Completely green synthesis of silver nanoparticle decorated MWCNT and its antibacterial and catalytic properties

Introduction

Introducing the topic of Green Chemistry has been emphasized in recent research techniques in order to fabricate environmentally benign synthetic routes for chemical products that will reduce or eliminate the use and generation of hazardous substances [1]. In recent years, nanotechnology has successfully applied Green Chemistry principles in the synthesis and applications of various nanomaterials. In the green synthetic strategy for nanoscale materials, utilization of non-toxic chemicals and environmentally friendly solvents have attracted considerable attention due to their advantage in reducing environmental risks [2, 3]. Among various metal nanoparticles, silver nanoparticles (Ag-NPs) have received considerable attention among researchers and companies alike have long incorporated its use in various applications due to its outstanding plasmonic activity, broad spectrum of antibacterial activities, chemical stability, good thermal and electrical conductivity, catalytic properties and recently as sensors for detecting reactive oxygen species (ROS) that possess a serious threat to biological systems [4–9]. Though the antibacterial activity of Ag-NPs have led to their wide applications in various fields especially biomedical research and incorporation in many products by various companies, their potential hazards to health and environments have become a source of concern. Another critical challenge is the enhancement of the antibacterial properties while maintaining their biocompatibility. A scientific approach to this is the development of new hetero-structured nanomaterials with novel properties using nontoxic reagents via environmentally benign synthetic routes. Integration of Ag-NPs into multi-walled carbon nanotubes (MWCNTs) will not only enhance their antibacterial efficacy, biocompatibility and durability, it will also reduce their bio-toxicity and their absorption into the body as well as excretion into the environment [10].

Carbon nanotubes (CNTs) have attracted much attention due to their remarkable electronic and mechanical properties, thermal stability and high aspect ratio [11, 12]. In order to enhance their chemical compatibility and dispersability for potential applications in biomedicine and biotechnology, extensive research has been focused on the surface modification of CNTs [13, 14]. Multiwalled carbon nanotubes (MWCNTs) are more commonly used because of their low cost compared with single-walled carbon nanotubes (SWCNTs). The bulk viscosity of CNT-based materials is high due to its long and tangled structure and this phenomenon will limit the application of MWCNTs. Therefore, in order to solve these problems, it is very important to obtain short and functionalized MWCNTs. Functionalization of the pristine MWCNTs will result in the presence of various functional groups like carboxylic acid groups (–COOH) and amine group (-NH₂) on the surface of MWCNTs, while the raw MWCNTs will be shortened and debundled simultaneously [15, 16]. Furthermore, the defects and surface area of functionalized MWCNTs increase and this makes it possible to couple other materials with them [17–19]. The unique properties of both MWCNTs and nanoparticles make them desirable to create novel MWCNTs/nanocrystal nanohybrids in order to develop novel properties due to interactions between the two materials. In line with this innovation, different approaches have been developed to prepare such nanohybrids [20–24]. But they are usually characterized by lengthy synthetic protocols, many complicated steps, degradation of MWCNTs and the use of environmentally hazardous reducing agents and complexants such as NaBH₄, formamide, hydroxylamine, dimethyloformamide, ammonia or hydrazine. These are not suitable for commercial scale synthesis and will increase environmental impacts during application. Several studies on the preparation, characterization, and antibacterial activities of carbon nanostructures combined with Ag nanoparticles have been reported recently. Li et al., synthesized Ag-NPs supported on MWCNTs by calcinations of the complexes of Ag cation and acid-treated MWCNTs under sparging N₂ with favorable stability and bactericidal properties [22]. Liu et al., reported Ag–Fe-decorated CNTs using DC hydrogen arc discharge method [23]. Akhavan et al., prepared Ag-NPs entrapped into MWCNTs arrays by treating MWCNT with AgNO₃ and ethanol and which showed antibacterial activity against Escherichia coli [24]. Seo et al., reported the integration of Ag-NPs into MWCNTs using ethyl alcohol as the reducing agent [10]. The as-synthesized nanoparticles at 30 μg/mL showed high biocompatibility with negligible cytotoxicity to mammalian liver cells. However, the reaction was carried out at small scale and involved the degradation of MWCNTs during the transformation of Ag ions to Ag-NPs. Furthermore, the use of ethyl alcohol as reducing agent raises a lot of biological and environmental concerns for large scale synthesis. As far as authors know, the catalytic property of these hybrid structures are not yet exploited.

