Catalytic Reduction of 4-Nitrophenol using Biogenic Gold and Silver

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Catalytic Reduction of 4-Nitrophenol using Biogenic Gold and Silver Nanoparticles Derived from Breynia rhamnoides Abilash Gangula,† Ramakrishna Podila,‡ Ramakrishna M,† Lohith Karanam,† Chelli Janardhana,† and Apparao M. Rao*,‡,§ †

Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prashanthinilayam, India, 515134 Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634, United States § Center for Optical and Materials Science & Engineering Technologies, Clemson, South Carolina 29634, United States ‡

bS Supporting Information ABSTRACT:

A simple, green method is described for the synthesis of Gold (Au) and Silver (Ag) nanoparticles (NPs) from the stem extract of Breynia rhamnoides. Unlike other biological methods for NP synthesis, the uniqueness of our method lies in its fast synthesis rates (∼7 min for AuNPs) and the ability to tune the nanoparticle size (and subsequently their catalytic activity) via the extract concentration used in the experiment. The phenolic glycosides and reducing sugars present in the extract are largely responsible for the rapid reduction rates of Au3+ ions to AuNPs. Efficient reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of AuNPs (or AgNPs) and NaBH4 was observed and was found to depend upon the nanoparticle size or the stem extract concentration used for synthesis.

1. INTRODUCTION Noble metal nanoparticles (NPs) have shown remarkable potential for numerous applications in electronic, chemical, biological, and medical fields due to their distinctive properties, when compared to their bulk counterparts.1 3 Several physical properties of metal NPs (for example, surface plasmon frequency of Au and Ag NPs) can be tailored for a specific application by controlling their size, shape, and morphology.4 7 Wet-chemical synthesis technique has been widely used for producing metal NPs due to its simplicity, high growth rate, and throughput. However, extensive use of hazardous organic reagents in the wetsynthesis technique has raised concerns regarding the impact of chemically grown metal NPs on living organisms and the environment. Consequently, there has been a growing need to replace the chemical synthetic procedures with clean, nontoxic, and environmentally acceptable “green chemistry” methods. An environmentally benign solvent and eco-friendly reducing and capping agents are the three vital elements for a completely green synthesis technique. Accordingly, many researchers have turned toward biological systems such as microorganisms and plants to draw inspiration for green technologies.8 15 The use of plant r 2011 American Chemical Society

biomass and extracts for large-scale production of NPs has been gaining popularity over the use of microorganisms due to several factors such as (i) simple handling procedures, (ii) ready scalability, and (iii) preclusion of cell culture maintenance. However, longer reaction time has been a major drawback for many biosynthetic procedures compared to the chemical methods used for nanoparticle synthesis. In the current study, we synthesized AuNPs and AgNPs on time scales (∼7 min) that are faster than most or comparable to some of the existing chemical methods using stem extract of Breynia rhamnoides (a member of Euphorbiaceae family). In order to comprehend such fast reaction rates, the surface plasmon resonance of Au and Ag NPs was continuously monitored in situ using UV visible spectroscopy. Phytochemical screening of the stem extract has been done to comprehend the active compounds responsible for fast synthesis rates. Furthermore, we show that these biogenic Au and Ag NPs exhibit equivalent Received: September 15, 2011 Revised: October 24, 2011 Published: October 25, 2011 15268

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catalytic properties compared to commercially available (chemically grown) NPs. Lastly, we explored the dependence of the catalytic rate on parameters such as (i) the plant extract concentration used in NP synthesis, (ii) NP concentration, and (iii) the nature of metal (AuNPs vs AgNPs) in the catalytic conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP).

