The effect of voltage on a conductivity gradient in a nanochannel and its application to protein trapping

Studies in the last two decades have demonstrated the use of protein biomarkers for early disease detection. Increasingly, these biomarkers have been found at very low concentrations with a high background of abundant proteins. Recent work in nanofluidics has allowed for the possibility of high concentration enhancement and separation – two processes that are essential for the detection and discovery of trace biomarkers. With recent demonstrations of the trapping and detection of these biomolecules using nanochannels, there is a need to better model and appreciate the physical mechanisms that are involved in the trapping of these biomolecules. This paper investigates the effect that voltage has on a conductivity gradient established within a nanochannel. This conductivity gradient is used to focus proteins by balancing the opposing electrophoretic and viscous drag forces – and it is established in a nanochannel that connects two microchannels containing solutions of different salt concentrations and hence conductivity. A conductivity gradient is directly observed using a fluorescent salt, then the trapping behaviour of a fluorescent protein, in a phosphate buffered solution containing salt, is investigated, while varying the applied voltage. This study concludes that voltage appears to have a significant effect on the conductivity gradient inside a nanochannel, and that this effect may have important implications for protein trapping inside a nanochannel.