In this paper, we report a simple, straight-forward, faster, green and large scale synthesis of Ag-MWCNTs nanocomposites. Dextrose a biocompatible and biodegradable monosaccharide sugar was used as the reducing agent and water the universal solvent as the solvent. No organic solvent, accelerator (NaOH) or complexant (NH₃) is required and the degradation of MWCNTs which usually occur during the transformation of Ag-ions to Ag-NPs is absent. Thus, the synthetic route is environmentally friendly for commercial purposes. The as-synthesized Ag-MWCNTs showed better antibacterial activity against E. coli than F-MWCNTs and Ag-NPs. The catalytic activity of the Ag-MWCNTs for the reduction of P-nitrophenol to P-aminophenol occurs within 120 s, and follow first order kinetics with a rate constant of 5.18×10⁻¹ s⁻¹. It is believed that the synthesis of such hybrid nanocomposites via a simple environmental friendly benign method will encourage its enormous applications in nanoscience and nanotechnology.

Experimental procedures

Results and discussion

In pristine MWCNT, the possibility to form complexes with other nanostructures is less due to the absence of functional groups. Hence, functionalization of MWCNTs with various functional groups offers opportunity to form complexes with different metal cations. The acid functionalization of MWCNT was confirmed by FTIR spectroscopy. The FTIR spectra of the pristine MWCNTs, purified MWCNTs and F-MWCNTs are shown in Fig. 1. The Pristine MWCNTs spectrum shows a peak centered at 1570 cm⁻¹, which can be assigned to the C=C stretching of carbon nanotube backbones [25]. The F-MWCNTs spectrum shows peaks at 1706 cm⁻¹ and 1260 cm⁻¹ which were characteristic absorption peaks for C=O and C–O stretching bands of carboxylic acid groups [26, 27]. In addition, free hydroxyl (–OH) absorption was observed between 3300 and 3500 cm⁻¹ in the F-MWCNTs spectrum indicating successful functionalization of the MWCNTs. The presence of carboxylic acid groups on the sidewalls of the MWCNTs is very important because these groups can act as anchor groups to couple with other materials.

Fig. 1:

FTIR spectra of pristine-MWCNTs (a), purified MWCNTs (b) and F-MWCNTs (c).

Ag-NPs were synthesized using gelatin as stabilizing agent due to its biocompatibility and biodegradability in addition to the presence of carboxyl and amine groups in its structure. During the reaction, the color of the Ag+/gelatin solution changes from colorless to yellow, indicating the formation of Ag-NPs. After the addition of F-MWCNTs into the Ag solution, Ag functionalized MWCNT (Ag-MWCNT) was obtained. The schematic diagram indicating the mechanism for the formation of Ag-MWCNTs nanocomposites is shown in Scheme 1. MWCNTs tend to aggregate because of the high aspect ratio and strong van der Waals attraction between the MWCNTs. This results in poor solubility in most solvents and thus limits its practical applications [28]. Functionalization of the surface improves the dispersion of MWCNTs. After refluxing in concentrated HNO₃ and H₂SO₄, –COOH are attached to the defect sites of the MWCNTs [29]. After the addition of F-MWCNTs to the gelatin capped Ag-NPs, the Ag-NPs interact with F-MWCNTs through the -NH₂ group present in the gelatin to form a covalent bond with the carboxyl group of the F-MWCNT. The FTIR spectrum of the Ag-MWCNT given in Fig. 2 clearly shows the covalent bond formed between F-MWCNT and Ag-NPs. A broad peak between 2965 and 3664 cm⁻¹ shows the presence of –CO–NH– bond formed between the amine group of gelatin and carboxyl group of F-MWCNT. The presence of amide linkage is again confirmed by the peaks at 1549 cm⁻¹ which is due to the –NH bend. A sharp peak at 1296 cm⁻¹ is due to the amide linkage caused by the –C–N stretch plus –NH in phase bending [30]. A sharp peak at 1025 cm⁻¹ may be due to the –C–O stretching vibration from gelatin. A significant peak at 1619 cm⁻¹ is attributed to the carbonyl group (–C=O) vibration. Two small peaks at 588 cm⁻¹ and 666 cm⁻¹ corresponds to the deformation vibrations of metal oxygen bond between Ag-O [31].

