DNA-Based Platinum Nanozymes for Peroxidase ... - ACS Publications

Jul 10, 2014 - Catalytic Performance of Oligonucleotide-Templated Pt Nanozyme Evaluated by Laccase Substrates. Yu Wang , Chuan He , Wei Li , Jinli ...
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DNA-Based Platinum Nanozymes for Peroxidase Mimetics Yan Fu, Xuyin Zhao, Jinli Zhang, and Wei Li* Key Laboratory of Systems Bioengineering MOE; Key Laboratory for Green Chemical Technology MOE, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: G-/C-rich oligonucleotides are chosen as the nucleation templates to synthesize Pt nanozymes with the size distribution of 1.7−2.9 nm, showing high activity to mimic peroxidase. The physicochemical properties including charge states and particle sizes are greatly associated with the DNA templates, the precursor ions as well as the molar ratios of [precursor]/[DNA]. As increasing the average size from 1.8 to 2.9 nm, the fraction of Pt0 species increases while the proportion of Pt2+ decreases. In the meanwhile, the Km value toward H2O2 decreases by three times, whereas toward TMB the Km increases by two times. The most efficient Pt nanozyme consisting of approximate 66% metallic Pt0 is stabilized by the i-motif RET2 with the average diameter of 2.9 nm. These results pave a promising way to manufacture metal nanozymes with facile modulation of physicochemical properties through programming DNA sequences.

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INTRODUCTION Manufactured nanomaterials that mimic enzymes (nanozymes), including metal, metal oxide, bimetallic alloy, and carbon-based nanomaterials,1−4 have attracted increasing interests because of their promising applications in the fields of biosensing, immunoassays, cancer diagnostics, neuroprotection, stem cell growth, and pollutant removal,5−10 owing to the advantages of low cost, easy preparation, better stability, and tunable catalytic activity relative to native enzymes.11 Noble metal nanomaterials have recently emerged as promising candidates for enzyme mimetics owing to their unprecedented catalytic performance along with good biocompatibility and low toxicity. Up to now, several metal nanoclusters/nanoparticles have been explored to resemble catalase, superoxide dismutase, oxidase and peroxidase.12−14 Apoferritin-encapsulated Pt nanoparticles containing 30 Pt atoms per ferritin shell exhibited lower affinities to hydrogen peroxide (H2O2) substrate, but possessed higher stability against temperature and differential response to pH, compared with native catalase and horseradish peroxidase (HRP).15 BSA-stabilized Au nanoclusters exhibited more than 100-times higher affinity to 3,3′,5,5′-tetramethylbenzidine (TMB) than that of HRP, while they had 100-times lower affinity to H2O2 than that of HRP in the peroxidase mimic. Importantly, these BSA-Au clusters reacted faster than native HRP toward both TMB and H2O2, and can be utilized in the colorimetric detection of H2O2 and xanthine with the limits of 2 × 10−8 and 5 × 10−7 M, respectively.16 In the fields of bioinorganic chemistry, transition metal ions have been widely reported to favor Lewis acid−base interactions with the electron-rich nitrogen and oxygen atoms of DNA nucleobases.17 Therefore, it is intriguing to explore novel nanozymes with highly catalytic efficiencies based on another important nature biomolecule of DNA. © 2014 American Chemical Society

DNA can be self-assembled via unique base pairings into polymorphic structures including G-quadruplex, i-motif, and Watson−Crick duplex, etc. In the view of atom economy and environmental benign, polymorphic DNA has been recognized recently as an efficient template to synthesize metal nanoclusters/nanocrystals at ordinary temperature in aqueous solution.18−21 For example, platinum nanoclusters with a diameter of 1.4 nm were synthesized through deposition on genomic DNA−graphene oxide, and they had a mass activity of 2.6 times that of the Pt nanoparticles-graphene oxide (with an average size of 3 nm) for the oxygen reduction reaction.22 It is also suggested that the interaction mechanisms of DNA templates with metal ions potentially modulate the physicochemical properties of nanoclusters. For example, the fluorescent property of Ag nanoclusters prepared using series of DNA templates was greatly associated with the binding affinity between DNA and Ag+, i.e., the C-rich C4A4C3 with the highest binding constant (40.2 × 105 M−1) can stabilize the fluorescent emission of Ag nanoclusters for hundreds of hours, while the G-quadruplex G4T4G4 with the lowest binding constant (0.64 × 105 M−1) generated Ag with the shortest shelf life.23 Moreover, Pd8 ∼ Pd9 clusters, prepared using i-motif DNA as the template, exhibited extremely high catalytic activity toward the reduction reaction of 4-nitrophenol, whereas using the duplex DNA as the template the obtained Pd showed low catalytic activity.24 These studies shed light on a promising route to construct nanozyme through the modulation of physicochemical property of metal nanomaterials via polymorphic DNA template. Received: April 2, 2014 Revised: July 7, 2014 Published: July 10, 2014 18116

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In this article, it is the first report to synthesize a series of DNA-stabilized Pt nanoparticles for peroxidase mimetics. Grich oligonucleotide AG22 and C-rich oligonucleotide RET2 are chosen as the nucleation templates for Pt precursors including K2PtCl4 and H2PtCl6. By using the characterizations of CD, UV−vis, FT-IR, ITC, ICP, TEM and XPS, it is demonstrated that the physicochemical properties of Pt nanozymes are greatly associated with the DNA templates, the precursor ions as well as the molar ratios of [precursor]/ [DNA]. Interestingly, the peroxidase mimicking activities of these Pt-based nanozymes mainly depend upon the relative proportion of Pt0 against Pt2+ species. These results pave a promising way to manufacture metal nanozymes with facile modulation of physicochemical properties through programming DNA sequences.

v=

Km 1 1 = + v Vmax[S] Vmax

(4)

