Supercritical Fluid-Assisted Fabrication of Manganese (III) Oxide

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Supercritical Fluid-Assisted Fabrication of Manganese (III) Oxide Hollow Nanozymes Mediated by Polymer Nanoreactors for Efficient Glucose Sensing Characteristics Ze-Wen Kang,† Ranjith Kumar Kankala,†,‡ Biao-Qi Chen,† Chao-Ping Fu,†,‡ Shi-Bin Wang,†,‡ and Ai-Zheng Chen*,†,‡ Institute of Biomaterials and Tissue Engineering and ‡Fujian Provincial Key Laboratory of Biochemical Technology, Huaqiao University, Xiamen 361021, P. R. China

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S Supporting Information *

ABSTRACT: Despite their inherent efficacy in significantly accelerating the rate of chemical reactions in biological processes, the applicability of natural enzymes is often hindered because of their intrinsic limitations such as high sensitivity, poor operational stability, and relatively high cost for purification as well as preparation. Thus, the fabrication of catalytically active nanomaterials as artificial enzymes (nanozymes) has become a newly burgeoning area of research in bionic chemistry, aiming in designing functional nanomaterials that mimic various inherent properties of natural enzymes. To address these issues, we present the supercritical fluid (SCF)-assisted fabrication of discrete, monodisperse, and uniform-sized manganese (III) oxide (Mn2O3)-based hollow containers as high-efficiency nanozymes for glucose sensing characteristics. Initially, the core−shell nanoreactors based on polyvinylpyrrolidone (PVP)-encapsulated manganese (III) acetylacetonate (Mn(acac)3) as precursors are fabricated using the SCF technology and subsequent calcination resulted in the Mn2O3 hollow nanoparticles (MHNs). This eco-friendly approach has resulted in the PVP-coated Mn(acac)3 nanoreactors with an average diameter of 220 nm and subsequent calcined hollow products are about one-fifth to that of the precursor. Such MHNs conveniently catalyzed 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2) as a prominent peroxidase mimic, resulting in the oxidation products (TMB*+) at a specific absorption (UV−vis) maxima of 652 nm. Following typical Michaelis−Menten theory, this approach is further utilized to develop visual nonenzymatic sensing of glucose in a linear range of 0.1−1 mM at a detection limit of 2.31 μM. Collectively, this reliable as well as a cost-effective system with high precision potentially allows rapid detection of analytes, providing a convenient way for its utilization in diverse fields. KEYWORDS: supercritical fluid, polyvinylpyrrolidone, manganese (III) acetylacetonate, catalysis, nanozymes

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INTRODUCTION Metal oxide-based nanoarchitectures have garnered enormous interest from researchers because of their specific advantageous properties and tunable morphological attributes, which are of particular interest in diverse applications such as analysis and detection,1 biomedical applications,2−4 fluorescence sensors,5 catalysis,6−8 and information storage.9 Manganese is one such element from the first-row transition metals family with numerous oxide forms (MnOx). Hollow metal oxide nanomaterials have gained specific interest in adsorption,10 catalytic reaction,11,12 energy storage,13,14 and biomedical materials15 because of their advantages such as cavity-like inner space, low density, high specific surface area, and low thermal expansion coefficient.16 Owing to their substantial inner specific surface area, robustness, and excellent stability, the hollow architectures are highly beneficial for the catalysis application toward biomedical applications and clinical diagnosis.17 Several preparation methods have been used for fabricating such © XXXX American Chemical Society

nanostructured metal oxides, including polymer microsphere template method, colloidal SiO2 microsphere template method, mesoporous C sphere template method, micelle template method, emulsion method, and Ostwald curing effect, among others.18−20 Despite the success in fabricating nanostructured metal oxides with diverse morphologies, the end products from these approaches suffer from several shortcomings such as poor dispersion, as well as highly challenging in material separation and purification.21 Therefore, the convenient fabrication of hollow manganese oxide nanocontainers with a controllable morphology using a fast and green approach yet remained as a challenge. To this end, the supercritical fluid (SCF) technology has garnered significant interest from researchers because of its Received: April 1, 2019 Accepted: June 5, 2019 Published: June 5, 2019 A

DOI: 10.1021/acsami.9b05688 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematically illustrating the eco-friendly, SCF-assisted fabrication of MHNs mediated by polymeric nanoreactors and the nonenzymatic glucose detection.

