(Nanowerk Spotlight) Quantum dots are being intensively investigated for applications such as light-emitting diodes, solid-state lighting, lasers, solar cells, and fluorescent labels for biological imaging. These nanocrystals have great promise as light-emitting materials particularly for fluorescent tracking, because they are super-bright and photostable and the wavelength, or color, of light that the quantum dots give off can be very widely tuned simply by changing the size of the nanoparticles. However, one issue that has troubled scientists since the very beginning of quantum dot-based tracking (ca. early 2000s) is blinking – something of a nervous tic, where a quantum dot can be seen to randomly switch between bright and dark states.
Over the years, a number of approaches have been developed to eliminate blinking. Unfortunately as pointed out by a recent high-profile review paper ("Next-generation quantum dots"), while these "non-blinking" particles solve one problem, they also create another problem, i.e. lack of in situ methods to confirm particle aggregation status.
"The inability to confirm aggregation status can be extremely problematic," Jessica O. Winter, H.C. "Slip" Slider Assistant Professor in the Department of Biomedical Engineering at Ohio State University, tells Nanowerk. "For instance, when a cell biologist uses biomolecule-linked quantum dots to track biomolecules at the single molecule level, the first thing s/he needs to know before collecting data is that s/he is really looking at single molecules (or at least small molecule clusters). Nanoparticles are notoriously susceptible to aggregation because of their large surface area; thus an in situ aggregation-status indicator is very important."
Winter and Gang Ruan, a research scientist in her group, have found a creative solution to a paradox in quantum dot-based particle/molecule tracking: on one hand, as mentioned above, blinking is a problem for quantum dot-based tracking as it breaks up tracking trajectories, on the other hand, blinking is also a very useful indicator as it offers the best – and often only practical – in situ indication of aggregation status. Thus, when a researcher conducts a tracking experiment, s/he is annoyed by blinking but at the same time would be equally disappointed if blinking was not present to confirm single, or near single, particle status.
Previous strategies to manipulate blinking dynamics involve changing the structure (surface or core chemistry) of a quantum dot. Winter and Ruan, as they have reported in the February 15, 2011 online edition of Nano Letters ("Alternating-Color Quantum Dot Nanocomposites for Particle Tracking"), have taken a fundamentally different approach: quantum dots employed remain unchanged.
"Instead, we incorporate several quantum dots of different colors into a nano-sized container, e.g. micelle" says Ruan. "Because blinking dynamics of individual quantum dots are out of phase, the fluorescence of the composite nanoparticle has near-continuous intensity with a color changing effect, the latter of which serves as an indicator of aggregation status –large aggregates of composite nanoparticles exhibit constant color."
Color changes of composite nanoparticle. (Image: Winter lab, Ohio State University)
The two scientists developed a new class of composite nanoparticles (CNP), comprised of a few quantum dots with differing emission wavelengths (i.e., colors).
According to Ruan, because blinking dynamics are stochastic, a single CNP remains nearly continuously fluorescent, while the emission wavelength alternates between those of the constituent quantum dots and their combinations. "In contrast" he says, "large CNP aggregates display a nearly constant fluorescence emission color, permitting single (or very small clusters of) CNPs to be distinguished from large aggregates by their alternating-color emission. Thus, CNPs can be continuously tracked and identified as single (or very small clusters of) CNPs."
Winter notes that, in addition to solving the seemingly irreconcilable problems in particle tracking, this work also offers a new twist in several other areas.
"First, traditional quantum dots are well-known to change color with size, chemical composition, and crystal structure; with this new composite quantum dot structure, they can also change color continuously with time.
"Second, to the best of our knowledge, this is the first class of nanoparticles ever reported that change colors continuously (although FRET and related mechanisms do permit color changing in many settings, those color changes are not continuous).
"Third, there have been many efforts to microencapsulate large numbers of different colored quantum dots to form composite particles for molecular barcoding (the ground breaking paper: "Quantum-dot-tagged microbeads for multiplexed optical coding of biomolecules"); here, we show that encapsulating a very small number of different colored quantum dots is also very useful for a variety of applications.
This technology can be applied to track molecules – conjugated to composite nanoparticles – or particles. In cell biology, biologists are pursuing techniques that provide information about highly dynamic biological processes. With quantum dots' extraordinary brightness and photostability, quantum dots-based tracking has emerged as a state-of-the-art technology for such purposes, and has resulted in many successful applications published in high-profile journals.
According to Winter and Ruan, there are two areas where this technology can also be applied that may be less obvious, yet with great potential.
First, the composite nanoparticles can be used as a model particle to study the transport behavior of nanoparticles of different sizes and shapes in living cells. The size and shape of composite nanoparticles can be controlled by altering the composite nanoparticle assembly process. Information resulting from these studies will impact many areas in which nanoparticles are used in biological settings (e.g. drug delivery).
Second, the composite nanoparticles can be used as tracers in microfluidic devices to study flow mechanics in microfluidics. Conventional tracers used in flow mechanics studies are large beads. But since components in a microfluidic device are micron-sized, the conventional tracers are too big and will cause interference to the original flow mechanics. The composite nanoparticles, which are much smaller, are expected to offer many benefits in these situations.
A challenge of this technology is the fact that composite nanoparticles are about 2-3x in size compared to traditional quantum dot technologies, which could interfere with the biological processes being tracked in some situations. However, by careful optimization, composite nanoparticles as small as 10-15 nm should be possible, which is the size of water-soluble quantum dots currently on the market.
"Future research in our group will see exploration of the many potential applications of this technology" says Winter. "Additionally, we will try to optimize of the technology and perform fundamental studies of the physics and structure of the nanocomposites."