In the field of surface science, the LRS is developing an original approach that aims to reconcile a fundamental approach on single-crystal or model supports with their implementation at the solid-liquid interface, under conditions representative of industrial procedures (catalysis) or natural environment (prebiotic chemistry, biological environment).

This approach, which may be termed aqueous phase surface science, has received little or no national or international attention, since model approaches generally favor cleaner (usually UHV) but less realistic active species deposition procedures from an application point of view. However, this latter approach remains essential, particularly in the case of complex systems combining multiple compounds/materials for specific applications.

A case in point is the PhD thesis work by Elisa MERIGGIO (Labex Matisse) on the design of enantioselective heterogeneous catalysts. The aim of this study was to understand the interactions at molecular level between the various players in an enantioselective catalytic system, namely a chiral modifier, (R,R)-tartaric acid, an oxide support and a metal catalyst (Ni nanoparticles). The latter two elements were deposited by evaporation on a planar TiO₂(110) rutile support under clean and ultra-high vacuum controlled conditions. Using a combination of surface science techniques (HREELS, XPS, AFM, PM-IRRAS...) it was shown that no extended chiral domains were formed, in contrast to what is observed on metal surfaces such as Cu(110) or Ni(111). Overall, the adsorption of tartaric acid molecules takes place both on the rutile surface and on Ni nanoparticles, and enantioselective induction is therefore not limited to metal surfaces but can also be produced by the oxide support, which is rarely considered in the literature, thus opening up an original perspective.

A second aspect of the fundamental approach developed in this theme concerns the role of water as a (co)reactant (hydration or hydrolysis reactions), as a product (dehydration), as a competitive adsorbate or as a solvent (catalyst stability) in situations specific to biomass conversion (ambient or hydrothermal conditions, cavitation, etc.).

There are, for example, two types of variations on this approach: the first concerns the acid-base activation of trans-esterification reactions, in which the particular interest of magnesium silicates such as MSH and laponite has been demonstrated (Longfei LIN PhD thesis, Chinese Scholarship Council). The multiple effects of the solvent (methanol and water in this case) on the delamination of clays under reaction conditions, particularly in terms of the accessibility of active sites, have been highlighted.

A second example illustrating the central role of water in reactivity is provided by the field of chemistry at the origins of life, an original application of great fundamental and social interest. Indeed, many of the reactions that led to the self-organization of living molecules are condensation reactions involving the production of a water molecule; conversely, the destruction of biomolecules involves hydration/hydrolysis reactions. The diverse roles of water in this respect are recognized by the "origins of life" community, but the "surface science" approach to discriminating between them is original. Hagop ABADIAN's PhD thesis (ED 397), for example, focused on the study of peptide bond formation under abiotic conditions, revealing a novel intermediate species in the condensation of leucine in the presence of water adsorbed in controlled quantities on a silica surface. In this case, the amount of water on the surface could be quantified by FTIR spectroscopy using specific near-infrared bands.

In addition to aspects related to surface science, one of the major themes relating to liquid/solid interfaces is rooted in the prospect of developing projects compatible with a more sustainable chemistry, including in particular the problem of biomass transformation or environmental pollution control concerns, as well as the development of biomaterials.

We can illustrate this research orientation with two examples concerning the transformation of lignocellulosic biomass. The first concerns the use of this type of biomass (more specifically straw) to produce biomethane (Vincenzo CALCAGNO, post-doctoral researcher, MSTD SU Initiative). Unlike organic livestock waste, lignocellulosic biomass requires methanogenesis in solid form, after a pre-treatment stage. Interactions at the solid/liquid interface are essential, as they control the efficiency of the cellulose hydrolysis step, and therefore the yield of anaerobic fermentation in terms of methane production. In collaboration with colleagues at the Université Technologique de Compiègne (UTC), experts in the methanogenesis stage, the work carried out at the LRS focuses on establishing a correlation between the properties of the biomaterial and the nature of its pre-treatment, based on spectroscopic analyses such as solid state NMR or ATR-IR, but also NMR relaxometry or DRIFT, which allow us to characterize material porosity and water diffusion, as well as changes in the accessibility of surface OH groups.

