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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Facile and Green Approach To Prepare Nanostructured Au@MnO2 and Its Applications for Catalysis and Fluorescence Sensing of Glutathione in Human Blood Xu Zhou,†,‡ Genfu Zhao,† Muhan Chen,†,‡ Wei Gao,† Xiaojian Zhou,† Xiaoguang Xie,*,‡ Long Yang,*,† and Guanben Du*,† †

Key Lab for Forest Resources Conservation and Utilization in the Southwest Mountains, Ministry of Education, Yunnan Province Key Lab of Wood Adhesives and Glued Products, School of Materials Science and Engineering, Southwest Forestry University, Kunming 650224, China ‡ School of Chemical Science and Technology, Yunnan University, Kunming 650091, China S Supporting Information *

ABSTRACT: The increasingly serious environmental problems make it urgent to develop a new type of sustainable green material which can degrade pollutants and monitor human health. However, the traditional preparation methods are frequently limited by tedious operations, high-energy consumption, and massive pollution. Herein, we present a facile and green method for preparation of MnO2 nanoflakes mediated by macrocyclic molecule calix[8]arene. The MnO2 nanoflakes in situ grew on the preformed gold nanoparticles, forming an impressive core−shell Au@MnO2 flake-like nanocomposite. The catalytic properties of Au@MnO2 composite for reduction of 4-NP and degradation of MB were 2.4 and 187 times better than commercial Pd/C, respectively. Meanwhile, the assynthesized Au@MnO2 nanocomposite exhibited specially excellent sensitivity and selectivity for detection of GSH with a limit of detection (LOD) of 0.11 μM. The core−shell nanostructured Au@MnO2 shows great potential value for the sustainable development of the environment and human health. KEYWORDS: Nanostructured Au@MnO2, 4-NP reduction, MB degradation, Fluorescence sensing of GSH

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and bioimaging.9 Inspired by these fascinating applications, large quantities of efforts have been made to synthesize various 2D layered nanostructured MnO2 materials. Traditionally, plenty of bottom-up methods for synthesis of MnO2 often result in low dispersibility and stability in water, comparatively high cost-demanding, and time-consuming tedious synthetic procedures, including additional ion exchange and mechanical exfoliation.10,11 In recent years, the bottom-up wet chemistry synthesis approaches, such as hydrothermal, coprecipitation, electrodeposition, sol−gel, and microwave synthesis etc., have gotten further and wider attentions as a result of that nanoscale MnO2 in the geometric shapes of tubes, wires, plates, rods, belts, needles, flowers, spheres, and a variety of other morphologies can be obtained by these approaches.8 Multistep and sophisticated operations are needed frequently, although these methods provide accurate control of the morphological characteristics of MnO2 nanomaterials.

INTRODUCTION With the progressive deterioration of our natural environment, the amounts and species of various pollutants and diseases are also increasing.1 It is particularly crucial to develop a green and sustainable material that can degrade pollutants in the process of production and life and detect key biological components in the human bodys to maintain the health of the life systems.2−4 In varieties of materials, two-dimensional (2D) layered transition metal oxide and sulfide nanomaterials present a various kinds of predominant performances,5 owing to their large specific surface areas, magical light absorption, excellent electron transfer capacity, and captivating thickness at the atomic level, which have rendered them promising candidates for widespread applications in obtaining highly efficient catalysts, fluorescence sensor, high-energy supercapacitor, photocatalyst, drug delivery, new electronic materials, and electrochemical performances.6,7 As a typical example, manganese dioxides (MnO2) with 2D layered nanostructure have received much attention on account of popular prices, relative high catalytic activities,8 and splendid fluorescence quenching ability toward luminescent nanomaterials, which is significant potential for fluorescence sensing © XXXX American Chemical Society

Received: November 18, 2017 Revised: January 16, 2018

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DOI: 10.1021/acssuschemeng.7b04313 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

