Sonochemical enhancement of electrochemiluminescence

Sonochemical enhancement of electrochemiluminescence D.J. W a l t o n , S.S. Phull, D . M . Bates, J.P. Lorimer and T.J. M a s o n Department of Applied Physical Sciences, Coventry Polytechnic, Priory Street, Coventry CV1 5FB, UK The electrochemiluminescence of the tris-(2,2'-bipyridine) ruthenium (11) dication in aqueous oxalate and in acetonitrile is substantially enhanced by simultaneous irradiation with 4 0 - 6 0 kHz ultrasound. Edge effects and electrode patchiness are diminished, quantum efficiency is increased and lower cell voltages are required. Reproducibility and stability are improved and electrode fouling is minimized. Electrochemiluminescence (ECL) and sonoelectrochemiluminescence (SECL) seem to be of similar origin but a weak sonoluminescence (SL), also observed, is of a different origin. Preliminary experiments upon the luminol and 1-aminopyrene electrochemiluminescence systems suggest that different mechanisms for ultrasonic influence may operate in each case.

Keywords: sonochemistry; electrochemiluminescence; sonochemical enhancement

Electrochemiluminescence (ECL) is the emission of light from an electrolysis cell, as distinct from electroluminescence which is light emission from solid state phosphors and similar materials. In principle the potential range available to the electrochemist provides sufficient energy to produce the excited states necessary for ECL, but in practice thermal relaxation mechanisms predominate. Heat evolution is therefore a characteristic of electrolysis rather than light emission. ECL requires not only sufficient energetics, but also a reaction event of a very short duration, so that the necessary excited state is produced before bond vibration or other mechanical reorganization of energy can occur. ECL is not a widespread phenomenon, but several systems have been well established ~. ECL is presently attracting attention because of its potential applications. These include display devices, chromatographic detectors and the analysis of bioactive species. This latter application is particularly promising and covers immunoassays, the sensing of redox enzymes and the distribution, metabolism and pharmacokinetics of drugs and hormones 2. Thus in an example of an immunoassay, an ECL-active species may be attached to an antigen such that ECL is observed in the conditions employed. However, when combined with the corresponding antibody a large complex is formed in which access of the ECL-active species to the electrode is restricted leading to diminution in ECL which is quantified 3. There are, however, drawbacks to the development of these systems which result from the complex nature of the reaction mechanisms in many ECL processes, and from the influence of the kinetic regimes and diffusion characteristics which derive therefrom 4. Thus there are often edge effects at planar or disc electrodes, in which non-unEorm diffusion causes greater brightness near the edges; and patchiness across an electrode surface is 0041-624X/92/030186-06 0 1992 Butterworth-HeinemannLtd 186 Ultrasonics 1992 Vol 30 No 3

common. This patchiness has been used to map the spatial heterogeneity of, for example, carbon paste electrodes 5. There are thus restrictions upon the cell geometry and the small electrode areas demanded for reproducibility cause the emission of low light intensities which limit detection. In addition, electrode fouling with prolonged usage causes irreproducibility, lessened efficiency, shortened device lifetimes and demands cleaning procedures or other methods of reactivation 4. We have previously examined the influence of ultrasound upon electro-organic synthetic reactions, in particular the electro-oxidation of carboxylate anions, and have noted the following effects: enhanced diffusion characteristics, increased yields and current efficiencies, lessened cell voltage requirements, absence of electrode fouling and also altered reaction mechanism and shift in product distribution 6- 8. These effects are also of potential benefit to the development of ECL devices and we now report the influence of ultrasound upon representative ECL systems18. This is a topic which has been very little studied 9.

Electrochemiluminescent systems The requirements for ECL are demonstrated by the following systems: rubrene, luminol and tris-(2,2bipyridine) ruthenium (II) dichloride (structures are given in Figure 1). Rubrene

Many polyaromatic molecules display ECL when anion radicals generated at the cathode meet cation radicals from the anode. The corresponding annihilation reaction produces a ground state molecule and an excited state molecule, the latter decays to its ground state via the

Sonochemical enhancement of electrochemiluminescence : D.J. Walton et al.

