Journal of Electroanalytical Chemistry 682 (2012) 172–174

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Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

Short Communication

Substituent effects on the electrocatalytic oxidation of phenolsat preanodized screen-printed carbon electrodes

Hsueh-Hui Yang a,b,1, Mei-Hsin Chiu c,1, Ko-Ming Chang c, Ying Shih c,⇑a Department of Medical Research, Buddhist Tzu Chi General Hospital, Hualien 970, Taiwanb General Education Center, Tzu Chi College of Technology, Hualien 970, Taiwanc Department of Cosmetic Science, Providence University, Taichung 433, Taiwan

a r t i c l e i n f o

Article history:Received 30 May 2012Received in revised form 4 July 2012Accepted 4 July 2012Available online 14 July 2012

Keywords:Substituent effectPhenolsElectrocatalytic oxidationPreanodized screen-printed carbonelectrode

1572-6657/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jelechem.2012.07.001

⇑ Corresponding author. Tel.: +886 4 26328001x15E-mail address: yingshih@pu.edu.tw (Y. Shih).

1 These authors contributed equally to this work.

a b s t r a c t

Preanodized screen-printed carbon electrodes (SPCEs�) lower the oxidation overpotential and improvethe peak current of phenols because of the presence of hydroxyl groups on the surface of SPCEs�. Thesehydroxyl groups can form hydrogen bonds with the OH groups of the phenols. The oxidation peak poten-tial (Epa) is linearly related to the number of hydroxyl groups on the surface of SPCEs�. Moreover, the Epa ofphenols at SPCEs� was found to systematically shift depending on the substituents on the phenols. Theshifts in the Epa are strongly associated with Hammett’s constant.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Phenols are a group of molecules with important biologicalproperties, such as anti-inflammatory, antibacterial, and antioxi-dant properties [1,2]. In addition, these compounds are commonorganic pollutants with high toxicities found in the waste effluentsof industrial processes and agricultural activities. The quantifica-tion and characterization of phenols is an interesting and demand-ing subject in the field of electroanalysis. Therefore, an enormousnumber of modified or pretreated electrodes have been developedfor the detection of phenols. Examples of these treatments includeamino-functionalized mesoporous silica SBA-15 modification,poly(thionine) modification, and anodic pretreatment [3–11].

Most studies focus on the electroanalysis of phenols, and littleresearch is performed to investigate the mechanism by which phe-nols are detected. Previously, free amino functional groups on thesurface of modified electrodes were reported to form hydrogenbonds with the hydroxyl groups of the phenols [7,9]. These hydro-gen bonds contribute to the weakening of the AOH bond energyand facilitate electron transfer through the N� � �HAO linkage[7,9]. In contrast, at SPCEs�, the increase in the density of sur-face-bound carbon–oxygen functional groups and/or the genera-tion of edge plane sites through surface reorientation were found

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to facilitate electron transfer in the electrochemical oxidations ofNADH, dopamine, and creatinine [10,12,13].

The nature and the position of substituents greatly affectmolecular chemical and physical properties. In phenols, substitu-ents can influence the electron density of the hydroxyl group onthe aromatic ring through p-orbital interactions with the aro-matic ring [14]. As mentioned above, the hydroxyl group on thearomatic ring is involved in the mechanism of the electro-oxida-tion of phenols [7,9]. Therefore, the nature and position of a spe-cific substituent on the phenolic ring are important factors thatshould be considered during the electro-oxidation of phenols.To our knowledge, there is little discussion of these factors inthe literature.

Jiang et al. reported no obvious differences in the voltammetricbehaviors for the electrochemical oxidation of nitro-substitutedphenols, but the degradation of these phenols was significantlyinfluenced by the molecular structure [15]. Ahmed et al. observedthat the tendency of phenols to foul the electrode surface and toform new intermediates during the electro-oxidation was highlydependent on the type and position of the substituent [16]. Inthe present study, the cyclic voltammetric responses of chloro-,nitro-, and hydroxyl-substituted phenols were systematicallyinvestigated at SPCEs�. The effect of the presence of different sub-stituents at different positions on the ring on the electrocatalyticoxidation of the phenols is discussed. The results of the presentstudy could provide insight into the mechanism of the electrocat-alytic oxidation of phenols at SPCEs�.

