Capillary condensation in atomic scale friction: how water acts like a glue

Experimental evidence for ice formation at room temperature

The behavior of water under extreme confinement and, in particular, the lubrication properties under such conditions are subjects of long-standing controversy. Using a dedicated, high-resolution friction force microscope, scanning a sharp tungsten tip over a graphite surface, we demonstrate that water nucleating between the tip and the surface due to capillary condensation rapidly transforms into crystalline ice at room temperature. At ultralow scan speeds and modest relative humidities, we observe that the tip exhibits stick-slip motion with a period of 0.38+/-0.03 nm, very different from the graphite lattice. We interpret this as the consequence of the repeated sequence of shear-induced fracture and healing of the crystalline condensate. This phenomenon causes a significant increase of the friction force and introduces relaxation time scales of seconds for the rearrangements after shearing.

Tuning nanoscopic water layers on hydrophobic and hydrophilic surfaces with laser light

The evolutional function of ordered interfacial water near solid surfaces was postulated by Szent-Györgyi: "Life actually, may have started with building these water structures." Here we report their tunability with laser light on both hydrophobic and hydrophilic surfaces. On the former, the light caused their depletion--on the latter, an increase in fluidity--as measured by atomic force acoustic microscopy. Interfacial water layers play a key role in cellular recognition. Their tunability promises to revolutionize various fields in biomedical engineering and life sciences.

Antiinfectives and low-level light: a new chapter in photomedicine

OBJECTIVE

The purpose of this study was to identify synergistic effects in the interaction of light with biosystems in the presence of chemical agents. Their systematic analysis promises therapeutic strategies.

BACKGROUND DATA

Light intensities around 1000 Wm(2) potentially induce density variations in nanoscopic water layers adhering to surfaces in air or subaquatically. In permeable nanoscopic compartments in the interior of biosystems, this could result in powerful flow processes and bidirectional flows for repetitive applications of light. Consequently, external stimulation with light will force microorganisms and cells to incorporate a suitable antiinfective. Nanoscale biosystems, which respond to both light stimulation and antibiotics, are nanobacteria. Responses include growth, inhibition, and slime secretion. Slime secretion was provoked in vitro by gentamycin, an agent proposed for in vivo eradication, and blocked by light. Depending on the field of action, co-operative effects between light and an antiinfective can be exploited by considering two properties of the drug: transmission of light and resorption by the tissue. Antiinfectives can be administered in an active form or via drug delivery systems. In the latter case, a double action of the light could be exploited: stimulated release from the carrier and subsequent uptake by the targeted biosystem.

METHODS

The attenuation of laser light (670 nm) by antiinfectives was measured in films of different thickness of a vaginal suppository. The effect of 670-nm laser light - not absorbed by water - on nanoscopic water layers was examined by comparing the evaporation time of irradiated drops of water-based nanosuspensions with non-irradiated controls.

RESULTS

The 6-microm-thick suppository films were virtually transparent to the laser light, and the 1-mm-thick films totally attenuated it. Nanosuspension drops irradiated with 670-nm light needed more time to evaporate than controls.

CONCLUSION

Low-level light (LLL) therapy is compatible with antiinfectives, and even capable of boosting effects of superficially applied and/or absorbed antiinfectives. Temporal coordination between light treatment and drug administration maximizes drug effects and minimizes possible adverse effects. Irradiation should start when the drug concentration has reached its maximum in the desired field of action. Light-induced flow in nanoscale cavities could represent one mechanism of LLL therapy.

Understanding sub-20 nm breakdown behavior of liquid dielectrics

Nanoscale confinement of dielectric molecules is expected to influence their breakdown mechanism in applications such as nanoprobe based machining, molecular electronics, and other related technologies. This Letter presents the first experimental study of the breakdown of nonpolar, nonthiolated liquid dielectrics in the nanometer regime and develops a field emission assisted avalanche based approach to model such behavior. The studies show that dielectric breakdown in the sub-20 nm regime is independent of the cathode materials and is dominated by the electron emission and atomic cluster migration due to the "sub-20 nm scale confinement of the liquid dielectric."

