Formation of PtCuCo Trimetallic Nanostructures with Enhanced

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Formation of PtCuCo Trimetallic Nanostructures with Enhanced Catalytic and Enzyme-like Activities for Biodetection Weiwei He, Junhui Cai, Hui Zhang, Lixia Zhang, Xiaowei Zhang, Jing Li, and Jun-Jie Yin ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00109 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Nano Materials

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Formation of PtCuCo Trimetallic Nanostructures with

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Enhanced Catalytic and Enzyme-like Activities for

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Biodetection

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Weiwei He†*, Junhui Cai†, Hui Zhang§, Lixia Zhang†, Xiaowei Zhang‡, Jing Li†, Jun-Jie Yin §*

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†

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College of Advanced Materials and Energy, Institute of Surface Micro and Nano Materials, Xuchang

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University, Xuchang, Henan 461000, P. R. China

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‡

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Key Laboratory of Micro-Nano Materials for Energy Storage and Conversion of Henan Province,

Food and Bioengineering College, Xuchang University, Xuchang, Henan 461000, P. R. China

§

Division of Bioanalytical Chemistry and Division of Analytical Chemistry, Office of Regulatory

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Science, Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, College

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Park, Maryland 20740, USA

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ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: The unique enzyme-like properties of metal nanoparticles (NPs) hold great promise for

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chemical and biomedical applications; implementation relies on improvements in their catalytic

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efficiency. We have constructed PtCuCo nanostructures using hydrothermal treatment of Co3O4,

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Cu2+ and Pt2+ in aqueous solutions, resulting in PtCuCo trimetallic alloy NPs having hollow

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structures, as confirmed by TEM, XRD, XPS, and EDS analyses. These PtCuCo NPs are capable of

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oxidizing TMB in the either absence or presence of hydrogen peroxide, suggesting efficient

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peroxidase- and oxidase-like activity. These NPs could also catalyze the reduction of 4-nitrophenol

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by NaBH4. We believe the combination of Pt, oxidized Cu and Co species result in surfaces with rich

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active sites possessing a wide range of electronic variation, which promotes both generation of

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hydroxyl radicals and electron transfer. Consequently, the catalytic capabilities of our PtCuCo NPs

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are greater than either Pt or PtCu NPs. Building upon the peroxidase-like activity of PtCuCo NPs, we

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developed a colorimetric method capable of not only specifically detecting glucose, but also able to

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detect rongalite and HS-, based on the inhibitory effect of these substances upon PtCuCo

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nano-peroxidase.

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KEYWORDS: PtCuCo, trimetallic nanostructures, peroxidase-like activity, catalytic reduction,

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biodetection

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INTRODUCTION

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The superior catalytic properties of Pt based nanostructures have wide applications in fuel cells,

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chemical synthesis and biocatalysis.1-3 These catalytic properties are strongly dependent on the size,

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shape, structure, and composition of specific nanostructures; each feature provide additional avenues

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for optimizing catalytic performance in specific applications. Consequently, remarkable efforts have

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been made to design Pt-based nanoparticles (NPs) of specific sizes,4 morphologies (spherical, cubic,

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wire and dendrite-like shapes),5-7 composed of multi-metal alloys (AuPt, AgPt, PdPt, PtPdCu).8-11

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Another motivation for creating multi-metal alloys is cost: although Pt offers superior

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electrocatalytic activity, it is an expensive and limited resource. Rationally-designed catalysts should

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be optimized for high catalytic performance using as little Pt as possible. Therefore, multi-metallic

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hollow nanostructures may be ideal. These would provide good atom economy, low density, large

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specific surface areas, lower catalyst costs, high reactivity at both corners and edges, and additional

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synergetic effects may be brought by different components.12,13 Furthermore, by refining the

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chemical composition and surface states of such multi-metallic nanostructures, we can obtain

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important insights about the structure-activity relationships of advanced catalytic nanomaterials.

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Here we explore the catalytic properties intrinsic to hollow trimetallic nanostructures to elucidate

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catalytic properties of Pt-based materials that have been rarely reported in previous studies of

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Pt-based mono- and bi-metallic catalysts.

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Among the most intriguing applications of Pt NPs in the past decade is as artificial enzymes that

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could be useful for biocatalysis and medicine.14,15 Multiple studies have shown Pt NPs to exhibit

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enzyme-like activities, capable of mimicking natural enzymes including oxidase, peroxidase,

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superoxide dismutase (SOD) and catalase.16-18 Nanostructures containing two precious metals, such



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as AuPt NPs, retain valuable functions of both elements and can be used in enzyme-linked

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immunosorbent assay (ELISA) antigen detection kits, as well as new colorimetric methods for

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detecting glucose and cholesterol.16,19 Additional options for refining the enzyme-like activities of Pt

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NPs come from alloying these particles with Ag, Pd, or Au.20-22 However, most of the work done on

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enzymatic activity to this point has used either mono or bimetallic Pt NPs, rather than Pt-based

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trimetallic hollow structures. Our project specifically explores the synthesis and applications of

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Pt-based trimetallic hollow structures, as these could prove to be highly-efficient catalysts with

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additional enzyme-like properties.

