The light scattering and absorption properties of gold nanoparticles (GNPs) can be utilised for the detection of DNA.
Binding of molecules to the GNP influences the local refractive index. The increase in refractive index can be measured
as proportional red-shift of the GNPs extinction maximum; therefore GNPs are suitable for use as nanoparticle chemical
sensors. Utilizing this method it is possible to detect DNA in naturally occurring quantities.
In bulk measurements we have shown a red-shift of 7 nm of the absorption maximum (λ<sub>max</sub>) upon binding of thiolated
ssDNA. Subsequently, we were able to follow the interaction between two sets of GNPs functionalised with
Randomly immobilised GNPs were visualised with an inverted darkfield microscope. The use of a colour camera enables
us to analyse the colour change of each individual particle in the field of view. A change of λ<sub>max</sub> of 1 nm can be detected
by the colour camera, which corresponds to ~100 20mer ssDNA molecules. For the detection of a single DNA binding
events we are developing an assay for DNA detection, utilizing a second set of GNPs. The interaction of two GNPs
within a range of 2.5 times the radius of each other results in a shift of ~7 nm in λ<sub>max</sub> for the presence of one DNA strand.
This increased shift makes the method not only more accurate but also easier to detect.
Metal nanoparticles posses the property of changing their optical properties as a function of both internal
characteristics (size, shape, dielectric function) and refractive index of the local environment. A special class of
applications in the field of biosensing uses the dependency of the nanoparticle's plasmonic peak localization on the local
refractive index change. The response of this type of sensors is usually monitored by the change of the extinction
spectrum of an ensemble of nanoparticles where analytes interact with functionalized nanoparticles in solution or
immobilized at an interface; detection is done with a spectrophotometer. This type of sensors has a limited sensitivity.
This can be overcome by using single nanoparticle based biosensors. This type of sensors measures the changes of the
scatter spectrum of a collection of individually addressable functionalized nanoparticles in the presence of analytes.
Here we report on a new detection method of binding events of analytes to functionalized gold nanoparticle
using a standard colour camera attached to a darkfield microscopy setup. This setup is capable of parallel detection of
the spectral shifts of thousands of 60 nm antibody-functionalized gold spheres as a result of binding events of protein
analyte molecules. This setup can be the basis for multiplexing and quantification.