Metal-Organic Gels Hybrids

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Functional Nanostructured Materials (including low-D carbon)

In Situ Synthesis of Gold Nanoparticles/Metal-Organic Gels Hybrids with Excellent Peroxidase-Like Activity for Sensitive Chemiluminescence Detection of Organophosphorus Pesticides Li He, Zhong Wei Jiang, Wei Li, Chun Mei Li, Cheng Zhi Huang, and Yuan Fang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08768 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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ACS Applied Materials & Interfaces

In Situ Synthesis of Gold Nanoparticles/MetalOrganic Gels Hybrids with Excellent PeroxidaseLike Activity for Sensitive Chemiluminescence Detection of Organophosphorus Pesticides Li He,† Zhong Wei Jiang,† Wei Li,† Chun Mei Li*,‡ Cheng Zhi Huang, ‡ and Yuan Fang Li*,† †

Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest

University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, P. R. China. ‡

College of Pharmaceutical Sciences, Southwest University, Chongqing 400716, P. R. China.

KEY WORDS: AuNPs/MOGs (Fe) hybrids, catalysis, chemiluminescence, luminol, organophosphorus pesticides, detection

ACS Paragon Plus Environment

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ABSTRACT: Until now, despite much progress in the study of metal-organic gels (MOGs), the modification of transition metal containing MOGs with noble metal nanoparticles is far from fully developed. Herein, iron-based MOGs nanosheets hybrids with gold nanoparticles (AuNPs) immobilization were first synthesized by a facile in situ grown strategy at ambient conditions. It is found that the as-prepared AuNPs/MOGs (Fe) hybrids exhibited enhanced mimicking peroxidase-like

activity,

making

them

endowed

with

outstanding

performance

in

chemiluminescence (CL) field in the presence of H2O2. The remarkable CL enhancement by AuNPs/MOGs (Fe) hybrids was attributed to the modification of AuNPs on MOGs (Fe) nanosheets, which could synergistically accelerate the CL reaction by speeding up the generation of OH•, O2•−, and 1O2. Accordingly, a sensitive CL detection of organophosphorus pesticides was successfully achieved by the AuNPs/MOGs (Fe) hybrids CL enhancing system in the range of 5800 nM with a detection limit of 1 nM. We envision that this highly active and novel enzyme mimetic catalyst can be applicable to other extended AuNPs/MOGs (Fe) hybrids-based CL system for sensitive detection of various analytes.


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INTRODUCTION Nanozymes, the nanomaterials with enzyme-like activities, have drawn tremendous attention in the field of environmental treatment, biosensing and nanomedicine,1 owing to their low cost, simple synthetic routes and storage, high stability, durability and versatility, as well as tunable catalytic properties2 as compared to native enzyme. Since the first report that Fe3O4 ferromagnetic nanoparticles (NPs) were found to possess intrinsic peroxidase mimicking activity,3 various nanomaterials have been reported to exhibit unexpected peroxidase-like activity, for instance, transition metal dichalcogenides (TMDs),4-5 carbon-based nanomaterials,6-8 metal-based nanomaterials9-10 and metal-organic frameworks (MOFs).11-14 Especially, the sheetlike materials, including graphene, TMDs and MOFs, possess large surface areas and abundant active edges, as well as unique electrical, chemical, mechanical and thermal properties, making them potential materials to serve as enzyme mimics. Moreover, it is noteworthy that metallic NPs anchored nanosheets hybrids could achieve enhanced function of materials through synergistic effects.15-16 In particular, modifying nanosheets materials with one or more noble metal-based NPs could effectively decrease the Fermi energy, increase the electron transfer rate and then greatly improve the catalytic activity.17-18 For example, Quan et al. reported that graphene/AuNPs hybrids prepared by in situ grown “naked” AuNPs on graphene nanosheets exhibited excellent synergistic effect in peroxidase-like activity.19 Dravid’s group successfully synthesized Fe3O4/MoS2 composites via a facile one-pot method and the resulting composites have shown enhanced peroxidise-like activity over their individual components alone.20 Zeng’s group found that various noble metal NPs could be immobilized on MOFs nanosheets and the prepared MOFs-based composites showed good catalytic performance.21 However, despite the above progress and achievements, to our knowledge, the synthesis of supporting materials or


