Zirconium-Based Porphyrinic MetalâOrganic Framework (PCN-222

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A Zirconium Based Porphyrinic Metal-organic Framework (PCN-222): Enhanced Photoelectrochemical Response and Its Application for Label-free Phosphoprotein Detection Guang-Yao Zhang, Yu-Hong Zhuang, Dan Shan, Guo-Fang Su, Serge Cosnier, and Xueji Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03484 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 21, 2016

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Analytical Chemistry

A Zirconium Based Porphyrinic Metal-organic Framework (PCN-222): Enhanced Photoelectrochemical Response and Its Application for Label-free Phosphoprotein Detection

Guang-Yao Zhang,† Yu-Hong Zhuang,‡ Dan Shan,*† Guo-Fang Su,‡ Serge Cosnier,§ Xue-Ji Zhang,*† †

Sino-French Laboratory of Biomaterials and Bioanalytical Chemistry, School of

Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China ‡Department of gynecology and obstetrics, Zhongda Hospotal, Southeast University, Nanjing 210009, China §University

of Grenoble Alpes-CNRS, DCM UMR 5250, F-38000 Grenoble, France

*Corresponding author: Email: [email protected] (D. Shan) [email protected] (X.J. Zhang) Fax: 0086-25-84303107

1

ACS Paragon Plus Environment


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ABSTRACT: A simple and rapid photoelectrochemical (PEC) sensor is developed for the label-free detection of a phosphoprotein (α-casein) based on a zirconium based porphyrinic metal−organic framework (MOF), PCN-222, which exhibits an enhanced photocurrent response towards dopamine under the O2-saturated aqueous media. In this work, in terms of PEC measurements and cyclic voltammetry, the PEC behaviors of PCN-222 in aqueous media were thoroughly investigated for the first time. Additionally, in the virtue of the steric hindrance effect from the coordination of the phosphate groups and inorganic Zr–O clusters as binding sites in PCN-222, this biosensor shows high sensitivity for detecting α-casein and the limit of detection (LOD) is estimated to be 0.13 µg mL-1. Moreover, the proposed method provides a promising platform for clinic diagnostic and therapeutics.

KEYWORDS: metal−organic framework, photoelectrochemical sensor, porphyrin, dopamine, phosphoprotein

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INTRODUCTION Protein phosphorylation plays vital roles in the signal transduction, metabolic modulation, cell growth and other biological functions.1,2 The abnormal expression of phosphoprotein is highly relevant to a series of diseases including obesity,3 cancers and Alzheimer’s diseases.4-6 Hence, the detection of phosphoproteins is greatly significant and necessary in the fields of medicinal and bioanalytical chemistry. At present, the most commonly used methods for the detection of phosphorylated proteins and peptides include mass spectrometry,7 fluorescent labeling,8 radiolabeled methods,9 and immunoprecipitation.10 However, these methods mainly focus on the enrichment and sequence identification of phosphopeptides, and require expensive equipments and complicated signal probe labeling procedures. In contrast, due to the advantages of simple instrumentation, low cost, rapid analysis, and high sensitivity, photoelectrochemical (PEC) detection has developed into a quite promising analytical technique based on the generation of photoelectric signal from the electron transfer among analyte, photoactive species and electrode with photoirradiation.11 When the photoactive species are excited by light and generate electron–hole pairs, charge recombination may occurred, which obviously restrain the photoelectric conversion efficiency.12,13 But, the presence of electron donors, whose energy level is suitable, can enhance photocurrent via the scavenging holes.14,15 Dopamine (DA) is a small molecule neurotransmitter that can behave as an electron donor to photoactive species. It is easy to be oxidized when adsorbed to an electron acceptor.16-18 In this work, in order to realize the efficient and rapid detection for phosphoprotein, for the first time, a novel PEC sensor is developed for the label-free detection of a phosphoprotein (α-casein) based on a zirconium based porphyrinic metal-organic framework (MOF), PCN-222, which may exhibit excellent 3



