Chinese Chemical Letters  2015, Vol.26 Issue (11): 1421-1425   PDF    
The selectivity of triethylene glycol modified glassy carbon electrode for charged and uncharged pieces
Ju-Jie Rena , Na Suna, Min Cuia, Xue-Ping Jib    
a Department of Chemistry, School of Sciences, Hebei University of Science and Technology, Shijiazhuang 050018, China;
b Department of Medical Chemistry, Hebei Medical University, Shijiazhuang 050017, China
Abstract: A triethylene glycol modified glassy carbon electrode (TEG-GCE) was fabricated by a controlledpotential electrolysis procedure. The performance of the film on the modified electrode surface was investigated by cyclic voltammetry with different probes. It was firstly found that while neutral pieces could penetrate the TEG film on the GCE surface, the ionic pieces, whatever it is anion or cation, was blocked by the film. This property was successfully used for determining dopamine (DA) in the presence of ascorbic acid (AA) with differential pulse voltammetry (DPV).
Key words: Triethylene glycol     Glassy carbon electrode     Dopamine     Ascorbic acid     Electrochemical modification    
1. Introduction

Owing to its properties of high temperature resistance,extreme resistance to chemical attack and impermeability to gases and liquids,glassy carbon has been widely used as an electrode material in electrochemistry [1, 2]. Based on glassy carbon electrode (GCE),various electrochemical sensors and biosensors have been fabricated by modifying the electrode with chemical or biological materials. The modification can change the electrocatalysis property,or improve the selectivity [3, 4].

An electrochemical method for direct covalent modification of glassy carbon surfaces with 1-alkanol was proposed by Ohmori and co-workers [5, 6, 7]. They noticed that oxygen functionalities on the carbonsurface increasedwhena carbonelectrodewas oxidizedinan aqueousmedium. They considered that the carbocations formed by the oxidation of the electrode reacted with water to create hydroxyl group sites on the electrode surface and somewere further oxidized to carbonyl and carboxyl groups. Based on this consideration they proposed that alkoxy group could be immobilized on GCE surface by oxidizing the electrode in 1-alkanolmedium. The alkanol molecules could be fixed on the electrode surface via an ether-linkage in this way. With the similar procedure,they also studied the properties of GCE oxidized in α,ω-alkanediol and oligo (ethylene glycol) (OEG) medium,respectively [8, 9, 10]. Lin and co-worker reported a GCE covalently modified with poly(vinyl alcohol) by the similar procedure for simultaneous electroanalysis of dopamine,ascorbic acid and uric acid [11].

OEG is water soluble. OEG self-assembled monolayers have been widely used to prevent protein adsorption from biological media [12, 13, 14]. Ohmori and co-workers modified GCE with OEG by anodization of GCE in OEG,e.g. triethylene glycol (TEG) to eliminate protein adsorption,and applied the TEG modified GCE (TEG-GCE) in the electrochemical HPLC analysis of proteincontaining samples [9, 10]. In these studies,Ohmori and coworkers noticed that cationic and neutral pieces had good electrochemical performance on TEG-GCE while the electrochemical signal of anion was suppressed. They ascribed these phenomena to the anodization of the terminal hydroxyl groups to carboxylates on TEG-GCE surface.

However,we observed that cation was also blocked by the film on TEG-GCE surface. As far as we know,this property of TEG-GCE has not been noticed by other researchers. In this study,we first fabricated a TEG-GCE by an electrochemical oxidation procedure, and the performance of the film on electrode surface was investigated by cyclic voltammetry with different probes. As an application,the TEG-GCE was then used in detection of DA in the existence of ascorbic acid (AA) by differential pulse voltammetry (DPV) in neutral pH.

