Reversible Assembly and Dynamic Plasmonic Tuning of Ag

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Reversible assembly and dynamic plasmonic tuning of Ag nanoparticles enabled by limited ligand protection Luntao Liu, Zongpeng Gao, Baolai Jiang, Yaocai Bai, Wenshou Wang, and Yadong Yin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02325 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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Reversible assembly and dynamic plasmonic tuning of Ag nanoparticles enabled by limited ligand protection †

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Luntao Liu, Zongpeng Gao, Baolai Jiang, Yaocai Bai, Wenshou Wang*, Yadong Yin*

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†National Engineering Research Center for Colloidal Materials and School of Chemistry and Chemical Engineering, Shandong University, Ji’Nan 250100, P. R. China. ‡Department of Chemistry, University of California, Riverside CA 92521 USA.

*Corresponding Author: E-mail address: [email protected], [email protected].

ABSTRACT: Dynamic manipulation of optical property through the reversible assembly of plasmonic nanoparticles offers great opportunities for practical applications in many fields. The previous success, however, has been limited to Au nanoparticles. Reversible assembly and plasmonic tuning of Ag nanoparticles (AgNPs) have remained a significant challenge due to the difficulty in finding an appropriate surface agent that can effectively stabilize the particle surface and control their interactions. Here we overcome the challenge by developing a limited-ligandprotection (LLP) strategy for introducing polyacrylic acid with precisely controlled coverage to AgNP surface to not only sufficiently stabilize the nanoparticles but also enable effective control over the surface charge and particle interaction through pH variation. The as-synthesized AgNPs

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can be reversibly assembled and disassembled and accordingly display broadly tunable coupling of plasmonic properties. Compared with the Au-based system, the success in the reversible assembly of AgNPs represents a significant step toward practical applications such as colorimetric pressure sensing as they offer many advantages including broader spectral tuning range, higher color contrast, one-pot process, and low materials and production cost. This work also highlights LLP as a new avenue to controlling the interparticle forces, their reversible assembly, and dynamic coupling of physical properties.

KEYWORDS: Silver nanoparticles, assemblies, plasmon coupling, limited ligand protection, reversible self-assembly. Colloidal metal nanoparticles (NPs) have been investigated for a wide range of applications such as optoelectronics and biomedical diagnosis thanks to their unique optical properties induced by localized surface plasmon resonance (LSPR).1-7 Since LSPR is strongly dependent on particle size and shape, extensive efforts have been made conventionally to control the dimension and morphology of noble metal NPs.8-12

On the other hand, the assembly of

plasmonic NPs into secondary structures has been found to allow near-field coupling of surface plasmons among adjacent particles and therefore exhibit collective properties that are difficult to obtain in individual particles.13-20 An essential feature of such secondary structures is that the plasmonic properties of NP assemblies can be dynamically controlled by manipulating their interparticle separation, and therefore produce stimuli-responsive optical materials which hold great promises for applications such as color signage, bio- and chemical detection, and environmental sensing.21-27

It is therefore highly desirable to develop effective strategies


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towards reversible assembly and dynamic tuning of the surface plasmonic coupling of metal NPs. Reversible assembly has been demonstrated in Au nanoparticles by manipulating the particle interactions using various methods, for example, by changing the ionic strength of the solution, adding thiol-ligands, using a photoswitchable medium, or reversible linking through DNA molecules.28-32 It however has remained a great challenge to extend the success to Ag nanoparticles (AgNPs) mainly due to the relatively reactive nature of Ag and consequently low chemical stability of the nanoparticle surface.33,

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However, the assembly of AgNPs has its

significance as silver possesses the highest plasmonic activity in terms of quality factor across most of the spectrum.35 Furthermore, due to its high interband transition energy, Ag supports surface plasmons in the blue-UV region of the spectrum.36 Upon assembly, the plasmon resonance of AgNPs could expand across the whole visible spectrum, promising for applications requiring broad coverage in the visible/near infrared (NIR) range. The first challenge in achieving reversible assembly is to find an appropriate capping agent that can bind robustly to the surface of AgNPs. Although many ligands such as those containing thiol and amine groups can bind to Ag surface strongly, under oxidative conditions, they may promote the oxidation of Ag on the surface and therefore detach from there.37, 38 In this work, we address this issue by employing a polymeric ligand, polyacrylic acid (PAA), which attach to the AgNP surface robustly through its multidentate anchoring points. The relatively weak bond between Ag and individual carboxyl group ensures minimal surface oxidation and consequently no substantial ligand detachment when the system is exposed to air.39 In addition, the choice of PAA is also motivated by the ability that uncoordinated carboxyl groups on the polymer chains allow effective manipulation of the surface charge, e.g., by varying the pH of the solution,40 and


