Highly luminescent near-infrared-emitting gold nanoclusters with further natural etching: photoluminescence and Hg2+ detection
- Shuhong Lian†1, 2,
- Dehong Hu†2,
- Changchun Zeng1,
- Pengfei Zhang2,
- Songhao Liu1 and
- Lintao Cai2Email author
© Lian et al.; licensee Springer. 2012
Received: 29 March 2012
Accepted: 18 June 2012
Published: 27 June 2012
Highly luminescent near-infrared (NIR)-emitting gold nanoclusters (Au NCs) protected by glutathione with ultra-small size were prepared at high temperature following with a further natural etching at room temperature. The optical and surface properties of Au NCs were monitored by ultraviolet–visible and photoluminescence (PL) spectra, high-resolution transmission electron microscopy, and electrospray ionization mass spectrometry. The diameter of the etched Au NCs was reduced to approximately 1.35 nm with 30 % PL quantum yield. Interestingly, the PL of Au NCs was decreased obviously by the addition of Hg2+ and increased by the addition of Pb2+ at certain concentration. Our preliminary results illustrated that the highly luminescent NIR-emitting Au NCs would be an alternative probe for the detection of heavy metal ions in water and environmental monitoring.
Gold nanoclusters (Au NCs) have received extensive attention because of their interesting optical properties[1–3], fluorescence[4, 5], magnetism, and redox properties, as well as their potential applications in many fields such as optics[8–10] and catalysis[11, 12]. To synthesize these highly fluorescent Au NCs, several methods have been developed. A method of synthesizing Au NCs is based on the reducing Au3+ ions in the presence of thiol ligands. For example, Dyer et al. exemplified the application of this approach by synthesizing Au NCs encapsulated by polyamidoamine dendrimers. Ying and coworkers developed a new method of Au NC synthesis using protein bovine serum albumin (BSA) as sole reduction agent at high pH. The other method of synthesizing Au NCs is based on the etching process using polymers. For example, Duan and Nie report a ligand-induced etching process for preparing highly fluorescent and water-soluble Au NCs. Jin et al. found that gold nanoparticles could be etched to produce ultrasmall clusters under reflux at high temperature. Inspired by this discovery, we wonder whether the etching method can be applied to using thiol ligands other than polymers.
Glutathione (GSH), the most abundant low-molecular weight thiol with naturally defined chemical components and structures, has been extensively used as ligands to induce the nucleation and growth of nanocrystals based on biomineralization or as templates to synthesize nanocrystals with well-defined morphologies. Au NCs, protected with GSH, have been well studied[17, 18]. Recently, Shichibu et al. discovered that some GSH-protected clusters could be etched by GSH, which generated GSH-protected Au25 as the main product[3, 19]. However, the quantum yield of these reported gold nanoclusters is relatively low. Hence, a method of creating Au NCs with monodispersed, ultra-small size, high quantum yield and near-infrared (NIR) emission would be extremely valuable for extending their fundamental properties and investigation of a wider range of applications.
In this work, we reported a novel synthetic method, based on the capability of a common commercially available thiol ligands, GSH, for the preparation of ultra-small size highly luminescent Au NCs at high temperature following with a further natural etching at RT with NIR emission (λem max = 650 nm, quantum yield (QY) ≈ 30 %). Our process is similar to the biomineralization behavior of organisms in nature: sequestering and interacting with organisms, followed by providing scaffolds for the minerals formed. The optical and structural properties of the etched Au NCs were performed by ultraviolet–visible (UV–vis) and photoluminescence (PL) spectroscopies, high-resolution transmission electron microscopy (HRTEM), and electrospray ionization mass spectrometry. The PL of the etched Au NCs was decreased obviously by the addition of Hg2+ and increased by the addition Pb2+ at certain concentration. Such etched Au NCs have great potential application in heavy metal ion detection in water and environmental monitoring.
Chloroauric acid trihydrate (HAuCl4·3H2O, 99.99 %) was purchased from Alfa Aesar (Ward Hill, MA, USA). Sodium tetrahydroborate (NaBH4, 99 %) and GSH in the reduced (GSH, 98 %) form were obtained from ACROS ORGANICS (NJ, USA). All other chemicals used in this study were of analytical reagent grade and used without further purification. A JL RO100 Millipore-Q Plus water purifier (Millipore Co., Billerica, MA, USA) supplied deionized water with a resisitivity of 18.25 MΩ·cm.
Synthesis of Au NCs
Au NCs were synthesized using the reported protocol, with a few modifications. To a 50 mL methanol solution (0.5 mM) of HAuCl4·3H2O, 1.0 mM GSH (1:2 molar ratio, the total volume of methanol was 50 mL) was added. The mixture was cooled to 0 °C in an ice bath for 30 min. An aqueous solution of NaBH4 (0.2 M, 12.5 mL), cooled to 0 °C, was injected rapidly into the above mixture under vigorous stirring. The mixture was allowed to react for another hour. The resulting precipitate was collected and washed repeatedly with methanol through centrifugal precipitation. Finally, the Au NCs precipitate was dried and collected as a dark brown powder.
Etching of Au NCs with GSH
The above clusters (1 mg) were dissolved in 1 mL of deionized water that contained 0.0125 mol of GSH. The mixture reacted at 65 °C ± 5 °C for 24 h at 500 rpm in the reaction container then the supernatant was put in the dark at room temperature for 5 to 8 days. The products were precipitated by added methanol (volume ratio 1:1), then the precipitate was collected and washed repeatedly with methanol through centrifugation. Finally, the products were dried by a vacuum drier and can be dissolved in water easily.
