Mikkel Christian R Petersen

B Lotz

Getinet Woyessa

Nath Amar

Thibaut Ghosh

L Sylvestre

Johann Brilland

Mogens H Troles

Rafael Jakobsen

O Taboryski

Bang

Mikkel B Lotz

Amar N Ghosh

Thibaut Sylvestre

Laurent Brilland

Johann Troles

Mogens H Jakobsen

Rafael Taboryski

Ole Bang

Nanoimprinting and tapering of chalcogenide photonic crystal fibers for cascaded supercontinuum generation

come   L'archive ouverte pluridisciplinaire

SC light sources based on chalcogenide glass optical fibers are among the main candidates for enabling new and improved midinfrared photonic applications within broadband imaging and spectroscopy [1][2][3]. Currently, the most promising method for generating SC reaching far into the mid-infrared fingerprint region is through so-called cascaded generation using a series of concatenated silica, fluoride, and chalcogenide fibers that gradually extend the spectrum from the near-infrared to the mid-infrared [4][5][6][7].

One critical point in particular is the coupling to-and from the chalcogenide fiber. Due to the high refractive index of chalcogenide glasses, a significant part of the pump light is reflected at the end facets, which means that both the input-and output power is substantially reduced. One way to mitigate this is by using AR thin film coatings such as Al2O3 or diamond deposited on the fiber end facets [8]. However, deposition of thin films on fiber facets is an elaborate and time-consuming cleanroom process that requires careful sample preparation, and the resulting coating is typically narrowband, which is not ideal for cascaded generation with a broadband input source. Another way to achieve AR over a broad bandwidth is by direct thermal nanoimprinting [9][10][11]. Due to the low glass transition temperature (Tg) of chalcogenide glasses (~160-280 °C) the process can be performed using simple heating elements, and takes only a few minutes to complete.

Another critical aspect of coupling light to a chalcogenide fiber is avoiding damage when launching high peak power pulses. The simplest solution to allow for better power handling is to scale up the fiber core. However, when the core diameter is increased the nonlinearity of the fiber is reduced, thus hindering further extension of the spectrum. In order to increase the nonlinearity without sacrificing the damage threshold, the fiber diameter can be gradually reduced along the propagation direction through thermal tapering, thus improving the MIR generation efficiency [12,13].

In this work we investigate these two thermal post-processing techniques, nanoimprinting and tapering, in order to improve the output power and bandwidth of cascaded SC sources. We chose to work with single-material Ge10As22Se68 PCF in order to avoid issues with varying Tg's and viscosities between the core and cladding in step-index fibers (SIF). The pump source for the experiments was a SC source based on a master oscillator power amplifier (MOPA) configuration with a variable pulse repetition rate and 1 ns pulse envelope [14]. The MOPA was used to pump a 7 μm core diameter ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) glass fiber generating a spectrum covering 1-4.2 μm.

The principle of fiber nanoimprinting is depicted in Fig. 1(a). A nickel (Ni) mold with the negative structure is placed on a hot plate and heated to above the Tg of the chalcogenide glass (~175 °C [15]). For more details regarding the fabrication of the mold see refs. [9,16]. The fiber is then pressed against the mold for a brief period of time, after which the fiber is quickly released to allow for the imprinted structure to settle. Force was applied by manually moving a translation stage, and the combination of force, imprint time, and temperature enabled for varying degrees of structuring and deformation of the fiber. In general, higher temperatures allow for faster imprinting using low force, which also reduces the heat conduction to the rest of the fiber and thus reduces deformation. Fig. 1(b) shows the PCF used in the experiments, which has a core diameter of 15 μm and cladding diameter of 170 μm. The best imprints were achieved using a temperature between 230-250 °C applying a very small translation force for just a few seconds. Fig. 1(c) and 1(d) show high magnification scanning electron microscope (SEM) images of the imprinted structures, which were designed to have a pitch of 1050 nm and a height of around 1350 nm, providing excellent anti-reflection from 2.6-6.2 μm (<4 % reflection) peaking at 4 μm (<1 % reflection) [9,16]. The size of the structure meant that for wavelengths below ~2.2 μm the imprint acts as a diffraction grating, effectively scattering much of the light into the cladding. This effect is seen from the microscope image of Fig. 2(b), which shows the chalcogenide fiber lighting up near the butt-coupling interface with the ZBLAN fiber from the pump source. It is also apparent from the image that the imprinting procedure in this case caused significant deformation of the fiber tip, resulting in a so-called elephant's foot shape. However, this deformation affects mainly the outermost part of the cladding leaving the core area more or less intact except for some hole shrinkage, as is apparent from Fig. 1(c). The effect of fiber tip deformation and hole shrinking on the output beam profile is seen when comparing the beam profiles of Fig. 2(c) and 2(d), which shows the near fields of the non-imprinted and imprinted fibers, respectively. The near field images were captured using an uncooled PbSe camera covering from 1-5 μm wavelength (Tachyon 6400, NIT). In the nonimprinted fiber output beam the PCF structure is clearly seen, while the imprinted output beam shows a significant distortion of the air-cladding structure while maintaining a circular core-guided mode. Note that the intensity scale on the figures is logarithmic in order to clearly see the air-hole patterns. By optimizing the temperature, pressure, and imprinting time it was possible to reduce the fiber tip deformation while maintaining a full imprint of the structure, but it was found to have no influence on the transmission since the core area is seemingly unaffected by the deformation. To measure the output spectra the PCF output was butt-coupled to a 150 μm core diameter chalcogenide patch cable (IRF-Se-150, IR Flex) and guided to a grating-based spectrometer covering from 0.2-5 μm (Spectro 320, Instrument Systems). From comparing the red output spectrum of the non-imprinted fiber in Fig. 2(a) to the purple and blue output spectra of the 1-face (output) and 2-face imprinted fiber, respectively, it is clear that a significant fraction of the light below 2.2 μm is lost due to diffraction. Such properties could potentially be useful for reducing the linear and nonlinear absorption of shorter wavelengths, in particular twophoton absorption, which may improve the power handling. It is also evident from the spectra that the long-wavelength transmission was improved after imprinting. In order to quantify the improvement the total output power and output power after a 3.5 μm long-pass filter (LPF) was measured for the non-imprinted and the 2-face imprinted fibers. Many fiber samples with similar lengths around 20-30 cm were prepared and measured, and because the imprinting was performed manually the results varied from producing only minor or no improvement in transmission, to even reducing the output power compared to the nonimprinted reference. However, some samples also produced a significant increase in transmission, and these results are presented in Table 1. The total transmission improvement (ΔT) between the non-imprinted and 2-face imprinted fibers ranged from 7.0-27.9 % and the maximum total transmission (T) was 73.8 %. As seen from the orange curve of Fig. 2(a), a significant part of the pump spectrum is below the diffraction limit of the imprinted structure, so the transmission increase >3.5 μm was also measured and found to range from 12.3-60.2 % with a maximum transmission of 98.4 %. However, this extremely high value is attributed to a combination of both reduced reflection and increased red-shifting of wavelength components below 3.5 μm due to the higher transmission.

