Gold Nanoparticle Networks with Photoresponsive Interparticle

Jul 15, 2005 - Photoresponsive gold nanoparticle networks were prepared by functionalizing them with azobenzene derivatives. A network can be formed w...
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Langmuir 2005, 21, 7979-7984

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Gold Nanoparticle Networks with Photoresponsive Interparticle Spacings Deepti S. Sidhaye,† Sudhir Kashyap,‡ Murali Sastry,† Srinivas Hotha,*,‡ and B. L. V. Prasad*,† Nanoscience Group, Materials Chemistry Division, and Combi Chem-Bio Resource Center, Division of Organic Chemistry: Synthesis, National Chemical Laboratory, Pune 411 008, India Received April 27, 2005. In Final Form: June 13, 2005 Photoresponsive gold nanoparticle networks were prepared by functionalizing them with azobenzene derivatives. A network can be formed when a linker molecule constituting the azobenzene moiety suitably derivatized on either side with gold surface sensitive groups such as thiols and amines is added to the nanoparticle solution. It is shown that the interparticle spacing in the networks could be controlled by the reversible trans-cis isomerization of the azobenzene moiety induced by UV and visible light, respectively. The photoinduced variation in the interparticle spacings is inferred by the changes in the optical spectra of the gold nanoparticles which display a red or blue shift in the surface plasmon resonance peak depending on a decrease or increase in the interparticle spacing, respectively. Transmission electron microscopy images are in consonance with the evidence from the optical spectra.

Introduction Functional materials fabricated from nanoparticle building blocks are evincing great curiosity among researchers keeping future technological needs in mind.1 Nanoparticles, especially those of coinage metals such as gold and silver, are attracting more interest because of their striking colors in the visible region of the electromagnetic spectrum and hence their potential applications in the opto- and optoelectronic fields.2 These colors arise due to the collective oscillation of the conduction electrons in response to the alternating electric field of the incident electromagnetic radiation, and the wavelength at which resonance occurs is termed as the surface plasmon resonance. What is more appealing is that these optical properties can be easily modulated by varying the interparticle interaction/separation, the dielectric properties of the medium in which they are dispersed, the nature of the surface functionalization, and also by the size and shape of the nanoparticles.2f With the information on the dependence of optical properties on the above parameters known,2b-d focused efforts are being made to modulate the optical properties with specific applications in mind. Bio diagnostics and gas sensing are two of the areas where great amounts of research efforts are being focused currently.3 The underlying principle based on which the applications are perceived is the reduced interparticle spacing when DNA* To whom correspondence should be addressed. Ph: 91-2025893400 Ext: 24012260/2013. Fax: 91-20-/25893044. E-mail: [email protected]; [email protected]. † Nanoscience Group, Materials Chemistry Division. ‡ Organic Chemistry Division (Synthesis). (1) (a) Shipway, A. N.; Willner, I. Chem. Commun. 2001, 2035. (b) Nimeyer, C. M. Angew. Chem. Int. Ed. 2001, 40, 4128. (c) Davis, S. A.; Breulmann, M.; Rhodes, K. H.; Zhang, B.; Mann, S. Chem. Mater. 2001, 13, 3218. (d) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549. (e) Lu, Y.; Liu, G. L.; Lee, L. P. Nano Lett. 2005, 5, 5. (2) (a) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (b) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (c) Mulvaney, P. Langmuir 1996, 12, 788. (d) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (e) Taleb, A.; Petit, C.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 2214. (f) Storhoff, J. J.; Lazarides, A. A.; Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Schatz, G. C. J. Am. Chem. Soc. 2000 122, 4640.

