Advances in self-powered chemical sensing via a triboelectric nanogenerator

Congxi Huang , Guorui Chen , Ardo Nashalian and Jun Chen *
Department of Bioengineering, University of California, Los Angeles, Los Angeles, CA 90095, USA. E-mail: jun.chen@ucla.edu

Received 30th October 2020 , Accepted 16th December 2020

First published on 16th December 2020


Abstract

Chemical sensors allow for continuous detection and analysis of underexplored molecules in the human body and the surroundings and have promising applications in human healthcare and environmental protection. With the increasing number of chemical sensors and their wide-range distribution, developing a continuous, sustainable, and pervasive power supply is vitally important but an unmet scientific challenge to perform chemical sensing. Self-powered chemical sensing via triboelectric nanogenerators (TENGs) could be a promising approach to this critical situation. TENGs can convert mechanical triggers from the surroundings into usable electrical signals for chemical sensing in a self-powered and environment-friendly manner. Moreover, their simple structure, low probability of failure, and wide choice of materials distinguish them from other chemical sensing technologies. This review article discusses the working principles of TENGs and their applications in chemical sensing with respect to the role of TENGs as either a self-powered sensor or a power source for existing chemical sensors. Advances in materials innovation and nanotechnology to optimize the chemical sensing performances are discussed and emphasized. Finally, the current challenges and future prospect of TENG enabled self-powered chemical sensing are discussed to promote interdisciplinary field development and revolutions.


image file: d0nr07770d-p1.tif

Jun Chen

Dr Jun Chen currently is an assistant professor in the Department of Bioengineering at the University of California, Los Angeles. His research focuses on nanotechnology and bioelectronics for energy, sensing, and therapeutic applications in the form of smart textiles, wearables, and body area networks. He has already published 2 books, 140 journal articles and 90 of them are as first/corresponding authors in Chemical Reviews, Nature Energy, Nature Electronics, Nature Sustainability, Nature Communications, Joule, Matter, and many others. His works were selected as Research Highlights by Nature and Science 6 times and covered by world mainstream media over 1000 times in total, including NPR, ABC, NBC, Reuters, CNN, The Wall Street Journal, Scientific American, and Newsweek. He also filed 20 US patents and licensed 1. Beyond research, he is currently an Associate Editor of Biosensors and Bioelectronics, and an Editorial Board Member of The Innovations, Advanced Fiber Materials, Nano-Micro Letters, Frontiers in Pharmacology, Frontiers in Chemistry, Textiles, Biosensors, and Smart Materials in Medicine. With a current h-index of 70, he was identified to be one of the world's most influential researchers in the field of Materials Science by the Web of Science Group. Dr Chen is the recipient of the Highly Cited Researchers 2020/2019 in Web of Science, ACS Nano Rising Stars Lectureship Award, Frontiers in Chemistry Rising Stars, Okawa Foundation Research Award, MINE2020 Young Scientist Excellence Award, TenCate Protective Fabrics Award, Materials Research Society Graduate Student Award, National Award for Outstanding Students Abroad, National Scholarship of China, and many others.


1 Introduction

With the development of nanoscience and nanotechnologies, chemical sensing has become a hotspot for both researchers and consumers. As for health monitor devices, nano-size chemical sensors are often used to detect the changes of certain biomarkers in body fluids, such as glucose,1 uric acid,2 and certain neurotransmitters,3 as well as exocrine secretions such as sweat,4 which help monitor the condition of human health. As the attention on personal health is rising worldwide, the health industry has been developing rapidly in the past few years. As predicted by the Economist Intelligence Unit, health spending around the world is expected to rise at an annual growth rate of 5 percent in 2019–2023, up from 2.7 percent in 2014–2018.5 The high popularity of healthcare drives forward the demand for chemical sensors. Meanwhile, in sewage treatment, chemical sensors can help determine the type and quantity of pollutants, such as heavy metal ions6–8 and organic matters such as antibiotics.9,10 The treatment procedure could be specifically designed and continuously tracked, given the data collected by chemical sensors, which highly increases sewage treatment efficiency and performance. Other industries, such as the prevention of air pollution,11 waste degradation,12 and ecology,13 also benefit from the exact data of pollutants. Chemical sensors based on nanotechnology and nanoengineering also play a vital role in environmental protection by precise detection of pollutants, continuous detection of treatment effectiveness, and apparent elimination of costs. According to the EU in respect of these investments, its member countries have invested 34% more on environmental protection in 2019 than 2006,14 and so it is for countries outside of the EU. Such worldwide attention and development of environmental protection facilitate the development of chemical sensors.

Many chemical sensors, based on different materials and principles, have been explored and taken into use. As for sensing materials, carbon materials (nanotubes,15,16 nanowires,17 and graphene18,19), metal–organic frameworks,20 conjugated polymers,21,22 and ceramics23 have been adopted. Furthermore, optical,24 electrochemical,25 magnetic,26 and piezoelectric27 mechanisms were used as a basis to fabricate chemical sensors. In their development, chemical sensors have exhibited considerable sensitivity and detection range.18,20 However, these sensors more or less shared similar problems, such as poor degradation ability, lack of an ideal power source, complex signal transmission, etc.28 Chief among these problems is that of power sources. For chemical sensors, a sustainable power supply is one of the most decisive factors for large-scale use. The power supply is vital for sensors to stay active to target molecules and transmit electrical signals. Currently, there are many forms of power sources, such as batteries29–33 or fixed AC/DC power supplies transmitted from the major grid.34 However, these sources fail to perform well when integrated with widely-distributed chemical sensors in specific scenarios, e.g., aqueous, in vivo, or harsh environment, where common power sources, e.g., Li-ion batteries, are difficult to charge, repair or replace when they are exhausted;35 after every repair or replace, the unrecyclable materials used in old batteries will cause pollution.36 To provide enough power supply, the sources have to be large in size and mass or complicated in structure, which eliminates the portability of chemical sensors, especially nano-size ones. The additional function of receiving energy from the major grid also increases both the size and cost of the chemical sensors. As a result, traditional sensors are facing a large problem with the power supply. Working in a self-powered manner is a unique approach for the chemical sensor community to resolving the problem. Based on triboelectrification and electrostatic induction, a triboelectric nanogenerator is one of the best solutions among self-powered sensors for chemical sensing. The principle of TENGs allows their self-powering ability and dual-functionality, i.e. harvesting energy and sensing, which has been extensively discussed by previous researchers.37–57 By a simple contact electrification between two dissimilar materials with delicate surface nanoscale modification, TENGs can produce a variation of electrical quantities in the external circuit. This variation of electrical quantities can be adjusted by the amount of chemicals adsorbed on the triboelectric layers, giving TENGs potential to act as a chemical sensor.

