A simple and sensitive graphene oxide/gold nanoparticle surface plasmon resonance Rayleigh scattering-energy transfer analytical platform for detection of iodide and H₂O₂

Abstract

Graphene oxide/gold nanoparticle (GO/GN) composites were prepared by citrate reduction that exhibited a strong resonance RS (RRS) peak at 370 nm. When KIO₃ and H₂O₂ were added respectively, they reacted with KI to form I₃⁻ ions that adsorbed on the GO/GN surfaces and the RRS intensity decreased due to the RRS energy of GO/NG being transferred to the receptor I₃⁻ ions. So, a simple and sensitive GO/GN surface plasmon resonance (SPR) Rayleigh scattering (RS)-energy transfer (SPRRS-ET) analytical platform was fabricated and can be utilized to detect trace KIO₃ and H₂O₂.

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1. Introduction

Energy transfer (ET) was combined with fluorescence, chemical luminescence, electrochemical luminescence and bioluminescence to develop a series of new ET spectral methods.^1–6 GN is an ideal plasmon that has been used to study the cell environment and ET mechanism.^7–9 Based on the plasmon resonance energy transfer (PRET) of GNs, trace Cu(II), cytochrome C, 2,4,6-trinitrotoluene (TNT) and pH nanospectral detection technologies were developed by means of dark-field RS images of nanogold on a colour CCD camera.^10–14 Using a common fluorescence spectrophotometer, a simple and sensitive RRS was developed for protein, nucleic acid and metal ion analysis.^15–20 Recently, a selective SPRRS-ET spectral method was establish for detection of trace boron, based on the RRS-ET between nanogold and the complex of boron–methylene amine–H.²¹ GO is a kind of best water-soluble graphene nanomaterials and has been applied to nanoanalysis.^22–26 The nanocomposite of GO/GN is of GO superiority such as good water-soluble stability and GN superiority such as strong RRS effect, and has been utilized in sensor and analysis.^27–29 However, there are no reports about RRS-ET methods with GO/GN nanocomposite as donor and I₃⁻ as acceptor.

Iodine is one of the essential trace elements and iodine deficiency can lead to some diseases. To prevent and control the iodine deficiency disorders, the simplest way is addition of KIO₃ in salt to increase the body's iodine content.^30–32 In some countries, a certain amount of KIO₃ or KI was added compulsively in salt. Therefore, the detection of iodide is necessary in salt. The iodine analytical methods include spectrophotometry, ion selective electrode (ISE), chromatography and atomic absorption spectrumetry.^33–39 The iodine–starch spectrophotometry is simple and low-cost, but the sensitivity is low.³⁷ Iodine ISE can be used to monitor wide range of iodide concentration, with a detection limit (DL) of 0.63 μmol L⁻¹.³⁸ Ion chromatography-inductively coupled plasma/mass spectrometry is very sensitive foe the determination of iodide, with a DL of 0.8 nmol L⁻¹,³⁹ but the operation is complicated and the cost is high. Here, a new SPRRS-ET analysis platform was developed to detect iodide in salt, with high sensitivity, good selectivity and simple operation.

2. Experimental

A model of F-7000 fluorescence spectrophotometer (Hitachi, Japan), a model of TU-1901 double-beam UV-visible spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.), a model of DXR smart Raman spectrophotometer (Thermo Fisher, USA) with a power of 3.5 mW, a model of nanoparticle and Zeta potential analyzer (Malvern Company, England), a model of JEM-2100F field emission transmission electron microscope (Japanese Electronics), and model of SK8200LH ultrasonic reactor (Shanghai Guide Ultrasonic Instruments Co. Ltd.) were used.

