domain: spi.nano chim phys

text:
Pierre Moritz

Antoine Gonon

Thomas Blon

Nicolas Ratel-Ramond

Fabrice Mathieu

Pierre Farger

Juan-Manuel Asensio-Revert

Simon Cayez

David Bourrier

Daisuke Saya

Liviu Nicu

Guillaume Viau

Thierry Leïchlé

Lise-Marie Lacroix
email: lmlacroi@insa-toulouse.fr

Magnetophoresis-Assisted Capillary Assembly: A Versatile Approach for Fabricating Tailored 3D Magnetic Supercrystals

Keywords: permanent magnet, cobalt nanorods, magnetophoresis, MEMS, nanoparticle-based materials, magnetic nanoparticles, controlled assembly TOC :

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INTRODUCTION.

The fabrication and integration of sub-millimeter magnetic materials into predefined circuits is of major importance for portable devices used in telecommunications, automotive, biomedical and space applications. For instance, inductors are soft magnetic elements widespread in daily-life objects such as cell phones, where they act as passive filters to properly extract the signal of interest from the electromagnetic noise, or in chargers as passive elements for power electronic converters. 1 Furthermore, the expanding market of integrated sensors/actuators requires miniaturized power supplies. Permanent magnets (PM) represent the most attractive choice for such integrated power sources, and would benefit to numerous applications based on Micro-ElectroMechanical Systems (MEMS), such as actuators, 2,3 relays, 4 micromachines, 5 and sensors (magnetic, viscosity or biosensors). 6,7,[START_REF] Javor | PT/Cm Single-Point MEMS Magnetic Gradiometer from a Commercial Accelerometer[END_REF] A major scientific challenge therefore consists in providing submillimeter magnetic materials with optimized properties to address the massive demand for power supplies and converters. Up to now, the fabrication and integration of magnetic materials into portable devices is achieved via three main routes: [START_REF] Arnold | Permanent Magnets for MEMS[END_REF] -the sputtering deposition, which gives access to a wide range of materials and a good composition control, but is generally limited to thin films due to low deposition rates and built-in stress that can cause delamination; -the electrodeposition technique, which can lead to thick films but is restricted to certain materials (Fe, Co, Ni, Pt-based alloys); -the patterning of magnetic polymers by inkjet, imprinting or photolithography. [START_REF] Pallapa | A Review of the Hybrid Techniques for the Fabrication of Hard Magnetic Microactuators Based on Bonded Magnetic Powders[END_REF] However, in such granular materials, the dilution of the magnetic particles into a polymeric matrix drastically reduces the overall properties.

Nanoparticle-based materials represent a very appealing alternative approach to address both criterions of magnetic performances and submillimetric thickness. This recent class of materials can indeed exhibit enhanced properties with a high level of control compared to bulk material.

They benefit not only from the intrinsic properties of the individual nanoparticle building blocks, whose size and shape can be finely tuned thanks to the recent developments in nanochemistry, but also to the collective interactions which could be specifically engineered through the control of the superstructure. [START_REF] Chen | Systematic Electron Crystallographic Studies of Self-Assembled Binary Nanocrystal Superlattices[END_REF] The formation of nanoparticle super-lattices results from the delicate balance of attractive and repulsive forces and are often assisted by capillary forces. Supercrystals exhibiting enhanced catalytic, electronic or plasmonic properties could be prepared and integrated into optoelectronic devices. [START_REF] Talapin | Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications[END_REF] Surprisingly, magnetic supercrystals have been scarcely addressed and were mostly limited to superparamagnetic nanoparticles. [START_REF] Marino | Favoring the Growth of High-Quality, Three-Dimensional Supercrystals of Nanocrystals[END_REF] The controlled deposition of nanostructured materials into functional devices still represents a challenge to overcome. Presently, several approaches have been developed: inkjet printing allows a fast patterning of particles, but cannot achieve a thickness over a few micrometers. Serigraphy is suitable to deposit isotropic nanoparticles on a few hundreds of microns, but can hardly be considered to align anisotropic ones without inducing aggregations. Capillary assembly combined with topographic or chemical structuration assists the displacement of the meniscus and moves the particles to pre-defined areas, but was only achieved on micrometer thick films. Due to the fairly long-range of electromagnetic forces, (di)electrophoresis or magnetophoresis technique allows attracting polarizable or magnetic particles over a large volume on predefined substrate [START_REF] Fratzl | Magnetophoretic Induced Convective Capture of Highly Diffusive Superparamagnetic Nanoparticles[END_REF][START_REF] Tanase | Magnetic Trapping and Self-Assembly of Multicomponent Nanowires[END_REF] or within micro-channels. [START_REF] Alnaimat | Microfluidics Based Magnetophoresis: A Review[END_REF] In this paper we report a versatile strategy relying on a magnetophoresis-assisted capillary assembly approach for the fabrication of high-performance submillimeter magnetic supercrystals.

It consists in the specific attraction and further evaporation of a NP suspension, under an external magnetic field, towards predefined areas exhibiting large magnetic field gradients. The resulting nanostructured materials can be directly integrated into substrates, with well-controlled dimensions and precise positions, or fabricated as self-standing magnets. Their final properties are finely tuned thanks to the different nanoparticle constituents which can be used, such as:

-single-crystalline Co nanorods (NRs), which combine a high magnetization and a large magnetic anisotropy. These anisotropic particles exhibiting hard magnetic behaviour constitute building blocks of primary choice for permanent magnet applications as we previously reported at the macroscopic scale. [START_REF] Anagnostopoulou | Dense Arrays of Cobalt Nanorods as Rare-Earth Free Permanent Magnets[END_REF][START_REF] Ener | Consolidation of Cobalt Nanorods: A New Route for Rare-Earth Free Nanostructured Permanent Magnets[END_REF] -spherical and fairly air-stable iron-based nanoparticles (Fe2.2C NPs), which display soft magnetic properties (high magnetization and low anisotropy). [START_REF] Asensio | To Heat or Not to Heat: A Study of the Performances of Iron Carbide Nanoparticles in Magnetic Heating[END_REF] A theoretical overview of the different forces involved in the magnetophoresis process is presented to determine the critical magnetic field gradients to apply, depending on the particle size and shape. Then, the fabrication of individual Co NR magnet and magnet networks is presented using patterned substrate consisting of pairs or arrays of electrodeposited Ni blocks. The magnetically soft Ni blocks effectively serve as flux concentrators inducing local magnetic field gradients which guide Co NR assembly. The optimization of the experimental parameters is further discussed based on finite element simulations and detailed structural and magnetic characterization of the obtained magnets. The efficiency of such nanostructured PM is evidenced by the electromagnetic actuation of a MEMS device. Finally, the magnetophoresis assisted capillary assembly approach is extended towards i) millimetric self-standing PM using Fe bars as flux concentrators, and ii) integrated soft magnets using Fe2.2C nanoparticles as building blocks.

RESULTS AND DISCUSSION

GENERAL PRINCIPLES OF MAGNETOPHORESIS-DRIVEN CAPILLARY

ASSEMBLY.

As depicted in Figure 1, our patterning and assembly method to create localized magnetic supercrystals relies on:

(i)
the alignment of nanoparticles dispersed in a highly concentrated suspension thanks to an external magnetic field and their accumulation in zones of strong field gradient (magnetophoresis);

(ii) their compaction due to capillary forces during the solvent evaporation. which drives the particle towards the strong field gradient zones, ⃗ : solvent dragging force (in green), which is opposed to the particle motion and h) ⃗ : the capillary forces encountered during the solvent evaporation.

Combined with microfluidics, magnetophoresis has attracted tremendous interest for the separation and sorting of complex fluids, using a diluted solution of superparamagnetic beads. [START_REF] Alnaimat | Microfluidics Based Magnetophoresis: A Review[END_REF][START_REF] Alnaimat | Magnetophoresis and Microfluidics: A Great Union[END_REF] Therefore, the analytical description of the phenomenon is well documented for isotropic objects both in presence of high gradients, used in the High Gradient Magnetic Separation (HGMS) technique, and lower ones (Low Gradient Magnetic Separation). [START_REF] Leong | Unified View of Magnetic Nanoparticle Separation under Magnetophoresis[END_REF] However, only few reports deal with anisotropic particles, [START_REF] Lim | Magnetophoresis of Iron Oxide Nanoparticles at Low Field Gradient: The Role of Shape Anisotropy[END_REF][START_REF] Lim | Optical Imaging and Magnetophoresis of Nanorods[END_REF] thus we will briefly introduce the methodology and determine qualitatively the critical magnetic gradient allowing for magnetic convection.

To obtain a proper alignment and a selective assembly by magnetophoresis, the nanoparticle motion must be driven by magnetic convection and not by thermal diffusion. The Peclet number (Pe) allows determining the relative importance of these two contributions and reads as follow:

= .

(Eq. 1)

with the typical length of the magnetic nanoparticle (length for a nanorod, diameter for a sphere), the magnetically induced velocity and D the diffusivity of the NP in the solvent.

Convective magnetophoresis is predominant when Pe > 1, whereas thermal diffusion is stronger for Pe < 1. Thus, determining the Peclet number will allow ensuring that magnetophoresis conditions are adequate to drive the nanoparticle motion.

In order to determine the magnetic velocity umag, one needs to quantify the different forces called into play and use classical Newtonian dynamics (further details can be found in supplementary). [START_REF] Fratzl | Magnetophoretic Induced Convective Capture of Highly Diffusive Superparamagnetic Nanoparticles[END_REF] We will consider, at first, a perfectly dispersed colloidal solution composed of non-interacting nanorods (NRs). Though not realistic, this assumption will allow quantifying the different forces acting on a single Co NR (Length = L = 150 nm, diameter = 2r =15 nm). For comparison, the forces acting on a 15 nm Fe2.2C nanoparticle are given in supplementary.

The weight, which leads to sedimentation, and the opposed buoyancy can be neglected here compared to Brownian motion. The magnetic force ⃗ acts on the particle and moves them to the positive magnetic field gradient (Figure 1g). In our experimental conditions, the external magnetic field is applied along the ⃗ direction. The magnetization ⃗ of the Co NRs is considered uniform along their long axis and independent of ⃗ , due to its ferromagnetic character and high anisotropy.

The magnetic torque applied on the Co NRs will lead to their alignment along the ⃗ direction.

Therefore, the Co NR magnetization reads ⃗ = . ⃗ and thus leads to the simplified form of the magnetophoretic force:

⃗ = ⃗. ∇ ⃗ = . . . ! (Eq. 2)
The drag force ⃗ also applies on the particle due to the surrounding viscous medium. This force is always opposed to the particle motion and depends on the medium viscosity. Considering that the steady-state regime is reached, the drag force compensates the magnetophoretic force ( = . The magnetophoresis speed is then given by: # = $ % with C the drag force coefficient.

In the case of anisotropic particle, the drag force has two distinct coefficients, parallel (Eq.4)

with the length of the rod and ? its radius, @ the Boltzmann constant, A the temperature (@ . A = 4,11.10 -21 J at 300 K) and considering a magnetization m of the Co NR aligned along x, as schematized in Figure 1g. perpendicularly to it (Figure 2 and Table S1). As stated, this simplified model was applied for a single particle. Considering inter-particle interaction, which favors particle chains formation, leads to a decrease of the critical magnetic field gradients, as previously reported for isotropic nanoparticles. [START_REF] Leong | Unified View of Magnetic Nanoparticle Separation under Magnetophoresis[END_REF] As an example, the evolution of the Peclet number is reported for a pair of NRs aligned along their long axis, the critical values dropped to 180 and 60 T.m -1 respectively (dashed lines in Figure 2). The convection phenomenon induced by the momentum transfer from the particle to the surrounding fluid, [START_REF] Leong | Unified View of Magnetic Nanoparticle Separation under Magnetophoresis[END_REF] which tend to favor the magnetophoresis process, [START_REF] Fratzl | Magnetophoretic Induced Convective Capture of Highly Diffusive Superparamagnetic Nanoparticles[END_REF] is also neglected here. The critical magnetic field gradients determined through our simplified model should thus be seen as the maximal values mandatory to favor the magnetic convection of a Co NR suspension.

