MERcury Electrochemical SENSor for in situ trace determination – MERESENS

In order to improve the Hg sensor’s lifetime, it appeared necessary to stabilize the gold nanoparticles on the electrode surface. To reach this goal, a mixed, two-step functionalization strategy has been developed in the course of the MERESENS project:
- in the first step, organic films of several nanometers thickness based on diazonium salts have been grafted on the glassy carbon electrode surface.
- in the second step, gold nanoparticles have been electrodeposited on the diazonium-functionalized electrodes.
The organic films bear thiol groups (SH), the Sulphur atom of which strongly interacts with the Gold atoms of the nanoparticles, and thus help at their stabilization by limiting their release from the electrode surface and hindering their reactivity between themselves. The as-prepared mixed diazonium/nanoparticles interface should afford an increased lifetime to the sensor and thus allow Hg determination over a wider time period.
The mixed functionalization strategy is also very simple, since both the diazonium grafting and the nanoparticles electrodeposition are performed by using electrochemical route.

- A new surface functionalization strategy has been developed, based on organic films and metal nanoparticles.
- The new interfaces allow the Mercury sensor’s lifetime to increase from a few days to several weeks.
- Water samples of various compositions have been collected all around the world and will be tested for Mercury determination.
- A new international collaboration has been started between the universities of Toulouse and Bremen

The increase in sensor lifetime from a few days to more than 3 weeks should allow a mid-term sensor deployment in natural media. Moreover, the physicochemical characterizations already performed on the mixed organic films/metal nanoparticles provide meaningful information to help at further increase sensor lifetime.
First tests in natural water samples have proved the sensor capable to detect Hg traces and suggest it will be soon possible to quantify Hg.

To date, the MERESENS project gave rise to 1 paper in an international peer-reviewed journal, 5 oral and poster communications in international congresses and 4 and national congresses.
An invited, vulgarization talk including some MERESENS results has been also given.
Further income is expected in the forthcoming months: particularly, another paper will be submitted in an international peer-reviewed journal.

Due to their wide spreading in the natural media, including fresh and marine waters, air and soils, heavy metals and particularly mercury (Hg), represent a growing environmental and health concern. Hg is present as inorganic and organometallic species such as the very toxic and bioaccumulating methylmercury (MeHg) form. The appearance of this latter very much depends on the concentration and (bio-)availability of inorganic mercury(II) (Hg(II)). To date, Hg(II) concentration is mainly monitored by spectrometric techniques, such as coupled cold-vapor atomic fluorescence spectrometry (CV-AFS). These techniques require expensive materials associated to complex and time-consuming procedures, thus limiting any in situ or on line and operando analysis. Consequently there is an urgent need for the development of in situ, real-time and highly-sensitive sensors allowing multiple monitoring points dedicated to early warning pollution alert.

The MERESENS project aims at developing such a reliable tool for in situ determination of Hg(II) at environmentally relevant levels. Indeed, we propose to develop and test a novel electrochemical Hg sensor for ecosystem monitoring in line with both European Water Framework Directive (Directive 2000/60/EC) and Marine Strategy Framework Directive (Directive 2008/56/EC) and requirements of European member states to fund monitoring networks and programs at optimized costs.

In the MERESENS project we will develop an electrochemical sensor based on glassy carbon (GC) electrodes functionalized by gold nanoparticles (AuNPs) and diazonium compounds. These sensors will be tested for Hg(II) trace measurements in natural waters (lake, river, rain, sea water,…) and optimized to exhibit good sensitivity and selectivity as well as reproducibility and stability over time.

The use of AuNPs to functionalize the electrode surface will enhance the sensitivity for low Hg(II) levels, whereas diazoniums will ensure the AuNPs stabilization on the electrode by affording a covalent anchoring with GC surface. This latter point is of outmost importance for long-term deployment.
A large set of functionalized interfaces will be produced by varying parameters such as organic layer thickness and structuration, and AuNPs size and density. All the resulting electrodes will be fully characterized by electrochemistry (cyclic voltammetry, electron impedance spectroscopy and scanning electrochemical microscopy) and physicochemical techniques (field emission scanning electron microscopy, atomic force microscopy, grazing incidence small angle X-ray scattering, …). Their analytical performances (sensitivity, limit of detection, repeatability, and stability) will be evaluated by checking their electrochemical response to varying Hg(II) concentrations, both in synthetic and real natural waters. The influence of interfering species will be also checked. Correlations will be established between the analytical performances and the information brought by the characterization of the mixed diazonium/AuNPs interfaces structuration. If necessary, further optimization of the analytical performances will be achieved on the basis of these correlations.

A large panel of natural water samples (submarine groundwater discharge, seawater, sea ice, brine, and so on) will be used for testing the sensor in order to verify its capability to afford a reliable response in many matrix conditions. The obtained results will be verified by using reference technique such as CV-AFS.

At the end of the project, a highly-sensitive and selective electrochemical sensor which exhibits very good stability for possible long-term (over months) deployment should be achieved and available, and the Technology Readiness Level 3 (TRL 3) will be reached. From a fundamental point of view, major advances are expected in the understanding of the interactions of the AuNPs with the organic diazonium films and the GC surface.