Novel Iron(III)-Based MetalâOrganic Gels with Superior Catalytic

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Novel Iron(III)-Based Metal-Organic Gels with Superior Catalytic Performance Towards Luminol Chemiluminescence Li He, Zhe Wei Peng, Zhong Wei Jiang, Xue Qian Tang, Cheng Zhi Huang, and Yuan Fang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08476 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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ACS Applied Materials & Interfaces

Novel Iron(III)-Based Metal-Organic Gels with Superior Catalytic Performance Towards Luminol Chemiluminescence Li He,† Zhe Wei Peng,† Zhong Wei Jiang,† Xue Qian Tang,† Cheng Zhi Huang*,‡ and Yuan Fang Li*,† †

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

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

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

KEY WORDS: Metal-organic gels, catalysis, chemiluminescence, luminol, dopamine, detection

ACS Paragon Plus Environment

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ABSTRACT: Novel metal-organic gels (MOGs) consisting of iron (Fe3+) as central ions and 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) as ligands were synthesized by a mild facile strategy. The Fe(III)-containing metal-organic xerogels (Fe-MOXs), obtained after removing the solvents in MOGs, were found to exhibit outstanding performance on the catalysis of luminol chemiluminescence (CL) for the first time even in the absence of extra oxidants such as hydrogen peroxide. The possible CL mechanism was discussed according to the electro/optical measurements including electron paramagnetic resonance (EPR), UV-vis absorption and CL spectra, and the effects of radical scavengers on Fe-MOXs-catalyzed luminol CL system, suggesting that the CL emission of luminol might be originated from the intrinsic oxidase-like catalytic activity of Fe-MOXs on the decomposition of dissolved oxygen. Additionally, the potential practical application of the resulting luminol-Fe-MOXs system was evaluated by the quantitative analysis of dopamine. Good linearity over the range from 0.05 to 0.6 µM was obtained with the limit of detection (LOD, 3σ) of 20.4 nM and acceptable recoveries ranging from 98.6% to 105.4% in human urine. These results may open up the promising application of novel metal-organic gels as highly effective catalyst in the field of chemiluminescence.


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INTRODUCTION Metal-organic gels (MOGs), emerging as a new type of metal-organic hybrid materials, are rapidly built up by straightforwardly assembling organic ligands and metal ions through metalligand coordination and noncovalent interactions, wherein the formation of chemical bonds, hydrogen bonding, π-π stacking, van der Waals forces and ionic interaction play very important roles.1-4 Impressively, MOGs not only can respond sensitively to environmental stimuli such as the change of pH, sonication, light, and temperature, but also exhibit high surface area, porous structure and high thermal stability.5-6 Owing to their unique properties, MOGs have continuously received tremendous attention in diverse applications including sensing,7-9 adsorption,10-11 light-emitting diodes,12-14 drug delivery,15-16 and environmental pollution abatement.17-19 Especially in the field of catalysis, the transition metal-containing MOGs as catalysts displayed immense importance and outstanding performance in the field of catalysis due to their high efficiency, low-cost, easy availability and non-toxic nature.20-23 However, to the best of our knowledge, the utilization of MOGs as catalyst for luminol chemiluminescence (CL) has been rarely reported. CL assay, applied as a fast, facile, and cost-effective analytical methodology, has been widely investigated in chemical and biological applications since it has the advantages of simple device, high sensitivity, wide calibration range and low background interference.24 Despite these superiorities, traditionally, typical CL systems usually suffer from low efficiency of transforming the chemical energy into light, greatly limiting their further development in analytical applications. In this regard, enormous research interests over the past few decades have been dedicated to develop novel CL systems and fabricate functional materials with unique catalytic properties to enhance the CL efficiency. Lv’s group developed novel CL system with new


