Novel electrochemical preparation of gold nanoparticles decorated on a reduced graphene oxide–fullerene composite for the highly sensitive electrochemical detection of nitrite

Abstract

In this study, we report a novel amperometric nitrite sensor based on a glassy carbon electrode (GCE) modified with gold nanoparticles (AuNP) decorated reduced graphene oxide–fullerene (RGO–C60) composite. The RGO–C60/AuNPs composite modified electrode was prepared via the electrochemical reduction of a GO–C60 composite modified electrode in pH 3 solution containing 5 mM gold(III) chloride trihydrate at the constant potential of −1.4 V for 200 s. The as-prepared materials were characterized using scanning electron microscopy, and Raman and Fourier transform infrared spectroscopy. Cyclic voltammetry results confirm that the RGO–C60/AuNPs composite modified electrode has high catalytic activity for the detection of nitrite compared with other modified electrodes. The RGO–C60/AuNPs modified electrode exhibits a stable amperometric response for nitrite in the liner concentration range of 0.05–1175.32 μM and its detection limit was estimated to be 0.013 ± 0.003 μM. The modified electrode shows high selectivity towards the determination of nitrite in the presence of potentially active common metal ions. In addition, the fabricated sensor exhibits many advantages for the detection of nitrite such as fast amperometric response, excellent operational and storage stability and appropriate practicality.

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Introduction

The accurate and reliable determination of nitrite is of potential interest due to its serious effects on human health and environmental and physiological systems.¹ Nitrite is widely used for food preservation and is highly toxic to humans at a level of over 200 ppm in meat.² In addition, nitrite can easily react with blood and amines, which results in the oxidation of hemoglobin and the formation of carcinogenic nitrosamines.² According to the world health organization, the maximum permissible level of nitrite is 3 mg L⁻¹ in drinking water and an excess quantity of nitrite could cause different diseases such as blue baby syndrome and shortness of breath.^3,4 Therefore, the reliable and low level determination of nitrite in food and water is essential to assess the quality of food and water. To date, different methods have been reported for the determination of nitrite, including high-performance liquid chromatography,⁵ calorimetry,⁶ chemiluminescence,⁷ capillary electrophoresis⁸ and electrochemical methods.⁹ However, electrochemical methods are widely used for the detection of nitrite due to their specific advantages such as compactness, relatively low cost, reliability, sensitivity and real-time analysis.¹⁰ In general, the direct electro-oxidation method is widely used for the detection of nitrite due to the small interference effect with molecular oxygen.¹¹ However, the electrochemical oxidation of nitrite at unmodified carbon electrodes is still limited due to their high oxidation potential, low sensitivity and fouling of the oxidation signals.¹² Therefore, modified electrodes are commonly used as an alternative to rectify the aforementioned problems in the detection of nitrite. For instance, carbonaceous materials,¹³ metal oxides,¹⁴ metal phthalocyanines,¹⁵ metals and metal alloy nanoparticles^16,17 have been extensively used as selective probes for the sensitive and reliable detection of nitrite.

Recently, fullerene C60 (C60) has served as a potential candidate for diverse applications, which includes electrochemical sensors and biosensors.¹⁸ However, the electrochemical activity of C60 is poor in aqueous solutions and therefore different materials or approaches have been used on the C60 surface to improve its electrochemical performance and stability. For example, graphene oxide,¹⁹ reduced graphene oxide (RGO)²⁰ and carbon nanotubes²¹ have been widely used to improve the electrochemical properties of C60. In addition, the electrochemical activation method has also been used to enhance the electrochemical properties of C60.²² On the other hand, gold nanoparticles (AuNPs) has received foremost interest in electroanalytical chemistry due to their unique properties such as high electrocatalytic activity, high surface-to-volume ratio and high stability.²³ Modified AuNPs electrodes are known for their high catalytic activity towards nitrite.^24,25 In addition, composites of AuNPs, such as choline/AuNPs,²⁶ polyaniline–graphene/AuNPs,²⁷ multi-walled carbon nanotubes/zinc oxide–AuNPs²⁸ and graphene/AuNPs,²⁹ have been used for the sensitive detection of nitrite. Recently, we reported a simple electrochemical preparation of an RGO combined C60 composite, and showed the enhanced electrocatalytic activity and high stability of C60 in an aqueous solution.²⁰ In the present study, we fabricate a novel AuNPs decorated RGO–C60 (RGO–C60/AuNPs) composite via a single step electrochemical reduction method and the resulting composite is used as a probe for the detection of nitrite. To the best of our knowledge, there are no reports are available for the fabrication of an RGO–C60/AuNPs composite and its application in electrochemical sensors.

