www.rsc.org/analyst Volume 132 | Number 8 | August 2007 | Pages 709–824

Interdisciplinary detection science

ISSN 0003-2654

HIGHLIGHTMarcus D. Lay et al.Recent progress in chemical detection with single-walled carbon nanotube networks

PAPERJaromir Ruzicka et al.Automated capture and on-column detection of biotinylated DNA on a disposable solid support

Publ

ishe

d on

23

May

200

7. D

ownl

oade

d by

Fre

ie U

nive

rsita

et B

erlin

on

22/1

0/20

14 1

0:16

:27.

View Article Online / Journal Homepage / Table of Contents for this issue

Recent progress in chemical detection withsingle-walled carbon nanotube networksPornnipa Vichchulada, Qinghui Zhang and Marcus D. Lay*

DOI: 10.1039/b618824a

Single-walled carbon nanotubes (SWNTs) have had significant impact on thedevelopment of gas sensors in the last decade. However, useful applications ofSWNTs are limited by the lack of manufacturable routes to device formation. ThisHighlight article chronicles recent progress in this area and demonstrates thegreat promise of a new room temperature deposition method for SWNT networksin gas sensing applications. This liquid deposition technique allows the depositionof pre-treated, highly aligned SWNT networks on a wide variety of substrates. Asignificant advantage of SWNT-network sensors is that fluctuations in theelectrical response of individual SWNTs become less important as the size of thenetwork increases. Therefore, device properties can be controlled by the overalldensity of the network rather than the physical properties of any individual SWNT.At densities where semiconducting pathways dominate, highly sensitive thin-filmchemoresistive sensors can be fabricated. Such devices also have higher signal-to-noise ratios and are easier to fabricate than devices based on a single SWNT.

Introduction

Single-walled carbon nanotubes

(SWNTs) have many enhanced mechani-

cal, structural, and electronic properties

that have made them the focus of intense

research. Though it has long been known

that carbon fibers, along with soot, can

be produced with a carbon arc,1,2 it was

not until 1991, when Iijima observed that

these fibers were hollow, that the current

effusion of interest began.3 That report

demonstrated that the carbon fibers

observed in soot from an arc discharge

experiment consisted of several con-

centric tubes nestled inside each other.

These nanometer-scale structures are

known today as multi-walled carbon

nanotubes (MWNTs). Subsequently, in

1993 separate reports from the Iijima and

Bethune research groups independently

confirmed the existence of SWNTs.4 The

past decade has seen an exponential

increase in publications related to both

types of nanotubes.5

There has been great recent interest

in investigating SWNTs, due to their

unique physical and chemical charac-

teristics. These one-dimensional wires

are the strongest known material.6,7

Further, they also exhibit enhanced

electronic properties, including near bal-

listic transport.8–11 Therefore, SWNTs

show great potential as the building

blocks of nano- and micro-electronic

devices of the near future.12–16 There

are many other possible applications of

SWNTs, including hydrogen storage,17

field emission materials,18 tips for scan-

ning probe microscopy,19 chromatogra-

phic stationary phases,20 and sensors.21,22

There is a direct correlation between

the physical rolling vector (chirality) of a

carbon nanotube and its electronic pro-

perties; SWNTs may be either semicon-

ductors or metallic. In a bulk sample of

SWNT material, roughly one-third will

be metallic and two-thirds semiconduct-

ing.23–26 Absolute control of SWNT

properties is necessary to fabricate a

device based on a single SWNT.

Therefore, there has been great research

into methods of separating the different

types of nanotubes. Thus far, these

investigations have met with limited

success for exceedingly small masses of

material.27–31

However, separation of metallic and

semiconducting SWNTs will not result

in total control over device response

as the properties of semiconducting

SWNTs vary greatly with chirality and

diameter.32–34 Particularly, the bandgap

of semiconducting SWNTs varies

inversely with diameter. Consequently,

smaller diameter nanotubes have larger

bandgaps. Therefore, reproducibility in

SWNT device performance is limited not

only by the presence of metallic SWNTs,

but also by the inability to control all

physical properties of the nanotubes

during the growth process.

