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Gold Plating of Silver Nanoparticles for Superior Stability and Preserved Plasmonic and Sensing Properties Nimer Murshid, a Ilya Gourevich, b Neil Coombs b and Vladimir Kitaev a * 5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Gold-plated silver nanoparticles (Au@AgNPs, shell@core) with high stability in various environments detrimental to AgNPs have been produced.
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Silver and gold nanoparticles (AgNPs and AuNPs) are primary chemically stable plasmonic metal nanoparticles (PNPs) that feature diverse applications in catalysis, sensing, and biomedicine.1 PNPs are distinct in their properties from both bulk materials and smaller superatomic metal clusters2 in displaying surface plasmon resonance (SPR). SPR energy can be advantageously tuned by composition, size, and morphology of PNPs, as well as by a refractive index of an ambient dielectric medium.3 Among coinage metals, silver features the highest energy of d-sp transitions with a sharp and strong SPR that can span throughout the entire visible spectrum and NIR (ca. 395 nm to 1300+ nm).4 The strong and sharp SPR of silver makes AgNPs advantageous compared to AuNPs for applications in plasmonic sensing and surface enhanced Raman spectroscopy (SERS). Despite these advantages, many applications of AgNPs, especially in biological systems, are hindered by low AgNP stability in various detrimental environments, e.g. chloride ions commonly found in physiological fluids.5 In contrast, AuNPs feature superior chemical stability, but their major disadvantage is in dampened SPR peaks below ca. 650 nm due to Au d-sp transitions in the visible. Despite the SPR limitations, AuNPs currently remain the only choice of PNPs for a wide range of biomedical applications.6 Consequently, preparation of stable PNPs with strong SPR is essential for versatile applications. Several approaches to protect the surface of AgNPs by depositing a layer of silica,7 TiO28 or gold9 have been reported. Silica coating obtained by common solgel preparations in solution imparts excellent colloidal stability but is porous10 and does not offer chemical protection from small ions such as cyanide and chloride. Non-porous silica coating can be obtained by CVD11 but the process is generally less compatible with colloidal modification in dispersions. Due to galvanic replacement, deposition of Au at the surface of AgNPs commonly causes dissolution of AgNPs resulting in hollow gold particles that do not retain advantageous SPR characteristics of silver.12 In recent reports, Xue’s group achieved gold coating on silver nanoprisms using hydroxylamine as a reducing agent9a and Yin’s group reported a synthetic protocol to prepare stable silver nanoplatelets by lowering the reduction potential of the gold salt through complexation with iodide ions.13 Both approaches rely on the use of additional reagents and are appreciably complex to This journal is © The Royal Society of Chemistry [year]
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be readily adapted to silver morphologies other than nanoprisms. Herein we report a straightforward reliable synthetic protocol to significantly improve stability of AgNPs while retaining their advantageous plasmonic properties. The reported procedure relies upon slow addition of a gold precursor to achieve uniform gold plating of silver decahedral NPs (AgDeNPs) and pentagonal rod NPs (AgPRNPs). Au@AgNPs feature superior stability in media destructive to AgNPs, maintaining their sharp well-defined SPR. Fig. 1 shows the schematics of the gold plating, transmission electron microscopy (TEM) images representative of the smoothness and uniformity of Au@AgNPs, and UV-vis spectra demonstrating preservation of plasmonic properties. The key factor in controlling uniform deposition of gold and minimizing galvanic replacement is lowering the effective reduction potential of gold ions by using slower rates of gold addition to AgNPs.
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Fig. 1 (a) Schematic illustration of preparation of Au@AgDeNPs by slow addition of gold ions to AgDeNPs. TEM images of (b) Au@AgDeNPs and (c) Au@AgPRNPs treated with 0.5 M H2O2. UV-vis spectra§ of (d) precursor AgDeNPs and 10% Au@AgDeNPs; (e) precursor AgPRNPs and 2% Au@AgPRNPs. The scale bars are 100 nm for all TEM images.
Specifically, uniform smooth gold plating on both AgDeNPs and AgPRNPs has been achieved by low rates of gold addition (3.2 nmol/h). The TEM images (Fig. 1b,c and S2), show perfectly preserved morphologies of AgNPs with uniform thin layers of gold coating (ca. 1.5 nm gauging by comparison of EM images of Au-plated and non-plated AgDeNPs and are in agreement with the estimates for the size increase upon plating (see Fig. S2†)) without any pits or voids characteristic of galvanic replacement. Notably, the faceting is nicely preserved, including the most fragile edges between (100) planes in AgNPRs.14 Fig. 1d demonstrates preservation of strong SPR by comparing UV-vis spectra of original AgDeNPs and Au@AgDeNPs after deposition of 10%Au (molar percentage of deposited gold [journal], [year], [vol], 00–00 | 1
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The main cause of Au@AgNP deterioration in different etching environments was found to be colloidal instability due to high ionic strength rather than their chemical dissolution, as illustrated by experiments with 1.5 M L-arginine (Fig. S6†). In these solutions, NP aggregation occurs due to high ionic strength (impediment of charge stabilization). Using higher concentrations of a sterically stabilizing agent, PVP, alleviates aggregation significantly and improves NP stability (Fig. 2a, S5b†). Exposure to H2S and hydrosulphide ions as tarnishing media 2 | Journal Name, [year], [vol], 00–00
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were tested for Au@AgDeNPs. Noteworthy, the integrity of the particles are largely preserved (Fig. S14†) while the SPR is strongly dampened by highly absorbing sulphides forming on the surface, and thus limiting SPR applications in sulphide media. In order to determine an optimal amount of gold plating for both stability and SPR preservation of AgDeNPs, we have performed a series of experiments with varying amounts of Au relative to Ag (Au%). Fig. 3a shows the SPR changes of the Au@AgDeNPs, with the variation of Au%, monitored by UV-vis spectroscopy.
