Journal of Electroanalytical Chemistry 448 (1998) 79–86

Electrocatalytic oxidation of hydrazines at a 4-pyridyl hydroquinoneself-assembled platinum electrode and its application to amperometric

detection in capillary electrophoresis

Li Niu, Tianyan You, John Y. Gui 1, Erkang Wang, Shaojun Dong *

Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun,130022, People’s Republic of China

Received 4 August 1997; received in revised form 5 January 1998

Abstract

4-Pyridyl hydroquinone on a platinum electrode adsorbs through the pyridine nitrogen forming stable self-assembled layers.The electrocatalytical oxidation of hydrazines was performed by the modified electrode. The overpotential of hydrazines wasdecreased markedly at the self-assembled monolayer (SAM) electrode. The mechanism of hydrazine oxidation was alsoinvestigated. Amperometric detection of hydrazine under zero potential (vs Ag�AgCl�sat. KCl) was exhibited by the SAM electrodeused as an electrochemical detector in a flow system. © 1998 Elsevier Science S.A. All rights reserved.

Keywords: Electrocatalytic oxidation; Hydrazines; Self-assembled monolayer electrode; Amperometric detection

1. Introduction

Hydrazine compounds are widely used in many indus-trial and pharmaceutical fields such as herbicides andpesticides [1], particularly in fuel cells and rocket propel-lants because their electrochemical oxidation producesnitrogen and water that do not cause environmentalpollution [2–5]. Thus their detection and quantitationare always problems of considerable analytical interest.But it is difficult to analyse hydrazines by amperometricdetection because of their large overpotentials at elec-trodes. During the last decade, many efforts have beenmade to lower the oxidation potential of hydrazines tofacilitate the amperometric detection, by the use ofvarious chemically modified electrodes, for example, apretreated glassy carbon electrode [6], a polymeric por-phyrin film electrode [7], a single crystal metal electrode

[8], a metallophthalocyanine electrode [9], a metal-modified carbon fiber electrode [10], and so on.

A self-assembled monolayer has higher stability undervarious conditions due to the special chemical bondingbetween the organic molecule and the electrode surfaceatom. A rapid response to surroundings can occurthrough the self-assembled monolayer because it can actas a mediator of rapid electron transfer.

In this work, a newly synthesized molecule, 4-pyridylhydroquinone, was used to modify platinum based onthe strong covalent binding between Pt and the N atomsin the pyridyl group [11]. The redox couple of hy-droquinone needs to transfer a pair of protons, so thiscan help to oxidize catalytically some compounds fol-lowing the exchange of protons. Hydrazines can takeadvantage of this for the oxidation by the SAM elec-trode. The oxidation process of these species is anirreversible diffusion-controlled process accompanied byproton transfer [12]. The mechanism in detail and anapplication in which the SAM electrode acts as anamperometric detector in a flow system are presentedherein.

* Corresponding author. Fax.: +86 431 5685653; e-mail:dongsj@ns.ciac.ac.cn

1 Present address: General Electric Company, P.O. Box 8, Schenec-tady, NY 12301, USA.

0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.PII S0022-0728(98)00028-X

L. Niu et al. / Journal of Electroanalytical Chemistry 448 (1998) 79–8680

2. Experimental

2.1. Reagents

Hydrazine sulphate (HZ) was obtained from BeijingInstitute of Chemicals. Methylhydrazine (MHZ) waspurchased from Shanghai Reagent Factory. Allreagents were of analytical grade and used as received.All aqueous solutions were prepared using ultra purewater from a Millipore System and stored at 4°C beforeuse. 4-pyridyl hydroquinone was provided as a gift bythe laboratory of Professor A.T. Hubbard [11].

2.2. Instruments

Cyclic voltammetric measurements were performedwith a CHI 660 electrochemical system (CHI, USA) ina conventional three-electrode cell. A 1 mm diameterplatinum wire was used as the working electrode whichwas polished with 1.0, 0.3 and 0.05 mm a-Al2O3 pow-der, respectively, and cleaned ultrasonically. In the flowsystem experiments, electrophoresis in the capillary wasdriven by a high-voltage power supply (Spellman CZE1000R, USA). A 37 cm uncoated fused-silica capillarywith 25 mm I.D. was used (Yongnian Optical FiberFactory, Hebei, China). Before each run, the capillarywas flushed with 0.1 M sodium hydroxide solution,doubly distilled water and running buffer for 2, 2 and 5min, respectively. End column detection was employedusing a wall-jet configuration. The experiments werealso operated in a three-electrode cell with a 25 mmplatinum microdisk electrode as the working electrode.The constant-potential control and the amperometricdetection were carried out with a PAR Model 400electrochemical detector (USA). The electrophoregramdata were recorded by a Philip computer configuredwith Gilson 715 HPLC system controller software.

