Functionalization of carbon nanotubes regulated with amino acids

Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 16–22

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Functionalization of carbon nanotubes regulated with amino acids Jingting Hu a,1 , Lili Li a,1 , Wei Feng a,∗ , Peijun Ji b,∗ a b

Department of Biochemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 A new method for functionalization of carbon nanotubes has been demonstrated.  The adsorption of a sugar-based surfactant on MWNTs is carried out in amino acid solutions.  Amino acid solutions can promote the adsorption of the surfactant on MWNTs.  Amino acid solutions can regulate the amount and conformation of the assembled surfactant on MWNTs.  The functionalized MWNTs (f-MWNTs) have a good dispersibility in water and are biocompatible.

a r t i c l e

i n f o

Article history: Received 4 February 2012 Received in revised form 27 April 2012 Accepted 28 April 2012 Available online 6 May 2012 Keywords: Sugar-based amphiphile Carbon nanotubes Self-assembly Amino acids

a b s t r a c t A new approach has been developed for the functionalization of multi-walled carbon nanotubes (MWNTs). The self-assembly of a sugar-based amphiphile on MWNTs was carried out in amino acid solutions. The functionalization of MWNTs has been investigated by means of UV–vis, Raman spectra, FTIR, XPS, XRD, and HRTEM. It has been found that amino acids can promote the self-assembly of the amphiphile on MWNTs and regulate the amount and conformation of the assembled amphiphile. The effect of amino acids on the aggregation of the sugarbased amphiphile on MWNTs was analyzed through FTIR spectra. The specific interactions of f-MWNTs with the protein concanavalin A were investigated through the measurement of UV–vis spectra. This work demonstrates that, the functionalized MWNTs with disaccharide groups on their exterior surface have a good dispersibility in water and are biocompatible, and may have a potential application of biomolecular recognition. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Carbon nanotubes (CNTs) have extraordinary structural, mechanical, electrical, and thermal properties [1], and are being extensively investigated for their potential biological applications [2]. However, the potential applications of CNTs have been hampered by the poor aqueous solubility, due to highly hydrophobic surface of pristine CNTs and formation of bundles. To overcome

∗ Corresponding authors. Tel.: +86 010 64446249; fax: +86 010 64416406. E-mail addresses: [email protected] (W. Feng), [email protected] (P. Ji). 1 Both authors contributed equally to this work. 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.04.043

this problem, surface functionalization of CNTs has been regarded as an efficient method to improve the solubility of the material and their compatibility with the biomolecules [3]. Covalent and non-covalent modification methods have been developed to improve the aqueous solubility and surface functionality of carbon nanotubes [4]. Covalent approach has been shown to disrupt CNTs ␲-network, leading to possible loss of the mechanical, electrical, and biosensing properties [5]. Preserving the intrinsic electronic structure and properties requires noncovalent strategies [6]. A wide variety of substances have been used for functionalization of CNTs with noncovalent approaches, including polymers [7] and surfactants [8]. Carbon nanotubes wrapped by polysaccharides presented good water solubilities, such as starch-wrapped singlewalled carbon nanotubes (SWNTs) [9], dissolution of SWNTs by

J. Hu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 16–22

17

OH O

HO HO

OH OH O

O

NH_(n_C16H33)

HO OH Scheme 1. Molecular structure of N-hexadecyl-d-maltosylamine.

