Environmental and biochemical studies have identified hydrazine (N₂H₄) as a compound that negatively affects the liver, kidneys, and the blood [1]. The American Conference of Governmental Hygienists lowered the threshold limit concentration for hydrazine to 10 ppb for an 8-hour exposure time [1]. Industrial uses for hydrazine include corrosion inhibitors, pesticides, and rocket fuel, all of which can pollute soil and groundwater systems [2]. Consequently, there is a need to study hydrazine chemistry to develop simple, inexpensive, portable, and sensitive sensors and monitoring systems for the real-time detection and determination of this compound in trace amounts. Current efforts to detect hydrazine employ methods such as chemiluminescence with FIA, ion chromatography, and spectroscopy [3-6]. For example, a hydrazine detection method based on supressing luminol chemiluminescence in a flow system has been developed. Although the flow sensor is stable for over 400 analyses, a complete analysis requires 2 minutes [7]. Some of these methods and sensors are generally expensive, complex, and have long analysis and recovery times. Electrochemical studies of hydrazine chemistry have shown that some ruthenium complexes in the appropriate oxidation state reacts with hydrazine in bulk solution in an electrocatalytic cycle [8]. Electrochemical detection of this analyte provides detection limits comparable to other methods, but with a simplified detection scheme. Previously, we reported on some of the electrochemistry of ruthenium bis(1,10-phenanthroline)(4-methyl-4’vinyl-2,2’- bipyridine) polymer films in organic solvents, but not in aqueous environments [9]. Two important advantages of using polymeric thin films as components in chemical and electrochemical sensing are the ability to (1) effectively control the physical and chemical characteristics of the reaction system, and (2) provide a significantly higher “effective concentration” per surface area for reactions to occur. However, the challenge in designing polymeric films for sensing applications is structuring the molecular architecture of the monomer to form the polymer film to facilitate and direct electronic and ionic charge transfer reactions at the electrode|film|solution interfaces and within the polymer film [9]. The goal of this research was to develop a detection scheme for hydrazine using a ruthenium bis(1,10-phenanthroline)(4-methyl-4’vinyl-2,2’-bipyridine) polymer film, which is the electroactive polymer film in contact with hydrazine solutions in a flow injection analysis (FIA) system.

Solutions and Electrochemical Instrumentation

The concentrations of the ruthenium bis(1,10-phenanthroline)(4-methyl-4’vinyl-2,2’-bipyridine) complex and the supporting electrolyte solutions in propylene carbonate were 1.00 x 10-3 mol. L^-1 and 0.100 mol. L^-1, respectively. The ruthenium complex was synthesized according to published procedures with no deviations thereof [10]. Tetraethylammonium perchlorate (TEAP), the supporting electrolyte, obtained from GFS Chemicals (Columbus, OH), was dried under vacuum to remove its water content prior to use in electrochemical measurements. Buffered solutions (pH 7.00) prepared from 0.100 mol. L^-1 sodium phosphate were used in the cyclic voltammetric characterization of the films in the presence of hydrazine and in FIA measurements.

Electrochemical measurements in a three-electrode cell configuration were made with a CHInstruments 660 Series Potentiostat (CHInstruments, Texas). The three-electrode cell system for all cyclic voltammetric and hydrodynamic measurements were made using a glassy carbon working electrode (0.0760 cm² ), a platinum auxiliary electrode, and a silver/silver chloride reference electrode (3 M KCl). All solutions were deoxygenated with nitrogen prior to commencing with electrochemical measurements and a nitrogen blanket was maintained on the solutions during the measurements.

Preparation And Characterization Of Ruthenium bis(1,10-Phenanthroline)(4-methyl-4’vinyl-2,2’-bipyridine) Polymer Films Using Cyclic Voltammetry

Prior to using the glassy carbon and platinum electrodes, they were polished using 0.05 µm polishing alumina obtained from Bioanalytical Systems. Details of the polishing procedure are described elsewhere [11]. The potential window for the electropolymerization of the ruthenium complex and characterization of the polymer films was 1.600 V to -1.600 V. To determine the optimum film thickness for monitoring hydrazine, the number of cyclic voltammetric scans at 100 mV. sec^-1 varied from 10 to 90. The films were characterized in 0.100 mol. L^-1 TEAP in propylene carbonate using a scan rate of 10 mV. sec^-1.

Electrochemical Detection of Hydrazine Using Flow Injection Analysis

A Coulochem III electrochemical detector (ESA, Inc., Chelmsford, MA), equipped with an analytical flow cell (model 5040) and a guard cell (model 5020) was used in all FIA measurements.

