DEVELOPMENT OF OPTICAL SENSOR FOR SENSITIVE DETECTION OF NITROAROMATICS EXPLOSIVES

Abstract

As human populations and resources consumption increase, environmental monitoring and assessment are critically important. Hence, there is considerable interest in developing sensors for real-time detection of human hazardous environmental contaminants such as trace explosives. Numerous techniques, such as gas chromatography surface enhanced Raman spectroscopy colorimetric sensing, electrochemical, ion mobility spectrometry, mass spectrometry, etc., have been established and are currently being used for trace -level detection of nitroaromatic explosives. However, their large footprints and high-cost analytical instruments limit their use for field applications. An important example is ion mobility spectrometry, commonly used to detect explosives throughout airports in the USA and India. However, this technique lacks enough sensitivity and is susceptible to false-positive alarms for some medications containing nitroglycerine due to the similar molecular structure of nitro explosives. On the other hand, the last years have seen a growing development of optical sensors for various applications such as biomedical, clinical, biomedical, environmental monitoring, and process controlling. When real-time data is required with high sensitivity and selectivity, optical sensors are attractive and promising analytical tools. Typically, optical sensors can be subdivided into two main streams based on their sensing mechanisms: Absorbance (Colorimetric) and Fluorescence-based optical sensors. In recent years, optical sensors based on fluorescence quenching mechanisms have proved to be very effective and promising due to benefits such as excellent sensitivity, fast response time, selectivity, and a simple on/off detection method. Furthermore, onsite analyte detection could be easily achieved using a handheld device that utilized fluorescence quenching as a source and detector. Thus, the fluorescence quenching-based method has the great promise to be applied in the rapid, low-cost, and selective detection of analytes and thus becomes the focus of this thesis. Commercial sensors are now available based on this sensing mechanism, but further developments are required to apply the technique universally. So, this thesis is concerned with the design strategy of the analyte-specific optical sensor using fluorescence organic materials such as copolymers, small molecules, and nano-materials. The present thesis is divided into five chapters. The First Chapter, entitled "General Introduction" presents a general overview of the toxic environmental effect of explosives. The chapter also summarized a literature survey of fluorescence-based optical sensors for explosives detection based on a different fluorophore sensing moiety. A brief discussion about the design of fluorometric sensors and different sensing mechanisms has also been discussed. The chapter ends up with an outline of the objectives of the thesis work. Section 2A, entitled "Fluorescent Fe2O3-CdSe nanocomposite probe for selective detection and removal of picric acid," examined a possible magneto-fluorescent nanosensor, namely Fe2O3@SiO2 nanoparticles anchored with Triethoxylsilyl) propylcarbamoyl butyric acid and Cysteamine-capped-CdSe QDs that are strongly electrostatically attached to an anionic carboxyl group in the spacer arm of Fe2O3@SiO2 nanoparticles. These stable magneto-fluorescent nanosensor showed strong specific response to Picric Acid over a number of other explosive (NACs) in DMSO, making them promising fluorescence probes for PA detection and removal, simultaneously, for real-time application. The quenching constant (KPA) of nanosensor with PA was obtained to be 4.3×104 M-1 in DMSO with a LOD up to 2.2 μM as a consequence of turn off sensing. The nanosensor had also been shown to be capable of removing detected PA molecules using an external magnet. This could be a potential application for low-cost stand-off sensors. external magnet, making it a potential application for low-cost stand off sensors. In Section 2B, titled "A graphene xerogel nano-sensor anchored with Cysteamine capped CdSe QDs for simple onsite visual detection of TNT," a colorimetric sensor is presented for the visual detection of TNT using Cysteamine capped-CdSe quantum dots (QDs) decorated on graphene-chitosan xerogel (GSXS), which has a high signal-to-noise ratio. Steady-state and time-resolved spectroscopy, along with DFT, support the characterization of the Meisenheimer complex formation, which is a widely recognized sensing mechanism. A novel pathway for chemical sensing applications is provided by the suppression of QD-GSXS fluorescence when exposed to Green Fluorescent Protein (GFP), which is caused by the stable Meisenheimer complex. Under optimized conditions, the sensor displayed a broad linear range of 0.0 to 311.4 µM and a LOD of 9 µM. With its outstanding stability and selectivity, the developed chemo-sensor is a suitable tool for rapid and sensitive detection of NAEs in solution phase, including in clinical settings. The third chapter of the thesis titled "Fluorescent Electrospun Nanofibers Doped with 3,6 Diaminocarbazole for Highly Sensitive Detection of Nitroaromatics" investigates an affordable technique to fabricate composite nanofibers of PAN/DAC, which can be used for vapor sensing of DNT and TNT. Furthermore, the research investigates the photo induced electron transfer mechanism and sensing capability of DAC towards DNT and TNT in a solution. The study reveals that the self-condensation of DAC molecules results in a lower sensitivity of DAC in the solution phase. To address this issue, the researchers developed a fluorescent nanofiber probe made up of 3,6-Diaminocarbazole (DAC) and Polyacrylonitrile (PAN) that exhibits excellent sensitivity and selectivity in the vapor detection of DNT and TNT. The electrospinning technique is employed to cast PAN/DAC nanofibrous film, thereby reducing self-condensation and enhancing sensitivity. The progress made in this research on enhancing the sensitivity of carbazole molecular units is noteworthy and holds the potential to facilitate the creation of efficient and deployable vapor sensors for detecting nitroaromatic compounds. The applications of such sensors could range from explosive sensing to addressing environmental pollution concerns. Chapter Four, titled "Optical Sensor for Nitroaromatics Detection Using Perovskite Nanocrystals/Quantum Dots and Microfluidics," is comprised of two main parts. In Section 4A, a study was conducted to create a sensor for the detection of picric acid (PA) using luminescent organic-inorganic metal halide hybrid perovskites nanocrystals (CH3NH3PbBr3) with p-xylylene diamine as an additional capping agent. The sensor demonstrated high sensitivity and selectivity with a LOD of 0.3 μM. Upon adding 10.8 μM PA, the fluorescence of perovskite nanocrystals (PNCs) was significantly quenched (86%) due to the electrostatic interaction between picric acid and the capping ligand. In addition, the research confirmed the feasibility of using the newly created sensor for detecting PA with a 3D-printed device that incorporates paper microfluidics and surface mounting devices (SMDs). The resulting prototype device successfully detected PA via fluorescence turn-off without requiring expensive or complex equipment, thanks to the integrated excitation source. In Section 4B, a study was conducted on the efficient detection of picric acid (PA) using 2D RP-(BA)2(MA)2Pb3Br10 perovskite quantum dots

(PQD) synthesized using the LARP method. To determine the optical and structural characteristics of the PQD, several methods were employed, including XRD, TEM, UV Vis, fluorescence spectroscopy, and FT-IR spectroscopy. The study also calculated the Stern-Volmer constant (Ksv) for the sensing strategy to be 1.41 ×105 M−1 and established a detailed sensing mechanism through time-resolved and steady-state absorbance and fluorescence spectroscopy. The last part of the thesis Fifth Chapter will have the conclusion of the thesis and future outlook. This dissertation would provide new dimensions in the area of applied physics and materials science by for the already existing fluorophore.