Materials Science and Engineering C 40 (2014) 9–15
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Preparation and application of a novel electrochemical sensing material based on surface chemistry of polyhydroquinone Xueping Dang a,b,c, Yingkai Wang a, Chengguo Hu a,c,⁎, Jianlin Huang b, Huaixia Chen b, Shengfu Wang b, Shengshui Hu a,c,⁎ a
Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules & College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, PR China c State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing 10080, PR China b
a r t i c l e
i n f o
Article history: Received 3 September 2013 Received in revised form 2 March 2014 Accepted 18 March 2014 Available online 26 March 2014 Keywords: Hydroquinone Self-polymerization Surface chemistry Hydrophilicity Multiwalled carbon nanotubes (MWNTs)
a b s t r a c t A new analogue of polydopamine (PDA), i.e., polyhydroquinone (PH2Q), was polymerized and its surface chemistry was studied by different ways of characterization. PH2Q was produced by the self-polymerization of H2Q mediated by dissolved oxygen, and the self-polymerization process was strongly dependent on the type and the pH value of the buffer solutions. PH2Q can not only achieve surface hydrophilization of different substrates like polyethylene terephthalate (PET) film, graphite strip, C12SH/Au and wax slice, but also possess several unique properties like reversible adsorption, good solubility and low cost. These properties made PH2Q an ideal polymeric modifier for the noncovalent functionalization of some nanomaterials. By simply grinding with PH2Q, pristine multi-walled carbon nanotubes (MWNTs) can be readily dispersed in water with high solubility and good stability. The resulting MWNT–PH2Q composite exhibited excellent electrochemical performance, which was employed for the simultaneous determination of dopamine (DA) and uric acid (UA). © 2014 Elsevier B.V. All rights reserved.
1. Introduction The excellent surface chemical properties of solid state materials deeply rely on diverse methods of chemical modification, including self-assembled monolayer (SAM) [1,2], functionalized silanes [3,4], Langmuir–Blodgett deposition [5,6], and layer-by-layer assembly [7,8]. However, hydrophilic modification of the material surfaces is a necessary prerequisite since the surfaces of common materials are hydrophobic, such as polyethylene, polyvinylidene fluoride, raw glass, metals and carbon materials. Many methods can be used to improve the hydrophilicity of these materials, e.g., surface adsorption, grafting, and intercalation, mechanochemistry and material composites [9–14]. One of the most commonly used is surfactants, which may affect some applications of the materials, so a simple, fast and nondestructive method for hydrophilic modification has great significance. Inspired by the composition of adhesive proteins in mussels, Lee [15] used the self-polymerization of dopamine (DA) to form thin, surfaceadherent polydopamine (PDA) films onto a variety of inorganic and organic materials, e.g., noble metals, oxides, polymers, semiconductors,
⁎ Corresponding authors at: Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Luojia Hill No. 26, Wuhan 430072, PR China. Tel.: + 86 27 8788 1642; fax: +86 27 6875 4067. E-mail addresses: [email protected] (C. Hu), [email protected] (S. Hu).
http://dx.doi.org/10.1016/j.msec.2014.03.039 0928-4931/© 2014 Elsevier B.V. All rights reserved.
and ceramics. Indeed, the use of dopamine and related catecholic molecules has recently emerged as a promising method for surface chemistry and surface modification [16]. Their applications involve the fields of biology [17], chemistry [18], medicine [19], materials [20] and sensors [21, 22]. For example, carbon nanotubes can be coated by PDA without ruining sidewalls at room temperature [23]. New mussel-inspired surface chemistry to functionalize graphene oxide was developed based on PDA and poly-noradrenaline (PNA), which resulted in simultaneous surface functionalization and the reduction of graphene oxide [24,25]. Glucose oxidase (GOx) can effectively be entrapped by PDA and PNA, in order to construct new polymer–enzyme–metallic nanoparticle films with obvious electrocatalysis/enhancement effects for biosensing of glucose [26,27]. Polymerization of dopamine involves the oxidation of catechol to quinone, which further reacts with amines and other catechols/quinones to form an adherent polymer film [28]. The chemical composition of PDA coating is not precisely known. However, catechol and quinone functional groups are believed present, the latter of which are capable of covalent coupling to nucleophiles [29]. Here, we demonstrated that hydroquinone (H2Q), a kind of phenolic aromatic organic compound, can self-polymerize to form water-soluble polymers based on air oxidation in weak alkaline medium [30,31]. Its polymerization products, polyhydroquinone (PH2Q), not only had some properties in the surface chemistry similar to PDA, but also possessed several unique properties that may have promising applications in material fields. For instance, PH2Q can achieve the surface
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hydrophility of various hydrophobic substrates like polyethylene terephthalate (PET) film, graphite strip and wax slice. It also exhibits latent reactivity toward amine and thiol groups. As compared to PDA, solidstate PH2Q is easily soluble in most common polar solvents including water, which supply the reversibility of hydrophilic modification. Additionally, PH2Q with higher molecule weight can be easily collected from its solution by the technique of dialysis and rotary evaporation for the further applications. Beyond that, the monomer of DA is expensive and has biological effects [32], but H2Q is less expensive and has a wide range of applications in various fields [33,34]. Carbon nanotubes (CNTs) are one-dimensional carbon nanomaterials which have a variety of applications including composite material and biomedicine [35–37]. However, the hydrophilicity of CNTs limits its application. So far, diverse strategies including covalent and non-covalent modification on their surfaces were explored to overcome its hydrophilicity [38]. Among these, the most popular approach is deliberately grafting oxygen-containing functional groups into CNTs [39]. In our work, the pristine multi-walled carbon nanotubes (MWNTs) were noncovalently functionalized by PH2Q to form the MWNT–PH2Q composite, which can be dispersed in an aqueous solution. The resulting MWNT–PH2Q composite modified electrode can be used to detect DA and uric acid (UA) simultaneously. 