Figure 2. Schematically illustrating the instrument setup of the SEDS technology and relevant mechanism of PVP-encapsulated Mn(acac)3 nanoreactor formation.

economically promising character, environmentally benign nature, and mild operation conditions, that is, the temperature of 31.26 °C and pressure of 7.38 MPa.22,23 This eco-friendly technology offers enormous advantages, such as highly compatible in selecting wide range of excipients, precise control over the particle morphology, and acceptable limits of organic solvent residues in the end products. Different variants have been developed to fabricate the particulate systems for diverse applications.24 Considering the eventual sizes of the precursor nanoparticles, the solution-enhanced dispersion by SCF (SEDS) approach is the most convenient technique among various currently available SCF processes because of the high mass transfer rate that required for rapid nucleation, resulting in the discrete nanoarchitectures.25 Briefly, supercritical carbon dioxide (SC-CO2) and the solution containing dispersed components (polymer, therapeutic molecules, and other excipients) are simultaneously sprayed into the liquid chamber through a co-axial nozzle. This procedure often facilitates better control over the resultant particle morphology and stability of the end product.26,27

In recent times, nanomaterial-based artificial enzymes, often referred to nanozymes, have garnered enormous interest from researchers because of their tunable catalytic activity, excellent stability, and economically promising nature compared to natural enzymes.28,29 In this framework, the transition-metal oxide nanomaterials are one of such important classes of artificial enzymes, which are of particular interest in enzymemimicking catalytic activities.30 In this regard, tremendous progress has been evidenced by the advancements in utilizing these inorganic nanostructured metal oxides for catalysis. Yan and colleagues developed an immunoassay using the iron oxide nanoparticle-based peroxidase activity, in which the magnetite nanoparticles modified with an antibody offered three key efficient features of capture, separation, and detection.31 Qu and co-workers demonstrated the intrinsic peroxidase activity of carboxyl-modified graphene oxide to establish a colorimetric approach-based detection of glucose and hydrogen peroxide (H2O2). Moreover, the explicit confirmations based on kinetic analyses elucidated that the catalytic process followed a pingpong mechanism, in which the graphene oxide construct B


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Figure 3. Characterization showing the morphological attributes of fabricated PVP-coated Mn(acac)3 nanoreactors as well as their subsequent MHNs. (A) FE-SEM image of as-prepared PVP-coated Mn(acac)3 nanoparticles; (B) statistical analysis of PSD of nanoreactors. (C,D) TEM images of MHNs at different magnifications; (E) high-resolution TEM, and the inset showing the selected-area electron diffraction (SAED), and (F) TEM-based elemental mapping of MHNs; (G) N2 adsorption−desorption isotherms of PVP-coated Mn(acac)3 nanoparticles and the MHNs; and their (H) corresponding pore size distribution curves obtained by the BJH method.

was explored in the presence of H2O2. Meanwhile, the sensitive detection of glucose and H2O2 molecules was investigated using these hollow nanocontainers during the process of validating intrinsic peroxidase activity.

exhibited even higher efficacy toward 3,3′,5,5′-tetramethylbenzidine (TMB), compared to the natural enzyme horseradish peroxidase (HRP).32 In another approach, Lv and colleagues established the bovine serum albumin (BSA)-directed fabrication of biocompatible MnO2 nanoparticles with excellent dispersion, for exploring their catalase-, oxidase-, and peroxidase-like activities. Among the various catalytic efficacies tested, the oxidase-like activity of BSA−MnO2 nanoparticles toward TMB oxidation originated from their intrinsic feature of reducing the dissolved oxygen.33 Motivated by these considerations, we present the convenient fabrication of manganese (III) oxide (Mn2O3) hollow nanozymes mediated by the SCF-assisted polymeric nanoreactors for efficacious glucose sensing characteristics (Figure 1). Initially, the polyvinylpyrrolidone (PVP)-encapsulated manganese (III) acetylacetonate (Mn(acac)3) precursors as nanoreactors were fabricated using the SCF technology, and they were then calcined to generate the Mn2O3 hollow nanocontainers. It should be noted that the organic solvent residues can be rapidly extracted by SC-CO2, resulting in the precipitation of the polymer instantaneously over the metal nanoparticles. Further, the intrinsic peroxidase efficacy of TMB

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EXPERIMENTAL SECTION

Materials. 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), Mn(acac)3, methanol, o-phenylenediamine (OPD), and TMB were purchased from Sigma Co. Ltd (St. Louis, USA). Acetic acid (HAc), PVP-K30 (Mw = 40 000), sodium acetate (NaAc), H2O2, glucose (C6H12O6), and ethanol (C2H5OH) were purchased from Sinopharm Co. Ltd (Shanghai, China). CO2 of 99.9% purity was obtained from the Rihong Air Products Co. Ltd. (Xiamen, China). All other chemicals were utilized as received without further treatment unless otherwise stated. Preparation of MHNs. Initially, the typical SEDS (experimental apparatus from Waters, S.N. 3937782, Milford, USA) approach was used to synthesize the precursor, PVP-coated Mn(acac)3 constructs as nanoreactors (Figure 2). Herein, these nanoreactors were synthesized using SC-CO2 as an antisolvent, following the reported procedure.34 Initially, the CO2 gas was cooled to around 3 °C using the incessant circulation system to avoid the cavitation problems and warrant the liquefying of CO2 gas before entering the pump. Further, the liquefied CO2 was heated to the required working temperature of 45 °C. The C


