Light-Triggered Reversible Self-Assembly of Gold Nanoparticle

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Light-triggered Reversible Self-assembly of Gold Nanoparticle Oligomers for Tuneable SERS Youju Huang Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504365b • Publication Date (Web): 25 Dec 2014 Downloaded from http://pubs.acs.org on December 30, 2014

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Light-triggered Reversible Self-assembly of Gold Nanoparticle Oligomers for Tuneable SERS

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Langmuir la-2014-04365b.R1 Article 23-Dec-2014 Zhang, Lei; Ningbo Institute of Material Technology and Engineering,Chinese Academy of Science, Division of Polymer and Composite Materials, Dai, Liwei; Ningbo Institute of Material Technology and Engineering,Chinese Academy of Science, Division of Polymer and Composite Materials, Rong, Yun; Ningbo Institute of Material Technology and Engineering,Chinese Academy of Science, Division of Polymer and Composite Materials, Liu, Zhenzhong; Ningbo Institute of Material Technology and Engineering,Chinese Academy of Science, Division of Polymer and Composite Materials, Tong, Dingyi; Ningbo Institute of Material Technology and Engineering,Chinese Academy of Science, Division of Polymer and Composite Materials, Huang, Youju; Ningbo Institute of Material Technology and Engineering,Chinese Academy of Science, Division of Polymer and Composite Materials Chen, Tao; Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Department of Polymer and Composite

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Light-triggered Reversible Self-assembly of Gold Nanoparticle Oligomers for Tuneable SERS Lei Zhang, Liwei Dai, Yun Rong, Zhenzhong Liu, Dingyi Tong, Youju Huang∗ and Tao Chen* Division of Polymer and Composite Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Science, Ningbo, 315201, China ABSTRACT: A photoresponsive amphiphilic gold nanoparticle (AuNP) is achieved through the decoration on AuNP with hydrophilic poly (ethylene glycol) (PEG) and hydrophobic photoresponsive polymethacrylate containing spiropyran units (PSPMA). Owing to photoresponsive property of spiropyran units, the amphiphilic AuNPs can easily achieve the controllable assembly/disassembly behaviours under the trigger by light. Under visible light, spiropyran units provide weak inter-molecular interactions between neighbored AuNPs, leading to isolated AuNPs in the solution. While under UV light irradiation, spiropyran units in the polymer brushes transform into merocyanine isomer with conjugated structure and zwitter-ionic state, promoting the integration of adjacent AuNPs through π-π stacking and electrostatic attractions, further leading to the formation of Au oligomers. The smart reversible AuNP oligomers exhibited switchable plasmonic coupling for tuning surface-enhanced Raman scattering (SERS) activity, which is promising for the application of SERS based sensors and optical imaging.

Key

words: Gold nanoparticle; Self-assembly; Photoresponsive; Surface-enhance Raman

spectroscopy; ∗

Corresponding author: e-mail: [email protected]; [email protected] ACS Paragon Plus Environment

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1. Introduction Plasmonic coupling aggregates1 fabricated by the self-assembly of gold nanoparticles (AuNPs) have been widely explored in various fields including plasmonics,2 sensors,3 chemical amplifiers,4 photo thermal therapy5 and catalysts6 because of their unique collective properties different from discrete NPs.7 One of the important features of Au plasmonic coupling aggregates is the ‘hot spots’ created at the junction of adjacent NPs.8, 9, 10, 11, 12 The hot spot exhibits extraordinary surface-enhanced Raman scattering (SERS) activity and dramatic signal enhancement compared with those from individual NPs,8, 9, 10, 11, 12

which allows ultrasensitive detection of single molecules.13 Therefore, the designation and

controlled construction of plasmonic coupling AuNPs aggregates in a well-defined manner is critical for their properties and applications.14, 15 Self-assembly, as a powerful and effective approach, can not only avoid problems from lithography method such as high cost and low yield, but also provide a flexible means to control precisely the interparticle spacing in the assembly structures and consequent spectroscopic properties.16,

