Acidity sensor based on porphyrin self-assembled monolayers covalently attached to the surfaces of tapered fibres

Surface organization and photochemistry of free-base porphyrin self-assembled monolayers (SAMs) covalently bonded to the surfaces of tapered optical fibres and reference glass plates were studied. The surface modification was carried out in two steps: (i) surface activation by aminopropyltrimethoxysilane and (ii) porphyrin attachment via activated pentafluorophenyl esters. The measurements of surface activation time and real-time porphyrin adsorption monitoring revealed that each of the two steps required 1 h reaction time. Porphyrin immobilization on fibres was as good as on plane glass substrates. Fluorescence measurements were used to determine the optimum diameter of the coned part of the tapered fibre for sensor applications. For 125 mu m fibres with a 10 mu m cladding layer, this diameter was found to be 35-48 mu m. The mono-and diprotonated forms of free-base porphyrin have distinct fluorescence spectra. This phenomenon was employed to demonstrate a pH sensor application of the SAM-modified tapered fibre, which operates in the pH range of 0.6-3.8.