NANOSIZED OPTICAL BIOSENSORS
Silver nanoparticles serve as sensitive and inexpensive detectors
Tiny specs of silver could play a central role in a sensitive and inexpensive biological detection system, according to a study conducted at Northwestern University. The nanoparticle-based analytical procedure might be used to monitor ligand-receptor binding events, protein adsorption on self-assembled monolayers, and other types of analyte-surface interactions.
Based on a technique in which researchers measure changes in the extinction (absorption plus scattering) spectrum of microscopic metal particles, the new procedure provides a way to measure low concentrations of specific analyte molecules using simple UV-Vis spectroscopy instrumentation. Chemistry professor Richard P. Van Duyne and graduate student Amanda J. Haes demonstrated the method by detecting low picomolar quantities of biotin-streptavidin--a model ligand-receptor system [J. Am. Chem. Soc., published online Aug. 8, http://dx.doi.org/10.1021/ja020393x].
The detection principle is based on a variation of surface plasmon resonance (SPR) spectroscopy, Haes explains. In that method, centimeter-sized, thin gold films functionalized with receptor molecules serve as substrates for detection. Binding molecules to the surface causes a change in the system's index of refraction that, in turn, causes a shift in the SPR wavelength.
LOTS O' DOTS Prepared via simple lithographic methods, nanometer-sized silver triangular particles (pyramids) make sensitive biosensors.
In an analogous manner, the Northwestern group uses nanosized silver triangles--actually flat-topped pyramids--as substrates. By switching to the nanometer scale, the team has shown that the substrates' spectral properties are sensitive to the particles' size and shape. Those parameters are readily customized using a synthesis technique also developed by Van Duyne's group.
"The work is superb, in that it takes SPR methods to a whole new level," remarks Charles T. Campbell, a chemistry professor at the University of Washington, Seattle. Campbell adds that by using two-dimensional arrays of nanoparticles, the method's lateral resolution, surface sensitivity, and other properties are improved compared to traditional forms of SPR spectroscopy.
When light of arbitrary wavelength is shined on nanoparticles, most of the light is reflected. But at specific wavelengths, the particles absorb light. Van Duyne explains that in the case of the silver nanoparticles, absorption is due to a collective excitation of the roughly 10 million silver atoms in each particle.
Previous work in Van Duyne's laboratory, conducted by former graduate student Michelle Duval-Malinsky, showed that adsorbing a monolayer of long-chain alkanethiols on the metal nanoparticles caused an unexpectedly large shift in the peak of the SPR spectrum [J. Am. Chem. Soc., 123, 1471 (2001)].
"The conventional wisdom in the field was that there would be a resonance shift--but a tiny one," Van Duyne asserts. The 30- to 40-nm shift observed by his group came as quite a surprise, he says. And the shift now lies at the heart of a new detection method.
"When you get something very different than what you expect, that's when science becomes fun."