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Aggregation Driven Controllable Plasmonic Transition of Silica-Coated Gold Nanoparticles with Temperature Dependent Polymer-Nanoparticle Interactions for Potential Applications in Optoelectronic Devices Na Kyung Kwon, Tae Kyung Lee, Sang Kyu Kwak, and So Youn Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13123 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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ACS Applied Materials & Interfaces

Aggregation

Driven

Controllable

Plasmonic

Transition of Silica-Coated Gold Nanoparticles with Temperature Interactions

Dependent for

Polymer-Nanoparticle

Potential

Applications

in

Optoelectronic Devices Na Kyung Kwon, Tae Kyung Lee, Sang Kyu Kwak* and So Youn Kim* School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

KEYWORDS: silica-coated gold nanoparticles, polymer adsorption, colloidal stability, particle aggregation, solvent quality, localized surface plasmon resonance, plasmon hybridization

ABSTRACT: Localized surface plasmon resonance (LSPR) effect relies on the shape, size, and dispersion state of metal nanoparticles and can potentially be employed in many applications such as chemical/biological sensor, optoelectronics, and photocatalyst. While complicated

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synthetic approaches changing shape and size of nanoparticles can control the intrinsic LSPR effect, here we show that controlling interparticle interactions with silica-coated gold nanoparticles (Au@SiO2 NPs) is a powerful approach, permitting wide range of optical bandwidth of gold nanoparticles with great stability. The interparticle interactions of Au@SiO2 NPs are controlled through concentration-, temperature-, and time-dependent polymer-induced interactions. The polymer-induced interactions modulate the state of particle dispersion, resulting an effective plasmonic shift by more than 200 nm. We further explore the microstructure of particle aggregation and explain mechanisms of plasmonic shift based on the results of smallangle X-ray scattering (SAXS) and discrete dipole approximation (DDA) calculation. We show that an effective control of LSPR behavior is now available through trapped aggregation of Au@SiO2 NPs with temperature variation. We anticipate that the suggested strategy can be employed in many practical applications such as optical bio-imaging and optoelectronic devices.


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INTRODUCTION Gold nanoparticles (Au NPs) are of particular interests due to their unique optical properties with excellent stability1-4; thus, employed in many applications such as upconversion,5-6 chemical/biological sensor,7-8 optoelectronics,9-10 photocatalyst,11-12 drug delivery,13-15 and photothermal therapy.16-17 The application of Au NPs is fundamentally based on their intrinsic localized surface plasmon resonance (LSPR) effect. LSPR occurs from the collective coherent oscillations of conduction electrons near the metal/dielectric interfaces when the size of the NPs is much smaller than the wavelength of the incident light.18 To employ the LSPR effect with NPs, Au NPs are more frequently employed than other metal NPs because they have a very strong plasmonic absorption band with high sensitivity at visible wavelengths. In principle, the plasmon characteristics of Au NPs are based on the size, shape and the assembling structure of Au NPs. Further, an additional LSPR effect can be observed when the spacing between particles is small enough to delocalize surface electrons causing plasmon coupling.19-21 Therefore, numerous studies have been dedicated to explore the various LSPR effects of Au NPs over a wide range of wavelengths. Many synthetic approaches have been suggested since delicate changes in size and shape of NPs can modulate the LSPR effects.22-24 However, these approaches often require complicated experiments and processes, or difficult to provide LSPR effects over a wide range of wavelengths. On the other hand, the self-assembly of nanoparticles is another approach; LSPR can be effectively shifted to other wavelengths when Au NPs form clusters or aggregates.25-26 To be able to use this aggregation-induced LSPR effect in practical applications, one needs to control the shape and size of aggregates (the degree of aggregation) precisely. Many


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experimental approaches have been introduced to form the desired shape and size of aggregates using biomolecules, ligands, cross-linkers or polymer grafting.15, 27-31 A recent study has shown that Au NPs aggregation can be incorporated by mixing with block copolymers32-33; however, the degree of aggregation was not controlled. Also, Bootharaju et al. proposed a method of forming metallic nanoclusters with ligand-exchange by controlling ligand-metal binding energy29; however, it only worked with thiol-based ligands. Liu et al. also showed the precise control of LSPR wavelength with Au-DNA nanoconjugated system27. Despite the possibility of tuning shape and size of aggregates through self-assembly, these approaches require complicated recipes that are labor-intensive, time-consuming and often cause stability problems. Another problem with aggregation-associated LSPR effect is that it cannot cover a wide range of wavelengths. Van Haute et al. introduced a method for creating metallic nanoclusters with small molecule cross-linkers as capping agents15. The size and shape of clusters could be well-controlled with low polydispersity; but could not reach longer wavelengths of the visible range, only up to 552 nm. To increase the operating range of LSPR effect, surface-to-surface distance of NPs should be considered as well as the size and shape of aggregates. Generally, inter-surface distance of metal NPs is often controlled by creating inorganic shell layers; coating silica onto gold nanoparticles (Au@SiO2 NPs) tune LSPR effect and provide further stability.34-35 In this study, we introduce the controlled aggregation-driven LSPR effect of Au@SiO2 NPs mediated by the addition of polymer. Here, adding polymers provide a good stability when they are adsorbed onto NPs; however, it drives aggregation when adsorption is lost. We show that controlling interparticle interactions with polymers can form a variety of nanoparticle aggregates in the aqueous system. In addition, this polymer-mediated interparticle interactions are temperature-dependent and thus can effectively change the size and shape of aggregates


