Turn-on optomagnetic bacterial DNA sequence detection using volume-amplified magnetic nanobeads

Detection of a Vibrio cholerae DNA-sequence using an optomagnetic read-out exploiting the dynamic behavior of magnetic nanobeads along with two turn-on data analysis approaches is demonstrated. The optomagnetic method uses a weak uniaxial AC magnetic field of varying frequency applied perpendicular to the optical path and measures the modulation of laser light passing through a cuvette containing the sample with oligonucleotide-tagged magnetic beads and macromolecular coils of single-stranded DNA. The DNA coils are formed upon a padlock probe ligation followed by rolling circle amplification (RCA). The presence of target gives rise to a change of the 2nd harmonic component, V2=V2'+iV2'', of the transmitted light. We demonstrate that by using the phase angle ξ defined as ξ=arctanV2'/V2'' in the low-frequency region we obtain a limit of detection of 10 pM for an RCA time of only 20 min corresponding to a total assay time of 60 min. Moreover, we show that the approach based on ξ is significantly more robust than the analysis based on a turn-off of the signal due to free magnetic nanobeads used in previous work (Donolato et al., submitted for publication), where a limit of detection of 10 pM was obtained for an RCA time of 60 min. The increased robustness and the reduction in total assay time constitute significant steps towards the realization of a low-cost, rapid and sensitive biosensor platform suitable for pathogen detection in both human and veterinary medicine settings.