Optimising the enzyme response of a porous silicon photonic crystal via the modular design of enzyme sensitive polymers†
We describe the immobilization within the pores of a porous silicon photonic crystal of an enzyme degradable polymer network, for optical biosensing. A porous silicon (PSi) rugate filter is a one-dimensional photonic crystal with a high-reflectivity optical resonance that is sensitive to small changes in the refractive index of the pore space permeating through the structure. An enzymatically degradable polymer network was constructed by first “clicking” an antifouling copolymer, poly(oligo ethylene glycol-co-acrylic acid)-N3, to an alkyne functionalized PSi surface via copper(I)-catalysed alkyne–azide cycloaddition (CuAAC) reaction. MMP-2 or MMP-9 specific cleavable peptide sequences, with diamine functional groups, were then added, using a 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide/N-hydroxysuccinimide (EDC/NHS) chemistry to react with the acrylic acid group. The polymer network was completed by further attachment of a sacrificial polymer, poly(hydroxyethyl acrylate-co-N-hydroxysuccinimide ester acrylate). X-ray photoelectron spectroscopy (XPS) and optical reflectivity measurements reveal successful modification of the PSi with the polymer–peptide network. Exposure of the biosensor platform to solutions of matrix metalloproteinases, MMP-2 or MMP-9, caused a change in the average refractive index of the photonic crystal, resulting in a discernible blue shift in the reflectivity spectra. The blue shift indicated the degradation of the polymer network within the porous network. Selective detection of different MMPs was demonstrated, via the use of different peptide sequences, which are selectively digestible by different MMPs, to link the two polymers.