Article pubs.acs.org/JPCC
β‑Cyclodextrin−Graphite Oxide−Carbon Nanotube Composite for Enhanced Electrochemical Supramolecular Recognition Hien N. Le and Hae kyung Jeong* Department of Physics, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Republic of Korea ABSTRACT: The β-cyclodextrin−graphite oxide−carbon nanotube (βCD− GO−CNT) composite was synthesized by a simple chemical method and characterized for the supramolecular recognition by using the cyclic voltammetry and differential pulse voltammetry. Three kinds of biomolecules (dopamine, thioridazine, L-tyrosine) were used, and βCD−GO−CNT composite presented excellent capability for supramolecular recognition of the biomolecules compared to individual CNT and βCD−CNT composites, exhibiting the presence of GO in the βCD−GO−CNT composite could immobilize βCD molecules effectively. It is, therefore, demonstrated that the new composite, βCD−GO−CNT, provides a synergistic effect of high electric conductivity from CNT and high supramolecular recognition capability from βCD effectively immobilized in GO for future biosensor applications.
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INTRODUCTION Cyclodextrins (CDs) are oligosaccharides composed of six, seven, or eight glucose units, so-called α-, β-, or γ-CD, respectively, and have a toroidal shape with a hydrophobic inner cavity and a hydrophilic exterior. The interesting characteristics enable them to bind various organic, inorganic, and biological guest molecules selectively to their cavities, forming stable host−guest inclusion complexes or nanostructured supramolecular assemblies in a hydrophilic ambience.1 Carbon nanotubes (CNTs) are a good candidate of electrodes for the electrochemical double layer capacitor (EDLC) because they have excellent electric conductivities and high aspect ratios, resulting in a good network or matrix for easy attachment of electrolyte ions in the EDLC.2,3 Graphite oxide (GO) is prepared by treating graphite with strong aqueous oxidizing agents such as fuming nitric acid/ potassium chlorate or sulfuric acid/potassium permanganate.4,5 For this reason, graphite oxide has a hydrophilic surface and consists of covalently attached oxygen-containing groups such as hydroxyl, epoxide, carbonyl, and carboxyl groups. βCD among CDs can incorporate a good carbon matrix of CNTs, forming βCD−CNT composites, for the electrochemical biosensors in which the electrochemical signal could be enhanced by the synergistic effect of good electric conductivity of the CNT and excellent supramolecular recognition of βCD. The CNT itself, however, could only immobilize a few βCD molecules because βCD of the hydrophilic surface is easily detached from the hydrophobic surface of the CNT.6 It is, therefore, necessary to introduce something that could immobilize βCD molecules effectively. Here we introduce GO to solve the problem because GO of the hydrophilic surface might immobilize βCD of the hydrophilic exterior effectively. GO consists of oxygen-containing groups © XXXX American Chemical Society
such as hydroxyl, epoxide, carbonyl, and carboxyl groups, which could enhance wettability of the cyclodextrin onto GO surface. Dopamine hydrochloride or dopamine (DA) is an important neuron transmitter compound widely existing in the brain for message transfer in the mammalian central nervous system.7,8 Abnormal concentration levels of DA may lead to several diseases, such as Parkinson’s disease, schizophrenia, and human immunodeficiency virus infection.7,9 Therefore, it is of great significance to develop simple and rapid determination methods of DA. L-Tyrosine (4-hydroxyphenylalanine) is one of the aromatic amino acids, found in many protein products such as chicken, fish, milk, cheese, and soy products. It plays an important role in our body, related to dopamine, noradrenaline, adrenaline, thyroid hormones, and estrogen.10 A dearth of tyrosine in our body can lead to depression and Parkinson’s disease.11−13 An excess of it also generates diseases such as Basedow’s disease, thyrotoxicosis, and hyperthyroidism.14 Efficient determination of tyrosine levels, therefore, is crucial in the diagnosis and medical treatments. Thioridazine hydrochloride or thioridazine [10-2-(1-methyl2-piperidyl)ethyl]-2(methylthio) phenothiazine monohydrochloride] belongs to the antipsychotic phenothiazine group and is used mainly in the treatment of schizophrenia and the control of mania and agitation. It is also used for the short-term treatment of adults with major depression who have varying degrees of associated anxiety. It may be used in the management of anxiety states of children who have behavior problems.15 Thioridazine, however, can cause a serious type of irregular heartbeat that may cause sudden death. Studies have shown that older adults with dementia who take antipsychotics, Received: April 7, 2015 Revised: July 13, 2015
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DOI: 10.1021/acs.jpcc.5b03363 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. XRD results of graphite, GO, CNT, βCD−CNT, and βCD−GO−CNT.
