Volatile organic compounds optical fiber sensor based on lossy mode resonances

An optical fiber sensor to monitor volatile organic compounds (VOCs) has been developed. The sensing material used is the organometallic [Au ₂Ag ₂(C ₆F ₅) ₄(NH ₃) ₂] _n: its optical properties show reversible changes in presence of organic vapors. The main idea consists of using the sensing features of this material to prepare a sensor based on wavelength modulation: specifically, it meets the conditions required to induce lossy mode resonances (LMRs). The sensor works in transmission configuration, depositing the sensing compound along a segment of a plastic cladding silica (PCS) fiber where the cladding was previously removed. The method followed to deposit the sensing coating is the Layer by Layer (LbL), which allows the LMR spectral location to be tuned by means of controlling the sensing coating thickness. A spectral shift to blue is observed when the device is in presence of different organic solvents: actually, it shows a linear relation between the LMR spectral shift and the vapor concentration with a sensitivity of 0.131 nm ppm ^-1 from 0 to 250 ppm (v/v) of methanol. In this range, the total spectral shift is around 30 nm. The cross sensitivity to relative humidity is very low (a maximum shift of 1 nm from 40 to 60% R.H.) and negligible in the case of temperature between 20 °C and 60 °C.