Functionalized single walled carbon nanotube containing amino acid based hydrogel: a hybrid nanomaterial

Abstract

Fmoc-protected amino acid based (Fmoc-Phe-OH) hydrogel has been developed in our laboratory previously (S. Roy and A. Banerjee, Soft Matter, 2011, 7, 5300–5308). This hydrogel has been used to incorporate and disperse functionalized single-walled carbon nanotube (f-SWCNT) within the gel phase to make a hybrid hydrogel at physiological pH and temperature and this is a convenient procedure to create a f-SWCNT based hybrid nanomaterial. The hybrid hydrogel has been characterized using different microscopic studies including transmission electron microscopic imaging (TEM) and atomic force microscopic imaging (AFM) and rheological studies. The AFM study demonstrates a nice 1D alignment of f-SWCNTs on the surface of the gel nanofibers. Functionalized SWCNT containing hybrid hydrogels are more thermally stable than the native gel. Rheological experiments show that the hybrid hydrogel is a more elastic material than the native hydrogel obtained from the Fmoc-Phe-OH. Moreover, G′ (storage modulus) of the native hydrogel has been increased about 16 times upon the inclusion of f-SWCNT into the gel matrix. Conductivity of the f-SWCNT containing hydrogel has been found to be 3.12 S cm⁻¹ and conductivity of the f-SWCNT has dropped after incorporation of f-SWCNT into the hydrogel system.

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1. Introduction

Supramolecular gels have many fascinating applications in different fields.¹ Low molecular weight hydrogels are usually obtained by the self-assembly of small gelator molecules in aqueous medium through various non-covalent interactions including hydrogen bonding, van der Waals interactions, π–π stacking and others.²Peptide based hydrogelators are common in the literature.^1c,e,m,n However, there are relatively few examples of amino acid based low molecular weight hydrogelators in the literature.^3,4 Recently, our research group has been engaged in investigating amino acid based hydrogelators.^4e The hydrogel that can disperse carbon nanotubes (CNTs) and form a hybrid hydrogel with CNTs is a good candidate for making CNT based hybrid materials. This is because CNTs have revolutionized the field of nanoscience and nanotechnology due to their extraordinary mechanical properties, aspect ratio, enhanced electrical and thermal conductivity and other interesting properties.⁵ In particular, the organization of CNTs into films,⁶ fibers,⁷ gels⁸ or micropatterned materials⁹ may provide versatile opportunities for various applications as scanning probe tips,¹⁰ supercapacitors,¹¹ nanodevices,¹² nanoelectronics,¹³ photovoltaic devices,¹⁴ field emitters,¹⁵ electromechanical actuators,¹⁶ chemical sensors,¹⁷ flow sensors,¹⁸hydrogen-storage reservoirs,¹⁹catalyst support for fuel cell,²⁰ and nanocomposite materials.²¹ The combination of CNTs with a low molecular weight gel can be expected to exhibit different thermal, mechanical and electrical properties from CNTs within the gel matrix.²² CNT based nanocomposites have drawn considerable research interest over past few years due to their various applications including catalyst support for fuel cell, tissue engineering, electrode materials for lithium batteries, bioimaging, biosensor, drug and other delivery systems, bioscaffolds and others.²³ Aligned CNTs within the hydrogel matrix can be applied for field emission devices, thermal conducting pads, ion transportation, micro-electromechanical devices and also chemical vapor sensors.^7,24Carbon nanotube based hybrids have also been applied to humidity responsive behavior, light-emitting diodes and photovoltaic cells.²⁵ However, dispersity still remains a major problem for carbon nanotubes. There are several ways to disperse them in organic/aqueous medium.²⁶Single-walled carbon nanotube (SWCNT) dispersions have been achieved by noncovalent modification using surfactants and polymers with aromatic functionalities that assemble onto nanotube surfaces via π–π interactions.^21a,27

