NIR CdSeTe

A near-infrared-emitting alloy of cadmium, selenium, and tellurium, has been used to make a novel type of quantum dot that is of high-quality, water-soluble, and biocompatible. The quantum dots might have application in the analysis and study of biological systems.

Guo-Xi Liang, Miao-Miao Gu, Jian-Rong Zhang, and Jun-Jie Zhu of the Key Laboratory of Analytical Chemistry for Life Science (MOE), at Nanjing University, People's Republic of China, explain that fluorescent semiconductor nanocrystals or quantum dots are attracting much interest from diverse research communities. The reason for this is that quantum dots have unique but tuneable optical properties, such as brightness and narrow and symmetrical emission spectra. This means they are being investigated keenly as agents for ultrasensitive chemical analysis and cellular imaging.

Quantum dots that emit in the near-infrared (NIR) range of 650 to 900 nanometres are particularly interesting for biologists because at these wavelengths auto-fluorescence from the biological entities being studies is minimised; which means far less interference than with other analytical techniques. Unfortunately, high-quality NIR quantum dots have remained elusive at the far-end of NIR despite pioneering efforts devoted to the preparation of near-NIR quantum dots prepared using multi-step organometallic or aqueous approaches.

Other researchers have turned to alloyed materials, such as cadmium-selenium-tellurium based materials. They have had some success in creating quantum dots with NIR emission at the 900 nanometre extreme, however their preparative method was not amenable to biological studies.

Now, Zhu and colleagues have used water-loving, hydrophilic, capping agents to make CdSeTe alloyed quantum dots (AQdots) more water soluble and so open up their use in biology without resorting to noxious solvents, such as mercaptopropionic acid, in their preparation.

The Nanjing team recently reported a novel approach in the journal Nanotechnology, that produces high-quality NIR-emitting CdSeTe quantum dots, under aqueous conditions by using the amino acid L-cysteine as the stabilizing agent Their approach involves a relatively simple one-pot refluxing preparation, as opposed to complex multi-step approach.

They carried out their synthesis in oxygen-free water under nitrogen. The CdSeTe quantum dots were prepared by incorporating selenium ions into CdTe nanocrystals with a small amount of L-cysteine dissolved in the reaction mixture and pH adjusted to 11.5 under reflux at 95 Celsius. The as-prepared CdSeTe quantum dots were precipitated and washed with 2-propanol solvent and dried overnight at room temperature in a vacuum. The final product in was then re-suspended in water. They used UV-Vis spectroscopy and X-ray diffraction to verify the structure of the quantum dots.

The team explains that, "by changing the growth time and/or composition, the fluorescence emission of the quantum dots could be tuned from the visible to the NIR region." These quantum dots are highly water soluble, are stable, and crucially give high quantum yields for cellular imaging.

Using a fixed concentration of CdSeTe solution, the team successfully tested their viability in detecting different quantities of glucose or cholesterol in the presence of hydrogen peroxide. Fluorescence intensity of the solution was recorded at 750 nm with an excitation wavelength of 300 nm, they explain.

"The fluorescence of the quantum dots was sensitive to hydrogen peroxide, which was used for the determination of glucose and cholesterol," the team explains, "results that show that the CdSeTe quantum dots have great potential applications for biological sensing and imaging."