For comparison, the evolution of the Peclet number as a function of the magnetic field gradient is also reported considering non-interacting 15 nm Fe2.2C NPs further used in our study. The motion of these isotropic NPs could only be driven by diffusion according to our simplified model.

We will however see later that the magnetophoresis-driven convection did apply, revealing the importance of inter-particle interactions. [START_REF] Asensio | To Heat or Not to Heat: A Study of the Performances of Iron Carbide Nanoparticles in Magnetic Heating[END_REF]

DIRECTED DEPOSITIONS OF NANOPARTICLES ON PATTERNED SUBSTRATE

Ni blocks of typically 200 µm (length L) x 500 µm (width W) x 150 µm (thickness T) were electroplated on a silicon substrate in order to serve as flux concentrators once placed in a homogeneous external magnetic field (Figure S1a). The Ni blocks exhibit a very weak coercivity (µ0HC= 4 mT, Figures S1b) and a saturation magnetization MS close to the bulk value (MS = 480 ± 2 kA.m -1 ). The saturation field µ0Hsat, determined as the field at which the magnetization M = 0.95 MS, was evaluated at µ0Hsat = 285 mT.

Simulations and experimental observation of attractive and repulsive areas

The magnetic field gradients induced by the two Ni blocks were mapped using finite element The effect of the external magnetic field intensity was numerically studied (Figure S3). At 10 mT, the Ni blocks being far from fully magnetized the induced magnetic field gradients were very limited. Increasing the field above 50 mT leads to magnetic field gradients overpassing the critical values and thus the appearance of attraction and repulsion zones. The lateral extensions of these zones were characterized by plotting the magnetic field gradient profile on specific lines passing through the center of the Ni blocks (Figure S4). The distances onto which the magnetic field gradients overpass the critical values, and thus would induce the magnetic motion of the NRs, were considered. The lateral extension dx depicted in Figure 3a and 3d corresponds to the attraction zones observed in-between the Ni blocks and was characterized by a magnetic field gradient > 730 T.m -1 (Figure 3a andS4). The lateral extension dy and dz correspond to the repulsion zones observed on top of the Ni blocks where (or ! ) > 210 T.m -1 (Figure 3b, c and Figure S4).

Considering the symmetry of our experiments, dy = dz. As shown in Figure 3d, the attraction and repulsion zones spread rapidly with the magnetic field up to 400 mT. Increasing further the external field did not modify the induced magnetic field gradients, the Ni blocks being fully saturated as observed in Figure S1b. The larger extension of the repulsion zone (dy) compared to the attraction zone (dx) is due to the difference in the critical magnetic field gradients required for the magnetic motion of the Co NRs perpendicularly or parallel to the rod long axis, respectively. were already observed in absence of any applied field, evidencing the presence of strong interactions between the NRs (Figure 4a). The external magnetic field was progressively increased.

At low field, no motion was detected, in agreement with the simulations. At µ0Hext = 40mT, the agglomerates started to move towards the gap and the external attraction zones, as predicted.

Increasing the applied magnetic field induced a stronger attraction towards the Ni blocks (Figure 4b). Therefore, a depletion zone, nearly free of particles, progressively formed around the blocks.

This zone progressively extended from ~200 µm at 100 mT to ~700µm at 1T (Figure 4c). Similar results were observed at higher concentration (~40 g/L, Figure S6), but the depletion zone only

Supercrystal fabrication through magnetophoresis-driven controlled deposition

The capillary assembly assisted by magnetophoresis was implemented to fabricate a PM which fulfills the criterions of performance, miniaturization and integration into micro-electronic devices.

To fill the entire gap between individual pairs, a concentrated Co NR suspension in anisole ([Co] ~ 80 g/L) was used. The deposition process, consisting of the suspension drop casting, the magnetophoresis-driven assembly, and the washing of nonspecifically deposited Co NRs using an excess of solvent was repeated between two and six times depending on the Ni block thickness.

The capillary forces encountered during the complete evaporation of the solvent leads to the partial contraction of the assembly. Additional depositions could be done to fill the resulting free space.

For instance, conformal 150 µm thick magnets were classically prepared from six depositions and three drying steps (Figures 5a-c and S8, Video S2). Similar results were obtained using Co NR suspension in chloroform.

As previously observed for a dilute suspension, Co NR assembly occurred in the attraction zones, corresponding to high magnetic field gradient regions, located on both side of the Ni blocks.

Therefore, magnetic material was deposited not only in the gap but also at the extremity of the Ni/Co/Ni structure. The use of an additional resist mask surrounding the Ni blocks allows getting rid of this unwanted deposit after conventional lift-off process (Figure 5d-f). Thus, well-defined Co NR based magnet with controlled geometries and thicknesses were obtained.

The magnetophoresis-based process is well-suited for parallel fabrication. For instance, 144 Co-NR based magnets could be easily simultaneously fabricated using a predefined Ni block array (Figure S9). The 5 mm x 5 mm magnetic array was obtained after only two depositions and a single evaporation step. The Co-NR based magnets exhibit all lateral dimensions closed to the expected ones of 100 µm (L) x 100 µm (W) x 25 µm (T).

Structural characterization of the supercrystal : effect of the external magnetic field

The Co-NR based material prepared under an external magnetic field µ0Hext = 1T was characterized by SEM (Figure 6a-b). At the surface, nanorods were fairly well aligned along the external magnetic field direction, an angular dispersion of 16° being determined. The orientation distribution of the alignment was quantified on the whole material using XRD measurements in the so-called texture mode (Figure 6c). The pole figure obtained for the (101 F 0) reflection is strongly anisotropic with a maximum intensity at ϕ = 0° and 180° and for all χ values between 0 and 90° (Figure 6d). This figure shows that the c axis of the rods, which is also their long axis, is mostly parallel to the y axis, with a random orientation of the a and b axis in the (x, z) plane. In The effect of the external magnetic field value on the alignment quality was studied following the same method (Figure S10). The full width at half maximum, Δγ and Δα of the intensity profiles I(ϕ) and I(χ) of the pole figures are summarized in Table 1. For an applied field of 100 mT, the pole figure obtained was fairly isotropic, revealing a poor alignment of the Co NRs. The in-plane

Δγ and out-of-plane Δα were both evaluated at ca. 40°. At 400 mT, the characteristic anisotropic figure was observed. The alignment quality improved at higher fields, as revealed by Δγ which dropped down to 26° at 1T (Table 1). Δα remains fairly constant above 400 mT, thus a good compromise between alignment quality and external magnetic field constraint could be reached for a critical magnetic field of ~400 mT. This value further confirms the previous results obtained by simulations and preliminary experiments at low concentrations.

Magnetic characterization of the Co NR-based material

The alignment and compacity of the supercrystal prepared in the optimal conditions of µ0Hext = 1T were studied by Vibrating Sample Magnetometry. After Ni blocks removal, the room temperature magnetization was measured as a function of the applied field (Figure 7a). A broad hysteresis cycle, characterized by a large coercive field µ0HC = 590 mT was obtained.

The degree of magnetic alignment was characterized by the MR/MS ratio, as classically established. [START_REF] Pousthomis | Localized Magnetization Reversal Processes in Cobalt Nanorods with Different Aspect Ratios[END_REF] Experimentally, MR/MS = 0.85 evidencing a fairly good alignment of the rod long axes, in agreement with XRD results.

The magnetic volume fraction of the supercrystal could be determined from the absolute saturation magnetization measured and the total volume estimated from SEM and profilometric measurements (see Experimental section for details). The obtained magnetization was then compared to the bulk value (Ms Co = 1440 kA.m -1 25 ) to evaluate the magnetic volume fraction.

A magnetic volume fraction fv = 29 ± 2% was determined. Such a value is typically obtained for materials prepared with Co NRs. The Co NRs are indeed complex objects composed of a metallic cobalt core surrounded by a 1.2 nm-thick cobalt oxide shell and 3.5nm-thick surface ligands, the core being the only part contributing to the magnetic volume fraction. Considering perfectly aligned hexagonal arrays of Co NRs with similar features (2r = 15 nm, L = 150 nm) leads to a maximum magnetic volume fraction of fv MAX =37 %, not that far above the experimental values obtained here. [START_REF] Anagnostopoulou | Dense Arrays of Cobalt Nanorods as Rare-Earth Free Permanent Magnets[END_REF] Considering the experimental magnetic volume fraction, one could extrapolate the induction curve B(H) and the energy product, (BH)max. = 30 ± 3 kJ.m -3 (Figure 7b). Such energy product competes with the best values reported for comparable sub-millimeter sized magnets prepared with NdFeB-based magnetic polymers. 26 The magnetic induction generated by the Co NR supercrystal was measured via a Hall-effect micro-probe (figure 7c). The induction profile was determined as a function of the distance to the magnet D. At the minimal distance of 70 µm, a magnetic induction as high as 100 mT was reached.

Induction of 36 mT and 20 mT were still measured at D = 200 µm and 300 µm respectively.

The experimental induction profile has been fitted using finite element simulations, considering the supercrystal geometry, as determined by SEM, the experimental M(H) curve and the magnetic volume fraction as free parameter. The best adjustment was obtained for fv = 31 % in agreement with the magnetometry results.

The long-range extend of the induced stray field can be compared to the previous reported results. For instance, the induction generated by NdFeB powder-based micromagnets (25 µm x 25 µm x 10 µm) vanishes at D = 50 µm. [START_REF] Roy | Fabrication and Characterization of Polymer Membranes with Integrated Arrays of High Performance Micro-Magnets[END_REF] The outstanding NdFeB sputtered magnets (50 µm x 2000 µm x 38 µm) generated stray fields of 60 mT, measured at a distance D = 25 µm, but a 30% decay was faced on a 5 µm window (42 mT simulated at D = 30µm). [START_REF] Kustov | Magnetic Characterization of Micropatterned Nd-Fe-B Hard Magnetic Films Using Scanning Hall Probe Microscopy[END_REF]

Proof-of-concept : actuation of a gravimetric MEMS device using Lorentz force

To demonstrate the potentiality of Co-NR supercrystals as integrated permanent magnet, the actuation of a microelectromechanical system (MEMS) gravimetric sensor, 7 consisting of a freestanding vibrating Si microcantilever, was studied (Figure 8a). Such device can be used for e.g. rheological measurements or to detect molecule absorption where addition of a mass at the surface of the cantilever induces a shift in the MEMS resonance frequency. The actuation of this MEMS sensor relies on electromagnetic transduction that uses the Lorentz force and requires both an AC current i(t) circulating in metal lines patterned on the microcantilever and an external magnetic induction. The vibration of the cantilever is detected here by means of integrated piezoresistors.

The optimized Co NR-based magnet was placed in front of the cantilever at a distance d varying between 70 µm and 600 µm (Figure 8b). An actuation current of 30 mA supplied the device while the piezoresistors were polarized at 1 V. The figure 8c shows the first resonance peak at a frequency of 31. with an excess of solvent. A second addition of Co NR was performed to fill the entire gap volume.

After proper washing and complete evaporation of the solvent, a compact self-standing supercrystal with a length and a diameter of 2 mm was obtained, these dimensions being conformal to the geometry of the gap (Figure 9a-b). Though prepared solely from Co NRs, the material exhibits mechanical robustness allowing an easy separation from the two Fe bars. The millimeter magnet has a very shiny aspect, revealing fairly smooth surfaces and thus proper alignment of the rods.

