Ultrasmall Gold Cluster Arrays Encapsulated in Silica Nanospheres

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Ultrasmall Gold Cluster Arrays Encapsulated in Silica Nanospheres: Applications in Fluorescence Imaging and Catalysis Anupam Samanta,† Basab B. Dhar,‡ and R. Nandini Devi*,† † ‡

Catalysis and Inorganic Chemistry Division, National Chemical Laboratory, Pune 411008, India Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411008, India

bS Supporting Information ABSTRACT: Facile synthesis of ultrasmall gold nanoclusters of size 2 nm, surface plasmon resonance bands appear in the range of 500 1000 nm progressively red shifting as core size increases. In our system, we observed an exponential decay from the UV region into the visible region but characterized by a definite absence of surface plasmon resonance band indicative of polydisperse ultrasmall clusters (Figure 2). Absence of surface plasmon bands signals quantum confinement effects, characteristic of clusters of size 2 nm do not show fluorescence emission. Fluorescence spectrum (Figure 2) of our cluster system shows a broad emission band peaking in the NIR region at ∼720 nm while excited at 460 nm thus confirming the presence of ultrasmall gold clusters. Fluorophores emitting in NIR region are known to be advantageous for imaging of biological systems due to diminished scattering by such systems in that wavelength range.29,30 The quantum efficiency of the clusters was calculated to be 5.37 10 4 using Rhodamine 6G (QY 0.95) as a standard at 488-nm excitation. These values are comparable to reported monolayer protected Authiol clusters.31,32 These microscopy and spectroscopic studies have proved that the Au thiol clusters synthesized employing the novel ligand are quantum clusters exhibiting unique properties. Stabilization and encapsulation of such clusters in inert oxides like silica is 1750

dx.doi.org/10.1021/jp2093737 |J. Phys. Chem. C 2012, 116, 1748–1754

The Journal of Physical Chemistry C

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Figure 4. UV vis spectrum (black) and photoluminescence spectrum at excitation wavelength of 460 nm (blue) of aqueous solution of silicaencapsulated Au cluster. The spike at 630 nm is an artifact from the quartz cell.

Figure 3. TEM image of silica-encapsulated Au clusters showing the clusters of average size 1.5 nm individually covered inside the SiO2. White arrows point to examples of the clusters.

envisaged to make these inherently unstable and reactive systems more robust toward functionalization and device fabrication. However, the limitation for developing such a step lies in the same drawbacks since ligands, solvent systems, and coating conditions affect the stability and may lead to agglomeration. It is imperative that the silica coating techniques employed do not compromise the structural integrity of the clusters. Hence we proceeded to use these clusters as precursors and encapsulated them with silica using a simple hydrolysis method in a water alcohol mixture (1:4). The method involves an initial direction of TEOS to the hydrophobic region and a slow hydrolysis under basic conditions leading to the formation of silicate precursors which condense on further stirring to form the spheres. TEM analysis showed the presence of reasonably monodisperse silica spheres of diameter 25 30 nm. On magnification, it becomes clear that Au clusters of the precursor solution are encapsulated within the silica spheres (Figure 3). Interestingly, the average particle size remained intact at 1.5 nm (see Figure S4 of Supporting Information). The clusters seem to be arranged sufficiently space separated from each other and not agglomerated to form berrylike structures.19 An interesting pattern which is discernible from TEM is that the silica encompasses individual clusters and not aggregates in a core shell model.20 The hydrophobic propyl groups of the ammonium may have helped in directing TEOS to individual clusters preventing the formation of core shell architectures. This is advantageous in enhancing the stability at harsher conditions preventing severe agglomeration within the core. Moreover, the size of silica spheres could be controlled at a much lower range than other reported materials.19,20 This makes the composite even more advantageous by deploying the clusters very close to the surface and not buried deep inside thick silica layers. The ratio of Au:SiO2 was found by elemental analysis to be 1:50 amounting to a final composition of 6 wt % Au in silica. We believe that the structure of the ligand balancing the charge and hydrophobicity is responsible for the ease of silica encapsulation

Figure 5. Fluorescence images of (top) pristine Au thiol clusters for a scan area of 212 μm 212 μm and (bottom) silica-encapsulated clusters for a scan area of 140 μm 140 μm at an excitation wavelength of 488 nm. Signal intensity is color coded (inset).

in this case. The question which arises at this point is regarding the extent of encapsulation by SiO2; whether all the Au is present 1751



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Figure 6. TEM images of silica-encapsulated nanoreactors after 250 °C. Magnified image (right) shows that >90% of the particles are of size ∼3 nm. Inset shows the pore like features in silica.

