J. Phys. Chem. C 2008, 112, 3633-3640
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Self-Assemblies from RNA-Templated Colloidal CdS Nanostructures Anil Kumar* and Vinit Kumar Department of Chemistry and Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee - 247667, India ReceiVed: NoVember 19, 2007
RNA serves as an effective template for the synthesis of quantized CdS nanoparticles and mediates their growth to create novel assemblies. Unlike DNA-stabilized particles in aqueous medium, these particles display relatively strong emission at 2.34 eV, which is further enhanced by more than 2.5 fold and blue shifts to higher energy (2.39 eV) upon aging. Chelation of Cd2+ with RNA restricts the nucleation of CdS. A variation in the molar ratio of Cd/S from 2 to 6 produces different nanostructures with varied electronic properties. Unlike general colloidal systems, aging of these nanoparticles produced smaller crystalline nanocrystals as evidenced by their blue-shifted optical threshold and fluorescence maxima, and by atomic force microscopy and transmission electron microscopy analysis. Different nanostructures grow upon aging to yield self-assembly of different shapes, and the morphology and structure of these nanostructures could be manipulated by changing the molar ratio of Cd/S. The formation of organized structures could provide a basis for controlled fabrication of new nanostructures and nanodevices.
Introduction Natural biological systems have been observed to contain a large number of nanostructures.1-3 Interfacing of metals/ semiconductors with biomolecules possessing similar dimensions have lately aroused considerable interest because of their enhanced characteristic physicochemical properties and potential multidisciplinary applications.2,4,5 The multifunctionality of biopolymer could be utilized to produce building blocks for fabricating macromolecular structure by modifying its surface through hydrogen bonding6 or polar interactions with other moieties.7 These materials are finding increasing applications in medicine and biology,2 drug delivery,2,8 as biological sensors,9-11 imaging probes,12,13 biological labeling,14,15 intracellular monitors,16 and in the design of nanodevices with high precision and tunability.12,17 Coupling of biomolecules with inorganic metal/semiconductor colloids using wet chemistry is emerging as an important tool to design self-assembled functionalized superstructures with controlled morphology. The three-dimensional (3D) network of biomolecules18-23 has lately been exploited extensively as templates to produce organized quantum confined nanostructures with well-defined morphology. RNA is known to play a key role in the early stages of biological evolution.24 It functions both as a genetic information store and a replicative enzyme. In fact, it is RNA instead of DNA that is the first carrier of information.25 Semiconductor nanoparticles interfaced with biopolymers having a large number of anchoring sites may demonstrate quite different electronic, photonic, and biological applications. The interaction of Cd2+ with RNA is known in literature to be specific and selective.26 It binds to RNA at the γ′ and γ-position of subtype A DIS duplex. In view of this, in the present work torula yeast RNA matrix is used to bind Cd2+ in aqueous medium. Its interaction with SH- under the optimized reaction conditions produces highly soluble, crystalline fluorescent CdS nanoparticles. The * To whom correspondence should be addressed. E-mail: anilkfcy@ iitr.ernet.in. Tel.: +91-1332-285799. Fax: +91-1332-273560.
Figure 1. (A) Effect of [RNA] (g/100 mL) at 0.001 (a), 0.003 (b), 0.010 (c), and 0.015 (d) on the electronic spectra of CdS at pH 9.2. Inset: absorption spectrum of 2 months aged CdS particles under optimized conditions having Cd/S 6 (e). (B) Effect of [RNA] (g/100 mL) at 0.001 (a), 0.003 (b), 0.01 (c), and 0.015 (d) on the fluorescence spectra of CdS at pH 9.2. Fluorescence spectrum of 2 months aged CdS particles synthesized under optimized conditions having Cd/S 6 (e). [λex 380 nm].
