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w w w . s c i a m . c o m S C I E N T I F I C A M E R I C A N 67

SOPHISTICATED FORMS OF NANOTECHNOLOGY

WILL FIND SOME OF THEIR FIRST REAL-WORLD

APPLICATIONS IN BIOMEDICAL RESEARCH,

DISEASE DIAGNOSIS AND, POSSIBLY, THERAPYBY A. PAUL ALIVISATOS

The 1966 film Fantastic Voyage treated movie-goers to a bold vision of nanotechnology applied to medicine:through mysterious means, an intrepid team of doctors andtheir high-tech submarine were shrunk to minute size so thatthey could travel through the bloodstream of an injured patientand remove a life-threatening blood clot in his brain. In the past35 years, great strides have been made in fabricating complexdevices at ever smaller scales, leading some people to believethat such forms of medical intervention are possible and thattiny robots will soon be coursing through everyone’s veins. In-deed, in some circles the idea is taken so seriously that worrieshave emerged about the dark side of such technology: Could

self-replicating nanometer-scale automatons run amok and de-stroy the entire biological world?

In my view, shared by most investigators, such thoughts be-long squarely in the realm of science fiction. Still, nanotech-nology can potentially enhance biomedical research tools—forexample, by providing new kinds of labels for experimentsdone to discover drugs or to reveal which sets of genes are ac-tive in cells under various conditions. Nanoscale devices could,moreover, play a part in quick diagnostic screens and in genet-ic tests, such as those meant to determine a person’s suscepti-bility to different disorders or to reveal which specific genes aremutated in a patient’s cancer. Investigators are also studyingthem as improved contrast agents for noninvasive imaging andas drug-delivery vehicles. The emerging technologies may not beas photogenic as a platelet-size Raquel Welch blasting away ata clot with a laser beam, but they are every bit as dramatic be-cause, in contrast, the benefits they offer to patients and re-searchers are real.

HIGHLY MAGNIFIED VIALS contain solutions of quantum dots—semiconduc-

tor nanocrystals—of specific sizes. The precise size of a quantum dot deter-

mines the color it emits after exposure to light. By attaching different sizes

of dots to different biological molecules, investigators can track the activities

of many tagged molecules simultaneously.

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NANOMEDICINE


How exactly can nanotechnology doall these things? The answer hinges onone’s definitions. All of biology is ar-guably a form of nanotechnology. Afterall, even the most complicated creature ismade up of tiny cells, which themselvesare constructed of nanoscale buildingblocks: proteins, lipids, nucleic acids andother complex biological molecules. Butby convention the term “nanotechnol-ogy” is usually restricted to artificial con-structions made, say, from semiconduc-tors, metals, plastic or glass. A few inor-ganic structures of nanometer scale—

minute crystals, for instance—have al-ready been commercialized, notably ascontrast agents.

Magnetic AttractionNATURE ITSELF provides a beautifulillustration of the usefulness of such in-organic crystals in a biological context:humble magnetotactic (magnetic-sens-ing) bacteria. Such organisms, which livein bodies of water and their muddy bot-toms, thrive only at one depth in the wa-ter or sediment. Above this position, oxy-gen is too abundant for their liking; be-low, too scarce. A bacterium that driftsaway from the right level must swimback, and so, like many of its cousins, themicrobe wields a whiplike tail for pro-pulsion. But how does the buoyant celltell up from down when gravity has es-sentially no effect on it?

The answer is that this bacterium hasfixed within it a chain of about 20 mag-netic crystals that are each between 35and 120 nanometers in diameter. To-gether these crystals constitute a minia-ture compass. Because the magnetic fieldof the earth is inclined in most places (itpoints not only north but also down-ward in the Northern Hemisphere andupward in the Southern), a magnetotac-tic bacterium can follow a magnetic fieldline up or down to its desired destination.