The second example concerns the depolymerization of lignin, one of nature's most abundant polymers and the only renewable source of aromatic molecules (Louay AL HUSSAINI PhD thesis, doctoral program in process engineering). The oxidative cleavage of lignin is carried out in a basic medium, which generates a quantity of waste to be managed that is incompatible with the economic feasibility of the process. The approach taken at LRS was to use oxygen as the oxidizing agent, in the presence of the H₆[PMo₉V₃O₄₀] catalyst required for its activation. Two catalyst synthesis routes were explored, including an original one combining a ball milling step for vanadium and molybdenum oxides, before a hydrothermal step leading to very good performance in terms of oxidation of two lignin-model compounds.

A complementary aspect of the study of oxidation reactions using oxygen at the solid/liquid interface is being developed within the framework of Ana SCHUH FRANTZ's PhD thesis (ED 397), which aims to develop and study a method for depolluting aqueous effluents contaminated by so-called "emerging" organic pollutants. More specifically, the LRS is involved in the implementation of attenuated total reflection Fourier transform infrared spectroscopy (ATR-IR) to determine, in situ, the degradation kinetics of three priority model pollutants on the surface of Iron(II)-based substrates. Time-resolved ATR-IR (acquired with funding from Labex Matisse) is ideally suited to the study of interfacial phenomena for solid-liquid reactions, since it excludes most of the contribution of water used as a solvent.

Last but not least, we should mention biocatalysis, a scientific theme in the field of liquid/solid interfaces, which has a number of applications. In addition to bridging the gap between the laboratory's catalysis and biointerface activities, the use of enzymes as biocatalysts is part of a resolutely sustainable approach to chemistry. It is also particularly well suited to the challenges of transforming biomass, their natural substrate, as well as those of biomineralization.

Two examples of the use of biocatalysts in different applications can be cited. The first aims to synthesize enantiomerically pure amines from biomass-derived alcohols in two successive steps (PhD theses of Pryianka GIROLA and Yfan ZAN, ED397). In the first stage, using a metal nanoparticle catalyst, the alcohol is oxidized to aldehyde or ketone, which are then converted to amine by transaminase enzymes. The scientific challenges are: (i) to find conditions for the selective oxidation of alcohols in a solvent compatible with the subsequent use of the enzyme, i.e. water, so as to carry out both stages without intermediate purification, and (ii) to develop a common Metal Organic Framework (MOF)-type support enabling the catalysis to be "heterogenized".

Finally, in the field of biocatalysis, we can cite work using laccase, an oxidation-reduction enzyme catalyzing the oxidation of certain phenolic substrates or aromatic amines concomitantly with the reduction of dioxygen to water, with an application in the field of biopile/biocell (Achraf BLOUT PhD thesis, Labex Matisse). The efficiency of the electron transfer process at the solid electrode depends on both the amount of biocatalyst immobilized and the orientation of the protein at the solid/liquid interface. Consequently, several immobilization strategies have been tested on carbon electrodes, in particular after nanostructuring to form nanowalls on the graphite surface, leading to measured current densities approaching the highest current densities reported in the literature.

In 2021, the LRS welcomed four permanent researchers and teacher-researchers: Vincent VIVIER (Directeur de Recherche CNRS), Mireille TURMINE (Maîtresse de Conférences SU), Oumaïma GHARBI (Chargée de Recherche CNRS) and Kieu NGO (Maître de Conférences SU), whose scientific activities focus on electrochemistry, particularly surface corrosion and electrocatalysis.

