Polyamine-functionalized carbon nanodots: a novel chemiluminescence probe for selective detection of iron(III) ions

Abstract

We firstly studied the chemiluminescence behavior of branched poly(ethylenimine)-functionalized carbon dots (BPEI-CDs). The results demonstrated that BPEI-CDs can be used as a novel chemiluminescence probe in alkaline solution for rapid detection of iron(III) ions with high sensitivity and selectivity. A possible CL mechanism was studied by UV-Vis, fluorescence, CL, FTIR, XPS and EPR spectroscopy. Iron(III) could be selectively captured by the surface functional groups of BPEI and injected holes into the carbon dots which resulted in a great improvement of the BPEI-CDs' CL signal in alkaline solution. The work sheds new light on the characteristics and further application of functionalized carbon dots.

---

Carbon nanodots (CDs), as newcomers to the world of nanomaterials, have attracted intensive attention due to their alluring luminescence properties and wide application in many areas of fundamental and technical importance. Luminescent properties of carbon dots are usually investigated by photoluminescence (PL) produced using photoexcitation,^1–7 and electrochemiluminescence (ECL) generated by electron injection.^8–11 Chemiluminescence (CL), known as the emission of light from chemical reactions, has proved to be a useful phenomenon due to its ever increasing analytical application due to its high sensitivity and lack of background scattering light interference. Therefore, the CL behavior of CDs with their related analytical application has been paid increasing attention recently.

To date, the CL behavior of carbon dots has been investigated by many groups either in coexistence with oxidants (KMnO₄, Ce(IV), NaIO₄, NaClO, KFe(CN)₃)^12–16 or ultraweak CL systems (H₂O₂–NaHSO₃, H₂O₂–HNO₂ or Co²⁺–H₂O₂)^17–19 or CL reagents such as luminol.²⁰ However, these studies mainly focused on the CL behavior and mechanism of CDs. For the analytical application of CDs based on their CL, there are limited reports. To the best of our knowledge, only CDs-oxidants NaClO,¹⁶ KFe(CN)₃¹⁴ and CDs-ultraweak CL systems (H₂O₂–HNO₂¹⁷ or Co²⁺–H₂O₂¹⁹) have been reported for the detection of reducing substances such as dopamine, nitrites, chlorine, and cobalt ions. Intense research is still focused on exploring the new CD-based CL systems and obtaining new insight into the CDs' CL characteristics and their further application.

Recently, we studied the CL behavior of CDs in alkaline solution without the presence of any CL reagents, CL system or oxidants.²¹ The CD CL system had very good reproducibility. However, when it was used for analytical applications, there were two bottlenecks: firstly, the CL intensity was very low due to the low quantum yield of CDs (only 10%);⁸ secondly, the selectivity was very poor. In contrast, branched polyamine-functionalized CDs (BPEI-CDs) not only exhibit excellent optical properties (42.5% FL quantum yield), but also are envisioned to be applicable in chemical sensing because the polyamine has a broad complexing property for transition metal ions.^22,23 Based on these factors, the CL properties of BPEI-CDs in alkaline solutions were firstly studied. Similar to the unmodified CDs,²¹ BPEI-CDs can generate CL signals in strong alkaline solutions and the CL intensity was higher than that found for the unmodified CDs. Moreover, the effects of various coexisting ions in water samples on the CL signal were investigated. Interestingly, only iron(III) improved the CL intensity of BPEI-CDs in the alkaline solution. This suggested that BPEI-CDs can be used as a novel chemiluminescence probe for the rapid detection of trace ferric ions in water samples with high selectivity and sensitivity which has proven to be difficult in the past.²⁴

To perform the experiments, polyamine-functionalized carbon dots (BPEI-CDs) were synthesized by low temperature pyrolysis using citric acid (CA) as the carbon source due to its low carbonization temperature (<200 °C), and branched poly(ethylenimine) (BPEI) as the functional reagent according to the literature²⁵ with some modification. X-ray photoelectron spectroscopy (XPS) (Fig. S1 (ESI†)) showed that the as prepared BPEI-CDs contained carbon, nitrogen and oxygen, the characteristic peak of C–C sp² suggesting that abundant graphite structures formed (C 1s spectrum). Fourier transform infrared (FTIR) spectroscopy (Fig. S2 (ESI†)) showed that there were many characteristic absorption bands of BPEI and amide, but nearly no absorption of CA, which indicated that the CA was mostly carbonized, and BPEI was kept stable and coated at the surface of the CDs by the amide linkages as previously reported. High resolution transmission electron microscopy (HRTEM) results suggested that the BPEI-CDs were mostly of spherical morphology with an average diameter of 3.5–4.5 nm and lattice spacing of 0.31 nm which matches the 〈002〉 spacing of graphitic carbon (Fig. S3 (ESI†)).

