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View Article OnlineView Journal | View Issue

aDepartment of Chemical Engineering, Ben-G

Israel, 84105bThe Ilse Katz Institute for Meso and Nanos

University of the Negev, Beer-Sheva, Israel

+972 86472916

† Electronic supplementary informa10.1039/c2an36399b

Cite this: Analyst, 2013, 138, 1490

Received 27th September 2012Accepted 4th January 2013

DOI: 10.1039/c2an36399b

www.rsc.org/analyst

1490 | Analyst, 2013, 138, 1490–149

A simple solution for the determination of pristinecarbon nanotube concentration†

Michael Shtein,ab Ilan Pri-bara and Oren Regev*ab

Upon dispersant-assisted exfoliation, pristine carbon nanotubes (CNTs) are divided between the

supernatant and precipitate, which makes the determination of dispersant concentration a challenging

task. We have developed a thermogravimetric-spectroscopy-based approach to accurately determine the

dispersant-assisted CNT (or nanoparticles, in general) concentration in dispersion. A thermogravimetric

analysis of the filtered and washed precipitate, that is usually discarded after centrifugation, is used

here to accurately calculate the CNT mass in the precipitate and (through mass-balance) its mass in the

supernatant. Once the true CNT concentration has been determined, a conventional spectroscopy-based

concentration calibration plot is constructed for simple and swift use in further concentration

measurements. Such true concentration analysis is crucial for studying the concentration–property

relationship.

Introduction

Studies of nanoparticles (NPs), such as carbon nanotubes(CNTs),1 graphene sheets (GSs)2 or inorganic tungsten disuldenanotubes (INTs-WS2),3 have made an immense contribution toprogress in nanoscience and nanotechnology. Industrial appli-cations of pristine CNTs have been hampered due to aggrega-tion and formation of CNT bundles, which prevent theirefficient use.4–10 Individual CNTs are usually required to makefull use of their unique properties and to facilitate their inter-nalization in composite materials.6 The properties of compositematerials depend on the concentration of individual CNTs,rather than CNT bundles.

There are three major approaches to the preparation ofdispersions maximizing the quantity of individual CNTs:11 (i)covalently attaching hydrophilic groups to the CNT surface(chemical functionalization);12 (ii) dispersing pristine CNTs insolvents;4 and (iii) using non-covalently adsorbing dispersingmolecules (dispersants), which enhance CNT exfoliation.Representative examples of such dispersants are ionic surfac-tants;13 nonionic surfactants;6 polysaccharides14 and proteins.15

This work focuses on the determination of the CNT concen-tration in a dispersant-assisted dispersion (approach (iii)).

The CNTs in procedure (ii and iii) are sonicated by a high-energy tip (TS) or by a low-energy bath (BS) followed by

urion University of the Negev, Beer-Sheva,

cale Science and Technology, Ben-Gurion

, 84105. E-mail: oregev@bgu.ac.il; Fax:

tion (ESI) available. See DOI:

6

centrifugation to accelerate precipitation. The obtained super-natant contains exfoliated CNTs that are further being used asllers in composite materials and soluble dispersants, while theprecipitate consists of non-dispersed CNTs, dispersant residues,metal oxides and other impurities.16

The determination of the true post-centrifugation CNTconcentration in the supernatant of (iii) is an essential step inevaluating structure–property relationships (e.g., in CNTconcentration-dependent composite materials). However, onlypart of the initial CNT concentration is exfoliated and actuallycontributes to the polymer properties' enhancement. This pointis overlooked in some studies where the CNT concentration isevaluated only qualitatively.6,17–19 In such studies, the CNTconcentration-dependent properties are attributed to the initialCNT concentration and not to the effective (exfoliated) CNTconcentration. We argue that the determination of the true CNTconcentration of a centrifuged and decanted CNT dispersion(prior to integration into the polymer) would undoubtedly helpin evaluating its net inuence on the properties of the CNT–polymer composite into which it is integrated. Since the deter-mination of the exfoliated CNT concentration is rathercomplicated many choose to skip the centrifugation–decanta-tion steps, assuming that exfoliated and aggregated CNTscontribute equally to the enhancement of the compositematerial properties.

