THE PROCESS OF MAKING CORE NANOCRYSTALS

The first nanocrystals I worked with were the CdSe NCs. Cadmium is a member of the elements found in the 12th column, group II B, of the Periodic Table of Elements. Selenium is an element located in the 16th column, group VI A, of the table. (Like sulfur, tellurium, oxygen, and polonium, selenium is called a chalcogen.) Accordingly, the designation of II-VI is applied to semiconductor chalcogenides made up of corresponding elements in the two columns. Later, I will report on work I have done with other II-VI members (i.e., CdS, CdTe, HgS) as well as NCs designated as III-V (e.g., InP), IV-VI (e.g., PbS), etc.

The fundamental process employed for producing NCs is based on an initial, rapid crystal nucleation phase followed by a slower growth phase with controlled Ostwald ripening. This ripening is an example of the rich getting richer. The larger crystals will grow at the expense of the smaller ones. (Examples of this may be seen in the section on Microcrystals at this web site.) When crystal growth begins, clusters of crystals may form with some larger ones occurring by chance. Those larger crystals will grow, preferentially. In Ostwald ripening, smaller crystals that form will quickly redissolve, donating their molecules to the larger crystals. The rates of crystal formation and growth are just two of the many factors that need to be controlled to make a reliably consistent product. With the high temperatures involved in the chemistry, up to 360°C in some cases, 60 seconds one way or the other can radically alter the size and, accordingly, the fluorescent emission color of that product.

Previously, I mentioned a growing movement in the area of nanocrystal research (and other areas too) to develop greener chemistry to help reduce the toxic outflow from laboratories. To that end, several research groups have developed an NC production technique that uses environmentally friendly fatty acids to moderate the growth rate of the crystals. Oleic acid is one such fatty acid that, together with a relatively inert, non-coordinating solvent along with a suitable cadmium compound, is capable of producing high quality core NCs. The combination offers a controllable growth rate and good monodisperse properties with minimal size variations. By varying the solvents used in producing the selenium precursor, it is possible to eliminate the need for an inert atmosphere for many experiments. Also, by varying the fatty acid used, growth rate can be slowed or hastened, and ultimate NC size can be more easily controlled.

The NCs I first produced were core nanocrystals with a fatty acid capping the surface. Core-shell NCs, on the other hand, require calculated quantities of monolayers of other inorganic II-VI materials (e.g., ZnS) with a wider band gap than the core NC but, preferably, with somewhat similar crystal lattices (near-epitaxial growth). An outer shell of the desired semiconductor is formed around an inner core of, say, CdSe. This can be done by exposing previously prepared core NCs to the precursors of the shell and constructing the shell in place in the appropriate solvent(s) while maintaining the right conditions (temperature, pH, etc.). It is believed that the external shell serves to prevent the electrons and holes from wandering around and falling into energy traps on the surface of the core. These energy traps greatly reduce the emitted fluorescence by allowing the charge carriers to follow a path of radiationless recombination. It seems likely that traps form because of imperfections in the (nano)crystal lattice grown in solution. There are many benefits of the inorganic outer shell approach, including a much greater photostability over time and vastly improved quantum yields for the NCs. Core-shell NCs will be explored in future experiments.

Controlling the surface chemistry of the core (or core-shell, for that matter) nanocrystal is key in determining the behavior and usability of the product. The first NCs described here are dispersible (they are colloidal particles, not molecules, so they don't really dissolve) only in nonpolar solvents, such as toluene and chloroform, forming clear liquids. For researchers interested in staining cellular components, NCs will first need to have their surface chemistry altered to render them soluble in water. Next, by conjugating the NCs to suitable biomolecules, researchers can target specific cellular components for study. One of the most intriguing things about working with nanocrystals is the ability to exercise control over the surface behavior of particles no larger than protein molecules.

WARNING: Green chemistry or not, cadmium, selenium, tellurium, and mercury ions are highly toxic. Proper facilities (ventilation hood, glove box, particle masks, etc.) are mandatory. Virtually all the required solvents are hazardous and some are suspected carcinogens. The temperatures required for much of the work are quite high and very dangerous. This is especially true for liquids that tend to splash and splatter. Also, the nanocrystals produced are extremely minute and may be able to pass through the blood-brain barrier. Therefore, the NC material should never (read, NOT EVER) be handled in powder form, unless special particle masks are used. Alternatively, there are several companies that sell the finished products. These prepared NCs can be purchased capped with suitable bioconjugated molecules.

Having just designed and built a special microscope optimized to work with nanocrystals (refer to separate LED Fluorescence section for details), I am now resuming work with nanocrystal synthesis. My next series of steps will entail producing Zn chalcogenide shells to cap and protect the core NCs. Ways to conjugate the core-shell NCs to peptides and antibodies will then have to be adopted. Finally, the bioconjugated NCs will be used to fluorescently label individual cells and organisms for study and photography.