The optical diameter of gold naoparticles is several order of magnitude larger than its physical diameter. A Gold nanoparticles of few tens of nanometer diameter can be easily seen under a regular microscope using dark field condenser because of intense scattering due to surface plasmon resonance of these nanoparticles.
Dark field image of 40nm gold nanospheres on the glass substrate.
Inset: magnified diffraction-limited image of a single nanosphere. Scales correspond to the object plane
If another molecule with absorption band matching the resonance frequency of gold nanoparticle is brought into close proximity to nanoparticle, energy transfer happens quenching the scattering and making the nanoparticle disappear in dark field microscopy. The process called Plasmon resonance energy transfer (PRET) has been exploited for the first time to design a metal ion sensor with sensitivity much higher than conventional organic reporter based methods. To design a PRET sensor for Cu2+, gold nanoparticles of 50nm are functionalized with ethylenediamine ligand. In solution, two equivalent of ethylenediamine complex with Cu2+ to form a blue-purple solution with absorption band of 547nm that overlaps with scattering wavelength of 50nm gold nanoparticle. In the presence of Cu2+, gold nanoparticle functionalized with ethylenediamine complexes with Cu2+ resulting in PRET between absorbance band of metal complex and scattering spectra of gold nanoparticle. Under a dark field microscope, the scattering intensity decreases in proportion to amount of Cu2+ can be easily seen and quantitated using a CCD camera.
Sensitivity of the PRET sensor is 1nM for Cu2+ and the sensor is specific since other biologically and environmentally relevant metal ions like Mg2+, Ca2+, Na+, and K+ even at concentration of 100 µM don’t result in any PRET.
PRET sensor adds to a growing list of gold nanoparticle based label-free sensors!