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The ability to systematically modify the properties ofnanostructures by controlling their structure and their surfaceproperties at a nanoscale level makes them extremely attractivecandidates for use in biological contexts, from fundamentalscientific studies to commercially viable technologies.

Addresses*Department of Bioengineering, †Department of Electrical andComputer Engineering and Department of Chemistry, Rice University,6100 South Main, Houston, TX 77005-1892, USA*e-mail: jwest@rice.edu†e-mail: halas@rice.edu

Current Opinion in Biotechnology 2000, 11:215–217

0958-1669/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved.

IntroductionThe expanding availability of a variety of nanostructures withhighly controlled properties in the nanometer size range hassparked widespread interest in their use in biotechnologicalsystems. Size does matter — the fact that nanoparticles aresimilar in size range to many common biomolecules makesthem appear to be natural companions in hybrid systems.More importantly, however, are the new and unique proper-ties that nanostructures bring to biotechnological applications.By controlling structure precisely at nanoscale dimensions,one can control and tailor properties of nanostructures, such assemiconductor nanocrystals and metal nanoshells, in a veryaccurate manner. In addition, one can make modifications tonanostructures to better suit their integration with biologicalsystems; for example, modifying their surface layer forenhanced aqueous solubility, biocompatibility, or biorecogni-tion. With selected biomolecules bound to nanostructuresurfaces, new ‘hybrid’ nanostructures can be obtained forapplications such as biosensing and imaging, or nanostruc-tures can be embedded in other biocompatible materials tomodify material properties or impart new functionality.

The idea of merging biological and nonbiological systemsat the nanoscale level is not a new one. The broad field ofbioconjugate chemistry is based on combining the func-tionalities of biomolecules and non-biologically derivedmolecular species for specialized use in applications rang-ing from markers for research in cell and molecular biologyto biosensing, bioimaging and masking of immunogenicmoieties to targeted drug delivery [1]. Many current appli-cations of nanostructures in biotechnology are a naturalevolution of this approach. In fact, several of the ‘break-through’ applications recently demonstrated usingnanostructure–biomolecular hybrids are in fact traditionalapplications originally addressed by standard molecularbioconjugate techniques that have been revisited withthese newly designed nanostructure hybrids.

So one might argue, why replace conventional moleculartags, such as fluorescent chromophores, with nanostruc-tures? Typically, nanostructures possess properties farsuperior to the molecular species they replace — higherquantum efficiencies, greater scattering or absorbancecross sections, optical activity over more biocompatiblewavelengths, and significantly increased chemical or pho-tochemical stability. The systematic control ofnanostructure properties obtained by controlled variationsin particle size and dimension is in direct contrast to mole-cular tags, whose properties vary nonsystematicallybetween molecular species. This systematic variation ofproperties via structure variation not only improves tradi-tional applications, but also leads to new, uniqueapplications well beyond the scope of conventional molec-ular bioconjugates. A prime example is the opticalproperties of semiconductor nanocrystals and metalnanoshells, which are new and robust fluorophores,absorbers and scatterers in the near infrared, a region of theelectromagnetic spectrum where tissue is essentially trans-parent. The availability of these new nanostructures willgreatly facilitate new in situ probes and sensor methods.

In this article, we introduce several successful examplesof nanostructures that have been integrated with biomol-ecular species and applied to relevant problems inbiotechnology. The use of bioconjugate semiconductornanocrystals, or ‘quantum dots’, as fluorescent biologicallabels will be discussed. A new and powerful assay basedon the optical properties of bioconjugate gold nanoparti-cles, a new innovation on a traditional bioconjugatetechnology, will also be described. The biotechnological-ly friendly properties of gold nanoshells are summarized,and a novel photothermally triggered drug delivery sys-tem based on a new nanoshell–polymer composite willalso be discussed.