In the field of surface reactivity, the approach adopted by the LRS is original on a national scale, since it aims to combine ideal surfaces with controlled properties with preparation methods typical of the chosen application (catalysis, biosensors, etc.), usually in the liquid phase, in order to build a bridge between the world of ultra-high vacuum (clean, single-crystal surfaces) and that of surface chemistry (powdery materials, real surfaces, mainly in the aqueous phase).

In particular, an aqueous phase surface science approach is being implemented to monitor the effect of support on the genesis of the active phase (MoS₂) in hydrotreating catalysts. This approach is the subject of a sustained collaboration between LRS and IFPEN, which has funded a PhD thesis during the current contract (Ricardo GARCIA, 2017-2020). The chosen approach aimed to use α-Al₂O₃ single crystals in 4 orientations: C(0001), A(11 -2 0), M(10 -1 0), and R(1 -1 02) as model surfaces with the aim of studying MoS₂ genesis from the deposition of the active phase in solution while controlling the speciation of the support surface sites which is not achievable on the poorly crystalline industrial support γ-Al₂O₃.  The combination of ideal surfaces with methods for preparing catalysts in solution is completely original on a national scale. It enables us to combine the complexity of interfacial chemistry (surface charge, speciation at interfaces, etc.) with perfectly defined surfaces.

This approach showed that certain MoS₂ properties (sulfidation rate, dispersion and relative orientation with respect to the support) are controlled by Mo-support interactions, which in turn are determined by the type of surface OH intrinsic to α-Al₂O₃ faces. By integrating typical industrial catalyst promoters and additives (cobalt, phosphorus, triethylene glycol) into these model catalysts, it was also possible to study their catalytic activity in thiophene hydrodesulfurization by developing a catalytic set-up specially adapted to the extremely low active phase contents of catalysts on flat surfaces. These results showed that the catalytic activity followed a volcano curve as a function of the exposed surface (with a maximum for the A(11 -2 0) surface), which tends to prove that the strength of the metal-support interaction is a major descriptor of catalytic activity by controlling the strength of the Mo-sulfur bond of the active phase according to Sabatier's principle.

The adsorption of biomolecules onto inorganic surfaces is another original and promising direction for the laboratory. In this line of research, we are working on understanding the interaction mechanisms between small peptides and metal and oxide surfaces, with the aim of gathering information at the molecular level. For several years now, we have been studying the adsorption of these biomolecules in the gas phase under ultra-high vacuum (UHV) conditions, enabling us to fine-tune the characterization of the system under study. These experiments have enabled us to identify and quantify the interaction sites between peptides and a copper surface, and to reveal a novel interfacial process in the gas phase (monolayer vs. multilayer depending on peptide sequence).

A significant development for this UHV set-up has been the acquisition of an electro-spray that enables the adsorption of biomolecules from an aqueous solution. Such idea has been little explored for molecule adsorption, yet offers a major advantage linked to the preservation of the charge state of the solubilized molecule during adsorption in a UHV environment. Recent experimental results, combined with theoretical calculations, have shown that a peptide molecule (Glu-Cys-Gly) adsorbs intact in a deprotonated state on a copper surface. However, the interaction mode of the molecule is highly dependent on the crystal face (Cu(111) vs. Cu(110)). This includes, for example, the interaction mode of the thiol group (hollow vs. bridge) and the formation of an H bond between the surface and the amine group. This electro-spray adsorption technique has also enabled other complex organic molecules to be deposited in a controlled manner.

In parallel with the electro-spray studies under UHV, we set up adsorption experiments, of the same peptide, in liquid phase, with or without electrochemical control, to modulate the reactivity of the copper surface. For these experiments we explored the combination of measurements, carried out in aqueous media in situ by quartz crystal microbalance (QCM-D) or ex situ by atomic force microscopy (AFM), or in the "dried" phase by XPS and surface infrared (PM-IRRAS). The results showed that Cu⁺ ions, generated by the copper surface, play a major role in the adsorption mode of peptides, leading to the unexpected formation of multilayers. These experiments, supported by theoretical calculations, have yielded decisive results in reconciling phenomena observed under model conditions (UHV) and in aqueous media. In particular, they highlight the complex role of water at the oligopeptide/copper surface interface.