2. Materials and methods 2.1. Reagents Hydroquinone (H2 Q), dodecyl mercaptan (C12SH), chloroauric acid (HAuCl4 · 4H2O) and hydroxylamine hydrochloride (NH2OH · HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. Multi-walled carbon nanotubes (MWNTs) were obtained from Nanotimes Co., Chengdu, China (purity N 95%, diameter: 10–20 nm). Octadecyltrichlorosilane (OTS) was purchased from J&K Chemical. DA and UA were the products of Fluka. Tris(hydroxymethyl)aminomethane (Tris) was purchased from Shanghai Bioscience & Technology, Co., Ltd. All chemicals were of analytical grade and used as obtained. All solutions were prepared with twice-distilled water. Polyethylene terephthalate (PET) films were purchased from Mingnisuda Mining Manufacture Material Technology (Shanghai), Co., Ltd. Graphite strips were prepared by polishing graphite powder (spectral purity) on an adhesive tape by hand. Gold layer coated PET films were prepared by a seeded growth method [40]. Then, it was immersed in 5 mmol/L dodecyl mercaptan (C12SH) overnight, flushed by ethanol, and blow-dried by nitrogen gas flow. The resulting hydrophobic gold film was denoted as C12SH/Au. 2.2. Preparation of polyhydroquinone (PH2Q) To prepare PH2Q, 22.0 g H2Q was dissolved in 500 mL 0.1 mol/L phosphate buffer solution (PBS, pH 8.0) to form 400 mmol/L H2Q solution. After stirring for 50 h at room temperature, the solvent was removed by rotary evaporation. The squishy substance, containing both PH2Q and the buffer, was collected and dried at ambient temperature, which was ground into fine powder and sealed up after degassing with nitrogen. The PH2Q ink was also processed by the technique of dialysis to remove the smaller molecules. 2.3. Preparation of MWNT–PH2Q film modified GCE To achieve the surface modification of MWNTs by PH2Q, 10 mg of pristine MWNTs and 60 mg of the above PH2Q product were mixed and ground in an agate mortar for 3 h by hand. The mixture was dispersed in 20 mL ultrapure water by sonication, and washed by vacuum filtration with water on a mixed cellulose membranes (pore size, 0.22 μm) until the filtrate became colorless. The resulting MWNT–PH2Q composite was collected, and then it was dried in air for characterization
or redispersed in ultrapure water by sonication to form 1.0 mg/mL MWNT aqueous dispersion. A glassy carbon electrode (GCE, 3.0 mm i.d.) was polished to a mirror finish with 0.3 and 0.05 μm alumina slurries on a polish paper, and then cleaned thoroughly in an ultrasonic cleaner with nitric acid water (1:1) solution, alcohol and redistilled water, sequentially. Then, the GCE was coated with 6 μL of the resulting MWNT–PH2Q suspension, and allowed to evaporate water at room temperature in air, producing a MWNT– PH2Q composite modified glassy carbon electrode (MWNT–PH2Q/GCE). 2.4. Instruments UV–Vis spectroscopy was recorded on a UV–Vis Spectrophotometer TU-1901 (Beijing Purkinje General Instrument, China) and 10.0 mmol/L H2Q in PBS (pH 8.0) was measured directly at different exposure time in air. Fourier transform infrared (FTIR) spectra were measured on a Nicolet Magna-IR 550 spectrometer and the KBr disc technique was employed. Prior to sample preparation for FTIR measurements, PH2Q was washed with water by dialysis to remove the small molecules and then dried by water bath. Raman spectroscopy was carried out on the Renishaw RM1000 laser confocal Raman spectrometer and the excitation wavelength is 514.5 nm. X-ray diffraction (XRD) was measured on the X-ray powder diffraction meter (XRD, Rigaku D/max-rA, Japan). The powdered MWNT–PH2Q composite was used for these characterizations. The MWNT–PH2Q composite aqueous dispersion was characterized by transmission electron microscopy (TEM) on Tecnai G20, FEI, Holland. The contact angle was measured on the contact angle meter JC2000C1, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd. The electrochemical experiments were carried out with a CHI830 electrochemical workstation (CH Instruments, Shanghai, China). A three-electrode system was used, including a modified glassy carbon working electrode (GCE), a saturated calomel reference electrode (SCE), and a platinum wire counter electrode. Electrochemical impedance diagrams (EIS) were performed on the EG&G 283 electrochemical workstation and EG&G 5210 lock-in amplifier (PAR,US) controlled by Powersuit software. Unless noted otherwise, all electrochemical experiments were performed at room temperature. 3. Results and discussion 3.1. The self-polymerization condition of hydroquinone As a kind of phenolic aromatic organic compound, H2Q can selfpolymerize based on air oxidation in a weak alkaline medium [30]. We investigated the influences of different reaction conditions on the selfpolymerization of hydroquinone, such as reaction time, reaction media and dissolved oxygen. Fig. 1A shows that the color of 10.0 mmol/L H2Q gradually changes from colorlessness to brown after a long period storage in air (a to d), indicating the self-polymerization of H2Q. No precipitation is observed during this process, which implies the good solubility of PH2Q in water. Fig. 1B shows the influences of dissolved oxygen, solution pH and buffer type on the self-polymerization of H2Q. The color of H2Q becomes brown in PBS (pH = 8.0) in air, and no color change is observed under the conditions of acidic or anoxic conditions. Moreover, the self-polymerization of H2Q in different media, such as HCl–NaOH, Tris, HAc–NaOH, Na2CO3 and PBS with the same pH 8.0, indicates that Na2CO3 and PBS buffers produce the deepest color for the solution of PH2Q after storing in air for 50 h. Considering the instability of Na2CO3 in air, PBS is chosen as the suitable medium for preparing PH2Q. 3.2. Characterization of the self-polymerization of hydroquinone Fig. 2A illustrates the UV–Vis spectra of 10.0 mmol/L H2Q in PBS (pH 8.0) for different exposure time in air. It can be seen that there is no apparent absorption peak for the fresh H2Q solution (curve a). However, an absorption peak appears at about 530 nm after the storage of
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Fig. 1. The photos for the self-polymerization of 10 mmol/L H2Q under different conditions. A: Different time exposure in air for 50 h, 25 h, 5 h and 0 h (from left to right). B: Selfpolymerization for 50 h in deoxidized PBS (pH 8.0), in PBS (pH 8.0) under air condition, in ultrapure water and in PBS (pH 3.0) under air condition (from left to right). C: Selfpolymerization for 50 h in different buffers (with the same pH 8.0): HCl–NaOH, Tris, HAc–NaOH, Na2CO3 and PBS (from left to right).
