Portable multicolor molecularly imprinted chitosan gel beads for smartphone-assisted tetracycline screening in milk

Rapid screening of tetracycline (TC) residues in milk is important for dairy quality control and regulatory compliance. In this study, portable multicolor molecularly imprinted chitosan gel beads embedded with europium-doped carbon dots (Eu-CDs@MIP) were developed as a visual and smartphone-assisted screening platform for TC in pretreated milk samples. The beads produced a concentration-dependent fluorescence color transition from blue to red under 365 nm UV light, enabling both ratiometric fluorescence quantification and RGB-based image analysis. The method showed linear responses in the range of 0-200 nM, with limits of detection (LOD) of 7.0 ± 0.12 nM for F 620 /F 486 and 9.9 ± 0.09 nM for the R/B ratio. In spiked commercial milk samples with different fat contents, recoveries ranged from 92.3% to 106.65% with satisfactory reproducibility. Agreement with liquid chromatography-mass spectrometry (LC-MS) was confirmed by Passing-Bablok regression and Bland-Altman analysis. The visual color output, short assay time, and smartphone compatibility support the use of this method as a screening and triage tool with portable readout for laboratory-pretreated milk samples.