Hindawi Publishing CorporationJournal of SensorsVolume 2013, Article ID 741248, 7 pageshttp://dx.doi.org/10.1155/2013/741248
Research ArticleConductivity-Dependent Strain Response of CarbonNanotube Treated Bacterial Nanocellulose
S. Farjana,1, 2 F. Toomadj,1, 2 P. Lundgren,1, 2 A. Sanz-Velasco,1, 2
O. Naboka,1, 2 and P. Enoksson1, 2
1Micro- and Nanosystems Group, BioNano Systems Laboratory, Department of Microtechnology and Nanoscience (MC2),Chalmers University of Technology, 412 96 Göteborg, Sweden
2Wallenberg Wood Science Center, Chalmers University of Technology, 412 96 Göteborg, Sweden
Correspondence should be addressed to P. Enoksson; [email protected]
is paper reports the strain sensitivity of �exible, electrically conductive, and nanostructured cellulose which was preparedby modi�cation of bacterial cellulose with double-walled carbon nanotubes (DWCNTs) and multiwalled carbon nanotubes(MWCNTs).e electrical conductivity depends on themodifying agent and its dispersion process.e conductivity of the samplesobtained from bacterial cellulose (�NC) pellicles modi�ed with DWCNT was in the range from 0.034 S⋅cm−1 to 0.39 S⋅cm−1, andfor �NC pellicles modi�ed with MWCNTs it was from 0.12 S⋅cm−1 to 1.6 S⋅cm−1. e strain-induced electromechanical response,resistance versus strain, was monitored during the application of tensile force in order to study the sensitivity of the modi�ednanocellulose. A maximum gauge factor of 252 was found from the highest conductive sample treated by MWCNT. It has beenobserved that the sensitivity of the sample depends on the conductivity of the modi�ed cellulose.
1. Introduction
Recently, sensors based on nanostructured material haveattracted considerable attention due to their low powerconsumption, high sensitivity and selectivity, and promptresponse [1, 2]. Conventional sensors are restricted in theirapplication area by their rigidity and fragility. For this reason,development of sensor materials which are �exible andenvironmentally friendly has received a great deal of attention[3]. Comparing with ceramic and semiconducting materials,sensors which are based on organic nanostructured materialhave gained in signi�cance due to their attractive properties[3]. It has been reported that such materials can be obtainedby the introduction of nanoparticles with promising electricaland mechanical properties into a polymer matrix [4].
Among several nanostructures, carbon nanotubes(CNTs) have attracted a special interest because of theirunique electronic, mechanical, and thermal properties whichexpanded the application �eld of CNT to nanoelectronicsand biomedical devices [5]. Recently, the incorporation ofCNT to polymers has been investigated to reinforce themechanical properties of the polymers [4, 5]; it was shown
that the elastic modulus and the ultimate strength of polymercomposites increase even with the incorporation of smallamounts of CNT.
Cellulose, the most abundant natural polymer, is aninexhaustible raw material with fascinating structure andproperties [6]. e properties of cellulose allow obtaining ofenvironmentally friendly, biodegradable, and biocompatibleproducts. Recently, research related to cellulose demon-strated its value for diverse applications including actuatorsand sensors. e new class of �exible cellulose-based elec-troactive materials was named electroactive paper (EAPap)[7]. It has been discovered that the electric power con-sumption of EAPap is very low [8]. e actuation principleof cellulose-based EAPap has been analyzed in differentresearch works [7, 8]. Also, research has been conductedto investigate the mechanical properties of cellulose andthe effect of ambient conditions on these properties. In thepresent work, the strain-induced electromechanical responseof CNT-modi�ed cellulose has been characterized.
Cellulose could be derived from various sources suchas plants, bacteria, or even animals [9]. Recently, bacterial
2 Journal of Sensors
nanocellulose (BNC) has gained attention due to someexclusive properties which are not offered by plant cellulose.Since plant cellulose is part of a natural composite whichconsists of lignin, pectin, and hemicelluloses, the separationand puri�cation of cellulose is di�cult. BNC, on the contrary,belongs to the speci�c products of primary metabolism andcould easily be puri�ed [9]. Moreover, BNC consists of highlycrystalline nano�bril which leads to higher mechanicalstrength compared to plant cellulose [10].
