Characterization of electrografted 4-aminophenylalanine layers for low non-specific binding of proteins

Abstract

Novel diazonium salts based on the 4-amino derivative of phenylalanine were electrografted onto gold surfaces with the ultimate goal of formation of surfaces resistant to nonspecific adsorption of proteins. A pulsed potential deposition profile was used instead of the more conventional approaches in order to circumvent mass-transport limitations. The influence of the deposition parameters, including pulse potentials, pulse width and number of pulses, with respect to electrode coverage was evaluated as a function of the blocking effect towards the diffusion of Fe(CN)₆^4−/3− to electrode surfaces. By appropriately choosing the deposition parameters, peak potential differences (ΔE_p) of ∼750 mV (for ν = 100 mV s⁻¹), contrasting the ∼440 mV obtained for layers deposited via conventional electrochemical methods. FT-IR spectra confirm that the general structure of the electografted layers displays the same chemical functionality as the precursor molecule. Furthermore, the presence of the carboxylic acid group, characterized by the absorption feature at 1724 cm⁻¹, indicates that the layers retain the ability to undergo post-deposition functionalization with bioreceptors. Ellipsometric analysis demonstrates the versatility of this method by depositing layers ranging from ∼1 to ∼24 nm thick. Finally, surface plasmon resonance spectroscopy was used to monitor the modified surfaces upon exposure to highly fouling media (76 mg mL⁻¹ bovine serum albumin in phosphate buffered saline solution). A protein surface coverage equivalent to 62 ng cm⁻² was measured, representing a significant improvement compared with more established antifouling layers based on polyethylene glycol (100 ng cm⁻²) and alkanethiol self-assembled monolayers (268 ng cm⁻²). The resistance towards nonspecific adsorption may be associated with the hydration layer tightly bound by the ionic charges present in the organic layer at physiological pH.