Letter pubs.acs.org/ac
Gold Nanoparticles Bifunctionalized by Chemiluminescence Reagent and Catalyst Metal Complexes: Synthesis and Unique Chemiluminescence Property Mengxiao Liu, Hongli Zhang, Jiangnan Shu, Xiaoyang Liu, Fang Li, and Hua Cui* CAS Key Laboratory of Soft Matter Chemistry, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China S Supporting Information *
ABSTRACT: Despite much progress in functionalized gold nanomaterial (GNMs), chemiluminescent (CL) functionalized GNMs with high CL efficiency are far from fully developed. In this work, we report a general strategy for the synthesis of gold nanoparticles (GNPs) bifunctionalized by CL reagent and catalyst metal complexes (BF-GNPs) by taking N-(aminobutyl)-N-(ethylisoluminol) (ABEI) as a model of CL reagents. The complexes of 2-[bis[2-[carboxymethyl-[2-oxo-2-(2sulfanylethylamino)ethyl]amino]ethyl]amino]acetic acid (DTDTPA) with various metal ions, including Co2+, Cu2+, Pb2+, Ni2+, Hg2+, Cr3+, Eu3+, La3+, Gd3+, Sm3+, Er3+, Dy3+, Ce4+, and Ce3+, were grafted on the surface of ABEI functionalized GNPs (ABEI-GNPs) to form a series of BF-GNPs. These BF-GNPs exhibited excellent CL activity. In particular, the CL intensity of DTDTPA/Co2+-ABEI-GNPs was over 3 orders of magnitude higher than ABEI-GNPs. This work demonstrates for the first time that metal complexes grafted on the surface of GNPs have unique catalytic activity on the CL reaction, superior to that in the liquid phase. Such BF-GNPs may find future applications in bioassays, microchips, and molecular/cellular imaging.
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exhibited good CL activities. These functionalized gold nanomaterials have been successfully used as nanoprobes and nanointerfaces for immunoassays and DNA assays.18−20 However, CL functionalized gold nanomaterials with high CL efficiency are far from fully developed and cannot meet the requirement for the determination of analytes at ultratrace amounts. Since CL approaches have become increasingly important in clinical, pharmaceutical, environmental, and food safety fields due to its high sensitivity, wide linear range, and simple instrumentation,21 CF-GNMs with excellent CL activity are highly desired in order to achieve extremely high sensitivity. Metal ions, especially transition metal ions (e.g., Co(II), Fe(II), Cu(II)), or metal complexes (e.g., ferrum(II)/ethylenediaminetetraacetic acid, cobalt(II)/phthalocyanine, cobalt(II)/meso-tetrakis(4-sulfonatophenyl)porphine) have strong catalytic effect on various CL systems, such as luminol-H2O2, ABEI-H2O2, and lucignin-H2O2, dramatically improving CL intensity.22−25 Analytical applications and catalytic mechanisms have been studied for decades.26−34 However, the concept of grafting catalyst metal ions on the surface of CF-GNPs to promote new self-assembling features and to achieve new functional properties has not been reported, to the best of our knowledge. It is believed that the catalytic effect or enhance-
unctional nanomaterials have attracted considerable interest motivated by their promising applications ranging from chemistry to life sciences, materials, nanosciences, engineering sciences, and environmental sciences.1−4 Gold nanomaterials (GNMs) possess excellent stability, good biocompatibility, ease of self-assembly and unique optical, catalytic, redox-reactive, electrochemical, and surface properties.5 Combining these features with the intrinsic functionalities of various molecules will lead to the birth of novel hybrid materials. To date, a series of functionalized gold nanomaterials with novel magnetic, surface-enhance Raman scattering, localized surface plasmon resonance, fluorescent, phosphorescent, catalytic, and biological properties have been successfully synthesized.6−12 However, few studies have been reported regarding GNMs with novel chemiluminescent (CL) property. Xing and co-workers connected CL reagent Ru(II) complex with gold nanoparticles via a bridge molecule of DNA to form Ru(II) complex functionalized gold nanoparticles and applied them as labels for bioassays and biosensors.13,14 In our previous work, direct synthesis strategies were developed for the preparation of CL functionalized gold nanomaterials (CFGNMs) through one-pot or seed growth methods.15−17 CL reagents, such as luminol and N-(aminobutyl)-N-(ethylisoluminol) (ABEI), could directly reduce HAuCl4 in aqueous solution to form CF-GNMs. During synthetic process, a great number of the CL reagent molecules as stabilizers were directly coated on the surface of GNMs so that these CF-GNMs © 2014 American Chemical Society
Received: January 19, 2014 Accepted: March 3, 2014 Published: March 4, 2014 2857