Another route to incorporation of

SWNT material into functional electro-

nic devices is the use of networks of

SWNTs. In a SWNT network, a macro-

scopic material is formed by depositing

individual SWNTs on a surface at a

density such that there is significant

overlap. At densities between the perco-

lation threshold for semiconducting and

metallic SWNTs, this material behaves as

a thin-film semiconductor.35 This is due

to the fact that fully metallic conductive

pathways are highly unfavored below

their percolation threshold.

One application of SWNT networks is

in the field of sensing. There are two

common types of gas sensors: surface

acoustic wave (SAW) devices and che-

moresistors. The SAW device is com-

posed of a polymer-coated quartz crystal

micro balance chip which is oscillated

near its resonance frequency. The fre-

quency of oscillation changes during

Department of Chemistry, and NanoscaleScience and Engineering Center (NanoSEC),University of Georgia, Athens, GA 30602E-mail: mlay@chem.uga.edu

i-SECTION: HIGHLIGHT www.rsc.org/analyst | The Analyst

This journal is � The Royal Society of Chemistry 2007 Analyst, 2007, 132, 719–723 | 719

Publ

ishe

d on

23

May

200

7. D

ownl

oade

d by

Fre

ie U

nive

rsita

et B

erlin

on

22/1

0/20

14 1

0:16

:27.

View Article Online

mass uptake as this polymer specifically

absorbs an analyte. This change in

frequency results in a detectable signal.

Another common sensor configuration

is the chemoresistor, in which signal

transduction occurs via a change in

resistance observed upon analyte adsorp-

tion/absorption. Carbon black sensors, a

type of chemoresistor, are composed of a

thin film of micron-sized conducting

carbon particles embedded in a non-

conducting polymer matrix. The loading

of carbon black is such that tunnel

junctions between particles exist over

macroscopic dimensions, resulting in a

material with defined conductance.

Swelling of the polymer matrix during

exposure results in a change in conduc-

tivity that can be easily detected.

SWNTs are expected to work very

well in chemoresistive-type sensors due

to their nanometer-scale diameter and

incredibly high aspect ratio; effective

response can be achieved with thinner

polymer matrices, increasing the kinetics

of response. Further, the surface area of

individual SWNTs is about 1600 m2 g21,

the highest value for any known mate-

rial.36 This presents a huge effective

sensing area for the adsorption of

vapors. As a result, SWNT sensors show

a fast response and a significantly higher

sensitivity than that of existing room

temperature solid-state sensors.11

Sensors based on one SWNT

Early reports indicated great promise for

SWNTs in high-performance macro-

scale sensors. Dai and co-workers were

the first to report the use of SWNTs to

detect NO2 and NH3, where the resis-

tance of a single semiconducting SWNT

was observed to increase or decrease by

2–3 orders of magnitude upon exposure

to NO2 or NH3, respectively.22 This

change in resistance was attributed to

shifts in the valence band with respect

to the Fermi level. In a p-type semi-

conductor, like a SWNT, the depletion of

charge carriers by NH3 (an electron-

donating adsorbate) results in a shift of

the valence band away from the Fermi

level, and a corresponding decrease in

conductivity. In contrast, NO2 (an elec-

tron-withdrawing adsorbate) resulted in

shifting the valence band toward the

Fermi level, resulting in an observable

increase in conductivity. Subsequent

work by numerous researchers has

demonstrated the efficacy of SWNT

sensors for a wide variety of gas phase

analytes.37–40

Sensors composed of bundlesof SWNTs

While highly effective for specialized use,

sensors based on a single SWNT are

impractical for mass production due to

great variability in SWNT properties;

variations in chirality and diameter result

in irregular device function. As a result,

many investigators have studied the

sensor applications of bundles or ‘ropes’

of SWNTs. In fact, a recent study by

Manohar’s group41 demonstrated that

SWNTs bundles deposited onto poly-

(ethylene terephthalate) (overhead trans-

parency film) displayed a change in

conductance that was four times as

sensitive to toluene vapors as a similar

sensor composed of carbon black.

Bundles of purified SWNTs were found

to exhibit even greater sensor response.

This enhancement was attributed to

chemically bound oxygen on the purified

SWNTs. Furthermore, four point probe

studies showed that sensors composed of

carbon nanotube bundles demonstrated

enhanced response to a variety of organic

vapors. The reproducibility of this

response was demonstrated to be roughly

four times greater than that obtained for

similar carbon black sensors.

