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The Enhanced Electrochemiluminescence of OneDimensional Self-Assembled Porphyrin Hexagonal Nanoprisms Wen-Rong Cai, Guang-Yao Zhang, Kun-Kun Lu, Haibo Zeng, Serge Cosnier, Xue-Ji Zhang, and Dan Shan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 4, 2017

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ACS Applied Materials & Interfaces

The Enhanced Electrochemiluminescence of One-Dimensional Self-Assembled Porphyrin Hexagonal Nanoprisms Wen-Rong Cai,† Guang-Yao Zhang,† Kun-Kun Lu,† Hai-Bo Zeng,† Serge Cosnier,‡ Xue-Ji Zhang,† Dan Shan†* †

MIIT Key Laboratory of Advanced Display Materials and Devices, School of

Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China ‡

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

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

1

ACS Paragon Plus Environment


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ABSTRACT: In this work, we synthesized one-dimensional nanostructure of zinc 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine

(ZnTPyP)

via

a

self-assembly

technique. Using sodium dodecyl sulfate (SDS) as “soft template”, the self-assembled ZnTPyP (SA-ZnTPyP) had the morphology of hexagonal nanoprisms with the uniform size (diameter of 100 nm). The SA-ZnTPyP exhibited remarkably different spectral properties from the original ZnTPyP. The as-prepared SA-ZnTPyP was used to modify glassy carbon electrode (GCE) and the electrochemiluminescence (ECL) behaviors of the SA-ZnTPyP/GCE were investigated. The hydrophilic carbon dots (C-Dots) could efficiently prevent the dissolution of SA-ZnTPyP in DMF containing 0.1 mol L−1 TBAP, and simultaneously could accelerate electron transfer. Therefore, the enhanced ECL was realized by C-Dots/SA-ZnTPyP/GCE, using H2O2 as coreactant. This amplification of ECL was further studied by ECL spectroscopies and cyclic voltammetry, and the corresponding mechanism was proposed.

KEYWORDS: zinc porphyrin; one-dimensional structure; surfactant-assisted self-assembly; electrochemiluminescence; carbon dots.

2


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1. INTRODUCTION Nowadays,

electrochemiluminescence

(also

called

electrogenerated

chemiluminescence, abbreviated ECL) has become a powerful analytical tool and has been widely applied in many fields, on account of its advantages, such as high sensitivity, simplicity of operation, and good temporal and space control over the light emission.1-4 ECL is the process whereby species generated at electrodes undergo high-energy electron-transfer reactions to form excited states that emit light.5-6 In other words, ECL refers to chemiluminescence triggered by electrochemical methods. It represents a valuable marriage of electrochemistry and chemiluminescence. ECL reactions generally require a luminophore, also termed chemiluminescent reagent. Several classic ECL luminophores such as luminol,7 tris(2,2-bipyridyl) ruthenium(II) (Ru(bpy)32+),8-9 quantum dots,10 and porphyrin11 have been reported and studied extensively. However, ECL still has shortcomings like poor ability to repeatedly electrochemically cycle an individual luminophore, a broad emission spectroscopy, and signal loss due to ECL reagents leaving the detection zone.12-14 Therefore, further improvement of ECL signal is still of significance. Nanomaterials can be incorporated into ECL sensor to improve efficiency, but also represent a new class of ECL emitters. Porphyrins are 18π aromatic azaannulenes consisting of regularly arranged four methine carbons.15 Electrochemical and photochemical properties of porphyrins have attracted much attention for decades because of their unique aromatic structure and 3