TENGs are widely used in chemical sensing due to their outstanding advantages, among which self-powered ability is the most distinguishing characteristic, as proved by previous research studies on other TENG sensors.58–62 It reduces the need for an external power source, which is crucial in enhancing the portability and eliminating the inconveniences of recharging and replacement of batteries. This characteristic allows for miniaturization of sensors to even a nanoscale. Meanwhile, TENG sensors have really simple structures: basic TENG sensors only consist of a pair of triboelectric layers for chemical species modification and recognition, with a pair of electrodes and wires connecting them to the data processing and transmission system. These qualities largely reduce their fabrication cost, and make chemical sensing easier, cost-effective, and environmentally friendly. The materials used in TENG sensors hold a wide range of choices:63 metals,64 polymers,65 inorganics,66 oxides,67 and natural materials such as cellulose,68–70 skin71 and textile72,73 can be harnessed as the triboelectric layers of TENGs. This largely expands the usage of TENGs as chemical sensors in different environments.74

As TENGs bring revolutionary changes to chemical sensing together with the nanotechnology and nanoscience, as well as inspire novel sensor design and applications, this review article systematically summarizes the application of TENGs as a compelling technology to bring new blood into the chemical sensing community via two pathways: TENGs as a self-powered chemical sensor device, and TENGs as a renewable power source to sustainably drive a chemical sensor. With regard to the type of detectable chemical species, sensing of inorganic and organic chemicals is discussed (Fig. 1). As for TENG-based chemical sensor devices, two detection mechanisms via designing triboelectric layers or doped recognition materials are comprehensively included. Toward future field development, problems pressing for solutions and onward research directions of TENG enabled chemical sensing are also objectively discussed to deliver a coherent picture.


image file: d0nr07770d-f1.tif
Fig. 1 The pathways and detectable molecules of TENG based chemical sensing. The upper half shows one pathway: developing the TENG into a chemical sensor. The TENG receives chemical signals from molecules and produces electrical signals concerning chemicals. The lower half shows another pathway: developing a TENG as a power source to drive a chemical sensor. The TENG provides power to a chemical sensor. The left half shows that TENG sensors can detect organic chemical species, and the right half shows that TENG sensors can detect inorganic chemical species.

2 Principles of TENGs for chemical sensing

TENG devices work on the basic principles of triboelectrification and electrostatic induction. Wang et al.75 raised an explanatory theory on triboelectrification, in which the atoms on the interface of one material will attract the atoms on another through the Coulomb force, which is called adhesion.76 From a micro perspective, adhesion is defined as the overlap of electron clouds of two atoms or, in the perspective of quantum mechanics, the superposition of the wave function of electrons. When two materials come in contact with one another, the overlap of electron clouds appears, and the energy barrier between two atoms is lowered. As a result, the electrons tend to transfer between two atoms, as shown in Fig. 2a, and the inner surface of two triboelectric layers is charged. Thanks to the dielectric materials used as the triboelectric layers of TENGs, equivalent induced charges will occur on the back-coated electrodes as a result of electrostatic induction, and they may flow due to the difference in electric potential if the pair of electrodes are connected with a lead wire. Fig. 2b shows the distribution of charge on the triboelectric layers of the TENG and the current resulting from flow of electrons in the external circuit. This theory of triboelectrification and electrostatic induction can also explain the recognition and sensing of chemicals on the triboelectric layers. As some sites on triboelectric layers of TENGs may be active to certain molecules, they are more likely to react with or adsorb these molecules, referring to them as “target molecules”, on the surface, as shown in Fig. 2c.64,66 Besides triboelectric layers, sometimes external materials are introduced on the surface of the triboelectric layers of TENGs to complete the adsorption process.77,78 The specific mechanisms and cases of doping will be discussed in section 3. Since the electrons of adsorbed target molecules have different wave functions from that of the atoms on the triboelectric layer, the superposition process becomes different; therefore, electron cloud overlap is influenced. The extent of the overlap will change and influence the quantity of electrons on each triboelectric layer, and accordingly the charge distribution on the triboelectric layers will vary. Fig. 2c shows the case in which electrons accumulate due to adsorbed chemicals. Furthermore, Fig. 2d indicates the decrease of the electrical potential on the surface of triboelectric materials as a result of the electron accumulation. The more molecules are adsorbed, the more influence will be addressed to lower energy barriers and electrical potential. The change on potential on both triboelectric layers will be reflected in some electrical qualities, e.g., output voltage, output current, or inner impedance. Ultimately, some of these qualities will be used to determine the properties of chemicals, and thus the chemical sensing is realized.
image file: d0nr07770d-f2.tif
Fig. 2 Working principle of TENGs for chemical sensing. (a) The process of triboelectrification. The left part shows the overlap of electron clouds, and the right part shows the lowered energy barrier and transfer of electron as a result (reproduced with permission from ref. 75. Copyright 2019 Elsevier). (b) The charge distribution and current on the circuit-connected TENG after triboelectrification, shown in the longitudinal section. The charge on electrodes is generated from electrostatic induction, and the flow of electrons in circuit is the result of difference on electric potential. (c) The process of chemical recognition at the surface of triboelectric layers, shown in a magnified longitudinal section. When target molecules are recognized at certain sites, negative charge density on triboelectric layer 2 of the TENG increases and so does the induced positive charges on triboelectric layer 1. (d) Schematic electric potential distribution on the intersection of the surface and longitudinal section of triboelectric layer 2 before and after chemical sensing. At the recognition sites binding with target molecules, the electric potential drops, which implies information of chemicals.

Fundamentally, the TENG has four working modes:79 vertical contact-separation mode,80–83 lateral sliding mode,84,85 single electrode mode,86–88 and free-standing triboelectric-layer mode.89–92 The differences between each mode mainly lie in their different types of relative motion and the directions of the external force. The working mode of a particular chemical TENG sensor depends on the characteristics of target molecules and the environment of detection.

