A DNAzyme Catalytic Beacon Sensor for Paramagnetic Cu²⁺ Ions in Aqueous Solution with High Sensitivity and Selectivity

TLDR

Copper is a key environmental and biological metal ion, and high concentrations can cause adverse health effects, yet few fluorescent sensors exist for the paramagnetic Cu²⁺ ion despite progress with diamagnetic ions. The authors aim to develop a highly sensitive, selective turn‑on fluorescent sensor for Cu²⁺ based on an in vitro selected DNAzyme. The sensor uses a DNAzyme with a fluorophore‑quencher pair on the substrate strand and an additional quencher on the enzyme strand, enabling fluorescence activation upon Cu²⁺‑induced oxidative cleavage. The sensor exhibits a 13‑fold fluorescence increase upon Cu²⁺ addition, a detection limit of 35 nM, a dynamic range up to 20 µM, selectivity exceeding 2000‑fold over Fe²⁺ and UO₂²⁺ and 10,000‑fold over other metals, and demonstrates potential for extending the approach to other paramagnetic ions.

Abstract

Copper is a key metal ion both in environment monitoring and in biology, and exposure to high concentration of copper can cause adverse health effects. Although significant progresses have been made in designing fluorescent sensors for diamagnetic metal ions, few effective Cu2+ sensors are known because of the paramagnetic nature of the metal ion. We herein report a highly sensitive and selective "turn-on" fluorescent Cu2+ sensor based on an in vitro selected DNAzyme (also known as catalytic DNA or deoxyribozyme). The substrate strand of the DNAzyme was labeled with a fluorophore on the 3'-end and a quencher on the 5'-end, and the enzyme strand was labeled with a second quencher on the 5'-end. Initially, the fluorescence was quenched. Addition of Cu2+ induced oxidative cleavage of the substrate, and the fluorescence intensity increased by 13-fold. The sensor has a detection limit of 35 nM and a dynamic range up to 20 μM. The sensor selectivity is more than 2000-fold for Cu2+ over Fe2+ and UO22+ and more than 10 000-fold over any other metal ions. The DNAzyme catalytic beacon method demonstrated here can be applied to designing turn-on fluorescent sensors for other paramagnetic metal ions.