Ultrasonic enhancement of electrochemiluminescence

Electrochimica Acla, Vol. 38, No. 21% pp. M7-310, Prinkd in Great Britain.

1993 0

ULTRASONIC ENHANCEMENT ELECTROCHEMILUMINESCENCE

0013~4686/93 $6.00 + O.Co 1992 Pergamon Press Ltd.

OF

DAVID J. WALTON,* SUKHVINDERS. PHULL, DAMN M. BATES,J. PHILIP LORIMERand TIMOTHYJ. MASON

Department of Applied Physical Sciences, Coventry Polytechnic, Priory Street, Coventry CVl SFB, U.K. (Received 30 March 1992; in reoisedform 10 June 1992) Ah&a&-The electrochemiluminescence of the tris-(2,2’-bipyridine) ruthenium (II) dication in aqueous oxalate and in acetonitrile containing tetrabutylammonium tetratluoroborate is markedly enhanced by simultaneous irradiation with 40 and 6OkHz ultrasound: edge effects and electrode patchiness are diminished, reproducibility and stability are improved, electrode fouling is minimised, quantum efficiency is increased and lower cell voltages are required. Key words: ultrasound, electrochemiluminescence, ruthenium (II) dication.

INTRODUCTION Electrochemilumineacnce (ECL), the emission of light from an electrolysis cell, is a relatively uncommon phenomenon because not only must there be sufficient energetics to achieve a suitable excited state, but also a reaction event of very short duration is required to prevent thermal relaxation by bond vibration or other mechanical reorganisation of energy[ 11. Several electrochemiluminescent systems are well known and these often have complex mechanisms. Thus luminol (S-amino-2,3-dihydro-l&phthalhydraxide) produces light from the excited state of the final product 3-aminophthalate. The mechanism in aqueous alkaline solution exposed to oxygen or containing hydrogen peroxide involves electrochemical co-oxidation of the luminol anion and an oxygen derived species leading through a chain of intermediates to the emitting species[2]. Despite these complicated mechanistic pathways there is considerable interest in ECL systems at present because of their potential analytical applications, in particular the analysis of bioactive systems, immunoassays and the distribution, metabolism and pharmacokinetics of drugs[3]. For an immunoassay an ECL active species is attached for example to an antigen such that luminescence is still observed. Exposure to the antibody now produces a large complex in which access of the ECL-moiety to the electrode is restricted, then the diminution in luminescence is quantified[4, 51. There are drawbacks in the development of such ECL systems, arising from the reaction mechanisms and the kinetic regimes and diffusion characteristics that these demand. There are often “edge effects” at planar or disc electrodes in which non-uniform diffusion causes greater brightness near the edges. In addition, patchiness across an electrode surface is common, and this has been put to use to map the

sonoluminescence,

electrolysis, tris-(2,2’-bipyridine)

spatial heterogeneity of, for example, carbon paste electrodes[2]. These factors place restrictions upon cell configuration, and the small electrode areas required for reproducibility result in the emission of low light intensities which limit detection. Furthermore many ECL systems, particularly those involving electrooxidations, suffer from electrode fouling after prolonged usage. This is a problem which causes lessened efficiency, shortened irreproducibility, device lifetime and requires cleaning procedures or other methods of reactivation[SJ. We have previously examined the influence of ultrasound upon electro-organic synthetic reactions, in particular electro-oxidation of carboxylate anions, and have noted the following effects[6-81: enhanced diffusion characteristics, increased yields and current efficiencies, altered reaction mechanism, shifts in product distribution and lessened cell voltage requirements. These are all of potential benefit to the enhance.ment of ECL; and we have therefore addressed the influence of ultrasound upon a representative ECL system. This is a topic which has received little attention previously[9]. We chose to study the electrochemiluminescence from tris-2,2-bipyridine) ruthenium dichloride. This system produces light emission from the excited state of the t&(2,2-bipyridine) ruthenium dication. This species may be produced by several different procedures[ 10,111. Thus the tris-(2,2-bipyridine) ruthenium trication formed at the anode reacts with the ruthenium bipyridine monocation from the cathode :

CRu(b~y),l + + CRu(bpy)J’+ -, CRu(bpy)l’+ + CRu(bpy)J’+ * . Non-aqueous solvents such as acetonitrile with a wide reduction potential capability are required because of the reactivity of [Ru(bpy),]+. 307

D. J.

308

WALTON et al.

The reduced and oxidised species may be produced using two separate electrodes both poised at the respective potentials; alternatively a single working electrode may be sequentially pulsed between the positive and negative extremes. This pulsed technique is also known as the “regenerative” method. Alternatively [Ru(bpy)J3’ may be produced at the anode in the presence of a reducing agent such as the oxalate dianion. Here the anodic potential requirements allow the use of aqueous media, necessary in any case for oxalate solubility. The oxalate decays to carbon dioxide via a variety of intermediate species including CzO,- and CO,-[12].

