Miniaturized Piezoelectric Formaldehyde Sensor Using EPCo-COF and Pollen Carbon Composite as a High-Performance Sensing Probe.

Covalent organic frameworks (COFs), owing to their high surface area, well-defined porous structures, and abundant polar recognition sites, have shown great potential in trace gas detection. However, conventional COFs often suffer from aggregation, limited spatial distribution, and low structural utilization, which restrict their performance in gas sensors. To enhance the structural dispersity and gas sensing capability of COFs, we developed a miniaturized quartz tuning fork (MQTF) gas sensor based on a composite of EPCo-COF and pollen carbon (EPCo-COF@PC). In this hybrid sensing material, sheet-like COFs were grown in situ on a three-dimensional (3D) honeycomb-like pollen carbon scaffold, enabling effective spatial regulation of COF distribution and the formation of a stable and efficient gas transport network. The EPCo-COF@PC composite integrates abundant polar sites and 3D mass transport pathways, thereby enhancing selective adsorption of formaldehyde (HCHO), a common indoor air pollutant. Taking advantage of the high mass sensitivity of the MQTF, the adsorption-induced mass change is translated into a stable frequency shift signal. Experimental results revealed that the sensor exhibits rapid and reversible frequency responses even for ppb-level HCHO, with a broad linear detection range and excellent recovery behavior. It also shows superior selectivity against various common interfering gases and maintains 96.7% of its response after 40 days of storage at room temperature, indicating excellent long-term stability. This work provides an effective material strategy and design concept for the development of high-performance, low-cost, and integrable miniaturized gas sensors, and expands the potential applications of COF-based materials in environmental monitoring and portable sensing devices.