The Reactivity Toward Ag+: A General Strategy to Generate a New

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The Reactivity Toward Ag+: A General Strategy to Generate a New Emissive Center from the NIR-Emitting Gold Nanoparticles Yaping Wang, Lulu Liu, Lingshan Gong, Ying Chen, and Jinbin Liu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03295 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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The Journal of Physical Chemistry Letters

The Reactivity Toward Ag+: A General Strategy to Generate a New Emissive Center from the NIREmitting Gold Nanoparticles Yaping Wang, Lulu Liu, Lingshan Gong, Ying Chen, and Jinbin Liu* Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China. Corresponding Author *E-mail: [email protected]; Tel.: +86-20-22236846

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ABSTRACT

We report a facile strategy for the transformation of the single NIR-emitting AuNPs to dual-NIRemitting bimetallic Ag@AuNPs based on the robust reactivity toward Ag(I) ions under mild conditions. The reactivities toward Ag(I) ions were discovered significantly different between the visible- and NIR-emitting glutathione(GSH)-coated AuNPs: the high GSH surface coverage on the 610 nm-emitting AuNPs resulted in an reversible interaction due to enough surface steric hindrance to resist Ag(I) ions from interaction with the Au(0) core, whereas the low GSH surface coverage on the 810 nm-emitting AuNPs led to both the anti-galvanic reaction and Ag(I)carboxylate shell formation on surface of the AuNPs, which were responsible for the formation of a new emissive center at 705 nm. This strategy was also demonstrated to exhibit excellent generalization toward various NIR-emitting AuNPs with surface chemistries containing carboxyl groups, opening a new pathway of tailoring the optical properties of the metallic NPs through surface reactivity.

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The ultrasmall-sized luminescent gold nanoparticles (AuNPs) have become highly attractive to the research fields such as sensing and bioimaging due to the unique optical properties and good biocompatibility.1-5 Controllable synthesis of luminescent AuNPs with tunable optical properties can be achieved by strictly controlling the reaction conditions such as ligand to gold ratio, pH and temperature.6-10 Thus, the emission of the AuNPs could be adjusted from visible to


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near-infrared (NIR) window to facilitate the in-vivo bioimaging applications. For example, the glutathione (GSH)-coated AuNPs (GS-AuNPs) emitting at 610 nm or 810 nm was synthesized by controlling the GSH to gold ratio of 1.6:1 and 0.8:1, respectively.6, 11 However, the most reported probes based on the luminescent AuNPs were detected by the change of the emission intensity as the recognition signal, which would be interfered by the factors such as photobleaching, environmental conditions, instrumental stability or probe concentration.12,13 When being used in the various analytical applications, the dual-emissive probes with ratiometric capability are more attractive as compared to the single emissive probes due to the sensitivity and built-in calibration system from the two different emissive wavelengths for avoiding the irrelevant influence factors.14-18 A common strategy for fabricating ratiometric emissive probes is to fuse two distinct emissive fluorophores/species together and then utilize their separate responses as signal response group or reference.19-21 The construction of ratiometric emissive nanoprobe from the above strategy often involved in tedious multistep preparations and sophisticated coupling or chemical modification processes.22 Recently, we reported the creation of a dual-emissive GSAuNPs with ratiometric pH response by fine-tuning of the GSH surface coverage in a one-pot synthesis process,6 which opened up a new possibility for the design of ultrasmall-sized ratiometric nanoprobes (~ 2.5 nm) with simple construction process and avoiding the tedious chemical coupling. However, this one-step synthesis required strictly controlling the surface coverage and the local bonding environment during the synthesis process, which made this strategy limited to only a few surface chemistries. In addition, it would be a challenge to achieve multimodal imaging capabilities for this one single ultrasmall-sized AuNPs (~2.5 nm) using the precursor chloroauric acid as the only metal source. Since the optical properties of the AuNPs


