Semiconductor nanocrystal toxicity

Semiconductor nanocrystals are being used as fluorescent biological labels. It is likely that sensors based on nanotechnology will revolutionize health care, climate control and detection of toxic substances. It is quite possible that we will have nanochips to carry out complete chemical analysis. Such nano-total analysis systems will have to employ new approaches to valves, pipes, pumps, separations and detection. 

Recent studies revealed that cadmium-based semiconductor nanocrystals did not affect the biological functions if they were completely coated with organic ligands [58, 59]. After the ligands were detached, the nanocrystals became extremely toxic [59]. The detachment of the ligands will not only destabilize the colloidal system but also cause possible cytotoxic problems [56]. 

A dielectric oxide layer such as silica is useful as shell material because of the stability it lends to the core and its optical transparency. The thickness and porosity of the shell are readily controlled. A dense shell also permits encapsulation of toxic luminescent semiconductor nanoparticles. The classic methods of Stober and Her for solution deposition of silica are adaptable for coating of nanocrystals with silica shells [864,865]. These methods rely on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticles including hematite and magnetite [866-870]. The procedures are mostly adaptations of the Stober process. Oxide particles such as boehmite can also be coated with silica [871]. Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparticles is that due to Mulvaney and coworkers [872—875]. In this method, the smface of the nanoparticles is functionalized with aminopropyltrimethylsilane, a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Changes in the optical properties of metal nanoparticles with silica shells of different thicknesses were studied systematically [873 75]. This procedure was also extended to coat CdS and other luminescent semiconductor nanocrystals [542,876-879]. 

In 2002, Rosenthal and co-workers reported using semiconductor nanocrystals as visual aides for introducing the particle-in-a-box concept to juniors and seniors in physical chemistry (P). However, at that time, the best chemical procedure for producing large quantities of high quahty quantum dots was based on the work of Murray 10, 11). In this method, dimethyl cadmium, an extremely toxic and pyrophoric substance, was reacted at high temperatures under an inert atmosphere. The synthesis was expensive, difficult to control, and could potentially become explosive, certainly not a method suitable for an introductory laboratory with novice laboratoiy students. 

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