Palladium Supported on an Amphiphilic TriazineâUrea-Functionalized

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Palladium supported on Amphiphilic Triazine-Urea-functionalized Porous Organic Polymer as highly-efficient Electrocatalyst for Electrochemical Sensing of Rutin in Human Plasma A.T.Ezhil Vilian, Rajamanickam Sivakumar, Yun Suk Huh, Ji Ho Youk, and Young-Kyu Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00579 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Applied Materials & Interfaces

Palladium supported on Amphiphilic Triazine-Urea-functionalized Porous Organic Polymer as highly-efficient Electrocatalyst for Electrochemical Sensing of Rutin in Human Plasma

A.T.Ezhil Vilian,†a Rajamanickam Sivakumar,†b Yun Suk Huh*c Ji Ho Youk*b and Young-Kyu Han*a a

Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 04620, Republic of

Korea. E-mail: [email protected] (Y.-K. Han) b

Department of Applied Organic Materials Engineering, Inha University, Incheon 22212, Republic of

Korea. E-mail: [email protected] (J. H. Youk) c

Department of Biological Engineering, Biohybrid Systems Research Center (BSRC), Inha University,

Incheon 22212, Republic of Korea. E-mail: [email protected] (Y. S. Huh) †

These authors contributed equally to this work.

Corresponding Authors a

* E-mail: [email protected] (Y.-K. Han).

b

c

* E-mail: [email protected] (J. H. Youk)

* E-mail: [email protected] (Y. S. Huh)

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Abstract Metal nanoparticle-containing porous organic polymers have gained great interest in chemical and pharmaceutical applications owing to their high reactivity and good recyclability. In the present work, a palladium nanoparticle-decorated triazine-urea-based porous organic polymer (Pd@TU-POP) was designed and synthesized using 1, 3-bis (4-aminiphenyl) urea with cyanuric chloride and palladium acetate. The porous structure and physicochemical properties of this electrode material Pd@TU-POP were observed using a range of standard techniques. The Pd@TU-POP material on the electrode surface showed superior sensing ability for rutin (RT) because the Pd dispersion facilitated the electrocatalytic performance of TU-POP by reducing the overpotential of RT oxidation dramatically and improving the stability significantly. Furthermore, TU-POP provides excellent structural features for loading Pd nanoparticles, and the resulting Pd@TU-POP exhibited enhanced electron transfer and outstanding sensing capability in a linear range between 2 and 200 pM having low a detection value of 5.92×10-12 M (S/N = 3). The abundant porous structure of Pd@TU-POP not only provides electron transport channels for RT diffusion but also offers facile route for quantification sensing of RT with satisfactory recoveries in aqueous electrolyte containing human plasma and red wine. These data reveal that the synthetic Pd@TU-POP is an excellent potential platform for the detection of RT in biological samples. Keywords: Electrochemical sensor; Rutin detection; Pd nanoparticles; cyclic voltammetry; Porous organic polymers.

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Introduction Porous organic polymers (POPs) have become a valuable source in material science owing to their high specific surface area, abundant nitrogen content, tunable pore walls, high thermal stability, and excellent physicochemical properties.1-2 With their demand in assorted applications, different functionalities of POPs were designed by the conscious selection of building blocks containing suitable functional groups.3-4 Considerable efforts have been made on the design and synthesis of POPs for various applications, such as separation/storage of gas,5 chemical sensors,6 heterogeneous catalysis,7 and photocatalysis.8 In addition, various chemicals, such as 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX),9 hydrogen sulfide,

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fluoride ion,11 2,4,6-

trinitrophenol,12 and nitroaromatic and benzoquinone vapors13-15 have been successfully detected by the POPs in an attractive manner. In terms of a stable covalent bond, low skeleton density, and hydrophobic components, POPs are the most promising materials for the encapsulation of metal NPs and the stability of metal NPs has been sustained because of their excellent coordination ability with POPs. The decoration of Pd NPs both at the external surface of porous materials as well as inside the nanopores has been successful using various synthetic approaches, such as graphitic carbon nitride,16 nitrogen-enriched mesoporous carbon,17 metal-organic frameworks,18 nitrogen-rich heptazine-based porous framework,19 Click-based POPs, and polyphosphazene nanotubes.20 Furthermore, enormous efforts have been made in the design and synthesis of Pd NPs decorated on the POPs surface and used as a heterogeneous catalyst,21 but there are few reports on the detection of chemicals. Bandyopadhyay et al. reported the application of Pd-NPs containing POPs to the sensing of nitroaromatics and recently, Pd NPs were fabricated on porous aromatic 3



frameworks to detect vanillin with satisfactory results.22-23 In addition, POP-supported Au NPs were applied to the sensing of quercetin at very low concentrations.24 Furthermore, urea linkages which act as binding sites for transition metals,25 help to stabilize metal nanoparticles in POPs through coordination interactions. The majority of POPs are hydrophobic in nature, which prevents their effective dispersion and results in poor contact between the substrates and active sites.26

