Article pubs.acs.org/ac
Introducing Confinement Effects into Ultraweak Chemiluminescence for an Improved Sensitivity Shichao Dong, Jinpan Zhong, and Chao Lu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *
ABSTRACT: So-called confinement effects at the interface of nanomaterials could spring up unique properties in catalytical activities and optical amplifiers. There is apparently no good reason to disregard confinement effect-amplified chemiluminescence (CL). In this work, confinement effects were first introduced into CL field using cetyltrimethylammonium bromide (CTAB) bilayer aggregates confined at the interface of the CTAB−carbon dots, which were prepared by one-step microwave irradiation using glycerol as carbon source in the presence of CTAB. Interestingly, it was found that the CTAB bilayer confined at the interface of carbon dots can amplify H2O2 induced ultraweak CL emissions, such as the Co(II)-triggered Fenton-like reaction, the peroxynitrous acid (ONOOH) system, and the peroxymonocarbonate (HCO4−) system. The study of fluorescent properties of the as-prepared CTAB−carbon dots and the comparison with the CL efficiency of their analogues indicated that the CTAB bilayer confined in carbon dots could act as a special micelle microenvironment, helping the access of reactive intermediates to the central carbon core. Our findings opened up new possibilities in confinement-enhanced CL emissions. he field of molecular self-assembly in a confined space of a few nanometers has drawn considerable attention of physicists, chemists, and biologists in the past few years.1−3 Such spatial confinement can induce new structures and open pathways to materials with unique properties.4 Recently, an overwhelming number of studies have focused on the morphology and properties of surfactants under confinement.5−8 The experimental and theoretical studies demonstrated that surfactants undergo structural rearrangements to perform many advantageous properties when surfactants were confined into various inorganic host matrices, such as metallic phases, porous alumina, or silica.9 Surfactant microenvironments as unique chemiluminescence (CL) reaction media can solubilize, concentrate, and organize reactants, and thus have received much attention in amplifying CL emission in liquid-phase CL systems.10 In recent years, although a variety of nanoparticle materials have been successfully explored to enhance CL emission,11 there are very few reports of the surfactant-amplified CL-based nanoparticles in comparison to the micelle-enhanced CL emission in liquid-phase systems.12−14 For example, Li et al.12 found that positively charged cetyltrimethylammonium bromide (CTAB) molecules were bound onto the surface of negatively charged thioglycolic acid-capped CdTe nanoparticles via electrostatic attractions, facilitating the CL generation. Furthermore, we previously investigated that organo-modified layered double hydroxides could enhance the CL emission via making the disappearance of electrostatic repulsion between anionic surfactants and anionic CL reactants.13
T
© 2014 American Chemical Society
Fluorescent carbon dots, as an exciting new class in the nanocarbon family, have attracted growing interest in recent years, because of their remarkable advantages in stable photoluminescence, high aqueous solubility, low cytotoxicity, and excellent biocompatibility over traditional toxic metalbased semiconductor quantum dots.15−17 Since Lin’s group observed that carbon dots could amplify the ultraweak CL emission of peroxynitrous acid (ONOOH),18 intensive research has been concentrated on the synthesis of strong luminescent carbon dots with the objective of producing high CL efficiency.19−22 Nevertheless, carbon dots, as a new type of CL probe, did not exhibit obvious advantages over other nanomaterial-based CL probes, in terms of sensitivity and selectivity, mainly because of the fact that the capping ligands of carbon dots can strongly affect the rate of the hole injection and electron injection, resulting in the occurrence of inefficient CL emissions.23,24 Apparently, it is a great challenge to synthesize the special ligand-functionalized carbon dots to meet the requirement of high CL efficiency. H2O2-induced ultraweak CL systems have been popularly investigated, because of their extensive applications in a variety of fields.25,26 These ultraweak CL systems involving H2O2 mainly include Fenton/Fenton-like systems, ONOOH systems, and peroxymonocarbonate (HCO4−) systems. In this study, we first reported a new one-step microwave-assisted approach to Received: May 27, 2014 Accepted: July 10, 2014 Published: July 10, 2014 7947
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ments were performed on a Bruck (Germany) D8 Advance Xray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.54178 Å). The 2θ angle of the diffractometer was stepped from 10° to 70° at a scan rate of 10°/min. The CL spectrum was obtained using a F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) at a slit of 20.0 nm and a scanning rate of 600 nm/min. The fluorescence spectra were performed using a F-7000 FL spectrophotometer at a slit of 5.0 nm and a scanning rate of 1500 nm/min. UV−vis spectra were measured on a USB 4000 miniature fiber optic spectrometer in absorbance mode with a DH-2000 deuterium and tungsten halogen light source (Ocean Optics, Dunedin, FL). Thermogravimetric analysis (TGA) was carried out using a TA Instruments Model Q5000 system (TA Instruments, New Castle, DE, USA) under nitrogen atmosphere. About 5.0 mg of sample was heated from an initial temperature of 25 °C to 600 °C at a rate of 10 °C/min. All samples were dried under vacuum at 40 °C for 48 h prior to TGA measurements. Fourier transform infrared (FTIR) measurements were performed using a Perkin−Elmer Model 100 FTIR spectrometer (Waltham, MA, USA). Raman spectrum was gained by a Renishaw invia Reflex Laser Confocal Raman spectrometer with a 785-nm laser radiation source. Time-resolved fluorescence measurements were performed on an Edinburgh Instruments spectrometer in photocounting mode. The lifetime values were obtained from the reconvolution fit analysis (Edinburgh F980 analysis software). The quantum yield was measured with an integrating sphere in a fluorescence spectrophotometer (Edinburgh Instruments spectrometer) according to a reported procedure. Zeta potentials were determined using a Malvern Zetasizer 3000HS nanogranularity analyzer. A microwave reactor (Discover SP, CEM, Matthews, NC, USA) was used for the preparation of carbon dots. The CL detection was conducted on a BPCL luminescence analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China). A CHI model 660E electrochemical workstation (Shanghai Chenhua Instruments, China) was used for the electrochemical measurements. Preparation of Carbon Dots. As shown in Figure S1 in the Supporting Information, the CTAB−carbon dots were synthesized using a rapid one-step procedure with a microwave reactor by pyrolyzing glycerol in the presence of CTAB. Briefly, 1.0 g of CTAB and 15 mL of glycerol were mixed in a 100-mL crucible. Then, it was treated by microwave reactor at 100 W for 6 min. When cooled to room temperature, a yellow-colored solution was obtained. Serine−carbon dots were prepared from 1.0 g serine in the presence of 15 mL glycerol to form a transparent solution, followed by heating in microwave reactor for 6 min. The solution changed from colorless to dark brown over the time course of the reaction. The as-prepared carbon dots were purified by the dialysis membrane with a molecular weight cutoff of 1000. The obtained carbon dots were stored in a refrigerator at 4 °C for further use. CL Measurements. For the Co(II)-triggered Fenton-like reaction, 100 μL 0.01 M NaOH solution was injected into a mixture solution of 60 μL 0.1 mM Co(II) solution, 60 μL 1.0 mM H2O2 solution, and 100 μL carbon-dot solution. For the ONOOH CL system, ONOOH solution was freshly prepared by the reaction of the mixed solution (150 μL) containing 0.05 M H2O2 and 0.02 M HCl with 0.01 M NaNO2 (200 μL). Then, 100 μL of ONOOH solution was injected into a 100-μL carbon-dot solution. For the HCO4− CL system, 100 μL 0.1 M H2O2 solution was injected into a mixture solution of 100 μL of
produce water-soluble carbon dots within minutes, using glycerol as carbon source in the presence of CTAB bilayer. Second, we found that the as-prepared CTAB−carbon dots can greatly enhance the ultraweak CL emissions from Fenton-like systems, ONOOH systems, and HCO4− systems. Finally, the enhanced mechanism of the as-prepared CTAB−carbon dots on these ultraweak CL systems was investigated by using the Fenton-like CL reaction as a model system. Significantly, it has been recognized that abundant CTAB bilayer aggregates were completely confined at the interface of carbon dots, helping the access of reactive intermediates to the central carbon core (Figure 1). These findings could obtain novel insights into the
Figure 1. Schematic illustration of the CTAB−carbon-dot-amplified ultraweak CL emissions.