wrapping with single-chain schizophyllan and s-curdlan [10], chitosan functionalized CNTs [11]. Carbohydrate derivatives exhibit amphiphilic properties suitable for the interactions with CNTs, including the pyrene-based glycoconjugates [3], glycopolymers [12]. Sugar-based amphiphiles have been interesting due to their significance in areas of self-assembly and molecular recognition in biological systems [13]. In this work, N-hexadecyl-dmaltosylamine (HDMA), a sugar-based amphiphile, is used to functionalize multi-walled carbon nanotubes (MWNTs) in amino acid solutions of leucine and serine. One of the advantages utilizing amino acids is that they can also be produced greenly by the fermentation method using natural materials [14], the enzymatic reaction converting renewable sources [15,16], and the hydrothermal extraction method [17]. On the other hand, amino acids have been demonstrated to form thermodynamically stable associates with the sugars [18]. They can favorably interact with sugar-based amphiphiles in aqueous solution and adjust the self-assembly of the amphiphiles. This work demonstrates the promotion and regulation of the self-assembly of HDMA on MWNTs by the amino acids. In addition, HDMA has a maltose head group and the alkyl tail. Maltose can specifically bind to sugar-binding proteins. Upon interaction of the amphiphile with the MWNTs sidewalls through hydrophobic interaction, the aggregates expose the maltose head group to the water surface, promoting the interaction with its specific receptor. The specific interactions of functionalized MWNTs with the protein concanavalin A are investigated through the measurement of UV–vis spectra. 2. Experimental 2.1. Chemicals Multi-walled carbon nanotubes (MWNTs) were purchased from Nanotech Port Co. (Shenzhen, China). The purity of the MWNTs is higher than 95%, and the catalyst residue was less than 0.2%. The diameters are in the range of 40–60 nm. Leucine, serine, dmaltose, hexadecylamine, concanavalin A (Con A) were purchased from Sigma–Aldrich Chemical Co. (Shanghai, China). 2-Propanol and ethanol were obtained from Sinopharm Chemical Reagent Co. (Beijing, China). All chemicals were of analytical reagent grade and were used as supplied. 2.2. Synthesis of N-hexadecyl-d-maltosylamine HDMA (Scheme 1) was synthesized according to the method as described elsewhere [13]. A brief description is as following.

3.0 mmol of d-maltose was dissolved in 6.0 ml of deionized water; 5.0 mmol of hexadecylamine was dissolved in 10.0 ml of 2-propanol. The two solutions were mixed and stirred at room temperature. When the solution turned turbid, it was heated to 60 ◦ C to dissolve the precipitate; and then the solution was stirred at room temperature. The mixture was stirred for 24 h with periodic heating to 60 ◦ C at regular intervals as and when the solution turned turbid. The crude residue was dried under vacuum, and then recrystallized from ethanol and then again freeze-dried to eliminate traces of water. The obtained HDMA was monitored by measuring the 1 H NMR data, which are in accordance with the paper [13].

2.3. Functionalization of MWNTs with HDMA MWNTs were purified as reported elsewhere [19]. MWNTs were refluxed in an aqueous solution of 2.6 M HNO3 at 70 ◦ C for 45 h. The nanotube suspension was diluted and washed with double-distilled water, then filtered through a 0.8 ␮m polycarbonate membrane. The samples were dried at 80 ◦ C under vacuum. The purified MWNTs were functionalized with HDMA in amino acid solutions, including leucine and serine. Before used for the functionalization, the amino acid solutions were neutralized with sodium hydroxide. 30.0 mg of HDMA was added to 100.0 ml of the neutralized solutions and sonicated for 10 min to produce a clear homogeneous solution. 10.0 mg of purified MWNTs was then added and sonication was continued for 1 h followed by centrifugation at 12,000 rpm for 30 min, yielding wellsuspended MWNTs coated with HDMA. The suspension was filtered through a 0.45 ␮m polycarbonate membrane and rinsed with water to remove unbound HDMA. The aqueous dispersibility of thus functionalized MWNTs (f-MWNTs) was monitored by measuring ultraviolet–visible (UV–vis) spectra, which were recorded on a Shimadzu UV2550-PC spectrophotometer.

2.4. Measurements Ultraviolet–visible absorption spectroscopy measurements were performed in a Shimadzu UV2550-PC spectrophotometer using 1-cm-path length quartz cuvettes. Spectra were collected within a range of 190–800 nm. The X-ray diffraction (XRD) patterns were obtained with a diffractometer of X’Pert PRO MPD using a Gu anode at 40 kV, wavelength 0.154 nm. The diffractograms were operated at a scan rate of 1◦ /min from 2 = 5◦ to 2 = 90◦ . Raman spectroscopy measurements were recorded using a Renishaw InVia equipment (514.5 nm, Elaser = 2.41 eV).

