Palladium Supported on an Amphiphilic Triazine–Urea-Functionalized

May 23, 2018 - Metal nanoparticle-containing porous organic polymers have gained great interest in chemical and pharmaceutical applications owing to t...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 19554−19563

Palladium Supported on an Amphiphilic Triazine−UreaFunctionalized Porous Organic Polymer as a 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 2018.10:19554-19563. Downloaded from pubs.acs.org by ST FRANCIS XAVIER UNIV on 09/01/18. For personal use only.

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Department of Energy and Materials Engineering, Dongguk University, Seoul 04620, Republic of Korea Department of Applied Organic Materials Engineering and §Department of Biological Engineering, Biohybrid Systems Research Center (BSRC), Inha University, Incheon 22212, Republic of Korea

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S Supporting Information *

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-aminophenyl)urea with cyanuric chloride and palladium acetate. The porous structure and physicochemical properties of the 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 a low 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 a 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 materials 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,10 fluoride ion,11 2,4,6-trinitrophenol,12 and nitroaromatic and benzoquinone vapors,13−15 have been successfully detected by 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 nanoparticles (NPs), and the stability of metal NPs has been sustained because of © 2018 American Chemical Society

their excellent coordination ability with POPs. The decoration of Pd NPs both at the external surface of porous materials and 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 POP 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 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 Received: January 11, 2018 Accepted: May 23, 2018 Published: May 23, 2018 19554

DOI: 10.1021/acsami.8b00579 ACS Appl. Mater. Interfaces 2018, 10, 19554−19563

Research Article

ACS Applied Materials & Interfaces

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

Furthermore, urea linkages which act as binding sites for transition metals25 help to stabilize metal NPs in POPs through coordination interactions. The majority of POPs are hydrophobic in nature, which prevents their effective dispersion and results in poor contact between 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 (RT) (3,3′,4′,5,7-pentahydroxyflavone-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, antitumor, 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 and 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 lowcost 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.

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 the urea-functionalized POP improved the adsorption of RT at the electrode surface. The TU-POP 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 electrode material were attributed to the uniform decoration of Pd NPs over the POP support, as proven from the transmission electron microscopy (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.

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MATERIALS AND METHODS

Chemicals and Materials. 1,4-Phenylenediamine (>98%), urea (98%), cyanuric chloride (98%), sodium bisulfite, acetic acid, and N,Ndiisopropylethylamine (99%) were purchased from TCI-Japan. All other reagents were bought from Merck and used without further purification. The phosphate-buffered saline (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. 19555

DOI: 10.1021/acsami.8b00579 ACS Appl. Mater. Interfaces 2018, 10, 19554−19563

Research Article

ACS Applied Materials & Interfaces

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), and pore size distribution curve of TU-POP and Pd@TU-POP. Instruments and Characterization. The morphology of Pd@ TU-POP was acquired using 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/ max-2500). Solid-state 13C 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 an Autolab PGSTAT302F system (Eco Chemie, The Netherlands) with glassy carbon electrode (GCE), Ag/AgCl, and Pt as the working, reference, and counter electrodes, respectively. Synthesis of TU-POPs. Urea-based amphiphilic POP (TU-POP) was synthesized as follows (Figure 1): Step-1: Synthesis of 1,3-bis(4-aminophenyl)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 °C. The final product was obtained after filtration and repeated washing with copious amount of hot water, which removed the residual urea, followed by drying in a vacuum oven. Step-2: Synthesis of TU-POPs. 1,3-Bis(4-aminophenyl)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 °C 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. A solid was formed by the addition of ice to the crude product, which was then filtered and undergone sequential washing with dichloromethane, tetrahydrofuran, and ethyl acetate to remove the oligomers. The obtained product was confirmed by FT-IR and solid-state 13C NMR spectroscopy. Synthesis of Pd@TU-POP. The TU-POP (1 g) was mixed with a 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 portionwise, and stirring was continued for another 18 h. The resulting product was isolated through filtration, washed sequentially with water, ethyl acetate, dichloromethane, and tetrahydrofuran, and then dried for 12 h at 80 °C under vacuum to obtain Pd@TU-POP (1 g). Fabrication of Pd@TU-POP-Based Electrochemical Sensors. Initially, the GCE (3 mm diameter) was cleaned using 0.5 μm alumina, 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 followed by ultrasonication 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 was prepared using a similar procedure. Finally, the Pd@TU-POP/GCE obtained was used to detect RT in human plasma and red wine samples.

