Lignin film was deposited at the surface of glassy carbon electrode potentiostatically. In contrast to the unmodified glassy carbon electrode, an oxidative peak with an improved current and overpotential for caffeine at modified electrode showed catalytic activity of the modifier towards oxidation of caffeine. Linear dependence of peak current on caffeine concentration in the range
Alkaloids are broad category of nitrogen containing organic metabolites produced by plants; the plants that produce these alkaloids make their leaves unattractive to eating by insects and higher animals [
However, reports on human and animal studies showed that caffeine produces mental and behavioral effects that are similar to those of typical psychomotor stimulant drugs such as amphetamine and cocaine [
The word coffee is believed to be derived from
Coffee beans, which are seeds of Rubiaceae botanic family and
Among the coffee varieties in the country, coffees cultivated at four localities of Ethiopia, Finoteselam, Wolega, Zegie, and Wonbera, are commonly consumed by the consumers in Bahir Dar City, North West Ethiopia. Of these, the coffee cultivated in Wonbera locality is the most expensive and preferred while the Zegie coffee is the cheapest and least preferred, the basis for which might be psychological, aroma, otherwise the caffeine content. Thus, the aim of this research was to check whether the users’ preference can be related to the caffeine content or not which needs determination of the caffeine content of the four coffee varieties using a sensitive, selective, fast, and environmentally friendly method.
High performance liquid chromatography [
In contrast to the conventional analytical methods, electrochemical methods are powerful and versatile analytical techniques that offer high sensitivity, accuracy and precision, relatively wider linear dynamic range and low-cost instrumentation, specificity for a particular oxidation state of an element, relative inexpensiveness, and being usually environmentally friendly (use no or minimum amount of organic solvent) [
In recent years, working electrodes including boron doped diamond [
Carbon materials have broad potential window, low background current, rich surface chemistry, comparative chemical inertness, relatively easy electrode preparation, and low cost, making them electrodes of choice [
Lignin is a natural polymer contained in most woody trees and shrubs [
Caffeine (Fischer Scientific), H3PO4 (Cipla Ltd., India), K2HPO4 (Wagtech International Ltd., UK), CH3COOH and NaOH (Landmark chemicals PVT., India), HCl (Riedel-De Haen, Germany), lignin (Riedel-De Haen, Germany), HNO3 (65–70%), and H2SO4 (99%) (both from Loba chemie Pvt., Ltd.) were used. All reagents were of analytical grade and were used directly without further purification. Glassware was cleaned using chromic solution prepared by dissolving 1 g of potassium dichromate in 1 L of H2SO4 followed by rinsing with distilled water. Distilled water was used for the preparation of all solutions.
Voltammetric experiments were carried out using CHI760D Electrochemical Workstation (Austin, Texas, USA) connected to a personal computer. All electrochemical experiments were performed, employing a conventional three-electrode system with a glassy carbon electrode (3 mm in diameter) or a Lignin modified glassy carbon electrode as the working electrode, platinum coil as an auxiliary electrode, and Ag/AgCl as a reference electrode. All experiments were carried out at
Coffee samples were purchased from the respective local markets (Wolega, Finoteselam, Zegie, and Wonbera). Aqueous extracts of the coffee samples were prepared following the procedure outlined elsewhere [
The lignin modified glassy carbon electrode (LGCE) was prepared following a procedure reported elsewhere [
Lignin polymer was then deposited at the electrode surface at a potential of +0.9 V for 2 minutes. The lignin modified electrode was then rinsed with distilled water to remove physically adsorbed and unreacted species from the electrode surface. Subsequently, the modified electrode was stabilized in pH 7 PBS by scanning the potential between −0.2 V and +1.0 V until a steady cyclic voltammogram was obtained. Finally, the modified electrode was dried in air and made ready for use.
For cyclic voltammetric studies, 10 mM stock solution of caffeine was prepared by dissolving 0.1941 g of caffeine in 100 mL of 0.1 M pH 5 Acetate buffer solution (ABS) [
Cyclic voltammetric and differential pulse voltammetric techniques were used to investigate the electrochemical behavior of caffeine and determine caffeine content in coffee samples, respectively.
Figure
CVs of unmodified GCE (a and a′) and LGCE (b and b′) in pH 5 ABS containing no (a and b) and 1 mM (a′ and b′) caffeine.
As can be seen from the figure, no peak was observed at both the unmodified and modified glassy carbon electrodes in caffeine-free buffer solution. On the contrary, well resolved oxidative peak was recorded at the unmodified GCE and LGCE at a potential of 1.578 and 1.510 V, respectively. Absence of a reductive peak indicated that caffeine undergoes an irreversible oxidation at both the modified and unmodified electrodes which is in agreement with previously reported works [
Appearance of an oxidative peak with a relatively larger peak current and improved peak potential at the lignin modified electrode compared with the unmodified GCE also showed the catalytic activity of the lignin modified electrode which might be due to an increased electrode surface area and/or improved electron exchange at the electrode surface [
To investigate the type of reaction kinetics the caffeine follows at the surface of lignin modified glassy carbon electrode, the determination coefficients (
Cyclic voltammograms of lignin modified GCE at pH 5 ABS containing 1 mM caffeine at different scan rates ((a)–(m): 10, 20, 40, 60, 80, 100, 125, 150, 200, 250, 300, 350, and 400 mV s−1, resp.). Inset: plot of oxidative peak current as a function of the square root of scan rate.
