Fluorescence Detection of Endotoxin Using Cadmium Complexed with Dipicolylamine-type Pyrene Probe

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Endotoxin (lipopolysaccharide: LPS) is a cell wall component of Gram-negative bacteria. A rapid detection technique for LPS, a potent immune stimulant, is highly desired as the current detection method using the limulus amebocyte lysate (LAL) assay is time-consuming. Against this backdrop, we developed a novel analytical method to detect LPS using cadmium complexed with a dipicolylamine (dpa)-type pyrene probe (Cd-dpa-C4Py). This probe showed greater fluorescence enhancement at 474 nm in response to LPS compared with a previously reported zinc-type probe (Zn-dpa-C4Py). Moreover, the selectivity of Cd-dpa-C4Py for LPS over other phosphate derivatives, i.e., biomolecules that often coexist with LPS, was further improved; no phosphate derivatives other than LPS gave a substantial fluorescence response. It was demonstrated that the one-point replacement of the central metal complexed with dpa-C4Py played an important role in changing the fluorescence properties, including sensitivity and selectivity, possibly because the ionic radius of Cd²⁺ is larger than that of Zn²⁺, and because the two differ in hardness as Lewis acids according to the hard and soft acids and bases (HSAB) theory. Cd-dpa-C4Py is a promising chemical sensor especially for use in the rapid detection of LPS in water samples that may contain other phosphate derivatives, given its prominent selectivity.