Research Article | Open Access
Jianping Yong, Xiaoyu Jiang, Xiaoyuan Wu, Shuijin Huang, Qikai Zhang, Canzhong Lu, "Synthesis and Characterization of Ferrocene Derivatives and Preliminarily Electrocatalytic Oxidation of L-Cysteine at Nafion-Ferrocene Derivatives Modified Glassy Carbon Electrode", Advances in Chemistry, vol. 2014, Article ID 987481, 7 pages, 2014. https://doi.org/10.1155/2014/987481
Synthesis and Characterization of Ferrocene Derivatives and Preliminarily Electrocatalytic Oxidation of L-Cysteine at Nafion-Ferrocene Derivatives Modified Glassy Carbon Electrode
Five new structural ferrocene derivatives (2a~2e) were firstly synthesized and characterized by 1H NMR, 13C NMR, ESI-MS, and XRD. Subsequently, the preliminarily electrocatalytic oxidation of L-cysteine (L-Cys) at nafion-ferrocene derivatives modified glassy carbon electrode (GCE) has also been investigated by cyclic voltammetry. The results showed that 2e can dramatically electrocatalyze the oxidation of L-cysteine at its modified GCE in 0.1 mol L−1 NaNO3 aqueous solution with a quasireversible process with mV.
Ferrocene derivatives have attracted considerable attention for their potential applications as nonlinear optical devices, functional materials in electrochemical sensor [1–4], and chiral catalysts [5–7]. In addition, ferrocene derivatives have also attracted wide interest for their considerably biological activities [8–15]. For example, the ferroquine (FQ, SSR97193, Figure 1) is about to complete phase II clinical trials as a treatment for uncomplicated malaria .
L-Cysteine (L-Cys) plays a crucial role in both bio- and environmental chemistry and can be applied in many biochemical processes and diagnosis of disease states. Especially, L-Cys provides a modality for the intramolecular crosslinking of proteins through disulfide bonds to support their secondary structures and functions. Therefore, it is very important to develop simple and effective methods to trace L-Cys detection. Many methods, such as spectrometric method , chromatography , and electrochemical method [19–23], have been used to detect the trace content of L-Cys. However, electrochemical method has attracted considerable attention for simple operation, fast response, and sensitive in situ detection.
The oxidation of L-Cys at Hg, Au, Ag, Pt, and diamond electrodes has been reported [24–27]. However, the direct oxidation of L-Cys at GCE is very sluggish. Ferrocene derivatives have been used as the selective probes for detecting the trace ions, biomacromolecules [28–32]. Nafion is a special material, which possesses widespread applications in the field of electrochemistry analysis, chemical sensor, and nanomaterials [33, 34]. According to our survey, no electrocatalytic oxidation of L-Cys at the nafion-ferrocene derivatives modified GCE has been reported. In this work, five new structural ferrocene derivatives (2a~2e) (Scheme 1) were firstly synthesized and then confirmed by IR, 1H NMR, 13C NMR, MS, and XRD. Subsequently, their preliminarily electrocatalytic oxidation of L-Cys at the nafion-ferrocene derivatives modified GCE has also been investigated.
2. Experimental Details
2.1. Materials and Apparatus
2-Chloro-4,6-dimethoxy-[1,3,5]triazine (CDMT), N,N′-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt), 6-Chloro-1-hydroxybenzotriazole (6-Cl-HOBt), 1-hydroxyl-7-azabenzotriazole (HOAt), 4-dimethylaminopyridine (DMAP), and N-methylmorpholine (NMM) were purchased from Aladdin Reagent company. 5 wt% Nafion solution (DuPont, USA) and other chemicals were commercially available and used without further purification. All melting points were determined on X-4 micro-melting points apparatus and values are uncorrected. 1H (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a 400 MHz Bruker AVANCE III spectrometer in CDCl3. The chemical shifts are expressed in ppm relative to tetramethylsilant (TMS) as the internal standard; ESI-MS was performed on a DECAX-30000 LCQ Deca XP (70 Ev). FTIR spectra were recorded on Spectrum One with KBr disks in the 600–3600 cm−1 region. All electrochemical experiments were carried out using an electrochemistry workstation CHI760 (CH Instrument Inc., USA). The working electrode used in cyclic voltammetry experiments was a CHI104 glassy carbon electrode with a geometric area of ca. 0.15 cm2; a twisted platinum wire auxiliary electrode and Ag/AgCl served as the reference electrode. Tetrahydrofuran (THF) was distilled over sodium and benzophenone before being used; all electrolyte solutions were prepared with doubly distilled water.
