Multiwalled carbon nanotube hybrids derived from ... · promising applications as electronics, catalysts, biosensors, imaging and therapeutics [22-24]. Recent advances have revealed

International Journal of Advanced Chemical Science and Applications (IJACSA)

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ISSN (Print):2347-7601, ISSN (Online): 2347-761X, Volume -3, Issue -2, 2015

19

Multiwalled carbon nanotube hybrids derived from functionalization

of amphiphilic poly(amidomine) dendrimers for improved

dispersibility and electrical conductivity

1G. Vimala,

2E. Murugan

1Department of Chemistry Pachaiyappa’s College for Women, Kanchipuram – 631 501. Tamil Nadu, India. 2Department of Physical Chemistry, University of Madras Guindy, Chennai – 600 025, Tamil Nadu, India

Email: [email protected],

[email protected]

[Received :15th Feb.2015; Accepted: 15th April 2015]

Abstract - Multiwalled carbon nanotubes (MWCNTs)-

amphiphilic poly(amidoamine) (PAMAM) dendrimers

(APAMAM) based hybrids were developed through

individual functionalization of PAMAM with generation 4

(G4) and generation 5 APAMAM (G5) on MWCNTs. The

two hybrids were used as matrices and deposited with

silver (Ag) nanoparticles (NPs) to develop another two

types of new MWCNT based hybrids viz., MWCNT-

APAMAM (G4)-AgNPs, and MWCNT-APAMAM (G5)-

AgNPs. The degree of covalent functionalization of

APAMAM (G4)/(G5) in MWCNTs and deposition of AgNPs

in APAMAM (G4)/(G5) were examined by Fourier

transform infrared spectroscopy, Raman spectroscopy,

thermogravimetric analysis, zeta potential, scanning and

high-resolution transmission electron microscopy, and

energy-dispersive spectroscopy. Further, based on the

results of x-ray diffraction and high-resolution

transmission electron microscopy, the size of AgNPs

present in MWCNTs-APAMAM (G4)-AgNPs was

determined to be in the range of 6 nm, and 2 nm for

MWCNTs-APAMAM and (G5)-AgNPs respectively. The

MWCNTs-APAMAM (G5) showed better dispersibility in

aqueous and various polar and nonpolar organic solvents,

and especially the dispersed homogeneous solution formed

from water and DMSO was stable for 9 months. The

MWCNTs-APAMAM (G4)/(G5)-AgNPs showed an

electrical conductivity 13-19 times higher than pristine

MWCNTs.

[Key words: MWCNTs, amphiphilic poly(amidoamine),

AgNPs Raman spectroscopy, thermogravimetric analysis,

zeta potential ]

I. INTRODUCTION

The research on carbon nanotubes (CNTs) is an

attracting topic nowadays due to their unique structures,

excellent mechanical, electrical properties and

considerable potential applications [1-3]. In practice, the

insolubility and weak dispersibility of CNTs in common

solvents have limited their applications especially in the

fields of development of composite materials and

bottom-up hybrid nanomaterials or devices [4,5]. Few

researchers have focused their attention on tube

functionalization and modification in order to improve

the solubility and surface functionality. Especially,

functionalization also introduces the field of CNTs-

based nanochemistry. Much progress has been made in

this area, based on the fundamental work performed by

Smalley [3] and Haddon [4]. To date, two

methodologies, namely noncovalent and covalent, have

been developed to functionalize CNTs with a variety of

organic, inorganic metallic, biochemical and polymeric

structures [6]. Generally, the attachment of small or

large molecules to the CNTs by covalent methods is

more stable and effective. With regard to the covalent

methods, carboxylic acid groups formed at the ends and

defect sites of CNTs are commonly used for precursor

functionalization in the formation of amine, ester and

organometallic structures [7]. Among the various

covalent CNT functionalization, binding polymers to the

CNTs is a very attractive area, because the individual

properties of the two materials can be combined to give

one hybrid material.

Particularly, functionalization of nanotubes with

symmetrical polymer like dendrimers represents a

promising strategy to introduce sufficient amount of

functional groups onto the CNT surfaces for succeeding

in processing with a limited number of sp2 carbon

atoms attacked [8]. Dendrimers are globular, highly

branched macromolecules with a three-dimensional

dendritic architecture. A Poly(amidoamine) (PAMAM)

dendrimer can introduce a dense outer amine shell

through a cascade-type generation [9-11]. Because of

their low melting viscosity, high solubility and

abundance of functional groups dendrimers have

potential applications in a wide range of fields from drug

delivery to material coatings [12-14]. Amphiphilic

dendrimers act as unimolecular micelles that attract non-

polar compounds to their hydrophobic regions and

counter ions to their hydrophilic charged regions [15,

16]. Ford et al [17] reported hydrophobically modified

poly(propyleneimine) (PPI) dendrimer containing

quaternary ammonium and tertiary amine functionality

and the same used for catalysis. Shim et al [18] reported

that poly(styrene) and poly(4-vinylpyridine) brushes can

be functionalized with MWCNTs through solution


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20

polymerization. Further, they also observed that the said

polymer fabrication derived from MWCNTs was

dispersed only in hydrophobic solvent such as toluene. It

may be mentioned here that the importance of

solubilization of CNTs in aqueous/organic phase has

gained much attention due to its numerous applications,

specifically in biomedical applications which include

biosensors, antimicrobial, anticancer activity,

nanoprobes, and nanotweezers, etc. Similarly, several

conducting CNT polymer composites, viz., CNT poly

(3-octylthiophene), CNT polyaniline (CNT-PANI), and

MWCNT sulfonated polyaniline (MWCNT-SPAN)