3 TENGs as chemical sensors

After the in-depth discussion of the mechanism of chemical sensing, this section will demonstrate the practical TENG-based self-powered chemical sensors regarding the type of target molecules and the mechanism of chemical sensing.

3.1 Inorganic species detection

Detection of inorganic species is a common application of chemical sensing in fields such as air pollution monitoring or sewage treatment. These applications require detection of inorganic gas, e.g. NO2[thin space (1/6-em)]93 and NH3,94 and inorganic liquid, especially the ions in the liquid, such as Cu2+ and Pb2+.77 Self-powered TENG chemical sensors are qualified in the detection of both matters with a wide detection range and good sensitivity. The molecules can be recognized either by the triboelectric layers of TENG or external doped materials. These two mechanisms are displayed in the following typical examples, while more applications are shown in Table 1.
Table 1 Applications of TENGs as a self-powered chemical sensor
Mechanism Structure Sensing performancea Ref.
Materials Detected object Detection range Sensitivity
Minb Max
a “—”in the table implies that the data were not recorded in research. “∼”in the front of data means that the data are an approximation. b Some research studies do not show the exact lower detection limit of the sensors. However, the detection limit is very near to zero which is suggested in other parts of the research.
Inorganic species
Doped materials PDMS Ammonia 0.1 ppm 25 ppm 0.137 ppm−1 94
PANI with Ce-doped ZnO (0.1–1 ppm),
1.11 × 10−2 ppm−1
(1–25 ppm)
Doped materials PTFE Cr3+ 4 nmol (Cr3+) 200 μmol L−1 4.38 × 10−3 μmol−1 (Cr3+) 77
Agents doped AAO Cu2+ 5 nmol (Cu2+) 5.16 × 10−3 μmol−1 (Cu2+)
Pb2+ 3 nmol (Pb2+) 3.25 × 10−3 μmol−1 (Pb2+)
Triboelectric layers PANI Ammonia 500 ppm 10[thin space (1/6-em)]000 ppm 2.60 × 10−4 ppm−1 103
PVDF (500–3000 ppm)
Triboelectric layers PDMS CO2 30[thin space (1/6-em)]000 ppm 1.11 × 10−4 nC ppm−1 (70% RH, static) 104
PEI
Triboelectric layers PEI CO2 10 wt% 105
FEP
Triboelectric layers PET H2 5 vol% 0.750 (0–2 vol%) 106
ITO with Pd deposition 0.200 (2–5 vol%)
Triboelectric layers PI H2 0.001 vol% 1 vol% 2.70 × 103 per vol% (0–0.1 vol%) 64
MC
Both with Pd deposition
Triboelectric layers Au Hg2+ 30 nmol L−1 100 μmol L−1 1.57 × 10−4 nmol−1 107
PDMS
Triboelectric layers Al Multiple inorganic liquid/solution e.g., NaCl, CrO3 5 wt% 5.38 × 10−3 V per wt% (NaCl) 108
PDMS 2.00 × 10−3 V per wt% (CrO3)
Triboelectric layers Al Multiple Ions e.g., Na+, NO3 ∼2.5 mg L−1 109
PTFE
Triboelectric layers Latex NO2 80 ppm 0.0416 ppm−1 93
Synthesized sensitive material
Triboelectric layers FEP H+ 1 × 10−7 mol L−1 1 × 10−2 mol L−1 7.4 V per pH unit 110
Triboelectric layers PTFE PEDOT:PSS NaCl 0.9 wt% 111
Triboelectric layers GO Water 20% RH 99% RH 1.69 × 10−2 per % RH (20%–85% RH) 66
Kapton
Triboelectric layers FEP Multiple vapors, e.g., water vapor 112
PFSA
Triboelectric layers PTFE Water 20% RH 100% RH 0.219 V per % RH (flow rate 15 L min−1) 113
Glass
Organic species
Doped materials ZnO-doped Ag Acetylene 30 ppm 1000 ppm 1.94 × 10−2 V ppm−1 114
PDMS
Doped materials TiO2 doped PET Ethanol 20 vol% 0.550 μA per vol% 115
Doped materials PI VOCs 0.1 vol% 10 vol% 102
NiO doped ZnO
Triboelectric layers Al Aniline 1200 ppm 4.00 × 10−3 mol−1 67
rGO-In2O3
Triboelectric layers PTFE Dopamine 500 μmol L−1 1.11 nA ln−1 (μmol L−1) 116
Triboelectric layers PA Ethanol 100 vol% 0.300 V per vol% 117
Al
Triboelectric layers Cu Multiple organic compounds e.g. ethanol, ethylene glycol 101
FEP
Triboelectric layers ZIF-8 Tetracycline 5 μmol L−1 80 μmol L−1 3.12 V μm−1 118
Kapton


3.1.1 Recognition by triboelectric layers. Target inorganic molecules can simply be adsorbed and remain on the triboelectric layers of the TENG. An example raised by Uddin et al.64 demonstrates a TENG-based hydrogen sensor. The structure of the chemical sensor is shown in Fig. 3a. PI and MC form two triboelectric layers of the TENG, and the double-layer wavy structure allows a larger contact area and larger output. Pd is deposited on both triboelectric layers and forms a layer with 10–15 nm thickness. A unique characteristic of Pd to absorb hydrogen molecules95 allows H2 to diffuse into the lattice of Pd crystals during the sensing process. This diffusion influences the electron affinity of triboelectric layers and thus negatively affects the response. As shown in Fig. 3b, the sensitivity of the sensor reaches 2.70 × 103 per vol%, though it begins to move significantly lower in high hydrogen concentrations.
image file: d0nr07770d-f3.tif
Fig. 3 Examples of TENG based chemical sensors for inorganic species detection. (a) Structure (left) and longitudinal section (right) of a H2 sensor. (b) Curve of the output voltage response of the sensor in (a) to H2 concentration. (Reproduced with permission from ref. 64. Copyright 2017 Elsevier.) (c) Structure and sensing mechanism (shown as inset) of a humidity sensor. (d) Curve of output open-circuit voltage of the sensor in (c) response to RH. (Reproduced with permission from ref. 66. Copyright 2020 Nature Publishing Group.) (e) Structure and sensing mechanism of an ion sensor. (f) Curve of output voltage response of the sensor in (e) to ion concentration. (Reproduced with permission from ref. 77. Copyright 2020 Wiley-VCH.)