Characterizations. UV−vis Spectroscopy. UV−vis spectra were recorded by Varian Cary300 spectrophotometers at 25 °C using a 4 mL quartz glass cuvette with 1 cm path length. The reaction kinetics was monitored at the wavelength of 652 nm. Circular Dichroism Spectroscopy (CD). CD spectra were obtained with a Jasco J-810 spectropolarimeter at 25 °C using a quartz glass cuvette with 0.1 cm path length. All the CD spectra were measured from 350 to 200 nm at a scan speed of 200 nm/ min, and each CD spectrum was an average of three scans. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR spectra of DNA as well as DNA with Pt precursors were performed with a Thermo Scientific Nicolet iS10 FT-IR spectrometer. DNA-Pt samples were prepared by adding concentrated K2PtCl4/H2PtCl6 solutions to annealed DNA samples (strand concentration of 200 μM) at the [precursor]/ [base] ratio of 0.5. Then 100 μL sample was dropped on ZnSe transparent support and dried in a vacuum drying oven. All the spectra were recorded from 1800 to 800 cm−1. Isothermal Titration Calorimetry (ITC). ITC measurements were performed using a VP-isothermal titration calorimeter (Microcal, Northampton, MA). Titration was carried out by injecting 10 μL aliquots (20 mM K2PtCl4) per injection to 40 μM DNA solution at 25 °C, for a total of 28 injections with 180 s intervals. Titration curves were corrected for heat of dilution by injecting 20 mM K2PtCl4 solution into 10 mM MES buffer (pH 5.0). Transmission Electron Microscopy (TEM). Electron microscopy was performed on JEM-2010FEF equipment (JEOL, Japan). Specimens were prepared by applying 10 μL solutions of DNA-Pt on a carbon-coated grid for 5 min, and then excess liquid was removed with filter paper. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were carried out on the PHI5000 Versa Probe XPS spectrometer. The whole processes of sample preparations were conducted under nitrogen gas atmosphere. Thirty μL of DNAPt solution was dropped onto a clean silicon wafer (6 mm × 6 mm), which was rinsed by water, chloroform and ethanol before being used. After the wafer dried, another 30 μL solution was dropped and this operation was repeated thrice. Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The concentration of unbound platinum ions was determined by ICP-OES (Varian 725-ES). Specimens were prepared by filtrating DNA-Pt complexes through a Nanosep omega spin column (3K molecular weight cut off, PALL Life Sciences). The filtrate was analyzed after treating by nitric acid.

EXPERIMENTAL SECTION Chemicals. DNA oligonucleotides AG22 (5′-A(G3T2A)3G33′) and RET2 (5′-GC5(GC4)3T-3′) were purchased from Takara Biotechnology Co. (Dalian, China) and purified by high-performance liquid chromatography (HPLC). All the DNA samples (final strand concentration of 10 μM) were denatured in MES buffer (pH 5.0) at 95 °C for 5 min, and then slowly cooled down to room temperature. K2PtCl4 and H2PtCl6·6H2O were purchased from Alfa Aesar with the purity of 99.9%. Hydrogen peroxide (30 wt %), 3,3′,5,5′-tetramethylbenzidine (TMB) and dimethylamine borane (DMAB) were purchased from J&K Scientific, Heowns and SigmaAldrich, respectively. Preparation of Platinum Nanocatalysts. Platinum nanocatalysts were synthesized through the reduction of K2PtCl4 or H2PtCl6 by DMAB in the presence of DNA template. Oligonucleotides (strand concentration of 10 μM) dissolved in 10 mM MES buffer (pH 5.0) were first incubated with K2PtCl4/H2PtCl6 at the [precursor]/[DNA] ratio of 15 and 30 respectively. After incubation for 12 h, freshly prepared DMAB solution was added to the mixture at the [DMAB]/ [precursor] ratio of 5, to start the reduction reaction. The obtained DNA-Pt complex can be used as nanozyme after 24 h reaction. Reaction Kinetics. Time-dependence of absorption was monitored at 652 nm using UV−vis spectroscopy at 25 °C. In order to detect the peroxidase-like activities of different Pt nanocatalysts, the concentration of TMB and H2O2 was fixed at 125 μM and 125 mM respectively, in citrate buffer at pH 4.0. Then certain amount of DNA-Pt solution was added into the working solution to initiate the reaction. All the experiments were repeated thrice for reproducibility. The initial velocities (v) were calculated according to eqs 1 and 2: A Cp = 652 (1) εL dCp dt

(3)

where v was the initial velocity of the reaction, Vmax was the maximal rate of reaction, [S] was the substrate concentration, and Km was the Michaelis−Menten constant. Km and Vmax were obtained by Lineweaver−Burk plot method according to eq 4:

■

υ=

Vmax[S] K m + [S]

(2)

where Cp represented the concentration of oxTMB, ε was the extinction coefficient of oxTMB, ε = 3.9 × 104 M−1 cm−1, L was the optical path length of 1 cm. In order to calculate the enzymatic parameters of different Pt nanocatalysts, serial solutions of various TMB or H2O2 concentrations were done using citrate buffer of pH 4.0. The kinetic parameters were determined via Michaelis−Menten eq 3:

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RESULTS AND DISCUSSION Interactions of Platinum Precursors with G- and CRich Oligonucleotides. Platinum favors Lewis acid−base interactions with the electron-rich nitrogen atoms of the nucleobases, especially the N7 site of guanine base as well as 18117