the solution in air for 1 h (curve b). The absorbance is enhanced with increasing time, and the absorption peak has a trend of red shift (curves c, d and e). Fig. 2B shows the FTIR spectra of H2Q and PH2Q. It is clear that H2Q has several characteristic IR absorptions. The peaks at 1510 and 1470 cm−1 can be assigned to the vibrations of C_C bonds in the aromatic rings. The peak at 1200 cm− 1 can be assigned to the phenolic OH groups. Peaks at 825 and 760 cm−1 can be assigned to the out-ofplane vibrations of the C\H bonds of the aromatic ring (curve a). After H2Q was exposed in air for 50 h, these featured absorptions
become weak, which is consistent with the reported reference [41]. However, the absorptions associated with the carbonyl C_O stretch vibration (1640 cm− 1) are enhanced, indicating that the hydroxyl group attached to the benzene ring has been oxidized to the carbonyl (curve b). Moreover, the wide absorption associated with the hydroxyl group O\H stretching vibration (3250 cm−1) becomes sharper, associated with a little displacement to high frequency (3430 cm− 1). This change fits with the results that the intermolecular hydrogen bond O\H is blocked by the self-polymerization. It is known that H2Q is very sensitive to the action of aqueous alkali and gives polymeric condensation products known as synthetic humic acids (SHA) or synthetic melanins. The structure of SHA can be seen as a co-polymer having hydroxyhydroquinone and hydroxybenzoquinone monomeric units [30], which match up with the results of the above characterizations. It indicates that PH2Q is composed of phenol and quinone structure units, which is similar to PDA. These phenolic hydroxyl groups are easily adsorbed on various surfaces, and they can graft with amidogen and sulfydryl, which is helpful to various surface functionalizations. 3.3. The hydrophilicity of polyhydroquinone (PH2Q) We investigated the hydrophilicity of PH2Q on the surface of different materials, such as polyethylene terephthalate (PET) film, graphite strip, C12SH/Au and wax slice (A, B, C and D in Fig. 3). Table 1 shows the values of the contact angle of the water droplet on these hydrophobic materials before and after treatment with PH2Q for 2 h. The contact angle apparently decreases after treatment with PH2Q. The photos of E and F are the captured images on the PET film in the contact angle measurement. It predicts that PH2Q may possess good hydrophilicity, which may be helpful to the surface functionalization of different materials. Additionally, the good hydrophilicity of PH2Q allows it be used as an ink to print hydrophilic arrays on hydrophobic PET by ink-jet printing (Fig. 3, G). 3.4. Characterization of water-soluble MWNT–PH2Q
Fig. 2. A: UV for the self-polymerization of 10 mmol/L H2Q for different times: 0 h, 1 h, 10 h, 25 h, and 50 h. B: FTIR for the self-polymerization of 10 mmol/L H2Q for different times: 0 h (a) and 50 h (b).
The hydrophobicity of carbon materials inevitably restricts their processability and limits the practical applications. It is important to improve their hydrophilicity by the surface modification. Fig. 4A is the TEM image of the MWNT–PH2Q composite. MWNTs grinded with PH2Q maintain their tube structure, while their surface is wrapped by PH2Q. The π–π conjugation between MWNTs and PH2Q may contribute to the wrapping, improving the hydrophilicity of MWNTs. Fig. 4B shows the photos of MWNTs treated by different hydrophilic substances, including PDA, SDS and PH2Q. Obviously, MWNTs have the best dispersion stability in water with the modification of PH2Q, indicating a
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Fig. 3. Photos of water on different substrates before (left) and after (right) PH2Q treatment: PET film (A); graphite strip (B); C12SH/Au(C); wax slice (D); the captured images on PET film before (E) and after (F) PH2Q treatment in the contact angle measurement; the hydrophilic array used PH2Q as ink on hydrophobic PET (G).
better hydrophilic surface modification performance of PH2Q for the surface modification of carbon nanomaterials for solution dispersion. Different techniques were used to characterize the MWNT–PH2Q composite. Fig. 5 A shows the FTIR spectra of PH2Q and MWNT–PH2Q composite. It can be seen that the characteristic absorption peaks at 1640 cm− 1 and 3430 cm−1 for PH2Q appear in the FTIR of MWNT– PH2Q (curve a, b). These peaks are apparently repressed after washing the composite by ethanol (curve c), and the dispersibility of MWNTs completely disappears, indicating the reversible adsorption of PH2Q on MWNTs. Raman spectroscopy is widely used in the characterization for geometry and the electronic structure properties of carbon materials. As can be seen in Fig. 5 B, there are two scattered bands in the Raman spectrum of MWNTs (curve a). The scattered band at about 1580 cm− 1 is the G band, associated with C\C tangential stretching vibration mode. The scattered band at about 1350 cm−1 is the D band, which is associated with the scattering of the defect location in MWNTs. Two broad bands appear at the same position for PH2Q, resulting from the stretching and deforming of aromatic ring. The Raman spectra of the MWNT–PH2Q composite combine the characteristics of MWNTs and PH2Q (curve c), suggesting the successful synthesis of the composite. X-ray powder diffraction (XRD) is also used for characterizing the MWNT–PH2Q composite (Fig. 5 C). There is no obvious diffraction peak found for PH2Q (curve a). Three graphite characteristic peaks of MWNTs appear at 26.12°, 42.97°, and 54.54°, which are respectively attributable to the crystal face C (002), C (100) and C (004) (curve
b). There is no change for the MWNT–PH2Q composite except C (002) at 26.12° increases after the modification of MWNTs by PH2Q (curve c). It may be attributed to the PH2Q properties of the stripping and surface modification. If the aggregation of MWNTs is stripped, its diffractive surface will become larger and the corresponding peaks will increase. 3.5. Electrochemical properties of MWNT–PH2Q film The electron transfer kinetics of the MWNT–PH2Q film is investigated by cycle voltammetry (CV) and electrochemical impedance spectra (EIS) (Fig. 6). The background current of the probe K3Fe(CN)6 on MWNT–PH2Q/GCE (curve b) is higher than that on the bare GCE (curve a), but the peak current and peak-to-peak separation hardly
Table 1 The values of the contact angle of the water droplet on different hydrophobic materials before and after treatment with PH2Q for 2 h. Different materials a
Before treatment After treatmenta a
PET film
Graphite strip
C12SH/Au
Wax slice
76.4 58.6
68.2 51.6
82.6 74.4
110.0 104.6
Average of three measurements.
