Nanoscale particles are not new in either nature or science. However, the recent leaps in areas such as microscopy have given scientists new tools to understand and take advantage of phenomena that occur naturally when matter is organized at the nanoscale. In essence, these phenomena are based on “quantum effects” and other simple physical effects such as expanded surface area (more on these below). In addition, the fact that a majority of biological processes occur at the nanoscale gives scientists models and templates to imagine and construct new processes that can enhance their work in medicine, imaging, computing, printing, chemical catalysis, materials synthesis, and many other fields. Nanotechnology is not simply working at ever smaller dimensions; rather, working at the nanoscale enables scientists to utilize the unique physical, chemical, mechanical, and optical properties of materials that naturally occur at that scale.
Computer simulation of electron motions within a nanowire that has a diameter in the nanoscale range.
Scale at which Quantum Effects Dominate Properties of Materials
When particle sizes of solid matter in the visible scale are compared to what can be seen in a regular optical microscope, there is little difference in the properties of the particles. But when particles are created with dimensions of about 1–100 nanometers (where the particles can be “seen” only with powerful specialized microscopes), the materials’ properties change significantly from those at larger scales. This is the size scale where so-called quantum effects rule the behavior and properties of particles. Properties of materials are size-dependent in this scale range. Thus, when particle size is made to be nanoscale, properties such as melting point, fluorescence, electrical conductivity, magnetic permeability, and chemical reactivity change as a function of the size of the particle.
Nanoscale gold illustrates the unique properties that occur at the nanoscale. Nanoscale gold particles are not the yellow color with which we are familiar; nanoscale gold can appear red or purple. At the nanoscale, the motion of the gold’s electrons is confined. Because this movement is restricted, gold nanoparticles react differently with light compared to larger-scale gold particles. Their size and optical properties can be put to practical use: nanoscale gold particles selectively accumulate in tumors, where they can enable both precise imaging and targeted laser destruction of the tumor by means that avoid harming healthy cells.
A fascinating and powerful result of the quantum effects of the nanoscale is the concept of “tunability” of properties. That is, by changing the size of the particle, a scientist can literally fine-tune a material property of interest (e.g., changing fluorescence color; in turn, the fluorescence color of a particle can be used to identify the particle, and various materials can be “labeled” with fluorescent markers for various purposes). Another potent quantum effect of the nanoscale is known as “tunneling,” which is a phenomenon that enables the scanning tunneling microscope and flash memory for computing.
Scale at Which Much of Biology Occurs
Over millennia, nature has perfected the art of biology at the nanoscale. Many of the inner workings of cells naturally occur at the nanoscale. For example, hemoglobin, the protein that carries oxygen through the body, is 5.5 nanometers in diameter. A strand of DNA, one of the building blocks of human life, is only about 2 nanometers in diameter.
Drawing on the natural nanoscale of biology, many medical researchers are working on designing tools, treatments, and therapies that are more precise and personalized than conventional ones—and that can be applied earlier in the course of a disease and lead to fewer adverse side-effects. One medical example of nanotechnology is the bio-barcode assay, a relatively low-cost method of detecting disease-specific biomarkers in the blood, even when there are very few of them in a sample. The basic process, which attaches “recognition” particles and DNA “amplifiers” to gold nanoparticles, was originally demonstrated at Northwestern University for a prostate cancer biomarker following prostatectomy. The bio-barcode assay has proven to be considerably more sensitive than conventional assays for the same target biomarkers, and it can be adapted to detect almost any molecular target.i
Growing understanding of nanoscale biomolecular structures is impacting other fields than medicine. Some scientists are looking at ways to use nanoscale biological principles of molecular self-assembly, self-organization, and quantum mechanics to create novel computing platforms. Other researchers have discovered that in photosynthesis, the energy that plants harvest from sunlight is nearly instantly transferred to plant “reaction centers” by quantum mechanical processes with nearly 100% efficiency (little energy wasted as heat). They are investigating photosynthesis as a model for “green energy” nanosystems for inexpensive production and storage of nonpolluting solar power.ii
Scale at which Surfaces and Interfaces Play a Large Role in Materials Properties and Interactions
Nanoscale materials have far larger surface areas than similar masses of larger-scale materials. As surface area per mass of a material increases, a greater amount of the material can come into contact with surrounding materials, thus affecting reactivity.
A simple thought experiment shows why nanoparticles have phenomenally high surface areas. A solid cube of a material 1 cm on a side has 6 square centimeters of surface area, about equal to one side of half a stick of gum. But if that volume of 1 cubic centimeter were filled with cubes 1 mm on a side, that would be 1,000 millimeter-sized cubes (10 x 10 x 10), each one of which has a surface area of 6 square millimeters, for a total surface area of 60 square centimeters—about the same as one side of two-thirds of a 3” x 5” note card. When the 1 cubic centimeter is filled with micrometer-sized cubes—a trillion (1012) of them, each with a surface area of 6 square micrometers—the total surface area amounts to 6 square meters, or about the area of the main bathroom in an average house. And when that single cubic centimeter of volume is filled with 1-nanometer-sized cubes—1021 of them, each with an area of 6 square nanometers—their total surface area comes to 6,000 square meters. In other words, a single cubic centimeter of cubic nanoparticles has a total surface area one-third larger than a football field!
Illustration demonstrating the effect of the increased surface area provided by nanostructured materials
One benefit of greater surface area—and improved reactivity—in nanostructured materials is that they have helped create better catalysts. As a result, catalysis by engineered nanostructured materials already impacts about one-third of the huge U.S.—and global—catalyst markets, affecting billions of dollars of revenue in the oil and chemical industries.iii An everyday example of catalysis is the catalytic converter in a car, which reduces the toxicity of the engine’s fumes. Nanoengineered batteries, fuel cells, and catalysts can potentially use enhanced reactivity at the nanoscale to produce cleaner, safer, and more affordable modes of producing and storing energy.
Large surface area also makes nanostructured membranes and materials ideal candidates for water treatment and desalination, among other uses. It also helps support “functionalization” of nanoscale material surfaces (adding particles for specific purposes), for applications ranging from drug delivery to clothing insulation.
i For example, see C.S. Thaxton, R. Elghanian, A.D. Thomas, S.I. Stoeva, J.S. Lee, N.D. Smith, A.J. Schaeffer, H. Klocker, W. Horninger, G. Bartsch, and C.A. Mirkin. Nanoparticle-based bio-barcode assay redefines “undetectable” PSA and biochemical recurrence after radical prostatectomy. Proc. Nat. Acad. Sci. U. S. A. 106(44):18437–18442, 2009, doi:10.1073/pnas.0904719106.
ii For more detail, see http://newscenter.lbl.gov/feature-stories/2010/05/10/untangling-quantum-entanglement/ and associated links.
iii As of 2003, catalyst technologies accounted for over $1 trillion of revenue in the U.S. economy and about a third of the material GDP (M.E. Davis and D. Tilley, Future Directions in Catalysis Research, Structures that Function on the Nanoscale, NSF Workshop, Caltech, June 19-20, 2003; http://www.che.caltech.edu/nsfcatworkshop/#Reports).