Joseph F Moore

Erwan Paineau

Pascale Launois

Milo S P Shaffer

Continuous Binder-Free Fibers of Pure Imogolite Nanotubes

Keywords: nanotube, fiber, imogolite, alignment, humidity, jamming, spinning

Imogolite nanotubes display a range of useful properties and provide an ideal material system to study the assembly of nanomaterials into macroscopic fibers. A method of wet spinning pure, binder-free imogolite fibers has been developed using double-walled germanium imogolite nanotubes. Nanotube aspect ratio can be controlled during the initial synthesis and is critical to the spinning process. Fibers made from short nanotubes (<100 nm) have very low gel-strengths, whilst dopes with longer nanotubes (500-1000 nm) are readily spinnable.

The tensile behaviour of the resulting imogolite nanotube fibers is strongly influenced by relative humidity (RH), with a modulus of 30 GPa at 10% RH compared to 2.8 GPa at 85% RH, as well as a change in failure mode. This result highlights the importance of 2 inter-nanotube interactions in such assemblies and provides a useful strategy for further exploration. Interestingly, in the absence of a matrix phase, a degree of misorientation appears to improve load transfer between the individual INTs within the porous fiber, likely due to an increase of the number of inter-particle contacts. Imogolite nanotubes are an appealing analogue to other nanotube fiber systems, and it is hoped that learnings from this system can also be used to improve carbon nanotube fibers.

INTRODUCTION

Nanotubes and nanorods are promising candidates for producing high performance fibers, due to their excellent intrinsic mechanical properties and compatible 1D morphology.

However, the mechanical properties of macroscale fibers are typically significantly lower than those of the constituent nanomaterials, due to a combination of processing challenges and weak inter-molecular interactions. To date, most studies have focused on carbon nanotube (CNT) based fibers, using both dry and wet spinning methods; 1-4 fiber properties have typically been improved by increasing the aspect ratio and alignment of the nanotubes.

CNT processing has proved particularly challenging given their poor solubility in convenient solvents, and tendency to agglomerate. Understanding and optimizing these spinning processes is generally hindered by the difficulty in measuring CNT orientation within the fibers. Strong light absorbance limits the use of optical methods, whilst polydispersity, irregular packing, and weak scattering makes it challenging to study single filament samples with X-rays efficiently. [START_REF] Mikhalchan | A Perspective on High-Performance CNT Fibres for Structural Composites[END_REF] Imogolite nanotubes (INTs) are an appealing analogue for CNTs due to their monodisperse diameter and spontaneous dispersion in deionized water, which enables simple solution processing. [START_REF] Paineau | Imogolite Nanotubes: A Flexible Nanoplatform with Multipurpose Applications[END_REF] The archetypal INT is single-walled and comprises a curved outer gibbsite Al(OH) 3 sheet inside which isolated O 3 SiOH tetrahedra are located. [START_REF] Cradwick | Imogolite, a Hydrated Aluminium Silicate of Tubular Structure[END_REF][START_REF] Paineau | Nanomaterials from Imogolite: Structure, Properties and Functional Applications[END_REF] However, a variety of INTs can be synthesized via sol-gel methods using different precursors, particularly ones with Ge substituting for Si, [START_REF] Konduri | Controlling Nanotube Dimensions: Correlation between Composition, Diameter, and Internal Energy of Single-Walled Mixed Oxide Nanotubes[END_REF] in either double-walled or single-walled form. [START_REF] Maillet | Evidence of Double-Walled Al-Ge Imogolite-Like Nanotubes. A Cryo-TEM and SAXS Investigation[END_REF][START_REF] Thill | Physico-Chemical Control over the Single-or Double-Wall Structure of Aluminogermanate Imogolite-Like Nanotubes[END_REF] Similarly to solutions of CNT polyelectrolytes, [START_REF] Jiang | Increased Solubility, Liquid-Crystalline Phase, and Selective Functionalization of Single-Walled Carbon Nanotube Polyelectrolyte Dispersions[END_REF][START_REF] Davis | True Solutions of Single-Walled Carbon Nanotubes for Assembly into Macroscopic Materials[END_REF][START_REF] Kleinerman | Direct Imaging of Carbon Nanotube Liquid-Crystalline Phase Development in True Solutions[END_REF] INTs form liquid crystalline mesophases which can be reoriented either by electrical or flow fields, [START_REF] Paineau | A Liquid-Crystalline Hexagonal Columnar Phase in Highly-Dilute Suspensions of Imogolite Nanotubes[END_REF][START_REF] Paineau | Role of Initial Precursors on the Liquid-Crystalline Phase Behavior of Synthetic Aluminogermanate Imogolite Nanotubes[END_REF] potentially leading to highly aligned fibers.

In contrast to CNTs, INT solutions are optically transparent which greatly aids the use of polarized optical microscopy (POM) in their characterization. Additionally, Al and Ge atoms have much greater X-ray scattering cross-sections than carbon, which facilitates the use of lab-source X-rays for studying the structure of both dispersions and macroscale fiber assemblies. The hydrophilicity of INTs also enables the tuning of nanotube interactions by varying humidity. INTs have been shown to adsorb up to 80% of their weight in water at 95% relative humidity (RH) [START_REF] Ohashi | Characterization of Synthetic Imogolite Nanotubes as Gas Storage[END_REF] with the water adsorbed in the internal cavity, interstitially between the walls of double-walled nanotubes and crucially on the outer surface. [START_REF] Liao | Water Adsorption in Single-and Double-Walled Inorganic Nanotubes[END_REF] In principle, by varying the humidity, the strength of inter-nanotube hydrogen bonding should be modified, enabling the influence of shear interactions on the mechanical properties of nanotube-based fibers to be studied. [START_REF] Li | Organic-Inorganic Hybrid Films Fabricated from Cellulose Fibers and Imogolite Nanotubes[END_REF][START_REF] Benitez | Humidity and Multiscale Structure Govern Mechanical Properties and Deformation Modes in Films of Native Cellulose Nanofibrils[END_REF] In addition to their appeal as a model system, INTs are increasingly of interest in their own right, with potential applications in molecular filtration, catalysis and molecular transport and release. [START_REF] Paineau | Imogolite Nanotubes: A Flexible Nanoplatform with Multipurpose Applications[END_REF] Whilst INTs have lower absolute mechanical properties than CNTs, the strength and stiffness are still significant, with predicted elastic modulus between 100 and 400 GPa. [START_REF] González | Mechanical Response of Aluminosilicate Nanotubes under Compression[END_REF][START_REF] Da Silva | Structural, Electronic, and Mechanical Properties of Inner Surface Modified Imogolite Nanotubes[END_REF][START_REF] Lourenço | Nanotubes with Well-Defined Structure: Single-and Double-Walled Imogolites[END_REF][START_REF] Liu | Influence of Interwall Interaction in Double-Walled Aluminogermanate Nanotubes on Mechanical Properties[END_REF] More generally, they have complementary properties to CNTs in electrical conductivity, color and hydrophilicity. Nanocomposite and hybrid films of INTs with polymers and biomaterials have previously been produced [START_REF] Kang | Solution-Processed Ultrathin Aluminosilicate Nanotube-Poly(Vinyl Alcohol) Composite Membranes with Partial Alignment of Nanotubes[END_REF][START_REF] Kang | Single-Walled Aluminosilicate Nanotube/Poly(Vinyl Alcohol) Nanocomposite Membranes[END_REF][START_REF] Ma | Application of Imogolite Clay Nanotubes in Organic-Inorganic Nanohybrid Materials[END_REF] and recently continuous INT-polyvinyl alcohol composite fibers have been prepared which show a degree of self-healing at remarkably high absolute strengths. [START_REF] Lee | Inorganic Nanotube Mesophases Enable Strong Self-Healing Fibers[END_REF] However, as yet, no macroscale fibers have been reported from pure INTs. This study, therefore, investigates the possibility of using wet spinning to create pure, binder-free imogolite nanotube fibers and explores the effects of nanotube length, alignment, and hydration on fiber processability and mechanical properties.

