Synthesis of Quantum Dot Based Probe for Hydrogen Sulfide Sensing

Abstract

The thesis focuses on the development of a quantum dot-amphiphilic polymer composite for hydrogen sulfide sensing. Hydrogen sulfide (H2S), which is colorless poisonous gas is mainly generated by the degradation of organic compounds. This gas is a gasotransmitter together with carbon monoxide and nitric oxide (NO) which regulates the physiological effects in human bodies. However, excess amounts of H2S in living systems can cause various diseases like Alzheimer’s, cirrhosis of the liver damage to the gastric mucosa, Parkinson’s disease, and pulmonary arterial hypertension. Therefore, developing methods for H2S detection in biological systems is important. The general techniques for H2S detection are using methylene blue, gas chromatography (GC), and electrochemical methods. However, these methods have low biocompatibility and require a long detection time. Among various detection methods, using fluorescent probes is capable of real-time imaging with high sensitivity and biocompatibility. Most of the reported fluorescent probe use only a single emission of the fluorescent probe, however, it is difficult to detect in a biological environment due to the light scattering of the analyte, and fluctuations of excitation light intensity. To overcome these disadvantages, ratiometric sensing has been developed. Ratiometric sensing uses two fluorescence signals which have a self calibration function. It doesn’t affect by the fluctuation of the excitation light or other external factors. Therefore, measuring the concentration of the analyte with high accuracy is possible. In general, for the ratiometric sensing materials, Quantum dots (QDs), and organic fluorescent probes are usually used. Quantum dots are semiconductor nanocrystals which is applicable as a diagnostic fluorescent probe due to their broad range of excitation wavelength, tunable wavelength, high quantum yield (QY), and superior photostability. In this study, the QDs amphiphilic polymer composite for H2S ratiometric sensing was designed. Using two fluorescence signals, CdSe/CdS/ZnS QDs which emits red fluorescence, and reaction-based H2S detection fluorescent probe 7-azido-4-methylcoumarin (AzMC) which emits blue fluorescence are encapsulated in PEI-based amphiphilic polymer. The fluorescence signal of the AzMC was sensitive to H2S, the 7-azido-4-methylcoumarin converted to 7-amino-4-methylcoumarin (AMC) by azide-amine reduction, and fluorescence intensity was increased. By measuring the two fluorescence intensities, detecting the hydrogen sulfide ratiometrically with high accuracy and selectivity is possible. Through this study, it is expected that the ratiometric H2S sensing ratiometric probe using a QDs is an established and accurate method to monitor H2S in a biological condition. This can also be applied to diseases related to H2S.