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Chemical analysis and cellular imaging with quantum dots

Andrew M. Smith and Shuming Nie*Departments of Biomedical Engineering and Chemistry, Emory University and Georgia Institute ofTechnology, 1639 Pierce Drive Suite 2001, Atlanta, GA 30322, USA

Received 24th March 2004, Accepted 20th April 2004First published as an Advance Article on the web 10th June 2004

Quantum dots are tiny light-emitting particles on the nanometer scale. They are emerging as a new class of biologicallabels with properties and applications that are not available with traditional organic dyes and fluorescent proteins.Their novel properties such as improved brightness, resistance against photobleaching, and multicolor light emission,have opened new possibilities for ultrasensitive chemical analysis and cellular imaging. In this Research Highlightarticle, we discuss the unique optical properties of semiconductor quantum dots, surface chemistry and bioconjugation,current applications in bioanalytical chemistry and cell biology, and future research directions.

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1. IntroductionSemiconductor quantum dots (QDs) havecaptivated scientists and engineers over thepast two decades due to their fascinatingoptical and electronic properties that arenot available from either isolatedmolecules or bulk solids. Nanoscience, thestudy of materials with nanometer-scaledimensions, has revealed that theirproperties are strongly dependent on theparticle size. Nanotechnology has recentlyaimed to utilize these unique properties tocreate useful nanoscale devices. Becausethe size-dependent properties of quantumdots are so pronounced in fluorescence,their use as biological probes has attractedconsiderable attention. Indeed, QDs areoften used as a model system fornanoscience, and have become one of thefirst commercial applications ofnanotechnology.

QDs are nanocrystals of inorganicsemiconductors that are restricted in threedimensions to a somewhat spherical shape,typically with a diameter of 2–8 nm (onthe order of 200–10000 atoms).1,2 Bulk-phase semiconductors are characterized byvalence electrons that can be excited to ahigher-energy conduction band. The energydifference between the valence band andthe conduction band is the band gap energyof the semiconductor. The excited electronmay then relax to its ground state throughthe emission of a photon with energy equalto that of the band gap. When asemiconductor is of nanoscale dimensions,the band gap is dependent on the size ofthe nanocrystal. As the size of asemiconductor nanocrystal decreases, theband gap increases, resulting in shorterwavelengths of light emission. Thisquantum confinement effect is analogousto the quantum mechanical “particle in abox,” in which the energy of the particleincreases as the size of the box decreases.Cadmium selenide (CdSe) is theprototypical QD, and its size-tunablefluorescence throughout the visible light

spectrum is depicted in Fig. 1. Othersemiconductor materials display

fluorescence in different spectral ranges, sothat QDs can be synthesized to emit atwavelengths between 400–2000 nm bychanging their composition and size.3–7 Animportant consequence of this quantumconfinement effect for biologists is thatthese size-tunable properties occur at thesame size regime as biologicalmacromolecules like proteins and nucleicacids.

2. Quantum dots vs. organicfluorophoresFluorescent dyes have been valuable in thestudy of biological phenomena due to theirinherent high sensitivity of detection andease of use. QDs may provide a new classof biological labels that could overcomethe limitations of organic dyes andfluorescent proteins. With size-tunablefluorescence emission, QDs can begenerated for any specific wavelength,

from the UV through the near infrared(Fig. 1). QD emission peaks are narrow

(FWHM typically 25–35 nm) andsymmetric compared to organicfluorophores, making them ideal forapplications involving the simultaneousdetection of multiple fluorophores (Fig.2).8 In addition, the broad absorptionspectra of QDs allow the excitation ofmultiple fluorophores with a single lightsource, at any wavelength shorter than theemission peak wavelength. QDs are highlyresistant to photobleaching, a commonproblem for organic fluorophores, thusmaking them useful for continuousmonitoring of fluorescence.9 QDs havevery large molar extinction coefficients andhigh quantum yields, resulting in brightfluorescent probes in aqueous solution.10

Moreover, QDs have long fluorescencelifetimes on the order of 20–50 ns, whichmay allow them to be distinguished frombackground and other fluorophores forincreased sensitivity of detection.11

It should be noted, however, that QDs

Fig. 1 Size-tunable fluorescence spectra of CdSe quantum dots (A), and illustration of the relativeparticle sizes (B). From left to right, the particle diameters are 2.1 nm, 2.5 nm, 2.9 nm, 4.7 nm, and7.5 nm.