Herein, we report the simple electrochemical fabrication of an RGO–C60/AuNPs composite modified electrode for the first time. The as-prepared composite modified electrode is further used for the electrochemical detection of nitrite. The composite modified electrode is also applied for the determination of nitrite in different water samples.

Experimental

Materials and methods

Graphite powder (98.0% purity) and fullerene–C60 (99.5% purity) were obtained from Sigma-Aldrich, Taiwan (http://www.sigmaaldrich.com/catalog/product/aldrich/282863?lang=en%26region=TW). Gold(III) chloride trihydrate (HAuCl₄·3H₂O) was purchased from Aldrich, Taiwan (http://www.sigmaaldrich.com/catalog/product/aldrich/520918?lang=en%26region=TW). The supporting pH 5 and pH 3 electrolytes were prepared using 0.05 M Na₂HPO₄ and NaH₂PO₄ solutions. All other chemicals were of analytical grade and used without further purification. All solutions were prepared using Millipore DI water.

Electrochemical measurements, including cyclic voltammetry (CV) and amperometry, were carried out using a CHI 1205b electrochemical analyzer (CH Instruments, Austin, TX, USA). A glassy carbon electrode (GCE) was used as the working electrode, and platinum wire and sat. Ag/AgCl were used as the counter and reference electrodes, respectively. Raman spectra were obtained using a Raman spectrometer (Dong Woo 500i, Korea) equipped with a 50× objective and a charge-coupled detector. Surface morphological studies of the fabricated composites were conducted using a Hitachi S-3000 H (Japan) scanning electron microscope (SEM). Energy-dispersive X-ray (EDX) spectra were obtained using a HORIBA EMAX X-ACT coupled with the Hitachi S-3000 H scanning electron microscope. Fourier transform infrared spectroscopy (FT-IR) was carried out using a Thermo SCIENTIFIC Nicolet iS10 (USA) instrument. A rotating disc electrode (RDE) was used for amperometric i–t experiments. All electrochemical experiments were performed in an N₂ atmosphere at room temperature.

Preparation of RGO–C60/AuNPs composite modified electrode

Graphene oxide and the graphene oxide–C60 (GO–C60) composite were prepared using our previously reported method.^20,30 The as-prepared GO–C60 composite was re-dispersed in DI water by sonication for 30 min. Approximately, 8 μL (optimum) of GO–C60 dispersion (1 : 2, v/v%) was drop casted on a pre-cleaned GCE and dried at room temperature. Then, the GO–C60 composite modified electrode was transferred to a pH 3 solution containing 5 mM HAuCl₄·3H₂O and a constant potential of −1.4 V was applied for 200 s, and this led to the formation of RGO–C60/AuNPs composite. Finally, the RGO–C60/AuNPs composite modified electrode was gently rinsed in DI water and dried at room temperature. A schematic representation for the fabrication of RGO–C60/AuNPs composite is shown in Scheme 1. For comparison, the RGO–C60 composite was prepared via the electrochemical reduction of the GO–C60 modified electrode in pH 5 solution by applying a constant potential of −1.4 V for 200 s. It should be noted that we used a C60 containing GO aqueous dispersion (1 : 2, v/v%) as the optimum for the preparation of the RGO–C60/AuNPs composite, which showed the best sensitivity and a low LOD for the detection of nitrite (not shown). We further optimized the effect of AuNP deposition on RGO–C60 composite using different concentrations of HAuCl₄·3H₂O towards the detection of nitrite, and the results are shown in Fig. 3A (inset). It is clearly revealed that the best nitrite response was observed for the AuNPs electrodeposited in the pH 3 solution containing 5 mM H(AuCl₄·3H₂O) and therefore this was used as the optimum for the composite preparation. It should also be noted that we have not optimized the concentration of HAuCl₄·3H₂O for the loading of AuNPs on RGO–C60 composite, although we have optimized it for the detection of nitrite.

Scheme 1 Schematic representation for the preparation of RGO–C60/AuNPs composite.

