Polymorph-Dependent Electrogenerated Chemiluminescence of Low

Feb 21, 2017 - The fluorescence emission spectrum was measured with fluorescent spectroscopy (HITACHI F-4500). The absolute quantum yield was tested w...
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Polymorph-Dependent Electrogenerated Chemiluminescence of Low-Dimensional Organic Semiconductor Structures for Sensing Jianmin Gu,*,† Yahui Gao,† Jingxiao Wu,† Qing Li,§ Aixue Li,† Wei Zhang,∥ Haiyun Dong,∥ Bin Wen,‡ Faming Gao,*,† and Yong Sheng Zhao*,∥ †

Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China ‡ State Key Laboratory of Metastable Materials Science and Technology (MMST), Yanshan University, Qinhuangdao 066004, China § College of Chemistry and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China ∥ Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: A sensitive electrogenerated chemiluminescence (ECL) sensor with an organic semiconductor as active material for detecting trace amounts of molecules has been highly desired. However, the crystal structure responses of the ECL properties of the organic semiconductor materials, that is, structure−property relationship, is not clear, which limits the development of the sensitive ECL sensors. Herein, for the first time, we reported a novel concept for molecular-stacking-arrangement-dependent electrogenerated chemiluminescence properties of organic semiconductor rubrene microstructures. The rubrene 1D microwires and 2D hexagonal plates with different polymorphs (triclinic and monoclinic) were controllably constructed with the reprecipitation method. The supersaturation of the rubrene molecules plays an important role in the thermodynamically and kinetically dominated process of growth, which affects not only the polymorphs but also the morphology of the obtained microstructures. These microstructures show good optoelectronic properties, which are used as active ECL materials for the construction of ECL sensors. The ECL sensors exhibited distinct electrogenerated chemiluminescence properties, probably related to different inherent crystal-structuredependent triplet−triplet annihilation rate and charge-transfer rate. The sensors manifested electrogenerated chemiluminescence responses in broad linear range for the monitoring of creatinine molecules. KEYWORDS: molecular self-assembly, organic semiconductor nanomaterials, polymorph, electrogenerated chemiluminescence, sensing

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of emitters, and stable radical anions and cations.26−28 Hence, to develop sensitive ECL sensors with organic micromaterials would be a significant challenge. The main issue of organic micromaterials for the ECL application is the slow rate of charge transfer, which could result in the weak ECL signal for the sensors. Organic semiconductor micromaterials with large carrier mobility29 have recently attracted intensive attention in optoelectronics fields because of the unique size- and morphology-dependent optoelectronic properties,30−32 which have been used as ECL sensors that have exhibited high sensitivity and stability.33,34 For sensors with organic semiconductors as active materials, the performance of the sensors is dominated by both molecular structures and how organic molecules assemble themselves in the solid state.35,36 When organic molecules are used as building blocks for construction of functional materials, their

INTRODUCTION Electrogenerated chemiluminescence is a well-studied phenomenon that bridges the traditional fields of analytical electrochemistry and luminescence spectroscopy.1−4 The principle of ECL involves the generation of light from energetic electrontransfer reactions between electrogenerated species, which could be manipulated by the introduction of the specific species.5 As a valuable detection method, ECL has been increasingly acknowledged for trace detection because of its versatility, high stability, low background signal, and the simplicity of photonic detection.6−8 In recent years, lowdimensional materials with large specific surface areas have been used to fabricate ECL sensors9−12 for simple and sensitive analysis of specific targets.13−16 Most of the established ECL sensors rely on inorganic materials for ECL generation;8,16,17 however, these sensors could be used only to detect H2O218,19 and some enzymes,20,21 which are not used to detect the organic molecules. Organic micro/nanomaterials are attractive ECL materials22−25 with small molecules as coreactant because of their high fluorescence quantum efficiencies, broad spectrum © 2017 American Chemical Society

Received: December 15, 2016 Accepted: February 21, 2017 Published: February 21, 2017 8891