Although sensors composed of carbon

nanotube bundles have demonstrated

interesting properties, two-dimensional

networks of SWNTs exhibit heightened

response due to the fact that the presence

of metallic SWNTs in a bundle will

mitigate the electrical response of the

semiconducting SWNTs. Therefore, two-

dimensional networks of unbundled

SWNTs may present the best com-

promise between ease of fabrication and

heightened signal-to-noise ratio.

High temperature formationof SWNT networks

Initial methods of SWNT-network

fabrication involved chemical vapor

deposition (CVD). This method involves

catalytic decomposition of hydrocarbon

precursors and subsequent formation of

SWNTs at metal nanoparticles. Owing to

the high temperatures involved, CVD

growth of SWNT networks typically

involves the use of silicon wafer frag-

ments as substrates. Growth conditions

vary widely (i.e. catalyst, carbon source,

and substrate),42–48 but remain variations

on the method reported by Lieber and

co-workers.49

Si wafer fragments are dipped in an Fe

salt/2-propanol solution and rinsed with

solvent. This is followed by placing the

substrate in a tube furnace under Ar flow

at 600–800 uC. Next, H2 is flowed into

the system to reduce surface-bound metal

catalyst nanoparticles to elemental metal.

Finally, a hydrocarbon source is intro-

duced and SWNTs grow from the metal

nanoclusters, which act as catalysts for

the decomposition of the hydrocarbon

gas. This process flow results in the

growth of a random network of

unbundled SWNTs. Owing to the high

aspect ratio of SWNTs, they overlap at

low densities (ca. 2 SWNTs mm22),

resulting in an electrically continuous

network.

CVD-grown SWNT-network-based

sensors were first reported by Dai and

co-workers.50 It was demonstrated that

SWNT networks in aqueous solution

were highly sensitive protein detectors.

These devices were composed of SWNT

networks bridging to metal electrodes on

a dielectric surface. Subsequent reports

confirmed the high sensitivity of SWNT

networks to a variety of gases.41,48,51–53

However, though highly effective, this

procedure requires a substrate that can

withstand the high temperatures neces-

sary for catalytic decomposition of

hydrocarbon sources. Additionally, a

by-product of this growth method is the

growth of amorphous carbon. Another

source of contamination is the remaining

catalyst used in the growth process. The

effect of these impurities on device

reproducibility and performance cannot

be neglected. Therefore, exploration of

liquid deposition techniques has recently

increased.

Room temperature depositionof two-dimensional networksof SWNTs

The major advantage of liquid deposition

of SWNT networks is that it occurs at

room temperature. This facilitates the

inclusion of heat-sensitive substrates, like

various polymers, plastics, and glass.54

720 | Analyst, 2007, 132, 719–723 This journal is � The Royal Society of Chemistry 2007

Publ

ishe

d on

23

May

200

7. D

ownl

oade

d by

Fre

ie U

nive

rsita

et B

erlin

on

22/1

0/20

14 1

0:16

:27.

View Article Online

Another very important advantage is

that as the SWNT growth and deposition

processes occur in two separate steps,

purification protocols can be applied

prior to the formation of the SWNT

networks. This becomes extremely

important in cases where purified, or

otherwise modified SWNTs, are desired.

Further, liquid deposition methods

allow inexpensive and rapid development

of custom patterns that are fully compa-

tible with current optical lithography

techniques.

Networks of SWNT bundles have

been deposited at room temperature by

various techniques such as spin-casting,

spray-coating or filtering followed by a

contact printing step.55,56 It was recently

reported that networks of highly aligned

SWNTs could be formed using a facile

liquid deposition technique (Fig. 1).35

The alignment of SWNTs in the

deposition process was directed by the

laminar N2 flow used. This discovery

presented a new opportunity for the

creation of enhanced sensing materials

from SWNTs.

In a method similar to molecular

combing of DNA,57–63 unidirectional

air flow is applied to the air–liquid

interface in a manner which imparts

order to the SWNTs in aqueous suspen-

sion and then deposits them on a surface

(i.e. Si/SiOx coated with a silane, over-

head transparency film, polyimide, etc.).

