Microstructured Magnetoactive Elastomers for Switchable Wettability.

We demonstrate the control of wettability of non-structured and microstructured magnetoactive elastomers (MAEs) by magnetic field. The synthesized composite materials have a concentration of carbonyl iron particles of 75 wt.% (≈27 vol.%) and three different stiffnesses of the elastomer matrix. A new method of fabrication of MAE coatings on plastic substrates is presented, which allows one to enhance the response of the apparent contact angle to the magnetic field by exposing the particle-enriched side of MAEs to water. A magnetic field is not applied during crosslinking. The highest variation of the contact angle from (113 ± 1)° in zero field up to (156 ± 2)° at about 400 mT is achieved in the MAE sample with the softest matrix. Several lamellar and pillared MAE structures are fabricated by laser micromachining. The lateral dimension of surface structures is about 50 µm and the depth varies between 3 µm and 60 µm. A systematic investigation of the effects of parameters of laser processing (laser power and the number of passages of the laser beam) on the wetting behavior of these structures in the absence and presence of a magnetic field is performed. In particular, strong anisotropy of the wetting behavior of lamellar structures is observed. The results are qualitatively discussed in the framework of the Wenzel and Cassie-Baxter models. Finally, directions of further research on magnetically controlled wettability of microstructured MAE surfaces are outlined. The obtained results may be useful for the development of magnetically controlled smart surfaces for droplet-based microfluidics.

Reconfigurable Surface Micropatterns Based on the Magnetic Field-Induced Shape Memory Effect in Magnetoactive Elastomers.

A surface relief grating with a period of 30 µm is embossed onto the surface of magnetoactive elastomer (MAE) samples in the presence of a moderate magnetic field of about 180 mT. The grating, which is represented as a set of parallel stripes with two different amplitude reflectivity coefficients, is detected via diffraction of a laser beam in the reflection configuration. Due to the magnetic-field-induced plasticity effect, the grating persists on the MAE surface for at least 90 h if the magnetic field remains present. When the magnetic field is removed, the diffraction efficiency vanishes in a few minutes. The described effect is much more pronounced in MAE samples with larger content of iron filler (80 wt%) than in the samples with lower content of iron filler (70 wt%). A simple theoretical model is proposed to describe the observed dependence of the diffraction efficiency on the applied magnetic field. Possible applications of MAEs as magnetically reconfigurable diffractive optical elements are discussed. It is proposed that the described experimental method can be used as a convenient tool for investigations of the dynamics of magnetically induced plasticity of MAEs on the micrometer scale.

Molecular recognition of a lipophilic guanosine derivative in Langmuir films at the air-water interface.

Molecular recognition of a lipophilic deoxyguanosine derivative at the air-water interface was investigated by film balance experiments and Brewster Angle Microscopy. Results showed that guanosine, despite strong tendency towards self-assembly, interacts with both complementary and noncomplementary liponucleosides (lipophilic derivatives of deoxycytidine and deoxythymidine). At surface pressures below 17mN/m, attractive and repulsive interactions were present in case of both mixed monolayers and were the strongest at guanosine molar fractions of 0.5 and 0.75. At higher values of surface pressure, deoxyguanosine-deoxycytidine interactions were strictly attractive and were present only for monolayers with guanosine molar fraction of 0.75. On the contrary, attractive and repulsive interactions remained present in case of deoxyguanosine-deoxythymidine mixed monolayers. This indicates that interactions between guanosine and cytidine are much stronger than guanosine-thymidine interactions. Interactions for none of the nucleoside pairs, however, are specific and π-stacking interactions between the aromatic planes of liponucleoside derivatives probably dominate over hydrogen bonding interactions. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.

Ion-specific self-assembly of a lipophilic guanosine derivative in thin surface films.

We investigated the effect of various ions on the surface assembly of a guanosine derivative with one hexadecanoyl chain at the air-water interface. The ions were added to the water subphase prior to spreading of the surface film. Like in bulk water, also at the air-water interface, K(+) ions exhibit the strongest influence on the assembly features as they induce structural transformation from lamellar to mosaic-like assembly. In contrast, Li(+) and Na(+) ions only slightly modify the properties of the assembled film with respect to those observed on pure water. The nature of anions plays an important role in the surface self-assembly as well. We found that (Pic(-)) is 2 orders of magnitude more effective for assembly regulation than Cl(-). All surface assemblies observed in our study are very stable and robust, and consequently they remain practically unperturbed after Langmuir-Blodgett transfer onto a solid support.

