SWNTs nanocomposite with prominent peroxidase

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POMOFs/SWNTs nanocomposite with prominent peroxidasemimicking activity for L-cysteine ‘on-off switch’ colorimetric biosensing Xiao Li, Xi-Ya Yang, Jing-Quan Sha, Tao Han, Chun-Jiang Du, Yuan-Jie Sun, and Ya-Qian Lan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00872 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Materials & Interfaces

POMOFs/SWNTs Nanocomposite with Prominent Peroxidase-Mimicking Activity for L-cysteine ‘On-Off Switch’ Colorimetric Biosensing

Xiao Li,† Xi-Ya Yang,† Jing-Quan Sha,*,† Tao Han,† Chun-Jiang Du,† Yuan-Jie Sun,† and Ya-Qian Lan*,‡ †

The Talent Culturing Plan for Leading Disciplines of Shandong, Department of

Chemistry and Chemical Engineering, Jining University, Shandong 273155, China. ‡

Key Laboratory of Biofunctional Materials of Jiangsu, School of Chemistry and

Materials Science, Nanjing Normal University, Nanjing 210023, China.

ABSTRACT: In order to explore novel colorimetric biosensors with high sensibility and

selectivity,

two

new

Keggin

polyoxometalates

(POMs)-based

Cu-trz

(1,2,4-triazole) metal-organic frameworks (MOFs) with suitable specific surface areas and multiple active sites were favorably fabricated, then single-walled carbon nanotubes (SWNTs) were merged with new POMOFs to construct POMOFs/SWNTs nanocomposites. Herein, POMOFs/SWNTs nanocomposites as peroxidase mimics were explored for the first time, and the peroxidase-mimicking activity of the prepared POMOFs/SWNTs nanocomposites is heavily dependent on the mass ratio of POMOFs and SWNTs, in which the maximum activity is achieved at the mass ratio of 2.5 : 1 (named as PMNT-2). More importantly, PMNT-2 exhibits the lowest limit of detection (0.103 μM) among all reported materials to date and the assumable selectivity towards L-cysteine (L-Cys) detection. With these findings, a convenient, sensitive and effective ‘on-off switch’ colorimetric platform for L-Cys detection has been successfully developed, providing a promising prospect in the biosensors and clinical diagnosis fields. KEYWORDS: Polyoxometalate; Metal-organic framework; SWNTs; Colorimetric biosensing; L-cysteine

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INTRODUCTION L-cysteine (L-Cys), as an semi-essential amino acid in biological fluids, plays an important role in living systems,1,2 and the disordered L-Cys levels can lead to retarded growth of children, hair decolorization, liver and skins damage, and even cancers.3-6 Therefore, its detection and quantification is of vital important in health examination and medical diagnosis.7 Thereafter diverse technical methods including fluorescence spectroscopy,8 electrochemical technique,9 HPLC,10 MS and GC11 have been explored. However, due to their complex procedures and time-wasting, accompany with need of skilled personnel and complicated instruments,12 these aforementioned methods were hindered in practical applications. In contrast, the colorimetric biosensing approach can break through the abovementioned limitations to some extent,13,14 because of its easy operation, cheapness, satisfactory sensitivity and selectivity, and so on. Recently, the peroxidase-mimicking catalytic reaction system has been employed as a typical colorimetric biosensor towards L-Cys detection, showing a gratifying prospect in bio-technology.15-17 Polyoxometalates (POMs) are a class of excellent inorganic nanoclusters with tunable acid/base and excellent redox properties,18-22 which recently have attracted research interests in colorimetric biosensing. For instance, PW12O40,23 SiW12O40,24 and tetranuclear zirconium-substituted POMs25 showed excellent peroxidase-mimicking activities, namely, POMs can act as catalysts oxidate 3,3’,5,5’-tetramethylbenzidine (TMB) in the presence of H2O2 through multi-step electron transfer processes to generate a colored substance. Nevertheless, the relatively small surface areas (< 10 m2.g-1), poor recovery issues and environmental contamination hindered their application.26,27 Simultaneously, metal-organic frameworks (MOFs) have become a very interesting study focus owing to their big specific surface areas, regulatable pore shapes and broad applications in catalysis, sensors, drug delivery, sorption, separation, etc.28-31 Recently, MOFs as colorimetric biosensors have been also explored,32-35 such as, porphyrinic Fe(III) centralized PCN-222,36 MIL-53 and MIL-68 based on Fe(III),37,38 and MOF-808 containing Zr−OH(OH2) groups.39 Thus, the high-performance colorimetric biosensor might be gained if we can combine the