@

NH2

dication ~°. This species may be produced by several procedures. Thus the ruthenium (III) bipyridine derivative formed at the anode reacts with the ruthenium(I) bipyridine analogue from the cathode as shown below

I-Aminopyrene

[ R u ( b p y ) 3 ] + + [ R u ( b p y ) 3 ] 3+ - . [ R u ( b p y ) ] 2+ Rubrene (9, 10, II, 12 tetraphenyl napthacene) NH2

O

+ [ R u ( b p y ) 3 ] 2+* CI 2

~NH

c% o Luminol

Figure1

tris-(2,2'bipyridine) ruthenium ( ]I ) dichloride

Su bstrate molecules for electrochemilu minescent systems

emission of light 1. The electrodes may be kept poised at the respective cell potentials, in which case attention must be paid to the diffusion characteristics of the cell, to maximize the annihilation process. Alternatively the working electrode may be sequentially pulsed to the positive and negative potential extremes, producing both required species which diffuse in sequence from the electrode and undergo the annihilation reaction when they meet. Here parameters such as pulse duration and duty cycle are also available for manipulation. Hybrid systems may also be employed in which for example either of these ion-radicals are produced to react with the corresponding redox species from another substrate molecule, such as a cation radical from N,N,N'N'tetraphenyl-p-phenylene diamine ( T M P D ) and an anion radical from rubrene. 1-Aminopyrene

An ostensibly similar system is 1-aminopyrene, favoured by biochemists because of the capability for attachment to biologically important molecules via the amino group. Here, however, the mechanism of ECL has not been identified with certainty. The intervention of peroxides is postulated in aqueous solution and oxygen sensitivity is observed 3.

L uminol This well-known chemiluminescent, bioluminescent and electrochemiluminescent system follows a complex mechanism 5 given in Figure 2. Here electro-oxidation in alkaline solution of the monoanion of luminol (5amino-2,3-dihydro-l,4-phthalhydrazide) accompanies the simultaneous electro-oxidation of anionic species derived from oxygen or hydrogen peroxide; leading to nitrogen evolution and the formation of the 3-aminophthalate dianion in the excited state. Such complexity is suggested because maximum light emission does not accompany maximum electrolysis current. Tris- ( 2,2'-bipyridine ) ruthenium (11) dichloride This more recent ECL system concerns light emission from the excited state of the ruthenium bipyridine

[ R u ( b p y ) 3 ] 2+* ~ [ R u ( b p y ) 3 ] 2+ + hv Non-aqueous solvents with wide potential windows are required because of the reactivity of these species, particularly [ R u ( b p y ) 3 ] +. Reduced and oxidized species may be formed using two separate electrodes poised at the respective positive and negative potentials; alternatively the working electrode is pulsed between positive and negative extremes as described above for rubrene. This pulsed technique is also known as the regenerative method. Alternatively [Ru(bpy)3] + may be cathodically produced in the presence of an oxidizing species such as HO2; simultaneously formed in oxygenated water or else in the presence of another oxidizing agent 1~. This is termed the 'reductive-oxidation' system. Finally [-Ru(bpy)3] 3+ may be anodically produced in the presence of a reducing agent, particularly organic species such as oxalate ions. This is termed the 'oxidative reduction' system. Here the anodic potential requirements allow the use of an aqueous medium which is also necessary for oxalate solubility. The oxalate decays to CO 2 via a variety of species; and the following mechanism has been proposed~2 [Ru(bpy)3] 2+ --~ [Ru(bpy)3] 3+ + e rRu(bpy)3] 3+ + C2042- - . [Ru(bpy)3] 2+ + C 2 0 4 C 2 0 4 --* C O ; +

CO 2

[Ru(bpy)3] 2+ + CO 2 --. [Ru(bpy)3] + + C O 2 [Ru(bpy)3] 3+ + [Ru(bpy)3] + ~ [Ru(bpy)3] 2+* + [Ru(bpy)3] 2+ [Ru(bpy)3] 2+* -* [Ru(bpy)3] z+ -I- hv Oxalate is therefore consumed and the ruthenium complex regenerated. In this paper we examine the influence of power ultrasound and the ultrasonic parameters of power and frequency upon, in the main, the ruthenium bipyridine system, using either the oxidative reduction procedure in aqueous oxalate or the regenerative procedure in acetonitrile. Both these procedures are known to be insensitive to the presence of oxygen 4'1°.