SYNTHESIS OF DW Ge-INTs

It is challenging to spin fibers from nanomaterials such as CNTs and nanocellulose, if the aspect ratio is too low. [START_REF] Tsentalovich | Influence of Carbon Nanotube Characteristics on Macroscopic Fiber Properties[END_REF][START_REF] Clemons | Nanocellulose in Spun Continuous Fibers: A Review and Future Outlook[END_REF] Therefore, in order to enhance spinnability in this work, the INT synthesis was developed to provide high aspect ratio feedstocks. Compared to the conventional synthesis route using sodium hydroxide, [START_REF] Farmer | Synthesis of Imogolite: A Tubular Aluminium Silicate Polymer[END_REF] the in-situ production of hydroxyl ions by thermal decomposition of urea produces significantly longer double-walled germanium imogolite nanotubes (DW Ge-INTs) after 5 days of reaction; the inner and outer diameters remain consistently 1.6 nm and 4.3 nm, respectively. [START_REF] Amara | Single-Step Formation of Micron Long (OH) 3 Al 2 O 3 Ge(OH) Imogolite-Like Nanotubes[END_REF] Although several studies have been devoted to the mechanisms of nanotube growth during synthesis, [START_REF] Du | Insights into the Formation Mechanism of Imogolite from a Full-Range Observation of its Sol-Gel Growth[END_REF][START_REF] Yang | Growth Mechanism of Synthetic Imogolite Nanotubes[END_REF] INT-5d is dominated by short nanotubes (<100 nm). With further incubation, these nanotubes were consumed to produce a bimodal distribution with average lengths centred at 150 nm and 600 nm. Beyond 20 days incubation, little further change to the distribution was identified. Oriented attachment (OA) of formed nanotubes has previously been proposed as a key mechanism of INT growth after consumption of the initial precursors and topological rearrangment of protoimogolite nanostructures. [START_REF] Du | Insights into the Formation Mechanism of Imogolite from a Full-Range Observation of its Sol-Gel Growth[END_REF][START_REF] Yucelen | A Generalized Kinetic Model for the Formation and Growth of Single-Walled Metal Oxide Nanotubes[END_REF] However, this growth mechanism is expected to occur extremely slowly at larger nanotube lengths due to the dependency of tip-tip collision frequency, explaining the lack of meaningful further growth after 20 days. [START_REF] Maillet | Growth Kinetic of Single and Double-Walled Aluminogermanate Imogolite-Like Nanotubes: An Experimental and Modeling Approach[END_REF] IR spectroscopy indicates no significant difference in the chemical bonding within the INTs for the different reaction periods (Figure S1).

SPINNING DW Ge-INT FIBERS

Binder-free imogolite fibers were spun by injecting an aqueous spinning dope solution of DW Ge-INTs (9 g/L) through a needle into a coagulation bath of calcium chloride in water (300 g/L). The spinning dopes exhibited strong birefringence (Figure S2), indicative of an ordered mesophase observed previously in both carbon and imogolite nanotube dispersions, [START_REF] Paineau | Role of Initial Precursors on the Liquid-Crystalline Phase Behavior of Synthetic Aluminogermanate Imogolite Nanotubes[END_REF][START_REF] Paineau | Colloidal Stability of Imogolite Nanotube Dispersions: A Phase Diagram Study[END_REF][START_REF] Chang | Liquid Crystallinity of Carbon Nanotubes[END_REF] which should be ideal for liquid crystalline wet spinning. The high ionic strength of the coagulant (I = 8.1 M) enabled quick gelation of the spinning dope. At slow injection rates, a transparent gel monolith formed at the tip of the needle. Increasing the injection velocity led to the formation of a gel-like proto-fiber that could be manipulated and then collected on a winding wheel to provide draw during the spinning process (Supplementary Movie 1). The fracture strength of the proto-fibers, and thus their ease of manipulation, depended strongly upon the length distribution of the nanotube feedstock. The longer nanotube feedstocks (12d-40d) could be easily spun, whereas frequent filament breakage occurred with the INT-5d feedstock. It was observed that the filament breakage consistently occurred at the coagulant/air interface, indicating that the surface tension of the coagulant liquid imparts sufficient force to overcome the gel strength of the fiber. The stress in a cylindrical fiber being withdrawn from a liquid is inversely proportional to its radius (Figure S3). Thus, increasing the radius of the gel fiber should reduce the probability of filament breakage. In accordance with this theory, the INT-5d feedstock was found to be unspinnable using a 24 gauge needle (internal diameter, ID 311 m) but spinnable using a 21 shown fracture strengths of 20 kPa and 100 kPa respectively. [START_REF] Park | Design and Characterization of Hybrid Hydrogels Composed of Imogolite Fibrous Nanotubular Clay and Hyaluronic Acid[END_REF] Unlike polymers, the rigid rod INT system is held together only by friction and jamming, rather than molecular entanglement.

After washing in deionized water and drying in ambient conditions for 48 hours, the resulting INT fibers were robust and readily handled with an average diameter of ~20 m, textured surfaces and irregular cross-sections typical of nanomaterial fibers [START_REF] Jiang | Macroscopic Nanotube Fibers Spun from Single-Walled Carbon Nanotube Polyelectrolytes[END_REF] (Figure 2a-c and S4). Energy dispersive X-ray spectroscopy indicates that the dry fibers are mostly comprised of aluminium, germanium and oxygen with a small residue of calcium (3 at%) and chlorine (6 at%) remaining after the washing procedure (Figure S4). As expected from the Ge-INT structural formula (GeAl 2 O 7 H 4 ), the measured atomic ratio of aluminium to germanium was 2:1. Although, it is challenging to image the internal structure by SEM, due to the low electrical conductivity and small INT diameter, the fibre microstructure appears uniform with no evidence of a skin-core texture. The packing density of the fibers can be estimated from the dope concentration, needle area, draw ratio and fiber cross-sectional area and density of a DW Ge-INT (Supplementary Note 1). This estimate yields packing densities around 45-55%, similar to that of floating catalyst chemical vapor deposition CNT fibers (~35-60%), [START_REF] Khoshnevis | Effect of Alignment and Packing Density on the Stress Relaxation Process of Carbon Nanotube Fibers Spun from Floating Catalyst Chemical Vapor Deposition Method[END_REF] but lower than highly optimized wet spun CNT fibers (~75%). X-ray scattering (XRS) patterns were used to quantify the orientation within the fibers (typical example in Figure 3a); in these pure fibers, all the observed scattering features can be attributed to the INTs. The intensity scattered by a single nanotube is located in reciprocal planes perpendicular to its long axis, at , where is the period of the nanotube atomic structure along its long axis (T≈8.5Å) and is an integer. In these patterns the scattering features from the fibers are modulated angularly with the scattering signal located on the equator of the fiber scattering pattern and the (002) feature centered in the fiber direction, indicating the preferred orientation of the nanotubes along the fiber axis. [START_REF] Lee | Inorganic Nanotube Mesophases Enable Strong Self-Healing Fibers[END_REF] The intensity on the plane (Figure 3b and S7) exhibits oscillations characteristic of the squared form factor of DW Ge-INTs. [START_REF] Amara | Single-Step Formation of Micron Long (OH) 3 Al 2 O 3 Ge(OH) Imogolite-Like Nanotubes[END_REF] Although the broad modulation around 2.6 nm -1 is flattened, the lack of sharp Bragg peaks indicates that any local structural organization is limited to bundles of a small size. [START_REF] Amara | Hexagonalization of Aluminogermanate Imogolite Nanotubes Organized into Closed-Packed Bundles[END_REF][START_REF] Paineau | Effect of Ionic Strength on the Bundling of Metal Oxide Imogolite Nanotubes[END_REF] The formation of larger, more ordered bundles is presumably hindered during the fast coagulation process, given the relatively large size and mass of the INTs (~2-17 GDa), compared to typical polymer molecules which may crystallize. The angular distribution of the signals is used to quantitatively determine the orientation of nanotubes within the fibers. [START_REF] Lee | Inorganic Nanotube Mesophases Enable Strong Self-Healing Fibers[END_REF][START_REF] Pichot | Structural and Mechanical Properties of Single-Wall Carbon Nanotube Fibers[END_REF][START_REF] Alemán | Strong Carbon Nanotube Fibers by Drawing Inspiration from Polymer Fiber Spinning[END_REF] The angular distribution of scattering intensity in reciprocal space fits well to a Lorentzian distribution (Figure 3c and S8) which, when transformed to an orientation distribution function (ODF) in direct space, results in a Lorentzian function to the power 1.5. [START_REF] Lee | Inorganic Nanotube Mesophases Enable Strong Self-Healing Fibers[END_REF] The Hermans order parameter, or , is used to characterise orientation between perfectly uniaxial ( or randomly oriented (

; it was calculated from the direct space ODF as . Calculated values range from 0.49 to 0.74 for the eight fiber samples studied by XRS (Table S1).

