Acridinium Ester-Functionalized Carbon Nanomaterials: General

Jun 23, 2016 - ... (Agilent 8453, Santa Clara, CA). XPS was carried on an ESCALABMK II electron spectrograph (VG Scientific, UK) with Al Kα radiation...
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Acridinium Ester-Functionalized Carbon Nanomaterials: General Synthesis Strategy and Outstanding Chemiluminescence Zhili Han,†,⊥ Fang Li,‡,⊥ Jiangnan Shu,† Lingfeng Gao,† Xiaoying Liu,† and Hua Cui*,† †

CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China ‡ Department of Applied Chemistry, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P.R. China S Supporting Information *

ABSTRACT: In this work, three different kinds of acridinium ester (AE)-functionalized carbon nanomaterials, including AEfunctionalized carbon nanoparticles (AE-CNPs), AE-functionalized graphene oxide (AE-GO), and AE-functionalized multiwalled carbon nanotubes (AE-MCNTs), were synthesized for the first time via a simple, general, and noncovalent strategy. AE molecules were assembled on the surface of carbon nanomaterials by electrostatic interaction, π−π stacking interaction, and amide bond. The synthesized AE-CNPs, AEGO, and AE-MCNTs with 5.0 × 10−8 mol·L−1 of synthetic AE concentration, which was very low compared with other chemiluminescence (CL) reagents such as luminol, N-(aminobutyl)-N-(ethylisoluminol), and lucigenin at the concentration of 3.3 × 10−4 to 5.0 × 10−6 mol·L−1 used for the synthesis of CL-functionalized nanomaterials, exhibited outstanding CL activity and good stability. It was found that carbon nanomaterials as nanosized platforms could efficiently immobilize AE molecules and facilitate the formation of OH• and O2•−, leading to strong light emission. Moreover, the CL intensity of AE-GO was the highest, which was about 8.7 and 3.7 times higher than that of AE-CNPs and AE-MCNTs, respectively. This mainly resulted from a difference in the amount of adsorbed AE molecules on the surface of different carbon nanomaterials. Additionally, the prepared AE-CNPs demonstrated excitation-dependent fluorescence property and good fluorescence stability against photobleaching. On the basis of the excellent CL and special fluorescence properties of AE-CNPs, a dual-mode array strategy has been proposed for the first time and seven kinds of transition-metal ions could be successfully discriminated. KEYWORDS: acridinium ester, chemiluminescence, carbon nanomaterials, functionalization, fluorescence, array

1. INTRODUCTION Carbon nanomaterials, including carbon nanoparticles (CNPs), graphene, graphene oxide (GO), and carbon nanotubes (CNTs), have attracted great attention due to their unique optical, catalytic, electrochemical, and biocompatible properties.1−5 The synthesis and application of functionalized carbon nanomaterials with special properties are developing rapidly.6−8 Chemiluminescence (CL) as a valuable analytical tool has been widely used in immunoassays and nucleic acid assays because of its high sensitivity, wide dynamic range, and simple formats.9 Recently, CL-functionalized graphene, GO, and CNTs with good CL properties were successfully synthesized by noncovalent and covalent methods. The used CL reagents included luminol, isoluminol, N-(aminobutyl)-N-(ethylisoluminol) (ABEI), lucigenin, and ruthenium(II) complexes.10−13 The obtained CL-functionalized carbon nanomaterials have demonstrated good CL activity, which has been successfully applied to bioassays. However, CL-functionalized carbon nanomaterials with high CL efficiency are far from fully developed to meet the increased demand for ultrasensitive analytical methods. Acridinium ester (AE) is another kind of classical CL reagent with high CL efficiency, which has been widely used as signal © XXXX American Chemical Society

