Three-Dimensional Graphene Supported Bimetallic Nanocomposites

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Three-dimensional graphene supported bimetallic nanocomposites with DNA regulated-flexibly switchable peroxidase-like activity Fang Yuan, Huimin Zhao, Hongmei Zang, Fei Ye, and Xie Quan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00306 • Publication Date (Web): 28 Mar 2016 Downloaded from http://pubs.acs.org on March 30, 2016

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ACS Applied Materials & Interfaces

Three-dimensional graphene supported bimetallic nanocomposites with DNA regulated-flexibly switchable peroxidase-like activity Fang Yuan, Huimin Zhao*, Hongmei Zang, Fei Ye, Xie Quan

Key Laboratory of Industrial Ecology and Environment Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

*Corresponding author *e-mail: [email protected] Tel. 86-411-84706263 Fax. 86-411-84706263

1

ACS Paragon Plus Environment


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Abstract: A synergistic bimetallic enzyme mimetic catalyst, three-dimensional (3D) graphene/Fe3O4-AuNPs, was successfully fabricated which exhibited flexibly switchable peroxidase-like activity. Compared to the traditional 2D graphene-based monometallic composite, the introduced 3D structure, which was induced by the addition of glutamic acid, and bimetallic anchoring approach dramatically improved the catalytic activity, as well as the catalysis velocity and its affinity for substrate. Herein, Fe3O4NPs acted as supporters for AuNPs, which contributed to enhance the efficiency of electron transfer. Based on the measurement of Mott-Schottky plots of graphene and metal anchored hybrids, the catalysis mechanism was elucidated by the decrease of Fermi level resulted from the chemical doping behavior. Notablely, the catalytic activity was able to be regulated by the adsorption and desorption of single-stranded DNA molecules, which laid a basis for its utilization in the construction of single-stranded DNA-based colorimetric biosensors. This strategy not only simplified the operation process including labeling, modification and imprinting, but also protected the intrinsic affinity between the target and biological probe. Accordingly, based on the peroxidase-like activity and its controllability, our prepared nanohybrids was successfully adopted in the visualized and label-free sensing detections of glucose, sequence-specific DNA, mismatched nucleotides and oxytetracycline. Keywords: three-dimensional graphene; bimetallic; peroxidase-like; colorimetric; single-stranded DNA; activity regulation 2


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1. Introduction Metal-loaded nanomaterials have attracted much interest due to their practical applications in many fields such as catalyst, energy, capacitors, sensor detection, 1-6

pollutants degradation

. With respect to the selection of the appropriate supporter,

graphene, a kind of single-atom-thick sp2-bonded carbon material which possess low-dimensional surface, has shown great potential in anchoring various metal nanoparticles based on its huge surface area, Dirac-fermions and quantum Hall effect, especially the plenty of functional groups or defects on the surfaces of graphene oxide for metal anchoring

7-9

. Because of the spatial confinement and synergetic electronic

interactions between supporting graphene and loaded metal elements

10

, the new

nanohybrids are capable of harnessing the original properties of respective elements to gain some novel features. Among them, enzyme mimics catalytic property, the capacity of non-biological materials for mimicking the function of natural enzymes, has recently drawn extensive attention due to its potential applications in the development of catalysis, sensors and smart material devices 11-13. Currently, two-dimensional (2D) planar graphene-based materials are still suffering from agglomeration which would shield abundant active sites on the surface, resulting in the significant drop of the catalytic activity. Considering this problem, arisen efforts have been focused on the construction of three-dimensional (3D) structure

14-16

. This

spatial structure with higher surface-to-volume ratio improves not only the stability of graphene sheet, but also the functional activity due to the promotion of mass transfer and substrate capture

17-18

. On this basis of the structural improvement of graphene, 3



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the approach of bimetallic anchoring has also been introduced which is demonstrated to be capable of enhancing the original catalytic activity because of the synergistic effect between different metals

