Biosensors and Bioelectronics 21 (2005) 758–767
Spore and micro-particle capture on an immunosensor surface in an
ultrasound standing wave system
Stacey P. Martin
, Rosemary J. Townsend
, Larisa A. Kuznetsova
, Kathryn A.J. Borthwick
, Martin B. McDonnell
, W. Terence Coakley
School of Biosciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3TL, UK
School of Engineering Science, Electromechanical Research Group, University of Southampton, Southampton SO17 1BJ, UK
Dstl Porton Down, Salisbury, Wiltshire SP4 0JQ, UK
Received 22 October 2004; received in revised form 17 December 2004; accepted 20 January 2005
Available online 16 February 2005
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 (3MHz) 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 m diameter biotinylated ﬂuorescent microparticles. A mean spatial acoustic pressure amplitude of
460kPaand 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 35s to bring
95% of particles to the coated surface. A suspension ﬂow rate of 0.1ml/min and a reﬂector thickness of 1.01 mm gave optimal capture in a
2min 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 2min assay when the suspending phase concentration was 10
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
© 2005 Elsevier B.V. All rights reserved.
Keywords: Ultrasound; Biosensor; Bacillus subtilis var. niger; BG spores; Immunoassay; Bio-terrorism
Bacteria-detecting sensors include immunosensors where
antibodiesinintimate contact withatransduceractas speciﬁc
capture elements for the cells (Perkins and Squirrell, 2000).
Highly sensitive amperometric immunosensors based on an-
tibody immobilization on membranes have been developed
forrapiddetection of bacteria (Shah etal., 2003). Membrane-
ﬁlter-based approaches can be limited by blockage problems
Corresponding author. Tel.: +44 29 20874287; fax: +44 29 20874305.
E-mail address: firstname.lastname@example.org (W.T. Coakley).
WD23 2BW, UK.
that rely on molecular capture of bacteria ﬂowing past a sen-
sor surface are limited to sampling that part of the suspension
(2004) described a system in which sample ﬂowed through a
membrane-free rectangular cross-section channel. An ultra-
sound transducer and an antibody-coated glass slide, acting
as an acoustic reﬂector, formed the longer sides of the rect-
angle. The depth of the channel (coupling layer to reﬂector
distance) was less than one half an acoustic wavelength, i.e.
less than 0.25 mm at the driving frequency of 3MHz. Forces
associated with the acoustic ﬁeld drove particles from the
ﬂowing suspension onto the reﬂector surface. Hawkes et al.
(2004) reported a 200-fold ultrasound-induced enhancement
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