Probing the Use of Homemade Carbon Fiber Microsensor for Quantifying Caffeine in Soft Beverages

Abstract

In the development of electrochemical sensors, carbon micro-structured or micro-materials have been widely used as supports/modifiers to improve the performance of bare electrodes. In the case of carbon fibers (CFs), these carbonaceous materials have received extensive attention and their use has been proposed in a variety of fields. However, to the best of our knowledge, no attempts for electroanalytical determination of caffeine with CF microelectrode (µE) have been reported in the literature. Therefore, a homemade CF-µE was fabricated, characterized, and used to determine caffeine in soft beverage samples. From the electrochemical characterization of the CF-µE in K₃Fe(CN)₆ 10 mmol L^-1 plus KCl 100 mmol L^-1, a radius of about 6 µm was estimated, registering a sigmoidal voltammetric profile that distinguishes a µE indicating that the mass-transport conditions were improved. Voltammetric analysis of the electrochemical response of caffeine at the CF-µE clearly showed that no effects were attained due to the mass transport in solution. Differential pulse voltammetric analysis using the CF-µE was able to determine the detection sensitivity, concentration range (0.3 to 4.5 µmol L^-1), limit of detection (0.13 μmol L^-1) and linear relationship (I (µA) = (11.6 ± 0.09) × 10^-3 [caffeine, μmol L^-1] - (0.37 ± 0.24) × 10^-3), aiming at the quantification applicability in concentration quality-control for the beverages industry. When the homemade CF-µE was used to quantify the caffeine concentration in the soft beverage samples, the values obtained were satisfactory in comparison with the concentrations reported in the literature. Additionally, the concentrations were analytically determined by high-performance liquid chromatography (HPLC). These results show that these electrodes may be an alternative to the development of new and portable reliable analytical tools at low cost with high efficiency.

Keywords: beverages; caffeine; carbon fiber; cyclic voltammetry; microelectrode.

Figure 1

Scheme for the CF-µE homemade fabrication.

Figure 2

Cyclic voltammogram recorded with the CF-µE using K₃[Fe(CN)₆] 10 mmol L⁻¹ in KCl 100 mmol L⁻¹ solution; ν = 30 mV s⁻¹; r = 6 µm.

Figure 3

(a) CVs recorded at CF-µE in 25 mL of 0.5 mol L⁻¹ H₂SO₄ solution (black curve) and 100 µL of 0.1 mol L⁻¹ caffeine in 25 mL of 0.5 mol L⁻¹ H₂SO₄ solution (red curve), scan rate: 10 mV s⁻¹; (b) scan rate effect (20 (pink line), 40 (blue line), 60 (green line), 80 (orange line) and 100 mV s⁻¹ (red line)) as a function of the electrochemical response of caffeine using 100 µL of 0.1 mol L⁻¹ caffeine in 25 mL of 0.5 mol L⁻¹ H₂SO₄; (c) oxidation peak current (I_pa) values versus the scan rates (v), I_pa (µA) = 1.3 × 10⁻⁴ (µA mV s⁻¹) + 0.098, r² = 0.9928; (d) CVs recorded at CF-µE in 100 µL of 0.1 mol L⁻¹ caffeine in 25 mL of 0.5 mol L⁻¹ H₂SO₄ solution for the first, tenth, and after 30 days; r = 6 µm.

Figure 4

Mechanism of overall oxidation of caffeine.

Figure 5

DPV profiles at CF-µE in 0.5 mol L⁻¹ H₂SO₄ as supporting electrolyte (a) in absence (dashed line) or presence (orange full line) of caffeine in solution, (b) standard additions of caffeine solution (0.1 mol L⁻¹): (1) 0.40, (2) 0.79, (3) 1.19, (4) 1.57, (5) 1.96, (6) 2.34, (7) 2.72, (8) 3.10, (9) 3.47, (10) 3.84, (11) 4.21, and (12) 4.58 µmol L⁻¹. DPV parameters were of initial potential = 0.5 V; final potential = 1.8 V; potential scan rate = 10 mV s⁻¹, pulse amplitude = 50 mV and slow agitation. (c) Linear calibration plot of caffeine concentration in solution versus current peak, based on the data collected from (b), using CF-µE in acidic (0.1 mol L⁻¹ H₂SO₄) medium; r = 6 µm. (d) Graphic displays weighted residuals.

Figure 6

DPV analysis of (a) some drink samples ((1) caffeine-free soft drink, (2) cola–soft drink 1 and (3) cola–soft drink 2), and (b) the standard addition procedure for the cola–soft drink 2 (plot with the DPV profile of supporting electrolyte), the soft beverage sample as well as the 1° addition, 2° addition, and 3° addition of 0.1 mol L⁻¹ caffeine, with CF-µE. DPV parameters were of initial potential = 0.5 V; final potential = 1.8 V; potential scan rate = 10 mV s⁻¹, pulse amplitude = 50 mV.

Figure 7

DPV profiles at CF-µE in 0.5 mol L⁻¹ H₂SO₄ as supporting electrolyte for contructing the analytical curve of caffeine (standard additions of caffeine solution (0.1 mol L⁻¹): (1) 0.40, (2) 0.79, (3) 1.19, (4) 1.57, (5) 1.96, (6) 2.34, (7) 2.72, (8) 3.10, (9) 3.47, (10) 3.84, (11) 4.21, and (12) 4.58 µ mol L⁻¹) in presence of ascorbic acid (50 µmol L⁻¹) in solution. Inset: Linear calibration plot of caffeine concentration in solution versus current peak, based on the data collected from the analytical curve.