| Semiconductor quantum dots (QDs) are promising optical labels for sensing and biosensing since QDs offer many advantages over conventional organic fluorphores such as high photoluminescence efficiency, size-dependent emission wavelengths, broad absorption and good photostability. In recent years, QDs-based optical sensors have become one of the most exciting forefront fields in analytical chemistry. In this dissertation, our goal is to explore to construct optical sensors based on quantum dots for the detection of TNT explosives and organophosphorus pesticide residues such as chlorpyrifos. The use of superior optical properties of quantum dots and the theory of fluorescence resonance energy transfer, two types of QDs-based optical sensing method were designed and built: ratiometric fluorescence probe and ligand replacement-induced fluorescence switch of quantum dots. The novel sensing methods were established for rapid, sensitive, visual detection of TNT and chlorpyrifos. The main parts of the results are summarized briefly as follows:1. Semiconductor quantum dots with high quality are a prerequisite for constructing optical sensors. Firstly, different thiol molecules as a stabilizer, high luminescence efficiency, stable, and monodisperse CdTe quantum dots were prepared in aqueous solution. The different color and size of the CdTe QDs growing further is controlled by the duration of reflux and can easily be monitored by absorption and PL spectra. The photoluminescence efficiency of quantum dots can be further improved by controlling UV irradiation time. The preparation of quantum dots is simple, easy to operate, good repeatability and stability. One-step modification of the functional groups (e.g., hydroxyl and amino groups) on the surface of quantum dots in the synthesis process enables their direct coupling with relevant molecules and construction of functionalized fluorescent probes, offering solid grounds for the design and application of optical sensors in the analysis field.2. To detect trace trinitrotoluene (TNT) explosives deposited on various surfaces instantly and on-site still remains a challenge for homeland security needs against terrorism. This work demonstrates a new concept and utility for visual detection of TNT particulates on various package materials. The concept takes advantages of the superior fluorescent properties of quantum dots (QDs) for visual signal output via ratiometric fluorescence, the feasibility of surface grafting of QDs for chemical recognition of TNT, and the ease operation of the fingerprint lifting technique. Two-sized CdTe QDs emitting red and green fluorescences have been hybridized by embedding in silica nanoparticles and covalently linking on the silica surfaces, respectively, to form a dual-emissive fluorescent hybrid nanoparticle. The fluorescence of red QDs in the silica nanoparticles keeps constant, whereas the green QDs functionalized with polyamine can selectively bind TNT by the formation of Meisenheimer complex, leading to the green fluorescence quenching due to resonance energy transfer. The variations of the two fluorescence intensity ratios display continuous color change from yellow-green to red upon exposure to different amounts of TNT. By immobilization of the probes on a piece of filter paper, a fingerprint lifting-like technique has been innovated to visualize trace TNT particulates on various surfaces by the appearance of a different color against yellow-green background under a UV lamp. This method shows high selectivity and sensitivity with a detection limit as low as 5 ng/mm2 on a manila envelope with the naked eye.3. The development of a simple and on-site assay for the detection of organophosphorus pesticide residues is very important for food safety and ecosystem protection. This paper reports the finding of a surface coordination-originated fluorescence resonance energy transfer (FRET) of CdTe quantum dots (QDs) and a simple ligand-replacement fluorescence turn-on mechanism for the highly sensitive and selective detection of organophosphorothioate pesticides. It has been demonstrated that the coordination of dithizone at the surface of CdTe QDs in basic media can strongly quench the green emission of CdTe QDs by a FRET mechanism. Upon the addition of organophosphorothioate pesticides, the dithizone ligands at CdTe QDs surface were replaced by hydrolyzate of organophosphorothioate, and thus the fluorescence will be turned on with the pesticides. The fluorescence on is immediately fast and the limit of detection for chlorpyrifos is as low as ~0.1 nM. Two consecutive linear ranges allow a wide determination of chlorpyrifos concentrations from 0.1 nM to 10μM. Importantly, the fluorescence turn-on chemosensor can directly detect the residue of chlorpyrifos in apples with a limit of 5.5 ppb that is under the maximal residue limit prescribed by U.S. Environmental Protection Agency. The very simple strategy reported here will open a new window to make fluorescence turn-on chemosensors for chemo/bio detections.
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