NH2

O"

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O

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2 - ca

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Mechanism of luminol e l e c t r o c h e m i l u m i n e s c e n c e 5

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Sonochemical enhancement of electrochemiluminescence: D.J. Walton et al. Experimental

All electrolyses were carried out in an undivided cell, either under potentiostatic control (versus SCE reference electrode throughout) using an E G & G Princeton potentiostat (model 273) and 1 cm / Pt flag electrodes, or under galvanostatic control from a Thurlby PL310 power supply and 2 cm 2 Pt electrodes. Solutions were degassed with nitrogen except where stated. Ultrasound was mainly 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 EMI 9871 R photomultiplier tube ( P M T ) and dedicated interface outputting to a Tektronix digital storage oscilloscope (model 2221A). Preliminary experiments employed a Walker cleaning bath (20 kHz, 15 W) with visual data inspection. Spectroscopic measurements ( 3 0 0 - 7 0 0 n m ) were made using a custom built monochromator with photodetector ( P M T ) system interfaced to a BBC microcomputer with manual scanning of wavelength.

Oxidative-reduction procedure Sodium oxalate, 0.03 M (BDH AnalaR) was used in pH 5.0 phosphate buffer with tris-(2,2-bipyridine)ruthenium(II) dichloride (Aldrich, purified by recrystallization from distilled water) at various concentrations from 0.0001 to 0.005 M. Potentiostatic control was employed at 1.2 V, or galvanostatic control at up to 100 mA cm-2.

Regenerative procedure Tetrabutylammonium tetrafluoroborate, 0.1 M, was used (Fluka puriss) in acetonitrile ( H P L C grade, BDH) and tris-(2,2-bipyridine)ruthenium(II) dichloride at 0.0001M only. Potentiostatic control, where employed, was at 1.5 V then - 1.9 V, at various duty cycles (1 5 Hz). Results

Oxidative-reduction procedure aqueous oxalate system Both the potentiostatic and galvanostatic electrooxidations of [Ru(bpy)3] 2+ (at 0.0001 M) in aqueous oxalate produce orange ECL in the region of the anode. Over the relatively large surface areas of > 1 cm 2, the edge effects and patchiness are clearly visible. Under simultaneous ultrasonic irradiation in the cleaning bath, however, the ECL is markedly brighter and there is greater uniformity of emission across the electrode surface. These results are quantified by use of the horn probe a n d P M T system. Figure 3 shows oscilloscope traces for potentiostatic ECL and sonoelectrochemiluminescence (SECL) compared to the background. A substantial increase in light intensity under SECL is evident. There is also a weak sonoluminescence (SL) in the absence of an applied potential. This SL is similar to that observed for the aqueous oxalate alone without [ Ru (bpy)3 ] 2 +. Sonoluminescence of aqueous solutions is well established and is thought to be due to the generation of hydroxylic radical species and their reaction products 13, though there is some doubt regarding the role of ultrasound in this process 14. The enhancement of quantum efficiency 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 100mAcm -2 without ultrasound; while with ultrasound the same intensity

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requires 5 7 m A c m -2. Since ECL quantum efficiency represents the ratio of photons emitted to the number of electrons consumed, this represents an enhancement of (100/57), or almost a doubling in the quantum efficiency of the system. The cell potential required to maintain the constant current also lowers under the influence of ultrasound. This phenomenon has been observed throughout our sonoelectrochemistry studies. To maintain a fixed current of 100mAcm -2, the cell potential drops from 1.9v without ultrasound to 1.4 v with ultrasound. The effect of the concentration of [Ru(bpy)3] 2+ in aqueous oxalate solution under potentiostatic control is shown in Figure 4. There is a steady increase in light emission with increasing [Ru(bpy)3] 2 + concentration for SECL, ECL from a still solution and ECL of a solution agitated mechanically. Of these, SECL produces the greatest light output and at high [Ru(bpy)3] 1+ concentrations the photodetection system becomes saturated. Mechanical agitation does not produce such a pronounced enhancement. The SL shows different behaviour and if anything diminishes slightly at higher concentrations of [Ru(bpy)3] 2 +. This suggests that the ruthenium species

a) b) c)

Background SL ECL

d)