Therefore, the introduction of hydrophilic groups, such as the urea unit into the

hydrophobic aromatic framework may improve the dispersity and efficiency of POPs.27-28 Rutin (3,3′4′5,7-pentahydrohyflavone-3-rhamnoglucoside), one of the most abundant bioactive flavonoid glycosides, is found widely in many plants and drugs.29 The promising physiological activities, including anti-inflammatory, antiviral, anti-tumor, and anticancer activities, allow it to be used as a therapeutic medicine.30 Therefore, the accurate detection of RT in drugs is of practical importance. High-performance liquid chromatography (HPLC) and capillary electrophoresis are highly efficient techniques to detect low concentrations of RT; however, they are expensive, time consuming, and may require complicated pretreatment, which limit their further use.31-32 The development of a simple, sensitive, and accurate analytical method to diagnose RT is still a significant and challenging task. Other alternative analytical techniques for the determination of RT at low concentrations have been explored over the last few decades, including electrogenerated chemiluminescence, and electrochemical sensors.33 The electrochemical method is the one of the best approaches for the detection of RT, because it is a simple, fast, sensitive, and low-cost detection approach. To the best of our knowledge, there have been no reports on the applications of Pd NP-supported amphiphilic POPs as an electrochemical sensor. 4


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In the present study, a novel triazine-urea-containing POP decorated with Pd NPs was used as a sensor for the electrochemical determination of RT. The triazine units were connected with urea linkages as the binding sites for the Pd NPs, which is beneficial for stabilizing the Pd NPs through coordination interactions. The distinguished performance of the Pd NPs supported on urea-functionalized POP improved the adsorption of RT at the electrode surface. The TUPOP promoted the electrochemical activity by accelerating electron transfer and providing a suitable pathway during the electrochemical detection of RT. The electrochemical performance and stability of the electro material were attributed to the uniform decoration of Pd nanoparticles over the POP support, as proven from the TEM images. We observed that the Pd@TU-POP material exhibited an outstanding electrochemical RT detection with a low detection limit of 5.92×10-12 M and a wide linear range from 2 to 200 pM. The Pd@TU-POP material is an outstanding platform for containing a large number of electrocatalytic sites that can be used for the electrochemical sensing of RT in red wine and human plasma samples.

Materials and methods Chemicals and Materials 1,4-Phenylenediamine (>98%), Urea (98%), cyanuric chloride (98%), sodium bisulfite, acetic acid, and N, N-diisopropylethylamine (99%) were purchased from TCI-Japan. All other reagents were bought from Merck and used without further purification. The PBS buffer solution (0.05 M) was used as an electrolyte. Ultra-pure water (>18 MΩ cm) from a Millipore system (US) was used to prepare the supporting electrolyte solutions. 5



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Instruments and Characterization. The morphology of Pd@TU-POP was acquired transmission electron microscopy (TEM, JEOL-JEM-2010F). Fourier transform infrared (FT-IR,

FT/IR-6600, JASCO, Japan)

spectroscopy was performed over the region, 4000–400 cm-1, using KBr pellets. The elemental compositions were studied using elemental analysis (Thermo Scientific elemental analyzer, Flash EA 1112). The crystal structures were determined using X-ray diffraction (XRD, Rigaku D/max2500). Solid-state

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C nuclear magnetic resonance spectroscopy was carried out using a 4 mm

double resonance magic angle spinning (NMR, Bruker AVANCE III 600-MAS) probe. X-ray photoelectron spectroscopy (XPS, Thermo Scientific-Alpha instrument,) was performed to analyze the surface elemental states. The specific surface areas (m2 g-1) of Pd@TU-POP and TU-POP were measured on a Micromeritics Tri-Star ASAP 2020 instruments (77 K). Electrochemical analyses were demonstrated using Autolab PGSTAT302F system (Eco Chemie, The Netherlands) with GCE, Ag/AgCl, and Pt as working-, reference- and counter-electrode, respectively. Synthesis of TU-POPs Urea based amphiphilic porous organic polymer (TU-POP) was synthesized as follows (Figure 1): Step-1: Synthesis of 1,3-bis(4-aminiphenyl) urea. According to the literature procedure 33

, urea (3.3 g, 55.5 mmol), 39% sodium bisulfite (2.75 mL), and acetic acid (2.3 mL) were

added to a stirred solution of 1,4-phenylenediamine (5 g, 46.2 mmol) in water (50 mL), and the reaction mixture was kept under constant stirring for 30 h at 100 οC. The final product was 6


