Marie-Charlotte Tatry

Yating Qiu

Véronique Lapeyre

Patrick Garrigue

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

Valérie Ravaine
email: vravaine@enscbp.fr

Sugar-responsive Pickering emulsions mediated by switching hydrophobicity in microgels

Keywords: Responsive polymers, Microgels, Pickering emulsions, Sugar-sensitive materials, Boronic acids

come

1) Introduction

Microgels are soft and deformable particles. When constituted with amphiphilic chains, they adsorb at interfaces. These features are now well-known for poly(N-isopropylacrylamide) (pNIPAM) microgels [START_REF] Zhang | Poly(N-isopropylacrylamide) microgels at the air-water interface[END_REF][START_REF] Monteux | Poly(Nisopropylacrylamide) microgels at the oil-water interface: Interfacial properties as a function of temperature[END_REF] and other poly(N-alkylacrylamides) derivatives [START_REF] Wiese | Microgel stabilized emulsions: Breaking on demand[END_REF] but they can also apply to other kinds of polymeric structures, such as poly(N-vinylcaprolactam) (pVCL) [START_REF] Maldonado-Valderrama | Thermoresponsive microgels at the air-water interface: the impact of the swelling state on interfacial conformation[END_REF][START_REF] Wu | Behavior of Temperature-Responsive Copolymer Microgels at the Oil/Water Interface[END_REF] or proteins such as whey protein or lactoglobulin microgels to name a few [START_REF] Benetti | SPI microgels applied to Pickering stabilization of O/W emulsions by ultrasound and high-pressure homogenization: rheology and spray drying[END_REF][START_REF] Jiao | High-Internal-Phase Pickering Emulsions Stabilized Solely by Peanut-Protein-Isolate Microgel Particles with Multiple Potential Applications[END_REF][START_REF] Destribats | Emulsions stabilised by whey protein microgel particles: towards food-grade Pickering emulsions[END_REF][START_REF] Murphy | Dynamic and viscoelastic interfacial behavior of βlactoglobulin microgels of varying sizes at fluid interfaces[END_REF][START_REF] Gonzalez-Jordan | Influence of the Protein Particle Morphology and Partitioning on the Behavior of Particle-Stabilized Water-in-Water Emulsions[END_REF][START_REF] Dickinson | Biopolymer-based particles as stabilizing agents for emulsions and foams[END_REF][START_REF] Matsumiya | Soybean protein isolate gel particles as foaming and emulsifying agents[END_REF][START_REF] Xu | Interfacial and emulsion stabilized behavior of lysozyme/xanthan gum nanoparticles[END_REF] . pNIPAM and pVCL are two examples of thermoresponsive microgels, which are swollen in water at temperatures below the volume phase transition temperature (VPTT) and collapse above. Bearing small hydrophobic groups and amide functions, the polymer is in good solvent and swells at low temperature when hydrogen bonds are established between water and amide, but collapses at high temperature, when hydrogen bonds break down, due to the predominance of hydrophobic interactions. Thus, microgel thermoresponsiveness is intimately linked to their amphiphilicity.

Thanks to their softness, microgels are deformed at liquid interfaces [START_REF] Geisel | Unraveling the 3D localization and deformation of responsive microgels at oil/water interfaces: A step forward in understanding soft emulsion stabilizers[END_REF][START_REF] Geisel | New Insight into Microgel-Stabilized Emulsions Using Transmission X-ray Microscopy: Nonuniform Deformation and Arrangement of Microgels at Liquid Interfaces[END_REF][START_REF] Style | Adsorption of soft particles at fluid interfaces[END_REF] , a behavior which is also predicted theoretically [START_REF] Rumyantsev | A polymer microgel at a liquid-liquid interface: theory vs. computer simulations[END_REF] . They undergo some conformational changes from spread to less deformed at the interface that are visible under compression in Langmuir films [START_REF] Pinaud | Adsorption of microgels at an oil-water interface: correlation between packing and 2D elasticity[END_REF][START_REF] Geisel | Highly ordered 2D microgel arrays: compression versus selfassembly[END_REF][START_REF] Picard | Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density[END_REF][START_REF] Rey | Interfacial arrangement and phase transitions of PNiPAm microgels with different crosslinking densities[END_REF] but these conformational changes also occur over time during spontaneous adsorption [START_REF] Murphy | Dynamic and viscoelastic interfacial behavior of βlactoglobulin microgels of varying sizes at fluid interfaces[END_REF][START_REF] Pinaud | Adsorption of microgels at an oil-water interface: correlation between packing and 2D elasticity[END_REF][START_REF] Tatry | Kinetics of spontaneous microgels adsorption and stabilization of emulsions produced using microfluidics[END_REF][START_REF] Monteillet | Ultrastrong anchoring yet barrier-free adsorption of composite microgels at liquid interfaces[END_REF][START_REF] Deshmukh | Equation of state and adsorption dynamics of soft microgel particles at an air-water interface[END_REF] . Soft microgels were found to efficiently stabilize Pickering emulsions or foams in good solvent conditions, i.e. at low temperature but these emulsions demulsify on demand upon deswelling of the particles [START_REF] Tsuji | Thermosensitive pickering emulsion stabilized by poly(Nisopropylacrylamide)-carrying particles[END_REF][START_REF] Kwok | Correlating the effect of co-monomer content with responsiveness and interfacial activity of soft particles with stability of corresponding smart emulsions[END_REF][START_REF] Ngai | Novel emulsions stabilized by pH and temperature sensitive microgels[END_REF][START_REF] Brugger | Emulsions stabilized by stimuli-sensitive poly(Nisopropylacrylamide)-co-methacrylic acid polymers: Microgels versus low molecular weight polymers[END_REF][START_REF] Horiguchi | Temperature-responsive Pickering foams stabilized by poly(N-isopropylacrylamide) nanogels[END_REF][START_REF] Wang | Pickering Emulsions Stabilized by pH-Responsive Microgels and Their Scalable Transformation to Robust Submicrometer Colloidoisomes with Selective Permeability[END_REF][START_REF] Sun | On-chip thermotriggered coalescence of controllable Pickering emulsion droplet pairs[END_REF] . The microgel deformability is an important criterium to control the stability of emulsions [START_REF] Destribats | Soft microgels as Pickering emulsion stabilisers: role of particle deformability[END_REF] . Indeed, emulsions prepared from microgels with a higher cross-linker contents were mechanically less stable than those prepared with poorly cross-linked microgels. A tentative explanation is the interfacial elasticity caused by entanglements between adjacent microgels [START_REF] Destribats | Soft microgels as Pickering emulsion stabilisers: role of particle deformability[END_REF] . At drop surface, flattened microgels assemble into a continuous 2D network, connected together side by side. We suggested that less deformable microgels were less prone to the formation of a cohesive network and the more brittle interface could be broken upon mechanical disturbances. Thus, the flocculation state of emulsions is also directly linked to microgel conformation [START_REF] Schmitt | Surface compaction versus stretching in Pickering emulsions stabilised by microgels[END_REF][START_REF] Destribats | Origin and control of adhesion between emulsion drops stabilized by thermally sensitive soft colloidal particles[END_REF][START_REF] Destribats | Pickering Emulsions Stabilized by Soft Microgels: Influence of the Emulsification Process on Particle Interfacial Organization and Emulsion Properties[END_REF] . Indeed, the homogeneity of the polymer layer depends on the packing and conformation of microgels: compressed microgels result in a thicker polymer layer, whereas flattened ones lead to a more uneven coverage. The latter ones are more prone to bridging than the first, owing to their tendency to share microgels between two adjacent interfaces with poor coverage area [START_REF] Schmitt | Surface compaction versus stretching in Pickering emulsions stabilised by microgels[END_REF] . We have previously demontrated that this parameter could be tuned via the size of the microgels [START_REF] Destribats | Impact of pNIPAM Microgel Size on Its Ability To Stabilize Pickering Emulsions[END_REF] and process parameters such as the stirring rate [START_REF] Tatry | Kinetics of spontaneous microgels adsorption and stabilization of emulsions produced using microfluidics[END_REF][START_REF] Destribats | Pickering Emulsions Stabilized by Soft Microgels: Influence of the Emulsification Process on Particle Interfacial Organization and Emulsion Properties[END_REF] or the initial swelling state of the microgels [START_REF] Destribats | Origin and control of adhesion between emulsion drops stabilized by thermally sensitive soft colloidal particles[END_REF] . Similar features have been observed in foam films which could be either adhesive in case of low microgel concentration/flattened conformation or non adhesive in case of excess concentration/compressed conformation, as the result of different kinetics of spontaneous adsorption [START_REF] Keal | Drainage dynamics of thin liquid foam films containing soft PNiPAM microgels: influence of the cross-linking density and concentration[END_REF] .

