Preferential killing of bacterial cells by hybrid carbon nanotube-MnO2 nanocomposite synthesized by novel microwave assisted processing

Accepted Manuscript Preferential killing of bacterial cells by hybrid carbon nanotubeMnO2 nanocomposite synthesized by novel microwave assisted processing

D. Sivaraj, K. Vijayalakshmi PII: DOI: Reference:

S0928-4931(17)32087-8 doi: 10.1016/j.msec.2017.08.027 MSC 8232

To appear in:

Materials Science & Engineering C

Received date: Revised date: Accepted date:

2 June 2017 10 July 2017 10 August 2017

Please cite this article as: D. Sivaraj, K. Vijayalakshmi , Preferential killing of bacterial cells by hybrid carbon nanotube-MnO2 nanocomposite synthesized by novel microwave assisted processing, Materials Science & Engineering C (2017), doi: 10.1016/ j.msec.2017.08.027

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ACCEPTED MANUSCRIPT Preferential killing of bacterial cells by hybrid carbon nanotube- MnO2 nanocomposite synthesized by novel microwave assisted processing D. Sivaraj and K. Vijayalakshmi* Research Department of Physics, Bishop Heber College, Tiruchirappalli, Tamilnadu620 017, India.

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Abstract

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We present a comprehensive study on the enhanced effect of CNT addition on the

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structural, optical, morphological and antibacterial properties of CNT-MnO2

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nanocomposite synthesized by microwave assisted processing. X-ray diffraction pattern of the hybrid CNT/MnO2 nanocomposite revealed the shifting of highly

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oriented MnO2preferential planes. SEM images show porous MnO2nanospheres

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uniformly and discretely attached on the walls of carbon nanotube network.

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Significant variation in the peak shape and IR absorption intensity with CNT addition, indicate the interaction of CNT ions with MnO2 nanoparticles. Optical

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studies reveal a decrease in energy band gap caused by a significative reduction of

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electron-hole recombination in manganese activated CNT nanocomposite favouring photocatalytic activity. The antibacterial activity of pure MnO2 and the CNT/MnO2hybrid was investigated by using both Gram positive and Gram negative bacteria as test organisms. The hybrid nanocomposite revealed higher antibacterial activity compared with pure MnO2 nanoparticles, due to reduced particle size and high specific surface area of CNT. Also, compared to Gram positive bacteria, Gram negative bacteria shows enhanced antibacterial activity due to the improved particle 1

ACCEPTED MANUSCRIPT surface reactivity to visible light through effective charge transfer between MnO2 and CNT content of the nanopowder synthesized through microwave processing. -----------------------------------------------------------------------------------------------------------*Corresponding author. Tel: +91 9994647287(mobile). E-mail address: [email protected] (K. Vijayalakshmi).

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Owing to the high surface area, large inner volume and other important physico-

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interestingly be studied in the near future.

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chemical properties, the antibacterial activity of carbon-based nanocomposites may

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ACCEPTED MANUSCRIPT 1. Introduction Bacterial contamination in public can result in human’s serious illness and even death under poor air circulation, since it can get into the air very easily. In present days, the major health issues are abdominal cramps, nausea, vomiting, diarrhoea and

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fever caused by both Gram-positive and Gram-negative bacterial strains. Hence, there is a persistent demand for next-generation anti-infective materials for

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shielding wounds, contact lenses and medical devices against microbial

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contaminations [1, 2]. Polymers and peptides are extensively used as antibacterial

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materials. However, there are limitations connected with both peptides and

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polymers such as molecular weight, hydrophobicity, scale up issues, enzyme susceptibility and unknown pharmacokinetics [3-8]. Hence, development of

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environmentally benign nano sized functional materials with high antimicrobial

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properties has been the subject of interest by many researchers [9]. Nanosized particles exhibit high antimicrobial activity due to their exclusive physical and

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chemical properties. Nanomaterials has been paying attention in molecular tagging,

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drug delivery, food technology, biosensors, optogenetics, bioimaging, textile manufacturing, antimicrobial coatings, energy and environment, etc, due to its small dimension and exceptional surface properties[10, 11]. Nanomaterials are generally used in gene and drug delivery, because it can kill or reduce the activity of numerous microorganisms within minutes of contact [12]. As the surface area of nanomaterials is exceedingly high relative to their size it may be deliberately beneficial as active antibacterial groups [13]. Small dose of nanosized particles provide high 3

ACCEPTED MANUSCRIPT antibacterial activity. Researchers have reported the combination of hydrogels and nanoparticles, functional inorganic nanomaterial, upconversion nanoparticles, magnetic Janus particles for biosensors, therapy and drug delivery [14-16]. Xian Jun Loh et al and Z. Li et al investigated the properties of hybrid material and functional inorganic nanomaterial for drug delivery and gene delivery [17,18]. The combination

