Biomolecule-Functionalized Carbon Nanotubes: Applications in Nanobioelectronics

Biomolecule-Functionalized Carbon Nanotubes: Applications in Nanobioelectronics
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  1084  ¹ 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  DOI: 10.1002/cphc.200400193  ChemPhysChem  2004  , 5, 1084±1104  Biomolecule-Functionalized Carbon Nanotubes:Applications in Nanobioelectronics Eugenii Katz and Itamar Willner* [a] 1. Introduction The rapid progress in nanotechnology and nanoscience intro-duced a scientific momentum that involves the fundamentalunderstanding of the properties of nanostructures, the synthe-sis of nanoscale materials, the imaging of nanostructures, andthe assembly of functional nanoscale devices. [1] The discoveryof unique optical, [2] photophysical, [1a] electronic, [3] and catalyt-ic [4] properties of nanoparticles or nanorods consisting of metals (e.g., Ag, Au, Cu) [5] or semiconductors (e.g., CdS, CdSe,TiO 2 , InP) [6] has established nanoscale building blocks for theassembly of functional nanostructures and devices. [1a,b] The chemical functionalization of nanoparticles or nanorodshas enabled the fabrication of two-dimensional or three-di-mensional architectures of composite nanostructures on surfa-ces. [1a] Functional devices such as specific sensors, [1a,7] systemsexhibiting tunable electrochemiluminescence [8] or enhancedphotoelectrochemistry, [9] nanoparticle-based switches, orsingle-electron transistors [10] have been reported. Within thesegeneral activities, the use of biomaterial±nanoparticle hybridsystems for biosensor, bioelectronic, and circuitry applicationshas substantially advanced, and these efforts have establishedthe rapidly developing field of nanobioelectronics and nano-biotechnology. [11] For example, the integration of metallicnanoparticles with biomaterials [12] (e.g., enzymes, [12a] nucleicacids, [12b,c] or antigens/antibodies [12d,e] ) has led to the develop-ment of electrochemical or optical biosensors. Similarly, the in-tegration of biomaterials [13] (e.g., enzymes, [13a] DNA, [13b±d] or an-tigens/antibodies [13e,f] ) with semiconductor nanoparticles hasled to the development of optical or photoelectrochemical bi-osensor systems. Also, nanoparticles have been incorporatedinto biomaterials that act as templates, and the resulting struc-tures have been grown into metallic or semiconductor nanocir-cuitry. [14] This latter approach was suggested as a ™bottom-up∫miniaturization method for fabricating nanostructures with di-mensions that are smaller than the presently achievable pat-terns using lithography.The unidirectional growth of materials to form nanowires ornanotubes has attracted substantial interest in the past fewyears, and nanowires (or tubes) consisting of organic or inor-ganic materials exhibiting conductive, semiconductive, or insu-lating properties have been prepared. [15] By modifying nano-wires or nanotubes, nanostructures and nanocircuitry of highercomplexity and hierarchical functionality have been compo-sed. [15c] Shortly after the successful laboratory synthesis of full-erenes, [16] carbon nanotubes (CNTs) were discovered, [17] andsince then they have become a material that has attracted sub-stantial theoretical and experimental interest. [18] Single-walledcarbon nanotubes (SWCNTs) [17±19] are one-dimensional molecu-lar wires (1±2 nm diameter) exhibiting special structural fea-tures and unique electronic properties that have focused inter-est in their application as active components in future solid-state nanoelectronics [20±24] and optoelectronics. [25] Different methods of synthesizing, isolating, and purifyingCNTs have been developed in the last few years. [26] CNTs canbe cut into smaller segments by sonication (agitation using ul-trasound) in concentrated acid mixtures, and the resultingfragmental CNTs can be separated into tubes of narrow lengthdistributions using chromatography. [19b] Aligned assemblies of CNTs with controllable length and density have been generat-ed. [27] Chemical strategies to tune the nanotube electronicproperties have been developed. The selective functionaliza-tion of SWCNTs (e.g., with thiol groups) and their attachmentto preorganized