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loading noble metal nanoparticles require complex and time-consuming procedures. Therefore, the facile and efficient preparation strategy of novel NPs-nanosheet hybrids with prominent peroxidase-like activity still remains a challenge. Metal-organic gels (MOGs) are a newly emerging class of intelligent soft materials materials which are self-assembled from metal ions and organic ligands through metal-organic coordination and intermolecular forces.22-23 Benefiting from their unique magnetic, electronic, optical, and catalytic properties,24-27 MOGs exhibited excellent performance in the field of adsorption,28-29 sensing,30-31 catalysis,32-33 wearable devices,34 drug delivery,35-36 white-lightemitting LEDs (WLED)37-38 and environmental pollution abatement.39-40 In general, shapes of the studied MOGs are mostly fibrous network,41 nanoparticle,42 microflower,39 nanosheet32 or spongelike morphology.43 Similar to MOFs nanosheets, MOGs nanosheets also possess remarkable characteristics of large specific surface area and abundant exposed metal active sites, which could complex directly with reductants, realizing the facile in situ synthesis of the noble metal NPs, making them promising novel supporting materials for synthesis of functional composites. In addition, MOGs could be synthesized in a very gentle and facile manner (namely, just by blending metal ions and organic ligands at room temperature for seconds or minutes).44-45 Therefore, the study on preparation of metal NPs supported on MOGs nanosheets is highly appealing. Inspire by above idea, a simple, facile and green strategy was designed to prepare iron-based MOGs nanosheets supported gold nanopaticles hybrids (AuNPs/MOGs (Fe)) by in situ grown AuNPs on outer surface of MOGs (Fe) nanosheets. Intriguingly, the as-prepared AuNPs/MOGs (Fe) hybrids exhibited synergistically enhanced effect in mimicking peroxidase-like activity, as indicated by a typical luminol chemiluminescence experiment in the presence of H2O2.


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Furthermore, based on the enhanced peroxidase-like activity of the as-prepared AuNPs/MOGs (Fe) hybrids, sensitive detection of organophosphorus pesticides can be achieved by an enzymebased inhibition CL biosensing strategy. Scheme 1. Schematic illustration of the preparation process and CL behavior of AuNPs/MOGs (Fe) hybrids.

EXPERIMENTAL SECTION Materials and Reagents. The organic ligand 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) (Figure S1) was purchased from Zhengzhou Alfa Co., Ltd (China). Ethoprophos (EP) standard substances were from Macklin Biotech Co., Ltd. (Shanghai, China). Acetylcholinesterase (AChE) and cholin oxidase (ChOx) were supplied by Yuanye Biotech Co., Ltd. (Shanghai, China). Iron (III) chloride, acetylcholine chloride (ACh) and luminol were supplied by Sigma-


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Aldrich (Shanghai, China). All other reagents were directly available from suppliers and used without further purification. Apparatus. The overall morphology of the samples were investigated through Hitachi S-4800 scanning electron microscope (SEM). The specific morphology, energy dispersive spectroscopy (EDS) and elemental mapping analysis were carried on field-emission transmission electron microscope (TEM, JEM-2100). The Fourier transform infrared FTIR spectrometer (Shimadzu, Japan) was used for recording the FTIR spectrum. X-ray photoelectron spectroscopy (XPS) analysis were measured by an Escalab 250 Xi XPS system with Al Kα source (hν = 1486.6 eV). X-ray diffraction (XRD) measurements were determined on a D8 Advance X-ray diffractometer (Bruker, Germany) using Cu Kα radiation (λ=1.5406 Å). The 300E electron spin resonance (ESR) spectrometer (Bruker, Germany) was employed to record the ESR spectra at room temperature. All chemiluminescence (CL) signals were collected using an ultra weak luminescence analyzer (BPCL, China). UV-vis absorption analysis was performed on a U-3010 spectrometer (Hitachi, Japan). Mott-Schottky plots were carried out with a CHI 660E electrochemical workstation (Shanghai, China) in Na2SO4 electrolyte (0.1 M), in which the three-electrode system was constructed with a modified glassy carbon electrode (GCE) as working electrode, a saturated Ag/AgCl as reference electrode and a Pt wire as counter electrode. Preparation of AuNPs/MOGs (Fe) Hybrids. MOGs (Fe) nanosheets were prepared according to a previously reported method.32 The immobilized AuNPs were synthesized by employing tannic acid (TA) as a reductant via in situ grown method. First, 15 mg of obtained MOGs (Fe) nanosheets were homogeneously suspended in 9.5 mL of ultrapure water via ultrasonication and interacted with 0.5mL of 50 mg/mL TA solution for 1 h under vigorous stirring at room temperature. Then, the obtained mixture was centrifuged