H2 forms chemical bonds with Pd in the example above, but hydrogen atoms can also form hydrogen bonds. This characteristic is used as the recognition mechanism of some chemical sensors. For example, Ejehi et al.66 designed a humidity sensor based on GO. As shown in Fig. 3c, Kapton and GO are used as the triboelectric layers of the vertical contact-separation mode TENG. Water molecules can be adsorbed by GO at active sites through hydrogen bonds, such as hydrophilic groups and vacancies. This negatively influences the charge quantity on the triboelectric layers and finally decreases the output voltage. As the concentration of water vapor increases, hydrogen bonds among water molecules may increase and therefore create a continuous induction barrier, which positively contributes to the response of the output voltage. Fig. 3d shows that the response of the output voltage increases linearly to RH between 20% and 85% RH. When RH is higher than 85%, the response of the output voltage increases quickly. This phenomenon can be due to the ability of water molecules to penetrate into the internal layers of the GO under high concentration. Also, the response time between the applications of external impulse and the occurrence of peak short-circuit current Isc§ is less than 0.05 s. The chemical sensor, attributed to its low cost of materials, high linearity of output curves, and fast response time, may be a good choice for portable domestic humidity sensing.

3.1.2 Recognition by doped materials. Doping is commonly defined as introducing impurities into a material of interest.96 Here, it refers to the attachment of extra materials onto the surface of a TENG for complete recognition. Generally, the doped materials are grown on the triboelectric layers during the synthesis process. This is the case in the study by Wang et al.94 and other examples which will be discussed in different categories. However, special structures of triboelectric layers may allow for doping. For example, Li et al.77 designed a TENG-based chemical sensor to detect multiple ions. Fig. 3e shows the structure of the TENG, which works on vertical contact-separation mode. PTFE and AAO are the two triboelectric layers of the TENG. The AAO is porous at its surface, with an average pore diameter of 80 ± 5 nm and a pore depth of 40 ± 10 nm. Therefore, recognition molecules can be located in these pores and can specifically capture target molecules. Three ions, Cu2+, Cr3+, and Pb2+, are studied in this work. The three ions can be recognized and captured through chemical reactions by sodium diethyldithiocarbamate, diphenylcarbazide, and dithizone respectively. As shown in Fig. 3f, the voltage response is measured under the same external impulse, and the results show that the sensor has a constant and high sensitivity. The study also shows that the sensor has good selectivity and reliability: the response of the Pb2+ sensor to Pb2+ is 10 times higher than that of other ions, as an example. Such a sensor has high potential for future applications in sewage treatment, because the doped materials have been common agents to purify water. As an example, diphenylcarbazide, which is used in this sensor to detect Cr3+, has been a common agent in its treatment.97 TENG chemical sensors combined with these agents can potentially fabricate self-powered integrated sensors that also serve the purpose of water purification.

3.2 Organic species detection

Detection of organic species is a common application of chemical sensing in fields like air pollution monitoring,11 human healthcare,1 or marine ecology.98 Common organic species include VOCs such as isopropanol99 and antibiotics, i.e. tetracycline.100 Organic compounds usually have more complex structures and phase transformations. However, self-powered TENG sensors can detect them in either gas78 or liquid101 phases. Like the case of inorganic species detection, the molecules can be recognized either by the triboelectric layers of the TENG or external doped materials. These two mechanisms are displayed in the following typical examples, while more applications are shown in Table 1.
3.2.1 Recognition by triboelectric layers. An example of such a method is a sensor to multiple organic species that was developed by Wang et al.101 As shown in Fig. 4a, two semi-circular Cu electrodes cover a FEP tube with little vacancy, and two electric brushes, connected by an external circuit, are fixed and kept in contact with Cu electrodes. The electrodes are mounted on a rotating acrylic base. A solution of target molecules is injected into the tunnel, filling part of its lower half. Pre-friction is done prior to the detection, in order to positively charge the solution and negatively charge the electrode. As the FEP tube starts rotating, friction occurs and the net charge on the part of electrodes covering the lower half of the tube increases. This increase has different impacts on the two electrodes as they account for different portions of the lower half. The continuous rotation creates a cyclic change of charges on the two electrodes, as shown in Fig. 4a, and results in the generation of an output current and a voltage in the external circuit. The response is relevant to adsorbed target molecules due to their different polarities and contact angles. As shown in Fig. 4b, the output open-circuit voltage is different among different materials, such as hexane, isopropanol, ethanol, acetone, and ethylene glycol. This chemical sensor is prominent for its simple use: detection of chemicals is achieved by simply injecting their solutions into the tube, rotating the tube, and reading the output voltage. However, this sensor cannot give observers the exact concentration of target molecules. This problem is addressed in the sensor that is discussed below.
image file: d0nr07770d-f4.tif
Fig. 4 Demonstration of a TENG based chemical sensor for organic species detection. (a) Structure and working process of a self-recognition organic liquid sensor (shown in the longitudinal section). (b) Chart of output open-circuit voltage of the sensor in (a) with six different organic liquids. (From 1 to 6: hexane, isopropanol, ethanol, acetone, ethylene glycol, and DI water.) (Reproduced with permission from ref. 101. Copyright 2019 American Chemistry Society.) (c) Sensing mechanism of an alcohol sensor. (d) Curve of output voltage of the sensor in (c) to concentration of alcohol. (Reproduced with permission from ref. 78. Copyright 2015 Elsevier.) (e) Structure and sensing mechanism of a VOC sensor. (f) Curve of output voltage of the sensor in (e) under cyclic flow of methanol. (g) Curve of output voltage of the sensor in (e) under cyclic flow of ethanol. (Reproduced with permission from ref. 102. Copyright 2015 Wiley-VCH.)
3.2.2 Recognition by doped materials. Wen et al.78 designed an alcohol sensing TENG unit that works in free-standing triboelectric layer mode, with Cu and FEP as its triboelectric layers. p-Type semiconductor Co3O4, grown by a hydrothermal method, is doped on the triboelectric layers to detect alcohol molecules. Fig. 4c explains the principles of the recognition process to alcohol molecules. In a vacuum, the main charge carriers of Co3O4 are holes. After being exposed to air, oxygen can be negatively charged and adsorbed by the surface of Co3O4. As a result, an increase in the conductivity emerges due to the generated holes; thus it forms a charge accumulation layer on the surface of Co3O4 in order to neutralize added negative charges. Then, once the alcohol is introduced, it will be oxidized. As the consumption of oxygen decreases the concentration of oxygen cathodes on the surface of Co3O4, the charge accumulation layer becomes thinner correspondingly. As the concentration of alcohol changes, the charge density on the surface of TENG triboelectric layers also changes, consequently changing the output voltage as well. When the sensor is active, a constant rotation speed is applied to induce friction. As shown in Fig. 4d, the response of an open-circuit output voltage is almost linear to the concentration of alcohol, with a sensitivity of 0.016 V ppm−1. Notably, the detection range is 10–2000 ppm, wider than the linearity range; the response time of the sensor is within 5 s, which is an optimal parameter. The high sensitivity, wide detection range, and fast response time provide the chemical sensor with broad applications in detecting alcohol in daily lives such as self-powered breath analysers.

