In this article, we focus on the use of graphene in water sensors, exploring how these sensors detect common contaminants, including heavy metals, microbes and irregular pH.
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Various types of sensors are employed during batch and continuous water quality assessment in various applications, measuring a diverse range of properties. The materials from which these sensors are constructed, particularly during continuous monitoring, must be durable and hard-wearing, and importantly, must not impart contaminants into the water supply itself.
The high mechanical, chemical, and electronic robustness of single or multilayer graphene sensors has given the material promising applications in sensing common water contaminants.
How do Graphene Sensors Work?
Graphene is composed of carbon arranged in a planar hexagonal lattice, each carbon bonded to its nearest three neighbors by σ-bonds and a delocalized ∏-bond, which allows sheets to be stacked along the z-axis. Graphene sheets are flexible and can be bent into a desired shape while maintaining high-strength properties.
The delocalized electron network of graphene makes it an excellent electrical conductor with applications in modern electronics.
Most graphene sensors typically exploit this innate conductivity by passing a consistent charge through a graphene sheet and measuring the current reaching the other side, and thus gaining an indication of the resistance of the sheet.
When molecules, preferably in a liquid medium, adsorb to the high energy surface of the graphene sheet, they cause a change in resistance, owing to changes in electronic arrangement. The type and quantity of molecules adsorbed to the surface can, therefore, be inferred.
The numerous novel properties of graphene can be exploited in other designs of sensor, such as those intended to detect changes in magnetism, electronic field, and the application of pressure, though these are less relevant to water quality monitoring.
Typically, some additional chemical modifications are made to the graphene sheet in order to adapt its sensing function to a single or small range of parameters, such as the presence of heavy metal contaminants, pH, or microbial invasion.
Some of the specific ways in which graphene-based water sensors have been adapted to purpose will be discussed in more detail below.
How do Graphene Sensors Detect pH?
pH can be indicative of a water supply’s contamination levels. It can be detected using graphene sensors as excess H+ and OH- ions in close contact with the sensor surface influence the electronic properties of graphene. Specifically, this occurs due to the electrostatic gating effect, wherein H+ ions close to the graphene surface will attract electrons, and OH- ions will attract electron holes. H+ ion adsorption decreases conductance, while OH- ion adsorption increases it. This phenomenon can be exploited in the detection of various charged species, such as heavy metal ions and salts, though it often requires some modification to the graphene sensor surface to promote interaction.
How do Graphene Heavy Metal Sensors Work?
Heavy metals are a common water pollutant, with anthropogenic activities being increasingly linked to the presence of heavy metals in water supplies.
Zhang et al. (2010) functionalized single sheet graphene with 1-octadecanethiol, which self-assembled in-plane with the graphene surface. When exposed to only 10 ppm Hg2+ strong interaction with the thiol groups of these molecules causes a positive shift in Dirac point, the transition point between the valence and conduction bands.
Similarly, Wang et al. (2016) bound nucleic acid strands to graphene non-covalently by introducing an aromatic pyrene group to the 5’ end that was held in the carbon p-orbital of graphene electrostatically, in this case detecting Pb2+, exclusively in the presence of several other ions including Na2+, K+, Mg2+, and Ca2+, and causing a shift towards negative Dirac values upon successful bonding.
The reason for the selectivity exhibited by certain molecules towards specific ions is largely related to differences in ionic charge and atomic radius, as they relate to the atomic arrangement of the probe molecule.
How do Graphene Sensors Detect Microbes?
Microbe-contaminated water supplies are known to be hazardous to health, contributing to infectious diseases worldwide.
Chang et al. (2014) were able to adapt graphene sensors to the detection of E. coli by non-covalently functionalizing the surface with gold nanoparticles decorated with glutaraldehyde, to which was covalently bound the E. coli O157 antibody. This antibody bonds with antigens present on the bacterial cell wall, which bears a slight negative charge.
Once sufficient E. coli cells have built up on the surface, the cumulative negative charge causes a change in conductance, thus indicating the presence of this specific strain of bacteria. In a similar application, the sensitivity of this type of graphene sensor was further enhanced by Thakur et al. (2018) by passivating the sensor surface with electron rich Al2O3. This decreases the concentration of charge carriers in the channel during the accumulation of E. coli, acting as a charge gate.
A shift in the behavior of the p-type semiconductor properties of the sensor occurred only at sufficient E. coli concentration, indicating that a minimum threshold has been reached. Specificity towards particular strains of bacteria or even viruses is imparted by modification of the antibody utilized within the probe, or other chemical modification, potentially allowing any variety of microorganisms to be detected. For example, Li et al. (2021) combined graphene with MXene materials and influenza specific antibodies in the detection of the virus, achieving a resolution of only 125 virus copies per mL medium.
References and Further Reading
Zubiarrain-Laserna, A. & Kruse, P. (2020). Review—Graphene-Based Water Quality Sensors. Journal of the Electrochemical Society, 167(3), p. 037539. doi.org/10.1149/1945-7111/ab67a5
Li, Y., et al. (2021). MXene–Graphene Field-Effect Transistor Sensing of Influenza Virus and SARS-CoV-2. ACS Omega, 6(10), pp. 6643–6653. doi.org/10.1021/acsomega.0c05421
Zhang, T., et al. (2010). Selfassembled 1-octadecanethiol monolayers on graphene for mercury detection. Nano Letters, 10, p. 4738.
Chang, J., et al. (2014). Graphene-based sensors for detection of heavy metals in water: a review. Analytical and Bioanalytical Chemistry, 406, p. 3957.
Thakur, B., et al. (2018). Rapid detection of single E. coli bacteria using a graphene-based field-effect transistor device. Biosensors and Bioelectronics, 110, p. 16.
Zamora-Ledezma et al. (2021) Heavy metal water pollution: A fresh look about hazards, novel and conventional remediation methods. Environmental Technology & Innovation, 22. doi.org/10.1016/j.eti.2021.101504