Well crystalline gold nanoparticles (AuNPs) of different sizes were fabricated using sundried Coffea arabica seed (CAS) extract at room temperature by controlling the pH of the green extract. The size, shape and crystallinity of the nanoparticles have been studied using electron microscopy and X-ray diffraction. The presence of phenolic groups (revealed through FT-IR studies) from the CAS extract are responsible both for the reduction of Au ions and stabilization of the formed AuNPs. The efficiency of the CAS extract mediated green synthesis technique for the production of AuNPs has been compared to the conventional chemical Turkevich technique, which not only uses a toxic reductant such as NaBH 4 , but also operates around the boiling point of water. It has been observed that the CAS extract mediated synthesis process produces relatively bigger AuNPs at similar pH values of the reaction mixture in comparison to the AuNPs produced in the Turkevich process. Although the AuNPs synthesized using CAS extract are relatively larger and polydisperse in nature, their catalytic efficiencies for the degradation of an aromatic nitro compound (4-nitrophenol) are found to be comparable to the chemically fabricated AuNPs. Probable mechanisms associated with the formation of AuNPs and their size control in the CAS extract mediated green synthesis process have been discussed. Fig. 2 Typical SEM images of GAuNPs (left column) and CAuNPs (right column) synthesized at different pH values of the reaction mixture. Insets show histogram reflecting the size dispersion corresponding to each pH; average size of the NPs vs. number of particles. 24822 | RSC Adv., 2018, 8, 24819-24826 This journal is

Noble metal (silver (Ag), gold (Au), platinum (Pt), and palladium (Pd)) nanoparticles have gained increasing attention due to their importance in several research fields such as environmental and medical research.

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In this study, we report a simple method for the fabrication of carbon dots sensitized zinc oxide-porous silicon (ZnO-pSi) hybrid structures for carbon dioxide (CO 2 ) sensing. A micro-/nanostructured layer of ZnO is formed over electrochemically prepared pSi substrates using a simple chemical precipitation method. The hybrid structure was structurally and optically characterized using scanning electron microscopy, X-ray diffraction, fluorescence, and cathodoluminescence after the incorporation of hydrothermally prepared nitrogen-doped carbon dots (NCDs) by drop casting. With respect to the control sample, although all the devices show an enhancement in the sensing response in the presence of NCDs, the optimal concentration shows an increase of ∼37% at an operating temperature of 200 • C and a response time <30 s. The increment in the CO 2 -sensing response, upon the addition of NCDs, is attributed to an increase in CO 2 -oxygen species reactions on the ZnO surface due to an increment in the free electron density at the metal-semiconductor-type junction of NCD clusters and ZnO micro-/nanorods. A significant increase in the sensing response (∼24%) at low operating temperature (100 • C) opens the possibility of developing very large-scale integrable (VLSI), low operational cost gas sensors with easy fabrication methods and low-cost materials.

Fluorescent nitrogen-doped graphene oxide dots (NGODs) have been demonstrated as an on–off nanosensor for the detection of Hg 2+ , Au 3+ , and H 2 O 2 . As compared to l -cystine, where the luminescence signal recovery results from the detachment of Hg 2+ from the NGODs, signal recovery through l -ascorbic acid (turn-off–on model) has been attributed to the reduction of Hg 2+ to Hg 0 . The sustainable recovery of the photoluminescence signal is demonstrated using common citrus fruits containing vitamin C ( l -AA), suggesting a promising practical usage of this sensing system. Additionally, the sensitivity of NGOD- and AA-originated signal recovery from the Hg(II)–NGODs mixture has been successfully tested in Hg 2+ ion-spiked tap water from three different places. Mimic devices were executed and verified on the basis of characteristic spectral changes, and the possible utility of this system in electronic security and memory element devices has also been demonstrated. Considering an easy synthesis process and excellent performance of NGODs, this investigation opens up new opportunities for preparing high-quality fluorescent NGODs to meet the requirements of many applications.