biosensors capable of detecting unlabelled molecules have become
in life sciences as well as in drug discovery over the last 10 years.
et al., (2002) provided proof-of-principle for a biosensor, claimed to
unprecedented sensitivity for molecular detection that exploited shifts
optical resonances in a single transparent dielectric microparticle
a 300 mm diameter spherical glass bead) to detect adsorption of bovine
serum albumin protein deposition. The potential of the system for
type applications was demonstrated by detection of streptavidin binding
biotin-coated surface of a microsphere. The group went on to show
that microspheres as small as 7.2 mm
diameter could be effectively exploited for
protein detection and that the system was particularly sensitive to
adsorbing at particular locations on the microsphere surface (Arnold et
2003). The theoretical understanding of the sensitivity of the
system is also progressing (Teraoka et al., 2003). Double microsphere
were shown to allow for multiplexed DNA quantiﬁcation (Vollmer, Arnold
2003) Also micro-disc resonators using the same detection principle
predicted to be sensitive (Boyd et al., 2001) , and polystyrene
resonators were shown to have a high sensitivity as chemical (glucose)
adsorption sensors (Chao et al., 2003).
The sensitivity of adsorption bioassays can be improved by ultrasonic surface wave excitation (Wang et al., 1999). The effect is attributed to streaming induced at the transducer fluid interface. The streaming patterns at a sensing surface in a quarter wavelength bulk acoustic wave resonator have recently been characterised by particle image velocimetry techniques. Induction of small Rayleigh type vortices was observed at many points (Kuznetsova and Coakley, 2004). Three-fold enhancement of the immuno-capture of 200nm fluorescent latex particles (a size so small that acoustic streaming rather than radiation pressure would be solely responsible for increased adsorption) provides promising data (Kuznetsova et al., in preparation) on how molecular adsorption might also be enhanced by streaming.
Arnold et al. Optics
Letters 28,272 (2003), Fig.1:
Nanoscopic protein molecule at position ri on the surface of a
sphere near an eroded optical fiber core. The sphere and fiber
are surrounded by an aqueous solution.