Polyethylenimine-Capped Silver Nanoclusters as a Fluorescence

Article pubs.acs.org/JAFC

Polyethylenimine-Capped Silver Nanoclusters as a Fluorescence Probe for Highly Sensitive Detection of Folic Acid through a TwoStep Electron-Transfer Process Jian Rong Zhang,† Zhong Ling Wang,† Fei Qu,†,§ Hong Qun Luo,*,† and Nian Bing Li*,† †

Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of Education), School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China § Shandong Provincial Key Laboratory of Life-organic Analysis, College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, Shandong 273165, People’s Republic of China S Supporting Information *

ABSTRACT: A highly sensitive folic acid (FA) detection method based on the fluorescence quenching of polyethyleniminecapped silver nanoclusters (PEI-AgNCs) was put forward. In the sensing system, FA and PEI-AgNCs were brought into close proximity to each other by electrostatic interaction, and a two-step electron-transfer process, in which the electron was transferred from FA to AgNCs through PEI molecule, led to fluorescence quenching. The fluorescence quenching efficiency of PEI-AgNCs was linearly related to the concentration of FA over the range from 0.1 nM to 2.75 μM. Good linear correlation (R2 = 0.9981) and a detection limit of 0.032 nM were obtained under optimum conditions. Moreover, the proposed method was used for the determination of FA in real samples with satisfactory results, and those coexistent substances could not cause any significant decrease in the fluorescence intensity of AgNCs. Therefore, the proposed research system is of practical significance and application prospects. KEYWORDS: silver nanoclusters, folic acid, fluorescence, electron transfer

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INTRODUCTION

demonstrated to significantly reduce the incidence of neural tube defects resulting in malformations of the spine (spina bifida), skull, and brain (anencephaly).4−7 The lack of FA will cause many diseases from several types of cancer, to dementia, to affective disorders and Down’s syndrome.8−10 In addition, the ability of FA to lower homocysteine suggests it might have a positive influence on cardiovascular disease. Moreover, the U.S. Food and Drug Administration together with the U.K. Department of Health introduced mandatory corroboration of grain products with FA at concentrations of 140 and 240 mg 100 g−1, respectively.11,12 Other countries, such as Australia, Canada, New Zealand, and Chile, also mandated folate fortification of staple foods.13 Therefore, FA is a very significant component for human health, and the determination of FA has received a great deal of attention; a simple, rapid, reliable, and sensitive detection method is highly anticipated. FA is usually present at minor concentration in many systems and exhibits low stability, which makes the analysis of folic acid not easy. Nevertheless, several methods for the determination of FA have been developed in recent years, including enzymelinked immunosorbent assay,14 liquid chromatography−tandem mass spectrometry,15,16 capillary electrophoresis−chemiluminescence,17 microemulsion electrokinetic chromatography,18 high-performance liquid chromatography,19,20 high-performance liquid chromatography−fluorescence,21 fluorescence,22−27

Folic acid (FA), one of the water-soluble B group vitamins, by the name of (N-[p-{[(2-amino-4-hydroxy-6-pteridinyl)methyl]amino}benzoyl]-L-glutamic acid (see Scheme 1 for structure), exists widely in most animal and plant foods. FA acts as a coenzyme in the transfer and utilization of one-carbon groups, and it is also essential for the biosynthesis of nucleic acid, hemoglobin, and methyl compounds.1−3 Adequate supplementation of FA before and during pregnancy has been Scheme 1. Molecular Structures of both PEI and FA Together with Schematic Representation of FA Detection

Received: Revised: Accepted: Published: © 2014 American Chemical Society

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December 5, 2013 June 19, 2014 June 27, 2014 June 27, 2014 dx.doi.org/10.1021/jf4054534 | J. Agric. Food Chem. 2014, 62, 6592−6599

