General Strategy for Enhancing Electrochemiluminescence of

Chem. , 2015, 87 (24), pp 12372–12379. DOI: 10.1021/acs.analchem.5b03890. Publication Date (Web): November 13, 2015. Copyright © 2015 American Chem...
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General Strategy for Enhancing Electrochemiluminescence of Semiconductor Nanocrystals by Hydrogen Peroxide and Potassium Persulfate as Dual Coreactants Pan-Pan Dai, Tao Yu, Hai-Wei Shi, Jing-Juan Xu,* and Hong-Yuan Chen* State Key Laboratory of Analytical Chemistry for Life Science and Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China S Supporting Information *

ABSTRACT: Semiconductor nanocrystals usually suffer from weak electrogenerated chemiluminescence (ECL) emissions compared with conventional organic emitters. In this work, we propose, for the first time, a very convenient but effective way to greatly enhance ECL emission of semiconductor TiO2 nanotubes (NTs) by H2O2 and K2S2O8 as dual coreactants, generating ECL emission ca. 6.3 and 107 times stronger than that of K2S2O8 or H2O2 as an individual coreactant, respectively. Scanning electron microscopy, X-ray diffraction, and electron paramagnetic resonance spectral studies were carried out to investigate the ECL enhancement mechanism. The ECL enhancement of TiO2 NTs by the K2S2O8−H2O2 system was supposed to originate from the coordination of H2O2 to the TiO2 surface and the synergy effect between H2O2 and K2S2O8 in the ECL process. The coordination of H2O2 to the surface of TiO2 could stabilize the electrogenerated coreactant-related radical OH• (hydroxyl radical), which could obviously promote the amount of sulfate radical anion (SO4•) near the electrode surface by inducing decomposition of K2S2O8 into SO4• or inhibiting the consumption of SO4• by its reaction with H2O. The holes (h+) released from SO4• were injected into the valence band of TiO2, resulting in more TiO2+, which combined with the electrons coming from the conduction band with an enhanced light emission. Moreover, this enhancement effect was also applicable to ECL of a CdS nanocrystal film on a glass carbon electrode, with ca. 2.74- and 148.3-fold enhanced ECL intensity correspondingly, indicating wide applications in the development of semiconductor nanocrystal-based ECL biosensors.

E

emission exhibited a ca. 5- and 4-fold increases, respectively. Both of them were accompanied by positive movement of the ECL onset potential. Recently, our group achieved ca. 58-fold enhanced ECL emissions of the CdS NC film by in situ activation in the presence of H2O2 and citric acid.14 The application of coreactants is another effective approach to improve the ECL intensity. Since the first report by Bard’s group in 1977,15 coreactants have been almost exclusively applied in modern ECL systems.1,16 Compared with ECL emission realized through annihilation, coreactants can help to overcome either the limited potential window of a solvent or poor anion or cation stability.9 To date, S-NC-based ECL is usually generated with R−O emission in the presence of different coreactants such as hydrogen peroxide or peroxydisulfate.1 Generally, their intermediates are the real participants in generating the excited states of nanomaterials just as the hydroxyl radical (OH•) for H2O214 and the sulfate radical anion (SO4•) for S2O82−.17 S2O82− has been popularized in immunoassay and cytosensing due to the strongly oxidizing intermediate SO4•, generated during the

lectrochemiluminescence (ECL) has become a powerful analytical technique that combines the unique advantages of electrochemistry and chemiluminescence, such as rapidity, high sensitivity, and a simplified optical setup.1−3 Since the ECL emission of Si in an organic solvent was first reported in 2002,4 the ECL behaviors of various semiconductor nanocrystals (S-NCs) such as CdS, CdSe, CdTe, and TiO2 have been extensively studied.5−8 Compared with conventional organic emitters, S-NCs have controlled luminescence and good stability against photobleaching.9 ECL based on S-NCs has been widely applied in biosensing and biaoanalysis.1−3 However, S-NCs usually suffer from weaker emissions than that of Ru(bpy)32+ or luminol, which may restrict their analytical applications. Thus, it is very significant to find a way to obtain enhanced ECL emission of S-NCs. Element doping, compositing with other nanomaterials, or in situ activation is efficient in improving the ECL of S-NCs through changing the surface states, reducing the electron relay barrier, or accelerating the electron/hole injection rate, etc.10−14 For example, N-doped TiO2 nanotubes (NTs) exhibited a 10.6-fold reductive−oxidative (R−O) ECL intensity enhancement, which was associated with the valence band edges shifting upward.10,11 With compositing of carbon NTs12 and Au nanoparticles13 with CdS nanocrystals (NCs), the ECL © XXXX American Chemical Society

