Stability and Catalytic Activity of PEG-b-PS-Capped Gold

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Stability and Catalytic Activity of PEG‑b‑PS-Capped Gold Nanoparticles: A Matter of PS Chain Length Yurong Que, Chun Feng,* Sen Zhang, and Xiaoyu Huang* Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China S Supporting Information *

ABSTRACT: Gold nanoparticles (AuNPs) covered with a series of well-defined poly(ethylene glycol)-b-polystyrene (PEG-b-PS) amphiphilic diblock copolymers containing a thiol group at the end of PS block were prepared to explore the influence of chain length of PS segment on the colloidal stability and catalytic activity of AuNPs. PEG-b-PS amphiphilic diblock copolymers with different PS chain lengths and narrow molecular distributions (Mw/Mn ≤ 1.15) were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization employing a PEG-based macromolecular chain transfer agent (Mn ≈ 2000 g/mol), followed by transforming the thiocarbonate end functionality into a thiol group in the presence of 2-aminoethanol and tributylphosphane. PEG-b-PS-stabilized gold nanoparticles (Au@PEG-bPS) were prepared by ligand exchange reaction between citrate-stabilized AuNPs and the thiol end group of PEG-b-PS diblock copolymer. The presence of the hydrophobic PS layer not only improved the stability of Au@PEG-b-PS against electrolyte-induced aggregation but also greatly promoted the resistance of Au@PEG-b-PS against competitive displacement of dithiothreitol. Au@PEG-b-PS showed excellent catalytic activity in the reduction reaction of 4-nitrophenol into 4-aminophenol, and the catalytic activity increased with the decrease in the chain length of PS block. In addition, the high stability imparted by the PS layer endowed Au@PEG-b-PS with good reusability in catalysis without the loss of catalytic activity.

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INTRODUCTION Metal nanoparticles, particularly gold nanoparticles (AuNPs), have attracted significant attention in the last couple of decades due to their unique properties that are distinct from bulk gold.1−8 These nanoparticles are being considered in a variety of applications, such as photonics, information storage, electronic and optical detection system, therapeutics, diagnostics, photovoltatics, and catalysis.2−5,9−13 In these fields, one of major problems for nanoparticles is their limited colloidal stability and resulting tendency to form aggregates in aqueous media. In order to explore and realize the applications of AuNPs in biomedicine and catalysis, great of efforts have been made to improve the colloidal stability of AuNPs.14−19 Previous studies indicated that the colloidal stability of gold nanoparticles can be significantly promoted by the coverage of gold nanoparticles with a relatively thick shell of polymers, including poly(N-vinyl-2-pyrrolidone) (PVP), polyethylenimine (PEI), PEG, and so on.14−19 Among of these polymers, PEG-based ligand is the most attractive one because it can endow AuNPs with good water solubility, high colloidal stability, and excellent inhabitation to nonspecific absorption of peptide and protein.20,21 To attach polymeric chains onto the surface of AuNPs, thiol is widely employed as an anchoring group via a Au-to-thiol bond.22−29 However, the Au-to-thiol bond is thought to be a Lewis acid-based interaction, not a covalent © 2015 American Chemical Society

bond, and be able to be affected by electrolyte (NaCl), pH, and external thiol compound leading dissociation of the Au-to-thiol bond.22,26−29 The applications in biomedicine and catalysis usually require that the nanoparticles are able to be stable in solutions rich in salt and thiol compounds. However, monothiol-PEG-covered AuNPs would lose their colloidal stability and aggregate under some harsh conditions.22,27−29 One strategy to enhance the stability is to increase the number of anchoring groups at polymer chain ends.22,26,28−30 For example, Mattoussi et al. synthesized water-soluble nanoparticles by coating AuNPs with PEG chains through multiple thiol groups, which significantly improved colloidal stability compared to AuNPs coated with monothiol-terminated PEG chains.26 In addition, a previous study showed that the stability of nanoparticles could also be improved by adjusting the chemical structure of the spacer connected to the thiol group.27 For instance, Schulz et al. prepared AuNPs with shells of different monodentate PEG-SH ligands, in which hydrophobic spacers of phenylene, a long (C10), and short (C2) alkylene groups were used to connect the thiol group with the PEG moiety.27 Their results demonstrated that AuNPs covered by Received: November 27, 2014 Revised: January 7, 2015 Published: January 9, 2015 1960

DOI: 10.1021/jp511850v J. Phys. Chem. C 2015, 119, 1960−1970

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The Journal of Physical Chemistry C

a Bruker Avance 800 spectrometer (800 MHz); tetramethylsilicone was used as internal standard. Relative molecular weights and molecular weight distributions were measured by a conventional gel permeation chromatography (GPC) system equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (HR3 (500−30 000), HR4 (5000−600 000), and HR5 (50 000−4 000 000), 7.8 × 300 mm, particle size 5 μm). GPC measurements were carried out at 35 °C using THF as eluent with a flow rate of 1.0 mL/min. The system was calibrated with linear polystyrene standards. Hydrodynamic diameter (Dh) was measured by dynamic light scattering (DLS) with a Malvern Nano-ZS90 Zetasizer. TEM images were obtained by a JEOL JEM-1230 instrument operated at 80 kV. UV−vis spectra were measured by a Hitachi U-2910 spectrophotometer with a rate of 400 nm/min. Measurements of inductively coupled plasma mass spectrometry (ICP-MS) were carried out on a PerkinElmer Elan 9000 instrument operating under normal Ar plasma conditions (1400 W forward plasma power, 17 L/ min Ar plasma gas flow, 1.2 L/min auxiliary Ar flow, and 0.95 L/min nebulizer Ar flow). Stability of Au@PEG-b-PS against Electrolyte-Induced Aggregation. Sodium chloride (117 mg, 4 mmol) added was to 2.0 mL of AuNPs sample, and after thorough mixing of each sample, photographs were taken after 30 min. The concentration of Au atom in the dispersions was 27.2 mg/L. The sample was diluted three times for UV−vis measurements for monitoring the microscopic change of AuNPs. Stability of Au@PEG-b-PS at pH = 12. NaOH aqueous solution (10 μL, 10 M) was added to 1.0 mL of AuNPs sample, and after thorough mixing of each sample, photographs were taken after 30 min. The concentration of the Au atom in the dispersions was 27.2 mg/L. The samples were diluted three times for UV−vis measurements for monitoring the microscopic change of AuNPs. Stability of Au@PEG-b-PS against Competitive Displacement of DTT. NaOH aqueous solution (30 μL, 1.0 M) was added to 3.0 mL of AuNPs sample, and the aqueous DTT and NaCl solutions were freshly prepared to yield concentrations of 1.5 M DTT and 800 mM NaCl in the cuvette by dissolving 693 mg of DTT (4.5 mmol) and 140.4 mg of NaCl (2.4 mmol) simultaneously in AuNPs. The concentration of the Au atom was 27.2 mg/L. UV−vis spectra were recorded in the sequence of time, collecting absorbance spectra in the range of 400−900 nm. During the experiments, the samples were stirred in a UV−vis spectrophotometer. Catalytic Reduction of 4-Nitrophenol. The reduction of 4-NP by NaBH4 was selected as a model reaction to test the catalytic activity of the as-prepared AuNPs. 4-NP (0.3 mL, 0.05 M) and NaBH4 (22.7 mg, 0.6 mmol) were added to deionized water (3.25 mL) in a cuvette under stirring. After adding AuNPs (0.65 mL), the bright yellow solution gradually faded as the reaction proceeded. The concentration of the Au atom in the solution was 6.8 mg/L. During the experiment, 120 μL of sample was diluted 25 times for UV−vis measurement to determine the conversion of 4-NP after different waiting periods. Recyclable Catalysis Reduction of 4-Nitrophenol. 4NP (0.1 mL, 0.15 M) and NaBH4 (45.4 mg, 1.2 mmol) were added to deionized water (2.8 mL) in a cuvette under stirring, and AuNPs (1.3 mL) was mixed under stirring to initiate the catalytic reduction. The concentration of Au atom in the solution was 13.6 mg/L. During the experiment, 60 μL of