surfaces (e.g., gold) allow the assembly of CNTs in devices and, most importantly, provide low-resistancecontacts of CNTs to other electronic components. With suchtechniques in hand, CNTs should find applications in both the [a]  Dr. E. Katz, Prof. I. Willner Institute of Chemistry, The Hebrew University of Jerusalem Jerusalem 91904 (Israel)Fax: (    972)2-6527715E-mail: willnea@vms.huji.ac.il  Carbon nanotubes (CNTs) revealing metallic or semiconductive properties depending on the folding modes of the nanotubewalls represent a novel class of nanowires. Different methods toseparate semiconductive CNTs from conductive CNTs have beendeveloped, and synthetic strategies to chemically modify the sidewalls or tube ends by molecular or biomolecular componentshave been reported. Tailoring hybrid systems consisting of CNTsand biomolecules (proteins and DNA) has rapidly expanded and attracted substantial research effort. The integration of biomate-rials with CNTs enables the use of the hybrid systems as activefield-effect transistors or biosensor devices (enzyme electrodes,immunosensors, or DNA sensors). Also, the integration of CNTswith biomolecules has allowed the generation of complex nano-structures and nanocircuitry of controlled properties and func-tions. The rapid progress in this interdisciplinary field of CNT-based nanobioelectronics and nanobiotechnology is reviewed by summarizing the present scientific accomplishments, and ad-dressing the future goals and perspectives of the area. ChemPhysChem  2004  , 5, 1084±1104  DOI: 10.1002/cphc.200400193  ¹ 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  1085  construction and use of novel nanoscale devices such as bio-sensors, fuel cells, and in molecular electronics. Though graph-itic sidewall functionalization will unavoidably lead to somemodifications of the nanotube delocalized  p -system, this canoffer a convenient and controllable means of tethering molec-ular species. [28] The integration of CNTs with biological systems to formfunctional hybrid assemblies is, however, a new and relativelyunexplored area. [29] Carbon nanotubes as well as other nano-tube structures, such as self-assembled lipid microtubes orpeptide nanotubes, have been explored for possible applica-tions in nanobiotechnology. [30] Also, biomedical applications of biomaterial-functionalized CNTs are envisaged. [25] Functional-ized CNTs are able to cross cell membranes and accumulate inthe cytoplasm, and even reach the nucleus, without being cy-totoxic (in concentrations up to 10 m m) . [31] Thus, CNTs couldact as carriers that transport and deliver other bioactive com-ponents into cells. In fact, the effective delivery of biomole-cules into cells has been used for their immunization and en-hanced generation of antibodies. [32] Pioneering studies have re-ported on the use of SWCNTs as atomic force microscopy(AFM) imaging tips of biomacromolecules, such as antibodies,DNA, and  b -amyloid protofibrils (a constituent of the amyloidplaques that are characteristic of Alzheimer's disease). [33] Inthese applications, the unique mechanical properties of CNTsare utilized. [34] The integration of biomaterials (e.g., proteins/enzymes, anti-gens/antibodies, or DNA) with CNTs provides new hybrid sys-tems that combine the conductive or semiconductive proper-ties of CNTs with the recognition or catalytic properties of thebiomaterials, Figure 1. This may yield new bioelectronic sys-tems (biosensors, field-effect-transistors) or templated nanocir-cuitry. The development of this research topic includes numer-ous challenging issues, such as the synthesis of site-specificand structurally-defined biomaterial±CNT hybrids, improvedmethods for the separation and characterization of conductiveand semiconductive CNTs, the ordered and controlled assem-bly of addressable biomaterial±CNT systems on surfaces, andthe development of microscopic imaging techniques to char-acterize the structures and functions of the nanoscale devices.The recent advances in the fabrication of biomaterial±CNThybrid systems and their potential application in differentnanobioelectronic systems are discussed in the followingReview article. Itamar Willner was born in 1947 andcompleted his Ph.D. studies in