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The para-sulfonated calixarene, one of the functionalized calixarenes modified by sulfonate group, the third generation of macrocyclic supramolecules after crown ethers and cyclodextrins, has appropriate reducibility and nonsubstitutable water-solubility.12,13 Liu et al. reported a bottom-up method that sodium dodecyl sulfate (SDS) was used as reductant and regulating agent to prepare single-layer MnO2 nanostructure applied to high-performance pseudocapacitors in 2015.14 Glutathione (GSH), as the largest amount of sulfhydryl reagent in a living being system, plays pivotal roles in plenty of vital biological functions, such as elimination of internal toxins and free radicals and maintenance of reversible redox reaction.15 Upon its excellent reducibility, GSH can be briskly converted to glutathione disulfide (GSSG), the dipolymer of GSH, in response to oxidative forces within the human body.16 It has been certified that the content of GSH (or GSH/GSSG ratio) in human blood suggests the health level in some degree, which is closely associated with several diseases, including aging, Alzheimer’s disease, and cancers.17,18 It has been reported that organic probes exhibited relatively good performances for detection of GSH.19,20 However, most synthetic methods of the organic probes often require specific temperatures, organic solvents, and tedious synthetic steps, which will lead to energy consumption, environmental pollution, and lower efficiency.21 Therefore, simpler, faster, more effective, and selective approaches for detection of GSH in human whole blood are urgently needed to better analyze its physiological and medical functions.22 Herein, inspired by these studies, we report a green approach (Scheme 1) with the preponderances of simplicity of operation

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EXPERIMENTAL SECTION

Chemicals and Materials. Sodium citrate (>98%), citric acid (CA, >99%), gold chloride hydrate (HAuCl4, 99.999%), SCX8 (>98%), potassium permanganate (KMnO4, >99%), L-glutamine (>99%), L-cysteine (Cys, 99%), D,L-homocysteine (Hcys, >95%), Lglutathione (GSH, 99%), 4-nitrophenol (4-NP, 99%), methylene blue (MB, >98%), and Pd/C (10%) were obtained from Adamas-beta Reagent Co., Ltd. (Shanghai, China). All chemicals were analytical grade and were used as received without further purification. Human whole blood samples were permitted and collected from healthy volunteers in the hospital of Southwest Forestry University. The electrical resistance of ultrapure water prepared by a Millipore Milli-Q water purification system was 18.25 MΩ in all experiments. Synthesis of Au@MnO2 Composite. The AuNPs were prepared by a modification of the classical reported approach.23 In this method, 39 mL of ultrapure water was stirred in the bottom of a 100 mL round bottomed flask and 3 mL of 10 mM HAuCl4 solution was added into the container when the ultrapure water was heated up to boiling point. After that, 3 mL of 10 mg mL−1 sodium citrate solution was added and the solution was refluxed for about 30 min. AuNPs solution was received after the resultant solution cooled down to room temperature and stored in 4 °C. Next, 6 mL of the AuNPs solution was added into a 50 mL beaker. With sufficient stirring, 6 mL of 1.0 mM SCX8 and 4 mL of 2.0 mg mL−1 KMnO4 were added into the resultant AuNPs solution, respectively. The mixture needed to react for several minutes until the color of the mixture changed from amaranth to dark brown. Then, the Au@MnO2 composite were obtained after being centrifuged with 18000 rpm for 30 min and washed for 3 times. The Au@MnO2 powder was obtained by freeze-drying. Apparatus and Characterization. The prepared Au@MnO2 samples were centrifuged by a Hitachi Himac CR21G high speed refrigerated centrifuge (Hitachi, Japan) with 18000 rpm for 30 min and washed three times to obtain the precipitates. Ultraviolet−visible light (UV−vis) absorption spectra of 4-NP and MB were measured using a Hitachi U-2900 UV−vis spectrometer (Hitachi, Japan) in the range of 200−600 nm and 400−800, respectively. Fluorescence spectra were gained on a Hitachi F-4600 fluorescence spectrophotometer (Hitachi, Japan). The transmission electron microscopy (TEM) images of the Au@MnO2 composite were characterized by a JEM 2100 TEM instrument (JEOL, Japan) with an accelerating voltage of 200 kV. The X-ray diffractometry (XRD) spectrum with a 2θ angle range from 10° to 80° was obtained from a Rigaku TTR III X-ray diffractometer (Rigaku, Japan). X-ray photoelectron spectroscopy (XPS) was performed on Thermo fisher Scientific K-Alpha+ with multiprobes using Mono Al Kα as the radiation source where the wavelength λ is 1486.6 eV. The values of ζ-potential and size obtained by dynamic light scattering (DLS) were collected at room temperature using a model Zetasizer Nano ZS instrument (Malvern, UK), performed with a He−Ne laser (633 nm). UV−vis Spectroscopic Measurements. To study the catalytic activity of the prepared Au@MnO2 composite, UV−vis spectroscopic measurements for reduction of 4-NP and degradation of MB were operated. In the measurement of reduction of 4-NP, with the adding of 1.5 mL of 0.2 mM 4-NP and 1.5 mL of 20 mg mL−1 NaBH4 aqueous solution into quartz cuvette having 1 cm path length, 30 μL of 320 μg mL−1 Au@MnO2 composite were mixed up with the above solution and time-dependent UV−vis absorption spectra in the range of 200− 600 nm were recorded to monitor the change in the reaction mixture at a regular time interval of 1 min. In order to reveal the catalytic performance of the prepared Au@MnO2 composite, 30 μL of 320 μg mL−1 commercial Pd/C was also used as catalyst to reduce 4-NP for comparison. Similarly, in order to degradation of MB, 10 μL of 320 μg mL−1 Au@MnO2 composite or Pd/C solution was put into the mixture solution after 2 mL of 50 μM MB and 0.5 mL of 0.1 M NaBH4 aqueous solution being added into the same quartz cuvette. In the same way, the degradation progress of MB can be monitored by timedependent UV−vis absorption spectra in the range of 400−800 nm. Preparation of the GQDs−Au@MnO2 Conjugation for Fluorescent Sensing Platform. GQDs were prepared by directly

