Ultra-sensitive immunoassay using VCSEL detection system - Electronics Letters

Introduction: Protein-protein interaction is one of the most important subjects in molecular biology [1]. Bioassays have been developed to detect and quantify the biomolecular interactions with high sensitivity. There are two general categories of techniques: labelling with compounds and direct molecule identification. Label-free biosensor technologies are among the latter, which are desirable because of their non-intrusive nature and greatly simplified assay preparation. Recently, we proposed a compact label-free biosensor system utilising a vertical cavity surface emitting laser (VCSEL) as the new detection system [2]. The biosensor (molecular binding surface) used here is a guided mode resonant (GMR) filter, which is made from a plastic sheet bonded to a bottomless 96-wells microplate [3]. The readout system consisted of one singlemode VCSEL and two pin detectors, which recognises and quantifies the binding event of the biomolecules. The biosensor configuration is shown in Fig. 1. The GMR has a narrow reflection wavelength (lpeak), which varies greatly with the optical thickness (i.e. multiple of refractive index and physical thickness) of the biomolecules attached on top of its surface. The VCSEL and the two detectors are used to track this wavelength shift as materials are being deposited onto the GMR surface. In our previous publication [2], we showed that the VCSEL readout system could detect a minute variation of refractive index by immersing the GMR top surface with fluids having precise refractive index. In this Letter, we report the use of this system in immunoassay with a record high sensitivity to antigen concentration.