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are unlikely to replace organic dyes.Although QDs are commercially available,they are currently expensive compared toorganic dyes, and changing an alreadyestablished biological detection systemfrom dyes to QDs will require time andoptimization. Also QDs are an order ofmagnitude larger than organic dyes.Therefore, applications such as real-timemonitoring of biomolecular interactions (inwhich steric hindrance is of concern) mayrequire the use of organic dyes, as QDssmaller than 1 nm are inherently unstable.In addition, most organic dyes are ofsimilar sizes, so that fluorophores ofdifferent emissions are similar sterically,compared to the large difference in QDsize required to tune their wavelength.However, it has been reported that theemission wavelengths of alloy QDs may betuned by altering the alloy composition,while keeping the size constant.5

3. Chemical synthesis andbioconjugationA wide variety of QD synthesis protocolshave been developed, and the synthesis ofCdSe has become the most studied andbest understood. Nearly all synthesesinvolve the combination of a cadmiumprecursor and a selenium precursor in the

presence of a ligand that binds to theresulting nanoparticle surface for steric orelectrostatic stabilization, and to preventgrowth into bulk semiconductor. Thehighest quality QDs have been synthesizedat high temperature in a coordinatingsolvent, in which the entire solvent is ananoparticle ligand.12,13 Trioctylphosphineoxide (called TOPO) is the mostcommonly used ligand, and others such astrioctylphosphine (TOP) andhexadecylamine (HDA) are frequentlyused together. In comparison withcoordinating solvent syntheses, aqueousprocedures have so far not yielded particlesof comparable monodispersity andphotoluminescence efficiency.

QDs are normally modified aftersynthesis to improve their opticalproperties and for conjugation tobiomolecules. A shell of wide-bandgapsemiconductor (usually zinc sulfide) istypically grown on the core CdSe in orderto protect the QD, resulting in higherstability and increased brightness.14 Unlikethe aqueous syntheses, QDs synthesized inhigh temperature organic solvents have thedrawback of only being soluble in organic,nonpolar solvents, due to the alkyl chainsof the ligands that extend from the QDsurface. Therefore use in aqueous solutionsrequires a phase-transfer technique that

attaches polar functional groups to thesurface of the QD. Two general methodshave been developed to overcome thisproblem (Fig. 3). Hydrophobic ligandsmay be exchanged with bifunctionalligands, such as mercaptoacetic acid,which contains a thiol functional groupthat binds to the QD surface, allowing thecarboxylic acid group to extend away fromthe QD.10 The resulting nanoparticle issoluble in aqueous and polar solvents. Asecond phase transfer method utilizesamphiphilic polymers that containhydrophilic groups to render watersolubility, and hydrophobic alkyl chains tointerdigitate with the alkyl chains of QDligands.9,15 This later procedure has beenshown to yield QDs that are brighter andmore stable than surface-exchanged QDsdue to the retention of the native TOPO,HDA, or TOP ligands. Once soluble inwater, QDs may be conjugated tobiological probes such as nucleic acids,proteins, or small molecules. It has beenshown that QDs may be bioconjugatedelectrostatically,16 covalently,10 or throughlinker interactions such as biotin-avidin.11

4. Biological applicationsSince the first major reports using QDs forbiological labeling in 1998, the rate ofpublications in this field has dramaticallyincreased.10,11 Many reports haveconcentrated on simply replacing organicdyes with QDs, without utilizing theirunique properties. This analysis will focuson the publications that have exploitedtheir resistance to photobleaching andpotential for multiplexed detection.

Bioanalytical assaysOrganic fluorophores are commonly usedas reporters in a large number of in vitrobioassays, such as quantitativeimmunoassays and fluorescence quenchingassays for macromolecular interactions.High sensitivity has been realized with theuse of organic dyes, but the spectralproperties of QDs could lead to furtherimprovements. Research in the applicationof QDs for in vitro bioanalysis has beenadvanced by Mattoussi and his coworkersat the US Naval Research Laboratory,17,18

and can be divided into two areas:immunoassays and biosensors.

Immunoassays typically involve thespecific binding of a labeled antibody to ananalyte, followed by physical removal ofunbound antibody to allow thequantification of the bound label. QDshave been conjugated to antibodies for usein an assortment of thesefluoroimmunoassays for detection ofproteins and small molecules.19,20 Theresults of these studies proved that QDsmay be used as “generalized” reporters inimmunoassays, but did not demonstrate an

Fig. 2 Comparison of optical absorption (A) and fluorescence emission (B) spectra betweensemiconductor quantum dots and organic dyes. The data shown above are for CdSe QDs andfluorescein isothiocyanate (FITC). The emission peaks have nearly identical wavelengths but the QDpeak is narrower and more symmetric (~25 nm FWHM).