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carbon nitride quantum dots for selectively and sensitively detecting free chlorine in natural water.25 Lin et al. fabricated CdTe/CdS/ZnS quantum dots-based novel CL systems26 and investigated mimicking hydrogen peroxide properties of fluorescent carbon nanoparticles in CL system.27 Fe3O4 magnetic nanoparticles,28 CuO nanoparticles,29 and noble metal nanoparticles (NPs), such as AuNPs30 and AgNPs31, were all demonstrated to have outstanding performance toward luminol CL. More recently, metal-organic hybrid materials, such as metal-organic frameworks (MOFs),32-33 have been reported to exhibit excellent catalytic activity for luminol CL as well. Similar to MOFs, another type of metal-organic hybrid materials, MOGs, not only possess porous structure, large surface area and abundant active metal sites, but also can be synthesized by a mild facile strategy, making them promising catalyst candidates. As a consequence, it seemed worthwhile to study the application of MOGs in luminol CL fields. With this in mind, the Fe-containing MOGs was synthesized at first, and then Fe(III)-based metal-organic xerogels (Fe-MOXs) were obtained by further freeze-drying. The as-prepared FeMOXs exhibited outstanding performance on catalyzing luminol CL even if without extra oxidants. Since dopamine (DA) could greatly inhibit the Fe-MOXs-catalyzed luminol CL, a sensitive detection method of DA was proposed (Scheme 1). This is the first example of MOGs as catalyst for sensing platform in CL field as far as we know, and the resulting Fe-MOGs are unique in that: (1) Comparing with the previously reported catalysts, Fe-MOGs can be obtained in a very facile way (only mixing at room temperature for 30 s). (2) With freeze-drying treatment, the obtained porous Fe-MOXs were endowed with strong catalytic activity towards luminol CL system.


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Scheme 1. Schematics to Show the Preparation of Fe-MOGs (A) and the CL Detection of DA with Fe-MOXs (B).

EXPERIMENTAL SECTION Materials and Reagents. All reagents were commercial available and used without further pretreatments or purifications. Iron (III) chloride (Aladdin, Shanghai, China) and 1,10phenanthroline-2,9-dicarboxylic acid (PDA) (Alfa, Zhengzhou, China) were used to prepare metal-organic gels. Other reagents, including dimethyl sulfoxide (DMSO), ascorbic acid (AA), thiourea, superoxide dismutase (SOD) and sodium azide (NaN3) were commercially obtained from

Chongqing

Chemical

Reagent

Company

(Chongqing,

China).

Luminol

(3-

aminophthalhydrazide) stock solution (0.01 M) was prepared by dissolving required luminol (Sigma, America) in 100 mL of 0.01 M NaOH solution and kept in dark place for at least one


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week before use. All of the experimental solutions were prepared using Mili-Q purified water (18.2 MΩ). Apparatus. S-4800 scanning electron microscope (SEM) (Hitachi, Japan) was used to characterize the morphology of the as-prepared Fe-MOGs, while transmission electron microscope (TEM) measurements were carried out on a JEM-1200EX (120KV) TEM instrument (JEOL, Japan). Elemental analyses were recorded on an energy dispersive scanning (Model 550i) attached to the S-4800 scanning electron microscope. The infrared spectra in the range of 5003500 cm-1 were recorded by a Prestige-21 Fourier transform IR spectrometer (Shimadzu, Japan) with KBr pellet. D8 Advance X-ray diffractometer (Bruker, Germany) was employed for powder X-ray diffraction (PXRD) study by keeping at 40 kV, 40 mA with Cu Kα radiation (λ=1.5406 Å). An Escalab 250 Xi X-ray photoelectron spectrometer (Thermo Fisher Scientific) coupled with an Al Kα source (hν = 1486.6 eV) was used to achieve X-ray photoelectron spectrometry (XPS) analysis. CL signals were detected by using a computerized BPCL ultra weak luminescence analyzer (Institude of Biophysics, Chinese Academy of Science, Beijing, China). The Micromeritics ASAP 2460 (Micromeritics Instrument Shanghai Ltd., Shanghai, PRC) was employed for the study of the specific surface area of Fe-MOXs using nitrogen gas adsorption/desorption at 77K. UV-vis absorption spectra were measured on a Hitachi U-3010 spectrometer (Hitachi, Japan). The ESP-300E spectrometer (Bruker, Germany) was used for recording electron spin resonance (EPR) spectra at room temperature. Preparation of the Gels. In a typical preparation of metal-organic gels, 250 µL DMSO solution of PDA (13.4 mg; 0.2 M) was placed in glass vials. Then, 250 µL of metal salt aqueous solution (from 0.1 to 1.0 molar equivalent with respect to the PDA concentration) were added into the vials. Subsequently, yellow color opaque metal-organic gels were observed within 30s under


