Abstract
As an alternate for the conventional glass-based pH sensor which is associated with problems like fragile nature, alkaline error, and potential drift, the development of a new redox-sensitive pH probe-modified electrode that could show potential, current-drift and surface-fouling free voltammetric pH sensing is a demanding research interest, recently. Herein, we report a substituted carbazole-quinone (Car-HQ) based new redox-active pH-sensitive probe that contains benzyl and bromo-substituents, immobilized multiwalled carbon nanotube modified glassy carbon (GCE/MWCNT@Car-HQ) and screen-printed three-in-one (SPE/MWCNT@Car-HQ) electrodes for selective, surface-fouling free pH sensor application. This new system showed a well-defined surface-confined redox peak at an apparent standard electrode potential, Eo′ = − 0.160 V versus Ag/AgCl with surface-excess value, Γ = 47 n mol cm−2 in pH 7 phosphate buffer solution. When tested with various electroactive chemicals and biochemicals such as cysteine, hydrazine, NADH, uric acid, and ascorbic acid, MWCNT@Car-HQ showed an unaltered redox-peak potential and current values without mediated oxidation/reduction behavior unlike the conventional hydroquinone, anthraquinone and other redox mediators based voltammetry sensors with serious electrocatalytic effects and in turn potential and current drifts. A strong π–π interaction, nitrogen-atom assisted surface orientation and C–C bond formation on the graphitic structure of MWCNT are the plausible reasons for stable and selective voltammetric pH sensing application of MWCNT@Car-HQ system. Using a programed/in-built three-in-one screen printed compatible potentiostat system, voltammetric pH sensing of 3 μL sample of urine, saliva, and orange juice samples with pH values comparable to that of milliliter volume-based pH-glass electrode measurements has been demonstrated.
Highlights
The determination of solution pH is one of the fundamental analyses that is related to several practical applications including biomedical, clinical, industrial, environmental pollution, food processing, agricultural, pharmaceutical systems etc[1,2,3]
There are some stability (Fig. 3a) and selectivity complications due to the redox-mediated electrocatalytic oxidation of certain biochemicals like NADH, Hyd and CySH, HQ-based chemically modified electrodes has been rarely used for reagent-less pH-sensing applications[1, 3]
In this report following effort was taken to design a new redox-probe molecule that can be immobilized on graphitic carbon surface and can be used effectively; (i) HQ with increased aromatic units: An additional aromatic ring may provide a strong π–π interaction between the aromatic πe−s and sp[2] carbon of graphitic structure for improvement of the stability. (ii) Introduction of a labile halogen atom (C– Br) on the molecular structure: Aim of this part of the work is to generate potential assisted carbo-cation, > C(+) and the nucleophilic addition of MWCNT resulted in the Carbon–Carbon linked new redox-mediated modified electrode surface. (iii) Introduction of a nitrogen atom: It may provide different adsorption and spatial orienta
Summary
The determination of solution pH is one of the fundamental analyses that is related to several practical applications including biomedical, clinical, industrial, environmental pollution, food processing, agricultural, pharmaceutical systems etc[1,2,3]. We introduce, a synthetically prepared benzyl and bromo substituted carbazole-quinone (Fig. 1) immobilized MWCNT modified screen-printed electrode, designated as SPE/MWCNT@Car-HQ as an efficient and interference-free redox-probe system for elegant voltammetric pH sensing applications (Fig. 2). The slope value is closer to the response (− 45.2 mV pH−1) obtained with pH effect by conventional GCE modified electrode in the CV analysis verifying the applicability of the analytical protocol on the screenprinted electrode surface.
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