Surface-induced hydrogelation

The first azo dye that gels from its aqueous solution was synthesized and its moist hydrogel was investigated with electron microscopy and atomic force microscopy; the anionic azo dye in aqueous solution forms a hydrogel on cationic surfaces even at concentrations 50 times below the minimal gelation concentration.     

Surface-induced droplet fusion in microfluidic devices

Here we demonstrate a new method for droplet fusion based on a surface energy pattern on the walls of a microfluidic device, that does not require active elements nor accurate synchronization of the droplets.     

Surface-induced self-assembly of dipeptides onto nanotextured surfaces

There is an increasing interest for the utilization of biomolecules for fabricating novel nanostructures due to their ability for specific molecular recognition, biocompatibility, and ease of availability. Among these molecules, diphenylalanine (Phe-Phe) dipeptide is considered as one of the simplest molecules that can generate a family of self-assembly based nanostructures. The properties of the substrate surface, on which the self-assembly process of these peptides occurs, play a critical role. Herein, we demonstrated the influence of surface texture and functionality on the self-assembly of Phe-Phe dipeptides using smooth silicon surfaces, anodized aluminum oxide (AAO) membranes, and poly(chloro-p-xylylene) (PPX) films having columnar and helical morphologies. We found that helical PPX films, AAO, and silicon surfaces induce similar self-assembly processes and the surface hydrophobicity has a direct influence for the final dipeptide structure whether being in an aggregated tubular form or creating a thin film that covers the substrate surface. Moreover, the dye staining data indicates that the surface charge properties and hence the mechanism of the self-assembly process are different for tubular structures as opposed to the peptidic film. We believe that our results may contribute to the control of surface-induced self-assembly of peptide molecules and this control can potentially allow the fabrication of novel peptide based materials with desired morphologies and unique functionalities for different technological applications.     

Surface-induced phase transition of asymmetric diblock copolymer in selective solvents

Surface-induced phase transition of asymmetric diblock copolymer in selective solvents is first theoretically investigated by using the real-space version of self-consistent field theory (SCFT). By varying the distance between two parallel hard surfaces (or the film thickness) W and the block copolymer concentration f(p), several morphologies are predicted and the phase diagram is constructed. Self-assembly morphologies of the diblock copolymer in dilute solution are found to change significantly with different film thickness. In confined systems, stable morphologies found in the bulk solution become unstable due to the loss of polymer conformation entropy. We find that in a very dilute block copolymer solution, phase separation can be induced through polymer depletion as the solution becomes more confined. Our findings provide an interesting starting point for a renewed effort in both experimental and theoretical investigations of confined block copolymer solutions.     

Surface-induced dissociation of peptide ions in Fourier-transform mass spectrometry

Peptide molecular ion species up to m/z 3055 introduced into a Fourier-transform mass spectrometer can be made to undergo extensive fragmentation by electrically floating the ion cell. The proportion of ions dissociated increases with increasing voltage, with 48 eV producing the highest absolute abundance of fragment ions above m/z 200. At this energy, spectra closely resemble those from photodissociation at 193 nm, indicating an internal energy deposition of 6–7 eV; change of product abundances with kinetic energy resembles a conventional breakdown curve. The precursor ions apparently are electrostatically attracted to strike screen wires across the ion cell entrance, producing daughter ions of low kinetic energy.

     

Surface-induced phase transitions in ultrathin films of block copolymers

We study theoretically the lamellar-disorder-lamellar phase transitions of AB diblock and tetrablock copolymers confined in symmetric slitlike pores where the planar surface discriminatingly adsorbs A segments but repels B segments, mimicking the hydrophobic/hydrophilic effects that have been recently utilized for the fabrication of environmentally responsive "smart" materials. The effects of film thickness, polymer volume fraction, and backbone structure on the surface morphology have been investigated using a polymer density-functional theory. The surface-induced phase transition is manifested itself in a discontinuous switch of microdomains or a jump in the surface density dictated by the competition of surface adsorption and self-aggregation of the block copolymers. The surface-induced first-order phase transition is starkly different from the thickness-induced symmetric-asymmetric or horizontal-vertical transitions in thin films of copolymer melts reported earlier.     

