Microwave-Assisted Fabrication of Bimetallic PdCu Nanocorals with

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Microwave-Assisted Fabrication of Bimetallic PdCu Nanocorals with Enhanced Peroxidase-Like Activity and Efficiency for Thiocyanate Sensing Yanfang He, Xiangheng Niu, Longhua Li, Xin Li, Wenchi Zhang, Hongli Zhao, Minbo Lan, Jianming Pan, and Xifeng Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00578 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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ACS Applied Nano Materials

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Microwave-Assisted Fabrication of Bimetallic PdCu Nanocorals

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with Enhanced Peroxidase-Like Activity and Efficiency for

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Thiocyanate Sensing

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Yanfang He,a Xiangheng Niu,a,* Longhua Li,a Xin Li,a Wenchi Zhang,a Hongli Zhao,b

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Minbo Lan,b,* Jianming Pan,a Xifeng Zhanga,*

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a

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Chemical Engineering, Jiangsu University, Zhenjiang 212013, China

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b

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and Molecular Engineering, East China University of Science and Technology,

Institute of Green Chemistry and Chemical Technology, School of Chemistry and

Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry

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Shanghai 200237, China

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* Corresponding author. E-mail: [email protected]; [email protected];

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[email protected]

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1 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: Nanomaterials with enzyme-like characteristics (nanozymes) are an

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emerging alternative of traditional bio-enzymes that holds great promise in a variety

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of fields. The activity and efficiency of most current nanozymes, however, are lower

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than those of natural enzymes, which will inevitably hinder their wider applications.

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Thus, development of new nanozymes with favorable catalytic properties is urgently

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desired. In this work, we reported a coral-structured bimetallic PdCu nanozyme that

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exhibited enhanced peroxidase activity and efficiency. The coral-like nanozyme

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fabricated by a microwave-assisted wet-chemical method was composed of assembled

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PdCu branches. In comparison with pure Pd, the bimetallic PdCu had an enhanced

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ability to catalyze the reaction of colorless tetramethylbenzidine (TMB) to its blue

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oxide (TMBox) in the presence of H2O2. X-ray photoelectron spectroscopy and

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theoretical calculation indicated that insertion of Cu atoms into the Pd lattice would

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increase the Pd0 proportion and decrease the adsorption difficulty of H2O2, both of

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which finally resulted in the improvements in catalytic activity and efficiency. It was

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further found that thiocyanate (SCN-) could inhibit the color reaction by decreasing

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the activity of PdCu selectively. On the basis of this principle, SCN- in the much wide

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range of 0.001–100 μM could be detected, along with a limit of detection down to 1

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nM.

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Keywords: PdCu; Nanozyme; Improved peroxidase-like activity; Thiocyanate;

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Colorimetric detection

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1. Introduction

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Since Fe3O4 nanoparticles were discovered to possess intrinsic peroxidase-like

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properties in 2007 by Yan’s group,1 nanomaterials with peroxidase mimetic activity

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have drawn intensive research interest in the past decade.2-5 A variety of materials

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with peroxidase-like characteristics, including non-previous transition metal-based

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nanostructures (Fe3O4,1,6 Co3O4,7 CeO2,8 V2O5,9 CuS,10 Prussian blue,11-12 et al.),

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carbon-based materials (GO,13 g-C3N4,14 et al.), and noble metals (Pt,15 Pd,16 Au,17 et

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al.), have been successively developed. Among these mimics, noble metal-based

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nanozymes attract special attention. On the one hand, compared with non-previous

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transition metal-based nanostructures like Fe3O4 NPs, noble metal-based nanozymes

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show better stability especially in strong acid or alkaline solutions; On the other hand,

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compared with current carbon-based materials, noble metals exhibit higher catalytic

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activity. Besides, noble metal-based nanozymes have excellent bioconjugation and

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biocompatibility than other nanozymes.18 These superiorities have endowed them

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with extensive applications in biochemical sensing and medical therapy.19-25

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In contrast to homogeneous bio-enzyme reactions, nanozymes often catalyze a

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reaction via their exposed surface atoms and sites.2 The heterogeneous nature of

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nanozymatic catalysis leads to the lower activity and efficiency of most current

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nanozymes compared with natural counterparts.26 Besides, nanosized materials are

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prone to aggregation to decrease active surfaces, which also results in their sluggish

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activity. In addition, the cost of noble metal-based nanozymes is relatively high. These

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defects will severely limit their wider applications. Thus, exploitation of new noble



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metal-based nanozymes with ideal catalytic properties is urgently desired from both

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the application and economic aspects.