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advantages and overcome the shortcomings of POMs and MOFs synchronously, i.e. POM-based

MOFs

(POMOFs).

To

date,

some

POMOFs

including

Cu6(Trz)10(H2O)4[H2SiW12O40] and Na4H2[Cu4(im)14][Cu3(H2O)3(BiW9O33)2] have been used as colorimetric biosensors,

40,41

and the synergistic effect among the

components does improve the peroxidase-mimicking activities. It is a pity that the poor conductivity of POMOFs results in relatively lower peroxidase-like activity and higher limit of detection (LOD). Single-walled carbon nanotubes (SWNTs), as a new allotropic form of carbon have been considered to be extremely useful leading candidates to prepare an unprecedented colorimetric biosensor based on POMOFs and SWNTs,42-46 because organic molecules in MOFs may enhance the affinity of POMs and substrate SWNTs by the non-covalent interactions including π-π stacking or H-bonding, thus enhance their peroxidase-mimicking activity. Especially, SWNTs have been traditionally found to be excellent colorimetric biosensors, which guides the promising prospect for the design of SWNTs-based biosensor.47,48 Herein, we present the fabrication of colorimetric biosensor by integrating POMs, MOFs and SWNTs for the first time. In the work, 1,2,4-triazole (trz) was selected as organic molecule because of its vital role in affinity enhancement and charge transfer, thus two new [PW12O40]3- based Cu-trz MOFs were successfully fabricated through pH transformation, formulated as [Cu18(trz)12Cl3(H2O)2][PW12O40] (Cu18PW12)

and

[CuI2CuII4(trz)8Cl2][PW12O40]

(Cu6PW12).

Then

the

POMOFs/SWNTs nanocomposites (PMNTs) were prepared by employing a simple sonication-driven

periodic

functionalization

strategy.

The

results

of

the

peroxidase-mimicking catalytic activity evaluation of PMNTs indicating that the integration of SWNTs and POMOFs enhanced greatly the peroxidase-mimicking activity due to the short interactions between POMOFs and SWNTs and the higher affinity of PMNTs with substrates. Most importantly, a simple, highly sensitive and selective ‘on-off switch’ colorimetric approach for L-Cys detection has been developed. Note that PMNT nanocomposite exhibits the best performance among all reported materials as L-Cys colorimetric biosensors to date (Scheme 1).



Scheme 1. Schematic illustration of the POMOF NC/SWNTs nanocomposite (PMNT) and its peroxidase-mimicking activity for L-Cys ‘on-off switch’ colorimetric biosensing.

EXPERIMENTAL SECTION Materials and general methods All reagents purchased commercially were used without purification, and deionized water was used in the process of experiments. And the methods of material characterization were provided in supporting information. Syntheses of POMOFs and Nano-POMOFs The detailed synthesis experiment of materials are provided in supporting information. Fabrication of Cu18PW12NC/SWNTs 20 mg purified SWNTs were dispersed in 15 mL methanol under 60 kHz ultrasonication for 1 h. Then 40 mg Cu18PW12NC in 2 mL acetonitrile was transferred to above SWNTs methanol solution, and the resulting mixture was sonicated for another 1 h. Then the composite was isolated by centrifugation and rinsed using dry methanol and dried at 40 °C overnight. Three experiments were carried out under the different mass ratios of Cu18PW12NC : SWNTs (2 : 1, 2.5 : 1 and 3 : 1), and corresponding three nanocomposites were obtained and abbreviated as PMNT-1, PMNT-2 and PMNT-3, respectively. Fabrication of Cu18PW12NC/SWNTs(M)