H.-H. Yang et al. / Journal of Electroanalytical Chemistry 682 (2012) 172–174 173

2. Materials and methods

All chemicals used were of ACS-certified reagent grade. Aque-ous solutions were prepared with Millipore de-ionized water(18 MX/cm) throughout this investigation. A stock solution of ana-lytes was prepared in 1.0 M phosphate buffer solution (PBS, pH 7).Electrochemical measurements were performed with a CHI 621belectrochemical workstation (CH Instruments, Austin, TX) in athree-electrode cell assembly. A bare screen-printed carbon elec-trode (SPCE) or SPCE� working electrode, an Ag/AgCl/3 M KCl refer-ence electrode and a platinum disk auxiliary electrode were usedto complete the cell setup. The SPCE, with a geometric area0.196 cm2, was purchased from Zensor R&D (Taichung, Taiwan).The measured average resistance on the product data sheet was85.64 ± 2.10 X/cm. The SPCE� was prepared by scanning 10 cyclesin the potential range of 0–1.3 V and applying different preanodi-zation potentials for 180 s in 0.1 M PBS (pH 7). X-ray photoelectronspectroscopy analysis (XPS) was performed according to previousprocedures [10].

SPCE*/2.0V

SPCE*/1.6V

SPCE*/1.4V

SPCE*/1.2V

SPCE

Epa

/ V

(vs

. Ag/

AgC

l)

0.1

0.2

0.3

0.4

0.5(A)

3. Results and discussion

To study the effect of the preanodization treatment on the oxi-dation of phenols, hydroquinone (4-hydroxyphenol, 4HP) was se-lected as a model compound. Fig. 1 shows the typical cyclicvoltammetric responses of 10 ppm 4HP at the SPCE and SCPEs� in0.1 M PBS (pH 7.0) at a scan rate of 50 mV/s. At the SPCE, the redoxpeaks of 4HP appeared at 0.37/�0.118 V. At SPCEs�, the redoxpeaks shifted to 0.314/�0.089 V, 0.108/0.027 V, and 0.086/0.053 V for SPCE�/1.2 V, SPCE�/1.6 V, and SPCE�/2.0 V, respectively.In addition to decreasing the potential differences between redoxpeaks, the preanodization treatment also resulted in a decreasein the peak potential and in a large increase in the peak current.The results above indicated that SPCEs� exhibited improved elec-trocatalytic performance and higher sensitivity for 4HP.

It has been shown that the pretreatment of a carbon electrodeaffects the formation of edge planes and the density of surface car-bonyl groups on the graphite edge planes [17,18]. Other studiesfurther demonstrated that the increase in surface carbon–oxygenfunctional groups and/or the generation of edge plane sites wereinvolved in the electrocatalytic oxidation of NADH, dopamine,and creatinine at SPCEs� [10,12,13]. Therefore, the physical proper-ties of the electrodes’ surfaces were characterized. As expected, thedeconvoluted C1s XPS spectra of SPCE and SPCE� reveal three peaks

Potential / V (vs. Ag/AgCl)-0.6-0.4-0.20.00.20.40.60.81.0

Cur

rent

/ μA

-60

-40

-20

0

20

40

60

SPCE*/2.0V

SPCE*/1.6V

SPCE*/1.2V

SPCE

Fig. 1. CV responses of 10 ppm HP at SPCE, SPCE�/1.2 V, SPCE�/1.6 V, and SPCE�/2.0 V (the number after slash denoted the preanodization potential) in 0.1 M PBS(pH 7) at a scan rate of 50 mV/s.

corresponding to CAC (284.1 eV), CAOH (285.7 eV), and C@O(287.6 eV) (data not shown) [19].

