Selective On-Off-On Polymer Bonded Quantum Dot-Nitroxide Radical Fluorescent Sensor to Detect Cd²⁺ Ion in Aqueous Media

One of the most important problems of recent years is heavy metal pollution, which has great harm to the environment and human health. Especially, Cd²⁺ ion selectivity of quantum dot (QD)-nitroxide radical nanoprobe developed in this study was found as higher than the other heavy metal ions (Al³⁺, Co²⁺, Cu²⁺, Mn²⁺, Ag⁺, Fe³⁺, Ni²⁺, Pb²⁺, Cr³⁺, Hg²⁺, Mg²⁺, Zn²⁺). Cadmium is widely used in industry, agriculture and many other fields. Recently, serious environmental and health problems caused by Cd²⁺ ion have made it a great need to develop methods for detecting and monitoring cadmium levels. Due to the simplicity and high sensitivity features, fluorescent methods have become powerful tools to monitor target heavy metal ions in different sources. The main aim of this study is to develop cross-linked copolymer-based fluorescent sensor with high mechanical strength, sensitivity and selectivity that can detect Cd²⁺ ion in aqueous media. In the literature studies, different types of fluorophores have shown selective properties against different heavy metal ions, and in general, only turn-off and turn-on sensors have been developed. Compared to the studies in the literature, in this study, it is planned to go one step further than the liquid-phase and disposable sensors that have been made so far, and develop a fluorescent sensor with a more functional and high mechanical strength and on-off-on feature. Today, solventless vapor phase deposition methods are most commonly used techniques for production of polymeric thin film coatings. Initiated chemical vapor deposition (iCVD) is a suitable method for the fabrication of crosslinked copolymer thin film coatings and it has many advantages such as low production cost, low temperature, 3D geometry coating performance and high deposition rate. Therefore, production of crosslinked copolymer thin films of glycidyl methacrylate (GMA) and 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4D4) via iCVD system was performed in this study. Also, experimental studies for investigations of quantum dot-nitroxide radical nanoprobe formation, attachment of this nanoprobe on iCVD deposited crosslinked copolymer film, interaction between the nanoprobe and target heavy metal ion and some parameter effects on the Cd²⁺ ion detection with developed fluorescent sensor were carried out. Fluorescence Spectroscopy, Fluorescence Microscopy, UV-Vis Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and Electron Paramagnetic Resonance Spectroscopy (EPR) analyses were performed to examine the formation of QD and nitroxide radical nanoprobe due to electron transfer between them, QD-nitroxide radical nanoprobe attachment to the thin polymeric film surfaces and interaction between polymer bonded nanoprobe and target heavy metal ion. Under optimal conditions, a good linear relationship between the fluorescence response and concentration of Cd²⁺ ion was obtained in the range from 0.5 to 15 μM. The detection limit (LOD) of Cd²⁺ ion with QD-nitroxide radical nanoprobe was found as 0.223 μM showed that sensor nanoprobe developed in this study can be comparable with the other Cd²⁺ ion detection studies made so far in literature. Cd²⁺ ion detection study was performed with nanoprobe connected to the developed cross-linked copolymer film and according to the results, Cd²⁺ ion detection was successfully performed with the novel polymer-bonded fluorescent sensor nanoprobe because the desired fluorescence recovery (~ 88%) was obtained from analysis results.