 605 nm (2.05 eV), is more significant for Au(I)-SR made in neutral conditions, while at pH=3.1, the longer wavelength band, which is centered at 663 nm (1.87 eV), is more pronounced. Under a highly acidic environment, the two bands are of similar intensities. In the following, we will combine the morphology and the Au local structure of these Au(I)-SR to analyze the origin of these two emission bands. The morphologies of the Au(I)-SR are shown in Figure 2. At pH=1.6, 3.1 and 11.9, small particles can be easily located under TEM and the average size of individual particles grows slowly with the increase of pH: 2.7 nm at pH=1.6, 2.9 nm at pH=3.1 and 3.0 nm at pH = 11.9. There were multiple attempts to locate Au(I)-SR prepared at pH=7.1 under TEM, but only scattered particles


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with very small sizes could be found (Figure 2(c)), and the number of the particles was too low to give a reliable size distribution analysis. The detailed size distribution can be found in the Supporting Information Figure S4. Another significant difference is the degree of aggregation of these particles. At pH = 1.6, the particles are aggregated into large clusters of irregular shapes with sizes extended up to several hundred nms. When the pH increases to 3.1, the aggregated clusters form circular shape with diameters around a few tens of nm. At pH=11.9, particles are welldispersed with a narrower size distribution, and no aggregation is observed. The pH-dependent aggregation is also examined with dynamic light scattering (DLS) measurement (see Supporting Information Figure S5). The average size of the clusters prepared under acid environment is much larger than the one under basic environment, which is consistent with the TEM observation.

Figure 2. TEM images of Au(I)-SR at (a) pH = 1.6, (b) pH = 3.1, (c) pH =7.1 and (d) pH=11.9. The insets of (a), (b) and (c) show magnified views of the Au(I)-SR. Scale bars in main Figures: 20 nm; scale bars in insets: 5 nm.


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The four pH levels used for Au(I)-SR synthesis were chosen to fall between the range of different pKa values of GSH. When the solution pH is less than pKa1 or greater than pKa3, the SG ligands on the surface of an Au(I)-SR are positively or negatively charged, respectively (see Supporting Information Scheme S1). Interaction between thiol ligands in these cases is mostly repulsive. At acidic conditions however, the presence of COOH groups could also facilitate the formation of hydrogen bonding. This could explain the appearance of large aggregates in Figure 2a. When the solution pH=3.1, the pH is close to the pKa2 value. In these conditions, SG ligands on the surface of an Au(I)-SR are isoelectric. Although hydrogen bonding is still present, the electrostatic interaction is the dominant attraction force, and unlike the irregular shapes formed at pH=1.6, the Au(I)-SR at pH=3.1 are aggregated into circular clusters. It is interesting that only aggregation through electrostatic attraction results in enhanced PL. At neutral pH, the two carboxylic groups are deprotonated, but the amine group is still protonated, so the Au(I)-SR are separated by a repulsive force. To further correlate the observed PL with the Au local structure, the coordination environment of Au was examined using EXAFS at the Au L3-edge. Figure 3 shows the Fourier transformed EXAFS spectra of the four Au(I)-SR samples in solution. The spectra in k-space are included in the Supporting Information Figure S6. We attempted to fit all spectra with a two-shell scattering model (i.e., Au-S and Au-Au shell), but this only worked for pH=3.1 and pH=11.9. Therefore, fitting results reported here for pH=1.6 and pH=7.1 only show the Au-S shell. Table 1 summarizes the fitting parameters, and the fitted spectra are shown in the Supporting Information Figure S7. All Au(I)-SR samples contain a Au-S coordination number (CN) close to 2, suggesting they all contain mostly SG-Au(I)-SG structures. If these were AuNCs with a small metallic core, the AuS staple structures on the surface would yield a Au-S CN lower than 2.49,52 Meanwhile, Au(I)-


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Au(I) bonding is also detected in the sample with strong AIE, indicating that AIE could also facilitate aurophilic interactions.

