Immobilization of proteins to a carboxymethyldextran-modified gold surface for biospecific interaction analysis in surface plasmon resonance sensors

A method for fast and simple covalent immobilization of proteins to a carboxymethyldextran-modified gold surface intended for surface plasmon resonance sensors is described. The method utilizes the formation of N-hydroxysuccinimide esters from a fraction of the carboxyl groups of the carboxymethyldextran matrix via reaction with N-hydroxysuccinimide and N-ethyl- N′-(dimethylaminopropyl) carbodiimide hydrochloride in water. In a second step the protein is passed over the surface in a solution of low ionic strength with a pH value below the isoelectric point of the protein. The protein is thereby concentrated in the matrix by electrostatic attraction forces and a simultaneous reaction with the active esters takes place. In a final step, the remaining active esters are transformed into amides via reaction with ethanolamine. This sequence is performed automatically in a system comprising an integrated microfluidic cartridge and an autosampler. Typical reaction times of less than 30 min are required for the immobilization of proteins at surface concentrations in the region of 70 fmol mm −2. Parameters such as protein concentration, protein solution ionic strength, pH, reaction times, and reagent concentration can be varied in order to control the immobilized amount of ligand. The biospecific interaction of the immobilized ligand with its biological counterpart is illustrated by the effects on the interaction of immunoglobulins with immobilized Staphylococcus aureus protein A for various amounts of protein A.