Potential-Resolved Multicolor Electrochemiluminescence of N-(4

Engineering, Shandong University, Jinan, Shandong 250100, China. Anal. Chem. , Article ASAP. DOI: 10.1021/acs.analchem.7b04175. Publication Date (...
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Potential-resolved Multi-color Electrochemiluminescence of N-(4-aminobutyl)N-ethylisoluminol/tetra(4-carboxyphenyl)porphyrin/TiO Nanoluminophores 2

Jiangnan Shu, Zhili Han, Tianhua Zheng, Dexin Du, Guizheng Zou, and Hua Cui Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04175 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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Analytical Chemistry

Potential-resolved Multi-color Electrochemiluminescence of N-(4aminobutyl)-N-ethylisoluminol/tetra(4carboxyphenyl)porphyrin/TiO2 Nanoluminophores Jiangnan Shu,† Zhili Han,† Tianhua Zheng,† Dexin Du,† Guizheng Zou‡ and Hua Cui*† †

CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, China * Hua Cui, Fax: +86-551-63600730, E-mail: [email protected] ABSTRACT: Most electrochemiluminescence (ECL) studies involve single luminophore with a unique emission process, which severely limits its applications. Recently, multi-color ECL has attracted considerable interests. Herein, we report a novel nanoluminophore prepared by coating 5,10,15,20-tetrakis(4-carboxyphenyl)-porphyrin (TCPP) and N-(4-aminobutyl)-N-ethylisoluminol (ABEI) on the surface of TiO2 nanoparticles (TiO2-TCPP-ABEI), which exhibited unique potential-resolved multi-color ECL emissions using H2O2 and K2S2O8 as coreactants in an aqueous solution. Three ECL peaks, ECL-1 at 458 nm, ECL-2 at 686 nm and ECL-3 at 529 nm, were obtained with peak potentials of 1.05, -1.65 and -1.85 V, which were attributed to the ECL emission of ABEI, TCPP and TiO2 moiety of the nanoluminophores, respectively. Potential-resolved multi-color ECL from a nanoluminophore was observed for the first time in an aqueous solution. It opens a new research area of multi-color ECL of nanoluminophores, which is of great importance in ECL field from fundamental studies to practical applications.

Electrochemiluminescence (ECL) is light emission from the excited states of an ECL luminophore produced at an electrode surface through electron-transfer reactions.1 ECL does not require external light sources, which can dramatically improve the signal-to-noise ratio and sensitivity, allows temporal, spatial and electrode potential control on light emission and avoids electrochemical interference.2 ECL has been widely used in many fields, including optoelectronics,3 light-emitting devices,4 biosensing,5 immunoassays,6 DNA assays7 and so on. Especially, ECL has now become a powerful and commercialized technique in clinical diagnostics for the detection of tumor biomarkers,8 bacterials,9 viruses,10 hormones11 and so on. However, most ECL studies involve single luminophore with a unique emission process, which severely limits its applications. Recently, multi-color ECL has attracted considerable interests, with the enticing prospect of simultaneously detecting multi-analytes, built-in self-calibration for a precise and quantitative analysis and constructing multi-color emitting devices, and so on.12 The first type of multi-color ECL is from a mixture consisting of ruthenium(II) (Ru2+) and iridium(III) (Ir3+) complexes.13 For example, Francis, Hogan et al. tuned ECL emission color by using two concomitant electrochemiluminophores, including [Ru(bpy)2(L)]2+ ((L= N4,N4′-bis((2S)-1methoxy-1-oxopropan-2-yl)-2,2′-bipyridyl-4,4′dicarboxamide) and either Ir(ppy)3 or [Ir(df-ppy)2(BPS)](BPS= bathophenanthroline-disulfonate), and by controlling the electrode potential in the presence of tripropylamine.14 Later on, a series of mixed Ru2+ and Ir3+ complexes coreactant ECL systems have been reported, demonstrating that the ECL emission colors of concomitant metal complexes

