Facile Fabrication of Branched Gold Nanoparticles by Reductive

Feb 3, 2011 - High yield synthesis of branched gold nanoparticles as excellent catalysts for the reduction of nitroarenes. Kalvakunta Paul Reddy , Kan...
0 downloads 12 Views 4MB Size
Download PDF
ARTICLE pubs.acs.org/Langmuir

Facile Fabrication of Branched Gold Nanoparticles by Reductive Hydroxyphenol Derivatives Yuhan Lee and Tae Gwan Park* Department of Biological Sciences and Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea

bS Supporting Information ABSTRACT: In nature, polyphenol is one of the most important chemicals in many reductive biological reactions widely found in plants and animals. In this study, we demonstrated that hydroxyphenol compounds and their derivatives could be used as versatile reducing agents for facile one-pot synthesis of gold nanoparticles with diverse morphological characters by reducing precursor Au(III) ions into a gold crystal structure via a biphasic kinetically controlled reduction process. We found that the biphasic reduction of hydroxyphenols generated single-crystalline branched gold nanoparticles having high-index facets on their surface. The kinetically controlled self-conversion of hydroxyphenols to quinones was mainly responsible for the generation of morphologically different branches on the gold nanoparticles. Different hydroxyphenol derivatives with additional functional groups on the aromatic ring could produce totally different nanostructures such as nanoprisms, polygonal nanoparticles, and nanofractals possibly by inhibiting the self-conversion or by inducing self-polymerization. In addition, polymeric hydroxyphenol derivatives generated stably polymer-coated spherical gold nanoparticles with controlled size, usefully applicable for biomedical applications.

’ INTRODUCTION Controlled synthesis of anisotropic nanomaterials with tunable size and shape is critically important for diverse applications such as optics, nanoelectronics, catalysis, biosensors, and therapeutics. Optoelectronic and physicochemical properties of various inorganic nanoparticles are strongly dependent on their size, shape, composition, crystallinity, and structure.1-4 During the past decade, gold nanoparticles having various morphologies such as platonic nanoparticles, nanocubes, nanorods, nanowires, nanobelts, and nanosheets have been synthesized.5-10 Especially, polyhedral and branched gold nanoparticles possessing high-index facets have been of great interest since they show enhanced signals in surface-enhanced Raman spectroscopy (SERS) and exhibit superior catalytic activities due to high surface-to-volume ratio.11,12 However, controlled crystal growth of gold nanoparticles with high-index planes is difficult because the stable formation of high-index planes having higher surface energy are thermodynamically unfavorable compared to the lowindex planes such as {111}, {100}, and {110}. Recently, several studies reported the synthesis of polyhedral gold nanoparticles bounded by high-index facets (e.g., {730}, {210}, and {221}) using electrochemical or solution-phase methods with specific capping of cetyltrimethylammonium chloride (CTAC).13,14 In addition, pentapod star-shaped gold nanoparticles bounded by {331} facets were fabricated using deep eutectic solvents (DES).15 In contrast, controlled synthesis of highly branched gold nanoparticles is still challenging because little is known about the exact mechanism of the generation of branch structures on the surface of gold nanoparticles. Herein, we demonstrate the r 2011 American Chemical Society

use of reductive hydroxyphenolic compounds and their polymeric derivatives for facile morphological control of various gold nanostructures including branched nanoparticles, highly branched nanostructures (nano-snowflakes), nanoprisms, and nanospheres. Hydroxyphenols, especially di- and trihydroxyphenols, are commonly observed structures in polyphenolic compounds widely found in animals (e.g., melanins and mussel-binding proteins (Mefps)) and plants (e.g., tannins and flavonoids). Recently, hydroxyphenols are vigorously studied for their unique reductive properties such as antioxidant effects and photoprotection from UV irradiation in diverse biological processes16 and also for exceptional binding affinities to various substrates including metals, metallic oxides, and organic surfaces.17-20 The key feature of hydroxyphenols is rapid oxidative selfconversion into their quinone forms by releasing protons and electrons under mild reductive conditions (Figure 1a). In contrast to the reduced form (hydroxyphenols), the oxidized quinones exhibit less reductive potential and significantly decreased binding affinity to metal surfaces.21 However, the quinones are highly reactive with amine and thiol compounds via Michael-type addition or Schiff base formation, resulting in further oxidative self-polymerization.22 Several previous studies have suggested that various biomolecules containing phenolic compounds could be utilized as reducing agents. Polyphenols (e.g., tannin), extracts Received: November 5, 2010 Revised: January 12, 2011 Published: February 03, 2011 2965