Results and discussion

Characterization

The surface morphology of the as prepared materials was characterized using SEM. Fig. 1 shows the SEM images of (A) C60, (B) GO–C60, (C) RGO–C60 and (D) RGO–C60/AuNPs. The SEM image of C60 Fig. 1(A) clearly reveals its ultra-rod shape morphology with an association micro rod bundles. The SEM image of GO–C60 Fig. 1(B) shows that the micro rods of C60 are firmly attached on the ultra-thin surface of GO (indicated by yellow arrow), whereas the RGO–C60 Fig. 1(C) composite shows that the C60 rods are enfolded by RGO nanosheets.²⁰

Fig. 1 SEM images of C60 (A), GO–C60 (B), RGO–C60 (C) and RGO–C60/AuNPs (D). Inset of (D) shows the EDX quantitative results of the RGO–C60/AuNPs composite.

The SEM image of RGO–C60/AuNPs Fig. 1(D) composite shows that the AuNPs are uniformly decorated on the surface of RGO–C60 composite and the average particle size of the AuNPs on the RGO–C60 composite was 41 ± 2 nm (Fig. 1D). The presence of AuNPs on RGO–C60 composite was confirmed by elemental analysis and the corresponding quantitative results are shown in the inset of Fig. 1D. It can be observed that the metallic Au, carbon and oxygen were clearly observed on the RGO–C60 composite. This result further confirms the presence of AuNPs on the RGO–C60 composite.

The formation of RGO–C60/AuNPs composite was confirmed by FTIR and Raman spectral analyses. Fig. 2A shows the FTIR spectra of GO–C60 (a) and the RGO–C60/AuNPs composite (b). The broad bands at around 3500–4000 cm⁻¹ are due to the –OH stretching vibrations of the C–OH group and water.³¹ The bands at 1042, 1285 and 1671 cm⁻¹ are due to the presence of C–O (epoxy) groups, C–OH stretching vibrations and stretching vibration modes of C=O, respectively.^30,31 The band at 1420 cm⁻¹ is due to the C=C skeletal vibrations of unoxidized graphitic domains in graphite.

Fig. 2 (A) FTIR spectra of GO–C60 (a) and RGO–C60/AuNPs (b). (B) Raman spectra of GO–C60 (black color), RGO–C60 (red color) and RGO–C60/AuNPs (green color).

On the other hand, the bands at around 3500–4000 cm⁻¹ for the –OH stretching vibrations of the C–OH group are significantly reduced after the electrochemical reduction of GO–C60 composite, which indicates the removal of oxygen functional groups from the GO–C60 composite and successful transformation to RGO–C60 composite.³⁰ It should also be noted that the reduction of GO–C60 composite was not affected by the simultaneous electrodeposition of AuNPs on the RGO–C60 composite. This result confirms the formation of the RGO–C60/AuNPs composite.

Raman spectroscopy is used to confirm the transformation of GO to RGO and the representative Raman spectra of GO–C60 (black), RGO–C60 (red) and RGO–C60/AuNPs (green) are shown in Fig. 2B. The Raman spectrum of GO–C60 (black color) shows three prominent peaks at 1342, 1467 and 1585 cm⁻¹. The characteristic D and G peaks at 1342 and 1585 cm⁻¹ are attributed to the vibrations of sp³ carbon atoms of disordered graphene nanosheets and the vibrations of sp² carbon atom domains of graphite,³¹ whereas the characteristic peak at 1467 cm⁻¹ is due to the Ag mode of C60 in GO–C60 composite.³² Usually, the Hg mode of C60 is observed at 1680 cm⁻¹; however, in our case the Hg mode of C60 not observed in GO–C60 (black color), RGO–C60 (red color) and the RGO–C60/AuNPs composite (green color). This is possibly due to the overlapping of Hg mode of C60 with the G-band of GO and RGO.²⁰ The peak at 866 cm⁻¹ in GO–C60, RGO–C60 and the RGO–C60/AuNPs composite is derived from the underlying quartz substrate.³³ The I_D/I_G ratio of RGO–C60 and the RGO–C60/AuNPs composite was calculated to be 1.03 and 1.01, respectively. The I_D/I_G ratio was greatly increased in the RGO–C60 composite than that of GO–C60 composite (0.92), which clearly indicates the transformation of GO–C60 to RGO–C60 composite.

