Abstract
The concept of a reversible polymer displacement sensor mechanism for electrochemical glucose monitoring is demonstrated. A pyrene-derivatised boronic acid chemo-receptor for glucose is adsorbed onto a graphene foam electrode. Spontaneous oxidative polymerisation of nordihydroguaiaretic acid (NHG) onto the graphene foam electrode leads to a redox active film (poly-NHG) covalently attached to the boronic acid receptors. Oxidation of poly-NHG frees the boronic acid receptors to interact with glucose from the solution phase, which is detected due to competitive binding when reduced poly-NHG re-binds to the boronic acid functional groups. The sensor shows the anticipated boronic acid selectivity of fructose > glucose. The ratio of charges under the voltammetric peaks for poly-NHG unbound and bound is employed for glucose sensing with an approximately linear analytical range from 1 to 50 mM glucose in aqueous pH 7 buffer. The new methodology is shown to give apparent saccharide - boronic acid binding constants and to work in human serum. Therefore, in the future it could be developed further for glucose monitoring.
Highlights
Diabetes is a chronic condition in which the sufferer can no longer naturally regulate their own blood glucose levels, either due to not being able to produce insulin or due to a loss of cell responsiveness to insulin
Nordihydroguaiaretic acid (NHG) is poorly soluble in aqueous media but can be readily dispersed by initially preparing 5 mM ethanolic solution followed by dilution in 0.1 M phosphate buffer pH 7
The symmetric shapes of these cyclic voltammograms differ from classic diffusion-controlled signal shapes. This shape is indicative of a surface-immobilised redox system, which is linked to spontaneous polymerisation of nordihydroguaiaretic acid (NHG) onto graphene foam
Summary
Boronic acids have previously been explored for their potential in glucose detection, often in optical or fluorescence methods.[6,7,8,9] Electrochemical detection methods based on boronic acids have been developed[10] for a range of analytes. The chosen boronic acid for this work was 4-borono-1-( pyren-2-ylmethyl)pyridin-1-ium bromide (abbreviated T1; see Fig. S1A†), a pyrene-appended pyridinium boronic acid receptor that has been employed previously.[12] Both the boronic acid and the glucose analyte are not redox active and not detected at electrodes under physiological conditions. Decoupling of the boronic acid–polymer complex on the graphene foam electrode creates ‘free’ boronic acid binding sites, which can interact competitively with relevant diols (such as glucose) that may be present in solution, reducing the number of boronic acid binding sites for the polymer to re-bind to (after ortho-quinone reduction back to ortho-quinol). This is shown here to cause voltammetric peak signals, which are related to the diol/glucose concentration
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