A pH 2.8 Na₂HPO₄–citric acid buffer solution: A 15.8 mL 0.2 mol L⁻¹ Na₂HPO₄ solution and 84 mL 0.1 mol L⁻¹ citric acid solution were mixed and diluted to 100 mL, with a concentration of 31.7 mmol L⁻¹ Na₂HPO₄. A pH 3.4 Na₂HPO₄–citric acid buffer solution: A 28.5 mL 0.2 mol L⁻¹ Na₂HPO₄ solution and 71.5 mL 0.1 mol L⁻¹ citric acid solution were mixed and diluted to 100 mL, with a concentration of 57 mmol L⁻¹ Na₂HPO₄. A 1% HAuCl₄ (National Pharmaceutical Group Chemical Reagents Company, China), 0.02 mol L⁻¹ KI solution, 0.0100 mol L⁻¹ KIO₃ standard solution, 0.400 mol L⁻¹ H₂O₂ standard solution were prepared.

GO: the Hummer procedure was used to prepare GO.²⁹ A 1.0 mg mL⁻¹ (0.01%) GO solution: a 100 mg GO was dissolved in 100 mL water by ultrasonic dispersion.

GN: GN was synthesized through reduction of HAuCl₄ by trisodium citrate. A 50 mL water was added into a flask, heated to boil. Then 0.5 mL 1% HAuCl₄ and 3.5 mL 1% trisodium citrate were added rapidly into the boiling water successively. After boiling for 10 min with stirring, the color became from colorless to wine red. The mixture was continued stirring to room temperature, and then diluted to 50 mL. The GN concentration was 48.7 μg mL⁻¹ Au, with a size of about 10 nm.

GO/GN: a 50 mL water was added into a flask, heated to boil. Then 0.5 mL 1% HAuCl₄, 250 μL 0.1% GO and 3.5 mL 1% trisodium citrate were added rapidly into the boiling water successively. After boiling for 10 min with stirring, the color became from colorless to wine red. The mixture was continued stirring to room temperature, and then diluted to 50 mL. The concentration, counting as GN, was 47.8 μg mL⁻¹ GO/GN. All reagents were of analytical grade and the water was doubly distilled.

Procedure for RRS detection of KIO₃: into a 5 mL marked tube, a 150 μL pH 2.8 Na₂HPO₄–citric acid buffer solution, a 400 μL 0.02 mol L⁻¹ KI, a certain amount of KIO₃ solution, and 200 μL 47.8 μg mL⁻¹ GO/GN were added orderly and diluted to 2 mL with water. The RRS spectrum was recorded, using synchronous scanning technique (λ_ex − λ_em = Δλ = 0), volt of 450 V, both excited slit and emission slit of 5.0 nm, and emission filter of 1% T attenuator. The RRS intensity at 370 nm (I_{370 nm}) and a blank value without KIO₃ (I_{370 nm})_b were measured, and the ΔI_{370 nm} = (I_{370 nm})_b − I_{370 nm} was calculated.

Procedure for RRS detection of H₂O₂: into a 5 mL marked tube, a 100 μL pH 3.4 Na₂HPO₄–citric acid buffer solution, a 300 μL 0.02 mol L⁻¹ KI, a certain amount of H₂O₂ solution, 300 μL 47.8 μg mL⁻¹ GO/GN were added and diluted to 2 mL with water. Then it was heated 15 min in 80 °C water bath and cooled with tap-water. The RRS spectrum was recorded, using synchronous scanning technique (λ_ex − λ_em = Δλ = 0), volt of 450 V, both excited slit and emission slit of 5.0 nm, and emission filter of 1% T attenuator. The RRS intensity at 370 nm (I_{370 nm}) and a blank value without H₂O₂ (I_{370 nm})_b were measured, and the ΔI_{370 nm} = I_{370 nm} − (I_{370 nm})_b was calculated.

Procedure for surface-enhanced Raman scattering (SERS) detection of KIO₃: into a 5 mL marked tube, a 150 μL pH 2.8 Na₂HPO₄–citric acid buffer solution, a 400 μL 0.02 mol L⁻¹ KI, a certain amount of KIO₃ solution, 100 μL 5.0 μmol L⁻¹ VBB, 200 μL 47.8 μg m⁻¹ LGO/GN were added orderly and diluted to 2 mL with water. Taken the mixture into a quartz cell and recorded the SERS spectra. Measured the SERS intensity at 1609 cm⁻¹ (I₁₆₀₉) and the blank without KIO₃ (I₁₆₀₉)₀, the ΔI₁₆₀₉= (I₁₆₀₉)₀ − I₁₆₀₉ was obtained.