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Considering that the activity of a nanozyme depends on its size, shape, and

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component, one has designed a few of high-efficiency nanozymes by tailoring these

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intrinsic characteristics. For instance, Yan’s group found that Fe3O4 particles with

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various sizes showed different levels of peroxidase-like activity.1 Gao et al.

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synthesized high-index faceted Pt concave nanocubes (HIF-Pt-CNCs) and

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demonstrated that the unique shape of HIF-Pt-CNCs could provide 4-fold

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peroxidase-like activity higher than Pt nanospheres.27 Recently, Nagvenkar and

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Gedanken reported that doping of Zn into CuO was able to improve the

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peroxidase-mimicking activity of CuO.28 These successful examples suggest that

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modulating the intrinsic properties (size, shape, component, et al.) of nanozymes is an

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effective strategy to harvest high-performance peroxidase mimics for promising

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applications.

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In this work, we fabricated a coral-like bimetallic PdCu architecture as a promising

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peroxidase mimic that provided enhanced catalytic activity and efficiency. A

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microwave-assisted wet-chemical synthetic method was employed to rapidly prepare

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the nanozyme composed of assembled PdCu nanobranches, in which Cu atoms were

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well doped into the Pd lattice. A series of experiments were carried out to demonstrate

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the stronger ability of PdCu, in comparison with pure Pd, to catalyze the color

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reaction of tetramethylbenzidine (TMB) in the presence of H2O2. Physicochemical

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characterization and theoretical calculation were conducted to reveal the underlying


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mechanisms for the improved activity observed in PdCu. Interestingly, it was found

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that thiocyanate (SCN-) was able to suppress the color reaction by decreasing the

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activity of PdCu selectively. Based on this principle, sensing of SCN- with high

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sensitivity, selectivity, and reliability was further demonstrated.

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2. Experimental section

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2.1. Chemicals and materials

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K2PdCl4 and NaBH4 were obtained from Shanghai Aladdin Biochemical

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Technology Co., Ltd. CuSO4, H2O2, TMB, terephthalic acid (TA), NaSCN, glucose,

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and uric acid were provided by Sinopharm Chemical Regent Co., Ltd. Deionized

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water was utilized throughout the study. All other chemicals were of analytical grade

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and used as received.

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2.2. Synthesis of the bimetallic PdCu

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A microwave-assisted wet-chemical synthetic approach was established to prepare

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the bimetallic PdCu, as illustrated in Scheme S1. In detail, 10 mL of the metal

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precursor mixture consisting of 5 mL 0.1 M K2PdCl4 solution and 5 mL 0.1 M CuSO4

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solution was quickly injected into a 250 mL three-neck flask containing 50 mL 0.1 M

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NaBH4 with a wild stir. The three-neck flask was in an XH-100A microwave

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synthesizer (Beijing Xianghu Science and Technology Development Co., Ltd.) for

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reaction. The reaction was performed at 60oC for 5 min, along with a microwave

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power of 600 W. The black PdCu product was collected by centrifugation and washed

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with water and ethanol alternately for several times. For comparison, pure Pd was also

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prepared under identical conditions, with no CuSO4 in the synthesis system.



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2.3. Characterization

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X-ray diffraction (XRD) measurements were conducted on a 6100 diffractometer

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(Shimadzu) with a Cu Kα radiation. A scanning electron microscope (SEM, S4800,

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JEOL) was utilized to observe the morphology of materials with a working voltage of

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15 kV. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM),

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and elemental mapping measurements were carried out on a JEM-2100F microscope

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(JEOL) with an accelerating voltage of 100 kV. X-ray photoelectron spectroscopy

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(XPS) measurements were performed on an ESCALAB-MKII spectrometer

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(Thermo-Fisher Scientific Co., Ltd.) with an Al Kα radiation as the excitation source.

