Ethylenediamine-bound magnetite nanoparticles as dual function colorimetric sensor having charge transfer and...

A reusable, low-cost, and convenient ethylenediamine (EDA)-bound magnetite nanoparticles (MNPs)-based colorimetric sensor has been developed for dual function colorimetric determination of nitroaromatic explosives such as TNT and tetryl. Colorimetric detection of analytes may occur through two independent routes: (1) nano-Fe₃O₄- EDA- NH₂ as σ-donor may interact with the σ- and π-acceptor aromatic-poly(NO₂) groups to produce a colored charge-transfer (CT) complex; (2) nano-Fe₃O₄-EDA-NH₂ as a Fenton-type nanozyme may generate reactive species that comprise hydroxyl radicals (^•OH) with H₂O₂ to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to a blue-colored diimine (oxTMB-TMB) CT complex, where this color is bleached with TNT/tetryl because of donor-acceptor interactions between the explosive -NO₂ groups and the -NH₂ group of Fe₃O₄-EDA nanoparticles of restricted nanozyme activity. Both methods can quantify TNT well below the EPA recommended TNT residential screening level in soil, LOD being in the micromolar range. As EDA was covalently bound to MNPs, the same sensor can be separately reused six times for TNT and eight times for tetryl determination, using method (1). Common metal ions, anions, energetic materials, several camouflage materials, and soil components such as humates did not interfere with the nanosensor performance for TNT and tetryl. The combination of charge-transfer and nanozyme ability of Fe₃O₄- EDA-NH₂ nanoparticles may bring a new approach to dual function colorimetric sensor design. To the best of our knowledge, this is the first dual function colorimetric sensor for TNT and tetryl using the same nanoparticles as sensing elements in two different detection systems involving either formation or bleaching of colored species. The proposed colorimetric sensor can determine nitroaromatic explosives in two different ways: method-1 for TNT and tetryl sensing with EDA-MNPs relies on the donor-acceptor interaction between the electron-deficient nitroaromatics and electron-rich amine groups covalently functionalized on MNPs to produce an absorbance at 512 nm. In method-2, EDA-MNPs having nanozyme activity react with H₂O₂ to form reactive species that can oxidize TMB to its blue-colored charge-transfer (CT) complex, where TNT and tetryl addition may partially inhibit the nanozyme activity of EDA-MNPs and cause color bleaching (decrement of 650 nm absorbance) by disrupting the CT complex formed from TMB. This is the first dual function colorimetric sensor for nitro explosives uniquely combining charge-transfer and nanozyme ability of EDA-Fe₃O₄ nanoparticles in the same nano-sensor.