Silver–Gold Alloy Nanoclusters as a Fluorescence-Enhanced Probe

The as-prepared MSA–AgAu NCs were characterized using spectroscopic and ... Probably based on the deposition of Al3+-enhanced fluorescence, the PEGy...
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Silver−Gold Alloy Nanoclusters as a Fluorescence-Enhanced Probe for Aluminum Ion Sensing Ting-yao Zhou,† Li-ping Lin,† Ming-cong Rong,† Ya-qi Jiang,† and Xi Chen*,†,‡ †

The MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China ‡ State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, Fujian 361005, China S Supporting Information *

ABSTRACT: In this paper, the development of a simple method is described for preparing highly red fluorescent mercaptosuccinic acid stabilized AgAu alloy nanoclusters (MSA−AgAu NCs) through the core etching of Ag nanoparticles (NPs) and a galvanic exchange reaction using nonorganic solvent and no multistep centrifuge washing. The as-prepared MSA−AgAu NCs were characterized using spectroscopic and microscopic techniques. After covalently attaching methoxy-poly(ethylene glycol)NH2 (m-PEG-NH2), PEGylated MSA−AgAu NCs were still stable even in 1 M NaCl. Probably based on the deposition of Al3+-enhanced fluorescence, the PEGylated MSA−AgAu NCs offered highly selective and sensitive sensing of Al3+ in aqueous solution with a detection limit of 0.8 μM.

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with the target ions. The use of bovine serum albumin (BSA)− Au NCs is reported as a chemical sensor for Hg2+,11b as is glutathione (GSH)−Au NCs for Ag+ detection,12a and also BSA−Au NCs as a fluorescent sensor for CN−.13 Chemical sensing for other ions are also reported using fluorescent Au NCs or Ag NCs, such as Pb2+, Fe3+, Cu2+, Cr3+, S2−, and I−.14 Until now, no study for the determination of Al3+ using metal NCs has been reported, and the design of a selective and sensitive metal NCs for Al3+ would be challenging research due to the low coordination ability of Al3+. Recently, a new Ag7Au6 alloy NC is prepared by interfacial etching of mercaptosuccinic acid (MSA)-stabilized Ag nanoparticles (NPs) and a galvanic exchange reaction.15 However, the synthesis process requires harmful toluene and multistep centrifuge washing. In our study, we synthesized highly fluorescent AgAu alloy NCs that were applied as a fluorescent probe for the sensing of Al3+ based on their fluorescence enhancement in aqueous solution. Compared with the previous report,15 the AgAu alloy NCs protected with MSA could be prepared by core etching of Ag NPs and a galvanic exchange reaction (as shown in Figure 1a) using nonorganic solvent and no multistep centrifuge washing. The MSA-stabilized AgAu alloy NCs (MSA−AgAu NCs) exhibited highly fluorescence ability, a large Stokes shift, and good photostability. In order to subject the MSA−AgAu NCs to hostile environments, they were derivatized with poly(ethylene glycol) (PEG) using 1-

s the most abundant metal in the earth crust, aluminum is extensively used in modern daily life and industrial fields. Excessive amounts of Al3+ can hamper plant performance,1 kill fish in acidified water,2 and induce a range of human health issues, such as Parkinson’s disease3 and Alzheimer’s disease.4 It is important to develop a facile Al3+ sensing approach for environmental monitoring and biological assays. In comparison to inductively coupled plasma mass spectrometry5 and atomic absorption spectrometry,6 the fluorescence sensing approaches have recently received considerable attention because of their excellent properties of high sensitivity, specificity, cost efficiency, and real-time monitoring. In the past few decades, various fluorescence approaches have been reported for the determination of Al3+ using organic dyes, but they suffer from small Stokes shift, short lifetime, and poor photostability. In addition, most of them only work well in organic solvents.7 Thus, it is highly desirable to develop a new approach for the sensing and detection of Al3+ in aqueous solutions. Metal nanoclusters (NCs), consisting of several to tens of metal atoms, have attracted a great deal of attention in recent years due to their unique physical, electrical, and optical properties.8 As their sizes approach the Fermi wavelength of an electron, they exhibit a strong luminescence property due to quantum confinement.9 Metal NCs have emerged as a class of promising optical probes for the construction of excellent chemical sensors because of their ultrasmall size, strong luminescence, good photostability, and low toxicity.10 Recently, there have been a few reports concerning the sensing of Hg2+,11 Ag+,12 and CN−13 using Au NCs or Ag NCs based on the metal core or ligand shell of the NCs providing specific interaction © 2013 American Chemical Society