2.2. Synthesis Parts
2.2.1. Synthesis of 2-Ferrocene Carboxylic Acid-1,3-dicyclohexyl-isourea (2a) 
Ferrocene carboxylic acid 1 (0.46 g, 2 mmol) and DCC (0.453 g, 2.2 mmol) were added into a 100 mL one necked round bottom flask with 20 mL dry THF. This mixture was stirred under cold bath and a THF solution (10 mL) containing DMAP (0.268 g, 2.2 mmol) was added dropwise by syringe, the mixture was stirred under cold bath for 30 min, and then temperature rose naturally to room temperature. The completion of reaction was judged from the simple TLC analysis. The mixture was evaporated under reduced pressure and the residual directly purified by column chromatography (EtOAc/petroleum ether: 5 : 1 to 2 : 1) to give the desired compound 2a.
Light yellow solid (0.75 g, 86% yield). M.p. 173°C, IR (cm−1): 3235 (NH), 3039 (=C–H), 2931 (C–H), 1701 (O=C–O–), 1598 (C=C), 1542 (C=N), 1182 (C–O); 1H NMR (CDCl3) (m, 8H), 1.35–1.66 (m, 5H), 1.79–1.85 (m, 6H), 1.99–2.09 (m, 2H), 3.48–3.53 (m, 1H), 4.19 (s, 5H, C5H5), 4.28 (t, Hz, 2H of C5H4), 4.67 (t, Hz, 2H of C5H4), 6.04 (d, Hz, 1H, O=C–NH); 13C NMR (CDCl3) , 25.4, 25.5, 26.3, 30.9, 31.1, 31.2, 31.3, 32.6, 47.7, 49.8, 56.8, 70.4, 77.9 (C5H4), 70.3 (C5H5), 154.9 (C=N), 171.9 (–C=O); MS (ESI): m/z 436 [M]+.
2.2.2. Synthesis of Ferrocene Carboxylic Acid Benzotriazol-1-yl Ester (2b)
Ferrocene carboxylic acid 1 (0.46 g, 2 mmol), DCC (0.453 g, 2.2 mmol), and HOBt (0.337 g, 2.5 mmol) were added into a 100 mL one necked round bottom flask with 20 mL dry THF. This mixture was stirred under cold bath and a THF solution (10 mL) containing DMAP (0.268 g, 2.2 mmol) was added dropwise by syringe, the mixture was stirred under cold bath for 30 min, and then temperature rose naturally to room temperature. The completion of reaction was judged from the simple TLC analysis. The mixture was evaporated under reduced pressure and the residual purified directly by column chromatography (EtOAc/petroleum ether: 5 : 1 to 2 : 1) to give the desired compound 2b.
Light yellow solid (0.638 g, 92% yield). M.p. 141-142°C, IR (cm−1): 3035 (=C–H), 1776 (O=C–O–), 1441 (C=C), 1261 (C=N), 1054 (C–O), 768 (Ph); 1H NMR (CDCl3) (s, 5H, C5H5), 4.67 (t, Hz, 2H of C5H4), 5.07 (t, Hz, 2H of C5H4), 7.39–7.45 (m, 2H), 7.52 (d, Hz, 1H), 8.08 (d, Hz, 1H); 13C NMR (CDCl3) , 70.8, 73.5 (C5H4), 71.0 (C5H5), 108.4, 120.5, 124.7, 128.6, 128.9, 143.6 (benzotriazol-ring), 168.3 (–O–C=O); MS (ESI): m/z 347[M]+.
