Biomolecule Confinement and Preconcentration Using Nanofluidic Filters

With increasing demands from researchers on discovering novel drug targets and early disease markers, people found current proteomics technologies fall short on dealing with protein complexities and abundance variation. Since human proteome has more than 10,000 different proteins, with high abundant proteins having 109 higher concentrations than low abundant ones, identifying low abundant proteins (biomarkers) in complex mixtures is one of the major challenges in proteomics. As a result, before one can identify any target proteins, at least one separation step must be performed.

This project focuses on studying and applying the physiochemical nanofluidic channels (~40 nm). We have developed a nanofluidic preconcentrator that can concentrate biomolecular samples up to 10 million fold. Due to the electrical double layer overlapping, sub 100 nm nanochannels have preferential transfer over counter-ions (or counter-ion current). As a result, a well known phenomenon called concentration polarization can be observed. However, once a higher bias is applied, the system will be driven into the over-limiting current regime, where the charge neutrality in the bulk no longer exists and the extended space charge layer (SCL) is formed. The detail mechanism is not well understood so far. However, by coupling a tangential field across the SCL, we can have a fast accumulation of charged molecules in front of it. In short, this device collects charged biomolecules based on two features: (i) the energy barrier for charged biomolecules generated by the induced space charge layer near the nanofluidic filter; (ii) a faster nonlinear electroosmotic flow for sample deliveries. Currently, we are able to achieve more than a million fold enhancement factor in 30 mins. The preconcentration factors and collection speed are close to those of the PCR for nucleic acids, which is an essential step for many genomics researches. Applications included biomolecular preconcentration and fluid pumping using electroosmotic flow (EOF) of the 2nd kind.

Nanofluidic channels can be used to enhance surface binding reactions, since the target molecules are closely confined to the surfaces which are coated with specific binding partners. Moreover, diffusion-limited binding can be significantly enhanced if the molecules are steered into the nanochannels via either pressure-driven or electrokinetic flow. By monitoring the nanochannel impedance, which is sensitive to surface binding, low analyte concentrations have been detected electrically in nanofluidic channels within response times of 1-2 hours. This represents a ~54 fold reduction in the response time using convective flow compared to diffusion-limited binding. At high flow velocities the presented method of reaction kinetics enhancement is potentially limited by force-induced dissociations of the receptor-ligand bonds. Optimization of this scheme could be useful for label-free, electrical detection of biomolecule binding reactions within nanochannels on a chip.