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Significant progress has been made on the fabrication of Au plasmonic coupling aggregates by selfassembly. Tang and co-workers20 reported self-assembled Au aggregated superstructures with altered size, shape and arrangement pattern of AuNPs. Duan and co-workers21 showed that amphiphilic polymers modified AuNPs can spontaneously self-assemble into high-purity plasmonic vesicles in selective solvents. Liu et al22, 23, 24 indicated the polymer-modified Au nanorods could self-assembled into linear, branched, and cyclic nanostructures. These reports focus on the fabrication of coupling aggregates with large size, which is ineffective to keep stable in water and high biocompatibility. 25 Apart from challenge in control of the aggregation size, controlling reversibly the dynamic selfassembly of AuNPs by external stimulus is another important issue. The reversible modulation provides the precise manipulation of NP interactions in the assembled nanostructures, which is significantly important for applications such as controlled drug release/delivery,26, 27, 28 switchable sensors8, 29, 30 and ACS Paragon Plus Environment

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adjustable catalysts.6 We have previously investigated that micro- or nanoparticles could be manipulated to move by solvent responsive polymer brushes.31, 32 Duan et al33 reported a pH-responsive amphiphilic AuNPs and controlled reversibly the assembly of AuNPs into dimer or trimer aggregates via tuning pH value of solution. pH stimuli modulation usually introduces inevitably exotic acidic or basic compound in solution. In previous work,34, 35, 36 light stimuli has shown distinctive advantages such as easy operation, remote manipulation, non-invasiveness and non-destructiveness of solution system, which would be a convenient alternative to tune the self-assembly of AuNPs.37, 38 Grzybowski et al reported AuNPs decorated with azobenzene ligands that could aggregate reversibly under light irradiation.6 Nie et al prepared a kind of AuNPs capped with dodecanethiol and oleylamine which could self-assemble into vesicles by UV-induced oxidation of mercapto groups to sulfonic groups.39 Despite these great progresses on reversible AuNP aggregates, there is no report on a small sized reversible AuNP aggregates tuned by light (e.g., reversible Au oligomers). Herein, we report a novel type of AuNPs decorated with hydrophilic poly (ethylene glycol) (PEG) and hydrophobic photoresponsive polymethacrylate containing spiropyran units (PSPMA) is achieved via sequential “grafting to” and “grafting from” strategies. The introduction of PEG chains provides strong hydrophilicity to stabilize AuNPs in solution and the steric hindrance to avoid the formation of big sized aggregates.21, 33 While PSPMA chains offers hydrophobility and photo responsive functional groups,40, 41

displaying close-cycled spiropyran and zwitter-ionic merocyanine isomers under visible light and UV

light, respectively. This amphiphilic AuNPs, denoted as Au@PEG/PSPMA NPs, can reversibly selfassemble into aggregates with controlled numbered NPs (oligomers) by light irradiation. The obtained novel smart reversible AuNP oligomers were further explored as a tunable SERS-active material.

2. Material and methods 2.1. Materials and instrument Chloroauric acid (HAuCl4·4H2O) and methoxy-poly (ethylene glycol)-thiol (PEG) with a molecular weight of 2000 (Mn) were purchased from Sigma Aldrich. Bis (2-hydroethyl) disulfide and 2-bromo-2ACS Paragon Plus Environment

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methylpropionyl bromide were obtained from Energy Chemical in Shanghai. CuBr, sodium citrate, 4mercaptopyridine (MPy) and N,N,N’,N’’,N’’-pentamethyldiethylenetriamine (PMDETA) were obtained from Aladdin Company in Shanghai. CuBr was refined by washing in glacial acetic acid and followed by methanol. Spiropyran-containing methacrylate monomer (SPMA) was synthesized according to previous reported work42,

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and characterized by 1H-NMR spectrum (shown in Figure S1). Other