The magnet density could be determined thanks to the direct measurement of its weight (m = 24.7 ± 0.1 mg) and its volume (V = 6.3 ± 0.6 mm 3 ). A value of ρmagnet = 3.9 ± 0.4 g.cm -3 was determined corresponding to 44 ± 4 % of the density of bulk Co (ρCo = 8.9 g.cm -3 ). The magnetic volume fraction, which is only sensitive to the Co core, was determined at fv = 36 ± 3 % (see experimental section for details). The density and the magnetic volume fraction values are in agreement with perfectly aligned 15 nm diameter Co NRs exhibiting a 1.2 nm thick oxide shell, and separated by a 3.5 nm distance resulting from the organic surfactant corona. [START_REF] Anagnostopoulou | Dense Arrays of Cobalt Nanorods as Rare-Earth Free Permanent Magnets[END_REF] The magnet exhibits a broad magnetization cycle at room temperature which is comparable with the submillimeter integrated magnet's one (Figure 9c and Table 2). The magnetic alignment and coercivity are fairly similar (MR/MS = 0.81 and 0.85, respectively; µ0HC = 590 mT in both cases).

The difference in magnetization between the two type of magnets arises from the magnetic volume fraction, which is slightly higher in the millimeter-scale magnet (fv = 36 % and 29 %, respectively).

Thanks to this improvement, the maximal energy product reaches (BH)max = 37 ± 3 kJ.m -3 for the millimetric magnet. Due to the good magnetic properties and the large volume of this millimetric magnet, intense stray fields were induced on a large-range distance. The induction profile was mapped from few hundred of microns to few millimeters using Hall-microprobe (Figure 9f). Induction field larger than 20 mT were measured up to a 2 mm distance. This long-range extension was further confirmed by the actuation of MEMS gravimetric sensor. As evidenced in Figure 9e, the resonance of the vibrating microcantilever is clearly observed up to magnet-to-cantilever distance of 2 mm. could be estimated (Figure S17).

CONCLUSION.

We report here a fairly simple and fast fabrication process for integrated magnetic materials based on the controlled assembly of individual nanoparticles. NRs and nanospheres are efficiently arranged into supercrystals through a magnetophoresis-driven capillary assembly process leading to performant rare-earth free magnets and soft magnetic supercrystals. The fabrication process, based on pre-patterned ferromagnetic flux concentrator structures is suited to the fabrication of integrated submillimetric magnets with tuneable geometries as well as millimetric self-standing structures. The overall fabrication process is highly cost efficient, since the magnetophoresis forces attracts most of the nanoparticles diluted in the drop-cast solution to assemble them in the final supercrystal. This process is also time efficient, since only a few minutes is needed to obtain NPbased materials with the designed outer shape and density. The efficiency of the rare-earth based permanent magnet prepared from Co NRs was evidenced through the electromagnetic actuation of a MEMS cantilever. Soft magnets were efficiently prepared from Fe-based nanoparticles. In all the examples described, the particles were handled in air. The magnetophoresis-driven assembly process can however be performed under inert atmosphere to avoid the NP surface oxidation and to increase the magnetic volume fraction of the assemblies. Being not limited by the NP size, shape or chemical composition, this fabrication process can be as well extended to other magnetic NPs.

EXPERIMENTAL SECTION.

Fabrication of the decorated substrate 1. Electroplating of Ni blocks 25 to 150 µm thick nickel blocks were electroplated onto a silicon substrate using conventional microfabrication procedure. The fabrication process is further detailed for 150 µm thick Ni blocks.

First, a conductive seed layer constituted of 100 nm Ti / 200 nm Cu was evaporated onto a 4-inches Si wafer with a thickness of 525 µm. Then, a 200 µm thick dry film was elaborated by the lamination at 100°C of two 100 µm negative films of WBR 2100 (Dupont) on the substrate. The resist was then exposed by photolithography at a power of 1.5 J/cm² and a wavelength of 365 nm.

The resist was developed via its immersion in Na2CO3 developer at room temperature during 20 min. This 200 µm thick film acts as a mold for the electrochemistry, the film is thicker than the targeted Ni in order to improve the deposition homogeneity on the wafer. Then, nickel was electroplated inside Yamamoto-MS cell plating with a NB Semi Plate Ni 100 bath (NB Technology) applying 3 A.dm -2 equivalent to a plating speed of 0.7 µm/min. The resist was finally stripped by NF52 at 70 °C during 25 min and the Cu/Ti seed layer was etched using a mixture of sulfuric acid and hydrogen peroxide at 1% of each, and a 5 % hydrofluoric acid solution to remove the Ti layer.

Imprint of a resist mask

A secondary resist was deposited around the Ni blocks to prevent the particle aggregation at their extremity using the imprinting technique under pressure. As previously, two 100 µm thick Fe2.2C nanoparticles (NPs) were synthesized using a carbidization process previously reported. [START_REF] Asensio | To Heat or Not to Heat: A Study of the Performances of Iron Carbide Nanoparticles in Magnetic Heating[END_REF] Fe0 NPs were dispersed in mesitylene before being pressurized at 150 °C for 24 h with CO/H2 (3 bar of a 1:1 mixture of both gases) for 4 days. Then, the NPs were decanted with the help of a magnet, washed with 3x10 mL of toluene and dried under vacuum to form a black powder. They were re-dispersed in degassed mesitylene just before the deposition step.

Preparation of the concentrated NP suspension

Highly concentrated NP suspension were prepared by dispersing the magnetic nanoparticle of interest (Co NRs or Fe2.2C) into the solvent of choice (anisole, chloroform or mesitylene). Stored in the reaction medium, Co NRs were magnetically extracted and purified using chloroform.

Typically, 20 mL of chloroform is added to 10 mL of the mother liquor solution. After 5 min of sonication, the particles are magnetically attracted. The procedure is repeated 4 times, until the supernatant was colourless. Based on Gas chromatography-mass spectrometry (GC-MS) of the discarded supernatant, thermogravimetric analysis (TGA) and magnetometry measurements (VSM) of the obtained Co NR, we could determine that four washing/precipitation with chloroform allowed removing the solvent (butanediol and the product of its oxidation(1-hydroxy-2-butanone)) and the surfactant in excess (laurate). After such treatment, the mass fraction of the laurate surfactant was of 8%, matching with the theoretical value expected for a monolayer of surfactant at the rod surface. It allows stabilizing the suspension of Co NR during the few minutes of the magnetophoresis-assisted assembly without altering too much the magnetic volume fraction in the final supercrystal. The obtained NRs were redispersed into 600 µL of anisole leading to a

Co suspension with an estimated concentration of approximatively 80 g/L. In the case of the 25 µm thick magnetic array, a suspension diluted twice is used. ([Co] ~ -40g/L following the same procedure but starting from only 5 mL of the mother liquor solution).

The generated magnetic field gradients were simulated using Comsol Multiphysics 4.3. First, the magnetic induction was calculated by solving the first Maxwell equation in stationary regime using the Magnetic Field No Current (MFNC) module. A 50 µm mesh size was used as a good compromise between resolution and extended computational time. The uniform external field generated by the electromagnet was implemented through magnetic potentials of 1600 W/m and 0 W/m, respectively on the opposite faces of a 4 mm 3 air box. Two Ni blocks were placed in the middle of the box and were defined via magnetic flux conservation. Their magnetization was specified using experimental hysteresis cycles. Then, the 3D magnetic induction gradients were calculated using the Coefficient Form PDE module considering three dependent variables u1, u2, u3 which correspond to each spatial component of the magnetic induction. These Lagrangian elements were derived spatially at the first order to obtain the magnetic field gradient in the air box.

Structural and magnetic characterizations

Scanning electron microscopy (SEM) were performed using a JEOL JSM 6700F microscope at an accelerating voltage of 15 kV and a current of 8 µA. Powder X-ray diffraction measurements were performed on a PANalytical Empyrean diffractometer using Co-Kα radiation and equipped with a linear Pixcel1D detector. Texture characterization of rods assembly were performed with a Brucker D8 Discover diffractometer equipped with a Co micro-source, a 2D Vantec-500 detector and a 300 µm collimator. The Bragg-Brentano angle 2θ was set to 48.9° to probe the (10-10) orientation plane of the rod assembly. The sample holder was rotated in plane of an angle φ varying between 0° and 359° through successive 20° steps while three rotations out of plane were performed at χ values of -13.9°, -47,4° and -80.9°. The 2D images were processed by the software Maud and fitted using a harmonic spherical model to yield the pole figure.

The magnetic properties of the rod assemblies were characterized using a Quantum Design Physical Property Measurement System (PPMS) with the Vibrating Sample Magnetometer (VSM) configuration. The hysteresis measurements in the parallel configuration (applied field along the material anisotropy axis) were done at room temperature between -3 and +3 T.

The magnetic volume fraction J K within the supercrystal was calculated from the saturation magnetization MS recorded at 3 T (expressed in emu), the supercrystal volume estimated by SEM (expressed in cm 3 ) and the expected bulk magnetization (for Co : L M,%N O :8 = 1440 kA.m -1 , [START_REF] Jackson | Integration of Thick-Film Permanent Magnets for MEMS Applications[END_REF] for Fe2.2C : L M,$P% O :8 = 1200 kA.m -1 [START_REF] Asensio | To Heat or Not to Heat: A Study of the Performances of Iron Carbide Nanoparticles in Magnetic Heating[END_REF] ) as follow :

J K = Q R,S T 8U. VW Q R,XYZ[ 8U. VW (Eq.5)
Where L M,P \ @]. 3^ = L M,P \ . _ 3` = Q R P a b (Eq. 6)

The energy product (BH)max was determined from the B(H) loop, which was extrapolated from the M(H) loop considering: = µ 4 d + L (Eq. 7)

Magnetic characterizations were realized via a Hall-effect micro-probe. [START_REF] Shaw | A Scanning Hall Probe Microscope for High Resolution, Large Area, Variable Height Magnetic Field Imaging[END_REF]

Forces acting on nanoparticles during the magnetophoresis-driven assembly

The detail analysis is inspired by the magnetophoresis review, 1,2 and the specific reports using anisotropic nanoparticles. 3,4 The values of the forces are summarized in Table S1.

Graviational force and buoyancy

The weight ⃗ , which lead to sedimentation, and the opposed buoyancy ⃗ can be neglected compared to the Brownian motion considering single nano-objects. The corresponding sedimentation energy is indeed given by: 5

( -ρ ) ℎ ≤ (Eq. 1)

with ρ the particle density ( , = 8900 . # $% , &' = 7200 . # $% ), the density of the medium ( ,*+,-' = 995 . # $% , ,/'-,01 '+' = 864 . # $% ), the particle volume, the acceleration due to the gravity and ℎ the height of the particle compared to the substrate (estimated to 100 µm here).

Magnetophoresis force

As described in the paper, the magnetic force 4 / ⃗ is expressed by:

4 / ⃗ = (# ⃗. ∇)6 ⃗
with # ⃗ the magnetization of the nanoparticle and ∇6 ⃗ the magnetic field gradient. We suppose that the magnetization lays parallel to the magnetic field gradient ∇6 ⃗ = 100 . # $8 .

The magnetization of the nanoparticle is calculated by:

# = 9 -, × *;0,< '
where Ms,bulk corresponds to the saturation magnetization of the material considered (9 =, = 1440 kA.m -1 , 6

Magnetophoresis speed

The magnetophoresis speed is given by : K /*L = & M with C the drag force coefficient.

For a sphere : N = 6AOB with µ the dynamic viscosity of the solvent (µanisole = 0.99 mPa.s, µmesitylene = 0.72 mPa.s) and r the particle radius (r = 15 nm here).

