The rapid development of sensing technology has created an urgent need for chemical sensor systems that can be rationally integrated into efficient, sustainable, and wearable electronic systems. In this case, the triboelectric nanogenerator (TENG) is expected to be a major impetus to such innovation because it can not only power the sensor by scavenging mechanical energies and transforming them into electricity but also act as the chemical sensor itself due to its intrinsic sensitivity towards the chemical reaction that occurs at the triboelectric interface.
Over the past decades, chemical sensors have played a significant role in clinical, industrial, and biomedical analyses because of their remarkable detection capability, simplicity, and low cost 
. Meanwhile, in keeping with the trend of microminiaturization, networking, and intellectualization 
, chemical sensors are finding wider applications in flexible electronic equipment and wearable intellectual devices 
. Nevertheless, due to the intrinsic drawbacks of traditional chemical batteries, such as insufficient capacity and limited-service life, the development of efficient, sustainable, and autonomous wearable chemical sensing systems still faces severe challenges 
. Energy harvesting strategies that can bridge the integration of chemical sensors with energy storage units and promote the design of reliable and sustainable autonomous chemical sensing systems are urgently needed 
Among the various renewable energy harvesting technologies, such as triboelectric, thermoelectric, and photochemical generators 
, triboelectric nanogenerators (TENGs) are receiving tremendous focus due to their ubiquitous advantages in self-powered facilities, where they can act as both power source and smart sensor, including dynamic force sensors and chemical sensors, due to their intrinsic sensitivity toward chemical reactions at the triboelectric interface 
. Theoretically, TENGs collect mechanical energies from the surroundings by interfacial contact electrification and electrostatic induction, and the magnitude of the electrical signal is highly related to the charge density in the friction electrode surface 
. In recent years, TENG-based chemical sensors have been advancing significantly, which allows reliable, practical, and energy-efficient chemical sensor systems (CSSs) to be established.
2. Basic Principle of TENGs
2.1. Working Mode of TENGs
By varying the relative motion of the contact initiation layer, TENGs are generally classified into four basic operating modes: the vertical contact separation mode, the sliding mode, the single-electrode mode, and the free-standing friction layer mode 
The most typical device structure in TENGs is the vertical contact separation mode (Figure 1
a), which has two electrodes—one attached to the top of the dielectric membrane and the other on the back. These two electrodes move towards each other in the vertical direction of the device, generating an electrical charge as a result of an externally applied force. The potential between the electrodes will change during contact or separation, resulting in a current output from the external circuit 
Figure 1. Four modes of TENGs: (a) vertical contact separation mode; (b) lateral sliding mode; (c) single-electrode mode; (d) free-standing triboelectric layer mode.
Next is the sliding mode (Figure 1
b), which operates similarly to the vertical contact separation mode. The device is driven by an external force parallel to the interface, resulting in a relative displacement between the electrode material surfaces, which generates a dense frictional charge. The magnitude of the frictional charge varies periodically with the increase or decrease of the contact surface area. The potential difference between the electrodes changes, resulting in an external current to balance the potential difference. Both modes of operation require moving the object with electrodes 
Given that most movements in real-world scenes are irregular, researchers recommend the single-electrode mode (Figure 1
c), in which the object can move freely without being constrained by electrodes. In this mode, the device only requires one electrode to be grounded, which acts as a reference electrode. After contact with a free-moving object, an electrical charge is generated at the interface. During the motion, the potential distribution caused by the charged surface will change, resulting in the flow of external circuit current 
The free-standing triboelectric layer mode requires only one free-moving part (Figure 1
d). In this case, the device consists of a dielectric layer and a pair of symmetrical electrodes, and the free-moving part generates an asymmetric potential distribution by changing its position. Subsequently, electrons move continuously between the two electrode materials, which results in electrical output 
2.2. Working Principle
Triboelectrification is a common phenomenon that people encounter daily. While its origin is still unclear, it is widely believed that frictional electricity is caused by the overlap of electron clouds inside the material when the electrode surfaces are in contact. In this case, the energy barrier in the interior of the atom is lowered 
. From a theoretical point of view, the charges of the two friction electrodes are equal. When no external force is applied, the two friction electrode surfaces are separated from each other, leading to the establishment of an internal potential in the device 
. Additionally, in this process, the internal potential can induce the flow of carriers in the material from one electrode to the other, thus counteracting the change in electric potential caused by electrostatic induction and, finally, generating a positive current 
. When there is an external force, the two electrodes of the TENG move toward each other, which can cause the carriers in the material to flow in opposite directions, resulting in a change in the polarity of the electric potential as well 
. In addition, when the two friction electrodes are in contact again, this process will cycle, resulting in a periodic output with positive and negative properties 
3. Progress of TENG-Based Chemical Sensor Systems
3.1. Chemical Sensor Powered by TENG
As a renewable energy harvesting technology, TENGs have promoted the development of self-powered sensors and portable electronic equipment to meet the growing demand for autonomy and sustainability 
. Here, the basic principle, design strategy, and recent progress of TENGs acting as a power source in chemical sensors are illustrated.
