Preparation and preliminary evaluation of anhydrous tris(2,2′-bipyridyl)ruthenium(III) perchlorate as a temporally stable reagent for analytical chemiluminescence
Introduction
Since its discovery by Hercules and Lytle [1], the chemiluminescence produced from the reduction of the tris(2,2′-bipyridyl)ruthenium(III) ([Ru(bipy)3]3+) cation has proven to be a sensitive and versatile detection system when employed with either flow analysis or liquid chromatography [2]. The detection chemistry (summarised in Scheme 1) is common to all applications and relies upon the oxidation of [Ru(bipy)3]2+ to produce the reagent, [Ru(bipy)3]3+ which in turn is reduced by a suitable analyte leaving the ruthenium(II) complex in an electronically excited state that subsequently returns to its ground state via the emission of a photon [1].
In the majority of analytical applications reported to date the generation of the [Ru(bipy)3]3+ has been accomplished using electrochemical oxidation [2]. The primary reason for this preference was the facility to conveniently produce the reagent immediately prior to reaction with the analyte, thus, overcoming the inherent instability of [Ru(bipy)3]3+ in aqueous solution due to its ability to oxidise water [3]. Recently, Gerardi et al. [4] described two chemical approaches to the production of stable solutions of [Ru(bipy)3]3+. The first utilised highly acidic reagent solutions (up to 2.0 M H2SO4), which prevented the oxidation of water by [Ru(bipy)3]3+[5]. Such low pH reaction conditions necessitated the use of appropriate buffers with commensurate concentrations in order to avoid any decrease in the chemiluminescence intensity or degradation of the flow analysis response profiles [4]. The other method continuously recirculated the reagent solution through an immobilised solid phase oxidant (lead dioxide), however, this required an extra pump that somewhat complicated the analytical manifold.
This paper describes the preparation of the reagent in the solid state as anhydrous tris(2,2′-bipyridyl)ruthenium(III) perchlorate ([Ru(bipy)3](ClO4)3) and the use of dry acetonitrile as the solvent to increase the temporal stability of [Ru(bipy)3]3+ in solution thereby extending its analytically useful lifetime.
Section snippets
Instrumentation
Absorbance measurements of [Ru(bipy)3]3+ were carried out using a Hitachi U3200 double beam spectrophotometer (Hitachi) and pH measurements were made with a Jenco Electronics pH-Vision 6071 (CHK Engineering). The analytical performance of the [Ru(bipy)3]3+ was evaluated on a simple two line automated flow injection analysis (FIA) manifold. Carrier and buffered standard solutions were delivered using a peristaltic pump (Gilson Miniplus 3) at 1.5 ml min−1 per line using Masterflex Viton tubing (1.6
Preliminary experiments
Previously [8], [9], [10], we have employed solid lead dioxide for the oxidation of an acidic (typically 0.05 M H2SO4) aqueous solution of [Ru(bipy)3]2+ to generate the desired reagent with the excess oxidant then being filtered off. Taking a similar approach, a small amount of solid lead dioxide was added to a solution of [Ru(bipy)3]Cl2·6H2O (1×10−3 M) in acidic acetonitrile. After a few minutes agitation, the solution had undergone the characteristic colour change from orange to green
Conclusions
The preliminary results presented here have demonstrated the ease of preparation, long-term temporal stability and analytical utility of [Ru(bipy)3](ClO4)3. The additional advantages of this new reagent are minimal buffer concentrations, which in turn, reduces blank responses and that no prior oxidation step is required. The performance of acetonitrile solutions of [Ru(bipy)3](ClO4)3 are currently being evaluated for the chemiluminescence detection of a wide range of analytes [2] using both
Acknowledgements
The authors are most grateful to Dr. Peter Ash of Johnson Matthey Ltd., UK, for the generous gift of the ruthenium(III) chloride.
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