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the presence of Pt precursors, the detectable exothermicity mainly attributes to the coordination of Pt2+ to guanine (N7). For the i-motif RET2, the magnitude of exothermic response is 0.04 kcal mol−1, which attributes to the coordination of Pt2+ to cytosine (O2) as well as a partial breaking of C H+ C base pairings. ICP measurements show that the ratios of DNAbound ions and total precursor ions are approximately 38% for AG22-Pt2+ and 17% for RET2-Pt2+, respectively. The binding proportion of Pt4+ is much lower than that of Pt2+, i.e., 6% for AG22 and 5% for RET2 respectively. Combining with CD, FTIR, ITC and ICP characterizations, the binding affinity of Pt2+ ions with G-rich AG22 is much stronger than that of C-rich RET2. Controllable Synthesis of Nanosized Platinum with Different Physicochemical Properties. Dimethylamine borane (DMAB) was utilized as a mild reductant to synthesize Pt cluster chains on long double-stranded DNA template.28 In this study, after adding reductant DMAB to K2PtCl4 and H2PtCl6 solutions, black precipitates form after 1 h incubation, suggesting that precursor ions can be reduced by DMAB to large aggregates without DNA template (shown in Figure S4). According to TEM images, the formed Pt nanoparticles agglomerate into >20 nm large in MES buffer without the protection of DNA template (Figure S5). In contrast, under the protection of DNA, the resulting solutions exhibit light yellow after incubation with DMAB for 24 h. As-prepared Pt vary significantly in their sizes, which are greatly associated with DNA template, precursor, and the molar ratio of precursor ions and DNA strand. Using K2PtCl4 as the precursor, the reduced AG22-Pt shows the average diameter of 1.9 and 2.1 nm respectively, at the [precursor]/[strand] ratio of 15 and 30 (Figure 2a,b). Adopting H2PtCl6 as the precursor at the ratio of 15 and 30 (Figure 2c,d), the average sizes of 1.7 and 1.8 nm are detected for the AG22-Pt, respectively. The RET2-Pt exhibits the average sizes of 2.9 and 2.0 nm respectively, prepared with K2PtCl4 and H2PtCl6 at the [precursor]/[strand] of 30 (Figure 2f,h). Therefore, we denote these Pt nanoparticles as RET2Pt2.9, RET2-Pt2.7, RET2-Pt2.0, RET2-Pt1.9, AG22-Pt2.1, AG22-Pt1.9, AG22-Pt1.8 and AG22-Pt1.7, corresponding to the nanoparticles synthesized at [Pt2+]/[RET2] = 30, [Pt2+]/ [RET2] = 15, [Pt4+]/[RET2] = 30, [Pt4+]/[RET2] = 15, [Pt2+]/[AG22] = 30, [Pt2+]/[AG22] = 15, [Pt4+]/[AG22] = 30 and [Pt4+]/[AG22] = 15 respectively. Compared with ICP and ITC results, G-rich oligonucleotide that provides N7 sites of guanines is susceptible to produce small nanoparticles in this study. Moreover, the average sizes of Pt nanoparticles synthesized by H2PtCl6 are smaller than those synthesized by K2PtCl4. As shown in Figure 3a, UV−vis spectrum of the mixture of AG22 and Pt2+ exhibits a maximal absorbance peak at 258 nm. After incubation with DMAB for 24 h, an apparent increase in absorbance at longer wavelength occurs, indicating the reduction of precursor ions. The absorbance band at 260 nm of the AG22-Pt4+ mixture significantly decreases after adding DMAB (Figure 2b). A disappearance of the shoulder peak around 380 nm and a slight increase at longer wavelength suggest that Pt4+ can be reduced by DMAB in the presence of DNA (inset of Figure 3b). In the presence of i-motif RET2, strong absorption at longer wavelength is detected for Pt2+ after reduction (Figure 3c). Upon addition of DMAB, UV−vis spectrum of the RET2-Pt4+ mixture shows a significant decrease in intensity at 260 nm and a red-shift to 275 nm (the maximal peak of RET2 alone). In the meanwhile, the shoulder peak near

the N3 site of cytosine base, therefore, we choose G-rich oligonucleotide AG22 (5′-A(G3 T2A) 3G3-3′) and C-rich oligonucleotide RET2 (5′-GC5(GC4)3T-3′), as the nucleation templates for Pt precursors including K2PtCl4 and H2PtCl6. AG22 adopts random coil at pH 5.0 without monovalent cations (shown in Figure S1). Upon addition of K2PtCl4 and H2PtCl6 respectively, no secondary structures such as Gquadruplexs can be detected. CD spectrum of RET2 at pH 5.0 shows a positive peak at 290 nm and a negative band around 260 nm, corresponding to the i-motif conformation (Figure S2). The characteristic band at 290 nm exhibits a detectable decrease after incubation with K2PtCl4 and H2PtCl6 respectively, suggesting that the i-motif structure undergoes a conformational change upon addition of precursor ions. Furthermore, FT-IR spectra are collected to deeply investigate the binding sites of precursor ions with DNA. As shown in Figure 1, upon addition of K2PtCl4 to single-stranded AG22,

Figure 1. FT-IR spectra of 200 μM AG22 (a) and RET2 (b) in the absence and presence of Pt2+ and Pt4+ respectively.

the C8N7 vibration of guanine at 1488 cm−1 disappears. In the meanwhile, the absorption bands at 1691 cm−1 assigned to the C6O6 vibration of guanine remains unchangeable, suggesting that Pt2+ ions bind to N7 sites of guanine bases of AG22 sequence.25 The 1250−1000 cm−1 region which represents sugar−phosphate vibrations maintains, indicating that Pt2+ specifically binds to bases instead of sugar and phosphate backbone. In the presence of H2PtCl6, the band at 1488 cm−1 is visually weakened, while the band at 1691 cm−1 maintains. FT-IR spectrum of RET2 at pH 5.0 exhibits a band at 1727 cm−1 and a broad absorption centered at 1665 cm−1, corresponding to the C2O2 stretching of cytosine in the imotif formation.26 Upon addition of K2PtCl4 and H2PtCl6, the band at 1727 cm−1 blue-shifts to 1719 and 1725 cm−1, while the band around 1665 cm−1 shift to 1656 and 1655 cm−1 respectively, suggesting that both Pt2+ and Pt4+ primarily interact with O2 sites of cytosine bases. In ITC measurements, the extent of exothermicity recorded with the first few injections is recognized as the measure of different strength of interaction. 27 The magnitude of exothermic response is detected as 0.38 kcal mol−1 for singlestranded AG22, at the time of the first injection of K2PtCl4 to DNA. The heat evolved during the subsequent binding events decreases and reaches saturation at the equimolar ratio of DNA strand and K2PtCl4, which is an indicative of complete binding (Figure S3). Since no secondary structure forms for AG22 in 18118

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Figure 2. TEM images of Pt nanoparticles synthesized through Pt2+and Pt4+ reduced by DMAB in the presence of AG22 and RET2 at pH 5.0: (a) AG22-Pt1.9, (b) AG22-Pt2.1, (c) AG22-Pt1.7, (d) AG22-Pt1.8, (e) RET2-Pt2.7, (f) RET2-Pt2.9, (g) RET2-Pt1.9, (h) RET2-Pt2.0; [DMAB]/ [precursor] = 5, DNA strand concentration = 10 μM.

fraction of Pt was in the ionic form. A new band near 290 nm was detected in the UV spectrum, which might be assigned to the formation of binuclear Pt2+-Pt2+ clusters. Since DNA exhibits strong UV absorption in the range of 290−300 nm, it is

380 nm disappears and the absorption at longer wavelength occurs, suggesting the reduction of Pt4+ (Figure 3d). Somorjai and co-workers reported that bound-Pt2+ inside PAMAM G4OH was only partially reduced by NaBH4 and the largest 18119

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Figure 3. UV−vis spectra of 10 Mm AG22 (a, b) and RET2 (c, d) after incubation with 300 mM Pt2+ and Pt4+ for 12 h, and DNA-Pt after reduction for 24 h ([DMAB]/[precursor] = 5); the blue and the cyan lines represent the spectra of precursor ions and DMAB.