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Cu18PW12NC and SWNTs were directly mixed in the different mass ratios (2 : 1, 2.5 : 1 and 3 : 1), and other three nanocomposites were obtained and abbreviated as PMNT(M)-1, PMNT(M)-2 and PMNT(M)-3, respectively. Peroxidase-like activity evaluation of as-prepared samples Typically, TMB (150 μM), H2O2 (100 μM) and catalyst (0.6 mg/mL) were added into the acetate buffer solution (pH 3.5) in turn, and the resulting mixture was stirred at 40 °C, then filtered using a syringe filter immediately. The UV absorbance of the filtrates at 661 nm was measured to evaluate the oxTMB content. In addition, the influence of pH (2.5 to 6.0) and temperature (25 to 55 ℃) on the activity were also investigated in a similar assay. Steady-state kinetics assay The catalytic activity of PMNT-2 was evaluated using the Michaelis– Menten model,12,23 and the apparent steady-state kinetic parameters were determined under the optimal condition by changing the concentrations of TMB (0.04 mM; 0.08 mM; 0.12 mM) with the constant concentration of H2O2 (0.1 mM). Conversely, changing the concentrations of H2O2 (0.03 mM; 0.06 mM; 0.1 mM) with the constant concentration of TMB (0.04 mM). H2O2 and L-Cys detection assay For H2O2 detection, the standard curve of H2O2 was established at 40 ℃ for 5 min. The colorimetric detection assay of L-Cys was also carried out, more specifically, TMB (150 μM), H2O2 (100 μM), catalyst (0.6 mg/mL) and a certain amount of L-Cys were added into the acetate buffer solution (pH 3.5) in turn, then the mixture solutions were shaken and equilibrated for 60 s and filtered using a syringe filter immediately. The oxTMB content was tested through UV spectra at 661 nm. Selectivity evaluation in L-Cys detection Several potential interfering solutions coexisting in real samples, including glutamic acid (Glu), alanine (Ala), lysine (Lys), glycine (Gly), tyrosine (Tyr), Na+, Ca2+, K+, Mg2+, Cl-, and NO3- were prepared (200 μM), respectively. For L-Cys determination in real samples, the urine and serum samples were treated by centrifugation (12000 rpm, 30 min) firstly to obtain the supernatants for the next-step



measurement. RESULTS AND DISCUSSION Structural description of Cu18PW12 and Cu6PW12 Cu18PW12 and Cu6PW12 samples were synthesized under the similar conditions except for pH value. Structural analysis reveals that Cu18PW12 crystallizes in monoclinic C2/c space group and displays a glamorous 2D-in-3D organic-inorganic hybrid framework, while Cu6PW12 crystallizes in triclinic P-1 space group and exhibits a POM-pillared 3D framework. Cu18PW12 consists of a half [PW12O40]3(hereafter PW12), six trz ligands, ten Cu ions, two Cl anions and one lattice water molecule (Figure S2a). Cu6PW12 is composed of one PW12 polyoxoanion, eight trz ligands, six Cu ions and two Cl ions (Figure S4a). Bond-valence sum (BVS) calculations49 (Tables S2 and S3) suggest that all W atoms are in the full oxidation states in both compounds, and all Cu atoms are in the +I in Cu18PW12, while one Cu atom is in +I in Cu6PW12. The coordination states of Cu ions and PW12 are illustrated in Figures S2 and S4. Moreover, each trz ligand adopts fully μ1,2,4-coordination mode in Cu18PW12 (Figure S2d), while trz ligands in Cu6PW12 adopt μ1,2,4- and μ1,2-coordinated modes (Figure S4c). The bond lengths range from 2.403 to 2.731 Å for Cu-O and 1.842 to 2.012 Å for Cu-N in Cu18PW12, and 2.382-2.604 Å for Cu−O and 1.873-2.032 Å for Cu−N in Cu6PW12. On one hand, two types of [Cu4(trz)4Cl] subunits with “pyrazole-like” bridging mode exists in Cu18PW12 (denoted as subunit A and subunit B in Figure S3a). Adjacent subunits A are fixed by Cu7 and Cu9 ions to give metal-organic layer A, meanwhile adjacent subunits B are connected by Cu6 ions to form metal-organic layer B (Figures 1 and S3b). On the other hand, PW12 polyoxoanions are fused together through Cu9 and Cu6 ions, giving birth to the inorganic layer (Figure S3c). And adjacent inorganic layers are further connected by Cu6 ions, generating the 3D inorganic framework comprises 1D channels along the crystallographic a-, b- and c-axis (Figure 1). As a consequence, the metal-organic layers are inserted into 3D inorganic frameworks in an A-B-A-A-B-A parallel staggering mode via Cu-O bonds, leading to the formation of a well closed 2D-in-3D hybrid framework (Figure 1 and


