ordered mesoporous carbon as a high-performance electrocatalyst for efficient detection of baicalein

Journal Pre-proof Calix[4]arene-based [Co4 ] complex/ordered mesoporous carbon as a high-performance electrocatalyst for efficient detection of baicalein Chang Liu, Wen-Yuan Pei, Jian-Fang Li, Jin Yang, Jian-Fang Ma

PII:

S0925-4005(20)30024-1

DOI:

https://doi.org/10.1016/j.snb.2020.127677

Reference:

SNB 127677

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

1 November 2019

Revised Date:

24 December 2019

Accepted Date:

5 January 2020

Please cite this article as: Liu C, Pei W-Yuan, Li J-Fang, Yang J, Ma J-Fang, Calix[4]arene-based [Co4 ] complex/ordered mesoporous carbon as a high-performance electrocatalyst for efficient detection of baicalein, Sensors and Actuators: B. Chemical (2020), doi: https://doi.org/10.1016/j.snb.2020.127677

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Calix[4]arene-based [Co4] complex/ordered mesoporous carbon as a high-performance electrocatalyst for efficient detection of baicalein

Chang Liu, Wen-Yuan Pei,* Jian-Fang Li, Jin Yang, Jian-Fang Ma*

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University, Changchun 130024, People’s Republic of China

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Key Lab for Polyoxometalate Science, Department of Chemistry, Northeast Normal

*

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Corresponding author

E-mail: [email protected] (W.-Y. Pei).

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E-mail: [email protected] (J.-F. Ma).

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Graphical abstract A new [Co4] based metal-organic hybrid complex, [Co4(TCA)2]·xsolvents (Co-TCA), was combined with ordered mesoporous carbons (OMC) to give Co-TCA@OMC composites, which was used as an efficient sensor for detection of baicalein with high

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selectivity and sensitivity.

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Highlights 

►A dumbbell-shaped calixarene coordination complex Co-TCA was designed and synthesized.



► By a simple one-step method Co-TCA was embedded and dispersed in OMC to give Co-TCA@OMC composites.



► The Co-TCA@OMC/GCE was applied for the electrochemical detection of

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ABSTRACT

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baicalein with high selectivity and sensitivity.

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Rational design and preparation of efficient electrochemical sensor with great stability and environmentally friendly performance is a critical step for electrochemical detection.

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Herein, a new dumbbell-shaped metal-organic hybrid complex, [Co4(TCA)2]·3DMF·MeOH

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(Co-TCA), was synthesized with the bowl-shaped p-tert-butylthiacalix[4]arene (TCA). By a simple one-step solvothermal reaction method, Co-TCA was embedded and dispersed in

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ordered mesoporous carbon (OMC) to give Co-TCA@OMC composites. The synergistic effects between Co-TCA and OMC drastically enhanced the adsorption capacity of the

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resulting materials. In contrast to Co-TCA, the introduction of OMC resulted in the increased specific surface area, porosity and conductivity. The glassy carbon electrode modified with Co-TCA@OMC composite (Co-TCA@OMC/GCE) was used in the electrochemical detection of baicalein with a high sensitivity of 8.956 μA μM−1 and an ultra-low detection

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limit of 1.6 nM. This work developed a simple method with potential applications for constructing high-performance electrochemical sensing systems.

Keywords: Calixarene-based complex; Ordered mesoporous carbon; Electrochemical sensor;

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Baicalein

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1. Introduction

Flavonoids have attracted great attention for their valuable effects on human health[1, 2].

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In this regard, baicalein (5, 6, 7-trihydroxyflflavone), as a natural polyhydroxy flavonoid,

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plays a vital role in prevention and treatment of physical health on account of its different medicinal values, such as anti-cancer, anti-HIV and anti-oxidant et al [3-5]. Nevertheless,

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excessive supplementation of baicalein will lead to adverse effects including spasm, dizziness and coma [6]. Thus, it is of great importance to develop an accurate and sensitive technique

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for quantitative determination of baicalein in clinical analysis. To the best of our knowledge, several traditional techniques, such as spectrometry [7], gas chromatography (GC) [8], capillary electrophoresis (CE) [9], electrochemistry [10] and high performance liquid chromatography coupled to mass spectrometry (HPLC/MS) [11], have been applied for the analysis of baicalein. Most of the existing analytical methods are sensitive and effective for 4

the detection of baicalein, but they almost need expensive devices, complicated sample pretreatment and skilled technicians [7-9, 11]. Noticeably, the baicalein possesses a flavone structure of trihydroxy benzene, and the phenolic hydroxyls are easily oxidized to quinones [12]. Therefore, the electrochemical sensing methods are potential candidates for the baicalein detection owing to their low cost, time-saving operation and fast response [10, 13].

understood with the electrochemical analytical method [14].