not possible to detect binuclear clusters in these experiments. Tetranuclear clusters can not be found in this study since these compounds exhibit characteristic absorption band at 347−375 nm.29 However, the proportion of precursor ions binding to DNA sequence in the incubation time can not be quantitatively determined by UV spectroscopy. To understand deeply the surface properties of as-prepared DNA-Pt complexes, the charge states of Pt species are investigated by X-ray photoelectron spectroscopy (XPS). In analyzing spectra, it was assumed that the doublets have Gaussian shaped (5/2−7/2) components of equal half-widths with an intensity ratio of ca. 3:4, and the Pt(4f) spin splitting energy was about 3.3 eV.30 As shown in Figure 4, the binding energy of the electron on the Pt 4f 7/2 orbitals of AG22-Pt1.8 can be deconstructed into Pt2+ and Pt0 components with binding energies of 72.8 eV (∼84%) and 71.4 eV (∼16%), respectively, suggesting that Pt4+ ions are reduced by DMAB to Pt2+ and Pt0. In the case of RET2-Pt2.9, the fraction of Pt0 and Pt2+ is determined as 66% and 34% respectively. The molar ratios of [Pt0]/[Pt2+] increase significantly with increasing the particle sizes. In order to eliminate the influence of unbound platinum ions, ICP analysis after filtration show that approximately 92% and 56% of precursor ions participate in the formation of Pt nanoparticles after the reduction of RET2Pt2+ and AG22-Pt4+ respectively, resulting in the particle sizes of 2.9 and 1.8 nm. Therefore, it is suggested that the unbound precursor ions are involved in the formation of Pt nanoparticles. Although it is difficult to determine the binding affinity of reduced Pt atom with DNA by experimental method, it is reasonable to conclude that the reduced Pt can bind to the DNA template without further agglomeration since the electron-rich sites have not been totally occupied by the precursor ions. It also can be deduced that a certain proportion of incompletely reduced platinum are associated with DNA templates since Pt2+-coordination with electron-rich nitrogen/ oxygen atoms of DNA bases hinders the reduction process.

Figure 4. Pt 4f XPS spectra of different Pt nanoparticles: black, red, blue and cyan lines represent the raw curve, the fitted curve, the Pt2+ and the Pt0 components’ curves, respectively.

The reduction of Pt2+ species has been reported to be hindered by the multidentate coordination of Pt2+ with amide groups.29,31 Three types of Pt were detected for Pt nanoclusters templated by PAMAM G4OH: Pt2+ cations that chelate strongly with the stabilizing agent, binuclear Pt2+-Pt2+ as well as Pt colloids. For other metal nanoclusters, Desireddy et al. reported that p-mercaptobenzoic acid-stabilized silver nanoclusters contain a shell of protecting layer through coordination interactions between silver and thiolate, consisting Ag2S5 capping structures.32 Lysozyme-stabilized gold fluorescent nanoclusters with an average size of 1 nm was a mixture of Au0 and Au+, of which the Au+ species (about 24%) could be 18120

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assigned to the gold atoms on the surface ligated with lysozyme.33 Moreover, a core−shell model of Ag(0)NCs@ Ag(I)-carboxylate complex comprised of 30% Ag(I) and 70% Ag(0), was recently proposed for fluorescent silver nanoclusters templated by poly(methacrylic acid) with the sizes of 2−5 nm.34 Combining with XPS and ICP studies, it is demonstrated that Pt2+ species exist both on the DNA template and the particle surface. Size-Dependent Activities of Pt Nanocatalysts as Enzyme Mimetics. In order to investigate peroxidase mimicking activities of DNA-Pt complexes with different physicochemical properties, we choose 3,3′,5,5′-tetramethylbenzidine (TMB) and hydrogen peroxide (H2O2) as the peroxidase substrates, and all the reaction kinetics are monitored at 652 nm corresponding to oxidized TMB. The as-prepared Pt nanoparticles possess peroxidase-like activities in the pH range of 3−6, and have the highest activity at pH 4.0 (Figure S6). Moreover, these nanoparticles exhibit highly peroxidase-like activities in the temperature range of 37−60 °C (Figure S7). As shown in Figure 5, the reaction rates of these Pt nanocatalysts decrease in the following order: RET2-Pt2.9 > RET2-Pt2.7 > AG22-Pt2.1 > RET2-Pt2.0 > RET2-Pt1.9 > AG22-Pt1.9 > AG22-Pt1.8 > AG22-Pt1.7. It is suggested that peroxidase-like activities of Pt nanocatalysts are greatly dependent upon the preparation conditions. The initial rates sharply decrease as the particle size decreases. For example, the v value of RET2-Pt2.9 prepared with RET2 at the [Pt2+]/ [strand] of 30 is determined as 0.492 μMs1−, which is almost 50 times higher than that of AG22-Pt1.7 prepared with AG22 at [Pt4+]/[strand] of 15. In contrast, the reaction rates are calculated as 0.003, 0.001, 0.0016, and 0.0007 μMs1− for AG22Pt2+, AG22-Pt4+, RET2-Pt2+ and RET2-Pt4+ without reduction, respectively. Therefore, it is reasonable to conclude that the reduced Pt0 species in the DNA-Pt complexes mainly contribute to their catalytic activities. For further analyzing the catalytic mechanism and acquiring the kinetic parameters, we further investigate steady-state kinetics for the oxidation of TMB in the presence of hydrogen peroxide catalyzed by DNA-Pt nanozymes. Within the suitable range of TMB and H2O2, typical Michaelis−Menten curves are received for three kinds of Pt nanoparticles with the sizes of 2.9, 2.1, and 1.8 nm respectively (Figure 6, Figure S8 and S9). The data are fitted to Lineweaver−Burk equation to obtain the important enzyme kinetic parameters such as Michaelis− Menten constant (Km) and maximal velocity (Vmax) listed in Table 1. The apparent Km value of RET2-Pt2.9 with TMB as the substrate is eight times lower than that for HRP, suggesting that RET2-Pt2.9 possesses a higher affinity to TMB than the native enzyme.35 The Km with H2O2 as the substrate is ten times higher than that for HRP. With decreasing the size from 2.9 to 1.8 nm, the Km with TMB decreases from 0.056 to 0.0162 mM. Oppositely, the Km with H2O2 increases from 48.0 to 117.2 mM. Therefore, it is suggested that small-sized Pt has high affinity to TMB and low affinity to H2O2. Since Michaelis−Menten constant is independent of the enzyme’s concentration, free Pt2+ ions have little influence on these enzymatic kinetic parameters. The Vmax value of RET2-Pt2.9 is detected as 30 times higher than that of AG22-Pt1.8 with TMB as the substrate, indicating that RET2-Pt2.9 exhibits the highest catalytic efficiency among these DNA-based Pt nanozymes. For other peroxidase nanomimetics, a kind of irregular-shaped Pt nanoparticles with a mean length of 7.0 nm and a narrowing width from 2.0 to 5.0 nm along longitudinal axes, exhibited the