J. Hu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 16–22

3. Results and discussion The functionalized MWNTs (f-MWNTs) were obtained by the functionalization of MWNTs with HDMA in water and in amino acid solutions. On the surface of f-MWNTs, only HDMA molecules were adsorbed through self-assembly. Amino acids were rinsed off in the washing step and the water molecules were eliminated through freeze drying. The analysis of the atomic composition of f-MWNTs through XPS spectra confirmed that amino acids are not present in f-MWNTs. The obtained f-MWNTs were redispersed in water, and the suspensions were stable for more than two months. The dispersibility of f-MWNTs in water is higher than 15 mg/ml. The comparison between the dispersibilities of f-MWNTs was investigated by measuring UV–vis spectra. A higher UV–vis absorbance means a larger dispersibility of the f-MWNTs in water. Fig. 1 shows the fact that the f-MWNTs obtained by the functionalization in amino acid solutions have a larger dispersibility than the f-MWNTs obtained by the functionalization in water. From the results of UV–vis spectra, we can conclude that the functionalization of MWNTs is promoted by the amino acids. The amphiphile HDMA has a hydrophilic carbohydrate head group and a long hydrophobic chain. Due to the hydrogen bonding and hydrophobic interactions, the amphiphile molecules self-assemble and aggregate in water (opalescent masses can be observed). The amino acids, each having a carboxyl group and an amino group, have been

a

3.2

f-MWNTs were obtained by the functionalization

Absorbance

XPS spectra were acquired using a Thermo VG ESCALAB250 X-ray photoelectron spectrometer, which was operated at the pressure of 2 × 10−9 Pa using Mg K␣ X-ray as the excitation source. Analysis of the data was carried out with Thermo Avantage XPS software. All XPS spectra were referenced to the main C 1s hydrocarbon peak at 284.9 eV binding energy. A Shirley background [20] was used in all curve-fitting [21]. The elemental compositions were calculated using an O 1s relative sensitivity factor (RSF) of 2.50 relative to C 1s [21], using N 1s RSF of 1.73 relative to C 1s. Infrared spectra were collected using a Fourier transform infrared (FTIR) spectrometer (Bruker TENSOR 27) equipped with a horizontal, temperature-controlled attenuated total reflectance (ATR) with ZnSe Crystal (Pike Technology). Infrared spectra from 1000 to 4000 cm−1 were collected using a liquid-nitrogen–cooled mercury–cadmium–telluride detector that collected 128 scans per spectrum at a resolution of 2 cm−1 . All spectra were corrected by a background subtraction of the ATR element spectrum. Ultrapure nitrogen gas was introduced at a controlled flow rate to purge water vapor. The recorded infrared spectra were analyzed using Peak Fit software (SPSS Inc., Chicago, IL) [22]. Deconvolution was performed using the Lorentzian or Gaussian peak shape and a full width at half-maximum (fwhm; cm−1 ) of 18–30 cm−1 to ensure good resolution of peaks without overfitting. Prior to deconvolution the second-derivative spectra were analyzed to determine the number of hidden peaks. All data were analyzed and compared on the basis of peak areas. Selection of peaks and calculations of peak areas as a measure of spectral intensity were performed by maximum likelihood peak fitting, and all data were fit with r2 values of greater than 0.99. Specific interaction of f-MWNTs with Con A was measured by addition of Con A (0.02 mg/ml) into the aqueous solution of fMWNTs (0.07 mg/ml). The mixtures were then shaken at 20 ◦ C in an incubator shaker at 150 rpm for 4 h. For the competitive dissociation of the protein Con A from the f-MWNTs, Con A (0.02 mg/ml) and glucose (0.5 mg/ml) were added into the aqueous solution of fMWNTs (0.07 mg/ml), the mixtures were then shaken at 20 ◦ C in an incubator shaker at 150 rpm for 4 h. The solutions were measured using a Shimadzu UV2550-PC spectrophotometer.