2. EXPERIMENTAL METHODS Materials. Silver nitrate (AgNO3) and chloroauric acid (HAuCl4) were obtained from SRL Chemicals, India, and used as received. All solutions were prepared in double-distilled water and all apparatus were rinsed with aqua regia (3:1 solution of HCl:HNO3) and then washed with double-distilled water before use. All reagents and solvents used in this study were of guaranteed reagent grade. Preparation of the Extract. The stems of Breynia rhamnoides were collected from the wild, cut into small pieces, thoroughly washed, shade-dried for 2 3 days, and machine-ground into fine powder. About 15 g of this stem material was soaked in 100 mL of double-distilled water and refluxed for 30 min. The extract was then filtered hot for further use in experimentation. Before adding to the metal salt solution, the extract was centrifuged at 4500 rpm for 10 15 min to separate any plant debris. When not in use, the extract was stored at 4 °C. Synthesis of AuNPs Using the Extract. Varying amounts (0.5 3.5 mL or 5 35% vol fraction) of the stem extract of Breynia rhamnoides were added separately to 6 mL solutions of 1 mM aqueous HAuCl4 under dark conditions. The final volume of salt and plantextract solution was adjusted to 10 mL by adding appropriate amount of double-distilled water. Synthesis of AgNPs Using the Extract. Five milliliters of stem extract was added to the solution containing 5 mL of 10 mM AgNO3 and 2.5 mL of 30% ammonia under dark conditions, and the final volume was increased to 50 mL by adding appropriate amount of double-distilled water so that the final concentration of AgNO3 is 1 mM. Characterization Methods. All UV visible spectra were recorded on a Hitachi U-2001 spectrophotometer with 1 cm quartz cells. For transmission electron microscopy (TEM) studies, the aqueous nanoparticle suspension was sonicated for ∼30 s and a drop of sonicated sample was placed on carbon-coated Cu grids (200 mesh) and allowed to dry overnight. TEM measurements were carried out on a Hitachi H-60 instrument operated at an accelerating voltage of 75 kV. Fourier transform infrared (FTIR) spectra were collected with a Shimadzu spectrophotometer in the transmittance mode in the range 400 4000 cm 1. The aqueous suspension of nanoparticles was centrifuged at 10 000 rpm for 15 min, and the pellet was redispersed in doubledistilled water to eliminate any unadsorbed biomolecules of the extract. The process of centrifugation and redispersion was repeated twice in order to ensure better separation of uncoordinated entities from the surface of metal nanoparticles. The purified pellet was then freeze dried to obtain dry powder. Subsequently, ∼5 mg of dry powder was mixed with ∼100 mg of KBr to form pellets for FTIR studies.

3. RESULTS AND DISCUSSION Time-Dependent UV visible Spectroscopy of AuNPs. Figure 1 shows the UV visible absorption spectrum of 5% vol fraction of the extract and 1 mM Au salt. Clearly, a distinct surface plasmon resonance (SPR) peak at ∼528 nm can be seen ∼30 s (black curve) after the addition of stem extract to Au salt indicating instantaneous reduction of HAuCl4 to form AuNPs. The kinetics of this reduction was followed by monitoring the intensity of SPR peak as a function of time (Figure 1a). The SPR peak appears immediately after the addition of the plant extract

Figure 1. (a) Time evolution of UV visible spectra during the formation of AuNPs from 0.5 mL of extract (5% vol fraction) and (b) the corresponding plot of absorbance at λmax showing 90% completion of reaction within 7 min. (c) UV visible spectra of AuNPs from 0.5 mL of extract (5% vol fraction) showing no significant difference even after 5 months.

and increases rapidly in intensity as a function of time with no significant peak shift (Figure 1b). This behavior is indicative of the rapid bioreduction of Au3+ ions and thus a swift increase in the number of AuNPs. We observe that the increase in SPR intensity saturates within 7 min. Any incubation time beyond 7 min shows almost no further increase in SPR intensity suggesting the completion of the reaction within 7 min (Figure 1b). As mentioned earlier, longer reaction time is a major drawback for several biosynthetic procedures. The most widely applied chemical procedures to obtain Au hydrosols are variations of the classic Turkevich Frens16 citrate reduction route and the twophase Brust method.17 In these methods, the reaction time ranges from ten to several minutes and involves continuous heating and stirring. In the present investigation, we see an instantaneous macroscopic change and saturation in the color from faint yellow (HAuCl4) to maroon red (AuNPs). Thus, this clean and nontoxic approach for formulating NPs not only is faster than other chemical methods, but also proceeds without the need for external heating/stirring. The sharp drop in the reaction time from several hours8,13,14 to a few minutes for biosynthetic methods is a promising result and can be potentially useful for various commercial applications. The SPR peak did not change in position or intensity even after 5 months after AuNP preparation (see Figure 1c), which suggests that the presence of capping agents in the stem extract prevents any possible agglomeration of as-prepared NPs. Effects of Stem Concentration on the AuNP Size. We mixed different extract concentrations (ranging 5 35% vol fraction) to 1 mM Au salt to study the effects of stem extract concentration on the biosynthesis of AuNPs. Figure 2 shows that the SPR peak for AuNPs concomitantly undergoes a red shift with increase in the concentration of extract, suggesting a possible increase in the particle size.18 The dynamic light scattering (DLS) and TEM 15269

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Figure 2. (a) Absorption spectra of AuNP systems synthesized from different extract concentrations (5 35% vol fraction). (b) The corresponding plot of λmax values against volume of extract showing red shift with increase in the concentration of extract.