Quantum mechanical calculations of charge effects on gating the KcsA channel

A series of ab initio (density functional) calculations were carried out on side chains of a set of amino acids, plus water, from the (intracellular) gating region of the KcsA K(+) channel. Their atomic coordinates, except hydrogen, are known from X-ray structures [D.A. Doyle, J.M. Cabral, R.A. Pfuetzner, A. Kuo, J.M. Gulbis, S.L. Cohen, B.T. Chait, R. MacKinnon, The structure of the potassium channel: molecular basis of K(+) conduction and selectivity, Science 280 (1998) 69-77; R. MacKinnon, S.L. Cohen, A. Kuo, A. Lee, B.T. Chait, Structural conservation in prokaryotic and eukaryotic potassium channels, Science 280 (1998) 106-109; Y. Jiang, A. Lee, J. Chen, M. Cadene, B.T. Chait, R. MacKinnon, The open pore conformation of potassium channels. Nature 417 (2001) 523-526], as are the coordinates of some water oxygen atoms. The 1k4c structure is used for the starting coordinates. Quantum mechanical optimization, in spite of the starting configuration, places the atoms in positions much closer to the 1j95, more tightly closed, configuration. This state shows four water molecules forming a "basket" under the Q119 side chains, blocking the channel. When a hydrated K(+) approaches this "basket", the optimized system shows a strong set of hydrogen bonds with the K(+) at defined positions, preventing further approach of the K(+) to the basket. This optimized structure with hydrated K(+) added shows an ice-like 12 molecule nanocrystal of water. If the water molecules exchange, unless they do it as a group, the channel will remain blocked. The "basket" itself appears to be very stable, although it is possible that the K(+) with its hydrating water molecules may be more mobile, capable of withdrawing from the gate. It is also not surprising that water essentially freezes, or forms a kind of glue, in a nanometer space; this agrees with experimental results on a rather different, but similarly sized (nm dimensions) system [K.B. Jinesh, J.W.M. Frenken, Capillary condensation in atomic scale friction: how water acts like a glue, Phys. Rev. Lett. 96 (2006) 166103/1-4]. It also agrees qualitatively with simulations on channels [A. Anishkin, S. Sukharev, Water dynamics and dewetting transitions in the small mechanosensitive channel MscS, Biophys. J. 86 (2004) 2883-2895; O. Beckstein, M.S.P. Sansom, Liquid-vapor oscillations of water in hydrophobic nanopores, Proc. Natl Acad. Sci. U. S. A. 100 (2003) 7063-7068] and on featureless channel-like systems [J. Lu, M.E. Green, Simulation of water in a pore with charges: application to a gating mechanism for ion channels, Prog. Colloid Polym. Sci. 103 (1997) 121-129], in that it forms a boundary on water that is not obvious from the liquid state. The idea that a structure is stable, even if individual molecules exchange, is well known, for example from the hydration shell of ions. We show that when charges are added in the form of protons to the domains (one proton per domain), the optimized structure is open. No stable water hydrogen bonds hold it together; an opening of 11.0 A appears, measured diagonally between non-neighboring domains as glutamine 119 carbonyl O-O distance. This is comparable to the opening in the MthK potassium channel structure that is generally agreed to be open. The appearance of the opening is in rather good agreement with that found by Perozo and coworkers. In contrast, in the uncharged structure this diagonal distance is 6.5 A, and the water "basket" constricts the uncharged opening still further, with the ice-like structure that couples the K(+) ion to the gating region freezing the entrance to the channel. Comparison with our earlier model for voltage gated channels suggests that a similar mechanism may apply in those channels.

Surface molecular view of colloidal gelation

We investigate the phase behavior of surface-functionalized silica colloids at both the molecular and macroscopic levels. This investigation allows us to relate collective properties such as aggregation, gelation, and aging directly to molecular interfacial behavior. By using surface-specific vibrational spectroscopy, we reveal dramatic changes in the conformation of alkyl chains terminating submicrometer silica particles. In fluid suspension at high temperatures, the interfacial molecules are in a liquid-like state of conformational disorder. As the temperature is lowered, the onset of gelation is identified by macroscopic phenomena, including changes in turbidity, heat release, and diverging viscosity. At the molecular level, the onset of this transition coincides with straightening of the carbon-carbon backbones of the interfacial molecules. In later stages, their intermolecular crystalline packing improves. It is the increased density of this ordered boundary layer that increases the van der Waals attraction between particles, causing the colloidal gas to aggregate. The approach presented here can provide insights into phase transitions that occur through surface modifications in a variety of colloidal systems.

Hydrothermally shrunk alumina nanopores and their application to DNA sensing

Hydrothermal treatment of anodized alumina membranes has been known for years and is believed to seal the pores by transforming aluminium oxide into lower density hydroxides. We demonstrate that, at least for 60 nm diameter pores grown from anodization in oxalic acid at 40 V, the hydrothermal treatment significantly shrinks but does not fully seal the nanopores. The pores shrink to a neck of less than 10 nm in diameter and 2-4 microm in length, in which the diffusion coefficient of ions is five orders of magnitude smaller than in the bulk. Because of a high electrolyte resistance through hydrothermally treated shrunken nanopores, they can be used for electrical sensing applications, as demonstrated using the example of DNA sensing. Hybridization of target DNA with a complementary ssDNA covalently immobilized inside the nanopores causes an increase in impedance by more than 50% while a noncomplementary ssDNA has no measurable effect.