Muhammad Iqbal Zaman

Biologically synthesized silver nanoparticle-based colorimetric sensor for the selective detection of Zn2+

A simple, selective and cost effective colorimetric sensor has been investigated for the detection of Zn2+ using biologically synthesized silver nanoparticles (AgNPs). The AgNPs were prepared from the leaf extract of Amomum subulatum via two different procedures i.e., at room temperature and by a heat treatment procedure. The AgNPs prepared through the heat treatment procedure exhibited efficient results. The as synthesized AgNPs were studied by simple UV-vis spectroscopy which showed an intense absorption band at 425 nm which was further confirmed by FT-IR and SEM analysis. The synthesized AgNPs exhibited a good colorimetric sensing property towards Zn2+ by changing the color of the solution from yellowish-brown to colorless accompanying a decrease in absorption intensity. The proposed detection mechanism of the sensor has been discussed. The sensor showed an excellent linear response towards Zn2+ in the concentration range from 1 × 10−5 to 8 × 10−5 M with a correlation coefficient (R2) of 0.996. The detection limit of the method was found to be 3.5 × 10−6 M. There was no interference effect observed for Zn2+ detection in the presence of other heavy metal ions. The proposed sensor was successfully applied for the detection of Zn2+ in drinking water samples.

Thiamine-functionalized silver nanoparticles for the highly selective and sensitive colorimetric detection of Hg2+ ions

Hg2+ contamination is a serious threat to the environment; hence, the development of methods for its trace level detection is urgently required. To contribute in this domain, we report for the first time the application of a highly sensitive and selective colorimetric sensing platform for Hg2+ ions using thiamine-functionalized silver nanoparticles (Th-AgNPs). Upon the addition of Hg2+ ions to the solution, the AgNPs exhibited a noticeable color change from yellow to violet-blue. This color change was monitored by the UV-vis spectrophotometric observation of a significant decrease in the absorption band at 395 nm along with the appearance of a new peak at 550 nm. The designed sensor demonstrated good sensitivity in the concentration range of 1 × 10−8 to 5 × 10−6 M with a detection limit of 5 nM. The sensor also showed high selectivity for Hg2+ when tested in the presence of several competing metal ions. Moreover, the method was found to be applicable to river water samples with satisfactory percentage recoveries.

Density functional theory calculations for the investigation of (Ag, N) codoping effect on the electronic and optical properties of anatase TiO2

Efficient absorption of light in visible range and enhance separation of photoexcited electron-hole pairs (EHPs) are crucial for improving the photoactivity of metal nonmetal codoped TiO2. By using density functional theory (DFT) calculations, an effective metal (Ag) and nonmetal (N) codoping approach is described to modify the photoelectrochemical properties of titanium dioxide ( TiO2). Nitrogen (N) doping introduces isolated N-2p states above the valence band maximum (VBM) which acts as an electron trap to promote EHP recombination. For Ag-doped TiO2, Ag-4d states are introduced above the VBM which leads to the band gap narrowing. Silver (Ag) and nitrogen codoped TiO2 possess stable configuration, narrowed band gap and best visible light absorption. Defect pair binding energy calculation shows that individual dopants, located at a distance of 8.951 A bind each other, which indicates that the defect pair is stable compared to the isolated impurities in the host lattice. Ag and N codoped TiO2 shows better visible light absorption as compared to other doped models due to the reduced band gap. N doping reduces the band gap of TiO2 while Ag doping enhances the EHPs separation, so their combined presence in a sample would improve the photocatalytic activity due to their synergistic codoping effect. Our calculations provide reasonable explanation for the experimental findings.

Spectroscopic and Thermodynamic Analysis of Cd2+ Ion Sorption Kinetics by Manganese Dioxide in the Presence of Oxyanions

The present study reports the effect of contact time, nature of electrolyte, and temperature on the sorption kinetics of Cd2+ by manganese dioxide, which is the most active oxide in soils and sediments. The sorption kinetics of Cd2+ by manganese dioxide is evaluated at pH 6 in different electrolytes in the temperatures range 293-323 K. The solid samples are equilibrated in 0.45 mmol.L−1 Cd2+ and different electrolytes at pH 6. The results indicate that sorption of Cd2+ increases with time and temperature and the system attains equilibrium within 60 min in KNO3 as the electrolyte. However, in the presence of 0.001 M KH2PO4 sorption of Cd2+ increases and the time for equilibrium shifts to 90 min. The data second-order kinetics model and the calculated rate constant k and initial sorption rate h increases with increasing temperature phosphate treatment. Among the calculated thermodynamic activation parameters the positive values of ΔH‡ and ΔG‡ show the sorption process to be endothermic and nonspontaneous, while the ΔS‡ being negative indicates a decrease in randomness of the system during sorption process at the solid-liquid interface. The free energy of activation decreases from 15.95 kJ.mol−1 in nitrate to 8.76 kJ.mol−1 in phosphate. These observations suggest that the rate controlling step in Cd2+ sorption is diffusionally controlled, a fact that has been proved by application of Fick’s law.

Effect of Cu2+ doping on the physicochemical properties and chromate adsorption capacity of goethite

ABSTRACT Laboratory-prepared pure and 0.32% Cu2+ ion-doped goethite samples have been characterized for surface area, pH of point of zero charge (pHpzc), X-ray diffraction, thermogravimetric and differential thermal analyses, transmission electron microscopy, and Fourier-transform infrared (FTIR) analysis. The analyses show that the Cu2+ ions get incorporated into the crystals of goethite by forming a solid solution. Due to the presence of Cu2+ ions, the surface area and pore volume of goethite are increased from 34.69 to 126.52 m2 g−1 and from 0.02 to 0.06 cm3g−1, respectively. The results of chromate adsorption experiments, carried out in the concentration range 0.25–2.01 mmol.L−1, show that chromate adsorption remarkably increases at pH 3 and 5, with Cu doping in goethite, and decreases with increasing pH. However, unlike pure goethite, chromate uptake by Cu-doped goethite decreases with increasing temperature. The data of chromate adsorption by both the samples of goethite fit well into the Langmuir model. The values of isosteric heats of adsorption (), being positive for pure goethite and negative for Cu-codoped goethite, show the processes of adsorption to be endothermic in case of pure and exothermic in case of Cu-codoped goethite. FTIR analysis reveals that innersphere complexation is the dominant mechanism for chromate adsorption by Cu-codoped goethite.