PEG-based ligands with a long alkylene spacer showed better stability against cyanide etching and dithiothreitol (DTT) displacement. Therefore, these results might indicate that a hydrophobic and thick inner layer near the thiol group could protect the relatively weak S−Au bond from “attacking” by external electrolyte, cyanide, and DTT. In addition, previous reports also indicated that peptide conjugation, immune response, protein adsorption, and macrophage uptake of AuNPs were related with the length of alkyl spacer for PEG-stabilized AuNPs.31−33 However, to the best of our knowledge, previous studies on the influence of spacer on properties of AuNPs were limited to the territory of small alkyl compounds.27,31−33 In order to get better understanding of the important role of the hydrophobic spacer, especially its length, in the colloidal stability and surface-related properties of PEGstabilized AuNPs, it is necessary to extend the study on the influence on the spacer from the small compound territory to the polymeric realm. Specifically, in the present work, we prepared a series of PEG-b-PS-SH amphiphilic block copolymers, in which PS segments with different chain lengths were used as hydrophobic spacers to connect the PEG moiety with a thiol anchoring group. After replacing citrate of AuNPs by PEG-b-PS-SH via ligand exchange to afford PEG-b-PScoated AuNPs (Au@PEG-b-PS), the influence of PS chain length on the stability of Au@PEG-b-PS against electrolyte and DTT-induced aggregation and catalytic activity on reduction of p-nitrophenol (4-NP) of Au@PEG-b-PS were investigated. The results demonstrated that the presence of PS layer could greatly improve the stability of Au@PEG-b-PS and the stability against electrolyte and DTT-induced aggregation increased with the decrease in PS chain length; Au@PEG-b-PS exhibited excellent catalytic activity, and their catalytic activity increased with the decrease in PS chain length. Furthermore, the high stability of Au@PEG-b-PS originating from the presence of the PS layer endowed Au@PEG-b-PS with reusability in catalysis without the loss of catalytic activity.

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EXPERIMENTAL SECTION Materials. Styrene (St, Aldrich, 99%) was washed with 5% aqueous NaOH solution to remove the inhibitor and then with water, dried over MgSO4, and distilled twice from CaH2 under reduced pressure prior to use. 2,2′-Azobis(isobutyronitrile) (AIBN, Aldrich, 98%) was recrystallized from anhydrous ethanol. Poly(ethylene glycol) methyl ether (PEG-OH, Mn ≈ 2000 g/mol, Aldrich) was purified by precipitation in ethyl ether prior to use. Tetrahydrofuran (THF, Aldrich, 99%), dichloromethane (CH2Cl2, Aldrich, 99.5%), and toluene (Aldrich, 99%) were dried over CaH2 and distilled from sodium and benzophenone under N2 prior to use. N,NDimethylformamide (DMF, Aldrich, 99%) was dried over MgSO4 and distilled under reduced pressure prior to use. 4Nitrophenol (4-NP, Aldrich, 99%), sodium borohydride (NaBH4, Aldrich, 99%,), dicyclohexylcarbodiimide (DCC, Aldrich, 98%), 4-(dimethylamino)pyridine (DMAP, Aldrich, 99%), 2-aminoethanol (Aldrich, 98%), tributylphosphane (Aldrich, 99%), HAuCl4·H2O (Aldrich, 99%), citric sodium (Aldrich, 99%), and dithiothreitol (DTT, Aldrich, 98%) were used as received. RAFT chain transfer agent, 4-cyano-4(dodecylsulfanylthio carbonyl-sulfanyl)pentanoic acid, was prepared according to previous literature.34,35 Measurements. All 1H NMR analyses of polymers were performed on a Bruker Avance 500 spectrometer (500 MHz), while 1H NMR analyses of Au@PEG-b-PS were performed on 1961

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The Journal of Physical Chemistry C sample was diluted 50 times for UV−vis measurement to determine the conversion of 4-NP after different waiting periods. After the complete conversion of 4-NP, 4-NP and NaBH4 were added to the solution again to start another cycle of reaction. The same procedures were performed in the additional cycles. Transmission Electron Microscopy. Structural characterization of AuNPs was carried out using a JEOL JEM-1230 transmission electron microscope operated at 80 kV. Samples for TEM were prepared by pipetting a drop of AuNP dispersion onto Formvar/carbon-coated copper grid and letting it dry for at least 24 h. Quantitative analyses of AuNP size distributions based on TEM measurements were performed using ImageJ software. UV−Vis Spectroscopy. Absorption spectra were recorded using a Hitachi U-2910 spectrophotometer with a rate of 400 nm/min. The spectra were collected using quartz cuvettes loading 3 mL of solution with a 1 cm optical path length. Dynamic Light Scattering. DLS measurements were carried out using a Malvern Nano-ZS90 Zetasizer. The sample temperature was maintained at 25 °C. For each sample, the autocorrelation function was the average of three runs of 11 s each, and the graphical presentations of the results show the average of these measurements as intensity percentages. Inductively Coupled Plasma Mass Spectrometry. ICPMS measurements were carried out on a PerkinElmer Elan 9000 instrument. A cross-flow double- pass spray chamber was used in all instances. Samples for ICP-MS were prepared by adding 500 μL of aqua regia into 1.5 mL of AuNP dispersion, heating at 70 °C for 20 min, and then diluting with deionized water to 5 mL. All experiments were performed using an autosampler (PerkinElmer AS93) modified for operation with Eppendorf 1.5 mL tubes. Sample volume was fixed at 1.5 mL, and the sample uptake rate was adjusted depending on the particular experiment, typically 100 μL/min. Standards were prepared from 1000 μg/mL PerkinElmer pure Single- Element Standard solutions by sequential dilution with high-purity HNO3. High-purity HNO3 was measured as well in each experiment to be used as blank signal. The blank signals were subtracted from the sample signals, which were then normalized to the signals of standard solutions (1 ppb).