Scheme 1. Illustration for Synthesis of the Nanostructured Au@MnO2 and Its Applications for 4-NP Reduction, MB Degradation, and GSH Fluorescence Sensing

and high efficiency of synthesis to prepare MnO2 nanoflakes grown on gold nanoparticles (AuNPs) without any organic solvent or high temperature, recorded as Au@MnO2, where manganese source is KMnO4 and para-sulfonated calix[8]arene (SCX8) is both as a reductant and a controlling agent. To the best of our knowledge, this is the first time to report that the macrocyclic supramolecule is used to mediate and control the synthesis of nanostructured MnO2 materials. The Au@MnO2 produced in this work exhibits extremely appealing applications to catalytic reduction of 4-nitrophenol (4-NP), degradation of methylene blue (MB), and fluorescence sensing of GSH in the sustainable development of the environment and human health. B


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Figure 1. XRD pattern of Au@MnO2 composite (A); ζ-potentials of AuNPs (B) and Au@MnO2 composite (C); TEM image (D) and HRTEM images (E and F) of Au@MnO2. in the literature.25,26 For the sake of inhibiting γ-glutamyltranspeptidase (GGT) activity, 20 μL of 100 mM serine−borate complex was needed to put into 1 mL of human whole blood. Hemocytes of the sample were removed by centrifugation at 12000 rpm for 15 min at room temperature. The supernatant liquid was diluted 500-fold and added into the GQDs−Au@MnO2 sensing system to measure their fluorescence spectra as mentioned.