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through temperature changes. More importantly, this aggregation-driven LSPR effect with polymers was found in a wide range of visible wavelengths, exhibiting distinctive color changes. Au NPs are synthesized and thinly coated with silica. A low molecular weight of poly(ethylene glycol) (PEG) is then added to aim to control the degree of nanoparticle aggregation. First, we present the effect of LSPR on polymer concentration, time, and temperature with ultraviolet-visible (UV-Vis) spectroscopy and confirm the corresponding microstructure of aggregates with small-angle X-ray scattering (SAXS) experiments. Next, the aggregation induced LSPR effect is explained theoretically with discrete dipole approximation (DDA) calculation results. Finally, we show that the state of Au@SiO2 aggregations with corresponding LSPR effects can be controlled through temperature quenching. In addition, we discuss the stability of these trapped nanoaggregates for a wide range of pH and high ionic strength changes. Various polymer molecular weight and the polymer end-function effects on the polymer-driven aggregation are also discussed.

EXPERIMENTAL Materials. Poly(ethylene glycol) (PEG) with number average molecular weights (Mn) of 400, 3350 and 20,000 g/mol, dimethyl terminated PEG (Mn = 500 g/mol) and tetraethyl orthosilicate (TEOS) were purchased from Sigma-Aldrich. Tetra-chloroauric (III) acid (HAuCl4˖3H2O) and trisodium citrate dihydrate (Na-cit) were purchased from ACROS Organics. Sodium chloride (NaCl, 99.5%), hydrochloric acid (HCl, 35.0-37.0%), ammonia solution (NH4OH, 28-30%) and ethanol (99.8%) were purchased from SAMCHUN chemical. All chemicals were used as received with no further purification.


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Particle Synthesis and sample Preparation. Silica coated-gold nanoparticles (Au@SiO2 NPs) used in this study were synthesized following two steps. First, Au NPs were synthesized based on the method of E. Mine et al.,36-37 which accompanies the reduction of HAuCl4 with Na-cit. Freshly prepared 0.94 ml of 0.34 M Na-cit solution was quickly added to 200 ml of 0.24 mM HAuCl4 solution at 80 ºC under vigorous stirring. Then, the color of mixture gradually changed from dilute yellow to red wine within a few minutes, indicating synthesis of 15 nm Au NPs. Exact particle diameter was determined from TEM and SAXS measurements; TEM image analysis by Image J yielded a diameter of 15.1 ± 1.1 nm averaged by 41 NPs and a scattering fitting to the form factor of spherical particles yielded a diameter of 14.9 ± 1.8 nm. Next, the surface of Au NPs was coated with silica based on the modified Stöber method.38-39 The 50 mL of Au NPs solution prepared from the first step was mixed with 500 mL of ethanol, followed by injection 5 mL of 35.7 mM TEOS with 5 µL/sec speed. The reaction was run at 50 ºC for 4 hours after adding 30 mL of ammonia solution. Final solution was transferred to aqueous system with excess deionized water and concentrated by 15 times by heating and stirring in a ventilation hood. After solvent transfer, thick silica shell was washed away leaving a thin silica shell. The total diameter of synthesized Au@SiO2 NPs was 15.9 ± 2.8 nm with shell thickness of 0.49 ± 0.19 nm determined from the form factor fitting of SAXS data and TEM micrograph (Figure S2). The color of the Au@SiO2 NPs solution remains red wine, similar to Au NP solutions. The concentration of Au@SiO2 NPs was approximately 8.31×10-10 M. Au@SiO2 solutions at various polymer concentrations were prepared by mixing polymers on a vortex mixer. All polymers were completely miscible with Au@SiO2 solutions at room temperature and