19-22

. However, despite the above achievements,

applications of the active hybrids based on regulation of the catalytic activity still remain great challenges. Benefiting from the π-rich conjugation surface domains, graphene are able to interact with biomolecular receptors through electrostatic or noncovalent interactions, which is expected to develop label-free biosensors through regulating the synergistic interfacial catalytic activity of the nanocomposites. In this work, we fabricate a synergistic 3D graphene-supported bimetallic Fe3O4, Au nanoparticles (3D graphene/Fe3O4-AuNPs) enzyme mimetic catalyst through an eco-friendly one-step solvothermal reaction. It has been demonstrated that the hybrids possess excellent peroxidase-like activity via a typical color reaction experiment (containing 3, 3’, 5, 5’-tetramethylbenzidine (TMB) as chromogenic substrate) in the presence of H2O2 (Scheme 1A), while corresponding monometallic hybrids have weaker or little activity. Kinetic analysis indicates that the catalytic behavior is in accord with typical Michaelis-Menten kinetics. Meanwhile, the mechanism of the whole catalytic process and the detailed functions of main elements are discussed. Furthermore, the catalytic activity can be flexibly regulated by biological stimuli (Scheme 1B), which lays a basis for its applications in the construction of label-free colorimetric biosensors. Based on this tunable feature, our prepared nanocomposites are also expected to be employed in the designs of label-free colorimetric platforms for the detection of sequence-specific DNA, mismatched nucleotides and monitoring 4


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the targets which have specific aptamers.

Scheme 1. Schematic illustrations of (A) peroxidase-like activity of 3D graphene/Fe3O4-AuNPs nanohybrids, and (B) Flexible regulation of peroxidase-like activity by single-stranded DNA.

2. Experimental Section 2.1 Materials DNA oligomers were provided by Takara Biotechnology Co. (Dalian, China) and purified by high-performance liquid chromatography (HPLC). Graphite powder, NaOH, citric acid, NaH2PO4·2H2O, Na2HPO4·12H2O were purchased from Tianjin Bodi Chemicals Co., Ltd. (Tianjin, China). Glucose, H2SO4, H3PO4, H2O2, glutamic acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). KMnO4, ethylene glycol were purchased from Tianjin Fuyu Fine Chemicals Co., Ltd. (Tianjin, China). FeCl3·6H2O was purchased from Tianjin Damao Chemical Reagent 5



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Factory (Tianjin, China). TMB, glucose oxidase (GOx), oxytetracycline (OTC), tetracycline (TET), doxycycline (DOX), chlortetracycline (CTE) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). HAuCl4 was purchased from Shenyang Jinke Chemical Reagent Factory (Shenyang, China). TMB working solution was prepared through dilution of its stock solution by citrate buffer (20.0 mM, pH 4.0, prepared by mixing the stock solution of Na2HPO4 and citric acid). All reagents were of analytical reagent grade and used as received without further treatment. All glassware was thoroughly cleaned with chromic acid and rewashed with the ultrapure water. Ultrapure water obtained from a Millipore water purification system (resistivity >18.0 MΩ•cm-1, Laikie Instrument Co., Ltd., Shanghai, China) was used throughout the experiments. 2.2 Instruments Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) images were gained from field emission SEM (Hitachi S-4800) equipped with EDS (IXRF systems 550i). Transmission electron microscopy (TEM) images were obtained through high-magnification TEM (FEI Tecnai G2 F30 S-Twin). X-ray diffraction (XRD) patterns were measured on Shimadzu LabX XRD-6000 with Cu Kα radiation (λ = 0.154056 nm). Raman spectra were recorded on a RenishawMicro-Raman system 2000 spectrometer with HeNe laser excitation. X-ray photoelectron spectroscopy (XPS) was employed to analyze the chemical state of respective element with a VGESCALAB 250 spectrometer. Ultraviolet-visible (UV-Vis) absorption spectra were recorded on a Jasco V-550 spectrometer. Mott-Schottky plots were 6


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measured in Na2SO4 electrolyte (0.1 M) at a frequency of 1 kHz on a CHI 650B electrochemical workstation (CH Instruments, Inc., Shanghai) in a conventional three-electrode configuration. 2.3 Preparation of 3D graphene/Fe3O4-AuNPs hybrids Graphene oxide was prepared through an improved method which provided a greater amount of hydrophilic oxidized graphene material as compared to Hummers’ method or Hummers’ method with additional KMnO4

23

. The concentration of prepared

graphene oxide aqueous solution was measured to be 51.49 mg mL-1. In the typical experiment,

a

one-step

solvothermal

reaction

was

used

to

prepare

3D

graphene/Fe3O4-AuNPs hybrids. 75 µL graphene oxide aqueous solution, 18.8 mg glutamic acid, 34.5 mg FeCl3·6H2O, 1.28 mL HAuCl4 aqueous solution (95 mM) were stepwise added into 3.75 mL ethylene glycol. The pH value of the mixture was adjusted to 10 by NaOH (0.1 M) under vigorous stirring. After being sonicated for 2.5 h at room temperature, the solution was transferred to a Teflon lined autoclave for solvothermal reaction for 12 h at 180 °C which was the common used temperature for graphene oxide reduction. The product cooled naturally to room temperature was subsequently washed by ultrapure water followed by freeze drying which could prevent the aggregation of graphene sheets. The final dark brown powder was the expected 3D graphene/Fe3O4-AuNPs hybrids. Corresponding monometallic hybrids were prepared by the same steps without the addition of HAuCl4 or FeCl3 in the reaction solution. 2.4 Colorimetric assay for peroxidase-like catalytic activity 7