making CA being pyrolyzed just reported.24 Briefly, 2 g of CA was added into a 10 mL beaker at 200 °C with the help of an oven for about 10 min. With the ongoing liquidation of CA, GQDs were obtained when the color of the liquid turned achromatous into orange. The GQDs solution was obtained after 110 mL of 10 mg mL−1 NaOH solution was added drop by drop into the GQDs, under adequate agitation, to transfer the pH of the liquid mixture to neutral. Then, various concentrations of Au@MnO2 (0 to 39 μg mL−1) were added to the cuvette with 1 cm path length containing 2 mL of GQDs solution to obtain a GQDs−Au@MnO2 conjugation for constructing the fluorescence sensing platform. Detection of GSH in Buffer Solution and Human Whole Blood Samples. In the fluorescent sensing of GSH, various concentrations of GSH (0−56 μM, which were diluted with 0.1 M pH 7.0 PBS) was added into 2 mL of GQDs solution containing 78 μg Au@MnO2, then the mixture was incubated at room temperature for 5 min. After that, the fluorescence spectra of the mixture solutions above were measured. In order to analyze selectivity of the sensing platform toward GSH, a series of common components of metal ions (NaCl, KCl, Na2SO4, MgSO4, CaCl2, MnCl2), amino acids (glutamine, cysteine, and homocysteine), and other interferences (PBS, glucose, sucrose, BSA) were widely investigated. The human whole blood samples were kept in a heparin anticoagulated tube and treated according to the procedure reported

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RESULTS AND DISCUSSION Characterization of the Au@MnO2 Composite. The XRD was employed to investigate the crystal structure Au@ MnO2 composite. The XRD pattern of the Au@MnO2 composite is provided in Figure 1A. The characteristic diffraction peaks of Au(111), Au(200), Au(220), Au(311), and Au(222) crystal faces (JCPDS No. 04-0784) were observed obviously;27 nevertheless, the MnO2 nanoflakes gave relatively weak peaks assigned to (220) and (002) reflections (JCPDS No. 44-0141), indicating the amorphous nature of the prepared MnO2.28,29 To illustrate the stability and dispersity of the Au@MnO2 composite, the ζ-potential measurement was operated. As shown in Figure 1B,C, the ζ-potential of AuNPs and Au@ C


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Figure 2. XPS survey spectrum of Au@MnO2 (A); high-resolution XPS spectra of C 1s (B), O 1s (C), Mn 2p (D), and Au 4f (E).

MnO2 were −40.7 and −50.8 mV, respectively, demonstrating that both of them possessing excellent stability and dispersity. Compared with AuNPs, Au@MnO2 had greater superiority owing to it lower value of ζ-potential, which could be attributed to the negative charge of the sulfonate group (−SO3−) of SCX8. Thus, the as-prepared Au@MnO2 showed exceptional water-solubility and colloidal stability in aqueous solutions.30,31 The size of the Au@MnO2 composite was also determinated by DLS. From the size distribution histogram of Figure S1, an average size of 57 nm for Au@MnO2 was obtained. The morphological image of the Au@MnO2 composite was recorded by TEM as given in Figure 1D. The TEM image exhibited that a shell of amorphous MnO2 nanoflakes grew on the surface of monodispersed AuNPs to form a core−shell nanostructure. Furthermore, all the AuNPs were embedded in the MnO2 nanoflakes and almost no free AuNPs were observed. The generated MnO2 was regulated and controlled to the form of nanoflakes via noncovalent interactions between the generated MnO2 and SCX8. In the present work, the key to growing MnO2 nanoflakes on the AuNPs was the utilization of citrates-capped AuNPs as a nucleation center. In this case, hydrogen bonding interactions could be formed between the citrates molecules on the surface of AuNPs and the hydroxyl groups of SCX8 on the surface of MnO2 nanoflakes. Moreover, TEM demonstrated that amorphous MnO2 nanoflakes were covered on the surface of AuNPs, which further confirmed the feeble crystallinity of MnO2 nanoflakes. It was this kind of amorphous structure that made it more competitive for improvement of the catalytic performance as a powerful catalyst of reduction of 4-NP and degradation of MB. According to the results of high resolution TEM (HRTEM, Figure 1E), the average diameter of the AuNPs was approximately 10 nm coverd by MnO2 nanoflakes and the