High-resolution transmission electron microscope
Au NCs dissolved in deionized water were used directly. HRTEM specimens were prepared by drop costing one or two drops of the solutions onto an ultra-thin carbon-coated film supported on a copper grid. Bright field HRTEM images were acquired with an electron microscope operated at 200 kV (JEM-2100HR, JEOL, Japan). Typical magnifications of the images were × 200,000 and × 400,000. Moreover, energy-dispersive X-ray spectroscopy (EDS) was taken three points a sample at the same time using the affix of JEM-2100HR IET250.
Optical absorption and photoluminescence spectroscopy
UV–vis absorption of the Au NCs was recorded in aqueous solution at ambient temperature using a PerkinElmer Lambda 25 spectrophotometer (PerkinElmer, Waltham, MA, USA). The PL spectra were obtained by a spectrofluorometer (F900, Edinburgh Instruments Ltd., West Livingston, UK). The sample solutions were not deaerated prior to the measurement. The cluster concentrations for the PL measurement were typically <10 μM, where the intensities increase linearly with the cluster concentrations. The PL quantum yield was determined according to.
Electrospray ionization mass spectrometry
Mass spectrometric studies were conducted using an electro spray (electrospray ionization mass spectrometry (ESI-MS)) system (LC-6A, Japan). Samples with a concentration lower than 10 ppm, taken in deionized water, were electrosprayed at a low rate of 10 μL/min and an ion spray voltage of 5 kV.
Assay of metal ions
A series concentration of aqueous Hg2+ was added to cluster solutions one by one and equilibrated at room temperature for 10 min prior to measurement of the PL with an excitation wavelength at 470 nm. All other ions (Pb2+, Cu2+, Ca2+, Mg2+, K+, and Na+) tested in interference studies using the same method.
As Hg2+ and Pb2+ are highly toxic and have adverse effects on human health, all experiments involving heavy metal ions should be performed with protective eyeglasses, chemical safety gloves, and protective clothing to prevent skin exposure. The waste solutions containing heavy metal ions should be collectively reclaimed to avoid polluting the environment.
Results and discussion
The optical property of Au NCs and its etching effect
Time-dependent-characterized optical spectra
The structural properties of Au NCs characterized by HRTEM
Element analysis of the etched Au NCs with EDS
Weight percentage (wt.%)
Atomic percentage (at.%)
ESI-MS analysis of the clusters
Effect of Hg2+ on the etched Au NCs
Possible reaction mechanisms
A PL quenching was observed in the presence of Hg2+, which could be explained in terms of electron transfer between the excited etched Au NCs to the Hg2+. The mechanism can be explained using a photoinduced electron transfer process. First, the etched Au NCs and Hg2+ ion form a complex. Second, the electrons within the etched Au NCs are firstly excited to the excited state under photo-irradiation. Third, Hg2+ can directly intercept one of the charge carriers and is reduced to Hg+, which can disrupt the radiative recombination of the holes and the excited electrons and quench the PL of the etched Au NCs. This assumption is supported by the following facts: In this test, GSH was used in competition with Au NCs for Hg2+. The PL of Au NCs was not quenched after the addition of the intermixture (1 mM Hg2+, 0.1 mM GSH). In a control experiment, GSH showed no influence on the PL of Au NCs in the absence of Hg2+, which confirmed that the complex formation occurred between GSH and Hg2+. Based on the finding, we concluded that the complex of Au NCs and Hg2+ and a photoinduced electron transfer process were the primary mechanism for PL quenching.
The reason of enhanced fluorescence in the presence of Pb2+ was very complicated. The spatial confinement of free electrons in the etched Au NCs results in discrete and size-tunable electronic transitions, leading to molecular-like properties. Therefore, the luminescent mechanism for the etched Au NCs would involve relaxed luminescence across the HOMO-LUMO gap. When the Pb2+ was added into the etched Au NCs, HOMO-LUMO gap would be decreased. Therefore, the enhanced luminescence of the etched Au NCs was observed. Due to the complicated reaction process, the precise luminescence enhancement mechanism remains unclear, and the detail reaction mechanism was under the way.
We report herein a further etching and characterization of well-defined Au NCs protected by GSH ligands. The spectroscopic measurement of these clusters revealed that a new method of etching Au NCs was developed, and high PLQY clusters were obtained. After etching, the diameter of the Au NCs decreased to approximately 1.35 nm with 30 % PLQY. These features contributed to the excellent selectivity toward Hg2+ and Pb2+, rendering the GSH-Au NCs highly suitable for the analysis of environmental samples containing other metal ions.
The study was conducted by the research team of the Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. Prof. LC is the head of the team, and the team carries out the world's leading research on a range of problems of biomedical nanotechnology (nanotechnology, functional materials, surface analytical chemistry, optoelectronics, polymer science, and electrochemistry). Our aim is dedicated to studying multifunctional and nanostructured composite materials, providing highly sensitive and selective detection methods through molecular probes for medical imaging and molecular diagnosis in nanoscale and single molecular level, and exploring new device concepts and self-assembly techniques for the development of biomedical nanodevices and sensors for biosensing, environmental monitoring, information processing, energy utility, and other applications.
The authors gratefully acknowledge the support for this research from the National Basic Research Program of China (973 program #2011CB933600), the ‘Hundred Talents Program’ of Chinese Academy of Sciences, the National Natural Science Foundation of China (grant numbers 81071249, 20875062, and 20905050), Science and Technology Key Project of Guangdong (2009A030301010), and Guangdong Innovation Team of Low-cost Healthcare.
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