The magnetic nanoparticles
contain recognition agents for a target molecule. If the target is present,
it will adhere to the magnetic nanoparticle. The nanoparticle probes with DNA
strands (the bio-barcode strands) are added to the solution and form a sandwich
complex with the target and the magnetic microparticle probes. A magnet is
used to pull the particles to the side of the test tube and hold the captured
target. The remainder of the sample is removed and the barcodes are then released
by heating. For every target recognition event,
1000s of barcodes are released, thereby amplifying the signal.
Gold nanoparticles can catalyze the conversion of silver salts into silver
metal, which when combined with standard photographic processing techniques
yields an assay that is able
to detect extremely small concentrations of the target. For example, Bacillus
anthracis (anthrax) target DNA sequences were detected as low as 500 zeptomolar
(zepto = 10-21).