Figure 3. Au L3-edge EXAFS spectra of products at various pH levels.

pH 1.6 3.1 7.1 11.9

CN 2.3(2) 2.1(1) 2.0(1) 1.9(1)

Table 1. Au L3-edge EXAFS Fitting Results Au-S Au-Au 2 2 R(Å) σ (Å ) ΔE0(eV) CN R(Å) σ2(Å2) ΔE0(eV) 2.306(4) 0.0035(7) 0.5(8) 2.316(2) 0.0025(4) 2.1(5) 0.7(5) 3.05(1) 0.007(6) 2.1(5) 2.301(2) 0.0030(4) 1.1(5) 2.309(2) 0.0020(3) 1.0(3) 1(1) 3.04(3) 0.024(11) 1.0(3)

Since GSH acts as both the reducing agent and the capping ligand, the negatively charged end groups of GSH at pH=11.9 not only facilitate the formation of a more definite Au(I)-SG cluster structure but also prevent the formation of aggregated clusters. The well-dispersed Au(I)-SR then produce more of a single-component long-wavelength PL (Figure 1b and Figure S2). On the other hand, the presence of Au-Au interaction in Au(I)-SR prepared at pH=3.1 due to AIE also shows higher intensity in the 663 nm band, which further suggests the origin of this band is due to the


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Au(I)-Au(I) interaction. Such interaction is therefore responsible for the production of microsecond-long PL decay component (Supporting Information Table S2). The shorter wavelength PL, on the other hand, can be attributed to Au-S LMCT, and as such this band is present in all Au(I)-SR regardless of their morphologies.

pH 1.6

Table 2. Summary of Au(I)-SR Structure-PL Correlation Degree of HDegree of Au(I)Charge on SG ligand PL intensity bonding Au(I) interaction +1 High Weak Lowest

3.1

0

Medium

Strong

Highest

7.1

-1

Low

Weak

Low

11.9

-2

Low

Strong

High

Table 2 summarizes the correlation between the structure of Au(I)-SR and the corresponding PL property. The PL due to LMCT is always present in the Au(I)-SR, but only the samples with more Au(I)-Au(I) interaction show enhanced PL intensity. Au(I)-SR in a mildly acidic environment exhibit aggregated form and produce increased PL due to AIE mechanism, in which the intramolecular motions of ligand molecules are restricted due to the strong electrostatic force (oppositely charged end groups). However, aggregation due to a high degree of H-bonding will decrease the PL intensity. This is because when the photoexcited electrons are produced, it is more likely (compared to other pH conditions) to interact with solvent molecules, and the energy is dissipated non-radiatively.


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Figure 4. Evolution of PL during pH adjustment. The pH of the labelled data points are measured to be: a. 11.9; b. 2.3; c. 1.6; d. 2.4; e. 11.9.

We then selected Au(I)-SR formed under basic conditions (i.e., pH=11.9) to further examine the effect of pH on the PL after Au(I)-SR have formed. This is done by gradually adding HCl to the Au(I)-SR solution until the pH is lower than the pKa1 of GSH (i.e., pH=1.6), and then add NaOH to bring it back to the original pH. Figure 4 shows the change of overall PL intensity throughout this process. Full PL spectra showing each data point can be found in the Supporting Information Figure S8. As can be seen in Figure 4, the original Au(I)-SR (stage a) exhibits the lowest PL, and as the solution turns acidic, the PL starts to increase. The first PL maximum appears when pH=2.3 (stage b). This value is slightly above pKa2 of GSH, at which point GSH is isoelectric. Further increasing pH decreases the PL until it reaches the minimum at the most acidic environment tested in this work (pH=1.6, stage c). However, when the solution is tuned from acidic to basic, a second PL maximum appears at pH=2.4 (stage d), while further increasing pH will lead to a decrease of intensity. It should be noted that although the final solution has the same pH as the starting solution, the PL intensity is slightly higher than the initial value (stage e). This process was repeated four times and the PL can be found in the Supporting Information Figure S9. We found that this trend


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remains the same during these pH adjustment cycles: the PL is most intense at pH=2.4, but lower at other pH levels. The most significant enhancement appears at the first cycle, and in the next few cycles the degree of enhancement reduces but still exhibits similar trend. The appearance of the Au(I)-SR in solution due to scattering of visible light also undergoes significant change during the pH adjustment. Figure 5 shows photographs and TEM images of the five representative stages during pH adjustment (stages a-e). Au(I)-SR solutions with higher PL intensities are more opaque in appearance, and TEM reveals that they aggregate into clusters with sizes of a few tens of nms. These cluster sizes are directly proportional to the PL intensity. The DLS measurements also reveal that the Au(I)-SR exhibit a much larger cluster size at pH=2.3 than pH=11.9 (See Supporting Information Figure S10). It should be noted that the solutions at each pH tuning stage are reasonably stable: even more opaque solutions (e.g., pH=2.3 in Figures 5b and 5d) remains stable with no significant PL change for a period of at least 3 days. Infrared spectra of the Au(I)-SR during the pH adjustment stage also exhibit identical features, suggesting that there is no new bond formation between the GSH ligands.(Supporting Information Figure S11) It is interesting that although between pKa2 (2.12) and pKa3 (3.53), GSH is isoelectric, the degree of aggregation as well as the PL intensity does not remain the same, and this is true for both the acidbase and baseacid directions. As a result, the end-group charge distribution is not the only factor that controls the degree of aggregation as well as the corresponding PL. The presence of excessive hydrogen bonding at acidic environment decreases the PL intensity. When the pH of the solution returns to 11.9, it becomes monodisperse, and the average size of the Au(I)-SR drops noticeably.