Up to now, the applicability of microgel-stabilized emulsions has been mainly dedicated to food science using protein microgels [START_REF] Dickinson | Biopolymer-based particles as stabilizing agents for emulsions and foams[END_REF] and the range of targeted stimuli was rather narrow, pH and temperature being the most common. Recently, new applications in hydrogen storage were investigated using carbon dioxide-sensitive microgels [START_REF] Zhang | Programming Hydrogen Production via Controllable Emulsification/Demulsification in a Switchable Oil-Water System[END_REF] . In the present paper, our goal is to pave the way towards broader applications, using functional microgels having specific responsive behavior. In fact, we address the following question: what is the effect of introducing a functional group into the microgel on their ability to stabilize emulsions ? This question was raised long ago for pH-sensitive microgels bearing carboxylic functions [START_REF] Kwok | Correlating the effect of co-monomer content with responsiveness and interfacial activity of soft particles with stability of corresponding smart emulsions[END_REF][START_REF] Brugger | Emulsions stabilized by stimuli-sensitive poly(Nisopropylacrylamide)-co-methacrylic acid polymers: Microgels versus low molecular weight polymers[END_REF][START_REF] Massé | Impact of electrostatics on the adsorption of microgels at the interface of pickering emulsions[END_REF] , i.e. hydrophilic functions bringing electrostatic contributions. It was demonstrated that the amount of charges did not affect emulsion stability, whereas pH could indirectly modify internal cross-links due to hydrogen bonds and thus emulsion stability [START_REF] Destribats | Soft microgels as Pickering emulsion stabilisers: role of particle deformability[END_REF][START_REF] Massé | Impact of electrostatics on the adsorption of microgels at the interface of pickering emulsions[END_REF] .

In this study, we focus our research on the use of sugar-responsive microgels to stabilize Pickering emulsions, obtained via the introduction of phenylboronic acid (PBA) groups in the structure (Figure 1). PBA is known to bind diols of saccharides to make a cyclic ester, a reversible bond classified among the dynamic covalent bonds [START_REF] Lapeyre | Monodispersed glucose-responsive microgels operating at physiological salinity[END_REF][START_REF] Ancla | Designed Glucose-Responsive Microgels with Selective Shrinking Behavior[END_REF] . PBA function also exhibits a hydrophobic character, which can be switched to more hydrophilic upon sugar binding. It has been many times exploited to build glucose-responsive hydrogel architectures [START_REF] Bull | Exploiting the Reversible Covalent Bonding of Boronic Acids: Recognition, Sensing, and Assembly[END_REF][START_REF] Springsteen | A detailed examination of boronic acid-diol complexation[END_REF][START_REF] Lorand | Polyol complexes and structure of the benzeneboronate ion[END_REF][45][START_REF] Ma | Phenylboronic acid-based glucose-responsive polymeric nanoparticles: synthesis and applications in drug delivery[END_REF] . Other glucose-responsive materials have been developed by dissembling a structure linked by boronate esters in the presence of a competing monosaccharide [START_REF] Wu | Organization of glucose-responsive systems and their properties[END_REF] . A few glucose-responsive emulsions have been reported, all based on PBA derivatives as glucose-sensing elements [START_REF] Chen | Boronic Acid Linear Homopolymers as Effective Emulsifiers and Gelators[END_REF][START_REF] Guo | Responsive Emulsions Stabilized by Amphiphilic Supramolecular Graft Copolymers Formed in Situ at the Oil-Water Interface[END_REF][START_REF] Guo | Dual responsive Pickering emulsions stabilized by constructed core crosslinked polymer nanoparticles via reversible covalent bonds[END_REF] .

Sumerlin et al. [START_REF] Chen | Boronic Acid Linear Homopolymers as Effective Emulsifiers and Gelators[END_REF] exploited the soluble to insoluble transition of a PBA polymer as a function of glucose to turn a Pickering emulsion into an amphiphilic polymer-stabilized one. In another study, Cheng and Gao took advantage of interfacial complexation between diol polymers and PBA-ones, each polymer being soluble in a different phase of the emulsion [START_REF] Guo | Responsive Emulsions Stabilized by Amphiphilic Supramolecular Graft Copolymers Formed in Situ at the Oil-Water Interface[END_REF] . In the presence of a competing monosaccharide, the interfacial complex was disrupted and the emulsion was destroyed. Based on a similar approach, Gao used self-assembled micelles cross-linked by PBA-diol reversible covalent bonds to stabilize emulsions, which disrupted upon glucose addition [START_REF] Guo | Dual responsive Pickering emulsions stabilized by constructed core crosslinked polymer nanoparticles via reversible covalent bonds[END_REF] .

Figure 1: Scheme illustrating the swelling states of the microgels in the absence or presence of saccharide. For the sake of clarity, the mechanism is simplified. In the absence of saccharide, a small fraction of the phenylboronic acids is actually under the form of boronates. Upon saccharide addition, the formation of the boronate ester subsequently increases the fraction of charged species.

In the present paper, a detailed study of the link between the swelling properties of various boronate-functional microgels in aqueous solution, the microgel adsorption at a liquid interface (air-water) and their ability to stabilize Pickering emulsions is presented. The role of sugar addition on both interfacial behavior and the resulting emulsion stability is investigated. Thanks to the modulation of hydrophobic interactions inside the microgels and thus their deformability, the outcome of this work is the development of sugar-responsive emulsions.

2) Materials and methods

Chemicals

All reagents were purchased from Sigma-Aldrich unless otherwise noted. Nisopropylacrylamide (NIPAM) and N-isopropylmethacrylamide (NIPMAM) were recrystallized from hexane (ICS) and dried under vacuum overnight prior to use. N,N'methylenebis(acrylamide) (BIS), potassium persulfate (KPS), D-(+)-glucose (>99.5%, Sigma-Aldrich), D-(-)-fructose (>99%, Fluka BioChemika) and n-dodecane (purity >99%) were used as received. 3-acrylamidophenylboronic acid (AAPBA) was achieved adapting the procedure previously described by Kitano and co-workers. [START_REF] Schmitt | Surface compaction versus stretching in Pickering emulsions stabilised by microgels[END_REF][START_REF] Kitano | A novel drug delivery system utilizing a glucose responsive polymer complex between poly (vinyl alcohol) and poly(N-vinyl-2-pyrrolidone) with a phenylboronic acid moiety[END_REF] Milli-Q water was used for all synthesis reactions, purification and solution preparation.