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ambient conditions with the addition of 0.5 to 1.0 equiv of metal salt (Figure S1). It was found that the as-prepared Fe-MOGs in sealed vials were stable within a few months at room temperature. Fe-MOXs were further acquired after removing the solvent of Fe-MOGs with freeze-drying treatment. Catalytic Activity of Fe-MOXs for the Luminol CL System. To investigate the luminol-FeMOXs CL system, the CL activity was conducted on the BPLC luminescence analyzer. In a typical CL measurement, 50 µL of 0.2 M Britton-Robinson (BR) buffer solution (pH 9.27) and 50 µL of 1mg/mL ultrasonic dispersion of Fe-MOXs aqueous solution were premixed in the cuvette. As soon as 250 µL of 1.4 mM luminol solution injected quickly, the CL profile and intensity were measured and integrated at intervals of 1 s at -900V. Detection of DA in Aqueous Medium and Urine. DA analysis was realized as follows: 50 µL of 0.2 M BR buffer solution (pH 9.27) and 50 µL ultrasonic dispersion of Fe-MOXs aqueous solution (1mg/mL) were premixed in a 3-mL quartz cuvette, and then a certain amount of DA or urine were added. After that, 250 µL of 1.4 mM luminol solution was quickly injected, followed by the collection of CL signals with the luminescence analyzer. Urine samples were obtained from healthy volunteers. RESULTS AND DISCUSSIONS Characterization. SEM and TEM images clearly revealed that both Fe-MOGs (Figure 1a, b) and Fe-MOXs (Figure 1c, d) are platelet-like morphology. Moreover, it was found that the platelet-like morphology of Fe-MOXs could be kept over a wide pH range (Figure S2). XPS, energy dispersive X-ray (EDX), and elemental mapping analysis confirmed the presence of Fe, C, N, and O in the Fe-MOXs (Figure 1e, S3). The Fe 2p core-level photoelectron spectrum


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(Figure 1f) indicated that XPS pattern of the Fe-MOXs have the typical Fe 2p3/2 and Fe 2p1/2 peaks at binding energies of 710.3 and 724.2 eV, respectively, confirming the coordination of Fe3+ to PDA.34 The Brunauer-Emmett-Teller (BET) isotherm measurements disclosed that FeMOXs exhibit typical type-III isotherm characteristics of microporous materials (Figure S4). Further, Fe-MOXs present large specific surface area and porous structure, endowing them with outstanding catalytic activity towards luminol CL.

Figure 1. Characterization of Fe-MOGs and Fe-MOXs: (a) SEM and (b) TEM images of FeMOGs, (c) SEM and (d) TEM images of Fe-MOXs; XPS survey spectra of Fe-MOXs (e) and the Fe 2p (f).


