Real-time monitoring of bovine serum albumin at femtogram/mL levels on antibody-immobilized tapered fibers

We report the use of a long wavelength (1550 nm) laser and a tapered fiber optic biosensor (TFOBS) to monitor the real-time attachment of model protein bovine serum albumin (BSA) to the antibody-immobilized surface of the taper. Continuous biconical tapers were fabricated at various waist diameters in the range of 6–12 μm. The surfaces of the tapers were modified with an amine group to allow for the formation of a covalent bond between the amine and one of the carboxylic groups of the antibody. A 1550 nm distributed feed back laser was used to launch light at one end of the fiber with the optical throughput monitored on an optical spectrum analyzer. While cuvette measurements established that BSA was non-absorbing at 1550 nm showing no significant changes in optical throughput through the cuvette, tapered fibers with antibody-immobilized surfaces showed changes in optical throughput at bulk concentrations down to10 fg/mL of BSA. The kinetics of attachment is appropriately fitted to the Langmuir adsorption model and the estimated time required for attachment corresponds to those obtained in earlier studies. We postulate that the adsorption of the protein to the tapered fiber leads to changes in the optical characteristics of the taper. This affects the evanescent field leading to changes in optical throughput. Compared to the traditional use of short wavelength (300–400 nm) biosensors, longer wavelength (1550 nm) sensors offer a highly sensitive means of observing protein–protein interaction because of the increase in penetration depth of the evanescent field at 1550 nm.