Size-specific concentration of DNA to a nanostructured tip using dielectrophoresis and capillary action.

One of the critical challenges in the fields of disease diagnostics and environmental monitoring is to concentrate extracellular DNA from a sample mixture rapidly. Unlike genomic DNA in normal cells, extracellular DNA dissolved in a biological sample can potentially offer crucial information about pathogens and toxins. The current concentration methods, however, are not able to directly concentrate extracellular DNA due to aggressive sample preparation steps. This paper presents a concentration mechanism of extracellular DNA onto a nanostructured tip using dielectrophoresis (DEP) in conjunction with capillary action. DNA immersed in a solution is captured onto a nanotip by two sequential actions: (1) attraction of DNA and other bioparticles in the vicinity of a nanotip by DEP and (2) size-specific capture of DNA onto the nanotip by capillary action. To investigate the size-specific capturing mechanism, an analytical model for the capillary action on a nanotip is presented, which is compared to the experiment for capturing polystyrene nanospheres. This analysis predicts the capture of a spherical particle smaller than 0.39 times a nanotip diameter, whereas our experiment shows that polystyrene spheres smaller than 0.84 times a nanotip diameter are captured. This discrepancy can be caused by the increase of the capturing force due to attractive DEP force. In addition, the diameter of the captured spheres can be increased by other experimental conditions including the tip geometry, the multiple particle interaction, and the contact angles. When a nanotip is used for concentrating lambda-DNA, 6.7 pg/mL (210 aM) of DNA is selectively extracted from a sample mixture containing lambda-DNA and Drosophila cells in one minute. The captured DNA is investigated by fluorescence microscopy, scanning electron microscopy (SEM), and X-ray analysis. This nanotip-based DNA concentrating method is a rapid and highly sensitive technique to detect extracellular DNA from a sample mixture.