Direct measurement of chain order in single lipid mono- and bilayers with multiplex CARS.

We present direct measurements of the thermodynamic state of single phospholipid mono- and bilayers based on the intrinsic vibrational properties of the molecules. The increased, shot-noise limited, sensitivity of multiplex coherent anti-Stokes Raman scattering permits the acquisition of vibrational spectra of single lipid bilayers with a signal-to-noise ratio of 4 in 20 ms. The lipid chain order for both liquid-crystalline and gel phase supported lipid bilayers is determined. Using deuterated lipids, vibrational spectra can be measured selectively in either of the leaflets of an asymmetric bilayer.