DOPA-Mediated Reduction Allows the Facile Synthesis of Fluorescent

Article pubs.acs.org/ac

DOPA-Mediated Reduction Allows the Facile Synthesis of Fluorescent Gold Nanoclusters for Use as Sensing Probes for Ferric Ions Ja-an Annie Ho,*,† Heng-Chia Chang,‡ and Wen-Ta Su† †

BioAnalytical and Nanobiomedicinal Laboratory, Department of Biochemical Science and Technology, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan ‡ Department of Chemistry, National Tsing Hua University, No. 101, Sec. 2, Kuang-Fu Road, Hsinchu, 30013 Taiwan S Supporting Information *

ABSTRACT: In this paper, we describe a simple one-pot method, employing L-3,4-dihydroxyphenylalanine (L-DOPA) as a reducing/capping reagent, for the synthesis of fluorescent gold nanoclusters (AuNCs). Within a short reaction time of 15 min (excluding the time required for purification), this strategy allows the fabrication of homogeneous AuNCs having the capability to sense ferric ions (Fe3+). The as-prepared AuNCs exhibited a fluorescence emission at 525 nm and a quantum yield of 1.7%. On the basis of an aggregation-induced fluorescence quenching mechanism, these fluorescent AuNCs offer acceptable sensitivity, high selectivity, and a limit of detection of 3.5 μM for the determination of Fe3+ ions, which is lower than the maximum level (0.3 mg L−1, equivalent to 5.4 μM) of Fe3+ permitted in drinking water by the U.S. Environmental Protection Agency.

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range.5,7,8 Many methods have been developed for the synthesis of fluorescent AuNCs through the use of reducing agents exhibiting both stabilizing and protecting behavior. 8−13 For example, AuNCs accommodating poly(amidoamine) (PAMAM) dendrimers have been found to emit blue light.9 Fluorescent AuNCs with red emissions can be synthesized with the aid of bovine serum albumin (BSA).10 In addition to these relatively large species, several small molecules, including glutathione (GSH), 11 N,Ndimethylformamide (DMF), 8 , 1 2 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES), and 2-(Nmorpholino)ethanesulfonic acid (MES)13 have been used to assist the synthesis of AuNCs, due to their inherent reducing properties. More recently, the amino acid histidine was used as both a reductant and stabilizer to produce fluorescent AuNCs in a simple reaction.14 Those as-described gold-based fluorophores have demonstrated a variety of applications in the sensing of heavy metal ions,11,15 bioimaging,16,17 and

old nanoparticles (AuNPs) are employed widely in environmental, biological, and pharmaceutical fields because of their interesting electronic, catalytic, and optical properties. Many colorimetric assays based on AuNPs have been developed for the sensing of metal ions, single molecules, DNA, and proteins; typically, they involve the observation of a color change, from a monodistributed “red” AuNPs solution to an aggregation-induced “purple” solution, resulting from distance-dependent surface plasmon resonance (SPR) phenomena.1−3 Over the past decade, the syntheses of gold nanoclusters (AuNCs), which vary in size from several atoms to subnanometer-sized particles, have been described extensively, as have their applications arising from their unique fluorescence characteristics. When the sizes of AuNCs are decreased to approach the Fermi wavelength of the electrons in the conduction band, the continuous density of states breaks into discrete energy levels, resulting in the observation of molecule-like size-dependent fluorescence.4−6 The electrons undergo excitation from the valence band (filled 5d10) to the conduction band (6sp1), resulting in a strong size-dependent fluorescence emission that can extend from the visible to the near-infrared (near-IR) © 2012 American Chemical Society

