Molecular vibrational spectroscopy is extensively used in imaging and sensing applications, which provides molecular level intrinsic information. In an article recently published in the journal Analytical Chemistry, researchers developed a coherent Raman spectroscopy platform, termed time-resolved coherent Raman scattering (TCRS) spectroscopy, which focused on molecular vibrational dephasing.
Study: Enhanced Chemical Sensing with Multiorder Coherent Raman Scattering Spectroscopic Dephasing. Image Credit: Golden Wind/Shutterstock.com
High dynamic range detection allowed the demonstration of environmental effects and molecular vibrational dynamics with multidimensional spectroscopic sensing.
Detection of Molecular Vibrations
Intrinsic properties of a chemical bond drive molecular vibrational detection, which is essential for label-free optical imaging and spectroscopy to explore the fields of chemistry, material science, and biomedicine.
Stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) are non-linear molecular vibrational techniques. The coherent excitation in these techniques offers unprecedented detection speeds and enhances the specificity and optical sectioning capacity in 3D imaging. Coherent Raman scattering (CRS) is advantageous over conventional imaging techniques such as fluorescence imaging.
Although vibrational dephasing removes nonresonant background and separation overlapping spectral peaks in CRS imaging, the ultrashort pulse consistently excites molecular vibrations. An additional femtosecond probe pulse can interrogate the dephasing dynamics with spatial resolution and enhanced sensing at the molecular level.
Despite the development of many time-resolved techniques to investigate the excited-state relaxation, they have limitations with the chemical-selective and time consumption capacity to use for imaging.
High temporal and spectral resolution are restricted mutually due to the uncertain relation. To this end, femtosecond stimulated Raman spectroscopy (FSRS) possesses temporal and spectral resolutions. Despite the detection of electronic relaxation in transient absorption (TA) spectroscopy, the electronic and vibrational state interaction can help in guiding photochemistry.
In the present work, the team demonstrated T-CRS which includes simultaneous time-resolved coherent Stokes Raman scattering (T-CSRS), T-CARS, and higher-order time-resolved (HOT)-CARS, to describe molecular vibrational dephasing.
The frequency difference between the Stokes beam (ω2) and pump beam (ω1) should be equivalent to resonant frequency (Ω) in target T-CARS and T-CSRS vibrational mode. The probe beam (ω3) with time delay (T) allowed the timing and excitation frequencies between pulses which were independently controlled, providing a frequency and time-resolved spectroscopic approach.
The reorganization of molecular polarization generated a non-linear scattering which is a result of ω3 rearrangement. Moreover, delaying the ω3 decreases the photon echo due to pure dephasing processes and depopulation of the intra- and intermolecular interaction sensitive superposition state. Thus, molecular vibrational dephasing time provides dynamic information through T-CRS for an interrogating nano-environment.
Initially, the carbon-hydrogen (C−H) bending mode of dimethyl sulfoxide (DMSO) at various concentrations demonstrated the spectral properties by T-CRS spectroscopy. DMSO solution simulates the hydrogen-bonding model in biological systems and reagents in chemical studies. Raman transitions of DMSO were indicated by the peaks at 2912 and 2999-centimeter inverse, corresponding to C-H stretching mode, and the peak at 1417- centimeter inverse corresponds to C−H bending. Time delay tuning of prob pulses helps obtain the series of time-resolved spectra. Integration of each specific peak resulted in acquiring time-delayed signal intensity.
The T-CRS modalities developed in the present work had unique advantages. The sidebands in the decay profiles of CSRS and CARS help avoid systematic imperfections. This kind of cross-validation helps design an accurate and robust system for dephasing measurements and spectroscopy. Moreover, the signals obtained from T-CRS modalities were collected without any complication in the existing system or equipment.
Comparison of each modality in the T-CRS platform allows the analysis of spectral profiles and dephasing system. The team observed that each signal resided in different spectral windows, which helped avoid the parasitic signals, including two-photon autofluorescence.
Although HOT-CARS had lower photon counts, it could boast broader excitation windows with a given laser source bandwidth. The multidimensional vibrational dephasing provided a unique advantage in imaging applications and molecular spectroscopy. In addition to CARS microscopy applications in different fields, a new dimension of molecular dephasing allows T-CRS use in imaging applications.
The HOT-CARS experiments cannot study photodamage, which is a limitation in biological samples. This damage is due to higher intensity laser power critical for high-order nonlinear processes. Further, HOT-CARS multiphoton damage was reduced by separating the probe beam from pump/Stokes beams. The time-resolved technique allows the enhancement of signal-to-background contrast and a better signal-to-noise ratio in a shorter time.
In conclusion, the researchers demonstrated enhanced sensing of molecular vibrational coherent dephasing. Multi-order T-CRS, which includes CARS, CSRS, and HOT-CARS signals, has provided insights into the molecular nano-environment. Measuring the variations of the vibrational dephasing time helped reveal the relation of macroscopic properties, providing a new dimension to investigate the dynamics of intra- and intermolecular interactions in material science, medicine, and biology.
Zhu, H., Xu, C., Wang, D. W., Yakovlev, V. V., & Zhang, D. (2022). Enhanced Chemical Sensing with Multiorder Coherent Raman Scattering Spectroscopic Dephasing. Analytical Chemistry. https://pubs.acs.org/doi/abs/10.1021/acs.analchem.2c01060