Fig. 4. A: TEM of MWNT–PH2Q composite; B: photos of MWNTs in water stored for 24 h with treatment by PH2Q (upper), SDS (middle) and PDA (down).
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Fig. 5. A: FTIR characterization of PH2Q (a), MWNT–PH2Q (b) and MWNT–PH2Q washed by ethanol (c); B: Raman of MWNTs (a), MWNT–PH2Q (b) and PH2Q (c). Excitation wavelength: 514.5 nm, exposure time: 50s, scan times: 3; C: XRD of PH2Q (a), MWNTs (b) and MWNT–PH2Q (c).
Fig. 6. Electrochemical responses of 5.0 mmol/L [Fe(CN)6]3−/4− at GCE (a) and MWNT– PH2Q/GCE (b). A: CV; B: Nyquist plots; C: Bode plots.
change (Fig. 6 A). Thus, the MWNT–PH2Q composite has a good filmforming property and can act as a stable modifier to increase the surface area of the electrode. Because the used MWNTs are pristine and without any treatment, it can't increase significantly the electron transfer rate of the probe K3Fe(CN)6. Fig. 6 B shows that the Nyquist diagram (Z″ vs. Z′) on the bare GCE comprises an imperfect circular arc in the high frequency zone and a sloping line in the low frequency zone (curve a), corresponding to the electron transfer and the diffusion processes, respectively. The semicircle is unobvious on the MWNT–PH2Q film but has a small diameter as compared with the naked GCE, indicating a faster electron transfer rate (curve b). The Bode diagram (θ vs. logf) shows that the height of the relaxation peak on the bare GCE is apparently higher than that on the MWNT–PH2Q film, reflecting the much rough surface property and a large surface area of the composite film modified electrode (Fig. 6 C).
3.6. Application for the simultaneous detection of dopamine and uric acid on the MWNT–PH2Q film DA and UA are two important biomolecules, which are widely present in human body and play important roles in human health. The MWTN modified electrode was often employed to catalyze the oxidation of DA and UA [42–45]. However, the MWNTs used must be pretreated with acid to obtain good water dispersibility [44,45]. In our work, the pristine MWNTs were noncovalently functionalized by PH2Q to form the MWNT–PH2Q composite, which has favorable dispersibility and good film-forming property. The differential pulse voltammetric (DPV) responses of DA and UA were examined at the MWNT–PH2Q film modified GCE. As shown in Fig. 7, DA and UA exhibit an oxidation peak at 0.136 V and 0.265 V, respectively. The oxidation peak currents gradually increase with their concentration in the range of 1.0 × 10−7 mol/L to 2.0 ×
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Acknowledgment This research is supported by the National Nature Science Foundation of China (Nos. 20805035, 31070885 and 61301048), the Natural Science Fund for Creative Research Groups of Hubei Province of China (No. 2011CDA111) and the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University. References
Fig. 7. Differential pulse voltammetry of DA and UA at MWNT–PH2Q/GCE with various concentrations. Insets a and b showed the relationship between oxidation peak currents and concentrations of DA; inset c showed the relationship between oxidation peak currents and concentrations of UA.
10−4 mol/L. Two linear regression equations of DA were obtained at different concentration ranges. One is in the range of 1.0 × 10−7 mol/L to 5.0 × 10−6 mol/L by Ipa (μA) = 1.059 + 1.115C (μmol/L), R = 0.998 (Inset a in Fig. 7), and the other is in the range of 1.0 × 10−5 mol/L to 2.0 × 10−4 mol/L by Ipa (μA) = 7.208 + 0.2146C (μmol/L), R = 0.998 (Inset b in Fig. 7). The detection limit is 5.0 × l0−8 mol/L (S/N = 3). A linear regression equation of UA in the range of 1.0 × 10−6 mol/L to 2.0 × 10−4 mol/L was also obtained by Ipa (μA) = 0.2597 + 0.07834C (μmol/L), R = 0.999 (Inset c in Fig. 9), with a detection limit of 5.0 × l0−7 mol/L (S/N = 3). The performance parameters of the proposed modified electrode based on the MWNT–PH2Q composite were better than or comparable to that of other modified electrodes (Table 2). Thus, it provides an alternative to determine DA and UA simultaneously.
4. Conclusions In this work, we demonstrated that the oxidation-based selfpolymerization of hydroquinone only occurred in neutral or weakly basic aqueous solutions in the presence of dissolved oxygen. The product PH2Q was readily dissolved in water for forming stable hydrophilic monolayers on universal substrates, such as PET film, graphite strip, C12SH/Au, wax slice and OTS/ITO. The rapid adsorption of PH2Q on the surface of MWNTs by noncovalent π–π interactions provided a simple but effective approach to the preparation of water-soluble carbon nanomaterials with high stability and solubility, which can be achieved either by physical grinding or vigorous stirring of the mixture of MWNTs and PH2Q. The resulting MWNT–PH2Q composites possessed excellent electrochemical activity toward a variety of electrochemical probes. For instance, the MWNT–PH2Q composite exhibited sensitive electrochemical responses for DA and UA and can be used to determine them simultaneously.