Scheme 1:

Schematic diagram indicating the mechanism for the formation of Ag-MWCNTs nanocomposites.

Fig. 2:

FTIR spectra of Ag-MWCNTs.

The X-ray diffraction patterns of the F-MWCNTs and the as-synthesized Ag-MWCNTs are shown in Fig. 3. The diffraction pattern for the F-MWCNTs in Fig. 3a shows two peaks at 2θ=25.8° and 42.7° assigned to the graphite crystalline phase of MWCNTs which corresponds to the (002) and (110) planes, respectively. Compared to the normal graphite peak at 2θ=26.5°, this peak shows a downward shift; which is attributed to an increase in the sp², C=C layers spacing [32]. The diffraction pattern for Ag-MWCNTs nanocomposite (Fig. 3b) shows four diffraction peaks at 2θ=38.48, 44.32, 64.2, and 77.64° corresponding to (111), (200), (220), and (311) crystalline planes of the face centered cubic (fcc) crystalline structure of metallic silver, respectively (JCPDS file No. 04-0783). In addition, the characteristic Bragg diffraction peak of MWCNTs which is at about 25.8° is also retained in the composite. The XRD results showed that the composites composed of Ag and MWCNTs and confirmed its crystallinity.

Fig. 3:

X-ray diffraction patterns of (a) F-MWCNTs and (b) Ag-MWCNTs.

Figure 4 shows the Raman spectra of MWCNTs, F-MWCNTs and Ag-MWCNTs. Three characteristic bands corresponding to the D band (defect), G band (graphite band) and G′ band (D overtone) of the MWCNTs were observed at 1324 cm⁻¹, 1574 cm⁻¹ and 2644 cm⁻¹, respectively (Fig. 4a). The D′ band which indicates the structural defect of the MWCNTs, F-MWCNTs and Ag-MWCNTs hybrid nanocomposites were found at 1601 cm⁻¹, 1608 cm⁻¹ and 1611 cm⁻¹, respectively (Fig. 4a–c). The shift in D′ band of the Ag-MWCNTs to higher wavenumber compared to the MWCNTs is attributed to both structural defects after functionalisation and broadening caused by modification with Ag-NPs [26]. The G′ peak of the Ag-MWCNTs hybrid nanocomposites shifted to higher wavenumber (2688 cm⁻¹) compared with MWCNTs and F-MWCNTs (2644 cm⁻¹ and 2680 cm⁻¹, respectively). The shift in the G′ peak is attributed to a substantial charge transfer interaction between the Ag-NPs and F-MWCNTs [33]. Zhang et al. [34] and Santangelo et al. [35] had shown that information related to the amount of structural defects and sp³ hybridized carbon atom and thus, the degree of side wall functionalization can be obtained from the relative intensity of the D and G bands (I_D/I_G). The degree of crystallinity and the scale on which the graphitic order extends can be understood from the relative intensity of the G′ and D bands (I_G′/I_D) and relative intensity of the G′ and G bands (I_G′/I_G), respectively [35]. The relative intensities of the D/G, G′/D and G′/G ratios are presented in Fig. 5. The D/G ratios are 1.176, 1.233, and 1.499 for the MWCNTs, F-MWCNTs, and Ag-MWCNTs hybrid nanocomposites, respectively. The increase in the D/G ratio of F-MWCNTs compared with that of the MWCNTs confirms successful introduction of functional groups on the MWCNTs surface and also indicated that, the outer layers of the MWCNTs were chemically modified [34]. An increase in the D/G ratio of Ag-MWCNTs hybrid nanocomposites when compare with that of F-MWCNTs can be attributed to the introduction of Ag nanoparticles on the surface of the F-MWCNTs. The G′/G ratios are 0.78, 0.76, and 0.48 for MWCNTs, F-MWCNTs, and Ag-MWCNTs hybrid nanocomposites, respectively, while the G′/D ratios are 0.66, 0.62, and 0.32 for the MWCNTs, F-MWCNTs and Ag-MWCNTs hybrid nanocomposites, respectively. The decrease in the G′/G and G′/D ratios of the F-MWCNTs compared with those of the MWCNTs has been attributed to the presence of higher density of lattice defects in the F-MWCNTs [35]. The intensity of the G′ peak of the Ag-MWCNTs hybrid nanocomposites increases due to the increase in mass fraction of MWCNTs in the hybrid material. The G′ peak arises from a two-phonon process, and the increase in G′ intensity shows the sample becoming more ordered by, not interfering on the coupling effect that is required for the two-phonon process. This effect is different compared to the earlier reports which shows normal decrease in G′ after its surface functionalization [36]. The G′/G and G′/D ratios of the Ag-MWCNTs hybrid nanocomposites showed the lowest values because of the formation of structural defects due to the interaction between the Ag-NPs and the MWCNTs.