In carbon nanotube containing gels,²⁸carbon nanotubes (CNTs) are well dispersed within the gel. Gelators with aromatic moieties can interact with single-walled carbon nanotubes as they can provide π–π stacking interactions with these nanotube walls. This helps to disperse the nanotubes in the water/organic medium. It has been demonstrated that there is a successful way to disperse single carbon walled nanotubes (SWCNTs) within hyaluronic acid (HA) solution and induce the formation of reinforced hydrogel in the presence of the cross-linking reagent divinyl sulfone (DVS).^28d Recently, Bhattacharya and co-workers have demonstrated a luminescent organogel containing a conjugated p-phenylenevinylene based aromatic chromophore, which acts as an excellent host for both pristine SWCNTs (Pr-SWCNTs) and n-hexadecyl amide functionalized SWCNTs (C16-SWCNTs).^22b Aida and co-workers have discovered imidazolium-ion-based ionic liquids as a new class of dispersants for CNTs.^28a Ajayaghosh and co-workers have also demonstrated carbon nanotube triggered self-assembly of oligo(p-phenylene vinylene)s to make stable hybrid π-gels.^28f Functionalized β-cyclodextrins (β-CD) have been used to prepare SWCNT based hybrid hydrogels using host–guest interactions between β-CDs of pyrene incorporated β-CD and SWCNTs. This SWCNT based hydrogel has also shown gel to sol transition with the addition of a competitive host as well as guest molecules.^28g Li and co-workers have demonstrated the preparation of a SWCNT–CdS nanoparticle hybrid using an easy chemical route. This hybrid exhibits interesting optoelectronic properties including the regulation of photo-response mediated by amines.²⁹ It has been reported that a carbon nanotube–platinum nanoparticle hybrid nanocomposite system has shown excellent catalytic activity under heterogeneous and homogeneous conditions.³⁰ However, there is a crucial need for the preparation of functionalized single-walled carbon nanotube–low molecular weight hydrogel based hybrid nanomaterials at physiological pH and temperature and they may exhibit semiconductivity behavior at room temperature.

In this report we demonstrate the formation and dispersion of functionalized single-walled carbon nanotubes (f-SWCNTs) within the Fmoc-protected amino acid (Fmoc-Phe-OH) based hydrogel at pH 7.46 in phosphate buffer and at physiological temperature. The amino acid based hydrogel has a nanofibrillar network structure with micro-porous pockets that are filled up with a lot of solvent molecules and these pockets can accommodate nanomaterials like carbon nanotube (CNT). Atomic force microscopic images of the SWCNT-containing hydrogel reveals a nice 1D alignment of f-SWCNTs over the outer surface of the Fmoc-amino acid based hydrogel nanofiber. Rheological study reveals the more rigid and elastic properties of the hybrid hydrogel containing f-SWCNT compared to that of the native hydrogel. Moreover, the f-SWCNT reinforced hybrid material exhibits conductivity 3.12 S cm⁻¹ at room temperature (26 °C).

2. Experimental

2.1 Reagents and materials

9-Fluorenylmethyl chloroformate (Fmoc-Cl) and L-phenylalanine were purchased from local chemical SRL. 1,4-Dioxane, sodium carbonateetc. were purchased from Merck, Germany. SWCNTs were purchased from Cheap Tubes, USA having an outer diameter 1–2 nm, length 5–30 μm and inner diameter 0.8–1.6 nm.

