Surface Engineering of Phenylboronic Acid-Functionalized Poly(3,4

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Surface Engineering of Phenylboronic AcidFunctionalized Poly(3,4-ethylenedioxythiophene) for Fast Responsive and Sensitive Glucose Monitoring Po-Chun Huang, Mo-Yuan Shen, Hsiao-hua Yu, Shu-Chen Wei, and Shyh-Chyang Luo ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00060 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 17, 2018

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ACS Applied Bio Materials

Surface Engineering of Phenylboronic Acid-Functionalized Poly(3,4-ethylenedioxythiophene) for Fast Responsive and Sensitive Glucose Monitoring Po-Chun Huang, † Mo-Yuan Shen, ‡ Hsiao-hua Yu, ‡ Shu-Chen Wei,§ and Shyh-Chyang Luo†,*

†

Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4,

Roosevelt Road, Taipei 10617, Taiwan

‡

Smart Organic Material Laboratory, Institute of Chemistry, Academia Sinica, Nankang, Taipei,

Taiwan

§

Department of Internal Medicine, National Taiwan University Hospital and College of Medicine,

No.1 Jen Ai Road, Section 1, Taipei 10051, Taiwan

KEYWORDS: conducting polymer, glucose sensor, quartz crystal microbalance (QCM), phenylboronic acid, nanostructure, surface engineering

ABSTRACT

1 ACS Paragon Plus Environment

ACS Applied Bio Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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In this study, we have successful demonstrated a nanostructured phenylboronic acid-grafted poly(3,4-ethylenedioxythiophene), poly(EDOT-PBA), platform for fast and sensitive glucose monitoring. The poly(EDOT-PBA) films of well-organized tubular nanostructures can be fabricated by direct electropolymerization without templates. Compared to the smooth poly(EDOT-PBA), the nanotubular poly(EDOT-PBA) shows enhanced glucose sensitivity and different adsorption process of bovine serum albumin (BSA). Besides, the BSA blocking and low concentration fructose and galactose do not affect the sensitivity of this platform. Both quartz crystal microbalance (QCM) and electrochemical impedance spectroscopy (EIS) methods are used and compared for glucose monitoring by applying nanotubular poly(EDOT-PBA) as conductive substrates. Compared to QCM analysis, EIS has higher sensitivity to glucose and the detection limit is about 50 µM. Besides, the binding with glucose on poly(EDOT-PBA) is highly reversibly. Based on these observations, the nanotubular poly(EDOT-PBA) has great potential for enzyme-free electrodes targeting continues glucose monitoring applications.

INTRODUCTION

Glucose is an important energy source for maintaining daily life and function of human bodies. However, people who lose functions for glucose metabolism and regulation, generally suffer from diabetes mellitus and uncontrolled blood glucose concentration, which leads to serious health


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problems, such as strokes, heart attacks, high blood pressure, and even blindness or death.1-2 Based on the clinic trials, the normal level of blood glucose is 3.3-6.1 mM and the diabetes patients usually have higher blood glucose concentration.3 The diabetes patients need continuous measure and regulation of the blood glucose concentration. Most current commercial available glucose sensors are disposable enzyme-based sensors, which were first invented by Leland C. Clark in 19544. Electrodes immobilized with glucose oxidase (GOx) can oxidize glucose and produce hydrogen peroxide, which leads to indirectly monitoring of glucose level by the redox reaction of water and hydrogen peroxide. The enzyme-based glucose sensors provide high sensitivity for glucose detection. However, it might be instable to be become invalid mainly because GOx lose their activities caused by temperature or pH change.5 In contrast to bio-receptors such as GOx, the chemical receptors are more durable and can be stored for a long period more easily. Because the continuous glucose monitoring is critical for better controlling the blood glucose through diabetes therapy, new electrode materials and sensing designs are highly needed to meet the requirements of point-of-care purposes and continuous glucose monitoring.6-7

