MWCNT/perylene bisimide water dispersions for miniaturized temperature sensors

Abstract

We report on a new ionic surfactant based on extended polycyclic aromatic perylene bisimides (PBI), namely N,N′-bis(2-(1-piperazino)ethyl)-3,4,9,10-perylenetetracarboxylic acid diimide dichloride (PZPERY), suitable for the exfoliation of multi-walled carbon nanotubes (MWCNTs). The ultrasonication (400 W for 5 min) of MWCNT/PZPERY water mixtures followed by centrifugation (4000 rpm for 30 min) provided disentangled and undamaged MWCNTs, which interact with the PZPERY molecules through π–π stacking interactions. The deposition of MWCNT/PZPERY dispersions on plastic film supporting gold electrodes allowed the fabrication of temperature sensors showing reproducible electrical resistances and typical semiconducting (activated) electrical transport with decreased resistivity when heated from 20 to 40 °C. The resistivity–temperature profile was very reproducible and with a negative temperature coefficient of about −0.02 K⁻¹, a value comparable to that found in common thermistors. Our results suggest that MWCNT/PZPERY based films have utility for the formation of sensitive, stable and reproducible sensors to measure body temperature.

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Introduction

Carbon nanotubes (CNTs) are a third allotropic form of carbon, which have become a dominant class of nanostructured carbon-based materials with unique mechanical, electrical and thermal properties that depend critically on their structural perfection and high aspect ratio (typically >10²).^1–4 Single-walled CNTs (SWCNTs) consist of single graphene sheets (monolayer of sp² bonded carbon atoms) wrapped into cylindrical tubes with a diameter ranging from 0.7 to 2 nm and lengths up to micrometres. Multi-walled CNTs (MWCNTs) consist of concentric assemblies of SWCNTs and are therefore characterized by larger average diameters. The exceptional properties shown by SWCNTs and MWCNTs have led to an expanding number of applications including the production of strong and lightweight materials, the fabrication of electronic nano-devices and electrochemical sensors.^2,3,5 However, the strong van der Waals interactions between individual nanotubes hinder uniform dispersion at the nanoscale level and make their large-scale utilization problematic. Ultrasonication,⁶ as well as covalent^7–10 and non-covalent functionalization methods,^11–17 have proven to be highly effective for the exfoliation and dispersion of CNTs in several media and in polymer matrices. More specifically, non-covalent functionalization by surfactants has attracted considerable attention as it is highly scalable, reversible and preserves the structural integrity of the graphitic network, which is a fundamental requisite to preserve the electronic properties of CNTs.

Recently, extended polycyclic aromatic perylene bisimides (PBI) have enabled the dispersion of graphitic materials, such as CNTs, by means of the non-covalent functionalization through strong π–π stacking interactions.^16,18–22 PBI was found to constitute an exceptionally well-suited anchor unit to the sp² carbon network of CNTs and allowed their effective dispersion in solution.^20,23,24 In this process, the energy dissipated with sonication disassembles CNT bundles that are immediately surrounded by PBI molecules. The PBIs bind via strong π–π interactions between CNTs and prevent their re-bundling. PBIs are widely used as pigments and dyes, with colours ranging from red to black depending on the particle size and crystal packing of the molecules.^25–28 In addition to optical features such as strong absorption in the visible region, high quantum yields and excellent photostability, PBIs also display a low reduction potential,²⁹ which enables their use as electron-acceptors and n-type semiconductors in photoinduced charge–transfer reactions. For these appealing properties, perylene derivatives have been extensively studied for the fabrication of field-effect transistors,³⁰ luminescent solar concentrators,³¹ dye lasers,³² photovoltaic cells³³ and organic light-emitting diodes (OLEDs).³⁴ These properties and the strong associations with CNTs make integrated CNT/perylene materials systems attractive candidates for the generation of nanostructured electronic devices and sensors.

