Voltammetric Behavior of Carbon Paste Electrodes with Native and Chemical Modified Porphyrans

Porphyra species are important edible red algae abundantly cultivated; they are commonly known as ‘nori’ and used to prepare sushi. P. tenera and P. yezoensis are the species cultured and processed into this sheet type dried food. Porphyran is a linear sulfated water soluble polysaccharide that can be obtained from Porphyra species^1,2. It is constituted by units of (6-O-methyl)-β-D-galactose, 3,6-anhydro-α-L-galactose and α-L-galactose 6-sulfate^1,2. Carbon paste electrodes (CPE) containing natural ionic polysaccharides (pectic and alginic acid and heparin) have been studied³ and biosensors like these are generally developed for the detection of analytes particularly in aqueous phase⁴. The present work describes the voltammetric behavior of porphyran-modified CPE obtained before and after chemical desulfation of the polysaccharides. Porphyran was obtained from dried milled nori by water extraction (1.5 %w/v, for 16 h, at room temperature) and precipitation with ethanol (three volumes). The native fraction (PN) was purified by redissolution in water, dialysis and lyophilization. The desulfated porphyrans fractions were obtained by solvolysis⁵ (PD) and alkaline treatment⁶ (PA) from the native fraction. The polysaccharides structures were confirmed by ¹³C NMR analysis. The carbon paste was prepared by hand-mixing the porphyran with graphite powder (100 mg) and mineral oil (30 µL). Electrodes containing 10, 20, 40, 60, 80 and 100 µg of the native (EN) and chemically modified porphyran (ED and EA) were studied. The carbon paste was packed into the hole of the electrode body (working electrode) and an Ag/AgCl (reference electrode) and a platinum wire (auxiliary electrode) completed the three-electrode system in the measurement cell. Cyclic voltammograms (CV) were recorded in the potential range of -0.6 – 1.0 V (PBS buffer 0.15 mol L^-1, pH 6.5) with a scan rate of 50 mV s^-1 using K₃Fe(CN)₆/K₄Fe(CN)₆ 0.1 mol L^-1 as redox reference solution. The measurements were performed using a PalmSens potentiostat (Palm Instruments BV). The NMR spectra obtained for PN, PD and PA confirmed the porphyran structural pattern as they also showed that the chemical treatments produced desulfated polysaccharides. Especially for PA fraction, the NMR spectrum indicated an increase in the 3,6-anhydrogalactose content as a result of cyclization of the galactose-6-sulfate, as expected by the alkaline treatment⁷. The CVs was used to characterize the porphyran-modified CPEs and the voltammograms indicated an increase in the cathodic and anodic current peaks in the same potential range for all electrodes as compared to unmodified CPE. Despite this, the increase in current values did not correlate directly with the porphyran concentration used to modify the electrodes, so the CPEs with 10, 20 and 40 µg showed the most intense anodic peaks for EN (3,648.81 µA), EA (3,838.67 µA) and ED (4,452.00 µA), respectively (Figure 1). The higher values of redox current peaks were found for the electrode ED at all porphyrans concentrations studied. This behavior can be attributed to electrostatic repulsion between the K₃Fe(CN)₆/K₄Fe(CN)₆ and the ionic polysaccharide. As the porphyran PD has a lower number of sulfate groups, the electrode ED may have a better electrochemical response. In addition, the peak currents increased and the cathodic and anodic peak potential exhibited a small shift along with the increase of scan rate for EN (10ug), EA (20ug) and ED (40ug). At the same time, the anodic peak current increased linearly with the square root of the scan rate (correlation coefficient of 0.9711, 0.9975 and 0.9943 for EN, EA and ED respectively). This phenomenon suggested that the redox process was diffusion-controlled. In conclusion, the CVs demonstrated that the electron transfer on the porphyran-modified CPEs surface, especially on ED, are more efficient, conducting higher current and offering lower resistivity.

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Figure 1: Cyclic voltammogram of EN (10 µg), EA (20 µg) and ED (40 µg), in PBS buffer 0.15 mol L^-1, pH 6.5, potential range of -0.6 – 1.0 V, using K₃Fe(CN)₆/K₄Fe(CN)₆ 0.1 mol L^-1 as redox reference solution.