CL enhancement characteristics of surfactants in confined spaces. The unique CL properties of the as-prepared CTAB− carbon dots enabled them to serve as promising materials for the improvement of CL assay sensitivity.
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EXPERIMENTAL SECTION Chemicals and Solutions. All chemicals used were of analytical grade and were used as received without any further purification. CoCl2·6H2O, glycerol, NaOH, and Triton X-100, HCl, and NaHCO3 were purchased from Beijing Chemical Reagent Company (Beijing, China). NaNO2 was purchased from Tianjin Chemical Reagent Company (Tianjin, China). CTAB, tetradecyltrimethylammonium bromide (TTAB), dodec yl t r i m e t hy l a m m on i u m b r o mi d e (D T A B ) , 1 , 4 diazabicyclo[2.2.2]octane (DABCO), serine, and sodium dodecyl sulfonate (SDS) were purchased from Sigma−Aldrich Chemical Co. (St. Louis, MO, USA). Working solutions of H2O2 were prepared daily from 30% (v/v) H2O2 (Beijing Chemical Reagent Company, China). A 0.1 M CoCl2 stock solution was prepared by dissolving CoCl2 with deionized water (18.2 MΩ cm, Milli-Q, Millipore, Barnstead, CA, USA). The working solutions of CoCl2 were freshly prepared by diluting the stock solution with deionized water. A mixed working solution of 0.05 M H2O2 and 0.02 M HCl was freshly prepared by volumetric dilution of commercial 30% (v/v) H2O2 and 36% (v/v) HCl with deionized water, respectively. Nitro blue tetrazolium chloride (NBT) was purchased from Nacalai Tesque Inc. (Tokyo, Japan). Uranine was purchased from Acros. Apparatus. Transmission electron microscopy (TEM) photographs were taken on a Tecnai G220 TEM from USA FEI Company. The powder X-ray diffraction (XRD) measure7948
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Figure 2. (a) TEM image of the CTAB-carbon dots. (b) Fluorescence emission spectra (with progressively longer excitation wavelengths from 400 nm in 20 nm increment) of the CTAB−carbon dots; insets show photographs of CTAB−carbon dots under ambient light (left) and UV light (365 nm) (right). (c) Normalized fluorescence emission spectra of the CTAB−carbon dots at different excitation wavelengths progressively increased from 400 nm with a 20-nm increment to 500 nm. (d) A typical time-resolved fluorescence-decay curve of the CTAB−carbon dots (λex = 340 nm) measured at 420 nm.
carbon-dot solution and 100 μL of 0.1 M NaHCO3. The CL signals were monitored by a photomultiplier tube (PMT) adjacent to the CL quartz vial. The data integration time of the BPCL analyzer was set at 0.1 s per spectrum, and a work voltage of −1100 V was used for the CL detection. The signals were imported to the computer for data acquisition. The schematic diagram of the flow injection analysis has been illustrated in Figure S2 in the Supporting Information. It consisted of two peristaltic pumps (BT-100M, Baoding, China), a 80 μL loop injector, a six-way injection valve (Shimadzu, Tokyo, Japan), a quartz tube, and a BPCL luminescence analyzer. The mixed solution of carbon dots and Co(II) standard or sample solution was injected into the carrier stream (H2O2, 3.5 mL/min) through a valve injector with a 80 μL sample loop, and mixed with NaOH (2.5 mL/ min) through a three-way connector. The signal output from the CL reaction was detected by the PMT. The signals were imported to the computer for data acquisition. Cyclic Voltammetry Measurements. For the cyclic voltammetry experiments, a conventional three-electrode system was employed with a glassy carbon electrode as the working electrode, a platinum wire as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. The solutions were prepared by adding 7.5 mM 3.5 mL H2O2, 0.1 μM 3.5 mL Co(II), and the CTAB−carbon dots to 5.0 mM 2.5 mL NaOH, then ultrapure N2 was passed through the solution for 5 min to remove dissolved oxygen. The scanning voltage ranged from −0.2 V to 1.2 V.
