ZIF-8-Based Surface Plasmon Resonance Gas Sensor for Volatile Organic Compound Detection

Surface plasmon resonance (SPR) sensors are capable of real-time detection of gas, chemical and biological analytes due to the probe behaviour of surface plasmons to boundary conditions. Because of gas molecules are much smaller than biomolecules, conventional SPR sensors are not sensitive enough to detect gas adsorption at the surface of the sensor. Consequently, the functionalization of the surface with a sensitive coating is a frequent strategy to enhance their sensitivity, which relies on the refractive index (RI) change of the sensitive film caused by the adsorption of analytes. Metal-organic frameworks (MOFs) are advanced materials with a crystalline structure which is appealing as sensitive coatings due to their intrinsic porosity and high surface-to-volume ratio. Zeolitic Imidazolate Frameworks-8 (ZIF-8) is a particularly interesting MOF for gas sensing applications due to its hydrophobic properties, which can heavily reduce cross-sensitivity with relative humidity (RH) – a major issue that decreases the sensitivity of many gas sensors.<br/><br/>Based on the previous description, we fabricated a ZIF-8-based SPR gas sensor to monitor the presence of volatile organic compounds (VOCs) by tracking the red-shift of the SPR wavelength due to the adsorption-dependent RI of the MOF. The substrates contain 4 diffraction gratings (DG) with periods of L = 400 nm and L = 500 nm which have been covered by a ZIF-8 layer, obtained through a layer-by-layer deposition method. We present the fabrication process of both SPR substrates and ZIF-8 film, the MOF characterization and its integration in the final device, and gas sensing results when exposing them to different concentrations of VOCs.<br/><br/>We present the calibration of the SPR substrates with liquids as external media to obtain the bulk sensitivity of the DG, which is around 387 nm/RIU and 527 nm/RIU for L = 400 nm and L = 500 nm, respectively. We describe the environmental conditions to fabricate reproducible ZIF-8 nanolayers of controlled thickness regardless of the substrate. We report, as well, the gas sensing study when exposing the sensors with various ZIF-8 thicknesses to different VOCs, showing that the dip in the SPR resonance red-shift is independent of the thickness if the film is thicker than the penetration depth of the evanescent wave into the nanolayer, and independent of the period of the DG used, which allows to fine tune the working SPR wavelength without strongly affecting sensitivity in a set-up miniaturization using LEDs and photodetectors. Gas sensing experiments show adequate sensitivity to the studied VOCs (red-shifts of around 35 nm) and, particularly noticeable, no sensitivity to RH thanks to the hydrophobicity of ZIF-8.<br/><br/>In this presentation we will discuss these aspects as well as the benefit of the planar geometry of the SPR substrates, which potentially allow multiplexed gas sensing on an array of DG with different surface functionalization coatings and film thicknesses.