Poly(ethylene glycol)- and Carboxylate-Functionalized Gold

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PEG and Carboxylate-Functionalized Gold Nanoparticles by Polymer Linkages: Single-Step Synthesis, High Stability, and Plasmonic Detection of Proteins Garam Park, Daeha Seo, Im Sik Chung, and Hyunjoon Song Langmuir, Just Accepted Manuscript • DOI: 10.1021/la402315a • Publication Date (Web): 03 Oct 2013 Downloaded from http://pubs.acs.org on October 5, 2013

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PEG and Carboxylate-Functionalized Gold Nanoparticles using Polymer Linkages: Single-Step Synthesis, High Stability, and Plasmonic Detection of Proteins Garam Park,†, ‡ Daeha Seo,† Im Sik Chung,*, § and Hyunjoon Song*,†, ‡ †

Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305701, Korea

‡

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 305-701, Korea §

Research Center for Integrative Cellulomics, Korea Research Institute of Bioscience and Biotechnology, Daejeon 305-806, Korea

KEYWORDS gold nanoparticles · surface functionalization · poly(ethylene glycol) · polymer linkage · protein adsorption

ACS Paragon Plus Environment

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ABSTRACT

Gold nanoparticles with suitable surface functionalities have been widely used as a versatile nanobio-platform. However, functionalized gold nanoparticles using thiol-terminated ligands have a tendency to aggregate, particularly in many enzymatic reaction buffers containing biological thiols, due to ligand exchange reactions. In the present study, we developed a onestep synthesis of PEGylated gold nanoparticles using poly(dimethylaminoethyl methacrylate) (PDMAEMA) in poly(ethylene glycol) (PEG) as a polyol solvent. Due to the chelate effect of polymeric functionalities on the gold surface, the resulting PEGylated gold nanoparticles (Au@P-PEG) are very stable under the extreme conditions at which the thiol-monolayer protected gold nanoparticles are easily coagulated. Using the solvent mixture of PEG and EG and subsequent hydrolysis, gold nanoparticles bearing mixed functionalities of PEG and carboxylate are generated. The resulting particles exhibit selective adsorption of positivelycharged chymotrypsin (ChT) without nonselective adsorption of bovine serum albumin (BSA). The present nanoparticle system has many advantages, including high stability, simple one-step synthesis, biocompatibility, and excellent binding specificity; thus, this system can be utilized as a versatile platform for potential bio-related applications, such as separation, sensing, imaging, and assays.


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INTRODUCTION

Gold nanoparticles have been widely used as a versatile platform for a variety of applications. Their tuneable size and morphology with high uniformity, and unique photophysical properties based on localized surface plasmon resonance (LSPR) render them very attractive for electronics and optics.1-9 In addition, their chemically inert and biocompatible features lead to potential biorelated applications such as sensing, imaging, drug delivery, and photothermal therapy.6,10-19 For diverse applications, surface functionalization with suitable ligands or surfactants is critical to ensure particle stability against aggregation.20-23 Particularly in biological applications, nonspecific binding with biomolecules diminishes the sensitivity of assays for specific targets. Huang et al. addressed this issue with the introduction of poly(ethylene glycol) (PEG) units on the surface, i.e., PEGylation, leading to the material’s sufficient stability in aqueous media and strong resistance of the binding against various biomolecules.24 The most common method of preparing PEGylated Au nanoparticles is by covalent attachment of thiol-modified PEG onto the Au nanoparticle surfaces, because thiols can be used as anchoring groups that are directly chemisorbed to the gold surface.14,16,23,25-30 Moreover, selective binding to target molecules has also been achieved using a mixed monolayer formation consisting of both PEG and capture units.31-33 It should be noted, however, that these Au nanoparticles exhibit poor stability against ligand exchange reaction by thiol-containing molecules (e.g., mercaptoethanol34 and dithiothreitol (DTT)35). These molecules are involved in many enzymatic reaction buffers36,37, and, thus, Au nanoparticles functionalized via a Au-S bond undergo rapid and irreversible aggregation. Mattoussi et al. successfully utilized multiple thiol ligands such as di- and tetrathiols to greatly enhance the colloidal stability under extreme conditions.38,39 Recently, Lee


