Dependence of the dielectrophoretic upper crossover frequency on the lipid content of microalgal cells

High frequency dielectrophoresis (> 20 MHz) was used to demonstrate that the upper crossover frequency of microalgal cells is reduced as lipids accumulate within the cytoplasm. Currently, the majority of AC dielectrophoresis applications differentiate cells by exploiting differences in the lower crossover frequency, typically between 10 and 500 kHz. However, the single shell model also predicts another crossover in the 20-200 MHz range that is dependent upon the dielectric properties of the cytoplasm. We demonstrate this effect with microalgal cells due to the relative ease in which the properties of the cytoplasm can be altered. Chlamydomonas reinhardtii cells were cultured in regular media and were observed to have an upper crossover frequency of  ~ 75 MHz. The same cells, when cultured under nitrogen-free conditions, began to accumulate neutral lipids. The lipid content was verified via fluorescence microscopy and the upper crossover frequency was measured to be ~ 40 MHz. To measure the upper crossover frequency, two needle shaped electrodes were patterned onto a glass slide and the motion of the cells was observed as an AC signal was swept from 10-110 MHz at ~ 30 voltage peak-to-peak (Vpp). We conclude that the increase in lipid content reduces the effective conductivity of the cytoplasm thus reducing the upper crossover frequency. High frequency dielectrophoresis may be a label free technique to sort algae cells on the basis of lipid content.