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Highly active graphene oxide-supported cobalt single-ion catalyst for chemiluminescence reaction Jue Wang, Wenhui Zhong, Xiaoying Liu, Tongtong Yang, Fang Li, Qi Li, Weiren Cheng, Chen Gao, Zheng Jiang, Jun Jiang, and Hua Cui Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03873 • Publication Date (Web): 23 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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Analytical Chemistry

Highly active graphene oxide-supported cobalt single-ion catalyst for chemiluminescence reaction Jue Wang,†,§ Wenhui Zhong,†,‡,§ Xiaoying Liu,† Tongtong Yang,† Fang Li,† Qi Li,† Weiren Cheng, ∥ Chen Gao,∥Zheng Jiang,⊥ Jun Jiang,†,* Hua Cui†,* †

CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Institute of Applied Physics, GuizhouSynergetic Innovation Center of Scientific Big Data for Advanced Manufacturing Technology, Guizhou Normal College,Gaoxin Road 115, Guiyang, Guizhou 550018, P. R. China ∥ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ⊥ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Zhang Heng Road No. 239, Pu Dong District, PO Box 918, Shanghai 201204, P. R. China

* Corresponding author: Prof. H. Cui, Prof. J. Jiang, E-mail: [email protected]; [email protected] Fax: +86-551-63600730; +86-551-63602969

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ABSTRACT: Graphene or graphene oxide (GO)-supported metallic nanoparticles and single metal atom as potentially effective catalysts for chemical reactions have recently received extensive research interests. However, metal utilization in nanoparticle catalysts is limited and metal atoms are readily to drift on graphene surface and consequently form aggregated large particles, making practical applications limited. Here, we report metal ions directly immobilized on GO as a novel GO-supported single-ion catalyst for chemiluminecence (CL) reactions. It is found that GO-supported cobalt ions with good stability could catalyze strongly luminol-H2O2 and lucigenin-H2O2 CL reactions, accompanied by dramatically enhanced CL emission. Theoretical studies reveal that the coupling between Co2+ and GO induces effective polarization charges, improving chemical activity of reaction site, which promotes the generation of intermediate radicals and accelerates the CL reactions. This work may be generalized to other GO-supported metal ions as catalysts for a wide range of chemical reactions. The developed GO-supported cobalt single-ion nanocomposites as nanointerfaces may find future applications in CL bioassays.

Two-dimensional (2D) graphene or graphene oxide (GO) materials have emerged as a rapidly rising star in the field of catalysis research mainly because of their superior charge mobility, high surface-to-volume ratio, and excellent compatibility with other materials, to name a few.1,2 For instance, graphene and GO have been shown as good catalysts for oxidation and hydration reactions.3-5 However, the key drawback of such carbon-catalysts is their low catalytic activity.6-8 High-efficiency catalysts involving graphene-supported metallic nanoparticles have recently attracted much attention. For example, palladium nanoparticles deposited on GO were demonstrated as efficient electro-catalysts in methanol electro-oxidation and hydrogen fuel cells as well as good chemical catalysts in oxidation, C-C coupling and hydration reactions.9,10 However, metal utilization in these catalysts is limited, as the active sites are only located on the surface of the nanoparticles. A recent exciting breakthrough in graphene or GO-supported catalyst development is to attach a single metal atom on the surface of graphene or GO in order to create efficient chemical activation centers with maximized efficiency by utilizing most of the metal atoms. For example, atomic cobalt and platinum on nitrogendoped graphene as highly active and robust catalysts could be used for hydrogen evolution reactions.11,12 However, metal atoms are readily to drift on graphene surface and consequently form aggregated large particles, which severely reduce active catalytic sites and damage catalyst stability, making practical applications limited. An alternative method is to bind metal atom with graphene through strong chemical interactions such as replacing C atom with metal, which however requests extra steps and sophisticated techniques to decorate the perfect graphene.7 Therefore, new concept is needed for the development of highly efficient graphene-supported catalysts. In this work, we report metal ion directly immobilized on the surface of GO as a novel GO-supported metal single-ion catalyst (Mx+/GO) for chemiluminecence (CL) reactions. Employing luminol-H2O2 and lucigenin-H2O2 CL reactions as models of catalytic reactions, it was found that GO-supported cobalt ions could catalyze strongly the CL reactions, accompanied by dramatically enhanced CL emission.

