This study presents the modification of cassava root husk-derived biochar (CRHB) with ZnO nanoparticles (ZnO-NPs) for the simultaneous adsorption of As(III), Cd(II), Pb(II) and Cr(VI). By conducting batch-mode experiments, it was concluded that 3% w/w was the best impregnation ratio for the modification of CRHB using ZnO-NPs, and was denoted as CRHB-ZnO3 in this study. The optimal conditions for heavy metal adsorption were obtained at a pH of 6–7, contact time of 60 min, and initial metal concentration of 80 mg L⁻¹. The heavy metal adsorption capacities onto CRHB-ZnO3 showed the following tendency: Pb(II) > Cd(II) > As(III) > Cr(VI). The total optimal adsorption capacity achieved in the adsorption of the 4 abovementioned metals reached 115.11 and 154.21 mg g⁻¹ for CRHB and CRHB-ZnO3, respectively. For each Pb(II), Cd(II), As(III), and Cr(VI) metal, the maximum adsorption capacities of CRHB-ZnO3 were 44.27, 42.05, 39.52, and 28.37 mg g⁻¹, respectively, and those of CRHB were 34.47, 32.33, 26.42 and 21.89 mg g⁻¹, respectively. In terms of kinetics, both the pseudo-first-order and the pseudo-second-order fit well with metal adsorption onto biochars with a high correlation coefficient of R², while the best isothermal description followed the Langmuir model. As a result, the adsorption process of heavy metals onto biochars was chemisorption on homogeneous monolayers, which was mainly controlled by cation exchange and surface precipitation mechanisms due to enriched oxygen-containing surface groups with ZnO-NP modification of biochar. The FTIR and EDS analysis data confirmed the important role of oxygen-containing surface groups, which significantly contributed to removal of heavy metals with extremely high adsorption capacities, comparable with other studies. In conclusion, due to very high adsorption capacities for metal cations, the cassava root husk-derived biochar modified with ZnO-NPs can be applied as the alternative, inexpensive, non-toxic and highly effective adsorbent in the removal of various toxic cations.

In this work, we have carried out systematic studies on the critical role of polyvinyl pyrrolidone (PVP) and citrate in the well-known chemical reduction route to synthesize silver nanodecahedra (AgND). Silver nitrate (AgNO₃) was used as silver source, which can be directly converted to metallic silver after being reduced by sodium borohydride (NaBH₄) under blue light-emitting diode (LED) irradiation (λ_max = 465 nm), and polyvinyl pyrrolidone (PVP) as a capping agent to assist the growth of AgND. The obtained products were silver nanodecahedra of excellent uniformity and stability with high efficiency and yield. The results showed that PVP acted as a capping agent to stabilize the silver nanoparticles, prolonging the initiation time required for nanodecahedra nucleation, thus inducing anisotropic growth, allowing the size and morphology of the AgND to be controlled successfully. This improved understanding allows a consistent process for the synthesis of AgND with significantly enhanced reproducibility to be developed and the formation mechanism of these nanostructures to be elucidated. This is a simple, cost-effective and easily reproducible method for creating AgND. The typical absorption maxima in the UV-vis spectroscopy of Ag seeds was λ_max ∼400 nm and that of AgND was λ_max ∼480 nm. The size of the prepared AgND was in the range of 60–80 nm. SEM images confirmed the uniform and high density of AgND when the concentration of PVP was 0.5 mM. The XRD pattern showed that the final product of AgND was highly crystallized. In addition, the prepared AgND can be used to detect methylene blue (MB) in a sensitive manner with good reproducibility and stability using Surface-Enhanced Raman Scattering (SERS) phenomenon. Out of the obtained products, the AgND prepared with 50 min blue LED light irradiation (AgND-50) displayed the strongest SERS signal. Interestingly, MB in diluted solution can be detected with a concentration as low as 10⁻⁷ M (the limit of detection, LOD) and the linear dependence between SERS intensity and the MB concentration occurred in the range from 10⁻⁷ to 10⁻⁶ M. The enhancement factor (EF) of the SERS effect was about 1.602 × 10⁶ with a MB concentration of 10⁻⁷ M using 532 nm laser excitation.

Eu-doped CdS quantum dots (QDs) with the Eu dopant concentration in the range of 0.5–10% and zinc blende (ZB) structure were successfully synthesized by a wet chemical method. The fabricated Eu-doped CdS QDs exhibited emissions in the visible window approximately at 465, 590, 618 and 696 nm, which correspond to the excitonic emission of CdS QDs and the electronic transitions of the intra 4f⁶ configuration from the ⁵D₀ level to ⁷F₁, ⁷F₂ and ⁷F₄ levels of Eu³⁺ dopant ions, respectively. Judd–Ofelt theory was used to estimate the properties of ligand field and luminescence quantum efficiency of the material. The interaction mechanism and the efficiency of the energy transfer process from CdS QDs to Eu³⁺ ions were found by using Reisfeld's approximation formulas. The luminescence quenching of Eu³⁺-doped CdS QDs was studied through analysis of emission spectra and decay curves. The dominant interaction mechanism between Eu³⁺ ions and energy transfer parameters have been found by fitting the decay curves to the Inokuti–Hirayama model. The cross-relaxation channels leading to the luminescence quenching of Eu³⁺ have also been predicted.