Chemis-try in 1978 at The Hebrew University of Jerusalem. After a postdoctoral staywith M. Calvin at the University of Cali-fornia, Berkeley, he joined the Instituteof Chemistry at The Hebrew Universityof Jerusalem in 1982. In 1986 he was ap-pointed as Professor at The Hebrew Uni-versity. He acts as a member of severaleditorial boards and is the recipient of the Kolthoff Award, The Max-Planck Re-search Award for International Cooperation, the Israel Chemical So-ciety Award (2001) and the Israel Prize in Chemistry (2002). He is amember of Israel Academy of Sciences and a member of the Euro-pean Academy of Sciences. His research interests include molecularelectronics and optoelectronics, nanotechnology, bioelectronicsand biosensors, optobioelectronics, nanobiotechnology, supra-molecular chemistry, nanoscale chemistry, and monolayer and thin-film assemblies, light-induced electron-transfer processes and artifi-cial photosynthesis.Eugenii Katz was born in 1952 inMoscow. He completed his Ph.D. in1983 at the Frumkin Institute of Electro-chemistry, Moscow, and until 1991 actedas senior scientist at the Institute of Photosynthesis, Pushchino, Russia. In1991 he performed postdoctoral re-search at The Hebrew University of Jeru-salem and later, as a recipient of theHumboldt scholarship, he worked at theTechnische Universit‰t, M¸nchen, 1993.He joined the research group of I. Will-ner at the Hebrew University in 1994 as a Senior Research Associ-ate. His research interests include electroanalytical chemistry, func-tionalized monolayers, functionalized nanoparticles, biosensors, bi-ofuel cells, and bioelectronics. Figure 1.  The conceptual generation of biomolecules±carbon nanotube conju-gates, and their assembly to yield functional devices. 1086  ¹ 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  www.chemphyschem.org  ChemPhysChem  2004  , 5, 1084±1104 Biomolecule-Functionalized Carbon Nanotubes  2. Structure and Electronic Properties of Carbon Nanotubes A single-walled carbon nanotube (SWCNT) is formed when onesingle layer of graphite is wrapped onto itself and the resultingedge is joined. The structure of a SWCNT can be defined usinga roll-up vector  r  and a chiral angle  q  (Figure 2A). [19c,23] Theroll-up vector can be defined according to Equation (1) as alinear combination of base vectors  a  and  b  of the basic hexa-gon, r ¼ n a þ m b  ð 1 Þ where  m  and  n  are integers. The roll-up vector is perpendicularto the axis of the nanotube. In Figure 2A, the zone betweenthe dashed lines is the area which is rolled up along an axisperpendicular to the roll-up vector. Different types of nano-tubes are defined by the values of   m  and  n  thus chosen. Basedon theoretical predictions, [35] SWCNTs can be either metallic orsemiconducting, depending on their diameters and helical ar-rangement. Whether a SWCNT is metallic or semiconducting isbased on the band structure of a two-dimensional graphitesheet and periodic boundary conditions along the circumfer-ence direction. Depending on the roll-up vector, SWCNTs maybe metallic (when  n  m = 3  p , where  p  is integer) or semicon-ducting (all other  n  and  m  values). Figure 2B shows idealizedimages of defect-free SWCNTs ( n , m ) with open ends when theyform metallic conductive structures: ™armchair∫ state (10,10) or™zigzag∫ state (15,0) (images ™a∫ and ™c∫, respectively) and asemiconducting structure: chiral state (12,7) (image ™b∫). CNTscan be multiwalled with a central tubule of nanometric diame-ter surrounded by several graphitic layers separated by about3.4±3.6ä (Figure 3). Historically discovered first, [17,19c] multiwal-led carbon nanotubes (MWCNTs) have a higher complexity of their structure than SWCNTs and thus they are mostly used asa bulk material in applications where the ordered structuringof the systems is less important.CNTs exhibit excellent structural flexibility and fluidity andthus may be bent, collapsed, or deformed into various shapessuch as buckles, rings, or fullerene onions, providing a varietyof shape-controlled physical properties of the nanostructures.Varying the geometric structure of CNTs could allow controlover their electronic properties, such as electrical conductivityor electron emission