I use the standard protocol of adding gluteraldehyde (50% in water, excess) to NH2-SiO2 in PBS 50 (7.4) for two hours and after purification adding NH2-DNA (excess) and overnight stirring. Later I use glycine and Sodium cyanoborohydride to reduce the imine bonds to stable amide bond. My DNA loading on the particles are low and I would appreciate any inputs regarding reaction conditions for maximum DNA loading on the particles.