This article discusses the rise of transition metal dichalcogenides(TMDs) in sensing devices, covering specific examples of these sensitive systems.
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Transition metal dichalcogenides (TMDs) are layered materials in which a single layer contains metal atoms sandwiched between chalcogen atoms. The metal is bonded to the neighboring chalcogens through covalent bonds. Moreover, in a stack, such layers are bonded to each other through weak van der Waals forces of attraction.
TMDs have reported chemical, physical, mechanical and electronic properties similar to that of graphene. In addition, they have semiconducting properties that make them more suitable for many optoelectronic applications.
A few examples of TMDs are molybdenum disulfide (MoS2), tungsten disulfide (WS2) and molybdenum ditelluride (MoTe2).
TMDs exist in three polytypes, 2H, 3R and 1T, depending on the crystal structure. H, R and T represent hexagonal, rhombohedral and trigonal crystal structures, respectively. 2H is the most thermodynamically stable form showing semiconducting properties, whereas 1T is a metastable form having metallic properties.
Many strategies are employed to produce TMDs, such as chemical vapor deposition (CVD), mechanical exfoliation, hydrothermal/solvothermal method and liquid-phase exfoliation.
Different morphologies of TMDs are reported, such as nanoribbons, nanosheets, and nanospheres. TMDs are exploited for sensing different moieties such as heavy metals, biomolecules, toxic gases and many more.
What Makes TMDs Suitable for Sensing Applications?
TMDs have a high surface area to volume ratio that provides large active sites for surface adsorption of external moieties. This provides an excellent platform for the adsorbents for anchoring on the sites. In addition, TMDs are surface-sensitive materials making them very sensitive to environmental changes. Their properties change with the surface adsorbent, surface doping, stress/strain or under any such conditions.
TMDs bandgap can be tuned through various techniques that further provide appropriate bandgap and band alignment with its composite. Moreover, defect engineering in TMDs can be easily performed to make them more suitable for sensing applications.
TMDs for Heavy Metal Detection
The properties of TMDs such as high surface area, high electrical conductivity and tunable electrical/ optical properties make them suitable for sensing heavy metals. Few layered MoS2-based field-effect transistors (FET) were studied by Jiang et al. for the detection of Hg2+ ions.
These ions have strong binding towards the sulfur of MoS2 with partial electron transfer from MoS2 to Hg2+. The conductivity, as well as the photoluminescence, was found to be modulated with the addition of the ions.
Various other studies have reported TMDs to be efficient in detecting heavy metals like Cu2+, Ag2+, Cd2+, Zn2+, Pb2+, Cr2+ and Sn2+, even at deficient concentrations. Detection and removal of such heavy metals are significant as they bring detrimental effects to the health of living beings.
TMDs for Biomolecules Detection
Detection of glucose levels in blood samples of diabetic patients using a simple and sensitive device is in high demand. In work by Rohaizad et al., the metallic phase of WSe2 was studied to show improved signaling as glucose biosensors.
The presence of metallic 1T phase of WSe2 was reported to increase the rate of heterogeneous electron transfer. This was further reported to improve the electrocatalytic performance and signal transduction in glucose biosensors.
The incorporation of metallic nanoparticles in TMDs has also reported to increase the response time of the sensor.
Detection of DNA is of significant importance in the field of molecular diagnosis, cancer detection and forensic investigation. In research by Wang et al., ssDNA is immobilized over exfoliated TMDS for probing the dsDNA. The hybridization of dsDNA at the surface of TMDs causes a change in the physical properties of TMDs used for sensing the DNAs.
A peptide nucleic acid-based Lab-on-PCB microsystem was developed by Jolly et al. for the rapid detection of DNAs. The increase in the concentration of complementary DNA was detected through the increase in charge transfer resistance observed through a Nyquist plot.
TMDs-based FET biosensors have been reported to show advantages of small size, high sensitivity, selectivity, low detection limit and fast response.
Mei et al. reported a MoS2 based FET biosensor functionalized with phosphorodiamidate morpholino oligos (PMO) probe for the detection of DNA, which could detect DNA at a concentration of 6.0 fM.
Gas Sensing Applications of TMDs
Gas sensing utilizing TMD sensors is mainly through the direct interaction of gas molecules with the surface of TMDs. WS2-based gas sensors have reported high sensitivity at room temperature for NH3 sensing based on the mechanism of physical adsorption.
Heterostructure formation and chemical functionalization of TMDs have been reported to show enhanced performance in sensing.
Outlook for TMD Based Sensors
TMDs are an emerging class of two-dimensional materials having properties of high surface to volume ratio, high in-plane carrier mobility, bandgap tunability and high surface sensitivity making them a suitable material for sensor applications. Water contamination detection, toxic gas detection and biomolecules detection are a few of the notable areas where TMDs have been explored to show efficient performances.
However, there is plenty of room to explore the practical application of TMDs as sensors. Further research is required to better understand the optimization mechanisms and material modifications.
Large-scale production with uniform deposition of two-dimensional TMDs is an obstacle to the commercialization of these sensors. Furthermore, theoretical concepts on the TMD heterostructure, analyte interaction and the working media have to be developed before TMD based sensors become commercially available.
Continue reading: What are Transition Metal Dichalcogenides (TMDs)?
Reference and Further Reading
Rahman, M.T., Kumar, R., Kumar, M. and Qiao, Q., (2021). Two-dimensional transition metal dichalcogenides and their composites for lab-based sensing applications: Recent progress and future outlook. Sensors and Actuators A: Physical, 318, p.112517.
Zhang, X., Teng, S.Y., Loy, A.C.M., How, B.S., Leong, W.D. and Tao, X., (2020). Transition metal dichalcogenides for the application of pollution reduction: A review. Nanomaterials, 10(6), p.1012.
Jiang, S., Cheng, R., Ng, R., Huang, Y. and Duan, X., (2015). Highly sensitive detection of mercury (II) ions with few-layer molybdenum disulfide. Nano Research, 8(1), pp.257-262.
Rohaizad, N., Mayorga-Martinez, C.C., Sofer, Z. and Pumera, M., (2017). 1T-phase transition metal dichalcogenides (MoS2, MoSe2, WS2, and WSe2) with fast heterogeneous electron transfer: application on second-generation enzyme-based biosensor. ACS applied materials & interfaces, 9(46), pp.40697-40706.
Wang, X., Nan, F., Zhao, J., Yang, T., Ge, T. and Jiao, K., (2015). A label-free ultrasensitive electrochemical DNA sensor based on thin-layer MoS2 nanosheets with high electrochemical activity. Biosensors and Bioelectronics, 64, pp.386-391.
Jolly, P., Rainbow, J., Regoutz, A., Estrela, P. and Moschou, D., (2019). A PNA-based Lab-on-PCB diagnostic platform for rapid and high sensitivity DNA quantification. Biosensors and Bioelectronics, 123, pp.244-250.
Mei, J., Li, Y.T., Zhang, H., Xiao, M.M., Ning, Y., Zhang, Z.Y. and Zhang, G.J., (2018). Molybdenum disulfide field-effect transistor biosensor for ultrasensitive detection of DNA by employing morpholino as probe. Biosensors and Bioelectronics, 110, pp.71-77.
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