The CNT concentration in the supernatant is usually deter-mined via UV-visible (UV-vis) spectroscopy,20 which is an accu-rate and well-established technique for the concentrationdetermination of a single component in a dispersion. Func-tionalized CNTs (i) may indeed get fully dispersed in solution(with no precipitate phase) and their concentration may bedirectly and accurately determined by UV-vis adsorption

This journal is ª The Royal Society of Chemistry 2013

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measurement.21 In other cases (ii and iii), the UV-vis determi-nation of the CNT concentration in the presence of a solvent ordispersant is more difficult, either due to the dispersant over-lapping absorbance22 or due to complex mixture of individualand bundled CNTs23–25 in both the supernatant and theprecipitate phases. Therefore, in this study we complementUV-vis measurements with the thermogravimetric technique tosolve the above difficulties and establish an accurate standardmethod for the concentration determination of CNTs. In anattempt to overcome the above-mentioned difficulties in CNTconcentration measurement, some groups do not centrifuge(bundle-removing) the sonicated samples for calibration curvepreparation,4,26–30 assuming that either all the dispersed CNTsare individual or the contribution of the bundled CNTs to theUV-vis absorption is negligible. Nevertheless, the sonicateddispersion is a complex mixture of both bundled and individualCNTs, while its centrifugation does decrease the absorbancespectra of CNTs.23 The latter insinuates that calibration samplesof non-centrifuged dispersions may severely impair the accu-racy of CNT concentration analysis.

In some other cases, the calibration curve preparations inaqueous solution were based on the CNT extinction coefficientsin non-aqueous solvents.31 Here, one should demonstrate thatthese sonicated CNT aqueous mixtures are identical to thoseobtained in the non-aqueous dispersions. The use of thermog-ravimetric analysis (TGA) for the determination of the CNTconcentration of the dried CNT supernatant phase has beenreported.32 However, the measurement error was rather large,due to the relatively low CNT and high dispersantconcentrations.

In this paper, we present a facile and accurate analyticalmethod for the determination of CNT concentration. Thenovelty of our proposed method is that it is based on the TGA ofthe precipitate to obtain the CNT concentration in the superna-tant and it is complemented by UV-vis spectroscopy measure-ments. The measurement error is consequently minimized,since the washed precipitate has a higher CNT weight than thesupernatant phase. The obtained accurate CNT concentrationin solution is of extreme importance in generating concentra-tion–property dependence. Our straightforward method is notlimited solely to the determination of CNT concentration, but isalso applicable to most NPs (e.g., graphene), regardless of theNP dispersion state.23,24

ExperimentalMaterials

Pristine multi-wall carbon nanotubes (MWCNTs) werepurchased from Nanocyl (NC7000) (Nanocyl MWCNTs) andCheaptubes (batch number 802.254.6969) (CheaptubesMWCNTs). Functionalized MWCNTs were purchased fromNanolab (PD15L-5-COOH) (Nanolab MWCNTs batch number111909). Tungsten disulde inorganic nanotubes werepurchased from Nanomaterials (INTs-WS2). Single-wall carbonnanotubes (SWCNTs) (CAS 308068-56-6), graphite akes (CAS7782-42-5) (GFs), triton X-100 (CAS 9002-93-1) (TX100), tetrae-thylorthosilicate (CAS 78-10-4) (TEOS), bovine serum albumin