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Fig. 2 Stability of Au@AgDeNPs in aqueous solutions of (a) 0.150 M NaCl (with 1.5 mM PVP), (b) 0.5 M NH3, and (c) 0.5 M H2O2 monitored by UV-vis spectra. (d) Comparative stability of AgDeNPs in 0.5 M H2O2. The insets in (a-c) are TEM images (all scale bars are 100 nm) of Au@AgDeNPs after 10-day exposure to the corresponding conditions.
The SPR intensity of AgDeNPs decreased with the increased Au%. The red shift of the SPR maximum progressed slowly (from ca. 472 to 500 nm) upon increasing Au% up to ca. 40%, whereas subsequent SPR shift increased dramatically to ca. 700 nm after ca. 50% (Fig. S8†). Below the critical Au% boundary of ca. 40%, shown in Fig. S9†, minimal galvanic replacement of silver occurred (virtually none below ca. 20%). In the absence of galvanic replacement, the plasmonic properties of AgNPs are not significantly affected with the expected moderate red shift due to the presence of gold. In addition to the preservation of beneficial SPR properties, Au@AgDeNPs retained AgNP functionality as a substrate for Raman spectroscopy (Fig. S10†). SERS efficiency of Ag@AgDeNPs remains very close to that of AgDeNPs for Au% up to 10% and starts to reduce significantly upon further increase in Au%, dropping almost 100 times above 40%Au (Fig. 3b).
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Fig.3 (a) UV-vis spectra of precursor AgDeNPs and Au@AgDeNPs prepared by addition of various Au%. (b) Comparison of surface enhanced Raman scattering (SERS) efficiency for AgDeNPs and Au@AgDeNPs with different Au% and same Ag concentration (see ESI).
Changes in SPR and SERS efficiency of Au@AgDeNPs for different Au% can be directly related to the uniformity of gold plating and galvanic replacement. 10%Au has been found to be an optimal amount of gold plating based on the combination of chemical stability, minimization of galvanic replacement, and preservation of both SPR and SERS properties (Fig. 1b and S2b). Fig. S9† shows representative TEM images of Au@AgDeNPs This journal is © The Royal Society of Chemistry [year]
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relative to silver in precursor AgNPs). Upon gold deposition, SPR maximum shifts from 472 to 485 nm due to the presence of gold with its longer SPR wavelength. Plasmonic properties of AgDeNPs were retained with a low reduction in intensity (ca. 15%) of the main SPR peak. The transverse SPR peak at ca. 400 nm was affected more greatly due to the dampening of gold d-sp transitions. Preservation of SPR peak intensities and a relatively small SPR shift are indicative of the minimization of galvanic replacement and uniform deposition of a thin layer of gold. The gold layer deposition at the AgDeNP was quantitatively confirmed by EDX (Fig. S3†). Given the plasmon excitation depth of ca. 3 nm,15 alloying on the surface of Au@AgDeNPs cannot be excluded and requires further studies. AgDeNPs surface is confined by stable (111) facets. We have further investigated applicability of the developed plating protocol to AgPRNPs with (100) facets that are known to be more sensitive to degradation.16 The stability and strong SPR were indeed preserved for coating of AgPRNPs, as evident from UV-vis spectra of AgPRNPs before and after deposition of 2%Au (Fig. 1e). Similarly to AgDeNPs, the transverse SPR peak at ca. 400 nm has been more strongly affected. AgDeNPs and AgPRNPs were synthesized in high yield with shape- and size- selectivity using new-generation synthetic protocols developed from our previous work.16 Uniform gold deposition has been accomplished by controlled slow addition of HAuCl4 solution to AgNP solution containing citrate and polyvinylpyrrolidone (PVP) as charge and sterically stabilizing reagents, respectively. Detailed procedures for the synthesis of AgDeNPs, AgPRNPs and Au@AgDeNPs are provided in ESI†. SPR changes can be sensitively monitored by UV-vis spectroscopy to characterize the stability of Au@AgDeNPs in etching media. Fig. 2a-c shows the long-term stability of the prepared Au@AgDeNPs in aqueous solutions of NaCl (150 mM), NH3 (0.5 M), and H2O2 (0.5 M), respectively. The stability was tested for several weeks to assure compatibility with long-term applications. As shown in Fig. 2d and Fig.S4, uncoated AgDeNPs dissolve completely after several minutes in the etching reagents used, while Au@AgDeNPs remain stable with only some minor degradation observed for 0.15 M chloride solution (one of the main components of biological fluids and buffers). The stability of the prepared Au@AgDeNPs is further corroborated by their TEM images shown in the inset of Fig. 3a-c. Additional TEM evidence illustrating the stability of Au@AgDeNPs at higher concentrations of NaCl (0.30 M) and NH3 (1.5 M) solutions are given in Fig. S5†. As an demonstration of Au@AgDeNPs stability of in aggressive basic medium, such as NH3, silica encapsulation of these MNPs with perfect shape preservation has been achieved (Fig. S13†). Au@AgPRNPs chemical stability can be attested by their resistance to 0.5 M H2O2 solution (Fig. S7†).
