Electrogenerated chemiluminescence emission from cadmium germanate nanoparticles

Abstract

The ECL phenomenon of nano Cd₂Ge₂O₆, both amorphous and crystallized, is described in this paper. This is the first report on the ECL of semiconductor nanocrystals involving three elements and also the first paper dealing with the amorphous nanomaterials, which shed some light on the exploration of new ECL nanomaterials. The ECL mechanism could be ascribed to the cadmium-rich surface state of amorphous Cd₂Ge₂O₆. The ECL intensity linearly increased along with the square of the H₂O₂ concentration. Therefore, it was employed to detect hydrogen peroxide, and the linear range was 5.0 × 10⁻⁶ to 4.0 × 10⁻⁴ mol L⁻¹.

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1. Introduction

Semiconductor nanocrystals have been widely investigated due to their size dependent optical and electronic properties.¹ Electrochemiluminescence (ECL) is a useful detection method because of its promising advantages,² such as high sensitivity, low background and easy controllability compared with fluorescence and chemiluminescence. Recently, ECL of semiconductor nanocrystals has attracted great attention, and it is widely applied to the determination of all kinds of analytes, such as, H₂O₂,^3,4 Cu²⁺,⁵ DNA,^6,7 protein.^8–10 To date, the studies of ECL semiconductor nanocrystals have mainly focused on IIA–VIB semiconductor nanomaterials, such as CdS,^11,12 CdSe,^13,14 CdTe,¹⁵ etc. In addition, some other nanomaterials such as C,^16,17 Si,¹⁸ Ge,¹⁹ even Au nanocluster^20,21 are also studied. However, the reported nanomaterials with ECL properties are still quite limited. Therefore, there is great potential to explore new nanomaterials possessing ECL properties beyond the limit of IIA–VIB or elemental semiconductor nanomaterials.

Herein, for the first time, we describe ECL from nano Cd₂Ge₂O₆. As far as we know, this is the first report of ECL of amorphous semiconductor nanomaterials composed of three elements. As a demonstration of its application, it was used to detect H₂O₂.

2. Experimental

2.1. Apparatus

Electrochemical measurements were performed at an electrochemical working station VMP3 (Princeton Applied Research Co., Ltd, USA). ECL signals were recorded by an Ultra-weak Chemiluminescence Analyzer (BPCL-K, Institute of Biophysics, Academia Silica, Beijing, China) controlled by a personal computer with 0.1 s sample interval. While collecting ECL signals, the cell was placed directly in front of the photomultiplier (PMT operated at −800 V unless mentioned) and the PMT window was opened towards the working electrode only. The counter electrode was a platinum foil (area ca. 200 mm²), while an Ag/AgCl (3 M KCl) electrode was used as a reference electrode.

2.2. Preparation of Cd₂Ge₂O₆

The Cd₂Ge₂O₆ was synthesized similar to the method reported by Fu et al.²² Briefly, 2.66 g of Cd(CH₃COO)₂·2H₂O, and 1.04 g Ge₂O were added to 30.0 ml of water. The resulting mixture was adjusted to pH 8 by adding NaOH (30 wt%). The mixture was stirred for 1 h and then transferred to a stainless Teflon-lined autoclave of 100 mL inner volume. Five of the same autoclaves were heated to 90 °C, 100 °C, 110 °C, 120 °C, 140 °C separately and then tempered at these temperatures for 24 h, followed by cooling to room temperature. The products were centrifuged, filtered, and rinsed with alcohol and D.I. water several times until the number of ionometer was smaller than 5 ppm. Finally, the products were dried overnight.

2.3. Preparation of Cd₂Ge₂O₆ – modified glassy carbon electrode

The Cd₂Ge₂O₆ solid was milled to powder and then dissolved in Nafion solution (0.5%) to form 5 mg mL⁻¹ suspending liquid. Three microlitres of the suspending liquid were dropped to glassy carbon electrode. The as-prepared electrode was dried in the air and was used as working electrode.

3. Results and discussion

3.1. Characterization

As shown in Fig. 1, the XRD patterns of the samples in Fig. 1c indicate that the products prepared at temperature lower than 110 °C are amorphous, and crystallized over 120 °C. As can be seen in ESI Fig. 1,† the products are considered to be Cd₂Ge₂O₆ by comparing to the powder diffraction standard card (JCPDS: 43-0468; a = 10.1835, b = 9.6519, c = 5.3771).