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obtained after filtration and repeated washing with copious amount of hot water which removed the residual urea followed by drying in vacuum oven. Step-2.: Synthesis of TU-POP. 1,3-Bis(4-aminiphenyl) urea (1.98 g, 8.17 mmol) was mixed with a stirred solution of cyanuric chloride (1 g, 5.42 mmol) in dry DMSO (10 mL). The reaction mixture was kept at 15 οC with drop-wise addition of N, N-diisopropylethylamine (2.3 mL, 16.2 mmol). The product obtained was kept under constant stirring for 36 h. Solid was formed by the addition of ice to the crude product which was then filtered and undergone sequential washing with dichlormethane, tertrahydrofuran and ethyl acetate to remove the oligomers. The obtained product was confirmed by FT-IR and solid-state

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C NMR

spectroscopy. Synthesis of Pd@TU-POP. TU-POP (1 g) was mixed with stirred solution of palladium acetate (0.4 g) in tetrahydrofuran (20 mL) and ethanol (5 mL) mixture which was kept under constant stirring for 2 h. Then, Sodium borohydride (0.19 g) was added to the solution portion wise and stirring was continued for another 18 h. The resulting product was isolated through filtration and washed sequentially with water, ethyl acetate, dichloromethane, and tetrahydrofuran and then dried for 12 h at 80 οC under vacuum to obtain Pd@TU-POP (1 g). Fabrication of Pd@TU-POP -Based Electrochemical Sensors. Initially, the glassy carbon electrode (3 mm diameter) was cleaned using 0.5 µm alumina and washed repeatedly with deionized water, and dried before use. Suspensions of Pd@TU-POP were prepared by adding 10 mg of the corresponding Pd@TU-POP material to 1 mL of ethanol 7



followed by ultra-sonication for 30 min. Subsequently, 8 µL of Pd@TU-POP dispersion was dropped onto a cleaned electrode surface and dried at room temperature. The Pd@TU-POP/GCE were prepared using a similar procedure. Finally, the Pd@TU-POP /GCE obtained was used to detect the RT in human plasma and red wine samples.

Figure 1. (a) Scheme for the fabrication of Pd@TU-POP and (b-d) TEM images of Pd@TUPOP.

Results and discussion 8


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Structurally and morphologically Characterization of the Pd@TU-POP material Following the synthetic route outlined in Figure 1a, TU-POP was synthesized by treating 1,4-phenylenediamine with urea followed by cyanuric chloride in the presence of N, Ndiisopropylethylamine. The solubility of TU-POP in water and organic solvents are very poor. Therefore, it was treated with Pd(OAc)2 in a tetrahydrofuran and ethanol mixture to produce the Pd (II)@ TU-POP. With the help of Sodium borohydride the Pd(II) was reduced to Pd (0) NPs with an average size of 6 nm which were encapsulated on the surface of the TU-POP and the resulting product was characterized by FT-IR spectroscopy, Solid-state 13C NMR spectroscopy, TEM, elemental analysis, PXRD, XPS, and thermogravimetric analysis (TGA).The FT-IR spectra of the cyanuric chloride and the synthesized compounds such as 1,3-bis (4-aminophenyl) urea, TU-POP and Pd@TU-POP are depicted in Figure 2a and S1. The FT-IR spectrum of TUPOP (Figure 2a) revealed a stretching vibration band at 1665 cm−1 for the carbonyl group (C=O) of the urea linkage. Other absorption bands around 1490, 1300, and 810 cm−1 indicated the characteristic triazine unit in all the materials. The solid-state

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C NMR spectrum (Figure S2)

revealed that the peak at δ=158 ppm could be attributed to the presence of a carbonyl carbon atom of the urea moiety and the rest of the sp2 carbons appears at δ =165, 163, 156, 155, 135,133, 123, and 121 ppm. Figure 1b-d present TEM images of Pd@TU-POP showing that the porous polymer (TU-POP) on the external surface are embedded with Pd NPs due to the coordination ability of the urea which contain carbonyl group and the 1,3,5-triazine backbone. Moreover, the compositions of Pd@TU-POP was examined using EDS elemental mapping which confirm the distribution of the Pd NPs on the TU-POP surface (Figure S3). TEM- EDX pattern confirmed that C, N, O and Pd contents are occur in Pd@TU-POP. A TEM image of TUPOP provided in Figure S4 in the Supporting Information. Due to the presence of the trapped 9



guest molecules, TU-POP elemental analysis resulted in slightly lower experimental values of carbon and nitrogen than the corresponding theoretical values, which is common in porous materials. The TGA (Figure S5) analysis showed the presence of guest molecules; initial losses of 3% up to 100 οC were observed and the material was stable up to 210 οC. XRD was used to examine the crystalline structures of the cyanuric chloride and in house prepared materials like 1,3-bis (4-aminophenyl) urea, TU-POP and Pd@TU-POP and the obtained patterns are shown in Figure 2b and S6. The wide-angle powder X-ray diffraction (PXRD) pattern of Pd@TU-POP displayed characteristic peaks at values (2θ) of 39°, 44.5°, and 65.9 ° correspond to the reflections from (111), (200), and (220) planes which shows the face centered cubic arrangement of Pd NPs (JCPDS no 46-1043).34 This XRD reveals a distinct pattern, showing the dispersion of Pd NPs over the outer surface of the TU-POP (Figure 2b).35 The Brunauer-Emmet-Teller (BET) surface area of TU-POP was 145 m2g-1, this unexpectedly low surface area could be due to the intermolecular hydrogen bonding between the urea groups during framework development.25 The Nitrogen adsorption-desorption isotherms for TU-POP and Pd@TU-POP showed hysteresis loop pertaining to the capillary condensation and the existence of mesopores in the material and followed type-IV isotherm (Figure 2c and d).