2.2 Synthesis of functionalized SWCNT and characterizations

In 150 mL of 3 (M) HNO₃, 200 mg of SWCNTs were suspended by sonication. The mixture was then refluxed for about 2 days, sonicated for 1 h, and refluxed for another 2 days.^31a Then, 50 mL of 3 (M) HNO₃ was added, and after sonication for 2 h, the mixture was again refluxed for 12 h. The resultant suspension was then diluted by deionized water, filtered through a polycarbonate filter (Isopore, pore size 100 nm), and rinsed thoroughly with deionized water several times until neutral. The resulting SWCNTs were resuspended in deionized water and sonicated for 2 min. The suspension was then again filtered. The obtained black product was dried and characterized by FTIR (see Fig. S1, †ESI), Raman spectroscopy (Fig. 1), AFM (see Fig. S2, †ESI) and TGA^31a (see Fig. S3, †ESI). A solid state FT-IR spectrum of the f-SWCNT is shown in Fig. S1 (†ESI). The FT-IR spectrum shows the presence of transmittance peaks at 1400 cm⁻¹, 1745 cm⁻¹, 3421 cm⁻¹ corresponding to the different vibrational frequencies of the carboxylic acid group of the functionalized SWCNTs. Moreover, a very small peak at ∼1200 cm⁻¹ corresponding to the O–H bending,^31b two peaks at 2922 cm⁻¹ and 2852 cm⁻¹ corresponding to the asymmetric vibration of C–H groups are also observed. So, it can be concluded that the functionalized SWCNT contains mainly –COOH groups and a very small amount of –OH functionality.

Fig. 1 Raman spectroscopic analysis of functionalized SWCNT and functionalized SWCNT containing hybrid hydrogel based nanomaterial.

To measure the surface area of the f-SWCNTs, we investigated the N₂ gas desorption–adsorption isotherm (77 K) for f-SWCNT and it is shown in Fig. S4. For that purpose we first degassed the sample at 353 K for 1.6 h. Gas adsorption data reveal that the f-SWCNTs have a BET surface area of 166.21 m² g⁻¹. The adsorption isotherm is close to the type IV adsorption isotherm.^31c The pore volume of the f-SWCNT is 0.1478 cc g⁻¹. The pore size of the f-SWCNT has been calculated to be 3.78 nm.

3. Results and discussion

3.1 Formation of functionalized SWCNT–hydrogel hybrid materials

There are a few examples of CNT containing hybrid gels. Recently, Coleman and co-workers demonstrated that pyridine-functionalized SWCNTs were able to act as cross-linkers and could form hydrogen bonds to poly(acrylic acid) to form SWNT containing hydrogels.^28h Bhattacharya and co-workers have reported that pristine and long-chain functionalized SWCNTs have been incorporated successfully in supramolecular organogels formed by an all-transtri(p-phenylenevinylene) bis-aldoxime.^22b Chen and his co-workers have discovered that well-dispersed SWCNTs have been incorporated into a supramolecular hydrogel by forming an inclusion complex with R-cyclodextrin (R-CD).²⁸ⁱ Our previous study has demonstrated that the Fmoc-protected amino acid (Fmoc-Phe-OH) has formed hydrogels (Fig. 2) in 50 mM of phosphate buffer within a very narrow pH range of 7.00 to 7.66, with a minimum gelation concentration of 0.1% w/v without any sonication and this has been reported elsewhere.^4e Now, 0.3 mg of SWCNT–CO₂H has been incorporated into the Fmoc-Phe-OH based gel (0.5% w/v) with a tip sonication of about 5 min and slight heating in a sealed tube at physiological pH (7.46). Then this has been subjected to sonication of about 30 s. 0.5% w/v of the native hydrogel has been used to make the f-SWCNT containing hydrogel and it has been observed that the maximum capacity of the native hydrogel that can tolerate the incorporation of f-SWCNT is 0.1% w/v with respect to solvent to obtain a homogenous hybrid gel system (Fig. 2). If the concentration of f-SWCNT is increased further, a heterogeneous phase separated system has been obtained. These f-SWCNT–hydrogel hybrid nanomaterials have been characterized at pH 7.46 (physiological pH) using TEM, AFM, rheology and X-ray diffraction analyses.

Fig. 2 Photographs of the hydrogel (0.5% w/v) and hydrogel containing functionalized SWCNT (0.1% w/v) hybrid material.

3.2 UV-Vis-NIR study

Vis-NIR study has been carried out by taking the f-SWCNT containing hydrogel system in a UV cuvette. The Vis-NIR spectrum of the synthesized hybrid hydrogel shows absorption over a wide range of the absorption spectrum. The Vis-NIR profile is shown in Fig. S5 (see †ESI). The peak at 1200–1500 nm clearly demonstrates the presence of a characteristic feature of a CNT. This is due to the presence of the characteristic peak between 1200–1500 nm corresponding to the electronic transition between the van Hove singularities.³² The band gap of the hybrid nanomaterial has been calculated as about 1 eV.