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the samples were kept separately at 20 ºC and 70 ºC in a water bath to prevent solvent evaporation. Diluted HCl and NH4OH aqueous solutions were used to adjust the pH of Au@SiO2 polymer solution. The 10, 50, and 100 mM of NaCl aqueous solutions were prepared and mixed with the Au@SiO2 polymer solution to meet the desired ionic strength. Ultraviolet-Visible (UV-Vis) Spectroscopy. The absorbance spectra of Au@SiO2 NPs and PEG mixture at each time were obtained by V-760 UV-visible/NIR spectrometer (JASCO Analytical Instruments) using 10 mm optical glass cuvette cells. The light source was halogen and deuterium lamp with double monochromator and photomultiplier tube detector was used. The scanning speed was 1000 nm/min with a fast response. The intensity was normalized at the wavelength of 450 nm where no LSPR changes were found Small-Angle X-ray Scattering (SAXS) Measurements. SAXS experiments were performed at the 9A beamline of the Pohang Accelerator Laboratory (PAL) to explore the dispersion of Au@SiO2 NPs in the polymer solutions; a sample-to-detector distance of 2 m and radiation wavelength, λ, of 0.7994 Å were employed. The scattered X-rays were recorded with a Mar charge-coupled device (CCD) area detector. The two-dimensional SAXS patterns were then azimuthally averaged and the relative one-dimensional scattering intensity was plotted as a function of the scattering vector, q (q = (4π·sin(θ/2))/λ), where θ is the scattering angle. Zeta Potential and pH Measurements. The zeta potential (ζ-potential) of Au NPs and Au@SiO2 NPs solutions with polymers were measured with a Zetasizer Nano ZS90 (Malvern Instrument) using disposable folded capillary cells. The light source was a He-Ne laser (λ = 633 nm, maximum power = 4 mW) and the light scattered at an angle of approximately 13º was detected. All measurements were conducted at 25 ºC. The pH of each solution was measured


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with a S220 SevenCompact pH/ion meter (METTLER TOLEDO) using an InlabExpert Pro-ISM electrode. Transmission Electron Microscopy (TEM). TEM images were obtained with JEOL 2100F microscope operating at 200 kV acceleration voltages with 0.102 nm lattice resolution. The samples were dropped on a square mesh carbon-coated copper grid from Electron Microscope Sciences and dried in a vacuum oven for 1 day. Discrete dipole approximation (DDA) calculation. In this study, the discrete dipole approximation (DDA) calculation40-42 method was used to estimate the optical properties (i.e., extinction, absorption, and scattering spectra) and the electric field distributions of Au@SiO2 NPs. The DDA calculation can be applied to an arbitrary shape of target material, which has dielectric function(s). In this calculation, dipole moment (Pi) is solved for the target system, which is divided into a finite array of dipole points at constant spacing (i.e., dipole spacing, d). Notably, there is an essential criterion to follow; |m|kd < 1, where |m| is complex refractive index, k is reciprocal value of the incident wavelength, and d is the dipole spacing between adjacent dipole points. To perform the DDA calculation, we used the DDSCAT program (ver 7.3.0) developed by Drain and Flatau.41 Model systems are discussed in details with necessary information on quantum effect of Au NP and electron tunneling effect between Au NPs (see Calculation details for the discrete dipole approximation (DDA) calculation of Supporting Information).

RESULTS AND DISCUSSION The Au NPs were firstly synthesized with a diameter of 15 nm and showed a characteristic absorbance maximum at 521 nm (Figure S1) in agreement with previous studies.38-


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39

After surface coating with silica and following treatment described in the Experimental

Section, Au@SiO2 NPs were formed with a very thin silica layer terminated by silanol groups (SiOH). The diameter and layer thickness of Au@SiO2 NPs were characterized with SAXS form factor fitting to the scattering intensity and TEM images: 14.9 nm and 0.49 nm, respectively. (see the details in section 1 of Supporting Information, Figure S2 and Table S1). Absorbance spectrum of Au@SiO2 NPs is broader than that of Au NPs while the peak position only moved subtly. Despite a very thin layer of silica coating, there was a dramatic change in their surface charge such that zeta-potential has decreased from -43 mV to -70 mV with silica coating, due to the dissociated silanol groups on silica surfaces in aqueous solution. The synthesized Au@SiO2 NPs were highly stable and did not show any aggregation at room temperature for months, because of the highly repulsive electrostatic interaction between particles. To control the interparticle interactions, we added low molecular weight of PEG, 400 g/mol. In many colloidal systems, polymers often drive additional interactions such as entropic depletion attractions when polymers are non-adsorbing43-44 or enthalpic steric repulsions when polymers are adsorbing45-46. While PEG is well-known to adsorb to silica surface providing steric repulsion, remaining PEG in bulk can still act as a depletant47-48; thus one expect that the Au@SiO2 NPs may experience complicated polymer-induced interactions in the presence of PEG. The overall aggregation process with adding polymers is demonstrated in Scheme 1. In the absence of polymers, aggregation is not observed at room temperature. The addition of polymers induces particle aggregation, but the degree of aggregation at a fixed polymer concentration cannot be controlled at room temperature. Higher temperature leads to higher


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degree of particle aggregation; however, the sizes of aggregates continuously grow over time. We note that the formed aggregates at high temperature can be trapped at a desired level of aggregation with temperature drop.

Scheme 1. Adding polymer-, temperature- and time-dependent Au@SiO2 NPs aggregation.