As with other carbon dots, the photoluminescence (PL) emission of BPEI-CDs shifted as the excitation wavelength increased with the centered PL wavelength at 450 nm (Fig. S4 (ESI†)). UV-Vis absorption results showed that the BPEI-CDs have two absorption bands centered at 245 nm and 353 nm (Fig. S5 (ESI†)). However, the addition of Fe(III) into the BPEI-CDs solution gave rise to a new broad band found in the range from 270 nm to 330 nm, while the absorption bands centered at 245 nm of BPEI-CDs and the absorption bands at 224 nm of Fe(III) diminished or even disappeared. The same changes in the absorption spectrum were also observed when Fe(III) ions were added into BPEI. This indicated that a complex reaction happened between the BPEI capped on the surface of the CDs and the Fe(III) ions.

We then tested whether the BPEI-CDs could act as a chemiluminescence probe for Fe(III) ions in alkaline solutions. As demonstrated in Fig. 1A, the BPEI-CDs exhibited weak chemiluminescence only in NaOH solution. In the presence of Fe(III) ions, the chemiluminescence signal increased drastically and reached its maximum within 1.5 s (Fig. 1B). Moreover, with the increase of Fe(III) concentration, the CL intensity improved linearly (Fig. 1C). The CL intensity was also dependent on the concentration of NaOH and carbon dots in a certain range (Fig. S6 (ESI†)). In order to obtain a low detection limit and a good recovery, 0.1 M NaOH and 7.5 mg ml⁻¹ BPEI-CDs were selected.

Fig. 1 (A) Improvement of the CL intensity with Fe(III) ions added into the BPEI-CDs–NaOH CL system. (B) CL kinetic curve of the BPEI-CDs–NaOH–Fe(III) system. (C) CL signals in the presence of different concentrations of Fe(III) ions. Inset: the calibration curve for Fe(III) ions. (D) CL spectrum of the BPEI-CDs–NaOH–Fe(III) system. The concentrations of carbon dots, NaOH and Fe(III) ions were 7.5 mg ml⁻¹, 0.1 M and 1 mM, respectively.

Based on these conditions, the capability of BPEI-CDs as a chemiluminescence probe for quantitative detection of Fe(III) was evaluated. As shown in Fig. 1C, there are two linear regions in the plot of CL intensity and Fe(III) concentration, one from 1 × 10⁻⁷ M to 1 × 10⁻⁶ M with a correlation coefficient of 0.993, and the other from 1 × 10⁻⁶ M to 1 × 10⁻⁵ M with a correlation coefficient of 0.992. The relative standard deviations (RSD) (n = 9) of the analysis were 2.3%, 3.0%, 1.9% for Fe(III) concentrations of 5.0 × 10⁻⁷ M, 1.0 × 10⁻⁶ M and 5.0 × 10⁻⁶ M, respectively. The limit of detection (S/N = 3) for Fe(III) was 6.67 × 10⁻⁸ M which is far lower than the WHO guideline recommendation of 0.3–3.0 mg L⁻¹ in drinking water.

The selectivity of the sensing method for Fe(III) ions was evaluated before its application in real water samples. The influence of various environmentally relevant coexisting ions in water on the CL was tested (Fig. 2). It can be seen that only Fe(III) ions effectively improved the chemiluminescence signal of BPEI-CDs in NaOH solution, whereas no obvious CL signal changes were observed for other relevant ions. These results demonstrated that the BPEI-CDs can be a CL probe which is highly selective toward Fe(III) ions over other relevant ions.

Fig. 2 Selectivity of the BPEI-CDs-based CL sensing for Fe(III) ions in alkaline solution. The concentrations of carbon dots, NaOH and Fe(III) were 7.5 mg ml⁻¹, 0.1 M and 1 μM, respectively; the concentrations of other common ions in water were all 10 μM.

The excellent specificity combined with high sensitivity and fast response of BPEI-CDs to Fe(III) ions suggested that our method might be directly applied to the detection of Fe(III) ions in real samples. Therefore, we further examined the practicality of the assay by testing Fe(III) ions in natural water (tap water and river water). The results are shown in Table S1 (ESI†). It can be seen that the recoveries for three samples were satisfactory, thus further demonstrating the applicability of the proposed methodology.