Bioconjugate quantum dots as fluorescentbiological labelsSemiconductor nanocrystals are highly light absorbing,luminescent nanoparticles whose absorbance onset andemission maximum shift to higher energy with decreas-ing particle size, due to quantum confinement effects [2].These nanocrystals are in the size range of 2–8 nm indiameter. Unlike molecular fluorophores, which typicallyhave very narrow excitation spectra, semiconductor quan-tum dots absorb light over a very broad spectral range.This makes it possible to optically excite a broad spec-trum of quantum dot colors using a single excitation laserwavelength, which enables one to simultaneously probeseveral markers. Although the luminescence properties ofsemiconductor nanocrystals have historically been sensi-tive to their local environment and nanocrystal surface

Applications of nanotechnology to biotechnologyCommentaryJennifer L West* and Naomi J Halas†

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preparation, recent core-shell geometries where thenanocrystal is encased in a shell of a wider band gap semi-conductor have resulted in increased fluorescencequantum efficiencies (> 50%) and greatly improved pho-tochemical stability. In the visible region, CdSe–CdScore-shell nanocrystals have been shown to span the visi-ble region from 550 nm (green) to 630 nm (red). Othermaterials systems, such as InP and InAs, provide quan-tum dot fluorophores in the near infrared region of theoptical spectrum, a region of high physiological transmis-sivity. Although neither II-VI nor III-V semiconductornanocrystals are water soluble, let alone biocompatible,surface functionalization with molecular species such asmercaptoacetic acid or the growth of a thin silica layer onthe nanoparticle surface facilitate aqueous solubility [3].Both the silica layer and the covalent attachment of pro-teins to the mercaptoacetic acid coating permit thenanoparticles to be biocompatible. Specific binding tocell surfaces, insertion into cells, and binding to cellnuclei have all been demonstrated following conjugationof the nanoparticle with the appropriate targeting protein [2].

A gold nanoparticle bioconjugate-basedcolorimetric assayThe use of gold colloid in biological applications began in1971, when Faulk and Taylor invented the immunogoldstaining procedure. Since that time, the labeling of target-ing molecules, especially proteins, with gold nanoparticleshas revolutionized the visualization of cellular or tissuecomponents by electron microscopy [4]. The optical andelectron beam contrast qualities of gold colloid have pro-vided excellent detection qualities for such techniques asimmunoblotting, flow cytometry, and hybridization assays.Conjugation protocols exist for the labeling of a broadrange of biomolecules with gold colloid, such as protein A,avidin, streptavidin, glucose oxidase, horseradish peroxi-dase, and IgG [5].

Gold nanoparticle conjugation was recently applied topolynucleotide detection in a manner that exploited thechange in optical properties resulting from plasmon–plas-mon interactions between locally adjacent goldnanoparticles [6]. The characteristic red of gold colloid haslong been known to change to a bluish-purple color uponcolloid aggregation. In the case of polynucleotide detection,mercaptoalkyloligonucleotide-modified gold nanoparticleprobes were prepared. When a single-stranded targetoligonucleotide was introduced into solution, a polymer net-work was formed consisting of the target oligonucleotideand the conjugated nanoparticles. This condensed networkbrought the nanoparticles into close enough vicinity toinduce a dramatic red-to-blue macroscopic color change.Because of the extremely strong optical absorption of goldcolloid, this colorimetric method can be used to detect~10 fmol of an oligonucleotide, which is 50 times more sen-sitive than sandwich hybridization detection methods basedon fluorescence detection.

A gold nanoshell–polymer compositephotothermally triggered drug delivery systemGold nanoshells are new composite nanoparticles that com-bine infrared optical activity with the uniquelybiocompatible properties of gold colloid. Metal nanoshellsare concentric sphere nanoparticles consisting of a dielectric(typically gold sulfide or silica) core and a metal (gold) shell[7]. By varying the relative thickness of the core and shelllayers, the plasmon-derived optical resonance of gold can bedramatically shifted in wavelength from the visible regioninto the infrared over a wavelength range that spans theregion of highest physiological transmissivity (see Figure 1)[8]. By varying the absolute size of the gold nanoshell, it canbe made to either selectively absorb (for particle diameters<~75 nm) or scatter incident light. Because the gold shelllayer is deposited using the same chemical methods used togrow gold colloid, the surface properties of gold nanoshellsare virtually identical to those of gold colloid. The same con-jugation protocols used to bind a wide variety ofbiomolecules to gold colloid are therefore readily transfer-able to gold nanoshell conjugation. Successful goldnanoshell conjugation with enzymes and antibodies hasbeen demonstrated. Gold nanoshells also take advantage ofthe inherent biocompatibility of gold, not requiring furthersurface functionalization or protective layer growth.