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RESULTS AND DISCUSSION Structural and Morphological Characterization of the Pd@TU-POP Material. Following the synthetic route outlined in Figure 1a, the TU-POP was synthesized by treating 1,4phenylenediamine with urea followed by cyanuric chloride in the presence of N,N-diisopropylethylamine. The solubility of TU-POP in water and organic solvents is very poor. Therefore, it was treated with Pd(OAc)2 in a tetrahydrofuran and ethanol mixture to produce Pd(II)@TU-POP. With the help of sodium borohydride, 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, powder X-ray diffraction (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 Figures 2a and S1. The FT-IR spectrum of the 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 materials. The solid-state 13 C NMR spectrum (Figure S2) revealed that the peak at δ = 19556

DOI: 10.1021/acsami.8b00579 ACS Appl. Mater. Interfaces 2018, 10, 19554−19563

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ACS Applied Materials & Interfaces

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) electrochemical impedance spectroscopy (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).

hysteresis loop pertaining to the capillary condensation and the existence of mesopores in the material and followed type-IV isotherm (Figure 2c,d). The XPS spectra 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 five 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 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 Pd@TU-POP compared to the TU-POP material suggested that 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 interaction of Pd NPs, which are embedded on the surface of the porous material, and the oxygen atom of the urea units (Figure 3e). The Pd 3d spectra reveal 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. Electrochemical Behaviors of Pd@TU-POP. EIS and cyclic voltammetry (CV) are useful techniques for examining the changes in the electrochemical interfacial behaviors of the

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 presents the TEM images of Pd@TU-POP showing that the porous polymer (TU-POP) on the external surface is embedded with Pd NPs because of the coordination ability of the urea, which contains carbonyl group and the 1,3,5triazine backbone. Moreover, the composition of Pd@TU-POP was examined using EDS elemental mapping, which confirms the distribution of the Pd NPs on the TU-POP surface (Figure S3). The TEM−EDX pattern confirmed that C, N, O, and Pd contents exist in Pd@TU-POP. A TEM image of the TU-POP is provided in Figure S4 in the Supporting Information. Because of the presence of the trapped guest molecules, TUPOP elemental analysis resulted in slightly lower experimental values of carbon and nitrogen than the corresponding theoretical values, which is common in porous materials. TGA (Figure S5) showed the presence of guest molecules; initial losses of 3% up to 100 °C were observed, and the material was stable up to 210 °C. XRD was used to examine the crystalline structures of the cyanuric chloride and in-house prepared materials such as 1,3bis(4-aminophenyl)urea, TU-POP, and Pd@TU-POP, and the obtained patterns are shown in Figures 2b and S6. The wideangle PXRD pattern of Pd@TU-POP displayed characteristic peaks at values (2θ) of 39°, 44.5°, and 65.9° corresponding to the reflections from the (111), (200), and (220) planes, respectively, 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− Emmett−Teller (BET) surface area of TU-POP was 145 m2 g−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 19557

DOI: 10.1021/acsami.8b00579 ACS Appl. Mater. Interfaces 2018, 10, 19554−19563

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ACS Applied Materials & Interfaces

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 and (b) pH = 5.0 PBS (0.05 M) containing 10 μM RT. Scan rate: 50 mV s−1. (c) CV curves of Pd@TU-POP/GCE in pH = 5.0 PBS (0.05 M) solution containing 10 μM RT with various scan rates (10−100 mV s−1) and (d) calibration plot of anodic and cathodic current peaks vs scan rate.

POP. The addition of electrocatalytically active Pd NPs 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 ΔEp of 0.106 V). Therefore, the faradaic processes assisted by the electrode surface modification of the Pd@TU-POP were confirmed. The electrochemical performance of the synthesized Pd@ TU-POP-modified electrode was investigated by CV to evaluate its sensing ability to RT. Figure 4b presents the 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, TU-POP/GCE, and Pd@TUPOP/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 the TU-POP/GCE was slightly higher compared to the GCE, which had no sufficient electrochemical response. Well-defined redox peaks of RT were shown by the Pd@TU-POP/GCE electrode at approximately 0.402 V (Epa) and 0.361 V (Epc), and the peak potential separation (ΔEp) was 41 mV. In addition, the redox peak current was twice the value exhibited by the bare GCE and TU-POP/GCE electrodes. The Pd@TU-POP-modified GCE showed an excellent electrochemical response compared to the other modified electrode, which was attributed to the combined activities of TU-POP and Pd. This is because the larger surface area of 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 the Pd@TU-POP electrode, which provides opportunities for the enhanced electrochemical detection of RT. Finally, Pd NPs maintained

