One step preparation of proton-functionalized photoluminescent graphitic carbon nitride and its sensing applications

Abstract

Graphitic carbon nitride (g-C₃N₄) has aroused extensive attention in the field of catalysis, sensing and bioimaging due to its unique structure and superior optical properties. Reported synthesis procedures for g-C₃N₄ usually need high reaction temperatures and long reaction times; in addition, the macro size and poor water solubility of bulk g-C₃N₄ significantly limit its practical applications. Herein, we report for the first time a fast one-step approach for the preparation of proton-functionalized g-C₃N₄ under microwave with thiourea and HNO₃ as source materials. After ultrasonic stripping of the raw material for 4 h, well dispersed g-C₃N₄ nanosheets in solution are readily obtained. The g-C₃N₄ nanosheets are positively charged and ultrathin, with a planar size of ca. 60–90 nm and a thickness of 3.3 nm. The presence of hemin causes a significant quenching on the fluorescence of g-C₃N₄, and a sensitive and selective hemin sensing approach is thus developed, with a detection limit of 0.17 μg mL⁻¹. The method is further utilized for the analysis of hemin in a series of biological samples.

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1 Introduction

Graphitic carbon nitride (g-C₃N₄), with a two-dimensional graphene-like structure, is the most stable allotrope of carbon nitride.^1,2 It has attracted great research attention recently due to its unique semiconductor properties.^3,4 Apart from its successful application in catalysis,⁵ its favourable photoluminescence nature makes it a potential candidate for bioimaging and sensing.^6–8 However, the macro size and poor water solubility of bulk g-C₃N₄ largely restricted such applications. It is not until recently that significant breakthrough is achieved in preparing ultrathin g-C₃N₄ nanosheets with good water solubility.⁹ g-C₃N₄ nanosheets with the thickness ranging from 0.4 to 4 nm can be achieved by ultrasonic exfoliation, some of which even down to atomic single layer (0.4–0.5 nm).¹⁰ The nanosize and stable photoluminescence facilitate fast and sensitive sensing of some specific analytes in aqueous media, e.g., metal ions,^11,12 biological molecules,^13,14 and nitroaromatic explosives.¹⁵ In comparison with its carbon counterpart, i.e., carbon quantum dots (CQDs), the fields of applications for the fluorescent g-C₃N₄ nanosheets in biological sensing still need to be further exploited.^16,17

In general, the bulk g-C₃N₄ materials can be simply obtained by thermal polycondensation of nitrogen rich precursors including melamine, cyanamide, dicyandiamide, urea and thiourea.^18–22 However, traditional methods always need to react at high temperature, e.g., 400–600 °C, for a long time of 2–6 h. Microwave irritation is an ideal approach to facilitate the rapid and uniform heating of the reaction medium, which can dramatically shorten the reaction time and increase the product yields.²³ We have previously reported a two-step hydrothermal approach for a yellow emitted pyrrole-ring functionalized graphene quantum dots (GQDs) under microwave irritation, which largely shortened the whole synthesis procedure to 1 h.²⁴ We further reported a one-step microwave approach for the preparation of highly luminescent boron-doped GQDs. Microwave heating avoided the usage of acid in traditional GQD preparation steps and reduced the preparation time to only 30 min.²⁵ Recently, Yuan et al. reported a microwave heating procedure for the synthesis of bulk g-C₃N₄.²⁶ However, a microwave absorber, CuO, is needed to produce a high temperature for the polymerization of urea into g-C₃N₄, and 15 min microwave irradiation is necessary to get highly crystalline g-C₃N₄.