2.2.3. Synthesis of Ferrocene Carboxylic Acid 6-Chloro-benzotriazol-1-yl Ester (2c)
The detailed process for synthesis of 2c is the same as of 2b. Red crystal (0.723 g, 75% yield). M.p. 112-113°C, IR (cm−1): 3100 (=C–H), 1792 (O=C–O–), 1445 (C=C), 1286 (C=N), 1029 (C–O), 881 (C–Cl), 827 (Ph); 1H NMR (CDCl3) (s, 5H, C5H5), 4.72 (t, Hz, 2H of C5H4), 5.10 (t, Hz, 2H of C5H4), 7.41 (dd, , 1.6 Hz, 1H), 7.49 (d, Hz, 1H), 8.03 (d, Hz, 1H); 13C NMR (CDCl3) , 71.3, 73.9 (C5H4), 71.4 (C5H5), 108.6, 121.8, 126.4, 129.9, 135.5, 142.4 (benzotriazol-ring), 168.5; MS (ESI): m/z 482[M]+.
2.2.4. Synthesis Ferrocene Carboxylic Acid [1,2,3]triazolo[4,5-b]pyridin-3-yl Ester (2d)
The detailed process for synthesis of 2d is the same as synthesis of 2b. Light yellow solid (0.557 g, 80% yield). M.p. 136-137°C, IR (cm−1): 3108 (=C–H), 1775 (O=C–O), 1448 (C=C), 1265 (C=N), 1058 (C–O), 887; 1H NMR (400 MHz, CDCl3) (s, 5H, C5H5), 4.61 (s, 2H of C5H4), 5.03 (s, 2H of C5H4), 7.38 (dd, , 4.8 Hz, 1H), 8.38 (d, Hz, 1H), 8.68 (d, Hz, 1H); 13C NMR (CDCl3) , 71.1, 73.5 (C5H4), 71.3 (C5H5), 129.5, 135.1, 140.9, 151.7 ([1,2,3]triazolo[4,5-b]pyridin-ring), 168.1 (–O–C=O); MS (ESI): m/z 348[M]+.
2.2.5. Synthesis of Ferrocene Carboxylic Acid 4,6-Dimethoxy-[1,3,5]triazin-2-yl Ester (2e)
Ferrocene carboxylic acid 1 (0.46 g, 2 mmol) and CDMT (0.438 g, 2.5 mmol) were added into a 100 mL one necked round bottom flask with 20 mL dry THF. This mixture was stirred under cold bath and a THF solution (10 mL) containing NMM (0.253 g, 2.5 mmol) was added dropwise by syringe, the mixture was stirred under cold bath for 30 min, and then temperature rose naturally to room temperature. The completion of reaction was judged from the simple TLC analysis. The mixture was evaporated under reduced pressure and the residual purified directly by column chromatography (EtOAc/etroleum ether: 5 : 1 to 2 : 1) to give the desired compound 2e.
Light yellow solid (0.863 g, 92% yield). M.p. 90–92°C, FTIR (cm−1): 3104 (=C–H), 2951 (C–H), 1747 (O=C–O), 1577 (C=C), 1469 (C=N), 1366 (C–O); 1H NMR (400 MHz, CDCl3) δ (ppm): 4.08 (s, 6H, 2OCH3), 4.39 (s, 5H, C5H4), 4.56 (s, 2H of C5H4), 4.95 (s, 2H of C5H4); 13C NMR (CDCl3) (OCH3), 70.6, 71.3, 73.1 (C5H4), 71.4 (C5H5), 167.9 (–C=O), 171.2, 174.5 ([1,3,5]triazin-ring); MS (ESI): m/z 369[M]+.