could not only be spectrally resolved, but also be controlled by applied potential.15-17 The second type of multi-color ECL is from multi metal (e.g. Ru2+ and Ir3+) centers within a single molecule.18 For example, Peng et al. designed a series of heterodinuclear Ir/Ru complexes, [(bpy)2Ru(bpy)(CH2)n(bpy)Ir(df-ppy)2]3+ (n= 10, 12, 14), with the ability to tune the intensity ratio of red ECL emission (from the Ru moiety) and green ECL emission (from the Ir moiety) by alternating electrode potentials, enabling the ECL color to be tuned from green to red.19 The third type of multi-color ECL is from light-emitting devices, which combines solid-state ECL materials (metal complexes) with organic light-emitting diodes (e.g. small organic molecules and polymers).20 For example, De Cola et al. reported the fabrication of a simple ECL device with bias-polarity-controlled two-color emission (red and green) by combining the light emitting diode material polyphenylenevinylene derivative with a dinuclear ruthenium complex, which served as the dual roles of an emitter and an electron transfer mediator.4 Up to date, multi-color ECL has been limited to concomitant metal complexes, single metal complex molecule with multi metal centers in the presence of coreactants in organic solvents, and an assembly of metal complexes with organic light-emitting diode materials (small organic molecules and polymers). Potential- and spectralresolved ECL of nanoluminophores has rarely been explored. Recently, Ding et al. synthesized BODIPY dye capped PbS nanocrystals with dual-color ECL, which were produced in the same potential range in organic solvents.21 Herein, we report for the first time unique potential-resolved multi-color ECL of a novel nanoluminophore in an aqueous solution. The nanoluminophore was obtained by coating 5,10,15,20-tetrakis(4-

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carboxyphenyl)-porphyrin (TCPP) and N-(4-aminobutyl)-Nethylisoluminol (ABEI) on the surface of TiO2 nanoparticles (TiO2-TCPP-ABEI). A schematic illustration for the synthesis of TiO2-TCPPABEI nanoluminophores is shown in Figure 1a. Firstly, TiO2TCPP nanocomposites were obtained as described in the Secondly, hydrochloride and Nliterature.22 hydroxysulfosuccinimide were added into the TiO2-TCPP nanoparticle suspension under magnetic stirring with 15 min for activating the carboxyl groups of TCPP. Finally, the activated TiO2-TCPP reacted with ABEI to obtain the TiO2TCPP-ABEI nanoluminophores.

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applied potential and emission wavelength, when H2O2 and K2S2O8 were used as coreactants in 0.2 M pH 12 Brittonrobison (BR) buffer in the potential range of 0.0 ~ 1.5 ~ -2.0 ~0.0 V. Three ECL peaks were observed during the forward scan. ECL-1 was obtained at positive potential range, whereas ECL-2 and ECL-3 at negative potential range. The effect of coreactants on 3D ECL emissions of TiO2-TCPP-ABEI nanoluminophores was examined. When only K2S2O8 was used as a coreactant (Figure 2b), ECL-1 was very weak and ECL-2 and ECL-3 did not show obvious change. When H2O2 was used as a coreactant, only a well-defined ECL-1 could be observed and no ECL emission could be found on the negative scan (Figure 2c). The results suggested that ECL-1 was dominated by coreactant H2O2, whereas ECL-2 and ECL-3 were mainly related to K2S2O8. The effect of pH of BR buffer was investigated and pH 12 was finally adopted for higher ECL intensities (Figure S4). Moreover, the TiO2-TCPP-ABEI nanoluminophores were stable in at least 50 days (Figure S5).

Figure 1. (a) Schematic illustration for fabrication of TiO2-TCPPABEI nanoluminophores. (b) TEM image of TiO2-TCPP-ABEI nanoluminophores. (c) UV-vis absorption spectra of TiO2 nanoparticles, ABEI, TCPP, TiO2-TCPP nanocomposites and TiO2TCPP-ABEI nanoluminophores.