10.1021/jp7109803 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/20/2008
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Figure 2. XRD patterns of freshly prepared CdS having Cd/S molar ratio of 4 (A) and 6 (B). Additional unmarked peaks are due to free Cd2+/ Cd(OH)2.
growth of these colloidal particles has been probed by using transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and spectroscopic techniques. It results in the formation of a variety of novel metastable self-assembled superstructures. An understanding of the formation of self-assembly may provide a key to design nanoscale devices. Experimental Section Materials. Cadmium perchlorate and RNA torula yeast (Sigma), NaOH and coumarin 152 (Aldrich), and perchloric acid and FeS (Merck) were all of analytical grade. All these chemicals were used without any further purification. Nitrogen and oxygen with purity >99.9% were procured from Sigma. N2 was used for deoxygenating the solution during synthesis. Equipment. Electron microscopy was performed on a FeiPhilips Morgagni 268D and FEI-TECNAI 200kV Digital TEM having variable magnification up to 280 000× and 1 100 000×, respectively. The surface morphology of colloidal particles was determined by FEI-QUANTA 200F FESEM coupled with energy dispersive X-ray (EDX) analysis. AFM images in both 2D and 3D mode were recorded on NTEGRA (NTMDT). X-ray diffraction (XRD) patterns were recorded on a Philips DW 1140/ 90 X-ray diffractometer using Cu KR line (1.5418 A°) of the X-ray source. Electronic and emission spectra measurements were made on a Shimadzu UV-2100S spectrophotometer and Shimadzu RF-5301-PC spectrofluorophotometer, respectively. IR spectra were obtained on a Thermo Nicolet Nexus FTIR spectrophotometer. The 1H NMR spectra have been recorded on a 500 MHz Bruker Avance 500 spectrometer in aqueous media. The fluorescence lifetime experiments were carried out on a Horiba Jobin Yvon-IBH single photon counter using a NanoLEDs and LDs as excitation source. Decay curves were analyzed by iterative reconvolution technique using multiexponential fitting program provided from IBH. Methodology. Experimental conditions for synthesis of RNAcapped CdS nanoparticles were optimized by varying the amount
of RNA (0.007 g to 0.02 g/100 mL), pH (9.2-9.8), and the amount of Cd2+ (1 × 10-4 to 6 × 10-4 mol dm-3). CdS colloidal nanoparticles were prepared by injecting freshly prepared SH- (1 × 10-4) to the deaerated aqueous solution of RNA (0.015 g/100 mL) and 2 × 10-4 mol dm-3 of Cd(ClO4)2 at pH 9.2 (Figure 1A,B). The absorption spectra of RNA-capped CdS having a Cd to S molar ratio of 2 and 4 were monitored over a period of 2 months by varying the concentration of RNA from 0.003 to 0.015 g/100 mL H2O at pH 9.2. An increase in the concentration of RNA blue shifts the onset of absorbance from 2.67 to 2.8 eV but did not influence its exciton absorption. A further increase in RNA did not demonstrate any change in its absorption thereafter. Surface topography of fresh and aged particles was analyzed with an AFM operating under semi-contact mode. The resonance frequency used was kept at 300 kHz. Images were recorded at room temperature. Surface morphology was also examined by employing FESEM coupled with EDX. Samples for AFM and FESEM analysis were prepared by applying a drop of the solution on to a glass plate and then drying it under vacuum at room temperature. The AFM has been calibrated with a standard specimen (diffraction grating TDG01) with average surface roughness g50 nm. Results and Discussion The absorption and fluorescence spectra of CdS produced in RNA matrix exhibit a regular blue shift in the onset of absorption, exciton, and emission peaks with an increase in the concentration of RNA. For the optimized concentration of RNA (0.015 g/100 mL H2O), a prominent exciton peak and the absorbance threshold due to CdS were observed at 380 nm (3.3 eV) and 440 nm (2.8 eV), respectively (Figure 1A), which are significantly blue-shifted compared to those observed with bulk CdS. These particles depict the band gap emission centered at 530 nm (2.34 eV) with a full width at half-maximum (FWHM) of 160 nm (Figure 1B), quite different from bulk CdS that
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Figure 3. (a) TEM images and their particle size distribution of CdS with Cd/S 2 having amount of RNA [g/100 mL]: 0.001 (A, A′), 0.015 (B, B′). Cd/S ratio 4: fresh (C, C′); aged (D, D′) (scale bar is 200 nm). (b) SAED of CdS having Cd/S 4: fresh (A), 2 months aged (B).