In this paper, we present a �exible electrically conductivenanocomposite based on BNC cellulose and CNT. Double-walled carbon nanotubes (DWCNTs) and multiwalled car-bon nanotubes (MWCNTs) have been used to modify BNCpellicles. Different dispersed CNT solutions with differentvolumes and concentrations have been used to modify thecellulose in order to �nd optimum conditions for makingappropriate BNC �lms. e strain-oriented electromechan-ical properties of DWCNT and MWCNT treated cellulosehave been measured and characterized.
2. Experimental
2.1. Sample Preparation. e BNC used in this work wasproduced by Gluconacetobacter xylinum bacteria in a staticmedium. DWCNT (+90% purity, Nanocyl S A, Belgium) andMWCNT (+95% purity, Nanocyl S A, Belgium) modi�edwith carboxyl groups have been used as a conductive agentsfor modi�cation of BNC.e dispersion of CNT was carriedout using cetyltrimethylammonium bromide (CTAB) as asurfactant (Fluka, assay ≥ 96%). e dispersion process con-sisted of a combination of heating, stirring, and sonication,followed by centrifugation to remove undispersed CNT.Dispersions of DWCNT and MWCNT with concentrationof 1mg/mL and 2mg/mL were used. Also, 3 × 3 cm2 BNCpellicles were immersed in the CNT dispersions (15mL of1mg/mL, 15mL of 2mg/mL, and 30mL of 2mg/mL) for24–72 h under mild shaking. Aer �nishing the treatmentstep, all samples were washed carefully with deionized waterin order to remove free surfactant and CNT residue. epellicles were dried in a fume hood between polytetra�uo-roethylene plates. Figure 1 shows the steps involved to prepareBNC samples.
e total thicknesses of the dried BNC �lms were25–65 μm asmeasured by a standardmicrometer with ±1 μmaccuracy. A scanning electron microscope (SEM) has beenused to study the surface morphology of the samples (LeoUltra 55 FEG SEM). e electrical conductivity measure-ments were performed by using a four-point probe system(CMT-SR2000N, AIT, Korea).
2.2. Experimental Setup for Electromechanical Characteri-zation. e strain-induced electromechanical response oftreated cellulose samples has been monitored by using anInstron Material Testing Instrument (Series 5500). Constanttensile force can be applied by this instrument. Samples usedin the Instron instrument were 3 cm in length and 1 cm inwidth. Electrical contact was provided by an aluminum foilthat was pressed to the sample by the Instron clamp. Aninsulating layer of plastic was put between the clamp and
the aluminum foil. e resistance of the sample has beenmeasured along the same direction as the applied force. eexperimental setup for tensile test is shown in Figure 2.e samples were placed in the Instron machine, and theload was increased to the desired level and held there forat least two hours at room temperature and humidity. Adigital multimeter (Agilent 34401A) was used to measure theresistance change with respect to the extension of the sample.
It has been observed before that the mechanical proper-ties of nanocellulose samples have been changed at differ-ent environmental conditions [7]. On the other hand, theelectrical and mechanical properties of CNTs are sensitive totemperature. To ignore the environmental effect on treatednanocellulose sample, all samples have been kept in the lab atleast 24 hours to get the samples adjusted with the humidityand temperature of the lab, and all tests were performed inthe same room.
2.3. Experimental Procedure. To calculate the fractionalincrement in resistance (Δ𝑅𝑅𝑅𝑅𝑅0), where Δ𝑅𝑅 is the differencebetween the momentary resistance (𝑅𝑅) and the initial resis-tance (𝑅𝑅0), a constant force has been applied for at least3 hrs. To calculate the fractional increment in resistance, theresistance change during the �rst 20 minutes was not con-sidered, as this period was considered as the initial stabilityperiod for the sample aer applying the load. e calculatedinitial resistance was the value of the resistance which wasmeasured aer 20 minutes once the load was applied. Tocalculate the fractional increment in length (Δ𝐿𝐿𝑅𝐿𝐿0), whereΔ𝐿𝐿 is the difference between the length of the sample aerstrain (𝐿𝐿) and the initial length of the sample (𝐿𝐿0), the initiallength of the sample was considered to be 3mm, less thanthe original length as some part of the sample was inside theclamp which was not affected by the load.
