Metal-Enhanced Fluorescence Immunosensor Based on Plasmonic Arrays of Gold Nanoislands on an Etched Glass Substrate

The recent advances in nanofabrication processes offer encouraging opportunities for designing highly sensitive detection tools. One example is metal-enhanced fluorescence (MEF)-based biosensors: by effectively coupling the metal nanostructures with the fluorescent dye used for the detection of the target molecule, MEF-based sensors exhibit higher sensitivity and lower limit of detection in comparison to traditional optical biosensors. Ordered arrays of nanostructures with coupled fluorophores can potentially achieve thousand-fold enhancement in fluorescence intensity. However, nanofabrication techniques required for ordered nanostructures tend to be time consuming and costly. On the other hand, with moderate enhancement, randomly assembled nanoplasmonic arrays on large-scale substrates can be realized by easy, cost-effective, and reliable methodologies in a relatively short time, thus being more suitable for mass production. In this work, we develop a MEF-based immunosensor involving randomly assembled plasmonic arrays of gold nanoparticles fabricated by a simple and versatile three-step process to modulate the size, the interparticle distance, and the optical properties of the gold nanostructures to achieve the optimal fluorescence enhancement. Specifically, we have tested three different fluorescent dyes (Alexa Fluor 488, Alexa Fluor 546, and PE-Cy7) coupled with optimized gold nanostructures and achieved up to ≈170-fold enhancement in the fluorescence emission, significantly better than those achieved by metal nanostructures in solutions and by randomly assembled nanoplasmonic arrays. Our MEF immunosensor is then employed for detecting immunoglobulins in a model antigen–antibody system, achieving a limit of detection of 4.3 ng/mL. This value is lower than that of similar immunoglobulin detection assays and offers promising opportunities for a wide range of biosensing applications.