## 3 Results and Discussion
### Performance Testing of Synthesized Membranes  
The fouling tests that were conducted using BSA solutions were conducted as described above in the methods section. Figure 10a indicates the membranes' fluxes at different stages of the fouling test. The M7 sample showed a much higher initial DW flux in the order of three folds higher than the neat membrane's initial flux with DW. This is expected as it is well documented that membranes with higher hydrophilicity, total porosity and average pore size possess higher flux [33]. All membranes show a decline in flux once BSA filtration takes place. Interestingly, the recovery flux (a DW flux after BSA filtration) varied significantly. Importantly, M7 samples showed much higher flux recovery compared to the neat membrane. The normalized flux data that is shown in Figure 10b sheds more light on the varied recovery ability across each membrane sample. For instance, M8 had a normalized  
Figure 8: ATR-FTIR (Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy) spectra for synthesized PES neat and PES/GO (M8) membranes. Figure 9: Average Young’s modulus values for different PES/GO membranes. flux which was more than double that of M1 (neat PES membrane). The enhanced performance in terms of flux recovery can be attributed to the increased hydrophilicity and surface charge of the PES/GO membranes. As can be seen in Figure 11a, BSA rejection decreases as the loading of GO increases. The observed decrease can be accounted for by the increase in pore size as the GO content increases (see Figure 5). This inverse relationship between pore size and rejection performance has been reported across many studies [39; 52; 53]. Conversely, the FRR values in Figure 11b show an opposite trend with a general increase in the flux recovery with an increase of GO content in PES/GO membranes. Remarkably, the FRR value more than doubles when M8 is compared with the neat PES membrane (M1) (36 and 75%, respectively). The increase in membrane surface hydrophilization, coupled with the increased negative surface charge of PES/GO membranes would naturally cause an electrostatic repulsion effect against the negatively charged BSA molecules at a neutral pH (the isoelectric point of BSA being between 4.5 and 5.0) [54]. This would result in less BSA adsorption on the membrane surface and in the pores, explaining the decrease in irreversible fouling that is seen in Figure 11c. Figure 10: BSA (bovine serum albumin) fouling test with different membrane samples (**a**) The DW (deionized water) flux was measured for the first 15 min of filtration, then DW was replaced with BSA solution and the flux was evaluated for the next 15 min, after which time the flux for DW filtration was measured for the last 15 min. (**b**) Normalized flux for different PES membranes, calculated as the \(J_{f}/J_{0}\) ratio, after filtration of BSA solution. PES neat and PES/GO membranes that were incubated in _E. coli_ suspension or _B. subtilis_ (as a model for Gram-negative and Gram-positive bacteria, respectively) for 24 h at 37 \({}^{\circ}\)C on nutrient agar fared very differently in terms of bacterial colonization on the membrane surface (see Figure 12). Impressively, the FESEM images reveal a clearance of _E. coli_ bacterial colonies on all membranes which incorporated GO for both tests. In contrast, neat M1 samples (without GO) did not show the inhibition of bacterial growth, showing a carpet-like appearance of bacterial colonies. Similar results were seen in the tests involving membranes that were incubated with Gram-positive bacteria (_B. subtilis_) as shown in Figure 12. A clear inhibitory effect is noticeable on all membranes bar the M1 samples which did not contain GO. The apparent inhibition of bacteria on these membranes can be explained in relation to the contact angle and zeta potential data that were obtained. The increased hydrophilicity and negative surface charge that were exhibited by the PES/GO membranes could act in an additive manner to cause repulsive forces against similar charged bacteria.