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photocurrent response towards DA under the O2-saturated aqueous media. As a dye sensitizer, porphyrins received much attention because of their widespread occurrence in nature, strong optical absorption and emission, and electrochemical properties applicable for electron transfer reactions related to light harvesting,19,20 applied in photoelectric devices to improve photoelectric conversion efficiency.21-23 And, the generation of cathodic and anodic photocurrent via electrodes modified with porphyrin compounds have been widely studied.24-28 In particular, porphyrinic MOFs, constructed from porphyrinic or metalloporphyrinic ligands, have attracted continuous research interest based on the remarkable features of porphyrin molecules, such as enzymatic, biochemical, and photochemical functions.29 As a direct result, the chemical stability of a porphyrin compound may be enhanced when it is incorporated into the framework of a MOF. This can bring the following advantages: (1) In comparison with porphyrin ligand, the HOMO–LUMO energy gap of porphyrinic MOFs will become smaller, which is beneficial for a photoinduced electron transfer (PET) process;30 (2) the extended conjugation in the framework can prolong the electron–hole recombination time within the framework, which can enhance the photoelectric conversion efficiency. Additionally, thanks to high specific surface areas, well-ordered porous structures and structural tenability,31,32 especially as an electrocatalyst in oxygen reduction reaction (ORR),33,34 MOF can play a decisive role as an oxygen container and facilitate some interactions between the surface of the pores and the oxygen molecules simultaneously.35,36 Therefore, MOFs provide a platform to prepare new heterogeneous catalysis with highly accessible external and internal surface and evenly distributed active sites.37 On the other hand, some Zr-based materials including Zr-based MOFs have been implemented in the fields of the selective enrichment of phosphopeptides and the 4


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assembled immobilization of DNA probe with the aid of the coordination binding zirconium to phosphate groups of biomolecules.38-41 In this work, the tetrakis(4-carboxyphenyl)porphyrin (TCPP) ligand involving PCN-222 is able to act as a visible-light-harvesting unit, and the high porosity and tunable structures might facilitate the enrichment of oxygen and DA molecules around the PEC-active TCPP ligands, thereby enhancing the photoelectric conversion efficiency. Furthermore, as shown in Scheme 1, based on the steric hindrance effect from the coordination of the phosphate groups and inorganic Zr–O clusters as binding sites in PCN-222, the PCN-222 modified electrode could be applied for the label-free detection of phosphoproteins. It is envisioned that the PCN-222 and other similar porphyrinic MOFs would have potential applications in ultrasensitive chemical sensing and even in accurately clinic diagnostic.

EXPERIMENTAL SECTION Materials

and

Zirconium(IV)

Reagents.

chloride,

benzoic

acid,

N,

N-dimethylformamide (DMF), cysteine (Cys), ascorbic acid (AA), dopamine (DA),

α-casein,

β-galactosidase

albumin,

and

N-(2-Hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid), sodium salt (HEPES) was purchase

from

Sigma-Aldrich

Chemical

Co.,

Ltd.

(Shanghai,

China).

Meso-tetra(4-carboxyphenyl)porphyrin (TCPP) was ordered from J&K Scientific Ltd. (Shanghai, China). All other reagents were of analytical grade and were used without further purification. Ultrapure water obtained from a Millipore water purification system (≥18 MΩ, Milli−Q, Millipore) was used as the water source throughout the work. 10 mM pH 7.4 HEPES containing 0.3 M KCl solution was employed as aqueous

electrolyte

solution

for

photoelectrochemical 5


and

electrochemical


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measurements. Instrumentation. The morphology was investigated with a FEI Quanta 250F field emission scanning electron microscope (FESEM) (Houston, USA). UV−vis absorption spectra were obtained using a UV-3600 UV−vis−NIR spectrophotometer (Shimadzu Co. Kyoto, Japan). Photoluminescence (PL) spectra were recorded at room temperature in a quartz cuvette on a FLSP920 fluorescence system. Powder X-ray diffraction patterns (PXRD) were recorded on a Bruker D8-Focus Bragg−Brentano X-ray Powder diffractometer equipped with a Cu sealed tube (λ=1.54178 Å) at room temperature.