All platinum working electrodes used were scannedin 1 M H2SO4 solution until stable cyclic voltam-mograms were obtained. A platinum wire was used asthe counter electrode, and an Ag�AgCl�sat. KCl elec-trode as the reference electrode, against which all po-tentials were reported.

2.3. Procedures

The polycrystalline platinum disk working electrodewas cleaned electrochemically before every modifica-tion[11,13]. Such a clean platinum electrode was im-mersed into a 10 mM KF+HF(pH=3) buffer solutioncontaining 5 mM 4-pyridyl hydroquinone and the po-tential was kept at −0.1 V versus Ag�AgCl�sat. KCl for5 min. Then the electrode was washed thoroughly with10 mM KF+HF (pH=3) buffer and distilled water.

3. Results and discussion

3.1. Surface electrochemistry of 4-pyridyl hydroquinoneSAM Pt electrode

A layer of 4-pyridyl hydroquinone (PHQ) can beassembled easily on a platinum surface. A model of thehydroquinone group forming a pendant structure onthe self-assembled monolayer surface is exhibited inFig. 1A [11]. Because of the existence of proton trans-port in the red-oxidation of hydroquinone, there is anobvious pH effect on the peak shape, peak potentialand peak-to-peak potential difference. The protontransfer rate coming from bulk solution controls therelation between peak current and scan rate. Thus, it isdisadvantageous to examine the surface wave featurefor such an immobilized molecule by measurement ofcyclic voltammetry under higher pH conditions. Here, 1M H2SO4 is used as supporting electrolyte. It providesan environment allowing easy proton exchange, and thestability of the SAM modified electrode was also exam-ined under such an acid condition.

The SAM electrode shows a typical quasi-reversiblecyclic voltammogram with a small, although not zero,DEp assigned to the redox reaction of the hydroquinonegroup on the electrode surface, as shown in Fig. 1A.The broad voltammetric shape may be attributed to theintermolecular repulsion of the 4-pyridyl hydroquinoneon the platinum surface [14]. Furthermore, the currentis directly proportional to the scan rate in the range of10 and 800 mV s−1, confirming a surface reaction (Fig.1B).

With increasing pH, DEp increased, the apparentpotential shifted negatively and the relation betweenscan rate and peak current lost the properties of asurface wave. These changes were attributed mainly tothe fact that redoxidation of 4PHQ was accompaniedby proton transfer and the proton transport from bulksolution was the rate determining step at higher pHvalues (data not shown).

The stability of the 4PHQ-modified electrode in 1 MH2SO4 and pH=7.2 phosphate buffer solution isshown in Fig. 1C and D. In several cyclic potentialscans, no obvious desorption can be monitored (onlyabout 20% desorption can be seen after the SAMmodified electrode was immersed in 1 M H2SO4 for 24h and ca. 30% after 6 days). Compared with the previ-ous work on monolayers of pyridyl derivatives [15,16],the stability of pyridyl hydroquinone monolayer in thiswork is higher. It is necessary to get firm adsorption ofpyridyl group at a platinum surface so that the pyridylgroup forms a slightly tilted conformation and showssmall steric repulsion in the chain and tail group [17].Here, the small 4PHQ molecule does not obstruct theformation of the tilted conformation, and the smallertail group than that in previous work [16] will also

L. Niu et al. / Journal of Electroanalytical Chemistry 448 (1998) 79–86 81

Fig. 1. (A) Schematic diagram of 4-pyridyl hydroquinone adsorbed on a platinum surface and cyclic voltammograms of the 4-pyridylhydroquinone SAM platinum electrode in 1 M H2SO4; curves a to h correspond to scan rates of 100, 200, 300, 400, 500, 600, 700 and 800 mVs−1, respectively; (B) Plot of anodic peak current vs scan rate. Plot of charge vs time for the desorption of a monolayer of 4PHQ; (C) in 1 MH2SO4; and (D) in pH=7.2 phosphate buffer solution.

result in slight steric repulsion. In addition, strongintermolecular interaction based on the hydrogen bondof hydroxyl groups may be a possible factor for increas-ing the anchored stability [18]. Thus, the 4PHQ-modified platinum surface shows excellent stabilityduring our research period and pH condition.

