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Ling, X. Graphene-Based Environmental Sensors. Encyclopedia. Available online: https://encyclopedia.pub/entry/9300 (accessed on 04 September 2024).
Ling X. Graphene-Based Environmental Sensors. Encyclopedia. Available at: https://encyclopedia.pub/entry/9300. Accessed September 04, 2024.
Ling, Xi. "Graphene-Based Environmental Sensors" Encyclopedia, https://encyclopedia.pub/entry/9300 (accessed September 04, 2024).
Ling, X. (2021, May 04). Graphene-Based Environmental Sensors. In Encyclopedia. https://encyclopedia.pub/entry/9300
Ling, Xi. "Graphene-Based Environmental Sensors." Encyclopedia. Web. 04 May, 2021.
Graphene-Based Environmental Sensors
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This entry is adapted from the peer-reviewed paper 10.3390/molecules26082165

Graphene-Based Environmental Sensors means use graphene as a sensing platform for environmental applications. Especially, we highlight the electrical and optical sensing devices developed based on graphene and its derivatives.

2D materials sensing FETs spectroscopy sensitivity selectivity

1. Introduction

Simple and reliable sensors for trace species detection are highly desirable in a spectrum of applications ranging from medical diagnosis, environmental monitoring, industrial and agricultural processes control, and lab-on-a-chip. In the past two decades, various nanomaterials have been explored for sensing applications, including nanoparticles, nanowires, nanotubes, and nanosheets [1][2][3][4][5][6]. Among these nanomaterials, graphene, a two-dimensional (2D) carbon layer, has drawn significant attention in sensing applications due to its novel properties. The unique structure and electronic properties of graphene (e.g., atomically-thin thickness, large specific surface area, high electron mobility, and high sensitivity to electronic perturbations from foreign molecules) are the cornerstones for the development of graphene-based materials as sensing platforms [7][8][9][10][11]. The general working principle of graphene-based sensors rely on detecting perturbations in electrical and optical signals caused by interactions between graphene and a target molecule. To improve these interactions for selective sensing, probes are usually introduced on the surface of a graphene sheet through surface modifications (e.g., chemical linkages) that act as specific binding sites to the target species.

2. Graphene-Based FET Sensors and Graphene-Based Optical Sensors

2.1. Graphene-Based Optical Sensors

The versatility of graphene is also showcased in optical sensors, which enjoy a plethora of sensing mechanisms that can provide more information about the target species than field-effect-transistors. In essence, optical sensors are instruments capable of measuring the interaction between electromagnetic radiation and matter; here, we discussed how spectroscopies can benefit from graphene. A major advantage of optical sensors arises from the fact that they are capable of probing energetic states (e.g., electronic, vibrational, rotational) which are intrinsic to the target species, paving way for directly detecting the presence of the target analyte. In stark contrast to GFET devices that, instead, rely on indirect measurements (i.e., conductivity changes in the graphene-molecule complex) to detect the target analyte.

There are numerous optical sensors with distinct sensing mechanisms, and while all are not presented here, we highlight the use of graphene in a variety of spectroscopic modalities ranging from colorimetric sensors, that require little sample preparation and almost no post analysis processing, to more complex techniques like Raman spectroscopy that provide specific spectral signatures but require specialized instrumentation and knowledge to operate. We begin by discussing sensors that detect changes in the optical properties of the graphene-analyte complex.

Graphene-based electrical sensors are based on FET devices, and work by detecting changes in the conductance of the transducing material (i.e., graphene). Due to the unique linear band structure around the K point, the conductance of the graphene channel is very sensitive to molecular adsorptions on the FET device. Figure 2a shows a typical graphene-based back-gated FET (GFET), which is composed of a source and drain metallic electrodes bridged by a graphene channel and is usually supported by a conductive silicon substrate coated with an insulating dielectric SiO2 layer as the back gate. In such devices, the carrier concentration, and thus the conductivity of graphene, can be tuned by the gate voltage. Figure 2b shows a typical measurement where a constant bias voltage (Vsd) between the graphene channel and the source is applied, and changes in the source-drain current (Isd) are recorded. By changing the back-gate voltage (Vg), the electrochemical potential of the charge carriers (i.e., the Fermi energy) can be modulated. The type of carrier can continuously be tuned from holes (red curve) to electrons (gray curve), with the Isd change following a “V” shape, where the minimum current point marks the transition between p- and n-type, also known as the charge neutrality point (CNP). This behavior is the so-called “ambipolar behavior”.