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their beneficial characteristics such as high molar absorptivity, as well as the vital roles in biological processes such as photosynthesis, oxygen transport, and biocatalysis.16-18 Porphyrins have a flexible and adjustable molecular structure in chemical modification.19-21 The introduction of molecular-recognition motifs (e.g., hydrogen bonding, metal-ligand bonds and π-π stacking) into the porphyrin building blocks will make them form new structures, such as fibers, cubes, sheets, simple micelles and wheels.22-25 In our previous work, we have developed several ECL sensors based on porphyrin-nanocomposite. Proto-porphyrin IX (CoPPIX) was combined with ultrathin carbon nitride nanosheets (C3N4). CoPPIX@C3N4 complex possessed much higher catalytic activity.26 Laponite nanosheets were utilized to manage the assembly and immobilization of proto-porphyrin IX (PPIX). A new eco-friendly emitter: singlet oxygen produced in the vicinity of nanoclay-supported zinc proto-porphyrin IX (ZnPPIX).27 We synthesized zirconium-based porphyrinic metal−organic framework (MOF) for the detection of a phosphoprotein. The active center ZnTCPP in MOF-525-Zn as electron media reacts with O2 in the 3D nanocage to produce 1O2, resulting in enhanced ECL signal.28 Due to the structure properties of porphyrin and its derivatives, much work so far has focused on the synthesis of porphyrin self-assembled structures. Wang’s group proved that robust porphyrin nanotubes can be prepared by ionic self-assembly of two oppositely charged porphyrins in aqueous solution.29 Cai et al obtained the porphyrin nanostructures

of

zinc

meso-tetraphenylporphyrin

meso-tetraphenylporphyrin (CuTPP)

by

a

solvent

4


(ZnTPP)

and

exchange

copper method.30

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Surfactant-assisted nanospheres and nanorods of meso-tetra(4-pyridyl)porphyrins (H2TPyP) have also been reported in a recent publication.31 These stable materials have an open structure, indicating the better photoelectric activity. Encouraged by above-mentioned knowledge, in present work, we synthesized a novel nanostructure of

zinc

5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine

(ZnTPyP)

via

a

surfactant-assisted self-assembly method. Sodium dodecyl sulfate (SDS) was used as “soft template”. The self-assembled ZnTPyP (SA-ZnTPyP) was initially used to modify electrode and the ECL behavior of SA-ZnTPyP was for the first time examined. 2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Zinc 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (ZnTPyP),

tetra-n-butylammonium

perchlorate

(TBAP),

and

N-(2-hydroxyethyl)piperazine-N’-(2-ethanesulfonic acid) (HEPES) sodium salt were purchased from J&K Scientific Inc. (Shanghai, China). Sodium dodecyl sulfate (SDS) and N,N-Dimethylformamide (DMF) were purchased from Sigma-Aldrich Chemical Co., Ltd. (Shanghai, China). Potassium ferricyanide [K3Fe(CN)6] and potassium ferrocyanide [K4Fe(CN)6] was purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). The carbon dots (C-Dots) were synthesized according to the protocol reported previously.32 All other chemicals were of analytical reagent 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. All experiments were carried out under the condition of nitrogen 5



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saturation. 2.2. Measurements and Apparatus. A CHI 660D electrochemical workstation (CH Instrument) was used for cyclic voltammetry (CV) measurements. ECL measurements were carried out using a MPI-EII multifunctional electrochemical and chemiluminescent analytical system (Xi’an Remex Analytical Instrument Co., Ltd., China) in which the emission window lays above the photomultiplier tube biased at 800 V. ECL spectra were collected with an Edinburgh FLS920 fluorescence spectrometer (Livingston, UK) and CHI 660D electrochemical workstation. Above-mentioned studies were performed with a conventional three electrode system. Glassy carbon electrodes (GCE) (diameter 5 mm) were polished to a mirror finish mechanically by 0.3 and 0.05 µm alumina powders and sequentially cleaned in alcohol, acetone and doubly distilled water by sonication, separately. An Ag/AgCl electrode and a Pt wire electrode were used as reference and counter electrodes, respectively. The ECL behaviors were investigated at a scan rate of 50 mV s−1 within the scan range from 0 to -1.6 V. The morphologies of the modified electrode were investigated with a XL-30E scanning electron microscope (SEM). Scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDS) was used for characterize morphology and element distribution. 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. The water contact angle (CA) measurements were performed by the sessile drop technique using an Optical Tensionmeter (Theta Lite, Finland) under ambient 6