Such an effect of semiconductor oxides can be further applied to different chemical sensors with a wider choice of target molecules, therefore, combining the advantages of two organic sensors mentioned above. Here is an example. Kim et al.102 designed a TENG-based VOC sensor. PTFE and ZnO nanowires are its two triboelectric layers, and NiO particles with a radius of 20 nm are doped on the top of the ZnO nanowires, as shown in Fig. 4e. As NiO is a p-type semiconductor, the particles achieve recognition in the same way the alcohol sensor discussed earlier does. Results show that the device can distinguish different organic gases efficiently due to the different voltage changes after exposure to the various gases. Fig. 4f shows the curve of output voltage to time, in a repeated period of about 3 min exposure to methanol gas and about 10 min lapse until the voltage returns. Fig. 4g is the curve of ethanol under the same conditions. The difference of the two curves mainly lies in their response time. In addition, decreasing the rate and the ratio of the output voltage can distinguish different gases and their concentrations as well. A potential application of this chemical sensor is an electrical nose, if a signal processing system which can separate signals of different gases is implemented.

There are many other TENG-based chemical sensors listed in Table 1 with those discussed above.103–119 The table shows their characteristics and performances. Their performances are highlighted by the detection range and sensitivity. Generally, TENG-based chemical sensors show the following characteristics: First, polymers are often chosen as the triboelectric layers of TENGs, partly because of their good dielectric and mechanical performances; Second, the sensors show good detection capability at lower concentration compared with traditional sensors;17 Third, the selectivity of the sensors within the detection range is high,77 thanks to the specific detection ability of the triboelectric layers or recognition molecules.

4 TENGs as a power source for chemical sensing

As mentioned above, TENGs have been widely used in harvesting and providing energy. Therefore, they have become an ideal choice as a power source for chemical sensors in fields like pollution sensing120 and humidity detection.121 To become efficient in powering chemical sensors, the TENG has to meet the following criteria: firstly, the output should be direct, stable and constant in order to continuously drive the sensors; secondly, the maximum output power and energy conversion efficiency should be high; thirdly, it should be able to resist the influence of working conditions. With slight design modifications, TENGs have been proved to meet different requirements, support the work of different chemical sensors, and be ideal power sources for multiple chemical sensors to detect both inorganic and organic species with a stable, easily tunable, and broad range of output currents and voltage signals. Designs and performances of the TENG powered sensors are discussed below by their target molecules.

4.1 Inorganic species detection

As discussed above, TENGs can be connected to inorganic gas and ion sensors and effectively power them by leveraging pervasive mechanical energy. Possible target molecules of the sensor include gases like CO2,122 and NH3,120 and ions like H+.123 Below are some typical examples, while more applications are shown in Table 2. Lee et al.124 designed a polymer-based TENG to power a multi-inorganic gas sensor. They focus on 6FDA-APS, an organic material which has high charge density and as a result high output voltage, current, and power. Its charge density reaches about 512 μC m−2, the open-circuit voltage is 281.6 V, and the current density increases to 75.1 mA m−2 under 3 Hz external impulse. Notably, the maximum output power of the material is 9.3 μW. Its high electrical properties are contributed by its excellent charge-retention characteristics and enhanced charge transfer capability. Thanks to its high charge and power density, the material can be used as the triboelectric layers of a TENG, with Cu as another triboelectric layer. As shown in Fig. 5a, when it is used for powering a gas sensor, it is connected to a capacitor to store the energy temporally. Experiments show that the output voltage increases as the frequency of external force applied to the TENG increases, and the output voltage and current vary to different types and concentrations of target gas molecules, as shown in Fig. 5b. According to the figure, 10 min of power generation from the TENG can effectively power the sensor for 20 min. It displays potential for this TENG to power other chemical sensors durably and stably.
image file: d0nr07770d-f5.tif
Fig. 5 Demonstration of a TENG powered chemical sensor for inorganic species detection. (a) Structure of the TENG, and powering circuit of a NO2 sensor. (b) Curve of output open-circuit voltage and short-circuit current of the sensor in (a) during the whole working process of the sensor under different frequencies of impulse and choices of materials of sensors. (Reproduced with permission from ref.124 Copyright 2019 Wiley-VCH.) (c) Structure of the TENG, and powering circuit of a CO2 sensor. (d) Curve of output current of the sensor in (d) during the whole working process of the sensor under different concentrations of target gas. (Reproduced with permission from ref. 122. Copyright 2018 Elsevier.) (e) Structure and working process of the TENG of an ion sensor. (f) Structure of the sensor in (e). (g) Curve of output open-circuit voltage of the sensor in (e) to concentration of multiple ions. (Reproduced with permission from ref. 125. Copyright 2019 Elsevier.)
Table 2 Applications of TENGs as power sources for a chemical sensor
Materials Detected object Output performancea Detection range Ref.
Maximum I/μA Maximum U/V Minb Max
a “—”in the table implies that the data were not recorded in research. “∼”in the front of data means that the data are an approximation. b Some research studies do not show the exact lower detection limit of the sensors. However, the detection limit is very near to zero which is suggested in other parts of the research.
Inorganic species
PTFE CO2 19.8 2 × 105 ppm 122
Cu
6FDA-APS PI H2, CO, NO2 282 5 ppm 100 ppm 124
Cu
Chitosan-glycerol H2O ∼15 130 20% RH 80% RH 119
PTFE
Chitosan-glycerol Antigen 250 1.05 μg mL−1 8.4 μg mL−1 130
PDMS
PTFE Multiple ions e.g. Na+, SO42− 105 10 μmol L−1 125
Cu
PDMS NH3 15.0 10.0 5 ppm 60 ppm 120
Al
PTFE NO2 ∼10 ∼75 100 ppm 131
Al
PTFE NO2 ∼2 ∼20 100 ppm 132
Al
PE H+ ∼40 ∼300 123
Al
PTFE Water 1.86 33.7 7% RH 97.3% RH 121
Al
PTFE Water ∼500 97% RH 133
Cu
Organic species
Al Ethanol ∼2.4 ∼26 5 ppm 200 ppm 127
Kapton
Al Glucose ∼100 0.1 mmol L−1 1 mmol L−1 134
PDMS
Cu Glucose 317 153 135
PTFE
PMMA Glucose ∼20 ∼13 4.9 × 10−3 g L−1 1.5 g L−1 126
Kapton
PDMS Lactose 500 1 × 10−5 mol L−1 1 × 10−2 mol L−1 128
Gelatin
FEP Organic compounds from degummed fibers 3.50 × 103 10.0 129
Cu