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and washed with ultrapure water for three times to remove free TA. The obtained TA/MOGs (Fe) intermediates were re-dispersed in 9.5 mL of ultrapure water and the pH value of the suspension was tuned to 5.2 with 0.05 M K2CO3, followed by addition of 0.5 mL HAuCl4 aqueous solution (0.08% (w/v)) drop by drop and reacting for 30 min under a stirring speed of 600 rpm at room temperature. The obtained AuNPs/MOGs (Fe) hybrids were isolated three times by centrifuging and washing with ultrapure water, and dried by freeze-drying treatment to obtain AuNPs/MOGs (Fe) hybrids powder. As a comparison, AuNPs-MOGs (Fe) hybrids were prepared by adsorbing positively charged AuNPs synthesized according to a established method46 on negatively charged MOGs (Fe) nanosheets via electrostatic interaction. Chemiluminescence Assay for Peroxidase-like Catalytic Activity. For a typical CL measurement, 50 µL of Tris-HCl buffer (0.02 M, pH 8.5), 100 µL of 0.1 mM H2O2 and 50 µL of 1 mg/mL AuNPs/MOGs (Fe) hybrids solution were orderly added into a 3 mL quartz cuvette. As soon as 250 µL of 1.9 mM luminol solution were added into the quartz cuvette rapidly, the CL profile and intensity of the resulting assay solution was collected by the BPLC luminescence analyzer. Measurement of Pesticides. Firstly, various concentrations of ethoprophos were incubated with AChE solution (25 µL, 5 U/mL) for 30 min in 0.02 M Tris-HCl buffer (pH 8.5) at room temperature. Then, ACh (50 µL, 5 mM) and ChOx (25 µL, 5 U/mL) were added and incubated for another 30 min. The CL emissions were collected when AuNPs/MOGs (Fe) hybrids (50 µL, 1 mg/mL) dispersion was added and luminol (250 µL, 1.9 mM) solution was quickly injected. To test the applicability of the assay, the concentrations of ethoprophos in tap water and river water was determined with different spiking.


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RESULTS AND DISCUSSIONS Synthesis and Characterization of the AuNPs/MOGs (Fe) Hybrids. The detailed synthetic process of AuNPs/MOGs (Fe) hybrids is illustrated in scheme 1. First, tannic acid (TA), a green reductant with multiple phenolic hydroxyl groups, can be easily decorated on MOGs (Fe) nanosheets via the complexation of unsaturated hydroxyl groups in TA with exposed coordination unsaturated Fe(III) in MOGs (Fe) nanosheets. Then, the obtained TA/MOGs (Fe) intermediates were isolated by centrifugation and washed with ultrapure water to remove residual TA. After that, with the addition of chloroauric acid, Au ions were continuously reduced by the complexed TA on the surface of MOGs (Fe) nanosheets, leading to the formation of AuNPs/MOGs (Fe) hybrids. As Figure. 1 presented, the SEM image (Figure 1A) and TEM image (Figure 1B) demonstrated that AuNPs were successfully in situ decorated on MOGs (Fe) nanosheets without aggregation, which cause negligible influence on the morphology of MOGs (Fe) nanosheets (Figure S2). The inset size distribution of AuNPs in Figure 1A clearly depicted that most of the AuNPs were evenly distributed on the surface of MOGs (Fe) nanosheets with the average size of 23 nm. Moreover, elemental mapping analysis (Figure 1C) and EDS (Figure 1D) of the AuNPs/MOGs (Fe) hybrids indicate that C, O, N, Fe and Au were distributed evenly across the entire hybrids.


