Microfiber Devices Based on Carbon Materials

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Microfiber devices based on carbon materials

Sun, Gengzhi; Wang, Xuewan; Chen, Peng

2014

Sun, G., Wang, X., & Chen, P. (2014). Microfiber devices based on carbon materials. Materials Today, 18(4), 215‑226. https://hdl.handle.net/10356/79436 https://doi.org/10.1016/j.mattod.2014.12.001

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Materials Today Volume 00, Number 00 December 2014 RESEARCH

Review

Microfiber devices based on carbon

materials RESEARCH:

Gengzhi Sun, Xuewan Wang and Peng Chen*

School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore 637457, Singapore

Microﬁber devices are able to extend the micro/nano functionalities of materials or devices to the

macroscopic scale with excellent ﬂexibility and weavability, promising a variety of unique applications

and, sometimes, also improved performance as compared with bulk counterparts. The ﬁber electrodes in

these devices are often made of carbon materials (e.g. carbon nanotubes and graphene) because of their

exceptional electrical, mechanical, and structural properties. Covering the latest developments and

aiming to stimulate more exciting applications, we comprehensively review the preparation and

applications of carbon-microﬁber devices on energy conversion and storage, electronics, sensors and

actuators.

Introduction esting microﬁber devices will be discussed, including energy con-

Emerging microﬁber based devices are promising for wearable version and storage devices, electronics, sensors, and actuators.

electronics, smart textiles, ﬂexible electronics, micro-probes or

sensors, among others. Such miniaturized devices often also dem- Preparation of carbon-based microﬁbers

onstrate improved performance (e.g. output power density, detec- As an industrial product, CF can be manufactured at the ton-scale

tion sensitivity) compared to their bulk counterparts. Although by graphitization of precursor ﬁbers based on pitch or polyacrylo-

micro- or nano-structured, tangible microﬁbers ease the fabrica- nitrile. Despite the high mechanical strength, CF is too fragile for

tion challenges encountered by the individual, small, constituent certain applications. Interested readers may refer to a recent review

components while preserving their intrinsic properties. article for further information [3]. Here, we place the emphasis on

Because of high conductivity and ready availability, metallic CNTF and GF which, as compared to solid CF, have excellent

wires have traditionally been employed for ﬁber-based devices [1]. mechanical properties, light-weight, good electrical conductivity,

But the tendency to oxidize under ambient conditions, mediocre porous structure, the feasibility to be integrated with various guest

mechanical strength and ﬂexibility, and heavy weight greatly limit nanomaterials, as well as other unique merits inherited from CNT

their practical applications, such as, in wearable electronics [2]. and graphene including large-surface area and tunable physico-

Carbon-based microfibers, specifically, carbon fibers (CFs), carbon chemical properties by chemical and heteroatom doping [7,8].

nanotube ﬁbers (CNTFs) and graphene ﬁbers (GFs) are excellent These merits allow much more complex systems to be made and

alternatives due to their outstanding mechanical properties, good signiﬁcantly richer applications than that of CFs. The properties of

electrical conductivity, the ease with which they may be function- microﬁbers largely depend on the fabrication processes. Usually,

alized or hybridized with other materials, and the possibility to CNTFs show superior mechanical properties and conductivity

align the functionalities on a large scale in an axial direction [3–6]. while GFs have a higher speciﬁc surface and are easier to functio-

In this article, the spinning techniques to fabricate CNTF and GF nalize or hybridize with other nanomaterials.

will be considered, and the latest developments for various inter-

CNT ﬁbers

Recent progress in continuous spinning CNTFs has demonstrated

the possibility of retaining the exceptional properties of individual

*Corresponding author:. Chen, P. ([email protected]) CNTs at larger and more practical scales [4,5,9]. Various methods

1369-7021/ß 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). http://dx.doi.org/10.1016/

j.mattod.2014.12.001 1

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RESEARCH Materials Today Volume 00, Number 00 December 2014

have been developed to assemble tiny CNTs into macroscopic ﬁber polyvinyl alcohol (PVA) coagulation bath; while the surfactant is

structures [10–15], which could be generally categorized into two displaced by PVA, a gel ﬁber forms from the collapsed CNTs (Fig. 1a).

classes: wet-spinning and dry-spinning. The resultant CNT/PVA composite ﬁber (CNT, 60 wt%) gives a

tensile strength of 0.1 GPa and a Young’s modulus of 9–15 GPa.