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In the past, several synthetic approaches have been used to synthesize hollow Pt-based

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trimetallic nanostructures. Among these, galvanic replacement reaction (GRR) has been the most

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commonly used; has been successfully used to create a variety of hollow, polymetallic

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nanoparticles.23-26 For example, Lin and coworkers prepared Pd-Cu-Pt ternary nanodendrites through

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GRR between Pt precursor and a PdCu template in aqueous solution: these nanodendrites exhibited

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enhanced electrocatalytic performance in oxygen reduction reactions.24 However, GRR is not the

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only method for constructing these NPs. Syntheses using a one pot approach in an aqueous reaction

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solution have also produced a variety of interesting hollow trimetallic nanostructures, for example,

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Pt–Pd–Ru, Pt-Pd-Cu, and Au-Pt-Pd nanodendrites.27-29 This alternative method also provides many

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opportunities for tuning the entire composition and morphology of the resulting nanoparticles;

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careful selection of surfactants and reagents that can regulate crystal growth. Further, the trimetallic

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nanodendrites obtained in this manner exhibit superior electrocatalytic activity compared to their

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GRR-derived counterparts.

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In this study, we prepared hollow PtCuCo trimetallic nanostructures to investigate their


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enzyme-like activities and suitability for use in colorimetric detection. Both PtCu and PtCo

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bimetallic NPs are excellent catalysts for electrochemical reactions, and the abundant chemical

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valence states of Cu and Co offer synergistic interactions with hydrogen peroxide. Those synergistic

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effects could be harnessed to create novel detection assays, and such capabilities have not, to our

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knowledge, yet been demonstrated by PtCuCo trimetallic nanostructures. We prepared our hollow

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PtCuCo nanostructures using glycine and Ni2+ mediated hydrothermal treatment of Co3O4, Cu2+ and

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Pt2+ in aqueous solution. We will describe the derivation of these PtCuCo NPs and the methods by

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which we can demonstrate their extremely high peroxidase- and oxidase-like capabilities, including

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catalyzing the reduction of 4-nitrophenol by NaBH4. Finally, we put the peroxidase-like activity of

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our PtCuCo NPs to practical use, demonstrating a novel colorimetric method for detecting

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biologically-relevant molecules.

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EXPERIMENTAL SECTION

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Chemicals and Materials

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The following materials were obtained from Alfa Aesar (location): Potassium platinochloride

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(K2PtCl4), Nickel Chloride (NiCl2·6H2O), Cupric Chloride (CuCl2•2H2O), polyvinyl pyrrolidone

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(PVP, K30), cobalt acetate (Co(Ac)2·4H2O), Glycine, hydrogen peroxide (H2O2), α-D-glucose,

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D-Fructose,

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1,1-diphenyl-2-picrylhydrazyl (DPPH), 3,3′,5,5′-tetramethylbenzidine (TMB), L-Ascorbic acid (AA)

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and glucose oxidase (from Aspergillus niger, GOx), cetyltrimethylammonium bromide (CTAB), and

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Ethylene glycol (EG). Milli-Q water (18 MΩ cm) was used for all the experimental preparations. All

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glassware and the autoclaves used in the following procedures were cleaned using an aqua regia

Maltobiose,

D-Galactose,

Sodium


borohydride,

p-nitrophenol,


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solution (HNO3/HCl = 1 : 3 v/v).

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Synthesis of PtCuCo and PtCu Nanostructures.

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The PtCuCo nanostructures were prepared using a two-step method. The first step was

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synthesizing the necessary templates, in this case, Co3O4 particles, using a hydrothermal method. We

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dissolved 0.6 g Co(Ac)2 and 2.2 g CTAB in 60 mL Ethylene glycol (EG) and 11 mL H2O and stirred

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this mixture magnetically for 20 min, before transferring the mixture to a 100 ml Teflon-lined

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stainless steel autoclave. This autoclave was sealed and heated at 180 °C for 48 h, then air-cooled to

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room temperature. The resulting black precipitate was collected and washed, twice with ethanol and

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once with water, then calcined at 350 ° C for 15 minutes. These cleaned Co3O4 particles were used as

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the templates for the synthesis of our PtCuCo nanostructures.

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The second step in producing PtCuCo nanostructures was mixing 90 mg Co3O4 particles with

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the following substances: 440 mg PVP, 180 mg glycine, 1.0 mL of 25.6 mM NiCl2 solution, 1.0mL

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of 25.6 mM CuCl2, and 2.6 mL of a 20 mM H2PtCl6 solution. This mixture was stirred for 5 min and

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then sonicated for 5 min at room temperature. The resulting suspension was transferred to a 20 mL

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Teflon-lined stainless-steel autoclave, heated at 200 °C for 6 h before being allowed to cool to room

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temperature naturally. The PtCuCo particles were collected via centrifugation at 10000 rpm for 15

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min, then purified using ethanol and water. We also prepared bimetallic PtCu NPs as controls; these

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NPs were produced in a similar procedure to the trimetallic NPs, except the Co3O4 templates were

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not used.

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For synthesis of Pt nanoparticles, 440 mg PVP and 180 mg glycine was dissolved with 6.7 mL

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water in a 50 mL round bottom flask, then 2.6 mL of a 20 mM H2PtCl6 solution was added and

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stirred magnetically. The flask was placed in a 60 °C water bath during the stirring. After 5 min, 1.3


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mL 1.0 M L-ascorbic acid solution was injected into the mixture and stirring was continued. Two

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hours later, we centrifuged the resulting Pt NPs twice with ethanol and water. The precipitates were

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re-dispersed in 4 mL water for further use.