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et al. reported a synthetic method of enhancing the chemical stability of monothiol-modified oligonucleotide-Au nanoparticle conjugates using a thin silica coating.40 Although these approaches have been successfully employed in bio-applications, there is further demand for highly stable colloids with various functionalities on their surface that can work with any biological media. Polymer surfactants are known to be effective for the confinement of the nanoparticle surface and for control of the particle morphology. Thus, such surfactants are referred to as ‘surface regulating polymers’.2,41,42 With the help of abundant functionality, polymeric ligands are strongly bound to the particle surface and enhance the particle stability. We synthesized various morphologies of gold nanoparticles in the presence of surface regulating polymers using a modified polyol process.43-46 Changing the polymer units could provide useful functionalities such as hydroxyl and carboxylic acid groups on the surface and make it possible to adjust the solution behavior of the nanoparticles.47-49 In the present study, we

developed

a

one-step

synthesis

of

PEGylated

gold

nanoparticles

using

poly(dimethylaminoethyl methacrylate) (PDMAEMA) in PEG as a polyol solvent for the reduction of metal precursor. The resulting nanoparticles were very stable toward DTT due to a Au-carbonyl group bond in PDMAEMA, where the thiol-monolayer protected gold nanoparticles easily coagulated. The use of a PEG/ethylene glycol (EG) mixture was used to generate mixed functionalities on the gold surface, and basic hydrolysis on the EG groups yielded gold nanoparticles consisting of both PEG and carboxylate groups. These nanoparticles exhibited the selective adsorption of positively charged α-chymotrypsin (ChT) without the nonselective binding of bovine serum albumin (BSA); this result was readily monitored using UV-VIS spectroscopy and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis.


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EXPERIMENTAL SECTION

Chemicals. Tetrachloroaurate trihydrate (HAuCl4·3H2O, 99.9+ %), silver nitrate (AgNO3, 99+ %), poly(ethylene glycol) (PEG, Mn = 300 and 600), ethylene glycol (EG, 99 %), sodium hydroxide (NaOH, 99.998 %), dithiothreitol (DTT, 98 %), sodium chloride (NaCl, 99.999 %), sodium citrate (99.0 %), and 3-mercapto-1-propanol (HS(CH2)3OH, 95 %) were purchased from Aldrich. Poly(dimethylaminoethyl methacrylate) (PDMAEMA, 18.45 % solution in toluene) was purchased from Scientific Polymer Products and used after solvent evaporation. O-(2Mercaptoethyl)-O’-(methyl)polyethylene glycol (5000) (HS-PEG-OCH3, 5000) was purchased from NOF corporation. Albumin from bovine serum (BSA) and α-chymotrypsin (type II from bovine pancreas, ChT) were purchased from Sigma. Phosphate buffered saline (PBS, 10×, pH 7.4) was purchased from Invitrogen and used after 10 times dilution to 1×. All chemicals not described here were used as received. Synthesis of Au@P-PEG nanoparticles. Several drops of water were added to the dried PDMAEMA (0.072 g, 0.45 mmol based on the repeating unit) to disassemble the polymer chains, and PEG (10.0 mL) was added to this slurry. To remove residual water, the solution was heated under vacuum at 100 °C for 6 h. A AgNO3 solution in PEG (0.15 mL, 0.020 M) was injected into the solution at 250 °C, and a HAuCl4 solution in PEG (3.0 mL, 0.050 M) was periodically added every 20 s over 5 min. The mixture was allowed to stir at 250 °C for 30 min under an argon atmosphere. After cooling the reaction mixture, the product was thoroughly washed several times by repetitive dispersion-precipitation cycles with ethanol; finally the particles were dispersed in deionized water (30 mL).