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ment by some substances of the luminol and its analogue CL reactions is due to that catalysts or enhancers greatly simulate the generation of oxygen-related radicals such as OH•, O2•−, and other radical derivatives, accelerating the CL reactions.35,36 Gold nanoparticle is an ideal nanosized platform for the generation of oxygen-related free radicals and radical-involved CL reactions.36 Therefore, by virtue of gold nanoparticle as a platform, if both of CL reagent and catalyst metal ions are coated on the surface of gold nanoparticles, GNMs with unique CL features might be obtained. Herein, we report a general strategy for the synthesis of gold nanoparticles bifunctionalized by CL reagent and catalyst metal complex (BF-GNPs) and their novel CL properties by taking ABEI as a model of CL reagent. It has been reported that Gd3+ chelates such as Gd3+/ 2-[bis[2-[carboxymethyl-[2-oxo-2-(2-sulfanylethylamino)ethyl]amino]ethyl]amino]acetic acid (DTDTPA) were successfully grafted to GNPs by the Au−S bond in the study of contrast agents for magnetic resonance imaging (MRI).37−41 Therefore, DTDTPA was chosen as a chelator because of its outstanding and far-ranging chelating ability, ease of assembly with GNPs via the Au−S bond, as well as easy and simple synthesis and purification procedures. The studied metal ions include transition metal ions Co2+, Cu2+, Pb2+, Ni2+, Hg2+, Cr3+, and rare earth metal ions Eu3+, La3+, Gd3+, Sm3+, Er3+, Dy3+, Ce4+, and Ce3+. A schematic illustration for the synthesis of the BF-GNPs is shown in Figure 1. To start with, ABEI functionalized GNPs
complexes were grafted to the surface of ABEI-GNPs by adding DTDTPA/Co2+ complex aqueous solution to ABEI functionalized GNPs colloid (Figure 1c). After stirring overnight, a dark purple color solution was obtained. The resulting solution was centrifuged at a speed of 12 500 rpm for 15 min. After the supernatant was removed, the soft precipitant was dispersed in 24 mL of water or NaOH solution to gain the purple GNPs bifunctionalized by ABEI and metal complexes. Figure 2a showed transmission electron microscopy (TEM) images of DTDTPA/Co2+-ABEI-GNPs, confirming the formation of GNPs. The DTDTPA/Co2+-ABEI-GNPs are monodispersed sphericity with the diameter of 22.5 ± 2.2 nm, which is similar to the diameter of ABEI-GNPs (22.0 ± 2.2 nm). Since monomolecular layer modification normally is undetectable by TEM, the results reflect the size of the gold core. Moreover, the DTDTPA/Co2+-ABEI-GNPs (Figure 2a) had a better dispersibility than ABEI-GNPs (Figure 2b) as the DTDTPA/Co2+ complexes are negatively charged, increasing the electrostatic repulsion of GNPs. The surface composition of the DTDTPA/Co2+-ABEI-GNPs was studied by UV−visible spectra (Figure S4 in the Supporting Information) and X-ray photoelectron spectroscopy (XPS, Figure S5 in the Supporting Information). The results showed that ABEI, its oxidized product N-(aminobutyl)-N(ethylphthalate) as well as DTDTPA/Co2+ complex coexisted on surface of BF-GNPs by the Au−N and the Au−S bond, respectively. Considering the fact that DTDTPA/Co2+-ABEI-GNPs contain CL reagent ABEI molecules and catalyst DTDTPA/ Co2+ complexes, it may open up a novel class of functionalized gold nanomaterials with CL activity. The CL behaviors of DTDTPA/Co2+-ABEI-GNPs were studied by static injection on a microplate luminometer as shown in Figure 3. When 50 μL of 0.1 M H2O2 aqueous solution was injected to 90 μL of NaOH-adjusted pH = 12 BF-GNPs dispersion in a microwell, strong CL emission was observed as shown in Figure 3a. The CL intensity of DTDTPA/Co2+-ABEI-GNPs was over 3 orders of magnitude higher than ABEI-GNPs (1333 times) (Figure 3e). Although the CL intensity of DTDTPA grafted ABEIGNPs without metal ions (Figure 3b) and ABEI-GNPs containing Co2+ (Figure 3d) was also enhanced by 67 and 3 times, respectively, compared with ABEI-GNPs, their CL intensities are much weaker than that of DTDTPA/Co2+-ABEIGNPs. The CL intensity of DTDTPA/Co2+-ABEI-GNPs was 20 and 500 times of DTDTPA grafted ABEI-GNPs without Co2+ and ABEI-GNPs containing Co2+, respectively. Diethylene triamine pentacetate acid (DTPA) is the precursor of DTDTPA without −SH group, DTPA/Co2+ complexes would be difficult to be grafted on the surface of ABEI-GNPs due to the lack of −SH and exist in a solution. Thus the CL intensity of DTDTPA/Co2+-ABEI-GNPs was also compared with ABEI-GNPs containing DTPA/Co2+ complexes not immobilized on the surface of ABEI-GNPs (Figure 3c). The CL intensity of the former is 164 times of the later. As DTPA/ Co2+ complexes were almost removed after the centrifugationdispersion procedure, the CL intensity was the same as that of ABEI-GNPs (Figure 3f), indicating that DTPA/Co2+ complexes existed in the solution, rather than on the surface of ABEI-GNPs. These results indicated that the chelator DTDTPA, Co2+ and DTPA/Co2+ complexes not immobilized on the surface of ABEI-GNPs could catalyze the CL reaction to some extent, but their catalytic ability was much weaker than DTDTPA/Co2+ immobilized on the surface of ABEI-GNPs.
