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Porphyrin Based Porous Organic Frameworks as a Biomimetic Catalyst for Highly Efficient Colorimetric Immunoassay Xiao Deng, Yishan Fang, Sha Lin, Qian Cheng, Qingyun Liu, and Xiaomei Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15637 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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ACS Applied Materials & Interfaces

Porphyrin Based Porous Organic Frameworks as a Biomimetic

Catalyst

for

Highly

Efficient

Colorimetric Immunoassay Xiao Deng,† Yishan Fang,§Sha Lin,† Qian Cheng, † Qingyun Liu,* ‡ Xiaomei Zhang* † †

School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100,

China ‡

School of Chemistry and Environmental Engineering, Shandong University of Science and

Technology, Qingdao, Shandong 266590, China §

School of Food Science and Engineering, Qilu University of Technology, Jinan, Shandong

250353, China

KEYWORDS: porphyrin, porous organic frameworks, biomimetic catalyst, peroxidase-like enzyme, colorimetric immunoassay

ABSTRACT: Herein, we synthesized a cost-effective porphyrin based covalent organic polymer, namely FePor-TFPA-COP, through an easy aromatic substitution reaction between pyrrole and tris(4-formylphenyl)amine (TFPA). The triangular pyramid shaped, N-centric structure of TFPA facilitated the formation of FePor-TFPA-COP with three-dimensional porous structure, larger surface area and abundant surface catalytic active sites. FePor-TFPA-COP

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exhibited strong intrinsic peroxidase activity toward a classical peroxidase substrate 3,3’,5,5’tetramethylbenzidine (TMB) in the presence of H2O2. Compared with Horseradish peroxidase (HRP), FePor-TFPA-COP exhibited several advantages such as easy storage, high sensitivity, and prominently chemical and catalytic stability under the harsh conditions, which guaranteed the accuracy and reliability of measurements. Utilizing the excellent catalytic activity, a FePorTFPA-COP based colorimetric immunoassay was first established for AFP detection and showed high sensitivity, stability and acceptable reproducibility. The linear responses range for AFP was 5 pg/mL to 100 ng/mL and the detection limitation is 1 pg/mL. The routine provided a brilliant biomimetic catalyst to develop the non-enzyme immunoassay. More importantly, the high chemical and catalytic stability and sensitivity facilitated the future practical applications under various conditions.

INTRODUCTION Enzyme has been studied and applied in biochemistry, pharmaceutics and food industry for many years due to its high catalytic efficiency, strong specificity, and mild reaction conditions.1-3 However, the low physical/chemical stability under harsh temperature and pH conditions, susceptibility to protein denaturation, and high cost associated with their low content in organisms and laborious separation and purification, seriously limit their practical application.4 To relieve the limitation of enzyme applications and reduce costs, researchers are committed to develop stable enzyme substitutes. Over the past decades, mimic enzymes such as porphyrins, 5-7 host

reagents,

imprinting

polymers,8-10

membrane

systems,11

nanoparticles12

and

nanocomposites13 have drawn much attention, even part of them have owned analogous catalytic efficiency and selectivity as natural enzymes.


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As the active centers of many natural enzymes in vivo, porphyrins have been artificially synthesized to simulate the catalytic function of biological enzyme under mild conditions.14 However, the toilless formation of peroxo-bridged Fe-porphyrin dimers with poor catalytic performance severely limits their direct applications of porphyrin monomer during the oxidation reaction.15 To solve this problem and make better use the biomimetic catalytic properties of porphyrin, various stable methods have been developed, including covalent bond connection, ion-pair combination, introduction of axial ligands, immobilization or embedment of porphyrins in heteroid supports. These strategies authentically enhance the catalytic activity of porphyrin catalysts, but bring some new problems at the same time, such as deficient contact between the catalyst and substrate, reduced activity by porphyrin axial ligand saturation, catalyst detachment from the supports, and indistinct catalytic mechanism caused by the interplay of porphyrin and the supports, etc.16-18 In recent years, a promising strategy is to immobilize or encapsulate porphyrins into porous framework materials to produce heterogeneous catalysts.19-22 Due to the distinct macrocyclic structure and easily modified molecular structure, they can be facilely incorporated into organic frameworks by covalent bond linkage.23 Crystalline porphyrinic metal organic frameworks (MOFs)