This compass is a marvel of naturalnanoscale engineering. For one, it is madeof the perfect material—either magnetiteor greigite, both highly magnetic ironminerals. The use of multiple crystals isalso no accident. At very small scales, thelarger a magnetic particle is, the longerit stays magnetized. But if the particle be-comes too large, it will spontaneouslyform two separate magnetic domainswith oppositely directed magnetizations.Such a crystal has little overall magneti-zation and does not make for a very ef-fective compass needle. By building itscompass out of crystals that are of justthe right size to exist as stable, singlemagnetic domains, the bacterium makesthe best use of every bit of iron it laysdown. Interestingly, when people designmedia for hard-disk storage, they followexactly the same strategy, using magnet-ic nanocrystals that are of the proper sizeto be both stable and strong.

Artificial magnetic crystals of similardimension might soon serve biomedicalresearch in a novel way. Two groups,one in Germany and the other at my in-stitution, the University of California atBerkeley, are exploring the use of mag-netic nanoparticles to detect particularbiological entities, such as microorgan-isms that cause disease.

Their method, like many of the tech-niques applied today, requires suitableantibodies, which bind to specific targets.The magnetic particles are affixed, as la-bels, to selected antibody molecules,which are then applied to the sample un-der study. To detect whether the anti-bodies have latched onto their target, aninvestigator applies a strong magneticfield (which temporarily magnetizes theparticles) and then examines the speci-men with a sensitive instrument capableof detecting the weak magnetic fields em-anating from the probes. Labeled anti-bodies that have not docked to the sam-ple tumble about so rapidly in solutionthat they give off no magnetic signal.Bound antibodies, however, are unableto rotate, and together their magnetictags generate a readily detectable mag-netic field.

Because the unbound probes produceno signal, this approach does away withthe time-consuming washing steps usual-ly required of such assays. The sensitivi-ty demonstrated with this experimentaltechnique is already better than with stan-dard methods, and anticipated improve-ments in the apparatus should soon boostsensitivity by a factor of several hundred.

Despite these advantages, the mag-netic method probably will not com-pletely replace the widespread practice oflabeling probes with a fluorescent tag,typically an organic molecule that glowswith a characteristic hue when it is ener-gized by light of a particular color. Col-ors are very useful in various diagnosticand research procedures, such as whenmore than one probe needs to be tracked.

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The technologies may not be as photogenicas a minute Raquel Welch blasting away at a clot, but they are

just as dramatic because their benefits are real.

� Nanometer-scale objects made ofinorganic materials can serve inbiomedical research, diseasediagnosis and even therapy.

� Biological tests measuring thepresence or activity of selectedsubstances become quicker, moresensitive and more flexible whencertain nanoscale particles are put towork as tags, or labels.

� Nanoparticles could be used todeliver drugs just where they areneeded, avoiding the harmful sideeffects that so often result frompotent medicines.

� Artificial nanoscale building blocksmay one day be used to help repairsuch tissues as skin, cartilage andbone—and they may even helppatients to regenerate organs.

Overview/Nanomedicine


The world of modern electronics isalso full of light-emitting materials.Every CD player, for instance, reads thedisc with light from a solid-state laserdiode, which is made of an inorganicsemiconductor. Imagine carving out avanishingly small piece of that material,a scoop the size of a protein molecule.The result is a semiconductor nanocrys-tal, or, in the talk of the trade, a “quan-tum dot.” Like nanoscale magnetic crys-tals, these minuscule dots have much tooffer biomedical researchers.

As the name suggests, quantum dotsowe their special properties to the weirdrules of quantum mechanics, the same

rules that restrict the electrons in atomsto certain discrete energy levels. An or-ganic dye molecule absorbs only photonsof light with just the right energy to lift itselectrons from their quiescent state toone of the higher levels available to them.That is, the incident light must be exact-ly the right wavelength, or color, to dothe job. The molecule will subsequently

emit a photon when the electron fallsback to a lower energy level. This phe-nomenon is quite different from whathappens in bulk semiconductors, whichallow electrons to occupy two broadbands of energy. Such materials can ab-sorb photons in a broad range of colors(all those that have enough energy tobridge the gap between these two bands),


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R A. PAUL ALIVISATOS is a professor in the department of chemistry at the University of Cal-ifornia, Berkeley, where he received his doctorate in physical chemistry in 1986. A Fellowof both the American Association for the Advancement of Science and the American Physi-cal Society, Alivisatos has received many awards for his research on the physical proper-ties of nanocrystals. He is a founder of Quantum Dot Corporation, which is working to com-mercialize the use of semiconductor nanocrystals as fluorescent labels in biomedical tests.