For a rod, two coefficients parallel to the long axis (Cpara) and perpendicular to the long axis (Cperp) are found:

N *;* = >PQR STU V WX Y$ .Z> , N '; = >PQR STU V WX Y[ .\
with L the length (L = 150 nm) and 2r the diameter of the rod (2r = 15 nm).

Diffusivity

The diffusivity is given for a sphere by : ] = _Pµ; = ^ with kT the thermal energy Magnetophoresis speed (m.s -1 ) for gh ⃗ = ijjk. l $i 6.5 x 10 -6 11.5 x 10 -6 1.4 x 10 -6

Péclet for gh ⃗ = ijjk. l $i 0.13 0.48 4.9 x 10 -4 gh ⃗ (T.m -1 ) for Péclet = 1 732 207 2 x 10 5

Figure 1 .

 1 Figure 1. a,c,e) Schematic views of the successive steps of the fabrication process of magnetic

Figure 2 .

 2 Figure 2. Evolution of the Peclet numbers as a function of the magnetic field gradient applied

  calculations (Comsol Multiphysics, Figure 3a-c and S2). For an external magnetic field µ0Hext = 1T, classically used in this study, values as high as 3000 T.m -1 were found at the extreme vicinity of the magnetized Ni blocks. Considering the analytical model previously developed, convection of Co NR will be observed in the regions where the magnetic field gradients overpass the critical values of 730 T.m -1 ( ) and 210 T.m -1 ( , ! . Large attraction zones in the three spatial directions were evidenced as highlighted on Figure 3. Along the ⃗ direction, the zones were located on both side of each Ni blocks, inducing a potential accumulation of Co nanorods in three main regions: in the gap, which is the targeted area, and at the two extremities. Along the B ⃗ and C ⃗ directions, attractive gradients were observed in the same three regions, allowing for the Co NR supply through a motion perpendicular to the rod long axis. Repulsion zones were observed laterally to the Ni blocks.

Figure 3 .

 3 Figure 3. Simulated mapping of the magnetic field gradients a) , b) and c) ! induced

  covered ~400 µm at 1T. The evolution of the extension of the depletion zone as a function of the applied field is reported in Figure4d. To compare the results obtained on a set of experiments at different concentrations, the extensions of the depletion zones have been normalized with respect to the maximal length measured at 1T. They all follow a similar trend: a fast growth above the critical field of 40 mT to reach a quasi-plateau above 500 mT. The extension of the repulsion zone, determined theoretically was compared. While the simulation predicted a saturation above 400 mT in the case of non-interacting Co NR, the depletion zones kept spreading in our experiments. The difference can be attributed first to the presence of Co NR agglomerates in the suspension, which promote the magnetic convection. The progressive assembly of Co NRs around the Ni blocks can also be invoked. Co NRs exhibit a saturation field of 1.2 T (figureS7), far above the Ni blocks'one (285 mT for 200µm-length Ni blocks). They do contribute to the overall magnetic field gradients, leading to stronger magnetic-convection which kept increasing up to high external field.

Figure 4 .

 4 Figure 4. Snapshots taken during the deposition of a dilute suspension of Co NRs in anisole

Figure 5 :

 5 Figure 5: a-c) Snapshots taken during the fabrication process of a Co NR-based PM consisting of

  Figure 6d), and by the intensity profile I(χ) plotted as a function of χ for ϕ=90 and 270° (i.e. along

Figure 6 :

 6 Figure 6: a-b) SEM images of Co NR-based supercrystal deposited between two individual Ni

Figure 7 :

 7 Figure 7: a) Magnetic measurement recorded at 300K of the c.a. 500 µm x 500 µm x 150 µm

  1 kHz, detected by means of the output piezoresistor voltage (Vout). The amplitude of the resonance peak decreases with the distance between the cantilever and the Co NR-based magnet as evidenced for D = 100, 200 and 300 µm. The evolution of the resonance peak amplitude as a function of D was reported on Figure 8d. The decay observed perfectly matches the magnetic induction profile of the Co NR supercrystal, confirming that the nanostructured magnet is responsible for the MEMS actuation up to distances of 600 µm. The submillimeter Co NR-based magnet successfully replaced the centimetric NdFeB magnets typically used for the device actuation (Figure S11).

Figure 8 :

 8 Figure 8: a) Schematic view of a MEMS gravimetric sensor consisting of a Si microcantilever

Figure 9 :

 9 Figure 9: a-b) Optical images of the self-standing 2 mm thick magnet prepared solely from 15 nm

Figure 10 .

 10 Figure 10. SEM images of Fe2.2C-based supercrystal integrated as a) a magnetic array of 100 µm

  This sensor was instrumented via an acquisition card Ni 6341 from National Instruments which polarized an electronic circuit, providing a constant current of 50 µA to the probe and measuring the Hall voltage in real-time. The Hall constant of the micro-probe was preliminary calibrated using a coil and the gaussmeter BGM 101 (Brockhaus Measurements). A Hall constant k = 200 Ω.T -1 was determined. The Hall-effect micro-probe was mounted on a (XY) plane, while the supercrystal was placed on a moving stage (A2V Mécatronique) to control the z position.

Figure S1 : 5 Figure S2 : 6 Figure S3 :

 S15S26S3 Figure S1: SEM image and magnetization curve of pairs of Ni blocks electroplated 5 Figure S2: Schematic views of the nickel blocks used for simulation 6 Figure S3: Evolution of the magnetic field gradients as a function of the external magnetic field

Figure S4 : 8 Figure S5 : 9 Figure S6 :

 S48S59S6 Figure S4: 2D maps and evolution profile of and 8 Figure S5: Snapshots of the magnetophoresis of a dilute suspension of Co NRs (20 g/L) 9 Figure S6: Snapshots of the magnetophoresis of a suspension of Co NRs (40 g/L) 10 Figure S7: TEM image of Co NR and the hysteresis cycle at 300K after magnetic alignment Figure S8: Snapshots of the magnetophoresis yielding to a full magnet 12 Figure S9: SEM images of a magnetic array and the corresponding thickness profile 13 Figure S10: Effect of the magnetic field applied on the pole figure and the intensity profiles.Figure S11: Image of the gravimetric MEMS sensor actuated by NdFeB magnets 15 Figure S12: 2D maps of magnetic field gradients induced by magnetized Fe bars. 16 Figure S13: TEM image of Fe2.2C NPs used and the corresponding size distribution 17 Figure S14: SEM view of the Fe2.2C assembly and the corresponding hysteresis cycle 17

Figure S11 :

 S11 Figure S4: 2D maps and evolution profile of and 8 Figure S5: Snapshots of the magnetophoresis of a dilute suspension of Co NRs (20 g/L) 9 Figure S6: Snapshots of the magnetophoresis of a suspension of Co NRs (40 g/L) 10 Figure S7: TEM image of Co NR and the hysteresis cycle at 300K after magnetic alignment Figure S8: Snapshots of the magnetophoresis yielding to a full magnet 12 Figure S9: SEM images of a magnetic array and the corresponding thickness profile 13 Figure S10: Effect of the magnetic field applied on the pole figure and the intensity profiles.Figure S11: Image of the gravimetric MEMS sensor actuated by NdFeB magnets 15 Figure S12: 2D maps of magnetic field gradients induced by magnetized Fe bars. 16 Figure S13: TEM image of Fe2.2C NPs used and the corresponding size distribution 17 Figure S14: SEM view of the Fe2.2C assembly and the corresponding hysteresis cycle 17

  9 =,&'>.> = 1200 kA.m -1 7 ) and Vparticle the volume of the nanoparticle ( ?@ = AB > C = 26506D# % , &'>.> ?E = F % AB % = 1767D# % ). The magnetization value obtained for Co NR and Fe2.2C NP are # = 38.3 × 10 $8H I. #² and # &'>.> = 1.3 × 10 $8H I. #²

  and for a rod by two coefficients : ] *;is finally given by the ratio : c = Mde .R f

Figure S1 :

 S1 Figure S1: a) Scanning Electron Microscopy (SEM) image of pairs of Ni blocks electroplated separated by a 500 µm gap. The lateral dimensions of the blocks were 200 µm (length) x 500 µm (width) x 150 µm (thick). b) The corresponding magnetization curve recorded at room temperature as a function of the magnetic field applied along the x direction indicated in a).

Figure S2 :

 S2 Figure S2: Schematic views of the geometry of the two nickel blocks used in the Comsol Multiphysics simulations evidencing a) the lateral dimensions (W: width = 500 µm; T: thickness = 150 µm; L: length = 200 µm; S: spacing = 500 µm) and b) the two symmetry planes xy (in green) and xz (in yellow) which contain the block center and onto which the magnetic field gradients are mapped. The measured magnetic properties of the Ni blocks (Fig. S1b) are introduced in the simulations.

Figure S3 :

 S3 Figure S3: Evolution of the magnetic field gradients (left column), (middle column) and m (right column) as a function of the external magnetic field µ0Hext applied along the n ⃗ direction. The magnetic field amplitude was varied from 10 mT to 1 T. The maps were obtained on the two symmetry planes (xy) and (xz) containing the Ni block centers as shown in Figure S2. The color scales have been adjusted to the critical magnetic field gradients values determined by the simple model ( : 730 T/m : left side; and m : 210 T/m, right side). White arrows indicated on the 100 mT maps are guide to the eye for the attractive and repulsive zones felt by Co NR.

Figure S4 :

 S4 Figure S4: a-b) 2D maps simulated at 100 mT and c-d) the evolution of the magnetic field gradients a-c) and b-d) along the profile line depicted by the white dashed line on a-b) for different external magnetic field. The spatial extension of the attractive and repulsive areas at the vicinity of two Ni blocks of 200 x 500 x 150 µm 3 separated by a 500 µm gap were determined at the intersection of the simulated curve with the critical gradient value, represented by the dashed blue line on c-d).

Figure S5 :

 S5 Figure S5: Snapshots taken during the deposition of a dilute suspension of Co NRs ([Co] ~ 20 g/L) on top of two Ni blocks (200 µm (L) x 500 µm (W) x 150 µm (T)) and placed in an external magnetic field of a) 0, b) 40 mT, c) 60 mT, d) 100 mT, e) 150 mT and f) 500 mT (Video S1). The white arrows are guide to the eye for the Co NR motion. For clarity they are only presented for the left-hand side Ni block.

Figure S6 :

 S6 Figure S6: Snapshots taken during the magnetophoresis of a Co NR suspension ([Co] ~ 40 g/L) by two Ni blocks (200 µm (L) x 500 µm (W) x 150 µm (T)) under an external magnetic field of a) 50 mT, b) 80 mT, c) 140 mT, d) 200 mT, e) 300 mT, f) 400 mT, g) 500 mT and h) 600 mT. Scale bar = 200 µm.

Figure

  Figure S7: a) TEM image and b) 300K Hysteresis cycle of aligned Co NRs. The Co-NR based material obtained exhibit a ratio MR/MS = 0.9.

Figure S8 :

 S8 Figure S8: Snapshots taken during the fabrication process of a Co NR-based PM consisting of successive deposition and magnetophoresis-driven assembly, washing and solvent drying steps (Video S2). Assembly obtained after a) 1 st deposition, b) washing of unspecifically deposited Co-NRs; c) 2 nd deposition and d) the corresponding washing; e) 3 rd deposition, f) drying; g) 4 th deposition, h) drying i) 5 th and 6 th deposition and j) final drying leading to Co NR assembly in the gap and at the two extremities. Scale bar = 200 µm.