24-28

and covalent organic frameworks (COFs)

29-32

are two representative examples

which consist of highly ordered porphyrin arrays. However, the catalytic sites could be easily blocked owing to axial coordination as in case of MOFs, or the active sites become inaccessible to the substrate as in case of two-dimensional COFs due to the close stacking layer structures.3334

Base on this consideration, amorphous three-dimensional covalent organic polymers (COPs)

seems to be a brilliant candidate to solve this problem. Bhaumik’s group designed a bottom-up approach using pyrrole and dialdehydes to construct a highly porous porphyrinic network in


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situ.35 The porous framework skeleton can isolate each porphyrin ligand in a fixed position to stabilize the catalytically active metal center.36 By embedding catalytic centers into the porous frameworks, the active sites could be adequately exposed, and the thermal and chemical stability of porphyrin catalysts would be promoted. Meanwhile, some porphyrinic COPs exhibit goodish specific surface area and pore volumes which can be comparable with porphyrin MOFs and COFs.19,22 Although there are a few reports about porphyrin-based organic polymers as biomimetic catalysts, such as PPN-24(Fe),15 CHF-1,20 por-POF-8-FeCl,37 investigations about incorporating porphyrin into three-dimensional COPs are rare. For all we know, porphyrin-based COPs have not been applied as biomimetic catalysts for the colorimetric immunosensor. In this paper, we report a novel porphyrin-functionalized covalent organic polymer, namely FePor-TFPA-COP, produced by the reaction between pyrrole and tris(4-formylphenyl)amine (TFPA) with the presence of FeCl3 (Scheme 1). The triangular pyramid shaped, N-centric structure of the building block (TFPA) can facilitate the formation of FePor-TFPA-COP with three-dimensional structure. The chemical composition, structure and morphology of the material were characterized by IR, solid state

13

C NMR, elemental analysis, and TEM. The catalytic

behavior of this porous material as an artificial mimetic enzyme was explored and compared with HRP. Kinetic analysis indicated that the peroxidase-like catalytic characteristic of FePorTFPA-COP satisfied typical Michaelis-Menten kinetics and the Michaelis constant (Km) showed strong affinity between the material and the substrate. Finally, based on FePor-TFPA-COP, a colorimetric biosensor on enzyme-linked immunoassay was established to detect alphafetoprotein (AFP) for the first time and applied to detect AFP in real serum samples, which exhibited acceptable specificity, sensitivity, experimental reproducibility and stability. Compared


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with other AFP immunosensors, 38-40 these results indicated that the FePor-TFPA-COP could be used as a promising enzyme mimetic candidate.

Scheme 1. The preparation of FePor-TFPA-COP by the condensation reaction between pyrrole and TFPA.

Scheme 2. Fabrication of FePor-TFPA-COP-based colorimetric immunoassay.


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EXPERIMENTAL SECTION

Reagents and Materials. Human AFP Antigen (Ag), McAb to human alpha fetoprotein monoclonal capture antibody (Ab1, 2.5 mg/mL) and monoclonal mouse anti-human AFP detection antibody (Ab2, 3.1 mg/mL) were purchased from Linc-Bio Science Co. Ltd. (Shanghai, China). Hydrogen peroxide solution (30%) and chloroauric acid (HAuCl4·4H2O) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Pyrrole, bull serum albumin (BSA) and tris base were purchased from Aladdin Bio-chem Technology Co. Ltd. (Shanghai, China). Horseradish peroxidase (HRP, 300 u/mg) and 3,3',5,5'-Tetramethylbenzidine (TMB) were purchased from ShangHai YuanYe Bio-technology Co. Ltd. The serum samples were acquired from the Second Hospital of Shandong University (Shandong, China). All other reagents and solvents were of analytical grade and used as received without further purification. Self-made Dulbecco's phosphate buffered saline (D-PBS) (0.01 M, pH 7.4) containing 2.7 mM KCl, 8.0 mM Na2HPO4, 1.8 mM KH2PO4, 136.7 mM NaCl was kept at 4 oC before use. Ultrapure water from a UPR-II-10T purification system was used throughout the experiment (18.25 MΩ cm−1, Ulupure).

