Metal Complex as an Optical Sensing Platform for Rapid Multimodal Recognition of a Pathogenic Biomarker in Real-Life Samples

Anthraimidazoledione-based charge transfer dyes have been designed for multimodal detection of a pathogenic biomarker, dipicolinic acid (DPA), at physiological pH. A change in visible color from yellow to red was observed along with an appearance of red luminescence when the probe-Eu3+ complex was exposed to DPA. Conversely, with the probe-Cu2+ complex, the solution color turns into orange in the presence of DPA with blue-colored fluorescence. Thus, the present sensory system can achieve naked-eye detection of DPA, which is very rare for metal complex-based probes. Mechanistic investigations revealed that variations in the metal ion center influence the nature of the DPA interaction, which subsequently dictates the output optical signal. DPA forms a ternary complex with the probe-Eu3+ conjugate, while in the case of Cu2+, it dissociates the preformed probe-metal ion conjugate. Interestingly, at a given concentration of DPA, the probe with the 2,2'-(phenylazanediyl)diacetic acid group (1) at the receptor end shows a more prominent change with DPA as compared to the probe with the 2,2'-((2-(carboxymethoxy)phenyl)azanediyl)diacetic acid (2) functional group. Subsequently, the system is involved in the screening of DPA in complex real-life samples, such as human blood serum, urine, natural water and soil samples, etc. In addition, the present assay can be employed for quantitative evaluation of Bacillus subtilis spores, and as low as 2.2 x 10(4) spores/mL was detected. Further, to extend the practical implication, low-cost paper-based devices are developed as an eco-friendly alternative for on-location detection of DPA.