Figure S9 :

 S9 Figure S9: a) Optical and SEM images of b) the electroplated Ni array and c) the hybrid Co/Ni magnetic array obtained after the magnetophoresis-driven assembly. Inset: large view. d) evolution of the thickness profile measured on the Co NR assembly (red line in c) and on the Ni blocks (black line in c).

Figure S10 :

 S10 Figure S10: a) Pole figures of the reflection (101 o 0) considering the assemblies of Co NRs realized at 100 mT, 400 mT, 700 mT and 1 T. The corresponding b) ϕ and c) χ angle distribution allowing to extract the in-plane (Δγ) and out-of-plane (Δα) disorientations.

Figure S11 :

 S11 Figure S11: Image of the classical set-up used to actuate the gravimetric sensor. 1cm x 1cm squared NdFeB magnets are placed at the vicinity of the device.

Figure S12 :

 S12 Figure S12: a) Schematic views of the geometry of the two Fe bars used in the Comsol Multiphysics simulations (d: diameter = 2 mm; S: gap = 2 mm). As inset the two symmetry planes xy (in green) and xz (in yellow) which contain the Fe bar center and onto which the magnetic field gradients are mapped. 2D maps of the magnetic field gradients b) , c) and d) m induced by the Fe bars magnetized under an external field of µ0Hext = 1T. The color scales have been adjusted to the critical magnetic field gradients values determined by the simple model ( : 730 T/m; and m : 210 T/m). White arrows are guide to the eye for the attractive and repulsive zones felt by Co NR.

Figure

  Figure S13: a) Representative TEM images of the Fe2.2C nanoparticles used in the study b) and the corresponding size histogram obtained on 100 NPs. A mean diameter of 15.4 nm was determined, with a size distribution σ = ± 7%.

Figure S14 :

 S14 Figure S14: a) SEM image showing a large view of Fe2.2C NP assemblies. b) Magnetization as a function of the applied field measured at 300K for a Fe2.2C supercrystal of 500 µm (L) x 500 µm (W) x 145 ± 5 µm (T).

Table 1 :

 1 Evolution of the FWHM of the in plane (IP) and out of plane (OOP) orientation distribution determined on the pole figures as a function of the external magnetic field applied during the magnetophoresis.

		100 mT	400 mT	700 mT	1T
	GH (IP)	40°	33°	31°	26°
	GI (OOP)	39°	16°	20°	17°

Table 2 :

 2 Comparison of the magnetic properties of the Co-NR based material fabricated as submillimeter magnet or self-standing millimeter magnet.

		Dimensions	µ0Hc (mT)	MR/MS	fv -VSM (%)	(BH)max (kJ.m -3 )	B (mT)	fv -Hall (%)
	Integrated magnet	500 µm x 500 µm x 150 µm	590	0.85	30 ± 2 30 ± 3	B100µm 80 mT	31
	Self-standing magnet	2r = 2mm L = 2mm	600	0.80	36 ± 3 39 ± 3	B400µm 100 mT	36

Table S1 :

 S1 Forces acting on Co NR and Fe2.2C NP 4 Deposition of a dilute suspension of Co NRs ([Co] ~ 20 g/L) on top of two Ni blocks Video S2: Fabrication process of a 150µm thick Co NR-based permanent magnet

	Table of Content:
	Forces acting on nanoparticles during the magnetophoresis-driven assembly	2
	Video S1:

Table S1 .

 S1 Values determined for a Co NR (2r = 15 nm, L = 150 nm) and a Fe2.2C NP (2r = 15 nm) dispersed in anisole and mesitylene, respectively.

		Parallel	Co NR Perpendicular	FeCx NP
	kT/Sedimentation energy		20	610
	Drag force coefficient C	5.9 x 10 -10	3.3 x 10 -10	1.0 x 10 -10
	Diffusivity D (m 2 .s -1 )	0.8 x 10 -11	0.4 x 10 -11	4.1 x 10 -11
	Magnetophoresis force (N) for gh ⃗ = ijjk. l $i	38.3 x10 -16	1.3 x10 -16

ACKNOWLEDGMENT PM thanks the Région Occitanie and the Université Fédérale de Toulouse for phD funding. This study has been partially supported through the French national project POMADE (ANR 19-CE09-0021-01), the EUR grant NanoX n° ANR-17-EURE-0009 in the framework of the « Programme des Investissements d'Avenir, the prematuration program of the Région Occitanie (AimCap) and the SATT TTT (Soft Magneto). This work has been supported by the technology platform of LAAS-CNRS, Member of the French RENATECH network. JMA thanks the ERC Advanced Grant (MONACAT 2015-694159) for financial support. Marc Respaud and Sébastien Pinaud are warmly thanked for fruitful discussion and technical support.

VERSATILITY OF THE MAGNETOPHORESIS ASSISTED FABRICATION PROCESS

The fabrication process based on magnetophoresis assisted assembly was extended to i) self-standing permanent magnets with larger dimensions and to ii) soft magnets based on dense arrays of spherical nanoparticles.

Fabrication of a millimeter sized permanent magnet

Magnetophoresis-assisted capillary assembly of Co NRs was studied for the fabrication of millimeter sized self-standing magnets. The electrodeposited Ni blocks which so far were used as flux concentrators were replaced by two commercial Fe bars embedded in a Teflon mold and facing each other (inset Figure 9a). The diameter of the Fe bars and the distance between them could be adjusted to tune the final dimension of the nanostructured magnet. We focus here on 2 mm diameter Fe bars separated by a 2 mm gap and magnetized by an external magnetic field of µ0Hext = 1 T. The magnetic field gradients induced by the bars were simulated using Comsol Multiphysics (Figure S12). Local magnetic field gradients up to 3000 T.m -1 were predicted, far above the critical values of 730 T.m -1 and 210 T.m -1 required to attract the Co NRs according to our simplified model.

The mold cavity was filled with 600 µL of a Co NR suspension in anisole ([Co] ~ 16 g/L) prepared from the same Co NRs (2r = 15 nm, L = 150 nm) as the one used for the submillimeter magnet previously described. The magnetophoresis was performed under the optimized external magnetic field of µ0Hext = 1T. Within only few seconds, all the Co NRs present in the suspension were attracted in the gap. The colorless solvent was discarded and the Co NRs material washed

Preparation of soft magnetic materials

We investigated the potentiality of the magnetophoresis-driven capillary assembly to fabricate soft magnetic materials. For that, monodisperse iron carbide nanospheres (Fe2.2C NPs) with a mean diameter of 15 nm and a size dispersion σ = 7% were used (Figure S13). As previously described in section I, the simple theoretical model we developed considering non-interacting NP predicted a motion only driven by diffusion in our experimental conditions. Indeed, the magnetic field gradients generated by the magnetized 200 µm-length Ni blocks reach at most 3000 T.m -1 (Figure 3), far below the 2.10 5 T.m -1 field gradient required to establish the convection-regime (see supplementary information).

Experimentally, the Fe2.2C NPs contained in the colloidal solution ([Fe2.2C] ~ 15g/L in mesitylene) were however rapidly and selectively attracted towards the Ni blocks once magnetized under an external magnetic field µ0Hext = 1T. Thus, the presence of magnetic interaction within the fairly concentrated colloidal solution promoted the magnetophoresis convection over the Brownian motion.

Magnetic arrays and individual soft magnets, with conformal geometries compared to the Ni blocks could be prepared as shown in Figure 10a-b. SEM images of the supercrystal surface evidence nearly defect-free hexagonal closed packed structures on micrometric areas (Figure 10c). films of WBR 2100 were laminated together to form a film of 200 µm. This thick film was then imprinted under vacuum to avoid the accumulation of air bubbles between the substrate and the resist by the NX-2500 Nanoimprintor at 85 °C at 30 Psi during 120 s. The resist was then exposed by photolithography at a power of 1.5 J/cm² and a wavelength of 365 nm. The resist was developed by immersing it in Na2CO3 developer at room temperature during 20 min. Finally, the substrate was diced with a diamond saw on a chip presenting 1x1 cm 2 lateral dimensions.

Magnetophoresis experiment

Nanoparticle synthesis

Co nanorods (NRs): Cobalt (II) acetate hydrate (Alfa Aesar), NaOH (Acros), RuCl3.xH2O (SigmaAldrich, 99.98%), 1,2-butanediol (Sigma-Aldrich), Na-(C11H23COO) (Aldrich, ≥99%) were purchased and used without any further purification. Cobalt nanorods were synthesized by the polyol process following a protocol previously described. [START_REF] Anagnostopoulou | Dense Arrays of Cobalt Nanorods as Rare-Earth Free Permanent Magnets[END_REF] First, cobalt (II) laurate was prepared by mixing equimolar cobalt acetate and sodium laurate to obtain a pink precipitate. This one was washed with deionized water and dried at 50 °C during one week. Then, Co(C12H23O2)2 was mixed in 120 mL of a basic solution of 1,2 butanediol at 0.08 mol.L -1 ([Co] = 4.7 g/L).

Ruthenium chloride salt is added ([Ru]/[Co]=2.5%) and the mixture heated at 175°C. After 20 min, the reduction of Co(II) complexes to metal was complete. Then, the black suspension was cooled down at room temperature and the Co NRs were separated from the butanediol by magnetic attraction. Particles were washed with absolute ethanol till the supernatant was colourless.

Additional washing with chloroform allows removing the unreacted species and the ligand excess. The Fe2.2C NPs were stabilized by a mixture of acid (palmitic acid) and amine (hexadecylamine).

After three magnetic-assisted washing with toluene, TGA and VSM revealed a mass fraction of surfactants of 20%. Once redispersed in mesitylene, the NPs were fairly stable in suspension, even at high concentration.

Magnetophoresis-driven capillary assembly

The 1 x 1 cm² samples consisting of Si chip patterned with Ni blocks were placed in the center of a 2 cm length cavity of a Teflon mold. The mold was set in an electromagnet generating a uniform magnetic field varying between 0 and 1 T (see supplementary figure S1). The local deposition of the magnetic nanoparticles was performed following a three-step impregnation process. i) In absence of any magnetic field, 10 µL of the dense colloidal suspension was deposited on the substrate using a micropipette. ii) A 1 T magnetic field was then applied, leading to the alignment of the magnetic nanoparticles and their attraction by the Ni blocks. iii) After 30 s, the suspension was washed away with 500 µL of chloroform. The impregnation was classically repeated three times, before allowing the solvent evaporating at room temperature and atmospheric pressure. Depending on the suspension concentration and the targeted magnetic layer thickness, the deposition process (3 impregnations and 1 evaporation) can be repeated several times. In the case of the deposition through a resin mask, a lift-off process was finally realized, the sample was immersed 5 min in an acetone bath and then rinse with ethanol and DI-water.

Finite Element simulations MEMS actuation

MEMS devices, fabricated for rheological applications, consisted of two 840 µm long, 100 µm wide and 20 µm thick cantilevers: a reference cantilever fixed to the handling substrate and a freestanding cantilever able to vibrate using electromagnetic force actuation. Lorentz force was generated upon injection of a sinusoidal current f to the conducting path patterned onto the cantilevers (scheme in supplementary) and perpendicular to a static magnetic field generated by a neighboring permanent magnet. The vibration amplitude reaches a maximum when the frequency of the actuation current equals the mechanical resonance frequency of the microsystem. To detect this resonance peak, piezoresistors were integrated at the clamped-end of the cantilevers, where the stress induced by the deformation is maximal. Mechanical oscillations were converted by the piezoresistors to a variation of resistance ∆R/R and measured by a vector network analyzer which generates two complementary voltages and measures the output signal of a transimpedance amplifier.