3. Results and Discussion
3.1. Morphology of Conductive BNC. Nanocellulose samplesmodi�ed with CNT are characterized by the same �exibilityas native cellulose. e cross-section of the BNC pellicleshows no deep penetration of CNT (Figure 3(a)). Toosmall pores in the native BNC matrix (Figure 3(b)) preventCNT from penetration into the cellulose. As a result, anasymmetric conductive layer was formed on the surface ofthe BNC pellicles.
According to investigations using SEM, MWCNTsare more homogeneously distributed on the BNC surface(Figures 3(d) and 4(a)) compared to DWCNT (Figures 3(c)and 4(b)), which is consistent with results of a visual check ofsamples (the surface of BNC �lms modi�ed with MWCNTis uniformly black, whereas the surface of BNC pelliclesmodi�ed with DWCNT contains colourless parts, Figure 1).is observation points to better dispersion of MWCNTin water which is probably caused by higher ratio of CTABweight to CNT speci�c surface area for MWCNT (speci�csurface area 115m2/g [11]) compared to DWCNT (speci�csurface area > 500m2/g).
To observe the effect of strain on the BNC samples, atensile force of 4N has been applied on bothDWCNT treated
Journal of Sensors 3
DWCNT
DWCNT
or MWCNT
CTAB,
H O
BNC
Immersion
during 24–72 hours
Drying
Heating, stirring,
sonication,
centrifugation
Heating, stirring
MWCNT
2
F 1: Steps involved in sample preparation.
Force
Insulator
Sample
Electrical
connection
Aluminium foil insulatedfrom microtester
Electricalconnection
Force
(a) (b)
F 2: Experimental setup for tensile testing.
BNC sample and MWCNT treated BNC samples for threehours. According to SEMmorphology of the conductive BNCsamples, there were no change aer the application of strain(Figure 4).
3.2. Electrical Conductivity Measurement. e electrical con-ductivity of the nanocellulose modi�ed with DWCNT andMWCNT is affected by changing the volume and the concen-tration of the dispersions and by increasing the immersiontime. In the case of the sample which is modi�ed with thelowest concentration and volume, an increase of the immer-sion time did not give any signi�cant effect on the electricalconductivity of the modi�ed BNC pellicles (Figure 5(a)).erefore, one could conclude that saturation capacity ofcellulose for DWCNT in 1mg/mL dispersions is not enoughto form the conductive layer using small volume of dispersion(15mL). Indeed increasing the CNT concentration and vol-ume, the conductivity rises signi�cantly with the immersiontime (Figure 5(a)), indicating the substantial increase of thesaturation capacity.
e conductivity of the BNC pellicles has been increasedby one order of magnitude when the modifying agent waschanged from DWCNT to MWCNT (Figure 5(b)). eseresults could be explained by the formation of more homo-geneous layers on the surface of BNC by MWCNT thanDWCNT. e highest conductivities have been obtained forthe pellicles modi�ed in the 30mL of 2mg/mL solutions:0.39 S⋅cm−1 for DWCNT and 1.6 S⋅cm−1 for MWCNT, whichis signi�cantly higher than previously reported �12].
3.3. Electromechanical Response. Strain sensors can operateon the principle that as the sensing material is strained, theresistance of the material changes in a well-de�ned way.To observe the strain sensitivity of treated BNC, differenttypes of samples treated with different concentrations ofDWCNTorMWCNTwere evaluated under a �xed stretchingforce (4N). We measured the resistance value at 10 minuteintervals. It has been observed that the sample continuouslyextends under a �xed tensile force (Figure 6).