3.3 Thermal study

Thermal study has been performed to examine the phase transition of the f-SWCNT containing hybrid hydrogel. Fig. S6 (see †ESI) shows that the gradual addition of % w/v of f-SWCNT incorporated into the native hydrogel vs. temperature keeping the gelator concentration of 0.5% w/v. This clearly indicates that upon incorporation of SWCNT–CO₂H into the native hydrogel, the T_gel increases gradually. The T_gel of the native hydrogel has been calculated to be 38 °C at minimum gelation concentration (MGC),^4e whereas at 0.5% w/v the T_gel is 69 °C. The incorporation of f-SWCNT into the native hydrogel triggers the increase of the T_gel of the hybrid hydrogel system. The T_gel obtained from 0.01% w/v of f-SWCNT containing hydrogel hybrid nanomaterials is determined to be 71 °C and it gradually increases up to 78 °C at the addition of 0.1% w/v f-SWCNT into the native hydrogel. This observation indirectly provides the proof that the incorporation of SWCNT–CO₂H into the hydrogel enhances the thermal stability of the f-SWCNT containing hybrid hydrogel. The thermal stability of the hybrid hydrogel is more than that of the native hydrogel. This may be due to the presence of more cross-linked structure in the hybrid hydrogel than that of the native hydrogel.³³

3.4 Raman spectroscopic studies

Raman spectroscopic analyses have been performed using freeze dried f-SWCNT containing gel as well as the synthesized SWCNT–CO₂H powder. The Raman spectra of f-SWCNT powder and freeze dried hydrogel–SWCNT hybrid nanomaterials are shown in Fig. 1. Raman spectroscopic analysis clearly shows that the hybrid nanomaterials retain the carbon nanotube's one dimensional properties. The peak in the low frequency region around 170–240 cm⁻¹ is attributed to the radial breathing mode (RBM), whose frequency depends mainly on the tube diameters.³⁴ The peak in the high frequency region with a maximum at 1590 cm⁻¹ represents the tangential mode (TM). There are some reports in the literature which demonstrated that upon incorporation of SWCNT the luminescent material can overlap with the specific Raman signal of the SWCNT in the hybrid system.³⁵ However, in the present study the specific Raman signals (TM and RBM) are clearly visible even after the inclusion of f-SWCNT into the luminescent hydrogel system. The peak at 2635 cm⁻¹ corresponds to the second order Raman scattering from D-band vibrations. However, the second order Raman scattering from D-band vibrations in the case of f-SWCNT (Fig. 1) is calculated to be 2650 cm⁻¹i.e. after incorporation of f-SWCNT into the gel the second order Raman scattering from D-band vibrations has been blue shifted by about 15 cm⁻¹ and this may be due to the interaction of the conducting f-SWCNT with the nonconducting amino acid based gelator molecules.

3.5 X-Ray powder diffraction studies

Fig. 3 presents the XRD pattern of xerogel of hydrogel–SWCNT hybrid nanomaterials. Peaks of 2θ around 10.3, 26.2, and 43.0 are linked to the characteristics of CNTs. An obvious diffraction peak at 2θ = 20° to 30° indicates the amorphous nature of the carbon.^28c The intensity of the native xerogel peak^4e at 2θ = 25.269 in XRD is very low. However, the intensity of that peak for the hybrid hydrogel is very high. It may be indirect proof of the better π–π interactions of the hydrogelator with the f-SWCNT wall. The X-ray diffraction pattern of the wet gel containing f-SWCNT hybrid nanomaterial is shown in Fig. S7(a) (see †ESI). The XRD data of the hybrid xerogel containing f-SWCNT is shown in Fig. S7(b) (see †ESI) and the X-ray diffraction pattern of the f-SWCNT shows a major peak at 2θ = 26.2° indicating its crystalline nature.