Measurements performed at varying positions along the length of a single fiber section indicates that the degree of alignment is constant within each section, and the differences in orientation between samples are likely to arise from the manual handling of each fiber section. elements such as cellulose nanofibers (tensile strength ~10-500 MPa, elastic modulus ~2-30 GPa [START_REF] Clemons | Nanocellulose in Spun Continuous Fibers: A Review and Future Outlook[END_REF][START_REF] Siró | Microfibrillated Cellulose and New Nanocomposite Materials: A Review[END_REF] ), and to carbon nanotube fibers assembled from surfactant dispersions, [START_REF] Poulin | Films and Fibers of Oriented Single Wall Nanotubes[END_REF] but lower than highly optimized, denser carbon nanotube fibers spun from liquid crystalline mesophases which can have tensile strengths exceeding 4 GPa. In addition to the increased intrinsic strength and modulus, the significantly higher strengths of the optimized CNT fibers may be due to the much larger aspect ratio of the nanotubes. The best performing CNTs 49 had an aspect ratio around 6700, while the INTs in this work had an aspect ratio of ~200. An approximately linear relationship between fiber tensile strength and CNT aspect ratio has been reported previously, [START_REF] Tsentalovich | Influence of Carbon Nanotube Characteristics on Macroscopic Fiber Properties[END_REF] as might be expected below the critical stress transfer length in a discontinuous nanotube assembly. [START_REF] Henry | Semi-Analytical Simulation of Aligned Discontinuous Composites[END_REF] In this regime, the fibers fail by shear slip of nanotubes, which is illustrated by electron micrographs of fracture surfaces showing a 'finger-like' pull-out structure (Figure 4b). This fibrillar failure, due to nanotube slip, has previously been noticed in carbon nanotube fibers with a similar packing density. The importance of shear interactions in determining the fiber mechanical properties is illustrated by the effect of humidity (Figure 5a). The modulus, in particular, is strongly dependent upon the humidity during the test, with a modulus of 30 GPa at 10% RH compared In order to explore the scatter in fiber properties in more detail, a larger number of tensile samples were tested for INT-20d. The stress-strain curves exhibit the typical shape seen in nanomaterial fibers. There is a short initial take-up related to the straightening of fiber followed by an elastic region and then plastic deformation characteristic of inter-nanotube slip, [START_REF] Park | Mathematical Model for the Dynamic Mechanical Behavior of Carbon Nanotube Yarn in Analogy with Hierarchically Structured Bio-Materials[END_REF] indicated by a prominent 'elbow' at around 25 MPa in most cases (Figure 6a). Perhaps surprisingly, higher degrees of alignment correlate strongly with lower elastic modulus and a higher strain-to-failure (Figure 6b,c). This result initially seems to be counter-intuitive, (c) strain-at-failure as a function of Hermans order parameter However, this trend may be explained by considering the microstructure and load transfer within these relatively porous fibers, in the absence of any matrix. The fibers are comprised of stiff, straight rods which have a low interfacial shear strength. In highly aligned fibers, load is transferred between the nanotubes by relatively mobile point contacts, which can slide without arrest, leading to ductile behaviour and a higher strain-to-failure. However, in less aligned packings of high aspect ratio rods, load can also be supported by mechanical interlocking which occurs through a jamming transition when the number of independent contacts per rod exceeds a critical value of around ten. [START_REF] Philipse | The Random Contact Equation and Its Implications for (Colloidal) Rods in Packings, Suspensions, and Anisotropic Powders[END_REF][START_REF] Wouterse | On Contact Numbers in Random Rod Packings[END_REF] This mechanism of load transfer has been observed in systems across a variety of length scales including bird nests, bamboo skewers and colloidal gels. [START_REF] Weiner | Mechanics of Randomly Packed Filaments-the "Bird Nest" as Meta-Material[END_REF][START_REF] Solomon | Microstructural Regimes of Colloidal Rod Suspensions, Gels, and Glasses[END_REF][START_REF] Blouwolff | The Coordination Number of Granular Cylinders[END_REF] As the number of contacts between rigid rods increases with increased misorientation, [START_REF] Toll | Note: On the Tube Model for Fiber Suspensions[END_REF] it is expected that jamming occurs more frequently in the less aligned INT fibers and leads to more efficient stress transfer within the fiber and hence a higher modulus. Deformation of these fibers may be considered as analogous to the shear of granular assemblies of frictional rigid rods. Such assemblies are modelled to undergo extensive shear alignment in order to reduce inter-particle contact and reduce the stress required for deformation. [START_REF] Guo | Granular Shear Flows of Flat Disks and Elongated Rods without and with Friction[END_REF] As the solid volume fraction of these models increases above 50%, a rapid increase in shear stress occurs due to mechanical percolation through the formation of rigid clusters. We propose that the INT fibres are in this mechanical percolation regime, with a percolation threshold that increases with alignment. [START_REF] Rahatekar | Modelling Percolation in Fibre and Sphere Mixtures: Routes to More Efficient Network Formation[END_REF] At a constant volume fraction, within this regime, less oriented systems are more networked. The granular modelling also highlights the importance of friction in determining the extent of network formation, which here correlates with the effects of humidity on mechanical response.

The orientational dependence is more manifest in these INT fibers than in analogous CNTs or nanocellulose fibers, due to the comparably short aspect ratio and high rigidity of the INTs. The individual INTs are less able to bend and form the necessary contacts for frictional load transfer, and do not kink or fibrillate in the manner of CNT and nanocellulose bundles.

The coagulation / gelation process delivers porous fibres near the percolation threshold, and as a result, the jamming behaviour is most important to the fiber mechanical properties. At higher packing fractions, or with a composite matrix, [START_REF] Lee | Inorganic Nanotube Mesophases Enable Strong Self-Healing Fibers[END_REF] a more conventional increase in mechanical properties with alignment can be anticipated.

CONCLUSIONS

Continuous macroscopic fibers of pure imogolite nanotubes can be prepared by lab scale wet spinning which, as a form of conventional coagulation spinning, can be readily optimized and scaled-up. In the future, these binder free fibers can be converted into more complex macroscale constructs by weaving, braiding, or other textile processes. They may find applications in catalysis or molecular filtration by providing a robust structure that enables reagent access to the monodisperse nanotubes and their nanoscale pores. In addition, once optimised, INT fibers may be used as reinforcements in structural composites. Whilst the absolute performance will be lower than carbon nanotube-based fibres, the improved surface interaction with the matrix, oxidation stability and optical transparency may offer advantages, just as glass fibres are often used in preference to carbon fibres in many composite applications. The ease of fiber spinning and the resulting fiber properties are both strongly linked to the length distribution of the INT feedstocks. Short INTs, less than 100 nm, were challenging to spin, due to the low gel strength of the proto-fibers. However, longer INTs formed more robust gels, as well as stronger and stiffer dried fibers. The formation of a liquid crystal mesophase in the spinning dope likely contributes to the alignment of the INTs due to shear during spinning. However, in contrast to typical models of nanotube yarns, fiber strengths and stiffnesses were lower for more aligned fibers. This unusual finding can be attributed to the relatively low aspect ratio of the INTs and their high rigidity, which makes them much straighter than equivalent CNTs or biological nanofibrils, and less able to bend to form contacts. They are, therefore, much less likely to entangle or interlock in aligned fibers.

Combined with relatively low strength inter-tube interactions, which are easily disrupted by ambient water, misorientation is needed to transfer load across the individual INTs within the porous fiber. Ductile failure occurs due first to frictional sliding of the INTs, followed by pull-out. The most significant factor currently affecting the mechanical properties of these INTs fibers is the relative humidity during tensile testing. Reducing the humidity removes the water from the inter-tube space, suppressing the plasticity associated with INT sliding at elevated humidity. As a result, the tensile strength and modulus of the fibers increase dramatically from 20 to 155 MPa and 2.7 to 30 GPa, respectively, when reducing humidity from 85 to 10% RH. This result highlights the importance of controlling shear interactions in nanomaterial fibers, in general. The INT fibers provide a useful model system in which the shear interactions can be systematically explored through humidity. Through this system, an optimized relationship between aspect ratio, alignment, and interfacial shear properties may be identified which can be matched with other material systems. With further optimization of feedstock and spinning process, denser, stronger INT fibers may usefully complement CNT fibers in structural composites.

METHODS

Synthesis of DW Ge-INTs

Double-walled Ge-INTs were synthesized using aluminum perchlorate nonahydrate (Reagent grade, Alfa Aesar), tetraethoxygermane (TEOG, ≥ 99.95%, Sigma Aldrich) and urea (>99%, Sigma Aldrich) following the procedure described elsewhere. [START_REF] Amara | Single-Step Formation of Micron Long (OH) 3 Al 2 O 3 Ge(OH) Imogolite-Like Nanotubes[END_REF] TEOG was mixed in a PFTE beaker with an aqueous solution of aluminum perchlorate (C = 0.2 mol.L -1 ) and a urea solution with a molar ratio [Ge]:[Al]:[urea] = 1:2:2. After mixing, the PFTE beaker was placed in an acid digestion bomb (Zeoclave, Maximator, France) for hydrothermal treatment at 140°C. The solution was recovered after 5 days and then dialyzed against ultrapure water using semi-permeable membranes (Spectra/Por ® , cut-off = 10 kDa) until the conductivity of bath drops below 0.5 mS m -1 . 37

Preparation of INT fibers

INT fibers were prepared by injecting each INT feedstock through a 21 gauge needle at 5 mL/hr (linear velocity 6.7 mm s -1 ) into a coagulation bath of aqueous calcium chloride (300 g L -1 ). The fibers were collected on a rotating PTFE wheel (diameter 100 mm, surface linear velocity 33.5 mm s -1 ) with a spin draw ratio of 5. After collection on the wheel, the fibers were cut into 15 cm lengths and washed by dipping in deionized water (3 x 2 seconds).