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could be controlled by their surface ligands and metal compositions,23,24 the reaction of the AuNPs towards metal ions would provide a specific route for improving their optical properties. Significant emission enhancement has been observed from the few-atom gold nanoclusters (AuNCs) after the reaction with Ag(I) ions.25-27 For instance, highly red-emissive bimetallic nanoclusters Au@AgNCs at 667 nm could be produced by doping Ag(I) ions as linkers to bridge the small Au(I)–thiolate motifs on the weakly luminescent ultrasmall sized AuNCs, and red emission of the Au@AgNCs was immediately quenched upon the addition of cysteine to remove the Ag(I) ions.28 The anti-galvanic reduction (AGR) has been already recognized through the reaction between Ag(I) ions and the AuNCs, resulting in 3.4-fold enhancement of the emission at 670 nm.29 These interesting observations demonstrated that the reactivity towards Ag(I) ions was pivotal to the control of the emissions from the few-atom sized AuNCs. However, the above reported methods mainly focused on the synthesis of single-emissive bimetallic NCs, and the creation of dual-emissive bimetallic Ag@AuNPs with few-nanometer size (2~3 nm) has not been achieved. To realize the scope of possible transformations of single-emissive AuNPs to dual-emissive bimetallic NPs, it is important to further understand and elucidate the reactivity of the different emissive AuNPs towards Ag(I) ions. Herein we propose a facile and general strategy for achieving dual-NIR-emitting bimetallic Ag@AuNPs from the 810 nm-emitting AuNPs resulted from the robust reactivity toward Ag(I) ions under mild conditions. The reactivities toward Ag(I) ions were discovered significantly different between the visible- and NIR-emitting GS-AuNPs: the high GSH surface coverage on the 610 nm-emitting AuNPs resulted in an reversible interaction due to enough surface steric hindrance to resist Ag(I) ions interacted with the Au(0) core, whereas the low GSH surface coverage on the 810 nm-emitting AuNPs led to both the anti-galvanic reaction (AGR) and Ag(I)-


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carboxylate shell formation on the surface of AuNPs, which were responsible for the creation of a new emission center at 705 nm. Interestingly, upon the introduction of the Ag(I) ions, the generated dual-NIR-emitting Ag@GS-AuNPs with both 705 and 810 nm emissions in a single small sized AuNPs (~ 2.3 nm) showed unique linearly ratiometric pH-dependent emissions. Not limited to GSH, this strategy was also adaptable to other thiolate surface ligands with carboxyl groups, demonstrating the generality of this proposed strategy. The dual-emissive Ag@GS-AuNPs with both emissions at 705 and 810 nm was synthesized from the single 810 nm-emitting GS-AuNPs by introducing trace amount of Ag(I) ions (Figure 1A). The reaction took place at room temperature within 5 seconds. After the introduction of Ag(I) ions with concentration from 10.0 to 200.0 µM, the GS-AuNPs showed an increased emission peak at 705 nm meanwhile a slight enhancement of the intrinsic emission peak at 810 nm (Figure 1B). Interestingly, as the Ag(I) ions concentration increased from 30.0 to 100.0 µM, the emission spectra showed an isosbestic point at 765 nm, indicating the formation of the dualNIR-emitting bimetallic Ag@GS-AuNPs (Figure S1). After the introduction of the increasing concentration of Ag(I) ions, the Ag@GS-AuNPs showed an increased red-colored fluorescence under the ultraviolet light while the color of GS-AuNPs showed no significant changes under the room light (Figure S2A). The quantum yields of Ag@GS-AuNPs were measured to be 1.06% at the 705 nm emission peak and 1.03% at the 810 nm emission peak, respectively, using Nile Blue A as the reference standard. The UV-Vis spectra of the Ag@GS-AuNPs showed strong absorption in the UV region, which were similar as those of the GS-AuNPs (Figure S2B). No surface plasma resonance absorption was observed after the introduction of the Ag(I) ions, suggesting that the size of the Ag@GS-AuNPs has negligible variation as compared to that of the GS-AuNPs (Figure S3). We then used transmission electron microscopy (TEM) to characterize


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the synthesized Ag@GS-AuNPs (Figure 1C). The core size of the Ag@GS-AuNPs was measured to be 2.3 ± 0.3 nm, similar to that of the GS-AuNPs (2.5 ± 0.3 nm) (Figure S3A). In addition, only one distinct band was observed in the agarose gel electrophoresis (2%) of the synthesized Ag@GS-AuNPs (Figure 1D), and the band showed strong increased red-colored emission under UV illumination with the increase of Ag(I) ions. These results suggested that the dual emissions after the reaction with Ag(I) ions were emitted by the as-synthesized Ag@GSAuNPs instead of the impurities generated during the reaction process. In addition, we also studied the stability of the as-synthesized Ag@GS-AuNPs at room temperature (Figure S4), and the results showed that little emission decrease was observed after more than one week, demonstrating the good stability of Ag@GS-AuNPs.