Table 1. Synthesis of PEG-b-PS-CTA Amphiphilic Diblock Copolymer P1 P2 P3

NEG

time (h)

Mn,GPCa (kDa)

Mw/Mna

Mn,NMRb (KDa)

NStc

45 45 45

3.5 8 24

5.2 10.6 13.4

1.09 1.15 1.15

3.2 5.2 8.8

12 30 65

a

Measured by GPC in THF. bObtained from 1H NMR. cThe number of styrene repeated unit obtained from 1H NMR

b-PS-CTA was measured by GPC and 1H NMR (Figure S3, Supporting Information). The molecular weight obtained on the basis of the GPC system was a relative value, which was calibrated with linear polystyrene standards. On the contrary, the molecular weight of the copolymer and length of PS segment (NSt) were determined on the basis of the 1H NMR integral area ratio of typical protons attributed to PS and PEG segments, and the absolute molecular weight of PEG was measured by MALDI-TOF. Thus, the molecular weight and NSt of PEG-b-PS-CTA determined by 1H NMR were used in this work as summarized in Table 1. The thiocarbonate group in PEG-b-PS-CTA chain end was transformed into a thiol group in the presence of 2aminoethanol and tributylphosphane according to a previous report.36 The characteristic peaks originating from the protons of CTA completely disappeared in Figure S2B, Supporting Information. Additionally, the color of product turned white after the transformation (Figure S4, Supporting Information). These observations indicated that all thiocarbonate chain ends were reduced to thiol groups. It needs to be pointed out that all GPC curves of PEG-b-PS-SH after the transformation exhibit unimodal and symmetrical elution peaks and almost overlap with those of PEG-b-PS-CTA before the transformation (Figure S5, Supporting Information). This fact demonstrated that neither cross-linking reaction nor chain broken reaction occurred. Preparation of Au@PEG-b-PS. Citrated-stabilized AuNPs (Au@citrate) were first prepared by the classic Turkevich method.37 Figure S6, Supporting Information, shows a TEM image and diameter distribution histogram of the obtained Au@citrate. Most Au@citrate had diameters in the range from 10.9 to 15.6 nm, and the number-average diameter of AuNPs was 12.9 nm. Subsequently, ligand exchange of native citrate ligands on AuNPs with PEG-b-PS-SH was performed in the mixture of DMF/THF/water (VDMF:VTHF:Vwater = 2:2:1). We need to point out that no aggregate was formed for all PEG-bPS-SH on the basis of DLS results, which indicated that PEG-bPS was unimolecularly dissolved in the mixture. Moreover, excess amounts of ligands with a ratio of thiol-to-Au surface of about 100 were added to enhance the replacement of original citrate ligands by PEG-b-PS-SH. After the ligand exchange, free ligands in the solution were removed by three cycles of centrifugation and dispersion in THF, followed by another three cycles of centrifugation and dispersion in water. GPC results (Figure S7, Supporting Information) showed that all free ligands were removed. Furthermore, PEG-stabilized AuNPs were also prepared as a control by a similar method. Figure 1 shows TEM images of PEG and PEG-b-PSstabilized AuNPs (Au@PEG and Au@PEG-b-PS). One can notice that Au@citrate exhibited signs of aggregation or clumping in Figure S6A, Supporting Information, whereas Au@PEG and Au@PEG-b-PS presented a well-dispersed fashion regardless of the length of PS chain as shown in Figure

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RESULTS AND DISCUSSION Synthesis of PEG-b-PS-SH Ligand. On the basis of previous works about the preparation of polymers with a thiol chain end by RAFT polymerization, we first synthesized PEGbased macromolecular chain transfer agent via esterification reaction between commercially available PEG-OH and 4-cyano4-(dodecylsulfanylthiocarbonylsulfanyl)pentanoic acid according to previous reports.34,35 Subsequently, the obtained PEGbased macroCTA was employed in RAFT polymerization of styrene to give PEG-b-PS-CTA diblock copolymers. Through changing the polymerization time, three PEG-b-PS-CTA diblock copolymers with different PS chain lengths were prepared as listed in Table 1. Figure S1, Supporting Information, shows GPC curves of purified PEG-b-PS-CTA diblock copolymers. One can see that all GPC curves exhibit unimodal and symmetrical elution peaks with narrow molecular weight distributions (Mw/Mn ≤ 1.15), indicating the mediation of PEG-CTA for the polymerization of styrene. The typical proton resonance signals of PS and PEG segments were observed in 1H NMR spectrum of PEG-b-PS-CTA (Figure S2A, Supporting Information). The molecular weight of PEG1962

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attributed to the protons of PEG and PS blocks were observed in 1H NMR spectrum of the obtained Au@PEG-b-PS (Figure S8, Supporting Information), indicative of the replacement of citrate ligands with PEG-b-PS ligands. In summary, the aforementioned results demonstrated that citrate ligands on the surface of AuNPs were replaced by PEG and PEG-b-PS ligands without affecting the integrity of AuNPs and inducing the formation of aggregates. Estimation of Polymer Grafting Density on AuNPs. ICP-MS was employed to measure the content of Au (mAu), and solid concentration (Msolid) was obtained by weighting after lyophilization. Thus, the content of AuNPs (NAuNP) can be estimated by eq 1 NAuNP = (mAu/ρAu )/VAuNP

(1)

where ρAu and VAuNP are the density of AuNP and the average volume of AuNP, respectively. We assume that the density of AuNP is close to that of Au (11.4 g/cm3), and the average volume of AuNP was estimated on the basis of average diameter of AuNP measured by TEM (12.9 ± 2.3 nm). Then, the average number of polymer chains on each AuNP (Npolymer) was calculated from eq 2 Npolymer = [(Msolid − mAu)/M n)]NA /NAuNP

Figure 1. TEM images of Au@PEG (A), Au@PEG45-b-PS12 (B), Au@ PEG45-b-PS30 (C), and Au@PEG45-b-PS65 (D).