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The nanozyme concentrations were determined by inductively coupled plasma optical

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emission spectroscopy (ICP-OES) on an IRIS-1000 spectrometer (Thermo-Fisher

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Scientific Co., Ltd.).

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2.4. Peroxidase-like activity of the bimetallic PdCu

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The peroxidase-like activity of PdCu was studied by the catalytic oxidation of TMB

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in the presence of H2O2. All the reactions were monitored in the standard mode using

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a Cary-8454 Ultraviolet-Visible (UV-Vis) spectrometer (Agilent Technologies Co.,

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Ltd.). Unless otherwise stated, each measurement was repeated for three times. A 5

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mM TMB stock solution was prepared with ethanol for use. H2O2 stock solutions with

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various concentrations were daily prepared. Typically, the colorimetric measurements

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were performed in a 5 mL cylinder with 3 mL NaAc-HAc buffer (0.2 M, pH 4.0)

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consisting of 10 μg/mL PdCu, 0.327 M H2O2, and 0.167 mM TMB. The

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time-dependent absorbance changes for different reaction systems were recorded with


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a 30 s interval. The steady-state kinetics measurements were carried out by recording

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the absorbance at 652 nm at a 5 s interval within 1.5 min. The apparent kinetics

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parameters were calculated based on the equation ʋ = Vmax × [S]/(Km + [S]), where ʋ is

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the initial velocity, Vmax is the maximum reaction velocity, [S] is the substrate (H2O2

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or TMB) concentration, and Km is the Michaelis-Menten constant.

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To investigate the catalytic mechanism of the PdCu peroxidase mimic, we used TA

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as a fluorescent probe to capture hydroxyl radicals.29-30 To be specific, 10 μg/mL

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PdCu was incubated with 0.25 mM TA and 0.327 M H2O2 in 3 mL NaAc-HAc buffer

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(0.2 M, pH 4.0) at 30oC for 4 h. The fluorescence measurements were carried out on a

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Cary-Eclipse spectrometer (Varian Co., Ltd.) with an excitation wavelength of 315 nm.

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For comparison, control experiments were also conducted under identical conditions.

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2.5. Theoretical calculations

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All geometry optimizations and energy calculations were performed with periodic

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slab models using the Vienna Ab-Initio Simulation Package (VASP).31 The Pd(111)

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surface was modeled by a five-layered slab in the (111) direction and a p(3×3) unit

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cell in the lateral directions. During geometry optimizations, the top three layers for

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Pd(111) was fully relaxed, and the bottom two layers was frozen at the corresponding

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bulk face-centered-cubic (fcc) lattice positions. The adsorption energies were

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calculated using the equation Eads = Eslab+mol – (Eslab + Emol), where Eslab+mol is the total

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energy of the Pd slab with the adsorbate on it, and Eslab and Emol are energies of the

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isolated slab and adsorbate, respectively.

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2.6. Colorimetric detection of SCN- based on the inhibition of the PdCu



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peroxidase activity

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For the sensing of SCN-, SCN- stock solutions with various concentrations were

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first prepared with 0.2 M NaAc-HAc buffer (pH 4.0). Then, the mixture of SCN-,

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PdCu, H2O2, and TMB was incubated at 25oC for 5 min. Afterwards, UV-Vis spectra

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were recorded under standard conditions.

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The SCN- levels in tap water and human urine (provided by two 26-year-old

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smoker and non-smoker) were detected by our developed SCN- sensor. The tap water

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sample was directly used for colorimetric measurements. For each urine sample, 4.5

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mL 2% (v/v) HAc was first added into a 500 μL urine sample and centrifuged at

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10,000 rpm for 30 min to remove possible proteins. Then, 400 μL of the supernate

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was used for measurements.