GOAL: Superior Implants

The National Nanotechnol-ogy Initiative includes

among its goals, or “grandchallenges,” a host offuturistic improvements inthe detection, diagnosis and treatment of disease.Some are depicted here. Thegoals, many of which are farfrom being realized, alsofeature new aids for visionand hearing, rapid tests for detecting diseasesusceptibility and responsesto drugs, and tiny devicesable to find problems—suchas incipient tumors,infections or heart problems—

and to relay the informationto an external receiver or fixthem on the spot.

Nanoparticles would deliver treatments to specifically targeted sites, including places thatstandard drugs do not reach easily. For example, gold nanoshells (spheres) that were targeted totumors might, when hit by infrared light, heat upenough to destroy the growths.

A GRAND PLAN FOR MEDICINE

GOAL: New Ways to Treat Disease

Nanometer-scale modifications of implant surfaceswould improve implant durability and biocompat-ibility. For instance, an artificial hip coated withnanoparticles might bond to the surrounding bonemore tightly than usual, thus avoiding loosening.

GOAL: Improved ImagingImproved or new contrast agentswould detect problems at earlier,more treatable stages. They might,for instance, reveal tumors (red)only a few cells in size.

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but they emit light only at one specificwavelength, corresponding to the band-gap energy. Quantum dots are an inter-mediate case. Like bulk semiconductors,they absorb photons of all energies abovethe threshold of the band gap. But thewavelength of light a quantum dotemits—its color—depends very stronglyon the dot’s size. Hence, a single type ofsemiconducting material can yield an en-tire family of distinctly colored labels.

Physicists first studied quantum dotsin the 1970s, thinking that they mightone day fashion new electronic or opticaldevices. Few of the pioneering investiga-tors had any idea that these objects couldhelp diagnose disease or discover new

drugs. And none of them would havedreamed that the first real-world appli-cations of quantum dots would be in bi-ology and medicine. Making quantumdots that would function properly in bi-ological systems did indeed require yearsof research, but they are now a reality.

The Rainbow CoalitionTHE COMPANY LEADING the pushto commercialize this technology, Quan-tum Dot Corporation, has licensed tech-niques developed in my lab and at theMassachusetts Institute of Technology,Indiana University, Lawrence BerkeleyNational Laboratory and the Universityof Melbourne in Australia. I helped tofound this company, so my assessmentsmay be biased, but I view the prospectsfor quantum dots as, well, bright.

Semiconductor nanocrystals haveseveral advantages over conventional dyemolecules. Small inorganic crystals canwithstand significantly more cycles of ex-citation and light emission than can typ-ical organic molecules, which soon de-compose. And this stability allows inves-tigators to track the goings-on in cellsand tissues for longer intervals than cannow be achieved. But the greatest benefitsemiconductor nanocrystals offer is lesssubtle—they come in more colors.

Biological systems are very complex,and frequently several components mustbe observed simultaneously. Such track-ing is difficult to achieve, because eachorganic dye must be excited with a dif-ferent wavelength of light. But quantumdots make it possible to tag a variety ofbiological molecules, each with a crystalof a different size (and hence color). Andbecause all of these crystals can be ener-gized with a single light source, they canall be monitored at once.

This approach is being pursued ac-tively, but quantum dots offer even moreinteresting possibilities. Imagine a smalllatex bead filled with a combination ofquantum dots. The bead could, for in-

stance, contain five different sizes of dots,or five colors, in a variety of concentra-tions. After the bead is illuminated, it willgive off light, which when spread out bya prism will produce five distinct spectrallines with prescribed intensities—a spec-tral bar code, if you like. Such beads al-low for an enormous number of distinctlabels (billions, potentially), each ofwhich could be attached, say, to DNAmolecules composed of different se-quences of genetic building blocks.