The TiO2-TCPP-ABEI nanoluminophores were characterized by transmission electron microscopy (TEM), UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction spectroscopy (XRD), respectively. TEM image in Figure 1b revealed that the as-synthesized TiO2-TCPP-ABEI nanoluminophores possessed good dispersibility with the average diameter of 25 nm. UV-vis spectra (Figure 1c) showed the characteristic absorption of TCPP around at 420 nm in TiO2TCPP and TiO2-TCPP-ABEI nanocomposites, which was not observed in the spectrum of TiO2. These results revealed that TCPP was coated on the surface of TiO2 nanoparticles. The characteristic absorption peaks of ABEI were around 290 nm and 325 nm, respectively, which also appeared in the spectrum of TiO2-TCPP-ABEI nanolumonephores. The results demonstrated that ABEI molecules were also coated on the surface of TiO2-TCPP nanocomposites. Moreover, the results of XPS spectra (Figure S1) and FT-IR spectra (Figure S2) also strongly supported that TiO2 nanoparticles are successfully functionalized by TCPP and ABEI molecules in sequence. Furthermore, the crystal form of TiO2 was studied by XRD, as shown in Figure S3. The results demonstrated that the major crystalline phase of the TiO2 nanoparticles was pure anatase and the crystalline characteristics of the TiO2 nanoparticles before and after the reactions remained unchanged. The ECL behavior of TiO2-TCPP-ABEI nanoluminophores was studied under cyclic voltammetry (CV) condition by dropping the suspension of the nanoluminophores onto the Fdoped tin oxide (FTO) electrode. Figure 2a shows the threedimensional (3D) ECL spectrum of the TiO2-TCPP-ABEI modified FTO electrode, i.e. ECL intensity as a function of

Figure 2. 3D ECL spectrum of TiO2-TCPP-ABEI modified FTO electrode using (a) H2O2 and K2S2O8, (b) K2S2O8 and (c) H2O2 as coreactants in 0.2 M BR buffer (pH 12) under CV condition at air atmosphere. H2O2, 5×10-4 M; K2S2O8, 0.01 M; scan rate 0.05 V/s. Each spectrum was acquired for 1 s, with a time interval of 1 s.

In order to elucidate various ECL peaks generated by TiO2TCPP-ABEI nanoluminophores in CV conditions when H2O2 and K2S2O8 were used as coreactants. The stacked ECL spectra in different potential windows are shown in Figure 3. ECL-1 centered at 458 nm on the positive scan appeared at 0.55 V (vs AgQRE) and reached a maximum at 1.05 V (Figure 3a and b). ECL-2 centered at 686 nm on the negative scan appeared at -1.20 V and reached a maximum at -1.65 V in the forward scan (Figure 3c and d),. ECL-3 centered at 529 nm on the negative scan started at an onset potential of −1.60 V and reached a maximum at -1.85 V (Figure 3e and f). As a result, ECL-1, ECL-2 and ECL-3 could be resolved by the electrode potential and ECL spectra. The results revealed that potentialresolved multi-color ECL of TiO2-TCPP-ABEI could be easily obtained in a potential scan and the emission colors could also be adopted by the choice of coreactants. The assignments of the three ECL peaks were studied by comparing the ECL spectra from ECL-1, ECL-2 and ECL-3 with that of pure ABEI, TCPP and TiO2 nanoparticles under the same conditions, as shown in Figure S6. The results demonstrated that ECL-1 at 458 nm, ECL-2 at 680 nm and

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Analytical Chemistry

ECL-3 at 529 nm were attributed to ABEI, TCPP and TiO2 moiety of TiO2-TCPP-ABEI nanoluminophores, respectively. In addition, these assignments of three ECL peaks were further corroborated by analyzing the FL spectra of TiO2-TCPP-ABEI nanoluminophores. The TiO2-TCPP-ABEI nanoluminophores showed maximal emission peaks of ABEI moiety at 440 nm and TCPP moiety at 677 nm (Figure S7a and b), which were consistent with those of ECL spectra from ECL-1 and ECL-2. The results supported that ECL-1 at 458 nm and ECL-2 at 686 nm corresponded to ABEI and TCPP moiety of TiO2-TCPPABEI nanoluminophores, respectively. The FL spectrum of TiO2 moiety in TiO2-TCPP-ABEI nanoluminophores showed a strong FL band peaking at 378 nm and a broad one ranging from 400 to 500 nm. The former arose from the radiative annihilation of excitons (band-to-band recombination), while the latter was attributed to the electron transition mediated by defect levels such as oxygen vacancies in the band gap.23 By comparing the spectrum from ECL-3 with the ECL spectra of TiO2 nanoparticles, it was suggested that ECL-3 at 529 nm was from TiO2 moiety of TiO2-TCPP-ABEI nanoluminophores. It is obvious that the ECL-3 at 529 nm was substantially red-shifted from the FL peak of TiO2, owning to that ECL depended more sensitively on surface states and FL mainly occurred through excitation and emission within the TiO2 interior.24