The effect of pH on the electrochemical response of lignin modified electrode for caffeine was studied in the pH range of 3 to 6. Figure
CVs of LGCE in ABS of different pH values (3.0–6.0) containing 1.0 × 10−3 mol L−1 of caffeine. Inset: plot of Ipa versus pH in pH range 3.0–6.0 at a scan rate of 100 mV s−1.
Since it is an effective voltammetric method with established advantages, including good discrimination against background current and low detection limit, differential pulse voltammetry (DPV) was used for the quantification of caffeine content of coffee samples cultivated in different localities of Ethiopia. Figure
Differential pulse voltammograms (corrected for background) of unmodified (a) and modified (b) GCE in pH 4.0 ABS containing 1 mM caffeine.
The effect of differential pulse voltammetric parameters such as the step potential and pulse amplitude on the peak current response of lignin modified glassy carbon electrode for 1 mM caffeine under the optimized pH 4 ABS was investigated (Figure
DPVs of lignin modified GCE in pH 4 ABS containing 1 mM caffeine at (a) different step potentials ((a)–(c): 4, 8, and 12 mV, resp.) and pulse amplitude of 50 mV and (b) different amplitudes ((a)–(d): 25, 50, 75, and 100 mV, resp.) and step potential of 8 mV.
After optimizing the method and solution parameters, DPV voltammograms were obtained for the LGCE in pH 4.0 ABS containing no (a), 1 mM caffeine (b), and the corrected voltammogram for the blank (c) (Figure
DPVs of LGCE in pH 4 ABS containing (a) no, (b) 1 mM caffeine, and (c) 1 mM caffeine corrected for the blank under optimized conditions.
As can be observed from the figure, closeness of the peak current of the subtracted (c) and unsubtracted (b) voltammograms of the LGCE showed low capacitive current of the modified electrode and hence fitness of the electrode for caffeine determination.
Under the optimized solution and method parameters, anodic peak current of caffeine at lignin modified GCE was linearly proportional to the caffeine concentration in the range of 6 to 100 × 10−6 mol L−1 (Figure
DPVs of LGCE in pH 4.0 ABS containing various concentrations of caffeine ((a)–(j): 6, 8, 10, 20, 30, 40, 50, 60, 80, and 100
Aqueous coffee extract samples were prepared as described in the procedure under the experimental part. To determine the concentration of caffeine in real coffee samples cultivated in four different localities of Ethiopia, the differential pulse voltammetric peak current for each extract sample was recorded which then was converted to concentration units using the regression equation of the calibration curve. Figure
Summary of concentration of caffeine in aqueous coffee extract and corresponding amount of caffeine per gram of roasted coffee of coffee samples from different localities of Ethiopia.
Locality of coffee sample | DPV oxidative peak current ( |
Caffeine concentration ( |
Caffeine |
---|---|---|---|
Wonbera | 1.45 | 44.41 | 10.78 |
Wolega | 1.17 | 36.18 | 8.78 |
Finoteselam | 0.83 | 26.18 | 6.35 |
Zegie | 0.76 | 24.12 | 5.85 |
DPVs of LGCE in pH 4 ABS containing aqueous extracts of coffee samples cultivated in different localities of Ethiopia ((a)–(d): Zegie, Finoteselam, Wolga, and Wonbera, resp.).
To evaluate the accuracy of the developed DPV method using lignin modified GCE, the recovery of spiked standard caffeine in aqueous extracts of Wolega coffee, which showed an intermediate amount of caffeine among the studied coffee samples, was checked. Figure
DPVs of aqueous Wolega coffee extract samples spiked with different volumes of 60
As can be seen from the figure, the DPV anodic peak current increased with increasing volume of the spiked standard, indicating the sensitivity of the method developed. Excellent recovery results in the range 93.79–102.17% (Table
Summary of percentage recoveries of spiked standard caffeine in aqueous coffee extract.
Sample | Initial |
Spiked standard ( |
Expected, |
% recovery |
---|---|---|---|---|
Coffee extract (a) | 26.31 | — | — | — |
Coffee extract (b) | 26.31 | 5.43 | 5.45 | 99.63 |
Coffee extract (c) | 26.31 | 8.00 | 7.83 | 102.17 |
Coffee extract (d) | 26.31 | 12.99 | 13.85 | 93.79 |
Coffee extract (e) | 26.31 | 15.33 | 15.56 | 98.52 |
This work presents the differential pulse voltammetric determination of water extracted caffeine content of coffee samples cultivated in different localities of Ethiopia using a lignin modified glassy carbon electrode. The electrode modifier reported is relatively cheap, environmentally friendly, easily deposited at the electrode surface exhibiting low capacitive current, and easy to clean.
Differential pulse voltammetric current response showed linear dependence on the concentration in the range 6–100
The authors declare that there are no conflicts of interest regarding the publication of this paper.