In this paper, graphene-like carbon materials were prepared from waste paper and modified by surfactants. The carbonized paper (CP) and the carbon material modified with laundry detergent (CP/LD) were applied to the detection of ammonia (NH₃) and formaldehyde (CH₂O). The experimental results show that the CP sensor shows the characteristics of p-type semiconductor and the CP/LD sensor shows the characteristics of n-type semiconductor, but the response of CP sensor is small for both gases, and the response of CP/LD sensing for both gases is significantly improved after modification, with the response size of 2 021.59% and 756.73% respectively, which is 1 168 and 176 times better, and 85% of relative humidity (85% RH) showed better resistance to interference. In addition, both sensors exhibit good reproducible properties.

Covalent organic frameworks (COFs) are an emerging type of porous crystalline polymers formed by combining strong covalent bonds with organic building blocks. Due to their large surface area, high intrinsic pore space, good crystallization properties, high stability, and designability of the resultant units, COFs are widely studied and used in the fields of gas adsorption, drug transport, energy storage, photoelectric catalysis, electrochemistry, and sensors. In recent years, the rapid development of the Internet of Things and people’s yearning for a better life have put forward higher and more requirements for sensors, which are the core components of the Internet of Things. Therefore, this paper reviews the recent progress of COFs in synthesis methods and sensing applications, especially in the last five years. This paper first introduces structure, properties, and synthesis methods of COFs and discusses advantages and disadvantages of different synthesis methods. Then, the research progress of COFs in different sensing fields, such as metal ion sensors, gas sensors, biomedical sensors, humidity sensors, and pH sensors, is introduced systematically. Conclusions and prospects are also presented in order to provide a reference for researchers concerned with COFs and sensors.