Pre-concentration is the procedure used to obtain a high local protein concentration at the sensor chip surface. Pre-concentration makes the immobilization of the protein to the sensor chip surface more efficient. Pre-concentration is done with the dextran-carboxyl group based sensor chips. For sensor chips NTA, SA and hydrophobic based sensor chips pre-concentration is not beneficial because the ligand binding is based on pre-immobilized tags. Pre-concentration is aided by a low ionic strength and a solution pH slightly lower than the pI of the ligand (1). The pH of the coupling buffer, which will change the surface and ligand charges, regulates the rate and amount of the ligand that goes towards the sensor chip surface (pre-concentration). If the buffer has a high ionic strength, it will mask the charges of the surface and ligand and thereby diminish the pre-concentration effect. Therefore a buffer with low ionic strength (e.g. 10 mM) is used.
In general, an immobilization solution with a pH between 4.0 and 6.0 is used. The dextran matrix will lose its pre-concentration capacity at a pH lower than 3.0. The buffer components of the flow and immobilization buffer should not react with the activated surface. In addition, additives such as azide must be avoided during the immobilization because they can react with the activated surface sites and therefore lower the immobilization efficiency.