Figure 1. A) Schematic diagram of the formation of the dual-emissive Ag@GS-AuNPs. B) Luminescent spectra of the dual-emissive Ag@GS-AuNPs formed with various concentrations of Ag(I)) ions. C) Typical TEM images of the dual-emissive Ag@GS-AuNPs formed with 100.0 µM of Ag(I) ions (left panel), and the core-size distributions of the 810 nm-emitting GS-AuNPs


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and dual-emissive Ag@GS-AuNPs (right panel), respectively. D) Agarose gel electrophoresis pictures of the dual-emissive Ag@GS-AuNPs formed with increasing concentration of Ag(I) ions (from lane 1 to lane 6: 0, 30.0, 40.0, 50.0, 60.0 and 70.0 µM) under room light (left panel) and UV light (right panel), respectively.

The emergence of the 705 nm emission peak in the luminescence spectra of the 810 nmemitting GS-AuNPs indicated that the surface electron energy of the GS-AuNPs has been changed after the reaction with Ag(I) ions. The structure of the luminescent GS-AuNPs was generally considered as a model of Au(0) core protected by the polymeric GS-Au(I) structures (e.g., GS-Au-SG-Au-SG staple motifs),30 which govern their optical properties. In order to study the reaction between GS-AuNPs and Ag(I) ions, we added a small thiolate ligand cysteine that could interact strongly with Ag(I) ions via the formation of cysteine-Ag(I) complexes to the purified Ag@GS-AuNPs solution. The 705 nm emission of the Ag@GS-AuNPs increased significantly rather than restored to those of the GS-AuNPs (Figure 2A), which indicated that the emission change was irreversible, and cysteine should insert into the gold surface to further increase the surface ligand density of the Ag@GS-AuNPs instead of removing the Ag(I) ions (Figure 2B). In addition, the ethylenediaminetetraacetic acid (EDTA, Figure S5A) with strong coordination binding with the Ag(I) ions could not cause the change of the emissions of the Ag@GS-AuNPs. However, the iodides could cause the decreases from both the 705 nm and 810 nm emissions due to the highly binding with both the gold and silver in the Ag@GS-AuNPs (Figure S5B). The above results clearly indicated that Ag(I) ions should have strong binding with the Au(0) core inside the GS-AuNPs resulted from the low surface coverage of the 810 nmemitting GS-AuNPs. The low surface coverage of the 810 nm-emitting GS-AuNPs could not provide enough steric hindrance to resist Ag(I) ions interacted with the Au(0) core.


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Figure 2. A) Luminescence spectra of the original 810 nm-emitting GS-AuNPs and dualemissive Ag@GS-AuNPs before or after the addition of 500.0 µM cysteine, respectively. B) Schematic illustration of the luminescence enhancement of the dual-emissive Ag@GS-AuNPs after the introduction of cysteine. C) Luminescence spectra of the original 610 nm-emitting GSAuNPs and 630 nm-emitting Ag@GS-AuNPs before or after the addition of 500.0 µM cysteine, respectively. D) Schematic illustration of the luminescence quenching of the 630 nm-emitting Ag@GS-AuNPs after the introduction of cysteine. E) Au 4f XPS spectra of 810 nm-emitting GSAuNPs after reaction with different concentrations of Ag(I) ions.

To verify this deduction, we synthesized the similar sized (2.6 ± 0.3 nm) 610 nm-emitting GS-AuNPs (Figure S6), which have high GSH coverage on the surface.6 The 610 nm-emitting GS-AuNPs showed a similar red emission increase around 630 nm after the introduction of Ag(I) ions (Figure S7), but the emission of the single-emissive Ag@GS-AuNPs reverted back to that of


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the 610 nm-emitting GS-AuNPs after the addition of cysteine (Figure 2C), significantly different from the results of the 810 nm-emitting GS-AuNPs with low GSH coverage (Figure S8), indicating that the Ag(I) ions on the surface of 610 nm-emitting GS-AuNPs could be removed due to the high affinity of cysteine with Ag(I) ions (Figure 2D). The different reactivities toward Ag(I) ions between 610 nm and 810 nm-emitting GS-AuNPs further demonstrated that Ag(I) ions could interact with Au(0) core inside the 810 nm-emitting GS-AuNPs. We then used X-ray photoelectron spectroscopy (XPS) to investigate the gold valence states of the 810 nm-emitting GS-AuNPs after Ag(I) reaction. As shown in Figure 2E, the binding energies of Au 4f7/2 increased with the Ag(I) ions from 0 to 0.9 mM, demonstrating that Au(0) were oxidized to Au(I) by Ag(I) ions, an anti-galvanic reduction (AGR) could occur inside the Au(0) core of the 810 nm-emitting GS-AuNPs.31,32 After the deconvolution into Au(0) and Au(I) species with binding energy of 84.0 eV and 84.6 eV,33-35 respectively, the Au(I) species increased from 30% to 43% with the increase of the concentration of Ag(I) ions. It should be noted that there is no much difference in the binding energies between Ag(0) and Ag(I) species, different from the cases of Au.29, 32 The binding energy of the Ag 3d was measured to be 367.8 eV (Figure S9), indicating that the dominant Ag species were univalent in the synthesized [email protected] The hydrogen peroxide (H2O2) as an oxidizing agent and dimethylamine borane (DMAB) as a reducing agent were then added to the synthesized Ag@GS-AuNPs, respectively (Figure S10). Both of the 810 nm-emitting GS-AuNPs and dual-emissive Ag@GSAuNPs were stable at the presence of H2O2, while the DMAB cause a significant quenching in both the 810 nm-emitting GS-AuNPs and dual-emissive Ag@GS-AuNPs, respectively, which demonstrated that both the Au(I) and Ag(I) species played importance roles in governing their optical properties. XPS spectra of S 2p and N 1s were then investigated, respectively. The