functionalized nanoparticles are exposed to complementary sequences of DNA3a or the greater/smaller interparticle interaction when they are exposed to certain gaseous species leading to a change in the color or conductivity of the nanoparticles films.3f-l In this context, it is also worth mentioning that a plethora of research reports have appeared on the optical switching phenomenon in organic molecules, and the trans-cis-trans isomerization of azobenzenes has been a subject of great interest due to their possible application in energy storage systems or in photochemical devices.4 Many theoretical and spectroscopic studies have been carried out to understand the mechanism and the time scales involved in the photoinduced isomerization of these molecules.5 Several pathways have been proposed for the isomerization mechanism, and it is in general accepted that the functional groups attached to the azobenzene moiety and the medium in which the isomerization studies were carried out have great bearing on both of these factors. However, one common factor in all of the studies reported so far is that at room temperature the trans isomer is the most stable one, and by irradiating this with UV light, a trans to cis transformation can be brought about. Although this particular transformation happens quite rapidly, the reverse i.e., cis to trans transformation, can happen either at room temperature (3) (a) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547. (b) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (c) Briglin, S. M.; Gao, T.; Lewis, N. S. Langmuir 2004, 20, 299. (d) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959. (e) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger R. L.; Mirkin, C. A. Science 1997, 277, 1078. (f) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Mullen, K.; Yasuda, A.; Vossmeyer, T. Nano. Lett. 2002, 2, 551. (g) Evans, S. D.; Johnson, S. R.; Cheng, Y. L.; Shen, T. J. Mater. Chem. 2000, 10, 183. (h) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856. (i) Han, L.; Daniel, D. R.; Maye, M. M.; Zhong, C.-J. Anal. Chem. 2001, 73, 4441. (j) Schlupp, M.; Heil, C.; Koch, A.; Mu¨ller-Albrecht, J.; Bargon, U. Sens. Actuators, B 2000, 71, 9. (k) Shipway, A. W.; Lahav, M.; Willner, I. Adv. Mater. 2000, 12, 993. (l) Kharitonov, A.; Shipway, A. W.; Willner, I. Anal. Chem. 1999, 71, 5441. (4) (a) Sekkat, Z.; Dumont, M. Appl. Phys. B 1992, 54, 486. (b) Willner, I.; Rubin, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 367. (5) (a) Biswas, N.; Umapathy, S. J. Chem. Phys. 2003, 118, 5526. (b) Biswas, N.; Umapathy, S. J. Chem. Phys. 1997, 107, 7849. (c) Sanchez, A. M.; de Rossi, R. H. J. Org. Chem. 1996, 61, 3446. (d) Wildes, P. D.; Paciifici, J. G.; Irick, G., Jr.; Whitten. D. G. J. Am. Chem. Soc. 1971, 93, 2004.

10.1021/la051125q CCC: $30.25 © 2005 American Chemical Society Published on Web 07/15/2005

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Scheme 1. (A) Chemical Structures of the Linker in the trans Form 1 and the cis Form 2 and (B) Schematic Representation of the Transformations Occurring in the Nanoparticle Networks by Effecting the trans-cis Isomerization of the Linker Molecule

over a wide range of time scales or can be brought about by irradiating the cis isomer with visible light. Many functional molecules and materials were synthesized exploiting this trans-cis-trans isomerization principle.4b,6 For instance, azobenzene tethered DNAs have been used to photoregulate the formation and dissociation of the duplex or triplex DNAs by irradiating with UV or visible light and observed that trans-azobenzene (visible light) stabilizes formation of duplexes or triplexes and cisazobenzene (UV light irradiation) destabilizes the duplex.7 Furthermore, azobenzene molecules have been used to alter the conformation of linear peptides to β-hairpins,8a the sense of a peptide helix,8b the changes in the peptide aggregation,8c and the content of peptide helix,8d β-sheet,8e and coil structures.8f Recently, it has been shown that an azobenzene incorporated 16 amino acid peptide adopts a helical structure when the azobenzene is in the trans form and goes to a less helical form when it is in the cis form.9 Flint et al. have shown that peptides containing an azobenzene cross-linker between cysteine residues un(6) (a) Ichimura, K. Chem. Rev. 2000, 100, 1847. (b) Mallia, V. A.; Tamaoki, N. Chem. Soc. Rev. 2004, 33, 76, 2. (c) Jousselme, B.; Blanchard, P.; Gallego-Planas, N.; Delaunay, J.; Allain, M.; Richomme, P.; Levillain, E.; Roncali, J. J. Am. Chem. Soc. 2003, 125, 2888. (d) Shinkai, S.; Minami, T.; Kasano, Y.; Manabe, O. J. Am. Chem. Soc. 1983, 105, 1851. (7) (a) Zettu, N.; Ubukata, T.; Seki, T.; Ichimura, K. Adv. Mater. 2001, 13, 1693. (b) Asanuma, H.; Takarada, T.; Yoshida, T.; Tamaru, D.; Liang, X.; Komiyama, M. Angew. Chem., Int. Ed. 2001, 40, 2671. (c) Liang, X.; Asanuma, H.; Komiyama, M. J. Am. Chem. Soc. 2002, 124, 1877. (d) Liang, X.; Asanuma, H.; Kashida, H.; Takasu, A.; Sakamoto, T.; Kawai, G.; Komiyama, M. J. Am. Chem. Soc. 2003, 125, 16408. (8) (a) Aemissegger, A.; Krautler, V.; van Gunsteren, W. F.; Hilvert, D. J. Am. Chem. Soc. 2005, 127, 2929. (b) Ueno, A.; Takahashi, K.; Anzai, J.-I.; Osa, T. J. Am. Chem. Soc. 1981, 103, 6410. (c) Pioroni, O.; Fissi, A.; Houben, J L.; Ciardelli, F. J. Am. Chem. Soc. 1985, 107, 2990. (d) Pioroni, O.; Houben, J. L.; Fissi, A.; Costantino, P.; Ciardelli, F. J. Am. Chem. Soc. 1980, 102, 5913. (e) Kumita, J. R.; Smart, O. S.; Wooley, G. A. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 3803. (f) Flint, D. G.; Kumita, J. R.; Smart, O. S.; Woolley, G. A. Chem. Biol. 2002, 9, 391. (9) Chen, E.; Kumita, J.; Woolley, G. A.; Kliger, D. S. J. Am. Chem. Soc. 2003, 125, 12443.