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In the typical experiment, 1.05 mM TMB and 0.15 mM H2O2 were orderly added into 0.1 mg mL-1 hybrids. After reaction for 2 min (25 °C), the UV-Vis absorption spectra of solution were recorded on spectrometer under absorbance-dependent style. 2.5 Assay for catalytic activity inhibition In the typical experiment, 1.0 µM single-stranded (ssDNA) or double-stranded (dsDNA) was added into 0.1 mg mL-1 graphene/Fe3O4-AuNPs and incubated for 20 min. Then the solution was mixed with 1.05 mM TMB and 0.15 mM H2O2. After reaction for 2 min (25 °C), the mixture was transferred to a quartz cell for absorbance-dependent absorbance measurement. 2.6 Potential application 2.6.1 Detection of glucose Firstly, 0.1 mL GOx (1.0 mg mL-1) and 0.1 mL different concentrations of glucose in PBS buffer (pH 7.4) were incubated at 37 °C for 30 min. Then 0.1 mg mL-1 graphene/Fe3O4-AuNPs was added into 50.0 µL above glucose reaction solution and followed by the addition of 1.05 mM TMB. After reaction for 2 min (25 °C), the UV-Vis absorption spectra of solution were recorded on spectrometer under absorbance-dependent style. 2.6.2 Detection of sequence-specific DNA or mismatched nucleotides The probe S1 (1.0 µM) was hybridized with its complementary sequence C1 or mismatched sequences in PBS buffer (20.0 mM, pH 7.4, prepared by mixing the stock solution of Na2HPO4 and NaH2PO4) under the experimental condition of 10 cycles of 95 °C for 60 s and 25 °C for 120 s. After being cooled to room temperature, the 8


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solution was mixed with 0.1 mg mL-1 graphene/Fe3O4-AuNPs and incubated for 20 min. Then 1.05 mM TMB and 0.15 mM H2O2 was added into the mixture. After reaction for 2 min (25 °C), the mixture was transferred to a quartz cell for absorbance-dependent absorbance measurement. 2.6.3 Detection of OTC OTC aptamer (1.0 µM) was mixed with various concentrations of OTC for 30 min at room temperature. Subsequently, 0.1 mg mL-1 graphene/Fe3O4-AuNPs was added into the solution and incubated for 20 min. Then 1.05 mM TMB and 0.15 mM H2O2 was added into the mixture. After reaction for 2 min (25 °C), the mixture was transferred to a quartz cell for absorbance-dependent absorbance measurement.

3. Results and discussion 3.1 Characterization We fabricated porous 3D graphene/Fe3O4-AuNPs hybrids through an eco-friendly solvothermal reaction. The oxygen functional groups and defects on the graphene sheets provided anchoring sites for reduced metal nanoparticles. From the SEM image of the prepared hybrids in Figure 1A, the graphene-based material with 3D structure on which some nanoparticles were uniformly deposited on the surface was clearly observed. And the edge of curly graphene sheets was observed at high magnification in Figure 1B. Combined with the data of EDX (Figure S1 in Supporting Information), these nanoparticles were inferred as gold and iron particles. The HRTEM images were showed in Figure 1C. The lattice fringes could be clearly observed of which the 9



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spacing values was calculated to be (1) 0.24 nm, corresponding to the cubic Au (111) crystal plane (0.236 nm, PDF#04-0784); (2) 0.20 nm, 0.26 nm and 0.29 nm, matching well with the (400), (311) and (220) crystal planes of cubic Fe3O4 nanoparticles respectively (0.209 nm, 0.253 nm, 0.297 nm, PDF#19-0629). Moreover, the interlaced lattice fringe indicated the overlapping or adjacent in situ growth of these two kinds of nanoparticles, which was helpful for the improvement of its catalytic activity. Combined with the characterization results of the morphologies in the HRTEM images of monometallic graphene/AuNPs (Figure S2A in Supporting Information) and graphene/Fe3O4NPs (Figure S2B in Supporting Information) hybrids, it was concluded that the black particles with a diameter of about 15 nm was AuNPs and the light gray particles around with an average diameter of about 7 nm were Fe3O4NPs.