When optically absorbing gold nanoshells are embedded ina matrix, illuminating them at their resonance wavelengthcauses the nanoshells to transfer heat to their local envi-ronment. This photothermal effect can be used to optically‘remote control’ drug release in a nanoshell–polymer com-posite drug delivery material [9]. Copolymers ofN-isopropylacrylamide (NIPAAm) and acrylamide (AAm)

216 Commentary

Figure 1

Optical resonances of gold-shell–silica-core nanoshells, as a functionof their core : shell ratio, for particles of 120 nm core diameter.Respective spectra correspond to the nanoparticles depicted beneath.Reproduced from [8] with permission.

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exhibit a critical solution temperature (LCST) that isslightly above body temperature. When the temperature ofthe copolymer exceeds the LCST, the material collapsescausing a burst release of any soluble material held withinthe polymeric matrix. Gold nanoshells that had beendesigned to strongly absorb near infrared light have beenincorporated into poly(NIPAAm–co-AAm) hydrogels; lightat these wavelengths (800–1200 nm), which can be trans-mitted through tissue with relatively little attenuation, isabsorbed by the nanoparticles and converted to heat, thuscausing the copolymer to collapse as its temperatureexceeds its LCST. Significantly enhanced drug release hasbeen achieved in response to irradiation by Nd:YAG laserlight at 1064 nm. The triggered release of methylene blueand proteins of varying molecular weight has also beendemonstrated (Figure 2). The nanoshell–composite hydro-gels can also release multiple bursts of protein in responseto repeated near IR irradiation.

ConclusionNanotechnology in the form of nanoparticles whose proper-ties can be precisely tailored by chemical methods is rapidlybecoming an important new tool in the arsenal of thebiotechnologist. Nanostructures will no doubt lead to newand improved assays and sensing methods, new optically

controlled functional materials, new highly specific color-coded probes of cellular function, and new optically basedtherapeutic methods.

References 1. Hermanson GT (Ed): Bioconjugate Techniques. San Diego: Academic

Press; 1996.

2. Bruchez M Jr, Moronne M, Gin P, Weiss S, Alivisatos AP:Semiconductor nanocrystals as fluorescent biological labels.Science 1998, 281:2013-2016.

3. Chan WCW, Nie S: Quantum dot bioconjugates for ultrasensitivenonisotopic detection. Science 1998, 281:2016-2018.

4. Hayat M (Ed): Colloidal Gold: Principles, Methods and Applications.San Diego: Academic Press; 1989.

5. Anonymous: Immunochemistry Labfax. Oxford, UK: BIOS ScientificPublishers; 1994.

6. Elghanian R, Storhoff JJ, Mucic RC, Letsinger RL, Mirkin CA:Selective colorimetric detection of polymucleotides based on thedistance-dependent optical properties of gold nanoparticles.Science 1997, 277:1078-1081.

7. Averitt RD, Sarkar D, Halas NJ: Plasmon resonance shifts ofAu-coated Au2S nanoshells: insight into multicomponentnanoparticle growth. Phys Rev Lett 1997, 78:4217-4220.

8. Oldenburg SJ, Averitt RD, Westcott SL, Halas NJ: Nanoengineeringof optical resonances. Chem Phys Lett 1998, 288:243-247.

9. Sershen S, Westcott SL, Halas NJ, West JL: Temperature-sensitivepolymer-nanoshell composites for photothermally modulateddrug delivery. J Biomed Mater Res 2000, in press.

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Figure 2

(a) Release of bovine serum albumin (BSA) from non-irradiated(diamond), irradiated NIPAAm–co-AAm hydrogels (triangle), andirradiated nanoshell-composite hydrogels (square). Irradiation was at

1064 nm. (b) Release of BSA from nanoshell-composite hydrogels inresponse to sequential irradiation at 1064 nm.

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