Supporting Information Available

Details


Wala Abou Saoud

Abdoulaye Kane

Pierre Le Cann

Anne Gérard

Lina Lamaa

Laure Peruchon

Cedric Brochier

Abdelkrim Bouzaza

Dominque Wolbert

Aymen Amine Assadi
email: aymen.assadi@ensc-rennes.fr

Anne Gerard

Innovative photocatalytic reactor for the degradation of VOCs and microorganism under simulated indoor air conditions: Cu-Ag/TiO2-based optical fibers at a pilot scale

Keywords: Photocatalysis, Indoor air treatment, mixture (VOCs/E. coli), Cu-Ag/TiO2-based optical fibers, continuous reactor

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

Introduction

Indoor air contaminants (such as Volatile Organic Compounds, VOCs and Bacteria), generated from food processing and preparation [START_REF] Scotto | Olfactory pollution in urban environment: special case of odors in restaurants[END_REF], can cause serious risks to public health. Moreover, the indoor air of all public places (Food industry, Hospitals, libraries,) were in the range above highly contaminated according to European Commission classification and the most isolates are considered as potential candidates involved in the establishment of sick building syndromes and often associated with clinical manifestations like allergy, rhinitis, asthma and conjunctivitis [START_REF]Biological Pollutants' Impact on Indoor Air Quality[END_REF]. This can be due to the bacteria, viruses, molds and pollens. Thus, attention must be given to control those environmental factors which favor the growth and multiplication of microbes in indoor environment of public places to preserve the health of users and workers. Therefore, bacterial inactivation and VOCs removal remained very important approach for health and safety reasons [START_REF][END_REF][START_REF]Guidelines on air handling in the food industry-trends[END_REF]. The promising advanced technologies (Table 1.), provide interesting solutions at low cost with good removal efficiencies such as photocatalysis [START_REF] Ramesh | Application of ultraviolet light assisted titanium dioxide photocatalysis for food safety: A review[END_REF][START_REF] Huang | Removal of Indoor Volatile Organic Compounds via Photocatalytic Oxidation: A short Review and Porspect[END_REF], cold plasma with/without catalysis [START_REF] Holzer | Non-thermal plasma treatment for the elimination of odorous compounds from exhaust air from cooking processes[END_REF][START_REF] Thirumdas | Cold Plasma: A novel Non-Thermal Technology for Food Processing[END_REF][START_REF] Karatum | A comparative study of dilute VOCs treatment in nonthermal plasma reactor[END_REF] and catalytic ozonation [START_REF] Alejandro-Martín | Catalytic Ozonation of Toluene Using Chilean Natural Zeolite: The Key Role of Brønsted and Lewis Acid Sites[END_REF][START_REF] Da Costa Filho | Ozonation and ozone-enhanced photocatalysis for VOC removal from air streams: Process optimization, synergy and mechanism assessment[END_REF]. Among the Advanced Oxidation Processes including chemical and microbiological contaminants, have been investigated by many researchers in recent years [START_REF] Rtimi | Microstructure of Cu-Ag Uniform Nanoparticulate Films on Polyurethane 3D Catheters: Surface Properties[END_REF][START_REF] Xie | Efficient photocatalytic oxidation of gaseous toluene over F-doped TiO2 in a wet scrubbing process[END_REF]. As a result, TiO2/UV-light oxidation was considered as a clean process for VOCs degradation with higher performance and complete mineralization [START_REF] Héquet | Photocatalytic oxidation of VOCs at ppb level using a closed-loop reactor: The mixture effect[END_REF][START_REF] Malayeri | Modeling of volatile organic compounds degradation by photocatalytic oxidation reactor in indoor air: A review[END_REF][START_REF] Shayegan | Photocatalytic oxidation of volatile organic compounds for indoor environment applications: Three different scaled setups[END_REF][START_REF] Rochetto | Degradation of volatile organic compounds in the gas phase by heterogenous photocatalysis with titanium dioxide/ultraviolet light[END_REF][START_REF] Boyjoo | A review on photocatalysis for air treatment: From catalyst development to reactor design[END_REF]. Innovative catalysts can be used for increasing bacterial inactivation and VOCs degradation: Antibacterial metals such as Ag and Cu [START_REF] Joshi | Efficient inactivation of Staphylococcus aureus by silver and copper loaded photocatalytic titanate nanotubes[END_REF][START_REF] Gupta | Ag and CuO impregnated on Fe doped ZnO for bacterial inactivation under visible light[END_REF][START_REF] Pham | Advanced removal of toluene in aerosol by adsorption and photocatalytic degradation of silver-doped TiO2/PU under visible light irradiation[END_REF][START_REF] Rao | Enhanced disinfection of Escherichia coli and bacteriophage MS2 in water using a copper and A c c e p t e d m a n u s c r i p t silver loaded titanium dioxide nanowire membrane[END_REF] coatings on semiconductors, usually titanium dioxide (TiO2), on glass fiber tissue [START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF][START_REF] Assadi | Isovaleraldehyde elimination by UV/TiO2 photocatalysis: comparative study of the process at different reactors configurations and scales[END_REF][START_REF] Assadi | Photocatalytic oxidation of Trimethylamine and Isovaleraldehyde in an annular reactor: Influence of the Mass Transfer and the relative humidity[END_REF][START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF], cellulosic paper [START_REF] Rochetto | Degradation of volatile organic compounds in the gas phase by heterogenous photocatalysis with titanium dioxide/ultraviolet light[END_REF] and polymer support [START_REF] Dhanasekar | Ambient light antimicrobial activity of reduced graphene oxide supported metal doped TiO2 nanoparticles and their PVA based polymer nanocomposite films[END_REF] have been studied for indoor air treatment

The Novelty of this work is to follow the degradation mechanism of contaminants (VOCs/bacteria mixture) using pilot scale continuous photocatalytic reactor. Butane-2,3-dione (C4H6O2), Heptan-2-one (C7H14O) (i) and Escherichia coli (E. coli) (ii) were chosen as a VOCs (i) and bacteria strain (ii), specifically found in indoor air of food industries. The second novelty is to use a specific technology of textile fibers (TiO2, TiO2-Cu and TiO2-Ag), suitable for chemical/microbiological applications. The aim of this configuration is to provide a new and compact process, and no external UV lamps needed. To our knowledge, this is the first investigation on this combination (catalyst-reactor-semi-real conditions) which has been carried out at pilot scale.

Materials and methods

Polluted-contaminated air generation

Two targeted compounds like Butane-2,3-dione (C4H6O2) and Heptan-2-one (C7H14O), belonging to ketone groups, were used. Butane-2,3-dione (99% purity) was purchased from Janssen Chimica, Belgium and Heptan-2-one of 98% purity, was supplied from Acros Organics, France. The objective of this study was to validate the photocatalysis process efficiency in a case closer to the real indoor air conditions.

Butane-2,3-dione and Heptan-2-one were selected following the building air analysis campaign in a food industry (Brittany region of France-June 2019). The exposure limit values of Butane-2,3-dione is around 11 mg.m -3 [START_REF]Valeurs limites d'exposition aux postes de travail 2016 : www[END_REF]. The polluted air flow was generated continuously by means of a syringe pump system (Kd Scientific Model 100, USA) in order to obtain a stable VOC concentration during the experiment (from 5 to 20 mg.m -3 ). To create a VOCs/air mixture, two similar injection systems are used for each pollutant [START_REF] Abou Saoud | Abatement of ammonia and butyraldehyde under non-thermal plasma and photocatalysis: Oxidation for the removal of mixture pollutants at pilot scale[END_REF][START_REF] Abou Saoud | Integrated process for the removal of indoor VOCs from food industry manufacturing: elimination of Butane-2,3-dione and Heptan-2-one by cold plasma-photocatalysis combination[END_REF][START_REF] Costa | Study of butyraldehyde degradation and by-products formation by using a surface plasma discharge in pilot scale: Process modeling and simulation of relative humidity effect[END_REF]. A static mixer at upstream ensures a proper mixing of air .

The (a)

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

Reactor design and catalyst form

The pilot reactor, with its new configuration, was constructed to operate in photocatalytic mode using a novel catalytic material constructed with Copper (Cu)/Silver (Ag)-based optical fiber, double-sided optical fiber with a metal wire (Figure 2. (c)), as performed by Brochier Technologies Company (UVtex®) [32]. The characteristics of Optical Fiber (OF) are presented in Table 2. The optical fiber (Figure 2. (a)) is composed of (i) polymer fibers (polyester, Trévira CS TM fibres) and

(ii) polyester fibers (polymethyl methacrylate, MMA CK-20 Eska TM fibres) and (iii) metallic wire. These elements are weaved following the Jacquard process meaning the interlacing, in a same plane, of fibers disposed in the direction of the chai n (textile fiber) and some fibers disposed perpendicular to chain fibers, in the direction of the weft (optical fibers and metallic wire). These optical fibers have a mean diameter of 500 µm, its core is made in polymethyl methacrylate resin with a 480 µm mean diameter and covered with 10 µm of thick fluorinated polymer [START_REF] Bourgeois | Characterization of a new photocatalytic textile for formaldehyde removal from indoor air[END_REF]. In the absence of micro texturing the surface of the optical fibers, light is transmitted from to the end of the fabric. Since this fabric is intended to be the UV irradiation source, the whole surface of the textile must be bright. Micro texturing is a necessary treatment allowing a uniform light output across the surface of the optical fibers [START_REF] Brochier | Nappe textile présentant des propriétés dépolluantes par photocatalyse[END_REF][START_REF] Brochier | [END_REF]. As described on our previous paper [START_REF] Assadi | Pilot scale degradation of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: investigation of competition and synergism[END_REF], the photocatalyst used is TiO2 Degussa P25. The textile fibers, with/without metal wires (Cu/Ag), are coated with titanium dioxide in a bath of a concentrated aqueous suspension of TiO2 (50 g L -1 ). The coating was carried out by dipping the optical fiber textile in the bath maintained at 70 • C during 1h. Then, the sample is dried during 1h at 70 • C. Luminous textile is coated by a first layer by silica to protect optical fiber aging. In absence of silica, the optical fiber could be attacked A c c e p t e d m a n u s c r i p t by photocatalysis. However, silica is chosen because it allows the passage of UV, and it is not an organic compound therefore is not degraded by photocatalysis [START_REF] Indermühle | Modelling of UV optical ageing of optical fibre fabric coated with TiO2[END_REF].

Photocatalytic analyses system 2.3.1. VOCs monitoring

Gas Chromatography (GC), fitted with a flame ionization detector (FID), was used to monitor the VOCs concentration at the reactor downstream. The analysis was carried out using a Clarus GC-500 chromatograph fitted with a 60 m 0.25 mm polar DB-MS capillary column (film thickness, 0.25 µm).. The inlet/outlet samples (500 μL) are injected via a gas syringe.

Microorganisms monitoring

The performance of E. coli inactivation by each photo-catalyst was monitored over time by using agar-based culture method. Firstly, E. coli samples were collected on a polycarbonate membrane filter (Millipore, France) in a closed cassette 35 mm in diameter by using a membrane pump (KNF lab N86k18, France) with a 300 L.h -1 flow (Figure 3). Secondly a desorption step of E. coli collected on membrane filter was realized using a sterile aqueous solution (Tween 80 0.01% Tryptone 0.1% H2O). The suspension consisting of bacteria was diluted in Tryptone medium and subsequently mixed thoroughly using a Vortex. 100 µL aliquots were plated on Tryptone Soya Agar (TSA) medium in Petri dishes. Then, all agar Petri dishes were incubated at 37°C for 24 h to allow bacterial growth.

