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Thioflavin T (ThT) is a typical dye used to visualize the aggregation and formation of fibrillar structures, e.g., amyloid fibrils and peptide nanofibrils. ThT has been considered to produce stable fluorescence when interacting with aggregated proteins. For single-molecule localization microscopy (SMLM)-based optical super-resolution imaging, a photoswitching/blinking fluorescence property is required. Here we demonstrate that, in contrast to previous reports, ThT exhibits intrinsic stochastic blinking properties, without the need for blinking imaging buffer, in stable binding conditions. The blinking properties (photon number, blinking time, and on-off duty cycle) of ThT at the single-molecule level (for ultralow concentrations) were investigated under different conditions. As a proof of concept, we performed SMLM imaging of ThT-labeled α-synuclein fibrils measured in air and PBS buffer.

Mineral-water interfaces play an important role in many natural as well as technological fields. Fundamental properties of these interfaces are governed by the presence of the interfacial water and its specific structure at the surface. Calcite is particularly interesting as a dominant rock-forming mineral in the earth's crust. Here, we combine atomic force microscopy, sum-frequency generation spectroscopy, and molecular dynamics simulations to determine the position and orientation of the water molecules in the hydration layers of the calcite surface with high resolution. While atomic force microscopy provides detailed information about the position of the water molecules at the interface, sum-frequency generation spectroscopy can deduce the orientation of the water molecules. Comparison of the calcite-water interface to the interfaces of magnesite-water, magnesite-ethanol, and calcite-ethanol reveals a comprehensive picture with opposite water orientations in the first and second layer of the interface, which is corroborated by the molecular dynamics simulations.

Perfluorinated acids (PFAs) are widely used synthetic chemical compounds, highly resistant to environmental degradation. The widespread PFA contamination in remote regions such as the High Arctic implies currently not understood long-range atmospheric transport pathways. Here, we report that perfluorooctanoic acid (PFOA) initiates heterogeneous ice nucleation at temperatures as high as -16 °C. In contrast, the eight-carbon octanoic acid, perfluorooctanesulfonic acid, and deprotonated PFOA showed poor ice nucleating capabilities. The ice nucleation ability of PFOA correlates with the formation of a PFOA monolayer at the air-water interface, suggesting a mechanism in which the aligned hydroxyl groups of the carboxylic acid moieties provide a lattice matching to ice. The ice nucleation capabilities of fluorinated compounds like PFOA might be relevant for cloud glaciation in the atmosphere and the removal of these persistent pollutants by wet deposition.

Ice-nucleating proteins (INPs) found in bacteria are the most effective ice nucleators known, enabling the crystallization of water at temperatures close to 0 °C. Although their function has been known for decades, the underlying mechanism is still under debate. Here, we show that INPs from Pseudomonas syringae in aqueous solution exhibit a defined solution structure and show no significant conformational changes upon cooling. In contrast, irreversible structural changes are observed upon heating to temperatures exceeding ∼55 °C, leading to a loss of the ice-nucleation activity. Sum-frequency generation (SFG) spectroscopy reveals that active and heat-inactivated INPs impose similar structural ordering of interfacial water molecules upon cooling. Our results demonstrate that increased water ordering is not sufficient to explain INPs' high ice-nucleation activity and confirm that intact three-dimensional protein structures are critical for bacterial ice nucleation, supporting a mechanism that depends on the INPs' supramolecular interactions.

Measurements of the friction coefficient of steel-on-ice over a large temperature range reveal very high friction at low temperatures (-100 °C) and a steep decrease in the friction coefficient with increasing temperature. Very low friction is only found over the limited temperature range typical for ice skating. The strong decrease in the friction coefficient with increasing temperature exhibits Arrhenius behavior with an activation energy of E a ≈ 11.5 kJ mol -1 . Remarkably, molecular dynamics simulations of the ice-air interface reveal a very similar activation energy for the mobility of surface molecules. Weakly hydrogen-bonded surface molecules diffuse over the surface in a rolling motion, their number and mobility increasing with increasing temperature. This correlation between macroscopic friction and microscopic molecular mobility indicates that slippery ice arises from the high mobility of its surface molecules, making the ice surface smooth and the shearing of the weakly bonded surface molecules easy.