Lamellar versus compact self-assembly of lipoguanosine derivatives in thin surface films.

We performed a comparative study on the self-assembling properties of four guanosine derivatives with one and two lipophilic chains of two different lengths at the air-water interface and after Langmuir-Blodgett (LB) deposition onto various solid supports (mica, silicon wafer, graphite). At the air-water interface the derivatives with one lipophilic chain exhibit surface compression behaviour with a profound first order phase transition from the liquid-expanded to the liquid-condensed phase. They assemble into lamellar surface formations, whose structural characteristics remain practically unmodified after their transfer onto the solid substrates. Domain regions with orientationally aligned lamellar formations of sizes up to 150µm(2) can be obtained. The compression behaviour of double-chain derivatives is more diverse. While the derivative with two decanoyl chains exhibits the liquid-expanded as well as the liquid-condensed phase, the derivative with two hexadecanoyl chains reveals only the condensed-analogous phase with a relatively high collapse pressure. LB films of double chain derivatives show formation of very homogeneous and compact surface structures with high surface coverage.

Two novel ternary dicopper(II) µ-guanazole complexes with aromatic amines strongly activated by quantum dots for DNA cleavage.

Two novel (µ-guanazole)-bridged binuclear copper(II) complexes with 1,10-phenanthroline (phen) or 2,2'-bipyridine (bipy), [Cu2(µ-N2,N4-Hdatrz)(phen)2(H2O)(NO3)4] (1) and [Cu2(µ-N1,N2-datrz)2(µ-OH2)(bipy)2](ClO4)2 (2) (Hdatrz = 3,5-diamino-1,2,4-triazole = guanazole), have been prepared and characterized by X-ray diffraction, spectroscopy, and susceptibility measurements. Compounds 1 and 2 differ in the aromatic amine, which acts as a coligand, and in the Cu···Cu'-bridging system. Compound 1, which contains two mono-bridged copper ions, represents the first example of a discrete Cu-(NCN-trz)-Cu' complex. Compound 2, with two triply bridged copper ions, is one of the few compounds featuring a Cu-[(NN-trz)2 + (O-aquo)]-Cu' unit. Both compounds display antiferromagnetic coupling but of different magnitude: J (µ2,4-triazole) = -52 cm(-1) for 1 and J (µ1,2-triazolate) = -115 cm(-1) for 2. The DNA binding and cleavage properties of the two compounds have been investigated. Fluorescence, viscosimetry, and thermal denaturation studies reveal that both complexes have high affinity for DNA (1 > 2) and that only 1 acts as an intercalator. In the presence of a reducing agent like 3-mercaptopropionic acid, 1 produces significant oxidative DNA cleavage, whereas 2 is inactive. However, in the presence of very small quantities of micelles filled with core-shell CdSe-ZnS quantum dots (15 nM), 1 and 2 are considerably more active and become highly efficient nucleases as a result of the different possible mechanisms for promoting cooperative catalysis (metal-metal, metal-hydrogen bonding, metal-intercalation, and metal-nanoparticle). Electrophoresis DNA-cleavage inhibition experiments, X-ray photoelectron spectroscopy studies, and fluorescence ethidium bromide displacement assays reveal that in these novel nucleases the QDs act as redox-active protein-like nanoparticle structures that bind to the DNA and deliver electrons to the copper(II) centers for the generation of Cu(I) and reactive oxygen species.

Liponucleoside thin films: the special behaviour of guanosine.

We performed a comparative study of self-assembling properties of liponucleoside materials containing different nucleobases in their hydrophilic headgroups, but possessing the same type of lipophilic moiety in their hydrophobic tails. Surface pressure versus area isotherms (π(A)) of monolayers formed at the air-water interface show that guanine-based materials form stable monolayer films exhibiting a profound first order phase transition from the liquid expanded to the condensed phase. Materials based on other nucleobases form less stable films exhibiting a liquid-like behaviour. Analysis of surface structures of Langmuir films with Brewster angle microscopy (BAM) and of Langmuir-Blodgett (LB) films deposited on mica surface by atomic force microscopy (AFM) confirms surface organization as resolved from the isotherms. The specific behaviour of molecules with a guanine head group is attributed to their optimal hydrophilic/hydrophobic balance, which is associated with the low water solubility of guanine derivatives.