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advantage over organic fluorophores, inthat their sensitivity was comparable tothat of commercial assays (proteinconcentrations down to 2 ng mL21, or 100pM).19 The main advantages of QDs forimmunoassays are their narrow, symmetricemission profiles and the excitability ofmany different QDs with a single lightsource, allowing the detection of multipleanalytes simultaneously. Taking advantageof these spectral properties, Goldman etal.18 simultaneously detected four toxinsusing four different QDs, emitting between510 and 610, in a sandwich immunoassayconfiguration. Although there was spectraloverlap of the emission peaks,deconvolution of the spectra revealedfluorescence contributions from all fourtoxins. This assay was far fromquantitative, however, and it is apparentthat fine-tuning of antibody cross-reactivitywill be required to make multiplexedimmunoassays useful.

Whereas immunoassays require thephysical separation of unbound QDconjugates prior to analysis, biosensors can

be developed to detect biomolecular targetson a real-time or continuous basis. QDs areideal for biosensor applications due to theirresistance to photobleaching, allowing forcontinuous monitoring of a signal.Fluorescence resonance energy transfer(FRET) has been the major proposedmechanism to render QDs switchable froma quenched “off” state to a fluorescent“on” state.21 FRET is the nonradiativeenergy transfer from an excited donorfluorophore to an acceptor.22 The acceptorcan be any molecule (another nanoparticle,a nonemissive organic dye, or fluorophore)that absorbs radiation at the wavelength ofdonor emission. QDs are promising donorsfor FRET-based applications due to theircontinuously tunable emissions that can bematched to any desired acceptor, and theirbroadband absorption, allowing excitationat a short wavelength that does not directlyexcite the acceptor.

It has been confirmed that QDs can beFRET donors, quenchable with efficienciesup to 99%, using organic fluorophores,17,23

nonemissive dyes,24 gold nanoparticles,25

or other QDs as acceptors.25,26 Medintz etal.24 used QDs conjugated to maltosebinding proteins as an in situ biosensor forcarbohydrate detection. Adding a maltosederivative covalently bound to a FRETacceptor dye caused QD quenching, andfluorescence was restored upon addition ofnative maltose, which displaced the sugar-dye compound. A key element of this workwas that the physical orientation andstoichiometry of the maltose receptors onthe QDs were controlled so that therestoration of QD fluorescence uponmaltose addition could be directly relatedto maltose concentration. Although theFRET quenching efficiency was low, thiswork demonstrates the potential of QD-based in situ biosensing. QD biosensorshave also been assembled that do notrequire binding and dissociation tomodulate quenching and emission.Medintz et al.27 conjugated a donor QD toa photoresponsive dye that becomes anacceptor after exposure to UV light, andbecomes FRET-inactive following white-light exposure. In this system, lightexposure acts as an on–off switch, but thequenching efficiency was only ca. 60%and decreased after repeated cycles ofmodulation. Nonetheless, QD biosensorscould become an important tool forstudying biological interactions if efficientquenching is used to improve the detectionsensitivity.

QD-encodingRather than using single QDs forbiological detection schemes, it has beenproposed that different colors of QDs canbe combined into a larger structure, such asa microbead, to yield an “opticalbarcode.”28 With the combination of 6 QDemission colors and 10 QD intensity levelsfor each color, one million different codesare theoretically possible. Biologicalmolecules may be optically encoded byconjugation to these beads, opening thedoor to the multiplexed identification ofmany biomolecules for high-throughputscreening of biological samples.Pioneering work was reported by Han etal.28 in 2001, in which 1.2 mm polystyrenebeads were encoded with three colors ofQDs (red, green, and blue) and differentintensity levels. The beads were thenconjugated to DNA, resulting in differentnucleic acids being distinguished by theirspectrally distinct optical codes. Theseencoded probes were incubated with theircomplementary DNA sequences, whichwere also labeled with a fluorescent dye asa target signal. The hybridized DNA wasdetected through colocalization of thetarget signal and the probe optical code,via single-bead spectroscopy, using onlyone excitation source. The bead codeidentified the sequence, while the intensityof the target signal corresponded to the

Fig. 3 Methods for rendering hydrophobic QDs water soluble and biocompatible. Organiccompounds are drawn disproportionately large for clarity. (A) Ligand exchange method: schematicrepresentation of TOPO-coated QD demonstrates the displacement of TOPO ligands by bifunctionalhydrophilic ligands. This method may be used to modify the surface of QDs with different terminalfunctional groups such as carboxylic acids or silica. (B) Amphiphilic polymer method: nativehydrophobic ligands may be retained and used to interact with an amphiphilic polymer, such asoctylamine-modified polyacrylic acid. The surfaces of water-soluble QDs may be conjugated tobiomolecules as noted in the text.

