MetalâOrganic Framework Nanomaterials as Novel Signal Probes for

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Metal–organic frameworks nanomaterials as novel signal probes for electron transfer mediated ultrasensitive electrochemical immunoassay Ting-Zhi Liu, Rong Hu, Xi Zhang, Kun-Lei Zhang, Yi Liu, Xiao-Bing Zhang, Ru-Yan Bai, De-lei Li, and Yunhui Yang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04191 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 17, 2016

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Analytical Chemistry

Metal–organic frameworks nanomaterials as novel signal probes for electron transfer mediated ultrasensitive electrochemical immunoassay Ting-Zhi Liua, Rong Hu*a, Xi Zhanga, Kun-Lei Zhanga, Yi Liua, Xiao-Bing Zhangb, Ru-Yan Baia, De-lei Lia, Yun-Hui Yang*a a. College of Chemistry and Chemical Engineering, Yunnan Normal University, Yunnan, Kunming 650092, P.R. China b. Molecular Science and Biomedicine Laboratory, State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Collaborative Innovation Center for Molecular Engineering for Theronastics, Hunan University, Changsha, 410082, China

*To whom correspondence should be addressed. E-mail: [email protected], [email protected] Phone: 86-871-65941087. Fax: 86-871-65941086

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Abstract A novel and simple electrochemical immunoassay for C-reactive protein was developed using metal-organic frameworks (Au-MOFs) as signal unit. In this study, we found MOFs could be used as signal probe. And this new class of signal probe differs from traditional probe. The signal of the copper ions (Cu2+) from MOFs could be directly detected without acid dissolution and preconcentration, which would greatly simplify the detection steps and reduce the detection time. Moreover, MOFs contain large amounts of Cu2+ ions, providing high electrochemical signals. Our report represents the first example of using MOFs themselves as electrochemical signal probe for biosensors. Platinum nanoparticle modified covalent organic frameworks (Pt-COFs) with high electronic conductivity was employed as the substrate, which is the first time demonstrating the use of Pt-COFs for electrochemical immunoassay. Under the optimized experimental conditions, the proposed sensing strategy provides a linear dynamic ranging from 1 ng/mL to 400 ng/mL. A detection limit of 0.2 ng/mL was obtained, indicating an improved analytical performance. With these merits, this stable, simple, low-cost, sensitive and selective electrochemical immunoassay shows promise for applications in the point-of-care diagnostics of dieses and environmental monitoring.


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Introduction The development of efficient methods for highly sensitive and rapid detection of proteins is essential for the early diagnosis of serious diseases. The use of electrochemical method is one of the most attractive techniques for the detection of clinical samples by virtue of its low cost, strong flexibility, high sensitivity, fast and portability. For an electrochemical biosensor, two important factors should be considered in bioanalysis: the sensitive and distinguishable signal tags, high electronic conductivity of the modified electrode. In the past decade, various labeling methods integrated with signal amplification strategies such as nanoparticles-based amplification1-4, enzymatic signal amplification5,6 and metal ion-functionalized amplification7,8 have been developed. The prepared labels could greatly amplify the signal in bioassays. However, the nanoparticles-based signal tags were usually suffered from integrating with the biomolecules indirectly, which needed coupling agents to covalently bind biomolecules. Nevertheless, these coupling agents will be easily hydrolyzed during storage or reaction, resulting in the low efficiency of bioconjugation. While irreversible denaturing of protein enzymes under such harsh conditions will be occurred9. Currently, some metal ions based nanotracers were also used as signal probe, such as Cd2+, Pb2+ ions7, 8. This nanotracer should be soaked in solution for 24 h, which were generally time-consuming and laborious. Therefore, the development of a simply, stable, sensitive and selective electrochemical biosensor is highly desirable. Metal-organic frameworks (MOFs) as well as coordination polymers, consisting of metal ions or clusters linked by organic bridging ligands, have received tremendous attention in recent years due to the favorable properties such as high surface areas, tunable physicochemical properties, and high density of metal sites10-13. In particular, increasing interest has been focused on the sensing of inorganic and organic molecules using functionalized MOFs14-16. For instance, Ling et al. reported an electrochemical sensor based on mimetic catalysis of metal-organic framework and allosteric switch of hairpin DNA for DNA detection14. Cui et al. constructed electrochemical DNAzyme biosensor for lead detection sensitized with porphyrinic metal-organic framework mimic peroxidase15. In this study, we ACS Paragon Plus Environment