GO was synthesized by the Brodie method17 from graphite, and GO of 10 mg was mixed with DI water of 20 mL followed by the sonication for 2 h to form well dispersed GO solution. βCD (200 mg) and CNT (20 mg) was then mixed with the GO solution followed by the same procedure after that. Scanning electron microscopy (SEM, Ltd., S-4300, JEOL, Japan) and transmission electron microscopy (TEM, 120 kV, HITACHI, Ltd., H-7600, Japan) at different magnifications were performed to view the surface morphology of the composites. Thermogravimetric analysis (TGA, TGA Instruments, Q600) was used to measure the component and weight of the elements. X-ray diffraction (XRD, Rigaku Rotaflex D/ MAX System, Rigaku, Japan) at 40 kV with Cu Kα (1.54 Å) was characterized for the crystal structure of the composites. Electrochemical properties of the obtained sample were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using EC-Lab (Bio-Logic, sp-150, France) in a three-electrode cell. An Ag/AgCl electrode (BAS) was used as the reference electrode, and a platinum wire was employed as the counter electrode. The working electrode was prepared by the following method. Each sample of 0.5 mg was dispersed well in 1 mL of 2-propanol and then the 10 μg mixture was dropped onto the glassy carbon electrode (GCE) and dried completely. CV was performed at a scanning rate of 50 mV s−1, and DPV was measured in the potential ranges between 0.05− 0.3, 0.45−0.65, and 0.55−0.65 V. A pulse amplitude of 2.5 mV, pulse width of 100 ms, and scan rate of 10000 mV s−1 were applied for DPV measurement.
medications for mental illness, such as thioridazine have an increased chance of death during treatments.16 Detection of the presence of dopamine, thioridazine, and Ltyrosine, therefore, is an interesting subject now in the point of life, business, and manufacturing. In this report, the synthesis of βCD−GO−CNT and βCD−CNT composite is introduced by the simple chemical method, and electrochemical behaviors of the composites with the biomolecules mentioned above are characterized. The electrode of each composite without the biomolecules displays typical EDLC behavior. With the biomolecules, new redox peaks appeared in the composites, and especially the βCD−GO−CNT composite shows the highest capability of supramolecular recognition compared to that of βCD−CNT composite.
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EXPERIMENTAL SECTION Graphite powder (200 mesh) was purchased from Alfa Aesar, and multiwalled carbon nanotube (20 μm of length, 10 nm of diameter) was purchased from Hanwha Nanotech. βCD, DA, thioridazine, and L-tyrosine were purchased from Sigma-Aldrich and used without any pretreatment as it was. βCD−CNT composite was synthesized by mixing of βCD (200 mg) and CNT (20 mg) in deionized (DI) water of 20 mL. Then the mixture was sonicated for 180 min at room temperature. After the sonication was finished, the mixture was filtered by a cellulose paper (ADVANTEC, 0.45 μm of pore size, 47 mm of diameter). Obtained βCD−CNT composite film was dried at 60 °C in a vacuum oven for overnight and used for the characterization. βCD−GO−CNT composite was also synthesized through the similar processes except for adding GO into the composite. B
DOI: 10.1021/acs.jpcc.5b03363 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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interlayer distance is 6.3 Å.18 This implies that graphite is expanded when oxidized due to oxygen functional groups.19 The βCD−CNT composite has multiple peaks originating from βCD and CNT, and the βCD−GO−CNT composite has peaks from βCD, CNT, and GO, indicating the new composites included all components and combined them successfully. TGA result is shown in Figure 2. The βCD−CNT composite has one big drop of the weight from 265 to 340 °C, which corresponds to the weight loss by βCD. It is worth noting that the melting point of βCD is 265 °C, and it was found that βCD of 4.9 wt % was included in the βCD−CNT composite. Although the βCD−GO−CNT composite has two big weight losses near 100 and 300 °C. The first weight loss (4.7 wt %) is due to water evaporation originated from GO because GO is very hydrophilic, and the second one (15.5 wt %) started the decomposition from 230 to 340 °C due to both GO and βCD weights. The weight loss by GO is 4.7 wt % because the decomposition of GO started from 230 to 265 °C, and then the weight loss by βCD is 10.8 wt % between 265 and 340 °C. More βCD is incorporated in βCD−GO−CNT composite than in βCD−CNT by adding GO. The introduction of GO to increase the immobilization of βCD effectively, therefore, succeeded. It is worthy of note that the resistivities of βCD− CNT and βCD−GO−CNT thin film are very similar and measured 0.12 and 0.14 Ω mm, respectively. Figure 3 shows the SEM (a−f) and TEM (g, h) images of the samples. GO has a relatively smooth and flat surface, and CNT exhibits a tangled long and thin one-dimensional structure. The
Figure 2. TGA results of βCD−CNT and βCD−GO−CNT.