Figure 1. Schematic Illustration for synthesis of (a) ABEI-GNPs, (b) DTDTPA/Co2+, and (c) BF-GNPs.
(ABEI-GNPs) were prepared as described previously, by reducing HAuCl4 with ABEI in aqueous solution at room temperature through a seed growth method (Figure 1a).17 The synthesis of chelator DTDTPA was carried out according to ref 41 with some modification. Co2+ was chosen as a model metal ion. DTDTPA/Co2+ complexes were prepared by mixing 6 mM of DTDTPA aqueous solution with 6 mM of cobalt chloride solution with the same volume (Figure 1b). DTDTPA/Co2+ 2858
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Figure 2. TEM images of (a) BF-GNPs and (b) ABEI-GNPs. The inset is the histogram of size distribution of BF-GNPs and ABEI-GNPs.
The excellent CL efficiency of DTDTPA/Co2+-ABEI-GNPs might be due to the synergistic effect of DTDTPA, Co2+ and gold nanoparticles on the ABEI-H2O2 CL system. Earlier studies showed that the CL catalysis and enhancement related to the generation of oxygen-related radicals such as OH•, O2•−, and other radical derivatives.35,36 The luminol-H2O2-Co2+ CL mechanism has been documented.25,42,43 Burdo and Seitz have proposed that Co2+ could react with H2O2 to form a highly reactive hydroxyl radical OH•, followed by the reaction with luminol in basic aqueous media to produce a luminol radical, accelerating the CL reaction.42 DTDTPA is a carboxyl-rich organic molecule. It has been reported that phenolic compounds and amino acids containing −COO- groups could enhance the CL from the luminol-H2O2-Co2+ at higher pH.43 The CL enhancement might be due to the reaction of −COO- in their molecules with O2•− generated in the CL reaction to form −CO4•2‑, which could react with luminol to facilitate the formation of luminol radicals. Resembling these compounds, the CL enhancement by DTDTPA may follow the same mechanism. It has been reported that gold nanoparticles as catalysts could facilitate radical generation and electron transfer in chemical reactions.44,45 Earlier work demonstrated that gold nanoparticles could catalyze the CL of the luminolH2O2 system. The O−O bond of H2O2 might be broken up into double HO• radicals in situ by virtue of the catalysis of gold nanoparticles, which reacted with luminol anion and HO2− to facilitate the formation of luminol radicals and O2•− on the
Figure 3. CL kinetic curves for reaction of (a) DTDTPA/Co2+-ABEIGNPs, (b) DTDTPA-ABEI-GNPs, (c and d) ABEI-GNPs with 3 mM DTPA/Co2+ and Co2+ in the solution, respectively, (e) ABEI-GNPs, and (f) centrifuged-dispersed ABEI-GNPs containing 3 mM DTPA/ Co2+ with H2O2. Inset (left): CL spectrum of DTDTPA/Co2+-ABEIGNPs -H2O2. Inset (right): magnification of curves c−f. Reaction conditions: 0.1 M H2O2, NaOH-adjusted pH = 12 GNPs dispersion.
The CL spectrum of the DTDTPA/Co2+-ABEI-GNPs- H2O2 system showed a peak centered at ∼430 nm as shown in Figure 3, inset (left), which was consistent with that of the CL reaction of ABEI with H2O2.
