Defect Engineering of Graphene to Modulate pH Response of Graphene Devices

Abstract

Graphene-based pH sensors are a robust, durable, sensitive, and scalable approach for the sensitive detection of pH in various environments. However, the mechanisms through which graphene responds to pH variations are not well-understood yet. This study provides a new look into the surface science of graphene-based pH sensors to address the existing gaps and inconsistencies among the literature concerning sensing response, the role of defects, and surface/solution interactions. Herein, we demonstrate the dependence of the sensing response on the defect density level of graphene, measured by Raman spectroscopy. At the crossover point (ID/IG = 0.35), two countervailing mechanisms balance each other out, separating two regions where either a surface defect induced (negative slope) or a double layer induced (positive slope) response dominates. For ratios above 0.35, the pH-dependent induction of charges at surface functional groups (both pH-sensitive and nonsensitive groups) dominates the device response. Below a ratio of 0.35, the response is dominated by the modulation of charge carriers in the graphene due to the electric double layer formed from the interaction between the graphene surface and the electrolyte solution. Selective functionalization of the surface was utilized to uncover the dominant acid–base interactions of carboxyl and amine groups at low pH while hydroxyl groups control the high pH range sensitivity. The overall pH-sensing characteristics of the graphene will be determined by the balance of these two mechanisms.

Detection of free chlorine in water using graphene-like carbon based chemiresistive sensors

“…cations 57,68,69 or pH. 61 In addition, we have demonstrated the use of two different sensor geometries, which expands the possibilities of sensor deployment. The manual lab-based device fabrication method employed in this study does not lend itself to achieving narrow tolerances, resulting in variations in the magnitude of response between devices.…”

Defect Density-Dependent pH Response of Graphene Derivatives: Towards the Development of pH-Sensitive Graphene Oxide Devices

“…Since the holes are the majority carrier in graphene due to the presence of electron-withdrawing oxygen atoms, n-doping the surface reduces the charge carriers in the chemiresistor and makes it more resistive. In contrast, electrostatic p-doping of OH − aq ions accumulated in the Stern layer increases the current at a high pH [ 26 ]. The schematic illustrations of the formation of the electrochemical double layer (EDL) in acidic solutions are shown in Figure 2 d. It should be noted that the charge transfers through the formation of the EDL by H 3 O + and OH − are considered fully nonfaradaic.…”

“…This means that decreasing the pH reduces the current, while increasing the pH toward the basic solution increases n-doping. We have recently shown that the defect-induced pH response of graphene originates from the protonation/deprotonation of carboxyl and amine at a low pH and hydroxyl groups at a high pH [ 26 ]. For example, in carboxyl groups, upon decreasing the pH to 3 (below the pK a = 3.1), the is protonated to -COOH, and the surface becomes p-doped.…”

“…Knowing that the type and density of the defects in graphene derivatives determine their pH response, the development of a pH-sensitive device can be achieved by selective functionalization. For this purpose, noncovalently attached pyrene derivatives with various pH-sensitive functional groups were employed as a model system to resemble the graphene surface terminated with pH-sensitive groups [ 26 ]. The charge transfer upon protonation/deprotonation of the functional groups is directly transduced to the FLG [ 46 ] via the interactions of the π-electron system of the pyrene ring with the FLG surface [ 54 ].…”

“…Therefore, the concept of pH sensors based on graphene derivatives can be fully developed only if the aqueous electrolyte’s impact on graphene is entirely investigated. Moreover, the pH-sensing mechanism of graphene has been shown to be defect-dependent [ 26 ], but there is still no clarity on the role of defects in the pH response of graphene devices.…”