Previous work with fiber nanoimprinting has focused on large-core multi-mode As2S3 SIFs (50-100 μm core diameters) [11], and so it was not clear whether the micron scale structures and especially the fiber tip deformation would affect transmission of smaller core fibers. In that respect, this work represents the first demonstration of AR nanoimprinting of a single-mode chalcogenide PCF, achieving increased transmission in a fiber suitable for SC generation without significantly affecting the core mode of the fiber. Despite the increased transmission and red-shifting in the imprinted fibers the spectral extent was limited to 5 μm, so in order to increase the bandwidth even further, fiber tapering was for the first time to the authors' knowledge investigated for cascaded SCG. In a previous study on SC in tapered chalcogenide PCFs [12] using direct pumping it was found that the length of uniform fiber before (Lb) and after (La) the tapered fiber section (Lt) greatly influenced the bandwidth obtained, but the influence of the tapered core diameter was never fully explored. Here we have investigated the influence of the tapered core diameter (dc) in five different tapered fibers, whose parameters are summarized in Table 2. All tapers were designed to have around 2.5 cm adiabatic taper transitions. When the fiber core is reduced the zerodispersion wavelength (ZDW) is shifted towards shorter wavelengths from around 4.9 μm at the initial 15 μm diameter to around 3.6-4 μm at 6-7.5 μm diameter, respectively [12]. But while the reduced ZDW allow for more pump power to couple into the anomalous dispersion regime for improved SCG efficiency, the confinement loss edge is also pushed towards shorter wavelengths, which introduces a trade-off between efficiency and long-wavelength loss. Fig. 3(a) shows the resulting spectra from pumping tapers A-E with around 270 mW of average power at 1 MHz pulse repetition rate. The spectra were measured using a fibercoupled Fourier-tranform infrared (FTIR) spectrometer (FOSS, custom) covering from 2.8-14 μm. The total transmission of the non-imprinted tapers A-D varied from 31-38 %, with the highest output power of 103 mW achieved for taper D. The improvement in transmission is defined as ΔT = T(2-face)/T(non). Square brackets denote values >3.5 μm.

b Calculated using the output power >3.5 μm for Non #2 and 2-face #4.

3

A c c e p t e d M a n u s c r i p t

Despite the differences in taper parameters, tapers A-C produced very similar output spectra reaching up to around 8 μm at the -30 dB level, while taper D was limited to 7.5 μm and exhibited significantly lower flatness compared to the three others. For a 7.5 μm core diameter the ZDW is estimated to be around 4 μm, which is exactly where we see a dip in the output spectrum of taper D. Such a dip followed by a peak or pedestal of lower intensity is commonly observed when pumping in the normal dispersion region close to the ZDW [17,18]. Since the SC pump starts to cut off at around 4 μm the solitons coupled into the PCF will initially experience normal dispersion and therefore only the part that broadens into the anomalous dispersion via self-phase modulation (SPM) will then contribute to the longwavelength edge. As a result the amount of power >3.5 μm is only ~21 % in taper D (22.2 mW) compared to ~29 % in taper C (25.8 mW).

To investigate whether the spectral extension was limited by the taper length, another taper denoted Taper E was tested. Taper E has the same 6 μm core diameter as taper A, but twice the tapered section length and a longer length of uniform fiber before the taper. It is seen in Fig. 3(a) that the longer taper improves the long-wavelength generation, but reduces the output power significantly due to increased losses. Fig. 3(b) shows the entire taper E output spectrum at ~350 mW pump power (41 mW output power), where maximum broadening was observed covering from 1.07-7.94 μm at the -20 dB level. Further increasing the pump power did not improve the long-wavelength edge, as seen from the trend in Fig. 4(a), which shows that for these pump and taper parameters the broadening is limited to around 8 μm. This behavior was found in all of the tapers tested, which suggests that further broadening is limited mainly by the soliton dynamics of the MOPA source and ZBLAN fiber rather than confinement loss or dispersion in the tapers [5,7]. Lastly, the output power stability was tested. Note that at the point of this measurement (4 months later) taper C had degraded significantly due to exposure to ambient lab con-ditions [15], and so the obtainable bandwidth and output power was reduced. The insert shows the degradation in output power during a 116.5 hour continuous power stabil-ity measurement, and adjusting the coupling did not im-prove the output power. This highlights the need for proper sealing of the optical facets, which could for instance be realized by either fully collapsing the air-holes, or by splicing an unstructured fiber end-cap onto the PCF prior to imprinting.

In conclusion, we have demonstrated for the first time improved transmission and broadening in cascaded SC generation through nanoimprinting and tapering of Ge10As22Se68 chalcogenide PCFs. The maximum total transmission of a nanoimprinted uniform fiber was increased by 27.9 % relative to the non-imprinted fiber, and the average power >3.5 μm was increased by up to 60.2 %. By tapering a non-imprinted 15 μm core PCF down to a core diameter of 7 μm over a 12 cm section a spectrum covering up to ~8 μm with 90 mW output power and 25.8 mW >3.5 μm was achieved. By combining the two post-processing methods we expect that the output power can be increased to more than 120 mW with more than 40 mW above 3.5 μm without damaging the fiber. Such power levels would enable applications such as optical coherence tomography to push further into the mid-IR for improved inspection of ceramics, coatings, polymers, and fiber-reinforced composites [19].

Funding. European Union H2020 (722380, 732968); Innovation Fund Denmark (4107-00011A).

1

 1

Fig. 1 .

 1 Fig. 1. (a) Principle of fiber nanoimprinting using a heated Ni mold. (b) SEM image of the PCF prior to imprinting. (c) SEM image of the imprinted core and air-clad region. (d) High magnification SEM of the imprinted structures in the core.

Fig. 2 .

 2 Fig. 2. (a) Comparison between the input spectrum (orange) and the non-imprinted (red), output imprinted (purple), and 2face imprinted (blue) output spectra, respectively. (b) Microscope image of the ZBLAN to PCF butt-coupling revealing scattered light from diffraction caused by the imprinted structure. (c,d) Output beam profile of the non-imprinted (c) and imprinted (d) fiber. Note that log scale was used on the intensity axis to better visualize the air-cladding.

2

 2

A c c e

  p t e d M a n u s c r i p t

Fig. 3 .

 3 Fig. 3. (a) FTIR spectra of the long-wavelength edge obtained for tapers A-E for a pump power of ~270 mW. The corresponding measured total output power and power >3.5 μm (brackets) is indicated in the legend (*=estimated). (b) Stitched total output spectrum for taper E using 350 mW pump power and generating a 41 mW continuum with a 20 dB bandwidth from 1.07-7.94 μm.

Fig. 4 .

 4 Fig. 4. (a) Spectral broadening in taper E with increasing output power (brackets: power >3.5 μm). (b) Output power degradation in Taper C during 116.5 hour testing.

Table 2 . Taper parameters.

 2

	Taper ID	dc (μm)	Lb (cm)	Lt (cm)	La (cm) Ltot(cm)
	A	6	6	11	18	35
	B	6.5	10	15	11	36
	C	7	2	12	19	33
	D	7.5	9	15	18	42
	E	6	27	30	12	69

Table 1 . Measured transmission (T=Pout/Pin) of non-imprinted and 2-face imprinted PCFs a

 1

	Fiber sample	Pin (mW)	Pout (mW)	T (%)	ΔT a (%)
	Non #1	139.8 [31.9]	74.4 [18.9]	53.2 [59.2]	--
	2-face #1	139.8 [31.9]	83.5 [23.3]	59.7 [73.0]	10.9 [18.9]
	2-face #2	139.8 [31.9]	86.5 [25.8]	61.9 [80.9]	14.1 [26.8]
	2-face #3	133.5 [31.2]	98.5 [30.7]	73.8 [98.4]	27.9 [60.2]
	Non #2	160.5 [--]	72.5 [19.9]	45.2 [--]	--
	2-face #4	160.5 [--]	78.0 [22.7]	48.6 [--]	7.0 [12.3] a
	a


Yara Aceta

Philippe Hapiot

Yann R Leroux

Investigation of Protective Properties of Organic Layers Towards Reactive Oxygen Species (ROS)

Keywords: Reactive Oxygen Species (ROS), Scanning Electrochemical Microscopy (SECM), Aryl Diazonium, Surface Functionalization

The antioxidant protective properties of polyaromatic organic layers were evaluated towards Reactive Oxygen Species (ROS) using Scanning Electrochemical Microscopy (SECM) in a foot-printing strategy. The layers were prepared by electrografting of aryldiazonium salts. Where p-(methyl)phenyl films show only weak protective properties towards ROS, p-(ethynyl)phenyl films evidence an efficient protection of the covered surfaces.

