J. Phys. Chem. C 2010, 114, 799–802
799
Highly Sensitive Emission Sensor Based on Surface Plasmon Enhanced Energy Transfer between Gold Nanoclusters and Silver Nanoparticles Chih-Wei Chen, Chun-Hsiung Wang, Chih-Ming Wei, Chun-Yi Hsieh, Yung-Ting Chen, Yang-Fang Chen,* Chih-Wei Lai, Chien-Liang Liu, Cheng-Chih Hsieh, and Pi-Tai Chou Departments of Physics and Chemistry, National Taiwan UniVersity, No. 1, Sect. 4, RooseVelt Rd., Da-an District, Taipei 106, Taiwan ReceiVed: August 31, 2009; ReVised Manuscript ReceiVed: December 4, 2009
In the study of interaction between emissive gold nanoclusters and silver nanoparticles, we explored that the emission of gold nanoclusters is very sensitive to the presence of silver nanoparticles. Quite remarkably, the quenching ratio can reach more than several hundred times. We demonstrate that the underlying mechanism can be attributed to the surface energy transfer with the quenching efficiency following the expression χ ) 1/[1 + (d/d0)4], where d is the distance between gold nanoclusters and silver nanoparticles, and d0 is the characteristic length of energy transfer. This highly sensitive behavior in the composite consisting of relatively nontoxic gold nanoclusters and silver nanoparticles may find a powerful potential in developing biomedical applications, such as biosensors and drugs delivery. Introduction Noble metal nanoparticles (NPs) such as gold and silver with sizes below 2 nm, which is commonly termed as nanoclusters (NCs) have been attracting great attention. Relevant research is ubiquitously seen, ranging from fundamental properties such as photoluminescence (PL),1–5 and optical chirality6–8 to potential applications such as sensing9,10 and single molecule optoelectronics.11,12 The high electron density, strong electronelectron coupling, and efficient screening make change of the electronic structure of noble metals that depend strongly on size and geometry of their nanoclusters. Theoretical work indicates that PL of noble metals originates from the electronic transitions from the occupied d bands into the states above the Fermi level.13 The photoexcited electrons from an occupied sp band then recombine with holes and photons are emitted under the constraint of conservation of energy and momentum. It is also known that the metal NCs, with sizes comparable to the Fermi wavelength of electrons (∼0.7 nm),14 possess molecule-like properties including discrete size-dependent electronic states and fluorescence.15 Desipite that PL of the noble metals is a wellknown phenomenon, so far, studies devoted to the fluorescence of metal NCs are scant.16 Moreover, in view of bioapplication, unlike commonly used CdSe/ZnS or CdTe/ZnS etc., semiconductor quantum dots with high cytotoxicity, relatively much less toxic metal (e.g., Au) NCs make them potential candidates in labels for biologically motivated experiments.17 In yet another approach, it has been known that the enhanced local electrical field resulting from the surface plasmons of metal NPs can increase the absorption cross section and radiative recombination rate of fluorophores.18,19 Such a local electrical field can also increase the strength of donor-acceptor interaction and hence enhance the efficiency of Fo¨rster resonance energy transfer (FRET).20,21 Due to the large surface-to-volume ratio, the nanometal surface follows the expression given by the surface energy transfer (SET) * To whom correspondence should be addressed at the Department of Physics. Fax: +886-2-23639984. Phone: +886-2-33665125. E-mail:
[email protected] instead of the usual dipole-dipole type of FRET.22 The length scale for the detection of the FRET-based process is typically in the order of