Conducting polymers (CPs) have been successfully demonstrated as versatile biomaterials used for various biomedical applications, such as implanted electrodes,8-10 biosensor,11-13 controlled release,14-15 biointerfaces,16-18 etc. Compared to traditional metallic materials or semiconductors, the fabrication of conducting polymers and their nanostructures can be simply achieved by chemical or electrochemical polymerization.19 Poly(3,4-ethylenedioxythiophene) (PEDOT) show



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superior ionic and electronic transporting properties20, which have been used for various bioelectronics application.10, 21-22 In recent years, it has been getting popular to use functionalized PEDOT to mimic biological membranes, which allows the biorecognition events found in biological systems, such as controlled capture and release of circulating tumor cells by Shen et al.23 as well as the large enhancement of neurite outgrowth by Zhu et al.24 In this study, we developed an enzyme-free platform for monitoring glucose by using a phenylboronic acid (PBA)-grafted PEDOT film. Boronic acid can reversibly bind with cis-1,2-diols or cis-1,3-diols to form stable cyclic boronate ester in the aqueous phase.25 Besides, in an alkaline environment, boronic acids convert to boronates, which improves their affinity to diols. Therefore, several recent studies demonstrated the idea to use grafted boronic acid groups as capture probes as biosensors to detect glucose level using various techniques, including electrochemical impedance spectroscopy (EIS),5 surface plasmon resonance (SPR),26 field-effect transistor (FET),27-28 and quartz crystal microbalance (QCM)29. Caroline et al.29 applied surface RAFT polymerization to graft poly(3-methacrylamido phenylboronic acid) brushes on the surface of QCM sensor for glucose sensing. It showed nice linear response over human’s glucose level with minimum interferance by fructose. Alex et al.25 modified gold surface with bis-boronic acids derivative and tri(ethylene glycol)-terminated thiol by self-assembled monolayer to make electrodes used for glucose electrochemical sensors. The results showed good sensitivity (from 0.01 to 10.0 µM). Kajisa et al.27 used a FET mordified with PBA funtionalized hydrogels to fabricate a highly


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sensitive and biocompatible glucose sensor. Phenylboronic acid has also been applied for fluorescence sensor as reported by Wu et al.30 The sensor used stilbeneboronic acidγ-cyclodextrin complex as the capture probes for glucose which also showed good sensitivity and selectivity. Based on these studies, PBA has long-term stability in aqueous buffers, which is critical for continue glucose monitoring.

In this study, we first synthesized PBA-functionalized EDOT (EDOT-PBA) monomers. By applying electropolymerization at three different solution systems, including acetonitrile, ionic liquids and dichloromethane, we fabricated poly(EDOT-PBA) films of both smooth and tubular surface structures of as shown in Scheme 1. We used QCM to investigate the interaction between glucose and PBA groups on PEDOT films at two pH buffers. We further evaluated the binding of bovine serum albumin (BSA) on poly(EDOT-PBA) films and examined the glucose detection when the films are under the BSA blocking and interference of fructose and galactose. The EIS was applied to demonstrate an electrochemical glucose sensor using poly(EDOT-PBA) films as conducting substrates. The results showed both high sensitivity to glucose concentration and reversible binding to glucose from nanotubular poly(EDOT-PBA) films, which indicates a potential application for continuous monitoring of glucose in bloods.



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BSA

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EDOT-PBA

acetonitrile [emim][triflate]

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Scheme 1. To engineering nanostructured poly(EDOT-PBA) films for reversible binding with glucose under BSA blocking.

MATERIALS AND METHODS

Martials D-glucose, D-fructose, phosphate buffered saline (PBS) powder, galactose, sodium hydroxide, potassium nitrate, dichloromethane (DCM, 99.8%), bovine serum albumin (BSA, 96%), hydrogen peroxide (30%). were purchased from Sigma-Aldrich. Sulfuric acid (97%) was purchased from showa chemical company. Tetrabutylammonium perchlorate (TBAP, 99%) Acetonitrile (99%), potassium hexacyanoferrate(Ⅱ) trihydrate (98%) was purchased from Alfa Aesar. Potassium chloride (KCl, 99%) was purchased from fisher scientific. Potassium ferricyanide (98%) was purchased from Acros Organics. 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate ([emim][triflate]) was purchased from Uni-region Bio-Tech Company.