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Fig. 4 UV-vis spectra and TEM images of 60%Au@AgDeNPs (shell@core) produced (1) with citrate and (2) without citrate. Optical photograph insets in TEM images illustrate the actual colour of Au@AgDeNP solutions. The scale bars are 100 nm for TEM images.
We have preliminarily explored the conditions to form the most uniform gold shells of AgDeNPs and found that slow addition of gold tetrachloroaurate together with citrate produces high-quality shells (Fig. 4). The sharpest SPR peaks of these gold shells (at ca. 700 nm) were attained using 60-80Au%. For the explored system, samples with >80%Au had secondary nucleation of gold (Fig. S12 in ESI†). The future research will be directed at the optimization of plasmonic properties of the gold shells templated from AgDeNPs and AgNPRs and achieving uniform gold plating at higher Au% and modelling plasmonic properties in the system. In summary, a reliable synthetic protocol to produce protective gold plating on AgNPs has been developed and described. The plating procedure was performed reliably in many hundreds of runs and can be readily scaled. The developed 10%Au@AgDeNPs both retain sharp and strong SPR and SERS enhancement, and display superior stability in conditions detrimental to AgNPs including 1.5 M NH3, 0.5 M H2O2, and 150 This journal is © The Royal Society of Chemistry [year]
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mM NaCl. The reported preparation of gold-plated AgNPs has achieved an advantageous combination of properties that should prove useful for different biochemical and optical applications. The authors are grateful to NSERC and the Government of Ontario (ERA) for financial support. Dilyn Keogh, Rachel Keunen, and Matthew McEachran are acknowledged for initial developments of the project. Krysten Hobbs is acknowledged with the manuscript proofreading.
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Chemistry Department, Wilfrid Laurier University, 75 University Ave. W, Waterloo, Ontario, Canada, N2L 3C5. E-mail: [email protected] b Centre for Nanostructure Imaging, Chemistry Department, University of Toronto, 80 St George Street, Toronto, Ontario, Canada M5S 3H6. † Electronic Supplementary Information (ESI) available: Experimental details and additional UV-vis spectra, EDX data and electron microscopy images. See DOI: 10.1039/b000000x/ § UV-vis spectra were measured in a 0.5-cm path length cell with the total silver concentration of 0.064 mM for all samples. 1 (a) J. Zeng J., Q. Zhang, J. Chen and Y. Xia, Y., Nano Lett., 2010, 10, 30–35; (b) Y. Xia, W. Li, C. M. Cobley, J. Chen, X. Xia, Q. Zhang, M. Yang, E. C. Cho and P. K. Brown, Acc. Chem. Res. 2011, 44, 914–924; (c) N. G. Greeneltch, M. G. Blaber, J. A. Dieringer, G. C. Schatz and R. Van Duyne, J. Phys. Chem. C, 2013,117, 2554-2558; (e) N. Cathcart and V. Kitaev, Nanoscale, 2012, 4, 6981-6989. 2 Hakkinen, H.; Whetten, R. L. J. Phys. Chem. C 2011, 114, 1587715878. 3 (a) B. J. Wiley, S. Im, Z. Li, J. McLellan, A. Siekkinen and Y. Xia, J. Phys. Chem. B, 2006, 110, 15666-15675; (b) A. Frank, N. Cathcart, K. Maly and V. Kitaev, J. Chem. Educ., 2010, 87, 1098-1101, (c) M. McEachran, and V. Kitaev, Chem. Commun., 2008, 44, 5737-5739. 4 (a) B. Wiley, Y. Sun, B. Mayers, Y. Xia, Chem. Eur. J., 2005, 11,454 – 463; (b) I. Pastoriza-Santos, L. M. Liz-Marzan, J. Mater. Chem., 2008, 18, 1724 – 1737; (c) N. Cathcart, A. J. Frank and V. Kitaev, Chem. Commun., 2009, 46, 7170-7172. (d) M. Rycenga, C. Cobley, J. Zeng, W. Li, C. Moran, Q. Zhang, D. Qin, Y. Xia, Chem. Rev. 2011, 111, 3669-3712. 5 J. An, B. Tang, X. Zheng, J. Zhou, F. Dong, S. Xu, Y. Wang, B. Zhao, W. Xu, J. Phys. Chem. C, 2008, 112, 15176 – 15182. 6 C. M. Cobley, J. Chen, E. C. Cho, L. V. Wang and Y. Xia, Chem. Soc. Rev., 2011, 40, 44–56. 7 (a) C. Xue, X. Chen, S. J. Hurst, C. A. Mirkin, Adv. Mater., 2007, 19, 4071-4074; (b) G. Sotiriou, T. Sannomiya, A. Teleki, F. Krumeich, J. Vo ro s and S. E. Pratsinis, Adv. Funct. Mater., 2010, 20, 4250−4257. 8 S. Standridge, G. C. Schatz, and J. T. Hupp, Langmuir 2009, 25, 2596-2600. 