N2

adsorption-desorption

isotherms

were

measured

using

a

Micrometritics ASAP 2020 system at 77 K. A CHI 660D electrochemical workstation (CHI Co., USA) was used for cyclic voltammetry (CV) measurements. Electrochemical impedance spectroscopy (EIS) measurements were carried out using an Autolab PGSTAT30 (Eco chemie, The Netherlands) controlled by NOVA 1.10 software. The EIS measurements were performed in 0.1 M KCl containing 5 mM [Fe(CN)6]3-/4-. The amplitude of the applied sine wave potential was 5 mV. The impedance measurements were recorded at a bias potential of 290 mV within the frequency range of 0.1 Hz to 10 kHz. The photoelectrochemical measurements were performed on a Zahner workstation (Zahner, German) with a LW405 light (10 mW cm-2) as the accessory light source. All electrochemical and photoelectrochemical studies were performed with a conventional three electrode system. A modified indium tin oxide (ITO) electrode (1×0.5 cm2) was used as working electrodes. A Ag/AgCl (sat. KCl) electrode and a Pt wire electrode were used as reference and counter electrodes, respectively. All the photocurrent responses were collected at open circuit potentials. Preparation of PCN-222. The PCN-222 was synthesized based on the previous 6


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report with slight modifications.42 Typically, ZrCl4 (75 mg), TCPP (30 mg), and benzoic acid (1750 mg) in 10 mL of DMF were ultrasonically dissolved in a 20 mL pyrex vial. The mixture was heated to 120 ºC in an oven for 48 h. After cooling down to room temperature, purple needle shaped crystals were harvested by centrifugation. Preparation of ITO/PCN-222. Before modification, the ITO glass plate was washed with acetone, ethanol, and water in ultrasonic bath sequentially. Then ITO glass plate was immersed in a solution of 1:1 (v/v) ethanol/NaOH (1 M) for 15 min to active the surface. After rinsed with pure water and dried under N2 flow, 20 µL of 1 mg mL-1 PCN-222 power was spread on the surface of ITO glass plate, and dried at ambient temperature, obtained ITO/PCN-222 modified electrode. In addition, for comparison, ITO/TCPP was prepared following the same procedure. Label-free Detection of Phosphoprotein. The ITO/PCN-222 modified electrode was soaked in HEPES buffer solution (10 mM, pH 7.4) containing different concentration of α-casein for 1 h at room temperature. After rinsed with pure water, the ITO/PCN-222/α-casein modified electrode was obtained. Finally, the assembled electrode would generate PEC signal in the O2-saturated aqueous solution containing dopamine and gave the quantitative criteria for the proposed phosphoprotein sensor.

RESULTS AND DISCUSSION Characterization of the PCN-222. The morphology of the as-synthesized PCN-222 was characterized by scanning electron microscopy (SEM). As shown in Figure 1A, the morphology of the PCN-222 is uniform and exists as rod-shaped single crystal with 2.4 µm in length. The powder X-ray diffraction (PXRD) pattern of the PCN-222 powder was shown in Figure 1B. The main diffraction peaks at 2.4º, 4.8º, 7.1º, and 9.8º can be observed in the pattern of the PCN-222 powder, which match 7