This journal is ª The Royal Society of Chemistry 2013

(CAS 9048-46-8) (BSA), b-lactoglobulin (CAS 9045-23-2) (BLAC),dodecylbenzene sulphonic acid sodium salt (CAS 25155-309-0)(SDBS), cetyl trimethylammonium bromide (CAS 57-09-0)(CTAB), pluronic F-127 (CAS 9003-11-6) (F127) and sodiumdodecyl sulphate (CAS 151-2-31) (SDS) were all purchased fromSigma-Aldrich and used as received. Sodium diisopropylnaph-thalene sulphonate (CAS 1322-93-6) (Aerosol OS), sodium dio-ctylsulphosuccinate (CAS 577-11-7) (Aerosol OT) and a mixtureof sulphonates in water (sodium monododecyl phenoxy-benzene-disulphonate 31.5–33.5% (CAS 28519-02-0) andsodium didodecyl phenoxybenzene-disulphonate 11.5–13.5%(CAS 25167-32-2) (Aerosol DPOS-45)) were received as a gi fromCytec (see Table S1 in the ESI†). Deionized (DI) water with aresistivity of 18.2 MU cm and acetone of analytical grade wereused. Cellulose acetate/cellulose nitrate ester lter membraneswith a 0.22 m pore size (CA lters) were purchased from MF-Millipore. Nylon lter membranes with a 0.2 m pore size (nylonlters) were purchased from Whatman.

Sonication methods

Bath sonication (BS) was performed in an Elma sonic bath(model S10; 30 W, 37 kHz, Singen). The water level in the bathwas kept constant and the vial (20 mL) was placed in the centerof the bath. Tip sonication (TS) was performed in a VCX 400instrument (400W, 20 kHz, mtip, Sonics &Materials Inc.) at 38%intensity. The bath temperature was kept at 0 �C by the additionof ice. Tip–bath–tip (TBT) sonication, a consecutive three-stepprocedure, was performed. TS was followed by BS and then byanother TS process.

NP dispersion

CNTs and graphite akes (GFs) were mixed with a pre-preparedsolution of dispersant in DI water, as indicated in Table 1. INTs-WS2 were mixed with a solution of dispersant and acetone.These dispersions were then sonicated and centrifuged (Mega-fuge 1.0, Heraues) for 20 minutes at 4000g to accelerateprecipitation. We found that longer centrifugation does notchange the CNT concentration. Following sonication andcentrifugation, a phase separation of exfoliated (supernatant)and aggregated (precipitate) NPs is performed by decantation.

Filtration, drying and weight determination

Both supernatant and precipitate phases were ltered (nylon orCA lter) and the solid residue was washed with DI water(50 mL) and acetone (50 mL). The wet powders on the lterswere dried at 120 �C for 1 hour or lyophilized for 48 hours. Theexact weights of the dried precipitate and dried supernatantwere determined by weighing the respective loaded lters (andsubtracting the lter weights).

UV-vis spectroscopy

A double-beam UV-vis spectrophotometer (JascoV-630) in the350–700 nm range was used. The supernatant phase adsorptionfor water was measured in plastic cuvettes (10 mm width) andfor acetone in quartz cuvettes (2 mm width). These dispersions

Analyst, 2013, 138, 1490–1496 | 1491

Table 1 Initial concentration of NPs in sonicated mixtures

NP type Dispersant typeDispersant concentration[mg mL�1]

NP concentration[mg mL�1]

Sonication procedureand (duration) [min]

Nanolab MWCNTs F127 2.9 2.7 BS (1800)SWCNTs F127 3.2 3.1 BS (1800)Nanocyl MWCNTs F127 1.5 2.0 TS (15)Cheaptubes MWCNTs F127 7.0 12 TS (10)SWCNTs BSA 2.9 1.5 BS (1200)Cheaptubes MWCNTs SDBS 12 10 TS (10)Cheaptubes MWCNTs TX100 7.0 12 TS (10)SWCNTs TX100 7.0 5.0 TS (10)GFs TX100 15 15 TBT (30:60:30)INTs-WS2 TEOS 15 15 BS (30)Nanocyl MWCNTs BLAC 1.5 3.0 BS (120)Nanocyl MWCNTs SDS 12 3.1 TS (10)Cheaptubes MWCNTs SDS 12 7.0 TS (10)Cheaptubes MWCNTs Aerosol OT 12 7.0 TS (10)Cheaptubes MWCNTs Aerosol OS 7.0 7.0 TS (10)Cheaptubes MWCNTs DPOS-45 7.0 14 TS (10)Cheaptubes MWCNTs CTAB 5.0 7.0 TS (10)

Fig. 1 Cryo-TEM micrographs of the aqueous dispersion of (a) MWCNT–F127(2.0 mg CNT and 1.5 mg F127 per mL water) and (b) SWCNT–TX100 (5 mgSWCNTs and 7 mg TX100 per mL water) aqueous dispersions.