reporters in immunoassays and nucleic acid assays because of its high CL efficiency and stability.14−16 However, to date, AEfunctionalized nanomaterials with CL activity have not been reported. Considering that AE molecules contain rigid aromatic structure, they may be decorated on the basal planes of GO and MCNTs through π−π interactions.17,18 On the other hand, the AE molecule is a trifluoromethanesulfonate, which can be ionized to positively charge the AE+ with aromatic structure and CF3SO3− in aqueous solution. Thus, positively charged AE+ might also be immobilized onto the surface of negatively charged carbon nanomaterials through electrostatic interaction. Accordingly, AE may be directly assembled on the surface of carbon nanomaterials to form AE-functionalized carbon nanomaterials (AE-CNMs). In this work, three different kinds of AE-functionalized carbon nanomaterials, including AE-functionalized carbon nanoparticles (AE-CNPs), AE-functionalized graphene oxide (AE-GO), and AE-functionalized multiwalled carbon nanoReceived: April 5, 2016 Accepted: June 23, 2016

A

DOI: 10.1021/acsami.6b04055 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces tubes (AE-MCNTs), were synthesized for the first time via a simple, general, and noncovalent strategy. The resulting AEfunctionalized carbon nanomaterials were characterized by high-resolution transmission electron microscopy (HRTEM), UV−visible spectrophotometry, and X-ray photoelectron spectroscopy (XPS), and the assembly mechanism was discussed. The CL behavior of AE-functionalized carbon nanomaterials when reacted with H2O2 was studied by a static injection method. Surprisingly, the obtained AE-functionalized carbon nanomaterials exhibited outstanding CL activity as well as good stability. The effects of oxygen, nitrogen, and the radical scavengers on the CL intensity of AE-GO-H2O2 were investigated. The CL mechanism has been proposed. Additionally, the fluorescence (FL) property of the AE-functionalized carbon nanomaterials was explored. The synthesized AECNPs demonstrated special fluorescence properties. Finally, sensing arrays based on dual-mode measurement of the CL and FL intensity from AE-CNPs in the presence of transition-metal ion was developed for the first time for the discrimination of transition-metal ions.

12 and 6 h, respectively. Then the GO mixture and MCNTs mixture were centrifuged twice at a speed of 15 000 rpm for 15 min. Finally, the AE-GO and AE-MCNTs precipitants were dispersed in aqueous solution to obtain final AE-GO and AE-MCNTs dispersions. 2.4. CL Measurements. The CL activity was conducted on a microplate luminometer (Centro LB 960, Berthold, Germany). For a typical CL measurement, 100 μL of AE-CNPs, AE-GO, or AEMCNTs was injected into each well of a microtiter plate, respectively, and then 100 μL of 0.01 mol·L−1 H2O2 in 0.1 mol·L−1 NaOH (pH 13) solution was injected into each well. The light emission was collected by the microplate luminometer.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of CNPs and AECNMs. A schematic illustration for the synthesis of AE-CNMs is shown in Scheme 1. In the preparation of AE-CNMs, a Scheme 1. Schematic Illustration for Synthesis of AE-CNPs, AE-GO, and AE-MCNTs

