Fabrication of Luminol and Lucigenin Bifunctionalized Gold

Article pubs.acs.org/JPCC

Fabrication of Luminol and Lucigenin Bifunctionalized Gold Nnanoparticles/Graphene Oxide Nanocomposites with DualWavelength Chemiluminescence Yi He and Hua Cui* CAS Key Laboratory of Soft Matter Chemistry, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: A facile, fast, and reliable method was proposed to prepare luminol and lucigenin bifunctionalized AuNPs/GO nanocomposites via absorption of AuCl4− by positively charged lucigenin functionalized GO and the subsequent reduction of AuCl4− by luminol at room temperature for the first time. The morphology and surface composition of the nanocomposites were characterized by transmission electron microscopy, X-ray powder diffraction, and mass and fluorescence spectra. The results indicated that AuNPs with a uniform size are fairly well monodispersed on the surface of GO. The size of AuNPs in the nanocomposites was tunable from 8 to 18 nm by changing the amount of AuCl4−. Moreover, it was found that luminol and lucigenin coexisted on the surface of the nanocomposites. A formation mechanism of AuNPs/GO nanocomposites is proposed. It is suggested that lucigenin molecules and AuNPs were located at the surface of GO by π−π stacking and electrostatic force respectively, and luminol existed on the surface of AuNPs by virtue of Au−N covalent interaction in the as-prepared nanocomposites. Because luminol and lucigenin were attached to the surface of the nanocomposites, the obtained nanocomposites could react with hydrogen peroxide resulting in a good dual-wavelength chemiluminescence activity. Also, the nanocomposites could also react with silver nitrate giving rise to chemiluminescence emission. Besides, the nanocomposites exhibited fluorescence properties. The nanocomposites could be considered as not only functionalized materials but also as a promising platform for multipurpose sensing and bioassays.

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properties, and good biocompatibility.5,6 The AuNPs/GO nanocomposites have been prepared by different approaches to date and used for electrochemical biosensing,7 catalytic reactions,8,9 and optical imaging.10 For example, Gao and coworkers reported a facile method for synthesizing AuNPs/GO nanocomposites. The obtained nanocomposites exhibited unexpected catalytic activity for the Suzuki−Miyaura coupling reaction of chlorobenzene and phenylboronic acid.9 Huang’s group developed a noncovalent approach to fabricate AuNPs/ GO hybrid, which was used to directly illuminate graphene for optical imaging by employing the strong localized surface plasmon resonance light scattering of AuNPs as an effective signal reporter.10 Recently, the attention has been paid to chemiluminescence (CL) functionalized metal nanoparticles/ GO for ultrasensitive bioassays. In our previous work, luminol functionalized sliver nanoparticles/GO, and lucigenin functionalized Pt nanoparticles/reduced GO nanocomposites have been

INTRODUCTION Graphene is a flat monolayer of carbon atoms tightly packed into a 2D honeycomb lattice.1 Graphene oxide (GO), an important graphene derivative, has received more and more attention because of its good dispersibility, convenient chemical modification, good mechanical and thermal properties, high surface area, and unique plane structure.2 These interesting properties make GO a promising material for extraordinary applications in the fields such as catalysis, sensors, smart equipments, and high-performance batteries.3,4 To further expand their application fields, the preparation of GO based nanocomposites with unique properties is very significant. Until now, various organic molecules (porphyrin and polymers) and inorganic nanoaprticles such as TiO2, SiO2, Fe3O4, and metal nanoparticels have been employed to synthesize GO-based nanocomposites.5 The nanocomposites built from assembly of metal nanoparticles and GO have stimulated intense research in recent years due to their potential to combine desirable properties of them. Gold nanoparticles (AuNPs) are widely used metal nanoparticles. They exhibit unique optical properties, catalytic © 2012 American Chemical Society

Received: April 6, 2012 Revised: May 16, 2012 Published: May 22, 2012 12953

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successfully prepared and used for detection of glutathione.11,12 Despite the recent progress, it is still a great challenge for the design and preparation of AuNPs/GO nanocomposites with novel CL. Moreover, GO-based nanocomposites with multiwavelength CL have not yet been explored. In the present work, we develop a simple, fast, and reliable approach for preparing luminol and lucigenin bifunctionalized AuNPs/GO nanocomposites at room temperature. The morphology and surface state of the resulting AuNPs were characterized by TEM, X-ray powder diffraction (XRD), mass spectra, and fluorescence spectra. The results demonstrated that luminol and lucigenin coexisted on the surface of the nanocomposites and luminol and lucigenin bifunctionalized gold nanoparticles/graphene oxide nanocomposites were formed. Furthermore, a formation mechanism of AuNPs/GO nanocomposites was investigated. Finally, the CL properties of the AuNPs/GO nanocomposites with some oxidants, such as hydrogen peroxide and silver nitrate, were explored by a static injection method. The CL reaction mechanism involving the two CL systems above were also discussed.