Herein, we report a one-step microwave approach for the preparation of proton-functionalized g-C₃N₄, with thiourea and nitric acid as source materials, the entire process takes only 3 min. To the best of our knowledge, the proton-functionalized g-C₃N₄ prepared by direct microwave treatment has not been reported previously and this is the hitherto reported shortest reaction time for the preparation of g-C₃N₄. Moreover, the obtained g-C₃N₄ is protonated, which can cause its preliminary delamination and significantly shorten the ultrasonic time to get g-C₃N₄ nanosheets. By ultrasonic exfoliation of the proton-functionalized g-C₃N₄ for only 4 h, ultrathin g-C₃N₄ nanosheets with an excellent dispersion is obtained, which is stable for 6 months in aqueous medium. Under UV excitation at 365 nm, the g-C₃N₄ nanosheets emit strong blue fluorescence, which is highly stable within a wide range of pH 1–12, and an ionic strength up to 2 mol L⁻¹. The application of the proton-functionalized g-C₃N₄ nanosheets in biosensing is further exploited with hemin as a model analyte as it is an essential cofactor for a variety of enzymes including catalases and peroxidases.^27,28 Based on the quenching effect of hemin on the fluorescence of g-C₃N₄ nanosheets, a sensitive and fast sensing approach for the measurement of hemin is developed, which can be further used for the analysis of hemin in biological samples.

2 Experimental

2.1 Chemicals and materials

Unless otherwise specified, all the reagents used in this study were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China) and used as received without any further purification. Hemin capsule was obtained from Guoguang Pharmaceutical Co. (Jiangxi, China). Fetal bovine serum was obtained from Nanjing KeyGEN Biotech Co. (Nanjing, China). Human whole blood and urine samples of healthy volunteers were provided by the Hospital of Northeastern University. Pork livers were obtained in BHG supermarket. Deionized water (DI water) of 18 MΩ cm⁻¹ was used throughout.

2.2 Instrumentations

The transmission electron microscopy (TEM) images were taken on a Tecnai G²20 field-emission electron microscope operating at 200 kV (FEI, America) for the characterization of the morphology of the prepared samples. The thickness of the g-C₃N₄ nanosheets were measured by a Bruker dimension icon atomic force microscopy (Bruker, Germany) using tapping mode with a tip type of Scanasyst air. The X-ray diffraction (XRD) patterns were obtained by using a MPDDY2094 X-ray diffractometer (PANalytical B.V., Netherlands) with Cu-Kα irradiation in the range of 2θ from 5 to 80°. The chemical compositions and functional groups of the samples were investigated using an ESCALAB 250 X-ray photoelectron spectrometer (XPS, Thermo Instruments Inc, USA) with an Al-Kα 280.00 eV excitation source and a Nicolet-6700 FT-IR spectrophotometer (Thermo Instruments Inc, USA) from 400 to 4000 cm⁻¹. The surface charge properties and planar size of g-C₃N₄ nanosheets in aqueous media were measured by a ZS90 Nano Zetasizer (Malvern, UK). The UV-vis absorption spectra were recorded on a U-3900 spectrophotometer (Hitachi High Technologies, Japan) with a 1.0 cm optical path. The photoluminescence (PL) measurements were performed using an F-7000 fluorescence spectrophotometer (Hitachi High Technologies, Japan) providing a 0.5 cm quartz cell. The excitation and emission slits were both set as 5.0 nm, with a scan speed of 1200 nm min⁻¹. The quantum yield of the material was recorded on a Quantaurus-QY absolute photoluminescence quantum yield measurement system (Hamamatsu Photonics, Japan).

2.3 Preparation of g-C₃N₄ nanosheets

The bulk g-C₃N₄ was prepared by direct polymerization of thiourea in a microwave system. In detail, 0.761 g (10 mmol) of thiourea was ground for 10 min in an agate mortar and then transferred to a beaker and dissolved in 10 mL of water, followed by the addition of 0.1 mL of nitric acid. The reaction mixture was then irradiated under microwave for 3 min in a domestic microwave oven (G70F23CN2P-BM1(S0), Galanz, China) at 100% MW power (700 W). A cream yellow powder, which was afterwards confirmed to be g-C₃N₄, was obtained and rinsed thoroughly with DI water to remove any residual acid on the sample surface.

The g-C₃N₄ nanosheets were obtained by exfoliating the bulk g-C₃N₄ as in the following: 100 mg of bulk g-C₃N₄ powder was dispersed in 100 mL of DI water and exfoliated by ultrasound for 4 h. The suspension was then centrifuged at 5000 rpm for 15 min to remove the un-exfoliated g-C₃N₄ with large size.