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laboratory conditions. Electrochemical impedance spectroscopy (EIS) measurements were carried out using an Autolab PGSTAT30 (Eco chemie, The Netherlands) controlled by NOVA 1.10 software. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on K-Alpha (Thermo Fisher Scientific Co., USA). The ultraviolet absorption spectra were measured by UV-vis spectrophotometry (UV-3600, Shimadzu, Japan). The N2 saturated aqueous solution was premade by constantly bubbling pure N2 to preserve the atmospheric pressure. 2.3. Preparation of self-assembled ZnTPyP (SA-ZnTPyP). The sample of SA-ZnTPyP was prepared according to the report in the literature.33 0.5 mL of ZnTPyP solution (0.01 M, 0.2 M HCl) was added into 9.5 mL of SDS solution (0.05 M, containing 0.02 M NaOH) with continuous stirring, and the mixture was stirred at room temperature (25 ˚C) for 48 hours and the final product was centrifuged and washed three times with deionized water and finally dried in the vacuum freeze dryer. 2.4. Preparation of SA-ZnTPyP based electrode. Initially, 1.0 mg SA-ZnTPyP was dispersed in 1 mL deionized water with sonication for 5 min. SA-ZnTPyP/GCE was prepared by coating GCE with 10 µL of SA-ZnTPyP solution and dried at ambient temperature. The C-Dots aqueous solution was obtained by dispersing 2.0 mg C-Dots in 1 mL ultrapure water under sonication. Subsequently, 20 µL of C-Dots solution was spread on the surface of the modified electrode and dried at ambient temperature. The final modified electrode was denoted as C-Dots/SA-ZnTPyP/GCE. For the control experiments, ZnTPyP/GCE and C-Dots/ZnTPyP/GCE were also prepared following the same procedure with ZnTPyP dispersion (1mg mL−1). 7



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3. RESULT 3.1. Characterization of as-synthesized SA-ZnTPyP. Practically, to prepare the ZnTPyP-based nanostructures (SA-ZnTPyP), an acidulated aqueous solution of ZnTPyP (0.01 M) was added dropwise into alkalified colloidal solution of SDS, after which the system was aged for 48 h with stirring, The morphology of the thus-produced nanostructures was then investigated by means of SEM. Figure 1A and Figure 1B show the morphology of original ZnTPyP and SA-ZnTPyP, respectively. ZnTPyP precursor exhibits the “rod-like” structure with the random size ranging from 2∼8 µm (Figure 1A). Nevertheless, the as-synthesized SA-ZnTPyP is confirmed to be nanoprism with the uniform size (Figure 1B). As illustrated in inset of Figure 1B, the cross section of the nanoprims is hexagon. This average diameter of the hexagonal nanoprisms is 100 nm (relative standard deviation 10%) and the aspect ratio varies from 1 to 3. The structure of SA-ZnTPyP was further characterized by X-ray diffraction (XRD) (Figure S1). The data shows single crystal patterns with major peaks between 5° and 30°, which is consistent with the results reported in the literature, indicating that we have successfully synthesized the porous nanostructure of ZnTPyP.33 The SA-ZnTPyP exhibits remarkable optical properties that are quite different from the original ZnTPyP (Figure 1C). The UV-visible spectrum of the ZnTPyP precursor displays that the ZnTPyP has a strong Soret band (B-band) at 425 nm tailing with two Q bands at 557 nm and 596 nm (curve a of Figure 1C). As for that of SA-ZnTPyP (curve b of Figure 1C), the Soret band locates at 417 nm and 450 nm. The Q bands of 8


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SA-ZnTPyP engender a red-shifted band arising and ultimately locates at 573 nm and 611 nm. [Figure 1] 3.2. ECL Investigation. SA-ZnTPyP/GCE was feasibly prepared by casting 10 µL of SA-ZnTPyP solution (1 mg mL−1) on the surface of GCE and dried at ambient temperature. The ECL behaviors of SA-ZnTPyP/GCE were investigated in 10 mmol L−1 pH 7.4 HEPES containing 0.3 mol L−1 KCl solution in the absence and presence of 0.05 M H2O2, respectively. As shown in Figure 2A, no obvious ECL signal can be observed in the absence of H2O2 (Figure 2A, curve a). In the presence of 0.05 M H2O2, a cathodic ECL signal whose light intensity is about 400 a.u. occurred and the ECL peak is located at -1.4 V (Figure 2A, curve b). However, the ECL intensity decreases obviously upon continuous cyclic scans (Figure 2B). After scanning five cycles, the ECL signal decreases about 15%, and the intensity is just 59% of the original intensity after nine cycles. [Figure 2] Figure 2C displays the ECL response of SA-ZnTPyP/GCE in DMF containing 0.1 mol L−1 TBAP solution in the absence and presence of 0.05 M H2O2. Similar to the result in aqueous media, the ECL signal did not appear until 0.05 M H2O2 was added. The intensity of the peak is about 4300 a.u., which is quite stronger than that in aqueous solution. Unfortunately, as displayed in Figure 2D, with the increase of the scanning time, the ECL intensity sharply decreases. The intensity of the ECL signal 9



