Electrochemiluminescence of dipicolinic acid (DPA) and (bpy)2Ru(DPA)+ (bpy = 2,2′-bipyridine)

72 ORIGINAL RESEARCH J. Byrd, J. G. Bruno and M. M. Richter

Copyright © 2005 John Wiley & Sons, Ltd. Luminescence 2006; 21: 72–76

Luminescence 2006; 21: 72–76Published online 6 October 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/bio.886 ORIGINAL RESEARCH

Electrochemiluminescence of dipicolinic acid (DPA) and(bpy)2Ru(DPA)+++++ (bpy ===== 2,2′′′′′-bipyridine)

Jessica Byrd1, John G. Bruno2 and Mark M. Richter1*1Department of Chemistry, Missouri State University, Springfield, MO 65897, USA2Operational Technologies Corporation, 4100 NW Loop 410, Suite 230, San Antonio, TX 78229-4253, USA

Received 20 May 2005; revised 26 July 2005; accepted 2 August 2005

ABSTRACT: The spectroscopic and electrochemiluminescence (ECL) properties of dipicolinic acid (DPA), (bpy)2Ru2+ (bpy = 2,2′-bipyridine) and the species formed when DPA and (bpy)2Ru2+ [abbreviated to (bpy)2Ru(DPA)+] are allowed to react are reported.The UV-Vis absorption maxima for (bpy)2Ru2+ and (bpy)2Ru(DPA)+ are 493 and 475 nm, respectively, indicating the in situformation of a complex between DPA and (bpy)2Ru2+. DPA, (bpy)2Ru2+ and (bpy)2Ru(DPA)+ display ECL upon oxidation in thepresence of the oxidative–reductive co-reactant tri-n-propylamine (TPrA). The ECL of (bpy)2Ru(DPA)+ is at least two-fold higherthan either of the parent species. An ECL spectrum of (bpy)2Ru(DPA)+ displays a peak maximum 40 nm red-shifted from thephotoluminescence peak maximum, suggesting that the excited state formed electrochemically is different from that formedspectroscopically. Copyright © 2005 John Wiley & Sons, Ltd.

KEYWORDS: Electrochemiluminescence; dipicolinic acid; ruthenium bipyridyl; coreactant

Copyright © 2005 John Wiley & Sons, Ltd.

*Correspondence to: M. M. Richter, Department of Chemistry,Missouri State University, Springfield, MO 65897, USA.E-mail: [email protected]/grant sponsor: American Chemical Society PetroleumResearch Fund, USA.Contract/grant sponsor: Camille and Henry Dreyfus Foundation,USA.Contract/grant sponsor: National Science Foundation, USA; Contract/grant number: DUE-0124367.Contract/grant sponsor: Southwest Missouri State University, USA.

The ECL of aminopeptidase and esterase cleavageproducts have been reported by covalently attach-ing these species to bis(bipyridine)ruthenium (II)[(bpy)2Ru2+]. (bpy)2Ru2+ has little to no reportedintrinsic ECL, but attachment of a third ligandleads to ECL enhancement (19). For example, anon-electrochemiluminescent molecule was formedbetween picolinic acid ethyl ester and (bpy)2Ru2+ (19).Hydrolysis of this ester by pig liver esterase resultedin the formation of an electrochemiluminescentcomplex, presumably (bpy)2Ru(DPA)+ (DPA = 2,6-pyridinedicarboxylic acid or dipicolinic acid; Fig. 1) andpermitted the in situ determination of (bpy)2Ru(DPA)+

as well as the hydrolysis reaction.DPA is also important to biological defence, since

DPA appears to be unique to the composition ofbacterial endospores (20). Endospores are producedas a survival mechanism during periods of dehydrationor nutritional stress by such genera as Bacillus andClostridium. The most notorious species in these generainclude pathogenic strains of B. anthracis (anthrax) andC. botulinum (responsible for various types of botulinumneurotoxin). Since DPA can be linked to the presenceof anthrax, it is considered to be a preliminary or pre-sumptive indicator of anthrax in mass spectral analysis

