Direct Determination of Ascorbic Acid in a Grapefruit: Paving the Way for In Vivo Spectroelectrochemistry

Abstract

The study of real samples is more complicated than the study of other systems. However, the inherent advantages of UV-vis absorption spectroelectrochemistry should overcome some difficulties related to direct measurements in complex matrices. For this reason, a singular spectroelectrochemistry device has been fabricated and validated. The novel cell is based on single-walled carbon nanotubes, which are filtered and subsequently press-transferred on a polyethylene terephthalate support using a stencil with a custom design. With this new methodology, working, counter, and reference electrodes are completely flat on the surface, where two optical fibers are fixed in a long optical path length configuration. To demonstrate the usefulness of this device and the power of spectroelectrochemistry techniques to solve problems of the current world, this device is used to quantitatively detect the concentration of ascorbic acid in a complex matrix such as a fruit, directly, without any previous sample pretreatment. The ease to fabricate the device, the advantages related to its use, and the excellent results obtained not only with univariate but also with multivariate analysis, shed more light on the analysis of samples as they occur in nature. According to the particular features of this cell, to the best of our knowledge this is the first spectroelectrochemical sensor that can be inserted directly in a biological matrix, laying the groundwork to perform in vivo measurements in a near future.

Spectroelectrochemistry of Quantum Dots

“…Two of the most important drawbacks of this configuration were the difficulty to align the light beams and the complex assembly of the cells; but they have been overcome with the use of optical fibers with diameters close to the dimension of the diffusion layer. Bare optical fibers without jacketing material extraordinarily simplify the measurements in parallel configuration because they can be directly fixed to the WE without the need of a particular SEC cell, sampling the first microns of the solution adjacent to the electrode surface …”

“…As can be observed, optical fibres extremely simplify the design of SEC cells. Moreover, with their help, SEC has been performed even inside a grapefruit …”

“…Bare optical fibers without jacketing material extraordinarily simplify the measurements in parallel configuration because they can be directly fixed to the WE without the need of a particular SEC cell, sampling the first microns of the solution adjacent to the electrode surface. [47] The combination of normal and parallel configuration in the same experiment is called bidimensional SEC (BSEC) and it provides information about the electrochemical processes taking place at the electrode surface and that ones in the solution adjacent to the electrode. [43,44] Figure 5 shows a BSEC cell and illustrates how to simplify experimental setups for UV/VIS/NIR-SEC measurements.…”

“…Moreover, with their help, SEC has been performed even inside a grapefruit. [47] In ATR configuration, [40] Figure 4.e, the light beam that hits on a surface is bent normal from the surface as it passes from a higher to lower refractive index material, undergoing an internal reflection. In ATR, a high angle of incidence and a high refractive index crystal must be used.…”

Spectroelectrochemical Sensing: Current Trends and Challenges

“…From a quantitative point of view, the most used SEC techniques are UV/Vis absorption SEC , fluorescence SEC and Raman SEC . For this reason, and taking into account the objectives of this review, these three techniques are those that will be described in more detail below.…”