The Polymerization Process Research Group of the Polymat Institute of the UPV/EHU–University of the Basque Country has efficiently encapsulated semiconductor nanocrystals or quantum dots of various sizes into polymer particles. Great stability in terms of their optical properties and good fluorescence control when combining different quantum dots have been achieved. The possible applicability of these materials as sensors of volatile organic compounds has also been explored.

Microscopic image of quantum dots (dark points) encapsulated in polymer particles (Credit: Dr. Mariano Barrado/UPV/EHU]

Nanotechnology and nanoscience are disciplines in which minute molecular structures with special physical and chemical properties are designed, manufactured and studied. One of the types of particles that are studied in these disciplines are quantum dots; they are semiconductor nanocrystals, which ranges between 2 and 10 nm and which have excellent optical and electronic properties. Worthy of mention is the fact that they emit light in different colors depending on their size. In other words, “the emission wavelength is varied just by varying the size of the nanocrystal, without modifying its composition,” explains Alicia de San Luis, a Polymat researcher and author of a paper on this work.

The properties of quantum dots render them potentially useful for a range of applications, including detection in biomedicine. Yet, “their drawbacks also need to be taken into consideration: they are difficult to handle owing to their small size, and are toxic, given that the quantum dots of higher quality mostly consist of heavy metals,” she says.

To get the most out of the excellent optical properties of these nanoparticles, while not forgetting the toxicity problems they have, at the UPV/EHU's Polymat Institute of Research, they have managed to efficiently encapsulate commercial quantum dots into polymer particles dispersed in water while maintaining the fluorescence of the quantum dots over long periods of time. “The main aim was to encapsulate the quantum dots into slightly larger polymer particles to protect them and, at the same time, be able to handle them without them losing their properties,” as the author of the research pointed out. “We have implemented a simple method yielding good results: polymer particles with fluorescence stable over a minimum of nine months,” she adds.

Different Combinations and Applications

Having achieved the first aim, “the second step was to encapsulate combinations of quantum dots of varying sizes to create a bar code that could be used for multiple detection in biological systems,” she explains. That way they managed to control the fluorescence of these combinations, since by using quantum dots that emit at different wavelengths, “their signals can be detected simultaneously without one being superimposed on another one.” This could be useful for biomedical detection because there is a possibility of modifying the surface of the polymer particle with different analytes (or different antibodies or antigens). In the researcher's view, “it is a pretty powerful, straightforward, fast detection technique. Most labs have a fluorometer and, what is more, one would not have to wait several days to process the sample.”

They also explored the combining of quantum dots with other inorganic nanoparticles (CeO2) by co-encapsulating them into the same polymer particles. In this study, they were able to see an increase in the emission of fluorescence during the time they were exposed to sunlight.

Finally, they tackled the possible applicability of a range of synthesized combinations, such as optical and electrical sensors of volatile organic compounds (VOCs) by producing nanofibers and subsequently putting them in contact with VOCs. This part of the research is being carried out in collaboration with Tecnalia. “In this case we are working on fluorescence as well as on conductivity measurements of the nanofibers,” says de San Luis.

This research was conducted by Alicia de San Luis-González as part of her PhD thesis at the UPV/EHU's Polymat Institute of Research. The thesis, entitled “Nano-structured polymeric aqueous dispersions containing quantum dots,” was supervised by Jose Ramon Leiza, professor of the UPV/EHU's Faculty of Chemistry and director of Polymat, and by María Paulis, tenured lecturer at the UPV/EHU's Faculty of Chemistry. The final part of the work is being conducted in collaboration with Tecnalia.

For more information, visit here.