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report that for the first time, MOFs, HKUST-1 could be used as signal probe, in which the signal of the copper ions (Cu2+) could be easily detected in buffer solutions. This new class of signal probe differs from traditional probe. It could be detected directly without acid dissolution and preconcentration, greatly simplifying the detection steps. Moreover, the subsequent conjugation step is simple and stable because metal nanoparticles could be easily incorporated into MOFs. Although metal ions have been used as signal tags, the modification processes were generally time-consuming. For example, Gao et al. utilized the encapsulated Cu2+ ions into the dendrimer as the signal tags for sensitive electrochemical detection of DNA hybridization17. Given the unique properties of this signal probe, it is highly beneficial to construct novel electrochemical systems for the early diagnosis of serious diseases. Another important issue is to design a high-conductive and reusable sensing interface for development of an electrochemical biosensor. Covalent organic frameworks (COFs) that are organic analogues of MOFs are a series of crystal organic porous materials, which are constructed solely from organic building units via covalent bonds18-20. Recently, COFs have emerged as new porous materials for gas adsorption and storage because of their high porosity, high surface area, robust thermal stability, and low densities21. Unlike most of the organic polymers, COFs have an ordered crystalline structure, which makes them a promising new class of templates for hosting nanoparticles22. Especially, the layered structure of 2D COFs provides periodic arrays of π clouds that can greatly facilitate charge-carrier transport23-25. The high electronic conductivity is highly suitable for electrochemical biosensors. Until now, a large number of COFs have been synthesized over the past few years, but these materials have only rarely been explored as chemosensors26. Therefore, it is interesting use COFs as the substrate, allowing high electronic conductivity and therefore low detection limits. Herein, we designed a signal probe, the Au NPs and MOFs composite (Au-MOFs), to label the signal antibody (Ab2), and developed an ultrasensitive immunoassay method by Pt-COFs immobilized with capture antibodies (Ab1) to facilitate the signal amplification (Scheme 1). C-reactive protein (CRP) was used as the model analyte, because CRP is the correlative inflammatory factors of coronary heart disease. And CRP is a main acute phase protein, the concentration will remarkable increase when ACS Paragon Plus Environment

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infected or the tissue is injured27-29. In this study, Pt-COFs with high electronic conductivity were employed as the substrate, and this is the first time demonstrate the use of Pt-COFs for electrochemical immunoassay. CRP antibody was conjugated with Au-MOFs via noble metal NPs. The conjugation step is simple and stable. Moreover, MOF composite (Au-MOFs) contain large amounts of Cu2+ ions, providing high electrochemical signals. The metal ions in the bioconjugates (Au-MOFs-Ab2) could be detected directly without acid dissolution and preconcentration, which would greatly simplify the detection steps and reduce the detection time. The strategy offers an excellent signal transduction platform for the detection of CRP, and could be integrated with other recognition elements to broaden its applications in bioassay.

Scheme 1 Schematic illustration of the electrochemical immunosensor

Experimental Materials and Apparatus CRP was obtained from Express Technology Co., Ltd (Beijing, China). 1, 3, 5-triformylbenzene, 1, 4-dioxane, 1, 4-diaminobenzene, phosphate buffer solution (PBS, pH 7.4), chitosan (CHIT) (85% deacetylated), 1, 3, 5-benzenetricarboxylic acid and BSA were obtained purchased from Sigma Co., Ltd (Missouri, USA). H2PtCl6 was purchased from Kunming Boren Precious Metal Co., Ltd (Kunming, China). All other reagents were of analytical reagent grade and used without further purification. Doubly distilled water (resistance >18 MΩ cm−1) was employed throughout all experiments. ACS Paragon Plus Environment