product was collected by centrifugation and washed two times with water and redispersed in water for further measurements. CL Activity of Luminol and Lucigenin Bifunctionalized AuNPs/GO Nanocomposites. As-prepared nanocomposites (0.3 mL) were added to a CL cell. Then, 0.3 mL of 1 mM H2O2 containing 0.1 M NaOH was injected into the cell to initiate the CL reaction and record the CL intensity during the reaction along with time. The CL detection was conducted on a BPCL Luminescence Analyze (Beijing, China) with a fixed voltage of −800 V.

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RESULTS AND DISCUSSION The strategy that we developed to synthesize the luminol and lucigenin bifunctionalized AuNPs/GO nanocomposites is illustrated in Scheme 1. First, the lucigenin functionalized GO Scheme 1. Schematic Diagram of Preparing AuNPs/GO Nanocomposites

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EXPERIMENTAL SECTION Materials and Instruments. Luminol was purchase from Sigma-Aldrich. Silver nitrate, absolute ethanol, sodium hydroxide, and chloroauric acid were obtained from Shanghai reagent (Shanghai, China). A stock solution of lucigenin (5 mM) was prepared by dissolving lucigenin (Tokyo Chemical Industry Co., Ltd., Japan) in ultrapure water. Working solutions of H2O2 were prepared fresh daily from 30% (v/v) H2O2 (Xin Ke Electrochemical Reagent Factory, Bengbu, China). Water used throughout all experiments was purified with the Mill-Q system. TEM, high-resolution TEM (HRTEM) images, and energy dispersive spectroscopy (EDS) were taken on a JEM-2100F field emission transmission electron microscope operating at 200 KV. An XRD pattern was obtained from a Rigaku D/max X-ray diffractometer. Fluorescence spectrum was measured on a model F-7000 spectrofluorometer. Mass spectrum was performed on a ProteomeX-LTQ mass spectrometer in a positive mode. The CL spectra were measured using highenergy cutoff filters at wavelengths of 360, 380, 400, 420, 470, 490, 510, and 530 nm. ΔIfλ was calculated as shown: for example, ΔI360 = Iblank (without filter) − I360, ΔI380 = I360 − I380, and so forth.13 Synthesis of Lucigenin Functionalized GO Composite. The lucigenin functionalized GO was prepared as described previously.12 Typically, 100 μL of 5 mM lucigenin (Figure S1 of the Supporting Information) aqueous solution was added to 100 mL of the homogeneous graphene oxide dispersion (0.1 mg mL−1), followed by stirring for 48 h at room temperature. Finally, the yellow dispersion was centrifuged two times and redispersed in 100 mL of ultrapure water. Synthesis of Luminol and Lucigenin Bifunctionalized AuNPs/GO Nanocomposites. The as-prepared dispersion of lucigenin functionalized GO (3.5 mL), 5 mL of absolute ethanol, and 9 mL of H2O were added to a 50 mL flask. Meanwhile, different volumes (1.0, 1.5, and 2.0 mL) of 10 mM HAuCl4 aqueous solution was then added to the mixture under magnetic stirring at room temperature, followed by stirring for 10 min. Subsequently, 0.6 mL of 0.01 M luminol (0.1 M NaOH) was quickly added to the above solution. The solution was maintained at room temperature for 1 h, during which time a color change from yellow to red- or pink-colored. The

nanocomposites with positive charge are prepared as described previously.12 The resulting positively charged lucigenin functionalized GO nanocomposites are used to absorb the AuNPs precursor (AuCl4−) by electrostatic interactions. Afterward, luminol as a reducing agent is used to reduce the precursors in situ to form luminol functionalized AuNPs on the surface of GO at room temperature. The successful preparation of AuNPs/GO nanocomposites was confirmed by TEM. Part A of Figure 1 shows TEM images of the as-prepared nancomposites. It can be seen that AuNPs with a uniform size of 8 nm are fairly well monodispersed on the surface of GO. The diameter of AuNPs is tunable though the change of HAuCl4 amount. The size of AuNPs increases from 8 to 11 nm (part B of Figure 1) and 18 nm (part C of Figure 1) with the increase of HAuCl4 amount. The high-resolution TEM image (Figure S2 of the Supporting Information) of the AuNPs shows that the interplanar spacing of the particle lattice is 0.23 nm, which agrees well with the (111) lattice spacing of facecentered cubic (fcc) Au. Furthermore, the energy dispersive spectroscopy taken on the nanoparticles (part D of Figure 1) reveals the existence of Au element in the composite, whereas Cu element results from the copper grids. The AuNPs/GO nanocomposites were also characterized by the XRD spectrum as shown in Figure 2. The peaks at 38.37°, 44.37°, 64.83°, and 77.80° are assigned to fcc buck gold (111), (200), (220), and (311) respectively, which is in agreement with the standard values of gold (JCPDS 04−0784). The spectrum also shows the characteristic peak of GO at 10.7°. Thus, the results of XRD 12954