solvents and regents are obtained from Sinopharm Chemical Reagent Co., Ltd and used as received. The structures of AuNP were observed by transmission electron microscopy (TEM) conducted on the JEOL JEM2010 electron microscope. UV-Vis absorption spectra of AuNP solution were recorded by virtue of TU-1810 UV/Vis spectrophotometer from Purkinje General Instrument Co. Ltd, Beijing. The SERS measurement is conducted on the Renishaw inVia-Reflex micro-Raman spectrometer equipped with 633 nm laser. The Raman spectrum is calibrated using silicon substrate (520 nm calibration peak). The sample is exposed by the laser for 2 s every time and is scanned for 5 times under 1 % total laser power. 2.2. Synthesis of 2, 2’-dithiobis [1-(2-bromo-2-methyl-propionyloxy)] ethane (DTBE) The initiator DTBE was synthesized according to previous work.44 Briefly, 0.32 g (2.07 mmol) bis(2hydroethyl) disulfide and 1.06 g (10.5 mmol) triethylamine were dissolved in 50 mL dichloromethane and stirred at 0°C under argon atmosphere. Then, 1.13 g (4.93 mmol) 2-bromo-2-methylpropionyl bromide was added. The mixed solution was stirred for 1 h at 0 °C and then for another 2 h at room temperature. The resultant mixed solution was filtered and the organic phase was extracted with 2 M Na2CO3 aqueous solution saturated with NH4Cl. Dichloromethane in the solution was removed by rotary evaporation. The resultant viscous liquid product DTBE was collected and characterized by 1HNMR spectrum (shown in Figure S2). 2.3. Synthesis of AuNPs45 AuNPs were synthesized by sodium citrate reduction of HAuCl4 in deionized water. Typically, 10mL aqueous solution containing 0.1 g sodium citrate was rapidly added into a boiled 100 mL aqueous ACS Paragon Plus Environment

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solution containing 10 mg HAuCl4 under vigorous stirring. After boiling for 30 min, the color of the solution changed to wine red. The obtained AuNPs solution was cooled down and was characterized by UV/Vis absorption spectra and TEM observation (shown in Figure S3 and Figure 1 Aa). 2.4. Synthesis of amphiphilic AuNPs coated with polymer brush (Au@PEG/PSPMA)33, 44 Typically, 30 mg PEG and 10 mg DTBE were dissolved into 2 mL DMF solvent. The resultant solution was added slowly into 100 mL AuNPs aqueous solution (0.25 mM) and stirred for 20 h at room temperature. The obtained Au@PEG/DTBE solution was purified by repeated centrifugation and redispersion in DMF. Subsequently, amphiphilic AuNPs were prepared by surface-initiated atom transfer radical polymerization (ATRP) method. 10 mg SPMA monomer was dissolved in the Au@PEG/DTBE solution in DMF (4 mL, 2.1 mM). The mixed solution was bubbling with nitrogen for 30 min to remove oxygen. 4 mg CuBr and 18 µl PMDETA were introduced into the deoxygenated solution under nitrogen atmosphere. The reaction was conducted at 40 °C for 2.5 h. The resultant mixed solution was refined by repeated centrifugation before obtaining wine red Au@PEG/PSPMA NPs solution. 2.5. The self-assembly/disassembly of amphiphilic Au@PEG/PSPMA induced by UV or visible light irradiation Assembly process: In order to promote the self-assembly of Au@PEG/PSPMA induced by UV irradiation, 0.2 mL deionized water was added slowly into 2 mL Au@PEG/PSPMA solution (0.63 mM) in dimethylformamide (DMF) and mixed by gentle stirring. The resultant AuNPs solution was irradiated by UV light (8 W) for 4 h. During the UV light irradiation, the UV light was turned off every 30 min and solution was mixed uniformly by 1 min ultrasonication. The color of AuNPs solution changed from wine red color to violet color, indicating the formation of self-assembled aggregates.


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Disassembly process: The uniform aggregate solution was irradiated by visible light (40 W) for 15 h. During the visible light irradiation, the visible light was turned off every 30 min and solution was sonicated for 1 min to allow the AuNPs disperse uniformly in the solution. The light-driven self-assembly process was conducted in a specially designed box-type instruments in which the temperature was nearly kept at room temperature by virtue of a recycling air blower. 2.6. The surface-enhanced Raman scattering measurement The SERS properties of the AuNPs were characterized from solution samples (hold in a glass capillary, (Figure S4)) and the glass substrate. Characterization in solution: the reversible Raman signal variation of AuNPs in the assembly/disassembly process was recorded in situ under UV or visible irradiation; Characterization on substrate: MPy was selected as Raman probe. AuNPs solution (about 0.25 mM) in different assembly/disassembly stage under UV or visible light irradiation was extracted respectively and mixed with MPy aqueous solution (10-4 M). Every time 10 µL relevant AuNPs solution was mixed with 10 µL MPy aqueous solutions. After the solvent water dried completely, a solid mixed film consisting of AuNPs and MPy on the substrate was obtained. The above mixing process was repeated three times before SERS characterization. 3. Results and discussion AuNPs with the average size about 25 nm was prepared according to our previous method,45, 46 which was further decorated with mixed polymer brushes via sequential “grafting onto" and "grafting from” reactions (Scheme 1 A). The initiator of DTBE for ATRP and hydrophilic PEG were grafted onto the surface of AuNPs through Au-S bond interaction. Photo responsive polymer of PSPMA was subsequently grafted from surface initiator DTBE by surface initiated polymerization of spiropyrancontaining methacrylate monomer (SPMA).25 The obtained amphiphilic AuNPs coated with PEG and PSPMA brushes were purified by repeated centrifugation. The AuNPs coated with PEG (2 KDa) and PSPMA brushes (Mn=10 KDa) were characterized by 1H-NMR spectrum and thermogravimetric ACS Paragon Plus Environment