Electrocatalytic oxidation of nitrite

The electrocatalytic behaviour of different modified electrodes was investigated towards the oxidation of nitrite by CV. Fig. 3A displays the CV response of GO–C60/AuNPs (a), GO/AuNPs (b), bare/AuNPs (c), RGO/AuNPs (d) and RGO–C60/AuNPs (e) modified electrodes in pH 5 solution containing 500 μM nitrite at a scan rate of 50 mV s⁻¹. The bare/AuNPs and GO/AuNPs modified electrodes show an obvious oxidation current response to nitrite. However, the observed current response was lower than that observed at GO–C60/AuNPs and RGO/AuNPs modified electrodes. The electrocatalytic activity of GO/AuNPs modified electrode was greatly improved in the presence of C60, although the response current did not change compared to the RGO/AuNPs modified electrode. The GO–C60/AuNPs modified electrode shows an oxidation peak for nitrite at 0.817 V, which was 0.036, 0.131 and 0.142 V lower than that observed at the AuNPs decorated GO, RGO and bare modified electrodes, respectively. This result clearly indicates that the unique properties of C60 in GO–C60/AuNPs result in high catalytic activity and low oxidation potential towards the detection of nitrite. On the other hand, the RGO–C60/AuNPs modified electrode exhibits a sharp oxidation peak for nitrite at 0.847 V. The peak current response of nitrite was 2.0 folds higher than that observed at the GO–C60/AuNPs modified electrode. Usually, a negative shift in the oxidation peak is a result of the electron transfer ability of the modifier towards the electrode surface. It should be noted that the RGO–C60/AuNPs modified electrode shows high oxidation potential towards nitrite when compared to GO–C60/AuNPs and GO/AuNPs modified electrodes. However, GO–C60/AuNPs and GO/AuNPs modified electrodes show a lower oxidation peak current response to nitrite when compared with the RGO–C60/AuNPs modified electrode. The combined unique properties of RGO, C60 and AuNPs result in enhanced electrocatalytic activity towards the detection of nitrite. This result further indicates that the RGO–C60/AuNPs modified electrode is more suitable for the detection of nitrite than the other modified electrodes.

Fig. 3 (A) Cyclic voltammetry response of GO–C60/AuNPs (a), GO/AuNPs (b), bare/AuNPs (c), RGO/AuNPs (d) and RGO–C60/AuNPs (e) modified electrodes in pH 5 solution containing 500 μM nitrite at a scan rate of 50 mV s⁻¹. Inset shows the effect of different concentrations of HAuCl₄·3H₂O towards the detection of nitrite. (B) Under the same conditions, the cyclic voltammetry response of GO (a), bare (b), C60 (c), GO–C60 (d), RGO (e) and RGO–C60 (f) modified electrodes for the addition of 500 μM nitrite.

We also investigated the electrocatalytic activity of different modified electrodes in the absence of AuNPs towards the oxidation of nitrite. Fig. 3B shows the CV response of GO (a), bare (b), C60 (c), GO–C60 (d), RGO (e) and RGO–C60 (f) modified electrodes in 500 μM nitrite solution of pH 5 at a scan rate of 50 mV s⁻¹. The RGO modified electrode showed good catalytic activity towards the oxidation of nitrite and the oxidation peak potential was observed at 1.12 V, whereas the oxidation peak potential of nitrite was 0.12 V lower than that observed for the RGO modified electrode. In addition, the RGO–C60 modified electrode showed an enhanced current response for nitrite compared to that of the bare, GO, C60 and GO–C60 modified electrodes. It should also be noted that the electrocatalytic activity of RGO–C60 modified electrode was quite similar to that observed at the RGO modified electrode. Therefore, the RGO–C60 modified electrode is more suitable for further electrode modification with AuNPs.

