• reductive amination
• Polysaccharides
• Pickering emulsion
• photo-responsive
• Janus particles
• CuAAc
• coating
• click chemistry
• azidated polysaccharides
• smart materials
• spiropyran

In the present work, innovative smart polysaccharide-base materials were successful developed. Three polysaccharides, composed by glucopyranose repetitive units but with different configuration of the glycosidic bonds or degree of branching have been selected: amylose, amylopectin and dextran. Amylose and amylopectin were functionalized with an alkynyl derivative of spiropyran (SPCC) and/or propargyl alcohol by a two steps procedure consisting in the derivatization of glucopyranose units with azide at the C6 position followed by Cu(I)-catalysed azide-alkyne cycloaddition (CuAAc). Substitution degrees with spiropyran (DSSP) between 0.10 and 0.95 were achieved by simply modulating the molar ratio between alkyne species and polysaccharide in the CuAAc reaction, as it was confirmed by FT-IR and NMR spectroscopy characterization. The UV-Vis spectra of SPCC-modified amyloses in DMF solution kept in the dark exhibited an absorbance band with maximum at ~346 nm, ascribed to spiropyran units. The intensity of the absorbance was found to increase linearly with the DSSP. Upon irradiation with UV light, a band at ~560 nm appears, due to the conversion of spiropyran to zwitterionic merocyanine, its intensity linearly increasing with DSSP too. ICP-MS analysis showed the presence of small amount (< 270 ppm) of residual Cu from the catalyst in the functionalized polysaccharides. However, EPR spectroscopy indicated that Cu2+ species are bonded to different moieties and they do not significant interfere in the optical behaviour. In order to evaluate the combined effect of solvent and photo-isomerization on the solubility and solvatochromism of the spiropyran functionalized polysaccharides, ASP40 was selected and solubilized/dispersed in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), water, 2,2,2-trifluoroethanol (TFE), ethanol, DMSO, DMF, and tetrahydrofuran (THF). Four distinct photochromic behaviours depending on the solubility were observed which on turn depend on the solvent polarity: i) good solvents (DMF, DMSO) where the most stable form is a mixture of spiropyran in closed conformation and a not well identify open conformer exhibiting yellow colour, ii) ad solvent (water and alcohols) that are not able to solubilize the functionalized polysaccharide but allow the photo-isomerization of the spiro form to merocyanine, iii) intermediate solvent (THF) that almost solubilizes the functionalized polysaccharide when the grafted groups are in the closed apolar conformation but allows a switchable solubility reduction by irradiation with UV-Vis light, iv) the very polar HFIP that induced inverse photochromism.
The filmability property of the functionalized amyloses was tested by three different coating methods on glass and paper as substrates. Coatings on glass were prepared by solvent casting and spin coating and they were characterized by UV/Vis spectroscopy that showed the isomerization of SPMC occurring, even if it was slower than in solution. AFM analysis of the films obtained by spin coating from HFIP solutions evidenced the presence of pores whose volume increased with the increase of the substitution degree of the polysaccharide. The contact angle analysis of films prepared by solution casting on glass showed a higher hydrophilicity for the film with SP groups than for the azidated precursor and the difference was higher after irradiation of the films with UV light. A similar result was observed for films on paper that were obtained by dip coating. The treated natural substrate showed photochromic properties and polarity increase after irradiation with UV light. These findings indicate that the proposed synthetic strategy can be exploit to prepare photochromic materials that can be used as coating for paper to provide the natural substrate with photochromism as well as waterproof properties.
The second part of this work was focused on the fabrication of Janus particles with polysaccharide grafted on part of the surface. Indeed, anisotropically functionalized particles are attractive for their peculiar characteristics such as hierarchical self-assembling and tuneable amphiphilicity. In particular, this work was focused on the development of an efficient grafting method allowing to prepare Janus particles with the largest superficial concentration of polysaccharide. In order to prepare the Janus silica particles (JP), silica nanoparticles prepared by Stöber method (SNPs) were chosen after comparison with commercial silica particles (CSPs). Then, they were selectively modified at one side after immobilization at wax-water interface in a wax-in-water emulsion. Under the assumption that the particles at the solid wax particle surface had only one side exposed to water, the hydroxyl groups thereon were modified by reaction with functionalized alkyl trialkoxy silane in order to graft amine and azide moieties on the particles surface. Dextran (a water soluble, linear α-D-1,6-glucose-linked glucan) was selected for this study. In order to graft polysaccharides on functionalized silica Janus particles, two well-known coupling reactions were compared: reductive amination with aldehyde and CuAAc reaction. The reductive amination reaction was carried out between amine groups on silica Janus particles and aldehydes groups on dextran. Either natural occurring aldehyde at the reducing chain-end of the polysaccharide or aldehyde groups produced by oxidation along the polymer backbone were exploited. CuAAc reaction was carried out between dextran bearing alkyne moieties and azidated Janus particles. All the prepared Janus particles were characterized by spectroscopic analysis, calorimetry and the grafting degree was evaluated by spectrophotometric titration. The effective asymmetrical nature of the obtained Janus particles was assessed by functionalization with a citrate-stabilized Au nanoparticle dispersion followed by TEM analysis. Indeed, the stained samples clearly show the specific absorption of the Au nanoparticles onto one of the two differently functionalized side of the surface of each Janus particle. Such an occurrence was ascribed to the different nature of the functional groups at the two sides of the Janus particle surface, the one at one side only interacting with the gold nanoparticles.