A small tag of aluminium foil (~20 mg) was attached to one end of each fiber and they were hung to dry in ambient conditions.

Tensile testing of INT fibers

INT fiber samples were tested following the standard BS ISO 11566:1996. Fiber samples were mounted onto card frames with a gauge length of 15 mm with the ends fixed with epoxy adhesive (Araldite Rapid, Huntsman Advanced Materials Ltd., GB). The tensile tests were conducted on a TST350 tensile stress tester (Linkam Scientific Instruments Ltd., GB) with a 2 N load cell and a crosshead speed of 1 mm min -1 . The cross-sectional area for each sample was determined using the observed diameter in transmission optical microscopy.

Characterization of INT fibers

A small portion of INT feedstock was sampled and dried for further infrared (IR) characterization between KBr pressed pellets (~ 1 wt % of dry DW Ge-INTs powder). IR spectra were acquired in transmission mode (Nicolet iS50), by averaging 256 scans at a resolution of 4 cm -1 . Transmission electron microscopy (TEM) was performed on highly dilute dispersions of feedstock (1 mg L -1 ) prepared in ethanol and deposited in carbon-coated copper grid. TEM micrographs were recorded with a JEOL 1400 operating at 80 kV. X-ray scattering (XRS) experiments were carried out on a rotating anode (Model RU H3R, Rigaku Corporation, JP) using CuKα radiation (λ = 0.154 nm) delivered by a multilayer W/Si optics.

Pieces of single fibers were mounted on cardboard struts and placed in the beam path. Some samples were imaged in lab atmosphere (for which a background due to scattering from air was subtracted) and others imaged using a vacuum chamber equipped with a collimator, with entrance and exit windows made of 500 mm-thick Mylar film. The fiber was kept perpendicular to the incident X-ray beam. Two-dimensional patterns were recorded on a MAR345 detector (marXperts GmbH, DE) with 150 µm pixel size. Extraction of the scattered intensity I as a function of the scattering modulus Q (Q = 4πsin =  where 2 is the scattering angle) or azimuthal angle  were performed with home-developed software.

Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) were carried out using a JEOL 6010LA microscope on Au sputter-coated samples at an accelerating voltage of 20 kV. Cross-sections were imaged using a Zeiss Auriga Crossbeam focused ion beam scanning electron microscope (FIB-SEM) after milling with a 30 kV Ga+ ion beam at a beam current of 30 nA. The water sorption behavior of INT fibres was investigated through the Dynamic Vapour Sorption (DVS) technique using DVS

  the effect of long reaction times (>5 days) remains largely unexplored. In this work, the ageing time of the nanotube synthesis was varied from 5 to 40 days (feedstocks INT-5d, INT-12d, INT-20d etc.) to create spinning dopes with an expected further increase in length. The length distribution of each INT feedstock was determined by measuring the length of at least 200 nanotubes over several TEM images (Figure 1).

Figure 1 .

 1 Figure 1.(a) Length number distributions of INT feedstocks determined by measurements in

  gauge needle (ID 514 m). The gel strength of the INT-5d proto-fiber was therefore estimated to be approximately 500 Pa. The other feedstocks, from INT-12d to INT-40d, were successfully drawn from the coagulant without breakage using a 27 gauge needle (ID 210 m) implying a gel strength of at least 1.5 kPa. These gel strengths are relatively low, requiring careful handling of the proto-fibers before collection and drying. Polymer/INT hybrid hydrogels typically have significantly higher fracture strengths; for comparison polyacrylamide hydrogels with a 5% INT loading have been produced with a fracture strength of 220 kPa[START_REF] Shikinaka | Strain-Induced Reversible Isotropic-Anisotropic Structural Transition of Imogolite Hydrogels[END_REF] and hyaluronic acid/INT hydrogels with 1% and 10% INT loading have

29

 29

Figure 2 .

 2 Figure 2. (a) Photograph of fiber section on spool, (b) SEM image of focused ion beam

Figure 3 .

 3 Figure 3. (a) False color X-ray scattering pattern of dry fiber from INT-20d, (b) Radial

Figure 4 .

 4 Figure 4. (a) Tensile strength of fibers for each feedstock, dots show individual test results,

to 2 . 8

 28 GPa at 85% RH. Due to the strongly hydrophilic nature of both the INTs and the remnant CaCl 2 , increasing humidity leads to significant adsorption of water, with uptake reaching 150% by mass at 90% RH (Figure 5b and S6). The presence of water is expected to modify the interactions between the surface hydroxyl groups, leading to a reduction of both the shear strength and shear modulus of the inter-nanotube interactions, without affecting the intrinsic INT axial properties. The tenfold decrease in tensile strength and modulus with INT-5d INT-12d INT-20d INT-30d INT-therefore, indicates that the fiber properties are driven by inter-nanotube interactions, which are weakened by incorporation of water into the fiber structure. The change in failure mode of these INT fibers with relative humidity is clearly shown by the fracture surfaces (Figure5c-e). Fibers tested at low relative humidity show a flat brittle fracture surface. As humidity is increased more pull-out is evident and at high relative humidity local necking and shear deformation is visible. The humidity dependence is broadly reversible, with fibers conditioned at 85% RH and then tested at 40% RH showing similar tensile behaviour to those simply conditioned at 40% (FigureS9).

Figure 5 .

 5 Figure 5. (a) Stress-strain curves for fiber samples of INT-30d at controlled humidity, (b)

54 Figure 6 .

 546 Figure 6. (a) Stress-strain curves of fiber samples from INT-20d, (b) elastic modulus and

ACKNOWLEDGMENTS

The authors would like to thank DSTL for the funding received for this project through the UK-France PhD Scheme. The authors would like to acknowledge use of characterization facilities within the Harvey Flower Electron Microscopy Suite, Department of Materials, Imperial College London. The present work has benefited from Imagerie-Gif core facility supported by l'Agence Nationale de la Recherche (ANR-11-EQPX-0029/Morphoscope; ANR-10-INBS-04/FranceBioImaging; ANR-11-IDEX-0003-02/Saclay Plant Sciences). We are grateful to C. Goldmann for her support with TEM observations. X-ray scattering experiments were performed on the MORPHEUS platform of experiments at Laboratoire de Physique des Solides. We thank S. Rouzière and P.-A. Albouy for their support with the Xray scattering experiments and V. Karde and J. Heng for conducting the DVS measurement.

Advantage apparatus (Surface Measurement Systems, UK). Several strands of INT fibre (~13 mg) were manually shaped into a ball of sample and loaded in an aluminium pan. The moisture sorption was analyzed over a range of pre-set RH conditions starting from 0% RH and increasing to 90% RH in different RH steps at 25°C, before decreasing to 0% RH to obtain the desorption data. The moisture sorption measurements were recorded using the DVS Analysis Suite software. of DW Ge-INTs; radial profiles of scattered X-ray intensity; azimuthal profiles of scattered X-ray intensity; stress-strain curves of fibers tested at 40% RH with and without initial conditioning at 85% RH; table of structural parameters and mechanical properties; note on calculation of packing density. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *pascale.launois@universite-paris-saclay.fr *m.shaffer@imperial.ac.uk

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


Valentin Maingret

Véronique Schmitt
email: veronique.schmitt@crrp.cnrs.fr

Valérie Héroguez
email: heroguez@enscbp.fr

Spatio-temporal control over destabilization of Pickering emulsions stabilized by light-sensitive dextran-based nanoparticles

Keywords: Stimuli-responsive Pickering emulsions, light-responsive particles, natural polysaccharide, on-demand release, photolysis, photo-induced coalescence

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

Introduction

Stimuli-responsive Pickering emulsions are of high interest for the design of smart systems [START_REF] Dupont | New Insights into the Formulation and Polymerization of Pickering Emulsions Stabilized by Natural Organic Particles[END_REF][START_REF] Maingret | Dextran-Based Nanoparticles to Formulate pH-Responsive Pickering Emulsions: A Fully Degradable Vector at a Day Scale[END_REF][START_REF] Tang | Stimuli-responsive Pickering emulsions: Recent advances and potential applications[END_REF]. In this respect, light-sensitive trigger remains one of the most convenient switches because it is non-invasive and easy to set up. It is also the most challenging system to obtain. One reason may lie in the fact that many energy dissipative phenomena can happen in an enlightened emulsion: multi scattering and refraction for instance. That is why the response of the Pickering emulsion stabilizers must be as fast and efficient as possible, to lower the required light intensity and irradiation time.