With these kinds of beads, technicianscould easily compare the genetic materi-al in a sample against a library of knownDNA sequences, as might be done if aninvestigator wanted to find out whichgenes were active in certain cells or tis-sues. They would simply expose the sam-ple to the full beaded library and read thespectral bar codes of the library DNAsthat bind to sequences in the sample. Be-cause binding takes place only when ge-netic sequences match closely (or moreprecisely, when one sequence comple-ments the other), the results would im-mediately reveal the nature of the genet-ic material in the sample.

Semiconductor quantum dots shouldsoon serve biomedical researchers in thisway, but they are not the only nano-structures useful for optically sensing thegenetic composition of biological speci-mens. Another example emerges fromthe work of Chad A. Mirkin and RobertL. Letsinger of Northwestern University,who recently developed an ingeniousmethod to test for the presence of a spe-cific genetic sequence in solution. Theirscheme employs 13-nanometer gold par-ticles studded with DNA.

The trick here is to use two sets ofgold particles. The first set carries DNAthat binds to one half of the target se-quence; the second set carries DNA thatbinds to the other half. DNA with thecomplete target sequence readily attach-es to both types of particles, linking themtogether. Because each particle has mul-


For therapy, one might encapsulate drugs within nanometer-scale packages that control the

medicines’ release in sophisticated ways.

LATEX BEADS filled with quantum dots of single

colors glow at nearly the same wavelengths as

the dots themselves. Researchers have also

loaded selections of different dots into single

beads. Their aim is to create a huge variety of

distinct labels for biological tests. (See also “Nano

Bar Codes” in the box on the opposite page.)



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GOLD PARTICLESGold nanoparticles studded with short segments of DNA couldform the basis of an easy-to-read test for the presence of a genetic sequence (black) in a sample under study. DNAcomplementary to half of such a sequence (red) is attached to one set of particles in solution, and DNA complementary to theother half (blue) is attached to a second set of particles. If the sequence of interest is present in the sample, it will bind to the DNA tentacles on both sets of spheres, trapping the balls in a dense web. This agglomeration will cause the solution tochange color ( from red to blue).

NANO BAR CODESLatex beads filled with several colors of nanoscale semiconductorsknown as quantum dots can potentially serve as unique labels forany number of different probes. In response to light, the beadswould identify themselves (and, thus, their linked probes) byemitting light that separates into a distinctive spectrum of colorsand intensities—a kind of spectral bar code.

CLEVER CANTILEVERSBiological samples can be screened for the presence ofparticular genetic sequences using small beams (cantilevers)of the type employed in atomic force microscopes. The surfaceof each cantilever is coated with DNA able to bind to oneparticular target sequence. A sample is then applied to thebeams. Binding induces a surface stress, which bends theaffected beams by nanometers—not much, but enough to revealthat the bent beams found their specific targets in a sample.

BIO-NANOTECH IN ACTION

The items here could one day enhance the speed and power of biomedical tests, such as those used to

screen small samples of material for the presence of particular genetic sequences. For clarity, the imageshave not been drawn to scale.

MAGNETIC TAGSMany tests reveal the presence of a molecule or disease-causing organism by detecting the binding of an antibody tothat target. When antibodies labeled with magnetic nano-particles bind to their target on a surface (foreground), briefexposure to a magnetic field causes these probes collectively to give off a strong magnetic signal. Meanwhile unbound anti-bodies tumble about in all directions, producing no net signal.This last property makes it possible to read the results withoutfirst washing away any probes that fail to find their target.

MAGNETICNANOPARTICLES

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tiple DNA tentacles, bits of genetic ma-terial carrying the target sequence canglue many particles together. And whenthese gold specks aggregate, their opticalproperties shift markedly, changing thetest solution from red to blue. Becausethe outcome of the test is easy to seewithout any instrumentation at all, sucha system might be particularly useful forhome DNA testing.

Feeling the ForceNO DISCUSSION of bio-nanotechnol-ogy would be complete without at leasta brief mention of one of the hottest in-struments in science today—the atomicforce microscope. Such devices probematerials in the same way an old-fash-ioned phonograph reads the grooves in arecord: by dragging a sharp point overthe surface and detecting the resulting de-flections. The tip of an atomic force mi-croscope is, however, much finer than aphonograph needle, so it can sense farsmaller structures. Regrettably, fabricat-ing tips that are both fine and sturdy for

these microscopes has proved to be quitedifficult.