2.4 Hemin sensing procedure

The hemin stock solution (1 mg mL⁻¹, pH 8.61) was prepared by dissolving appropriate amount of hemin solid in NH₃·H₂O directly, and a series of hemin standard solutions with various concentrations were obtained by dilution with PBS (0.1%, pH 7.04). 200 μL of the g-C₃N₄ nanosheets dispersion was then added into 200 μL of different concentrations of hemin solutions. The fluorescent emission spectra were recorded under excitation at 270 nm after shaking at room temperature for 2 min.

2.5 Real sample preparation

The pharmaceutical sample powders enclosed in capsules were dissolved in NH₃·H₂O and stored at 4 °C. The human serum samples were separated from the red blood cells by centrifugation of fresh human blood samples (3000 rpm, 5 min, 4 °C) and then diluted for 50-fold and stored at 4 °C for further use. The fetal bovine serum and urine samples were diluted for 50-fold directly without any pretreatment before applying the sensing procedure. The pork livers were first washing thoroughly with DI water to remove blood on the surface and then mashed and centrifuged at 5000 rpm for 15 min. The obtained supernatant was then diluted for 200-fold and stored at 4 °C for future use. Unless otherwise specified, all the above samples were diluted by PBS (pH 7.04).

3 Results and discussion

3.1 Preparation and characteristic of g-C₃N₄

The preparation of the proton-functionalized g-C₃N₄ powder under microwave assist is illustrated in Scheme S1.† The thermal condensation of thiourea in acidic medium results in a positively charged g-C₃N₄ powder, with a zeta potential of +32.9 mV. It should be emphasized that the bulk g-C₃N₄ cannot be obtained in the presence of excessive amount of solvent, e.g., DI water in the present case. That is, the polycondensation process only begins when the solution dries out. The molten materials cause a rapid and uniform heating, and thus it is not necessary to include a microwave absorber, e.g., CuO powder, as reported previously.²⁶ By further ultrasonic exfoliation, well dispersed g-C₃N₄ nanosheets suspension is obtained. With an ultrasonic stripping for 4 h, a product yield of 16.8% is obtained, which is comparable to that achieved as reported (10 h, 14.5%)¹¹ yet much more time saving. The as-exfoliated g-C₃N₄ nanosheets suspension is stable within 6 months.

TEM images of the obtained g-C₃N₄ nanosheets indicate an irregular lamellar structure (Fig. 1A), and the individual g-C₃N₄ nanosheets is nearly transparent with a diameter of <100 nm (Fig. 1B). The average diameter of the g-C₃N₄ nanosheets is further measured by dynamic light scattering (DLS) to be 60–90 nm (Fig. 1C). The AFM result in Fig. 1D proves that the thickness of g-C₃N₄ nanosheets is about 3.3 nm, i.e., only 7–8 g-C₃N₄ layers.

Fig. 1 TEM images of the g-C₃N₄ nanosheets (A and B); size distribution of g-C₃N₄ nanosheets obtained by dynamic light scattering analysis (C); AFM image of g-C₃N₄ nanosheets (D), with the height profile along the line in the AFM image inset in (D).

X-ray diffraction (XRD) patterns of the bulk g-C₃N₄ and the g-C₃N₄ nanosheets are shown in Fig. 2A. There is only a single apparent peak in the XRD pattern of the g-C₃N₄ nanosheets while there are two for the bulk g-C₃N₄. The peak of (002) at 27.6°, attributing to a distance of d = 0.32 nm between layers, corresponds to the stacking of the conjugated aromatic system in graphitic-like structures of g-C₃N₄.²⁹ The low-angle diffraction observed at 13.3° with an interlayer distance of d = 0.66 nm is indexed to the (100) plane of g-C₃N₄ and corresponds to the in-plane tri-s-triazine structure repeating.³⁰ The reduced peak intensity at high-angle (27.6°) of the g-C₃N₄ nanosheets corresponds to few layers with respect to those in the bulk g-C₃N₄. On the other hand, the disappearance of the low-angle peak (13.3°) implies a decreased planar size. These observations confirm the successful exfoliation of the g-C₃N₄.³¹

Fig. 2 XRD patterns (A) and FT-IR spectra (B) of bulk g-C₃N₄ and the g-C₃N₄ nanosheets.

