Concentration-dependent emission of nitrogen-doped carbon dots and its use in hazardous metal-ion detection
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Carbon Letters https://doi.org/10.1007/s42823-020-00182-6
ORIGINAL ARTICLE
Concentration‑dependent emission of nitrogen‑doped carbon dots and its use in hazardous metal‑ion detection Jayasmita Jana1 · Seung Hyun Hur1 Received: 2 June 2020 / Revised: 16 August 2020 / Accepted: 28 August 2020 © Korean Carbon Society 2020
Abstract Nitrogen-doped carbon dots (CDts) with tunable fluorescence properties in aqueous media were synthesized hydrothermally. The excitation wavelength variation to obtain the maximum emission produced a blue shift in the emission peaks upon dilution in an aqueous solution. The shift can be explained by a re-absorption phenomenon in a concentrated solution. The interparticle interaction within was responsible to show dilution-dependent optical behavior. The as-synthesized solution of CDts did not show any prominent absorption peak over a wide range. However, upon dilution, two peaks became predominant. The concentration-dependent behavior was observed during the interaction with metal cations. Cationic salts of Co(II) and Hg(II) caused quenching at different dilutions of CDts. This might be explained by the exposure of different surface functional groups during dilution and metal-ion–CDts charge transfer. The quenched fluorescence of CDts was rescued using ascorbic acid. Therefore, the one-pot detection of Co(II)/Hg(II) and ascorbic acid was designed through a ‘Turn Off/On’ phenomenon. Keywords Carbon dot · Excitation dependence · Dilution · Sensing · Hazardous metal ions
1 Introduction In the recent years, carbon dots (CDs) have emerged as an efficient fluorescent probe owing to their low cytotoxicity, water solubility, resistance towards photobleaching, wide range of absorption, tunable emission, and rich surface functional groups [1]. Since their discovery in 2004 [2], the origin and mechanism of the fluorescence of CDs have been studied widely. The process of radiative recombination, surface functionalization, quantum confinement, and doping with a hetero atom [3–5] might cause the intriguing fluorescence of CDs. The associated fluorophore has also been investigated in search of the origin of the emission. CDs show an unusually large Stokes shift and mostly the excitation wavelength-dependent emission [6, 7]. Fu et al. [7] reported that the absorption and fluorescence spectra of CDs Electronic supplementary material The online version of this article (https://doi.org/10.1007/s42823-020-00182-6) contains supplementary material, which is available to authorized users. * Seung Hyun Hur [email protected] 1
School of Chemical Engineering, University of Ulsan, Daehak‑ro 93, Nam‑gu, Ulsan 44610, Republic of Korea
are similar to that of a mixture of small polycyclic aromatic hydrocarbons, namely, pyrene, coronene, and pyrelene. The large Stoke’s shifts could be explained by the formation of excimer aggregates of these hydrocarbons. Therefore, the optical properties of CDs could be explained by considering the associated hydrocarbon subunit as a fluorophore base. Another study showed th
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