Efficient Quenching of Two-Photon Absorption Induced Photoluminescence in Carbon Nanodots for Fe 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 (σ) as large as 1.4 × 10 Goeppert-Mayer (GM) at the excitation wavelength of 720 nm and the quantum yield (QYs) of 28%. By analyzing the σ 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 Fe 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 Fe 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 Fe 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.