properties, thus providing modified elec-tronic characteristics of the functional devices based on theCNTs. Although it is difficult to control the geometric structureof CNTs by conventional synthetic methods, recent techniqueshave been developed to generate CNTs with controlled shapesand properties. For example, SWCNTs with a triangular cross-section have been produced using electrochemically fabricatedporous alumina as the template. [36] Figure 4A shows SEMimages of triangular SWCNTs inside the template pores. Theside length of the SWCNTs was estimated to be about 100 nm.The dimensions of the resulting SWCNTs were found to becontrolled by the cross-section area of the porous template. Inanother example, CNTs were blown into large carbon bulbsupon fast exothermal decomposition of explosives (e.g., a mix-ture of picric acid and nickel formate) generating temperaturesup to 1200 K and a pressure jump of up to 40  M Pa. [37] In-situgenerated nickel particles were found to catalyze the forma- Figure 2.  A) Scheme showing the folding procedure to create nanotube cylin-ders from planar graphene sheets:  a  and   b  are the primitive lattice vectors of the hexagonal lattice,  r   is a roll-up vector and   q  is a chiral angle. B) Idealized representation of defect-free SWCNTs (  n  , m  ) with open ends: a) A metallic con-ducting (10,10) tube (™armchair∫); b) a semiconducting (12,7) tube (chiral); and c) a conducting (15,0) tube (™zigzag∫). (Figure 2B was adapted from ref. [81],Figure 1, with permission). Figure 3.  Computer simulated idealized representation of defect-free MWCNTs.(Adapted from ref. [19c], Figures 1 and 4, with permission). Figure 4.  A) SEM images of SWCNTs with a triangular cross-section fabricated in a porous alumina template: a) low magnification view, b) high magnifica-tion view. B) SEM images of the carbon bulbs generated by the explosionmethod: a) The freestanding carbon bulb; b) The assembly with a flasklikestructure composed of the interconnected carbon bulb and MWCNT. (Figure 4Awas adapted from ref. [36], Figure 3; Figure 4B was adapted from ref. [37], Fig-ures 3c and 4b, with permission). ChemPhysChem  2004  , 5, 1084±1104  www.chemphyschem.org  ¹ 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  1087 I. Willner and E. Katz   tion of MWCNTs, whereas the fast expansion of a hot gas mix-ture yielded the carbon bulbs. Most of the produced bulbswere freestanding and separated from the CNTs, but some of them were connected chemically with the CNTs resulting inbulb±tube assemblies (Figure 4B). In these assemblies, thebulbs were located at the ends of the tubes and formed flask-like structures. The blown spherical bulbs, characterized bylarge dimensions, thin walls, and fully hollow cores, are expect-ed to exhibit new physical and chemical properties and to findsome special applications as microcontainers or microreactors.The dependence of electron transport in CNTs on theirunique topological structure has been extensively studied ex-perimentally, [38,39] particularly in CNT bundles [39a] and ropes. [39b,c] Electron transport measurements have become possible for anindividual MWCNT [40] and an individual SWCNT. [41] Theoreticalpredictions of the CNTs conductivity, based on band structurecalculations, have recently been confirmed in a direct way bytunneling spectroscopy of individual SWCNTs using low-tem-perature scanning tunneling microscopy. [42] The conductiveSWCNTs have shown exceptional current-carrying capabil-ity [39b,41a] (electrical conductivity up to a 1000 times greaterthan copper) [43] and have recently been assembled into transis-tors, [41b,44] diodes, [45] bits of memory, [46] and logical gates. [47] However, the formation of CNTs with predetermined conduc-tive or semiconductive properties, or the effective separationof the conductive and semiconductive CNTs from their mix-tures is still a challenge. Besides physical means for separatingconductive and semiconductive CNTs, [48] for example, using di-electrophoresis, [49] their chemical separation appears promis-ing. [50±54] For example, free-base porphyrins could specificallybind to semiconductive SWCNTs, resulting in their solubiliza-tion and, therefore, separation from a mixture containing