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Characterization

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To characterize these NPs, a Hitachi S-4800 electron microscope was used to capture SEM

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images and XRD patterns were collected by X-ray diffraction (XRD, Bruker D8 Advance

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diffractometer) using monochromatized Cu Kα radiation (λ = 1.5418 Å). Transmission electron

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microscopy (TEM) images were captured on a Tecnai G2 F20 U-TWIN electron microscope with an

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accelerating voltage of 200 kV. That same microscope was used to perform high-resolution TEM

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(HRTEM), selected-area electron diffraction and energy dispersive X-ray spectrometry. Elemental

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composition and element distribution were verified by the spectrum-imaging method, using

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dedicated scanning transmission electron microscope (STEM) and energy dispersive X-ray

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spectroscopy in the high-angle annular dark field (HAADF) mode. X-ray photoelectron spectroscopy

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was conducted using a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer

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using 150W Al Kα radiation and base pressure of approximately 3×10-9 mbar. The binding energies

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were calibrated to the C1s line at 284.8 eV. UV-visible absorption spectra were obtained by a

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UV-VIS-NIR Spectrometer (Varian Cary 5000). The electron spin resonance (ESR) spectroscopy was

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recorded using a Bruker EMX ESR spectrometer at ambient temperature. Specifically, ESR spectra

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were captured from mixtures containing 25 mM BMPO, 0.1 mM H2O2 and 0.1 mg/mL PtCuCo NPs

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under several different pH conditions and time intervals. During the ESR measurements, the settings

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were 20 mW microwave power, 100 G scan range and 1 G field modulation.

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Measurements of peroxidase-like and oxidase-like activities



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The peroxidase-like activities of the trimetallic vs bimetallic NPs was studied by catalyzing the

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oxidation of TMB in the presence of hydrogen peroxide. First, we mixed 20 µL of 20 mM TMB and

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20 µL of 0.1 M H2O2 in 3 mL of H2O, then added 5 µL 25.0 mg/mL of our PtCuCo NPs suspensions

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to initiate the oxidation of TMB. Both TMB oxidation and its accompanying color changes were

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monitored and quantified using UV-Vis absorption spectroscopy. The reaction kinetics for the

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catalytic oxidation of TMB were evaluated by recording absorption spectra at 2 min intervals using

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the scanning kinetics mode. We used similar techniques to assess the oxidase-like activity of these

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NPs; hydrogen peroxide was not used in that procedure.

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Detection of glucose, bisulfide and formaldehyde hydrosulfite

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Next, we used the peroxidase-like capabilities of PtCuCo NPs to detect glucose. In this

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procedure, glucose was first oxidized by glucose oxidase to produce gluconic acid and H2O2 in a pH

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7.4 buffer solution, then we used the color change which occurs during TMB oxidation to detect the

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presence of H2O2 catalyzed by PtCuCo NPs. Specifically, glucose oxidase (20 µL, 100 Unit mL−1)

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and glucose aqueous solution (480 µL) at varied concentrations were incubated at 37 ºC for 30 min.

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Then, 2.5 mL H2O and 20 µL 20 mM TMB were added. 10 µL PtCuCo NPs was added to trigger the

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color-change reaction. After 30 minutes incubation at 37 ºC for 30 min, samples from the final

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mixture were assessed by UV-Vis spectra test. To determine the selectivity of this test, we also ran

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experiments using other sugars, specifically maltose (1 mM), galactose (1 mM), and fructose (1 mM),

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instead of glucose. The optimized pH value for glucose oxidase to catalyze the glucose is around 5.2.

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In current work, we use the pH 7.4 is to meet their physiological environment for real applications.

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This pH does not affect largely the glucose oxidase activity during the detection, we observed that

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the glucose oxidase exhibit high activity to catalyze the glucose oxidation at pH 7.4 within 30 min of


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reaction time. This condition was also verified in other systems.19 By using different concentrations

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of inhibitors, ascorbic acid, bisulfide, or formaldehyde hydrosulfite, we could evaluate their effects

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on the peroxidase-like activity of PtCuCo NPs.

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Catalytic reduction of 4-nitrophenol

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To compare the catalytic capacities of the three types of NPs (trimetallic, bimetallic, and

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monometallic), a 9.0 mL aliquot of 4×10-4 M 4-nitrophenol aqueous solution and 1 mL of 1.2 M

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NaBH4 solution were mixed and stirred for 10 min at room temperature. We placed 0.5 mL of this

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mixture, diluted with 2.5 mL H2O, into separate quartz cells. Then, to initiate the reduction reaction,

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we added a 5 µL 25.0 mg/ml suspension of one type of NP per each quartz cell. Reaction kinetics

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were recorded at 1.0 min intervals.

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Scheme 1 Schematic illustration for synthesis of PtCuCo trimetallic nanostructures.

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Figure 1. Typical SEM (a), TEM (b), HRTEM (c), and STEM-HAADF (d) image of PtCuCo NPs;

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STEM-EDS element mappings of Co, Cu and Pt in selected PtCuCo particle (e-g). The inset in c is

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the corresponding electron diffraction pattern.

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RESULTS AND DISCUSSION

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Formation and characterization of hollow PtCuCo nanostructures.

