Adaptation of a surface plasmon resonance biosensor with microfluidics for use with small sample volumes and long contact times.

The efficient delivery of sample to surface-immobilized sites is a key element in biosensing. For a surface plasmon resonance (SPR) biosensor, this has been addressed by constant flow through a microfluidic system with a sample injection loop (Sjölander, S.; Urbaniczky, C. Anal. Chem. 1991, 63, 2338-2345). The present study describes an alternative mode of sample delivery without constant unidirectional flow. It was implemented on a commercial Biacore X SPR biosensor equipped with a microfluidic cartridge, but with the fluidic handling performed by an externally computer-controlled syringe pump. We demonstrate that sample volumes as low as 2 microL can be reproducibly positioned to cover the sensor surfaces, manipulated in a serial fashion, efficiently mixed by applying an oscillatory flow pattern, and fully recovered. Compared to the traditional continuous unidirectional flow configuration, we found very similar kinetic responses at high analyte concentrations and slightly slower responses at low concentrations, most likely due to depletion of analyte from the small sample volumes due to surface binding. With the antibody-antigen systems tested, binding parameters were obtained that are generally within 10% of those from conventional experiments. In the new configuration, biosensor experiments can be conducted without the usual constraints in the surface contact time that are correlated with sample volume and mass transport rate. This can translate to improved detection limits for slow reactions and can facilitate kinetic and thermodynamic binding studies.