Todd Krauss Shines as Brightly as his Quantum Dots
Associate Professor Todd Krauss is one of several young investigators in the Department of Chemistry. Currently supervising twelve graduate students, three postdoctoral research fellows, and two undergraduates, Krauss' research spans many areas:
- Fundamental studies of the optical and electronic properties of carbon nanotubes and semiconductor quantum dots
- Biomedical and renewable energy applications based on carbon nanotubes and semiconductor quantum dots
- Underlying forces responsible for protein folding
A carbon nanotube is one sheet of graphene rolled into a very tiny cylinder. While the nanotube may be only one nanometer in diameter, it can extend up to one centimeter in length.
Depending on how they're made, some nanotubes are semiconductors that fluoresce. Krauss and his group are trying to understand basic physical properties of carbon nanotubes by studying their fluorescence one nanotube at a time. His first step is to dilute a solution containing nanotubes, and then pour the solution onto a rapidly spinning surface. This tends to isolate individual nanotubes, which hit the surface with a coverage of one nanotube per square micron. When a laser hits them, these single pieces of carbon semiconductor photo luminesce in the infrared.
Krauss' group was the first to discover that individual single-walled carbon nanotubes exhibit robust and unwavering fluorescence in the near-infrared (see "Simultaneous Fluorescence and Raman Scattering from Individual Single-Walled Carbon Nanotubes," Science 2003, 301, 1354). In June 2007, Krauss and PhD student Lisa J. Carlson reported their findings in "Photophysics of Individual Single Walled Carbon Nanotubes," Acc. Chem. Res 2008, 41, 235-243.
Krauss also studies macroscopic samples (i.e. many nanotubes) with very short pulses to try and understand how they behave. As he explains, "The rapid photophysics of these nanotubes can be very fast, and you miss critical events if you look at the molecules over a long time scale."
Because few molecules fluoresce in the infrared, nanotubes can be used in a variety of future applications spanning biology to photonics. For example, nanotubes can be used to deliver drugs and by following the fluorescence of nanotubes inside living organisms, it can be confirmed that the drugs arrived at the correct target locations. The surfaces of cells or the insides of cells can also be fluorescently tagged with nanotubes, which seem to enter cells fairly easily.
Nanotubes are not perfect, as they fluoresce weakly, but their ability to fluoresce lasts a long time compared to traditional fluorophores such as dye molecules. Professor Krauss postulates that stronger fluorescence requires pristine carbon-carbon nanotubes with no defects. "There's great conduction and symmetry along the nanotube," he explains. "A chemical defect on the tube means that the perfect carbon-carbon symmetry is broken at that point. If you then excite an electron in the nanotube, it gets stuck where the symmetry breaking occurs and it doesn't fluoresce."
Originally, the best nanotubes had a fluorescence efficiency of approximately 1%, meaning that, for every one hundred photons absorbed by the nanotube, only one was reemitted. Typical nanotubes fluoresced even more weakly than this. By contrast, a dye molecule has an efficiency of about 50%. Recently, Krauss and his team boosted the nanotube fluorescence efficiency by over an order of magnitude to over 20% percent, and this work will be published soon.
Semiconductor Quantum Dots
Professor Krauss and his team are also looking into the photophysics of semiconductor quantum dots, also called nanoparticles or nanocrystals. They've published several papers in this field, including “Ultrabright PbSe Magic-Sized Clusters” (Nano Lett. 2008, 8, 2896-2899) and “Uncovering Forbidden Optical Transitions in PbSe Nanocrystals” (Nano Lett. 2007, 7, 3827-3831). A recently published paper pushes the field yet further by describing "non-blinking" semiconductor nanocrystals. “Non-blinking Semiconductor Nanocrystals,” Nature, 2009, 459, 686-689. See the University of Rochester Press Release here.
"Basically," explains Krauss, "we ask similar questions about semiconductor quantum dots that we ask about nanotubes: What is their fluorescence efficiency? What does the fluorescence spectrum of a single particle look like? And what does that tell us about the nanoparticle?"
In the October 2007 paper mentioned above, Krauss and his team looked at the actual electron states of the PbSe nanoparticle. What they discovered was a surprise: the optically allowed transitions of PbSe nanocrystals occur at energies that should be formally forbidden based on quantum mechanics.
Says Krauss, "We don't yet know why this is happening. All of our theories were ultimately proved not to be responsible. But we've experimentally seen it happen. One hypothesis is that the current understanding of the states of semiconductor quantum dots is entirely wrong."
While continuing to explore fundamental physics, the group is also creating new nanoparticles that haven't been discovered before. In the August 2008 paper mentioned above, they discovered a simple, and scalable synthesis of ultrasmall PbSe quantum dots called magic-sized clusters.
A normal nanoparticle (or quantum dot), which might be 500-1000 atoms in size, typically grows by adding atoms one at a time. But when these nanoparticles are really small--approximately 50 atoms in size--it is not energetically favorable to add single atoms. For example, if you add an atom to a cube, the new atom will "hang off" the original cube and cost the structure a lot of energy.
The nanoparticles created by Krauss and his team are approximately 50 atoms in size, but they are very stable structures. While the paper describes PbSe, Krauss has also seen these small, stable structures with CdSe.
The fluorescence of Krauss' nanoparticles is at infrared wavelengths around 900 nanometers (image below right). With PbSe, he obtains a fluorescence about 100 times brighter than CdSe clusters of a similar size. In the image below left, the intensity of the fluorescnce from CdSe is multiplied by 30 so it can be seen. However, Krauss' PbSe fluorescence efficiency is much higher, comparable to a fluorescent dye molecule. Compared to typical semiconductor quantum dots, the combination of high fluorescence efficiency, infrared emission and small size is unprecedented.