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All electrochemical measurements were carried out using a CHI 660D electrochemical workstation (Chenhua Instruments Co., Ltd., Shanghai, China). A three-electrode system was used which consisted of a glassy carbon (GC) electrode, a saturated calomel reference electrode (SCE) and a platinum wire auxiliary electrode. All potentials throughout experiments were reported versus the saturated calomel reference electrode. Preparation of Pt NPs modified covalent organic frameworks (Pt-COFs) COFs were prepared according to the literature with some modification22. Briefly, 0.16g (15 mmol) of 1, 4-diaminobenzene and 0.16g (10 mmol) of 1, 3, 5-triformylbenzene were first added to a reaction tube. Then, 10.0 mL of 1, 4-dioxane was employed to dissolve the above solid. Afterwards, 2.0 mL of 3M acetic acid was dropped to the tube slowly until the yellow solid was obtained. And the obtained products were transferred to the drying oven for 3 days at 120 °C. After them cooling down to the room temperature, COFs were washed with DMF and THF respectively. The resulting solid was isolated by centrifugation and washed with THF using Soxhlet extraction for 24 h, and then dried at 80 °C under vacuum for 12 h. 50 mg COFs were ultrasonically dispersed in 50.0 mL H2O for 30 min, followed by the addition of 1.0 mL 1 % H2PtCl6. After heating at reflux to 90 °C, 3.5 mL of 1 % sodium citrate solution was added quickly, and the reaction mixture was refluxed for 12 h. The above solution was centrifuged, washed with the water. The obtained products were resuspended in water to preserve. Preparation of MOFs HKUST-1 was synthesized according to the literature30. Briefly, 1, 3, 5-benzenetricarboxylic acid was dissolved in a methanol/H2O mixture, and then CuSO4 ·7H2O solution was dropped into the above solution. The mixture was allowed to react for 2 h to produce a blue precipitate. Then, the precipitates were extensively washed with water via centrifugation until there was no red precipitation appeared when one drop of washing solution was added to the mixture of 6 mol/L HAc and K4[Fe(CN)6], which meant no free Cu2+ existed on MOFs, and resuspended in methanol at 4 °C. ACS Paragon Plus Environment

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Preparation of Au-MOFs The gold nanoparticles employed in this work were prepared according to literature with a slight modification31. Briefly, 50 mL of 0.01 % HAuCl4 solution was heated under reflux. Then, 2 mL of 1 % trisodium citrate solution was added under reflux for 10 min. When the color of the solution changed to red indicated that the gold nanoparticles were formed. The solution was stirred continuously until it was cooled to room temperature. The solution was stored in the refrigerator at 4 °C until use. Then, 50 mg cleaned HKUST-1 were added to 15 mL of gold nanoparticles with constant stirring at room temperature. The mixture was allowed to react for 48 h to produce a purple precipitate. Finally, the precipitate was washed with ethanol and water, separately. Preparation of signal-MOFs-metal ion probes 1 mL of Au-MOFs nanoparticles was redispersed in 1 mL of PBS. Then, 200 mM Na2CO3 was added to adjust pH to 9. Then, 10 µL of 1 mg/mL CRP antibody was pipetted to the mixed solution by five times and each internval was 3 min. After the solution was left to stand overnight at 4 °C, 25 µL of BSA (1 %) was added to the solution to block nonspecific binding sites. At last, the precipitates were extensively washed with water via centrifugation, and resuspended in PBS buffer at 4 °C. Fabrication of immunosensor The GC electrode was pretreated according to the previous reported work32-34. First, 0.5 % CHIT was mixed with COFs with 1:1 volume ratio to form a CHIT/COFs solution. Then, 10 µL of the above CHIT/COFs solution was dropped onto the electrode surface, and dried at room temperature to allow it to chemisorb onto the surface. After washing with distilled water, 10 µL of 40 µg/mL anti-CRP was spread on the GC electrode at 4 °C overnight. Afterward, 10 µL of 1 % BSA solution was added to the electrode surface to block the nonspecific sites. After reacting for 1 h, the electrode was incubated with 10 µL of different concentrations of CRP at 37 °C for 1 h. Measurement Procedure



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The electrochemical measurement was carried out in pH 4.5 HAc/NaAc. A differential pulse voltammetric (DPV) scan from -0.3 to 0.3 V with pulse amplitude of 25 mV, pulse frequency of 15 Hz, and quiet time of 2 s was performed to record the electrochemical responses at -0.02 V for quantitative measurement of CRP.