dx.doi.org/10.1021/la1044078 | Langmuir 2011, 27, 2965–2971

Langmuir

ARTICLE

Figure 1. Fabrication of branched gold nanoparticles using reductive hydroxyphenols. (a) Chemical scheme of oxidative self-conversion of benzene-1,2diol into catecholquinone and schematic illustration of the resultant branched gold nanoparticle structure. (b) Representative FE-SEM image of gold nanoparticles fabricated by benzene-1,2-diol. (c-h) TEM images of gold nanoparticles fabricated by (c) benzene-1,2,3-triol, (d) benzene-1,4-diol, (e) benzene-1,2,4-triol, (f) benzene-1,2-diol, (g) benzene-1,3,5-triol, and (h) benzene-1,3-diol. Scale bar: (b) 500, (c-f) 200, and (g, h) 50 nm.

from plants (e.g., soybean extract and lemongrass leaf extract), and enzymes have been used to fabricate gold nanoparticles with various shapes.23-28 However, shapes and morphologies of the resultant AuNPs were limited only to the spheres and nanoprisms partly due to the insufficient in-depth understanding on the chemistry of reductive hydroxyphenol moieties in the redox reactions. In this study, we hypothesized that various hydroxyphenol compounds and their derivatives could be used as versatile reducing agents for the formation of gold nanoparticles with diverse morphological characters by converting precursor Au(III) ions into a gold crystal structure via a biphasic kinetically controlled reduction process. The gold crystal seeds could be fabricated by hydroxyphenols in the primary phase of reduction, and subsequently various branches on the seed structure could be generated in a controlled manner in the second phase by in-situoxidized quinones and their polymers.

’ RESULTS AND DISCUSSION To fabricate branched gold nanoparticles, di- and trihydroxyphenols were first reacted with hydrogen tetrachloroaurate (HAuCl4, 1 mM in distilled and deionized water) into Au(s) in aqueous solution at room temperature. As shown in a representative scanning electron microscopy (SEM) image and transmission electron microscopy (TEM) images in Figure 1b-f (also in Figures S1 and S2), highly branched gold nanoparticles were readily fabricated by hydroxyphenols having ortho- and/or parahydroxyl groups: benzene-1,2,3-triol (c: all in ortho-position), benzene-1,4-diol (b and d: para-position), benzene-1,2,4-triol (e: ortho- and para-position), and benzene-1,2-diol (f: orthoposition). The product yields of the four types of branched nanoparticles were over 80%. Selected-area electron diffraction (SAED) patterns of the fabricated nanoparticles (bottom-right inset of each image) could be indexed to the [011] zone axis of a 2966

dx.doi.org/10.1021/la1044078 |Langmuir 2011, 27, 2965–2971

Langmuir single crystal of fcc gold, evidencing that the nanoparticles were composed of a single-crystal structure. The most possible mechanism of the nanoparticle branching is that the branches of gold nanoparticles fabricated using o- and p-hydroxyphenols were directly grown from the surface of the primary seed structures, rather than physically attached on the surface. On the other hand, hydroxyphenols having two meta-hydroxyl groups (benzene-1,3-diol) and three meta-hydroxyl groups (benzene-1,3,5-triol) showed remarkably different morphological features. In principle, m-hydroxyphenols cannot oxidize to quinone forms due to the lack of electron resonance in the aromatic ring. Unlike reductive reactions of other hydroxyphenols, both benzene-1,3-diol (Figure 1g) and benzene-1,3,5-triol (Figure 1h) generated non-single-crystalline gold nanoparticles in a very slow reduction process (∼10 min in our experimental setting). SAED patterns of both structures showed that the gold

