Tracking and viewing molecular interactions inside a cell with great detail is invaluable for understanding how organisms operate and to the future of medicine. Quantum dots (QDs), semiconductor crystals on the nanoscale with confined electron excitations, have almost all the right properties for this task. They have high absorption constants, are small enough to sneak inside cells, and can be tailored to release at different wavelengths. Additionally, they can carry therapeutic proteins on their coating. However, they are toxic. The current leading QDs are composed of hazardous elements like cadmium, selenium, and tellurium. Recently, advances have made more biocompatible crystals out of safer materials, such as zinc, silver, and indium. Yet, these crystals have long reaction times and are difficult to customize and produce en masse.
Researchers at Rutgers University have developed a method to generate an entire library of safe QDs quickly and efficiently. They used the molecular structure ZnS-AgInS2 (ZAIS), a version of the previous non-toxic crystals. They placed a powdery bulk chemical precursor into a vial and blasted it with 20 kHz of ultrasound for five minutes. The sound waves broke up the powder into QDs with a uniform size of about 12nm. The crystals also had a property unique among other types of dots. Instead of size controlling the color of emission, the ratio of elements in the compound did. The more zinc or silver added to the precursor, the more blue-shifted the resulting QDs were. By tuning the stoichiometry, the researchers synthesized QD samples across the entire visible spectrum.
They then tested to see how the dots impacted a biological environment. The ZAIS dots were compared against the standard and toxic cadmium selenide (CdSe) QDs. Both were placed with brain tumor cell, marrow stem cell, and mouse fibroblast samples to see how each fared in their presence. In all trials, the ZAIS QDs had negligible toxic effects, even when in high concentration or oxidized by four hours of UV exposure. In contrast, the CdSe QDs failed the tests miserably, practically killing off half the sample at high concentration. Moreover, the ZAIS dots were extremely stable, lasting for two months in storage without losing any of their photoluminescence.
Lastly, the researchers tested how well these QDs could carry out a multifunctional purpose. They mutated a sample of the tumor neurons to create a fluorescent protein that made them glow green. They then attached the silencing RNA that targets and destroys the protein-creating gene to the QD surface. The researchers watched the dots enter the cells, as they were easy to track due to their glow. After three days, about 80% of the green fluorescence disappeared.
These results open up the possibility of safely using QDs in humans. Furthermore, different colored crystals can simultaneously carry out an array of therapeutic and imaging functions, depending on their surface polymers. The researchers believe that their ultrasound technique can be used to rapidly create and characterize the toxicity of other nanoparticles as well.