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were diluted to keep their measured absorbance values in the0.1–2 range throughout the entire wave-length range.20

TGA

The dried powders and pure dispersants were analyzed using aMettler Toledo Star System (Mettler TGA/STDA85) under N2 at aow rate of 50 mL min�1. The N2 atmosphere minimizes CNToxidation (a common result if an O2 atmosphere is used) andthus improves the accuracy of TGA. The sample (in a 70 mLalumina crucibles) was rst heated at a rate of 10 �Cmin�1 from30 �C to 430 �C, aer which the temperature remained constantat 430 �C for 30 minutes. The NPs and dispersant weightpercentages were then calculated from the thermograms.

Electron microscopy

Transmission electron microscope (TEM) micrographs wereobtained by using a FEI Tecnai 12 G2 TWIN TEM, operated at 120kV. The cryogenic TEM (Cryo-TEM) samples were prepared usinga Vitrobot (Mark 1) at room temperature. A drop of the solutionwas deposited on holey carbon-coated copper grids (300 mesh,lacey carbon, Ted Pella), automatically blotted with lter paperand then plunged into liquid ethane at its freezing point. Thesevitried samples were stored in liquid nitrogen before beingtransferred to a TEM, using aGatanworkstation and a cryoholder(626) for imaging at 98 K. Themicroscope was operated at 120 kVin a low electron-dose mode (to reduce radiation damage) and ata few micrometers under focus in order to increase phasecontrast. The images were recorded by a Gatan 794 CCD cameraand analyzed by Digital Micrograph 3.6 soware.

Results and discussion

Here, we present direct imaging of the NPs by means of cryo-TEM. This is followed by TGA and ltration results. Finally, thedata obtained by the complementary UV-vis spectroscopy tech-nique are detailed and the combined approach for estimating

1492 | Analyst, 2013, 138, 1490–1496

the NP concentration is depicted. Calibration curves for variousNPs, as developed from this analysis, conclude this work.

Dispersion characterization

To verify CNT exfoliation we rst imaged the dispersant-assisteddispersion (iii) by cryo-TEM. The studied systems are denoted ina “CNT–dispersant” format. For example, aqueous dispersionsof MWCNT–F127 and SWCNT–TX100 aqueous solutions (seeTable 1 and Experimental section) are imaged in Fig. 1a and b,respectively. Indeed, mostly exfoliated CNTs are found. Thehydrophobic parts of F127 and TX100 are believed to beattached at the CNT–water interface, hence screening thehydrophobic carbon surface from water, whereas the hydro-philic part is extended out into the aqueous phase, providing asteric barrier and preventing CNT aggregation.33

Concentration determination

Aer verifying that the CNTs in solution are exfoliated we focuson the concentration problem; following sonication andcentrifugation, the dispersion is separated (decanted) to

This journal is ª The Royal Society of Chemistry 2013

Fig. 3 Thermograms of the Nanolab MWCNT–F127 system. (a) Pure F127dispersant. Theweight-loss at ca.385 �C corresponds to thedecompositionof F127.(b) Dried precipitate of the Nanolab MWCNT–F127 system. The first step is due todispersant decomposition, which is followed by an almost linear slope with rela-tively slow MWCNT decomposition. The blue curve indicates the sample weight-loss in the ordinate (dashed – stepwise) with respect to time or temperature(abscissae). The red curve is the time derivative of the weight-loss curve (mg s�1).