Fig. 4:

Raman spectra of MWCNT, F-MWCNTs and Ag-MWCNTs nanocomposite.

Fig. 5:

Relative intensity ratios of D/G, G′/G, and G′/D peaks of MWCNTs, F-MWCNTs and Ag-MWCNTs hybrid nanocomposites.

The electron microscope images of the Ag-NPs and Ag-MWCNTs are shown in Fig. 6. The TEM image of the Ag-NPs (Fig. 6a) indicates that the particles are small, spherical and well dispersed with an average particles size of 9.88±1.23 nm. The TEM image of Ag-MWCNTs hybrid nanocomposites shown in Fig. 6b clearly shows the presence of Ag-NPs grafted on the surface of F-MWCNTs. Ag-NPs are homogeneously dispersed with no agglomeration and are strongly adhered to the surfaces of the F-MWCNTs. The aveage diameter of the Ag-NPs grafted onto the surface of the F-MWCNTs was found to be 9.68±1.44. The diameter and aspect ratio of the F-MWCNTs was about 32.9±1.37 nm and 6 nm, respectively. Figure 6c shows the high resolution image of a single silver nanoparticle on the surface of the MWCNTs with inset showing the SAED image of the Ag-MWCNT nanocomposite. The SAED diffraction pattern [Fig. 6c (inset)] confirmed the crystallinity of the Ag-MWCNTs nanocomposite. During the functionalization process, some Ag-NPs were also formed inside the F-MWCNTs (Fig. 6d) and the size of the Ag-NPs were 4.68±1.22 nm.

Fig. 6:

TEM images of (a) Ag-NPs (b) Ag-MWCNTs (c) aligned Ag-NPs on MWCNTs and (d) single Ag-NP on the MWCNTs surface (inset: SAED image of Ag-MWCNTs).

Antibacterial activity

The antibacterial activities of Ag-MWCNTs nanocomposites were assessed with the paper-disk diffusion method. Ten μg/mL of the samples were used to test the antibacterial activity As shown in Fig. 7, Ag-MWCNTs exhibit significant inhibitory effects on E. coli and P. aregunosa. For comparison, antibacterial activities of Ag-NPs, and F-MWCNTs were studied and the results of inhibition zones are given in Table 1. The results show that both F-MWCNTs and Ag-NPs show lower activities towards E. coli than Ag-MWCNTs while in the case of P. aregunosa, the Ag-MWCNTs is the least effective. Compared to the previously reported antibacterial actvities which showed lesser inhibition zone even at higher concentration of 50 μg/mL [37, 38], this work gave high inhibition zone at 10 μg/mL indicating the efficiency of our synthetic method in producing highly active hybrid nanocomposite. The inhibitory result indicates that Ag-MWCNTs could hinder the growth of bacteria by direct contact, in which the cell membrane was ruptured and surface charge interactions between Ag-MWCNTs and the bacteria were initiated [10]. The higher antibacterial activity of Ag-MWCNTs compared to that of F-MWCNTs and Ag-NPs individually is attributed to the synergic effect of MWCNTs and Ag-NPs in the nanocomposite.

Fig. 7:

Results of the antibacterial test carried out using paper-disk diffusion method against (a) Gram-positive Pseudomonas and (b) Gram-negative E. coli bacteria.

Table 1

Average diameter of inhibition zone for functionalised CNT, Ag-MWCNT and silver nanoparticles against E. coli and P. aeruginosa.