Research on reactivity in the liquid phase is based on a range of techniques available in the laboratory or through collaborative projects. It should be noted that a particular effort has been made during this term to develop techniques adapted to the specificity of the solid-liquid interface, either by extending the scope of pre-existing laboratory techniques to in situ analyses in the presence of solvent, or by deploying new experimental resources.

Among the techniques dedicated to the study of the solid/liquid interface most recently deployed in the laboratory, thanks to the acquisition of the appropriate equipment, is that of light scattering (DLS) (ANR UPPHOTOCAT), used to study in real time the growth of calcium phosphate controlled by enzymatic catalysis. By combining the information provided by DLS and static light scattering (SLS), it has also been possible to characterize both the nucleation and growth of these minerals, enabling a more systematic study of the biomineralization model systems developed in the laboratory.

As many systems at the liquid/solid interface involve aqueous phases, the state of charge at the interface and the study of how this varies according to the nature of the solution (pH, ionic strength, adsorption, etc.) are essential elements of the interaction involved. Acquiring the zeta potential of the surface by measuring the flow current is an essential element of characterization. The experimental set-up (Surpass, Anton Parr, ANR SlimCat) initially acquired to study the charging of single-crystal alumina surfaces, was subsequently implemented with catalytic supports (collaboration with IFPEN), functionalized gold surfaces (PhD thesis of Yacine MAZOUZI) and in collaboration with external laboratories. The expertise acquired in the laboratory using this little-used technique, thanks in particular to the recruitment of Clément GUIBERT, a senior lecturer at the LRS since the end of 2017 recruited specifically for the "study of the solid/liquid interface" profile, is thus a highlight testifying to the laboratory's growing strength in the further development of this theme.

We have already mentioned the development of an innovative method for introducing reagents by electrospray, enabling us to study the adsorption of molecules from the aqueous phase onto surfaces, a very important adaptation of XPS in the context of characterizing the solid/liquid interface. In the field of spectroscopy, an FTIR spectrometer acquired specifically for the study of reaction mechanisms at the solid/liquid interface (Matisse funding) at the start of this term has been added to the range of tools around which the LRS is developing its expertise. The device, which operates in transmission or ATR mode, has a "rapid scan" option capable of recording up to 50 spectra per second. Studies using this time-resolved FTIR device include those in the field of prebiotic chemistry, such as the formation of peptides from amino acids or the reactivity of carbamyl phosphate, a nucleic base precursor, in aqueous solution or on inorganic surfaces, as well as biomineralization phenomena. In addition, FTIR "light minus dark" difference spectra were obtained, at progressive illumination times on the seconds time scale, for photoactivatable proteins of the Orange Carotenoid Protein family. Scanning speed was a key factor in this study, enabling very good signal-to-noise difference spectra to be obtained. Finally, a specific ATR module enables the study of fast kinetic reactions triggered by perfusion, such as the degradation of organic pollutants in aqueous phase by oxidation in the presence of an iron-based catalyst deposited on the ATR crystal (Ana SCHUH FRANTZ PhD thesis, ANR DEPOLECO).

Some of the work carried out in the field of biointerfaces, particularly that relating to biosensors and biomineralization, involves complex phenomena involving several "objects": proteins, surfaces, nanoparticles, themselves potentially pre-functionalized, and which need to be characterized individually and under operating conditions, in solution. In this case, a combination of different techniques is often used. This approach is illustrated in the following two examples. In the context of biomineralization, we are interested in the mechanisms by which proteins generate and structure mineral matter through regulatory processes such as homeostasis and compartmentalization.