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3. Results and discussion

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3.1. Fabrication and characterization of the bimetallic PdCu architecture

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In the traditional incipient wetness impregnation approach that is usually utilized to

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synthesize multi-component metal materials, separated entities of these metals are

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readily formed due to their differences in the reduction potential and lattice

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parameters.32 In the present study, microwave irradiation was employed to avoid this

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deficiency. Microwave-assisted synthesis can be performed in shorter reaction times

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and at lower temperatures compared with conventional heating methods.33 More

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importantly, microwave irradiation can provide ultrahigh temperature within

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molecules.34 This can facilitate the penetration of different metal atoms into each

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other.35-36 In our established microwave-assisted wet-chemical approach, NaBH4 was


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used as a reductant, and no surfactant or capping agent was employed (Scheme S1).

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Thus, clean material surface is expected to be obtained, which is beneficial for the

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free access of substrates to active sites.10, 37 In the present study, a 1:1 ratio of the

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K2PdCl4 and CuSO4 precursors was utilized to synthesize the bimetallic PdCu.

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ICP-OES measurements reveal that the atomic ratio of Pd to Cu in the prepared PdCu

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is 44:56, which basically corresponds to the feeding ratio of precursors.

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XRD measurements were first conducted to differentiate the synthesized bimetallic

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PdCu from the pure Pd counterpart. As shown in Figure 1, the pure Pd XRD pattern

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presents several diffraction signals with the 2θ positions of 40.12◦, 46.65◦, and 68.12◦,

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which are ascribed to the (111), (200), and (220) planes of the face-centered cubic

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structure of Pd (JCPDS No. 46-1043). These peaks are well defined with narrow full

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width at half maximum (FWHM). In comparison with pure Pd, the synthesized PdCu

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only produces recognizable Pd diffraction signals with much wider FWHM and lower

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intensity, and no other impurity peak, such as metallic Cu and its oxides, is observed

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in the PdCu XRD pattern. This phenomenon indicates that the Cu entity has been

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immersed into the Pd lattice facilitated by microwave irradiation. The insertion of Cu

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atoms with a smaller atomic radius causes the crystal defects of Pd, which reduces the

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crystallinity of PdCu, thus resulting in the lower intensity and wider FWHM. Besides,

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these diffraction peaks have an apparent shift to larger 2θ positions compared with the

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pure Pd, which also results from the incorporation of Cu atoms into the Pd lattice in

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place of Pd atoms. Taken together, a bimetallic PdCu material with Cu and Pd atoms

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interpenetrated well has been obtained via the microwave-assisted synthetic method.



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Figure 1. XRD patterns of the as-prepared pure Pd and bimetallic PdCu.

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The morphology of the as-prepared PdCu was examined by TEM and SEM.

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Similar to the pure Pd counterpart (Figure S1(A), Supporting Information), a

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well-defined coral-like (Figure 2(B)) architecture is clearly observed in the

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synthesized PdCu (A), which is in good agreement with the bimetallic PdCo material

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prepared by a similar method.36 The SEM image (C) also suggests a

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three-dimensional (3D) structure assembled by numerous protuberances and branches

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in the bimetallic PdCu. Similar to the coral-like PdCo,36 this PdCu structure is also

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formed by the interconnection and stacking of nanoparticles. In pure Pd, long-range

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order lattice fringes are found (Figure S1(B), Supporting Information), indicative of

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its excellent crystallinity. The 0.225 nm lattice spacing matches well with the (111)

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plane of face-centered cubic Pd. In PdCu, short-range order lattice fringes are

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captured (Figure 2(D)), suggesting that the bimetallic PdCu still remains a certain

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crystal structure. The lattice spacing of Pd decreases to 0.218–0.219 nm for the

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Pd(111) facet. This phenomenon is ascribed to the intercalation of Cu atoms into the 10 ACS Paragon Plus Environment

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Pd lattice. As Cu has a smaller atomic size than Pd, the intercalation of Cu causes

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crystal defects, which leads to the decrease of the Pd lattice spacing. Elemental

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mapping images (C–E) demonstrate the uniform distribution of Pd and Cu in the

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synthesized PdCu architecture, which also supports the formation of bimetallic PdCu

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with Cu atoms inserted in the Pd lattice. The coral-like structure of PdCu has the

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following advantages: (1) in comparison with independent nanoparticles that are

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prone to agglomeration, the interconnected structure in the synthesized PdCu

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nanozyme makes it more stable; (2) the branches in the coral-like structure provide

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abundant active sites and surfaces for reaction; (3) besides, the unique structure can be

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easily obtained by the microwave-assisted wet-chemical approach, with no surfactant

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or capping agent required.

