Self-Assembly of Biologically Inspired Complex Functional Materials
C. Jeffrey Brinker, Sandia National Laboratories and the University of New Mexico
Nature combines hard and soft materials, often in hierarchical architectures, to get synergistic, optimized properties with proven, complex functionalities. Emulating such natural designs in robust engineering materials using efficient processing approaches represents a fundamental challenge to materials chemists. This presentation will review progress on understanding so-called ‘evaporation-induced silica/surfactant self-assembly’ (EISA) as a simple, general means to prepare porous thin-film nanostructures. Such porous materials are of interest for membranes, low-dielectric-constant (low- k ) insulators, and even ‘nano-valves’ that open and close in response to an external stimulus. EISA can also be used to simultaneously organize hydrophilic and hydrophobic precursors into hybrid nanocomposites that are optically or chemically polymerizable, patternable, or adjustable.
In constructing composite structures, a significant challenge is how to controllably organize or define multiple materials on multiple length scales. To address this challenge, we have combined sol-gel chemistry with molecular self-assembly in several evaporation-driven processing procedures collectively referred to as evaporation-induced self-assembly (EISA). EISA starts with a silica/water/surfactant system diluted with ethanol to create a homogeneous solution. We rely on ethanol and water evaporation during dip-coating (or other coating methods) to progressively concentrate surfactant and silica in the depositing film, driving micelle formation and subsequent continuous self-assembly of silica/surfactant thin film mesophases. One of the crucial aspects of this process, in terms of the sol-gel chemistry, is to work under conditions where the condensation rate of the hydrophilic silicic acid precursors ( º Si-OH) is minimized. The idea is to avoid gelation that would kinetically trap the system at an intermediate non-equilibrium state. We want the structure to self-assemble then solidify, with the addition of a siloxane condensation catalyst or by heating, to form the desired mesostructured product. Operating at an acidic pH (pH = 2) minimizes the condensation rate of silanols to form siloxanes Si-O-SiIn addition, hydrogen bonding and electrostatic interactions between silanols and hydrophilic surfactant head groups can further reduce the condensation rate. These combined factors maintain the depositing film in a fluid state, even beyond the point where ethanol and water are largely evaporated. This allows the deposited film to be self-healing and enables the use of virtually any evaporation-driven process (spin-coating, inkjet printing, or aerosol processing) to create ordered nanostructured films, patterns, or particles.
Understanding EISA - To understand EISA, we have taken advantage of the steady-state nature of dip-coating. When a substrate is withdrawn vertically from a solution reservoir at a constant rate, a balance of upward moving liquid flux and evaporation causes the film thickness profile to become steady in the lab frame. Measuring this profile allows calculation of the concentrations of the non-volatile components. Spatially resolved structural measurements allow the complete EISA process (from the dilute disordered state near the reservoir surface to the highly ordered dry state) to be followed in situ. Using spatially resolved grazing incidence small-angle x-ray scattering, we interrogate the periodic nanometer-scale structure as a function of thickness/composition, ultimately allowing the mapping of structureonto the appropriate ethanol/water/surfactant phase diagram. We find that the structure forms by an interfacially mediated process where, above the critical micelle concentration, oriented intermediate lamellar and correlated micellar structures form in compositional regions that are expected (on the basis of the ethanol/water/surfactant phase diagram) to be isotropic. We attribute these discrepancies to the presence of interfaces and the introduction of silica, which serves as a second hydrophilic component. Enhanced solvent evaporation at the liquid/vapor interface promotes self-assembly there to form incipient lamellar silica/surfactant mesostructures that, based on d-spacing, largely exclude ethanol and water. Further evaporation results in a correlated structure composed of wormlike cylindrical micelles that organize and align at the solid and vapor interfaces in a proto-hexagonal arrangement. Finally, an oriented hexagonal mesophase forms with its cylinder axes oriented parallel to the substrate surface. The hexagonal mesophase appears to be nucleated at the solid/liquid and liquid/vapor interfaces and, based on x-ray reflectivity experiments, grows inward toward the film interior through conversion of the correlated wormlike mesostructure. Perhaps surprisingly, GISAXS studies on evaporating surfactant/water/ethanol systems prepared with and without oligosilicic acids as secondary hydrophilic components showed that silicic acid promoted the formation of ordered mesophases. This is explained by the silicic acid oligomers serving as nonvolatile hydrophilic components. By maintaining fluidity, they avoid kinetic barriers to self-assembly as experienced in the silica-free system. Concerning thermodynamic factors, we found that by considering silicic acid to be equivalent (on a per mole silicon basis) to water, the first appearance of the hexagonal mesophase upon evaporation occurred near the isotropic/hexagonal phase boundary of the corresponding surfactant, cetyltrimethylammonium bromide (CTAB), in the surfactant/water/ethanol system. This suggests that under acidic conditions, the oligosilicic acids have similar interactions with the hydrophilic head groups as water. To date, the appropriate quaternary silicic acid/water/ethanol/surfactant phase diagrams have not been determined experimentally.
Responsive Sytems - Responsive Systems
Living systems are able to sense and respond to their environments through various chemical- and physical-based transduction mechanisms. From an engineering perspective, robust materials that respond predictably to pH, temperature, light, biomolecules, or electric fields could find applications in controlled release of drugs or corrosion inhibitors, sensors, optical storage, and optomechanical actuation. So the question arises, how do we impart responsive lifelike qualities to robust engineering materials?
