Quantum dots: the Maxwell's demons of biology
WHILE DISCUSSING ideas in thermodynamics, on heat flow from one place to another, physicists talk of a mythical fellow called Maxwell's demon.
He is supposed to be the size of atoms and molecules (more an imp or dwarf than a demon!) who acts as a gatekeeper, allowing hot atoms or molecules to go across while denying cold ones.
In a while, he thus creates order through selective gating. Well, with the discovery and fabrication of materials called nanoparticles, a Maxwell's demon of sorts has left the world of fantasy and jumped into real life.
Monitoring cell events
Nanoparticles are increasingly being used to monitor events that happen within biological cells, as they occur in real time. They are thus reporters of what happens within cells and tissues in vivo and in situ.
A particular kind of these, called semiconductor nanocrystals or quantum dots (qdots for short), have evolved over the past 20 years from electronic materials science to biological applications.
Two recent reviews by pioneers of these applications have appeared, one from the group of Shuming Nie of Emory University (Photochemistry and Photobiology, December 2004) and the other from the California group of Shimon Weiss (Science, 28 Jan 2005).
Capturing attention
As the name suggests, nanoparticles are a collection of atoms or molecules with sizes in the range of 10 to 100 nanometres or nm. They could even be a single molecule such as C-60 or fullerene, a protein or DNA, or a collection of many molecules such as those of cadmium sulphide (CdS) or cadmium selenide (CdSe).
It is these nanoparticles of inorganic materials that have captured the attention of scientists because of the wide range of applications that they can be put to.
Novel and creative ways of assembling them in definite numbers and sizes have been reported, and our own Professor C. N. R. Rao is a well-known expert in the creation and use of such nanoparticles and assemblies.
Using molecules to identify the component parts of biological cells is not new.
It was first introduced by Heinrich Waldeyer, the teacher of Paul Ehrlich, in 1863. He used the extract of logwood (haematoxylon campechianum) to stain cells and biological specimen. (Logwood itself has a hoary and bloody history, since it was an excellent and cheaper alternative to the ones then in use in Europe.
When colonial Spaniards and Englishmen found them in Central America, they began a flourishing trade shipping logwood from the New World to Europe. Colonials used boatloads of slaves in the Yucatan, Belize and the Caribbean to cut and ship logwood, and the conditions of these slaves were inhuman.
These early woodcutters, called baymen, exported 13,000 tonnes of logwood yearly. After the colonials left, a new nation called Belize was born, and to this day its flag carries the images of baymen and the logwood tree.)
Dyes that specifically attach to the cell nucleus, or membranes and other parts, and impart colour there or emit fluorescence, have been known. A whole branch of biology called histochemical staining deals with this.
But these are by and large static reporters and mostly do not report `live' action. They also `bleach' and lose their colour (and chemical identity) after a while, and can also enter into chemical reactions with some molecules in the cell.
It is in this area that qdots have emerged as versatile, and multiplex, yet non-interfering reporters of events in cells and tissues. A favourite qdot of biology is the nanocrystal made of CdSe.
Assembling molecules
During the last two decades, it has become possible to assemble molecules of CdSe (and similar material) in precisely controlled numbers to produce nanocrystals of well-defined sizes and surface. Typically around 10 nm in diameter, nanocrystals are larger than molecules but smaller than bulk solids.
It is this `intermediacy' or the Trishanku state that allows them to exhibit physical and chemical properties somewhere in between. The fact that a nanocrystal is virtually all surface and no interior makes its properties vary considerably as the crystals grow in size.
First of all, nanocrystals of CdSe (and similar materials) are semiconductors. In a solid, the energy states of electrons in the constituent atoms form continuous `bands.'
The valence band is the highest electronic level occupied; when a small amount of energy is provided (heat, light, magnet), these electrons can go from the valence band to the `conduction band,' generating electric conduction.
In an insulator, this gap between the valence and conduction band is way too large and no electric conduction is possible. In a semiconductor, the situation is in between.
The valence and conduction bands do not overlap as in a conductor, but the band gap is small enough that some electrons may be excited at room temperature to form charge carriers.
But what is fascinating about semiconductors is that the band gap is not only dependent on the composition of the material but also the particle size. When the size is reduced to the nanometre regime, or the `quantum well' dimension, the material has energy levels that allow it to absorb light over a broad range but fluoresce at a definite wavelength region.
The fluorescent wavelength emission depends on the particle size. In other words, the colour the materials emit is `tuneable' through their particle sizes.
The consequences
It is for this reason that they are called quantum dots — thanks to the quantum well effect and the dot-like size. As the size of CdSe nanocrystal is tuned between 2-7 nm, its fluorescence can be tuned to be between 450 and 650 nm (from bluish to orange).
Imagine the consequences. We can make CdSe in three or four different dot sizes, illuminate them all with the same light — and they emit different colours. They can thus be used as `reporters.' Great, but they should be made water- soluble so that they can enter cells as inert reporters.
Next, they should be made to report on specific zones or regions of the cell. Dr. Nie's group, as those of others, has achieved both.
They attach some organic acids or amine to the surface, and then attach chosen proteins or peptides on them. Dr. Shiman Weiss's group has specialised in the latter attachment strategies, making qdot reporting very versatile. Of course, qdots can be used as histological markers and diagnostic tools.
Nie's group recently linked monoclonal antibodies to qdots and were able to detect the membrane antigen present specifically on the cell surface of prostate cancer. They injected living mice with these qdots and detected the cancer cells at the very initial stage. Diagnosis of this kind allows very early intervention and treatment.
Whether benign or toxic
Dr. E. Rouslalti's group loaded three different kinds of peptides on qdots and monitored lung cells, brain cells and human breast carcinoma cells in vivo and in vitro. Targeting qdots to specific molecules in the cell has thus become possible.
Qdots are being used increasingly in living animals — to image and monitor blood vessels, liver, bone marrow and lymph nodes.
Whether the introduction of qdots in live animals is benign or toxic is currently being investigated. While so far, no adverse effects on cell viability, size and shape, function or development have been reported for several days, extensive scrutiny is called for since these are but initial and sparse results.
As better and more specific coatings, optimal dosage of injection, and range of qdot size are examined, safe dosages will emerge.
The perspectives
The perspectives of qdots in imaging, diagnostics and even site-specific drug delivery into cells and live animals are exciting and extensive. They will emerge as biosensors, bioanalytical agents, monitors of events and development stages, as surgical and therapeutic agents.
Multiplex monitoring of cells by using qdots of varying sizes and coatings, to report on what is happening at different cellular locales through colour coding, is still in its infancy. When this is developed in the next several years, we shall truly have a rainbow of applications of these biological Maxwell's demons.
D. Balasubramanian
dbala@lvpei.org
Printer friendly
page
Send this article to Friends by
E-Mail
Sci Tech
