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http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.snb.2011.03.040mailto:mshabbir@dso.org.sgmailto:alps@ntu.edu.sghttp://www.elsevier.com/locate/snbhttp://www.sciencedirect.com/science/journal/09254005http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.snb.2011.03.040

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2 D.W.H. Fam et al. / Sensors and Actuators B 157 (2011) 17

Table 1

Various platforms for detection of chemical and biomolecules.

Transducer platforms LOD Specificity Portability Commercialization

efforts

PSA DNA NOx CO DMMP

Bulk acoustic wave ng/ml [17] fM [18] ppb [19] N.A. ppb [20] Receptor dependant Yes Successful [21]

Surface acoustic wave fg/ml [22] nM [23] ppb [24] ppm [25] Sub ppm [26] Receptor dependant Yes On going [27]

Mass spectrometry N.A. N.A. ppb [28] N.A. ppb [29] Highly specific Limited Successful [30]

Opto-chemical sub n g/ml [31]

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D.W.H. Fam et al. / Sensors and Actuators B 157 (2011) 17 3

Fig. 1. Trend of patents filed over the last decade [27].

in length. They can essentially be thought of as a layer of graphite

rolled up into a cylinder [47] and depending on the arrangement,

they could either be single walled carbon nanotubes (SWCNTs) ormulti-walled carbon nanotubes (MWCNTs). SWCNT comprises one

single layer which is favorable for the effective functionalization

leading to high specificity to analytes while MWCNTs have many

layers (approximately 50) [47].

Ever since the discovery of CNTs, key milestones for sensor

development have been reached. CNTs have a huge potential as

sensors due to its high surface area to volume ratio that allows

for high sensitivity,high thermal stabilitythat makes it viable to be

deployed in most environmentsand yetthey canbe modified easily

to imbue selectivity. There are many different types of CNT-based

electronic sensors including ionization sensors, capacitors, resis-

tors and transistors. In a review by Gruner, CNT based platforms

were citedas biocompatiblesensors because of the similarityin size

with analytes such as cells, proteins and even DNA [48]. Further-more, the electrostatic and electron transfer phenomena occurring

on thetransducer surface as a consequence of thechemicaladsorp-

tion or the biological recognition processes induces shifts in the

sensor response that is readily detectable by the electronic device.

Thus, there is a huge potential in monitoring these processes using

CNT based electronic transducers.

3.2. Morphology

There are two different methods, to date, capable of fabricat-

ing these CNT devices; the first being the dispersion of CNT on

pre-patterned substrates and the second methodology is to syn-

thesize CNT directly on the substrates. The latter is considered to

be themost promising route in terms of outputand reproducibility.Efforts have been taken in controlling the growth morphology to

improve the device performance. The initial growth of CNTs using

chemical vapor deposition (CVD) was uncontrolled and resulted

in a random bush-like network [49]. However, in recent years it

has become apparent that the device performance could be signifi-

cantlyimproved bycontrollingthe morphology of CNT. Rogerset al.

demonstrated this by adopting a large-scale synthesis methodol-

ogy to generate aligned nanotube architecture [50] on a ST (stable

temperature) cut quartz wafer via CVD using patterned iron cat-

alyst and methane as the feedstock for growth. The synthesis of

aligned tubes improved the device performance in terms of a

reduced device resistanceand improved signal to noiseratio, which

ultimately lead to an increased sensitivity [51]. This formed the

impetus for the synthesis and fabrication of devices with differ-

ent morphologies for different applications. In another work by

the Rogers group, crossbar devices, random network devices and

high-density aligned nanotube devices were fabricated by combin-ing CVD growth on ST cut quartz with a two-step transfer process

using poly(dimethylsiloxane) (PDMS) stamping. This gave a rela-

tively good control over the CNT network morphology and opened

an avenue for controlling CNT device characteristics.

