The challenge of creating visible and nontoxic nanomaterials for sensing

(Nanowerk Spotlight) Nanotechnologies opened a new door towards the development of novel techniques and devices for probing biological systems such as biomolecules and single cells. The most reliable bioprobes today rely on fluorescent or radioactive labeling.
Phosphorescent emitters are preferable for use in sensing or biological labeling schemes because the emission occurs over a very long timescale (for nanoscientists, 'very long timescale' is a relative term; here we are talking about 1 microsecond).
Especially semiconductor nanocrystals (quantum dots) possess several properties that make them very attractive for fluorescent tagging: broad excitation spectrum, narrow emission spectrum, precise tunability of their emission peak, longer fluorescence lifetime than organic fluorophores and negligible photobleaching.
Scientists have discovered that these nanocrystals can enable researchers to study cell processes at the level of a single molecule and may significantly improve the diagnosis and treatment of diseases such as cancers. However, the band gap of most emissive semiconductors, with the exception of cadmium-containing materials, is either too high or too low to easily make visible emitting quantum dots. Unfortunately, cadmium is quite toxic and therefore not really suitable for medical applications.
In a step towards circumventing the issues with cadmium toxicity, researchers have make progress in demonstrating visible phosphorescence from doped nanocrystal systems. A recent example is the synthesis of a nanoscopic material composed of the non-toxic elements zinc, selenium, sulfur and manganese which displays efficient visible emission.
It has been demonstrated before that nanocrystal/organic dye combinations may be used as ratiometric, fluorescent sensors ("A Ratiometric CdSe/ZnS Nanocrystal pH Sensor"). For example, pH sensing is achieved by measuring the ratio of nanocrystal emission versus organic dye emission from a conjugated nanocrystal-dye complex, which is self-calibrating and thus very preferable. However, this sensing scheme requires the measurement of the emission spectrum which most commercially available microscopes cannot do.
"With our new nanoscopic material – synthesized from zinc, selenium, sulfur and manganese and displaying efficient visible emission – researchers may use the lifetime of the phosphorescent emission as the observable that is related to, say, the pH of a solution" Dr. Preston T. Snee tells Nanowerk. "The lifetime of a phosphor is a much more simple measurement to make using a microscope."
Snee, an Assistant Professor in Physical and Inorganic Chemistry at the University of Illinois at Chicago, explains that the phosphorescence arises from excitation of the core ZnSe nanocrystal which transfers energy to doped Mn(2+) ions in the ZnS shell of the material.
"Several researchers have shown emission from doped elements in the core and shell of nanoparticles, however our material system is completely non-toxic and is very easy to synthesize and functionalize with organic molecules" he says. "We have also shown that these materials may be solubilized in water with minimal loss of emission intensity."
Snee and his group have also tethered organic dyes to the surface of the doped nanoparticle which allows them to transfer energy from the core ZnSe to the Mn2+ doped shell to the surface bound organic dyes. In the future, such complex control of energy transfer may allow researchers to synthesize nanoscopic sensing materials where the transfer of energy is enhanced or reduced by the presence of targeted analytes.
Nanocrystals under room and UV light
ZnSe/ZnMnS/ZnS nanocrystals solubilized in water under ambient light and under UV irradiation (Image: Dr. Snee, University of Illinois at Chicago).
While core and shell doped nanocrystals have been previously reported, the specific material system developed by Snee's group is unique and preferable because of the non-toxic nature of the composition. Furthermore, this is the first demonstration that a phosphorescent chromophore embedded in a nanocrystal can function as energy donors to surface bound organic dyes.
In their recent paper in Nano Letters ("Efficient Emission from Core/(Doped) Shell Nanoparticles: Applications for Chemical Sensing") Snee and his team describe how their development of efficient phosphorescent nanoscopic materials from core/doped shell nanocrystals relied on several previous observations:
"First, small semiconductor clusters are more resistant to impurity doping than larger nanocrystals, which is why we examined doping of the shell of a core/shell quantum dot.
"Further, it is known that the binding, and thus residence time, of guest impurities on a growing nanocrystal surface has a large effect on the ability to envelop that impurity within a nanocrystal framework. As it has been predicted from modeling that the manganese has a greater interaction with a zinc sulfide surface versus zinc selenide, doping a ZnS shell should result in better incorporation of Mn2+ into the nanocrystal system.
"Impurity emitters are also known to be more efficient if they are diluted within the outer edges of a nanocrystal (or located in shell of a core/ shell material)."
"Our method combines all of these effects in the approach to impregnate impurity phosphors within the shell of a strongly binding material" says Snee. He points out that their process is robust and insensitive toward variations in the reaction conditions.
This new class of nontoxic emissive nanocrystals is very promising for biological applications and is a strong candidate for future medical devices and materials.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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