Liquid acetonitrile at a silica surface is known to organize in a manner that, in some respects, resembles a supported lipid bilayer. Here we use broadband vibrational sum-frequency generation (VSFG) spectroscopy to study how this organization depends on temperature. We find that VSFG spectra in the methyl-stretching region of the spectrum decrease in magnitude and shift to the blue for all polarization combinations (SSP, SPS, and PPP) as the temperature is raised from 25 to 60 °C. The observed decrease in intensity with increasing temperature is unexpected and suggests that the dynamics of the second sublayer of acetonitrile molecules plays an important role in the spectral behavior of this system. We propose that the decrease in intensity with increasing temperature arises largely from the more static first sublayer becoming more disordered. This picture is supported by the observed blue shift at high temperature, which is consistent with faster reorientation-induced spectral diffusion. The rapid dynamics and average orientation of the second sublayer are biased by the first sublayer, but this bias is not influenced substantially by increasing the temperature.

Nitriles are important solvents not just for bulk reactions but also for interfacial processes such as separations, heterogeneous catalysis, and electrochemistry. Although nitriles have a polar end and a lipophilic end, the cyano group is not hydrophilic enough for these substances to be thought of as prototypical amphiphiles. This picture is now changing, as research is revealing that at a silica surface nitriles can organize into structures that, in many ways, resemble lipid bilayers. This unexpected organization may be a key component of unique interfacial behavior of nitriles that make them the solvents of choice for so many applications.The first hints of this lipid-bilayer-like (LBL) organization of nitriles at silica interfaces came from optical Kerr effect (OKE) experiments on liquid acetonitrile confined in the pores of sol–gel glasses. The orientational dynamics revealed by OKE spectroscopy suggested that the confined liquid is composed of a relatively immobile sublayer of molecules that accept hydrogen bonds from the surface silanol groups and an interdigitated, antiparallel layer that is capable of exchanging into the centers of the pores.This picture of acetonitrile has been borne out by molecular dynamics simulations and vibrational sum-frequency generation (VSFG) experiments. Remarkably, these simulations further indicate that the LBL organization is repeated with increasing disorder at least 20 Å into the liquid from a flat silica surface. Simulations and VSFG and OKE experiments indicate that extending the alkyl chain to an ethyl group leads to the formation of even more tightly packed LBL organization featuring entangled alkyl tails. When the alkyl portion of the molecule is a bulky t-butyl group, packing constraints prevent well-ordered LBL organization of the liquid. In each case, the surface-induced organization of the liquid is reflected in its interfacial dynamics.Acetonitrile/water mixtures are favored solvent systems for separations technologies such as hydrophilic interaction chromatography. Simulations had suggested that although a monolayer of water partitions to the silica surface in such mixtures, acetonitrile tends to associate with this monolayer. VSFG experiments reveal that, even at high water mole fractions, patches of well-ordered acetonitrile bilayers remain at the silica surface. Due to its ability to donate and accept hydrogen bonds, methanol also partitions to a silica surface in acetonitrile/methanol mixtures and can serve to take the place of acetonitrile in the sublayer closest to the surface.These studies reveal that liquid nitriles can exhibit an unexpected wealth of new organizational and dynamic behaviors at silica surfaces, and presumably at the surfaces of other chemically important materials as well. This behavior cannot be predicted from the bulk organization of these liquids. Our new understanding of the interfacial behavior of these liquids will have important implications for optimizing a wide range of chemical processes in nitrile solvents.

Molecular dynamics simulations have been used to study the orientational dynamics of methanol at the liquid/vapor and liquid/silica interfaces. Orientational time-correlation functions for the symmetric and asymmetric methyl stretches of methanol have been calculated to assess the role of reorientation in the vibrational sum-frequency generation (VSFG) spectroscopy of these modes at the two interfaces. We find that internal methyl rotation plays a significant role in suppressing the intensity of the asymmetric methyl stretches at the liquid/vapor interface. The broad orientational distribution of the methyl groups and the properties of the Raman spectra of the asymmetric stretches are also major contributors to the low intensity of these modes in VSFG spectra at this interface. We find that after an initial rapid, inertial decay, there is little coupling among the orientational degrees of freedom of the methyl group, which suggests that time-dependent VSFG spectroscopy may be used to probe interfacial orientational dynamics in this and other hydrogen-bonded liquids.

