Measurement of Minute Liquid Volumes of Chiral Molecules Using In-Fiber Polarimetry

Abstract

We report an optofluidic method that enables to efficiently measure the enantiomeric excess of chiral molecules at low concentrations. The approach is to monitor the optical activity induced by a Kagome-lattice hollow-core photonic crystal fiber filled with a sub-μL volume of chiral compounds. The technique also allows monitoring the enzymatic racemization of R-mandelic acid.

Figure 1

(a) Micrograph of the Kagomé-lattice HC-PCF used for the experiment. Concept of the experiment. The unit cell of the Kagomé lattice cladding is indicated in blue. The near-field of the output end of the fiber, recorded with a CCD camera, is shown on the SEM of the fiber. (b) Concept of the experiment. The molecules that fill the entire length of the HC-PCF are probed by the laser beam. Analysis includes polarization, mode profile, and spectrum.

Figure 2

Experimental setup. A Kagome fiber (SEM on Figure 1) is clamped with both ends into two liquid cells. The solution to be analyzed is filled into a pressure vessel and pressed into the fiber with 12 bar N₂ gas. The pump is a 532 nm CW microchip laser. A set of waveplates control the input polarization at the input of the fiber. Polarization after the fiber is analyzed with a polarimeter. A CCD camera monitored the near-field of the fiber.

Figure 3

Measured optical activity for pure 2-butanol over enantiomeric excess for different fiber lengths; points are the measured values; and lines correspond to eq 1; T = 20 °C; λ = 532 nm (continuous wave).

Figure 4

Absolute value for the measured optical activity for R- and S-2-butanol dissolved in methanol over enantiomeric excess; the ee of the component in excess was used; points are the measured values; and lines correspond to eq 1; c_2-But, tot = 0.044 g mL^–1, L = 70 cm; T = 20 °C; λ = 532 nm (continuous wave).

Figure 5

Measured enantiomeric excess over reaction time for a R-mandelic acid/mandelate racemase solution. Enzyme concentration: 0.64 μg mL^–1; mandelic acid concentration: 0.015 g mL^–1; L = 100 cm; T = 20 °C; λ = 532 nm (continuous wave).