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with the simulated XRD pattern of PCN-222 reported previously.42 The porosity of PCN-222 were also measured by N2 adsorption–desorption experiments (Figure 1C), which revealed a typical type IV isotherm43 and a Brunauer-Emmett-Teller (BET) surface area of 2217 m2 g−1. The experimental total pore volume was as high as 1.202 cm3 g−1. Additionally, from inset of Figure 1C, two types of pores were estimated with sizes of 1.3 nm and 3.2 nm by the density functional theory (DFT) simulation, assigned to triangular microchannels and hexagonal mesochannels, respectively. Based on this, PCN-222, formulated as Zr6(µ3-OH)8(OH)8-(TCPP)2, exhibits a three-dimensional (3-D) network based on Zr–O clusters connected by TCPP ligand, shown in Scheme 1A.42 Such highly regular porous structure and high density of 3-D open channels enable this material to possess high gas storage capacity and are beneficial to the enrichment of reaction substrates. Likewise, the adsorption of oxygen molecules by depending on porous materials characteristically derived from much interactivity between the surface of the pores and oxygen molecules, and from the fact that they are in an unique state arising from an electronic effect and a confinement effect.35 On the basis of porphyrin and its derivatives as catalysts in some redox-active reactions involving molecular oxygen,44 porphyrinic MOFs are particularly appropriate for this challenge over other solid-state materials in virtue of the high surface and intrinsic open frameworks.45 The spectroscopic properties of PCN-222 were investigated via the UV-visible and PL emission spectra (Figure 1D). The maximum Soret band absorption of PCN-222 at 436 nm (curve b, solid) in ethanol was red-shifted in comparison with that of free TCPP at 416 nm (curve a, solid). The 20 nm red shift could be attributed to the hydrophobic property of the octahedral cavity and the sensitivity of the Soret band to the dielectric constant of the solvent.46 This result indicates PCN-222 possesses the 8


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smaller HOMO–LUMO gaps,30 which is beneficial for a PET process. The fluorescence spectrum of PCN-222 excited at 420 nm showed a strong emission peak at 652 nm and a lesser at 718 nm (curve b, dashed); this result is in agreement with the fluorescence spectrum of TCPP monomer (curve a, dashed), confirming that the spectroscopic properties of porphyrin and even the PEC properties,28 can also be reflected in PCN-222. PEC Investigations. Precisely since it is well known that Zr-based porphyrinic MOFs maintain the wide-pH-range stability in the aqueous solutions and the dye characteristic of involved porphyrin molecules,47 the PCN-222 is expected to be suitable for electrochemical or PEC applications in aqueous media. Recently, in our laboratory, on the basis of the excellent electrochemical activity of zinc porphyrin as electron media,48,49 zinc porphyrin based MOF (MOF-525) has been designed as a three-in-one platform possessing oxygen nanocage, electron media, and bonding site for electrochemiluminescence (ECL) biosensor for highly sensitive protein kinase activity assay.50 Hereon, the PEC properties of PCN-222 were investigated on ITO electrode in a 0.1 M HEPES, pH 7.4 buffer solution. The device of PEC measurements was displayed in Figure 1S. The effect of dissolved oxygen on PEC behaviors of ITO/PCN-222 was shown in Figure 2A. Specifically, a slight and insignificant photocurrent response of modified electrode was observed in the N2-saturated atmosphere (curve a). With the increase of dissolved oxygen concentration, the cathodic photocurrent gradually increased (curve b, c), demonstrating that oxygen molecules serve as electron acceptors in the porphyrin-based PEC system. This similar phenomenon was also observed at the ITO/TCPP modified electrode (Figure 2E, curve a, b). Under light irradiation, the TCPP in PCN-222 behaved as a visible-light-harvesting unit, and electrons 9