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thus enable reversible tuning of the surface plasmon coupling. For effective regulation of the particle interactions, we have developed a limited ligand protected (LLP) growth strategy to control the density of surface ligands (and therefore surface charges) of AgNPs and further demonstrated the assembly of the resulting nanoparticles into secondary structures with tunable plasmonic properties ranging from 400 nm to NIR. The assembled AgNPs can be disassembled reversibly through the manipulation of surface charge by pH variation, which enables the reversible tuning of surface plasmon coupling. The LLP growth strategy was previously reported for the synthesis of metal oxide complex nanostructures by reducing the degree of ligand protection to induce three-dimensional (3D) oriented attachment of NPs.41, 42 In our design, the LLP growth is employed to decrease the density of ligand (charge) distribution on AgNPs’ surface during their growth, thus weaken the interparticle electrostatic repulsion and promote their self-assembly. A typical synthesis involves the injection of a solution containing 100 mg (0.59 mmol) of AgNO3 in 3 mL of ethylene (EG) into a mixture containing 32 mg (0.018 mmol) PAA and 15 mL ethylene glycol preheated to ~195 °C under N2 protection (see details in the Experimental Section). With sufficient amount of PAA (> 180 mg), this process leads to the formation of AgNPs that can be dispersed well in water due to sufficient interparticle repulsion. In the LLP growth model, however, by limiting the amount of PAA on the particle surface, we can reduce the interparticle repulsion so that their dispersion in water becomes unstable and they aggregate into secondary structures. The varying amount of PAA on the nanoparticle surface also determines the interparticle distance which, in turn, controls the plasmonic coupling of the AgNPs and the color of the solution. RESULTS AND DISCUSSION


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As shown in Figure 1, with a limited amount of PAA (32 mg), the LLP growth leads to AgNP assemblies in water with broadly tunable plasmonic bands depending on the reaction time. At a short reaction time of 2 min, AgNPs were produced with a sharp plasmonic peak around at 400 nm corresponding to the excitation of SPR of isolated spherical AgNPs (Figure 1a). Increasing the reaction time from 2 to 12 min gradually red-shifted the absorption peak of the products from ~ 400 nm to 730 nm with absorption shoulder extended to NIR (~1000 nm). Figure 1b shows the corresponding digital photographs of the dispersions exhibiting bright colors of yellow, brown, red, purple, and blue. As evidenced by dynamic light scattering (DLS) measurements, the sizes of Ag samples increased from 9 to 165, 295 and 395 nm with increasing reaction time from 2 to, 4, 8, and 11 minutes (Figure 2a and Figure S1a). Inspection by transmission electron microscopy (TEM) showed that non-assembled AgNPs with an average size of ~ 9 nm were formed at a short reaction time of 2 minutes, then the AgNPs gradually grew in size and simultaneously assembled into aggregates with increasing sizes (Figure 1c-e). The redshift of the spectra can, therefore, be attributed to the SPR coupling43-48 among neighboring AgNPs. As discussed in detail later, the aggregates can disassemble into discrete AgNPs upon adding NaOH solution. We further measured the size of the disassembled AgNPs using DLS (Figure S1b), which suggested a dramatic increase in AgNP size from the initial value of 9 to 21, 34 and 52 nm with increasing reaction time (Figure 2a). Prolonging the reaction time significantly promoted the growth and assembly degree of AgNPs (Figure S2), which is consistent with the TEM observation. The Xray diffraction (XRD) patterns of the samples show that all diffraction peaks can be indexed to metallic silver (JCPDF 04-0783) (Figure S3). The Fourier transform infrared (FT-IR) spectrum shows that there are obvious carboxylate groups of PAA on AgNP assemblies (Figure S4),40 which impart the Ag samples an overall dispersibility in water. Thanks to the stabilizer of PAA,


