Yes, the dextran matrix will lose its pre-concentration ability below pH 3.5. At a pH below the pI of a protein, it will have an overall positive charge. At a pH above approximately pH 3.0 the carboxyl groups (pKa COOH = 2.19) will have an overall negative charge. The matrix is fully negatively charged at pH values above 7(1).
I am not familiar with silica particles, so I do not know if it will work.
Iam not sure what you mean with gluteraldehyde chemistry, but the aldehyde protocol used with Biacore depends on ligands having aldehyde groups. By activating the dextran surface to a hydrazide the aldehyde will react forming a covalent bond. The reaction is done a pH 4.0.
One reference about peptide immobilization: Houseman, B. T., Huh, J. H., Kron, S. J., and Mrksich, M. Peptide chips for the quantitative evaluation of protein kinase activity Nat.Biotechnol. 20: 270-274 (2002).
Reference List
1. BIACORE AB BIACORE Technology Handbook (1998) .