2.3.1. The Process for Preparation of the Solutions of Nafion-Ferrocene Derivatives
Compound 2a (6.6 mg, 0.015 mmol) and 50 μL 5% nafion solution were added in a 2 mL centrifugal tube, then 1 mL of absolute alcohol was added, and the mixture was sonicated for 10 min to obtain the solution of mol L−1 2a.
The detailed process for preparing the solutions of 1.5 ×10−2 mol L−1 2b, 2c, 2d, and 2e is the same as 2a.
2.3.2. General Process for Preparing the Modified Glassy Carbon Electrode and Preliminarily Electrocatalytic Oxidation of L-Cysteine
A GCE was polished with 0.1 μm α-Al2O3, rinsed with doubly distilled water, sonicated in 1 mol L−1 H2SO4 for 5 min, and rinsed with doubly distilled water again prior to each experiment. Then, it was dried naturally.
A modified GCE was prepared by placing 5 μL of solution prepared in Section 2.3.1 onto the dried GCE surface by microsyringe. The electrode was dried naturally to obtain the modified GCE.
The modified GCE, a twisted platinum wire electrode, and Ag/AgCl electrode were dipped into 20 mL (V/V, 1 : 1) water solution of 6.0 × 10−3 mol L−1 L-Cys and 0.1 mol L−1 NaNO3. The Ag/AgCl electrode was connected to the main body of the cell through a luggin capillary whose end was connected on the modified GCE and positioned close to the electrode surface. The solution was thoroughly flushed with high purity nitrogen for 5 min before each run to remove the oxygen from the solution in the electrochemical cell. All experiments were carried out at room temperature.
3. Results and Discussion
Ferrocene derivatives (2a~2e) were firstly synthesized in higher yield and their structures were confirmed by IR, 1H and 13C NMR, and mass spectrometry.
There showed three kinds of proton signals for ferrocene core of the ferrocene derivatives (2a~2e) in 1H NMR spectra (Figure 2), the protons of C5H5 showed a singlet, the C5H4 showed two triplets of 2a~2c, but the C5H4 showed two singlets of 2d. Three signals of the carbon atoms of C5H4 and one signal of carbon atom of C5H5 appear in 13C NMR spectra, while two carbon signals of C5H4 appear upfield and one carbon signal of C5H4 appears downfield compared to the carbon signals of C5H5.
3.2. X-Ray Crystallographic Study
X-ray diffraction data for 2c was collected on a Siemens Smart CCD diffractometer equipped with a graphite-monochromated CuKa radiation ( Ǻ). The structure was solved by direct method using the program SHELXL-97 and refined by full-matrix least-squares techniques on with SHELXL-97 program package [36, 37]. The molecular structure (Figure 3) and crystal data of 2c are listed in Tables 1 and 2.
3.3. Cyclic Voltammetric Behavior of Compounds 2a~2e and Nafion Modified GCE Electrocatalyze the Oxidation of L-Cys
The preliminarily electrochemical oxidation of L-Cys with the nafion-ferrocene derivatives modified GCE in 0.1 mol L−1 different supporting electrolyte solutions, such as NaCl, NaNO3, NaAc, and Na2SO4, has been investigated by cyclic voltammetry. The results (Figure 4) showed that 2e can promote the electrochemical oxidation of L-Cys dramatically in 0.1 mol L−1 NaNO3 solution with a quasireversible process with mV (curve (d) in Figure 4), while direct oxidation of L-Cys at a GCE is very sluggish (cruve (b) in Figure 4).