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Particularly, the pharmacological action and reaction mechanism of the baicalein could be

Recently, discrete metal-organic complexes have attracted great concerns as electrocatalysts for their diverse structures and abundant active sites [15-18]. Calix[4]arenes

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are a type of effective ligands to fabricate metal-organic complexes with adjustable sizes and

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shapes [19-21]. In this facet, p-tert-butylthiacalix[4]arene (TCA), featuring a bowl-shaped aromatic cavity, provides a platform for the selective adsorption of guest molecules [22-25].

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Usually, TCA analogues are inclined to form a series of tetranuclear M(II) 4-thiacalixarene (M

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= Fe, Co, Ni, Cu, etc.) complexes via their phenoxy O atoms and S atoms [26]. Thus, the TCA-based complexes possess potentials to become a class of electrocatalysts with efficient

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electrocatalytic activity and molecular recognition ability [27]. However, the direct use of the single component TCA-based complex in electrochemical detections is greatly limited owing

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to their poor electrical conductivity and inferior mechanical stability [28, 29]. To overcome these issues, a valid strategy that the calixarene-based complex was combined with conductive materials to enhance the overall conductivity and stability was proposed [30]. Generally, diverse materials, including carbon materials [31], metal/metal oxide nanoparticles [32], conductive polymers [33] and graphene-based materials [34], have been applied as 5

conductive materials. In this regard, ordered mesoporous carbons (OMC), as representative carbon materials, are a potential support platform for calixarene-based complexes in virtue of stheir high specific surface area and electrical conduction [30]. In this study, a bowl-shaped thiacalix[4]arene-based [Co4] complex (Co-TCA) was rationally designed and synthesized. Further, the Co-TCA@OMC composites were fabricated by in-situ growth of Co-TCA crystals on the OMC platform through a simple

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one-step solvothermal reaction approach. Single crystal X-ray diffraction (SC-XRD), powder X-ray diffraction (PXRD), elemental analysis (EA), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FT-IR), X-ray

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photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption analyses were applied

electrode

(GCE)

Co-TCA@OMC/GCE,

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shows

a

with

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remarkable

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carbon

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to characterize the resulting morphologies and structure features of these materials. The glassy layer

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adsorption

Co-TCA@OMC, capacity

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namely, excellent

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electro-catalytic activity in the electrochemical detection of baicalein. Finally, the developed sensor was used to detect baicalein in baicalein aluminum capsules with satisfying recovery.

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2. Experimental

2.1. Reagents and instruments

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All related reagents and instruments were given in the Supporting Information.

2.2. Sample pretreatment Baicalein aluminum capsules (Xiuzheng Pharmaceutical Group Co., Ltd. China) was achieved from a local drugstore. The pretreatment was conducted according to the previous report [10]. First, precise quantification powder of baicalein aluminum capsule (5 mg) was 6

dipped in 20 ml methyl alcohol with ultrasonication for 2 h to give the homogenized solution. Afterward, the suspension solution was centrifuged to obtain the supernatant. Finally, the supernatant was stored at 4 ℃ in darkness for reservation. Before determination, the baicalein solution was diluted quantitatively with 0.1 M phosphate buffer solution (PBS, pH = 3.0). 2.3. Synthesis of OMC SBA-15 silica template was prepared with amphiphilic poly (alkylene oxide)-type

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triblock copolymers in water media [35]. OMC were prepared with SBA-15 silica as the template and sucrose as the carbon source [36, 37]. 2.4. Synthesis of TCA

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A mixture of S8 (51.3 g, 1.6 mol), p-tert-butylphenol (120 g, 0.8 mol) and NaOH (16.0 g,

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0.4 mol) was stirred in tetraethylene glycol dimethyl ether (80 mL) under nitrogen atmosphere [38, 39]. The mixture was gradually heated to 230 °C over a period of 4 h. After the resulting

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hydrogen sulfide was removed by a slow nitrogen stream the mixture was maintained at

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230 °C for further 3 h. Afterward, ethyl acetate (200 mL) and glacial acetic acid (30 mL) were added to the mixture under ultrasonic oscillation. The crude product TCA was achieved after

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filtering and washing the precipitation with water and methanol. The pure TCA was collected by recrystallization from chloroform.

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2.5. Synthesis catalyst of Co-TCA and Co-TCA@OMC Hybrid Co-TCA@OMC composite was prepared with TCA, CoCl2·6H2O and OMC of

different mass ratios. Typically, TCA (0.072 g, 0.1 mmol), CoCl2·6H2O (0.1 g, 0.4 mmol) and OMC (0.012 g) were mixed in 40 mL of DMF/EtOH (V: V = 1: 1). Then, the mixture was sealed in a Teflon-lined autoclave and stored at 110 °C for 72 h. Subsequently, the mixture 7

was cooled to 25 °C at a rate of 5 °C/h. Afterward, the composite material of Co-TCA@OMC(1:1) was achieved. The resulting Co-TCA@OMC samples with different mass ratios were denoted as Co-TCA@OMC(x:y) (x:y = 2:1, 1:1, and 1:2, w/w). 2.6. Construction of modified electrodes As the primary step, the bare GCE electrode was carefully burnished with alumina slurry (particle size 0.05 µm) to create a shiny surface, and then rinsed with absolute ethyl alcohol

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and distilled water in sequence. Afterward, the GCE was dried in the air. Meanwhile, the uniform and stable dispersion solution of electrode material was prepared by dispersing 3.0 mg Co-TCA@OMC(1:1) into 1.0 mL Nafion diluent (0.5 wt%) under the ultrasonic

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vibration for 15 min. Before testing, 5 μL of the above-prepared dispersion solution (3 mg

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mL−1) was dropped on the pretreated bare GCE surface and then dried under an infrared lamp. OMC/GCE, Co-TCA/GCE, Co-TCA@OMC(2:1)/GCE and Co-TCA@OMC(1:2)/GCE

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2.7. Electrochemical method

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were prepared with the similar steps.