were also reported [19-21]. Recently, it was realized that

the integration of CNTs with metal nanoparticles (NPs)

generates a new family of novel materials with more

advanced properties and applications than the pristine

precursors. As a result, hybrid materials of CNTs with

metal NPs have attracted greater attention due to their

promising applications as electronics, catalysts,

biosensors, imaging and therapeutics [22-24]. Recent

advances have revealed that dendrimers can be

covalently functionalized on the surface of CNTs for

subsequent metal or metal oxide nanoparticle synthesis

and assembly for further electrical and biological

applications. This implies that by combining the

dendrimers surface functionality and unique molecular

recognition ability with the electronic properties of

CNTs, it may be possible to generate various complex

composite nanodevices for a wide range of electrical and

biomedical applications [25-32]. Although few studies

on amphiphilic hybrid nanospheres and macromolecular

nanowires were reported, particularly those based on

covalent linkage of polymer building blocks, have been

rarely studied [33-35]. More particularly, studies

pertinent to amphiphilic dendrimers functionalized CNT

composite with metal nanoparticles are accountable. In

our previous study, we developed new nanohybrids viz.,

MWCNTs functionalized with amphiphilic

poly(propyleneimine) dendrimer carrying silver

nanoparticles (AgNPs) and demonstrated that they have

better dispersibility and antimicrobial activity [36].

Similarly, in our another study, we reported that an

amphiphilic multiwalled carbon nanotube polymer

hybrid with improved conductivity and dispersibility

produced by functionalization with poly (vinyl benzyl)

triethyl ammonium chloride [37]. Further, we developed

novel MWCNTs hybrid catalysts for effective catalysis

of 4-nitrophenol reduction [38]. In this study, we have

developed MWCNTs nanohybrids via effective

functionalization of amphiphilic PAMAM dendrimer

(APAMAM) with generation number 4 (G4) and

generation 5 (G5) and thus produced 2 types of

corresponding hybrids with integration of all 3 smart

nanomaterials viz., MWCNTs-APAMAM (G4) and

MWCNTs-APAMAM (G5). These two hybrids in turn

were used as a individual matrices and deposited with

AgNPs, and thus yielded the another two types of

corresponding hybrids viz., MWCNTs-APAMAM (G4)-

AgNPs, and MWCNTs-APAMAM (G5)-AgNPs. The

resulting MWCNTs hybrids were thoroughly

characterized with spectral, thermal, microscopic,

electrical conductivity, and dispersibility studies so as to

identify the superior amphiphilic MWCNTs-APAMAM

hybrid material with high functionalized yield, good

dispersibility, and improved electrical conductivity

which can be used for fabrication of electronic materials,

sensors, and materials for biomedical applications.

II. MATERIALS AND METHODS

A. Materials

MWCNTs with purity greater than 95% were purchased

from Sigma-Aldrich. Hydrochloric acid (HCl, Merck),

potassium permanganate (KMnO4, Merck), methylene

chloride (CH2Cl2, Merck),

tetrabutylammoniumbromide (TBAB, Alfa aesar), acetic

acid (CH3COOH, 99.8%, Merck), Thionyl chloride

(Merck), N,N’-dicyclohexylcarbodiimide (DCC,

Aldrich), PAMAM (G4 & G5) dendrimers (Symo-Chem,

Netherland), tributylamine (Merck), silver acetate

(Merck), toluene (SRL), tetrahydrofuran (THF, Merck),

chloroform (CHCl3, Merck), dimethylsulphoxide

(DMSO, Merck) were of analar grade of 99% purity and

were used as such for the reactions.

B. Characterization

To ascertain the functionalization of amphiphilic

dendrimers on MWCNTs, the pristine MWCNTs and 4

types of MWCNTs hybrids viz., MWCNTs-APAMAM

(G4), MWCNTs-APAMAM (G5), MWCNTs-APAMAM

(G4)- AgNPs, and MWCNTs-APAMAM (G5)-AgNPs

were characterized with spectroscopy, thermal and

microscopic techniques. Fourier transform infrared

spectra (FTIR) were recorded on a Bruker Tensor-27

FTIR spectrophotometer with OPUS software in the

range 4000 to 400 cm-1. The pellet for analysis was

made by taking equal amount of each MWCNTs hybrids

and KBr (1:1 ratio). Similarly, all the MWCNTs hybrids

were used for the thermogravimetric analysis and

Raman studies. The thermogravimetric analysis (TGA)

was carried out on SDT Q600 V20.5 Build 15

instrument at a heating rate of 10oC/min from 50 to 800

o C under nitrogen atmosphere. Raman spectra were

recorded on a Witec Confocal Raman instrument (CRM

200) with Argon ion laser (514.5 nm). X-ray diffraction

(XRD) patterns were recorded on a X’perts Highscore

plus/Pan Analytical, Philips X-ray diffractometer,

equipped with Cu Kα photon source (40 KeV, 20 mA,

λ= 1.5418 Ao) and scanned at the rate of 10o min-1 over

the range of 10o - 80o (2θ). Scanning electron

microscopy (SEM) and energy dispersive spectroscopy

(EDS) measurements were carried out on a HITACHI S-

3000H scanning electron microscope instrument

interfaced with EDS DX – 4 energy diffraction

spectrometer and both the analyses were performed

using the MWCNTs hybrid through accelerating voltage

of 2 kV. That is, the samples for analyses were prepared

by taking equal amount and were spread on the surface

of double-sided adhesive tape, one side being already

adhered to the surface of a circular copper disc pivoted

by a rod and the spread samples were sputtered with


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gold prior to SEM observation. After the SEM