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S3d). Compared with Cu18PW12, two types of Cu-trz motifs can be also extracted in Cu6PW12 (denoted by subunit* A and subunit* B). Then subunits* are linked by sharing Cu2 ions, resembling an infinite 1D chain (Figure S5a). The adjacent chains interconnect with Cu1 ions to form a 2D layer (Figure 1). As we can see, these layers are further stacked parallel through the interactions between PW12 polyoxoanions and Cu centers to fabricate a 3D POM-pillared layer framework (Figure 1). It is noticeable that two kinds of channels intersect with each other in Cu6PW12 with the dimension of ca. 13.486 × 6.500 Å2 and ca. 11.245 × 7.975 Å2 along a- and b-axis, respectively (Figures S5b and S5c). Applying topological analysis, Cu18PW12 can be reduced to a (2,4,8)-connected net with the Schläfli symbol of (44·62)(46)2(412·612·84)(88), in which Cu7 acts as two-connected nodes, subunit-A, subunit-B, Cu6 and Cu9 as four-connected nodes, and the PW12 polyoxoanions as eight-connected nodes (Figure S3e). The framework of Cu6PW12 can be reduced to a (2,3,4,6)-connected net with the Schlafli symbol of (3·8·9)2(32·102·112) (32·84·94·102·112·12)(84·10·12)(8)2, in which Cu1 acts as two-connected nodes, Cu2 as three-connected nodes, PW12 polyoxoanions and subunit* A as four-connected nodes, and subunit* B as six-connected nodes (Figure S5d).

Figure 1. Combined ball/stick and polyhedral structures of compounds Cu18PW12 and Cu6PW12.

PXRD, FT-IR, Raman, SEM, TEM and EDX analyses The structures of as-prepared Cu18PW12, Cu6PW12, Cu18PW12NC, Cu6PW12NC,



PMNT-1, PMNT-2 and PMNT-3 are characterized by powder X-ray diffraction patterns (PXRD) (Figures 2a and S6). Cu18PW12NC, PMNT-1, PMNT-2 and PMNT-3 exhibit similar diffraction patterns with Cu18PW12, confirming the good phase purities and crystallinities. Note that the peaks of SWNTs are invisible, maybe due to the small amount of SWNTs. Their FT-IR spectra are shown in Figures S7 and S8. The characteristic bands at ca. 1060, 962 and 800 cm-1 are ascribed to the P-O, Wt-O, Wb-O-Wb stretching vibrations. In PMNT-1, PMNT-2 and PMNT-3, the peaks of Cu18PW12NC and SWNTs show slight movements, indicating the effective hydrogen bonds and π–π interactions between Cu18PW12NC and SWNTs (Scheme S1). Moreover, the Raman spectra of PMNT-1, PMNT-2 and PMNT-3 (λex = 633 nm) present characteristic bands at 1326 (D band), 1586 (G band) and 2628 cm-1 (D* band), confirming the presence of SWNTs (Figure 2b). Scanning electron microscopy (SEM) images of Cu18PW12NC and Cu6PW12NC show that both possess uniform shapes and average sizes at ca. 100 nm (Figures S9 and S10). SEM images of PMNT-1, PMNT-2 and PMNT-3 are provided and the SEM image of pristine SWNTs is also provided as control (Figures 2c and S9). It can be seen that the SWNTs bundles loosened upon treatment with Cu18PW12NC, indicating the improved dispersion of SWNTs in the nanocomposite, meanwhile the intrinsic structure of Cu18PW12NC is well retained. Transmission electron microscopy (TEM) images of SWNTs, PMNT-1, PMNT-2, and PMNT-3 were provided (Figures 2d and S11), and the results indicate that a large number of SWNTs are homogeneously located around Cu18PW12NC, and the content of SWNTs are decreased with the increasing mass ratios of Cu18PW12NC : SWNTs from 2 : 1 to 3 : 1. The uniformity of PMNT-2 is also verified by the elemental mapping, and the equidistribution of C, Cu, N, O, P, W and Cl (Figure 2e) are consistent with EDX spectrometry (Figure S12).