Optical Kerr effect (OKE) spectroscopy is a widely used technique for probing the low-frequency, Raman-active dynamics of liquids. Although molecular simulations are an attractive tool for assigning liquid degrees of freedom to OKE spectra, the accurate modeling of the OKE and the motions that contribute to it relies on the use of a realistic and computationally tractable molecular polarizability model. Here we explore how the OKE spectrum of liquid benzene, and the underlying dynamics that determines its shape, are affected by the polarizability model employed. We test a molecular polarizability model that uses a point anisotropic molecular polarizability and three other models that distribute the polarizability over the molecule. The simplest and most computationally efficient distributed polarizability model tested is found to be sufficient for the accurate simulation of the many-body polarizability dynamics of this liquid. We further find that the atomic-to-molecular polarizability transformation approximation [Hu et al. J. Phys. Chem. B 2008, 112, 7837–7849], used in conjunction with this distributed polarizability model, yields OKE spectra whose shapes differ negligibly from those calculated without this approximation, providing a substantial increase in computational efficiency.

Many biological and physiological processes depend upon directed migration of cells, which is typically mediated by chemical or physical gradients or by signal relay. Here we show that cells can be guided in a single preferred direction based solely on local asymmetries in nano/microtopography on subcellular scales. These asymmetries can be repeated, and thereby provide directional guidance, over arbitrarily large areas. The direction and strength of the guidance is sensitive to the details of the nano/microtopography, suggesting that this phenomenon plays a context-dependent role in vivo. We demonstrate that appropriate asymmetric nano/microtopography can unidirectionally bias internal actin polymerization waves and that cells move with the same preferred direction as these waves. This phenomenon is observed both for the pseudopod-dominated migration of the amoeboid Dictyostelium discoideum and for the lamellipod-driven migration of human neutrophils. The conservation of this mechanism across cell types and the asymmetric shape of many natural scaffolds suggest that actin-wave-based guidance is important in biology and physiology.

Photolithography is a crucial technology for both research and industry. The desire to be able to create ever finer features has fuelled a push towards lithographic methods that use electromagnetic radiation or charged particles with the shortest possible wavelength. At the same time, the physics and chemistry involved in employing light or particles with short wavelengths present great challenges. A new class of approaches to photolithography on the nanoscale involves the use of photoresists that can be activated with one colour of visible or near-ultraviolet light and deactivated with a second colour. Such methods hold the promise of attaining lithographic resolution that rivals or even exceeds that currently sought by industry, while at the same time using wavelengths of light that are inexpensive to produce and can be manipulated readily. The physical chemistry of 2-colour photolithography is a rich area of science that is only now beginning to be explored.

Molecular dynamics (MD) simulations of propionitrile have been performed to assess the influence of reorientation on vibrational sum-frequency-generation (VSFG) spectra at the liquid/vapor (LV) and liquid/silica (LS) interfaces. Orientational time–correlation functions (TCFs) are derived for the VSFG spectroscopy of the symmetric and asymmetric stretches of functional groups such as methylene groups and rotationally hindered methyl groups. The MD simulations are used to compute VSFG orientational TCFs for the methyl, methylene, and cyanide groups of propionitrile at the LV and LS interfaces. Although propionitrile exhibits relatively fast reorientation in the bulk liquid, we find that for symmetric stretching modes at these interfaces, reorientation only plays a significant role in VSFG spectra under SPS polarization conditions. For asymmetric stretches, reorientation affects the VSFG spectra significantly under all polarization conditions. Azimuthal dynamics tend to dominate the orientational TCFs.

Synthetic nanostructures, such as nanoparticles and nanowires, can serve as modular building blocks for integrated nanoscale systems. We demonstrate a microfluidic approach for positioning, orienting, and assembling such nanostructures into nanoassemblies. We use flow control combined with a cross-linking photoresist to position and immobilize nanostructures in desired positions and orientations. Immobilized nanostructures can serve as pivots, barriers, and guides for precise placement of subsequent nanostructures.

Complementary experimental and theoretical studies presented in this work examine the structure, organization, and solvating properties of methanol at a silica/methanol, solid/liquid interface. Findings from these experiments illustrate how strong association between a silica substrate and methanol solvent creates a distinctly nonpolar solvation environment for adsorbed solutes. Resonance-enhanced second-harmonic spectra and time-resolved fluorescence emission in a total internal reflection geometry both show that adsorbed solutes sample an interfacial environment having properties resembling those of a nonpolar solvent. Molecular dynamics simulations identify the origin of this effect. Strong hydrogen bonding between the first layer of methanol and silica’s silanol groups creates what is effectively a methyl-terminated surface that leads to a second layer having significantly reduced density and hydrogen bonding compared to bulk solution. The calculated solvent reorientation times in these first two layers is significantly slower than in bulk, implying slow dielectric relaxation and supporting both second-harmonic and time-resolved fluorescence results. Collectively, these studies illustrate how surface-induced changes in solvent structure change the chemistry at strongly associating solid/liquid interfaces as compared to bulk solution limits.