dergo photoisomerization from trans-cis by modulating the wavelength of the light source.8f In this paper, we report our results on the reversible manipulation of interparticle spacing in gold nanoparticle networks connected by molecules comprising an azobenzene functional group. Our strategy depends on the reversible photoisomerization of the azo functionality incorporated into nanoparticle interconnecting molecules. At room temperature, the azo functionality is known to exist in the trans form, which can be transformed to the cis form by irradiation with UV light (for example, at 360 nm). In a typical molecule like 1 (Scheme 1A), the endto-end distance in the trans form is ∼3 nm, which shrinks to ∼2.0 nm in the cis form.10 Then, it can again be isomerized to the trans form by irradiating with a visible light or by keeping it at room temperature for sufficiently long time. Therefore, we envisaged that if suitably derivitized azobenzene molecules can be tagged on to gold nanoparticles they would lead to the formation of a photoresponsive network with changeable interparticle spacings. The spacers we chose, between the nanoparticles surface and the azobenzene moiety, were peptide-based ligands with two cysteine end groups synthesized via a bidirectional synthetic methodology using Boc (tertbutoxycarbonyl) chemistry and HBTU (O-benzotriazol1-yl-N,N,N′,N′-tetramethyluronium hexaflurophosphate) as the coupling reagent. The choice of spacers was based on the robustness of the peptide functional groups and the convenience with which they can be prepared. Although there are plenty of reports on the covalent and noncovalent self-assembly of nanoparticles,11 very few (10) The molecular lengths were estimated from optimized geometries of each molecule obtained by first drawing the molecule in CS Chemdraw (ver 6.0.1) and then optimizing it by the MOPAC routine available in CS Chem 3D (ver 6.0). Particular care was taken to draw the initial structures according to standard bond lengths and bond angles. (11) (a) Sastry, M.; Rao, M.; Ganesh, K. N. Acc. Chem. Res. 2002, 35, 847. (b) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (c) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549.

