Unprecedented Two-Step Chemiluminescence of Polyamine-Functionalized Carbon Nanodots Induced by Fenton-Like System
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We reported an unprecedented chemiluminescence (CL) behavior of polyamine-functionalized carbon dots induced by Fe3+–H2O2 Fenton-like system. The first-step CL intensity increased with the increasing of the concentration of H2O2 and Fe3+, when the Fe3+ concentration came to 10−3 M, the unprecedented two-step CL behavior appeared. The CL intensity of BPEI-CDs induced by Fenton-like system was about 10 times stronger than that of naked CDs. The possible two-step CL mechanism was speculated based on the photoluminescence spectra, CL emission spectra, and the effects of radical scavengers on the CL intensity. Radiative recombination of the injected holes by strong oxidant perferrate formed through Fe3+–H2O2 reaction and the ·OH generated from successive Fenton reaction with the thermally excited electrons was proposed, which further facilitate full understanding about the optical properties of carbon dots.
KeywordsPolyamine-functionalized carbon nanodots Two-step chemiluminescence Fenton-like system Fe3+–H2O2 reaction
Carbon dots (CDs), as a new class of carbon-based luminescence materials, have attracted much attention in recent years. Different from the carbon nanotubes or carbon nanodiamonds, CDs are not of pure carbon composition but proved to be generally oxygen-containing carbonaceous nanoparticles which typically contain many carboxylic acid moieties, hydroxyl moieties at their surface and they are water soluble and can be subsequently functionalized with various chemical groups [1, 2, 3]. Compared with the fluorescent heavy metal-containing semiconductor-based quantum dots, CDs are not only superior in chemical inertness, biocompatibility and environmental benign, but also exhibit the fascinating optical properties, such as the interesting photoluminescence phenomenon dependent on the size and excitation wavelength, photoinduced electron transfer, etc. Therefore, CDs may become very promising alternative to the semiconductor-based QDs for the promising applications and attracted increasing attention.
Up to date, many researches focused on the exploration of new approaches to synthesis carbon dots including functionalized carbon dots, which covered chemical oxidation of arc-discharge SWCNTs  or candle soot , laser ablation of graphite , electrochemical oxidation of multi-walled carbon tubes , thermal decomposition of molecular precursors [8, 9, 10], and microwave hydrothermal proper carbon sources [11, 12], and the luminescence properties of carbon dots, which frequently be used in analytic detection , bio-imaging  and photo-catalyst . However, the origin of the luminescence of carbon dots is still a matter of current debate, intense research still focused on the optical properties and its related mechanism.
Luminescent properties of carbon dots was usually investigated by photoluminescence (PL) produced using photoexcitation [16, 17, 18, 19], electrochemiluminescence (ECL) generated by electron injection [20, 21]. Recently, chemiluminescence (CL) generated from chemical energy excitation through a chemical reaction is also used to study the optical properties of carbon dots. For example, Lin [22, 23] and Cui  group described the CL behavior of carbon dots when it coexists with oxidants (KMnO4, Ce(IV), NaIO4) or Ultra-weak chemiluminescence system (H2O2–NaHSO3 or H2O2–HNO2) in the acid conditions. Lu et al.  reported the CL behavior of surfactant-modified CDs when it coexisted with Co2+–H2O2 system. They proposed the reaction between oxidants related species and carbon dots was the main pathway for the CL. More recently, we firstly observed the CL phenomenon of carbon dots in the presence of strong alkaline solutions without the presence of any CL reagent or CL system or oxidants . The possible CL mechanism involving the radiative recombination of the injected electrons by “chemical reduction” of carbon dots with the thermally excited generated holes was proposed. Although the proposed CL emission intensity and mechanism was different probably due to the different synthesized method, materials as well as the surface modification, these CL reactions were all generated one-step CL. Herein, we surprisingly found an unprecedented two-step CL phenomenon of CDs and polyamine-functionalized-CDs (BPEI-CDs) induced by Fenton-like reaction such as Fe3+–H2O2 system. Especially for BPEI-CDs, the CL intensity was higher than the naked CDs probably due to its high quantum yield . Therefore, using BPEI-CDs as model, The two-step CL mechanism of BPEI-CDs–Fe3+–H2O2 CL system was preliminarily studied through fluorescence, CL emission spectrum and the effects of radical scavengers on CL intensity, which may further facilitate full understanding the physical–chemical and optical properties of the carbon dots.
2 Experimental Section
2.1 Materials and Reagents
All the chemicals and solvents were of analytical grade. Ethylene imine polymer (C2x+4kH5x+10kNx+2k, M.W. 1800, 99%) was obtained from Aladdin Industrial Corporation. Citric acid monohydrate (C6H8O7·H2O) was purchased from Beijing Chemical Reagents Company. Fe(III) chloride hexahydrate (FeCl3·6H2O), HCl (36% ~ 38%), H2O2, NaN3, isopropanol and thiourea were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). Dimethylcarbinol (C3H8O) and superoxide dismutase were from Sigma-Aldrich. Water used for the preparation of solutions was from a Millipore Milli-Q (Biocel) water purification system.
