Photo-reduction assisted synthesis of MnO₂/reduced graphene oxide/P25 for electrochemical detection of hydrogen peroxide

Abstract

In this work, we report the synthesis of MnO₂/RGO (reduced graphene oxide)/P25 nanocomposites for a non-enzymatic hydrogen peroxide sensor. MnO₂/RGO/P25 nanocomposites were synthesized with a photo-reduction approach and characterized by field emission scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. A non-enzymatic hydrogen peroxide sensor was fabricated by dropping MnO₂/RGO/P25 nanocomposites on the surface of a glassy carbon electrode. Electrochemical measurements of the MnO₂/RGO/P25 modified electrode were carried out on an electrochemical workstation. The as-prepared sensor exhibited high electrocatalytic activity, selectivity and stability towards the oxidation of H₂O₂. Under optimum conditions, the calibration curve for H₂O₂ determination was linear in the range from 1.0 × 10⁻⁶ to 4.0 × 10⁻³ M (R² = 0.999) with a detection limit of 3 × 10⁻⁷ M (S/N = 3).

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1. Introduction

Hydrogen peroxide is one of the most important and common oxidizing agents widely utilized in clinical areas, biochemistry, the pharmaceutical industry and environmental fields.¹ Therefore, accurate and fast detection of H₂O₂ via highly sensitive and cost-effective sensors is of great practical significance. Thus, many detection methods have been developed, such as titrimetry,² chemiluminescence,^3–4 fluorescence,⁵ spectrophotometry⁶ and the electrochemical method.^7–9 Among them, the electrochemical method is the most attractive because of its high sensitivity, low cost and operational simplicity. Over the past decades, intensive interests have been directed to electrochemical detection of H₂O₂ based on enzymes due to their high sensitivity and selectivity.^10–12 However, the drawbacks of enzymes based sensors such as limited lifetime, instability, poor reproducibility and complicated fabrication procedures have impeded them from being prevailing.¹³ In recent years, much effort has been devoted to non-enzymatic electrochemical detection of H₂O₂ to overcome these disadvantages due to the intrinsic fragility nature of enzymes.¹⁴ Various materials have been successfully applied extensively in fabricating high efficient non-enzymatic electrochemical sensors, such as metals,^15–18 alloys,¹⁹ transition metal oxide nanoparticles²⁰ and graphene.^21–23 In particular, as one outstanding electrocatalytic material towards H₂O₂, manganese dioxide has drawn intensive attentions in virtue of excellent physicochemical properties, low cost, environmental benignity, safe operation voltage and non-toxic.^24–26

Reduced graphene oxide (RGO) is a kind of attractive material in the scientific community owning to its excellent mobility of charge carriers, high thermal conductivity, great chemical stability, high mechanical flexibility and large specific surface area.^27–29 So far, various synthesis methods have been developed to prepare RGO such as chemical reduction, electrochemical reduction and thermal reduction methods.^30–33 In recent years, photo-reduction methods turning graphene oxide into RGO in the presence of TiO₂ and avoiding drawbacks caused by chemical reduction methods and thermal reduction methods have been developed by researchers.³⁴ Compared with the chemical and thermal reduction of GO, photo-reduction is a facile and green reduction method to prepare RGO.

TiO₂ is one of the most promising materials and has been well studied over the years because of its photocatalytic properties, non-toxicity, high chemical stability and huge potential application value.^35–37 When TiO₂ is irradiated with UV lights, the electrons will be excited from the valence bands to the conduction bands, leaving holes in valence bands and generating photo electron–hole pairs in this process.³⁸ Photogenerated electrons and holes migrate on surface of TiO₂, triggering reductive reaction and oxidative reaction, respectively. P25 is a kind of commercial TiO₂ nanoparticle consisting 80% anatase phase and 20% rutile phase. The mixed phases suppress the recombination of the photo-electrons and photo-holes, which promote its photocatalytic efficiency. Based on this, many kinds of TiO₂–RGO nanocomposites have been developed to improve the photocatalysis property of TiO₂.

In this work, we successfully synthesized MnO₂/RGO/P25 nanocomposites using photo-reduction method. MnO₂/RGO/P25 nanocomposites were characterized by field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. Cyclic voltammetry (CV) of MnO₂/RGO/P25 modified sensor were conducted on electrochemical workstation. The as-prepared sensor exhibited high electrocatalytic activity, good selectivity and stability toward the oxidation of H₂O₂. To the best of our knowledge, this is the first report on photo-reduction assisted synthesis of MnO₂/RGO/P25 nanocomposites and their use in electrochemical detection of hydrogen peroxide.

