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Inducing a phase transition of a self-organized object may trigger its structural transformation. Here, we demonstrate local control of the morphology and shape of self-organized microparticles with a nanoflake outer surface by nanoplasmonic heating. To increase the photothermal efficiency of the microparticles, gold nanoparticles (AuNPs) or single-walled carbon nanotubes (SWCNTs) were incorporated. AuNPs and SWCNTs, which have excellent photothermal activity, acts as photoresponsive heat converters. Because they have distinct absorption characteristics, visible or near-infrared lasers can be used to induce local heating. The photothermal effect was used to spatially confine the melting to the space within the particle and the aggregate; as a result, microparticles with various shapes and morphologies have been fabricated. Such morphological changes lead to a superhydrophobic-hydrophobic wetting transition, which was confirmed by the films constituting the microparticles. The work presented is seen useful for anisotropic particle synthesis, local wetting control, lithography, and morphological control of functional materials.

We demonstrate a silanization reaction-based simple and reproducible method of converting a standard microscope glass slide to a transparent superhydrophobic surface with careful controlling of the temperature, concentration, and reaction time. By optimizing the preparation conditions, the two step treatment of a glass slide (reaction with dilute methyltrichlorosilane in toluene followed by ethanol extraction) leads to ultrahigh water contact angles (> 150°), i.e., water droplets sit atop the surface in the Cassie–Baxter state while the glass slide is still optically transparent. UV–vis absorption spectra confirm that the optical transmittance of the superhydrophobic glass slides can be as high as 99% (λ = 550 nm), which is superior to untreated slides (93%). Both the superhydrophobicity and optical transparency are dictated by the microscopic morphology resulting from the controlled silanization of the glass surface, for which in this study the reproducibility was achieved to be about 90%.

The long-term stability (gradual oxidation) of two structurally different organic monolayers, C6H5(CH2)3−Si≡ (C3Ph−Si≡) and CH3(CH2)11−Si≡ (C12−Si≡), covalently bonded to n-type silicon(111) was st...

The water structure at superhydrophobic solid/bulk water interfaces has been investigated by vibrational sum frequency generation (SFG) spectroscopy. We have adapted a simple and facile chemical modification procedure (methyltrichlorosilane/toluene treatment followed by extraction with ethanol) to prepare transparent quartz crystal surfaces with varied wetting properties, from hydrophilic to hydrophobic and superhydrophobic. In comparison with those obtained on bare quartz, the SFG spectra on polymethylsiloxane-modified surfaces showed significant changes of the relative intensities of the two broad OH stretching modes of hydrogenbonded water at ～3200 and ～3450 cm^[-1], which can be correlated with surface morphology and molecular variations. Intriguingly, on superhydrophobic quartz these bands are very weak and replaced by a characteristic "free OH" (not hydrogen-bonded) stretching (>3600 cm^[-1]) band that is typically observed at water/air interfaces. These results suggest that hydrogen bonding between water molecules weakens as hydrophobicity increases on rough heterogeneous solid surfaces. More importantly, this study provides direct evidence for the existence of stable solid/air/water three-phase interfaces when a superhydrophobic solid is in contact with bulk water.

The conformation of self-assembled monolayers (SAMs) buried at metal-semiconductor interfaces plays a crucial role in the functioning of the molecular junctions thus formed. We have studied by vibrational sum frequency generation (SFG) spectroscopy the influence of deposited gold films on the molecular orientation of a series of n-alkyl monolayers that are covalently bonded to silicon(111) via Si-C linkages (≡Si-CnH_[2n+1],n = 10, 12, 14, and 18). The SFG spectra indicate that the gold contacts (overlayers) introduce significant gauche defects within the initially highly oriented alkyl chains. The methyl tilt angles of the buried n-alkyl monolayers depend on the gold deposition method (vacuum evaporation vs sputtering) and the alkyl chain length. These observations are supported by solid-state electrical measurements.