In this study, five ferrocene derivatives (2a~2e) were firstly synthesized and structurally characterized using spectroscopic methods. The preliminarily electrochemical oxidation of L-Cys with the nafion-ferrocene derivatives modified GCE in 0.1 mol L−1 NaNO3 solution has also been investigated by cyclic voltammetry. The results showed that 2e can promote the electrochemical oxidation of L-Cys dramatically at its modified GCE with a quasireversible process with mV, while L-Cys itself showed a sluggish electrochemical response at GCE. Thus, 2e modified GCE can be used to detect the trace L-Cys content. (The quantitative analysis of trace L-Cys and electrochemical kinetics of 2e modified GCE are undergoing.) Based on this research, more ferrocene derivatives will be synthesized and the oxidation of L-Cys and other biological molecules at the modified GCE will be studied soon.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by the 973 Key Program of the MOST (2010CB933501, 2012CB821705), the Chinese Academy of Sciences (KJCX2-YW-319, KJCX2-EW-H01), the National Natural Science Foundation of China and the Natural Science Foundation of Fujian Province (2007HZ0001-1, 2009HZ0005-1, 2009HZ0006-1, and 2006L2005), and Postdoctoral Foundation of Fujian Province.
- J. S. Miller and A. J. Epstein, “Organic and organometallic molecular magnetic materials—designer magnets,” Angewandte Chemie (International Edition in English), vol. 33, no. 4, pp. 385–415, 1994.
- P. Nguyen, P. Gómez-Elipe, and L. Manners, “Organometallic polymers with transition metals in the main chain,” Chemical Reviews, vol. 99, no. 6, pp. 1515–1548, 1999.
- A. Sola, A. Tárraga, and P. Molina, “A ferrocenyl-guanidine derivative as a highly selective electrochemical and colorimetric chemosensor molecule for acetate anions,” Dalton Transactions, vol. 41, no. 27, pp. 8401–8409, 2012.
- Y. Wang, X. He, K. Wang, X. Ni, J. Su, and Z. Chen, “Ferrocene-functionalized SWCNT for electrochemical detection of T4 polynucleotide kinase activity,” Biosensors and Bioelectronics, vol. 32, no. 1, pp. 213–218, 2012.
- L.-Z. Du, J.-F. Gong, C. Xu, Y. Zhu, Y.-J. Wu, and M.-P. Song, “Synthesis and structures of novel diastereomeric cyclopalladated ferrocenylimines derived from chiral β-amino alcohols,” Inorganic Chemistry Communications, vol. 9, no. 4, pp. 410–414, 2006.
- J. Zhang, L. Zhao, M. Song, T. C. W. Mak, and Y. Wu, “Highly efficient cyclopalladated ferrocenylimine catalyst for Suzuki cross-coupling reaction of 3-pyridylboronic pinacol ester with aryl halides,” Journal of Organometallic Chemistry, vol. 691, no. 6, pp. 1301–1306, 2006.
- F. Yang, X. Cui, Y.-N. Li, J. Zhang, G.-R. Ren, and Y. Wu, “Cyclopalladated ferrocenylimines: efficient catalysts for homocoupling and Sonogashira reaction of terminal alkynes,” Tetrahedron, vol. 63, no. 9, pp. 1963–1969, 2007.
- D. R. van Staveren and N. Metzler-Nolte, “Bioorganometallic chemistry of ferrocene,” Chemical Reviews, vol. 104, no. 12, pp. 5931–5985, 2004.
- C. Herrmann, P. F. Salas, B. O. Patrick et al., “1,2-disubstituted ferrocenyl carbohydrate chloroquine conjugates as potential antimalarial agents,” Dalton Transactions, vol. 41, no. 21, pp. 6431–6442, 2012.
- K. N. Tiwari, J.-P. Monserrat, A. Hequet et al., “In vitro inhibitory properties of ferrocene-substituted chalcones and aurones on bacterial and human cell cultures,” Dalton Transactions, vol. 41, no. 21, pp. 6451–6457, 2012.
