Mesoporous Pt Nanotubes as a Novel Sensing Platform for Sensitive

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Mesoporous Pt Nanotubes as a Novel Sensing Platform for Sensitive Detection of Intracellular Hydrogen Peroxide QIurong Shi, Yang Song, Chengzhou Zhu, Haipeng Yang, Dan Du, and Yuehe Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08146 • Publication Date (Web): 14 Oct 2015 Downloaded from http://pubs.acs.org on October 15, 2015

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ACS Applied Materials & Interfaces

Mesoporous Pt Nanotubes as a Novel Sensing Platform for Sensitive Detection of Intracellular Hydrogen Peroxide Qiurong Shi†,§, Yang Song†,§, Chengzhou Zhu†, Haipeng Yang†, Dan Du†,‡, ‡‡ and Yuehe Lin†,‡,‡‡* †

School of Mechanical and Material Engineering, Washington State University, Pullman, WA

99164, United States ‡ Key Laboratory of Pesticide and Chemical Biology, Ministry of Education of the P.R. China, College of Chemistry, Central China Normal University, Wuhan, 430079, China ‡‡

Paul G. Allen School for Global Animal Health, Washington State University, Pullman, WA

99164, United States

Keywords: hollow nanotubes; mesoporous; colorimetric method; H2O2 detection; living cell

ABSTRACT: Controlling the shape, structure and surface morphology of nanomaterials is of great significance in optimizing sensitivity and catalytic performances in biosensing applications. The main goal of employing Pt-based nanomaterials is to increase their utilization efficiency due to their high cost. Herein, we report the synthesis of mesoporous Pt nanotubes (NTs) using Pluronic P123 as soft templates and Ag nanowires (NWs) with 50 nm in diameter as hard

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templates. The resultant materials with unique structures show high sensitivity and stability toward H2O2 detection with low cellular cytotoxicity. The high sensitivity and catalytic properties are attributed to the mesopores and hollow structures making the inner Pt surfaces accessible to reaction media and enlarging the total surface area, and one-dimensional (1D) structure facilitating the mass diffusion rate.

1. Introduction Rationally designed one-dimensional (1D) nanostructured materials, especially metallic nanowires (NWs), nanotubes (NTs), and nanorods (NRs) has always been attractive research subjects for its unique anisotropic morphology that could be potentially used in optical devices, electronic devices, sensors, catalysts and so on.

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Nowadays, Pt-based 1D nanomaterials have

been extensively studied in various applications in electrode materials for fuel cell, detection of hydrogen peroxide and glucose, gas sensors, etc.9-12 It is because that the anisotropic structure of 1D material with high aspect ratio could not only improve mass transfer efficiencies,13 but also avoid dissolution, Ostwald ripening, and aggregation problems that could probably appeared in nanoparticles.14 However, as one of the noble metals, the high cost of Pt is always a big challenge. Therefore, improving the utility of the Pt-based 1D materials via modifying their shapes and morphologies is critical in various practical applications. Synthesizing Pt-based 1D nanomaterials with mesoporous shells and hollow structures is one of effective approaches toward improving the comprehensive performances of sensors or catalysts. Mesoporous metal nanomaterials have been very popular in recent years mostly due to their extremely high surface to volume ratio and unique exposed atomic structures.15-19 The mesoporous shells with controlled thicknesses could also improve the efficiency of materials because the meso-sized pores is much larger than the mean free path of many reactant molecules, which could facilitate the inner active


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sites accessible to reactants and decrease diffusion resistance remarkably.20 Besides, hollow structures in nanomaterials provide both exterior and interior surface areas of Pt nanomaterials available that could significantly enlarge the active surface areas. Moreover, hollow structures could effectively reduce material density, which is beneficial for employing materials in many light-weighted devices, drug delivery and nanoreactors.21-23 Reactive oxygen species (ROS) were related to various biophysiological reactions and processes in aerobic life due to they serve as the biochemical mediators in cellular pathology and the marker of pathological events, such as cell injury of cancer.24-31 They have raised lots of attention on the early diagnosis for cytotoxic32 and metabolism disorders.33 H2O2, one kind of ROS in human body, has received much consideration for its stable penetrability through cell membranes.30, 34, 35 It involves with various pathological effects in organisms and takes part in different cellular metabolism processes in the concentration dependent performance.36, 37 It can inversely damage living cell structures, resulting in inflammatory diseases.38,