The ability of PZPERY molecules to generate π–π stacking interactions among their aromatic backbones has been previously utilized by our group for the preparation of chromogenic plastic materials as temperature indicators.³⁵ The effective π-associations between PZPERY and MWCNTs suggested to us that these nanocomposites should function as temperature sensors, since both semiconducting and metallic CNTs are characterized by temperature sensitive resistivities.³⁶ Previous investigations have demonstrated the possibility of fabricating flexible, miniaturized MWCNT/polymer temperature sensors in the form of thin films to be deposited on different support materials. This kind of device was reported to find successful application wherever low cost, large surface and low volume of the sensor are fundamental issues. For example, wireless, flexible and disposable temperature sensors could be used in the monitoring of body temperature in the range 20–40 °C, and in particular the bed temperature of chronic wounds.^6,37

However, these systems displayed resistance sensitivity to temperature of only −0.004 K⁻¹, a value that is comparable to that found in metals.^6,37,38 Such low sensitivity was ascribed to the polymeric dispersants, such as poly(styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS)⁶ or poly(vinylbenzyl chloride) derivative with triethylamine,³⁷ and the insulating nature of these components limit the thermal and electronic conductivities of the nanocomposite. In the present paper, we explore the possibility that the highly conjugated core of the PZPERY surfactant would be able to produce higher performance (resolution) temperature sensors.

In this work, we report on the preparation and characterization of MWCNTs water dispersions by using the N,N′-bis(2-(1-piperazino)ethyl)-3,4,9,10-perylenetetracarboxylic acid diimide dichloride (PZPERY) as ionic surfactant (Fig. 1).

Fig. 1 Chemical structure of the N,N′-bis(2-(1-piperazino)ethyl)-3,4,9,10-perylenetetracarboxylic acid diimide dichloride (PZPERY).

The MWCNT/PZPERY dispersions were obtained by ultra-sonicating their mixtures in water for five minutes followed by centrifugation, and the quantity of dispersed CNTs was quantified by gravimetric analyses. Raman spectroscopy and transmission electron microscopy (TEM) were employed to assess the structural integrity of MWCNTs. The electrical resistance of films, obtained by drop casting aliquots of MWCNT/PZPERY dispersions, was investigated over the range 20–40 °C to explore their potential for the development of sensitive, stable, and reproducible temperature sensors.

Experimental

Materials

Perylene-3,4,9,10-tetracarboxydianhydride (PTCDA), imidazole, and 1-(2-aminoethyl)piperazine were supplied by Aldrich and used without further purification. Multi-walled carbon nanotubes (MWCNTs, Baytubes C150 P), a generous gift from Bayer Material Science, were used as received. These nanotubes are vapour grown and typically consist of 3–15 graphitic layers wrapped around a hollow 4 nm core. Typical diameters range from 13 to 16 nm, and the lengths are between 1 and 10 μm.

Synthesis of N,N′-bis(2-(1-piperazinyl)ethyl)-3,4,9,10-perylenetetracarboxylic acid diimide dichloride (PZPERY)

PTCDA (1 g, 2.55 mmol) was dissolved in 20 mL of 1-(2-aminoethyl)piperazine at 160 °C and stirred for 22 h under nitrogen. After cooling to room temperature, the solution was treated with 100 mL of 2 N HCl. The dark violet solution was stirred for 12 h. The hydrochloride salt reaction products precipitated in acetone, were separated by filtration, and then washed thoroughly with distilled water until the pH of the washings were neutral. The dried product 1.3 g (1.92 mmol, yield: 75.6%) was a deep-red solid.

FT-IR (KBr): 2926 (νCH aliph.), 1695, 1656, 1593 (νCO imide), 1508, 1441, 1401, 1362 (νCC ring) cm⁻¹.

¹H NMR δ: 8.8–8.9 (d, 4H arom., J = 15 Hz), 4.7–4.9 (m, 4H, N_imm–CH₂), 4.4–4.5 (m, 4H, N_pip–CH₂), 4.0 (m, 16H, CH₂) ppm.

¹³C NMR δ: 31.2, 37.8, 45.5, 52.8, 117.5, 120.4, 122.5, 125.4, 129.3, 132.5, 162.5 ppm.