(2)

where NA and Mn are Avogadro’s constant and the molecular weight of the polymer, respectively. Correspondingly, the grafting density (σ) of polymer chains on each AuNP on the basis of the average diameter of AuNP (12.9 ± 2.8 nm) can be estimated from eq 3 (D is the diameter of the gold nanoparticle)

1. No size change of AuNPs was observed after ligand exchange, which could exclude the surface etching reaction during the ligand exchange. Figure 2A shows UV−vis absorption spectra of Au@citrate, Au@PEG, and Au@PEG-b-PS. A slight red shift of the surface plasma resonance peak was observed for Au@PEG and Au@ PEG-b-PS compared to Au@citrate. This phenomena might originate from the change of dielectric environment at the gold nanoparticle surface after replacement of citrate ligands by PEG or PEG-b-PS.22,26 DLS data shows that the hydrodynamic diameters of Au@PEG, Au@PEG45-b-PS12, Au@PEG45-b-PS30, and Au@PEG45-b-PS65 were 45.7, 62.1, 64.6, and 70.4 nm, respectively (Figure 2B), which were higher than that of Au@ citrate (18.2 nm). Since the sizes of Au@PEG and Au@PEG-bPS were shown as intensity-average hydrodynamic diameters, these values might be overestimated.38 However, the sizes of Au@PEG-b-PS increased with the increase of PS chain length, and no peak attributed to larger aggregates was observed in DLS profiles. This point further confirmed the replacement of citrate with PEG or PEG-b-PS, along with well dispersion of Au@PEG and Au@PEG-b-PS in aqueous media. Typical peaks

σ = Npolymer /π D2

(3)

Table 2 shows the values of Npolymer and σ for Au@PEG and Au@PEG-b-PS. One can see that the grafting densities were 2.8 Table 2. Polymer Coverage on Au Surface

PEG45-SH PEG45-bPS12−SH PEG45-bPS30-SH PEG45-bPS65−SH

average polymer chains on AuNPs (chains/particle ×103)

grafting density (chains/nm2)

1.5 ± 0.1 2.2 ± 0.1

2.8 ± 0.1 4.2 ± 0.1

1.5 ± 0.1

2.9 ± 0.1

0.9 ± 0.1

1.8 ± 0.1

Figure 2. (A) Absorption spectra and (B) hydrodynamic diameter distributions of Au@citrate, Au@PEG, and Au@PEG-b-PS in aqueous media. 1963

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The Journal of Physical Chemistry C ± 0.1, 4.2 ± 0.1, 2.9 ± 0.1, and 1.8 ± 0.1 chains/nm2 for PEG, PEG45-b-PS12, PEG45-b-PS30, and PEG45-b-PS65, respectively. Widrig et al. studied the monolayer of alkanethiol (CH3(CH2)nSH, n = 3−18) on the surface of gold films, and the coverage of alkanethiolate was estimated to be 5.0 and 4.6 alkanethiol/nm2 for all alkanethiol by experiment (scanning tunneling microscopy) and theory, respectively.39 Oh et al. investigated the coverage of monothiol-PEG (Mn = 750 g/mol) on AuNPs with an average diameter of 15 nm by X-ray photoelectron spectroscopy (XPS), and the density of coverage was estimated to be about 4.4 PEG/nm2.22 However, they pointed out that this value seemed to be overestimated, considering the results obtained by scanning tunneling microscopy. In the current case, the grafting density of PEG is about 2.8 ± 0.1 chains/nm2, assuming that AuNPs were spherical, which was close to the value estimated by Oh et al. It is interesting to notice that the grafting density of Au@PEG45b-PS12 is higher than that of Au@PEG, which is 4.2 ± 0.1 chains/nm2. This value is very close to those of monolayers of alkanethiol on flat Au surfaces (4.6 alkanethiol/nm2), which might suggest more compact packing of PEG 45 -b-PS 12 compared to that of PEG. Previous studies indicated that a hydrophobic long alkyl chain linking a thiol to a polymer or a functional group could improve the polymer coverage on the surface of gold nanoparticle.22,40,41 Thus, we speculated that the higher coverage of PEG45-b-PS12 compared to that of PEG might be attributed to the presence of a relatively short PS chain linker (12 repeated units). It was found that the grafting density decreased with the increase of PS chain length, which might be due to the increase in steric repulsion with increasing PS chain length. Stability against Electrolyte and DTT-Induced Aggregation. Electrolyte- and DTT-induced aggregation of AuNPs were commonly employed to examine the stability of AuNPs. In order to test our hypothesis that a relatively longer hydrophobic spacer linking thiol group and hydrophilic polymer might improve the stability of AuNPs, the stability of Au@citrate, Au@PEG, and Au@PEG-b-PS against electrolyte-induced aggregation was first tested in the presence of 2.0 M NaCl. Since the aggregation of AuNPs commonly leads to a red shift and broadening of surface plasma band of AuNPs due to electromagnetic coupling of gold nanoparticle upon aggregation, a red shift of the absorption in the UV−vis spectrum and blue shift of solution’s transmission are typical phenomena for the aggregation. One can clearly see that the color of solutions of Au@citrate and Au@PEG turned black and purple from pink after storing in the presence of NaCl for 30 min (Figure 3A), respectively, whereas the color of solutions of Au@PEG-b-PS remained pink. These observations were supported by UV−vis measurements as shown in Figure 3B. The absorbance peaks of Au@citrate and Au@PEG were shifted to 656 and 585 nm, respectively. On the contrary, the absorbance peak of all Au@PEG-b-PS did not show any obvious change after storing for at least 1 month as shown in Figure 3B. The colloidal stability of Au@citrate originated from electrostatic repulsion between Au@citrate due to the negatively charged surface of Au@citrate, while for Au@PEG and Au@PEG-b-PS their colloidal stability was provided by the steric repulsion between AuNPs, which was imparted by a compact hydrated PEG layer on the surface of AuNPs. Thus, a high concentration of NaCl can screen the surface charge of Au@citrate so that the electrostatic repulsive force and colloidal

Figure 3. Photographs (A) and UV−vis absorbance spectra (B) of Au@citrate, Au@PEG, and Au@PEG-b-PS in 2.0 M NaCl.