Figure 5. UV−vis absorption-time course curves of TMB-H2O2 reaction system (pH 4.0) catalyzed by different AG22-Pt (a) and RET2-Pt (b) At 25 °C, Pt concentration in the catalytic system is fixed at 900 nM (calculated from Pt precursors); (c) the initial velocities (V) of this reaction catalyzed by different DNA-Pt, the concentration of TMB and H2O2 is 0.125 mM and 125 mM respectively, error bars represent standard deviations from three repeated experiments.

affinities of 769 mM and 0.12 mM for H2O2 and TMB, respectively.36 Apoferritin-encapsulated Pt nanoclusters of 1−2 nm had a Km of 0.22 mM toward TMB and 187.25 mM toward H2O2.15 Therefore, RET2-Pt2.9 stabilized by i-motif DNA exhibits much higher affinities toward H2O2 and TMB compared to other Pt nanozymes. Combining with previous reports on other metals and metal oxides (listed in Table 2), Pt nanomaterials templated by G-/C-rich oligonucleotide are promising nanozymes for peroxidise mimetics. In the absence of H2O2, TMB can also be oxidized to a blue color product in the presence of RET2-Pt2.9, RET2-Pt2.7 and AG22-Pt2.1 respectively, demonstrating oxidase mimicking activities of DNA-Pt with the size >2 nm (Figure S10 and S11). In nature, catalase is employed as the most efficient enzyme for the conversion of hydrogen peroxide to molecular oxygen. A further experiment is performed to study oxygen generation as a product of H2O2 decomposition by different DNA-Pt using a dissolved oxygen meter (Figure 7). Adopting RET2-Pt2.9, the dissolved oxygen concentration sharply increases in the first 5 min and reaches to a level of 38.2 18121

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Figure 6. Steady-state kinetic assay and catalytic mechanism of RET2-Pt2.9, the initial velocities in the oxidization of TMB in the presence of H2O2 are measured using 900 nM Pt (calculated from Pt precursors) at pH 4.0 at 25 °C: (a) the concentration of H2O2 is fixed at 125 mM and the TMB concentration is varied, (c) the concentration of TMB is fixed at 0.125 mM and the H2O2 concentration is varied, (b) and (d) are the doublereciprocal plots of (a) and (c), respectively.

Table 1. Kinetic Parameters of Different DNA-Pt Nanozymes Prepared in This Study As Peroxidase Mimetics nanozyme

substrate

Km (mM)

RET2-Pt2.9

TMB H2O2 TMB H2O2 TMB H2O2

0.0560 48.0 0.0329 74.4 0.0162 117.2

AG22-Pt2.1 AG22-Pt1.8

Vmax (Ms−1) 5.82 5.68 1.19 3.05 1.93 5.19

× × × × × ×

10−7 10−7 10−7 10−7 10−8 10−8

Table 2. Comparison of the Kinetic Parameters of Different Nanomaterials That Mimic Peroxidase enzyme (temp/°C) Pt nanotubes (20 °C) [email protected] (30 °C)38 Cu nanoclusters (30 °C)14 37

Au nanoclusters (40 °C)16 DNA-Pt complexes (25 °C)39 Fe3O4 nanoparticles (40 °C)35 CuO nanoparticles (37 °C)40 Co3O4 nanocubes (25 °C)41 RuO2 nanoparticles42 RET2-Pt2.9 (25 °C, this study)

substrate

Km (mM)

TMB TMB TMB H2O2 TMB H2O2 TMB TMB H2O2 TMB H2O2 TMB H2O2 TMB H2O2 TMB H2O2

0.0186 0.027 0.648 29.16 0.002 53 25.3 4.88 0.098 154 0.016 41.0 0.037 140.07 0.236 212 0.0560 48.0

Vmax (Ms−1) 1.18 1.81 5.96 4.22 6.23 7.21 3.44 9.78

6.27 1.21

5.82 5.68

× × × × × × / × × / / × × / / × ×

Figure 7. Effect of different pt nanoparticles on the generation of O2 through decomposition of H2O2, reaction conditions: 250 mM H2O2 and 3 μM Pt (calculated from Pt precursors) in 10 mM PBS buffer (pH 7.4).

10−7 10−7 10−8 10−8 10−8 10−8

mg/L at 10 min, providing direct evidence on the catalase-like activity of RET2-Pt2.9. At 20 min, the O2 concentrations are detected as 14.9, 11.4, and 7.3 mg/L for AG22-Pt2.1, RET2Pt2.0 and AG22-Pt1.8, respectively. Therefore, the particle sizes and the catalytic activities exhibit similar relationships among peroxidase, oxidase and catalase mimetics, further confirming the critical role of metallic Pt0 species on the activation of hydrogen peroxide or molecular oxygen. Relationship between Catalytic Performance and Physicochemical Property of Pt Nanozyme. Though hydrogen peroxide is stable and less active, it can be converted into highly active •OH through Fenton chemistry. An Eley− Rideal mechanism was previously proposed for Pt nanoparticles as peroxidase mimetics: First, H2O2 molecules were initially

10−8 10−8

10−8 10−7

10−7 10−7

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Pt4+ ion mainly binds to N7 site of guanine of the singlestranded AG22, while it primarily interacts with O2 site of cytosine of the i-motif RET2. The physicochemical properties of as-prepared Pt nanoparticles including charge states and particle sizes are greatly associated with the DNA templates, the precursor ions as well as the molar ratios of [precursor]/ [DNA]. In the presence of DNA template, precursor ions can be partially reduced to Pt nanoparticles consisting of Pt0 and Pt2+. Since the affinity of Pt2+ to N7 site of guanine of AG22 is higher than that to O2 site of cytosine of RET2, AG22-Pt complex contains higher proportion of Pt2+ species and exhibits smaller particle size. As increasing the average size from 1.8 to 2.9 nm, the fraction of metallic Pt0 atoms increases, which mainly contribute to enzymatic activities of Pt nanozymes. In the meanwhile, the Km value toward H2O2 decreases by three times, whereas toward TMB the Km increases by two times. The most efficient Pt nanozyme is stabilized by i-motif RET2 with the average size of 2.9 nm.