Elemental analysis: calcd (C₃₆H₃₈Cl₂N₆O₄): 62.9% C, 5.3% H, 12.2% N; found: 63.5% C, 5.7% H, 12.4% N.

Preparation of the MWCNT/PZPERY dispersion

Stock solutions (1 mg mL⁻¹) of PZPERY were prepared in ultra-pure water (purified by a Millipore S.A. 670120 water purification system) and kept in the dark at 4 °C until use within two days. The concentration (either mmol mL⁻¹ or mg mL⁻¹) of PZPERY is indicated as C_PERY. Aliquots of this solution (10 mL) were poured in vials containing 1 mg of MWCNTs. Uniform dispersions were then obtained by ultra-sonicating (UP 400 S, Hielscher) the MWCNTs/PZPERY mixtures for 5 minutes at full power (about 400 W) and a frequency of 24 kHz. Cooling with a stirred water/ice bath limited the increase in temperature and the loss of water during sonication to negligible levels (less than 0.4% by weight). The ultrasonic device was fixed so that its sonotrode could be reproducibly dipped into the dispersions, which were prepared in Falcon™ 50 mL conical centrifuge tubes. This procedure and the accurate weighing allowed limiting the variability of the process, evaluated as the fraction of dispersed nanotubes, to within 2%. The resulting dispersions were centrifuged at 4000 rpm for 30 min and then filtered to remove the residual MWCNT agglomerates. The final MWCNT/PZPERY weight ratio in the solution was measured by thermogravimetric analysis (TGA) of the dried materials. Three different MWCNT/PZPERY dispersions, namely, I, II and III, were prepared according to the same procedure. Two 10 μL aliquots of each MWCNT/PZPERY dispersion were cast onto each gold electrodes supported on a plastic film (Fig. S1†) and then left to evaporate at room temperature for 2 h to fabricate the sensing film. Once dried, their electrical resistance was then measured at different temperatures.

Apparatuses

¹H- and ¹³C-NMR spectra were recorded by a Varian Gemini 300 MHz instrument using 5–10% CF₃COOD (Aldrich, 99.5 atom% D) solutions. NMR spectra were recorded at 20 °C, and the chemical shifts were assigned in ppm using the solvent signal as a reference. FT-IR spectra were recorded with the help of a Perkin Elmer Spectrum One spectrometer in KBr dispersions. The UV-vis spectra were recorded on a Shimadzu UV-2450 double beam spectrophotometer having temperature control to within ±0.1 °C. Two different sets of experiment were performed for inspection of broad dye concentration ranges: for the diluted dye a cell with 1.0 cm path was used (until Abs < 1.0), whereas for concentrated solutions a 1.0 mm thick cell was used. In order to process and display data, the first data set was normalised to a 1.0 mm path to be merged together with the second set. Fluorescence spectra were recorded using a Perkin Elmer LS55 spectrofluorometer with temperature control (±0.1 °C, quartz cell of 1 cm path length). Increasing amounts of either the PZPERY solution or the MWCNT/PZPERY mixture were made using a Hamilton syringe connected to a micrometric screw enabling to dispense volumes down to 0.166 μL. All measurements were performed at 25.0 °C.

Thermogravimetric scans were carried out by a Mettler Toledo Starc System (TGA/SDTA851e). Samples were heated from 100 to 850 °C at 10 °C min⁻¹ under a nitrogen flow. Samples were annealed at 100 °C for 10 min before measurement to remove water. The morphologies of the MWCNT/PZPERY films were examined with a Philips CM12 transmission electron microscope (TEM). Raman spectra of solid dispersion obtained by drop casting the supernatant of each sample (after centrifugation) onto glass substrates were measured with a Horiba LabRAM HR Raman Spectrometer at an excitation wavelength of 532 nm. Sensors were fabricated by casting MWCNT/PZPERY dispersions in water onto supports (Cad Line, Pisa, Italy) consisting of a polyimide film (Kapton®, thickness 50 μm) and suitable electrodes. Kapton® was chosen due to its flexibility, chemical inertness, and low permeability to water and vapours, which helps the protection of the sensing film. Copper tracks were prepared by photolithography and then electroplated with nickel and gold for electrodes fabrication (dimensions: length 7 mm, width 1 mm, distance 2 mm; thickness of copper 35 μm, nickel 3.0 μm, gold 1.2 μm). Sensor calibration in the range 20–40 °C was performed by placing the sensors in a temperature controlled hot stage (±0.1 °C) and measuring the electrical resistance with a digital multimeter (KEITHLEY Mod. 2700).