observation, using the respective MWCNTs hybrid the

elemental analysis was also performed with EDS and the

percentage of elements was observed (semi-

quantitative). High resolution transmission electron

microscopy (HRTEM) analysis was performed on JEOL

3010 transmission electron microscope operating at 200

kV. The MWCNTs hybrid samples to be analyzed was

initially sonicated with acetone for few minutes and then

one drop of each sample (suspension) was placed on a

glow discharged carbon-coated grid and the sample used

for HRTEM observation after evaporating the solvent.

Electrophoretic measurement was carried out with a zeta

potential analyzer (Zetaplus 3+). The electrical

conductivity of MWCNT was measured using a four-

probe resistivity/Hall measurement system (HL5500PC,

Bio-Rad). Sheet samples of 40-60 μm thickness were

prepared by pumping 5 mg of MWCNTs between two

iron plates at a pressure of 150 KN/cm2. The bulk

electrical conductivity of each MWCNTs hybrid was

measured at room temperature using a programmable

curve tracer (Sony Tektronix 370A). Specimens were

polished on both sides into a thickness of 1 mm, and a

very small amount of silver paste (of thickness about

0.05 mm) was applied on the sample surface to reduce

the contact resistance between the samples and

electrodes. To minimize any potential problems

associated with silver paste, the samples were heated at

40oC to remove solvent quickly. Then, the edges of the

samples were ground again to remove silver paste

attached on them. The dispersibility of all the

amphiphilic hybrids were examined with water and

different organic solvents.

C. Synthesis

1) Functionalization of MWCNTs

Pristine MWCNTs (200 mg) and 15 mL CH2Cl2 were

taken in a 100 mL round-bottomed flask and the mixture

was dispersed in ultrasonicator (Cole Parmer) for 10

min. Then, 0.25 g of TBAB in 5 mL H2O, 5 mL acetic

acid and 0.065 g KMnO4 in 5 mL H2O were mixed

together and the resulting solution was added to the flask

[37]. Then the resulting mixture was stirred vigorously

at 25oC for 48 hrs. Then it was diluted with 1000 mL of

deionized water and the resulting product was filtered

under vacuum by 0.2 m Teflon membrane. Then

washing and centrifugation of resulting functionalized

materials were performed continuously until the pH of

the filtrate showed 7 (at least 10 cycles were required).

The resulting filtrate was dried in vacuum and thus

obtained 0.198 g of functionalized MWCNTs

(MWCNTs-COOH) (2).

2) Synthesis of MWCNTs-APAMAM (G4), MWCNTs-

APAMAM (G5), MWCNTs- APAMAM (G4)-AgNPs

and MWCNTs-APAMAM (G5)-AgNPs hybrids 0.08 g

of MWCNTs-COOH (2) was dispersed in 5 ml of DMF

by ultrasonication for 10 min and the dispersed solution

was refluxed with 10 ml of solution ; washed with

acetone, filtered, and dried in vacuum for 24 hrs yielding

the 0.092 g of acid chloride functionalized MWCNTs

product (MWCNTs-COCl) (3). This product in turn was

transferred to two different 100 ml RB flasks and

dispersed separately in 5 ml of DMF through sonication

for 3 min.

Then 100 mg of PAMAM (G4) and PAMAM (G5) was

added dropwise to the respective containers and 0.96 g

of DCC was also added to each container. The

respective mixture solution was refluxed for 48 hrs in

dry nitrogen atmosphere. The solutions were washed

with acetone, filtered, and dried in vacuum at 60oC and

thus yielded black powders viz., 0.188 g of MWCNTs-

PAMAM (G4) (4) and 0.275 g of MWCNTs-PAMAM

G5(5). These two hybrids were converted into

amphiphilic form through quaternization reaction. For

quaternization reaction, 0.1 g of MWCNTs-PAMAM

(G4) and MWCNTs-PAMAM (G5) hybrids were taken

individually in two different 100 ml RB flasks and

stripped in 5 ml of DMF, 10 ml of tributylamine and 10

ml of methyl iodide followed by deaeration under N2

atmosphere and again the reaction mixtures were gently

refluxed for 72 hrs under nitrogen atmosphere at 80oC.

The resulting quaternized product was centrifuged and

dried under vacuum to obtain MWCNTs-APAMAM

(G4) (6) with 0.138g and MWCNTs-APAMAM (G5)

hybrid (7) with 0.245 g (Scheme 1).

Hundred milligrams of MWCNTs-APAMAM (G4) (6)

and MWCNTs-APAMAM (G5) (7) was taken

individually in two different 50 ml RB flask and

dispersed with addition of 10 ml deionized water in each

container. To this 10 ml of silver acetate aqueous

solution (0.01 mol L-1) was added dropwise in each

container. The resulting solution was magnetically

stirred for 24 hrs, washed, filtered and dried and thus

obtained a respective blackish product in the form of

powder, viz. MWCNTs-APAMAM (G4)-AgNPs (8) and

MWCNTs-APAMAM (G5)-AgNPs (9) hybrids

(Scheme 1). In order to remove the loosely adsorbed

AgNPs, the MWCNTs-APAMAM (G4)/(G5)-AgNPs

hybrids were thoroughly washed individually with

deionized water and then centrifuged [39].