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of hybrid materials and inorganic materials are upcoming materials for delivery,

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therapy, sensors, etc. Inorganic metal oxides nanoparticles are used as antimicrobial

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agents because of safety, robustness, re-usability, long shelf life and stability under

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high temperature and pressure [19]. The application of metal oxides as catalysts has attracted more attention due to their high catalytic activity and improved selectivity

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[20,21]. In particular, metal-oxide nanocrystals are of practical interest for technological applications owing to their unique size-dependent properties and

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exceptional process capability. Researchers have reported the use of colloidal and supported transition metal nanoparticles as catalysts for a diversity of organic and

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inorganic reactions [22, 23]. To date, much effort has been committed to the latter using transition-metal oxides including MnO2, Co3O4, NiO, V2O5, and CuO , because

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they bear a much higher specific capacitance and energy density [24-28]. Among various transition metal oxide nanostructures, manganese dioxide (MnO2) has been paying attention because of its low cost, low band gap, environmental compatibility and abundant availability nature. Moreover, they have a variety of oxidation states so as to allow the efficient redox charge transfer. Manganese oxide can exist in α, β,  and  structure which differ in the way of MnO6 octahedral. They have interlayers 4

ACCEPTED MANUSCRIPT with gaps of different magnitudes with large surface area [29, 30]. Different rearrangement of MnO6 octahedral gives rise to manganese oxide with tunnel and layered structure, which exhibit cation exchange and molecule adsorptive, like zerolitrs and clay minerals. Nanostructured MnO2 with different physical and chemical properties, such as crystallinity makes the MnO2 material unique.

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Generally, the antimicrobial activity of the metal oxide nanoparticles depend on their

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surface modification, intrinsic properties, and the type of microorganism [31, 32].

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The antibacterial activity of MnO2 can be improved by modifying the surface area,

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surface defect and ultraviolet (UV) absorption. Doping by non-metal elements, such as nitrogen, sulfur, and also carbon increase the surface defects leading to higher

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biological activities and also affects the optical and electronic properties. Carbonbased nanoparticles exhibit enhanced antimicrobial activity due to the high oxidative

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stress. Carbon nanotubes (CNTs) have incited significant attention for their potential application as the antimicrobial materials in the recent years [33]. P. LináChee et al

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reported that the multifunctional carbon dots showed excellent antibacterial activity [34]. Additionally, CNTs used as drug delivery vehicles have shown potential in

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targeting specific cancer cells CNTs exhibited high bacterial adsorption capacity and the microbial adsorption capacities [35]. It has crucial effect on the antimicrobial activity, because of the direct contact between CNTs and target pathogens [36]. There is some evidence in the literature that the combination between bacterial cells and carbon nanomaterials cause direct contact between the cells and carbon nanomaterials which in turn lead to cell death [37-39]. The nanosized level of 5

ACCEPTED MANUSCRIPT inorganic MnO2 and bioactive CNT can be combined into a single material, which could create entirely new advanced compositions with truly unique properties for drug delivery. Moreover, hybrid CNTs act as nanovehicle to deliver the drug in specific cells. CNTs possessing large surface area provide more landing site for MnO2 for active sites, and thereby improve the antibacterial activity. Many studies

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reported the antibacterial activity of CNTs and MnO2 separately but there are no

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reports on CNT/MnO2 nanocomposites for improved antibacterial activity. The

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hybrid nanostructured manganesedioxide/CNT composite is belived to provide

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superior performance, exhibiting synergistic effect between the metal oxide and CNT phase. The main role of CNT is to stabilize highly dispersed metal oxide/ metal

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nanoclusters, which may result in a high specific area. However, in some case, hybrid composite show improved properties other than the specific area, such as higher

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electron conductivity due to the presence of CNTs. Various methods have been used to synthesis manganese dioxide, such as self-

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reacting microemulsion, chemical co-precipitation, room-temperature solid reaction,

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sonochemical, thermal decomposition, sol-gel method, microwave assisted method, colloidal method and hydrothermal methods [40-50]. In the present work, we report a novel microwave assisted method for the synthesis of hybrid MnO2/CNT nanocomposite, because it provides exact parameter control in a short time, efficient and less hazardous energy source. Because of molecular homogeneity, the magnitude of reaction time is reduced by several orders. Also, the microwave heating takes place mainly due to ionic conduction and dipolar polarization. Compared to 6

ACCEPTED MANUSCRIPT conventional methods, microwave heating results in less energy consumption, high product yield, higher uniformity in product and better properties. To the best of our knowledge, the utilization of microwave processing for the synthesis of MnO2 nanocomposite has not been investigated yet.