Wet-spinning In contrast to conventional CF, such a CNTF can be heavily bent and

Similar to synthesizing polymer ﬁbers, CNTFs can be produced tightly tied without breaking. By optimizing the spinning process or

using ‘coagulation spinning’. When a polymer dispersed in solvent introducing post-treatments (e.g. stretching or hot drawing), the

A is extruded through a capillary tube into solvent B (in which A is mechanical properties of CNTFs can be further improved [16–19].

soluble but not the polymer), the polymer will condense and form a However, retention of a considerable amount of polymers unavoid-

RESEARCH:

ﬁber because of phase separation. Such a spinning approach was ably compromises the thermal and electrical conductivity of the

initially adopted for CNTF spinning by Poulin’s group [10]. In brief, resultant ﬁber. In an attempt to tackle this issue, Kozlov et al.

single-walled CNTs (SWNTs) are ﬁrst dispersed by a surfactant in an developed a polymer-free spinning method but, to an extent, with

aqueous solution; this is followed by injection into an aqueous a sacriﬁce of mechanical properties [20]. Review

FIGURE 1

The experiment setup of a wet-spinning process: (a) coagulation-based and (b) LC-based. Adopted from Refs. [10,11]. Dry-spinning process: (c) CNTFs spun

from vertically aligned array. Adopted from Ref [15]. (d) CNTFs directly spun from CVD furnace. Adopted from Ref [14].

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Owing to their high-aspect ratio, CNTs exhibit liquid crystalline improving the performance of GFs (e.g. mechanical strength,

(LC) behavior in fuming sulfuric acid [21]. By extruding such SWNT electrical conductivity and functional uses). Increasing the size

solutions into a coagulant bath (ether diethyl ether, 5% sulfuric of GO sheets, stretching the ﬁber, coordinating cross-linking (with

acid, or water) (Fig. 1b), Ericson et al. made well-aligned pure CNTFs divalent ions), hybridizing with other materials (e.g. polymers)

with a mechanical strength of 116 MPa and conductivity of 5 kS/ have been found to be beneﬁcial to the mechanical strength and

cm [11]. But strong acid causes protonation of SWNTs, leading to, electrical conductivity [36,38,39]. Thus far, the maximum

for example, impaired electrical conductivity. This process is very strength and conductivity of GF has been reported to be

sensitive to water. Minimal moisture can cause phase separation and 500 MPa and 40 kS/m [39], respectively.

precipitation of discrete needle-like crystal solvates. Moreover, this

superacid route is not effective for multi-walled CNTs (MWNTs) Applications

[11]. In comparison, chlorosulfonic acid was found to be a good Although the application of carbon-based microﬁber devices is still

Review

solvent for both SWNTs and MWNTs at high concentration (2– in its infant stage, the ﬁeld is quickly evolving. By bridging the gap

6 wt%) [22,23]. The obtained ﬁber exhibits an improved strength of between macroscopic processing/handling and micro-/nano-

1.0 GPa and a very high conductivity of 2.9 MS/m. scopic materials, they have demonstrated great potential for wear-

able, ﬂexible, or miniaturized devices for energy conversion and

RESEARCH:

Dry-spinning storage, sensing, actuation, among others.