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presence and abundance of the target DNAsequence.

The high-throughput potential of thisseminal report was realized in 2003 withthe use of a similar system to detect DNAsequences that differed by only onenucleotide (single nucleotidepolymorphisms).29 In this work, 194samples of 10 different DNA sequencesfrom specific alleles of the humancytochrome P450 gene family werecorrectly identified by hybridization toencoded probes. High-throughput analysiswas achieved by the use of flow cytometryto identify spectral codes, rather thansingle-bead spectroscopy. Thisidentification would have beenconsiderably more difficult with organicfluorophores due to the fact that theiremission peaks overlap, obscuring thedistinct codes, and the fact that multipleexcitation sources would be required.

Once encoded libraries have beendeveloped for identification of nucleic acidsequences and proteins, solution-basedmultiplexing of QD-encoded beads couldquickly produce a vast amount of genomicand protein expression data. Anotherapproach to gene multiplexing has been theuse of planar chips, but bead-basedmultiplexing has advantages of greaterstatistical analysis, faster assaying time, theflexibility to add additional probes at lowercosts.8

Biological labelingFluorescent dyes are used routinely fordetermining the presence and location ofbiological molecules in cultured cells andtissue sections. Two original papers in1998 demonstrated the feasibility of usingQDs for cell labeling, displaying distinctadvantages over organic dyes (Fig. 4).Bruchez et al.11 demonstrated dual-colorlabeling of fixed mouse fibroblasts,staining the nucleus with green QDs, andlabeling the F-actin filaments in thecytoplasm with red QDs. Chan and Nie10

showed that QDs maintained their brightfluorescence in live cells, by imaging theuptake of transferrin-conjugated QDs byHeLa cells. These studies showed that QDswere brighter and more photostable thanorganic fluorophores, a claim that has beenverified by independent reports.9,30

In 2003, QDs were used for the firsttime to visualize cellular structures at highresolution, as Wu et al.9 illustratedimmunocytochemical stains of membrane,cytoplasmic, and nuclear antigens in fixedcells. Although imaging of fixed cells isuseful and sufficient for many applications,live cell microscopy is ideal for visualizingcellular processes, but is considerably moredifficult. It has been shown that many celltypes naturally engulf QDs through anonspecific uptake mechanism.31–33 Thismechanism was used to track the migration

of breast tumor cells on a substrate coatedwith red QDs; the fluorescence inside thecells increased as the cells transversed andengulfed the QDs, leaving behind a darkpath.31 This and other studies32

demonstrated that QDs can be imagedinside living cells for long periods of time(over a week), a task that is not possiblewith organic fluorophores due tophotobleaching. Indeed, QDs have openedup a new avenue for studying biomolecularprocesses inside living cells.

Two true marvels of real-time live cellimaging have recently been demonstratedusing QDs. Dahan et al.34 labeled glycinereceptors on neuronal membranes withQDs. Imaging of the cells revealed theability to observe the motion of singleQDs, in real time (single, isolated QDs canbe identified visually because they“blink”). This first example of singlemolecule detection using QDs in livingcells produced remarkable movies ofglycine receptor diffusion. In a secondreport, Lidke et al.35 used QDs conjugatedto epidermal growth factor (EGF) tomonitor the interactions between EGF andthe erbB/HER receptor on living cellmembranes. Single molecules of EGF werevisualized, in real time, as they bound toreceptors and were endocytosed. Thisallowed the study of receptor interactions,revealing a new cellular filopodia transportphenomenon.

Because the use of QDs as reporters in

living cells is quickly becomingconventional, the possibility of QDcytotoxicity is of interest and concern.Almost all of the reports of QDs in livingcells have revealed little or no obviouscytotoxicity or changes in cellulardifferentiation.15,31–33,36 QDs contain toxicelements, most importantly divalentcadmium, but an in-depth study of thecytotoxicity of QDs in liver hepatocytesrevealed that appropriate protection of theQD surface from oxidation preventsrelease of cadmium ions, thus minimizingtoxicity.37 Cytotoxicity issues may onlybecome relevant for truly long term(months to years) visualization of QDs incells, a time period in which QD oxidationcould become significant.