The electrochemical measurement was accomplished through electrochemical impedance

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spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The detailed parameter was given in the Supporting Information.

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3. Results and discussion 3.1. Characterization of as-prepared materials The

crystal

sample

of

the

tetranuclear

Co(II)4−thiacalixarene

complex,

[Co4(TCA)2]·3DMF·MeOH (Co-TCA), was synthesized with the multidentate ligands TCA and CoCl2·6H2O under the solvothermal conditions. The final molecular formula was 8

established on the basis of thermogravimetric analysis, electron diffraction density and elemental analysis. As shown in Fig. 1A, four Co(II) cations in a square arrangement are linked by two bowl-shaped TCA ligands via their eight phenoxy O atoms and eight S atoms, resulting in the dumbbell-shaped tetranuclear Co-TCA complex. The outer dimension of the Co-TCA

Co-TCA was a promising candidate for guest captures.

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molecule is approximately 13.1 × 13.1 × 13.3 Å3 (opposite Cbutyl···Cbutyl atoms). As a result,

The OMC, as typical mesoporous materials, possess ordered pore structures and tunable pore spaces, and therefore they are ideal accommodations for Co-TCA. The scanning electron

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microscopy (SEM) image of the OMC demonstrates that the carbon material has abundant

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ordered and interconnected mesopores of 3–5 nm in diameter (Fig. 2A). As shown in Fig. 2B, the crystal sample of Co-TCA shows rod-like morphologies. Noticeably, the SEM of

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Co-TCA@OMC(1:1) composite indicates that the defined rod-like Co-TCA crystals with

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homogeneous scatters were successfully embedded in the OMC (Fig. 2C). Particularly, the morphology of Co-TCA is well remained after the introduction of the OMC.

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The samples of Co-TCA, OMC and Co-TCA@OMC were characterized by PXRD patterns. As shown in Fig. 3A, the peak position of the as-synthesized sample of Co-TCA are

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well consistent with the simulated pattern without characteristic diffraction peaks from impurities, proving the successful synthesis of the highly purified Co-TCA crystals. For the inset of Fig. 3A, the weak and broad diffraction peaks in the range of 20-27° belong to the (002) crystal plane of the carbon material OMC. All the characteristic diffraction peaks of Co-TCA@OMC(1:1) composite (Fig. 3A(c)) well correspond to the ones of Co-TCA, 9

indicating that the formed samples of Co-TCA were coupled with the OMC material. For Co-TCA@OMC composites with different ratios of Co-TCA and OMC (2:1, 1:1 and 1:2), the structure of Co-TCA was well maintained though the diffraction peak intensities decrease slightly, as illustrated in Fig. S2(a-c). The FT-IR spectroscopy was utilized to further characterize the Co-TCA@OMC(1:1) composite. As shown in Fig. 3B(a), the symmetric and asymmetric −CH3 stretching vibrations

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appeared at 2959 cm−1 and 2868 cm−1, respectively, for the TCA ligand. The stretching bands at 1589 cm−1 and 1457 cm−1 are ascribed to the phenyl plane bending vibrations of the TCA ligand [40]. In addition, for free TCA, the strong absorption peak at high frequencies around

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3331 cm−1 corresponds to the −OH vibration. The strong absorption peak in Co-TCA

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disappeared (Fig. 3B(b) and (c)), suggesting that the hydroxyl group (−OH) was involved in interactions with Co(II) cations [40]. Other characteristic peaks remained unchanged (Fig.

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3B). The result indicates that Co-TCA and Co-TCA@OMC(1:1) were successfully

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combined together under the solvothermal condition. Surface elemental compositions of Co-TCA and Co-TCA@OMC(1:1) were

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characterized with X-ray photoelectron spectroscopies (XPS). The XPS spectra indicate that both Co-TCA and Co-TCA@OMC contain Co, S, C and O elements (Fig. 3C and the inset).