Polymer Concentration-, Temperature-, Time-Dependent Plasmonic Transition. Figure 1a shows the UV-Vis spectra for Au@SiO2 NPs at various polymer concentrations (cp) at 20 °C after 6 hours from preparation. The characteristic peak of Au@SiO2 NPs progressively shifts to higher wavelength (up to 550 nm) and broadens the width with adding PEGs. While the intensity of characteristic peak of single Au@SiO2 NP progressively decreases, a new peak (secondary peak) emerges at a higher wavelength. The clear shift of peak positions is


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also consistent with the color changes of Au@SiO2 solutions; color changes from red at cp = 0 to purple at cp = 20 wt.%, and bluish at higher cp than 33 wt.%. The observed peak shifts at low temperature are believed to be the result of particle aggregation as polymer induces depletion attractions. Note that depletion attraction becomes stronger with polymer concentrations.49-50 The critical overlap polymer concentration is about 23 wt% as calculated in Supporting Information. The surface-to-surface distance of Au NPs is close enough to delocalize electrons, but not zero because of thin layer of silica.


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Figure 1. Normalized absorption spectra in UV-Vis range and optical images of Au@SiO2 NPs in PEG solutions at varying cp after being (a) 6 hours and (b) 28 days at 20 °C. The corresponding spectra and images are in (c) and (d) at 70 °C, respectively. The change of UV-Vis spectra was quite instantaneous with adding polymers and did not substantially changes with time at 20 °C. Figure 1b shows absorption spectra for the same samples obtained after 28 days. The peak positions were maintained at the same wavelengths, indicating the aggregates did not grow further as illustrated in Scheme 1. While aggregated particles can settle down due to the density differences between Au@SiO2 NPs and aqueous polymer solution, which decreases absorbance intensity, they can be readily re-dispersed by shaking solutions. Previously, similar polymer concentration dependent aggregation was studied in PEG-silica systems, indicating that depletion attraction is indeed strongly concentration dependent47 (see the depletion calculation in section 6 of Supporting Information and Figure S14e). The UV-Vis spectra of Au@SiO2 NPs change more dramatically at high temperature, 70 °C (Figure 1c and d). First, a distinguishable secondary peak separated from the original single particle LSPR peak emerges near 700 nm at concentrations higher than 20 wt.%. Compared to the spectra at 20 °C at the same cp, the peak shifts to a higher wavelength as the color changes accordingly; thus, the size and shape of aggregates at 70 °C would be significantly different from those of at 20 °C at a given polymer concentration. Second, a strong timedependent aggregation is predicted based on the rapidly changing absorption spectra over time. Figure 1d shows the spectra after 28 days. At higher concentrations (cp > 20 wt.%), NPs almost lost their characteristic peaks, and as it turned into a clear solution with settling, hardly absorbed. At low concentrations (cp < 10 wt.%), NPs retain the characteristic single nanoparticle peak but


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the secondary peaks were noticeably widened at around 650 nm. The NPs aggregate continuously at 70 °C, changing the plasmon characteristics of NPs over time. The reported temperature dependent- aggregation is expected to be found at other temperatures as the solvent quality/polymer adsorption decreases with temperatures. In the similar systems, silica nanoparticles can be stable up to 40 °C, but aggregated at 60 °C.48, 51-52 The Au@SiO2 NPs can be aggregated at temperatures below 70 °C; however, the rate of aggregation is expected to be slower. Thus we chose 70 °C to observe the dynamic aggregation and its LSPR effect within a certain amount of time.

Figure 2. (a) Absorption spectra of cp = 10 wt.% stored at 70 °C with times in UV-Vis range. (b) Time-resolved secondary LSPR peaks are plotted as a function of polymer concentration


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dependent on temperatures; black squares and red spheres for 20 °C and 70 °C, and solid lines also dashed lines for initial state and a month later, respectively. Experimental scattered intensities, I(q), versus q at (c) 20 °C and (d) 70 °C are plotted with Au@SiO2 NPs form factor at cp = 33 wt.%. Inset figure represents power law exponents in the Guinier region over times at the same cp; black spheres and orange stars for 20 °C and 70 °C stored samples, respectively.

Time-Dependent Plasmonic Transition of Au@SiO2 with Aggregation. Time-resolved UV-Vis spectra of Au@SiO2 in 10 wt.% of PEG solution at 70 °C is shown in Figure 2a. The characteristic peak of single NPs retains over time; however, peak width becomes broad with a separated secondary peak around 650 nm. The color change was also observed: it was red within a few hours after preparation and became purple after 28 days (Figure S3a). Figure 2b shows how the secondary peak position varies with cp at different times and temperatures. The higher wavelength where the peak is located indicates that more aggregated particles are present. As shown in Figure 2b the secondary peak moves to the higher wavelengths indicating higher degree of aggregation with increasing cp.26, 53 The peaks from 70 °C are located at much higher wavelength than those from 20 °C as shown in Figure 2b. We confirm that as the time increases, the peak position hardly changes at 20 °C and there is a significant increase at 70 °C. While SEM/TEM images are commonly necessary for studies of nanoparticle aggregation, imaging the state of dispersions at high concentration of polymer solutions was not available in this system. Because of the low Tg of PEG 400 (-5 °C), the sample could not be