Wet-spinning approaches rely on a good dispersion of CNTs,

which is a challenge particularly at high concentrations. Alterna- Energy devices

tively, neat CNTFs can be spun directly from dry-state CNT assem- Solar cells

blies: vertically aligned CNT arrays or CNT aerogels. Fiber-shaped photovoltaic devices, including polymer solar cells

Dry spinning was ﬁrst demonstrated by Jiang et al. in 2002 [12]. (PSC) and dye-sensitized solar cells (DSSC), are light-weight, wea-

They draw a CNT yarn (30 cm long) out from a vertically aligned vable/wearable, and integratable with other micro-devices [40,41].

CNT array grown by chemical vapor deposition (CVD). By intro- Using SWNTs for the counter electrode, co-axial and twisted PSCs

ducing twisting, Zhang et al. assembled such a CNT yarn into a were fabricated by Cao and co-workers (Fig. 2a–c) [42], achieving

long microﬁber with a strength of 460 MPa (Fig. 1c) [15]. Differ- an efﬁciency of 2.31% and 2.11%, respectively. Another PSC was

ent strategies have been proposed to improve the ﬁber perfor- similarly fabricated by twisting MWNT ﬁber with Ti/TiO2 wire

mances, for example, by adjusting the length [24], packing density coated with photoactive materials [43]. But the current ﬁber-based

[25], or morphology [26] of CNTs. The maximum mechanical PSCs suffer from poor efﬁciency (typically <3%).

strength reported so far is 3.3 GPa. In spite of the fascinating As shown in Fig. 2d, a DSSC was fabricated by twisting CF with

properties of thus-spun ﬁbers [27–32], the growth of a spinnable Ti/TiO2 wire [44]. CF replaces the conventional platinum (Pt) wire

À À

CNT array is however technically nontrivial and the spinning as the counter electrode for the redox reaction of I /I3 . Using a CF

mechanism is still under debate [15,25]. with a diameter of 230 mm, 2.7% efﬁciency was realized (Fig. 2e).

Neat CNTFs could also be spun from a CNT aerogel formed in The same group later developed an all-carbon DSSC using TiO2-

CVD reaction zone (Fig. 1d) [14]. Studies have shown that a range coated CF and ink-carbon (or Pt) coated CF as the working and

of oxygen-containing carbon sources can serve as CVD precursors counter electrode, respectively [45,46]. By wrapping a thin-layer of

to produce CNT aerogels with the assistance of thiophene [13,14]. CNTs on a TiO2 nanotube modiﬁed Ti wire, a co-axial DSSC with

Koziol et al. optimized the spinning process and achieved a ﬁber an efﬁciency of 1.6% was demonstrated [47]. A DSSC made by

strength of 10 GPa by drawing the aerogel at a winding speed of twisting CNT fiber with CNT@TiO2 fiber gave an efficiency of 2.6%

20 m/min [13]. But the ﬁber’s mechanical strength quickly (Fig. 2f) [48]. The CNT ﬁber shows a higher electrocatalytic activity

decreases with increasing gauge length. As shown by Wang than the conventional Pt wire because of the larger surface area

et al., an additional step of pressurized rolling can produce com- (Fig. 2g). Interestingly, remote controllable and stretchable ﬁber-

pact CNTFs with high strength nearly independent of gauge shaped DSSCs have been developed [49,50]. The maximum energy

length (4.23 and 3.71 GPa for 1 and 20 mm gauge length, respec- conversion efﬁciency of 8.45% has been attained by using a Pt-

tively) [33]. nanoparticle decorated graphene ﬁber as the counter electrode

[51].