Figure 4. CL behaviors of reaction of various BF-GNPs with H2O2. Reaction conditions: 50 μL of 0.1 M H2O2 aqueous solution was injected to 90 μL of NaOH-adjusted pH = 12 BF-GNPs dispersion in a microwell. 2859
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surface of gold nanoparticles.36 ABEI is an analogue of luminol. It is believed that the CL reaction of ABEI with H2O2 in the presence of catalyst follow a similar mechanism. The excellent CL activity of DTDTPA/Co2+-ABEI-GNPs with H2O2 might be due to that DTDTPA, Co2+, and gold nanoparticles facilitate the formation of −CO4•2‑, O2•−, and HO• in the CL reaction and gold nanoparticles as nanosized platform promote radicalinvolved CL reaction, resulting in strong light emission. On the other hand, both ABEI and DTDTPA/Co2+ are highly concentrated on the surface of GNPs. It is also possible that the actual concentration of catalyst in the microenvironment near/on GNPs surface is much higher than that in bulk solution, leading to better catalytic efficiency. This may also contribute to such a strong light emission to some extent. Further work regarding the mechanism is needed. It has been reported that various metal ions such as transition metal ions Co2+, Cu2+, Pb2+, Ni2+, Hg2+, and Cr3+ and rare earth metal ions Eu3+, La3+, Gd3+, Sm3+, Er3+, Dy3+, Ce4+, and Ce3+ could catalyze the CL reactions of luminol and its analogues with H2O2.22 DTDTPA is also a universal chelator for such metal ions. Therefore, the proposed strategy was generalized to the synthesis of gold nanoparticles bifunctionalized by ABEI and metal complexes. DTDTPA first interacted with various metal ions to form DTDTPA/metal complexes, and then DTDTPA/metal complexes were grafted to the surface of ABEI-GNPs by the Au−S bond. The CL behaviors of various as-synthesized BF-GNPs were studied as shown in Figure 4. All the tested DTDTPA/metal complexes coated on the surface of ABEI-GNPs showed 1−3 orders of magnitude higher CL intensity than ABEI-GNPs. The CL intensity follows the order: Co2+ > Cu2+ > Er3+ > Pb2+ > Ni2+ > Hg2+ ∼ Gd3+ ∼ La3+ > Sm3+ > Er3+ > Cr3+ > Dy3+ > Ce4+ > Ce3+. DTDTPA/Co2+ABEI-GNPs exhibited the strongest CL intensity, which is in good agreement with that Co2+ has the strongest catalytic effect on the CL reactions of luminol and its analogues with H2O2.25 In summary, we report a general strategy for the synthesis of gold nanoparticles bifunctionalized by ABEI and metal complexes by simply stirring DTDTPA/metal complexes with ABEI-GNPs. The complexes of DTDTPA with various metal ions, including Co2+, Cu2+, Pb2+, Ni2+, Hg2+, Cr3+, Eu3+, La3+, Gd3+, Sm3+, Er3+, Dy3+, Ce4+, and Ce3+, were grafted on the surface of ABEI-GNPs via the Au−S bond to form a series of BF-GNPs. These BF-GNPs exhibited excellent CL activity, showing 1−3 orders of magnitude higher CL intensity than ABEI-GNPs. In particular, the CL intensity of DTDTPA/Co2+ABEI-GNPs was over 3 orders of magnitude higher than ABEIGNPs. This work demonstrates for the first time that metal complexes grafted on the surface of gold nanoparticles have unique catalytic activity on the CL reaction, superior to that in the liquid phase. It also provides a new class of functionalized gold nanomaterials with high CL efficiency. These BF-GNPs with excellent CL activity may find future applications in biosensors, bioassays, microchips, and molecular/cellular imaging. The proposed strategy might be generalized to the synthesis of gold nanoparticles bifunctionalized by other CL reagents and metal complexes. Further work is under investigation.