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Figure 4. (a) IR spectra of pure RNA (A), RNA-Cd2+(B), and CdS on RNA matrix (C). (b) 1H NMR of pure RNA (A), RNA-Cd2+ (B), CdS on RNA matrix (C), and expended portion of C (D).
displays emission at 650 nm (2.8 eV) with a FWHM > 200 nm. These observations are consistent with the formation of quantized particles of CdS. XRD patterns for different samples of RNA-capped CdS containing varied ratio of Cd/S (4 and 6) are given in (Figure 2). The XRD pattern for the sample containing Cd/S ratio 4 exhibits only two peaks centered at approximate 2θ values of 28 and 47°. However, for Cd/S ratio of 6 the observed d spacing shows CdS to be produced in the hexagonal phase (Figure 2). The broadening of the diffraction pattern at low Cd/S suggests that the CdS sample prepared under these conditions is amorphous and the particles produced may be fairly small. The structure and size of these particles were further probed by TEM (Figure 3). The size and size histograms of colloidal particles were recorded as a function of varied amount of RNA (0.001-0.015 g/100 mL) at different molar ratios of Cd/S. These images depict the formation of spherical particles in all the cases. At lower amount of RNA (0.001 g/100 mL) and Cd/S molar ratio of 2, CdS particles are produced with an average size of
11 nm with a size distribution ranging from 7 to 17 nm. An increase in the amount of RNA to 0.015 g/100 mL reduced the average size to 6 nm along with a decrease in size distribution from 3 to 10 nm. Selected area diffraction pattern (SAED) of these particles depicts them to be polycrystalline in nature (Figure 3b). The indexing of SAED patterns correlates to four diffraction peaks, (100), (102), (110), and (202), corresponding to the hexagonal phase of CdS. These peaks match to those observed by XRD (Figure 2) and thus confirm CdS to be produced in hexagonal phase. Figure 4a presents IR spectra of pure RNA (A), RNA-Cd2+(B), and CdS on RNA matrix (C). IR spectrum of pure RNA depicts various vibration bands (cm-1) at 1686 (G and U); 1649 (sh) (U, G, A, and C); 1508 (C); and 1238 (sh, PO2- asymm. Str.) as reported in literature.28A comparison of IR spectra of pure RNA, with Cd2+-RNA reveals that interaction of RNA with Cd2+ influences vibrational bands due to (U, G, A and C)
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J. Phys. Chem. C, Vol. 112, No. 10, 2008 3637 SCHEME 1: CdS on RNA Matrix at Low Cd/S Molar Ratio 2
SCHEME 2: CdS on RNA Matrix at Higher Cd/S Molar Ratio 4 Figure 5. Decay curves of CdS having Cd/S molar ratio 2: fresh (a), aged (a′); Decay curves of CdS having Cd/S molar ratio 4: fresh (b).
and (G and U). The asymmetric stretching due to phosphate became prominent and is displayed at 1237 cm-1. With CdS sample supported on RNA, all vibrations due to purines and pyrimidine bases depict an interaction. Specifically, the band due to (U, G, A, and C) became more intense and shifted slightly to lower energy (1643 cm-1), and the asymmetric stretching due to phosphate ions remained unaffected. IR studies thus indicate that Cd2+ interacts strongly with phosphate attached with sugar moiety of RNA and depict some mild interaction with purine and pyrimidine bases, whereas CdS shows interaction with all the base moieties and not with the PO2-. Proton NMR spectra of pure RNA, Cd2+-RNA, and CdS grown on RNA matrix samples have been shown in Figure 4b. NMR spectrum of pure RNA depicted all characteristic peaks corresponding to protons of the sugar (H2′, H3′, H4′, H5′, H5′′), sugar and purine bases (H1′, H5) and aromatic