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The peaks at 284.6, 531.8 and 165.3 eV were assigned to C 1s, O 1s, and S 2p, respectively [41]. The high-resolution Co 2p spectra of Co-TCA (Fig. S3A) and Co-TCA@OMC(1:1) (Fig. S3B) show two main peaks at 783.5 and 804.2 eV, which correspond to the Co 2p3/2 and Co 2p1/2, respectively, indicting the formation of the Co(II) complex [41]. Moreover, the carbon ratio of Co-TCA@OMC(1:1) is higher than that of Co-TCA, and the sulfur and 10

cobalt ratios of Co-TCA@OMC(1:1) are much lower than those of Co-TCA, which are consistent with the PXRD patterns. The porosity of OMC and Co-TCA@OMC(1:1) were investigated with N2 adsorption-desorption measurements. As shown in Fig. 3D, the adsorption-desorption isotherms of OMC and Co-TCA@OMC(1:1) belong to type IV isotherm curves [42]. The pore size distributions suggest the existence of mesopores (Fig. 3D inset). The BET specific

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surface area of OMC and Co-TCA@OMC(1:1) are calculated to be 1362 and 1083 m2 g−1 and the pore sizes are 3.50 and 3.35 nm, respectively. The incorporation of Co-TCA with mesoporous OMC increased the electron and proton transfer. Further, the large BET specific

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surface area improved the accessibility for the electrocatalytic sites of Co-TCA@OMC

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composite.

3.2. Electrochemical characterization for the modified electrodes

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CV and EIS were used to characterize the electrochemical behaviors of the different

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sensors in the electrolyte of 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) in total and 0.1 M KCl at ambient temperature. As shown in Fig. 4A, the bare GCE displays a couple of invertible redox

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peaks with a scan rate of 50 mV s−1. The anodic and cathodic peak currents of [Fe(CN)6]3−/4− remarkably decreased at Co-TCA/GCE, indicating that the channel of electron migration is

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obstructed on the Co-TCA/GCE surface. The redox peak currents at OMC/GCE and Co-TCA@OMC(1:1)/GCE were drastically enhanced relative to the bare electrode, leading to a significant change in the CV curves. The result can be attributed to the good conductivity of the OMC. Particularly, a small peak-to-peak separation (ΔEp) as well as a high peak current response appeared at Co-TCA@OMC(1:1)/GCE, demonstrating that the electron transfer 11

rates are improved on the surface of Co-TCA@OMC(1:1)/GCE on the basis of Nicholson's theory [43]. Above results suggest that the electron transfer process controlled by kinetics and thermodynamics is favored on the modified electrode surface of Co-TCA@OMC(1:1) [44]. EIS is an effective mean to further evaluate the interface property and impedance change of different electrodes. The semicircle of the Nyquist diagram, corresponding to the interface layer resistance, can demonstrate the electron transport behavior on the electrode interface

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[45]. The Nyquist plots for the prepared electrodes and the Randles equivalent circuit model of the EIS data are shown in Fig. 4B. Based on the diameters of the semicircles, the bare GCE features a relatively low electron transfer resistance (Rct) of 232.5 Ω, and Co-TCA/GCE

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shows a large semicircle domain with Rct value of 479.9 Ω, demonstrating a poor electrical

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conductivity. In contrast, the semicircle of OMC/GCE is nearly invisible with the electron transfer resistance (Rct) of 3.2 Ω due to the high electrical conductivity and the low electron

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transfer resistance of the OMC [37]. Co-TCA@OMC(1:1)/GCE exhibits a small semicircle

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diameter with the electron transfer resistance (Rct) 108.6 Ω. Obviously, the introduced OMC with the excellent electronic transmission capability dramatically expedited the electron

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transfer at the electrode. The result also indicates that the successful incorporation of Co-TCA into OMC effectively increased the charge transfer kinetics of the Co-TCA@OMC

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composite.

3.3. Electrocatalytic oxidation of baicalein The electrochemical behaviors of baicalein (50 μM) at various electrodes were initially investigated using CV in 0.1 M PBS (pH = 3.0). As illustrated in Fig. 5A, the oxidation peaks of baicalein were observed at 0.416 and 0.422 V for the unmodified GCE and Co-TCA/GCE, 12

respectively. For OMC/GCE, a couple of broad and symmetrical redox peaks appeared at 0.432 and 0.386 V, demonstrating the low adsorption capacity and negligible electrochemical process of baicalein at OMC/GCE. Nevertheless, a couple of regular redox peaks could be clearly observed at Co-TCA@OMC(1:1) modified electrode, with the anodic (Epa) and cathodic potentials (Epc) of 0.436 and 0.382 V, respectively. Though the peak-to-peak separation (ΔEp) value of Co-TCA@OMC(1:1)/GCE is 54 mV which is close to that of

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OMC/GCE (46 mV), the anodic peak current of Co-TCA@OMC(1:1)/GCE is approximately threefold higher than that of OMC/GCE. The enhanced electrochemical behaviors may account for the synergistic effect of Co-TCA and OMC. First, the OMC offer

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a high conductive network that facilitates the electron transfer. Second, Co-TCA possesses

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the bowl-shaped cavity for baicalein adsorption as well as rich active centers for electrocatalysis. Third, the Co-TCA@OMC composite with large specific surface area (1083

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m2 g−1) and wide pore-size distribution (3.35 nm) provides a pathway to reach the active site

