Gold Nanoparticle Layers on Polystyrene Microspheres of Controlled

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Gold nanoparticle layers at polystyrene microspheres of controlled structure and electrokinetic properties Magdalena O#wieja, Dawid Lupa, and Zbigniew Adamczyk Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01491 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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Fig. 1. Size distribution of a) PSMs600 and b) AuNPs derived by SEM imaging. The inset in part a) shows the PSM layer at silicon wafer (deposition conditions: cPSMs600= 60 mg L-1, I=10-3 M NaCl, pH 5.6); the inset in part b) shows the AuNP layers at silicon wafers (deposition conditions: cAuNPs= 64 mg L-1, I=10-3 M NaCl, pH 5.6).

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Fig. 2. Dependences of hydrodynamic diameter of a) PSMs600 and b) AuNPs on pH determined for (●) I=104 M NaCl, (■) I=10-3 M NaCl and (▲) I=3x10-3 M NaCl. Measurements conditions: cPSMs600=60 mg L-1, cAuNPs=50 mg L-1, T=298 K. The solid lines represent fits of experimental results. 151x119mm (300 x 300 DPI)


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Fig. 2. Dependences of hydrodynamic diameter of a) PSMs600 and b) AuNPs on pH determined for (●) I=104 M NaCl, (■) I=10-3 M NaCl and (▲) I=3x10-3 M NaCl. Measurements conditions: cPSMs600=60 mg L-1, cAuNPs=50 mg L-1, T=298 K. The solid lines represent fits of experimental results. 151x119mm (300 x 300 DPI)


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Fig. 3. Dependence of the zeta potential of the PSMs600 and the AuNPs on pH determined for 1 (●) I=10-4 M NaCl, 2 (■) I=10-3 M NaCl and 3 (▲) I=3x10-3 M NaCl. Measurement conditions: cPSMs600=60 mg L-1, cAuNPs=50 mg L-1, T=298 K. The lines represent nonlinear fits of experimental results. 152x119mm (300 x 300 DPI)


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Fig. 4. Dependence of zeta potential of the PSMs600 on the AuNP concentration in the suspension (the upper horizontal axis) and the nominal coverage of nanoparticles at microparticles calculated from Eq. (2) (the lower horizontal axis). Deposition conditions: cPSMs600=60 mg L-1, T=298 K, pH 5.6, ionic strength: a) 104 M, b) 3x10-3 M. The solid lines denote theoretical results calculated from the general electrokinetic model Eq. (3). 152x125mm (300 x 300 DPI)


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Fig. 4. Dependence of zeta potential of the PSMs600 on the AuNP concentration in the suspension (the upper horizontal axis) and the nominal coverage of nanoparticles at microparticles calculated from Eq. (2) (the lower horizontal axis). Deposition conditions: cPSMs600=60 mg L-1, T=298 K, pH 5.6, ionic strength: a) 104 M, b) 3x10-3 M. The solid lines denote theoretical results calculated from the general electrokinetic model Eq. (3). 152x125mm (300 x 300 DPI)


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Fig. 5. SEM micrographs presenting the AuNPs layers at the PSMs600 characterized by the coverage of: a) θ=0.24 (Ns= 2123 µm-2) and b) θ=0.33 (Ns= 2919 µm-2). 119x95mm (271 x 271 DPI)


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Fig. 5. SEM micrographs presenting the AuNPs layers at the PSMs600 characterized by the coverage of: a) θ=0.24 (Ns= 2123 µm-2) and b) θ=0.33 (Ns= 2919 µm-2). 1393x1000mm (28 x 28 DPI)


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Fig.6. Dependence of the AuNP coverage at the PSMs600 on their initial concentration in the mixture. The solid line shows the nominal coverage of AuNPs calculated from Eq. (2). The points represent experimental results obtained for ionic strength equal to 3x10-3 M, pH 5.6 and the PSMs600 concentration of 60 mg L-1; (●) concentration depletion method (AFM), results calculated from Eq.(6), (□) direct particle enumeration (SEM), (▲) direct measurements of electrophoretic mobility, particle coverage calculated from Eq.(3).

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Fig. 7. Dependence of the AuNP coverage at the silica substrate derived from AFM imaging (points) on their initial concentration in the suspension, pH 5.6, deposition time 30 min, The concentration of PSMs600 60 mg L-1. 1 (●) reference results obtained for pure AuNP suspension, 2 (●) 10-3 M and (●) 3x10-3 M. The solid lines show the linear extrapolation of the experimental data. The inset shows a typical AFM micrographs for the surface concentration of AuNPs equal to 530 µm-2. 155x121mm (300 x 300 DPI)


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Fig. 8. Dependence of zeta potential on pH at ionic strength 3x10-3 M: 1 (●) zeta potential derived from Eq.(7) for the AuNP layer deposited at microparticles of the coverage equal to 0.33. The line shows the interpolated results derived from the measurements of electrophoretic mobility of the AuNPs in the bulk. The dashed line shows the interpolated zeta potential of microparticles in the bulk . 152x119mm (300 x 300 DPI)


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Fig. S2. a) The AuNP size distribution derived from TEM micrographs (marked in grey, average particle size 13±3 nm) and SEM micrographs of the AuNPs deposited at the PSMs600 (marked in black, average particles size 12±3 nm), b) TEM micrograph presenting the AuNPs deposited on copper grid from the drop of suspension, c) SEM micrograph of the AuNPs deposited on the PSMs600. 150x118mm (300 x 300 DPI)


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Gold nanoparticle layers at polystyrene microspheres of controlled structure and electrokinetic properties

Magdalena Oćwieja*, Dawid Lupa, Zbigniew Adamczyk*

Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, PL-30239 Krakow, Poland

* Corresponding author Magdalena Oćwieja, Zbigniew Adamczyk Jerzy Haber Institute of Catalysis and Surface Chemistry PAS Niezapominajek 8 PL-30239 Krakow, Poland phone: +48126395104 e-mail: [email protected], [email protected]