A

B

C

D

Figure 1. (A, B) SEM images of 3D graphene/Fe3O4-AuNPs hybrids under different 10


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magnifications. (C) HRTEM image of prepared hybrids. (D) XRD spectra of a) graphene/Fe3O4NPs, b) graphene/AuNPs, c) bimetallic graphene/Fe3O4-AuNPs hybrids. XRD pattern was employed to analyze the crystal structure of the prepared 3D graphene/Fe3O4-AuNPs hybrids (Figure 1D). Compared to the XRD patterns of monometallic graphene/Fe3O4NPs and graphene/AuNPs, the typical diffraction peaks of the bimetallic material at 2θ values of 35.4° and 43.1° corresponded to the (311) and (440) crystal planes of the cubic Fe3O4 nanoparticles (PDF#19-0629). The peaks at 2θ values of 38.2°, 44.4°, 64.6°, 77.5° and 81.7° could be assigned to (111), (200), (220), (311) and (222) crystal planes of cubic Au nanoparticles (PDF#04-0784). Based on the characterization results above, it was concluded that the porous 3D graphene/Fe3O4-AuNPs was successfully synthesized. Figure 2A showed the Raman spectra of graphene and 3D graphene/Fe3O4-AuNPs hybrids. In the typical Raman spectrum of the bimetallic hybrids, two prominent peaks at the wavenumbers of 1353 cm-1 and 1605 cm-1 belonged to the D and G bands of graphene respectively. Compared to the spectrum of pure graphene, not only the ratio of the intensity of D and G (ID/IG) was increased, but also the peaks were shifted from 1350 cm-1 to 1353 cm-1 and 1620 cm-1 to 1605 cm-1. In the graphitic structure, G band represented the in-plane vibration of sp2-hybridized carbon, whereas D band represented the sp3-hybridized carbon and defects which were associated with vacancies and grain boundaries defects 24. The increasing of ID/IG was attributed to the localized sp3-hybridized defects in the sp2-hybridized carbon network 25 caused by the 11



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metallic doping behavior, meanwhile the shift of the peaks was seen as a symbol of charge transfer between graphene sheets and the anchored metal nanoparticles resulted from their robust covalent interfacing

26

, which contributed to the

enhancement of the catalytic activity 27. Furthermore, a relatively weak peak related to Fe3O4NPs appeared at 688 cm-1 which belonged to the A1g vibration mode (symmetric stretch of oxygen atoms along Fe-O bonds)

28

, indicating the oxidation

state of Fe3O4 in the nanocomposite. The XPS spectra were characterized to further clarify the composition and surface information of the nanohybrids (C 1s set to 284.6 eV for calibration). The two broad peaks around the binding energies of 710.5 eV and 724.0 eV (Figure 2B) were assigned to Fe 2p3/2 and Fe 2p1/2 respectively which further confirmed the oxidation state of Fe3O4 29. The two peaks centered at the binding energies of 83.9 eV and 87.6 eV (Figure 2C) were corresponded to Au 4f7/2 and Au 4f5/2 respectively 30, indicating Au existed in a metallic state almost without surface oxidation. In the high-resolution C 1s XPS spectrum (Figure 2D), a peak appeared at the binding energy of around 285.0 eV, which was associated with oxygenated functional groups. Through peak fitting analysis, the residual group species were mainly C-C (284.6 eV), C-OR (285.5 eV), C-O-C (286.6 eV) and O=C-OR (288.3 eV), which were conducive to the in situ growth of metal particles on graphene sheets 21.

12


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A

B

C

D

Figure 2. (A) Raman spectra of a) graphene, and b) graphene/Fe3O4-AuNPs. (B) High-resolution Fe 2p XPS spectra of hybrids. (C) High-resolution Au 4f XPS spectra of hybrids. (D) High-resolution C 1s XPS spectra of graphene/Fe3O4-AuNPs. 3.2 Peroxidase-like enzyme mimetic property of graphene/Fe3O4-AuNPs In the following, the peroxidase-like enzyme mimetic property of the prepared 3D graphene/Fe3O4-AuNPs hybrids was assessed. Peroxidase was able to degrade H2O2 to produce hydroxyl radicals (·OH), which later oxidized the relative substrates 31. As shown in Figure 3A, in the existence of the nanohybrids, it was observed that the typical peroxidase substrate TMB could be catalytically oxidized by H2O2 to produce a blue color with a typical absorption wavelength of 652 nm

32

, while no obvious

color change occurred in the absence of the hybrids. It demonstrated the peroxidase-like activity of 3D graphene/Fe3O4-AuNPs. Figure 3B indicates that the 13