DOI: 10.1021/acsami.6b16118 ACS Appl. Mater. Interfaces 2017, 9, 8891−8899

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Molecular structure of rubrene. (b) XRD patterns of as-obtained microwires, hexagonal plates and the calculated XRD of triclinic and monoclinic powders. (c) SEM image of 1D rubrene microwires. (d) TEM image of a single rubrene microwire. Inset is the SAED pattern of 1D microwire. (e) SEM image of the 2D hexagonal plate. (f) TEM image of a single rubrene hexagonal plate. Inset is the SAED pattern of the hexagonal plate.

molecular-stacking arrangement and external crystal shape affect the optical and electrical characteristics of organic semiconductor materials,37−42 which makes the materials a good platform for studying the structure−property relationship. Herein, for the first time, we reported molecular-stackingarrangement-dependent electrogenerated chemiluminescence properties using organic semiconductor rubrene microstructures with distinct polymorphs as model ECL active materials. The rubrene 1D and 2D microstructures with distinct polymorphs are prepared through modulating the kinetic assembly process of rubrene molecules. At low supersaturation, 1D rubrene microwires with triclinic polymorph are obtained; however, at high supersaturation, 2D rubrene hexagonal plates with monoclinic polymorph are obtained, which could be thermodynamically and kinetically controlled. These microstructures show good optoelectronic properties, which might offer a model system for construction of ECL sensors. The ECL sensors with the two kinds of rubrene microstructures as active ECL materials exhibited distinct electrogenerated chemiluminescence properties, which is probably attributed to their different molecular-packing-mode-dependent triplet−triplet annihilation rate and charge-transfer rate. The sensors manifested electrogenerated chemiluminescence response in broad linear range, when it was used for the detection of creatinine molecules. The results might offer a comprehensive

understanding of the ECL reaction mechanism and provide insight and guidance for the development of organic-materialbased optoelectronic devices.

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RESULTS AND DISCUSSION

An ideal ECL sensor requires sensitive materials with high fluorescent quantum yield and good electrical conductivity as ECL active materials, which could help to detect the various molecules. The π-conjugated 5,6,11,12-tetraphenylnaphthacene (rubrene), a tetraphenyl derivative of tetracene (Figure 1a), is an important organic semiconductor molecule,43 which has shown good carrier mobility and light-emitting performances for optoelectronics applications.32,44 Moreover, the crystal structures of the rubrene materials exhibit multiple polymorphic forms45−47 with triclinic and monoclinic crystal phases. Therefore, rubrene micromaterials with different polymorphs were selected as the model ECL active materials for the construction of the ECL sensor. This will facilitate the study of the crystal structure responses of the ECL properties of the rubrene semiconductor materials because molecularstacking arrangement is a key factor for optoelectronics applications. The rubrene microstructures were prepared with a facile reprecipitation method. In a typical preparation, 50 μL of 30 mM rubrene stock solution in chloroform was quickly added 8892

DOI: 10.1021/acsami.6b16118 ACS Appl. Mater. Interfaces 2017, 9, 8891−8899

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Figure 2. (a) Schematic depiction for formation mechanism of two kinds of the rubrene microstructures and their corresponding fluorescence microscopy images. (b) Schematic illustration of energy-reaction coordination diagram for two kinds of polymorphs. (c) Schematic representation of the relationships between the relative nucleation and growth rates of the rubrene crystals and degree of supersaturation. (d) Schematic illustration for dependence of the fraction of triclinic and monoclinic crystals of the rubrene microstructures on the degree of supersaturation.

into different volume of methyl alcohol, which could lead to the formation of the rubrene microstructures. The formation of the rubrene microstructures is determined by the supersaturation of rubrene molecules (σ), which could be altered as a function of the volume of methyl alcohol when the total amount of rubrene molecules is fixed. At low supersaturation (the volume of methyl alcohol is 1.45 mL), the 1D wire-shaped rubrene (called Rub MWs) were obtained on a large scale as shown in Figure 1c. It can be seen that the Rub MWs have lengths of ∼100 μm and widths of ∼2 μm. When the supersaturation of rubrene molecules became large (the volume of methyl alcohol is 0.08 mL), 2D hexagonal plates (called Rub HPs) with average edge lengths of hexagonal plates about 10−20 μm are obtained (Figure 1e). The crystal phase of the as-synthesized rubrene microstructures were examined using XRD (Figure 1b). It can be seen that the Rub MWs belong to triclinic crystalline phase, whereas the Rub HPs are in monoclinic crystalline structure. Combined with the SAED patterns in Figure 1d, the Rub MWs are single-crystalline and grow along the [20−1] direction of rubrene crystals. SAED pattern in Figure 1f shows that the Rub HPs are single-crystalline and the homogeneous growth along the [011] and [01−1] directions leads to the formation of the hexagonal plates. On the basis of the above observation, the possible effect of supersaturation of rubrene molecules on the polymorphs of rubrene microstructures is schematically illustrated in Figure 2a. The self-assembly process of rubrene molecules was controlled by the change of the solvent surroundings with turbulent mixing of good and poor solvent, which could lead to the formation of the rubrene nuclei. In the nucleation progress, the corresponding rubrene nuclei transforms from the triclinic to the monoclinic phase with increasing supersaturation, which

