Scientists at the Sanford Burnham Prebys Medical Discovery Institute (SBP) in San Diego, California have devised a way to optically image tumors with unprecedented clarity using quantum dots. These nano structures are tiny particles, only a few nanometers wide, that generate light of a specific wavelength when they’re themselves stimulated by a light beam. On their own quantum dots are quite bright, but their signal gets washed out by other nearby quantum dots. To clean the signal and be able to see tumors better, the SBP team used a so-called “etchant”.
The technique works by injecting quantum dots into the blood stream to circulate through the body, some of which penetrate cancer cells. While the quantum dots inside the cells would be easy to spot, the quantum dots left circulating in the blood stream are also visible essentially as noise. By injecting the etchant into the blood stream that turns off the fluorescence of quantum dots, the free floating quantum dots go dark while the ones inside of cells continue to be seen.
Some details according to SBP:
The etchant and the QDs undergo a “cation exchange” that occurs when zinc in the QDs is swapped for silver in the etchant. Silver-containing QDs [quantum dots] lose their fluorescent capabilities, and because the etchant can’t cross membranes to reach tumor cells, the QDs that have reached the tumor remain fluorescent. Thus, the entire process eliminates background fluorescence while preserving tumor-specific signals.
The method was developed using mice harboring human breast, prostate and gastric tumors. QDs were actively delivered to tumors using iRGD, a tumor penetrating peptide that activates a transport pathway that drives the peptide along with bystander molecules—in this case fluorescent QDs—into cancer cells.
Here’s an SBP video with one of the lead researchers of the study:
Study in Nature Communications: In vivo cation exchange in quantum dots for tumor-specific imaging…