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of the electrocatalyst. Further, the effect of OMC content on the electrocatalytic property was also studied. At the beginning, the oxidation peak currents increased, and then decreased with

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the increased contents of OMC. The maximum peak current value appeared at Co-TCA@OMC(1:1)/GCE, indicating its optimal electrocatalytic ability and kinetic

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property for baicalein oxidation (Fig. S4). Hence, the Co-TCA@OMC(1:1)/GCE was applied for the electrochemical detection of baicalein. 3.4. Optimization of experimental parameters for determination of baicalein 3.4.1. Influence of pH

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The electrochemical redox of baicalein is a proton-electron coupled reaction. Therefore, the determination of baicalein was performed in the presence of protons. In other words, the acidic analytical condition was selected to determine the baicalein [46]. The different pH values (2.0−6.0) of PBS on the electrochemical behavior of baicalein were studied using CV at Co-TCA@OMC(1:1)/GCE electrode. As depicted in Fig. 5B, the oxidation peak potential (Epa) shifts negatively with increased pH values. Based on the equation Epa = −0.0645 pH +

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0.5439 (R2 = 0.9954), a good linear correlation between Epa and pH was observed with the slope of −64.5 mV/pH (Fig. 5C). The slope value is near to the theoretical one (−59 mV/pH) of Nernst equation, demonstrating that the electrochemical behavior is a redox process, in

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which equal number of electron and proton are involved in electro-oxidation process of

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baicalein [10, 47]. Meanwhile, the maximal current response of baicalein appeared at pH =

3.4.2. Influence of scan rate

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3.0 (Fig. 5C). Hence, the supporting electrolyte at pH = 3.0 was used for the further study.

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To further explore the reaction mechanism of baicalein at Co-TCA@OMC(1:1)/GCE, the CVs of 50 μM baicalein were determined at Co-TCA@OMC(1:1)/GCE with different

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scan rates (Fig. 5D). As illustrated in Fig. 5E, both the anodic and cathodic peak currents (Ipa and Ipc) obey the linear relationships with scan rates in the range of 10 to 300 mV s-1. The

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result demonstrates that the redox of baicalein at the Co-TCA@OMC(1:1)/GCE is a representative surface-controlled process [10, 13]. In Fig. 5F, redox peak potentials (Epa, Epc) shift linearly with the logarithm of scan rate (log v) at the high scan rates from 0.2 to 0.3 V s-1. Based on the Laviron's equation [48]

14

Epa  E 0 

2.303RT log v (1   )nF

(1)

Epc  E 0 

2.303RT log v  nF

(2)

the electron transfer number (n) and transfer coefficient (α) are calculated to be 1.88 and 0.52 from the slope of Ep vs logv. As a result, we can deduce that the redox reaction of baicalein at Co-TCA@OMC(1:1)/GCE belongs to a two-electron and two-proton process. Notably, a

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reversible electrochemical oxidation process occurred on the ortho-position phenolic hydroxyls of baicalein (Scheme 2), which is consistent with the previous reports [10, 12, 13]. 3.4.3. Influence of the accumulation potential and time

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Accumulation is a simple and useful method for the improvement of sensitivity in electrochemical detection [49]. Particularly, the accumulation potential and time greatly

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influence the accumulation efficiency of baicalein at Co-TCA@OMC(1:1)/GCE. As

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illustrated in Fig. S5A, the accumulation potentials negatively vary from 0.1 to −0.4 V, and the peak current values gradually increase and attain the maximum value at −0.2 V. When the

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more negative potentials are used, the peak current values gradually decrease. Thereby, the accumulation potential of −0.2 V was adopted for the following investigation. Meanwhile, the

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effect of the accumulation time was studied with a fixed accumulation potential of −0.2 V

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(Fig. S5B). When the accumulation time is prolonged to 300 s, the peak current increase rapidly, and almost remains unchanged after 300 s, which may be caused by the accumulation of baicalin and the saturation of active sites on the surface of Co-TCA@OMC(1:1)/GCE. As a result, the accumulation time of 300 s was employed for the detection of baicalein. 3.5. Analytical performance of Co-TCA@OMC(1:1)/GCE to baicalein 15

Under the optimized test conditions, different pulse voltammogram (DPV) was applied for quantitatively analyzing baicalein at Co-TCA@OMC(1:1)/GCE because of its high sensitivity and efficient analytic signal. As shown in Fig. 6A, the oxidation peak current responses were enhanced with increased baicalein concentrations. Two excellent linear plots of peak currents versus concentrations of baicalein are represented as Ipa1 (μA) = 8.956 C (μM) + 2.700 (0.005–1.5 μM, R2 = 0.9932) and Ipa2 (μA) = 1.974 C (μM) + 13.127 (1.5–17.0 μM,

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R2 = 0.9970) with the sensitivity values of 8.956 and 1.974 μA/μM, respectively (Fig. 6B). The change in the slope of the fitting curves may be attributed to the inevitable adsorption of baicalein or its reaction product on the surface of the Co-TCA@OMC(1:1)/GCE, which

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hinds the diffusion process of baicalein [50, 51]. Notably, the detection limit for baicalein (1.6

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nM) is determined based on 3σ/b, where σ is the standard deviation for the peak currents of 11 times in the blank PBS buffer solution and and b represents the slope of the calibration plot.