3.2. Electrocatalytic acti6ity of 4-pyridyl hydroquinoneSAM Pt electrode in the oxidation of hydrazine

Fig. 2 shows the catalytic activity of 4-pyridyl hy-droquinone in the oxidation of hydrazine at pH 7.2. Itcan be seen that the oxidation peaks appear at ca.

L. Niu et al. / Journal of Electroanalytical Chemistry 448 (1998) 79–8682

−0.1 to −0.2V. This value is more negative than thatat other modified electrodes [6–10,12,19,20]. The SAMshows an exceptional decrease of the overpotential ofhydrazine oxidation and makes it easy to determinehydrazine by amperometric detection quantitativelyand qualitatively.

The cyclic voltammogram for hydrazine oxidation atthe SAM electrode is shown in Fig. 3A, as a function ofscan rate, showing a square-root dependence and be-havior characteristic of a totally irreversible diffusion-controlled process (Fig. 3B). The equation for the peakcurrent of an irreversible diffusion controlled electrodeprocess is [12,21],

Ip=2.99×105n(an)1/2Ac0D1/20 n1/2

Here the diffusion coefficient of hydrazine (D0=1.4×105 cm2 s−1) is known, n is the total number ofelectrons involved in the oxidation, an is a parameterreflecting the irreversibility of the reaction and can bedetermined using the equation [12],

Ep/V= (0.03/(an))log(n)+Constant

From the linear relationship between the peak potentialand the logarithm of scan rate (Fig. 3C), an=0.58 wasobtained. Considering c0=10−3 M and the geometricalarea of the electrode is 0.785 cm2, it is estimated thatthe number of electrons involved in the hydrazine oxi-dation is n=3.5(90.4), namely, a four electron trans-fer for the whole process is probably occurring.

In many previous studies [22–25], it has been demon-strated that the electrode surface modified by incorpo-rating quinone-containing materials was effective inpromoting proton transfer from adsorbed species suchas hydrazines, NADH and ascorbic acid. Recent results[23,26] also support a process in which hydrazinespreadsorbed on a surface active site were oxidized andthen a proton was desorbed.

Fig. 3. (A) Cyclic voltammograms of 1 mM hydrazine solution at a4-pyridyl hydroquinone SAM electrode under different scan rates, (a)20 mV s−1; (b) 40 mV s−1; (c) 60 mV s−1; and (d) 80 mV s−1; (B)The plot of peak current in (A) as a function of the square-root ofscan rate; (C) The plot of peak potential in (A) as a function of thelogarithm of scan rate.

Fig. 2. Cyclic voltammograms of hydrazine with different concentra-tions at a 4-pyridyl hydroquinone SAM Pt electrode in 0.1 Mphosphate buffer (pH=7.2). (a) buffer; (b) 0.5 mM; (c) 1 mM; (d) 0.2mM; and (e) 4 mM hydrazine. Scan rate: 50 mV s−1.

After potential scanning in hydrazine solution, theSAM electrode does not recover immediately. To re-store the modified electrode, multiple cyclic scans inphosphate buffer solution are necessary. Combining the

L. Niu et al. / Journal of Electroanalytical Chemistry 448 (1998) 79–86 83

Fig. 4. Cyclic voltammograms of hydrazine oxidation at the SAM Pt electrode under different pH values of (a) 3, (b) 4, (c) 5, (d) 6, (e) 7, and(f) 8. Scan rate: 50 mV s−1, inset: plot of Epa vs pH.

adsorbed oxidation process and proton exchange func-tion of the immobilized hydroquinone, we assume thatthe possible overall reaction of hydrazine oxidation atthe 4-pyridyl hydroquinone SAM electrode involves thefollowing steps (Scheme 1).