spherical in morphology with a narrow size distribution of 3.0− 5.0 nm in diameter (see Figure 2a). The XRD pattern of the CTAB−carbon dots (Figure S3 in the Supporting Information) showed a single broad peak centered at 2θ = 20.228°, similar to that observed previously by Giannelis et al.27 In addition, the carbon structures of the CTAB−carbon dots were confirmed by Raman spectral measurement (Figure S4 in the Supporting Information). The broad peak at 1379 cm−1 was assigned to the D-band, corresponding to the sp3 defects in carbon dots; on the other hand, the peak at 1532 cm−1 was matched well with the first-order G-band related to the in-plane bond-stretching motion of C sp2 atoms.28 The emission band maximum shifted to longer wavelengths and the intensity decreased gradually as the excitation wavelength increased from 400 nm to 500 nm (see Figures 2b and 2c).29 The inset photograph of Figure 2b showed that the CTAB−carbon dot colloidal solution was light yellow, transparent, and clear under ambient light and exhibited strong blue luminescence under UV light (365 nm). The fluorescence quantum yield of the CTAB−carbon dots was determined to be 5.28% with an integrating sphere in a fluorescence spectrophotometer.30 Figure 2d represented the fluorescence double exponential decay profile of the CTAB−carbon dots (see Table S1 in the Supporting Information). The average lifetime, ⟨τ⟩, was calculated to be 3.34 ± 0.02 ns, indicating the observed fluorescence from the radiative recombination of the excitons.31 Furthermore, the excellent photostability of the CTAB−carbon dots was demonstrated by continuously exciting the asprepared carbon dots for more than 4 h using a xenon lamp, obtaining 4% decay of the emission peak intensity (see Figure S5 in the Supporting Information). The fluorescence emission of the CTAB−carbon dots was highly salt-resistant, and the
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RESULTS AND DISCUSSION Characterization of CTAB−Carbon Dots. TEM images illustrated that the as-prepared CTAB−carbon dots were near 7949
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maximum emission wavelength did not shift in the pH range of 3−11 (see Figures S6 and S7 in the Supporting Information). Finally, the as-prepared CTAB−carbon dots remained stable for approximately three months. The chemical structure of the CTAB−carbon dots was characterized using Fourier tranform infrared (FTIR) spectroscopy (see Figure S8 in the Supporting Information). In comparison with the FTIR spectrum of CTAB, the bands of C−H stretching vibration in the FTIR spectrum of the CTAB− carbon dots slightly shifted from 2919 cm−1 to 2935 cm−1 and from 2849 cm−1 to 2883 cm−1, respectively. The weak peak at 3010 cm−1 (asymmetric stretching vibrations of CH3 and CH3−N+) in the FTIR spectrum of the CTAB−carbon dots was covered by the vibration bands of O−H (3379 cm−1), indicating that the hydrophilic headgroups of CTAB molecules were attached to the surface of the CTAB−carbon dots.32 On the other hand, the double peaks at 1487 and 1463 cm−1 attributed to scissoring vibrations of CH2 in CTAB molecules became a singlet peak at 1460 cm−1 in the CTAB−carbon dots. These obvious differences for asymmetric and symmetric C−H scissoring vibrations of CH3−N+ moiety between CTAB molecules and the CTAB−carbon dots also revealed that CTAB molecules were attached to the CTAB−carbon dot surface.33 The decreased stretching vibration bands of C−N+ (961, 937, 908 cm−1) as well as the appearance of a series of new bands in the range of 750−1300 cm−1 demonstrated that the surface of CTAB−carbon dots can affect stretching modes of C−N+.34 Geometrical Configuration of CTAB at the Interface of Carbon Dots. To further explain the origin of the attractive CL amplification of the CTAB−carbon dots, we investigated the surface structure of CTAB at the interface of the CTAB− carbon dots using zeta potential measurements and TGA measurements. The zeta potential of the CTAB−carbon dots was positively charged (32.1 mV), indicating that the stable CTAB bilayer bunches were formed on the surface of the CTAB−carbon dots via hydrophobic chain−chain interaction with hydrophilic head groups exposed to the continuous aqueous phase.35 However, the zeta potential of the CTABcapped serine−carbon dots (i.e., the CTAB solution was mixed with the as-prepared serine−carbon dots) was positively charged (2.48 mV), indicating that the CTAB molecules could not be easily attached to the surface of the serine−carbon dots. In addition, the TGA curves for CTAB and the CTAB− carbon dots are shown in Figure 3. For the TGA curve of CTAB, the weight loss occurred sharply from 240 °C to 300 °C, revealing thermal decomposition of the CTAB molecules within this temperature range. In contrast, the differential thermogravimetry (DTG) curve of the CTAB−carbon dots indicated that three weight loss steps (Figure 3). The first weight loss within 150−250 °C was corresponded to the decomposition of free CTAB and CTAB outer layer with low desorption energy.33 The second weight loss from 250 to 350 °C was attributed to decomposition of CTAB inner layer, in which the higher energy barriers came from the strong interactions between the confined CTAB and carbon dots.33 The third weight loss gradually between 350 and 500 °C was attributed to the combustion of the carbon-based core.36 The DTG curve of the CTAB−carbon dots showed three endothermic peaks centered at 235, 298, and 378 °C, corresponding well to those of three weight loss in its TGA curve. These findings were in accordance with those obtained
Figure 3. TGA and DTG plots for CTAB and the CTAB−carbon dots. Inset: possible configuration of the CTAB bilayer at the surface of the CTAB−carbon dots.
by zeta potential measurements. Therefore, it is confirmed that the CTAB molecules at the interface of the CTAB−carbon dots were packed in a bilayer structure. CTAB−Carbon Dots-Amplified CL in Co(II)-Triggered Fenton-Like Reaction. At first, we investigated the influence of the CTAB−carbon dots on the Co(II)−H2O2−OH− CL reaction. As shown in Figure 4, the CTAB−carbon dots can induce a significant increase in the CL intensity of the Co(II)− H2 O2−OH− system. Under the optimum experimental conditions (i.e., 10 mM NaOH, 7.5 mM H2O2, 1.0 g CTAB, and 1.6 g/L CTAB−carbon dots; see Figure S9 in the Supporting Information), we examined the analytical performances of the proposed CL method for sensing Co(II) with the flow injection system illustrated in Figure S2 in the Supporting Information. As shown in Figure S10 in the Supporting Information, there was a good linear relationship in a ln−ln plot between the relative CL intensity and Co(II) concentration in the range from 0.1 nM to 500 nM with a correlation coeffcient of 0.9945. The relative standard deviation (RSD) values (n = 11) for 0.1 μM Co(II) were 1.6%. The limit of detection (S/N = 3) for Co(II) was 0.07 nM. In contrast, a weak CL enhancement was observed in the presence of the CTAB solution, the serine−carbon dots, and the CTAB-capped serine−carbon dots. These interesting results demonstrated that the CL enhancement of the CTAB−carbon dots on the Co(II)−H2O2−OH− system was not merely attributed to the hydrophobic microenvironment of the CTAB micelle. Moreover, the other surfactants including SDS, Triton X-100, DTAB and TTAB were used instead of CTAB for the preparation of carbon dots. The corresponding carbon dots were applied to examine the Co(II)−H2O2−OH− CL system. As shown in Figure 4, the weak CL emissions from 7950
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did not contribute to the observed CL.18 However, the CL signals were greatly quenched by 10 mM thiourea or 0.1 mM ascorbic acid (scavengers of •OH radicals), demonstrating the existence of •OH radicals.37 When 1.0 mM NBT was added to the proposed CL system, an obvious decrease in the CL intensity occurred, meaning the generation of superoxide radicals (•O2−).21 On the other hand, the CTAB−carbon dot−Co(II) + H2O2 + OH− system could lead to an obvious decrease in the fluorescence intensity of 4.0 μM uranine (Figure S11a in the Supporting Information), indicating the formation of the larger amount of •OH radicals in the presence of the CTAB−carbon dot−Co(II) + H2O2 + OH− system.38 Moreover, the absorbance of 40 μM NBT at 557 nm could be significantly increased in the presence of the CTAB−carbon dot−Co(II) + H2O2 + OH− system (Figure S11b in the Supporting Information). These results demonstrated that the CTAB−carbon dot−Co(II) + H2O2 + OH− system could generate abundant •O2−.39 Finally, the cyclic voltammetry was employed for acquiring qualitative information about electrochemical reactions from the CTAB−carbon dot−Co(II) + H2O2 + OH− system. As shown in Figure S11c in the Supporting Information, the anodic peak in the CTAB−carbon dot−Co(II) + H2O2 + OH− system was strong, indicating the rapid decomposition of H2O2 in the CTAB−carbon dot− Co(II) + H2O2 + OH− system.40 In conclusion, the CTAB bilayer aggregates were confined at the interface of the asprepared CTAB−carbon dots, helping the access of the reactive intermediates (i.e., •O2− and •OH radicals) to the central carbon core through electrostatic interaction and the hydrophobic microenvironment. As a result, the hole-injected and electron-injected carbon dots were easily formed. Subsequently, the electron-transfer annihilation of hole-injected and electroninjected carbon dots could generate the excited-state carbon dots (see Figure S12 in the Supporting Information).18 Universality of Confinement Effect-Amplified Ultraweak CL Emissions. In order to verify the generality of the confinement effect-amplified CL emissions, we investigated the other two commonly used ultraweak CL systems: ONOOH and HCO4−. As shown in Figure 5, the CL signals of the two ultraweak CL systems were remarkably increased in the presence of the as-prepared CTAB−carbon dots. These results demonstrated that the CTAB−carbon dots exhibited the generality in amplifying H2O2-induced ultraweak CL emissions.
