New screen-printed electrodes for Raman spectroelectrochemistry. Determination of p-aminosalicylic acid.

BACKGROUND: The availability of new surface enhanced Raman scattering (SERS) substrates is essential to develop quantitative analytical methods. Electrochemistry is an easy, fast and reproducible methodology to prepare SERS substrates on screen-printed electrodes (SPEs). RESULTS: This work proposes new SPEs based on a three-electrode system all made of silver. Using the same ink for the whole electrode system facilitates the fabrication process, reduces production costs, and leads to excellent analytical performance. The results showed that Raman enhancement depends strongly on the type of silver ink. To demonstrate the capabilities of the new electrodes developed, 4-aminosalicylic acid was determined in complex matrices and in the presence of strong interfering compounds such as salicylic acid and acetylsalicylic acid. The proposed analytical method is based on the electrochemical surface oxidation enhanced Raman scattering (EC-SOERS) strategy. AgCl nanocrystals are generated on the working electrode surface, which amplify the Raman signal of 4-aminosalicylic acid. Good figures of merit were obtained both in the absence and in the presence of the interfering compounds, achieving a correct estimation of a 4-aminosalicylic test sample in complex matrices. SIGNIFICANCE: The new SPEs have been demonstrated to be very sensitive and reproducible which, together to the high specificity of the Raman signal, makes this methodology very attractive for chemical analysis.

Bidimensional Spectroelectrochemistry with Tunable Thin-Layer Thickness.

Bidimensional spectroelectrochemistry (Bidim-SEC) is an instrumental technique that provides operando UV/vis absorption information on electrochemical processes from two different points of view, using concomitantly a parallel and a normal optical configuration. The parallel configuration provides information about chemical species present in the diffusion layer, meanwhile the normal arrangement supplies information about changes occurring both in the diffusion layer and, mainly, on the electrode surface. The choice of a suitable cell to perform Bidim-SEC experiments is critical, especially while working under a thin-layer regime. So far, most of the proposed Bidim-SEC cells rely on the use of spacers to define the thin-layer thickness, which leads to working with constant thickness values. Herein, we propose a novel Bidim-SEC cell that enables easy-to-use micrometric control of the thin-layer thickness using a piezoelectric positioner. This device can be used for the study of complex interfacial systems and also to easily measure the key parameters of an electrochemical process. As a proof of concept, the study of the roughening of a gold electrode in KCl medium is performed, identifying key steps in the passivation and nanoparticle generation on the gold surface.

Fe-MOF Catalytic Nanoarchitectonic toward Electrochemical Ammonia Production.

Electrochemical reduction of nitrate into ammonia has lately been identified as one among the promising solutions to address the challenges triggered by the growing global energy demand. Exploring newer electrocatalyst materials is vital to make this process effective and feasible. Recently, metal-organic framework (MOF)-based catalysts are being well investigated for electrocatalytic ammonia synthesis, accounting for their enhanced structural and compositional integrity during catalytic reduction reactions. In this study, we investigate the ability of the PCN-250-Fe3 MOF toward ammonia production in its pristine and activated forms. The activated MOF catalyst delivered a faradaic efficiency of about 90% at -1 V vs RHE and a yield rate of 2.5 × 10-4 mol cm-2 h-1, while the pristine catalyst delivered a 60% faradaic efficiency at the same potential. Theoretical studies further provide insights into the nitrate reduction reaction mechanism catalyzed by the PCN-250-Fe3 MOF catalyst. In short, simpler and cost-effective strategies such as pretreatment of electrocatalysts have an upper hand in aggravating the intrinsic material properties, for catalytic applications, when compared to conventional material modification approaches.

Raman, UV-Vis Absorption, and Fluorescence Spectroelectrochemistry for Studying the Enhancement of the Raman Scattering Using Nanocrystals Activated by Metal Cations.

Raman signal enhancement is fundamental to develop different analytical tools for chemical analysis, interface reaction studies, or new materials characterization, among others. Thus, phenomena such as surface-enhanced Raman scattering (SERS) have been used for decades to increase the sensitivity of Raman spectroscopy, leading to a huge development of this field. Recently, an alternative method to SERS for the amplification of Raman signals has been reported. This method, known as electrochemical surface oxidation-enhanced Raman scattering (EC-SOERS), has been experimentally described. However, to date, it has not yet been fully understood. In this work, new experimental data that clarify the origin of the Raman enhancement in SOERS are provided. The use of a complete and unique set of combined spectroelectrochemistry techniques, including time-resolved operando UV-vis absorption, fluorescence, and Raman spectroelectrochemistry, reveals that such enhancement is related to the generation of dielectric or semiconductor nanocrystals on the surface of the electrode and that the interaction between the target molecule and the dielectric substrate is mediated by metal cations. According to these results, the interaction metal electrode-nanocrystal-metal cation-molecule is proposed as being responsible for the Raman enhancement in Ag and Cu substrates. Elucidation of the origin of the Raman enhancement will help to promote the rational design of SOERS substrates as an attractive alternative to the well-known SERS phenomenon.

Determination of uric acid in synthetic urine by using electrochemical surface oxidation enhanced Raman scattering.

In this work, a new and easy methodology to determine uric acid in relevant samples using Raman spectroelectrochemistry is presented. The spectroelectrochemistry experiment is based on the in-situ formation of a suitable substrate that enables the enhancement of the Raman signal of an analyte during the oxidation stage of a silver electrode. This phenomenon is known as electrochemical surface oxidation enhanced Raman scattering (EC-SOERS) and has proved to be useful in quantitative analysis using disposable screen printed electrodes. The successful combination of EC-SOERS with PARAFAC analysis allows the determination of uric acid in a relevant complex sample avoiding the use of standard addition method and without using a baseline correction, which simplifies the application of such methodology in routine analysis.

Formic acid electrooxidation on thallium-decorated shape-controlled platinum nanoparticles: an improvement in electrocatalytic activity.

Thallium modified shape-controlled Pt nanoparticles were prepared and their electrocatalytic activity towards formic acid electrooxidation was evaluated in 0.5 M sulfuric acid. The electrochemical and in situ FTIR spectroscopic results show a remarkable improvement in the electrocatalytic activity, especially in the low potential region (around 0.1-0.2 V vs. RHE). Cubic Pt nanoparticles modified with Tl were found to be more active than the octahedral Pt ones in the entire range of Tl coverages and potential windows. In situ FTIR spectra indicate that the promotional effect produced by Tl results in the inhibition of the poisoning step leading to COads, thus improving the onset potential for the complete formic acid oxidation to CO2. Chronoamperometric experiments were also performed at 0.2 V to evaluate the stability of the electrocatalysts at constant potential. Finally, experiments with different concentrations of formic acid (0.05-1 M) were also carried out. In all cases, Tl-modified cubic Pt nanoparticles result to be the most active. All these facts reinforce the importance of controlling the surface structure of the electrocatalysts to optimize their electrocatalytic properties.