Detection of Nitroaromatic Compounds through Fluorescence Quenching of Pyrene Labeled Starch Nanoparticles

Abstract

The aim of this research was to demonstrate the potential application of starch nanoparticles (SNPs) labeled with 1-pyrenebutyric acid (PBA) as the active component in optical sensors used for the detection of minute quantities of nitroaromatic compounds such as 2,4,6-trinitrotoluene (TNT). Pyrene-labeled SNPs (Py-SNPs) with pyrene content ranging from 0.06 to 39 mol% were prepared. Quenching of the Py-SNPs by nitromethane (NM), 4-mononitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and TNT was investigated in dimethyl sulfoxide (DMSO) and water. The bimolecular quenching rate constant for Py-SNPs in DMSO (kqSNP) remained constant with pyrene content and was found to equal 1.6 (± 0.1), 3.9 (± 0.2), 2.2 (± 0.2), and 1.6 (± 0.2) Mns for NM, MNT, DNT, and TNT, respectively, averaged across all Py-SNPs for a given quencher. These quenching experiments revealed that the quenching efficiency in DMSO increased according to the sequence MNT>DNT>TNT=NM, where MNT was the most efficient quencher and TNT and NM were the worst quenchers. Quenching studies conducted in water for Py-SNPs quenched by NM showed increased protective quenching with increased pyrene content, suggesting that hydrophobic aggregation of the pyrene labels induced a collapse of the Py-SNPs in water that hindered the diffusion of NM to access the pyrene labels. Hardly any diffusive quenching was observed in water for the quenching of the Py-SNPs by MNT, DNT, and TNT. Rather these quenchers targeted hydrophobic aggregates of pyrene labels, which emitted as excimer. The binding of the quenchers to the pyrene aggregates of the Py-SNPs increased with increasing pyrene content and followed the sequence TNT >> DNT > MNT. In addition to quenching studies conducted in solution, quenching studies were repeated with MNT, DNT, and TNT on solid films of Py-SNPs coated filter papers (Py-CFPs). Vapour quenching studies yielded response times of 0.48 (± 0.05) and 3.6 (± 0.3) min for air saturated with MNT and DNT, respectively. Detection limits for the quenching of Py-CFPs by MNT, DNT, and TNT deposited onto the filter paper were found to equal 30 (± 9), 11 (± 5), and 1.4 (± 0.6) ng per mm. In terms of detection limits, these results place the Py-CFPs among the best optical sensors currently available for the detection of TNT.