Figure 2. HR-TEM analysis of branched gold nanoparticle surface. (a, b) TEM images of branched gold nanoparticles fabricated by benzene1,4-diol. (c, d) HR-TEM images of branched surface of (b). Scale bar: (a) 50, (b) 10, and (c, d) 2 nm.

ARTICLE

nanoparticles possessed multiple crystal structures, possibly suggesting that they were the aggregates of small seeds with different crystalline structures such as nanospheres and polyhedron structures. High-resolution TEM (HRTEM) image analysis revealed that the surface of highly branched gold nanoparticles was composed of a variety of high-index facets. The images taken from the [110] zone axis displayed a variety of facets in the single-crystal fcc structure (Figure 2). Figure 2c shows the representative projection of benzene-1,4-diol-reduced branched gold nanoparticles (Figure 2a,b) having (331) facet along the [110] direction. The corresponding d spacing values between the adjacent lattice planes were 0.94 Å, indicating the generation of {331} plane. It was also observed that the surface of the convex branch in a single-crystal fcc structure was bound by multiple high-index facets. As shown in Figure 2d, the two part of the branch surface was bounded by (221) and (552) facets (d spacing: 1.36 and 0.55 Å, respectively). Further HRTEM analysis of other surfaces also revealed that the hydroxyphenol-reduced gold nanoparticles possessed various high-index facets (Figure S3). The oxidative self-conversion of hydroxyphenols into quinone forms (pyrocatechol) was kinetically analyzed during the time course of gold nanoparticle formation (Figure 3). On the basis of the change of UV-vis absorbance spectra, the whole reaction could be divided into two kinetically separated stages. In the first stage of the reaction (∼6 s), a benzene-1,2-diol peak at 306 nm rapidly diminished, and two new peaks at 389 nm (quinone) and 560 nm (gold nanoparticle seed) were generated, indicating that the catechols released electrons to Au(III) ions and generated gold nanoparticle seeds. Since hydroxyphenols have high reduction potential (e.g., Eo-benzene-1,2-diol = -795 mV), they could give electrons to Au(III) ions to rapidly grow gold seeds in the first stage (EoAu(III) = þ1.52 V) and convert themselves into quinones. Considering that the plasmon resonance maximum value is highly dependent on the diameter of AuNPs,29,30 the relatively longer peak wavelength observed in the UV-vis spectra (560 nm for AuNP seeds and 520 nm for 20 nm AuNPs) might imply that this rapid reaction generated spherical AuNPs having a size of ∼100 nm. During the second stage of reaction (6-16 s), the catecholquinone peak intensity was steeply enhanced, and the surface plasmon resonance peak of AuNPs was also red-shifted to 674 nm, resulting from the production of branches on the seed structure. It is highly possible that the less reductive quinones might induce the growth of branches on

Figure 3. Kinetic analysis of gold nanoparticle fabrication process of benzene-1,2-diol using UV-vis spectroscopy: (a) UV-vis spectra observed every 1 s; (b) kinetic analysis of peak intensities of catechol (306 nm) and catecholquinone (398 nm); (c) kinetic analysis of the peak wavelengths of surface plasmon resonance of the resulting gold nanoparticles. 2967

