(234b) Electroosmotic Mixing inside Polyacrylamide Gels Via Immobilized Silica Nanoparticles

The dynamics of biosensor response to a change in analyte concentration represent a problem of chemical reaction with coupled mass transfer. The designs of biosensor devices often call for biomolecular recognition elements, such as enzymes, antibodies, or oligonucleotides, to be immobilized in the sensor substrate. Immobilization in gels is commonly used to protect the sensing elements in applications involving exposure to harsh or fouling environments. This introduces significant barriers to the mass transfer of the analyte molecules to the sensing elements. Additionally, the structure of the gel makes mechanical mixing schemes inappropriate in devices based on molecular recognition within gels. We present an internal pumping strategy to enhance solute fluxes in polymer gels. The method is based on electroosmotic flow driven by an electric field applied across a polyacrylamide gel that has been doped with charged colloidal silica inclusions. The homogeneity of the charged particle distribution in the gels was investigated by small angle neutron scattering. Comparisons of the scattering intensity of gels with silica particles with the scattering from dilute aqueous suspensions indicated that silica particles did not aggregate in the gels. The appearance of a local minimum in the scattering intensity and the structure factor for the particle-laden gels suggested that the particle positions may be correlated in the gel. The effect of the particle volume fraction on the gel density was studied by a gravimetric analysis and we found that an increase in osmotic pressure results in lower polymer volume fractions with increasing particle load. We demonstrated the feasibility for convective mixing inside the gels by the measurement of fluorescein mass transport in particle loaded gels under the application of nonuniform ac electric fields. Florescence recovery after photobleaching (FRAP) was used to test electrokinetic mixing. Electrokinetic phenomena provided convection mechanisms for the enhancement of mass transport. The experiments with applied electric fields were compared with control diffusion experiments. The spot deformation in gels with silica particles under designed oscillatory electric potential gradients provided an electroosmotic mixing mechanism that enhanced the spot recovery rate sometimes by more than an order of magnitude when compared to electrophoretic mixing in gels that did not contain silica inclusions. The development of this novel electroosmotic pumping technique can potentially impact the design of new biosensing techniques or biomedical devices.