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Figure 2. (A) shows the TEM image of the as-synthesized bimetallic PdCu. (B) is the

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photograph of the real coral. (C) presents the SEM image of the coral-like PdCu.

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(E)-(G) are the elemental mapping profiles of Pd-L, Cu-K, and Pd-L+Cu-K, 11 ACS Paragon Plus Environment


1

respectively.

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3.2. Evaluation of the peroxidase-like activity of PdCu

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Previously experimental and theoretical studies have suggested that Pd-based

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nanomaterials may have intrinsic peroxidase-like activity at acidic conditions.16,31,38

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To verify this feature in our synthesized PdCu, colorimetric experiments were

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performed with TMB as a peroxidase substrate. As presented in Figure 3(A), the

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bimetallic PdCu can catalyze the oxidation of colorless TMB into its blue oxide

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(TMBox) in the presence of H2O2, with a maximum absorbance at around 652 nm

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observed in the UV-Vis spectra. The absence of anyone of H2O2, PdCu and TMB will

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not trigger the color reaction, which is also demonstrated by the time-dependent

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absorbance changes for different reaction systems (Figure S2, Supporting

12

Information). This phenomenon is the direct evidence that the synthesized PdCu

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possesses the peroxidase-mimicking activity.

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Figure 3. (A) shows UV-Vis spectra for different combinations of TMB, H2O2, and

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PdCu. The inset presents the photograph of different reaction systems. (B) compares

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the time-dependent absorbance changes resulted from different peroxidase-like

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nanozymes. The inset shows the corresponding photographs.


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Similar to natural bio-enzymes, the peroxidase reaction catalyzed by the bimetallic

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PdCu is affected by reaction conditions. With the increase of the PdCu content, a

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faster color change process is observed (Figure S3, Supporting Information). The

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relationships between the buffer pH and temperature and the enzymatic activity

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exhibited by PdCu were also studied by varying the pH values from 2 to 7 and the

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temperature from 10 to 60°C (Figure S4, Supporting Information). The activity is

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observed to be optimal at pH 4 and at room temperature. In comparison with natural

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bio-enzymes, one of the most remarkable merits of nanozymes is their stronger

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resistance against harsh environments. To check this, we first incubated the PdCu

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nanozyme in solutions with various pH values or at different temperatures for 2 h, and

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then measured its activity under standard conditions. Our previous studies have

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revealed that natural horseradish peroxidase (HRP) exhibits high activity only in

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neutral media.10-11 With pH decreases or increases, its activity is sharply reduced, and

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no activity is found in strong acid solutions. With regard to temperature, the activity

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of HRP rapidly decreases when the incubation temperature exceeds 45℃. In contrast,

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as depicted in Figure S5 (Supporting Information), the PdCu nanozyme exhibits no

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remarkable change in activity when it is incubated in buffers with a wide range of pH

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from 3 to 14 (A). Its activity has also no notable change upon temperature (B). These

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results demonstrate the excellent robustness of the PdCu peroxidase mimic.

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It is generally considered that the peroxidase reaction can occur through two

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different paths:39-40 one is the hydroxyl radical mechanism, and the other is the

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electron transfer mechanism. To uncover the possible path for PdCu, TA was utilized



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as a fluorescence probe to track hydroxyl radicals, because it can capture the radicals

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and generate the unique fluorescence at around 425 nm.13 As displayed in Figure S6

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(Supporting Information), in comparison with the control groups, addition of PdCu

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into H2O2 can increase the fluorescence intensity originated from the oxidation of TA

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by hydroxyl radicals. This result reveals the hydroxyl radical mechanism for the

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peroxidase reaction catalyzed by the synthesized PdCu.