2. Experimental section

2.1 Materials and apparatus

Graphene oxide was purchased from Institute of Coal Chemistry, Chinese Academy of Sciences (Shanxi, China). The commercial TiO₂ (Degussa P25) was obtained from Degussa Co., Ltd. (Germany). KMnO₄, KH₂PO₄, K₂HPO₄·3H₂O, hydrogen peroxide (H₂O₂, 30%), CH₃CH₂OH and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 0.1 M phosphate buffer solutions (PBS, pH 6–10) were prepared from NaH₂PO₄, Na₂HPO₄·3H₂O and NaOH and used as supporting electrolytes. All chemicals were of analytical grade. Ultra-pure water was used throughout the experiment. UV lamp (16 W) was purchased from Hangzhou Yaguang Lamp Co., Ltd. (Hangzhou, China).

The morphologies of GO, RGO/P25 and MnO₂/RGO/P25 were characterized by field emission scanning electron microscopy (Zeiss, Germany). The chemical states of elements in MnO₂/RGO/P25 were analyzed by WSCALAB 250 Xi X-ray photoelectron spectroscopy (England) using a source of Al Kα radiation with energy of 1486.6 eV. Infrared spectra of GO, RGO/P25 and MnO₂/RGO/P25 were carried out on an AVATAR 370 Fourier transform infrared spectrometer (FTIR, America) in the region of 400–4000 cm⁻¹. Raman spectra of GO, RGO/P25 and MnO₂/RGO/P25 were characterized by inVia Reflex (Renishaw, England) with an excitation laser wavelength of 514.5 nm. All electrochemical measurements were performed on a CHI660C Electrochemical Workstation (Shanghai Chenhua, China) with a conventional three-electrode system, in which a saturated calomel electrode (SCE), platinum foil and MnO₂/RGO/P25 modified GCE were served as the as reference, counter and working electrode, respectively.

2.2 Synthesis of MnO₂/RGO/P25 nanocomposites

MnO₂/RGO/P25 nanocomposites were synthesized using photo-assisted reduction method. In brief, 0.2 mg P25 was dispersed in 2 mL GO (1.0 mg mL⁻¹) solution under magnetic stirring at room temperature (25 °C) and the suspension solution was vigorously stirred for 1 h under UV irradiation to obtain RGO/P25 homogenous dispersion. After that, a series of KMnO₄ (0.2, 0.4, 0.6, 0.8, and 1.0 mg) solutions with different amount were dropwise added into the same volume RGO/P25 homogenous dispersion solutions which had been irradiated with UV light to synthesize MnO₂/RGO/P25 nanocomposites. Finally, MnO₂/RGO/P25 nanocomposites were centrifuged and washed with ultrawater for 3 times to further analysis.

2.3 Preparation of MnO₂/RGO/P25 modified electrodes

A glassy carbon electrode (GCE, 3 mm in diameter) was polished with 0.05 μm alumina slurries using Buehler polishing kit, followed by washing with deionized water, ethanol and finally deionized water under ultrasound conditions (each for 3 min), and drying at room temperature to remove any adsorbed impurities on the electrode surface. 5 μL of as-prepared MnO₂/RGO/P25 nanocomposites was dropped onto the surface of GCE and dried in air under the infrared lamp for 15 min. For comparison, 5 μL of GO and RGO/P25 were dropped onto GCE and dried at the same conditions as MnO₂/RGO/P25–GCE to obtain GO–GCE and RGO/P25–GCE.

3. Results and discussion

3.1 Photo-reduction synthesis of MnO₂/RGO/P25 nanocomposites

As shown in Scheme 1, when P25 was first irradiated with UV lights, the electrons would be excited from the valence bands to the conduction bands, leaving holes in valence bands and generating photo electron–hole pairs in this process (eqn (1)). Then, the photo holes reacted with H₂O to generate oxygen and GO was photo-reduced to RGO with photo electrons (eqn (2) and (3)). Lastly, KMnO₄ solution was drop-wise added into RGO/P25 homogenous dispersion solution irradiated with UV light to synthesize MnO₂/RGO/P25 nanocomposites (eqn (4)). The photo-assisted reduction mechanism might be proposed as follows:^34,39–43

P25 + hν → P25 (3h⁺) + P25 (3e⁻) (1)

P25 (3h⁺) + [/]H₂O → P25 + ¾O₂ + 3H⁺ (2)

P25 (3e⁻) + GO → RGO/P25 (3)

RGO (3e⁻)/P25 + MnO₄⁻ + 4H⁺ → MnO₂/RGO/P25 + 2H₂O (4)

Scheme 1 Photo-reduction synthesis of MnO₂/RGO/P25 nanocomposites.

