Lanthanide-Free Infinite Coordination Polymer Nanoparticles for Real-Time Monitoring of Alkaline Phosphatase and Its Application for Digital Algal Bloom Detection

In this study, 1,1,2,2-tetra(4-carboxylphenyl)ethylene (H4TCPE) with the aggregation-induced emission (AIE) feature and sulforhodamine 101 (SR101) with the aggregation-caused quenching (ACQ) feature were rationally selected and introduced into a lanthanide-free infinite coordination polymer (ICP) host consisting of copper ions (Cu2+) and guanosine-5-monophosphate (GMP). The host–guest effect empowered the H4TCPE/SR101/Cu-GMP ICP nanoparticles with opposite emission characteristics and a blue color. Under the stimuli of alkaline phosphatase (ALP), the ligand GMP was hydrolyzed and the network structure of the Cu-GMP ICP host was destructed, resulting in the state change of both guests and the corresponding ratiometric emission changes: the AIE of H4TCPE at 450 nm (blue fluorescence) was quenched, while the ACQ of SR101 at 625 nm (red fluorescence) was recovered. As the signal response directly stemmed from the substrate, the H4TCPE/SR101/Cu-GMP ICP nanoparticles not only allowed for ALP detection with high sensitivity and specificity but also enabled ALP activity monitoring in a real-time fashion. Based on the great analytical performance, the H4TCPE/SR101/Cu-GMP ICP nanoparticles could be successfully applied for the dynamical monitoring of ALP in environmental samples under different spatial–temporal conditions. Furthermore, besides the spectral response, the blue color H4TCPE/SR101/Cu-GMP ICP nanoparticles turned pink, while dual-wavelength emissions led to a purple-blue to red fluorescence color change. These color changes could be recognized using a smartphone in the form of the R/B ratio, which offers great field application potential for the establishment of a new digital analytical platform with reliability in algal bloom warning with surging ALP as an effective indicator.