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exfoliated (supernatant) and aggregated (precipitate) CNT pha-ses. Bymeasuring only the CNT weight in the precipitate, we cancalculate its concentration in the supernatant through mass-balance. The supernatant is rst removed by decantation. Then,the wet precipitate powder is washed with solvents to removethe excess dispersant, hence increasing TGA accuracy. The driedprecipitate is weighed and the percentage of residual dispersantand CNT in the precipitate is determined by TGA. A blockdiagram summarizing the proposed analytical approach isshown in Fig. 2.

The advantages of the proposed analytical method are clearlyshown in the concentration determination results for a specicsystem: Nanolab MWCNTs dispersed by F127 (Fig. 3), and arevalid for all other systems in this work as well.

The precipitate contains only 19.6% of the dispersant(Fig. 3b), while the supernatant contains as much as 65%, asmeasured by TGA (Table 2, in bold letters). Note that thedifference between these values is more pronounced whencomparing the dispersant fraction of the initial dispersantweight: there is only 13% (3.9 mg/29 mg) dispersant in theprecipitate, while there is 74% (21.5 mg/29 mg) in the super-natant. The remaining 12% is washed out during the ltrationstep. These results substantiate our assumption that TGAmeasurement in the precipitate is, indeed, much more accuratedue to the lower dispersant and higher CNT mass in theprecipitate (Table 2).

The TGA of the dispersant alone (Fig. 3a) indicates itscomplete decomposition (98.9% weight-loss, dispersantdecomposition fraction 4d ¼ 0.99).

The mass of the dispersant in the precipitate is lower by anorder of magnitude compared to the supernatant, since most ofthe soluble dispersant remains in the supernatant (Table 2) andthe excess dispersant in the precipitate is removed during thewashing step. The validity of this technique is established bymass-balance calculations comparing the initial CNT weight

Fig. 2 Concentration determination of CNTs in the precipitate (right branch) andin the supernatant (left branch). The supernatant CNT concentration determina-tion is performed for validation (left branch). The construction of a calibrationcurve by UV-vis spectroscopy is indicated in the left branch (dashed lines). Inset:the supernatant and precipitate phases after centrifugation.

This journal is ª The Royal Society of Chemistry 2013

with the sum of the CNT weight in the precipitate and thesupernatant (right and le branch of Fig. 2, respectively).

The CNT concentration in the supernatant is determined byeqn (1) or (2):

CNTs ¼�Ms �

�1� 4s

4d

��1

Vs

(1)

CNTs ¼�CNTinitial �Mp �

�1� 4p

4d

��1

Vs

(2)

where: CNTs – the CNT concentration in the supernatant [mgmL�1]. Ms – the dried supernatant mass [mg]. 4s – the mass

Table 2 CNT concentration determination calculations for the NanolabMWCNT–F127

Initial Nanolab MWCNT mass (CNTinitial) 27 mgInitial F127 mass (Dinitial) 29 mgDried supernatant mass (Ms) 33 mgDried precipitate mass (Mp) 20 mgDispersant decomposition fraction (4d) 0.99Mass fraction of F127 in supernatant (4s)(TGA of supernatant)

0.65 (F127: 21.5 mg,CNT: 11.5 mg)

Mass fraction of F127 in precipitate (4p)(TGA of precipitate)

0.2 (F127: 3.9 mg,CNT: 16.1 mg)

CNT concentration in supernatant (CNTs) 1.2 mg mL�1

Analyst, 2013, 138, 1490–1496 | 1493

Table 3 Calculated concentrations and dispersion efficiency for various NPs–dispersant mixtures

NP type Dispersant typeNP supernatant concentration(mg mL�1) Dispersion efficiencyc (%)