Figure 3. (a) TEM image and (b) dynamic light scattering spectrum for Au NPs obtained with 5% vol fraction. (c) Histogram for 5% vol fraction AuNP size distribution obtained from the TEM images. (d) Trend of increasing particle size as a function of stem extract concentration obtained from the DLS spectra.

studies clearly show an increase in the particle size with increasing stem extract concentration (SI Figures S1 and S2). Such an observation is expected since the presence of more reducing moieties (at higher extract concentrations) results in an additional interaction between the surface capping molecules and the secondary reduction process on the surface of preformed nuclei giving rise to bigger particles.

Figure 3 shows typical TEM image of AuNPs biosynthesized using 5% vol fraction of stem extract with its corresponding size distribution histogram (Figure 3c). To determine the NP size distribution, more than 150 particles were analyzed using ImageJ software,19 and the resultant data were plotted in histograms. As can be seen from Figure 3a, the particles are predominantly spherical in shape with very few particles having anisotropic 15270

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Figure 5. FTIR spectra of stem extract, AuNPs, and AgNPs.

Figure 4. (a) Time evolution of UV visible spectra during the formation of AgNPs from 0.5 mL of extract (5% vol fraction) and (b) the corresponding plot of absorbance at λmax showing much longer time of 120 min for AgNPs for 90% completion of the reaction. (c) TEM image of AgNPs (scale bar = 100 nm) and (d) the corresponding particle size distribution.

structures of irregular shape. The DLS spectra for 5% vol fraction of stem extract concurs with the TEM particle size of ∼25 nm (Figure 3b). Furthermore, hydrodynamic size (Figure 3d) obtained from DLS spectra of AuNPs also shows a consistent increase in NP size at higher extract concentrations. Synthesis of AgNPs. The reducing and capping moieties in the stem extract can also be employed to reduce other metal salts such as Ag. In order to prove the presence of strong reducing moieties in the stem extract, we synthesized Ag NPs by adding AgNO3. The reduction of AgNPs is expected to be more difficult and slower than Au NPs due to lower reduction potential (Ag+/Ag0 = 0.80 V and Au3+/Au0 = 1.50 V versus SCE). Figure 4 shows the SPR peak of Ag NPs at ∼428 nm as a function of time. Clearly, the number of AgNPs in the system gradually increases with reaction time and reaches a maximum after 2 h (Figure 4b) indicating completion of the reaction. Here, ammonia acts as an accelerating agent and catalyzes the reduction process.13 TEM analysis show that the AgNPs are relatively larger and more polydispersed with an average particle size of 64 nm (Figure 4c and d). In summary, the formation of Ag NPs at 5% vol fraction of stem extract concentration suggests that the strong reducing and capping agents present in Breynia rhamnoides can be universally used with any metal salt for producing NPs. FTIR Studies. As mentioned in the discussion of Figure 1c, the agglomeration of AuNPs is prevented due to the capping agents present in the stem extract. The IR spectra of purified Au and Ag NPs very closely resemble the dried mass spectrum (Figure 5) confirming that the biomolecules present in the extract are responsible for capping and efficient stabilization of nanoparticles. The FTIR spectrum of the dried stem extract (before nanoparticle synthesis) shows no peaks in the amide I and amide II regions (1660 cm 1 and 1535 cm 1, respectively). The absence of amide bands in the initial extract implies the minimal role of proteins in the reduction of Au3+ and Ag+ ions unlike the

