A pH-sensitive nanotip array imaging sensor Original Research Article

Nanotip arrays have been fabricated on the distal faces of coherent fiber-optic bundles. A typical 270 μm diameter nanotip array comprised ∼6000 individual optical fibers that were etched chemically. Individual conical nanotips were 5.5 μm long with radii of curvatures as small as 15 nm. A combination spin-coating and photopolymerization technique was developed to deposit a chemical sensing layer (polymer+fluorescent dye) across the array such that the nanotip architecture was retained. PVA-SbQ was chosen as the model photopolymer and FITC-dextran was chosen as the pH-sensitive dye. The resulting nanotip array imaging sensor (NAIS) comprised individual pH-sensing tips with ∼100 nm radii of curvature. NAIS fluorescence images were transmitted through the fiber-optic bundle and acquired by an epifluorescence microscope/charged-coupled device imaging system. The pKa of the PVA-SbQ immobilized FITC-dextran was 6.2 and the NAIS time response to a 0.3 pH unit change was 1.5 s. Fluorescence images acquired from a NAIS inserted in a rat liver indicated that the sensing layer was robust and that the NAIS’s analytical performance was not altered following insertion.