Study On Sensing Behavior Of Explosive By Fluorescence Sensor Regulated By Surfactant Assembly

It is importantly significant to detect explosives in terms of national security, military activities, and ecological environment protection. Explosives are not only the chief culprit of rampant terrorism activities worldwide, but also one of the reasons of environmental pollutions. Explosives are usually small organic compounds including nitro-aromatics, nitro-aliphatics, nitroamines, nitrate esters and peroxides. Their pollution to environments has attracted increasingly widespread attention, not only because they are the main ingredients of the left weapons from wars, but also one of the main components of discharged industrial wastewater. When the explosive ingredients divulged or discharged into natural water systems, they will pollute water and soil. Fluorescent sensors have proven to be an effect method to detect explosives because they possess remarkable advantages like high sensitivity and selectivity, easy preparation and multiple fluorescence signals. However, fluorescent sensors are usually used in organic solvents or organic solvent-water mixtures for explosive detections due to the fact that most fluorophores are organic small molecules or polymers, which limit the applications in actual sample detections.Surfactants, composed of hydrophilic head groups and hydrophobic tail chains, can spontaneously form various heterogeneous supramolecular assemblies, such as micelle, vesicle and microemulsion, etc. in aqueous solutions. These self-assemblies can be considered as nanoscale microcontainers with hydrophobic cavities in aqueous environment, which can encapsulate hydrophobic fluorophores. It not only significantly enhance the water solubility and stability of fluorophores in water as well as their fluorescence quantum yields, but also easily modulate the sensing behaviors of the encapsulated fluorophores to analytes by simply adjusting surface charge, concentration and hydrophobic chain length of surfactants.Based on the urgent practical needs on detection of trace explosives in the water phase, and the prominent advantages of using surfactants in fluorescent sensors, the objectives of the present thesis are to develop surfactant assemblies-modulated fluorescent sensors for detecting explosives in aqueous solutions. In the first part of the research work, the effect of surfactant micelles on the photophysical properties of a cationic bis-pyrene fluorophore, Py-dilM-Py, was systemically examined. The results from series of measurements including UV-vis absorption, steady-state fluorescence emission, quantum yield, fluorescence lifetime, and time-resolved emission spectra revealed that the cationic fluorophore is only encapsulated by the anionic sodium dodecyl sulfate (SDS) surfactant micelles, and not incorporated in the cationic dodecyl trimethyl ammonium bromide (DTAB) and neutral Triton X-100(TX100) surfactant micelles. This different fluorophore location in the micellar solutions significantly influences its sensing behavior to various explosives. Fluorescence quenching studies reveal that the simple variation of micellar systems leads to significant changes in the sensitivity and selectivity of the fluorescent sensor to explosives. The sensor exhibits an on-off response to multiple explosives with the highest sensitivity to picric acid (PA) in the anionic SDS micelles. In the cationic DTAB micelles, it displays the highest on-off responses to PYX. Both the sensitivity and selectivity to PYX in the cationic micelles is enhanced compared with that to PA in the anionic micelles. However, the poor encapsulation in the neutral surfactant TX100micelles leads to fluorescence instability of the fluorophore and fails to function as a sensor system. This work demonstrates that the electrostatic interaction between the cationic fluorophore and differently charged micelles plays a determinative role in adjusting its distribution in micellar solutions, which further influences the sensing behavior of the obtained micellar sensor systems.In the second part of the research work, a neutral bis-pyrene fluorophore, Py-TOA-Py, was introduced into four differently concentrated DTAB micellar solutions to fabricate fluorescent sensor systems for detecting explosives in aqueous environments. The effect of surfactant concentration on the sensing behaviors of such systems was investigated. The fluorescence study results show that the sensing system at critical micelle concentration exhibits the highest sensitivity to PA and some extent responses to PYX and HNS. When DTAB concentration is increased to30mM, the sensor system displays ratiometric responses to HNS, where monomer emission intensity increases along the decrease of the excimer emission. Whereas, the increasing addition of PYX leads to not only obvious fluorescence quenching, but also notable red-shift of excimer maximum emission. However, the gradual addition of PA brings only fluorescence quenching of the sensor system. These results indicate that the sensor with DTAB concentration at30 mM exhibits cross-reactive responses to explosives. The results from investigation of the fluorescence responses of this sensor system to nine explosives illustrate that it can produce recognition pattern to distinct explosive analytes by combining the fluorescence variations at four typical emission wavelengths. Thus, this single sensor system can provide multiple-type signal responses to nine nitro-explosives by only adjusting the concentration of micelles. The present work has shown strengths in fabrication of fluorescent sensors such as easy preparation, rich signals, and potentials in practical detecting of real samples (e.g., trace explosives in ground water and soil-treated solutions). It is expected to expand the methods of fabricating fluorescent sensor arrays and the actual applications.