A portable colorimetric-fluorometric dual-mode paper-based sensor based on NCDs@NH2-MIL-88B@rMIP for the detection of oxytetracycline in water.

A novel bifunctional nanozyme composite with enzyme-mimetic activity and fluorescence characteristics was prepared utilizing a dual-solvent method, which nitrogen-doped carbon dots (NCDs) were in-situ confined within the pores of NH2-MIL-88B (NCDs@NH2-MIL-88B), and then an oxytetracycline (OTC) molecularly imprinted polymer film is synthesized on the surface of NCDs@NH2-MIL-88B, using 3-Aminopropyltriethoxysilane (APTES) as the functional monomer and Tetraethyl orthosilicate (TEOS) as the cross-linking agent. The fluorescence intensity of NCDs@NH2-MIL-88B was enhanced based on the restricted intramolecular motion (RIM) effect and fluorescence resonance energy transfer (FRET). In addition, the complexation reaction between OTC and Fe3+ on NCDs@NH2-MIL-88B@rMIP promoted the electron transfer rate, improved the enzyme-mimetic activity, and thus achieved the enhancement of dual signals. Furthermore, a portable dual-mode colorimetric-fluorometric sensor was constructed by combining with a paper-based device for the on-site detection of OTC. Using a colorimeter and a smartphone as signal output devices, portable detection of OTC in water is achieved. The colorimetric mode exhibits a detection range from 1.00 × 10-9 mol L-1 to 1.00 × 10-5 mol L-1, while the fluorometric mode demonstrates a broader range of 1.00 × 10-10 mol L-1 to 1.00 × 10-5 mol L-1. Detection results of the same samples using the dual-mode sensor and the LC-MS method confirmed the accuracy of the proposed approach. This work offers the potential possibility to meet the requirements for the on-site analysis.