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analysis (TGA) (Figure S5 and S6). According to the equation reported by Duan, the grafted density of polymer brush on the AuNPs is calculated about 0.6 chains/nm2 based on the NMR and TGA results. The photo responsive behaviors of Au@PEG/PSPMA NPs were investigated primitively by UV/Vis absorption spectra. Figure 1 (A) shows the absorption spectra of Au@PEG/PSPMA NP solution in the process of self-assembly/disassembly induced by UV or visible light. The AuNP solution in DMF showed a single plasmonic peak at 525 nm (Figure 1 Aa) which was ascribed to characteristic surface plasmonic resonance from isolated Au@PEG/PSPMA NPs. The addition of a small amount of water (the volume fraction of water in the mixed solution is 10%) into Au@PEG/PSPMA NPs solution resulted in the reduction of absorption intensity around 525 nm and slight increasing in the range from 640 to 800 nm (Figure 1 Ab).44 This indicated the formation of a small number of assembled aggregates.33,

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The addition of water allowed hydrophobic PSPMA brushes shrink slightly and

shortened the distance between adjacent Au@PEG/PSPMA NPs.33 On the other hand, the effect of small amount of water on the AuNPs stability was monitored by UV/Vis absorption spectra (Figure S7). There was no clear change after three days in the UV/Vis absorption spectra, which indicated the mixed solution containing small amount of water kept stable. Au@PEG/PSPMA NP solution in DMF/H2O was subsequently exposed to UV light and tracked by UV/Vis absorption spectra at different irradiation time to record the dynamic self-assembly process (Figure 2). After about 4 h UV light irradiation, a new shoulder peak at 670 nm appeared in the absorption spectra (Figure 2 and Figure 1 Ac), indicating the formation of some AuNP self-assembled structures.33 These self-assembled structures showed similar spectra features with Au nanorod characterized by longitudinal and transverse peaks.47, 48 It was inferred dimer or trimer shaped structures according to previous work.33, 49 Unlike Au nanorod, the self-assembled AuNP oligomers showed weak longitudinal peak and strong transverse peak, which was ascribed to the presence of the nanometer-sized gap between AuNPs.50, 51 When Au@PEG/PSPMA NP solution with double plasmonic absorption peaks was further irradiated by visible light, the shoulder peak in the absorption spectra decreased gradually and disappeared nearly upon long time irradiation (Figure 1 Ad), indicating reversible self-assembly ACS Paragon Plus Environment

