Abstract
We report for the first time a procedure in which Nafion/Graphite nanoplatelets (GNPs) thin films are fabricated using a modified layer-by-layer (LbL) method. The method consists of dipping a substrate (quartz and/or glassy carbon electrodes) into a composite solution made of Nafion and GNPs dissolved together in ethanol, followed by washing steps in water. This procedure allowed the fabrication of multilayer films of (Nafion/GNPs)n by means of hydrogen bonding and hydrophobic‒hydrophobic interactions between Nafion, GNPs, and the corresponding solid substrate. The average thickness of each layer evaluated using profilometer corresponds to ca. 50 nm. The as-prepared Nafion/GNPs LbL films were characterized using various spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), FTIR, and optical microscopy. This characterization highlights the presence of oxygen functionalities that support a mechanism of self-assembly via hydrogen bonding interactions, along with hydrophobic interactions between the carbon groups of GNPs and the Teflon-like (carbon‒fluorine backbone) of Nafion. We showed that Nafion/GNPs LbL films can be deposited onto glassy carbon electrodes and utilized for the voltammetric detection of caffeine in beverages. The results showed that Nafion/GNPs LbL films can achieve a limit of detection for caffeine (LoD) of 0.032 μM and linear range between 20‒250 μM using differential pulse voltammetry, whereas, using cyclic voltammetry LoD and linear range were found to be 24 μM and 50‒5000 μM, respectively. Voltammetric detection of caffeine in beverages showed good agreement between the values found experimentally and those reported by the beverage producers. The values found are also in agreement with those obtained using a standard spectrophotometric method. The proposed method is appealing because it allows the fabrication of Nafion/GNPs thin films in a simple fashion using a single-step procedure, rather than using composite solutions with opposite electrostatic charge, and also allows the detection of caffeine in beverages without any pre-treatment or dilution of the real samples. The proposed method is characterized by a fast response time without apparent interference, and the results were competitive with those obtained with other materials reported in the literature.
Highlights
Electrochemical sensors have received widespread attention in the analytical sciences due to their high selectivity and sensitivity, along with fast response time, ease of fabrication and low cost [1]
There are two advantages to using such a method: (1), the addition of Nafion in the solution prevents the aggregation of the graphite nanoplatelets (GNPs), giving good dispersion even in water; (2) the multilayer assembly is faster since it requires fewer steps, as the LbL layers are formed using the same composite solution in each deposition cycle
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Summary
Electrochemical sensors have received widespread attention in the analytical sciences due to their high selectivity and sensitivity, along with fast response time, ease of fabrication and low cost [1]. Several methods have been developed to produce graphene, including mechanical graphite exfoliation [7], carbon nanotubes unzipping [8,9], chemical vapor deposition of methane gas (CVD) [10,11] and reduction of graphene oxide [12,13] These methods have shown low reproducibility, with high costs associated with the synthesis processes. Polymers are preferable to surfactants because, in helping the dispersion, especially in benign solvents such as alcohols and water, they confer on the composite material the physicochemical properties associated with the polymer An example of this is Nafion, a sulfonic-based ionomer with cation exchange properties that is widely used in electroanalysis to preconcentrate positively charged species while repelling anions [15]. As a result of this preconcentration ability to detect cations, we tested the Nafion/GNPs composite films for the detection of caffeine, a natural alkaloid widely present in food such as chocolate, coffee, soft drinks
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