In this paper, the surprising sensing performance of fiber-optic near-field chemical sensors, based on wavelength scale tin dioxide particle layers, against chemical pollutants in air environment at room temperature are reported. The layers were deposited upon the distal end of standard single-mode optical fibers by means of the very simple, versatile, and low-cost electrostatic spray pyrolysis technique. The morphologic and optical features of the deposited layers were characterized by means of a complex scanning probe system constituted by simultaneous atomic force microscope (AFM) and near-field scanning optical microscope (NSOM). Particle layers composed by tin dioxide grains, with wavelength and subwavelength dimensions, are very promising because they are able to significantly modify the optical near-field profile emerging from the film surface. As matter of fact, a local enhancement of the evanescent wave contribute occurs leading to a strong sensitivity to surface effects induced by the analyte interaction. Here, for the first time to our best knowledge, experimental results on the sensing capability of the proposed chemical probes in air environment are reported. Also, a preliminary study on the effects of the processing stage and the post processing thermal annealing on the film morphology and near-field behavior are presented.
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