Orientational time correlation functions (TCFs) are derived for vibrational sum-frequency generation (VSFG) spectroscopy of the symmetric and asymmetric stretches of high-symmetry oscillators such as freely rotating methyl groups, acetylenic C–H groups, and cyanide groups. Molecular dynamics simulations are used to calculate these TCFs and the corresponding elements of the second-order response for acetonitrile at the liquid/vapor and liquid/silica interfaces. We find that the influence of reorientation depends significantly on both the functional group in question and the polarization conditions used. Additionally, under some circumstances, reorientation can cause the VSFG response function to grow with time, partially counteracting the effects of other dephasing mechanisms.

Previous experiments and simulations have shown that acetonitrile organizes into a lipid-like bilayer at the liquid/silica interface. Recent simulations have further suggested that this bilayer structure persists in mixtures of acetonitrile with water, even at low acetonitrile concentrations. This behavior is indicative of microscopic phase separation of these liquids near silica interfaces and may have important ramifications for the use of acetonitrile in chromatography and heterogeneous catalysis. To explore this phenomenon, we have used vibrational sum-frequency-generation spectroscopy to probe acetonitrile/water mixtures at a silica interface. Our spectra provide evidence that acetonitrile partitions to the hydrated silica interface even when the mole fraction of acetonitrile is as low as 10%. A blue shift is observed in the spectrum of the methyl symmetric stretch upon increasing water mole fraction, in agreement with vibrational spectra of bulk mixtures. Line shape analysis suggests that acetonitrile may exist in the form of bilayer patches at high water mole fractions.

There is a growing appreciation that dynamic processes play an important role in determining the line shape in surface-selective, nonlinear spectroscopies such as vibrational sum-frequency-generation (VSFG). Here we analyze the influence that reorientation can have on VSFG spectra when the vibrational transition frequency is a function of orientation. Under these circumstances, reorientation-induced spectral diffusion (RISD) causes the underlying spectral line shape to become time dependent. Unlike previously reported mechanisms through which reorientation can contribute to the VSFG signal, RISD influences the line shape regardless of the degree of polarization of the Raman transition that is probed. We assess the impact of RISD on VSFG spectra using a model system of liquid acetonitrile at a silica interface. Comparison of delay-time-dependent VSFG spectra with simulations that employ static line shapes suggests that RISD contributes substantially to the spectra, particularly at delay times that are comparable to or greater than the probe pulse duration. The observed behavior is in qualitative agreement with a two-state RISD model that uses orientational distributions determined from previous molecular dynamics simulations.

Efficient multiphoton radical generation chemistry has been developed for use in aqueous media. Through a combination of multiphoton absorption polymerisation (MAP) and optical tweezers, this chemistry has been applied to the fabrication, manipulation, and assembly of 3D polymeric and biomolecular structures. Combining MAP and optical tweezers allows for the direct assembly of 3D structures from microscale objects as well as for the realisation of structures, such as tape-like and rope-like microthreads, that can be used for unconventional microfabrication techniques including microbraiding and microweaving. These capabilities significantly expand the toolbox of methods available for the creation of functional microstructures in aqueous media.

Propionitrile has been studied at the liquid/vapor, silica/vapor, and silica/liquid interfaces using vibrational sum-frequency generation (VSFG) spectroscopy and optical Kerr effect (OKE) spectroscopy. VSFG studies show that the alkyl tail of propionitrile tends to point into the vapor phase at the liquid/vapor and silica/vapor interfaces. At the silica/liquid interface, all vibrational resonances except for the methylene symmetric stretch exhibit strong cancellation in the VSFG signal. This result supports the existence of a lipid-bilayer-like organization at the silica/liquid interface, in agreement with simulation. OKE data for propionitrile confined in porous sol–gel glasses indicate that there is a surface layer, with a thickness of roughly 4 Å, that experiences inhibited orientational dynamics. The OKE data thus corroborate the picture of interfacial organization suggested by the VSFG results.

