Analytica Chimica Acta 912 (2016) 24e31

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Electrogenerated chemiluminescence induced by sequential hot electron and hole injection into aqueous electrolyte solution €ivi Kuosmanen a, Matti Pusa a, Oskari Kulmala b, Markus Håkansson a, Kalle Salminen a, Pa Sakari Kulmala a, * a b

Aalto University, Department of Chemistry, Laboratory of Analytical Chemistry, P.O. Box 16100, FI-00076 Aalto, Finland University of Helsinki, Department of Physics, P.O. Box 64, FI-00014, Finland

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Hot electrons injected into aqueous electrolyte solution.  Generation of hydrated electrons.  Hole injection into aqueous electrolyte solution.  Generation of hydroxyl radicals.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 October 2015 Received in revised form 15 December 2015 Accepted 12 January 2016 Available online 27 January 2016

Hole injection into aqueous electrolyte solution is proposed to occur when oxide-coated aluminum electrode is anodically pulse-polarized by a voltage pulse train containing sufficiently high-voltage anodic pulses. The effects of anodic pulses are studied by using an aromatic Tb(III) chelate as a probe known to produce intensive hot electron-induced electrochemiluminescence (HECL) with plain cathodic pulses and preoxidized electrodes. The presently studied system allows injection of hot electrons and holes successively into aqueous electrolyte solutions and can be utilized in detecting electrochemiluminescent labels in fully aqueous solutions, and actually, the system is suggested to be quite close to a pulse radiolysis system providing hydrated electrons and hydroxyl radicals as the primary radicals in aqueous solution without the problems and hazards of ionizing radiation. The analytical power of the present excitation waveforms are that they allow detection of electrochemiluminescent labels at very low detection limits in bioaffinity assays such as in immunoassays or DNA probe assays. The two important properties of the present waveforms are: (i) they provide in situ oxidation of the electrode surface resulting in the desired oxide film thickness and (ii) they can provide one-electron oxidants for the system by hole injection either via F- and Fþ-center band of the oxide or by direct hole injection to valence band of water at highly anodic pulse amplitudes. © 2016 Elsevier B.V. All rights reserved.

Keywords: Hole injection Electron injection Hot electron-induced electrochemiluminescence HECL Electrogenerated chemiluminescence ECL Tb(III) chelates

1. Introduction

* Corresponding author. E-mail address: (S. Kulmala). 0003-2670/© 2016 Elsevier B.V. All rights reserved.

Hot carrier injection in electronic devices is sometimes beneficial phenomenon, like in hot electron transistors and in novel