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ACS Applied Materials & Interfaces CO2 gas was then sprayed along with the solution containing the dispersed components into the high-pressure vessel at the gradually maintained optimum supercritical conditions by adjusting the positioning of the pressure and temperature valves. In addition, the required temperature in the high-pressure vessel was regulated by externally covered with a gas bath. Further, the specific conditions were maintained for fabricating the polymeric nanoreactors. Initially, the total concentration of the Mn(acac)3 precursor (0.25 wt %) was fixed at a mass ratio of 5:1 with PVP. The high-pressure vessel was maintained at constant pressure and temperature of 150 bar and 45 °C, respectively. Further, the flow rates of SC-CO2 and the precursors in the absolute ethanol solution were maintained at 45 g min−1 and 1 mL min−1, respectively. During SEDS processing, the ethanol solution dispersed with the precursor tended to be broken by rapid SC-CO2 flow and subsequently atomized into small-sized particles at high pressure.35 Accordingly, the expansion of the contact area between the solution and SC-CO2 could result in several consequences: (i) enhancing the mass transfer between the gas and liquid; (ii) improving the contact efficiency between the solution and gas; (iii) accelerating the evaporation speed of the organic solvent; and (iv) enabling the solution to reach the state of rapid supersaturation and precipitate the polymer over the nanoparticles.36 Furthermore, the fabricated PVP-coated Mn(acac)3 nanoreactors were subjected to calcination at 600 °C (heating rate of 2 °C min−1) for 2 h to sinter the deposited PVP completely. Finally, the PVP-encapsulated Mn(acac)3 nanoparticles and the resultant powdered Mn2O3 hollow nanoparticles (MHNs) were collected and subsequently characterized using various techniques. Characterization. To observe the surface morphology, energydispersive X-ray spectroscopy (EDS, OXFORD 51-XMX, Oxford, UK)-assisted field-emission scanning electron microscopy (FE-SEM, HITACHI S-4800, Tokyo, Japan) and transmission electron microscopy (TEM, FEI Tecnai G2 F30, Hillsborough, USA) were used. The samples for FE-SEM observations were prepared by dispersing them onto the conductive adhesive and spur-coated using an ultrathin layer of gold. The samples for TEM were prepared by dispersing the aqueous solution of the sample onto the carbon film, and the images were captured. Further, the crystalline patterns of the material were observed using the powder X-ray diffraction (PXRD, D8 Advance, Bruker AXS, Karlsruhe, Germany) analysis by scanning the powdered sample in the 2θ range of 15−75° at a 1°/min scanning speed. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, New York, USA) was used to measure the energy of photoelectrons and analyze the chemical valence of the elements. The chemical functionalities were explored by Fourier-transform infrared spectroscopy (FT-IR, Thermo a Nicolet AVATAR 360, New York, USA) using the KBr pellet method. A laser confocal Raman spectrometer (Renishaw, inVia, London, UK) was also used to demonstrate the molecular structures of the designed polymeric nanoreactors. Nitrogen adsorption−desorption isotherms were recorded using Micrometric (ASAP 2020 apparatus, Georgia, USA). Further, the Barrett−Joyner−Halenda (BJH) method was used to estimate the pore size distributions of the designed samples. The absorbance changes were recorded on a UV−vis spectrophotometry (UV-2600, Shimadzu, Tokyo, Japan). Intrinsic Peroxidase-like Activity In Vitro. The intrinsic peroxidase-like activity of MHNs was performed by mixing 10 μL of nanoparticle solution (0.5 mg mL−1) into the reaction system containing H2O2 (100 μL, 10 M), TMB (500 μL, 1.2 mM), and HAc−NaAc buffer (0.2 M, pH 3.5). The time-dependent UV−vis spectra (λmax of 652 nm) of this reaction system were then recorded automatically to explore the reaction process for every 2 min at room temperature. Moreover, the catalytic mechanism was observed based on the typical Michaelis−Menten theory with respect to the concentrations of TMB or H2O2. Subsequently, the effects of pH (3.0−5.5) and temperature (25−50 °C) of the system were also investigated. The colorimetric reaction-based experiment was performed independently in triplicate. Determination of GOx-like Activity In Vitro. In addition, the designed MHNs were also subjected to investigate the GOx-like

activity using TMB and glucose as substrates, where glucose could be oxidized to gluconic acid in the absence of glucose oxidase, and then the generated H2O2 could further oxidize the substrate TMB to its corresponding tinted end product (TMB*+). Briefly, the reaction solution in the HAc−NaAc composite buffer (0.2 M, pH 3.5) containing MHN sample (0.5 mg mL−1) and glucose (1 mM) was incubated at room temperature. Then, the solution was added into the reaction system containing TMB (1.2 mM), and the changes in the absorbance values of TMB*+ at 652 nm were recorded.

