Ji-Xin Cheng

Coherent anti-Stokes Raman scattering (CARS) microscopy permits vibrational imaging with high-sensitivity, high speed, and three-dimensional spatial resolution. We review recent advances in CARS microscopy, including experimental design, theoretical understanding of contrast mechanisms, and applications to chemical and biological systems. We also review the development of multiplex CARS microspectroscopy, which allows high-speed characterization of microscopic samples, and CARS correlation spectroscopy, which probes fast diffusion dynamics with vibrational selectivity.

Coherent anti-Stokes Raman scattering (CARS) microscopy, which allows vibrational imaging of myelin sheath in its natural state, was applied to characterize lysophosphatidylcholine (lyso-PtdCho)-induced myelin degradation in tissues and in vivo. After the injection of lyso-PtdCho into ex vivo spinal tissues or in vivo mouse sciatic nerves, myelin swelling characterized by the decrease of CARS intensity and loss of excitation polarization dependence was extensively observed. The swelling corresponds to myelin vesiculation and splitting observed by electron microscopy. The demyelination dynamics were quantified by the increase of g ratio measured from the CARS images. Treating spinal tissues with Ca2+ ionophore A23187 resulted in the same kind of myelin degradation as lyso-PtdCho. Moreover, the demyelination lesion size was significantly reduced upon preincubation of the spinal tissue with Ca2+ free Krebs' solution or a cytosolic phospholipase A2 (cPLA(2)) inhibitor or a calpain inhibitor. In accordance with the imaging results, removal of Ca2+ or addition of cPLA(2) inhibitor or calpain inhibitor in the Krebs' solution remarkably increased the mean compound action potential amplitude in lyso-PtdCho treated spinal tissues. Our results suggest that lyso-PtdCho induces myelin degradation via Ca(2+) influx into myelin and subsequent activation of cPLA(2) and calpain, which break down the myelin lipids and proteins. The current work also shows that CARS microscopy is a potentially powerful tool for the study of demyelination.

Abstract Plasmon‐enhanced stimulated Raman scattering (PESRS) microscopy has been recently developed to reach single‐molecule detection limit. Unlike conventional stimulated Raman spectra, dispersive‐like vibrational line shapes were observed in PESRS. Here, we propose a theoretical model together with a phasor diagram to explain the observed dispersive‐like line shapes reported in our previous study. We show that the local enhanced electromagnetic field induced by the plasmonic nanostructure interferes with the molecular dipole‐induced field, resulting in the dispersive profiles of PESRS. The exact shape of the profile depends on the phase difference between the plasmonic field and the molecular dipole field. We compared plasmon‐enhanced stimulated Raman loss (PESRL) and plasmon‐enhanced stimulated Raman gain (PESRG) signals under the same pump and Stokes laser wavelength. The PESRL and PESRG signals exhibit similar signal magnitudes, whereas their spectral line shapes show reversed dispersive profiles, which is in an excellent agreement with our theoretical prediction. Meanwhile, we verify that the nonresonant background in PESRS mainly originates from the photothermal effect. These new insights help the proper use of PESRS for nanoscale bio‐imaging and ultrasensitive detection.

Gap-enhanced Raman tags (GERTs) have been widely used for surface-enhanced Raman scattering (SERS) imaging due to their excellent SERS performances. Here, we reported a synthetic strategy for novel gap-enhanced dumbbell-like nanoparticles with anisotropic shell coatings. Controlled shell growth at the tips of gold nanorods was achieved by using cetyltrimethylammonium bromide (CTAB) as a capping agent. A mechanism related to the shape-directing effects of CTAB was proposed to explain the findings. Optimized gap-enhanced gold dumbbells exhibited highly enhanced SERS responses compared to rod cores, with an enhancement ratio of 101.5. We further demonstrated that gap-enhanced AuNDs exhibited single-particle SERS sensitivity with an acquisition time as fast as 0.1 s per spectrum, showing great potential for high-speed SERS imaging.