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To understand the formation mechanism of Fe-MOGs, PXRD and FT-IR spectra were measured to study the interactions between PDA and Fe3+. Weak and broad signals were observed in PXRD (Figure S5a), indicating that most of the gel scaffold is amorphous. A broad peak at 2θ = 23.62° in PXRD, which corresponds to the d-spacing value of 3.76 Å, confirms the involvement of π-π stacking interactions between 1, 10-phenanthroline groups in the gelation process. FTIR spectra clearly showed the characteristic carbonyl antisymmetric stretching vibration peak at 1736 cm-1 (Figure S5b), indicating the presence of carboxylates in PDA. After the formation of gels, double intense bands at 1639 and 1578 cm-1 were presented for Fe-MOXs, which could be assigned to the symmetric and antisymmetric O-C-O stretching vibrations of carboxylates (∆ν = ∆vas - ∆νs = 61 cm-1), indicating bidentate coordination modes between PDA and Fe3+.35 Thus, in view of the results of PXRD and FT-IR spectra, the presence of the π-π stacking between 1, 10-phenanthroline building blocks and the coordination between Fe3+ and COO- of PDA drives the formation of Fe-MOGs. Catalytic Activity of Fe-MOXs for the Luminol CL System. For the luminol-Fe-MOXs system, strong CL emission was observed with the injection of Fe-MOXs into luminol alkaline solution (Figure 2a), as compared to the hardly detectable CL emission in the absence of FeMOXs, indicating the excellent catalytic performance of Fe-MOXs towards luminol CL. Meanwhile, the catalytic activity of Fe-MOXs at the molar ratio of FeIII/PDC from 0.5 to 1.0 were all demonstrated to be excellent (Figure S6), where the molar ratio of 0.5 was chosen to further study their catalytic performance. In order to better understand the Fe-MOXs-catalyzed luminol CL properties, we initially studied the emitting species of CL spectrum through high-energy cut off filters (230-640 nm). As Figure 2b illustrated, the maximum emission wavelength of luminol CL centered at ~440nm in


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either the absence or the presence of Fe-MOXs, suggesting that the excited state 3aminophthalate anions (3-APA*) in both cases are luminophors, which was consistent with previous reported catalysts on luminol CL system.36-38 Then, we investigated the role of FeMOXs in this luminol-Fe-MOXs system by UV–visible absorption measurement. Figure 2c showed that there is no appreciable change of the maximum absorption position of luminol-FeMOXs system, in comparison to the individual Fe-MOXs and luminol system, revealing that the addition of Fe-MOXs did not result in the generation of new species, further proving that the remarkable luminol CL emission should be attribute to the catalytic activity of Fe-MOXs.

Figure 2. CL features of Fe-MOXs-catalyzed luminol. (a) CL response curves and (b) CL spectra of luminol (black line) and luminol-Fe-MOXs (red line). (c) UV-vis absorption spectra of Fe-MOXs (black line), luminol (red line) and luminol-Fe-MOXs (blue line). (d) The comparison of CL behavior for Fe-MOXs (black line) and free Fe3+ (red line) (with equivalent Fe3+ content).


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Experimental conditions: Fe-MOXs, 0.1 mg/mL; luminol, 0.7 mM in 0.2 M BR buffer solution (pH 9.27). Additionally, considering the fact that a large amount of Fe-containing materials, such as Fe3O4,28 ZnFe2O4,39 Fe-MOF,40 have been demonstrated to show high catalytic activity, Fe3+ in the Fe-MOXs structure thus might play an important role in the Fe-MOXs-catalyzed luminol CL system. As illustrated from the comparison of CL behaviors between Fe3+ and Fe-MOXs (Figure 2d), free Fe3+ can catalyze the luminol CL emission, but the corresponding CL intensity was much weaker than that by Fe-MOXs under the same conditions. Such conspicuous differences were mainly due to the fact that Fe3+ is generally unstable under alkaline conditions, which is much more prone to form its hydroxide. By constrast, Fe-MOXs, not only have large specific area, but also display platelet-like morphology with amounts of exposed active sites, thus resulting for the outstanding catalytic activities toward luminol CL system in alkaline solutions. Besides, the SEM images of Fe-MOXs (Figure S7) showed that its morphology could be kept before and after the CL reaction. CL Mechanism. It is well-known that traditional CL system usually requires the involvement of extra oxidants, such as H2O2. In this study, as no extra oxidants involved, the CL emission was possibly associated with dissolved oxygen, which may react with Fe-MOXs to generate reactive oxygen species (ROS). To obtain evidence for supporting this hypothesis, we investigated the effects of nitrogen on the CL intensity of luminol-Fe-MOXs system. Experiments showed that the CL signal was obviously weakened in nitrogen-saturated solution as compared with that in air-saturated solution (Figure S8), clearly confirming that the dissolved oxygen was responsible for the original source of ROS, which played a crucial role in the Fe-MOXs-catalyzed luminol CL system.


