Paramagnetic particles with amino functional groups can be derivatized using glutaraldehyde and thiourea in order to immobilize silver ions. The silver immobilization procedure did not alter the surface morphology of the particles according to Tapping mode AFM imaging. Rutherford backscattering showed that silver resides on the particle surface while iron is present throughout the particle. Particle-Induced X-Ray Emission spectroscopy determined that the derivatized particles have a silver capacity of 7 wt%. Paramagnetic particles containing immobilized silver ions have an affinity binding constant of the order 10(8) for mono- and tri-biotin-labeled oligonuceotide with primary sequence 3＇-GCCCCTTTTTAAAAACCCCG-5＇ while the original amino particles have little affinity for the oligonucleotide. Measurement of binding and release was enabled by attaching either fluorescein isothioocyanate (FITC) to the 3＇-end or Texas Red or fluorescein phosphoramidite (FAM) to the 5＇-end of the oligonucleotide. Up to a 97% release of the FAM labeled biotinylated oligonucleotide from the particle surface can be achieved using an aqueous solution of thiodiglycol. (C) 2000 Elsevier Science B.V. All rights reserved.