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Journal of Analysis and Testing

, Volume 1, Issue 4, pp 315–321 | Cite as

Unprecedented Two-Step Chemiluminescence of Polyamine-Functionalized Carbon Nanodots Induced by Fenton-Like System

Original Paper
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Abstract

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.

Keywords

Polyamine-functionalized carbon nanodots Two-step chemiluminescence Fenton-like system Fe3+–H2O2 reaction 

1 Introduction

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 [4] or candle soot [5], laser ablation of graphite [6], electrochemical oxidation of multi-walled carbon tubes [7], 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 [13], bio-imaging [14] and photo-catalyst [15]. 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 [24] 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. [25] 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 [26]. 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 [27]. 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.

2.2 Apparatus

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

The CL kinetic characteristics of carbon dots were obtained by batch experiments (as shown in Fig. 1), which were achieved by a static system consisted of a glass cuvette and the BPCL ultra-weak luminescence analyzer. 100 µL of BPEI-CDs and H2O2 solution were added into the glass cuvette and then Fe3+ was injected by a 100 µL micro syringe from the upper injection pore. The CL signal was monitored by the photomultiplier tube (PMT).
Fig. 1

The procedure of batch experience

3 Results and Discussion

3.1 Characterization of BPEI-CDs

The polyamine-functionalized carbon dots (BPEI-CDs)were synthesized by the pyrolysis of citric acid and BPEI according to the manuscript with some modification [27]. The high resolution TEM image (Fig. 2a) revealed that the synthesized BPEI-CDs were all quasi-spherical shape and monodisperse quantum dots with a diameter of 4.4–6.0 nm. The UV–Vis absorption bands results showed that the BPEI-CDs have two absorption bands centered at 245 and 353 nm. The photoluminescence (PL) emission was also investigated. The maximum emission of 450 nm was obtained with an excitation wavelength of 360 nm. Moreover, the emission wavelength red-shifted with increasing excitation wavelength, which revealed a distribution of the different surface energy traps of the synthesized BPEI-CDs (Fig. 2b) [16].
Fig. 2

Characterization of carbon dots (a HR-TEM, b fluorescence and UV–Vis spectrogram, c FT-IR, d XPS)

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 [28]. 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

The preliminary studies indicated that the very weak chemiluminescence of CDs can be generated when it mixed with the oxidants such as H2O2 and KMnO4 [20]. In the previous research, we have also found Fe3+ can induce the BPEI-CDs to generate the weak chemiluminescence in alkaline solution, and BPEI-CDs can be used CL probe for selective detection of Fe3+ [29]. But it is not good for the detection of Fe3+ in alkaline solution because Fe3+ can be easily converted into Fe(OH)3 precipitation. Then, if using HCl instead of NaOH in this system, would the situation be changed? In fact, there was no observable CL signal in BPEI-CDs–Fe3+ system under the acidic conditions until H2O2 was added into the glass cuvette. Therefore, the BPEI-CDs CL behavior induced by the Fenton-like system was established which can emit distinct chemiluminescence signal. With the increasing of Fe3+ or H2O2 concentration (Fig. 3a, b), the CL intensity increased, when the concentration of Fe3+ came to 1 × 10−3 M, an unprecedented two-step chemiluminescence phenomenon appeared (Fig. 3c). Although the BPEI-CDs were purified by dialysis, a control experiment with the reagents used for the preparation of BPEI-CDs (CA and BPEI) was also carried out. There was almost no CL signal for this CL system. This confirmed that the CL was raised from BPEI-CDs. The similar CL behavior of naked CDs induced by Fenton-like reaction was also appeared, but the CL intensity was lower about 10 times than that of BPEI-CDs (Fig. 3d), which probably because BPEI capped on the surface of CDs can capture and enrich the Fe3+ ions [27]. Additionally, the BPEI-CDs had a higher quantum yield than naked CDs as described before [28]. The kinetic profiles of BPEI-CDs–Fe3+–H2O2 CL system showed that the CL reaction was very fast and the first-step CL intensity reached a maximum value within 1 s after injection of Fe3+ into the BPEI-CDs–H2O2 solutions, and then decayed dramatically in 1 s; the second-step CL peaks increased slowly to its maximum within 15 s and then decayed gently.
Fig. 3