Nanolab MWCNTs F127 1.2 � 0.01a 43 � 1.0SWCNTs F127 3.0 � 0.1a 97 � 3.0Nanocyl MWCNTs F127 1.9 � 0.05a 95 � 3.0Cheaptubes MWCNTs F127 9.0 � 0.1a 75 � 1.0SWCNTs BSA 0.7b 42Cheaptubes MWCNTs SDBS 6.5 � 0.2a 64 � 2.0Cheaptubes MWCNTs TX100 7.8 � 0.3a 65 � 3.0SWCNTs TX100 2.0b 40GSs TX100 4.0b 27INTs-WS2 TEOS 6.0 � 0.1a 40 � 1.0Nanocyl MWCNTs BLAC 0.3 � 0.01a 9.0 � 1.0Nanocyl MWCNTs SDS 1.9b 65Cheaptubes MWCNTs SDS 6.0b 85Cheaptubes MWCNTs Aerosol OT 5.7b 82Cheaptubes MWCNTs Aerosol OS 5.7b 81Cheaptubes MWCNTs DPOS-45 9.8b 69Cheaptubes MWCNTs CTAB 5.4b 77

a NP supernatant concentration determined with validation mass-balance calculations comparing NP weights in the precipitate and thesupernatant. b NP supernatant concentration determined by precipitate weight only. In these cases the validation process for the supernatant istechnically complicated, due to NP adhesion to the lter or insufficient mass for TGA. c NP weight in the supernatant � 100/initial NP weight.

Fig. 4 UV-vis absorption spectra with and without NPs in DI water/acetone. (a)MWCNT–F127 in DI water UV-vis absorption spectra, showing a featurelessspectrum and negligible adsorption of TX100, F127 and BLAC dispersants atmaximum concentration; (b) INTs-WS2–TEOS in acetone UV-vis absorptionspectra with peaks at l ¼ 652 nm, 543 nm and 485 nm and negligible TEOSdispersant adsorption. Samples with different NP concentrations were preparedby diluting the supernatant with DI water (a) and acetone (b) to keep themeasured values of absorbance within the range of 0.1–1 during the entirewave-length range.

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fraction of the dispersant in the supernatant. 4d – the disper-sant decomposition fraction. Vs – the supernatant volume [mL].CNTinitial – the initial CNT mass [mg].Mp – the dried precipitatemass [mg]. 4p – the mass fraction of the dispersant in theprecipitate.

An example of concentration determination for the NanolabMWCNT–F127 system is detailed in Table 2.

The accuracy of the mass loss of various dispersants in TGAis tested by measuring a thermogram of a pure dispersant(without CNTs). The dispersants being studied undergo thermaldegradation in nitrogen within the temperature range of 300–430 �C. Some dispersants undergo complete degradation intovolatile products 4d � 1 (e.g., TX100, F127, see Fig. 3a); in otherdispersants some non-volatile decomposition products remainin the TGA crucible (4d < 1), e.g., SDS and SDBS. If the puredispersant (control-Fig. 3a) is not completely removed duringthe TGA (due to non-volatile residues, e.g. inorganic salt moie-ties in the decomposition of sodium sulfonate derivatives), thecalculation of the weight of the residual dispersant is correctedaccordingly in eqn (1) and (2).

A blank TGA of CNTs demonstrates a negligible change inmass up to 430 �C (e.g., the MWCNT mass loss at 430 �C is lessthan 0.1% min�1), aer which the isotherm is linear with time(Fig. S2 in the ESI†) and may be a result of oxidative decar-bonylation caused by oxygen impurity in the TGA chamber.

Our concentration measurement method could be used toevaluate the concentration of various NPs (e.g. graphene sheets(Fig. S1†), INTs-WS2). The NP concentrations in the supernatantand the dispersion efficiency of the systems listed in Table 1 areshown in Table 3. The NP weight in the precipitate and thesupernatant adds up to the initial weight of the NPs, within�3%, consequently validating the proposed analytical methodfor concentration determination.

1494 | Analyst, 2013, 138, 1490–1496 This journal is ª The Royal Society of Chemistry 2013

Fig. 5 Calibration curves of NPs: UV-vis absorption vs. NP concentration. (a)MWCNT–F127 system calibration curves at l ¼ 660 nm, 500 nm and 400 nm; (b)INTs-WS2–TEOS system calibration curves at l ¼ 652 nm, 543 nm and 485 nm.