2. Experimental 2.1. Reagents and apparatus

Dopamine hydrochloride (DA) and ascorbic acid (AA) were purchased from Wako Pure Chemicals (Osaka,Japan). Hexaamminecobalt( III) chloride,ruthenium hexaammine trichloride,potassium ferrocyanide,hexaamminecobalt(III) chloride,3,4- dihydroxytoluene (DHT) and 3,4-dihydroxyphenylacetic acid (DOPAC) were all purchased from Sigma-Aldrich. CuSO4·5H2O was purchased from Tianjin HengXing Chemical Reagent Co.,Ltd. (NH4)2Fe(SO4)2·6H2O was obtained from Tianjin Damao Chemical Reagent Factory and FeNO3·9H2O was purchased from Tianjin Yongda Chemical Reagent Co.,Ltd. The reagents in different buffer solutions were prepared immediately before use. Buffer solutions were 0.1 mol/L KCl + 0.05 mol/L (citric acid + NaOH + HCl) buffer (pH 2.2),0.05 mol/L KCl + 0.05 mol/L (citric acid + sodium citrate) buffer (pH 5.2),0.1 mol/L phosphate buffer solution (PBS) (pH 7.2), 0.1 mol/L phosphate buffer solution + 0.05 mol/L NaOH (pH 9.2) and 0.05 mol/L Na2HPO4·12H2O + 0.1 mol/L NaOH (pH 11.2). Triethylene glycol (TEG) was obtained from Nacalai Tesque INC. Glassy carbon (GC) disk was provided by Tokai Carbon Co.,Ltd. The GCE was electrochemically modified by a potentiostat/galvanostat (HA301,Hokuto Denko,Tokyo,Japan) connected to a coulomb ampere hour meter (HF201,Hokuto Denko,Tokyo,Japan). Cyclic voltametric experiments were performed at room temperature using an arbitrary function generator (HB105 Hokuto Denko) and a potentiostat/galvanostat (HA150). All differential pulse voltammetrys (DPVs) and electrochemical impedance spectroscopy (EIS) measurements were performed with a CHI 660D electrochemical workstation (Chenhua,Shanghai,China). The electrochemical cell was assembled with a three-electrode system using a GC disk (with or without electrochemical modifications) as working electrode,a saturated calomel electrode (SCE) as reference electrode and a platinum wire as counter electrode.

2.2. Preparation of TEG-GCE

The cleaned GCE was subjected to controlled-potential electrolysis in TEG containing a supporting electrolyte (0.1 mol/kg LiClO4) at 2.0 V [10]. The electric quantity flowing through the cell was controlled so as to control the deposition amount of TEG on GCE surface. Reference electrode was AgCl/Ag wire. After the treatment,the modified electrodewaswashedwith water,MeOH andwater,and then itwas electrochemically treated in 0.1 mol/kg KCl solution by repetitively scanning the electrode potential between 0 and -0.5 V at rate 0.1 V[3TD$DIF]/s[1TD$DIF] for five cycles. After the electrochemical treatment,it was washed successively as the above steps. Then the modified electrode was ready for use.

3. Results and discussion 3.1. EIS characterization of the modified electrodes

Fig. 1 shows the Nyquist plots of the impedance spectroscopy of the bare and modified GCE. At a bare GCE,only a very small semicircle could be observed,showing a low charge transfer resistance (Fig. 1(a),Rct = 0.470 kΩ). When TEG film was electrodeposited on GCE,the charge transfer resistance increased greatly (Fig. 1(b),Rct = 30.2 kΩ),implying that the layer of TEG film obstructed charge transfer of the redox probe. As the increase of deposition electric quantity,the values of Rct increased gradually (Fig. 1c-e). The results indicate that the TEG film inhibits the penetration of the [Fe(CN)6]3-/[Fe(CN)6]4- redox pieces toward the electrode,and this inhibition gradually strengthens with the increase of modifying electric quantity.

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Fig. 1.Nyquist plot recorded in 5.0 mmol/L [Fe(CN)6]3-/[Fe(CN)6]4- containing 0.5 M KCl over a frequency range of 1.0 × 101–1.0 × 105 Hz on bare GCE (a), TEG– GCE with the electric quantity of 1.0 C (b), 3.0 C (c), 6.0 C (d), and 10.0 C (e).
3.2. Cyclic voltammetric response of TEG-GCE to ruthenium hexaammine and ferrocyanide

Ruthenium hexaammine trichloride and potassium ferrocyanide are hydrophilic inorganic compounds. Ruthenium hexaammine and ferrocyanide are positively and negatively charged respectively in PBS at pH 7.2,5.2 and 2.2. Fig. 2(I) shows the cyclic voltammograms of 2 mmol/L ruthenium hexaammine trichloride solutions at pH 7.2,5.2 and 2.2,respectively. Curve aa' shows the voltammogram on bare electrode,curves bb'' and cc0 show the voltammograms on GCE modified by TEG with electric quantity of 1.0 C and 3.0 C,respectively. As shown in Fig. 2(I),the voltammetric response of ruthenium hexaammine on bare electrode was almost reversible. However,the voltammetric response of ruthenium hexaammine on the electrode,which was treated in TEG with the electric quantity above 3.0 C,was almost completely blocked by the modified surface film.

