Leaky waveguide biosensors for label-free measurement of human serum albumin

Early diagnosis of diseases such as kidney disease relies on the successful measurement of albumin concentration in urine. We report label-free detection of human serum albumin (HSA) using a leaky waveguide (LW) optical biosensor. The LW reported in this work comprised a few microns-thick mesoporous polyacrylamide hydrogel film deposited on a glass substrate by casting and, for the first time, copolymerized with N-(3-aminopropyl)methacrylamide (APMAA) to provide functional amine groups required to immobilise recognition elements, half-antibody fragments. Furthermore, this is an unprecedented report on the use of a high molecular weight (3700 D) poly(ethylene glycol) diacrylamide in contrast to previously reported low molecular weight bis-acrylamide crosslinkers to increase the porosity of waveguide films. Equally, other parameters such as molar ratio of APMAA to acrylamide and total weight of (monomers and crosslinker) to volume ratio were optimised to obtain hydrogel films with pore size and amine groups required to immobilise half-antibody fragments in hydrogel films. Three different strategies for immobilisation of recognition elements; two based on streptavidin biotin interactions and the third based on half fragments of antibody were studied. The third immobilisation strategy resulted in the most reproducible results and hence was used to measure the equilibrium dissociation constant of HSA and its corresponding half-antibody fragments. Using the LW-based label-free optical biosensor, HSA was successfully detected with a limit of detection of 28 ng mL^-1 in buffer and the lowest concentration of HSA measured in this work was 66.5 ng mL^-1. This capability of quantitation of HSA by the LW can be built upon to realise a LW biosensor for early detection of diseases including kidney disease.