Lanthanide magneto-luminescent and plasmonic (Gd₂O₃:Eu@AuNR) nanoassembly for the turn-on fluorescence detection of nitro aromatic compound

In this study, we design a nanoassembly-based chemosensor possessing the fluorescence in the visible region which comes into play for analyte detection in aqueous medium. Here, Mercaptopropionic acid-functionalised nanophosphor (Gd₂O₃:Eu @ MPA) acts as donor, and the Cysteamine functionalised gold nanorod (AuNR @ Cysteamine) acts as the acceptor molecule. The working principle of this nanoassembly is the FRET phenomenon which happens between nanophosphors and gold nanorods through amine-carboxyl attractive interactions (Turn-Off) followed by the Meisenheimer complex formation between –NO₂ groups of TNT and primary amines of the Cysteamine functionalised AuNR (Turn-On) which gives corresponding fluorescent signals in the visible regions. The fluorescence turn-on is immediate, and the limit of detection is as low as 11.88 x 10⁻⁹ M. The above-mentioned phenomena were substantiated using the UV–Visible, Photoluminescence, and Time-Correlated Single Photon Counting spectroscopic techniques. The size, morphology, particle interactions, charge, and functionalisations were substantiated through TEM, DLS, Zeta potential, and FTIR techniques. The size variations happened to the AuNR in three different stages are evident from the TEM images. (i) when AuNR (Gold nanorod) is present alone, i.e. LnNp and TNT free system, the average size of AuNR was 15.17 nm (ii) When LnNp (Lanthanide Nanophosphor) was added (attached), i.e. AuNR + LnNp involved state, the average size of AuNR was increased to 23.05 nm (iii) When TNT was introduced to AuNR + LnNp system (Analyte attachment and LnNp detachment happening state) i.e. AuNR + LnNp + TNT involved state, the average size of AuNR was decreased to16.3 nm as it was in its pristine form. The same trend was obtained for the DLS measurements.