could be schematically illuminated from thermodynamic and kinetic theories. Figure 2b demonstrates an energy-reaction coordination diagram, which shows that the monoclinic nucleus is calculated to be thermodynamically stable (ΔGM>ΔGT and G0 is free energy per mole of a solute in a supersaturated solution).48,49 The triclinic nucleus might have a lower nucleation barrier than a monoclinic nucleus, and therefore, the triclinic structure is favored due to the low nucleation barrier at low supersaturation. From kinetic analysis, the formation of different polymorphs is not only related to the relative nucleation rate but also related to the relative crystal growth rates.50 The nucleation rates are given by eq 1 according to the Volmer’s theory:51 JT JM

= exp

3 γM3 ⎞ 16πv 2 ⎛ γT ⎜ ⎟⎟ + ⎜ 3(kBT )3 ⎝ (lnσT)2 (lnσM)2 ⎠

(1)

where kB represents Boltzmann constant, v represents molecular volume, γ represents surface free energy, T represents temperature, σ represents degree of supersaturation. The mechanism for crystalline growth follows the Burton− Cabrera−Frank (BCF) model.52 According to this model, the effects of degree of supersaturation on growth rates, R, are determined by eq 2,

( ) ( )

⎛ σ − 1 ⎞2 tanh RT =⎜ T ⎟ RM ⎝ σM − 1 ⎠ tanh

D σT − 1

D σM − 1

(2)

where D represents a temperature-dependent constant. The schematic kinetic process of the rubrene polymorph (Figure 2c) shows the supersaturation dependence of the relative 8893

DOI: 10.1021/acsami.6b16118 ACS Appl. Mater. Interfaces 2017, 9, 8891−8899

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Figure 3. Illustration of the closet molecular stacking plane in two crystal structures: (a) triclinic, (b) monoclinic crystals. Large-scale FLIM images with the relevant fluorescence decay curve, instrumental response function, and fitted lifetime curve of (c) triclinic MWs and (d) monoclinic HPs with 375 nm picosecond-pulse laser. (e) Cyclic voltammograms of the bare ITO electrode (black curve), the triclinic Rub MWs/ITO electrode (red curve), the monoclinic Rub HPs/ITO electrode (blue curve). Supporting electrolyte: 1.0 × 10−3 M K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixtures in 0.1 M KCl aqueous solution, scan rate: 300 mV/s. (f) EIS of bare ITO electrode (black curve), triclinic Rub MWs/ITO electrode (red curve) and monoclinic Rub HPs/ITO electrode (blue curve) with frequency range of 0.01−100 000 Hz, signal amplitude of 5 mV and scan rate of 100 mV/s. Inset is fitted equivalent circuit. 1: electron-transfer resistance; 2: finite-length Warburg element; 3: constant phase element; 4: uncompensated resistance.

plates are linked to form the butterfly-shaped plate. When the supersaturation became much higher, the butterfly shaped plates grow further into the monoclinic hexagonal plates (E of Figure 2a). It can be seen that the supersaturation affects the relative growth rate of distinct crystalline face, which could modify the morphology of the rubrene materials from wire to hexagonal plate. Finally, the effect of supersaturation on the fraction of the triclinic and monoclinic polymorphs was schematically illustrated in Figure 2d. With the increase of supersaturation, the fraction of the monoclinic polymorphs in the final products increases. To determine the structural characteristics and intermolecular interactions of the rubrene microstructures, we simulated the closest molecular stacking of different kinds of crystalline structures. (Figure 3a,b) Figure 3a shows the packing diagrams of the triclinic rubrene crystals, where the red facet indicates the (20−1) crystal planes. The rubrene molecules are arranged along the [20−1] crystal direction with a distance of 3.871 Å, in which cofacial π-stack interactions and electronic couplings are quite efficient. For the monoclinic crystals, the overlapping of