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products (3-aminophthalate), respectively. The ion at m/z 178.8 corresponds to lucigenin (Scheme S1 of the Supporting Information). Part A of Figure 4 shows the fluorescence

Figure 4. Fluorescence excitation and emission spectra (A) of lucigenin (black line) and luminol (red line), fluorescence excitation and emission spectra (B) of AuNPs/GO nanocomposites.

excitation and emission spectra of lucigenin and luminol. It is observed that the maximum excitation and emission wavelengths of lucigenin are around 364 and 479 nm, respectively, while the maximum excitation and emission wavelengths of luminol locate near 353 and 425 nm, respectively. The two couples of peaks are also observed in the solution of AuNPs/ GO nanocomposites (part B of Figure 4). Moreover, the synchronous fluorescence spectrum of AuNPs/GO nanocomposites is also obtained by scanning simultaneously the excitation and emission monochromators with the wavelength interval Δλ = 150 nm. The synchronous fluorescence spectrum (Figure S3 of the Supporting Information) shows two peaks at 250 and 288 nm, which are ascribed to the excitations from luminol and lucigenin, respectively. These results indicate that the nanocomposites are successfully functionalized by luminol and lucigenin. On the basis of the experimental results above, we proposed the formation mechanism of AuNPs/GO nanocomposites as follows. Lucigenin was first immobilized on the surface of GO by π−π stacking, which altered the surface charge of GO from negative to positive. When AuNPs precursor (AuCl4−) aqueous solution mixed with positively charged lucigenin functionalized GO, strong electrostatic attraction occurred between them. AuCl4− was anchored on the surface of lucigenin functionalized GO via electrostatic force. Subsequently, luminol was introduced as a reductant to in situ reduce AuCl4− to AuNPs at room temperature and luminol and its oxide product (3aminophthalate) coexited on the surface of AuNPs via Au−N covalent interaction. Finally, the nanocomposites functionalized by luminol and lucigenin were obtained. In summary, lucigenin molecules and AuNPs were located at the surface of GO, and luminol existed on the surface of AuNPs by virtue of Au−N covalent interaction in the as-prepared nanocomposites. Because of the presence of two kinds of classic CL reagents, lucigenin and luminol, the CL property of the AuNPs/GO nanocomposites was investigated by the static injection method. As it well-known, both luminol and lucigenin can react with hydrogen peroxide giving rise to CL emission. Therefore, hydrogen peroxide was used to initiate the CL of AuNPs/GO nanocomposites. When the nanocomposites solution was mixed with hydrogen peroxide solution, CL response was observed (Figure 5). The CL spectrum for AuNPs/GO nanocomposites−H2O2 system displays two peaks centered at 425 and 490 nm (insert in Figure 5) indicating that the original luminophors are excited-state 3-aminophthalate anion from the luminol system (part A of Figure S4 of the

Figure 1. TEM images of AuNPs/GO nanocomposites obtained with the use of 1.0 mL (A), 1.5 mL (B), and 2.0 mL (C) of HAuCl4; EDS spectrum (D) of the resulting AuNPs/GO nanocomposites.

Figure 2. XRD pattern of AuNPs/GO nanocomposites.

further demonstrate the successful synthesis of AuNPs/GO nanocomposites. In our previous work, it was demonstrated that luminol could reduce HAuCl4 to AuNPs at boiling temperature. It was found that luminol and its oxide product (3-aminophthalate) coexited on the surface of AuNPs via Au−N covalent interaction.14 Hence, in the present work, luminol and lucigenin may coexit on the surface of AuNPs/GO nanocomposites. Mass spectrum and fluorescence spectra are employed to validate this assumption. Figure 3 displays the mass spectrum of the asprepared AuNPs/GO nanocomposites. The peaks that appear at m/z 175.9 and 183.9 correspond to the luminol and its oxide

Figure 3. Mass spectrum of AuNPs/GO nanocomposites. 12955

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Figure 5. CL kinetic curves for the reaction of AuNPs/GO nanocomposites with different size AuNPs with H2O2. Insert: CL spectrum for the AuNPs/GO nanocomposites−H2O2 system.

