Adsorption kinetics of proteins in plastic microfluidic channels: Real-time monitoring of lysozyme adsorption by pulsed streaming potentials

A proof-of-concept study is presented illustrating the real-time monitoring of lysozyme (Ly) adsorption on the surface of microchannels made with cyclic olefin copolymer (COC). The signal arises from changes in the surface charge upon adsorption of Ly which is monitored by pulsed streaming potentials. Because streaming potentials are electrochemical potentials resulting from the pressure-driven flow of the liquid and the surface charge of the microchannel, this approach is ideal for microfluidics. Initial adsorption rates showed a linear correlation with the bulk concentration of [Ly] in the range between 7.0 and 350 nM. Fitting of the adsorption isotherms allowed the estimation of equilibrium and rate constants of adsorption. The influence of phosphate ions on the adsorption kinetics of Ly was also investigated. Unlike the steady flow used in conventional streaming potential measurements, the present approach incorporates pulsed flow and the ability to inject finite samples into the microfluidic stream. The pulsed flow allows the use of non-reference electrodes which removes the need for special electrolytes to stabilize the electrode potentials. Likewise, the injection permits monitoring of adsorption and desorption events in real time. The label-free monitoring of these events and the high sensitivity of the adsorption kinetics of Ly to solution species found in this work indicates that this method could be applicable to study protein–protein interactions.