nucleation and growth rates of the rubrene polymorph according to above classical theories. JT/JM and RT/RM represent the relative nucleation and growth rates of monoclinic and triclinic polymorph of rubrene, respectively. At low supersaturation, relative nucleation rate, JT/JM, is larger than relative growth rate, RT/RM, and thus, the crystalline phase of the polymorph could be confirmed via JT/JM.50 The nucleation rate of triclinic phase is larger than that of monoclinic phase, which might result from low interfacial tension of triclinic phase.53 Therefore, the complete triclinic Rub MWs were obtained at relatively low supersaturation (A of Figure 2a). However, with the increase of supersaturation, the effects of supersaturation for the nucleation rates become predominant, which could lead to the formation of the triclinic and monoclinic nuclei. The triclinic MWs decreased gradually; on the contrary, the rhombic plate appeared gradually with the increase of the supersaturation (B and C of Figure 2a). At high supersaturation, JT/JM → 1, but RM is higher than RT, which indicates that the polymorph of rubrene might favor form monoclinic nuclei. From D of Figure 2a, two or more rhombic 8894

DOI: 10.1021/acsami.6b16118 ACS Appl. Mater. Interfaces 2017, 9, 8891−8899

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Figure 4. (a) Schematic reaction cell for ECL measurement. The conventional three-electrode system was applied, where ITO, platinum wire, and Ag/AgCl electrode (sat. KCl) was used as working electrode, counter electrode, and reference electrode, respectively. The electrolyte solution is 0.1 M KCl solution. (b) ECL intensities comparison of triclinic Rub MWs (black curve) and monoclinic Rub HPs (blue curve). The ECL intensity of the triclinic microwires is stronger than that of the monoclinic plates. Inset: ECL emission from the Rub MWs/ITO in 0.1 M KCl aqueous solution. Scan rate: 300 mV/s. (c) Schematic representation of possible ECL mechanism of these rubrene microstructures without the presence of coreactant. (d) Calibration curve of the dependence at wide creatinine concentrations of triclinic microwires. K represents slope of line; R represents Pearson product-moment correlation coefficient. Inset: ECL intensity of the Rub MWs/ITO with various creatinine concentrations from A to F: 0, 1.0 × 10−13 M, 1.0 × 10−11 M, 1.0 × 10−9 M, 1.0 × 10−7 M, 1.0 × 10−5 M. Scan rate: 300 mV/s.

IP = 2.69 × 105AD1/2n2/3v1/2C

adjacent molecules will be very weak, and the large stacking distance leads to the weak intermolecular π−π stacking (Figure 3b) These results help us to consider the effects of the molecular-packing arrangement on luminescence and electrical properties of the materials. The normalized fluorescence decays of single rubrene microstructure are measured with fluorescence lifetime imaging microscopy (FLIM), which could provide evidence of the radiation decay process for a single particle on recording the fluorescence lifetimes of chromophores at a microscope image (Figure 3c,d).54 The decay curves of the Rub MWs in Figure 3c were well fitted by threeexponent fluorescence lifetimes (t1 = 0.227 ns, t2 = 0.835 ns, t3 = 4.313 ns). The decay curves of the Rub HPs in Figure 3d were also fitted by three-exponent fluorescence lifetimes (t1 = 0.994 ns, t2 = 0.217 ns, t3 = 8.560 ns). The triclinic microwires have faster decay (tav = 2.62 ns) than that of monoclinic plates (tav = 3.61 ns). The monoclinic crystals possess good optical properties with fluorescence quantum yield of 23.4%, higher than that of triclinic crystals 14.5%. (Supporting Information, Figure S1) It could be thought that the intermolecular π−π packing of monoclinic crystalline structure is weaker than that of triclinic crystal, which could lead to the changes of fluorescence quantum yield and decay kinetics of the two kinds of polymorphs. These rubrene microstructures, as active materials, were immobilized onto a cleaned-ITO substrate to construct an electrode to study the electrical properties of these materials. The electroactive surface area of the electrode can play a crucial role for improvement of the electrode activity because it is beneficial for providing many binding sites for the analyte on the electrode.16,33,55 The electroactive surface area is determined by Randles−Sevcik equation:17