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Scheme 1 illustrates the procedure for synthesis of PtCuCo trimetallic nanostructures. Many

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factors appear to be involved in the formation of PtCuCo hollow nanostructures, and further studies

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will be required to understand how each factor functions. Here we will discuss what we have been

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able to establish so far. Figure 1 shows the typical SEM and TEM images of PtCuCo nanostructures.

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The diameter of these PtCuCo NPs, averaged across 70 randomly selected particles, was calculated

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to be 32.2±5.1 nm. The TEM indicates the PtCuCo NPs we produced included both hollow and solid

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structures (Figure 1b); the yield of hollow NPs was calculated to be about 82%. The HRTEM

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displays well-defined lattice planes (Figure 1c). The adjacent lattice spacing was calculated to be


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0.21 nm (marked in red), which matched well with (111) planar distance of PtCuCo and falls

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between the values for Pt (0.228 nm) and Cu (0.208 nm) or Co (0.204 nm). The direction of the (111)

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facet differs in one particle (marked in double red line), which suggests that PtCuCo NPs have a

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polycrystalline structure (Figure S1). This hypothesis is supported by the pattern of diffraction spots

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which, when superimposed on the rings in ED pattern, also indicate that each entire PtCuCo particle

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has a polycrystalline structure (Figure 1c inset). Energy-dispersive X-ray spectroscopy (EDS)

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analysis verified the co-existence of element Pt, Cu and Co, but not Ni, in the resultant products

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(Figure S2), the atomic ratio of Pt/Cu/Co was measured to be ~1:2.8:0.2.

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The elemental distribution of Pt, Cu and Co in our NPs was analyzed by using STEM-EDS

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mapping (Figures 1f-g); this confirms that Pt, Cu, and Co are homogeneously distributed throughout

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each entire particle, forming a trimetallic alloy nanostructure composed of Pt, Cu, and Co. The

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STEM-EDS line profiles of elemental composition verify the higher percentage of Pt, Cu and Co at

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the wall than center, indicating the hollow structure of PtCuCo NPs (Figure S3). Although the

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surface of these PtCuCo NPs are very rough and appear to have multiple defects, such as lattice

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disorders, dislocations, and interior boundaries, such defects might serve as highly active catalytic

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sites, which could be beneficial.

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We observed that the successful formation of hollow PtCuCo nanostructures was dependent

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upon the addition of glycine, Co3O4 NPs, and Ni2+ templates. To understand what roles each of these

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materials play in the process, performed an additional series of controlled experiments. Previously,

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synthetic procedures similar to ours have been used to prepare Pt, PtCu, and PtNiCu

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nanocrystals.30-32 Those experiments demonstrated that glycine acted as a co-reductant, cooperating

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with PVP and a surface regulator to mediate the reduction kinetics of the different metal ions in a one



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pot synthesis. We proposed that the reduction of Co3O4 to Co may go through two steps: (1) the

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coordination between Co precursors and glycine accelerating the decomposition of Co3O4,33 this was

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evidenced by less Co3O4 precipitate remained after the hydrothermal treatment of Co3O4 and glycine;

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and (2) the reduction of the Co (II, III)/glycine complex by PVP under hydrothermal condition. Our

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experiments suggest that in the absence of glycine, the reductions necessary to form trimetallic

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PtCuCo nanostructures cannot occur; instead, only Co3O4 NPs are produced. As shown by the SEM

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image and XRD patterns in Figure S4, we found that in the absence of Co3O4 NPs or other Co3+ ions,

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only bimetallic PtCu NPs formed. In the absence of Ni ions, pure PtCuCo NPs cannot be obtained:

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the only products that form are a mixture of Co3O4 and PtCu NPs, indicating the significant effect of

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Ni2+. Intriguingly, elemental Ni was not detectable in the resultant products, even though the same

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amounts of Ni2+ as Cu2+ were fed into the procedure. Since the glycine have coordination interaction

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with various metal ions, including Cu2+, Ni2+, Co2+ and Pt2+,33 the coordination binding may

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influence at varying degree on the reduction potentials of different metal ions. As a consequence, the

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resulting Ni2+/glycine complex become more difficult to be reduced. In addition, Ni2+ may mediate

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the development of PtCuCo NPs by altering the surface formation energies of the UPD layer as

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reported by Ma, et al.34


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Figure 2. XRD patterns of as-prepared PtCuCo and PtCu NPs, along with the standard PDF card of

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Pt, Cu, and Co, inserted for comparison.

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These XRD patterns indicate that PtCuCo nanostructures have a cubic phase, consistent with the

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phases of elemental Pt, Cu, Co and PtCu NPs (Figure 2). The diffraction peaks can be indexed to the

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planes (111), (200) and (220); no additional peaks were detected from pure Cu, Co and Co3O4

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indicating the high purity of these PtCuCo NPs. The diffraction peaks (111) and (200) of PtCuCo

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NPs was, as expected, located between the corresponding peaks in Pt and Cu (Co) NPs. The d-values

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of PtCuCo (111) fell between the Pt (111) of 0.228 nm and the Cu (111) of 0.208 nm, Co (111) of

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0.204 nm, providing confirmation of the formation of a trimetallic alloy, and agreeing with the

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results of HRTEM. More details about the surface and alloy structures of our PtCuCo NPs were

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revealed by X-ray photoelectron spectroscopy (XPS) (Figure 3). A comparison of the XPS spectra of

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PtCuCo and PtCu nanostructures is provided in Figure 3. The XPS survey spectra from the PtCuCo