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catalytic reaction of system was enhanced with the increasing of the catalyst concentration. Furthermore, the synergistic catalytic property of the hybrids was investigated in Figure 3C. Compared to monometallic hybrids graphene/AuNPs and graphene/Fe3O4NPs, the prepared bimetallic 3D catalyst exhibited much more enhanced catalytic activity, which proved the bimetallic system was contributing to improvement of the catalytic activity. AuNPs, Fe3O4NPs or graphene alone exhibited little peroxidase-like activity towards decomposition of the existed H2O2 in system, and it was noted that the simple physical mixtures of graphene and the metal nanoparticles also performed relatively poor catalytic activity as well. It indicated that the enhanced catalytic activity was ascribed to the synergetic coupling effect that occurred at the interface of graphene and these metal nanoparticles, which was seen as a result of metal precursor in situ growth on the graphene sheet 11, 33.

A

B

C

14


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Figure 3. (A) Ultraviolet-visible absorption spectra of TMB reaction solutions with and without graphene/Fe3O4-AuNPs. (B) Ultraviolet-visible absorption spectra of TMB

reaction

solutions

catalyzed

by

various

concentrations

of

graphene/Fe3O4-AuNPs. (C) Comparison of catalytic activities of prepared hybrids and graphene, AuNPs, Fe3O4NPs, graphene/Fe3O4NPs, graphene/AuNPs, physical mixtures of graphene with Fe3O4NPs and AuNPs under the same condition. The maximum point was set as 100%. 3.3 Mechanism analysis Firstly, we devoted to investigate the kinetic mechanism of peroxidase-like catalytic activity by determining the apparent steady-state kinetic parameters for this color change reaction. Within certain concentration ranges of TMB (Figure 4A) and H2O2 (Figure

4B),

the

typical

Michaelis-Menten

curves

of

the

prepared

3D

graphene/Fe3O4-AuNPs were fitted. The Michaelis-Menten constant (Km) and the maximum reaction velocity (Vmax) were calculated from Michaelis-Menten equation (Eq. 1):

ܸ଴ =

௏೘ೌೣ [ௌ]

(1)

௄೘ ା[ௌ]

Where V0 was initial velocity, Vmax was maximum reaction velocity, [S] was substrate concentration, and Km was the Michaelis-Menten constant which equaled to the substrate concentration when the reaction velocity reached half of Vmax. Fundamentally lower Km and higher Vmax indicated stronger affinity between substrate and catalyst and higher catalytic activity respectively. Table 1 listed the comparison of Vmax and Km of natural enzyme HRP and those of 3D 15



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graphene/Fe3O4-AuNPs. As expected, the Vmax of the inorganic catalyst with TMB and H2O2 as substrates were about 6.1 and 1.1 times higher than those of HRP, which suggested higher peroxidase-like activity towards the catalytic reaction. Meanwhile, the values of Km with TMB and H2O2 as substrates were about 1.1 and 23 times lower than those of HRP, suggesting stronger affinity between substrate and catalyst. Compared to the apparent kinetic parameters of other researched inorganic enzyme memitics, especially the corresponding monometallic hybrids (Table S1 in Supporting Information), the Km of this catalyst with H2O2 as substrate was about 20 times lower than that of graphene oxide, even 3.6 times and 3 orders of magnitude lower than those of graphene/Fe3O4 and graphene/Au respectively. It was worth noting that the Km value with H2O2 as substrate of our prepared catalyst was higher than that of 3D graphene-supported bimetallic Fe3O4, Pd nanoparticles (3D graphene/Fe3O4-Pd)

21

,

which indicated that relatively higher concentration of H2O2 (compared to that of graphene/Fe3O4-Pd) was adequate for reaching the maximum of the catalytic activity.

A

B

Figure 4. Kinetic analysis of prepared 3D graphene/Fe3O4-AuNPs hybrids by double reciprocal plots with substrate of (A) TMB, (B) H2O2. Table 1. Comparison of kinetic parameters of prepared 3D graphene/Fe3O4-AuNPs 16


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hybrids and HRP. Substrate

Vmax (10-7 Ms-1)

Km (mM)

3D graphene/Fe3O4-AuNPs

TMB

4.6

0.20

3D graphene/Fe3O4-AuNPs

H2 O2

1.5

0.20

HRP

TMB

0.8

0.21

HRP

H2 O2

1.4

4.57

Theoretical investigations further interpreted the mechanism of the whole catalytic reaction. The oxidation of the benzidine derivative TMB by H2O2 could be achieved only when there existed suitable catalyst in this reaction system, otherwise the electrons of the non-bonding orbital (NBO) of TMB could not be transferred to the lowest unoccupied molecular orbital (LUMO) of H2O2 with the higher energy by fulfilling the redox potential of H2O2

