Catechol, 1,2-dihydroxybenzene (1,2-DHB), is a well-known organic redox molecule and a potent biological electron-transfer mediator widely used in electrocatalysis, bio-electrocatalysis, energy-storage and pH sensing applications. Functionalization of catechol (CA) over the carbon-electrode is an essential step to achieving the desired application. There are several studies reported for the chemical synthesis of functionalized CA derivatives following surface immobilization procedure [1-3]. Indeed, preparation of a simple and stable catechol functionalized electrochemically modified electrode is a challenging task. For instance, 4-[2-(2-naphthyl)vinly]catechol and 4-[2-(9,10-ethanoanthracen-9-yl)vinlyl]catechol grafted graphite electrodes and a library of 26 different dihydroxy benzene derivatives covalently attached to glassy carbon via ethylenediamine and C₆H₄-CH₂-NH linkers have showed serious surface fouling during their voltammetric experiments [1,4]. In this work, we report a simple in-situ electrochemical method for functionalizing catechol derivative (ferulic acid, FA) on a low-cost carbon black (CB) modified electrode surface suitable for voltammetric pH sensing. The precursor organic compound, FA has been electrochemically oxidized over the CB modified glassy carbon electrode, GCE/CB, by performing the electrochemical-potential treatment in pH 7 phosphate buffer solution that leads to a high redox-active surface-confined FA-catechol derivative (GCE/CB@FA-Redox). Cyclic voltammetric response of the GCE/CB@FA-Redox showed a well-defined and stable redox peak at a standard electrode potential, E^o’ = 0.160 V vs Ag/AgCl with a peak-to-peak potential difference, ∆E = 0.044 V vs Ag/AgCl and surface excess value, Γ = 1.88×10^-7 mol cm^-2. Unlike the previously reported CA-based chemically modified electrodes which showed a well-defined mediated oxidation reaction of cysteine (CySH), hydrazine and sulfide ion [5-7], the GCE/CB@FA-Redox modified electrode didn’t show any interference to common electroactive biochemicals and chemicals such as ascorbic acid, glucose, cysteine, caffeic acid, hydrazine, hydrogen peroxide, uric acid, dopamine, creatinine, urea, nitrite, sulfide and sulfate ions. There was no alteration in the peak potential and peak current responses in the presence of the chemicals mentioned above. It is a clear advantage of using this new redox electrode for the voltammetric pH sensing application. The surface orientation of the FA-Redox system on the CB surface is the likely reason for the non-selective voltammetry response.

In further, scanning electrochemical microscopy (SECM) technique along with feedback current mode was adopted for the electrochemical mapping of the hot spot of the active site. In the typical experimental procedure, the CB@FA-Redox modified electrode was chosen as a substrate, and an ultramicro-platinum electrode (25µm) was used as a probe for the surface imaging using potassium hexacyanoferrate as the redox couple (in 0.1 M KCl) (Figure 1D). Figure 1E showed a typical SECM study of the present system showing a cloud-like surface morphology of the active site.

The GCE/CB@FA-Redox modified electrode was subjected to various pH solutions by performing CV studies at a fixed scan rate, 50 mV s^-1. (Figure 1B). The voltammetric peak potential, E_pa and E_pc values were found to shift systematically against an increase in pH from 3 to 11, with a slope value of (∂E_p/∂pH) -58 ± 3 mV pH^-1 (Figure 1C), matching with the ideal Nernstian value obeying the Nernstian equation, (E = E^o - ((0.0591/n)ln(RT/F)).

Selective and low volume pH sensing of specific clinical samples like saliva, urine and and tears will provide vital information about the health disorder [8-9]. For instance, alteration in urine and saliva samples' urinary pH values and its associated clinical diseases like kidney stones and oral health disruption. Similarly, variation in the pH value is observed during the E.coli growth, wherein, acidic by-products such as acetic acid is released into the medium, which can cause specific decrement in the solution pH. As an independent study, a three-in-one screen-printed electrode modified with CB@FA-Redox has been developed for the pH sensor applications. As a practical application, selective voltammetric pH monitoring of real biological samples like urine, saliva and E.coli culture grown has been demonstrated.

References

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