Graphene ﬁbers The efﬁciency of the current PSC and DSSC is poor when com-

While inheriting various intrinsic properties of pristine graphene pared with conventional planar counterparts. This problem worsens

(e.g. extremely high speciﬁc surface area), graphene oxide (GO) as the ﬁber length increases. Thus, effort is required to optimize the

sheets gain other interesting features [34,35]. In particular, the device structures and improve the material properties.

amphiphilic characteristic makes it well-soluble in various sol-

vents, and therefore amenable for solution-based ﬁber fabrication. Nano-generators

CNTs exhibit LC behavior in strong acids whereas GO sheets Mechanical energy can be harvested using a ﬁber-shaped nano-

behave the same in various solvents (except acids) [36]. The high generator to produce electrical power. Wang and co-workers dem-

orderliness of GO dispersions permits self-assembly of individual onstrated this concept by coating CFs with a thin layer of ZnO ﬁlm

micrometer-sized GO sheets into highly aligned ﬁbers. Subsequent (Fig. 3) [52]. Under compressive strain, the textured structure of

chemical or thermal reduction will transform GO ﬁbers into GFs ZnO ﬁlm can establish a piezopotential across its inner and exte-

with good ﬂexibility and electrical conductivity. Since the dem- rior surfaces thereby producing an electric current in the external

onstration by Gao’s group [37], much effort has been focused on load (Fig. 3a). The power generation can even be realized in a non-

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FIGURE 2

(a) Illustration of the fabrication of ﬁber-shaped PSC. (b) SEM image of co-axial PSC. (c) SEM image of twisted PSC. Adopted from Ref [42]. (d) Schematic of

DSSC fabricated from CF. (e) Current-voltage curves of the ﬁber-shaped DSSCs using CF (different diameter) or Pt wire as counter electrode. Adopted from

Ref [44]. (f) SEM image of a twisted ﬁber-shaped DSSC (inset shows the cross-sectional structure). (g) Cyclic voltammograms of a CNTF and a Pt wire in I /

I3 electrolyte (scan rate of 50 mV/s). Adopted from Ref [48].

contact mode, that is, responding to air breeze (Fig. 3b). Repeated electrodes show smaller over-potentials than the conventional Pt

exhaling gives an AC output. The average output voltage and electrode [55]. However, the small speciﬁc surface area of CF limits

current is 1.5 mV and 0.5 nA, respectively. When attached onto the power output and operational lifetime. Mano and co-workers

the wrist, it can also be driven by a heartbeat pulse (Fig. 3c). used porous microﬁbers assembled by oriented CNTs, which was

Although this device is not able to power regular electronics, it modiﬁed with glucose oxidase (GOx) as the anode or modiﬁed

could be useful to drive nano-devices or for force detection. with bilirubin oxidase (BOD) as the cathode (Fig. 4a) [56]. The

CNTF cathode exhibits a fourfold higher current density of oxygen

Bio-fuel cells reduction and more positive onset potential than the convention-

Bio-fuel cells (BFCs) are able to harvest energy from green and al CF electrode (Fig. 4b). The improved performance is attributed

sustainable fuels using biocatalysts (enzymes or microorganisms) to the nano-porosity of CNTF which ensures a large active surface

[53,54]. In comparison with bulk counterparts, the miniaturized area and high catalyst loading. The over-potential of the BOD/

ﬁber-shaped BFCs are desirable as implantable power sources for CNTF cathode is lower than that of the Pt wire and CF. The

medical devices. In addition, microﬁber electrodes have been maximum power density of such a BFC is 740 mW/cm , which

employed to overcome the issues of poor electron transfer and is fourfold higher than that of CF-based BFC at 15 mM glucose

slow mass transport normally existing in miniaturized BFCs. CF concentration (Fig. 4c). The improved open-circuit voltage bene-

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FIGURE 3

(a) Schematic of a fiber nanogenerator based on a carbon fiber coated by a ZnO thin film and working principle of the fiber nanogenerator, where the ‘+/À’

signs indicate the polarity of the local piezoelectric potential created on the inner and outer surfaces of the ZnO thin ﬁlm. (b) The open-circuit output

voltage. The inset is a photograph of ﬁber-based nanogenerator. (c) Schematics of the experimental set-up and current outputs generated by heartbeat

pulses. Adopted from Ref [52].