3.2 Field emission scanning electron microscopy (FESEM)

Morphologies of GO, RGO/P25 and MnO₂/RGO/P25 were characterized by FESEM. As shown in Fig. 1A, GO displays a tentative structure with affluent wrinkles. From the image of RGO/P25 nanocomposites (Fig. 1B), P25 nanoparticles with diameter in the range of 20–30 nm were wrapped by RGO and attached on the surface of photo-reduced graphene oxide sheets. Fig. 1C shows MnO₂/RGO/P25 nanocomposites were formed after MnO₄⁻ ions were photo-reduced and anchored on the surface of RGO/P25. RGO/P25 is likely to provide the nucleation sites for growing preferably to form MnO₂ nanoplatelets. Most of MnO₂ nanoparticles have a size of ∼100 nm and disperse on the surface of RGO/P25 nanocomposites. Fig. 1D–F correspond to real samples of GO, RGO/P25 and MnO₂/RGO/P25 composites, respectively. Obviously, the colors of GO, RGO/P25 and MnO₂/RGO/P25 visibly change from brownish to black after irradiation.

Fig. 1 FESEM images of GO (A), RGO/P25 (B) and MnO₂/RGO/P25 (C); photos of GO (D), RGO/P25 (E) and MnO₂/RGO/P25 (F).

3.3 X-ray photoelectron spectroscopy

The surface chemical states of GO, RGO/GO, MnO₂/RGO/P25 were examined by XPS. Fig. 2A shows the XPS of MnO₂/RGO/P25. Different C1s binding energies imply the existence of different chemical environments of carbon in GO (Fig. 2B), RGO/GO (Fig. 2C), MnO₂/RGO/P25 (Fig. 2D). As shown in spectra, four peaks with binding energies of 284.8, 286.7, 287.4 and 288.9 eV, can be assigned to the contributions of the C–C (sp²) bonds, C–O bonds, C=O bonds and O=C–O bonds,⁴⁴ respectively. Compared to GO, peak intensity at 284.8 eV in RGO/GO and MnO₂/RGO/P25 increased after illumination, indicating GO were reduced into RGO. Fig. 2E shows the Mn 2p XPS of MnO₂/RGO/P25. The peaks observed at 641.9 and 653.7 eV with a BE difference of 11.8 eV were ascribed to Mn 2p^3/2 and Mn 2p^1/2, respectively.⁴⁵ These results agree well with Mn(IV) characters in phase of MnO₂.

Fig. 2 XPS of MnO₂/RGO/P25 nanocomposites (A); C1s XPS spectra of GO (B), RGO/P25 (C) and MnO₂/RGO/P25 nanocomposites (D); Mn 2p XPS spectra of MnO₂/RGO/P25 nanocomposites (E).

3.4 Raman and FTIR spectroscopy

Samples of GO, RGO/P25 and MnO₂/RGO/P25 were also characterized by Raman spectroscopy recorded between 100 and 2000 cm⁻¹ at the same laser power and acquisition time. As depicted in Fig. 3A, GO shows two broadened characteristic frequencies at 1344 and 1597 cm⁻¹ corresponding to the D and G bands, respectively.^46–47 Higher intensity ratio means higher defects in GO comprising less sp² hybridization. The intensity ratio of D and G bands (I_D/I_G) of GO is 1.06. After photo-reduction reaction process, this value decreases to 1.03 in RGO/P25 and 1.02 in MnO₂/RGO/P25 indicating more graphitization of GO. Compared MnO₂/RGO/P25 with RGO/P25, there is no significant change of the I_D/I_G value after further UV-light irradiation, which indicates no obvious sp² hybridization after further photo-reduction reaction.⁴⁸ Raman results are also in agreement with the X-ray photoelectron spectra in Fig. 2. Raman spectra of RGO/P25 and MnO₂/RGO/P25 further show frequencies at 398 and 507 cm⁻¹ ascribing to the B_1g and A_1g vibrational modes of TiO₂ anatase phase, respectively. The presence of rutile phase is characterized by peaks at 153 and 630 cm⁻¹ which correspond to the modes B_1g and A_1g. The band at 680 cm⁻¹ of MnO₂/RGO/P25 is always seen in MnO₂ materials and corresponds to the symmetric stretching vibration (Mn–O) of the octahedral [MnO₆] groups⁴⁹ indicating that MnO₂ were introduced. The red shift in the G band from 1597 to 1591 cm⁻¹ is possibly due to increased numbers of isolated carbon double bonds in disordered GO after photo-reduction reaction.