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spintronics [1,2], but normally an unwanted issue which often gradually degrades the performance of some of the electronic devices and shortens their lifetime [3e7]. In general, hot carriers are treated as particles that attain a very high kinetic energy from being accelerated by a high electric field or due to their emission/injection/generation mechanism [1e7]. In electronics, the term hot hole is often used analogously to the term hot electron, although on the basis of chemist's view, an electron can be considered as a particle with a single elemental charge but all the aspects of a hole are not intuitively comprehendible. The hot (energetic) carriers can be injected into normally inhibited regions of the device, e.g. to the gate dielectric, where they can get trapped or cause interface states to be formed at the contact areas of the other phases with the dielectric material. These defects/interface states then typically lead to problems in metal oxide semiconductor (MOS) devices [1e9]. Hot holes have their mean free paths in materials just like electrons and can also be created by high energy irradiation [8]. In the field of physics of electronics holes are treated as fermions [9,10], e.g. holes can just like electrons have (i) a spin and (ii) a certain effective mass depending on the material where they act as charge carriers [11]. In addition, when an electron and a hole produce an exciton the result can be either a triplet or singlet depending on the spin combinations (and finally result in either fluorescence or phosphorescence emission, respectively) [12]. Trapped electrons and holes in solid insulators are easy to understand also from the point of view of chemistry, although some of the aspects of holes raise questions. A hole, simply put is an electron missing from the valence band of a solid or liquid substance and it can move in an electric field unless it becomes localized and trapped. In solid ionic materials electrons are normally trapped in anion vacancies. E.g. in the cases of typical insulators in electronics, silica and alumina, the different types of trapped electrons, electron centers, are situated in energy somewhere between the conduction band edge and mid band region energy levels in the solid [13e21]. The energy levels of trapped holes, hole centers, are typically somewhere between valence band edge and mid band region values (normal trapping sites are cation vacancies or impurity sites within the band gap) [15e22]. Trapped electrons and holes can be applied in analytical chemistry e.g. by dissolving easily dissolvable solids containing trapped electrons and holes into a solution containing species that can be reduced/oxidized with the dissolution uncovered trapped electrons and holes. In this way e.g. aromatic Tb(III) labels, Ruthenium(II)tris-(2,20 -bipyridine) and luminol can be sensitively detected on the basis of chemiluminescence where dissolutionuncovered trapped electrons/solvated electrons act as strong reductants and trapped holes as strong oxidants [22e24]. Physicists classify holes in the valence bands as heavy and light holes [11,25,26] depending on their effective masses, in general the heavy holes are energetically situated closer to the top of valence band [11]. In electronics a hot hole normally means holes situated at energy levels considerably below the valence band edge of the solid. However, if these hot holes can be transferred or injected into an electrolyte solution they will either behave as extremely strong one-electron oxidants towards solutes or, in the case of aqueous electrolyte solution, injection of sufficiently hot holes in water would result in hole injection in the valence band of water. Chemically this also means producing localized valence band holes, H2Oþ ions, the fate of which is well-known on the basis of pulse radiolysis studies to be the rapid decomposition to proton and hydroxyl radical [27]. Photoemission of holes to insulating oxides and organic solids has been observed and studied for a long time [28e31]. Also evidence of hole photoemission into solutions have been obtained [32,33]. Electrochemical hole injection from solution species to


solid electrodes has been under research for an extended period of time [34e39]. Electrical hole injection from a conductor into organic solids is an indispensable process in the present organic light emitting diodes (OLED) and therefore hole injection to OLED materials has been extensively studied [40]. According to Ma et al. hole injection from silicon to organic solids occurs by thermionic emission or by a modified thermionic emission mechanism but hole injection into organic insulators, such as, polyethylene have been reported to occur by FowlereNordheim-type of tunneling [41]. Hole injection into aqueous electrolyte is in principle an analogous process to that of hole injection in OLEDs, and into insulators [41,42] with the energetics only being somewhat different. Electron injection to OLEDs is often carried out using aluminum cathode due to its suitable work function and sometimes with oxide-coated aluminum cathodes [43] but the anodes providing hole injection are typically high work function materials such as indium tin oxide (ITO) which is also suitable optically transparent material for anodes in aqueous electrolyte solutions [43]. In our first ECL studies with aluminum electrodes we utilized an excitation pulse train that contained both anodic and cathodic voltage pulses (either symmetric double step (SDS) or asymmetric double step (ASDS) waveforms) [44e46]. We thought that anodic pulses were useful since they could repair possible damages induced by the cathodic pulses and also adjust in situ the oxide film thickness by anodic oxidation. In these studies we did not realize that we could have injected both hot electrons and holes into the aqueous electrolyte solution. However, we have later demonstrated that hot electrons can be injected into aqueous electrolyte solution from thin insulating filmcoated pulse-polarized cathodes (no anodic pulses in the stimulus waveform) [47,48]. The injection of hot electrons can be utilized in hot electron-induced electrochemiluminescence (HECL) detection of luminescent labels such as Tb(III) chelates and Ruthenium(II)tris(2,20 -bipyridine) with the detection limits close to picomolar levels [49,50]. Luminescent labels displaying long-lived emission and allowing time-resolved HECL (TR-HECL) detection provide the best performance due to the unavoidable intrinsic emission of the thin insulating films [49]. Recently, exactly the same tunneling phenomena and materials have been exploited in hot electron transistors [1,2] which are being developed due to the need of high speed electronic devices. The present work was carried out to study the effects of anodic pulses in the excitation pulse train during HECL experiments and the assumed possibility of hole injection into aqueous electrolyte solution. An aromatic Tb(III) chelate was utilized as a luminophore that cannot be excited using traditional electrochemistry at active metal electrodes in aqueous solution but is known to yield strong chemiluminescence in the simultaneous or successive presence of hydrated electron and strongly oxidizing radicals [22,51]. As far as we know, there is only a single related earlier report of electrochemical hole injection/transfer into aqueous solution. This was suggested to occur from ZnO electrode at highly anodic potentials and to result in oxidation of hydroxide ions at the electrode surface [52]. 2. Experimental Our luminescence generation and measuring system has been described elsewhere [46,49,53,54] The potentiostat was a Pine Instruments RD4 and an in-laboratory-built programmable pulse generator was used for the creation of the stimulus waveforms. The HECL cell consisted of disposable Al-cup working electrode and a Pt-wire counter electrode. The resistance of the cell was 9.5 U with 1.0 M Na2SO4 in 0.05 M Na2B4O7 buffer at pH 9.2. According to