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Hence, in the present work, pure and hybrid CNT activatedMnO2 nanoparticles were synthesized by microwave assisted method. The synthesised nanoparticles were

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characterized using XRD, FTIR, UV-vis, FESEM, and EDAX techniques. The

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antibacterial activity of pure and nano composited CNT/MnO2 were studied using

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well diffusion method, the degree of inhibition of Gram positive and Gram negative bacteria treated with pure and CNT doped MnO2 nanoparticles were compared for

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better antibacterial activity.

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2. 1 Experimental Procedure

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All chemical reagents were of analytical grade purchased from Aldric and used

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without any purification. The functionalized MWCNT samples were purchased commercially (purity>90%, aldrich). Firstly,manganese dioxide nanoparticles were

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prepared using 0.l M concentration of KMnO4 (purity>90%, aldrich) in deionised water. The reaction was performed under vigorous stirring for 2 hr. The resulting aqueous solution was then taken in an alumina crucible and subjected to microwave irradiation for 5 min at a power of 500 W in a microwave accelerated reaction system in the temperature controlled mode. The microwave synthesis instrument (Model: MC8088NRH) could be operated under both temperature-controlled and pressure controlled mode. The system uses non-pulsed microwave to heat the 7

ACCEPTED MANUSCRIPT suspension continuously. The power could be varied by automatic frequency conversion with the variation of temperature. The system can be operated with the maximum power and frequency of 1350 W and 2450 MHz, respectively, and can be controlled continuously at different power ranges, according to the selected temperature. In a similar procedure, the hybrid MnO2/CNT nanopowder was

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prepared. The CNTs were multi–walled carbon nanotube (MWCNT) powders of 95%

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purity with outer diameter of 10–30 nm and length of <5–15 mm. 100 mg of CNT

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was treated with nitric acid at 110◦C for 3 h in order to remove iron catalyst from

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CNT growth and to improve the wetting ability of the CNT surface. The CNT–nitric acid solution was added dropwise to the alkoxide–ethanol solution and stirred for 2

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h. The resulting solution was added dropwise into the MnO2 precursor and stirred for 1 h. The suspension was subjected to microwave irradiation for 5 min at the same

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power of 500 W in the oven in the temperature controlled mode. Finally, the solution was filtered and washed with deionised water and acetone several times to remove

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the residual KMnO4. The final colloidal solution was filtered, to collect the powder product. The resultant powder samples were annealed for 2 h in a muffle furnace at

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500 C in air atmosphere to remove the organic by products of the microwave generated chemical reactions.Fig. 1 shows the schematic diagram of the preparation process of pure MnO2 and hybrid CNT-MnO2 nanocomposite.

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Fig. 1 Schematic of the experimental set up used for the preparation of pure MnO2 and hybrid CNT- MnO2 nanocomposite by microwave processing. The synthesised nano powder samples were further investigated for its structural, compositional, vibrational, optical and antibacterial activity. The structural analysis was made using X pert Pro X-ray diffractometer (XRD). Vibrational spectral analysis of the nanoparticles was done using Fourier transform infrared (FTIR) spectrophotometer. The optical characterization of the nanoparticles was made 9

ACCEPTED MANUSCRIPT using ultra violet (UV)-visible spectrophotometer. The scanning electron micrographs of the samples were recorded using a field emission scanning electron microscope (FESEM). 2. 2 Bacterial sensibility assay of pure MnO2 and CNT- MnO2 nanocomposite

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Antibacterial activity of the synthesized nanoparticles was determined using the disc diffusion method. In this technique, 6.0 mm diameter discs were impregnated with

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nanoparticles (0–500 μg/disc) followed by sterilization. The media plates were

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streaked with bacteria for 2–3 times by rotating the plate at 60 angles for each

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streak to ensure homogeneous distribution of inoculum. Overnight grown 0.1 mL

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culture (inoculum) of Bacillus subtilis, Staphylococcus aureus, Proteus Mirabilis and Escherichia coli were then spread on Muller–Hinton agar plates separately. After

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inoculation, nanoparticle impregnated discs were placed over them followed by

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incubation at room temperature for 24 h. The bacterial growth, and variation in zone of inhibition for nano-sized MnO2 and the hybrid CNT-MnO2 nanocomposite showing

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antibacterial activity against test pathogens were recorded. The zone of inhibition on

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the agar surface was measured to determine the sensitivity of the bacterial species to the synthesized MnO2 and CNT- MnO2 nanocomposite.