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binding energies of S 2p were about 162.6 eV, which could be assigned to the S atom bound to the gold in both GS-AuNPs and Ag@GS-AuNPs (Figure S11A). No typical oxidized sulfur with the peak of 168.2 eV was observed, indicating the completely formation of Au(I)–GS complex on the surface of the [email protected] In addition, the binding energy of N 1s at 399.6 eV corresponding to the amine species did not show significant change (Figure S11B).37 However, the binding energy of O 1s from the carboxyl group of the GSH on the surface of 810 nmemitting GS-AuNPs showed significant changes after the reaction with Ag(I) ions (Figure S11C). After the decomposition into C-O- and C=O with binding energy of 531.2 eV and 532.7 eV,38 respectively, the C-O- species increased with the increase of the Ag(I) ions, accompanied by the formation of emission at 705 nm from dual-emissive Ag@GS-AuNPs, suggesting that the emission at 705 nm should be related to the Ag(I)-carboxylate shell formed on the surface of the 810 nm-emitting GS-AuNPs besides the AGR between the Ag(I) ions and the Au(0) core in the formation of the metallic bond.38 The contribution from the type of surface ligands in the formation of dual-NIR-emitting Ag@GS-AuNPs was subsequently investigated. The carboxyl and amino groups are typical groups with substantial electron-rich properties in the surface ligand of GSH. To avoid interference by the organic surface ligands on the surface of the AuNPs, we synthesized a series of stable 810 nm-emitting AuNPs with core sizes of ~ 2.0 - 2.3 nm coated with thiolate PEG ligands (1 KDa) including HS-PEG, HS-PEG-NH2 and HS-PEG-COOH. The detailed synthesis and characterization were described in the supporting information (Figure S12). The PEGlyated AuNPs or PEGlyated AuNPs with amino group showed a significant emission quenching at 810 nm upon the addition of Ag(I) ions (Figure 3A,B), but the PEGlyated AuNPs with carboxyl group exhibited similar optical properties as the 810 nm-emitting AuNPs synthesized using GSH


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(Figure 3C). Since Ag(I) ions has no specific sites with the surface ligands of the PEG-AuNPs or NH2-PEG-AuNPs, Ag (I) ions could interact with the Au core of the PEG-AuNPs or NH2-PEGAuNPs. Therefore, d10-d10 interactions between Au and Ag could happen,39 resulting in the emission quenching of the PEG-AuNPs or NH2-PEG-AuNPs.40 The different emission responses among the different 810 nm-emitting AuNPs (PEG-AuNPs, NH2-PEG-AuNPs and HOOC-PEGAuNPs) towards Ag(I) ions indicated the surface chemistries have significant effect on the formation of dual-emissive Ag@AuNPs although all of the AuNPs have similar particle sizes. In addition, the new emission peak at 615 nm were formed from the HOOC-PEG-AuNPs after the Ag(I) reaction, distinct from those of the PEG-AuNPs or NH2-PEG-AuNPs with emissions quenching upon the additions of Ag(I) ions, demonstrating the importance of the carboxyl group. To further verify the effect of carboxyl group, we used lipoic acid (LA) which have two carboxyl groups as the thiolate surface ligand. The 810 nm-emitting AuNPs synthesized using LA showed a similar core size of 2.2 ± 0.3 nm (Figure S13). As expected, the LA-AuNPs with a single emission peak at 810 nm transformed into the dual-emissive Ag@LA-AuNPs with two emission peaks at 690 nm and 810 nm after the reaction with Ag(I) ions (Figure 3D), which clearly demonstrated that the carboxyl group of surface ligands was important in the formation of dualemissive bimetallic NPs through ligand-to-metal-metal charge transfer (LMMCT) or ligand-tometal charge transfer (LMCT) in the Ag(I)-carboxylate shell on the surface of the single NIRemitting AuNPs.41,42


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Figure 3. Luminescence spectra of the PEG-AuNPs (A), NH2-PEG-AuNPs (B), HOOC-PEGAuNPs (C) and LA-AuNPs (D) after reacting with various concentrations of Ag(I) ions, respectively.