This room temperature technique for the

formation of electrically continuous net-

works of highly aligned SWNTs is

compatible with a wide variety of sub-

strates (including polymer substrates)

and can be combined with existing

optical lithography methods. This pro-

vides a facile route to the formation of

chemical sensing devices.

The properties of this material can be

explained thus: although individual

SWNTs average 1–3 mm in length, at

sufficiently high density, many of the

tubes overlap at some point. This causes

electrical conductivity over macroscopic

lengths (wafer-scale and beyond) at low

SWNT coverage. The result is the crea-

tion of a transparent, flexible, thin-film

semiconductor that can be deposited on a

wide variety of substrates.

This liquid deposition technique allows

the formation of highly parallel SWNT

networks having arbitrary density and

orientation. The density of the SWNT

network is determined by the density of

the SWNTs in the suspension and the

number of deposition cycles. Fig. 2 is an

atomic force microscopy (AFM) micro-

graph obtained after one deposition cycle

from an aqueous suspension of approxi-

mately 0.1 mg mL21 SWNTs. This

large-scale micrograph demonstrates the

level of control in deposition that is

possible without any photolithographic

modification of the surface. AFM

indicated that this surface was covered

with a uniform layer of highly aligned

SWNTs over its entire area. This process

has been observed to occur with high

efficiency for shortened HNO3 treated

SWNTs, as well as with longer as-

produced nanotubes.

The number of SWNTs on a surface

increases linearly with the number of

deposition cycles, with macroscopic net-

work formation occurring between three

and five deposition cycles, depending on

the SWNT suspension concentration.

Fig. 3 shows a typical deposit formed

from three deposition cycles of the same

solution used in Fig. 2. This low-density

network of aligned SWNTs is trans-

parent in the visible range and electrically

conductive over macroscopic scales.

It has been previously determined that

this material has anisotropic electrical

properties.35 The on/off ratio of thin-film

transistors composed of highly aligned

carbon nanotube networks varies accord-

ing to the direction of the nanotube

network with respect to the electrical

contacts. This results in another layer

of control over the response to vapor

phase analytes. This group is further

investigating this effect and how this

may be used to obtain further enhanced

chemical sensors. Further, the molecular

combing method used to deposit

Fig. 1 Two-dimensional networks of highly aligned SWNTs are fabricated on Si/SiOx

wafers (or any flat substrate) using a molecular combing process. A suspension of SWNTs is

deposited on a prepared surface, and then unidirectional air flow is applied to the solid–

liquid interface in a manner to effect laminar flow of the suspension and deposition of highly

aligned SWNTs.

Fig. 2 AFM micrograph of a two-dimensional network of SWNTs deposited using a

laminar flow liquid deposition technique. The density of SWNTs deposited is easily

controlled by varying the solution concentration and number of deposition cycles.

This journal is � The Royal Society of Chemistry 2007 Analyst, 2007, 132, 719–723 | 721

Publ

ishe

d on

23

May

200

7. D

ownl

oade

d by

Fre

ie U

nive

rsita

et B

erlin

on

22/1

0/20

14 1

0:16

:27.

View Article Online

horizontally aligned SWNTs has been

demonstrated as a very capable method

for the formation of higher order con-

structs with the formation of SWNT

crossbar nanostructures.64

The count of impurities on the surface

increases with deposition cycles. This

necessitates the development of purifica-

tion steps. Therefore, this group is

investigating numerous purification pro-

tocols to remove amorphous carbon,

catalyst nanoparticles, and other impu-

rities. The removal of these impurities is

expected to increase the reproducibility

of sensor response. The harshness of a

purification method (and its effect on a

given substrate) is not a concern as

network formation occurs in a subse-

quent step.

In addition, this group is also investi-

gating the cause of 1/f noise (which is a

factor that affects the detection limit of a

sensor) in chemoresistors composed of

nano-scale molecular wires. As it has

been demonstrated that the level of 1/f

noise is strongly dependent on device

geometry and application of a gate bias,

studies into the effect of device dimen-

sions on the detection limit are expected

to result in even lower detection limits.65

Additionally, non-covalent modification

with metal nanoclusters and chemoselec-

tive polymers present routes to enhanced

SWNT sensors. Liquid deposition is

compatible with most SWNT modifica-

tion protocols.