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To thoroughly understand the CL mechanism of luminol-Fe-MOXs system, different radicals that may be formed in this CL system including superoxide radical anion (O2•―), singlet oxygen (1O2), and hydroxyl radical (OH•) were identified with the help of active oxygen radical scavengers and ESR spectra measurements. The great inhibition on CL intensity with increasing the concentration of AA (Figure S9a), a well-known effective radical scavenger, indicates that active oxygen radicals were generated and played a critical role in luminol-Fe-MOXs CL system. Meanwhile, the CL intensity was effectively inhibited upon addition of SOD (Figure S9b), a commonly considered effective scavenger of O2•― radical,41 which suggests that Fe-MOXs did exhibit prominent catalytic activity to catalyze the decomposition of dissolved oxygen on their surface to produce a high yield of O2•―. NaN342 and tetramethyl-4-piperidine (TEMP)43 were effective detection reagent for 1O2. The addition of 5 mM NaN3 could cause 58% decrease of CL emission (Figure S9c), while the addition of TEMP could yield a strong 1:1:1 triplet signal of the TEMPO nitroxide radical (Figure 3a). These results indicated that 1O2 was formed during the FeMOXs-catalyzed luminol CL process. Similarly, OH• radical could also be identified by thiourea44 and 5,5-Dimethyl-1-pyrroline N-oxide (DMPO).45 The results showed that 5 mg/mL of thiourea can cause more than 50% decrease of the CL signal (Figure S9d) and the 1:2:2:1 quartet characteristic peak of typical DMPO-OH• adduct was obviously observed when DMPO was introduced into luminol-Fe-MOXs system (Figure 3b), indicating that OH• was involved in CL emission process.


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Figure 3. ESR spectra of (a) 1O2 and (b) OH• radicals in luminol (black line) and luminol-FeMOXs (red line) systems. Experimental conditions: Fe-MOXs, 0.1 mg/mL; TEMP, DMPO, 0.05 M; luminol, 0.7 mM in 0.2 M BR buffer solution (pH 9.27). In such case, the possible CL mechanism of luminol-Fe-MOXs system could be concluded as follows: the dissolved oxygen got decomposed firstly, forming O2

•―

radical at the surface of Fe-

MOXs platelet (reaction 1), owing to the intrinsic oxidase-like catalytic activity of Fe-MOXs; then, the as-formed O2•― radical would quickly convert into 1O2 and H2O2 as it is unstable in aqueous solution (reaction 2).46 Furthermore, OH• generated from the evolved O2•― and H2O2 (reaction 3);47 finally, luminol molecules would be oxidized by the resultant ROS to produce luminol radical in the alkaline solution, which further reacted with O2•― to generate excited-state 3-aminophthalate anions (3-APA*), accompanying the light emission at 440nm.48


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Fe − MOXs

•−

→ O2 O2  

1 •− •− − O2 + O2 + 2H 2O→ O 2 + H 2 O 2 + 2OH

•− − O2 + H 2O 2 → O 2 + OH • + OH

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(1) (2) (3)

CL Detection of DA in Human Urine. Since DA is readily reacted with the active oxygen radicals, resulting in the inhibition of luminol oxidation process in the luminol-Fe-MOXs system, DA could be detected based on decreased CL intensity. Under the optimal assay conditions including pH, the concentrations of luminol and Fe-MOXs (Figure S10), a good linear relationship was available between the CL intensity (∆I) and the dopamine concentration in the range from 0.05 to 0.6 µM (Figure 4), which can be expressed as ∆I = 8027.4 + 52233.9 cdop with R2 = 0.9928, and the detection limit of 20.4 nM (signal/noise=3). Comparing to fluorescence, colorimetry, electrochemical and electrochemiluminescence (Table S1), this new developed method is simple and sensitive for DA detection.