surface modification of the stepwise construction path of Pd@ TU-POP. Electrochemical characterization of the bare electrode, TU-POP-, and Pd@TU-POP/GCE modified electrodes was 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 in the high-frequency region was assigned to the charge-transfercontrolled process and the linear segment of the low-frequency region corresponded to the diffusional step of the electrochemical process. The experimental impedance data were obtained by fitting the Randles equivalent circuit (inset of Figure 3f), which consisted of charge-transfer resistance (Rct), solution resistance (Rs), Warburg impedance (Zw), and doublelayer 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 because of the partial blocking of interfacial charge transfer to the electrode surface. In contrast, when the GCE was modified with Pd@TU-POP, the semicircle diameter of the electrochemical sensor was decreased significantly (174 Ω), suggesting that the effective surface area of unique porousarchitecture 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 the 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/GCEmodified 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 ΔEp value of 0.20 V, which reveals a quasi-reversible process. In contrast, a pair of redox peaks was observed for the TU-POP/GCE with a slightly higher current (ΔEp) of 0.281V with respect to bare GCE, which may be due to the poor electrical conductivity of TU19558

DOI: 10.1021/acsami.8b00579 ACS Appl. Mater. Interfaces 2018, 10, 19554−19563

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ACS Applied Materials & Interfaces

Figure 5. (a) CV curves of Pd@TU-POP/GCE in RT (10 μM) in PBS solution at various pHs (pH = 1.0, 3.0, 5.0, 7.0, and 9.0); scan rate 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) and (d) influences of the accumulation potential on the anodic peak current of 10 μM RT in 0.05 M PBS solution at Pd@TU-POP/GCE.

investigated for other pH solutions (pH 1.0−9.0). A linear relationship was observed between pH values and peak potentials with the calibration equation Epa (V) = 0.0497 − 0.6475pH, with 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 the 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 NPs, and it allows 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 a 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 the 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 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 the Pd@TU-POP/GCE (Figure S7)

their efficient electrocatalytic response of RT on the Pd@TUPOP surface, and the high electrical conductivity of Pd NPs would favor the transfer of electrons. Therefore, it is expected that the Pd NPs (3.56%) loaded on the TU-POP electrode could exhibit an excellent electrocatalytic response toward RT. Optimization of the Experimental Parameters. Influence of Scan Rate. 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 currents of RT are linearly proportional to the scan rate. The linear equations are found to be 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 was found to be an adsorption-controlled process (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 m2 g−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 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@TU-POP/GCE 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 to be the optimal solution for the electrochemical oxidation of RT. The oxidation peak potential of RT was also 19559

DOI: 10.1021/acsami.8b00579 ACS Appl. Mater. Interfaces 2018, 10, 19554−19563

Research Article

ACS Applied Materials & Interfaces

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 vs 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) Two hundred 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 mV s−1 (inset: TEM image of Pd@TU-POP after 200 cycles).

Table 1. Comparison of the Analytical Sensing Performance of Various Electrodes toward RT Detectiona electrode Au−Ag NTs/NG PDDA-Gr/GCE SH-β-CD-Gr/PdNPs Cu2O−Au/NG/GCE PSSA/CNTs/MBT/Au PdPc-MWCNTs-Nafion/GCE GO-Cs/GCE GPH-AuNP/CSPE NiCo2O4/rGO Pd-UOF-1/GCE

method DPV DPV DPV DPV DPV CV DPV SWV DPV SWV

linear range (M) −7

1.0 × 10 −0.00042 4.0 × 10−10−1.0 × 10−6 1.0 × 10−9−3.0 × 10−5 6.0 × 10−8−0.0005129 1.0 × 10−8−8.0 × 10−7 and 8.0 × 10−7−1.0 × 10−5 1.0 × 10−7−5.1 × 10−5 9.0 × 10−7−9.0 × 10−5 0.10 × 10−6−15.0 × 10−6 1.0 × 10−7−0.00015 2.0 × 10−12−2.0 × 10−10