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readily dried and adsorbed polymer on particles were too sensitive to E-beam even at low eV and low temperature. Instead, the detailed time-dependent structural evolution of aggregates was observed with extensive SAXS experiments. The scattered intensity was considered to arise only from the Au@SiO2 NPs assuming single component system and can be written as I(q)~Pc(q)Scc(q,ϕc) after background subtraction, where ϕc is the nanoparticle volume fraction, Pc(q) is the single-particle form factor, and Scc(q,ϕc) is the nanoparticle structure factor. Figure 2c and 2d show the scattering profiles of Au@SiO2 NPs at cp = 33 wt.% over time at 20 °C and 70 °C, respectively. The cp = 33 wt.% was chosen for the scattering experiments, which showed a progressive aggregation in a given time of 48 hrs. The trends of time- dependent aggregation will remain same at other concentrations on a different time scale. The scattering intensities of Au@SiO2 NPs at 20 °C increased at low q compared to that of form factor, which means a certain degree of nanoparticle aggregation occurred due to the adding polymer. On the other hand, the shape and curvature of the scattering intensities did not change for 120 hours. Figure 2d shows that nanoparticles were rapidly aggregated at 70 °C, indicated by continuous slope changes with time, from p = 1.4 to p = 1.7. In the Guinier (low q) region, scattering intensity of primary particles follows power-law decay function such that I(q)~NSq-p, where S is the surface area, N is the number density and p corresponds to the mass fractal dimension of aggregates. The value of p was less than 2 in all cases, implying diffusion limited aggregation.54-55 The exponents remain constant in the low q region at 20 °C implying formed aggregates do not grow, while it continuously increases at 70 °C indicating aggregates grow into more compact structures as shown in the inset figure in Figure 2d. These changes are also easily observed with UV-Vis absorption spectra and photographs (Figure S3). Here we note that the


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transition to aggregates is not a simple dimers or trimers, but a transition to a massive fractal structured aggregate, which is different from previous study.56 The temperature- or time- dependent aggregation is mainly based on the temperature dependent interactions between silica surface and polymer. Noting the polymer molecular weight of polymer is low, possible bridging aggregations between the particles can be excluded. Therefore, we expect that the temperature dependent aggregation remains same at high particle concentrations. Overall, one observes clearly different temperature effect with time. Fast aggregation explores the LSPR effect at broad wavelengths at 70 °C. However, it steadily grows over time and thus could not maintain a certain size of aggregate and its spectrum. On the other hand, when small aggregates were formed at 20 °C, additional aggregation at 20 °C was too slow or absent to observe a dynamic change in the LSPR effect.

Mechanism of Aggregation Driven Plasmonic Transition: DDA Calculation. To understand the plasmon characteristics of Au@SiO2 NPs, we calculated extinction spectra and electric field distribution for different types of aggregated nanoparticles through the DDA calculation (see section 3, 4 and Figure S6 of Supporting Information). These observations imply that the peak separation and red-shift results shown in experimental UV-Vis absorption spectra (Figure 1) are due to the large and compact structure of particle aggregates.

Aggregation Driven Controllable Plasmonic Transition with Temperature.


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We have noted that the aggregation driven plasmonic transition of Au@SiO2 NPs depends on temperature. In this regard, we assumed that the structure and size of aggregated nanoparticles at 70 °C can be kinetically trapped by lowering temperature quickly to 20 °C. To confirm this hypothesis, we first grew the aggregates in the presence of polymers at 70 °C for certain hours, and then cooled the system. (Scheme 1) Figure 3a shows scattering profiles of Au@SiO2 NPs with controlled aggregation. At 70 °C, Au@SiO2 NPs in 33 wt.% of PEG solution rapidly aggregated for 14 hours; the scattering intensity lost the form factor curvature and exponent p increased from 1.4 to 1.6 at low q, which showed diffusion limited aggregation once again. Then, we quenched the sample and stored it at 20 °C. (see the curve “70 °C 14 h 20 °C 2 h” and “70 °C 14 h 20 °C 14 h”) The scattering curves showed no change after cooling 20 °C, indicating the formed aggregates did not grow further. Figure 3b shows UV-Vis absorption spectra for growing and trapped aggregation of Au@SiO2 NPs at cp = 33 wt.%. At 70 °C without quenching, plasmonic shifts was observed over time as aggregates continued to grow, confirmed by the clear red-shift after 28 days. However, when the further aggregation was trapped after 12 hours being at 70 °C by removing the sample and restoring at 20 °C, no plasmonic changes were found even after 28 days.