9 (a) M. M. Shahjamali, M. Bosman, S. Cao, X. Huang, S. Saadat, E. Martinsson, D. Aili, Y. Y. Tay, B. Liedberg, S. C. J. Loo, H. Zhang, F. Boey, C. Xue, Adv. Funct. Mater., 2012, 22, 849 – 854; (b) P. Dong, Y. Lin, J. Deng and J. Di, ACS Appl. Mater. Interfaces, 2013, 5, 2392-2399. 10 S. Poovarodom, J.D. Bass, S. J. Hwang, A. Katz. Langmuir, 2005, 21,12348-12356. 11 B. Hatton, J. Aizenberg, V. Kitaev. D. D. Perovic, G. A. Ozin, J. Mater. Chem. 2010, 20, 6009-6013. 12 (a) Y .Ma, W. Li, E. C. Cho, Z. Li, T. Yu, J. Zeng, Z. Xie and Y. Xia, ACS Nano, 2010, 4, 6725-6734; (b) X. Xia and Y. Xia, Front. Phys., 2013, DOI 10.1007/s11467-013-0318-8 (c) Y. Choi, S. Hong, L. Liu, S. K. Kim and S. Park, Langmuir, 2012, 28, 6670-6676. 13 C. Gao, Z. Lu, Y. Liu, Q. Zhang, M. Chi, Q. Cheng and Y. Yin, Angew. Chem. Int. Ed., 2012, 51, 1–6. 14 M. McEachran, D. Keogh, B. Pietrobon, N. Cathcart, I. Gourevich, N. Coombs and V. Kitaev, J. Am. Chem. Soc., 2011, 133, 8066-8069. 15 Y.Y Ma, W.Y. Li, E.C. Cho, Z.Y. Li, T.K. Yu, J. Zeng, Z.X. Xie, Y.N. Xia, ACS Nano, 2010, 4, 6725-6734. 16 N. Murshid, D. Keogh, V. Kitaev, Part. Part. Syst. Charact. 2013, DOI: 10.1002/ppsc.201300225.
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prepared using varied Au% and treated with H2O2. For Au% < 10%, the amount of gold is not sufficient for uniform deposition to protect the entire surface of AgDeNPs which results in dissolution of unprotected silver upon exposure to H2O2. Increasing Au% above 10% leads to increasing formation of hollow Au-AgNPs due to galvanic replacement. The galvanic replacement becomes noticeable as minor pitting at ca. 20Au%, and hollow nanoparticles become the predominant structures starting at >40Au% (Fig. S9e,f†). To counteract the galvanic replacement of silver and to attain smooth uniform coating at higher Au%, several routes have been explored including the use of mild reducing agents, such as ascorbate and formate; complexing agents, such as KI and citrate; and adjusting the pH. At higher pH (with added KOH), addition of tetrachloroaurate led to increased reduction of the excess gold ions in solution and secondary seeding with the formation of worm-like AuNPs (Fig. S11a†). When ascorbate was used as a reducing agent, the reduction of gold ions occurred predominantly at the edges of AgDeNPs, as well as in the solution. As a result, ill-defined gold frames were the main product after etching with hydrogen peroxide (Fig. S11b†). Similarly, addition of formate could not prevent the galvanic replacement and formation of hollow nanoparticles (Fig. S11c†). In another series of experiments, KI was introduced to form complexes with the gold ions13 prior to their addition to AgDeNPs. We could not achieve uniform protective coating under these conditions: less-defined morphologies were observed after treatment of Au@AgDeNPs with H2O2 (Fig. S11d†). We attribute the difficulties to achieve uniform gold coating at higher Au% to increasing amounts of chloride and enhanced rates of gold deposition after the formation of an initial gold coating.
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Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: N. Murshid, I. Gourevich, N. Coombs and V. Kitaev, Chem. Commun., 2013, DOI: 10.1039/C3CC46075D.
Volume 46 | Number 32 | 2010
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This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication. Chemical Communications www.rsc.org/chemcomm
Volume 46 | Number 32 | 28 August 2010 | Pages 5813–5976
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Accepted Manuscripts are published online shortly after acceptance, which is prior to technical editing, formatting and proof reading. This free service from RSC Publishing allows authors to make their results available to the community, in citable form, before publication of the edited article. This Accepted Manuscript will be replaced by the edited and formatted Advance Article as soon as this is available. To cite this manuscript please use its permanent Digital Object Identifier (DOI®), which is identical for all formats of publication.