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reports on the photocontrollable linkers in nanoparticle networks and vesicles have appeared in the literature. Mikami et al. studied the reversible photoswitching of the magnetization of γ-Fe2O3 particle networks with 8-[4{4-butoxy-phenyl(azo)-phenoxy}octan-1-ol].12 Einaga et al. reported on their investigations of the reversible photoinduced switching of magnetic properties of iron oxide particles in self-assembled films containing azobenzene molecules.13 In an earlier paper, they also reported on the magnetic properties of Prussian blue intercalated into photoresponsive organic vesicles.14 Apart from these studies, we are not aware of any other reports wherein nanoparticle networks with azobenzene moieties have been constructed and their photoresponsive optical properties have been studied. It is worth mentioning here that tethering of ligands with azobenzene moieties incorporated in them on nanoparticle and supramolecular surfaces has been the subject of many previous papers.15 However, the subject of those reports is the influence of self-assembly on the photoresponsiveness of the trans-cis isomerization of azobenzene moieties. Therefore, those ligands were designed in such a way that they can be attached only on one nanoparticle surface and is thus fundamentally very different from the studies reported here. The photoisomerization of azobenzene derivatives (in particular the trans-to-cis isomerization) is accompanied by an increase in molecular volume, and the reaction gets greatly inhibited due to the close packing of the chromophores. To tackle this problem Mikami et al. decorated the γ-Fe2O3 particles with n-octylamine molecules.12 Here, we studied this situation in a more systematic way. We decorated the gold nanoparticles with three different types of ligands; (a) benzyldimethylstearylammoniumchloride (BDSAC), (b) octadecylamine (ODA, C18H39N), and (c) dodadecylamine (DDA, C12H27N). These three differ in their propensity to bind with gold nanoparticle surfaces (ODA ) DDA > BDSAC) and in their molecular length (BDSAC > ODA > DDA).10 It was observed that BDSAC coated nanoparticles show the greatest photoresponsiveness, followed by ODA and DDA capped ones. These results are rationalized by considering that BDSAC can probably be replaced and reattached on the nanoparticles surface easily facilitating the nanoparticles to come together or go apart due to the pulling or pushing effect of the cis or trans azobenzene. BDSAC is also lengthier than ODA and DDA making more space available for the nanoparticles to come together through interdigitation of the capping ligands during the trans-cis isomerization of the linker molecules. In DDA capped gold nanoparticles, the capping ligands preclude the trans-cis isomerization of the azobenzene moiety due to steric repulsions, and hence, no photoresposnsive behavior is observed in those networks. Presented below are the details of the investigation. (12) Mikami, R.; Taguchi, M.; Yamada, K.; Suzuki, K.; Sato, O.; Einaga, Y. Angew. Chem., Int. Ed. 2004, 43, 6135. (13) Einagaa, Y.; Gub, Z.-Z.; Hayamib, S.; Fujishimaa, A.; Sato, O. Thin Solid Films 2000, 374, 109. (14) Einaga, Y.; Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. J. Am. Chem. Soc. 1999, 121, 3745. (15) (a) Manna, A.; Chen, P.-l.; Akiyama, H.; Wei, T.-X.; Tamada, K.; Knoll, W. Chem. Mater. 2003, 15, 20. (b) Zhang, J.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2001, 13, 2323. (c) Hu, J. H.; Liu, F.; Kittredge, K.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2001, 123, 1464. (d) Whitten, D. G.; Chen, L.; Geiger, H. C.; Perlstein, J.; Song X. J. Phys. Chem. B 1998, 102, 10098. (e) Song X.; Perlstein, J.; Whitten, D. G. J. Phys. Chem. A 1998, 102, 5440. (f) Song X.; Perlstein, J.; Farahat, M.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 9144. (g) Song, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7816.