CWbM2 CWhvh13 + +

C20:

+ -, [Ru(bpy)13+ + e-

CWWI +

-,

+ c,o,c20,CWwM2 + +

CO2

+ co,-

-,

+ c,o,-

CWwhl

+

+ co, CRu@w),l + + CR@tvh13 + -, CWm’)12 +

+ CWWM2 +* CWwyM2 +* -+ [Ru(bpy)12+ + hv. Oxalate is therefore consumed and the ruthenium dicationic complex regenerated. This is termed the “oxidative-reduction” technique.

EXPERIMENTAL Electrolyses were carried out in an undivided cell, under potentiostatic control from an EG & G Princeton Model 273 potentiostat with 1 cm2 platinum flag electrodes, or under galvanostatic control from a Thurlby PL310 power supply and 2cm2 Pt electrodes. Solutions were degassed with nitrogen except where stated. Ultrasound was provided by an Undatim SonoReactor variable power/frequency source with a horn probe (1 cm tip diameter) normally operating at 40 kHz. Light emission was measured by a Thorn EM1 98711 photomultiplier tube and dedicated interface outputting to Tektronix model 2221A digital storage oscilloscope. Preliminary experiments employed a Walker cleaning bath (20kHz, 15 watts) with visual data inspection. Spectroscopic measurements (350-700 nm) were made using a custom-built monochromator with a photodiode interfaced to a BBC microcomputer with manual wavelength scanning. In the oxidative-reduction procedure ruthenium bipyridine dichloride, purified by recrystallisation from distilled water, and in the concentration range 0.0001-0.005 M was dissolved in 0.03 M sodium oxalate in distilled deionised water. Potentiostatic control was employed at 1.2V (vs. see) or galvanostatic control at up to 100mA/cm2.

In the regenerative procedure ruthenium bipyridine dichloride at 0.0001 M only was dissolved in redistilled HPLC grade acetonitrile containing 0.1 M tetrabutyl ammonium tetrafluoroborate. Potentiostatic control was employed with pulsing to + 1.5 V then - 1.9 V (vs. see), at various duty cycles (l-5 Hz).

RESULTS

AND DISCUSSION

For the oxidative reduction procedure both the potentiostatic and galvanostatic electro-oxidations of CWbyM2 + at 0.0001 M in aqueous oxalate produce orange ECL in the region of the anode. Over the relatively large electrode area of > 1 cm2 the edge effects and patchiness are visible to the naked eye. Under simultaneous ultrasonic irradiation in the cleaning bath the ECL is clearly brighter and there is greater uniformity of emission across the electrode surface. This result is quantified by use of the horn probe and photomultiplier system. Figure 1 shows oscilloscope traces for potentiostatic electrochemiluminescence (ECL) and sonoelectrochemiluminescence (SECL) compared to the background baseline. A substantial increase in light intensity under SECL is evident. In addition there is a weak sonoluminescence (SL) under ultrasound in the absence of an applied field. This is similar to the level of SL from aqueous oxalate without [Ru(bpy)3]2’. Sonoluminescence from aqueous solutions is well known, though there remains discussion regarding the exact role of ultrasound in the processC13, 141. The enhancement of quantum efficiency (Q.E.EcL) was estimated from galvanostatic experiments. Thus for 0.0001 M [Ru(bpy)3]2’ in aqueous oxalate a fixed light intensity (arbitrary units) is produced at 100 mA/cm2 without ultrasound, but at only 57 mA/ ultrasonic irradiation. cm2 under simultaneous Q.JLx. is the ratio of photons emitted to the electrons consumed; and this therefore represents an enhancement of 100/57, or almost a doubling in the quantum efficiency of the insonated system.

a) Background b) SL C) ECL

d) SECL

Fig. 1. Light emission from 0.0001 M tris-(2,2-bipyridine)ruthenium (II) dichloride in 0.03 M aqueous sodium oxalate solution. (a) background zero line, (b) sonoluminescence (SL) from 40 kHz probe, no applied electrode potential, (c) electrochemiluminescence (ECL) at + 1.2 V (vs. see), potentiostatic control, Pt electrodes, no ultrasound, (d) sonoelectrochemiluminescence (SECL) at + 1.2 V (vs. see) under ultrasound from 40 kHz probe.

Ultrasonic enhancement of electrochemiluminescnce

309

Fig. 2. Effect of Ru(bpy),CI, concentration on luminescence in aqueous oxalate. 0 SECL, + 1.2 V potentiostatic 49 kHz probe; x ECL, + 1.2 V potentiostatic, no ultrasound; n ECL, mechanical agitation with nitrogen gas; and 0 SL, 40 kHz probe, no applied potential.