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Stability test. PEG-OCH3-terminated gold nanoparticles were synthesized using the Turkevich-Frens method40 and the ligand exchange reaction with HS-PEG-OCH3.26-28 A HAuCl4 aqueous solution (50 mL, 0.25 mM) was refluxed. A sodium citrate aqueous solution (0.50 mL, 0.034 M) was added to the solution and vigorously mixed for 2 min. The gold nanoparticles were isolated by centrifugation and redispersed in deionized water (50 mL). The pH of the gold nanoparticle dispersion (30 mL, 7.6 mmol) was adjusted to ~10 by adding a few drops of the 1 M NaOH solution. HS-PEG-OCH3 solution (23 mmol) was added to the dispersion while stirring and the mixture was sonicated for 1 h. The excess ligands were removed by centrifugation and the final product was dispersed in deionized water (15 mL). For the stability test, A DTT aqueous solution (1.4 mL, 3.0 M) was mixed with a NaCl aqueous solution (0.60 mL, 2.0 M), and the solution was diluted with a PBS buffer solution. The nanoparticle dispersion was added to the solution to adjust the optical density to 0.3 at the maximum of the surface plasmon band. The resulting dispersion was thoroughly mixed for 30 s, and periodically checked by UV-VIS spectroscopy every 2 min for 60 min. Synthesis of Au@P-PEG/EG nanoparticles. PDMAEMA (0.072 g, 0.45 mmol based on the repeating unit) was dissolved in a mixture of EG (5.0 mL) and PEG (5.0 mL). A AgNO3 solution in PEG (0.15 mL, 0.020 M) was injected into the solution at 190 °C, and a HAuCl4 solution in EG (3.0 mL, 0.050 M) was periodically added every 20 s over 5 min. The mixture was stirred at 190 °C for 30 min under an argon atmosphere. After cooling the reaction mixture, the product was thoroughly washed with ethanol and was finally dispersed in deionized water (30 mL). Formation of Au@P-PEG/COO- by basic hydrolysis. A NaOH aqueous solution (14 mL, 1.0 M) was added to the Au@P-PEG/EG nanoparticle dispersion in deionized water (1.0 mL, 76 µmol with respect to the gold precursor). The mixture was sonicated for 60 min and heated at 70


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ºC for 2 d in order to ensure complete hydrolysis of the EG groups on the gold surface. Then, the product was thoroughly washed with deionized water, and was finally dispersed in deionized water (15 mL). Nonselective and selective adsorption test. Au@S(CH2)3OH nanoparticles were prepared using a synthetic procedure identical to that used for the Au@S-PEG-OCH3 nanoparticles. A HS(CH2)3OH solution (0.020 mmol) was added to the gold nanoparticle dispersion (30 mL, 7.6 mmol with respect to the gold precursor concentration) at pH ~10. The mixture was sonicated for 1 h and the excess ligands were removed by centrifugation. This procedure was repeated three times to ensure effective exchange of the thiols, and the final product was dispersed in deionized water (15 mL). For the adsorption test, a BSA (0.10 mL, 2.6 mg) or a ChT (0.10 mL, 1.0 mg) aqueous solution was added to the particle dispersion in the PBS solution (2.0 mL, 0.51 µmol with respect to the gold precursor concentration), and the UV-VIS extinction was measured 10 min after the addition. Identification of proteins by SDS-PAGE analysis. After the adsorption test using UV-VIS extinctions, the unbounded proteins were removed by centrifugation. The sediment containing protein-bound nanoparticles was dispersed in a PBS solution and the proteins were denatured and released using heat treatment with a 2% SDS solution for 5 min. Molecular weight markers, free BSA and ChT, and the sediment solutions were subjected to 15% SDS-PAGE at 160 V for 90 min according to the standard procedures. Characterization. Scanning electron microscopy (SEM) images were obtained using a Philips XL30S FEG operated at 10 kV. Dynamic light scattering (DLS) measurements were performed using a 90 Plus Particle Size Analyzer (Brookhaven Instruments Corporation) with the sample


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aqueous dispersions at room temperature. Fourier transform infrared (FTIR) spectroscopy data were collected on a BRUKER EQUINOX55 spectrometer. The samples were prepared using a few drops of the colloidal solutions on silicon wafers (P-100), followed by drying in air. X-ray photoelectron spectroscopy (XPS) studies were carried out using a VG ESCA2000 with a Mg Kα source. The UV-VIS extinction data were recorded on a Jasco V530 UV-VIS spectrophotometer using colloidal dispersions in water or in the 1×PBS solution. SDS-PAGE was performed using the bio rad mini protean tetra system at a constant voltage of 160 V for 90 min against pre-stained protein markers (Dokdo MarkTM). Gels were stained with coomassie brilliant blue R-250 solution.