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Table 2 The characteristic comparison of some modified electrodes based on MWNTs used for the simultaneous detection of DA and UA. Modified film
MWCNTs [42] Iron(III)-porphyrin/MWCNTs [43] Poly(acrylic acid) - MWNTs [44] Methylene blue- MWNTs [45] This work
MWNTs
Linear range (μmol/L)
Detection limit (μmol/L)
Sensitivity (μA/μmol L−1)
Acid treatment
Dispersion medium
DA
UA
DA
UA
DA
UA
No No Poly(acrylic acid) Nitric acid No
DMF DMF Water Water Water
10–200 0.7–3600 0.04–3 0.4–10 0.1–200
57–200 5.8–1300 0.3–10 0.2–200 1–200
0.67 0.09 0.02 0.2 0.05
1.65 0.3 0.11 1.0 0.5
0.0237 0.0112 43.96 59.25 Not reported 0.5959 0.3101 1.115 0.0783
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[42] H.R. Zare, N. Nasirizadeh, J. Iran. Chem. Soc. 8 (2011) S55–S66. [43] C. Wang, R. Yuan, Y. Chai, S. Chen, Y. Zhang, F. Hu, M. Zhang, Electrochim. Acta 62 (2012) 109–115. [44] A. Liu, I. Honma, H. Zhou, Biosens. Bioelectron. 23 (2007) 74–80. [45] S. Yang, G. Li, R. Yang, M. Xia, L. Qu, J. Solid State Electrocem. 15 (2011) 1909–1918.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Preparation and application of a novel electrochemical sensing material based on surface chemistry of polyhydroquinone Xueping Dang a,b,c, Yingkai Wang a, Chengguo Hu a,c,⁎, Jianlin Huang b, Huaixia Chen b, Shengfu Wang b, Shengshui Hu a,c,⁎ a
Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules & College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, PR China c State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing 10080, PR China b
a r t i c l e
i n f o
Article history: Received 3 September 2013 Received in revised form 2 March 2014 Accepted 18 March 2014 Available online 26 March 2014 Keywords: Hydroquinone Self-polymerization Surface chemistry Hydrophilicity Multiwalled carbon nanotubes (MWNTs)
a b s t r a c t A new analogue of polydopamine (PDA), i.e., polyhydroquinone (PH2Q), was polymerized and its surface chemistry was studied by different ways of characterization. PH2Q was produced by the self-polymerization of H2Q mediated by dissolved oxygen, and the self-polymerization process was strongly dependent on the type and the pH value of the buffer solutions. PH2Q can not only achieve surface hydrophilization of different substrates like polyethylene terephthalate (PET) film, graphite strip, C12SH/Au and wax slice, but also possess several unique properties like reversible adsorption, good solubility and low cost. These properties made PH2Q an ideal polymeric modifier for the noncovalent functionalization of some nanomaterials. By simply grinding with PH2Q, pristine multi-walled carbon nanotubes (MWNTs) can be readily dispersed in water with high solubility and good stability. The resulting MWNT–PH2Q composite exhibited excellent electrochemical performance, which was employed for the simultaneous determination of dopamine (DA) and uric acid (UA). © 2014 Elsevier B.V. All rights reserved.
1. Introduction The excellent surface chemical properties of solid state materials deeply rely on diverse methods of chemical modification, including self-assembled monolayer (SAM) [1,2], functionalized silanes [3,4], Langmuir–Blodgett deposition [5,6], and layer-by-layer assembly [7,8]. However, hydrophilic modification of the material surfaces is a necessary prerequisite since the surfaces of common materials are hydrophobic, such as polyethylene, polyvinylidene fluoride, raw glass, metals and carbon materials. Many methods can be used to improve the hydrophilicity of these materials, e.g., surface adsorption, grafting, and intercalation, mechanochemistry and material composites [9–14]. One of the most commonly used is surfactants, which may affect some applications of the materials, so a simple, fast and nondestructive method for hydrophilic modification has great significance. Inspired by the composition of adhesive proteins in mussels, Lee [15] used the self-polymerization of dopamine (DA) to form thin, surfaceadherent polydopamine (PDA) films onto a variety of inorganic and organic materials, e.g., noble metals, oxides, polymers, semiconductors,
⁎ Corresponding authors at: Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Luojia Hill No. 26, Wuhan 430072, PR China. Tel.: + 86 27 8788 1642; fax: +86 27 6875 4067. E-mail addresses: [email protected] (C. Hu), [email protected] (S. Hu).
http://dx.doi.org/10.1016/j.msec.2014.03.039 0928-4931/© 2014 Elsevier B.V. All rights reserved.
and ceramics. Indeed, the use of dopamine and related catecholic molecules has recently emerged as a promising method for surface chemistry and surface modification [16]. Their applications involve the fields of biology [17], chemistry [18], medicine [19], materials [20] and sensors [21, 22]. For example, carbon nanotubes can be coated by PDA without ruining sidewalls at room temperature [23]. New mussel-inspired surface chemistry to functionalize graphene oxide was developed based on PDA and poly-noradrenaline (PNA), which resulted in simultaneous surface functionalization and the reduction of graphene oxide [24,25]. Glucose oxidase (GOx) can effectively be entrapped by PDA and PNA, in order to construct new polymer–enzyme–metallic nanoparticle films with obvious electrocatalysis/enhancement effects for biosensing of glucose [26,27]. Polymerization of dopamine involves the oxidation of catechol to quinone, which further reacts with amines and other catechols/quinones to form an adherent polymer film [28]. The chemical composition of PDA coating is not precisely known. However, catechol and quinone functional groups are believed present, the latter of which are capable of covalent coupling to nucleophiles [29]. Here, we demonstrated that hydroquinone (H2Q), a kind of phenolic aromatic organic compound, can self-polymerize to form water-soluble polymers based on air oxidation in weak alkaline medium [30,31]. Its polymerization products, polyhydroquinone (PH2Q), not only had some properties in the surface chemistry similar to PDA, but also possessed several unique properties that may have promising applications in material fields. For instance, PH2Q can achieve the surface