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GRAPHICAL ABSTRACT


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ABSTRACT Formation of positively charged gold nanoparticle layers at polystyrene microparticles (PSMs600) was studied using the electrokinetic and the concentration depletion methods based on atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging. Primarily, the dependence of electrophoretic mobility of microparticles on the gold nanoparticle concentration in the suspension was measured. These results were quantitatively interpreted in terms of the 3D electrokinetic model. This allowed to derive a formula for calculating the coverage of nanoparticles under in situ conditions whose validity was confirmed by direct SEM imaging of deposited gold nanoparticles. Additionally, the maximum coverage of gold nanoparticles for various ionic strengths was determined using a concentration depletion method based on AFM imaging of residual particles deposited at the silica substrate. The maximum coverage increased with ionic strength attaining value of 0.35 for the ionic strength of 3x10-3 M. This effect was attributed to the decreasing range of lateral electrostatic interactions among deposited particles. The electrokinetic properties of the gold nanoparticle layers were also evaluated in pH cycling experiments that confirmed their stability. Beyond significance to basic science, the new data acquired in this work confirm the feasibility of preparing gold nanoparticle layers at polymer microparticles characterized by a controlled structure, coverage and electrokinetic properties.


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1. INTRODUCTION Colloid particles composed of polymer core and noble metal nanoparticle layer are of a considerable practical interest in catalysis Raman spectroscopy (SERS)

8-10

1-5

, for biosensing

6,7

and in surface enhanced

. For preparing such composite particle suspensions,

polystyrene microparticles (polystyrene beads) are often applied as the core material because they are readily available in a wide range of sizes, surface properties (charge) and low dispersity

9,11

. Additionally, polystyrene microparticles are biocompatible

12

and inert under

various conditions, which ensures a broad spectrum of their application in immunological diagnostics 13,14. Among various types of nanoparticles which can be immobilized on polystyrene microparticles, gold nanoparticles attract a special attention because of their unique electrical, optical, catalytic and biological properties

15,16

. Moreover, due to their biocompability, gold

nanoparticles are especially desirable for transporting and unloading of various drugs and pharmaceuticals 17. They are also applied in biological imaging and in cancer diagnostics 18. Preparation of gold nanoparticle-coated polystyrene microparticles is carried out using surface precipitation reactions surface seeding

10

2,9,19-21

, surface reactions

and a combination of these methods

22

, self-assembly processes

6,12,23,24

,

8,25

. One can observe that the surface

precipitation method plays a dominant role in the fabrication of gold nanoparticle-coated polystyrene microparticles. Lee et al.

21

described a direct metallization of the surface of

sulfonated-polystyre beads of an average diameter of 2.7 µm via the incorporation of cationic gold ligands into the microparticles and their subsequent reduction by sodium borohydride. Using scanning electron microscopy (SEM) it was found that the average diameter of deposited particles varied between 1 and 4 nm. The gold loading in a single polystyrene bead, determined by inductively coupled plasma mass spectrometer (ICP-MS), was equal to 16.8 wt. %.


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Kim et al.

20

investigated the nucleation of gold nanoparticles on polystyrene

microparticle surfaces functionalized by sulfonate or imidazole groups. Additionally, the influence of reducing agents on the coverage of gold nanoparticles was studied. It was determined that the formation of gold nanoparticles was more effective in the case of imidazole-functionalized microparticles than for sulfonate-modified beads. It was also demonstrated that application of sodium borohydride or dimetylamine borane complex caused the formation of well dispersed gold nanoparticles with a diameter of 1-5 nm. On the other hand, aggregates of gold nanoparticles were produced when the process was induced by the ascorbic acid or hydroxylamine. Formation of homogeneous layers of gold nanoparticles of an average size varying from 1.3 to 13 nm of various polymer beads was described by Ishida et al. 2. It was shown that the gold nanoparticle-coated polystyrene microparticles obtained in the precipitation process using a strong reducing agent such as sodium borohydride, possess shell structure rather than raspberry-like morphology. In the approach proposed by Ji et al.

26

, layers of gold nanoparticles at polystyrene

microparticles were obtained via two following processes: a self-assembly of negatively charged gold nanoparticles (3 or 15 nm) on the polyethyleneimie (PEI)-modified surface of microparticles (700 nm) and a subsequent seeding reactions of hydroxylamine and hydrogen tetrachloroaurate(III). It was found that the coverage of the layers obtained in the selfassembly process attained 25% whereas the subsequent seeding reaction allowed to increase this value to 90%. The deposition of positively charge gold nanoparticles (of the size of 6 nm) on polystyrene microparticles (640 nm in diamter) modified by the adsorption of poly(allylamine hydrochloride) (PAH) and polystyrenesulfonate (PSS) was also studied by the Gittins et al. 23.


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It was shown that the immobilization carried out at pH 10.5 and ionic strength of 0.01 M allowed to obtain the AuNP coverage equal to 30%. Li et al.

24

confirmed that the structure and coverage of gold nanoparticle-coated

polystyrene microparticles, obtained via the blending of gold suspension with polystyrene microspheres, strongly depend on various parameters. The influence of pH and the amounts of polyvinylpyrrolidone (PVP) on the deposition of citrate stabilized gold nanoparticles of an average size of 25 nm was studied. The measurements conducted using thermogravimetric analysis (TGA) indicated that the coverage of gold nanoparticles on the surface of polystyrene microspheres decreased as the amount of PVP increased. It was determined that the mass loading of gold nanoparticles was equal 11.7, 7.3, 4.7 and 0.7 wt.% for the PVP concentration equal to 0, 0.2, 0.4 and 0.7 wt.%, respectively. In order to irreversibly attach gold nanoparticles to polystyrene microparticle surface, organic mercapo derivatives were also used by some researchers

8,25

. In order to obtain

peptide bonds between carboxylic acid and amine groups and thiol-terminated surface, Shi et al. 8 functionalized caboxylate-terminated polystyrene microspheres with 2-aminoethanethiol hydrochloride (cysteamine hydrochloride). Then, citrate-stabilized gold nanoparticles were covalently immobilized on the surface of modified microparticles. Deposited nanoparticles served as seeds for the growth of a continuous gold layers by a reduction of additional gold precursor. The maximum coverage of gold nanoparticles attained in this work was about 50%. Similar approach was used by Xu et al.

