Analysis of protein adsorption and binding at biosensor polymer interfaces using x-ray photon spectroscopy and scanning electrochemical microscopy

We describe a method, based on X-ray photoelectron spectroscopy (XPS) measurements, to assess the extent of protein adsorption or binding on a variety of different μTAS and biosensor interfaces. Underpinning this method is the labeling of protein molecules with either iodine- or bromine-containing motifs by using protocols previously developed for radiotracer studies. Using this method, we have examined the adsorption and binding properties of a variety of modified electrodeposited polymer interfaces as well as other materials used in μTAS device fabrication. Using polymer interfaces modified with poly(propylene glycol) (PPG) chains, our results indicate that a chain of at least ∼30 monomer units is required to inhibit non-specific adsorption from concentrated protein solutions. The XPS methodology was also used to probe specific binding of avidins and enzyme conjugates thereof to biotinylated and mixed biotin/PPG-modified polymer interfaces. In one example, using competitive binding, it was established that the mode of binding of a peroxidase-streptavidin conjugate to a biotinylated modified polymer interface was primarily via the streptavidin moiety (as opposed to nonspecific binding via the enzyme conjugate). XPS evaluation of nonspecific and specific peroxidase-streptavidin immobilization on various functionalized polymers has guided the design and fabrication of functionalized interdigitated electrodes in a biosensing μTAS device. Subsequent characterization of this device using scanning electrochemical microscopy (SECM) corroborated the adsorption and binding previously inferred from XPS measurements on macroscale electrodes.