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transformed from the valence band (HOMO) to conduction band (LUMO) of the semiconductor-like PCN-222, and generated electron–hole pairs. Then electrons transformed from LUMO of PCN-222 to O2 to produce O2•−, realizing the electron circuit.51 It is worth noting that the presence of electron donors, whose energy level is suitable, can enhance photocurrent via the scavenging holes.14,15 Furthermore, several active molecules, cysteine, ascorbic acid, DA and H2O2 were chosen as electron donors for the PEC investigations. As seen in Figure 2B, in the O2-saturated environment, the order of the photocurrent response intensities is DA≫ascorbic acid>cysteine≈O2≫H2O2, demonstrating that DA is the most suitable candidate as electron donor, which greatly promotes the generation of photocurrent. In order to explain this phenomenon, cyclic voltammogram and photovoltage generation measurements were displayed in Figure 2C, D. The redox peaks of DA was significantly stronger than that of others (Figure 2C) and the oxide peak was at 0.42 V, which was just potential response excited by light (Figure 2D). This phenomenon just confirms this conclusion: DA can effectively be oxidized to provide more electrons to occupy on the LUMO of PCN-222, and to promote the generation of electron-hole pairs under the excitation of light, resulting in the enhanced photocurrent response. Interestingly, the electrochemical and PEC behaviors of PCN-222 towards H2O2 were just opposite to that of DA. As for the reason for this result, we conclude that H2O2 can promote the majorities of electrons back to the HOMO of PCN-222 and result in the radiative electron−hole recombination, decreasing the photoelectric conversion efficiency. In addition, we made a comparison of the photocurrent performance of PCN-222 and TCPP ligand. After starting the light, the electrolytic cell equipped with 10


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ITO/TCPP modified electrode was filled with fluorescent (Figure 1S), derived from the TCPP ligand on the surface of electrode gradually dissolved in the aqueous electrolytic solution, attributed to the hydrophilic carboxylic groups of TCPP. As a result, the continuous reduced photocurrent signal was obtained in the O2-saturated 0.1 M pH 7.4 HEPES buffer solution containing DA (inset of Figure 2E, curve c). And, under the same TCPP molar concentration, the photocurrent intensity of PCN-222 is around 5 times higher than that of TCPP by calculation. In comparison with this instability photocurrent response of TCPP ligand itself, PCN-222 possesses a stronger and more stable photoelectric conversion ability, confirming that the PEC activity can be greatly enhanced by assembling the TCPP ligand onto MOFs. It should be attributed to the enrichment of dissolved oxygen in the 3-D nanocage of PCN-222 frameworks. Of course, the importance of oxygen molecules as electron acceptors can not be ignored in this PEC system. As shown in Figure S2, despite the presence of DA, but the photocurrent signal was still weak under N2-saturated condition (Figure S2, curve c). Meanwhile, the variation of photocurrent for the ITO/PCN-222 modified electrode with incident wavelength was illustrated in Figure S3. The increase in photocurrent observed at shorter wavelengths fairly follows the visible absorption spectra of PCN-222 peaking at ca. 430 nm (see Figure 1D). On the basis of our above results, the mechanism of cathodic photocurrent generation on ITO/PCN-222 towards DA under the O2-saturated aqueous media was proposed and illustrated in Figure 2F. Specifically, under light irradiation, electron transfer takes place from PCN-222 (TCPP∗) (-0.4 V vs NHE),30 yielding the porphyrin radical cation in PCN-222, PCN-222 (TCPP•+), and generate electron–hole pairs, while make the enriched O2 in the 3-D nanocage to produce O2•−; DA as the electron donor whose energy level 11



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(-4.91 eV)17,18 lies between the LUMO and HOMO of PCN-222, can scavenge the holes, resulting in more electrons occupied on the LUMO of PCN-222. These procedures lead to more electrons transforming from LUMO to dissolved oxygen molecules and generate an enhanced cathodic photocurrent. The Interreaction Investigations between PCN-222 and Phosphoprotein. In order to make full use of the function of another structure unit of PCN-222, Zr-O clusters, we attempted to investigate the interaction between the PCN-222 and