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and PtCu NPs indicates that both sample types have signals from Pt 4f, Pt 4d, Cu 2p, C 1s, N 1s, and

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O 1s. These PtCuCo NPs also exhibit the signal from Co 2p that is absent from the spectra of PtCu



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NPs, further confirming the presence of elemental Pt, Cu and Co in PtCuCo NPs. The atomic ratio of

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Pt/Cu/Co from the PtCuCo particle surface was measured by XPS is 2/1/1. The C 1s at 284.8 eV and

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O 1s at 532 eV comes primarily from the additive chemicals (e.g., glycine and PVP) used to prepare

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the catalysts. Figure 3b-d shows the high resolution XPS spectra of Pt 4d, Cu 2p and Co 2p of

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PtCuCo catalysts, respectively. The binding energy of Pt 4d 5/2 (315.3 eV) and Pt 4d 3/2 (315.3 eV)

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indicate the Pt existed in the chemical state of Pt0. The peaks of Cu 2p at 932.8 eV (Cu 2p 3/2) and

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952.5 eV (Cu 2p 1/2) can be understood as corresponding to zerovalent Cu, while two additional

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peaks appear at 934.8 eV and 955.1 eV reveal the presence of oxidized Cu. In the spectra of Co 2p,

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the peaks at 779.2 eV and 794.8 eV were assigned to 2p3/2 and 2p1/2 of zero valent state of Co, the

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peaks at slightly higher energy (783.9 eV and 800.1 eV) stand for the existence of oxidized Co.35,36

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Neither Co and Cu were detectable in their oxide forms by XRD, suggesting only trace amounts of

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oxidized species on the particle surface.

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Figure 3. (a) XPS spectra of PtCu NPs and PtCuCo NPs. The high resolution spectra of (b) Pt 4d, (c)

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Cu 2p and (d) Co 2p of PtCuCo NPs.

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Figure 4. Peroxidase-like activity of PtCuCo nanostructures. (a) UV-Vis spectra evolution of TMB

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over time after the mixing with hydrogen peroxide and PtCuCo NPs; the inset shows the color

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evolution of TMB and OPD catalyzed by PtCuCo nanostructures. (b) The absorbance change at 650

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nm as a function of reaction time during the TMB oxidation catalyzed by Co3O4, Pt, PtCu and

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PtCuCo nanostructures in the presence of hydrogen peroxide.



1 2

Enhanced Enzyme-like activity of PtCuCo nanostructures.

3

We expected these PtCuCo nanostructrues to exhibit high levels of enzyme-like activity, due to

4

their deep cavities, abundant crystal defects, and rich chemical valence states. To confirm these

5

hypotheses, we used, 3,3,5,5-Tetramethylbenzidine (TMB), a commonly used chromophoric

6

substrate for peroxidase and oxidase mimetic studies, to evaluate the enzyme-like activity of PtCuCo

7

NPs. First, we found that PtCuCo NPs can quickly catalyze the oxidation of TMB and OPD to

8

generate a typical blue and yellow color either in the presence or in the absence of hydrogen

9

peroxide (Figure 4A and Figure S5). In contrast, the control condition, which did not include the

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PtCuCo NPs, showed negligible color changes over the same time period, indicating the

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peroxidase-like and oxidase–like activity of PtCuCo alloy NPs.

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Next, we compare the ability of PtCuCo nanostructures to catalyze the oxidation of TMB to the

13

capabilities of Pt and PtCu NPs to perform the same reaction. These Pt and PtCu NPs were prepared

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by a similar glycine-mediated method. The resulting Pt NPs had a dendritic structure, with an

15

average diameter of 31 nm (Figure S6). The bimetallic PtCu NPs had a spherical shape with scaggy

16

surface, the diameter was calculated to be ~44 nm (Figure S7). The EDS analysis verified the

17

coexistence of element Pt and Cu with Pt/Cu molar ratio of 1.5. As expected, the right panel of

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Figure 4 shows the oxidation of TMB with H2O2 was evidently accelerated by either Pt or PtCu NPs,

19

indicating their strong peroxidase-like activity. It appears that alloying Pt with Cu leads to higher

20

peroxidase-like activity than what is possible using only pure Pt. This may explain how PtCuCo NPs

21

demonstrate an impressive increase in the oxidation rate of TMB, ~2 times higher than that of PtCu

22

or Pt NPs.


Page 16 of 34

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1

These data support our hypothesis that hollow PtCuCo nanostructures will exhibit superior

2

enzyme-like capabilities. In order to better understand the nature of this enhancement, we performed

3

two sets of additional experiments. In previous studies, we have demonstrated that a serial of metal

4

NPs (e.g. Au, Ag, Pt and Pd) interacting with hydrogen peroxide could generate hydroxyl radials, the

5

ability to produce hydroxyl radicals were associated with their catalytic oxidative capability .37-40

6

Therefore, we used the similar ESR technique to test the generation of hydroxyl radical by PtCuCo

7

nanostructures. To confirm the generation of these radicals, we used the standard spin trap for

8

hydroxyl radicals, BMPO. Figure 5a shows the ESR spectra obtained for solutions at pH 6.0 in the

9

absence and presence of either PtCu or PtCuCo NPs. In the control samples containing BMPO only

10

or only H2O2 and BMPO, no ESR signal was observed. In contrast, in the samples to which we

11

added either PtCu NPs or PtCuCo NPs, we could observe ESR spectra having four lines with relative