crystal lattice spacing of AuNPs was 0.235 nm (Figure 1F), which was corresponding to the (111) faces of AuNPs.32−34 The average diameter of the Au@MnO2 nanocomposite was approximately 50 nm, which was in agreement with the result of DLS. In order to analyze the chemical constitution and functional group of the as-prepared Au@MnO2 composite, XPS was conducted. The XPS pattern of Au@MnO2 composite distinctly showed Mn 2p peaks, O 1s peaks, C 1s peaks, and Au 4f peaks (Figure 2). In the survey pattern (Figure 2A), the intensity of Au 4f peaks was much feebler than that of others, illustrating that AuNPs might be embedded in MnO 2 nanoflakes to compose a core−shell nanostructure.7 The high-resolution XPS spectra of Au 4f, Mn 2p, and C 1s of Au@MnO2 composite are given in Figure 2B−E. The Au 4f7/2 peak (84.38 eV) and Au 4f5/2 peak (88.07 eV) of Au@MnO2 composite illustrated a metallic Au(0) state. It was the same consequence as reported before,35,36 Mn 2p3/2 peak with a 642.40 eV of binding energy and Mn 2p1/2 peak with a 654.10 eV of binding energy were unfolded by the high-resolution Mn 2p spectrum. The high-resolution XPS spectrum of C 1s revealed that 3 types of carbon bonds, including CC (284.75 eV), CO (286.39 eV) and OCO (288.50 eV), were contained in the resulting composite.37−40 The presence of OCO (288.50 eV) could be ascribed to the citrates capped on the surface of AuNPs. All the above characterizations fully certified the success in fabrication of Au@MnO 2 composite. Catalytic Reduction of 4-NP and Degradation of MB. The catalytic activities of the Au@MnO2 composite were analyzed by the reduction of 4-NP to 4-AP and degradation of MB in the presence of NaBH4. In the UV−vis spectroscopic measurement for reduction of 4-NP, the spectra of the bright D


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Figure 3. Time-dependent UV−vis absorption spectra for catalytic reduction of 4-NP in the presence of Au@MnO2 (A) and Pd/C (B) catalysts; plots of ln[Ct/C0] as function of the reaction time for reduction of 4-NP (C); time-dependent UV−vis absorption spectra for catalytic degradation of MB in the presence of Au@MnO2 (D) and Pd/C (E) catalysts; plots of ln[Ct/C0] as function of the reaction time for degradation of MB (F).

cavity. And electrostatic effects played an important role in molecular recognition. The electrostatic potential of the nitro group at the paraposition was significantly negative because the nitro moiety was an electron-withdrawing group. The electrostatic potential of the amino group at the paraposition was significantly positive because the amino moiety was an electrondonating group. Thus, the 4-AP could be easily included into the cavity of SCX8 due to the electrostatic attraction between the sulfonate group (−SO3−) of SCX8 and the amino group (−NH2) of 4-AP. And the 4-NP could not be included into the cavity of SCX8 due to the electrostatic repulsion between the sulfonate group (−SO3−) of SCX8 and the nitro group (−NO2) of 4-NP. The reaction process of catalytic reduction of 4-NP to 4-AP could be recorded accurately by the timedependent absorption spectra at 1 min intervals regularly. About 10 min later, as the peak of nitro compounds came into tending to be smooth, the color of mixed solution of 4-NP and NaBH4 change from bright yellow to colorless because of the termination of catalytic reduction of 4-NP. Based on a pseudofirst-order reaction kinetics, − ln[Ct/C0] = kt (eq 1),42 where Ct and C0 are the concentration of 4-NP at time t and 0,

yellow mixed solution of 4-NP and NaBH4 had a strong absorption peak at 400 nm, which was in agreement with the previous report.41 The intensity of the adsorption peak at 400 nm would not change in the absence of catalyst. After the addition of Au@MnO2 nanocomposite, the peak of 4-NP at 400 nm decreased sharply, and a new absorption peak ascribed to 4-AP at about 300 nm appeared (Figure 3A). However, the time-dependent variance for the two peaks was in disagreement. Additional experiments were performed in order to investigate this issue. A series of concentrations of SCX8 were added into 4-NP (0.1 mM) and 4-AP (0.1 mM) solution, respectively, and the related UV−vis spectral data were obtained. Figure S2 illustrates that the intensity of the absorbance peak of 4-NP at 400 nm did not change with the increase concentrations of SCX8. Figure S3 reveals that the intensity of the absorbance peak of 4-AP at approximately 300 nm decreased with the increase of SCX8. From the UV−vis spectral data, we could draw a conclusion that the reason of the disagreement for the two peaks was that the generated 4-AP was included into the cavity of SCX8. Besides, the SCX8 has an excellent ability of molecular recognition owing to its specific E