inside the spheres. To address this question, we carried out XPS and energy-dispersive X-ray analysis (EDAX); both surfacesensitive techniques with probe depth limitations. Scanning electron microscopy (SEM)-EDAX analysis showed the presence of Au and an EDAX analysis with depth profiling using Focused ion beam milling indicated a discernible but not substantial increase in Au content after etching (see Figure S5 of Supporting Information). However, this may not be conclusive since the X-ray probe depth is more than the nanometer scale of the material under consideration. XPS could be more effective in understanding these structures of encapsulated materials, since the probe depth is in nm length scale. In our case, no surface concentration of Au was detected proving that the Au clusters are completely encapsulated within the silica spheres (see Figure S6 of Supporting Information). Since the most interesting and unique properties of these systems are related to their optical characteristics, any process carried out for stabilization should not lead to deterioration of these properties. Further experiments to probe the optical properties of the silica-encapsulated clusters provided evidence that the silica coating technique used by us has led to a system in which these properties are intact (Figure 4), and this could be achieved without effecting any agglomeration. The absorption bands in the UV vis absorption spectrum of the silica coated clusters did not show any change from the pristine clusters. Fluorescence emission also was found to be intact after silica coating. The apparent reduction in fluorescent intensity when compared to pristine clusters is due to reduced concentration of Au clusters in the silica coated sample solution. To confirm the absence of quenching, fluorescence emission of the coated clusters was followed with progressing duration of stirring with TEOS. The resulting spectra revealed that no quenching mechanisms are at play (see Figure S7 of Supporting Information). This observation also gives credence to the possibility of using these materials in fluorescence imaging of biological systems. Fluorescence images of drop cast films of pristine clusters as well as the silica coated clusters are shown in Figure 5. This shows that the clusters are fluorescent even in solid state increasing its potential for imaging applications many fold. Very interestingly, silica-encapsulated clusters also presented bright fluorescence images. Silica encapsulation increases stability, widening the range of test matrices in which this material can

be used. Moreover, silica encapsulation renders the clusters more amenable for further functionalization and biocompatibility. As mentioned earlier, structural characteristics of the silicaencapsulated arrays whereby individual Au clusters are protected by silica layers, is conducive for minimizing agglomeration by coalescence at higher temperatures, when compared to core shell structures of multiple clusters. This property can be effectively exploited in catalysis since stability in terms of particle size under harsh conditions is of paramount interest in this application. However, for encapsulated Au systems to be used efficiently in catalysis, the active nanoparticles embedded inside the matrix need to be accessible for reactant molecules. Hence the silica coating should be porous, and this has been achieved recently by postsynthetic silica etching by base.33 However, this has so far been reported in case of bigger nanoparticles of size >10 nm. We have used the silica-encapsulated Au clusters as precursors to synthesize Au nanoreactors within porous silica in a simple one step calcination at 250 °C to remove the thiol ligands. HRTEM studies reveal that, as expected, agglomeration is highly controlled, and on magnification, it becomes clear that >90% of the particles are of size 2 nm particles (Figure S8 of the Supporting Information). Another interesting feature which is visible in TEM is the porosity of the silica surrounding the nanoparticle. It is evident that the abundant ligand molecules within the silica spheres help in creating porosity through which molecules can diffuse and access the active surfaces. N2 adsorption studies also indicated the presence of microporosity (Figure S9 of Supporting Information). The pore size distribution curve calculated from the adsorption branch of the isotherm exhibited a maximum at 0.97 nm and the BET surface area of the sample calcined at 250 °C was estimated to be 550 m2 g 1, indicating the highly porous nature of the silica matrix. Catalytic activity of this material was tested for H2O2 sensing. Glucose oxidase is an enzyme responsible for oxidizing glucose thereby producing H2O2. The presence of H2O2 can be detected by its reaction with peroxidase substrate TMB, which is recently known to be catalyzed by Au nanoparticles acting as peroxidase mimetic.34 This is an ideal reaction for testing the accessibility of reactant molecules to the Au NPs encapsulated inside the silica spheres since the reaction proceeds via H2O2 adsorption on Au 1752



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while being able to physically manipulate them. We have also evidenced utilization of these materials as precursor for synthesizing encapsulated nanoreactors. The advantage of the presence of arrays of multiple clusters separately coated by silica is that at high temperatures, agglomeration is minimized. Moreover, abundance of ligand molecules ensures in situ formation of micropores during calcination. These observations emphasize that this system forms an ideal platform for multifunctional materials.

