Optical hybrid nanocomposite sensors for selective explosive detection

Resumen

Over the last decade, the detection of improvised explosive devices (IEDs) in both military and civil fields has become a strategic priority in homeland security due to the increasing terrorist threat. Although conventional techniques such as gas chromatography coupled to mass spectrometry or X-ray diffraction show notable advantages such as high sensitivity and selectivity, most of them have many drawbacks such as time-consuming processes and expensive, complex, and cumbersome instrumentation, which limit continuous and real-time sampling. For that reason, there is constant research for a sensing platform suitable for different environments. The role of nanoscience has been greatly determining the development of chemical sensors with enhanced performance because of their outstanding properties. The design of low-cost, easy-to-fabricate and portable sensors with a low limit of detection (LOD), good selectivity, high sensitivity and short response time is very challenging. The goal of this PhD thesis is to synthesize different optical solid-state sensing platforms based on a nanocomposite of different nanoparticles embedded in a polymer matrix for the detection and quantification of some explosive taggants and explosive-like molecules in the vapour phase. Firstly, the synthesis of a plasmonic sensor based on a nanocomposite of Ag nanoparticles (NPs) embedded in a molecularly imprinted polymer (MIP) for selective detection of 3-NT, an explosive taggant for 2,4,6-trinitrotoluene. In our approach, the in-situ synthesis of Ag NPs and the molecular imprinting with 3-NT as a template take place simultaneously inside the polyethyleneimine (PEI) thin film during the baking step after spin coating. The chemosensing capabilities of Ag-PEI MIP nanocomposites to 3-NT using the localized surface plasmon resonance band intensity decay as a sensing parameter were demonstrated. Moreover, the molecular imprinting approach results in an enhancement of the sensor sensitivity and selectivity to 3-NT. As a result, these plasmonic sensors can be easily implemented with portable reading platforms into remote explosive detection and bomb disposal robots. In the second part, chemical sensors based on fluorescent nanoparticles such as quantum dots (QDs) and metal halide perovskites (PVKs) have been used because of their excellent optical and electronic properties. One of the sensors is based on an array containing either green-emitting or red-emitting CdSe QDs embedded in polycaprolactone (PCL) as a polymer host matrix. The sensing capability of the nanocomposites by exposing both sensors to vapours of explosive taggants, explosive-like molecules and some common solvents was evaluated. They exhibit a very fast response time of <30s and low LOD. Moreover, the sensor array constitutes a powerful tool to discriminate between explosive taggants (3-nitrotoluene, 4-nitrotoluene and 2,3-dimethyl-2,3-dinitrobutane) and shows specific molecular recognition towards picric acid. The other sensor is based on a nanocomposite of CsPbBr3 nanocrystals (NCs) embedded in a molecularly imprinted polymer (MIP) using 3-nitrotoluene (3-NT) or nitromethane (NM) as template molecules. The straightforward and low-cost molecular imprinting process occurs inside the nanocomposite of CsPbBr3-PCL during the baking step after spin-coating. The sensing capability of the MIP sensors was evaluated and compared to that of the non-imprinted polymer (NIP) by monitoring the photoluminescence (PL) upon exposure to vapours of different explosive taggants, nitro-containing molecules and some organic solvents. The nanocomposite sensors show a fast response time to analytes below 5 s. Moreover, molecular imprinting enhances the PL response of MIP sensors and the specificity to 3-NT. An excellent selectivity towards nitro containing molecules is also exhibited, particularly when NM is used as the template molecule. In the light of the reported results, this PhD thesis proposes that these sensing platforms are potential candidates for effective explosive detection in the vapour phase.