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and 1H NMR results; characterization of BF-GNPs including UV−visible and XPS results. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-551-63606645. Fax: +86-551-63600730. E-mail: [email protected]. Author Contributions
M.X. Liu and H.L. Zhang 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 Support of this research by the National College Students’ Innovative Training Project, the National Natural Science Foundation of P. R. China (Grant Nos. 21173201, 21075115, and 20625517), the Fundamental Research Funds for the Central Universities (Grant No.WK2060190007) and the Opening Fund of State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, CAS (grant no. SKLEAC201110) is gratefully acknowledged. We are also grateful to Tingting Wang for her help in organic synthesis.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Experimental section including chemical and solutions, synthesis of DTDTPA and BF-GNPs, and CL detection of BFGNPs; characterization of DTDTPA including ES-MS, FT-IR, 2860
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Gold Nanoparticles Bifunctionalized by Chemiluminescence Reagent and Catalyst Metal Complexes: Synthesis and Unique Chemiluminescence Property Mengxiao Liu, Hongli Zhang, Jiangnan Shu, Xiaoyang Liu, Fang Li, and Hua Cui* CAS Key Laboratory of Soft Matter Chemistry, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China S Supporting Information *
ABSTRACT: Despite much progress in functionalized gold nanomaterial (GNMs), chemiluminescent (CL) functionalized GNMs with high CL efficiency are far from fully developed. In this work, we report a general strategy for the synthesis of gold nanoparticles (GNPs) bifunctionalized by CL reagent and catalyst metal complexes (BF-GNPs) by taking N-(aminobutyl)-N-(ethylisoluminol) (ABEI) as a model of CL reagents. The complexes of 2-[bis[2-[carboxymethyl-[2-oxo-2-(2sulfanylethylamino)ethyl]amino]ethyl]amino]acetic acid (DTDTPA) with various metal ions, including Co2+, Cu2+, Pb2+, Ni2+, Hg2+, Cr3+, Eu3+, La3+, Gd3+, Sm3+, Er3+, Dy3+, Ce4+, and Ce3+, were grafted on the surface of ABEI functionalized GNPs (ABEI-GNPs) to form a series of BF-GNPs. These BF-GNPs exhibited excellent CL activity. In particular, the CL intensity of DTDTPA/Co2+-ABEI-GNPs was over 3 orders of magnitude higher than ABEI-GNPs. This work demonstrates for the first time that metal complexes grafted on the surface of GNPs have unique catalytic activity on the CL reaction, superior to that in the liquid phase. Such BF-GNPs may find future applications in bioassays, microchips, and molecular/cellular imaging.
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exhibited good CL activities. These functionalized gold nanomaterials have been successfully used as nanoprobes and nanointerfaces for immunoassays and DNA assays.18−20 However, CL functionalized gold nanomaterials with high CL efficiency are far from fully developed and cannot meet the requirement for the determination of analytes at ultratrace amounts. Since CL approaches have become increasingly important in clinical, pharmaceutical, environmental, and food safety fields due to its high sensitivity, wide linear range, and simple instrumentation,21 CF-GNMs with excellent CL activity are highly desired in order to achieve extremely high sensitivity. Metal ions, especially transition metal ions (e.g., Co(II), Fe(II), Cu(II)), or metal complexes (e.g., ferrum(II)/ethylenediaminetetraacetic acid, cobalt(II)/phthalocyanine, cobalt(II)/meso-tetrakis(4-sulfonatophenyl)porphine) have strong catalytic effect on various CL systems, such as luminol-H2O2, ABEI-H2O2, and lucignin-H2O2, dramatically improving CL intensity.22−25 Analytical applications and catalytic mechanisms have been studied for decades.26−34 However, the concept of grafting catalyst metal ions on the surface of CF-GNPs to promote new self-assembling features and to achieve new functional properties has not been reported, to the best of our knowledge. It is believed that the catalytic effect or enhance-
unctional nanomaterials have attracted considerable interest motivated by their promising applications ranging from chemistry to life sciences, materials, nanosciences, engineering sciences, and environmental sciences.1−4 Gold nanomaterials (GNMs) possess excellent stability, good biocompatibility, ease of self-assembly and unique optical, catalytic, redox-reactive, electrochemical, and surface properties.5 Combining these features with the intrinsic functionalities of various molecules will lead to the birth of novel hybrid materials. To date, a series of functionalized gold nanomaterials with novel magnetic, surface-enhance Raman scattering, localized surface plasmon resonance, fluorescent, phosphorescent, catalytic, and biological properties have been successfully synthesized.6−12 However, few studies have been reported regarding GNMs with novel chemiluminescent (CL) property. Xing and co-workers connected CL reagent Ru(II) complex with gold nanoparticles via a bridge molecule of DNA to form Ru(II) complex functionalized gold nanoparticles and applied them as labels for bioassays and biosensors.13,14 In our previous work, direct synthesis strategies were developed for the preparation of CL functionalized gold nanomaterials (CFGNMs) through one-pot or seed growth methods.15−17 CL reagents, such as luminol and N-(aminobutyl)-N-(ethylisoluminol) (ABEI), could directly reduce HAuCl4 in aqueous solution to form CF-GNMs. During synthetic process, a great number of the CL reagent molecules as stabilizers were directly coated on the surface of GNMs so that these CF-GNMs © 2014 American Chemical Society
