Triangular silver nanoparticles ( approximately 100 nm wide and 50 nm high) have remarkable
optical properties. In particular, the peak extinction wavelength, lambda(max) of
their localized surface plasmon resonance (LSPR) spectrum is unexpectedly sensitive
to nanoparticle size, shape, and local ( approximately 10-30 nm) external dielectric
environment. This sensitivity of the LSPR lambda(max) to the nanoenvironment has allowed
us to develop a new class of nanoscale affinity biosensors. The essential characteristics
and operational principles of these LSPR nanobiosensors will be illustrated using
the well-studied biotin-streptavidin system. Exposure of biotin-functionalized Ag
nanotriangles to 100 nM streptavidin (SA) caused a 27.0 nm red-shift in the LSPR lambda(max).
The LSPR lambda(max) shift, DeltaR/DeltaR(max), versus [SA] response curve was measured
over the concentration range 10(-)(15) M < [SA] < 10(-)(6) M. Comparison of the data
with the theoretical normalized response expected for 1:1 binding of a ligand to a
multivalent receptor with different sites but invariant affinities yielded approximate
values for the saturation response, DeltaR(max) = 26.5 nm, and the surface-confined
thermodynamic binding constant K(a,surf) = 10(11) M(-)(1). At present, the limit of
detection (LOD) for the LSPR nanobiosensor is found to be in the low-picomolar to
high-femtomolar region. A strategy to amplify the response of the LSPR nanobiosensor
using biotinylated Au colloids and thereby further improve the LOD is demonstrated.
Several control experiments were performed to define the LSPR nanobiosensor's response
to nonspecific binding as well as to demonstrate its response to the specific binding
of another protein. These include the following: (1) electrostatic binding of SA to
a nonbiotinylated surface, (2) nonspecific interactions of prebiotinylated SA to a
biotinylated surface, (3) nonspecific interactions of bovine serum albumin to a biotinylated
surface, and (4) specific binding of anti-biotin to a biotinylated surface. The LSPR
nanobiosensor provides a pathway to ultrasensitive biodetection experiments with extremely
simple, small, light, robust, low-cost instrumentation that will greatly facilitate
field-portable environmental or point-of-service medical diagnostic applications.