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The analytical performance of Co-TCA@OMC(1:1)/GCE for the detection of baicalein is

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comparable to other sensors presented in Table 1. Clearly, in contrast to the known related sensors, the Co-TCA@OMC(1:1)/GCE features several advantages for the baicalein

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detection, such as high sensitivity and wide linear range and ultra-low detection limit [10, 12, 13, 52-56].

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Noticeably,

the

excellent

performance

for

electrocatalysis

of

baicalein

at

Co-TCA@OMC(1:1)/GCE may account for the particular physicochemical characteristics of Co-TCA and OMC as well as the extraordinary synergy effect of Co-TCA@OMC. Structurally, electroactive Co-TCA is uniformly embedded and dispersed in OMC. This hybrid architecture provides open channels for electrolyte transportations and improves 16

electron transfers between the catalyst and electrode. Additionally, the bowl-shaped cavity of TCA provides a platform for the adsorption of baicalein and in turn effectively catalyzes baicalein molecules with the amplified current signals and improved sensitivity. Moreover, the introduced OMC also increased the electrical conductivity of the composite, resulting in the efficient electron and proton transfers between the baicalein and electrode surface. Consequently, Co-TCA@OMC(1:1) composite features efficient electrocatalytic activities

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for baicalein.

3.6. Reproducibility, stability and selectivity for detection of baicalein

It is noteworthy that the reproducibility, selectivity and stability are important

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characteristics for electrochemical sensors [13]. To explore the reproducibility, six identical

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Co-TCA@OMC(1:1) modified GCEs were applied for the determination of 10 μM baicalein in 0.1 M PBS (pH = 3.0). As shown in Fig. 7A, all the independent determination system of

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baicalein shows basically equal electrochemical signals with the relative standard deviation of

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3.65%. Further, this sensor was determined continuously for three weeks and recorded at time intervals of two days. Approximately, the current value retains 95.32% of the original current

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value for baicalein detection after covering the electrode with a glass beaker for three weeks at 4 °C in the refrigerator (Fig. 7B). This finding indicates the long-term storage and

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operational stability of Co-TCA@OMC(1:1) modified GCE. To investigate the selectivity of Co-TCA@OMC(1:1)/GCE, the interferents including either common biomolecules or inorganic ions (100-fold K+, Al3+, Na+, SO42-, Cl-, PO43- and 50-fold glucose, sucrose, dopamine, ascorbic acid, glycine, tryptophan, and urea) were mixed in 10 μM baicalein. From Fig. 7C, the signal change for the detection is less than 5.0% at the Co-TCA@OMC(1:1) 17

modified electrode, indicating the high selectivity of this sensor towards the detection of baicalein. 3.7. Analysis of real sample To verify the practical application of the sensor Co-TCA@OMC(1:1)/GCE, the detection of baicalein in real drug sample (baicalein aluminum capsules) was conducted. The baicalein measurements for the sample was performed in 10 mL 0.1 mol L-1 PBS (pH = 3.0)

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containing 50 μL sample. The current values of three different concentrations of baicalein were recorded with the successive injection of standard baicalein solution. The detection results of baicalein in the drug sample are listed in Table 2. The largest relative standard

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deviation among all the measurements is 2.25% and the acceptable recoveries range from

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98.00% to 102.20%. These satisfying data imply that this established sensing method is highly accurate and feasible for quantitative determination of baicalein in real sample.

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4. Conclusion

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We have developed an effective one-step method for the preparation of Co-TCA@OMC composite by dispersing the electroactive Co-TCA into ordered mesoporous carbon OMC.

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Electrochemical performance analysis demonstrated that the synergistic effects between Co-TCA and OMC drastically enhanced the properties of Co-TCA@OMC composite, such

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as catalytic ability, conductivity and surface area et al. The resulting electrochemical sensor Co-TCA@OMC(1:1)/GCE can be successfully used to detect baicalein with ultra-low detection limit, high sensitivity and excellent selectivity. In real sample tests, the sensor was successfully utilized to monitor the concentration of baicalein in baicalin aluminum capsules. The proposed detection method will be helpful for drug monitoring in clinical and 18

pharmaceutical fields. Also, the present work offers a feasible way for the design and fabrication of highly efficient electrochemical sensing systems.

Declaration of Interest Statement

conflict of interest in connection with the work submitted.

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Acknowledgments

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We declare that we do not have any commercial or associative interest that represents a

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21471029) is highly appreciated.

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Financial support from National Natural Science Foundation of China (Grant No.

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Author Biographies

Chang Liu received her bachelor degree from Jilin University in 2017. Now she is a doctoral candidate at Northeast Normal University (NENU) and her research focuses on

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electrochemical sensors and catalysts.

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Wen-Yuan Pei received her PhD from Northeast Normal University (NENU). Her current research interests focus on coordination chemistry.