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hydrophility of various hydrophobic substrates like polyethylene terephthalate (PET) film, graphite strip and wax slice. It also exhibits latent reactivity toward amine and thiol groups. As compared to PDA, solidstate PH2Q is easily soluble in most common polar solvents including water, which supply the reversibility of hydrophilic modification. Additionally, PH2Q with higher molecule weight can be easily collected from its solution by the technique of dialysis and rotary evaporation for the further applications. Beyond that, the monomer of DA is expensive and has biological effects [32], but H2Q is less expensive and has a wide range of applications in various fields [33,34]. Carbon nanotubes (CNTs) are one-dimensional carbon nanomaterials which have a variety of applications including composite material and biomedicine [35–37]. However, the hydrophilicity of CNTs limits its application. So far, diverse strategies including covalent and non-covalent modification on their surfaces were explored to overcome its hydrophilicity [38]. Among these, the most popular approach is deliberately grafting oxygen-containing functional groups into CNTs [39]. In our work, the pristine multi-walled carbon nanotubes (MWNTs) were noncovalently functionalized by PH2Q to form the MWNT–PH2Q composite, which can be dispersed in an aqueous solution. The resulting MWNT–PH2Q composite modified electrode can be used to detect DA and uric acid (UA) simultaneously. 2. Materials and methods 2.1. Reagents Hydroquinone (H2 Q), dodecyl mercaptan (C12SH), chloroauric acid (HAuCl4 · 4H2O) and hydroxylamine hydrochloride (NH2OH · HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. Multi-walled carbon nanotubes (MWNTs) were obtained from Nanotimes Co., Chengdu, China (purity N 95%, diameter: 10–20 nm). Octadecyltrichlorosilane (OTS) was purchased from J&K Chemical. DA and UA were the products of Fluka. Tris(hydroxymethyl)aminomethane (Tris) was purchased from Shanghai Bioscience & Technology, Co., Ltd. All chemicals were of analytical grade and used as obtained. All solutions were prepared with twice-distilled water. Polyethylene terephthalate (PET) films were purchased from Mingnisuda Mining Manufacture Material Technology (Shanghai), Co., Ltd. Graphite strips were prepared by polishing graphite powder (spectral purity) on an adhesive tape by hand. Gold layer coated PET films were prepared by a seeded growth method [40]. Then, it was immersed in 5 mmol/L dodecyl mercaptan (C12SH) overnight, flushed by ethanol, and blow-dried by nitrogen gas flow. The resulting hydrophobic gold film was denoted as C12SH/Au. 2.2. Preparation of polyhydroquinone (PH2Q) To prepare PH2Q, 22.0 g H2Q was dissolved in 500 mL 0.1 mol/L phosphate buffer solution (PBS, pH 8.0) to form 400 mmol/L H2Q solution. After stirring for 50 h at room temperature, the solvent was removed by rotary evaporation. The squishy substance, containing both PH2Q and the buffer, was collected and dried at ambient temperature, which was ground into fine powder and sealed up after degassing with nitrogen. The PH2Q ink was also processed by the technique of dialysis to remove the smaller molecules. 2.3. Preparation of MWNT–PH2Q film modified GCE To achieve the surface modification of MWNTs by PH2Q, 10 mg of pristine MWNTs and 60 mg of the above PH2Q product were mixed and ground in an agate mortar for 3 h by hand. The mixture was dispersed in 20 mL ultrapure water by sonication, and washed by vacuum filtration with water on a mixed cellulose membranes (pore size, 0.22 μm) until the filtrate became colorless. The resulting MWNT–PH2Q composite was collected, and then it was dried in air for characterization
or redispersed in ultrapure water by sonication to form 1.0 mg/mL MWNT aqueous dispersion. A glassy carbon electrode (GCE, 3.0 mm i.d.) was polished to a mirror finish with 0.3 and 0.05 μm alumina slurries on a polish paper, and then cleaned thoroughly in an ultrasonic cleaner with nitric acid water (1:1) solution, alcohol and redistilled water, sequentially. Then, the GCE was coated with 6 μL of the resulting MWNT–PH2Q suspension, and allowed to evaporate water at room temperature in air, producing a MWNT– PH2Q composite modified glassy carbon electrode (MWNT–PH2Q/GCE). 2.4. Instruments UV–Vis spectroscopy was recorded on a UV–Vis Spectrophotometer TU-1901 (Beijing Purkinje General Instrument, China) and 10.0 mmol/L H2Q in PBS (pH 8.0) was measured directly at different exposure time in air. Fourier transform infrared (FTIR) spectra were measured on a Nicolet Magna-IR 550 spectrometer and the KBr disc technique was employed. Prior to sample preparation for FTIR measurements, PH2Q was washed with water by dialysis to remove the small molecules and then dried by water bath. Raman spectroscopy was carried out on the Renishaw RM1000 laser confocal Raman spectrometer and the excitation wavelength is 514.5 nm. X-ray diffraction (XRD) was measured on the X-ray powder diffraction meter (XRD, Rigaku D/max-rA, Japan). The powdered MWNT–PH2Q composite was used for these characterizations. The MWNT–PH2Q composite aqueous dispersion was characterized by transmission electron microscopy (TEM) on Tecnai G20, FEI, Holland. The contact angle was measured on the contact angle meter JC2000C1, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd. The electrochemical experiments were carried out with a CHI830 electrochemical workstation (CH Instruments, Shanghai, China). A three-electrode system was used, including a modified glassy carbon working electrode (GCE), a saturated calomel reference electrode (SCE), and a platinum wire counter electrode. Electrochemical impedance diagrams (EIS) were performed on the EG&G 283 electrochemical workstation and EG&G 5210 lock-in amplifier (PAR,US) controlled by Powersuit software. Unless noted otherwise, all electrochemical experiments were performed at room temperature. 3. Results and discussion 3.1. The self-polymerization condition of hydroquinone As a kind of phenolic aromatic organic compound, H2Q can selfpolymerize based on air oxidation in a weak alkaline medium [30]. We investigated the influences of different reaction conditions on the selfpolymerization of hydroquinone, such as reaction time, reaction media and dissolved oxygen. Fig. 1A shows that the color of 10.0 mmol/L H2Q gradually changes from colorlessness to brown after a long period storage in air (a to d), indicating the self-polymerization of H2Q. No precipitation is observed during this process, which implies the good solubility of PH2Q in water. Fig. 1B shows the influences of dissolved oxygen, solution pH and buffer type on the self-polymerization of H2Q. The color of H2Q becomes brown in PBS (pH = 8.0) in air, and no color change is observed under the conditions of acidic or anoxic conditions. Moreover, the self-polymerization of H2Q in different media, such as HCl–NaOH, Tris, HAc–NaOH, Na2CO3 and PBS with the same pH 8.0, indicates that Na2CO3 and PBS buffers produce the deepest color for the solution of PH2Q after storing in air for 50 h. Considering the instability of Na2CO3 in air, PBS is chosen as the suitable medium for preparing PH2Q. 3.2. Characterization of the self-polymerization of hydroquinone Fig. 2A illustrates the UV–Vis spectra of 10.0 mmol/L H2Q in PBS (pH 8.0) for different exposure time in air. It can be seen that there is no apparent absorption peak for the fresh H2Q solution (curve a). However, an absorption peak appears at about 530 nm after the storage of
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Fig. 1. The photos for the self-polymerization of 10 mmol/L H2Q under different conditions. A: Different time exposure in air for 50 h, 25 h, 5 h and 0 h (from left to right). B: Selfpolymerization for 50 h in deoxidized PBS (pH 8.0), in PBS (pH 8.0) under air condition, in ultrapure water and in PBS (pH 3.0) under air condition (from left to right). C: Selfpolymerization for 50 h in different buffers (with the same pH 8.0): HCl–NaOH, Tris, HAc–NaOH, Na2CO3 and PBS (from left to right).
