Electrochemical polymerized DL-phenylalanine modified carbon nanotube sensor for the selective and sensitive determination of caffeic acid with riboflavin

Abstract

In this study, DL-phenylalanine modified with a multiwall carbon nanotube paste electrode is used as advanced electrochemical sensor for analysing of 0.1 mM caffeic acid (CFA) with simultaneous detection of riboflavin (RFN). The developed sensors include electrochemically polymerized DL-phenylalanine (DL-PA) modified multiwall carbon nanotube paste electrode [DL-PAMMCNTPE] and bare multiwall carbon nanotube paste electrode [BMCNTPE]. The increasing stability in the developed electrochemical sensor for the quantification of CFA is highlighted in detail, along with its characterization using voltammetric techniques such as electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), differential pulse voltammetry (DPV), and linear Sweep Voltammetry (LSV). Scanning electron microscopy (SEM) technique was used to studied the structural analysis of BMCNTPE and DL-PAMMCNTPE surface. The investigation of 0.1 mM CFA in 0.2 M phosphate buffered solution (PBS) using a 7.0 pH at 0.1 V/s scan rate was highlighted using DL-PAMMCNTPE, which shows good electrochemical responses compared to BMCNTPE. This work characterizes the voltammetric responses by inspecting the pH effect, scan rate effect, and concentration difference of CFA at the DL-PAMMCNTPE surface. The CFA responses specify that the scan rate progress is adsorption controlled. The concentration of CFA detection was started from 20 μM to 600 μM using DPV method, with lower limit of detection (LOD) of 0.280 μM and limit of quantification (LOQ) of 0.936 μM. And for CV method concentration range 20 to 550 μM, with LOD of 0.198 μM and LOQ of 0.702 μM. Furthermore, the developed electrochemical sensor responses are shows good stability, repeatability, and reproducibility, for CFA. The analytical applicability of CFA in apple juice and coffee powder samples was also evaluated.

Keywords: Caffeic acid; DL-phenylalanine; Electrochemical analyser; Multiwall carbon nanotube paste sensor; Voltammetry.

Fig. 1

(a) 10 Cyclic voltammograms (CVs) of polymerized DL-PA at the surface of BMCNTPE in PBS pH 7.0. at the scan rate of 0.1 V/s. Graph (b) plotted number of cycles vs current.

Fig. 2

SEM pictures of (a) BMCNTPE (b) DL-PAMMCNTPE.

Fig. 3

EIS curves of (a) DL-PAMMCNTPE and (b) BMCNTPE.

Fig. 4

CVs of 1.0 mM K₄[Fe(CN)₆] in 0.1 M KCl on DL-PAMMCNTPE (cycle-a) and BMCNTPE (cycle-b) surface at the scan rate 0.1 V/s.

Fig. 5

(a) CVs of 0.1 mM CFA on DL-PAMMCNTPE surface in 0.2 M PBS with different pH from 5.5 to 7.5. at 0.1 V/s scan rate. Plot of (b) I_pa vs pH and (c) E_pa vs pH.

Fig. 6

(a) The CVs of 0.1 mM CFA on the surface of DL-PAMMCNTPE in PBS pH 7.0 at changed scan rate from 0.025 to 0.450 V/s ranges. Plot of (b) log υ vs log I, (c) υ vs Current, and (d) log υ vs Ep.

Fig. 7

Electrochemical redox reaction of CFA.

Fig. 8

CVs of 0.1 mM CFA for the presence and absence in PBS of pH 7.0 on the surface BMCNTPE (cycle-a) DL-PAMMCNTPE (cycle-c) and (blank, cycle-b) at 0.1 V/s scan rate.

Fig. 9

(a) Differential pulse voltammograms were recorded for CFA across a concentration range spanning from 20 to 600 μM. these measurements were carried out in 0.2 M PBS at a pH of 7.0, at 0.1 V/s scan rate. Plot of (b) concentration of CFA vs current.

Fig. 10

(a) CVs recorded for CFA concentration range 20 to 550 μM in 0.2 M PBS a pH 7.0, at scan rate of 0.1 V/s. Plot of (b) current vs concentration of CFA.

Fig. 11

Graph of % of peak signal vs interferents.

Fig. 12

Simultaneous analysis of CFA and RFN at BMCNTPE and DL-PAMMCNTPE (6.0 pH, 0.2 M PBS) at the scan rate of 0.1 V/s, using (a) CV method. (b) DPV method. (c) concentration variation of CFA and RFN ranges from 10 μM to 90 μM at DL-PAMMCNTPE surface using LSV method.