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Figure 4. Fluorescence spectra of the GQDs upon the addition of different concentrations of Au@MnO2 (A, 0−39 μg mL−1), insets show the photos of GQDs before and after addition of 39 μg mL−1 Au@MnO2; the quenching efficiency (%) vs Au@MnO2 concentrations (B); the fluorescence spectra of the GQDs−Au@MnO2 conjugation upon the addition of different concentrations of GSH (panel C is 0−56 μM, panel D is 0−2 μM), insets in panel C show the photos of GQDs before and after addition of 56 μM GSH; plot of the relative fluorescence intensity [(F − F0)/ F0] vs GSH concentrations (from 0.33 to 56 μM, inset is from 0.33 to 2.0 μM) (E); the selectivity represented by the relative fluorescence intensity [(F − F0)/F0] in the presence of different analytes, 0.25 mM for NaCl, KCl, Na2SO4, MgSO4, CaCl2, MnCl2, PBS, glucose, sucrose, and glutamine, 0.25 mg mL−1 for BSA, and 0.05 mM for Cys, HCys, and GSH (F).

conversion after five cycles. In addition, the turn over efficiencies (TOEs) were also calculated and the values of Au@MnO2, MnO2, and Pd/C catalysts for reduction of 4-NP are 9.06 × 10−3, 4.68 × 10−3, and 3.75 × 10−3 mol g−1 min−1, respectively, indicating the higher catalytic activity of Au@ MnO2 than those of MnO2 and Pd/C. Besides, in the process of degradation of MB, the mixture solution showed no visible color change as NaBH4 aqueous solution was added to MB. However, after the addition of Au@ MnO2, the mixed solution in cuvette began to fade quickly. The reaction process of catalytic degradation of MB could be monitored accurately by the time-dependent absorption spectra at 30 s intervals regularly. The UV−vis spectra (Figure 3D) showed that the intensity of characteristic peaks at 664 nm (the monomer of MB) and 613 nm (the dimer of MB) of MB decreased gradually until completely disappeared within 150 s

respectively, the reaction rate constant k could be gotten by fitting the data from the rate equation in Figure 3C. For comparison, MnO2 and commercial Pd/C with the same mass concentration were also used to catalyze the reduction of 4-NP (Figure S4 and Figure 3B). The rate constants k for the reduction of 4-NP catalyzed by the Au@MnO2, MnO2, and commercial Pd/C were 0.29, 0.15, and 0.12 min−1 (Figure 3C), respectively. The excellent catalytic performance of a minuscule amount of Au@MnO2 arised from the glorious synergistic effect (high dispersion among AuNPs and MnO2 nanoflakes and amorphous structural defects of MnO2 nanoflakes), which were beneficial to adsorption of BH4− and 4-NP on the Au@ MnO2 composite surface and electron transfer from BH4− to 4NP via a surface-hydrogen species.43,44 The reusability of the Au@MnO2 catalyst was also studied (Figure S5) showing similar catalytic performance without obvious reduction in F


