In this study, by rationally designing the stimulus response of graphene quantum dot (GQD)-sensitized terbium/guanine monophosphate (Tb/GMP) infinite coordination polymer (ICP) nanoparticles, we have constructed a smartphone-based colorimetric assay with ratiometric fluorescence, which could be applied for the detection of acetylcholinesterase (AChE) and organophosphorus pesticides (OPs) directly. First, GQDs with abundant functional groups were chosen as the guest, which not only could be used as one of the signal readouts but also served as the antenna ligand to further sensitize the fluorescence of the host Tb/GMP. Upon being excited at 330 nm, the green fluorescence of the Tb/GMP host is highly enhanced, while the blue fluorescence of GQDs is suppressed due to the confinement of the ICP host. With the presence of thiocholine (TCh), an enzymatic product hydrolyzed from acetylthiocholine (ATCh) by AChE, the competitive coordination of Tb3+ between GMP and TCh results in the collapse of the ICP network and thereby the release of GQDs into the solution; thus, the fluorescence of Tb/GMP turns off and the fluorescence of GQDs turns on. The dual-responsive ratiometric fluorescent intensity change leads to the corresponding green-to-blue fluorescent color change obviously, which constitutes a novel mechanism for the colorimetric analysis of AChE. Moreover, when OPs are subsequently introduced, the activity of AChE is blocked, thus preventing the stimulus response of GQD@Tb/GMP ICP nanoparticles, leading to the fluorescent color change from greenish-blue to green, which could also be employed for OP detection. Benefitting from the high sensitivity, good reliability, and the obvious color changes, the method demonstrated here is a promising candidate to realize smartphone-based point-of-use applications, which is of great importance for timely clinical diagnosis and treatment of OPs related to health issues with AChE as an exposure biomarker in less industrialized countries, in remote settings, or even in home care services.
Keywords: GQD@Tb/GMP ICPs; acetylcholinesterase; dual-responsive ratiometric fluorescence; organophosphorus pesticides; point-of-use analysis.