In an article recently published in the journal ACS Applied Polymer Materials, researchers constructed three sensing materials based on luminescent conjugated microporous polymers (CMPs) with truxene (Tx) cores for highly selective and sensitive detection of picric acid (PA), a nitroaromatic explosive (NAE) compound, via Suzuki-Miyaura cross-coupling reaction.
Study: Luminescent Conjugated Microporous Polymers for Selective Sensing and Ultrafast Detection of Picric Acid. Image Credit: luchschenF/Shutterstock.com
The mode of detection was based on the degree of quenching of the fluorescence intensity of the Tx-CMP compound with an increase in the concentration of PA.
Trinitrophenol, also known as PA, is an NAE compound used in munitions, explosives, dyes, matches, electric batteries, extraction of insulin from human tissues, and medicines as an antiseptic and astringent agent. It is a highly flammable oxidizing chemical that contaminates soil and water. Thus, the detection and removal of PA are essential for the safety of the public, military, and aquatic biosystems.
Organic CMPs have extended perpetual pore channels due to continuous π-conjugated building blocks and linkages. Additionally, several polymeric reactions can perform C-C coupling reactions to form highly stable covalent bonds. Due to these reasons, CMPs have a high specific surface area with high thermal, physical, and chemical stability and have earned their place in various applications such as gas and energy storage, sensing, and separation.
CMPs with Tx cores are mostly fluorescent due to the presence of electron-rich flexible aryl linkers. The interaction of electron-rich aryl linkers of Tx with electron-deficient PA results in fluorescent quenching of the Tx-based compound. This method can be used for the ultra-fast detection of PA in both solids and liquids.
Fluorescent quenching means a decrease in the emissivity or fluorescence intensity of fluorescent materials, which may occur due to energy transfer, electron transfer, formation of excited-state complexes, formation of ground-state complexes, and collision. Aggregation-caused quenching (ACQ) is a part of collisional quenching, in which molecules aggregate due to distortion of π-π stacking of layers, which leads to a decrease in the intensity of fluorescence.
About the Study
In the present study, the researchers synthesized three CMPs with Tx cores via a facile one-step palladium-catalyzed Suzuki−Miyaura cross-coupling reaction to detect the presence of a PA-based on electron or energy transfer-type fluorescence quenching. The three Tx-CPM compounds were designated as Tx-CMP-1, Tx-CMP-2, and Tx-CMP-3 by cross-coupling the Tx precursor, i.e. 3,8,13-tribromo-5,5,10,10,15,15-hexamethyltruxene with corresponding diboronic acids, namely, (1,4-phenylene) diboronic acid, (2,5-dimethoxy-1,4-phenylene) diboronic acid, and (1,1'-biphenyl) -4,4'-diyldiboronic acid, respectively.
Subsequently, several methods were used to analyze all Tx-CPMs samples as follow: Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) were used to analyze the chemical composition; N2 absorption method was used to calculate the Brunauer-Emmett-Teller (BET) surface area; Thermogravimetric analysis (TGA) for thermal stability; Nonlocal density functional theory (NLDFT) for pore size distribution (PSD); UV-vis and photoluminescence (PL) spectroscopy for photophysical properties; and powder X-ray diffraction (PXRD) and field emission scanning electron microscopy (FE-SEM) for morphology.
The FT-IR spectra of all Tx-CPMs had weak absorption peaks between 2900 and 3100 cm−1, corresponding to the C−H stretching of the benzene and alkyl groups of the Tx core. Similarly, the peaks around 1500 cm−1 were representing C=C bonds of Tx.
All Tx-CPMs demonstrated high degrees of thermal stability with two-step degradation peaks in the TGA plot. The first degradation was due to the loss of the methyl group of Tx, whereas the second degradation was due to the decomposition of the C=C polymer backbone at higher temperatures.
The BET surface area of Tx-CMP-1, Tx-CMP-2, and Tx-CMP-3 were 915.5, 788.7, and 826.2 m2. g-1, respectively. Meanwhile, the average PSDs of the polymers were 1.38, 1.41, and 1.41 nm, respectively.
PXRD and FE-SEM results revealed a completely amorphous structure of all Tx-CPMs with a very negligible presence of the palladium catalyst residue from the cross-coupling polymerization reaction.
All Tx-CMPs demonstrated strong fluorescence in the solid-state as the aryl linkers prevent chromophore aggregation. Additionally, they showed a broad absorption in visible light regions, which is helpful in easy detection with naked human eyes. The Stern-Volmer constant for PA was measured to be 3.97 × 104, 7.35 × 104, and 2.39 × 104 M−1, which showed that Tx-CMP-2 was the most efficient and selective in detecting PA.
The researchers of this study synthesized three CMPs with Tx cores using palladium-catalyzed Suzuki−Miyaura cross-coupling reaction to trace hazardous PA waste in soil and water-based on energy transfer-type fluorescence quenching. All samples exhibited high specific surface areas; however, Tx-CMP-1 exhibited the highest surface area.
Nevertheless, Tx-CMP-2 exhibited the highest efficiency and selectivity for PA detection. The three major factors for their high efficiency were photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), and molecular electrostatic interactions. Hence, fluorescent Tx-CMPs are promising materials for PA detection.
Naiwal, Y., Devi, M., Pal, S., Luminescent Conjugated Microporous Polymers for Selective Sensing and Ultrafast Detection of Picric Acid, ACS Applied Polymer Materials, 2022, https://pubs.acs.org/doi/10.1021/acsapm.1c01905?ref=pdf