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decreases from 4387 a.u. to 2615 a.u. after just three cycles, and continuing to scan for two cycles, the intensity is just 28% of the original intensity. To improve the stability of the SA-ZnTPyP/GCE, the C-Dots were introduced to the proposed system. It is great that the C-Dots/SA-ZnTPyP/GCE presents excellent stability in DMF containing 0.1 mol L−1 TBAP (Figure 2E). The ECL responses of the modified electrode, which is obtained during a continuous potential scan, show well constant and stable signals and the relative standard deviation (RSD) is just 1.5%, demonstrating wonderful stability and reproducibility of the proposed ECL system. 4. DISCUSSION 4.1. Formation of the self-assembled ZnTPyP. In this work, SA-ZnTPyP was prepared via ZnTPyP self-assembling with SDS working as the “soft template”. To demonstrate that SDS did not participate in the self-assembling of porphyrin, the XPS survey scans of ZnTPyP precursor and SA-ZnTPyP were measured (Figure S2). As we can see, after the assembly, the peaks did not change significantly. Meanwhile, the spectrum shows that no sulfur is detected. Thus, we can conclude that the resulting nanostructures are the result of porphyrin self-assembly. According to the published literature, we can learn that the concentration of SDS and the concentration ratio of ZnTPyP and SDS affect the configuration and the size of the nanostructures.34 The concentration of SDS solution is 0.05 M in this experiment, which is about 6 times higher than its cmc (critical micelle concentration), making the micelle of O/W appears a form of a circular hexametric cage,35 as illustrated in Scheme 1. In an

10


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acidulated aqueous solution, the original ZnTPyP is protonated. Due to the dominant π-π stacking interaction in the aqueous solution, ZnTPyP molecules are stacked compactly with the embedded active sites (Scheme 1). After injecting into alkalified colloidal solution of SDS, deprotonation happens and the hydrophobic ZnTPyP transfers spontaneously into the organic phase of SDS micelle. The further self-assembly of ZnTPyP occurs within the special circular hexametric cage created by SDS. In organic phase, π-π stacking is not dominant.36 ZnTPyP molecules can be connected with each other through other cooperative interactions, e.g., ligand coordination. Every single ZnTPyP molecule is bound to four neighbors by Zn-N axial ligations and π-π stacking between molecules and finally forming hexagonal prism (Scheme 1).33 This makes porphyrin molecules be shored up and form an open structure, resulting in available active sites exposed. Since such a microreactor created by SDS molecules has a fixed size, the SA-ZnTPyP with uniform size was synthesized. [Scheme 1] Figure S3A shows the SEM-EDS image of SA-ZnTPyP and the dispersion mapping for C, Zn, and N elements are presented in Figure S3B-D. These images indicate that the distribution of Zn and N is relatively uniform and the surface of SA-ZnTPyP contains relatively rich zinc element, which further demonstrates that there are abundant active sites on the surface of SA-ZnTPyP. The formation of SA-ZnTPyP was further confirmed by UV-visible measurement. The split of the Soret band could be due to the J-aggregation. This is beneficial since it is the exciton coupling 11



