Sílvia C. Nunes

Water-mediated structural tunability of an alkyl/siloxane hybrid: from amorphous material to lamellar structure or bilamellar superstructure

Mono-amide cross-linked alkyl/siloxane hybrids (classed as mono-amidosils and represented by the notation m-A(x), where m and A stand for mono and amide, respectively, and x is the number of methylene repeat units) with water-mediated tunable structures have been prepared by means of sol–gel chemistry and self-direct assembly routes from the organosilanes CH3(CH2)x–C(O)NH–(CH2)3–Si–(OCH2CH3)3. The amorphous sample m-A(8) has been produced under the stoichiometric conditions (molar ratio Si : ethanol : water = 1 : 4 : 2) used previously to obtain the lamellar bilayer highly organized m-A(14) hybrid material, showing that the length of the pendant alkyl chains affects the degree of order of the materials. Three structurally ordered samples identified as AC-m-A(8) (1 : y) (where AC represents acid catalysis and y is the number of moles of water per mole of Si) have been obtained using the acid catalyzed hydrolytic route and different water contents (y = 600, 300 and 100). Water plays a unique role in the organized mono-amidosils: it not only reverses the natural tendency of the precursor molecule with x = 8 to yield a disordered material, but it also allows the induction of order, leading to the formation of a lamellar structure exclusively in AC-m-A(8) (1 : 100). The presence of a higher water content promotes extra ordering in AC-m-A(8) (1 : 300) and AC-m-A(8) (1 : 600), where the lamellar structure coexists with a bilamellar superstructure. Upon heating the AC-m-A(8) (1 : 600) sample to 120 °C and then cooling it to room temperature, the lamellar structure remained unaffected, while the superstructure was destroyed. The occurrence of the superstructure, although basically associated with the preferential entrapment of water molecules every two lamellae at the siliceous nanodomains, is also correlated with the amide–amide hydrogen bonded array. The m-A(8) and AC-m-A(8) (1 : 600) hybrids are multi-wavelength emitters under UV/VIS excitation. The emission spectra exhibit a broad band (300–650 nm) resulting from two contributions: a “blue” band, due to electron–hole recombinations in the NH/CO groups of the amide cross-links, and a “purplish-blue” band, due to oxygen-related defects •O–O–Si≡(CO2) in the siliceous nanoclusters. The absolute emission quantum yields determined for m-A(8) and AC-m-A(8) (1 : 600) in the 360–380 nm excitation wavelength range were 0.10 ± 0.01 and 0.15 ± 0.01, respectively. While the lifetime values of the high and low wavelength components of the siliceous-related emission are practically the same for both samples (∼3.5 ms), the lifetime value of the NH/CO component for m-A(8) is higher than that of AC-m-A(8) (1 : 600) (∼292 and ∼156 ms, respectively), suggesting a lower non-radiative transition probability for the former hybrid.

Matrix assisted formation of ferrihydrite nanoparticles in a siloxane/poly(oxyethylene) nanohybrid

Matrix-assisted formation of ferrihydrite, an iron oxide hydroxide analogue of the protein ferritin core, in a sol–gel derived organic–inorganic hybrid is reported. The hybrid network (named di-ureasil) is composed of poly(oxyethylene) chains of different average polymer molecular weights grafted to siloxane domains by means of urea cross-linkages and accommodates ferrihydrite nanoparticles. Magnetic measurements, Fourier transform infrared and nuclear magnetic resonance spectroscopy reveal that controlled modification of the polymer molecular weight allows the fine-tuning of the ability of the hybrid matrix to assist and promote iron coordination at the organic–inorganic interface and subsequent nucleation and growth of the ferrihydrite nanoparticles whose core size (2–4 nm) is tuned by the amount of iron incorporated. The polymer chain length, its arrangement and crystallinity, are key factors in the anchoring and formation of the ferrihydrite particles.