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process controlled by different light irradiation. Meanwhile, the color of Au@PEG/PSPMA NPs solution hold in a capillary was changed clearly from pink (Figure 1 Ba and b) to purple under UV light (Figure 1 Bc), then reverted to pink under visible light (Figure 1 Bd), which also confirmed the reversible self-assembly process. To gain a direct insight into the reversible self-assembly process, transmission electron microscope (TEM) was used to investigate the morphology of Au@PEG/PSPMA NPs self-assembled structures. Initially, Au@PEG/PSPMA NPs dispersed individually in DMF solution (Figure 3 A), since both PEG and PSPMA polymer chains can be completely dissolved and extended freely in DMF solution. After the addition of a small amount of water,33 the hydrophobic PSPMA brushes shrunk slightly to form unsubstantial hydrophobic domains and further resulting into the formation of a small number of selfassembled AuNP oligomers.44 But most AuNPs still keep isolated form (Figure 3 B). Subsequently, Au@PEG/PSPMA NP solution was irradiated by UV light for 4 h. It was surprised to find that almost all the isolated AuNPs disappeared and only AuNP oligomers such as dimers and trimers existed (Figure 3 C). Under UV light, spiropyran units in PSPMA chains transformed into zwitter-ionic merocyanine isomers with conjugated structure, which provided electrostatic attractions and strong π-π stacking40, 52 for the strong interaction between PSPMA brushes (Scheme 1 B). In order to investigate the light triggered aggregation resulted from the interaction of spiropyran units, a homogenous polymer containing spiropyran was synthesized and its UV/Vis absorption spectra are shown in Figure S8. After UV irradiation, a new peak attributed to open-cycled merocyanine isomer around 570 nm appeared, which indicated the spiropyran transform into merocyanine form under UV light.52 The absorption peak around 380 nm was the featured peak of merocyanine aggregates formed by π-π stacking and electrostatic force.52 The absorption around 380 nm was strengthened generally with increasing UV irradiation time, which confirmed more merocyanine aggregates were formed. This phenomenon is in good line with previous related work.43, 52 The UV light triggered photoisomerization of spiropyran to merocyanine isomer resulted in the increase of attractive interaction sites in polymer brushes, prompting adjacent AuNPs to form ACS Paragon Plus Environment

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interparticle junction. Meanwhile, PEG brushes at nanoparticle junction reorganized gradually toward the non-contacting area of the two NPs, which increases the density of PEG brushes in the noncontacting area.33 Kinetically, when the density of PEG brushes is high enough, the integration of additional NPs into assembled structures was inhibited due to the steric hindrance of PEG brushes. Therefore, large-sized aggregated assembly could not exist in the solution and only oligomers were formed upon UV irradiation (Figure 3 C). The population of self-assembled AuNP oligomers with different amount of NPs was studied through statistical analysis based on TEM images. Figure 4 shows the TEM images of AuNP oligomers observed in different position on the same copper grid. The population of various oligomers such as dimer, trimer and tetramer in the TEM images was qualified statistically (Figure 4 E, F, G and H). Au oligomers such as dimer, trimer and tetramer were dominated in the random filed of copper grid and up to 80 percent of the total assembled aggregates. When small amount of water was added into AuNPs solution, hydrophobic PSPMA brushes shrunk slightly to form unsubstantial hydrophobic domains, which led to the close distance between isolated AuNPs. Subsequently, the spiropyran in PSPMA brushes converted to zwitterionic merocyanine isomer under UV light. These adjacent AuNPs tend to assemble into oligomer aggregates based on long-ranged electrostatic attractions. In the assembled oligomer, polymer brushes in the interparticle junction were expelled out because of limited space. The density of PEG brushes in the non-contact area increased obviously, which limited the interactions between merocyanine units from different AuNPs. This avoided the formation of large-sized aggregates (shown in Scheme S1). Interestingly, these Au oligomers could disassemble into individual AuNPs under visible light irradiation (Figure 3 D), displaying reversible self-assembly behaviors. Under visible light irradiation, merocyanine isomers in the brushes revert to spiropyran form and strong molecular interactions disappeared.

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PEG and PSPMA grafts stretched freely and distributed randomly on the surface of

AuNPs, leading to the disassembly of Au oligomers to isolated AuNPs (Figure 3 D). The AuNPs oligomers at different assembly/disassembly stages were observed by TEM images. After 1h UV irradiation, original individual AuNPs started to assemble into dimers or trimers. But there ACS Paragon Plus Environment

