How are 2D Heterostructures Used for Chemical Sensing?

By Chinmay ChariReviewed by Susha Cheriyedath, M.Sc.Oct 13 2022

In a paper recently published in the journal Advanced Functional Materials, researchers reviewed the recent developments in 2D van der Waals heterostructure (VDWH)-based chemical sensors, providing an overview of the sensing techniques, challenges, and future opportunities.

Study: 2D Van der Waals Heterostructures for Chemical Sensing. Image Credit: luchunyu/Shutterstock.com

Background

Sensors can be used to examine bodily motions or find residues of hazardous gases, pesticides, biomolecules, antibiotics, and microorganisms in meals, beverages, and the air. Downscaling miniaturized devices are now conceivable due to the exceptional charge transport properties of 2D materials that are preserved in sub-nanometer-thick sheets. Due to their accessibility and distinctive qualities, transition metal dichalcogenides (TMDs) as well as graphene have been thoroughly researched for their utility in sensing.

Graphene exhibits various benefits for creating sensors, including high carrier mobility, a high surface-to-volume ratio, and remarkable electrical and thermal properties. TMDs, like graphene, have a sizable specific area, which makes them excellent platforms for enhancing sensor performance. Due to its abundance in nature, molybdenum disulfide (MoS₂) is a thoroughly studied compound. Given the range of 2D material qualities and their availability, combining them in hybrid structures can be a potent tactic for taking advantage of their complementary strengths.

VDWH Synthesis Strategies

By using the weak VDW interactions that hold together adjacent 2D layers, VDWH can be created by vertically superimposing two or more 2D materials. The first instance of VDWH was reported in a study in 2010. Exfoliated monolayer graphene was layered on top of a 2D hexagonal boron nitride (h-BN) layer that was synthesized on a SiO₂/Si substrate by the group in a vertical heterostructure.

Although graphene/h-BN heterostructures have been initially researched, most papers have primarily studied graphene/TMD heterostructures. Mechanical transfer, liquid phase assembly, and chemical vapor deposition (CVD) are all used in the manufacturing of VDWHs. By combining various suspensions of single- and few-layer 2D materials generated by liquid-phase exfoliation, heterostructures with random stacking can be synthesized. Utilizing 2D materials created by CVD allows for the fabrication of vertical heterostructures having large areas.

VDWH-Based Gas Sensors

Utilizing a variety of mechanisms, such as the development of p-n junctions, the improvement of electrical properties during interactions with sensory materials, the photoelectric effect, and the tuning of Schottky contacts, VDWHs may improve gas sensing performances. A graphene/MoS₂ heterostructure, which was based on an NH₃ and NO₂ chemo-resistive sensor, was created in a study in 2015. Additionally, VDWHs can be employed to increase chemical stability and inhibit the active sensory material from deteriorating.

The first example of a p-n VDWH-based gas sensor was used to detect NO₂ and was built on a heterostructure that was placed between MoS₂ flakes and few-layered phosphorene (BP). Recently, a 2D heterojunction between p-type and n-type MoS₂ was used to create a NO₂ sensor. The sensor that was n-type MoS₂-based had the highest sensitivity toward triethylamine (TEA), and the p-type MoS₂-based sensor had the highest selectivity toward NO₂.

Electrochemical and Biosensors

Graphene-based materials can combine their benefits and enhance their electrochemical performance by hybridizing with TMDs, particularly MoS₂. Sensing devices have hybrid graphene/MoS₂ integration for more sensitive NO₂ detection. A variant of the organophosphorus insecticide methyl parathion was discovered, and sensors based on MoS₂/graphene nanocomposite were created to detect it. Common drugs and their precursors can also be detected using MoS₂/graphene-based sensors.

Electrochemical biosensors are simple-to-use analytical tools that enable the rapid, accurate, and selective detection of a variety of analytes at a low cost with great sensitivity. To create mediator-free biosensors that can detect H₂O₂, hemoglobin (Hb) having flower-like MoS₂-modified graphene oxide (GO) nanocomposite has been produced. In order to identify antibiotics, an electronic tongue (e-tongue) which was based on MoS₂ and GO was described. Due to their high specific surface area and strong electric conductivity, graphene and MoS₂ biosensors can be used to detect a variety of biomolecules, including l-ascorbic acid (AA), dopamine (DA), folic acid (FA), and uric acid (UA).

Due to their quick response time and excellent sensitivity, surface plasmon resonance (SPR) biosensors have gained growing attention for biosensing applications like DNA hybridization. Other 2D materials, including BP, WS₂, MoSe₂, and WSe_2, have also shown the ability to boost SPR sensitivity because of their distinctive electrical and optical properties.

Conclusions

To summarize, the researchers outlined the possible uses of hybrids comprising two or more 2D materials, more commonly referred to as VDWHs for sensing various substances. Due to their high surface-to-area ratios, 2D materials make interesting sensing candidates. Graphene can act as a shield for MoS₂, which is air-sensitive in biosensors, increasing the stability of the produced devices.

However, the process utilized to create many of the VDW entails time-consuming and non-scalable stages. Additionally, it is still unclear how VDWHs function in terms of biosensing. The researchers must create industrially scalable and reasonably priced manufacturing methods in order to use 2D VDWHs in practical applications. In order to produce point-of-care devices to be applied in medical diagnosis, research on the integration of developed sensors in portable and flexible technologies would be crucial.

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