Compositions and methods for covalently immobilizing an oligonucleotide onto a polymer-coated bead or similar structure are described. Specifically, the polymer-coated bead or similar structure possesses a large number of activatable moieties, preferably primary and secondary amines. An oligonucleotide is activated with a monofunctional or multifunctional reagent, preferably the homotrifunctional reagent cyanuric chloride. The resultant covalently immobilized oligonucleotides on the beads or similar structures can serve as nucleic acid probes on solid supports, and hybridization assays can be conducted wherein specific target nucleic acids are detected in complex biological samples. The beads or similar structures can be employed free in solution, such as in a microtiter well format; in a flow-through format, such as in a column; or in a dipstick, including a multisite indicator card wherein multiple beads possessing oligonucleotides with different sequences or specificities can be closely aligned whereby a multiplicity of pathogens can be detected in a single biological sample. Additionally, dichlorotriazine oligonucleotides and processes for activating oligonucleotides by treatment with cyanuric chloride are included in the present invention.