static control. These data is of a preliminary nature, and point values are given from manually scanned wavelengths. The vertical scales have been displaced for clarity. The main emission peak occurs at 580 nm for SECL, for ECL under nitrogen and also for ECL in oxygen saturated solution, and has been assigned to the excited state of [Ru(bpy)J*‘. These results confirm the known lack of sensitivity of the [Ru(bpy)J]2’ system to oxygen, which is in contrast to many other ECL systems, including luminol[2]. The SL emission was too weak to record a spectrum with the apparatus employed. Further experiments are in hand to investigate the nature of this SL emission. Additional discrimination between the types of emission was obtained by differential masking of the electrode region. This showed that SECL and ECL originate near the anode while SL is a phenomenon of the bulk solution. Furthermore the presence of oxalate was found to be essential for both SECL and ECL, suggesting similarities in mechanism; but oxalate was not required for sonoluminescence to be observed. The effect of ultrasound upon electrode fouling is shown in Fig. 4 for 0.0001 M [Ru(bpy)J*’ in aqueous oxalate under potentiostatic control. When the electrode is pulsed on and off (1 Hz duty cycle) in the absence of ultrasound there is a rapid loss in light output. However, when the procedure is repeated under simultaneous ultrasonic irradiation there is no appreciable loss of light output over a number of consecutive pulses. The loss in light output for a stationary solution of Ru(bpy),Cl, could be due to oxalate depletion near the electrode surface. However, it is known that the [Ru(bpy)J*’ system is prone to electrode fouling. This has been demonstrated by previous studies in which a platinum electrode employed for luminescence in these conditions and then removed to a separate cell and used for the redox voltammetry of ferrocene monocarboxylic acid exhibited a marked loss of activity depending on the time period of prior luminescence usage[YJ. Ultrasonic enhancement is also observed in the regenerative procedure for [Ru(bpy)J*’ ECL. This is demonstrated in Fig. 5 for 0.0081 M [Ru(bpy)J*’ in acetonitrile/O.l M Bu,N+BF,_ . Again SECL produces a substantial increase in emission compared to ECL (the noise originates in the pulsing technique).

Fig. 3. Emission spectra from luminescence of [Ru(bpy)J*’ in aqueous oxalate. 0 ECL, + 1.2 V potentiostatic, no ultrasound; A SECL, + 1.2V potentiostatic, 40 kHz probe, oxygen degassed; and x SECL, + 1.2V potentiostatic, 4OkHz probe, nitrogen degassed.

Fig. 4. Electrode fouling study: effect of ultrasound on CRu(hpy),l‘+ luminescence in aqueous oxalate for successive potentiostatic pulses (1 Hz duty cycle). + SECL, + 1.2V potentiostatic, 4OkHz probe; 0 ECL, + 1.2V potentiostatic, no ultrasound.

In addition the cell potential required to maintain a fixed current is lessened under ultrasound. We have observed this phenomenon throughout our sonoelectrochemical studies[6-81. Here to maintain a current of 100 mA/cm* requires 1.9 V without and 1.4 V with ultrasound. Figure 2 gives the effect of [Ru(bpy)J*’ concentration in aqueous oxalate under potentiostatic control. The conditions and timescales are similar to those for Fig. 1. There is a steady increase in light output with concentration for SECL, ECL agitated mechanically with nitrogen gas and for ECL in stationary solution. Of these SECL produces greatest emission and at high [Ru(bpy)J*’ concentrations the photodetector saturates. Mechanical agitation does not produce such a pronounced enhancement. The SL shows a different behaviour, being hardly affected by a considerable increase in [Ru(bpy),]*’ concentration. The origins of SECL and SL therefore appear to be different, whereas SECL and ECL involve similar processes. This observation is further supported by the emission spectra given in Fig. 3, obtained from CWbwM* + in aqueous oxalate under galvano-

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100

200

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400

500

600

Concentration Ru(bpy)J Cl2 pmol dms3

D. J. WALTONet al.

310

Acknowledgements-We

thank SERC for funding (to S. S. P. and D. M. B.) and Prof. P. N. Bartlett, University of Bath, for assistance.

SWkg,O”“d

ECL

SECL

I Fig. 5. Regenerative procedure for electrochemiluminescence. Ruthenium bipyridine in acetonitrile with 0.1 M tetrabutyl ammonium tetrafluoroborate. Platinum electrode, potentiostatic control (vs. see) 1.5 V then - 1.9 V with 1 Hz duty cycle. 40 kHz probe for SECL.

SL also occurs, though this is not shown in the figure, and has been well-established as a phenomenon in non-aqueous solutions as well as aqueous ones[ 151. We have shown that the electrochemiluminescence is notably enhanced by simultafrom [Ru(bpy)J” neous irradiation with ultrasound in the 40 and 60 kHz frequency range; and this enhancement is observed in two different reaction media using different experimental procedures. Edge effects and electrode patchiness are diminished, reproducibility and stability are improved, electrode fouling is minimised, quantum efficiency is increased and lower cell voltages are required. To provide a mechanistic account of these phenomena we are examining the effects of ultrasonic power and frequency upon this phenomenon and are extending the study to other electrochemiluminescent systems. We have found from preliminary experiments that luminol ECL systems also shows an enhancement under ultrasound, but the laminopyrene ECL system is not enhanced[16], suggesting that an account of the sonoelectrochemiluminescent effect must reflect the individuality of each system.

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