The development of surfaces that have switchable properties, also known as smart surfaces, have been actively pursued in the past few years. The recent surge of interest in these switchable systems stems from the widespread number of applications to many areas in science and technology ranging from environmental cleanup to data storage, micro- and nanofluidic devices. Moreover, the ability to modulate biomolecule activity, protein immobilisation, and cell adhesion at the liquid–solid interface is important in a variety of biological and medical applications, including biofouling, chromatography, cell culture, regenerative medicine and tissue engineering. Different materials have been exploited to induce such changes in surface biological properties that are mostly based on self-assembled monolayers or polymer films. This critical review focuses on the recent progress in the preparation of these switchable surfaces, and highlights their applications in biological environments. The review is organized according to the external stimuli used to manipulate the properties of the substrate—chemical/biochemical, thermal, electric and optical stimuli. Current and future challenges in the field of smart biological surfaces are addressed (189 references).

Stimuli-responsive surfaces for bio-applications.

Ferrite magnetic nanoparticles (MNPs) were functionalized with a variety of silanes bearing different functional endgroups to render them stable with respect to aggregation and keep them well-dispersed in aqueous media. The MNPs were prepared by the thermal decomposition method, widely used for the synthesis of monodisperse nanoparticles with controllable size. This method makes use of a hydrophobic surfactant to passivate the surface, which results in nanoparticles that are solely dispersible in nonpolar solvents. For use in biological applications, these nanoparticles need to be made water-dispersible. Therefore, a new procedure was developed on the basis of the exchange of the hydrophobic surface ligands with silanes bearing different endgroups to decorate ferrite magnetic nanoparticles with diverse functionalities . By this means, we could easily determine the influence of the endgroup on the nanoparticle stability and water-dispersibility. Amino-, carboxylic acid- and poly(ethylene glycol)-terminated...

/pdf/silane-ligand-exchange-to-make-hydrophobic-superparamagnetic-2fe5s89xou.pdf

Silane Ligand Exchange to Make Hydrophobic Superparamagnetic Nanoparticles Water-Dispersible

This critical review outlines the current state-of-the-art research on the reversibly switchable wettability of surface brought about by external stimuli and the exchange of counterions. Chemical composition and surface topography are the two key factors in the wettability of solid substrates. Applying external stimuli and exchanging counterions of ionic liquids and polyelectrolyte films are valuable approaches for rendering the change in surface chemistry and/or topography, and for driving the transition between hydrophilicity and hydrophobicity of surfaces. Through the combination of stimuli-responsive films and micro-/nanostructural surfaces, smart surfaces with reversible switching between superhydrophobicity and superhydrophilicity have been achieved. As an important advancement in reversibly switchable wettability, this review briefly introduces ionic liquids (ILs) as on–off systems to obtain reversibly switchable wettability and then discusses in more detail the methods to induce the reversibly switchable wettability of surfaces modified by ILs, additives, or thin films. In addition to reversibly switchable wettability mechanisms, open problems and potential solutions are discussed (157 references).

Reversibly switchable wettability

Scandium trifluoride maintains a cubic ReO(3) type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ∼300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, α(l) ≈ -14 ppm K(-1)). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW(2)O(8) and positive thermal expansion above ∼1100 K. While the cubic ReO(3) structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF(3) is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.

