The nature of carbon quantum dot (CD) luminescence is still broadly investigated based on different CD synthesis processes. This study aims at investigating the effect of microwave power on optical characteristics of CDs using microwave radiation techniques. CDs are synthesized with citric acid as a source of carbon and urea as a passivation agent. CDs have been successfully synthesized, and an increase in the CD burn-off percentage was observed with an increase in the microwave power. The TEM test results show that the dimensions of the CDs resulted are in the range of 3.4–9.5 nm with an average size of 6 nm at 450 W. The results of the FTIR functional group structural analysis show that a CD has N—H and O—H stretching bonds. According to the results of UV–Vis analysis, the CDs have absorption peaks at 335 nm and 407 nm, which indicates a π → π^* electronic transition from the C=C bond and an n → π^* transition from the C=O bond. Using the Tauc-plot method, the CD energy bandgap values were found to be 2.47 eV, 2.51 eV, 2.51 eV, 2.52 eV, and 2.53 eV at CD 300, CD 450, CD 600, CD 850, and CD 1000, respectively. The optical multichannel analyzer test results show that the peak emission waves produced by CDs are 536 nm and 532 nm with the strongest bright green light at CD 1000. Time-resolved photoluminescence testing shows that the CD decay time generally ranges from 5 ns. Overall, an increase in the microwave power causes an increase in the percentage of burn-off, energy bandgap, absorption intensity, and CD emission intensity.

Silver (Ag) substituted ZnO thin films were successfully deposited onto glass substrates by spray coating technique. Structure, morphology, and optical properties were evaluated by X-ray diffractometer (XRD), scanning electron microscopy (SEM), and UV-Vis spectrophotometer, respectively. XRD spectra had polycrystalline wurtzite structure; SEM images showed that thin films had different surface morphology at different Ag doping concentration. From transmittance spectra, thin films transparency decreased as well as band gap energy along with increase of Ag doping concentration. Methylene blue (MB) solution was used as a pollutant in the photodegradation studies. Under UV light irradiation, the optimal Ag doping is 25%, with 83% of the decolorizing efficiencies after 3 h irradiation time and apparent constant (kapp) about 9.69 × 10⁻³ min⁻¹.

Early detection of heavy metals in drinking water is a fundamental step that must be taken to prevent adverse effects on health. This research aims to develop a heavy metal ion detector by utilizing the fluorescence properties of carbon dots. Cdots were synthesized using the microwave irradiation method based on the central composite design: urea mass 0.31–3.68 gr; reactor power 200–1000 W; synthesis time is 13–46 min, and the response is quantum yield. Material characterization includes PL, TEM, UV-VIS, XRD, and FTIR. The selectivity and sensitivity of Cdots as detectors were tested for Ag⁺, Bi³⁺, Ni²⁺, Al³⁺, Co²⁺, Pb²⁺, Fe³⁺, Zn²⁺, Zr⁴⁺, and Hg²⁺ ions at concentrations of 0–10 μM. The results showed that Cdots were successfully synthesized by fluorescent light green at 544 nm. An adequate response model is quadratic with the formulation QY = +58.36 + 10.41X₁ + 14.06X₂ +13.59X₃–5.57X₂X₃–4.89X₁ ²−8.60X₂ ²–5.40X₃ ². The best Cdots were obtained in the formulation of R9 (3 g, 800 W, 40 min), which resulted in a QY of 74.39%. The characteristics of Cdots are spherical, diameter 6.6 nm, the bandgap of 2.53 eV, and having an amorphous structure. The surface of Cdots contains various functional groups such as O–H, C–H, C=O, C–N, and C=C. In the heavy metal detection test, Cdots showed specific sensitivity to Fe³⁺ ions. The addition of Fe³⁺ concentration and the extinction of Cdots fluorescence intensity formed a linear correlation F₀/F = 0.08894[Fe³⁺]+0.99391 (R² = 0.99276). The detection ability of Cdots for Fe³⁺ ions reaches a concentration of 0.016 ppm, much lower than the regulatory threshold limit of SNI, WHO, and IBWA. The detection of Fe³⁺ ions in drinking water uses a fluorescence technique consistent with the SSA and ICP-OES. Based on these results, the fluorescence technique using Cdots can be an instrument for quality control of the final drinking water product.

The aim of this study was to develop fly ash-supported CaO catalyst derived from waste mollusk shell of Anadara granosa and Paphia undulata for transesterification of palm oil to yield biodiesel. The active precursor Ca(OH)₂ of the catalyst has been developed from calcination of waste mollusk shell, while the mesoporous has been prepared using the wet impregnation method. The results demonstrated that the mollusk shell-derived catalyst could gain the yield of biodiesel of 92 and 94 % for A. granosa shell and P. undulata shell, respectively. The obtained yield was comparable with the yield of biodiesel gained from CaO catalyst derived from reagent grade CaCO₃ (yield = 94 %). The optimum condition was achieved at catalyst loading of 6 %, reaction temperature of 60 °C and reaction time of 2 h. The reusability study showed that the developed catalysts exhibited higher recycle degree characteristic indicated by 50 % yield reduction after three reuse cycles.