In addition, the new nanoparticles can be transferred from organic solvents into water. In the image below are two vials, both containing water on the top and solvent on the bottom. However, the right vial contains the PbSe nanoparticles, or quantum dots that Krauss has designed for organic solvents, while the left vial contains the nanoparticles that he's designed for water.
Given that the ideal biological label is very bright, very small, and able to enter and exit cells--like a dye--yet robust like a nanoparticle, Krauss and his team may have found a new type of label that's superior to the current dyes. In fact, the team has patented the new nanoparticle and hopes to commercialize the discovery.
Biomedical and Energy Applications Based on Carbon Nanotubes and Semiconductor Quantum Dots
Professor Krauss collaborates with URMC researchers to devise biomedical and energy applications for carbon nanotubes and quantum dots. Many projects are underway in both areas.
For example, in the area of biomedical applications, Krauss's group is working with Dr. Gunter Oberdorster and his team at URMC trying to learn what happens when people ingest or breathe in the nanotubes and nanoparticles. Projects focus on determining whether nanoparticles containing cadmium cross the blood-brain barrier, and how the body might excrete the particles.
Preliminary studies working with Drs. Tim Mosmann and Martin Zand from URMC attempt to use quantum dots of different fluorescent colors to label various proteins on the surface of the cell that originate from an immune response. If a cell expresses a protein in very small amounts on the cell surface, dye molecules simply aren't bright enough to enable scientists to see the protein. However, quantum dots are bright enough to use for this purpose.
As for energy applications, Krauss and his team are funded by the DOE to study the potential of nanometer materials to develop inexpensive and efficient solar energy. It currently costs approximately $4.00 to make a watt of energy with a silicon solar cell (i.e. over $20,000 for just the solar panels on your roof). Therefore, new materials are needed to push the cost of solar cells into the realm of affordability for the average person.
However, in a traditional solar cell, energy efficiency cannot exceed 32% because (1) infrared photons aren't absorbed, and (2) electrons produced by the absorption of UV and visible photons posses a large amount of energy that rapidly decays away as heat and thus is lost. Thus, one route to making solar cells affordable is to increase the efficiency of them using nanometer scale materials.
In a normal semiconductor, if a high energy photon enters, a highly excited electron-hole pair is created, and this electron-hole pair loses energy through generation of a lot of heat. With nanometer scale materials such as carbon nanotubes and semiconductor quantum dots, something different happens. When a high energy photon is absorbed, there is a chance that a second extra hole-electron pair is created, rather than generating only heat (see figure at left). Thus, a single photon can produce potentially several electrons, which can ultimately increase the solar cell efficiency by 10-15%.
Semiconductor carbon nanotubes are a potentially elegant way to solve the problem of inexpensive and efficient solar cells. Nanotubes have a high aspect ratio, excellent conductivity, a macroscopic size, and the ability to create charge carriers upon photon absorption. Arrayed correctly to span two collection electrodes, carbon nanotubes might someday capture sunlight and convert that light energy to electrical energy. Related to this process, Krauss is studying how nanotubes absorb light, how electrical charge carriers are created, and how the energy is transported through the nanotube. As he says, "When you photo-excite an electron-hole pair, how does that pair get transported down the nanotube, and is possible for us to split the pair?"
Biophysics of Protein Folding
About a quarter of Krauss' group works on problems related to understanding the mechanisms behind efficient protein folding. A sequence of approximately 100 amino acids tends to fold within a second or less in nature, yet a protein with the same 100 amino acids would require 1023 years to find that same structure using random tries. The main question, therefore, is: how do proteins know how to fold into the correct sequences so quickly in nature? Krauss collaborates closely on this project, including jointly advising students and postdoctoral research fellows, with Professor of Chemistry Kara L. Bren (left), a bio-inorganic and biophysical chemist with almost two decades of experience related to protein structure and function.
For many years, scientists have tried to predict protein structures based on their amino acid sequences. Uncovering the mystery of how an amino acid sequence adopts its natural fold, says Krauss, may lead to key discoveries about diseases that involve misfolded proteins such as mad cow disease, Parkinson's, Alzheimer's, and other serious illnesses.
"What Kara and I are trying to understand ," he explains, "is how the protein folds a little, stops, somehow realizes that the current folding may be incorrect, undoes that incorrect folding, then starts folding itself again. So the protein folds, stops, rewinds, folds, stops, rewinds, and so forth until it has successfully folded itself correctly. And all of this happens within a second or less."
To measure the folding mechanism, Krauss and Bren place fluorescent labels on the protein at various places and then monitor the protein as it folds through changes in fluorescence intensity. In March 2008, Bren and Krauss published a paper (Amy A. Ensign, Iris Jo, Ilyas Yildirim, Todd D. Krauss, and Kara L. Bren, "Zinc porphyrin: A fluorescent acceptor in studies of An-cytochrome c unfolding by fluorescence resonance energy transfer," PNAS, 2008, 105, 10779-10784), that explores the use of dye molecules on seven points around a protein to obtain a crude measure of how it's unfolding relative to a central heme.
Other Projects
In addition to many other projects, Professor Krauss joined forces in 2005 with University of Rochester researchers Christopher M. Strohsahl and Benjamin L. Miller to found Lighthouse Biosciences, Inc. The company's NanoLantern™ technology comprises immobilization of one or more fluorescent DNA probes that light up when the DNA of a targeted pathogen is detected. This start-up company is the result of several years of academic research predicated upon the development of in vitro molecular diagnostics for the rapid and accurate detection and screening of pathogenic organisms.