Inspired by the synergistic effect of the two emissive centers integrated in one single nanoparticle, we studied the pH response of the synthesized Ag@GS-AuNPs formed with Ag(I) ions from 30.0 to 100.0 µM, respectively (Figure S14). The results showed that the emission intensity at 705 nm increased along with the increase of the pH values (Figure 4A), but the emission of 810 nm did not show significant change, which were different from our previous reported 610/810 nm dual-emissive GS-AuNPs,6 indicating the different formation mechanisms of the two dual-emissive NPs. Interestingly, the ratios of the intensities of 705 nm and 810 nm (I705nm/I810nm) were dependent on the pH values from 4.0 to 12.0, and the I705nm/I810nm were linearly dependent on the pH values from pH 5.0 to 10.0, and the response slopes could be controlled by the amount of Ag(I) ions involved in the formation of Ag@GS-AuNPs (Figure 4B). Therefore, the reaction toward the Ag(I) ions not only formed the dual-emitting Ag@GS-AuNPs,


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but also generated a good ratiometric pH nanoprobe, which would have various potential bioapplications.

Figure 4. A) Luminescence spectra of the dual-emissive Ag@GS-AuNPs formed with 40.0 µM of Ag(I) ions in B-R buffer at different pH values. B) The pH-induced ratiometric luminescence responses of the dual-emissive Ag@GS-AuNPs formed with different concentrations of Ag(I) ions from 30.0 to 100.0 µM.

In summary, we have developed a novel strategy for the facile synthesis of dual-NIRemitting bimetallic Ag@AuNPs from the single NIR-emitting AuNPs through the reaction toward Ag(I) ions under mild conditions. This strategy was based on the low GSH coverage of the NIR-emitting GS-AuNPs that resulted in both AGR reaction and Ag(I)-carboxylate shell formation on the surface of AuNPs, which was responsible for the transformation from single NIR-emitting GS-AuNPs to dual-NIR-emitting Ag@GS-AuNPs. The synthesized dual-NIRemitting Ag@GS-AuNPs exhibited stable optical properties even in the present of ligands that have strong affinity to Ag(I) ions including cysteine and EDTA. In addition, this strategy was demonstrated to exhibit excellent generalization toward the single NIR-emitting AuNPs and can be applicable to various NIR-emitting AuNPs with diversified thiolate surface chemistries containing carboxyl group such as HOOC-PEG-SH and LA. The synthesized dual-NIR-emitting bimetallic Ag@GS-AuNPs was also demonstrated to be a good ultrasmall-sized ratiometric pH


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probe, which showed great potential in the ratiometric imaging and real-time multimode monitoring in the biosystems. Furthermore, the introduction of Ag(I) ions not only transformed the single NIR-emitting GS-AuNPs to dual-NIR-emitting Ag@GS-AuNPs, but also would provide a good potential platform for the design of antimicrobial agents due to the robust antimicrobial activities of the Ag-related nanoparticles.43-45 Therefore, we believe this smart strategy provide an important approach in the tailoring of the optical properties of the metallic or bimetallic NPs to facilitate their potential bioapplications. ASSOCIATED CONTENT ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (Grant 21573078), the 1000 Young Talent Program, Guangdong Natural Science Funds for Distinguished Young Scholars (Grant 2016A030306024), and the Fundamental Research Funds for the Central Universities. Supporting Information. Extra experimental details and supplementary data on the characterizations of the synthesized different AuNPs and Ag@AuNPs. Notes There are no conflicts to declare. REFERENCES (1) Du, B.; Jiang, X.; Das, A.; Zhou, Q.; Yu, M.; Jin, R.; Zheng, J. Glomerular Barrier Behaves as an Atomically Precise Bandpass Filter in a Sub-Nanometre Regime. Nat. Nanotechnol. 2017, 12, 1096-1102. (2) Yao, Q.; Yuan, X.; Fung, V.; Yu, Y.; Leong, D. T.; Jiang, D.; Xie, J. Understanding SeedMediated Growth of Gold Nanoclusters at Molecular Level. Nat. Commun.2017, 8, 927.


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The Reactivity Toward Ag+: A General Strategy to Generate a New

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