Batch CL experiments were carried with a BPCL ultra-weak luminescence analyzer (Institute of Biophysics, Chinese Academy of Sciences, Beijing, China). Fluorescence measurements were performed on a FluoroMax-4 spectrofluorometer (Horiba JobinYvon, Edison, NJ, USA), using 350–580 nm excitation and a slit width of 5 nm. UV–Vis absorption spectra were measured on an Agilent 8453 UV–Visible spectrophotometer (Palo Alto, CA, USA). High-resolution transmission electron microscopy (HRTEM) images were recorded by an electron microscope operating at 120 kV (JEM-2010, JEOL, Japan). Surface chemical bonding states were analyzed by X-ray photoelectron spectroscopy (ESCALAB250Xi, Thermo Scientific, USA). Fourier transform infrared (FT-IR) measurements were carried out with a FT-IR spectrometer (6100, JASCO, Japan). The CL spectra were examined by a series of high-energy optical filters (425, 440, 460, 475, 490, 520, 535, 555, 575, 590, 605 nm).
2.3 Preparation of BPEI-CDs
The polyamine-functionalized carbon dots (BPEI-CDs)was synthesized by the pyrolysis of citric acid and branched poly (ethylenimine) (BPEI). Firstly, 2.0 g BPEI and 4.0 g citric acid were dissolved uniformly with 40 ml hot water in a 100 ml beaker, and then the above mixture was poured into a stainless steel autoclave with a Teflon liner of 100 ml capacity and heated at 195 °C for 3 h. Finally, the reactor was automatically cooled to room temperature. The resulting brown solution was centrifuged at 7000 rpm for 10 min to remove the weight precipitate and agglomerated particles.
Then the brown aqueous solution of BPEI-CDs was dialyzed for 8 h in super-purified water. The cut-off of the dialysis membrane was equal to molecular weight of 2000. Then the concentration of the acquired uniform aqueous about 30 mg/ml was determined by freeze–dry method.
2.4 Procedure for CL Detection
3 Results and Discussion
3.1 Characterization of BPEI-CDs
The FT-IR spectra (Fig. 2c) were used to identify surface functional groups of carbon dots. The peak centered at 3369 cm−1 contributed to the O–H stretch vibration of the carboxylic moiety, the shoulder at 2878 cm−1 to C–H vibration, the bands at 1705 and 1624 cm−1 attributed to C=O and C=C vibrations, respectively, the stretching vibration band of C–O–N–H (1698 cm−1) and N–H (3369 cm−1) indicated the surface of carbon dots was partly azotized during the polysis treatment of CA. In additional, there were many characteristic absorption bands of BPEI and amide, which indicated that BPEI was kept stable and coated at the surface of CDs by the amide linkage as previously reported . XPS results (Fig. 2d) indicated that there are abundance N performs which will increase the optical properties such as fluorescence and chemiluminescence.
3.2 Two-Step CL of BPEI-CDs Induced by Fenton-Like Reaction
3.3 Possible CL Mechanism
3.4 Analytical Performance
To explore the potential analytical applicability of the BPEI-CDs-Fenton-Like CL system, we preliminarily evaluated the capability of this system for the detection of Fe3+ and H2O2. The results showed that linear relationship between first-step CL intensity and the analytes concentration were in the range from 1 × 10−6 to 1 × 10−5 M with a correlation coefficient of 0.993 for Fe3+ and from 1 × 10−4 to 1 × 10−3 M with a correlation coefficient of 0.992 for H2O2. The relative standard deviation (RSD) (n = 9) of the analysis were 2.0 and 3.0% for Fe3+ concentration of 5.0 × 10−5 M and H2O2 concentration of 1.0 × 10−3 M, respectively. The limit of detection (S/N = 3) for Fe3+ was 6.7 × 10−6 M which is far lower than the WHO guideline recommendation of 0.3–3.0 mg/L in drinking water. In additional, several reducing substances, such as ascorbic acid, iodide ions on the reaction were examined. The two-step CL intensity can be inhibited by the ascorbic acid and iodide ions due to their competitive reaction with oxidants which reduced the CL reaction between BPEI-CDs and oxidants or hydroxyl radical, which indicated that combined with chromatographic separation method, the CL system can be applied the simultaneous analysis of several compounds.
In this work, we firstly demonstrated that the unprecedented two-step CL phenomenon of BPEI-CDs induced by the Fenton-Like system. The characteristic and possible CL mechanism of BPEI-CDs induced by Fenton-Like system was investigated based on the fluorescence spectrum, CL emission, FT-IR, XPS spectroscopy and the effects of radical scavengers on the CL intensity. Based on the obtained results, the first CL emission was probably due to the recombination of injected holes by unstable intermediate perferrate formed by Fe3+ and H2O2 and thermally excited electrons to form the excited-state CDs, which acts as the final emitter in the system. Then perferrate was reduced to ferrate which react with the H2O2 to generate ·OH, the ·OH can inject the hole as another oxidants into the valence band of CDs to generate second CL emission. Moreover, the potential analytical application was proposed for the detection of Fe3+, H2O2 and some other reducing species including ascorbic acid that could inhibit the CL signals of the BPEI-CDs–Fe3+–H2O2 system.
The authors gratefully acknowledge financial support from the National Key Research and Development Program of China (2016YFA0203102), the Chinese Academy of Sciences (XDB14040100), and the National Natural Science Foundation of China (Nos. 21677152 and 21177138).
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