34

. In this work, our prepared naked graphene

was proved to be incapable of catalyzing the decomposition of H2O2 to oxidize TMB while the prepared hybrids were quite qualified for the catalysis (Figure 3C). This was due to the hindering of the electrons transfer resulted from the higher Fermi energy of naked graphene than the NBO of TMB. On the basis of the calculations of density functional theory, it could be inferred that the difference of the work functions 35

between graphene (4.5 eV for Au

38

) and metals (5.52 eV for Fe3O4 (111)

36-37

and 5.54 eV

) would change the electronic structure and Fermi level of graphene during

the chemical interaction process at the graphene-metal interface, facilitating doping with electrons or holes

39

. Factually in situ growth of Fe3O4NPs and AuNPs on 17



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graphene indeed resulted in the decrease of Fermi energy of graphene, which was attributed to the plethora of electrons in conduction band of the metal nanoparticles, especially the AuNPs, caused underlying p-type 3D hybrids matrix. This deduction was further confirmed in Figure 5A and 5B, which showed the Mott-Schottky plots of graphene and our prepared 3D graphene/Fe3O4-AuNPs, characterizing their Fermi levels were about -0.32 eV and 0.2 eV (vs SCE) respectively. In addition, the results of corresponding monometallic hybrids (Figure S3A and S3B in Supporting Information) also proved that bimetallic anchoring was benefited to the decrease of Fermi level. Subsequently, the 3D hybrids with decreased Fermi energy could effectively receive the lone pair electrons from NBO of TMB followed by the rise of the original Fermi level 40, which was higher than the LUMO of H2O2 to facilitate the electron transfer, successfully processing the oxidation of the chromogenic substrate TMB. Furthermore, we attempted to gain more insight into the synergetic catalytic behaviors of the graphene-bimetallic nanomaterial. After the chemical interaction process, the catalytic activity of the fabricated hybrids was obviously enhanced. The difference between the work functions of the metals promoted the electron transfer from Fe3O4NPs to AuNPs, enhancing the catalytic activity of the hybrids 21. XPS was also employed to investigate the difference of electronic structures of AuNPs in the prepared nanohybrids and the corresponding monometallic hybrids (Figure 5C). The main Au 4f peak of 3D graphene/Fe3O4-AuNPs negatively shifted to lower binding energy (about 0.2 eV) as compared to that of graphene/AuNPs (C 1s set to 284.6 eV 18


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for calibration), which signified the electrons transferred from Fe3O4NPs to AuNPs 41-42

, improving the catalysis for oxidation of TMB. This was because the chemical

doping increased the charge carrier density and mobility, accelerating the whole catalytic reactions

43

. Herein, Fe3O4NPs, acting as supporters for AuNPs

21

which

could also be observed from the interlaced lattice fringes in the HRTEM (Figure 1C), contributing to improve the efficiency of electron transfer. Graphene, with 3D structure which benefited to more efficient contact with H2O2

44

, provided a huge

plane for loading metal nanoparticles. Carbon vacancy defects and oxygenated functional groups on the original graphene oxide surface helped the connection of graphene and metal nanoparticles based on the interaction between π orbitals in graphene and d orbitals in metals 45-47.

A

B

C

Figure 5. Mott-Schottky plots of (A) graphene, (B) graphene/Fe3O4-AuNPs hybrids. 19



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(C) Comparison of high-resolution Au 4f XPS spectra between a) monometallic graphene/AuNPs and b) bimetallic graphene/Fe3O4-AuNPs. 3.4 Optimization and flexible regulation of catalytic activity by DNA Experiments observed that the catalytic activity of this peroxidase mimic was also dependent on pH, hydrothermal time, concentration of glutamic acid and molar ratio of Au:Fe during the hybrids fabricating process, and temperature and concentrations of H2O2 and TMB respectively during the color developing reaction (Figure S4 in Supporting Information). What was noteworthy was that glutamic acid influenced the peroxidase activity via influencing the 3D structure of the hybrids. Figure 6A showed the SEM image of the hybrids fabricated without the addition of glutamic acid, of which the 3D structure was almost disappeared. Their peroxidase-like activities were also characterized in Figure 6B. The impaired activity also proved the 3D structure was helpful to increase the catalytic capability. Accordingly, it suggested that glutamic acid played an essential role in the formation of the 3D structure. Interestingly, we found that the catalytic interface of this bimetallic 3D catalyst could be flexibly “switched off” based on interaction with ssDNA nucleotides. As shown in Figure 6C, the catalytic activity was inhibited upon the addition of a random ssDNA sequence (S1, 22 mer, Table S2 in Supporting Information) in the reaction system. With the increase of concentration of S1, the activity of the catalyst became more inactive. Due to the strong π-stacking interactions between the ring structure in nucleobases and the hexagonal cells of graphene 48, ssDNA could be easily adsorbed onto the surface of the synergistic catalyst to occupy the active catalytic sites of the 20


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hybrids. Thereby the formation of self-assembled complex allows the original catalytic activity to be inhibited. Furthermore, it had been reported that the ssDNA could also easily stick to gold nanoparticles because of its flexible structure

11, 49

.

