Micro-liquid volume measurements realized by changing the plasmonic conditions via specialty optical fibers

In this work, a novel sensing approach used to develop a micro-liquid volume measurement system is exploited. This sensing methodology exploits the effects induced by a small liquid volume in contact with a patch of a plastic light-diffusing fiber (LDF) connected at the input of a surface plasmon resonance (SPR) sensor realized in a D-shaped plastic optical fiber (POF). More in detail, with a fixed solution present on the SPR-POF sensor, when a different micro-liquid volume is in contact with the LDF patch, the mode profile of the light into the patch and, so, in the SPR-POF sensor changes, producing a shift of the SPR spectra. As a proof of concept, the described sensor system has been tested in a volume range from $0\ \mu \mathrm{l}$ to 5 μl with a step size of 1 $\mu\mathrm{l}$. The obtained results have highlighted a good linear response with a sensitivity and a resolution equal to about 0.337 $\text{nm}/\mu\mathrm{l}$ and 0.59 $\mu \mathrm{l}$, respectively.