Journal of Agricultural and Food Chemistry

Article

chemiluminescence,28,29 and electrochemical methods.30,31 However, most of them have some limitations such as poor selectivity, insufficient sensitivity, and time-consuming procedure, which limit their applications to real sample analysis. Especially, although fluorescence analysis can provide high sensitivity and good selectivity, those developed fluorescent measurements based on oxidation-enhanced fluorescence,23,24 lanthanide-sensitized fluorescence,25,26 and quantum dots,27 remained poorly sensitive for FA. Consequently, further research is needed to develop a highly sensitive fluorescence sensor for folic acid. Metal nanoclusters, especially Au and Ag nanoclusters, have received much attention during the past decades due to their ultrasmall size, nontoxicity, biocompatibility, and high fluorescence properties, and various fluorescence sensors using metal nanoclusters have been developed.32−45 However, until now, fluorescence sensors for detecting FA on the basis of metal nanoclusters have not been reported. Herein, polyethylenimine-capped silver nanoclusters (PEI-AgNCs) were introduced as a new fluorescent platform for sensing FA. The interactions between PEI and FA bring FA and AgNCs into close proximity, and a two-step electron transfer from FA to AgNCs through PEI results in the decrease in fluorescence (Scheme 1). A two-step electron-transfer quenching mechanism was proposed and discussed. As expected, this sensing system has been applied successfully to the detection of FA in real samples. Those substances coexisting with FA could not lead to any significant fluorescence decrease of AgNCs. Consequently, our research would be a promising candidate for practical application.

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Sample Preparation. One tablet of folic acid was fully dissolved in 5 mL of 0.10 M NaOH solution,46 and the resulting mixture was centrifuged for 10 min at 12000 rpm. The supernatant was used as sample solution. Wheat flour was obtained from a local market. The sample preparation was done as described by Xiao,47 with a slight modification. First, wheat flour (0.5 g) or wet fermented dough (1.0 g) was dispersed in 5.0 mL of 0.1 M NaOH solution. Then, 10 min of centrifugation was performed at 12000 rpm. Finally, the supernatant was filtered using a 0.22 μm micropore filter membrane, and the filtrate was collected and used as the sample solution. A urine sample was stored in a refrigerator immediately after collection.48 After 10 min of centrifugation at 12000 rpm at 10 °C, an appropriate volume of 0.1 M AgNO3 solution was added to the supernatant and the mixture was shaken well. Another centrifugation later, the pH value of the supernatant was adjusted to 12 with NaOH solution. The pH-adjusted system was further centrifuged, and the supernatant was filtered using a 0.22 μm micropore filter membrane. Liquid milk and milk powder were first deproteinized according to the literature.49 Milk powder (0.5 g) or liquid milk (1.0 g) was uniformly dispersed in 0.05 M HAc solution, and the mixture was centrifuged for 20 min at 12000 rpm at 10 °C. The supernatant was collected, in which appropriate amounts of 0.1 M BaCl2 and AgNO3 solutions were added in turn. After thorough shaking, the following treatments, including centrifuging, pH-adjusting, centrifuging, and filtering, were the same as those for the urine sample.

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RESULTS AND DISCUSSION Establishment of the Fluorescence Sensing System for Folic Acid. The fact that the fluorescence of PEI-AgNCs is quenched by FA was observed in an exploratory experiment. To achieve the best sensing performance, several conditions such as pH, buffer system, concentration of buffer solution, and reaction time were optimized, respectively. The effect of the pH value on the sensing system was investigated first because the fluorescence quenching efficiency (1 − F/F0) of PEI-AgNCs is pH-dependent (Supporting Information Figure S1). F0 and F denote the fluorescence intensity of PEI-AgNCs in the absence and presence of FA, respectively. To cover a wide pH range, BR buffer solutions were used as the medium for optimizing the pH value. Figure S1 in the Supporting Information shows that, in the presence of FA, high-efficiency fluorescence quenching was observed at pH >7.0, and the highest quenching efficiency was obtained at pH 11.0. Therefore, a basic medium at pH 11.0 would ensure the sensitive detection of FA. The influence of different buffer systems on FA analysis was studied as the result of the influence of the ionic species on the stability of PEI-AgNCs.42 Apart from BR buffer solution (50 μL mL−1: 1 mL of solution contains 50 μL of stock BR buffer solution), another four buffer solutions, including Na2B4O7− NaOH (10 mM), Na2B4O7−Na2CO3 (10 mM), Na2HPO4− NaOH (10 mM), and NH2CH2COOH−NaOH (20 mM), were employed to test the quenching of the PEI-AgNCs fluorescence by FA. In consideration of the better stability of the diluted PEI-AgNCs and almost zero ionic strength of ultrapure water as well as the low ionic strength of sodium hydroxide solution at pH 11.0, ultrapure water and sodium hydroxide solution at pH 11.0 were also used as the medium for investigating the changes in fluorescence intensity of PEIAgNCs (Supporting Information Figure S2 ). The quenching efficiencies of PEI-AgNCs are higher, and only a little difference was observed for ultrapure water, sodium hydroxide solution, and BR buffer. Therefore, the other four buffers were excluded at first. Meanwhile, the BR buffer can give a greater ability to resist interference than ultrapure water and the pure acid or

