Photochemistry and Photobiology, 2014, 90: 1099–1107

Photoactive titania float for disinfection of water; evaluation of cell damage by bioanalytical techniques.

Shwetharani R, Jyothi M S, Laveena P D’souza and R. Geetha Balakrishna*

Center for Nano and Material Sciences, Jain University, Jakkasandra (P), Kanakpura (T) Bangalore – 562112, India

 

ABSTRACT:

A photoactive float was fabricated with the modified titania to cause a feasible disinfection of water, contaminated water. The commercially available titania was doped with neodymium by pulverization technique to enhance its activity in sunlight and a multi approach technique was used to evaluate the extended efficiency of the doped sample. Doping induces semiconductor behavior with lower band energy that could respond to visible light and exhibit better disinfection activity. The amount of malondialdehyde, protein, DNA and RNA released on destruction of E.coli was observed to be 0.915 × 10-3 µg/mL, 859.912 µg/mL, 20.173 µg/mL and 1146.073 µg/mL respectively. The above analytical methods along with standard plate count method substantiated the enhanced disinfection efficiency of the doped sample in sunlight.

PMID: 24689654

 

SUPPLEMENT:

Background:

The availability of decontaminated water is a serious problem in rural areas of developing countries. 37.7 million Indians are affected by waterborne diseases annually; 1.5 million children are estimated to die of diarrhea (1). The major pathogenic organisms responsible for water borne diseases in India are bacteria (E.coli, Shigella, V. cholera), viruses (Hepatitis A, Poliovirus, Rotavirus) and parasites (E. histolytica, Giardia Hook worm). E.coli is a gram negative microorganism and is used as an indicator of fecal contamination. Waterborne diseases caused from this organism involve flu-like symptoms such as diarrhea, nausea, fever, and vomiting.

The classical techniques such as ultraviolet light irradiation, ozone treatment, low frequency ultrasonic irradiation and reverse osmosis processes have their disadvantages and advantages. A simple disinfection method which can work in sunlight and without usage of non renewable source of energy is interesting. Titania is a well known photocatalyst for its use in oxidative, photodestructive and disinfection processes in the presence of solar light (2). TiO2 due to its large band energy absorbs in the near UV and generate excess electrons in the conduction band and positive holes in the valence band. At the TiO2 particle surface the holes react with surface OH- groups to form HO. radicals. Excess electrons in the conduction band react with molecular oxygen to form superoxide ions, which further disproportionate to form more HO. radicals. Hence, the photocatalyst mediated reactions cause the generation of a number of reactive oxygen species (ROS) and hydroxyl radicals (HO.). The generated ROS interacts with the cell wall components like peptidoglycan, amino acids and lipids and cause the damage of the bacterial cell wall. TiO2 utilizes only a small fraction of the solar spectrum and this led to the search of a photocatalyst which could utilize the most abundant energy source-the sunlight. We thus have investigated the doping effect of inner transition metal, neodymium into TiO2 by solid state technique to induce changes in its electronic structure for the effective absorption of a broader spectrum of sunlight, which results in a higher amount of reactive oxygen species for an improved and feasible photoinactivation of E.coli in sunlight.

Method:

The present study investigates an economic and effective method for inactivation of pathogenic microbe using a photoactive float in the presence of solar light. The photocatalysts (TiO2 and 1 % Nd doped TiO2) was coated on a polymer sphere using Spektron Dip Coater and it was used for disinfection of exponential phase of E. coli. Irradiation of photocatalyst with photons of energy equivalent to its band gap causes excitation of electrons. The photogenerated holes formed in the process react with surface water to produce hydroxyl radicals (Fig 1).

 

Fig-1 Figure 1. Mechanism of disinfection

The ROS reacts with cytoplasmic membrane which led to peroxidation of membrane lipid and destruction of cell wall followed by leakage of intracellular components like DNA, RNA and protein. The extent of inactivation by ROS was evaluated by lipid peroxidation, standard plate count method, protein, RNA, DNA estimation and morphological studies through scanning and transmission electron microscope (Fig 2 and 3). The doped photocatalyst shows highest antibacterial activity attributed to increased range of light absorption due to substitutional impurity Nd in TiO2. Highlight:

Titania and its doped analogues were investigated in powder form for bactericidal activity with respect to both gram positive and gram negative bacteria (3). But the suspended powders seemed to pose a problem of separation before usage of disinfected water, limiting it to practical applications. To solve the problem, an immovable photocatalyst was envisaged and fabricated to act as a better and recyclable disinfectant. The highlight of the present study lies in the use of the concept of float wherein a spherical polymeric object has been coated with the photocatalyst and allowed to float on contaminated water to effectively deactivate bacteria.

Fig-2Figure 2. Scanning electron micrographs of untreated, control and treated samples after 160 min of irradiation under different experimental conditions

 

Fig-3Figure 3. Transmission electron micrographs of untreated, control, CA-TiO2 and Nd-CA-TiO2 treated samples after 160 min of irradiation under different experimental conditions.

Conclusions:

Nd as a substitutional impurity in TiO2 facilitates the better utilization of a much broader spectrum of solar radiation. The sunlight activity of doped sample is better than the undoped sample. The effective shallow trapping and de-trapping, with lower band energy, increases the disinfection process and release of inner cell components like protein, DNA and RNA will takes place due to splintering of cell membrane. The lipids of E.coli cell membrane easily gets peroxidised by ROS initiating the destruction process. This study demonstrates the significance of the process to use the photoactive float of Nd-doped TiO2 to act as a better disinfectant in sunlight.

 

References:

[1] Montgomery, M. A., and Elimelech, M., (2007). Water and Sanitation in Developing Countries: Including Health in the Equation. Environ. Sci. Technol. 41. 17-24.
[2] Adesina, A. A., (2004). Industrial exploitation of photocatalysis: progress, perspectives and prospects. Catalysis Surveys from Asia. 8. 4. 265-273.
[3]. Swetha. S., Maheshwari Kumari Singh, Minchitha K. U. and Geetha Balakrishna . R., (2012). Elucidation of Cell Killing Mechanism by Comparative Analysis of Photoreactions on Different Types of Bacteria. Photochem Photobio., 88: 414-422

 

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