Interbead interactions within oligonucleotide functionalized ferrofluids suitable for magnetic biosensor applications

Abstract

Interactions in oligonucleotide functionalized ferrofluids were investigated by measurements of frequency dependent complex magnetization in a superconducting quantum interference device. The ferrofluids consisted of aqueous suspensions of single-stranded oligonucleotide functionalized 130 nm sized magnetic beads, suitable for further use in magnetic biosensor applications based on changes in the Brownian relaxation behaviour of oligonucleotide-coated beads. The interbead interactions were found to depend strongly on the surface coverage by oligonucleotide molecules. At low surface coverage, aggregates of beads within the ferrofluid were formed, most likely due to interbead SS-crosslinking reactions. When the surface coverage increased, the entropic interbead repulsion arising from the thermal motion of oligonucleotide molecules began to stabilize the ferrofluid by preventing aggregation, and the crosslinking probability decreased. At a surface coverage, of ∼40 oligonucleotides per bead, determined by a radioactive labelling analysis, an optimal configuration was obtained for which the ferrofluid behaved as consisting of almost non-interacting beads. At higher oligonucleotide functionalization degrees, the high coverage induced less thermal motion flexibility of the oligonucleotide chains which decreased the entropic repulsion effect. This in turn led to a higher crosslinking probability, as well as an increase in the hydrodynamic size of the beads. Thus, the ferrofluid sample of nearly non-interacting nature, in which an optimal degree of stabilization and the highest Brownian relaxation frequency are achieved in the low surface coverage region, may be considered as the most appropriate one for further use in magnetic biosensor applications, from both a functional and an economic point of view.