Particles synthesis and purification

The microgels were obtained by an aqueous free-radical precipitation polymerization [START_REF] Pelton | Preparation of aqueous latices with N-isopropylacrylamide[END_REF] , which was adapted for the incorporation of the PBA ligand [START_REF] Lapeyre | Monodispersed glucose-responsive microgels operating at physiological salinity[END_REF][START_REF] Denisov | Saccharide-induced modulation of photoluminescence lifetime in microgels[END_REF] . In brief, NIPAM or NIPMAM and BIS as a cross-linker agent were copolymerized with AAPBA. . The total monomer concentration was held constant at 70 mM and the co-monomer ratio (87.5:2.5:10) (NIPAM or NIPMAM : BIS : AAPBA) was varied according to the desired receptor concentration. NIPAM or NIPMAM and BIS were introduced into 47.5 mL of water. AAPBA was dissolved in 0.5 mL of methanol and added to the previous solution. The whole solution was heated to 70 °C and thoroughly purged with nitrogen prior to initiation. Free radical polymerization was then initiated with a 2 mL of degassed KPS solution (1.5 mM). The solution reacted for 6 h under nitrogen atmosphere. After polymerization completion, the microgels were purified by several centrifugation-redispersion steps in water or phosphate buffer pH 8.5 containing 20 mM fructose (16 000 tr.min -1 corresponding to 29 220 gr where gr is the terrestrial acceleration, for 1 hour, at 24°C) until the surface tension of supernatant was above 70 mN/m for 1 min, a proof that the microgel dispersions were almost free of surface active impurities (approx. 5 cycles).

The resulting microgels were named pNIPAM-PBA or pNIPMAM-PBA and are represented in Elemental analysis. The boron contents in dilute suspensions of microgels were determined by inductive coupled plasma-optical emission spectroscopy (Varian ICP/OES 720 ES). The molar percentages of PBA monomeric units were calculated knowing the dried mass of polymer in the solution. It was 14.4 mol% for pNIPAM-PBA and 12.6 mol% for pNIPMAM-PBA.

Particle characterization

Dynamic Light Scattering. Microgel hydrodynamic diameters as well as polydispersity index were determined by dynamic light scattering (DLS) with a Zetasizer Nano S90 Malvern Instruments equipped with a HeNe laser at 90°, using the software supplied by the manufacturer. The samples contained about 0.01 wt% of polymer in various aqueous solutions (pure water, phosphate buffer 4 mM at pH 8.5, glucose (100 mM) or fructose solutions (20 mM) in the phosphate buffer).

Zeta potential. Electrophoretic mobility measurements were performed using the Zetasizer Nano ZS (from Malvern Instruments, UK) at a given temperature and pH, with or without glucose in phosphate buffer, after allowing the mixture to equilibrate for 10 min. Each measurement was performed at least 5 times.

Transmission Electron Microscopy (TEM). The microgel morphology was studied by TEM (TEM JEOL JEM 1400 (120 kV)), after drying on a carbon-coated copper grid and staining with uranyl acetate.

Atomic Force Microscopy (AFM)

The dried microgels were observed after deposition on a silicon substrate. Two preparation routes were used: firstly, by drop casting, secondly after transfer to the solid silicon substrate of a microgel monolayer built at the liquid air-water interface using a Langmuir trough. The samples were analysed by AFM (Veeco Multimode 8). Topographic images were obtained in Scanasyst-in Air mode using a silicon tip on nitride lever (Scanasyst-Air, Bruker) with a spring constant of 0.4 N.m -1 and a resonance frequency 70 kHz. 10x10 µm 2 or sometimes larger, images were taken at different areas of the samples, distant from at least 3 mm, the maximum distance allowed by the screw of the set-up. The spatial organization remained the same over the full-explored range. As a usual treatment in AFM, the height images were flattened to remove slight tilt of the sample with respect to the AFM tip.

Emulsion preparation and characterization

Emulsification. Emulsions composed of 30 wt% of dispersed phase were prepared. Typically, 14 g of aqueous phase containing a given amount of microgels were mixed with 6 g of dodecane, upon stirring with an Ultra-Turrax T25 mixer equipped with a S25 KV-25F shaft, at constant speed (9500 rpm) for 30 s, while being cooled at T=4°C. This cooling step was performed in order to maximize the swelling state of the microgels. Emulsion characterization was then performed at 20°C. Emulsion stability was checked by visual inspection. Microgel concentrations Cp in the range between 0.01 and 0.05 wt% with respect to aqueous weight were chosen in order to be in the limited coalescence domain. This concentration range is the socalled particle poor regime that leads to more monodisperse emulsions and total microgel adsorption at the oil/water interface thanks to the limited coalescence phenomenon occurring in Pickering emulsions [START_REF] Arditty | Some general features of limited coalescence in solid-stabilized emulsions[END_REF] .

Optical microscopy. The size distributions of emulsions were estimated by direct imaging using inverted optical microscope (Zeiss Axiovert X100) and a video camera.

CryoSEM observations. Cryo-SEM observations were carried out with a SEM-FEG HR JEOL 6700F electron microscope equipped with liquid nitrogen cooled sample preparation and transfer units, as already described in previous papers [START_REF] Destribats | Soft microgels as Pickering emulsion stabilisers: role of particle deformability[END_REF] .

Dynamic surface tension

Dynamic interfacial tension of the air/water and dodecane/water interfaces was measured using the pendant-drop technique (Teclis). An 10 µL aqueous drop (approximately 22 mm² ) containing a known concentration of microgels in either phosphate buffer (4 mM, pH 8.5) or fructose (20 mM) solution in the same phosphate buffer, was suspended in air or in dodecane.

The tension was deduced from the axisymmetric drop shape by fitting with the Laplace equation. A constant drop volume was maintained over 10 000 seconds and the tension was recorded as a function of time.

Compressed Langmuir film

Compression isotherms were recorded at 22°C using a home-made Langmuir-trough (trough dimensions: 145 mm × 690 mm, compressible area: 1000 cm 2 ). The surface pressure was recorded by a balance equipped with a paper Wilhelmy-plate. The microgels were dispersed in a mixture of pure water and isopropanol (5:1). Prior to spreading, it was verified by DLS that the dispersion did not contain any aggregates. The subphase could be composed of pure water, phosphate buffer (4 mM, pH 8.5) or fructose (20 mM) solution in buffer. The deposition method, compression and transfer on solid silicon substrate was performed as described previously [START_REF] Picard | Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density[END_REF] . To study the film stability at different surface pressures, a setpoint of the surface pressure was applied. If the film was stable, the Langmuir barriers automatically stopped leading to a constant surface area corresponding to the required surface pressure. If not, the barriers continued to move in order to maintain the surface pressure constant, showing that the microgels did not remain adsorbed at the interface and probably dived in the subphase. .

3) Results and discussion

3.1. Microgel synthesis pNIPAM and pNIPMAM-based microgels incorporating phenylboronic acid (PBA) groups as a glucose ligand were synthesized by copolymerization of NIPAM or NIPMAM respectively with 3-acrylamidophenylboronic acid (AAPBA) (Figure 2). The synthesis of the first ones was reported earlier by some of us [START_REF] Lapeyre | Monodispersed glucose-responsive microgels operating at physiological salinity[END_REF] . Here, we propose to study two different architectures as two prototypical microgels, having different amplitudes in the response to saccharides at a given temperature. The choice of the main monomer is important to control the VPTT of the microgel, being respectively 32°C and 42°C for pNIPAM and pNIPMAM [START_REF] Pelton | Temperature-sensitive aqueous microgels[END_REF] . The incorporation of PBA function in a thermoresponsive structure, generally decreases its VPTT, because PBA is hydrophobic [START_REF] Lapeyre | Monodispersed glucose-responsive microgels operating at physiological salinity[END_REF][START_REF] Kataoka | Totally synthetic polymer gels responding to external glucose concentration: Their preparation and application to on-off regulation of insulin release [15[END_REF] . The change in the VPTT depends on the amount of PBA, on its ionization state and on its distribution in the structure [START_REF] Ancla | Designed Glucose-Responsive Microgels with Selective Shrinking Behavior[END_REF][START_REF] Hoare | Engineering Glucose Swelling Responses in Poly(N-isopropylacrylamide)-Based Microgels[END_REF] , which is itself a consequence of its copolymerization rate with the main monomer and the cross-linker. Based on previous results, we chose to introduce 10 mol% of PBA monomer relative to NIPAM or NIPMAM. Elemental analysis confirmed an effective incorporation of 14.4 and 12.6 mol% for pNIPAM-PBA and pNIPMAM-PBA respectively. The discrepancies between effective and targeted values, are attributed to the different reactivity ratios of NIPAM and NIPMAM with respect to AAPBA.