3. Results and discussion

GO nanosheets contain epoxide, carboxyl, and hydroxyl groups on their basal planes and edges, the abundance carboxyls coordinate with Au³⁺ to form stable GO–Au_n³⁺ complex. The GO can not be reduced by citrate, and the Au³⁺ reduced by citrate to form stable GNs on the GO surface that GO/GNs are stable in the solution. The IO₃⁻ oxidized to excess I⁻ to generate I₃⁻ in Na₂HPO₄–citric acid buffer solution. The receptor of I₃⁻ ions with strong hydrophobicity closed to the donor of GO/GN with strong SPR RS, the RRS-ET take place, owing to the overlapping of GO/GN RRS and I₃⁻ absorption spectra. When I₃⁻ increased, the RRS intensity decreased due to more I₃⁻ adsorbed on the surface of GO/GN that caused the RRS-ET (Fig. 1). So we can establish a simple and rapid GO/GN-I₃ SPR analytical platform for detection of trace of KIO₃ and H₂O₂.

Fig. 1 The principle of catalytic-SERRS detection of NO₂⁻ using Rh6G as probe.

The 48.7 μg mL⁻¹ GN⁻¹ and the 48.7 μg mL⁻¹ GO/GN solutions are all in red color that exhibited a SPR absorption peak at 520 nm. There are no obvious SPR peaks for the pH 2.8 Na₂HPO₄–citric acid–4 mmol L⁻¹ KI–4.78 μg mL⁻¹ GO/GN because the GN concentration is low. When KIO₃ or H₂O₂ (Fe³⁺) added, it reacted with KI to form anion I₃⁻ that appeared two absorption peaks at 290 nm and 350 nm (Fig. 1S†). The GN and GO systems all exhibited two absorption peaks at 290 nm and 350 nm that both are ascribed to I₃⁻ (Fig. 2S and 3S).

In pH 2.8 Na₂HPO₄–citric acid–KI–GO/GN system, when KIO₃ or H₂O₂ (Fe³⁺) added, it reacted with KI to form anion I₃⁻ receptor that accepted the RRS-ET of GN aggregation donor, led to the RRS signal quenched (Fig. 4S, 5S and 6S†). With increasing the concentration of KIO₃ or H₂O₂, more yellow complex anion I₃⁻ formed, and RRS signal at 370 nm decreased linearly, although their RRS spectra are different that the peak at 290 nm is blue-shifted about 10 nm and the peak at 370 nm is red-shifted weakly about 30 nm due to formation of low concentration of I₃⁻ (Fig. 5S†). For the GN-KI–H₂O₂–Fe³⁺ catalytic system, the peak at 290 nm disappeared and the red-shifted of 370 nm is about 80 nm due to formation of high concentration of I₃⁻ (Fig. 6S†). Thus, the wavelength was selected for detection of trace KIO₃ and H₂O₂. GO nanoparticles have three strong RRS peaks at 290 nm, 340 nm and 370 nm (Fig. 7S†). With the concentration of KIO₃ increasing, the RRS energy transferred to the I₃⁻ that make the RRS signal significantly reduced at 290 nm and 350 nm that are ascribed to the absorption of I₃⁻ receptor. And the peak at 290 nm is blue-shifted about 10 nm and the peak at 370 nm is red-shifted weakly about 35 nm due to formation of low concentration of I₃⁻ with low absorption. Fig. 2 shows that GO/GN nanoparticles have three clear RRS peaks at 295 nm, 370 nm and 520 nm, the latter is ascribed to GNs. With the concentration of KIO₃ increasing, the RRS energy transferred to the I₃⁻ that make the RRS signal significantly reduced at 292 nm, 370 and 520 nm. A peak wavelength of 370 nm was selected to detect KIO₃ because it is most sensitive and best linear relationship, with a DL of 8 nmol L⁻¹ and linear range (LR) of 0.025–7 μmol L⁻¹ as in the inseted Figure.