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Experimental Section Chemicals. Chloroauric acid (HAuCl4), octadecylamine (ODA, C18H39N), dodadecylamine (DDA, C12H27N), benzyldimethylstearylammoniumchloride (BDSAC, C6H5CH2N[(CH2)17CH3](CH3)2Cl), and diaminoazobenzene were obtained from Aldrich Chemicals and used as received. Protected amino acids and HBTU were purchased from Novabiochem. In a typical experiment, aqueous gold nanoparticles were synthesized by reducing 10-4 M aqueous HAuCl4 solution using sodium borohydride as the reducing agent using the available procedures. The optical spectrum of the gold nanoparticles displays a single peak around 520 nm corresponding to the surface plasmon resonance peak (Figure S1 of the Supporting Information). Phase transfer of aqueous gold nanoparticles in chloroform was carried using three different phase transferring agents BDSAC, ODA, and DDA. A total of 100 mL of the aqueous gold nanoparticles solution was taken in three separate conical flasks. Then, in one conical flask, 100 mL of a 10-3 M solution of BDSAC in chloroform was added. To the second one was added 10 mL of a 10-2 M solution of ODA in chloroform, and to the third one was added 10 mL of a 10-2 M solution of DDA in chloroform. In each case, immiscible layers of the red colored gold hydrosol on top of the colorless organic solution was observed. The three conical flasks were then subjected to vigorous stirring. This process resulted in the transfer of the gold nanoparticles into chloroform as evidenced by the red coloration of the organic phase (and a corresponding loss of color from the aqueous phase). The organic layer from each of the conical flasks was separated and was used for further experiments. The phase-transferred gold was subjected to optical measurements and transmission electron microscopy (TEM) analysis. The optical measurements of the organic phases clearly indicated that there are no major changes in the UV-vis spectra between these three cases (Figure S1 of the Supporting Information). Synthesis of Linker Molecule with Azo Moiety. The synthesis of the linker molecule (Scheme 1A, 1) was accomplished following standard protocols used for peptide synthesis (Supporting Information, S1). This molecule was fully characterized by various spectroscopic techniques including 1H NMR and 13C NMR spectra. Formation of Nanoparticle Networks and Photoresponsive Measurements. To 10 mL of solution of Au nanopaticles in chloroform (phase transferred using BDSAC) was added 10-5 M solution of molecule 1 (Scheme 1A) in chloroform. The mixture was irradiated by UV light for 30 min (Pyrex filter, >280 nm, 450-W Hanovia medium-pressure lamp). UV-vis absorbance spectra were recorded before and after irradiation. Samples of the mixture before and after irradiation were prepared for TEM measurements. The mixture was then illuminated by a normal incandescent lamp for 30 min. The sample’s UV absorbance spectra was recorded, and the sample was prepared for TEM. The same procedure was repeated for ODA and DDA capped Au nanoparticles. We mention here that the temperatures of the nanoparticles dispersions were maintained at room temperature during their exposure to UV and visible lights by keeping them in water baths or by circulating cool water around the tubes. Measurement Techniques. UV-Vis Spectroscopic Studies. The optical properties of the different gold nanoparticle solutions were measured using a Jasco UV-Vis spectrophotometer (V570 UV-Vis-NIR) operated at a resolution of 2 nm. Transmission Electron Microscopy Measurements. TEM measurements were performed on a JEOL model 1200EX instrument operated at an accelerating voltage at 120 kV. Each sample was prepared by placing a drop of the nanoparticle solution on a carbon-coated copper grid. The films on the TEM grids were allowed to dry in air prior to measurement.

Results and Discussion We start our discussions by taking the case of BDSAC capped nanoparticles interconnected by linker molecule 1 (Scheme 1A). The whole experimental procedure to investigate the photoresponsiveness of the system can be divided into four steps. Step 1 is the phase transfer of gold

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Figure 1. UV-visible spectra of BDSAC capped nanoparticles at various stages. Curve 1: spectra of phase transferred nanoparticles. Curve 2: Immediately after the formation of network by the addition of linker molecule to BDSAC capped nanoparticles. Curve 3: Spectra recorded after irradiating the network with UV light. Curve 4: Spectra recorded after the network has been irradiated with visible light.

nanoparticles into the organic medium by BDSAC and their UV-visible spectrum displays a sharp peak centered around 520 nm indicative of the presence of individual and separate nanosized gold particles in the solution (curve 1, Figure 1). In step 2, molecule 1 is added to the BDSAC capped gold nanoparticles and the UV-vis absorption spectrum obtained from this solution is shown in curve 2, Figure 1. An appreciable red shift in the surface plasmon peak to ∼550 nm occurs along with a peak at 365 nm attributed to the azobenzene moiety in the trans conformation.15b Curve 3 in Figure 1 corresponds to the UV-vis absorption spectrum of the gold nanoparticle solution after step 3, where the above solution has been irradiated with UV light for about 30 min. A clear and observable shift in the surface plasmon resonance peak now to 640 nm is seen along with the development of a small peak around 320 nm which is attributed to the cis conformer of the azobenzene moiety.15b In step 4, the solution was subsequently irradiated with visible light, and the UV-vis spectrum of the resulting solution is shown in curve 4 of Figure 1. Irradiating this sample with visible light clearly brings the optical absorption of the sample back to 560 nm very close to the value we get from the gold nanoparticles networks as in step 2. Notably, The peak at 320 nm ascribed to the cis conformer is also absent in this spectrum suggesting that all the cis conformers have been reverted back to the trans conformer. The TEM images obtained after each step described above are well in accordance with the observations of UV-vis spectra. The phase transferred and BDSAC capped nanoparticles are well separated and isolated from each other (Figure 2A). Immediately after the addition of the linker molecule, the nanoparticles are brought together and form a network (Figure 2B). The gold nanoparticles in the network get even closer when the solution is subjected to UV irradiation (Figure 2C), and the closepacked structure opens up subsequently by simple irradiation with visible light (Figure 2D). ODA capped nanoparticles more or less follow the same course as described above; namely, the phase-transferred nanoparticles show a single absorbance peak at 520 nm (curve 1, Figure 3A), which undergoes a red shift to 530 nm after the addition of the compound 1 (curve 2, Figure 3A). This peak is further red shifted to ∼550 nm upon irradiation with UV light (curve 3, Figure 3A) that subsequently can be reversed back to ∼540 nm by exposing it to visible light (curve 4, Figure 3A). The TEM images obtained after each step also are as expected. Here, the initial phase transferred particles are well separated from