3.1.1. Basic Principle
As shown in Figure 2, the TENG serves as a power source that can directly drive a chemical sensor system (CSS). In this process, the output of the TENG is processed by a power management device in the system, which typically consists of a rectifier and various types of energy storage devices. In this case, the stability of the TENG output can only affect the charging efficiency of the energy storage devices in the system and has no effect on the detection abilities of the sensor.
Figure 2. TENG serves as a power source in the TENG-based chemical sensor system.
3.1.2. Design Criteria of TENG-Based Power Source
To be used as a power supply, the design of TENGs needs to consider the following aspects. First, a direct and stable output TENG should be designed to continuously drive the sensor. Second, it is essential to increase the maximum output power and energy conversion efficiency of the device and, finally, improve the reliability performance of the device in harsh operating conditions.
Power Management Approach
It knows that conventional TENGs produce a transient peak AC output when in operation, which is one of their most significant limitations; therefore, they cannot be directly used as a power source for sensors. Nowadays, there are two main methods of achieving conversion from AC power to DC power in a circuit. The first method is to connect a rectifier to the TENGs to obtain a stable, constant DC voltage, thus maintaining the daily operation of the sensor.
Performance Enhancement Strategy
Although TENGs are a promising power source due to their ability to collect energy from the environment, the relatively low output power limits the broad application of TENGs. The following are several effective ways to increase the output of TENGs: firstly, increase the surface charge density of the material; subsequently, increase the area of electrode contact separation; in addition, optimizing the structural design is also a solution.
Resistance to Harsh Conditions
In practical applications, self-powered chemical sensor systems based on nanogenerators always face complex operating environments such as acid, alkali, temperature, corrosive, high humidity, high salinity, and strong magnetic fields.
3.1.3. TENG-Powered Chemical Sensors
TENG can be constructed from a wide range of materials with virtues such as biocompatibility, flexibility, permeability, and so on 
. These features offer a foundation for the further development of CSSs that are powered by TENGs.
Biosensors are important in healthcare and diagnostic treatments. Nevertheless, the battery life of these instruments is limited 
. Battery replacement is not only an expensive and risky process for patients but also may be painful and inconvenient for them. Batteries are not ideal power sources for medical devices because they contain toxic chemicals 
. Therefore, self-powered sensors are a promising strategy for the sustainable operation of biosensors.
Recently, TENGs have been extensively utilized as a complementary power source for gas sensors, which helps the continuous operation toward the online detection of the environmental atmosphere.
Ion sensors are widely used at present to either detect the environmental chemical composition or monitor physiochemical conditions. For autonomous operation, TENGs should provide the necessary electric power and complement electrochemical power sources.