QDs have been used as labels forstudying single molecules that interact withthe membranes of living cells. Performingthe same task with intracellular targets willbe much more difficult, but is essential forvisualizing processes in living cells.Advanced delivery methods are needed todeliver QD probes into living cells, and thedelivered probes most be available forbinding to intracellular targets, not trappedin endosomes, lysosomes, or otherorganelles. Until this becomes possible,intracellular processes can only bemodeled by using isolated macromoleculesunder in vitro conditions. For example,QD-actin bioconjugates have been used toobserve single molecule motorized motionof actin filaments sliding across myosinproteins in an ATP-driven reaction.38 Alsochaperonin proteins have been shown toenclose QDs in vitro, and were capable ofreleasing them upon addition of ATP.39

These model systems should bevisualizable intracellularly once a protocolfor the translocation across the cellularmembrane is established.

In vivo imagingThe progression from optical microscopyof cells in vitro to optical imaging of entireorganisms has mainly been hampered bypoor penetration of visible light throughtissue.40 Due to this attenuation problem,QDs were initially used as optical imagingcontrast agents only in simple modelsystems. In 2002 Akerman et al.41

conjugated QDs to peptides for targetingendothelial cell receptors in specific tissues(lung, tumor blood vessels, or tumorlymphatic vessels). Intravenous injectionof these bioconjugated nanoparticles into amouse revealed accumulation of QDs inthe targeted tissue, visualizedhistologically. Whole organism imagingwas not performed in this work, but wasachieved by Dubertret et al.15 on smallXenopus embryos containing intracellularQDs. Microinjection of more than a billionQDs into single cells allowed cell lineagetracking and real time imaging of stably

Fig. 4 Immunofluorescence labeling of humanbreast tumor cells with antibody-conjugatedquantum dots, and comparison of signalbrightness and photostability with organic dyes.(a) Cancer cells labeled with antibody-conjugated QD or Texas Red (TR) targeting cellsurface antigen uPAR. (b) Cancer cells labeledwith antibody-conjugated QD or FITC targetingcell surface antigen Her-2/neu. Excitation froma 100 W mercury lamp caused negligiblephotobleaching of QDs, compared to the twoorganic fluorophores. (Courtesy of Dr XiaohuGao, Emory University.)

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fluorescent QDs.To solve the attenuation problem for

optical imaging in larger organisms and indeeper tissue, it has been shown that thefar-red and near-infrared (NIR) spectralregions are characterized by less scatteringand absorption by biological tissue.40 Forsensitive detection, wavelengths must bechosen so that excitation light canpenetrate tissue to the desired depth, andthe emitted light must be able to travelback to a photodetector. A theoretical studyof the ideal wavelengths for fluorescenceimaging revealed that the tissue type willlikely dictate which wavelengths are ideal,but biological tissue is generally moretransparent at wavelengths longer than thatof visible light (>650 nm).42 Therefore,Larson et al.43 used NIR radiation to excitevisible QDs in mouse capillaries usingmultiphoton fluorescence imaging.Multiphoton fluorescence is the excitationof one QD with multiple longerwavelength photons for the emission of asingle photon of shorter wavelength. Thisangiographic imaging system allowed real-time imaging with better detail and greaterbrightness than conventional fluorophores.However because the emission light was inthe visible spectrum, it is likely that thistechnique would not allow detection indeep tissue.

For deep tissue penetration, QDs thatemit in the NIR will be required. Severalsemiconductor materials have been used togenerate bright QDs that emit between650–2000 nm.5,6,44–46 There are noconventional dyes that are bright andphotostable that can emit fluorescence lightbeyond ~850 nm,47,48 which is why QDsare expected to provide substantialadvantages for NIR optical imaging. NIRQDs with emission maxima between750–860 nm were used to image coronaryvasculature in a rat model,42 and tovisualize sentinel lymph nodes in a pig, in1 cm deep tissue.48

Most imaging systems generate imagecontrast based on attenuation of radiationthrough tissue. Imaging with contrastbased on molecular differences in tissue iscalled molecular imaging.49 Organicfluorescent dyes and fluorescent proteinshave already been used as contrast agentsfor fluorescent molecular imaging inanimal models.47,50 Indeed NIR QDs willbe powerful tools for molecular imagingbecause they can be imaged in real timewith multiplexed detection to monitorbiomolecular phenomena in vivo. In ourown lab we have recently been able toperform molecular imaging for thedetection of subdermal tumors. Targetingof antibody-conjugated QDs to tumors hasallowed generation of whole-bodyfluorescence images of mice with contrastbased on biomolecular differences betweennormal and cancerous tissue (Fig. 5).