Applied potentials and electrolytes used during ORR are critical parameters to control, to prevent or reduce the influence of ROS production and hence enhancing devices lifetime.

Introduction

In a general manner, protective strategies towards Reactive Oxygen Species (ROS) have to be found in order to prevent damages of various devices where ROS could be generated during operation. Biological systems have developed several antioxidant defense strategies to regulate the production of ROS, as superoxide dismutase, 1 glutathione 2 or selenoproteins [START_REF] Steinbrenner | Protection against reactive oxygen species by selenoproteins[END_REF] to name of few. In the literature, many research groups have proposed various protecting strategies towards ROS. As an example, glutathione has been successfully immobilized onto surfaces, [START_REF] Griveau | Electrochemical grafting by reduction of 4-aminoethylbenzenediazonium salt: Application to the immobilization of (bio)molecules[END_REF] showing high sensitivity towards ROS. [START_REF] Latus | Scanning Electrochemical Microscopy Studies of Glutathione-Modified Surfaces. An Erasable and Sensitive-to-Reactive Oxygen Species Surface[END_REF] Also, polydopamine nanoparticles (NPs) have been shown to act as a radical scavenger [START_REF] Ju | Bioinspired Polymerization of Dopamine to Generate Melanin-Like Nanoparticles Having an Excellent Free-Radical-Scavenging Property[END_REF] and have been used to efficiently scavenge ROS in periodontal disease [START_REF] Bao | Polydopamine Nanoparticles as Efficient Scavengers for Reactive Oxygen Species in Periodontal Disease[END_REF] or during ischemic stroke. [START_REF] Liu | Comprehensive Insights into the Multi-Antioxidative Mechanisms of Melanin Nanoparticles and Their Application to Protect Brain from Injury in Ischemic Stroke[END_REF] Finally, as a last example, cerium oxide NPs were found also to exhibit antioxidant properties, and have been utilized in biomedical applications [START_REF] Dhall | Cerium Oxide Nanoparticles: A Brief Review of Their Synthesis Methods and Biomedical Applications[END_REF] or to enhance polymer exchange membrane (PEM) fuel cells lifetime, where the loss of performance has been especially attributed to the degradation of the membrane by ROS generated during operation. [START_REF] Rodgers | Fuel Cell Perfluorinated Sulfonic Acid Membrane Degradation Correlating Accelerated Stress Testing and Lifetime[END_REF] Even if some of these strategies allowed protecting or enhancing lifetime devices, there is still missing studies about new and cheap protective layers towards ROS. Few years ago, we have found that some organic films deposited on carbon surfaces show irreversible degradations when exposed to ROS. [START_REF] Noel | Evidence for OH Radical Production during Electrocatalysis of Oxygen Reduction on Pt Surfaces: Consequences and Application[END_REF][START_REF] Lhenry | Use of Catechol As Selective Redox Mediator in Scanning Electrochemical Microscopy Investigations[END_REF] More recently, Bard and coworkers observed similar degradations of organic films, and used ROS to effectively clean an electrode surface from the adsorption of 4aminothiophenol. [START_REF] Percival | Cathodically dissolved platinum resulting from the O2 and H2O2 reduction reactions on platinum ultramicroelectrodes[END_REF] Mirkin et al. also demonstrated that platinum dissolves during ORR in organic and aqueous media, [START_REF] Noël | Dissolution of Pt at Moderately Negative Potentials during Oxygen Reduction in Water and Organic Media[END_REF] and that this dissolution generates platinum nanoparticles. [START_REF] Bae | Dissolution of Pt during Oxygen Reduction Reaction Produces Pt Nanoparticles[END_REF] From these observations, in this study, our aim is to investigate the protective properties of organic films towards Reactive Oxygen Species (ROS) generated during Oxygen Reduction Reaction A c c e p t e d M a n u s c r i p t (ORR) on platinum. For that purpose, we use Scanning Electrochemical Microscopy (SECM) in a foot-printing strategy.

SECM [START_REF] Bard | Scanning electrochemical microscopy. Introduction and principles[END_REF][START_REF] Kwak | Scanning electrochemical microscopy. Theory of the feedback mode[END_REF] has demonstrated all its interest for probing at the local scale a variety of biological, chemical and electrochemical processes occurring at interfaces. [START_REF] Polcari | Scanning electrochemical microscopy: a comprehensive review of experimental parameters from 1989 to 2015[END_REF] In feedback mode, SECM provides a chemical view of a surface from the solution side and is complementary to other electrochemical methods such as cyclic voltammetry. [START_REF] Kwak | Scanning electrochemical microscopy. Theory of the feedback mode[END_REF] Here, the foot-printing strategy is based on the use of SECM in feedback mode when the substrate under investigation is not electrically connected (unbiased conditions). The process involves three main steps: (i) the SECM tip is placed in the close proximity of the organic layer under investigation by registering approach curves with the aid of a redox probe in solution. Because the modified surface is insulating, the tip-substrate distance is derived from the approach curve and compared with the theoretical current-distance curve that only depends on the tip geometry for an insulating surface; (ii) once the SECM tip is in position, ROS are generated at the SECM tip. To do so, the solution with the redox probe is replaced with a clean aqueous electrolytic solution and a potential sufficiently negative to generate ROS (during Oxygen Reaction Reduction, ORR) is applied to the SECM tip ; (iii) finally, to evidence the impact of the ROS on the organic layer, the latter is probed with an indifferent redox mediator either by moving the tip in solution to register an image of the area in the x-y plane or by registering approach curves before and after ROS interaction. Different types of approach curves can be obtained (from positive to negative feedback) that sense the ability of the probe to pass through the layer, providing a view of the modifications.

In previous studies, we [START_REF] Noel | Evidence for OH Radical Production during Electrocatalysis of Oxygen Reduction on Pt Surfaces: Consequences and Application[END_REF][START_REF] Lhenry | Use of Catechol As Selective Redox Mediator in Scanning Electrochemical Microscopy Investigations[END_REF] and other research groups [START_REF] Ktari | Patterning of polystyrene by scanning electrochemical microscopy. Biological applications to cell adhesion[END_REF][START_REF] Ktari | Local oxidation of polystyrene by scanning electrochemical microscopy[END_REF][START_REF] Shiku | Microfabrication of alkylsilanized glass substrate by electrogenerated hydroxyl radical using scanning electrochemical microscopy[END_REF] observed that ROS produced during O 2 reduction on a Pt electrode could irreversibly damage various polyphenylene layers for A c c e p t e d M a n u s c r i p t patterning purpose. These investigations have also demonstrated the production of extremely reactive radicals in organic and neutral aqueous media during ORR. [START_REF] Noel | Evidence for OH Radical Production during Electrocatalysis of Oxygen Reduction on Pt Surfaces: Consequences and Application[END_REF] Here, we focus on the protective properties of two different organic layers, a p-(methyl)phenyl and a p-(ethynyl)phenyl film. These two organic layers were selected as they can easily be electro-grafted from their respective aryl diazonium salt parents and form thin and compact organic layers on many substrates. Both organic layers are derivate of hydrocarbons, but their respective chemical nature should induce different reactivity towards oxidation reactions. The study was first performed in basic medium, which avoid the presence of anions other than OH -in the electrolytic solution that are known to react with ROS. [START_REF] Keene | Kinetics of Radiation-induced Chemical Reactions[END_REF][START_REF] Adams | Spectroscopic Studies of Reactions of the OH radical[END_REF][START_REF] Adams | Transient Spectra of Some Inorganic Radical Anions Produced by Reactions of the Hydroxyl Radical[END_REF] These experimental conditions allow longer radical lifetime and cleaner test media by limiting the formation of secondary radicals. For comparison, same experiments were also performed in neutral aqueous solution. The role of the different applied potentials is also examined.