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Figure 1. Representative (A) SEM image, (B)TEM image, (C) elemental map and (D) EDS spectrum of AuNPs/MOGs (Fe) hybrids. (E) UV-Vis absorption spectra of MOGs (Fe) nanosheets and AuNPs/MOGs (Fe) hybrids in an aqueous solution. Inset: Histogram of size distribution of AuNPs. By comparing the UV-vis absorption spectra of MOGs (Fe) nanosheets and AuNPs/MOGs (Fe) hybrids dispersion in Figure 1E, an obvious new peak was appeared at about 530 nm for AuNPs/MOGs (Fe) hybrids, which is the typical surface plasmon resonance absorption of AuNPs,47-48 further demonstrating the successful decoration of AuNPs on MOGs (Fe) nanosheets. Furthermore, the FTIR spectra of TA, MOGs (Fe) nanosheets, TA/MOGs (Fe) intermediates and AuNPs/MOGs (Fe) hybrids were also used to characterize the formation of AuNPs/MOGs (Fe) hybrids (Figure S3). The double bands at 1578 and 1639 cm-1 can be


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assigned to the antisymmetric and symmetric carboxylate vibrations after bidentate coordination modes between Fe3+ and PDA, which are the typical characteristic bands of MOGs (Fe) nanosheets32. The infrared spectra of TA/MOGs (Fe) and AuNPs/MOGs (Fe) both exhibited the stretching vibrations of ester carbonyl bending mode at 1609 cm-1 in TA, suggesting that TA was successfully immobilized on MOGs (Fe) nanosheets. Meanwhile, the characteristic in-plane bending mode of -OH group at 1450 cm-1 in TA was shifted to 1469 cm-1 for TA/MOGs (Fe), while disappeared for AuNPs/MOGs (Fe), indicating that the hydroxyl groups in TA have been successfully complexed with unsaturated Fe(III), and the residual hydroxyl groups have been oxidized to quinone, which donate electrons to reduce Au ions to yield AuNPs.49 To give further insight into the structure and chemical status of the as-synthesized AuNPs/MOGs (Fe) hybrids materials, XRD and XPS were carried out. As Figure 2A indicated, the weak and broad peak at 2θ values of 23.62° in XRD corresponded to the d-spacing value of 3.76Å, demonstrating that π-π stacking interaction was involved in the gel formation, which was consistent with previous report.32 A serious of diffraction peaks at 2θ values of 38.2°, 44.3°, 64.6°, and 77.5° are attributed to the (111), (200), (220), and (311) lattice planes of Au, respectively,50 indicating that the AuNPs grown in situ on MOGs (Fe) nanosheets were exist in highly crystalline state. Besides, as Figure 2B shown, the XPS surface analysis of AuNPs/MOGs (Fe) hybrids revealed the same elemental composition as EDS analysis. The typical Au 4f5/2 and Au 4f7/2 peaks at the binding energies of 87.7 and 83.9 eV (Figure 2C), respectively, confirm the formation of Au0.51 And the characteristic XPS peaks of Fe 2p1/2 and Fe 2p3/2 were observed at 724.2 and 710.7 eV (Figure 2D), respectively, indicating that Fe existed as tervalent in MOGs nanosheets.52 Based on all of the aforementioned results, we conclude that AuNPs have been successfully in situ grown on MOGs (Fe) nanosheets through this facile strategy.


