Characterization of porous alumina membranes for efficient, real-time, flow through biosensing

Nanofluidic sensing devices promise high performance by overcoming issues of mass transport of analyte molecules to the sensing surface, whilst micro-porous membranes promise high sensitivity due to a large surface for their capture. Anodic alumina (AAO) filter membranes allow the flow through of samples, and could be used as a convenient and readily available fluidic platform for the targeted delivering of analytes to bioreceptors immobilized on the pore walls. The relatively small pore dimensions, compared to fluidic diffusion lengths, promise highly efficient capture of analytes from the whole sample volume, enabling relatively fast sensing response times and the use of small sample volumes (<100μL). In this paper we use polarimetry as the real time readout method and characterize the response of commercially available filter membranes, with 200nm pore size, to bulk solution refractive index changes and to determine their suitability for biosensing (LOD 5×10^-6RIU). The membrane surface loading was characterized using the adsorption of an amino-polymer, polyethylenimine, followed by the physisorption of biotinylated BSA and specific binding of streptavidin coated CdSe quantum dots (SA-QD). This method provides an approach for the comparison of responses across a wide range of sensing techniques. The capture efficiency was determined with a fluorescent flow-through capture assay using Cy3 labeled streptavidin which, combined with modeling, was also used to provide pore size distribution information for the porous membrane. Compared to a conventional planar biosensors, this AAO membrane device shows much higher efficiency analyte capture from solution (17% vs 32%), which is ultimately more determined by the distribution of pore sizes than the average or nominal pore size. The combination of porous membranes and SA-QD detection also raises the potential for other transduction mechanisms to be explored in these devices, such as fluorescence, colorimetric or back-pressure measurement.