Fig. 3 Raman spectra of GO, RGO/P25 and MnO₂/RGO/P25 (A); FTIR spectra of GO, RGO/P25 and MnO₂/RGO/P25 (B).

FTIR spectra of GO, RGO/P25 and MnO₂/RGO/P25 nanocomposites were recorded in 400–4000 cm⁻¹. As shown in Fig. 3B, all samples present a broad absorption peaks around 3300 cm⁻¹, which are attributed to the surface absorbed water.⁵⁰ In the FTIR spectrum of GO, the absorption peaks at 1732 and 1622 cm⁻¹ correspond to the C=O stretching and C=C stretching vibrations, respectively (curve a). The band at 1223 cm⁻¹ can be ascribed to the epoxy groups vibrations of GO. Compared to GO, the peaks at 1732 and 1223 cm⁻¹ of RGO/P25 and MnO₂/RGO/P25 nanocomposites disappeared, which indicate reduction of GO and its transformation into RGO (curve b and c). The peak appearing at 1053, 1070 and 1093 cm⁻¹ are assigned to the vibrations of C–O in alkoxy of GO (curve a), RGO/P25 (curve b) and MnO₂/RGO/P25 (curve c), respectively. For RGO/P25 nanocomposites, the broad absorption at low frequency of 663 cm⁻¹ and 519 cm⁻¹ (below 1000 cm⁻¹) are attributed to the vibration of Ti–O–Ti bonds in P25. FTIR spectrums of MnO₂/RGO/P25 nanocomposites also show two peaks at 617 and 509 cm⁻¹ corresponding to the stretching vibrations of Mn–O–Mn and Mn–O bonds, respectively.

3.5 Electrochemical behavior of MnO₂/RGO/P25 modified glassy carbon electrode

In order to evaluate the electrocatalytic properties of MnO₂/RGO/P25 to oxidation of H₂O₂, the performance of bare GCE, GO–GCE, RGO/P25–GCE and MnO₂/RGO/P25–GCE was evaluated by cyclic voltammetry in 0.1 M PBS (pH 8) at the scan rate of 100 mV s⁻¹. Fig. 4 shows CVs of bare GCE (A), GO–GCE (B), RGO/P25–GCE (C) and MnO₂/RGO/P25–GCE (D) in the absence (curve a, b, c and d) and presence (curve a′, b′, c′ and d′) of 1 mM H₂O₂ in 0.1 M PBS (pH 8) at scan rate of 100 mV s⁻¹. In the absence of H₂O₂, no oxidation peaks were observed at bare GCE (curve a) and GO–GCE (curve b). A very weak oxidation peak around +0.61 V in the absence of 1 mM H₂O₂ in 0.1 M PBS (pH 8) was observed at RGO/P25–GCE (Fig. 4C, insert) which might be ascribed to oxidation of RGO.⁵¹ In contrast MnO₂/RGO/P25–GCE (curve d) displayed a pair of broad but weak peaks in the potential range of 0–1.0 V in the absence of H₂O₂, which might be attributed to the reduction of MnO₂ to Mn(II or III) and the reoxidation of Mn(II or III) back to MnO₂.^52–53 When 1 mM H₂O₂ was added into 0.1 M PBS (pH 8), no obvious electrochemical oxidation peaks were observed at bare GCE (curve a′) and GO–GCE (curve b′). A weak electrochemical oxidation peak was observed at RGO/P25–GCE (curve c′). Compared with bare GCE (curve a′), GO–GCE (curve b′) and RGO/P25–GCE (curve c′), a broad and strong oxidation peak was observed on MnO₂/RGO/P25–GCE (curve d′) at around +0.7 V in 0.1 mM H₂O₂ + 0.1 M PBS (pH 8). Considering the standard potential values of Mn species, we can make a conclusion that reductive peaks and oxidative peaks are assigned to the reduction of MnO₂ to Mn(II, III) (eqn (5) and (6)) and the oxidation of Mn(II, III) to Mn(IV) (eqn (7) and (8)),^52–53 respectively. The electrochemical reaction mechanism is proposed as following equations:

2MnO₂/RGO/P25 (s) + H₂O₂ (aq) → Mn₂O₃/RGO/P25 (s) + O₂ (g) + H₂O (5)