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Hence,

developing a rapid, sensitive, and accurate method to detect intracellular H2O2 quantitatively is crucial in studying the biological effect of H2O2 and preventing relative diseases associated with human inflammatory.40-44 Herein, we have successfully synthesized mesoporous Pt NTs at room temperature so as to enhance their sensitivity toward H2O2 detections. The detailed synthesis procedures were illustrated in Scheme 1. We employed Ag NWs as hard template, Pluronic P123 as soft template, ascorbic acid (AA) as reducing agent and chloroplatinic acid (H2PtCl6) as precursors. The asobtained mesoporous Pt NTs is about 70 nm in diameter with shell thickness of about 9.5 nm. They combined three prominent advantages of 1D nanomaterials of fast mass transfer, mesoporous shells and hollow structures for favoring mass diffusion and utilization of both


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exterior and inner surface areas. Clearly, the unique structure and morphology of mesoporous Pt NTs were expected to boost the enhanced peroxidase-like catalysis for intercellular H2O2 detection.

2. Experimental Section Chemical and materials. Ascorbic acid (AA), Polyvinylpyrrolidone (PVP) (MW=360,000 and MW=5,5000), Pluronic P123 were all purchased from Sigma-Aldrich Co. Ltd. Silver nitrate (AgNO3, 99 +%) was purchased from Alfa Aesar. Chloroplatinic acid hexahydrate (H2PtCl6· 6H2O) were purchased from STREM Chemicals. N-for-mylmethionyl-leucylphenylalanine (fMLP), adenosine5-diphosphate (ADP), phorbol myristate acetate (PMA), o-phenylenediamine dihydrochloride (OPD) and 3,3,5,5-tetramethylbenzidine (TMB) were purchased from SigmaAldrich. Commercial Pt/C (20%) was purchased from Alfa Aesar. The MCF-7 cell line was ordered from ATCC. For cell cultural, collagen G, Eagle’s medium, L-glutamine, gentamycin, penicillin, fetal bovine serum (FBS) and streptomycin were purchased from ATCC. All glasswares and stirring bars were cleaned with aqua regia (3:1 v/v HCl (37 %): HNO3 (65 %) solutions) and then rinsed thoroughly with H2O before use. (Caution: aqua regia solutions are dangerous and should be used with extreme care; never store these solutions in closed containers.) The water in all experiments was prepared in a three-stage Millipore Milli-Q plus 185 purification system and had a resistivity higher than 18.2 MΩ cm. Synthesis of Ag NWs and mesoporous Pt NTs. Silver nanowires (Ag NWs) with diameter of about 50 nm were synthesized via ethylene glycol (EG) reduction reported by Ye’s group.16 Typically, 0.08 g PVP (MW=5,5000) and 0.12 g PVP (MW=360,000) were dissolved in 22 mL of EG. Subsequently, 2.5 mL of FeCl3 solution (600 µM in EG) and AgNO3 solution (0.180 g in


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3 mL of EG) were added rapidly into PVP solution within 1 min. The mixed solution were stirred under 140 ℃ for 2hrs. After the reaction finished, Ag NWs were washed with ethanol and water for 3 times, respectively. Mesoporous Pt NTs were synthesized using as-obtained Ag NWs as hard template (Ag NWs were dissolved in water with a concentration of 7.2 mg/mL). 100 mg Pluronic P123 were dissolved in 4.3 mL water, followed by adding 400 µL of Ag NWs, 300 µL H2PtCl6 (0.01 M) and 1 mL AA (0.01 M). The mixed solution was under stirring for 5 hrs and washed with ethanol and water for 3 times, respectively. Then the as-synthesized Ag@Pt NWs were treated with concentrated nitric acid for 12 hrs, followed by washing with water for three times. Detection procedure of H2O2. For measuring the H2O2 standard solution, the H2O2 concentration-dependent UV absorbance was studied. Several different concentration of H2O2 was mixed with the same volume of TMB (20 mM), and then the same volume of Pt NTs was added. Then the mixture solution was incubated at room temperature for 5 min to allow for obtaining the blue color. The UV absorbance could be determined by microplate UV reader. To measure the H2O2 release from living cells, MCF-7 cancer cell was cultured in 96 well plates with cell density of 1.0 x105 cells/mL for 24 hrs. After removing the cultural medium, 140 µL PMA (200 ng/mL) was added and incubated for 1 min by shaking. Then 30 µL TMB and 30 µL Pt NTs were further added and incubated at room temperature for 5 min to finish the reaction. The UV absorbance could be determined by microplate UV reader. Cytotoxicity assay. The cytotoxicity of Pt NTs for MCF-7 cell in vitro was performed by the standard MTT assay. MCF-7 cells were incubated in the 96-wells plate with the cell density of 1.0x105 per mL for 24 hrs. Then the medium was discarded and treated with DMEM medium