The role of oxygen functionalities in the electrocatalytic oxida-tion of 4HP was next studied at SPCEs�. As shown in Fig. 2A, theCAOH/CAC ratio was found to increase with increasing preanodi-zation potential, and interestingly, a linear relationship betweenthe Epa and the ratio of CAOH/CAC was observed. In contrast, theC@O/CAC ratio shifted only at SPCE�/2.0 V, and there was no obvi-ous relationship between the Epa and the ratio of C@O/CAC. There-fore, CAOH, and not C@O, is proposed to participate in theelectrocatalytic oxidation mechanism. Because the oxygen inCAOH and the nitrogen in CANH2 possess similar properties, it issuspected that the surface CAOH groups on SPCEs� form hydrogenbonds with the hydroxyl groups of the phenols; these hydrogenbonds catalyze the electrooxidation of phenols, as previously re-ported [7,9].

Finally, to study the effects of substituents on the aromatic ringon the electrocatalytic oxidation of phenols, the cyclic voltammetricresponses of chloro-, nitro-, and hydroxyl-substituted phenols wereinvestigated at SPCEs�. As shown in Table 1, the observed Epa valuesfollowed this order: 4HP > 3-hydroxyphenol (3HP) > 4-chlorophe-nol (4CP) > 3-chlorophenol (3CP) > 3-nitrophenol (3NP) > 4-nitro-phenol (4NP). Hammett constants (r) were introduced in the1930s and have been used as the primary means for quantifyingsubstituent effects [14]. Fig. 3 shows that there is a linear relation-ship, Epa = 0.4144 + 0.6167r (r = 0.9552), between the r valuesand the measured Epa of phenols. The Epa increases with increasesin the r value.

Substituents have two primary electronic effects on the polari-zation and charge redistribution in aromatic compounds: inductiveeffects and resonance effects. Inductive effects are those that occur

Ratio (C-OH/C-C)

0.60 0.65 0.70 0.75 0.80 0.85 0.90E

pa /

V (

vs. A

g/ A

gCl)

0.0

Ratio (C=OH/C-C)

0.00 0.05 0.10 0.15 0.20

SPCE*/2.0V

SPCE*/1.6V

SPCE*/1.4V

SPCE*/1.2V

SPCE

0.0

0.1

0.2

0.3

0.4

0.5(B)

Fig. 2. The effect of preanodization potential on the ratio of (A) CAOH/CAC and (B)C@O/CAC for SPCE� and the relationship between the Epa of 10 ppm 4HP and theratio of (A) CAOH/CAC and (B) C@O/CAC for SPCE�.

σσ value-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Epa

/ V

(vs

. Ag/

AgC

l)

0.0

0.2

0.4

0.6

0.8

1.0

4HP

3HP

4CP3CP

3NP4NP

Fig. 3. Linear correlation between r values and the measured Epa of phenols.

Table 1Summary of observed Epa, r [14], and natural charges on the (deprotonated)phenoxide oxygen [14] for substituted phenols. Conditions are the same as Fig. 1.

Substituent Epa (V) r Qn (O�)

4-OH 0.086 �0.37 �0.76553-OH 0.57 0.12 �0.74854-Cl 0.628 0.23 �0.73933-Cl 0.705 0.37 �0.72953-NO2 0.799 0.71 �0.72654-NO2 0.833 0.78 �0.6767

174 H.-H. Yang et al. / Journal of Electroanalytical Chemistry 682 (2012) 172–174

through the r system due to electronegativity-related effects. Res-onance effects are those that occur through the p system and can berepresented by resonance structures. Through a resonance donat-ing/withdrawing effect, hydroxyl groups/nitro groups increase/decrease the electron density on the ring at the ortho- and para-positions. As for chloro groups, both inductive withdrawing andresonance donating exist, resulting in partial cancellation of the ef-fects. Therefore, the natural charges on the (deprotonated) phenox-ide oxygen followed this order: 4HP < 3HP < 4CP < 3CP < 3NP < 4NP(Table 1).