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All the chemicals were used without further purification. The phenylboronic acid-functionalized 3,4-ethylenedioxythiophene (EDOT-PBA) was synthesized following procedures recently reported23. The procedure is briefly introduced here. EDOT-OH (861 mg), NaH (240 mg), and NaI (150 mg) were loaded in a round-bottom flask filled with Ar. THF (20ml) was then slowly added and the reaction mixture was stirred for 15 min in an ice bath. Methyl bromoacetate (0.57 mL) was injected into the flask and the mixture was stirred for 18 hr. The crude product that was extracted from THF solutions by a rotary evaporator and then purified by silica gel column (hexane/ethyl acetate = 5:1) to produce the EDOT-COOH as synthetic precursors of EDOT-PBA. After dissolving EDOT-COOH (1.00 g) and 3-aminophenylboronic acid hydrate (0.59 g) into a 20 mL of THF and 20 mL of DCM mixed solution at room temperature, EDC-HCl (1.25 g) was added into the flask with stirring at room temperature for 12 hr. The crude product was purified 1

by column chromatography (DCM /ethanol = 15:1) to receive EDOT-PBA. H-NMR (400 MHz, DMSO-d6, 25 °C): δ 7.82 (s, 1H), 7.73 (d, 1H, J = 7.8 Hz), 7.52 (d, 1H, J = 7.5 Hz), 7.27 (dd, 1H, J = 7.5, 7.8 Hz), 6.60 (m, 2H), 4.44 (m, 1H), 4.33 (m, 1H,), 4.12-4.03 (m, 3H), 3.78 (d, 2H, J = 5.1 Hz), 3.63 (s, 1H). All chemical used for the synthesis of EDOT-PBA were purchased form Sigma-Aldrich and used without further purification, including 3-aminophenylboronic acid hydrate (98%), N’-ethylcarbodiimide hydrochloride (EDC-HCl, 99%), hydroxymethyl EDOT (EDOT-OH, 95%), NaH (60% suspension in mineral oil), NaI (99%), tetrahydrofuran (THF; 99%), and methyl bromoacetate.



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Synthesis of Poly(EDOT-PBA) Films The poly(EDOT-PBA) thin films were prepared by electropolymerization, which was carried out with an Autolab PGSTAT128N potentiostat in a typical three-electrode glass cell at 15 0C. The work electrodes were Au chips used for QCM studies and platinum wire was the counter electrode. Ag/AgCl and Ag/Ag+ were used as reference electrodes in aqueous and organic solutions, respectively. The poly(EDOT-PBA) thin films were prepared in three different kinds of monomer solutions and procedures for comparison: 1. 10 mM EDOT-PBA in acetonitrile contained 100 mM TBAP as electrolytes. A cyclic potential was applied form -0.6V to 1.4V (vs. Ag/Ag+) at a scan rate of 100 mV/s for three cycles; 2. EDOT-PBA dissolved in [emim][triflate] to make a 50 mM monomer solution without adding electrolytes. A cyclic potential form -0.6 V to 1.5 V (vs. Ag/AgCl) at scan rate of 50 mV/s for one cycle; 3. 10 mM EDOT-PBA in DCM contained 100 mM TBAP as electrolytes. A constant voltage at 1.5 V (vs. Ag/Ag+) was applied for 10 s for polymerization.

Surface Characterization of Poly(EDOT-PBA) Films Scanning electron microscopy (SEM) were conducted by a scanning electron microscope (Jeol JSM-7800F, at accelerating voltage of 10 keV) to investigate surface morphology of poly(EDOT-PBA) films. A thin Pt layer (~5 nm) was coated on polymer films by a sputter coater (Cressington, 108 Auto) prior to SEM experiments. Water contact angle measurement was conducted by using a contact angle meter (Model 100 SB, Sindatek) to determine the static contact angle of samples, which is the angle between the air/DI water interface and the DI water/polymer interface. Each sample was


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measured by three times to receive average values and standard deviations. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface compositions of poly(EDOT-PBA) films. The experiment was conducted by using a XPS (VG scientific ESCALAB 250) system with a monochromatic Al Kα (1486.6 eV) source under 10-8 torr and a spherical sector analyzer with multi-channeltron array. The topography and roughness were determined by an atomic force microscopy (AFM, Bioscope resolve, Bruker). The measurement is done with a silicon probe (Scanasyst-air, Bruker) under PeakForce Tapping mode at a scan rate of 0.5 Hz.

