Sensitive circular dichroism spectroscopy based on nonlinear degenerate four-wave mixing.

Degenerate four-wave mixing is demonstrated as an effective and sensitive laser analytical spectroscopic method for circular dichroism measurements. A forward-scattering degenerate four-wave mixing optical setup is used to obtain simple optical alignment, highly efficient wave mixing, and very effective use of low laser power. This nonlinear laser-based circular dichroism method offers many advantages, including easy and efficient optical signal collection, use of very short analyte path lengths (e.g., 0.1 mm), and excellent detection sensitivity that is comparable or better than conventional laser-based or non-laser-based circular dichroism methods. Using an analyte path length of only 0.1 mm, and a probe volume of 98 pL, a circular dichroism mass detection limit of 0.68 pg or 2.8 fmol is reported for (+)Co(en)3(3+).