Figure 4. CL intensity of Co(II)−H2O2−OH− system in the presence of CTAB, serine−carbon dots (serine−CDs), CTAB-capped serinecarbon dots (serine−CDs+CTAB) and CTAB−carbon dots (CTAB− CDs). Inset: CL intensity of Co(II)−H2O2−OH− system in the presence of the carbon dots derived from SDS, Triton X-100, DTAB, TTAB, and CTAB.
the SDS−carbon dots and Triton X-100−carbon dots were observed. On the other hand, the stronger CL signals appeared in the presence of the DTAB−carbon dots and TTAB−carbon dots. The obvious CL enhancement of cationic surfactant− carbon dots may be ascribed to the fact that the produced HO2− anions from the dissociation of H2O2 in alkaline environment were not easily bound onto the surface of anionic or nonionic micelles via the electrostatic attraction, which was easy to take place for cationic micelles.37 Note that the CTAB− carbon dots had a stronger effect than the DTAB−carbon dots and TTAB−carbon dots, which was attributed to the lowest critical micelle concentration of CTAB among three cationic surfactants, resulting in the most stable micelle state at the interface of the carbon dots. Enhanced CL Mechanism. In order to clarify the mechanism of the striking CL, the effects of different active oxygen radical scavengers on the CL intensity of CTAB− carbon dot−Co(II)−H2O2−OH− system were investigated. As shown in Table S2 in the Supporting Information, 5.0 mM NaN3 and 1.0 mM DABCO (scavengers for 1O2) had no inhibition on the CL of the present system, indicating that 1O2
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CONCLUSIONS In summary, the new carbon dots were prepared via one-step microwave irradiation, using glycerol as the carbon source in
Figure 5. CL intensity of (a) the ONOOH system or (b) the HCO4− system in the presence of CTAB, serine−carbon dots (serine−CDs), CTABcapped serine−carbon dots (serine−CDs+CTAB) and CTAB−carbon dots (CTAB−CDs). 7951
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the presence of CTAB surfactant. Unlike the conventional surface-modified carbon dots, the CTAB bilayer aggregates were confined at the interface of the as-prepared CTAB− carbon dots, thereby facilitating the access of reactive intermediates to the central carbon core through the hydrophobic microenvironment of the confined CTAB bilayer. These striking performances of the as-prepared carbon dots were responsible for the noticeable enhancement in the CL emissions of the H2O2-induced ultraweak CL emissions. This observation not only provided useful information on the elucidation of the relationship between the geometrical configuration of surfactant microenvironment-confined carbon dots and its CL enhancement nature but also offered a novel approach for improving CL assay sensitivity under confinement. To the best of our knowledge, this is the first report to study the CL behavior in confinement effects. Such new hybrid architecture may provide promising applications in the fields of CL biosensing and imaging. We are confident that more confined nanocomposites will be easily obtained to extend the CL advancements in the future.
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ASSOCIATED CONTENT
S Supporting Information *
Diagram of the microwave preparation of carbon dots from glycerol and CTAB, schematic diagram of the flow injection analysis for Co(II)-triggered Fenton-like reaction, characterization of the as-prepared CTAB−carbon dots by XRD and Raman spectrum, effects of photoirradiation time, salt concentration and pH on the fluorescence intensity of the CTAB−carbon dots, FTIR spectra of CTAB and the CTAB− carbon dots, effects of NaOH, H2O2, CTAB and the CTAB− carbon dots on the CL intensity, CL signals of the proposed system by adding different concentrations of Co(II) and the calibration curve for Co(II), fluorescent spectra of uranine in the absence or presence of the Co(II)−H2O2−OH−−CTAB− carbon-dot system, absorbance spectra of NBT in the absence or presence of the Co(II)−H2O2−OH−−CTAB−carbon-dot system, cyclic voltammetry curves of Co(II)−H2O2−OH−− CTAB−carbon-dot system, fluorescence spectra for the CTAB−carbon dots before and after the CL reaction and CL spectrum for the CTAB−carbon dot−Co(II)−H2O2−OH− system, experimental procedures lifetime data obtained using the double exponential model for the CTAB−carbon dots and effects of radical scavengers on the CTAB−carbon dot− Co(II)−H2O2−OH− system. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel./Fax: +86 10 64411957. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program, No. 2014CB932103), the National Natural Foundation of China (No. 21375006), and the Fundamental Research Funds for the Central Universities (No. JD1311). We also thank Prof. Jin-Ming Lin (Tsinghua University) for his valuable discussions. 7952
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(35) Dong, S. C.; Liu, F.; Lu, C. Anal. Chem. 2013, 85, 3363−3368. (36) Bourlinos, A. B.; Zboril, R.; Petr, J.; Bakandritsos, A.; Krysmann, M.; Giannelis, E. P. Chem. Mater. 2012, 24, 6−8. (37) Zhang, L. J.; Zhang, Z. M.; Lu, C.; Lin, J.-M. J. Phys. Chem. C 2012, 116, 14711−14716. (38) Moore, J.; Yin, J.-J.; Yu, L. L. J. Agric. Food Chem. 2006, 54, 617−626. (39) Xue, W.; Lin, Z.; Chen, H.; Lu, C.; Lin, J.-M. J. Phys. Chem. C 2011, 115, 21707−21714. (40) Liu, R.; Goodell, B.; Jellison, J.; Amirbahman, A. Environ. Sci. Technol. 2005, 39, 175−180.