stability of Au@citrate was reduced. In contrast, the neutral hydrated PEG layer is thought to be insensitive to electrolyte and can resist a high salt concentration to a certain extent. Although both Au@PEG and Au@PEG-b-PS were covered by a compact hydrated PEG layer, Au@PEG showed poor stability, whereas Au@PEG-b-PS exhibited excellent stability in the presence of 2.0 M NaCl. Mattoussi et al. examined the colloidal stability of AuNPs with an average diameter of 15 nm covered by monothiol- and dithiolane-terminated PEG (Mn ≈ 750 g/mol) in the presence of 1.0 M NaCl.28 They found that monothiol-terminated PEG-stabilized AuNPs precipitated after 2 days of storage, while AuNPs covered with dithiol-terminated PEG were stable for at least 6 months. Oh et al. estimated the coverage density of monothiol- and dithiol-terminated PEG (Mn ≈ 750 g/mol) on the surface of AuNPs with an average diameter of 15 nm by XPS.22 Their results showed that sulfur coverage for monothiol- and dithiol-terminated PEG was about 4.4 and 6.8 S/nm2, respectively, that is, 4.4 and 3.4 PEG chains/ nm2. Therefore, monothiol-PEG-stabilized AuNPs should have a more compact PEG hydrated layer and weaker affinity of the anchoring group to the surface of AuNPs than those of dithiolPEG-coated AuNPs. These results indicated that the stability of monothiol-PEG- and dithiol-PEG-covered AuNPs should be dominated by the affinity of the anchoring group onto the surface of AuNPs, not the density of the hydrated PEG layer. The Au-to-thiol bond is thought to be a Lewis acid−base interaction, not a covalent bond, and able to be affected by intermittent dissociation (“on” and “off” binding).26 The high content of electrolyte (NaCl) might induce dissociation of the Au-to-thiol bond and reduce the stability of AuNPs. Since the affinity of the thiol-to-Au bond should be the same, the possibility of the dissociation of thiol-to-Au bond was supposed to be equal under certain conditions. The possibility for the PEG chain leaving the surface would decrease with increasing number of anchoring group if other factors, presumably affecting the strength of thiol-to-Au bond, such as microenvironment of thiol-to-Au bond and structure of substituted group to thiol functionality, were assumed to be not dominant. This is might be the reason why the stability of AuNPs 1964

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Figure 4. Time-dependent absorbance spectra of Au@citrate (A), Au@PEG (B), Au@PEG45-b-PS12 (C), Au@PEG45-b-PS30 (D), and Au@PEG45-bPS65 (E) in the presence of 1.5 M DTT, 0.8 M NaCl, and 10 mM NaOH. (F) Plot of the normalized aggregation factor (AF) extracted from plots of B−E.

was further tested in the presence of DTT. We monitored the change of absorbance of Au@citrate, Au@PEG, and Au@PEGb-PS in the presence of 1.5 M DTT, 0.8 M NaCl, and 10 mM NaOH, where basic pH made DTT more reactive and DTTinduced aggregation can be accelerated in the presence of an excess of NaCl according to previous reports.22,26,27 In order to exclude the possibility of NaOH-induced aggregation, the stability of AuNPs in the presence of 10 mM NaOH (pH = 12.0) was examined first. No visible sign of a red shift and broadening in the absorbance spectra was observed over 30 min (Figure S9, Supporting Information), indicative of a high stability of Au@citrate, Au@PEG, and Au@PEG-b-PS in basic environment. Then, the absorbance spectra of AuNPs were recorded for each sample from 0 to 30 min after adding DTT solution (Figure 4). Precipitation was observed for the solution of citrate-stabilized AuNPs immediately after adding DTT solution, and the original red color attributed to the surface plasma resonance of AuNPs completely disappeared. The absorbance spectra of Au@PEG showed an obvious red shift, broadening, and decrease in the intensity at 530 nm with time, while those of Au@PEG-b-PS just showed a decrease in the intensity at the peak of 535 nm with time. Although the electrolyte could induce the aggregation of Au@PEG in the presence of 2.0 M NaCl, only a slight decrease in absorbance

increased with the increase in the number of anchoring groups. In the current work, both PEG and PEG-b-PS were attached to the surface of AuNPs via a monothiol-to-Au bond; thus, the affinity to AuNPs should be similar. Although the coverage of Au@PEG45-b-PS65 is about 1.8 ± 0.1 chains/nm2, much lower than that of Au@PEG (2.8 ± 0.1 chains/nm2), the stability of Au@PEG45-b-PS65 against NaCl was higher than that of Au@ PEG. Thus, the stability of Au@PEG and Au@PEG-b-PS should be also mainly determined by the possibility of dissociation of the thiol-to-Au bond, not the density of PEG chains on the surface in accordance with previous reports.22,26,27 We speculated that the hydrophobic PS layer or collapsed PS chain near the thiol-to-Au bond was likely to cover the local surface where the thiol-to-Au bond was formed and prevent the “invasion” of polar Na+ and Cl− through the hydrophobic PS layer. This would decrease the local concentration of NaCl and retard NaCl-induced dissociation of the Au-to-thiol bond. Thus, the stability of Au@PEG-b-PS against NaCl-induced aggregation was greatly improved. Moreover, since the affinity/stability of the ligand to AuNP surface could be reflected by competitive displacement of the ligands with DTT, which can replace weakly to modestly bound ligands from the surface of AuNP and lead to the aggregation of AuNPs, the enhanced stability of PEG-b-PS-covered AuNPs 1965

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Figure 5. Absorbance spectra of 4-NP solutions containing Au@citrate (A), Au@PEG (B), Au@PEG45-b-PS12 (C), Au@PEG45-b-PS30 (D), and Au@PEG45-b-PS65 (E). (F) dependence of ln(At/A0) on reaction time; At was extracted from absorbance spectra in A, C, D, and E. [NaBH4] = 0.14 M, [4-NP] = 3.6 mM, [AuNPs] = 3.3 × 1011 nanoparticles/L.