The biomimetic model recently developed in the laboratory combines two key factors: the in situ generation of phosphate ion precursors of the mineral phase via an enzyme, alkaline phosphatase, and the co-immobilization of enzymes and type I collagen. This model made it possible to monitor mineralization in real time using light-scattering techniques (DLS and SLS), as described at the beginning of this paragraph, and to quantify the kinetic parameters relating to the nucleation and growth of calcium phosphate particles. Furthermore, the association of collagen with enzyme-controlled mineralization at the solid/liquid interface showed a major effect on mineralization. These findings were obtained by combining characterization techniques to probe events in liquid phase (AFM) and in real time (QCM-D), as well as the properties of adsorbed layers (morphology, rigidity, viscoelasticity in particular).

In the field of biosensors, in-situ monitoring of biomolecules adsorbed on flat surfaces has been made possible thanks to QCM/surface plasmon resonance (SPR) coupling. Two such applications include the detection of diclofenac, considered an emerging pollutant in surface waters, by controlling protein adsorption and hydration levels during biosensor design, and the exploration of the viscoelastic properties of extracellular vesicles (EVs) derived from mouse striatal cells expressing either a mutated or wild-type allele of huntingtin (Htt), the Huntington's disease gene.

Data collected during adsorption of these specific EV subpopulations can serve as particularly effective biomarkers for detecting intercellular changes associated with the neurodegenerative condition.

The unit has undergone several changes during this term, in particular the arrival of a group comprising two teacher-researchers, two researchers and their students. This integration, which was the subject of scientific discussions between the various parties, resulted in a transfer effective June 1, 2021, based on a project concerning the operando characterization of surface reactivity by coupling electrochemical measurements to spectroscopic techniques. This arrival has enabled the LRS to extend its field of expertise, which has already resulted in the publication of 13 articles. Thanks to this fruitful association, several projects directly involving LRS know-how have just been accepted, including a PEPRH2 project and a PEPR DIADEM project, which are also in line with the priority orientations of our institutes, namely hydrogen and artificial intelligence.

Finally, thanks to the many interactions that took place during the preparation of this integration, it has already led to several effective collaborations:

- a PhD thesis begun in late 2022 (Yakoub SMATI) involving J. BLANCHARD, J. REBOUL, M. TURMINE and V. VIVIER on the characterization of MOFs dedicated to applications in electrocatalysis (water splitting), from their performance to their degradation,
- a project starting in September 2022 (Mélanie DE VOS, ATER) involving C. THOMAS, M. TURMINE and V. VIVIER on the study of the electrocatalytic reactivity of Rh clusters of controlled sizes
- a project starting in 2023 with an IUT trainee and a CSC PhD thesis application involving J. SCHNEE, M. TURMINE and V. VIVIER on the exsolution synthesis of catalysts for water splitting,
- a project starting in 2022 (Salvador RAMIREZ RICO, post-doc) on the development of an EPR/electrochemistry coupling cell to study the degradation of ionic liquids when used as electrolytes (F. AVERSENG, O. GHARBI, M. TURMINE and V. VIVIER)
- a project starting in 2022 (Ekatarina KURCHAVOVA, ANR thesis) on NMR/electrochemistry coupling to study the degradation of ionic liquids when used as electrolytes (liquid NMR) and, in the longer term, the electrode of proton batteries (solid-state NMR) (V. HERLEDAN, Y. MILLOT, M. TURMINE and V. VIVIER)

In addition, a number of projects have been submitted to various calls:

- ANR WaterSens (S. BOUJDAY, J-M. KRAFFT, J. BLANCHARD, C. METHIVIER, A. WILSON, K. NGO and A. MICHE)
- ANR CoCo (O. GHARBI, J. LANDOULSI, C. METHIVIER, V. VIVIER)
- Access to large instruments: coupling electrochemistry / EXAF via two applications to study the formation of corrosion-inhibiting films in ionic liquids (O. GHARBI, K. NGO, M. TURMINE, V. VIVIER and A. WILSON) and to characterize the organization of molecules at an electrified interface (O. GHARBI, K. NGO, M. TURMINE, V. VIVIER and A. WILSON).