Fig. 1 SEM images of (a) the crystallized Cd₂Ge₂O₆ sample prepared at 140 °C (b) the amorphous Cd₂Ge₂O₆ sample prepared at 90 °C. (c) XRD patterns of the Cd₂Ge₂O₆ samples prepared at 90 °C, 100 °C, 110 °C, 120 °C, 140 °C for 24 h.

Fig. 1a and b show the SEM images of crystal Cd₂Ge₂O₆ and amorphous Cd₂Ge₂O₆. The results indicate that crystallized Cd₂Ge₂O₆ presents a cosh shape with 100–200 nm length and 30–50 nm width. The size of amorphous Cd₂Ge₂O₆ is less than 10 nm and it is difficult to distinguish from the SEM.

ESI Fig. 2† shows the UV-vis spectrum of the samples for germanium acid cadmium. Two peaks (P1, P2) are appeared at 275 nm and 395 nm for the crystal type of Cd₂Ge₂O₆. P1 corresponds to Cd₂Ge₂O₆ band gap absorption.²³ P2 is not clear, which may be absorption of defect or impurity level. For amorphous Cd₂Ge₂O₆ samples, there are two similar peaks. But compared with the crystal type of Cd₂Ge₂O₆, two absorption peaks are flat and peak positions are not clear. That may be caused by incomplete crystallization of Cd₂Ge₂O₆, leading to the broadening of absorption peaks.

3.2. The ECL behaviors of Cd₂Ge₂O₆

The ECL curves of Cd₂Ge₂O₆ samples in 0.1 mol L⁻¹ NaClO₄ solution are shown in Fig. 2. The onset ECL potential of amorphous Cd₂Ge₂O₆ is more negative and the ECL intensity is stronger (expect the sample prepared at 90 °C) than the crystallized. This phenomenon may be due to quantum size effect and the surface effect, as the crystallized Cd₂Ge₂O₆ is in larger size and less surface area compared with the amorphous samples as observed by the SEM images. The ECL intensity of as Cd₂Ge₂O₆ prepared at 100 °C was much stronger than that prepared at other temperatures. So further investigations was performed for the Cd₂Ge₂O₆ prepared at 100 °C hereinafter (The reason why the sample prepared at 90 °C is weaker will be explained below).

Fig. 2 The ECL curves of Cd₂Ge₂O₆ samples prepared at different temperature in 0.1 M NaClO₄ solution at 20 mV s⁻¹. The insert: magnified graph of Cd₂Ge₂O₆ samples prepared at 140 °C.

The CV property of Cd₂Ge₂O₆-modified glassy carbon electrode was also investigated. As shown in Fig. 3c, two distinct reduction peaks located at −0.87 V and −0.75 V. Compared with the CV curve of glassy carbon electrode in 1 × 10⁻⁴ M CdCl₂ solution (Fig. 3b), it was concluded that the peak at −0.87 V was a reduction peak of Cd²⁺. The reduction peak of at −0.35 V (Fig. 4a) was assigned to the reduction of dissolved oxygen as it can be observed at the glassy carbon electrode. However, after the modification of Cd₂Ge₂O₆, the reduction peak of dissolved oxygen shifted 400 mV negatively, which can be explained by the blockage of electron transfer by the Cd₂Ge₂O₆-modified layer.

Fig. 3 The CV curves of (a) glassy carbon electrode in 0.1 M NaClO₄ solution. (b) Glassy carbon electrode in 1 × 10⁻⁴ M CdCl₂ solution. (c) Cd₂Ge₂O₆–modified glassy carbon electrode in 0.1 M NaClO₄ solution. The scan rate is 50 mV s⁻¹.

Fig. 4 Fluorescence spectrum and ECL spectrum of Cd₂Ge₂O₆. (1) Florescence spectrum of Cd₂Ge₂O₆; (2) ECL spectrum of Cd₂Ge₂O₆.

3.3. The possible ECL mechanism of Cd₂Ge₂O₆

As can be seen in Fig. 4, the PL peak of Cd₂Ge₂O₆ was 500 nm, while the ECL peak of Cd₂Ge₂O₆ was 525 nm. The tinny difference in the spectra suggested that the ECL of Cd₂Ge₂O₆ was related to its shallow surface state.

