A novel sensor for detecting particulate matter 2.5 (PM<sub>2.5</sub>, particles with a diameter smaller than 2.5 μm) concentration in environment air is presented by using modal interference in a photonic crystal fiber (PCF). The sensor is composed of a single mode-multimode-PCF-multimode-single mode optical fiber, and the corresponding polypyrrole (PPy) sensing nanofilm with a light-inducing characteristic is synthesized onto the outside surface of a PCF in-situ by an interfacial ploymerization method. The experimental result shows that the thickness of the sensing film is within the range of 100~150 nm by a scanning electron microscope. When the sensor is placed in PM<sub>2.5</sub> air flow, the PM<sub>2.5</sub> particles are absorbed onto the surface of a PPy sensing film due to a light-inducing electrostatic effect, resulting in the refractive index (RI) change of a sensing film. For PM<sub>2.5</sub> air flow with a concentration of 55 μg/m<sup>3</sup> and a sampling time of 30 min, the characteristic wavelength of the interference spectra has a blue shift with 1 nm. After turning off the light source, the characteristic wavelength of the sensor is back to the initial value owing to no light-inducing electrostatic effect and the PM<sub>2.5</sub> particles desorbing. The sensor has a good reversibility.
Various sensors based on the piezoelectric crystal have been widely used in the process of research. Currently, the research and application of the piezoelectric sensors are mostly based on the "mass responses" and "non-mass responses" of the piezoelectric crystal. But in the researches and applications of the piezoelectric crystal sensors, many phenomenons have been found which can not be explained and can not accord with the Sauerbrey theory based on the "mass responses". And the scholars often explained those phenomenons with the "non-mass response of the piezoelectric crystal" <sup></sup>. In our researches, we improved the detectors and got some primary experimental results. And the results
showed that the non-mass respones has no relation to the piezoelectric sensor, and it related to the characteristics of the
liquor itself. Based on the results, it can be reckoned that this is a new physical phenomenon, that is to say, the liquor has
the electrical resonance characteristic which likes the characteristic of the piezoelectric crystal, and can identify the
characteristic of the liquor using the frequency of the electrical resonance. As soon as any of the characteristics of the
liquor changed such as component, and density, the resonance frequency will change accordingly.