FT-IR spectra of the bulk g-C₃N₄ and the g-C₃N₄ nanosheets are shown in Fig. 2B. The absorption band at about 806 cm⁻¹ is originated from the breathing mode of tri-s-triazine units which is the characteristic band for the bone structure of g-C₃N₄ and that in the range of 1000–1800 cm⁻¹ belongs to the characteristic stretching modes of CN heterocycles containing bridging C–N(–C)–C (full condensation) and C–NH–C (partial condensation), whereas the broad bands within 3000–3500 cm⁻¹ are attributed to the primary (–NH–) and secondary amines (–NH₂).^11,32,33 The typical IR bands of the g-C₃N₄ nanosheets are analogical to those of the bulk g-C₃N₄ (Fig. 2B), demonstrating that the ultrasonic process does not destroy the basic structure of g-C₃N₄. Meanwhile, the absorption at about 2058 cm⁻¹ is contributed by the stretching vibration of –N=C=S connected with tri-s-triazine units, indicating the presence of sulfur element in the prepared g-C₃N₄.

The chemical state and constitution of the as-exfoliated g-C₃N₄ nanosheets are further characterized by X-ray photoelectron spectra (XPS). The survey XPS spectrum in Fig. 3A shows that the g-C₃N₄ nanosheets are mainly composed of C, N, O and S elements, with atomic percentage of 41.20%, 46.57%, 10.02% and 2.21%, respectively.

Fig. 3 XPS spectrum (A), C1s spectrum (B), N1s spectrum (C) of the as-prepared g-C₃N₄ nanosheets and the possible structure of protonated g-C₃N₄ nanosheet (D). Inset of (A) is the atomic percentage of elements.

As can be seen from Fig. 3B, the dominant C1s peak at 288.0 eV corresponds to the sp²-bonded carbon (N–C=N) of g-C₃N₄, while the 284.6 eV peak is ascribed to the standard reference carbon (graphitic carbon are usually observed on the XPS spectra for carbon nitrides).^34,35 The N1s spectrum of g-C₃N₄ nanosheets can be mainly divided into two peaks centered at 399.7 eV and 398.5 eV, respectively (Fig. 3C). The major peak at 398.5 eV is assigned to sp² hybridized aromatic N bonded to carbon atoms (C=N–C), while the peak located at 399.7 eV is identified as the bridging N atoms bonded with C or H atoms.³⁶ Furthermore, the very weak peak at 404.3 eV is attributed to the positively charged CN cycle, resulting from the protonation of g-C₃N₄ nanosheets.^37,38 The calculated molar ratio of C/N for the prepared g-C₃N₄ nanosheets is about 0.88, which is close to the stoichiometric molar ratio of g-C₃N₄ (0.75).³⁹

The results of XRD, FT-IR and XPS analyses give rise to a conclusion that both the bulk material and the nanosheets are graphitic-like carbon nitride with protonated sites, and the ultrasonic treatment does not destroy the planar structure of g-C₃N₄. A postulated structure for the protonated g-C₃N₄ nanosheets is proposed in Fig. 3D.

3.2 Optical properties of g-C₃N₄

The optical properties of the as-exfoliated g-C₃N₄ nanosheets are verified by the UV-vis absorption spectra and PL emission spectra. As shown in Fig. 4A, the UV-vis absorption spectrum exhibits a strong peak at 320 nm. It also shows a PL emission peak centered at 380 nm with excitation at 270 nm, which exhibits a blue shift compared with those reported in the literatures,^40–42 while this observation is similar to that previously reported for the acid-ultrasonic treated g-C₃N₄ nanosheets.⁴³ Moreover, PL emission of the g-C₃N₄ nanosheets is excitation-independent with the excitation wavelength ranging from 270 to 370 nm (Fig. 4A), suggesting that the PL properties of the g-C₃N₄ nanosheets are dependent on their surface states rather than their morphology.⁴⁴

Fig. 4 (A) UV-vis absorption spectrum and PL spectra for the g-C₃N₄ nanosheets at different excitation wavelengths. (B) The PL emission spectra of g-C₃N₄ nanosheets (HNO₃) and g-C₃N₄ nanosheets (HCl) at an excitation wavelength of 270 nm (both are dispersed in water).