The organization of liquid propionitrile at a hydrophilic silica surface and at the liquid–vapor interface has been studied by molecular dynamics simulations. Analysis has been performed on particle densities and the orientations of different bond vectors. At the silica/liquid interface, propionitrile adopts a lipid-bilayer-like structure in which the carbon–nitrogen bond vector has opposite orientations in two sublayers. To explore the influence of the alkyl group on the interfacial structure, we have also studied the methylene–methyl vector orientations and locations of the methyl groups for molecules in the different sublayers. This analysis reveals that liquid propionitrile forms a compact, entangled bilayer at a silica surface, in contrast to the interdigitated bilayer that has been observed previously for acetonitrile.

Optical spectroscopy has been used to probe the interfacial organization and dynamics of trimethylacetonitrile (TMACN). Molecular orientation at the silica/liquid, silica/vapor and liquid/vapor interfaces of TMACN has been studied using vibrational sum-frequency generation (VSFG) spectroscopy. These studies reveal that TMACN exhibits appreciable organization at each of these interfaces, despite the bulky nature of its tert-butyl group. VSFG spectra measured from the silica/liquid interface suggest that TMACN does not form the sort of well-organized bilayer that has been observed previously for acetonitrile and propionitrile. Optical Kerr effect studies of TMACN confined in porous silica glasses demonstrate that this liquid forms a dynamically inhibited surface layer that is roughly one molecule thick, which is consistent with the organizational model suggested by the VSFG data.

We demonstrate a technique for the precise immobilization of nanoscale objects at accurate positions on two-dimensional surfaces. We have developed a water-based photoresist that causes nanostructures such as colloidal quantum dots to segregate to a thin layer at surfaces. By combining this material with electroosmotic feedback control, we demonstrate the ability to position selected, individual quantum dots at specific locations and to immobilize them with 130 nm precision via localized UV exposure.

We demonstrate the use of high-speed, multiphoton absorption polymerization (MAP) for the fabrication of large-area microfluidic master structures. High-speed fabrication in SU8 without laser-induced damage is made possible by the use of a novel photoacid generator with a high two-photon-absorption cross-section. Master structures fabricated with MAP can be used to produce polydimethylsiloxane microchannels with high aspect ratios and/or arbitrary cross-sections. Microchannels with different cross-sections and heights can be combined readily in a single device. This fabrication technique significantly increases the diversity of channel architectures available for microfluidic devices.

Molecular dynamics simulations and vibrational sum frequency generation (VSFG) experiments in the methyl-stretching spectral region have been used to study acetonitrile at the silica/liquid, silica/vapor, and liquid/vapor interfaces. Our simulations show that, at the silica/liquid interface, acetonitrile takes on a considerably different structure than in the bulk liquid. The interfacial structure is reminiscent of a lipid bilayer, and this type of ordering persists for tens of Ångstroms into the bulk liquid. This result has important implications for processes involving solid/acetonitrile interfaces, such as heterogeneous catalysis and chromatographic separations. Fitting of VSFG data that have an extremely low nonresonant background contribution provides strong evidence for interfacial populations pointing in opposite directions at these interfaces, in agreement with our simulations. The picture developed from our simulations and experiments reconciles conflicting interpretations of data from previous experimental studies of interfacial acetonitrile.

Optical Kerr effect (OKE) spectroscopy allows for the acquisition of high-quality, Bose−Einstein-corrected, low-frequency Raman spectra in liquids. However, the assignment of a molecular interpretation to these spectra remains an open problem. To address this issue, here we present an OKE study of benzene and four of its isotopologues. Our results indicate that hindered rotations are the major contributor to the OKE reduced spectral density (RSD) over the entire intermolecular spectral region (0−250 cm−1). We also have found that isotopologues with six 13C atoms have RSDs that are enhanced at frequencies below 30 cm−1. We further demonstrate that the collective orientational correlation time of these liquids scales with the inverse square root of the tumbling moment of inertia, indicating that there is strong translation−rotation coupling in benzene.

Metal-enhanced multiphoton absorption polymerization (MEMAP) is studied using gold nanowires with a set of three different photoresists. Photoresists that employ radical and cationic polymerization are investigated. In all cases, MEMAP is strongly correlated with multiphoton-absorption-induced luminescence (MAIL) of the nanowires. Wavelength-dependent studies indicate that the dominant mechanism for MEMAP is not field-enhanced two-photon absorption of the photoinitiator, but rather single-photon excitation of the photoinitiator by the broadband MAIL emission.

Industrial approaches to improving lithographic resolution rely upon using radiation or charged particles of ever shorter wavelengths. Working with such light or particles is difficult and expensive. However, recently developed techniques show promise for performing nanoscale photolithography using visible light. The ability to employ visible light for nanoscale photolithography may facilitate considerable reductions in the difficulty and expense involved in the continued increase of transistor density in integrated circuits. In this Perspective, we review the physical chemistry of several of these new techniques and discuss future prospects for visible-light nanoscale photolithography.

