Adenine-stabilized carbon dots for highly sensitive and selective sensing of copper(II) ions and cell imaging

Adenine-stabilized carbon dots (A-CDs) are shown to be a viable fluorescent probe for highly sensitive detection and imaging of Cu²⁺. The probe has a linear fluorometric response in the 1-700 nM concentration range and a 0.3 nM detection limit. The probe, with excitation/emission maxima at 380/435 nm, is highly selective for Cu²⁺ over other metal ions, anions, amino acids, and biomolecules. The fluorescence quenching mechanism of the A-CDs by Cu²⁺ is investigated using transmission electron microscopy images coupled with elemental mapping, X-ray photoelectron spectroscopy, X-ray-excited Auger electron spectroscopy, fluorescence lifetime, UV-visible spectroscopy, and cyclic voltammetry. The experimental results show that the fluorescence quenching is caused by the combination of Cu²⁺-coordination-induced aggregation of the A-CDs, the reduction of Cu²⁺ by the A-CDs, and the nonradiative photoinduced electron transfer process from the A-CDs to Cu²⁺ or metallic Cu. The high sensitivity and high selectivity of the sensor are ascribed to the chemical interactions between the A-CDs and Cu²⁺, the photophysical process between the A-CDs and Cu²⁺, and the high fluorescence quantum yield of the A-CDs (44.6%). The A-CDs have excellent water solubility, good stability to variation of pH values, high photostability, fast response time, and low cytotoxicity. They are successfully employed for intracellular imaging of Cu²⁺ in HepG2 cells and Cu²⁺ detection in the tap water samples.