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Scheme 1: Synthesis of MWCNTs-APAMAM (G4),

MWCNTs-APAMAM (G5), MWCNTs-APAMAM

(G4)-AgNPs and MWCNTs-APAMAM (G5)-AgNPs

hybrids

III.RESULTS AND DISCUSSION

1) FTIR

The FTIR spectra of MWCNTs-COOH, MWCNTs-

COCl, MWCNTs-PAMAM (G4), MWCNTs-APAMAM

(G4), MWCNTs-PAMAM (G5), MWCNTs-APAMAM

(G5), MWCNTs-APAMAM (G4)-AgNPs, and

MWCNTs-APAMAM (G5)-AgNPs are shown in Fig. 1

a-i respectively. The FTIR spectrum for MWCNTs-

COOH is shown in Fig. 1a, and it shows new broad and

intense peaks at 3400, 1730, and 1262 cm-1 and these

peaks were due to O-H, C=O, and C-O groups and thus

indicate the surface functionalization of COOH onto the

MWCNTs. The FTIR spectrum for MWCNTs-COCl is

shown in Fig. 1b and the characteristic peaks viz.,

C=O(str) and C-Cl(str) were noticed at 1725 cm-1 and

642 cm-1, respectively. In the spectrum of MWCNTs-

PAMAM (G4) and MWCNTs-PAMAM (G5), the

condensation of surface amino group of PAMAM

(G4)/(G5) dendrimers with MWCNTs-COCl showed the

characteristic peaks of N-H(str) and C=O(str) at 3430

and 1642 cm-1, respectively and are shown in Fig. 1c

and 1e, respectively. The formation of amphiphilic

character on the MWCNTs-APAMAM (G4) and

MWCNTs-APAMAM (G5) was evident through the

appearance of intense peak for C-N+ (str) at 1154 cm-1,

the decreased peak intensity of N-H(str) at 3430 cm-1,

and increased of peak intensity for C-H2(str) at 2922

and 2851 cm-1 (Fig. 1 d & f). After deposition of

AgNPs on MWCNTs-APAMAM (G4) and MWCNTs-

APAMAM (G5), the characteristic peaks were shifted

from 3430 to 3445 cm-1 and from 1154 to 1160 cm-1 is

certainly an indication for the formation/deposition of

AgNPs on respective MWCNTs amphiphilic hybrids

(Figure 1 g & h). Similar observations were also

reported by Li et al [40] in which they stated that the

characteristic amine group band was shifted from 3368

cm-1 to 3420 cm-1 and thus established the formation of

AgNPs in the preparation of stable silver colloids. A

analog study was also performed by Cao et al [41] and

they observed that the N-H band observed at 1655 cm-1

for d-MWCNTs was shifted to 1565 cm-1 in d-

MWCNTs/ Ag thus confirming the binding of Ag onto

the –NH2 groups

Fig. 1. FTIR spectra of (a) MWCNTs-COOH, (b)

MWCNTs-COCl, (c) MWCNTs-PAMAM (G4), (d)

MWCNTs-APAMAM (G4), (e) MWCNTs-PAMAM

(G5), (f) MWCNTs-APAMAM (G5), (g) MWCNTs-

APAMAM (G4)-AgNPs, and (h) MWCNTs-APAMAM

(G5)-AgNPs.

2) TGA Analysis

The MWCNTs-APAMAM (G4) and MWCNTs-

APAMAM (G5) hybrids were characterized with TGA

to quantify the amount of APAMAM (G4) and

APAMAM (G5) functionalized onto MWCNTs. The

weight loss curves for pristine MWCNTs, MWCNTs-

COOH, MWCNTs-PAMAM (G4), MWCNTs-

APAMAM (G4), MWCNTs-PAMAM (G5) and

MWCNTs-APAMAM (G5) hybrids are shown in Fig. 2.

For pristine MWCNTs (Fig. 2a), no weight loss was

noticed up to 800oC. For MWCNTs-COOH (Fig. 2b),

10% weight loss was observed due to the decomposition

of carboxyl groups from 300 to 500oC [42-44]. In the

case of MWCNTs-PAMAM (G4) (Fig. 2c) and

MWCNTs-PAMAM (G5) (Fig. 2e), 32 and 47 % weight

loss was observed due to the decomposition of

dendrimers at 200 to 400oC. Chan Park et al [39]

reported that the functionalization of MWCNTs with

PAMAM dendrimers with –NH2 surface groups having

generation number 2 and 3 were found to be grafted

with 29% and 45% respectively. Hence,

functionalization of dendrimers increased linearly with

the increase of generation number from 4 to 5 [28]. The

amphiphilic hybrid viz., MWCNTs-APAMAM (G4) and

MWCNTs-APAMAM (G5), showed weight loss of

66.5% (Fig. 2d) and 68.5% (Fig. 2f) at 330-400oC and

this is due to the decomposition of amphiphilic


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quaternary ammonium groups. Analogous study

reported that the chloromethyl and quaternary groups

were decomposed in the range of 300-400oC [45].

Fig. 2. TGA curves of (a) Pristine MWCNTs, (b)

MWCNTs-COOH, (c) MWCNTs-PAMAM (G4), (d)

MWCNTs-APAMAM (G4), (e) MWCNTs-PAMAM

(G5) and (f) MWCNTs-APAMAM (G5).