25

for the preparation of gold nanoparticle-coated

polystyrene microparticles exhibiting a dense structure of layers. Uniform polystyrene microparticles modified by polypyrrole (PPy) were synthesized by a chemical oxidative polymerization. The surface of microparticles was modified by mercaptoacetic acid and then gold nanoparticles were deposited on the surfaces. Finally, the gold coatings were improved


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by the continuous growth of nanoparticles induced via the reduction of hydrogen tetrachloroaurate(III) by trisodium citrate. Although various methods have been developed to produce gold nanoparticle-coated polystyrene microparticles, the amount of the gold coating layer was not well controlled 12. It is also worth mentioning that in most cases the deposition of gold nanoparticles on the microparticles was confirmed by micrographs obtained from scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Coverage of gold nanoparticles at microparticles was rarely determined with the use of other experimental techniques

21,24

.

Apart from the optical properties of gold nanoparticle-coated polystyrene microparticles 8,12,23, other physicochemical properties of the composites, such as the electrokinetic charge or stability 25, were not investigated. Moreover, the deposition of gold nanoparticles on polymer microparticles was not quantitatively evaluated. Considering the limitations, the main goal of this paper is to determine mechanisms of gold nanoparticle deposition on polystyrene microparticles, especially the role of electrostatic interactions, using combination of various experimental techniques including electrokinetic measurements, concentration depletion methods and direct imaging techniques. In this way, a proper calibration of the direct methods is realized that can be used for a quantitative in situ determination of the nanoparticle layer coverage. Another goal of the work is to evaluate the electrokinetic properties of gold nanoparticle layers at microparticles under controlled conditions of pH. It is worth mentioning that this kind of measurement was not performed before. Except significance for basic science, the results obtained in this work allow one to prepare gold nanoparticle layers at polymeric carrier microspheres of well-defined structure, coverage and electrokinetic properties. One can also expect that the results obtained for the


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nanoparticles can be used as useful reference data for a quantitative analysis of protein adsorption on polymeric carrier particles. 2. MATERIALS AND METHODS

2.1. Chemicals Tetrachloroauric (III) acid trihydrate (HAuCl4·3H2O), sodium borohydride (NaBH4), cysteamine hydrochloride (HSCH2CH2NH2·HCl), potassium persulfate (K2S2O8), sodium chloride, hydrochloric acid, sodium hydroxide were commercial products of Sigma Aldrich. Styrene was supplied by Acros Organics. Silica wafers, used in the deposition experiments, were supplied by Siegert Wafer GmbH. Before the experiments, the silica wafers were cleaned in a mixture of 95% sulfuric acid and 30% hydrogen peroxide in volume ratio 1:1 for 30 minutes. Afterward, the wafers were rinsed by ultrapure water at 80oC for 30 minutes and dried out in a stream of inert gas. The Milli-Q Elix & Simplicity 185 purification system from Millipore SA Molsheim was used in order to obtain ultrapure water of conductivity 0.06 µS cm-1 which was used in the experiments.

2.2.Synthesis of polystyrene microparticles (PSMs600) Negatively charged sulfate polystyrene microparticles, hereafter referred to as PSMs600, were synthesized using the emulsion polymerization of styrene. Briefly, 150 mL of 0.017 M sodium chloride solution was placed into reactor equipped with mechanical stirrer and reflux condenser. The mixture was heated to 70oC and then 10 g of styrene was added and dispersed. Then, 0.22 g of potassium persulfate dissolved in 5 mL of water was added. The synthesis was carried out under argon in 70oC for 26 hours with stirring at 300 rpm. After this period of time, obtained suspension was purified using centrifugation at 8000 rpm. The obtained


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particles were redispersed in 140 mL of deionized water. The above procedure was repeated until the conductivity of suspension reached a value of 15 µS cm-1.

2.3.Synthesis of gold nanoparticles (AuNPs) The gold nanoparticles were obtained according to the chemical reduction method involving sodium borohydride and cysteamine hydrochloride. Briefly, an aqueous solution of sodium borohydride (50 mL, 2.3 mM) was added dropwise to 150 mL of 1 mM tetrachloroauric (III) acid solution according to the procedure described in detail in our previous work

27

. After the addition of the reducing agent, the mixture was stirred for 10

minutes and then 25 mL of 0.9 mM aqueous solution of cysteamine hydrochloride was introduced. The mixing of a burgundy red suspension was carried out for 30 minutes. After this period of time, the AuNP suspension was purified using ultrafiltration method according to the procedure described previously 27.

2.4. Preparation of gold nanoparticle-coated polystyrene microspheres

The stock suspension of PSMs600 of well-defined concentration was diluted priori to experiment to a desired concentration ranging from 60 to 500 mg L-1. The ionic strength of the PSMs600 suspension was regulated by the addition of sodium chloride whereas pH was controlled by the addition of sodium hydroxide or hydrochloric acid. In similar manner the pH and ionic strength of the AuNP suspension of concentration from 5 to 100 mg L-1 were fixed. Then, the PSMs600 and AuNP suspensions of the same ionic strength and pH were mixed in a proper volume ratio in order to obtain 5 mL of the mixture with a constant concentration of PSMs600 equal to 60 mg L-1 and diverse values of the AuNP concentration. The mixing was continued over 20 minutes. This period of time was sufficient for electrostatically driven deposition of AuNPs on PSMs600 as discussed later on 28,29. 9 ACS Paragon Plus Environment

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2.5.Deposition of particles at the silicon substrate

The kinetics of AuNP deposition from native suspensions and from the mixtures with microparticles was thoroughly studied in order to calibrate the concentration depletion method. Accordingly, small fragments of freshly cleaned silicon wafers were vertically immersed into AuNP suspension of a desired concentration (1-50 mg L-1). Particle deposition proceeded under diffusion-controlled transport at the temperature of 298 K over the time up to 300 min. The silicon substrates with deposited particles were rinsed with ultrapure water in order to remove unbounded particles and the excess of sodium chloride. Finally, the samples were air dried and imaged using different microscope techniques (AFM, SEM) as described below. In an analogous way the deposition of AuNPs from mixtures with the microparticles was carried out over the time of 30 min. It should be mentioned, that the deposition of polymer microparticles was negligible over this time due to their much lower diffusion coefficient compared to the AuNPs.