α-casein, which is one of the phosphoproteins, particularly exists in mammalian milk.52 The XPS survey scans of PCN-222 before and after α-casein adsorption was shown in Figure 3. As compared to the PCN-222, apart from the C, N, O, Zr, and Si (derived from the glass substrates) elements shown, the new P2p peak at 132.7 eV can be observed the spectra of α-casein-loaded PCN-222 (Figure 3A,B), corresponding to phosphate groups from α-casein. It indicates that the phosphoprotein, α-casein was successfully adsorbed on the PCN-222 frameworks. In the XPS spectrum for the PCN-222, the Zr3d exhibits two main peaks at 181.3 and 183.7 eV (Figure 3C). After

α-casein adsorbed, a slight peak shift to higher binding energy was observed for Zr3d spectra (Figure 3D), and two other components centered at 181.6 and 184.4 eV are gained by the decomposition procedure attributed to Zr−O−P from the coordination of the PCN-222 and α-casein. This means that α-casein can be specifically assembled on the surface of ITO/PCN-222 modified electrode. Meanwhile, the specific adsorption capacity of ITO/PCN-222 modified electrode was evaluated by EIS. The diameter of the semicircle in the impedance spectra represents the charge-transfer resistance (Rct), related to the difficulty of electron transfer of a ferricyanide-redox probe between the solution and the electrode. As shown in the inset of Figure 4, the Rct value decreased significantly after modified 12


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PCN-222 (curve b), indicating that PCN-222 can significantly improve the electron transfer capacity of the modified electrode. As expected, the obvious Rct value changed was obtained upon addition of α-casein (curve c). In addition, only a small increase in Rct value to albumin and β-galactosidase was observed (curve d, e), caused by nonspecific adsorption to the surface of ITO/PCN-222. The significant difference of these responses indicates that steric hindrance effect based on the interaction between the Zr-O clusters of PCN-222 on the electrode and the phosphorylated sites of α-casein is the main driving force of these changes in the impedance. These results indicate the label-free sensor has excellent selectivity for α-casein. PEC Sensor for Phosphoprotein Detection. After the above research work, the PEC sensor was developed for label-free detection of α-casein. Although α-casein is a good protein source, it is also known as an allergen which is supposed to bind human IgE or IgG.53 Therefore, accurate identification of α-casein is greatly significant and necessary in food processing. The ITO/PCN-222 modified electrode was immersed into the HEPES, pH 7.4 buffer solution with 0.1 M NaCl containing various concentrations of α-casein. The obtained α-casein linkage electrodes were measured in the PEC system. As shown in Figure 5, the photocurrent signal was significantly decreased with the increasing amounts of α-casein, indicating that steric hindrance effect of biological macromolecules prevented the interface electron transfer among PCN-222, dissolved oxygen and DA (Scheme 1B). And the resulting calibration curve is illustrated in the inset of Figure 5. The photocurrent was proportional to the value of the α-casein concentration in a range of 0 to 2 µg mL-1. The liner regression equation was (I0-I)/I0 = 0.313C (µg mL-1) with a correlation coefficient of 0.995. The limit of detection (LOD) was estimated to be 0.13 µg mL-1 (signal-to-noise ratio of 3), which is lower than that of the previously reported study.54 13



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Compared with the sensitivity of ELISAs for α-casein (0.05 µg mL-1),53 our above estimated LOD was slightly higher, but the proposed protocol for the label-free PEC sensor is much simpler, more efficient, and lower cost than those of conventional methods.

CONCLUSIONS In conclusion, we have successfully developed a label-free PEC sensing of a phosphoprotein using a novel Zr-based porphyrinic MOFs (PCN-222). Specifically, compared with TCPP ligand, in the virtue of the enrichment of oxygen molecules from the high porosity and tunable structures of MOFs, PCN-222 exhibits much better photoelectronic activity towards DA under the O2-saturated aqueous media. The substance DA can effectively inhibit charge recombination of electron–hole pairs, that is, the recombination suppression effect, leading to enhanced photoelectric conversion efficiency. Furthermore, on the basis of steric hindrance effect from the coordination of the phosphate groups and inorganic Zr–O clusters as binding sites in PCN-222 structures, PCN-222 can be further used as signal probes for highly selective PEC label-free phosphoprotein assay. This proposed approach not only provides a deeper understanding of the photocurrent generation mechanism involved in MOFs, but also provides a valuable strategy for clinic diagnostics and therapeutics.

