Electrochemical assay for Heavy Metals trace in Marine Environment by The Peaks ShifT Analysis during catalysis – EMMETT

The metal detection method consists of 3 stages: i) modification of the electrode surface by molecules that accumulate the metal ions present in the sample; ii) change of medium, i.e. replacement of the sample to be analysed by a medium optimised for the specific detection of the desired metal; and iii) electrochemical analysis of the surface using the cyclic voltammetry technique.

The surface of the carbon electrodes is modified by chemical reactions that do not use toxic or harmful solvents. Commercial products are used; no chemical synthesis is carried out during this project.

The electrocatalysis reactions envisaged for metal revelation exploit the electrolysis reaction of water or the presence of oxygen in the medium.

Surface analysis by XRD, XPS, and/or RAMAN spectroscopy is envisaged to study the evolution of metal deposits on electrodes. Tailor-made experimental devices will be produced for this purpose (e.g., an electrochemical cell for in situ X-ray analysis or electrochemical modification of MET grids).

The production of carbon inks uses commercial carbon products (carbon black, graphite, graphene microparticles, and glassy carbon), a cellulose-based binder and a solvent based on solvents classified as non-carcinogenic (cyrene, water, and cyclohexanon). These inks may be functional, i.e., they contain selective metal chelating agents to enable them to accumulate passively and be detected. These inks will be deposited on substrates using dispensing equipment (DMD100 from Kelenn Technology or Proplus4 from Nordson).

Firstly, we wondered whether the preliminary results obtained for the ultrasensitive detection of copper were dependent on the nature of the receptor grafted to the working electrode. To answer this question, we applied the same detection protocol to a carbon electrode modified with aminobenzoic acid or 4-aminophenyl phosphate. We were then able to show that the proton reduction catalysis reaction that reveals and enables the copper assay is independent of the nature of the ligand. This result raises a more fundamental question: What is the copper-based catalyst? According to the literature, it is supposed that it forms nanoparticles on the surface of the electrodes during the detection stage and that the potential at which proton reduction occurs depends on the size of the copper nanoparticles (in other words, the amount of copper accumulated on the sensor). The smaller the particles (i.e. the less copper on the sensor), the lower the proton reduction potential. To answer this question, we are developing experimental devices that will enable XRD and TEM analysis of copper nanoparticles. One of the difficulties with these approaches is the availability of materials compatible with the analysis technique (XRD/TEM) and electrochemical analysis methods. This work is currently in progress. These results were the subject of two presentations at an international and a national congress.

At the same time, we were interested in the detection of iron(III). We used the same modified electrodes as for copper detection. It turned out that only the 4-aminophenyl phosphate-modified electrode allowed iron to accumulate. Detection of this element is therefore straightforward down to a hundred nanomolar (i.e. around 30 ppb when the detection threshold set by the WHO for drinking water is 300 ppb, for example). We were able to demonstrate that using the electrocatalysis property of iron with the reduction of hydrogen peroxide reduces the detection limit for iron by a factor of 2. Given the results obtained with copper, we were interested in the literature protocols for the formation of iron nanoparticles. It turned out that the deposits formed have an electroactivity that has never been reported before. To identify their nature, analysis by XPS and TEM is once again envisaged.

Finally, three formulations of carbon ink based on carbon black and/or graphene aggregates were developed and successfully printed using the motorised dispensing technique. The electrodes showed fairly good electrochemical reactivity. However, printability remains difficult, and surface modifications of insufficient quality to achieve our objectives. Optimisation of these formulations is in progress.

Results that will be obtained by DRX, TEM (and XPS if necessary) should enable the publication of a first article, which will propose an alternative approach for the detection of trace metals, thanks to their shaping into nanoparticles. This article could use the results obtained with iron unless these results can be enhanced by a second publication. A publication strategy will need to be put in place with our collaborators (Ifremer, UBO).

The extension of these results to the detection of manganese, as envisaged at the start of the project, will also be studied in the medium term.

An initial cyclam-functionalised carbon ink for copper detection has been formulated and deposited on a flexible substrate. The device produced is awaiting analysis. With this first trial, we hope to demonstrate that it is possible to detect a metal using the classic ‘stripping’ method.

The EMMETT project proposes the development of a new electrochemical method for metal-trace analysis in the marine environment. This method is more sensitive and more specific than usual stripping techniques and will be investigated in a lab-on-chip with a view for long-term and in situ applications. To do this, the project is based on recent results obtained at the laboratory for a marine copper sensor development. This work has highlighted that following copper passive accumulation on modified electrodes, the electrocatalytic properties of the metal towards protons reduction could be used for its dosage. This analytical way presented a better sensitivity, more than 10 times compared to classical stripping technique. The uniqueness of this method is the use of the catalytical regime to measure the catalyst rather than the substrate: this is the potential measurement at the highest catalytical current density that permits to obtain the catalyst quantity pre-concentrated at the electrode. Since most of heavy metals are known to show catalytical properties for water electrocatalysis, the EMMETT project proposes to extend this methodology for iron and manganese detection, which are metals essential to phytoplankton production and usual searched elements in the marine environment. Moreover, as in situ electrochemical measurements of these elements lack alternatives to methods employing mercury electrodes, it is proposed to develop a homemade graphite-based material, that could be included in a microfluidic module made by stereolithography. The final objective of the project is to obtain a complementary module to the existent CHEMINI marine in situ chemical microanalyzer in order to detect trace metals thanks to their electrocatalytic properties, with eco-friendly material and increased sensitivity, compared to the conventional methods.