Different strategies of covalent attachment of oligonucleotide probe onto glass beads and the hybridization properties.

The glass bead is a new biochip support material for immobilization biomolecules, due to its independence and convenient rearrangement. In order to optimize the immobilization efficiency of oligonucleotides onto glass beads and obtain the highest hybridization efficiency, three commonly used coupling strategies have been studied for covalently attaching oligonucleotides onto large glass beads. Glass beads with 250 microm diameter were amino-silaned with 2% 3-aminopropyltrimethoxysilane (APTMS) and then reacted separately with glutaraldehyde, succinic anhydride and 1,4-phenylene diisothiocyanate (PDITC) to derive CHO beads, COOH beads and isothiocyanate-modified beads (NCS-Beads) accordingly. Afterwards, amino-terminal oligonucleotides were covalently attached onto the surface of beads achieved by three strategies mentioned above. The immobilization efficiency were studied to compare the three strategies, which turned out 2.55 x 10(13) probes/cm2 for CHO-Beads, 3.21 x 10(13) probes/cm2 for COOH beads and 6.68 x 10(13) probes/cm2 for NCS beads. It meant that the immobilization efficiency based on NCS beads was most acceptable. And the method, developed by attaching amino-terminal oligonucleotides onto these cyanate active beads, could be regarded as an efficient one for immobilizing oligonucleotides onto a solid surface. Moreover, in this paper, the hybridization properties of NCS bead-based oligonucleotides have been studied by employing Cy5-tagged complementary oligonucleotides. It was found that the high probe density NCS beads led to low hybridization efficiency possibly due to the existence of steric crowding. In addition, the equilibrium binding constant KA was determined by employing Langmuir isotherm model, which was 7.0 x 10(6) M(-1) for NCS beads with the density of 6.7 x 10(13) probes/cm2. Furthermore, it only took 60 min to reach hybridization equilibrium. These large microspheres (>100 microm) can be employed in the mesofluidic systems for automated heterogeneous assays.