According to the above steps, the hydrazine moleculeadsorbed on the monolayer surface forms surface com-plexes by hydrogen bonding and is successively dehy-drogenated through proton transfer from hydrazine toquinone, so quinone is reduced to hydroquinone. Sucha transfer through adsorption decreases the Gibbs en-ergy of the oxidation reaction and the molecule under-goes very fast oxidation. Each hydrazine molecule canbridge with a maximum number of four hydroxylgroups of the surrounding quinones and each quinonemolecule can be linked to the nearest two hydrazines,as is known from previous work [5,12,21,27]. Such anadsorbed model leads to a crosslinking-like network. Soa hydrazine molecule simultaneously carries on the fourabove procedures according to the stoichiometric ratioof two quinone molecules to one hydrazine molecule.The number of hydroxyl groups linked with each hy-drazine molecule is controlled by spatial configurationof the surface immobilized quinone molecule and thearray of adsorbed hydrazine molecules. Different num-bers of adsorbed hydroxyls to every hydrazinemolecule, affecting the number of electrons transfered,would result in different oxidized products. The disor-der of a monolayer on a rough polycrystalline platinumsurface is an important reason that lead to small ad-

L. Niu et al. / Journal of Electroanalytical Chemistry 448 (1998) 79–8684

Fig. 5. Cyclic voltammograms of methylhydrazine with different concentrations at a 4-pyridyl hydroquinone SAM Pt electrode in 0.1 M phosphatebuffer (pH=7.2). (a) Buffer, (b) 0.5 mM, (c) 1 mM, (d) 2 mM and (e) 4 mM methylhydrazine. Scan rate: 50 mV s−1. Inset: relationship betweenthe anodic peak current and the square-root of scan rate.

sorbed numbers. Under the neutral pH conditions,proton transfer was the rate determining step of redox-idation of the quinone moiety. Here, the proton wasprovided through the adsorbed hydazine. Thus, theoxidation and dehydrogenation of hydrazine results inthe position of the rate determining step. The result ofn=3.5(90.4) leads to the conclusion that a majorityof the adsorbed hydrazines basically fit the maximumbinding number and the hydrazine can be easily oxi-dized to produce nitrogen and water.

3.3. The pH effect on hydrazine oxidation at theSAM Pt electrode

The electrochemistry of hydrazine at the 4-pyridylhydroquinone SAM electrode depends strongly on pH.The oxidation peak shifts positively with decrease ofpH (shown in Fig. 4) because the oxidation of thecomplexes of the hydroquinone moiety with hydrazinecorresponds to a proton transfer process. The lower theacidity, the easier the proton transfer throughadsorption.

The plot of Epa versus pH has a discontinuity in thisregion of two linear segments with slopes of −44 and−48 mV per pH unit, respectively. This can be ex-plained by the assumption that the condition of pH55is unfavorable to the oxidation of hydrazine and thedecrease of the oxidation current also indicates thedecrease of the degree of the reaction. The oxidation of

hydroquinone involves two electrons, ca. two protonsare added to the immobilized species in the two pHregions. The big jump in the Epa versus pH plot (with aslope of ca. 180 mV per pH unit) should be related tothe acidity of the surface complexes of quinone andhydrazine. In this pH region, the complexes are formed,so the reaction involves transfer of multiple protons.

3.4. The electrocatalytical oxidation of methylhydrazineat the SAM electrode

Methylhydrazine is very difficult to oxidize at anordinary electrode, and its overpotential is very higheven at some CMEs [6,10]. 4-Pyridyl hydroquinone at aplatinum electrode was found to be also effective indecreasing the overpotential of methylhydrazine. InBaldwin’s group [6], an oxidation potential of ca. 0.55V (versus SCE) was obtained. No result of oxidation ofmethylhydrazine under lower potential could be found,the oxidation current in that work could be obtainedonly at E\0.8V. From cyclic voltammograms ofmethylhydrazine at this SAM electrode (Fig. 5), a sig-nificant oxidation peak at ca. 0.45–0.55 V can be seen.A small shoulder at a more positive potential (ca. 0.65V) is also shown in Fig. 5. Referring to the literature[8], the process at ca. 0.5 V would correspond to theoxidation of methylhydrazine diffusing from solution.Thus the rate of the process increases steadily with thebulk concentration of methylhydrazine. The shoulder

L. Niu et al. / Journal of Electroanalytical Chemistry 448 (1998) 79–86 85

(ca. 0.65 V), which is overlapped by the main peak (ca.0.5 V), seems to be related to oxidation of the dissoci-ated species of strongly adsorbed methylhydrazine on a

Fig. 7. Electrophoregram of (A) 10 mM hydrazine and 10 (M methyl-hydrazine; (B) 1 mM hydrazine and 3 mM methyldrazine; (C) 10 mMhydrazine. Conditions: capillary, 25 mm i.d., 37 cm long; buffer, 10mM phosphate (pH=7.0); injection, 2 s at 15 kV; separation poten-tial, 15 kV; working electrode, 25 mM 4-pyridyl hydroquinonemodified microdisk Pt electrode at 0.5 V (A), (B) and 0.0 V (C),respectively.