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Fig. 2.Cyclic voltammograms of (I) 2 mmol/L ruthenium hexaammine at pH 7.2 (A), 5.2 (B) and 2.2 (C) obtained on bare GCE (curve aa') and TEG–GCE treated with the electric quantity of 1.0 C (curve bb'') and 3.0 C (curve cc0) and (II) 2 mmol/L ferrocyanide at pH 7.2 (A), 5.2 (B) and 2.2 (C) obtained on bare GCE (curve aa') and TEG–GCE treated with the electric quantity of 0.10 C (curve bb'') and 1.0 C (curve cc0) at scan rate of 0.1 V s-1.

Fig. 2(II) A,B,and C show the cyclic voltammograms of 2mmol/L potassium ferrocyanide solution at pH 7.2,5.2 and 2.2,respectively. Curve aa' shows the voltammogram of ferrocyanide on bare electrode,curves bb'' and cc0 show the voltammograms of ferrocyanide on GCE modified by TEG with electric quantity of 0.10 C and 1.0 C,respectively. The response of ferrocyanide on bare GCEwas reversible,while itwas completely blocked by themodified surface film when the electrode was modified by TEG with the electric quantity above 1.0 C.

The above results demonstrate that TEG film on GCE surface can suppress the response of analytes in water solution whether it is negatively or positively charged. In this case,the pH of the solution ranging from 7.2 to 2.2 had no significant influence on the electrochemical property of electrodes. These results were different from the reports [8, 10],in which the authors proposed that the terminal hydroxyl group of TEG on GCE was oxidized to carboxyl group during the anodic modification,thus the TEG film was negatively charged at pH 7.2 and it just suppressed the response of anion,whereas cation had well response on the modified electrode.

To further confirm the resistance of TEG film to cation,cyclic voltammetric experiments of TEG-GCE to Cu2+,Fe3+,Fe2+ and hexaamminecobalt(III) were carried out at pH 7.2. The cyclic voltammetric responses of TEG-GCE to all of these cations were well blocked. (The figures are shown in supplementary data.) TEG- GCE showed a similar performance at higher pH than pH 7.2.

3.3. Cyclic voltammetric response of TEG-GCE to DHT

The ionization degree of DHT in water solution is very small, and decreases weakly with the increase of the pH from 2.2,5.2,to 7.2. DHT almost all exists neutrally at the three pH.

Fig. 3 shows that the presence of TEG film on GCE surface just slow down a little the heterogeneous electron transfer of DHT. There was no obvious change on the anodic peak currents for modified electrode even if the electrode was modified by TEG with the electric quantity of 10.0 C or more (not shown here). The electrochemical response of DHT was a little bigger in lower pH solution than that in high pH solution. These results demonstrate that neutral analytes can penetrate the TEG film on the GCE surface and can be detected by the electrode even if it was modified by TEG with large electric quantity.

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Fig. 3.Cyclic voltammograms of 2 mmol/L DHT at pH 7.2 (A), 5.2 (B) and 2.2 (C) obtained on bare GCE (curve aa') and TEG–GCE treated with the electric quantity of 10.0 C (curve bb'') at scan rate of 0.1 V[3TD$DIF]/s[1TD$DIF].

As the neutral DHT is more hydrophobic than charged ruthenium hexaammine and ferrocyanide,the results shown in Figs. 2 and 3 also demonstrate that the TEG layer on GCE surface is hydrophobic. That may because the layer modified on GCE surface is hydrophobic,although TEG itself is hydrophilic. Thus,while the hydrophobic compounds in water solution are willing to enter and penetrate this layer,the hydrophilic compounds will prefer remaining in water solution. The above results show that,while the responses of hydrophilic inorganic analytes can completely be blocked by TEG modification surface film,hydrophobic organic compounds present high responses at TEG-GCE even if it is modified by TEG with large electric quantity.

3.4. Cyclic voltammetric response of TEG-GCE to AA,DOPAC and DA

The charge and hydrophobicity of AA,DOPAC and DA are influenced by the pH of the solution. At higher pH,AA[2TD$DIF] (pKa = 4.17) and DOPAC (pKa = 4.22) exist as ascorbic and 3,4-dihydroxyphenylacetic anion respectively,while DA[2TD$DIF] (pKb = 8.87) exists as a neutral compound. At lower pH,DOPAC and AA exist as neutral pieces,while DA exists as a positively charged one. Typical cyclic voltammograms of 2 mmol/L DOPAC and 2 mmol/L AA at pH 7.2, 5.2,2.2 are shown in Fig. 4(I) and (II),respectively.