The solution appeared in 1996, whenworkers at Rice University affixed a slen-der carbon nanotube to the tip of anatomic force microscope, making it pos-sible to probe samples just a few nano-meters in size. In 1998 Charles M. Lieberand his co-workers at Harvard Univer-sity applied this approach to probing bio-molecules, providing a very high resolu-tion means to explore complex biologi-cal molecules and their interactions at themost basic level.

But atomic force microscopy maysoon be applied to more than just mak-ing fundamental scientific measurements.Last year James K. Gimzewski, then atthe IBM Zurich Research Laboratory,showed with collaborators at IBM andthe University of Basel that an array ofmicron-scale arms, or cantilevers, muchlike the ones employed in atomic forcemicroscopes, could be used to screensamples for the presence of certain ge-netic sequences. They attached short

strands of DNA to the tops of the can-tilevers. When genetic material carryinga complementary sequence binds to theanchored strands, it induces a surfacestress, which bends the cantilevers sub-tly—by just nanometers—but enough tobe detected. By fabricating devices withmany cantilevers and coating each witha different type of DNA, researchersshould be able to test a biological sam-ple rapidly for the presence of specificgenetic sequences (as is now done rou-tinely with gene chips) by nanomechan-ical means without the need for labeling.

This example, like the others de-scribed earlier, illustrates that the con-nections between nanotechnology andthe practice of medicine are often indi-rect, in that much of the new workpromises only better research tools oraids to diagnosis. But in some cases, nano-objects being developed may themselvesprove useful for therapy. One might, forinstance, encapsulate drugs within nano-meter-scale packages that control themedicines’ release in sophisticated ways.

Consider a class of artificial mole-cules called organic dendrimers. Twodecades ago Donald A. Tomalia of theMichigan Molecular Institute in Mid-land fashioned the first of these intrigu-ing structures. A dendrimer moleculebranches successively from inside to out-side. Its shape resembles what one wouldget by taking many sprigs from a tree andpoking them into a foam ball so that theyshot out in every direction. Dendrimersare globular molecules about the size ofa typical protein, but they do not comeapart or unfold as easily as proteins do,because they are held together withstronger chemical bonds.

Like the lush canopies of maturetrees, dendrimers contain voids. That is,they have an enormous amount of inter-nal surface area. Interestingly, they canbe tailored to have a range of differentcavity sizes—spaces that are just perfectfor holding therapeutic agents. Den-drimers can also be engineered to trans-port DNA into cells for gene therapy,and they might work more safely thanthe other leading method: geneticallymodified viruses.

Other types of nanostructures pos-


Petite Plumbing JobsMicrofluidics enhances biomedical research

Most of the nanotechnologies now being developed for biomedical use take theform of minute objects immersed in comparatively large quantities of fluid, be it

water, blood or a complex experimental concoction. But investigators are alsobuilding devices to manipulate tiny amounts of such liquids. These so-calledmicrofluidic systems pump solutions through narrow channels, controlling the flowwith diminutive valves and intense electric fields.

The ability to handle vanishingly small quantities of a solution in this way allowsbiomedical researchers to carry out many different experiments on what might beonly a modest amount of sample—and to do so in an efficient manner, with hundredsof tests being performed, say, on the surface of a single glass slide. Microfluidicdevices also offer researchers the means to carry out experiments that could nototherwise be done; for example, to deliver test solutions of specific compositions todifferent parts of a cell under study.