2.8

with HDMA in the aqueous solutions of LEU with

2.4

various ratios of LEU to HDMA (mol: mol)

2.0 1.6 1.2

0.5:1.0

0.4:1.0

0.3:1.0

0.6:1.0

0.0:1.0

0.2:1.0

0.1:1.0

0.8 0.4 200

300

400

500

600

700

800

Wavelength / nm

3.2 2.8

b

f-MWNTs were obtained by the functionalization with HDMA in the aqueous solutions of SER with various ratios of SER to HDMA (mol: mol)

2.4

Absorbance

18

0.1:1.0 2.0 1.6

0.2:1.0

0.05:1.0

0.0:1.0

0.3:1.0

0.4:1.0

0.5:1.0 1.2 0.8 0.4 200

300

400

500

600

700

800

Wavelength / nm Fig. 1. UV–vis absorbance of f-MWNTs dispersed in water. LEU, leucine; SER, serine.

demonstrated to form thermodynamically stable associates with the sugars due to hydrophilic–ionic, hydrophilic–hydrophilic, and hydrophilic–hydrophobic interactions [18]. When HDMA is added to the solutions of amino acids, aforementioned interactions may involve in the interactions between amino acids and HDMA. As a consequence of the interactions, opalescent masses due to the HDMA aggregation are greatly reduced. More free (not aggregated) HDMA molecules are available to contact and interact with MWNTs, resulting in more MWNTs that are functionalized. Amino acid concentrations have an effect on the functionalization of MWNTs. For each amino acid solution, there is an optimum amino acid concentration, at which the obtained f-MWNTs have the highest UV–vis absorbance (the largest solubility of f-MWNTs in water), as indicated by the black lines (Fig. 1). For the functionalization of MWNTs, the media of amino acid solutions are advantages over pure water. The adsorption of HDMA on MWNTs has been confirmed by Raman spectra as shown in Fig. 2. The D-band at 1348 cm−1 and G-band at 1576 cm−1 were obviously observed for the purified MWNTs. For the MWNTs attached with HDMA, the D- and G-bands are blue shifted. This is due to the interactions between HDMA and the nanotubes, which increase the energy necessary for vibrations and shift the Raman band to the higher frequency [23]. Functionalized MWNTs were imaged with high-resolution transmission electron microscopy (HRTEM). As shown in Fig. 3, thick layers of the amphiphile confirm the self-assembly of HDMA on the MWNTs. Fig. 4 presents the X-ray diffraction (XRD) patterns of f-MWNTs. Compared to the XRD pattern of the CNT-free HDMA, when HDMA

J. Hu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 16–22

19

Purified MWNT f-MWNT-LEU f-MWNT-SER

1200

1300

1400

1500

1600

1700

1800

Wavenumber /cm-1 Fig. 2. Raman spectra of f-MWNTs. f-MWNTs were obtained by the functionalization with HDMA in the amino acid solutions with molar ratios of amino acid to HDMA (mol:mol). f-MWNT-LEU: leucine to HDMA (0.5:1.0); f-MWNT-SER: serine to HDMA (0.1:1.0).