As mentioned above, it is suspected that hydrogen bonds be-tween surface CAOH at SPCEs� and the hydroxyl groups of the phe-nols participate in the catalysis of phenol electro-oxidation. Thisinteraction weakens the AOH bond energy to facilitate electrontransfer through CAO� � �HAO. The decrease in the density of theelectron cloud results in a decrease in the strength of the hydrogenbonds. As the strength of the hydrogen bonds decreases, the

electroactivity decreases and the Epa increases. As shown in Table 1,the observed experimental results are consistent with this theory.

4. Conclusions

The present study demonstrates that SPCEs� could effectivelylower the oxidation overpotential and improve the peak currentthrough surface CAOH at SPCEs�. Surface CAOH groups on SPCEs�

could form hydrogen bonds with the OH groups of the phenols.This interaction weakens the AOH bond energy, and the electronswould be transferred through CAO� � �HAO. The Epa is linearly re-lated to the number of surface CAOH groups on SPCEs�. Moreover,characteristic potential shifts are observed for the different substit-uents on phenols at SPCEs�. The shifts in the peak potential arestrongly associated with Hammett’s constant.

Acknowledgments

The authors would like to thank the National Science Council ofRepublic of China (Taiwan) for financial support under Contract No.NSC 98-2113-M-126-005-MY3.

References

[1] L. De Martino, V. De Feo, F. Fratianni, F. Nazzaro, Natl. Prod. Commun. 4 (2009)1741–1750.

[2] Y.C. Kim, Arch. Pharm. Res. 33 (2010) 1611–1632.[3] M.D.T. Sotomayor, A.A. Tanaka, L.T. Kubota, Anal. Chim. Acta 455 (2002) 215–

223.[4] J.J. Yu, W. Du, F.Q. Zhao, B.Z. Zeng, Electrochim. Acta 54 (2009) 984–988.[5] A.J.S. Ahammad, S. Sarker, M.A. Rahman, J.-J. Lee, Electroanalysis 22 (2010)

694–700.[6] S.-M. Wang, W.-Y. Su, S.-H. Cheng, Int. J. Electrochem. Sci. 5 (2010) 1649–1664.[7] A.J.S. Ahammad, M.M. Rahman, G.R. Xu, S. Kim, J.J. Lee, Electrochim. Acta 56

(2011) 5266–5271.[8] H. Yin, Q. Zhang, Y. Zhou, Q. Ma, T. Liu, L. Zhu, S. Ai, Electrochim. Acta 56 (2011)

2748–2753.[9] X. Zhang, S.O. Duan, X.M. Xu, S.A. Xu, C.L. Zhou, Electrochim. Acta 56 (2011)

1981–1987.[10] K.S. Prasad, G. Muthuraman, J.-M. Zen, Electrochem. Commun. 10 (2008) 559–

563.[11] Y.-P. Ding, W.-L. Liu, Q.-S. Wu, X.-G. Wang, J. Electroanal. Chem. 575 (2005)

275–280.[12] K.S. Prasad, J.-C. Chen, C. Ay, J.-M. Zen, Sens. Actuators B Chem. 123 (2007)

715–719.[13] J.-C. Chen, A.S. Kumar, H.-H. Chung, S.-H. Chien, M.-C. Kuo, J.-M. Zen, Sens.

Actuators B Chem. 115 (2006) 473–480.[14] K.C. Gross, P.G. Seybold, Int. J. Quantum Chem. 85 (2001) 569–579.[15] Y. Jiang, X. Zhu, H. Li, J. Ni, Chemosphere 78 (2010) 1093–1099.[16] S. Ahmed, M. Ahmad, S.B. Butt, Res. Chem. Intermediat. 38 (2012) 705–722.[17] S. Ranganathan, T.-C. Kuo, R.L. McCreery, Anal. Chem. 71 (1999) 3574–3580.[18] K. Ray, R.L. McCreery, Anal. Chem. 69 (1997) 4680–4687.[19] E.A. Hoffmann, T. Körtvélyesi, E. Wilusz, L.S. Korugic-Karasz, F.E. Karasz, Z.A.

Fekete, J. Mol. Struct. (Theochem) 725 (2005) 5–8.