One novel humidity-resistance formaldehyde molecular probe based hydrophobic diphenyl sulfone urea dry-gel: Synthesis, sensing performance and mechanism

- Through linking effectively sensing probe and a hydrophobic group, the formaldehyde sensor exhibits excellent response stability and gas selectivity under a wide relative humidity range.
- We discovered that the amount of probe molecule can be tuned by controlling their dispersity under ultrasonic irradiation.
- The calculation results reveal Schiff base interaction between the amine group and aldehyde group is more responsible for formaldehyde sensing instead of hydrogen bond interaction between carbamido and aldehyde group.
- We design an adsorption isotherm experiment to extract the ΔH of formaldehyde adsorption. The adsorption isotherm experiments reveal that adsorption enthalpy change between DDS urea dry-gel and HCHO molecules locates the range of chemical adsorption, endowing the sensor satisfactory selectivity and reversibility.

The work designs a novel hydrophobic organic dry-gel named diamino diphenyl sulfone (DDS) urea for selectively detecting formaldehyde in air. It is synthesized by using octadecylisothiocyanate and diamino diphenyl sulfone as reactants in tetrahydrofuran solution at 70 °C. The dry-gel is obtained by freezing drying. Sulfanilamide urea dry-gel acts as the contrastive material which is synthesized in a similar condition. The dry-gel coated Quartz Crystal Microbalance (QCM) sensor exhibits a rapid and reversible selective response towards formaldehyde gas. Repeated measurement results show high sensitivity and low detection limit (1 ppm) for formaldehyde detection. The gas sensing and contact angle tests indicate its stability under different humidity conditions. Based on the adsorption isotherm experiments, it reveals that adsorption enthalpy change between DDS urea dry-gel and HCHO molecules locates the range of chemical adsorption, endowing the sensor satisfactory selectivity and reversibility. Sensing mechanism of formaldehyde sensor has been proved by a simulation calculation based on a quantum chemistry software Gaussian though comparing the interaction between amine group or carbamido and formaldehyde. The calculation results reveal that Schiff base interaction between the amine group and aldehyde group is more responsible for formaldehyde sensing instead of hydrogen bond interaction between carbamido and aldehyde group.