Self-assembly Gold Nanoislands for Localized Surface Plasmon Resonance Biosensing
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Self-assembly Gold Nanoislands for Localized Surface Plasmon Resonance Biosensing Chi-Man Lawrence Wu and Siu-Pang Ng Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, P. R. China ABSTRACT Localized surface plasmon resonance (LSPR) is a label-free biosensing technique employing plasmonic nanostructures to detect local refractive index change induced by biomolecules in the vicinity of these nanostructures. In analogy to surface plasmon resonance (SPR) sensor in a cuvette, LSPR is resistant to bulk refractive index fluctuation yet remains comparably sensitive for biosensing purpose. LSPR has the advantage over SPR in that the overall system size is smaller, and not affected by normal temperature fluctuations during measurement. However, mass production of a cheap but effective LSPR substrate remains challenging. In this paper, a self-assembly gold nanoisland structure was synthesized on transparent glass substrate by a simple two-step deposition-growth process. The first step involved depositing an ultra thin film of gold with nominal thickness of 5 nm by thermal evaporation at 1× 10-7 torr. Then the gold coated substrate was placed into a high temperature oven and annealed at 450°C for 10 hours. By first observation, the annealed substrate turned from pale green to dark pink. Upon scanning with atomic force microscopy, it was revealed that nanoislands of about 100 nm to 150 nm wide with average height of 60 nm were formed. Optical extinction measurements showed that the absorption peak was about 560 nm with fullwidth-half-maximum of 100 nm, so dark pink color was observed. For the biosensing demonstration, Bovine serum albumin (BSA) and Anti-BSA bio-affinity interaction was measured using the self-assembly gold nanoisland LSPR sensor. Anti-BSA was functionalized onto the sensing site and BSA of known concentrations, i.e. 1 ug/ml was injected. The results showed LSPR spectral intensity change of 650 counts at the resonance slope of 634 nm. With standard deviation of spectral intensity fluctuation at 7 counts, the detection limit of BSA was estimated at about 0.5 nM which was comparable with that of LSPR systems with more elaborate nanostructures. The limit of detection of the present system can be further improved by implementing phase measurement and further nanostructure improvement. INTRODUCTION The key element of all LSPR biosensors is the plasmonic nanostructure which supports the extremely intense and highly confined electromagnetic field resonance induced by photonnanostructure interaction [1]. The resulting collective electron gas oscillation is highly sensitive to the dielectric properties of the immediate surrounding medium. Any change of the dielectric property due to biomolecular interaction, i.e. antigen-antibody adhesion, alters the resonance condition thus the optical signal can be detected in the far field. The fabrication of these plasmonic nanostrucutres is generally divided into two approaches [2], 1) top-down via clean-room technique such as lithography and 2
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