Design of Colorimetric Sensor Employing CdSe QDs Decorated Graphene Xerogel for Onsite Visual Detection of TNT
Authors: Kumar Vishal , Kumar Anshu , Nath Prathul , Satapathi Soumitra 
Published: July 2021

In today's world, extensive use of various organic explosives such as nitroaromatics, nitramines and peroxide can be seen in both military and terrorism. Among the different explosives, 2,4,6-Trinitrotoluene (TNT) and its degraded compound 2,4-Dinitrotoluene (DNT) are the most used energetic material in different legitimately produced explosives and improvised explosive devices (IEDs). The detonation of explosives usually causes damages to the environment, which in turn affects humans’ health and safety. Owing to the importance of explosive detection for the security of land and the environment, the exploration of new methodologies for sensing electron-deficient nitroaromatics explosives (NAEs) is urgently imperative. [1,2] In recent years, a number of fluorescent chemosensors/probes have been developed and widely used in chemical sensing of DNT and TNT, especially quantum dots (QDs) and nanoparticles, because of their comprehensive investigation in various applications such as optical sensor, bio-imaging, bio-labeling, biomolecular and NAEs detection. [3] Here, our research efforts are to promote fluorescent Cysteamine capped-CdSe QDs for decorating porous graphene xerogel (GSXS) to design a robust sensor with a new and elegant methodology. Consequently, this sensor can provide good opportunities for visual TNT detection in wearable applications.
Graphene oxide (GO), a single monolayer of graphite, has recently attached great scientific attention; besides, the 3D-porous structure of GO has shown impulsive properties, such as large surface areas, high compressibility, ultralow density and adsorption potential. On the other hand, chitosan (CS), alkaline deacetylation of chitin, owned many fascinating properties such as biocompatibility, biodegradability and antimicrobial ability. The 3D-network of GO and CS blend scaffold is expected to have high porosity, surface area, electrochemical properties and high adsorption potential because of hydrogen-bond linking of epoxy groups of GO with the amino group of CS. This nanocomposite could effectively adsorb the QDs and NPs via covalent and non-covalent interactions. Inspired by this top-notch property of GO-CS nanocomposite and the excellent fluorescence property of CdSe QDs, a simple and elegant approach is introduced to design a colorimetric sensor based on QDs decorated graphene xerogel (QD-GSXS), also called GO-CS nanocomposite. Its dual benefit can provide a novel path of sensitive and selective sensing for chemical and biological applications. The preparation of this fluorescent QD-GSXS nanosensor would be a challenge of relevance because of the fluorescent material is usually quenched by graphene. [4] So, this work aims to successfully synthesis fluorescent QD-GSXS nanosensor and exhibits its application for on-site and visual detection of TNT via the formation of the Meisenheimer complex.
Herein firstly, we synthesized porous graphene xerogels wrapped with an optimized mass ratio of chitosan. This xerogel surface is further decorated with highly fluorescent Cysteamine capped-CdSe QDs. These composite were characterized using XRD, FE-SEM, FTIR, Raman, BET, UV-Vis and fluorescence spectroscopy. This spongy fluorescent QD-GSXS nanosensor probe was employed for the sensitive and selective detection of TNT. The sensing mechanism is further stabilized via Time-correlated Single Photon Counting (TCSPC) measurement and with the frontier-molecular energy level analysis, calculated by Density-Functional-Theory (DFT). More importantly, we further demonstrated the utility of this designed nanosensor probe as on-site visual TNT sensor by observing a rapid fluorescence change in optical images captured under the illumination of Green Fluorescent Protein (GFP). The design chemosensor showed several advantages, including good selectivity, excellent stability and having linearity with the concentration of TNT. Such a simple colorimetric sensor allows the visual detection of TNT at µM level without any sophisticated instrument. In addition, CdSe-NH2 QDs were also exhibited sensing with DNT and TNT in solution with LOD 18.2 and 9.7 µM, respectively. Considering their structure versatility and design flexibility, we anticipate that our efforts were to combine the electron-donor unit, cystamine capped-CdSe QDs, with an intrinsic porosity of GSXS and make it functional for NAEs detection. This novel and elegant approach gives a new direction to make a prototype chemosensor for visual detection of different NAEs with the advantages of simplicity, ease of operation and high sensitivity. However, there is a wide scope to increase the sensitivity further. This approach will also benefit future research on developing fluorescent hybrid nanosensors based on fluorescent nanoparticles and quantum dots for various chemical and biological applications.

References:

[1] V. Kumar, B. Maiti, M.K. Chini, P. De, S. Satapathi, Multimodal Fluorescent polymer sensor for Highly sensitive Detection of Nitroaromatics, Scientific Reports 9(1), 7269 (2019); doi: 10.1038/s41598-019-43836-w
[2] V. Kumar, N. Choudhury, A. Kumar, P. De, S. Satapathi, Poly-tryptophan/carbazole based FRET-system for sensitive detection of nitroaromatic explosives, Optical Materials 100, 109710 (2020); doi: 10.1016/j.optmat.2020.109710
[3] V. Kumar, A. Kumar, M.K. Chini, S. Satapathi, Fluorescent Fe2O3-CdSe nanocomposite probe for selective detection and removal of picric acid, Materials Chemistry and Physics 260, 124130 (2020); doi: 10.1016/j.matchemphys.2020.124130
[4] V. Kumar, A. Kumar, P. Nath, S. Satapathi, Fabrication of Cysteamine capped-CdSe QDs anchored graphene xerogel nanosensor for facile onsite visual detection of TNT, Nano-Structures & Nano-Objects 25, 100643 (2020); doi: 10.1016/j.nanoso.2020.100643

Figure 1