adsorbed on the surface of Pt nanocrystals, and the oxygen− oxygen bond of H2O2 was rapidly broken by catalytic active Pt to give •OH, which was stabilized on the nanoparticles’ surface via partial electron exchange interactions between the unpaired electrons of the adsorbed radicals and the conduction-band electrons of the metallic particles.36,43−45 Following that, TMB can be oxidized by •OH to form a blue color product. According to XPS results and kinetic parameters, it can be concluded that the affinity to H2O2 increases with the proportion of Pt0 species on the Pt surface. Pt0 species are mainly involved in the activation of H2O2 since the DNA-Pt containing a larger proportion of Pt0 exhibits higher affinity to H2O2. In contrast, toward another substrate TMB (a typical HRP substrate), the affinity increases with the proportion of Pt2+ species. For DNA-based Pt nanozymes, the Km values toward H2O2 and TMB change oppositely with the particle size, however, the catalytic performance in the oxidation of TMB mainly depends upon the efficiencies on the H2O2-activation. Platinum-based nanomaterials have been recognized as one of the most effective catalysts for oxidation reaction, therefore, many efforts are devoted to control the particle sizes and the nanostructures of platinum by using small ligands, polymer, dendrimer, protein, nucleic acids. Previous studies have reported several approaches on the synthesis of Pt nanoparticles/nanoclusters templated by oligonucleotides including [A]20, [G]20, [C]20, [T]20, etc., as well as natural doublestranded DNA.46−48 For example, Kim and co-workers synthesized Pt-nanodendrite/DNA/reduced-graphene-oxide hybrid by the NaBH4 reduction of H2PtCl6 in the presence of double-stranded DNA, and these hybrids displayed higher ORR catalytic activities than industrially adopted catalysts.49 For other investigations on the size effects of Pt nanocatalysts, Arnby et al. synthesized two Pt nanoparticles dispersed on Al2O3 with the average diameter of 5.6 and 2.0 nm, respectively. In the oxidation reactions such as those for CO and different kinds of hydrocarbons, larger nanoparticles exhibited more active in oxidizing environments since more Pt species remained metallic compared to smaller ones.50 In Crooks’s studies, the largest dendrimer-Pt nanoparticles with 240 atoms on average showed the highest specific activities for the oxygen reduction reaction, whereas the smaller nanoparticles exhibited slower ORR kinetics.51 In the case of the charge states of Pt nanoparticles, Şen et al. reported that a positive shift from Pt0 to Pt4+ of Pt catalyst resulted in a decrease in catalytic activity of methanol oxidation reaction.30 In this research, the substantial difference among DNA-based Pt nanozymes is the charge state of platinum on the surface. The fraction of metallic Pt atoms mainly contributes to their peroxidase-like activities. Compared to the i-motif RET2, Grich AG22 chelates with more precursor ions, therefore, after reduction the proportion of Pt2+ species is higher, which result in lower affinity toward H2O2. Therefore, it is demonstrated that Pt nanoparticles consisting of more metallic Pt0 species possess higher enzymatic activities compared to the smaller nanoclusters. Although DNA is a promising candidate for the synthesis of Pt nanozyme, more attention should be paid for balancing the charge state and the particle size.

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ASSOCIATED CONTENT

S Supporting Information *

CD spectroscopy, ITC measurements as well as enzymatic kinetics studies of DNA-based Pt nanozymes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-22-27890643. Fax: +86-22-27890643. E-mail: liwei@ tju.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21276179, 21206107). REFERENCES

(1) He, W.; Zhou, Y.-T.; Wamer, W. G.; Hu, X.; Wu, X.; Zheng, Z.; Boudreau, M. D.; Yin, J.-J. Intrinsic Catalytic Activity Of Au Nanoparticles With Respect To Hydrogen Peroxide Decomposition And Superoxide Scavenging. Biomaterials 2013, 34, 765−773. (2) Liu, S.; Lu, F.; Xing, R.; Zhu, J.-J. Structural Effects Of Fe3O4 Nanocrystals On Peroxidase-Like Activity. Chem.Eur. J. 2011, 17, 620−625. (3) Zhang, W.; Liu, X.; Walsh, D.; Yao, S.; Kou, Y.; Ma, D. CagedProtein-Confined Bimetallic Structural Assemblies With Mimetic Peroxidase Activity. Small 2012, 8, 2948−2953. (4) Song, Y.; Qu, K.; Zhao, C.; Ren, J.; Qu, X. Graphene Oxide: Intrinsic Peroxidase Catalytic Activity And Its Application To Glucose Detection. Adv. Mater. 2010, 22, 2206−2210. (5) Liu, M.; Zhao, H. M.; Chen, S.; Yu, H. T.; Quan, X. Interface Engineering Catalytic Graphene For Smart Colorimetric Biosensing. ACS Nano 2012, 6, 3142−3151. (6) Guo, A.; Wu, D.; Ma, H.; Zhang, Y.; Li, H.; Du, B.; Wei, Q. An Ultrasensitive Enzyme-Free Electrochemical Immunosensor For CA125 Using Au@Pd Core−Shell Nanoparticles As Labels And Platforms For Signal Amplification. J. Mater. Chem. B 2013, 1, 4052− 4058. (7) Tao, Y.; Lin, Y.; Huang, Z.; Ren, J.; Qu, X. Incorporating Graphene Oxide And Gold Nanoclusters: A Synergistic Catalyst With Surprisingly High Peroxidase-Like Activity Over A Broad Ph Range And Its Application For Cancer Cell Detection. Adv. Mater. 2013, 25, 2594−2599. (8) Chen, J. P.; Patil, S.; Seal, S.; Mcginnis, J. F. Rare Earth Nanoparticles Prevent Retinal Degeneration Induced By Intracellular Peroxides. Nat. Nanotechnol. 2006, 1, 142−150.