LOD (M) −8

1.5 × 10 4.0 × 10−11 3.0 × 10−10 3.0 × 10−8 1.80 × 10−3 7.5 × 10−8 5.6 × 10−7 1.10 × 10−8 1.0 × 10−8 5.92 × 10−12

refs 29 30 31 32 39 40 41 42 43 this work

a

PSSA/CNTs/MBT/Au: poly (sulfosalicylic acid) gold electrode, 2-mercaptobenzothiazole, and multiwalled carbon nanotubes; PdPc: palladium phthalocyanine; GO-Cs: graphene oxide/chitosan; GPH-AuNP: graphene and gold nanoparticle; CSPE: carbon screen-printed electrode; NTsnanothorns; NG: N-doped graphene; PDDA-Gr: poly(diallyldimethylammonium chloride)-functionalized graphene; SH-β-CD-Gr/PdNPs: thio-βcyclodextrin-functionalized graphene/palladium nanoparticles; and DPV: differential pulse voltammetry.

Analytical Performance. Square-wave voltammetry (SWV) was performed for the rapid and accurate level sensing of RT at the Pd@TU-POP electrode due to the low detection limit [limit of detection (LOD)] owing to the very low nonfaradaic 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@TUPOP electrode showed an outstanding sensitivity of 0.1157 μA pM−1 cm−2 and a low detection limit of 5.92 × 10−12 M (S/N = 3). The Pd@TU-POP/GCE prepared in this study showed a higher sensitivity, a lower detection limit, and a 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

suggested that the electrochemical oxidation Ipa of RT at the 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@TUPOP/GCE 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 ascorbic acid (AA), glucose (Glu), 4-nitrophenol, uric acid (UA), hydroquinone, vanillin (VA), 2,4-dinitrophenol, and dopamine (DA) had no significant effect on the peak current of RT (signal change below 5%), as shown in Figure S8. These results indicated that the Pd@TU-POP/GCE has excellent anti-interference ability. 19560

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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 99.22−100.07% was obtained, showing that the Pd@TU-POP/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 a rapid increment in RT current with incubation time, is provided in 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). The results are in good agreement with those obtained using the proposed method. The experimental results showed that the Pd@TUPOP/GCE is effective and accurate for the detection of RT in commercial pharmaceutical samples.

Pd@TU-POP/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 toward the electrolyte solution at the interface, which enhances the RT oxidation activity. The Pd NPs are grown in situ on TU-POP, which resulted in the combination of the porous material with Pd, facilitating 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. 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 the Pd@TU-POP/GCE, representing the suitable sensor performance. The stability of the Pd@TU-POP/GCE was also tested at 3 day intervals over a period of 5 weeks. The 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 the Pd@TUPOP/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 the Pd@TU-POP/GCE revealed ∼5 nm particles after 200 cycles of RT oxidation. The above results show that the Pd@TU-POP/GCE has excellent stability during the oxidation of RT. The reusability of Pd@TUPOP/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 were compared with those of the asprepared 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 N 1s (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@TUPOP/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 via electrostatic interactions. Furthermore, without any loss of activity or obvious leaching, the Pd NPs supported on TU-POP can be repeatedly used four times with ease. Real Sample Analysis. To assess the real applications, the Pd@TU-POP/GCE 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 real sample was added into the supporting electrolyte and examined. The anodic peak current increased linearly with increasing concentration of RT in the range 2−120 (Figure

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CONCLUSIONS Using a conventional method, a large-specific area Pd@TUPOP 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 a satisfactory recovery. The Pd@TU-POP showed a significant reduction in overpotential, and the electrochemical sensing of RT was considerably more sensitive than those of the TU-POP and bare electrodes. The electrode showed excellent electrocatalytic performance for RT with an acceptable linear range of 2−200 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 microchannels to allow electrochemical detection of RT in the presence of interfering substances. Moreover, four times reusability of the 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@TUPOP porous material is a promising electrode material for nextgeneration biomedical devices and is expected to have applications in various fields, such as catalysts, food industry, clinical applications, and energy storage.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b00579. 19561

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Description of solid-state 13C NMR spectrum of TUPOP, additional TEM image of TU-POP, interference study, and the effect of accumulation time (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.S.H.). *E-mail: [email protected] (J.H.Y.). *E-mail: [email protected] (Y.-K.H.). ORCID

Yun Suk Huh: 0000-0003-1612-4473 Young-Kyu Han: 0000-0003-3274-9545 Author Contributions ∥

A.T.E.V. and R.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation grant funded by the Ministry of Science, ICT and Future Planning of Korea (nos. 2017M2A2A6A01020938 and 2018R1A2B2006094) and the R&D Convergence Program of the National Research Council of Science and Technology (CAP-15-02-KBSI).

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