4 Journal of Sensors
100 �m
(a)
300 nm
(b)
100 nm
(c)
100 nm
(d)
F 3: SEM images of BNC. (a) Cross�sectional optical microscope image of BNC pellicle modi�ed �ith DWCNT. (b) SEM micrographof native untreated BNC. (c) SEMmicrograph of DWCNT treated nanocellulose. (d) SEMmicrograph of MWCNT treated nanocellulose.
500 nm
Before strain
(a)
500 nm
After strain
(b)
500 nm
Before strain
(c)
500 nm
After strain
(d)
F 4: SEM images of BNC samples modi�ed �ith MWCNT (a) and DWCNT (b) before and a�er application of strain.
Journal of Sensors 5
0
0.1
0.2
0.3C
on
du
ctiv
ity
(S/c
m)
24 48 72
Immersion time (hr)
15 mL of 1 mg/mL DWCNT dispersion
30 mL of 1 mg/mL DWCNT dispersion
30 mL of 2 mg/mL DWCNT dispersion
(a)
Co
nd
uct
ivit
y (S
/cm
)
24 48 72
Immersion time (hr)
30 mL of 2 mg/mL DWCNT dispersion
30 mL of 2 mg/mL MWCNT dispersion
0
0.5
1
1.5
2
(b)
F 5: �lectrical conductivities of BNC samples modi�ed with DWCNT (a) and MWCNT (b) as a function of immersion time.
0
0.2
0.4
0.6
0.8
Str
ain
(%
)
0 50 100 150 200
Time (min)
F 6: Strain versus time plot for DWCNT treated BNC sample.Sample shows continuous e�tension under a ��ed tensile force.
We tested samplesmodi�ed with different concentrationsof modifying agent and yielding different conductivitiesbetween 0.05 S/cm and 0.395 S/cm. As discussed in Section3.2, the conductivity of the treated samples depends on thetreatment parameters such as concentration of themodifying
agent and the dispersion time; the BNC samples used in thiscase are prepared under different conditions.
e gauge factor is the parameter which is used to de�nethe sensitivity of a sensor. It measures the ratio of relativechange in electrical resistance to the mechanical strain 𝜀𝜀,which is the relative change in length, of the sensor [13].
In this case, the sensitivity factor (gauge factor) 𝑆𝑆 can bede�ned as
𝑆𝑆 𝑆Δ𝑅𝑅𝑅𝑅𝑅0Δ𝐿𝐿𝑅𝐿𝐿0
× 100%. (1)
It has been observed that as the conductivity of treatedBNC sample treated with DWCNT increases, the strainsensitivity of the sample also increases. Figure 7 shows thata treated BNC sample of conductivity 0.05 S/cm has a lowergauge factor than one with a conductivity of 0.395 S/cm.Some reports claim that the sensitivity of a strain gaugeincreases with the conductivity of the sample unless theconductivity of the sample reaches a certain level [14].e correlation between conductivity and gauge factor maydepend on the percolation threshold level of the CNTnetwork. It can be presumed that aer reaching percolationthreshold, sensitivity will not change with conductivity. Inthis work, percolation threshold level was not reached sincewe could observe continuous change of sensitivity in thestudied range of conductivity.
When MWCNT treated BNC samples have been tested,the same results have been obtained. Comparing Figures 7and 8, it has been observed that the MWCNT treatedcellulose shows the same type of response as that of DWCNTtreated BNC. e highest gauge factor has been obtained
6 Journal of Sensors
0
0.05
0.1
0.15
0.2
0 0.001 0.002 0.003 0.004
𝑆 = 66.45𝑥
𝑆 = 32.89𝑥
𝑆 = 20.16𝑥
𝑆 = 6.61𝑥
𝑆 = 2.01𝑥
Δ𝑅/𝑅0
Δ𝐿/𝐿0Conductivity 0.395 S/cm (15 mL of 1 mg/mL, 72 hr treatment)
Conductivity 0.145 S/cm (30 mL of 2 mg/mL, 72 hr treatment)
Conductivity 0.125 S/cm (30 mL of 1 mg/mL, 24 hr treatment)
Conductivity 0.07 S/cm (30 mL of 2 mg/mL, 72 hr treatment)
Conductivity 0.05 S/cm (30 mL of 2 mg/mL, 48 hr treatment)
F 7: Fractional increase in resistance versus fractional increasein length plot for BNC samples subject to different DWCNTtreatments resulting in different conductivities.
by MWCNT treated BNC sample since it has the highestconductivity.