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contained the Pt NTs with the concentration from 1.0 to 10 µg mL-1 for 24 hrs. After that, the cells were incubated with 20 µL of MTT (5 mg mL-1 in PBS) for 4 hrs. After incubation process, 150 µL DMSO was added into each well. The optical density was recorded by microplate reader at the wavelength of 490 nm. Characterization. Transmission electron microscopy (TEM) images were obtained by Philips CM200 UT (Field Emission Instruments, USA). FEI sirion field emission scanning electron microscope (FESEM) was used for imaging and energy-dispersive X-ray analysis (EDX). The tube was operated at 40 KV accelerating voltage and 15 mA current. X-ray Diffraction (XRD) characterization was carried out by Rigaku Miniflex 600. UV-vis spectra were obtained from Tecan Safire2 microplate reader. Cellular imaging was performed with a confocal laser fluorescence microscopy (Leica SP8 Point Scanning Confocal).

3. Results and Discussion 3.1. Characteristics of mesoporous Pt nanotubes In this work, Ag NWs were firstly synthesized via EG reduction of AgNO3 and employing PVP as capping agent.16 Figure 1A shows TEM images of the as-obtained uniform Ag NWs with diameter of about 50 nm and length of about 30 µm. In the second step, we used Pluronic P123 as a soft template mixing with hard templates to generate micelles on the surface of Ag NWs. And then Pt precursors were reduced and thus evolved into mesoporous nanostructures onto the surface of Ag NWs by AA using generated micelles as soft templates. We then removed Ag with concentrated nitric acid for about 12 hrs. The detailed synthesis procedure was vividly illustrated in Scheme 1. We can clearly see the uniform and high yield of hollow and mesoporous structures from TEM images in Figure 1B and 1C. The thickness of Pt shell is about 9.5 nm. The different lengths of Pt NTs were mostly caused by ultrasound when dealing with washing processes.


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In this process, Pluronic P123 plays a key role in creating the mesoporous Pt shells when Pt4+ ions were reduced by AA. It is noted that Pluronic P123 is a typical mesostructural micelle that is widely applied in synthesizing mesoporous silicon, carbon and metallic nanostructures.4548

Pluronic block copolymers favors the formation of mesopore walls generated from the

aggregation of small sized Pt nanoclusters by surface capping mechanism.49,

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Control

experiment was conducted without adding any surfactant to verify the effect of Pluronic P123. TEM image in Figure 1D exhibits the resultant Pt NTs with a very smooth shell that quite different from mesoporous Pt NTs using Pluronic P123 as stabilizing agent. We also replaced Pluronic P123 with Brij58 (Figure S1), which is another widely used mesostructural soft template, to further investigate the effect of stabilizing agent. From TEM image of Pt NTs in Figure S1, Pt shells indeed shows mesoporous structures but displays disordered and nonuniform morphologies. Therefore, Pluronic P123 could effectively facilitate the formation of uniform and mesoporous Pt shells. The thickness of Pt shell could be readily adjusted by controlling the concentration of Pt precursors. We tuned the thickness from ~7.2±0.5 nm to ~9.5±0.3 nm, ~11.5±0.4 nm, and ~12.8±0.4 nm with amount of Pt4+ ions increased from 2 µmol to 3 µmol, 4 µmol, and 5 µmol, respectively. The resultant TEM images of mesoporous Pt NTs were shown in Figure 2(A-D). The X-ray energy dispersive spectroscopy (EDS) result in Figure 2E shows that the major component is Pt, and only about ~10% Ag residual after acid treatment. The composition of asprepared mesoporous Pt NTs was further characterized by using X-ray diffraction (XRD) in Figure 2F. The diffraction peaks of mesoporous Pt NTs could be indexed as the (111), (200), (220), (311), (222) planes of a typical face-centered cubic (fcc) lattice.51, 52 The peak position of Pt and Ag is in accordance with the JADE PDF#65-2868 and PDF#04-0783, respectively. XRD


