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
a,1
, Rosemary J. Townsend
b
, Larisa A. Kuznetsova
a
, Kathryn A.J. Borthwick
a
,
Martyn Hill
b
, Martin B. McDonnell
c
, W. Terence Coakley
a,∗
a
School of Biosciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3TL, UK
b
School of Engineering Science, Electromechanical Research Group, University of Southampton, Southampton SO17 1BJ, UK
c
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
Abstract
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
4
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.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Ultrasound; Biosensor; Bacillus subtilis var. niger; BG spores; Immunoassay; Bio-terrorism
1. Introduction
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: coakley@cf.ac.uk (W.T. Coakley).
1
Presentaddress:SmithsDetection,ParkAvenue,Bushey, Watford,Herts
WD23 2BW, UK.
whenmonitoringenvironmentalsamples.Moreopensystems
that rely on molecular capture of bacteria flowing past a sen-
sor surface are limited to sampling that part of the suspension
flowingincloseproximitytothesensorsurface.Hawkesetal.
(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
0956-5663/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.bios.2005.01.013