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Figure 2. (A) XRD pattern of AuNPs, MOGs (Fe) and AuNPs/MOGs (Fe) hybrids. XPS spectra of (B) survey, (C) Au 4f, and (D) Fe 2p,regions of AuNPs/MOGs (Fe) hybrids. CL activity of AuNPs/MOGs (Fe) Hybrids as Catalyst. To explore the peroxidase-like enzyme mimetic catalytic activity of the obtained AuNPs/MOGs (Fe) hybrids, CL experiment was performed by static injection (Figure 3). It was easily found that, comparing to the negligible CL signal of luminol (curve a), the signal was significantly enhanced in the presence of AuNPs/MOGs (Fe) hybrids (curve f), which was around 3 orders of magnitude (411 times) higher CL intensity than pure AuNPs (Figure S4) which prepared by same method with average size of 25-30 nm (curve b, e). Although the pure MOGs (Fe) nanosheets exhibited catalytic activity, its CL intensity was just half of that AuNPs/MOGs (Fe) hybrids displayed (curve c, f). In this case, the enhanced catalytic activity might be attributed to the positive synergistic coupling effect of AuNPs and MOGs (Fe) nanosheets. To verify this assumption, the synergistic catalytic property of AuNPs/MOGs (Fe) hybrids was systematically investigated. As is revealed, the simple physical mixing of pure MOGs and AuNPs showed relatively equivelent catalytic activity to that pure MOGs (Fe) nanosheets (curve d). Remarkably, the AuNPs-MOGs (Fe)


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Figure 3. CL kinetic curves for the reaction of (a) luminol, (b) AuNPs and luminol, (c) MOGs (Fe) nanosheets and luminol, (d) physical mixtures of MOGs (Fe) nanosheets with AuNPs and luminol, (e) AuNPs-MOGs (Fe) hybrids prepared by electrostatic interaction and luminol and (f) AuNPs/MOGs (Fe) hybrids prepared by in situ grown method and luminol in alkaline conditions (0.1 mg/mL for each nanomaterials). The inset is the magnification of curves a-b. Reaction conditions: 1 mM luminol and 0.1 mM H2O2 in 0.02 M Tris-HCl buffer solution (pH=7.5). hybrids which prepared by adsorbing positively charged AuNPs on this negatively charged MOGs (Fe) nanosheets via electrostatic interaction (Figure S5) also showed performance inferior to that of AuNPs in situ grown on MOGs (Fe) nanosheets (curve e, f). What’s more, the CL behavior of AuNPs/MOGs (Fe) hybrids was also compared with the remaining in situ synthesized AuNPs by disintegrating MOGs (Fe) nanosheets in strong alkaline condition (Figure S6). After disintegration, the AuNPs alone exhibited very weak catalytic activity, namely, only the AuNPs/MOGs (Fe) hybrids material which formed by in situ grown AuNPs on MOGs nanosheets exhibit excellent catalytic activity. Furthermore, to better understand the reason for the enhanced catalytic activity of the as-synthesized hybrids, the Mott-Schottky plots measurement was carried out. As Figure 4A and B presented, the Fermi levels of MOGs (Fe)


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nanosheets and AuNPs/MOGs (Fe) hybrids were about -0.25 and 0.16 eV respectively, which demonstrated that the Fermi energy decreased after in situ grown AuNPs on MOGs (Fe) nanosheets. Thus the AuNPs/MOGs (Fe) hybrids with decreased Fermi energy could effectively receive electrons from luminol, followed by transferring more electrons to H2O2 to accelerate the decomposition of H2O2 to produce more reactive oxygen radicals, successfully oxidize luminol to produce enchanced CL emission.53 So, the AuNPs/MOGs (Fe) hybrids with decreased Fermi energy facilitated electron transfer, leading to the enhanced catalytic activity. Moreover, it is reported that AuNPs as nanosized platform could facilitate electron transfer and promote radicalinvolved CL reaction.54-55 Taken together, the in situ grown AuNPs on MOGs (Fe) nanosheets indeed significantly improved the mimicking peroxidase-like catalytic activity.