Pronounced Negative Thermal Expansion from a Simple Structure: Cubic ScF3

The change in shape inducible in some photo-reversible molecules using light can effect powerful changes to a variety of properties of a host material. This class of reversible light-switchable molecules includes molecules that photo-dimerize, such as coumarins and anthracenes; those that allow intra-molecular photo-induced bond formation, such as fulgides, spiro-pyrans, and diarylethenes; and those that exhibit photo-isomerization, such as stilbenes, crowded alkenes, and azobenzenes. The most ubiquitous natural molecule for reversible shape change, however, and perhaps the inspiration for all artificial bio-mimics, is the rhodopsin/retinal protein system that enables vision, and this is the quintessential reversible photo-switch for performance and robustness. Here, the small retinal molecule embedded in a cage of rhodopsin helices isomerizes from a cis geometry to a trans geometry around a C=C double bond with the absorption of just a single photon. The modest shape change of just a few angstroms is quickly amplified and sets off a cascade of larger shape and chemical changes, eventually culminating in an electrical signal to the brain of a vision event, the energy of the input photon amplified many thousands of times in the process. Complicated biochemical pathways then revert the trans isomer back to cis, and set the system back up for another cascade upon subsequent absorption. The reversibility is complete, and many subsequent cycles are possible. The reversion mechanism back to the initial cis state is complex and enzymatic, hence direct application of the retinal/rhodopsin photo-switch to engineering systems is difficult. Perhaps the best artificial mimic of this strong photo-switching effect however in terms of reversibility, speed, and simplicity of incorporation, is azobenzene. Trans and cis states can be switched in microseconds with low-power light, reversibility of 105 and 106 cycles is routine before chemical fatigue, and a wide variety of molecular architectures is available to the synthetic materials chemist, permitting facile anchoring and compatibility, as well as chemical and physical amplification of the simple geometric change. This review article focuses on photo-mechanical effect taking place in various material systems incorporating azobenzene. The photo-mechanical effect can be defined as reversible change in shape by absorption of light, which results in a significant macroscopic mechanical deformation, and reversible mechanical actuation, of the host material. Thus, we exclude simple thermal expansion effects, reversible but non-mechanical photo-switching or photo-chemistry, as well as the wide range of optical and electro-optical switching effects for which good reviews exist elsewhere. Azobenzene-based material systems are also of great interest for light energy harvesting applications across much of the solar spectrum, yet this emerging field is still in an early enough stage of research output as to not yet warrant review, but we hope that some of the ideas put forward here toward promising future directions of research, will help guide the field.

Azobenzene photomechanics: prospects and potential applications

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 isocyanate-functionalized 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.

Azobenzene-containing monolayer with photoswitchable wettability.

Structure and raman spectroscopy of czochralski-grown barium yttrium and barium ytterbium fluorides crystals

IR and Raman active modes have been measured in several fluoride scheelites, namely the laser matrix (studied as a reference) and four compounds (Ln = Ho, Er, Tm and Yb). The observed phonons have been assigned in accordance with the various irreducible representations of the point group of the crystal. In three modes never observed before have been measured. The polarized experimental IR reflection spectra have been analysed through a `four-parameters' model to give the mode frequencies and dampings. For the whole set of the compounds, the frequency dependence of the dielectric function, optical indices and absorption coefficient have been deduced from the IR study. The electronic Raman effect has been shown in the low-temperature Raman spectra for Ln= Tm, Ho and Yb. The corresponding measured wavenumbers are compared with the calculated energies between crystal-field levels of L SJ states, each being associated to an irreducible representation of the ion-site group .

https://iopscience.iop.org/article/10.1088/0953-8984/9/32/016/pdf

Lattice dynamics of fluoride scheelites: I. Raman and infrared study of and ( and Yb)

The cubic-rhombohedral structural phase transition that occurs in M${\mathrm{F}}_{3}$ compounds (M=Al,Cr,Ga,V,Fe,In) is studied by calorimetric measurements (differential scanning calorimetry, differential thermal analysis) and by Raman and by Raman scattering. The transition temperatures and the hysteresis are determined with precision. The Raman spectra of M${\mathrm{F}}_{3}$ fluorides have been collected at room temperature and high temperature. An assignment of the four Raman-active modes of the rhombohedral phase is proposed on the basis of group-theory analysis in the cubic phase. The temperature study of Raman spectra indicates the presence of two soft modes due to the condensation of the triply degenerate ${\mathit{R}}_{5}$ mode of the cubic phase [at the R(1/2,1/2,1/2) point of the first cubic Brillouin zone]. The Raman data are used to adjust the parameters of a rigid-ion model and to deduce short-range interaction parameters assuming constant ratio ${\mathit{A}}_{\mathit{i}}$/${\mathit{B}}_{\mathit{i}}$. The F-F short-range interactions are obtained for a wide range of interionic distances, and their behavior is physically reasonable. The calculated phonon spectra and the phonon density of states are presented. The latter exhibits an important discontinuity at the soft-mode frequencies.

A. Bulou

Papers

Azobenzene-containing monolayer with photoswitchable wettability.

Structure and raman spectroscopy of czochralski-grown barium yttrium and barium ytterbium fluorides crystals

Lattice dynamics of fluoride scheelites: I. Raman and infrared study of and ( and Yb)

Raman-scattering study of crystallized MF3 compounds (M=Al,Cr,Ga,V,Fe,In): An approach to the short-range-order force constants.

Structural characterization and AC conductivity of bis tetrapropylammonium hexachlorado-dicadmate, [N(C3H7)4]2Cd2Cl6