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result demonstrates that the composition of the as-synthesized mesoporous Pt NTs was almost pure Pt since no shift of peak positions was observed because most of Ag on the surface was removed by acid treatment. 3.2. Colorimetric Investigations of Peroxidase–like Catalytic Activities of Pt NTs To investigate the peroxidase-like activity of mesoporous Pt NTs, the catalysis of peroxidase substrate TMB and OPD was tested in the presence of H2O2. As shown in Figure 3A, it was observed that mesoporous Pt NTs could catalyze H2O2-induced oxidization of TMB or OPD, and the solution exhibited dark blue color and yellow color within 10 minutes. These results indicated that Pt NTs behaved as peroxidase toward TMB and OPD oxidation with H2O2. These reactions are ascribed to the charge-transfer complexes derived from the one electron oxidation of TMB and OPD. The related reaction is described in Figure 4A, in which H2O2 served as electron accepters. These results indicated that the mesoporous Pt NTs showed the intrinsic peroxidase-like activity. We further investigated the catalytic activity of mesoporous Pt NTs. Figure 3B showed the UV absorbance spectra obtained after TMB oxidation by adding mesoporous Pt NTs and commercial Pt/C nanopowders with the same amount of Pt (5 µg/mL). Each solution exhibited blue color, corresponding to the absorbance at 652 nm. Compared with commercial Pt/C nanopowders, the TMB-H2O2 assay catalyzed by mesoporous Pt NTs achieved stronger absorbance intensity. This means mesoporous Pt NTs could oxidize more TMB than commercial Pt/C nanopowders under the same concentration of Pt. In addition, Figure 3C exhibited the UV-vis spectra with mesoporous Pt NTs synthesized via adding different amount of Pt4+ ions. The absorption peaks of all the Pt NTs were centered at


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652 nm. The mesoporous Pt NTs with thickness of about 9.5 nm exhibited the strongest absorption intensity, suggesting the highest peroxidase-like activity among Pt NTs with different thicknesses. The inner active sites of Pt NTs with a thicker shell may be hindered and hardly accessible to the molecules, which leads to their poorer peroxidase-like activities. While mesoporous Pt NTs with a proper shell thickness of about 9.5 nm possess the best peroxidaselike activity via taking full advantage of the active surface area. But Pt NTs, except whose shell thickness of 12.8 nm, still show better performances than that of commercial Pt/C nanopowders. Therefore, it has the potential to elude intrinsic disadvantages of natural enzyme. The steadystate kinetic parameters of mesoporous Pt NTs were further characterized by keeping one concentration of TMB or H2O2 constant and changing the other’s concentration simultaneously. According to the Lineaweaver-Burk equation, the Michaelis constant, Km was obtained in Figure 4. The Km of mesoporous Pt NTs and Pt/C nanopowders were shown in Table 1, as compared with HRP. Clearly, it was found that the Km value of mesoporous Pt NTs with a H2O2 substrate (5 mM) is lower than the one of HRP and other enzyme mimics, indicating that it could have stronger affinity toward H2O2. Besides, the Km value of mesoporous Pt NTs for TMB was also lower than that of Pt/C nanopowders, suggesting that mesoporous Pt NTs exhibited higher affinity for TMB than other mimics. These results supported the hypothesis that the assynthesized mesoporous Pt NTs have stronger peroxidase-like activity than commercial Pt/C, which most possibly attributed to its unique mesoporous, hollow and 1D structure resulting in the higher active surface area and mass transportation speed. The thermal and pH stability of mesoporous Pt NTs were further studied and the results were shown in Figure 3D. Pt NTs exhibited a stable enzymatic catalytic activity toward H2O2 in the temperature range from 4 to 90 °C, whereas the enzymatic activity of HRP drastically decreased over 40 °C due to the