Fig. 4A shows the effect of scan rate on the response to nitrite at the RGO–C60/AuNPs modified electrode. It is observed that the oxidation peak current of nitrite increases with increasing the scan rate from 20 to 200 mV s⁻¹. The oxidation peak current of nitrite was plotted against the square root of scan rate, which was linear over the scan rate range of 20–120 mV s⁻¹ with a correlation coefficient of 0.9818. This result indicates that the oxidation of nitrite is a typical diffusion-controlled kinetic process on the RGO–C60/AuNPs modified electrode.²⁶

Fig. 4 (A) Cyclic voltammetry response of RGO–C60/AuNPs modified electrode in pH 5 solution containing 500 μM nitrite at different scan rates from 20 to 200 mV s⁻¹ (a–j). Inset shows the linear plot for (scan rate)^1/2 vs. I_pa of nitrite. Error bar is relative to the standard deviation for 3 measurements. (B) Cyclic voltammetry response of the RGO–C60/AuNPs modified electrode in 500 μM nitrite containing different pH (pH 3, 5, 7, 9 and 11) at a scan rate of 50 mV s⁻¹. Inset is the linear plot for pH vs. I_pa of nitrite. Error bar is relative to the standard deviation for 3 measurements.

The effect of pH on the oxidation peak current response of nitrite was studied because pH plays an important role and can greatly affect the electrocatalytic activity of nitrite. Fig. 4B shows the CV response of RGO–C60/AuNPs modified electrode in 500 μM nitrite containing different pH at a scan rate of 50 mV s⁻¹. The pH was tested in the range of pH 3, 5, 7, 9 and 11. It can be observed that a distinct oxidation peak of nitrite was clearly observed in each pH, and the maximum oxidation peak current of nitrite was observed at pH 5. The oxidation peak current of nitrite decreased when the pH was above or below pH 5, which clearly indicates that the modified electrode exhibits higher sensitivity at pH 5 than that of other pH. In general, the electroactive species of nitrite is highly unstable when the pH is below 4, whereas higher pH leads (pH > 7) to inhibition of the oxidation of nitrite.³⁴ Therefore, pH 5 was used as the optimum for further electrochemical studies.

The electro-oxidation mechanism of nitrite involves a two-step process and the oxidation phenomenon is based on the CE mechanism. The overall oxidation mechanism of nitrite at the RGO–C60/AuNPs modified electrodes is shown in Fig. 5.

Fig. 5 Electro-oxidation mechanism of nitrite at the RGO–C60/AuNPs composite modified electrode.

Amperometric determination of nitrite

The amperometric i–t method was further used for the determination of nitrite owing to its high sensitivity, low background current and high precision compared with other voltammetric methods. Fig. 6A shows the typical amperometric response of RGO–C60/AuNPs modified RDE for successive additions of nitrite into a constantly stirred N₂ saturated pH 5 solution at the operating potential of 0.807 V. A stable and well-defined amperometric response was observed for each addition of nitrite into the pH 5 solution. It can be observed that a noteworthy amperometric i–t response was observed for the addition of 0.05 μM (a), 0.1 μM (b), 1 μM (c) and 5 μM (d) of nitrite (Fig. 6A inset), which indicates the excellent electro-oxidation of nitrite at the RGO–C60/AuNPs modified electrode. The response of nitrite reaches a steady state within 4 s, which indicates the fast diffusion of nitrite on the RGO–C60/AuNPs composite modified electrode surface. The amperometric response of nitrite was linear over the concentration range of 0.05–1175.32 μM with the correlation coefficient of 0.9952 (Fig. 6B). The LOD was calculated to be 0.013 ± 0.003 μM based on S/N = 3. The electroanalytical properties (LOD and linear range) of the nitrite sensor are comparable with previously reported nitrite sensors,^{12,26–29,34–39} as shown in Table ST1.† This result indicates that the fabricated RGO–C60/AuNPs composite modified electrode is very suitable for the determination of nitrite.

Fig. 6 (A) Amperometric i–t response obtained at the RGO–C60/AuNPs modified RDE for the addition of nitrite from 0.05 to 1275.315 μM into a constantly stirred pH 5 solution. Working potential = 0.807 V. Inset shows the amperometric i–t response of RGO–C60/AuNPs modified RDE for the addition of 0.05 μM (a), 0.1 μM (b), 1 μM (c), 5 μM (d), 10 μM (e), 25 μM (f), and 50 μM (g and h) of nitrite into a constantly stirred pH 5 solution. (B) Calibration plot for [nitrite] vs. current response. Error bar is relative to the standard deviation for 3 measurements.