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Jian-Fang Li she is a master candidate at Northeast Normal University (NENU).

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Jin Yang was born in China (1976). He received his MS degree in 2003 from Northeast Normal University (NENU), and his PhD in 2007 from Jilin University. Currently, he is a

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Professor at NENU and mainly focuses on coordination polymers and their properties. Jian-Fang Ma (born 1966) obtained his BS degree from Nankai University in 1987, and then

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received his PhD (1993) under the direction of Professor Jia-Zuan Ni from Changchun Institute of Applied Chemistry (CIAC). In 1993, he joined CIAC as an assistant professor. He first worked as a visiting scholar at The Chinese University of Hong Kong and then as a postdoctoral research fellow at Toho University, Japan, during 1995–1997. Since 2001 he has been a professor in Department of Chemistry, Northeast Normal University (NENU). His 27

current research interests focus on the coordination polymers and organotin cluster chemistry.

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Figure and Table Captions

Scheme 1. Schematic representative for the synthesis of Co-TCA/OMC composite via a

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simple one-pot solvothermal method.

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Scheme 2. The proposed mechanism of redox behaviors of baicalein at Co-TCA@OMC

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(1:1)/GCE.

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Fig. 1. (A) Crystal structure of the dumbbell-shaped Co-TCA complex. (B) View of the

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packing structure of Co-TCA along the a axis.

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Fig. 2. SEM images of the materials (A) OMC, (B) Co-TCA and (C) Co-TCA@OMC (1:1).

Fig. 3. (A) XRD patterns of the simulated Co-TCA (a), the as-synthesized Co-TCA (b) and the Co-TCA@OMC (1:1) (c). (B) FT-IR spectra (KBr pellets) of the prepared TCA (a), Co-TCA (b) and Co-TCA@OMC (1:1) (c). (C) XPS spectrum of the prepared Co-TCA@OMC (1:1) (Inset: XPS spectrum of the prepared Co-TCA). (D) Nitrogen 28

adsorption-desorption isotherm of OMC and Co-TCA@OMC (1:1) (Inset: pore-size distribution plots of OMC and Co-TCA@OMC (1:1)).

Fig. 4. (A) CVs of the bare GCE, Co-TCA/GCE, OMC/GCE and Co-TCA@OMC (1:1)/GCE in the solution of 5 mM K3[Fe(CN)6]/ K4[Fe(CN)6] (1:1) in total and 0.1 M KCl with the scan rate of 50 mV/s. (B) Nyquist plots of EIS spectra of GCE, Co-TCA/GCE, OMC/GCE and

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Co-TCA@OMC (1:1)/GCE in the electrolyte solution of 5 mM K3[Fe(CN)6]/ K4[Fe(CN)6] (1:1) in total and 0.1 M KCl (Inset: Randles equivalent circuit model).

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Fig. 5. (A) CV curves of 50 μM baicalein for GCE, Co-TCA/GCE, OMC/GCE and

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Co-TCA@OMC (1:1)/GCE in 0.1 M PBS (pH = 3.0) electrolyte. (B) CVs of 50 μM baicalein for Co-TCA@OMC (1:1)/GCE in 0.1 M PBS buffer with different pH values (from left to

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right: pH = 6.0, 5.0, 4.0, 3.0 and 2.0). (C) The linear dependence relationship of oxidation

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peak currents (Ipa) and oxidation peak potentials (Epa) vs pH. (D) CV curves of 50 μM baicalein for Co-TCA@OMC (1:1)/GCE in 0.1 M PBS (pH = 3.0) with different scan rates

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(from inner to outer: 10−300 mV s−1). Plots of cathodic and anodic peak currents (Ipa, Ipc) and

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potentials (Epa, Epc) against v(E) or logv(F), respectively.

Fig. 6. (A) The DPV responses at Co-TCA@OMC (1:1)/GCE in 0.1 M PBS (pH = 3.0) with different baicalein concentrations. (B) The linear relationships between the oxidation peak currents versus baicalein concentrations (0.005–17 µM).

29

Fig. 7. (A) The DPV responses of six identical Co-TCA@OMC (1:1)/GCEs for the oxidation of 10 μM baicalein. (B) Stability test of the Co-TCA@OMC (1:1)/GCE for 10 μM baicalein detection every 3 days in three weeks. (C) Column graph of the percentage change of current signal for 10 μM baicalein at Co-TCA@OMC (1:1)/GCE in presence of large amounts of

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interfering compounds.

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Scheme 1. Schematic representative for the synthesis of Co-TCA/OMC composite via a simple one-pot solvothermal method.

30

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Scheme 2. The proposed mechanism of redox behaviors of baicalein at Co-TCA@OMC

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lP

re

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(1:1)/GCE.

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Fig. 1. (A) Crystal structure of the dumbbell-shaped Co-TCA complex. (B) View of the

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packing structure of Co-TCA along the a axis.

31

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na

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Fig. 2. SEM images of the materials (A) OMC, (B) Co-TCA and (C) Co-TCA@OMC(1:1).