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COMMUNICATION J. Fraser Stoddart et al. Directed self-assembly of a ring-in-ring complex
FEATURE ARTICLE Wenbin Lin et al. Hybrid nanomaterials for biomedical applications
1359-7345(2010)46:32;1-H
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
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Gold Plating of Silver Nanoparticles for Superior Stability and Preserved Plasmonic and Sensing Properties Nimer Murshid, a Ilya Gourevich, b Neil Coombs b and Vladimir Kitaev a * 5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Gold-plated silver nanoparticles (Au@AgNPs, shell@core) with high stability in various environments detrimental to AgNPs have been produced.
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Silver and gold nanoparticles (AgNPs and AuNPs) are primary chemically stable plasmonic metal nanoparticles (PNPs) that feature diverse applications in catalysis, sensing, and biomedicine.1 PNPs are distinct in their properties from both bulk materials and smaller superatomic metal clusters2 in displaying surface plasmon resonance (SPR). SPR energy can be advantageously tuned by composition, size, and morphology of PNPs, as well as by a refractive index of an ambient dielectric medium.3 Among coinage metals, silver features the highest energy of d-sp transitions with a sharp and strong SPR that can span throughout the entire visible spectrum and NIR (ca. 395 nm to 1300+ nm).4 The strong and sharp SPR of silver makes AgNPs advantageous compared to AuNPs for applications in plasmonic sensing and surface enhanced Raman spectroscopy (SERS). Despite these advantages, many applications of AgNPs, especially in biological systems, are hindered by low AgNP stability in various detrimental environments, e.g. chloride ions commonly found in physiological fluids.5 In contrast, AuNPs feature superior chemical stability, but their major disadvantage is in dampened SPR peaks below ca. 650 nm due to Au d-sp transitions in the visible. Despite the SPR limitations, AuNPs currently remain the only choice of PNPs for a wide range of biomedical applications.6 Consequently, preparation of stable PNPs with strong SPR is essential for versatile applications. Several approaches to protect the surface of AgNPs by depositing a layer of silica,7 TiO28 or gold9 have been reported. Silica coating obtained by common solgel preparations in solution imparts excellent colloidal stability but is porous10 and does not offer chemical protection from small ions such as cyanide and chloride. Non-porous silica coating can be obtained by CVD11 but the process is generally less compatible with colloidal modification in dispersions. Due to galvanic replacement, deposition of Au at the surface of AgNPs commonly causes dissolution of AgNPs resulting in hollow gold particles that do not retain advantageous SPR characteristics of silver.12 In recent reports, Xue’s group achieved gold coating on silver nanoprisms using hydroxylamine as a reducing agent9a and Yin’s group reported a synthetic protocol to prepare stable silver nanoplatelets by lowering the reduction potential of the gold salt through complexation with iodide ions.13 Both approaches rely on the use of additional reagents and are appreciably complex to This journal is © The Royal Society of Chemistry [year]
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be readily adapted to silver morphologies other than nanoprisms. Herein we report a straightforward reliable synthetic protocol to significantly improve stability of AgNPs while retaining their advantageous plasmonic properties. The reported procedure relies upon slow addition of a gold precursor to achieve uniform gold plating of silver decahedral NPs (AgDeNPs) and pentagonal rod NPs (AgPRNPs). Au@AgNPs feature superior stability in media destructive to AgNPs, maintaining their sharp well-defined SPR. Fig. 1 shows the schematics of the gold plating, transmission electron microscopy (TEM) images representative of the smoothness and uniformity of Au@AgNPs, and UV-vis spectra demonstrating preservation of plasmonic properties. The key factor in controlling uniform deposition of gold and minimizing galvanic replacement is lowering the effective reduction potential of gold ions by using slower rates of gold addition to AgNPs.
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Fig. 1 (a) Schematic illustration of preparation of Au@AgDeNPs by slow addition of gold ions to AgDeNPs. TEM images of (b) Au@AgDeNPs and (c) Au@AgPRNPs treated with 0.5 M H2O2. UV-vis spectra§ of (d) precursor AgDeNPs and 10% Au@AgDeNPs; (e) precursor AgPRNPs and 2% Au@AgPRNPs. The scale bars are 100 nm for all TEM images.