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Introducing Confinement Effects into Ultraweak Chemiluminescence for an Improved Sensitivity Shichao Dong, Jinpan Zhong, and Chao Lu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *
ABSTRACT: So-called confinement effects at the interface of nanomaterials could spring up unique properties in catalytical activities and optical amplifiers. There is apparently no good reason to disregard confinement effect-amplified chemiluminescence (CL). In this work, confinement effects were first introduced into CL field using cetyltrimethylammonium bromide (CTAB) bilayer aggregates confined at the interface of the CTAB−carbon dots, which were prepared by one-step microwave irradiation using glycerol as carbon source in the presence of CTAB. Interestingly, it was found that the CTAB bilayer confined at the interface of carbon dots can amplify H2O2 induced ultraweak CL emissions, such as the Co(II)-triggered Fenton-like reaction, the peroxynitrous acid (ONOOH) system, and the peroxymonocarbonate (HCO4−) system. The study of fluorescent properties of the as-prepared CTAB−carbon dots and the comparison with the CL efficiency of their analogues indicated that the CTAB bilayer confined in carbon dots could act as a special micelle microenvironment, helping the access of reactive intermediates to the central carbon core. Our findings opened up new possibilities in confinement-enhanced CL emissions. he field of molecular self-assembly in a confined space of a few nanometers has drawn considerable attention of physicists, chemists, and biologists in the past few years.1−3 Such spatial confinement can induce new structures and open pathways to materials with unique properties.4 Recently, an overwhelming number of studies have focused on the morphology and properties of surfactants under confinement.5−8 The experimental and theoretical studies demonstrated that surfactants undergo structural rearrangements to perform many advantageous properties when surfactants were confined into various inorganic host matrices, such as metallic phases, porous alumina, or silica.9 Surfactant microenvironments as unique chemiluminescence (CL) reaction media can solubilize, concentrate, and organize reactants, and thus have received much attention in amplifying CL emission in liquid-phase CL systems.10 In recent years, although a variety of nanoparticle materials have been successfully explored to enhance CL emission,11 there are very few reports of the surfactant-amplified CL-based nanoparticles in comparison to the micelle-enhanced CL emission in liquid-phase systems.12−14 For example, Li et al.12 found that positively charged cetyltrimethylammonium bromide (CTAB) molecules were bound onto the surface of negatively charged thioglycolic acid-capped CdTe nanoparticles via electrostatic attractions, facilitating the CL generation. Furthermore, we previously investigated that organo-modified layered double hydroxides could enhance the CL emission via making the disappearance of electrostatic repulsion between anionic surfactants and anionic CL reactants.13
T
© 2014 American Chemical Society
Fluorescent carbon dots, as an exciting new class in the nanocarbon family, have attracted growing interest in recent years, because of their remarkable advantages in stable photoluminescence, high aqueous solubility, low cytotoxicity, and excellent biocompatibility over traditional toxic metalbased semiconductor quantum dots.15−17 Since Lin’s group observed that carbon dots could amplify the ultraweak CL emission of peroxynitrous acid (ONOOH),18 intensive research has been concentrated on the synthesis of strong luminescent carbon dots with the objective of producing high CL efficiency.19−22 Nevertheless, carbon dots, as a new type of CL probe, did not exhibit obvious advantages over other nanomaterial-based CL probes, in terms of sensitivity and selectivity, mainly because of the fact that the capping ligands of carbon dots can strongly affect the rate of the hole injection and electron injection, resulting in the occurrence of inefficient CL emissions.23,24 Apparently, it is a great challenge to synthesize the special ligand-functionalized carbon dots to meet the requirement of high CL efficiency. H2O2-induced ultraweak CL systems have been popularly investigated, because of their extensive applications in a variety of fields.25,26 These ultraweak CL systems involving H2O2 mainly include Fenton/Fenton-like systems, ONOOH systems, and peroxymonocarbonate (HCO4−) systems. In this study, we first reported a new one-step microwave-assisted approach to Received: May 27, 2014 Accepted: July 10, 2014 Published: July 10, 2014 7947
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ments were performed on a Bruck (Germany) D8 Advance Xray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.54178 Å). The 2θ angle of the diffractometer was stepped from 10° to 70° at a scan rate of 10°/min. The CL spectrum was obtained using a F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) at a slit of 20.0 nm and a scanning rate of 600 nm/min. The fluorescence spectra were performed using a F-7000 FL spectrophotometer at a slit of 5.0 nm and a scanning rate of 1500 nm/min. UV−vis spectra were measured on a USB 4000 miniature fiber optic spectrometer in absorbance mode with a DH-2000 deuterium and tungsten halogen light source (Ocean Optics, Dunedin, FL). Thermogravimetric analysis (TGA) was carried out using a TA Instruments Model Q5000 system (TA Instruments, New Castle, DE, USA) under nitrogen atmosphere. About 5.0 mg of sample was heated from an initial temperature of 25 °C to 600 °C at a rate of 10 °C/min. All samples were dried under vacuum at 40 °C for 48 h prior to TGA measurements. Fourier transform infrared (FTIR) measurements were performed using a Perkin−Elmer Model 100 FTIR spectrometer (Waltham, MA, USA). Raman spectrum was gained by a Renishaw invia Reflex Laser Confocal Raman spectrometer with a 785-nm laser radiation source. Time-resolved fluorescence measurements were performed on an Edinburgh Instruments spectrometer in photocounting mode. The lifetime values were obtained from the reconvolution fit analysis (Edinburgh F980 analysis software). The quantum yield was measured with an integrating sphere in a fluorescence spectrophotometer (Edinburgh Instruments spectrometer) according to a reported procedure. Zeta potentials were determined using a Malvern Zetasizer 3000HS nanogranularity analyzer. A microwave reactor (Discover SP, CEM, Matthews, NC, USA) was used for the preparation of carbon dots. The CL detection was conducted on a BPCL luminescence analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China). A CHI model 660E electrochemical workstation (Shanghai Chenhua Instruments, China) was used for the electrochemical measurements. Preparation of Carbon Dots. As shown in Figure S1 in the Supporting Information, the CTAB−carbon dots were synthesized using a rapid one-step procedure with a microwave reactor by pyrolyzing glycerol in the presence of CTAB. Briefly, 1.0 g of CTAB and 15 mL of glycerol were mixed in a 100-mL crucible. Then, it was treated by microwave reactor at 100 W for 6 min. When cooled to room temperature, a yellow-colored solution was obtained. Serine−carbon dots were prepared from 1.0 g serine in the presence of 15 mL glycerol to form a transparent solution, followed by heating in microwave reactor for 6 min. The solution changed from colorless to dark brown over the time course of the reaction. The as-prepared carbon dots were purified by the dialysis membrane with a molecular weight cutoff of 1000. The obtained carbon dots were stored in a refrigerator at 4 °C for further use. CL Measurements. For the Co(II)-triggered Fenton-like reaction, 100 μL 0.01 M NaOH solution was injected into a mixture solution of 60 μL 0.1 mM Co(II) solution, 60 μL 1.0 mM H2O2 solution, and 100 μL carbon-dot solution. For the ONOOH CL system, ONOOH solution was freshly prepared by the reaction of the mixed solution (150 μL) containing 0.05 M H2O2 and 0.02 M HCl with 0.01 M NaNO2 (200 μL). Then, 100 μL of ONOOH solution was injected into a 100-μL carbon-dot solution. For the HCO4− CL system, 100 μL 0.1 M H2O2 solution was injected into a mixture solution of 100 μL of
produce water-soluble carbon dots within minutes, using glycerol as carbon source in the presence of CTAB bilayer. Second, we found that the as-prepared CTAB−carbon dots can greatly enhance the ultraweak CL emissions from Fenton-like systems, ONOOH systems, and HCO4− systems. Finally, the enhanced mechanism of the as-prepared CTAB−carbon dots on these ultraweak CL systems was investigated by using the Fenton-like CL reaction as a model system. Significantly, it has been recognized that abundant CTAB bilayer aggregates were completely confined at the interface of carbon dots, helping the access of reactive intermediates to the central carbon core (Figure 1). These findings could obtain novel insights into the
Figure 1. Schematic illustration of the CTAB−carbon-dot-amplified ultraweak CL emissions.