2.6. Physicochemical characteristics of particles

The mass concentration of the PSMs600 in the stock (native) suspension was determined using the dry mass method where 1 g of the microsphere suspension was dried until no noticeable changes in sample mass were observed. Independently, the concentration was determined using the Anton Paar DMA 5000M densitometer according to the previously described procedure

29,30

. This method was also applied for the determination of the bulk

concentration of AuNPs in the stock suspension 27.


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The size distribution and morphology of the PSMs600 and the AuNPs were determined using the JEOL JSM-7500F Field Emission scanning electron microscope working in scanning (SEM) and transmission (TEM) mode. This apparatus was also used for the visualization of the AuNPs@PSMs600 composites and for the determination of the structure and surface concentration (Ns) (coverage (θ )) of the AuNP layers deposited on silicon wafers. The drop (60 µL) of colloidal suspension containing the AuNPs@PSMs600 was placed on the freshly cleaned silicon wafer. After water evaporation, the samples were investigated using SEM (method I), similarly such as the specimens of silica wafers with the AuNP layers deposited under diffusion-controlled conditions as well as the AuNPs@PSMs600 composites (method II). Additional measurements were also done using high-resolution scanning electron microscope FEI Versa 3D at an accelerating voltage of 20 kV and a beam current of 30 pA. Number averaged mean diameter and size distributions of the particles were determined by analysis of obtained TEM and SEM micrographs using a MultiScan Base software according to the procedure described elsewhere 30. The AuNP and the AuNPs@PSMs600 composite layers were also examined using NTMDT Solver Pro atomic force microscope (AFM) equipped with the SMENA SFC050L scanning head. The images were done in semi-contact mode using composite probes possessing a siliconbody, polysilicon levers, and high resolution silicon tips. AFM and SEM images were used for determination of the number of the AuNPs per unit area of the layers. Typically, the number of particles was determined over 10–15 equally sized areas randomly chosen over the silicon wafer 27. The size distribution and the average size of the PSMs and AuNPs were directly determined in the suspensions using dynamic light scattering (DLS) technique via the diffusion coefficients measurements carried out using Zetasizer Nano ZS instrument from


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Malvern company. From the diffusion coefficients (D) of nanoparticles, the hydrodynamic diameters were calculated from the Stokes-Einstein relationship 30. The electrophoretic mobility (µe) of the particles and the AuNPs@PSMs600 composites under controlled conditions of pH and ionic strength was measured using the laser Doppler velocimetry (LDV) technique using Zetasizer Nano ZS instrument. Knowing the values of the electrophoretic mobility, zeta potential was calculated using the Henry’s model 30.

3. RESULTS AND DISCUSSION

3.1.Characteristics of the PSMs600 and AuNPs As mentioned, the morphology and the size distributions of the PSMs600 and AuNPs were determined from SEM and TEM micrographs. Typical images of the particle layer deposited on silicon wafers are shown in Fig. 1 (insets). The particle size distributions obtained from the measurements of their surface area and diameters

30

are shown in this

Figure in the form of histograms (additional data was shown in Supporting materials). From the histograms, it was calculated that the average size of the PSMs600 particles was equal to 660±20 nm whereas the average size of AuNPs was equal to 12±4 nm. The diffusion coefficients of PSMs600 measured at pH 5.6 was equal to 7.1x10-9 cm2 s-1. Interestingly, the diffusion coefficient was independent on ionic strength (for the range 10-4 – 3x10-3 M). The hydrodynamic diameter of PSMs600 determined from the Stokes-Einstein relationship 30 was equal to 690±6 nm. This is slightly larger than the value obtained by SEM imaging and can be attributed to the presence of protruding polystyrene chains in aqueous suspensions

31,32

. It was also demonstrated that the hydrodynamic diameter of the PSMs600

was practically independent on pH within the range 4-9, which indicates that the polymer microparticle suspension remained stable (Fig. 2a). 12 ACS Paragon Plus Environment

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The diffusion coefficient of the AuNPs at pH 5.6 was equal to 4.1x10-7 cm2 s-1 that corresponds to the hydrodynamic diameter of AuNPs equal to 12±3 nm for all investigated ionic strengths and pH up to 6.3. This value well correlate with the size obtained from SEM micrographs. However, a significant increase in the hydrodynamic diameter was observed at pH > 7 for ionic strength above 3x10-3 M (Fig. 2b). Independently on ionic strength the hydrodynamic diameter of AuNPs was stable up to the pH value of 6.2. Under basic conditions, the AuNPs aggregated which was indicated by the increase of their hydrodynamic diameter. Furthermore, it was observed that larger aggregates of AuNPs were formed at the highest ionic strength. This fact arises from the decrease of repulsive interactions between positively charged nanoparticles.


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a)

b)



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a)

b)

Fig. 2. Dependences of hydrodynamic diameter of a) PSMs600 and b) AuNPs on pH determined for (●) I=10-4 M NaCl, (■) I=10-3 M NaCl and (▲) I=3x10-3 M NaCl. Measurements conditions: cPSMs600=60 mg L-1, cAuNPs=50 mg L-1, T=298 K. The solid lines represent fits of experimental results.