The ECL intensity of Cd₂Ge₂O₆ became weaker and extincted along with several times of scanning. Interestingly, it could be recovered and became even stronger while soaked it in 10 mM H₂O₂ solution for several minutes, as shown in Fig. 5. It illuminates that the ECL behaviour of Cd₂Ge₂O₆ is completely irreversible, while H₂O₂ participated in chemical reaction at the Cd₂Ge₂O₆ surface which recovered the ECL.

Fig. 5 ECL graph of Cd₂Ge₂O₆ after immersed in 10 mM H₂O₂ solution for different time. The scanning rate is 20 mV s⁻¹.

In order to further research the ECL mechanism of Cd₂Ge₂O₆, the effect of some surface passivation agent and activation agent were investigated. CV was carried out continuously. As shown in ESI Fig. 1.† By adding Cd²⁺, the ECL intensity of Cd₂Ge₂O₆ was enhanced remarkably. Additionally, it was found that the ECL intensity get weaken distinctly with the addition of PO₄³⁻, as shown in ESI Fig. 2.† This is can be attributed to the adsorption of PO₄³⁻ at the surface of Cd₂Ge₂O₆, as Cd²⁺can be bound to PO₄³⁻ according the solubility produce of phosphate cadmium. The ECL spectrum of Cd₂Ge₂O₆ did not change before and after soaked in CdCl₂ solution in ESI Fig. 3.†

The above results illuminate that the ECL process is probably related to the surface cadmium ions, i.e. cadmium-rich surface state.^23,24 This conclusion is also in good agreement with the reason why the ECL intensity of the sample prepared at 90 °C is weaker than that at 100 °C, as it was found that, while washing the samples, a lot of Cd²⁺ was leached out of the sample prepared at 90 °C than any other temperatures due to the poor crystallization. Therefore, less Cd²⁺ remains at the surface compared with that prepared at 100 °C.

According to the results upwards, it is presumed that the ECL of Cd₂Ge₂O₆ is related to the cadmium-rich surface state, i.e. surface oxygen vacancy, which locates at 525 nm, ca. 20–30 nm red shift from the PL spectrum.

While there is no clear evidence on how the ECL of the amorphous Cd₂Ge₂O₆ can be recovered by soaking in H₂O₂. There are still some sideways that lead us to conclude the ECL mechanism. Firstly, as indicated in Fig. 5, the ECL intensity increased significantly with the soaking time. This result suggests the chemical reaction of H₂O₂ with the reduced Cd₂Ge₂O₆ species, which can recover the ECL. Secondly, after soaked in H₂O₂ solution, extremely strong ECL can be observed in the NaClO₄ solution without H₂O₂, suggesting that the coreactant was adsorbed on the Cd₂Ge₂O₆ surface. Finally, the ECL is related to the cadmium-rich surface state, i.e. surface oxygen vacancy, which is likely to adsorb H₂O₂.²⁴ Therefore, the ECL mechanism is conjectured as follows: H₂O₂ was adsorbed on the surface oxygen vacancy (V_O²⁺), then electrochemically reduced to ˙OH, which can react with the reduced the surface oxygen vacancy (V_O⁺) to produce ECL. However, since V_O⁺ is not stable, it can be further reduced to V_O. V_O can be oxidized to V_O²⁺ in the presence of H₂O₂, thus recover the ECL. The whole processes are listed in the following equations.

V_O²⁺ + H₂O₂ → V_O²⁺·H₂O₂ (adsorption)

V_O²⁺·H₂O₂ + 2e → V_O⁺ + ˙OH + OH⁻

V_O⁺ + ˙OH → V_O²⁺ + OH⁻ + ECL

V_O²⁺ + 2e → V_O (irreversible electrochemical reduction)

V_O + H₂O₂ → V_O²⁺ + 2OH⁻ (recovery)

3.4. Analytical application

The ECL intensity of Cd₂Ge₂O₆ increased with the increasing concentration of hydrogen peroxide solution with the same soaking time. Therefore, the relationship between the ECL intensity and the concentration of hydrogen peroxide was investigated. Interestingly, as shown in Fig. 6, we found that the ECL intensity was linearly increased along with of the square of H₂O₂ concentration, which agrees with the mechanism proposed above, where two H₂O₂ involved in the ECL process. Based on this, a novel biosensor to detect the concentration of hydrogen peroxide was developed. When soaking time is two minutes, the linear range is 5.0 × 10⁻⁶ to 4.0 × 10⁻⁴ mol L⁻¹, and the linear relation curve is: I_ECL = −87.35758 + 3 × 10¹¹ C², where I_ECL is the intensity of ECL, and C is the concentration of hydrogen peroxide.