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the AgNPs and assemblies can remain stable at room temperature for at least half a year without notable changes in their optical properties (Figure S5). In the LLP growth process, PAA plays a key role in protecting the AgNPs from significant fusion and determining their assembly. At the early stage of the reaction, more PAA is available to protect AgNPs due to the small size of the particles. The sufficient protection prevents the aggregation of AgNPs and allows them to be dispersed individually in water. Since the total amount of PAA is limited, PAA cannot effectively limit the growth of AgNPs as the reaction continues. The area density of PAA on the surface of AgNPs gradually decreases upon the growth of AgNPs, which can be confirmed by measuring the surface charge or the zeta (ξ) potential of the particles. As shown in Figure 2b, the ξ-potential of the products changes from 38 to -13 mV when the reaction time is extended from 2 min to 12 min, indicating the decrease of surface charge. Accordingly, thermogravimetric analysis (TGA) shows that the amount of PAA decreases from 8.8 to 2.8 wt.% during the particle growth (Figure 2c), further demonstrating that decrease of PAA coverage density on AgNPs’ surface. These results clearly confirmed that under limited ligand protection, the AgNPs can progressively increase their sizes which, in turn, decrease the ligand density on their surfaces. The reduced surface charge and thus weakened electrostatic interparticle repulsion eventually trigger the self-assembly of AgNPs into large aggregates. The assembly degree of AgNPs can be simply achieved by changing the amount of PAA, which controls the coverage density of PAA on the surface of AgNPs. The plasmonic peak of the Ag samples gradually blue-shifts from 510 nm to 400 nm with increasing amount of PAA from 32 to 800 mg while keeping other reaction conditions the same as the typical synthesis (Figure S6). The sizes of both primary AgNPs and their assemblies decrease significantly with


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increasing the amount of PAA in the reaction system (Figure 2a and Figure S7). Introducing a large amount of PAA into the reaction system can significantly increase the PAA coverage, which effectively limit the growth of AgNPs and prevent them from aggregation. On the contrary, reducing the amount of PAA (~22 mg) in the reaction system can lead to large particle size, decrease the PAA coverage, and therefore promote aggregation (Figure S7), resulting in a red-shift of the plasmonic peak of about 60 nm compared to that of typical Ag sample synthesized with 32 mg PAA (Figure S6a). The self-assembled degree of AgNPs to assemblies can also be easily tailored by controlling the amount of AgNO3. Injecting an increasing amount of AgNO3 precursor into the reaction system significantly promotes the growth of AgNPs to larger sizes, and thus decreases the PAA coverage density on AgNPs’ surface and enhances their aggregation into assemblies with tunable plasmonic bands from ~ 400 to 630 nm with a shoulder extending to NIR region (Figure 2a and Figure S8, S9). TEM images further confirm the morphology change from dispersed AgNPs to assemblies of increasing sizes (Figure S10). Increasing the reaction temperature also promotes the growth of AgNPs, therefore has the same effect as increasing the concentration of AgNO3 or decreasing the amount of PAA (Figure S11). The charge dissociation of the carboxyl groups on the AgNP surface can be dynamically manipulated by pH variation,40 thereby enables reversible assembly of the nanoparticles. Upon adding NaOH solution into the dispersion of the typical AgNP assemblies, the plasmonic peak originally at ~ 510 nm gradually decreased in intensity, while the plasmonic peak at ~ 400 nm started to develop and eventually evolved into a distinct peak (Figure 3a), indicating the disassembly of AgNP assemblies into discrete NPs. Accordingly, the dispersion changed color from red to brown, and yellow during the disassembled process (Figure 3c, Figure S12). Subsequent addition of H3PO4 solution into the same dispersion gradually reversed the