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(proportional to the oscillator strength) between these states that are relevant for exciton transport.37 We succeeded in synthesizing nanomaterials with uniform size from the porphyrin precursor. The as-prepared SA-ZnTPyP has a porous and open structure. Nano-materials with this structure usually have fantastic photoelectric properties and are expected to construct the enhanced ECL sensor. 4.2. The role of C-dots. Based on the above results we can know that hydrogen peroxide, as a coreactant, is indispensable in this system. In the presence of 0.05 M H2O2, the light intensity of SA-ZnTPyP/GCE in HEPES is just one-tenth of the modified GCE with the same amount of SA-ZnTPyP in DMF. In addition, the ECL peak gradually blue shifted during the continuous potential scan in aqueous media while in DMF it is stable. Considering the weak light intensity and the instable ECL emission in aqueous media, ECL behaviors of SA-ZnTPyP in DMF were chosen as the emphasis of this paper. Figure 2D displays the instability of the light intensity of the SA-ZnTPyP/GCE in DMF. It may be attributed to the lipophilic property of the as-prepared SA-ZnTPyP. The water contact angle (CA), which was carried out through dropping a drop of distilled water on the surface of modified glass plates within 30 s of the contact, was measured to be 89° (Figure 3A, image a), proving the hydrophobicity of SA-ZnTPyP. Thus, in the process of scanning, the SA-ZnTPyP modified on the electrode will gradually dissolve in DMF, making the effective constituent on the electrode be less and less and the intensity of ECL signal be weaker and weaker. To solve this problem, the C-Dots were introduced to the proposed system. The 12


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C-Dots are hydrophilic and the presence of C-Dots layer on the surface of SA-ZnTPyP/GCE can prevent the SA-ZnTPyP from gradually dissolve into DMF. The water contact angle of C-Dots/SA-ZnTPyP on the glass plates is 35° (Figure 3A, image b). The value of CA reveals the hydrophobicity/hydrophilicity property of the electrode surface. After covering the hydrophilic C-Dots, the CA decreased substantially, suggesting that C-Dots enclosed SA-ZnTPyP/GCE not only demonstrate hydrophily, which means the SA-ZnTPyP will not be in direct contact with DMF solution and then dissolve in it, but also makes the H2O2 be allowed to infiltrate through it. Curve b in the inset of Figure 2C has no ECL signal, illustrates that the C-Dots do not participate in the ECL reaction. The C-Dots just plays the role of fixing the SA-ZnTPyP on the surface of electrode. Simultaneously, EIS measurements were carried out to further study its role with Fe(CN)64−/3− as a sensitive redox probe (Figure 3B). As shown, the charge-transfer resistance (Rct), that is, the diameter of the semicircle in the impedance spectra, which is related to the capability of electron transfer of a ferricyanide-redox probe between the electrolyte and the electrode, significantly decreased after the C-Dots modified on the SA-ZnTPyP/GCE surface. This demonstrates that the electron transfer can be highly improved by C-Dots. [Figure 3] 4.3. Electrochemical investigation. To evaluate the enhancement of ECL by the proposed SA-ZnTPyP, the GCE was modified by original ZnTPyP, and the ECL experiment by C-Dots/ ZnTPyP/GCE was also performed. As you can see in inset of Figure 2C, the C-Dots/ZnTPyP/GCE has a cathodic ECL emission at -1.58 V (curve 13



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c). The light intensity can achieve around 2050 a.u.. The peak intensity of C-Dots/SA-ZnTPyP/GCE is about two times higher than that of C-Dots/ZnTPyP/GCE. To

further

reveal

the

ECL

signal

amplification

mechanism

by

C-Dots/SA-ZnTPyP/GCE, electrochemical properties of different modified electrodes were studied. Figure 4 shows the comparative CV curves of different modified electrodes in DMF containing 0.1 mol L−1 TBAP solution in the absence and presence of 0.05 M H2O2. All the studies were carried out under the condition of N2-saturated. As shown, no apparent redox peaks were observed on GCE (Figure 4A, curve a) and C-Dots/GCE (Figure 4B, curve a). Curve a in Figure 4C displays that there are two reduction processes of C-Dots/ZnTPyP/GCE, located at -1.15 V and -1.55 V, respectively, and three oxidation processes at -1.38 V, -1.08 V and -0.27 V. Based on our previous knowledge,38 we supposed that the two reduction peaks and the first two oxidation peaks (Peak at -1.38 V and peak at -1.08 V) should be derived from zinc coordination with porphyrin ring (Zn2+TPyP→Zn•+TPyP→Zn0TPyP→Zn•+TPyP →Zn2+TPyP). In comparison, the reduction peaks of C-Dots/SA-ZnTPyP/ GCE are found at -1.26 V and -1.47 V, while the corresponding oxidation peaks are at -1.39 V and -1.09 V (Figure 4D, curve a). The peak at -0.27 V of C-Dots/ZnTPyP/ GCE and C-Dots/SA-ZnTPyP/ GCE is attributed to the electro-dehydroxylation of central zinc (HO-Zn2+TPyP→O=Zn2+TPyP).