Besides endurable and stable power supply, good power sources also require rapid performance when demand on current or voltage changes suddenly. TENGs are an ideal solution to chemical sensors of such cases. Zhao et al.122 developed a CO2 sensor powered by a TENG unit. As shown in Fig. 5c, Cu and PTFE are applied as two triboelectric layers of the TENG. When impulse is applied, the TENG creates voltage between the tungsten needle and the steel sheet, which are set in a little distance. Therefore, the gas undergoes a discharge process and produces a high voltage peak. As is shown, the TENG can power the sensor under different conditions. Fig. 5d shows the graph of the output current under different concentrations in complete charge and discharge cycles, given the distance between the tungsten needle and steel sheet being 0.15 mm. As CO2 concentration is increased from 0 to 200[thin space (1/6-em)]000 ppm, the current peak produced by the TENG increases from 12.5 to 20 μA, and the frequency of discharge also decreases. Under different CO2 concentrations and distance between the needle and the sheet, the TENG can produce different currents. As a result, the sensor can effectively detect CO2 in some situations such as an industrial gas purification process.

The TENG is also able to provide power to some chemical sensors which require both stable and sudden-changing voltage or current supply. An example is a TENG to supply power for ion concentration sensing in water designed by Chen et al.125 The structure of the TENG is shown in Fig. 5e. Cu and PTFE are applied as two electrodes of a lateral-sliding-mode-based TENG. The ion type and concentration in solution affect the impedance of the part shown in the dotted rectangle in Fig. 5f, because of their different mobility and degree of ionization. Given a moderate rotation speed of the TENG which generates power in a moderate frequency, the change of impedance is relative to the type and concentration of the target ion. Research shows that the maximum output voltage reaches 105 V, and the maximum output current is 112 μA. Fig. 5g shows the relationship between open-circuit voltage and the concentration and type of ions, tested by adding a specific amount of solution containing target molecules and waiting for about 10 s. The result suggests that the TENG can not only power the sensor within its whole detection range, but also respond when the concentration of ions either is stable or undergoes a sudden change.

4.2 Organic species detection

The TENG can also power several organic species sensors effectively. Possible organic species include glucose,126 ethanol,127 and lactose,128 which are discussed below. More applications are shown in Table 2. Li et al.129 developed a TENG-powered multiple organic compound sensor based on electrolysis. According to Fig. 6a, a lateral-sliding-mode-based TENG serves as a power source to the electrolytic cell. Powered by the TENG, organic compounds undergo oxidation or reduction reactions on the electrodes under a suitable pH value, and they synthesize molecules such as CO2 and water. So, the sensor can not only sense the on-line concentration of organic compounds, but also help degrade them. During the sensing procedure, the TENG shows a maximum output short-circuit current of 3.5 mA and an open-circuit voltage of 10 V. Fig. 6b shows the COD of a waste water sample, which indicates that the organic compound concentration decreases with respect to time and the increase of current. Under a 3.5 mA output current, the COD value decreases from 4589 mg L−1 to 420 mg L−1 in 150 min. This TENG-powered chemical sensor could be developed into an integrated water quality sensing and treatment device.
image file: d0nr07770d-f6.tif
Fig. 6 Demonstration of a TENG powered chemical sensor for organic species detection. (a) Structure of the TENG and powering circuit of an organic sensor. (b) Curve of COD to degradation time in (a) under different output currents of the TENG. (Reproduced with permission from ref. 129. Copyright 2016 Elsevier.) (c) Structure of the TENG of a glucose sensor. (Reproduced with permission from ref. 164. Copyright 2013 Wiley-VCH.) (d) Curve of output current to glucose concentration under different strains. (Reproduced with permission from ref. 126. Copyright 2013 Wiley-VCH.)

Yu et al.126 developed a TENG-powered glucose sensor. The TENG used in the sensor depends on vertical contact-separation mode, whose two triboelectric layers are Kapton and PMMA, as shown in Fig. 6c. In the sensing process, the resistance of the sensor changes as the concentration of glucose adsorbed on its surface changes, thanks to the characters of piezoelectric ZnO materials. As shown in Fig. 6d, the current increases as the concentration of glucose increases, and the output voltage, determined by strain, increases as well. The output of the TENG is stable and tunable from 0 to 0.2 μA under different conditions of glucose concentration and strain. The sensor could be used in in vivo glucose detection if the materials of triboelectric layers are more biocompatible.

There are many other TENG-powered chemical sensors listed in Table 2 along with those discussed above.121,130–135 The table shows their design, target molecules, and performances of powering and sensing. Generally, TENG-powered chemical sensors have satisfactory performances: As for the output, a rectifier is often connected to the TENGs to maintain a stable, constant direct current and voltage input to sensors, as a method to meet the requirement of stable power supply for TENGs.130,132,133 As for the energy conversion, the TENG can effectively harness the ambient mechanical energy. Table 2 shows that the TENG has large output voltage or current in many cases. As for influence of working conditions, the TENG proves its resilience to changing working conditions such as humidity.123 As for the chemical sensing process, generally, TENGs can be a good power source to different sensors regardless of their working principles, e.g., electrochemistry,129 piezotronic,126 gas discharging/ionization,122etc. This proves the wide applications of TENGs.

