A study on the photophysical properties of strong green-fluorescent N-doped carbon dots and application for pH sensing

Abstract

Color tuneable effective pH sensor in both acidic and alkaline medium is important for environment and bioanalytical purpose. In this report, natural biomass derived N-doped fluorescent carbon dots are utilized as pH sensor. The fast microwave irradiation method was employed for synthesis using natural sweet lime juice extract. Ethylenediamine (EDA) was used for nitrogen doping with different amounts. The prepared N-doped carbon dots are comprehensively studied by various characterization methods to analyse the effect of dopant concentrations on photophysical and luminescence properties. Improved thermal stability from 900 °C to 1000 °C with EDA content is observed. The fluorescence spectra of prepared N-doped carbon dots exhibited stable green emission with high quantum yield 83 % under ultraviolet and 440 nm irradiation wavelengths for an optimal concentration of EDA. The color of emission confirmed by CIE plot and about 3.47 ns decay time is found by PL decay curve. The anti-photo bleaching ability and good photostability for 10 days are found. The prepared N-doped carbon dots exhibit high ionic stability and bio compatibility. Further, pH dependent color tuneable (green to yellow) emission spectra of prepared N-doped carbon dots displayed high fluorescence in acidic medium at pH -5. Although, emission was quenched and red shifted in alkaline medium. The effective pH-sensitivity, and good reversibility of fluorescent N-doped carbon dots samples were tested with real water samples having different pH values. The reasonable mechanism is discussed. It depicts the prepared N-doped carbon dots have potential to serve as pH sensor for physiochemical and bioanalytical measurements.