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of the prepared nanocomposite for quantitative analysis was also investigated. The linear relationship in Figure 4E between the concentrations of GSH ranging from 0.33 to 16 μM and the values of fluorescence recovery (F − F0)/F0 provided a strong evidence for quantitative detection of GSH. Correlation coefficient (R2) and linear equation are 0.993 and (F − F0)/ F0 = 0.14C + 0.0092 (eq 3), respectively. The detection limit of 0.11 μM for GSH according to the 3σ rule was obtained by the linear relationship, which was very competitive compared with the reported methods.50−53 Selectivity of the fluorescent sensing system for detection of GSH in buffer solution was evaluated. A large quantity of interferents were widely investigated, including a series of common components of metal ions (NaCl, KCl, Na2SO4, MgSO4, CaCl2, MnCl2), amino acids (glutamine, Cys, and HCys) and other interferences (PBS, glucose, sucrose, BSA). As shown in Figure 4F, GSH (50 μM) presented a pretty obvious increase of fluorescence intensity ratio (F − F0)/F0, whereas other interferents exhibited no or only slight fluorescence intensity ratios. It should be noted that high concentrations of Cys and Hcys could also contribute to fluorescence recovery. But, the as-prepared GQDs−Au@MnO2 fluorescent sensing platform still showed a pretty satisfying result toward GSH in human whole blood due to the very low concentrations of these interferents in human whole blood samples.25,26,54 Practical application in complex biological media of human whole blood still needed to be investigated although the prepared sensing platform had high performance for detection of GSH. In the GSH detection of human whole blood, 500-fold diluted human whole blood and GQDs−Au@MnO2 were added into the cuvette to measure. Similar with that in buffer solution, fluorescence recoveries were observed. According to the working curve and dilution ratio, we could get that the concentrations of GSH in three human whole blood samples are 920, 880, and 960 μM, respectively, which was in agreement with the reported result of 900 ± 140 μM.53,55 Furthermore, in standard addition experiments added with different concentrations of GSH in the human whole blood sample, recoveries ranging from 95.55 to 102.23% and the relative standard deviations (RSD) ranging from 2.76 to 4.02% were obtained (Table 1). All these results implyed that the GQDs−Au@ MnO2 fluorescent sensing system could be successfully applied to detection of GSH in human whole blood samples.

with the process of the reaction, indicating the ultrahigh catalytic ability of the Au@MnO2 nanocomposite toward the degradation of MB. Just like the reduction of 4-NP, for comparison, the catalytic performance of MnO 2 and commercial Pd/C with the same mass concentration were also investigated toward degradation of MB (Figure S6 and Figure 3E). The rate constants k were collected to be 1.59, 0.28, and 0.0085 min−1 for the reactions catalyzed by Au@MnO2 nanocomposite, MnO2, and commercial Pd/C (Figure 3F), respectively. The rate constant k for degradation of MB catalyzed by Au@MnO2 was 187 higher than that of commercial Pd/C. The superior catalytic activity for degradation of MB catalyzed by Au@MnO2 nanocomposite could be attributed to the following reasons: (I) benzene rings of SCX8 functionalized on Au@MnO2 composite, benefited from the green synthetic method and the unique structure, had an intense adsorption effect toward MB by reason of the strong π−π stacking interactions;45 (II) the local electron concentration was enhanced by electron transfer from the BH4− (on the surface of MnO2 nanoflakes) to the AuNPs and then the electrons were accepted by nitro groups. And electron transfer promoted MB molecules to absorb the electrons of BH4−;46,47 (III) exceptional water-solubility and colloidal stability increased the contact area and interaction between Au@ MnO2 composite and MB; (IV) just like 4-NP, amorphous structural defects of Au@MnO2 composite also played a vital role in accelerating adsorption of BH4− and MB and electron transfer from BH4− to MB.47 And all of the reasons above led to Au@MnO2 composite extremely outstanding catalytic performance. The reusability of the Au@MnO2 catalyst for degradation of MB was also studied (Figure S5) and similar catalytic performance without obvious decrease in conversion after five cycles. Besides, the TOEs were also calculated and the values of Au@MnO2, MnO2, and Pd/C catalysts for degradation of MB are 49.7 × 10−3, 8.75 × 10−3, and 0.265 × 10−3 mol g−1 min−1, respectively, indicating the higher catalytic activity of Au@ MnO2 than those of MnO2 and Pd/C. Detection of GSH in Buffer Solution and Human Whole Blood Samples. To further explore the superior performance of Au@MnO2 composite, we put forward the proposal of building a fluorescent sensing platform for GSH detection owing to the splendid reducing capacity of GSH toward MnO2 nanoflakes. In the process, GQDs were selected as fluorescence probes and the fluorescence of GQDs was quenched by Au@MnO2 due to the fact that both AuNPs and MnO2 were outstanding fluorescence quenchers. Then, the MnO2 flakes were reduced to Mn2+ when GSH was added into the GQDs−Au@MnO2 conjugation and GSH itself was oxidized into GSSG. The following is the chemical equation of the reaction mechanism: 2GSH + MnO2 + 2H+ → GSSG + Mn2+ + 2H2O (eq 2).48,49 Thus, the concentration of GSH could be detected facilely according to the changes in the intensity of the fluorescence signal of GQDs. The experiment results of fluorescent measurement showed that GQDs had a strong fluorescence peak at 475 nm. With the addition of Au@MnO2 composite, the fluorescence intensity declined substantially as the fine quenching ability of Au@ MnO2 until the concentration of Au@MnO2 reached to 39 μg mL−1 (Figure 4A). The quenching efficiency curve of Au@ MnO2 was illustrated in Figure 4B. With the continuously addition of GSH in the cuvette, the fluorescence intensity of the GQDs was found to recover continuously (Figure 4C,D), indicating that MnO2 was reduced to Mn2+ by GSH. The ability

