Efficient Quenching of Two-Photon Absorption Induced Photoluminescence in Carbon Nanodots for Fe³⁺ Ion Detection

This study reports on the linear and nonlinear optical (NLO) properties of water-dispersed carbon nanodots (CNDs) fabricated via a rapid one-step hydrothermal microwave-assisted technique. The CNDs exhibit two-photon excited luminescence, which was characterized with spectrally tunable femtosecond laser pulses as involving the two-photon absorption (TPA) cross sections (σ₂) as large as 1.4 × 10³ Goeppert-Mayer (GM) at the excitation wavelength of 720 nm and the quantum yield (QYs) of 28%. By analyzing the σ₂ spectra, specific wavelength ranges optimal for excitation via the two-photon process were identified. In addition, the potential of the CNDs as sensors for the selective and sensitive detection of Fe³⁺ ions through one- and two-photon induced fluorescence quenching was investigated. To gain deeper insights into the mechanism underlying the observed decrease in fluorescence intensity upon addition of Fe³⁺ ions, potentially involving dynamic quenching, fluorescence quenching experiments across various temperatures were conducted, being the first such study in both one-photon and two-photon excitation regimes for this sensor. The possibility of energy transfer between CNDs and Fe³⁺ ions was investigated by analyzing the luminescence kinetics using time-correlated single-photon counting (TCSPC) and streak camera techniques, in one- and two-photon regime, respectively. The pronounced nonlinear optical response of the CNDs highlights their potential as active optoelectronic materials for optical sensors operating in the near-infrared (NIR) region. Cytotoxicity studies of the water-dispersed CNDs revealed no observable toxicity, even at high concentrations, making them suitable for biorelated applications.