Fig. 6. Chronoamperometric curves of the steady state response at a4-pyridyl hydroquinone SAM Pt electrode (A) on increasing theconcentration of hydrazines in 2×10−6 M steps, Operating poten-tial, −0.15 V; (B) on increasing the concentration of methylhy-drazine in 1×10−6 M steps, Operating potential, 0.55 V. All ofsolutions are kept at pH 7.

SAM electrode at this potential [8]. Thus, the peakpotential of the shoulder is constant. The same linearrelationship between the current of the main oxidationpeak and the square root of scan rate as that ofhydrazine can also be obtained. It indicates an irre-versible diffusion-controlled process in oxidation ofmethylhydrazine.

The mechanism of this oxidation was also studied.Similarly, a result of n=4.7(90.7) was obtained as-suming that the diffusion coefficient of hydrazine is thesame as that of methylhydrazine. It is estimated that amethylene oxide may be produced referring to theabove reaction process of hydrazine and according toabout a five electron loss.

L. Niu et al. / Journal of Electroanalytical Chemistry 448 (1998) 79–8686

3.5. The steady state responses of 4-pyridyl hydroquinoneSAM Pt electrode towards oxidation of hydrazines

Under our experimental conditions, the chronoam-perometric responses of hydrazine oxidation on theSAM electrode are very rapid. A steady current isobtained almost instantaneously. The results of steadystate responses of hydrazine are shown in Fig. 6A witha detection range from 10−6 to 10−3 mol l−1. Thelower working potential (−0.15 V) is a substantialadvantage for amperometric detection especially in aflow system. Fig. 6B shows that the current response ofmethylhydrazine is also as rapid as that of hydrazine. Ahigher sensitivity can be obtained using this electrode.

3.6. Capillary electrophoresis electrochemistry by usingthe 4-pyridyl hydroquinone SAM microdisk Ptelectrode

Compared to a conventional electrode, a microdiskelectrode offers several practical advantages includingthe higher signal to noise ratio, easier electrode capil-lary alignment and greater stability and reproducibility.Therefore, a 4-pyridyl hydroquinone modified mi-crodisk platinum electrode was also used in this work.

Electrophoregrams of 10 mM hydrazine and 10 mMmethylhydrazine in 10 mM phosphate buffer at pH 7.0at +0.5 V (versus Ag�AgCl) are shown in Fig. 7A. Fig.7B is the electrophoregram of a lower concentration of1 mM hydrazine and 3 mM methylhydrazine. A perfectseparation of the two kinds of hydrazines has beenperformed. Hydrazine has a short migration time. Evenat the potential of 0 V, hydrazine can also be detectedeasily (as shown in Fig. 7C). Because many species arenot easily oxidized at so low a potential, the detectorusing a 4-pyridyl hydroquinone modified electrodeavoids interference of other species and improves theselectivity towards hydrazine.

A few interfering species, like methanol, ethanol andascorbic acid, have been examined. Under the condi-tions of detecting potential, especially zero potential,they cannot be catalytically oxidized. Thus, such aSAM modified electrode exhibits an excellent selectivityto hydrazines, especially, to hydrazine at zero potential.

4. Conclusions

4-Pyridyl hydroquinone can form a self-assembledmonolayer on a platinum electrode, and it retains theredox reactivity of the hydroquinone group. The SAMelectrodes modified with this molecule exhibit electro-catalytic behaviour for the oxidation of hydrazine andmethylhydrazine under a much lower potential than

that found in previous work. The results indicate thefour-electron oxidation process with proton transfer ofthe adsorbed species. This study has also demonstratedthat the SAM electrode can be applied to the detectionof hydrazines in a flow system with excellent sensitivityand rapid response. Detection of hydrazine of zeropotential has been performed with higher selectivitytoo. Such SAM electrodes show great promise in ex-tending voltammetric techniques to detect hydrazines inflow systems, especially hydrazine.

Acknowledgements

We thank the laboratory of Professor Arthur T.Hubbard for their help in providing 4-pyridylhydroquinone.

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