Quartz Crystal Microbalance (QCM) Measurements The adsorption process between biomolecules and polymer films was investigated by a QCM system (QCM-D E1, Biolin scientific) at 25 oC in this study. The flow rates were set to 7.5 µL/min with a tubing pump (Ismatec ISM 829, Germany). Au sensors coated with poly (EDOT-PBA) films were first installed in the chamber. The system reached an equilibrium with 1X PBS buffer solutions before measurements. For glucose detection, the mobile phase was changed to the glucose solutions of various concentrations ranginged from 0.1 mM to 50 mM. When the adsorption of glucose reached an equilibrium, the flow changed to pure PBS buffer solutions for rinsing.

The

frequency shifts were recorded by Qsoft401 software to illustrate the adsorption of glucose on the surface of polymer films. The pH value of PBS buffer solutions was adjusted from 7.4 to 9.0 by using a 0.1 M KNO3 and 0.1 M NaOH mixed solutions. The Au sensors were cleaned with a piranha solution consisting of 3:1 mixture of sulfuric acid with hydrogen peroxide.



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Electrochemistry Impedance Spectroscopy (EIS) Measurements EIS was performed on an Autolab PGSTAT128N potentiostat with a FRA32M module. The three-electrode setup was made of a Au electrode coated poly(EDOT-PBA) as working electrode, a Ag/AgCl as reference electrode and a platinum wire as counter electrode in [Fe(CN)6]3-/4- solution. [Fe(CN)6]3-/4solution contain 5 mM K3Fe(CN)6, 5 mM K4Fe(CN)6 and 0.125 M KCl in deionized water. The EIS spectra were recorded at frequencies ranging from 10−1 to 105 Hz at 0.2 V vs. Ag/AgCl.

RESULTS AND DISCUISSION

Surface Properties of Poly(EDOT-PBA) Films The surface morphologies of poly(EDOT-PBA) films were investigated by SEM, and the water contact angle measurement was carried out by a contact angle meter, as shown in Figure 1. The surface morphology was highly dependent on the solutions used for electropolymerization. In Figure 1(a), the film whose surface was made up of many tubular nanostructures was prepared in DCM in the presence of TBAP. Compared to these tubular-type structure prepared in DCM, the poly(EDOT-PBA) film prepared in acetonitrile and in an ionic liquid, [emim][triflate], were much smoother, as shown in Figure 1(b) and 1(c), respectively. Figure 1(d) showed the results of water contact angle on polymer films prepared in DCM, acetonitrile, and [emim][triflate] solutions. The contact angle on the films prepared in


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DCM solutions was 35.760 ± 2.950, which is lower than those prepared in acetonitrile (45.030 ± 3.400) and [emim][triflate] (50.300 ± 1.200). This indicates that the film prepared in DCM solutions is more hydrophilic mainly because of the tubular nanostructures. The XPS from three poly(EDOT-PBA) films also showed clear B and N elements in addition to the C, O and S as shown in Figure S1 and their atomic percentage were shown in Table S1. We also used AFM to observe the surface morphology of poly(EDOT-PBA) films as shown in Figure S2. The calculated root-mean-square roughness of poly(EDOT-PBA) films prepared in DCM, acetonitrile, and [emim][triflate] solutions were 482.0 nm, 5.3 nm, and 20.2 nm, respectively. The thicknesses of the polymer film prepared in DCM, acetonitrile and [emim][triflate] were roughly 1.00 µm, 100 nm and 150 nm, respectively.

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Figure 1. SEM images of poly(EDOT-PBA) polymer films prepared in (a) DCM, (b) acetonitrile, and (c) [emim][triflate]. The scale bar is 1 µm and 300 nm in figures and the inset figures, respectively. (d) Water contact angle results.