Lamellar mono-amidosil hybrids incorporating monomethinecyanine dyes

Mono-amide cross-linked alkyl–siloxane hybrids synthesized through the sol–gel process and the self-directed assembly have been doped with variable concentrations of monomethinecyanines SSCH2(C6H5)I, SSCH3Cl and SSC2H5ClO4. The host matrix m-A(14) is a lamellar structured hybrid composed of short methyl-capped alkyl chains grafted to the siliceous framework through amide groups. The doped mono-amidosil hybrid materials were obtained as solid powders. The bulky SSCH2(C6H5)I dye inhibited the condensation of the silica network. In the SSCH3Cl- and SSC2H5ClO4-doped m-A(14)-based materials the original lamellar structure coexists with a new lamellar ordered phase with a lower interlamellar distance in which the alkyl chains adopt essentially all-trans conformations. The efficient encapsulation of these two dyes by m-A(14), which ensures their complete dissolution, is due to the coordination of the S atoms of SSCH3Cl and SSC2H5ClO4 to the carbonyl oxygen atoms of the amide cross-links. This process leads to a major breakdown of the disordered and ordered hydrogen-bonded aggregates of m-A(14), but does not affect globally the strength of the hydrogen-bonded array. The most dilute SSC2H5ClO4-doped mono-amidosil studied displays room temperature emission in the visible spectral region (380–680 nm) due to the convolution of the intrinsic emission (blue spectral region) of m-A(14) with that of the SSC2H5ClO4 dye (green and red spectral regions) due to the formation of fluorescent J-dimers. The excitation spectra (monitored along the emission spectra) reveal the occurrence of host-to-monomethinecyanine energy transfer.

Fractality and metastability of a complex amide cross-linked dipodal alkyl/siloxane hybrid

A novel room-temperature white light emitter amide-cross linked alkyl/siloxane hybrid material (amidosil A) was produced by self-organization through the rational design of the precursor. This hybrid displays a highly complex hierarchical architecture composed of two lamellar bilayer structures, the relative spatial arrangement of which yields a multiplicity of ordered nanodomains with variable shapes and sizes, some of them persisting at the microscale. Macroscopically A was obtained as clusters of hydrophobic hemispherical and spherical micro-objects exhibiting a lettuce coral-like pattern, which represent unprecedented pieces of evidence illustrating the principles of self-similarity and demonstrating that the time scale of biomimetic morphogenesis in this non-bridged silsesquioxane is similar to that in biological systems. Heating metastable A above the order/disorder phase transition acted as an external quake driving the material to another metastable state, which has persisted for more than 12 months, and was manifested as a marked change of all the macroscopic properties. The occurrence of the self-organization process operating on A, instead of a self-directed assembly, is primarily associated with the formation/rupture of hydrogen bonds, therefore supporting that these interactions are critical factors dictating on what side of the self-assembly/self-organization boundary a non-bridged silsesquioxane system will evolve.

Novel Highly Luminescent Amine-Functionalized Bridged Silsesquioxanes

Amine-functionalized bridged silsesquioxanes (BSs) were synthesized from bis[(3-trimethoxysilyl)propyl] amine via a solvent-mediated route. BS-1 and BS-2 were obtained at neutral pH with sub- and stoichiometric amounts of water, respectively, and high tetrahydrofuran content. BS-3 was prepared with hyperstoichiometric water concentration, high tetrahydrofuran content, and hydrochloric acid. BS-4 was synthesized with hyperstoichiometric water concentration, high ethanol content, and sodium hydroxide. BS-1 and BS-2 were produced as transparent films, whereas BS-3 and BS-4 formed white powders. Face-to-face stacking of flat or folded lamellae yielded quasi-hydrophobic platelets with emission quantum yields of 0.05 ± 0.01 (BS-1 and BS-2) or superhydrophilic onion-like nanoparticles with exciting emission quantum yields of 0.38 ± 0.03 (BS-3) and 0.33 ± 0.04 (BS-4), respectively. The latter two values are the largest ever reported for amine-functionalized siloxane-based hybrids lacking aromatic groups. Fast Grotthus proton hopping between = NH2+/ = NH groups (BS-3) and = N−/ = NH groups (BS-4), promoted by H+ and OH− ions, respectively, and aided by short amine-amine contacts provided by the onion-like morphology, account for this unique optical behavior.