Received: October 14, 2015 Accepted: November 13, 2015

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Analytical Chemistry reduction of S2O82−.16 Though, having weaker oxidizing capacity and lower efficiency than S2O82−,18 H2O2 is also an important coreactant for S-NC-based ECL biosensors on the basis of enzyme-catalytic reactions, because O2 and H2O2 both are the substrates of oxidases and peroxidases.19,20 However, even using S2O82− as a coreactant, S-NCs usually exhibit relatively weak ECL emissions, which may restrict their analytical applications. Thus, finding appropriate ways to obtain enhanced ECL of S-NCs is the constant driving force of this area. Here, we found a very convenient but effective way to greatly enhance ECL emissions of S-NC/K2S2O8 and S-NC/H2O2 systems by adding H2O2 and K2S2O8 as dual coreactants simultaneously (Scheme 1), resulting in ca. 6.3- and 107-fold

Fabrication of CdS NC and TiO2 NT Electrodes. The utilized CdS NCs were synthesized according to the previous report.21 The TiO2 NTs were fabricated by an electrochemical anodic oxidation technique according to the literature with a slight modification.22 Prior to anodization, a Ti sheet was thoroughly cleaned by sequential ultrasonication in ethanol and distilled water followed by a chemical polishing using a solution containing HF/HNO3/H2O with a volume ratio of 1:4:5 for 1 min. It was then rinsed with ethanol and distilled water, and finally dried in a nitrogen stream. The anodization process was performed in a two-electrode system consisting of the Ti sheet and a platinum electrode as the anode and cathode, respectively, using 0.5 wt % hydrofluoric acid as the electrolyte at a constant potential of 20 V at room temperature for 2 h. The distance between the electrodes was fixed at 3.0 cm. The top part of the titanium ribbon was used for the electrical contact. The efficient area of the anodized ribbon was 0.7 × 2.0 cm. The anodized Ti sheet was washed with ultrapure water and then annealed at 500 °C for 1 h in the ambient atmosphere to crystallize the amorphous tubes to obtain a well-ordered and uniform TiO2 NT array. The as-prepared TiO2 NT electrode was thoroughly cleaned by sequential ultrasonication in ethanol and distilled water for 30 s before use.

Scheme 1. Schematic Representation of the Strategy for Enhanced Electrochemiluminescence of TiO2 NTs Based on the Application of Hydrogen Peroxide and Potassium Persulfate as Dual Coreactants

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RESULTS AND DISCUSSION Characterization of the TiO2 Electrode. Figure 1A shows the scanning electron microscopy (SEM) image of the TiO2 NT array, which exhibits a self-organized porous structure with good conformity on a large scale on the surface of the Ti foil. Such well-aligned one-dimensional features would allow increased light emission and efficient directional charge transport within the arrayed tubes. The magnified SEM image (inset) further demonstrates that the average inner and outer pore diameters are approximately 90 and 130 nm, respectively. X-ray diffraction (XRD) analysis (Figure 1B) verified the as-prepared TiO2 NTs are a mixture of anatase and rutile phases. TiO2 NTs were chosen and used in the ECL biosensor in recent years due to their regular oriented structure and larger specific surface area.10,17 K2S2O8 is an important coreactant for TiO2 R−O ECL emission. By referencing the proposed model,23 as the electrode potential became sufficiently negative, S2O82− was reduced to a strong oxidant, SO4•. The formed SO4• injected holes (h+) into the valence band of TiO2, and the semiconductor was excited. Then the holes combined with the electrons from the conduction band, resulting in the

enhanced ECL for a TiO2 NT electrode and 2.74- and 148.3fold enhanced ECL for CdS NCs, respectively, when enhanced ECL emission reached the strongest intensity. The ECL enhancement mechanism was investigated in detail. The present work provides fundamentals to the understanding of the effects of coreactants in S-NCs’ ECL and promotes the development of S-NCs in analytical applications.