dx.doi.org/10.1021/la1044078 |Langmuir 2011, 27, 2965–2971

Langmuir AuNP seeds in the second stage of reaction. As shown in the UV-vis spectra (Figure 3b,c), the evolution of catecholquinone peak (λmax = 389 nm) was strongly correlated with the AuNP SPR peak red shift, evidencing that the remnant Au(III) ions were reduced into Au(s) on the surface of AuNP seeds directly or indirectly by quinones. The other p- and o-hydroxyphenols showed similar behaviors during the reaction (Figure S3). In addition, it is also noteworthy to mention that m-hydroxyphenols (benzene-1,3,5-triol and benzene-1,3-diol) generated different types of AuNPs having wide size distribution and low reproducibility compared to other hydroxyphenols. Since the m-hydroxyphenols cannot form quinones due to the lack of electron resonance in the aromatic ring, they are less susceptible to oxidation than others, resulting in the lower extent of reaction with Au(III) with poor stability of the resultant AuNPs. The TEM results of AuNPs prepared by benzene-1,3-diol reaction (Figure 1h and Figure S1f) exhibited random coagulation of threadlike gold structures with polycrystalline characters, supporting the low stability of intermediate products during the reaction. Most branched nanoparticles showed well-dispersed characters and great stability in water for a week except for m-hydroxyphenols (benzene-1,3-diol and benzene-1,3,5-triol). However, the AuNPs showed low stability under high-salt conditions, suggesting that further surface modifications (e.g., poly(ethylene glycol) (PEG), phosphine, lipids) might be required for practical applications such as drug delivery, bioimaging, catalyst, or surface-enhanced Raman spectroscopy (SERS). Figure 4 shows that an additional functional group introduced in the structure of hydroxyphenols dramatically altered the morphology of nanoparticles. Dopamine (2-(3,4-dihydroxyphenyl)ethylamine) was examined as a model self-polymerizable molecule (Figure 4a). It is known that, under reductive conditions, a dopamine molecule is oxidized to a dopaminequinone form that is highly reactive with an adjacent amine group, resulting in oxidative polymerization to form a highly branched polydopamine structure.22 When a dopamine solution (10 μM) was dropped into Au(III) solution, the solution color rapidly turned pale yellow to ruby red within 2 s, suggesting the instantaneous formation of gold nanostructures. Interestingly, the TEM image analysis revealed that the fabricated gold nanostructures showed highly branched morphology with secondary branching on the primary branches, resembling the shape of snowflakes (nano-snowflakes) (Figure 4b,c). It is most likely that the secondary branches were due to the third mode of reduction exerted by self-polymerized products followed by the primary (seed generation) and secondary (primary branching) reduction. Although terminal primary amines on dopamine molecules could also reduce Au(III) ions, this amine-based reduction is kinetically unfavorable due to the high redox potential of catechol groups (638 mV).31 UV-vis spectrum analysis showed that a catechol peak at 280 nm was significantly decreased and a broad shoulder peak from 300 to 500 nm was generated, revealing that dopamine was oxidized into polymerized products during the reaction. It should be noted that this change of optical behavior is distinct from hydroxyphenols (Figure 3a) where they generated a sharp peak at the wavelengths of their quinone forms with no following shoulder peak. This suggests that, unlike unmodified hydroxyphenols, the polymerization reaction of amine-containing dopamine might be kinetically enhanced under similar oxidative conditions. In contrast to dopamine, 3,4-dihydroxybenzoic acid (DHBA) (Figure 4d) produced polyhedral gold nanoparticles and nanosheets at DHBA

ARTICLE

Figure 4. Effect of additional functional groups of catechol on the morphology of fabricated gold nanoparticles. (a) Chemical scheme of oxidative self-conversion of dopamine into dopaminequinone and further oxidative polymerization product. The cartoon below is the schematic illustration for the generation of gold nanoparticles according to the serial self-conversion and polymerization. (b) TEM images of highly branched gold nanoparticles using dopamine (scale bar: 0.5 μm). (c) Higher magnification images of a primary branch (scale bar: 0.1 μm). (d) Chemical scheme of oxidative self-conversion of DHBA into its quinone form that requires higher energy than catechol conversion. (e, f) TEM images of gold nanoparticles fabricated by (e) 32 μM DHBA (scale bar: 0.2 μm) and (f) 3.2 μM DHBA (scale bar: 1 μm).