These adsorptions all prevented the diffusion and combination of the peroxidase substrate to the hybrids interface with catalytic activity. Subsequently, dsDNA (obtained from the hybridization of S1 and its complementary sequence C1 (Table S2 in Supporting Information)) was employed to further prove this hypothesis. The slow decrease of activity compared to that of ssDNA indicated no significant inhibition effect to the catalysis, which was the result of the weak binding interactions between graphene sheet and dsDNA brought by the stiffer structure of dsDNA. On the other hand, positively charged TMB was liable to complexed with dsDNA characterized with repetitive units due to the electrostatic interaction

11

, which also hindered the

adsorption of dsDNA onto the graphene. Moreover, the inhibition was even effected by the length of the sequence. Five ssDNA sequences with varying length (from 7 mer to 76 mer) were investigated in Figure 6D which showed that the 22 mer ssDNA exhibited the strongest impact on the catalytic activity (the sequence details were listed in Table S2 in Supporting Information). The interaction between graphene and shorter nucleotide was relatively weak for stable adsorption, while longer nucleotide would easily form secondary structure itself thus preventing its adsorption onto graphene 50. Together with these observations about the controllable interface of the peroxidase-like catalyst, it was possible to develop a series of visible and label-free colorimetric sensors for specific targets in 21



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homogeneous solution system.

A

B

C

D

Figure 6. (A) SEM image of 3D graphene/Fe3O4-AuNPs fabricated without glutamic acid. (B) Comparison of catalytic activities of prepared hybrids a) without, b) with glutamic acid. (C) Comparison of decreased catalytic activity of prepared hybrids inhibited by ssDNA and dsDNA. (D) Catalytic activity of prepared hybrids inhibited by ssDNA with varying lengths. 3.5 Application 3.5.1 Detection of glucose Considering the fact that the 3D graphene/Fe3O4-AuNPs catalyzed color variation of TMB oxidation was dependent on the H2O2 concentration, we simply used this property in the quantitative detection of H2O2. As expected, the color of reaction system became blue along with the addition of H2O2. The results in Figure 7A showed 22


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the absorbance at 652 nm (the characteristic absorption peak of oxidized TMB) was proportional to the concentration of H2O2. The linear detection range was 0.02-0.19 µM with the limit of quantitation (LOQ) of 0.02 µM based on the experimental data, and the limit of detection (LOD) of 0.012 µM based on the 3δ/slope rule. It suggested that our prepared material was able to develop a simple and rapid method to detect H2O2 in solution. Thus it was inferred that this method could be extended to the detection of substrate which was able to generate H2O2. For instance, glucose oxidase (GOx) could catalyze the reaction of glucose and O2, generating gluconic acid and H2O2 (the reaction equation was C6H12O6+O2→C6H12O7+H2O2). Under the catalytic action of GOx, the proposed colorimetric method was expected to be employed in the determination of glucose which was seen as a significant marker for the diagnosis of diabetes mellitus. As shown in Figure 7B, the absorbance at 652 nm linearly increased with the increasing of glucose concentration. The LOQ was 0.015 µM and the LOD was calculated to be 0.012 µM (the inset of Figure 7B) based on the linear detection range of 0.015-0.5 µM. Considering that the concentration ranges of serum glucose in healthy and diabetic individuals were 3-8 mM and 9-40 mM respectively

51

, this

developed method was also promising to detect the glucose in diluted human serum. Compared to other available methods for H2O2 and glucose detections based on inorganic enzyme mimics (Table S3 in Supporting Information), the strategy with our prepared hybrids was comparative or even more sensitive, which proved its feasibility of practical application in the future.