Microgel characterization

To characterize the result of the synthesis, microgels were probed by different techniques: in the hydrated state, by DLS (Figure 3) and Zeta potential (Figure S1) measurements, and by imaging in the dried state, by TEM (Figure 4) and AFM (Figure 5). In all cases, the effect of saccharide addition was investigated. Since the microgels were thermoresponsive, the effect of temperature on their hydrodynamic diameter was also investigated. Phenylboronic acids (PBA) are known to make reversible cyclic ester bonds with 1,2-and 1,3diols [START_REF] Lorand | Polyol complexes and structure of the benzeneboronate ion[END_REF][45] , among which saccharides (Figure 1). They have been extensively investigated as synthetic ligands for glucose [START_REF] Wu | Organization of glucose-responsive systems and their properties[END_REF][START_REF] Ma | Phenylboronic Acid-Containing Glucose-Responsive Polymer Materials: Synthesis and Applications in Drug Delivery[END_REF][START_REF] Elshaarani | Synthesis of hydrogel-bearing phenylboronic acid moieties and their applications in glucose sensing and insulin delivery[END_REF] . The addition of saccharides provokes the formation of an ester boronate with a binding constant which depends on sugar configuration. Complexation of the trigonal acidic form of PBA with diols is unstable because it is highly susceptible to hydrolysis [START_REF] Lorand | Polyol complexes and structure of the benzeneboronate ion[END_REF] . Therefore, at pH close the pKa of the PBA, the addition of saccharide induces a shift of the ionization equilibrium towards a higher concentration of charged entities. The translation of diol-binding to hydrogel swelling exploits the transition from hydrophobic/neutral trigonal acidic form to hydrophilic/charged tetrahedral form of the diolboronate ester PBA. In fact, swelling changes are the consequences of both solvency effecti.e. the increasing hydrophilicity of the polymer chain improves the polymer-solvent affinity in water -and Donnan pressure effects -i.e. the change in charge density increases the local concentration of mobile counterions, which increases the inner osmotic pressure in the microgels [START_REF] Rika | Swelling of ionic gels: Quantitative performance of the Donnan theory[END_REF] . Regarding these principles, microgel swelling in response to saccharide addition were studied at pH 8.5, close to the pKa of the PBA derivative (8.2 for AAPBA [START_REF] Matsumoto | Glucose-responsive polymer bearing a novel phenylborate derivative as a glucose-sensing moiety operating at physiological pH conditions[END_REF] ). Two monosaccharides were investigated: glucose for its interest in the medical field, in particular diabetes, and fructose as a model, because fructose presents a higher association constant with PBA (Kass = 160 M -1 ) than glucose (Kass = 4.6 M -1 ) [START_REF] Springsteen | A detailed examination of boronic acid-diol complexation[END_REF] . The evolution of the hydrodynamic diameter as a function of temperature was followed in various buffer conditions: pure water (pH 6), phosphate buffer (pH 8.5) and buffer containing either glucose (100 mM) or fructose (20 mM). It was known from previous results that these concentrations give the highest amplitude of swelling [START_REF] Denisov | Saccharide-induced modulation of photoluminescence lifetime in microgels[END_REF] . The two microgel samples were monodisperse, with a low polydispersity index, as indicated by DLS analysis (Table 1). In pure water, the PBA moieties were mostly in their neutral form, as also proved by zeta potential measurements (Figure S1). The microgels deswelled as temperature increased and reached a full collapsed state above 31°C and 36°C for pNIPAM-PBA and pNIPMAM-PBA respectively. The transition was very smooth, over a temperature range comprised between 10 and 15°C. As expected, the presence of PBA decreased the VPTT but the large width of the transition indicated an inhomogeneous structure. For instance, linear thermoresponsiveness was observed in the literature as the result of a delicate interplay between thermodynamics and mechanical properties in core-shell microgels incorporating different LCST polymers, the collapsed shell exerting a corset effect on the more swollen core [START_REF] Zeiser | Linearly thermoresponsive core-shell microgels: Towards a new class of nanoactuators[END_REF] . Similar phase transition characteristics were observed in PBA-grafted microgels with grafts localized in the outer shell [START_REF] Hoare | Engineering Glucose Swelling Responses in Poly(N-isopropylacrylamide)-Based Microgels[END_REF] and in core-shell microgels bearing PBA groups in the shell [START_REF] Lapeyre | Glucose-responsive microgels with a core-shell structure[END_REF] , whereas phase transitions were sharp in the case of hydrogels with randomly distributed PBA moieties [START_REF] Kataoka | Totally synthetic polymer gels responding to external glucose concentration: Their preparation and application to on-off regulation of insulin release [15[END_REF] . In the present case, the PBA-functionalized microgels could have a more hydrophobic shell, bearing a higher concentration of PBA monomeric units. When compared in the same hydrated state, i.e. in the collapsed state (50°C), pNIPMAM-PBA microgels were smaller than pNIPAM-PBA.

290

(0.182) [START_REF] Zhang | Poly(N-isopropylacrylamide) microgels at the air-water interface[END_REF] The relative swelling is defined as the microgel volume in presence of fructose to the microgel volume in absence of fructose ratio. dH is the hydrodynamic diameter measured by DLS in the various conditions.

The monodispersity criterion was confirmed by TEM (Figure 4a, e) and AFM imaging (Figure 5a, c), although AFM imaging revealed the presence of a few doublets. These two observations were performed in the dried state, possibly leading to different apparent diameters depending on the microgel deformation on the substrate. In TEM, grainy structures were observed for pNIPAM-PBA particles (Figure 5a).

AFM observations revealed the compact structure of the particles. They exhibited heights from about 2 to 3.5 times larger than those of pNIPAM microgels having similar hydrodynamic diameter in solution. Indeed, for instance, corresponding pure pNIPAM microgels had a height below 100 nm [START_REF] Picard | Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density[END_REF] , indicating their marked flattening on solid substrate [START_REF] Horecha | Ordered surface structures from PNIPAM-based loosely packed microgel particles[END_REF][START_REF] Serpe | Colloidal hydrogel microlenses[END_REF][START_REF] Schmidt | Thermoresponsive surfaces by spincoating of PNIPAM-co-PAA microgels: A combined AFM and ellipsometry study[END_REF] . In order to quantify the deformation of the microgel on the solid substrate, the height was compared to the full width at half maximum (FWHM) (Table 2). The height over FWHM ratio were found to be about 0.75, to be compared with 0.1 obtained for pNIPAM with the same cross-linking ratio and taken in the same conditions (note that for a non-deformed microgel this value should be equal to 1).

These data show that the PBA-modified microgels were hardly deformed on the solid substrate.

The fact that the diameter at base was very close the FWHM for both PBA functionalized microgels, also confirmed the low deformability of the microgels studied in the present paper. At pH 8.5, although part of the PBA was ionized, the hydrodynamic diameters were very close to those at pH 6 whatever the temperature below the collapsed state (Figure 3). However, once the collapsed state was reached, the microgels aggregated due to the presence of electrolyte in the buffer (4 mM), which caused a decrease of the Debye length (about 4 nm) and consequently screened the electrostatic repulsions. The addition of saccharides provokes a swelling of both microgels whatever the temperature below the VPTT and a shift of the VPTT towards higher temperatures. These two phenomena were both explained by the increased affinity of the polymer chain for the solvent and an increase of the counterion Donnan osmotic pressure inside the microgels. The response to fructose was stronger than that to glucose, in accordance with the binding constant order. For the sake of simplicity, we focused our study on the role of fructose on emulsion stabilization and interfacial properties, considering that glucose plays an equivalent role but to a less extent.