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Figure 2. Characterization of the as-prepared samples: PXRD patterns (a) and Raman spectra (b) of the as-prepared samples; SEM (c) and TEM images (d) of PMNT-2; (e) EDX elemental mapping of C, Cu, N, O, P, W and Cl in PMNT-2.

Peroxidase-like catalytic activity and optimal reaction conditions A typical method to the catalytic oxidation of TMB is adopted to explore the peroxidase-mimicking activities of as-synthesized samples. As shown in Figure 3a, TMB can be oxidized to produce a green color (λ = 661 nm) under the existence of PMNT-1 as catalyst, demonstrating the peroxidase-mimicking property of PMNT-1. However, solution color does not change in the absence of PMNT-1, or H2O2, or TMB. The results indicate PMNT-1 has the high peroxidase-mimicking activities when H2O2 serves as oxidant. In addition, the peroxidase-mimicking properties of Cu18PW12NC and SWNTs were also conducted and exhibited lower catalytic performances (Figures 3b, S13 and S14), indicating that the synergetic effect between Cu18PW12NC and SWNTs can improve the peroxidase-mimicking performance of nanocomposites.

More

specifically,

the

non-covalent

interactions

between

Cu18PW12NC and SWNTs including H-bonding or π−π stacking can generate a smart



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microenvironment for the access of H2O2 and TMB to drive the movement of TMB and H2O2 toward PMNT-1. Moreover, the loose packing of SWNTs provides a rational pathway for H2O2 moving toward active centers. As a consequence, the stronger affinity of PMNTs with H2O2 and TMB can be guaranteed and is beneficial in enhancing the peroxidase-mimicking activity. It should also be mentioned that the introduction of SWNTs may improve the biocompatibility of PMNT-1 for bio-applications. Considering that Cu6PW12NC shows poor peroxidase-mimicking activity than Cu18PW12NC (Figures 3b and S13), Cu18PW12NC was selected to construct POMOFs/SWNTs nanocomposites for subsequent activity analysis. Similar to most reported peroxidase mimics,39,50 the influences of pH, temperature and catalyst dosage on peroxidase-like catalytic activities of PMNT-1 were also explored (Figures 3c and S15). The response curves show the highest peroxidase-mimicking activities at pH 3.5, 45 ℃

and 0.6 mg/mL catalyst.

Considering that PMNT-1 also possesses excellent catalytic activity at 40 ℃ , and higher temperature can cause the fast decomposition of H2O2, so 40 ℃ was adopted as

the

first-rank

reaction

temperature

to

conduct

the

subsequent

peroxidase-mimicking activity. In addition, we found that the peroxidase-mimicking activities of the PMNTs are heavily dependent on the mass ratios of Cu18PW12NC:SWNTs due to the synergistic effect with different levels. As shown in Figures 3d and S16, PMNT-2 (Cu18PW12NC:SWNTs = 2.5:1) exhibits the highest peroxidase-mimicking catalytic activity among three PMNTs. Therefore PMNT-2 was selected for the subsequent peroxidase-mimicking activity evaluation. To further illustrate the synergetic effect of Cu18PW12NC and SWNTs in PMNTs nanocomposites, PMNTs(M) (PMNT(M)-1, PMNT(M)-2, and PMNT(M)-3) were adopted as the contrasts to study the peroxidase-mimicking activity (Figure S17). The results show that their peroxidase-mimicking activities are considerably less than that of PMNTs nanocomposites, which further indicates that the synergetic effect between Cu18PW12NC and SWNTs is vital to the peroxidase-mimicking activity advancement.