Although many of the components being created for these systems areconsiderably larger than a micron, some experimental devices include nanoscaledimensions. Notably, Harold G. Craighead’s team at Cornell University has devisedmethods for sorting different sizes of DNA fragments in water according to how fastthe fragments traverse passages measuring 100 nanometers across or travelthrough microchannels that repeatedly narrow to a depth of 75 to 100 nanometers.These or other nanofluidic devices could potentially increase the speed and reducethe costs of separating DNA molecules for sequencing and could in theory be adaptedfor separating proteins or other molecules. —A.P.A.


sess high surface area, and these too may prove useful for delivering drugswhere they are needed. But dendrimersoffer the greatest degree of control andflexibility. It may be possible to designdendrimers that spontaneously swelland liberate their contents only when the appropriate trigger molecules arepresent. This ability would allow a cus-tom-made dendrimer to release its loadof drugs in just the tissues or organsneeding treatment.

Other drug-delivery vehicles on thehorizon include hollow polymer capsulesunder study by Helmuth Möhwald of theMax Planck Institute of Colloids and In-terfaces in Golm, Germany. In responseto certain signals, these capsules swell orcompress to release drugs. Also intrigu-ing are so-called nanoshells, recently in-vented by the researchers at Rice.

Nanoshells are extremely small beadsof glass coated with gold. They can befashioned to absorb light of almost anywavelength, but nanoshells that captureenergy in the near-infrared are of mostinterest because these wavelengths easilypenetrate several centimeters of tissue.Nanoshells injected into the body cantherefore be heated from the outside us-ing a strong infrared source. Such ananoshell could be made to deliver drugmolecules at specific times by attaching itto a capsule made of a heat-sensitivepolymer. The capsule would release itscontents only when gentle heating of theattached nanoshell caused it to deform.

A more dramatic application envi-sioned for nanoshells is in cancer thera-py. The idea is to link the gold-platedspheres to antibodies that bind specifi-cally to tumor cells. Heating the nano-shells sufficiently would in theory destroythe cancerous cells, while leaving near-by tissue unharmed.

It is, of course, difficult to know forcertain whether nanoshells will ultimate-ly fulfill their promise. The same can besaid for the myriad other minuscule de-vices being developed for medical use—

among them, one-nanometer buckyballsmade from just a few dozen carbonatoms. Yet it seems likely that some ofthe objects being investigated today willbe serving doctors in the near future.

Even more exciting is the prospect thatphysicians will make use of nanoscalebuilding blocks to form larger struc-tures, thereby mimicking the naturalprocesses of biology. Such materialsmight eventually serve to repair dam-aged tissues. Research on these boldstrategies is just beginning, but at leastone enterprise already shows that thenotion has merit: building scaffoldingson which to grow bone. Samuel I. Stuppof Northwestern is pioneering this ap-proach using synthetic molecules thatcombine into fibers to which bone cellshave a strong tendency to adhere.

What other marvels might the futurehold? Although the means to achievethem are far from clear, sober nanotech-

nologists have stated some truly ambi-tious goals. One of the “grand challenges”of the National Nanotechnology Initia-tive is to find ways to detect canceroustumors that are a mere few cells in size.Researchers also hope eventually to de-velop ways to regenerate not just boneor cartilage or skin but also more com-plex organs, using artificial scaffoldingsthat can guide the activity of seeded cellsand can even direct the growth of a va-riety of cell types. Replacing hearts orkidneys or livers in this way might notmatch the fictional technology of Fan-tastic Voyage, but the thought that suchmedical therapies might actually becomeavailable in the not so distant future isstill fantastically exciting.


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Ultrasensitive Magnetic Biosensor for Homogeneous Immunoassay. Y. R. Chemla, H. L. Grossman,Y. Poon, R. McDermott, R. Stevens, M. D. Alper and J. Clarke in Proceedings of the National Academy ofSciences USA, Vol. 97, No. 27, pages 14268–14272; December 19, 2000.

The author’s Web site is at www.cchem.berkeley.edu/~pagrp/

Information on using gold nanoparticles for DNA testing is available atwww.chem.nwu.edu/~mkngrp/dnasubgr.html

Information on nanoshells is available at www.ece.rice.edu/~halas/

Information about quantum dots and their use in biomedicine is available at www.qdots.com

M O R E T O E X P L O R E

ORGANIC DENDRIMER, shown in an artist’s conception, could be roughly the size of a protein

molecule. Dendrimers harbor many internal cavities and are being eyed as drug-delivery vehicles.


copyright 2001 scientific american, inc

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