Figure 4. Mott-Schottky plots of (A) MOGs (Fe) nanosheets and (B) AuNPs/MOGs (Fe) hybrids. To obtain the best CL performance, the effects of three important parameters including reaction pH, the concentration of TA and HClO4 on CL intensity in the synthesis of AuNPs/MOGs (Fe) hybrids were studied by orthogonal experiments. As Table S1 and S2 showed, Range B > Range C > Range A, revealing that the key factor on CL properties was the reaction pH, followed by the concentration of HClO4 and TA. Moreover, A3B2C2 was finally chosen after comparing the three


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factors of k1, k2 and k3 values, indicating that the optimum experimental conditions were as follows: 50mg/mL TA, the reaction pH 5.2 and 0.08% HClO4. CL Mechanism. To explain the CL phenomenon, the CL spectrum of the AuNPs/MOGs (Fe)luminol-H2O2 system were measured. As Figure S7 presented, the maximum emission wavelength in both cases were 440 nm, implying that the excited 3-aminophthalate anions (3APA*) was the luminophores.56 According to earlier studies, the generation of reactive oxygen species is closely related to the CL catalysis and enhancement of luminol-H2O2 system. Therefore, in order to confirm whether the reactive oxygen species generated in the CL reaction process, the effects of radical scavengers on CL intensity of AuNPs/MOGs (Fe) hybrids catalyzed luminol-H2O2 system were first studied. As Figure 5A indicated, the CL intensity significantly decreased with the addition of thiourea, an effective scavenger of OH•,57 confirming the involvement of OH• in the reaction. Superoxide dismutase (SOD) is a specific scavenger of O2•−.58 The CL emission of AuNPs/MOGs (Fe) catalyzed luminol-H2O2 system were effectively inhibited in the presence of SOD (Figure 5B), which indicated that O2•− was also took part in this CL reaction. Furthermore, as demonstrated in Figure 5C, the inhibited percentage increased with increasing the concentration of NaN3, an effective scavenger of

1

O2.59 These results

demonstrated that OH•, O2•− and 1O2 were all the critical intermediates in the AuNPs/MOGs (Fe) hybrids catalyzed luminol-H2O2 system. To further confirm the radicals formed in the AuNPs/MOGs (Fe) hybrids catalyzed luminolH2O2 CL reaction system, ESR experiments were carried out. Based on the characteristic signals of different spin adducts of 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), a spin trap that can be applied to identify radicals,46 a variety of free radicals can be distinguished and detected. In this research, DMPO was employed to capture OH• and O2•− during the CL reaction process via


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respective characteristic signals. As presented, the characteristic signals of both DMPO–OH• adduct (Figure 5D) and DMPO–O2•− adduct (Figure 5E) were observed, indicating that both OH• and O2•− were generated in this study. 2,2,6,6-Tetramethyl-4-piperidine (TEMP), a typical scavenger of 1O2, can react with 1O2 to produce a stable, ESR measurable 2,2,6,6-Tetramethyl-4piperidine-N-oxide (TEMPO) adduct product.60 Figure 5F shows a characteristic 1:1:1 triplet signal of TEMPO, demonstrating that 1O2 was produced in this AuNPs/MOGs (Fe) hybridsluminol-H2O2 CL system. Accordingly, OH•, O2•−, and 1O2 were indeed generated and played a leading role in this CL system.

Figure 5. Effects of the radical scavengers of (A) thiourea, (B) SOD, and (C) NaN3 on the AuNPs/MOGs (Fe)-luminol-H2O2 CL system. The ESR spectra of (D) DMPO-OH•, (E) DMPOO2•−, and (F) TEMP-1O2 adduct in AuNPs/MOGs (Fe)-luminol-H2O2 CL system Reaction conditions: 1 mM luminol, 0.1 mg/mL AuNPs/MOGs (Fe) hybrids and 0.1 mM H2O2 in 0.02 M Tris-HCl buffer solution (pH=7.5).


