Part I: From Conducting Polymers to Carbon Nanotubes: … · able through a reduction in size. Besides, ... revealed the significance of the use ... From Conducting Polymers to Carbon

Vol. 16, No. 2, 2005

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Liming DaiUniversity of Dayton, Dayton, Ohio

The recent development ofnanotechnology has opened up novelfundamental and applied frontiers inmaterials science and engineering.At the nanometer scale, the wave-likeproperties of electrons inside matterand atomic interactions are influencedby the size of the material. As aconsequence, changes in the size-dependent properties (e.g. meltingpoints, magnetic, optic, and electronicproperties) may be observed evenwithout any compositional change.Due to the high surface-to-volumeratio associated with nanometer-sizedmaterials, a tremendous improvementin chemical properties is also achiev-able through a reduction in size.Besides, new phenomena, such as theconfinement-induced quantizationeffect could also occur when the sizeof materials becomes comparable tothe deBroglie wavelength of chargecarriers inside. By creating nano-structures, therefore, it is possible tocontrol the fundamental propertiesof materials through the surface/sizeeffect. This should, in principle,allow us to develop new materialsand advanced devices of desirableproperties and functions fornumerous applications.

Polymers offer a unique class ofmaterials, with the natural lengthof polymer chains and theirmorphologies in the bulk lyingprecisely at the nanometer-lengthscale. This, together with the largenumber of possible configurations/conformations available to a macro-molecular chain, indicates considerable

room for creating polymeric materialsof new properties and functions evenwithout any change in their chemicalcomposition. The above descriptionis particularly true for a class ofmacromolecules having conjugatedstructures of alternating single anddouble C-C bonds with differentmolecular symmetries, includingconjugated polymers, (buckminster)fullerenes, and carbon nanotubes(Figure 1).

Polymers have been used traditionallyas electrically insulating materials.After all, metal cables are coated inplastic to insulate them. The visit ofMacDiarmid to Shirakawa at TokyoInstitute of Technology in 1974 and,later, Shirakawa to MacDiarmidand Heeger at The University ofPennsylvania, however, led to thediscovery of conducting polyacetylenein 1977 __ a prototype conjugatedconducting polymers (Figure 1a). Thisfinding opened the important new

field of polymers for electronicapplications and was recognized bythe 2000 Nobel Prize in Chemistry.The subsequent discovery of theelectroluminescent light emissionfrom conjugated poly(p-phenylenevinylene), by Friend�s group atCavendish Laboratory in 1990,revealed the significance of the useof conjugated polymers in photonicdevices. Various conjugated poly-mers can now be synthesized to showthe processing advantages of plasticsand the optoelectronic properties ofinorganic semiconductors or metals,with a conductivity even up to1.7×105 S/cm - comparing favorablywith the value of 106 S/cm for copperor silver (Table 1).

Sometimes history repeats itself.The visit made by Kroto in 1985 toSmalley and Curl at Rice Universityalso led to a Noble-Prize-winningdiscovery of buckminsterfullereneC60 __ a conjugated molecule with a

Figure 1. Conjugated structures with different dimensions: (a) polyacetylene; (b)buckminsterfullerene C60; (c) single- and double-wall carbon nanotubes.

Part I: From Conducting Polymers toCarbon Nanotubes: A Revolution of SensorsBased on Architectural Diversity of the

(a)

(b) (c)

(a)

(b) (c)

-Conjugated Structure

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From Conducting Polymers to Carbon Nanotubes, (cont.)From Conducting Polymers to Carbon Nanotubes, (cont.)From Conducting Polymers to Carbon Nanotubes, (cont.)From Conducting Polymers to Carbon Nanotubes, (cont.)From Conducting Polymers to Carbon Nanotubes, (cont.)

soccer-ball like structure consisting of12 pentagons and 20 hexagons facingsymmetrically (Figure 1b). AlthoughOsawa had theoretically predictedthe structure of C60 in 1970 andHuffman might have produced the C60molecules in his �graphite smoke� asearly as 1973, it was the 1985 discoverythat generated a great deal of interestand created an entirely new branch ofcarbon chemistry. The subsequentdiscovery of carbon nanotubes byIijima in 1991 opened up a new era inmaterial science and nanotechnology.Carbon nanotubes may be viewed asa graphite sheet that is rolled into ananoscale tube form (single-walledcarbon nanotubes, SWNTs) or withadditional graphene tubes around thecore of a SWNT (multi-walled carbonnanotubes, MWNTs) (Figure 1c).

