Although label-free methods for detection
of biomolecular binding are attractive, they are limited
by complex and expensive instrumentation and by their limited
ability to perform high-throughput assays. Motivated by
these limitations of current label-free sensors, we are
developing a label free biosensor –which we call nanoSPR–
that exploits the change in color of immobilized noble metal
nanoparticles in response to the changes in the local refractive
index that accompanies receptor-ligand binding at the nanostructure-liquid
interface. The fabrication of this “chip-based’
nanoSPR sensor is simple and flexible and can be easily
implemented in an array format.
The nanoSPR chip consists of immobilized
gold nanoparticles onto glass slides, and the nanoparticles
are functionalized surface with biological “receptors”.
My group demonstrated for the first time, that nanoparticle
decorated surfaces enable receptor-ligand interactions to
be detected in real-time by the shift in the absorbance
spectrum of individual colloids in a conventional UV-vis
spectrometer [Anal
Chem 2002 (74) 504-509].
This assay is analogous to conventional surface plasmon
resonance (SPR) with the added advantage of being performed
in widely available, low-cost UV-visible spectrophotometers.
A US patent has been filed that covers this invention.
The primary advantage of this sensor is
its simplicity and flexibility at several different levels
in our implementation. First, gold nanoparticles are easily
prepared, and can be easily and reproducibly deposited on
glass (or other optically transparent substrate) by solution
self-assembly. Second, the spontaneous self-assembly of
alkanethiols on gold allows convenient fabrication of surfaces
with well-defined interfacial properties and reactive groups,
which allows the chemistry at the interface to be easily
tailored for a specific application of interest, an advantage
this sensor shares with conventional SPR on gold or silver
films. Third, this sensor enables label free detection of
biomolecular interactions. The biosensor can also be easily
multiplexed to enable high-throughput screening in an array-based
format for applications in genomics, proteomics and drug
discovery.
The design of biosensors using metal
nanoparticles is an exciting area of research, with many
opportunities to translate fundamental research findings
into new sensing technology. An important area of fundamental
research –and one that we believe has not received
sufficient attention is the development of theoretical models
that can enable in silico design of nanoparticles with the
direct goal of optimizing sensor response. At the simplest
level, such a theoretical model would allow input of nanoparticle
composition, size and shape into the computational model,
with outputs that are directly relevant to biosensing, such
as the sensing volume and sensitivity to the refractive
index of the surrounding medium. At an increased level of
sophistication that would allow reverse engineering of the
biosensor from first principles, such a model would provide
the necessary design parameters of nanoparticles that would
maximize the sensitivity and dynamic range for target-receptor
pairs of known sizes and binding constants. Our group has
launched a collaboration with Anne
Lazarides
(MEMS, Duke), Adam
Wax
(BME) and L.
Jay Guo
(EECS, Univ. Michigan) that
takes a combinatorial, high-throughput approach to this
problem by the fabrication of a matrix of different, discrete
nanostructures by e-beam lithography, measurement of their
individual SPR spectrum in a single snapshot on an imaging
spectrometer as a function of perturbation of the surrounding
medium, followed by theoretical modeling of the response.
These studies will develop a fundamental understanding of
how sensor performance is controlled by nanostructure composition,
shape, and size. Together, these fundamental studies will
provide the design rules for nanoparticle sensors optimized
for specific interactions hat are characterized by a known
binding constant, size of interaction partners and the likely
concentration that will be encountered in actual use.
Another area of research that is
of great importance to nanoparticle biosensors is new methods
to reproducibly synthesize anisotropic nanostructures. This
is an active area of research as seen by new methods for
synthesis of anisotropic nanostructures such as nanoprisms,
cubes, wires, and belts that have recently appeared in the
literature. However, the optimization of these synthetic
procedures to control the shape and size dispersity, scale
up of the synthesis, and long-term stability of the nanostructures–
issues that, to date, have received significantly less attention–
are an active area of research in my group to enable noble
metal nanostructures to make the transition from basic science
to technology.