EXPERIMENTAL SECTION

Chemicals and materials. GO was purchased from XFNANO Materials Tech Co., Ltd. (Nanjing, China). Luminol was obtained from Sigma-Aldrich (U.S.A.) and a stock solution of luminol (10 mM) was prepared by dissolving luminol in NaOH solution (0.1 M). Working solutions of H2O2 were prepared fresh daily from 30% (v/v) H2O2 (Xinke Electrochemical Reagent Factory, Bengbu, China). All other reagents were of analytical grade. Ultrapure water was prepared by a Milli-Q system (Millipore, France) and used throughout. Synthesis procedure. 0.1 mM CoCl2 solution interacted with 0.1 mg/mL GO suspension with a volume ratio of 1:4 for 20 minutes. Then, the obtained solution was purified twice by centrifugation at 13,000 rpm for 15 min to remove free Co2+. The precipitate was re-dispersed in ultrapure water to form Co2+/GO hybrids. Similar method is used to synthesize other Mx+/GO hybrids. Metal ions, including Cd2+, Ce2+, Cr3+, Cu2+, Fe3+, Hg2+, La3+, Mn2+, Ni2+ and Pb2+, were mixed with GO and reacted for 20 min. Centrifugation and re-dispersion procedures were carried out to remove free ions. Characterizations. As-prepared Co2+/GO hybrids were subsequently characterized by high-resolution transmission electron microscopy (HRTEM, JEM-2010, Hitachi, Japan), atomic force microscopy (AFM, DI Innova, Veeco), energy dispersive spectroscopy (EDS, JEM-2010, Hitachi, Japan), inductively coupled plasma mass spectrometry (ICP-MS, PlasmaQuad 3, Thermo VG Elemental, UK), atomic fluorescence spectrometry (AFS-230Q, Beijing) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo-VG Scientific, UK) with Al Kα radiation as the X-ray source. Co2+/GO hybrids were purified by centrifugation and the precipitates were redispersed in the ultrapure water, so that true information about the surface of Co2+/GO could be obtained by various characterization methods. Extended X-ray absorption fine structure (EXAFS) was collected in transmission mode at the vicinity of the Co-K edge (7.709 keV) at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics (SINAP), China, operated at 3.5Gev with a maximum current of 240 mA. A Si (111) double-crystal monochromator was used to reduce the harmonic component of the monochrome beam. All samples were ground and sieved through 400 mesh and brushed onto tapes that were stacked to give approxi-


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Analytical Chemistry mately one X-ray absorption length at the Co K-edge. Metallic cobalt foil and cobaltous carbonate (CoCO3) were measured as standard references and for energy calibration. Analysis of EXAFS data follows the standard procedures using IFEFFIT software. CL measurements. The catalytic behavior of Co2+/GO on CL reaction was studied using a static injection CL system with an ultra-weak luminometer (BPCL-K, China) or a Centro LB 960 microplate luminometer (Berthold, Germany). Specifically, 50 μL of Co2+/GO solution containing 8 μM luminol, 8 μM lucigenin or control solutions were added into a CL cell. CL emission was recorded when 50 μL of 1 mM H2O2 in 0.04 M B-R buffer (pH=12) or 0.01 mM H2O2 in 0.04 M B-R buffer (pH=11) was injected into the CL cell quickly. The measurement time was optimized to be 15 s with a time interval of 0.1 s. CL spectra were measured on a fluorescence spectrometer (HITACHI F-7000, Hitachi, Japan) operated with the lamp off. Theoretical methods. First-principles simulations were implemented with Gaussian09 software package via density functional theory (DFT) using the hybrid functional B97D.13 The basis sets LANL2DZ and 6-31G were chosen for metal and non-metal atoms, respectively. The graphene was modeled by a 7 × 7 supercell with 96 carbons and unit cell parameters of a=b=2.46Å, c=6.80Å; α=β=90 °, γ=120°. The graphene oxide (GO) was built with oxygen atom locating at the carbon-carbon bridge site. The adsorption/bonding energy (Ea) was calculated to evaluate the structural stability, with Ea = EA + EB –ETotal (EA, EB, and ETotal represent energies of separated parts and the hybrid structure, respectively). Transition states for reactions were scanned and optimized with CalcFC in Transit-Guided QuasiNewton (STQN) methods.14