Results and discussion

First, the photocatalytic removal of Butane-2,3-dione, Heptan-2-one alone and E. coli have been studied separately. Then, the main novelty: mixture of VOCs and E. coli study at pilot scale, was also performed. The effect of all photo-catalysts on the efficiency of oxidation was evidenced under real operating conditions (VOCs/E. coli concentration, flow rate and humidity). Finally, the results observed are discussed (i) -3 , results in a decrease of the degradation rate from 66% to 38%. Our previous studies confirm this behavior for any other VOCs or groups of VOCs either at pilot or at industrial scale [START_REF] Bouzaza | Photocatalytic degradation of some VOCs in the gas phase using an annular flow reactor: determination of the contribution of mass transfer and chemical reaction steps in the photo-degradation process[END_REF][START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF][START_REF] Assadi | Pilot scale degradation of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: investigation of competition and synergism[END_REF][START_REF] Zadi | Indoor air treatment of refrigerated food chambers with synergetic association between cold plasma and photocatalysis: Process performance and photocatalytic poisoning[END_REF][START_REF] Gharib-Abou Ghaida | Association of surface dielectric barrier discharge and photocatalysis in continuous A c c e p t e d m a n u s c r i p t reactor at pilot scale: Butyraldehyde oxidation, by-products identification and ozone valorization[END_REF][START_REF] Assadi | Photocatalytic oxidation of Trimethylamine and Isovaleraldehyde in an annular reactor: Influence of the Mass Transfer and the relative humidity[END_REF][START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF]. Therefore, photocatalysts containing metal wires e.g. Copper (Cu)/Silver (Ag) revealed a slight decrease compared to TiO2 alone in terms of Butane-2,3-dione degradation performance but remains adequate for photocatalytic application. Similarly, it is readily seen in Figure 3 & 4 that the performance of degradation decreased with very high air flow (in our case 12 m 3 .h -1 ). This higher level of flow condition causes a decrease of the contact time between the Butane-2,3-dione and the active sites of catalyst and consequently impede the interesting adsorption/oxidation on the photocatalytic sites. For example, it has been noted that increasing VOCs/air flow from 2 to 12 mg.m -3 , performance of oxidation showed a decrease from 55 to 33% in the case of TiO2 study alone. Several studies confirm these experiment observations like Trichloromethane [START_REF] Zadi | Treatment of hospital indoor air by a hybrid system of combined plasma with photocatalysis: Case of trichloromethane[END_REF], inorganic compounds (e.g. Ammonia) [START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF], ketones (e.g.

Heptan-2-one) [START_REF] Abou Saoud | Integrated process for the removal of indoor VOCs from food industry manufacturing: elimination of Butane-2,3-dione and Heptan-2-one by cold plasma-photocatalysis combination[END_REF], aldehydes (e.g. Butyraldehyde) [START_REF] Abou Saoud | Abatement of ammonia and butyraldehyde under non-thermal plasma and photocatalysis: Oxidation for the removal of mixture pollutants at pilot scale[END_REF] and related the slowdown in the performance. When VOCs inlet concentration is increased, due to the saturation and/or deactivation of the active sites by pollutants and /or oxidation by-products has been observed

This new configuration of material integrating photocatalyst (TiO2) and fiber optic lighting has also demonstrated its efficiency to remove the formaldehyde [START_REF] Bourgeois | Characterization of a new photocatalytic textile for formaldehyde removal from indoor air[END_REF]. A good distribution of light on catalyst surface through fiber filaments improves the photocatalytic activity. A similar behavior (with concentration and flow rate) was also observed in the case of Heptan-2-one.

A comparative study of Heptan-2-one and Butane-2,3-dione behavior's was exhibited in our previous work (Glass Fiber Tissue as catalyst) [START_REF] Abou Saoud | Integrated process for the removal of indoor VOCs from food industry manufacturing: elimination of Butane-2,3-dione and Heptan-2-one by cold plasma-photocatalysis combination[END_REF] and the removal In order to improve the performance of this photocatalytic reactor and achieve higher levels of VOC removal, it is interesting to investigate(i) the effect of the number of the catalyst plates and (ii) the effect of space/air gap between the plates.

Effect of VOC mixing on removal efficiency

In order to investigate the effect of (Butane-2,3-dione/Heptan-2-one) mixture on removal efficiencies, initial experiments were performed continuously with each VOCs alone and then with their equimolar mixture. Removal efficiencies were studied in the same operational conditions: input flow rate was set to 2 m 3 .h -1 with ~ 5% of humidity and initial concentration was 20 mg.m -3 for each VOCs. The objective of this experiments study with the combination TiO2/metallic wire was to validate the photocatalysis process efficiency for a simultaneous treatment (VOCs/bacteria) by using a specific catalyst exhibit promising antibacterial efficiency. Results of mixture study (with TiO2-Ag and with TiO2-Cu) are presented in Figure 5, interesting removal efficiency was obtained with TiO2-Cu followed by TiO2-Ag either for single or for mixture VOCs oxidation. First, it is readily seen (Figure 5) that oxidation performance of heptan-2-one remains low, removal efficiency ∼ 19% with TiO2-Cu, compared to Butane-2,3-dione, removal efficiency ∼ 41% with TiO2-Cu. As for mixture, a positive oxidation effect (under low intensity) is observed for Butane-2,3-dione experiments in the case of TiO2-Cu and TiO2-Ag but not obviously observed on Heptan-2-one. This trend clearly demonstrates that Heptan-2-one presents a long molecular chain (C7H14O), harder to degrade via photocatalytic oxidation. Moreover, in our previous work, a classical configuration of the reactor was used with a TiO2-GFT catalyst (Glass Fiber Tissue) and illuminated with external UV Lamp [START_REF] Abou Saoud | Integrated process for the removal of indoor VOCs from food industry manufacturing: elimination of Butane-2,3-dione and Heptan-2-one by cold plasma-photocatalysis combination[END_REF], oxidize by-products exhausts from C4H6O2/C7H14O mixture are identified. Indeed, for a classical A c c e p t e d m a n u s c r i p t configuration: the catalyst was illuminated with an external lamp. In this case, a glass annular reactor was used (2 concentric Pyrex cylinders, 100 cm length): the catalyst, Glass Fiber Tissue, is maintained on the inner reactor wall and the UV lamp (100 cm length) is placed in the inner concentric cylinder in order to have a homogeneous irradiation. The results showed that conventional by-products were created under photocatalytic treatment namely: Carbon dioxide, Acetone, Acetaldehyde, Acetic acid and Acid compounds (Formic acid, Propionic acid,…). In addition, the degradation pathway explaining what happens during the photocatalysis process has been added (Figure 6(a)). We suggest the following reactional scheme for Butane-2,3dione/Heptan-2-one degradation, where Acetone (C3H5O) and Propionic acid (C3H6O2) comes directly through Butanoic acid (C4H8O2), Pentanoic acid (C5H10O2) and Hexanoic acid (C6H12O2) and could dissociate into acetic acid (C2H4O2), acetaldehyde (C2H4O) and Formic acid (CH2O2). This last step (i) could be a common degradation pathway. Oxidation by-products (Figure 6 Butane-2,3-dione, (3) Heptan-2-one, (4) acetic acid, (5) formic acid, propionic acid, butanoic acid, pentanoic acid, hexanoic acid and Heptane2,6-dione.

Bacterial inactivation: case of Escherichia coli

Before oxidation experiments via specific catalysts (TiO2-Cu, TiO2-Ag), a preliminary test (with TiO2 alone) was conducted to verify its effectiveness. The experiment was performed in the same photocatalytic reactor, replacing a syringe pump system (VOCs) by a nebulizer system (E. coli), at a 1 m 3 .h -1 input flow and E. coli initial concentration was 3 10 3 CFU.mL -1 (Colony Forming Unit per milliliter). Escherichia coli can be encountered as air contaminant in food industry [START_REF] Whyte | Distribution and Prevalence of Airborne Microorganisms in Three Commercial Poultry Processing Plants[END_REF]. Bacterial inactivation under UV irradiation was then calculated as (Eq. 3):

(3)

where, A: antibacterial activity and CFUcatalyst inlet, CFUcatalyst outlet : number of CFU at the inlet and outlet of the photocatalytic reactor.

Results (Figure 7) showed that there is no inactivation of E. coli with TiO2 alone during our experiments with continuous configuration. As for TiO2-Cu and TiO2-Ag, experimental results were presented in Figure 7. First, it is noticeable that E. coli inactivation increase when the photo-catalyst tissue is woven with TiO2-Ag and TiO2-Cu. Performance of E. coli inactivation under UVA light was estimated at around zero in the case of TiO2 alone and in contrast to the other photo-catalysts, E. coli inactivation was 0.25 and 0.37 log CFU with TiO2-Ag and TiO2-Cu, respectively. This behavior indicates that the contact between Silver (Ag), Copper (Cu) and bacteria can play an important role to improve bactericidal activity [START_REF] Assadi | Isovaleraldehyde elimination by UV/TiO2 photocatalysis: comparative study of the process at different reactors configurations and scales[END_REF]. It was shown by Rtimi

The synergistic mechanism for the photocatalytic inactivation of bacteria (E. of the semiconductor (in our case TiO2). As for Silver, Ag ions escape from Silve R.STAT® fibre, enter the bacterial wall; destroy its cellular structure thus preventing bacteria from developing and multiplying. In order to show the photocatalysis stability, the reusability of catalyst (with TiO2-Cu and with TiO2-Ag) was studied (results on Figure 7). A stability of the performance of metal-fiber was confirmed after during the cyclic studies. In addition, experiment with (i) fibers alone without light and (ii) light alone without photocatalyst were also performed. Bacterial inactivation was negligible around 0.013 CFU (i) and < 0.01 CFU (ii). We observed, in our previous work [START_REF] Abidi | Bessais Simultaneous removal of bacteria and volatile organic compounds on Cu2O-NPs decorated TiO2 nanotubes: competition effect and kinetic studies[END_REF], a similar trend of bacterial removal efficiency in a batch reactor.

Combination treatment: VOC/microorganisms removal

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

The removal of specific pollutants/microorganisms (as Butane-2,3-dione and E. coli in our case) is one of the principal objectives of indoor air treatment. In our study, the idea was to investigate, for the first time, the performance of photocatalytic application for simultaneous removal of VOCs and E. coli at pilot scale. Combination experiments were conducted with Butane-2,3-dione (10 mg.m -3 ) and E. coli (3 10 3 CFU.ml -1 ) at dry conditions (air ~ 5% of Relative Humidity, RH) with a flow rate around 1 m 3 .h -1 . The pollutant degradation over time was monitored as well as the bacterial inactivation (Figure 8). First, three experiments were performed by using various catalysts (TiO2, TiO2-Ag and TiO2-Cu). Results show an interesting efficiency in the case of TiO2-Ag and TiO2-Cu, indicating that these two metals oxides (in our case Silver and Copper) seem to be efficient for bacteria inactivation, which is attributable mainly to a damage of bacteria protein by the illumination of the photocatalyst surface [START_REF] Li | Antibacterial activity of TiO2 nanotubes: influence of crystal phase, morphology and Ag deposition[END_REF][START_REF] Abidi | Photocatalytic indoor/outdoor air treatment and bacterial inactivation on CuxO/TiO2 prepared by HiPIMS on polyester cloth under low intensity visible light[END_REF][START_REF] Liu | A progressive approach on inactivation of bacteria using silver-titania nanoparticles[END_REF]. However, in the case of TiO2-Ag, higher performance of Butane-2,3-dione removal (47%) was observed and in contrast to the E. coli inactivation was only around 0.25 CFU. As for TiO2-Cu, results exhibit simultaneously (i) higher E. coli inactivation (1.27 CFU) and (ii) higher Butane-2,3-dione removal (52%). In our case, (i) this could be explained by the impact of by-products resulting from the degradation of VOCs on the behavior of E.coli. The LED-UVA intensity applied to the lamp is low (only 1.5 W m -2 ) and in view of this value (low-cost), (ii) a 52% VOC removal is an interesting result. Thus, a compromise must exist between the removal rate of the target molecule and the inactivation rate of bacteria with a metallic wire. On the other hand, the inactivation value was always zero via TiO2 alone, as shown in Figure 8. Thus, by comparing the inactivation efficiency obtained in the case of E. coli alone by photocatalytic process (Figure 7) to the results obtained in the case of E. coli/VOCs mixture (Figure 8). It is readily seen that the E.

