Layer-by-layer assembly of magnetic-core dual quantum dot-shell nanocomposites for fluorescence lateral flow detection of bacteria

Lateral flow immunoassay (LFA) strips are extensively used for rapid tests of various biochemical molecules, but these strips still have some limitations in bacterial detection due to their low sensitivity and poor stability in complex samples. In this study, we reported a highly sensitive and quantitative fluorescent LFA strip for bacterial detection by using novel magnetic-core@dual quantum dot (QD)-shell nanoparticles (Fe₃O₄@DQDs) as multifunctional fluorescent labels. The Fe₃O₄@DQDs were prepared through a polyethyleneimine (PEI)-mediated layer-by-layer (LBL) assembly method, and they possess monodispersity, high magnetic responsiveness, good stability, and superior fluorescence properties. Based on these merits, the Fe₃O₄@DQDs were used to capture and enrich bacteria from complex samples and then used as advanced fluorescent labels of LFA strips for the quantitative detection of bacteria. Under optimal conditions, the assay ultra-sensitively detected Streptococcus pneumoniae with a low limit of detection of 8 cells per mL and a wide dynamic linear range of 10 cells per mL to 10⁷ cells per mL. Systematic comparison revealed that the fluorescence detection limit of the Fe₃O₄@DQD-based strip was 55 and 1000 times higher than those of Fe₃O₄-core@QD-shell nanocomposite (Fe₃O₄-QD)-based and conventional QD microsphere-based strips, respectively. The proposed method also exhibited high specificity and selectivity for biological samples (human whole blood and sputum) and is thus a promising tool for real bacterial sample testing.