We study the resonant interaction of the OH stretch vibrations of water molecules at the surfaces of liquid water and ice using heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. By studying different isotopic mixtures of H 2 O and D 2 O, we vary the strength of the interaction, and we monitor the resulting effect on the HD-SFG spectrum of the OH stretch vibrations. We observe that the near-resonant coupling effects are weaker at the surface than in the bulk, for both water and ice, indicating that for both phases of water the OH vibrations are less strongly delocalized at the surface than in the bulk.

We study the signatures of the OH stretch vibrations at the basal surface of ice using heterodyne-detected sum-frequency generation and molecular dynamics simulations. At 150 K, we observe seven distinct modes in the sum-frequency response, five of which have an analogue in the bulk, and two pure surface-specific modes at higher frequencies (∼3530 and ∼3700 cm -1 ). The band at ∼3530 cm -1 has not been reported previously. Using molecular dynamics simulations, we find that the ∼3530 cm -1 band contains contributions from OH stretch vibrations of both fully coordinated interfacial water molecules and water molecules with two donor and one acceptor hydrogen bond.

Understanding the structure of proteins at surfaces is key in fields such as biomaterials research, biosensor design, membrane biophysics, and drug design. A particularly important factor is the orientation of proteins when bound to a particular surface. The orientation of the active site of enzymes or protein sensors and the availability of binding pockets within membrane proteins are important design parameters for engineers developing new sensors, surfaces, and drugs. Recently developed methods to probe protein orientation, including immunoessays and mass spectrometry, either lack structural resolution or require harsh experimental conditions. We here report a new method to track the absolute orientation of interfacial proteins using phase-resolved sum frequency generation spectroscopy in combination with molecular dynamics simulations and theoretical spectral calculations. As a model system we have determined the orientation of a helical lysine-leucine peptide at the air-water interface. The data show that the absolute orientation of the helix can be reliably determined even for orientations almost parallel to the surface.

Harvesting multiexcitons from semiconductor quantum dots (QDs) has been proposed as a path toward photovoltaic efficiencies beyond the Shockley-Queisser limit. Although multiexciton generation efficiencies have been quantified extensively in QD structures, the challenge of actually collecting multiple excitons at electrodes-a prerequisite for high-efficiency solar cell devices-has received less attention. Here, we demonstrate that multiexciton collection (MEC) at the PbS QD/mesoporous SnO 2 interface can be boosted 5-fold from ∼15 to reach ∼80% quantum yield, by partial localization of holes in a QD molecular capping shell. The resulting weakened Coulombic interactions give rise to reduced Auger recombination rates within the molecularly capped QDs, so that biexciton Auger relaxation, competing with MEC, is strongly suppressed. These results not only highlight the importance of surface chemistry and energetics at QD/ligand interfaces for multiexciton extraction but also provide clear design principles for realizing the benefits of MEG in sensitized systems exploited in solar cells and fuel geometries.

The interfacial structure of water in contact with TiO 2 is the key to understand the mechanism of photocatalytic water dissociation as well as photoinduced superhydrophilicity. We investigate the interfacial molecular structure of water at the surface of anatase TiO 2 , using phase-sensitive sum frequency generation spectroscopy together with spectra simulation using ab initio molecular dynamic trajectories. We identify two oppositely oriented, weakly and strongly hydrogen-bonded subensembles of O-H groups at the superhydrophilic UV irradiated TiO 2 surface. The water molecules with weakly hydrogen-bonded O-H groups are chemisorbed, i.e. form hydroxyl groups, at the TiO 2 surface with their hydrogen atoms pointing toward bulk water. The strongly hydrogen-bonded O-H groups interact with the oxygen atom of the chemisorbed water. Their hydrogen atoms point toward the TiO 2 . This strong interaction between physisorbed and chemisorbed water molecules causes superhydrophilicity.