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Figure 2. (a) FTIR curves of TU-POP and Pd@TU-POP. (b) XRD patterns of TU-POP and Pd@TU-POP, (c) N2 adsorption/desorption isotherm of TU-POP and Pd@TU-POP (d), poresize-distribution curve of TU-POP and Pd@TU-POP. The X-ray photoelectron spectroscopy (XPS) of TU-POP and Pd@TU-POP revealed three binding energy peaks at 284.5, 399.3, and 533 eV, which were assigned to C 1s, N 1s, and O 1s, respectively (Figure 3a). The C 1s spectrum of Pd@TU-POP was deconvoluted into three components: C-H, C=N, C-N, C=C, and C=O as shown in Figure 3b. For Pd@TU-POP, one new peak appeared at 337 eV corresponding to the Pd 3d binding energy. Figure 3c presents the N 1s spectra of TU-POP and Pd@ TU-POP. The N 1s binding energy peaks of the TU-POP material 11



at 399.2 and 399.5 eV were ascribed to two types of nitrogen species: triazine and urea unit. N 1s spectrum analysis for Pd@TU-POP was carried out to confirm the coordination of N atoms with Pd0 NPs. The positive shift in the binding energies of N 1s spectrum at 399.7 and 400.3 eV for the Pd@TU-POP compared to the TU-POP material suggested the N atoms from the triazine and urea unit were weakly coordinated with Pd.36 The higher binding energy in the O 1s spectrum at 531.9 eV for Pd@TU-POP compared to that of TU-POP at 530.5 eV showed strong Pd NPs interaction which are embedded on the surface of the porous material and the oxygen atom of the urea units (Figure 3e). The Pd 3d spectra reveals the oxidation state of Pd in the Pd@TU-POP material (Figure 3d). The two different peaks at binding energy values of 335.5 (Pd 3d5/2) and 340.7 eV (Pd 3d3/2) are assigned to Pd(0) species.37 In addition, the atomic percentages of carbon, nitrogen, oxygen and palladium in TU-POP were analyzed from the XPS spectra and the values are given in Table S1. These results clearly show that the O atoms of the urea units are coordinated more strongly to Pd than to the N atoms of the triazine unit in the Pd@TU-POP polymer.

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Figure 3. XPS survey (a), and high-resolution C 1s (b), N 1s (c), Pd 3d (d), and O 1s (e) XPS spectra of Pd@TU-POP and (f) EIS of bare GCE, TU-POP GCE and Pd@TU-POP/GCE at the frequencies swept from 10,000 to 0.01 Hz, Inset: Randles circuit.

Electrochemical behaviors of Pd@TU-POP EIS and CV are useful techniques for examining the changes in the electrochemical interfacial behaviors of the surface modification of the stepwise construction path of the Pd@TU-POP. Electrochemical characterization of the bare electrode, TU-POP, and Pd@TUPOP/GCE modified electrodes were conducted in 10 mM [Fe(CN)6]3-/4- containing 0.1 M KCl with scanning frequencies ranging from 0.01 to 100,000 Hz (Figure 3f). In the Nyquist plot, the semicircle region at high frequency region was assigned to the charge transfer controlled process and the linear segment of the low frequency region correspond to the diffusional step of the electrochemical process. The experimental impedance data were obtained by fitting the Randle's equivalent circuit (inset of Figure 3f), which consisted of charge transfer resistance (Rct), solution 13