protons of purine and pyrimidine bases (H2, H8, H6) and exhibit resonance absorption between 4 to 4.5, 5 to 6, and 7 to 8.3 ppm, respectively. Upon addition of Cd2+, resonance due to these moieties is still observed at the same frequencies except that peaks due to different protons become more resolved. Similar observations have earlier been made with other ions in the literature.23 On the contrary, in the presence of CdS frequencies due to protons of the sugar and purine bases resonated at higher frequencies but the peak corresponding to 2′ proton of the hydroxyl group shifts to lower frequency and is split into two peaks. Obviously, there is an increased deshielding of protons due to different base nuclei and splitting of 2′ proton of -OH after the formation of CdS due to the presence of neighboring -CH proton of sugar. It suggests that the interaction of -OH with CdS reduces the exchange of -OH protons significantly compared to when RNA is complexed with Cd2+ alone. Among different components of RNA, namely, cytosine monophosphate (CMP), uracil monophosphate (UMP), adenosine monophosphate (AMP) and guanosine monophosphate (GMP), CMP and UMP did not stabilize CdS particles. Both AMP- and GMP-mediated particles were fluorescing and depict emission maxima at 2.0 eV (600 nm) and 2.1 eV (580 nm), respectively. A comparison of these data with those of obtained with RNA-mediated CdS (Figure 1) indicates that the exciton absorption and emission maxima due to AMP- and GMPstabilized particles are fairly different from those observed with RNA-capped CdS particles. AMP- and GMP-mediated CdS
nanoparticles were, however, fairly stable for month(s). These findings evidently manifest that the observed absorption and emission effects are not contributed by the individual component of RNA, and it is rather the combined effect due to its different moieties. Details of these systems will be published elsewhere. Nanostructures of RNA-templated CdS have been manipulated by varying the amount of precursor Cd2+ ion added to RNA followed by the precipitation of CdS by adding fixed amount of SH- (1 × 10-4 mol dm-3). Growth of these particles was then followed upon aging them over a period of 2 months by recording their electronic properties and surface architecture under different experimental conditions. An increase in molar ratio of Cd/S from 2 to 6 simply red shifts the onset of absorption and excitonic peaks of CdS in electronic spectra, but it enhances the emission intensity by a factor of 2 without appreciably influencing the emission maximum (Figure 1B). Aging of these colloids for about 2 months at 5 °C, however, causes a blue shift in both the absorption threshold and the fluorescence maximum due to these
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TABLE 1: Effect of Variation in Amount of Cd2+(mol dm-3) on the Lifetime of Fresh and Aged CdSa components
1
2
3
amount of Cd2+ (mol dm-3)
τ1 (ns)
emission (%)
τ2 (ns)
2 × 10-4
0.1 (3.000) 0.5 (0.2711) 0.2 (0.2945)
87.7
1.0 (0.3200) 2.4 (0.0707) 1.3 (0.0671)
(a) fresh samples 9.4 6.9 (0.0599) 18.7 12.9 (0.0197) 17.4 8.9 (0.0152)
0.7 (0.4333) 0.4 (0.8214) 0.6 (0.1077)
72.8
3.1 (0.0901) 1.9 (0.2385) 2.9 (0.0291)
(b) aged samples 15.1 20.3 (0.0353) 20.0 9.3 (0.0673) 18.8 13.0 (0.0074)
4 × 10-4 6 × 10-4 2 × 10-4 4 × 10-4 6 × 10-4 a
72.0 76.4
69.0 69.5
emission (%)
τ3 (ns)
4 emission (%)
τ4 (ns)
emission (%)
(ns)
χ2
1.8
50.7 (0.0383) 73.2 (0.0154) 67.6 (0.0087)
1.1
33
1.1
4.1
51
1.2
2.2
48
1.0
6.1
86
1.0
5.3
46
1.1
6.9
58
1.0
5.2 4.0
6.0 5.7 4.8
110.2 (0.0366) 61.4 (0.0630) 74.3 (0.0107)
λex ) 375 nm; λem ) 530 nm.