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Figure 3. Time-resolved (a) scattering profiles, (b) absorption spectra, and (c) summarized secondary LSPR peak positions of aggregation controlled Au@SiO2 NPs under various keeping conditions. Different degrees of aggregations are expected depending on the temperature and quenching time as illustrated in the right panel of (c). All data was obtained at cp = 33 wt.%. The summarized changes of secondary LSPR peak position are given in Figure 3c. A continuous plasmonic transition is found with particles being aggregated at 70 °C, observing the changed location of the secondary LSPR peaks. The peak shifts to the higher wavelength indicate that the size of aggregates become larger and the particles distance becomes more compact with time whereas the constant peak position implies the further aggregations is prevented as formed cluster becomes stable. The original surface plasmon band of Au@SiO2


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NPs at cp = 33 wt.% was located at 594 nm from initial aggregation and keeping this sample at 20 °C did not change the peak position. After 5 hours at 70 °C, the secondary peak moved to 665 nm and by quenching to 20 °C, its aggregation was trapped; the peak position did not change anymore. Likewise, after 12 hours at 70 °C, the secondary peak moved to 680 nm. If the aggregation was trapped at this state by quenching to 20 °C, peak position did not change. Therefore, by changing the temperature, one can effectively tune the LSPR effect with controlled aggregation. Aggregation can be easily trapped with quenching the temperature and trapped aggregates become stable and do not grow at the quenched temperature. If temperature is increased again, the formed aggregate will start to grow again. Same results were observed at other polymer concentrations (Figure S4 and S5). The dispersion state can be trapped even when dried if the system can be properly quenched without a time delay. Note the Tg of PEG 400 is about -5 °C; thus, the system requires a low enough temperature to be trapped. Temperature Dependent- Total Pair-Wise Interaction Potentials of Au@SiO2 NPs. Kinetically trapped aggregation with temperature quenching implies that interparticle interactions are qualitatively different at different temperatures. To understand the intrinsic nature of trapped aggregation, we calculated pair-wise interaction potentials at two different temperatures based on the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory57-58 and polymer induced-interactions theory.59-61 The total interaction potentials are plotted in Figure 4 and can be written as,

V (r ) = VA (r ) + VR (r ) + VP (r ) + VS (r ) where VA(r) is van der Waals attraction, VR(r) is electrostatic repulsion, VP(r) is polymermediated depletion attraction, and VS(r) is steric repulsion as a function of surface-to-surface, r.


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The detailed procedures and relevant equations are given in section 6 of Supporting Information. We found that the electrostatic repulsions and van der Waals attractions do not change significantly with temperature, based on the measured zeta potential (ζ-potential) and calculated Hamaker constants. Thus, we hypothesized that the substantial change of particle interaction arises from the polymer-induced interactions. We previously studied the stability of silica NPs in PEG 400 solutions with varying temperature52. Based on the similarity between the previous and present studies, we hypothesized that the strong temperature-dependent polymer adsorption causes the stability of Au@SiO2 NPs. Polymer adsorption at 20 °C brings additional repulsive interaction while dramatic reduction of steric repulsions was found at 70 °C. Total interparticle potentials at all polymer concentrations are shown in Figure S14 (details in section 6 of Supporting Information).

Figure 4. Calculated total pair-wise interaction potentials, V(r)/kT, of Au@SiO2 NPs at cp = 33 wt.% (a) 20 °C and (b) 70 °C, are plotted as a function of surface-to-surface distance, r. The cutoff distance was 0.1 nm. Pair-wise interaction potentials for Au@SiO2 NPs at cp = 33 wt.%, as a function of surface-to-surface distance for van der Waals attraction, electrostatic repulsion, depletion


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attraction, steric repulsion and total interactions, are given in Figure 4 at two temperatures, 20 °C and 70 °C. At 20 °C, NPs undergo attractive inter-particle potentials; however, there is an energy barrier at a close proximity by purely repulsive nature of electrostatic and steric repulsions. An attractive well creates small aggregates, which can remain stable. We note that the depth of the attractive well is only 1 kT; thus, formed aggregates can remain stable even after vigorous vortex mixing. The depth of the well may vary in the order of a few kT. At 70 °C, NPs experience much stronger attractions due to the absence of steric repulsions. This is consistent with the fast aggregation observed at 70 °C. When the temperature drops from 70 °C to 20 °C, NPs form stable aggregates by regaining steric repulsion and stopping aggregation. Therefore, plasmonic behavior can be effectively controlled by temperature dependent polymer-nanoparticle interactions. The calculated pair-wise interaction potentials at different cp show similar temperaturedependent interactions (Figure S14). Depletion attraction is effectively varied with cp in the order of only -1 ~ -3 kT. Electrostatic repulsion is more strongly dependent on cp such that surface charge and Debye length decreases with cp, which attribute to the different degree of aggregations occurring at 20 °C. The details are given in section 6 of Supporting Information and Ref.47, 60