The chemiluminescence characteristics of the BPEI-CDs were investigated. A CL spectrum of the BPEI-CDs–NaOH–Fe(III) system was measured by a fluorescence spectrometer using high-energy cutoff filters of various wavelengths and compared with the PL spectrum. As shown in Fig. 1D, the maximum CL peak was located in the wide range of 430–600 nm with a center at 530 nm. The wide range was similar to that seen in the fluorescence emission spectrum of the BPEI-CDs. Hence, it was reasonable to attribute the CL to the various surface energy traps that existed on the BPEI-CDs. The CL peak was red-shifted in comparison to the most intense PL peak (centered at 450 nm), 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 PL.²⁶ Furthermore, superoxide dismutase (SOD), thiourea and histidine, the quenchers of superoxide ion, hydroxyl free ion and singlet oxygen respectively, caused no inhibition of the CL signal of the system. De-oxygenated solutions also had no inhibitory effect on the CL, which indicated that the dissolved oxygen had little effect on the CL and the singlet oxygen emitter could be excluded in the BPEI-CDs–NaOH–Fe(III) system.

EPR was utilized to investigate the ground-state properties of luminescent species in the BPEI-CDs. The carbon dots showed an EPR signal at g = 2.0022 ± 0.000076 (ESI†), which revealed a singly occupied orbital in ground-state BPEI-CDs. The singly occupied orbital indicated that BPEI-CDs could be electron donors or acceptors during the reaction. The EPR spectra of the BPEI-CDs in NaOH and in the NaOH–Fe(III) system were also presented in Fig. S7 (ESI†). The g value of the BPEI-CDs increased from 2.0022 ± 0.000029 to 2.0031 ± 0.000050 in a NaOH solution, so electron transfer was suggested after the “the chemical reduction” reaction, similarly to the hydroxyl-CDs.²¹ With the addition of Fe(III), the g value was reduced from 2.0031 to 2.0027 which suggested that Fe(III) in basic solutions can act as an oxidant to inject holes into the BPEI-CDs through oxidation similarly to KMnO₄.¹²

In order to verify the CL mechanism, the pristine BPEI-CDs, NaOH-treated BPEI-CDs and Fe(III)–NaOH treated BPEI-CDs were characterized through Fourier transform infrared spectroscopy (FTIR) (Fig. 3). The results revealed that many functional groups such as –C–H–, C–N, –CONH– (amide), and N–H, were detected on the BPEI-CDs, in agreement with previous studies. For NaOH-treated BPEI-CDs, there was a significant reduction in the signal from carbon–oxygen double bonds in amide groups at 1698 cm⁻¹ and the appearance of a signal from carbon–oxygen single bonds at 1158 cm⁻¹, which indicated that the surface carbon–oxygen double bonds in amide groups were probably partly reduced to carbon–oxygen single bonds. With the addition of Fe(III), the surface functional groups did not change; however, the X-ray photoelectron spectroscopy (XPS) spectrum of Fe (1s) (Fig. S8 (ESI†)) showed the appearance of Fe²⁺ and Fe⁰ peaks around 709.9 eV and 711.2 eV (Fe 2p) after the CL reaction, which indicated that the Fe(III) acted as the oxidant in the CL reaction.

Fig. 3 FTIR spectrum of pristine BPEI functionalized carbon dots (black line), NaOH-treated BPEI-CDs (green line) and BPEI-CDs after reaction with NaOH and Fe(III) (blue line).

Based on the above study, a possible CL mechanism for a BPEI-CDs–NaOH–Fe(III) system is proposed and illustrated as shown in Fig. 4. The single orbital of BPEI-CDs detected by EPR spectra could serve as the electron or hole traps. In the presence of NaOH, the injected electrons through “chemical reduction” annihilate thermally generated holes to produce the weak CL signal. Fe(III), which can be captured by the surface functional groups of BPEI, could inject holes into the BPEI-CDs as the oxidant. The injection of holes through the oxidation reaction accelerated the electron–hole annihilation, which resulted in a higher CL intensity.

Fig. 4 Schematic illustration of possible FL and CL mechanism of the BPEI-CDs–NaOH–Fe(III) system.

In conclusion, polyamine-functionalized CDs were found to act as chemiluminescence probes for Fe(III) detection with high selectivity and sensitivity in water solution. The method relied on the facts that BPEI-CDs can generate CL signals in alkaline solution, and that Fe(III) ions captured on the surface of the BPEI-CDs can act as an oxidant to inject holes into the BPEI-CDs resulting in a great improvement of the BPEI-CDs CL signal. The assay platform has two important features. Firstly, the method provides a convenient “mix-and-detect” procedure for homogeneous and rapid detection of Fe(III). Additionally, this assay exhibits high sensitivity and selectivity toward Fe(III) versus other metal ions, wide linear range and low cost. It has been demonstrated to have promising applications in the detection of Fe(III) in environmental water samples. It is evident from the present work that functionalized CDs can also generate chemiluminescence in strong alkaline solution, and trace levels of certain metal ions can be rapidly and selectively detected by using appropriate surface functionalized CDs as the chemiluminescence probes, suggesting promising analytical applications of CDs.

Acknowledgements

This work was supported by 973 Program (no. 2011CB936001, 2010CB933502) and the National Natural Science Foundation of P.R. China (Grant no. 21177138, 21377142, 21277158, 21207146).

