Gold nanoparticle-based colorimetric assay for the direct detection of cancerous cells

Early detection of tumors is one of the Holy Grails of cancer research, an achievement that would greatly improve cancer therapy and prognosis. Two new reports describe different but promising approaches to solving this problem.

At the University of Florida, Weihong Tan, Ph.D., and colleagues used gold nanoparticles linked to aptamers, which are short, synthetic molecules of deoxyribonucleic acid (DNA) that bind to specific targets much like antibodies. In work published last year, Dr. Tan’s group described the methods it developed to create aptamers that bind specifically to cancer cells. The current paper, published in the journal Analytical Chemistry, shows how the combination of these aptamers with gold nanoparticles produces a diagnostic optical signal when they cover targeted cancer cells.

Gold nanoparticles are efficient optical beacons whose light-absorbing properties depend strongly on the size of the nanoparticle. In this work, the investigators used gold nanoparticles that show peak absorption of light with a wavelength of about 500 nanometers when linked to an aptamer. However, when these same aptamer-conjugated nanoparticles bind in large numbers to a targeted cell, their absorption spectrum changes dramatically, producing a visible shift in color from green to red. Although this change is visible to the human eye, the researchers used a microplate spectrophotometer to increase the sensitivity of the assay to a lower limit of 90 malignant cells.

Taking a different approach, Gary Maki, Ph.D., and collaborators at the University of Idaho have developed a nanowire transistor capable of detecting very low levels of DNA methylation. DNA methylation plays a critical role in silencing tumor suppressor genes and thus could serve as a very early indicator of tumor development.

Conventional methods of detecting DNA methylation are complex and time-consuming, two limitations that Dr. Maki and his team set out to address. The heart of their device is a nanowire transistor, formed using standard electron-beam photolithography, that is coated with a antibody that binds to methylated cytosine, one of the four bases of DNA. When DNA containing methylated cystein passes over the nanowires, the antibodies bind the DNA, generating a measurable electrical signal.

To isolate the target DNA (in this case, the promoter region of a tumor suppressor gene known as p16INK), the investigators use magnetic beads connected via a breakable linker to the complementary DNA sequence. The beads are added to a mixture of genes—imagine all the DNA extracted from a biopsy sample—and the target gene is extracted by applying a magnetic field and washing away all DNA that does not bind to the magnetic beads. Then, the captured DNA is released from the beads by severing the breakable linker, and the resulting solution is applied to the nanowire sensor. Devices with 28- to 80-nanometer-long nanowires were capable of detecting as few as 25,000 molecules of methylated DNA without any false-positives. This level of sensitivity is sufficient to eliminate the need to use polymerase chain reaction amplification to detect trace levels of methylated DNA.

Dr. Tan and colleagues’ work is described in the paper “Gold nanoparticle-based colorimetric assay for the direct detection of cancerous cells.” This work was funded in part by the NCI. An abstract of this paper is available through PubMed.

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The research from Dr. Maki and colleagues appears in the paper “Nanowire-transistor based ultra-sensitive DNA methylation detection.” An abstract of this paper is available through PubMed.

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http://nano.cancer.gov