Experimental section.

2.1.Chemicals and reagents.

Commercially available reagents were used as received. 4-(ethynyl)benzenediazonium tetrafluoroborate (Eth-ArN 2 + BF4 -) [START_REF] Evrard | Electrochemical functionalization of carbon surfaces by aromatic azide or alkyne molecules: a versatile platform for click chemistry[END_REF] and 4-(methyl)benzenediazonium tetrafluoroborate (Me-ArN 2 + BF4 -) [START_REF] Combellas | Steric effects in the reaction of aryl radicals on surfaces[END_REF] were synthesized according to published procedures. All aqueous solutions were made employing ultrapure water (18.2 MΩ cm).

2.2.Pyrolytic Photoresist Films (PPFs).

The fabrication of PPFs consists of a thermal processing of a standard positive photoresist (AZ4562, supplied by Microchemicals), which was previously deposited by spin coating (6000 r.p.m. during 30 s) over a silicon wafer (Si (100)) and heated in an inert atmosphere (95 % Ar/ 5

A c c e p t e d M a n u s c r i p t % H 2 ) at rate of 10 °C/min up to a temperature of 1100 °C. For complete graphitization of the photoresist material, the annealing was performed during 1 h. [START_REF] Del Campo | Fabrication of PPF electrodes by a rapid thermal process[END_REF] Finally, the PPFs with typical dimensions of 15 x 15 mm² were cleaned in an ultrasonic bath in acetone, methanol and 2propanol, then dried under Argon and stored at room temperature for later use. The resulting PPFs show electrochemical properties close to that observed in glassy carbon electrodes, and low resistivity.

2.3.Preparation of modified substrates.

All carbon electrodes were modified in a 10

2.4.Electrochemical instrumentation.

All surface modifications were performed using a standard three-electrode setup at a controlled potential with a potentiostat Autolab PGSTAT 302N (Metrohm). The set of electrodes consisted of glassy carbon (GC) disk electrodes (IJ Cambria, surface area 0.03 cm²) or small squares (15x15 mm²) of PPFs as working electrodes, a platinum mesh as the auxiliary electrode, and a saturated calomel electrode (SCE, KCl sat.) as reference. The latter was isolated from the working solution employing a salt bridge, which contained the electrolyte solution. When SECM experiments were performed with a three-electrode setup, comprising a 10 µmdiameter Pt UME (IJ Cambria) with a RG = 7, a Pt wire as the auxiliary electrode, and a homemade Ag/AgCl (0.1 mol L -1 KCl) electrode as reference. Measurements were performed using a homemade setup similar to that described in reference [29]. The SECM setup is equipped with an adjustable stage for the tilt angle correction and is controlled by the SECMx software written by G. Wittstock et al. [START_REF] Sklyan | SECMX Software[END_REF] For SECM imaging, the tip was positioned at a fix distance of 5 µm, recording approach curve in the top left corner. SECM images and approach curves were recorded using a CHI910 potentiostat from CH Instruments.

2.5.Atomic Force Microscopy.

Atomic force microscopy (AFM) experiments were performed on modified PPF substrates using a NT-MDT Ntegra microscope. Topographic images were obtained in semi-contact mode using ACTA tips (AppNano Inc.) for which the resonance frequency is around 300 kHz. The images were treated using Gwyddion open-source software. [START_REF]Gwyddion[END_REF] The roughness of the organic films deposited on PPF substrates was estimated, according to the standard ISO 4287/1-1997, as the average of the measured height deviations taken within the image (Root Mean Square (RMS) roughness).

2.6.Spectroscopy Ellipsometry.

The thickness of the organic films was measured by Spectroscopy Ellipsometry with an SE ellipsometer (J.A. Woollam, Co.) using a model-based analysis approach. Prior to modification with the aryldiazonium molecule, the SE data of the bare substrate were registered to build a substrate optical model which represents the nominal structure of the non-modified material; the We considered two different poly-aromatic films that were electrodeposited on carbon substrates (pyrolized photoresist films, PPF). They differ by the presence of an unsaturated bond: a p-(methyl)phenyl and a p-(ethynyl)phenyl films. In both cases, a polyaromatic structure is obtained as confirmed by our analysis and according to the literature [START_REF] Belanger | Electrografting: a powerful method for surface modification[END_REF][START_REF] Chehimi | Aryl Diazonium Salts: New Coupling Agents in Polymer and Surface Science[END_REF] (long time electrolysis and large concentration of precursor, see Experimental section for details). In these conditions, thick organic layers with strong blocking properties towards electron transfer of redox probes in solution are obtained. The formation of multi-layers by reduction of these compounds has been reported in the literature but the quality of the film is highly dependent on the experimental conditions. [START_REF] Leroux | Efficient covalent modification of a carbon surface: use of a silyl protecting group to form an active monolayer[END_REF][START_REF] Combellas | Steric effects in the reaction of aryl radicals on surfaces[END_REF] To determine the characteristics of these films, we examined their blocking properties by cyclic voltammetry, and their homogeneity, roughness and thickness by Atomic Force Microscopy (AFM) and spectroscopic ellipsometry (SE). Dopamine and ferrocene were used as redox probes in solution to investigate the blocking properties of the modified surfaces. Dopamine undergoes a reversible oxidation to o-quinone in acidic medium but it needs to be absorbed onto the electrode surface for rapidly transferring its charge, which makes dopamine a very sensitive and suitable redox species for probing the quality of organic films (presence of pinholes or defects), by discriminating between direct and indirect charge transfer. [START_REF] Mccreery | Advanced carbon electrode materials for molecular electrochemistry[END_REF][START_REF] Duvall | Control of catechol and hydroquinone electron transfer kinetics on native and modified glassy carbon electrodes[END_REF] Ferrocene also undergoes a reversible oxidation but contrarily to dopamine, it could transfer its charge by tunneling through the organic layer, meaning that ferrocene does not require to be in contact with the electrode substrates for transferring its charge. [START_REF] Mccreery | Advanced carbon electrode materials for molecular electrochemistry[END_REF] As shown in Figure 1, after modification of the PPF surfaces with p-(methyl)phenyl and p-(ethynyl)phenyl diazonium salts, both ferrocene and dopamine oxidation are totally inhibited on the modified PPF surfaces. This indicates that both organic films are multilayers, free of defects or pinholes, and that they are thick enough to inhibit the electron transfer by tunneling of ferrocene. In connection with the observed electrochemical behavior, the formation of defect-free covering was confirmed by AFM imaging on both modified PPF surfaces (Figure 2). The topography and surface roughness increase after the grafting associated to the disordered and dendritic growth of these organic films. Analyses of thickness by ellipsometry yielded similar values for both p-(methyl)phenyl and p-(ethynyl)phenyl films of 5.6 nm and 6.8 nm The protective properties of the organic films were investigated in two different aqueous electrolyte solutions: 10 -2 mol L -1 KOH and 10 -2 mol L -1 KPF 6 aqueous solutions for comparison.