Figure 2. (a) Schematic of a back-gated GFET. (b) Typical ambipolar transfer characteristics showing that the type of carriers in graphene can continuously be modulated from holes (on the left, in red) to electrons (on the right, in gray) using the field effect. (c) The charge carriers in single-layer graphene exposed to different concentrations of NO2. Upper inset: Scanning electron micrograph of this device. Lower inset: Characterization of the graphene device by using the electric-field effect. (d) Changes in resistivity, Δp of graphene by exposure to various gases diluted to 1 ppm. T is response time. Region I: device in vacuum; II: exposure to diluted chemicals; III: evacuation of the experimental setup; and IV: annealing at 150 °C. Inset shows an optical micrograph of the graphene device. I Optical image of flexible graphene/MoS2 heterostructured sensor on a bent polyimide substrate. SEM image of the MoS2 sensor with patterned graphene electrodes. MoS2 flake is bridged by two graphene lines. Panels (c,d) are reprinted with permission from [12] Copyright 2007, Nature Materials. Panel (e) is reprinted with permission from [13] Copyright 2015, American Chemical Society.

When exposed to a gaseous environment, gas molecules adsorb on graphene, causing a doping effect to graphene, which, in turn, affects the carrier concentration in graphene. The CNP consequently shifts positively or negatively, depending on whether the gas molecules are p-type or n-type dopant [14]. The degree of the shift can be used to quantify the concentration of gas molecules in the environment. Such devices have been shown useful in assessing gas by-products in manufacturing plants, such as carbon dioxide and ammonia gas—well-known greenhouse gases [15]. Schedin et al. realized the first micrometer-sized graphene sensor and demonstrated its potential by detecting a single gas molecule adsorbed on graphene’s surface (Figure 2c), the highest sensitivity among any detection techniques at the time. To achieve this feat, the authors used a ME graphene, for its inherently low Johnson noise, which led to a high signal-to-noise ratio. The device showed concentration-dependent changes in electrical resistivity when adsorbing NO2, H2O, NH3, and CO gases, allowing for quantitative analysis, shown in Figure 2d. As CO and NH3 act as electron donors and NO2 and H2O act as electron acceptor gases, they were found to strongly adsorb on graphene at room temperature. Moreover, the authors demonstrated the robustness of these devices by recovering them through vacuum annealing at 150 °C suggesting the potential for multiple measurements.

The rise in temperatures around the globe over the past several decades has significantly affected the water cycle. A consequence of these changes is an erratic precipitation pattern which will challenge, among others, the agricultural industry, natural ecosystems, and potable water supplies. Therefore, there is great interest in continuing the development of new technologies in water quality assessment. Following the realization of the GFET gas sensors, liquid-gated GFET sensors were also developed for sensing liquid samples to assess changes in pH, ion concentrations, and contaminants in water samples [15]. The change of Isd versus Vref of the liquid-gated GFETs has similar characteristics to back-gated GFETs, but the gate bias is applied to the liquid medium through a reference electrode (often Ag/AgCl), instead of a dielectric material (e.g., SiO2) as in the back-gated configuration.

2.2. Graphene-Based Optical Sensors

The versatility of graphene is also showcased in optical sensors, which enjoy a plethora of sensing mechanisms that can provide more information about the target species than field-effect-transistors. In essence, optical sensors are instruments capable of measuring the interaction between electromagnetic radiation and matter; here, we discuss how spectroscopies can benefit from graphene. A major advantage of optical sensors arises from the fact that they are capable of probing energetic states (e.g., electronic, vibrational, rotational) which are intrinsic to the target species, paving way for directly detecting the presence of the target analyte. In stark contrast to GFET devices that, instead, rely on indirect measurements (i.e., conductivity changes in the graphene-molecule complex) to detect the target analyte.

There are numerous optical sensors with distinct sensing mechanisms, and while all are not presented here, we highlight the use of graphene in a variety of spectroscopic modalities ranging from colorimetric sensors, that require little sample preparation and almost no post analysis processing, to more complex techniques like Raman spectroscopy that provide specific spectral signatures but require specialized instrumentation and knowledge to operate.

3. Conclusions

We have highlighted the recent progress in the development of graphene-based electrical and optical sensors for environmental applications, for example, in assessing pH and humidity levels, and the presence and concentration of various molecules. Graphene-based FET sensors can be made to detect target molecules in both gas and liquid samples, and due to their similarity with FETs in operation, their incorporation in existing sensing systems is viable. While graphene-based optical sensors require more intricate setups (e.g., light source and detector), they offer the advantage of being non-contact and are promising for multiplexing in the case of optical-fiber sensors and G-SERS.