the solution in air for 1 h (curve b). The absorbance is enhanced with increasing time, and the absorption peak has a trend of red shift (curves c, d and e). Fig. 2B shows the FTIR spectra of H2Q and PH2Q. It is clear that H2Q has several characteristic IR absorptions. The peaks at 1510 and 1470 cm−1 can be assigned to the vibrations of C_C bonds in the aromatic rings. The peak at 1200 cm− 1 can be assigned to the phenolic OH groups. Peaks at 825 and 760 cm−1 can be assigned to the out-ofplane vibrations of the C\H bonds of the aromatic ring (curve a). After H2Q was exposed in air for 50 h, these featured absorptions
become weak, which is consistent with the reported reference [41]. However, the absorptions associated with the carbonyl C_O stretch vibration (1640 cm− 1) are enhanced, indicating that the hydroxyl group attached to the benzene ring has been oxidized to the carbonyl (curve b). Moreover, the wide absorption associated with the hydroxyl group O\H stretching vibration (3250 cm−1) becomes sharper, associated with a little displacement to high frequency (3430 cm− 1). This change fits with the results that the intermolecular hydrogen bond O\H is blocked by the self-polymerization. It is known that H2Q is very sensitive to the action of aqueous alkali and gives polymeric condensation products known as synthetic humic acids (SHA) or synthetic melanins. The structure of SHA can be seen as a co-polymer having hydroxyhydroquinone and hydroxybenzoquinone monomeric units [30], which match up with the results of the above characterizations. It indicates that PH2Q is composed of phenol and quinone structure units, which is similar to PDA. These phenolic hydroxyl groups are easily adsorbed on various surfaces, and they can graft with amidogen and sulfydryl, which is helpful to various surface functionalizations. 3.3. The hydrophilicity of polyhydroquinone (PH2Q) We investigated the hydrophilicity of PH2Q on the surface of different materials, such as polyethylene terephthalate (PET) film, graphite strip, C12SH/Au and wax slice (A, B, C and D in Fig. 3). Table 1 shows the values of the contact angle of the water droplet on these hydrophobic materials before and after treatment with PH2Q for 2 h. The contact angle apparently decreases after treatment with PH2Q. The photos of E and F are the captured images on the PET film in the contact angle measurement. It predicts that PH2Q may possess good hydrophilicity, which may be helpful to the surface functionalization of different materials. Additionally, the good hydrophilicity of PH2Q allows it be used as an ink to print hydrophilic arrays on hydrophobic PET by ink-jet printing (Fig. 3, G). 3.4. Characterization of water-soluble MWNT–PH2Q
Fig. 2. A: UV for the self-polymerization of 10 mmol/L H2Q for different times: 0 h, 1 h, 10 h, 25 h, and 50 h. B: FTIR for the self-polymerization of 10 mmol/L H2Q for different times: 0 h (a) and 50 h (b).
The hydrophobicity of carbon materials inevitably restricts their processability and limits the practical applications. It is important to improve their hydrophilicity by the surface modification. Fig. 4A is the TEM image of the MWNT–PH2Q composite. MWNTs grinded with PH2Q maintain their tube structure, while their surface is wrapped by PH2Q. The π–π conjugation between MWNTs and PH2Q may contribute to the wrapping, improving the hydrophilicity of MWNTs. Fig. 4B shows the photos of MWNTs treated by different hydrophilic substances, including PDA, SDS and PH2Q. Obviously, MWNTs have the best dispersion stability in water with the modification of PH2Q, indicating a
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Fig. 3. Photos of water on different substrates before (left) and after (right) PH2Q treatment: PET film (A); graphite strip (B); C12SH/Au(C); wax slice (D); the captured images on PET film before (E) and after (F) PH2Q treatment in the contact angle measurement; the hydrophilic array used PH2Q as ink on hydrophobic PET (G).
better hydrophilic surface modification performance of PH2Q for the surface modification of carbon nanomaterials for solution dispersion. Different techniques were used to characterize the MWNT–PH2Q composite. Fig. 5 A shows the FTIR spectra of PH2Q and MWNT–PH2Q composite. It can be seen that the characteristic absorption peaks at 1640 cm− 1 and 3430 cm−1 for PH2Q appear in the FTIR of MWNT– PH2Q (curve a, b). These peaks are apparently repressed after washing the composite by ethanol (curve c), and the dispersibility of MWNTs completely disappears, indicating the reversible adsorption of PH2Q on MWNTs. Raman spectroscopy is widely used in the characterization for geometry and the electronic structure properties of carbon materials. As can be seen in Fig. 5 B, there are two scattered bands in the Raman spectrum of MWNTs (curve a). The scattered band at about 1580 cm− 1 is the G band, associated with C\C tangential stretching vibration mode. The scattered band at about 1350 cm−1 is the D band, which is associated with the scattering of the defect location in MWNTs. Two broad bands appear at the same position for PH2Q, resulting from the stretching and deforming of aromatic ring. The Raman spectra of the MWNT–PH2Q composite combine the characteristics of MWNTs and PH2Q (curve c), suggesting the successful synthesis of the composite. X-ray powder diffraction (XRD) is also used for characterizing the MWNT–PH2Q composite (Fig. 5 C). There is no obvious diffraction peak found for PH2Q (curve a). Three graphite characteristic peaks of MWNTs appear at 26.12°, 42.97°, and 54.54°, which are respectively attributable to the crystal face C (002), C (100) and C (004) (curve
b). There is no change for the MWNT–PH2Q composite except C (002) at 26.12° increases after the modification of MWNTs by PH2Q (curve c). It may be attributed to the PH2Q properties of the stripping and surface modification. If the aggregation of MWNTs is stripped, its diffractive surface will become larger and the corresponding peaks will increase. 3.5. Electrochemical properties of MWNT–PH2Q film The electron transfer kinetics of the MWNT–PH2Q film is investigated by cycle voltammetry (CV) and electrochemical impedance spectra (EIS) (Fig. 6). The background current of the probe K3Fe(CN)6 on MWNT–PH2Q/GCE (curve b) is higher than that on the bare GCE (curve a), but the peak current and peak-to-peak separation hardly
Table 1 The values of the contact angle of the water droplet on different hydrophobic materials before and after treatment with PH2Q for 2 h. Different materials a
Before treatment After treatmenta a
PET film
Graphite strip
C12SH/Au
Wax slice
76.4 58.6
68.2 51.6
82.6 74.4
110.0 104.6
Average of three measurements.