Received: December 18, 2011 Accepted: February 26, 2012 Published: February 27, 2012 3246

dx.doi.org/10.1021/ac203362g | Anal. Chem. 2012, 84, 3246−3253

Analytical Chemistry

Article

the fabrication of biosensors.18,19 The drawbacks of most of the existing synthetic methods, however, are that they require multiple steps and reaction times ranging from several hours to days. Herein, we report a simple and rapid approach for the synthesis of fluorescent AuNCs. Measuring the level of ferric ions (Fe3+) is a significant factor in the evaluation of water quality. In addition, Fe3+ ions also play important roles in biological systems. As an essential component of heme groups and iron sulfur clusters, iron is an important element for electron transport in the respiratory chain and for a variety of enzymatic reactions. A lack of iron can lead to anemia, a condition in which there are too few red blood cells.20 In contrast, when present in excess, iron can harm biological systems because, in its redox-active form, it catalyzes the generation of highly reactive oxygen species, leading to iron poisoning, with such symptoms as severe vomiting, diarrhea, abdominal pain, and heart/liver damage.21 Because iron deficiency and overload can both impair cellular functions, the quantitation of iron is of considerable interest. At present, it can be achieved using an assortment of analytical methods, including flame atomic absorption spectroscopy (FAAS),22 inductively coupled plasma mass spectrometry (ICP-MS),23 and spectrophotometric detection using organic dyes or quantum dots.24,25 The drawbacks of these techniques are that they often require a difficultto-synthesize fluorescent detecting probe, time-consuming sample pretreatment procedures, and sophisticated instrumentation. L -3,4-Dihydroxyphenylalanine ( L -DOPA) is a neurotransmitter commonly known for its use in the treatment of neural disorders (e.g., Parkinson’s syndrome).26 The Willner group employed the reducing properties of DOPA to generate AuNPs, noting the concurrent production of subnanometer-sized AuNCs (2−6 nm) during this process.27 Inspired by their results, in this present study we developed a simple and rapid (18 MΩ·cm) prior to use. The fluorescent AuNCs were synthesized through L -DOPA-mediated reduction of HAuCl 4. Briefly, HAuCl 4 solution (8.6 × 10 −4 M, 7.0 mL) was brought to a vigorous boil with stirring and then DOPA solution (2.3 × 10 −3 M, 1.0 mL) was added all at once. The solution was stirred for an additional 10 min under a vigorous boil and then it was removed from heat and cooled slowly to room temperature. After centrifugation (16,000 rpm, 10 min) to remove byproducts with larger particle sizes, the light-brown supernatant containing the AuNCs was collected. A dialysis membrane (MWCO: 1 kDa; pore size: ca. 1.0 nm) was then used to separate the AuNCs from any residual unreacted species. The as-obtained purified AuNC solution was stored at 4 °C in the dark until required for further use. Fluorescence Detection of Fe3+ Ions. An aliquot of AuNCs (0.5 mg/mL, 20 μL) was added to sodium acetate buffer (50 mM, pH 4.5, 1.0 mL) containing various concentrations of Fe3+ ions. The solution was mixed thoroughly and left to react at room temperature for 5 min. The fluorescence quenching spectra was then recorded (excitation 360 nm; maximum emission 525 nm); the slit widths for the excitation and emission were set at 5.0 and 10.0 nm, respectively. The interfering effects of other metal ions were investigated individually in the presence of the fluorescent AuNCs. Tap water and lake water samples were collected from our laboratory and from the Cheng Kung Lake on the campus of National Tsing Hua University, respectively. All the raw samples were centrifuged (16,000 rpm) and filtered through a 0.45-μm membrane to remove any suspended particles. A series of samples was prepared by spiking standard solutions (0.5 mL) containing various concentrations of Fe3+ (from 5 to 1280 μM) in 100 mM sodium acetate buffer (pH 4.5) into the tap water or lake water (0.5 mL). The resulting solutions were further mixed with AuNC solutions (0.5 mg/mL, 20 μL). After a 5-min incubation period, the fluorescence spectra were recorded (excitation wavelength: 360 nm).