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ACCEPTED MANUSCRIPT 3. Results and Discussion 3.1 Variation in crystallite size and phase of pure MnO2 and CNT- MnO2 nanocomposite Fig. 2 shows the XRD patterns of the pure MnO2 and hybrid CNT-MnO2

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nanocomposite prepared by microwave assisted method. The diffraction peaks for

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the as-formed MnO2 samples are at 2θ = 13.76°, 18.23°, 25.92°, 29.20°, 37.52°,

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39.95°, 41.47°, 50.05°, 56.05° and 59.77°, which matches with the reflection planes of (110), (200), (310), (211), (420), (310), (411), (600), and (521), respectively. The

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sharp and narrow diffraction peaks indicate that the pure MnO2 nanoparticles are

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well crystallized. All the diffraction peaks of the nanopowder can be indexed to a pure tetragonal phase of α- MnO2 (JCPDS Card, no. 53-0633). It can be seen that the

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(211) peaks are stronger than other peaks, indicating that α- MnO2 nanoparticles

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exhibit an orthorhombic structure with preferred orientation.

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Fig. 2 XRD patterns of pure MnO2 and hybrid CNT- MnO2 nanocomposite

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prepared by microwave processing. In CNT/ MnO2 nanocomposite, all the peaks are consistent with those in pure MnO2

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nanoparticles, except for the peak at 26 due to CNTs. The 2θ peak centred at 26.0°

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correspond to the (002) reflections of the graphite from MWCNTs, indicating the successful addition of MnO2 on nanotube surface [51]. The broad diffraction peaks observed for CNT-MnO2 nanocomposite could be attributed to various reasons, namely, small particle sizes together with strain induced due to defects in the hybrid nanomaterials [52]. It is also observed that the peak position of MnO2 (101) diffraction shifts to lower angles after CNT addition, which may result in the alteration of lattice parameters, due to the successful impregnation of MnO2 on CNT 12

ACCEPTED MANUSCRIPT lattice. No diffraction lines associated with impurities were detected. The crystallite size (D) was calculated from Scherrer formula [53]. 

D = 0.9/cos

where is the wavelength of X-ray source, is full-width at half maximum (FWHM)

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in radians and is Bragg’s diffraction angle. The average crystallite size of pure MnO2

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was found to be 40–60 nm, which decreased significantly to 30–40 nm for CNT–

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MnO2 nanocomposite, dependent on the increasing broadening of peaks with CNT addition. The decrease in crystallite size of MnO2 with CNT addition can be attributed

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to atomic diffusion caused during the stepwise relocation of atoms from one lattice

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site to another lattice site. This is due to the fact that the atoms are in continual motion in a solid material with quickly varying positions. However, the atom should

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have adequate energy to make such movement and to break bonds with its

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neighbour atoms, which inturn results in some lattice alteration during the displacement [54]. The smaller crystallite size obtained in the present study for the

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hybrid MnO2/CNT nanocomposite is feasible because of microwave processing

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which accelerate the hydrolysis process, leading to the formation of small individual particles. The small size of the particles in the nanocomposite may exhibit different chemical or biological features, as they posses more differential toxicity profiles than normal crystals. The size of the particles and hence the surface area play major role in the antibacterial activity of MnO2/CNT nanocomposite [55].

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ACCEPTED MANUSCRIPT 3. 2 Variation in morphology of pure MnO2 and CNT/MnO2 nanocomposite

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The morphologies of the MnO2 and hybrid CNT/MnO2 nanocomposite were

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characterized by FESEM, as shown in Fig. 3 (a) and (b). SEM images revealed

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different morphology for pure MnO2 and the hybrid CNT/MnO2 nanocomposite because of the stepwise diffusion of functionalized CNTs on MnO2 atoms. The oxygen containing carboxyl or hydroxyl groups on the walls of the CNTs, acts as nucleation sites for the growth of MnO2 on CNTs [56]. Fig. 3 FESEM images and EDX spectra of (a) pure MnO2 and (b) hybrid CNTMnO2 nanocomposite.

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ACCEPTED MANUSCRIPT The metal ions adsorbed on these sites were further oxidised by KMnO4 upon microwave irradiation.

During the microwave

synthesis

of CNT/ MnO2

nanocomposite, CNTs served not only as support but also as reducer and hence, no additional reducer was needed. Such preparation technology meets the demand of green preparation and green chemistry and it is easy to be scaled up. The

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nanocomposite samples are rather heterogeneous, consisting of small MnO2 in

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carbon nanotubes network, however, the single MnO2 components are intimately

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mixed together. Indeed, at a closer examination, it appears that nanotubes are

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partially covered by MnO2 grafted on nanotubes wall, resulting in a strong interfacial adhesion and a better dispersion of surface modified CNT in the metal oxide matrix.