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More interestingly, the bimetallic PdCu peroxidase mimic exhibits higher activity

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and efficiency, compared with the pure Pd counterpart, to induce the color reaction of

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TMB. As demonstrated by Figure 3(B), the results depict higher absorbance values

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and faster color changes for the bimetallic PdCu, indicating the enhanced performance

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of this nanozyme. To further evaluate the catalytic activity of the bimetallic PdCu in

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contrast to that of pure Pd, steady-state kinetics measurements for the two nanozymes

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were performed. As displayed in Figure S7 (Supporting Information), typical

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Michaelis-Menten curves are observed for both pure Pd and bimetallic PdCu. The

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data were further fitted to the Michaelis-Menten model to obtain the kinetics

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parameters, as listed in Table 1. The obtained Km values for PdCu are a little larger

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than those for pure Pd. The larger Km for PdCu may be related to its larger Vmax.

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Surprisingly, the kcat values of PdCu are over 2-fold larger than those of the pure Pd

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counterpart for both TMB and H2O2. The catalytic efficiency, defined by kcat/Km, is

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also higher for PdCu than for pure Pd. These data conclude that the bimetallic PdCu is

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a desirable nanozyme with improved enzymatic activity and efficiency upon pure Pd

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with either TMB or H2O2 as the substrate.


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Table 1. Comparison of the kinetics parameters for pure Pd and bimetallic PdCu. Nanozyme Substrate Km (mM) Vmax (M s-1) kcat (s-1)

kcat/Km(s-1 M-1)

TMB

0.14

0.69×10-8

7.34×10-5

0.51

H2O2

1.52

3.96×10-6

4.21×10-2

27.66

TMB

0.25

1.19×10-8

20.20×10-5 0.81

H2O2

3.05

6.25×10-6

10.61×10-2 34.75

Pd

PdCu

2

The question why enhanced catalytic activity and efficiency are observed in the

3

bimetallic PdCu inspires us to explore the underlying mechanism. Since the catalytic

4

activity of nanozymes highly relies on their surface chemistry, we first carried out the

5

surface characterization for both Pd and PdCu by XPS. As depicted in Figure 4(A),

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the Pd 3d XPS patterns for both nanozymes can be de-convoluted to the Pd0 and Pd2+

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species. The ratio of the area sums for Pd0 and Pd2+ can be employed to assess the

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relative amounts of the two species presented on surface.41 As a result, the proportion

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of Pd0 in the bimetallic PdCu is calculated to be 68.6%, higher than that of the pure

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Pd (57.6%). This means that insertion of Cu atoms into the Pd lattice increases the Pd0

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percentage. According to the peroxidase mechanism for noble metal (M) nanozymes,

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metallic M0 participates in the activation of the H2O2 substrate to release hydroxyl

13

radicals.15,42-43 Therefore, the increase of the Pd0 ratio in PdCu will rationally improve

14

its activity and efficiency. On the other hand, the adsorption energy (Eads) of H2O2

15

onto nanozyme surface was investigated by density functional theoretical (DFT)

16

calculation, because the adsorption energy between H2O2 and metals can be used as a

17

convenient descriptor to predict the relative enzyme-like activity of the metals with a 15 ACS Paragon Plus Environment


1

similar surface morphology.31 Figure 4(B) illustrates the optimized structure of H2O2

2

adsorbed on Pd or PdCu surface. The H2O2 molecule can initiatively adsorb onto the

3

surfaces of both Pd and PdCu. The lower Eads of PdCu (-0.318 eV) than Pd (-0.295 eV)

4

also indicates the higher activity of the former to catalyze the peroxidase reaction in

5

acidic solutions.31

6 7

Figure 4. (A) compares the Pd 3d XPS patterns of the synthesized Pd and PdCu. (B)

8

shows the structure of H2O2 adsorbed on the surfaces of pure Pd and bimetallic PdCu.