12

intensities of 1:2:2:1 and hyperfine splitting parameters of aN = aH = 14.9 G, which was characteristic

13

for the spin adduct of BMPO/•OH. These results verify the production of hydroxyl radicals from

14

hydrogen peroxide assisted by PtCu NPs or PtCuCo nanostructures. In the previous publication, we

15

have demonstrated that Pt NPs can generate hydroxyl radicals by catalyzing the decomposition of

16

hydrogen peroxide through the reaction (H2O2 ሱۛۛሮ 2•OH).40 The ability to generate hydroxyl

17

radicals was affected by particle size, hydrogen peroxide concentration and environmental pH. At

18

present, we proposed that this may be also one contributor to the hydroxyl radicals produced by

19

PtCuCo NPs. Interestingly, over the course of the same amount of time and number of observations,

20

ESR signal intensity generated by PtCuCo nanostructures was calculated to be about 10 times

21

stronger than the signal from PtCu NPs, which indicates these PtCuCo nanostructures are

22

significantly more effective for generating hydroxyl radicals. It is possible that the hollow structure

Pt NPs



1

of these trimetallic NPs have a larger effective surface area, each providing more reactive sites which

2

can accelerate the generation of hydroxyl radicals. Additionally, the abundant chemical valence of

3

Cu, Cu oxide, Co and Co oxide on the particle surface will facilitate the generation of hydroxyl

4

radicals by effectively combining the Fenton reaction and catalytic effects of elemental Pt in the

5

presence of hydrogen peroxide.

6

The electronic structure of Pt would be modified after alloying with other metals, such as Cu

7

and Co. It was predicted by the Hammer−Nørskov reactivity model that when a metal with larger

8

lattice constants (Pt) is alloyed with metal with smaller lattice constants (Co and Cu), the d-band

9

center will shift down.41-42 Therefore, it might be expected that the downshift of d-band center are the

10

main contributors to the facilitated electron transfer that likely enhances catalytic performance. To

11

explore this phenomenon beyond the ROS production induced by PtCuCo nanostructures, we have

12

evaluated their ability to facilitate electron transfer by scavenging 2, 2-diphenyl-1-picrylhydrazyl

13

(DPPH), a well-known stable radical with unpaired electron. Importantly, DPPH has a deep violet

14

color in solution with an absorption band centered at around 520 nm (Figure 5b). When DPPH loses

15

an electron, that distinctive color is no longer present; this decoloration and accompanying

16

disappearance of the absorption peak at 520 nm, provide convenient way to study the interaction

17

between NPs and DPPH. We found that both PtCu and PtCuCo nanostructures can scavenge DPPH

18

radical in a time and concentration dependent manner (Figure 5c and 5d). PtCuCo nanostructures

19

were approximately 9 times more effective than PtCu NPs in reduction of DPPH. This indicated the

20

electronic change of Pt nanostructures after alloying with Cu and Co does have a positive effect in

21

accelerating electron transfer. Earlier research had indicated that the electronic charge transfer

22

between adjacent metal atoms in bi- or trimetallic nanoparticles also played an important role in


Page 18 of 34

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1

enhancing the catalytic activity.43-45 Our findings fit this hypothesis; our PtCuCo trimetallic NPs, the

2

ionization energy for Pt, Cu and Co is 8.96, 7.72 and 7.88 eV, respectively. Each of Cu and Co is

3

more electronegative than Pt in PtCuCo NPs. Pt atoms could accept the electronic charges from

4

either neighboring Cu or Co atoms at the same time, exhibiting superior capabilities for facilitating

5

electron transfer. Our DPPD scavenging results also supported this view. Therefore, it is our

6

conclusion that the combination of hollow structure, surface defects, electronic variation, and

7

electronic charge transfer effects are responsible for superior catalytic capabilities of PtCuCo

8

nanostructures.

9 10

Figure 5. a) The generation of hydroxyl radicals by PtCu and PtCuCo nanostructures in the presence

11

of hydrogen peroxide at pH 6.0. These ESR spectra were recorded from the sample solution that

12

contained 25 mM BMPO and 0.1 mM hydrogen peroxide in either the absence (control) or the

13

presence of PtCu or PtCuCo nanostructures after 10 min mixing. Panels b), c) and d) represent the

14

spectra evolution of DPPH in the absence and presence of PtCu and PtCuCo nanostructures,

15

respectively.

16 17 18

Enhanced catalytic activity of PtCuCo nanostructures for reduction of 4NP. Besides these enzyme-like activities, the unique structure of PtCuCo nanoparticles may provide



1

benefits for the enhancement of other redox reactions. For example, we found that PtCuCo NPs

2

exhibited outstanding catalytic activity for reducing 4-nitrophenol (4NP) to produce 4-aminophenol

3

by sodium borohydride. We also compared the catalytic performance of Pt, PtCu and PtCuCo NPs

4

for 4NP reduction. Conveniently, 4NP in sodium borohydride solution displays a characteristic

5

absorption at 400 nm, which is decreased after reduction of 4NP. By monitoring the absorption

6

change over time we can observe the rate and extent of that reduction. Figure 6 plots the change of

7

A400 of 4NP over time in the presence of different catalysts. The baseline spectra of 4NP, without

8

addition of any catalyst, remained unchanged; however, adding a trace amount of PtCuCo can

9

quickly accelerate the reduction of 4NP (Figure 6 inset). Pt NPs show good catalytic activity for the

10

reduction of 4NP. However, when we tested pure Co3O4 NPs for comparison, these were inactive in

11

this system. As expected, PtCu and PtCuCo NPs both greatly enhanced the reduction of 4NP.

