‘Giant’ Nanoparticles May Offer Improved Performance

Michael A. Greenwood

Although nanocrystal quantum dots offer a host of advantages and continue to be explored for a range of imaging applications, these tiny optical probes do come with some baggage.

They are, for instance, prone to changes in their quantum yield, photobleaching and blinking, a phenomenon that is particularly irksome for investigators who are trying to monitor a single biomolecule.

A research team from Los Alamos National Laboratory in New Mexico led by Jennifer A. Hollingsworth is reporting that it has developed “giant” quantum dots that appear to overcome many of these issues.

This high-resolution transmission electron microscope image of CdSe/CdS giant nanocrystal quantum dots (g-NQD) has 19 layers (a). Recent blinking data for a CdSe/CdS sample showed further improved statistics: More than 40 percent of these nanocrystal quantum dots showed virtually no blinking (b). Courtesy of Jennifer A. Hollingsworth.

The quantum dots were fashioned by Yongfen Chen from CdSe cores measuring 3 to 4 nm in diameter. To this, Chen sequentially — and very slowly — added monolayers of inorganic shells consisting of CdS, ZnS or Cd_xZn_yS alloys, creating finished particles that measured between 15 and 20 nm. This multilayering approach created a thick inorganic shell that effectively isolated the wave function of the core from changes that might be experienced on the surface.

The investigators tested this nanocrystal against traditional quantum dots and found that its chemical stability was enhanced. Changes in ligand concentration and identity on the nanocrystal’s surface had no apparent effect on the core’s behavior, and quantum yield showed no noticeable change. Such changes on traditional quantum dots can have wide-ranging consequences.

In terms of photostability, the researchers found that their giant quantum dots remained stable under continuous laser illumination. Even after hours of irradiation, no photobleaching was observed. In the control samples, meanwhile, signs of photobleaching were evident even after very limited exposure.

Under continuous excitation, the giant quantum dots also exhibited a reduced incidence of blinking. The investigators found that more than 20 percent of them exhibited no blinking behavior and that 40 percent had an on-time fraction of >0.8. By comparison, 70 percent of the control nanocrystals had on-time fractions of <0.2.

The scientists said that the relatively large size of their nanocrystal quantum dots should not prevent their use with single-particle tracking and other applications. Work is under way, meanwhile, to further explore the relationship between the giant quantum dots’ structure and function.

Journal of the American Chemical Society, April 16, 2008, pp. 5026-5027.