The electrophoretic mobility measurements showed that at pH 5.6 for the entire investigated range of ionic strength, the PSMs600 were negatively charged. Accordingly, the zeta potential decreased with ionic strength assuming the value of –54 to –73 mV for ionic strength equal to 10-4 and 3x10-3 M, respectively (see Fig. 3). One can also observe in Fig. 3 15 ACS Paragon Plus Environment

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that

the

zeta

potential

of

the

PSMs600

Page 32 of 52

decreased

with

pH

attaining

–83 mV for ionic strength 3x10-3 M and pH 8.5. On the other hand, the zeta potential of AuNP was positive for the entire range of ionic strength but a significant decrease of zeta potential with pH (see Fig. 3) is observable. Thus, at pH 4.2, it was equal to 57 mV and 51 mV for ionic strength of 10-4 M and 10-3 M, respectively whereas at pH 8.5 the zeta potential was 35 mV and 31 mV for these ionic strengths. This decrease in the AuNP zeta potential induces their aggregation resulting in an increase in their hydrodynamic diameters (Fig. 2) 27. Taking into account that the AuNPs were stabilized by cysteamine molecules

27

, one can conclude that that the decrease in the zeta

potential resulted from a deprotonation of amine moieties 33,34.

Fig. 3. Dependence of the zeta potential of the PSMs600 and the AuNPs on pH determined for 1 (●) I=10-4 M NaCl, 2 (■) I=10-3 M NaCl and 3 (▲) I=3x10-3 M NaCl. Measurement conditions: cPSMs600=60 mg L-1, cAuNPs=50 mg L-1, T=298 K. The lines represent nonlinear fits of experimental results. Considering the physicochemical characteristics of the particlesit was predicted that the deposition of the AuNPs on the PSMs600 should be most effective for ionic less than 3x10-3 M and at pH of 5.6.


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3.2. Formation of gold nanoparticle-coated polystyrene microspheres (AuNPs@PSMs600 composites)

The deposition of the AuNPs on the PSMs600 was studied using the direct electrophoretic mobility measurements, microscope (SEM) imaging and the concentration depletion method based on AFM imaging. The characteristic time of the AuNP layer formation under these conditions can be calculated from the formula 28,29,35:

2

t Au

2 Φ ρ 3 d = 104  mx PSMs 600  PSMs−600  cPSMs 600  4 D

(1)

where Φmx is the maximum packing (volume fraction) of the microspheres in 3D

36

,

dPSMs600 is the diameter of PSMs600, ρPSMs 600 is specific density of PSMs600 equal to 1.05 g cm-3

−

37

, cPSMs600 is the PSM mass concentration in the suspension (mg L-1) and D is the

mutual diffusion coefficient of the AuNPs, which is equal to the sum of the individual diffusion coefficients of AuNPs and PSMs600, i.e., 4.2x10-7 cm2 s-1 29. Using the values relevant to the experimental conditions one can calculate from Eq.(1) that

tAu=4.5 s. Thus, this relaxation time is much shorter than the time of the AuNP

adsorption experiments that was equal to 30 min. It is worth mentioning that the electrophoretic mobility of the AuNPs@PSMs600 was also determined after 2, 5 and 15 hours of the mixing of colloidal suspensions. In this way the stability of the AuNPs@PSMs600 characterized by various surface coverage of the AuNPs

was

evaluated.

No

changes

in

the

electrophoretic

mobility

of

the


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AuNPs@PSMs600 measured for prolonged times were observed that confirmed the stability of the composites. Knowing the electrophoretic mobility of the AuNPs@PSMs600 composites, their zeta potential was calculated from the Henry’s formula 30. On the other hand, knowing the sizes of both types of particles, their specific densities and their mass concentration in the suspensions, one can calculate the nominal coverage of AuNPs at the PSMs600 from the following equation 28,29,35:

θ AuNP s =

1 ρ PSMs 600 d PSMs 600 c AuNPs 4 ρ AuNP d AuNPs cPSMs 600

(2)

where θ AuNPs = Np Sg, Np is the surface concentration of deposited AuNPs (the number of 2 particles per unit of the microparticle surface), S g = π d PSMs 600 is the characteristic cross-

section area of the microparticle and ρ AuNP is specific density og gold equal to 19.3 g cm-3 38. By defining θ AuNPs one can express the experimental results acquired in these experiments as the dependence of zeta potential of the microparticles on the nominal coverage of the AuNP layers (Fig.4, lower horizontal axis). The deposition experiments were conducted for the concentration of PSMs600 equal to 60 mg L-1, at pH 5.6 and for various ionic strengths equal to 10-4 M (Fig. 4a) and 3x10-3 M (Fig. 4b). Analyzing the results shown in Fig. 4, one can notice that the zeta potential of the AuNPs@PSMs600 composites abruptly increases with the concentration of the AuNPs. It was found that for both

ionic strengths, the inversion of the negative zeta potential of the

PSMs600 took place for the AuNP concentration exceeding 10-15 mg L-1 ( θ AuNPs = 0.10.15). For larger AuNP concentration, exceeding 25 mg L-1 ( θ

AuNPs

= 0.3), a limiting zeta

potential of the AuNPs@PSMs600 is attained that is equal to 25 and 32 mV for I=10-4 M and 3x10-3 M, respectively. It should be mentioned that these values are markedly lower 18 ACS Paragon Plus Environment

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than the bulk zeta potentials of the AuNPs, which was equal to 52 mV and 48 mV at I=10-4 M and 3x10-3 M, respectively. These experimental data were theoretically interpreted by applying the 3D electrokinetic model developed in Ref.

36

where the hydrodynamic flow field and the electrostatic potential

distribution in the vicinity of particles adsorbed at solid/electrolyte interface are considered in an exact way. This model allows one to formulate analytical expressions connecting the zeta potential changes of surfaces with the dimensionless particles coverage θ AuNPs defined above. The details of the derivation of this equation are given in the Supporting Information. One can observe in Fig. 4 that the results derived from this model, shown as solid lines, adequately reflect the experimental data for both ionic strengths. By confirming this, one can invert the theoretical dependence formulated within the scope of this model that yields the following formula for the particle coverage as a function of the experimentally determined zeta potential 36

θ AuNPs =

ζi −ζ∞ 1 ln Ci ζ (θ AuNP s ) − ζ ∞

(3)

where Ci is the dimensionless constant (Supporting Information), which depends on ionic strength, ζ i is the zeta potential of bare PSMs600 and ζ ∞ is the limiting zeta potential strictly related to the bulk zeta potential of the AuNPs.