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were still lots of isolated AuNPs in the solution (Figure S9A). Upon 3 h irradiation, most of AuNPs formed oligomer and only few isolated AuNPs (Figure S9B) were found. Subsequently, these assembled structures were exposed to the visible light and oligomer generally disassembles. After 7 h visible light irradiation, some isolated AuNPs fell off the assembly but there were still some assembled oligomer existed in the solution (Figure S9C). The density of PEG brushes also had remarkable effect on the selfassembly of AuNPs. We synthesized Au@PEG/PSPMA NPs coated by PEG brushes with different density (about 0.2 and 0.1 chains /nm2). Their self-assembled nanostructures under same lightcontrolled processes were observed by TEM images, as shown in Figure S10. It was found that the AuNPs grafted with lower density of PEG brushes were apt to assemble into large-sized aggregates while the sample with high density of PEG brushes were apt to form mixed structures containing oligomer and some isolated AuNPs. Previous work has shown that plasmonic coupling aggregates such as dimers, trimers and superstructures can significantly improve SERS signals due to enhancement effect of hotspots on local electromagnetic field.9, 12, 20, 33, 51, 53 Herein, the interparticle gaps in the AuNP oligomers were analyzed statistically based on TEM images (Figure S11). The averaged interparticle gap in the AuNP oligomers was about 2.2 ± 0.3 nm, which is in line with theoretically and experimentally optimized the molecular-sized gaps (less than 10 nm) to maximize the electric field.10 Therefore, the switchable plasmonic coupling and Raman signal enhancement of Au@PEG/PSPMA NPs in the reversible assembly and disassembly process were studied. The SERS measurement of Au@PEG/PSPMA NP solution hold in a capillary was carried in situ. The Raman bands at 1661, 1440, 1407, 1092, 880 cm-1 are originated from indoline and chromene structure of spiropyran.54 The bands at 1440 and 1407 cm-1 are assigned to stretching modes of ring-closed spiropyran and merocyanine isomer (MC), respectively.54 The Raman band at 1407 cm-1 are the most intense assigned to MC and selected as featured band to study Raman signal enhancement. The initial Au@PEG/PSPMA NP solution shows SERS intensity at 1407 cm-1 around 170 (Figure 5 Aa). Under identical SERS test conditions,


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Au@PEG/PSPMA NP solution upon UV light irradiation shows improved SERS intensity around 249 at 1407 cm-1, which is nearly 1.5 times of initial intensity (Figure 5 Ab). The low SERS enhancement possibly can be ascribed to the real efficiency of the enhanced molecules. PSPMA chains was homogeneously modified onto AuNPs surfaces, only a few limited molecules located at the gap between adjacent NPs in oligomers contributes to the signal enhancement ,while the other spiropyran units on the non-gap areas kept instant SERS signals, which dominated SERS signals. This leads to the low collaborative SERS enhancement in Au@PEG/PSPMA NP solution. When Au@PEG/PSPMA NP solution was irradiated subsequently by visible light, the Raman signals can be returned to the original value (Figure 5 Ac), since AuNP oligomers were disassembled into individual NPs in solution. In addition, Raman signals can be tuned to “up” and “down” many times (Figure 5B) by controlling light-driven reversible self-assembly/disassembly of AuNP oligomers. In order to further study the SERS enhancement effect of the assembled oligomer, 4mercaptopyridine (MPy) is selected as another Raman probe molecule. Figure 6 shows the Raman enhancement efficiency of Au@PEG/PSPMA in different assembly/disassembly stages. Pure MPy molecules only showed weak Raman peaks (Figure 6 Aa). After mixed with AuNPs solution, the Raman signal intensity was enhanced obviously and featured Raman peaks of MPy can be seen clearly (Figure 6 Ab) including 718, 1011, 1062, 1100, 1217, 1581, and 1612 cm−1, which agreed well with previous work.55 The Raman peak at 1011 cm-1 was selected as featured band. The SERS enhancement factor (EF) was calculated by a formula, EF= ISERS/IO ×NO/NSERS,46, 56 where ISERS and IO represented Raman band intensity of MPy absorbed on AuNPs and free MPy, respectively. NO and NSERS are the number of MPy absorbed on AuNPs and free MPy molecules.46 With increasing the UV irradiation time, the EF was increased generally (Figure S12). Upon 4h UV light irradiation, Raman peaks were enhanced by an EF of approximately 1.8×105 (Figure 6 Ac). This Raman enhancement can account for the formation of more hotspots in the oligomer produced by UV light. Subsequently, the AuNPs solution was exposed to visible light. The resultant AuNPs solution was mixed with fresh MPy again. The EF value of AuNPs ACS Paragon Plus Environment

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oligomer generally decreased with the disassembly of oligoemr under visible light (Figure S12B). After 15 h visible light irradiation, the EF value reduced to 6×103. In addition, Raman signals enhancement efficiency of AuNPs can be tuned to “up” and “down” many times (Figure 6 B) on the solid substrate. The smart tunable plasmonic materials have the promising opportunity for SERS based bioimaging and biosensing.