5. Future directionsQDs have already fulfilled some of theirpromise as ground-breaking biologicallabels. The tremendous amount of interestin QDs is sure to quickly improve theprevious applications and inspire newones. Organic fluorophores may never becompletely supplanted due to theirinherently small size, but research in thepast 5 years has shown that QDs offerremarkable advantages.

In the near future, the development ofefficient QD biosensors may make QDsinto powerful tools not just for in vitrobiosensing, but also for living cell studiesand in vivo imaging. Biosensors based onorganic fluorophores have already shownpromise in vivo, as tumors in mice weredetected with “stealth” quenched probes,activatable upon exposure to proteases inthe tumor microenvironment.51 The abilityof a biochemical signal to switch a QDfrom an “off” to an “on” state would be animportant and powerful tool for studyingintracellular signaling. The development ofa “quantum dot beacon” would be amonumental advance. For this to become areality, however, biosensing QDs must betranslocated across the cell membrane.Although microinjection and nonspecificuptake have been used,15,32 widespreadapplications must wait for a generalizablemethodology for efficient delivery of QDprobes into living cells. Recent research inour lab has shown that delivery peptidescan be used for rapid intracellular QDtranslocation, and other groups havealready demonstrated their efficacy for thedelivery of other types of nanoparticles.52

The QD combination of real time imaging,multiplexing capabilities, single moleculedetection, and biological sensing on thenanoscale should allow scientists toaddress a broad array of analyticalproblems and biological questions.

A major goal in nanotechnologyresearch is to develop smartmultifunctional devices with nanometerdimensions. Although this is a lofty goalseemingly for the distant future, many

multifunctional devices have already beencreated, and many tools have beendeveloped for the assembly of QDs intocomplex, ordered structures.53–55 Oneproposed multifunctional device is ananoscale contrast agent for multi-modalityimaging. QDs are fluorescent contrastagents, but can also be used as markers forelectron microscopy due to their highelectron density.56 Multimodal imaging hasalready been performed in cells to correlatefluorescence staining with electronmicrographs using QDs.34,56 Further, QDshave been combined with MRI contrastagents (superparamagentic iron oxidenanoparticles) (X. Gao and S. Nie,personal communication). By correlatingthe deep imaging capabilities of magneticresonance imaging (MRI) withultrasensitive optical fluorescence, asurgeon could visually identify tiny tumorsor other small lesions during an operationand remove the diseased cells and tissuecompletely. Medical imaging modalitiessuch as MRI and PET (positron emissiontomography) can detect diseasesnoninvasively, but they do not provide avisual guide during surgery. Thedevelopment of magnetic or radioactiveQD probes could solve this problem.

Another desired multifunctional devicewould be the combination of a QDimaging agent with a therapeutic agent.Not only would this allow tracking ofpharmacokinetics, but diseased tissuecould be treated and monitoredsimultaneously and in real time.Surprisingly QDs may be innatelymultimodal in this fashion, as they havebeen shown to have potential activity asphotodynamic therapy agents.57 With QDsbridging the gap between nanotechnologyand biomedicine it will be thrilling towatch how these fluorescent nanoparticleschange these fields in the near future.58

AcknowledgementsThis work was supported by a grant fromthe National Institutes of Health (R01GM60562), the Georgia Cancer Coalition(Distinguished Cancer Scholar Award toS.N.), and the Coulter TranslationalResearch Program at Georgia Tech andEmory University. We are grateful to Dr.Xiaohu Gao for stimulating discussionsand for providing Figs. 4 and 5 for thisarticle. A.M.S. acknowledges the WhitakerFoundation for generous fellowshipsupport.

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Fig. 5 In vivo tumor imaging with quantumdots. This picture shows a fluorescence imageof a live mouse with QD-tagged cancer cellsinjected into its right flank (orange glow). Notethat the tumor fluorescence signals do notoverlap with the background fluorescence (skinautofluorescence) due to the large Stokes shiftsthat are possible with QDs. (Courtesy of DrXiaohu Gao, Emory University.)

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