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FIGURE 4

(a) Schematic of BFC. (b) Dependence of current density on voltage for a modified CNT fiber electrode and a modified carbon fiber electrode. (c)

Dependence of power density on operating voltage for a BFC made with CF and CNTF in a quiescent phosphate-buffered saline buffer, under air, 15 mM

glucose at 37 8C. Adopted from Ref [56].

ﬁts from the intimate electrical coupling between the catalysts and supercapacitor was prepared by twisting two Pt-wire-plied

individual CNTs. The performance of a ﬁber-shaped BFC was PEDOT/CNT ﬁbers with PVA/H2SO4 electrolyte as a separator

recently improved to 2.18 mW/cm using biscrolled CNT-PEDOT [65]. The volumetric capacitance of thus-fabricated ﬁber electro-

composite ﬁber [57]. des reaches up to 179 F/cm . As an indication of the capability for

rapid charging, its discharge current linearly scales with the

Li-ion battery voltage scan rate up to 80 V/s and 20 V/s for liquid and solid

Wearable electronics require the incorporation of wearable energy electrolytes, respectively. Besides the twisted structure, a core-

storage units such as Li-ion batteries (LIBs) which offer high energy sheath ﬁber-shaped supercapacitor was realized by scrolling an

density. Ren et al. prepared a ﬁber-shaped half-cell LIB by twisting a aligned CNT sheet onto a CNTF with a gel electrolyte in between

MnO2/CNT composite ﬁber with a Li wire which reached a speciﬁc [66]. Superior to the twisted counterpart, such a coaxial super-

3 2

capacity of 100 mAh/cm at a discharge current of 0.5 mA [58]. By capacitor exhibits a speciﬁc capacitance of 8.66 mF/cm (59 F/g)

depositing Si (38.1%) onto CNTF, the performance of the composite at a discharge current of 0.1 mA without any degradation during

ﬁber was further improved to 1670 mAh/g [59]. A ﬁber-shaped bending. The same research group also developed a stretchable

full-cell LIB with coaxial structure was recently devised by sequen- ﬁber-based supercapacitor by sequentially wrapping aligned

tially winding a composite fiber cathode (CNT/lithium manganate CNT sheets on an elastic fiber, which gave a specific capacitance

(LMO)) and anode (CNT/Si) onto a cotton ﬁber, with a gel electrolyte of 18 F/g after stretch by 75% for 100 cycles [67]. Various

as separator (Fig. 5a) [60]. This LIB shows an output voltage of 3.4 V CNT-based composite ﬁbers have been fabricated for supercapa-

with a speciﬁc discharge capacity of 106.5 mAh/g, and a capacity citors which exhibit volumetric capacitance ranging from 25 to

retention of 87% after 100 cycles (Fig. 5b,c). It can illuminate a light 48.5 F/cm [69–71].

emitting diode and be woven into a ﬂexible textile (Fig. 5d,e). By Qu and co-workers fabricated a GF-based supercapacitor, in

replacing Si with Li4Ti5O12 as the anodic material, a stretchable which GF was covered with a sheath of porous graphene structure

ﬁber-shaped LIB was fabricated (wrapping on an elastic heat-shrink- [62]. The resultant device exhibits an areal capacitance of 1.2–

able tube). The resulting full cell achieves a speciﬁc capacity of 1.7 mF/cm at a scan rate of 50 mV/s. Kou et al. proposed a coaxial

138 mAh/g at 0.01 mA and 84% capacity retention after stretching wet-spinning approach to simultaneously wrap graphene/CNT

for 200 cycles at a strain of 100% [61]. ﬁbers with a sheath of polyelectrolyte [72]. The obtained ﬁber

2 3

electrode shows an areal capacitance of 177 mF/cm (158 F/cm ) at

Supercapacitors a current density of 0.1 mA/cm . Sun et al. recently demonstrated a