ACKNOWLEDGMENT This research was supported by National Natural Science Foundation of China (Grant No.21675086), the Fundamental Research Funds for the Central Universities (30915015101), and a project founded by the priority academic program development 14


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of Jiangsu Higher Education Institutions (PAPD).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. -

Photos for photoelectrochemical (PEC) cell, the effect of oxygen molecules on photocurrent responses of ITO/PCN-222, and the effect of excitation light wavelength on photocurrent responses of ITO/PCN-222 (PDF).

Notes The authors declare no competing financial interest.

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Figure Captions Figure 1 (A) SEM image of at different scales. Inset A: Enlarged PCN-222 morphology. (B) Powder XRD patterns for simulated and experimental PCN-222. (C) The nitrogen adsorption–desorption isotherm of PCN-222 at 77 K, 1 atm. Inset C: the DFT pore size distribution of PCN-222. (D) UV-visible spectra and PL emission (λex = 420 nm) spectra of TCPP (a) and PCN-222 (b) in ethanol. Figure 2 (A) Photocurrent responses of ITO/PCN-222 in the N2-saturated (a), air-saturated (b) and O2-saturated (c) 0.1 M pH 7.4 HEPES buffer solution. (B) Photocurrent responses of ITO/PCN-222 in the O2-saturated 0.1 M pH 7.4 HEPES buffer solution with the absence (a) and presence of 1 mM cysteine (b), 1 mM ascorbic acid (c), 1 mM DA (d), and 1 mM H2O2 (e). (C) CVs and (D) photovoltage generation of ITO/PCN-222 in the O2-saturated 0.1 M pH 7.4 HEPES buffer solution with the absence (a) and presence of 1 mM cysteine (b), 1 mM ascorbic acid (c), 1 mM dopamine (d), and 1 mM H2O2 (e). (E) Photocurrent responses of ITO/TCPP in the N2-saturated (a), O2-saturated 0.1 M pH 7.4 HEPES buffer solution with the absence (b) and presence of 1 mM dopamine (c). Inset: enlarged curve c. (F) Proposed mechanism of cathodic photocurrent generation based on ITO/PCN-222. All the photocurrent responses were collected at open circuit potentials. Figure 3 XPS survey scans (A) and the high-resolution XPS response of P2p (B) core energy levels for ITO/PCN-222 and ITO/PCN-222/α-casein. The high-resolution XPS response

of

Zr3d

core

energy

levels

for

ITO/PCN-222

ITO/PCN-222/α-casein (D). 21


(C)

and


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Figure 4 EIS of bare ITO (a), ITO/PCN-222 (b), ITO/PCN-222/α-casein (c), ITO/PCN-222/albumin (d) and ITO/PCN-222/β-galactosidase (e) in 0.1 M KCl solution containing 5 mM Fe(CN)64-/3-. Inset A: Equivalent circuit and amplified curves a, b, d and e. Figure 5 Photocurrent responses of the phosphoprotein sensor at various concentration of α-casein. Inset: The photocurrent change of different α-casein concentrations ranging from 0 to 6 µg mL-1. Scheme 1 Schematic illustration for the construction of PCN-222 (A) and the mechanism of charge recombination suppression-based photoelectrochemical strategy for detection of phosphoprotein (B).

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Figure 1

23



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Figure 2

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Figure 3

25



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Figure 4

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Figure 5

27



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Scheme 1

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For TOC only

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Zirconium-Based Porphyrinic MetalâOrganic Framework (PCN-222

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