In view of the large number of well-defined carbon-carbon single anddouble bonds in their molecularstructure, fullerenes and carbonnanotubes are polymeric in essence.Just as conjugated polymers havebeen widely regarded as quasi-one-dimensional semiconductors,fullerenes and carbon nanotubes canbe considered as a quantum dot andquantum wires, respectively. Havinga conjugated all-carbon structure withunusual molecular symmetries,fullerenes and carbon nanotubes have

also been shown to possess somesimilar optoelectronic properties toconjugated polymers.

Just like metals have high conductiv-ity due to the free movement ofelectrons through their structure, inorder for organic materials to beelectronically conductive, they mustpossess not only charge carriers butalso an orbital system which allowsthe charge carriers to move. Theconjugated structure can meet thesecond requirement through acontinuous overlapping ofMost organic materials, however, donot have intrinsic charge carriers.

The required charge carriers maybe provided by partial oxidation(p-doping) of the organic (macro)molecules with electron-acceptors(e.g. I2, AsF5) or by partial reduction(n-doping) with electron-donors(e.g. Na, K) (Figure 2). Through such adoping process, charged defects areintroduced, which could then beavailable as the charge carriers.

The early work on the conductivitymeasurements of polyacetylene filmsupon doping with vapors of iodine,bromine, or AsF5, and subsequentcompensation with NH3, therefore,constitutes the simplest conductingpolymer gas sensors. Indeed, metal(e.g. copper or palladium)-dopedelectrodeposited polypyrrole andpoly-3-methylthiophene films recentlyhave been used to detect reducinggases such as NH3, H2 and CO. Besides,the interaction of a doped conjugatedconducting polymer (e.g. d,l-camphor-sulfonic acid-doped polyanilineemeraldine) with certain organicsolvents (e.g. m-cresol) could causea conformational transition of thepolymer chain from a �compact coil�to an �expanded coil� through theso-called �secondary doping� process,which was found to be accompaniedby a concomitant change in conductiv-ity. The conformation-inducedconductivity changes also have beenobserved when conjugated conductingpolymers were exposed to somecommon organic vapors (methanol,hexane, chloroform, THF, benzene,toluene, acetone, etc.), providingthe basis for developing conjugatedconducting polymer-based sensors forthe detection of hydrocarbon vapors.

Just as conjugated conducting poly-mers have been demonstrated to showgood sensing performance, the high

# The range of conductivities listed is from that originally found to the highest values obtained to date.

Table 1. Some conjugated conducting polymers.

Figure 2. A schematic description of the doping of trans-polyacetylene chain: (a) p-dopingand (b) n-doping.

(a) (b)

-orbitals.

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surface area and good electronicproperty provided by carbonnanotubes is also an attractive featurein the advancement of a chemical-/bio-sensors. Indeed, carbon nanotubesand their derivatives have recentlybeen reported as gaseous sensingmaterials. In most of these studies,nonaligned carbon nanotubes are usedfor gas sensing, and the gas detectionis accomplished by measuring thechange in electrical properties ofthe carbon nanotubes induced by thecharge transfer with the gas molecules(e.g. O2, H2, CO2) or the mass changedue to physical adsorption of the gasmolecules. Consequently, the numberof analytes that can be determinedusing a carbon nanotube-based sensoris hampered by the limited transduc-tion mechanisms employed. It willbe a significant advancement for acarbon nanotube based biosensor,if we can prepare perpendicularly-aligned carbon nanotubes as thesensing material. This will providea well-defined large surface area for abiosensor with an enhanced sensitiv-ity. These sensors can be built on eachof the constituent-aligned carbonnanotubes while they can be ad-dressed collectively through thecommon electrode. The alignmentstructure will also facilitate surfacemodification of the carbon nanotubeswith various transduction materialsfor broadening the scope of analytesto be detected by the nanotube sensor.We have prepared aligned carbon