Surface-induced dissociation of singly and multiply protonated polypropylenamine dendrimers

The ease of fragmentation of various charge states of protonated polypropylenamine (POPAM) dendrimers is investigated by surface-induced dissociation. Investigated are the protonated diaminobutane propylenamines [DAB(PA)n] DAB(PA)8 (1+ and 2+), DAB(PA)16 (2+ and 3+), and DAB(PA)32 (3+ and 4+). These ions have been proposed to fragment by charge-directed intramolecular nucleophilic substitution (SNi) reactions. Differences in relative fragment ion abundances between charge states can be related to the occupation of different protonation sites. These positions can be rationalized based on estimates of Coulomb energies and gas-phase basicities of the protonation/fragmentation sites. The laboratory collision energies at which the fragment ion current is approximately 50% of the total ion current were found to increase with the size, but to be independent of charge state of the protonated POPAM dendrimers. It is suggested that intramolecular Coulomb repulsion within the multiply protonated POPAM dendrimers selected for activation does not readily result in easier fragmentation, which is in accordance with the proposed fragmentation mechanism.     

Kinetics of unfolding the human telomeric DNA quadruplex using a PNA trap

The kinetics of opening of the DNA quadruplex formed by the human telomeric repeat have been investigated using real-time fluorescence resonance energy transfer (FRET) measurements with a peptide nucleic acid (PNA) trap. It has been found that this opening is zero-order with respect to PNA, indicating that the initial step is a rate-limiting internal rearrangement of the quadruplex. A study of the temperature dependence of the rate of quadruplex opening was performed and the activation energy of the process estimated to be 98 +/- 8 kJ mol(-1).     

Surface-induced patterns from evaporating droplets of aqueous carbon nanotube dispersions

Evaporation of aqueous droplets of carbon nanotubes (CNTs) coated with a physisorbed layer of humic acid (HA) on a partially hydrophilic substrate induces the formation of a film of CNTs. Here, we investigate the role that the global geometry of the substrate surfaces has on the structure of the CNT film. On a flat mica or silica surface, the evaporation of a convex droplet of the CNT dispersion induces the well-known "coffee ring", while evaporation of a concave droplet (capillary meniscus) of the CNT dispersion in a wedge of two planar mica sheets or between two crossed-cylinder sheets induces a large area (>mm(2)) of textured or patterned films characterized by different short- and long-range orientational and positional ordering of the CNTs. The resulting patterns appear to be determined by two competing or cooperative sedimentation mechanisms: (1) capillary forces between CNTs giving micrometer-sized filaments parallel to the boundary line of the evaporating droplet and (2) fingering instability at the boundary line of the evaporating droplet and subsequent pinning of CNTs on the surface giving micrometer-sized filaments of CNTs perpendicular to this boundary line. The interplay between substrate surface geometry and sedimentation mechanisms gives an extra control parameter for manipulating patterns of self-assembling nanoparticles at substrate surfaces.     

Surface-induced morphologies in thin films of a rod-coil diblock copolymer

A block copolymer containing a rodlike block is studied for its adsorption and formation of nanostructured thin films on the substrate surface. The block copolymer is poly(styrene-b-3-triethoxysilylpropylisocyanate) (PS-b-PIC) of which the PIC chain consists of repeating amide units with triethoxysilyl side groups. As the copolymer chains are adsorbed onto silica surfaces, the PIC blocks pack laterally on the plane in a smectic manner, and the PS chains segregate along the ordered PIC chains, resulting in stripe patterns. The width of the stripes formed on the silica surface appeared to be much larger that on the carbon surface. This was accounted for by the bilayered smectic packing of the rod blocks that is induced by rod-surface attractive interaction. The periodicity of the stripe pattern on the carbon surface indicates that interdigitated packing is preferred by the copolymers on the hydrophobic surface in a manner similar to those in the bulk state of rod-coils. Excess rod-coils on the bilayered smectic layer resulted in a terraced morphology due to large difference in the periodicity between the bilayered smectic layer at the substrate surface and the interdigitated smectic layer in the bulk.     

Surface-induced enantiomorphic crystallization of achiral fullerene derivatives in thin films

The chirality of organic semiconductors is important for various applications in optoelectronics and spintronics. Here, we propose a new strategy to induce structural chirality in achiral organic semiconductors in thin films. Enantiomeric fullerene derivatives (S)-pSi and (R)-pSi, which have oligo(dimethylsiloxane) as a low-surface-energy moiety, were synthesized and used as surface-segregated monolayers (SSMs) in spin-coated films of several achiral fullerene derivatives. Upon thermal annealing, the presence of the chiral SSMs led to the crystallization of the fullerenes in the films as an SSM-induced crystal phase at lower temperatures. The crystallized films showed circular dichroism ascribed to the fullerene absorption, the sign and the intensity of which depended on the handedness of the SSM molecules and the film thickness, respectively. These results indicate that the achiral fullerene derivatives in the films were induced by the SSMs to crystallize into enantiomorphic crystals. Our approach to inducing chirality in organic thin films is compatible with many device applications.