Summary

Novel nano-scale electronic materials

will play a role of great significance in

sensing transducers of the near future. As

single-walled carbon nanotubes are com-

posed entirely of surface atoms, they are

exquisitely sensitive to gas phase phy-

sisorption events and have demonstrated

great promise in chemical sensing appli-

cations. Such sensors work for a wide

variety of volatile organics. Furthermore,

chemoresistors present a greatly simpli-

fied route to sensor fabrication without

sacrificing sensitivity. Recent advances

in the chemical modification of SWNTs

present an opportunity to obtain

enhanced selectivity and sensitivity in

chemoresistor sensors.

The optical transparency and flexibi-

lity of these networks will likely result in

numerous novel sensor applications.

Further, liquid room temperature routes

to fabrication present great advantages

over high temperature methods, includ-

ing ease of fabrication, pre-deposition

purification of SWNT material and

compatibility with heat-sensitive sub-

strates. These advantages facilitate

the development of numerous novel

applications.

A novel deposition technique for the

formation of unbundled networks of

highly aligned SWNTs is currently being

investigated as a tool for achieving

greater gas sensor reproducibility and

performance. This deposition process, in

combination with various purification

and functionalization techniques, is

expected to hasten the development of

functional device structures composed of

flexible, transparent SWNT thin films.

References

1 J. Abrahamson, P. G. Wiles and B. L.Rhoades, Carbon, 1999, 37, 1873–1874.

2 P. G. Wiles and J. Abrahamson, Carbon,1978, 16, 341–349.

3 S. Iijima, Nature, 1991, 354, 56–58.4 D. S. Bethune, C. H. Kiang, M. S. Devries,

G. Gorman, R. Savoy, J. Vazquez andR. Beyers, Nature, 1993, 363, 605–607;S. Iijima and T. Ichihashi, Nature, 1993,363, 603–605.

5 M. Terrones, Annu. Revi. Mater. Res.,2003, 33, 419–501.

6 B. Lukic, J. W. Seo, R. R. Bacsa,S. Delpeux, F. Beguin, G. Bister,A. Fonseca, J. B. Nagy, A. Kis, S. Jeney,A. J. Kulik and L. Forro, Nano Lett.,2005, 5, 2074–2077.

7 P. J. F. Harris, Int. Mater. Rev., 2004, 49,31–43.

8 B. Obradovic, R. Kotlyar, F. Heinz,P. Matagne, T. Rakshit, M. D. Giles,M. A. Stettler and D. E. Nikonov, Appl.Phys. Lett., 2006, 88, 142102.

9 Y. Ouyang, Y. Yoon, J. K. Fodor andJ. Guo, Appl. Phys. Lett., 2006, 89,203107.

10 J. Guo, E. C. Kan, U. Ganguly andY. G. Zhang, J. Appl. Phys., 2006, 99,084301.

11 H. C. d’Honincthun, S. Galdin-Retailleau,J. See and P. Dollfus, Appl. Phys. Lett.,2005, 87, 172112.

12 J. Appenzeller, R. Martel, V. Derycke,M. Radosavjevic, S. Wind, D. Neumayerand P. Avouris, Microelectron. Eng., 2002,64, 391–397.

13 S. Saito, Science, 1997, 278, 77–78.14 P. Avouris, Acc. Chem. Res., 2002, 35,

1026–1034.15 A. Bachtold, P. Hadley, T. Nakanishi and

C. Dekker, Science, 2001, 294, 1317–1320.16 R. P. Raffaelle, B. J. Landi, J. D. Harris,

S. G. Bailey and A. F. Hepp, Mater. Sci.Eng., B, 2005, 116, 233–243.

17 A. C. Dillon, K. M. Jones, T. A. Bekkedahl,C. H. Kiang, D. S. Bethune andM. J. Heben, Nature, 1997, 386, 377–379.

18 S. J. Tans, A. R. M. Verschueren andC. Dekker, Nature, 1998, 393, 49–52.

19 S. S. Wong, E. Joselevich, A. T. Woolley,C. L. Cheung and C. M. Lieber, Nature,1998, 394, 52–55.

20 L. M. Yuan, C. X. Ren, L. Li, P. Ai,Z. H. Yan, M. Zi and Z. Y. Li, Anal.Chem., 2006, 78, 6384–6390.

21 P. G. Collins, K. Bradley, M. Ishigami andA. Zettl, Science, 2000, 287, 1801–1804.

Fig. 3 AFM micrograph of a two-dimensional network of SWNTs deposited using three

deposition cycles from the same solution as in Fig. 2. This network, which is electrically

continuous over macroscopic dimensions, exhibits anisotropic electrical response.