5 Conclusion and perspectives

5.1 Field expansion of TENGs for chemical sensing

Since the pioneer research of the TENG in 2012,37 its potential on self-powered chemical sensing has been widely discovered and developed by worldwide researchers. TENGs have been highly praised for their unique advantages in many aspects such as low cost,136,137 easy fabrication,138–140 and diverse choice of materials.141,142 Self-powering capability largely cuts the need of repairing and changing power sources for chemical sensors, eliminates environmental pollution caused by the content of batteries such as toxic electrolytes,36 and prolongs the working time of the sensor. A simple structure, i.e., two triboelectric layers, electrodes and connecting wires to the data processing and transmission system, allows for easy, rapid, and precise detection of chemical molecules, simplifies the fabrication process, decreases the cost, and expands the usage in multiple environments. Meanwhile, by changing the materials of the triboelectric layers or introducing different doping materials, sensing systems of different chemicals can be easily produced. High sensitivity and selectivity give TENG based chemical sensors opportunities to be used in numerous fields such as environmental protection102,104 and human healthcare.128,135 Low response time and a wide detection range allow instant, or on-line detection of chemicals, and therefore can be applied to fields with high demand of punctuality, such as a real time air pollution monitor102 and an alcohol breath analyser.78

The current applications of TENG based chemical sensors were discussed above, and are abbreviated in Fig. 7. The discussion was progressed by classifications of the different roles of the TENGs, molecule species, and detection principles. The sensors have been used to detect multiple chemical molecules from inorganic species such as CO2,122 NH3,103 and Cu2+ (ref. 77) to organic species such as ethanol127 and glucose.135 As the development of technology and industry increases, TENG based chemical sensors can be applied into many more fields, as shown in Fig. 7. In human healthcare, Internet of Things (IoT) working together with TENG sensors, including chemical sensors, may give human health monitoring a fresh sight. In the IoT paradigm, many objects surrounding people will be connected, and in this way IoT is on the verge of transforming the current static Internet into a fully integrated future Internet.143 Connection of TENG sensors to IoT can expand the application to a new, broad phase. For example, if a glucose sensor is connected to a computer through IoT, doctors will have access to live glucose levels in patients and write prescriptions; patients will monitor their own health status and change their diet accordingly; an automatic program may also be set to purchase hypoglycemic agents and be sent to the house of patients if his glucose level rises. An in vivo heavy metal ion sensor connected to IoT may work with sirens to monitor poisoning.


image file: d0nr07770d-f7.tif
Fig. 7 A summary of current progress and future perspectives of TENG enabled chemical sensing. From inside to outside, the following aspects of TENG enabled chemical sensing are shown in different layers: (1) The TENG can efficiently work as a chemical sensor, as well as a power source for chemical sensors. (2) When they work as chemical sensors, the TENG can recognize chemicals by either chemically modified triboelectric layers or doped materials. (3) The TENG has been developed into different designs and materials. (4) TENG related chemical sensors have been used to sense multiple organic and inorganic molecules. (5) In the future, enhancing the sensitivity, improving the selectivity, and maintaining the stability are highly desired and require more research inputs for TENG-based chemical sensors as ways to improve their overall performances and reliability. (6) The TENG is projected to have extensive application prospects in health care, environmental protection, and manufacturing procedures.

In the field of environmental protection, TENGs can be used to detect the concentration of toxic and greenhouse gases. Current gas sensors find it difficult to work under harsh conditions,144 but TENG gas sensors can solve this question by their stable performance. The simple structure and easy-to-get materials allow their use in cheap gas sensors such as domestic gas sensing. In addition, TENGs can play an important role in the manufacturing procedure. They have good prosperity in sewage recycling and treatment. Current usage of sewage treatment methods has met problems, e.g., vulnerability to changing working conditions, unstable performance, and difficulty in detecting the accurate amount of pollutants during treatment.145,146 The TENG chemical sensor can not only perform well under complex conditions, but also accomplish detection and removal of pollutants in one step, as discussed above. Currently, TENGs can still be optimized to reach better performance in chemical sensing in the following aspects.

5.2 Sensitivity

Sensitivity is one of the most important parameters of a sensor, which means the variation of measured quantity in unit variation of detected quantity. Improving the sensitivity requires a higher electrical feedback change in response to the increase of target molecule concentration. This may be achieved by increasing the amplitude of external impulse or choosing more electrical-sensitive triboelectric layer materials with higher charge densities, such as 6FDA-APS,102 and active carbon-doped PVDF.147 A carefully designed sensing mechanism may also eliminate possible interferences by quantities which are hard to control, such as internal resistance of the sensor.93 It has also been discussed before that environmental factors, such as humidity and temperature,148 may influence the sensitivity of chemical sensors. An appropriate working environment is required for the TENG based sensors.

During the chemical sensing process, it is important to control errors in order to maintain a stable and accurate sensitivity. One possible source of errors is mechanical stimuli to triboelectric layers. This always happens during the injection or flow of chemicals into triboelectric layers of the TENG. There are generally two solutions to this problem: one is, to fix the layers on a solid, rigid material to prevent unwanted mechanical triggers;102 another is, to add a constant, higher impulse on triboelectric layers in the detection period to weaken the effects of the mechanical by-product.66 Other possible sources of errors include interferences of environmental factors and non-target chemicals, which will be discussed below.

It is equally important to expand high sensitivity to a broader detection range. The principle of TENGs suggests that the minimum and maximum of the detection range of the sensor are determined by the minimum and maximum output voltage or current of the TENG. Since the number of adsorbed target molecules affects the response, and friction is a vital issue to the initial value of electrical quantities, increasing the maximum number of adsorbed target molecules and extent of friction will be beneficial to expand the upper limit. Doping leads to better recognition and capture of target molecules, which contributes to the former strategy.102,107 Energy band engineering, which has been a hotspot, may positively contribute to doping techniques.149 As for the latter strategy, besides enhanced impulse, a larger contact area will contribute to it. Specially designed structures, such as coarse,150 porous77 or wavy64 surfaces with a complex nanostructure, are useful for enlarging the contact area,151 which has been proved rather effective in the alternative application of TENGs as an energy harvester.82,89,152 Moreover, an appropriately designed external circuit or rectifier will decrease the loss of electrical energy and thus expand the lower detection limit.153

Sometimes dynamical factors influence linearity and sensitivity of sensors in very low or very high concentration of target molecules. For example, in the example of Uddin et al.,64 saturation of H2 in Pd causes the elimination of sensitivity when the concentration is very high. This is because the adsorption and recognition processes become different, as discussed above. The solution may be modification or replacement of triboelectric layer materials.