Received: January 19, 2014 Accepted: March 3, 2014 Published: March 4, 2014 2857
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Letter
ment by some substances of the luminol and its analogue CL reactions is due to that catalysts or enhancers greatly simulate the generation of oxygen-related radicals such as OH•, O2•−, and other radical derivatives, accelerating the CL reactions.35,36 Gold nanoparticle is an ideal nanosized platform for the generation of oxygen-related free radicals and radical-involved CL reactions.36 Therefore, by virtue of gold nanoparticle as a platform, if both of CL reagent and catalyst metal ions are coated on the surface of gold nanoparticles, GNMs with unique CL features might be obtained. Herein, we report a general strategy for the synthesis of gold nanoparticles bifunctionalized by CL reagent and catalyst metal complex (BF-GNPs) and their novel CL properties by taking ABEI as a model of CL reagent. It has been reported that Gd3+ chelates such as Gd3+/ 2-[bis[2-[carboxymethyl-[2-oxo-2-(2-sulfanylethylamino)ethyl]amino]ethyl]amino]acetic acid (DTDTPA) were successfully grafted to GNPs by the Au−S bond in the study of contrast agents for magnetic resonance imaging (MRI).37−41 Therefore, DTDTPA was chosen as a chelator because of its outstanding and far-ranging chelating ability, ease of assembly with GNPs via the Au−S bond, as well as easy and simple synthesis and purification procedures. The studied metal ions include transition metal ions Co2+, Cu2+, Pb2+, Ni2+, Hg2+, Cr3+, and rare earth metal ions Eu3+, La3+, Gd3+, Sm3+, Er3+, Dy3+, Ce4+, and Ce3+. A schematic illustration for the synthesis of the BF-GNPs is shown in Figure 1. To start with, ABEI functionalized GNPs
complexes were grafted to the surface of ABEI-GNPs by adding DTDTPA/Co2+ complex aqueous solution to ABEI functionalized GNPs colloid (Figure 1c). After stirring overnight, a dark purple color solution was obtained. The resulting solution was centrifuged at a speed of 12 500 rpm for 15 min. After the supernatant was removed, the soft precipitant was dispersed in 24 mL of water or NaOH solution to gain the purple GNPs bifunctionalized by ABEI and metal complexes. Figure 2a showed transmission electron microscopy (TEM) images of DTDTPA/Co2+-ABEI-GNPs, confirming the formation of GNPs. The DTDTPA/Co2+-ABEI-GNPs are monodispersed sphericity with the diameter of 22.5 ± 2.2 nm, which is similar to the diameter of ABEI-GNPs (22.0 ± 2.2 nm). Since monomolecular layer modification normally is undetectable by TEM, the results reflect the size of the gold core. Moreover, the DTDTPA/Co2+-ABEI-GNPs (Figure 2a) had a better dispersibility than ABEI-GNPs (Figure 2b) as the DTDTPA/Co2+ complexes are negatively charged, increasing the electrostatic repulsion of GNPs. The surface composition of the DTDTPA/Co2+-ABEI-GNPs was studied by UV−visible spectra (Figure S4 in the Supporting Information) and X-ray photoelectron spectroscopy (XPS, Figure S5 in the Supporting Information). The results showed that ABEI, its oxidized product N-(aminobutyl)-N(ethylphthalate) as well as DTDTPA/Co2+ complex coexisted on surface of BF-GNPs by the Au−N and the Au−S bond, respectively. Considering the fact that DTDTPA/Co2+-ABEI-GNPs contain CL reagent ABEI molecules and catalyst DTDTPA/ Co2+ complexes, it may open up a novel class of functionalized gold nanomaterials with CL activity. The CL behaviors of DTDTPA/Co2+-ABEI-GNPs were studied by static injection on a microplate luminometer as shown in Figure 3. When 50 μL of 0.1 M H2O2 aqueous solution was injected to 90 μL of NaOH-adjusted pH = 12 BF-GNPs dispersion in a microwell, strong CL emission was observed as shown in Figure 3a. The CL intensity of DTDTPA/Co2+-ABEI-GNPs was over 3 orders of magnitude higher than ABEI-GNPs (1333 times) (Figure 3e). Although the CL intensity of DTDTPA grafted ABEIGNPs without metal ions (Figure 3b) and ABEI-GNPs containing Co2+ (Figure 3d) was also enhanced by 67 and 3 times, respectively, compared with ABEI-GNPs, their CL intensities are much weaker than that of DTDTPA/Co2+-ABEIGNPs. The CL intensity of DTDTPA/Co2+-ABEI-GNPs was 20 and 500 times of DTDTPA grafted ABEI-GNPs without Co2+ and ABEI-GNPs containing Co2+, respectively. Diethylene triamine pentacetate acid (DTPA) is the precursor of DTDTPA without −SH group, DTPA/Co2+ complexes would be difficult to be grafted on the surface of ABEI-GNPs due to the lack of −SH and exist in a solution. Thus the CL intensity of DTDTPA/Co2+-ABEI-GNPs was also compared with ABEI-GNPs containing DTPA/Co2+ complexes not immobilized on the surface of ABEI-GNPs (Figure 3c). The CL intensity of the former is 164 times of the later. As DTPA/ Co2+ complexes were almost removed after the centrifugationdispersion procedure, the CL intensity was the same as that of ABEI-GNPs (Figure 3f), indicating that DTPA/Co2+ complexes existed in the solution, rather than on the surface of ABEI-GNPs. These results indicated that the chelator DTDTPA, Co2+ and DTPA/Co2+ complexes not immobilized on the surface of ABEI-GNPs could catalyze the CL reaction to some extent, but their catalytic ability was much weaker than DTDTPA/Co2+ immobilized on the surface of ABEI-GNPs.
