Quantitative evaluation of the depletion efficiency of nanofractures generated by nanoparticle-assisted junction gap breakdown for protein concentration

Abstract

Sample preconcentration is crucial to the accuracy of biochemical or clinic detection efforts, particularly for samples with extremely low protein concentrations. Overlapped electrical double layers in nanofluidic channels can generate concentration polarization through the application of an electric field. This induces nonlinear electro-kinetic flow and results in the exclusion-enrichment effect: the rapid accumulation of proteins in front of the induced ionic depletion zone. This study created nanofractures to preconcentrate proteins via the exclusion-enrichment effect, in which protein samples are driven by electro-osmotic flow to accumulate at a specific location. A preconcentration chip was fabricated by standard soft lithography using a polydimethylsiloxane replica. Nanofractures were formed by nanoparticle-assisted electrical breakdown. This study also developed a simple method of quantitatively evaluating the depletion efficiency of nanofractures, whereby proteins are stacked in the area at which the microchannels intersect by balancing the depletion force with the driving force produced by electro-osmotic flow (EOF). The proteins leak into the left reservoir when the driving voltage of EOF is higher than that necessary for stacking; i.e., the depletion force is less than the driving force of EOF. Increasing the depletion force works to repel the proteins toward the lower corner of the measurement region, close to the intersection of the microchannels. An increase in depletion force expands the area without proteins and the relative size of this area can be measured using grayscale and binary images to evaluate the depletion efficiency. The proposed deposition of gold nanoparticles at the junction gap between microchannels greatly reduces the electrical breakdown voltage required for the formation of nanofractures.