PEG. Furthermore, the length of the PS chain length also affected the stability against competitive displacement by DTT, and the stability of Au@PEG-b-PS increased with the decrease in PS chain length. The highly improved stability against DTT for Au@PEG-bPS, compared to Au@PEG, was also supposed to be imparted to the presence of a hydrophobic PS layer. The hydrophobic PS chains would collapse in aqueous media; thus, hydrophobic “caps” would form near the Au-to-thiol bond and cover the surface of AuNPs. The collapsed PS domains were likely to decrease the accessibility of polar DTT to the surface of AuNPs and the possibility of “attacking” by DTT. The grafting densities were 4.2 ± 0.1, 2.9 ± 0.1, and 1.8 ± 0.1 chains/nm2 for Au@PEG45-b-PS12, Au@PEG45-b-PS30, and Au@PEG45-bPS65, respectively. Thus, the PS layer became much more compact with the decrease in PS chain length, though the thickness of the PS layer should become thinner with the decrease in PS chain length, The more compact PS layer might result in a much lower local concentration of DTT, that is, lower possibility in the displacement of PEG-b-PS by DTT. Therefore, the presence of a hydrophobic PS chain improved the stability of Au@PEG-b-PS against DTT-induced aggregation, and the stability was enhanced with the decrease in PS chain length.

was observed in the presence of 0.8 M NaCl with a time frame of 30 min as shown in Figure S10, Supporting Information. Thus, NaCl-induced aggregation could be excluded for Au@ PEG in the current DTT-induced aggregation experiment. For quantitative comparison of microscopic changes of AuNPs, we defined the aggregation factor (AF) in eq 4 AF = (I − I0)/A

(4)

where I0 and I are the total area from 500 to 800 nm in the absorbance spectra of AuNPs before and after adding DTT solution, respectively. A is the absorbance at 535 nm after adding DTT solution. The absorbance spectra of strongly capped and stable AuNPs will show no or small changes in AF. Conversely, if the capping ligands are weakly bound, AuNPs become unstable upon addition of DTT, AF will increase with time. AF as a function of time for all DTT-induced aggregation experiments are plotted in Figure 4F. AF of Au@PEG exhibited the steepest increase with time and reached 717 within 30 min. In contrast, AF for all Au@PEG-b-PS showed a gradual increase with time. AF increased to 0.5, 75, and 112 within 30 min for Au@PEG45-b-PS12, Au@PEG45-b-PS30, and Au@PEG45-b-PS65, respectively, much lower than that of Au@PEG within the same time frame. These results indicated that all Au@PEG-b-PS had a higher resistance against DTT-induced aggregation than Au@ 1966

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The Journal of Physical Chemistry C Catalytic Activity of AuNPs. The results obtained in electrolyte- and DTT-induced aggregation experiments could reflect the influence of the PS chain length on the accessibility of electrolyte and DTT to the microenvironment near the thiolto-Au bond to a certain extent, which should be “protected” by hydrophobic PS domains, but these results seemed to be not able to reflect the accessibility of electrolyte and DTT to the surface “uncovered” by PS domains. Thus, in order to further evaluate the influence of PS chain length on the accessibility of surface “uncovered” by PS domains to polar compounds and examine if the presence of hydrophobic PS segments would affect the catalytic activity of Au@PEG-b-PS in aqueous media, we performed Au-catalyzed reduction of 4-NP in the presence of NaBH4 considering that the surface accessible to BH4− and 4-nitrophenol should be related to the catalytic activity of AuNPs. We chose this reaction because the substrates and products of this reaction were easily monitored by spectroscopic methods, and there was no formation of appreciable byproduct. According to previous reports, 4-nitrophenolate ions have a characteristic absorbance peak at 400 nm.42−45 As the reduction reaction proceeded, the absorbance at 400 nm would decrease and the absorbance at 295 nm would increase gradually, indicating the reduction of 4-NP and formation of 4aminophenol (4-AP), respectively. Thus, the reduction reaction can be tracked by the change in intensity of those two peaks. The concentrations of AuNPs in mother solution for each sample were first normalized by the absorbance at 450 nm according to a previous report, assuming that the content of gold nanoparticle has a linear relationship with its absorbance at 450 nm.46 The content of Au atoms was about 6.8 mg/L estimated by ICP-MS, that is, the content of AuNPs in each solution was about 3.3 × 1011 nanoparticles/L. After 0.65 mL of each AuNPs solution was added into 3.55 mL of aqueous solution containing 0.6 mmol of NaBH4 and 0.015 mmol of 4NP, the conversion of 4-NP after different waiting periods was determined by UV−vis measurements. Figure S11, Supporting Information, shows the typical photographs of solutions containing Au@citrate, Au@PEG, and Au@PEG45-b-PS65 after the reaction continued for 60 min. The color of the solution with Au@citrate changed from bright yellow to colorless, and some purple sediment was found on the bottom of the flask and the surface of the stirring bar. The purple sediment was likely to be aggregates of AuNPs. For the solution with Au@PEG, the color remained bright yellow and did not obviously fade. We also noticed some aggregates on the surface of the stirring bar after several minutes. On the contrary, the color of the solution containing Au@PEG-b-PS changed from yellow to pink as the reaction carried on. Additionally, no sediment or sign of aggregation was observed at the end of the reaction. Felice et al. studied the adsorption of cysteine on Au(111) surface of gold by periodic supercell density functional theory calculations and showed that the N−Au bond strength was estimated to be 6 kcal/mol (47 kcal/mol for Au−S bond).47 In addition, Trout et al. reported that amine could be bound to the Au surface via interaction with a single Au atom to form a Au−N bond. Moreover, the bond could be enhanced significantly for AuNPs.48 On one hand, the product of 4-AP might displace the citrate and PEG-SH, thus, and induce the aggregation of AuNPs; on the other hand, the concentration of NaBH4 was about 0.14 M, which can also screen the surface charge of citrate-stabilized AuNPs and induce the aggregation. For all Au@PEG-b-PS-catalyzed reaction systems, although the reagents of BH4− and 4-NP can access the surface of AuNPs

and make the Au-catalyzed reduction reaction go smoothly, the hydrophobic PS domains still endowed the nanoparticle with a high colloidal stability. Overall, these results were consistent with the observations in NaCl- and DTT-induced aggregation experiments. In order to investigate the influence of PS chain length on the catalytic activity of AuNPs quantitively, the reaction process was tracked by UV−vis measurement as shown in Figure 5. The reaction was analyzed by a first-order rate law due to the fact that an excess of NaBH4 was used and an excess of catalytic Au was assumed to be present. On the basis of Figure 5, the ratio of the absorbance of 4-NP at a given time (At) to its initial value (A0) measured at t = 0 directly gave the corresponding concentration ratio C0/Ct of 4-NP (Ct and C0 are the concentrations of 4-NP at time t and t = 0, respectively). Thus, the kinetic equation of the reaction can be shown in eq 5 (kapp is the apparent rate constant and t is reaction time) ln(C0/C t) = ln(A 0 /A t ) = kappt

(5)