Non-Controlled Aggregation with Bare Au NPs. The trapped aggregation and controllable LSPR effect is originated from a strong temperature dependent-PEG adsorption onto silica. Therefore, bare Au NPs could not exhibit the presented trapped aggregation (see the section 7 in Supporting Information and Figure S15). Compared to Au@SiO2 NPs in PEG 400 solution, the temperature dependent-stability of Au NPs


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in PEG 400 solutions is substantially different. First, bare Au NPs are less stable than Au@SiO2 NPs at 20 °C because of lower surface charge and the absence of steric repulsion.34, 62-63 Second, no distinctive secondary peak is observed but the peak is red-shifted with adding polymers, indicating small aggregation of Au NPs. Third, the peak is continuously red-shifted over time at 20 °C, presumable due to progressed aggregation. Last, the peak is slightly blue-shifted at 70 °C, which is in the opposite direction to the Au@SiO2 system. Au NPs become more stable with increasing temperature as predicted from general colloidal phase diagram.64 Thus, we emphasize that the unique temperature-dependent aggregation and corresponding LSPR properties are only available with silica coated gold nanoparticles.

Stability of Au@SiO2 Aggregates for Various pH and Ionic Strength Changes. We further examined the stability of trapped Au@SiO2 aggregates at various environmental conditions. Au@SiO2 NPs solution at cp = 20 or 33 wt.% was left at 70 °C for 12 hours and quenched to 20 °C, which showed the first and secondary peaks in UV-Vis absorption spectra from single particle and aggregates, respectively. We confirmed that these aggregates remain stable and retain the same optical property for more than 28 days at room temperature (Figure 3b). Noting the position of the secondary LSPR peak indicates the degree of aggregation, we also monitored the position of secondary LSPR peak at different pH and ionic strengths. pH range was adjusted from 4.1 to 8.5 and ionic strength was varied up to 100 mM by adding NaCl.


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Figure 5. Normalized absorption spectra of trapped Au@SiO2 aggregates (a) for varying pH at cp = 20 wt.% and (b) for varying ionic strength at cp = 33 wt.% 13 days after the preparation. Figure 5 confirms a great stability of trapped aggregates at various pH and ionic strength with non-changing the position of the secondary LSPR peaks. Only subtle peak shift was found at the highest ionic strength after 13 days. Thus, we conclude that the stability of Au@SiO2 NPs is insensitive to pH and ionic strength, unlike bare Au or silica NPs.30, 65 Temperature-dependent aggregation was originated from temperature-dependent polymer adsorption; PEG can adsorb onto silica surface via hydrogen bonding (H-bonding)52 and pH or ionic strength do not affect the adsorption ability.

Polymer Molecular Weight and End-Group Effect on Controlled Plasmonic Transition. Temperature-dependent particle stability is qualitatively different with increasing PEG molecular weight, where PEG adsorption is much less sensitive to temperature as predicted from the previous studies66-67. Increasing molecular weight of PEG creates bigger steric repulsion which enhanced the stability of Au@SiO2 NPs. Figure S16 shows that UV-Vis spectra of Au@SiO2 NPs in PEG 3350 and 20,000 g/mol solutions only exhibit a peak from single


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Au@SiO2 NPs at both temperatures with no peak shift and separation dependent on cp. Even at 70 °C, particles remain stable, consistent with reddish color of solution. Because temperature-dependent polymer adsorption relies on the H-bonding ability of PEG68-69, varying end-group of PEG changes temperature-dependent Au@SiO2 NP stability. Figure 6a and 6b show absorption spectra of Au@SiO2 NPs in dimethyl terminated PEG (DMTPEG) 500 g/mol solution. Both qualitative and quantitative differences were found between hydroxyl and dimethyl terminated systems. First, aggregation in dimethyl terminated PEG solution is more pronounced compared to that in hydroxyl terminated PEG solution at the same cp at 20 °C. Thus, the secondary LSPR peaks of DMTPEG system are located at higher wavelengths as shown in Figure 6c. Additionally, the secondary LSPR peak continuously increases to 723 nm at 70 °C with time (Figure S17a). The adsorption ability of H-bonding of dimethyl terminated PEG is significantly reduced because H-bonding between SiOH and OH terminated groups is stronger than that between SiOH and methoxy groups.70 Thus, reduced steric repulsions destabilized NPs at a given cp in DMTPEG system. Second, for the reason described, Au@SiO2 NPs solutions with DMTPEG are weakly dependent on temperature compared to that with hydroxyl terminated PEG. Figure 6c shows the location of secondary LSPR peaks as a function of cp at different temperatures compared with both PEGs. From 20 °C to 70 °C, the peak was shifted by 40 – 60 nm from hydroxyl terminated PEG whereas only shifted by 10 – 25 nm from dimethyl terminated PEG. Thus, reduced absorption ability could contribute to weakened temperature dependency. Third, the adsorption amount decreased, but showed similar temperature-dependent stability. Thus, one can also tune the LSPR effect with dimethyl terminated PEGs and obtain the desired optical property with temperature (see the section 9 in Supporting Information and Figure S17b).