23



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A

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B

Figure 7. (A) Linear calibration curve of corresponding absorbance of 652 nm as a function of H2O2 concentration. (B) Absorbance change of 652 nm as a function of glucose concentration. Inset: linear calibration curve. 3.5.2 Detection of sequence-specific DNA and mismatched nucleotides Taking advantage of the unique DNA species-responsive feature of the 3D hybrids, we investigated its potential application in the analysis of sequence-specific DNA oligonucleotide based on DNA hybridization. S1 was chosen as the probe for the detection of its complementary sequence C1. As shown in Figure 8A, in absence of C1, the activity of the catalyst was first inhibited by the treatment of S1 adsorption. With the addition of the target C1, S1 was hybridized with C1 to form the corresponding dsDNA based on the base complementation pairing rule, which lead to the escape of these DNA from hybrids, resulting in the recovery of the passivated catalytic capability. The color change, which was dependent on the concentration of C1, was produced with desorption of the dsDNA. The LOQ was measured to be 0.02 µM, and the LOD was calculated to be 0.011 µM based on the equation of 3δ/slope (the inset of Figure 8A). We further tested this colorimetric system for the detection of several mismatched ssDNA nucleotides (from one to four bases mismatches) (Figure 24


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8B). The relatively unapparent activity recovery suggested that the system based on the interaction between the peroxidase-like catalyst and ssDNA was expected to be used for sequence-specific ssDNA target or mismatched nucleotides.

B

A

Figure 8. (A) Absorbance change of 652 nm as a function of C1 concentration. Inset: linear calibration curve. (B) Catalytic activity recovery based on mismatched ssDNA nucleotides with concentration of 0.4 µM (from one to four bases mismatches). 3.5.3 Detection of OTC based on its aptamer Given the fact that aptamer, ssDNA or RNA nucleotide, was capable of exhibiting ultrahigh affinity for the specific target based on the target-induced conformational change 52, we applied the constructed active catalyst in the detection of OTC, a small molecule environmental pollutant of which the aptamer was a 76 mer ssDNA sequence, to develop a simple sensing method without any label or modification process. In the existence of the OTC aptamer, the catalyst display negligible activity to catalytically oxidize TMB. Upon the addition of OTC, the color change got stronger with the increase of the concentration of added OTC (Figure 9A). The LOQ was measured to be 0.01 µM, and the limit of detection (LOD) was calculated to be 0.008 µM (the inset of Figure 9A) based on the linear detection range of 0.01-0.25 25



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µM. Table S4 in Supporting Information summarized several available sensing approaches for OTC detection. It indicated that the LOD achieved in the present assay was comparable to the majority of listed methods. The specific binding of OTC and its aptamer resulted in more folds of the aptamer, which released the OTC/aptamer complex from the catalyst active interface. In direct contrast, no obvious color producing in the presence of three other structurally similar interferences including tetracycline (TET), doxycycline (DOX), and chlortetracycline (CTE) (Figure 9B). The anticipated result indicated our prepared hybrids were in principle appropriate candidates for constructing simple sensing platform for detecting a wide range of analytes with certain aptamers.

A

B

Figure 9. (A) Absorbance change of 652 nm as a function of OTC concentration. Inset: linear calibration curve. (B) Comparison of activity recovery respectively in the presence of 0.1 µM OTC, 5 µM TET, 5 µM DOX, 5 µM CTE.

4. Conclusions In conclusion, we fabricated a synergistic bimetallic 3D graphene/Fe3O4-AuNPs enzyme mimetic catalyst with flexibly switchable peroxidase-like activity. Compared 26


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to the traditional 2D graphene-based monometallic composite, the glutamic acid-induced 3D structure and bimetallic anchoring approach dramatically improved the catalytic activity, as well as the catalysis velocity and its affinity for substrate. Moreover, the catalytic activity was able to be regulated by the adsorption and desorption of ssDNA molecules. Accordingly, the attractive feature laid a basis for its utilization in the construction of ssDNA-based colorimetric biosensors. This strategy not only simplified the operation process including labeling, modification and imprinting in other methods, but also protected the intrinsic affinity between the target and biological probe. Subsequently, our prepared nanohybrids was successfully adopted in the quantitative analysis of glucose based on the peroxidase-like activity, and further realized the visualized and label-free detections of sequence-specific DNA, mismatched nucleotides and OTC. We anticipated that the smart graphene-based material would facilitate its potential applications in more fields such as environmental monitoring, gene detection and molecular diagnosis in the future.

Acknowledgment This work was supported by the National Nature Science Foundation of China (No. 21477012), Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R05).

Supporting Information Oxidation reaction of TMB to oxidized TMB. EDS pattern, HRTEM images and 27



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Mott-Schottky plots of prepared nanohybrids. Comparison of kinetic parameters of reported inorganic enzyme mimic. Optimization of experimental conditions. Nucleotide sequences used in this work. Comparison of available methods for H2O2, glucose and oxytetracycline detection.

28


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Three-Dimensional Graphene Supported Bimetallic Nanocomposites

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