A c c e p t e d M a n u s c r i p t

We used low concentrations of electrolyte to extend the lifetime of radicals in solution that could react with the electrolyte but also with the impurities present in the salts to form secondary radicals. ORR was performed with the concentration of O 2 naturally dissolved in these media under air. The experimental concentration of oxygen was derived from the diffusion-limited current at the platinum ultramicroelectrode (UME) during the reduction of O 2 by cyclic voltammetry in these media (Figure 3). The steady-state limit current at a disk ultramicroelectrode is related to the concentration of the electroactive species by the equation:

i lim = 4nFDaC where a is the radius of the platinum UME (5 µm radius), C is the concentration of oxygen and D is the diffusion coefficient of O 2 (1.97 x 10 -5 cm² s -1 from the literature). [START_REF] Ferrell | Diffusion coefficients of nitrogen and oxygen in water[END_REF][START_REF] Davis | The solubility and diffusion coefficient of oxygen in potassium hydroxide solutions[END_REF] It was 4c,d). Figures 4e-h show the SECM experiments performed in 10 -2 mol L -1 KPF 6 , before (Figures 4e,f) and after ORR (Figures 4g,h). SECM images on the left side

show the experiments performed on the p-(ethynyl)phenyl film (Figures 4a,c,e,g); SECM images on the right side, the ones performed on the p-(methyl)phenyl film (Figures 4b,d,f,h). In each media, we studied the effect of the applied potential during the ORR performed at the Pt tip electrode that is a polycrystalline surface. The half-wave potential of oxygen reduction in the basic solution was -0.28 V vs Ag/AgCl, and -0.44 V vs Ag/AgCl in the neutral solution (10 -2 mol L -1 KPF 6 ). We used a transient method for the production of the ROS. Each applied potential to the tip electrode was composed of a sequence of two pulses in chronoamperometry, for a total duration of 20 seconds, all started from 0 V to the applied potential. Four different applied potentials ranging from 0 V (before ORR, not shown) to the plateau of the ORR were considered. All potentials studied by SECM in each media are marked in the voltammograms of vs Ag/AgCl in 10 -2 mol L -1 KPF 6 solution (g, h). Y-axes display the normalized tip current (i/i inf ).

Note that the background current was subtracted for comparison, where the background current corresponds to the value of normalized tip current when the UME tip is at 5 µm to the organic film.

As shown in Figure 4, before ORR, all organic films show a totally insulating behavior toward electron transfer when Fc(MeOH) 2 is used as probe, displaying a fairly homogeneous electrochemical response on the surface of the samples. No surface defects or large current variations were evidenced in both neutral and basic solution. Dopamine was not used in these experiments as it is too sensitive to oxygenated species and can lead to a misinterpretation of the For a more quantitative comparison between experiments, it is interesting to examine the currentdistance profiles. Figure 5 shows the normalized current (i/i inf ) versus the distance covered by the SECM tip on the x-axis (400 µm) in the two aqueous electrolyte solutions after ORR. As previously indicated, no modification of the p-(ethynyl)phenyl film was observed under any of the experimental conditions studied whatever the applied potentials and electrolyte solutions (Figure 5a). On contrast, the p-(methyl)phenyl film shows a different behavior towards ROS in solution, and this behavior varies with both the applied potential and the electrolyte solution. As seen on Figure 5b, the extent of the modification is much more important in alkaline medium (red) than in neutral solution (black). The normalized current associated to the electron transfer of Fc(MeOH) 2 is more than three time larger when the applied potentials are in the plateau of the ORR.

A c c e p t e d M a n u s c r i p t (black) solutions. Profiles drawn from Figures 4 c,d, g and h. Data analysis was performed with Gwyddion software. [START_REF]Gwyddion[END_REF] Note that the background current was subtracted for comparison, where the background current corresponds to the value of normalized tip current when the UME tip is at 5 µm to the organic film.

Discussion.

The p-(ethynyl)phenyl film does not undergo modification after exposition to ROS, while p-(methyl)phenyl film is considerably affected. Applying a more cathodic potential to the platinum UME, clearly enhanced the damage on the organic film since the rate of generation of ROS increases. Notice that the size of the damages remains unchanged and in the order of the diffusion cone of the SECM tip while the contrast (normalized tip current) of the damages increase when applying more cathodic potential. This is in agreement with the behavior expected in a steady state diffusing system. [START_REF] Bard | Scanning Electrochemical Microscopy[END_REF] It is first noticeable that the small difference in layer thickness between the two films cannot explain the extent of the modification observed. The p-(ethynyl)phenyl film is 20 % thicker than the p-(methyl)phenyl one, while degradations of the latter is largely enhanced (normalized tip current increases by at least 300 %) when ORR is performed in the plateau of oxygen reduction (see Figure 4 and5). The different behaviors between the organic films could be ascribed to changes of reactivity of the films towards radicals produced during ORR on Pt. As demonstrated in our earlier studies, superoxyde (O 2

•-

) [START_REF] Latus | Scanning Electrochemical Microscopy Studies of Glutathione-Modified Surfaces. An Erasable and Sensitive-to-Reactive Oxygen Species Surface[END_REF] is not responsible for the degradation of any organic films. Furthermore, the effects of hydrogen peroxide (H 2 O 2 ) on both organic layers were evaluated (see supporting information) and any degradation was observed. From here, we concluded that the most likely responsible for the degradation of the organic films is the radical HO • , being one of the most reactive radical among all the ROS formed during ORR at the platinum electrode. [START_REF] Noel | Evidence for OH Radical Production during Electrocatalysis of Oxygen Reduction on Pt Surfaces: Consequences and Application[END_REF] Reactions between OH • and aromatic molecules have been extensively studied by pulse radiolysis [START_REF] Anderson | Reactions induced by hydroxyl radical attack on acetylene in aqueous solution. A pulse radiolysis study[END_REF][START_REF] Senosiain | The reaction of acetylene with hydroxyl radicals[END_REF][START_REF] Anbar | Reactivity of aliphatic compounds towards hydroxyl radicals[END_REF] including molecules which incorporate terminal acetylene and methyl functions. Three possible reactions between HO • and organic molecules were generally identified: (i) the abstraction of H atoms, (ii) the electrophilic addition on unsaturated bonds, and (iii) the electron transfer process. 47 By analogy, it is expected that the reaction between the HO • and the acetylene groups in the p-(ethynyl)phenyl film mainly proceeds by electrophilic addition to the carbon-carbon triple bond leading to the corresponding enol form of the formylmethyl radical that is stabilized by resonance; [START_REF] Anderson | Reactions induced by hydroxyl radical attack on acetylene in aqueous solution. A pulse radiolysis study[END_REF][START_REF] Senosiain | The reaction of acetylene with hydroxyl radicals[END_REF] while hydrogen abstraction is the dominant reaction pathway for the attack of HO • on the terminal methyl function in the p-(methyl)phenyl film. [START_REF] Anbar | Reactivity of aliphatic compounds towards hydroxyl radicals[END_REF][START_REF] Kopinke | What Controls Selectivity of Hydroxyl Radicals in Aqueous Solution? Indications for a Cage Effect[END_REF] In this case, the A c c e p t e d M a n u s c r i p t not enough reductive to reduce H 2 O 2 to OH • . On the contrary, a longer lifetime of the superoxide radical in an alkaline medium (less favorable hydrogen peroxide formation) would allow the production of OH • or other secondary radicals by interaction with the solvent, explaining the highest extent of the phenomena in alkaline media and the different reactivity between alkaline and neutral media. Also, as quoted in the introduction, similar anions than PF 6 -in solution are known to react strongly with ROS. [START_REF] Keene | Kinetics of Radiation-induced Chemical Reactions[END_REF][START_REF] Adams | Spectroscopic Studies of Reactions of the OH radical[END_REF][START_REF] Adams | Transient Spectra of Some Inorganic Radical Anions Produced by Reactions of the Hydroxyl Radical[END_REF] In our conditions, these reactions could lead to an important decrease of the ROS lifetime in the medium. These secondary reactions compete with the reaction of the ROS with the deposited organic layer leading to an apparent enhancement of the protective properties in this media.