Received: July 30, 2013 Accepted: September 9, 2013 Published: September 9, 2013 9839

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aqueous solution under vigorous stirring. The Ag NPs were obtained after stirring overnight. An amount of 147.0 mg of MSA in powder form was added to the as-prepared Ag NPs solution, and then the MSA−Ag NCs were obtained by incubating for 48 h under stirring in a 70 °C oil bath. Preparation of MSA−AgAu NCs. The MSA−AgAu NCs were prepared using a procedure modified from that reported earlier.15 In a typical synthesis, 1.0 mL of the as-prepared MSA−Ag NCs was added into 2.65 mL of water under stirring. Then 350 μL of 10 mM HAuCl4 was added dropwise into this aqueous solution under vigorous stirring at room temperature, and the reaction mixture was allowed to proceed for another 15 min under vigorous stirring. After dialysis in ultrapure water through cellulose membrane dialysis tubing (MWCO 1000) for 24 h, the product was collected and stored at 4 °C for further use. Preparation of PEGylated MSA−AgAu NCs. The PEGylated MSA−AgAu NCs were prepared through covalently attaching methoxy-PEG-NH2 to the MSA−AgAu NCs via EDC.16 Briefly, 0.5 mL of 80 mM m-PEG-NH2 was added into 4 mL of the dialyzed MSA−AgAu NCs under stirring. An excess amount of EDC was added into this solution, and it was stirred overnight. After being dialyzed against ultrapure water for 24 h using cellulose membrane dialysis tubing (MWCO 3500), the resulting solution was stored at 4 °C for further use. Fluorescence Sensing of Al3+. AlCl3 aqueous solutions at different concentrations together with other metal ion solutions were freshly prepared before use. To evaluate the sensitivity toward Al3+, different concentrations of Al3+ were added into 0.2 M acetate buffer (pH 4.0) containing the same amount of PEGylated MSA−AgAu NCs and the mixed solutions were equilibrated for 10 min before spectral measurement. To investigate the selectivity to Al3+, the same amount of PEGylated MSA−AgAu NCs but with different metal ion solutions were mixed in 0.2 M acetate buffer (pH 4.0) and equilibrated for 10 min before spectral measurement. The fluorescence spectra were recorded by operating the fluorescence spectrophotometer with an excitation wavelength of 390 nm.

Figure 1. (a) Schematic illustration of the preparation process for MSA−AgAu NCs. TEM images of Ag NPs (b), MSA−Ag NCs (c), and MSA−AgAu NCs (d).

ethyl-3-(3-dimethylaminopmpyl) carbodiimide (EDC) chemistry.16 The PEGylated MSA−AgAu NCs showed highly selective and sensitive sensing of Al3+ in aqueous solution, and a 3-fold fluorescence enhancement was observed upon the addition of 2 mM Al3+. The signal-generation mechanism was investigated in the study. To the best of our knowledge, this is the first report concerning fluorescence-enhanced sensing for the determination of Al3+ using fluorescent metal NCs.

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EXPERIMENTAL SECTION Materials. Silver nitrate (ACS, 99.9+%) was purchased from Alfa Aesar; MSA, EDC·HCl, and 2,6-pyridinedicarboxylic acid (PDCA) were from Sigma-Aldrich; methoxy-PEG-NH2 (mPEG-NH2, MW 750) was from Biomatrik Int.; chloroauric acid and other chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd. All the reagents were used as received without further purification. Cellulose membrane dialysis tubing (MWCO 1000 or 3500) was purchased from Sangon Biotech Co., Ltd. Ultrapure water with a resistivity 18.2 MΩ· cm−1 obtained from a Millipore purification system was used for the experiments. Instrumentations. UV−vis absorption spectra were obtained with a UV2550 UV−vis spectrophotometer (Shimadzu, Japan), and fluorescence spectra were obtained with an F4500 spectrophotometer (Hitachi, Japan). Fourier transform infrared (FT-IR) spectra were recorded with a Nicolet 330 spectrophotometer (Thermo Electron Corp., U.S.A.). The fluorescence lifetime was determined on a FluoroMax-4 spectrofluorometer (Horiba JobinYvon, France). Transmission electron microscope (TEM) images were collected using a TECNAI F-30 (Philips-FEI, Netherlands). Dynamic light scattering (DLS) and ζ-potential were determined using a Nano-ZS (Malvern Instruments, U.K.). X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS system (Physical Electronics, U.S.A.); all the binding energies were calibrated by C 1s as reference energy (C 1s = 284.6 eV). Preparation of Citrate-Stabilized Ag NPs and MSA− Ag NCs. The Ag NPs and the MSA−Ag NCs were synthesized based mainly on a protocol described previously.17 Briefly, 2.5 mL of 100 mM AgNO3 and 2.5 mL of 100 mM trisodium citrate were added into 39 mL of water under stirring. Then, 6 mL of freshly prepared 50 mM NaBH4 was added into this