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RESULTS AND DISCUSSION Characterization. From the FE-SEM observations (Figure 3A), it is evident that the fabricated polymer-coated nanoreactors by the SEDS approach were discrete globular architectures with uniform distribution (250 nm) and smooth surface. Further, the particle size distribution (PSD) of the nanoreactors was calculated by statistically analyzing 200 nanoparticles from the scanning electron microscopy observation (Figure 3B), which has shown an average size of 224.2 ± 28.4 nm. Moreover, the uniform dispersion of particles could be attributed to the formation of liquid bridges by SC-CO2 residues. Owing to the interactions between the polar pyrrolidone functional groups of PVP and hydroxyl groups of methanol, the dissolution process of CO2 might reduce the interfacial tension between the hydrophilic group of PVP and the formed droplet at the tip of the co-axial nozzle, resulting in reduced agglomeration and a good dispersion of particles.37 These experimental results validated that the prepared SCCO2-assisted PVP-coated Mn(acac)3 precursors were uniform and discrete, demonstrating that the SEDS method is a convenient and stable process. Moreover, the elemental distribution from the EDS-based mapping images (Figure S1) showed that the PVP-coated Mn(acac)3 nanoreactors possessed the highest content of C element over O and Mn species (Table S1). Further, to verify the eco-friendly nature of the SEDS method, we detected the amounts of residual methanol in precursor PVP-coated Mn(acac)3 nanoparticles using the headspace gas chromatography. As shown in Table S2, the average residual amount of methanol was around 121.73 ppm, which was extremely low and acceptable for biomedical applications, indicating the environmental-friendly nature of the SCF technology. These anticipated consequences were due to the appropriate selection of the solvent, herein methanol possessed excellent miscibility with SC-CO2, enabled its diffusion into the precursor and subsequent extraction during the fabrication.38 Owing to their advantageous physicochemical properties, the nanostructured metal oxides exhibit exceptional optical as well as electrical properties, which are of particular interest in diverse applications. To elucidate these facts, we further explored the structural attributes of the MHNs, the calcination product of PVP-coated Mn(acac)3 nanoreactors. Surprisingly, these polymer-coated metal nanoreactors after subjecting to calcination at 600 °C for 2 h have resulted in the formation of hollow metal oxide nanoparticles (Figure 3C,D). Notably, it was evident from the TEM observations that the calcination resulted in the significant reduction of the final particle size of these hollow architectures, which was around one-fifth to that of the polymer-coated precursors. Moreover, the hollow nanoparticles possessed an outer shell of about 10−15 nm thickness. The HR-TEM image (Figure 3E) shows the lattice stripe-like structures of the hollow nanoparticles, which were uniform with the spacings of 0.27, 0.38, and 0.13 nm, corresponding to (222), (211), and (444) crystal surfaces of D


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Figure 4. Characterization of PVP-coated Mn(acac)3 precursors and subsequent MHNs. (A) PXRD analysis; (B) FT-IR spectra; (C) Raman spectra of MHNs; and (D−H) XPS analysis of MHNs (D) full spectra; (E) C 1s; (F) O 1s; (G) Mn 2p; and (H) Mn 3s.

mechanism behind the formation of hollow architectures from polymer-coated metal nanoreactors under high-temperature treatment-assisted calcination might be due to the combustion of organic matter (PVP). Thus, the metal salts in the nanoreactors were successfully transformed into their corresponding oxide forms. As a result, a large amount of combustion gas might be generated and filled in the interior of the nanoparticles, which subsequently resulted in the formation of hollow structures with reduced irreversible particle agglomeration.34 Furthermore, the existing phase and purity of the fabricated precursor PVP-coated Mn(acac)3 nanoparticles and MHNs after calcination were demonstrated by PXRD patterns. As shown in Figure 4A, it was observed that the precursor PVPcoated Mn(acac)3 nanoreactors prior to the calcination were in amorphous nature with almost no crystallinity features. To this end, the diffraction peaks of MHNs generated after calcination confirmed the existence of crystalline Mn2O3 nanoparticles (JCPDS no. 24-0508), indicating that the nanoreactors have shown significant changes in their crystallinity before and after calcination. Further, FT-IR and Raman spectra were used to demonstrate the chemical functionalities exploring the molecular structure of the MHNs. From the FT-IR spectral recordings (Figure 4B), it was also evident that the calcined MHNs have shown significant changes in the peaks in 500− 800 cm−1 region compared to the precursor PVP-coated Mn(acac)3 nanoparticles. The peak at 610.4 cm−1 could be attributed to the Mn−O−Mn asymmetric stretching vibration, and 531.2 cm−1 could be ascribed to the Mn−O bending vibration of Mn2O3.39 The peaks at 3438.6 and 1637.5 cm−1 could be assigned to the O−H stretching vibration and bending vibration of water molecules adsorbed by the MHNs, respectively.40 The minor peak at 1290.3 cm−1 could be attributed to the C−N stretching vibration of PVP,41 which has not existed after calcination. In the Raman spectra (Figure 4C), a total of 3 peaks, 307, 345, and 640 cm−1 were detected. The peak at 307 cm−1 could be attributed to the out-of-plane