resistance (Rs), Warburg impedance (Zw), and double layer capacitance (Cdl). The Rct value of the bare GCE was calculated to be 321 Ω. When the bare electrode surface was modified with the TU-POP film, the Rct values increased to 629 Ω due to the partial blocking of interfacial charge transfer to the electrode surface. In contrast, when the GCE was modified with Pd@TUPOP, the semicircle diameter of the electrochemical sensor was decreased significantly (174 Ω), suggesting that the effective surface area of unique porous-architecture TU-POP supported with the excellent electrochemical conductivity of Pd NPs played a key role in the electron-pumping channel through fast kinetics of electron mobility between the probe molecules and the modified electrode surface. Furthermore, successful modification of Pd@TU-POP on GCE surface was confirmed by the EIS data. CV was performed using [Fe(CN)6]3-/4- as a redox probe at the bare GCE, TU-POP /GCE, and Pd@TU-POP/GCE modified electrodes (Figure 4a). For the unmodified GCE electrode, the oxidation and reduction peak potentials appeared at 0.32 and 0.12 V, respectively, with a ∆Ep value of 0.20 V, which reveals a quasi-reversible process. In contrast, a pair of redox peaks were observed for TU-POP/GCE with a slightly higher current (∆Ep) of 0.281V with respect to bare GCE, which may be due to the poor electrical conductivity of TU-POP. Finally, the addition of electrocatalytically active Pd NP with the porous TU-POP material, microenvironment was facilitated to preserve the adsorbed RT. In addition, the Pd@TU-POP/GCE-modified electrodes exhibited higher electrocatalytic activity with a 1-fold increase in the current response at the Pd@TU-POP /GCE (lower ∆Ep of 0.106 V). Therefore, the faradaic processes assisted by the electrode surface modification of the Pd@TU-POP were confirmed.

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Figure 4. (a) CV curves of bare electrode, TU-POP, and Pd@TU-POP modified GCE recorded in 5 mM Fe(CN)6 3-/4- 0.1 M KCl solution, (b) pH = 5.0 PBS (0.05 M) containing 10 µM of RT. Scan rate 50 mV s-1. (c) CVs of the Pd@TU-POP/GCE in pH = 5.0 PBS (0.05 M) solution containing 10 µM of RT with various scan rates (10-100 mV s-1), (d) Calibration Plot of anodic and cathodic current peaks versus scan rate. The electrochemical performance of the-synthesized Pd@TU-POP modified electrode was investigated by CV to evaluate their sensing to RT. Figure 4b presents CV traces of the bare GCE, TU-POP/GCE, and Pd@TU-POP/GCE-modified electrodes in PBS in the presence and absence of 10 µM RT at a scan rate of 50 mV s-1. No redox peak was observed for the bare GCE, 15



TU-POP/GCE, and Pd@TU-POP /GCE in 0.05 M PBS without 10 µM RT, which suggests that there was no electrocatalytic activity by the electrode materials in the particular potential scanning region in the absence of RT. With the introduction of RT, only the oxidation peak current of RT at TU-POP/GCE was slightly higher compared to GCE, which had no sufficient electrochemical response. Well-defined redox peaks of RT were shown by The Pd@TUPOP/GCE electrode at approximately 0.402 V (Epa) and 0.361 V (Epc), and the peak potential separation (∆Ep) was 41 mV. In addition, the redox peaks current was twice the value exhibited by bare GCE and TU-POP/GCE electrodes. The Pd@TU-POP-modified electrode GCE showed an excellent electrochemical response compared to the other modified electrode, which was attributed to the combined activities of the TU-POP and Pd. This is because the larger surface area of the Pd@TU-POP can increase the adsorbed amount of RT greatly and facilitate the accessibility of RT with the porous material surface. Initially, the RT was efficiently adsorbed and accumulated on the charged functional groups at the surface of Pd@TU-POP electrode, which provides opportunities for the enhanced electrochemical detection of RT. Finally, Pd NP maintained its efficient electrocatalytic response of RT on the Pd@TU-POP surface and the high electrical conductivity of Pd NP would favor the transfer of electrons. Therefore, it is expected that the Pd NPs (3.56%) loading on TU-POP electrode could exhibit an excellent electrocatalytic response towards RT.

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Figure 5. (a) CVs of the Pd@TU-POP/GCE in RT (10 µM) in PBS solution at various pH (pH = 1.0, 3.0, 5.0, 7.0, and 9.0; scan rate of 50 mV s-1. (b) Plot of anodic peak potential (Epa) and pH. (c) Influences of the volume of Pd@TU-POP suspension (10 mg mL-1), (d) Influences of the accumulation potential on the anodic peak current of 10 µM RT in 0.05 M PBS solution at the Pd@TU-POP/GCE.