Overcoming such challenges is worth investigation as these systems have enormous potential as tools for controlled-delivery, colloidal engineering or reaction catalysis [START_REF] Chen | Light controlled reversible inversion of nanophosphor-stabilized pickering emulsions for biphasic enantioselective biocatalysis[END_REF][START_REF] Xie | Light and Magnetic Dual-Responsive Pickering Emulsion Micro-Reactors[END_REF]. While offering great stability in time, they can be destabilized on-demand like other stimuli-responsive systems. In addition, light can be switched on/off as an external stimulus and be precisely focused in space. Thus, light stimulus enables a spatio-temporal control over the response of the system. Very few examples of such systems can be found in the literature and most of them imply the use of non-degradable inorganic stabilizers [START_REF] Bai | Light-Triggered Release from Pickering Emulsions Stabilized by TiO2 Nanoparticles with Tailored Wettability[END_REF][START_REF] Jiang | Pickering Emulsions Responsive to CO2/N2 and Light Dual Stimuli at Ambient Temperature[END_REF], which may hinder the future development of such systems in fields where bio-compatibility or (bio)degradation are required [START_REF] Ren | Temperature and CO2 Dual-Responsive Pickering Emulsions Using Jeffamine M2005-Modified Cellulose Nanocrystals[END_REF]. Physical or chemical modification of bio-based materials is needed to provide photo-responsiveness, compared to some naturally light-sensitive inorganic materials like TiO 2 . In this respect, many photo-sensitive groups with reversible or irreversible response can be found in the literature and thus could be used as Pickering emulsion stabilizer modifiers like nitrobenzyl derivatives, which are well known as cleavable units under UV light (365 nm) (H. [START_REF] Zhao | O-Nitrobenzyl alcohol derivatives: Opportunities in polymer and materials science[END_REF]. Specifically, Soliman et al. grafted a polyNitroBenzylAcrylate (PNBA), a photo-sensitive and hydrophobic polymer, onto dextran, a biocompatible and biodegradable hydrophilic natural polysaccharide [START_REF] Soliman | Amphiphilic photosensitive dextran-g-poly(o-nitrobenzyl acrylate) glycopolymers[END_REF]. They were then able to elaborate nanoparticles via nanoprecipitation. They observed that the photo-cleavage of PNBA to produce Poly(Acrylic Acid) (PAA) led, at a higher scale, to the photolysis of the PNBA grafted dextran (Dex-g-PNBA) nanoparticles [START_REF] El Founi | Light-sensitive dextran-covered PNBA nanoparticles as triggered drug delivery systems: Formulation, characteristics and cytotoxicity[END_REF][START_REF] Soliman | From photosensitive glycopolymers to smart drug delivery systems[END_REF][START_REF] Soliman | Light-sensitive dextran-covered PNBA nanoparticles to continuously or discontinuously improve the drug release[END_REF]. As another example, the use of PNBA as surface modifier of stabilizing cellulose nanocrystals has been recently presented in the work of Tajmoradi et al. [START_REF] Tajmoradi | Cellulose nanocrystalgrafted multi-responsive copolymers containing cleavable o-nitrobenzyl ester units for stimuli-stabilization of oil-in-water droplets[END_REF]. However, light stimulus was used prior to emulsification and did not induce the destabilization of the emulsion. Zhao et al. and Zeng et al. employed alginate and cyclodextrin for the design of light-sensitive Pickering emulsion stabilizers and are thus part of the only examples which promote the use of bio-friendly brick materials. They both used photoisomerization of azobenzene derivatives to induce the disassembly of their particles based on host-guest interactions with cyclodextrin [START_REF] Zeng | Triple-Responsive Pickering Emulsion Stabilized by Core Cross-linked Supramolecular Polymer Particles[END_REF][START_REF] Zhao | Light-tuning amphiphility of host-guest Alginate-based supramolecular assemblies for photo-responsive Pickering emulsions[END_REF]. In this work, we present an original strategy to formulate for the first time Pickering emulsions stabilized by light-sensitive particles based on PNBA-grafted dextran. The purpose was to prove that such nanoparticles could stabilize Pickering emulsions, but also that their photolysis under UV light (365 nm) could lead to the coalescence of the droplets and the destabilization of the Pickering emulsion. As a result, Pickering emulsions were successfully obtained with an average drop size governed by the nanoparticle concentration (as a result of the limited coalescence phenomenon). Then, irradiation time and alkaline pH of the continuous phase were proved to be the key elements for the destabilization of these Pickering emulsions. To widen the scope of applications, visible light (405 nm) was also used to induce the spacecontrolled destabilization of the Pickering emulsions in an alkaline medium. In this case, destabilization took more time to occur than with UV irradiation but remained fast (< 2h), mainly because of the lower energy of the visible irradiation compared to UV light. This strategy, combining both temporal and spatial control over the coalescence process opens interesting perspectives: Pickering emulsion droplet reactors, on-demand release, colloidal and reaction engineering.

Experimental section

Materials

Dextran (M w = 34,000 g/mol, M n = 24,000 g/mol, Ð = 1.4 as measured by the authors using size-exclusion chromatography (SEC)) and dodecane (99%) were purchased from ABCR.

All used solvents were of analytical grade.

Synthesis of Dex-g-PNBA

Dex-g-PNBA was synthesized following previous works from Soliman et al. [START_REF] Soliman | From photosensitive glycopolymers to smart drug delivery systems[END_REF][START_REF] Soliman | O-nitrobenzyl acrylate is polymerizable by single electron transfer-living radical polymerization[END_REF]. Briefly, dextran was modified with alkyne moieties to operate a clickchemistry reaction with azide end-chain modified Poly(NitrobenzylAcrylate) (PNBA) (Figure S1). On one side, NBA monomer was synthesized from o-nitrobenzyl alcohol and acryloyl chloride (Eq.I, FigureS1) then polymerized through Single Electron Transfer-Living Radical Polymerization (SET-LRP) using ethyl 2-bromoisobutyrate as initiator (Eq.II, Figure S1).

Finally, bromide end-chain was modified thanks to sodium azide (Eq.III, Figure S1). On the other side, modification of dextran (Eq. V, Figure S1) was performed through esterification of the hydroxyl moieties with activated 5-hexynoic acid. Full details on the synthesis products and their characterization by 1 H NMR (400 MHz NMR instrument Bruker 400 UltraShield), FTIR, and SEC can be found in the supporting information section (Figure S1-S10). Final grafting of PNBA to modify dextran was checked and quantified thanks to SEC measurements.

Dex-g-PNBA nanoparticles obtained by nanoprecipitation

Dex-g-PNBA nanoparticles (NPs) were obtained through nanoprecipitation using a syringe pump (CHEM-YX Fusion 101 Syringe Pump). Specifically, Dex-g-PNBA chains were dissolved in a 20 mL mix of THF/H 2 O (90/10 v/v) (3 g/L) and injected in a 40 mL water bath (non-solvent) at 0.4 mL/min. At the end of this process, further 40 mL of water were added to the dispersion. THF was then removed with rotary evaporator (30°C bath). The resulting aqueous dispersion was poured in a cellulose dialysis bag with 100 kDa pore size. 5 liters of water were used as counter-dialysis solution and were changed after 6, 12 and 24 hours.

Finally, the recovered dispersion was concentrated using a rotary evaporator (40 °C bath) to eliminate water. The mass concentration of nanoparticles into dispersions was determined using the residual dry weight technique. The dispersions can be stored for several months without stability issues. HITACHI H-600 Transmission Electron Microscopy (TEM) instrument (HV=75 kV) was used for the shape characterization of the nanoparticles. The volume-averaged diameter D[4,3] of nanoparticles was calculated from TEM pictures using ImageJ software as follows (eq.1):

D [ 4 , 3]= ∑ i n d i 4 ∑ i n d i 3 (1)
where d i is the diameter of a nanoparticle from an n-nanoparticles population (n≥200).

NanoZS90 Dynamic light scattering (DLS) instrument (Malvern Instrument Ltd, UK) was used at a scattering angle of 90° to characterize the size distribution of the nanoparticles after dilution in water and their average diameter (z in nm). Three measurements of 10 runs (30 s each) at 25°C were carried out unless otherwise noticed. The concentration of the sample was set low enough to avoid multiple scattering and is given when necessary. Polydispersity index (PDI) was computed by the instrument and is expressed as follows (eq.2):

PDI = ( σ z ) 2 ( 2
)
Where z is the average diameter (nm) and σ the typical standard deviation (nm) of the sample.