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Based on the above study, the possible CL mechanism of AuNPs/MOGs (Fe)-luminol-H2O2 system could be concluded as follows: under alkaline conditions, H2O2 and luminol would first decompose to produce LH− and HO2−. Moreover, owing to the synergetic mimicking peroxidaselike catalytic activity of AuNPs and MOGs (Fe), H2O2 would rapidly decompose to generate large amount of OH• and O2•−. Then, the AuNPs/MOGs (Fe) hybrids could promote the reaction of LH− and HO2− with OH• to form L− and O2•−. Furthermore, the generated O2•− would further react with OH• to produce 1O2 under the catalysis of AuNPs/MOGs (Fe) hybrids. Subsequently, both O2•− and 1O2 could react with L−, generating a vital unstable excited 3-APA*, followed by the strong light emission at 440 nm when it returned to the ground-state. Overall, benefiting from the positively synergetic effects between AuNPs and MOGs (Fe) nanosheets, AuNPs/MOGs (Fe) hybrids with excellent peroxidase-like catalytic activity facilitated the electron transfer and radical generation in the CL chemical reaction, thus leading to the apparently enhanced CL emission. H 2 O 2 + OH − → HO 2− + H 2 O

(1)

LMU + OH − → LH − + H 2 O

(2)

•

•−

H O + AuNPs → OH + O 2

2

2

(3)

H 2 O 2 + MOGs (Fe ) → OH • + O •2−

(4)

OH • + HO −2 + LH − → O •2− + L•−

(5)

O •2− + OH • + AuNPs / MOGs (Fe )→1 O 2

(6)

L•− + O •2− → 3 − APA *

(7)

L•− +1 O 2 → 3 − APA *

(8)

3 − APA * → 3 − APA + hv


(9)

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A AuNPs/MOGs (Fe) Hybrids-Based CL Sensor for Pesticides Detection. As is known, in typical AChE involved enzymatic reaction, the substrate acetylcholine can be hydrolyzed to choline by AChE, and the produced choline can directly react with the dissolved O2 to generate H2O2 under the catalysis of ChOx (eqs 10 and 11). Furthermore, organophosphorus pesticide (OPs) can irreversibly inhibit the enzymatic activity of AChE, thus inhibiting the production of H2O2. In this case, a chemiluminescence sensor for OPs detection was proposed. As Figure 6 illustrated, when AuNPs/MOGs (Fe) hybrids dispersion was added to luminol system in the presence of acetylcholine, AChE and ChOx, the CL emission showed 110-fold enhancement than that in the absence of the hybrids. While after incubation with ethoprophos (EP), a kind of OPs compounds, the inhibition of AChE activity leads to less generation of H2O2, resulting in a significant decrease of the CL signal. Moreover, there are no obvious changes on CL intensity when adding different concentrations of EP to the AuNPs/MOG(Fe) hybrids-catalyzed luminolH2O2 system (Figure S8), indicating that organophosphorus pesticide itself does not affect the catalytic activity of AuNPs/MOG(Fe) hybrids. These results demonstrated that AuNPs/MOGs (Fe) hybrids could be used as a potential CL sensing platform for EP detection. Under the optimized experimental conditions (Figure S9), the CL intensity decreased with increasing concentration of EP (Figure 7A), and the CL intensity (∆I) was linear to the logarithm of EP concentration from 5 nM to 800 nM as given with the equation ∆I =53423lgc－20006, R2 = 0.999 (Figure 7B). Here, ∆I = I0 – Is represents the difference between the CL intensity in the absence (I0) and presence (Is) of EP, and c is the concentration of EP. The limit of detection was calculated to be 1 nM at signal-to-noise ratio S/N = 3. Compared with pure MOGs (Fe) nanosheets catalyzed luminol-H2O2 system for the detection of EP (Figure S10), the hybrids could provide obviously wider linear detection range and much lower LOD value.


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acetylcholin + H 2 O AChE → choline

(10)

choline + O 2 ChOx → H 2 O 2

(11)