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Figure 6. Normalized UV-Vis absorption spectra of Au@SiO2 NPs in DMTPEG solutions at varying cp with optical images at (a) 20 °C and (b) 70 °C after several hours from the preparation. (c) The position of secondary LSPR peaks of Au@SiO2 NPs in DMTPEG solutions are plotted at different temperatures in comparison to that in PEG solutions.

CONCLUSION In this study, we demonstrated that the plasmonic property of Au@SiO2 NPs can be effectively tuned with temperature-dependent aggregations. While Au@SiO2 NPs rapidly form clusters at high temperature, they remain stable at low temperature. Therefore, the aggregation can be effectively trapped by lowering temperature. The merit of this proposed approach is that one can obtain a desired plasmonic transition via controlled aggregation, without complicated synthesis and toxic materials. Based on the universality of steric and depletion forces, we believe the proposed study can be extended or combined with other existing methods for further exploration of LSPR effect. We have theoretically shown that the plasmon coupling induced by the plasmon hybridization of Au NPs and the convolution effect of different types of Au NPs (e.g., monomer and dimer) are the primary factors of peak splitting of plasmonic spectra, where the secondary


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LSPR peak is mainly caused by the BDP mode of two Au NPs whereas the main peak is induced by the dipole resonance of monomer and the HHP mode of two Au NPs. Effective control for particle stability is realized with temperature-sensitive adsorption ability of low molecular weight PEG onto silica. Not only LSPR effect is found in a wide range of wavelengths showing peak shifts up to near infrared, but formed aggregates at each level of aggregation showed an exceptional stability against pH and ionic strength change. More precise control of wavelength with narrow peaks remains future work. Therefore, we believe this study can provide a new strategy for an effective control of LSPR, based on the fundamental understandings of colloidal dispersions. We further anticipate that the suggested strategy can be employed in many practical applications such as optical bioimaging and optoelectronic devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail [email protected] (S. Y. K.); +82 52 217 2558, [email protected] (S. K. K.); +82 52 217 2541


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by Ministry of Education (NRF-2014R1A1A2056774) and by the year of 2017 Research Fund of the Ulsan National Institute of Science and Technology (UNIST) (1.170009.01). SAXS experiments were performed at 9A UNIST-PAL Beamline of the Pohang Accelerator Laboratory. S. Y. K. and S. K. K. also acknowledge financial support from NRF-2016H1A2A1907114 and NRF-2015H1A2A1033828 (Global Ph. D Fellowship Program) and computational support from UNIST-HPC and KISTI-PLSI. REFERENCES 1. Jones, S. T.; Walsh-Korb, Z.; Barrow, S. J.; Henderson, S. L.; del Barrio, J.; Scherman, O. A., The Importance of Excess Poly(N-isopropylacrylamide) for the Aggregation of Poly(Nisopropylacrylamide)-Coated Gold Nanoparticles. ACS Nano 2016, 10 (3), 3158-3165. 2. Jain, P. K.; El-Sayed, M. A., Plasmonic Coupling in Noble Metal Nanostructures. Chem. Phys. Lett. 2010, 487 (4-6), 153-164. 3. Gao, J.; Huang, X.; Liu, H.; Zan, F.; Ren, J., Colloidal Stability of Gold Nanoparticles Modified with Thiol Compounds: Bioconjugation and Application in Cancer Cell Imaging. Langmuir 2012, 28 (9), 4464-4471. 4. Pregent, S.; Lichtenstein, A.; Avinery, R.; Laser-Azogui, A.; Patolsky, F.; Beck, R., Probing the Interactions of Intrinsically Disordered Proteins Using Nanoparticle Tags. Nano Lett. 2015, 15 (5), 3080-3087. 5. Schietinger, S.; Aichele, T.; Wang, H. Q.; Nann, T.; Benson, O., Plasmon-Enhanced Upconversion in Single NaYF4:Yb3+/Er3+ Codoped Nanocrystals. Nano Lett. 2010, 10 (1), 134138. 6. Wu, D. M.; Garcia-Etxarri, A.; Salleo, A.; Dionne, J. A., Plasmon-Enhanced Upconversion. J. Phys. Chem. Lett. 2014, 5 (22), 4020-4031. 7. Sherry, L. J.; Jin, R. C.; Mirkin, C. A.; Schatz, G. C.; Van Duyne, R. P., Localized Surface Plasmon Resonance Spectroscopy of Single Silver Triangular Nanoprisms. Nano Lett. 2006, 6 (9), 2060-2065.


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Aggregation-Driven Controllable Plasmonic ... - ACS Publications

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