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EXPERIMENTAL SECTION Reagents and Apparatus. For details, refer to the Supporting Information.

Figure 1. (A) SEM images at low magnification and high magnification (inset) and (B) XRD patterns of the Ti foil (pink) and TiO2 NT substrate (blue) with annealing at 500 °C for 1 h. A, R, and Ti are anatase, rutile, and titanium peaks, respectively. B

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Figure 2. (A) CV curves of the Ti foil in 0.1 M PBS (pH 7.5) containing 0.1 M K2S2O8−10 mM H2O2 (a) and the TiO2 electrode in 0.1 M PBS (pH 7.5) containing 10 mM H2O2 (b), 0.1 M K2S2O8 (c), and 0.1 M K2S2O8−10 mM H2O2 (d). (B) Corresponding ECL−potential curves of (a)−(d) (scan rate, 100 mV/s; voltage of PMT, 500 V).

reduction of H2O2 to HO•. However, as for the corresponding ECL curve (Figure 2B, curve b), there are two ECL peak signals located at −1.55 and −0.70 V, respectively. First, the bigger ECL peak signal located at −1.55 V is assigned to the radiative deactivation of TiO2*, based on a previous report.25 Then the other one located at −0.70 V could be assigned to the radiative deactivation of the singlet-state oxygen (1(O2)2*) into the basic triplet state (3O2) through the following reaction process. First, the HO2• radicals on the electrode can exist alone or at the same time in the process of the recombination of OH• radicals resulting from the reduction of H2O2 on the surface of the electrode26 and the reduction reaction of dissolved oxygen.27 Then the recombination of two HO2• radicals or the recombination of HO2• with a HO• radical results in the excited-state species (1(O2)2*), which emits light,26 as shown in the following reactions:

excited-state species (TiO2*), which emitted light. The ECL mechanism17,24 can be elucidated as follows: S2 O82 − + e− → SO4•− + SO4 2 −

(1)

SO4•− → SO4 2 − + h+

(2)

TiO2 + h+ → TiO2+

(3)

TiO2+ + e− → TiO2 *

(4)

TiO2 * → TiO2 + hν

(5)

H2O2 is another important coreactant for S-NC-based R−O ECL emissions. It can also be used as a coreactant for TiO2 ECL emission.25 Since OH• can be formed on the electrode as a result of reaction 6, generation of TiO2 R−O ECL can also proceed during reaction 7 and with the further reactions 3−5. In general, compared with conventional organic emitters, semiconductor TiO2 usually exhibits relatively weak ECL emissions in the presence of S2O82− or H2O2 as a coreactant, which may restrict its analytical applications. Thus, finding appropriate ways to obtain enhanced ECL of semiconductor nanomaterials for bioanalysis is the constant driving force of this area. In this study, we find that K2S2O8 and H2O2 demonstrate synergy, which greatly increases the TiO2 ECL intensity. H 2O2 + e− → OH− + OH• •

−

+

OH → OH + h

OH• → 1 3 HO2• + 1 3 H 2O −

−

O2 + H 2O + e → OH + HO2

(8b)

HO2• + HO2• → H 2O2 + 1 2 1(O2 )2 *

(9a)

HO2• + OH• → H 2O + 1 2 1(O2 )2 *

(9b)

(O2 )2 * → 23O2 + hν

(10)

1

(6)

(8a) •

Similarly, the CV curve of the TiO2 electrode in 0.1 M K2S2O8 solution (Figure 2A, curve c) shows the reduction current increases significantly when the potential is more negative than −0.8 V, accompanied by a reduction of S2O82− to SO4•. Meanwhile, the corresponding ECL curve (Figure 2B, curve c) was also recorded where ECL was observed when the potential was more negative than −0.8 V, with the strongest ECL signal appearing at −1.55 V, which was assigned to the radiative deactivation of TiO2* resulting from the TiO2/S2O82− system. As for Figure 2B, we were amazed to find that the TiO2 NT electrode exhibited an outstanding ECL property using dual coreactants (curve d), resulting in ca. 6.3- and 107-fold enhanced ECL intensity compared with that of 0.1 M K2S2O8 (curve c) or 10 mM H2O2 (curve b) as an individual coreactant, respectively. This result indicates that dual coreactants could

(7)