(3)

where Ip represents peak current, n represents number of electrons of the reactions (n = 1), A represents electrode surface area (cm2), D represents diffusion coefficient of the redox probe (D = 6.70 ± 0.02 × 10−6 cm2/s at 25 °C), v represents scanning rate of cyclic voltammtry (V/s), and C represents concentration of [Fe(CN)6]3−/K4[Fe(CN)6]4− (C = 1 × 10−6 mol/cm3). Figure 3e and Figure S2 (Supporting Information) present the calculative electrode area value of bare ITO, Rub MWs/ITO, and Rub HPs/ITO based on the Randles−Sevcik equation. The rubrene microstructures (MWs and HPs) could enlarge electroactive area of the electrode, which could enhance the contact between the analytes and ITO electrode. The electroactive surface area of Rub MWs/ITO is almost the same as that of Rub HPs/ITO. The charge-transfer capability of different electrode was studied with electrochemical impedance experiments (Figure 3f). The electrochemical impedance spectra (EIS) were presented as Nyquist plots,12 fitting to a simulated model of simple equivalent circuit in the inset of Figure 3f. The radius of the semicircle of the triclinic Rub MWs/ITO electrode is shorter than that of monoclinic Rub HPs/ITO electrode. It is shown that the triclinic Rub MWs/ITO electrode has decreased the solid-state interface layer and charge-transfer resistances, which indicates that triclinic Rub MWs/ITO electrode has a faster charge transfer than monoclinic Rub HPs/ITO electrode. These obvious electrical properties differences of the rubrene materials should be attributed to the different molecular arrangements of the crystals. In the triclinic polymorph, the molecular planes of adjacent molecules are all parallel and laterally shifted with respect to each other with an overlap in the stacking direction, which could led to the strong 8895

DOI: 10.1021/acsami.6b16118 ACS Appl. Mater. Interfaces 2017, 9, 8891−8899

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ACS Applied Materials & Interfaces interactions between neighboring molecules with efficient π−π overlap (Figure 3a). This packing would facilitate more efficient charge migration than that of the monoclinic form with the weak intermolecular π−π stacking. The semiconductor rubrene micromaterials with good optical property and fast charge-transfer ability may offer a model system for the construction of the ECL sensors. The ITO-substrate-loaded rubrene microstructures, platinum wire, and Ag/AgCl electrode (sat. KCl) were applied as working electrode, counter electrode, and reference electrode, which constitute a convention three-electrode system in a typical ECL cell (Figure 4a). The ECL process of the sensors was investigated without the presence of ECL coreactant using the above ECL cell. The ECL emission spectra of the sensors shown in Figure 4b indicate that both the Rub MWs and Rub HPs provide noticeable ECL emission without the presence of coreactants. The electrogenerated chemiluminescence intensity of triclinic microwires is stronger than that of monoclinic plates. The sensor reveals good ECL stabilities when consecutive scans from −0.95 to 0.95 V were performed for several cycles, which indicates that the sensors are appropriate for quantitative electrogenerated chemiluminescence detection (see inset of Figure 4b). Figure 4c illustrates the possible ECL mechanism of rubrene microstructures without the presence of the coreactant. There might be a special light emission approach that is called the hot electron-induced electrogenerated chemiluminescence.56 Here, hot electron is defined as electron with thermal energy larger than thermal energy of a phase or as electron at energy far above the Fermi energy of a phase.57,58 Under an applied electric field, current flow across an ITO electrode surface can generate a hot solution-phase electron, which could be emitted into electrolyte solution. Because of the strong reductive abilities of the hot electron, the RUB+• radical that is oxidized on the electrode could be reduced and generate 3 RUB*. The excited-state singlet 1RUB* will be produced with subsequent annihilation of 3RUB* (triplet−triplet annihilation, TTA), which decays to produce orange emission. It is thought that the ECL intensity is primarily determined by the optoelectronics process, including the chemiluminescent and electrical process.30 In the chemiluminescent process, faster diffusion of 3RUB* would make 3RUB* meet more frequently and decay faster to 1RUB*. As we know, the diffusion rate of 3 RUB* becomes fast as intermolecular distances decrease, hence improving the 3RUB* reaching quenching site.59 Therefore, the triplet−triplet annihilation rate increases with decreasing intermolecular spacing, indicating that the luminescent efficiency of Rub MWs is larger than that of Rub HPs because of the intermolecular distance of different microstructures (Figure 3a,b). In the electrical process, Rub MWs/ ITO has a faster charge transfer than that of Rub HPs/ITO, which indicates that the electrochemical redox process of the Rub MWs is better than that of the Rub HPs.12 These factors give rise to the strong ECL signals of Rub MWs at almost the same electroactive surface area of the work electrodes (Figure 3e). For evaluating the performance of the obtained sensors, we selected tripropylamine (TPrA) as ECL coreactants to study the sensitivity of the sensors. As shown in Figure S3 (Supporting Information), the sensors give two near-linear detection ranges from 1.0 × 10−10 M to 7.0 × 10−6 M and from 1.0 × 10−5 M to 1.0 × 10−3 M. ECL emission spectra of asprepared rubrene microstructures were examined subsequently on account of the strong ECL emission signals (Supporting