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the X-ray fluorescence measurements the purity of the aluminum band (Merck Art. 1057) used as the material of Al-cups and Alsheets in this work was close to 99.98%; iron (130 ppm) and copper (120 ppm) were the main impurities and we did not find any other elements neither with this method nor by atomic absorption spectroscopy. Transient potential measurements were made using HewlettePackard HP-54503A oscilloscope (separation transformer connected to mains was used) with 1 MU input impedance and 1:10 probe against saturated calomel electrode by applying voltage pulses between the working and counter electrodes with Pine Instruments RD4 potentiostat in two electrode mode. The reference electrode Luggin capillary was positioned as close as possible to the working electrode surface and due to the high conductivity of the electrolyte solution the IR drop between the Luggin capillary and working electrode can be considered practically negligible.

3. Results and discussion 3.1. Effects of anodic and cathodic potential pulses on HECL In our older electrogenerated luminescence studies we applied SDS or ASDS waveforms as stimulus. The anodic pulse rapidly anodizes aluminum and produces anodic oxide film, the thickness of which is dependent on pulse voltage as described elsewhere [46,53,55]. When only cathodic pulses were applied it was observed that optimal aluminum oxide thickness was ca. 4e5 nm [49,50,55]. However, when the effect of in situ pulse-anodization during measurements was studied with Tb(III)-L (Tb(III)) chelated by 2,6bis[N,N-bis(carboxymethyl)aminomethyl]-4-benzoylphenol, i.e. 4benzoyl(1-hydroxybenzene)2,6-diyl)-bis(methylenenitrilo)tetrakis(acetic acid)), it was surprisingly observed that HECL could be now efficiently produced despite the resulting relatively thick oxide layers at higher anodic pulse amplitudes (Fig. 1). TR-HECL increased slightly above anodic pulse voltage of about 4 V corresponding to ca. 5 nm oxide film thickness [55]. Once formed, these pulseanodized oxide films, as well as, the DC-anodized specimens with thicker oxide films failed to produce Tb(III)-L specific HECL, if the

Fig. 1. Effect of applied anodic pulse voltage on TR-HECL of Tb(III)-L chelate with fixed cathodic pulse voltage when the electrodes were initially covered by a thin natural oxide film. (a) Cathodic pulse EL (squares); (b) TR-HECL, delay time 50 ms, time window 4.0 ms (circles). Conditions: 1.0  106 M Tb(III)-L, 1.0 M Na2SO4 in 0.2 M boric acid buffer at pH 9.2, cathodic pulse voltage e 10.0 V, pulse time 200 ms, pulse frequency 100 Hz.