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CONCLUSION G-rich AG22 (5′-A(G3T2A)3G3-3′) and C-rich RET2 (5′GC5(GC4)3T-3′) are chosen as the nucleation templates to synthesize Pt nanozymes with the size distribution of 1.7−2.9 nm, showing high activity to mimic peroxidase. Either Pt2+ or 18123

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(9) Huang, D. M.; Hsiao, J. K.; Chen, Y. C.; Chien, L. Y.; Yao, M.; Chen, Y. K.; Ko, B. S.; Hsu, S. C.; Tai, L. A.; Cheng, H. Y.; et al. The Promotion Of Human Mesenchymal Stem Cell Proliferation By Superparamagnetic Iron Oxide Nanoparticles. Biomaterials 2009, 30, 3645−3651. (10) Zhang, J. B.; Zhuang, J.; Gao, L. Z.; Zhang, Y.; Gu, N.; Feng, J.; Yang, D. L.; Zhu, J. D.; Yan, X. Y. Decomposing Phenol By The Hidden Talent Of Ferromagnetic Nanoparticles. Chemosphere 2008, 73, 1524−1528. (11) Wei, H.; Wang, E. Nanomaterials With Enzyme-Like Characteristics (Nanozymes): Next-Generation Artificial Enzymes. Chem. Soc. Rev. 2013, 42, 6060−6093. (12) Wang, X.; Zhang, Y.; Li, T.; Tian, W.; Zhang, Q.; Cheng, Y. Generation 9 Polyamidoamine Dendrimer Encapsulated Platinum Nanoparticle Mimics Catalase Size, Shape, And Catalytic Activity. Langmuir 2013, 29, 5262−5270. (13) Hamasaki, T.; Kashiwagi, T.; Imada, T.; Nakamichi, N.; Aramaki, S.; Toh, K.; Morisawa, S.; Shimakoshi, H.; Hisaeda, Y.; Shirahata, S. Kinetic Analysis Of Superoxide Anion Radical-Scavenging And Hydroxyl Radical-Scavenging Activities Of Platinum Nanoparticles. Langmuir 2008, 24, 7354−7364. (14) Hua, L.; Yuan, Y.; Zhang, L.; Zhao, J.; Majeed, S.; Xua, G. Copper Nanoclusters As Peroxidase Mimetics And Their Applications To H2O2 And Glucose Detection. Anal. Chim. Acta 2013, 762, 83− 86. (15) Fan, J.; Yin, J.-J.; Ning, B.; Wu, X.; Hud, Y.; Ferrari, M.; Anderson, G. J.; Wei, J.; Zhao, Y.; Nie, G. Direct Evidence For Catalase And Peroxidase Activities Of Ferritine Platinum Nanoparticles. Biomaterials 2011, 32, 1611−1618. (16) Wang, X.-X.; Wu, Q.; Shan, Z.; Huang, Q.-M. BSA-Stabilized Au Clusters As Peroxidase Mimetics For Use In Xanthine Detection. Biosens. Bioelectron. 2011, 26, 3614−3619. (17) Berti, L.; Burley, G. A. Nucleic Acid And Nucleotide-Mediated Synthesis Of Inorganic Nanoparticles. Nat. Nanotechnol. 2008, 3, 81− 87. (18) Fu, Y.; Wang, X.; Zhang, J.; Li, W. Nanomaterials And Nanoclusters Based On DNA Modulation. Curr. Opin. Biotechnol. 2014, 28, 33−38. (19) Fu, Y.; Zhang, J.; Chen, X.; Huang, T.; Duan, X.; Li, W.; Wang, J. Silver Nanomaterials Regulated By Structural Competition Of G-/CRich Oligonucleotides. J. Phys. Chem. C 2011, 115, 10370−10379. (20) Wu, J.; Fu, Y.; He, Z.; Han, Y.; Zheng, L.; Zhang, J.; Li, W. Growth Mechanisms Of Fluorescent Silver Clusters Regulated By Polymorphic DNA Templates: A DFT Study. J. Phys. Chem. B 2012, 116, 1655−1665. (21) Wang, Z.; Tang, L.; Tan, L. H.; Li, J.; Lu, Y. Discovery Of The DNA “Genetic Code” For Abiological Gold Nanoparticle Morphologies. Angew. Chem., Int. Ed. 2012, 51, 9078−9082. (22) Tiwari, J. N.; Nath, K.; Kumar, S.; Tiwari, R. N.; Kemp, K. C.; Le, N. H.; Youn, D. H.; Lee, J. S.; Kim, K. S. Stable Platinum Nanoclusters On Genomic DNA−Graphene Oxide With A High Oxygen Reduction Reaction Activity. Nat. Commun. DOI: 10.1038/ Ncomms3221. (23) Li, W.; Liu, L.; Fu, Y.; Sun, Y.; Zhang, J.; Zhang, R. Effects Of Polymorphic DNA On The Fluorescent Properties Of Silver Nanoclusters. Photochem. Photobiol. Sci. 2013, 12, 1864−1872. (24) Zhang, J.; Wang, X.; Fu, Y.; Han, Y.; Cheng, J.; Zhang, Y.; Li, W. Highly Active Subnano Palladium Clusters Embeded In I-Motif DNA. Langmuir 2013, 29, 14345−14350. (25) Banyay, M.; Sarkar, M.; Gräslund, A. A Library Of IR Bands Of Nucleic Acids In Solution. Biophys. Chem. 2003, 104, 477−488. (26) Rosu, F.; Gabelica, V.; Joly, L.; Grégoire, G.; Pauw, E. D. Zwitterionic I-Motif Structures Are Preserved In DNA Negatively Charged Ions Produced By Electrospray Mass Spectrometry. Phys. Chem. Chem. Phys. 2010, 12, 13448−13454. (27) Shukla, S.; Sastry, M. Probing Differential Ag+-Nucleobase Interactions With Isothermal Titration Calorimetry (ITC): Towards Patterned DNA Metallization. Nanoscale 2009, 1, 122−127.

