Fire hazards are manifested in property loss, casualties, ecological damage, etc., which seriously threaten the survival and development of human beings. Various measurements are utilized to mitigate the caused harm. Briefly, these measurements are divided into passive fire-control strategies, including the addition of flame retardants, and active fire-control strategies containing early fire warning and fire-fight management.

Early fire-warning sensors are one efficient strategy. They mainly work by monitoring and response to the features of combustion behavior in the early stage, which earns more available time to take timely action before fire propagation to achieve efficient fire management. This fire-control approach presents a salient advantage in mitigating fire hazards. Early fire warning sensors have recently been advanced, and their development is still ongoing. Aiming for further optimization, different fire-warning sensors are fabricated in this thesis to achieve smart low-temperature fire warnings, showing sensitivity in response temperature, response time, and intelligent transmission of detected messages. Specifically, these developments of fire-waring sensors are present in the graphene oxide (GO)-based early fire-warning sensors (Chapter 3), cellulose paper (CP) -based early fire-warning sensors (Chapter 4), and further explores the reused low-temperature early fire- warning sensor under various humidity condition (Chapter 5).

In chapter 3, a novel early thermo-sensitive fire sensor based on P/Si-GO film by layer-by- layer technology is proposed. This early fire-warning sensor achieves an ultrafast response within 1 s at a low temperature of 250 °C or an actual fire. Intriguingly, the cooperation of an intelligent custom-made wireless communication system allows this sensor to show warning messages on multiple local and long-range computer screens at distances up to 20 km from the target fire area. Furthermore, the pre-designed functional solution works as fire-warning nano-coating wrapped into a flexible polyurethane foam, indicating the feasibility of fire-warning nano-coating. Concerning the limitation of thermal reduction of GO at low temperatures, conductive MXene expected as a bridge is introduced to prepare another MXene-modified GO-based fire-warning sensor. This sensor implements a low-temperature alert at 250 °C within 1 s and is coupled with a small self-designed wireless signal conversion device to improve its intelligence. Moreover, the warning signal transmission contains the warning message on a computer monitor and precise real-time fire luminosity value on a local and a remote computer monitor via the associated luminosity sensor.

In chapter 4, CP substrate is applied to prepare fire-warning sensors promising for indoor fireproofing. In detail, a novel fire alarm is fabricated by using flame-retardant CP loaded with GO and MXene. Owing to the excellent temperature-dependent electrical resistance switching effect of GO, the modified paper acts as an electrical insulator at room temperature and becomes electrically conductive at high temperatures. Introducing conductive MXene enhances the fire detection speed, leading to a low-temperature fire warning of 2 s, which is first calculated by a novel and quantifiable technique. Moreover, the designed fire alarm sensor is coupled to a wireless communication interface to conveniently transmit fire signals “FIRE DANGER” to a liquid crystal display screen remotely. For achieving the balance between flexibility and flame retardancy, one flexible functional CP-based fire-warning system produced via a simple dip-coating technique provides flexibility, rapid low-temperature warning, and a novel local and remote wireless signal conversion capability. The proposed fire-warning system meets the critical requirement of a fast low-temperature warning, presenting warning messages within 2 s under 250 °C and mechanical properties. Moreover, the luminosity and temperature data collected is sent to local and remote computers by wireless conversion. These designed smart fire alarm papers are promising for fire-warning wallpaper for interior houses without sacrificing decoration.

In chapter 5, a salts-modified CS-based sensor is proposed using a facile assembling method responding to further lower temperatures, especially those caused by household electric appliances. This sensor responds quickly to a low temperature of 50 °C and a flame within 2 s. In addition, this sensor is reused to warn of danger 17 times, resulting in improved practicality. Furthermore, this sensor responds to relative humidity above 50 % at 50 °C and 75 °C sensitively due to its humid sensitivity, improving this early fire warning sensor’s ability to work under various humidity conditions. The particular property provides a potential application in harsh environments, especially on rainy days. The comparison between different salts-modified CS films is carried out to elucidate the mechanism of the formation of electric current under the joint driven by temperature and humidity. Moreover, real-time temperature and relative humidity monitoring can be achieved with a wireless transmission section. This design shows a promising approach for multifunctional CS-based sensors and paves a path to developing a new generation of smart fire-warning detectors.