(516d) Modeling of Nano-EIS in a High Peclet Number Packed Microfluidic Biosensor

Microfluidic Biosensors are characterized by both, low Peclet and Reynolds number laminar flow. Traditionally the three well-known transduction methodologies used in microfluidic biosensors are electrochemical methods, optical methods including fluorescence and electrical impedance-based techniques. Here we demonstrate the different physics in the electrochemical and electrical impedance based (EIS) transduction method arising from a microfluidic geometry that promotes convective transport. This is achieved by packing Carbon Nanotubes (CNTs) into a microfluidic channel. The high packing density of the CNT’s promotes convective transport. This is verified using cyclic voltammetry (CV), using ferric ferricyanide in 0.1 X PBS buffer. Further, the observed CV curve for our microfluidic system shows a pattern, unlike traditional CV curves which give us interesting perspective into the ionic reactions at the CNT interface. CV curves are plotted for bare CNT, CNT surface activated using NHS-EDC chemistry and with attached monoclonal antibody to CNT surface in 0.1x PBS aqueous at scan rates of 25 to 200 mV/s. It is interesting to note that in the absence of CNT, our device demonstrates regular CV characteristics where the peak current analysis is carried out using the Nicholson method. However, post CNT packing in the device, the CV curve deviates from the traditional CV curves. A thorough analysis of the CV indicates that convective transport dominates over diffusion. Further, the analysis clearly shows the interplay between surface coverage and charge transfer which results in different peaks in the CV sweep from CNT, CNT surface activated using NHS-EDC chemistry and with attached monoclonal antibody to CNT surface. The two competing phenomenon of surface coverage and charge transfer is illustrated in more details using electrochemical impedance spectroscopy or EIS. On addition of a secondary antibody to the monoclonal antibody-CNT system, it was observed that the EIS shift (chiefly the charge transfer resistance) decreased from bare CNT to EDC activated CNT to monoclonal antibodies and increased on the addition of secondary antibodies. Current literature studies show that on the addition of monoclonal antibody and then a polyclonal antibody always leads to an increase in the charge transfer resistance due to a decrease in the electrode surface area available to the buffer. This occurs from the increased electrode surface coverage from the large antibody molecules. It is important to note that, CNTs, as they are touching the electrodes on the glass surface, behave as extended electrodes here. Here, the addition of more charge to the CNT surface from bare CNT to EDC-NHS surface activated CNT to antibody functionalized CNT decreases charge transfer resistance. Initially, the addition of charge dominates (decrease in charge transfer resistance) while on the addition of the secondary antibodies, charge transfer resistance increases due to the decreasing CNT surface area. We ascribe this departure from classical formulations based on the nano-ordered packing structure of the CNT inside the channel. Here, the nanostructured CNT results in the ionic flux being confined in local nanodomains. This is modeled using a new EIS circuit which also allows for a new convective transport path. The EIS model further predicts the change in the EIS measurements due to the attachment of the monoclonal antibodies in the nano-ordered CNT structure. This study paves the way for increasing the sensitivity and selectivity of a packed biosensor.