Electrochemical behavior of rhodium acetamidate immobilized on a carbon paste electrode: a hydrazine sensor

Journal of the Brazilian Chemical Society

Kubota, Lauro T.¹  Universidade Estadual de Campinas, Campinas, Brazil¹  Gil, Eric de S.¹ 

关键词: rhodium acetamidate;    electrochemical sensors;    hydrazine determination;    carbon paste electrode;    polyethylenimine;   

DOI  :  10.1590/S0103-50532000000300018

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The researches for the development of new sensors have been a subject of great interest1-3. The greatest challenge in this field is the combination of the selectivity, sensitivity, simplicity, rapidity4-5.These characteristics are very important in analytical chemistry, mainly in complex samples6. In order to improve these characteristics the use of new materials in sensors has included many species such as metal complexes7, enzymes8 and DNA9. These modifications consist in immobilizing some catalyst or electron mediator species to electrocatalyze the oxidation or reduction of target analytes without application of high overpotentials7-9. For this purpose, the use of Rhodium Acetamidate Modified Electrode (RAME) has been described10. This complex presents few electrochemical studies, all of which in non-aqueous solution. Thus, it is very important in the sensors construction, to study the electrochemical behaviour, specially that of the immobilized complex that is unleachable in aqueous solutions. Furthermore, this complex belongs to an excellent class of catalysts and potential antitumour agents11-13. Concerning the cytostatic activity of these species, the most commonly proposed interaction is the axial (L) coordination (Figure 1) through nitrogen, oxygen or sulfur atoms of enzymes or DNA11,12. Hence, the affinity of rhodium complexes for nitrogenated groups could be exploited to increase the selectivity for nitrogen-bearing analytes, while their catalytic power can improve the sensitivity12. On the other hand, the biological significance of hydrazine compounds requires versatile methods for environmental and industrial monitoring for its determination at low levels14,15. Besides, this molecule presents great reactivity and high affinity by metal centers, where it can be oxidized16. Based on this context, the electrochemistry of rhodium acetamidate immobilized on carbon paste was studied and its possible aplication for hydrazine sensing investigated.    ExperimentalChemicalsRhodium acetamidate was synthesized by stepwise exchange reaction of [Rh2(O2CCH3)4 ] with acetamide as previously described17. The carbon powder was purchased from BDH (Poole UK). Polyethylenimine and hydrazine were from Sigma Chemical Co. All salts used for the electrochemical studies were of analytical grade.Electrochemical behavior of rhodium acetamidate solubilized in aqueous solutions The midpoint potential for 1.0 x 10-3 mol dm-3 [Rh2(HNOCCH3)4] aqueous solution with 0.5 mol dm-3 KCl, pH 7, was determined by scanning the potential in a range of 0-800 mV with scan rate of 25 mV s-1.Preparation of RAMEThe carbon paste modified electrode was prepared in three steps: 1) mixing 80 mg of graphite powder (99.9%) with 6 mg of rhodium acetamidate and 100 mm-3 of PEI 1% (water solution), 2) drying of this mixture at room temperature and 3) adding enough mineral oil to form a paste. This paste was put into a cavity of the platinum disk of 1 mm deep and 5 mm diameter sealed in the extremity of a glass tube.Studies of supporting electrolyte effectsThe effects of the nature of supporting electrolyte investigation was carried out by using 0.5 mol dm-3 KCl, NaCl, LiCl, KAc, KNO3 and K2SO4 solutions at pH 7. The cyclic voltammograms were recorded in the potential range of 0-800 mV at a scan rate of 10 mV s-1. The studies of the electrolyte concentration influence were carried out for KCl and KNO3 in the concentration range of 0.1 up to 1.0 mol dm-3.pH influenceThe studies of solution pH were evaluated over the range from 0 up to 800 mV vs SCE, in 0.5 mol dm-3 KNO3 and KCl. The solution pH between 2 and 8 was adjusted with dilute HCl or NaOH solutions. The scan rate for these studies was 10 mV s-1, keeping the ionic strength in 0.5 mol dm-3.Studies of the scan rate and stabilityThese studies were carried out in 0.5 mol dm-3 KCl and KNO3 at pH 7, with scan rates of 2, 5, 10, 20, 30, 40, 50 and 60 mV s-1 (0-800 mV). In order to verify the stability of this system, 40 cycles were carried out at a scan rate of 10 mVs-1 and 10 cycles at 2 mVs-1, immediately after the preparation of the paste and after 4 months of storage.Catalytic activitiyThe catalytic activity was initially evaluated by means of cyclic voltammetry in the same potential range used for all other studies (0-800 mV), by dropwise addition of 50 mm3 of 0.2 mol dm-3 hydrazine solution, in an electrochemical cell containing 5 cm3 of 0.5 mol dm-3 KCl solution at pH 7. The scan rate was 10 mV s-1.Chronoamperometric studiesFor the calibration curves, the electrochemical cell was filled with 5 cm3 of 0.5 mol dm-3 KCl solution at pH 7, fixing the potential in 300 mV vs. SCE and the rotation speed of the electrode in 200 rpm. The addition of 50 mm3 of a 0.02 mol dm-3 hydrazine solution was carried out step by step. InstrumentationThe electrochemical measurements were made on a Princeton Applied Research (PAR) 273A potentiostat/galvanostat and a Pine AFMSRX 002 rotatory system. The solutions pH were adjusted with the aid of a Corning 450 pH/ion meter. The cyclic voltammetry measurements were carried out utilizing a three-electrode system. The working electrodes were a platinum-disk electrode (to study of free rhodium acetamidate in aqueous solution) and Rhodium Acetamidate Modified Electrode (RAME). The reference electrode was a saturated calomel electrode (SCE), and a platinum wire was used as the counter electrode.  