concentrations of 32 and 3.2 μM, respectively, without showing any branch structures (Figure 4e,f). This suggests that very different gold nanostructures could be fabricated via a single mode reduction of DHBA under a kinetically controlled reduction process. Under oxidative stress, the carboxyl acid group in DHBA is first slowly oxidized into its deprotonated form,32 followed by subsequent oxidation of o-hydroxyphenol to orthoquinone. Thus, the p-carboxyl acid group could serve as an effective kinetic barrier for inhibiting the formation of branch structures in the second reduction phase, leading to the production of polyhedral and prismlike gold nanoparticles. This total 2968

dx.doi.org/10.1021/la1044078 |Langmuir 2011, 27, 2965–2971

Langmuir

ARTICLE

Figure 5. Templated gold nanoparticle fabrication using PEI-C micelles. (a) Schematic illustration of PEI-C micelles and templated fabrication of spherical gold nanoparticles. (b-d) TEM images of spherical gold nanoparticles fabricated by PEI-C having substitution degree of (b) 6.5%, (c) 15.4%, and (d) 28.4%. Scale bar: 50 nm. (e) Size distribution of gold nanoparticles fabricated using PEI-C having substitution degree of 6.5% (black bar), 15.4% (red bar), and 28.4% (blue bar). (f) DLS size distribution of PEI-C micelles for substitution degree of 6.5% (black bars), 15.4% (red bars), and 28.4% (blue bars).

control over gold nanostructures and resultant optical properties is highly desirable for practical applications (e.g., drug delivery and bioimaging) using polymeric conjugates of the reductive hydroxyphenols. To further demonstrate the gold fabrication reaction of polymer-hydroxyphenol conjugates, hydrocaffeic acid ((3,4dihydroxyphenyl)ethylpropionic acid) was conjugated to branched polyethylenimine (PEI) and used as a polymer template for the crystal growth of gold nanoparticles (Figure 5). PEI is an amine-rich and highly positively charged polymer that is widely used for the delivery of nucleic acids (e.g., plasmid DNAs

and siRNAs) into intracellular components. PEI grafted with catechol groups was self-assembled to produce spherical micelles (∼50 nm) in aqueous solution because moderately hydrophobic catechol groups were buried inside via hydrophobic and π-π stacking interactions to form an inner core while highly ionic PEI chain was surrounded around it as a shell layer. The catecholenriched inner core can serve as a highly reductive reservoir for the seed formation and growth of gold nanoparticles. In addition, further cross-linking reactions between oxidized quinones and amines in PEI could produce gold nanoparticles coated with PEI that was anchored on the gold surface via catechol-gold interactions. 2969

dx.doi.org/10.1021/la1044078 |Langmuir 2011, 27, 2965–2971

Langmuir Figure 5 shows that spherical gold nanoparticles with different sizes could be fabricated by controlling the substitution degree of dopamine onto the PEI backbone: 15.3 ( 8.7, 28.4 ( 6.4, and 50.6 ( 5.5 nm for substitution degree of 6.5%, 15.4%, and 28.4%, respectively (Figure 5b-e). The higher degree of catechol substitution generated the larger gold nanoparticles, probably because the micelle template size was larger due to the higher aggregation number. The dynamic light scattering (DLS) sizes of micelle templates were 13.3 ( 0.9, 30.8 ( 5.2, and 54.6 ( 6.7 nm for substitution degree of 6.5%, 15.4%, and 28.4%, respectively (Figure 5f). The gold nanoparticles that were fabricated using the catechol-grafted PEI as a spherical micelle template were spherical in contrast to other branched, snowflake, and nanosheet gold nanoparticles synthesized in the presence of hydroxyphenol derivatives in aqueous solution. This was due to the fact that the hydrophobic micelle interior could provide a more favorable microenvironment within a very confined volume for spherical growth of gold crystals in a more spatial and temporal manner. It is also notable that spherical gold nanoparticles have highly positive surface charge values ranging from þ4.9 to þ51.3 mV, suggesting that PEI was stably coated on the surface. This suggests the possibility that multifunctional gold nanoparticles coated with biocompatible polymers and cell-targeting ligands could be readily fabricated for further applications such as biosensing and drug delivery.