DLS allows a reliable statistically significant measure of the nanoparticle size in the suspension.

Formulation of O/W Pickering emulsions

Emulsions volume was set to 4 mL using water phase/oil phase (dodecane) ratio of 50/50 to initially assess the type of emulsion. This 50/50 volume ratio was chosen to determine the preferred emulsion type (O/W or W/O) dictated by the particles without interaction from the composition. For all other experiments the water/oil ratio was fixed to 80/20 v/v. Thanks to dry weight technique results, a precise amount of nanoparticles could be introduced in the water phase. This quantity was computed as a concentration relative to the volume of the oil dispersed phase. Nanoparticles were not dispersible in the oil phase, but since the total interfacial area, S, to be stabilized by the nanoparticles is directly linked to the dispersed oil volume, V d, and to the drop size, D[3,2] through S=6V d /D[3,2], it is more logical to compare the amount of particles to the one of oil. Dispersion of oil into the aqueous suspension was operated thanks to an Ultra Turrax (IKA T25) with a shaft (S25N 10G) at 20 000 rpm for 2 minutes. First, formulation of Pickering emulsion was done at a water phase/oil phase ratio of 50/50 v/v to assess the kind of emulsion stabilized by the nanoparticles (O/W or W/O). For that, after emulsification, droplets from the emulsion were put in two different vials containing either water or oil. The aliquots diluted easily into water, not in oil, proving that in all cases direct oil-in-water (O/W) Pickering emulsions were obtained. In all this work, only O/W Pickering emulsions were obtained, and the water phase/oil phase ratio used in the following was always set to 80/20 v/v. An optical microscope (Zeiss Axioskop 40) in bright field using x5 and x10 objectives was employed to observe the emulsions and pictures were taken using a digital camera device linked to the microscope. Size measurements of at least two hundred drops when possible were obtained by analysing the pictures using the ImageJ software. Such a high amount of drops allows to get a statistical significance. The mean drop size of the emulsions was computed according to the Sauter diameter (eq.3):

D [3 ,2]= ∑ i n d i 3 ∑ i n d i 2 (3)
where d i is the diameter of a drop from a n-drops population. The Sauter diameter is defined as the surface-average diameter of the drops. Finally, to describe the drop size distribution width of the emulsions, a typical standard deviation  d was calculated.

Light-induced destabilisation of O/W Pickering emulsions

Nanoparticles photolysis

Nanoparticle dispersions were loaded in a DLS cuvette and diluted to reach 0.2 mg/mL. A tiny stirring bar was added to ensure homogenization during the irradiation process. UV irradiation was performed with a LIGHTNINGCURE® series apparatus from HAMAMATSU equipped with a light guide and a 365 nm wavelength filter. The DLS cuvette was placed at a 3 cm distance (center-to-center) of the light guide. Irradiance, determined using a Power Puck II radiometer from EIT ® was usually equal to approximately 140 mW/cm 2 unless otherwise noticed. Regular DLS acquisitions were performed to measure the size distribution of the nanoparticles. Three measurements of 6 runs (10 s each) were taken to ensure data reliability.

Derived Count Rate (DCR) signal was also acquired and used for analysis. DCR depends both on the radius of the nanoparticles (r) and their concentration (c p ) (Eq.4):

DCR ∝ c p * r 6 (4)
If the size distribution remains unchanged, the DCR is directly proportional to the concentration (calibration curve can be found in the supporting information section (Figure S11)). In the photodegradation experiments, it was then possible to quantify the relative decrease of the nanoparticle amount when their size did not change with time by normalizing all DCR values by the one obtained at t 0 . However, it happened that the average size of nanoparticles increased in time, meaning that the DCR value measured may be higher than expected. In these cases, as the DCR always still decreased with irradiation time, the normalization allowed to major the relative population of nanoparticles meaning that the amount of remaining particle is then overestimated. Degradation of the nanoparticles was considered as complete when the relative amount of nanoparticles was equal or lower than 10%. From that point, no additional measurement nor irradiation step was done. Absorption scans in UV-visible light of irradiated samples were carried out using a Cary 100 UV-visible Spectrophotometer from Agilent Technologies. The samples were diluted by a factor three with pure water before analysis. Absorbance at 265 nm (indicating presence of nitrobenzyl acrylate moieties) and at 325 nm (indicating presence of nitrosobenzaldehyde moieties) were extracted from the scans.

Time and space O/W Pickering emulsion destabilization

In order to observe the destabilization under optical microscopy, a tiny part of a Pickering emulsion cream was deposited into a quartz slide (4.5 cm per 1.2 cm) with a small cuvette (3.8 cm per 0.8 cm, 0.1 cm deep) that was then filled with various aqueous phases. The hollow quartz slide was then recovered with a flat quartz lamella of the same size. Capillary forces made the slide and the lamella sticking together and ensured sample preservation with time. Then, the same conditions as for the nanoparticle photolysis were applied, that is 3 cm (center-to-center) between the light guide and the sample and an irradiance of about 140 mW/ cm 2 . The impact of the continuous phase pH was studied by using various aqueous phases in the quartz cuvette. Coalescence of the emulsion droplets was observed thanks to an optical microscope in bright field using a x5 objective. To irradiate the sample with visible light, a fluorescence microscope with an integrated 50 mW blue laser (405 nm) at 70% of its power intensity was used. The irradiation was focused on a precise area of the sample and regular pictures were taken. To assess the presence of nanoparticles at the interfaces, cryo-SEM was performed on emulsion droplets without irradiation and after a long irradiation time.

Dodecane-in-water emulsion were formulated with a nanoparticles suspension where the amount of particles was set to 3 mg/mL of oil without control over the pH of the continuous phase. Cryo-SEM allowed direct observation of the samples without the use of a replica.

Results and discussion

Dex-g-PNBA synthesis

Dextran was modified with alkyne moieties and grafted with PNBA-N 3 chains thanks to a click-chemistry reaction (Figure 1). According to SEC and 1 H NMR measurements, PNBA block length was about 7000 g/mol (DP = 33) and 12 grafts over 100 glucosidic units of a dextran chain could be found in average (Figure S10). As a result, Dex-g-PNBA chains were composed of 83%wt of PNBA in average. Complete characterizations of the different products obtained through the different synthesis steps can be found in the supporting information section (Figure S1-S10).

Figure 1. Scheme representing the grafting of PNBA photo-sensitive polymer onto modified dextran thanks to a copper catalyzed click-chemistry reaction.

3.2.

Dex-g-PNBA nanoparticles

Nanoparticles obtained via nanoprecipitation were characterized thanks to TEM observations, assessing their spherical shape (Figure 2). Size distribution of the Dex-g-PNBA nanoparticles from DLS and from TEM pictures analysis were determined in addition to their zeta potential without and in presence of salt (Table 1). As a result, nanoparticles exhibit spherical shape and possess a narrow monomodal size distribution (PDI < 0.2). In absence of salt, they are negatively charged (< -20 mV), however these charges can be easily screeened by slightly increasing the ionic strength of the medium (10 mM NaCl). It can be seen that this increased ionic strength made the nanoparticles size distribution a little bit broader and centered on bigger size which may be the result of partial swelling or slight aggregation. The effects of the ionic strength and so of the zeta potential of the nanoparticles on the stabilization of Pickering emulsions are discussed in the next section. After full characterization, nanoparticle dispersions were stored hidden from the light in a cupboard. They could be stored for more than 3 months without any stability issue (Figure S12).

Formulation of O/W Pickering emulsions

Determination of emulsion type

Water phase/oil phase ratio 50/50 v/v Pickering emulsions were first formulated to assess the type of obtained emulsion: O/W emulsions were always formed. Because of the large diameter of the droplets and the low density of dodecane (0.75 g.cm -3 ) compared to water, all emulsions creamed quickly. In the following water phase/oil phase ratio used was always 80/20 v/v.