A c c e p t e d m a n u s c r i p t coli inactivation is not influenced by the presence of VOCs. For example, we note that the inactivation of E. coli alone was around 0.25 log CFU (in the case of TiO2-Ag, Figure 7) and reaches 0.29 log CFU (in the case of E. coli/VOCs mixture, Figure 8).

In this case, we can indicate also that the competition reactions of photo-catalyst between bacteria, VOCs and VOCs by-products are negligible. Moreover, E. coli inactivation kinetics as well as Butane-2,3-dione degradation (Figure 9) are seen to be more dependent on the humidity level of air stream.

The same methodology as the first one (combined simultaneous Butane-2,3-dione and E. coli with dry input air) was conducted but with humid air. However, a result (in the case of TiO2-Cu) shows that the efficiency of removing/destroying contaminants (under low UVA-LED light intensity) was lowered by increasing Relative Humidity from dry (~ 5% of RH) to wet air (~ 70% of RH). It is observed that when the water vapor amount is higher (~70%), the effect of the competitive adsorption/oxidation phenomena between VOCs, H2O and E. coli becomes more important which causes the VOCs/E. coli removal decrease [START_REF] Abou Saoud | Integrated process for the removal of indoor VOCs from food industry manufacturing: elimination of Butane-2,3-dione and Heptan-2-one by cold plasma-photocatalysis combination[END_REF][START_REF] Abou Saoud | Synergism between non-thermal plasma and photocatalysis: Implications in the post discharge of ozone at a pilot scale in a catalytic fixed-bed reactor[END_REF][START_REF] Huang | Controlling Indoor Bioaerosols Using a Hybrid System of Ozone and Catalysts[END_REF][START_REF] Sánchez | Photocatalytic elimination of indoor air biological and chemical pollution in realistic conditions[END_REF]. However, indoor air in the food industry is usually humid (around 75-95% of humidity). As already noted that this study deals with indoor air treatment in simulated real conditions and which is why we studied a high level of humidity (~70%). Butane-2,3-dione removal and E. coli inactivation behavior were 52% and 1.27 log CFU (with dry flow), respectively and decreased to become 38% and 0.73 log CFU after the addition of water vapor into the photocatalytic reactor.

TiO2-Cu (dry air ~ 5% of RH) TiO2-Cu (wet air ~ 70% of RH)

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

Catalytic performance with real indoor air conditions

In order to confirm the good performance observed for destroying Escherichia coli

Conclusion

This new work aims to study the performance of photocatalytic application for the degradation/inactivation of contaminants (e.g. chemical and microbiological pollution) from indoor air (as Butane-2,3-dione/Heptan-2-one and Escherichia coli in our case).

The selection of photo-catalyst that will be used simultaneously for VOCs and bacteria removals represents one of the key parameters towards efficiency of photocatalysis. Three new configurations of photo-catalyst (TiO2, TiO2-Ag and TiO2-Cu) were selected and their effect regarding oxidation performance was evaluated under a continuous system. Copper (Cu)/Silver (Ag) wires exhibit promising

Figure 1 .

 1 Photocatalytic textile

Figure 1 .

 1 Figure 1. (a) shows of the image of the photocatalytic planar reactor. The reactor

Figure 2 .

 2 Figure 2. SEM images of the textile fiber surface: (a) and (b) SEM images of the textile fiber

  Figure 3 and 4 show the evolution of Butane-2,3-dione removal according to [Butane-

  A c c e p t e d m a n u s c r i p t 2,3-dione]Inlet from 5 to 20 mg.m

A

  c c e p t e d m a n u s c r i p t efficiency would be much more remarkable in the case of Butane-2,3-dione which is why presently we are working with Butane-2,3-dione alone.

Figure 3 .

 3 Figure 3. Variation of Butane-2,3-dione degradation (%) with inlet concentration (mg m -3 ) for diff erent photo-catalysts (1 mg cm -2 TiO2, 0.4% Ag, pure Cu). Operational conditions: T=20°C, LED-UVA intensity= 1.5 W m -2 , Flow rate= 2 m 3 .h -1 , dry air ~ 5% of RH.

Figure 4 .

 4 Figure 4. Variation of Butane-2,3-dione degradation (%) at different flow rates (m 3 .h -1 ) with

Figure 5 .Figure 6 .

 56 Figure 5. Variation of the removal efficiency of Butane-2,3-dione (C4H6O2) alone, Heptan-2-

A c c e p t e d m a n u

  s c r i p t et al. that Silver and Copper accelerated bacterial inactivation due to its disinfecting properties[START_REF] Rtimi | Advances in catalytic/photocatalytic bacterial inactivation by nano Ag and Cu coated surfaces and medical devices[END_REF]. The efficiency of E. coli inactivation under photocatalytic treatment is attributed to the oxidative damage mainly induced by the photogenerated reactive oxygen species/radicals ROS (e.g. O2 °-, H2O2 and HO°)[START_REF] Verdier | Antibacterial Activity of TiO2 Photocatalyst Alone or in Coatings on E. coli: The Influence of Methodological Aspects[END_REF][START_REF] Xiong | Bifunctional photocatalysis of TiO2/Cu2O composite under visible light: Ti3+ in organic pollutant degradation and water splitting[END_REF][START_REF] Joshi | Efficient inactivation of Staphylococcus aureus by silver and copper loaded photocatalytic titanate nanotubes[END_REF], produced by redox reactions between adsorbed species (e.g. water vapor and oxygen) and electrons/holes photo-generated.Xiong et al. and Joshi et al. showed that the light irradiation leads to generate the ROS and this may improve bacteria inactivation[START_REF] Xiong | Bifunctional photocatalysis of TiO2/Cu2O composite under visible light: Ti3+ in organic pollutant degradation and water splitting[END_REF][START_REF] Joshi | Efficient inactivation of Staphylococcus aureus by silver and copper loaded photocatalytic titanate nanotubes[END_REF]. The reaction mechanism of Ag-NPs and Cu-NPs, under light,[START_REF] Abidi | Photocatalytic indoor/outdoor air treatment and bacterial inactivation on CuxO/TiO2 prepared by HiPIMS on polyester cloth under low intensity visible light[END_REF][START_REF] Liu | A progressive approach on inactivation of bacteria using silver-titania nanoparticles[END_REF] can affect bacteria activity, as shown in Eqs. 4-5, under ROS (Eqs.6-9).

  coli) from indoor air in real condition (i) and the feasibility of simultaneous photo-oxidation of VOCs (Butane-2,3-dione & Heptan-2-one) is suggested in section 3.4 below. VOCs, VOCs by-products and E. coli suggested be adsorbing and/or oxidizing via ROS generated via Cu/Ag-doped TiO2, under UVA irradiation. By heating Cu as a metal in air, Cu Oxide I and II are formed and consequently the formation of copper oxides can be formed during the fabric drying step following the step of the surface modified

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

Figure 7 .

 7 Figure 7. Bacterial inactivation (E. coli) via photocatalytic application with different photocatalysts: TiO2, TiO2-Cu and TiO2-Ag. Operational conditions: T=20°C, Q= 1 m 3 .h -1 , [VOCs]= 0 mg.m -3 , [E. coli] = 3 10 3 CFU.ml -1 , LED-UVA intensity= 1.5 W m -2 , dry air ~ 5% of RH.

Figure 8 .

 8 Figure 8. Bacterial inactivation (E. coli) and Butane-2,3-dione degradation using TiO2,TiO2-Ag and TiO2-Cu as catalyst, with dry air. Operational conditions: T=20°C, Q= 1 m 3 .h -1 , [E. coli] = 3 10 3 CFU.ml -1 , [C4H6O2] = 10 mg.m -3 , LED-UVA intensity= 1.5 W m -2 , RH= 5%.

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

Figure 9 .

 9 Figure 9. Bacterial inactivation (E. coli) and Butane-2,3-dione degradation using TiO2-Cu as catalyst, with dry and wet air. Operational conditions: T=20°C, Q= 1 m 3 .h -1 , [E. coli] = 3 10 3 CFU.ml -1 , [C4H6O2] = 10 mg.m -3 , LED-UVA intensity= 1.5 W m -2 , RH= 5% and 70%.

(

  E. coli) under UVA-LED light in the presence of organic compounds (in our case Butane-2,3-dione), another experiences, with simulated real indoor air were performed. Here, a photocatalytic treatment with mixture of VOCs (Heptan-2-one) and E. coli, were performed. The photocatalytic reactor was continuously supplied with 1 m 3 .h -1 of a dry air (RH∼5%). The initial concentrations were: 3 10 3 CFU.ml -1 of the E. coli, 10 mg.m -3 of the Butane-2,3-dione and 13.25 mg m -3 of the Heptan-2-one. This value reflects the Escherichia coli concentration in real conditions (Building indoor air analysis campaign in a food industry (Brittany region of France-June 2019)). Regarding E. coli, with TiO2-Cu under UVA-LED (1.5 W m -2 ), it can be noted that the bacterial inactivation was around 0.37 for single oxidation (E. coli alone), reaches 1.27 and 1.77 for Butane-2,3-dione/E. coli and Heptan-2-one/E. coli, respectively (Figure10). The reaction medium, combining simultaneous UVA-LED, TiO2-Copper, VOCs and intermediate by-products, seem to be a powerful disinfectant for bacteria. About organic compounds behavior, a positive effect on oxidation performance was observed for both VOCs. Performance of VOCs degradation increases from 47% (Butane-2,3-dione) and 39% (Heptan-2-one), for single oxidation (VOCs alone), to 52% (Butane-2,3-dione) and 50% (Heptan-2-one) when both VOCs and bacteria mixed together. Indeed, the objective of this experiments study was to validate the photocatalysis process efficiency in a case closer to the real indoor air conditions where several pollutants/bacteria must be eliminated simultaneously and to see if there is always improvement of pollutants/bacteria removal in the case of mixing. Thus, in the best of cases (mixture target molecule/bacteria), a compromise must exist between the removal rate of the target molecule and the inactivation rate

A c c e p t e d m a n u

  s c r i p t of bacteria, and this is what was obtained during the photocatalysis, from which we concluded this promising technology for real applications.

Figure 10 .

 10 Figure 10. Bacterial inactivation (E. coli) and VOCs degradation using TiO2-Cu as catalyst. Operational conditions: T=20°C, Q= 1 m 3 h -1 , [E. coli] = 3 10 3 CFU.ml -1 , [VOCs] = 2.6 ppm, LED-UVA intensity= 1.5 W m -2 , dry air ~ 5% of RH.

Figure 11 .

 11 Figure 11. Suggested mechanism for E. coli inactivation and VOCs degradation under

A c c e p t e d m a n u s c r i p t antibacterial efficiency and

  photocatalytic activity either for VOCs and/or for E. coli. Simultaneously to bacterial

Table 1 .

 1 Oxidation technologies for VOC/bacteria removal (advantages and disadvantages).

Table 2 .

 2 Material specifications of photocatalyst.

	metal/weaving	Surface illuminating Allowable bending	OF
			radius of OF	diameter
	 -Textile wire of pure copper (Cu)
	 -Textile wire with silver (0.4% Ag)	Width (100 mm)	9 mm	500 µm
	 -Bath coating of titanium dioxide (50 g L -1 )	Length (800 mm)