anodic pulses were not utilized in the excitation pulse train (see Fig. S1 from Supplementary Material; with DC-preanodized electrodes, maximum TR-HECL was obtained with applied anodic pulse voltage of about 9 V). Electrogenerated luminescence (EL) during the cathodic pulse in our case is the sum of intrinsic high field solid state electroluminescence inside the oxide film, its F-center luminescence at the oxide/electrolyte interface [56,57] and the Tb(III)-L specific HECL [49]. Fig. 1 shows that the cathodic pulse EL continued growing also after the above-mentioned 4 V threshold anodic pulse voltage although TR-HECL did not. Fig. S2 in the supplementary material shows that even the naturally formed ultra-thin aluminum oxide films could collect trapped electrons during simultaneous irradiation of 6.7 eV and 4.88 eV photons and that long-lived recombination emission after the irradiation could be observed. These irradiations should not have been able to induce band-to-band transitions but only caused the filling of electron trapping sites within the band gap (and interface/surface states) by photoemission and promoting already existing trapped electrons to the oxide conduction band. However, it very clearly demonstrates that this ultra-thin oxide film (2e3 nm) can already provide trapping and recombination sites for electrons and holes. Next, the anodic and cathodic pulse potentials were measured as explained in the experimental section for to calibrate applied pulse voltages to actual potentials against SCE and then finally calculated to SHE scale. When Al-cups were used as working electrodes and were pulse-anodized with SDS voltage in advance with cathodic and anodic pulse amplitudes of 10 and 10 V, the threshold of measured anodic pulse potential for efficient TR-HECL production was observed to be between 4 and 6 vs. SHE, with a “half wave” anodic potential of about 4.3 V vs. SHE. The EL and TRHECL intensity were almost independent of anodic pulse amplitude beyond ca. 6 V vs. SHE, the EL slightly increasing and TR-HECL decreasing (Fig. 2). The measured achievable cathodic pulse potential (i.e. aluminum metal Fermi level during the cathodic pulse) was dependent on the applied anodic pulse amplitude and the measured high anodic pulse potentials vs. SHE (i.e. aluminum metal Fermi level during anodic pulse). Both polarity applied voltage pulses were as high as this system could handle, i.e. 10 and þ10 V pulses. The measured pulse potentials were e 6.7 V vs. SHE

Fig. 2. Cathodic pulse EL and TR-HECL intensity of Tb(III)-L chelate at a 10 V pulsepreanodized aluminum electrode, and measured cathodic pulse potential as a function of measured anodic pulse potential. (a) Cathodic EL (circles); (b) TR-HECL, delay time 50 ms, time window 4.0 ms (squares). Conditions: pulse-anodized electrodes, otherwise as in Fig. 1.

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and þ10 V vs. SHE, providing 16.7 eV energy window to induce different phenomena in the oxide film and its interfaces, and a kind of “half wave” potentials were ca. e 5.5 V vs. SHE and 4.3 V vs. SHE. Thus, exceedingly cathodic pulse potentials could be measured during cathodic pulses (Fig. 2). The TR-HECL maximum was achieved at about 6 V vs. SHE anodic pulse potential, which is about the valence band edge value of water (see Fig. 5 presented later). It is therefore tempting to propose that this peak potential is an indication of the occurrence of anodic hole injection from aluminum metal to the valence band of water by tunneling through the anodic oxide film or via the top of the valence band of aluminum oxide. In case of DC-anodized specimens, the maximum TR-HECL intensity could not be obtained until a potential of about 9 V vs. SHE pulse potential (Fig. S1) The oxide film quality must have been better in specimens that were DC-anodized in neutral ammonium borate, which is known to produce high quality barrier type anodic oxide film [55], i.e. probably much better than on the SDS-anodized specimens and, therefore, the actual anodic pulse potential must have been practically the same as applied anodic pulse voltage in experiments of Fig. S1. On the physical energy scale 9 V vs. SHE is about e 13.4 eV below vacuum level [58]. Applying fixed 10 V anodic voltage pulses within an ASDS waveform where cathodic pulse amplitude was increased, the cathodic pulse EL also increased steadily at measured low value cathodic potentials as a function of decreasing pulse potential vs. SHE, but the Tb(III) specific TR-HECL had almost the same threshold potential for the onset of TR-HECL as observed earlier without anodic voltage pulses (Fig. S3 in supporting material). The different shapes of cathodic EL and TR-HECL curves in Fig. 3 clearly show that Tb(III)-L specific TR-HECL cannot be due to the energy transfer excitation from the intrinsic emission centers. However, Fig. S2 in the supplementary material clearly points out that, if electrons and holes are created in the band gap of aluminum oxide it will result in luminescence emission, i.e. it is clear that, if electrons are injected into the conduction band of aluminum oxide and holes are injected into the valence band (or states within the band gap of aluminum oxide) successively or simultaneously, luminescence is a natural consequence due to the trapping/recombination sites of pure aluminum oxide. In addition, the impurity centers may provide recombination sites since our aluminum oxide was not of high purity.