As shown in Figure 5F, a linear relation between ln(C0/Ct) and reaction time t can be obtained for Au@citrate and Au@ PEG-b-PS. This fact indicated that the reaction did follow the first-order rate kinetics and verified our assumption that an excess of catalytic Au was present for all reactions catalyzed with Au@citrate and Au@PEG-b-PS. The apparent rate constant, kapp, can be obtained from the slope of the fitting lines (Figure 5F). One can see that among these Au-catalyzed systems, the catalytic activity of Au@citrate was the highest with a highest kapp of 4.6 × 10−3 S−1 and the reaction finished just in 12 min, although aggregation was formed during the reaction (Figure 5A). On the contrary, the catalytic activity of Au@PEG was the lowest and only about 61% of 4-AP was formed in 60 min, mainly due to a fast aggregation of Au@PEG (Figure 5B). For Au@PEG-b-PS-catalyzed reaction systems (Figure 5C, 6D, and 6E), the reaction finished in 22, 30, and 40

Figure 6. Conversion of 4-NP in five successive cycles of reduction catalyzed by Au@citrate, Au@PEG, and Au@PEG45-b-PS65.

min with kapp = 2.2 × 10−3, 1.9 × 10−3, and 1.5 × 10−3 s−1 for Au@PEG45-b-PS12, Au@PEG45-b-PS30, and Au@PEG45-b-PS65, respectively. The reaction catalyzed by AuNPs might follow a Langmuir− Hinshelwood mechanism, where both borohydride and 4-NP were absorbed on the surface first, and the borohydride absorption led to a surface-hydrogen species.42 Subsequently, the surface hydrogen species reacted with 4-NP absorbed on the surface, and the formed product of 4-AP was desorbed from the surface. Besides the rate for surface reaction between 1967

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The Journal of Physical Chemistry C surface hydrogen species provided by borohydride and 4-NP absorbed on the surface, the diffusion of both reactants and product also could determine the reaction rate.42 Ballauff et al. investigated the catalytic activity of palladium nanoparticles embedded in spherical polyelectrolyte brush shell and microgel shell for the reduction of 4-NP. The results demonstrated that the catalytic activity of nanoparticles in the microgel shell was lower than that in spherical polyelectrolyte brush shell since the cross-linked microgel restricted the diffusion of reactants and product.42 Studies on the catalytic activity of palladium and gold nanoparticles stabilized by dendrimer poly(propyleneimine) (PPI) and dendrimer poly(amidoamine) (PAMAM) by Esumi et al. also showed that the catalytic activity of palladium and gold nanoparticles covered by PPI with a smaller size was higher than that of palladium and gold nanoparticles covered by PAMAM.49,50 The smaller size of PPI led to faster diffusion of both reactants and product, which resulted in a higher catalytic activity. In the current case, the presence of a hydrophobic PS layer should also retard the diffusion of both reactants and product due to their high polarity. We plotted kapp for Au@PEG-b-PS against the number of styrene repeat units in PEG-b-PS and found that kapp decreased linearly with the number of styrene repeat units as shown in Figure S12, Supporting Information. Since the diffusion of both reactants and product was assumed to be related to the thickness of the PS layer, this observation further indicated the catalytic activity of AuNPs should be dominated by the diffusion, that is, the thickness of the PS layer. Thus, the catalytic activity of Au@PEG-b-PS increased with decreasing PS chain length. We need to point out that the stability of Au@PEG-b-PS was significantly improved due to the inner PS layer, compared to Au@citrate and Au@PEG. This property invoked our investigation on recovery of Au@PEG-b-PS in the catalytic reaction. To demonstrate the recovery of Au@citrate, Au@ PEG, and Au@PEG-b-PS, we increased the concentration of reagents and catalysts by 2 times so that the reaction can complete within 10 min (Figure 6). For Au@citrate, although the reaction can finish just within 12 min in the first cycle, aggregates of AuNPs were formed during the first cycle and the conversion of 4-NP steeply dropped to 0 in the second cycle of reaction. For Au@PEG, the conversion was just 61% in the first cycle even let the reaction last 60 min. On the contrary, the reaction completed within 8 min used Au@PEG45-b-PS65 as catalyst, and the catalyst could be reused in four successive reactions, all with conversions of about 100% within 8 min (Figure S13, Supporting Information). Apparently, the high stability of Au@PEG-b-PS due to the presence of hydrophobic PS layer can endow Au@PEG-b-PS with reusability in catalysis.

the grafting density of PEG45-b-PS12-coated AuNPs was higher than that of PEG-stabilized AuNPs since the relatively short PS chain (12 repeated units) might facilitate the order packing of polymer chains. In addition, the grafting density of PEG-b-PScoated AuNPs decreased with increasing PS chain length because the steric repulsion would become stronger with increasing PS chain length. Since the hydrophobic PS layer might retard the invasion of polar compounds into the PS layer covered surface, all PEG-b-PS-coated AuNPs showed a better stability in the presence of 2.0 M NaCl or 1.5 M DTT along with 0.8 M NaCl and 0.01 M NaOH, compared to PEG and citrate-coated AuNPs. The resistance of PEG-b-PS-coated AuNPs against DTT-induced aggregation increased with the decrease in PS chain length owing to a more compact coverage. Although the catalytic activity of PEG-b-PS-coated AuNPs were not as high as that of citrate-stabilized AuNPs in the reduction of 4-NP into 4-AP by NaBH4, all PEG-b-PS-coated AuNPs exhibited excellent stability, which endowed the nanoparticles with well reusability in catalysis without the loss of catalytic activity. Additionally, the catalytic activity of PEG-b-PS-coated AuNPs increased with decreasing PS chain length. It was thought that the reduction reaction might be determined by the diffusion of polar reactants of 4-NP and BH4− into the surface and product of 4-AP from the surface; thus, the catalytic activity decreased with the increasing thickness of the PS layer. Although previous studies underlined the importance of the structure or length of the alkyl spacer on the properties of semiconductor and metallic nanoparticles, the influence of structure and chain length of hydrophobic polymer spacer was not examined. In this study, we used PEG-b-PS-coated AuNPs as a model to investigate the influence of the PS chain length on the stability and catalytic activity of AuNPs. The information present in this study not only exhibited the importance of chain length of hydrophobic polymer spacer for amphiphilic copolymer-stabilized inorganic nanoparticles but also provided some experimental guides for rational design of amphiphilic polymer ligands to obtain optimal stability and catalytic activity. Given the versatile and robustness of RAFT polymerization, the improved understanding of chain length of polymer spacer on nanoparticles’ properties, such as stability- and surface-related behaviors, seems to be valuable for extending the application of other types of nanoparticles, even if some other factors involving in the particular application of interest and modification strategy might also need to be taken into account.