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EXPERIMENTAL SECTION Reagents and Materials. All chemicals were of reagent grade or better. L-DOPA, copper chloride, hydrochloric acid, lead nitrate, manganese sulfate, sodium acetate, sodium borohydride, sodium periodate, and zinc nitrate were purchased from Sigma−Aldrich (St. Louis, MO). Ferrous chloride was obtained from J. T. Baker (Phillipsburg, NJ). Ferric chloride was purchased from Mallinckrodt (Phillipsburg, NJ). Cadmium nitrate, cobalt nitrate, and nickel nitrate were obtained from Showa (Tokyo, Japan). Mercuric nitrate and nitric acid were obtained from Fluka 3247



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Iron supplement tablets (Zenyaku Kogyo, Tokyo, Japan) were purchased locally; the packaging claimed that they contained 79.5 mg Fe3+ per 400-mg tablet. The individual tablets were finely powdered in a porcelain mortar and then a portion of the powder was weighed and subsequently dissolved in an appropriate volume of 50 mM sodium acetate buffer (pH 4.5) to prepare a stock solution having a concentration corresponding to 5.3 mM Fe3+. The solution was sonicated for 1 h to ensure complete dissolution and subsequently subjected to centrifugation and filtration to remove any suspended particles. Working solutions were prepared through dilution of the stock solutions to concentrations of 50, 200, and 500 μM Fe3+. The fluorescence detection of Fe3+ ions was then performed as described above.

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RESULTS AND DISCUSSION Characterization of AuNCs. In this study, we prepared fluorescent AuNCs in boiling aqueous solutions incorporating DOPA as both a reducing and protecting agent. The two phenolic OH groups in DOPA underwent two-electron oxidation to form the o-hydroquinone derivative, concurrently reducing Au3+ to Au0. The carboxyl groups of the DOPA units, on the other hand, served as stabilizers for the nuclei of reduced Au atoms.28 We used boiling solutions for this hydrothermal synthesis to accelerate the reduction of Au3+ ions more homogeneously. During the course of the reaction, the simultaneous production of large gold nanoparticles and small AuNCs in the reduction of HAuCl4 with DOPA occurred, and the color of solution changed from an original light-yellow to a final dark-gray. Similar phenomenon has been observed by Willner et al., 27 and they proposed that the small-sized Au nanoclusters may be generated by the direct reduction of the HAuCl4 or from the detachment of small AuNP from large particles. Relatively large particles were removed at the end of reaction through centrifugation, providing a light-brown supernatant containing the fluorescent AuNCs. The whole process required only 15 min (for chemical reaction only, excluding the time required for purification); a longer reaction time did not increase the fluorescence intensity of the AuNCs (Figure 1A). Square wave voltammetry (SWV; Figure S1, Supporting Information) revealed the presence of the AuNCs after purification against dialysis (membrane pore size ∼1.0 nm) versus Milli-Q water for 24 h. Moreover, HRTEM images (Figure 1B) confirmed the monodispersity of as-obtained AuNCs (average diameter 2.0 ± 0.5 nm). Thus, both HRTEM and SWV confirmed that ultrasmall sized Au materials had been prepared successfully using the proposed method. Optical Properties of DOPA-Synthesized Fluorescent AuNCs. A distinct peak at 274 nm (solid line) appeared in the UV−Vis absorption spectra of the AuNCs (Figure 2A); we attribute this signal, which differs from that of DOPA at 280 nm (dash line), to the absorption of the units of dopaquinone, the oxidized product of DOPA,29 capped on the surface of the AuNCs through gold nitrogen bonds. In addition, we suspect that the broad band having its onset near 700 nm resulted from the multiple absorptions of the AuNCs. The absence of the typical SPR peak for larger AuNPs (ca. 520 nm) indicates that most of the AuNCs had dimensions of less than 2.5 nm.30 Strong emission of the AuNCs appeared at 525 nm, with an excitation maximum at 360 nm (Figure 2B). In contrast to organic fluorophores,

Figure 1. (A) Optimization of the reaction time for synthesizing the AuNCs under reflux. (B) HRTEM image of the AuNCs prepared through DOPA-mediated synthesis (scale bar = 10 nm).