Although these methodologies proved useful in controlling the

morphologies of CNTs, there are still concerns regarding the extent

of surface area enlargement and morphological control, which are

critical factors determining the performance of the devices. This

led to a focus of research on single-CNT devices, as it would elim-

inate the inter-tube resistance associated with random network

devices. One of the significant contributions to the development of

single-CNT devices was based on dielectrophoresis (DEP) to fabri-

cate large-scaled (several millions per square meter), devices with

electrodes bridged by a single nanotube [52]. Although the controlover the CNT morphology has increased, there are unfortunately

substantial differences in performance between the batches; hence

it is still a challenge to produce sensors with reproducible device

characteristics. Therefore, there is a need to improve the synthesis

protocol to accurately control the characteristics of each individ-

ual nanotube including its contact resistances with the electrode

materials.

3.3. Controlled synthesis

There are generally three techniques used for producing CNTs,

namely, carbon arc-discharge technique [53,54], laser-ablation

technique [55] and growth by either (CVD) [56,57] or other meth-

ods [58]. Iijima was the first toobserve fine threads (MWCNT) in anelectron microscope image of a sample prepared by arc-discharge

technique [46]. The arc-discharge technique was initially used for

the production of fullerenes and is a relatively simple method for

producing CNTs. This method involves passing a direct current

between two carbon rods, which are biased to create an arc dis-

charge at high temperature. This high temperature arc discharge

would then melt and vaporize one of the carbon rods and the vapor

would be collected as thread-like deposits on the other rod. The

threads were very thin and long tubes of pure carbon. However,

there is a lack of control in this method of synthesis as it produces

soot and other forms of carbon apart from CNTs and therefore,

chemical refinement is necessary to obtain the pure CNTs. In 1996,

bundles of aligned SWCNTs were produced by the laser ablation

technique [55]. This method of producing CNTs uses a pre-heated

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4 D.W.H. Fam et al. / Sensors and Actuators B 157 (2011) 17

(1200 C) horizontal tube under a controlled pressure using a flow

of inert gas with a target inside. This target consists of a composite,

which is a mixture of graphite and metal catalysts such as Co or Ni.

A laser is then introduced onto the target and this causes vaporiza-

tion to occur. The plume from the vaporization is then collected at

the cooler downstream end of the horizontal tube that is outside

the heating zone forming the CNTs. However, this method of syn-

thesis also produces soot and other carbon compounds apart from

CNTs. Following this, Jose-Yacaman et al., producing MWCNTs by

catalytically growing them using CVD [56]. MWCNTswere first dis-

covered in the buildup on the cathode of the carbon arc that was

used in a He environment to produce fullerenes such as C60. The

outer diameter of MWCNTs varies between 2 and 20nm typically

and the inner diameter is about 13 nm. The interlayer distance is

similar to the one layer of graphite (0.30.4nm). SWCNT, however,

have a smaller inner diameter of about 1.21.4 nm [54,58,59].

Studies have shown that the chirality of the carbon nanotube

plays an important role in determining the electronic structure

of the nanotubes [60,61]. Although not much is known quantita-

tively about the effects of the chirality, diameter and length of the

nanotubes on the gas sensing properties of the CNTs, these physi-

cal properties will affect the electronic properties of the CNTs, for

instance, modulation of the workfunction thus changing its capa-

bility for sensing. The chirality of the nanotube is the direction ofroll of the sheet of graphene to form a helical nanotube with the

helix direction being defined by the Hamada vectors (n,m). Calcu-

lations also have been done to show that the chiral armchair (n,n)

tubes display a metallic behavior whereas all(n,m) tubes, withnm

an integral multiple of 3 could be a small-gap semiconductor or

semimetallic tube [61]. The tubes which do not fall within these

categories should be semiconducting with a band gap roughly pro-

portional to the reciprocal of the tube radius [62]. This was found

to be qualitatively true for tubes with a large diameter where the

sp interaction is negligible [63,64].

Therefore, to obtain CNTs with reproducible electronic proper-

ties, the synthesis and sorting steps of CNTs must be controlled.