Results and discussionCyclic voltammetry behavior of soluble [Rh2 (O2CCH3)4] in aqueous solution. The electrochemical behavior of rhodium acetamidate and other dimeric analogues found in the literature until now have been investigated only in non aqueous systems. There is an agreement that the oxidation processes are metal centered and that they occur in two steps with distinct formal potentials (Em1 and Em2)12,17,18:Em1 RhII/RhII,III: [Rh2(HNOCCH3)4(S) 2] «                                       [Rh2 (HNOCCH3)4(S)2 ]+ + 1e-Em2 RhII,III/RhIII: [Rh2(HNOCCH3)4(S) 2]+ «                                       [Rh2(HNOCCH3)4(S) 2]2+ + 1e-Comparison between [Rh2(acetate)4] and [Rh2 (acetamidate)4] shows that these processes are shifted to lower potentials when the bridging acetate groups from the equatorial positions are changed by acetamidates, due to the stronger basicity of the amide. This will stabilize the oxidized form, since the electron density on the two metal centers is increased. Indeed, for each equatorial group exchanged, a decrease of 225 mV in its midpoint potential Em [where: Em=(Ea +Ec)/2] is observed18. Less intense effects are also observed upon changing ligangs in the axial positions17-19.Table 1 shows the values for both midpoint potentials (Em1 and Em2) obtained for rhodium acetamidates under the same conditions in different solvents. A linear inverse correlation between electron donor power and Em was observed, except for dimethylsulfoxide (DMSO) and triphenylphosphine (PPh3), in CH2Cl2. This fact was assigned to the p back-donation mechanism from the Rh-Rh to DMSO or PPh3 which withdraws some electron density from the metal centers19. The influence of axial ligands was also observed for aqueous systems in our experiments. The axial coordination with chloride anions (Table 2) resulted in a significant shift towards lower potentials, when compared to the literature data (Table 1). These observations agree with the literature data, that show that redox processes are favored by axial coordination which stabilizes the intermediate species or makes the oxidation process easier by increasing the electron density17-19. Furthermore, in the presence of higher Cl- levels a decrease in DE or about 100 mV for Em1 and 50 mV for Em2 was observed (scan rate of 25 mV s-1). These facts show that the chloride ion enhances the electron transfer.      Electrochemical behavior of RAME in different electrolytesThe electrochemical experiments were undertaken in aqueous solutions, since the aim of this work was the development of sensors, which could be useful for biological applications. The midpoint potentials were determined in aqueous electrolyte solutions (Figure 2) and they are listed inTable 3.    The most interesting fact to be higlighted is that coordinating anions like chloride can induce the appearance of the first oxidation peak. It should be correlated with the formation of an intermediate species Rh2II,III, which appears only after the second cycle when the complex is immobilized (Figure 3). For the other non-coordinating electrolytes, the first anodic peak is not well visualized while the resolution of the second reduction peak is slightly improved (Figure 2). This behavior can be explained by the interaction between immobilized complexes and electrolytes being more difficult than those observed for soluble ones. Hence, the first cycle is necessary to form [Rh2III]2+ species, which has higher affinity for anionic species. Furthermore, the interaction between complex and electrolytes may be more effective through the axial position17-19. For this reason, voltammograms in the presence of coordinating anions such as chloride may exhibit different patterns from those with non-coordinating anions. Thus, the chloride anions can form adducts with these dimers, due to its stronger interaction in the axial positions. The formation constants reported for the first coordination are much higher than for the second, and upon decreasing the concentrations of the ligand the coordination become more difficult12. Thus, different species can be formed depending on the chloride concentration in the medium. For soluble complexes larger shifts in the midpoint potentials were observed depending on the supporting electrolytes. In the particular

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