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Acridinium C2 NHS Ester was purchased from Sigma-Aldrich (St. Louis). GO was purchased from XFNANO Materials Tech Co. Ltd. (Nanjing, China). Carboxylated MCNTs were obtained from Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China). Glutamine was purchased from Solarbio (Beijing, China) and used without further purification. Working solutions of H2O2 were prepared fresh daily from 30% (v/v) H2O2 (Xinke Electrochemical Reagent Factory, Bengbu, China). All other chemicals were of analytical-reagent grade. Ultrapure water was prepared by a Milli-Q system (Millipore, Guyancourt, France) and used throughout. 2.2. Apparatus. HRTEM images of the prepared CNPs, GO, MCNTs, and AE-CNMs were recorded on an electronic microscope (JEOL, JEM-2100F, Tokyo). UV−visible absorption spectra were obtained with an UV−visible spectrophotometer (Agilent 8453, Santa Clara, CA). XPS was carried on an ESCALABMK II electron spectrograph (VG Scientific, UK) with Al Kα radiation as the X-ray source. All fluorescent spectra of prepared CNPs, GO, MCNTs and AE-CNMs were obtained by a fluorescence spectrophotometer (Hitachi, F-7000, Japan). The zeta potential was measured by using a zeta potential analyzer (Nano ZS90 Zetasizer, Malvern Instruments, Malvern, U.K.). 2.3. Synthesis of CNPs and AE-CNMs. CNPs were prepared from hydrophilic amino acids by a one-step alkali-assisted microwave treatment.19 In a typical procedure, 2 g of glutamine was dissolved in 20 mL of NaOH (0.5 mol·L−1) solution and the mixture was heated in a domestic microwave oven for about 3 min. After cooling, the obtained brownish-black solid powder was dissolved in 40 mL of ultrapure water and centrifuged at a speed of 13 000 rpm for 15 min. The supernatant was collected and dialyzed against ultrapure water through a dialysis membrane (molecular weight cutoff = 1000) for 72 h to remove the excess precursor and free small molecules. The resultant CNPs were finally dried under vacuum freeze-drying equipment. To prepare AE-CNPs, 100 μL of 5.0 × 10−6 mol·L−1 AE solution were added to 10 mL of 0.2 mg·mL−1 CNPs solution. The mixture was vigorously stirred for 18 h at room temperature. A homogeneous brown dispersion was obtained. Dialysis procedure was used to remove free AE and coexisting free molecules to obtain final AE-CNPs dispersion. To make sure that excess AE molecules have been removed successfully, CL intensity of the last dialysate was examined. The CL intensity of the last dialysate was below 400 arb unit, which was ignorable compared with that of AE-CNPs (120 000 arb unit). AE-GO and AE-MCNTs were prepared by simply mixing 100 μL of 5.0 × 10−6 mol·L−1 AE with 10 mL of 0.2 mg·mL−1 GO suspension and 0.2 mg·mL−1 MCNTs aqueous solution at room temperature for

general strategy with similar synthesis procedures was developed. First, CNPs were synthesized according to a previous work.19 Then AE-CNPs, AE-GO, and AE-MCNTs were prepared by simply mixing AE with CNPs, GO, and MCNTs aqueous solution, and reacted at room temperature for 18, 12, and 6 h, respectively. Purification steps, including centrifugation and dialysis procedure, were carried out to remove free AE and other coexisting free molecules before characterizations. The morphologies of CNPs, GO, MCNTs, AE-CNPs, AE-GO, and AE-MCNTs were investigated by HRTEM as shown in Figure S1 of the Supporting Information and Figure 1. It can be seen that the synthesized CNPs were approximately spherical with average diameter of 19.36 nm. In comparison, the average diameter of AE-CNPs was about 30.50 nm, which was about 11.14 nm larger than that of CNPs, indicating that special interaction occurred between AE and CNPs, leading to the slight aggregation of CNPs. The maximum UV−visible absorption peak of AE-CNPs was redshifted and broadened slightly in contrast with that of CNPs (Figure S2a), further proving that special interaction occurred between AE and CNPs.20 The HRTEM images of the synthesized AE-GO and AE-MCNTs are shown in Figure 1b,c, which were similar to those of GO and MCNTs (Figure S1b, S1c), indicating that AE-GO and AE- MCNTs remained stable after the immobilization of AE molecules. Particularly, all of the three obtained AE-CNMs were quite stable and could be kept at 4 °C for several months. The surface compositions of as-prepared AE-CNMs were characterized by XPS, as shown in Figures S3−S5. In the XPS B

DOI: 10.1021/acsami.6b04055 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. HRTEM images with different amplification times of AE-CNPs (a-1, a-2; inset: the histogram of size distribution of AE-CNPs), AE-GO (b-1, b-2), and AE-MCNTs (c-1, c-2).