Nucleic acids possess charged phosphate groups in their backbones, which require counterions to reduce the repulsive Coulombic interactions between the strands. Herein we report how different mono- and divalent metal cations influence the molecular orientations of DNA molecules on silicon surfaces upon immobilization and hybridization. Our sum frequency generation (SFG) spectroscopy studies demonstrated that the degree of conformational variation of DNA self-assembled monolayers on silicon depends on the type of metal cations present. The molecular orientation change of immobilized single-stranded oligonucleotides correlates with DNA-cation affinity (Mg(2+)Ca(2+)K(+) approximately Na(+)): metal cations with the strongest affinity disrupt the structure of the underlying linker monolayer the most. Upon hybridization the trend is reversed, which is attributed to the greater ability of divalent cations to mask the negative charges on the DNA backbone. These findings provide useful information for the construction of more sensitive DNA biosensors, particularly the optimization of on-chip hybridization performance.

Conventionally, amphiphiles are composed of hydrophobic and hydrophilic units. They are able to exhibit a wide variety of structures depending on the environment. Such features have been applied in supramolecular chemistry, by which apolar and polar groups are implemented in the molecular design. Here we present an attractive approach to introduce unique amphiphilicity. Relatively simple fullerene (C(60)) derivatives that bear long aliphatic chains behave as uncommon surfactants in organic media. Although two hydrophobic units are used to assemble the derivatives, slight differences in their polarity and chemical nature may make them incompatible and thus arrange them in microphase-separated mesostructures as lamellar ones. These assemblies are maintained by relatively weak forces, pi-pi interactions among C(60) moieties and van der Waals forces between alkyl chains. Therefore, the derivatives can undergo "supramolecular polymorphism" by which different supramolecular assemblies arise by changing the conditions of assembly. A simple modification in their substituent motif of derivatives influences the intermolecular interactions and provides a wide variety of supramolecular materials.

Carboxylic acid-terminated monolayers on crystalline silicon surfaces can be readily modified with biological macromolecules for the fabrication of semiconductor-based biosensing devices. They were prepared by acid-catalyzed hydrolysis of alkoxycarbonyl (ester)-terminated monolayers and studied by vibrational sum frequency generation (SFG) spectroscopy. The C-H vibration region of the SFG spectra consists of strong methyl bands with significant contributions from methylene stretching modes, indicating that these monolayers are generally ordered but with considerable gauche defects in the alkyl chains in comparison with n-alkyl monolayers. After hydrolysis, the methylene stretching modes prevail, with "residues" of the methyl bands, indicating incomplete hydrolysis and disruption of the monolayer structure. This work demonstrates that SFG is capable of providing quantitative information on structure-reactivity correlations in organic monolayers.

Carboxy-terminated crystalline silicon surfaces are of fundamental importance for biochip fabrication because of their reactivity toward biological macromolecules. To explore the feasibility of direct attachment of bifunctional molecules (e.g., omega-alkenoic acids) to hydrogen-terminated silicon crystal (H-Si) via Si-C linkages, we have investigated the photoreactivities of the alkene (-CH=CH(2)) and carboxy (-COOH) terminal groups of 1-dodecene, undecanoic acid, and undecylenic acid toward H-Si. The alkene terminus was found to react substantially faster than the carboxy terminus under UV irradiation (at 350 nm). By controlling the reaction time, high-quality carboxy-terminated monolayers, comparable to those formed by ester hydrolysis, can be obtained from a direct, one-step photochemical reaction between H-Si and undecylenic acid.

Plasmonic heating to trigger release of oligonucleotides from nanoconjugates is potentially useful for therapeutic purposes and designed assembly of DNA nanostructures. In the past, great controllability has been achieved by introducing distinctive absorption nanoparticle centers, where the anchoring bond (e.g., sulfur-gold bond) has been selectively broken. Instead of releasing the surface-bound duplex DNA via breakage of the gold-sulphur anchor bond, selective and non-destructive dehybridization of DNA under a "mild" condition on different gold nanoconjugates is demonstrated in this work. This finding will permit sequential dehybridization/release of DNA at specific regions of a complex system; thus it can be extended to control gene expression and to manipulate an assembly of highly organized DNA constructs. Particularly we show herein the feasibility of selectively dehybridizing DNA-Au nanoconjugates via localized plasmonic heating, which is accomplished by controlling the laser wavelength, power, and irradiation time.

We describe a versatile portable prototype device for electrical measurements of “soft” materials. It consists of a custom-made micromanipulator and a gas-tight syringe to hold a mercury mini-drop to make reliable and controllable electrical contacts with solid-state materials of interest. Compared with the conventional method (deposition of thin metal films under vacuum), the proposed experimental approach is simpler and more cost-effective.