722 | Analyst, 2007, 132, 719–723 This journal is � The Royal Society of Chemistry 2007

Publ

ishe

d on

23

May

200

7. D

ownl

oade

d by

Fre

ie U

nive

rsita

et B

erlin

on

22/1

0/20

14 1

0:16

:27.

View Article Online

22 J. Kong, N. R. Franklin, C. W. Zhou,M. G. Chapline, S. Peng, K. J. Cho andH. J. Dai, Science, 2000, 287, 622–625.

23 M. Liebau, A. P. Graham, G. S. Duesberg,E. Unger, R. Seidel and F. Kreupl,Fullerenes, Nanotubes Carbon Nanostruct.,2005, 13(Suppl. 1), 255–258.

24 M. S. Dresselhaus and H. Dai, MRS Bull.,2004, 29, 237–239.

25 H. J. Dai, Surf. Sci., 2002, 500, 218–241.26 W. Kim, H. C. Choi, M. Shim, Y. M. Li,

D. W. Wang and H. J. Dai, Nano Lett.,2002, 2, 703–708.

27 K. Balasubramanian, R. Sordan,M. Burghard and K. Kern, Nano Lett.,2004, 4, 827–830.

28 H. J. Huang, R. Maruyama, K. Noda,H. Kajiura and K. Kadono, J. Phys.Chem. B, 2006, 110, 7316–7320.

29 M. Zheng, A. Jagota, M. S. Strano, A. P.Santos, P. Barone, S. G. Chou, B. A. Diner,M. S. Dresselhaus, R. S. McLean, G. B.Onoa, G. G. Samsonidze, E. D. Semke,M. Usrey and D. J. Walls, Science, 2003,302, 1545–1548.

30 M. S. Strano, C. A. Dyke, M. L. Usrey,P. W. Barone, M. J. Allen, H. W. Shan,C. Kittrell, R. H. Hauge, J. M. Tour andR. E. Smalley, Science, 2003, 301,1519–1522.

31 R. Krupke, F. Hennrich, H. vonLohneysen and M. M. Kappes, Science,2003, 301, 344–347.

32 L. C. Venema, J. W. Janssen, M. R.Buitelaar, J. W. G. Wildoer, S. G. Lemay,L. P. Kouwenhoven and C. Dekker, Phys.Rev. B, 2000, 62, 5238–5244.

33 L. C. Venema, V. Meunier, P. Lambin andC. Dekker, Phys. Rev. B, 2000, 61,2991–2996.

34 J. W. G. Wildoer, L. C. Venema,A. G. Rinzler, R. E. Smalley andC. Dekker, Nature, 1998, 391, 59–62.

35 M. D. Lay, J. P. Novak and E. S. Snow,Nano Lett., 2004, 4, 603–606.

36 M. Cinke, J. Li, B. Chen, A. Cassell,L. Delzeit, J. Han and M. Meyyappan,Chem. Phys. Lett., 2002, 365, 69–74.

37 J. Andzelm, N. Govind and A. Maiti,Chem. Phys. Lett., 2006, 421, 58–62.

38 J. J. Zhao, Curr. Nanosci., 2005, 1,169–176.

39 X. Feng, S. Irle, H. Witek, K. Morokuma,R. Vidic and E. Borguet, J. Am. Chem.Soc., 2005, 127, 10533–10538.

40 E. Bekyarova, M. Davis, T. Burch,M. E. Itkis, B. Zhao, S. Sunshine andR. C. Haddon, J. Phys. Chem. B, 2004,108, 19717–19720.

41 K. Parikh, K. Cattanach, R. Rao, D. S.Suh, A. M. Wu and S. K. Manohar, Sens.Actuators, B, 2006, 113, 55–63.

42 H. J. Dai, in Carbon Nanotubes, ed. M. S.Dresselhaus, G. Dresselhaus andP. Avouris, Springer, Berlin, 2001, vol. 80,pp. 29–53.