SECL

Figure 3 Luminescence from aqueous ruthenium bipyridine/ oxalate system. Light emission from 0.0001 M ruthenium bipyridyl dichloride in 0.03M aqueous sodium oxalate solutions. (a) Background, base line; SL, sonoluminescence from 40 kHz probe, no applied electrode potential; ECL, electrochemiluminescence at + 1 . 2 V (versus SCE), potentiostatic control, Pt electrodes, no ultrasound; SECL, sonoelectrochemiluminescence at + 1.2 V (versus SCE) under ultrasound from 40 kHz probe

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300

,

i

400

,

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500

600

Concentration of [ Ru (bpy) 3 ] (/amol dm-3 } Figure 4 Effect of Ru (bpy)3 ' el2 concentration on luminescence in aqueous oxalate. R, SECL, + 1.2 V potentiostatic, 40 kHz probe; X, ECL, + 1.2 V potentiostatic, no u l t r a s o u n d ; . , ECL, mechanical agitation with nitrogen gas; £~, SL, 40kHz probe, no applied electrode potential

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Wavelength (nm) F i g u r e 5 Emission spectra from luminescence of [ R u ( b p y ) 3 ] 2+ in aqueous oxalate. I-3, ECL, + 1.2 V potentiostatic, no ultrasound; A , SECL, + 1 . 2 V potentiostatic, 40 kHz probe, oxygen degassed; X, SECL, + 1.2 V potentiostatic, 40 kHz probe, nitrogen degassed

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while SL is a bulk solution phenomenon, thus providing further mechanistic discrimination. The effect of ultrasound upon electrode fouling is shown clearly in Figure 6 for 0.0001 M [Ru(bpy)3] 2+ in aqueous oxalate under potentiostatic control. In the absence of ultrasound, sequential positive pulsing (l Hz duty cycle) of the electrode rapidly lessens the light output. The [Ru(bpy)3] 2+ oxidative-reduction procedure is known to be prone to electrode fouling. However, when the procedure is repeated in the presence of ultrasound there is no appreciable change in light output with consecutive pulses. This is a significant result for sensor technology. The effect of ultrasonic power on SECL is shown in Fieure 7a for 0.0001 M [Ru(bpy)3] z+ in aqueous oxalate solution under galvanostatic control. Two relative power levels are shown, low and high, the latter is noticeable by a distinctly different note from the horn and visual streaming from the horn tip. SL displays a different behaviour as shown in Figure 7b. Low power produces greater emission than higher power (note the different intensity scales for Figures 7a and 7b). It has previously been noted that SL of water under stable cavitation conditions may produce greater overall light intensity than under unstable cavitation. In the latter case bright spots may be localized, for example only near the tip of the horn 15. These SL effects require further examination. Varying the ultrasonic frequency from 40 to 60 kHz produced no appreciable change in light emission for a 0.0001 M [Ru(bpy)3] 2+ oxalate aqueous solution under galvanostatic control. The Undatim SonoReactor can

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~ 0

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= 10

Pulse number Figure 6 Electrode fouling study: effect of ultrasound on [ R u ( b p y ) 3 ] 2+ luminescence in aqueous oxalate for successive potentiostatic pulses (1 Hz duty cycle). 17, SECL, + 1 . 2 V , 4 0 k H z p r o b e ; . , ECL, + 1 . 2 V

is not involved in the key processes of sonoluminescence and that there might be low grade quenching processes occurring. The origins of SECL and SL are clearly different, but SECL and ECL appear to involve similar processes. The emission spectra from 0.0001 M [Ru(bpy)3] 2+ in aqueous oxalate under galvanostatic control are shown in Figure 5. This data is of a preliminary nature and point values are given at wavelengths that were scanned manually. The comparison shows that the main emission peak occurs at 580 nm for both ECL and SECL. This emission is due to the [Ru(bpy)3] 2+* excited state decaying to its ground state. Degassing with oxygen produces little effect on the SECL emission intensity. The [Ru(bpy)3] 2+ ECL system is known to be insensitive to oxygen 4'1° and contrasts with the behaviour of, for example, the luminol system. The SL emission was too weak to record a spectrum with the apparatus employed and further experiments are at hand to expedite spectroscopic analysis. Differential masking of the electrode region to the P M T shows that SECL and ECL originate near the anode

