## 3 Results and Discussion  
A yield of 3.52% \(\pm\) 0.94 SPs was obtained from mangrove leaves. It constituted 44% \(\pm\) 0.01 carbohydrates, and had a phenolic content of 50.62% \(\pm\) 0.1 mgGA/g. As for the biosynthesized Cu NPs, they constituted 3.3% \(\pm\) 0.004 carbohydrates and 1.44% \(\pm\) 0.1 mgGA/g. The presence of carbohydrate and phenolic contents in Cu NPs could be owed to the formation of the SPs capping layer that coats the Cu NPs. As analyzed by HPLC, the SPs extract was mainly comprised of 49.3% glucose, 24.6% galactose and 20.5% glucouronic acid by mol (Table S1 and Figure S1).  
To confirm the formation of Cu NPs, UV-Vis spectroscopy and TEM imaging were used. Figure 2a demonstrates the UV-visible spectrum of Cu NPs where an absorbance peak appears at the range of 250-350 nm. A similar range was reported in the previous literature for Cu NPs biosynthesized using gooseberry fruits [30] and _Vitis vinifera_ leaves [31]. In Figure 2b, the TEM micrographs show regular spheres with an average diameter of approximately 2.11 \(\pm\) 0.64 nm. Larger sized copper oxide  
Figure 1: Schematic outline of the work conducted in this study.  
nanoparticles of dimensions 5-45 nm have been synthesized from brown alga (_Bifurcaria bifurcata_) as reported in the previous literature [13]. In addition, the SEM images in Figure 2d reveal homogenous and spherical particles, with an elemental composition of 54.97% O, 19.20% C, 13.64% S and 12.18% Cu by mass as analyzed by EDX (Figure 2c). DLS measurements showed that the produced Cu NPs are slightly negative in charge with a zeta potential of 11 \(\pm\) 0.46 mV at the working pH range of 6-7. This charge aids to reduce the agglomeration between the Cu NPs and it could be attributed to the SPs functional groups present in the coating layer.  
The XRD pattern for the produced Cu NPs is depicted in Figure 3a, where major peaks at about 12\({}^{\circ}\), 19.5\({}^{\circ}\) and 23.5\({}^{\circ}\) could be observed. The 12\({}^{\circ}\) peak could be ascribed to Cu(OH)\({}_{2}\), while the latter two peaks could be attributed to Cu\({}_{2}\)O. In addition, minor peaks at 32\({}^{\circ}\), 37\({}^{\circ}\) and 45\({}^{\circ}\) appeared and could also be assigned to Cu\({}_{2}\)O. Hence, the observed peaks could indicate the presence of two main Cu species [32; 33], the orthorhombic phase of Cu(OH)\({}_{2}\) (JCPDS: #80-0656), and the cubic phase of Cu\({}_{2}\)O (ICCD-JCPDS: #78-2076) which correspond to the lattice planes of (020), (021), (111) and (200), respectively. According to the Pourbaix diagram, copper would predominantly be present in the form of Cu (I) species at pH 6-7 (solution pH).  
The FTIR spectra of SPs and Cu NPs are presented in Figure 3b. Both SPs and Cu NPs show bands at wavelengths of 3400, 1625, 1450-1420 and 1109 cm\({}^{-1}\) that correspond to the stretching vibration of the hydroxyl (O-H) group, the bending vibration of C-O of uronic acids, (S=O) sulfate ester bonds and acidic polysaccharides, respectively [34; 35]. These functional groups are bound to the surface of the Cu NPs and act as stabilizing agents.  
Figure 2: Characterization of sulfated polysaccharides (SPs) and Cu\({}_{2}\)O/Cu(OH)\({}_{2}\) nanoparticles (Cu NPs) using ultraviolet-visible (UV-Vis) spectroscopy (**a**), Transmission Electron Microscopy (TEM) imaging (**b**), Elemental-dispersive (EDX) spectroscopy analysis (**c**) and Scanning Electron Microscopy (SEM) imaging (**d**).