Fig. 2 RRS spectra of the GO/GN–KI–KIO₃ system. (a) pH 2.8 Na₂HPO₄–citric acid–4 mmol/LKI–4.78 μg mL⁻¹ GO/GN; (b) a + 0.5 μmol L⁻¹ KIO₃; (c) a + 1.0 μmol L⁻¹ KIO₃; (d) a + 2 μmol L⁻¹ KIO₃; (e) a + 4 μmol L⁻¹ KIO₃; (f) a + 8 μmol L⁻¹ KIO₃.

The relationship between the acceptor absorption peak and the donor RS valley was studied. As a receptor of I₃⁻ exhibited two absorption peaks at 290 nm and 350 nm, it can accepted the SPRRS energy from the donor such as GN aggregation, GO/GN and GO. As the best example, GO was selected to discuss the SPRRS-ET. It has strong RS in the range of 250 nm–400 nm that covered the absorption of receptor of I₃⁻. Due to both spectra overlapping, the SPRRS energy strongly transferred to the acceptor from the donor at 290 nm and 350 nm that led to the RRS signal quenched greatly and formed the two valleys (Fig. 3). Thus, with increasing the concentration of KIO₃ or H₂O₂, more yellow complex anion I₃⁻ formed, and RRS valleys at 290 nm and 370 nm enhanced greatly due to the SPRRS-ET enhanced. Thus, the valley wavelength at 370 nm was selected for detection of trace KIO₃ and H₂O₂.

Fig. 3 The relationship between the acceptor absorption peak and the donor RS valley.

In this paper, the molecular probes of rhodamine 6G, malachite green and Victoria blue (VBB) were studied. The result showed that only VBB existed in strong SERS effect in the GN nanosol substrate. There are ten strong SERS peaks at 190, 430, 790, 1160, 1200, 1290, 1360, 1390, 1560 and 1609 cm⁻¹ (Fig. 8SA†). With the concentration of KIO₃ increasing, the strongest SERS peak intensity reduced linearly at 1609 cm⁻¹. A 1609 cm⁻¹ was chosen to detect KIO₃. Using the GO/GN as substrate, KIO₃ can be also detected (Fig. 8SB†) that is inferior to the GN.

To obtain the transmission electron microscope (TEM), a 1.0 mL the nanoparticle solution was taken into a 1.5 mL centrifuged tube, centrifuged for 20 min in 15 000 r/min and discarded the supernatant. Then a 1 mL water was add in the centrifuge tube and dispersed by ultrasonic 30 min, and centrifuged again. The operation was repeated two times, the dispersed sample solution was dropped onto a silicon wafers and dried naturally, the TEM was recorded. Fig. 4a shows that it is spherical GNs in size of 10 nm. Fig. 4b is the TEM of GO/GN particles that there are 10 nm GNs and large aggregations. The laser scattering results (Fig. 9S†) show that the size distribution of GO particles and the average size is 230 nm. When HAuCl₄ solution containing GO reduced to form GO/GNs and little GNs that exhibited two peaks (Fig. 9S-b†), the size of nanoparticle increased greatly, is about 510 nm. In pH 2.8 Na₂HPO₄–citric acid buffer solution containing 4 mmol L⁻¹ KI, the GN/GOs were aggregated weakly that the size is about 600 nm.

Fig. 4 TEM of GN and GO/GN particles a:4.8 μg mL⁻¹ GN⁻¹; b: pH 2.8 Na₂HPO₄–citric acid buffer solution–4.0 mmol L⁻¹ KI–4.8 μg mL⁻¹ GO/GN.