Figure 2. (a) CL kinetic curves of (1) AE-GO, (2) AE-MCNTs, (3) AE-CNPs, (4) CNPs, (5) GO, and (6) MCNTs with H2O2. Insert: magnification of curves 4−6. (b) CL spectra of 1.0 × 10−8 mol·L−1 AE-H2O2 (black line), AE-GO-H2O2 (red line), AE-MCNTs-H2O2 (blue line), and AE-CNPs-H2O2 (green line). Reaction conditions: 100 μL of 0.01 mol·L−1 H2O2 in 0.1 mol·L−1 NaOH (pH 13.0) was injected into 100 μL of AE and AE-CNMs dispersion in a microwell, respectively.

structure could also be adsorbed on GO and MCNTs through π−π stacking. Thus, π−π stacking interactions were also involved in the assembly of AE-GO and AE-MCNTs in addition to electrostatic interaction. Considering that acridinium C2 NHS Ester was used in this work, which could easily react it with an amino group to form an amide bond. And CNPs contained a C−N bond according to the results of XPS. Therefore, AE may bind to CNPs not only by electrostatic interaction and π−π interactions but also by amide bond. 3.3. CL Property of AE-CNMs. Since AE is a kind of CL reagent with high CL efficiency, the three synthesized AECNMs containing AE molecules were speculated to have CL activity. Thus, the CL behaviors of the three synthesized AECNMs were studied. As shown in Figure 2a, strong CL emissions were observed when 100 μL of 0.01 mol·L−1 H2O2 solution in 0.1 mol·L−1 NaOH (pH 13) was injected into 100 μL of the purified AE-CNMs dispersion. In comparison, the CL signals of CNPs, GO, and MCNTs suspension were very weak. AE-CNMs demonstrated outstanding CL activity. The concentration of AE used for the preparation of AE-CNMs was 5.0 × 10−8 mol·L−1, which was rather low compared with other CL reagents such as luminol, ABEI, and lucigenin at the concentration of 3.3 × 10−4 to 5.0 × 10−6 mol·L−1 used for the synthesis of CL-functionalized nanomaterials.21,22 Nevertheless, the CL emission of AE-CNMs was more than 1 order of magnitude higher than those of functionalized nanomaterials by

survey of AE-CNPs (Figure S3b), the area of the XPS peak located at around 285.32 eV attributed to the C−N bond was larger than that of CNPs (Figure S3a), indicating that some AE molecules were immobilized on the surface of AE-CNPs, and leading to the enhancement of XPS peak attributed to the C−N bond. Compared with the XPS survey of GO (Figure S4a) and MCNTs (Figure S5a), additional XPS peaks at around 285.1 eV attributed to the C−N bond from AE molecules were observed in the XPS survey of AE-GO (Figure S4b) and AEMCNTs (Figure S5b), indicating the existence of AE molecules on the surface of AE-GO and AE-MCNTs, respectively. 3.2. Assembly Mechanism of AE-CNMs. The assembly mechanisms of AE-CNMs were investigated by zeta-potential measurements. The measured zeta potentials of CNPs, GO, and MCNTs in aqueous solution were −9.82, −51.1, and −39.4 mV, respectively. In comparison, the zeta potentials of AECNPs, AE-GO, and AE-MCNTs were −9.71, −49.2, and −38.5 mV, respectively, which were slightly positively shifted compared with those of CNPs, GO, and MCNTs, respectively. It indicated that AE molecules were assembled on the surface of three negatively charged CNMs (CNPs, GO, and MCNTs) via electrostatic interaction. The presence of positively charged AE might induce the slight aggregation of negatively charged CNPs, which was in good agreement with the HRTEM results of AE-CNPs above. Both GO and MCNTs have large π−π conjugated structures.17,18 AE molecules with aromatic C