Results and discussion

A MWCNT/PZPERY dispersion was prepared in a 1 : 10 MWCNT : PZPERY weight ratio: 1.0 mg of CNT was added to 10.0 mL of aqueous solution of PZPERY 1.0 mg mL⁻¹ (C_PERY = 1.46 mmol mL⁻¹). This mixture was sonicated (400 W for 5 min) and then centrifuged (4000 rpm for 30 min). The fluorescence of its precipitate, which had been removed and re-suspended, was negligible, meaning that all of the PZPERY remained in solution.

A thermogravimetric analysis permitted the quantification of the amount of MWCNTs included in the MWCNT/PZPERY dispersion (Fig. 2).

Fig. 2 TGA analyses of PZPERY (red curve) and MWCNT/PZPERY dispersion (black curve).

The thermal degradation of PZPERY was observed to occur at temperature of about 20 °C higher in the presence of MWCNTs (i.e. 338 instead of 315 °C), suggesting extra stability via the existence of effective interactions between the dye and the graphitic material. A weight percentage of 16.5% was determined for MWCNTs from the difference in weight between the residuals of the same amount of MWCNT/PZPERY and PZPERY at 850 °C, i.e. after the complete degradation of the organic matrix. This allowed us to conclude that the dispersion contained 1.0 mg mL⁻¹ and MWCNT 0.0165 mg mL⁻¹ (PZPERY : MWCNT ≈ 61) (Fig. S2†).

Spectroscopy

UV-vis and fluorescence spectroscopies were used to study the stabilizing effect of PZPERY in aqueous suspensions of MWCNTs. Spectra were recorded for different concentrations of the MWCNT/PZPERY dispersion. In these mixtures the C_PERY/C_MWCNT ratio remains constant, so that the appearance of a dye aggregation is indicative of condensation of the dyes placed on the surface of MWCNTs, a process that can lead to MWCNTs aggregation/precipitation via PZPERY bridges.

As concerns PZPERY alone, Fig. S3 of the ESI† shows that the UV-vis spectra recorded with increasing concentrations of the perylene dye did not display new bands or shoulders appearance. This result suggests that PZPERY dye persists as a stable monomeric form over the explored range. Fig. 3a shows a plot of the recorded absorbance vs. C_PERY (squares): the linear trend further confirms that no aggregation of the PZPERY (in the absence of MWCNTs) occurs under the explored conditions. To further investigate possible auto-aggregation effects, the Franck–Condon progression can also be used (Abs_0–0/Abs_0–1), where Abs_0–0 is the absorption at λ = 500 nm and Abs_0–1 is the absorption at λ = 550 nm. A decrease in the Abs_0–0/Abs_0–1 value is considered indicative of an aggregation process.¹¹ Fig. 3b (squares) shows a constant (within the experimental error) Abs_0–0/Abs_0–1 ratio over our concentration range and confirms the absence of auto-aggregation of PZPERY under the explored conditions.

Fig. 3 (a) Plot of absorbance vs. concentration for PZPERY alone in water (squares, λ = 548 nm) and of the MWCNT/PZPERY dispersion (circles, λ = 538 nm, C_CNT ≈ C_PERY/61); (b) evolution of the Franck–Condon correlation (Abs_0–0/Abs_0–1) at different concentrations of the dye (squares) and the dye/MWCNT dispersion (circles).