Fig. 3 X-Ray powder diffraction pattern of the dried hydrogel-SWCNT hybrid nanomaterial.

3.6 FT-IR studies

Evidence for the interaction of SWNTs with Fmoc-Phe-OH hydrogelator is obtained from FTIR analysis of the hydrogel–f-SWCNT hybrid nanomaterial. The frequency of the aromatic C–H bending mode between 700–900 cm⁻¹ in the FT-IR spectrum of Fmoc-Phe-OH has almost disappeared in the presence of f-SWNTs.³⁶ This finding clearly indicates a strong interaction between the π-conjugated backbone of the gelator Fmoc-Phe-OH with the SWNTs (Fig. 4). The B3LYP/6-31+G** energy minimized structure of Fmoc-Phe-OH is shown in Fig. S8 (see †ESI) along with the HOMO and LUMO structures. The energy minimized structure reveals that the fluorenyl and L-phenylalanine moieties are almost perpendicular to each other. As both fluorenyl of Fmoc group and phenyl ring of the amino acidL-phenylalanine are π moieties, they can interact with the SWCNT wall via π–π interactions. The evidence for the interaction between the gelator and the f-SWCNT has been studied and the peak of aromatic C–H bending has almost vanished in the region of 750 cm⁻¹.^28f,36c This clearly suggests that there is an interaction between the π-system of f-SWCNT with the π-system of the gelators in the hybrid gel.

Fig. 4 FT-IR spectra of the native xerogel and the hybrid xerogel.

3.7 ¹H NMR study

We have also performed ¹H NMR studies on the gelator as well as f-SWCNT containing gelators in the near gel state in D₂O by adjusting the pH using disodium hydrogen phosphate and sodium dihydrogen phosphate to about 7.46. The ¹H NMR data (see Fig. S9, †ESI) indicates that the aromatic proton peaks of the gelator molecules are broadened upon their interaction with the f-SWCNTs.^28f,36c

3.8 Morphology of different gels

The field emission scanning electron microscopic (FE-SEM) analysis of the freeze-dried hydrogel reveals the existence of a three-dimensional cross-linked network structure formed by the nanofibers, which has been reported elsewhere.^4eFE-SEM analysis reveals that the hydrogel fibers are of 20 to 35 nm in width and these nanofibers are several hundred of micrometres long. Atomic force microscopic (AFM) analysis^4e also exhibits that the hydrogel nanofibers are not straight fibers, moreover, they are helical in nature with a fibrillar width of 6 to 10 nm. The AFM study shows that the pitch length of the helical nanofibers (Fig. S10, see †ESI) is 88 nm. The picture of the helical nanofiber is uniform for a particular nanofiber. However, it varies from one nanofiber to another nanofiber. The magnitude of widths of different gel nanofibers obtained from FE-SEM and AFM analyses are different and this may be due to the fact that different procedures have been adopted for preparing samples for FE-SEM and AFM experiments.