’ ASSOCIATED CONTENT

bS

Supporting Information. Extra TEM images, XPS traces, IR spectra, SEM-EDAX data, and N2 adsorption data. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Figure 7. Linear calibration plot of the absorbance at 650 nm against concentration of H2O2. The inset shows the dependence of the absorbance at 650 nm on the concentration of H2O2 in the range 10 μM to 10 mM.

surface and O O bond cleavage and subsequent electron transfer. Further oxidation of the peroxidase substrate TMB turns the reaction mixture blue and the absorbance at 650 nm corresponding to the oxidized intermediate of TMB can be followed easily by UV vis spectroscopy. The activity of the catalyst was first tested with appropriate concentrations of H2O2 and TMB to observe the blue color. Calibration studies were carried out with various concentrations of H2O2 to estimate the potential of using this material in H2O2 detection (Figure 7). It was found that concentrations as low as 10 μM could be detected, which is comparable to other Au-based systems reported.34 The ability of sensing H2O2 has further implications in glucose detection and estimation. The activity of this catalyst points to the advantages of using the silica-encapsulated clusters as precursors. To test this hypothesis, a core shell-based Au catalyst with a thick shell of silica-encapsulating multiple nanoparticles was prepared (TEM image given in Figure S10 of the Supporting Information) and catalytic activity for TMB oxidation in presence of H2O2 was studied. This was found to be inactive showing that arrays of Au clusters within silica as in our case rather than core shell architectures form better precursors for fabricating metal nanoreactors. In this case also, further functionalization of the silica with moieties which enhance the catalytic activity can be envisaged.

’ CONCLUSIONS A synergistic effect of the novel ligand combining hydrophobicity, and charge has enabled us to achieve the synthesis of water-dispersible ultrasmall gold ligands with narrow size distribution. We have developed a simple method for surface silica coating of these nanoclusters whereby structural and optical properties of the clusters remain intact. The fluorescence property of the pristine clusters is found to be stable in solid state as well as after silica coating. Stabilization of the clusters in silica also makes it possible for surface modifications and further functionalization of silica, which can enhance biocompatibility and versatility of these materials. Such systems can open up new avenues for exploiting the advantages of Au quantum clusters

Corresponding Author

*E-mail: [email protected]. Phone: +912025902271. Fax: +912025902633.

’ ACKNOWLEDGMENT A.S. and B.D. acknowledge research fellowship from CSIR, India. The authors thank Dr. B. L. V. Prasad and Dr. Sayam Sen Gupta, National Chemical Laboratory, Pune, India, and Dr. Chithra Manikandan, Dow Chemicals, Pune, India, for useful discussions and Mr. Anal. Kr. Ganai and E. Venugopal, NCL, for assistance with UV and fluorescence experiments. ’ REFERENCES (1) Zhu, M.; Qian, H.; Jin, R. J. Am. Chem. Soc. 2009, 131, 7220. (2) Bao, Y.; Yeh, H.-C.; Zhong, C.; Ivanov, S. A.; Sharma, J. K.; Neidig, M. L.; Vu, D. M.; Shreve, A. P.; Dyer, R. B.; Werner, J. H.; Martinez, J. S. J. Phys. Chem. C. 2010, 114, 15879. (3) Zhu, M.; Aikens, C. M.; Hendrich, M. P.; Gupta, R.; Qian, H.; Schatz, G. C.; Jin, R. J. Am. Chem. Soc. 2009, 131, 2490. (4) Qian, H.; Jin, R. Nano Lett. 2009, 9, 4083. (5) Zhang, Q.; Xie, J.; Yu, Y.; Lee, J. Y. Nanoscale 2010, 2, 1962. (6) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (7) Jin, R. Nanoscale 2010, 2, 343. (8) Liu, S.; Han, M.-Y. Chem. Asian J. 2010, 5, 36. (9) Guerrero-Martinez, A.; Perez-Juste, J.; Liz-Marzan, L. M. Adv. Mater. 2010, 22, 1182. (10) Liu, S.; Han, M. Adv. Funct. Mater. 2005, 15, 961. (11) Joo, S. H.; Park, J. Y.; Tsung, C.-K.; Yamada, Y.; Yang, P.; Somorjai, G. A. Nat. Mater. 2009, 8, 126. (12) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (13) Pastoriza-Santos, I.; Perez-Juste, J.; Liz-Marzan, L. M. Chem. Mater. 2006, 18, 2465. (14) Obare, S. O.; Jana, N. R.; Murphy, C. J. Nano Lett. 2001, 1, 601. (15) Botella, P.; Corma, A.; Navarro, M. T. J. Mater Chem. 2009, 19, 3168. (16) Wang, H.; Schaefer, K.; Moeller, M. J. Phys. Chem. C. 2008, 112, 3175. (17) Han, Y.; Jiang, J.; Lee., S. S.; Ying, J. Y. Langmuir 2008, 24, 5842. (18) Wu, S.-H.; Tseng, C.-T.; Lin, Y.-S.; Lin, C.-H.; Hung, Y.; Mou, C.-Y. J. Mater. Chem. 2011, 21, 789. (19) Habeeb Muhammed, M. A.; Pradeep, T. Small 2011, 7, 204. 1753



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Ultrasmall Gold Cluster Arrays Encapsulated in Silica Nanospheres

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