Actuator-driven, purge-free formaldehyde gas sensor based on single-walled carbon nanotubes

Formaldehyde vapor (HCHO) is a harmful chemical substance potentially contaminated in the air, with an indoor permissible level below 0.08 ppm (80 ppb). Thus, highly sensitive gas sensors that are capable of continuously monitoring HCHO are in demand. The electric conductivity of semiconducting nanomaterials (e.g., single-walled carbon nanotubes (SWCNTs)) is susceptible to chemical substances adsorbed on the surface; thereby portable and highly sensitive chemiresistive gas sensors, including ones for HCHO, have been developed so far. However, when monitoring very trace gases (<1 ppm) involved in ambient air, most chemiresistive sensors could face practical issues, including false responses to interfering effects (e.g., fluctuations of room temperature and humidity), baseline drift, and the necessity to access purge gas. Here we report an actuator-driven, purge-free chemiresistive gas sensor that is capable of reliably detecting 0.05 ppm HCHO in the air. The sensor is composed of an HCHO→HCl converter (powdery hydroxylamine salt, HA), an HCl detector (SWCNT-based chemiresistor), and an HCl blocker (thin plastic plate). Upon exposure to HCHO, the HA emits HCl vapor, which diffuses onto the adjacent SWCNT and increases its electric conductivity through p-doping. Meanwhile, inserting a plastic plate between HA and SWCNTs makes the conductivity of SWCNTs insensitive to HCHO. Thus, under periodic actuation (insertion and removal) of the plastic plate, HCHO can be reliably detected in a wide concentration range (0.05-15 ppm), with excellent selectivity over other volatile organic compounds. The actuator-driven system is beneficial because the purge gas is unnecessary for sensor recovery and baseline correction. Moreover, since the response to HCHO is synchronized with the actuation timing of the plate, even small (~0.8%) responses to 0.05 ppm HCHO can be clearly separated from larger noise responses (>1%) caused by interfering effects and baseline drift. We believe that this work provides substantial insights into the practical implementation of nanomaterial-based chemiresistive gas sensors.