Visualization Method Based On Silver Stain Enhancement For The Detection Of Ultratrace Lead And Cadmium Ions

Heavy metal pollution has become one of the environmental issues of global concern due to the serious harm to human health. For example, lead and cadmium ions may cause high blood pressure, kidney damage, nervous system damage, etc. Especially lead can result in irreversible neurological damage in children. United States Environmental Protection Agency?EPA? has defined the maximum concentrations of lead and cadmium ions in drinking water are 0.015 mg/L and 0.005 mg/L, respectively. Traditional methods with high sensitivity have been used in the laboratory for the detection of lead and cadmium ions, such as graphite furnace atomic absorption spectrometry?GFAAS?, atomic emission spectroscopy?AES?, inductively coupled plasma mass spectrometry?ICP-MS?, etc. However, almost all of these methods require expensive instruments, professional personnel and high costs.In recent years, nanomaterials show great applications in bioanalysis. Some new methods based on quantum dots?QDs?, gold nanoparticles?AuNPs? or grouping have been developed for the rapid and simple detection of ultratrace lead and cadmium ions, such as colorimetric, fluorescence and electrochemical analysis methods. Colorimetric methods show great potential in popular detection, which takes advantage of simple operation, visualizing result and easy to on-site detection.Silver stain technique is an excellent means of signal amplification, which can greatly improve the sensitivity of detection methods. Aptamers are single-strand DNA or RNA selected in vitro that bind targets with high affinity and specificity. Aptamers offer remarkable convenience in the design and modification of their structures. Compared with antibodies, aptamers are more stable and easier to save. Taking advantage of these unique characteristics, aptamers are widely used in biological analysis. For this reason, a new method for visualization detection of lead and cadmium ions was proposed by means of combing chemical bath deposition and silver stain technique together, which shows great potential for on-site detection of lead and cadmium ions in environmental samples. The contents mainly include:?1? A new membrane based visualization method has been developed for the detection of ultratrace lead and cadmium ions. The basic steps were mainly as follows. Dithizone?Dz? was adsorbed on cellulose acetate membrane?CAM? and formed Dz-Pb²⁺/Cd²⁺ complexes in the presence of lead and cadmium ions. Then the complexes were converted into Dz-PbS/CdS composites after further processing with sulfide ions. While the new formed Dz-PbS/CdS composites could act as a catalyst in the subsequent silver stain reaction. With the silver nanoparticles depositing on CAM, the color of the membrane turned gray to black, which could be observed by naked eye. Semi-quantitative analysis of Pb²⁺/Cd²⁺was performed according to the ?Average gray value of CAM grayscale images.?2? Results showed that under the optimal conditions, ?Average gray value had a good linear relation with the logarithm of the Pb²⁺/Cd²⁺concentration in the range from 1 nmol/L to 100 nmol/L?R²=0.984?, and the naked eye detection limit could reach 1 nmol/L?⁰.0002 mg/L?. Besides, other common metal ions had no significant interference with the detection, indicating that the method had a good selectivity. Satisfactory results were obtained for the detection of real samples,the recovery ranged between 95% and 115% for the spiked samples.?3? A chip-based visualization method has been proposed for the detection of lead ions by combining specific aptamer with silver stain together. The main steps were as follows. The specific aptamer PS2.M was immobilized onto the surface of activated slides, then aptamer PS2.M recognize and bind to lead ion and lead ion were converted into PbS by adding S2-, silver stain could amplify the signal which could achieve qualitative detection. The analysis of the optical darkness ratio?ODR? could achieve semi-quantitative detection of Pb²⁺. Experimental results show that when the aptamer PS2.M concentration of 2 ?mol/L, silver staining time of 20 min, and the concentration of Pb2 + in the range from 0.5 ?mol/L to 10 ?mol/L, ODR value showed a good linear relationship with lgC?R² = 0.951?.