In examining biological systems, we note that responsiveness often derives from hierarchical assemblies that position environmentally sensitive moieties onto a 3D framework. In an attempt to emulate such biological designs, our idea was to organize optically and thermally responsive azobenzene molecules within a robust, ordered 3D nanostructure using self-assembly. In this synergistic design, photo or thermal energy is transduced into a 3D mechanical response that can control pore size and transport behavior.
Azobenzene derivatives were selected because of their well-studied response to light. Trans « cis isomerization changes the molecular dimension (molecular length of the cis isomer is ~3.4 Å shorter than that of the trans isomer) and also the dipole moment (0.5–3.1 Debyes). Azobenzenes have been used previously to achieve switchable properties. However, in order to accurately control pore size, we needed to anchor azobenzene ligands to the pore surfaces of a nanostructured scaffold composed of mono-sized pores. This allows the rigid inorganic scaffold to precisely position azobenzene ligands in 3D configurations, where switching results in a well-defined change in pore size. The 3D composite architecture should also enhance mechanical and thermal stability of the switchable ligands, important for their integration into devices.
Our synthesis scheme employed an azobenzene-modified silane, 4-(3-triethoxysilylpropylureido)azobenzene (TSUA) designed to serve as an amphiphilic cosurfactant after hydrolysis of the ethoxy groups. During EISA, the hydrophobic propylureidoazobenzene groups are positioned in the hydrophobic micellar cores. The hydrophilic head groups co-organize with added silicic acid oligomers at the hydrophilic micellar exteriors. After catalytic or thermally promoted siloxane condensation, azobenzene ligands are anchored to the surfaces of mono-sized pores, with the azobenzene ligands oriented toward the pore interiors. Subsequent surfactant extraction produces the target nanocomposite.
As determined with UV–visible spectroscopy, UV irradiation of the trans isomer causes transformation to the cis isomer. Removal of the UV radiation, irradiation with a longer wavelength, or heating switches the system back to the trans form. As a control experiment, we made films with the same sol but prepared without surfactant. In this case, the azobenzene ligands were randomly incorporated in a microporous silica matrix(pore diameter less than 1-nm) and exhibited no detectable photoisomerization. Similarly, we observed no photoisomerization for the ordered self-assembled films prior to solvent extraction of the surfactant templates. Only upon surfactant removal did we create the pore volume required (estimated as 127 Å 3) for photoisomerization. These results unambiguously locate the photosensitive azobenzene ligands along with surfactant within the uniform nanopores of the self-assembled films. They emphasize the need to accommodate the steric demands of the photoisomerization process by engineering the pore size and positioning the photoactive species on the pore surfaces
To demonstrate optical control of mass transport, we performed a chronoamperometry experiment using an azobenzene-functionalized nanocomposite membrane to modify the working electrode in an electrochemical cell. The chronoamperometry experiment uses ferrocene dimethanol (FDM) as a molecular probe and provides a measure of mass transport through the nanocomposite membrane by monitoring the steady-state oxidative current at constant potential for the reaction taking place on the working electrode surface, which is indium tin oxide (ITO). At constant potential, the effective pore size limits the diffusion rate of probing molecules to the electrode surface during electrolysis. Under dark conditions, the azobenzene moieties are predominately in their extended trans form. Upon UV irradiation ( l = 360 nm), the azobenzene moieties isomerize to the more compact cis form, which we expected would increase the diffusion rate and, correspondingly, the oxidative current. Likewise, exposure to visible light ( l = 435 nm) triggers the reverse cis ® trans isomerization of the azobenzene moieties, which should decrease the current to the pre-UV exposure level.
We observed that upon UV irradiation, the current increases progressively due to trans ®cis isomerization, which increases the pore size. It then reaches a new plateau, corresponding to the photostationary state of trans ® cis isomerization. Visible-light exposure decreases the oxidative current due to cis ® trans isomerization, which decreases the pore size. After 200 s, the current is reduced to its pre-UV exposure level. This corresponds to the photostationary state of cis ® trans isomerization. We performed three cycles of alternating UV/visible light exposure to show the reversibility and repeatability of this process. Control experiments were performed on a bare ITO electrode and one coated with mesoporous silica coated. However, no photoresponse was observed in either of these systems. This demonstrates that the photoresponse is not an artifact of the ITO electrode or the mesoporous silica film, but a true effect arising from the isomerization of azobenzene moieties attached to the nanopore surfaces.
To correlate the observed current changes with the actual isomerization state, a UV/visible spectroscopy study was performed, where the nanocomposite film used for chronoamperometry was immersed in the electrolyte solution and illuminated exactly as for the first I–t cycle in the chronoamperometry experiment. We observed that the absorbance of the nanocomposite film at 356 nm, attributed to the π–π * transition of azobenzene in the trans form, decreases progressively under UV irradiation, reaching a plateau corresponding to the photostationary state of trans ® cis isomerization. The following visible-light exposure causes the reverse cis ® trans isomerization, increasing the absorbance gradually before reaching the pre-UV state. These data exactly correlate the conformational changes of azobenzene moieties in the nanocomposite film with the oxidative current changes measured in the chronoamperometry experiment. Thus, we clearly demonstrate control of mass transport on the nanoscale.