Fig. 3. Pulse EL and TR-HECL of Tb(III)-L as a function of measured cathodic pulse potential. (a) Cathodic EL (squares); (b) TR-HECL, delay time 50 ms, time window 4.0 ms (circles). Conditions: Anodic pulse voltage fixed at 10.0 V and cathodic pulse voltage varied, pulse anodized electrodes, otherwise as in Figs. 1 and 2.


Fig. 4. Effect of free radical scavengers on TR-HECL of Tb(III)-L on SDS excitation. Electron and hole scavengers: Nitrate ion (solid circles), Iodide ion (open diamonds), azide ion (open down triangles), bromide ion (open up triangles), chloride ion (open circles), formate ion (solid squares). Conditions: Cathodic and anodic pulse amplitudes both 10.0 V, otherwise as in Figure.

3.2. Effects of free radical scavenger on HECL The free radical scavenger measurements with nitrate and Co(NH3)2þ 6 (latter not shown in Fig. 4 for clarity) demonstrated that the HECL could be prevented from forming with fast hydrated electron scavengers as in earlier studies that were performed by applying only cathodic pulses [49]. The effects of hole scavengers were studied with wider variety of species (Fig. 4). In pulse radiolysis studies, a hydroxyl radical is often used to generate halogen or pseudohalogen radicals according to reaction (1) where R· denotes hydroxyl radical and X halide or pseudohalide ion.

R þ X / R þ X 


Fig. 5. Energy diagram of Tb(III)-L chelate HECL at oxide-covered aluminum. Energy diagram of EL of oxide-covered aluminum cathode. The diagram is constructed on the following basis: (i) 0 V vs. SHE corresponds to 4.4 eV on the physical scale where the energy level of electron in vacuum is the zero point [58], (ii) the conduction band edge of aluminum oxide is at 1.1 eV [68]. (iii) the conduction band edge of water is reported to lie about 1.3 eV below the vacuum level [69] (ii) the energy level of hydrated electron lies at 1.5 eV [27]. (iii) the band gap of water is close to 8.9 eV [70,71] which results in a valence band edge of ca. 10.2 eV. (iv) Aluminum work function is 4.2 eV [72] (v) The redox potentials of inorganic couples and formal potentials of oxyradicals and hydrogen peroxide are placed at appropriate energy levels according to ref. [6,59,60] and (vi) those of the chelate ref [49]. (vii) The F-center band energy values are taken from Refs. [56,73].


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The standard potentials of the used halide and pseudohalide radicals have been collected in a table elsewhere [23,59]. The reaction (1a) is reported to occur nearly at diffusion controlled rates for many ions, e.g., for bromide, thiocyanate, azide and iodide, the second order rate constants of the reaction are close to 1  1010 M1 s1 [27]. Supposing a strong excess of halide or pseudohalide ion in the sample solution, successive reactions occur:

X þ X /X2


 X2 þ X2 /X 3 þX


Thus, weaker and weaker oxidants are produced in each series along the increasing parent ion concentration in the solution. As Fig. 4 shows, chloride had no effect but the more easily oxidized species quenched the luminescence strongly at high concentrations. It is first assumed that hydroxyl radical is produced by three one-electron reduction steps from dissolved oxygen since electrolyte solutions were in equilibrium with air. At pH over 7, other oxyradicals than hydroxyl radical are not capable of oxidizing the tested hole scavengers, and also the hydroxyl radical is a milder oxidant at pH 9 than fluorine or chlorine atom [27,59,60]. Therefore it is accepted that the oxidant of the system is, or at least behaves like, a hydroxyl radical. Now the effect of hole scavengers can be rationalized in the following way: hydroxyl radical is “converted” to milder oxidants by hydroxyl radical scavenging reactions 1a-4. The addition of iodide, bromide and azide increased TR-HECL at low hole scavenger concentrations. Therefore, secondary radicals I·, Br· and ·N3 seem to be better oxidants for the HECL system than the original oxidant/hydroxyl radical, but dihalide radical ions with (halide atoms complexed with their parent ions) and especially the products of equation (1c) are too weak oxidants for efficient HECL production. 

OH þ Br /HOBr ;

k ¼ 1:3  1010 l mol1 s1


HOBr /OH þ Br

Br þ Br /Br2 ;

k ¼ 1:1  1010 l mol1 s1

OH þ N 3 / N3 ;

k ¼ 1:2  1010 l mol1 s1


 N3 þ N 3 /N6 ;

k ¼ 0:33  1010 l mol1 s1


(2b) (2c)

The aromatic moiety of the present chelate has properties that are related both to the behavior of benzophenone and substituted phenols. Benzophenone can be rapidly reduced at least by hydrated electron [27] and hydroxyl radical can be added readily onto it.  e aq þ ðC6 H5 ÞCO/ðC6 H5 ÞCO ;

k ¼ 1:0  1010 l mol1 s1 (5)

OH þ ðC6 H5 ÞCO/ HOC6 H5 COC6 H5 ; k ¼ 8:8  1010 l mol1 s1 

(6) Phenolic compounds are not readily reduced by hydrated electron, but react rapidly with hydroxyl radical, the rate constants for p-cresol are known but the exact products are not known in aqueous solution [27].

k ¼ 4:2  107 l mol1 s1

e aq þ p  cresol/p  cresolred ;


OH þ p  cresolred /p  cresolox ;

k ¼ 1:2  107 l mol1 s1 (8)

It is typical for the hydroxyl radical that it readily reacts with organic compounds also by addition to unsaturated bonds or by abstracting hydrogen. On the other hand, halide and pseudohalide radicals act more clearly as one electron oxidants than the hydroxyl radical. For instance, p-hydroxyacetophenone is relatively fastly oxidized by dibromine radical ion. At low bromide ion concentrations the bromine atoms react with ligand before complexation reaction with the parent ion has time to take place and they probably are even more clearly one-electron oxidants without significant side reactions. However, also dibromide ion radicals prevailing in a bit higher concentration of bromide solutions are able to react/oxidize hydroxybenzophenones:

Br2 þ HOC6 H4 COCH3 /2 Br þ OC6 H5 COCH3 þ Hþ ; 

k ¼ 1:1  107 l mol1 s1


It is known that both aromatic moiety oxidation, and reductioninitiated pathway is energetically possible from the studies of hydrated electron induced chemiluminescence of Tb(III) chelates in the presence of appropriate oxidizing and reducing agents [22,51,61]. With this particular chelate ligand's aromatic moiety reduction potential is approximated to be equal to benzophenone one-electron reduction potential, i.e. ca. 1.0 to 1.1 V vs. SHE [62,63]. On the other hand, ligands aromatic moiety one-electron oxidation potential is approximated to be equal to phenolates oxidation potential, i.e. close to 0.8 V vs. SHE [64]. According to our PL measurements conducted with Gd(III)-L at 77 K, the lowest excited singlet and triplet states of ligand L are close to 3.7 and 2.8 eV, respectively [65]. The energy diagram of ligand L is added in the diagram in Fig. 5 below on this basis. The essential differences in the conditions of the present scavenger experiments with SDS voltage and with the earlier experiments (Fig. S4) with coulostatic excitation pulses are: (i) a high concentration of sodium sulfate is presently added as a supporting electrolyte, (ii) thick anodic oxide is pulse-anodized in boric acidbuffered rather concentrated sodium sulfate solution during HECL measurements, (iii) aluminum has a highly anodic rest potential between cathodic pulses with SDS and ASDS potential, (iv) the aluminum metal potential during cathodic pulses is more cathodic with SDS and ASDS voltage waveform than without anodic pulses, (v) cathodic pulse time is now fixed and the pulse charge slightly varied at the highest concentrations of scavengers, but it was usually ca. 110 mC/pulse which is quite close to the same to that was used earlier [49]. The earlier corresponding scavenger effect plot without anodic pulses in the pulse train is reproduced in Fig. S4 for easier comparison [49]. It is worth noticing, that if in this older study when the cell was connected in series with a diode which prevented anodic current peak, just after the cathodic pulse, totally from flowing (and emptying the F-center band) the HECL intensity drastically diminished [49]. Applying the series diode had also another effect: it shifted the aluminum rest potential between cathodic pulses from þ0.6 V vs. SHE to e 1.3 V vs. SHE. 3.3. The energetics of the Al/Al2O3/aqueous electrolyte interfaces The energetics of the interfaces and aqueous solution with redox