Sl. no.	Sample	Diameter zone of inhibition (mm)
		E. coli (ATCC 25922)	P. aeruginosa (ATCC 27853)
1	MWCNTs COOH	11	16
2	Ag-MWCNTs	15	10
3	Spherical Ag-NPs	14	11

Catalytic activity

Catalytic activities of Ag-MWCNTs for the conversion of p-nitrophenol (p-NP) to p-aminophenol (p-AP) in the presence of NaBH₄ were studied using the UV-Vis absorption spectrophotometer. Figure 8(a) shows the absorption spectra for the reduction of P-nitophenol at different reaction time. In the absence of Ag-MWCNTs nanocomposites, the solution containing p-NP and NaBH₄ shows absorption band maximum at ~400 nm. This band correspond to the p-nitrophenolate ion under alkaline condition. After addition of a small amount of Ag-MWCNTs nanocomposite to the above reaction mixture under continous stiring, the color of the solution changes gradually from yellow to colorless within 120 s. A time-dependent UV-Vis spectra of this reaction shows gradual reduction in the absorbance of the peak at 400 nm accompanied by a gradual development of a new peak at 270 nm. The new band at 270 nm is attributed to the formation of p-AP.

Fig. 8:

(a) UV-Vis spectra of p-nitrophenol at different time interval during its reduction by NaBH₄ in presence of Ag-MWCNTs as catalyst (b) plots of ‘ln At ’ vs. time ‘t’ (s) for the reduction of p-nitrophenol to paminophenol by NaBH₄ in presence of Ag-MWCNTs.

The catalytic conversion of p-NP to p-AP follows the first order kinetics. In this reaction, the reaction rate constant has been evaluated by plotting the ‘ln At’ versus time ‘t’; where At is absorbance at any time, ‘t’ and the calculated value is 5.18×10⁻¹ s⁻¹. The approximate linear relations of ln At versus ‘t’ observed for all the reaction times as shown in Fig. 8(b), support the first-order kinetics. We proposed that hydrogen from sodium borobydride is transferred to the Ag-MWCNTs by the decomposition of borohydride and this hydride species then reacts with p-NP to yield the product p-AP [39]. The Ag-MWCNTs here act as a template for the transfer of H⁺ from NaBH₄ and initiate the reduction process. The Langmuir–Hinshelwood mechanism can be used to model the kinetics of the reaction where both reactants need to be adsorbed on the surface prior to reaction [40]. The mechanism is schematically displayed in Scheme 2. The rate-determining step is governed by the reaction of the adsorbed species. The adsorption/desorption equilibrium is assumed to be much faster and is modeled in terms of a Langmuir isotherm. The rate of reduction is independent of NaBH₄ concentration because when this reagent was used in excess compared to p-NP, there was no change in the absorption spectra. The isosbestic points in the UV-vis spectra demonstrate with high precision that p-NP is converted to p-AP with no side reaction.

Scheme 2:

Mechanistic model of the Langmuir–Hinshelwood mechanism for the reduction of p-NP to p-AP by sodium borohydride in presence of Ag-MWCNTs nanoparticles.

Conclusion

We have successfully synthesized Ag-MWCNTs nanocomposite via a simple, straightforward and facile method without degradation of the MWCNTs. The method involves the use of biodegradable and biocompatible reagents and is suitable for large scale synthesis without additional chemicals, complexant, irradiations, ultrasound, optical and electric treatments. The surface chemistry investigated using FTIR and Raman spectroscopy analyses confirmed the formation of Ag-MWCNTs. The as-synthesized Ag-NPs were homogenously dispersed in the MWCNTs while the XRD and TEM confirmed the crystalline nature of the Ag-MWCNTs. The Ag-MWCNT nanocomposites show high antibacterial activity against E. coli compared to the Ag-NPs and MWCNTs. The catalytic activity of the hybrid nanocomposites demonstrated by studying its effect in the conversion of p-nitrophenol to p-aminophenol (p-AP) in presence of NaBH₄ follows first order kinetics with a rate constant of 5.18×10⁻¹ s⁻¹. By this environmental friendly technique, Ag-MWCNTs nanocomposites with enhanced antibacterial efficacy, durability and biocompatibility as well as reduced biotoxicity and environmental impacts can be easily synthesized on a large scale. This will further improve its practical applications in the areas of biomedical research, catalysis, environmental engineering, and water purification.