Development of a depolarized Raman spectrometer for potential surface-enhanced Raman optical activity (SEROA) measurements

摘要

Raman optical activity (ROA) spectroscopy is a chiral sensitive vibrational spectroscopic technique that measures the difference of molecular responses to either left or right circularly polarized photon radiation. As a structural sensitive probe, ROA has great potential applications in fabricating bio-molecular sensors due to its capability of capturing small changes in molecular structure. However, ROA is a weak effect on the order of 10-3 or 10-4 smaller than its parent spontaneous Raman. Extension of ROA to widely sensing applications can be accelerated by combining it with surface plasmon resonance effect, surface-enhanced Raman spectroscopy (SERS), allowing for trace chemical detections. However, it has been shown to be very challenging to combine SERS with ROA to obtain a reliable surface-enhanced Raman optical activity measurement by several independent ROA research groups. This is because of the complexity of interaction of chiral molecules with metal surfaces. In addition, another main factor for the difficulty is the lack of a depolarized ROA spectrometer which can remove effectively huge polarized Raman bands, generally the major sources of artifacts in ROA. In this paper, we will discuss the development of a depolarized Raman spectrometer. This newly developed depolarized Raman instrument is based on the BioTools' ChiralRAMAN-2X scattered circular polarization (SCP) ROA spectrometer and has been modified into a dual circular polarization ROA spectrometer with the introducing circularity conversion in the incident radiation path. Combing the simultaneous detection of left and right circularly polarized scattering photons, newly developed Raman instrument are capable of modulate both incident and scattered light simultaneously resulting dual circular polarization (DCP) ROA measurements from depolarized parent Raman scattering. The performance of the Raman spectrometer has been initially evaluated with standard sample of small molecules for the proof-of-principle measurements of DCP-ROA, and its potential application in SEROA measurement has also been discussed