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disassembly process, and the color changed from yellow to brown, and finally pink color, suggesting the re-assembly of the AgNPs (Figure 3b, c, Figure S13). TEM images confirmed the disassembly of AgNP aggregates into discrete particles upon the addition of NaOH (Figure S14a, b, Figure S15), and their re-assembly after adding H3PO4 solution (Figure S14c, d). We believe that the reduced electrostatic repulsion force upon protonation by H3PO4 drives the re-assembly of AgNPs. Compared with the initial typical AgNP assemblies, the dispersion of re-assembled AgNP assemblies has a pink color and the extinction spectrum red-shifts from 510 to 530 nm (Figure S16). The disassembly and re-assembly behavior was further supported by the DLS results measured by adding different volumes of NaOH and H3PO4 solutions into the dispersion of typical AgNP assemblies (Figure S17). The average size of AgNP assemblies decreased from about 165 to 90 and 20 nm during disassembly (Figure S18a), while the average size increased from 20 to 190 and 395 nm during re-assembly (Figure S18b). The difference in the color and size of the initial and reassembled samples might be attributed to the increased ionic strength of the reassembled sample resulting from the neutralization reaction, which reduces the thickness of the electric double layer and causes easier aggregation (assembly). FT-IR spectra show that the carboxyl group of AgNPs can reversibly disappear and reappear upon the addition of NaOH and H3PO4 solution, respectively (Figure 3d), confirming the deprotonation and protonation in response to the pH variation, which allows successful manipulation of the dissociation of surface charges and subsequently the reversible disassembly and assembly of AgNPs. The ξ-potential increased significantly from the initial -31 to -57 mV by adding NaOH solution into the dispersion of AgNP assemblies (Figure 3e), indicating that deprotonation of PAA imparted sufficient charge on AgNPs’ surface, which enhanced electrostatic interactions among AgNPs and led to the disassembly of AgNP assemblies. On the


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contrary, introducing H3PO4 solution into the disassembled dispersion reversibly protonated PAA, resulting in the decrease of surface charge, as confirmed from the decrease of ξ-potential from -57 to -6 mV (Figure 3e). The weakened electrostatic interactions among AgNPs triggered the re-assembly of discrete AgNPs to assemblies. In addition, the AgNPs synthesized with a reaction time of 3 and 6 min behaved in almost the same way as the typical samples (synthesized with a reaction time of 4 min) upon adding NaOH and H3PO4 solution, respectively (Figure S1922). To investigate the stability and reversibility of the AgNP assemblies, we cycled a similar dispersion five times by repetitively adding NaOH and H3PO4 solutions. As shown in the UVvis spectra in Figure 3f, two absorption spectra corresponding to the extinction profiles could be switched by adding NaOH and H3PO4 solution into the dispersion. Figure 3g plots the peak positions of the coupled surface plasmon band of the dispersion adding with NaOH and H3PO4 solution, respectively, exhibiting impressive reversibility and reproducibility of plasmonic optical switching of the AgNP assemblies. Finally, considering that the disassembly of AgNP assemblies leads to a pronounced color change, we constructed a colorimetric pressure sensor by embedding the AgNP assemblies in a thin solid film of polymer matrix (such as polyvinyl alcohol (PVA)). When the film is subjected to a pressure, it deforms and induces the disassembly of AgNPs, displaying a color change that can be correlated to the applied pressure. Although a similar pressure sensor has been developed previously based on the AuNP assemblies,49 the use of AgNPs has the advantage of higher color contrast and broader spectral range as the plasmonic band of isolated AgNPs is located at ~ 400 nm (instead of ~ 520 nm for AuNPs). Specifically, the composite solid film was made by casting a homogeneous mixed aqueous solution of PVA and AgNP assemblies on glass substrate followed by slow removal of the solvent by evaporation. The solid film shows a dark blue color


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with a strong coupling peak at about 655 nm and a shoulder extending to NIR region (Figure 4a). Upon the application of pressure on the solid film, the plasmonic coupling band generally blue shifted from initial 655, to 605, 558, 474 and 400 nm with increasing the applied pressures from 0 to 2, 5, 10 and 15 MPa, respectively (Figure 4a). Accordingly, the color changed from initial dark blue to purple, red, brown, and finally yellow, significantly extending the optical window (covering the visible region) for potential applications. The blue-shift of the plasmonic band could be attributed to the lateral flow of PVA chains which increased the separation of AgNPs in the assemblies.49 Figure 4b plots the dependence of the coupling peak position on the pressure applied on the film, showing that the coupling peak almost blue shifts linearly with increasing pressure.

