New chemical sensors based on lanthanide emission that enable sequential detection of two targets in live cells have been developed. The indicator molecules (unsaturated europium complexes) were covalently immobilized on carbon quantum dots (CQDs), and a water soluble fluorescent nano-probe (CQDs-DTPA-Eu) with biocompatibility was fabricated. A novel stepwise assembly method was used to generate unique responses to two different targets sequentially. Rapid and highly selective detection of tetracycline (Tc) in water and milk (detection limit 32 nM) were demonstrated. The in situ generated CQDs-DTPA-EuTc was non-emissive, but, upon addition of hydrogen peroxide (detection limit 21 nM) intense red luminescence was observed. Highly reproducible results were obtained in Plat-E cells, demonstrating the identification of tetracycline and H<inf>2</inf>O<inf>2</inf> in cellular environment with high accuracy.
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