2b shows Co2p spectrum of Co2+/GO hybrids. The Co 2p1/2 and Co 2p3/2 peaks could be curve-fitted into four components. The components at 796.8 eV and 781.0 eV were attributed to cobalt element in the Co–O group, and the components at 802.4 eV and 785.6 eV were attributed to the multielectron excitation intense satellite lines of the high spin cobalt(II) core, indicating that Co element on the surface of Co2+/GO was divalent cobalt ion. The surface of GO contained a rich variety of functional groups such as hydroxyl and carboxyl groups,15 which coordinated with cobalt(II) ion via Co-O bond to form Co2+/GO hybrids.

Figure 1. Synthetic scheme and characterizations. (a) Sche2+ matic illustration of the preparation of Co /GO hybrids. (b) 2+ Tapping mode AFM images of Co /GO hybrids. (c) HRTEM 2+ 2+ images of Co /GO hybrids. (d) EDS spectrum of Co /GO hybrids.

RESULTS AND DISCUSSION Synthesis and characterizations. The preparation of Co2+/GO hybrids is illustrated schematically in Figure 1a. To start with, CoCl2 solution interacted with GO suspensions with a volume ratio of 1:4 for 20 minutes. Then, the resulting solution was purified by centrifugation to remove free Co2+. The precipitate was re-dispersed in ultrapure water to form Co2+/GO hybrids. The as-prepared Co2+/GO hybrids were characterized by HRTEM and AFM. Comparing with Figure S1, the AFM (Figure 1b) and HRTEM images (Figure 1c) of Co2+/GO show that the morphology of Co2+/GO is the same as that of GO, which are essentially single-layered carbon structure with wrinkles and foldings at the surfaces and edges. Besides, EDS spectrum (Figure 1d) revealed that the Co signal from Co2+/GO as expected, while the Cu signal was derived from the Cu grid. Particularly, the obtained Co2+/GO hybrids were quite stable and could be kept at 4 °C for several months. The surface composition of as-prepared Co2+/GO was characterized by XPS (Figure 2 and Figure S2). As shown in Figure 2a, Co 2p peak spanning from 780 to 810 eV was observed in the survey of Co2+/GO, demonstrating the existence of Co element on the surface of Co2+/GO. Figure

Figure 2. XPS analysis. (a) Survey XPS data of GO (black) 2+ and Co /GO (red). (b) Deconvolution of Co2p spectrum of 2+ Co /GO.

The coordination environments of Co in the Co2+/GO hybrids was further studied by EXAFS experiments. As shown in Figure S3, the spectrum of the Co2+/GO hybrids closely resembled CoCO3 but was markedly different from metallic cobalt, implying that the chemical environment of Co in Co2+/GO hybrids was similar to that in CoCO3. Fourier transform of the XAFS function, as shown in Figure 3, further provided information on the structural evolution of the process. The Co2+/GO hybrids showed a main peak around 1.63 Å corresponding to the Co–O shells.16 Unlike CoCO3, the high-order coordination peaks was not obtained, indicating that amorphous Co was formed in the Co2+/GO hybrids. Notably, Fourier



transforms (FT) analysis of metallic cobalt showed a Co−Co bond distance of about 2.13 Å.17 However, no Co−Co contribution was found in Co2+/GO hybrids. Moreover, the XPS analysis showed that no Cl 2p electron peaks around 200 eV was found, indicating that Cl element did not participate in the coordination with Co. Thus, we could conclude that Co2+ existed as single ion site in the as-prepared Co2+/GO hybrids and Co-O coordination contributed mostly to the structure of Co2+/GO.

Figure 3. EXAFS measurements. Fourier transforms (FT) 3 2+ k [χ(k)] of Co /GO (blue), CoCO3 (red) and metallic cobalt (black).