A c c e

  p t e d M a n u s c r i p t necessary, a conventional gas inlet was used for bubbling argon inside the electrochemical cell to displace oxygen.

A c c e p t e d M a n u s c r i p t

  refractive index (n) and the extinction coefficient (k) were 2.4 and 1.1 for bare PPF electrodes. This substrate model was used after modification to establish the contribution of the substrate to the SE experimental data and thus, estimate the thickness of the organic layer according to the Cauchy model. Finally, the goodness of the fit was quantified by the Mean Squared Error (MSE) to validate our results. In all cases, the fitting reproduced the structures and features of the SE experimental data. The values of thicknesses obtained are the average of ten measurements performed on three different PPF substrates.

A c c e p t e d M a n u s c r i p t 3 .

 3 Results and Discussion.3.1. Characterization of the organic layers.

Scheme 1 .

 1 Scheme 1. Schematic representation of the structure of the organic films deposited on carbon

A c c e

  p t e d M a n u s c r i p t Both organic films are easily prepared on PPF surfaces by electro-reduction of the corresponding aryldiazonium cations in acetonitrile (MeCN) + 0.1 mol L -1 ntetrabutylammonium hexafluorophosphate (nBu 4 NPF 6 ) solutions under self-inhibition conditions

Figure 1 . 9 A

 19 Figure 1. Cyclic voltammograms of a glassy carbon electrode in an (a, c) aqueous solution of 10 - 3 mol L -1 dopamine + 0.1 mol L -1 H 2 SO 4 and (b, d) an acetonitrile solution of 10 -3 mol L -1

A c c e p t e d M a n u s c r i p t

Figure 2 .

 2 Figure 2. AFM topographic images of (a) bare PPF, (b) p-(methyl)phenyl-modified PPF and (c)

  estimated a concentration of O 2 of 8.50 x 10 -4 mol L -1 in 10 -2 mol L -1 KOH and 8.92 x 10 -4 mol L -1 in 10 -2 mol L -1 KPF 6 . Solubility of O 2 in aqueous electrolyte solutions range between 10 -3 and 10 -6 mol L -1 , depending on the partial pressure of O 2 and the concentration of electrolyte at 298 K.[START_REF] Lingane | Chronopotentiometric study of oxygen reduction at a platinum wire cathode[END_REF][START_REF] Narita | Solubility of oxygen in aqueous electrolyte solutions[END_REF]

Figure 3 .

 3 Figure 3. Steady-state cyclic voltammetry of O 2 in (a) 10 -2 mol L -1 KOH and (b) 10 -2 mol L -1

Figure 3

 3 Figure 3 as blue solid squares.

A c c e p t e d M a n u s c r i p t

Figure 4 .

 4 Figure 4. SECM images (400 x 200 µm²) registered with a Pt UME (5 µm-radius) in 10 -3 mol L - 1 Fc(MeOH) 2 as redox mediator in 0.1 mol L -1 H 2 SO 4 before (a, b, e, f) and after (c, d, g, h)

A c c e

  p t e d M a n u s c r i p t results. After exposition to ROS, no modification was observed on the p-(ethynyl)phenyl film for all applied reductive potentials; the layer remains totally insulating and homogenous, still blocking the electron transfer of Fc(MeOH) 2 in solution. This behavior is observed in both neutral and basic media. On the contrary, on p-(methyl)phenyl films, areas where the electron transfer rate of Fc(MeOH) 2 increases are clearly visible after exposure to ROS, for all applied reductive potentials in KOH solution and for the two most reductive potentials in KPF 6 . This increase in electron transfer rate reflects a change in the topology of the film where the areas under the SECM tip are now sufficiently thin to allow electron transfer from Fc(MeOH) 2 cation in solution to the underneath carbon substrate by electron tunneling, or direct transfer. This evidences the degradation of the organic layer by the ROS electro-generated at the SECM tip.

Figure 5 .

 5 Figure 5. Current-distance profiles of (a) a p-(ethynyl)phenyl film and (b) a p-(methyl)phenyl film deposited onto PPF substrates after ORR in 10 -2 mol L -1 KOH (red) and 10 -2 mol L -1 KPF 6

A c c e p t e d M a n u s c r i p t

  abstraction of H atoms results in the formation of benzylic methylene radicals, which are species much more reactive than the stabilized intermediate in the p-(ethynyl)phenyl film. These radicals could therefore induce other radical reactions (propagation reactions) through the organic film, thus contributing to the further oxidative degradation of the p-(methyl)phenyl film and leading to the different reactivity observed for the two films. Note that the conjugated polyaromatic A c c e p t e d M a n u s c r i p t structure of both organic films contributes to the stabilization of both benzylic methylene and phenyl acetylene radicals. Concerning the polyaromatic structure, hydroxyl radicals are strong electrophilic species that can also directly attack the aromatic ring to produce phenolic derivatives. In this case, the inductive effect and the πdonor and acceptor character of the substituent in the aromatic ring should be considered. The greater donor character of the methyl group favors to a larger extent the electrophilic attack of the HO • to the aromatic ring, reinforcing the evidences of a faster degradation of the p-(methyl)phenyl film by comparison with the p-(ethynyl)phenyl film. Hence, the unsaturated bonds integrated in the p-(ethynyl)phenyl films seem to prevent or delay the oxidative damage of the polyaromatic structure during the exposition to ROS; while p-(methyl)phenyl films is more easily damaged by species generated during the ORR on platinum.Furthermore, there are some noticeable differences between the reactions performed in the two different electrolytes. The degradation of p-(methyl)phenyl films is largely enhanced (normalized tip current increases by at least 300 %) in alkaline solution by comparison with the same electrochemical procedure when performed in the neutral solution (KPF 6 ). This can be explained in terms of the mechanism of the oxygen reduction reaction in the different media studied. In alkaline solution, a 1-electron mechanism from O 2 to O 2•-has been proposed. In this medium, superoxide exhibits a significant lifetime since high concentration of OH -promote the existence of superoxide according with the Equation 1, while the lifetime of superoxide in presence of protons is too short and should disappears by protonation to form HO

A c c e p t e d M a n u s c r i p t 4 .

 4 Conclusion.The antioxidant properties of two different organic films, a p-(ethynyl)phenyl and a p-(methyl)phenyl films, were investigated towards ROS in basic and neutral media. Non-intuitive differences in their reactivity were observed. p-(Ethynyl)phenyl film does not undergo modification after exposure to ROS, while p-(methyl)phenyl film is considerably affected and in different media. These different behaviors could be attributed to a "radical scavenging" property of the unsaturated bonds present in the p-(ethynyl)phenyl film. It seems to prevent or delay the oxidative damages of the polyaromatic structure during the exposition to ROS, unlike that observed in the p-(methyl)phenyl film. It will be interesting in further investigations to probe other polymer systems bearing unsaturated bonds, checking if they present or not similar effect.These findings open the way to new protection layers and materials towards ROS. ASSOCIATED CONTENT Supporting Information. SECM methodology is described in detail. Control experiments were performed to rule out possible damage by H 2 O 2 and a possible Fenton-like reaction on both organic films. The following file is available free of charge.

Table 1 .