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Figure 3. (a) UV-vis absorbance curves of the different reaction systems (pH 3.5 acetate buffer solution, 40 ℃, 5 min), the inset presents the optical photo of corresponding systems; (b) UV-vis absorbance curves and the pertinent photographs of absorption mode with different catalysts in 5 min (pH 3.5, 40 ℃ ); (c) Relationship about the peroxidase-mimicking activity of PMNT-1 against various pH values; (d) UV-vis absorption plots and the pertinent photographs of diverse systems containing PMNTs with different ratios of Cu18PW12NC: SWNTs in the presence of fixed TMB (150 μM) and H2O2 (100 μM).

Steady-state kinetics and fundamental catalytic mechanism assays of PMNT-2 For the purpose to study the peroxidase-mimicking catalytic efficiency of PMNT-2, a steady-state kinetic experiment was performed by changing one substrate concentration with the constant concentration of other substrate. As shown in Figures S18a and S18b, the data were fitted to the Michaelis-Menten equation, in which Michaelis constant Km represents the affinity of the enzyme towards the substrate and the value is equivalent to the substrate concentration when the reaction rate is half of Vmax.51 As we known that Km value is smaller, the affinity between the enzyme (PMNT-2) and the substrate (H2O2 and TMB) is stronger.52,53 Herein, the Km values of the PMNT-2 as catalyst towards H2O2 and TMB are 0.206 mM and 0.021 mM, respectively, which is lower than reported materials, and the Vmax value is higher than most reported materials



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(Table 1). The results demonstrate that PMNT-2 possesses higher affinity with TMB and H2O2 and superior catalytic efficiency than that of Cu18PW12NC and SWNTs (Figures S19 and S20), which further testify our speculation. Table 1. Km and Vmax of PMNT-2, Cu18PW12NC, SWNTs and reported enzyme mimics. Substrate

H2O2

Enzyme mimics HRP54 PW1223

Km (mM) 3.7 15.89

Vmax (10-8 M s-1) 8.71 4.24×10-4

MOF-80839

1.06

1.39

0.214

10.5

0.23

7.33

SWNTs

0.513

7.44

Cu18PW12NC

0.46

8.47

PMNT-2

0.206

10.37

HRP54 PW1223

0.434 0.11

10.00 43.1

MOF-80839

0.0796

3.12

0.033

10.8

0.03

5.25

SWNTs

0.045

4.47

Cu18PW12NC

0.038

5.36

PMNT-2

0.021

7.43

@GO55

FF@PW12

]41

Na4H2[Cu4(im)14][Cu3(H2O)3(BiW9O33)2

TMB

@GO55

FF@PW12

]41

Na4H2[Cu4(im)14][Cu3(H2O)3(BiW9O33)2

The double reciprocal plots of initial velocity against one substrate concentration were obtained over a range of the second substrate concentrations shown in Figures S18c and S18d. The almost parallel lines indicates the catalytic reaction follows the ping-pong mechanism.54,55 In addition, the nature of TMB catalytic reaction may originate from the formation of the hydroxyl radical (•OH). So the probing technique of the terephthalic acid (TA) photoluminescence was employed to determine •OH formation (Scheme S2). As expected, the fluorescence of TA was switched on upon the introduction of H2O2 and PMNT-2, and the fluorescence intensity of the PMNT-2 solution was above 10 times higher than that of the same solution except for no PMNT-2. The fact indicates that PMNT-2 indeed can activate H2O2 to produce •OH (Figure S21). As a comparison, the fluorescence intensities of the solutions containing Cu18PW12NC and SWNTs are lower than that of PMNT-2,


