A Self‐Powered Triboelectric Nanosensor for Mercury Ion Detection

Abstract

Mercury is a highly toxic metal that can pose serious dangers to the environment and human health. Therefore, monitoring the mercury concentration is an extremely important issue to prevent such a toxic metal from endangering human life. Hg ions are by far the most stable inorganic form of mercury, which are non-biodegradable and bioaccumulable. Even at a very low concentrations, they can still be fatal to human brain, heart, and kidney. Multiple approaches, such as atomic absorption/emission spectrometry and inductively coupled plasma mass spectrometry (ICP-MS), have been applied to detect the Hg ions in environmental and biological samples. Among these approaches, ICP-MS is the method that has the highest sensitivity and the widest linear range. However, sample preparation before measurement is rather complicated and time-consuming, and requires expensive instrumentation and the use of noble gas. Another drawback of this method is the difficulty in performing infield analysis. In consideration of the above disadvantages, researchers have instead developed a number of optical methods (colorimetric or fluorometric assays, and systems based on surface plasmon resonance or surfaceenhanced Raman scattering) and electrochemical sensors for the detection of Hg ions. Optical approaches are advantageous in high sensitivity and selectivity, and practicable for in-field analysis, yet they are involved with sophisticated chemistry in incorporating organic probes, such as crown ethers, porphyrins, specific proteins, DNA, and polymers, which substantially limits their application range. To overcome the limitations of these methods, the concept of self-powered nanosensors has recently been proposed and tested for its potential toward toxic pollutant detection. The working principle of self-powered nanosensors is based on combining/integrating nanogenerators with sensors. The nanogenerators harvest energy from the environment to power the sensors. Owing to its convenient monitoring mechanism, the self-powered nanosensors could be the most desirable and promising prototype for environmental protection/detection in the near future because no battery is needed to power the device. For the time being, the major challenge is how to develop a fully integrated, stand-alone and selfpowered nanosensor. The triboelectric effect is an old but well-known phenomenon in daily life. Recently, it has been utilized as an effective way to harvest mechanical energy. Contact between two materials with different triboelectric polarity yields surface charge transfer. The periodic contact and separation of the oppositely charged surfaces can create a dipole layer and a potential drop, which drives the flow of electrons through an external load in responding to the mechanical agitation. As for triboelectric nanogenerator (TENG), maximizing the charge generation on opposite sides can be achieved by selecting the materials with the largest difference in the ability to attract electrons and changing the surface morphology. For the plate-structured TENG, it needs more time and stronger applied force to ensure the contact and separation of the two oppositely charged material surfaces upon pressing and releasing are complete, especially under the electrostatic attraction between them. Adding spacers between two plates or using arch-shaped substrates have been demonstrated to improve the output of TENG. Herein, we show that the principle of the TENG can be used as a sensor for the detection of Hg ions. The first step is to improve the performance of the TENG through the assembly of Au nanoparticles (NPs) onto the metal plate. These assembled AuNPs not only act as steady gaps between the two plates at the strain-free condition, but also enable the function of enlarging the contact area of the two plates, which will increase the electrical output of the TENG. Through further modification of 3-mercaptopropionic acid (3-MPA) molecules on the assembled AuNPs, the high-output nanogenerator can become a highly sensitive and selective nanosensor toward Hg ions detection because of the different triboelectric polarity of AuNPs and Hg ions. On the basis of this unique structure, the output voltage and current of the triboelectric nanosensor (TENG) reached 105 V and 63 mA with an effective dimension of 1 cm 1 cm. Under optimum conditions, this TENG is selective for the detection of Hg ions, with a detection limit of 30 nm and linear range from 100 nm to 5 mm. A commercial LED lamp was tested as the indicator to replace the expensive electrometer and showed the possibility to simplify the detection system. Our study demonstrates an innovative and unique approach toward selfpowered detection of Hg. [*] Dr. Z.-H. Lin, G. Zhu, Y. S. Zhou, Dr. Y. Yang, P. Bai, J. Chen, Prof. Z. L. Wang School of Material Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332-0245 (USA) E-mail: zlwang@gatech.edu