CL enhancement characteristics of surfactants in confined spaces. The unique CL properties of the as-prepared CTAB− carbon dots enabled them to serve as promising materials for the improvement of CL assay sensitivity.
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EXPERIMENTAL SECTION Chemicals and Solutions. All chemicals used were of analytical grade and were used as received without any further purification. CoCl2·6H2O, glycerol, NaOH, and Triton X-100, HCl, and NaHCO3 were purchased from Beijing Chemical Reagent Company (Beijing, China). NaNO2 was purchased from Tianjin Chemical Reagent Company (Tianjin, China). CTAB, tetradecyltrimethylammonium bromide (TTAB), dodec yl t r i m e t hy l a m m on i u m b r o mi d e (D T A B ) , 1 , 4 diazabicyclo[2.2.2]octane (DABCO), serine, and sodium dodecyl sulfonate (SDS) were purchased from Sigma−Aldrich Chemical Co. (St. Louis, MO, USA). Working solutions of H2O2 were prepared daily from 30% (v/v) H2O2 (Beijing Chemical Reagent Company, China). A 0.1 M CoCl2 stock solution was prepared by dissolving CoCl2 with deionized water (18.2 MΩ cm, Milli-Q, Millipore, Barnstead, CA, USA). The working solutions of CoCl2 were freshly prepared by diluting the stock solution with deionized water. A mixed working solution of 0.05 M H2O2 and 0.02 M HCl was freshly prepared by volumetric dilution of commercial 30% (v/v) H2O2 and 36% (v/v) HCl with deionized water, respectively. Nitro blue tetrazolium chloride (NBT) was purchased from Nacalai Tesque Inc. (Tokyo, Japan). Uranine was purchased from Acros. Apparatus. Transmission electron microscopy (TEM) photographs were taken on a Tecnai G220 TEM from USA FEI Company. The powder X-ray diffraction (XRD) measure7948
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Figure 2. (a) TEM image of the CTAB-carbon dots. (b) Fluorescence emission spectra (with progressively longer excitation wavelengths from 400 nm in 20 nm increment) of the CTAB−carbon dots; insets show photographs of CTAB−carbon dots under ambient light (left) and UV light (365 nm) (right). (c) Normalized fluorescence emission spectra of the CTAB−carbon dots at different excitation wavelengths progressively increased from 400 nm with a 20-nm increment to 500 nm. (d) A typical time-resolved fluorescence-decay curve of the CTAB−carbon dots (λex = 340 nm) measured at 420 nm.
carbon-dot solution and 100 μL of 0.1 M NaHCO3. The CL signals were monitored by a photomultiplier tube (PMT) adjacent to the CL quartz vial. The data integration time of the BPCL analyzer was set at 0.1 s per spectrum, and a work voltage of −1100 V was used for the CL detection. The signals were imported to the computer for data acquisition. The schematic diagram of the flow injection analysis has been illustrated in Figure S2 in the Supporting Information. It consisted of two peristaltic pumps (BT-100M, Baoding, China), a 80 μL loop injector, a six-way injection valve (Shimadzu, Tokyo, Japan), a quartz tube, and a BPCL luminescence analyzer. The mixed solution of carbon dots and Co(II) standard or sample solution was injected into the carrier stream (H2O2, 3.5 mL/min) through a valve injector with a 80 μL sample loop, and mixed with NaOH (2.5 mL/ min) through a three-way connector. The signal output from the CL reaction was detected by the PMT. The signals were imported to the computer for data acquisition. Cyclic Voltammetry Measurements. For the cyclic voltammetry experiments, a conventional three-electrode system was employed with a glassy carbon electrode as the working electrode, a platinum wire as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. The solutions were prepared by adding 7.5 mM 3.5 mL H2O2, 0.1 μM 3.5 mL Co(II), and the CTAB−carbon dots to 5.0 mM 2.5 mL NaOH, then ultrapure N2 was passed through the solution for 5 min to remove dissolved oxygen. The scanning voltage ranged from −0.2 V to 1.2 V.