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RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of graphite, GO, CNT, βCD, βCD−CNT, and βCD−GO−CNT. Graphite exhibits a sharp diffraction peak centered at 2θ = 26.5° corresponding to the (002) graphite plane composed of well-ordered graphene with an interlayer spacing of 3.4 Å. This peak disappears in GO and a relatively low peak appears at 2θ = 14°, corresponding to the diffraction from the (0 0 2) graphite oxide plane and the
Figure 3. SEM images of (a) GO, (b) CNT, (c, d) βCD−CNT, and (e, f) βCD−GO−CNT at different scales and TEM images of (g) βCD−CNT and (h) βCD−GO−CNT. C
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Figure 4. Cyclic voltammogram of the bare GCE, CNT, βCD−CNT, and βCD−GO−CNT in 0.1 M phosphate buffer solution (PBS) [H2PO4]−/ [HPO4]2− (pH = 7.5) at two different potential windows of (a) −0.2−0.5 V and (b) 0.4−0.8 V (scan rate 50 mV s−1).
Figure 5. Cyclic voltammogram of bare GCE, and CNT, CD−CNT, and CD−GO−CNT in 0.1 M phosphate buffer solution (PBS) [H2PO4]−/ [HPO4]2− (pH = 7.5) with (a) 10 mM dopamine, (b) 10 mM thioridazine, and (c) 5 mM L-tyrosine.
βCD−CNT composite shows the tangled CNTs filled with βCD between them. CNT could immobilize a few βCD molecules so that the distribution of βCD is sparsely in the matrix of CNT. βCD was not easy to attach to the CNT surface because CNT has the hydrophobic surface and βCD has a hydrophilic exterior. Although GO has the hydrophilic surface because of oxygen functional groups on the surface,19 GO could immobilize the βCD molecules effectively. It is obvious in Figure 3e,f that more βCD molecules are filled between CNTs and located on top of GO sheets. The TEM images of Figure 4g,h clarify different densities of βCD molecules on βCD− CNT and βCD−GO−CNT composites. More βCD molecules are distributed uniformly and densely in βCD−GO−CNT composite, consistent with the TGA result.
Figure 4 presents CV results of the bare GCE, CNT, βCD− CNT, and βCD−GO−CNT in 0.1 M phosphate buffer solution (PBS) [H2PO4]−/[HPO4]2− (pH = 7.5) at two different potential windows. All samples present a rectangular shape of the CV of EDLC behavior, indicating no faradaic interaction occurred in the potential windows. βCD−CNT and βCD−GO−CNT show higher current intensity than GCE and CNT due to the CNT matrix connected well by βCD and/or GO, resulting in enhancement of electric conductivity in the CV. Moreover, βCD−GO−CNT has a higher current intensity compared to βCD−CNT, indicating that GO improved the electrochemical performance of βCD−CNT by immobilizing more βCD molecules onto the CNT matrix. The characteristics enable βCD−GO−CNT to be a good electrode for electroD
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Figure 6. Proposed mechanism of the redox reaction for (a) dopamine, (b) thioridazine, and (c) L-tyrosine.