Supercapacitors are energy storage devices with high power den- general strategy to spin hybrid ﬁbers consisting of rGO and single

sity and cycling ability. Fiber-shaped supercapacitors commonly layered transition metal dichalcogenides (TMDs, including MoS2,

use CF, CNTF or GF as electrodes and PVA-H3PO4 (or PVA-H2SO4) TiS2, TaS2 and NbSe2) (Fig. 6e) [73]. As compared with rGO-ﬁber

gel as the electrolyte [62–68]. A CF-based supercapacitor was based supercapacitors, incorporation of TMDs signiﬁcantly

fabricated by Zou’s group (Fig. 6a), in which CF was clothed with improves the capacitive performance (Fig. 6f).

a thin ﬁlm of carbon nanomaterials by dip-coating in pen ink

(Fig. 6b) [64]. The as-fabricated supercapacitor exhibits an areal Multi-functional energy devices

capacity of 19.5–11.9 mF/cm at a discharge current density of Implementing multi-functions into a single ﬁber device can great-

0.083–16.6 mA/cm (Fig. 6b). The improved performance is at- ly extend its applicability. A few interesting works on integrating

tributable to the coated carbon nanoparticles. The fragile nature energy harvesting and energy storage have been reported thus far

of CF limits its adoption in wearable devices whereas ﬂexible [74–79]. In a solar cell and supercapacitor hybrid device, a Ti wire is

CNTF is more suitable. In the recent work of Lee et al., PEDOT coated with photoactive materials on one segment and gel elec-

was introduced into CNTF by a biscrolling approach and the trolyte on the other to simultaneously realize the photovoltaic

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FIGURE 5

(a) Illustration of the fabrication of the coaxial fiber full LIB. (b,c) Voltage profiles and long-life performance of the fiber-shaped LIB between 2.0 and 4.3 V at

1 C. (d) Photograph of a ﬁber-shaped LIB to lighten up a light emission diode. (e) Fiber-shaped full LIBs being woven into a textile. Adopted from Ref [60].

conversion and energy storage, respectively (Fig. 7a). Subsequently, charged to 2.4 V in 5 s, and a stepwise voltage increase is ob-

two CNT ﬁbers are individually wrapped around each functional served with each movement of the hand. The fully charged super-

segment to complete the device with dual functionalities [74]. Upon capacitor is able to power a commercial liquid crystal display and

exposure to light, the supercapacitor segment is charged by the in- light emitting diode as shown in Fig. 7g.

series connected photovoltaic segment (Fig. 7b). The device can

then discharge the storage energy when the two functional parts are Electronic devices

disconnected (Fig. 7c). Fig. 7d presents the charge–discharge behav- In order to realize smart textiles, ﬁber-electronics are needed. This

ior of a device integrated with DSSC and supercapacitor under the challenge, however, remains largely unexplored. Thus far, only a

light irradiation of 100 mW/cm [74]. The light illumination rapidly non-volatile memory device and a photo detector have been made

increases the voltage output to 0.6 V. The energy conversion efﬁ- on carbon microﬁbers. It is, however, noteworthy that transistor

ciency and storage efﬁciency were calculated to be 2.2% and 68.4%, devices have already been realized on polyamide microﬁber

thus giving a complete photoelectric conversion and storage efﬁ- (10 mm) and silk ﬁbers [80–82].

ciency of 1.5%. A coaxial ﬁber combining both PSC and super-

capacitor was recently fabricated using P3HT/PCBM as the Non-volatile memory

photoactive material [76]. The energy conversion and storage efﬁ- As a proof-of-concept demonstration, Sun et al. reported a ﬁber-

ciency was calculated to be 1.01% and 81.2%, producing an overall based, non-volatile memory device [83], in which CNTF and GO

photoelectric conversion and storage efﬁciency of 0.82%. were used as the conductive electrode and active material, respec-