Therefore, the current solid film can be used as colorimetric pressure sensor to

memorize the mechanical stress experienced by the film. Moreover, when the film was pressed by a relief stamper, the pattern on the stamper can be printed onto the film with a pronounced color contrast. As shown in Figure 4c, he compressed region in contact with the “M” stamp exhibited a red color, while the rest of uncompressed region remained its initial dark blue color, resulting in the red “M” on the film. After applying different pressures on the entire film, it changed color from dark blue to purple and red, and the “M” gradually disappeared. Finally the film completely changed color to bright yellow. Compared to the previous Au-based system, the higher contrast of the color change and the relatively lower cost of the Ag-based one offers wider potential applications in pressure-sensitive information storage, anti-counterfeiting, and etc. CONCLUSIONS In summary, we have developed a limited-ligand-protection route to the synthesis of AgNP assemblies with tunable plasmonic properties ranging from 400 nm to NIR region. The limited presence of PAA ligand plays a critical role in the formation of AgNP assemblies in aqueous


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solution: it reduces surface charge of the particles, weakens electrostatic interparticle repulsion, and promotes their spontaneous assembly. Moreover, the PAA on AgNPs’ surface confers highly reversible assembly and dynamic color change to the system as the surface charge can be conveniently manipulated by controlling the protonation of the carboxyl group through pH variation. Further, we demonstrate that the AgNP assemblies can be incorporated into a polymer film to fabricate a colorimetric pressure sensor for detecting and memorizing the applied pressure. Compared with the previous Au-based system, we believe the AgNP assemblies will significantly broaden the scope of practical applications as they offer significant advantages such as higher color contrast, wider spectral tuning range, simpler fabrication procedure, and lower materials production cost. In addition, our work highlights the vast potential of the LLP strategy, which was initially proposed for nanoparticle synthesis, for manipulating the nanoparticle interactions, enabling their reversible assembly, and controlling the dynamic coupling of various physical properties. EXPERIMENTAL SECTION Synthesis of Ag NP assemblies: Typically, a mixture containing 32 mg (0.018 mmol) of PAA and 15 ml of ethylene glycol in a 50 mL flask was heated to ~195 °C under vigorous stirring with N2 protection. A mixture containing 100 mg (0.59 mmol) of AgNO3 and 3 mL of EG was injected quickly. The resulting mixture was kept at 195 °C for 4 min and then quickly cooled to room temperature by a water bath. The product was centrifuged by adding acetone to remove residuals at 11,000 rpm for 10 minutes, and then redispersed in water via ultrasound. No purification step is needed for all the as-synthesized products. DEG was used to instead of EG for the Ag samples synthesized at temperature of 215 and 235 °C while keeping other reaction conditions the same as the typical synthesis.


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Disassembly of AgNP assemblies: The typical AgNP assemblies were firstly dispersed in water (~0.8 mL) in a quartz cuvette with a concentration of about 30 µg•mL-1. Then, 7 µL of NaOH solution (0.5 M) was added into the dispersion of AgNP assemblies under manual stirring. The UV-vis spectrometer (HR2000+CG-UV-NIR, Ocean Optics) was used to measure the realtime spectra changing during the disassembled process. Assembly of AgNPs: 5 µL of H3PO4 solution (0.5 M) was added into the disassembled dispersion of typical AgNP assemblies under manual stirring. The UV-vis spectrometer (HR2000+CG-UV-NIR, Ocean Optics) was also used to measure the real-time spectra changing during the re-assembled process. Fabrication of AgNP assemblies/PVA solid film: A PVA/H2O stock solution (12 wt%) was firstly prepared by dissolving PVA in H2O at 90 °C. In a typical procedure, PVA/H2O stock solution (11 g), 0.26 g of EG, and the typical AgNP assemblies (3.5 mg•mL-1, 400 µL) were mixed together to form a homogeneous solution. After vacuuming for 1 hour at the atmosphere pressure of -0.1 MPa to remove the gas bubbles, the solution was drop casted on a glass beaker (50 mL) and dried at 30 oC in oven for about 48 hours to form a solid film. Characterizations: The absorption UV-Vis spectral measurements were measured by UV-Vis spectrophotometer (HR2000+CG-UV-NIR, Ocean Optics). Transmission electron microscopy (TEM) images were collected by a Hitachi-7700 microscope with an accelerating voltage of 100 kV. For TEM observation, the as-synthesized samples were dispersed in water by ultrasonic treatment, and then a small drop of the dispersion was cast onto a carbon-coated copper grid, followed by evaporation under vacuum at room temperature. Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku D/Max 2200pc diffractometer equipped with graphite