Figure 2. TEM images of BDSAC capped nanoparticles (A) after phase transferring them to chloroform, (B) immediately after the addition of linker molecule, (C) after irradiating the network with UV light, and (D) after the network has been irradiated with visible light.

Figure 3. UV-visible spectra of (A) octadecylamine capped nanoparticles at various stages. Curve 1: spectra of phase transferred nanoparticles. Curve 2: Immediately after the formation of network by the addition of linker molecule. Curve 3: Spectra recorded after irradiating the network with UV light. Curve 4: Spectra recorded after the network has been irradiated with visible light. (B) Dodecylamine capped nanoparticles at various stages. Curve 1: spectra of phase transferred nanoparticles. Curve 2: Immediately after the formation of network by the addition of linker molecule. Curve 3: Spectra recorded after irradiating the network with UV light. Curve 4: Spectra recorded after the network has been irradiated with visible light.

each other (Figure 4A) that show some sort of networking once the linker molecule is added (Figure 4B). The particles come further closer once the network is UV irradiated (Figure 4C) that can be reversed back by visible light irradiation (Figure 4D). Here again, it could be seen that a feature around 320 nm develops once the nanoparticles networked with azobenzene linkers are exposed to UV light, which is otherwise absent. However, the extent to which we observe the photoresponsiveness in this case is not as great as that in the BDSAC case. DDA capped nanoparticles behave very differently from the above two scenarios. First they do not show any difference in the optical spectra even though these were

Photoresponsive Gold Nanoparticle Networks

Figure 4. TEM images of octadecylamine capped nanoparticles (A) after phase transferring them to chloroform, (B) immediately after the addition of linker molecule, (C) after irradiating the network with UV light, and (D) after the network has been irradiated with visible light.

Figure 5. TEM images of dodecylamine capped nanoparticles (A) after phase transferring them to chloroform, (B) immediately after the addition of linker molecule C) after irradiating the network with UV light, and (D) after the network has been irradiated with visible light.

subjected to the same kind of UV and visible light exposure as was done to BDSAC capped and ODA capped ones. Here again the phase transferred nanoparticles with DDA show a single peak around 520 nm. The peak moves to 535 nm when the linker molecule is added, and retains its position without any further shifts even after UV irradiation and subsequent visible irradiation were carried out exactly in the same manner as they were carried out in the BDSAC and ODA capped cases (curves 1-4, Figure 3B). The TEM images are very similar to each other, and no distinguishable changes in the interparticle spacing could be observed (Figure 5A-D). One point worth noting here is that we do see the peak from the azobenzene linker at 360 nm after it is added to the gold nanoparticles indicating the incorporation of this linker molecule into the networks. Then, the fact no peak associated to the cis