At a fixed temperature below the VPTT, the effect of saccharide addition on the polymeric structure could be visualized by microscopy means. The microgels, initially immersed in fructose solution, were deposited and dried on a TEM grid and further imaged. Figure 4 gives a comparison of both microgels, with and without fructose. Obviously, additionally to the overall diameter increase also seen by DLS, the morphology has changed. After fructose addition, the particles appear to be surrounded by a corona of side chains. The differences are even more pronounced for pNIPMAM-PBA microgels, where the area covered by peripheral chains exceeds that of the cores. Without fructose, the particles are compact. In fructose solution, the dimensions of the cores are smaller than that of the initial particles but the side chains are widely extended. It suggests that the microgels are constituted by a non-responsive core and a sugar-responsive shell made of dangling chains, which undergo a collapse-to-coil transition upon fructose addition.

Emulsion characteristics

The as-prepared saccharide-responsive microgels were further used as emulsion stabilizers.

Emulsions were prepared by mixing oil -here dodecane-and an aqueous phase. The aqueous phase was either pure water or buffer in presence or absence of fructose. The solutions were kept at a temperature as low as possible below the VPTT of the microgels, in order to maximize the swelling state of the microgels. The conditions were chosen according to previous studies involving pure pNIPAM microgels, in which it was shown that non-swollen microgels could not stabilize emulsions. Various concentrations of microgels were also tested ranging from 0.01 wt% up to 0.05 wt% in aqueous phase, which is the usual range for this size of microgels in the limited coalescence process, where all the microgels adsorb irreversibly at the interfaces and the amount of oil-water interfacial area depends on microgel concentration [START_REF] Destribats | Soft microgels as Pickering emulsion stabilisers: role of particle deformability[END_REF] . This process generally yields monodisperse emulsions, because the drops undergo several coalescence processes until they reach a sufficient coverage [START_REF] Arditty | Some general features of limited coalescence in solid-stabilized emulsions[END_REF] . For PBA-functionalized microgels, oil-inwater emulsions were obtained in all conditions (Figure 6 and S2). The drops were quite large and rapidly creamed due to the low density of oil. Typical examples are given in Figure 6.

However, contrarily to pure pNIPAM microgels, it was not possible to prepare monodisperse emulsions based on limited coalescence process, whatever the microgel concentration (Figure 6c). Their kinetic stability was assessed at 4°C and at room temperature (20°C). At rest, pNIPMAM-PBA samples in pure water and buffer with or without fructose, presented the same behavior. However, as soon as the vials were gently shaken, those without fructose underwent a rapid coalescence leading to macroscopic phase separation (Figure 6). This mechanical instability was also detected in microscopy observations, where the simple transfer on the microscope slide induced drop coalescence (shown in Figure 6 A and B). The merging drops did not relax their shape. In the case of pNIPAM-PBA, emulsions were not stable at rest in the absence of fructose (Figure S2). Conversely, all the samples prepared in the presence of fructose remained stable at rest or under mechanical disturbance, although no limited coalescence could be observed again. These emulsions were highly flocculated and did not flow because of bridging between adjacent drops (Figure 6C). This phenomenon is classical in microgel-stabilized emulsions prepared with such large microgels [START_REF] Destribats | Origin and control of adhesion between emulsion drops stabilized by thermally sensitive soft colloidal particles[END_REF] . It is generally exalted by increasing the cross-linking density and size of the microgels, as a result of their slow adsorption [START_REF] Destribats | Impact of pNIPAM Microgel Size on Its Ability To Stabilize Pickering Emulsions[END_REF] . Therefore, the organization of microgels was further characterized in these conditions.

CryoSEM imaging (Figure 6E, F and S2c) revealed that the microgels were organized in a hexagonal array, with a lattice parameter of 1210 nm for pNIPMAM-PBA microgels and 1160 nm for pNIPAM-PBA, higher than their diameter in solution. It means that the microgels were flatten at the drop surface, in accordance with previous results obtained with pure pNIPAM [START_REF] Destribats | Soft microgels as Pickering emulsion stabilisers: role of particle deformability[END_REF] .

It is also worth noting that the microgels were more flattened on the drop surface than after drop casting in AFM, meaning that the organization on flat solid substrate, which is dryingdependent, does not reflect the behavior at drop surfaces in emulsions, which is processdependent.

Finally, the effect of the sugar concentration on the emulsion stability was studied (Figure 7) in the range of fructose concentration (0 -20 mM). It can be observed that below 5 mM the emulsions are only partially stable against a mechanical disturbance, the mechanical stability being even more pronounced than the fructose concentration increases. Above 5 mM, the emulsions are stable. This behavior can be correlated to the microgels' swelling behavior.

Indeed in the (0-5 mM) range, the swelling ratio is sugar-concentration dependent. [START_REF] Ancla | Designed Glucose-Responsive Microgels with Selective Shrinking Behavior[END_REF][START_REF] Denisov | Saccharide-induced modulation of photoluminescence lifetime in microgels[END_REF] . So there is a good accordance between the emulsions mechanical stability and the microgels' behavior, Note that no attempt was made to cross-correlate the droplet size because, in any case, emulsions were polydisperse.

Last but not least, these emulsions could be considered as dual-responsive systems. Indeed, they were also thermoresponsive (Figure S3). In this case, macroscopic phase separation occurred at rest by heating the emulsion above the VPTT. While all the emulsions creamed rapidly, only emulsions in the presence of fructose remained stable against coalescence despite a mechanical disturbance. CryoSEM views of pNIPMAM-PBA microgels at the surface of an oil drop (oil-water interface). Conditions: dodecane-inaqueous phase emulsion, the aqueous phase contained buffer, fructose (20 mM) and 0.01 wt% of microgels. Scale bar is 10 µm (D) and 1 µm (E).

Behavior of microgels at model interfaces

To better understand the role of the microgel structure on emulsion stability, it is necessary to get a better insight into the behavior of the particles at a flat liquid interface. We focused on the air-water interface because it avoids the adsorption of some impurities coming from the oil phase. Moreover, it was shown previously that the features are qualitatively the same as for oilwater interface, the only difference being the value of the surface pressure [START_REF] Picard | Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density[END_REF][START_REF] Tatry | Kinetics of spontaneous microgels adsorption and stabilization of emulsions produced using microfluidics[END_REF] .

We first studied the kinetics of adsorption using a pendant drop tensiometer. The kinetics of adsorption of soft microgels being dependent on microgel concentration range, the experiments were carried out at a constant concentration of microgels, selected in the domain where the interfacial tension has become independent of the microgel concentration, based on previous studies on pNIPAM microgels [START_REF] Tatry | Kinetics of spontaneous microgels adsorption and stabilization of emulsions produced using microfluidics[END_REF] . Comparison was made between solutions without and with fructose (Figure 8). For both pNIPAM-PBA and pNIPMAM-PBA, the surface tension progressively decreased and reached a quasi-plateau with a value 𝛾 independent of fructose concentration: about 45 mN/m and 50 mN/m respectively (values at 10,000 s). The surface pressure which is defined as the difference between 0 the pristine surface tension and that of the solution 𝜋(𝑡) = 𝛾 -𝛾(𝑡), could be deduced from these values. At infinite time, this surface pressure reaches a quasi-plateau at a value f equal to 25.9 mN/m for pNIPAM-PBA and 22.4 mN/m for pNIPMAM-PBA. The 3 mN/m lower interfacial pressure of pNIPMAM compared to pNIPAM may be due to its lower hydrophobicity. In both cases, as can be seen from Figure 8, the adsorption kinetics were accelerated in the presence of fructose. To quantify this effect, the characteristic time 𝑡 / was defined as the time required to increase the surface pressure by half of its final value. In both cases, the characteristic time decreased by one order of magnitude in the presence of fructose (Table 3). A similar behavior was qualitatively obtained at the oil-water interface (Figure S4). Quantitatively, the only difference was the value of the surface pressure at equilibrium, which was slightly higher than that at the air-water interface (up to 34 mN/m instead of 26 mN/m for pNIPAM-PBA). Such difference is in agreement with previous studies on pNIPAM [START_REF] Picard | Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density[END_REF] . Finally, the compression isotherm was recorded using a Langmuir trough. The deposition of the microgels at the air-water interface was performed in the presence of isopropanol as a spreading agent. Isopropanol is known to act as a co-nonsolvent for pNIPAM [START_REF] Geisel | Highly ordered 2D microgel arrays: compression versus selfassembly[END_REF] . Thus, the microgels, initially dispersed in buffer with or without fructose, were supplemented with isopropanol (15 vol% compared to aqueous phase) and deposited at the air-water interface.