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Figure 5. TEM images of Au(I)-SR at five stages during pH adjustment post-synthesis. (a) pristine Au(I)-SR synthesized at pH=11.9, (b) adding HCl until pH=2.3, (c) adding HCl until pH=1.6, (d) adding NaOH until pH=2.4, (e) adding NaOH until pH=11.9. The insets are the corresponding photographs of the Au(I)-SR solution. (f) PL of the Au(I)-SR solutions at stages a-e.

Figure 5f compares the PL of stages a-e. The single-component emission peak at stage a shifts to shorter wavelength as the solution turns acidic, and a shoulder feature appears. The blue-shift is a common observation for AIE-induced Au(I)-SR. Moreover, it is actually quite similar to Au(I)SR synthesized in a mildly acidic environment (Figure 1b). The maximum QY is measured to be 2.35% at pH=2.4 compared to Rhodamine 6G (QY≈99%). The complete QY values of Au(I)-SR at each stage can be found in Supporting Information Table S1. The asymmetric PL profile remains throughout the rest of the pH adjustment. Even when the AuNC sample resumes its original pH and its dispersed morphology, the original PL is not recovered. The observation above demonstrates that the addition of acid to the as-formed Au(I)-SR not only changes the charge


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distribution of the end groups, but also leads to a change in the structure or organization of Au(I)SR. Further cycling the pH values does not change the PL spectra (Supporting Information Figure S12), suggesting that the Au(I)-SR after the first cycle possess a pH-controlled reversible aggregated-dispersed property.

CONCLUSIONS In summary, using GSH as both a reducing and capping ligand, Au(I)-SR are mainly composed of oligomeric Au(I)-SG. Our results reveal that these Au(I)-SR are luminescent regardless of pH, even though none of the Au(I)-SR samples possess a definite Au(0) core. Our work suggests that the presence of Au(0) is not essential for producing luminescent nanoclusters. By tuning the pH environment during and post-synthesis, the degree of interaction between the capping ligands can be altered. Au(I)-SR exhibit dual emission bands from both Au(I)-SG charge transfer and Au(I)Au(I) aurophilic interaction, respectively. Aggregation of Au(I)-SR occurs under both hydrogen bonding and electrostatic interaction, but only the latter creates enhanced PL (i.e. AIE effect). In addition, well-dispersed Au(I)-SR can change into AIE-type Au(I)-SR with enhanced PL intensity when the pH is adjusted to slightly higher than pKa2 (i.e., for GSH: pH=2.3~2.4). This work also correlates the observed PL property with the Au local structure providing insights on strategies for optimizing Au(I)-SR with desired luminescence properties.

ASSOCIATED CONTENT Supporting Information. Illustration of the protonation/deprotonation of GSH, Table of PLQY of all Au(I)-SR, PL lifetime decay components, detailed optical spectra of Au(I)-SR prepared under different pH conditions, size distribution of Au(I)-SR measured by TEM and DLS, PL


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peak deconvolution, the Au L3-edge XANES of Au(I)-SG, EXAFS spectra in k-space, EXAFS fit, full evolution of PL during the pH adjustment process, FT-IR spectra of Au(I)-SR. These materials are available free of charge via Internet at http://pubs.acs.org. ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (project number 21403147) and Natural Science Research Programme of Higher Education of Jiangsu Province (project number 18KJB150026) is gratefully acknowledged. The authors acknowledge support from Soochow University-Western University Center for Synchrotron Radiation Research and Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), Soochow University. D. M. C. was supported by the NSERC CGS-Alexander Graham Bell scholarship during this work. P. Z. acknowledges NSERC Discovery Grants for funding.

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