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conformer develops even after UV irradiation suggests that this molecule is now spatially restricted and cannot undergo the trans-cis isomerization as it was happening in the other two cases. This interesting situation is further discussed below. In all of the three cases the gold nanoparticles phase transferred into an organic medium have an average size of 10 nm and are well separated in solution as evidenced by the optical absorption spectra (Figure 1A, 3A, and 3B, curve 1) and the TEM images (Figures 2A, 4A, and 5A). The subsequent steps in our photoresponsive studies for the BDSAC and ODA capped systems can be summarized as represented in Scheme 1B. As illustrated here, each molecule 1 can interconnect two gold nanoparticles leading to a network described as breathe-out, in which the interconnecting molecules are in the trans form (Figures 2B and 4B). The formation of this network itself is seen to induce a red shift in the surface plasmon resonance peak. The surface plasmon peak in gold nanoparticles is highly sensitive to the dielectric constant of the surrounding medium and the interparticle distances.2 When the interparticle distance is reduced there is a change in the local dielectric constant surrounding the nanoparticles, which causes a red shift in the surface plasmon resonance peak. Then, by irradiating this with UV light, the trans to cis isomerization can be effected causing a further reduction in the interparticle spacing (breathe-in type in Scheme 1B and Figure 2C and 4C) and hence a further red shift in the surface plasmon peak (curve 3, Figures 1A and 3A). Subsequently the linker molecules can be reverted back to the trans form bringing back the network to the breathe-out type by irradiating it with visible light (Figures 2D and 4D). This results in a concomitant blue shift in the surface plasmon peak of the nanoparticles (curve 4, Figures 1 and 3A). The development of a small feature around 320 nm (Figures 1 and 3A curve 3) only when the networks are exposed to UV light15b clearly support the contention that it is the trans-cis-trans isomerization of the linker molecules that is bringing about this breathe-out to breathe-in type changes in the network. One feature worth mentioning here is that in both the BDSAC capped and ODA capped cases the photoresponsiveness could be recycled few times, typically 3-4 cycles with each UV exposure resulting in the red shift followed by the blue shift on visible light exposure. However, we also wish to mention here that each cycle is accompanied by an appreciable loss in the intensity of the surface plasmon peak restricting the breathe-out to breathe-in type transitions to only few cycles. This could be due to the irreversible aggregation of nanoparticles that come closer and the simultaneous loss of surface capping and interconnecting molecules. This hypothesis is also supported by the TEM images observed in the BDSAC and ODA capped cases (Figures 2D and 4D) where coalescence of few nanoparticles is seen after they are exposed to the visible light. As mentioned previously, the photoresponsiveness is seen to the maximum extent in the BDSAC capped nanoparticle networks. BDSAC is the longest capping molecule used in this study (∼2.35 nm from the N+ site to the CH3 end) and also the bulkiest. Therefore, these nanoparticles capped with BDSAC exist as separate and isolated in solution. Once the linker molecule (end to end distance in the trans conformer ∼3.03 nm) is added, the nanoparticles come together forming a close-knit network as each linker molecule gets attached to two gold nanoparticles through the cysteine groups present at either end of the molecule. When this network is exposed to the UV light, the trans to cis isomerization of the azobenzene