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Calibration curves

The above TGA-based concentration measurement is expensiveand time consuming. Since UV-vis spectroscopy has a goodsignature of NP concentration (following the Beer–Lambertlaw), linking it with TGA-based concentration measurementsmakes the concentration determination fast, cheap and simple.In the 350–700 nm range, carbon-based NPs (SWCNTs,MWCNTs, GSs) show a featureless UV-vis spectrum,21 whileINTs-WS2 (inorganic NT) shows a spectrum with three distinc-tive peaks (Fig. 4b). A reference solution, containing only thedispersant at high concentration (5 wt%), has a negligibleadsorption (Fig. 4a); therefore, the measured absorbance isattributed to the NPs alone and gives a relative information onNP concentration.

The combination of the UV-vis absorption measurementswith NP concentrations (determined by TGA) yields a calibra-tion curve based on the Beer–Lambert law (absorbance versusconcentration) at any random wave-length (here we choose todemonstrate the absorbance at 3 values (l ¼ 660 nm, 500 nmand 400 nm for carbon-based NPs and l¼ 652 nm for INTs-WS2(Fig. 5)). These calibration curves allow for the concentrationdetermination of an unknown NP dispersion by a simple andfast UV-vis spectroscopy measurement. This method is appli-cable to aqueous or organic (acetone, ethanol, methanol)solvents prepared with dispersants that do not absorb at somepart of the UV-vis spectrum and decompose at lower tempera-tures than the NPs.

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Conclusion

The present work presents a general method for the accuratedetermination of NP concentration in solution, while over-coming the various limitations of other common techniques,namely, the superposition of the NP and the dispersantabsorption in UV-vis spectroscopy or the large measurementerror in the conventional TGA of the supernatant. The intro-duction of ltration–washing steps and precipitate analysisdrastically reduces the dispersant mass in both supernatantand precipitate, hence improving the accuracy of NP concen-tration measurements. The mass-balance calculations obtainedfor various NPs, dispersants and solvents validate the proposedtechnique and prove its generality (Table 3). Cryo-TEM obser-vations of various NP–dispersant solutions clearly indicate thatmostly individual NPs are dispersed in the supernatant phase. Acalibration plot constructed by means of UV-vis spectroscopy isan accurate, convenient and rapid characterization tool formonitoring NP concentrations in NP–dispersant solutions.

Acknowledgements

The authors thank Dr Ronit Bitton for critical reading of themanuscript. The nancial support of the MAGNET Program,nanotubes consortium no. 41680, by the Chief Scientist of theIsrael Ministry of Industry, Trade and Labour is appreciated. Wewould like to thank Cytec Industries Inc for providing freesamples of anionic surfactants.

Notes and references

1 S. Iijima, Nature, 1991, 354, 56–58.2 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang,S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004,306, 666–669.

3 R. Tenne, L. Margulis, M. Genut and G. Hodes, Nature, 1992,360, 444–446.

4 Z. Sun, V. Nicolosi, D. Rickard, S. D. Bergin, D. Aherne andJ. N. Coleman, J. Phys. Chem. C, 2008, 112, 10692–10699.

5 G. S. Duesberg, M. Burghard, J. Muster, G. Philipp andS. Roth, Chem. Commun., 1998, 435–436.

6 Y. Geng, M. Y. Liu, J. Li, X. M. Shi and J. K. Kim, Composites,Part A, 2008, 39, 1876–1883.

7 R. Murphy, J. N. Coleman, M. Cadek, B. McCarthy, M. Bent,A. Drury, R. C. Barklie and W. J. Blau, J. Phys. Chem. B, 2002,106, 3087–3091.

8 V. Weiss, R. Thiruvengadathan and O. Regev, Langmuir,2006, 22, 854–856.

9 M. Zheng, A. Jagota, E. D. Semke, B. A. Diner, R. S. McLean,S. R. Lustig, R. E. Richardson and N. G. Tassi, Nat. Mater.,2003, 2, 338–342.

