Biosensor based on hydrogel optical waveguide spectroscopy

A novel label-free biosensor based on the measurement of binding-induced refractive index changes by hydrogel optical waveguide spectroscopy (HOWS) is reported. This biosensor is implemented by using a surface plasmon resonance (SPR) optical setup in which a carboxylated poly(N-isoproprylacrylamide) (PNIPAAm) hydrogel film is attached on a metallic surface and modified by protein catcher molecules through amine coupling chemistry. The swollen hydrogel with micrometer thickness serves both as a binding matrix and optical waveguide. We show that compared to regular SPR biosensor with thiol self-assembled monolayer (SAM), HOWS provides an order of magnitude improved resolution in the refractive index measurements and enlarged binding capacity owing to its low damping and large swelling ratio, respectively. A model immunoassay experiment revealed that HOWS allowed detection of IgG molecules (molecular weight 150 kDa) with a 10 pM limit of detection that was 5-fold lower than that achieved for SPR with thiol SAM. For the high capacity hydrogel matrix, the affinity binding was mass transport limited. Therefore, we envisage that HOWS will provide further improved detection limit for low molecular weight analytes or for assays employing lower affinity catcher molecules.