Cal State investigator Feimeng Zhou, Ph.D., led the team that developed this method, which relies on the optical detection system known as surface plasmon resonance (SPR) to achieve such low levels of detection. While surface plasmon resonance is used commonly by analytical chemists, its sensitivity has not rivaled that of techniques that use fluorescent or radioactive probes to detect specific gene sequences.
The new assay uses a flow cell coated first with a thin layer of gold and then with a layer of dextran, which are attached to oligonucleotides, whose sequence is complementary to one end of the target DNA sequence. The dextran layer proved essential for reducing non-specific binding of DNA to the underlying gold layer, an event that has plagued other attempts to create such a device.
To find the target gene, a DNA sample is flowed through the cell and any target DNA binds to the dextran-bound capture probe. Next, gold nanoparticles carrying a second oligonucleotide, this one complementary to the other end of the target DNA, are added to the flow cell. The result is a sandwich in which the dextran-attached capture probe and the nanoparticle-bound "detection probe" form the outer layers of the sandwich and the target DNA makes up the inside layer of the sandwich.
The electronic interaction between the thin gold layer in the flow cell and the gold nanoparticles creates a powerful surface plasmon resonance signal that is detected easily using standard optical equipment. Tests with the device showed that it could identify accurately target DNA at concentrations as low as 1.38 femtomolar, some 39 times lower than previously obtained using surface plasmon resonance technology and comparable to other high-sensitivity assay techniques.
To test the clinical utility of this method, the investigators used it to detect mutations in the p53 gene. Mutations in p53 are present in about half of all malignant cells, and these experiments showed that this technology was capable of detecting p53 sequences at concentrations of 100 femtomolar and 300 femtomolar, well within a clinically useful range.