Information, Figure S4). The generated ECL spectra are almost identical with the fluorescence from the rubrene microstructures (inset of Figure S4, Supporting Information), which results from the excited-state species originated from electrochemical reactions of the rubrene materials. The possible ECL reaction mechanism12 with the presence of TPrA as ECL coreactants was schematically shown in Figure S5 (Supporting Information). Due to their outstanding properties, these ECL sensors could be used for detecting various biological molecules. Creatinine, an important indicator of renal health, was selected for acting as the model analytes. The ECL intensities of the two kinds of sensors were monitored as a function of the creatinine concentrations from 3.0 × 10−14 M to 1.0 × 10−4 M for Rub MWs and 1.0 × 10−13 M to 1.0 × 10−4 M for Rub HPs. The ECL intensities increased with the increasing creatinine concentration as shown in the inset of Figure 4d. The typical derived calibration curve60 of Rub MWs is demonstrated in Figure 4d, which indicates the near-linear detection at wide range over several orders of magnitude. The experimentally confirmed limit of quantitation of Rub MWs and Rub HPs is 3.0 × 10−14 M and 1.0 × 10−13 M, respectively (Figure 4d and Figure S6, Supporting Information). The possible electrogenerated chemiluminescence reaction mechanism with the presence of creatinine as an ECL coreactant is shown in Figure S7 (Supporting Information). Another molecule, dopamine, an important neurotransmitter, was also selected for acting as analytes to assess the performance of the sensor (Figure S8, Supporting Information). The dopamine could obviously enhance the ECL intensity of the sensor, exhibiting a concentration-dependent signal response, which is similar to that of creatinine. These results demonstrate that the asobtained sensor can be applied to detect various kinds of molecules at low concentration.

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CONCLUSIONS In conclusion, two kinds of organic semiconductor rubrene microstructures with different polymorphs, ranging from microwires to hexagonal plates, were synthesized through altering the supersaturation of rubrene molecules. The crystalline structures of 1D microwires and 2D plates were identified to be triclinic and monoclinic phase, which are used to study the molecular-stacking-arrangement-dependent electrogenerated chemiluminescence properties. The ECL sensors with two types of rubrene microstructures exhibit different ECL responses, which are determined by different solid-statestructure-dependent triplet−triplet annihilation rate and charge-transfer rate. The sensors manifested good ECL responses to biological molecule creatinine in a broad linear range with experimentally confirmed limit of quantitation of Rub MWs and Rub HPs 3.0 × 10−14 M and 1.0 × 10−13 M, respectively. These results of controlling distinct structures and electrogenerated chemiluminescence properties shed new light on the development of novel ECL materials for optoelectronic devices.

