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Developing new conjugated polymeric microporous architectures as solid state sensors for detection of harmful nitroaromatic derivatives
Project status: Ended
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Stage I (2022) of the project had as specific objective the synthesis and characterization of oligomers with conjugated structure and star/branched architecture, containing triphenylamine units connected by different π-spacers, and their electropolymerization in order to obtain microporous conjugated polymers.

In the stage I, the following results were achieved:

  • Synthesis of three oligomers with p-phenylene-vinylene and p-phenylene-vinylene-ethynylene conjugated structures, and star-shaped architectures. The synthetic approach involved a convergent synthesis method, via coupling reactions between derivatives with linear structure (containing peripheral triphenylamine units) and a central triphenylamine unit.
  • Structural confirmation of the obtained oligomers was achieved through NMR spectroscopy (1H and 13C) and FT-IR techniques, elucidating their chemical composition. UV-Vis and fluorescence spectroscopies were employed to assess their optical properties, while cyclic voltammetry helped to investigate their electrochemical features. These experiments highlighted the influence of the π-spacers between the triphenylamine units on the physico-chemical properties of the oligomers.
  • The optimization step of the electropolymerization protocols involving the oligomers resulted in the identification of optimal electrodeposition conditions; at the end of the successive electro-oxidation/reduction processes, conjugated polymers were obtained as uniform coatings (without cracks), adhering to the working electrode used;
  • Structural characterization of the obtained conjugated polymers was achieved by using FT-IR spectroscopy, while their optical properties were investigated by using UV-Vis and fluorescence spectroscopies. Electrochemical analysis of the electrodeposited polymer layers revealed good electrochemical stability and color shifts upon applied potential, prompting evaluation of their electrochromic properties. Surface morphology of the electrodeposited coatings was scrutinized via SEM spectroscopy.

Part of the obtained results were disseminated in form of 1 presentation at an international scientific event; and 1 scientific paper published in ISI journals, which brings acknowledgments to the financial support offerd by this project.

Stage II (2023) of the project had as specific objective the synthesis and characterization of oligomers with conjugated structure and star/branched architecture, used as precursors in polymerization processes (electrochemical and emulsion) with the aim of obtaining microporous conjugated polymers and the further evaluation of their efficiency in the detection of nitroaromatic harmful derivatives.

In the stage II, the following results were achieved:

  • Synthesis of two oligomers with conjugated structure and star/branched architecture, containing triphenylamine units (peripheral and central) and benzothiadiazole. Structural confirmation of the synthesized oligomers was performed by using 1H NMR and FT-IR spectroscopies; the optical properties were investigated by UV-Vis and fluorescence spectroscopies, while the cyclic voltammetry helped to evaluate their electrochemical characteristics. The structure-property correlations made on the basis of the obtained experimental data highlighted the influence of the π-spacers between the triphenylamine units;
  • Synthesis of two series of polymers by using two distinct polymerization protocols: electrochemical polymerization and emulsion polymerization; the optimization of polymerization conditions leads to the obtaining of polymers both in the form of uniform layers adhering to the support, and in powder form (soluble and insoluble in common organic solvents). FT-IR spectroscopy confirmed the structure of the obtained polymers, while optical properties were evaluated both in solid-state and solution mediums.
  • The surface morphology evaluation by using SEM spectroscopy revealed insights into the organization/ordering of polymer chains based on precursor structures and the significant impact of the polymerization method used. The BET gas adsorption isotherms allowed a more detailed assessment of porosity.
  • The evaluation of the detection capability for nitroaromatic derivatives via electrochemical methods and fluorescence spectroscopy demonstrated that obtained polymers, either in the form of electrogenerated layers or as solid particles in solution, can function effectively as receptors for 2,4-dinitrotoluene and trinitrophenolic acid.

Part of the results obtained in the Stage II were disseminated in form of 2 presentations and 1 poster presentation at 3 international scientific events, and 2 scientific papers published in ISI journals, which bring acknowledgments to the financial support offered by this project.

Stage III (2024) of the project aimed to test the efficiency of CMPs (obtained in Stage II) in the electrochemical/fluorescent detection of harmfull nitroaromatic derivatives and a critical discussion on the project results and preparation of the final report. 

In the stage III, the following results were achieved:

  • Four series of polymers obtained by electrochemical and oxidative chemical emulsion polymerization of two compounds: the oligomer with mixed structure of p-phenylene-vinylene-ethylene and a star architecture (TPAS2), and the polymer with mixed structure of p-phenylene-(CN)vinylene-ethylene and containing alternating triphenylamine and benzothiadiazole units (PTPA-P4), have been investigated for the electrochemical/fluorescent detection of nitroaromatic derivatives.
  • Electrochemical tests were performed to evaluate the sensitivity of two polymers generated electrochemically and deposited on ITO/glass electrodes for detecting two nitroaromatic derivatives: trinitrophenol (TNP, picric acid), and 2,4-dinitrotoluene (DNT). The recorded E(V) vs i(mA) curves, plotted against nitroderivative concentration, demonstrated a satisfactory response with the PTPAS2 polymer (obtained by electropolymerization of TPAS2 oligomer). In case of this polymer, specific cathodic reduction peaks characteristic of nitroaromatic derivatives were observed, with a detection limit of 49.0 μM for TNP which was lower than 147 μM recorded for the PTPA-P4 polymer (generated electrochemically). Nonlinear responses of cathodic peak current intensities vs [TNP] concentration were observed in both cases. For the DNT derivative, the detection limit was higher, and the cathodic reduction peaks were less pronounced.
  • The conclusion of this activity: At a fixed concentration of 98 μM TNP, the sensitivity of the electrogenerated polymers investigated in this study/project varies significantly, depending on several factors: the chemical structure, the morphology of the electrogenerated polymer films, and the type of nitroaromatic derivative. Among the studied polymers, the ability to detect the nitroderivative ranks as follows: PTPAS1 > PTPAS2 > PTPAS3 > PTPA-P4.
  • Two polymers obtained by emulsion polymerization were tested as solid particles for fluorescent detection of nitroaromatic derivatives (TNP and DNT) in both solution and vapor phase. The detection mechanism was based on the quenching phenomenon of the fluorescent emission of polymers in the presence of nitroaromatic derivatives, recording the emission curves and plotting the Stern-Volmer graphs, which allowed the determination of the quenching constant, KSV. The quenching efficiency of TNP and DNT derivatives on P2em polymer fluorescence is different, having a fluorescence quenching efficiency of 85 % (TNP) and 76 % (2,4-DNT), and quenching constant values of 10.7 x 103 M-1 (for TNP) and 6.0 x 103 M-1 (for DNT), and these values are average values compared to those obtained for the other polymers. The film of P2em polymer particles drop-cast onto the surface of a quartz substrate was exposed to TNP vapor for different time intervals, and the fluorescence emission spectra were recorded after each exposure. The spectral response reveals a decrease in fluorescence emission intensity even after 1 second, with a quenching efficiency of 65 % after 2 h.
  • The conclusion of this activity: The polymers synthesized and investigated in this study/project (P1em-P3em), whether as drop-cast films or as particles dispersed in solution, demonstrate the capability to detect nitroaromatic derivatives (especially TNP) in both solution and vapor phases through fluorescence quenching. However, both the quenching efficiency and the determined quenching constants (KSV) indicate that the sensitivity of the polymers depends on the polymer structure, the chosen polymerization method, and the type of nitroderivative.

The dissemination of the results obtained within the stage III, consisted in 1 presentation at an international scientific event and 1 scientific article (under review in a ISI journal), which brings acknowledgments to the financial support offered by this project.


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