the AuNCs exhibited a large Stokes shift (165 nm), which avoided crosstalk between the excitation and emission signals. Fluorescence lifetime measurements revealed that the fluorescence decay spectra (excitation at 360 nm) could be fitted with biexponential curves, suggesting the possible existence of two components with lifetimes of 1.2 (81%) and 8.7 μs (19%), respectively (Figure 2C). Long lifetimes and large Stokes shifts are common characteristics of fluorescent AuNCs.7,9,11,31,32 Through comparison with fluorescein (QY = 95%, in 0.1 N NaOH)33 we determined the QY of the AuNCs to be 1.7%, a great improvement, by at least 8 orders of magnitude, relative to that of bulk gold (QY = 10−8%)34 and higher than that of most previously reported fluorescent AuNCs.7,32,35−38 According to the article described by Wu and Jin,39 surface plays the most important role in enhancing the fluorescence of gold nanoparticles. They also mentioned that the surface ligands specifically affect the quantum yield in two different ways: (i) charge transfer from the ligands to the core of metal nanoparticle via the gold thiol bonds, and (ii) electron-rich atoms or groups of the ligands donate the delocalized electrons directly to the metal core.39 We thus speculate the electron-rich group, such as primary amine groups in our case, interacted with the surface of AuNCs and donated the delocalized electron density directly to the gold core, resulting in the larger enhancement of fluorescence. 3248



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Figure 2. (A) UV−vis absorption spectra of the AuNCs prepared through DOPA-mediated synthesis (black line) and DOPA (dashed line). (B) Normalized fluorescence excitation spectra (dashed line) and emission spectra (solid line) of the AuNCs prepared through DOPA-mediated synthesis. Inset: Photographs of the AuNCs prepared through DOPA-mediated synthesis, under room light (left) and a hand-held UV lamp with excitation at 365 nm (right). (C) Fluorescence lifetimes of the AuNCs prepared through DOPA-mediated synthesis. Data were obtained after excitation at 360 nm. The fluorescence lifetime data were fitted to a biexponential decay.

AuNCs. These two experiments suggested that the observed fluorescence arose mainly from the AuNCs themselves, and not to the oxidation products of DOPA. We suspect that our gold-based AuNC fluorophores would function with superior biocompatibility and lower toxicity relative to most semiconductor quantum dots because no toxic heavy metals (e.g., Cd) were involved in the synthetic process.44 As a result, these less-hazardous and more-environmentally friendly AuNCs with acceptable QYs might have high applicability in biological systems. Moreover our present method for the preparation of AuNCs is not time-consuming, does not require complicated reactions, and does not feature toxic reductants (e.g., NaBH4). Fluorescence Probe for Detection of Fe3+. Because o-quinone−containing ligands are known to form complexes with Fe3+ ions,45,46 we examined the practicality of using our AuNCs for the sensing of Fe3+ ions, through a mechanism based on the complexation of the Fe3+ ions in solution with the o-quinone units of the dopaquinone moieties on the

To exclude the possibility that the fluorescence originated from the oxidation of DOPA, we reacted DOPA with NaOH and NaIO4, respectively, as substitutes for Au3+ ions, at ambient temperature overnight or at 100 °C for 10 min. Although NaOH and NaIO 4 are reagents capable of oxidizing DOPA, we observed no significant fluorescence from either reaction product (Figure 3A).40,41 Moreover, to confirm that the observed fluorescence resulted from the AuNCs, we employed aqua regia (HNO3/HCl, 1:3) and KCN individually to digest and etch the AuNCs.42,43 Figure 3B reveals that the fluorescence intensity of 1.0 mL of the 0.1 mg/mL AuNCs solution decreased by approximately 90% in the presence of 50 μL of aqua regia, which oxidized the AuNCs to form gold ions.42 KCN, on the other hand, is known to etch AuNPs and AuNCs oxidatively to form Au(CN)4− ions.43 After reacting 1.0 mL of the 0.1 mg/mL AuNC solution with 0.5 mg of KCN for 30 min, the fluorescence intensity decreased by approximately 64% (Figure 3C), presumably as a result of etching of the 3249



