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Label-free Imaging

Although single-frequency coherent Raman microscopy allows high-speed vibrational imaging of known species, it was considered impossible to record a vibrational fingerprint spectrum as fast as fluorescence spectroscopy. Based on a tuned amplifier array invented at Purdue, the Cheng team, for the first time, pushed the speed of Raman spectral acquisition to microsecond scale (Light: Sci & Appl 2015, 4: e265), covering a spectral window of a few hundred wavenumbers. His method is able to record dispersed stimulated Raman scattering signals at speed of 5 microseconds per spectrum. This development represents a conceptual advance of the field, because it allows the spectral profile to be used as a contrast for functional imaging of live cells. This development also enabled high-throughput, high-content single cell analysis in a flow cytometry setting.
In an effort to overcome the limited imaging depth of CARS microscopy, which is on the scale of 0.1 millimeter, the Cheng group invented a deep-tissue imaging platform by “listening to chemical bond vibrations” (Phys Rev Lett 2011, 106: 238106). In this method, pulsed near infrared light induces overtone or combinational vibrations where spectral windows of minimal water absorption exist. Through the heat dissipation process, such vibrational absorption converts to detectable transient waves. Based on this concept, Cheng and coworkers demonstrated intravascular vibrational imaging with a depth of 7 millimeters and vibrational photoacoustic tomography with a depth of 3 centimeters. These studies are enabling clinical use of vibrational spectroscopy for molecular diagnosis.
A most recent work by the Cheng group demonstrated a paradigm-shifting approach for in vivo vibrational spectroscopic imaging (Science Advances 2015, 1: e1500738). As a gold standard for optical spectroscopy, the spectrometer employs a prism or grating to disperse the collimated signal light on a detector array, and requires a slit to reject scattered photons in order to achieve the desired spectral resolution. This scheme becomes highly inefficient for epi-detected in vivo spectroscopic imaging, where nearly all photons are scattered. Cheng and coworkers overcame this barrier through spatial frequency multiplexing of incident light and single photodiode detection of a stimulated Raman scattering spectrum. This approach enables spectroscopic viewing of residual disease in a surgical room and development of deep-tissue Raman spectroscopy towards molecular level diagnosis.
The Cheng team also spearheaded the development of high-speed, high-sensitivity transient absorption spectroscopic imaging for characterization of nanomaterials including the single-walled carbon nanotubes (Nature Nanotechnology 2012, 7: 56-61) and single layer graphene (Scientific Reports 2015, 5: 12394). The Cheng group was the first to use phase-sensitive transient absorption microscopy to separate semiconducting and metallic nanotubes (Phys Rev Lett 2010, 105: 217401). The Cheng team further demonstrated saturated transient absorption microscopy (Nature Photonics 2013, 7: 449) for super-resolution imaging of non-fluorescent species. These studies offer a label-free imaging platform for biomedical applications of nano-materials.