Optical responses, permeability and diol-specific reactivity of thin polyacrylamide gels containing immobilized phenylboronic acid

Thin semitransparent gels were prepared by radical copolymerization of N-acryloyl-m-aminophenylboronic acid (NAAPBA) and acrylamide (AAm) taken in molar ratios from 8:92 to 16:84, respectively, in water. The gels were characterized by the content of immobilized NAAPBA and monomer conversion. Scanning electron microscopy revealed the micrometer size pores in the dried gels. The wet gels displayed a linear optical response to sugars with sensitivity decreasing in the series: d-fructose, d-galactose, d-glucose, d-mannose, N-acetyl-d-glucosamine in the sugar concentration range from 1 to 40-60 mM at pH 7.3. Cross-linking of the gels with N,N-methylene-bis-acrylamide decreased the strength of optical response. Specific binding capacity of a diol-containing dye Alizarin Red S in the gels at pH 7.0 coincided with the content of immobilized NAAPBA indicating the 1:1 stoichiometry of the reaction and, therefore, good accessibility of the boronic acid ligands for water-soluble diols. Permeability of the gels was studied with a non-interacting dye Ethyl Orange exhibiting the pore diffusion coefficient of 1.4 × 10−7 cm2/s. The rate of optical response of the gels to glucose was found to be determined by diffusion of sugar into the relatively thick gels (l = 0.35-1 mm) with effective diffusion coefficients of 2 × 10−7 cm2/s. In the thinner gels (l = 0.1 mm) the input of other kinetic processes, such as affinity binding or structural rearrangements of the gel, was noticeable.