Influence of ionic strength in the formulation step

As seen before, ionic strength influences the zeta potential of the particles. To evaluate the impact of the latter on Pickering emulsion characteristics, Pickering emulsions at a nanoparticle concentration of 0.8 mg/mL were formulated without and with 10 mM carbonate buffer (pH 10). As a result, the emulsion droplets stabilized without the presence of salt were much bigger (around 650 µm) than the one from the emulsion with 10 mM of salt (436 µm)

(Figure 3). Moreover, it was observed that the subnatant of the first emulsion (without salt)

was far more bluish than the second one (which was rather clear). Thus, it indicated that not all the nanoparticles took part in the stabilization process, probably owing to the charges they bear which made them too hydrophilic so that a part of the nanoparticles remained in the aqueous continuous phase. More salt (30 mM) led to a slight further decrease of the droplet size (383 µm) but it induces rapid aggregation because of the very low zeta potential of the nanoparticles (~ 0mV). In the following, 10 mM of carbonate buffer (pH 10) were systematically introduced for the formulation step to increase the amount of nanoparticles involved in the stabilization of interfaces. It can be noticed that the emulsified creamed volume represents 33% of the total sample volume (see figure S14) showing that the drops are randomly close packed in the cream (between 64 and 65 %v/v). This is in complete agreement with the optical microscopy picture (Figure 3) showing noninteractive drops.

Influence of nanoparticle concentration -limited coalescence phenomenon

Coalescence is usually an unwanted deleterious phenomenon leading to the emulsion destabilization. However, in Pickering emulsions, a so-called limited coalescence phenomenon happens right after the homogenization step and is, on the contrary, beneficial.

Indeed, the emulsification step produces a large excess of interfacial area compared to the area that can be covered by the stabilizing nanoparticles. Consequently, drops, which are not enough covered, coalesce to decrease the interfacial area. This coalescence phenomenon stops when the drops are sufficiently covered (stabilizing particles are irreversibly anchored at the interface). Thus, one can take benefit from this phenomenon to produce a narrow monomodal size distribution of droplets with great reproducibility despite the turbulent flow that has been used for emulsification. Limited coalescence generally happens in the low-particle concentration range and in this case, for geometrical reasons, the inverted Sauter diameter of the emulsion droplets is linearly related to the amount of nanoparticles used to stabilize them, as described in eq. 5 [START_REF] Schmitt | Colloidal particles as liquid dispersion stabilizer: Pickering emulsions and materials thereof[END_REF]:

1 D [3,2] = c p 4 C ρ p d p (5)
where c p is the concentration of particles relative to the volume of the dispersed phase, C is the surface coverage, that is, the fraction of the droplet interfacial area covered by the particles, ρ p is the particle density and d p is the stabilizing particle diameter. Various O/W Pickering emulsions were formulated by varying the concentration of nanoparticles relative to the volume of the oil phase. In our case, optical microscopy observations confirmed that the average diameter of the droplets linearly decreased when increasing the amount of particle up to a concentration of 7 mg/mL showing that limited coalescence phenomenon is operative in these Pickering emulsions (Figure 4). Detailed measured sizes of Pickering emulsions are given in the Supporting Information section (Figure S13). This allowed varying the drop size from 55 to 623 µm (more than a decade) keeping a narrow drop size distribution. For the following induced destabilization experiments we chose to set the nanoparticles concentration equal to 2.4 mg/mL of oil. Pickering emulsions were stored in the same conditions as the stabilizing nanoparticles (protected from light). Various emulsions formulated at different nanoparticle concentrations showed good stability at room temperature for more than 5 months (Figure S14).

Light-induced destabilization of O/W Pickering emulsions

NBA moieties of Dex-g-PNBA chains are sensitive to UV light ( =365 nm) as described in the literature [START_REF] Soliman | Amphiphilic photosensitive dextran-g-poly(o-nitrobenzyl acrylate) glycopolymers[END_REF]. Thus, under irradiation, NBA units are photo-cleaved to release nitrosobenzaldehyde (NSBZ) and to generate acrylic acid units (AA) on the grafts (Figure S15). Soliman et al. showed that these AA units could be deprotonated in presence of phosphate buffer saline (PBS) [START_REF] Soliman | Light-sensitive dextran-covered PNBA nanoparticles to continuously or discontinuously improve the drug release[END_REF]. This dramatically increased their solubility in water so that irradiated Dex-g-(NBA-co-AA) chains became also much more soluble, leading to the destabilization of the nanoparticles. Our goal was then to design such nanoparticles able to both stabilize and destabilize O/W Pickering emulsion by exposure of the emulsion to a light stimulus providing photolysis of nanoparticles. In a process of understanding the multi-scale phenomena that happen, we first investigated the degradation of the nanoparticles in aqueous dispersions. Influence of irradiation time and alkaline pH on the kinetics of degradation were studied. Then, light controlled destabilization of Pickering emulsions was studied. The question was to determine whether or not controlled coalescence of the Pickering emulsions could be achieved in alkaline media despite the various dissipation phenomena that can lead to a loss of light intensity.

Photolysis of Dex-g-PNBA nanoparticles

UV irradiation (at 365 nm) was used to degrade the nanoparticles in aqueous dispersions.

Nanoparticle aqueous dispersions of initially 0.2 g/L were prepared in a DLS cuvette with a little stirrer. Various aqueous conditions were tested: pure water, 10 mM NaCl, 10 mM buffered solution at pH 9 and 10, and 10 mM NaOH. The samples were placed at 3 cm (center-to-center) from the UV source and stirred to homogenize the dispersion (UV spot was placed at mid-height). Then, at regular irradiation times, size distributions and DCR values were obtained from DLS to quantify the relative amount of nanoparticles remaining as a function of irradiation time (Figure 5A). No control over the pH of the dispersion (pure water, 10mM NaCl) resulted in an incomplete degradation (~80%) of the nanoparticles after 20 minutes (1200 s) of irradiation. Longer irradiation time would maybe not be enough to fully degrade the nanoparticles as the results showed a plateau-like tendency. On the other side, in alkaline conditions, complete degradation (> 90%) was achieved after only 5 minutes of irradiation. Detailed final values of degradation can be found in the supporting information section (Figure S16). To investigate this different behavior depending on the aqueous conditions, absorptions of NBA units and NSBZ units (at 265 nm and 325 nm respectively) were determined (Figure 5B) thanks to UV-vis spectroscopy measurements on diluted irradiated samples in their final state.

These two absorption peaks are closely related, as the photo-cleavage of one NBA unit produces one unit of NSBZ. For pure water and NaCl 10 mM, where the irradiation time was the longer (20 min vs 5 min for the others), the absorption of NBA and NSBZ were lower and higher respectively than those from other experimental conditions, which was expected.

However, although the photo-cleavage was more advanced in these two cases, the photolysis of the nanoparticles was lower (Figure 5A). In the other samples where the pH conditions were alkaline, faster and complete photolysis (>90%) were obtained (5 min). In addition, the UV-vis results showed that the obtained absorbance of NSBZ units in such samples (directly related to the photo-cleavage advancement) were lower compared non-alkaline conditions. This is consistent with the lower irradiation time, but it also means that the photo-cleavage of NBA moieties is not the kinetically determinant step as nanoparticles could be better photolyzed at lower photo-cleavage rates. The pH of the medium, as well as the irradiation time are then synergic key parameters as they settle the maximum nanoparticle achievable photolysis efficiency. Finally, another sample with a 10 mM buffer at pH 9 was irradiated at half the used irradiance (70 mW/cm 2 ) (Figure S16-S17). Results showed that the degradation was initially slower but still exceeded 90% after 5 minutes meaning that irradiance may be lowered without a high loss of performances. However, to balance the possible light dissipative phenomena in emulsions, we chose to keep the irradiance at 140 mW/cm 2 for the following section.

Light-induced Pickering emulsion destabilization

Under UV-light ( = 365 nm)

From the previous part, it has been shown that nanoparticle degradation can be performed up to 80% after 20 minutes of irradiation in a non-controlled pH medium. Transposed into a

Pickering emulsion system, a degradation of 80% of the stabilizing nanoparticles should reasonably lead to coalescence events and likely to the full destabilization of the emulsion.

Two different experiments were carried out to verify this hypothesis. First, a freshly made Pickering emulsion without controlled pH was subjected to UV irradiation for 2 hours.

Release of NSBZ was assessed but no destabilization could be observed after more than a week. It then proved that the destabilization could not be easily achieved without control over the pH of the emulsion. This sample and a control emulsion (not irradiated) were both observed by Cryo-SEM. Nanoparticles anchored at the interface of droplets could be seen in the control sample (Figure S18) whereas surprisingly no one could be observed at the surface of the irradiated droplets (Figure S19). Thus, it meant that irradiated emulsions could remain stable even without particles stabilizers (it should be noticed that an initial emulsion prepared without nanoparticles was not stable). This behavior suggests that a secondary stabilization, post-irradiation occurred and hindered the coalescence. Another Pickering emulsion without controlled pH was subjected to UV irradiation for 2 hours under high stirring (which did not affect the emulsion stability) so that the subnatant became orange (evidencing the presence of NSBZ). After one night, the emulsion remained still stable as in the first experiment. Part of the subnatant (whose pH was equal to 4.7) was discarded and replaced with NaOH to adjust the pH to 12. In less than 5 minutes coarse coalescence happened under no further agitation than a slight homogenization. Finally, after 30 minutes, a full demixion of the two phases was observed (Figure S20). Again, this behavior could be explained by a secondary stabilization mechanism, post-irradiation, coming from protonated AA units produced during the irradiation step. Indeed, in addition to incomplete degradation of nanoparticles (as found out in the previous part), poorly soluble protonated PAA grafts may adsorb on the oil droplets and provide steric stabilization of these (Figure 6). When the pH is adjusted to higher values, the PAA chains became deprotonated and much more soluble into water, leading to their desorption and to bare droplets that coalesced. The coalescence can be then induced by the light if the pH of the emulsion is initially alkaline enough. Reciprocally, an emulsion whose pH is not controlled can become pH-sensitive after irradiation. In addition to irradiation time and intensity, pH could be wisely used to control the destabilization of the formulated Pickering emulsions: from partial to complete coalescence of the droplets.