Figure 3. Stacked ECL spectra of ECL-1, ECL-2 and ECL-3 in different potential windows extracted from Figure 2. (a) ECL-1: from 0.55 to 1.05 V, (b) ECL-1: from 1.05 to 1.50 V, (c) ECL-2: from -1.15 to -1.65 V, (d) ECL-2: from -1.65 to -2.00 V, (e) ECL3: from -1.60 to -1.85 V, (f) ECL-3: from -1.85 to -2.00 V.

Besides, three counter-peaks ECL-1′ centered at 458 nm with a peak potential of 0.85 V, ECL-2′ centered at 686 nm with a peak potential of -1.50 V and ECL-3′ centered at 529 nm with a peak potential of -1.65 V, corresponding to ECL-1, ECL-2 and ECL-3, respectively, were obtained at the similar potentials during the reversal scans (Figure 2a and S8). It is obvious that the maximal emission wavelengths of ECL-1′,

ECL-2′ and ECL-3′ are the same as ECL-1, ECL-2 and ECL3, respectively. According to our previous studies, the formation of the three counter-peaks is due to a continuous electro-oxidation/reduction coupled with a competitive chemical reaction mechanism.25 In the forward scan, the ECL intensity decreased due to the consumption of reactants luminophores and coreactants. During the reversal scan, the electrooxidation or electro-reduction of luminophores and coreactants slowed down and their amounts would increase because of the incessant electro-generation. Therefore, the formation of ECL1′, ECL-2′ and ECL-3′ depended on the balance between the accumulation and consumption of luminophores (ABEI, TCPP or TiO2) and coreactants (H2O2 or K2S2O8). These ECL mechanisms were explored by analyzing the CV curves of TiO2-TCPP and TiO2-TCPP-ABEI nanoluminophores. The stacked spectrum of ECL-1 in Figure 3a and b shows an onset potential at 0.55 V and a peak potential at 1.05 V. Corresponding to ECL-1, an anodic CV peak (cvp1) round at 0.73 V was observed in the CV curve (Figure S9, black line). Cvp1 was due to the oxidation of ABEI to ABEI radical (ABEI•–), because there was no obvious anodic CV peak in CV curve of TiO2-TCPP under the same conditions (Figure S9, red line).26 Then, ABEI•– reacted with O2•– generated by chemical conversion of O2 or OOH– and electrooxidation of OOH– to form excited-state oxidation product N-(aminobutyl)-N-(ethylphthalate) (ABEI-ox*) with the light emission of ECL-1.26 The stacked ECL spectra of ECL-2 and ECL-3 in Figure 3c, d, e and f show that ECL-2 and ECL-3 appeared at an onset potential of -1.15 and −1.60 V, respectively. Correponding to ECL-2 and ECL-3, a broad cathodic peak (cvp3) with a peak potential of −1.55 V in the CV curve (Figure S9, black line) was observed. Cvp3 might involve the reduction of TCPP and S2O82- to TCPP– and SO4•–, respectively.27 Subsequently, TCPP– reacted with SO4•– to form TCPP*, accompanying by light emission of ECL-2. Meanwhile, SO4•– could also inject a hole (h+) into the valence band of TiO2. The injected holes recombined with electrons from the conduction band with the light emission of ECL-3.28 Moreover, H2O2 could be electro-reduced to OH• in the negative potential range, which could induce decomposition of K2S2O8 into SO4•–, resulting in an increase in the intensity of ECL peaks during the negative potential scan.29 The proposed ECL mechanisms of TiO2-TCPP-ABEI nanoluminophores under CV conditions are illustrated in Figure 4.