3) Raman Spectral Study

A Raman spectrum can provide qualitative and

quantitative information about the structural change of

electronic properties of MWCNTs due to

functionalization of PAMAM (G4), PAMAM (G5),

APAMAM (G4) and APAMAM (G5). Hence, all

MWCNTs hybrids were analyzed by Raman

spectroscopy along with pristine MWCNTs. The

observed ID/IG values for all MWCNTs hybrids along

with the control are presented in Table 1. The spectra of

MWCNTs-APAMAM (G4), AMWCNTs-PAMAM (G5),

MWCNTs-APAMAM (G4)-AgNPs and MWCNTs-

APAMAM (G5)-AgNPs are shown in Fig. 3 a-d,

respectively. Generally, in Raman spectra the peaks

observed at 1331 and 1559 cm-1 correspond to a

defective carbon band due to disordered sp3 hybridized

carbons in the nanotubes walls D-band and a graphite

carbon band from the sp2-hybridized G-band of

MWCNTs, respectively. The area ratio of D-band to G-

band of MWCNTs is a direct indication for the degree of

modification of MWCNTs. The calculated ID/IG ratio

for MWCNTs-APAMAM (G4) (Fig. 3a) and MWCNTs-

APAMAM (G5) (Fig. 3b) was 1.4 and 1.6, respectively.

This observation confirms the covalent functionalization

of APAMAM (G4) and APAMAM (G5) onto the

MWCNTs. The quantum of covalent functionalization

of dendrimers in MWCNTs is directly related to the

damage of CNTs structure and thereby drastic

disturbance in electronic properties, and this can be

ascertained through increased values of ID/IG in the

Raman spectrum [28]. In fact, the nature of covalent or

non-covalent functionalization on MWCNTs is normally

identified based on the ID/IG value. In our case, the

enhancement of the ID/IG ratio from 0.3 (pristine

MWCNTs) to 1.3 and 1.5 for MWCNTs-PAMAM (G4)

and MWCNTs-PAMAM (G5) hybrid sufficiently

confirmed the higher degree of covalent

functionalization of PAMAM (G4)/(G5) and similarly

further enhancement to 1.4 and 1.6 supported the

covalent functionalization of APAMAM (G4)/(G5).

Similarly, the ID/IG ratio of MWCNTs-APAMAM

(G4)-AgNPs (Fig. 3c) and MWCNTs-APAMAM (G5)-

AgNPs (Fig. 3d) hybrids were 1.42 and 1.67

respectively. However, deposition of AgNPs in their

respective MWCNT hybrids has mildly increased the

values or defects.

TABLE 1. ID/IG RESULTS OF MWCNTs HYBRIDS.

Types of MWCNTs hybrids ID/IGa

Pristine MWCNTs 0.30

MWCNTs-COOH 0.60

MWCNTs-PAMAM (G4) 1.30


MWCNTs-APAMAM (G4) 1.40


MWCNTs-APAMAM (G4)-AgNPs 1.42


Fig. 3. Raman spectra of (a) MWCNTs-APAMAM (G4),

(b) MWCNTs-APAMAM (G5), (c) MWCNTs-

APAMAM (G4)-AgNPs and (d) MWCNTs-APAMAM

(G5)-AgNPs.

4) SEM, EDS, HRTEM and XRD

The surface morphologies of pristine MWCNTs,

MWCNTs-APAMAM (G4), MWCNTs-APAMAM (G5),

MWCNTs-APAMAM (G4)-AgNPs, and MWCNTs-

APAMAM (G5)-AgNPs were studied with SEM, EDS

and HRTEM techniques. The SEM image Fig. 4a

suggested that the pristine MWCNTs entangled together

with a distribution such as fine threads/ropes, whereas

the image of MWCNTs-APAMAM (G4), MWCNTs-

APAMAM (G5) (Fig. 4b, c) showed clear, intense, white

patches distributed homogeneously, and the existence of

MWCNTs was not visible. It confirmed that the

MWCNTs were homogeneously mixed with a complete

coverage of polymers onto the surface. It also strongly

supported the functionalization of APAMAM (G4) and

APAMAM (G5) onto the MWCNTs. The images

obtained from MWCNTs-APAMAM (G4)-AgNPs and

MWCNTs-APAMAM (G5)-AgNPs hybrids are shown


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24

in Fig. 4 d and e respectively. In Figure 4d and e

distribution of white dots onto the MWCNTs-

APAMAM G4/G5 is seen and these dots are certainly

due to the deposition of AgNPs.

Fig. 4. SEM images of (a) pristine MWCNTs, (b)

MWCNTs-APAMAM (G4), (c) MWCNTs-APAMAM

(G5), (d) MWCNTs-APAMAM (G4)-AgNPs, and (e)

MWCNTs-APAMAM (G5)-AgNPs.

The EDS analysis is one of the most effective surface

characterization techniques for identifying and

quantifying (semi-quantitative) the surface elements.

The percentage of elements in pristine MWCNTs,



APAMAM (G5)-AgNPs was determined with EDS and

the observed spectrum along with the percentage of

elements are shown in Fig. 5 a-e respectively. The

results suggested that the percentage of carbon gradually

decreased from pristine MWCNT to MWCNT hybrids

with the induction of sizable percentage of newer

elements in each step of the functionalization. The

carbon (C), nitrogen (N), oxygen (O), iodide (I), and

silver (Ag) peaks appeared in all the spectrum with

varied intensity and the results are given in Table 2.