It can be seen in Fig. S2† that the fluorescence intensity of g-C₃N₄ nanosheets remains virtually unchanged after irradiation with a Xe lamp (λ_ex 270 nm, 150 W) for 2 h. This indicates an excellent photo-stability of the as-exfoliated g-C₃N₄ nanosheets. As illustrated in Fig. S3,† the aqueous dispersion of g-C₃N₄ nanosheets is relatively stable under either acidic or alkaline circumstance within pH 1–12. Furthermore, the fluorescence behavior of g-C₃N₄ nanosheets is noted to exhibit virtually no variation at a high ionic strength, e.g., up to a KCl concentration of 2.0 mol L⁻¹ (Fig. S4†). These observations demonstrate a great potential for the g-C₃N₄ nanosheets as an excellent light-emitting probe in sensing, wherein a wide pH range and high saline sample matrixes are frequently encountered.

It is worthwhile to mention that during the preparation process the replacement of nitric acid with hydrochloric acid results in an obvious red shift of the photoluminescence emission for g-C₃N₄ nanosheets (Fig. 4B). At an excitation wavelength of 270 nm, the HNO₃-derived g-C₃N₄ nanosheets show a maximum emission at λ_em 380 nm, while that of the HCl-derived g-C₃N₄ nanosheets shifted to 420 nm. The quantum yields are measured to be 4.3% for the HNO₃-derived g-C₃N₄ nanosheets and 4.8% for the HCl-derived g-C₃N₄ nanosheets. The above observation might be due to the different oxidizing capability of concentrated HCl and HNO₃ during the preparation of g-C₃N₄, which causes a distinction in the conduction band and valence band.⁴³

3.3 Fluorescent sensing of hemin in real samples

Our experiments have indicated that the presence of hemin tends to cause photoluminescence quenching for the g-C₃N₄ nanosheets suspension. The changes in the PL intensity of g-C₃N₄ nanosheets upon gradual addition of hemin within a concentration range of 0–200 μg mL⁻¹ and an optimized reaction time of 2 min (Fig. S5†) are recorded and illustrated in Fig. 5A. The inset of Fig. 5B shows a linear relationship between the PL intensity of g-C₃N₄ nanosheets and hemin concentration from 0.5 to 6 μg mL⁻¹, along with a detection limit of 0.17 μg mL⁻¹ (3σ/s, n = 11).

Fig. 5 (A) PL spectra of g-C₃N₄ nanosheets in the presence of different concentrations of hemin (from top to bottom: 0, 0.05, 0.1, 0.2, 0.5, 1, 5, 10, 20, 50, 100 and 200 μg mL⁻¹); (B) the PL intensity as a function of hemin concentrations (from top to bottom: 0, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 10, 20, 50, 75, 100, 200 and 500 μg mL⁻¹). Inset of (B): the linear calibration curve between PL intensity and the hemin concentration within a range of 0.5 to 6 μg mL⁻¹.