INTRODUCTION

Electrochemiluminescence (often called electrogener-ated chemiluminescence and abbreviated to ECL)involves the formation of excited states at or nearelectrode surfaces and is a sensitive probe of electron-and energy-transfer processes at electrified interfaces(1, 2). It is also being commercially developed andmarketed for use in clinical analyses (e.g. immunoassays,DNA probes) using Ru(bpy)3

2+ (bpy = 2,2′-bipyridine)and a co-reactant to generate an ECL signal (3). ECLco-reactants are species that, upon electrochemicaloxidation or reduction, produce intermediates that reactwith other compounds to produce excited states capableof emitting light (4–6). For example, in the Ru(bpy)3

2+–TPrA (TPrA = tri-n-propyl amine) system (7), an anodicpotential oxidizes Ru(bpy)3

2+ to Ru(bpy)33+. The co-

reactant is also oxidized and, upon deprotonation of anα-carbon from one of the propyl groups, produces areducing agent [such as TPrA•, with the structure(CH3CH2CH•)N(CH2CH2CH3)2] (7). TPrA• can theninteract with Ru(bpy)3

3+ to form the excited state (i.e.*Ru(bpy)3

2+). Several excellent reviews on ECL haveappeared (1–3, 8–18).

Figure 1. Structure of 2,6-pyridinedicarboxylic acid (DPA).


Electroluminescence of dipicolinic acid ORIGINAL RESEARCH 73 ORIGINAL RESEARCH 73

(21, 22) and has also been used to suggest the presenceof anthrax spores by fluorescence spectroscopy (23–25).However, techniques such as ECL that are suitable foron-line monitoring of samples (15, 16, 18), and that mayresult in lower detection limits and higher sensitivity, arealso of interest.

Considering the potential importance of DPAchemistry, and the fact that there have been few studieson the in situ formation and subsequent ECL of metal–ligand complexes (26–29), this work reports on the ECLof DPA and the coordination complex formed betweenDPA and (bpy)2Ru2+ using TPrA as a co-reactant.

MATERIALS AND METHODS

Materials

Ru(bpy)3Cl2•6H2O and (bpy)2RuCl2 were obtained

from Strem Chemicals (Newbury Port, MA). DPAwas from Sigma (Milwaukee, WI). ECL assay buffer wasfrom BioVeris (Gaithersburg, MD) and consisted ofan ~0.20 mol/L potassium phosphate monobasic buffersolution with 0.18 mol/L tri-n-propylamine (TPrA)at a pH of 7.5. Deionized water that had been passedthrough a Barnstead/Thermolyne filtration system wasused throughout. pH adjustments were made using6.0 mol/L H2SO4 or 6.0 mol/L NaOH.

Experimental methods

Solutions containing DPA and (bpy)2Ru2+ were pre-pared in Bioveris Assay buffer, and this buffer was usedfor all dilutions. (bpy)2Ru(DPA)+ solutions were pre-pared by combining 130 µmol/L DPA and 130 µmol/L(bpy)2Ru2+ in assay buffer and heating to 65°C in awaterbath for 1 h. The solution was allowed to cool andthen diluted, as indicated in the text and figures, to thereported concentrations using assay buffer.

A Cary 100 UV-Vis spectrophotometer (Varian)was used for all absorbance studies. Photoluminescencestudies were conducted with a Shimadzu RF-5301 PCspectrofluorophotometer. Excitation (λexc) was at 452 nmfor Ru(bpy)3

2+, 473 nm for (bpy)2Ru(DPA)+ and 493 nmfor (bpy)2RuCl2 in Bioveris Assay Buffer with detectionbetween 500 and 700 nm.

Two instrumental systems were used to study electro-chemiluminescence. ECL intensity vs. concentration ofanalyte, e.g. DPA, (bpy)2Ru2+ and (bpy)2Ru(DPA)+,were conducted using an ORIGEN ECL analyser(Bioveris, Gaithersburg, MD) (30). Luminophoreconcentrations were in the µmol/L order. ECL spectrawere studied with a conventional three-electrode system,described previously (31) in conjunction with theShimadzu RF-5301 PC spectrofluorophotometer.ECL intensity vs. potential were obtained using a

CH Instruments 620 Electrochemical Analyzer anda Hamamatsu HC 135 photomultiplier tube (PMT)contained in a ‘light-tight’ box (29, 31). Solutions usedto obtain ECL data were 27 µmol/L (bpy)2Ru2+, DPAand/or (bpy)2Ru(DPA)+ in the Bioveris Assay Buffer.