The CL behavior of BPEI-CDs induced by the Fenton-like system. a The chemiluminescence phenomena of BPEI-CDs–Fe3+–H2O2 with different Fe3+ concentration (1) 1 × 10−6 M, (2) 1 × 10−5 M, (3) 1 × 10−4 M, (4) 1 × 10−3 M and (5) 1 × 10−2 M. b The effect of H2O2 on the CL intensity. c The obvious two-step chemiluminescence with Fe3+concentration 1 × 10−3 M. d Comparison of CL intensity of BPEI-CDs induced by Fenton-like reaction with that of naked CDs. In which the concentration of CDs, H2O2 and HCl are 1 mg/ml, 1 and 0.01 M

3.3 Possible CL Mechanism

To explore the two-step chemiluminescence mechanism of the BPEI-CDs induced by Fenton-like system, the CL spectrum of the BPEI-CDs–Fe3+–H2O2 system were studied using high-energy cut-off filters of various wavelengths. As shown in Fig. 4, the maximum CL spectrums of two-step CL was the same which were both located in the wide range of 480–600 nm and centred at 535 nm. The wide range was similar to the fluorescent emission wavelength of the BPEI-CDs. Hence, it was reasonable that CL could be attributed to the various surface energy traps that existed on the BPEI-CDs. The CL spectrum was little red-shifted in comparison to the most intense PL spectrum, which mainly occurred through excitation and emission within the core of the nanoparticles. The red-shift most likely resulted from the smaller energy separations of the BPEI-CDs surface states, compared with the energy for the most intense photoluminescence [30].
Fig. 4

The CL spectra of the BPEI-CDs–FeCl3–H2O2 system. a The first-step CL spectrum; b the second-step CL spectrum

Furthermore, different active oxygen radical scavengers such as superoxide dismutase (SOD), NaN3, thiourea and isopropanol, the quencher of superoxide ion, singlet oxygen and hydroxyl free ion, were used to verify the CL phenomena in the present study. As shown in Fig. 5, there were no inhibition effect for the first-step CL intensity, but isopropanol and thiourea that are known as quenchers of ·OH had quenched the second-step CL intensity. That is to say, the second-step CL behavior was involved with the ·OH reaction which usually generated in the Fenton or Fenton-like system, but the first-step CL has no relation with the reactive oxygen radicals. According to previous reports, BPEI-CDs could be the electron donor or acceptor during the reaction due to its EPR signal at 2.0022 [28], which revealed a singly occupied orbital in ground-state carbon dots. When it is mixed with the high concentration of Fe3+ ions and H2O2, the Fe3+ can be probably firstly oxidized to the instable intermediate perferrate (FeO4 2−) which can oxidize the BPEI-CDs and a hole transfer was suggested [31]. Then perferrate was reduced to ferrate or ferrous which react with the H2O2 to generate ·OH, the ·OH can inject the hole as another oxidants into the valence band of BPEI-CDs to generate second-step CL emission.
Fig. 5

Effect of radical scavenger on the two-step CL intensity of BPEI-CDs–Fe3+–H2O2 system

Based on the above study, the possible CL mechanism of the BPEI-CDs–Fe3+–H2O2 system was speculated, as shown in Fig. 6. Due to the redox property of the discrete electron and hole states of BPEI-CDs, a hole can be injected into the valence band by instable strong perferrate formed which convert BPEI-CDs to oxidized-state BPEI-CDs. The single orbital detected by EPR spectra could serve as the holes traps. And then the thermally excited electrons in the high energy band annihilate with the oxidant injected holes to produce the first-step CL. On the other hand, the ·OH generated from the reaction of Fenton or Fenton-like reaction of Fe3+ or Fe2+ and H2O2 which can also inject the hole into the valence band of BPEI-CDs to generate CL. However, due to the low radical production rates of Fenton-like reaction, when Fe3+ concentration increased 1 × 10−3 M, the hydroxyl radical produced enough to oxidize the BPEI-CDs to generate second CL [32].
Fig. 6

Schematic illustration of possible two-step CL mechanism of BPEI-CDs–Fe3+–H2O2 system

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.

4 Conclusion

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.

Notes

Acknowledgements

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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Copyright information

© The Nonferrous Metals Society of China and Springer Nature Singapore Pte Ltd. 2017

Authors and Affiliations

  1. 1.State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental SciencesChinese Academy of SciencesBeijingPeople’s Republic of China

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