Coatings for the Ultra-Sensitive Detection of Volatile Organic Compounds with High Immunity to Humidity

In this paper, we present a novel platform based on a hybrid photonic cavity with metal-organic framework (MOF) coatings for VOCs detection. We have fabricated a compact gas sensor with detection limitation ranging from 29 to 99 ppb for various VOCs including styrene, toluene, benzene, propylene and methanol. Our results demonstrate that MOF coatings have significant potential in enhancing the sensitivity of miniaturized gas sensors.

Our design fully utilizes the strong absorption properties of MOFs[1] and the ultra-high quality factor (Q-factor) of photonic resonator structures. We selected ZIF-8 as the external MOF coating material because of its high surface area (1840 m² g^-1), hydrophobicity, good water stability, and high light transmittance in NIR.^{[2, 3]} As shown in Fig 1, ZIF-8 pre-concentration coating was grown on the Si₃N₄ waveguide based micro-ring resonator (MRR) by a layer-by-layer intergrowth process, after which the coating was patterned using lithography and etched by sulfuric acid. An illustration of the experimental apparatus is shown in Fig 2.

The resonant wavelength of the MRR is strongly dependent on the changes of ZIF-8 RI induced by gas adsorption. As shown in Fig 3, the resonant wavelength exhibited a blueshift firstly, and then started to redshift upon exposure to each gas with a concentration of 100 ppm. The equilibrium during the exposure time is between 30 to 45 minutes for these five VOC vapours, and the final shifts of the resonance are 172 pm, 341 pm, 282 pm, 131 pm, and 101 pm for methanol, propylene, benzene, toluene, and styrene, respectively. Compared to a MRR without pre-concentration/absorption layers, the hybrid photonic-MOF devices can improve the sensitivity up to 568, 1025, 621, 300, and 220 times for methanol, propylene, benzene, toluene, and styrene, respectively, as shown in Fig 4. Besides, as the resonant wavelength variation induced by the environmental temperature variation and tunable laser instability is around +/- 0.1 pm in the experiments as previous mentioned,^[4] the detection limits for these five VOC vapours are determined in the range from 29 to 99 ppb.