Mn2O3, respectively. Moreover, the SAED patterns (Figure 3E inset) in the selected region were consistent and in agreement with the PXRD results. Further, the TEM mapping (high-angle annular dark field, Figure 3F) demonstrated that the obtained hollow nanoparticles after calcination were composed of Mn and O elements, indicating the complete removal of element C (PVP). To further verify the formation of hollow architectures, we measured the textural properties including overall surface area based on the N2 adsorption−desorption isotherms using Brunauer−Emmett−Teller (BET) and their corresponding pore size distribution curves based on the adsorption curves using the BJH method (Figure 3H). The BET isotherms (Figure 3G) of the generated hollow nanoparticles have shown a typical type-IV adsorption pattern based on the International Union of Pure and Applied Chemistry (IUPAC) classification with a hysteresis loop, which is a typical characteristic of porous materials. An additional defined step was observed in the adsorption−desorption curve above the relative pressure of 0.9 with a desorption cumulative pore volume distribution of 0.31 cm3 g−1 because of the high degree of extraction of PVP from the nanoreactors, resulting in the excess adsorption of gas into the porous architectures. Although it is difficult to observe the porosity in the TEM images of MHNs, however, the BJH isotherms indicated that the MHNs resulted in the small-sized mesopores with the diameter around 9 nm. These tiny pores were generated because of the escape of combusted polymer in the form of carbon dioxide during the calcination. Contrarily, the precursor PVP-coated Mn(acac)3 nanoparticles resulted in a concave-shaped, rare type-III isotherm with no inflection point. In addition, the cumulative pore volume distribution was observed as 0.068 cm3 g−1 for the precursor PVP-coated Mn(acac)3 nanoparticles. Interestingly, the specific surface area (BET) of the calcined hollow nanoparticles (SBET = 87.1655 m2 g−1) was approximately three times to that of the PVPcoated metal precursors (SBET = 29.3755 m2 g−1), indicating that the hollow spaces were successfully created. The plausible E


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Figure 5. Confirmation of dual-enzyme activities. (A) Typical UV−vis absorption curves of catalytic reactions based on the peroxidase-like activity of MHNs compared to the control experiment. (B) Typical UV−vis absorption curves of catalytic reactions based on the GOx-like activity of MHNs compared to the control experiment. (C) Time-dependent absorbance changes of TMB at 652 nm in different reaction systems of HAc− NaAc buffer (pH-3.5) at room temperature; (D) time-dependent UV−vis spectral changes of the MHN−TMB−H2O2 system. The inset showing the visible change in the color of the resultant solution; the optimum (E) pH and (F) temperature of peroxidase-like MHN-catalyzed TMB; steadystate kinetic assay of MHNs, (G) concentration of TMB was 1.0 mM and (H) concentration of H2O2 was 100 mM. Insets showing their corresponding Lineweaver−Burk plots of the double reciprocal of the Michaelis−Menten equation.

bending modes of MnOx.42 The low-intensity peaks at 345 and 307 cm−1 could be ascribed to the asymmetric stretching of bridging oxygen species (Mn−O−Mn) and the high-intensity peak at around 640 cm−1 could be corresponded because of the symmetric stretching of trivalent manganese ions in Mn−O bond in the occupied octahedral sites.43 In addition, these Raman spectral results demonstrated that all of the peaks were evenly distributed (Figure S2). Moreover, the surface electron state, chemical composition, and oxide forms of the synthesized hollow nanoparticles were analyzed by XPS analysis (Figure 4D−H). The existence of various elements C, O, and Mn was demonstrated in the XPS of MHNs (Figure 4D), in which the electron binding energy of C 1s was 284.8 eV (Figure 4E). As shown in Figure 4G, the most substantial peak of Mn at about 641.4 eV confirmed the existence of oxide forms, MnOx in the hollow nanoparticles. Further, the oxidation state of Mn and its respective oxide form (MnOx) was confirmed by analyzing the detailed spectrogram of Mn 2p orbital binding energy, which resulted in the highintensity peaks at 641.6 and 653.4 eV, attributing to Mn 2p3/2 and Mn 2p1/2, respectively. The obtained spin−orbital splitting value was approximately 11.8 eV, which was consistent with the value of manganese oxide. Similarly, the peaks at 529.7 and 531.7 eV in the analysis of the O 1s spectrogram (Figure 4F) could be corresponded to oxygen O2− and Mn−O−Mn metalbonded oxygen, respectively.44 These results were consistent with the asymmetric Mn−O−Mn stretching vibration of FTIR spectra at 610.4 cm−1 and the asymmetric stretching of bridge oxygen species (Mn−O−Mn) of Raman spectra at 345 cm−1. To be more convincing, the binding energy of Mn 3s was also investigated (Figure 4H), where ΔE = 5.5 eV. The above conclusion demonstrated that the chemical composition of the hollow nanoparticle oxidation form is Mn2O3, which was