The assembly mechanism of Co2+/GO was investigated. It was reported that GO nanosheets could be used as superior sorbents for the removal of heavy metal ions, such as Ni2+ and Co2+ from pollution managements by virtue of coordination and electrostatic interactions.18 GO is negatively charged at pH 4.0-11.0.19 Thus, positively charged Co2+ might be adsorbed on the surface of GO by electrostatic interaction. Zeta-potential measurements were carried out to verify the contribution of electrostatic interaction. The measured zeta-potentials of GO and Co2+/GO in aqueous solution were -50.2 and -37.5 mV, respectively. The zeta-potential of Co2+/GO was positive-shifted by 12.7 mV relative to that of GO, which indicated that positively charged Co2+ could be assembled on the surface of negatively charged GO via electrostatic interaction. The XPS and EXAFS results above indicated that Co2+ could also be attached onto GO through coordination interactions between Co2+ and hydroxyl and carboxyl groups from GO. Thus, Co2+ could be immobilized on the surface of GO by virtue of electrostatic interaction and coordination interactions. Further experiments were carried out to clarify whether electrostatic interaction or coordination reaction dominated the assembly of GO with Co2+. First, the influence of ion strength on the synthesis of Co2+/GO catalyst was studied as shown in Figure S4. NaClO4 is chosen as the background electrolyte to regulate the solution ion strength due to the low coordination ability with cobalt ion. Both the CL intensity and concentration of Co2+ decreased slightly when ion strength increased from 0 to 2.5 mM, demonstrating that ion strength has little influence on the interaction between Co2+ and GO. When ion strength was beyond 10 mM, both the CL intensity and concentration of Co2+ decreased obviously. When ion strength increased to 100 mM, the CL intensity decreased

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drastically because of the coagulation of GO caused by high ion strength as shown in Figure S5. Accordingly, ion strength had slight effect on the synthesis of Co2+/GO catalyst and an obvious decrease in CL intensity with high ion strength of NaClO4 may be due to coagulation of GO. Based on the fact that the CL intensity was decreased by 6% when the concentration of NaClO4 was 100 times that of Co2+ (ion strength is 2.5 mM), it is suggested that not electrostatic interaction but coordination reaction dominated the assembly of GO with Co2+. Moreover, the effect of ligand glycine with medium complexion ability with Co2+ (lgβCo2+ /glycine =10.76) and EDTA with highly strong complexion ability with Co2+ (lgβCo2+ /EDTA =16.31) on the synthesis of Co2+/GO catalyst was studied. As shown in Figure S6a, the effect of glycine concentrations on CL intensity is fairly negligible, indicating that the coordination interaction of Co2+/GO is stronger than that of Co2+/glycine. As shown in Figure S6b, the CL intensity was very weak when EDTA concentration is comparable to that of Co2+ on the catalyst (CEDTA=20 uM). The results revealed that the coordination interaction between Co2+ and GO was stronger than Co2+/glycine but weaker than Co2+/EDTA. Therefore, we concluded that coordination reactions rather than electrostatic interactions played a leading role in the synthesis of catalyst.

x+

Figure 4. Catalytic performance of M /GO on luminol-H2O2 reaction. (a) CL kinetic curves for reactions of luminol 2+ (black), GO+luminol (red), Co +luminol (blue) and 2+ Co /GO+luminol (green) with H2O2, respectively. Inset (left): 2+ CL spectrum of Co /GO-luminol-H2O2. Inset (right): magnification of black and red curves. (b) Catalytic effect of various metal ions on the surface of GO on luminol-H2O2 CL reaction. Blank is CL kinetic curves of luminol-H2O2 system in the presence of GO. Conditions: 8 μM luminol, 0.1 mg/mL GO, 1 mM H2O2 in 0.04 M B-R buffer (pH=12), metal ion concentration determined as shown in Table S1, BPLC luminometer at -560 V PMT.