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CONCLUSIONS In summary, we have synthesized a core−shell environmentfriendly Au@MnO2 nanocomposite, which displayed extremely appealing performances for catalytic reduction of 4-NP, Table 1. Detection of GSH in Three Human Whole Blood Samples by the GQDs−Au@MnO2 Fluorescence Sensing Platform

G

Samples

Measured (μM)

RSD (%)

Added (μM)

Found (μM)

RSD (%)

Recovery (%)

1

1.84

2.35

2

1.76

3.83

3

1.92

2.69

2.00 4.00 5.00 7.00 10.00 12.00

3.76 5.97 6.83 8.55 11.39 13.67

3.42 2.98 2.76 4.02 3.54 3.87

97.92 102.23 101.04 97.60 95.55 98.20


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ACS Sustainable Chemistry & Engineering degradation of MB, and fluorescence sensing of GSH in human whole blood. To the best of our knowledge, this is the first time to report that the macrocyclic supramolecule SCX8 is used to prepare Au@MnO2 nanomaterials. The catalytic capacity of Au@MnO2 composite toward 4-NP relied on the adsorption of BH4− and 4-NP and the increasing of electron transfer from BH4− to 4-NP via a surface-hydrogen species. The catalytic effect of Au@MnO2 composite to MB mainly depended on the case that the benzene rings of SCX8 functionalized on MnO2 nanoflakes had strong π−π stacking interactions toward MB. Moreover, GSH sensing in human whole blood relied on the fact that GSH has the ability to reduce MnO2 into Mn2+. The as-prepared Au@MnO2 composite showed specially excellent performance for detection of GSH in human whole blood with high sensitivity and amazing catalytic performances for reduction of 4-NP (2.4 times higher than commercial Pd/C) and degradation of MB (187 times higher than Pd/C), which demonstrated the vastly potential applications to biological detection and degradation of pollutants in the sustainable development of the environment and human health.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04313. Size distribution histogram of Au@MnO2 detected by DLS, UV−vis absorbance spectra of 0.1 mM 4-NP upon successive addition of SCX8, time-dependent UV−vis absorption spectra for catalytic reduction of 4-NP in the presence of MnO2 catalyst, reusabilities of the Au@ MnO2 catalyst for the reduction of 4-NP and degradation of MB in the presence of NaBH4, and time-dependent UV−vis absorption spectra for catalytic degradation of MB in the presence of MnO2 catalyst (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X. Xie). *E-mail: [email protected] (L. Yang). *E-mail: [email protected] (G. Du). ORCID

Xu Zhou: 0000-0002-6037-9281 Long Yang: 0000-0002-0286-1803 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Program for Leading Talents, Department of Science and Technology of Yunnan Province (grant no. 2017HA013).

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