Structuring of Amide Cross-Linked Non-Bridged and Bridged Alkyl-Based Silsesquioxanes

The development of sophisticated organized materials exhibiting enhanced properties is a challenging topic of the domain of organic/inorganic hybrid materials. This review, composed of four sections, reports the work we have carried out over the last 10 years on the synthesis of amide cross-linked alkyl/siloxane hybrids by means of sol-gel chemistry and self-directed assembly/self-organization routes relying on weak interactions (hydrophobic interactions and hydrogen bonding). The various as-produced lamellar structures displaying a myriad of morphologies, often closely resembling those found in natural materials, are discussed. The major role played by the synthetic conditions (pH, water content, co-solvent(s) nature/concentration and dopant presence/concentration), the alkyl chains (length and presence of ramification or not) and the number of the amide cross-links present in the precursor, is evidenced. Examples of highly organized hybrids structures incorporating ionic species (alkali and alkaline earth metal salts) and optically-active centers (organic dyes and lanthanide ions) are described. A useful qualitative relationship deduced between the emission quantum yield of the ordered hybrid materials and the degree of order of the hydrogen-bonded network is highlighted.

Structuring of di-alkyl-urethanesils

AbstractA novel organized di-urethane crossed-linked dodecyl/siloxane (di-alkyl-urethanesil) was synthesized by the sol–gel process and self-directed assembly, from the organosilane precursor (CH3CH2O)3-Si-(CH2)3-NHC( = O)O-(CH2)12-O(O = C)NH-(CH2)3-Si-(OCH2CH3)3, through a fine control of the reaction conditions (hyper-stoichiometric amount of water, minor amount of tetrahydrofuran, and acid catalysis; molar ratio Si:H2O:HCl:THF = 1:300:0.1:12.5). The new bridged silsesquioxane was identified by the notation d-Ut(CY)AC, where Y = 12 is the number of carbon atoms C of the bridging alkyl chains and AC represents acid catalysis. The d-Ut(C12)AC material exhibits a structured lamellar organization with medium long-range order, a texture composed of homogeneous lamellae immersed in a sponge-like matrix made of randomly distributed thin plates, and is thermally stable up to ca. 350 °C. Despite the hydrophobic nature of the dodecane chains, the weakness of the urethane–urethane hydrogen-bonded array formed led to the growth of moderately ordered assemblies of amphiphilic organo(bis-silanetriol) substructures comprising mainly all-trans conformers. A lamellar di-urethane cross-linked dodecyl/siloxane (di-alkyl-urethanesil) with medium long-range order, a texture composed of homogeneous lamellae immersed in a sponge-like matrix made of randomly distributed thin plates, and an average water contact angle of 94.17 ± 17.10°, was synthesized by the sol–gel process and self-directed assembly.HighlightsA di-urethane crossed-linked dodecyl/siloxane was produced using a controlled hydrolytic sol–gel route.The material exhibits a structured lamellar organization with medium long-range order.The morphology of the material includes homogeneous lamellae immersed in a sponge-like matrix.The material is thermally stable up to ca. 350 °C.

Ionic Liquid-Assisted Synthesis of Mesoporous Silk Fibroin/Silica Hybrids for Biomedical Applications

New mesoporous silk fibroin (SF)/silica hybrids were processed via a one-pot soft and energy-efficient sol–gel chemistry and self-assembly from a silica precursor, an acidic or basic catalyst, and the ionic liquid 1-butyl-3-methylimidazolium chloride, acting as both solvent and mesoporosity-inducer. The as-prepared materials were obtained as slightly transparent-opaque, amorphous monoliths, easily transformed into powders, and stable up to ca. 300 °C. Structural data suggest the formation of a hexagonal mesostructure with low range order and apparent surface areas, pore volumes, and pore radii of 205–263 m2 g–1, 0.16–0.19 cm3 g–1, and 1.2–1.6 nm, respectively. In all samples, the dominating conformation of the SF chains is the β-sheet. Cytotoxicity/bioactivity resazurin assays and fluorescence microscopy demonstrate the high viability of MC3T3 pre-osteoblasts to indirect (≥99 ± 9%) and direct (78 ± 2 to 99 ± 13%) contact with the SF/silica materials. Considering their properties and further improvements, these systems are promising candidates to be explored in bone tissue engineering. They also offer excellent prospects as electrolytes for solid-state electrochemical devices, in particular for fuel cells.