 1 Roughness

	Surface properties	Bare PPF	Me-Ar-PPF	Eth-Ar-PPF
	Roughness -AFM / pm	61 ± 3	132 ± 8	154 ± 10
	Thickness -SE / nm	-	5.6 ± 0.7	6.8 ± 0.3
	3.2. Protective properties of organic layers.

and thickness values of the p-(methyl)phenyl-modified PPF (Me-Ar-PPF) and p-(ethynyl)phenyl-modified PPF (Eth-Ar-PPF) and the bare PPF for comparison. Values are the average of three different measurements performed on three different samples.

  𝐻𝐻 2 𝑂𝑂 ⇌ 𝐻𝐻 2 𝑂𝑂 2 + 𝑂𝑂 2 + 2 𝑂𝑂𝐻𝐻 - (Eq. 1)

	2	• that in turn
	undergoes disproportionation to H 2 O 2 . 52
	2 𝑂𝑂 2 •-+ 2

AUTHOR INFORMATION

Corresponding Author * yann.leroux@univ-rennes1.fr

Author Contributions


Oula Amrouni
email: oula.amrouni@instm.rnrt.tn

Yu Fisjak

Saadi Abdeljaouad

Hakim Abichou

Abdallah Hattour

Nejmeddine Akrout

Chrystelle Bancon-Montigny

Gil Mahé

Saâdi Abdeljaouad

Thouraya Benmoussa

Kerim Ben Mustapha

Lassâad Chouba

Domenico Chiarella

Michel Condomines

Lau- Rent Dezileau

Claudine Dieulin

Nadia Gaâloul

Ahmed Ghadoum

Abderraouf Hzami

Nabil Khelifi

Samia Khsiba

Fatma Kotti

Mounir Medhioub

Hechmi Missaoui

François Sabatier

Alberto Sánchez

Alessio Satta

Abdelaziz Sebei

Wième Ouertani

Multidisciplinary approaches for the study of sediment discharges to the Mediterranean Sea to mitigate the impact of climate and anthropogenic activities on coastal environments

Keywords: RYSCMED, Mediterranean, Medjerda River, Sediment, Coast, Sensitivity

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The coastal areas of the Mediterranean Sea are densely inhabited and produce a valuable part of the national incomes of many countries. However, land-sea areas have suffered since the last 100 years from increased salinity, changes in geomorphology, coastline regression, coastal biodiversity reduction and increased pollution of ecosystems mostly driven by the high number of dams which reduce water and sediment flows to the sea by storing a great part of them inland. This situation is worsened by both a sustained rainfall reduction and the sea level rise due to the worldwide temperature increase. Indeed, Mediterranean areas are among the most vulnerable environments in the world due to the potential impact of climate changes [START_REF]WCRP: The World Climate Research Programme Accomplishment Report[END_REF] [START_REF]AFED: Arab Environment: Sustainable Development In A Changing Arab Climate[END_REF]. In this scenario, North Africa and the Middle East are most vulnerable areas [START_REF] Sadik | Etude de la vulnérabilité environnementale et socio-économiques du littorale tunisien face à une élévation accélérée du niveau de la mer due aux changements climatiques et identification d'une stratégie d'adaptation[END_REF] [START_REF] El Raey | Impact of Sea Level Rise on the Arab Region[END_REF] in the face of this global problem, which urgently need a protection of their coastal landscapes.

Many initiatives have been developed in recent years to improve the knowledge on this rapidly changing overpopulated environment, and to enhance international cooperation around the Mediterranean. However, only few programs deal with the full continuum from the river basins to the coastline, and still less involve many countries from the South and East Mediterranean regions.

Hence, a multidisciplinary approaches for the study of sediment discharges to the Mediterranean Sea to mitigate the impact of climate and anthropogenic activities on coastal environments should contribute to a better understanding of the coastal areas.

The RYSCMED project has been implemented in the pre-estuarine and coastal field of the most driven river, i.e. the Medjerda in the bay of the Gulf of Tunis (Western Mediterranean, Tunisia) (Fig. 1). Actually, the low Medjerda Delta which is the main agricultural region in Tunisia and hosting industrial and fishing activities, is perceived as the highly vulnerable area to sea level rise scenario. The main tributary system of the Medjerda River discharges up to 30 g.l -1 of sediment load during floods, for an annual total of 20,400 tons, of which 10% is composed of sand [START_REF] Esonni | Etude de la dynamique des sels nutritifs et des métaux lourds en relation avec relation avec la sédimentologie et l'hydrodynamique dans le large du Golfe de Tunis[END_REF]. However, the sedimentary contributions to the coastal zone from the Medjerda River is still unknown. It is also the aim of the RYSCMED project to provide the quantitative amount of sediment yield from the Medjerda River to the sea via the coastal zone during the last 50 to 100 years. Fig. 1. The location map of the study field area of the depositional deltaic system of Medjerda River, western bay of the Gulf of Tunis (Tunisia, Mediterranean). A/ Sampling position of the core and coastal subsurface sediment reconstruction.

Methods

We propose to analyze the sediment inlet fluctuation in regard of events recorded on the inland basins: dam constructions, changes in agricultural practices and in population, and to create a sediment transfer model to the sea. Data used are based on sediment discharges from large rivers and sediment subsurface samples and three cores in the pre-estuarine zone, downstream of the last dams and far enough three cores from the coast to avoid marine influence. In order to assess the impact of the reduction of sediment transport from the upper basins to the coastal area since the past 50-100 years (Fig. 1 A). The River mouth nearshore, lagoon and dredged sediment were subject to sedimentological, mineralogical, geochemical and floristic analyses (campaign carried out from 2016 to 2018). Sedimentological multi approaches were based on the identification of different Sedimentary Types (ST). Accordingly, using grain size indexes (i.e., Mz, SKI and Ku), Sediment Trend Analysis (STA) modeling tools were applied to define the seasonal sediment transport pathways throughout the nearshore of the Medjerda Delta. Both sediment cores were subjected to multiproxy approaches (major and trace elements and dating radiometric 210 Pb and 137 Cs). Surface sediment of the Medjerda River and nearshore sediment undertook geochemical study.

The evolution of the coastline of Medjerda was also estimated by satellite images carried out from SPOT1, SPOT4 and Sentinel A2 imageries (from 1936 to 2016 surveys) completed by the aerial and topographic surveys (missions: 1900, 1936, 1948, 1952, 1974, 1984, 1988, 1999 and 2016). The lagoon system of Kalâat Andalous (and the fishing harbor inside) was subject to ecosystemic study. We also used Fauna and flora as biologic tools to characterize physical and chemical parameters of coastal and paralic environments. The investigations were based on the study of the sedimentological facies, a floristic (Environmental Quality Ratio of macrophytes, EXCLAME index), faunistic identification and geochemical analysis (As, Cd, Cr, Cu, Ni, Pb, Zn).

Results

Hydroclimatic and sedimentological evolution

The result of the lower valley and the coastal area cores' analysis show a succession of sedimentary layers that likely correspond to different flood deposits that succeeded on this site. The dating with Pb and Cs of the cores shows that the selected area is an important deposition area. The thickest layers of sedimentary deposits were related to the exceptional events. They are mainly concentrated on the lower part of the core and are predominantly composed of sands. Since the construction of the Sidi Salem dam in 1981, all cores presented mostly a succession of small layers of fine material (silt or clay) without any sand deposits in the downstream river bed. A strong decrease in the accumulation rate of sediment was observed based on the length of the flood units and the number of years between flood events. A lower sedimentation rate (about 2.3 cm y - 1 ) is observed between 1982 and 2015, while it was much more important between 1963 and 1981 (about 4.75 cm y -1 ), and around 10 cm.y -1 between 1950 and 1962 (Fig. 2 A). Sedimentological results show a fluvial dominance marked by silt and clay deposits rich in terrigenous chemical elements since the 1950. Seasonal nearshore sediment dynamics show that grain size distribution (GSD) and Sediment Trend Analysis (STA) model pathways are determined by cross-shore geomorphology, location of the sediment-cell, seasonal incident wave and local terrestrial supply.