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species are presented in Fig. 5. The aluminum metal Fermi levels during cathodic pulse and anodic pulse are added in the “half wave conditions” of the electrochemiluminogram of Fig. 2 (dashed lines), and their potential of maximum TR-HECL (solid lines). Band bending is not represented here, since the basic ideas are similar as those in flat band conditions. It can be readily seen that highly cathodic potentials would allow the direct tunneling mechanism to be replaced by both Fowler-Nordheim tunneling and Schottky emission [66,67]. Hole injection to the valence band of water clearly cannot occur here in “half wave” condition but, if the anodic pulse amplitude is raised to much higher values hole emission/injection to the valence band of water becomes energetically possible by tunneling mechanisms. The energy diagram of Fig. 5 is oversimplified, since in practice, there will be interface states (electron and hole trapping sites) at the aluminum/oxide interface and also corresponding surface states at the oxide/electrolyte solution interface. It seems probable that just after the onset pulse potentials of HECL induced by SDS or ASDS pulse train the cathodic charge transport through the insulating barrier can be direct tunneling mixed with other transport mechanisms [49,67]. In this pulse potential range, anodic charge transport can be carried out via Fcenter and Fþ-center band by resonant tunneling, and hole injection in the valence band of water surely cannot occur yet. When the anodic pulse potential is considerably high, the cathodic pulse potential is more negative in the present experiments maintaining the charge tunneling/emission/injection to the conduction band of aluminum oxide, and depending on the pulse voltages and thickness of the oxide film, the electrons may gain energy in the high electric field in the conduction band but also lose it in collisions. Therefore, with the thicker oxide films the electrons will be transferred to the solution species from the bottom of the conduction band of aluminum oxide, or depending on the electron energy distribution gained in the electric field, somewhat above the energy of conduction band edge at the oxide/solution interface under strong band bending conditions. However, at the pulse potentials þ6.2 vs. SHE and 6.0 V vs. SHE corresponding to observed maximum TR-HECL (Fig. 2.) the anodic pulse potential reaches the energy value of the valence band edge of water (Fig. 5, solid lines of Fermi levels). At the most extreme pulse potential conditions obtainable with our instruments of Fig. 4 (cathodic pulse potential 6.7 V vs. SHE and anodic pulse potential 10 V vs. SHE) the high concentrations of hydrated electron scavengers still prevented the HECL generation in this system which means either that hydrated electrons are still largely mediating reductions or the tested hydrated electron scavengers were reactive directly with the hot electrons entering the solution from the aluminum conduction band. In the used conditions the anodic oxide film is so thick (of the order of 13e14 nm [55]) that practically the whole applied voltage is concentrated in the cell across the anodic oxide film which makes it possible for aluminum to reach such highly anodic pulse potentials. Hole scavenger experiments of Fig. 4 clearly point out that the system contains a powerful oxidant that is able to oxidize the pseudo halides and halides used in experiments. The production of secondary radicals from these series show that an oxidant with an oxidizing power of about 1.3 V vs. SHE or slightly more is more optimal for the present chemiluminescence generation than the primary oxidant species. Removing oxygen from our present cells is not a viable option since oxygen is generated at the platinum counter electrode when it is acting as an anode. Thus, there is always a one-electron reduction route with three steps available to produce hydroxyl radical from dissolved molecular oxygen [49]. The steps are proceeding near diffusion controlled rate [27] so this source of hydroxyl radical for the primary oxidant of the system