Apparatus. Fourier transform infrared spectra (FT-IR) were recorded in KBr pellets with 2 cm-1 resolution using an αALPHA-T spectrometer. Elemental analysis was performed on an Elementar Vavio El III elemental analyzer. 13C CP/MAS NMR spectra were recorded using a 4 mm MAS probe and a spinning rate of 14 kHz. Scanning electron microscopy (SEM) image was obtained using a JEOL JSM-6700F field-emission scanning electron microscopy and Au (1-2 nm) was sputtered onto the grids to prevent charging effects and to improve the image clarity.


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The transmission electron micrograph (TEM) was recorded on a JEOL JEM-2010 transmission electron microscope operating at an accelerating voltage of 200 kV. N2 adsorption-desorption isotherms were measured on an ASAP 2020 (V4.01G) apparatus at 77.3 K and the surface area of FePor-TFPA-COP was calculated by both the Brunauer–Emmett–Teller (BET) and Langmuir method. Thermogravimetric measurements were performed on a Mettler Toledo TGA/SDTA 851o under N2, by heating to 600 oC at a rate of 10 oC min-1. 1H NMR spectra were recorded on a Bruker DPX 400 spectrometer. Solid-state NMR experiments were performed on Bruker AVANCE III 600 spectrometer at a resonance frequency of 150.9 MHz. X-ray photoelectron spectroscopy (XPS) was carried out on ESCALAB 250 Xi spectrometer (Thermo Fisher Scientific, USA). The Fe and Au contents of the COP samples were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis with an IRIS Intrepid II XRP instrument. The colorimetric immunoassay measurements were carried out on EnSpire Multimode Plate Readers (PerkinElmer, USA).

Synthesis of tris(4-formylphenyl)amine (TFPA). TFPA was synthesized according to the literature with a little modification. 41 Under N2 atmosphere, POCl3 (47.5 mL, 0.5 mol) was dropped into DMF (36 mL) in ice bath and mechanically stirred for another 2 h at about 0 oC. Then, 4-(N,N-diphenylamino)benzaldehyde (5.5 g, 0.02 mol) was added and reacted at 100 oC for 6 h. The obtained mixture was poured into ice water, and the solution pH was adjusted to 7-8 by adding 20% NaOH. The precipitate was filtered, washed with ultrapure water and dried in vacuum at 70 oC for 12 h. The crude product was further purified by silica gel chromatography with CH2Cl2 as eluent, then evaporated and dried under vacuum for 24 h to afford TFPA (1.35 g, 20%) as a pale yellow solid. Rf (CH2Cl2) = 0.3, 1H NMR (300 MHz, CDCl3): δ = 7.19-7.16 (d, J = 9 Hz, 6H, Ar-H), 7.79-7.76 (d, J = 9 Hz, 6H, Ar-H), 9.88 (s, 3H, -CHO).


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Synthesis of FePor-TFPA-COP. Under N2 atmosphere, freshly distilled pyrrole (26 µL, 0.37 mmol), TFPA (122 mg, 0.37 mmol), FeCl3·6H2O (120 mg, 0.44 mmol) and glacial acetic acid (15 mL) were stirred at room temperature for 3 h, and transferred into a Teflon-lined autoclave to heat at 180 oC for 72 h. After cooling down to room temperature, the resulting dark precipitate was filtered and thoroughly washed with DMF, water and methanol until the filtrate became colorless, and then dried in vacuum at 80 oC for 24 h. FePor-TFPA-COP was obtained as a brownish black powder with 87% yield. Elemental analysis values obtained by combustion were C, 67.89; H, 5.38; N, 7.72%; calculated theoretical formula for FePor-TFPA-COP: C, 72.66; H, 4.40; N, 10.28%. ICP-AES analysis revealed that Fe content is 2.19%.