Figure 3. (a) Schematic representation of the micropattern formation of aligned carbon nanotubes by the photolithographic process.(b) Typical SEMmicrographs of patterned films of aligned nanotubes prepared by the pyrolysis of FePc onto a photolithographicallyprepatterned quartz substrate.

nanotubes, having a well-graphitizedstructure with ca.50 concentric carbonshells and an outer diameter of ca.40 nm,by pyrolyzing iron(II) phthalocyanine(FePc) under Ar/H2 atmosphere at 800-1100 °C. We have also developed severallithographic and plasma patterningmethods to generate the perpendicularly-aligned carbon nanotube arrays withresolutions down to a micrometer scale.Our methods allow not only the prepara-tion of micropatterned substrate-freefilms of the perpendicularly-alignednanotubes, but also their transfer ontovarious substrates, including those whichwould otherwise not be suitable fornanotube growth at high temperatures(e.g. conducting substrates for electro-chemistry and polymer films for flexibledevices). Figure 3a shows the steps ofthe photolithographic process, in whicha patterned polymer film, after anappropriate carbonization treatment,acts as a shadow mask for the patternedgrowth of the aligned carbon nanotubes(Figure 3b).

On our further investigation of thealigned carbon nanotubes produced byFePc pyrolysis, we have developed asimple, but effective, method forpreparing aligned carbon nanotube�DNA sensors by chemically couplingDNA probes on both the tip and wallof plasma-activated aligned carbonnanotubes in a patterned or non-patterned fashion. In particular, wecarried out acetic acid-plasma treatmenton gold-supported aligned carbon

nanotubes, followed by graftingsingle-strand DNA (ssDNA) chainswith an amino group at the 5'-phosphate end (i.e. [AmC6]TTGA-CACCAGACCAACTGGT-3', I) ontothe plasma-induced �COOH groupthrough the amide formation in thepresence of an EDC [1-(3-dimethyl-aminopropyl)-3-ethylcarbodiimidehydrochloride] coupling reagent.Complementary DNA (cDNA)chains pre-labeled withferrocenecarbox-aldehyde, FCA,(designated as: [FCA-C6]ACCAGTTGGTCTGGTGTCAA-3', II)were then used for hybridizingwith the surface-immobilizedoligo-nucleotides to form doublestrand DNA (dsDNA) heliceson the aligned carbon nanotubeelectrodes (Figure 4a).

The performance of the surface-bound ssDNA (I) chains on theplasma-treated nanotube electrodefor sequence-specific DNA diag-noses was demonstrated in Figure4b. The strong oxidation peak seenat 0.29 V in curve a of Figure 4bcan be attributed to ferrocene andindicates the occurrence of hybrid-ization of FCA-labeled cDNA (II)chains with the nanotube-supportedssDNA (I) chains, leading to along-range electron transfer fromthe FCA probe to the nanotubeelectrode through the DNA duplex.In contrast, the addition of FCA-labeled non-complementary DNA

(a) (b)

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chains (i.e. [FCA-C6]CTCCAGGAGTCGTCGCCACC-3', III) under thesame conditions did not show anyredox response of FCA (curve b ofFigure 4b). This indicates that, asexpected, there was no specific DNApairing interaction with the non-complementary DNA chains, and thatphysical adsorption of the FCA-labeled DNA chains, if any, wasinsignificant in this particular case.Subsequent addition of target DNAchains (i.e. 5'-GAGGTCCTCAGCAGCGGTGGACCAGTTGGTCTGGTGTC-AA-3', IV) into the above solution,however, led to a strong redoxresponse from the FCA-labeled DNA(III) chains (curve c of Figure 4b)because the target DNA (IV) chaincontains complementary sequences forboth DNA (I) and DNA (III) chains.More interestingly, the electrochemi-cal responses seen in Figure 4b wererevealed to be highly reversible. Theseresults demonstrated that specificDNA sequences could be covalentlyimmobilized onto plasma-activatedaligned carbon nanotubes for sensingcomplementary DNA and/or targetDNA chains of specific sequences witha high sensitivity and selectivity.Along with the techniques formicropatterning aligned carbon

nanotubes, these results should haveimportant implications not onlyfor the sequence-specific analyses/diagnoses of DNA chains but alsofor the use of carbon nanotubes inadvanced sensing chips.