For example, considerable efforts have been devoted to sort the

chirality and nanotube diameter in an effort to control its elec-tronic properties and several methodologies have been proposed

including dielectrophoresis [65] or using DNA [66]. However, one

of the most poignant strategies involved structure-discriminating

surfactants for the separation of nanotubes (in powdered form) by

their diameters, band gaps and electronic types (semiconducting

or metallic) [67]. This methodology for production of pure (semi-

conducting or metallic) solutions introduced by NanoIntegris Inc.

constitutes a tool for the up-scale production of nanotubes. How-

ever, complete refinement in separation of the different chiralities

has yet to be achieved in this process [67]. Furthermore, as these

nanotubes are wrapped with surfactants their sensing capabilities

might be impeded. Accordingly, there is still a need to develop

CVD based growth protocols in order to obtain the CNT layers of

consistent thickness, required length, diameter and chirality forconsistent device fabrication.

3.4. Fabrication of sensor devices

CNT sensor device fabrication essentially integrates the CNT

morphology obtained via the chirality, diameter and length sep-

aration using controlled synthesis techniques with a supporting

substrate and electrode interconnections in a transistor (Fig. 2) or

resistor configuration.

The ability to sense lies in the nature of the interaction

between the sensitive material and the analyte; whether the ana-

lyte molecules bind specifically to the sensitive material and hence

change its intrinsic properties, electronic or mechanical. Due to the

difficultyin optimization of such a device, commercialization is still

Fig. 2. Schematic of CNT transistor device.

elusive. However, one notable contribution to commercialization

could be found in the works of Gruner and Star et al. [38,42,48,68].

In their works they have successfully commercialized sensors to

detect troponin, nitric oxide (NO) and various industrial gases. This

was accomplished using assays of CNT transistors arranged in an

array format. These CNT transistors are based on random network

architectures that display relatively large tube-to-tube variations.

However, in a network configuration, the difference is averaged

and the device performance is defined by the mean properties ofthe CNT architecture. This would mean that the only difference

between devices would be due to the density variations of the nan-

otubes. Despite the apparent ease in fabrication there has not been

a significant increase in number of sensors produced for different

analytes. This might be due to the difficulty in achieving nanotubes

that are uniformly functionalized and decorated with recognition

molecules.

In the recent years, sensors research has become more materi-

als oriented and the emphasis has been on advanced functional

nanomaterials that serve as specific sensing layers. These novel

materials include organic and polymer complexes [16,6984] and

different oxides of Sn, Zn and Ba [8592], amongst others. A recent

work by our group demonstrates an electronic sensor based on Ag

nanoparticles of mono-distributed size (4 nm) which are uniformlydecorated onto SWCNT for the purpose of selective and real-time

detection of hydrogen sulfide in gaseous form (H2S) [93]. To date,

much work has been done to improve the selectivity of CNT sen-

sors and it could be observed that customizable CNT based sensors

decorated with suitable selective receptors may become accessible

on the market in the near future.

3.5. Commercialization efforts

Although the achievements in CNT based chemical, toxic and

biosensors are remarkable, substantial research efforts are still

needed to make commercialization possible. Over the recent years,

much of the emphasis is being laid on controlling homogeneity of

nanotube devices and monitoring their interaction with analytes.Though seemingly straight forward, the underlying sensing mech-

anism of nanotube-based sensors is multifaceted and continues to

be actively researched and debated. Interaction between various

analytes and the CNT network may also be complicated for gen-

eralization. Moreover, during real time sensing, various molecules

other than the analyte of interest may also be physically adsorbed

on to the CNTs thereby introducing non-specific binding (NSB).

There have been some attempts to minimize NSB via substrate

passivation using blocking agents that does not interfere with the

molecular recognition event nor contribute to a sensor response.

Studies have also reportedon theuse oflipidbilayers andliposomes

that functionsas a bio-interface to achieve reliable sensing of trans-

membrane and signaling phenomena [94]. These are particularly

attractive systems to work with as they suppress NSB.

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