presented in Figure 5c for L-tyrosine molecules with the oxidation peak near 0.61 V. The oxidation process is like in Figure 6c. Two hydroxyl groups (−OH) change to (−O), but two different oxidation processes could not be distinguished in CV results. It is obvious that βCD molecules on the surface of GO in the CNT matrix can form complexes with a high capability for supramolecular recognition of the biomolecules as compared to the others, suggesting that the βCD−GO−CNT composite is a good candidate electrode for the biosensor with excellent supramolecular recognition capability. DPV is applied for the investigation of two different oxidation peaks clearly. There are advantages of the DPV method as followings. First, DPV can study the redox properties of extremely small amounts of chemicals. Second, the effect of the charging current (EDLC properties) can be minimized, so high sensitivity of the redox characteristics is achieved. Last, only faradaic current (redox peak) is extracted, so chemical reactions can be analyzed more precisely. Typical DPV results of the four samples without the biomolecules are shown in Figure 7a, indicating no faradaic interaction of the sample in the base solution. There is a clear oxidation peak near 0.15 V in Figure 7b by adding DA into the solution. For the thioridazine molecule, the oxidation peaks divided into two peaks precisely near 0.61 and 0.73, distinguishing between first and second step of oxidation, as suggested in Figure 6b by two electrons and two protons.21 This is also consistent with the
chemical sensors. It should be noted that introduction of large numbers of βCD molecules into the composite can improve not only the stability and dispersion of the composite but also the sensitivity of biomolecules and drugs detection through the formation of supramolecular complexes between βCD and guest molecules. Three kinds of electroactive biomolecules, DA, thioridazine, and L-tyrosine, are applied for the formation of supramolecular complexes. The 0.1 M PBS solution (pH = 7.5) of 10 mL was mixed with 0.25 mL each of 10 mM DA, 10 mM thioridazine, and 5 mM L-tyrosine, respectively, for the investigation. Figure 5a shows CV results for the DA molecule. The sharp oxidation and reduction peaks appeared near 0.15 and 0.12 V because DA is oxidized to dopamine-o-quinone as the potential is ramped up, and dopamine-o-quinone is reduced back to DA as the potential is ramped down,20 as described in Figure 6a. The intensity of the peaks increased in the order GCE, CNT, βCD− CNT, and βCD−GO−CNT, indicating enhancement of the catalytic activity of βCD−GO−CNT. For thioridazine molecules, the peak intensities showed similar behaviors; that is, βCD−GO−CNT exhibits the highest intensity compared to the others. There was no reduction peak except for the oxidation peak near 0.65 and 0.75 V, as shown in Figure 5b. The proposed oxidation mechanism is suggested in Figure 6b. There are two steps for the oxidation corresponding to two oxidation peaks. The same trend of the peak intensities are E
DOI: 10.1021/acs.jpcc.5b03363 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 7. Differential pulse voltammogram of the bare GCE, βCD−CNT, and βCD−GO−CNT in 0.1 M phosphate buffer solution (PBS) [H2PO4]−/[HPO4]2− (pH = 7.5) for the response of (a) no biomolecules, (b) 10 mM dopamine, (c) 10 mM thioridazine, and (d) 5 mM L-tyrosine.
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previously proposed mechanism for the thioridazine oxidation.22,23 The βCD−GO−CNT composite also showed excellent sensitivity among the others. The response to Ltyrosine is also presented in Figure 7d. Two oxidation peaks at 0.54 and 0.72 V appeared, corresponding to the oxidation of carboxyl and hydroxyl group of phenol in L-tyrosine.24 All of CV and DPV results for the biomolecules recognition capability demonstrated that βCD−GO−CNT composite exhibited the highest response to the molecules, indicating that βCD molecules cross-linked by GO could improve their supramolecular recognition and exhibit the highest electrochemical performances compared to those of individual CNT and βCD−CNT composites.
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AUTHOR INFORMATION
Corresponding Author
* H. k. Jeong. Tel: +82-53-850-6483. E-mail address: outron@ daegu.ac.kr. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2013R1A1A3A04008714).
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CONCLUSIONS
βCD−CNT and βCD−GO−CNT composites were synthesized through the simple chemical method and characterized. The βCD−GO−CNT composite showed not only the typical EDLC behavior with the highest current intensity but also excellent supramolecular recognition capability than those of the others. It was found that GO of the hydrophilic surface could immobilize more βCD molecules compared to the βCD−CNT composite, resulting in the enhancement of supramolecular recognition. Therefore, the new composite, βCD−GO−CNT, provides a synergistic effect of high electric conductivity from CNT and high supramolecular recognition capability from βCD effectively immobilized in GO for future biosensor applications. F
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