Specifically, uniform smooth gold plating on both AgDeNPs and AgPRNPs has been achieved by low rates of gold addition (3.2 nmol/h). The TEM images (Fig. 1b,c and S2), show perfectly preserved morphologies of AgNPs with uniform thin layers of gold coating (ca. 1.5 nm gauging by comparison of EM images of Au-plated and non-plated AgDeNPs and are in agreement with the estimates for the size increase upon plating (see Fig. S2†)) without any pits or voids characteristic of galvanic replacement. Notably, the faceting is nicely preserved, including the most fragile edges between (100) planes in AgNPRs.14 Fig. 1d demonstrates preservation of strong SPR by comparing UV-vis spectra of original AgDeNPs and Au@AgDeNPs after deposition of 10%Au (molar percentage of deposited gold [journal], [year], [vol], 00–00 | 1
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The main cause of Au@AgNP deterioration in different etching environments was found to be colloidal instability due to high ionic strength rather than their chemical dissolution, as illustrated by experiments with 1.5 M L-arginine (Fig. S6†). In these solutions, NP aggregation occurs due to high ionic strength (impediment of charge stabilization). Using higher concentrations of a sterically stabilizing agent, PVP, alleviates aggregation significantly and improves NP stability (Fig. 2a, S5b†). Exposure to H2S and hydrosulphide ions as tarnishing media 2 | Journal Name, [year], [vol], 00–00
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were tested for Au@AgDeNPs. Noteworthy, the integrity of the particles are largely preserved (Fig. S14†) while the SPR is strongly dampened by highly absorbing sulphides forming on the surface, and thus limiting SPR applications in sulphide media. In order to determine an optimal amount of gold plating for both stability and SPR preservation of AgDeNPs, we have performed a series of experiments with varying amounts of Au relative to Ag (Au%). Fig. 3a shows the SPR changes of the Au@AgDeNPs, with the variation of Au%, monitored by UV-vis spectroscopy.
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Fig. 2 Stability of Au@AgDeNPs in aqueous solutions of (a) 0.150 M NaCl (with 1.5 mM PVP), (b) 0.5 M NH3, and (c) 0.5 M H2O2 monitored by UV-vis spectra. (d) Comparative stability of AgDeNPs in 0.5 M H2O2. The insets in (a-c) are TEM images (all scale bars are 100 nm) of Au@AgDeNPs after 10-day exposure to the corresponding conditions.
The SPR intensity of AgDeNPs decreased with the increased Au%. The red shift of the SPR maximum progressed slowly (from ca. 472 to 500 nm) upon increasing Au% up to ca. 40%, whereas subsequent SPR shift increased dramatically to ca. 700 nm after ca. 50% (Fig. S8†). Below the critical Au% boundary of ca. 40%, shown in Fig. S9†, minimal galvanic replacement of silver occurred (virtually none below ca. 20%). In the absence of galvanic replacement, the plasmonic properties of AgNPs are not significantly affected with the expected moderate red shift due to the presence of gold. In addition to the preservation of beneficial SPR properties, Au@AgDeNPs retained AgNP functionality as a substrate for Raman spectroscopy (Fig. S10†). SERS efficiency of Ag@AgDeNPs remains very close to that of AgDeNPs for Au% up to 10% and starts to reduce significantly upon further increase in Au%, dropping almost 100 times above 40%Au (Fig. 3b).
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Fig.3 (a) UV-vis spectra of precursor AgDeNPs and Au@AgDeNPs prepared by addition of various Au%. (b) Comparison of surface enhanced Raman scattering (SERS) efficiency for AgDeNPs and Au@AgDeNPs with different Au% and same Ag concentration (see ESI).
Changes in SPR and SERS efficiency of Au@AgDeNPs for different Au% can be directly related to the uniformity of gold plating and galvanic replacement. 10%Au has been found to be an optimal amount of gold plating based on the combination of chemical stability, minimization of galvanic replacement, and preservation of both SPR and SERS properties (Fig. 1b and S2b). Fig. S9† shows representative TEM images of Au@AgDeNPs This journal is © The Royal Society of Chemistry [year]
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relative to silver in precursor AgNPs). Upon gold deposition, SPR maximum shifts from 472 to 485 nm due to the presence of gold with its longer SPR wavelength. Plasmonic properties of AgDeNPs were retained with a low reduction in intensity (ca. 15%) of the main SPR peak. The transverse SPR peak at ca. 400 nm was affected more greatly due to the dampening of gold d-sp transitions. Preservation of SPR peak intensities and a relatively small SPR shift are indicative of the minimization of galvanic replacement and uniform deposition of a thin layer of gold. The gold layer deposition at the AgDeNP was quantitatively confirmed by EDX (Fig. S3†). Given the plasmon excitation depth of ca. 3 nm,15 alloying on the surface of Au@AgDeNPs cannot be excluded and requires further studies. AgDeNPs surface is confined by stable (111) facets. We have further investigated applicability of the developed plating protocol to AgPRNPs with (100) facets that are known to be more sensitive to degradation.16 The stability and strong SPR were indeed preserved for coating of AgPRNPs, as evident from UV-vis spectra of AgPRNPs before and after deposition of 2%Au (Fig. 1e). Similarly to AgDeNPs, the transverse SPR peak at ca. 400 nm has been more strongly affected. AgDeNPs and AgPRNPs were synthesized in high yield with shape- and size- selectivity using new-generation synthetic protocols developed from our previous work.16 Uniform gold deposition has been accomplished by controlled slow addition of HAuCl4 solution to AgNP solution containing citrate and polyvinylpyrrolidone (PVP) as charge and sterically stabilizing reagents, respectively. Detailed procedures for the synthesis of AgDeNPs, AgPRNPs and Au@AgDeNPs are provided in ESI†. SPR changes can be sensitively monitored by UV-vis spectroscopy to characterize the stability of Au@AgDeNPs in etching media. Fig. 2a-c shows the long-term stability of the prepared Au@AgDeNPs in aqueous solutions of NaCl (150 mM), NH3 (0.5 M), and H2O2 (0.5 M), respectively. The stability was tested for several weeks to assure compatibility with long-term applications. As shown in Fig. 2d and Fig.S4, uncoated AgDeNPs dissolve completely after several minutes in the etching reagents used, while Au@AgDeNPs remain stable with only some minor degradation observed for 0.15 M chloride solution (one of the main components of biological fluids and buffers). The stability of the prepared Au@AgDeNPs is further corroborated by their TEM images shown in the inset of Fig. 3a-c. Additional TEM evidence illustrating the stability of Au@AgDeNPs at higher concentrations of NaCl (0.30 M) and NH3 (1.5 M) solutions are given in Fig. S5†. As an demonstration of Au@AgDeNPs stability of in aggressive basic medium, such as NH3, silica encapsulation of these MNPs with perfect shape preservation has been achieved (Fig. S13†). Au@AgPRNPs chemical stability can be attested by their resistance to 0.5 M H2O2 solution (Fig. S7†).