ACKNOWLEDGEMENTI thank my colleagues, includingPinggang He, Vardhan Bajpai, SinanLi, Yangyong Yang, Mei Gao, GordonWallace, Meixiang Wan, MinooMoghaddam, and Maxine McCallfor their contributions to the worksummarized in the article. I amgrateful for financial support fromthe NSF, ACS-PRF, AFOSR, CSIRO andthe University of Akron. Thanks alsoto AFRL/ML, Wright BrothersInstitute, Dayton DevelopmentColations, and the University ofDayton for a WBI Endowed ChairProfessorship in Nanomaterials.

To be continued in the next issue ofEnergeia.

Figure 4. (a) A schematic of the aligned nanotube�DNA electrochemical sensor. The upper right SEM image shows the aligned carbonnanotubes after having been transferred onto a gold foil. For reasons of clarity, only one of the many carboxyl groups is shown at thenanotube tip and wall, respectively. (b) the ssDNA (I)-immobilized aligned carbon nanotube electrode after hybridization with FCA-labeledcomplementary DNA (II) chains (a), in the presence of FCA-labeled noncomplementary DNA (III) chains (b), and after hybridization withtarget DNA (IV) chains in the presence of the FCA-labeled noncomplementary DNA (III) chains (c).

(a) (b)

Dr. Dai received a Ph.D., in chemistryfrom the Australian National University,Canberra, Australia, 1991. Before comingto the University of Dayton as a WBI ChairProfessor in Nanomaterials, he was apolymer engineering faculty member at theUniversity of Akron. Prior to joining theUniversity of Akron, he was a PrincipalResearch Scientist, The CommonwealthScientific and Industrial ResearchOrganization (CSIRO), Clayton, Australia,1992-2002. Email: [email protected].

5Energeia Vol. 16, No. 2, 2005 © UK Center for Applied Energy Research (continued, page 6)

COMMENTCOMMENTCOMMENTCOMMENTCOMMENTARYARYARYARYARY

Roy PalkCEO, East Kentucky Power

As one of the fastest growing electricutilities in this part of the nation,East Kentucky Power Cooperative(EKPC) must have progressivestrategies for meeting continuallychanging generation needs.

Our system is in the midst of one ofthe most dynamic and industriousperiods in our history. In 2008, ourdistribution cooperative membershipwill grow with the addition ofWarren RECC, a 53,000-memberTouchstone Energy partner based inBowling Green, Kentucky. Warren�sadditional load, along with recordgrowth throughout the rest of oursystem, creates a need for substantialbaseload capacity in the next decade.

When EKPC began generatingelectric power in Kentucky morethan 50 years ago, coal was ourproverbial �ace in the hole.� Despiteincreasing federal environmentalcontrols and competition from otherfuels, coal will undoubtedly continueto carry the bulk of our generationrequirements well into the21st century.

Besides being a plentiful domesticenergy resource and less expensivethan its competitors, coal claims abrighter future with the development

and demonstration of cleaner and moreefficient technologies. As we implementthese technologies, EKPC supports avaluable Kentucky natural resource,produces cleaner electricity, and meetsthe power needs of our membersefficiently and affordably.

Coal-fired power plants account formore than 95 % of all electric genera-tion in Kentucky, far above the nationalaverage of just over 50 %. As we addthree circulating fluidized bed units(CFB) by 2009, and continue evaluatingother clean coal technologies for thefuture, we believe coal�s future inKentucky�s electric power generationstrategy is secure.