We

can

see

that

the

cathodic

peak

of

C-Dots/SA-ZnTPyP/ GCE, which is attributed to Zn2+/Zn•+, is much more obvious and the intensity is much higher than that of C-Dots/ZnTPyP/ GCE, indicating the 14


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more available exposure of zinc ions in SA-ZnTPyP. In the presence of 0.05 M H2O2, bare GCE and C-Dots/GCE show no great change (Figure 4A, B, curve b). As for C-Dots/ZnTPyP/ GCE and C-Dots/SA-ZnTPyP /GCE, in the presence of H2O2, the cathodic peak current of both the Zn2+/Zn•+ and the Zn•+/Zn0 increase (Figure 4C, D, curve b). Obviously, the Zn2+/Zn•+ of SA-ZnTPyP/C-Dots/GCE has better catalytic performance for H2O2, showing abundant generation of •OH. Combined with the above-mentioned ECL behavior, we can draw the conclusion that the existence of Zn•+/Zn0 transition and •OH groups, which are produced by Zn2+/Zn•+ transition, plays an essential role in the ECL system. [Figure 4] 4.4. The ECL Signal Amplification Mechanism. Innovative luminophore is one of the key factors to enhance ECL signal. To verify the role of SA-ZnTPyP as well as uncover the luminous mechanism of the proposed system, the luminophore has to be confirmed. Both the ZnTPyP precursor (Figure S4, image a) and the SA-ZnTPyP (Figure S4, image b) exhibit a homogeneously bright red color under ultraviolet radiation (λ = 365 nm). It indicates that porphyrin still maintained their fluorescent property after the self-assembling. Simultaneously, the ECL emission spectrum of SA-ZnTPyP/C-Dots in DMF solution containing 0.1 mol L−1 TBAP and 0.05 M H2O2 was measured by using a fluorescence spectrometer. As shown in Figure 5, the ECL emission occurred in the range of 400−700 nm and the maximum wavelength is at 550 nm (Figure 5, red curve), which is consistent with the characteristics of the porphyrin luminescence in the literature.39 Compared with the fluorescence spectrum 15



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of ZnTPyP (Figure 5, black curve), we can conclude that zinc porphyrin is the luminophore and the innovative luminophore was successful synthesized. The intensity of ECL emission depends on the photochemical properties and the amount of the luminophore. Analogy to our system, the more effective ZnTPyP exists, the stronger luminous intensity appears. As already mentioned, for the original ZnTPyP, π-π stacking works as a dominate influence factor, leading to the coverage of central metal ion Zn2+. Therefore, the active sites cannot have full access to the coreactant H2O2 and the ECL emission is not as strong. The driving force for the self-assembly of ZnTPyP is not only π-π stacking of porphyrin rings but also the axial ligations of Zn-N. These coexisting bonding forces result in the large amount of Zn2+ exposed, which further plays a significant role in ECL measurements. [Figure 5] Consequently, as illustrated in Scheme 2, according to the experimental results and based on the above comprehensive discussion, the possible ECL mechanism is described with the following response equation: Zn2+TPyP + e− → Zn•+ TPyP Zn•+ TPyP + H2O2 → Zn2+TPyP + •OH + OH− Zn•+TPyP + e− → Zn0TPyP H2O2 + e− → •OH + OH− Zn0TPyP + •OH → Zn*+TPyP + OH− 16