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a)

b)

Fig. 4. Dependence of zeta potential of the PSMs600 on the AuNP concentration in the suspension (the upper horizontal axis) and the nominal coverage of nanoparticles at microparticles calculated from Eq. (2) (the lower horizontal axis). Deposition conditions: cPSMs600=60 mg L-1, T=298 K, pH 5.6, ionic strength: a) 10-4 M, b) 3x10-3 M. The solid lines denote theoretical results calculated from the general electrokinetic model Eq. (3). Thus, Eq. (3) can be exploited for a robust determination of the coverage of AuNP layers on the microparticles directly under in situ conditions. In this way, knowing the 20 ACS Paragon Plus Environment

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zeta potentials of bare PSMs600 and the maximum values of the AuNPs@PSM600 zeta potential (Fig. 4), one can calculate that the maximum coverage of AuNP layers (θAgNPsmx) that

was

equal

to

0.20,

0.25

and

0.33

at

ionic

strength

10-4 M, 10-3 M and 3x10-3 M, respectively. However, as pointed out in previous works

28,29,35

, the precision of the electrokinetic

method decreases for larger coverage of nanoparticles. Therefore, in order to evaluate this parameter more precisely and to determine the morphology of the AuNP layers at the PSMs600 the above SEM method was applied for imaging the AuNPs@PSMs600 composites deposited at the silicon substrate. It is worth emphasizing that in contrast to the previously applied method

28,29,35

, where the polysterene composites were deposited at

mica, the use of silicon wafers eliminates the necessity of sputtering the samples by a subsidiary layer. This enables an efficient imaging of the AuNPs deposited at microparticles using high vacuum electron microscope. In this way the morphology, the structure and the coverage of AuNP layers can be determined via the direct counting of the number of deposited particles. SEM micrographs of the AuNPs@PSMs600 composites of different AuNP coverage are shown in Fig. 5. One can observe that the layers are formed by isolated and non-aggregated particles. This makes it possible to determine the adsorbed particle size distribution and the particle coverage according to the direct enumeration method previously used for silver nanoparticle deposition at planar substrates 30. In this way, it was determined that the average size of adsorbed particle derived from SEM is equal to 12 nm compared to the particle size in the bulk equal to 13 nm. This indicates that smaller sized particles preferably adsorb at the PSMs600, which confirms the dispersity effect.


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a)

b)

Fig. 5. SEM micrographs presenting the AuNPs layers at the PSMs600 characterized by the coverage of: a) θ=0.24 (Ns= 2123 µm-2) and b) θ=0.33 (Ns= 2919 µm-2).

Using this direct counting procedure it was determined that the maximum coverage of AuNP layers is equal to 0.19, 0.24 and 0.33 for ionic strength of 10-4 M, 10-3 M and 3x10-3 M, respectively. This agrees well with previous data derived from direct electrophoretic measurements (see Table 1). It should also be pointed out that such measurements have not been presented before in the literature. 22 ACS Paragon Plus Environment

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Additionally, the concentration depletion method was applied in order to determine the maximum coverage of the AuNPs at the PSMs600. According to procedure described in detail elsewhere

28,29

, silica wafer was immersed in the PSMs600/AuNP mixture acquired after the

adsorption step to deposit unbound AuNPs under diffusion transport conditions over a controlled time (typically 30 min). The surface concentration of the AuNPs can be determined using AFM imaging because the PSM600 deposition is negligible during this time. Knowing this parameter, one can calculate the coverage of the AuNPs at the silica substrate ( θ AuNPs / Si ) that is related to their concentration in the mixture by the formula 39

θ AuNPs / Si = K Au (cAuNPs − cdAuNPs )

(4)

where 1

K Au

1  Dt  2 = 3 d   π  ρ AuNPs d AuNPs

(5)

is the known constant, cAuNPs is the mass concentration of the AuNPs in the initial mixture before deposition, cdAuNPs is the mass concentration of the AuNP deposited at the PSMs600 and td is the deposition time of AuNPs at the silica substrate. One can predict from Eq. (4) that for θ AuNPs / Si = 0 the deposited AuNP concentration is equal to its initial concentration in the mixture. By combining Eqs.(4,5) with Eq.(2), one obtains the explicit dependence for calculating the coverage of AuNPS at the PSMs600 as a function of the coverage at silica determined by AFM:


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θ AuNPs =

1 ρ PSMs 600 d PSMs 600 4 ρ AuNPs d AuNPs

 θ AuNPs / Si   cAuNPs −  K Au   cPSMs 600

Page 40 of 52

(6)

It is interesting to compare the applicability of this indirect concentration depletion method with the direct enumeration of deposited particle method based on SEM imaging. In order to do this, in Fig. 6 the AuNP coverage at the PSMs600 calculated from Eq.(6) is plotted as a function of their initial bulk concentration in the suspension. One can observe that these concentration depletion results correlate with the data obtained by the direct particle enumeration method (SEM). Moreover, one can also notice, that the results derived from the electrophoretic method using Eq.(3) also agree with the previous two methods. This has significant practical implications indicating that the electrokinetic method, which is considerably more efficient than the tedious concentration depletion method, can be directly used for a in situ determination of nanoparticle coverage in the layer formed at polymer carrier particles.

Fig.6. Dependence of the AuNP coverage at the PSMs600 on their initial concentration in the mixture. The solid line shows the nominal coverage of AuNPs calculated from Eq. (2). The points represent experimental results obtained for ionic strength equal to 3x10-3 M, pH 5.6 and the PSMs600 concentration of 60 mg L-1; (●) concentration depletion method (AFM), 24 ACS Paragon Plus Environment

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results calculated from Eq.(6), (□) direct particle enumeration (SEM), (▲) direct measurements of electrophoretic mobility, particle coverage calculated from Eq.(3).

