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Fluorescence Enhancement of Cadmium Selenide Quantum Dots Assembled on Silver Nanoparticles and Its Application for Glucose Detection Yecang Tang, Qian Yang, Ting Wu, Li Liu, Yi Ding, and Bo Yu Langmuir, Just Accepted Manuscript • Publication Date (Web): 19 May 2014 Downloaded from http://pubs.acs.org on May 21, 2014

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Fluorescence Enhancement of Cadmium Selenide Quantum Dots Assembled on Silver Nanoparticles and Its Application for Glucose Detection Yecang Tang*, Qian Yang, Ting Wu, Li Liu, Yi Ding, Bo Yu College of Chemistry and Materials Science, Anhui Normal University, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, Wuhu 241000, China E-mail: [email protected]

ABSTRACT: In this work, a new assembled glucose sensor based on Ag nanoparticles (AgNPs)-enhanced fluorescence of CdSe quantum dots (QDs) was developed.

The

mercaptoglycerol-modified

AgNPs

and

aminophenylboronic

acid-functionalized CdSe QDs are assembled into AgNPs-CdSe QDs complexes through the formation of boronate ester bond. As compared with that of bare CdSe QDs, up to 9-fold fluorescence enhancement and a clear blue-shift of the emission peak for AgNPs-CdSe QDs complexes were observed, which is attributed to the surface plasmon resonance of AgNPs. In addition, the as-formed complexes are gradually disassembled in the presence of glucose molecules because they can replace the AgNPs by competitive binding with boronic acid groups, resulting in the weakening of fluorescence enhancement. The decrease of fluorescence intensity presents a linear relationship with glucose concentration in the range from 2 to 52 mM with a detection limit of 1.86 mM. Such a metal-enhanced QDs fluorescence system may have promising applications in chemical and biological sensors. KEYWORDS: CdSe quantum dots, Ag nanoparticles, metal-enhanced fluorescence, glucose

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1. INTRODUCTION The in situ and accurate detection of glucose has long been of great interest due to its important role in the diagnosis of diabetes mellitus. The most investigated method for glucose measurement is the enzyme glucose oxidase, which has already been commercialized for diabetic patients.[1-3] However, the instability and high cost for such sensors have become serious problems. Accordingly, there is widespread research interest in developing synthetic reagents for monitoring glucose. Boronic acid is one of the promising ligands. It is well known that boronic acid and its derivatives are capable of forming reversible covalent complexes with 1,2- or 1,3diols, such as glucose.[4] The resulting complexes have been used to develop sensors for glucose based on fluorescent,[5-9] holographic,[10,11] colorimetric,[12-14] and electrochemical methods.[15, 16] Among these, the fluorescence analysis method is a powerful tool for glucose assay since it is nondestructive and because of the advantages such as low cost andhigh sensitivity. Quantum dots (QDs), as a new type of fluorescent probe, have attracted a great deal of attention owing to their photostability, continuous absorption spectra, and size-controlled fluorescence properties.[17-22] Recently, QDs and fluorescent organic moieties have been coupled with boronic acid-based ligands for the optical detection of glucose. For example, Singaram and coworkers [23] were the first to utilize QDs for glucose sensing. The fluorescence of CdTe/ZnS QDs was quenched by boronic acid-substituted viologen, while it was recovered upon the introduction of glucose to the quenched QDs solution. Willner and coworks

[24]

developed a competitive assay

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for glucose using fluorescence resonance energy transfer (FRET) between phenylboronic acid-functionalized CdSe/ZnS QDs and fluorophore-labeled galactose. Zhou and coworks

[7]

prepared phenylboronic acid-modified CdTe/ZnTe/ZnS QDs

and used them to determine the intracellular glucose level. The QDs were found to be self-assembled in the presence of glucose, resulting in fluorescence quenching of QDs. The same group also developed a glucose sensing system based on the immobilization of CdS QDs in the interior of boronic acid based microgels. The fluorescence of CdS QDs could be reversibly quenched and antiquenched as the microgels underwent swelling and shrinkage according to the glucose concentration change.[8] As the detection sensitivity of the QDs-based sensor is closely related with its fluorescence intensity, it is reasonable to expect that QDs fluorescence could be further enhanced. Metal-enhanced fluorescence (MEF) is an alternative method. It is well established that metal nanoparticles (NPs), especially Au nanoparticles (AuNPs) and Ag nanoparticles (AgNPs), exhibit a strong surface plasmon resonance. The plasmon excitations produce a locally enhanced electromagnetic field in the vicinity of NPs, and finally result in the fluorescence enhancement of QDs within a certain distance. Recently, several attempts have been developed to enhance the emission of QDs by adjusting the distance between QDs and metal NPs because MEF effect is related to the interparticle distance. One common method is to insert spacer layers with well defined thickness of solid medium between QDs and metal NPs.[25-31] However, for some special cases, the fluorescence enhancement was also observed without presence of spacer layer.[28,32,33] Recently, the metal-enhanced QDs

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fluorescence was obtained in liquid medium, which possesses great potential applications for QDs sensors in bioscience.