Figure 9 shows the sensitivity of the modi�ed BNC �lmsas a function of conductivity. e plot contains sensitivityvalues for bothMWCNT impregnated samples and DWCNTimpregnated samples. From Figure 9, it is clear that thesensitivity of the sample depends on the conductivity of thesample. Since samples impregnated with MWCNT displaythe highest conductivity, they exhibit highest sensitivity.
3.4. Repeatability Test. If the input signal and other measure-ment conditions remain the same and the sensor provides thesame response for every measurement, then this ability of thesensor is called repeatability.is property is crucial to ensurethe availability of the sensor for a long period of time and thereliability of the obtained measurement.
e repeatability of any sample depends on its elasticmodulus and plastic modulus. If the applied stretching forceis within the limit of elastic strength of the sample, then it canbe expected that when the force will be removed, the samplewill return to its previous shape and give the same responsecontinuously. It has been found that when CNT treated BNCsamples have been repeatedly subjected to a 4N force, theresponse behaviour of the samples has not been changedduring the �rst �0 minutes� however, some changes wereobserved starting from 30 minutes (up to 16%) (Figure 10).is makes the results repeatable only to limited extend.istype of materials could be used as disposable (single use)
0
0.05
0.1
0.15
0.2
0 0.0002 0.0004 0.0006 0.0008
𝑆 = 252.45𝑥
𝑆 = 147.38𝑥
𝑆 = 41.65𝑥
Δ𝑅/𝑅0
Δ𝐿/𝐿0Conductivity 1.6 S/cm (30 mL of 2 mg/ml, 72 hr treatment)
Conductivity 0.603 S/cm (30 mL of 2 mg/ml, 72 hr treatment)
Conductivity 0.2 S/cm (30 mL of 2 mg/ml, 24 hr treatment)
F 8: Fractional increase in resistance versus fractional increasein length plot for BNC samples subject to different MWCNT treat-ments resulting in different conductivities.
1
10
100
1000
0 0.5 1 1.5 2
Sen
siti
vity
Conductivity (S/cm)
F 9: Conductivity versus sensitivity plot of DWCNT (�lledmark) and MWCNT (open mark) treated BNC.
sensors or as multiuse sensors where very high sensitivity isnot required.
4. Conclusion
�lectrically conductive bacterial nanocellulose (BNC) �lmswere prepared by treatment of BNC with dispersions of
F 10: Repeatability test ofDWCNT(�lledmark) andMWCNT(open mark) treated BNC sample.
double-walled carbon nanotubes (DWCNTs) and multi-walled carbon nanotubes (MWCNTs) in the presence ofcetyltrimethylammonium bromide (CTAB). It has beenobserved that the dispersion process for the modifying agentaffects the electrical conductivity of the treated nanocellulose.e electrical conductivity increased when MWCNTs wereused as modifying agent. e highest conductivity obtainedby the treatment of BNC with DWCNT was 0.39 S⋅cm−1, andby the treatment of MWCNT, highest conductivity obtainedwas 1.6 S⋅cm−1.
e strain-induced electromechanical response of treatedBNC �lms was investigated. MWCNT treated celluloseshowed higher sensitivity than DWCNT treated cellulose. Agauge factor of 252 was obtained from the most conductivesample treated with MWCNT. Comparing the strain sen-sitivity of samples with different conductivity, it has beenobserved that high strain sensitivity correlates with highconductivity.
Acknowledgments
e Knut and Alice Wallenberg Foundation is gratefullyacknowledged for �nancial support through the WallenbergWood Science Center. Professor Paul Gatenholm (Depart-ment of Biological and Chemical Engineering WallenbergWood Science Center, and Chalmers University of Technol-ogy) is gratefully acknowledged for providing the bacterialcellulose samples.
References
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