Notes and references

1. Y. P. Sun, B. Zhou, Y. Lin, W. Wang, K. A. S. Fernando, P. Pathak, M. J. Meziani, B. A. Harruff, X. Wang, H. F. Wang, P. J. G. Luo, H. Yang, M. E. Kose, B. L. Chen, L. M. Veca and S. Y. Xie, J. Am. Chem. Soc., 2006, 128, 7756–7757.
2. S. L. Hu, K. Y. Niu, J. Sun, J. Yang, N. Q. Zhao and X. W. Du, J. Mater. Chem., 2009, 19, 484–488.
3. R. L. Liu, D. Q. Wu, S. H. Liu, K. Koynov, W. Knoll and Q. Li, Angew. Chem., Int. Ed., 2009, 48, 4598–4601.
4. X. J. Mao, H. Z. Zheng, Y. J. Long, J. Du, J. Y. Hao, L. L. Wang and D. B. Zhou, Spectrochim. Acta, Part A, 2010, 75, 553–557.
5. S. Chandra, S. H. Pathan, S. Mitra, B. H. Modha, A. Goswami and P. Pramanik, RSC Adv., 2012, 2, 3602–3606.
6. L. P. Lin, X. X. Wang, S. Q. Lin, L. H. Zhang, C. Q. Lin, Z. M. Li and J. M. Liu, Spectrochim. Acta, Part A, 2012, 95, 555–561.
7. Z. C. Yang, M. Wang, A. M. Yong, S. Y. Wong, X. H. Zhang, H. Tan, A. Y. Chang, X. Li and J. Wang, Chem. Commun., 2011, 47, 11615–11617.
8. H. Zhu, X. L. Wang, Y. L. Li, Z. J. Wang, F. Yang and X. R. Yang, Chem. Commun., 2009, 5118–5120.
9. L. L. Li, J. Ji, R. Fei, C. Z. Wang, Q. Lu, J. R. Zhang, L. P. Jiang and J. J. Zhu, Adv. Funct. Mater., 2012, 22, 2971–2979.
10. L. Y. Zheng, Y. W. Chi, Y. Q. Dong, J. P. Lin and B. B. Wang, J. Am. Chem. Soc., 2009, 131, 4564–4565.
11. J. G. Zhou, C. Booker, R. Y. Li, X. L. Sun, T. K. Sham and Z. F. Ding, Chem. Phys. Lett., 2010, 493, 296–298.
12. Z. Lin, W. Xue, H. Chen and J. M. Lin, Chem. Commun., 2012, 48, 1051–1053.
13. M. Amjadi, J. L. Manzoori, T. Hallaj and M. H. Sorauraddin, Spectrochim. Acta, Part A, 2014, 122, 715–720.
14. M. Amjadi, J. L. Manzoori, T. Hallaj and M. H. Sorouraddin, Microchim. Acta, 2014, 181, 671–677.
15. J. Jiang, Y. He, S. Li and H. Cui, Chem. Commun., 2012, 48, 9634–9636.
16. Y. R. Tang, Y. Y. Su, N. Yang, L. C. Zhang and Y. Lv, Anal. Chem., 2014, 86, 4528–4535.
17. Z. Lin, W. Xue, H. Chen and J. M. Lin, Anal. Chem., 2011, 83, 8245–8251.
18. W. Xue, Z. Lin, H. Chen, C. Lu and J. M. Lin, J. Phys. Chem. C, 2011, 115, 21707–21714.
19. J. X. Shi, C. Lu, D. Yan and L. N. Ma, Biosens. Bioelectron., 2013, 45, 58–64.
20. Z. Lin, X. N. Dou, H. F. Li, Q. S. Chen and J. M. Lin, Microchim. Acta, 2014, 181, 805–811.
21. L. X. Zhao, F. Di, D. B. Wang, L. H. Guo, Y. Yang, B. Wan and H. Zhang, Nanoscale, 2013, 5, 2655–2658.
22. Y. Dong, R. Wang, G. Li, C. Chen, Y. Chi and G. Chen, Anal. Chem., 2012, 84, 6220–6224.
23. I. Y. Goon, C. Zhang, M. Lim, J. J. Gooding and R. Amal, Langmuir, 2010, 26, 12247–12252.
24. M. Lu and R. G. Compton, Electroanalysis, 2013, 25, 1123–1129.
25. Y. Q. Dong, R. X. Wang, H. Li, J. W. Shao, Y. W. Chi, X. M. Lin and G. N. Chen, Carbon, 2012, 50, 2810–2815.
26. L. Zheng, Y. Chi, Y. Dong, J. Lin and B. Wang, J. Am. Chem. Soc., 2009, 131, 4564–4565.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08071h

‡ These authors contributed equally to this work.

---