After isopropanol evaporation, the barriers were moved for compression and the isotherm was recorded (Figure 9a). In the absence of fructose, a steep increase of the surface pressure was observed. It resembles that of hard spheres [START_REF] Szekeres | Two-dimensional ordering of Stöber silica particles at the air/water interface[END_REF][START_REF] Reculusa | Langmuir-Blodgett films of micron-sized organic and inorganic colloids[END_REF] in agreement with the morphology observations.

In the presence of fructose, the film could be compressed over a broader range of area like soft microgels [START_REF] Pinaud | Adsorption of microgels at an oil-water interface: correlation between packing and 2D elasticity[END_REF][START_REF] Picard | Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density[END_REF][START_REF] Rey | Isostructural solid-solid phase transition in monolayers of soft core-shell particles at fluid interfaces: Structure and mechanics[END_REF] , again in accordance with the morphology observations. In this case, the onset of surface pressure increase occurred at normalized area five times larger than that in the absence of fructose showing the higher deformability of these microgels. Above 20 mN/m, the surface pressure continued to increase significantly upon barrier compression, up to 50 mN/m, and the isotherm exhibited a change in the slope resembling a phase transition. However, with or without fructose, the films were unstable, as the surface pressure dropped immediately after stopping the barriers.

In order to get a clear view of the interfacial organization of microgels at surface pressures below 20 mN/m, the film obtained at pH 8.5 in presence of fructose was transferred on a solid substrate and further imaged by AFM in the dried state (Figure S5). Figure 9b and9c show images corresponding to two different surface pressures. As for pNIPAM microgels, it could be observed that the microgels arrange in a hexagonal packing, with a lattice parameter decreasing upon compression, although a few defects were observed, arising from the presence of some doublets in the dispersion. The center-to-center distance between microgels was found to be 1600 nm and 1150 nm at 0.5 mN/m and 19.5 mN/m respectively.

Discussion

In this paper, our intention is to report a general method to build microgel-stabilized emulsions that can demulsify on demand by molecular recognition in solution. The functionalization of the microgels by a ligand is thus a prerequisite. The recognition of the target molecule should not only induce a change in the swelling behavior in solution but also a change in the conformation of microgels at liquid interfaces. The case of PBA-functionalization illustrates how these changes can occur through introducing hydrophobic and aromatic groups. In order to better understand the role of the ligand, we need to address first the role of hydrophobicity and will then compare PBA functionalized-microgels to non-functionalized-pNIPAM ones.

Effect of microgel functionalization by PBA

The set of data presented above unambiguously demonstrates that pNIPAM-PBA microgels are more compact than pNIPAM ones at room temperature. The VPTT decreased when introducing PBA groups, which imparts hydrophobicity to the polymeric structure. By analogy with previous works [START_REF] Zeiser | Linearly thermoresponsive core-shell microgels: Towards a new class of nanoactuators[END_REF] , the smooth volume phase transition indicates a core-shell structure, where the shell could be less swollen than the core and play a corset effect. This would mean a higher density of PBA groups in the shell, providing a less swollen shell than the core, instead of a homogeneous distribution in the polymeric network.

TEM and AFM imaging both show compact structures. In particular, the profile view of the microgels after drying on a solid substrate shows that the PBA-modified microgels deform less than its non-modified homologous. Moreover, a fine structure is observed by TEM, showing some graininess at the surface of the particles, with spherical domains having a denser electronic contrast. These spherical domains could be attributed to hydrophobic clustering, originating from PBA groups. We thus hypothesize that PBA functions introduce intramicrogels attractive interactions, acting as additional physical cross-links. This hypothesis is strongly supported by their behavior at liquid interfaces (flat air-water or drop surface). Here, we compare PBA-functionalized microgels with non-functionalized pNIPAM microgels, having the same chemical cross-linking density (2.5 mol% BIS).

In compression experiments, pNIPAM microgels exhibit two different behaviors: at low surface pressures, they pack into a hexagonal array of soft colloids, being compressible, whereas at high surface pressure, microgels reach an incompressible state and a second phase transition occurs, identified as the soft-to-hard sphere transition [START_REF] Picard | Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density[END_REF] . Increasing further the compression leads to film collapse [START_REF] Picard | Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density[END_REF] . In the first regime, when the packing density of the particle increases, the lattice parameter decreases simultaneously, but not monotonously because the microgels have an uneven distribution of chains and cross-linker. It was noticed that the compressibility is higher for microgels having the lowest cross-linking density [START_REF] Picard | Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density[END_REF] . Therefore, for PBAfunctionalized microgels in presence of fructose, the observed features are very similar to the one observed for pNIPAM microgels. On the contrary, in the case of PBA-functionalized microgels without fructose, the surface pressure exhibits a steep increase, resembling that of hard undeformable particles [START_REF] Reculusa | Langmuir-Blodgett films of micron-sized organic and inorganic colloids[END_REF][START_REF] Hunter | Adsorption of Submicrometer-Sized Cationic Sterically Stabilized Polystyrene Latex at the Air-Water Interface: Contact Angle Determination by Ellipsometry[END_REF] . This behavior can be expected when the cross-linking density of microgels increases significantly as a continuous evolution from soft deformable to hard undeformable particles. Indeed the shape of the compression curve of PBA-functionalized microgels in absence of fructose reveals their low compressibility in accordance with their compact structure observed upon imaging single particles deposited on solid substrates. More focusing on emulsions' stability, we observed that emulsions stabilized by these microgels are mechanically unstable as it was observed earlier for more cross-linked pNIPAM microgels of equivalent sizes -typically bearing more than 5 mol% BIS -that were efficient to stabilize emulsions at rest but inefficient to protect them upon mechanical disturbance [START_REF] Destribats | Origin and control of adhesion between emulsion drops stabilized by thermally sensitive soft colloidal particles[END_REF] . This means that the 2.5 mol% BIS pNIPAM-PBA microgels in absence of fructose confer the same kind of stability to the emulsion than 5 mol% BIS pNIPAM microgels. This effective cross-linking density range is in agreement with the PBA content. Indeed, if we consider that every crosslinking junction requires at least two PBA groups, the density of physical cross-linking would be at most ~6 mol%, which comes in addition to the 2.5mol% of BIS. Emulsion flocculation is also cross-linking dependent [START_REF] Destribats | Origin and control of adhesion between emulsion drops stabilized by thermally sensitive soft colloidal particles[END_REF] . Drop bridging phenomenon through microgel sharing is more pronounced for higher cross-linking rates for pNIPAM microgels. We also found a link between the extent of the limited coalescence process and the cross-linking density in pNIPAM microgels: the more cross-linked the microgels, the less extended the limited coalescence domain in terms of microgels concentration. In the case of PBA-microgels in absence of fructose, the limited coalescence domain could not be detected even at low microgels concentration.

