*2.4. Photocatalytic Disinfection Experiments*

Bacterial cells with a final cell concentration of 5 ×10<sup>6</sup> CFU/mL were put in 1 L of normal saline solution and multiple reactions were performed with varying concentrations of Ag@ZnO ranging from 1 to 5 mg/L. Photocatalytic disinfection reactions were carried out in 2 L reactor vessels under continuous and controlled agitation (500 rpm).The set up was kept under dark conditions for 30 min to attain equilibrium. After the dark phase, the system was exposed to sunlight for 120 min and samples were collected at 15 min intervals. To monitor and analyze the inactivation of microbes, 100 μL of collected samples were further diluted in 900 μL of sterile 0.9% NSS and a volume of 100 μL from the final diluted sample was spread on nutrient agar plates. The plates were left for overnight incubation at 37 ◦C. Following this, viable cell count was performed to obtain the results for the rate of disinfection [11,16]. The above steps were repeated using two commonly used catalysts ZnO and TiO2 (Degussa P25) for comparative studies and with the optimum catalyst concentration for proper disinfection as obtained by Ag@ZnO. Additionally two experimental controls were performed. (1) In light control, under only photolytic condition the microbial population was exposed to sun-light in absence of Ag@ZnO. (2) In dark control, microbial population was reacted with Ag@ZnO in absence any light. The Intensity of sunlight was measured by a digital lux meter and found to be 90,000 ± 5000 lux. To evaluate whether the sun-light/Ag@ZnO assisted photocatalytic disinfection system is applicable to natural water systems, samples of tap (municipal supply, Bhubaneswar, India), river, and pond water were collected. Results were compared with de-ionized water. All water samples were collected and transported in clean and autoclaved sample bottles (Tarsons, Kolkata, India) at 4 ◦C and immediately were filtered by using Whatman filter paper and centrifuged at 5000 rpm for 15 min to remove insoluble materials followed by autoclaving to eliminate any microbial contamination. To demonstrate the efficiency of the synthesized catalyst, a calculated amount (as mentioned earlier) of

targeted pathogens were spiked in the sterilized natural water samples and subjected to photocatalysis in presence of three different photocatalysts (Ag@ZnO, ZnO and TiO2). The concentration of catalyst used was the one which was obtained as the optimum for the respective bacteria from experiments conducted in saline solution.