9

3.3. Colorimetric sensing of SCN-

10

It is unexpectedly found that thiocyanate (SCN-) is able to suppress the color

11

reaction of TMB in the presence of H2O2 catalyzed by PdCu. As shown in Figure 5(A),

12

with no SCN- in the solution, a remarkable color reaction is observed. When a certain

13

amount of SCN- is added, the color reaction is seriously suppressed. In general, this

14

inhibition phenomenon may result from four possible factors: (1) SCN- may react

15

with H2O2 and leads to the decrease of the H2O2 content; (2) the SCN- species may

16

interact with the TMB chromogen and affects the color reaction;44-46 (3) the

17

interactions between SCN-and TMBox may suppress the color;47-49 (4) SCN- may


Page 16 of 32

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1

make some influences on the activity and efficiency of the nanozyme.42,

2

clarify which mechanism mentioned above plays a role in inhibiting the color reaction

3

by the SCN- species, we investigated the possible interactions of SCN- with H2O2,

4

TMB or TMBox by UV-Vis measurements. The UV-Vis results suggest no notable

5

interaction between SCN- and H2O2 (Figure S8, Supporting Information), TMB

6

(Figure S9, Supporting Information) or TMBox (Figure S10, Supporting Information).

7

Thus, it is deduced that the SCN- species suppresses the color reaction by decreasing

8

the peroxidase-like activity of PdCu. As illustrated in Figure 5(B), SCN- can be

9

adsorbed onto the surface of PdCu via the binding interaction of SCN- and the Pd

10

atoms.53 The adsorption decreases the active sites exposed in the PdCu peroxidase

11

mimic, thus resulting in the inhibition of the color reaction.

50-52

To

12 13

Figure 5. (A) demonstrates that addition of SCN- results in the suppression of the

14

H2O2-TMB-PdCu color reaction. (B) illustrates the mechanism for SCN- sensing.

15

On the basis of the above principle, colorimetric sensing of SCN- with excellent

16

analytical performance was further achieved. Thiocyanate (SCN-) is an important

17

chemical that is widely utilized in the textile dyeing, electroplating, and printing

18

processes. In biomedicine, accumulation of the species gives rise to a higher risk for



1

atherosclerosis, unconsciousness, and vertigo.54 Besides, SCN- can be converted to

2

highly toxic cyanides through irradiation and chlorination, producing great harm to

3

the environment. These aspects underline the significance of SCN- detection. Figure

4

6(A) presents UV-Vis spectra for the H2O2-TMB-PdCu color reaction inhibited by

5

SCN- at different concentrations. Obviously, the absorbance at 652 nm decreases with

6

the increasing content of SCN-. The relationships between the absorbance and the

7

analyte level are presented in (B), and a calibration curve of the absorbance upon the

8

logarithm of the SCN- concentration in the range of 0.001–100 μM is obtained. The

9

detection limit (LOD) is down to 1 nM. In comparison with previously developed

10

methods,53-62 as listed in Table S1 (Supporting Information), our assay has the

11

superiority in the aspects of both linear range and LOD. Compared with previously

12

reported nanomaterials used in thiocyanate sensing, the PdCu nanozyme developed

13

here has the following advantages: (1) unlike using all noble metals like Au-Pt and

14

gold nanoparticles, the insertion of inexpensive Cu atoms into the Pd lattice to form

15

the bimetallic PdCu can enhance the enzymatic activity, which will reduce the

16

economic cost; (2) the bimetallic PdCu also remains good bioconjugation and

17

biocompatibility, which will be beneficial for the sensing of thiocyanate in biological

18

matrices.


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1 2

Figure 6. (A) presents UV-Vis spectra for the H2O2-TMB-PdCu color reaction

3

inhibited by SCN- at different concentrations. The inset shows the corresponding

4

photograph. (B) displays the relationship between the absorbance at 652 nm and the

5

SCN- content. The inset indicates a linear relationship between the absorbance at 652

6

nm and the logarithm of the SCN- concentration. (C) compares the absorbance of the

7

H2O2-TMB-PdCu color reaction in the presence of various species. (D) records the

8

absorbance of the SCN--inhibited H2O2-TMB-PdCu reaction measured upon time.

