Biochemical stability of components for use in a DNA detection system

We have proposed a DNA detection system that relies on superparamagnetic nanoparticles (Co and Fe₃O₄) as biological labels and submicrometer spin valves as sensors. In sensor operation, the magnetic labels are subjected to annealing at 95°C-98°C, at which DNA denatures, and the spin valve sensor surface is exposed to various DNA containing buffer solutions. We have performed stability studies to validate that the components of our system will survive normal sensor operation. We demonstrate that our proposed biological labels are magnetically stable after five thermal cycles of 5 min (with T_max=98°C). By tracking the magnetic diameter (D_m) of the nanoparticles, obtained from a Langevin function fitting method, we observe that D_m decreases by about 20% after the fifth temperature cycle. Compliance of the submicrometer spin valve sensor with a width of 300 nm and an magnetoresistance (MR) ratio of ∼7 % is studied by immersion cycling in buffer solutions (pH=7.5-7.9) with a high DNA concentration. The spin valve sensor maintains its characteristic MR ratio after four 30 min exposures to DNA solution and continues to perform with a ∼ 7 % MR after 24 h in solution. Notably, the spin valve sensor demonstrates this survivability with an ultra thin passivation layer (∼ 4 nm). Thus, the biochemical stability of these components suggests that our DNA detection system is compliant with standard biological sensor operation.