The fluorescence quenching of the g-C₃N₄ nanosheets in the presence of hemin could be explained by static quenching. We have tested the variation of PL intensity of g-C₃N₄ nanosheets with various concentrations of hemin at 20 °C and 40 °C, respectively, and the results are shown in Fig. S6A.† According to the Stern–Volmer equation as given in the following:

F₀/F = 1 + K_qτ₀C_hemin = 1 + K_svC_hemin,

the K_sv values, i.e., the quenching constants, are derived to 7.2 × 10⁵ L mol⁻¹ and 6.8 × 10⁵ L mol⁻¹ at 20 °C and 40 °C, respectively, indicating an approximately similar quenching constant at different temperatures. Hence, the quenching mechanism is not dynamic quenching. Furthermore, the absorption spectra of hemin, g-C₃N₄ nanosheets, and their mixture are also recorded (Fig. S6B†). It is seen that the absorption spectrum of hemin–g-C₃N₄ adduct does not overlap with the sum of the absorption spectra of g-C₃N₄ nanosheets and hemin, this observation gives rise to a strong proof of static quenching.⁴⁵

To evaluate the selectivity of this fluorescence probe in hemin sensing, the PL intensity changes of the g-C₃N₄ nanosheets suspension (0.1 mg mL⁻¹) have been evaluated upon the addition of some representative cationic and anionic species as well as biomolecules which are commonly encountered in biological samples. The concentrations of the potential interfering cationic and anionic species are set at 1 mmol L⁻¹ while that for the biomolecules are fixed at 0.01 mg mL⁻¹, and all the detections were performed at pH 7 in a PBS buffer at room temperature. It is clearly observed in Fig. S7† that the presence of the aforementioned matters at the given concentration levels causes virtually no influence on the photoluminescence of the g-C₃N₄ nanosheets suspension. This demonstrates a favorable selectivity for quenching the photoluminescence by hemin.

The present fluorescence probe has been applied for the assay of hemin in pharmaceutical, human serum, fetal bovine serum, urine and pork liver samples, and the analytical results are shown in Table 1.

Table 1 Hemin measurement in pharmaceutical, serum, urine and pork liver samples (n = 3)

Samples	Found (mg L^−1)	Spiked (mg L^−1)	Recovered (%)
a The unit of pharmaceutical sample test is mg g^−1.b The standard value of hemin in hemin pharmaceutical is 125 mg g^−1.
Pharmaceutical sample^a	124 ± 2^b	100	100 ± 2.4
		200	103 ± 3.6
		300	97 ± 3.7
Human serum 1	186.5 ± 8.0	50	98 ± 2.1
		75	100 ± 1.9
		100	104 ± 1.2
Human serum 2	93.0 ± 6.5	50	96 ± 1.7
		100	99 ± 2.6
		150	102 ± 2.5
Human serum 3	110.0 ± 3.0	50	95 ± 2.2
		100	101 ± 2.9
		150	104 ± 2.1
Fetal bovine serum 1	55.5 ± 5.0	50	105 ± 3.3
		100	97 ± 3.2
		150	105 ± 1.7
Fetal bovine serum 2	56.0 ± 3.5	50	103 ± 4.2
		100	112 ± 7.3
		150	103 ± 1.7
Urine 1	175.5 ± 9.0	50	96 ± 1.8
Urine 2	197.5 ± 0.5	50	101 ± 0.8
Urine 3	223.5 ± 4.5	50	100 ± 1.3
Pork liver 1	660 ± 14	200	97 ± 1.4
		400	99 ± 1.8
Pork liver 2	404 ± 12	200	98 ± 1.7
		400	99 ± 1.7

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It is seen that the concentration of hemin in the hemin pharmaceutical is in well agreement with the standard value as claimed in the pharmaceutical instructions. In addition, the recoveries for the spiked hemin in the sample matrixes of pharmaceutical, human serum, fetal bovine serum, urine and pork liver are quite satisfactory, within 95–112% for all samples. These observations well illustrated the practical usefulness of the sensing system.

4 Conclusions

Proton-functionalized g-C₃N₄ has been prepared via a facile one-step microwave-assisted approach, which shorten the reaction time to only 3 min. Moreover, the protonation of g-C₃N₄ facilitates its preliminary delamination and shortened its ultrasonic stripping into nanosheets to only 4 h. We emphatically investigate the photoluminescence properties of the as-prepared g-C₃N₄ nanosheets. Based on the quenching effect of hemin on the fluorescence of the g-C₃N₄ nanosheets, a sensitive and fast sensing approach for the measurement of hemin is developed, which is successfully applied in the analysis of hemin in a series of biological samples, demonstrating the viability of this approach.