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Figure 3. (A) Normalized fluorescence spectra of (a) the AuNCs prepared through DOPA-mediated synthesis, (b, c) the NaOH-mediated oxidation of DOPA by at (b) room temperature overnight and (c) under reflux, and (d, e) the NaIO 4-mediated oxidation of DOPA at (d) room temperature overnight and (e) under reflux. (B, C) Normalized fluorescence spectra of 1.0 mL of 0.1 mg/mL AuNCs (solid lines) and in the presence of (B) 50 μL aqua regia (dashed line) and (C) 0.5 mg KCN (dashed line). Excitation wavelength: 360 nm.

rescence intensity decreased upon increasing the concentration of Fe3+. The fitted linear data (Figure 4D) could be expressed as

surfaces of the AuNCs. The HRTEM image in Figure 4A reveals that the AuNCs tended to aggregate in the presence of Fe3+ ions, resulting in the formation of huge complexes (dimensions ∼500 nm). As indicated in Figure 4B, a significant decrease of fluorescence occurred within 1.0 min after the addition of Fe 3+ ions to AuNCs-containing preparation, presumably because of aggregation-induced fluorescence quenching. The maximum quenching occurred after 5.0 min of incubation. A dose−response graph was employed to determine the relationship between the fluorescence intensity of the AuNCs and the concentration of the quenching Fe 3+ ions 47 that is, the dependence of the quenching effect (I 0 /I) on the concentration of the quencher. Figure 4C reveals the quenching effect of the Fe3+ ions on the fluorescence of the AuNCs; here, the fluo-

I0/I = 1 + 0.003[Fe3 +] (R2 = 0.986)

The LOD for Fe3+ ions was 3.5 μM (at an SNR of 3); the slope was 0.003 μM −1. This value of slope, which is higher than those of other tested metal ions (e.g., 0.0002 μM−1 for Fe2+; 0.0001 μM−1 for Cr3+), indicates that the AuNC probes had a high affinity for Fe3+ ions. The response toward Fe3+ was highly linear over the concentration range from 5 to 1280 μM (Figure 4D). The LOD was lower than the maximum level (0.3 mg/L, equivalent to 5.4 μM)48 of Fe3+ ions permitted in drinking water by the US Environmental Protection Agency, implying that AuNC probes 3250



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Figure 4. (A) HRTEM image of aggregated AuNCs prepared through DOPA-mediated synthesis in the presence of Fe3+ ions (scale bar = 500 nm). (B) Optimization of the incubation time for the fluorescence quenching of the AuNCs in the presence of various concentrations of Fe3+ ions. (C) Normalized fluorescence spectra revealing the quenching effects of Fe3+ ions at various concentrations on the fluorescence emission of the AuNCs. (D) The dose−response graph revealing the quenching effects of Fe3+ ions on the fluorescence emission of the AuNCs at 525 nm, the linear fitting could be expressed as I0/I = 1 + 0.003[Fe3+] (R2 = 0.986).

hold great potential for use in the detection of Fe 3+ ions. Unlike organic fluorophores, these fluorescent AuNCs can be prepared rapidly without the need for complicated synthetic procedures. The high dispersibility of the AuNC probes in aqueous solution allows the sensitive detection of Fe3+ ions in water, the solvent of choice for green chemistry.49−51 Next, we investigated the selectivity of our AuNCs toward Fe3+ ions. Because Figure 4B revealed that the maximum quenching of the AuNCs [i.e., the maximum change in the value of (I0 − I)/I0] occurred after 5.0 min of incubation with Fe3+ ions, all of the data presented in Figure 5 were collected after 5 min of incubation with Fe3+ (100 μM) or other metal ions (500 μM; i.e., 5-fold more concentrated). The results confirm the high selectivity of the AuNC probes toward Fe3+ ions. Other metal ions had minor or negligible quenching effects on the fluorescence intensity of the AuNCs. To test its practicality, we applied the proposed method to the analysis of the aqueous samples spiked with Fe3+ ions. For this purpose, we employed the standard addition method to eliminate any matrix effects and applied the method