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denaturalization of the enzyme under high temperature. Therefore, the peroxidase activity of Pt NTs could be maintained at far higher temperature in comparison with enzyme. Furthermore, Pt NTs also exhibited stable activity in the solution with the pH range from 1 to 12; whereas the activity of HRP was significantly inhibited when the pH was lower than 7.0 and totally denaturized when pH dropped to 2. In addition, even under long time storage (Figure S2), the designed Pt NTs still maintains strong peroxidase activity. Therefore, the robust peroxidase activity of Pt NTs promotes varieties of biological applications which require strong and sensitive labels to serve as ultrasensitive reporter. On the basis of peroxidase-like activity of Pt NTs, the relationship between the peroxidaselike activity of Pt NTs and H2O2 concentrations was further studied in Figure 5(B-D). Figure 5C shows a linear plot of concentration of H2O2 corresponding to the absorption intensity. It was clearly observed that the absorbance intensity strongly depended on the H2O2 concentration at low concentration range (5x10-6-10 mM). Moreover, we also observed that the color intensity changing from colorless to dark blue with increasing H2O2 concentration in Figure 5B. This could provide a potential visual detection method for H2O2-related detection without any complicated instrumentation design. Figure 5D shows the absorbance evolution for different concentrations of H2O2 over time. When increasing the reaction time, the color intensity of different H2O2 concentration increased and reached the maximum value after 30 min. 3.3. Analytical performance of cellular H2O2 detection The mesoporous Pt NTs are also applied for detection of H2O2 released from living MCF-7 cells by colorimetric detection. Here, MCF-7 cell was chosen as the model cancer cell, while PMA was used as the stimulant agent, which will induce the H2O2 generation from cancer cells with consistent chemotactic response. When the cancer cells were stimulated, larger amount of H2O2


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releasing would occur. We investigated the specificity of H2O2 detection, whereas 50 mM glucose and ethanol were introduced as detection sample in the same condition, respectively. In Figure 6B, the response of H2O2 was significantly higher than that of ethanol and glucose, indicating that the detection method could be used to specific detection of H2O2. Figure 6A shows that the colorimetric response of H2O2 released from cancer cell which is stimulated by PMA. The absorption intensity reached to 0.254, corresponding to the concentration of released H2O2 is 6.63 x 10-7 M. The calculated H2O2 released from single cancer cell is about 6.63 x 10-15 mol. This value matches well with the previous report (6.3-7.1 x 10-15 mol/cell), demonstrating that the colorimetric detection method based on mesoporous Pt NTs could be used in further practical application. For obtaining the optimal detection performance, certain kind of stimuli agent should be considered. The effect of different stimuli on the H2O2 releasing from cancer cells were performed by using ADP, fMLP, and PMA, respectively. The highest amount of H2O2 was obtained using PMA as stimuli agent compared with ADP and fMLP (Figure 6A). Thus, PMA was adopted as the optimal stimuli. The cytotoxicity of the Pt NTs was evaluated by using the standard MTT assay with MCF-7 cell. As shown in Figure 6C, almost 85% cell viability is observed after incubation of MCF-7 cells with Pt NTs at concentration range from 1.0 to 10 µg mL-1 for 24 hrs, demonstrating the low cytotoxicity of Pt NTs. In order to evaluate the biocompatibility of Pt NTs, the standard staining method was used by calcein-AM as cell stain. The calcein-AM could incorporate with living cell through cell membrane and green fluorescent could be observed on cytoplasm by fluorescent microscope. MCF-7 cell was incubated with mesoporous Pt NTs for 8 hrs, and then incubated with calcein-AM for 15 min. The control experiment was performed with the cell without being cultured with mesoporous Pt NTs. In Figure 6D and E, the morphology of MFC-7


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cells was similar with the cells without incubating with mesoporous Pt NTs, indicating that Pt NTs have no obviously toxicity for cell viability.