’ CONCLUSION In conclusion, we demonstrated the facile single-step fabrication of diverse gold nanoparticles using reductive hydroxyphenols. It was found that the rapid self-conversion of hydroxyphenols into their quinone forms was highly important for producing branched gold nanoparticles. It was also demonstrated that additional functional groups in polyhydroxyphenols significantly altered the morphology. When dopamine that undergoes oxidative polymerization was used, branched gold nanoparticles with secondary branches were fabricated. In contrast, polyhedral nanoparticles and nanosheets were fabricated using DHBA which has very slow rate of self-conversion into quinone. In addition, polymer-coated nanoparticles having various sizes ranging from 18 to 54 nm were synthesized using polymercatechol conjugates. Hydroxyphenols, their derivatives, and polymer conjugates could be used as versatile reducing agents to prepare a wide variety of gold nanostructures. ’ EXPERIMENTAL SECTION Materials. Hydrogen tetrachloroaurate, benzene-1,2,3-triol, benzene-1,2,4-triol, benzene-1,3,5-triol, benzene-1,2-diol, benzene-1,3-diol, benzene-1,4-diol, dopamine hydrochloride, 3,4-dihydroxybenzoic acid, PEI (branched, Mw 25 kDa), and hydrocaffeic acid were purchased from Sigma-Aldrich (St. Louis, MO). 1-Ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC) was purchased from TCI (Tokyo, Japan). Synthesis of Gold Nanoparticles. For the synthesis of gold nanoparticles, 0.2 mL of 1 mg/mL trihydroxyphenol (benzene-1,2, 3-triol, benzene-1,2,4-triol, and benzene-1,3,5-triol) dissolved in distilled and deionized water (DDW) was added into 2 mL of 1 mM hydrogen tetrachloroaurate solution at room temperature. For the synthesis of gold nanoparticles using dihydroxyphenols (benzene-1,2-diol, resorcinol, and benzene-1,4-diol), dopamine, and DHBA, the reaction was done in the same manner under boiling condition. For the synthesis of gold nanoparticles using PEI-C, 0.2 mL of 5 mg/mL PEI-C dissolved in

ARTICLE

DDW was added into 2 mL of 1 mM hydrogen tetrachloroaurate solution under boiling condition.

Synthesis of Polyethylenimine-g-Hydrocaffeic Acid (PEIC). Catechol moieties were conjugated onto PEI using EDC coupling chemistry. Briefly, a predetermined amount of hydrocaffeic acid (1.6, 0.8, and 0.3 g for 28.4%, 15.4%, and 6.5% substituted PEI-C, respectively) dissolved in pH 5.0 PBS solution (10 mL) was added into 200 mL of PEI (MW 25 kDa, 3 g) solution containing EDC (2.71 g, TCI, Japan). The reaction was carried out for 4 h, and the pH was adjusted to 5.0 during the reaction. The product was purified using extensive dialysis in pH 5.0 HCl solution, dialyzed again in DDW, and lyophilized. The degree of catechol substitution was determined using a UV-vis spectrophotometer (UV-1601, Shimadzu, Japan). The size of self-assembled micelles was determined using dynamic light scattering (90Plus particle size analyzer, Brookhaven Instruments). Measurements. The size, shape, and crystal structure of the fabricated gold nanoparticles were visualized using FE-SEM (S-4800, Hitachi, Japan), TEM (CM20, Phillips, Netherlands), and HR-TEM (Tecnai F20, Phillips, Netherlands). Kinetics analysis of gold nanoparticle fabrication was done using UV-vis spectroscopy (8453 diode array spectrophotometer, Agilent) by obtaining each spectrum every 0.5 s.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional HRTEM images of gold nanoparticles fabricated by hydroxyphenols and additional UV-vis spectra showing kinetics of gold nanoparticle synthesis by hydroxyphenols. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel þ82-42-350-2621; Fax þ82-42-350-2610; e-mail tgpark@ kaist.ac.kr.