4. Conclusions We have demonstrated that smart reversible AuNP oligomers (dimers and trimers, tetramer, etc) can be successfully constructed by light-controlled assembly and disassembly of photoresponsive amphiphilic Au@PEG/PSPMA NPs. AuNPs can be completely dispersed as individual NPs in the solution, due to the weak inter-molecular interaction between neighbored NPs. Upon the irradiation by UV light, spiropyran units in polymer was transformed into mecyanine isomer with conjugated structure and zwitter-ionic state. This leads to the self-assembled AuNP oligomers because of the strong intermolecular interaction such as π-π stacking and electrostatic attractions between adjacent NPs. Our strategy provides the advantage in controlling the AuNPs self-assembled structures in a clean, green and remote approach. The obtained reversible plasmonic oligomers display tunable SERS-activity, which is promising for the application of SERS based sensors and optical imaging.

5. Acknowledgments We thank Chinese Academy of Science for Hundred Talents Program, Chinese Central Government for Thousand Young Talents Program, the Natural Science Foundation of China (21404110, 51473179, 51303195, 21304105) and Excellent Youth Foundation of Zhejiang Province of China (LR14B040001). Supporting Information Available:


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H-NMR spectra of SPMA monomer and initiator DTBE; TEM image of 25 nm AuNPs; The picture of

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Figures

Scheme 1. Schematic illustration of the preparation of amphiphilic Au@PEG/PSPMA NPs and their reversible self-assembly/disassembly processes induced by UV and visible light.


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Figure 1. (A) The UV/Vis absorption of amphiphilic Au@PEG/PSPMA NP solution. Initial Au@PEG/PSPMA NPs in DMF (a), after adding H2O (b), following UV light irradiation (c) and visible light irradiation (d). (B) The photographs of Au@PEG/PSPMA NPs solution hold in capillary (a), after the addition of H2O (b), the irradiation of UV light (c) and visible light (d). (The inner diameter of the capillary is about 1mm with wall thickness and length in 0.1mm and 10cm respectively, as shown in Figure S6) The scale bar is 1 cm.

0.5

Absorption

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0 0.5 h 1.0 h 1.5 h 2.0 h 3.0 h 4.0 h

0.4 0.3 0.2 0.1 0.0 400

600

800

Wavelength /nm Figure 2. The UV/Vis absorption spectra of Au@PEG/PSPMA dispersion containing small amount water upon UV irradiation for different time.


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Figure 3. The TEM images of Au@PEG/PSPMA dispersion (A), after addition of small amount of H2O (B), exposure to UV light (C) and sequential visible light irradiation (D).

Figure 4. The TEM images of Au@PEG/PSPMA assembly oligomer in different position on the substrate (A, B, C, D). The inset high resolution image in (A) shows the dimer structure. The column graph E, F, G and H represent the statistical proportion of various oligomers in the TEM images A, B, C and D, respectively.


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Figure 5. (A) The surface-enhanced Roman spectroscopy of amphiphilic Au@PEG/PSPMA solution (a), after UV irradiation (b) and sequential visible light irradiation (c). (B) The peak intensity around 1407 cm-1(highlighted by a dash-lined frame in image A) as a function ofUV and visible light irradiation over five times cycles.


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Figure 6. (A) The Roman spectroscopy of MPy (a), MPy/Au@PEG/PSPMA (b) and MPy/Au@ PEG/PSPMA after UV irradiation (c) and sequential visible light irradiation (d). (B) The peak intensity around 1011 cm-1 (highlighted by a arrow in image (A) as a function of UV and visible light irradiation over five times cycles.


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Table of Contents Graphic

Light-triggered Reversible Self-assembly of Gold Nanoparticle Oligomers for Tuneable SERS Lei Zhang, Liwei Dai, Yun Rong, Zhenzhong Liu, Dingyi Tong, Youju Huang∗ and Tao Chen* Division of Polymer and Composite Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Science, Ningbo, 315201, China.

∗

Corresponding author: e-mail: [email protected]; [email protected] ACS Paragon Plus Environment

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Light-Triggered Reversible Self-Assembly of Gold Nanoparticle

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