Kinetic analysis and stability test of FePor-TFPA-COP. Kinetic experiments were carried out at 28 oC using 24 µg FePor-TFPA-COP or 0.2 ng HRP as a control experiment in a reaction volume of 2.0 mL NaAc buffer solution (0.2 M, pH 5.0) with fixed concentration of TMB (0.3 mM) for various concentrations of H2O2 substrate. Then the concentration of H2O2 was fixed as 25 mM for various concentrations of TMB substrate. Kinetic measurements were carried out in time scanning mode by monitoring the absorbance change at 652 nm for 3 min on UV-Vis spectrophotometer. As to comparing the catalytic stability of FePor-TFPA-COP and HRP for TMB, the two materials were dispersed or dissolved in water with various pH values (1-14) for 12 h, or the neutral dispersion or solution were placed at various temperatures (0 to 90 oC), respectively. The other experimental conditions were same as the kinetic analysis. After 3 min of reaction, the absorbance values were recorded in spot scanning mode by UV-Vis spectrophotometer.

Mechanism detection. We presume that the peroxidase-like activities of the FePorTFPA-COP may originate from their catalytic ability to decompose H2O2 into ·OH radicals.


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Therefore, a fluorometric spectra method was used to measure the ·OH radicals with terephthalic acid as a probe. H2O2 (10 mM), terephthalic acid (0.5 mM) and various concentrations of FePorTFPA-COP were incubated in acetate buffer (pH 3.8) at 40 oC for 2 h. The reaction system was then centrifuged at 10,000 rpm for 10 min, and the liquor supernatant was used for fluorometric measurement.

Preparation of Au@FePor-TFPA-COP. Firstly, 30.5 mg of FePor-TFPA-COP in the mixture of ethanol (20 mL) and NaOH (0.2 M, 1 mL) was sonicated for 3 min to obtain a uniform suspension. Then 15 mL of HAuCl4 aqueous solution (1.3 mg/mL) was slowly added into the solution under irradiation with a 500 W Xenon lamp and stirred for another 15 min until the solution turned reddish brown. The precipitate was filtered and rinsed with water and ethanol. After vacuum drying at 70 oC for 24 h, Au@FePor-TFPA-COP (35.9 mg) was obtained as a brown powder. ICP-AES analysis revealed that the Fe and Au contents are 1.70 and 14.40%, respectively.

Preparation of Ab2@Au@FePor-TFPA-COP conjugates. Au@FePor-TFPA-COP (5 mg) and Ab2 (50 µg/mL, 1 mL) were dispersed in 9 mL of PBS (0.01 M, pH 7.4) and ultrasonically vibrated for 15 s, and the mixture was incubated overnight at 4 oC with slight shaking for 24 h. Afterwards, 5 mL of BSA (5 wt%, diluted in PBS, pH 7.4) blocking buffer was added and incubated at 4 oC for 12 h to block the possible residual sites on Au@FePor-TFPACOP. Following that, the mixture was collected by centrifuged at 10000 rpm for 20 min and washed twice with PBS. The obtained Ab2@Au@FePor-TFPA-COP conjugates were redispersed in 9 mL PBS and stored at 4 oC for further use.