Optimization of the experimental parameters Influence of scan rate

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To examine the reaction kinetics, the electrochemical performance of the Pd@TU-POP /GCE was measured in 0.05 M PBS with 1.0 × 10-5 M RT at the different scan rates (v) in the range of 10-100 mV s-1. According to Figure 4c, the anodic (Ipa) and cathodic (Ipc) peak current of RT is linearly proportional to the scan rate. The linear equations are found as Ipa (µA) = 0.2441 + 0.84 (mV s-1) (R2 = 0.9994) and Ipc (µA) = -0.2149-0.2533 (mV s-1) (R2 = 0.9996). The electrochemical oxidation of RT at the Pd@TU-POP/GCE were found to be adsorption-control processes (Figure 4d). On the other hand, the best results were obtained for the Pd@TU-POP electrode, which has a BET surface area of 118 m2g-1 and the largest microspore volume. The large number of electroactive sites and good electrical conductivity of Pd@TU-POP are very promising for the electrochemical detection of RT. Effect of pH The electrochemical detection of analytes and the response in relation to faradaic processes were crucially influenced by pH. We monitor the influences of various pH solutions on the electrochemical responses of RT at the Pd@TU-POP/GCE electrode with 10 µM RT in 0.05 M PBS. PBS is an effective supporting electrolyte in the range of pH = 1.0-9.0 (Figure 5a). The anodic (Ipa) peak current and the peak potential (Epa) of RT were obtained at the Pd@TUPOP/GCE electrode in different pH solutions containing 10 µM RT. The CV experiment was performed at a scan rate of 50 mV s-1 in the potential range of 0.0 to 0.7 V. The anodic peak current of RT increased from low pH to high pH and reached the maximum oxidation peak current at pH 5.0. Consequently, pH 5.0 was considered the optimal solution for the electrochemical oxidation of RT. The oxidation peak potential of RT was also investigated for other pH solutions (pH 1.0-9.0). A linear relationship was observed between the pH values and 18


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peak potentials with the calibration equation, Epa (V) = 0.0497-0.6475 pH, R2 = 0.9999 (Figure 5b). The oxidation peak potential showed a linear relationship with pH 1.0-9.0, clearly indicating that the electrocatalysis of RT at Pd@TU-POP electrode was pH dependent. The slope was determined from the regression equation as Epa (V)-49 mV/pH. This slope was closely related to the theoretical value of two-proton coupled two-electron transfer (-59 mV/pH at 25 °C).38 The TU-POP can protect the Pd NP and it’s allow stable electrochemical behavior in aqueous media. Optimization of Various Pd@TU-POP Suspensions The Pd@TU-POP level had an impact on the oxidation peak current of RT. Figure 5c shows the peak current changes with increasing Pd@TU-POP volume. The oxidation peak current of RT increased remarkably from 2 to 16 µL of the Pd@TU-POP suspensions. This clearly shows that the active surface area in the Pd@TU-POP promoted high current with large accumulation efficiency. However, the peak current showed slight change upon persistent increase in the cast amount. In this measurement, 8 µL of the Pd@TU-POP suspension was chosen for the analytical experiment. Effect of the accumulation potential and accumulation time The oxidation peak current of RT was monitored using Pd@TU-POP/GCE to understand the effects of the accumulation potential and time. The anodic current signal increased with each positive shift in the accumulation potential, which might help increase the loading of RT on the electrode surface, and subsequently improve the sensitivity with enhanced adsorption, indicating strong adsorption between RT and Pd@TU-POP /GCE, as shown in Figure 5d. RT oxidation at a large negative potential may damage the electrode surface, which may inhibit the electrode 19



performance. Therefore, an optimal potential of zero voltage was maintained in all experiments. Furthermore, an examination of the effects of the accumulation time of RT oxidation on Pd@TU-POP/GCE (Figure S7) suggested that the electrochemical oxidation Ipa of RT at Pd@TU-POP/GCE reached a plateau after 125 s, which can be attributed to the adsorption of RT reaching saturation. Hence, 125 s was selected for the optimal accumulation time. Interference study An examination of the anti-interference capability of a sensor is essential for practical applications. A selectivity study was performed by dipping the Pd@TU-POP/GCE electrode into a fixed amount of 10 nM RT mixed with common interfering ions, such as KCl, Na2CO3, fructose, and other species commonly existing (e.g., dopamine ascorbic acid, glucose, vanillin, and uric acid). In 10 nM RT containing solution, 500-fold higher concentrations of KCl, Na2CO3, and 100-fold higher concentrations of AA, Glu, 4-nitrophenol (NP), UA, hydroquinone (HQ), VA, 2,4-dinitrophenol, and DA had no significant effect on the peak current of RT (signal change below 5%), as shown in Figure S8. These results indicated that Pd@TU-POP/GCE has excellent anti-interference ability.

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Figure 6. (a) SWV response of Pd@TU-POP/GCE with different concentrations of RT in pH 5.0 PBS (0.05 M) buffer. (b) Calibration plots obtained for the anodic peak current versus RT concentration. (c) SWV response of Pd@TU-POP/GCE with different concentrations of RT in pH 5.0 PBS (0.05 M) + human plasma samples (1:100). (d) 200 consecutive voltammetry cycles for Pd@TU-POP/GCE with 10 nM RT in pH 5.0 PBS (0.05 M) buffer at a scan rate of 50 mVs-1, insert: TEM image of Pd@TU-POP after 200 cycles.