12

PtCuCo NPs exhibited the highest catalytic activity: about 4 times higher than PtCu and 16 times

13

higher than Pt at 2 min. These results suggest again that alloying Pt with Cu and Co to form

14

trimetallic nanostructures significantly enhances catalytic activity.

15


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1 2

Figure 6. Catalytic activity to reduction of 4-nitrophenol in the presence of different catalysts. Insets

3

display the 4NP spectra evolution with time in the absence (control) and presence of PtCuCo NPs,

4

the concentration of PtCuCo NPs was diluted by 5 times for recording the spectra change.

5 6

Detection of glucose by the peroxidase-like activity of PtCuCo nanostructures.

7

Hydrogen peroxide is an important product involved in many biochemical reactions, for example,

8

the decomposition of acetylcholine in the presence of AchE and ChOx and the oxidation of some

9

biological active molecules (e.g. glucose, xanthine, uric acid and cholesterol). This means that a

10

colorimetric test sensitive to H2O2 can be used as an indirect method to detect biologic molecules of

11

interest. The oxidase- and peroxidase-like capabilities of PtCuCo nanostructures can catalyze the

12

TMB oxidation in both the absence and presence of hydrogen peroxide. This process is accelerated

13

by increasing the H2O2 concentration. The initial reaction rate for TMB oxidation was linearly

14

dependent on the concentration of H2O2 (Figure S8). At lower concentrations of H2O2, a linear

15

relationship between the reaction rate and H2O2 concentration was found. This linear response was



1

the basis for the measurement of hydrogen peroxide concentration. The limit of detection for H2O2 in

2

our TMB oxidation reaction is 0.03 mM, with linearity observed from 0.067 mM to 0.67 mM. By

3

combining the peroxidase-like activity of PtCuCo NPs and the specific catalytic activity of glucose

4

oxidase toward glucose, it is possible to selectively detect glucose. Figure 7 compares the

5

absorbance at 650 nm (A650) in detection of varying substrates. It clearly shows the change in A650

6

was produced only in the presence of glucose. It is noteworthy that a relatively strong background

7

signal was produced from the control experiment, which is caused by the oxidase-like activity of

8

PtCuCo NPs. In the control sample including TMB and PtCuCo NPs, PtCuCo NPs can catalyze the

9

oxidation of TMB in the presence of dissolved oxygen. When we tested glucose analogues at the

10

same concentration as glucose, i.e., galactose, fructose, lactose and maltose, the absorbance changes

11

were comparable to the control conditions that did not contain glucose or glucose analogues. These

12

results demonstrate the high specificity of our proposed glucose detection method. The A650 was

13

found to be dependent on the concentration of glucose; higher concentrations of glucose resulted in a

14

higher absorption. We used the absorbance change at 650 nm to build a standard curve of absorbance

15

versus glucose concentration. This curve showed a good linear relationship in the glucose

16

concentration range of 0.2 mM to 1.5 mM with a limit of detection (LOD) of 0.08 mM (S/N=3). This

17

LOD for glucose is slightly higher than that used different nanoparticles as peroxide mimics (Table

18

S1). Nonetheless, these results suggest that PtCuCo NPs can serve as a nano-peroxidase in the

19

colorimetric detection of glucose. It may be possible to identify other H2O2-related molecules that

20

could be used in clinical diagnostics and personal health care applications.


Page 22 of 34

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1 2

Figure 7. Selectivity of the proposed method for glucose detection. Inset shows the concentration

3

response for glucose detection using PtCuCo nanostructures as peroxidase mimics.

4 5

Colorimetric detection of rongalite and bisulfide by inhibiting the peroxidase-like activity of

6

PtCuCo nanostructures.

7

Rongalite, also called formaldehyde hydrosulfite, is widely used as a bleaching and reducing

8

agent in dye industry and for organic synthesis.46 However, in the food industry, rongalite is a banned

9

additive. The illegal addition of rongalite can not only damage the nutrition of food gradients, but

10

also can result in allergic reactions, intestinal disease and even certain cancer. Having a rapid and

11

sensitive test for the presence of rongalite would be extremely useful. We have found that rongalite

12

molecules can effectively inhibit the peroxidase-like activity of PtCuCo NPs in a dose dependent

13

manner, which provides a simple way to detect rongalite. Fig. 8 inset shows the color evolution of

14

H2O2-mediated TMB oxidation reactions catalyzed by PtCuCo nanostructures. Adding different

15

concentrations of rongalite affects the TMB oxidation process, which can be observed



1

colorimetrically. As shown in (Figure 8), higher concentrations of rongalite lower absorbance at both

2

370 nm and 650 nm, reflective of decreased oxidation of TMB. Our tests showed that a very wide

3

range of rongalite concentrations can be detected in this manner: about 3 orders of magnitude, from

4

0.3 µM to 100µM. We found a good linear relationship between the absorbance changes at 650 nm

5

and the concentration of rongalite, even at low concentrations (0.3 µM to 10 µM).