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suggesting that less •OH is generated, which is coincident to the peroxidase-mimicking activities of these three systems. Detection of H2O2 Given that the peroxidase-mimicking activities of PMNT-2 are greatly rely on H2O2 concentration, a convenient ‘turn on’ approach could be used for the quantitative evaluation of H2O2 (Scheme S3). The absorption changes at 661 nm by varying H2O2 concentrations were measured under the first-rank conditions (40 ℃ , pH 3.5) shown in Figure S22a. It is clear that the reaction rate rises with the H2O2 concentration increasing in the range of 1 to 1000 μM. The typical concentration-response curves are presented in Figures S22b and S22c, indicating a wide linear relationship of absorbance value vs H2O2 concentrations between 1-100 μM (R2 = 0.9943). Furthermore, LOD can be calculated as 0.092 μM (LOD = K.S0/S). The LOD is lower than that of MOF-808 (4.5 μM),39 MIL-68 (0.256 μM),38 and FF@PW12@GO (0.11 μM),55 implying the superiority for H2O2 colorimetric sensing. Synchronously, the color variation could be observed easily, which provides a facile and simple approach for H2O2 detection with the naked eye observation even under lower H2O2 concentration (Figure S22d). Detection and Selectivity assay of L-Cys As described in the introduction, L-Cys plays an important role in many physiological processes, so the exploration of more sensitive detecting and quantifying methods of L-Cys attaches great interest, although numerous detection approaches have been reported over the past decades.56-58 Because L-Cys can reduce oxTMB to TMB resulting in the fading of the green color, an efficient and simple ‘on-off switch’ colorimetric assay is established to detect L-Cys (Scheme S3). More specifically, the absorbance intensity reduces with L-Cys concentration increasing in the TMB oxidation system catalyzed by PMNT-2 (Figures 4a and 4c), suggesting that L-Cys can inhibit the production of oxTMB molecules. Moreover, the dose-response curves about the decreased absorption value (ΔA) vs L-Cys concentrations after the 5-min reaction were shown in Figure 4b, suggesting that the corresponding linear calibration plot ranges from 1 to 80 µM (R2 = 0.9963), and the LOD is about 0.103



μM, which is superior than LaMnO3+δ nanofibers (0.1098 μM) and FeCo-CNFs (0.15 μM).12,15 Note that the LOD value is the best among those reported materials as L-Cys biosensors to date (Table S4). Furthermore, to further confirm the reliability of our proposed colorimetric sensor, the detection of L-Cys in human urine and serum samples were also performed, and the results indicate that the recoveries of samples are 96.4%-102.9% for human urine samples and 98.9%-101.7% for human serum samples (Table S5), demonstrating that the developed approach for L-Cys determination is feasible and reliable.

Figure 4. (a) UV spectra of the oxidized TMB catalyzed by PMNT-2 with various concentrations of L-Cys as an inhibitor; (b) The dose-response curve and the linear calibration plot for L-Cys detection; (c) The pertinent photographs of the above systems; (d) The relative ΔA values and the corresponding photographs of PMNT-2–TMB–H2O2 sensing system in 5 min containing fixed TMB (150 μM), H2O2 (100 μM), catalyst solution (0.6 mg/mL) with L-Cys (100 μM) or other different interferential substances (200 μM).

Considering that other amino acids and ions in human body may cause interference during the L-Cys detection process, here, glutamic acid (Glu), alanine (Ala), lysine (Lys), glycine (Gly), tyrosine (Tyr), Na+, C a2+, K+, Mg2+, Cl-, and NO3ions were selected to assess their inhibitory effects. As shown in Figure 4d, the color and the absorption intensity at 661 nm almost remain unchanged when these interferential substances as inhibitors exist in the reaction system. However the