is self-assembled on the MWNTs, a new peak appears at 2 = 8.3◦ and the intensities of the peaks at 21.5◦ are significantly increased. These results suggest that the symmetry of the crystal structure of HDMA deceases after its self-assembly on MWNTs [24]. This is probably due to the interactions between the HDMA molecules when self-assembling on the MWNTs. The initial adsorption of HDMA on MWNTs is due to the hydrophobic interactions between the hydrophobic chains and the wall of CNTs. Further adsorption of HDMA molecules, which are in the forms of free HDMA and aggregated HDMA in the solution, is due to intermolecular hydrogen bonding and chain–chain interactions. Hydrogen bonding interactions are important for the selfassembly of HDMA on the CNT. To investigate the hydrogen bond types, which are helpful to better understand the conformational change of HDMA upon self-assembly on MWNTs, FTIR spectroscopy has been used to obtain an infrared spectrum of adsorption at the OH band between 3100 cm−1 and 3700 cm−1 . Infrared spectroscopy has been proven to be a highly effective means of investigating specific interactions within and between molecules [25]. FTIR has also been used to study the mechanism of intra- and inter-molecular interactions through hydrogen bonding [24]. Spectral differences between free HDMA and the HDMA selfassembled on MWNTs are observed in the hydroxyl stretching region (3700 and 3100 cm−1 ) as shown in Fig. 5. The band profile of free HDMA (Fig. 5a) is broader than that of HDMA-CNTs (Fig. 5b and c). This indicates that a wider variety of hydrogen bonds are formed in free HDMA. For the HDMA self-assembled on MWNTs in the amino acid solutions, the band profiles differ slightly (Fig. 5b and c). The OH band at ∼3500 cm−1 is due to the formation of OH-␲-type hydrogen bonds. The ␲ electrons on the graphitic surface of MWNTs are specific sites that are able to form weak hydrogen bonds with the hydroxyl in HDMA molecules [26]. This indicates that, except the hydrophobic chains of the self-assembled HDMA, its head groups can also directly interact with MWNTs through OH-␲-type hydrogen bonds. The intensities (the OH bands at 3330–3350 cm−1 ) are increased relatively. This implies that, when HDMA selfassembles on MWNTs, the self-association due to intermolecular hydrogen bonding interactions between hydroxyl groups becomes significant. The intensities (3450–3480 cm−1 , 3360–3420 cm−1 , 3360–3420 cm−1 ) are decreased relatively. This means that the intramolecular hydrogen bonding interactions (3450–3480 cm−1 ) within HDMA molecules, multiple hydrogen bonding interactions (3360–3420 cm−1 ), as well as the intermolecular hydrogen bonding interactions between OH and glycosidic oxygen (3200–3290 cm−1 )

Fig. 3. HRTEM images of f-MWNTs and purified MWNT.

are decreased. On the other hand, the self-assembly of HDMA on the CNTs promotes the formation of OH· · ·N type hydrogen bond, as indicated by the peaks at 3100–3200 cm−1 . The aforementioned types of hydrogen bonds have been summarized in Table 1.

20

J. Hu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 16–22

Table 1 Assignment of the FTIR (OH region) bands of the free HDMA and HDMA-CNTs. Wavenumber at band center (cm−1 )

Abbreviations

Hydrogen bond

3600–3700 ∼3500 3450–3480

Free hydroxyl groups in an alcoholic group OH-␲-type hydrogen bonds between the hydroxyl groups and the graphitic surface of MWNTs Intramolecular hydrogen bond within the head group of HDMA; free N H

3360–3420

Free OH groups OH· · ·␲ OH· · ·OH OH· · ·O N H OH· · ·OH

3330–3350 3200–3290 3100–3200

OH· · ·O OH· · ·OH OH· · ·O OH· · ·N

Multiple formation of an intermolecular hydrogen bond between hydroxyl groups and between hydroxyl and glycosidic oxygen Intermolecular hydrogen bond between hydroxyl groups Intermolecular hydrogen bond between hydroxyl and glycosidic oxygen Intermolecular hydrogen bond between OH and N H