With its established success in fuelefficiency, competitive cost and lowemissions, CFB is an excellent approachto generating steam for electricity.Such technology allows us to burn notonly different varieties of coal, butother supplemental feedstocks suchas tire-derived fuel and biomass. OurCFB units � two at the H.L. SpurlockStation in Maysville and one at theJ.K. Smith Station near Winchester �will be among the nation�s cleanestcoal-fired power plants.

To satisfy our projected power needsover the next five years, we�ve madea significant investment in this latestversion of proven technology.

We have already begun looking beyond2009 and are exploring the utilization ofintegrated gasification combined-cycle(IGCC), the next generation of cleancoal technology. Capable of cleansingas much as 99 % of coal impurities,IGCC appears to be one of the risingstars in power generation.

Indeed, coal-fired generation is nolonger the cut-and-dried powersupply choice it was when EKPCbuilt its existing fleet of power plantsfrom the 1950s through the 1980s.

Environmental regulations over thepast decade have changed virtuallyevery aspect of our fuel procurementprocedures and power supplyplanning. As a result, EKPC hasjoined utilities across the nation inspending several hundred milliondollars to ensure our existing powerplants meet strict emission controlstandards.

Factors we never encountered, suchas emissions cap-and-trade programs,shrinking emission caps, and volatilemarket conditions have made coalsupply management and generationplanning complex and multi-facetedventures.

We have not yet seen the end ofemission limitations for sulfurdioxide, nitrogen oxide and particu-late matter, and we are facing furtherregulations on mercury and carbondioxide.

Indeed, environmental strategy hasbecome one of the driving forcesin our cooperative�s daily business.Fortunately, the fast-paced emer-gence of cleaner coal technologies ishelping to lessen the impact of thedifficult transitions in our industry.We must strike a balance between thepremium costs of environmentalcompliance, and our commitmentand obligation to provide reliable,affordable electric power to ourmember cooperatives and theirconsumers across Kentucky.

East Kentucky PowerCooperative�s Future toDepend on Coal

Energeia Vol. 16, No. 2, 2005 © UK Center for Applied Energy Research 6

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Energeia is published six times a year by theUniversity of Kentucky's Center forApplied Energy Research (CAER). The publication features aspects of energyresource development and environmentally related topics. Subscriptions are free and may be requested as follows: Marybeth McAlister, Editor of Energeia,CAER, 2540 Research Park Drive, University of Kentucky, Lexington, KY 40511-8410, (859) 257-0224, FAX: (859)-257-0220, e-mail: [email protected] and past issues of Energeia may be viewed on the CAER Web Page at www.caer.uky.edu. Copyright © 2005, University of Kentucky.

East Kentucky Power Cooperative�s Future, (cont.)East Kentucky Power Cooperative�s Future, (cont.)East Kentucky Power Cooperative�s Future, (cont.)East Kentucky Power Cooperative�s Future, (cont.)East Kentucky Power Cooperative�s Future, (cont.)The combination of environmental,technological and financial demandspresents an ambitious agenda forenergy providers everywhere, but inKentucky, coal is a well-establishedicon in the power supply industry. TheAmerican Coal Council perhaps put itbest when it stated, �There is nosubstitute for electric power in theeconomy. And there is no substitutefor coal in electric power.�

As some industry experts point tocostly nuclear power as an option tobest meet future environmentalstandards, we here in Kentucky feelthe absence of nuclear power is one ofthe reasons we have the lowestaverage residential electricity rates inAmerica. Our abundance of coal isanother.

The EKPC power generation strategyover the next several decades willremain primarily coal based, using thelatest versions of proven technologies.

With America�s coal reserves exceed-ing 250 years, we believe that despiteenvironmental challenges, coalremains the most practical, cost-effective option for future electricitygeneration. As EKPC deals with theintricacies of our continually changingindustry, the member-based founda-tion we established in 1941 remainsunshaken.

Further, as our system grows andprogresses, East Kentucky PowerCooperative will continue to be aleader in environmental stewardshipand environmental compliance. Mostimportantly, we will continue toprove that environmental care andsound business practices can andshould go hand in hand.