RESULTS AND DISCUSSION

Gold nanoparticles were prepared via a simple polyol process. The gold precursor solution was added dropwise to a reaction mixture of PDMAEMA and a small amount of AgNO3 in a polyol solvent, followed by heating for 30 min (Figure 1a). PDMAEMA has functional moieties of carbonyl and tertiary amine,48 which are essential for the shape control of gold nanocrystals, as in PVP.49 After the formation of gold nanocrystals, the ester groups of PDMAEMA were hydrolyzed to form carboxylic acids, and thus the resulting gold nanoparticle surface was functionalized by carboxylic acids.48 Silver species generated from AgNO3 effectively restricted the growth of the Au{100} surface, and, over a certain concentration of AgNO3 in the reaction mixture, nearly spherical particles were obtained with an average particle size range of 50 – 80 nm.43,44


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The polyol solvent was selected from the EG family. Interestingly, the reactions in EG and PEG with high molecular weight (Mn = 600 gmol-1) yielded stable colloidal dispersions; otherwise, the products were found to suddenly precipitate in water, even after prolonged sonication (Figure 1b). Although the dispersion of the particles in EG was stable, the particles were not uniform and had wide size distributions of up to a few hundred nanometers. When PEG was used as a solvent, the resulting gold nanoparticles (Au@P-PEG) were fully characterized (Figure 2). Scanning electron microscopy (SEM) image and dynamic light scattering (DLS) data show that the particles were uniformly spherical with an average diameter of 29±3 nm and a hydrodynamic size of 35 nm. The large hydrodynamic size is due to the swelling of the polymeric units in water. High resolution transmission electron microscopy (HRTEM) images showed that the particles were polycrystalline (Figure 2c,d). The Fourier transform infrared (FTIR) spectrum of the Au@P-PEG nanoparticles had three distinctive peaks (Figure 3). The intense peak at 2874 cm-1, assignable to C-H stretching, was similar to that of free PEG at 2886 cm-1. The multiple signals at 2770 cm-1, corresponding to the C-H stretchings of the tertiary amine moiety in free PDMAEMA, completely disappeared in the Au@P-PEG spectrum. The peak at 1724 cm-1 matched the ester C=O stretching of the free PDMAEMA. The distinctive main peak at 1108 cm-1 and the characteristic patterns at 1550-850 cm-1 were close to the C-O stretching patterns of pure PEG. These FTIR data indicate that the dimethylaminoethyl groups in PDMAEMA were replaced by PEG units through an ester exchange reaction during the particle synthesis. The X-ray photoelectron spectroscopy (XPS) data also support the idea of the formation of PEG esters. The N(1s) peak of the free PDMAEMA disappeared and the C-O peak intensity at 286.6 eV largely increased compared to those of the C=O (288.2 eV) and C-C/C-H (285.1 eV)