- M. M. Abd-Elzaher, S. A. Moustafa, A. A. Labib, H. A. Mousa, M. M. Ali, and A. E. Mahmoud, “Synthesis, characterization and anticancer studies of ferrocenyl complexes containing thiazole moiety,” Applied Organometallic Chemistry, vol. 26, no. 5, pp. 230–236, 2012.
- K. Kowalski, A. Koceva-Chyła, A. Pieniazek et al., “The synthesis, structure, electrochemistry and in vitro anticancer activity studies of ferrocenyl-thymine conjugates,” Journal of Organometallic Chemistry, vol. 700, pp. 58–68, 2012.
- L. Soulère and J. Bernard, “Design, solid phase synthesis and evaluation of cationic ferrocenoyl peptide bioconjugates as potential antioxidant enzyme mimics,” Bioorganic and Medicinal Chemistry Letters, vol. 19, no. 4, pp. 1173–1176, 2009.
- C.-W. Wei, Y. Peng, L. Zhang et al., “Synthesis and evaluation of ferrocenoyl pentapeptide (Fc-KLVFF) as an inhibitor of Alzheimer's Aβ1-42 fibril formation in vitro,” Bioorganic and Medicinal Chemistry Letters, vol. 21, no. 19, pp. 5818–5821, 2011.
- J. Guillon, S. Moreau, E. Mouray et al., “New ferrocenic pyrrolo[1,2-a]quinoxaline derivatives: synthesis, and in vitro antimalarial activity,” Bioorganic and Medicinal Chemistry, vol. 16, no. 20, pp. 9133–9144, 2008.
- A. Mahajan, L. Kremer, S. Louw, Y. Guéradel, K. Chibale, and C. Biot, “Synthesis and in vitro antitubercular activity of ferrocene-based hydrazones,” Bioorganic and Medicinal Chemistry Letters, vol. 21, no. 10, pp. 2866–2868, 2011.
- W. Hao, A. McBride, S. McBride, J. P. Gao, and Z. Y. Wang, “Colorimetric and near-infrared fluorescence turn-on molecular probe for direct and highly selective detection of cysteine in human plasma,” Journal of Materials Chemistry, vol. 21, no. 4, pp. 1040–1048, 2011.
- X. Guan, B. Hoffman, C. Dwivedi, and D. P. Matthees, “A simultaneous liquid chromatography/mass spectrometric assay of glutathione, cysteine, homocysteine and their disulfides in biological samples,” Journal of Pharmaceutical and Biomedical Analysis, vol. 31, no. 2, pp. 251–261, 2003.
- N. Sattarahmady and H. Heli, “An electrocatalytic transducer for l-cysteine detection based on cobalt hexacyanoferrate nanoparticles with a core-shell structure,” Analytical Biochemistry, vol. 409, no. 1, pp. 74–80, 2011.
- L.-P. Liu, Z.-J. Yin, and Z.-S. Yang, “A l-cysteine sensor based on Pt nanoparticles/poly(o-aminophenol) film on glassy carbon electrode,” Bioelectrochemistry, vol. 79, no. 1, pp. 84–89, 2010.
- Y.-T. Lai, A. Ganguly, L.-C. Chen, and K.-H. Chen, “Direct voltammetric sensing of l-Cysteine at pristine GaN nanowires electrode,” Biosensors and Bioelectronics, vol. 26, no. 4, pp. 1688–1691, 2010.
- Y.-P. Hsiao, W.-Y. Su, J.-R. Cheng, and S.-H. Cheng, “Electrochemical determination of cysteine based on conducting polymers/gold nanoparticles hybrid nanocomposites,” Electrochimica Acta, vol. 56, no. 20, pp. 6887–6895, 2011.
- R. Ojani, J.-B. Raoof, and E. Zarei, “Preparation of poly N,N-dimethylaniline/ferrocyanide film modified carbon paste electrode: application to electrocatalytic oxidation of l-cysteine,” Journal of Electroanalytical Chemistry, vol. 638, no. 2, pp. 241–245, 2010.
