Mono- and Bimetallic Nanoparticles for Catalytic Degradation of Hazardous Organic Dyes and Antibacterial Applications

In the present work, gold (Au), silver (Ag), and copper (Cu) based mono- and bimetallic NPs are prepared using a cost-effective facile wet chemical route. The pH for the synthesis is optimized in accordance with the optical spectra and supported by the finite difference time domain simulation studies. FESEM and TEM micrographs are used to analyze the morphology of the prepared nanoparticles. TEM images of bimetallic nanoparticles (BMPs) verified their bimetallic nature. XRD studies confirmed the formation of fcc-structured mono- and bimetallic NPs. Photoluminescence studies of the as-synthesized NPs are in good agreement with the previous publications. These synthesized NPs showed enhanced catalytic activity for the reduction/degradation of 4-nitrophenol, rhodamine B, and indigo carmine dyes in the presence of sodium borohydride (NaBH4) compared to NaBH4 alone. For the reduction of 4-nitrophenol, Au, Cu, and CuAg nanoparticles exhibited good catalytic efficiency compared to others, whereas for the degradation of rhodamine B and indigo carmine dyes the catalytic efficiency is comparatively high for CuAg BMPs. Furthermore, the antibacterial assay is carried out, and Ag NPs display effective antibacterial activity against Klebsiella pneumoniae, Salmonella ser. Typhimurium, Acinetobacter baumannii, Shigella flexneri, and Pseudomonas aeruginosa.


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. (a) Schematic representation of the FDTD simulations. Synthesized NPs in the water medium background are being illuminated by a plane-wave source whose propagation vector is in the negative z-direction while the electric (E) field is oscillating along the x-axis. Here, d-diameter, P-periodicity E-electric field vector, k-propagation vector (b) Real simulation environment with applied symmetry boundary conditions (BCs). Here, NP-nanoparticle, Ttransmittance monitor, R-reflectance monitor  NPs indicating their λmax. The simulation parameters are taken as summarized in Table S1 based on the structural characterization obtained in Figure 2 of the main manuscript

Effect of pH on morphology and absorption spectra
The synthesis of metal nanomaterials is pH sensitive. Here, HCl and NaOH are used to vary the pH. At lower pH, as the H + increases, the number of electrons available for reduction reaction is less and hence reaction may not take place.
Hence SPR peak may not be observed in optical spectra. On the other hand, the poor balance between nucleation and growth results in bigger and irregular-shaped particles due to agglomeration, causing the broadening and redshift of the SPR peak. In general, for particles >15 nm as particle size increases, so does the spectral width and redshift of the SPR peak position due to increased scattering at higher wavelengths and vice versa for smaller particles. 1 Further, the lack of electrons results in a higher agglomeration tendency due to lower electrostatic repulsion. As the pH increases, the rate of reduction is increased and thus a balance between nucleation and growth is achieved due to the presence of an excess of electrons (OHions). As a result, the basic condition is the favorable environment for the synthesis of metal nanoparticles. In the present work, except for the synthesis of Au and AuAg NPs, the latter statement is true.
On the other hand, the repulsive electrostatic interaction reduces the possibility of agglomeration. Hence monodispersed small nanoparticles are formed resulting in intense sharp peak 1,2 . On further increasing the pH, the rapid nucleation compared to slow growth results in larger, non-uniform particles thus causing broadening of the SPR peak. 2-,4

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The contradictory behavior of Au and Ag nanoparticles i.e., narrow SPR peaks are observed, which indicate the formation of small, monodispersed particles at acidic pH. This can be explained on the basis of the reaction mechanism of reducing agent, trisodium citrate used in both the cases. During the reduction reaction of Au salt, citrate gets oxidized by yielding dicarboxy acetone. Meanwhile, by utilizing the electrons from the citrate oxidation reaction, Au(III) salt gets reduced into Au(I) salt. At lower pH, as the H + , the citrate oxidation rate may increase and hence the increased number of electrons results in the higher rate of reduction achieving a balance between nucleation and growth. However, on further decreasing the pH, large number of H + ions limit the availability of electrons for the reduction. 3 In the current study, during the synthesis of Au NPs, the concentration of trisodium citrate NPs is relatively high, yielding a greater number of electrons for the reduction reaction. Hence, small, monodispersed Au particles are formed at pH 2, which is evident from the sharp and intense SPR peak. However, in the case of AuAg BMPs, pH 4 is the optimum pH. At pH 2, a slight increase in spectral width, diminished and redshift of SPR peak is noticed. This may be due to the decreased reduction rate caused by the lack of electrons due to the low concentration of trisodium citrate compared to Au seed solution and the concentration of Ag salt solution resulting in slightly larger particles. 4 We sincerely apologize for our inability in obtaining slots for FESEM characterization. We approached a few of the central facilities for FESEM, unfortunately, we are not able to get the slots for analysis at this moment due to the nonworking condition of the instruments. However, we were successful in obtaining slots for TEM analysis for BMPs, which is detailed above.  Figure. S12(a-e)). The corresponding percentages of degradation of dye using Au, Ag, Cu, AuAg, and CuAg as catalysts are found to be 87.25%, 53.64%, 92.44%, 91.20% and 97% in 6 minutes, 21 minutes, 14 minutes, 6 minutes and 3 minutes respectively. Whereas only 4.64% degradation of rhodamine B for 20 minutes is observed without metal catalysts and rate constant is measured to be 0.00003 sec -1 ( Figure. S9f and S12f).  Figure. 10(a-e)). From Figure. S10f and S13f, it can be observed that, in the absence of metal catalysts, the percentage degradation is 51.38% in 20 minutes and the corresponding rate constant is measured to be 0.0005 sec -S14 Figure S14. Mueller-Hinton Agar diffusion method for evaluating antibacterial activity of Au, Ag, and Cu NPs  The inactivity of Ag NPs towards certain bacteria like Vibrio cholerae, Escherichia coli and Aeromonas hydrophila might be due to the reason that negatively charged trisodium citrate capping agent inhibits the physical interaction of NPs with cell wall due to electrostatic repulsion.