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reaction system with L-Cys as inhibitor converted to colorless and an obvious absorption intensity quenching at 661 nm could be observed. This result reveals that PMNT-2 possesses excellent selectivity towards L-Cys detection, which is comparable

with LaMnO3+δ nanofibers.12 Overall mechanism of L-Cys colorimetric biosensing on PMNT-2 Based on the theoretical and experiment results, we elaborate the L-Cys colorimetric biosensing processes on PMNT-2 and the possible mechanism is summarized in Figure 5. As proposed in the mechanism, during the colorimetric biosensing process, H2O2 was adsorbed onto the surface of PMNT-2 firstly, and PW12 polyanions donate electrons initially and transfer them to the metal centers through trz ligands and SWNTs. Then adsorbed H2O2 was activated by metal centers through the breaking up of O-O bond coupling with the proton-electron transfer, producing double active •OH stabilized by PMNT-2.59 Subsequently, TMB molecules were adsorbed on the surface of PMNT-2, donating lone-pair electrons of the amino groups and reacting with desorbed •OH through electron transfer to complete the catalytic oxidation of TMB and the corresponding color change from colorless to green, which is similar to other reported results. When added L-Cys into the abovementioned system as an inhibitor, oxTMB convert to TMB and L-Cys convert to L-Cystine with disulfide bond simultaneously, leading to the fading of green color.



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Figure 5. Proposed mechanistic scheme for the L-Cys colorimetric biosensing on PMNT-2 (POMs serve as electron-donating groups, trz ligands and SWNTs as electron transfering groups and Cu ions as electron collecting groups).

Stability and reproducibility evaluation of PMNT-2 The PXRD patterns and IR spectra of Cu18PW12NC and PMNT-2 have exactly similar peak position before and after peroxidase-mimicking activity evaluation (Figures S23 and S24), which confirm the ultrahigh stability of Cu18PW12NC and PMNT-2. Moreover, the recycled experiment of PMNT-2 was performed (Figure S25), and the result shows that PMNT-2 exhibits no significant activity decrease after five

runs

under

same

conditions,

which

manifests

that

PMNT-2

as

peroxidase-mimicking catalyst possesses excellent reproducibility. Furthermore, we tested the usage stability of PMNT-2 as peroxidase mimics. It is found that PMNT-2 still retains above 95% initial activity after immerse in aqueous solution for 10 days, demonstrating its eminent stability, which is much better than natural enzymes reported previously (Figure S26). CONCLUSIONS In summary, owing to the effective composition, appropriate surface areas, rich active sites, and synergetic effect of components, PMNT-2 nanocomposite


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(Cu18PW12NC : SWNTs = 2.5 : 1) as a novel enzyme mimics exhibits superior peroxidase-mimicking catalytic activity. Moreover, a convenient and effective ‘on-off switch’ colorimetric platform based on PMNT-2 has been successfully developed for L-Cys detection with the lowest LOD among all reported peroxidase mimics, and the crucial selectivity can also be assured under the interferences of other common amino acids and ions. This work indeed provides an effective, simple, and low-cost strategy for the fabrication of high-performance nanomaterial as peroxidase mimics, and possesses the promising potential applications to extend the diversity of the peroxidase mimics family in the biosensors and clinical diagnosis fields. ASSOCIATED CONTENT Supporting Information Crystallographic

data

and

CCDC

can

be

obtained

via

www.ccdc.cam.ac.uk/data_request/cif. Crystal data and structural information of Cu18PW12 and Cu6PW12 can be obtained in supporting information. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS Financial support from the Natural Science Foundation of China (No. 21271089), the Talent Culturing Plan for Leading Disciplines and University Scientific Research Project (J17KA118) and the Natural Science Foundation (ZR2017LB001) of Shandong Province. REFERENCES (1) Wang, W. H.; Rusin, O.; Xu, X. Y.; Kim, K. K.; Escobedo, J. O.; Fakayode, S. O.; Fletcher, K. A.; Lowry, M.; Schowalter, C. M.; Lawrence, C. M.; Fronczek, F. R.; Warner, I. M.; Strongin, R. M. Detection of Homocysteine and Cysteine. J. Am. Chem. Soc. 2005, 127, 15949–15958.



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SWNTs nanocomposite with prominent peroxidase

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