32

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Fig. 3. (A) PXRD patterns of the simulated Co-TCA (a), the as-synthesized Co-TCA (b) and

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the Co-TCA@OMC(1:1) (c). (B) FT-IR spectra (KBr pellets) of the prepared TCA (a),

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Co-TCA (b) and Co-TCA@OMC(1:1) (c). (C) XPS spectrum of the prepared Co-TCA@OMC(1:1) (Inset: XPS spectrum of the prepared Co-TCA). (D) Nitrogen

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adsorption-desorption isotherm of OMC and Co-TCA@OMC(1:1) (Inset: pore-size

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distribution plots of OMC and Co-TCA@OMC(1:1)).

33

4.

(A)

CVs

of

the

bare

GCE,

Co-TCA/GCE,

OMC/GCE

and

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Fig.

Co-TCA@OMC(1:1)/GCE in the solution of 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) and 0.1 M KCl with the scan rate of 50 mV/s. (B) Nyquist plots of EIS spectra of GCE,

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Co-TCA/GCE, OMC/GCE and Co-TCA@OMC(1:1)/GCE in the electrolyte solution of 5

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na

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mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) and 0.1 M KCl (Inset: Randles equivalent circuit model).

34

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Fig. 5. (A) CV curves of 50 μM baicalein at GCE, Co-TCA/GCE, OMC/GCE and

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Co-TCA@OMC(1:1)/GCE in 0.1 M PBS (pH = 3.0) electrolyte. (B) CVs of 50 μM

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baicalein at Co-TCA@OMC(1:1)/GCE in 0.1 M PBS buffer with different pH values (from left to right: pH = 6.0, 5.0, 4.0, 3.0 and 2.0). (C) The linear dependence relationship of

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oxidation peak currents (Ipa) and oxidation peak potentials (Epa) vs pH. (D) CV curves of 50 μM baicalein at Co-TCA@OMC(1:1)/GCE in 0.1 M PBS (pH = 3.0) with different scan

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rates (from inner to outer: 10−300 mV s−1). Plots of cathodic and anodic peak currents (Ipa, Ipc)

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and potentials (Epa, Epc) against v (E) or logv (F), respectively.

35

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Fig. 6. (A) The DPV responses at Co-TCA@OMC(1:1)/GCE in 0.1 M PBS (pH = 3.0) with different baicalein concentrations. (B) The linear relationships between the oxidation peak

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lP

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currents versus baicalein concentrations (0.005–17 µM).

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Fig. 7. (A) The DPV responses of six identical Co-TCA@OMC(1:1)/GCEs for the oxidation

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of 10 μM baicalein. (B) Stability test of Co-TCA@OMC(1:1)/GCE for baicalein (10 μM) detection every 3 days in three weeks. (C) Column graph of the percentage change of current signal for 10 μM baicalein at Co-TCA@OMC(1:1)/GCE in presence of large amounts of interfering compounds.

36

Table 1 Comparison with the known electrochemical methods for baicalein detection. Modified electrode a

Technique b

Linear range (μM)

RuO2-PDDA-rGO/GCE

DPV

0.002–0.4

RGO/PDA/Au/GCE

DPV

Ta2O5-Nb2O5@CTS-CPE

Ref.

9.48

0.6

[10]

0.01–15

6.27

3.1

[12]

DPV

0.08–8

2.88

50

[13]

Ta2O5-CTS-CPE

DPV

0.08–4

0.23

50

[52]

TRGO/GCE

DPV

0.01–1.0; 1.0–10

7.36; 0.97

BDD electrode

SWV

1–95

0.054

GR/DNA/GCE

DPV

0.73–117

DNA-LB/GCE

SWV

0.01–2

Co-TCA@OMC (1:1)/GCE

DPV

0.005–1.5; 1.5–17

6.0

[53]

260

[54]

320

[55]

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－

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LOD (S/N=3, nM)

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16.53 8.96; 1.97

6.0

[56]

1.6

This work

ruthenium oxide loaded on Poly-(dimethyldiallylammonium chloride) functionalized reduced graphene oxide; PDA:

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aRuO -PDDA-rGO: 2

Sensitivity (μA·μM-1)

polydopamine; Ta2O5-CTS: tantalum oxide particles and chitosan; Ta2O5-Nb2O5@CTS: tantalum oxide, niobium oxide particles and antiseptic chitosan; TRGO: reduced graphene oxide; BDD electrode: boron doped diamond electrode; DNA-LB: DNA–octadecylamine (ODA)

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Langmuir–Blodgett film; GR/DNA: graphene and DNA; b DPV: differential pulse voltammetry; SWV: square wave voltammetry;

37

Table 2 Determination of baicalein in baicalein aluminum capsule sample Original found a (μM)

Standard added (μM)

Total found a (μM)

Recovery (%)

R.S.D (%)

98.00 102.20 98.30

1.32 1.06 1.38 2.25

5.38 1.0 5.0 10.0

Average value of five replicate measurements.

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a

6.36 10.49 15.21

38