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DOI: 10.1039/C3CC46075D
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Fig. 4 UV-vis spectra and TEM images of 60%Au@AgDeNPs (shell@core) produced (1) with citrate and (2) without citrate. Optical photograph insets in TEM images illustrate the actual colour of Au@AgDeNP solutions. The scale bars are 100 nm for TEM images.
We have preliminarily explored the conditions to form the most uniform gold shells of AgDeNPs and found that slow addition of gold tetrachloroaurate together with citrate produces high-quality shells (Fig. 4). The sharpest SPR peaks of these gold shells (at ca. 700 nm) were attained using 60-80Au%. For the explored system, samples with >80%Au had secondary nucleation of gold (Fig. S12 in ESI†). The future research will be directed at the optimization of plasmonic properties of the gold shells templated from AgDeNPs and AgNPRs and achieving uniform gold plating at higher Au% and modelling plasmonic properties in the system. In summary, a reliable synthetic protocol to produce protective gold plating on AgNPs has been developed and described. The plating procedure was performed reliably in many hundreds of runs and can be readily scaled. The developed 10%Au@AgDeNPs both retain sharp and strong SPR and SERS enhancement, and display superior stability in conditions detrimental to AgNPs including 1.5 M NH3, 0.5 M H2O2, and 150 This journal is © The Royal Society of Chemistry [year]
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mM NaCl. The reported preparation of gold-plated AgNPs has achieved an advantageous combination of properties that should prove useful for different biochemical and optical applications. The authors are grateful to NSERC and the Government of Ontario (ERA) for financial support. Dilyn Keogh, Rachel Keunen, and Matthew McEachran are acknowledged for initial developments of the project. Krysten Hobbs is acknowledged with the manuscript proofreading.
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Chemistry Department, Wilfrid Laurier University, 75 University Ave. W, Waterloo, Ontario, Canada, N2L 3C5. E-mail: [email protected] b Centre for Nanostructure Imaging, Chemistry Department, University of Toronto, 80 St George Street, Toronto, Ontario, Canada M5S 3H6. † Electronic Supplementary Information (ESI) available: Experimental details and additional UV-vis spectra, EDX data and electron microscopy images. See DOI: 10.1039/b000000x/ § UV-vis spectra were measured in a 0.5-cm path length cell with the total silver concentration of 0.064 mM for all samples. 1 (a) J. Zeng J., Q. Zhang, J. Chen and Y. Xia, Y., Nano Lett., 2010, 10, 30–35; (b) Y. Xia, W. Li, C. M. Cobley, J. Chen, X. Xia, Q. Zhang, M. Yang, E. C. Cho and P. K. Brown, Acc. Chem. Res. 2011, 44, 914–924; (c) N. G. Greeneltch, M. G. Blaber, J. A. Dieringer, G. C. Schatz and R. Van Duyne, J. Phys. Chem. C, 2013,117, 2554-2558; (e) N. Cathcart and V. Kitaev, Nanoscale, 2012, 4, 6981-6989. 2 Hakkinen, H.; Whetten, R. L. J. Phys. Chem. C 2011, 114, 1587715878. 3 (a) B. J. Wiley, S. Im, Z. Li, J. McLellan, A. Siekkinen and Y. Xia, J. Phys. Chem. B, 2006, 110, 15666-15675; (b) A. Frank, N. Cathcart, K. Maly and V. Kitaev, J. Chem. Educ., 2010, 87, 1098-1101, (c) M. McEachran, and V. Kitaev, Chem. Commun., 2008, 44, 5737-5739. 4 (a) B. Wiley, Y. Sun, B. Mayers, Y. Xia, Chem. Eur. J., 2005, 11,454 – 463; (b) I. Pastoriza-Santos, L. M. Liz-Marzan, J. Mater. Chem., 2008, 18, 1724 – 1737; (c) N. Cathcart, A. J. Frank and V. Kitaev, Chem. Commun., 2009, 46, 7170-7172. (d) M. Rycenga, C. Cobley, J. Zeng, W. Li, C. Moran, Q. Zhang, D. Qin, Y. Xia, Chem. Rev. 2011, 111, 3669-3712. 