It should be mentioned that the AFM aided concentration depletion method can also be used to determine the maximum coverage of AuNPs at the PSMs600 by exploiting Eq.(6). In these series of experiments, the coverage of AuNPs at the silica substrate ( θ AuNPs / Si ) was determined via AFM imaging for a broad range of their initial concentration in the suspensions. The results of such measurements, performed for different ionic strength, are presented in Fig. 7 (curves 2 and 3). For comparison, the reference results are also shown where the coverage of the AuNPs, deposited from native suspensions of various concentration (without presence of the PSMs600), was determined by AFM (line 1). One can observe in Fig. 7 that the AuNP coverage at the silica substrate only increases after exceeding threshold cAuNP concentrations, which are dependent on ionic strength. This effect is well described by a linear dependence of the surface concentration on bulk concentration of particles characterized by the slope identical to the slope of the reference line 1. This confirms that the deposition of the AuNPs at the PSMs600 was irreversible. Therefore, the threshold concentrations can be accurately determined as the intersection of the straight line approximating the experimental data with the horizontal axis (see Fig. 7). They were equal to 16 and 26 mg L-1 for ionic strength of 10-3 M and 3x10-3 M, respectively. According to Eq.(6), this corresponds to the maximum coverage of AuNPs at the PSMs600 equal to 0.19 and 0.32 for ionic strength of 10-3 M and 3x10-3 M, respectively, which agrees with previous experimental data (see Table 1). Analyzing the results presented in Table 1, one can deduce that the coverage of the AuNPs significantly increase with ionic strength. This suggest that this parameter is mainly controlled by the lateral electrostatic interactions among deposited particles. It should be 25 ACS Paragon Plus Environment

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Page 42 of 52

mentioned that analogous behavior was observed in the case of protein adsorption 39, silver 28 and hematite

29

nanoparticle deposition at polymer microparticles. However, the maximum

coverage obtained in this work is markedly larger than the coverage determined for the same gold nanoparticle deposition at planar and smooth silica substrates (sensors) determined in Ref. 27 using the QCM method. It is interesting to mention that these latter results well agreed with theoretical prediction derived from the random sequential adsorption modeling.

Fig. 7. Dependence of the AuNP coverage at the silica substrate derived from AFM imaging (points) on their initial concentration in the suspension, pH 5.6, deposition time 30 min, The concentration of PSMs600 60 mg L-1. 1 (●) reference results obtained for pure AuNP suspension, 2 (●) 10-3 M and (●) 3x10-3 M. The solid lines show the linear extrapolation of the experimental data. The inset shows a typical AFM micrographs for the surface concentration of AuNPs equal to 530 µm-2.


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Table 1. The maximum coverage of AuNP layers at the PSMs600 derived for selected ionic strengths using various experimental methods (pH 5.6).

Ionic strength [M]

κa [1]

Experimental Methods Electrophoretic mobility method

Depletion method (AFM)

Direct counting (SEM imaging)

Planar surfaces QCM 27

10-4

0.20

0.20±0.02

-

0.19±0.01

0.08

10-3

0.62

0.25±0.02

0.23±0.01

0.22±0.02

0.17

3x10-3

1.08

0.33±0.03

0.35±0.02

0.33±0.02

0.29

This effect can be attributed to the presence of the polymer chains protruding from the core part of the microparticles often referred to as the hairy layer

31,32

. The thickness of this

layer decreases with ionic strength because of the decreasing range of the electrostatic interactions among them. The negatively charged hairs on the PSMs600 induces an additional screening effect decreasing the repulsive interactions between the AuNPs. In consequence, their maximum coverage is significantly larger for low ionic strength compared to the smooth silica substrates. On the other hand, for larger ionic strength, the screening effect of the hairs becomes negligible, hence the maximum coverages for microparticles and flat substrates become similar. This hypothesis can be supported by the results obtained by Gittins et al. 23 who studied the deposition of positively charged AuNPs (6 nm ) at PAH and PSS-modified microparticles of the size 640 nm. For ionic strength of 10-2 M NaCl the maximum coverage determined in their work was equal to 0.30.


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It is worth mentioning that an analogous ionic strength dependent effect of PSM hair layer in fibrinogen adsorption at negatively charged polystyrene particles was reported in Ref. 40. This confirms an analogy between nanoparticle and protein adsorption at polymer microparticles. In

the

final

series

of

experiments

the

electrokinetic

properties

of

the

AuNPs@PSMs600 composites ware thoroughly studied under controlled conditions of pH. In the first stage the AuNP layers of coverage 0.33 was deposited at the PSMs600 according to the procedure described above (at ionic strength of 3x10-3 M and pH 5.6). One part of the suspension was used for

the measurements of electrophoretic mobility of the

AuNPs@PSMs600 composites at decreasing values of pH, which was regulated by the hydrochloric acid. Another part was used for measurements at increasing pH, which was adjusted by the addition of sodium hydroxide. Knowing the values of electrophoretic mobility, and using the Henry’s model

30

, the zeta potential of the complexes at various pH

was determined.

Fig. 8. Dependence of zeta potential on pH at ionic strength 3x10-3 M: 1 (●) zeta potential derived from Eq.(7) for the AuNP layer deposited at microparticles of the coverage equal to 0.33. The line shows the interpolated results derived from the measurements of


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electrophoretic mobility of the AuNPs in the bulk. The dashed line shows the interpolated zeta potential of microparticles in the bulk .