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The BSA Adsorption on Poly(EDOT-PBA) Films We first examined the BSA adsorption on our poly(EDOT-PBA) as shown in Figure 2. BSA has been widely used as blocking agents to prevent non-specific adsorption of proteins. Besides, researchers also used BSA adsorption to evaluate the non-fouling properties from the surfaces. As shown in Figure 2, the adsorption of BSA is strongly dependent on the preparation methods of poly(EDOT-PBA), which indicates the surface morphology is critical to the amount of adsorption. Compared to the smooth poly(EDOT-PBA) films prepared in acetonitrile and ionic liquids as shown in Figure 2(a), the poly(EDOT-PBA) of tubular nanostructures showed more than three times adsorption of BSA as shown in Figure 2(b). We also evaluated the dissipation change during the adsorption of BSA. For both smooth poly(EDOT-PBA) films prepared in acetonitrile and [emim][triflate] solutions, the dissipation increased after the adsorption of BSA. These results indicate the adsorbed BSA promoted the energy dissipation during the oscillation of Au sensors, which is a common observation during the process of protein adsorption monitored by a QCM-D system. Interestingly, during the adsorption of BSA on the poly(EDOT-PBA) of tubular nanostructures prepared in DCM solutions, the dissipation dropped. This results indicates the binding of BSA made the poly(EDOT-PBA) films more rigid. This observation was highly reproducible. Although the mechanism is not so clear yet, it might be mainly due to the filling up of vacancies on nanostructured poly(EDOT-PBA) films or the release of water molecules attached in tubular structures during the binding of BSA.


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Figure 2. Frequency and dissipation readouts from QCM-D showing BSA binding on (a) smooth poly(EDOT-PBA) films prepared in acetonitrile (dot line) and [emim][triflate] (dash line) compared to (b) nanotubular poly(EDOT-PBA) films prepared in DCM solutions.

The Adsorption of Glucose on Poly(EDOT-PBA) Figure 3 illustrated the adsorption of glucose on Au sensors modified with three poly(EDOT-PBA) coating at pH = 7.0 compared to pH = 9.0. According to previous studies, the boronic acid groups convert into boronate anion groups at pH = 9.0, which have stronger binding affinity to diols in an alkaline environment. As shown in Figure 3(a), the shifts of the frequency decreased during the glucose binding were proportional to the glucose concentrations. Besides, the adsorption of glucose on poly(EDOT-PBA) can usually be released after rinsing with buffer. At pH = 7.0, compared to the glucose adsorption on poly(EDOT-PBA) prepared in [emim][triflate] and acetonitrile, the adsorption of glucose on



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nanostructured poly(EDOT-PBA) was about four times higher. This results indicates the adsorption of glucose on poly(EDOT-PBA) can be greatly enhanced owing to the surface morphology and nanostructures. Compared to the adsorption of glucose at pH = 7.0, the adsorption of glucose at pH = 9.0 was increased by 10% to 20% depending on the surfaces. This indicates the interaction between poly(EDOT-PBA) and glucose were stronger at pH = 9.0 because of the formation of boronate anion. Besides, the release of bound glucose from poly(EDOT-PBA) surfaces was much slower at pH = 9.0 compared to pH = 7.0, which also indicates a stronger interaction owing to the formation of boronate anion. For the adsorption of 50 mM glucose at pH = 9.0, the adsorption was even hardly releases entirely from the nanotubular poly(EDOT-PBA) films. The data of glucose adsorption between at pH = 7.0 and pH = 9.0 was summarized at Figure 3(c) and (d). Although the increase of pH value from 7.0 to 9.0 can enhance the glucose adsorption on poly(EDOT-PBA) films, the nanostructure effect played a dominating role on the glucose adsorption. Since the results show nanotubular poly(EDOT-PBA) films provide the best sensitivity to glucose detection, only poly(EDOT-PBA) films prepared in DCM were used for further studies.