In order to examine the morphology of f-SWCNT incorporated hybrid hydrogel, different microscopic analyses have been carried out. Transmission electronic microscopic (TEM) analysis of the f-SWCNT incorporated hybrid hydrogel shows the nanotape type assembly. TEM pictures (Fig. 5) also reveal that the average width of the hybrid hydrogel nanofibers is 34.2 nm and these fibers are several micrometres in length (Fig. 5 and see Fig. S11, †ESI). A field emission scanning electron microscopic (FE-SEM) image of the f-SWCNTs containing hybrid hydrogel is shown in Fig. S12† indicating the cross-linked nanofibrillar network structure. Atomic force microscopic imaging (AFM) shows that the f-SWCNTs are aligned along the surface of the hydrogel nanofibers (Fig. 6 and see Fig. S13, †ESI). The π–π interactions also helps these f-SWCNTs to disperse in the gel medium interacting with the π-moieties of the hydrogelator. The AFM image (Fig. 6) clearly suggests the 1D array of f-SWCNT along the surface of amino acid based gel nanofibers. This is an easy way to make a single-walled carbon nanotube based nanohybrid system, where the CNTs are regularly aligned in a 1D array on the gel nanofiber. Atomic force microscopic (Fig. 6 and see Fig. S14, †ESI) analysis reveals that the hybrid hydrogel nanofibers are 15–20 nm (see Fig. S13, †ESI) in width and the fibers are several micrometres in length. Winey and co-workers have demonstrated the alignment of SWCNTs using polarized Raman spectroscopy.³⁷ They have also reported the alignment of SWCNTs increases with a decrease in the fiber diameter. Ko and co-workers have reported the alignment of SWCNTs along the PAN polymer fiber using an electrospinning method.³⁸ However, in our study the 1D alignment of f-SWCNTs along the amino acid based thin gel nanofiber has been performed by the simple incorporation of f-SWCNTs into the amino acid based hydrogel. This clearly suggests that the alignment of SWCNTs along the hydrogel nanofiber is a different process than those of previously reported methods.

Fig. 5 Transmission electron microscopic (TEM) analysis of the hydrogel containing functionalized SWCNTs. This figure shows the presence of both nanotape type as well as nanofibrillar morphologies indicating successful incorporation of f-SWCNTs into the hydrogel based nanofibrillar system. Red arrows indicate the presence of nanotape type morphology and the blue arrows indicate the presence of nanofibrils.

Fig. 6 (A) Atomic force microscopic imaging shows the 1D alignment of functionalized SWCNTs on the gel nanofibers and (B) topographic 3D view of the functionalized SWCNT along the hydrogel nanofiber.

3.9 Rheological study

The rheological measurements of the native hydrogel and the hydrogel containing f-SWCNTs nanohybrid system are presented in Fig. 7. In Fig. 7, the dynamic mechanical properties (e.g., storage modulus G′ and loss modulus G′′) of the native hydrogel and the hybrid gel are plotted against angular frequency at room temperature. Theoretically, if G′(ω) is comparative to ω₀, G′′(ω) is comparative to ω₀ and, G′(ω) > G′′(ω) (in the low frequency region), the system is known as a gel.³⁹ The G′ (storage modulus) versus frequency plot (Fig. 7) is almost linear and invariant with the angular frequency (ω) at 26 °C. The exponent values of G′ (calculated from the slope of the log-log plot in the low frequency range ≤5 rad s⁻¹) at 26 °C for native hydrogel and f-SWCNT containing gel are 2.18 and 4.32. So, at 26 °C, the f-SWCNT incorporated hybrid hydrogel behaves as a more rigid and elastic nanomaterial compared to that of the native hydrogel. Interestingly, upon the incorporation of 0.1% of f-SWNT, both the moduli (G′ and G′′) have been increased more than 16 times compared to the native hydrogel. Both G′ and G′′ exhibit a distinct plateau over the total frequency range studied. This clearly suggests that the formation of a more elastic solid-like material upon doping of the f-SWCNT into the native gel system. This also indicates that the f-SWCNT containing hybrid hydrogel has greater resistance to flow behavior than that of the native hydrogel. Bhattacharya and co-workers have reported that both moduli (G′ and G′′) have been increased more than 20 times after the incorporation of f-SWCNT into the transtri(p-phenylenevinylene) bis-aldoxime based organogel.^22b It has also been reported that the G′/G′′ ratio has been increased after the inclusion of SWCNTs into the oligo(p-phenylene vinylene)s based organogel compared to that of the native organogel.^28f However, this study reveals that the increase of G′/G′′ ratio is from 1.74 to 4.91 upon the inclusion of f-SWCNTs into the native hydrogel. All these data indicate that stiffness of the gel has been increased considerably upon the incorporation of f-SWCNTs into the native hydrogel system.

Fig. 7 Storage moduli (G′) and loss moduli (G′′) as a function of frequency sweep for the supramolecular hydrogel and hybrid hydrogel containing functionalized SWCNTs at 26 °C.