MnO₂/RGO/P25 (s) + H₂O₂ (aq) → Mn(OH)₂/RGO/P25 (s) + O₂ (g) (6)

Mn₂O₃/RGO/P25 (s) + 2OH⁻ (aq) → 2MnO₂/RGO/P25 (s) + H₂O (l) + 2e⁻ (7)

Mn(OH)₂/RGO/P25 (s) + 2OH⁻ (aq) → MnO₂/RGO/P25 (s) + 2H₂O (l) + 2e⁻ (8)

Fig. 4 Cyclic voltammograms of bare GCE (A), GO–GCE (B), RGO/P25–GCE (C) and MnO₂/RGO/P25–GCE (D) in the absence (curve a, b, c and d) and presence (curve a′, b′, c′ and d′) of 1 mM H₂O₂ in 0.1 M PBS (pH 8) at scan rate of 100 mV s⁻¹; amperometric response of bare GCE, GO–GCE, RGO/P25–GCE and MnO₂/RGO/P25–GCE on a successive of 0.5 mM H₂O₂ added into 0.1 M PBS (pH 8) with applied potential of +0.6 V (E). Insets: the amplified cyclic voltammogram of GCE (a), GO–GCE (b), RGO/P25–GCE (c) and MnO₂/RGO/P25–GCE (d) in the absence (curve c) of 1 mM H₂O₂ in 0.1 M PBS (pH 8).

The amperometric response of bare GCE, GO–GCE, RGO/P25–GCE and MnO₂/RGO/P25–GCE toward a successive of 0.5 mM H₂O₂ were also investigated in 0.1 M PBS (pH 8) with applied potential of +0.6 V, as shown in Fig. 4E. MnO₂/RGO/P25 nanocomposites modified GCE exhibited highest sensitivity.

3.6 Optimization of experimental parameters

The content of MnO₂ is a key factor to influence the response current. The influence of KMnO₄ was investigated using amperometric response to 0.5 mM H₂O₂ in 0.1 M PBS (pH 8) at the scan rate of 100 mV s⁻¹. As shown in Fig. 5A, the response current of H₂O₂ increased with increase of KMnO₄ from 0 to 0.6 mg. However, the response current decreased with further increase of KMnO₄. At first, a large number of MnO₂ can be easily anchored on the surfaces of RGO, which provide more electrocatalytic sites and significantly increase the response current towards H₂O₂. However, further increases of KMnO₄ result in the agglomeration of MnO₂, which decrease conductivity of MnO₂/RGO/P25 and slows mass transport through the electrode surface.⁵⁴

Fig. 5 Effects of KMnO₄ (A) and applied potential (C) on the amperometric response of 0.5 mM H₂O₂ in 0.1 M PBS (pH 8) at scan rate of 100 mV s⁻¹ with applied potential of +0.6 V; effects of different pH value on the amperometric response of 0.5 mM H₂O₂ in 0.1 M PBS (pH 8) at scan rate of 100 mV s⁻¹ (B). Error bars represent the standard deviation for three independent measurements.

The pH value of the electrolyte is also a crucial parameter affecting the stability and activity of the modified electrode. In order to obtain optimal amperometric response to H₂O₂, different pH values on the amperometric response of 0.5 mM H₂O₂ in 0.1 M PBS were also tested with the applied potential of +0.6 V at the scan rate of 100 mV s⁻¹ Fig. 5(B). The response current of H₂O₂ rose strikingly upon the increase of pH 6–8 and reached the maximum for pH 8, and then sharply decreased. Thus, pH 8 was chosen as the optimal pH for amperometric H₂O₂ sensing.

It is well known that the applied potential strongly affects the amperometric responses. The influence of the applied potential of the electrocatalytic activity of MnO₂/RGO/P25–GCE was investigated using amperometric response to 0.5 mM H₂O₂ in 0.1 M PBS (pH 8) at the scan rate of 100 mV s⁻¹. The response current increased sharply with the increase of applied potential until the potential reached +0.6 V. The response current drastically decreased when the applied potential exceeded +0.6 V. Therefore, +0.6 V was selected for the amperometric detection of H₂O₂.