21



Analytical performance SWV was performed for the rapid and accurate level sensing of RT at the Pd@TU-POP electrode due to the low detection limit (LOD) owing to the very low non-Faradaic current. Figure 6a shows that the consecutive addition of RT into the pH 5 PBS solution parallelly increased the oxidation peak current of RT. As expected, the SWV results showed an increase in the anodic peak currents with increasing RT concentration from 2 to 200 pM, followed a linear relationship as: Ipa (µA) = 0.0081C + 0.046 with a correlation coefficient R2 = 0.9997 (Figure 6b). Furthermore, the Pd@TU-POP electrode showed outstanding sensitivity of 0.1157 µA pM-1 cm-2, low detection limit of 5.92 ×10-12 M (S/N=3). The Pd@TU-POP/GCE in prepared this study showed higher sensitivity, a lower detection limit, and wide linear range for the determination of RT, which is comparable to other previously reported analytical techniques listed in Table 1. Therefore, the excellent electrochemical performance of the Pd@TUPOP/GCE-modified electrode was ascribed to the fast electron transfer efficiency and large electroactive surface area. The Pd@TU-POP/GCE can be explained by its large SBET and micropore volume, providing a large number of electroactive sites that can adsorb more RT molecules. The interconnected network of porous Pd@TU-POP exposes a large surface towards the electrolyte solution at the interface, which enhances the RT oxidation activity. The Pd nanoparticles are grown in situ on TU-POP, which resulted in the combination of the porous material with Pd, facilitate electron transfer with ordered pore structure, large electroactive surface area, and excellent electrical conductivity for measuring the analytical parameters (linear range, detection limit, stability, and reproducibility) for RT determination.

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Table 1. Comparison of the analytical sensing performance of various electrodes towards RT detection.

Electrode

Method

Linear range (M)

LOD (M)

Ref.

Au-Ag NTs/NG

DPV

1.0×10-7 − 0.00042

1.5×10-8

29

PDDA-Gr/GCE

DPV

4.0×10-10 − 1.0×10-6

4.0×10-11

30

SH-β-CD-Gr/PdNPs

DPV

1.0×10-9 − 3.0×10-5

3.0×10-10

31

Cu2O-Au/NG/GCE

DPV

6.0×10-8 − 0.0005129

3.0×10-8

32

PSSA/CNTs/MBT/Au

DPV

1.0×10-8 − 8.0×10-7 and

1.80×10-3

39

8.0×10-7 − 1.0×10-5

CV

1.0×10-7 − 5.1×10-5

7.5×10-8

40

GO-Cs/GCE

DPV

9.0×10-7 − 9.0×10-5

5.6×10-7

41

GPH-AuNP/CSPE

SWV

0.10×10-6 − 15.0×10-6

1.10×10-8

42

PdPc-MWCNTsNafion/GCE

23



Page 24 of 35

NiCo2O4/rGO

DPV

1.0×10-7 − 0.00015

1.0×10-8

43

Pd-UOF-1/GCE

SWV

2.0×10-12 − 2.0×10-10

5.92×10-12

This work

Note:

PSSA/CNTs/MBT/Au-poly

mercaptobenzothiazole phthalocyanine;

and

(sulfosalicylic

multi-walled

GO-Cs-graphene

acid)

carbon

oxide/chitosan;

gold

nanotubes;

electrode,

2-

PdPc-palladium

GPH-AuNP-graphene

and

gold

nanoparticles; CSPE- carbon screen-printed electrodes; NTs-nanothorns; NG-N-doped graphene; PDDA-Gr-poly (diallyldimethylammonium chloride) (PDDA)-functionalized graphene;

SH-β-CD-Gr/PdNPs-thio-β-cyclodextrin functionalized

graphene/palladium

nanoparticles; DPV-differential pulse voltammetry;

Reproducibility, reusability, and stability The repeatability and stability of a fabricated sensor are key factors that influence the practical application. The reproducibility of Pd@TU-POP/GCE was examined by detecting the oxidation signal for five sequential mixed samples containing 10 nM RT in PBS buffer. The relative standard deviation (RSD) of the measurements was tested to be 2.14%. The repeatability was examined by measuring the 10 nM RT oxidation signal of five independent solutions. The RSD obtained was 2.73 for Pd@TU-POP/GCE, representing the suitable sensor performance. The stability of Pd@TU-POP/GCE was also tested at 3-day intervals over a period of five weeks. The 24