Varying amounts of f-SWCNT have been added into the native gel system to examine the effect of G′ and G′′ with the gradual addition of f-SWCNT into the native hydrogel system. Interestingly, it is noticed that upon the successive addition of f-SWCNT into the native hydrogel up to a maximum addition of 0.1% w/v (keeping the gelator concentration fixed at 0.5% w/v) the elastic properties of the hybrid hydrogel increase gradually (Fig. 8). A maximum elasticity is obtained when the ratio of gelator : f-SWCNT is 5 : 1. The G′/G′′ vs. % w/v of the f-SWCNT incorporated into the native gel system is shown in Fig. S15 (see †ESI). However, further addition of f-SWCNT into the hydrogel system disrupts the hybrid hydrogel, and a phase-separated system is observed.

Fig. 8 Various storage moduli (G′) and loss moduli (G′′) as a function of frequency for the native supramolecular hydrogel and hybrid hydrogels containing different % w/v of functionalized SWCNTs at 26 °C showing the gradual increase of G′ upon the gradual addition of functionalized SWCNT into the native gel system.

3.10 Electrical conductivity (current (I)–voltage (V)) measurements

It is interesting to measure the conductivity of the f-SWCNT containing gel based hybrid nanomaterial through I–V experiments (see Fig. S16, †ESI). For I–V experiments the hybrid hydrogel nanomaterial was drop casted on a glass slide and this was accordingly dried in air for 6 h and this was subject to vacuum drying for 3 days. The conductivity of the SWCNT–CO₂H itself is more than 100 S cm⁻¹ (obtained from Cheap Tube SWCNT data sheet). However, the conductivity of the hydrogel–SWCNT hybrid nanomaterial has been calculated to be 3.12 S cm⁻¹. This study clearly indicates that the f-SWCNT incorporated hybrid hydrogel is less conducting in nature than that of the f-SWCNT material at 26 °C. So, upon the incorporation of f-SWCNT into the hydrogel, the conductivity has been decreased and the hybrid nanomaterial is less conducting. The decrease in conductivity of the f-SWCNTs after incorporation into the native gel may be due to the fact that the conducting material (f-SWCNT) is interacting with electrically nonconducting gelator molecules in the hybrid system.

4. Conclusions

This study demonstrates the successful incorporation and homogeneous dispersion of functionalized single-walled carbon nanotubes within the amino acid based hydrogel matrix to create a nanohybrid system at physiological pH and temperature. TEM experiments clearly demonstrate the presence of two different types of morphologies (a) nanotape type morphology and (b) thin nanofibrillar morphology. This suggests that successful incorporation of functionalized SWCNTs within the native hydrogel based nanofibrillar network. Functionalized SWCNTs are not randomly oriented on the hydrogel nanofiber, instead they are aligned in a regular 1D fashion on the gel nanofiber. AFM study suggests the 1D alignment of functionalized SWCNTs on the surface of the hydrogel-based nanofiber. Incorporation of functionalized SWCNTs into the native hydrogel makes the hybrid hydrogel much more elastic and rigid than the native gel as it is evident from rheological studies. The hybrid hydrogel is also more thermally stable than the corresponding native gel. Conductivity of the hybrid hydrogel is less than that of the f-SWCNT itself. This type of functionalized SWCNT based hybrid nanomaterials (obtained from the biocompatible method) can make a future promise to use them for interesting applications.

Acknowledgements

S. R gratefully acknowledges CSIR, New Delhi, India for providing the fellowship. We gratefully acknowledge the financial support from DST (Project No. SR/S1/OC/73/2009), Government of India, New Delhi, India. We also acknowledge Mr. Debasish Mandal (Department of Spectroscopy, IACS) for B3LYP/6-31+G** calculations.

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, X-ray diffraction, AFM, TEM, FE-SEM, energy minimized structures, N₂ gas adsorption, IR, I–V profile, UV-Vis-NIR and TGA. See DOI: 10.1039/c2ra00763k

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