also consistent with the PXRD patterns. Meanwhile, XPS spectra of precursor PVP-coated Mn(acac)3 nanoparticles were also tested for comparison (Figure S3). In Figure S3, the 2p and 3s orbitals of Mn stated that the chemical valence state of the element was not changed while fabricating, and the intrinsic properties of the substance could be well retained with only slight changes in the morphological attributes, indicating that the eco-friendly SCF technology is a stable and convenient approach for processing inorganic constructs. Peroxidase- and GOx-like Activities. Nanostructured enzymes, nanozymes, have some definite similarities to natural enzymes, such as overall size, shape, and surface charge, which allow them to mimic the natural enzymes in their performance significantly.45 Thus, we used our designed MHNs for peroxidase-like and GOx-like activities by selecting TMB as the chromogenic base. To verify the dual enzyme activity, we studied the absorption spectra of MHNs in different systems (Figure 5A,B). The substantial changes in the absorbance values of different reaction systems with time were measured at 652 nm (Figure 5C). Interestingly, MHNs exhibited excellent both peroxidase-like and GOx-like catalytic activities compared to that of the control experiments (in the absence of MHNs and precursors). Further, the time-dependent absorbance values of the MHN-based reaction system were also determined. Moreover, the peroxidase-like activity of MHNs was happened to be conducive by taking H2O2 as the oxidant or electron acceptor and TMB as the reducing agent or electron donor, thus promoting the electron loss of TMB to form the chromogenic product TMB*+.46 Accordingly, we hypothesized that the reaction mechanism behind this oxidaselike reaction system of MHNs is the oxidation of TMB because of their catalytic ability in reducing dissolved oxygen. This could also be demonstrated as the H 2 O 2 molecules F


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ACS Applied Materials & Interfaces decomposed higher amounts of dissolved oxygen in the hollow containers of MHNs, thus promoting the oxidation process of TMB to form TMB*+ (Figure S5A). In addition to the reaction processing, the stability of the system also plays a crucial role during catalytic applications. To verify the stability of the reaction system, a spectral scanning was carried out automatically at a time lapse of every 2 min. Through the time-dependent absorbance changes (Figure 5D), we could conclude that the changes in the spectra were uniform, demonstrating that this continuous reaction system utilizing TMB as a chromogenic base and H2O2 as an oxidant was stable and appropriate. Simultaneously, we explored the effect of different concentrations of H2O2 and TMB on the reaction rate of peroxidase. As shown in Figure S4, the increase in the concentrations of both H2O2 and TMB has resulted in the substantial upsurge of peroxidase reaction rates, indicating that MHNs with peroxidase-like activity were highly suitable for detecting H2O2 through TMB degradation. To further demonstrate the peroxidase-like activity of MHNs, different chromogenic substrates such as ABTS and OPD were selected and the spectral changes in their chromogenic reactions were recorded (Figure S5B). The UV−vis spectra with ABTS as the colorimetric substrate have shown that there was a distinct absorption peak at 420 nm, demonstrating that colorimetric reaction within the Mn2O3 hollow architectures resulted in the change of color from colorless to ABTS*+ (green).47 Similarly, the broad UV−vis absorption peak of OPD at 450 nm could give us the scope in drawing similar conclusions. Plausibly, OPD reacted with H2O2 to form a yellow tinted product of diaminobenzidine in the presence of MHNs.48 Similar to natural enzymes, the performance of the nanozymes can also be altered by various external factors, out of which, the effects of pH and temperature of the reaction medium were investigated, and the responses could be correlated with their performance. To explore these facts, we investigated the influence of pH value as well as the reaction temperature on the catalytic activity of the peroxidase-like efficacy of designed MHNs. At altered pH (pH 3.0−6.5) and temperature (25−50 °C at constant pH of 3.5) conditions, it was observed from the results that the controlled optimal pH and temperature values of this enzymatic reaction system were pH 3.5 and 40 °C, respectively (Figure 5E,F), at which the peroxidase-like efficacy of the designed MHNs was relatively high over other corresponding values. To further investigate the speed of the catalyzed reactions by MHNs and various factors influencing the reaction speed, we studied the catalytic behavior of MHNs and obtained the steady-state kinetic parameters using typical Michaelis− Menten curves (Figure 5G,H) under the optimum pH and temperature for TMB and H2O2, respectively. The corresponding Vmax and Km values were calculated from Lineweaver−Burk plots (inset of Figure 5G,H). The equation 1/V0 = (Km/Vmax)· (1/[S]) + 1/Vmax, where Km represents the affinity of the enzyme for one substrate, where smaller the Km value, the stronger affinity between the substrate and the enzyme. Accordingly, we listed the values of Km and Vmax obtained in this study (Table 1) and compared them with other nanomaterials with intrinsic peroxidase-like activity. The Km value of MHNs that catalyzed the substrate TMB was 0.076 mM, which was lower than that of the CuS cluster,49 Co3O4− rGO,50 and GO−Fe3O4 nanocomposites,52 indicating their higher affinity for TMB compared to some of the reported nanomaterials. This augmented catalytic efficacy of MHNs