con-ductive SWCNTs. [51] The mechanism of the porphyrin-selectivebinding has not yet been fully understood, but a speculationhas been made that the semiconducting SWCNT is more like aconjugated macromolecule with the nanotube surface proper-ties similar to those found in radical ion pairs, thus providinginteractions with the free-base porphyrin molecules. In anotherapproach, diazonium salts have been used to functionalizeSWCNTs suspended in aqueous solution with a high preferencefor metallic species, thus providing their separation from a mix-ture containing semiconductive nanotubes. [52] The selective re-activity was attributed to the availability of electrons near theFermi level to stabilize a charge-transfer transition state pre-ceding bond formation with the diazonium salt. The chemicalreaction was reversible, allowing the regeneration of the srci-nal state of the SWCNTs after the metallic nanotubes were sep-arated from the mixture. Similarly, the preferential charge-transfer complex formation of bromine with metallic CNTs al-lowed their separation from semiconductive CNTs by means of centrifugation. [53] Wrapping SWCNTs with single-stranded DNA(ss-DNA) was found to be sequence-dependent. [54] A systemat-ic search of the ss-DNA library selected a sequence d(GT) n  ( n = 10 to 45; G = guanine, T = thymine; d = deoxy) that self-assem-bles into a helical structure around individual nanotubes insuch a way that the electrostatics of the DNA±SWCNT hybriddepends on the tube diameter and electronic properties, ena-bling nanotube separation by anion exchange chromatogra-phy. [54b] In order to use CNTs as building units for nanodevices, it isessential to develop reliable methods for controlling the chem-ical and physical properties of these materials. [55] In particular,functionalization of the CNTs with biomaterials has great prom-ise for their targeted applications in nanobiodevices. 3. Functionalization of Carbon Nanotubeswith Biomolecules Carbon nanotubes can be functionalized with various biomole-cules without their covalent coupling. Open-ended carbonnanotubes provide internal cavities (1±2 nm in diameter) thatare capable of accommodating organic molecules and biomo-lecules of respective sizes. For example, it has been shownthat open SWCNTs can accommodate fullerenes [56] and smallproteins such as cytochrome c inside their internal cavities,leading to the stable immobilization of the proteins in bioac-tive conformations. [57] DNA could also enter into the carbonnanotube cavities, and DNA transport through a singleMWCNT channel has been directly followed by fluorescencemicroscopy. [58] Molecular dynamics simulations have indicatedthat DNA could be encapsulated inside SWCNTs in a water±so-lute environment via an extremely rapid dynamic interactionprocess, provided that the tube size exceeds a certain criticalvalue (Figure 5). Both van der Waals and hydrophobic forceswere found to be important, with the former playing a moredominant role on the DNA±CNT interaction. [59] Chemical reac-tions could be performed on the encapsulated molecules, forexample, fullerenes have been polymerized in the CNT chan-nel. [60] Proteins and oligonucleotides can also be non-specificallybound to the external sides of the carbon nanotube walls, andvisualized by high-resolution transmission electron microsco-py. [61] Proteins adsorb individually, strongly, and noncovalentlyalong the nanotubes. These nanotube±protein conjugateshave been characterized at the molecular level by atomic forcemicroscopy. [62] For example, the prolonged incubation of SWCNTs with glucose oxidase (GOx) led to the coating of thenanotube with the enzyme through a nonspecific adsorptionof the enzyme on the SWCNTs sidewalls. The surface of the Figure 5.  DNA entering an SWCNT cavity: simulation snapshots of an oligonu-cleotide (eight adenine bases) interacting with a (10,10) carbon nanotube at 0,30, 100, and 500 ps. (Adapted from ref. [59], Figure 1, with permission. Copy-right American Chemical Society, 2003). 1088  ¹ 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  www.chemphyschem.org  ChemPhysChem  2004  , 5, 1084±1104 Biomolecule-Functionalized Carbon Nanotubes
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