Figure 2 showed cyclic voltammograms and ECL curves of the Ti and TiO2 electrodes obtained synchronously during the cyclic potential scanning between −1.6 and 0 V in different coreactant solutions. First, the cyclic voltammetry (CV) and ECL curves of the Ti foil in 0.1 M phosphate-buffered saline (PBS) containing 0.1 M K2S2O8 and 10 mM H2O2 (curve a) were tested as a control experiment. There are no obvious reduction current and ECL emission, indicating that K2S2O8 and H2O2 could hardly contribute to the ECL intensity without TiO2 NTs on the electrode surface. Then the CV curve of the TiO2 electrode detected in 10 mM H2O2 solution (Figure 2A, curve b) shows that the reduction current increases with potential more negative than −0.4 V, accompanied by a C

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wavelength is greatly increased from curve a to curve c, which contributes to the total ECL enhancement. Enhancement Mechanism Discussion. To investigate the mechanism of the synergistic effect, the effects of pH and K2S2O8 and H2O2 concentrations on the ECL enhancement of TiO2 were examined in detail. The effect of pH on the ECL of TiO2 NTs with different coreactants is shown in Figure 4A. With the concentrations of H2O2 (curve a) and K2S2O8 (curve b) fixed, the ECL emissions were enhanced as the pH was increased, which is due to more H2O2 and S2O82− being reduced at higher pH, because the reduction of coreactants is inhibited as the proton is easy to reduce to hydrogen at lower pH. Nevertheless, at pH above 7.5, the ECL enhancement of the TiO2 NTs/H2O2 system (curve a) decreased quickly; on the other hand, the ECL emission from the TiO2 NT/K2S2O8 system (curve b) increased slowly and reached the highest value at pH 9.0. This phenomenon is illustrative of the fact that a high pH made H2O2 unstable. Finally, the dependence of the ECL intensity of TiO2 NTs with dual coreactants on the solution pH, shown in Figure 4A, curve c, suggested that the synergistic effect was most effective in the range between pH 7.0 and pH 10. It is worth noting that the whole changing trend was similar to the results of the TiO2 NT/H2O2 system. The reasons for this are as follows: At pH < 7, the reduction of dual coreactants was inhibited by the proton. However, more basic solutions would make H2O2 unstable14 and lead to the consumption of SO4• by the scavenging reaction with OH¯.29 In both situations, the real participants (HO• and SO4•) in TiO2 NT R−O ECL emission decreased singly and simultaneously, resulting in a weaker synergistic effect and then lower ECL intensity. In general, pH 7.5 gave the strongest ECL emission. Moreover, the effect of the K2S2O8 and H2O2 concentrations on the ECL intensity of TiO2 NTs was investigated. As illustrated in Figure 4B, curve a, the ECL intensity increased quickly with the concentration of S2O82− ranging from 0.01 to 0.10 M owing to the greater amound ot TiO2+ generated by injecting holes into TiO2 by SO4•. The ECL intensity began to increase slightly and trend to be stable while the concentration was higher than 0.1 M, because of the limited electrode surface. The effect of the concentration of H2O2 on the ECL intensity is shown in Figure 4B, curve b. The ECL intensity increased with an increase of the amount of H2O2

exhibit synergy in TiO2 NT R−O ECL emission. Compared with HO•, the strong oxidant SO4• resulting from the reduction of K2S2O8 on the surface of the electrode had a stronger oxidizing capacity to inject holes into the valence band of TiO2, resulting in the formation of TiO2+.18 Therefore, the enhanced ECL intensity of the TiO2 NT electrode with dual coreactants might arise from the increase of SO4• by H2O2 on the electrode surface. Figure 3 shows the ECL spectra of the TiO2 electrode in 0.1 M PBS containing 10 mM H2O2 (curve a), 0.1 M K2S2O8

Figure 3. ECL spectrum of the TiO2 electrode in 0.1 M PBS containing 10 mM H2O2 (a), 0.1 M K2S2O8 (b), and 0.1 M K2S2O8− 10 mM H2O2 (c) obtained through filters under a cyclic potential scan from 0 to −1.6 V.