There are several aspects that make graphene an ideal sensing platform. A major one is graphene’s unique linear band structure around the K point that makes it sensitive to perturbations caused by adsorbed molecules; here we reviewed how these changes can be probed electrically and optically. The atomically thin nature of graphene allows the possible miniaturization of devices, so that more sensors could be packed in the same area. Additionally, we also detailed general steps towards performance improvements in terms of selectivity and sensitivity through the addition of surface modifiers that (1) preferentially bind to the target molecule and (2) increase graphene’s sensitivity to external perturbations, respectively. Further research on surface modifiers is likely to solidify graphene as a sensing platform.

The future of graphene sensors is promising but some challenges need to be overcome. While there has been significant effort on realizing proof-of-concept devices, the transition from laboratory to market primarily hinges on the performance of these sensors. Although high selectivity and sensitivity can be achieved in laboratory settings, it is not clear how these devices will fare in real-world scenarios. It is rare to have contaminant-free samples when dealing with environmental applications and the effect of contamination might hinder sensor readings. Although we have discussed that graphene-based sensors do not always need to come single crystal graphene samples and lower grades are usually sufficient, the biggest hurdle for widespread adoption of this new technology still rests on the availability of graphene, both in terms of quality and cost. We also anticipate the integration of wireless technology to graphene sensors will significantly expand the adoption of these devices in the age of the Internet-of-Things for their real-time measurement capabilities.

References

  1. Abdel-Karim, R.; Reda, Y.; Abdel-Fattah, A. Review—Nanostructured Materials-Based Nanosensors. J. Electrochem. Soc. 2020, 167, 037554.
  2. Vikesland, P.J. Nanosensors for water quality monitoring. Nat. Nanotechnol. 2018, 13, 651–660.
  3. Li, M.; Gou, H.; Al-Ogaidi, I.; Wu, N. Nanostructured sensors for detection of heavy metals: A review. ACS Sustain. Chem. Eng. 2013, 1, 713–723.
  4. Liu, Y.; Deng, Y.; Dong, H.; Liu, K.; He, N. Progress on sensors based on nanomaterials for rapid detection of heavy metal ions. Sci. China Chem. 2017, 60, 329–337.
  5. Willner, M.R.; Vikesland, P.J. Nanomaterial enabled sensors for environmental contaminants. J. Nanobiotechnol. 2018, 16, 1–16.
  6. Khin, M.M.; Nair, A.S.; Babu, V.J.; Murugan, R.; Ramakrishna, S. A review on nanomaterials for environmental remediation. Energy Environ. Sci. 2012, 5, 8075–8109.
  7. Chang, J.; Zhou, G.; Christensen, E.R.; Heideman, R.; Chen, J. Graphene-based sensors for detection of heavy metals in water: A review Chemosensors and Chemoreception. Anal. Bioanal. Chem. 2014, 406, 3957–3975.
  8. Coroş, M.; Pruneanu, S.; Stefan-van Staden, R.-I. Review—Recent Progress in the Graphene-Based Electrochemical Sensors and Biosensors. J. Electrochem. Soc. 2020, 167, 037528.
  9. Justino, C.I.L.; Gomes, A.R.; Freitas, A.C.; Duarte, A.C.; Rocha-Santos, T.A.P. Graphene based sensors and biosensors. TrAC Trends Anal. Chem. 2017, 91, 53–66.
  10. Nag, A.; Mitra, A.; Mukhopadhyay, S.C. Graphene and its sensor-based applications: A review. Sensors Actuators, A Phys. 2018, 270, 177–194.
  11. Yao, Y.; Ping, J. Recent advances in graphene-based freestanding paper-like materials for sensing applications. TrAC Trends Anal. Chem. 2018, 105, 75–88.
  12. Schedin, F.; Geim, A.K.; Morozov, S.V.; Hill, E.W.; Blake, P.; Katsnelson, M.I.; Novoselov, K.S. Detection of individual gas molecules adsorbed on graphene. Nat. Mater. 2007, 6, 652–655.
  13. Cho, B.; Yoon, J.; Lim, S.K.; Kim, A.R.; Kim, D.-H.; Park, S.-G.; Kwon, J.-D.; Lee, Y.-J.; Lee, K.-H.; Lee, B.H.; et al. Chemical Sensing of 2D Graphene/MoS 2 Heterostructure device. ACS Appl. Mater. Interfaces 2015, 7, 16775–16780.
  14. Fu, W.; Jiang, L.; van Geest, E.P.; Lima, L.M.C.; Schneider, G.F. Sensing at the Surface of Graphene Field-Effect Transistors. Adv. Mater. 2017, 29, 1–25.
  15. Singh, E.; Meyyappan, M.; Nalwa, H.S. Flexible Graphene-Based Wearable Gas and Chemical Sensors. ACS Appl. Mater. Interfaces 2017, 9, 34544–34586.
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