MATERIALS AND METHODS

Chemicals. Silver nitrate (AgNO3), hyperbranched polyethylenimine (PEI, Mw = 10000, 99%), formaldehyde (35 wt %), 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and folic acid as well as all amino acids were purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Saccharides, vitamins, creatine, creatinine, uric acid, and urea were obtained from Shanghai Sangon Biotechnology Co., Ltd., Shanghai, China. Other reagents were of analytical reagent grade, and all chemicals were used as received without further purification. The PEI-AgNCs stock solution was synthesized according to the previous paper from our group.42 This stock solution was diluted 10-fold with ultrapure water (18.2 MΩ cm), and the diluted AgNCs was used as the probe to measure the concentration of FA in this assay. The FA stock solution was prepared daily by adding 0.10 M NaOH solution to an opaque system containing FA and ultrapure water until clear. Then, the clear FA solution was serially diluted to required concentrations with ultrapure water. The stock Britton− Robinson (BR) buffer solutions were prepared with 0.20 M NaOH solution and the mixed acid solution containing 0.04 M phosphate solution, 0.04 M boric acid solution, and 0.04 M acetate solution. All solutions were freshly prepared before use, and ultrapure water was used throughout the experiments. Detection of Folic Acid. In the quenching studies, 900 μL of ultrapure water, 50 μL of BR buffer solution (pH 11.0), 10 μL of diluted AgNCs, and 40 μL of FA solution at different concentrations were vortex mixed for 1 min. After 20 min of incubation at room temperature, the fluorescence emission spectra of PEI-AgNCs were recorded on a Hitachi F-4500 fluorescence spectrophotometer (Tokyo, Japan) at an excitation wavelength of 375 nm, and the fluorescence intensity at an emission wavelength of 452 nm was monitored. Three trials for each sample were performed, and the results represent the average of measurements. In addition, 10 μL of diluted AgNCs and 6 μM FA were used in the optimization of the sensing system. 6593

dx.doi.org/10.1021/jf4054534 | J. Agric. Food Chem. 2014, 62, 6592−6599


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Figure 1. Fluorescence emission spectra (A) and fluorescence quenching efficiencies (B) of PEI-AgNCs upon addition of different concentrations of FA: 0, 0.1, 1.0, 10 nM; 0.1, 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.25, 2.5, 2.75, 3.0, 3.5, 4.0, 6.0, 10.0 μM. The inset in panel B displays the linear relationship between the fluorescence quenching efficiency (1 − F/F0) and the concentration of FA from 0.1 nM to 2.75 μM. F0 and F denote the fluorescence intensity of PEI-AgNCs in the absence and presence of FA, respectively.

Figure 2. Selectivity of PEI-AgNCs for the detection of FA over other potential coexisting constituents. The concentration of FA is 2.75 μM; the concentration of saccharides is 5 mM; the concentrations of amino acids, carnitine, pantothenate, biotin, taurine, inositol, urea, creatine, creatinine, uric acid, oxalic acid, Ba2+, Ca2+, Na+, NO3−, CH3COO−, P2O72−, IO3−, and NH3·H2O are 1 mM; the concentrations of choline, nadide, and ascorbic acid are 100 μM; the concentrations of riboflavin, Cu2+, Mg2+, Zn2+, Ag+, and Fe2+ are 1 μM, respectively; the concentrations of CTAB, SDS, and Triton X-100 are 0.2, 0.2, and 1 mM, respectively. F0 and F represent the fluorescence intensity of PEI-AgNCs in the absence and presence of FA (or other species), respectively; (1 − F/F0) is the fluorescence quenching efficiency of PEI-AgNCs by FA or other species.