Fig. 4. A: TEM of MWNT–PH2Q composite; B: photos of MWNTs in water stored for 24 h with treatment by PH2Q (upper), SDS (middle) and PDA (down).
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Fig. 5. A: FTIR characterization of PH2Q (a), MWNT–PH2Q (b) and MWNT–PH2Q washed by ethanol (c); B: Raman of MWNTs (a), MWNT–PH2Q (b) and PH2Q (c). Excitation wavelength: 514.5 nm, exposure time: 50s, scan times: 3; C: XRD of PH2Q (a), MWNTs (b) and MWNT–PH2Q (c).
Fig. 6. Electrochemical responses of 5.0 mmol/L [Fe(CN)6]3−/4− at GCE (a) and MWNT– PH2Q/GCE (b). A: CV; B: Nyquist plots; C: Bode plots.
change (Fig. 6 A). Thus, the MWNT–PH2Q composite has a good filmforming property and can act as a stable modifier to increase the surface area of the electrode. Because the used MWNTs are pristine and without any treatment, it can't increase significantly the electron transfer rate of the probe K3Fe(CN)6. Fig. 6 B shows that the Nyquist diagram (Z″ vs. Z′) on the bare GCE comprises an imperfect circular arc in the high frequency zone and a sloping line in the low frequency zone (curve a), corresponding to the electron transfer and the diffusion processes, respectively. The semicircle is unobvious on the MWNT–PH2Q film but has a small diameter as compared with the naked GCE, indicating a faster electron transfer rate (curve b). The Bode diagram (θ vs. logf) shows that the height of the relaxation peak on the bare GCE is apparently higher than that on the MWNT–PH2Q film, reflecting the much rough surface property and a large surface area of the composite film modified electrode (Fig. 6 C).
3.6. Application for the simultaneous detection of dopamine and uric acid on the MWNT–PH2Q film DA and UA are two important biomolecules, which are widely present in human body and play important roles in human health. The MWTN modified electrode was often employed to catalyze the oxidation of DA and UA [42–45]. However, the MWNTs used must be pretreated with acid to obtain good water dispersibility [44,45]. In our work, the pristine MWNTs were noncovalently functionalized by PH2Q to form the MWNT–PH2Q composite, which has favorable dispersibility and good film-forming property. The differential pulse voltammetric (DPV) responses of DA and UA were examined at the MWNT–PH2Q film modified GCE. As shown in Fig. 7, DA and UA exhibit an oxidation peak at 0.136 V and 0.265 V, respectively. The oxidation peak currents gradually increase with their concentration in the range of 1.0 × 10−7 mol/L to 2.0 ×
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Acknowledgment This research is supported by the National Nature Science Foundation of China (Nos. 20805035, 31070885 and 61301048), the Natural Science Fund for Creative Research Groups of Hubei Province of China (No. 2011CDA111) and the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University. References
Fig. 7. Differential pulse voltammetry of DA and UA at MWNT–PH2Q/GCE with various concentrations. Insets a and b showed the relationship between oxidation peak currents and concentrations of DA; inset c showed the relationship between oxidation peak currents and concentrations of UA.
10−4 mol/L. Two linear regression equations of DA were obtained at different concentration ranges. One is in the range of 1.0 × 10−7 mol/L to 5.0 × 10−6 mol/L by Ipa (μA) = 1.059 + 1.115C (μmol/L), R = 0.998 (Inset a in Fig. 7), and the other is in the range of 1.0 × 10−5 mol/L to 2.0 × 10−4 mol/L by Ipa (μA) = 7.208 + 0.2146C (μmol/L), R = 0.998 (Inset b in Fig. 7). The detection limit is 5.0 × l0−8 mol/L (S/N = 3). A linear regression equation of UA in the range of 1.0 × 10−6 mol/L to 2.0 × 10−4 mol/L was also obtained by Ipa (μA) = 0.2597 + 0.07834C (μmol/L), R = 0.999 (Inset c in Fig. 9), with a detection limit of 5.0 × l0−7 mol/L (S/N = 3). The performance parameters of the proposed modified electrode based on the MWNT–PH2Q composite were better than or comparable to that of other modified electrodes (Table 2). Thus, it provides an alternative to determine DA and UA simultaneously.
4. Conclusions In this work, we demonstrated that the oxidation-based selfpolymerization of hydroquinone only occurred in neutral or weakly basic aqueous solutions in the presence of dissolved oxygen. The product PH2Q was readily dissolved in water for forming stable hydrophilic monolayers on universal substrates, such as PET film, graphite strip, C12SH/Au, wax slice and OTS/ITO. The rapid adsorption of PH2Q on the surface of MWNTs by noncovalent π–π interactions provided a simple but effective approach to the preparation of water-soluble carbon nanomaterials with high stability and solubility, which can be achieved either by physical grinding or vigorous stirring of the mixture of MWNTs and PH2Q. The resulting MWNT–PH2Q composites possessed excellent electrochemical activity toward a variety of electrochemical probes. For instance, the MWNT–PH2Q composite exhibited sensitive electrochemical responses for DA and UA and can be used to determine them simultaneously.
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Table 2 The characteristic comparison of some modified electrodes based on MWNTs used for the simultaneous detection of DA and UA. Modified film
MWCNTs [42] Iron(III)-porphyrin/MWCNTs [43] Poly(acrylic acid) - MWNTs [44] Methylene blue- MWNTs [45] This work
MWNTs
Linear range (μmol/L)
Detection limit (μmol/L)
Sensitivity (μA/μmol L−1)
Acid treatment
Dispersion medium
DA
UA
DA
UA
DA
UA
No No Poly(acrylic acid) Nitric acid No
DMF DMF Water Water Water
10–200 0.7–3600 0.04–3 0.4–10 0.1–200
57–200 5.8–1300 0.3–10 0.2–200 1–200
0.67 0.09 0.02 0.2 0.05
1.65 0.3 0.11 1.0 0.5
0.0237 0.0112 43.96 59.25 Not reported 0.5959 0.3101 1.115 0.0783
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