Acknowledgements

Financial support from the Natural Science Foundation of China (21375013, 21235001, 21405010) and the Fundamental Research Funds for the Central Universities (N130105002, N140504003, N140505003, N141008001) are highly appreciated.

Notes and references

1. S. W. Cao, J. Low, J. Yu and M. Jaroniec, Adv. Mater., 2015, 27, 2150–2176.
2. J. H. Zhang, Y. Chen and X. C. Wang, Energy Environ. Sci., 2015, 8, 3092–3108.
3. X. P. Dong and F. X. Cheng, J. Mater. Chem. A, 2015, 3, 23642–23652.
4. Y. Chen, C. L. Tan, H. Zhang and L. Z. Wang, Chem. Soc. Rev., 2015, 44, 2681–2701.
5. J. Q. Tian, Q. Liu, A. M. Asiri, K. A. Alamry and X. P. Sun, ChemSusChem, 2014, 7, 2125–2132.
6. X. D. Zhang, X. Xie, H. Wang, J. J. Zhang, B. C. Pan and Y. Xie, J. Am. Chem. Soc., 2013, 135, 18–21.
7. J. Q. Tian, Q. Liu, C. J. Ge, Z. C. Xing, A. M. Asiri, A. O. Al-Youbi and X. P. Sun, Nanoscale, 2013, 5, 8921–8924.
8. J. Q. Tian, Q. Liu, C. J. Ge, A. M. Asiri, A. H. Qusti, A. O. Al-Youbi and X. P. Sun, Nanoscale, 2013, 5, 11604–11609.
9. L. C. Chen, D. J. Huang, S. Y. Ren, T. Q. Dong, Y. W. Chi and G. N. Chen, Nanoscale, 2013, 5, 225–230.
10. J. Xu, L. W. Zhang, R. Shi and Y. F. Zhu, J. Mater. Chem. A, 2013, 1, 14766–14772.
11. J. Q. Tian, Q. Liu, A. M. Asiri, A. O. Al-Youbi and X. P. Sun, Anal. Chem., 2013, 85, 5595–5599.
12. M. C. Rong, L. P. Lin, X. H. Song, Y. R. Wang, Y. X. Zhong, J. W. Yan, Y. F. Feng, X. Y. Zeng and X. Chen, Biosens. Bioelectron., 2015, 68, 210–217.
13. X. L. Zhang, C. Zheng, S. S. Guo, J. Li, H. H. Yang and N. Chen, Anal. Chem., 2014, 86, 3426–3434.
14. T. Y. Ma, Y. Tang, S. Dai and S. Z. Qiao, Small, 2014, 10, 2382–2389.
15. M. C. Rong, L. P. Lin, X. H. Song, T. T. Zhao, Y. X. Zhong, J. W. Yan, Y. R. Wang and X. Chen, Anal. Chem., 2015, 87, 1288–1296.
16. X. T. Zheng, A. Ananthanarayanan, K. Q. Luo and P. Chen, Small, 2015, 11, 1620–1636.
17. S. Y. Lim, W. Shen and Z. Gao, Chem. Soc. Rev., 2015, 44, 362–381.
18. F. Dong, Z. Y. Wang, Y. J. Sun, W. K. Ho and H. D. Zhang, J. Colloid Interface Sci., 2013, 401, 70–79.
19. E. Z. Lee, Y. S. Jun, W. H. Hong, A. Thomas and M. M. Jin, Angew. Chem., Int. Ed., 2010, 49, 9706–9710.
20. K. Maeda, X. C. Wang, Y. Nishihara, D. L. Lu, M. Antonietti and K. Domen, J. Phys. Chem. C, 2009, 113, 4940–4947.
21. S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2009, 25, 10397–10401.
22. G. G. Zhang, J. S. Zhang, M. W. Zhang and X. C. Wang, J. Mater. Chem., 2012, 22, 8083–8091.
23. 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.