Results and discussion Design strategy The principle of the MOFs-Cu2+ signal probe for electrochemical biosensing is represented in Scheme 1. The Pt-COFs were first immobilized on the GC surface. MOFs, HKUST-1 containing large amounts of Cu2+ ions were explored as signal probe. Pt-COFs have high surface area and electronic conductivity was employed as biosensor platforms for immobilization of Ab1, which could remarkably enhance the sensitivity of the proposed biosensor. In the presence of analyte, the binding between antibody and antigen will be happen. Then, Ab2-modified Au-MOFs composite could be further attached to the modified electrode as signal tags. The corresponding electrochemical current response of Cu2+ was used to quantify the concentration of protein. Immunoassay using MOF-metal ion probes Metal elements are commonly employed as labels in electrochemical biosensor which exhibit specific voltammetric characteristics at different applied potentials. Thus, it offers an effective way to detect analyte by determination of the metal component7,8,35. As can be seen in Scheme 1, the metal ions in the MOFs nanoprobes were directly detected by highly sensitive DPV. The peak at -0.02 is ascribed to the oxidation of copper. In order to trace the origin of the electrochemical signal, MOFs were kept washing until there was no red precipitation appeared when one drop of washing solution was added to the mixture of 6 mol/L HAc and K4[Fe(CN)6] in the experiment. As shown in Figure S1, the mixture of 6 mol/L HAc and K4[Fe(CN)6] appeared reddish - brown color in the presence of 10 µM Cu2+ because the following reaction took place: 2Cu2+ + [Fe(CN)6]4- = Cu2[Fe(CN)6] ↓(left). However, K4[Fe(CN)6] ACS Paragon Plus Environment

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solution containing 6 M HAc in the presence of the washing solution of MOFs

was colorless (right),

indicated that no free Cu2+ existed on the MOFs after washing. After those cleaned MOFs were modified on the GCE, the signal of Cu2+ still can be detected by DPV. The result was shown in Figure S2. Moreover, those cleaned MOFs were dispersed in 10 mM Cd(NO3)2 aqueous solution for 24 h to let ion exchange reaction occurred in order to check whether the signal of Cd2+ could also be detected. Afterwards, those MOFs were washed about 20 times to guarantee no free Cd2+ existed on MOFs. Transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and chromogenic experiment confirmed that the ion exchange reaction was successfully happened (Figure S3, S4, S5). Then, MOFs nanoprobe with exchanged Cd2+ were modified on the surface of GCE and detected by DPV. From the results (Figure S6a), one can see that signal at −0.8 V could not be detected before Cd2+ ion exchange. After the ion exchange, the current signal of Cd2+ at −0.8 V appeared (Figure S6b). Then, Ab2-modified MOFs before and after negative voltage scanning was examined using scanning electron microscope (SEM). From the results one can see that the topological structure of the used MOFs could be easily collapsed and released Cu2+ to generate the signal when negative voltage was used (Figure S7 and S8, SI). Therefore, it can be concluded from all those above results that electrochemical signal derived from MOFs themselves rather than free ions adsorbed on MOFs during synthesis. According, the position of peak could reflect the corresponding antigen. Our proposed immunoassay use HKUST-1 MOFs as a model. Other kinds of MOFs containing different of metal ion, such as cadmium (Cd2+) based MOFs ( Cd-MOF-74)36 and zinc (Zn2+) based MOFs (ZIF-8)37, could be also explored as alternative signal probes in bioanalysis of proteins and other molecules (Figure S9 and S10, SI). Characterization COFs were prepared according to the literature, and the as-prepared nanoparticles were monodispersed with an average size of 200 nm (Figure 1a). The XRD date shows that the as-prepared COFs have a characteristic diffraction peak at 5° which attribute to the (100) diffraction22 (Figure 1b), ACS Paragon Plus Environment


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indicating the formation of crystalline COFs material. Solid-state NMR spectroscopy was also used to confirm that COFs were successfully synthesized. The 13C NMR peak at ∼156 ppm corresponds to the carbon atom of the C=N bond22, the formation of which is characteristic for the condensation reaction of aldehyde and primary amine. The signals at∼122, 130, 136, and 148 ppm could be ascribed to the carbon atoms of the phenyl groups (Figure S11, SI). The surface of COFs was then decorated with Pt NPs. The −NH– and −NH2 groups inside of the COFs could provide a large number of anchoring sites in the formation of Pt NPs on the surface of COFs22. After addition of reducing agent, sodium citrate, Pt NPs were achieved via the reduction of PtCl6–. From the TEM image, one can see that a large number of black dots which were corresponding to the Pt NPs were uniformly loaded on the surfaces of COFs (Figure 1c and 1d). The energy spectrum (EDS) was employed to confirm the presence of Pt NPs on COFs (Figure S12, SI). No agglomeration of the Pt NPs was detected, indicating that the formed Pt NPs were firmly anchored to the inherent functional groups on the surface of COFs38.