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Figure 3. The monitoring of glucose adsorption on Au sensors coated with poly(EDOT-PBA) films prepare in DCM (black), acetonitrile (red) and [emim][triflate] (blue) at (a) pH = 7.0 and (b) pH = 9.0. The sensor response of each poly(EDOT-PBA) film as function of glucose concentration at (c) pH = 7.0 and (d) pH = 9.0. The concentrations of glucose were 1 mM, 5 mM, 10 mM, 25 mM and 50 mM.

The Glucose Detection in the Presence of Interferences We evaluated the interference of fructose and galactose on the detection of glucose by using nanotubular poly(EDOT-PBA) films



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with the blocking of BSA as shown in Figure 4. The presence of fructose and galactose in human’s blood might influence the detection of glucose because both sugars also bind with PBA groups on poly(EDOT-PBA) films. Although the concentrations of both fructose and galactose are relatively low compared to glucose (less than 10.0×10-6 M in general)31-32 it is necessary to evaluate the interferences caused by fructose and galactose on our poly(EDOT-PBA) platform. We first tested if the binding of low concentration of fructose and galactose can be observed during the binding of glucose as shown in Figure 4(a). The nanotubular poly(EDOT-PBA) film was first blocked with BSA in stage Ⅰ using a buffer solution containing 1 mg/mL BSA. In stage Ⅱ, we rinsed the chamber with pure buffer solutions for 15 mins. After rinsing with buffer solution, the frequency reached an equilibrium value and most of the BSA remained attached to the surface. We then tested the glucose binding in stage Ⅲ by shifting the mobile phase to a buffer solution containing 5.0 mM glucose. After the frequency reach an equilibrium under the flow of buffer solution containing glucose as the mobile phase, the mobile phase was subsequently shifted to a buffer solution containing 1.0 × 10-2 mM fructose and 1.0 × 10-2 mM galactose in addition to 5.0 mM glucose in stage Ⅳ. No frequency drop was observed, indicating no fructose and galactose adsorption onto the poly(EDOT-PBA) films. In Figure 4(b), the nanotubular poly(EDOT-PBA) film was first blocked with BSA at stage Ⅰ, together with 1.0 × 10-2 mM fructose and 1.0 × 10-2 mM galactose. After rinsing with buffer for 15 mins in stage Ⅱ, the frequency also reached an equilibrium value and most of the BSA remained attached to the


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surface. In stage Ⅲ, we subsequently shifted the mobile phase to a buffer solution dissolving 5.0 mM glucose, 1.0 × 10-2 mM fructose and 1.0 × 10-2 mM galactose. During the binding process, frequency dropped by 5.8 Hz, which similar to the test using pure glucose solution. This result indicates the presence of low concentration fructose and galactose did not influence the detection of glucose.

Figure 4. The monitoring of glucose adsorption on Au sensors coated with poly(EDOT-PBA) under the blocking of BSA and interference of fructose and galactose. (a) The binding of fructose (1.0 × 10-2 mM) and galactose (1.0 × 10-2 mM) was not observed. (b) The binding of glucose was not affected when solution containing fructose (1.0 × 10-2 mM) and galactose (1.0 × 10-2 mM).

The Comparison of Glucose Detection between QCM and EIS Methods We used nanotubular poly(EDOT-PBA) films to evaluate two analytical methods, including QCM and EIS, for the detection of glucose as shown in Figure 5. The analysis has been done with BSA blocking on poly(EDOT-PBA) films in advance and in the presence of fructose (1.0 × 10-2 mM) and 17 ACS Paragon Plus Environment


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galactose (1.0 × 10-2 mM). As shown in Figure 5(a), the drop of frequencies during the binding of glucose was proportional to the concentrations of glucose. The detection limit for glucose detection by applying QCM as a measurement tool was about 0.5 mM and the linear range of glucose detection is up to 10 mM. The binding of glucose at a concentration lower than 0.2 mM could hardly be detected by using QCM. Compared to the QCM results, the EIS measurements showed better detection limit as shown in Figure 5(b). The impedance data represented in a Nyquist plot showed the radius of semi-circle increased as the concentrations of glucose increased, which indicates the increase of resistance after glucose binding. A detection limit of 0.05 mM was achieved from EIS method. We used a Randles circuit as the equivalent circuit for modeling and quantitatively analysis as shown in Figure 5(c). The charge transfer resistance, RCT, increased quickly when the concentrations of glucose were lower than 1.0 mM. When the concentrations were higher than 1.0 mM, the RCT continued increased up to 10.0 mM.