(28) Seidel, R.; Ciacchi, L. C.; Weigel, M.; Pompe, W.; Mertig, M. Synthesis Of Platinum Cluster Chains On DNA Templates: Conditions For A Template-Controlled Cluster Growth. J. Phys. Chem. B 2004, 108, 10801−10811. (29) Borodko, Y.; Thompson, C. M.; Huang, W.; Yildiz, H. B.; Frei, H.; Somorjai, G. A. Spectroscopic Study Of Platinum And Rhodium Dendrimer (PAMAM G4OH) Compounds: Structure And Stability. J. Phys. Chem. C 2011, 115, 4757−4767. (30) Şen, F.; Gökağac, G. Different Sized Platinum Nanoparticles Supported On Carbon: An XPS Study On These Methanol Oxidation Catalysts. J. Phys. Chem. C 2007, 111, 5715−5720. (31) Knecht, M. R.; Weir, M. G.; Myers, V. S.; Pyrz, W. D.; Ye, H.; Petkov, V.; Buttrey, D. J.; Frenkel, A. I.; Crooks, R. M. Synthesis And Characterization Of Pt Dendrimer-Encapsulated Nanoparticles: Effect Of The Template On Nanoparticle Formation. Chem. Mater. 2008, 20, 5218−5228. (32) Desireddy, A.; Conn, B. E.; Guo, J.; Yoon, B.; Barnett, R. N.; Monahan, B. M.; Kirschbaum, K.; Griffith, W. P.; Whetten, R. L.; Landman, U.; Bigioni, T. P. Ultrastable Silver Nanoparticles. Nature Doi:10.1038/Nature12523. (33) Wei, H.; Wang, Z.; Yang, L.; Tian, S.; Hou, C.; Lu, Y. LysozymeStabilized Gold Fluorescent Cluster: Synthesis And Application As Hg2+ Sensor. Analyst 2010, 135, 1406−1410. (34) Chen, Y.; Yang, T.; Pan, H.; Yuan, Y.; Chen, L.; Liu, M.; Zhang, K.; Zhang, S.; Wu, P.; Xu, J. Photoemission Mechanism Of WaterSoluble Silver Nanoclusters: Ligand-To-Metal−Metal Charge Transfer Vs Strong Coupling Between Surface Plasmon And Emitters. J. Am. Chem. Soc. 2014, 136, 1686−1689. (35) Gao, L.; Zhuang, J.; Nie, L.; Zhang, J.; Zhang, Y.; Gu, N.; Wang, T.; Feng, J.; Yang, D.; Perrett, S.; Yan, X. Intrinsic Peroxidase-Like Activity Of Ferromagnetic Nanoparticles. Nat. Nanotechnol. 2007, 2, 577−583. (36) Gao, Z.; Xu, M.; Hou, L.; Chen, G.; Tang, D. Irregular-Shaped Platinum Nanoparticles As Peroxidase Mimics For Highly Efficient Colorimetric Immunoassay. Anal. Chim. Acta 2013, 776, 79−86. (37) Cai, K.; Lv, Z.; Chen, K.; Huang, L.; Wang, J.; Shao, F.; Wang, Y.; Han, H. Aqueous Synthesis Of Porous Platinum Nanotubes At Room Temperature And Their Intrinsic Peroxidase-Like Activity. Chem. Commun. 2013, 49, 6024−6026. (38) He, W.; Liu, Y.; Yuan, J.; Yin, J.-J.; Wu, X.; Hu, X.; Zhang, K.; Liu, J.; Chen, C.; Ji, Y.; Guo, Y. Au@Pt Nanostructures As Oxidase And Peroxidase Mimetics For Use In Immunoassays. Biomaterials 2011, 32, 1139−1147. (39) Higuchi, A.; Siao, Y.-D.; Yang, S.-T.; Hsieh, P.-V.; Fukushima, H.; Chang, Y.; Ruaan, R.-C.; Chen, W.-Y. Preparation Of A DNA Aptamer-Pt Complex And Its Use In The Colorimetric Sensing Of Thrombin And Anti-Thrombin Antibodies. Anal. Chem. 2008, 80, 6580−6586. (40) Chen, W.; Chen, J.; Feng, Y.-B.; Hong, L.; Chen, Q.-Y.; Wu, L.F.; Lin, X.-H.; Xia, X.-H. Peroxidase-Like Activity Of Water-Soluble Cupric Oxide Nanoparticles And Its Analytical Application For Detection Of Hydrogen Peroxide And Glucose. Analyst 2012, 137, 1706−1712. (41) Mu, J.; Wang, Y.; Zhao, M.; Zhang, L. Intrinsic Peroxidase-Like Activity And Catalase-Like Activity Of Co3O4 Nanoparticles. Chem. Commun. 2012, 48, 2540−2542. (42) Deng, H.; Shen, W.; Peng, Y.; Chen, X.; Yi, G.; Gao, Z. Nanoparticulate Peroxidase/Catalase Mimetic And Its Application. Chem.Eur. J. 2012, 18, 8906−8911. (43) Ma, M.; Zhang, Y.; Gu, N. Peroxidase-Like Catalytic Activity Of Cubic Pt Nanocrystals. Colloids Surf. A: Physicochem. Eng. Aspects 2011, 373, 6−10. (44) Xu, S.-L.; Cui, H. Luminol Chemiluminescence Catalysed By Colloidal Platinum Nanoparticles. Luminescence 2007, 22, 77−87. (45) Zhang, Z.; Berg, A.; Levanon, H.; Fessenden, R. W.; Meisel, D. On The Interactions Of Free Radicals With Gold Nanoparticles. J. Am. Chem. Soc. 2003, 125, 7959−7963. 18124

dx.doi.org/10.1021/jp503242e | J. Phys. Chem. C 2014, 118, 18116−18125

The Journal of Physical Chemistry C

Article

(46) Matsuoka, Y.; Onodera, T.; Kojima, T.; Chang, Y.; Chen, W.-Y.; Imanaka, T.; Fukushima, H.; Higuchi, A. Novel Enzymatic Properties Of DNA-Pt Complexes. Biomacromolecules 2007, 8, 2684−2688. (47) Chen, X.; Zhou, X.; Hu, J. Pt−DNA Complexes As Peroxidase Mimetics And Their Applications In Colorimetric Detection Of H2O2 And Glucose. Anal. Methods 2012, 4, 2183−2187. (48) Tang, L.; Guo, X.; Li, Y.; Zhang, S.; Zha, Z.; Wang, Z. Pt, Pd And Au Nanoparticles Supported On A DNA−MMT Hybrid: Efficient Catalysts For Highly Selective Oxidation Of Primary Alcohols To Aldehydes, Acids And Esters. Chem. Commun. 2013, 49, 5213− 5215. (49) Tiwari, J. N.; Kemp, K. C.; Nath, K.; Tiwari, R. N.; Nam, H.-G.; Kim, K. S. Interconnected Pt-Nanodendrite/DNA/Reduced-Graphene-Oxide Hybrid Showing Remarkable Oxygen Reduction Activity And Stability. ACS Nano 2013, 7, 9223−9231. (50) Arnby, K.; Assiks, J.; Carlsson, P.-A.; Palmqvist, A.; Skoglundh, M. The Effect Of Platinum Distribution In Monolithic Catalysts On The Oxidation Of CO And Hydrocarbons. J. Catal. 2005, 233, 176− 185. (51) Ye, H.; Crooks, J. A.; Crooks, R. M. Effect Of Particle Size On The Kinetics Of The Electrocatalytic Oxygen Reduction Reaction Catalyzed By Pt Dendrimer-Encapsulated Nanoparticles. Langmuir 2007, 23, 11901−11906.

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