’ ACKNOWLEDGMENT Authors thank Haeshin Lee (Department of Chemistry, KAIST) for thoughtful discussions on hydroxyphenols. This work was supported by Basic Science Research Program (2010-0027955), National Research Laboratory, and World Class University program from the Ministry of Education, Science and Technology, Republic of Korea. ’ REFERENCES (1) Alivisatos, A. P. Perspectives on the physical chemistry of semiconductor nanocrystals. J. Phys. Chem. 1996, 100, 13226–13239. (2) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Photoinduced conversion of silver nanospheres to nanoprisms. Science 2001, 294, 1901–1903. (3) Moreno-Manas, M.; Pleixats, R. Formation of carbon-carbon bonds under catalysis by transition-metal nanoparticles. Acc. Chem. Res. 2003, 36, 638–643. (4) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. A DNA-based method for rationally assembling nanoparticles into macroscopic materials. Nature 1996, 382, 607–609. (5) Sun, Y.; Xia, Y. Shape-controlled synthesis of gold and silver nanoparticles. Science 2002, 298, 2176–2179. (6) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Controlled growth of tetrapod-branched inorganic nanocrystals. Nature Mater. 2003, 2, 382–385. (7) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Synthesis and optical properties of “branched” gold nanocrystals. Nano Lett. 2004, 4, 327–330. 2970