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RESULTS AND DISCUSSION Synthesis and Characterization of MSA−AgAu NCs and PEGylated MSA−AgAu NCs. The synthesis of MSA− AgAu NCs included three steps, and the products in each step were characterized using UV−vis spectrometry, FT-IR spectrum, and TEM. The first step was the preparation of citrate-stabilized Ag NPs using NaBH4 as a reducing agent. The Ag NPs exhibited an intense surface plasmon resonance peak at 390 nm and had a polydisperse size in the range of 6−30 nm as shown in Figure 1b. After etching with MSA at 70 °C in the second step, the product showed no apparent optical absorption features and the surface plasmon resonance disappeared completely (Figure S1 in the Supporting Information), indicating the formation of MSA−Ag NCs. From the TEM image (Figure 1c), the as-prepared MSA−Ag NCs appeared to have an average diameter of 1.8 nm, and the larger particles were caused by electron beam induced aggregation. In the third step, MSA−AgAu NCs could be obtained through the addition of an appropriate amount of HAuCl4 to the as-prepared MSA−Ag NCs. It was observed that the MSA−AgAu NCs also exhibited strong and broad absorption features similar to those of MSA−AgAu NCs and just the absorption intensity increased. The result (Figure 1d) 9840

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NCs depicted the presence of the expected elements (see Figure S4a in the Supporting Information), and the Ag/Au/S atomic ratio measured using XPS was 1:0.54:0.88. As shown in Figure 3e, the binding energy of Au 4f7/2 was 84.7 eV located at the region between Au(I) thiolate (86.0 eV) and Au film (84.0 eV),20 suggesting the existence both of Au(0) and Au(I) in MSA−AgAu NCs. The Ag 3d5/2 peak at 368.1 eV supported the existence of only Ag(0) in the nuclear cluster (Figure 3f).21 The binding energy of S 2p3/2 (162.7 eV, see Figure S4b in the Supporting Information) compared very well with the typical value of chemisorbed S species,8b inferring the formation of metal−S bonds in the synthesis of the metal clusters. The amount of HAuCl4 played an important role in MSA− AgAu NCs synthesis. From a previous report,15 the mechanism of formation of the alloy clusters was that Au(I) thiolate, produced from the reaction of Au3+ with excess MSA or Ag thiolate, reacted with the MSA−Ag NCs to form alloy clusters. The amount of HAuCl4 was optimized to derive MSA−AgAu NCs with a higher fluorescence emission. With the addition of an increasing volume of 10 mM HAuCl4, the broad absorption gradually increased in the whole range under study, while corresponding emission intensity gradually increased with the addition of a volume of 10 mM HAuCl4 up to 350 μL. After that, the corresponding emission intensity gradually decreased as illustrated in Figure S5 of the Supporting Information. An excess amount of HAuCl4 led to fluorescence quenching, probably due to damage to the metal clusters formation. On the basis of the photograph under UV light (Figure S5d in the Supporting Information), it is obvious that 350 μL was the best volume of 10 mM HAuCl4 to be used for the preparation of the metal alloy clusters. To give the metal alloy clusters a more excellent biocompatibility and colloidal stability in a salt-containing solution, the PEGylated MSA−AgAu NCs were prepared through covalently attaching m-PEG-NH2 to the surface of the metal alloy clusters via EDC chemistry.16 Compared with the FT-IR spectra of m-PEG-NH2, MSA−AgAu NCs, and PEGylated MSA−AgAu NCs (Figure S6 in the Supporting Information), the characteristic features of m-PEG-NH2 could be observed in the spectrum of PEGylated MSA−AgAu NCs, and a new sharp peak appeared at 1558 cm−1, which was attributable to the amide II band. This result suggested that mPEG-NH2 was successfully modified on the surface of the metal alloy clusters and the PEGylated MSA−AgAu NCs were formed. The fact that the emitted wavelength of PEGylated MSA−AgAu NCs blue-shifted to 625 nm compared to that of MSA−AgAu NCs (Figure S7 in the Supporting Information) was probably caused by the etching action of Cl− introduced by the EDC·HCl. The TEM image indicated that PEGylated MSA−AgAu NCs were spherical in shape and well-dispersed with an average diameter of 2.6 nm (Figure S8 in the Supporting Information), while the hydrodynamic diameter measured using DLS was approximately 15 nm (Figure S9 in the Supporting Information), indirectly revealing that m-PEGNH2 was attached to the surface of the metal alloy clusters. To explore their potential application in high ionic strength environments, the stability of PEGylated MSA−AgAu NCs was investigated in the presence of various concentrations of NaCl. The fluorescence intensity of PEGylated MSA−AgAu NCs in the different salt-containing solutions, even in 1 M NaCl solution, remained almost unchangeable compared with that without NaCl (Figure 4), while fluorescence intensity of MSA−AgAu NCs without attaching m-PEG-NH2 changed