cannot be regarded to be without significance. However, we are convinced that in the range of conditions between the TR-HECL anodic onset potential and anodic potential of maximum emission (4 Ve5 V vs. SHE), the charge transfer through F- and Fþ-center band (Fig. 5) is at least important as hydroxyl radical generation from reduction of oxygen species, since the present chelate is in need of either a simultaneous or sequential presence of sufficiently strong reductant and oxidant with proper ratio of reducing and oxidizing equivalents [22,49,51,61]. We propose that here in the intermittent anodic pulse potential range each anodic pulse empties the trapped electrons from the anion vacancies (F- and Fþ-center band) and induces surface hydroxyl groups of the oxide to be oxidized to hydroxyl radicals, which are now in this pulse potential range probably the most significant primary oxidants. It is also possible that the ligands and ligand radicals are oxidized directly by heterogeneous electron transfer to anion vacancies. In the range of anodic pulse potentials coinciding or with higher values vs. SHE than the valence band edge of water, it is a thermodynamic possibility that hole injection into the valence band of water can occur. With good quality barrier-type aluminum oxide films produced in neutral ammonium borate solution, the emission increased all the time as anodic pulse amplitudes were increased (Fig. S1) this would suggest that we could have two hole transfer routes into aqueous electrolyte solution, one through the anion vacancies (F- and Fþ-center band) and another, by injection of holes into the valence band of water. If this is true, the “half wave” values of TR-HECL luminograms do not have any physical meaning, they just point out the mean intensity value of TR-HECL induced by two different hole injection mechanisms both of which are able to produce hydroxyl radicals as strongly oxidizing one-electron mediator, that can be used to produce a better oxidant for the presently utilized and studied system. The present system can probably be utilized also for other purposes besides the detection of electrochemiluminescent labels. If SDS pulse train is utilized and high concentration e.g. oxalate ions are added the solution conditions can be converted to plainly reducing, and if high concentration of peroxodisulfate or peroxodiphosphate is added the solution conditions can be converted to extremely strongly oxidizing, which are standard tricks of pulse radiolysis conventions. We believe that our system is quite close to a “low-cost pulse radiolysis system”, but without the problems and hazards associated with ionizing radiation. Perhaps, e.g. decomposition of many pollutants could be carried out with high amplitude SDS polarization of aluminum electrodes [74]. 4. Conclusions The present waveforms and aluminum electrodes can be utilized in sensitive detection of electrochemiluminescent labels that (i) can be excited by successive one-electron oxidation/reduction or reduction/oxidation steps and (ii) having sufficiently long-lived radical intermediates in aqueous solutions, such as, some of the Tb(III) chelates, Fluorescein and Rhodamine B. The anodic potential pulse exceeding 6 V in the excitation pulse train is proposed to have three effects: (i) it makes the oxide film too thick for direct tunneling (direct field-assisted tunneling) but simultaneously enables aluminum to gain much higher cathodic potential (i.e. more negative on the SHE scale) resulting in the cathodic tunneling mechanism to change to Fowler-Nordheim tunneling mechanism from direct tunneling. (ii) It empties the anion vacancies (F- and Fþcenter band) allowing hole injection into aqueous solution, i.e., anodic charge transfer via states inside the band gap of aluminum oxide to the solution species. (iii) Hole injection into the conduction band of water can occur with anodic potential pulses exceeding the


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Electrogenerated chemiluminescence induced by sequential hot electron and hole injection into aqueous electrolyte solution.

Hole injection into aqueous electrolyte solution is proposed to occur when oxide-coated aluminum electrode is anodically pulse-polarized by a voltage ...
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