spherical in morphology with a narrow size distribution of 3.0− 5.0 nm in diameter (see Figure 2a). The XRD pattern of the CTAB−carbon dots (Figure S3 in the Supporting Information) showed a single broad peak centered at 2θ = 20.228°, similar to that observed previously by Giannelis et al.27 In addition, the carbon structures of the CTAB−carbon dots were confirmed by Raman spectral measurement (Figure S4 in the Supporting Information). The broad peak at 1379 cm−1 was assigned to the D-band, corresponding to the sp3 defects in carbon dots; on the other hand, the peak at 1532 cm−1 was matched well with the first-order G-band related to the in-plane bond-stretching motion of C sp2 atoms.28 The emission band maximum shifted to longer wavelengths and the intensity decreased gradually as the excitation wavelength increased from 400 nm to 500 nm (see Figures 2b and 2c).29 The inset photograph of Figure 2b showed that the CTAB−carbon dot colloidal solution was light yellow, transparent, and clear under ambient light and exhibited strong blue luminescence under UV light (365 nm). The fluorescence quantum yield of the CTAB−carbon dots was determined to be 5.28% with an integrating sphere in a fluorescence spectrophotometer.30 Figure 2d represented the fluorescence double exponential decay profile of the CTAB−carbon dots (see Table S1 in the Supporting Information). The average lifetime, ⟨τ⟩, was calculated to be 3.34 ± 0.02 ns, indicating the observed fluorescence from the radiative recombination of the excitons.31 Furthermore, the excellent photostability of the CTAB−carbon dots was demonstrated by continuously exciting the asprepared carbon dots for more than 4 h using a xenon lamp, obtaining 4% decay of the emission peak intensity (see Figure S5 in the Supporting Information). The fluorescence emission of the CTAB−carbon dots was highly salt-resistant, and the
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RESULTS AND DISCUSSION Characterization of CTAB−Carbon Dots. TEM images illustrated that the as-prepared CTAB−carbon dots were near 7949
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maximum emission wavelength did not shift in the pH range of 3−11 (see Figures S6 and S7 in the Supporting Information). Finally, the as-prepared CTAB−carbon dots remained stable for approximately three months. The chemical structure of the CTAB−carbon dots was characterized using Fourier tranform infrared (FTIR) spectroscopy (see Figure S8 in the Supporting Information). In comparison with the FTIR spectrum of CTAB, the bands of C−H stretching vibration in the FTIR spectrum of the CTAB− carbon dots slightly shifted from 2919 cm−1 to 2935 cm−1 and from 2849 cm−1 to 2883 cm−1, respectively. The weak peak at 3010 cm−1 (asymmetric stretching vibrations of CH3 and CH3−N+) in the FTIR spectrum of the CTAB−carbon dots was covered by the vibration bands of O−H (3379 cm−1), indicating that the hydrophilic headgroups of CTAB molecules were attached to the surface of the CTAB−carbon dots.32 On the other hand, the double peaks at 1487 and 1463 cm−1 attributed to scissoring vibrations of CH2 in CTAB molecules became a singlet peak at 1460 cm−1 in the CTAB−carbon dots. These obvious differences for asymmetric and symmetric C−H scissoring vibrations of CH3−N+ moiety between CTAB molecules and the CTAB−carbon dots also revealed that CTAB molecules were attached to the CTAB−carbon dot surface.33 The decreased stretching vibration bands of C−N+ (961, 937, 908 cm−1) as well as the appearance of a series of new bands in the range of 750−1300 cm−1 demonstrated that the surface of CTAB−carbon dots can affect stretching modes of C−N+.34 Geometrical Configuration of CTAB at the Interface of Carbon Dots. To further explain the origin of the attractive CL amplification of the CTAB−carbon dots, we investigated the surface structure of CTAB at the interface of the CTAB− carbon dots using zeta potential measurements and TGA measurements. The zeta potential of the CTAB−carbon dots was positively charged (32.1 mV), indicating that the stable CTAB bilayer bunches were formed on the surface of the CTAB−carbon dots via hydrophobic chain−chain interaction with hydrophilic head groups exposed to the continuous aqueous phase.35 However, the zeta potential of the CTABcapped serine−carbon dots (i.e., the CTAB solution was mixed with the as-prepared serine−carbon dots) was positively charged (2.48 mV), indicating that the CTAB molecules could not be easily attached to the surface of the serine−carbon dots. In addition, the TGA curves for CTAB and the CTAB− carbon dots are shown in Figure 3. For the TGA curve of CTAB, the weight loss occurred sharply from 240 °C to 300 °C, revealing thermal decomposition of the CTAB molecules within this temperature range. In contrast, the differential thermogravimetry (DTG) curve of the CTAB−carbon dots indicated that three weight loss steps (Figure 3). The first weight loss within 150−250 °C was corresponded to the decomposition of free CTAB and CTAB outer layer with low desorption energy.33 The second weight loss from 250 to 350 °C was attributed to decomposition of CTAB inner layer, in which the higher energy barriers came from the strong interactions between the confined CTAB and carbon dots.33 The third weight loss gradually between 350 and 500 °C was attributed to the combustion of the carbon-based core.36 The DTG curve of the CTAB−carbon dots showed three endothermic peaks centered at 235, 298, and 378 °C, corresponding well to those of three weight loss in its TGA curve. These findings were in accordance with those obtained
Figure 3. TGA and DTG plots for CTAB and the CTAB−carbon dots. Inset: possible configuration of the CTAB bilayer at the surface of the CTAB−carbon dots.