Chiral induction: surface-segregated monolayers of chiral molecules induce the enantiomorphic crystallization of achiral fullerene derivatives in thin films.     

Surface-induced phase behavior of polymer/nanoparticle blends with attractions

In an athermal blend of nanoparticles and homopolymer near a hard wall, there is a first order phase transition in which the nanoparticles segregate to the wall and form a densely packed monolayer above a certain nanoparticle density. Previous investigations of this phase transition employed a fluids density functional theory (DFT) at constant packing fraction. Here we report further DFT calculations to probe the robustness of this phase transition. We find that the phase transition also occurs in athermal systems at constant pressure, the more natural experimental condition than constant packing fraction. Adding nanoparticle-polymer attractions increases the nanoparticle transition density, while sufficiently strong attractions suppress the first-order transition entirely. In this case the systems display a continuous transition to a bulk layered state. Adding attractions between the polymers and the wall has a similar effect of delaying and then suppressing the first-order nanoparticle segregation transition, but does not lead to any continuous phase transitions.     

Hydration in protein folding: thermal unfolding/refolding of human serum albumin

Human serum albumin (HSA) is known to undergo both reversible and irreversible thermal unfolding and refolding, depending upon the experimental conditions (end temperature) at neutral pH. In this report we have used high precision densimetric and ultrasonic measurements to determine the apparent specific volume (phi v) and compressibility (phi k) of HSA at different unfolded and refolded states at two different end temperatures, 55 degrees C and 70 degrees C. The unfolded and refolded states were characterized using dynamic light scattering (DLS), circular dichroism (CD), picosecond-resolved fluorescence decay, and anisotropy of the single-tryptophan residue in HSA (Trp214). Both the unfolded states were allowed to refold by cooling wherein the former and latter processes were found to be reversible and irreversible, respectively, in nature. The results obtained from the densimetric and ultrasonic measurements reveal that the apparent specific volume and compressibility of the protein in the reversible protein unfolding process is preserved upon restoration of HSA to ambient temperature. However, a significant change in phi v and phi k occurs in the process of irreversible protein refolding (from 70 to 20 degrees C). The experimental observation is rationalized in terms of the exposure of domain IIA to an aqueous environment, resulting in the swelling of the protein to a higher hydrodynamic diameter. Our studies attempt to explore the extent of hydration associated with the structural integrity of the popular protein HSA.     

Surface-induced morphologies of ABC star triblock copolymer in spherical cavities

The morphologies and phase diagrams exhibited by symmetric ABC star triblock copolymer nanoparticles are investigated on the basis of real-space self-consistent field theory. The ABC star triblock copolymers were chosen to be tiling-forming with fixed polymer parameter and the spherical boundaries were modeled using the masking technique. We first study a number of examples where the ABC triblock copolymers confined in spherical cavities with neutral surface. Then, two types of spherical cavity distinct preferential surfaces are considered, including both A-block attractive and repulsive preferential surfaces. We aim at the effects due to various spherical cavity diameters and the degree of interactions between the polymer and the spherical surface. A variety of morphologies, such as ring-like structures, concentric sphere, and irregular cylinder, were identified in phase diagrams. The results show that both the degree of interactions and spherical diameters can influence the formation of morphologies so that ring-like structures and other novel structures could be obtained.     

Thermal unfolding of proteins

Thermal unfolding of proteins is compared to folding and mechanical stretching in a simple topology-based dynamical model. We define the unfolding time and demonstrate its low-temperature divergence. Below a characteristic temperature, contacts break at separate time scales and unfolding proceeds approximately in a way reverse to folding. Features in these scenarios agree with experiments and atomic simulations on titin.     

Surface-induced pretransitional order in the isotropic phase near the isotropic-nematic phase transition

We have studied the formation of a nematic layer in the isotropic phase of members of the homologous series of alkylcyanobiphenyl liquid crystals (nCB, n = 5-12) near a polyamide-coated glass surface. From the temperature dependence near the isotropic-nematic transition of the standard ellipsometric quantity δ, which is directly related to the residual birefringence, the wetting behaviour of the nematic layer was investigated using a high precision rotating analyser ellipsometer. In contrast to the results of Chen et al., who observed a wetting transition for nCB-DMOAP-glass systems, our results indicate that for the nCB-polyamide-glass configurations the wetting is always partial.