Determination of fructose in fruit juices using an electrode modified with reduced graphene oxide and molecularly imprinted polymer poly(phenylboronic acid)

Fruit juices are basically constituted by water, sugars, organic acids, lipids, proteins and minerals. Fructose is the sugar found in bigger quantity in these juices and is related to several diseases, such as diabetes, obesity and cardiovascular problems1, being of fundamental importance the control of its concentration during the production process of these beverages. Thus, are necessary the development of simple, sensitive, selective and low cost analytical methodologies to determine such sugar in fruit juices. In this context, electrodes modified with reduced graphene oxide (RGO) and molecularly imprinted polymer (MIP) using phenylboronic acid (PBA) as functional monomer (FM) are a viable option for this purpose, since RGO and MIP provide the modified electrode a increase of sensitivity and selectivity respectively, when compared to unmodified electrodes. In addition, PBA has a high affinity for making covalent bonds between hydroxyl groups with sugars in generaF, being used for the development of several sensors for this class of compounds. Thus, the objective of this work was to develop a sensor based on a glassy carbon electrode (GCE) modified with RGO and MIP (using PBA as FM) for determination of fructose in fruit juices. Modification of the GCE with RGO was performed by applying a constant potential of 1.5 V for 500 seconds using an aqueous suspension of graphene oxide 0.50 mg mL-1 in Na2S04 0.10 mol L-1. For MIP electrosynthesis, the GCE-RGO electrode was immersed in a solution containing PBA 1.0x1 o-3 mol L-1 and fructose 1.0x1 o-3 mol L-1 (pH 8.0) and 35 consecutive voltammetric cycles were applied in a potential range of -0.2 to 1.2 V at a scan rate of 50 mV s-1. For removal of the template, the electrode was inserted in an acetonitrile-acetic acid solution 5:1 (v:v) for 60 seconds under low stirring rates. For the rebinding stud ies, the sensor was inserted in different fructose solutions for 5 minutes under low stirring rates. GCE-RGO-MIP sensor was characterized by techniques: cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy, and atomic force microscopy. Parameters that influence the performance of the GCE-RGO-MIP sensor were optimized and the responses obtained were measured by differential pulse voltammetry using the redox pair [Fe(CN)6]-31-4. The developed sensor presented two analytical linear ranges, 1.0x1 0-14 to 1.0x1 0-13 mol L-1 and 1.0x1 0-13 to 6.0x1 0-13 mol L-1. The figures of merit, as limit of detection and limit of quantification were obtained using the first linear range as being 9.5x1 0-14 mol L-1 and 3.2x1 0-14 mol L- 1 respectively. In addition, this sensor showed considerable inter-day and intra-day repeatability, with RSD values s 5.0%. In the presence of potentially interfering molecules that are present in the fruit juices, the GCE-RGO-MIP sensor showed up selective for recognition of the fructose molecule. The method developed was successfully applied in the determination of fructose in grape, orange and apple juices. Average recoveries varied from 95% to 105% with RSDs between 2.0% and 4.5%, indicating that the developed method had good accuracy for the determination of fructose.