Table 1. Comparison of the Michaelis−Menten Constants (Km) and Maximum Reaction Rates (Vmax) among Several Materials and Enzymes from the Reported Literaturea Vmax (10−8 M s−1)

Km (mM) catalyst

TMB

H2O2

TMB

H2O2

references

CuS cluster Co3O4−rGO Fe/CeO2 NRs GO−Fe3O4 BSA−MnO2 MnSe HRP MHNs

0.648 0.19 0.176 0.43 0.04 0.786 0.434 0.076

29.16 24.04 47.6 0.71 0.12 0.0697 3.70 4.904

5.96 10.71 8.60 13.08 578 17 10.0 1.96

4.22 10.19 16.60 5.31 5.71 9.1 8.71 12.02

49 50 51 52 33 53 31 present work

a

Abbreviations: rGOreduced graphene oxide; NRsnanorods; GOgraphene oxide; BSAbovine serum albumin; HRPhorseradish peroxidase.

over several novel materials could be attributed to their large surface area, strong adsorption ability, and the accelerated charge transfers between TMB and H2O2 by increasing the electronic density. However, the Km value of MHNs with H2O2 as the substrate is 4.904 mM, which was apparently higher compared to that of BSA−MnO233 and HRP,31 because of the fact that maximal levels of peroxidase-like activity of MHNs could require lower H2O2 concentrations. Moreover, we tested the reaction activity over a range of substrate (H2O2 and TMB) concentrations. As shown in Figure S5C,D, the slopes of all lines were almost parallel, which was an apparent characteristic feature of a ping-pong mechanism for the intrinsic peroxidase-like colorimetric reaction. This result further motivated us to infer that the mechanism of the intrinsic peroxidase-like activity might be as explained: MHNs reacted with H2O2 to generate intermediate species of peroxide, and then the active species further reached the nucleophilic attack of the TMB substrate, resulting in the TMB molecules, which could be oxidized to form TMB*+ species. These consequences could be evident by the change of color from colorless to blue. Here, the functionality of MHNs was predominantly focused toward accelerating the electrontransfer process and stimulating the generation of free radicals in the designed hollow architectures. It was also observed from the results that MHNs resulted in a strong intrinsic peroxidase-mimicking efficacy in the presence of H2O2. According to the principle of colorimetric reaction, we used this method to detect H 2 O 2 . Because the concentration of TMB oxidation product TMB*+ was proportional to the concentration of H2O2, the detection limit (LOD) of H2O2 in the reaction system could be calculated by detecting the absorbance change of TMB*+ at 652 nm. In Figure S5E, it was observed that a line with an excellent linear relationship could be observed, where the linear range of H2O2 was 0.1−1.0 mM. The LOD of H2O2 was calculated according to the colorimetric reaction through LOD = 3σ/N formula, where σ is the standard deviation of blank and N is the slope of linear. A LOD of H2O2 was calculated as 2.46 μM, which was lower than that of reported Au particles,54 GQDs,55 and BNNSs@CuS56 (Table 2). Glucose is the main energy source of the living organism. However, excess glucose levels in the body lead to hyperglycemia. Thus, an efficient, rapid, convenient, and accurate glucose detection system is of great significance in biomedical applications and clinical diagnosis. Therefore, we propose a G


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of MHNs and the dual-enzyme detection system, we compared the catalytic activities of the same group of MHNs placed at room temperature in different time periods, as shown in Figure S5F. The results (Figure S5F) indicated that the MHNs prepared by calcination of SEDS-assisted precursor PVPcoated Mn(acac)3 nanoparticles possessed excellent stability in the dual enzyme detection system. In addition, this ecofriendly could result in the MHNs at high yields. A summary of nanomaterial-based sensors for glucose detection is displayed in Table 3.

Table 2. Comparison of the Performances among Various Sensing Materials for H2O2 Detection through a Colorimetric Approacha sensing materials

LOD (μM)

linear range (μM)

reference

CuS cluster BNNSs@CuS MnSe CeO2−MMT Pt NCs (+)-Au NPs GQDs MHNs

10 4.7 0.085 7.8 0.46 5 9 2.46

10−1000 100−1000 0.17−10 9−500 0−200 2−200 10−100 100−1000

49 56 53 57 29 54 55 present work

Table 3. Performances of Various Modified Glucose Sensing Materials Using a TMB-Based Detection Approacha

a

Note: BNNSboron nitride nanosheets; MMTmontmorillonite; NCsnanoclusters; (+)-Au NPspositively-charged gold nanoparticles; GQDsgraphene quantum dots.

