Spore and micro-particle capture on an immunosensor surface in an ultrasound standing wave system

The capture of Bacillus subtilis var. niger spores on an antibody-coated surface can be enhanced when that coated surface acts as an acoustic reflector in a quarter wavelength ultrasonic (3 MHz) standing wave resonator (Hawkes, J.J., Long, M.J., Coakley, W.T., McDonnell, M.B., 2004. Ultrasonic deposition of cells on a surface. Biosens. Bioelectron. 19, 1021-1028). Immunocapture in such a resonator has been characterised here for both spores and 1 μm diameter biotinylated fluorescent microparticles. A mean spatial acoustic pressure amplitude of 460 kPa and a frequency of 2.82 MHz gave high capture efficiencies. It was shown that capture was critically dependent on reflector thickness. The time dependence of particle deposition on a reflector in a batch system was broadly consistent with a calculated time of 35 s to bring 95% of particles to the coated surface. A suspension flow rate of 0.1 ml/min and a reflector thickness of 1.01 mm gave optimal capture in a 2 min assay. The enhancement of particle detection compared with the control (no ultrasound) situation was ×70. The system detects a total of five particles in 15 fields of view in a 2 min assay when the suspending phase concentration was 104 particles/ml. A general expression for the dependence of minimum concentration detectable on; number of fields examined, sample volume flowing through the chamber and assay time shows that, for a practical combination of these variables, the threshold detection concentration can be two orders of magnitude lower.