Graphene, a single atomic layer of carbon with a hexagonal crystal structure, is especially appealing for use in sensor applications because of its high surface area and abundance of active sites for analyte capture, favorable semiconductor properties such as high carrier mobility, and biomolecular compatibility [1-3].
One challenge of working with a 2D material like graphene is the ability to modify the surface without compromising its structural integrity. With relatively low applied powers, Harrick Plasma cleaners are designed to affect surfaces on an atomic level only and are well-suited to gently treat single layer or multilayer graphene.
In sensors, it is critical to maximize the number of active sites to significantly improve chemical detection and efficiency. For graphene in particular, plasma is capable of introducing oxygen functionalities to further tailor detection of specific target analytes and creating defects to increase the number of active sites.
In this note, we explore two research studies that applied plasma treatment to alter graphene surface chemistry and morphology, which were then used to develop chemical and biological sensors with enhanced sensitivity and device performance.
Yuan et al. developed a graphene-based field effect transistor (FET) as a biosensor for detecting potassium (K+) ions in the body and applied oxygen (O2) plasma treatment to graphene to improve K+ signal response and selectivity.
The solution-gated FET utilized graphene as the conducting channel, and a guanine-rich DNA (G-quadruplex DNA) was immobilized on graphene. The G-quadruplex DNA has strong binding affinity to K+ ions, and can efficiently and selectively capture K+ ions, fixing them close to the graphene surface.
Using the Expanded Plasma Cleaner, O2 plasma was applied to graphene at LOW RF for varying times (0-180 s).
X-ray spectroscopy (XPS) indicated that oxygen containing functional groups (C=O, C-O) increased linearly with up to 120 s of plasma exposure, with oxygen content increasing from 15% to 25%. In addition, the water contact angle decreased from 87° (untreated) to 47° (120 s), as shown in Figure 1 (●). The increase in oxygen functional groups increased the surface hydrophilicity and improved the adhesion of G-quadruplex DNA to graphene.
While increasing hydrophilicity, O2 plasma also created more defects on the graphene layer and resulted in an increase in electrical resistivity. Figure 1 (♦) shows that the sheet resistance ratio, RS/RS0, where RS is sheet resistance of plasma-treated graphene and RS0 is sheet resistance of untreated graphene, increased with plasma exposure.
On the FET biosensors, drain current versus gate voltage (ID -VG) measurements were conducted to quantify the electrical characteristics with varying K+ concentration and the effect of plasma treatment on K+ detection. The ID -VG curve showed a local minimum, a signal of interest designated as the charge neutrality point (VCNP), where VCNP shifted proportionally with K+ concentration and correlated to a chemical change in graphene due to K+ absorption.
The sensors demonstrated a K+ detection limit of 0.74 pM (for untreated graphene). In comparison, FETs based on graphene that was given a short 10 s plasma treatment showed a significantly lower detection limit of 0.058 pM. The increased K+ sensitivity was attributed to a higher concentration of guanine-DNA immobilized on the plasma-treated graphene, which promoted K+ interaction and binding of K+ ions in aqueous solution to the active sites.
With longer plasma exposures of 30-60 s, the device sensitivity did not further improve and, in fact, the detection limit increased. The researchers suggested that the greater number of defects induced with longer plasma exposures caused an increase in electrical resistivity, resulting in degradation of electrical properties. Thus, 10 s was found to be the optimal plasma duration to enhance K+ sensitivity and improve device performance.
Figure 1. Water contact angle (●) and sheet resistance ratio, RS/RS0 (♦), of graphene as a function of plasma treatment time, used in graphene-based field effect transistor biosensors to measure K+ concentrations. Data from Yuan Q, Wu S, Ye C, Liu X, Gao J, Cui N, Guo P, Lai G, Wei Q, Yang M, Su W, Li H, Jiang N, Fu L, Dai D, Lin C and Chee K. Sensors Actuat. B-Chem. (2019) 285: 333-340. DOI: 10.1016/j.snb.2019.01.058
Increasing graphene active sites
Mastrapa et al. developed a graphene-based resistive gas sensor and applied air plasma to increase the active area on the graphene surface and improve sensor sensitivity. In this study, nitrogen dioxide (NO2) and ammonia (NH3) were used as the target analytes with the goal of detecting in the parts per million (ppm) level.
Using the Basic Plasma Cleaner, air plasma was applied to graphene surfaces for varying times (5, 10, 20, 30 s). Air plasma etched the graphene layer and introduced defects across the surface.
Raman spectroscopy was performed to infer the evolution of defects with plasma exposure and quantify the effect of surface defects on the sensor’s detection limit. Raman maps indicated that defects were introduced uniformily across the surface. The Raman spectra were further analyzed by comparing the peak intensity of the D band (1360 cm-1, associated with defects) to that of the 2D and G bands (2700 cm-1 and 1580 cm-1, respectively, associated with monolayer graphene). Figure 2 shows that the 2D/G peak intensity (●, monolayer graphene) decreased with plasma treatment while the D/G peak intensity (♦, defects) increased with ≥10 s plasma.
With increasing plasma treatment, the sensor resistance increased and demonstrated a linear response with NO2 and NH3 gas concentration. Following a 30 s plasma duration, the graphene-based sensor was able to detect the presence of gas in the ppm level, with the lowest detection limit of 2 ppm for NH3 and 18 ppm for NO2.
These results demonstrated that plasma treatment can readily enhance the sensitivity of sensors by increasing vacancy defects, thus increasing the surface area and number of active sites for analyte adsorption. However, excessive defects can adversely affect the electrical conductivity of graphene and can result in poor electrical contact and device performance. Therefore, it is critical to balance these competing effects.
Figure 2. Raman spectra peak ratio of plasma-treated graphene. I2D/IG (●, monolayer graphene) is compared with ID/IG (♦, defects) for varying plasma treatment times. Data from Mastrapa GC and Freire F. J. Sensors (2019) 2019: Article ID 5492583. DOI: 10.1155/2019/5492583
Relevant Articles from Harrick Plasma Users
- Yuan Q, Wu S, Ye C, Liu X, Gao J, Cui N, Guo P, Lai G, Wei Q, Yang M, Su W, Li H, Jiang N, Fu L, Dai D, Lin C and Chee K. “Sensitivity enhancement of potassium ion (K+) detection based on graphene field-effect transistors with surface plasma pretreatment.” Sensors Actuat. B-Chem. (2019) 285: 333-340. DOI: 10.1016/j.snb.2019.01.058
- Mastrapa GC and Freire F. “Plasma-treated CVD graphene gas sensor performance in environmental condition: the role of defects on sensitivity”. J. Sensors (2019) 2019: Article ID 5492583 (7 pp.). DOI: 1155/2019/5492583
Supplemental References (Do not report using Harrick Plasma Instruments)
 Nag A, Mitra A and Mukhopadhyay SC. “Graphene and its sensor-based applications: a review.” Sensors Actuat. A-Phys. (2018) 270: 177-194.
 Wang T, Huang D, Yang Z, Xu S, He G, Li X, Hu N, Yin G, He D and Zhang L. “A review on graphene-based gas/vapor sensors with unique properties and potential applications.” Nano-Micro Lett. (2016) 8:95-119.
 Choi W, Lahiri I, Seelaboyina R and Kang YS. “Synthesis of graphene and its applications: a review”. Crit. Rev. Solid State 2010 35(1): 52-71.