GOx-like detection system based on MHNs. To confirm the GOx-like activity of MHNs, we used an indirect colorimetric detection approach with various glucose concentrations (Figure 6A), resulting in the apparent color changes in the

sensing materials

detection mode

LOD (μM)

linear range (μM)

PDI-Co3O4 CuS cluster Co3O4 GO-COOH carbon (+)-Au NPs GO−Fe3O4 porous Co3O4 Au/MXene Co3O4-HND MHNs

TMB TMB TMB TMB TMB TMB TMB EC EC EC TMB

2.77 100 5 1.0 20 4 0.74 0.1 5.9 0.58 2.31

5−100 100−2000 10−10000 1−20 25−100 18−1100 2−200 1−300 100−18000 2−6060 10−100

reference 58 49 45 32 59 54 52 61 62 63 present work

a

Abbreviations: PDIperylene diamides; GOgraphene oxide; (+)-Au NPspositively-charged gold nanoparticles; MXeneTi3C2 nanosheets; HNDhollow nanododecahedra; ECelectrochemical. Figure 6. (A) UV−vis spectra of the mixed solutions of TMB and MHNs in pH 3.5 HAc−NaAc buffer at room temperature. The TMB concentration was 1.0 mM, and 20 μL MHN dispersions (0.5 mg mL −1 ) were used. (B) Linear standard curve for glucose determination. The inset image showing the corresponding photograph of the solutions containing different concentrations of glucose.

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CONCLUSIONS In summary, we successfully fabricated highly stable as well as efficient hollow nanozymes, MHNs using a simple calcination approach mediated by PVP-coated Mn(acac)3 nanoreactors, which were prepared by an environmental-friendly, low-cost, and simple separation approach based on the SCF technology. Further, the as-synthesized MHNs resulted in efficacious peroxidase- and GOx-like activities. Moreover, the effects of substrate (H2O2) concentration, and other factors such as pH and temperature, were also investigated. In comparison to the reported materials as novel, peroxidase-mimic nanozymes showed enhanced catalytic activity, following a ping-pong-like mechanism. In the absence of glucose oxidase, the MHNs exhibited interesting dual-enzyme activity in the presence of TMB as a substrate, where the colorimetric sensor showed an exceptional performance toward glucose determination with a broad linear range of 10−100 μM and a limit of detection (LOD) of 2.31 μM. Together, we conclude that these stable MHNs could be used as potential candidates for colorimetricbased detection in biomedical fields and clinical diagnosis.

reactions, which could be observed by a naked eye (Figure 6B, inset). The standard working curve (Figure 6B) with the regression equation, Y = 0.13459X + 0.41805 (R = 0.99477) in the linear range of 0.1−1 mM, was plotted. Considering this as a biosensor-based nanoenzyme, the LOD calculation, based on the method mentioned above, was around 2.31 μM, which was lower than that of PDI-Co3O4,58 Co3O4 NPs,45 and carbon NPs.59 This phenomenon could be apparently caused by the proximity effects and the reaction in situ: MHNs with both peroxidase-like and GOx-like dual enzyme efficacies promoted the cascade reactions substantially, and the products from the first step could be easily performed as the substrate in situ for the second step.60 In this framework, glucose was first chemically transformed in the presence of MHNs to gluconic acid and H2O2, which subsequently initiated a peroxidase-like reaction in transforming the TMB to its resultant radical species in MHNs. On the basis of these considerations, we could infer that the presence of dual enzyme activities of MHNs eliminated the mass transfer process of reactants and intermediates, which further hastening the apparent reaction rate of glucose and TMB, and subsequently toward gluconic acid and TMB*+. Therefore, MHNs with dual enzyme activities could offer obvious advantages in glucose colorimetric sensing detection, which is difficult to be achieved by currently available catalytic systems. Considering the stability

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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/acsami.9b05688. EDS-based elemental mapping, MHN Raman mapping, XPS spectra of PVP-coated Mn(acac)3 nanoparticles, effect of H2O2 and TMB on peroxidase, and effect of dissolved oxygen and N2 on TMB oxidation (PDF) H


Research Article


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +86 592 616 2326. ORCID

Ranjith Kumar Kankala: 0000-0003-4081-9179 Ai-Zheng Chen: 0000-0002-5840-3406 Author Contributions

The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from National Natural Science Foundation of China (NSFC, U1605225, 31570974, and 31800794), Program for Innovative Research Team in Science and Technology in Fujian Province University, and Subsidized Project for Cultivating Postgraduates’ Innovative Ability in Scientific Research of Huaqiao University is gratefully acknowledged.

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Supercritical Fluid-Assisted Fabrication of Manganese (III) Oxide

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