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monochromatized CuKα radiation (λ = 0.15418 nm). The Fourier transform infrared (FT-IR) spectra were measured with a Bruker Alpha spectrometer in the range 500-2000 cm-1. Thermogravimetric analysis (TGA) was measured on a TGA/ADTA851e (Mettler-Toledo) with a heating rate of 10 °C min-1 under nitrogen atmosphere. Dynamic light scattering (DLS) and Zeta potential measurements were recorded on a Zetasizer nano ZS90 (Malvern Instruments). The test of colorimetric pressure sensor was performed on a powder compressing machine (769Yp-24B). ASSOCIATED CONTENT Supporting Information Additional experimental details, DLS results, TEM images, XRD patterns, FT-IR spectra, UVvis absorption spectra, digital photographs, including Figures S1-S21 (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. ORCID Wenshou Wang: 0000-0003-2674-4203 Yadong Yin: 0000-0003-0218-3042 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (W.W., No. 21671120) and the U.S. National Science Foundation (Y.Y., No. CHE-1308587). Acknowledgment is also made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (55904-ND10).

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18. Shao, L.; Zhuo, X.; Wang, J. Adv. Mater. 2018, 30, 1704338. 19. Jiang, N.; Zhuo, X.; Wang, J. Chem. Rev. 2018, 118, 3054–3099. 20. Gong, J.; Newman, R. S.; Engel, M.; Zhao, M.; Bian, F.; Glotzer, S. C.; Tang, Z. Nat. Commun. 2017, 8, 14038-14047. 21. Bonacchi, S.; Cantelli, A.; Battistelli, G.; Guidetti, G.; Calvaresi, M.; Manzi, J.; Gabrielli, L.; Ramadori, F.; Gambarin, A.; Mancin, F.; Montalti, M. Angew. Chem. Int. Ed. 2016, 55, 1106411068. 22. Klajn, R.; Wesson, P. J.; Bishop, K. J. M.; Grzybowski, B. A. Angew. Chem. Int. Ed. 2009, 48, 7035-7039. 23. Pillai, P. P.; Kowalczyk, B.; Kandere-Grzybowska, K.; Borkowska, M.; Grzybowski, B. A. Angew. Chem. Int. Ed. 2016, 55, 8610-8614. 24. Montelongo, Y.; Sikdar, D.; Ma, Y.; McIntosh, A. J. S.; Velleman, L.; Kucernak, A. R.; Edel, J. B.; Kornyshev, A. A. Nat. Mater. 2017, 16, 1127-1136. 25. Zhang, M.; Magagnosc, D. J.; Liberal, I.; Yu, Y.; Yun, H.; Yang, H.; Wu, Y.; Guo, J.; Chen, W.; Shin, Y. J.; Stein, A.; Kikkawa, J. M.; Engheta, N.; Gianola, D. S.; Murray, C. B.; Kagan, C. R. Nat. Nanotechnol. 2017, 12, 228-232. 26. Sun, Z.; Ni, W.; Yang, Z.; Kou, X.; Li, L.; Wang, J. Small 2008, 4, 1287-1292. 27. Zhao, H.; Sen, S.; Udayabhaskararao, T.; Sawczyk, M.; Kučanda, K.; Manna, D.; Kundu, P. K.; Lee, J.-W.; Král, P.; Klajn, R. Nat. Nanotechnol. 2015, 11, 82-88. 28. Kundu, P. K.; Samanta, D.; Leizrowice, R.; Margulis, B.; Zhao, H.; Boerner, M.; Udayabhaskararao, T.; Manna, D.; Klajn, R. Nat. Chem. 2015, 7, 646-652. 29. Zhang, H.; Wang, D. Angew. Chem. Int. Ed. 2008, 47, 3984-3987. 30. Chen, Y.; Wang, Z.; He, Y.; Yoon, Y. J.; Jung, J.; Zhang, G.; Lin, Z. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 1391-1400. 31. Liu, Y.; Han, X.; He, L.; Yin, Y. Angew. Chem. Int. Ed. 2012, 51, 6373-6377. 32. Harimech, P. K.; Gerrard, S. R.; El-Sagheer, A. H.; Brown, T.; Kanaras, A. G. J. Am. Chem. Soc. 2015, 137, 9242-9245. 33. Yao, Y.; Jie, K.; Zhou, Y.; Xue, M. Chem. Commun. 2014, 50, 5072-5074. 34. Lewandowski, W.; Fruhnert, M.; Mieczkowski, J.; Rockstuhl, C.; Gorecka, E. Nat. Commun. 2015, 6, 6590. 35. Liu, Q.; Liu, Y.; Yin, Y. Nat. Sci. Rev. 2018, 5, 128-130. 36. Valenti, M.; Venugopal, A.; Tordera, D.; Jonsson, M. P.; Biskos, G.; Schmidt-Ott, A.; Smith, W. A. Acs Photonics 2017, 4, 1146-1152. 37. Lee, J.-S.; Kim, H.; Algar, W. R. J. Phys. Chem. C 2017, 121, 28566-28575. 38. Choi, S.; Park, S.; Yu, J. Chem. Commun. 2014, 50, 15098-15100. 39. Hu, Y.; Ge, J.; Lim, D.; Zhang, T.; Yin, Y. J. Solid State Chem. 2008, 181, 1524-1529. 40. Weidman, J. L.; Mulvenna, R. A.; Boudouris, B. W.; Phillip, W. A. J. Am. Chem. Soc. 2016, 138, 7030-7039. 41. Naravanaswamy, A.; Xu, H.; Pradhan, N.; Peng, X. Angew. Chem. Int. Ed. 2006, 45, 53615364. 42. Narayanaswamy, A.; Xu, H.; Pradhan, N.; Kim, M.; Peng, X. J. Am. Chem. Soc. 2006, 128, 10310-10319. 43. Van Haute, D.; Longmate, J. M.; Berlin, J. M. Adv. Mater. 2015, 27, 5158-5164. 44. Armao, J. J.; Nyrkova, I.; Fuks, G.; Osypenko, A.; Maaloum, M.; Moulin, E.; Arenal, R.; Gavat, O.; Semenov, A.; Giuseppone, N. J. Am. Chem. Soc. 2017, 139, 2345-2350.