other green synthesis methods utilizing plants or microorganisms.8,10,15 It is possible that prolonged boiling during the extract preparation would have caused denaturation of proteins/ enzymes present in the plant. Apart from the usual sp3 stretching mode at 2900 cm 1 and the peak at around 750 cm 1 due to aromatic protons, several other peaks observed in the IR spectra can be assigned to various functional groups as shown in Table S1 (see Supporting Information). Many of the observed peaks in the IR spectra of Au and Ag NPs (Table S1) are closely associated with flavonones and terpenoids (present in the plant extract) suggesting the adsorption of these molecules on the surface of the nanoparticles. In the presence of low amounts of other strong ligating agents, flavonones or terpenoids could be adsorbed on the surface (due to high surface to volume ratio of NPs), possibly by interaction through carbonyl groups or π-electrons.14 The presence of several functional groups can influence the reduction of metal ions into corresponding metal nanoparticles. For example, oxidation of the ketone group in flavonoids to carboxylic acid may effect the metal ion reduction. Similarly, the conversion of aldehyde groups of terpenoid molecules to carboxylic acids can also play a role in reduction of metal ions. Phytochemical Screening. We performed a series of phytochemical tests20 22 (see Supporting Information) in order to confirm the presence of various secondary metabolites (such as flavonoids, saponins, etc.) in the stem of Breynia rhamnoides. The phytochemical constituents of the stem of Breynia rhamnoides are summarized in Table S2 (Supporting Information). As shown in Scheme S1 (Supporting Information), the aqueous extract of the plant (mother extract) was fractionated with ether, ethyl acetate (EtoAc), and n-butanol (n-BuOH), respectively. Fractionation results in the distribution of various secondary metabolites as per their polarity. Further, the aqueous layer obtained after every fractionation (see Scheme S1) was employed for the synthesis of nanoparticles. The SPR peaks of AuNPs widened and red-shifted for all the aqueous layers in comparison to the mother extract (Figure 6) indicating the broadening of the size distribution of the nanoparticles. This can be attributed to the decrease in the concentration of stabilizing agents in the aqueous layers due to their extraction into organic layers. Each of the fractions listed in Scheme S1 was also tested for the presence of reducing compounds like phenolics (comprising polyphenols, flavonoids, etc.) and glycosides/reducing sugars (see Supporting Information for further details). The reducing moieties present in the extracts are outlined in Table S3 (see Supporting Information). The dried mass obtained from each of the organic extracts is redispersed in water and tested for its ability to reduce Au3+ ions. The ether and n-BuOH extracts were able to reduce Au3+ ions to give nanoparticles unlike the EtOAc extract concurring with the results 15271

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Figure 6. Comparative UV vis spectra of AuNPs prepared from mother extract and from different aqueous layers (obtained after fractionation with organic solvents).

shown in Table S3. We found that the ether and n-BuOH extracts contain phenolic compounds, whereas the corresponding aqueous layer showed the presence of phenolics and glycosides/ reducing sugars. Thus, our phytochemical analysis confirms that the mother extract is composed of compounds belonging to polyphenols, phenolic glycosides, and reducing sugars. These biomolecules enable the stem extract of Breynia rhamnoides to synthesize stable nanoparticles at rapid rates. Our results agree with the previous phytochemical studies on the aerial parts of Breynia rhamnoides leading to the identification of sulfur-containing spiroketal glycosides, terpenic, and phenolic glycosides.23 Catalytic Study. To test the efficacy of AuNPs and AgNPs, for their catalytic activity, reduction of 4-NP was monitored as follows. About 1.4 mL of water, 0.3 mL of 2 mM solution of 4-NP, and 1 mL of 0.03 M NaBH4 solution were mixed in a 3 mL standard quartz cuvette (path length 1 cm). To this reaction mixture, 0.3 mL of biogenic nanoparticles (AuNPs and AgNPs) solution was added. Though the reduction of 4-NP to 4-AP using aqueous NaBH4 is thermodynamically favorable (E0 for 4-NP/ 4-AP = 0.76 V and H3BO3/BH4 = 1.33 V versus NHE), the presence of the kinetic barrier due to large potential difference between donor and acceptor molecules decreases the feasibility of this reaction. It is well-known that the metal NPs catalyze this reaction by facilitating electron relay from the donor BH4 to acceptor 4-NP to overcome the kinetic barrier. The conversion from 4-NP to 4-AP occurs via an intermediate 4-nitrophenolate ion formation. The 4-NP shows an absorbance peak at ∼317 nm, which red shifts to 400 nm in the presence of NaBH4 due to the formation of 4-nitrophenolate ion in the alkaline medium caused by NaBH4.24 Thus, the progress of the reaction can be monitored by tracking the changes in the absorption spectra of 4-nitrophenolate ion at 400 nm. Figure 7a shows successive absorption spectra of 4-nitrophenolate ion at 400 nm. On addition of nanoparticles, there is a rapid decrease in the intensity of the absorption peak at 400 nm while there is a concomitant appearance of a new peak at 298 nm indicating the formation of reduction product, 4-AP.24 The formation of 4-AP was also confirmed from 1H NMR spectrum (in DMSO) of the product after its preparation in the large scale (see Supporting Information) using similar reaction conditions as in our catalysis using AuNPs. The NMR peaks (Figure S4) match with the peaks established for 4-AP in the NMR database.25 The appearance of signals at δ 6.45 and δ 4.03, in the spectrum, are due to the presence of aromatic protons and amino protons, respectively. The two peaks at δ 2.5 and δ 3.3 are DMSO and water peaks and can be ignored. The mass spectrum (Figure S5) also shows an

