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Silver nanoparticles capped with m-hydroxybenzoic acid and p-hydroxybenzoic have been successfully synthesized, but the long-term stability data of these silver nanoparticles are not available. In this paper, we report the stability of these two types of silver nanoparticles for a period of 40 weeks observation based on the change of surface plasmon resonance spectra of silver nanoparticles. Silver nanoparticles were synthesized by reduction of silver nitrate with m-hydroxybenzoic acid and p-hydroxybenzoic acid without addition of capping agent. The presence of silver nanoparticles was indicated by the appearance of yellow color due to the surface plasmon resonance of silver nanoparticles. The resulted silver nanoparticles were stored at room temperature and further UV-visible spectrophotometer was used to follow the change in surface plasmon resonance spectra. The surface plasmon resonance spectra of silver nanoparticles were overlapped for the first 18 weeks, followed by little change in the position of absorption maxima (lmax), peak intensity, and width of the absorption peak until the week of 40. Silver nanoparticles capped with m-hydroxybenzoic acid and silver nanoparticles capped with p-hydroxybenzoic acid were highly stable which should make them suitable for further applications. The results show the potential of m-hydroxybenzoic acid and p-hydroxybenzoic acid to become a new reducing agent in the synthesis of highly stable silver nanoparticles. The m-hydroxybenzoic acid and p-hydroxybenzoic acid appeared to act as both reducing and capping agent.

Effect of pH and determination of adsorption capacity of Cu(II), Ni(II) and Pb(II) heavy metal ions on adsorbent prepared from Eichhornia crassipes (eceng gondok) biomass has been investigated. The influence of media acidity on the adsorption characteristics was carried out by determining ions adsorbed at various pH in the range of 2-10, while an adsorption isotherm model of Langmuir was used to estimate the capacity of adsorption. Results showed that Cu(II) was optimally adsorbed at the range pH of 5-6, Ni(II) at 2-4, while Pb(II) reached an optimum adsorption at pH 2-3. The adsorption data of Cu(II), Ni(II) and Pb(II) for the adsorbent folowed quite well Langmuir isotherm model, confirmed that such chemisorptions involved on that process. The ions adsorption capacities (am) were 27.47, 16.69, and 15.04 mg/g for Pb(II), Cu(II), and Ni(II), respectively.   Keywords: adsorption, heavy metal, Eichhornia crassipes, pH, capacity

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Highly stable silver nanoparticles capped with p-hydroxybenzoic acid were synthesized by reducing silver ion with p-hydroxybenzoic acid and used for the detection of paraquat. The synthesized silver nanoparticles, which are yellow, exhibited an absorption peak at 420 nm when measured with a UV-visible spectrophotometer due to the surface plasmon resonance. In the presence of paraquat, the color of silver nanoparticles changed from yellow to purple accompanied by the appearance of a new peak at 580 nm in addition to the peak at 420 nm. In order to obtain optimum experimental conditions, temperature, and time of reaction were optimized, and the ratio of absorbance obtained at 580 nm and 420 nm (A580/A420) were monitored. The A580/A420 is proportional to the concentration of paraquat. Under the most favorable condition, the calibration curve showed a high level of linearity ranging from 6.0 × 10–4 to 1.0 × 10–3 M, and the detection limit was found to be 8.30 × 10–6 M. Silver nanoparticles capped with p-hydroxybenzoic acid was found to be useful for the colorimetric determination of paraquat in the aqueous medium.