Moving to MWCNT/PZPERY dispersions, Fig. 3a and b (circles) show the relevant absorbance values corrected from scattering effects using a procedure based on double logarithmic plots.^39,40 Linearity up to a concentration of 0.4 mg mL⁻¹ and a constant Abs_0–0/Abs_0–1 ratio are observed, which both indicate that the MWCNT dispersion is efficiently stabilised by PZPERY. The fluorescence emission data suggested similar conclusions. As shown in the ESI (Fig. S4†) the emission characteristics of PZPERY were not significantly affected by the presence of MWCNTs. This confirmed that the interaction of the excess PZPERY with MWCNTs does not result in an aggregation of the dye on the MWCNT surface but had a stabilizing effect on the dispersion. In conditions of a large excess of PZPERY, the interaction with MWCNTs is not reflected in any steady state absorption and emission spectra.

Notably, Raman spectroscopy is a versatile technique used for the structural characterization of CNTs, as well as for studying their interaction with the exfoliating agents. Fig. 4 shows the Raman spectrum of the MWCNT/PZPERY dispersion compared to a previously investigated MWCNT composite with a thermoplastic elastomer, i.e. the poly(styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS), which had been reported to be very effective in CNT exfoliation and dispersion.^6,38 The most prominent features in a Raman spectrum of MWCNTs are the G and D bands (Fig. 4). The G-band at 1560 cm⁻¹ is an intrinsic feature of CNTs related to the planar vibration of carbon atoms in most sp² graphitic materials. Conversely, the disorder-induced D-band at 1320 cm⁻¹ is attributed to the scattering from defects breaking the basic symmetry of the graphene sheet. MWCNTs typically show the highest ratio (I_D/I_G) between the intensities of these peaks among all carbon nanotubes, since a large number of structural defects are present within the multiple graphite layers.

Fig. 4 Raman spectra of MWCNT/PZPERY (solid line) and MWCNT/poly(styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS) (dashed line)⁶ films (λ_exc. = 532 nm). The wavenumbers (cm⁻¹) of the peaks are reported as insets.

In the spectrum of MWCNT/PZPERY films, the two lines of PZPERY at 1294 and 1372 cm⁻¹ are well resolved and superimposed on an underlying fluorescent background.⁴¹ The D band of MWCNTs is obscured by the large PZPERY scattering, and its low intensity may be also the result of a shielding effect from the dye coating and the CNTs defects.⁴² It is worth noticing that the G band in the MWCNT/PZPERY dispersion is split in two unresolved peaks, which are blue-shifted of about 15–20 cm⁻¹ compared to the SEBS based nanocomposite. This feature may be a consequence of the effective π-interactions between MWCNTs and the PZPERY molecules, which favour the exfoliation of CNTs by keeping them apart (i.e., isolated double bonds resonate at higher frequencies than in the G band of interacting CNTs).⁴³

Morphology

In order to evaluate if our procedure allowed the effective exfoliation of MWCNTs, the MWCNT/PZPERY mixtures were analysed by transmission electron microscopy (TEM). The TEM images (Fig. 5) clearly show the single (unbundled) nanotubes as the dominant species, thus confirming the good interactions of PZPERY with MWCNTs in water. Moreover, the average length of MWCNTs is comparable to their nominal length (1–10 μm), thus suggesting that the CNTs are not severely damaged by the exfoliation process.

Fig. 5 TEM images of the MWCNT/PZPERY dispersion at low (a) and high (b) magnification.

Resistance sensitivity to temperature

Aliquots (10 μL) of three replicate MWCNT/PZPERY dispersions (namely, I, II and III) were drop cast onto the previously described electrodes on plastic support to fabricate six temperature sensors (two duplicates for each dispersion). Their electrical resistivities were measured after the complete evaporation of water and ranged between 24 and 54 kΩ (Table 1). These variations may be ascribed to some minor limitations in the reproducibility of the experimental setup. However, reproducibility for replicate experiments was good. CNTs with either semi-conducting or metallic character show a temperature dependent resistivity, which makes them suitable for the fabrication of small-size temperature sensors. Table 1 reports the electrical resistances of the sensors at different temperatures. A typical semiconducting behaviour with decreasing resistance values with increasing temperatures from 23.7 to 40.0 °C was obtained. Fig. 6 plots the resistance versus temperature variation for the MWCNT/PZPERY IIIa sensor, showing the classic Arrhenius behaviour R = R₀e^−B/T, with B as an experimental parameter and T the absolute temperature.⁴⁴