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Analytical Chemistry Catalytic performance on luminol-H2O2 CL reaction. The catalytic effects of Co2+/GO on the luminolH2O2 CL reaction were investigated by static injection, as shown in Figure 4a. An outstanding integrated CL intensity in 0~15 s of about 180000 A.U. was observed when 50 μL of 1 mM H2O2 in pH=12 B-R buffer was injected to 50 μL of Co2+/GO dispersion (green). Furthermore, the CL performance of control solutions, including luminol and luminol with GO (GO+luminol) or Co2+ (Co2++luminol), were also examined under the same conditions. The integrated CL intensities for luminol, GO+luminol and Co2++luminol were measured to be 2203 A.U. (black), 5701 A.U. (red) and 88614 A.U. (blue), respectively. Although Co2+ and GO could catalyze the luminol-H2O2 CL reaction, their catalytic abilities were much weaker than that of Co2+/GO hybrids. Compared with the luminol-H2O2 CL reaction in the absence of catalyst, the integrated CL intensity using Co2+/GO, Co2+ and GO as catalysts enhanced by 115, 22 and 5 times, respectively. Accordingly, Co2+/GO hybrids exhibited unique catalytic activity with respect to the luminol-H2O2 CL reaction, superior to Co2+ and GO. Furthermore, the CL emission was reproducible with RSD of less than 5% within a month or more (Supporting Information S4), demonstrating great stability of the asprepared Co2+/GO hybrids. The effect of pH on the CL intensity was also studied and maximal CL intensity was obtained at pH 12.0 (Supporting information S5). It has been reported that various metal ions such as Co2+, Cu2+, Fe3+, Hg2+, Cd2+, Ni2+, Mn2+ and Pb2+ could catalyze the luminol-H2O2 CL reaction.20 Thus, the catalytic activities of various Mx+/GO on the luminol-H2O2 reaction were also studied (Figure 4b). The integrated CL intensity of Co2+/GO was much higher than that of the other Mx+/GO hybrids, which was consistent with the fact that Co2+ was the best catalyst of the luminol-H2O2 CL reactions.21 The concentrations of metal ions on the surface of the Mx+/GO hybrids were measured (Table S1), demonstrating that the concentration of Co2+ on Co2+/GO is comparable to that of other metal ions, indicating that the high catalytic effect of Co2+/GO on the luminol-H2O2 CL system was not due to the loading amount of metal ions on GO. Catalytic mechanism on luminol-H2O2 CL reaction. Theoretical investigations at the first-principles level were carried out to reveal the underlying mechanisms accounting for high catalytic efficiency of Co2+/GO. The deposition of metal ions on a GO supercell of 7×7 were simulated by structurally optimizing the possible formation of covalent bonds between metal and oxygen, and the stability and chemical properties of Mx+/GO hybrids were examined. It can be seen (Figure S9) that Pb2+, Co2+ and Mn2+ were hard to precipitate under high pH conditions, while the other metal ions precipitated easily in alkaline solutions and were thus unavailable for GO deposition. Accordingly, the interaction of Pb2+, Co2+ and Mn2+ with GO was further studied. It turned out that Pb2+ and Mn2+ could not achieve stable anchoring on GO, as their bindings to oxygen had to destroy the C-O bond of GO which normally costs high energy (Figure S10). In contrast, Co2+

could anchor stably on GO through bindings to oxygen, which was consistent with EXAFS results. Therefore, the following studies focused on the hybrid structures of Co and Co2+ anchoring on GO. The optimized structures of Co/GO and Co2+/GO in Figure 5a possessed a fourmembered ring configuration, where Co or Co2+ formed strong connection with carbon and oxygen atoms. The adsorption energies (Ea) (Table S2) demonstrate that Co2+/GO is much more stable than Co/GO (17.1 eV .vs. 3.07 eV). The couplings between Co or Co2+ and GO induced effective polarization charges at the deposition sites. As shown in Figure 5a, GO could extract ~0.39 and 0.50 e- electrons from Co and Co2+, respectively. Similar structures and polarization effects were observed in systems with smaller GO supercell of 5×5 (Figure S11). These polarization positive charges on Co or Co2+ would induce strong electrostatic field, which often promotes chemical activity.

Figure 5. Theoretical simulations. (a) Top and side views of optimized structures of Co/GO and Co2+/GO hybrids. GO in a 7×7 supercell, together with polarization charge distributions. (b) Optimized structures of H2O2 adsorbed to the hybrids, resulting in 2(OH•)-Co/GO and 2(OH•)Co2+/GO configurations. The polarized charges are computed by subtracting charges of the isolated metal and GO parts from those of the hybrid structures. Cyan and yellow bubbles represent positive and negative charges, respectively, with isovalue of 0.006 e Å-3. (c) Potential energy profile of reaction pathways of luminol- coupling with 2(OH•)-Co /GO (blue color) and 2(OH•)-Co2+/GO (red color) to produce H2O and luminol•−. From left to right: optimized geometries of initial state (IS), transition state (TS), and final state (FS). Then the adsorption and dissociation of H2O2 on Co/GO and Co2+/GO based on the 7×7 graphene supercell were investigated. The optimized structures of H2O2 adsorbed to Co/GO and Co2+/GO in Figure 5b (Figure S12 for the smaller supercell of 5×5) show that H2O2 dissociated



