The combination of microarray technologies with microfluidic sample delivery and real-time detection methods has the capability to simultaneously monitor 10-1000 s of biomolecular interactions in a single experiment. Despite the benefits that microfluidic systems provide, they typically operate in the laminar flow regime under mass transfer limitations, where large analyte depletion layers act as a resistance to analyte capture. By locally stirring the fluid and delivering fresh analyte to the capture spot, the use of passive mixing structures in a microarray environment can reduce the negative effects of these depletion layers and enhance the sensor performance. Despite their large potential, little attention has been given to the integration of these mixing structures in microarray sensing environments. In this study, we use passive mixing structures to enhance the mass transfer of analyte to a capture spot within a microfluidic flow cell. Using numerical methods, different structure shapes and heights were evaluated as means to increase local fluid velocities, and in turn, rates of mass transfer to a capture spot. These results were verified experimentally via the real-time detection of 20-mer ssDNA for an array of microspots. Both numerical and experimental results showed that a passive mixing structure situated directly over the capture spot can significantly enhance the binding rate of analyte to the sensing surface. Moreover, we show that these structures can be used to enhance mass transfer in experiments regarding an array of capture spots. The results of this study can be applied to any experimental system using microfluidic sample delivery methods for microarray detection techniques.
Affinity-based biosensing systems have become an important analytical tool for the detection and study of numerous biomolecules. The merging of these sensing technologies with microfluidic flow cells allows for faster detection times, increased sensitivities, and lower required sample volumes. In order to obtain a higher degree of performance from the sensor, it is important to know the effects of the flow cell geometry on the sensor sensitivity. In these sensors, the sensor sensitivity is related to the overall diffusive flux of analyte to the sensing surface; therefore increases in the analyte flux will be manifested as an increase in sensitivity, resulting in a lower limit of detection (LOD). Here we present a study pertaining to the effects of the flow cell height H on the analyte flux J, where for a common biosensor design we predict that the analyte flux will scale as J ≈ H(-2/3). We verify this scaling behavior via both numerical simulations as well as an experimental surface plasmon resonance (SPR) biosensor. We show the reduction of the flow cell height can have drastic effects on the sensor performance, where the LOD of our experimental system concerning the detection of ssDNA decreases by a factor of 4 when H is reduced from 47 μm to 7 μm. We utilize these results to discuss the applicability of this scaling behavior with respect to a generalized affinity-based biosensor.