In an interesting work, Wang and co-workers integrated a ﬁber- tively. By cross-stacking two core–shell structured CNT@GO ﬁbers

supercapacitor onto a ﬂexible triboelectric generator to harvest (inset of Fig. 8a), a ﬁber-based memory device is formed. When the

mechanical energy [77]. Using a bridge rectiﬁer, the mechanically applied voltage exceeds the switching threshold (3.5 V), an abrupt

produced alternating current is transformed into direct current to current increase occurs (phase I to II), suggesting a transition from

charge the supercapacitor (Fig. 7e,f). The supercapacitor can be high resistance state (OFF state) to the low resistance state (ON

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FIGURE 6

(a) Schematic of the CF-based supercapacitor. (b) Discharge current dependent areal capacitance. Inset is a SEM image of CF coated by pen ink. Adopted

from Ref [64]. (c) SEM images of a PEDOT/CNT biscrolled ﬁber plied with a Pt wire. (d) Scan rate dependent volumetric and areal capacitances of a ﬁber

supercapacitor in a liquid electrolyte (inset: solid electrolyte). Adopted from Ref [65]. (e) MoS2 concentration dependent volumetric capacitance of a hybrid

ﬁber supercapacitor. (f) Cycling stability of rGO-MoS2 (2.2 wt%) hybrid ﬁber supercapacitor. Adopted from Ref [73].

state) which is equivalent to a set/write process in a digital storage Photo-detectors

device. The low resistance state is sustained in the following A ﬁber-shaped photo-detector was demonstrated by Shen’s

voltage sweeping (phase III to IV). Such write-once-read-many- group [84]. A Co3O4 nanowire coated nickel ﬁber acts as the

times devices exhibit a good data-retention ability (Fig. 8b). positive electrode while a rGO coated CF functions as both the

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FIGURE 7

(a) Schematic of the integrated device for photoelectric conversion (PC) and energy storage. Schematic of the circuit connection during (b) charging and (c)

discharging. (d) Photocharging–discharging curve of the integrated device (the discharging current is 0.1 mA). Adopted from Ref [74]. (e) Schematic of an

integrated device for three ﬁber supercapacitors (in series) and a triboelectric generator. (f) Charging curve of ﬁber supercapacitors by a triboelectric

generator. The inset shows a stepwise increase of voltage with the movement of the hand. (g) LCD (left) and LED (right) can be powered by the integrated

device. Adopted from Ref [77].

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FIGURE 8

(a) Electrical behavior of ﬁber-based non-volatile memory devices (inset is the schematic of the memory), (b) the data-retention ability of ﬁber-based

memory device in OFF and ON states (reading voltage is 1 V). Adopted from Ref [83]. (c) Working mechanism of ﬁber-based photodetector. (d)

Photoresponsive leakage currents of fully charged device (1.5 V) under exposure to white light (25 and 40 mW/cm ). Adopted from Ref [84].

negative electrode and light collector (Fig. 8c). Once the device fragile and still planar in nature as conventional electrodes (hence,

is charged as a supercapacitor, the electron–hole pairs generated low speciﬁc surface area).

in rGO upon light illumination separate due to the electric In comparison, CNT ﬁbers offer larger speciﬁc surface area, im-

field. Electrons flowing into the anode and holes flowing into proved electrochemical performance, and better mechanical robust-

cathode cause a large increase in leakage current (Fig. 8d). ness [87–90]. The ﬁrst CNTF electrode (wet-spun) for the detection of

However, the uses of metallic wire and Co3O4 limit the device NaDH, dopamine and H2O2 is dated back to 2003 [91]. It exhibits

ﬂexibility. higher sensitivity and better resistance to NADH oxidation induced

surface fouling than a CF electrode. Similar improvement is also

Electrochemical sensors observed in a CNTF disk-microelectrode (Fig. 9b,c) [89]. In fast-scan