From the results (weight percentage), it is suggested that

the percentage of carbon noticed in pristine MWCNTs

to MWCNTs-APAMAM (G5) are gradually decreased

from 99% to 80%. Whereas, the percentage of nitrogen

increased from 9.3 % to 11.3%, and the same trend was

also observed for oxygen, iodide, silver and palladium.

Since the percentage of elements was determined by

taking equal amount of each MWCNTs hybrid, it is

logical to compare the quantum of elements (semi-

quantitative) in the analysis.

The generation of amphiphilic character was achieved

through quaternization in MWCNT-PAMAM (G4),

MWCNTs-PAMAM (G5) and it is confirmed from the

appearance of nitrogen and iodide in MWCNT-

APAMAM (G4) and MWCNT- APAMAM (G5), and

deposition of AgNPs in MWCNT- APAMAM (G4)-

AgNPs and MWCNT- APAMAM (G5)-AgNPs. In a

nutshell, the decreasing trend of carbon from MWCNTs

to MWCNTs hybrids and appearance of relevant

elements like nitrogen, iodide, and silver supported the

amount of functionalization and deposition of

APAMAM (G4), APAMAM (G5) and AgNPs onto the

corresponding MWCNTs.


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Fig. 5. EDS spectra of (a) pristine MWCNTs, (b)



MWCNTs-APAMAM (G5)-AgNPs.

TABLE 2. EDS RESULTS OF MWCNTs HYBRIDS

Types of

MWCNTs

hybrids

EDS (wt %)

C N O I Ag

a 99 - -

b 85 8.5 2.7 3.00 -

c 80 11.3 3.5 4.20 -

d 78 8.5 2.7 3.00 6.00

e 75.1 11.3 3.5 3.0 7.10

The functionalization of APAMAM (G4), and

APAMAM (G5) onto the MWCNTs and deposition of

AgNPs on MWCNTs-APAMAM (G4)/(G5) was also

visualized through the HRTEM images. The image of

the pristine MWCNTs (Fig. 6a) showed a smooth

surface. In contrast, the images of MWCNTs-

APAMAM (G4) (Fig. 6b) and MWCNTs-APAMAM

(G5) (Fig.6c) suggested well distributed/dispersed

MWCNTs with heterogeneous coverage of layers on the

surface, thus proving the functionalization of APAMAM

(G4)/(G5) onto the MWCNTs. In other words, the degree

of debundled CNTs was more in MWCNTs-APAMAM

(G5) due to functionalization of higher generation

APAMAM (G5). The distribution of dense black dots

seen in the Fig. 6d &e is a strong evidence for

deposition of AgNPs onto the surface of MWCNTs-

APAMAM (G4)/(G5). Similarly, from transmission

electron microscopy images the size of the AgNPs

available in MWCNTs-APAMAM (G4)-AgNPs and

MWCNTs-APAMAM (G5)-AgNPs hybrids were

determined as 6 nm 2 nm respectively.


_______________________________________________________________________________________________

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26

Fig. 6. HRTEM images of (a) pristine MWCNTs, (b)



MWCNTs- APAMAM (G5)-AgNPs.

The X-ray diffraction patterns of MWCNTs are

presented in Fig. 7a. The 2θ peaks noted at 26.02o and

42.00o are pertinent to (0 0 2) and (1 1 0) planes of

MWCNTs. Fig. 7b and c are the X-ray diffraction

patterns of the MWCNTs-APAMAM (G4)-AgNPs and

MWCNTs-APAMAM (G5)-AgNPs. According to the

JCPDS database No. 04-0783 the diffraction peaks at 2θ

26.81O, 38.51o , 44.67

o, and 64.8

o are indexed to (0 0 2),

(1 1 1), (2 0 0), and (2 2 0) planes which are the

reflections of AgNPs with face-centered cubic

symmetry. Further, the size of the AgNPs was

determined from the (1 1 1) peak using the Scherrer

equation [46] and found as 6 and 2 nm.

Fig. 7. XRD patterns of (a) pristine MWCNTs, (b)

MWCNTs-APAMAM (G4)-AgNPs, and (c) MWCNTs-

APAMAM (G5)-AgNPs.

5) Zeta potential measurements

The surface modification of MWCNTs was once again

established through zeta potential measurements. That

is, zeta potentials of the MWCNTs, MWCNTs-COOH,

MWCNTs-PAMAM (G4), MWCNTs-PAMAM (G5),



APAMAM (G5)-AgNPs at pH 7 were measured

individually with zeta plus instrument in 1 mM sodium

chloride (NaCl) solution and the values are given in

Table 3. The surface potential for MWCNTs is

measured as 20 mV. The surface potential of MWCNTs-

COOH is -9.5 mV and this value became positive in the

case of MWCNTs-PAMAM G4 (28.2 mV) for and

MWCNTs-PAMAM (G5) (36.3 mV). Further, the

hybrids converting into amphiphilic form viz.,

MWCNTs-APAMAM (G4) and MWCNTs-APAMAM

(G5) have lead to increase more positive charge to the

tune of 50 mV and 60.5 mV respectively and this is due

to presence of poly(quaternary) ammonium ions and

also the electrostatic attraction between the APAMAM

(G4)/(G5) and MWCNTs. Furthermore, by the deposition

of AgNPs onto the surface of MWCNTs-APAMAM

(G4)/(G5), the zeta potential has shifted from 50 mV to -

33.6 mV and 60.5 mV to -46.7 mV respectively. The

change of zeta potential at every step strongly supports

the surface modification of MWCNTs by APAMAM

(G4)/(G5) and AgNPs.