The geochemical signature shows a highly concentration of Rb, Ti, Zn and Pb. Sediment deposit facies associated to the geochemical component and the 137Cs / 210Pbex radiometric dating of recent alluvial deposits confirm the shoreline retreat due to terrestrial sediment reduction (Fig. 2 B).

Temporal evolution of the shoreline

The temporal shoreline evolution is established according to the aerial surveys from 1900 to 2016. Although the general trends of the shoreline reveal an increased rate, the mesoscale qualitative monitoring displays a nonlinear shoreline and plain flood evolution. Three periods can be individualized: 1-The natural beach progression between 1900 to 1952, 2-The shoreline progression at the mouth of the new artificial channel, from 1952 to 1981 (+13 m ± 0.15 m y -1 ), 3-The shoreline retreat rate after the manmade construction comprised between 1981 and 2016 (-20 m ± 0.15 m y -1 ) The progradation and retrogradation of the Medjerda Delta during the XX th century is strongly influenced by an anthropogenic factor (Fig. 2 C).

Ecological study of the wetland system

Spatial Kalâat Andalous sediment distribution revealed that the central lagoon area is dominated by fine sand and silty sediments. Medium sand is located mainly on the north and at the south-eastern part of the lagoon. Floristic study revealed the existence of 12 species among which five reference species whose percentage is equal to 26%. Reference species are Cymodocea nodosa Zostera nolti (marine phanerogams), Cystoseira barbata, Cystoseira compressa, Dictyota dichotoma (macrophytes; brown algae). This low percentage may reveal an environment quality loss and an eutrophication of the lagoon water. The latter is proved by the EXCLAME index which is equal to 0.24 indicating that the lagoon water is in a poor state. The study of microfauna and specially foraminifera has permitted to distinguish several associations related to hydrodynamic context. Some are of coastal marine affinity, others are more tolerant to stress conditions. The richness of reworked species in some samples of the lagoon denotes a strong continental impact in this humid area.

Dredged sediment of Kalaât Andalous lagoon are dominated by silts and clays, dark black in colour and relatively enriched in organic matter (with a TOC value of 1.30 %) and sulphur compounds. However, the sediments are moderately contaminated by inorganic and organic pollutants except for Cd which is present in relatively highly amounts (2.85 ppm) over the standard amounts generally found in the sediment.

Geochemical characterization of the Delta-coast sediment yield

The trace metals (Zn, Cd, Cu, Cr and Ni) analysed in subsurface sediments along the Medjerda River tributaries and along the coast are relatively high. Lead contents vary from 12 to 46 ppm. The zinc concentrations vary from 2 to 71 ppm, while cadmium concentrations vary from 0.56 to 4 ppm (mean of 1.2 ppm). Chromium contents are between 46 and 178 ppm (mean of 98 ppm). Copper contents vary from 27 to 67 ppm, while nickel concentrations vary from 14 to 51 ppm (mean of 30 ppm). The Kalâat Andalous lagoon-habour sediment reveals a variable concentration of the MTE component (Al, Fe, Zn, Ni, Co, Cu et Pb, As) which respectively varies from 0,5 to 5,5%; 0,8 to 2,5%; 29 to 158 ppm ; 7 to 29 ppm ; 2 to 9 ppm; 3 to 31 ppm, 7 to 70 ppm and 4 to 15 ppm.

Discussion

The RYSCMED project outcomes confirm a variable sensitivity of the deltaic transition area of the sea level rise. Morphosedimentary quantitative distribution reveals a change in the fluvial deposits facies feeding the deltaic plain with a decreasing grade and shortage of sedimentary inputs towards the sea [11]. About 10 dams have been created on the Medjerda basin, among which the largest one, Sidi Salem, with more than 1 billion m 3 not far from the sea. These have a major impact on the hydrosedimentary regime downstream and undergone significant changes in morphology of the lower depositional deltaic valley and the coastal area (i.e., reduction in average rates and in volumes of solid transport, altered pattern of seasonal flows) [START_REF] Kotti | Etude de l'impact des barrages sur la réduction des transports sédimentaires jusqu'à la mer par approche paléohydrologique dans la basse vallée de la Medjerda[END_REF].

Sedimentological studies indicate that the western bay of the Gulf of Tunis is characterized by two sediments facies: (i) mud coming from the discontinuous supplies of rivers; and (ii) sandy sediments originating from the continuous reworking of erosion of the cliffs and coastal drift throughout beach during storm events.

The dominance of clay deposits over sandy facies on the beaches results from the impact of the dam construction. Only the finer grain size fractions can feed the coast. The actual sediment balance of the Gulf of Tunis coast shows a shoreline retreat under the coarser sediment deficit blocked by upstream dam construction [START_REF] Hzami | Satellite images survey for the identification of the coastal sedimentary system changes and associated vulnerability along the western bay of the Gulf of Tunis (northern Africa)[END_REF]. Archaeological surveys and remains around the site of the ancient paleo bay of Utica, the eldest sister of Carthage, confirm a rapid change and shift in the shoreline [START_REF] Delile | The Geoarchaeology of Utica, Tunisia: The Paleogeography of the Mejerda Delta and Hypotheses Concerning the Location of the Ancient Harbor, Geoarhcaeology[END_REF] That initiates a regressive sequence of deltaic deposits over spatial and temporal variability in facies and proocess in reponse to fluvial and marine controls [START_REF] Benmoussa | The sedimentological changes caused by human impact at the artificial channel of Medjerda-River (Coastal zone of Medjerda, Tunisia)[END_REF]. The textural facies of coastal system sandy components (river mouths, sandy spits, submarine sandbars) are modified towards very fine to silty sediments, becoming contaminant traps. Fluvial inputs, reduced in volume and size, work as a pollutant vector (trace elements) to adjacent beaches through coastal currents. The contaminant burden to protected areas (lagoon, bays) is related to the anthropogenic and natural component inputs. The heavy metals present in river-coast sediment have two origins: a natural source "natural background" and an anthropogenic origin. The high levels detected are mainly related to human activities (i.e., mining activities) [START_REF] Helali | Géochime des métaux lourds dans les sediments marins de surface du delta de l'Oued Medjerda[END_REF]. The study of wetland ecosystems associated to estuary areas via biotops and biocenosis showed the high degree of coastal ecosystem sensitivity to human activities.

Conclusions

The reduction in sediment discharge to the littoral caused by the dam is the principal cause of the negative sediment budget leading to a shortage of sand sediment on the coast closest to the delta and to the dominant erosion of the coastline. The nearshore enrichment of mud particles carried by waves and tide current contaminates strongly the coastal ecosystem. The vulnerability of the coastal areas to the sea level rise will be better addressed due to the new integrated approach of the sediment yields. The link with the experience of the Tunisian and French observatories will help the countries to develop Mediterranean environmental monitoring on the long term, which is crucial to provide answers to a large array of environmental questions, in relation with socio-economic and governance issues. The focused goal of the RYSCMED outcome is addressed to the implantation of Nature Based Solution to enhance urbanized Tunisian coast resilience.

Fig. 2 .

 2 Fig. 2. A/ The downcore grain size distribution of the CEM-1 carried out on the delta front and the geochronology using 137 Cs activity-depth profiles. B/ The shoreline evolution analysis at the mouth of the new Medjerda River during the 1936-2016 period. C/ The diagram synthesis of morphological changes involving the development and the retrogradation of the new delta of Medjerda River during the XX th century