automatically into double OH• radicals without energy barrier, resulting in the formation of 2(OH•)-Co/GO and 2(OH•)-Co2+/GO with large bonding energies ranging from 6 to 7 eV (Table S2). Given the fact that the dissociation of H2O2 with the catalyst of a single Co2+ needs to overcome a high energy barrier of 0.94 eV (Figure S13), we can ascribe such amazingly easy dissociation of H2O2 on Co/GO and Co2+/GO to the positive charges extracted from GO to Co and Co2+. After H2O2 dissociation, two OH• radicals would extract about 0.55 e- electrons from the Co2+/GO surface, making it ready for luminol anions (luminol-) attraction. Figure 5c illustrates the adsorptions of luminol- on 2(OH•)-Co/GO and 2(OH•)-Co2+/GO, exhibiting adsorption energies of 2.08 eV and 7.20 eV, respectively. The luminol- then reacts with 2(OH•)-Co/GO or 2(OH•)-Co2+/GO. OH• takes a hydrogen atom from a N-H bond of luminol to form luminol radical (luminol•−) and H2O. The whole process with 2(OH•)-Co/GO catalyst is exothermic by 1.08 eV, and needs to overcome a barrier of 0.55 eV. In contrast, the same reaction of luminol- with 2(OH•)-Co2+/GO is exothermic by 0.36 eV, and only needs to conquer a much smaller barrier of 0.13 eV. Importantly, H2O products will no longer be bonded to Co or Co2+, making it active again and available for the next cycle of CL reaction. Therefore, the high catalysis performance of Co2+/GO system in the CL reaction is due to the automatic dissociation of H2O2 to OH• by Co2+/GO and conversion of luminol- to luminol•− with very small energy barrier. The CL catalytic mechanism of Co2+/GO was further investigated. The CL spectrum for the reaction of Co2+/GO with luminol-H2O2 (inset in Figure 4a) indicated that the CL emission was from the luminol-H2O2 system.22 Thus the enhanced CL signals were attributed to the catalysis of Co2+/GO hybrids. The Co2+-catalyzed luminol-H2O2 CL mechanism has been proposed that Co2+ reacted with H2O2 to form OH•, followed by the reaction with luminolto produce luminol•−, accelerating the CL reaction.23 In this case, the excellent CL efficiency may be due to synergistic catalytic effect of Co2+ and GO in Co2+/GO hybrids. Theoretical studies demonstrated that the coupling between Co2+ and GO induced effective polarization charges, improving chemical activity of reaction site. H2O2 adsorbed on the surface of Co2+/GO dissociated into double OH• radicals, resulting in the formation of (OH•)-Co2+/GO, which easily attracted luminol-. The OH• radical took a proton from the N-H bond of luminol to form luminol•−. It was also reported that OH• could react with the H2O2 to form O2•–.24 Thus, OH•, luminol•− and O2•– radicals may be formed in the CL reaction. The effects of O2, N2 and radical scavenges on the CL intensity of the lumino-H2O2Co2+/GO reaction (Figure S14) demonstrated that the dissolved O2, HO• and O2•- were involved in the CL reaction. Furthermore, the HRTEM images of Co2+/GO before and after the CL reaction (Figure S15) shows that the graphene framework of Co2+/GO was partially broken up into smaller pieces after the reaction and GO took part in the CL reaction. Moreover, HO• might simultaneously add to double bonds in the GO plane to generate π-C=C• radi-

cals.25 Accordingly, in the CL reaction, Co2+/GO as a nanocatalyst could facilitate the generation of several radicals, including OH•, O2•−, π-C=C•, which may react with luminol- to accelerate the formation of luminol•−.23-25 Finally, luminol•− further reacted with O2•− to produce excited-state 3-aminophthalate anions*, leading to strong CL emission. The differences in luminol-H2O2 CL reaction mechanism using Co2+ and Co2+/GO as catalysts are summarized as follows: 1) Difference in the energy barrier and product of H2O2 decomposition. For free Co2+, OH• was formed with a high energy barrier; for Co2+/GO, 2(OH•)Co2+/GO was formed without energy barrier. 2) Different radicals were involved in the CL reactions. Such mechanism differences led to a difference in CL kinetics.

