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

Abstract

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 1-700nM concentration range and a 0.3nM detection limit. The probe, with excitation/emission maxima at 380/435nm, 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.