0.012

a

0.010

3344

Absorbance

3411

0.008 3277

0.006

3476

0.004 0.002

3202

0.000 3100

3200

3300

3400

3500

3600

Wavenumber (cm) -1 3337

b

0.004

Absorbance

X-ray photoelectron spectroscopy (XPS), as a surface analysis technique to determine relative atomic composition, has turned out to be a powerful tool for the investigation of functionalized MWNTs [27]. Fig. 6 shows the XPS spectra of the f-MWNTs obtained in different amino acid solutions. Compared to that of purified MWNTs, the spectra of f-MWNTs show an increase of the intensity of oxygen. To investigate the origin of the oxygen atoms, we have performed the functionalization of MWNTs using the amino acids. It was found that the amino acids could not functionalize MWNTs due to their weak interactions with the MWNTs. The FTIR and XPS spectra (figures not shown here) confirmed that the amino acids could not adsorb onto MWNTs. Thus it can be concluded that, the increase in the intensity of oxygen as indicated in Fig. 6 is attributed to the oxygen atoms of HDMA. The atomic ratio of oxygen to carbon is illustrated by the insert. As can be seen, more HDMA molecules are self-assembled on MWNTs in the amino acid solutions compared to the functionalization in pure water. The amounts of HDMA self-assembled on MWNTs are in the order: f-MWNT-SER, f-MWNT-LEU, f-MWNT-WAT. The results of XPS spectra suggest that the amount of self-assembled HDMA on MWNTs can be regulated with amino acid solutions. The interaction of the functionalized MWNTs with Con A was assessed with optical absorption spectroscopy [28], and these results are shown in Fig. 7. The f-MWNT-SER and f-MWNT-LEU show absorption peaks at 255 nm (in red) that experience a red shift of 7 nm and 6 nm upon interacting with Con A (in blue), respectively. Addition of glucose in solution induces competitive dissociation of the protein Con A from the maltose head groups, and this dissociation is reflected in the spectra (in Oliver). It is a strong indication of competitive dissociation of Con A from f-MWNTs. This result shows the reversibility of the HDMA-Con A interaction and

0.003

0.002

3289 3504

0.001

3369 3111

0.000 3100

3465

3200

3300

3400

3500

3600

Wavenumber (cm) -1 3340

c

0.004

HDMA

Absorbance

Purified MWNT

f-MWNT-HDMA-LEU

0.003

0.002 3289

f-MWNT-HDMA-SER

3506

3378

0.001 3112

0.000 3100

3469

3200

3300

3400

3500

3600

Wavenumber (cm) -1 10

20

30

40

50

60

2θ Fig. 4. XRD patterns of HDMA, purified MWNT and f-MWNTs.

70

Fig. 5. FTIR spectra of the OH region of HDMA. (a) Free HDMA; (b and c) for the HDMA assembled on MWNTs in the amino acid solutions. (b) In the serine solution (SER:HDMA = 0.1:1.0); (c) in the leucine solution (LEU:HDMA = 0.5:1.0).

J. Hu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 16–22

conformation of the self-assembled amphiphile on MWNTs. XRD patterns have illustrated the conformational changes of HDMA upon self-assembly on MWNTs. FTIR spectra have shown the changes of inter- and intra-hydrogen bonding interactions when HDMA self-assembles on MWNTs, which are helpful to better understand the conformational changes. The functionalized MWNTs with disaccharide groups on their exterior surface can have a specific interaction with Con A.

0.2

O/C ratio

f-MWNT-LEU f-MWNT-SER f-MWNT-WAT

0.1

Purified MWNT 0.0

C1s

O1s

21

Acknowledgement

600

500

400

300

This work was supported by the National Science Foundation of China (21076018, 21176025).

200

Binding energy (eV) References Fig. 6. XPS spectra of f-MWNTs. Insert: atomic ratio of oxygen to carbon. f-MWNTWAT was obtained by the functionalization with HDMA in water.

1.50

a f-MWNT-SER + Con A f-MWNT-SER + Con A + glucose f-MWNT-SER

1.25

Absorbance

1.00 0.75 0.50 0.25 0.00

300

400

500

600

700

800

Wavelength / nm 1.50

b

f-MWNT-LEU + Con A + glucose f-MWNT-LEU + Con A f-MWNT-LEU

Absorbance

1.25

1.00

0.75

0.50 300

400

500

600

700

800

Wavelength / nm Fig. 7. Optical absorption spectra of f-MWNTs due to association after the addition of Con A and the dissociation with glucose. The concentration of Con A is 0.02 mg/ml, the glucose concentration is 0.5 mg/ml. f-MWNTs were obtained by the functionalization with HDMA in the amino acid solutions with molar ratios of amino acid to HDMA, f-MWNT-LEU: leucine to HDMA (0.5:1.0); f-MWNT-SER: serine to HDMA (0.1:1.0).

also indicates that the interaction of Con A with HDMA occurs by means of biospecific molecular recognition. 4. Conclusions In the solutions of amino acids, the self-assembly of the sugarbased amphiphile on MWNTs has been investigated. It has been demonstrated that the amino acids can promote the self-assembly of the amphiphile on MWNTs and regulate the amount and

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