The GN–KI–KIO₃ analytical system was optimized. The influence of pH Na₂HPO₄–citric acid buffer solution and concentration of KI and GN was examined. The result (Fig. 10S–12S†) shown that when the pH of Na₂HPO₄–citric acid buffer solution is 2.8, buffer solution concentration counting Na₂HPO₄ is 2.38 mmol L⁻¹, KI concentration is 4 mmol L⁻¹ and GN concentration is 4.78 μg mL⁻¹, the ΔI is the biggest. The ΔI is big when reaction time is 10 min. Thus, the above conditions were selected to detect KIO₃. In addition, GOs and GO/GNs were instead of GNs respectively, their concentration influence was considered. When 15 μg L⁻¹ GO (Fig. 13S†) and 4.78 μg mL⁻¹ GO/GN were chosen to obtain big ΔI (Fig. 14S†). The GN–KI–H₂O₂ analytical system was also optimized. The influence of the pH Na₂HPO₄–citric acid buffer solution, the concentration of KI and GN, the temperature and the time on the system has been examined. The result (Fig. 15S–19S†) shown that when the pH Na₂HPO₄–citric acid buffer solution was 3.4, KI was 3 mmol L⁻¹, and GN was 4.78 μg mL⁻¹, the ΔI was the biggest. When reaction temperature reached 80 °C, the ΔI reached maximum. The temperature increased further, the ΔI did not changed. When the reaction time was long than 15 min, the ΔI increased slowly. Thus, above conditions were selected to detect H₂O₂.

Under the optimal conditions, the analytical features of the GN–KI system for RRS detection of KIO₃, the GN–KI system for RRS detection of H₂O₂, the GO–KI system for RRS detection of KIO₃, GO/GN–KI system for RRS detection of KIO₃, the GN–KI system for SERS detection of KIO₃, and the GO/GN–KI system for SERS detection of KIO₃ were obtained (Table 1). In the five scattering spectral methods for detection of KIO₃, the working curve slope of the GO/GN–KI system for RRS detection of KIO₃ is the largest, with a LR of 0.025–5 μmol L⁻¹, DL of 0.008 μmol L⁻¹ KIO₃ and a relative standard deviation (RSD) of 5.2% for the blank value (Table 1S†). Comparing with the reported analysis methods for detection of iodine (Table 2S†),^30–39 this RRS-ET method possesses high sensitivity, simplicity and rapidity.

Table 1 Analytical features for the nanoparticle analytical platform

Analyte	System	Method	Regress equation	LR (μmol L^−1)	DL (μmol L^−1)
IO3^−	GN–KI	RRS	ΔIRS = 422.6C + 34.2	0.25–5	0.1
IO3^−	GO-KI	RRS	ΔIRS = 128C + 89	0.5–10	0.2
IO3^−	GO/GN–KI	RRS	ΔIRS = 473C + 94	0.025–7	0.008
IO3^−	GN–KI–VBB	SERS	ΔI1609 = 30.5C + 17	0.2–4	0.1
IO3^−	GO/GN–KI–VBB	SERS	ΔI1609 = 0.48C + 5.3	0.25–5.5	0.1
H2O2	GN–KI	RRS	ΔIRS = 44.0C + 88	0.5–100	0.2
Fe^3+	GN–KI–H2O2	RRS	ΔIRS = 2.4C + 28	2–1000	0.7

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According to the procedure, this paper has been investigated the influence of coexisting substance on the GO/GN–KI system for RRS detection of KIO₃. When the concentration of KIO₃ was 2.5 μmol L⁻¹ and the relative error within ±10%, 200 μmol L⁻¹ K⁻¹⁺, Ca²⁺, Zn²⁺, glucose, Co²⁺, Al³⁺, Ba²⁺, Mg²⁺, Cu²⁺, Ni²⁺, Mn²⁺, BrO₃⁻, MoO₄²⁻, 12.5 μmol L⁻¹ Cr³⁺ and SeO₃²⁻ did not interfered with the detection (Table 3S†). It shows that the method has good selectivity. This paper has been examined the influence of coexisting substance on the GN–KI system for RRS detection of H₂O₂. When the concentration of H₂O₂ is 25 μmol L⁻¹ and the relative error within ±10%, 500 μmol L⁻¹ Ca²⁺, K⁺, Zn²⁺, glucose, Ba²⁺, Mg²⁺, SeO₃²⁻, SO₄²⁻, Cu²⁺, BrO₃⁻, Cr³⁺, 250 μmol L⁻¹ BPO, Al³⁺, 75 μmol L⁻¹ Mn²⁺ did not interfered with the detection (Table 4S†). It shows that the RRS-ET method for H₂O₂ has good selectivity.