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Figure 1. (a) Extinction spectra showing the tuning of the plasmon bands and (b) digital photographs of AgNPs and assemblies obtained with different reaction times. (c-e) TEM images of AgNPs and assemblies obtained at reaction times of 2 (c), 3 (d) and 4 minutes (e). Scale bars: 50 nm.


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Figure 2. (a) The size of AgNP assemblies and discrete AgNPs versus the reaction time, amount of PAA, and amount of AgNO3, respectively. (b) Zeta potential of AgNPs obtained at different reaction times. (c) TGA of AgNP assemblies obtained at reaction times of (i) 4, (ii) 6, (iii) 8 and (iv) 11 minutes.


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Figure 3. (a, b) The extinction spectra of a typical dispersion of AgNP assemblies by adding NaOH (a) and subsequently adding H3PO4 solution (b). (c) Digital photographs of initial, disassembled and re-assembled AgNP assemblies. (d) FT-IR spectra of initial (i), disassembled (ii) and re-assembled AgNP assemblies (iii). (e) Zeta potential for samples obtained by adding different volumes of NaOH and H3PO4 into the dispersion of typical AgNP assemblies. (f) 3D UV-vis spectra of five cycles switching between disassembled (blue line) and assembled (red line) states. (g) Repeated changes of plasmonic peak positions in the five switching cycles.


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Figure 4. (a) The extinction spectra of a typical PVA film containing AgNP assemblies in response to different pressures. (b) Plot showing the pressure dependent shift of the coupling peak position. (c) Digital images of the film imprinted with an “M” responding with color change to varying pressure. Scale bar: 10 mm.


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TOC

A limited-ligand-protection strategy has been developed to synthesize Ag nanoparticles with controlled loading of polyacrylic acid on surface to enable their reversible assembly and dynamic tuning of plasmonic property in response to pH change.


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Reversible Assembly and Dynamic Plasmonic Tuning of Ag

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