Background

=mt=~

Low power = ~ High power

b Background High power ===~ml~

Low power

Figure 7 Effect of ultrasonic power on (a) electrochemiluminescence of ruthenium bipyridine in aqueous oxalate and (b) sonoluminescence. (a) SECL, + 1.2 V potentiostatic, 40 kHz. Horn probe at t w o power levels: low power (no visible streaming), high power (streaming clearly evident and different audible note). Scale 0 . 5 V c m -1. (b) Same power settings and system, no applied electrode potential. Scale 0.2 V cm 1

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Sonochemical enhancement of electrochemiluminescence: D.J. Walton et al. Background =====l=~

Background

ECL

= ~

62.0 kHz 63.0 kHz = ~ (superimposed)

62.5 kHz

= ~

SECL

= ~

F i g u r e 8 Effect of ultrasonic frequency on electrochemiluminescence of ruthenium bipyridine in aqueous oxalate. Potentiostatic, + 1.2 V (versus SCE). Ultrasound provided by Undatim variablefrequency SonoReactor using 6 0 k H z probe, maximum SECL observed at 62.5 kHz

Figure 9 Regenerative procedure for electrochemiluminescence. Ruthenium bipyridine in acetonitrile with 0.1 M tetrabutyl ammonium tetrafluoroborate. Platinum electrode, potentiostatic control (versus SCE) 1.5 V then -- 1.9 V with 1 Hz duty cycle, 40 kHz probe for SECL

provide minor changes in ultrasonic frequency for a fixed frequency probe. This results in SECL providing an effective test for the coupling of ultrasonic power to the system. Figure 8 shows maximum SECL at 62.5 kHz and a rapid drop in SECL at either side of this maximum. The corresponding maximum SECL intensity for the 40 kHz probe is observed at 39.17 kHz. SECL therefore provides a means of fine tuning the probe frequency.

ultrasound, showing another enhancement. This solution also showed an sonoluminescence intensity of approximately 30%. These preliminary results suggest that the influence of ultrasound upon ECL is not always consistent, and that each of these often complex reaction systems must be considered individually. A mechanism for SECL that involves simply the narrowing of the electrode double layer, a general alteration of diffusion characteristics and influence upon the behaviour of dissolved gases, may prove to be insufficient.

Regenerative procedure and non-aqueous solvent system Both potentiostatic and galvanostatic procedures of positive/negative pulse sequences produce ECL from 0.0001 M [Ru(bpy)3] 2 ÷ in acetonitrile, and a pronounced SECL enhancement is observed (Figure 9). A weak sonoluminescence also occurs (not shown). It is known that SL may be observed from non-aqueous solvents 16'17. Excited state species such as C2H and CH have been detected from SL of alkanes, and when oxygen is present there is emission from C O / and OH, while under nitrogen CN is seen. These species are thought to be formed during cavitational collapse. In preliminary experiments we have observed SL, ECL and SECL of [ R u ( b p y ) 3 ] 2+ in dimethylformamide and 1,3,5trichlorobenzene. As predicted from current theory, the light intensities are greater from the denser and less volatile solvents. Further studies of luminescence from non-aqueous solutions are in progress.

Other electrochemiluminescent systems The [Ru(bpy)3] 2+ system is unusual in that it is insensitive to oxygen. To test whether ultrasound would produce similar SECL enhancements in systems where oxygen sensitivity is observed, preliminary studies were performed on 1-aminopyrene and luminol. Thus 1-aminopyrene (0.0025 M) in pH 7.0 aqueous phosphate buffer (0.2 M) was pulsed potentiostatically between 2.0 V and -- 1.9 V (versus SCE, 1 Hz duty cycle) and ultrasound was provided by the horn probe at 40 kHz. Luminol at 0.002 M in 0.1 M aqueous sodium hydroxide was pulsed potentiostatically between + 1.0 V and - 1.0 V (1 Hz duty cycle) and ultrasound was provided by the horn probe at 40 kHz. The results showed that an ECL light intensity of 100% (arbitrary units) for 1-aminopyrene without ultrasound became only 69% for SECL with ultrasound. Whereas for luminol the 100% initial ECL became 150% with