4. Conclusion In summary, we have successfully synthesized mesoporous Pt NTs employing both soft and hard templates at room temperature. Using Pluronic P123 as stabilizing agent is significant in generating the mesoporous hollow structures. We also adjusted the shell thickness of Pt NTs via tuning the amount of precursors and find that the thickness of about 9.5 nm is appropriate for taking full advantage of its specific surface area. The mesoporous Pt NTs were used as peroxidase-like agent for detecting H2O2. The intercellular H2O2 detection with a detection limit reached as low as 5 nM was achieved.

The H2O2 releasing from living MCF-7 cell was

successfully studied by the colorimetric method. The results show a high sensitivity and stability for detection of intercellular H2O2. One reason for its superior performance is that both the exterior and inner surface can be used as active sites due to the mesoporous and hollow structure of Pt NTs facilitating the access of molecules. Besides, the 1D nanostructure also favors the mass transfer and thus improves the sensitivity of the intercellular H2O2 detection. Moreover, the mesoporous Pt NTs show extremely low cellular cytotoxicity, which could be potentially used for detection of H2O2 in vivo. Overall, the mesoporous Pt NTs offers a higher specific surface area and fast mass diffusion rate which is promising for different applications, such as chemical and biosensing, chemical catalysis, and energy conversion.


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Figure Caption Scheme 1. Schematic illustration of the synthesis procedure of mesoporous Pt NTs. Figure 1. TEM images of as-synthesized Ag NWs (A), mesoporous Pt NTs using Pluronic P123 as capping agent (B and C), and Pt NTs without using any capping agent (D). Figure 2. TEM images of the mesoporous Pt NTs via adding different amount of Pt4+ ions, (A) 2 µmol, (B) 3 µmol, (C) 4 µmol, and (D) 5 µmol, respectively. The EDS (E) and XRD patterns (F) of as-fabricated mesoporous Pt NTs. Figure 3. (A) Schemes of oxidation color reaction of TMB and OPD by H2O2 with and without mesoporous Pt NTs. (B) UV absorption spectra of TMB-H2O2 reaction catalyzed by Pt NTs and commercial Pt/C. (C) The UV absorption spectra of mesoporous Pt NTs obtained via adding different amount of Pt4+ ions in pH 4.5 HAc buffer. (D) The stability of mesoporous Pt NTs. Peroxide activity was measured after Pt NTs were incubated at pH 1-11 for 5 hrs (black curve). Peroxide activity was measured after Pt NTs were incubated at 4-90 °C for 5 hrs (red curve). All the conditions: 20 mM TMB, 5 mM H2O2, 0.2 mM HAc-NaAc buffer (pH 4.5) and 5 µg/mL catalysts. Figure 4. Plots of steady-state kinetic assay of Pt NTs with (A) the concentration of H2O2 (5 mM) fixed and TMB concentration varied, (B) the concentration of TMB (20 mM) fixed and the H2O2 concentration varied. (C and D) Double reciprocal plots of activity of Pt NTs with the concentration of one substrate fixed and the other varied.


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Figure 5. (A) Mechanism of TMB color reaction. (B) The optical photo of peroxidase-like reaction with different concentration of H2O2: 1 mM, 100 µM, 10 µM, 1 µM, 100 nM, 10 nM, 5 nM and control, respectively (from left to right). (C) The calibration curve plots on peroxidaselike reaction with different concentration of H2O2. All the conditions: 20 mM TMB, 0.2 mM HAc-NaAc buffer (pH 4.5) and 5 µg/mL Pt NTs. (D) Plot of the absorbance evolution at 652 nm over time at several H2O2 concentrations. Figure 6. (A) Comparison of H2O2 release from cell induced by 0.2 µg/mL ADP, fMLP, and PMA. (B) Sensitivity of Pt NTs based H2O2 assay. (C) Relative viability of MFC-7 Cells incubated with a series of gradient concentrations of Pt NTs (1.0-10 µg/mL). Fluorescence imaging of the MCF-7 cancer cell cultured with (D) and without (E) Pt NTs for culturing 8 hrs.

Scheme 1. Schematic illustration of the synthesis procedure of mesoporous Pt NTs.


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Figure 1. TEM images of as-synthesized Ag NWs (A), mesoporous Pt NTs using Pluronic P123 as capping agent (B and C), and Pt NTs without using any capping agent (D). The mesoporous Pt NTs were synthesized via adding 3 µmol Pt4+ ions, 10 mg/mL Pluronic P123 and 10 µmol AA.