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Zn*+TPyP → Zn•+TPyP + hυ (λ=550nm) [Scheme 2] 5. CONCLUSION In summary, the self-assembly of porphyrin was obtained through surfactant-assisted cooperative interactions (e.g., π-π stacking, ligand coordination, and so forth). Compared with the original ZnTPyP, the SA-ZnTPyP exhibited the following advantages: (1) SA-ZnTPyP appears in a shape of hexagonal nanoprism with uniform size, which can provide enormous specific surface area of reaction; (2) The interior structure of ZnTPyP is compact due to the dominant π-π stacking of porphyrin molecules. As for the case of SA-ZnTPyP, there exists a circular hexametric cage structure cross-linked by a Zn-N axial coordination of pyridyl ligands. This porous and open structure of SA-ZnTPyP results in more efficient luminophores of ECL exposed and facile mass transfer. In addition, the introduction of the hydrophilic C-Dots alters the surface property of SA-ZnTPyP/GCE from lipophilicity to hydrophilicity, enhancing the stability and as well as conductivity of the modified electrode. Thereby, the enhanced ECL could be obtained synergistically. Further works should be carried out to modify the as-synthesized nanomaterials and study the ECL behaviors in aqueous solution, which have potential application in biosensing.

■ ACKNOWLEDGMENT This research was supported by National Natural Science Foundation of China (Grant No.21675086), the Fundamental Research Funds for the Central Universities 17



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(30917014107), and a project founded by the priority academic program development of Jiangsu Higher Education Institutions (PAPD). Notes The authors declare no competing financial interest.

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Figure Captions Figure 1 SEM image of (A) ZnTPyP and (B, Inset B) SA-ZnTPyP. (C) UV-vis absorption spectra of ZnTPyP (a) and SA-ZnTPyP (b). Figure 2 (A) ECL-potential curve of SA-ZnTPyP/GCE in10 mmol L-1 pH 7.4 HEPES containing 0.3 mol L-1 KCl solution in the absence (a) and presence (b) 0.05M H2O2. (B) ECL responses of SA-ZnTPyP/GCE recorded in aqueous solution during a continuous potential scan between -1.6 and 0 V. (C) ECL behaviors of SA-ZnTPyP/GCE in the absence (a) and presence (b) 0.05M H2O2 in DMF containing 0.1 mol L-1 TBAP. Inset (C) ECL-potential curves of GCE (a), C-Dots/GCE (b) and C-Dots/ZnTPyP/GCE (c) in DMF containing 0.1 mol L-1 TBAP with 0.05M H2O2. (D) ECL behaviors of SA-ZnTPyP/GCE recorded in DMF containing 0.1 mol L-1 TBAP with 0.05M H2O2 during a continuous potential scan between -1.6 and 0 V. (E) ECL responses of C-Dots/SA-ZnTPyP/GCE recorded in DMF containing 0.1 mol L-1 TBAP during a continuous potential scan between -1.6 and 0 V. Figure 3 (A) Contact angle measurement images of SA-ZnTPyP (a) and C-Dots/SA-ZnTPyP (b) on the glass plates. (B) EIS for SA-ZnTPyP/GCE (a) and C-Dots/SA-ZnTPyP/GCE (b) in 0.1 M KCl solution containing 5 mM [Fe(CN)6]4−/3−. Rs: the resistance of the electrolyte solution; Rct: the charge-transfer resistance; Zw: the Warburg impedance; CPE: constant phase element. Figure 4 Cyclic voltammograms of (A) bare GCE, (B) C-Dots/GCE, (C) C-Dots/ZnTPyP/GCE

and (D) C-Dots/SA-ZnTPyP/GCE in DMF containing 0.1 25



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mol L-1 TBAP in the absence (a) and presence (b) of 0.05M H2O2, scan rate: 50 mV s-1. Figure 5 ECL and PL spectrum of C-Dots/SA-ZnTPyP. Scheme 1 Schematic illustrations for the construction of SA-ZnTPyP. Scheme

2

Schematic

Diagrams

for

the

ECL

Reaction

C-Dots/SA-ZnTPyP/GCE.

26


Mechanism

of

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Scheme 1

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Scheme 2

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TOC

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