Finally, the adsorption kinetics of microgels was found to be much slower than that of identical non-functionalized microgels. For instance, in a buffer having the same ionic strength (I=4 mM), the characteristic time of pNIPAM microgel adsorption is about 50 s for the same concentration of microgels [START_REF] Tatry | Kinetics of spontaneous microgels adsorption and stabilization of emulsions produced using microfluidics[END_REF] , which is more than one order of magnitude lower than that of pNIPAM-PBA. The adsorption kinetics is also affected by the cross-linking density. It is believed that this effect is the consequence of less dangling chains being present at the periphery of microgels when the cross-linker concentration increases. Regarding the structure of the film at equilibrium, i.e. the final adsorption state, it seems that the modification of the structure by substituting 10 mol% of the monomer by PBA does almost not change the interfacial activity.

Indeed, the final surface pressures are very close to that of non-functionalized ones, being slightly lower, which seems in contradiction with the expected increase of the hydrophobicity of PBA groups. This probably means that the hydrophobic groups are not directly responsible for polymer adsorption but rather contribute to rigidify the microgel structure. To fully address this question, further analysis about the internal distribution of PBA moieties would be necessary.

Regarding all the interfacial properties: emulsion stability, flocculation state, adsorption kinetics, PBA-functionalized microgels in absence of fructose behave like poorly deformable particle, confirming the interpretation of microgels having additional internal cross-links.

Effect of fructose on PBA-functionalized microgels

For all the listed above features, addition of fructose brings softness and thus deformability to the microgels, via a more hydrophilic structure and a better affinity between polymer chains and the solvent. In particular, TEM view evidenced the presence of a highly porous and extended shell surrounding a core particle, which was interpreted as an unfolding of the initially collapsed shell. The behavior of microgels at the interface indicates unambiguously that the structure behaves as less cross-linked microgels. 1) Although still flocculated and polydisperse, emulsions were mechanically more stable and stabilized by flattened microgels, as shown by Cryo-SEM experiments. 2) Microgels spontaneously adsorb more rapidly at liquid interface, despite the higher size of the microgels. It shows unambiguously that this adsorption is not limited by diffusion as was already concluded for pNIPAM microgels [START_REF] Tatry | Kinetics of spontaneous microgels adsorption and stabilization of emulsions produced using microfluidics[END_REF] . 3) A large compressibility of the Langmuir film could be observed, with a classical behavior for soft microgels, having low cross-linking density.

In conclusion, fructose acts on microgel swelling by yielding hydrophilicity and charges to PBA groups via the formation of boronate esters. At interfaces, in presence of fructose, the particles behave as less cross-linked microgels than without fructose, being either more deformable or having more dangling chains. Although indiscernible, these two features indicate that internal cross-links were released by fructose addition. Following this discussion, a schematic representation of the structure of sugar-responsive microgels can be drawn in Figure 10. It should also be noticed that the presence of fructose does not affect the equilibrium surface pressure, demonstrating once again the absence of correlation between emulsion stabilization and interfacial tension.

Conclusion

In this paper, Pickering emulsions that can be destabilized on demand by changing the sugar concentration were reported. Using prototypical poly(N-isopropylacrylamide) microgels, which were functionalized with phenylboronic acid functions as glucose sensing units, we could show that controlling emulsion stability using responsive microgels is a general process.

More specifically, through the study of these PBA systems, we studied the role of functionalization by hydrophobic/aromatic moieties in water-swollen microgels. Importantly, we showed that PBA-functionalized microgels behave like more cross-linked nonfunctionalized ones at liquid interfaces, likely due to the formation of additional physical intra-cross-links in the microgel structure. These hydrophobic grafts therefore reinforce the rigidity of the particles. As a consequence, the microgels seem to be more compact, they adsorb slowlier than non-functionalized ones, their Langmuir compression isotherm ressembles that of undeformable particles and the emulsions are fragile and flocculated by drop bridging. When saccharide is added, these physical cross-links are released. Adsorption becomes fast again, the Langmuir isotherm resembles that of soft deformable microgels and emulsions are mechanically robust, less flocculated, with less microgels being shared by neighboring drops.

Thus, the addition of sugar is a way to decrease the effective cross-linking density, rendering the particles more hydrophilic, and thus to modulate the deformability of the microgel.

In summary, the findings of this study should fully enlarge the applicability of micro-/nanogels to the stabilization of Pickering emulsions or foams. From a fundamental point of view, this work has demonstrated the unexpected role of hydrophobic/aromatic moieties on particle rigidity, which should give clues for a better understanding of the behavior of soft particles at interfaces. This parameter should be taken into account for the design of new emulsion/foam stabilizers, either fully synthetic systems or biopolymer-based ones, including protein-based assemblies. Further nanomechanics and nanorheology studies might help to quantify it properly [START_REF] Backes | Nanomechanics and Nanorheology of Microgels at Interfaces[END_REF] .

From an applicability perspective, these findings give insights into a way to exploit the hydrophilic-hydrophobic transition as a switch to prepare responsive Pickering emulsions or foams. In particular, the key components to prepare saccharide-sensitive emulsions based on boronate microgel technology were deciphered. Given the great advances in the area of saccharide-sensitive materials [START_REF] Ma | Phenylboronic acid-based glucose-responsive polymeric nanoparticles: synthesis and applications in drug delivery[END_REF][START_REF] Wu | Organization of glucose-responsive systems and their properties[END_REF] , with possibly biocompatible materials, reacting to required saccharide challenges, this concept could easily be exploited to develop all food-grade or pharmaceutical-grade formulations, for drug delivery purposes or food science technology.

More generally, it is of interest to develop systems that react selectively by molecular recognition in a solution via ligand incorporation in the microgel structure [START_REF] Muratalin | Preparation and characterization of microgels sensitive toward copper II ions[END_REF][START_REF] Gao | Cyclodextrin triggered dimensional changes of polysaccharide nanogel integrated hydrogels at nanometer resolution[END_REF][START_REF] Li | Bioresponsive Hydrogels and Microgels[END_REF] . Glucose sensing is not only of interest by itself, but studying the effect of hydrophobic group incorporation helps providing guidelines for the development of other analyte-sensitive systems. Finally, the modulation of microgel deformability, via hydrophobicity switching, appears as a generic route to develop emulsions or foams that can break on demand. It could be applied to a broad range of stimuli where hydrophobicity/hydrophilicity switches are involved, such as, for instance, light-sensitive hydrogels materials based on spiropyran [START_REF] Ziółkowski | Self-protonating spiropyran-co-NIPAM-co-acrylic acid hydrogel photoactuators[END_REF] or donor-acceptor Stenhouse adducts [START_REF] Helmy | Photoswitching Using Visible Light: A New Class of Organic Photochromic Molecules[END_REF] , but also redox hydrogels [START_REF] Khodeir | Synthesis and characterisation of redox hydrogels based on stable nitroxide radicals[END_REF] with potential applications in sensing, catalysis or energy storage applications. For all these applications, it should be noted that re-emulsification can be easily achieved after phase separation, since there is no loss of the microgel integrity after applying the stimulus. This is of considerable advantage compared to other types of deconstructible systems based on interfacial assembly between two linear polymers, which requires the availability of components from both oil and aqueous phase [START_REF] Guo | Responsive Emulsions Stabilized by Amphiphilic Supramolecular Graft Copolymers Formed in Situ at the Oil-Water Interface[END_REF] . Microgel-stabilized emulsions represent a promising route to design responsive Pickering emulsions or foams that can be destabilized on demand.