all measurements were carried out after the FA solution or real sample solution was incubated with PEI-AgNCs in the BR buffer (pH 11.0) for 20 min. Sensitivity of the Sensing System. The linearity and detection limit of this measurement were evaluated by varying the concentration of FA under optimum conditions discussed above. The fluorescence of PEI-AgNCs was quenched proportionately over the FA concentration range of 0.1 nM− 2.75 μM (Figure 1). A good linear correlation (R2 = 0.9981) was found, and a detection limit of 0.032 nM (3σ/slope) was obtained. Compared to other FA sensors reported in recent years (Supporting Information Table S1), our method provides a better linear range and a lower detection limit. Selectivity of the Sensing System. To investigate whether this sensing system is specific for FA, the fluorescence response of PEI-AgNCs to other potential coexistent substances was studied under optimum conditions. Figure 2 shows that 5 mM saccharides, 1 mM amino acids, carnitine, pantothenate, biotin, taurine, inositol, urea, creatine, creatinine, uric acid, oxalic acid, Ba2+, Ca2+, Na+, NO3−, CH3COO−, P2O72−, IO3−, and NH3·H2O, 100 μM choline, nadide, and ascorbic acid, 1 μM riboflavin, and 1 μM Cu2+, Mg2+, Zn2+, and Fe2+ (according to the solubility product constants (Kθsp) of the

basic solution. Therefore, to gain more outstanding sensing performance, BR buffer is the only medium that can be adopted in this work. According to the literature,42 not only do the ionic species have different influences on the stability of PEI-AgNCs, but also the ionic strength has a nonnegligible effect. The ionic strength is tied to the concentration of ions, so the effect of the concentration of BR buffer on sensing performance was also investigated. Figure S3 in the Supporting Information indicates that the fluorescence quenching efficiencies of PEI-AgNCs in the presence of FA were decreased by increasing the concentration of BR buffer solution (pH 11.0). To gain the most satisfactory results, 50 μL mL−1 BR buffer solution was used for sensing FA. Finally, under the optimum conditions mentioned above, the effect of reaction time on the fluorescence intensity of the system was investigated. The fluorescence intensity of PEIAgNCs in the presence of FA was monitored for 60 min. The maximum quenching and stable fluorescence intensity was observed after 20 min of interaction (Supporting Information Figure S4), indicating that the interaction between FA and PEIAgNCs was complete within 20 min and the fluorescence of PEI-AgNCs/FA was stable for at least 40 min. Consequently, 6594



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Table 1. Detection of FA in Real Samples (n = 3) sample tablet

wheat flour

wet fermented dough

human urine

liquid milk

milk powder 1

milk powder 2

specified

measured

0.400 mg tablet−1

0.401 mg tablet−1

original (μM)

spiked (μM)

found (μM)

recovery (%)

RSD %

0.50 0.50 0.50

0.50 1.00 1.50

1.00 1.51 2.02

100.0 100.1 101.3

1.8 0.4 1.9 0.3

1.26 1.26 0.36

0.50 1.00 1.00

1.74 2.23 1.41

96.0 97.0 105.0

2.6 1.3 3.1 2.0

0.24 0.62 1.05

1.50 0.50 1.00

1.70 1.10 2.10

97.33 96.0 105.0

1.4 3.4 3.5 1.3

1.68 1.68 0.81

1.0 0.5 1.0

2.65 2.20 1.79

97.0 104.0 98.00

2.0 2.5 0.7 2.6

0.89 0.89 0.89

0.5 1.0 1.5

1.40 1.93 2.41

102.0 104.0 101.3

2.3 0.5 1.1 2.2

0.31 0.31 0.31

0.5 1.0 1.5

0.83 1.34 1.78

104.0 103.0 98.0

1.8 1.5 2.1 1.7

0.03 0.03 0.03

0.5 1.0 1.5

0.55 1.05 1.51

104.0 102.0 98.67

2.0 2.4 1.1 2.0

74.8 mg 100 g−1

53.2 mg 100 g−1

1.86 μg mL−1

2.18 μg mL−1

545 μg 100 g−1

68.8 μg 100 g−1

Mechanism for the Recognition of Folic Acid. To understand the mechanism of the fluorescence quenching of PEI-AgNCs by FA, the absorption and the fluorescence spectra of PEI-AgNCs, FA, and PEI-AgNCs/FA were studied. Figure 3A shows three similar fluorescence excitation wavelengths and three similar fluorescence emission wavelengths, suggesting that fluorescence resonance energy transfer as a possible mechanism for AgNCs fluorescence quenching is ruled out. Moreover, the absorption spectrum of PEI-AgNCs/FA exhibits no new peak (Figure 3B) compared to those of both AgNCs and FA, which implies that the decrease in fluorescence intensity was not induced by the change in the excitation wavelength or the aggregation of AgNCs, thereby excluding possible mechanisms including static quenching upon formation of the ground-state complex and AgNCs aggregation-induced self-quenching.44,52 In addition, evidence from the IR spectral data and the highresolution transmission electron microscopy (HRTEM) image also support the above inference, because IR spectra of PEIAgNCs/FA were almost a simple superposition of those of both PEI-AgNCs and FA (Figure 3C) and AgNCs remained dispersed upon addition of FA, which were similar to the original PEI-AgNCs (Figure 3D). As for the quenching mechanism, theoretically, fluorescence quenching in this system could occur by the nonfluorescent ground-state complex, aggregation of AgNCs, energy transfer,