[27, 34-36]

For instance, MEF of CdTe QDs

in aqueous solution was observed by direct mixing negatively charged AuNPs and negatively charged CdTe QDs. The magnitude of fluorescence enhancement was modulated by controlling the concentration and feed ratio of QDs and AuNPs.

[34]

Although to date the mechanism for metal-enhanced QDs fluorescence has been extensively investigated, its application in fluorescence-based assay is quite limited. In the present work, a metal-enhanced QDs fluorescence system is designed for the direct determination of glucose by conjugating CdSe QDs with AgNPs through the formation of reversible boronate ester bonds. The overall detection strategy is illustrated in scheme 1. The aminophenylboronic acid (APBA)-functionalized CdSe QDs are assembled on the surface of mercaptoglycerol (MG)-modified AgNPs via the covalent bonds between the boronic acid groups and the diol goups, thereby resulting in significant fluorescence enhancement of CdSe QDs. In the presence of glucose, the competition

between

MG-modified

AgNPs

and

glucose

towards

APBA-functionalized CdSe QDs would induce the disassembly of AgNPs-CdSe QDs complexes, which leads to the decrease of fluorescence intensity. The AgNPs-CdSe QDs complexes with MEF effect show an application prospect in the field of chemical and biological sensors.

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Scheme 1. Schematic illustration of the detection of glucose based on fluorescence enhancement of CdSe QDs assembled on AgNPs 2. EXPERIMENTAL SECTION 2.1 Materials 3-Mercaptopropionic acid (MPA, 99%), mercaptoglycerol (MG, 95%), NaBH4, Se powder (~60 mesh, 99.999%), 3-aminophenylboronic acid monohydrate (APBA), N, N′-dicyclohexylcarbodiimide (DCC), and 4-dimethylaminopyridin (DMAP) were purchased from Aladdin Chemical Co. Ltd (Shanghai, China). CdCl2·2.5H2O, AgNO3, glucose, and other chemicals acquired from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) were of analytical grade and used as received. The buffer solution was freshly prepared with KH2PO4, Na2HPO4·12H2O and Na3PO4·12H2O. Deionized water was used throughout the work. 2.2 Characterization Fourier transform infrared (FT-IR) spectra were recorded on Perkin-Elmer PE-983 FT-IR spectrophotometer with KBr pellets. Transmission electron microscopy (TEM) images were obtained by using a Tecnai G20 transmission electron microscope (America) with an accelerating voltage of 200 kV. High-angle

annular dark

field

scanning

transmission electron microscopy

(HAADF-STEM) and the corresponding energy-dispersive X-ray spectroscopy (EDS) mapping images were recorded on a Tecnai G2F20 electron microscope operated at

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300 kV. Diluted aqueous dispersions were dropped on a carbon-coated copper TEM grid, followed by drying at room temperature. UV-vis absorption spectra were measured on a Hitachi U-4100 spectrophotometer (Japan). Fluorescence spectra were obtained by using a Hitachi F-4500 spectrofluorometer (Japan) equipped with R3896 red-sensitive multiplier and 1 cm quartz cuvette. The pH values were measured with a Model pHS-3C (Shanghai, China). Dynamic laser light scattering (DLS) was conducted on an ALV/DLS/SLS-5022F spectrometer with a multi-τ digital time correlation (ALV5000) and a cylindrical 22 mW UNIPHASE He-Ne laser (λ0 = 632 nm) as the light source at the scattering angle of 90°. Each sample filtered through 0.45 µm Millipore Millex-LCR filter to remove dust before the DLS measurement. 2.3 Preparation of APBA-functionalized CdSe QDs MPA-capped CdSe QDs were first prepared in aqueous solution according to the literature reported.[37] Briefly, the precursor Na2SeSO3 was produced by dissolve Se (20 mg) powder and Na2SO3 (1.0 g) in 10 mL water. CdCl2·2.5H2O (46.5 mg) and MPA (35 µL) were dissolved in 10 mL of water under stirring, which was then adjusted to pH 9.0 with 1.0 M NaOH and purged with N2 for 40 min. Freshly prepared oxygen-free NaSeSO3 (2 mL) was injected into the solution under vigorous stirring, and then the resulting solution was heated to 100 ºC and refluxed for 2 h. The product was obtained by centrifugation. MPA-capped CdSe QDs (8 mg) was dissolved in 10 mL DMF, to which 11 mg of DCC and 10 mg DMAP were added and incubated for 30 min with ice-water bath under continuous gentle stirring to activate the carboxylic acids. After that, 10 mg of APBA dissolved in 10 mL DMF was added dropwise and then stirred overnight at

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room temperature for the conjugation of CdSe QDs and APBA. The resulting sample was centrifuged and washed three times with pH 7.4 PBS, which was then dissolved in 200 mL H2O and kept at 4 ºC before use. The molar concentration of obtained CdSe QDs was calculated to be 1.07 µM based on Peng′s empirical equations.[38] The APBA-functionalized CdSe QDs solution could be stable for 3 months at least. 2.4 Preparation of MG-modified AgNPs AgNPs were synthesized according to the previous report with slight modification.