Figure 7. (a) Time-dependent UV visible spectra for the catalytic reduction of 4-NP by NaBH4 in the presence of AuNPs obtained from 0.5 mL of extract. (b) Plots of ln(At/A0) vs time for the reduction of 4-NP by NaBH4 in the presence of AuNPs obtained from different amounts of extract. (c) Comparative plots of ln(At/A0) for AgNPs and AuNPs toward the reduction of 4-NP. (d) Plots of ln(At/A0) for different concentrations of AuNPs toward the reduction of 4-NP.

Table 1. Rate Constants of AuNP Systems Prepared from Different Amounts of Extract for 4-NP Reduction (cf. Figure 7b) vol of extract

SPR peak of

sample

used (mL)

AuNPs (nm)

calculated rate

1

0.5

528

9.19

2

1.0

536

5.72

3

2.5

542

4.65

constant (10

3

s 1)

M-1 peak corresponding to 4-AP at 108 thus confirming the formation of 4-AP. Finally, the conversion of 4-NP to 4-AP is also confirmed by the UV visible spectrum of 4-AP (extracted from the reaction mixture), which shows similar absorption peaks as that of an authentic sample (Figure S3). The data in Figure 7a imply that the reaction terminates within a time frame of 10 20 min in the presence of nanoparticles, consistent with the disappearance of the yellow color of 4-NP at the end of the reaction. Control of the above reaction was performed by taking water instead of nanoparticle solution and the peak at 400 nm remained unaltered even after 5 days, thus confirming the catalytic role of nanoparticles for the above reduction reaction. It is important to note that the absorbance measurements of 4-NP are not affected by NPs due to relatively low concentration of NPs in the system. Also, the concentration of borohydride greatly exceeds 4-NP (or catalyst particles) concentration. The excess NaBH4 increases the pH of the system, thus retarding the degradation of BH4 , and the liberated hydrogen cleanses the air thereby preventing the aerial oxidation of 4-AP.26 Since the concentration of NaBH4 greatly exceeds that of 4-NP, the reduction rate can be assumed to be independent of NaBH4 concentration. Therefore, the catalytic rate constant (K) in this case can be evaluated by studying the pseudo-first-order kinetics with respect to 4-NP concentration.24 15272

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Table 2. Details of the Catalytic Study Done for Different Concentrations of AuNPs (cf. Figure 7d) λmax values of 4-NP at different time intervals amount of

calculated rate constant

approximate time of completion

3

of reaction

AuNPs (mL)