DOI: 10.1021/acsami.6b04055 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces other CL reagents.10,21 Moreover, the CL intensity of AE-GO was the highest, which was about 8.7 and 3.7 times higher than those of AE-CNPs and AE-MCNTs, respectively. The CL intensity of AE-CNMs might be related to the amount of adsorbed AE molecules on the surface of carbon nanomaterials. The amount of AE adsorbed on the surface of AE-GO, AEMCNTs, and AE-CNPs were further studied by CL analysis. As shown in Figure S6, the final amount of adsorbed AE on CNPs, GO, and MCNTs were determined to be 3.44 × 10−9, 3.27 × 10−8, and 3.09 × 10−8 mol·L−1, respectively. Since the synthetic concentration of AE was 5.0 × 10−8 mol·L−1, carbon nanomaterials as nanosized platforms could efficiently immobilize AE molecules. Moreover, the results demonstrated that the amount of adsorbed AE molecules on AE-GO and AEMCNTs was about 1 order of magnitude higher than that on AE-CNPs and the amount of adsorbed AE molecules on AEGO was slightly higher than that of AE-MCNTs. This is in good agreement with negatively charged degree of GO (−51.1 mV), MCNTs (−39.4 mV), and CNPs (−9.82 mV). AE molecules adsorbed on the surface of GO by electrostatic interaction were more than that on MCNTs and much more than that on CNPs. On the other hand, AE molecules could also be adsorbed on the surface of GO and MCNTs by π−π stacking interaction. The specific surface area of GO (calculated value 600−900 m2/g) was the highest, followed by MCNTs (calculated value 170−280 m2/g) and CNPs (calculated value 133−184 m2/g).23,24 As a result, the amount of adsorbed AE molecules follows the order: AE-GO > AE-MCNTs > AECNPs. Accordingly, the CL intensity of AE-CNMs increased with increasing the amount of adsorbed AE molecules on the surface of carbon nanomaterials. To obtain AE-CNMs with highest CL intensities, the reaction time between AE and the three CNMs (CNPs, GO, and MCNTs), the pH, and the concentration of H2O2 for CL reactions were also optimized. As illustrated in Figure S7, the optimal conditions were as follows: the reaction times were 18 h for AE-CNPs, 12 h for AE-GO, and 6 h for AE-MCNTs, the concentration of H2O2 was 0.01 mol·L−1, and the pH of H2O2 solution was 13. The stability of the AE-functionalized nanomaterials was further investigated by CL emission. As shown in Figure 3, the relative standard deviations (R.S.D.) of seven replicated CL measurements of AE-CNPs, AE-GO, and AE-MCNTs within a day (n = 7) were 3.56%, 1.78%, and 2.41%, respectively; the R.S.D. in 15 days (n = 7) were 4.10%, 2.04%, and 4.80%, respectively. The results demonstrated that the three prepared

AE-CNMs have good stability, showing great application potential in bioassays. 3.4. CL Mechanism of AE-CNMs. The CL mechanisms of the AE-CNMs were further investigated. As shown in Figure 2b, the CL spectra from the reactions of AE, AE-GO, AEMCNTs, and AE-CNPs with H2O2 exhibited the same maximum emission wavelength centered at ∼450 nm. The results demonstrated that the luminophore of AE-CNMs-H2O2 CL systems were the oxidation product of AE. Taking AE-GO with the highest CL activity as a model, the CL mechanism of the AE-GO-H2O2 system was discussed. The effects of oxygen (O2), nitrogen (N2), and the radical scavengers on the CL intensity of AE-GO-H2O2 were investigated. As shown in Figure 4a, the CL intensity increased in oxygen-saturated solution and decreased in nitrogen-saturated solution compared with that in air-saturated solution, indicating that O 2 participated in AE-GO-H2O2 CL reaction. Thiourea and superoxide dismutase (SOD) are effective radical scavengers of hydroxyl radicals (OH•) and superoxide radical (O2•−), respectively. As shown in Figure 4b,c, both of the CL intensities decreased with increasing the concentration of thiourea and SOD, indicating that both OH• and O2•− were involved in the CL process. Earlier study demonstrated that GO as a nanosized reaction platform could facilitate radical generation of OH• and O2•−.25 Thus, the reaction mechanism of AE-GO with H2O2 is proposed as shown in Figure 5. First, GO could facilitate the generation of O2•− and OH• on the surface of AE-GO. Then OH• reacted with HO2− to generate more O2•− on the surface of GO. Finally, O2•− reacted with AE to generate excited-state N-methylacridone (N-methylacridone*), accompanying an enhanced light emission at ∼450 nm.26 The effects of O2, N2, and the radical scavengers on the CL intensity of AE-H2O2, AE-CNPs-H2O2, and AE-MCNTs-H2O2 systems were similar to those of the AE-GO-H2O2 CL system (Figure S8−S10), indicating that O2•− and OH• were also involved in the AE-H2O2, AE-CNPs-H2O2, and AE-MCNTsH2O2 CL reactions. It has been reported that MCNTs and CNPs could also facilitate the formation of O2•− and OH•.27,28 Thus, the CL reactions of AE-CNPs and AE-MCNTs with H2O2 may follow a similar mechanism. 3.5. FL Activity of AE-CNMs. It has been reported that CNPs exhibited special fluorescence property, including broad photoluminescence spectra, excitation-dependent emission spectra, and excellent resistance to photobleaching.29,30 Accordingly, the fluorescence property of AE-CNPs was also studied. As shown in Figure 6a, AE-CNPs also exhibited fluorescence property. The fluorescence emission peak of AECNPs was red-shifted from 409 to 540 nm with the excitation wavelength varying from 320 to 480 nm, which was in good agreement with the λex-dependent fluorescence emission behavior of CNPs (Figure S11a). This λ ex -dependent fluorescence emission property might be attributed to the optical selection of different size of AE-CNPs (quantum effect) and different emissive energy trap sites on the surface of AECNPs.31,32 The fluorescence stability of AE-CNPs was also studied as shown in Figure 6b. No photobleaching was observed when AE-CNPs were excited at 320 nm 13 times, demonstrating the good fluorescence stability of AE-CNPs. The fluorescence properties of the obtained AE-GO and AEMCNTs were also investigated as shown in Figure S11b,c. AEGO and AE-MCNTs showed only very weak fluorescence. 3.6. Application of AE-CNPs to Sensing Arrays for Transition-Metal Ion. The results above showed that AE-