Vibrational coupling is relevant not only for dissipation of excess energy after chemical reactions but also for elucidating molecular structure and dynamics. It is particularly important for O-H stretch vibrational spectra of water, for which it is known that in bulk both intra- and intermolecular coupling alter the intensity and line shape of the spectra. In contrast with bulk, the unified picture of the inter/intra-molecular coupling of O-H groups at the water-air interface has been lacking. Here, combining sum-frequency generation experiments and simulation for isotopically diluted water and alcohols, we unveil effects of inter- and intramolecular coupling on the vibrational spectra of interfacial water. Our results show that both inter- and intramolecular coupling contribute to the O-H stretch vibrational response of the neat H 2 O surface, with intramolecular coupling generating a double-peak feature, while the intermolecular coupling induces a significant red shift in the O-H stretch response.

The surface-active ions tetraphenylarsonium (Ph4As(+)) and tetraphenylboron (Ph4B(-)) have a similar structure but opposite charge. At the solution-air interface, the two ions affect the surface tension in an identical manner, yet sum-frequency generation (SFG) spectra reveal an enhanced surface propensity for Ph4As(+) compared with Ph4B(-), in addition to opposite alignment of interfacial water molecules. At the water-oil interface, the interfacial tension is 7 mN/m lower for Ph4As(+) than for Ph4B(-) salts, but this can be fully accounted for by the different bulk solubility of these ions in the hydrophobic phase, rather than inherently different surface activities. The different solubility can be accounted for by differences in electronic structure, as evidenced by quantum chemical calculations and NMR studies. Our results show that the surface propensity concluded from SFG spectroscopy does not necessarily correlate with interfacial adsorption concluded from thermodynamic measurements.

The nature of the photoconductivity in solution-processed films of methylammonium lead iodide perovskite is investigated by determining the variation of the photoconductive response with temperature. Ultrabroadband terahertz (THz) photoconductivity spectra in the 0.3-10 THz range can be reproduced well by a simple Drude-like response at room temperature, where free charge carrier motion is characterized by an average scattering time. The scattering time determined from Drude fits in the 0.3-2THz region increases from ∼4 fs at 300 K (tetragonal phase; mobility of ∼27 cm(2) V(-1) s(-1)) to almost ∼25 fs at 77 K (orthorhombic phase, mobility of ∼150 cm(2) V(-1) s(-1)). For the tetragonal phase (temperature range 150 < T < 300 K) the scattering time shows a ∼T(-3/2) dependence, approaching the theoretical limit for pure acoustic phonon (deformation potential) scattering. Hence, electron-phonon, rather than impurity scattering, sets the upper limit on free charge transport for this perovskite.

We present a combined experimental sum-frequency generation (SFG) spectroscopy and ab initio molecular dynamics simulations study to clarify the structure and orientation of water at zwitterionic phosphatidylcholine (PC) lipid and amine N-oxide (AO) surfactant monolayers. Simulated O-H stretch SFG spectra of water show good agreement with the experimental data. The SFG response at the PC interface exhibits positive peaks, whereas both negative and positive bands are present for the similar zwitterionic AO interface. The positive peaks at the water/PC interface are attributed to water interacting with the lipid carbonyl groups, which act as efficient hydrogen bond acceptors. This allows the water hydrogen bond network to reach, with its (up-oriented) O-H groups, into the headgroup of the lipid, a mechanism not available for water underneath the AO surfactant. This highlights the role of the lipid carbonyl group in the interfacial water structure at the membrane interface, namely, stabilizing the water hydrogen bond network.