3.2. TENG-Based Active Chemical Sensor
3.2.1. Basic Principle
During operation, the magnitude of the electrical signals (Voc
) generated by the TENG device is closely related, in some cases, even quantitatively proportional to the triboelectric charge density 
. As frictional electricity is essentially a chemical potential difference between two electrode surfaces and as the absorption of some chemical substances and their chemical reactions at the surface affect the charge transportation and density, thus affecting the output of the TENG, it is possible to detect the absorbed target substance quantitively and develop TENG-based active chemical sensors (Figure 3
). This multifunctional design reduces power consumption, simplifies the system, and, above all, enables sustainable, straightforward, and reliable sensor operation.
Schematic illustration of TENG-based active CSSs.
3.2.2. Design Criteria of TENG-Based Active Sensors
TENGs can scavenge mechanical energy from the ambient environment and even the human body 
and, thus, are ideal for powering chemical sensors and building on-body healthcare systems with high sustainability and wearability. Meanwhile, TENGs may also act as chemical sensors due to their intrinsic sensitivity toward the chemical reactions that occur at the triboelectric interface 
In the self-powered sensing system, sensitivity is always considered the most basic index to evaluate the probability of commercialization 
. Improving sensitivity through structural design, signal processing, and power management to meet or exceed the current commercial requirement of chemical analysis sensors is a hot topic in current research.
In practical applications, TENGs need to be integrated with a chemical sensor and other information devices to realize the automatic analysis of collected data and provide scientific guidance.
3.2.3. Applications of TENG-Based Active Chemical Sensors
TENG-based active chemical sensors have received broad attention and undergone rapid development in response to stimuli from chemical molecules or other factors, for instance, ethanol, phenol, and pH, where the output performance of the TENG and the stimulation shows a linear relationship.
TENG-Based Active Biosensors
Human body physiological signals are important physiological indicators for public healthcare and health monitoring . Monitoring these small but complex physiological signals in real-time with simple, convenient, economical, and accurate methods remains a challenge for healthcare devices . As a result, new methods for analyzing these biological analytes are required.
TENG-Based Active Gas Sensors
It is essential to develop rapid and precise real-time safety monitoring systems, such as chemical gas sensors, to find gases such as alcohol, acetylene, ammonia, aniline, etc., considering the increasing worry about the security of places where people work and reside. Due to their ability to implement portable gas sensors without a separate power source, self-powered gas sensors with TENGs are drawing a lot of attention 
TENG-Based Active Ion Sensors
Chemical ion sensors are commonly used in sectors to detect inorganic, organic, and biological ions, particularly ions in liquids such as H+
, and so on. With high selectivity, great sensitivity, and a wide sensing range, self-powered TENG-based active ion sensors are able to detect various types of substances. The ions in the detection target can be identified by the friction layer surface of the TENG or the outside dopant material 
. The following typical examples demonstrate these applications and advances.
Other Active Sensors
Aside from TENG-based activity sensors, as discussed above, there is a solid–liquid interface-based TENG that can detect organic compounds in solution 
, such as ethanol and acetone in solution. The sensing mechanism is the same as that of gas detection; the effect of different molecules or concentrations on the friction layer will cause the performance of the TENG to change. The detection method is similar to the case of ion sensors; TENG-based chemical sensors detect target molecules in a solution by dropping the solution onto the surface of a frictional electric material and observing its reaction with the counter electrode after the surface of that material has dried, enabling the analysis of the solution.
TENG-based CSSs not only demonstrate the sensitive detection of physiological activity, gases, and ions but may also achieve independent, continuous, and reliable operation by scavenging mechanical energy from nature or biology. Although this TENG-based sensor system has achieved significant progress through the design strategy of TENG-based active sensors, it is still not systematic and comprehensive enough. The challenges that TENG-based power sources face are relatively simple; the enhancement of output performance is always the prime target, which can be effectively promoted by improved power management, material selection, and structure design. For TENG-based active sensors, the main starting point is the adsorption or chemical reaction of specific targets at the triboelectric interface, whose reliability is susceptible to environmental conditions and the TENG’s own signal interference. Nevertheless, in both cases, the coupling of different material properties can bring new insights into the design of TENG-based CSSs with higher performance, thus innovating their applications in a wide range of multidisciplinary areas, such as disease diagnosis, environmental monitoring, and the defense industry.