Electrochemical data was obtained using squarewave voltammetry and a conventional three-electrodesystem (29) with a silver/silver chloride quasi-referenceelectrode. Solutions were 27 µmol/L (bpy)2Ru2+, DPAand/or (bpy)2Ru(DPA)+ in 0.18 mol/L potassiumphosphate (pH 7.3) containing no TPrA. The platinumdisk working electrode was cleaned prior to and aftereach run by polishing on a felt pad containing alumina,followed by sonication in 2 mol/L nitric acid and rinsingin deionized water.

RESULTS AND DISCUSSION

Absorption and photoluminescence

DPA, (bpy)2RuCl2 and (bpy)2Ru(DPA)+ all displayabsorptions in the ultraviolet and visible regions ofthe spectrum. DPA is characterized by an emissionpeak at 274 nm (molar absorptivity = 3993.63 mol/L/cm). Visible absorption spectra for (bpy)2RuCl2 and(bpy)2Ru(DPA)+ in aqueous solution (0.18 mol/L potas-sium phosphate, 0.05 mol/L TPrA) are shown in Fig. 2.The absorption bands at 475 nm for (bpy)2Ru(DPA)+

and 493 nm for (bpy)2RuCl2 are most likely MLCTtransitions (32) with molar absorptivities of 620.73 mol/L/cm and 184.27 mol/L/cm, respectively. Interestingly,the peak maximum of the lowest energy band for(bpy)2Ru(DPA)+ is ~18 nm blue-shifted when comparedto (bpy)2RuCl2 (slit width = 2 nm), indicating thein situ formation of a complex between (bpy)2Ru2+ andDPA. Although the identity of the species formed whenDPA and (bpy)2RuCl2 are allowed to react was notreported (19), we will abbreviate it (bpy)2Ru(DPA)+ forconvenience.

Excitation into the broad visible absorption bands(λem ≅ 473 nm for (bpy)2Ru(DPA)+ and 493 nm for

Figure 2. UV-Vis absorption spectra of (A) 6.5 µmol/L(bpy)2Ru(DPA)+ and (B) 20 µmol/L (bpy)2RuCl2 in 0.2 mol/Lpotassium phosphate buffer solution containing 0.18 mol/LTPrA.



(bpy)2RuCl2) produces room temperature photolumine-scence in aqueous buffered solution. Surprisingly, theemission spectra of (bpy)2Ru(DPA)+ and (bpy)2RuCl2

are nearly identical (Fig. 3), with emission bandsobserved at ~600 nm. This suggests that the PL is pre-dominantly metal-to-ligand charge transfer (MLCT) innature centred with emission from the π* orbitals of thebpy ligands to the dπ ruthenium-based orbitals (33, 34).

Electrogenerated chemiluminescence

The combination of (bpy)2RuCl2 and DPA heated to65°C in assay buffer results in a complex that emits ECL(Fig. 4). Surprisingly, both DPA itself and (bpy)2RuCl2

also display ECL. Clearly, the overall ECL signal for(bpy)2Ru(DPA)+ is higher than that for either parentspecies, suggesting the in situ formation of a complex(19). Further evidence for the formation of a coordina-tion complex is provided by square-wave voltam-metry. DPA displays no oxidation waves in the regionfrom +0.0 to +1.5 V vs. Ag/AgCl quasi-reference,

while (bpy)2RuCl2 displays oxidation waves at +0.41and +0.96 V. The wave at +0.96 is assigned asRu(II)/(III), based on comparisons to other rutheniumsystems (35). (bpy)2Ru(DPA)+ oxididizes approximately300 mV more cathodic (+0.60 V) than that observedin (bpy)2RuCl2, clearly indicating the formation of acomplex.

