Controlling wettability by light: illuminating the molecular mechanism (original) (raw)
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Molecular design of photoswitchable surfaces with controllable wettability
Journal of Materials …, 2011
In this work, we developed a photocontrollable substrate which was prepared using an azobenzenecontaining self-assembled monolayer (SAM) on the silicon surface via the chemisorption of 3glycidoxypropyltrimethoxysilane (GPTS) and 4-(4 0 -aminophenylazo) benzoic acid (APABA). The prepared surfaces were chemically characterized by X-ray photoelectron spectroscopy (XPS). The reversible photoswitching performance of APABA molecules were investigated by UV spectroscopy in dimethylsulfoxide (DMSO) solution. To understand and control this reversible photoswitchable mechanism and wettability properties, contact angle measurements were performed by using a variety of liquids after UV and visible light irradiation. These contact angle results are used to approximate the components of the APABA-modified surface energy under UV and visible light using the Lifshitz-van der Waals/acid-base approach. The total surface energy (g s ) after visible light irradiation (for trans formation) was calculated to be 37.28 mJ m À2 , whereas the value after UV light exposure (for cis formation) was also calculated to be 36.95 mJ m À2 . All the results demonstrate the great potential to control molecular events within and on the surfaces of molecular constructs using light.
Physical Chemistry Chemical Physics, 2013
We have developed polyelectrolyte multilayer bio-films containing azobenzene chromophores that enhance reversible photo-orientation upon irradiation with linearly polarized light, to effect surface photoswitching of adjacent biological systems. When conditions of film preparation and irradiation were optimized, we could observe the highest measured birefringence to date in amorphous systems (Dn > 0.2). This birefringence change to probe orientation was also for the first time measured and determined to be stable completely underwater, permitting optimization for in situ applications immersed in biological conditions. Thin polymer coatings containing photo-isomerizable molecules, such as azobenzene dyes, have garnered great interest due to their capacity for reversible optical storage, 1,2 optical switching, 3 and most recently for photo-reversible bio-surfaces to control cell behaviour with visible light. The key to these applications is the material's ability to switch reversibly between distinct trans and cis geometries and their molecular orientations, readily measured as changes in birefringence. These molecular motions are mainly the result of the re-alignment and migration of azobenzene groups during repetitive trans-cis isomerization cycles with polarized light . When using linearly polarized light, the azobenzene chromophores will continue to cycle and re-align until their dipole moments lie perpendicular to the polarization direction, depleting all other orientations and resulting in a build-up of molecular anisotropy. Most recently, by incorporating azo chromophores into soft bio-compatible films based on polyelectrolyte multilayers (PEMs) we have observed that significant and stable changes in surface energy are inducible, which can modulate cell behaviour. Chromophore re-orientation at the film surface is implicated as a mechanism, but has not yet been proven. With this recent interest in using photo-isomerizable materials in wet in situ applications such as bio-compatible films in cell media, 4 it is especially important to understand how these molecular mechanisms work underwater, which can be markedly different than in the dry state where all previous measurements have been reported. There is also great interest in discovering conditions for both film assembly and irradiation that optimize this orientation.
Following the Wetting of One-Dimensional Photoactive Surfaces
Langmuir, 2012
This article aims toward a full description of the wetting conversion from superhydrophobicity to superhydrophilicity under illumination with UV light of high-density ZnO nanorods surfaces by (i) following the evolution of the clusters and superstructures formed by the nanocarpet effect as a function of the water contact angle (WCA); (ii) characterization of the superhydrophobic and superhydrophilic states with an environmental scanning electron microscope (ESEM); and (iii) using the nanocarpet effect as a footprint of both local and apparent water contact angles. Thus, the main objective of the article is to provide a general vision of the wettability of 1D photoactive surfaces. In parallel, the nanocarpet (NC) formation by clustering of vertically aligned ZnO nanorods (NR) when water is dripped on their surface and then dried is studied for the first time by taking advantage of the possibility of tuning the surface water contact angle of the ZnO NR structure under UV preillumination. As a result, we demonstrate the feasibility of controlling the size and other morphological characteristics of the NCs. Moreover, a strong anisotropic wetting behavior, characterized by a Δθ = θ ∥ − θ ⊥ = 30°, is shown on an asymmetrically aligned NC surface resulting from arrays of tilted NRs. The study of the condensation/ evaporation of water on/from an as-prepared (superhydrophobic) or a preilluminated (superhydrophilic) NR surface examined by an environmental scanning electron microscope has evidenced the formation of supported water droplets with polygonal shapes in the first case and the complete filling of the inter-NR space in the latter. The long-term stability of the NC clusters has been utilized as a footprint to track the penetration depth of water within the inter-NR space in the three borderline regions of water droplets. This analysis has shown that for moderately hydrophobic surfaces (i.e., water contact angles lower than 130°) water droplets do not present a well-defined borderline trace but a spreading region where water penetrates differently with the NR interspace. The transition from a Cassie−Baxter to a modified Cassie−Baxter to finish in a Wenzel wetting state is found on these surfaces depending on the UV preillumination time and is explained with a model where water interaction with the NR units is the critical factor determining the macroscopic wetting behavior of these surfaces.