As for TENG-powered chemical sensors, the output performance of the TENG will influence the sensitivity of sensors. High sensitivity requires low response time (i.e., hysteresis), and a large detection range requires high output of TENG and high energy conversion efficiency. Low hysteresis may be contributed from materials with high elasticity.154 The high output of TENGs may be contributed by special structures mentioned above which make a large contact area,151 or chemical functionalization to improve the charge density of TENGs.155 Energy conversion efficiency can be enhanced by the improved power management to match the impedance between TENGs and sensors or special material designs.137,142

5.3 Stability

Stability refers to the robustness of sensing performance, which has two meanings which are all discussed below. The first meaning is the stable status of the comprised materials of the sensor, which suggests that the working process, especially output quantities of sensors, is not affected by the change of chemical and physical properties of materials as well as environmental factors. It is also defined as reproducibility. Delicately chosen materials will improve the reproducibility of sensors. For example, PTFE has been widely chosen as a material for triboelectric layers in many sensors discussed above,77,109,116 for its stability given by its small maximum pore size and good pore size distribution. Prevention from corrosive factors is another method. Salts, organic compounds, and biological effects may negatively affect the stability of materials, and measures like pretreatment, flushing, gas bubbling, or surface modification will help.156 Enhanced mechanical properties such as intensity and malleability may also contribute to enhanced reproducibility.157 Meanwhile, delicately choosing a mild working environment will be effective.129

The second meaning is the stable status of the working process. It implies that the sensor should give identical results to identical quantities for detection. It is also defined as repeatability. Two influential factors may be considered to this: the recovering of materials after applied external force (pressure or friction) and the desorption of target molecules after each detection. High elasticity of materials may address the first problem as shown in some examples,158,159 while the second problem may be overcome by changing reaction conditions. For example, continuous flow of pure gas after each detection may promote the desorption by eliminating the partial pressure of target gas molecules. Choosing suitable materials is another method to propagate the desorption. As an example, Cui et al.103 chose PANI as the triboelectric layer material, which can desorb the NH3 molecules quickly and increase the repeatability and stability.

As for TENG-powered chemical sensors, high stability of the sensor is guaranteed by stable output performance of the TENG. The strategies discussed above can be applied, such as choosing elastic and anti-corrosion materials, and protection from corrosive or severe working conditions.

5.4 Selectivity

Improving the selectivity of sensors is critical in improving the accuracy of detection. Selectivity refers to the ability to show high sensitivity to target molecules but low to others. Examples discussed above show good selectivity. For example, in the design by Li et al. the Pb2+ sensor shows a voltage response about 10 times higher than other ions in sensing Pb2+.77 It is achieved by the specific recognition of dithizone. As an inspiration, the selectivity may be enhanced by choosing suitable materials which show high response to target molecules and low response to others. Like the example, doping suitable materials is a convenient way to easily increase the selectivity of the sensors. Other ways include installing filters before the sensing process to remove unwanted molecules,160 reducing the noise of the output electrical signal,161 or maintaining appropriate working condition of the sensors.

5.5 Reliability

As the applications of TENG chemical sensing become much broader, the problem of reliability has also drawn the attention of researchers. Reliability of a sensor refers to the overall ability of a product to perform a required function at or below a stated failure rate for a given period of time.162 Reliability is critical in the application of sensors: in environmental monitoring, low reliability may result in failure to control and manage pollutants and thus do harm to environments; in human healthcare, low reliability may lead to misdiagnosis.

Reliability is a complicated quality. According to Choudhury et al., reliability can be ascertained by sensitivity, zero error, range, resolution, precision, accuracy, linearity, hysteresis, repeatability, reproducibility, response time, and transmission loss.163 In the discussions above, the sensitivity, detection range, linearity, hysteresis, repeatability, reproducibility, response time, and transmission loss have been involved, and here the remaining aspects will be discussed. As for zero error, most of the sensors above avoid this problem by measuring the response of output voltage or current.64,66,77 The zero error will be offset by subtracting the reading value after detection from that before detection in the course of calculating response. As for resolution, it can be enhanced by choosing more electrical-sensitive triboelectric layer materials.102,147 As for precision or accuracy, the improvement of both resolution and stability will work positively.

In conclusion, the TENG has witnessed its large-scale applications in chemical sensing since its first development. Under different mechanisms, structures and materials design, the TENG has shown outstanding capability in sensing a broad range of materials from inorganic gases to organic metabolites in vivo. With the efforts of researchers, TENG based chemical sensors can perform better and find their applications in more domains with compelling and unique advantages. The development of TENG based chemical sensing will be further boosted by dedicated work of researchers.

Abbreviations

6FDA-APS4,4′-(Hexafluoroisopropylidene)diphthalic anhydride bis(3-aminophenyl)sulfone
AAOAnodic aluminium oxide
CODChemical oxygen demand
EUEuropean Union
FEPFluorinated ethylene propylene
GOGraphene oxide
IoTInternet of things
ITOIndium tin oxide
MCMicrocrystalline cellulose
PAPolyamide
PANIPolyaniline
PDMSPolydimethylsiloxane
PEDOT:PSSPoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
PEIPolyethylenimine
PETPolyethylene terephthalate
PFSAPerfluorosulfonic acid ionomer
PIPolyimide
PTFEPolytetrafluoroethylene
PVDFPoly(vinylidene fluoride)
rGOReduced graphene oxide
RHRelative humidity
TENGTriboelectric nanogenerator
VOCVolatile organic compounds
ZIFZeolitic imidazole framework

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This is an invited review article for the Emerging Investigator Special Issue of Nanoscale. The authors acknowledge the Henry Samueli School of Engineering & Applied Science and the Department of Bioengineering at the University of California, Los Angeles for the startup support. J. C. also acknowledges the 2020 Okawa Foundation Research Grant.

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Footnotes

These authors contribute equally to this work.
Here, “response” of voltage or current is defined as |XX0|/X0. X refers to changed current I or voltage V, and X0 refers to former current or voltage. Sic passim.
§ Isc refers to short-circuit current, and Voc refers to open-circuit voltage. Sic passim.

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