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The MnO2 nanospheres formed a uniform layer on the surface of CNTs and simultaneously the length of the CNTs became short due to its oxidation by KMnO4

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upon microwave irradiation. The nanospherical MnO2 wrap around the nanotube giving rise only to a very weak Van deer Waals interaction without chemical bonds.

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Thus, microwave processing converts the uniform MnO2 nanoparticles in to porous MnO2 nanospheres, threaded with CNT networks [57]. These porous MnO2 with finer

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nanostructure provides larger surface area for CNT/ MnO2 nanocomposite, enabling active site accessibility for higher photocatalytic efficiencies and antibacterial activity. The chemical composition analysis of the calcined pure MnO2 and the hybrid CNTMnO2 nanocomposite were made by EDX (chemical microanalysis technique) to know the concentration of dopant and defects in the sample.Fig. 3 (a) and (b) shows 15

ACCEPTED MANUSCRIPT the EDX spectra of pure and the hybrid CNT-MnO2 nanopowders synthesized by microwave processing. The chemical composition reveal Mn and O element alone present in the pristine MnO2 nanopowder, which confirms the absence of any other impurities in the synthesizednanoparticle. EDX spectra of CNT-MnO2 nanocomposite shows the presence of Mn, O and C element in the nanopowder, thus conforming the

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successful impingement of MnO2 atoms on the walls of CNTs. The absence of other

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elements in the EDX spectrum reveals that the transmittance play a great role for the

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grain boundaries, since the CNT-MnO2 nanocomposite shows small grain size.

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3. 3 Variation in optical absorption and bandgap characteristics of pure MnO 2

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and CNT-MnO2 nanocomposite

Optical absorbance characteristics of the nanoparticles were investigated. Fig. 4 (a).

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shows the optical absorption spectra of pure MnO2 and the hybrid CNT activated

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MnO2 nanoparticles recorded in the wavelength region 350–800 nm. The visible light absorption wavelength around 556 nm and 575 nm were recorded for the pure

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MnO2 and CNT-MnO2 nanocomposite, respectively. The light absorption edges of

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hybrid CNT-MnO2 show a small shift towards higher wavelength side in the visible range compared to pure MnO2. The presumable shift in the optical absorption towards visible region after CNT impregnation favour antibacterial activity. Using the optical spectra, the band gap energy (Eg) was evaluated from the equation [58].  = A(h ν -Eg)n/hν

-------------------(2)

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ACCEPTED MANUSCRIPT where  is the absorption coefficient, A is a constant, h Planck’s constant, n the photon frequency, Eg the energy band gap and n is 2 for the indirect band gap semiconductor. The indirect allowed transition dominates in the optical absorption of MnO2 just above the absorption edge due to weak strength of the direct forbidden

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transition. The band gap energy (Eg) was estimated by Tauc plot of h versus (h)2.

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Fig. 4 (a) Optical absorption and (b) transmission spectra of pure MnO2 and hybrid CNT- MnO2 nanocomposite prepared by microwave processing.

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The inset shows the energy band gap of pure MnO2 and hybrid CNT- MnO2

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nanocomposite.

Fig. 4 (a) inset shows the plot of h versus (h)2 for pure MnO2 and CNT activated MnO2 nanocomposite. The direct band gap obtained in this study for MnO2 was 2.43 eV, and it slightly decreased to 2.21 eV after CNT addition, suggesting that, CNT ions in the valance band work as defect sites to decrease the band gap. Introducing CNT 17

ACCEPTED MANUSCRIPT into MnO2 lattice to produce additional electronic states in the oxide energy band gap cause changes in optical transitions and a red-shift in absorption spectrum [59]. Generally, three significant approaches are realized for band structure modification viz. doping with metallic/non-metallic elements, introducing defects such as oxygen vacancies and surface modification [60,61]. Thus, the renovation of

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oxygen vacancies results in a modification in band gap with a progressive carbon

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metallization of manganese dioxide. Hence, the decrease in the band gap can be

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attributed to formation of carbonaceous, Mn2+, Mn–C and Mn–O– C bonds in the CNT

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activated MnO2 nanocomposite. Also, the differences in band gap changes in CNTs/ MnO2 sample may be caused by the difference in the size of MnO2 grains.

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Nevertheless, these results show that the hybrid CNT- MnO2 nanocomposite can well

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absorb the visible light irradiation for antibacterial activity.

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Fig. 4 (b) shows the optical transmission spectra of pure MnO2 and CNT activated MnO2 nancomposite synthesised by microwave processing.Pure MnO2 and the hybrid

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CNT/MnO2 nanocomposite samples exhibit good transmittance in the visible region.

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A high transmittance of about 60% was obtained for pure MnO2 nanoparticles, however, it decreased to 50% after CNT addition. The decrease in transmittance may be attributed to the increased scattering of photons by crystal defects created due to CNT addition. The scattering resulted from the existence of grain boundaries, the point defect and dispersal on the surface [62].

3.4 Variation in vibrational bands of pure MnO2 and CNT-MnO2 nanocomposite 18

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Fourier transform infrared (FT-IR) spectroscopy is known for its high sensitivity,

especially in detecting inorganic and organic species with low content. Fourier

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transform-infrared (FT-IR) spectra were recorded in the range of 400–4000 cm−1 is presented in Fig. 5. The strong absorption bands observed in the range of 400–950

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cm-1 is assigned to the O–Mn–O bonding which indicates the formation of manganese dioxide. The band in the range 1400-1600 cm−1 corresponds to C=C stretching. The

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peak observed at 1578 cm-1 can be assigned to the carbonyl group from quinine or ring structure of MnO2, which exhibit significant shift to 1564 cm-1 in CNT-MnO2 nanocomposite. Fig. 5 IR spectra of (a) pure MnO2 and (b) hybrid CNT- MnO2 nanocomposite prepared by microwave processing.

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ACCEPTED MANUSCRIPT This shift indicates that considerable interaction exists between MnO2 and functionalized CNTs, which may be particularly useful for antibacterial activity. The absorption peak around 1627 cm-1 in CNT-MnO2 nanocomposite is associated with the stretching of the C=C bonds. The band in the range 2926–2968 cm−1 is attributed to the C–H stretching vibrations. The composites show the peaks in the region of

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3100 cm-1 which is due to the C–C bond stretching frequencies. The broad absorption

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band around 3400 cm-1 corresponds to the surface absorbed water [63]. Compared

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with pure MnO2, the spectrum of MWCNTs/ MnO2 composite show new absorption

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peak occurring which corresponds to CNT. In addition, the changes in peak shape and absorption intensity are observed, which may be due to the spectra overlapping

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and the interaction between functionalized MWCNTs and MnO2. This gives a clear evidence that MnO2 nanoparticles are attached to the CNTs. L. Fu et al [64] have

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reported that spectral overlapping may change in peak shape and absorption intensity.

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nanocomposite

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3. 5 Variation in antibacterial activity of pure MnO2 and CNT/MnO2

The mechanism of reactive oxygen species (ROS) generation on the surface of pure MnO2 and the hybrid CNT-MnO2 nanocomposite is shown in the schematic of Fig. 6.

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Fig. 6 The mechanism of reactive oxygen species generation in hybrid CNT-

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MnO2 nanocomposite.

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When a photon of higher energy level or equal to the band gap energy is absorbed by a MnO2 catalyst, photocatalytic oxidation reactions are initiated promoting an

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electron (e‾) from the valence band to the conduction band with simultaneous

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generation of a positive hole (h+) in the valence band. The holes split H2O molecules into hydroxyl radical (OH•) and hydrogen ion (H+). The oxygen molecules react with H+ to generates HO2•, and react with hydrogen ions to produce molecules of H2O2. The generated H2O2can penetrate the cell membrane and kill the bacteria. The mechanism of reactive species generation in MnO2 can be explained as follows [65] MnO2 + hν → MnO2 (e‾ + h+)

-------------------------(3)

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ACCEPTED MANUSCRIPT h+(VB) + H2O→ OH•+H+

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e‾(CB) + O2 → O2•‾

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O2‾+ H+→

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HO2•

HO2• + H+→ H2O2

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ACCEPTED MANUSCRIPT Fig. 7 Antimicrobial activity of pure MnO2 and hybrid CNT- MnO2 nanocomposite against (a) Bacillus subtilis,(b) Staphylococcus aureus, (c) Proteus Mirabilis and (d) Escherichia coli The antibacterial activity of synthesized MnO2 and nanocomposited CNT/MnO2

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suspensions was studied on four clinically isolated strains namely, Bacillus subtilis, Staphylococcus aureus, Proteus Mirabilis and Escherichia coli by the disc diffusion

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agar methods. The advanced oxidation processes appear to be a good method for the

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destruction of the toxic pollutants into nontoxic substances [66]. It is believed that

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the metal oxides carry the positive charge while the microorganism carry negative charge, hence, electromagnetic attraction takes place between microorganism and

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the metal oxides, which in turn, leads to oxidization and finally death of

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microorganism. Fig. 7 shows the zone of inhibition of pure MnO2 and CNT/MnO2

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nanoparticles synthesised by microwave assisted processing. The images show clear inhibition zone which indicates that the mechanism of the biocidal action of pure

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MnO2 and CNT/MnO2 nanocomposite involves disruption of the membrane with high

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rate of generation of surface oxygen species and finally lead to the death of pathogens. Interestingly, the size of the inhibition zone is different for MnO2 and CNT/MnO2 nanoparticles according to the type of pathogens.

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Zone of inhibition (mm) Bacteria CNT- MnO2

Nil

7-9

Nil

7-10

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Bacillus subtilis

MnO2

Staphylococcus

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(G+)

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(G-)

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Proteus Mirabilis

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aureus (G+)

E. Coli (G-)

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Table. 1 The measured inhibition level of pure MnO2 and the hybrid CNT/MnO2

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nanocomposite against G+ and G- bacteria

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The zone of inhibition level measured from the disc for pure MnO2and CNTMnO2 nanocomposite are reported in Table. 1. It is obvious from the results that the

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MnO2 nanoparticles activated by carbon nanotubes show enhanced bacterial activity. The enhancement in the antibacterial capability of the MnO2/CNTs composites may ascribed to the synergistic effect of the high electron conductivity of the carbon nanostructures with the antibacterial and catalytic properties of the inorganic constituents materials [67]. Because, the oxidative stress generation and damage to the membrane transport channels caused by hybrid CNT/ MnO2 is high compared to pure MnO2. Interestingly, pure MnO2 nanoparticles reveal a slight antibacterial 24

ACCEPTED MANUSCRIPT activity against all the tested pathogens. The zone of inhibition recorded for pure MnO2 shows some significant value of around 7-8 mm. Chandran Krishnaraj et al [68] and R. K. Kunkalekar et al [69] reported that pure MnO2 nanoparticles synthesised by co-precipitation method did not show any antibacterial activity tested against Gram-positive and Gram-negative bacteria. But in our case, pure MnO2

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nanoparticles exhibit significant antibacterial activity which may be due to the

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microwave assisted preparation method adopted in the present study. Rapid heating,

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simultaneous heating, and penetration involved in the microwave method resulted

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in a narrow distribution of the particle size. Smaller particles with a larger surface area possess higher antibacterial effects compared to the larger particles. In addition,

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for CNT activated MnO2 nanoparticles the maximum zone of inhibition in the range from 12-15 mm was observed against the tested pathogens. This significant

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difference in the antibacterial activities of MnO2 and CNT/MnO2nanocomposite may be due to the size of MnO2 crystallites whichis larger than that of CNT/MnO2

-

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nanocomposite. The smaller size of the hybrid CNT/MnO2 generates the greater number of ROS which in turn may lead to better antibacterial activity. Also, the

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improved hydrophilicity exhibited by functionalized CNTs enhance the easy interaction between MnO2 and CNTs. The bar graph showing the zone of inhibition introduced by pure MnO2 and the hybrid CNT/MnO2 nanocomposite against Bacillus subtilis, Staphylococcus aureus, Proteus Mirabilis and Escherichia coli is shown in Fig. 8. It is also clear from the graph that the hybrid CNT-MnO2 nanocomposite is most affective with 11- 13 mm zone on plates against Proteus Mirabilis and 12-15 mm 25

ACCEPTED MANUSCRIPT against Escherichia coli, respectively. For Gram positive

Bacillus subtilis and

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Staphylococcusaureus it is only 7-10mm.

Fig. 8 The zone of inhibition produced by pure MnO2 and the hybrid CNT/MnO2

strains

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nanocomposite against both Gram-positive and Gram-negative bacterial

In particular, the CNT/MnO2 showed more photocatalytic activity on Gram-negative bacteria, because Gram-positive bacteria have more peptidoglycan than Gramnegative in the cell wall, which is negatively charged, and more carbon may get trapped to peptidoglycan in Gram-positive bacteria [70]. Also compared to Gram negative bacteria, the Gram positive bacteria is more resistant because their cell wall obstructs absorption of many molecules to move through the cell membrane. Hence, 26

ACCEPTED MANUSCRIPT the increased antibacterial activity was due to the increased ROS generation in MnO2 activated carbon nanotubes irradiated under visible light. The Mn–C and Mn–O–C carbonaceous bonds formed in the nanocomposite samples annealed at 450˚ C were effectively involved in visible light absorption, the charge transfer between the CNTs and the MnO2 nanoparticles, reduction in the electron-hole recombination rate, and

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finally, increase in the rate of formation of OH radicals for photoinactivation of the

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bacteria. Yamamoto and Makhluf et al. [71] have reported that the smaller size

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particle generates more number of ROS . Abdulrahman Syedahamed Haja Hameed et

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al [72] also reported that the greater number of ROS is mainly attributed to the small crystallite size of the nanoparticles and an increase in oxygen vacancies. On the other

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hand, the higher surface area to volume ratio of the composite increase the diffusion ability of the reactant molecules, and enhanced the bioactivity of hybrid CNT/MnO2.

