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.

The effect of electrochemical double layer (EDL) on the performance of graphene-based sensing platforms has been an area of controversy over the last two decades 1. The hydrophobic nature of bare graphene tends to repel the water and minimizes the solution/solid surface interactions 2. However, the presence of oxygen-based functional groups on graphene introduces negatively charged sites and causes a negative surface zeta potential. Hence, the instant formation of an EDL on a graphene surface in an aqueous solution is inevitable, affecting the graphene surface's chemical/physical interactions 3. Thus far, multiple theories and techniques have been developed to investigate the impact of EDL on graphene sensing performance; however, they either suffer from complexity in design or have ignored the co-existence of other solution parameters such as pH and oxidation-reduction potential 4. In this work, we propose the use of alkaline chloride salts to understand the impact of the ionic strength and EDL of the solution on the few-layer graphene (FLG) based chemiresistive sensors. Considering the full ionization of NaCl and the low redox potential of the generated ions, alkaline chlorides are considered good candidates to modulate the electronic band structure of FLG without altering its surface chemistry 5. By use of Helmholtz theory of EDL, it is postulated the Inner Helmholtz Layer (IHL) to be formed by Na+ ions due to the negative surface charge of FLG. Whereas the hydrated counter ions, Cl-, form the Outer Helmholtz Layer (OHL). Upon the formation of a positively charged immobile layer of Na+ ions on the surface (IHL), the electrostatic gating effect of EDL dopes the graphene with electrons. This charge separation can be modelled as two parallel plates of a capacitor with infinitely thin dielectric 6. Since the presence of oxygen-based functional groups on FLG is inevitable during the synthesis process, it is considered inherently p-doped. Accordingly, upon the addition of NaCl concentrations, the FLG surface current decreases. However, at higher concentrations (around 2000 ppm NaCl), the sensor response is inversed, and surface current increases by the addition of NaCl (Figure-left). In fact, high ionic strength increases the EDL compactness and decreases the charge screening length (Debye length). As a result of this sharper difference in solid/solution potentials (Figure-right), a more n-doped surface having electron as majority charge carriers is obtained, changing the nature of the response. We have also demonstrated that the formation of sodium-oxygen metal complexes on FLG is not likely in an aqueous environment. According to the Raman spectroscopy results of FLG, an increase in the intensity ratio of D (defect) to G bands is observed upon exposure to water and NaCl. Moreover, the noticeable right shift in the 2D band position demonstrates the p-doping of FLG 7. Furthermore, the reversible response of the sensor during multiple exposures to NaCl demonstrates the sensor response originates from EDL, not the formation of sodium-oxygen metal complexes. In fact, the full or half hydrated Na+ ions in IHL electrostatically interact with surrounding oxygen. However, since the oxygen atom in water is more negatively charged compared to the oxygen on the graphene surface, Na+ is unlikely to bound to the surface after hydration. Therefore, using NaCl could be a suitable medium to study the impact of EDL on the performance of graphene devices. Reference Jurado et al., Scientific Reports, 7(1), pp.1-12 (2017). Angizi, S et al., Langmuir, 37(41), pp.12163-12178 (2021). Jung et al., Nano Letters, 21(1), pp.34-42 (2020). Pak et al., The Journal of Physical Chemistry C, 118(38), pp.21770-21777 (2014). Lee et al., ACS Applied Materials & Interfaces, 11(45), pp.42520-42527 (2019). Kwon et al., The Journal of Physical Chemistry C, 116(50), pp.26586-26591 (2012). Bruna et al., ACS Nano, 8(7), pp.7432-7441 (2014). Figure 1

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