Localised plasmonic hybridisation mode optical fibre sensing of relative humidity

This work reports an optical fibre probe functionalised with 'cotton-shaped' gold-silica nanostructures for relative humidity (RH) monitoring. The sensor response utilises the localised surface plasmon resonance (LSPR) of self-assembled nanostructures: gold nanospheres (40 nm) surrounded by one layer of poly (allylamine hydrochloride) and hydrophilic silica nanoparticles (10-20 nm) on the end-facet of an optical fibre via a wavelength shift of the reflected light. Sensor optimisation is investigated by varying the density of gold nanoparticles on the end-facet of an optical fibre. It is demonstrated that the plasmonic hybridisation mode appearing when the average gold interparticle distance is small (Median: 7.5 nm) is more sensitive to RH after functionalisation than the singular plasmonic mode. The plasmonic hybridisation mode sensor demonstrates a high linear regression to RH with a sensitivity of 0.63 nm/%RH and excellent reversibility. The response time (T10-90%) and recovery time (T90-10%) are calculated as 1.2 +/- 0.4 s and 0.95 +/- 0.18 s. The sensor shows no measurable cross-talk to temperature in the tested range between 25 degrees C to 40 degrees C and the 95% limit of agreement is 3.1%RH when compared to a commercial reference sensor. Simulation with finite element analysis reveals a polarisation-dependent plasmonic hybridisation with a redshift of plasmonic wavelength as a decrease of the interparticle distance and a higher refractive index sensitivity, which results in a high sensitivity to RH as observed in the experiment.

Automated summary

This work presents a functionalized optical fibre probe with 'cotton-shaped' gold-silica nanostructures for monitoring relative humidity (RH). The sensor utilizes the localized surface plasmon resonance (LSPR) of self-assembled nanostructures and demonstrates a high sensitivity to RH after optimization. The plasmonic hybridization mode sensor exhibited excellent linearity, reversibility, and fast response and recovery times.