Figure 3. Stability of AE-CNMs. (a) Seven replicated CL measurements of AE-CNPs, AE-GO, and AE-MCNTs within a day. (b) Seven replicated CL measurements of AE-CNPs, AE-GO, and AE-MCNTs in 15 days. Reaction conditions: 100 μL of 0.01 mol·L−1 H2O2 in 0.1 mol·L−1 NaOH was injected into 100 μL of AE-CNMs aqueous solutions. D

DOI: 10.1021/acsami.6b04055 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) CL kinetic curves of AE-GO-H2O2 under oxygen-saturated (black line), air-saturated (red line), and nitrogen-saturated solutions (blue line), respectively. (b) CL kinetic curves of AE-GO-H2O2 in the absence (black line) and presence of different concentrations of thiourea: 0.1 mg· mL−1 (red line), 1 mg·mL−1 (blue line), 5 mg·mL−1 (green line), and 10 mg·mL−1 (pink line). (c) CL kinetic curves of AE-GO-H2O2 in the absence (black line) and presence of different concentrations of SOD: 0.01 μg·mL−1 (red line), 0.1 μg·mL−1 (blue line), 1 μg·mL−1 (green line), and 10 μg· mL−1 (pink line). Reaction conditions: 100 μL of 1.0 × 10−4 mol·L−1 H2O2 in 0.1 mol·L−1 NaOH was injected into 100 μL of AE-GO aqueous solutions. The light emission was collected by the microplate luminometer.

Figure 7. Fingerprints of seven transition-metal ions based on the patterns for the corresponding values of ΔI/I0 obtained from the CL and FL intensity of AE-CNPs. ΔI = I − I0, where I and I0 are the value of CL or FL intensity in the presence and absence of the target transition-metal ion, respectively. The concentration of transitionmetal ion is 5 μM.

Figure 5. Possible mechanism of CL reaction between AE-GO and H2O2.

transition-metal ion against the sensing array (Figure 8). The result indicated that the AE-CNPs could successfully discriminate seven kinds of transition-metal ions by virtue of change in CL and FL intensity.

Figure 6. (a) FL emission spectra (with progressively longer excitation wavelengths from 320 to 480 nm in 20 nm increments) of AE-CNPs. (b) Photostability test of AE-CNPs in a fluorescence spectrophotometer using 320 nm excitation, spacing 5 min (repeat three times). The negative voltage of the photomultiplier was −700 V.