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EXPERIMENTAL SECTION

Preparation of the Rubrene Microstructures. The chloroform and methyl alcohol were purchased from Beijing Chemical Agent, Inc., and the rubrene was obtained from Alfa Aesar. These reagents were used without further purification. The rubrene microstructures were prepared by the facile reprecipitation technology. First, a stock solution of 30 mM rubrene in chloroform was obtained. Then, 50 μL

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DOI: 10.1021/acsami.6b16118 ACS Appl. Mater. Interfaces 2017, 9, 8891−8899

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ACS Applied Materials & Interfaces of the above stock solution was quickly added into different volumes of methyl alcohol as a poor solvent at ambient conditions (35 °C) under vigorous sonication for 30 s, followed by aging in a closed tube for 2 h. Lastly, the rubrene micromaterials were prepared and dispersed in the solution, which could be employed for preparation of the sample for further characterizations by drop-casting. Electrogenerated Chemiluminescence Sensors. The ultrapure water (>17 MΩ·cm) was applied in whole experiment. The tripropylamine (TPrA), creatinine, and dopamine were purchased from Alfa Aesar and used without further treatment. The commercially available ITO was chosen as the working electrode of the ECL sensor. The surface of the ITO electrode was washed under the ultrasound bath with ultrapure water and absolute alcohol for 10 min, and it could be used as a substrate for ECL active materials. The as-prepared rubrene microstructures were immobilized onto a cleaned-ITO substrate by dropping the colloid solutions of the rubrene microstructures. The concentration of colloid solution of the two kinds of rubrene microstructures was fixed. After they were dried in air, these ECL sensors were prepared for further characterization. Measurements. The rubrene microstructures were characterized with SEM (Hitachi S-4800), TEM (JEOL JEM-2010), and XRD (Bruker AXS D8 diffractometer), respectively. The fluorescence emission spectrum was measured with fluorescent spectroscopy (HITACHI F-4500). The absolute quantum yield was tested with Hamamatsu Absolute Quantum Yields Spectrometers C11347. A fluorescent image was taken with a fluorescence microscope through exciting the sample with ultraviolet bands (330−380 nm) of the mercury lamp. A fluorescence lifetime image was taken with an FLIM equipment (PicoQuant) through scanning materials with lasers. FLIM equipment was made up of picosecond pulsed diode lasers (PDL 800D), fiber coupling unit (FCU II), laser scanning microscope (Olympus FV-1000), channel detector routers (PHR 800), and a photomultiplier detector assembly (PMA Series). All spectroscopic and optical measurements had been implemented at room temperature. EIS was performed by an electrochemical workstation (PARSTAT 4000). Cyclic voltammograms and ECL were simultaneously recorded with an electrochemical workstation coupled with a photomultiplier tube (PMT, hammamatsu R4220p). The electrogenerated chemiluminescence cells were composed of a platinum wire as the counter electrode. The ITO electrode, as working electrode, was rinsed by water and absolute alcohol at vigorous sonication, respectively. Ag/AgCl electrode was applied as reference electrode. The electrogenerated chemiluminescence emitting spectrum of rubrene microstructures was measured with ECL equipment coupled with a narrow-gap optical filter. The transmittance of central wavelength is 90%. The bandwidth is 13 nm.

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Yong Sheng Zhao: 0000-0002-4329-0103 Author Contributions

The manuscript was written by J.M.G. and Y.H.G. through contributions of all authors. The project was supervised by J.M.G., Q.L., F.M.G., and Y.S.Z. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos. 21403189, 21371149), China Postdoctoral Science Foundation (No. 2014M551047), Natural Science Foundation of Hebei Province (B2016208082) and Yanshan University Doctoral Foundation (No. B790).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16118. Absolute fluorescence quantum yield; dependence of anodic peak current; linear calibration plot; ECL emission spectra; schematic representation of the mechanisms for ECL reactions; and calibration curves of the dependence at wide molecular concentrations (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*(J.M.G.) E-mail: [email protected]. *(F.M.G.) E-mail: [email protected]. *(Y.S.Z.) E-mail: [email protected]. ORCID

Jianmin Gu: 0000-0002-4031-8385 Bin Wen: 0000-0001-9846-5452 8897

DOI: 10.1021/acsami.6b16118 ACS Appl. Mater. Interfaces 2017, 9, 8891−8899

Research Article

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DOI: 10.1021/acsami.6b16118 ACS Appl. Mater. Interfaces 2017, 9, 8891−8899