The analytical cell housed the target glassy carbon electrode (0.076 cm²), an auxiliary stainless-steel electrode, and a solid-state palladium reference electrode. The applied potential of the analytical cell was 0.700 V. The guard cell, consisting of a porous graphite electrode, a stainless-steel electrode, and a solid-state palladium reference electrode was used to remove any impurities existing in the carrier solution when potential was held at -0.800 V. The guard cell was placed between the pump and injector system, but before the analytical flow cell. Flow rates of 0.25 mL. min^-1-1.00mL. min^-1 were used to optimize the analytical signal. In addition, injection volumes of 25 µL -75 µL were used to study the impact of the injection volume on the analytical signal. Figure 1 shows a schematic block diagram of the FIA system wherein: P=pump, CS=carrier stream, GC=guard cell, AC=analytical cell, ED=electrochemical detector control station, and W=waste.

Figure 1: Schematic block diagram of the FIA system. CS and P=carrier stream and pump; GC= guard cell; AC= analytical cell containing the target glassy carbon electrode; ED=electrochemical detector; and w=waste.

For the flow system, a dual-piston pump (Perkin Elmer, LC Series 200) delivered the carrier stream phase. Injections were made using a Perkin Elmer Series 200 autosampler with a delivery precision <0.5% (RSD).

Electropolymerization And Characterization Of Ruthenium bis(1,10-phenanthroline) (4- methyl-4’vinyl-2,2’-bipyridine)

The electrochemistry and electropolymerization of ruthenium bis(1,10-phenanthroline)(4-methyl-4’vinyl-2,2’-bipyridine) was examined using cyclic voltammetry. Figure 2 shows the electropolymerization of the ruthenium complex after completing ninety cyclic voltammetric scans. Three reversible redox couples identified during the electropolymerization are represented by Equations (1) - (3). During the initial electropolymerization, the redox transformations are reversible with peak separations, ΔEp, approximately equal to 59 mV/n, where n=1. However, as the process continues, the system becomes quasireversible (ΔEp > 59 mV/n). All of the peaks corresponding to Equations (1) - (3) increase in peak currents, which is indicative of forming the polymer film. Beyond ninety cyclic voltammetric scans, the peak currents do not increase and as a result, the sequence of cyclic voltammetric scans was stopped. Peaks 1 and 2 correspond to the reduction and oxidation of ruthenium. However, peaks 3 and 4, and 5 and 6 redox couples, each correspond to one-electron reduction and oxidation of the vinyl-2, 2’-bipyridine (vbpy) substituent, such that the -vbpy is finally reduced to -vbpy^2-. The electropolymerization process is initiated through a reduction of the vinyl substituent attached to the ring system and 1, 10-phenanthroline ligand system. Details pertaining the reaction mechanism to form the polymer film are discussed by Anson and co-workers [10].

Figure 2: Electropolymerization of 1.00 x 10^-3 mol. L^-1 ruthenium complex with 0.100 mol. L^-1 TEAP in propylene carbonate.

The formal reduction potential, E^?’, of the redox couples is the average of the cathodic peak and anodic peak potentials. Figure 3 shows a characterization cyclic voltammogram of the polymer film on the glassy carbon electrode compared to a clean glassy carbon electrode, both in the supporting electrolyte solution. When the electrode was rinsed with fresh solvent and placed in the supporting electrolyte solution for cyclic voltammetric characterization at lower scan rates (i.e., 10 mV. s^-1), the redox couples shown in Figure 2 corresponding to equations (1) – (3) persist upon repetitive cycling. Table 1 provides a summary of the Eo’values for the redox couples of cyclic voltammograms in Figures 2 and 3.

Table 1: Summary of Formal Reduction Potentials of Figures 2 and 3.

The electropolymerization of ruthenium bis(1,10-phenanthroline)(4-methyl-4’vinyl-2,2’- bipyridine) produces a multilayered polymer film that shows good electrochemical response in an aqueous environment, which has not been reported before. Cyclic voltammetric characterization of polymer films in the presence of hydrazine produces a strong anodic peak current that increases with hydrazine concentration indicating that the polymer films do function as a redox mediator. Hydrodynamic studies confirm the results on the oxidation potential for hydrazine acquired by cyclic voltammetry. The films are capable of being used in FIA systems for the determination of hydrazine in water. Although there is a slight decrease (c.a. 2%) in the peak height of multiple hydrazine injections of the same concentration, the FIA system can process approximately 120 samples/hour.

Funding Statement

This research was supported by Hope College Department of Chemistry Endowed Research Funds.

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