corresponding hydroxides, the concentrations of these metal ions are not higher than 1 μM in this testing system) could not lead to any significant fluorescence decrease of PEI-AgNCs. The surfactants, including 0.2 mM hexadecyltrimethylammonium bromide (CTAB, cationic), 0.2 mM sodium dodecyl sulfonate (SDS, anionic), and 1 mM Triton X-100 (nonionic), also show only little effect on the determination of FA. These results indicate that the proposed assay has a good selectivity for FA. Analytical Applications in Real Sample. To assess the applicability of the sensing system, the proposed method was used for the analysis of FA in tablet, human urine, wheat flour, wet fermented dough, liquid milk, and milk powder samples. The results are listed in Table 1. The recovery from 96.0 to 105.0% and the relative standard deviation (RSD) smaller than 5% were satisfactory. Furthermore, the measured result of 205 μg FA 100 g−1 milk powder is in good agreement with the testing value of 183 μg 100 g−1 by official method50 in the Comprehensive Test Center of China Academy of Inspection and Quarantine. The found values of 0.407 mg tablet−1 and 1.68 μg mL−1 human urine according to the ref 25 also show a good correspondence with the values found by the proposed method (0.401 mg tablet−1 and 1.55 μg mL−1 human urine, respectively). Therefore, the proposed design is feasible for FA detection in practical applications. 6595



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Figure 3. (A) Fluorescence emission and excitation spectra of PEI-AgNCs, FA, and PEI-AgNCs/FA. Concentration: 10 μL mL−1 diluted PEIAgNCs; 2.5 μM FA. (B) Absorption spectra of PEI-AgNCs, FA, and PEI-AgNCs/FA. (C) IR spectra of (a) PEI-AgNCs, (b) FA, and (c) PEIAgNCs/FA. (D) HRTEM image of (a) PEI-AgNCs and (b) PEI-AgNCs/FA.

Figure 4. (A) Decay time profiles of (a) PEI-AgNCs, (b) PEI-AgNCs/FA, (c) FA, (d) FA/PEI-AgNCs, and (e) FA/PEI. Concentration: 10 μL mL−1 diluted PEI-AgNCs; 10 μM FA. The concentration of PEI was the same as that of PEI in 10 μL mL−1 diluted PEI-AgNCs. λex = 375 nm and λem = 452 nm for (a) and (b); λex = 367 nm and λem = 462 nm for (c), (d), and (e). (B) Cyclic voltammogram of FA and PEI in BR buffer solution (pH 11.0) on the glassy carbon electrode.

Table 2. Fitting Results Using Global Analysis for the Data of Figure 4a sample PEI-AgNCs PEI-AgNCs/FA FA FA/PEI-AgNCs FA/PEI a

a1 0.755 6.17 × 7.82 × 3.95 × 5.42 ×

10−2 10−2 10−2 10−2

τ1 (ns)

a2

τ2 (ns)

a3

τ3 (ns)

0.133 0.753 6.24 × 10−2 0.495 5.43 × 10−3

1.108 0.35 9.719 0.554 10.607

7.72 × 10−3 7.19 × 10−3 0.132 7.79 × 10−3 0.285

τ̅ (ns)

0.294 1.543 4.59 1.924 4.485

3.34 10.38 0.6589 10.806 0.529

0.798 2.45 7.22 2.78 3.94

ai represents the amplitudes of components i at t = 0; τi is the decay time of component i; τ̅ is the average fluorescence lifetime.

