Biosensing device based on optical waveguide spectrometry with fluorescent solvatochromic beads for label-free...

A biosensing device based on optical waveguide spectrometry was developed using fluorescent solvatochromic beads as a sensor material. This device facilitated the analysis of the avidin-biotin interaction without labeling avidin. The fluorescent solvatochromic beads were synthesized from the 4-iodobenzoic-acid-substituted Wang resin, 2-bromothiophene, and N-tert-butyloxycarbonyl (Boc)-protected phenylpiperazine boronic ester through Suzuki-Miyaura cross-coupling, followed by condensation with NHS-biotin after the deprotection of the Boc-protected piperazine. The fluorescence spectrum of the beads showed a blue shift, and the fluorescence intensity rapidly increased with the addition of the neutravidin-dissolved phosphate-buffered saline (PBS) solution. This phenomenon indicated that the fluorescent solvatochromic dye on the beads was incorporated into neutravidin. Additionally, the kinetic and equilibrium properties of the interaction were analyzed by measuring the fluorescence intensity of the beads and comparing it with that of bovine serum albumin (BSA). The observed binding rate constants were found to be 2.7 × 10^-2 s^-1 at 17 mg mL^-1 of neutravidin and 4.3 × 10^-4 s^-1 at 18 mg mL^-1 of BSA. Furthermore, the fluorescence intensity of the beads with the BSA solution decreased upon washing the beads with PBS. In contrast, the beads with the added neutravidin solution showed constant fluorescence intensity even after washing.