Table 1 Electrical resistances of the MWCNT/PZPERY based sensors at different temperatures

T (°C)	R (kΩ)
	Ia	Ib	IIa	IIb	IIIa	IIIb
23.7	46	27	24	54	46	43
27.0	37	25	22	46	42	38
29.0	32	24	21	40	38	34
30.2	30	23	20	38	37	34
31.5	29	22	19	35	35	33
34.0	27	20	18	33	33	31
35.6	26	19	18	32	32	30
40.0	24	18	16	29	30	28

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Fig. 6 Electrical resistance of the MWCNT/PZPERY IIIa sensor as a function of temperature, and exponential fit to the data (y = y₀ + Ae^−B/T; R² = 0.98).

Resistivity variations of more than 35% were recorded over the 23.7 to 40.0 °C temperature interval for all the sensors. Sensors from the dispersion III displayed an almost equivalent response to temperature (Table 1) and were further investigated in successive calibration cycles.

The resistance responses of the MWCNT/PZPERY III sensors were determined over four successive heating cycles (Table 2). Reproducible resistance variations were observed, with maximum amplitudes of 30 kΩ within the temperature interval of 20 °C. These features support the use of the MWCNT/PZPERY system as a resistive sensor for temperature variations within the physiological regime.

Table 2 Resistance variations for the MWCNT/PZPERY III sensors as a function of the temperature

1^st cycle			2^nd cycle			3^rd cycle			4^th cycle
T (°C)	R (kΩ)		T (°C)	R (kΩ)		T (°C)	R (kΩ)		T (°C)	R (kΩ)
	IIIa	IIIb		IIIa	IIIb		IIIa	IIIb		IIIa	IIIb
23.7	46	43	21.5	70	70	21.8	77	68	21.4	81	72
27.0	42	38	28.0	56	56	32.0	63	54	25.0	74	62
29.0	38	34	31.0	51	51	37.0	52	45	27.2	69	58
30.2	37	34	33.5	47	47	38.5	50	44	29.9	65	56
31.5	35	33	34.0	46	46	40.0	48	43	31.3	61	54
34.0	33	31	35.0	45	44				33.4	58	50
35.6	32	30	36.5	43	43				37.0	54	48
40.0	30	28	38.5	42	42				38.5	53	46
			39.5	41	41				41.5	51	44
			40.4	40	40

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It is worth noting that, plotting the natural logarithm of the resistance variation (R/R₀, with R₀ the resistance at the lowest temperature) against the inverse of the temperature in K, a linear Arrhenius type behaviour is evident in all the heating cycles (Fig. 7). Moreover, the linear correlation (R² ≈ 0.99) appears almost identical for all the experiments, thus confirming that the MWCNT/PZPERY system is a highly reproducible temperature sensor. The temperature coefficient of the MWCNT/PZPERY sensor was calculated according to eqn (1) and the values reported in Table 3:

R(T) = R(T₀)[1 + α(T − T₀)] (1)

where α is the temperature coefficient expressed in K⁻¹.

Fig. 7 Arrhenius plot of the natural logarithm of the resistance variation as a function of the reciprocal of the absolute temperature according to the Arrhenius equation for successive heating cycles of MWCNT/PZPERY III dispersions, and linear fit (R² = 0.99) of IIIb 2^nd cycle.

Table 3 Temperature coefficient (α) of MWCNT/PZPERY III dispersions after successive heating cycles

α (K^−1)
1^st cycle		2^nd cycle		3^rd cycle		4^th cycle
IIIa	IIIb	IIIa	IIIb	IIIa	IIIb	IIIa	IIIb
−0.021	−0.021	−0.023	−0.023	−0.023	−0.021	−0.019	−0.020

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The MWCNT/PZPERY sensor displayed sensitivity corresponding to a negative temperature coefficient of −0.02 K⁻¹, an absolute value that is comparable to the highest values found in common thermistors (0.044 K⁻¹) and an order of magnitude higher than those of metals (0.0037–0.006 K⁻¹; 0.00385 K⁻¹ for a Pt(100) sensor). This higher sensitivity compared to the previously reported MWCNT/polymer systems^6,37,38 likely originates from the extended polycyclic aromatic perylene cores, which enable a more efficient electrical and thermal conduction within the sensor device. Notably, α maintains a constant value for every measurement cycle, thus confirming the relevant properties of the prepared temperature sensor.