(curve b), and 0.1 M K2S2O8−10 mM H2O2 (curve c). As can be seen from curve a, there are two distinctive emission peaks appearing on the ECL spectrum. One peak appearing at around 500 nm belongs to the radiative deactivation of TiO2*,25 and the other one at around 575 nm is observed due to the excessive OH•, which can be attributed to the emission of 1 (O2)2* in the TiO2/H2O2 system.28 Then in 0.1 M K2S2O8 solution and 0.1 M K2S2O8−10 mM H2O2, only the emission peak at around 500 nm belonging to the emission of TiO2* exists. That is, there is hardly any emission of 1(O2)2* under these conditions. The position of the ECL peak located at 500 nm hardly changes, but the ECL intensity at each detected

Figure 4. Effects of the solution pH (A) on the ECL intensity of TiO2 NTs in 0.1 M PBS (pH 7.5) containing 10 mM H2O2 (a), 0.1 M K2S2O8 (b), and 0.1 M K2S2O8−10 mM H2O2 (c). Inset: amplified plot (a). (B) Effects of the K2S2O8 concentration (inset a) and K2S2O8 fixed at 0 M (b) 0.01 M (c), 0.05 M (d), and 0.1 M (e) with different concentrations of H2O2 (0, 1, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, and 20 mM) on the ECL intensity of TiO2 NTs. D

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Figure 5. (A) EPR spectra of 0.1 M PBS buffer (pH 7.5) containing 100 mM DMPO (a), 10.0 mM H2O2−100 mM DMPO (b), 0.1 M K2S2O8−100 mM DMPO (c), 0.1 M K2S2O8−10.0 mM H2O2−100 mM DMPO (d), and 0.1 M K2S2O8−20.0 mM H2O2−100 mM DMPO (e) recorded after application of a potential of −1.6 V on the TiO2 electrode for 150 s. (B) EPR spectral overlap of (c)−(e).

owing to the greater amount of TiO2+ generated by injecting holes into TiO2 by OH•. A concentration higher than 10 mM resulted in decreased ECL intensity because excessive OH• radicals could annihilate each other. Since H2O2 and K2S2O8 could demonstrate synergy, it was very necessary to investigate the effect of the relative amount of S2O82− and H2O2 on the ECL intensity. K2S2O8 concentrations fixed at 0.01 M (Figure 4B, curve c), 0.05 M (Figure 4B, curve d), and 0.1 M (Figure 4B, curve e) were chosen to investigate the influences of the amount of H2O2 on the ECL intensity; with an increase in the H2O2 concentration, the ECL emission of TiO2 NTs increased quickly and reached the maximum at 7.5, 7.5, and 10 mM, respectively. In summary, the strongest ECL emission of TiO2 NTs was achieved in 0.1 M PBS (pH 7.5) containing 0.1 M S2O82− and 10 mM H2O2, which was ca. 6.3 and 107 times greater than that of 0.1 M K2S2O8 or 10 mM H2O2 as the coreactant, respectively. From eq 3, we can see that the ECL intensity was highly dependent on the stability and amount of TiO2+. That is, the ECL intensity could be enhanced remarkably by means of increasing the concentration of SO4•. Previous works30,31 have demonstrated that the SO4 • concentration can be consumed by its reaction with H2O to form OH• at all pH values in the persulfate activation system (eq 11a), but increased by HO•-induced decomposition of K2S2O8 (eq 11b). SO4•− + H 2O → OH• + HSO4 −

(11a)

OH• + S2 O82 − → SO4•− + HSO4 − + 1 2 O2

(11b)

could accelerate the reduction of H2O2 because of the consumption of OH•, which could be proved by its increased reduction current. It could be summarized that the greater the amount of OH• resulting from the reduction of H2O2, the greater the amount of SO4• and eventually the greater the ECL intensity. As can be imagined, the ECL intensity of the TiO2 electrode increased greatly on the basis of the synergistic effect between K2S2O8 and H2O2. Afterward the ECL intensity decreased continuously, because SO4• would be scavenged by excessive OH• (eq 12), resulting in a decrease in the ECL intensity.30 OH• + SO4•− → HSO4 − + 1 2 O2

(12)

To sum up, we proposed an easy, direct but effective strategy to enhance TiO2 ECL emission based on the synergistic effect between H2O2 and K2S2O8, composed of the following processes: (i) electrochemical reduction S2 O82 − + e− → SO4•− + SO4 2 −

(1)

H 2O2 + e− → OH− + OH•

(6)

(ii) synergistic effect SO4•− + H 2O → OH• + HSO4 −

(11a)