particles. In a typical case for particles having a molar ratio of Cd/S 6, the emission intensity is enhanced by more than a factor of 2.5 in the entire recorded wavelength range along with an increase in the energy of the emission maximum by 0.05 eV compared to that of particles having molar ratio of Cd/S 2 (Figure 1B, curve e). This emission intensity corresponds to a quantum efficiency of 0.02. For CdS particles having molar ratio of Cd/S 4, the average size of particles is reduced upon aging to 9 nm compared to 10 nm recorded for freshly prepared particles (Figure 3a). From the size histogram of these particles it may be noticed that although the range of size distribution for aged particles is slightly increased from 5 to 17 nm, a large fraction of particles lie in the range of 7 to 11 nm. SAED patterns of aged particles also exhibit the formation of rings (Figure 3 b) correlating to diffraction planes (002), (110), and (201) of CdS having hexagonal phase. XRD patterns of this sample were similar to that of the fresh sample (Figure 2) thus supporting the formation of fairly small particles in this case as well. These findings are contrary to the general observations with colloidal systems, which are known to undergo Ostwald’s ripening upon aging. To understand the complex electronic behavior of these nanosystems, the relaxation kinetics of charge carriers for both fresh and aged samples of CdS containing varied ratio of Cd/S was monitored by carrying out time-resolved fluorescence measurements. All decay curves could be fitted in four exponential kinetics with reasonably good χ2 values. An examination of lifetime data (Figure 5, Table 1) reveals that for freshly prepared CdS particles with molar ratio of Cd/S from 2 to 6, major components contributing to the fluorescence decay (>90%) correspond to the subnanosecond and nanosecond time domain, and the component corresponding to the longer time constant has fairly small contribution (∼10%) to this process. It evidently suggests that the later decay arises from the trap states involved in nonradiative recombination. This observation supports the relatively low quantum efficiency of emission recorded in steady-state emission. A calculation of the average lifetime depicts a small increase in lifetime from 33 to 51 ns upon increasing the molar ratio of Cd/S from 2 to 6. Aging of particles with molar ratio of Cd/S 2 enhanced the lifetime significantly from 33 to 86 ns, but at higher molar ratios of CdS it did not exhibit any significant change (Table 1). These changes in lifetimes are understood by the formation of relatively less rigid structure at lower molar ratio of Cd/S (vide infra Scheme 1) allowing the formation of more trap states owing to
large interface, which does not become possible at higher molar ratio due to rigid network of particles (vide infra Scheme 2). Two-dimensional AFM images of all the fresh samples depicted these particles to be spherical (Supporting Information, Figure S1). An increase in molar ratio of Cd/S from 2 to 4 increases the maximum surface roughness of these particles from 4 to 14 nm, and the particles at the later molar ratio became crystalline (Figure 6). Interestingly, an aging of this sample for about 2 months reduced their maximum surface roughness from 14 to 3.5 nm, and these particles displayed an organized structure (Figure 6D). To further probe the growth of the self-assembly, FESEM images and EDX analysis of some of the fresh and aged samples of RNA-templated CdS were recorded under different experimental conditions (Figure 7). Aging shows the formation of starlike structures, snowflakes, and a network of nanowires at various stages of their growth. At Cd/S ratio of 2, it forms a flowerlike structure, whereas at Cd/S 4 a network of nanowires is formed with a diameter of 250 nm. This networking could be seen with solid as well as thin film obtained by applying the colloidal solution on a glass plate. EDX analysis of a portion of this nanowire shows a homogeneous distribution of Cd and S all along the wire (Figure 7C). The formation of these superstructures is also supported by AFM image where small portion of these microstructure appeared as snowflakes (Supporting Information, Figure S1B). The observation of size-dependent optical and fluorescence changes recorded with CdS produced in RNA matrix is consistent with the fact that CdS is produced in quantumconfined region. These findings correlate very well with the observed changes in the particle size by TEM and AFM studies and are also supported by a previous finding on tRNA-templated CdS,21 which demonstrated the wild type tRNA- and MT tRNAmediated products to contain 5 and 7 nm sized particles, respectively. CdS on RNA matrix is produced in hexagonal phase and is bound to purine, pyrimidine, and the sugar moiety of RNA through Cd2+ (Figure 4a,b). The blue-shifted emission band with increased quantum efficiency upon aging of particles at higher molar ratio of Cd/S is understood by the production of smaller particles (Figure S2 under Supporting Information) with high degree of crystallinity (Figure 6D); the latter would have reduced the defects responsible for nonradiative emission. The higher emission yield of RNA-mediated CdS (2 × 10-2) compared to that of CdS produced in DNA matrix (1 × 10-4)29 can be assigned to the structural difference between the two
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Figure 6. 3D AFM images of CdS having Cd/S molar ratio of 2: fresh (A); 2 months aged (B). 3D AFM images of CdS having Cd/S molar ratio of 4: fresh (C); 2 months aged (D).