The selectivity of RGO–C60/AuNPs composite modified electrode towards the detection of nitrite in the presence of common metal ions was studied via amperometry. Fig. 7A displays the amperometric i–t response of RGO–C60/AuNPs composite modified RDE for the addition of 1 μM nitrite (a) and 100 μM additions of Ni²⁺ (b), Co²⁺ (c), Mg²⁺ (d), Ca²⁺ (e), Fe²⁺ (f), Cl⁻ (g), SO₄²⁻ (h) and SO₃²⁻ (i) into a constantly stirred pH 5 solution at the working potential of 0.807 V. A noteworthy amperometric response was observed for the addition of 1 μM nitrite. However, the 100 fold addition of common metal ions into the electrolyte solution did not show any apparent amperometric response on the RGO–C60/AuNPs composite modified electrode. The result clearly indicates the high selectivity of RGO–C60/AuNPs composite modified electrode, which could be used for the selective detection of nitrite in real samples.

Fig. 7 (A) Amperometric i–t response obtained at the RGO–C60/AuNPs modified RDE for the addition of 1 μM nitrite (a) and 100 μM addition of Ni²⁺ (b), Co²⁺ (c), Mg²⁺ (d), Ca²⁺ (e), Fe²⁺ (f), Cl⁻ (g), SO₄²⁻ (h) and SO₃²⁻ (i) into a constantly stirred pH 5 solution. Working potential = 0.807 V. (B) Amperometric i–t response of RGO–C60/AuNPs modified RDE for the addition of 300 μM nitrite (a) into a pH 5 solution and its related current response up to 4000 s. The amperometric working conditions are similar to Fig. 6A.

To further verify the practical ability and feasibility of RGO–C60/AuNPs composite modified electrode towards the detection of nitrite in real samples, the RGO–C60/AuNPs modified electrode was used for the determination of nitrite in different nitrite containing water samples via the amperometric method. All experimental working conditions were similar to that of Fig. 6A. Three different water samples were used for real samples analysis, including tap, drinking and river water. The recovery of nitrite in different water samples was calculated using the standard addition method, as reported earlier.^34,35 The obtained recoveries of nitrite in different water samples are summarized in Table ST2.† The average recoveries of nitrite in tap, drinking and river water samples were 97.2%, 96.8% and 101.6%, respectively. The appropriate recovery of nitrite in water samples clearly reveals the potential ability of RGO–C60/AuNPs composite modified electrode towards the detection of nitrite in water samples.

The operational stability of the RGO–C60/AuNPs composite modified electrode was examined using amperometric method and the results are shown in Fig. 7B. It can be observed that the RGO–C60/AuNPs modified electrode retains 90.5% of its initial current response to nitrite after a continuous run of up to 4000 s in 300 μM nitrite at pH 5, which indicates the good operational stability of the RGO–C60/AuNPs modified electrode. The storage stability of the sensor was investigated periodically by CV (not shown) and the experimental conditions are similar to that in Fig. 3A. The RGO–C60/AuNPs modified electrode retains 96.2% of its initial current response towards nitrite after two days storage in 300 μM nitrite containing pH 5 solution. The reproducibility and repeatability of the fabricated RGO–C60/AuNPs modified electrode was evaluated in 300 μM nitrite containing pH 5 solution by CV and the experimental conditions are similar to Fig. 3A. The reproducibility of 5 successive RGO–C60/AuNPs modified electrodes had the RSD of 5.7% and the repeatability for a single RGO–C60/AuNPs electrode for 10 successive measurements was found to have the RSD of 4.1%. This result indicates the appropriate reproducibility and repeatability of the fabricated electrode towards the detection of nitrite.

Conclusions

In conclusion, a novel and sensitive amperometric nitrite sensor has been fabricated using an RGO–C60/AuNPs composite modified electrode for the first time. The average size of the AuNPs was found to be 41 ± 2 nm on RGO–C60/AuNPs composite. The resulting composite modified electrode shows excellent electroanalytical features towards the detection of nitrite such as a low LOD (0.013 ± 0.003 μM), fast response (4 s) and wide linear response range (0.05–1175.32 μM). The analytical performance of RGO–C60/AuNPs modified electrode is superior for the detection of nitrite than the other previously reported modified electrodes. The high selectivity and appropriate practicality of the RGO–C60/AuNPs modified electrode clearly reveals that it could be applied for the detection of nitrite in real samples.

Acknowledgements

This study was supported by the Ministry of Science and Technology (MOST) of Taiwan (Republic of China).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10690k

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