Figure 1. Schematic Illustration for synthesis of (a) ABEI-GNPs, (b) DTDTPA/Co2+, and (c) BF-GNPs.
(ABEI-GNPs) were prepared as described previously, by reducing HAuCl4 with ABEI in aqueous solution at room temperature through a seed growth method (Figure 1a).17 The synthesis of chelator DTDTPA was carried out according to ref 41 with some modification. Co2+ was chosen as a model metal ion. DTDTPA/Co2+ complexes were prepared by mixing 6 mM of DTDTPA aqueous solution with 6 mM of cobalt chloride solution with the same volume (Figure 1b). DTDTPA/Co2+ 2858
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Figure 2. TEM images of (a) BF-GNPs and (b) ABEI-GNPs. The inset is the histogram of size distribution of BF-GNPs and ABEI-GNPs.
The excellent CL efficiency of DTDTPA/Co2+-ABEI-GNPs might be due to the synergistic effect of DTDTPA, Co2+ and gold nanoparticles on the ABEI-H2O2 CL system. Earlier studies showed that the CL catalysis and enhancement related to the generation of oxygen-related radicals such as OH•, O2•−, and other radical derivatives.35,36 The luminol-H2O2-Co2+ CL mechanism has been documented.25,42,43 Burdo and Seitz have proposed that Co2+ could react with H2O2 to form a highly reactive hydroxyl radical OH•, followed by the reaction with luminol in basic aqueous media to produce a luminol radical, accelerating the CL reaction.42 DTDTPA is a carboxyl-rich organic molecule. It has been reported that phenolic compounds and amino acids containing −COO- groups could enhance the CL from the luminol-H2O2-Co2+ at higher pH.43 The CL enhancement might be due to the reaction of −COO- in their molecules with O2•− generated in the CL reaction to form −CO4•2‑, which could react with luminol to facilitate the formation of luminol radicals. Resembling these compounds, the CL enhancement by DTDTPA may follow the same mechanism. It has been reported that gold nanoparticles as catalysts could facilitate radical generation and electron transfer in chemical reactions.44,45 Earlier work demonstrated that gold nanoparticles could catalyze the CL of the luminolH2O2 system. The O−O bond of H2O2 might be broken up into double HO• radicals in situ by virtue of the catalysis of gold nanoparticles, which reacted with luminol anion and HO2− to facilitate the formation of luminol radicals and O2•− on the
Figure 3. CL kinetic curves for reaction of (a) DTDTPA/Co2+-ABEIGNPs, (b) DTDTPA-ABEI-GNPs, (c and d) ABEI-GNPs with 3 mM DTPA/Co2+ and Co2+ in the solution, respectively, (e) ABEI-GNPs, and (f) centrifuged-dispersed ABEI-GNPs containing 3 mM DTPA/ Co2+ with H2O2. Inset (left): CL spectrum of DTDTPA/Co2+-ABEIGNPs -H2O2. Inset (right): magnification of curves c−f. Reaction conditions: 0.1 M H2O2, NaOH-adjusted pH = 12 GNPs dispersion.
The CL spectrum of the DTDTPA/Co2+-ABEI-GNPs- H2O2 system showed a peak centered at ∼430 nm as shown in Figure 3, inset (left), which was consistent with that of the CL reaction of ABEI with H2O2.