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AgNCs quenched the fluorescence of AgNCs. Although interactions between FA and PEI shortened the distance between FA and AgNCs, the distance was still longer than that between PEI and AgNCs. To our knowledge, electron transfer is distance-dependent,55 so electrons are more easily transferred from PEI to AgNCs. Consequently, the two-step electrontransfer process was reasonably proposed, in which FA served as an electron donor, AgNCs as an electron acceptor, and PEI as an electron-transfer bridge. In conclusion, a highly sensitive method for the detection of FA based on the fluorescence quenching of PEI-AgNCs was developed in this study, and the mechanism of fluorescence quenching through a two-step electron-transfer process was proposed. We believe that the research is of importance in the development of metal nanoclusters and FA detection in real samples.

or electron transfer. As discussed above, only the electron transfer is expected in the quenching process. The proposed electron-transfer quenching mechanism was further examined using fluorescence decay kinetics. The fluorescence decay profiles of PEI-AgNCs in the absence and presence of FA together with FA in the absence and presence of PEI-AgNCs are shown in Figure 4A, respectively. The fluorescence decay of all samples could be fitted using a three-exponential function, and the fitted results are given in Table 2. The average fluorescence lifetime (τ)̅ was estimated using the equation51 τ̅ =

∑ aiτi2 ∑ aiτi

(1)

where ai represents the amplitude of the component i at t = 0 and τi is the decay time of the component i. As shown in Table 2, the average fluorescence lifetime estimated for FA was approximately 7.22 ns. In the presence of PEI-AgNCs, the average lifetime for FA decreased to 2.78 ns. On the basis of the literature,52 we can deduce that the decrease in lifetime is due to the efficient electron transfer from FA to PEI-AgNCs, wherein FA acts as an electron donor. In addition, before and after the addition of FA, the lifetime of PEI-AgNCs increased from 798 ps to 2.54 ns. This opposite change provided evidence for electron-transfer sequence from FA to PEIAgNCs. Interestingly, the fluorescence of FA could also be quenched by PEI. The fluorescence quenching mechanism based on electron transfer is speculated because we did not observe any absorption band in the absorption spectrum of PEI, and the absorption spectrum of FA did not vary after PEI was added (Supporting Information Figure S5). The feasibility of the electron-transfer quenching was examined by further measuring the fluorescence decay dynamics of FA solution in the presence of PEI (plot e in Figure 4A). An average lifetime of 3.94 ns was obtained, and considering the average lifetime of 7.22 ns for FA, we conclude that the excited electrons of FA can be transferred to PEI. Meanwhile, the oxidation potential of both PEI and FA was further adopted to confirm the direction of electron transfer. The oxidation potential of PEI was measured to be +0.68 V versus Ag/AgCl (saturated KCl) [0.90 V vs normal hydrogen electrode (NHE)], and FA exhibited a more positive oxidation potential of +0.83 V versus Ag/AgCl (1.05 V vs NHE), as shown in Figure 4B. According to the test results, the electron transfer from FA to PEI can also be concluded. On the basis of the conclusions of the foregoing discussion and the sequence of 7.22 ns (the average lifetime of FA) > 3.94 ns (the average lifetime of FA with addition of PEI) > 2.54 ns (the average lifetime of FA in the presence of PEI-AgNCs), we put forward a two-step electron-transfer process in the system comprising FA and PEI-AgNCs, in which an electron is transferred from FA to AgNCs through the PEI molecule. Moreover, we note that a similar energy transfer mechanism has been proposed for ethidium bromide in the complex comprising a cationic conjugated polymer, a fluorescein molecule linked to one terminus of the DNA, and ethidium bromide.53 FA is brought into close proximity to the template molecule PEI for AgNCs through the electrostatic interactions between FA and PEI, and electron transfer from FA to PEI changes the electronic properties of PEI. As metal nanoclusters are both excellent electron donors and acceptors, the fluorescence of the nanoclusters can be quenched by electron acceptors or donors.54 Therefore, electron transfer from PEI to

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S5 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(H.Q.L.) Phone: +86 23 68253237. Fax: +86 23 68253237. Email: [email protected]. *(N.B.L.) Phone: +86 23 68253237. Fax: +86 23 68253237. Email: [email protected]. Funding

This work was supported by the National Natural Science Foundation of China (No. 21273174, 20975083), the Municipal Science Foundation of Chongqing City (No. CSTC-2013jjB00002), and the Fundamental Research Funds for the Central Universities of China (No. XDJK2013C064). Notes

The authors declare no competing financial interest.

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ABBREVIATIONS USED FA, folic acid; PEI-AgNCs, polyethylenimine-capped silver nanoclusters; AgNCs, silver nanoclusters

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REFERENCES

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