indicates that the uniform MSA−AgAg NCs were spherical and highly dispersible with an average size of 1.5 nm. The FT-IR spectra of MSA, MSA−Ag NCs, and MSA−AgAu NCs are compared in Figure S2 of the Supporting Information. The characteristic features of MSA and the disappearance of the S− H stretching band at 2554 cm−1 were observed in both the MSA−Ag NCs and MSA−AgAu NCs spectra, confirming that MSA was modified on the surface of the two kinds of NCs through either Ag−S or Au−S interactions. Owing to the quantum confinement and the discrete energy levels, strong photoluminescence is considered an important property of NCs, and therefore fluorescence spectrometry was applied to characterize the MSA−AgAu NCs. The MSA−AgAu NCs exhibited a symmetric emission spectrum with excitation and emission maxima at 390 and 637 nm (Figure 2). The inset

Figure 2. UV−vis absorption spectrum and fluorescence spectra of the as-prepared MSA−AgAu NCs. Inset: photographs of MSA−AgAu NCs under visible light (a) and UV light at 365 nm (b).

of Figure 2 reveals that the MSA−AgAu NCs solution was light yellow in color under visible light, while it emitted a bright red color under UV light at 365 nm. The decay profile of MSA− AgAu NCs was monitored (Figure 3a), and the fluorescence intensity decay showed three components at 269.5 ns (88.17%), 42.8 ns (10.81%), and 5.1 ns (1.01%), which is related to the surface states that arise from interaction with the ligands.8b,18 The absolute quantum yield of MSA−AgAu NCs was 1.6%, determined by means of a comparison with the lysozyme-stabilized Ag NCs previously reported by our group.19 Moreover, the photostability of the MSA−AgAu NCs was studied through continuous irradiation of a halogen lamp. Fluorescein isothiocyanate (FITC), which is a widely used organic dye, was chosen as a reference and was quickly photobleached within 20 min, whereas MSA−AgAu NCs remained unchanged after 60 min of continuous irradiation (Figure 3b). These results indicated that the as-prepared MSA−AgAu NCs were of good photostability. Furthermore, the as-prepared MSA−AgAu NCs solution was stable and presented no obvious change (only a 20% decrease) in fluorescence intensity even after 6 weeks’ storage in the dark at 4 °C (Figure 3c). The hydrodynamic diameter of MSA−AgAu NCs measured using DLS was about 5 nm (Figure 3d), and the ζ-potential of MSA−AgAu NCs at neutral pH was found to be −32.6 mV due to strong electrostatic repulsion from the carboxyl groups of the NCs surface (see Figure S3 in the Supporting Information). XPS is a powerful tool to investigate the oxidation states of metal cores and the surface interaction between the metal core and ligands in NCs. The XPS survey spectrum of MSA−AgAu 9841

