Preparation and voltammetric characterization of electrodes coated with Langmuir-Schaefer ultrathin films of Nafion®

Voltammetric Detection of Caffeine in Beverages at Nafion/Graphite Nanoplatelets Layer-by-Layer Films

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.

Study on the structural and electrocatalytic properties of Ba2+- and Eu3+-doped silica xerogels as sensory platforms

This work reports on the sol–gel synthesis of barium- and europium-doped silica xerogel and its use as an electrocatalytic sensor.

Stripping voltammetric measurement of trace metal ions using screen-printed carbon and modified carbon paste electrodes on river water from the Eerste-Kuils River System

Screen-printed carbon electrodes (SPCEs) and carbon paste electrodes (CPEs) were prepared as "mercury-free" electrochemical sensors for the determination of trace metal ions in aqueous solutions. SPCEs were coated with conducting polymer layers of either polyaniline (PANI), or polyaniline-poly(2,2'-dithiodianiline) (PANI-PDTDA). Furthermore, CPEs containing electroactive compounds with reactivity towards metal ions were employed to obtain enhanced selectivity. Optimised experimental conditions for Hg(2+), Pb(2+), Ni(2+) and Cd(2+) determination included the supporting electrolyte concentration, deposition potential (E(d)) and accumulation time (t(acc)). For the modified carbon paste sensors (MCPEs) it was found that -400 mV is an adequate deposition potential and an accumulation time of 120 s was adequate for the determination using the different constructed electrodes. Initial results showed linearity in the examined concentration range between 1 × 10(-9) M and 1 × 10(-6) M using the SPCE/PANI-PDTDA sensor on laboratory prepared standard solutions, while good selectivity for the different metal ions were obtained. Furthermore, the limit of detection (LOD) was determined for each of the sensors and for the SPCE/PANI-PDTDA sensor it was found to be 2.2 × 10(-13) M, while for the SPCE/PANI sensor the LOD was determined to be 8.4 × 10(-11) M. The MCPE sensors also showed good linearity between the concentration range of 1 × 10(-3) to 1 × 10(-9) M. The LOD values for the various MCPE sensors, were found to be Hg(II) - 1.3 × 10(-7) M; Cd(II) - 2.9 × 10(-7) M; Ni(II) - 3.2 × 10(-7) M; and Pb(II) - 1.7 × 10(-7) M for the CPE/PANI-PDTDA sensor. For the CPE/PANI sensor the LOD values were Hg(II) - 1.5 × 10(-5) M; Cd(II) - 8.6 × 10(-7) M; Ni(II) - 9.5 × 10(-7) M; and Pb(II) - 1.3 × 10(-6) M. For the CPE/MBT sensor the LOD values were Hg(II) - 3.8 × 10(-5) M; Cd(II) - 1.4 × 10(-6) M; Ni(II) - 1 × 10(-6) M; and Pb(II) - 6.3 × 10(-5) M. Very low detection was obtained for the SPCE/PANI-PDTDA sensor in Hg(2+) determination, while the MCPE sensors delivered sensitive simultaneous detection for Hg(2+), Pb(2+), Ni(2+) and Cd(2+) metal ions.

Epifluorescence imaging of electrochemically switchable Langmuir-Blodgett films of Nafion

A combination of electrochemistry and luminescence methods was exploited to obtain information on the electrochemical activity and homogeneity of Nafion Langmuir-Blodgett films. The redox behavior of the Ru(bpy)3(2+) probe incorporated in the Nafion film was monitored by epifluorescence microscopy. The photoluminescent images, recorded by a charge-coupled device (CCD) camera, reflect the distribution of the probe in the film, which resulted as very uniform, particularly in comparison with spin-coated films. Apparent diffusion coefficients (Dapp) determined by cyclic voltammetry for films of less than 10 layers are in the range of 1 x 10(-12) to 8 x 10(-12) cm(2) s(-1), that is, 2 orders of magnitude lower than values reported in the literature for spin-coated Nafion films. The application to the electrode of a potential able to oxidize the luminescent Ru(bpy)3(2+) to the nonluminescent Ru(bpy)3(3+) switched off the photoluminescence with a response time that for the LB films was much shorter than that for the spin-coated ones. Experimental evidence and calculations indicate that lowering of the film thickness down to the nanometric level is very effective in shortening the switching time, notwithstanding the lowering of the Dapp value in LB films.

Measurement of apparent diffusion coefficients within ultrathin nafion Langmuir-Schaefer films: comparison of a novel scanning electrochemical microscopy approach with cyclic voltammetry

The use of scanning electrochemical microscopy (SECM) to evaluate the apparent diffusion coefficient, Dapp, of redox-active species in ultrathin Nafion films is described. In this technique, an ultramicroelectrode (UME) tip, positioned close to a film on a macroscopic electrode, is used to oxidize (or reduce) a species in bulk solution, causing the tip-generated oxidant (reductant) to diffuse to the film/solution interface. The oxidation (reduction) of film-confined species regenerates the reductant (oxidant) in solution, leading to feedback to the UME. A numerical model is developed that allows Dapp to be determined. For these studies, ultrathin films of Nafion were prepared using the Langmuir-Schaefer (LS) technique and loaded with an electroactive species, either the ferrocene derivative ferrocenyltrimethylammonium cation, FA+, or tris(2,2'-bipyridyl)ruthenium(II), Ru(bpy)32+. The morphology and the thickness of the Nafion LS films (1.5 +/- 0.2 nm per layer deposited) were evaluated using atomic force microscopy (AFM). For comparison with the SECM measurements, cyclic voltammetry (CV) was employed to evaluate the concentration of electroactive species within the Nafion LS films and to determine Dapp. The latter was found to be essentially invariant with film thickness, but the value for Ru(bpy)32+ was 1 order of magnitude larger than for FA+. CV and SECM measurements yield different values of Dapp, and the underlying reasons are discussed. In general, the Dapp values for these films are considerably smaller than for recast Nafion films, which can be attributed to the compactness of Nafion LS films. Nonetheless, the ultrathin nature of the films leads to fast response times, and we thus expect that these modified electrodes could find applications in sensing, electroanalysis, and electrocatalysis.

Electrochemistry of cytochrome c incorporated in Langmuir–Blodgett films of Nafion® and Eastman AQ 55®

Ultrathin films of Nafion and Eastman-AQ 55 loaded with cytochrome c (cyt c) were obtained and transferred on indium tin oxide (ITO) electrodes via the Langmuir-Blodgett (LB) technique. The pressure-area isotherms for mixed ionomer-protein films indicate that the miscibility of cyt c in the interfacial layer is better for Nafion than for AQ 55. Interestingly, these composite films maintain the electroactivity of cyt c without requiring the addition of promoters or mediators. Both for AQ 55-cyt c and Nafion-cyt c films, the half-wave potential for the reversible reduction of ferricytochrome c corresponds to the value expected for the weakly adsorbed protein. The modified electrodes show electrocatalytic reaction with ascorbate anion. Comparison with previous literature reports indicate that for Nafion the LB coating procedure is unique in keeping the electroactivity of cyt c.