- Z. Liu, H. Zhang, S. Hou, and H. Ma, “Highly sensitive and selective electrochemical detection of L-cysteine using nanoporous gold,” Microchimica Acta, vol. 177, no. 3-4, pp. 427–433, 2012.
- G. Hager and A. G. Brolo, “Adsorption/desorption behaviour of cysteine and cystine in neutral and basic media: electrochemical evidence for differing thiol and disulfide adsorption to a Au(1 1 1) single crystal electrode,” Journal of Electroanalytical Chemistry, vol. 550-551, pp. 291–301, 2003.
- Z. Samec, Z. Malysheva, J. Koryta, and J. Pradáč, “A contribution to the voltammetric study of cystine and cysteine at Pt electrodes in 0.5 M H2SO4,” Journal of Electroanalytical Chemistry, vol. 65, no. 2, pp. 573–586, 1975.
- J. Koryta and J. Pradáč, “Electrode processes of the sulfhydryl-disulfide system III. cysteine at platinum and gold electrodes,” Journal of Electroanalytical Chemistry, vol. 17, no. 1-2, pp. 185–189, 1968.
- A. Thakur, S. Sardar, and S. Ghosh, “Click-generated triazole based ferrocene-carbohydrate bioconjugates: a highly selective multisignalling probe for Cu(II) ions,” Journal of Chemical Sciences, vol. 124, no. 6, pp. 1255–1260, 2012.
- J. B. Raoof, R. Ojani, and H. Karimi-Maleh, “Electrocatalytic oxidation of glutathione at carbon paste electrode modified with 2,7-bis (ferrocenyl ethyl) fluoren-9-one: application as a voltammetric sensor,” Journal of Applied Electrochemistry, vol. 39, no. 8, pp. 1169–1175, 2009.
- J. M. Casas-Solvas, E. Ortiz-Salmerón, L. García-Fuentes, and A. Vargas-Berenguel, “Ferrocene-mannose conjugates as electrochemical molecular sensors for concanavalin A lectin,” Organic and Biomolecular Chemistry, vol. 6, no. 22, pp. 4230–4235, 2008.
- H. Karimi-Maleh, S. Mehdipour-Ataei, M. Hatami, and M. A. Khalilzadeh, “Voltammetric determination of captopril using a novel ferrocene-based polyamide as a mediator and multi-wall carbon nanotubes as a sensor,” Journal of Analytical Chemistry, vol. 69, pp. 162–168, 2014.
- W. Guan, X. Duan, and M. A. Reed, “Highly specific and sensitive non-enzymatic determination of uric acid in serum and urine by extended gate field effect transistor sensors,” Biosensors and Bioelectronics, vol. 51, pp. 225–231, 2014.
- L. Vayssieres, K. Keis, S.-E. Lindquist, and A. Hagfeldt, “Purpose-built anisotropie metal oxide material: 3D highly oriented microrod array of ZnO,” Journal of Physical Chemistry B, vol. 105, no. 17, pp. 3350–3352, 2001.
- J. Lee, Z. Li, M. Hodgson, J. Metson, A. Asadov, and W. Gao, “Structural, electrical and transparent properties of ZnO thin films prepared by magnetron sputtering,” Current Applied Physics, vol. 4, no. 2-4, pp. 398–401, 2004.
- J.-P. Yong and H. A. Aisa, “Chemical modification of rupestonic acid and preliminarily in vitro antiviral activity against influenza A3 and B viruses,” Bulletin of the Korean Chemical Society, vol. 32, no. 4, pp. 1293–1297, 2011.
- G. M. Sheldrick, SHELXS-97, Program for Solution of Crystal Structures, University of Gottingen, Göttingen, Germany, 1997.
- G. M. Sheldrick, SHELXS-97, Program for Refinement of Crystal Structures, University of Gottingen, Göttingen, Germany, 1997.
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