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Conclusion These studies have shown that ECL from the [ Ru (bpy)3 ] 2 + system, whether in aqueous oxalate solution or in acetonitrile is substantially enhanced under simultaneous ultrasonic irradiation in the frequency range 40-60 kHz. Edge effects and problems of patchiness are diminished across planar electrodes of larger areas. Greater light intensities are available for measurement and quantum efficiencies are enhanced. Lower cell voltages are required. Electrode fouling problems are minimized, and stability and reproducibility are improved. The SECL enhancement is greater than that produced by vigorous mechanical agitation 18. Luminescence behaviour as a function of [ Ru (bpy)3 ] 2 + concentration, supported by spectral data, suggests that ECL and SECL have a similar origin, thought to be the emission of light from the [Ru(bpy)3] z÷* excited species. Both ECL and SECL are unaffected by the presence or absence of oxygen. A weak sonoluminescence (SL) is observed in the absence of an applied electric field. This does not have the same behaviour as ECL and SECL, and appears to be derived from the known SL characteristics of the solvent medium. Some enhancement in SL seems to be due to the presence of the [Ru(bpy)3] 2÷. Greater ultrasonic power enhances SECL but does not directly increase SL. Changes in ultrasonic frequency shows SECL to be a sensitive probe for fine tuning of the ultrasonic system. The extension of studies to other systems shows that ECL from 1-aminopyrene is adversely affected by ultrasound, whereas ECL from luminol is enhanced. Interference in oxygen sensitive systems by ultrasonic degassing is not necessarily involved in a straightforward manner, and the mechanism of the ultrasonic effects may vary from system to system. Further studies to elucidate these matters are in progress.

Sonochemical enhancement of electrochemiluminescence." D.J. Walton et al.

Acknowledgements

6

We thank the SERC (for funding to SSP and DMB) and Professor P.N. Bartlett (University of Bath) for assistance.

7 8 9

References 1 2 3 4 5

Faulkner, L.R. and Bard, A.J., in: Creation and Detection of the Excited State ( Ed Ware, W.R.) Marcel Dckker, New York (1976) Turner, A.T.S., Kanube, I. and Wilson, G.S. (Eds) BiosensorsFundamentals and Applications Oxford University Press, Oxford (1989) Aizawa, M., Kunoh, H. and Ikariyana, Y. Biochem Biophys Res Commun (1985) 128 987 Phull, S.S. The development and evaluation of an electrochemically generated chemiluminescent immunoassay system PhD Thesis University of Warwick (1990) Vitt, J.E. and Johnson, D.C. J Electrochem Soc (1991) 138 1637

10 11 12 13

14 15 16 17 18

Walton, D.J., Chyla, A., Lorimer, J.P. and Mason, T.J. J Chem Soc Chem Commun (1989) 603 Walton, D.J., Chyla, A., Lorimer, J.P. and Mason, T.J. Synth Commun (1990) 20 1843 Walton, D.J., (~hyia, A., Lorimer, J.P. and Mason, T.J. Proceedings 1990 Ultrasonics Conference Butterworths, London (1990) 1241 Zhivnov, V.A., Rumyantsev, I.Yu. and Tomin, V.I. Spec Lett (1977) 10 763 Ege, D., Becket, W.G. and Bard, A.J. Anal Chem (1984) 56 2413 Igasashi, R., Nosaka, Y., Fujii, N. and Miyama, W. Bull Chem Soc Jpn (1989) 62 1405 Rubinstein, i. and Bard, A.J. J Am Chem Soc( 1981 ) 103 512 Sehgal, C. and Verall, R.E., in: Ultrasound; its Chemical, Physical and Bioloffical Effects (Ed Suslick, K.S.) VCH (1988) Margulis, M.M., Kurochkin, A.K., Smorodov, E.A. and Valitav, R.V. Russ J Phys Chem (1986) 60 731,734 Crum, L.A. and Reynolds, G.T. J Acoust Soc Am (1985) 78 137 Flint, E.B. and Suslick, K.S. J Am Chem Soc (1989) 111 6987 Flint, E.B. and Suslick, K.S. Nature (1987) 330 553 Walton, D.J., Phnll, S.S., Bates, D.M., Chyla, A., Lorimer, J.P. and Mason, T.J. J Chem Soc Chem Commun in press

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