The recently developed 2D sum frequency generation spectroscopy offers new possibilities to analyze the structure and structural dynamics of interfaces in a surface-specific manner. Its implementation, however, has so far remained limited to the pump-probe geometry, with its inherent restrictions. Here we present 2D SFG experiments utilizing a novel noncollinear geometry of four incident laser pulses generating a 2D SFG response, analogous to the triangle geometry applied in bulk-sensitive 2D infrared spectroscopy. This approach allows for background-free measurements of fourth-order nonlinear signals, which is demonstrated by measuring the fourth-order material response from a GaAs (110) surface. The implementation of phase-sensitive detection and broadband excitation pulses allows for both highest possible time resolution and high spectral resolution of the pump axis of a measured 2D SFG spectrum. To reduce the noise in our spectra, we employ a referencing procedure, for which we use noncollinear pathways and individual focusing for the signal and local oscillator beams. The 2D spectra recorded from the GaAs (110) surface show nonzero responses for the real and imaginary component, pointing to contributions from resonant electronic pathways to the χ((4)) response.

Ultrafast photoinduced carrier dynamics in prototypical low band gap polymer:fullerene photovoltaic blend films PTB7:PC70BM and P3HT:PC70BM is investigated using ultrafast terahertz (THz) spectroscopy. The subpicosecond and few-picosecond decays of THz-probed photoconductivities for both compounds are observed, attributed to the rapid formation of polaron pairs by exciton-exciton annihilation and subsequent polaron pair annihilation, respectively. The transient THz photoconductivity spectra of PTB7:PC70BM are well described by the Drude-Smith (DS) model, directly yielding the important charge transport parameters such as charge carrier density, momentum scattering time, and effective localization. By comparison with P3HT:PC70BM, we find that in PTB7:PC70BM the mobile charge carrier photoconductivity is significantly enhanced by a factor of 1.8 and prevails for longer times after charge formation, due to both improved mobile charge carrier yield and lower charge localization. In PTB7:PC70BM, a strong dependency of electron momentum scattering time on electron density was found, well parametrized by the empirical Caughey-Thomas model. The difference in ultrafast photoconductivities of both P3HT:PC70BM and PTB7:PC70BM is found to correlate very well with the performance of photovoltaic devices based on those materials.

We combine second-order nonlinear vibrational spectroscopy and quantum-chemical calculations to quantify the molecular tilt angle and the structural variation of a decanoic acid surfactant monolayer on water. We demonstrate that there is a remarkable degree of delocalization of the vibrational modes along the backbone of the amphiphilic molecule. A simulation-based on modeled sum frequency generation (SFG) spectra offers quantitative insights into the disorder of surfactant monolayers at the water-air interface. It is shown that an average of one gauche defect in the alkyl chain suffices to give rise to the methylene stretch intensity similar in magnitude to the methyl stretch.

We present a combined molecular dynamics simulation and experimental study on the water bending mode at the water-vapor interface using sum-frequency generation (SFG) spectroscopy. The SFG spectrum simulated using an ab initio-based water model shows good agreement with the experimental data. The imaginary part of the SFG response shows a negative peak at ∼1650 cm(-1) and a positive peak at ∼1730 cm(-1). Our results reveal that these widely (∼80 cm(-1)) separated peaks result from the interference of two closely spaced (∼29 cm(-1)) peaks of opposite sign. The positive peak at ∼1689 cm(-1) originates from water with two donor hydrogen atoms with the HOH angular bisector pointing down toward the bulk, and the negative peak at ∼1660 cm(-1) from water with free O-H groups, pointing up. The small frequency difference of 29 cm(-1) indicates that the HOH bending mode frequency of interfacial water is relatively insensitive to the number of hydrogen bonds.

We study the molecular rotation of the protic room-temperature ionic liquid ethylammonium nitrate with dielectric relaxation spectroscopy and femtosecond-infrared spectroscopy (fs-IR) of the ammonium N-H vibrations. The results suggest that the rotation of ethylammonium ion takes place via large angular jumps. Such nondiffusive reorientational dynamics is unique to strongly hydrogen-bonded liquids such as water and indicates that the intermolecular interaction is highly directional in this class of ionic liquids.