Conclusions

We have demonstrated that PZPERY, a water soluble PBI dye, is an effective surfactant for MWCNTs exfoliation. Spectroscopy studies (i.e., UV-vis, fluorescence and Raman) together with microscopic investigations (TEM) revealed that PZPERY molecules are able to generate non-covalent functionalization of the MWCNT graphitic materials through π–π stacking interactions, and that the exfoliation process does not do significant damage to the one-dimensional CNT structure. MWCNT/PZPERY based sensors showed reproducible resistance values, thereby confirming the reliability and validity of the exfoliation procedure. Notably, the resistance of the MWCNT/PZPERY films varied considerably with the increase of temperature (about 30 kΩ over the temperature interval of 20 °C) showing the semiconducting behavior between 20 and 40 °C. Measurements repeated over four successive heating cycles revealed highly reproducible resistance variations with negative temperature coefficient of about −0.02 K⁻¹, an absolute value comparable to the values found in common thermistors. All these features support the use of the MWCNT/PZPERY system as a highly sensitive resistive sensor for temperature variations within the physiological regime.

Acknowledgements

This research was partially funded from the European Union's Seventh Framework Program under grant agreement no. 317894-SWAN-iCARE and the MIT-UNIPI 2013–2014 project “Nanostructured Materials for Sensing Applications”. The Raman measurements were made at the Institute for Soldier Nanotechnologies.

