Concentration gradients probed in microfluidics by gate-array electrolyte organic transistor

We propose a device architecture termed gate-array electrolyte gated organic transistor (GA-EGOFET) that quantitatively measures the solute concentration gradient created in a spatially inhomogeneous solution, for instance a (biological) fluid. The integrated H-cell microfluidics yields a diffusive concentration profile along the microfluidics channel according to the flow rate of the input streams. We demonstrate this concept by monitoring the formation of self-assembly monolayers (SAMs) on top of an array of parallel Au gate electrodes exposed to a different local concentration of alkanethiols. The deposition rate and the coverage both increase from the entrance towards the H-cell end. The voltage change at each gate is transduced in the transfer curve acquired with the specific gate electrode. For short chain length SAMs (n = 3), the trend of the current hints to a diffusion-limited surface reaction. For longer thiols (n = 6, 9), instead, the slower surface diffusion or incorporation in the more stable and compact SAM yields a current that is independent of the longitudinal gradient. The microfluidics/GA-EGOFET platform is viable for constructing dose curves in reproducible manner and for validating different electrode functionalization strategies where the deposition rates are different at each substrate site.