OH• + S2 O82 − → SO4•− + HSO4 − + 1 2 O2

(11b)

or

(iii) ECL emission

Therefore, for improving S-NC R−O ECL emission, it is necessary to find a way to not only inhibit the consumption of SO4• by its reaction with H2O, but also induce decomposition of K2S2O8 into SO4•. Clearly, the problems can be effectively solved by increasing the amount of OH•, which can be achieved by use of H2O2. According to Figure 2A, curve d, with the scanning potential becoming sufficiently negative, H2O2 and K2S2O8 are successively reduced to the negatively charged radicals (OH• and SO4•). Besides the formed SO4• by electrochemical reduction of S2O82−, the formed OH• would obviously promote the amount of SO4• near the electrode surface by inhibiting the consumption of SO4• by its reaction with H2O (eq 11a) and inducing decomposition of K2S2O8 into SO4• (eq 11b). Therefore, coexistence of dual coreactants

SO4•− → SO4 2 − + h+ +

TiO2 + h → TiO2

+

(2) (3)

TiO2+ + e− → TiO2 *

(4)

TiO2 * → TiO2 + hν

(5)

(iv) excessive H2O2 OH• + SO4•− → HSO4 − + 1 2 O2

(12)

As discussed above, the ECL enhancement of the TiO2 NT electrode by dual coreactants arises from the principal effect of E

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Figure 6. (A) ECL−potential curves of CdS NCs/GCE in 0.1 M PBS (pH 7.5) containing 7.5 mM H2O2 (a), 0.1 M K2S2O8 (b), and 7.5 mM H2O2−0.1 M K2S2O8 (c). Inset: amplified plot (a). (B) Effects of the H2O2 concentration on the ECL intensity of CdS NCs/GCE with K2S2O8 fixed at 0.1 M and pH fixed at 7.5.

K2S2O8, actually in the form of its intermediate (SO4•) for generating the excited states of semiconductor TiO2 nanomaterials. The main source of SO4• is from electrochemical reduction and decomposition of S2O82− induced by OH•, known as the synergistic effect. As for Figure 4B, we can see the enhanced cathodic ECL emission of the semiconductor TiO2 NT electrode by 0.01, 0.05, and 0.1 M K2S2O8 and 7.5 mM H2O2 as dual coreactants, resulting in ca. 14.3-, 8.8-, and 5.6fold enhanced ECL intensity compared with that of K2S2O8 of the corresponding concentration as an individual coreactant, respectively. The results suggest that higher H2O2 relative contents in a dual coreactant system could cause a stronger synergistic effect. In Figure 2B, what remains to be explained is why the ECL peak at −0.70 V of the TiO2 electrode assigned to the radiative deactivation of 1(O2)2* disappeared in 0.1 M PBS containing 0.1 M K2S2O8 and 10 mM H2O2 (curve d), compared to that in 0.1 M PBS containing 10 mM H2O2. Here is the reason: OH• radicals resulting from the reduction of H2O2 on the surface of the electrode are consumed because of the synergistic effect between H2O2 and K2S2O8, which is hardly conducive to the formation of 1(O2)2* through the process of reactions 8a, 8b, 9a, 9b, and 10. This can be further proved in the corresponding ECL spectrum of TiO2 NTs in the H2O2 and K2S2O8 system (Figure 3, curve c), which gives almost no ECL peak at around 575 nm. Finally, this phenomenon, in turn, can manifest indirectly the synergistic effect between H2O2 and K2S2O8. To further confirm this enhanced synergism, the electron paramagnetic resonance (EPR) technique was utilized to identify radical species generated in the ECL process. The EPR spectra in Figure 5B show the hyperfine splitting constants of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) radical adducts in different systems. The hyperfine splitting constants aN = 13.8 G, aβH = 10.5 G, and aγH = 1.2 G are representative of SO4• radicals added to DMPO (DMPO−SO4).32 Meanwhile, another radical signal with aN = 14.9 G and aβH = 14.85 G was observed, which is characteristic of the DMPO−OH adduct,33 verifying the generation of OH• in this process. It is demonstrated that only the DMPO−OH adduct signal exists in 0.1 M PBS containing 10 mM H2O2 (Figure 5, curve b). Then, in 0.1 M PBS containing 0.1 M K2S2O8 (Figure 5, curve c),