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peaks after particle formation (Figures 4). Without the gold precursor, free PDMAEMA formed PEG esters via an ester exchange reaction in the presence of an excess of PEG under the present acidic conditions. The FTIR spectrum of the polymer showed three intense peaks and patterns nearly identical to those of the Au@P-PEG nanoparticles, confirming the ester formation with PEG (Figure 3d). The Au@P-PEG nanoparticles have multiple polymeric bonds of PEG groups on the gold surface; these nanoparticles are expected to be stable due to the chelate effect.38,39 Dithiotreitol (DTT) is a useful reagent that prevents cysteine oxidation in biomolecules;51 it replaces the ligands bound to the gold surface, leading to progressive aggregation.35,39,52 In order to address the stability issue, gold nanoparticles capped with PEG-OCH3-terminated monothiols (HS-PEGOCH3, Mw = 5000) were synthesized by citrate reduction of a gold precursor,50 followed by a place exchange reaction with the corresponding thiols.27,28 Under the extreme reducing environment of DTT and an excess of NaCl in the 1×PBS (phosphate buffered saline) solution at pH 7.4, the monothiol-protected gold nanoparticles within 60 min exhibited a large spectral change from a sharp peak at 530 nm to a broad signature at 735 nm; this change is a typical phenomenon induced by severe particle aggregation.32 Such aggregation could easily be checked by the color change from red to blue (Figure 5a). However, the Au@P-PEG nanoparticles showed a nearly negligible change at 533 nm, while the red color dispersion was maintained for more than several months (Figure 5b). This indicates that the Au@P-PEG nanoparticles ensure high stability in any biological environment. To allow selective binding with target molecules, an additional functionality, other than the PEG groups, should be introduced on the nanoparticle surface. The representative functionality used for this purpose is carboxylate (or carboxylic acid), which can form an amide group with


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various amines by means of 1-ethyl-3-(3-dimethylaminopopyl) carbodiimide HCL (EDC) coupling.31 We tried to generate carboxylate moieties by partial basic hydrolysis of the ester linkages of the Au@P-PEG nanoparticles; however, the particles were very stable, even after soaking them in the 1 M NaOH aqueous solution. There was no change in the FTIR spectrum of the Au@P-PEG nanoparticles before or after the NaOH treatment (Figure 6a). On the other hand, the Au@P-EG nanoparticles, which were synthesized with PDMAEMA in EG and formed a stable aqueous dispersion (Figure 1b), generated carboxylate groups by basic hydrolysis. After the NaOH treatment, the characteristic asymmetric and symmetric stretchings of the carboxylate groups emerged at 1560 and 1389 cm-1, respectively, with a decrement of the C=O stretching of the ester groups at 1705 cm-1 in the FTIR spectrum (Figure 6b). To assess the effect of such different reactivity characteristics between PEG and EG groups on basic hydrolysis, Au@P-PEG/EG nanoparticles were synthesized in the presence of PDMAEMA in a mixed solvent of PEG and EG (Figure 7a). After the basic hydrolysis, the particles were uniformly spherical, with an average diameter of 34±5 nm as measured by SEM, and had a hydrodynamic size of 39.8 nm according to DLS (Figure 7b,c). The surface functionalities were investigated by FTIR spectroscopy. The Au@P-PEG/EG nanoparticles showed several distinctive peaks, similar to those of the Au@P-PEG nanoparticles. After the hydrolysis step, the asymmetric and symmetric stretchings of the carboxylate groups appeared at 1571 and 1466 cm1

, respectively, although other peaks were nearly unchanged (Figure 8). This indicates the

formation of Au@P-PEG/COO-, in which the carboxylate (COO-) functionality is generated from the hydrolysis of the original EG groups. The presence of the carboxylate groups on the nanoparticle surface led to pH-dependent assembly/disassembly in aqueous solution.48,53 This process could be quantified by defining the


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aggregation factor (AF) as the ratio between the optical density at 650 nm and that at 529 nm, where the surface plasmon band maximum values of the aggregated and isolated gold nanoparticles appeared, respectively (Figure 9). The Au@P-PEG nanoparticles showed no AF change, with excellent colloidal stability over the wide pH range of 4-9. However, the Au@PPEG/COO- nanoparticles exhibited the typical behavior of the carboxylate moiety of particle aggregation below pKa (~6). The particles were stable above pH 6.4 due to interparticle repulsion of the negative surface charge of COO-. As the pH decreased, the surface charges were neutralized to induce particle aggregation, and the surface plasmon band shifted to lower energy. This pH-dependent assembly/disassembly process happened reversibly. Such pH-dependence of Au@P-PEG/COO- nanoparticles indicates that the carboxylate functional groups dominated the surface property, although the bulky PEG groups were coordinated together in the form of mixed surface layers. The resulting Au@P-PEG/COOnanoparticles could be employed as an ideal system for selective binding with a target molecule through carboxylates, without a non-selective binding of other molecules due to the PEG groups. It has been reported that carboxylate-monolayer protected gold clusters effectively led to the binding and inhibition of chymotrypsin (ChT) through electrostatic interaction.31-33 ChT has active sites surrounded by hydrophobic residue and, further out, by a ring of cationic residue, providing a binding surface for anionic scaffolds. On the other hand, bovine serum albumin (BSA) is a common protein that favorably binds to any gold surface.54,55 In the present study, the goal was selective binding with ChT, not non-selective binding with BSA using mixed functionalities on the gold surface. The protein binding was readily monitored by the surface plasmon band change of the gold nanoparticles, as the proteins have refractive indexes (> 1.4) higher than that of the buffer solution (1.334).56