CONCLUSIONS In this study, we tested the influence of the PS chain length on the stability and catalytic activity of PEG-b-PS-SH-coated AuNPs with an average diameter of 12.9 nm. PEG-b-PS-SH with different PS chain lengths were prepared by RAFT polymerization using PEG-based macromolecular chain transfer agent (Mn ≈ 2000 g/mol), followed by reduction of the thiocarbonate chain end into the thiol group in the presence of 2-aminoethanol and tributylphosphane. Subsequently, citratestabilized AuNPs were transformed into PEG-b-PS-covered AuNPs through ligand exchange reaction, and the grafting density of the PEG-b-PS chain on the surface of AuNPs was calculated on the basis of ICP-MS. The results indicated that

Details about the preparation and characterization of PEG-bPS, Au@PEG, and Au@ PEG-b-PS; figures showing photographs and UV−vis absorbance spectra of Au@citrate, Au@ PEG, and Au@PEG-b-PS in 10 mM NaOH; UV−vis absorbance spectra of Au@PEG in 0.8 M NaCl; photographs of Au@citrate, Au@PEG, and Au@PEG45-b-PS65 during the reduction of 4-NP; dependence of kapp of Au@PEG-b-PScatalyzed reduction of 4-NP on the number of styrene repeat units in PEG-b-PS; UV−vis absorbance spectra of 4-NP solutions containing Au@PEG45-b-PS65 during the first cycle, second cycle, third cycle, fourth cycle, and fifth cycle. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ASSOCIATED CONTENT

S Supporting Information *

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The Journal of Physical Chemistry C

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(16) Hussain, I.; Graham, S.; Wang, Z. X.; Tan, B.; Sherrington, D. C.; Rannard, S. P.; Cooper, A. I.; Brust, M. Size-controlled synthesis of near-monodisperse gold nanoparticles in the 1−4 nm range using polymeric stabilizers. J. Am. Chem. Soc. 2005, 127, 16398−16399. (17) You, C. C.; Miranda, O. R.; Gider, B.; Ghosh, P. S.; Kim, I. B.; Erdogan, B.; Krovi, S. A.; Bunz, U. H. F.; Rotello, V. M. Detection and identification of proteins using nanoparticle−fluorescent polymer ‘chemical nose’ sensors. Nat. Nanotechnol. 2007, 2, 318−323. (18) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Synthesis of nanosized gold-silica core-shell particles. Langmuir 1996, 12, 4329− 4335. (19) Kairdolf, B. A.; Nie, S. M. Multidentate-protected colloidal gold nanocrystals: pH control of cooperative precipitation and surface layer shedding. J. Am. Chem. Soc. 2011, 133, 7268−7271. (20) Zhang, G. D.; Yang, Z.; Lu, W.; Zhang, R.; Huang, Q.; Tian, M.; Li, L.; Liang, D.; Li, C. Influence of anchoring ligands and particle size on the colloidal stability and in vivo biodistribution of polyethylene glycol-coated gold nanoparticles in tumor-xenografted mice. Biomaterials 2009, 30, 1928−1936. (21) Uchida, K.; Hoshino, Y.; Tamura, A.; Yoshimoto, K.; Kojima, S.; Yamashita, K.; Yamanaka, I.; Otsuka, H.; Kataoka, K.; Nagasaki, Y. Creation of a mixed poly(ethylene glycol) tethered-chain surface for preventing the nonspecific adsorption of proteins and peptides. Biointerphases 2007, 2, 126−130. (22) Oh, E.; Susumu, K.; Makinen, A. J.; Deschamps, J. R.; Huston, A. L.; Medintz, I. L. Colloidal stability of gold nanoparticles coated with multithiol-poly (ethylene glycol) ligands: importance of structural constraints of the sulfur anchoring groups. J. Phys. Chem. C 2013, 117, 18947−18956. (23) Dong, H. C.; Zhu, M. Z.; Yoon, J. A.; Gao, H. F.; Jin, R. C.; Matyjaszewski, K. One-pot synthesis of robust core/shell gold nanoparticles. J. Am. Chem. Soc. 2008, 130, 12852−12853. (24) Kanaras, A. G.; Kamounah, F. S.; Schaumburg, K.; Kiely, C. J.; Brust, M. Thioalkylated tetraethylene glycol: A new ligand for water soluble monolayer protected gold clusters. Chem. Commun. 2002, 2294−2295. (25) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. Nanometer gold clusters protected by surface-bound monolayers of thiolated poly(ethylene glycol) polymer electrolyte. J. Am. Chem. Soc. 1998, 120, 12696−12697. (26) Stewart, M. H.; Susumu, K.; Mei, B. C.; Medintz, I. L.; Delehanty, J. B.; Blanco-Canosa, J. B.; Dawson, P. E.; Mattoussi, H. Multidentate poly(ethylene glycol) ligands provide colloidal stability to semiconductor and metallic nanocrystals in extreme conditions. J. Am. Chem. Soc. 2010, 132, 9804−9813. (27) Schulz, F.; Vossmeyer, T.; Bastús, N. G.; Weller, H. Effect of the spacer structure on the stability of gold nanoparticles functionalized with monodentate thiolated poly(ethylene glycol) ligands. Langmuir 2013, 29, 9897−9908. (28) Mei, B. C.; Oh, E.; Susumu, K.; Farrell, D.; Mountziaris, T. J.; Mattoussi, H. Effects of ligand coordination number and surface curvature on the stability of gold nanoparticles in aqueous solutions. Langmuir 2009, 25, 10604−10611. (29) Zopes, D.; Stein, B.; Mathur, S.; Graf, C. Improved stability of “naked” gold nanoparticles enabled by in situ coating with mono and multivalent thiol PEG ligands. Langmuir 2013, 29, 11217−11226. (30) Zhao, G. Y.; Tong, L.; Cao, P. P.; Nitz, M.; Winnik, M. A. Functional PEG-PAMAM-tetraphosphonate capped NaLnF4 nanoparticles and their colloidal stability in phosphate buffer. Langmuir 2014, 30, 6980−6989. (31) Maus, L.; Dick, O.; Bading, H.; Spatz, J. P.; Fiammengo, R. Conjugation of peptides to the passivation shell of gold nanoparticles for targeting of cell- surface receptors. ACS Nano 2010, 4, 6617−6628. (32) Simpson, C. A.; Agrawal, A. C.; Balinski, A.; Harkness, K. M.; Cliffel, D. E. Short-chain PEG mixed monolayer protected gold clusters increase clearance and red blood cell counts. ACS Nano 2011, 5, 3577−3584.

AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-21-54925520. Fax: +86-21-64166128. E-mail: [email protected]. *Phone: +86-21-54925310. Fax: +86-21-64166128. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the financial support from the National Natural Science Foundation of China (21274162, 51373196, and 21474127) and Shanghai Scientific and Technological Innovation Project (12JC1410500, 13ZR1464800, 14QA1404500, and 14520720100).

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