CNPs have both excellent CL and special fluorescence properties, which might be used as a dual-mode sensing array. Accordingly, transition-metal ions with similar ionic radius and electronic shell structure, including Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+, were chosen as the targets. The effect of various transition-metal ions on the CL and FL intensity of AE-CNPs was studied. It was found that transitionmetal ions could produce different CL and FL responses when the concentration of H2O2 was 10 mM, and the excitation wavelength of FL was 320 and 350 nm, respectively, as shown in Figure 7. Principal component analysis (PCA), as a statistical technique, was employed to qualitatively differentiate transition-metal ions via the CL and FL response of AE-CNPs. Three replicates were tested for each transition-metal ion, and the raw data were subjected to PCA to generate three canonical factors. The first two most significant discrimination factors were employed to produce a two-dimensional plot in which each point represented the response pattern for an individual

Figure 8. PCA plot for the discrimination of seven transition-metal ions (5 μM) based on the CL and FL properties of AE-CNPs.

4. CONCLUSIONS In summary, AE-functionalized CNPs, GO, and MCNTs have been successfully synthesized for the first time via a simple, general, and noncovalent strategy. AE molecules were immobilized on the surface of CNPs, GO, and MCNTs by electrostatic interaction, π−π interactions, and amide bond. The amount of adsorbed AE on CNPs, GO, and MCNTs were E

DOI: 10.1021/acsami.6b04055 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces determined to be 3.44 × 10−9, 3.27 × 10−8, and 3.09 × 10−8 mol·L−1, respectively. The three synthesized AE-CNMs with 5.0 × 10−8 mol·L−1 of synthetic AE concentration much lower than that used for the synthesis of other CL reagents functionalized nanomaterials exhibited outstanding CL activity. Their CL intensity followed the order: AE-GO > AE-MCNTs > AE-CNPs, which was due to the amount of adsorbed AE molecules on the surface of carbon nanomaterials. Carbon nanomaterials as nanosized platforms could efficiently immobilize AE molecules and facilitate the formation of OH• and O2•− and electron transfer, leading to strong light emission. The obtained nanocomposites also exhibited good stability. This work presented for the first time that AE-functionalized nanomaterials with outstanding CL activity could be obtained, which provided a new family member of CL-functionalized nanomaterials. Compared with the free AE molecules, AEfunctionalized carbon nanomaterials have the following advantages. (i) A chemiluminescence interface immobilizing CL reagents on a material for label-free CL bioassays is necessary. Thus, the obtained AE-functionalized carbon nanomaterials with excellent CL activity may be used as a nanointerface for development of label-free CL bioassays. (ii) AE-functionalized carbon nanomaterials carry a number of signal reporters so that the detection signals can be greatly amplified. And the presented simple synthesis strategy may be extended to the synthesis of other AE-functionalized materials. Additionally, the prepared AE-CNPs also demonstrated excitation-dependent fluorescence properties and good fluorescence stability against photobleaching. On the basis of the excellent CL and special fluorescence properties of AE-CNPs, a dual-mode sensing array was developed for the first time to discriminate seven kinds of transition-metal ions successfully.

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Chemistry, CAS (Grant No. SKLEAC201408), are gratefully acknowledged.

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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/acsami.6b04055. Additional information as noted in text, including S1; characterization of CNPs, GO, MCNTs, and AE-CNMs, S2; determination of AE concentration on the surface of the three AE-CNMs, S3; optimization of the synthetic and CL measurement conditions, S4; effects of oxygen, nitrogen, thiourea, and superoxide dismutase (SOD) on the CL intensities, S5; fluorescence property (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-551-63600730. Fax: +86-551-63600730. E-mail: [email protected]. Author Contributions ⊥

Zhili Han and Fang Li are co-first authors and contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of this research by the National Natural Science Foundation of P.R. China (Grant Nos. 21475120 and 21173201), the Opening Fund of State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied F

DOI: 10.1021/acsami.6b04055 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b04055 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX