Regular articleInactivation of Staphylococcus aureus in visible light by morphology tuned α-NiMoO4
Graphical abstract
Introduction
The evolution of increasingly multidrug-resistant or highly-drug-resistant pathogens has drawn significant public attention as a human health threat causing economic and social complications [1], [2]. The Staphylococcus aureus (S. aureus) is a major candidate as multidrug-resistant pathogens, reported to acquire resistant to almost all antibiotics [3]. The presence of S. aureus in drinking water serve as origin for colonizing residents exposed to contaminated water that causes the terribly harmful effect in human health [4]. To solve the water pollution problem that arises from the pathogens, photocatalytic disinfection is potential green technology that can inactive pathogens by utilizing the natural energy of sunlight via advanced oxidation processes (AOPs) [5], [6], [7].
Inorganic nanomaterials are rapidly developing field of nanotechnology that have been applied in the area of medical diagnostics, biotechnology, catalyst, sensor, degradation of environmental pollutants, and bacterial inactivation [3], [5], [6], [7], [8], [9], [10], [11]. Recently, photocatalytic inorganic nanomaterials, such as TiO2 and ZnO are popular choice for inactivating various bacteria [5], [6]. However, such photocatalytic materials absorb wavelengths in UV region that can utilize only 4% of the solar spectrum. Nowadays, metal (transition metal and rare earth) doped photocatalytic materials were also reported for bacterial disinfection [3], [12], [13]. In addition, the silver nanoparticles loaded photocatalyst are also frequently applied in the antibacterial applications to utilize visible spectrum [14]. However, they are expensive at the commercial point of view. So, the visible light induced α-NiMoO4 seems to be an effective strategy for enhancement of bacterial inactivation due to its low cost, optimum band gap (2.8 eV), optical centers, good electrical conductivity, stable at room temperature, low temperature phase, high BET surface area, and environmentally friendly features [15], [16]. In α-NiMoO4 structure, photon induced exciton can easily generated via transfer of charge (d–d transition) from O 2p to 3d orbitals of Ni2+ and Mo6+ groups. NiMoO4 has dual type of semiconductor properties. It shows p-type and n-type semiconductor (above 560 °C). Moreover, Ni2+ ions have strong tendency to generate reactive oxygen species (ROS) (OH, HO2, H2O2, and O2−) that can inactivate the pathogens [15], [16], [17]. Upon visible light irradiation, photocatalyst can produce ROS that may destroy bacterial pathogens by damaging the cell membranes, proteins and DNA [6]. The particle size and morphology of photocatalyst play a vital role in the antibacterial activity because the photocatalytic process is surface based phenomena [18]. Surfactants are excellent additives that affect both the shape and size of crystals through the charge transfer and surface tension phenomena [19]. The cheap price, wide commercial availability, and varying properties make positive aspects of surfactants [20]. So, tuning the morphology of photocatalyst by surfactants is perfect way to enhancement of bacterial inactivation.
Metal molybdate families are the most promising examples of mixed metal oxides that have a long history of practical applications in phosphors, laser materials, scintillators, photocatalyst, antibacterial, optical fibers, humidity sensors, and magnetic materials [3], [7], [16], [21], [22]. NiMoO4 has two polymeric phases i.e. low temperature α-phase and the high temperature β-phase. Monoclinic α-NiMoO4 has C2/m space group. It composed of an octahedral co-ordination of Mo6+ and Ni2+ ions with edge shared oxygen atoms [16].
In this study, we have reported the surfactant dependent morphologies of microwave synthesized α-NiMoO4. The inactivation of S. aureus was carried out in dark/visible light. The microscopic analysis was used to observe the extent of damage in bacteria cellular organization. The ROS were evaluated by using 2, 7-dichloro-dihydro-fluorescein diacetate (DCFA-DA) and nitro blue tetrazolium (NBT) assay. The DNA degradation and proteins degradation were performed. The surfactants dependent morphologies and their optical and antibacterial properties were evaluated.
Section snippets
Synthesis of samples
In our experiment, all the chemicals were used from Sigma-Aldrich of analytical grade. The aqueous solution of Nickel chloride hexahydrate (40 mL) and aqueous solution of Molybdic acid (40 mL) were prepared in the separate beakers. Polyethylene Glycol [OH (CH2CH2O)nCH2CH2OH, n = 165-210], Sodium Citrate (C6H5Na3O7) and Sodium Dodecyl Sulfate [CH3(CH2)11OSO3Na] were used as surfactants. The sample codes and the corresponding compositions are listed in Table 1. These two precursors were mixed
Structural and morphological analysis
The diffraction patterns of various samples confirmed the monoclinic structure of pure α-NiMoO4 that is well matched with the JCPDS card number 086–0361, as shown in Fig. 1 [16], [24]. The crystallinity of samples is increased by the addition surfactants because of growth controlling and an agglomeration inhibiting actions of surfactants [25]. The crystallite size is calculated on the basis of the Scherrer’s equation,Where, D is the size of crystals, λ is X-ray wavelength, K is
Conclusions
In conclusion, the morphologies of α-NiMoO4 were modulated to nanorod (without surfactant), large bundle of needles (with PEG), small bundle of needles (with SC), and nanowires (with SDS) by microwave hydrothermal process. Morphologies dependent optical properties were described. The bacterial inactivation in light/dark by α-NiMoO4 was found in following order: Nanowires > small bundle of needles > Nanorod > large bundle of needles. Nanowire morphology has strong tendency to inactivate S. aureus
Acknowledgments
This Research was supported by the Global Research Laboratory Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of Korea (Grant Number: 2010-00339).
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