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

Abstract

In this study, we demonstrate that a highly pH-sensitive substrate could be fabricated by controlling the type and defect density of graphene derivatives. Nanomaterials from single-layer graphene resembling a defect-free structure to few-layer graphene and graphene oxide with high defect density were used to demonstrate the pH-sensing mechanisms of graphene. We show the presence of three competing mechanisms of pH sensitivity, including the availability of functional groups, the electrochemical double layer, and the ion trapping that determines the overall pH response. The graphene surface was selectively functionalized with hydroxyl, amine, and carboxyl groups to understand the role and density of the graphene pH-sensitive functional groups. Later, we establish the development of highly pH-sensitive graphene oxide by controlling its defect density. This research opens a new avenue for integrating micro–nano-sized pH sensors based on graphene derivatives into next-generation sensing platforms.

Identification and Quantification of Aqueous Disinfectants Using an Array of Carbon Nanotube-Based Chemiresistors

“…It can be seen that the blank sensor responses appear to correlate reasonably well with the ORP of the solution. One possible reason for the imperfect correlation may be due to the presence of defects on the surface that interact with the analytes. , Most of the deviations occur for free chlorine at higher pHs (8.5 and 9.5). The pH-dependent nature of the different free chlorine species can result in the (co)­presence of charged OCl – or molecular HOCl and Cl 2 .…”

“…One possible reason for the imperfect correlation may be due to the presence of defects on the surface that interact with the analytes. 47,48 Most of the deviations occur for free chlorine at higher pHs (8. amounts are calculated from the total free chlorine following the ratio of the species present at a certain pH. A pK a of 7.58 (at 25 °C) was used for the calculation of the HOCl/OCl − equilibrium.…”

A reagent-free phosphate chemiresistive sensor using carbon nanotubes functionalized with crystal violet

“…When unfunctionalized SWCNT electrodes were immersed in phosphate buffer, a decrease in current was observed due to the electrostatic gating effect, resulting in n-doping of the SWCNT film with water molecules. 29 When the sensor was immersed in the crystal violet solution, an additional decrease in current was observed (Fig. 2a).…”

Graphene-Based Chemiresistor Sensors for Drinking Water Quality Monitoring

“…So far, four graphene-based chemiresistive devices have been reported in the literature, where three of the devices [ 56 , 57 , 58 ] relied on the direct interaction between the analyte and the graphene sensing surface, while the fourth device [ 59 ] leveraged the functionalization of the interaction surface with various pyrene derivatives. Each reported device employed a different form of graphene and different material composition for the electrical contacts.…”

“…Direct interaction designs [ 56 , 57 ] demonstrated repeatability between measurements as well as similar effective pH ranges, while the sensors tested in [ 58 ] demonstrated an effective range from pH 1 to 11. Recently a highly sensitive device [ 59 ] based on a thermally annealed GO interaction surface was also fabricated. By investigating the sensor response as a function of the type and defect density of the graphene or graphene derivative used, a better understanding of the mechanism of detection regarding pH was achieved.…”