Static contact-angle measurements have been used to study water and five organic liquids (n-decane, benzene, acetonitrile, octyl cyanide, and sebaconitrile) on bare silica and on a range of silanized silica surfaces. We have used these measurements to determine the polar and dispersive components of the surface energies of these liquids and of the modified silica surfaces. These data offer clues into the microscopic structuring of the liquids and how this structuring is influenced by solid interfaces. We have also observed a strong relationship between the polar and dispersive components of the surface energies of the modified silica surfaces.

We investigate the connections between two field-enhanced phenomena of gold nanoparticles: multiphoton-absorption-induced luminescence (MAIL) and metal-enhanced multiphoton absorption polymerization (MEMAP). We observe a strong correlation between the nanoparticles and aggregates that have high efficiency for each process. The results of our studies indicate that for this system, MEMAP is driven not by field-enhanced two-photon absorption of the photoinitiator but rather by single-photon excitation of the photoinitiator by the MAIL emission.

It has recently been shown [J. Chem. Phys. 2005, 122, 134506; J. Am. Chem. Soc. 2006, 128, 5119] that liquid tetrahydrofuran has an unusual structure that features voids of significant dimension. Such voids should affect other observable properties of this liquid. Here we present temperature-dependent, optical Kerr effect data for tetrahydrofuran and a number of related liquids (furan, cyclopentane, tetrahydropyran, cyclohexane, diethyl ether, and n-pentane) as well as hexamethylphosphoramide to test whether this technique can be used to reveal the presence of sizable voids in liquids.

In this paper, we review the state of the field of optical Kerr effect (OKE) spectroscopy of simple liquids, with a focus on results from our laboratory. We discuss the history and the theoretical underpinnings of this technique. We consider contemporary issues in the interpretation of OKE spectra, including the origin of the “intermediate” response and the factors affecting the shape of the reduced spectral density. We highlight some applications of the OKE spectroscopy of simple liquids, including the study of liquid mixtures and the behavior of liquids in nanoconfinement. We also discuss future prospects for OKE spectroscopy and related techniques.

We present a detailed, temperature-dependent, optical Kerr effect (OKE) study of pyridine, pyridine-d5, 2,4,6-trifluoropyridine, 2,4,6-trimethylpyridine, and 1,3,5-tris(trifluoromethyl)benzene. By combining these data with those for other aromatic liquids that we have studied previously (Loughnane, B. J.; Scodinu, A.; Fourkas J. T. J. Phys. Chem. B, 2006, 110, 5708), we are able to assess the relative importance of molecular shape and electrostatic forces in determining the form of the OKE reduced spectral density for such liquids.

There is growing interest in lithographic technologies for creating 3D microstructures. Such techniques are generally serial in nature, prohibiting the mass production of devices. Soft-lithographic techniques show great promise for simple and rapid replication of arrays of microstructures but have heretofore not been capable of direct replication of structures with closed loops. We demonstrate that 3D microstructures created with multiphoton absorption polymerization can be replicated by using microtransfer molding to afford complex daughter structures containing closed loops. This method relieves many of the topological constraints of soft lithography, paving the way for the large-scale replication of true 3D microstructures.

Photolithography is a crucial technology for both research and industry. The desire to be able to create ever finer features has fuelled a push towards lithographic methods that use electromagnetic radiation or charged particles with the shortest possible wavelength. At the same time, the physics and chemistry involved in employing light or particles with short wavelengths present great challenges. A new class of approaches to photolithography on the nanoscale involves the use of photoresists that can be activated with one colour of visible or near-ultraviolet light and deactivated with a second colour. Such methods hold the promise of attaining lithographic resolution that rivals or even exceeds that currently sought by industry, while at the same time using wavelengths of light that are inexpensive to produce and can be manipulated readily. The physical chemistry of 2-colour photolithography is a rich area of science that is only now beginning to be explored.

Efficient multiphoton radical generation chemistry has been developed for use in aqueous media. Through a combination of multiphoton absorption polymerisation (MAP) and optical tweezers, this chemistry has been applied to the fabrication, manipulation, and assembly of 3D polymeric and biomolecular structures. Combining MAP and optical tweezers allows for the direct assembly of 3D structures from microscale objects as well as for the realisation of structures, such as tape-like and rope-like microthreads, that can be used for unconventional microfabrication techniques including microbraiding and microweaving. These capabilities significantly expand the toolbox of methods available for the creation of functional microstructures in aqueous media.