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Fig. 4.Cyclic voltammograms of (I) 2 mmol/L DOPAC at pH 7.2 (A), 5.2 (B) and 2.2 (C) obtained on bare GCE (curve aa') and TEG–GCE treated with the electric quantity of 6.0 C (curve bb'') and (II) 2 mmol/L AA at pH 7.2 (A), 5.2 (B) and 2.2 (C) obtained on bare GCE (curve a) and TEG–GCE treated with the electric quantity of 5.0 C (curve b) at scan rate of 0.1 V s-1.

At pH 7.2,both DOPAC and AA exist as anions. They are hydrophillic and inhibited by the TEG modification surface film,so they can hardly be detected by TEG-GCE with enough electric quantity,5.0 C for AA and 6.0 C for DOPAC. Here,the difference of electric quantity between AA and DOPAC can be explained by the fact that AA has more hydrophilic hydroxyl groups than DOPAC and no hydrophobic benzol group as DOPAC has,which means AA is more hydrophilic than DOPAC,so AA was completely undetectable by TEG-GCE. The same consideration can be made in the case of pH 5.2. At pH 5.2,the anodic peak became extremely small in voltammograms of AA while a large and obvious peak in the voltammograms of DOPAC. At pH 2.2,both DOPAC and AA exist mainly as neutral compounds in solution and are therefore more hydrophobic,so they can penetrate the TEG modification surface and be detected.

As shown in Fig. 5,on bare electrode,the anodic peak of DA at each pH was all obvious. On GCE modified by TEG with 6.0 C,the film of TEG suppressed the response of DA,and the influence differed at different pH of the solution. On GCE modified by TEG with 6.0 C,the anodic peak currents of DA was well suppressed at pH 2.2,but gradually became apparent from pH 5.2 to 7.2. This is because DA exists almost all as cation in the solution at pH 2.2,and the cation pieces is inhibited by the TEG modification surface film. However,the concentration of neutral DA in solution became high at pH 5.2 and 7.2,neutral DA was more hydrophobic than the cation and therefore can penetrate the TEG modification surface film and be detected.

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Fig. 5.Cyclic voltammograms of 2 mmol/L dopamine hydrochloride at pH 7.2 (A), 5.2 (B) and 2.2 (C) obtained on bare GCE (curve aa') and TEG–GCEs treated with the electric quantity of 6.0 C (curve bb'') at scan rate of 0.1 V s-1.
3.5. Determination of DA in the presence of AA at pH 7.2

In pH 7.2 PBS,TEG-GCE had no voltammetric response to AA, thus,TEG-GCE can be used to individual determination of DA in the presence of AA. DPVs were used for these determinations. As shown in Fig. 6,keeping the concentration of AA constant at 0.3 mmol/L and increasing the concentration of DA gradually from 5 to 35 μmol/L,the peak current of DA increased while AA was completely undetectable. It can be obtained that we can determine DA individually in the mixed solution and eliminate the interference of AA in pH 7.2 PBS. A calibration plot was obtained for DA with a dynamic range between 5 and 35 μmol/L (Fig. 6). The linear regression equation is expressed as: ip = 0.548 + 0.034 [DA μmol/L],with a correlation coefficient of 0.996. The detection limit for DA was determined to be 1.7 μmol/L. The relative standard deviation (RSD) for 20 μmol/L DA was 3.1% (less than 5%,n = 5). Thus,the modified electrode exhibited good reproducibility.

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Fig. 6.DPVs of DA at TEG–GCE in the presence of 0.3 mmol/L AA in pH 7.2 PBS. DA concentrations (from a to g): 5, 10, 15, 20, 25, 30 and 35 mmol/L (ip means the peak current), scan rate: 0.1 V/s.
4. Conclusion

TEG-GCE shows a selectivity. The relatively strong hydrophobic analytes are able to penetrate the TEG modified surface film easily and can be detected by the electrode,whereas relatively strong hydrophilic analytes were suppressed by the modified surface film. This may be because the modification film of TEG on GCE surface is hydrophobic. The selectivity was shown to depend on the concentration of TEG on the electrode surface. This selectivity can be utilized for determining DA in the presence of AA in neutral pH.

Acknowledgment

This work was financially supported by the Natural Science Foundation of Hebei Province of China (No. B2010000844) and Research Foundation of Education Department of Hebei Province of China (No. ZH2012078).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.07. 028.

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