In another set of irradiation experiments, emulsion cream was poured into a quartz cuvette lamella and water phase was added to fill it. In a first approach, NaOH 1M was taken as continuous phase to ensure quick solubilization of the PAA grafts produced during irradiation.

Despite the high molarity and pH of the environment, the droplets of emulsion remained stable. Direct observation, under optical microscope, of the coalescence events of the droplets under irradiation was made (Figure 7). Time-lapse of this destabilization experiment can be found in the supporting files (Video S1). The coalescence events started quite quickly (90 secs) and ended after 5 minutes of irradiation likely because after some coalescence events, drops are no more at contact as required for coalescence to happen. To substitute motion that was absent due to the drops large size and confinement, the sealed cell was slightly stirred (low upside downs of the sealed quart lamellae). Then droplets easily coalesced into much bigger ones. Finally, instead of NaOH at 1M, an aqueous pH 10 buffer at 100 mM was used and also led to coalescence (data not shown). To better evidence the space control coalescence that could be achieved, the quartz lamellae system containing an emulsion was partially covered with an aluminum sheet and then irradiated. After 30 minutes, a clear difference could be observed between the covered and uncovered area (Figure 8). In addition, the transition area which delimits the two parts showed that a great spatial resolution was achieved. coalesced together to produce one droplet of 227 µm. However, some of them did not coalesce at all, mainly due to the absence of homogenization which may be necessary to make droplets coalesced together as seen before (Figure 7).

Under visible light ( = 405 nm)

Light responsiveness of nitrobenzyl derivatives is induced around 365 nm. This wavelength may be slightly shifted depending on the added modifying groups (H. [START_REF] Zhao | O-Nitrobenzyl alcohol derivatives: Opportunities in polymer and materials science[END_REF]. In the work of Del Campo et al. visible light ( = 411 nm) was successfully used to induce the light response of nitrobenzyl derivatives [START_REF] Del Campo | Surface modification with orthogonal photosensitive silanes for sequential chemical lithography and site-selective particle deposition[END_REF]. However, no other example on the use of visible light could be found in the literature. In order to widen the range of applications of our system, we tried to use visible blue light (405 nm) to photo-cleave, for the first time, NBA moieties and thus induce the destabilization of the Pickering emulsions.

As before, 1M NaOH was used to dilute the emulsion cream into the quartz cuvette lamella.

The laser beam was focused on a specific area of the sample (dot square on Figure 9) for 100 minutes and regular pictures of this irradiated area were taken. No destabilization due to the highly alkaline outer phase (which was not irradiated) was evidenced even after 100 minutes of observation (Figure 9). As a result, only the area which was subjected to the irradiation showed signs of destabilization (coalescence). Because of the coalescence events, some droplets moved so that they also took part later in the local coalescence phenomenon, although they were not initially in the region of interest. Destabilization took nearly 30 more minutes to occur than when with the UV source, this may be due to the lower energy of the photons (at 405 nm vs at 365 nm) but also from the intrinsic lower power of the laser from the fluorescence microscope (50 mW, 70% intensity). Oil volume conservation during the process was assessed by computing the total volume of the 21 blue droplets at t 0 and of the 2 blue droplets at t 100min . After comparing the values, an error of less than 3% was found, evidencing the conservation of the oil volume during the process and that all the droplets which coalesced were well identified (Figure S21). This interesting result gives a proof of concept for the use of this system as micro-droplet reactors which can be merged on demand and with a high spatial precision using visible light.

Conclusion

This

Figure 2 .NPs

 2 Figure 2. TEM images of Dex-g-PNBA nanoparticles.

Figure 3 .

 3 Figure 3. Droplets of emulsion obtained at 0.8 mg/mL of nanoparticles at different ionic strength values (0, 10 and 30 mM of pH 10 buffer).

Figure 4 .

 4 Figure 4. Optical microscopy pictures of emulsions with various nanoparticle concentrations with respect to oil from 0.4 (A) to 7 mg/mL (H), scale bar is equal to 300 µm for samples from A to C and to 150 μm for samples from D to H (top). Evolution of 1/D[3,2] as a function of the nanoparticle concentration with a linear fit (bottom).

Figure 5 .

 5 Figure 5. Degradation of nanoparticles as a function of irradiation time with an initial concentration of 0.2 g/L in different aqueous media. Dotted line represents the 10% threshold below which the degradation was considered as complete (detection threshold) (A). Absorbance of the irradiated samples at 325 nm (related to the presence of NSBZ) and at 265 nm (related to the presence of NBA). Irradiation time was set to 20 min for the first two samples and to 5 min for the three others (B).

Figure 6 .

 6 Figure 6. Destabilization of an O/W Pickering emulsion under irradiation depending on the pH conditions.

Figure 7 .

 7 Figure 7. Optical microscopy pictures of the droplets in 1M NaOH before irradiation (nanoparticle concentration was 2.4 mg/mL of oil) a), after 5 min of irradiation at 140 mW/cm 2 b) and after very gentle agitation of the irradiated sample c).

Figure 8 .

 8 Figure 8. Optical microscopy pictures of a Pickering emulsion subjected to a space-controlled irradiation. Pictures from the covered to the uncovered areas from left to right were taken after 30 minutes of irradiation. Emulsion was formulated at 2.4 mg/mL of nanoparticles and the continuous phase was composed of 100 mM pH 10 buffer. Left part of the sample was kept protected from UV

Figure 9 .

 9 Figure 9. Optical microscopy pictures of the sample at different times of irradiation with a 405 nm source from a fluorescence microscope. The irradiation was focused in the white dot square and the droplets which coalesced are coloured in blue. Main coalescence events and times are shown in the little inserts. Pickering emulsion was formulated at a nanoparticle concentration of 2.4 mg/mL of oil.

  work offers a proof of concept of the use of Dex-g-PNBA as a brick material for the design of Pickering emulsion stabilizers. Stable O/W Pickering emulsions with droplets of tunable and narrow size distribution could be obtained. Photo-responsiveness of the polymer, at larger scale of the nanoparticles and at an even larger scale of stabilized Pickering emulsion were assessed and studied. As a result, additionally to irradiation, control over the pH of the continuous phase allows enhancing the light-response of the nanoparticle dispersion and of the Pickering emulsions. Finally, precise time and space control over coalescence of the Pickering emulsion droplets could be achieved both using UV and visible light which may find interest and applications in various fields such as colloidal engineering, reaction design or for encapsulation purposes. nanoparticle dispersions at t 0 and t 3.5months , picture of a stable Pickering emulsion after more than 5 months, size measurements of the Pickering emulsions, photo-cleavage scheme of Dex-g-PNBA, irradiation results in different nanoparticle dispersions, irradiation results at half the irradiance, Cryo-SEM pictures, pH 12 adjustment of an irradiated emulsion, Oil volume conservation calculus. CRediT authorship contribution statement V. M.: Conceptualization, Investigation, Writing -original draft. V. S.: Conceptualization, Writing -review & editing, Supervision. V. H.: Conceptualization, Writing -review & editing, Supervision.

Acknowledgments

V. Maingret acknowledges support from the Ministère de l'Enseignement Supérieur, de la Recherche et de l'Innovation for his Ph.D. grant. The authors thank their respective academic institution for financial support. The authors also acknowledge Emmanuel Ibarboure for his help in the spatially controlled experiments on fluorescence microscope using visible light and Isabelle Ly for Cryo-SEM observations.

Supporting information

Reaction schemes of the synthesis of Dex-g-PNBA, 1 H NMR characterizations of the different products, conversion and DP computations of PNBA-Br, FTIR characterization of PNBA-N 3 , SEC chromatograms of PNBA-N 3 , Dex-Alkyne and Dex-g-PNBA, modification of dextran and grafting computations, DCR calibration curve, stacked size-distribution curves of