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Figure 3. (a) Time-resolved decay of MSA−AgAu NCs. Lifetime values obtained are given in the inset. (b) Photostability of MSA−AgAu NCs, PEGylated MSA−AgAu NCs, and FITC at the same conditions. (c) The stability of MSA−AgAu NCs and PEGylated MSA−AgAu NCs stored in the dark at 4 °C. (d) Hydrodynamic diameter measured using DLS of MSA−AgAu NCs at neutral pH. Au 4f (e) and Ag 3d (f) XPS spectra of MSA−AgAu NCs deposited on a silica wafer.

terized under the same conditions used for the study of MSA− AgAu NCs. The fluorescence intensity of PEGylated MSA− AgAu NCs decreased only 15% after 60 min of continuous irradiation, suggesting they were of good photostability (Figure 3b), and furthermore, the PEGylated metal alloy clusters presented good stability. No obvious change in their fluorescence intensity could be found even when the clusters were stored for 6 weeks in the dark at 4 °C (Figure 3c). Fluorescence Probe for Detection of Al3+. Interestingly, the fluorescence intensity of PEGylated MSA−AgAu NCs was found to be greatly enhanced in the presence of Al3+, indicating that the NCs could be applied as a turn-on fluorescence probe for the determination of Al3+. When different concentrations of Al3+ (2.0−2000 μM) were added to 0.2 M acetate buffer solution (pH 4.0), containing the same amount of PEGylated MSA−AgAu NCs, in order to evaluate the sensitivity toward Al3+, their fluorescence intensity was gradually increased with increasing Al3+ concentration and a 3-fold fluorescence enhancement was achieved upon the addition of 2 mM Al3+ (Figure 5). The response rate was very fast, and it would only

Figure 4. Stability of PEGylated MSA−AgAu NCs in the presence of various concentrations of NaCl.

obviously in the presence of different NaCl concentrations (Figure S10 in the Supporting Information). Moreover, the photostability of PEGylated MSA−AgAu NCs was charac9842

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Figure 5. (a) Emission spectra of PEGylated MSA−AgAu NCs in the presence of various concentrations of Al3+ in 0.2 M acetate buffer solution (pH 4.0). (b) Relationship between relative fluorescence intensity and Al3+ concentration. The inset shows the linear detection range for 2−30 μM of Al3+.

take 5 min to reach equilibrium in the investigated range (Figure S11 in the Supporting Information). A significant linear correlation (R2 = 0.9755) existed between the value of (F − F0)/F0 and the Al3+ concentration in the range of 2.0−30 μM. It should be noted that F0 and F were the fluorescence intensity of the PEGylated MSA−AgAu NCs before and after adding Al3+. The resulting linear equation was (F − F0)/F0 = 0.02353[Al3+] + 0.00894. The detection limit at a signal-tonoise ratio for Al3+ of 3 was 0.8 μM, which was much lower than the maximum level (7.4 μM) of Al3+ in drinking water permitted by the World Health Organization.22 To test the selectivity of the PEGylated MSA−AgAu NCs in the determination of Al3+, several metal ions were studied as a control: Fe3+, Fe2+, Hg2+, Cu2+, Pb2+, Mg2+, Zn2+, Ni2+, Mn2+, Cr3+, Cd2+, Co2+, and Ba2+. Among these ions, Pb2+ slightly enhanced the fluorescence of PEGylated MSA−AgAu NCs, while Fe3+, Fe2+, Cu2+, and Hg2+ revealed a quenching effect, especially Fe3+ and Cu2+ (Figure S12 in the Supporting Information). The TEM image of aggregated PEGylated MSA−AgAu NCs in the presence of Fe3+ or Cu2+ (Figure S8 in the Supporting Information) supported the notion that the fluorescence quenching phenomenon occurred through an iontemplated chelation process as reported previously.11a,23 The aggregation was due mainly to the interaction of Fe3+ or Cu2+ with the residual carboxylate groups present on the surface of the PEGylated MSA−AgAu NCs. To further improve the selectivity of the PEGylated MSA−AgAu NCs toward Al3+, we introduced two chelating agents, PDCA (for Cu2+ and Pb2+) and MSA (for Fe3+, Fe2+, and Hg2+), which showed effective masking ability for the interfering ions. As indicated in Figure 6, in the presence of the two chelating agents, all of the other ions showed only a slight effect on the fluorescence of PEGylated MSA−AgAu NCs, while Al3+ could effectively enhance the fluorescence. These results indicated that the PEGylated MSA− AgAu NCs exhibited excellent selectivity toward Al3+ ions with the assistance of the selected masking agents. We tried to understand the mechanism of PEGylated MSA− AgAu NCs for Al3+ sensing based on fluorescence enhancement. In general, the metal core and ligand shell of metal NCs serve as the recognition component for ions resulting in signal generation.10 As shown in the TEM image (Figure S8 in the Supporting Information), slight aggregation could be found for the PEGylated MSA−AgAu NCs. The hydrodynamic diameter obtained from DLS shifted from 15 to 72 nm in the presence of 50 μM Al3+ (Figure S9 in the Supporting Information), possibly

