3 research outputs found
Toward Label-Free Super-Resolution Microscopy
We
propose and implement a far-field spectroscopic system for imaging
below the diffraction limit without the need for fluorescence labeling.
Our technique combines concepts from Stimulated Emission Depletion
(STED) microscopy and Femtosecond Stimulated Raman Spectroscopy (FSRS).
The FSRS process generates signal through the creation of vibrational
coherences, and here we use a toroidal-shaped decoherence pulse to
eliminate vibrational signal from the edges of the focal spot. The
nonlinear dependence on decoherence pulse power enables subdiffraction
imaging. As in STED, the resolution is in theory infinitely small
given infinite decoherence pulse power. Here, we first experimentally
demonstrate that the photophysical principles behind our super-resolution
Raman imaging method are sound. We then prove that addition of the
decoherence pulse significantly improves the spatial resolution of
our microscope, achieving values beyond the diffraction limit. We
discuss future directions for this technique, including methods to
reach resolution on the order of ten nanometers
Determination of Resonance Raman Cross-Sections for Use in Biological SERS Sensing with Femtosecond Stimulated Raman Spectroscopy
Surface-enhanced Raman spectroscopy
(SERS) is a promising technique
for <i>in vivo</i> bioanalyte detection, but accurate characterization
of SERS biosensors can be challenging due to difficulties in differentiating
resonance and surface enhancement contributions to the Raman signal.
Here, we quantitate the resonance Raman cross-sections for a commonly
used near-infrared SERS dye, 3,3′-diethylthiatricarbocyanine
(DTTC). It is typically challenging to measure resonance Raman cross-sections
for fluorescent dye molecules due to the overwhelming isoenergetic
fluorescence signal. To overcome this issue, we used etalon-based
femtosecond stimulated Raman spectroscopy, which is intrinsically
designed to acquire a stimulated Raman signal without strong fluorescence
or interference from signals resulting from other four-wave mixing
pathways. Using this technique, we found that the cross-sections for
most of the resonantly enhanced modes in DTTC exceed 10<sup>–25</sup> cm<sup>2</sup>/molecule. These cross-sections lead to high signal
magnitude SERS signals from even weakly enhancing SERS substrates,
as much of what appears to be a SERS signal is actually coming from
the intrinsically strong resonance Raman signal. Our work will lead
to a more accurate determination of SERS enhancement factors and SERS
substrate characterization in the biologically relevant near-infrared
region, ultimately leading to a more widespread use of SERS for biosensing
and bioimaging applications
Lanthanum-Mediated C–H Bond Activation of Propyne and Identification of La(C<sub>3</sub>H<sub>2</sub>) Isomers
η<sup>2</sup>-Propadienylidenelanthanum
[LaÂ(η<sup>2</sup>-CCCH<sub>2</sub>)] and deprotioÂlanthanaÂcyclobutadiene
[LaÂ(HCCCH)] of LaÂ(C<sub>3</sub>H<sub>2</sub>) are identified from
the reaction mixture of neutral La atom activation of propyne in the
gas phase. The two isomers are characterized with mass-analyzed threshold
ionization spectroscopy combined with electronic structure calculations
and spectral simulations. LaÂ(η<sup>2</sup>-CCCH<sub>2</sub>)
and LaÂ(HCCCH) are formed by concerted 1,3- and 3,3-dehydrogenation,
respectively. Both isomers prefer a doublet ground state with a La
6s-based unpaired electron, and LaÂ(η<sup>2</sup>-CCCH<sub>2</sub>) is slightly more stable than LaÂ(HCCCH). Ionization of the neutral
doublet state of either isomer produces a singlet ion state by removing
the La-based electron. The geometry change upon ionization results
in the excitation of a symmetric metal–hydrocarbon stretching
mode in the ionic state, whereas thermal excitation leads to the observation
of the same stretching mode in the neutral state. Although the La
atom is in a formal oxidation state of +2, the ionization energies
of these metal–hydrocarbon radicals are lower than that of
the neutral La atom. Deuteration has a very small effect on the ionization
energies of the two isomers and the metal–hydrocarbon stretching
mode of LaÂ(η<sup>2</sup>-CCCH<sub>2</sub>), but it reduces considerably
the metal–ligand stretching frequencies of LaÂ(HCCCH)