The porosity of hydrogels is a significant factor in determining the sensitivity of our sensors because recognition elements can be immobilised throughout their volume, thereby increasing the number of available binding sites for an analyte. Mesoporous hydrogels with interconnected pores of about 10 nm offer a good compromise between being porous to macromolecules and scattering losses.

We have been working on both natural and synthetic hydrogels, and more details are provided below.

Effect of hydrogel porosity on the concentration of recognition elements that can be immobilised

Natural hydrogels:
Chitosan: We are the first group to report low refractive index mesoporous waveguides made of chitosan. We have also developed a facile method where, drying time between spin coating the polymer solution and subsequent rehydration was controlled, for the fabrication of chitosan films that are porous to molecules of molecular weight of at least 300 KDa. For more details, visit here.

Chitosan is a naturally-occurring polysaccharide with free amine groups, which is attractive for the facile immobilization of recognition elements such as antibodies. After purification, chitosan is dissolved in acetic acid, spin coated on glass substrates and crosslinked to obtain hydrogel films that act as waveguides.

Synthetic hydrogels:
(Poly)acrylamide: We use free radical polymerisation of acrylamide: bisacrylamide and casting to make 10-100 μm thick polyacrylamide films/ blocks. For making 1-10 μm thick polyacrylamide films, we spin coat linear (poly)acrylamide solution followed by crosslinking with glutaraldehyde in acidic conditions.

The type and concentration of co-monomers is selected to obtain the required functionality e.g. tune the pKa of hydrogels to make pH gradients in microchannels for sample preparation. We also develop chemistries and fabrication strategies to produce two- and three-dimensional patterns of functional groups in hydrogels (called photofunctionalisable hydrogels), which can then be reacted with recognition elements, for sensing applications. Finally, we are interested in photopolymerisable hydrogels.