Compared to macro-electrodes, microelectrodes offer an en- cyclic voltammetry mode, Schmidt et al. used CNTF (spun from array)

hanced mass transfer rate, decreased Ohmic drop, lower double- electrode to detect neurotransmitter ﬂuctuations in acute brain slices

layer capacitance, and faster response times. Fiber microelectrodes [92]. Compared to the conventional CF electrodes, CNTF provides

are particularly advantageous for detection in a small volume (e.g. improved selectivity, sensitivity, spatial resolution, as well as faster

microdroplet), probing small targets (e.g. a single cell), and serving apparent electron transfer kinetics.

as implantable electrodes for medical devices [85]. To detect electrochemically inactive compounds, biological or

CF electrodes have been widely adopted for the electrochemical inorganic catalysts can be functionalized on to carbon electrodes.

detection of neurotransmitters or hormones (e.g. dopamine, aden- For example, glucose oxidase modiﬁed CNTF electrodes have been

osine, serotonin), hydrogen peroxide, and local pH at a single cell utilized for sensitive detection of glucose [93,94]. By electrodopo-

level or in brain tissues. For example, an ultraﬁne CF electrode is siting Pd nanoparticles on CNTF (Fig. 9d), H2O2 can be detected at

able to resolve single vesicular release of dopamine from a single a low detection limit of 2 mM and with a wide dynamic range (up to

rat phenchromocytoma cell [86]. However, CF electrodes are 1.3 mM) (Fig. 9e,f) [95].

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MATTOD-449; No of Pages 12

Materials Today Volume 00, Number 00 December 2014 RESEARCH Review RESEARCH:

FIGURE 9

(a) Photograph of the detection of dopamine using a CF electrode. Adopted from Ref [86]. (b) SEM image of CNTF tip electrode and (c) the resistance of

CNTF and CF electrodes against the chemical fouling of DA at 0.8 V vs Ag/AgCl in PBS. Adopted from Ref [89]. (d) SEM images of a CNTF electrode. Insets

show the fibers before (bottom right) and after (upper left) Pd modification. (e) Amperometric response of the Pd/CNT fiber electrode upon successive

injection of H2O2 to 0.1 M N2-saturated PBS solution (pH 7.4) at À0.5 V. (f) Linear plot of current density vs. H2O2 concentration. Adopted from Ref [95].

Actuators actuator can provide reversible 15,0008 rotation and 590 revolu-

The first fiber-shaped actuator (artificial muscle) was demonstrated tions per minute, it only operates in an electrolyte solution under

by Baughman’s group [96]. This reversible torsional actuator is three-electrode conﬁguration.

made by tethering both ends of the ﬁber, where the portion The same group later designed guest-material-ﬁlled, twist-spun

immersed in the electrolyte acts as torsional actuator and the CNT ﬁbers as electrolyte-free actuators (torsional motors, contrac-

remaining ﬁber functions as a torsional spring (Fig. 10a). It was tile muscles) operational in open air [97,98]. Electrical, chemical,

proposed that the simultaneous lengthwise contraction and tor- or photonic excitation alters the volume of the guest materials,

sional rotation are caused by ﬁber volume increase because of therefore causing torsional rotation and contraction of the ﬁber

electrochemical double-layer charge injection [96]. Both torsional host. However, such electrothermally driven actuators require a

and tensile actuations depend on the size of the electrolyte ion high operating voltage, and exhibit low energy conversion efﬁ-

used to compensate electronic charge (Fig. 10b,c). Although this ciency (limited by Carnot efﬁciency) [99].

FIGURE 10

(a) A two-end-tethered conﬁguration for simultaneously measuring torsional and tensile actuation. (b,c) Actuation results for two-end-tethered CNT ﬁbers:

paddle rotation (normalized to the length of actuating ﬁber) and tensile actuation during a cyclic potential scan at 50 mV/s. The electrolytes are 0.1 M

BMPÁTFSI in propylene carbonate (b) and 0.2 M TBAÁPF6 in acetonitrile (c). Arrows indicate scan directions. Adopted from Ref [96].

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MATTOD-449; No of Pages 12

RESEARCH Materials Today Volume 00, Number 00 December 2014

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