Volatile Organic Compound Vapour Measurements Using a Localised Surface Plasmon Resonance Optical Fibre Sensor Decorated with a Metal-Organic Framework

A tip-based fibreoptic localised surface plasmon resonance (LSPR) sensor is reported for the sensing of volatile organic compounds (VOCs). The sensor is developed by coating the tip of a multi-mode optical fibre with gold nanoparticles (size: 40 nm) via a chemisorption process and further functionalisation with the HKUST-1 metal-organic framework (MOF) via a layer-by-layer process. Sensors coated with different cycles of MOFs (40, 80 and 120) corresponding to different crystallisation processes are reported. There is no measurable response to all tested volatile organic compounds (acetone, ethanol and methanol) in the sensor with 40 coating cycles. However, sensors with 80 and 120 coating cycles show a significant redshift of resonance wavelength (up to similar to 9 nm) to all tested volatile organic compounds as a result of an increase in the local refractive index induced by VOC capture into the HKUST-1 thin film. Sensors gradually saturate as VOC concentration increases (up to 3.41%, 4.30% and 6.18% in acetone, ethanol and methanol measurement, respectively) and show a fully reversible response when the concentration decreases. The sensor with the thickest film exhibits slightly higher sensitivity than the sensor with a thinner film. The sensitivity of the 120-cycle-coated MOF sensor is 13.7 nm/% (R-2 = 0.951) with a limit of detection (LoD) of 0.005% in the measurement of acetone, 15.5 nm/% (R-2 = 0.996) with an LoD of 0.003% in the measurement of ethanol and 6.7 nm/% (R-2 = 0.998) with an LoD of 0.011% in the measurement of methanol. The response and recovery times were calculated as 9.35 and 3.85 min for acetone; 5.35 and 2.12 min for ethanol; and 2.39 and 1.44 min for methanol. The humidity and temperature crosstalk of 120-cycle-coated MOF was measured as 0.5 +/- 0.2 nm and 0.5 +/- 0.1 nm in the humidity range of 50-75% relative humidity (RH) and temperature range of 20-25 degrees C, respectively.

Automated summary

A tip-based fiber optic localized surface plasmon resonance (LSPR) sensor is developed for detecting volatile organic compounds (VOCs) by coating the tip with gold nanoparticles and further functionalizing with HKUST-1 metal-organic framework. The sensitivity and response time of the sensor are influenced by the thickness of the MOF film and different cycles of coating. The sensor with 120 coating cycles shows higher sensitivity and reversible response to VOCs, with a limit of detection (LoD) of 0.005%-0.011% for acetone, ethanol, and methanol measurements.