Notes and references

1. P. M. Ajayan, Chem. Rev., 1999, 99, 1787–1799.
2. R. H. Baughman, A. A. Zakhidov and W. A. de Heer, Science, 2002, 297, 787–792.
3. W. A. de Heer, MRS Bull., 2004, 29, 281–285.
4. T. W. Odom, J.-L. Huang, P. Kim and C. M. Lieber, Nature, 1998, 391, 62–64.
5. J. M. Schnorr and T. M. Swager, Chem. Mater., 2011, 23, 646–657.
6. N. Calisi, A. Giuliani, M. Alderighi, J. M. Schnorr, T. M. Swager, F. Di Francesco and A. Pucci, Eur. Polym. J., 2013, 49, 1471–1478.
7. J. L. Bahr and J. M. Tour, J. Mater. Chem., 2002, 12, 1952–1958.
8. C. A. Dyke and J. M. Tour, J. Phys. Chem. A, 2004, 108, 11151–11159.
9. D. Tasis, N. Tagmatarchis, A. Bianco and M. Prato, Chem. Rev., 2006, 106, 1105–1136.
10. Y.-P. Sun, K. Fu, Y. Lin and W. Huang, Acc. Chem. Res., 2002, 35, 1096–1104.
11. R. F. Araujo, C. J. R. Silva, M. C. Paiva, M. M. Franco and M. F. Proenca, RSC Adv., 2013, 3, 24535–24542.
12. L. Vaisman, H. D. Wagner and G. Marom, Adv. Colloid Interface Sci., 2006, 128–130, 37–46.
13. R. Shvartzman-Cohen, Y. Levi-Kalisman, E. Nativ-Roth and R. Yerushalmi-Rozen, Langmuir, 2004, 20, 6085–6088.
14. A. Star, Y. Liu, K. Grant, L. Ridvan, J. F. Stoddart, D. W. Steuerman, M. R. Diehl, A. Boukai and J. R. Heath, Macromolecules, 2003, 36, 553–560.
15. A. Di Crescenzo, M. Aschi and A. Fontana, Macromolecules, 2012, 45, 8043–8050.
16. A. Di Crescenzo, V. Ettorre and A. Fontana, Beilstein J. Nanotechnol., 2014, 5, 1675–1690.
17. A. Di Crescenzo, R. Germani, E. Del Canto, S. Giordani, G. Savelli and A. Fontana, Eur. J. Org. Chem., 2011, 2011, 5641–5648.
18. V. Georgakilas, K. Kordatos, M. Prato, D. M. Guldi, M. Holzinger and A. Hirsch, J. Am. Chem. Soc., 2002, 124, 760–761.
19. A. A. Mamedov, N. A. Kotov, M. Prato, D. M. Guldi, J. P. Wicksted and A. Hirsch, Nat. Mater., 2002, 1, 190–194.
20. C. Backes, F. Hauke and A. Hirsch, Adv. Mater., 2011, 23, 2588–2601.
21. J.-H. Lee, S.-M. Yoon, K. K. Kim, I.-S. Cha, Y. J. Park, J.-Y. Choi, Y. H. Lee and U. Paik, J. Phys. Chem. C, 2008, 112, 15267–15273.
22. C. Oelsner, C. Schmidt, F. Hauke, M. Prato, A. Hirsch and D. M. Guldi, J. Am. Chem. Soc., 2011, 133, 4580–4586.
23. C. Backes, U. Mundloch, C. D. Schmidt, J. N. Coleman, W. Wohlleben, F. Hauke and A. Hirsch, Chem.–Eur. J., 2010, 16, 13185–13192.
24. C. Backes, C. D. Schmidt, F. Hauke and A. Hirsch, Chem.–Asian J., 2011, 6, 438–444.
25. E. Hadicke and F. Graser, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1986, 42, 189–195.
26. K. Hino and J. Mizuguchi, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2005, 61, o672–o674.
27. J. Mizuguchi and K. Tojo, J. Phys. Chem. B, 2002, 106, 767–772.
28. S. Haremsa, in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2000.
29. F. Wurthner, Chem. Commun., 2004, 1564–1579.
30. G. Horowitz, F. Kouki, P. Spearman, D. Fichou, C. Nogues, X. Pan and F. Garnier, Adv. Mater., 1996, 8, 242–245.
31. M. G. Debije and P. P. C. Verbunt, Adv. Energy Mater., 2012, 2, 12–35.
32. M. Sadrai and G. R. Bird, Opt. Commun., 1984, 51, 62–64.
33. B. E. Hardin, H. J. Snaith and M. D. McGehee, Nat. Photonics, 2012, 6, 162–169.
34. G. Li, Y. Zhao, J. Li, J. Cao, J. Zhu, X. W. Sun and Q. Zhang, J. Org. Chem., 2015, 80, 196–203.
35. F. Donati, A. Pucci and G. Ruggeri, Phys. Chem. Chem. Phys., 2009, 11, 6276–6282.
36. S. Coiai, E. Passaglia, A. Pucci and G. Ruggeri, Materials, 2015, 8, 3377.
37. A. Giuliani, M. Placidi, F. Di Francesco and A. Pucci, React. Funct. Polym., 2014, 76, 57–62.
38. G. Matzeu, A. Pucci, S. Savi, M. Romanelli and F. Di Francesco, Sens. Actuators, A, 2012, 178, 94–99.
39. S. J. Leach and H. A. Scheraga, J. Am. Chem. Soc., 1960, 82, 4790–4792.
40. T. Biver, N. Eltugral, A. Pucci, G. Ruggeri, A. Schena, F. Secco and M. Venturini, Dalton Trans., 2011, 40, 4190–4199.
41. A. Débarre, R. Jaffiol, A. Richard and P. Tchénio, Chem. Phys. Lett., 2002, 366, 274–278.
42. P. Kaur, M.-S. Shin, A. Joshi, N. Kaur, N. Sharma, J.-S. Park and S. S. Sekhon, J. Phys. Chem. B, 2013, 117, 3161–3166.
43. K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prud'homme, I. A. Aksay and R. Car, Nano Lett., 2008, 8, 36–41.
44. P. Atkins and J. de Paula, Atkins' Physical Chemistry, W. H. Freeman and Company, New York, 8th edn, 2006.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11544b

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