Figure 5. Nanotbular poly(EDOT-PBA) films as substrates for the detection of glucose by (a) QCM and (b) EIS methods. The concentrations of glucose were ranging from 0.1 mM to 10.0 mM for QCM measurement, and 0.05 mM to 10 mM for EIS measurement, respectively. (c) The 18 ACS Paragon Plus Environment

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equivalent circuit and modeling results showing the charge transfer resistance, RCT, versus glucose concentration from 0.05 mM to 10.0 mM. We finally examined the potential application for continue glucose monitoring by applying our poly(EDOT-PBA) platform as shown in Figure 6. We demonstrated a reversible glucose binding on poly(EDOT-PBA) by QCM in Figure 3. Here we continuously examined the EIS signals when the poly(EDOT-PBA) coated electrodes were immersed in 0.5 mM and 5.0 mM solutions alternatively. The EIS was measured immediately after the electrode was immersed into the solutions. As shown in Figure 6, we continued the EIS measurements for 8 times when the solutions were switched between 0.5 mM and 5.0 mM solutions. The signals were nicely switched, which indicates a potential application for continue glucose monitoring by using our poly(EDOT-PBA) platform.

Figure 6. EIS showing continuous monitoring of glucose fluctuating between 0.5 mM and 5.0 mM by using nanotubular poly(EDOT-PBA) films as substrates.



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CONCLUSIONS In conclusion, we have successful demonstrated a new phenylboronic acid-grafted and nanostructured poly(3,4-ethylenedioxythiophene) platform for glucose monitoring. Through selecting the solutions and electrolytes for electropolymerization, poly(EDOT-PBA) films of both smooth and nanotubular surface morphologies were fabricated successfully. Compared to the poly(EDOT-PBA) of smooth morphology, the poly(EDOT-PBA) of tubular nanostructure provided higher glucose sensitivity and a different adsorption process of BSA, which is mainly due to the large increases of surface area contributed from the tubular nanostructures. We selected the nanotubular poly(EDOT-PBA) films as our sensing platform for glucose monitoring by both QCM and EIS methods. The detection of glucose was not affected by BSA blocking and low concentration fructose and galactose. Compared to QCM analysis, the EIS can provide a better detection limit at 50 µM. Besides, the binding with glucose on poly(EDOT-PBA) is highly reversibly. Based on these results, the nanotubular poly(EDOT-PBA) has great potential for enzyme-free electrodes targeting continues glucose monitoring applications because of its high sensitivity, fast response and reversible binding.

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


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XPS results and AFM images of nanostructured and smooth poly(EDOT-PBA) films prepared from DCM, acetonitrile, and [emim][triflate]. (PDF)

AUTHOR INFORMATION

Corresponding Author * Email: [email protected] ORCID Shyh-Chyang Luo: 0000-0003-3972-1086

Funding Sources Ministry of Science and Technology (MOST) of Taiwan.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge the financial support provided by National Taiwan University under grant NTU-107L7824, the Ministry of Science and Technology of Taiwan under grant MOST 106-2113-M-002-017-MY2 and Academia Sinica under grant AS-107-TP-A09.



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ABBREVIATIONS

EDOT, 3,4-ethylenedioxythiophene; PBA, phenylboronic acid; DCM, dichloromethane; [emim][triflate], 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate; BSA, bovine serum albumin; TBAP, Tetrabutylammonium perchlorate; QCM, quartz crystal microbalance; SEM, scanning electron microscope; AFM, atomic force microscope; EIS, electrochemical impedance spectroscopy; FET, field-effect transistor.

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TOC Graphic


Surface Engineering of Phenylboronic Acid-Functionalized Poly(3,4

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