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moiety takes place, shrinking the end to end distance of this molecule to ∼2.0 nm. This brings the nanoparticles, attached at the either end of this molecule, further closer, which seems to be more facilitated in the BDSAC capped case. Tetra alkylammonium groups are known to weakly attach to the gold surface that can get removed from the surface or exchanged by other ligands very easily. Weakly attached molecules such as BDSAC when attached to the gold surfaces may not form well-ordered structures as is known to happen in the case of ligands such as amines and thiols. Therefore, it is also possible that BDSAC molecules lie flat on the gold surfaces providing the space necessary for two nanoparticles to come closer. Thus, the extra space provided by the BDSAC molecules along with the convenience with which they get detached allow the nanoparticles to come together and go apart conveniently paving the way for a smooth transition for the breatheout to the breathe-in transition in this network (Scheme 1B). Alkylamines are expected to be reasonably good attaching groups on gold surfaces.16 Among the two alkylamine molecules we used to phase transfer gold nanoparticles into organic medium, ODA is longer (∼2.25 nm from N terminus to CH3). This amounts to ∼4.5 nm space when two nanoparticles capped with ODA ligands come together with the ligand ends on the two particles touching each other. Once the linker molecule is attached, the ODA ligands on different nanoparticle surfaces can get interdigitated allowing the nanoparticles to come together.17 If nanoparticles were to come even closer as the linker undergoes the trans-cis isomerization, more interdigitation should occur. Alkylamine molecules are not known to facilitate interdigitation as well as thiol ligands.16b This is probably the reason we see weaker shifts in the surface plasmon resonance peak in the ODA capped nanoparticles case. DDA is much smaller than BDSAC and ODA (∼1.50 nm from N terminus to CH3). Therefore, there is just enough space for the linker molecule to attach to two nanoparticles surfaces capped by DDA ligands. Now, if two nanoparticles each passivated by DDA were to come together assisted by the trans-cis isomerization of the linker molecule the DDA ligands on adjacent nanoparticles have to interdigitate fully which in the case of amine ligands seems to be impeded.16b This probably precludes the trans-cis isomerization of the linker molecule in this case leading to no shifts in the surface plasmon peaks. A very interesting feature here is that we do see the peak corresponding to the trans isomer of the azobenzene molecule in the network formed with DDA capped nanoparticles. We can easily rule out the possibility of these molecules existing separately in solution for in this scenario they can easily undergo the trans-cis isomerization. However, the fact that despite exposing this nanoparticle suspension to UV light we do not see any peak corresponding to the trans isomer gives credence to the contention that the linker molecule is now spatially very restricted to undergo any trans-cis isomerization. It could be argued that several other factors such as size and shape variation and thermal effects can lead to (16) (a) Kumar, A.; Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277. (b) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935. (c) Brown, L. O.; Hutchison, J. E. J. Phys. Chem. B 2001, 105, 8911. (17) (a) Luedtke, W. D.; Landman, U. J. Phys. Chem. 1996, 100, 13323. (b) Luedtke, W. D.; Landman, U. J. Phys. Chem. B 1998, 102, 6566.

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the observed shifts in the UV-vis spectra.2 We would like to mention however that these effects will be similar to all of the three cases, and therefore, we should be seeing similar trends irrespective of the surface capping at least in case of primary amine bearing molecules such as ODA and DDA, even if we accept that the nature of binding is different for these molecules and a tertiary amine bearing molecule like BDSAC. We also would like to mention here that phase transfer of gold nanoparticles accomplished by these three molecules does not seem to bring major changes in the UV-vis spectra suggesting that it is more than just the surface modification that brings about the changes in UV-vis spectra as observed (Figure S1 of the Supporting Information). Though particular precautions were taken to minimize thermal effects during the exposure to UV and visible lights, we cannot completely rule out their existence and that they may have caused the observed peak shifts. Nevertheless, these can also be safely disregarded since if thermal effects are the reason the peaks should be shifting in only one direction by both UV and visible light irradiation as thermal effects would be same for both these exposures. Another feature that the average size remains the same as measured after each step in the DDA case (in other case as the particles formed very close networks it was very difficult to obtain the particle size distributions) again shows that either the phase transfer or the addition of linker molecule do not impart major changes to the size and shape of nanoparticles (Figure S5 in the Supporting Information). Based on all of the above facts and the mere fact that we are observing a reversible peak shift, our contention that the photo switchable molecular linkers are playing a pivotal role in the reversible photo responsiveness of these networks is unambiguously supported. Conclusion Photoresponsive networks of BDSAC and ODA capped gold nanoparticles were prepared by linking them with molecules containing an azobenzene moiety. Then, by reversibly bringing out the trans-cis-trans isomerization of this group, the gold nanoparticles in the networks can be brought closer to each other or can be driven apart. These effects can be easily followed by the red shift (close assembly of particles) and the blue shift (loose assembly of the gold nanoaprticles) of the surface plasmon peak position of the nanoparticle network. It is also shown that the facility with which isomerization can be effected depends on the initial capping ligand; the size of the capping ligand can either promote or hinder this isomerization based on the space available and the ease with which it can detached and reattached. Acknowledgment. B.L.V.P. thanks the DST for funding through the fast-track scheme for young scientists. DS and S.K. acknowledge a Junior Research fellowship from CSIR, Govt. of India. The TEM analyses were carried out at the Center for Materials Characterization (CMC), NCL Pune and are gratefully acknowledged. Supporting Information Available: Optical spectra of gold nanoparticles dispersion in aqueous media as well after phase transferring them to organic media using BDSAC, ODA and DDA. More representative TEM images to highlight the photo responsiveness in all the three cases. Synthetic and characterization details of linker molecule 1. This material is available free of charge via the Internet at http://pubs.acs.org. LA051125Q