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For control experiments, three gold nanoparticles with similar diameters (~30 nm) but with different functionalities were examined in 1×PBS solutions in the presence of the corresponding proteins. First, the Au@P-PEG nanoparticles showed an extinction maximum at 528 nm, which did not change at all in the presence of BSA or ChT (Figure 10). This finding reveals that the surface PEG groups effectively prevented the nonselective binding of any proteins. Second, the Au@S(CH2)3OH nanoparticles were synthesized by the reduction of the gold precursor with citrate ions and a subsequent place exchange reaction with 3-mercapto-1-propanol. The selected end functionality was –OH, due to its similarity to PEG. In the presence of BSA, the extinction maximum shifted from 524 nm to 527 nm due to the effective surface adsorption of the gold nanoparticles. The addition of ChT also changed the extinction to 528 nm. These results indicate that both proteins were non-selectively adsorbed on the Au@S(CH2)3OH nanoparticles (Figure 10b). For the Au@P-PEG/COO- nanoparticles, the maximum peak at 529 nm was not changed in the presence of BSA, whereas the addition of ChT induced a 4 nm redshift to 533 nm (Figure 10c). In this case, the PEG functionality on the particle surface effectively blocked the nonselective adsorption of BSA, but the negative surface charge of COO- provided effective interaction with positively-charged ChT, leading to a selective adsorption on the surface. Such a selective adsorption with a target molecule is critical for many biological applications. To show convincing evidence of the selective complexation of Au@P-PEG/COO- with ChT, we conducted sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Gel electrophoresis is typically utilized for protein separation according to molecular weight, but it is also useful for checking the bonding of gold nanoparticles with biomolecules.31 Gold nanoparticles with bound proteins were purified; the proteins were then denatured and released from the gold surface via a 5 min heat treatment with a 2% SDS solution. SDS causes the


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detachment of proteins from gold nanoparticles; thus, SDS-PAGE analysis consequently showed the presence of proteins anchored on the original gold surface. In Figure 11, lanes 2 and 3 show the molecular weights of the free BSA (~70 kDa) and ChT (26 kDa). Lanes 4-6 show the Au@PPEG nanoparticles without proteins, with BSA, and with ChT, respectively, all showing no protein adsorption on the surface. Lanes 7-9 are Au@S(CH2)3OH under the present conditions. The blue spots in lanes 8 and 9 indicate both proteins adsorbed on the nanoparticles. Lanes 10-12 are Au@P-PEG/COO- with a distinctive spot in lane 12, indicating the selective complexation of ChT with the gold nanoparticles. All results precisely matched the surface plasmon measurements.

CONCLUSIONS

We successfully demonstrated that surface-regulating polymers could strongly anchor the PEG functionality on a gold nanoparticle surface via a one-step polyol process. The Au@P-PEG nanoparticles were stable under extreme salt conditions, while the thiol-monolayer protected gold nanoparticles were easily coagulated. Using a solvent mixture of PEG and EG and a subsequent hydrolysis process, gold nanoparticles bearing mixed functionalities of PEG and carboxylate were generated and the resulting particles exhibited selective adsorption of positively charged ChT without the nonselective adsorption of BSA. These nanoparticles can be further functionalized on carboxylate groups by EDC coupling and can be used to introduce chemical probes that are exclusively bound to specific targets such as biotin-streptavidin pairs. The present nanoparticle system has many advantages, including simple one-step synthesis, high


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stability, biocompatibility, and excellent binding specificity. Therefore, it is expected to be utilized as a versatile platform for potential bio-related applications, such as separation, sensing, imaging, and assays.