by zeta potential measurements. Therefore, it is confirmed that the CTAB molecules at the interface of the CTAB−carbon dots were packed in a bilayer structure. CTAB−Carbon Dots-Amplified CL in Co(II)-Triggered Fenton-Like Reaction. At first, we investigated the influence of the CTAB−carbon dots on the Co(II)−H2O2−OH− CL reaction. As shown in Figure 4, the CTAB−carbon dots can induce a significant increase in the CL intensity of the Co(II)− H2 O2−OH− system. Under the optimum experimental conditions (i.e., 10 mM NaOH, 7.5 mM H2O2, 1.0 g CTAB, and 1.6 g/L CTAB−carbon dots; see Figure S9 in the Supporting Information), we examined the analytical performances of the proposed CL method for sensing Co(II) with the flow injection system illustrated in Figure S2 in the Supporting Information. As shown in Figure S10 in the Supporting Information, there was a good linear relationship in a ln−ln plot between the relative CL intensity and Co(II) concentration in the range from 0.1 nM to 500 nM with a correlation coeffcient of 0.9945. The relative standard deviation (RSD) values (n = 11) for 0.1 μM Co(II) were 1.6%. The limit of detection (S/N = 3) for Co(II) was 0.07 nM. In contrast, a weak CL enhancement was observed in the presence of the CTAB solution, the serine−carbon dots, and the CTAB-capped serine−carbon dots. These interesting results demonstrated that the CL enhancement of the CTAB−carbon dots on the Co(II)−H2O2−OH− system was not merely attributed to the hydrophobic microenvironment of the CTAB micelle. Moreover, the other surfactants including SDS, Triton X-100, DTAB and TTAB were used instead of CTAB for the preparation of carbon dots. The corresponding carbon dots were applied to examine the Co(II)−H2O2−OH− CL system. As shown in Figure 4, the weak CL emissions from 7950
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did not contribute to the observed CL.18 However, the CL signals were greatly quenched by 10 mM thiourea or 0.1 mM ascorbic acid (scavengers of •OH radicals), demonstrating the existence of •OH radicals.37 When 1.0 mM NBT was added to the proposed CL system, an obvious decrease in the CL intensity occurred, meaning the generation of superoxide radicals (•O2−).21 On the other hand, the CTAB−carbon dot−Co(II) + H2O2 + OH− system could lead to an obvious decrease in the fluorescence intensity of 4.0 μM uranine (Figure S11a in the Supporting Information), indicating the formation of the larger amount of •OH radicals in the presence of the CTAB−carbon dot−Co(II) + H2O2 + OH− system.38 Moreover, the absorbance of 40 μM NBT at 557 nm could be significantly increased in the presence of the CTAB−carbon dot−Co(II) + H2O2 + OH− system (Figure S11b in the Supporting Information). These results demonstrated that the CTAB−carbon dot−Co(II) + H2O2 + OH− system could generate abundant •O2−.39 Finally, the cyclic voltammetry was employed for acquiring qualitative information about electrochemical reactions from the CTAB−carbon dot−Co(II) + H2O2 + OH− system. As shown in Figure S11c in the Supporting Information, the anodic peak in the CTAB−carbon dot−Co(II) + H2O2 + OH− system was strong, indicating the rapid decomposition of H2O2 in the CTAB−carbon dot− Co(II) + H2O2 + OH− system.40 In conclusion, the CTAB bilayer aggregates were confined at the interface of the asprepared CTAB−carbon dots, helping the access of the reactive intermediates (i.e., •O2− and •OH radicals) to the central carbon core through electrostatic interaction and the hydrophobic microenvironment. As a result, the hole-injected and electron-injected carbon dots were easily formed. Subsequently, the electron-transfer annihilation of hole-injected and electroninjected carbon dots could generate the excited-state carbon dots (see Figure S12 in the Supporting Information).18 Universality of Confinement Effect-Amplified Ultraweak CL Emissions. In order to verify the generality of the confinement effect-amplified CL emissions, we investigated the other two commonly used ultraweak CL systems: ONOOH and HCO4−. As shown in Figure 5, the CL signals of the two ultraweak CL systems were remarkably increased in the presence of the as-prepared CTAB−carbon dots. These results demonstrated that the CTAB−carbon dots exhibited the generality in amplifying H2O2-induced ultraweak CL emissions.
Figure 4. CL intensity of Co(II)−H2O2−OH− system in the presence of CTAB, serine−carbon dots (serine−CDs), CTAB-capped serinecarbon dots (serine−CDs+CTAB) and CTAB−carbon dots (CTAB− CDs). Inset: CL intensity of Co(II)−H2O2−OH− system in the presence of the carbon dots derived from SDS, Triton X-100, DTAB, TTAB, and CTAB.
the SDS−carbon dots and Triton X-100−carbon dots were observed. On the other hand, the stronger CL signals appeared in the presence of the DTAB−carbon dots and TTAB−carbon dots. The obvious CL enhancement of cationic surfactant− carbon dots may be ascribed to the fact that the produced HO2− anions from the dissociation of H2O2 in alkaline environment were not easily bound onto the surface of anionic or nonionic micelles via the electrostatic attraction, which was easy to take place for cationic micelles.37 Note that the CTAB− carbon dots had a stronger effect than the DTAB−carbon dots and TTAB−carbon dots, which was attributed to the lowest critical micelle concentration of CTAB among three cationic surfactants, resulting in the most stable micelle state at the interface of the carbon dots. Enhanced CL Mechanism. In order to clarify the mechanism of the striking CL, the effects of different active oxygen radical scavengers on the CL intensity of CTAB− carbon dot−Co(II)−H2O2−OH− system were investigated. As shown in Table S2 in the Supporting Information, 5.0 mM NaN3 and 1.0 mM DABCO (scavengers for 1O2) had no inhibition on the CL of the present system, indicating that 1O2
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CONCLUSIONS In summary, the new carbon dots were prepared via one-step microwave irradiation, using glycerol as the carbon source in
Figure 5. CL intensity of (a) the ONOOH system or (b) the HCO4− system in the presence of CTAB, serine−carbon dots (serine−CDs), CTABcapped serine−carbon dots (serine−CDs+CTAB) and CTAB−carbon dots (CTAB−CDs). 7951
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the presence of CTAB surfactant. Unlike the conventional surface-modified carbon dots, the CTAB bilayer aggregates were confined at the interface of the as-prepared CTAB− carbon dots, thereby facilitating the access of reactive intermediates to the central carbon core through the hydrophobic microenvironment of the confined CTAB bilayer. These striking performances of the as-prepared carbon dots were responsible for the noticeable enhancement in the CL emissions of the H2O2-induced ultraweak CL emissions. This observation not only provided useful information on the elucidation of the relationship between the geometrical configuration of surfactant microenvironment-confined carbon dots and its CL enhancement nature but also offered a novel approach for improving CL assay sensitivity under confinement. To the best of our knowledge, this is the first report to study the CL behavior in confinement effects. Such new hybrid architecture may provide promising applications in the fields of CL biosensing and imaging. We are confident that more confined nanocomposites will be easily obtained to extend the CL advancements in the future.
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ASSOCIATED CONTENT
S Supporting Information *
Diagram of the microwave preparation of carbon dots from glycerol and CTAB, schematic diagram of the flow injection analysis for Co(II)-triggered Fenton-like reaction, characterization of the as-prepared CTAB−carbon dots by XRD and Raman spectrum, effects of photoirradiation time, salt concentration and pH on the fluorescence intensity of the CTAB−carbon dots, FTIR spectra of CTAB and the CTAB− carbon dots, effects of NaOH, H2O2, CTAB and the CTAB− carbon dots on the CL intensity, CL signals of the proposed system by adding different concentrations of Co(II) and the calibration curve for Co(II), fluorescent spectra of uranine in the absence or presence of the Co(II)−H2O2−OH−−CTAB− carbon-dot system, absorbance spectra of NBT in the absence or presence of the Co(II)−H2O2−OH−−CTAB−carbon-dot system, cyclic voltammetry curves of Co(II)−H2O2−OH−− CTAB−carbon-dot system, fluorescence spectra for the CTAB−carbon dots before and after the CL reaction and CL spectrum for the CTAB−carbon dot−Co(II)−H2O2−OH− system, experimental procedures lifetime data obtained using the double exponential model for the CTAB−carbon dots and effects of radical scavengers on the CTAB−carbon dot− Co(II)−H2O2−OH− system. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel./Fax: +86 10 64411957. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program, No. 2014CB932103), the National Natural Foundation of China (No. 21375006), and the Fundamental Research Funds for the Central Universities (No. JD1311). We also thank Prof. Jin-Ming Lin (Tsinghua University) for his valuable discussions. 7952
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