Sum-frequency generation of acetate adsorption on Au and Pt surfaces: Molecular structure effects

Molecular Orientation of Carboxylate Anions at the Water-Air Interface Studied with Heterodyne-Detected Vibrational Sum-Frequency Generation

The carboxylate anion group plays an important role in many (bio)chemical systems and polymeric materials. In this work, we study the orientation of carboxylate anions with various aliphatic and aromatic substituents at the water-air interface by probing the carboxylate stretch vibrations with heterodyne-detected vibrational sum-frequency generation spectroscopy in different polarization configurations. We find that carboxylate groups with small aliphatic substituents show a large tilt angle with respect to the surface normal and that this angle decreases with increasing size of the substituent. We further use the information about the orientation of the carboxylate group to determine the hyperpolarizability components of this group.

Memory effects in polymer brushes showing co-nonsolvency effects

Densely packed polymer chains grafted to a substrate, especially polymer brushes, have been studied intensively. Of special interest are systems that react to changes in external conditions or"remember" previous conditions. With this focus, we explore the properties of PNiPAAm brushes and relate published work to own results. The co-nonsolvency effect leads to a collapse of a PNiPAAm brush for a certain mixing ratio of ethanol in water. This also influences the wetting behavior of PNiPAAm brushes. We show that through prewetting of a brush with different liquids (water and ethanol), the contact angle of subsequent water drops changes significantly. To explain this change, the swelling of the brush was investigated with spectroscopic ellipsometry and the orientation of the molecules at the surface with sum-frequency generation (SFG). Only little change in swelling was found. The SFG measurements reveal in the ethanol prewetted case a well ordered hydrophobic methyl layer at the interface, which is consistent with the contact angle measurement.

Effects of Ca2+ Ion Condensation on the Molecular Structure of Polystyrene Sulfonate at Air-Water Interfaces

The structure of poly(sodium 4-styrenesulfonate) (NaPSS) polyelectrolytes at air-water interfaces was investigated with tensiometry, ellipsometry, and vibrational sum-frequency generation (SFG) in the presence of low and high CaCl₂ concentrations. In addition, we have studied the foaming behavior of 20 mM NaPSS solutions to relate the PSS molecular structure at air-water interfaces to foam properties. PSS polyelectrolytes without additional salt exhibited significant surface activity, which can be tuned further by additions of CaCl₂. The hydrophobicity of the backbone due to incomplete sulfonation during synthesis is one origin, whereas the effective charge of the polyelectrolyte chain is shown to play another major role. At low salt concentrations, we propose that the polyelectrolyte is forming a layered structure. The hydrophobic parts are likely to be located directly at the interface in loops, whereas the hydrophilic parts are at low concentrations stretched out into near-interface regions in tails. Increasing the Ca²⁺ concentration leads to ion condensation, a collapse of the tails, and likely to Ca²⁺ intra- and intermolecular bridges between polyelectrolytes at the interface. The increase in both surface excess and foam stability originates from changes in the polyelectrolyte's hydrophobicity due to Ca²⁺ condensation onto the PSS polyanions. Consequently, charge screening at the interface is enhanced and repulsive electrostatic interactions are reduced. Furthermore, SFG spectra of O-H stretching bands reveal a decrease in intensity of the low-frequency branch when c(Ca²⁺) is increased whereas the high-frequency branch of O-H stretching modes persists even for 1 M CaCl₂. This originates from the remaining net charge of the PSS polyanions at the air-water interface that is not fully compensated by condensation of Ca²⁺ ions and leads to electric-field-induced contributions to the SFG spectra of interfacial H₂O. A charge reversal of the PSS net charge at the air-water interface is not observed and is consistent with bulk electrophoretic mobility measurements.

Flexible approach to vibrational sum-frequency generation using shaped near-infrared light

We describe a new approach that expands the utility of vibrational sum-frequency generation (vSFG) spectroscopy using shaped near-infrared (NIR) laser pulses. We demonstrate that arbitrary pulse shapes can be specified to match experimental requirements without the need for changes to the optical alignment. In this way, narrowband NIR pulses as long as 5.75 ps are readily generated, with a spectral resolution of about 2.5  cm-1, an improvement of approximately a factor of 3 compared to a typical vSFG system. Moreover, the utility of having complete control over the NIR pulse characteristics is demonstrated through nonresonant background suppression from a metallic substrate by generating an etalon waveform in the pulse shaper. The flexibility afforded by switching between arbitrary NIR waveforms at the sample position with the same instrument geometry expands the type of samples that can be studied without extensive modifications to existing apparatuses or large investments in specialty optics.

In situ spectroscopy of ligand exchange reactions at the surface of colloidal gold and silver nanoparticles

Gold and silver nanoparticles with their tunable optical and electronic properties are of great interest for a wide range of applications. Often the ligands at the surface of the nanoparticles have to be exchanged in a second step after particle formation in order to obtain a desired surface functionalization. For many techniques, this process is not accessible in situ. In this review, we present second-harmonic scattering (SHS) as an inherently surface sensitive and label-free optical technique to probe the ligand exchange at the surface of colloidal gold and silver nanoparticles in situ and in real time. First, a brief introduction to SHS and basic features of the SHS of nanoparticles are given. After that, we demonstrate how the SHS intensity decrease can be correlated to the thiol coverage which allows for the determination of the Gibbs free energy of adsorption and the surface coverage.