Figure 4. ECL mechanisms of TiO2-TCPP-ABEI nanoluminophores in the presence of H2O2 and K2S2O8.

Inner ECL resonance energy transfer (ECL-RET) was also observed in 3D ECL spectrum of TiO2-TCPP-ABEI nanoluminophores. Figure S10 shows the enlarged 3D ECL spectrum of the nanoluminophores in the potential range of 0.55 to 1.5 V. When H2O2 and K2S2O8 were used as coreactants and ECL1 intensity increased with the potential, two weak emission

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peaks were found at 675 and 739 nm with the same peak potential of ECL-1. Figure S11a shows that the ECL emission spectrum of ABEI (red line) overlapped with the UV-vis absorption spectrum of TCPP (black line). As shown in Figure S11b, TCPP possessed two FL peaks at 660 nm and 710 nm (blue line). When TCPP was bonded on the surface of TiO2 by ester-like linkage, the two FL peaks in the TiO2-TCPP and TiO2-TCPP-ABEI nanocomposites red-shifted to 677 and 742 nm, respectively. This may be due to the interaction between TCPP and TiO2. The ECL emission at 675 and 739 nm on positive scan (red line) is consistent with the FL emission spectra of TCPP moiety in TiO2-TCPP or TiO2-TCPP-ABEI nanocomposites. The results indicated inner ECL resonance energy transfer happened between ABEI and TCPP. The ECL emission of ABEI was absorbed by TCPP to form the excited state TCPP*, which returned to the ground state, emitting light at 675 and 739 nm. This finding shows anticipation to obtain more ECL colors. In conclusion, a new strategy has been proposed to synthesize TiO2-TCPP-ABEI nanoluminophores by coating TCPP and ABEI on the surface of TiO2 nanoparticles. The asprepared nanoluminophores exhibited potential-resolved multi-color ECL emissions using H2O2 and K2S2O8 as coreactants in an aqueous solution through a potential scan. Three ECL peaks, ECL-1 at 458 nm, ECL-2 at 686 nm and ECL-3 at 529 nm, were obtained with peak potentials of 1.05, -1.65 and -1.85 V, which were attributed to the ECL emission of ABEI, TCPP and TiO2 moiety of the nanoluminophores, respectively. The 3D ECL mechanisms have been proposed as follows: ECL-1 was due to the reaction of electro-oxidized ABEI•– with O2•– by electro-oxidation or chemical conversion of H2O2 and O2; ECL-2 was due to the reaction of electroreduced TCPP– with SO4•– by electro-reduction and chemical conversion of S2O82-; ECL-3 was due to that the formed SO4•– injected h+ into the valence band of TiO2 nanoparticles, which recombined with electrons from the conduction band to produce light emission. The emission colors of TiO2-TCPPABEI nanoluminophores could not only be resolved by applied potential, but also adopted by the choice of coreactants. Finally, inner ECL-RET between ABEI and TCPP moiety of TiO2-TCPP-ABEI nanoluminophores were demonstrated, showing anticipation for more ECL colors. This work provides a novel ECL nanoluminophore. Potentialresolved multi-color ECL from a nanoluminophore in an aqueous solution was observed for the first time. It opens a new research area of multi-color ECL of nanoluminophores, which is of great importance in ECL field from fundamental studies to practical applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details; characterization of TiO2-TCPPABEI; ECL spectra; FL spectra; effect of pH; cyclic voltammograms; 3D ECL spectrum (PDF)

AUTHOR INFORMATION Corresponding Author * Fax: +86-551-63600730. E-mail: [email protected]

ORCID

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Hua Cui: 0000-0003-4769-9464

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The support of this research by the National Key Research and Development Program of China (Grant No. 2016YFA0201300) and the National Natural Science Foundation of China (Grant Nos. 21527807 and 21475120) are gratefully acknowledged.

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