In this paper, the analysis platform was applied in detection of different commercial salts that potassium iodate, potassium iodide and organic iodine were added. For the salts added potassium iodate such as sample 1 and sample 2 (Table 2), the procedure of sample solution preparation is simple and as follows, a 1.00 g salt sample was dissolved with water and dilute to 10 mL with water. According to the experimental methods, a 250 μL salt sample solution was used to detect the iodate content. For the other salts including sample 3–6, they were also analyzed by this RRS-ET method before the iodine including iodide and organic iodine as oxidized to iodate by bromine water.⁴⁰ Results (Table 2) showed that the RRS-ET results are agreement with that of the iodine–starch spectrophotometry (ISS),³⁷ with a recovery of 96.1–109% and a standard deviation (SD) of 0.128–0.200 μmol L⁻¹. The H₂O₂ content in waste water samples was also determined by this RRS method, and the results (Table 3) are agreement with that of the fluorescence spectrophotometry (FS),⁴¹ with a recovery of 94.0–105% and a SD of 0.51–1.2 μmol L⁻¹.

Table 2 Analysis results of iodide in salt sample

Salt sample^a	Average (μmol L^−1, n = 5)	SD^b (μmol L^−1)	Added KIO3 (μmol L^−1)	Found (μmol L^−1)	Recovery(%)	I content (mg kg^−1)	ISS results^c (mg kg^−1)
a Sample 1 and 2 were salts added potassium iodate, sample 3 and 4 were salts added potassium iodide, sample 5 and 6 were salts added organic iodine.b SD stands for standard deviation.c ISS stands for iodine–starch spectrohotometry.
1	2.868	0.132	1.5	4.31	96.1	29.14	28.9
2	2.180	0.146	1.5	3.77	106	22.15	22.6
3	2.756	0.200	1.5	4.21	96.9	28.0	27.4
4	2.146	0.165	1.5	3.68	102	21.8	22.5
5	2.411	0.128	1.5	3.88	97.9	24.5	24.9
6	2.352	0.194	1.5	3.98	109	23.9	24.2

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Table 3 Analysis results of H₂O₂ in waste water sample

Waste water sample	Content (μmol L^−1, n = 5)	SD (μmol L^−1)	Added H2O2 (μmol L^−1)	Found (μmol L^−1)	Recovery (%)	FS results^a (μmol L^−1)
a FS stands for fluorescence spectrohotometry.
1	9.54	0.51	10.0	9.40	94.0	9.12
2	14.6	1.2	10.0	10.5	105	15.2
3	16.2	1.1	10.0	9.75	97.5	16.9

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4. Conclusion

In summary, based on the SPRRS-ET analytical principle, the decreased RRS intensity (ΔI_{370 nm}) is linear to KIO₃ concentration in the range of 0.025–5 μmol L⁻¹, with a DL of 8 nmol L⁻¹. And the LR of H₂O₂ is 0.5–100 μmol L⁻¹, with a DL of 0.1 μmol L⁻¹ H₂O₂. The iodide in salt samples was analyzed by this SPRRS-ET analysis platform, with satisfactory results.

Acknowledgements

This work supported by the National Natural Science Foundation of China (no. 21165005, 21267004, 21307017, 21367005, 21467001, 2144706, 21477025, 21465006), the Research Funds of Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, the Natural Science Foundation of Guangxi (no. 2013GXNSFFA019003, 2014GXNSFAA118059, 2014GXNSFAA118050), and the Research Funds of Guangxi Education Department (no. 2013YB234, 2013YB035).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12685h

‡ These authors (XH Zhang and YH Wang) contributed equally to this work.

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