Figure 1 (a) XRD patterns of COFs; (b) TEM image of COFs; (c, d) TEM image of Pt-COFs.

To achieve highly sensitive detection of CRP, Au-MOFs were also fabricated. In this study, AuMOFs were not only used as the supporter carrying antibody, but also employed as the signal tags for target detection. As shown in Figure 2a, the synthesized MOFs showed good dispersity. The XRD date ACS Paragon Plus Environment

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was used to investigate the formation of MOFs. The as-prepared MOFs have a characteristic diffraction peak at 12° (Figure 2b). Au NPs were successfully incorporated into MOFs with an average diameter of 100 nm (Figure 2c and 2d). The energy spectrum was also employed to confirm the presence of Au NPs on MOFs (Figure S13, SI). Theses results indicated that they can serve as an excellent biocompatible supporter to load CRP-antibody on the surface.

Figure 2 (a) TEM image of MOFs; (b) XRD patterns of MOFs; (c, d) TEM image of Au-MOFs.

Electrochemical characterization of the modified GC electrode The AC impedance characteristics of the stepwise self-assembly process of the modified interfaces were investigated. As shown in Figure 3, on can see that the bare substrate appeared a very small impedance (curve a), revealing its good electric conductivity. After the Pt-COFs were added on the surface of the electrode, these structures induce the decrease of electrochemical impedance, which suggested that Pt-COFs with periodic arrays of π clouds owned high electronic conductivity (curve b). About 400% percentage of electronic conductivity was improved by using Pt-COFs (Figure S14, SI). However, when anti-CRP was dropped onto the modified GC electrode, the impedance was increased (curve c), because the anti-CRP could be attached onto the modified surface through the interaction between Pt NPs and mercapto (or primary amine) groups of anti-CRP. In the presence of the target ACS Paragon Plus Environment


antigen CRP, the antibody- antigen interaction led to the peak further increased (curve d). After the MOFs-conjugated secondary antibody was captured in the modified GC electrode by the formation of a sandwich immunocomplex, which led to an obvious increase in the resistance of the working interface (curve e). The obtained results demonstrate the accomplishment of antibody-based biorecognition interface.

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Z'/ohm Figure 3 Nyquist plots of impedance spectra obtained in 5 mmol/L [Fe(CN)6]3-/4-, (a) bare GCE, (b) Pt-COFs/GCE, (c) anti-CRP/Pt-COFs/GCE, (d) CRP/anti-CRP/Pt-COFs/GCE, (e) Au-MOFs-Ab2-CRP/anti-CRP/Pt-COFs/GCE.

We also recorded the cyclic voltammtric curves of Au/Cu( Ⅱ )-HKUST-1 labeled anti-CRP/CRP/anti-CRP/ Pt-COF/GCE at different scan rate varying from 40 to 180 mV/s and shown in Figure 4. The inset was the relationship between peak current and the square root of scanning rate. From the inset, one can see that peak current was proportional to the square root of scanning rate (v1/2), which indicated that the current was diffusion-controlled electrode processes.


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Optimization of the experimental conditions of the immunoassay In order to achieve the best sensing performance, the effects of pH, the concentrations of anti-CRP, antibody and antigen incubation time on the response of immunosensor were optimized, respectively. Experimental results showed that the pH of 4.5, the anti-CRP concentration of 40 µg/mL, the anti-CRP and antigen CRP incubation time with 60 min could provide maximum signal for the sensing system (Figure 5), and these values were chosen as optimized conditions for further investigation.



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Figure 5 The effects of pH (a), anti-CRP concentration (b), and anti-CRP incubation time (c), and CRP incubation time (d) on the response of immunosensor.

Analytical performance Under the optimized conditions, the sensitivity of the sensor was also investigated. Figure 6 shows the electrochemical response in the presence of a series of different target concentrations. One can see that the peak current increased gradually with an increase in the concentration of CRP. A linear relationship between peak current intensity and the concentration of CRP range from 1 ng/mL to 400 ng/mL. The linear equation is △I peak = 8.1135×10-6 lgC+9.796×10-7. The new system is very sensitive, with the detection limit of 0.2 ng/mL determined by 3δ/slope (δ, standard deviation of the blank samples). The low detection limit might be attributed to MOFs containing large amounts of Cu2+ ions, which greatly amplified the peak signals. Therefore, the sensitivity is comparable or better than that of