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The MWCNTs pocessing large surface area provide more landing sites for MnO2 and allow the photogenerated electrons to move easily which helps to improve the

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photocatalytic inactivation of bacteria. The highly porous MnO2 nanoparticles decorated on the walls of CNTs leads to efficient diffusion of reactant molecules, and

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hence, an approximately two fold increase in the first order rate constant of the photocatalytic activity of MnO2. The proposed reaction mechanism for the antibacterial activity of

the hybrid

CNT/MnO2 nanopcomposite can be stated as follows. MnO2/CNT+ hυ MnO2 (e-(CB)) + CNT(h+(VB))( electron-hole generation)----(8)

27

ACCEPTED MANUSCRIPT H2O →H++HO-•

----------------------(9)

h++OH−→HO•

----------------------(10)

O2 +e→•O2·−

----------------------(11)

The photon energy absorbed on the CNT/ MnO2 nanocomposite form the

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photogenerated charge carriers in MnO2 host lattice and are transferred to carbon

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nanotube surface. The electrons in CNTs are injected into conduction band of MnO2.

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These electrons in the conduction band react with oxygen with the formation of

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super oxide O2- radical ion instantaneously, a positive charged hole formed by electron migrating from MnO2 valence band to CNTs and reduction of the so formed

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hole (h+) by absorbed OH oxidation takes place. This reduces the optical band gap as well as the rate of electron–hole pair recombination. In addition, the intimate

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interphase contact between MnO2 and CNTs leads to the tuning of optical properties into visible region. When MnO2 nanospheres are joined to the surface of CNT to make

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a heterojunction, a space charge region of few hundred nanometres will be formed near the junction to equalize Fermi levels. The driving force originate from interior

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electric field of the charge space and separate the photogenerated pairs which results in reduction of the recombination rate of the pairs. The overall synergistic was due to more well-dispersed active MnO2 particles on the surface of CNTs resulting in better photoinhibation of bacteria. Thus, the hybrid nanocomposite can result in a better antibacterial activity as compared to pure MnO2 nanoparticles. Qi et al [73, 74] demonstrated that immobilizatrion of antibacterial molecules to MWCNTs 28

ACCEPTED MANUSCRIPT via covalent bonding could greatly improve the antimicrobial activity. The application of functionalized carbon nanomaterials as carriers for the ordinary antibiotics possibly decrease the associated resistance, enhance their bioavailability and provide their targeted delivery.

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4. Conclusion

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In summary, a novel microwave assisted processing was adopted for the synthesise

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of pure MnO2 and hybrid CNT/MnO2 nanocomposites. XRD patterns of MnO2 nanoparticles show polycrystalline nature with an enhanced peak intensity after

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CNT incorporation. Morphology of the nanocomposite shows the isolated MnO2

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nanospheres homogenously attached to CNT side walls. The shift in the UV–vis absorption of the hybrid composite to longer wavelengths and narrowing of band

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gap was because of the synergetic contribution from CNTs and MnO2 which facilitate

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the inhibition of bacterial growth. Introduction of CNT into MnO2 matrix enabled longer charge separation by trapping photogenerated electrons and increased the

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antibacterial activity. Gram negative Proteus Mirabilis and Escherichia coli exhibit the

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maximum zone of inhibition of 11-15 mm when treated with CNT/MnO2 hybrid. The functionalised multiwalled CNTs exhibited much critical function for efficient use of manganese oxide for improved antibacterial activity. The enhanced stability, positive charged surface and improved antimicrobial properties of hybrid CNT-MnO2 promote further use of hybrid as a powerful antibacterial agent in industrial and clinical applications.

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ACCEPTED MANUSCRIPT Highlights  First report on synthesis of pure MnO2 and hybrid CNT-MnO2 nanocomposite prepared by microwave assisted  Decreased crystallite size of hybrid CNT-MnO2 nanocomposite favors

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antibacterial activity.  SEM reveals spherical shaped MnO2 nanoparticles attached on the walls of CNT

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 CNT enhance the toxic effect of MnO2 to bacteria under UV light.

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