TABLE 3. ZETA POTENTIAL RESULTS OF

MWCNTs HYBRIDS.

Types of MWCNTs hybrids Zeta potential

(mV)


MWCNTs-COOH -9.5

MWCNTs-PAMAM (G4)

28.2



MWCNTs-APAMAM (G5)

60.5

MWCNTs-APAMAM (G4)-AgNPs -33.6

MWCNTs-APAMAM (G5)-AgNPs -46.7

6) Electrical Conductivity Measurements

To confirm the observations established in the zeta

potential study, all MWCNTs hybrids were again

studied for the determination of electrical conductivity

measurements using the four-probe method at room

temperature. The observed values of electrical

conductivity of pristine MWCNTs, MWCNTs-PAMAM

(G4), MWCNTs-PAMAM (G5), MWCNTs-APAMAM

(G4), MWCNTs-APAMAM (G5), MWCNTs-APAMAM

(G4)-AgNPs, and MWCNTs-APAMAM (G5)-AgNPs

are presented in Table 1. Electrical conductivity

measurements could provide information about the

geometric configuration of the MWCNTs that cannot be

extracted by other measurements such as thermal

conductivity or optical spectrum. The recent studies

showed that the MWCNTs were dispersed into polymers

to increase the electrical conductivity of the composites.

Liu et al [47] and Linsunova et al [48] studied the


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27

electrical conductivity of MWCNTs in aqueous solution

fluids.

From Table 4, it is seen that conductivities of the

pristine MWCNTs was determined as 5.2 S/cm. The

conductivity of the MWCNTs-PAMAM (G4) and

MWCNTs-PAMAM (G5) was 24.78 and 37.64 S/cm,

respectively. The conductivity of the MWCNTs-

APAMAM (G4) and MWCNTs-APAMAM (G5) reaches

a value of 48.98 and 86.12 S/cm respectively, which is 9

and 17 times higher than the pristine MWCNTs.

Because, MWCNTs is an excellent electron acceptor

[49], while APAMAM (G4)/(G5) can be considered as an

good electron donor-acceptor. Therefore, it is inferred

that the enhanced doping effect is associated with

MWCNTs (or) effective charge transfer from

APAMAM (G4)/(G5) to the MWCNTs through induced

chemical bonding.

This kind of interaction, homogeneity with higher

generation dendrimers and compatibility enabled

electron delocalization and enhanced the conductivity of

the amphiphilic hybrid. It was expected that the AgNPs

decoration would have beneficial effect on the electrical

conductivity of MWCNTs because the inherent

electrical conductivity of Ag is 6.3 S/cm is much higher

than that of the pristine MWCNTs. The electrical

conductivity of MWCNTs-APAMAM (G4)/(G5) was

increased from 5.2 to 48.98 S/cm and 86.12 S/cm

respectively, this enhancement was attributed to the

charge transfer between MWCNTs and APAMAM

(G4)/(G5) that were covalently bonded onto the

MWCNTs. The conductivity of MWCNTs increased

significantly to 69.32 and 98.29 S/cm after silver

deposition which is 13 and 19 times higher than the

pristine MWCNTs showing the effectiveness of Ag

favors more electronic transport thus enhanced the

electrical conductivity of the MWCNTs-APAMAM

(G4)/(G5)-AgNPs hybrid.

TABLE 4. ELECTRICAL CONDUCTIVITY

RESULTS OF MWCNTs HYBRIDS.

Types of MWCNTs hybrids

Electrical

Conductivity

(S/cm)


MWCNTs-COOH 5.48







7) Dispersibility of MWCNT-APAMAM (G4) and

MWCNT-APAMAM (G5)

Amphiphilic MWCNT hybrids, viz., MWCNTs-

APAMAM (G4) and MWCNTs-APAMAM (G5) were

prepared and subsequently they were dispersed in water

to study the degree of dispersibility without sonication

under identical experimental conditions at ambient

temperature. To ascertain the degree of dispersibility

and stability, the respective MWCNTs-APAMAM (G4)

and MWCNTs-APAMAM (G5) solutions were

periodically monitored under undisturbed condition upto

8 to 9 months. The dispersibilities of pristine MWCNTs,

MWCNTs-APAMAM (G4) and MWCNTs-APAMAM

(G5) hybrids were studied not only in aqueous phase but

also in various organic solvents, viz., toluene, THF,

chloroform, and dimethyl sulfoxide (DMSO), and the

corresponding photographs are shown in Fig. 8 (i) & (ii)

A-E.

The photograph of pristine MWCNTs dispersed in water

(Fig. 8 (i & ii) A) showed that the MWCNTs settled

down in water due to its higher surface energy, van der

Waals force, and high aspect ratio [50, 51]. In contrast,

the degree of dispersibility of MWCNTs derived from

APAMAM (G4) and APAMAM (G5) in aqueous phase

gradually improved. But the degree of dispersibility of

later was relatively higher than the MWCNTs-

APAMAM (G4) hybrid in irrespective of the solvent.