Figure 6. Comparison for CL kinetic curves of luminol-ACh-AChE-ChOx (curve a), luminolACh-AChE-ChOx-AuNPs/MOGs (Fe) (curve b), and luminol-ACh-AChE-ChOx-AuNPs/MOGs (Fe)-EP (curve c). Reaction conditions: 0.95 mM luminol, 0.5 mM ACh, 0.25 U/mL AChE, 0.25 U/mL ChOx, 0.1 µM EP and 0.1 mg/mL AuNPs/MOGs (Fe) hybrids in 0.02 M Tris-HCl buffer (pH 8.5). The specificity of the proposed platform was evaluated by investigating the anti-interference capability against other co-existing substances (Figure S11). Specifically, we compared the CL intensity in the presence of EP (0.1 µM) and its mixture with some common interfering metal ions or anions in the environmental water samples such as Na+, K+, Ca2+, Mg2+, Al3+, Fe3+, SO42-, PO43-, CO32-, Cl-, Br- at 10 µM under the same condition. The results demonstrated that the mixture with these common interfering compounds showed similar CL intensity with that of EP, indicating the good anti-interference capability of this AuNPs/MOGs (Fe) hybrids-based sensing platform for the detection of EP.


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Figure 7. (A) CL response of the biosensor to EP with different concentrations. (B) Calibration curve for EP. Inset: linear calibration curve from 5 to 800 nM, R2 = 0.999. Reaction conditions: 0.95 mM luminol, 0.5 mM ACh, 0.25 U/mL AChE, 0.25 U/mL ChOx, and 0.1 mg/mL AuNPs/MOGs (Fe) hybrids in 0.02 M Tris-HCl buffer (pH 8.5). Furthermore, to test the potential feasibility of the proposed sensing platform in complex environment, the concentrations of EP in water samples including tap water and river water were assessed via the standard addition method. As Table 1 showed, the recovery varied in the range of 95.5% - 106.5%, and the relative standard deviation (RSD) was lower than 4.8%, suggesting that our proposed platform can be efficiently applied to the analysis of EP in environmental samples.


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Table 1. Detection of EP in tap water and river water Samples

Tap water

River water

Spiked amount

Found amount

Recovery

RSD

(nM)

(nM)

(%)

(%, n=3)

50

48.7

97.5

4.3

150

156.5

104.3

2.9

500

496.7

99.3

2.5

50

53.2

106.5

1.2

150

149.8

99.8

4.8

500

477.7

95.5

0.5

CONCLUSIONS In summery, we have devised a simple, facile, and green method to synthesize AuNPs anchored MOGs (Fe) nanosheets (AuNPs/MOGs (Fe)) hybrids. As expected, the immobilization of noble metal nanoparticles on the surface of MOGs (Fe) nanosheets dramatically improved the peroxidase-like activity, thus exhibited outstanding CL performance in the presence of H2O2. The excellent CL efficiency of AuNPs/MOGs (Fe) hybrids was attributed to the fact that in situ grown AuNPs on MOGs (Fe) nanosheets could synergistically promote the generation of multiple radicals (eg. OH•, O2•−, and 1O2) and electron transfer in the CL chemical reaction. On the basis of these findings, the as-prepared AuNPs/MOGs (Fe) hybrids were successfully employed in quantitative analysis of pesticide, and the potential feasibility of the sensing platform in complex environment was further demonstrated by the detection of pesticide in real water samples. This method is versatile and can potentially be generalized to the facile synthesis of other noble metal nanoparticles immobilized MOGs hybrids as novel artificial enzymes for


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mimicking the complex nature system. We anticipated that this work would help open a new window for preparing high active mimic enzyme catalyst through surface modification of MOGs nanosheets with other noble metal nanoparticles and facilitate its potential applications in various fields. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publication website. SEM images, FT-IR spectroscopy characterization, CL kinetic curves, CL spectra, CL optimization experiment, linear calibration curves, orthogonal test, and supplementary figures and tables. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (C. M. L.) *E-mail: [email protected]. (Y. F. L.) ORCID Yuan Fang Li: 0000-0001-5710-4423. Cheng Zhi Huang: 0000-0002-1260-5934. Chun Mei Li: 0000-0002-3168-2900.


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60. Wang, D. M.; Gao, M. X.; Gao, P. F.; Yang, H.; Huang, C. Z., Carbon Nanodots-Catalyzed Chemiluminescence of Luminol: A Singlet Oxygen-Induced Mechanism. J. Phys. Chem. C 2013, 117, 19219-19225.


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Table of Contents (TOC)


31

Metal-Organic Gels Hybrids

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