Figure 6. Selectivity of PEGylated MSA−AgAu NCs toward Al3+ over other metal ions in the presence of certain masking agents in 0.2 M acetate buffer solution (pH 4.0). The concentration of the metal ions was 50 μM.

due to Al3+-induced NCs aggregation through the interaction of Al3+ with the ligand shell. However, in order to provide more chelating sites with Al3+ and compete with the ligand shell of metal NCs, as shown in Supporting Information Figure S13, the addition of free succinic acid (SA) or MSA into this reaction mixture did not cause any obvious decrease in the fluorescence intensity of PEGylated MSA−AgAu NCs, and as expected, their hydrodynamic diameters did not become smaller. These results indicated that the hypothetical mechanism for fluorescence enhancement based on Al3+induced NC aggregation was not acceptable. The fluorescence properties of NCs are highly dependent on the local environment, especially the surface properties of the metal core.10 Aluminum reacts with gold to allow AlAu alloy formation.24 It is probable that the deposition of Al3+ on the surface of metal core could result in fluorescence enhancement. To prove this hypothesis, XPS, DLS, and ζ-potential were used to characterize the change of PEGylated MSA−AgAu NCs. No change in the Ag 3d XPS spectra and a 0.1 eV shift to higher energy in the Au 4f spectra could be observed after adding 50 μM Al3+, suggesting the reaction between metal core and Al3+ might occur (see Figure S14 in the Supporting Information). We should note that the oxide of metal core alone could not enhance fluorescence in this case, which was proved by adding H2O2 as a control (see Figure S15 in the Supporting Information).12a Interestingly, the hydrodynamic diameter of PEGylated MSA−AgAu NCs in the presence of 1000 μM Al3+ reduced to a smaller level as that without Al3+ and the 9843

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corresponding TEM image showed that the PEGylated metal alloy clusters exhibited good dispersion and no obvious aggregation (Figure S8f in the Supporting Information), while the ζ-potential of PEGylated MSA−AgAu NCs gradually shifted from a negative to a positive charge with the increasing concentration of Al3+ (Figure S16 in the Supporting Information). These results indicated the probability of the deposition of Al3+ on the surface of metal core. However, the detail signal-generation mechanism requires further study.

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CONCLUSIONS In summary, we have reported a simple method for preparing highly red fluorescent MSA−AgAu NCs through the core etching of Ag NPs and a galvanic exchange reaction. The AgAu NCs exhibited their excellent fluorescence characteristics with a large Stokes shift, long lifetime, good photostability, and excellent stability. After covalently attaching m-PEG-NH2 to the surface of the metal alloy clusters, the PEGylated MSA− AgAu NCs had high stability even when exposed to high ionic conditions (up to 1 M NaCl). The PEGylated MSA−AgAu NCs showed highly selective and sensitive sensing of Al3+ based on fluorescence enhancement in aqueous solution, with a detection limit of 0.8 μM. The signal-generation mechanism for Al3+ sensing was probably attributed to the deposition of Al3+ on the surface of the metal core. To the best of our knowledge, this is the first report concerning a sensing approach based on fluorescence enhancement for the determination of Al3+ using fluorescent metal NCs.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. 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]. Fax: +86 592 2184530. Phone: +86 592 2184530. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was financially supported by the National Nature Scientific Foundation of China (nos. 21175112, 21375112), National Basic Research Program of China (2010CB732402), and NFFTBS (no. J1030415), which are gratefully acknowledged. Furthermore, we would like to extend our thanks to Professor John Hodgkiss of The University of Hong Kong for his assistance with English.

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dx.doi.org/10.1021/ac4023764 | Anal. Chem. 2013, 85, 9839−9844