The original data of these measurements are shown in the Supporting Information . It is confirmed that the zeta potential of the AuNPs layers is significantly smaller than the zeta potential of the native AuNPs in the bulk derived by the electrophoretic measurements. In the next series of experiments the stability of the layers upon pH cycling was studied (Supporting Information). It was observed that after completing three cycles the differences in the zeta potential of AuNP layer were negligible for pH up to 8.5. This indicates that the AuNP layers at the PSMs600 were more stable compared to the native AuNP suspensions, which are only stable for pH < 6 at ionic strength of 3x10-3 M (see Fig. 2). Additionally, knowing the zeta potential of AuNP layers at the PSMs600 one can determine the zeta potential of deposited AuNPs using the above described electrokinetc model that furnishes the following formula

ζ (θ ) − Fi (θ ) ζ i  ζp =  Fp (θ )

(7)

where for ionic strength equal to 3x10-3 M and for θAuNPs=0.33, Fi (θ ) =0.035 and Fp (θ ) =0.67.

The zeta potential of AuNPs calculated from Eq. (7) is shown in Fig. 8 as black points. One can observe that these results are consistent with the zeta potentials of the AuNPs directly determined from the electrophoretic mobility measurements of the native AuNP suspension (depicted by the solid line in Fig. 8). This confirms that the electrokinetic properties of the AuNPs deposited at the PSMs600 are similar to the bulk properties of free AuNPs. This has 29 ACS Paragon Plus Environment

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Page 46 of 52

practical implications confirming that the AuNPs preserve their surface properties upon deposition on the PSMs600. In consequence, instead of using tedious bulk electrophoretic measurements, efficient characteristics of metal nanoparticles can be acquired via their controlled deposition on polymer carried microparticles.

4. CONCLUSIONS

Mechanisms of gold nanoparticle deposition at polystyrene microparticles was determined using a combination of various experimental techniques including the electrokinetic, the concentration depletion and the direct microscope imaging methods. The validity of the electokinetic model used for the analysis of LDV measurements was confirmed using the direct particle enumeration method involving SEM imaging. This has significant implication given that the LDV (eletrokinetic) method is considerably more efficient than other experimental techniques and can be directly used for in situ determination of nanoparticle monolayer coverage at polymer carrier particles, The aforementioned methods also allowed to precisely determine the maximum coverage of AuNPs that increased with ionic strength. This effect was attributed to the reduced range of electrostatic repulsion among deposited nanoparticles. Thorough characteristics of the layers were also performed, which confirmed their larger stability over a wide pH range (3.5-8.5) compared to the bulk stability of the AuNPs in the suspensions. Except significance for basic science, the results obtained in this work facilitate developing a procedure of preparing gold nanoparticle layers at polymeric carrier microspheres of welldefined structure, coverage and electrokinetic properties.


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One can also expect that the results obtained for the nanoparticles can be applied as useful reference data for a quantitative analysis of protein adsorption on polymeric carrier particles.

ACKNOWLEDGEMENTS This work was financially supported by the National Science Centre under the Opus Project UMO-2015/19/B/ST5/00847. Dawid Lupa is grateful to the PhD Teresa Basińska for her care and invaluable support during his internship in the Center of Molecular and Macromolecular Studies Polish Academy of Sciences in Lodz. The authors also would like to express gratitude to PhD Elzbieta Bielanska and PhD Katarzyna Berent for their help in the microscopic measurements.

REFERENCES (1) Liu, W.; Yang, X.; Huang, W. Catalytic properties of carboxylic acid functionalizedpolymer microsphere-stabilized gold metallic colloids. J. Colloid Interface Sci. 2006, 304, 160-165. (2) Ishida, T.; Kuroda, K.; Kinoshita, N.; Minagawa, W.; Haruta, M. Direct deposition of gold nanoparticles onto polymer beads and glucose oxidation with H2O2. J. Colloid Interface Sci. 2008, 323, 105-111. (3) Cheval, N.; Gindy, N.; Flowkes, C.; Fahmi, A. Polyamide 66 microspheres metallised with in situ synthesized gold nanoparticles for a catalytic application. Nanoscale Res. Lett. 2012, 7, 182-190. (4) Das, S.; Asefa, T. Core–shell–shell microsphere catalysts containing Au nanoparticles for styrene epoxidation. Top. Cat. 2012, 55, 587-594.


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(5) Ren, Z-H.; Li, H-T.; Gao, Q.; Wang, H.; Han, B.; Xia, K-S.; Zhou, C-G. Au nanoparticles embedded on urchin-like TiO2 nanosphere: An efficient catalyst for dyes degradation and 4nitrophenol reduction. Mater. Des. 2017, 121,167-175. (6) Cao, Y. C.; Wang, Z.; Jin, X.; Hua, X. F.; Liu, M. X.; Zhao, Y. D. Preparation of Au nanoparticle-coated polystyrene beads and its application in protein immobilization. Colloids Sur. A 2008, 334, 53-58.

(7) Chen, L. Y.; Fujita, T.; Ding, Y.; Chen, M. W. A Three‐Dimensional Gold‐Decorated Nanoporous Copper Core–Shell Composite for Electrocatalysis and Nonenzymatic Biosensing. Adv. Funct. Mater. 2010, 20, 2279-2285. (8) Shi, W.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Gold nanoshells on polystyrene cores for control of surface plasmon resonance. Langmuir 2005, 21, 1610-1617. (9) Li, J. M.; Ma, W. F.; Wei, C.; You, L.J.; Guo, J.; Hu, J.; Wang, C. C. Detecting trace melamine in solution by SERS using Ag nanoparticle coated poly (styrene-co-acrylic acid) nanospheres as novel active substrates. Langmuir 2011, 27, 14539-14544. (10) Cai, W.; Wang, W.; Lu, L.; Chen, T. Coating sulfonated polystyrene microspheres with highly dense gold nanoparticle shell for SERS application. Colloid Polym. Sci. 2013, 291, 2023-2029. (11) Kawaguchi H. Functional polymer microspheres. Prog. Polym. Sci. 2000, 25, 1171-1210. (12) Gong, J.; Zu, X.; Mu, W.; Deng, Y. In situ self-assembly synthesis of gold nanoparticle arrays on polystyrene microspheres and their surface plasmon resonance. Colloid Polym. Sci. 2013, 291, 239-244.


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