2.5 min

4.5 min

1

0.1

3.21

2.61

1.86

18.5 min

2

0.3

1.51

0.80

7.66

10.5 min

3

0.5

1.09

0.49

8.84

6.5 min

We studied the catalytic activity of AuNPs obtained from three different stem-extract concentrations. Figure 7b shows that after the induction time there is a good linear relation between ln (At/A0) and time t, for almost 90% of the reaction. Here, At stands for absorbance at any time t and A0 for absorbance at time 0. Rate constants (K) are calculated from the slopes of the linear sections of the plots and are given in Tables 1 and 2. K is highest for AuNPs (K = 9.19  10 3 s 1) synthesized from 0.5 mL of extract and was observed to decrease with increasing extract concentration. Such diminishing catalytic rates with increase in extract concentration can be attributed to an increase in the particle size, as suggested by DLS measurements (Figure 3d). Furthermore, a decrease in K can occur due to increase in the surface coverage by the biomolecules with increase in the extract concentration. Such increased coverage retards the diffusion of 4-NP to the nanoparticle surface. Esumi et al. observed a similar decrease in the catalytic activity of dendrimer metal nanocomposites during the reduction of 4-NP with increase in the concentration of dendrimer.26 Similar catalytic studies were performed using AgNPs (K = 4.06  10 3 s 1), which exhibited lower catalytic rates relative to AuNPs (K = 9.19  10 3 s 1). Figure 7c shows difference in slopes for ln(At/A0) vs t at 5% vol fraction between AuNPs and AgNPs. This difference is attributed to the relatively larger nanoparticle size and the possible oxidization of AgNPs.26 In the presence of strong nucleophile like BH4 the reduction potential of AgNPs is further lowered24 and its surface becomes more vulnerable to oxidation. However, a backward reduction of this oxide layer due to NaBH4 activates the Ag surface. Subsequently, BH4 ions and 4-nitrophenolate ions coadsorb on Ag NP surface. Ag NPs relay electrons from donor BH4 ions to the acceptor 4-nitrophenolate to catalyze the reaction. Thus, poisoning of the surface with the oxide layer can adversely affect the catalytic properties of Ag NPs. Influence of Catalyst Dosage. We now turn our attention to the dependence of the catalytic rate on the nanoparticle concentration in the catalytic conversion of 4-NP to 4-AP. The AuNP concentration was varied, while other parameters were kept constant for three catalytic runs. As anticipated, K increased with an increase in AuNP concentration (Figure 7d and Table 2) due to an increase in the number of reaction sites.

4. CONCLUSIONS In summary, we synthesized mg/mL quantities of Au NPs and Ag NPs on time scales (∼7 min) faster than traditional green chemistry methods. As obtained, AuNPs (AgNPs) were observed to be spherical in shape, with a mean diameter of 27 nm (64 nm). Importantly, the Au NP size can be controllably varied (∼25 nm to 110 nm) by changing the concentration of Breynia rhamnoides stem extract (5% to 30% vol fraction). The phytochemical analysis suggests that stem extract is mainly composed

(10

s 1)

sample

of phenolic glycosides and reducing sugars, which are responsible for rapid reduction of metal salts. Furthermore, catalytic studies performed for 4-NP to 4-AP conversion indicate that the catalytic rate constant was highest (K = ∼9.2  10 3 s 1) for AuNPs prepared from a minimum extract concentration (5% vol fraction). K decreased to ∼4.6  10 3 s 1 as the extract concentration increased to 25% vol fraction, and the decrease is attributed to an increase in the size of the AuNPs. The Ag NPs (K = 4.06  10 3 s 1) exhibited lower catalytic rates relative to Au NPs (K = 9.19  10 3 s 1) at 5% vol fraction of stem extract. Slower catalytic rates observed in Ag NPs were attributed to their relatively large size and the formation of surface oxide layer.

’ ASSOCIATED CONTENT

bS

Supporting Information. More details regarding the DLS spectra and TEM images of Au and Ag NPs, and NMR spectra and scheme and qualitative results of phytochemical analysis.This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors are grateful to the Chancellor, Bhagawan Sri Sathya Sai Baba, Sri Sathya Sai Institute of Higher Learning, for his constant inspiration. A.G. acknowledges UGC, New Delhi, for granting him Junior Research Fellowship. Authors at SSSIHL thank S. Prathap Chandran for helping us in obtaining TEM and other related electron microscope characterization for the samples reported. Authors at Clemson thank Dr. Brian Powell (Environmental Engineering, Clemson University) and Dr. P.-C. Ke, Pengyu Chen (Laboratory of Single-Molecule Biophysics and Polymer Physics, Clemson University) for helping with the DLS measurements. ’ REFERENCES (1) Ozin, G. A. Adv. Mater. 1992, 4, 612–649. (2) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–34. (3) Kholoud, M. M.; El-Nour, A.; Eftaiha, A.; Al-Warthan, A.; Ammar, A. A. Arab. J. Chem. 2010, 3, 135–140. (4) Alivisatos, A. P. Science 1996, 271, 933–937. (5) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800–803. (6) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (7) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J. Nanocrystals: Synthesis, Properties and Applications; Springer: Berlin, 2007; pp 24 85. 15273

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