9

To evaluate the selectivity of the developed SCN- assay, possible interferences from

10

a number of common anions and biological molecules were studied. As depicted in

11

Figure 6(C), these species (except Cl-) with a concentration of 500 μM (50-fold of the

12

SCN- level) affect little on the specific sensing of the analyte with a content of 10 μM,

13

while the Cl- species shows a little impact because of its poising effect. Taken together, 19 ACS Paragon Plus Environment


Page 20 of 32

1

the result verifies the acceptable selectivity of the SCN- sensor against these species.

2

The long-term reproducibility of our analytical method was also investigated by

3

recording the inhibited color reaction with 10 μM SCN- within 20 days. As shown in

4

(D), good long-term reproducibility of the assay for SCN- sensing is observed. To

5

further demonstrate the practicability of the colorimetric method for detecting SCN-,

6

one tap water sample and two human urine samples (from a non-smoker and a smoker,

7

respectively) were tested. As summarized in Table 2, the assay provides satisfactory

8

recoveries (in the range of 90%–120%) for the determination of the target in real

9

samples, indicative of its great promise for practical applications.

10

Table 2. Results of our method for the detection of SCN- in real samples. Sample

Tap water

[SCN-] added (μM) [SCN-] detected (μM) Recovery (%) 0

0.03±0.01

―

10

10.03±0.27

100.00

20

18.08±0.64

90.25

50

52.59±5.41

105.12

80

83.18±5.66

103.94

0

0.66±0.07

―

9.68±0.01

90.20

50

49.98±1.20

98.64

0

1.77±0.38

―

10

11.87±3.49

101.00

20

25.37±1.80

118.00

Non-smoker’s urine 10

Smoker’s urine


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1

4. Conclusions

2

In summary, we have fabricated a coral-like bimetallic PdCu architecture with

3

enhanced peroxidase-mimicking properties via a facile microwave-assisted approach.

4

The nanozyme proposed exhibits both enhanced catalytic activity and efficiency over

5

pure Pd. This improved performance is mainly attributed to the increased active Pd0

6

proportion and the reduced adsorption difficulty of H2O2 induced by the insertion of

7

Cu atoms into the Pd lattice. It is further found that SCN- can inhibit the activity of

8

PdCu selectively. Based on the principle, a colorimetric assay has been developed for

9

the sensing of SCN- with much wide detection scope, excellent sensitivity, good

10

selectivity, and favorable reliability. Our work not only offers a simple method to

11

acquire bimetallic enzyme mimics with desired catalytic characteristics but also

12

broadens their promising applications in biochemical analysis.

13

Acknowledgements

14

This study was financially supported by the National Natural Science Foundation

15

of China (Nos. 21605061 and 31601549), the Natural Science Foundation of Jiangsu

16

Province (No. BK20160489), the Natural Science Fund for Colleges and Universities

17

in Jiangsu Province (No. 16KJB150009), the Postdoctoral Fund of China (No.

18

2016M600365), the Postdoctoral Fund of Jiangsu Province (No. 1601015B), the Open

19

Fund from the Shanghai Key Laboratory of Functional Materials Chemistry (No.

20

SKLFMC201601), the Open Fund from the State Key Laboratory of Bioreactor

21

Engineering, and the Cultivation Project for Excellent Young Teachers in Jiangsu

22

University.



1

Supporting Information Available: Illustration for the preparation of PdCu; TEM

2

and HRTEM images; time-dependent absorbance changes of different reaction

3

systems; effects of conditions; robustness of the PdCu nanozyme; fluorescence

4

spectra for the interactions between different species and TA; steady-state kinetics

5

measurements; UV-Vis spectra for different combinations; comparison of detection

6

performance.

7

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Microwave-Assisted Fabrication of Bimetallic PdCu Nanocorals with Superior Peroxidase-Like Activity and Efficiency for Thiocyanate 1938x440mm (96 x 96 DPI)

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Microwave-Assisted Fabrication of Bimetallic PdCu Nanocorals with

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