Metal-enhanced fluorescence of lanthanide chelates near silver nanostructured films

Abstract

Metal-enhanced fluorescence (MEF) from lanthanide chelates in close proximity to silver films was studied. Different thicknesses of silver films (20 and 50 nm) were deposited onto quartz substrates using magnetron sputtering, while silver island films were prepared using a previously published procedure. Obviously enhanced emissions from the lanthanide chelates were observed on the silver films. The effect of using different concentrations of aqueous solutions polyvinyl alcohol (PVA) on MEF was also investigated. Fluorescence close to a 20-nm-thick silver film showed that the enhancement of emission is a function of the thickness of the PVA film, reaching a maximum (~2.5-fold increase in intensity) at 83 nm. In conclusion, the MEF induced by the silver films is caused by an electric field effect arising from surface plasmon resonance. metal-enhanced fluorescence, surface plasmon resonance, silver films, lanthanide chelatesCitation: Yang G, Wang T, Wang Y H, et al. Metal-enhanced fluorescence of lanthanide chelates near silver nanostructured films. Fluorescence spectroscopy is widely used in the fields of medical diagnostics and biotechnology [1,2]. However, the detection of a fluorophore is usually limited by its quantum yield and photostability. It has been found that fluorescent emission can be affected by metal nanostructures and excitation intensity [3]. The radiative rates of fluorophores and the extent of resonance energy transfer can be enhanced by metal surfaces [4,5] in a process known as metal-enhanced fluorescence (MEF) [6]. To date, numerous applications involving MEF have been demonstrated, including increased detectability and photostability of fluorophores [7], improved DNA [8] and RNA [2] detection, microwave-triggered chemiluminescence [9], and metal-enhanced phosphorescence [10]. The interactions of MEF between metallic surfaces and particles are caused by at least two known mechanisms: (1) an electric field effect [6,11] and (2) a radiating plasmon model [1]. In the electric field effect, the incident electric field and the radiative rate are increased as a result of the metal increasing the local electric field through surface plasmon resonance (SPR). In the second mechanism, the excited fluorophores partially transfer the energy to the surface plasmons. The excited fluorophores can radiate together with the induced plasmon. Lanthanide chelates are well known fluorophores that are widely used in DNA hybridization assays [12] and high-sensitivity immunoassays [13]. Numerous novel lanthanide complexes have been synthesized by different methods [14][15][16]. It has been shown that the luminescence of lanthanide complexes can be enhanced by silver island films deposited on quartz or glass slides [17].In this paper, a detailed investigation of MEF from lanthanide chelates near continuous Ag thin films and noncontinuous Ag island surfaces is reported. The fluorescence emission of the lanthanide chelates was approximately 2.5 times more intense on a continuous silver thin film than on a quartz substr...