of multiple standard additions to synthetic samples prepared with tap water and lake water. The fluorescence intensity of AuNC probes decreased when the samples of tap water and lake water had been spiked with standard solutions containing 20, 40, and 80 μM Fe 3+ ions. The recoveries of the spiked water samples ranged from 101.1 ± 2.1 to 108.1 ± 0.2% for the tap water samples and from 98.7 ± 8.8 to 108.4 ± 4.1% for the lake water samples [Table 1A]. The low relative standard deviations (RSDs), ranging from 0.2% to 5.6%, confirmed the accuracy of this method; thus, the AuNC probes meet the test requirements for environmental analyses. We also used our AuNC-based assays to analyze the levels of Fe3+ ions in over-the-counter Fe supplement tablets. Table 1B summarizes the quantitative data obtained; the contents of Fe3+ ions correlated well with the values listed on the “nutrition facts” labels provided with the Fe supplements. Because less than 10 min was required to run a single assay, this approach appears to be suitable for the rapid and simple analyses of real samples without the need for complicated sample preconcentration or purification techniques. 3251



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extension of these simply prepared, water-soluble fluorescent AuNC probes in biomedical applications is currently under investigation.

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ACKNOWLEDGMENTS We thank the National Science Council of Taiwan for financing the projects 98-2113-M-002-025-MY3, 99-2923-M-002-008MY2, 100-2627-M-002-015, and Ministry of Education (Taiwan) Aim for Top University Program.

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ASSOCIATED CONTENT

S Supporting Information *

Additional material as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. Selectivity of the AuNCs toward 100 μM Fe3+; the concentrations of the other metal ions were 500 μM. All experiments were performed in pH 4.5 in 50 mM sodium acetate buffer; excitation wavelength = 360 nm.

Corresponding Author

*Fax: +886-2-3366-4438. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Table 1. Results of Recovery Studies from (A) Water Samples (Tap and Lake Water) Spiked with Fe3+ Ions at pH 4.5 in 50 mM Sodium Acetate Buffer and (B) Over-theCounter Fe Supplement Tablets

tap water

lake water

found amount (μM)

20 40 80 20 40 80

20.3 43.2 80.9 19.7 43.4 86.4

± ± ± ± ± ±

1.8 0.1 1.7 1.8 1.6 4.3

recovery (%) 101.8 108.1 101.1 98.7 108.4 108.0

± ± ± ± ± ±

8.8 0.2 2.1 8.8 4.1 5.4

(B) sample

added amount (μM)

found amount (μM)

recovery (%)

Fe tablets

50 200 500

56.5 ± 1.5 189.0 ± 8.0 490.9 ± 10.9

113.0 ± 3.1 94.5 ± 4.0 98.2 ± 2.2

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(A) spiked amount (μM)

AUTHOR INFORMATION

CONCLUSIONS

We have developed a simple one-pot method for the synthesis of fluorescent AuNCs, through DOPA-mediated reduction of HAuCl4 under hydrothermal conditions within 15 min. These water-soluble AuNC probes feature several unique characteristics and advantageous properties: (i) they were prepared through organic solvent−free synthesis (green chemistry); (ii) they exhibited a respectable QY of 1.7%; (iii) they allowed analyses to be performed in aqueous solutions, avoiding the use of hazardous organic reagents as cosolvents; and (iv) they allowed the sensing of Fe3+ ions, based on a mechanism involving aggregation-induced fluorescence quenching, without the need for further surface modification. A table comparing the required preparation time, size and quantum yield for various fluorescent AuNCs is shown in Table S1 (Supporting Information). Our AuNC probes provided highly sensitive analyses of Fe3+ ions, with an LOD of 3.5 μM, linearity of the response from 5 to 1280 μM, and excellent selectivity with only minor or negligible interference from other metal ions. A table that summarizes the key analytical attributes of various fluorescent ferric ion detection methods for easy comparison is shown in Table S2 (Supporting Information). Further 3252



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DOPA-Mediated Reduction Allows the Facile Synthesis of Fluorescent

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