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FIGURES

Figure 1. (a) Synthesis of PEGylated gold nanoparticles by one-step polyol process. (b) Aqueous dispersions of gold nanoparticles synthesized using polyol solvents including EG, di(ethylene glycol) (DEG), tetra(ethylene glycol) (TEG), and PEG with low (Mn = 300 gmol-1) and high (Mn = 600 gmol-1) molecular weights.


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Figure 2. (a) SEM image and (b) hydrodynamic size distribution of Au@P-PEG nanoparticles. (c) TEM and (d) HRTEM images of a single Au@P-PEG nanoparticle.


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Figure 3. FTIR spectra of (a) Au@P-PEG nanoparticles, (b) free PDMAEMA, (c) free PEG, and (d) the polymer (P-PEG) synthesized from the reaction of PDMAEMA in PEG.


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Figure 4. (a) XPS spectra of Au@P-PEG nanoparticles (red line) and free PDMAEMA (black line) from survey scans. The high resolution XPS spectra in the C(1s) region of (b) Au@P-PEG nanoparticles and (c) free PDMAEMA.


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Figure 5. Time-dependent absorption spectra (0–60 min) of (a) gold nanoparticles capped with monomeric thiols (HS-PEG-OCH3) and (b) the Au@P-PEG nanoparticles under extreme conditions. The insets show the typical color change of the colloidal dispersions.


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Figure 6. FTIR spectra of (a) Au@P-PEG and (b) Au@P-EG nanoparticles before (solid line) and after (dotted line) the NaOH treatment.


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Figure 7. (a) Synthesis of Au@P-PEG/COO- nanoparticles using mixture of PEG and EG and subsequent basic hydrolysis. (b) SEM image and (c) hydrodynamic size distribution of Au@PPEG/COO- nanoparticles.


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Figure 8. FTIR spectra of Au@P-PEG/EG and Au@P-PEG/COO- nanoparticles.


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Figure 9. Plots of the normalized aggregation factors (AF) extracted from pH-dependent extinction spectra of Au@P-PEG (black dot) and Au@P-PEG/COO- (white dot) nanoparticles. The inset shows the typical color change of the Au@P-PEG/COO- nanoparticle dispersion in the buffer solution.


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Figure 10. Schematic diagrams and surface plasmon band changes of (a) Au@P-PEG, (b) Au@S(CH2)3OH, and (c) Au@P-PEG/COO- nanoparticles, as the reference (black line), and in the presence of BSA (blue line) and ChT (red line) in the 1×PBS solutions.


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Figure 11. SDS-PAGE analysis of proteins bound to gold nanoparticles. Lane 1, marker; Lane 2, free BSA; Lane 3, free ChT; BSA (lanes 5, 8, and 11) and ChT (lanes 6, 9, and 12) detection test using Au@P-PEG (lanes 4-6), Au@S(CH2)3OH (lanes 7-9), and Au@P-PEG/COO- (lanes 1012) nanoparticles.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. *E-mail: [email protected].

ACKNOWLEDGMENT This work was supported by the Institute for Basic Science in Korea and the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (2012-005624, R112007-050-00000-0). ISC acknowledges the supports by the grant from KRIBB Research Initiative Program and the NRF grant funded by the Korea Government (MSIP) (2010-0021769).


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BRIEFS Functionalized gold nanoparticles bearing poly(ethylene glycol) units, synthesized via a one-step polyol process using poly(dimethyl aminoethyl methacrylate), exhibit extremely high stability and selective adsorption of chymotrypsin without nonselective bonding of other proteins.

SYNOPSIS_TOC


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Poly(ethylene glycol)- and Carboxylate-Functionalized Gold

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