Figure 7. FESEM images of CdS having Cd/S molar ratio of 2: two months aged (A) magnified image of encircled portion in A (A′). FESEM images of CdS having Cd/S molar ratio of 4: (B); distribution of Cd and S along the wire (C).
cases as arrived by IR and NMR studies (vide supra). Specifically, the binding of RNA to CdS occurs through 2′-proton of
hydroxyl group, as evidenced by NMR (Figure 4b; Schemes 1and 2), which is absent in DNA.
3640 J. Phys. Chem. C, Vol. 112, No. 10, 2008 Because RNA-supported CdS nanostructures obtained by varying Cd/S molar ratios exhibit different optical and fluorescing properties (vide supra) and aging of these colloids demonstrate the absorption and fluorescence behavior contrasting to the general colloidal systems (Figure 1), it suggests that the formation of different self-assemblies over this period might be influencing these properties. It is likely that during the growth process several CdS units become loosely bound through weak van der Waals and hydrogen bonding interactions involving various functional groups on the surface of particles to form nano- and micro-assemblies. As the variation in the amount of precursor Cd2+ ion demonstrates a drastic change in the nanoarchitecture of CdS (Figures 6, 7, and Supporting Information Figure S1), it evidently indicates the formation of network involving Cd2+ and different functionalities of RNA contributing to the depicted structures. At low Cd/S molar ratio, aging yields relatively less complex structure (Scheme 1), whereas at higher molar ratios it forms a rigid structure involving extensive interaction through Cd2+ (Scheme 2). The excess Cd2+ binds different units of RNA involving O- of phosphate and purines, and in the basic pH range Cd2+ may exist as Cd(OH)2. To explore the possibility of the formation of Cd(OH)2-coated CdS under the present conditions, an experiment was designed in which the particles formed at lower molar ratio of Cd/S (2) were activated by increasing pH followed by the addition of excess Cd2+. In a typical case, raising the pH of this solution to 11 followed by an increase in the molar ratio of Cd/S to 4 reduced the emission intensity by about 50% without developing any new emission peak (Supporting Information, Figure S3). This observation is quite different to the previous report30 in which coating of Cd(OH)2 on CdS induces the strong band gap emission and thus rules out this possibility in the present system. Spectroscopic studies have clearly revealed an interaction of CdS through sulfur with the -OH group of sugar moiety and purine bases, which modifies the surface of the particles and thus contributes to the increased fluorescence efficiency. At high Cd/S, these extended structures might form multiexcitons as observed lately for CdSe, PbS, and PbSe quantum dots,31,32 which might also contribute to the observed electronic changes and charge carrier dynamics. Conclusions In summary RNA having a complex 3D structure provides an excellent matrix for the functionalization of CdS nanoparticles and designing of nano- and micro- assemblies. The amount of Cd2+ controls the nature of interaction between different units of RNA so as to produce a variety of nanostructures with different size and shape. An increased networking upon aging results in the formation of crystalline structures with multiexcitonic states causing a blue shift in the excitonic absorption and emission maximum. The formation of smaller particles on aging possibly results due to the reorganization of these particles on its surface owing to the availability of large number of free active binding sites of multifunctional RNA. It yields relatively ordered nanosystems as is clearly evidenced by reduction in surface roughness and increased crystallinity by AFM studies. Such controlled organization of nanoparticles to yield 3D networking is a step forward to fabricate nanodevices using colloidal materials. This is the first report on the use of RNA to produce synthetic nanoassemblies. Acknowledgment. The financial support of DST, New Delhi is gratefully acknowledged to undertake this work. V.K. is