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peak current intensity of the electrode retained approximately 92.6% of its initial current signal, demonstrating the good stability of the proposed sensor. The electrochemical stability of Pd@TU-POP/GCE was demonstrated using CV for 20 CV cycles in 10 nM RT in PBS buffer. The electrode Pd@TU-POP/GCE showed a stability of 92% after 200 cycles (Figure 6d). TEM was performed to monitor the electrochemical oxidation of RT for 200 cycles. The TEM image of Pd@TU-POP/GCE revealed ∼5 nm particles after 200 cycles of RT oxidation. The above results show that the Pd@TU-POP/GCE have excellent stability during the oxidation of RT. The reusability of Pd@TU-POP/GCE surfaces was examined for electrochemical applications. The as-renewed Pd@TU-POP/GCE for the measurement of 10 nM RT could restore 85.4% of the initial value after four assay runs, showing high reusability (Figure S9). The reused Pd@TU-POP catalyst was subjected to XPS (Figure S10) analysis to measure the elemental composition percentages which was compared with the as prepared catalyst. The obtained XPS results indicated that the reused Pd@TU-POP exhibits a low peak intensity and less composition percentage of Pd 3d (0.7) and N1s (9.14) compared with the as prepared Pd@TU-POP (Figure S10), indicating that the Pd@TU-POP might be blocked by oxidized RT. The good stability and reproducibility of the Pd@TU-POP/GCE could result from the rich functional groups at the surface of Pd@TU-POP, which is helpful for the attachment of Pd@TU-POP onto the GCE electrode via electrostatic interactions. Furthermore, without any loss of activity or obvious leaching the Pd nanoparticles supported on TU-POP can be repeatedly used for four times with ease. Real sample analysis

25



To assess the real applications, the Pd@TU-POP/GCE electrode was used for the detection of RT in human plasma using the standard addition method (Figure 6c). The human blood plasma was diluted (1:100) with 0.05 M PBS (pH 5.0). The sample solution into the supporting electrolyte and examined. The anodic peak current increased linearly with increasing concentration of RT in the range, 2 to 120 (Figures S11); which is expressed as follows: Ipa (µA) = 0.0093C + 0.1179 (R² = 0.9996). Table S2 lists the results obtained for the human plasma samples. Acceptable recovery of 99.22 to 100.07 % was obtained, showing that Pd@TUPOP/GCE could be used for the detection of RT in clinical samples. Also, the effect of accumulation time on the detection of RT in human plasma samples which shows rapid increment in RT current with incubation time is provided as Figure S12. Moreover, the proposed technique was applied further to a beverage sample to verify the real applications, as shown in Table S2. Red wine was purchased from a supermarket (Incheon, South Korea). The red wine samples (1 ml) were dissolved with 100 mL of PBS buffer and ultrasonicated for 2 h at room temperature. Different amounts of RT were then added to the red wine samples to test the recoveries. The recovery of the spiked samples ranged from 92.5% and 108.0%, and all samples studied showed precision within a RSD between 1.86 % and 3.42%. The red wine samples were also examined by HPLC, and the contents of the RT samples were 5.2 and 9.46 nM (RSD: 2.93 and 3.2%, n = 3), respectively. The results are in good agreement with those obtained using the proposed method. The experimental results showed that Pd@TUPOP/GCE is effective and accurate for the detection of RT in commercial pharmaceutical samples.

26


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Conclusions Using a conventional method, large specific area Pd@TU-POP was fabricated with highly-dispersed Pd NPs anchored on a TU-POP support. The TU-POP porous skeleton not only works as a large specific surface area supporting material to anchor Pd, but also offers outstanding conductive channels to accelerate electron transfer. Moreover, the electrochemical determination of RT in real samples, such as human plasma and red wine, showed satisfactory recovery. The Pd@TU-POP showed a significant reduction in overpotential and the electrochemical sensing of RT was considerably more sensitive than that of the TU-POP and bare electrodes. Under optimal detection conditions, excellent electro catalytic performance of RT was obtained with an acceptable linear range of 2 to 200 pM and a sensitivity of 0.1157 µA pM-1 cm-2. The modified electrode showed a low detection limit of 5.92×10-12 M with an extraordinary stability and excellent anti-interference which may be due to the excellent conductivity of the Pd NPs assembled on the TU-POP micro channels to allow electrochemical detection of RT in the presence of interfering substances. Moreover, four times reusability of modified electrode was achieved without any distinct leaching or activity loss. Under the optimal conditions, TEM confirmed that the original network morphology of Pd@TU-POP had been maintained after continuous cycling and the reproducibility in the determination of RT was retained. This suggests that the fabricated Pd@TU-POP porous material is a promising electrode material for next-generation biomedical devices and is expected to have applications in various fields, such as catalysts, food industry, clinical, and energy storage.

27



Page 28 of 35

Acknowledgments This work was supported by the National Research Foundation grant funded by the Ministry of Science,

ICT

and

Future

Planning

of

Korea

(No.

2014R1A5A1009799

and

2017M2A2A6A01020938). Supporting Information Available: Description of Solid-state 13C NMR spectrum of TU-POP, additional TEM image of TU-POP, Interference study and the effect of accumulation time. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION †

These authors contributed equally to this work.

Corresponding Author Correspondence to Yun Suk Huh, Ji Ho Youk, and Young-Kyu Han. *E-mail: [email protected] (Y. S. Huh), [email protected] (J. H. Youk) and [email protected] (Y.-K. Han). Notes The authors declare no competing financial interest.

28


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Palladium Supported on an Amphiphilic TriazineâUrea-Functionalized

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