Development of a voltammetric method for the determination of rapamycin in pharmaceutical samples at pretreated pencil graphite electrode

Abstract

This paper describes an electroanalytical method for the determination of rapamycin using a pretreated pencil graphite electrode (P‐PGE) by differential pulse voltammetry. The electrodes were successfully characterized by cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy, and Fourier transform infrared. The influences of instrumental and chemical parameters were investigated and optimized for the determination of rapamycin. Oxidation peak current obtained for rapamycin at P‐PGE was 10.5 times higher than that obtained for PGE. In pH = 3.0 H3PO4, a well‐defined irreversible anodic peak occurred at 1.07 V, which was attributed to the electro‐oxidation of rapamycin at P‐PGE. Under optimized conditions, the oxidation peak current of rapamycin was linear with its concentration in the range of 0.01–250 μmol/L, and the detection limit was 0.0075 μmol/L (S/N = 3). Also, the P‐PGE exhibited good reproducibility (relative SD [RSD] = 2.17% for 10 electrodes at different times using the same procedure), repeatability (RSD = 2.09% for 10 measurements with same electrode), and stability (95.80% for 10 days). The P‐PGE was simple to prepare and the method was successfully applied for the determination of rapamycin in spiked and real samples. In addition, for comparison, high‐performance liquid chromatographic method with ultraviolet (UV) detector was used for the determination of rapamycin.

Recent Developments in Voltammetric Analysis of Pharmaceuticals Using Disposable Pencil Graphite Electrodes

“…An araldite mixture (epoxy resin, 1:1) [16,41,[43][44][45], bentonite [38,39], or cyanoacrylate glue [40] was used to fix the graphite lead into the support. Other PGEs were obtained by covering a part of the graphite lead with nail polish [53] or with Teflon tape [20,[54][55][56][57][58][59][60][61][62][63][64][65] (Figure 1e), leaving a constant length and area of nacked pencil mine to act as an electroative surface. The electrical contact with the potentiostat is provided by a metallic (copper) wire wrapped arround the end of the mine, which is not exposed to the analyzed solution.…”

“…Thus, scanning electron microscopy (SEM) images emphasized that the non-treated PGE had a smooth surface, whereas the electrochemical pretreated PGE (ET-PGE) presented porosity and an increased surface area [80]. CV investigations pointed out the increase in the ET-PGE effective electroactive surface area in comparison with the PGE (e.g., 0.0170 cm 2 for an HB type PGE and 0.0235 cm 2 for ET-PGE [43], 0.0355 cm 2 for a 2B type PGE and 0.1408 cm 2 for ET-PGE [56], or approximately a threefold increase [80]). The electrochemical impedance spectroscopic (EIS) studies emphasized a lower charge transfer resistance and thus a higher conductivity of the ET-PGE [56,80] due to good electrocatalytic activity and a high interfacial electron transfer rate at the ET-PGE, resulting in enhanced peak currents.…”

“…The electrochemical characterization of bare and modified electrodes was usually performed by CV [87,88,117,124,125,129] and electrochemical impedance spectroscopy (EIS) using [Fe(CN) 6 ] 3−/4− as a redox probe and KCl as a supporting electrolyte [17,20,37,41,43,45,47,53,56,57,59,[63][64][65]69,80,85,87,90,105,117,124,125,127,131,132,136,[141][142][143]147], but [Ru(NH 3 ) 6 ] 3+/2+ was also used [38].…”

“…CV carried out at different scan rates was most often employed to calculate the electroactive surface area (A ea ) of bare or different modified electrodes while considering the slope of the regression equation corresponding to the I p = f(v 1/2 ) plot, where I p (A) repre-sents the peak current and v is the potential scan rate (V/s) [16,20,38,39,41,[43][44][45][46][47][48][49]53,56,59,64,67,72,74,82,85,87,88,106,[124][125][126]128,141].…”

“…The Nyquist plot consists of a linear part at lower frequencies, which corresponds to the diffusion process, and a semicircle at higher frequencies, whose diameter can be used to calculate the charge transfer resistance at the electrode's surface (Rct). The charge transfer resistance of the electrode is obtained from a known equivalent circuit model (Randle's equivalent circuit) [10,44,45,47,56,68,87,111,117,124]. Thus, the larger semicircle indicates a high interfacial electron transfer resistance, whereas a small semicircle indicates a lower resistance, increased conductivity, and faster electron transfer.…”