Single-Particle Plasmon Voltammetry (spPV) for Detecting Anion Adsorption

Nanoparticle and thin film surface plasmons are highly sensitive to electrochemically induced dielectric changes. We exploited this sensitivity to detect reversible electrochemical potential-driven anion adsorption by developing single-particle plasmon voltammetry (spPV) using plasmonic nanoparticles. spPV was used to detect sulfate electroadsorption to individual Au nanoparticles. By comparing both semiconducting and metallic thin film substrates with Au nanoparticle monomers and dimers, we demonstrated that using Au film substrates improved the signal in detecting sulfate electroadsorption and desorption through adsorbate modulated thin film conductance. Using single-particle surface plasmon spectroscopic techniques, we constructed spPV to sense sulfate, acetate, and perchlorate adsorption on coupled Au nanoparticles. spPV extends dynamic spectroelectrochemical sensing to the single-nanoparticle level using both individual plasmon resonance modes and total scattering intensity fluctuations.

Protein adsorption at the electrified air-water interface: implications on foam stability

The surface chemistry of ions, water molecules, and proteins as well as their ability to form stable networks in foams can influence and control macroscopic properties such as taste and texture of dairy products considerably. Despite the significant relevance of protein adsorption at liquid interfaces, a molecular level understanding on the arrangement of proteins at interfaces and their interactions has been elusive. Therefore, we have addressed the adsorption of the model protein bovine serum albumin (BSA) at the air-water interface with vibrational sum-frequency generation (SFG) and ellipsometry. SFG provides specific information on the composition and average orientation of molecules at interfaces, while complementary information on the thickness of the adsorbed layer can be obtained with ellipsometry. Adsorption of charged BSA proteins at the water surface leads to an electrified interface, pH dependent charging, and electric field-induced polar ordering of interfacial H(2)O and BSA. Varying the bulk pH of protein solutions changes the intensities of the protein related vibrational bands substantially, while dramatic changes in vibrational bands of interfacial H(2)O are simultaneously observed. These observations have allowed us to determine the isoelectric point of BSA directly at the electrolyte-air interface for the first time. BSA covered air-water interfaces with a pH near the isoelectric point form an amorphous network of possibly agglomerated BSA proteins. Finally, we provide a direct correlation of the molecular structure of BSA interfaces with foam stability and new information on the link between microscopic properties of BSA at water surfaces and macroscopic properties such as the stability of protein foams.

Impact of oxygen plasma treatment on the device performance of zinc oxide nanoparticle-based thin-film transistors

Thin-films of zinc oxide nanoparticles were investigated by photoluminescence spectroscopy and a broad defect-related yellow-green emission was observed. Oxygen plasma treatment was applied in order to reduce the number of defects, and the emission intensity was quenched to 4% of the initial value. Thin-film transistors that incorporate the nanoparticles as active semiconducting layers show an improved device performance after oxygen plasma treatment. The maximum drain current and the charge carrier mobility increased more than 1 order of magnitude up to a nominal value of 23 cm(2) V(-1) s(-1) and the threshold voltage was lowered.

Tuning the molecular order of C60 functionalized phosphonic acid monolayers

Mixed self-assembled monolayers (SAM) of alkyl phosphonic acids and C(60) functionalized octadecyl phosphonic acids (C(60)C(18)-PA) are deposited on alumina substrates from solution and are shown to form well-ordered structures with an insulating layer of alkyl chains and a semiconducting layer that comprises mainly C(60). Such an ordered structure is a necessity for the application of SAMs in organic transistors but is difficult to obtain since C(60)C(18)-PA without additional support do self-assemble in dense packaging but not in a well-ordered fashion. To avoid disordering of the SAM and to gain a better control of the interfacial properties we have investigated the stabilizing effects of fluorinated dodecyl phosphonic acids (FC(12)-PA) on the C(60)C(18)-PA monolayer. Vibrational sum-frequency (SFG) spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, and electrical measurements were applied to study the mixed monolayers. Here, we make use of the differently labeled PA to determine surface coverages and molecular properties of the two species independently. Adsorption of FC(12)-PA gives rise to vibrational bands at 1344 cm(-1) and 1376 cm(-1) in SFG spectra, while a pronounced vibrational band centered at 1465 cm(-1) is attributable to C(60) vibrations. The coexistence of the bands is indicative for the presence of a mixed monolayer that is composed of both molecular species. Furthermore, a pronounced maximum in SFG intensity of the C(60) band is observed for SAMs, which are deposited from solutions with ~75% C(60)C(18)-PA and ~25% FC(12)-PA. The intensity maximum originates from successful stabilization of C(60) modified C(60)C(18)-PA by FC(12)-PA and a significantly improved molecular order. Conclusions from SFG spectra are corroborated by electric measurements that show best performance at these concentrations. Our results provide new information on the morphology and composition of C(60) modified SAMs and establish a route to fabricate well-defined layers for molecular scale organic electronics.