Fig. 6 The relation between the ECL intensity of Cd₂Ge₂O₆ and the concentration of H₂O₂. The soaking time was two minutes and four minutes separately. (The applied high voltage of PMT is 1000 V) Before soaked in the H₂O₂ solution, the electrode was continuously scanned until the ECL disappeared.

4. Conclusions

ECL of nano Cd₂Ge₂O₆, both crystallized and amorphous, are observed. The amorphous Cd₂Ge₂O₆ possessed extremely strong ECL phenomenon. The ECL mechanism can be ascribed to the cadmium-rich surface state of amorphous Cd₂Ge₂O₆. Based on this, it was used for the determination of hydrogen peroxide and the linear range was 5 × 10⁻⁶ to 4 × 10⁻⁴ mol L⁻¹.

Acknowledgements

The authors thank for the financial support from the National Science Foundation of China (Nos. 21275030 and 21475032), National Basic Research Program of China (No.2010CB732403), and Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1116).

References

1. A. P. Alivisatos, Science, 1996, 271, 933.
2. W. Miao, Chem. Rev., 2008, 108, 2506.
3. G. Zou and H. Ju, Anal. Chem., 2004, 76, 6871.
4. X. H. Wei, C. Liu and Y. F. Tang, Talanta, 2012, 94, 289–294.
5. L. Zhang, L. Shang and S. Dong, Electrochem. Commun., 2008, 10, 1452.
6. Y. Shan, J. Xu and H. Chen, Chem. Commun., 2009, 905.
7. R. Kurita, K. Arai, K. Nakamoto, D. Kato and O. Niwa, Anal. Chem., 2012, 84, 1799–1803.
8. X. Liu, Y. Zhang, J. Lei, Y. Xue, L. Cheng and H. Ju, Anal. Chem., 2010, 82, 7351.
9. G. Jie, P. Liu, L. Wang and S. Zhang, Electrochem. Commun., 2010, 12, 22.
10. B. V. Chikkaveeraiah, A. A. Bhirde, N. Y. Morgan, H. S. Eden and X. Chen, ACS Nano, 2012, 6(8), 6546–6561.
11. T. Ren, J. Z. Xu, Y. F. Tu, S. Xu, J. J. Zhu and H. Y. Chen, Electrochem. Commun., 2005, 7, 5.
12. Y. Shan, J. J. Xu and H. Y. Chen, Nanoscale, 2011, 3(7), 2916–2923.
13. N. Myung, Z. Ding and A. J. Bard, Nano Lett., 2002, 2, 1315.
14. M. Amelia, C. Lincheneau, S. Silvi and A. Credi, Chem. Soc. Rev., 2012, 41, 5728–5743.
15. N. Myung, Y. Bea and A. J. Bard, Nano Lett., 2003, 3, 1053.
16. L. Zheng, Y. Chi, Y. Dong, J. Lin and B. Wang, J. Am. Chem. Soc., 2009, 131, 4564.
17. L. Wu, J. Wang, J. Ren, W. Li and X. Qu, Chem. Commun., 2013, 49, 5675–5677.
18. Z. Ding, B. M. Quirin, S. K. Haram, L. E. Pell, B. A. Korgel and A. J. Bard, Science, 2002, 296, 1293.
19. N. Myung, X. Lu, K. P. Johnston and A. J. Bard, Nano Lett., 2004, 4, 183.
20. Y. M. Fang, J. Song, J. Li, Y. W. Wang, H. H. Yang, J. J. Sun and G. N. Chen, Chem. Commun., 2011, 47, 2369.
21. P. P. Dai, J. Y. Li, T. Yu, J. J. Xu and H. Y. Chen, Talanta, 2015, 141, 97–102.
22. J. H. Huang, K. N. Ding, X. C. Wang and X. Z. Fu, Langmuir, 2009, 25, 8313.
23. Y. M. Fang, J. J. Sun, A. H. Wu, X. L. Su and G. N. Chen, Langmuir, 2009, 25, 555.
24. Y. M. Fang, J. Song, R. J. Zheng, Y. M. Zeng and J. J. Sun, J. Phys. Chem. C, 2011, 115, 9117.

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Footnote

† Electronic supplementary information (ESI) available: The figures about ECL and spectrum of Cd₂Ge₂O₆ immersed solution. See DOI: 10.1039/c5ra15217h

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