Chemiluminescence of carbon dots under strong alkaline solutions: a novel insight into carbon dot optical properties

Abstract

We report the surprising chemiluminescence (CL) behavior of fluorescent carbon dots in the presence of a strong alkaline solution, such as NaOH or KOH. The CL intensity was dependent on the concentration of the base and carbon dots in a certain range. A possible CL mechanism was studied by UV-Vis, fluorescence, CL, FTIR, XPS and EPR spectroscopy. Radiative recombination of the injected electrons by "chemical reduction" of carbon dots with thermally excited generated holes was proposed, which sheds new light on the characteristics of carbon dots.As a rising star of the nanocarbon family, carbon nanodots have attracted increasing attention due to their excellent luminescent properties and wide applications in many areas of fundamental and technical importance.1 Intense research still focuses on their further application and intrinsic luminescence mechanism.Luminescent properties of carbon dots are usually investigated by photoluminescence (PL) produced using photoexcitation, 2-8 and electrochemiluminescence (ECL) generated by electron injection. 9-12However, chemiluminescence (CL) generated from chemical energy excitation through a chemical reaction has been rarely used to study the optical properties of carbon dots. To the best of our knowledge, only Lin 13-15 and Cui's 16 groups have recently described the CL behavior of carbon dots when they coexist with oxidants (KMnO 4 , Ce(IV), NaIO 4 ) or an ultraweak chemiluminescence system (H 2 O 2 -NaHSO 3 , H 2 O 2 -HNO 2 ) under acidic conditions. Herein, we observed for the rst time that CL was generated when only a strongly alkaline solution was injected into the carbon dots, without the presence of any CL reagent, CL system or oxidants. Our initial purpose was to study the CL behavior of carbon dots using an alkaline solution as a reaction medium to improve the CL intensity. Surprisingly, the addition of high concentration NaOH solution to the carbon dots was accompanied by a fast CL phenomenon. This investigation provided new insight into the physical-chemical and optical properties of carbon dots.The carbon dots with a diameter of 2-4 nm (Fig. S1 in the ESI †) were synthesized by the microwave treatment described in the literature with a slight modication (see ESI †), using glucose as the carbon source and PEG 1500 as the co-reactant.9 As with the other carbon dots, the synthesized nanomaterial exhibited a UV-Vis absorption band at 280 nm, attributed to the n-p* transition of the C]O band and a p-p* transition of the conjugated C]C band.17 The uorescence (FL) emission shied as the excitation wavelength increased. When the emission reached its maximum, the peak centered at 525 nm. Interestingly, with the addition of NaOH into the carbon solution, the colour of the carbon dot aqueous solution darkened from light yellow to yellow-brown. A new absorption peak at 272 nm appeared in the UV-Vis spectrum, but no appreciable absorption peak was observed under 280 nm for the original carbon Fig. 1 The UV-Vis absorption spectra of pristine carbon dots and NaOH-treated c...

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“…The ESR spectra of EDA-CDs showed a Lorentzian shaped signal at g-value ∼ 2.001 (Figure 2d), which indicated a singly occupied orbital in ground-state carbon nanodots. 27,28 This indicated that EDA-CDs could serve as electron acceptors to stabilize and scavenge reactive radicals during reaction.…”

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“…The CL intensity was dependent on the concentrations of CDs and hydroxyl ion (OH − ), but independent on the concentration of dissolved oxygen. The authors claimed that the CL was caused by the radiative recombination of the injected electrons by "chemical reduction" of CDs with thermally excited generated holes [222]. Amjadi and coworkers reported that CDs could be oxidized directly by K 3 Fe(CN) 6 in an alkaline solution, giving a strong CL emission [92].…”

Chemiluminescence of carbon dots induced by diperiodato-nicklate (IV) in alkaline solution and its application to a quenchometric flow-injection assays of paracetamol, L-cysteine and glutathione

“…The absorption spectrum of the prepared C-dots shows a peak at 245 nm (Fig. S2A, ESM), which is ascribed to the π-π* transition of nanocarbon [29]. The inserted photograph shows the prepared C-dots exhibited an yellow color under sun radiation and a blue color under ultraviolet radiation (365 nm).…”

“…The spectral range was similar to the fluorescence emission wavelength of the carbon dots. Hence, it was reasonable that CL could be attributed to the various surface energy traps that existed on the carbon dots [29].…”