both DMPO−SO4 (SO4•, formed by eq 1) and DMPO−OH (OH•, formed by eq 11a) signals exist. Although SO4−DMPO and DMPO−OH exist simultaneously in 0.1 M PBS containing 0.1 M K2S2O8 and 10 mM H2O2 (Figure 5, curve d), the DMPO−OH signal had not been obviously enhanced with additional H2O2, compared with that in 0.1 M PBS containing 0.1 M K2S2O8. This is because the OH• formed by electrochemical reduction would be consumed by the synergistic effect. In addition, 20 mM H2O2 resulted in an obviously enhanced DMPO−OH signal (Figure 5, curve e), showing excessive formation of OH•. This could account for why more than 10 mM H2O2 would cause the ECL intensity to decrease obviously in Figure 4B, curve e. This is because SO4• would be scavenged by excessive OH• according to eq 12, resulting in a decrease in the ECL intensity. To prove the above conclusion, the effect of the H2O2 concentration on the increment in the EPR signal of DMPO−OH that emerged at 3501 G before and after addition of H2O2 (ΔI) was explored. As shown in Figure S1, the DMPO−OH signal had not been obviously enhanced while the concentration of H2O2 was less than 10 mM, but was enhanced quickly when the amount of H2O2 was more than 10 mM. Moreover, it is important to note that, in aqueous solutions, formation of the DMPO−OH adduct dominates, as the stability of DMPO−SO4 is very limited (half-life only ∼30 s);34 thus, its spectrum intensity is not clearly evident, and the change of the SO4−DMPO signal based on different conditions can easily go undetected. Many previous works have demonstrated that ECL emissions of S-NCs mainly occur via surface electron−hole recombination resulting from electrodes and coreactants. Taking CdS NCs as an example, in the potential region for CdS reduction, SO4• is generated by electrochemically reducing peroxydisulfate, and then reacts with negatively charged NCs (CdS•) by injecting a hole into the highest occupied molecular orbital to produce an excited state (CdS*). This state decays to the ground state with light emission and produces an ECL signal. We could see that the amount of SO4• was a major factor influencing the ECL intensity. To further investigate that the enhancement mechanism was related to the synergistic effect between two coreactants, ECL emissions of CdS/glass carbon electrode (GCE) in 0.1 M PBS F

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Analytical Chemistry

Figure 7. (A) ECL−potential curves of the same TiO2 NT electrode first in 0.1 M PBS containing 0.1 M K2S2O8 (a), then 0.1 M K2S2O8−10 mM H2O2 (b), and finally 0.1 M K2S2O8 again (c). (B) ECL−potential curves of the same CdS NCs/GCE in 0.1 M PBS (pH 7.5) containing 0.1 M K2S2O8 (a), 0.1 M K2S2O8−7.5 mM H2O2 (b), and 0.1 M K2S2O8 again (c).

for H2O2, and the electrode was washed three times with ultrapure water before the next measurement. As shown in Figure 7A, the ECL in 0.1 M PBS containing 0.1 M K2S2O8 again (curve c) still exhibited a 1.53-fold enhancement compared with that of 0.1 M K2S2O8 initially (curve a). With this method, the ECL intensity of CdS NCs/GCE in 0.1 M PBS containing 0.1 M K2S2O8 again (Figure 7B, curve c) could almost completely get back to the original value quickly (Figure 7B, curve a). The above results could better show that TiO2 NTs had better adsorption properties for H2O2 than CdS NCs. In addition, from Figure S2, we can see that the ECL intensity of the TiO2 electrode increased quickly with the concentration of H2O2 ranging from 0 to 10 mM. One more thing to note is that an ECL peak at −0.70 V begins to appear when the concentration of H2O2 is above 5 mM, due to the formation of excessive OH•. However, for CdS NCs/GCE with a smaller electrode surface, a similar ECL peak still does not appear when the concentration of H2O2 is 7.5 mM (Figure 6A, curve a). This would also prove that TiO2 NTs had better adsorption properties for H2O2, which can stabilize the electrogenerated coreactant-related radical OH• and then promote the formation of 1(O2)2* through reactions 8a, 8b, 9a, 9b, and 10. The reproducibility and stability of the ECL enhancement effect of TiO2 NTs and CdS NCs were also evaluated. Six electrodes fabricated identically were explored in the optimum condition. The relative standard deviation (RSD) was 7.5% and 5.5%, respectively. These experimental results confirmed that the reproducibility of the enhancement approach by two coreactants was excellent. In addition, the excellent stability of the enhanced ECL was also evidence of the feasibility of this method for enhancing the ECL intensity, as shown in Figure S3.