Figure 4. CL behaviors of reaction of various BF-GNPs with H2O2. Reaction conditions: 50 μL of 0.1 M H2O2 aqueous solution was injected to 90 μL of NaOH-adjusted pH = 12 BF-GNPs dispersion in a microwell. 2859
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surface of gold nanoparticles.36 ABEI is an analogue of luminol. It is believed that the CL reaction of ABEI with H2O2 in the presence of catalyst follow a similar mechanism. The excellent CL activity of DTDTPA/Co2+-ABEI-GNPs with H2O2 might be due to that DTDTPA, Co2+, and gold nanoparticles facilitate the formation of −CO4•2‑, O2•−, and HO• in the CL reaction and gold nanoparticles as nanosized platform promote radicalinvolved CL reaction, resulting in strong light emission. On the other hand, both ABEI and DTDTPA/Co2+ are highly concentrated on the surface of GNPs. It is also possible that the actual concentration of catalyst in the microenvironment near/on GNPs surface is much higher than that in bulk solution, leading to better catalytic efficiency. This may also contribute to such a strong light emission to some extent. Further work regarding the mechanism is needed. It has been reported that various metal ions such as transition metal ions Co2+, Cu2+, Pb2+, Ni2+, Hg2+, and Cr3+ and rare earth metal ions Eu3+, La3+, Gd3+, Sm3+, Er3+, Dy3+, Ce4+, and Ce3+ could catalyze the CL reactions of luminol and its analogues with H2O2.22 DTDTPA is also a universal chelator for such metal ions. Therefore, the proposed strategy was generalized to the synthesis of gold nanoparticles bifunctionalized by ABEI and metal complexes. DTDTPA first interacted with various metal ions to form DTDTPA/metal complexes, and then DTDTPA/metal complexes were grafted to the surface of ABEI-GNPs by the Au−S bond. The CL behaviors of various as-synthesized BF-GNPs were studied as shown in Figure 4. All the tested DTDTPA/metal complexes coated on the surface of ABEI-GNPs showed 1−3 orders of magnitude higher CL intensity than ABEI-GNPs. The CL intensity follows the order: Co2+ > Cu2+ > Er3+ > Pb2+ > Ni2+ > Hg2+ ∼ Gd3+ ∼ La3+ > Sm3+ > Er3+ > Cr3+ > Dy3+ > Ce4+ > Ce3+. DTDTPA/Co2+ABEI-GNPs exhibited the strongest CL intensity, which is in good agreement with that Co2+ has the strongest catalytic effect on the CL reactions of luminol and its analogues with H2O2.25 In summary, we report a general strategy for the synthesis of gold nanoparticles bifunctionalized by ABEI and metal complexes by simply stirring DTDTPA/metal complexes with ABEI-GNPs. The complexes of DTDTPA with various metal ions, including Co2+, Cu2+, Pb2+, Ni2+, Hg2+, Cr3+, Eu3+, La3+, Gd3+, Sm3+, Er3+, Dy3+, Ce4+, and Ce3+, were grafted on the surface of ABEI-GNPs via the Au−S bond to form a series of BF-GNPs. These BF-GNPs exhibited excellent CL activity, showing 1−3 orders of magnitude higher CL intensity than ABEI-GNPs. In particular, the CL intensity of DTDTPA/Co2+ABEI-GNPs was over 3 orders of magnitude higher than ABEIGNPs. This work demonstrates for the first time that metal complexes grafted on the surface of gold nanoparticles have unique catalytic activity on the CL reaction, superior to that in the liquid phase. It also provides a new class of functionalized gold nanomaterials with high CL efficiency. These BF-GNPs with excellent CL activity may find future applications in biosensors, bioassays, microchips, and molecular/cellular imaging. The proposed strategy might be generalized to the synthesis of gold nanoparticles bifunctionalized by other CL reagents and metal complexes. Further work is under investigation.
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and 1H NMR results; characterization of BF-GNPs including UV−visible and XPS results. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-551-63606645. Fax: +86-551-63600730. E-mail: [email protected]. Author Contributions
M.X. Liu and H.L. Zhang 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 Support of this research by the National College Students’ Innovative Training Project, the National Natural Science Foundation of P. R. China (Grant Nos. 21173201, 21075115, and 20625517), the Fundamental Research Funds for the Central Universities (Grant No.WK2060190007) and the Opening Fund of State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, CAS (grant no. SKLEAC201110) is gratefully acknowledged. We are also grateful to Tingting Wang for her help in organic synthesis.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental section including chemical and solutions, synthesis of DTDTPA and BF-GNPs, and CL detection of BFGNPs; characterization of DTDTPA including ES-MS, FT-IR, 2860
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