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Colorimetric immunosensor based on FePor-TFPA-COP. The colorimetric immunoassay assembly procedure is shown in Scheme 2. Firstly, 50 µL of Ab1 (10 µg/mL) in 0.01 M PBS (pH 7.4) was injected into per well of a high-binding polystyrene 96-well microplate and incubated at 4 oC overnight with the sealing film covering on the microplate to avoid evaporation and pollution. Following that, the plate was thoroughly washed by PBS washing buffer to remove the unbound antibody and then blocked with 5% BSA (200 µL per well) for 24 h at 4 oC to engage the nonspecific adsorption sites. After washed with PBS, the plate was incubated with 100 µL of various concentrations of AFP antigen (Ag) at 4 oC for 24 h. After the unbound antigen was rinsed off, 50 µL of the Ab2@Au@FePor-TFPA-COP suspension was injected into each well and incubated at 4 oC for 24 h, and then the plate was washed again to remove the redundant catalyst. The measurement procedure is as follow: 100 µL of H2O2 solution (100 mM in pH 5.0 NaAc buffer) and 100 µL TMB solution (0.57 mM in pH 5.0 NaAc buffer) were added into each well; after 20 min incubation at room temperature, the absorbance value of each well was read out at 652 nm using a microplate reader. The BSA and PBS as distractors and negative control were used to study the specificity of the FePor-TFPA-COPbased colorimetric immunoassay.

RESULTS AND DISCUSSION Synthesis and characterization of FePor-TFPA-COP. FePor-TFPA-COP was synthesized through extensive aromatic electrophilic substitution of pyrrole with tris(4formylphenyl)amine under solvothermal condition with the presence of 1.2 equivalent of FeCl3 in acetic acid medium at 180 oC for 72 h (Scheme 1). Under acidic conditions, tris(4formylphenyl)amine was first activated through protonation, then the pyrrole ring was


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electrophilic aromatic substituted by the activated carbon atoms and further condensation to yield centers with three free -CHO groups per tris(4-formylphenyl)amine unit. Repeating condensation of another -CHO with another pyrrole resulted in the formation of extensive organic polymeric network structures. After the reaction, brownish black fluffy powder was obtained with 87 % yield based on the starting materials. FePor-TFPA-COP is insoluble in water and common organic solvents, and is chemically stable even upon exposure to dilute solutions of acid and base, indicating a highly cross-linked and robust network. The thermal stability of FePor-TFPACOP was examined by thermogravimetric analysis (TGA). Under nitrogen, FePor-TFPA-COP displayed great thermal stability and almost no mass loss is observed until heating up to 290 oC (Figure S1, see Supporting Information). High thermal stability would guarantee that FePorTFPA-COP can exhibit robust catalytic stability under high temperature. The successful formation of the FePor-TFPA-COP was assessed by Fourier transform infrared (FT-IR),

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C CP/MAS NMR and UV-vis absorption spectra. In IR spectrum, the C-H

and C=O stretching band of -CHO in tris(4-formylphenyl)amine appear at 2820, 2720 and 1689 cm-1, respectively, and they almost disappeared after polymerization, implying the complete conversion of the starting material into the polymer (Figure 1A). Similar to other FePor-based polymer,42-44 a moderate peak at 1009 cm-1 assigned to the N-Fe vibration band of FePor was observed, which indicates the formation of porphyrin macrocycles. The solid state 13C CP/MAS NMR displays four broad peaks at 146.4, 137.5, 130.3, and 122.9 ppm that are attributed to the porphyrin macrocycles and phenyl moieties (Figure 1B). Meanwhile, the UV-vis absorption spectrum displays the typical metal porphyrinato features with the Soret band at 402 nm and Qbands at 567, 617 and 650 nm, respectively, clearly implying the retention of porphyrin building


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blocks (Figure 1C). SEM image shows that FePor-TFPA-COP consists of relatively uniform cake-like particles with the average size of 200 nm in diameters (Figure 1D).

Figure 1. The structure and morphology characterization of FePor-TFPA-COP. (A) FT-IR spectra of (a) TFPA and (b) FePor-TFPA-COP; (B) 13C CP/MAS NMR spectra of FePor-TFPA-COP; (C)UV-Vis spectra of FePorTFPA-COP dispersion in DMF; (D) SEM image of FePor-TFPA-COP on a carbon supported copper grid.

The nitrogen adsorption isotherm was measured to assess the permanent porosity of FePorTFPA-COP. As is shown in Figure S2 (see Supporting Information), at low relative pressure (P/P0

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