Microscopic mechanisms influencing the volume amplified magnetic nanobead detection assay

The volume amplified magnetic nanobead detection assay [Strömberg, M., Göransson, J., Gunnarsson, K., Nilsson, M., Svedlindh, P., Strømme, M., 2008. Nano Letters 8, 816–821] was investigated with respect to bead size, bead surface coverage of probe oligonucleotides, bead concentration and rolling circle amplification (RCA) time, with the objective to improve the understanding of the microscopic mechanisms influencing the assay. The most important findings for future biosensor development were the following: (i) small beads exhibit a much reduced tendency to cross-link several RCA products, thus enabling use of both complex magnetisation turn-on and turn-off detection strategies, whereas larger beads only allow for turn-off detection; (ii) small beads exhibit faster immobilisation kinetics, thus reducing the time for diagnostic test completion, and also immobilise in larger numbers than larger beads. Finally, (iii) by demonstrating qualitative dual-target detection of bacterial DNA sequences using 130 and 250 nm beads, the bioassay was shown to allow for multiplexed detection.