Figure 6. Catalytic performance of GO-supported metal ions on lucigenin-H2O2 reaction. (a) Catalytic effect of metal ions immobilized on GO on lucigenin-H2O2 CL reaction, includ2+ 2+ 2+ 3+ 2+ 2+ 2+ 3+ 2+ 3+ ing Co , Ce , Cd , Fe , Ni , Mn , Cu , Cr , Pb , La and 2+ Hg . Blank is CL kinetic curves of lucigenin-H2O2 reaction. (b) CL kinetic curves for reactions of lucigenin (black), 2+ GO+lucigenin (red), Co +lucigenin (blue) and 2+ Co /GO+lucigenin (green) with H2O2, Inset: CL spectrum of 2+ Co /GO with lucigenin-H2O2. Reaction conditions: 8 μM lucigenin, 0.1 mg/mL GO, 1 mM H2O2 in 0.04 M B-R buffer (pH=12). CL measurement: microplate luminometer at -900 V PMT.

Generalization to lucigenin-H2O2 CL reaction. The results above demonstrated that Co2+/GO as hybrid catalyst exhibited excellent catalytic activity on the luminol CL reaction. Furthermore, the generalization of Mx+/GO hybrids as catalysts for CL reactions was investigated. Lucigenin-H2O2 CL reaction was selected as another example since some transition metal ions could catalyze the CL reaction.26 Strong CL emission was observed for the lucigenin-H2O2 system in the presence of Co2+/GO, while very weak CL emission for the lucigenin-H2O2 system in the presence of other Mx+/GO (Figure 6a). It was due to


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Analytical Chemistry the fact that Co2+ was reported to be the most efficient metal catalyst for lucigenin-H2O2 CL reaction.27 As shown in Figure 6b, it can be seen that the CL intensity of the reaction between lucigenin mixed with GO and H2O2 was much weaker than that of lucigenin with H2O2 owing to the quench effect of GO on the lucigenin CL. However, the CL intensity of Co2+/GO-catalyzed lucigenin-H2O2 CL reaction were 20 times higher than that of lucigenin-H2O2 CL reaction and 3 times higher than that of Co2+-catalyzed lucigenin-H2O2 CL reaction. The results indicated that Co2+/GO as a hybrid catalyst exhibited good catalytic effect on the lucigenin-H2O2 CL reaction.

The support of this research by the National Key Research and Development Program of China (Grant No. 2016YFA0201300) and the National Natural Science Foundation of China (Grant Nos. 21527807 and 21475120) are gratefully acknowledged. We also thank the BL14W1 beamline of SSRF for EXAFS measurements.

REFERENCES (1)

(2) (3)

CONCLUSION In conclusion, we have reported that GO-supported single metal ion can function as highly active hybrid catalyst in CL reactions. Co2+ was directly immobilized on the surface of GO by virtue of electrostatic and coordination interactions. And the coordination reaction dominated the assembly of GO with Co2+. The assemble method is simple, fast, environment-friendly and low-cost. The Co2+/GO hybrid catalyst exhibited excellent catalytic activities as well as good stability on luminol-H2O2 and lucigenin-H2O2 CL reactions. The integrated CL intensity of Co2+/GO-catalyzed luminol-H2O2 reaction was 115 times higher than that of the luminol-H2O2 and the CL intensity of Co2+/GO-catalyzed lucigenin-H2O2 reaction was 20 times higher than that of the lucigenin-H2O2. Theoretical studies revealed that the coupling between Co2+ and GO induced effective polarization charges, improving chemical activity of reaction site, which promoted the generation of intermediate radicals and accelerated the CL reactions. GO-supported single metal ion as a catalyst is superior to GO supported single metal atom or nanoparticles in synthesis and stability. This work may be generalized to GO-supported other metal ions as catalysts for a wide range of chemical reactions. The developed GOsupported cobalt single-ion nanocomposites as nanointerfaces may find future applications in CL bioassays.

ASSOCIATED CONTENT

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional characterizations, catalytical mechanism studies and stability testing.

AUTHOR INFORMATION Corresponding Author *

(19) (20) (21) (22) (23) (24)

*

[email protected]; [email protected]

Author Contributions

(25)

§These authors contributed equally.

(26) (27)

Notes

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