Figure 4. Linear calibration curves for the response of CL intensity to different concentration of dopamine based on the luminol-Fe-MOXs sensor. Experimental conditions: Fe-MOXs, 0.1 mg/mL; luminol, 0.7 mM in 0.2 M BR buffer solution (pH 9.27).


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Taking into account the great challenge of DA detection in complex urine, the interfering effects of other co-existing substances including biomolecules and ions that commonly presented in urine were investigated before the detection of urine samples. The results showed that neglectable change of CL intensity was observed even though the concentrations of co-existing substances were much higher than that of DA (Figure S11), indicating that the proposed CL assay based on the luminol-Fe-MOXs sensor was of excellent selectivity for the detection of DA, and thus held great promise for the application in real samples. Table 1. Determination of DA in Urine Samples. Samples 1

2

Spiked amount

Found amount

Recovery

RSD

(µM)

(µM)

(%)

(%, n=3)

0.080

0.079

98.6

1.3

0.100

0.107

103.1

1.5

0.200

0.211

105.4

0.6

0.400

0.403

100.8

4.0

0.080

0.084

104.5

2.7

0.100

0.102

101.9

3.4

0.200

0.202

101.0

1.6

0.400

0.397

99.4

2.7

Subsequently, to confirm the applicability of this method, we investigated the spiked recoveries of DA in urine by adding a certain amount of standard solution of DA to the diluted urine samples. The corresponding recovery values of 98.6-105.4% were obtained (Table 1), demonstrating that the proposed CL method was suitable for sensitive detection of DA in urine samples.


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CONCLUSIONS In summary, a novel Fe(III)-based metal-organic gel with platelet-like structure has been fabricated by the direct gelation of PDA and Fe3+. By employing a simple freeze-drying method, the obtained Fe-MOXs were first demonstrated excellent catalytic activity for prompting the decomposition of dissolved oxygen to yield oxygen-related radicals, leading to a dramatic CL emission. The CL mechanism for luminol-Fe-MOXs system was due to the intrinsic oxidase-like catalytic activity of Fe-MOXs. In addition, a quantitative analysis platform for dopamine detection was developed with excellent selectivity and sensitivity. Our work demonstrated that MOGs synthesized in a mild facile strategy could act as highly efficient catalyst for luminol CL reaction, which not only open a new window of interest for the exploration of novel MOGs as splendid catalysts, but also foresaw its potential applications in CL fields. ASSOCIATED CONTENT Supporting Information Additional data and information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] * Tel.: (+86) 23-68254059. Fax: (+86) 23-68367257. E-mail: [email protected].

* Tel.: (+86) 23-68254659. Fax: (+86) 23-68367257. E-mail: [email protected].

ORCID


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Yuan Fang Li: 0000-0001-5710-4423. Cheng Zhi Huang: 0000-0002-1260-5934. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (NSFC, No. 21575117). REFERENCES (1) Pang, X.; Yu, X.; Lan, H.; Ge, X.; Li, Y.; Zhen, X.; Yi, T. Visual Recognition of Aliphatic and Aromatic Amines Using a Fluorescent Gel: Application of a Sonication-Triggered Organogel. ACS Appl. Mater. Interfaces 2015, 7, 13569―13577.

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Table of Contents (TOC)

A novel iron(III)-based metal-organic gel has been fabricated with outstanding performance on catalyzing luminol chemiluminescence for sensitive CL detection of dopamine.


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Novel Iron(III)-Based MetalâOrganic Gels with Superior Catalytic

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Novel Iron(III)-Based MetalâOrganic Gels with Superior Catalytic