24. S. Chen, X. Hai, C. Xia, X. W. Chen and J. H. Wang, Chem.–Eur. J., 2013, 19, 15918–15923.
25. X. Hai, Q. X. Mao, W. J. Wang, X. F. Wang, X. W. Chen and J. H. Wang, J. Mater. Chem. B, 2015, 3, 9109–9114.
26. Y. P. Yuan, L. S. Yin, S. W. Cao, L. N. Gu, G. S. Xu, P. W. Du, H. Chai, Y. S. Liao and C. Xue, Green Chem., 2014, 16, 4663–4668.
27. E. Golub, R. Freeman and I. Willner, Angew. Chem., Int. Ed., 2011, 50, 11710–11714.
28. Y. Shi, W. T. Huang, H. Q. Luo and N. B. Li, Chem. Commun., 2011, 47, 4676–4678.
29. X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
30. H. Xu, J. Yan, X. J. She, L. Xu, J. X. Xia, Y. G. Xu, Y. H. Song, L. Y. Huang and H. M. Li, Nanoscale, 2014, 6, 1406–1415.
31. N. Y. Cheng, P. Jiang, Q. Liu, J. Q. Tian, A. M. Asiri and X. P. Sun, Analyst, 2014, 139, 5065–5068.
32. S. Y. Yang, Y. J. Gong, J. S. Zhang, L. Zhan, L. L. Ma, Z. Y. Fang, R. Vajtai, X. C. Wang and P. M. Ajayan, Adv. Mater., 2013, 25, 2452–2456.
33. P. Niu, L. L. Zhang, G. Liu and H. M. Cheng, Adv. Funct. Mater., 2012, 22, 4763–4770.
34. R. M. Balleste, C. G. Navarro, J. G. Herrero and F. Zamora, Nanoscale, 2011, 3, 20–30.
35. Y. G. Li, J. Zhang, Q. S. Wang, Y. X. Jin, D. H. Huang, Q. L. Cui and G. T. Zou, J. Phys. Chem. B, 2010, 114, 9429–9434.
36. B. Jurgens, E. Irran, J. Senker, P. Kroll, H. Muller and W. Schnick, J. Am. Chem. Soc., 2003, 125, 10288–10300.
37. R. C. Dante, P. M. Ramos, A. C. Guimareas and J. M. Gil, Mater. Chem. Phys., 2011, 130, 1094–1102.
38. Y. J. Cui, J. S. Zhang, G. G. Zhang, J. H. Huang, P. Liu, M. Antonietti and X. C. Wang, J. Mater. Chem., 2011, 21, 13032–13039.
39. Q. Li, J. P. Yang, D. Feng, Z. X. Wu, Q. L. Wu, S. S. Park, C. S. Ha and D. Y. Zhao, Nano Res., 2010, 3, 632–642.
40. J. Q. Tian, R. Ning, Q. Liu, A. M. Asiri, A. O. Al-Youbi and X. P. Sun, ACS Appl. Mater. Interfaces, 2014, 6, 1011–1017.
41. F. Dong, Y. J. Sun, L. W. Wu and Z. B. Wu, Catal. Sci. Technol., 2012, 2, 1332–1335.
42. L. Lin, P. Ye, C. Cao, Q. Jin, G. S. Xu, Y. H. Shen and Y. P. Yuan, J. Mater. Chem. A, 2015, 3, 10205–10208.
43. H. Q. Zhang, Y. H. Huang, S. R. Hu, Q. T. Huang, C. Wei, W. X. Zhang, L. P. Kang, Z. Y. Huang and A. Y. Hao, J. Mater. Chem. C, 2015, 3, 2093–2100.
44. Y. Q. Dong, H. C. Pang, H. B. Yang, C. X. Guo, J. W. Shao, Y. W. Chi, C. M. Li and T. Yu, Angew. Chem., Int. Ed., 2013, 52, 7800–7804.
45. S. Huang, L. M. Wang, C. S. Huang, J. N. Xie, W. Su, J. R. Sheng and Q. Xiao, Sens. Actuators, B, 2015, 221, 1215–1222.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22829a

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