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Figure 2. TEM images of mesoporous Pt NTs via adding different amount of Pt4+ ions, (A) 2 µmol, (B) 3 µmol, (C) 4 µmol, and (D) 5 µmol, respectively. The EDS (E) and XRD patterns (F) of as-fabricated mesoporous Pt NTs.

Figure 3. (A) Schemes of oxidation color reaction of TMB and OPD by H2O2 with and without mesoporous Pt NTs. (B) UV absorption spectra of TMB-H2O2 reaction catalyzed by Pt NTs (with shell thickness of 9.5 nm) and commercial Pt/C. (C) The UV absorption spectra of mesoporous Pt NTs obtained via adding different amount of Pt4+ ions in pH 4.5 HAc buffer. (D) The stability of mesoporous Pt NTs. Peroxide activity was measured after Pt NTs were incubated at pH 1-11 for 5 hrs (black curve). Peroxide activity was measured after Pt NTs were incubated at 4-90 °C for 5 hrs (red curve). All the conditions: 20 mM TMB, 5 mM H2O2, 0.2 mM HAcNaAc buffer (pH 4.5) and 5 µg/mL catalysts.


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Figure 4. Plots of steady-state kinetic assay of Pt NTs with (A) the concentration of H2O2 (5 mM) fixed and TMB concentration varied, (B) the concentration of TMB (20 mM) fixed and the H2O2 concentration varied. (C and D) Double reciprocal plots of activity of Pt NTs with the concentration of one substrate fixed and the other varied.

Figure 5. (A) Mechanism of TMB color reaction. (B) The optical photo of peroxidase-like reaction with different concentration of H2O2: 1 mM, 100 µM, 10 µM, 1 µM, 100 nM, 10 nM, 5 nM and control, respectively (from left to right). (C) The calibration curve plot on peroxidase-


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like reaction with different concentration of H2O2. All the conditions: 20 mM TMB, 0.2 mM HAc-NaAc buffer (pH 4.5) and 5 µg/mL Pt NTs. (D) Plot of the absorbance evolution at 652 nm over time at several H2O2 concentrations.

Figure 6. (A) Comparison of H2O2 release from cell induced by 0.2 µg/mL ADP, fMLP, and PMA. (B) Selectivity of Pt NTs based H2O2 assay. (C) Relative viability of MFC-7 Cells incubated with a series of gradient concentrations of Pt NTs (1.0-10 µg/mL). Fluorescence imaging of the MCF-7 cancer cell cultured with (D) and without (E) Pt NTs for culturing 8 hrs.

Table 1 Comparison of the Km of various enzyme mimics. Enzyme mimics

Pt NTs

HRP

Km [H2O2]/(mM)

0.08

3.70

Km [TMB] /(mM)

1.47

0.43


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ASSOCIATED CONTENT Supporting Information TEM images of Pt NTs with Brij 58 as capping agent and stability of peroxidase activity of Pt NTs. Supporting Information is available free of charge via the Internet at ACS Publications Website.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Note The authors declare no competing financial interest. Author Contributions §These authors are equally contributed to this work. ACKNOWLEDGEMENTS This work was partly supported by the Centers for Disease Control and Prevention/National Institute for Occupational Safety and Health (CDC/NIOSH) Grant R21OH010768. Its contents are solely the responsibility of the authors and do not necessary represent the official views of the federal government. We acknowledge the financial support by National Natural Science Foundation of China (21575047, 21275062). QS thanks China Scholarship Council (CSC) for the financial support. REFERENCES (1). Garcia, M.; Batalla, P.; Escarpa, A. Metallic and Polymeric Nanowires for Electrochemical Sensing and Biosensing. TrAC, Trends Anal. Chem. 2014, 57, 6-22. (2). Kariuki, N. N.; Khudhayer, W. J.; Karabacak, T.; Myers, D. J. Glad Pt-Ni Alloy Nanorods for Oxygen Reduction Reaction. ACS Catal. 2013, 3, 3123-3132.


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Mesoporous Pt Nanotubes as a Novel Sensing Platform for Sensitive

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