5 J. An, B. Tang, X. Zheng, J. Zhou, F. Dong, S. Xu, Y. Wang, B. Zhao, W. Xu, J. Phys. Chem. C, 2008, 112, 15176 – 15182. 6 C. M. Cobley, J. Chen, E. C. Cho, L. V. Wang and Y. Xia, Chem. Soc. Rev., 2011, 40, 44–56. 7 (a) C. Xue, X. Chen, S. J. Hurst, C. A. Mirkin, Adv. Mater., 2007, 19, 4071-4074; (b) G. Sotiriou, T. Sannomiya, A. Teleki, F. Krumeich, J. Vo ro s and S. E. Pratsinis, Adv. Funct. Mater., 2010, 20, 4250−4257. 8 S. Standridge, G. C. Schatz, and J. T. Hupp, Langmuir 2009, 25, 2596-2600. 9 (a) M. M. Shahjamali, M. Bosman, S. Cao, X. Huang, S. Saadat, E. Martinsson, D. Aili, Y. Y. Tay, B. Liedberg, S. C. J. Loo, H. Zhang, F. Boey, C. Xue, Adv. Funct. Mater., 2012, 22, 849 – 854; (b) P. Dong, Y. Lin, J. Deng and J. Di, ACS Appl. Mater. Interfaces, 2013, 5, 2392-2399. 10 S. Poovarodom, J.D. Bass, S. J. Hwang, A. Katz. Langmuir, 2005, 21,12348-12356. 11 B. Hatton, J. Aizenberg, V. Kitaev. D. D. Perovic, G. A. Ozin, J. Mater. Chem. 2010, 20, 6009-6013. 12 (a) Y .Ma, W. Li, E. C. Cho, Z. Li, T. Yu, J. Zeng, Z. Xie and Y. Xia, ACS Nano, 2010, 4, 6725-6734; (b) X. Xia and Y. Xia, Front. Phys., 2013, DOI 10.1007/s11467-013-0318-8 (c) Y. Choi, S. Hong, L. Liu, S. K. Kim and S. Park, Langmuir, 2012, 28, 6670-6676. 13 C. Gao, Z. Lu, Y. Liu, Q. Zhang, M. Chi, Q. Cheng and Y. Yin, Angew. Chem. Int. Ed., 2012, 51, 1–6. 14 M. McEachran, D. Keogh, B. Pietrobon, N. Cathcart, I. Gourevich, N. Coombs and V. Kitaev, J. Am. Chem. Soc., 2011, 133, 8066-8069. 15 Y.Y Ma, W.Y. Li, E.C. Cho, Z.Y. Li, T.K. Yu, J. Zeng, Z.X. Xie, Y.N. Xia, ACS Nano, 2010, 4, 6725-6734. 16 N. Murshid, D. Keogh, V. Kitaev, Part. Part. Syst. Charact. 2013, DOI: 10.1002/ppsc.201300225.
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prepared using varied Au% and treated with H2O2. For Au% < 10%, the amount of gold is not sufficient for uniform deposition to protect the entire surface of AgDeNPs which results in dissolution of unprotected silver upon exposure to H2O2. Increasing Au% above 10% leads to increasing formation of hollow Au-AgNPs due to galvanic replacement. The galvanic replacement becomes noticeable as minor pitting at ca. 20Au%, and hollow nanoparticles become the predominant structures starting at >40Au% (Fig. S9e,f†). To counteract the galvanic replacement of silver and to attain smooth uniform coating at higher Au%, several routes have been explored including the use of mild reducing agents, such as ascorbate and formate; complexing agents, such as KI and citrate; and adjusting the pH. At higher pH (with added KOH), addition of tetrachloroaurate led to increased reduction of the excess gold ions in solution and secondary seeding with the formation of worm-like AuNPs (Fig. S11a†). When ascorbate was used as a reducing agent, the reduction of gold ions occurred predominantly at the edges of AgDeNPs, as well as in the solution. As a result, ill-defined gold frames were the main product after etching with hydrogen peroxide (Fig. S11b†). Similarly, addition of formate could not prevent the galvanic replacement and formation of hollow nanoparticles (Fig. S11c†). In another series of experiments, KI was introduced to form complexes with the gold ions13 prior to their addition to AgDeNPs. We could not achieve uniform protective coating under these conditions: less-defined morphologies were observed after treatment of Au@AgDeNPs with H2O2 (Fig. S11d†). We attribute the difficulties to achieve uniform gold coating at higher Au% to increasing amounts of chloride and enhanced rates of gold deposition after the formation of an initial gold coating.
