A 55.0nm silver layer was coated on cylindrical polypropylene water filters by a modified Balzers 760 machine (Germany) using the electron beam gun system [28]. The Balzers machine was modified in order to enable coating of the cylindrical filters in a homogenous manner as explained before [28]. Bacterial attachment to the surface of the filters was visualised using a JEOL JSM-6400 scanning electron microscope (Japan). For this, square-shaped samples with the approximate size of 1cm by 1cm were cut from the filter surfaces and visualized by the SEM. Prior to SEM analysis of the filters containing bacteria, the samples were treated with 4% glutaraldehyde for 24h at 4ºC following by two rounds of washing with 0.1M sodium cacodylate buffer and a post-fixation in 1% osmium tetroxide (2h, 4ºC). The samples were then dehydrated using a series of different concentrations of acetone (35- 100%) following by drying and sputter-coating the samples with gold.
To examine the bactericidal effect of the nano silver-coated polypropylene water filters, a custom-made pilot plant was used as shown in Fig. 1. Initially 15L of distilled water was inoculated with 103cfu/mL E-coli bacteria. A Stainless steel centrifugal pump (1–phase, 1/2 suction/discharge, Max Head 20m), was used for feed and recirculation. The flow of water was measured by rotameter and the temperature was controlled by a cooling device inserted into the feed tank. Two gauges were used to measure the pressure at the entrance and exit to the housing of
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the silver coated polypropylene water filter (Fig. 1). The flow rate was adjusted to 3L/hr and the pressure difference before and after the water filter was 0.1bar The antibacterial property of the filters was evaluated using the membrane filter method [29]. The bacteria-loaded distilled water was circulated for 15min before filtration. An aliquot of 100mL of this water was taken for the bacterial assay and then the nano silver coated filter was put in place in the pilot plant (Fig. 1). After filtration for certain time intervals, appropriate volumes of the water samples (100mL) were taken and passed through 0.45μm pore size cellulose ester membrane filters (Millipore, USA) that retain the bacteria present in the samples. The membrane filters were placed on the 5mL plates of EMB agar and incubated at 35°C for up to 24 hours. The plates were then inspected for the presence of red-violet color which indicates presence of the E-coli cells. All red violet colonies were counted and recorded.
The inductively coupled plasma/mass spectrometry (ICP/MS) was used to determine any amount of nano silver particles in the water sample at the end of a 5h filtration process. An Optima 7300Dv instrument (Perkin Elmer Corporation, Norwalk, CT, USA) was used according to the reported procedure [30-32].