Figure 6. CL kinetic curves for the reaction of AuNPs/GO nanocomposites with different size AuNPs with AgNO3. Insert: (a) the magnification of CL kinetic curves of as-prepared nanocomposites with 11 and 18 nm AuNPs, respectively; (b) CL spectrum for AuNPs/ GO nanocomposites−AgNO3 system.

Supporting Information) and N-methylacridone from the lucigenin system (part B of Figure S4), respectively.15 The result demonstrated that AuNPs/GO nanocomposites−H2O2 system could produce the CL with dual-wavelength. Interestingly, the CL intensity is tunable via the control of the size of AuNPs on the surface of GO and increases with the increase of AuNPs diameters (Figure 5). The size-related CL activity of the nanocomposites is ascribed to the increase in electron density with increasing the size of AuNPs, which is agreement with literature reports.16 The CL mechanism for the nanocomposites−H2O2 system is involved in the CL of luminol and lucigenin with H2O2. It was reported that AuNPs could catalyze the CL of the luminol−H2O2 system. Accordingly, the CL emission at 425 nm is from the luminol−H2O2−AuNPs CL system and the major CL-generating mechanism follows the pathways proposed by our group previously:16 (1) H2O2 was absorbed by AuNPs and broken to form HO· radicals; (2) the HO· radicals reacted with luminol and HO2− to facilitate the formation of luminol radicals and superoxide radical anion; (3) further electron-transfer processes between luminol radicals and superoxide radical anion produced CL emission. For the lucigenin−H2O2 CL system, AuNPs as catalysts seem to be inefficient.17 The CL emission at 490 nm may be due to that lucigenin was oxidized by H2O2 to yield an unstable dioxetanetype intermediate and the decomposition of this intermediate gave the light emission.15 Furthermore, it was also found that the AuNPs/GO nanocomposites could react with silver nitrate resulting in CL emission (Figure 6). The CL spectrum of this system was measured as shown in Figure 6 (insert). The spectrum showed a broad peak around 425 nm suggesting that the CL was initiated by luminol molecules on the surface of AuNPs/GO nanocomposites. The CL intensity of this system also depends on the size of AuNPs. The AuNPs/GO nanocomposites with small size of AuNPs show obviously higher CL intensity than their counterparts (Figure 6). This may be due to that the AuNPs of the nanocomposites with small size have large surface and high particle concentration leading to higher CL intensity.18 For the CL generation of the luminol−AgNO3 system, AuNPs as catalysts are important. The CL may proceed following the CL mechanism of the luminol− AgNO3−AuNPs system.18 Luminol was oxidized by AgNO3 by virtue of catalysis of AuNPs to produce luminol radical, which reacted with the dissolved oxygen resulting in CL emission.

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CONCLUSIONS In conclusion, we have for the first time proposed a facile, fast, and reliable method for preparing luminol and lucigenin bifunctionalized AuNPs/GO nanocomposites via absorption of AuCl4− by positively charged lucigenin functionalized GO and the subsequent in situ reduction of AuCl4− by luminol at room temperature. TEM, high-resolution TEM, EDS, XRD, and mass and fluorescence spectra illustrated that the nancomposites were successfully prepared. The diameters of the AuNPs of the nanocomposites could be tunable from 8 to 11 nm and 18 nm with the increase of HAuCl4 amount. Lucigenin molecules and AuNPs were located at the surface of GO, and luminol existed on the surface of AuNPs by virtue of Au−N covalent interaction in the nanocomposites. The obtained nanocomposites can provide dual-wavelength CL emission due to the reaction of hydrogen peroxide with luminol and lucigenin, respectively. Also, the nanocomposites could also react with silver nitrate giving rise to CL emission. Moreover, they also exhibited fluorescence properties. This novel nanocomposites combine the advantageous properties of GO, AuNPs, luminol, and lucigenin resulting in large area, excellent biocompatibility, good and dual-wavelength CL activity, which can be considered as not only functionalized materials but also a promising platform for multipurpose sensing and bioassays.

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ASSOCIATED CONTENT

S Supporting Information *

Cleavage mechanism of lucigenin molecule in the mass spectrometer, molecular formulas of luminol and lucigenin, synchronous fluorescence spectra of AuNPs/GO nanocomposites, CL spectra for luminol−H2O2 and lucigenin−H2O2 systems, and high-resolution TEM image of AuNPs/GO nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 12956

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of P. R. China (Grant Nos. 21173201, 21075115, 20625517 and 20573101), the Fundamental Research Funds for the Central Universities (Grant Nos.WK2060190007), and the Overseas Outstanding Young Scientist Program of the Chinese Academy of Sciences.

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