containing H2O2 and K2S2O8 were also performed, shown in Figure 6A, resulting in (curve c) ca. 2.74- and 148.3-fold enhanced ECL intensity compared with those of 0.1 M K2S2O8 (curve b) and 7.5 mM H2O2 (curve a) as individual coreactants, respectively. Moreover, the effects of the H2O2 concentration on the ECL enhancement of the CdS NC film have been examined. As shown in Figure 6B, the ECL intensity increased with the amount of H2O2 and reached the highest value at 7.5 mM due to the synergistic effect. Afterward the ECL intensity decreased continuously beyond 7.5 mM. The reason is the same as the above: SO4• would be scavenged by excessive OH•, resulting in a decrease in the ECL intensity. That is, 0.1 M PBS (pH 7.5) containing 0.1 M S2O82− and 7.5 mM H2O2 would give the greatest ECL emission with the CdS film. Synergistic Effect Comparison between TiO2 NTs and CdS NCs. Compared with CdS NCs/GCE, the TiO2 electrode based on H2O2 and K2S2O8 as dual coreactants had a stronger ECL enhancement in contrast to that of K2S2O8 existing alone. This indicated that besides the synergistic effect of two coreactants, there was another factor that enhanced ECL emission. As described in Figure 1B, the as-prepared TiO2 NTs were a mixture of anatase and rutile phases. A recent work by Huang et al. showed that H2O2 could be adsorbed on TiO2 anatase(101) and rutile(110) surfaces by first-principles calculation based on the density functional theory in conjunction with the projected augmented wave approach, using the PW91, PBE, and revPBE functionals.35 Then, in accordance with a previous report,14 the adsorption of H2O2 on the electrode surface could stabilize the electrogenerated coreactant-related radical OH•, which would further increase the amount of SO4• by inducing decomposition of K2S2O8 into SO4• or inhibiting the consumption of SO4• by its reaction with H2O, resulting in enhanced ECL intensity. In this work, this well-aligned one-dimensional feature and higher specific surface of the resulting TiO 2 NTs would be advantageous for the adsorption of H2O2. Therefore, for the TiO2 electrode, another factor for enhanced ECL was the strong adsorption capacity for H2O2. Moreover, the ECL−potential curves of the same TiO2 NT electrode first in 0.1 M PBS containing 0.1 M K2S2O8, then 0.1 M K2S2O8−10 mM H2O2, and finally 0.1 M K2S2O8 again were investigated to further prove the adsorption ability of TiO2 NTs

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CONCLUSIONS In this work, we first provided an easy but effective approach to enhance the ECL intensity of TiO2 NTs, based on the dual coreactants of H2O2 and K2S2O8. According to the extensive experimental investigations (ECL process, XRD, EPR spectra), and the detailed investigation of the related literature, the enhancement mechanism of dual coreactants on TiO2 ECL was given. The ECL enhancement was attributed to the adsorption property of the electrode and the synergistic effect of two G

DOI: 10.1021/acs.analchem.5b03890 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

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coreactants. First, the adsorption of H2O2 on the electrode surface could stabilize the electrogenerated coreactant-related radical OH•, and then OH• arising in the process was able to induce the decomposition of persulfate into SO4• and inhibit the consumption of SO4• by its reaction with H2O, resulting in a vigorous ECL enhancement. Coincidentally, this synergistic effect was also applicable to other S-NCs such as CdS NCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b03890. Additional experimental details and figures showing EPR spectra, the effects of the H2O2 concentration on the ECL intensity of TiO2 NTs, and the stability of enhanced ECL of TiO2 NTs and CdS NCs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone/fax: +86-25-89687294. E-mail: [email protected]. *Phone/fax: +86-25-89684862. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (Grant 2012CB932600) and the National Natural Science Foundation of China (Grants 21327902, 21475058, and 21535003). This work was also supported by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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DOI: 10.1021/acs.analchem.5b03890 Anal. Chem. XXXX, XXX, XXX−XXX