Distance-Dependent Metal-Enhanced Intrinsic Fluorescence of Proteins Using Polyelectrolyte Layer-by-Layer Assembly and Aluminum Nanoparticles

• Published in: Journal of physical chemistry. C 2012-05, Vol.116 (19), p.10766-10773
• Main Authors: Akbay, Nuriye, Lakowicz, Joseph R, Ray, Krishanu
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp2122714

Previously reported studies indicate that aluminum nanostructured substrates can potentially find widespread use in metal-enhanced fluorescence (MEF) applications particularly in the UV or near-UV spectral region toward label-free detection of biomolecules. MEF largely depends on several factors, such as chemical nature, size, shape of the nanostructure and its distance from the fluorophore. A detailed understanding of the MEF and its distance-dependence are important for its potential application in biomedical sensing. Our goal is to utilize intrinsic protein fluorescence for label-free binding assays. This is made possible by the use of metallic nanostructures which provide localized excitation and enhanced fluorescence of UV fluorophores and will also provide a way to separate the surface-bound proteins from the bulk samples. We evaluated varied probe distances from plasmonic nanostructures by the well-established layer-by-layer (LbL) technique. The investigated proteins were adsorbed on different numbers of alternate layers of poly­(styrene sulfonate) (PSS) and poly­(allylamine hydrochloride) (PAH). Bovine serum albumin (BSA) was electrostatically attached to the positively charged PAH layer, and goat and rabbit IgG were attached to negatively charged PSS layer. We obtained a maximum of a ∼ 9 fold increase in fluorescence intensity from BSA at a distance of ∼9 nm from the Al nanostructured surface. Approximately 6- and 7- fold increases were observed from goat and rabbit IgG at a distance of ∼8 nm, respectively. The minimum lifetimes were about 3-fold shorter than those on bare control quartz slides for all three proteins. The time-resolved intensity decays were analyzed with a lifetime distribution model to understand the distance effect on the metal–fluorophore interaction in detail. The present study indicates the distance dependence nature of metal-enhanced intrinsic fluorescence of proteins and potential of LbL assembly to control the metal-to-fluorophore distance in the UV wavelength region.