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some previous correlative work39-43. The wide linear range for the analyte was also very important for practical application. Moreover, the detection procedure is very simpler. (a)

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To investigate the specificity of the proposed strategy, several interfering agents including carcino-embryonic antigen (CEA), human chorionic gonadotrophin (HCG), glycine (Gly), and glucose (Glu) were used, because selectivity is another important factor for evaluating the performance of a biosensor. The selectivity of the sensor was tested under the same conditions as those involved for CRP detection, and the results were shown in Figure 7. Only CRP (100 ng/mL) caused a dramatic current



response. The electrochemical signal did not show much change upon the addition of other interferents (1 µg/mL). In other words, our proposed strategy has sufficient selectivity towards other molecules.

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Figure 7 Specificity for the determination of CRP using the proposed immunosensor.

Precision, Reproducibility, Stability and Reusability of Immunosensor The intra-assay precision of our proposed strategy was evaluated through five replicative measurements. The intra-assay variation coefficient was 3.8 % with the same concentration of CRP (20 ng/mL), demonstrating its good repeatability. Stability is an important issue for the practical implementation of CRP detection. Thus, successive cyclic voltammograms (CV) scans were studied to investigate the stability of the proposed strategy. After 100 times of CV scans, 96 % of the initial response of the immunosensor still could be detected (date are not shown). Moreover, the immunosensor could retain 90 % of the initial value after storage in pH 7.4 PBS at 4 °C for one week, revealing accepted stability. Through soaking with urea solution, the proposed immunosensor for CRP could also be regenerated. These above results indicated our proposed immunosensor had acceptable stability. Application in analysis of serum samples


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To investigate the applicability and reliability of our proposed strategy for the clinical applications, we further conducted its CRP detection in serum samples. In order to avoid the interference of background signal from serum, varying amounts of CRP were added to the diluted 10 % serum samples. The analytical results are shown in Table 1. From the Table 1, we observed that the results obtained in real serum samples showed good recovery values. The above results demonstrated that our proposed strategy could be applied to the direct detection of CRP in serum samples. Table 1: Recovery experiments of CRP in human serum samples. Added (ng/mL)

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RSD

3.56%

Conclusion A novel and simple electrochemical immunoassay was developed using Au-MOFs as signal probe. C-reactive protein was used as the model analyte, because CRP is the correlative inflammatory factors of coronary heart disease. The signal of the Cu2+ from MOFs could be directly detected without acid dissolution and preconcentration, which would greatly simplify the detection steps and reduce the detection time. MOFs contain large amounts of Cu2+ ions, providing high electrochemical signals. In the presence of CRP, the current is proportional to the amount of analyte. Platinum nanoparticle modified covalent organic frameworks (Pt-COFs) with high electronic conductivity were employed as the substrate, which is the first time demonstrating the use of Pt-COFs for electrochemical immunoassay. Under the optimized experimental conditions, the proposed sensing strategy provides a linear dynamic ranging from 1 ng/mL to 400 ng/mL. A detection limit of 0.2 ng/mL was obtained, indicating an improved analytical performance. With these merits, this stable, simple, sensitive and low-cost electrochemical immunoassay shows promise for applications in clinical diagnosis. ACS Paragon Plus Environment


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ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (Grants 21465026, 21165023, J1210040, 20975034, 21177036, 21275044, 21605130), the National Key Scientific Program of China (2011CB911000, 01100205020503104).

Supporting Information Detail about the following are available: Chromogenic experiments used to confirm no free Cu2+ on the cleaned MOFs (Figure S1); typical DPV signal of

cleaned MOFs(Figure S2); chromogenic

experiment,transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and used to confirm the ion exchange reaction (Figure S3, S4, S5); typical DPV signal for MOFs before (a) and after (b) ion exchange reaction of Cd2+ (Figure S6); the SEM of Ab2-modified MOFs before and after negative voltage scanning (Figure S7 and S8); Typical DPV signal for cadmium (Cd2+) based MOFs (Cd-MOF-74) and zinc (Zn2+) based MOFs (ZIF-8) (Figure S9 and S10);

13

C CP/MAS NMR

spectra of COFs(Figure S11); the energy spectrum of Pt-COFs nanomaterial and Au-MOFs nanomaterial(Figure S12 and S13); the cyclic voltammograms of GC electrode and Pt-COFs-modified electrode in 10 mM PBS (pH 7.4) containing 0.1 M KCl and 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (Figure S14).

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