dx.doi.org/10.1021/la1044078 |Langmuir 2011, 27, 2965–2971

Langmuir (8) Seo, D.; Yoo, C. I.; Park, J. C.; Park, S. M.; Ryu, S.; Song, H. Directed surface overgrowth and morphology control of polyhedral gold nanocrystals. Angew. Chem., Int. Ed. 2008, 47, 763–767. (9) Hutchinson, T. O.; Liu, Y.-P.; Kiely, C.; Kiely, C. J.; Brust, M. Templated gold nanowire self-assembly on carbon substrates. Adv. Mater. 2001, 23, 1800–1803. (10) Sun, Y.; Mayers, B.; Xia, Y. Transformation of silver nanospheres into nanobelts and triangular nanoplates through a thermal process. Nano Lett. 2003, 3, 675–679. (11) Treguer-Delapierre, M.; Majimel, J.; Mornet, S.; Duguet, E.; Ravaine, S. Synthesis of non-spherical gold nanoparticles. Gold Bull. 2008, 42, 195–207. (12) Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Shape control in gold nanoparticle synthesis. Chem. Soc. Rev. 2008, 37, 1783–1791. (13) Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity. Science 2007, 316, 732–735. (14) Ma, Y.; Kuang, Q.; Jiang, Z.; Xie, Z.; Huang, R.; Zheng, L. Synthesis of trisoctahedral gold nanocrystals with exposed high-index facets by a facile chemical method. Angew. Chem., Int. Ed. 2008, 47, 8901–8904. (15) Liao, H.-G.; Jiang, Y.-X.; Zhou, Z.-Y.; Chem, S.-P.; Sun, S.-G. Shape-controlled synthesis of gold nanoparticles in deep eutectic solvents for studies of structure-functionality relationships in electrocatalysis. Angew. Chem., Int. Ed. 2008, 47, 9100–9103. (16) Seagle, B.-L. L.; Rezei, K. A.; Kobori, Y.; Gasyna, E. M.; Rezaei, K. A.; Norris, J. R., Jr. Melanin photoprotection in the human retinal pigment epithelium and its correlation with light-induced cell apoptosis. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8978–8983. (17) Lee, H.; Lee, B. P.; Messersmith, P. B. A reversible wet/dry adhesive inspired by mussels and geckos. Nature 2007, 448, 338–342. (18) Lin, Q.; Gourdon, D.; Sun, C.; Holten-Anderson, N.; Anderson, T. H.; Waite, J. H.; Israelachvili, J. N. Adhesion mechanisms of the mussel foot proteins mfp-1 and mfp-3. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 3782–3786. (19) Lee, H.; Lee, Y.; Statz, A. R.; Rho, J.; Park, T. G.; Messersmith, P. B. Substrate-independent layer-by-layer assembly by using musseladhesive-inspired polymers. Adv. Mater. 2008, 20, 1619–1623. (20) Lee, Y.; Lee, H.; Kim, Y. B.; Kim, J.; Hyeon, T.; Park, H. W.; Messersmith, P. B.; Park, T. G. Bioinspired surface immobilization of hyaluronic acid on monodisperse magnetite nanocrystals for targeted cancer imaging. Adv. Mater. 2008, 20, 4154–4157. (21) Lee, H.; Scherer, N. F.; Messersmith, P. B. Single-molecule mechanics of mussel adhesion. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 12999–13003. (22) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-inspired surface chemistry for multifunctional coatings. Science 2007, 318, 426–430. (23) Nadagouda, M. N.; Varma, R. S. Green synthesis of silver and palladium nanoparticles at room temperature using coffee and tea extract. Green Chem. 2008, 10, 859–862. (24) Nune, S. K.; Chanda, N.; Shukla, R.; Katti, K.; Kulkarni, R. R.; Thilakavathy, S.; Mekapothula, S.; Kannan, R.; Katti, K. V. Green nanotechnology from tea: phytochemicals in tea as building blocks for production of biocompatible gold nanoparticles. J. Mater. Chem. 2009, 19, 2912–2920. (25) Shukla, R.; Nune, S. K.; Chanda, N.; Katti, K.; Mekapothula, S.; Kulkarni, R. R.; Welshons, W. V.; Kannan, R.; Katti, K. V. Soybeans as a phytochemical reservoir for the production and stabilization of biocompatible gold nanoparticles. Small 2008, 4, 1425–1436. (26) Ostwalt, W. An Introduction to Theoretical and Applied Colloid Chemistry; John Wiley & Sons Inc.: New York, 1917; p 23. (27) Willner, I.; Baron, R.; Willner, B. Growing metal nanoparticles by enzymes. Adv. Mater. 2006, 18, 1109–1120. (28) Shankar, S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Biological synthesis of triangular gold nanoprisms. Nature Mater. 2004, 3, 482–488.

ARTICLE

(29) Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. Calculated absorption and scattering properties of gold nanoparticles of different size, shape, and composition: applications in biological imaging and biomedicine. J. Phys. Chem. B 2006, 110, 7238–7248. (30) Link, S.; El-Sayed, M. A. Size and Temperature Dependence of the Plasmon Absorption of Colloidal Gold Nanoparticles. J. Phys. Chem. B 1999, 103, 4212–4217. (31) Hamamoto, K.; Kawakita, H.; Ohto, K.; Inoue, K. Polymerization of phenol derivatives by the reduction of gold ions to gold metal. React. Funct. Polym. 2009, 69, 694–697. (32) Bodini, M.; Osorio, C.; del Valle, M. A.; Arancibia, V.; Munoz, G. Redox chemistry of 3,4-dihydroxy-2-benzoic acid, its oxidation products and their interaction with manganese(II) and manganese(III). Polyhedron 1995, 14, 2933–2936.

2971

dx.doi.org/10.1021/la1044078 |Langmuir 2011, 27, 2965–2971