The
iconic helical structure of DNA is stabilized by the solvation
environment, where a change in the hydration state can lead to dramatic
changes to the DNA structure. X-ray diffraction experiments at cryogenic
temperatures have shown crystallographic water molecules in the minor
groove of DNA, which has led to the notion of a spine of hydration
of DNA. Here, chiral nonlinear vibrational spectroscopy of two DNA
sequences shows that not only do such structural water molecules exist
in solution at ambient conditions but that they form a chiral superstructure:
a chiral spine of hydration. This is the first observation of a chiral
water superstructure templated by a biomolecule. While the biological
relevance of a chiral spine of hydration is unknown, the method provides
a direct way to interrogate the properties of the hydration environment
of DNA and water in biological settings without the use of labels.
Vibrational
sum-frequency generation spectroscopy (SFG) is a powerful
tool for studying noncentrosymmetric environments, particularly interfaces.
Conventional homodyne-detected SFG inherently detects the intensity
of the emitted light and thus forfeits the ability to directly measure
the complex components, that is, phase, of the second-order nonlinear
susceptibility, which contains the molecular response of interest.
Heterodyne-detected SFG (HD-SFG) has recently been employed to recover
this lost information, but has not been broadly adopted due to restrictions
in the technical implementation. Presented in this Article is a HD-SFG
geometry that fills a need for ease of use and increased versatility;
our flexible and convenient design provides the capability to probe
any interface in any polarization combination with exceptional phase
stability. We demonstrate this ability by collecting the SFG signal
from an octadecyltrichlorosilane monolayer on the front of a solid
fused silica substrate and determine, for the first time with broadband
HD-SFG, the complex spectrum of buried dry and solvated interfaces,
collected in both ppp and ssp polarization combinations. This experimental
design does not display any appreciable phase shift for over 10 h,
which is a necessity for inclusion in more advanced methods such as
time-resolved HD-SFG and 2D-HD-SFG.
Catalytic interfaces involving surface-bound
molecular catalysts
often exhibit a large structural heterogeneity from uncontrolled variation
in surface morphology. Conventional spectroscopic techniques typically
average over these different structural motifs within the sample,
making it difficult to link the underlying surface morphology to the
properties of the immobilized catalyst. Here we present the first
direct comparison of the vibrational dynamics of a CO2 reduction
catalyst bound to two different single-crystalline TiO2 surfaces, rutile (001) and (110), probed with transient surface-specific
sum-frequency generation spectroscopy. We find that the change in
surface structure between crystallographic faces alters both the vibrational
frequency and relaxation time of the symmetric carbonyl stretching
mode of the catalyst, with (001) displaying a lower frequency and
longer relaxation time. This results from a change in the catalyst
electronic structure and indicates that the molecular properties of
the catalyst, likely including the catalytic properties, depend on
the specific TiO2 surface to which it is bound. The comparison
of the molecular properties on these two single crystal surfaces is
an essential step toward understanding how semiconductor surface structure
influences catalyst behavior and identifying optimal surface structures
for improved catalytic performance.
Molecular monolayers exhibit structural and dynamical properties that are different from their bulk counterparts due to their interaction with the substrate. Extracting these distinct properties is crucial for a better understanding of processes such as heterogeneous catalysis and interfacial charge transfer. Ultrafast nonlinear spectroscopic techniques such as 2D infrared (2D IR) spectroscopy are powerful tools for understanding molecular dynamics in complex bulk systems. Here, we build on technical advancements in 2D IR and heterodyne-detected sum frequency generation (SFG) spectroscopy to study a CO reduction catalyst on nanostructured TiO with interferometric 2D SFG spectroscopy. Our method combines phase-stable heterodyne detection employing an external local oscillator with a broad-band pump pulse pair to provide the first high spectral and temporal resolution 2D SFG spectra of a transparent material. We determine the overall molecular orientation of the catalyst and find that there is a static structural heterogeneity reflective of different local environments at the surface.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.