43 N. R. Franklin, Y. M. Li, R. J. Chen,A. Javey and H. J. Dai, Appl. Phys. Lett.,2001, 79, 4571–4573.

44 Y. G. Zhang, A. L. Chang, J. Cao,Q. Wang, W. Kim, Y. M. Li, N. Morris,E. Yenilmez, J. Kong and H. J. Dai, Appl.Phys. Lett., 2001, 79, 3155–3157.

45 X. M. H. Huang, R. Caldwel l ,L. M. Huang, S. C. Jun, M. Y. Huang,M. Y. Sfeir, S. P. O’Brien and J. Hone,Nano Lett., 2005, 5, 1515–1518.

46 I. Radu, Y. Hanein and D. H. Cobden,Nanotechnology, 2004, 15, 473–476.

47 A. Ismach, L. Segev, E. Wachtel andE. Joselevich, Angew. Chem., Int. Ed.,2004, 43, 6140–6143.

48 E. S. Snow, F. K. Perkins andJ. A. Robinson, Chem. Soc. Rev., 2006,35, 790–798.

4 9 J . H . H a f n e r , C . L . C h e u n g ,T. H. Oosterkamp and C. M. Lieber,J. Phys. Chem. B, 2001, 105, 743–746.

50 R. J. Chen, S. Bangsaruntip, K. A.Drouvalakis, N. W. S. Kam, M. Shim,

Y. M. Li, W. Kim, P. J. Utz and H. J. Dai,Proc. Natl. Acad. Sci. U. S. A., 2003, 100,4984–4989.

51 J. Li, Y. J. Lu, Q. Ye, M. Cinke, J. Hanand M. Meyyappan, Nano Lett., 2003, 3,929–933.

52 E. S. Snow, J. P. Novak, M. D. Lay,E. H. Houser, F. K. Perkins andP. M. Campbell, J. Vac. Sci. Technol., B,2004, 22, 1990–1994.

53 C. Wei, L. M. Dai, A. Roy and T. B. Tolle,J. Am. Chem. Soc., 2006, 128, 1412–1413.

54 N. Saran, K. Parikh, D. S. Suh, E. Munoz,H. Kolla and S. K. Manohar, J. Am.Chem. Soc., 2004, 126, 4462–4463.

55 G. Gruner, J. Mater. Chem., 2006, 16,3533–3539.

56 Y. X. Zhou, L. B. Hu and G. Gruner,Appl. Phys. Lett., 2006, 88.

57 A. T. Woolley and R. T. Kelly, Nano Lett.,2001, 1, 345–348.

58 H. Yokota, J. Sunwoo, M. Sarikaya,G. van den Engh and R. Aebersold,Anal. Chem., 1999, 71, 4418–4422.

59 J. F. Allemand, D. Bensimon, L. Jullien,A. Bensimon and V. Croquette, Biophys.J., 1997, 73, 2064–2070.

60 Z. Q. Ouyang, J. Hu, S. F. Chen, J. L. Sunand M. Q. Li, J. Vac. Sci. Technol., B,1997, 15, 1385–1387.

61 J. Hu, M. Wang, H. U. G. Weier,P. Frantz, W. Kolbe, D. F. Ogletree andM. Salmeron, Langmuir, 1996, 12,1697–1700.

62 D. Bensimon, A. J. Simon, V. Croquetteand A. Bensimon, Phys. Rev. Lett., 1995,74, 4754–4757.

63 A. Bensimon, A. Simon, A. Chiffaudel,V. Croquette, F. Heslot and D. Bensimon,Science, 1994, 265, 2096–2098.

64 P. Vichchulada and M. D. Lay, manu-script in preparation, 2007.

65 E. S. Snow, J. P. Novak, M. D. Lay andF. K. Perkins, Appl. Phys. Lett., 2004, 85,4172–4174.

This journal is � The Royal Society of Chemistry 2007 Analyst, 2007, 132, 719–723 | 723

Publ

ishe

d on

23

May

200

7. D

ownl

oade

d by

Fre

ie U

nive

rsita

et B

erlin

on

22/1

0/20

14 1

0:16

:27.

View Article Online