6

We also explored the utility of another substance for inhibiting the peroxidase-like capabilities of

7

PtCuCo NPs. Specifically, HS- ion, which is the main state that H2S exists under physiological

8

conditions, have a wide variety of biologic functions and can also significantly inhibit the

9

peroxidase-like activity of PtCuCo NPs. Figure 9 shows the color evolution and the absorption

10

spectra changes which occur after the introduction of HS- to a sample containing TMB, H2O2 and

11

PtCuCo NPs, thus demonstrating how HS- ions are capable of even more efficient inhibition of TMB

12

than rongalite. 3 µM HS- can completely inhibit the color production of TMB oxidation while 100

13

µM rongalite was required. The inhibitory degree was dependent on the concentration of HS-

14

(Figure 9): a higher concentration of disulfide leads to a greater reduction in TMB oxidation rate.

15

Based on that inhibitory effect, we constructed a standard curve of absorbance versus disulfide

16

concentration. This curve showed a good linear relationship in the concentration range of 0.03 µM to

17

0.67 µM with a lower limit of detection of 0.01 µM (S/N = 3). Our results show this novel method is

18

about 30 times high more sensitive to bisulfide than rongalite.

19

As demonstrated above, we can develop a colorimetric method for rapid detection of rongalite

20

and bisulfide with high sensitivity based on their inhibiting effect on activity of PtCuCo

21

nano-peroxidase. Although rongalite and bisulfide show the similar inhibitory behaviors, these

22

substances may operate by different mechanisms. HS- can chemically bind with the metal surface


Page 24 of 34

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1

through metal-sulfur bond, thereby disabling the active sites of PtCuCo NPs, consequently resulting

2

greatly reduced catalytic activity. In contrast, rongalite, a reducing agent, may inhibit the oxidation of

3

TMB by a competitive reaction with hydrogen peroxide between rongalite and TMB: adding more

4

rongalite leads to less TMB being oxidized. We believe this explains why the limit of detection for

5

bisulfide is much lower than rongalite, as trace amount of bisulfide ions can greatly modify the

6

surface of the catalyst, disrupting catalytic activity.

7 8

Figure 8. The concentration dependence on inhibiting the peroxidase activity of PtCuCo NPs for

9

detection of rongalite. Inset shows the UV-Vis spectra and color evolution of TMB oxidation in the

10

presence of rongalite with different concentrations.



1 2

Figure 9. A dose-response for inhibitory effect of bisulfide on the peroxidase like activity of PtCuCo

3

nanostructures.

4 5

CONCLUSIONS

6

We have developed a hydrothermal approach that enables production of hollow PtCuCo

7

nanostructures. These trimetallic PtCuCo nanostructures possess intriguing surface features,

8

including rich crystal defects and abundant active surface states, which enable these NPs to

9

demonstrate superior abilities to facilitate electron transfer and generate hydroxyl radicals in the

10

presence of hydrogen peroxide. As a consequence these PtCuCo NPs have enhanced catalytic

11

capabilities. We have demonstrated these PtCuCo nanostructures exhibited enhanced oxidase- and

12

peroxidase-like activities including the reduction of 4-nitrophenol by NaBH4. The peroxidase-like

13

activity of PtCuCo nanostructures was employed to develop a platform for colorimetric detections of

14

glucose and nanoenzyme inhibitors (e.g. rongalite and HS-) based on their inhibitory effects.

15

Although the mechanism for the growth and catalytic enhancement of PtCuCo nanostructures may


Page 26 of 34

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1

need further investigations, we believe that our study provides insights that can guide the design of

2

future multi-metallic nanostructures with enhanced, highly-efficient enzyme-like capabilities and low

3

usage of precious metal, which should be extremely valuable for the development of new bioanalytic

4

applications.

5 6

Supporting Information

7

High resolution TEM image of PtCuCo NPs (Figure S1), EDS spectra of PtCuCo nanostructures

8

(Figure S2), the cross-section composition line profiles of one PtCuCo NP (Figure S3), SEM images

9

and XRD pattern of Co3O4 nanoparticles (Figure S4), PtCuCo nanostructures catalyze the oxidation

10

of TMB in the absence of hydrogen peroxide (Figure S5), TEM images of Pt nanostructures (Figure

11

S6), TEM image and EDS spectra of PtCu NPs (Figure S7), concentration response for hydrogen

12

peroxide using PtCuCo NPs as peroxidase mimics (Figure S8), comparison of the limit of detection

13

for glucose detection by using different metal based nano-peroxidase (Table S1). This material is

14

available free of charge via the Internet at http://pubs.acs.org.

15 16

AUTHOR INFORMATION

17

Corresponding Authors

18

*E-mail: [email protected] (W.H.).

19

*E-mail: [email protected] (J.-J.Y.).

20

Notes

21

The authors declare no competing financial interest.

22



1

ACKNOWLEDGMENTS

2

This work was supported by the National Natural Science Foundation of China (Grant No. 51772256

3

and 61504117), the Plan for Scientific Innovation Talent of Henan Province (174100510014) and

4

Basic and Advanced Technology Research Project in Henan Province (162300410047). This work

5

was also supported by a regulatory science grant under the FDA Nanotechnology CORES Program.

6

This paper is not an official US FDA guidance or policy statement. No official support or

7

endorsement by the US FDA is intended or should be inferred. The authors thank Dr. Lili Fox Vélez,

8

Office of Regulatory Science, for her scientific writing support.

9 10

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Formation of PtCuCo Trimetallic Nanostructures with Enhanced

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