Figure captions

Figure 1: Scheme illustrating the swelling states of the microgels in the absence or presence of saccharide. For the sake of clarity, the mechanism is simplified. In the absence of saccharide, a small fraction of the phenylboronic acids is actually under the form of boronates. Upon saccharide addition, the formation of the boronate ester subsequently increases the fraction of charged species. While all the emulsions creamed rapidly, only emulsions in the presence of fructose remained stable against coalescence despite a mechanical disturbance. CryoSEM view of pNIPMAM-PBA microgels at the surface of an oil drop (oil-water interface). Conditions: dodecane-inbuffer containing fructose emulsion containing 0.01 wt% of microgels. Scale bar is 10 µm (D) and 1 µm (E). Tables Table

290

(0.182) [START_REF] Zhang | Poly(N-isopropylacrylamide) microgels at the air-water interface[END_REF] The relative swelling is defined as the microgel volume in presence of fructose to the microgel volume in absence of fructose ratio. dH is the hydrodynamic diameter measured by DLS in the various conditions. The emulsion was prepared at 4°C with dodecane (30 vol%) and aqueous phase, composed of pNIPMAM-PBA microgels (Cp=0.05%wt.) in 20 mM of fructose (phosphate buffer, pH 8.5).

The emulsion was stable at room temperature but macroscopic phase separation occurred after heating above the VPTT of the microgels in the presence of fructose, at 60°C.

Figure 2 .

 2 Figure 2.

Figure 2 :

 2 Figure 2: Synthesis of PBA-functionalized microgels.

Figure 3 :

 3 Figure 3: Influence of temperature and saccharide concentration on the microgel hydrodynamic

Figure 4 :

 4 Figure 4: TEM views of pNIPAM-PBA and pNIPMAM-PBA microgels initially immersed in

Figure 5 :

 5 Figure 5: (a) AFM images of pNIPAM-PBA (A and B) and pNIPMAM-PBA (C and D) in the

Figure 6 :

 6 Figure 6: Stability of emulsions prepared with dodecane (30 wt%) and water containing

Figure 7 :

 7 Figure 7: Sugar-dependence of the emulsion stability. Emulsions were prepared with dodecane

Figure 8 :

 8 Figure 8: Influence of fructose on the adsorption kinetics at the air-water interface of pNIPAM-

Figure 9 :

 9 Figure 9: a) Compression isotherms of the pNIPMAM-PBA microgels on pure water (pH 6,

Figure 10 .

 10 Figure 10. Schematic representation of the sugar-induced hydrophobic-to-hydrophilic

Figure 2 :

 2 Figure 2: Synthesis of PBA-functionalized microgels.

Figure 3 :

 3 Figure 3: Influence of temperature and saccharide concentration on the microgel hydrodynamic

Figure 4 :

 4 Figure 4: TEM views of pNIPAM-PBA and pNIPMAM-PBA microgels initially immersed in

Figure 5 :

 5 Figure 5: (a) AFM images of pNIPAM-PBA (A and B) and pNIPMAM-PBA (C and D) in the

Figure 6 :

 6 Figure 6: Stability of emulsions prepared with dodecane (30 wt%) and water containing

Figure 7 :

 7 Figure 7: Sugar-dependence of the emulsion stability. Emulsions were prepared with dodecane

Figure 8 :

 8 Figure 8: Influence of fructose on the adsorption kinetics at the air-water interface of pNIPAM-

Figure 9 :

 9 Figure 9: a) Compression isotherms of the pNIPMAM-PBA microgels on pure water (pH 6,

Figure 10 .

 10 Figure 10. Schematic representation of the sugar-induced hydrophobic-to-hydrophilic

Figure S1 :

 S1 Figure S1: Electrophoretic mobility of pNIPAM-PBA microgels as a function of pH and

  Figure S2: (A) Stability of emulsions prepared with dodecane (30 vol%) and water containing

Figure S3 :

 S3 Figure S3: Thermoresponsiveness of emulsions stabilized with pNIPMAM-PBA microgels.

Figure S4 :

 S4 Figure S4: Influence of fructose on the adsorption kinetics of pNIPAM-PBA at the dodecane-

Figure S5 :

 S5 Figure S5: AFM views of the interfacial film after transfer on a silicon substrate, for

Table 1 :

 1 Summary of microgel swelling properties in solution.

		dH (nm) T=20°C (PDI)	dH (nm) fructose T=20°C (PDI)	Relative swelling in fructose 1 T=20°C	dH (nm) collapsed (PDI)	dH (nm) fructose collapsed (PDI)
	pNIPAM-PBA	525	907	5.1	395	n.a.
		(0.12)	(0.18)		(0.16)
	pNIPMAM-	492	935	6.8	294
	PBA	(0.035)	(0.036)		(0.151)

Table 2 :

 2 Summary of microgel characteristics in solution and after deposition and drying on solid substrate, obtained by AFM.

			Maximal Height H (nm)	FWHM (nm)	Width at base (nm)	Ratio H/FWHM
	pNIPAM-PBA	330	410	480	0.8
	pNIPMAM-PBA	200	286	310	0.7
	pNIPAM	50	493	870	0.1
	ii)	Microgels suspended in saccharide solutions

Table 3 :

 3 Summary of microgel adsorption characteristics at the air-water interface.

		pNIPAM-PBA	pNIPMAM-PBA	pNIPAM 1
		pH 8.5	pH 8.5 fructose with	pH 8.5	pH 8.5 fructose with	10 -2 M NaCl
	f (mN/m)	46.9	46.9	50.4	50.4	43
	πf (mN/m)	25.9	25.9	22.4	22.4	29.8
	t1/2 (s)	1630	365	27.4	<1	<50
	1 extracted from ref 22

  1: Summary of microgel swelling properties in solution.

		dH (nm) T=20°C (PDI)	dH (nm) fructose T=20°C (PDI)	Relative swelling in fructose 1 T=20°C	dH (nm) collapsed (PDI)	dH (nm) fructose collapsed (PDI)
	pNIPAM-PBA	525	907	5.1	395	n.a.
		(0.12)	(0.18)		(0.16)
	pNIPMAM-	492	935	6.8	294
	PBA	(0.035)	(0.036)		(0.151)

Table 2 :

 2 Summary of microgel characteristics in solution and after deposition and drying on solid substrate, obtained by AFM.

		Maximal Height H (nm)	FWHM (nm)	Width at base (nm)	Ratio H/FWHM
	pNIPAM-PBA	330	410	480	0.8
	pNIPMAM-PBA	200	286	310	0.7
	pNIPAM	50	493	870	0.1

Table 3 :

 3 Summary of microgel adsorption characteristics at the air-water interface. -Charlotte Tatry 1,2 , Yating Qiu 1 , Véronique Lapeyre 1 , Patrick Garrigue, 1 Véronique Schmitt 2 , Valérie Ravaine 1 1 Univ. Bordeaux, CNRS, Bordeaux INP, ISM, UMR 5255, F-33400, Talence, France 2 Centre de Recherche Paul Pascal, UMR 5031, Université de Bordeaux, CNRS, 115 Avenue du Dr A. Schweitzer, 33600 Pessac, France.

		pNIPAM-PBA	pNIPMAM-PBA	pNIPAM 1
		pH 8.5	pH 8.5 fructose with	pH 8.5	pH 8.5 fructose with	10 -2 M NaCl
	f (mN/m)	46.9	46.9	50.4	50.4	43
	πf (mN/m)	25.9	25.9	22.4	22.4	29.8
	t1/2 (s)	1630	365	27.4	<1	<50
	1 extracted from 22

Marie

Acknowledgements

The authors thank the ''Ministère de l'Enseignement supérieur, de la Recherche et de l'Innovation" for funding the fellowship of M.C.T., Michel Martineau and the PLACAMAT facility for cryoSEM imaging, and S. Ravaine for fruitful discussions about Langmuir films.

Supporting Information.

Figure S1: Electrophoretic mobility of pNIPAM-PBA microgels as a function of pH and glucose concentration; Figure S2: Stability of emulsions prepared with dodecane and water containing pNIPAM-PBA microgels; Figure S3: AFM views of the interfacial film after transfer on a silicon substrate, for pNIPMAM-PBA microgels spread on fructose solution, at various surface pressures.