The ECL of (bpy)2Ru(DPA)+ peaks at about +0.89 V,indicating that oxidation of both the ruthenium complexand TPrA (Ea ~ +0.95 V) (36) is necessary for the gen-eration of ECL. An ECL spectrum for (bpy)2Ru(DPA)+

is shown in Fig. 5. The maximum for ECL emission(λecl) is approximately 40 nm red-shifted when com-pared to λem (Fig. 3), indicating that the excitedstate formed electrochemically differs from that formedphotochemically; perhaps due to decomposition of(bpy)2Ru(DPA)+ upon electrochemical oxidation and/orreaction with TPrA. Unfortunately, repeated attemptsto measure ECL spectra for both DPA and (bpy)2RuCl2

to confirm the nature of the emitting state in eachspecies were unsuccessful in both air-saturated anddeoxygenated solutions.

Whatever the nature of the emitting specieswhen DPA and (bpy)2Ru2+ react, it is reproduciblyformed, since the emission is linear with respect to[(bpy)2Ru(DPA)+] [correlation coefficient (R2) = 0.9941;Fig. 4] with a theoretical detection limit (signal plus3× standard deviation of noise) of 0.87 µmol/L atpH = 7.28. The linearity of the ECL emission for DPA(R2 = 0.7989) and (bpy)2Ru2+ (R2 = 0.8842) are alsopresented in Fig. 4.

The ECL of all three species as a function of pHis presented in Table 1. The pH of the solutions wasvaried from 6 to 9, with maximum intensities occurringat different pHs based on the emitting species. Little tono ECL was observed for the complex or parent speciesat below pH 6. In general, the ECL of (bpy)2Ru(DPA)+

decreased as the pH increased, while (bpy)2RuCl2

increased slightly, with a dramatic increase in DPA itselfat higher pHs. It was not possible to test the pH of these

Figure 3. Photoluminescence emission spectrum of (A)20 µmol/L (bpy)2RuCl2 (λexc = 493 nm, slit width = 10 nm) and(B) 6.5 µmol/L (bpy)2Ru(DPA)+ (λexc = 473 nm, slit width =10 nm) in 0.2 mol/L potassium phosphate buffer solutioncontaining 0.18 mol/L TPrA.

Figure 4. ECL intensity vs. concentration of emitter for (�)DPA, (�) (bpy)2Ru2+ and (�) (bpy)2Ru(DPA)+ in ECL assaybuffer containing 0.18 mol/L TPrA at pH 7.28. Each point isthe average of at least three scans, with error bars omitted forclarity. The standard deviation of each point is approximately± 3%. ECL was generated using the Origen Analyzer incorpo-rating a Pt electrode, with a potential sweep from 0.0 to +2.0 Vwith a sweep rate of 2.0 V/s.

Figure 5. ECL spectrum of 27 µmol/L (bpy)2Ru(DPA)+

(slit width = 20 nm) in 0.2 mol/L potassium phosphate buffersolution containing 0.18 mol/L TPrA upon potential sweepfrom +0.0 to +2.0 V vs. Ag/AgCl (sweep rate = 0.200 V/s).


Electroluminescence of dipicolinic acid ORIGINAL RESEARCH 75 ORIGINAL RESEARCH 75

systems above approximately 9.1, since the solubility ofTPrA decreases at higher pHs (7). This does indicate,however, that it may be possible to test for DPA alonewithout the need for metal complex formation.

CONCLUSIONS

Electrochemical, spectroscopic and ECL data indicatethe in situ formation of a complex between DPA and(bpy)2Ru2+. The resulting complex displays greater ECLthan either DPA or (bpy)2Ru2+ alone. However, theexact nature of the coordination complex and excitedstate species are unknown. This work complementsother ECL studies of metal–ligand complexes formedin situ (37–40), without prior synthesis and isolation ofthe light-emitting species.

Acknowledgements

The authors are grateful to the American ChemicalSociety Petroleum Research Fund for partial support ofthis work and the Camille and Henry Dreyfus Founda-tion, in the form of a Henry Dreyfus Teacher-Scholaraward (MMR); also to the National Science Founda-tion’s Course ‘Curriculum and Laboratory ImprovementProgram’, under grant DUE-0124367, and SouthwestMissouri State University for the purchase of theelectrochemiluminescence analyser.

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Electrochemiluminescence of dipicolinic acid (DPA) and (bpy)2Ru(DPA)+ (bpy = 2,2′-bipyridine)