Design of Pyrimidine-Based Photoresponsive Surfaces and Light-Regulated Wettability
Langmuir, 2009
Photoresponsive surfaces were prepared by attaching pyrimidine-terminated molecules to flat gold substrates (as thiol self-assembled monolayers) or by dip-coating quartz surfaces. Both types of films underwent photodimerization (two pyrimidine rings react with one another and form a cyclobutane type dimer through the C5dC6 double bond) when irradiated with light of 280 nm wavelength. The reverse reaction was carried out by irradiating the dimerized surface with light of 240 nm wavelength. The photoinduced chemical changes are accompanied by a change in the physical properties of the surface (e.g., wettability and acidity), and are highly dependent on the structure of the photoactive molecules. The surface dimerization reaction follows a pseudo-first order reaction. The rate constant is determined by the structure of the pyrimidine headgroup. In self-assembled monolayers, uracil derivatives dimerize faster than thymine derivatives due to a reduced steric repulsion near the reaction center. In dip-coated films, however, uracil derivatives appear to be less ordered and, correspondingly, the efficiency of the reaction is lower. The reaction rate is also very sensitive to the ordering within the layer, which can be manipulated through the structure of the tail. In SAMs, faster dimerization occurs with molecules containing flexible chains. In dip-coated films, the presence of a polar group at the chain terminus favors dimerization.
Azobenzene-Containing Monolayer with Photoswitchable Wettability
Langmuir, 2005
A compact monolayer containing azobenzene has been prepared on silicon substrates. The elaboration route consisted of covalent grafting of freshly synthesized azobenzene moieties onto an isocyanatefunctionalized self-assembled monolayer (SAM). The highly packed and ordered isocyanate-functionalized SAM and the azobenzene-functionalized SAM were monitored and characterized by contact angle measurements and X-ray reflectivity (XR). Photoswitching of the wettability of the film induced by the reversible cis-trans isomerization of the azobenzene chromophores is experimentally shown from water and olive oil contact angle measurements.
Reversible Photoswitching of Azobenzene-Based Monolayers Physisorbed on a Mica Surface
Langmuir, 2010
The formation of compact and large-scale self-assembled monolayers (SAMs) adsorbed on a mica surface has been achieved by insertion of alkyl chains on azobenzene derivatives, leading to strong intermolecular van der Waals interactions and hydrogen bonding. The reversible photoswitching of monolayers was investigated by monitoring the variation of the thickness of the SAMs during the cis-trans isomerization of the azobenzene cores with an atomic force microscope (AFM). The absence of covalent bonds between molecules and substrate induces a molecular diffusion which leads to the complete isomerization of the molecules constituting the SAMs.
Langmuir, 2007
A series of polymers with 4-perfluoroalkyl-modified azobenzene side groups was investigated for its light-induced changes in surface properties. The ultraviolet (UV) light activated trans to cis isomerization of the azobenzene group, and the influence of molecular order and orientation on this process were studied using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and water contact angle measurements. Light-induced molecular reorganization in the near-surface region was studied by NEXAFS using in situ UV irradiation of polymer thin films. Differential scanning calorimetry and wide-angle X-ray scattering studies showed that sufficiently long fluoroalkyl groups formed well-ordered smectic mesophases in the bulk, as well as on the surface, which was evidenced by NEXAFS. The disruption of mesogen packing by photoisomerization was found to be influenced by the fluoroalkyl segment length. Surfaces with perfluorohexyl and perfluorooctyl groups that showed high orientational order were also highly resistant to light-induced changes. In such cases, the trans-cis isomerization resulted in greater lowering of the azobenzene phenyl ring order parameters than the perfluoroalkyl order parameters. UV exposure caused reorientation of the phenyl rings of the azobenzene group, but the terminal perfluoroalkyl segments remained more or less ordered.
Fast and reversible photo-responsive wettability on TiO 2 based hybrid surfaces
J. Mater. Chem. A, 2015
A hybrid surface exhibiting a fast and reversible switch between hydrophobic and hydrophilic states was prepared by spin-coating a porous TiO 2 layer by a mixture of TiO 2 nanoparticles with 11-(4-phenylazo) phenoxy undecanoic acid (denoted as the AzoC11 acid). The nanocomposite film corresponding to the 10% AzoC11 acid/TiO 2 mass fraction exhibited both a very important and fast variation of the contact angle (by more than 120 ), along with an excellent reversibility. The wettability switching is explained by the trans / cis / trans isomerization of the AzoC11 acid under light irradiation or heating. Upon trans / cis isomerization, optical absorption bands were found to exhibit drastic evolution, the origin of which is discussed in detail by means of a time-dependent density functional theory (TDDFT) study. Subsequent to these isomerization processes, various mechanisms explaining the wettability variations were investigated by comparing the results obtained by DFT calculations, wetting measurements for different surface ratios of the AzoC11 acid under UV irradiation, and by analyzing the molecular structure and molecular surfaces of trans and cis-isomers of the AzoC11 acid. † Electronic supplementary information (ESI) available: Theoretical absorption spectra of trans-and cis-AzoC11 acid in THF, absorption onset on the UV-vis spectra of AzoC11 acid solution, theoretical estimation of molecular-areas, expression of surface ratio, values of the wetting study, comparison of contact angle variations versus the irradiation conditions. See