10 R. Tenne, Angew. Chem., Int. Ed., 2003, 42, 5124–5132.11 Y. Dror, W. Pyckhout-Hintzen and Y. Cohen,

Macromolecules, 2005, 38, 7828–7836.12 V. Georgakilas, K. Kordatos, M. Prato, D. M. Guldi,

M. Holzinger and A. Hirsch, J. Am. Chem. Soc., 2002, 124,760–761.

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13 B. Vigolo, A. Penicaud, C. Coulon, C. Sauder, R. Pailler,C. Journet, P. Bernier and P. Poulin, Science, 2000, 290,1331–1334.

14 R. Bandyopadhyaya, E. Nativ-Roth, O. Regev andR. Yerushalmi-Rozen, Nano Lett., 2001, 2, 25–28.

15 D. Nepal and K. E. Geckeler, Small, 2006, 2, 406–412.16 V. C. Moore, M. S. Strano, E. H. Haroz, R. H. Hauge,

R. E. Smalley, J. Schmidt and Y. Talmon, Nano Lett., 2003,3, 1379–1382.

17 J. Rausch, R. C. Zhuang and E. Mader, J. Appl. Polym. Sci.,2010, 117, 2583–2590.

18 K. T. Lau, M. Lu, C. K. Lam, H. Y. Cheung, F. L. Sheng andH. L. Li, Compos. Sci. Technol., 2005, 65, 719–725.

19 H. E. Unalan, G. Fanchini, A. Kanwal, A. Du Pasquier andM. Chhowalla, Nano Lett., 2006, 6, 677–682.

20 J. L. Bahr, E. T. Mickelson, M. J. Bronikowski, R. E. Smalleyand J. M. Tour, Chem. Commun., 2001, 193–194.

21 Z. F. Li, G. H. Luo, W. P. Zhou, F. Wei, R. Xiang and Y. P. Liu,Nanotechnology, 2006, 17, 3692–3698.

22 E. Edri and O. Regev, Anal. Chem., 2008, 80, 4049–4054.23 K. K. Kim, S. M. Kim, Y. Cui, M. S. Jeong, J. H. Han,

Y. C. Choi, K. H. An, K. Oh and Y. H. Lee, Carbon Letters,2009, 10, 14–18.

24 M. J. Green, Polym. Int., 2010, 59, 1319–1322.

1496 | Analyst, 2013, 138, 1490–1496

25 A. J. Blanch, C. E. Lenehan and J. S. Quinton, Nanoscienceand Nanotechnology (ICONN), 2010 International Conferenceon, 2010.

26 Y. Bai, I. S. Park, S. J. Lee, T. S. Bae, F. Watari, M. Uo andM. H. Lee, Carbon, 2011, 49, 3663–3671.

27 J. C. Goak, S. H. Lee, J. H. Han, S. H. Jang, K. B. Kim,Y. Seo, Y.-S. Seo and N. Lee, Carbon, 2011, 49, 4301–4313.

28 J. P. Piret, S. Detriche, R. Vigneron, S. Vankoningsloo,S. Rolin, J. Mejia Mendoza, B. Masereel, S. Lucas,J. Delhalle, F. Luizi, C. Saout and O. Toussaint, J. Nanopart.Res., 2010, 12, 75–82.

29 S. Vankoningsloo, J.-p. Piret, C. Saout, F. Noel, J. Mejia,A. Coquette, C. C. Zouboulis, J. Delhalle, S. Lucas andO. Toussaint, Nanotoxicology, 2011, 6, 1–17.

30 R. Rastogi, R. Kaushal, S. K. Tripathi, A. L. Sharma, I. Kaurand L. M. Bharadwaj, J. Colloid Interface Sci., 2008, 328,421–428.

31 R. Krishnamoorti, M. D. Clark and S. Subramanian, J. ColloidInterface Sci., 2011, 354, 144–151.

32 S. Attal, R. Thiruvengadathan and O. Regev, Anal. Chem.,2006, 78, 8098–8104.

33 I. Szleifer and R. Yerushalmi-Rozen, Polymer, 2005, 46, 7803–7818.

This journal is ª The Royal Society of Chemistry 2013