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Second-order nonlinear spectroscopy is becoming an increasingly important technique in the study of interfacial systems owing to its marked ability to study molecular structures and interactions. The properties of such a system under investigation are contained within their intrinsic second-order susceptibilities which are mapped onto the measured nonlinear signals (e.g. sum-frequency generation) through the applied experimental settings. Despite this yielding a plethora of information, many crucial aspects of molecular systems typically remain elusive, for example the depth distributions, molecular orientation and local dielectric properties of its constituent chromophores. Here, it is shown that this information is contained within the phase of the measured signal and, critically, can be extracted through measurement of multiple nonlinear pathways (both the sum-frequency and difference-frequency output signals). Furthermore, it is shown that this novel information can directly be correlated to the characteristic vibrational spectra, enabling a new type of advanced sample characterization and a profound analysis of interfacial molecular structures. The theory underlying the different contributions to the measured phase of distinct nonlinear pathways is derived, after which the presented phase disentanglement methodology is experimentally demonstrated for model systems of self-assembled monolayers on several metallic substrates. The obtained phases of the local fields are compared to the corresponding phases of the nonlinear Fresnel factors calculated through the commonly used theoretical model, the three-layer model. It is found that, despite its rather crude assumptions, the model yields remarkable similarity to the experimentally obtained values, thus providing validation of the model for many sample classes. [ABSTRACT FROM AUTHOR]

Ion‐Protein‐Wechselwirkungen sind wegen der zahlreichen möglichen Proteinbindungsstellen schwierig zu verstehen. Um spezifische Ionen‐Effekte auf Proteinbindungsstellen zu erfassen, untersuchen wir den Einfluss von Salzen auf des zwitterionische Peptid Triglycin. Dielektrische Spektren zeigen, dass Salze die Rotationsdynamik des Peptids beeinflussen, wobei der Effekt von stark denaturierenden Kationen (Li+, Guanidinium (Gdm+)) und Anionen (I−, SCN−) ausgeprägter ist als der von schwach denaturierenden Ionen (K+, Cl−). Die Effekte von Gdm+ und Li+ sind ähnlich. Moleküldynamiksimulationen bestätigen, dass sich Gdm+ und Li+ verstärkt an Triglycin anlagern, jedoch mit unterschiedlicher Bindungsgeometrie: Während Li+ hauptsächlich an der Carboxylatgruppe bindet, ist eine zweizähnige Bindung zu einem O‐Atom der Carboxylatgruppe und zum O‐Atom der benachbarten Amidgruppe von besonderer Bedeutung für Gdm+. Diese Bindung beeinflusst die Konformation des Peptids verstärkt und könnte daher die hohe Denaturierungsaktivität von Gdm+‐Salzen erklären. Umarmung des Proteins: Das Guanidinium‐Kation kann gleichzeitig zwei Sauerstoffatome eines Peptids koordinieren, was seine ungewöhnlich hohe Proteindenaturierungseffizienz erklären kann. [ABSTRACT FROM AUTHOR]

Ion–protein interactions are important for protein function, yet challenging to rationalize owing to the multitude of possible ion–protein interactions. To explore specific ion effects on protein binding sites, we investigate the interaction of different salts with the zwitterionic peptide triglycine in solution. Dielectric spectroscopy shows that salts affect the peptide's reorientational dynamics, with a more pronounced effect for denaturing cations (Li+, guanidinium (Gdm+)) and anions (I−, SCN−) than for weakly denaturing ones (K+, Cl−). The effects of Gdm+ and Li+ were found to be comparable. Molecular dynamics simulations confirm the enhanced binding of Gdm+ and Li+ to triglycine, yet with a different binding geometry: While Li+ predominantly binds to the C‐terminal carboxylate group, bidentate binding to the terminus and the nearest amide is particularly important for Gdm+. This bidentate binding markedly affects peptide conformation, and may help to explain the high denaturation activity of Gdm+ salts. Embrace the protein: The guanidinium cation can simultaneously coordinate to two protein oxygen atoms, which might explain its peculiarly high protein denaturation activity. [ABSTRACT FROM AUTHOR]

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology. [ABSTRACT FROM AUTHOR]

The burgeoning field of 2D heterostructures targets the combination of 2D materials with 3D, 1D, or 0D nanomaterials. Among the most popular 2D materials, the 2H polytype of molybdenum disulfide (MoS 2 ) features a well-defined bandgap that becomes direct at the monolayer level, which can be exploited for photodetection. A notable limitation of 2H-MoS 2 is its curtailed absorbance beyond the visible range. Here, a covalently-linked Pd nanosheet (PdNS)/functionalized MoS 2 (f-MoS 2 ) heterostructure is introduced, leveraging PdNS infrared-absorbing properties to surmount this constraint. A bifunctional molecule, featuring a maleimide for attachment to MoS 2 and a phenyl bromide for connection to PdNS, enables the synthesis of the heterostructure. Comprehensive spectroscopic and microscopic characterization shed light on the structure of PdNS@f-MoS 2 and the electronic interaction between its components. Prototype devices show an enhancement in the width and intensity of the optoelectronic response of PdNS@f-MoS 2 in the infrared, up to 1700 nm. In comparison, a van der Waals heterostructure with the same components shows poorer photoresponse. The results prove that the covalent linkage of metal nanostructures to 2D materials is a promising approach to build mixed-dimensional heterostructures., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)