This is because, the APAMAM (G5) functionalized

MWCNTs increases the hydrophilic character and hence

promotes the effective dispersibility. To check the

dispersibility and stability of MWCNTs-APAMAM (G5)

hybrid was dispersed in various organic solvents and

compared with the pristine MWCNTs and MWCNTs-

APAMAM (G4). The results in toluene, THF,

chloroform and DMSO (Fig. 8 (ii) B-E) indicated that

the functionalized MWCNTs showed homogeneous

dispersibility due to the presence of alkyl groups present

in the MWCNTs hybrid. Whereas, in water it gives a

clear homogeneous solution (Fig. 8 (ii) F) is due to the

increased polarity (10.2) as well as hydrophilic

attraction due to poly(quaternaryonium ions) present in

the amphiphilic hybrid confirming the effective

dispersibility of MWCNTs. Thus the synergetic action

of (i) attraction of high polar solvents with amphiphilic

MWCNTs hybrid and enriched functionalization of

poly(quaternaryonium ions) led to improved

dispersibility. Hence, the MWCNTs-APAMAM (G5)

hybrid was stable for 9 months in aqueous and other

organic solvents.

Xu et al [52] showed dispersion of poly(N-

isopropylacrylamide) functionalized MWCNTs hybrid

only in aqueous solution. Wang et al [53] reported that

the functionalized MWCNTs with double-hydrophilic

block copolymer viz., poly(ethyleneoxide)-b-poly[2-

(N,N-dimethyl amino)ethylmethacrylate] showed

dispersibility in DMSO and ethanol-water mixtures.

Choi et al [54] reported that the carboxylic acid-

terminated hyperbranched poly(ether-ketone) on

MWCNTs dispersed in polar solvents. We reported that


_______________________________________________________________________________________________

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28

the MWCNTs functionalized with amphiphilic

poly(propyleneimine) dendrimer showed better

dispersibility in aqueous and organic solvents [36].

Similarly, we reported that the MWCNTs functionalized

with 4 wt% poly (vinylbenzyl) triethylammonium

chloride showed better dispersibility in aqueous and

organic solvents [37]. In the present study, the

MWCNT-APAMAM (G5) hybrid contains both more

hydrophobic and hydrophilic properties which enabled

dispersion of the MWCNTs effectively in

aqueous/organic media with improved conductivity.

Fig. 8. Photograph of dispersibility studies of

(i) MWCNTs-APAMAM (G4) and (ii) MWCNTs-

APAMAM (G5) hybrids in (A) pristine MWCNTs, (B)

toluene, (C) THF, (D) chloroform, (E) DMSO and (F)

water.

IV. CONCLUSIONS

The MWCNTs-amphiphilic poly (amido amine)

dendrimers based hybrids were developed through

individual functionalization of APAMAM (G4),

APAMAM (G5) on MWCNTs. Subsequently, the above

two hybrids were used as a matrices for deposition of

AgNPs to develop two types of new MWCNT-

amphiphilic dendrimers-deposited with AgNPs based

hybrids viz., MWCNT-APAMAM (G4)-AgNPs, and

MWCNT-APAMAM (G5)-AgNPs. The appearance of

C-N+ peak at 1154 cm-1 and shifting of –N-H band

from 3430 to 3420 cm-1 in FTIR indicated the

quarternization and the shifting of N-H and C-N+ peaks

from 3430 to 3445 cm-1, and from 1154 to 1160 cm-1

respectively confirmed deposition of AgNPs on the

respective MWCNTs hybrids. The increased ratio of

ID/IG in the Raman spectrum from MWCNTs-

APAMAM (G4) (1.4) to MWCNTs-APAMAM (G5)

(1.6) strongly supported the covalent functionalization

of APAMAM (G4)/(G5) on MWCNTs. The quantum and

nature of the functionalization of APAMAM (G4)/(G5)

on MWCNTs was established through percentage of

weight loss of dendrimers molecules in TGA. The

decreased weight percentage of carbon from pristine

MWCNTs (99%) to MWCNTs nanohybrids (75.1%)

and appearance of N, I, and Ag peaks in EDS, change of

surface morphology from smooth to heterogeneous with

more black dots observed in SEM and HRTEM

supported more functionalization of APAMAM (G5) and

deposition of AgNPs on MWCNTs-APAMAM hybrids.

Further, based on the results of XRD and HRTEM, the

size of AgNPs present in MWCNTs-APAMAM (G4)-

AgNPs was 6nm and 2 nm for MWCNTs-APAMAM

(G5)-AgNPs. Further, the MWCNTs-APAMAM

(G4)/(G5) hybrid has proved to be 9 and17 times superior

in electrical conductivity than pristine MWCNTs and

further MWCNTs-APAMAM (G4)/(G5)-AgNPs hybrid

has showed 13 to 19 times higher than that of pristine

MWCNT as shown by four-probe conductivity

measurements. The MWCNTs-APAMAM (G5) showed

better dispersibility in aqueous and various polar and

nonpolar organic solvents which is stable for 9 months.

In a nutshell, the MWCNTs-APAMAM (G5) based

hybrid was better in terms of (i) high functionalized

yield, (ii) higher electrical conductivity, and (iii) better

dispersibility in aqueous and organic solvents.

ACKNOWLEDGMENT

The authors gratefully acknowledge the DST-

Nanomission (DST-NSTI), New Delhi, Government of

India for financial assistance and Prof. C.N.R.Rao,

Honorary President of Jawaharlal Nehru Centre for

Advanced Scientific Research, Bangalore, India.

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