Kumar and Kumar thankful to CSIR, New Delhi for the award of SRF. Thanks are also due to Professor and Head, IIC, IITR, Roorkee for providing the facilities of Single Photon counter, AFM, FESEM, TEM, and NMR, and Director, AIIMS, Delhi for providing the facilities of TEM. Supporting Information Available: Details of some 2D AFM images (Figure S1), absorption and emission spectra under different experimental conditions (Figures S2 and S3). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Seeman, N. C.; Belcher, A. M. Proc. Nat. Acad. Sci. U.S.A. 2002, 99, 6451-6455. (2) Salata, O. V. J. Nanobiotechnology 2004, 2, 3. (3) Ajayan, P. M.; Schadler, L. S.; Braun, P. V. Nanocomposites Science and Technology; Wiley-VCH GmbH & Co. KGaA: Weinhein, Germany, 2003. (4) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042-6108. (5) Sun, Y.; Kiang, C.-H. Handbook of Nanostructured Biomaterials and Their Applications in Nanobiotechnology; Nalwa, H. S., Ed.; American Scientific Publisher: CA, USA, 2005; Vol. 1-2, Chapter V. (6) Baron, R.; Huang, C.-H; Bassani, D. M.; Onopriyenko, A.; Zayats, M.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 4010-4015. (7) Sastry, M.; Rao, M.; Ganesh, K. N. Acc. Chem. Res. 2002, 35, 847-855. (8) Gref, R. Synthesis, Functionalization and Surface Treatment of Nanoparticles; Baraton, M.-I., Ed.; American Scientific Publishers: CA, USA, 2003; Chapter 11, p 233. (9) Alivisatos, P. Nat. Biotechnol. 2004, 22, 47-52. (10) Drummond, T. G.; Hill, M. G.; Barton, J. K. Nat. Biotechnol. 2003, 21, 1192-1199. (11) Peng, H.; Zhang, L.; Kjallman, T. H. M.; Soeller, C.; Travas-Sejdic, J. J. Am. Chem. Soc. 2007, 129, 3048-3049. (12) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu. A. M.; Gambhir, S. S.; Weiss. S. Science 2005, 307, 538-544. (13) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nat. Mater. 2005, 4, 435-446. (14) Michalet, X.; Pinaud, F.; Lacoste, T. D.; Dahan, M.; Bruchez, M. P.; Allivisatos, A. P.; Weiss. S. Single Mol. 2001, 2, 261-276. (15) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (16) Winter, J. O.; Liu, T. Y .; Korgel, B. A.; Schmidt, C. E. AdV. Mater. 2001, 13, 1673-1677. (17) Willner, I.; Patolsky, F.; Wasserman, J. Angew. Chem., Int. Ed. 2001, 40, 1861-1864. (18) Gerion, D.; Parak, W. J.; Williams, S. C.; Zanchet, D.; Micheel, C. M.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7070-7074. (19) Liang, H.; Angelini, T. E.; Ho, J.; Braun, P. V.; Wong, G. C. L. J. Am. Chem. Soc. 2003, 125, 11786-11787. (20) Fu, A.; Micheel, C. M.; Cha. J.; Chang, H.; Yang, H.; Alivisatos, A. P. J. Am. Chem. Soc. 2004, 126, 10832-10833. (21) Ma, N.; Dooley, C. J.; Kelley, S. O. J. Am. Chem. Soc. 2006, 128, 12598-12599. (22) Gugliotti, L. A.; Feldheim, D. L.; Eaton, B. E. Science 2004, 304, 850-852. (23) Kumar, A.; Jakhmola, A. Langmuir 2007, 23, 2915-2918. (24) Nelson, D. L.; Cox, M. M. Lehniger, Principles of Biochemistry, 4th Ed.; Freeman: New York, 2005; p 1027. (25) Yanagawa, H.; Ogawa, Y.; Ueno, M.; Sasaki, K.; Sato, T. Biochemistry 1990, 29, 10585-10589. (26) Ennifar, E.; Walter, P.; Dumas, P. Nucleic Acid Res. 2003, 31, 2671-2682. (27) Coffer, J. L.; Bigham, S. R.; Pinizzotto, R. F.; Yang, H.; Nanotechnology 1992, 3, 69-76. (28) Arakawa, H.; Neault, J. F.; Tajimir-Riahi, H. A. Biophys. J. 2001, 81, 1580-1587. (29) Bigham, S. R.; Coffer, J. L. J. Cluster Sci. 2000, 2, 359-372. (30) Kumar, A.; Negi, D. P. S. J. Colloid Interface Sci. 2001, 238, 310317. (31) Kilmov, V. I. J. Phys. Chem. B 2006, 110, 16827-16845. (32) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Nano Lett.. 2005, 5, 865-871.