3.7 Linearity, selectivity and stability

Under the optimum conditions, MnO₂/RGO/P25–GCE exhibited high rapid and sensitive response for H₂O₂ as displayed in Fig. 6A. The current response is linear with H₂O₂ concentration in the range of 1–4000 μM with sensitivity of 297.2 μA mM⁻¹ cm⁻², as shown in Fig. 6B. The corresponding regression equation is I (μA) = 0.754 + 0.021C (μM) (R² = 0.999). The detection limit is estimated to be 0.3 μM (S/N = 3). Selectivity is also an important aspect for sensors. The interferences of various common interfering substances including 100-fold concentration inorganic species such as CO₃²⁻, NO₃⁻, CH₃COO⁻, Cl⁻, K⁺ and Na⁺ and organic species ascorbic acid (AA, 1-fold), uric acid (UA, 1-fold) and glucose (10-fold) were evaluated towards H₂O₂ analysis. The interferences with the amperometric determination of 0.25 mM H₂O₂ was implemented under optimal conditions in 0.1 M PBS (pH 8), as shown in Fig. 6C. The results show that all interfering agents had response currents are less than 4% of that produced by H₂O₂, indicating high selectivity of MnO₂/RGO/P25–GCE for detecting H₂O₂. We also used the amperometric response method to evaluate the stability of MnO₂/RGO/P25–GCE at +0.6 V in 0.1 M PBS (pH 8). As shown in Fig. 6D, the respond current change a little during 1000 s test period indicating that MnO₂/RGO/P25–GCE shows high stability (Table 1).

Fig. 6 Amperometric response of MnO₂/RGO/P25–GCE of different concentrations of H₂O₂ in 0.1 M PBS (pH 8) at applied potential of +0.6 V (A); corresponding calibration curve of MnO₂/RGO/P25–GCE (B); amperometric response of MnO₂/RGO/P25–GCE to 0.25 mM H₂O₂ and different interferences in stirring 0.1 M PBS (pH 8) with applied potential at +0.6 V (C); the current change of MnO₂/RGO/P25–GCE to 0.25 mM H₂O₂ during 1000 s test period in 0.1 M PBS (pH 8) with applied potential at +0.6 V (D).

Table 1 Performance comparison of the proposed MnO₂/RGO/P25–GCE with other glucose sensors

Modified electrode	Linear range (μM)	Detection limit (μM)	Sensitivity (μA mM^−1 cm^−2)	Reference
a NTA stands for nitrilotriacetic acid.b VACNTs stands for vertically aligned multiwalled carbon nanotubes.c CF stands for carbon foam.d CNT stands for graphene/carbon nanotubes.
Mn-NTA^a/Nafion	5–2500	0.2	78.9	9
MnO2 nanorods	—	5 ± 2.5	—	53
GO/MnO2	5–600	0.8	38.2	55
MnO2/VACNTs^b	1.2–1800	0.8	1080	56
MnO2/CF^c	2.5–2055	0.12	54	57
MnO2/graphene/CNT^d	1–1030	0.1	539.4	58
MnO2/RGO/P25	1–4000	0.3	297.2	This work

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3.8 Real sample analysis

The application for real sample analysis of MnO₂/RGO/P25–GCE was evaluated by the determination of H₂O₂ in toothpaste. Briefly, 1.0 g toothpaste mixed with 10 mL PBS (0.1 M pH 8) was centrifuged before test. H₂O₂ has been estimated directly by amperometric method under optimal conditions with standard addition method. The detection result is shown in Table 2. The results have demonstrated that MnO₂/RGO/P25 has potential applications in the determination of H₂O₂ in real samples.

Table 2 Determination of H₂O₂ in toothpaste samples (n = 3)

Toothpaste samples	Detected (μM)	Added (μM)	Founded (μM)	Recovery (%)	RSD (%)
1	18.24	20.00	38.38	100.3	2.9
2	20.05	20.00	38.81	96.7	4.7
3	17.95	20.00	37.10	97.8	3.4

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4. Conclusion

In this work, a non-enzymatic H₂O₂ sensor based on MnO₂/RGO/P25 nanocomposites has been successfully prepared using a novel photo-reduction assisted synthesis method. The as-prepared sensor exhibited highly electrocatalytic activity, selectivity and stability towards the oxidation of H₂O₂. Under optimum conditions, the calibration curve for H₂O₂ determination was linear in the range from 1.0 × 10⁻⁶ to 4.0 × 10⁻³ M (R² = 0.999) with a detection limit of 3 × 10⁻⁷ M (S/N = 3). The proposed synthesis method is mild and simple, and the synthesized MnO₂/RGO/P25 nanocomposites possess low toxicity. The present study provides a novel approach for the construction of non-enzyme sensors.

Acknowledgements

This research is supported by the National Natural Science Foundation of China (No. 61171033, 61571278, 61571280).

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