[39]

Briefly, 20 mL of ice-cold, freshly

prepared NaBH4 (2.0 mM) solution was added to 5 mL of AgNO3 (1.0 mM) under vigorous stirring and reacted for 30 min, and then 100 µL of MG (0.1 M ) and 50 µL MPA (0.1 M) were added. The resulting mixture was dialyzed against deionized water using a semipermeable membrane with a cutoff molar mass of 8~14 kDa to remove excess stabilizing agents. The final concentration of AgNPs was calculated to be approximately 38 nM. [40] The as-prepared AgNPs solution was kept at 4 oC. 2.5 AgNPs-enhanced fluorescence of CdSe QDs Typically, 0.4 mL of purified CdSe QD (1.07 µM), 0.1 mL of pH 7.4 PBS and various amounts of AgNPs (38 nM) were placed in a series of colorimetric tubes. Afterwards, each sample solution was diluted to 1.0 mL with deionized water and incubated at room temperature for 1 h. The fluorescence spectra were recorded within the range of 420 to 800 nm with excitation wavelength at 400 nm. The slit widths of excitation and emission were 5 and 10 nm, respectively. The fluorescence intensity at 525 nm was used for quantitative analysis. 2.6 Fluorescence detection of glucose 0.91 mL of AgNPs (38 nM) solution, 5.2 mL

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of CdSe QDs (1.07 µM) solution and 1.3 mL pH 7.4 PBS were mixed and stirred for 1 h. The mixture solution was divided into 13 equal portions. Subsequently, 0.1 mL of pH 7.4 PBS and different concentrations of glucose were added, respectively. Each portion was further diluted with deionized water to a final volume of 1.0 mL and reacted for 30 min. The fluorescence spectra were recorded under the same experimental conditions as described above. 3. RESULTS AND DISCUSSION 3.1 Characterizations of AgNPs and CdSe QDs

In order to introduce ABPA

ligand to CdSe QDs surface, MPA capped CdSe QDs were first prepared, and then reacted with APBA in the presence of DCC and DMAP. The binding of APBA onto the surface of CdSe QDs was confirmed by FT-IR spectroscopy, TEM, UV-vis absorption spectra and fluorescence spectra. As shown in Figure 1, the FT-IR spectrum of APBA-functionalized CdSe QDs (curve b) has the characteristic peaks of APBA at 1645, 1568, 1306, 1273 and 668 cm-1. The band at 1645 cm-1 is assigned to C=O stretching vibration, while the 1568 cm-1 and 1273 cm-1 bands arising from amide N–H bending vibration and C–N stretching vibration correspond to amide II and III, respectively. The peak at 1306 cm-1 is attributed to B-O stretching vibration. In addition, the aromatic C-H distortion vibration appears at 668 cm-1. These results observed here may suggest that APBA molecules have been successfully bound to the surface of CdSe QDs. Figure 2A clearly depicts that APBA-functionalized CdSe QDs are spherical with average diameter of about 5 nm, which is in good agreement with the result calculated by the empirical equation.[38] The UV-vis absorption spectrum

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and fluorescence emission spectrum of APBA-functionalized CdSe QDs are shown in Figure 3 (curves a and c), respectively. Clearly, the first excitonic absorption peak and fluorescent emission wavelength of CdSe QDs are 390 nm and 583 nm, respectively.

a

Transmittance

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b 1645 1273 1568 1402

3000

2000

668

1000 -1

Wavenumber / cm

Figure 1. FT-IR of CdSe QDs (a) and APBA-functionalized CdSe QDs (b).

Figure 2. Typical TEM images of APBA-functionalized CdSe QDs (A), MG-modified AgNPs (B), and AgNPs-CdSe QDs complexes (C). HAADF-STEM image of Ag NP-CdSeQD complexes (D), EDS elemental mapping images of Ag (E) and Cd (F).

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To prepare water-soluble and diol groups modified AgNPs, we initially designed to directly use MG as stabilizing agent, whereas the formed AgNPs were unstable. To overcome this problem, MPA and MG were used as co-stabilizing agents to impart stabilizing and the optimized ratio of MPA to MG was 1:2. Figure 2B shows a typical TEM image of MG-modified AgNPs. Monodispersed and spherical AgNPs with average diameter of about 14 nm are obtained. The UV-vis absorption spectrum of bare AgNPs displays an obvious absorption peak centered at 406 nm (Figure 3, curve b), a typical surface plasmon resonance band for AgNPs, indicating the formation of AgNPs. After modified with MG, the absorption peak is shrifted to 420 nm (Figure 3, curve c). This red-shifting phenomenon is attributed to an increase of the local

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Wavelength / nm Figure 3. UV-vis absorption spectra of APBA-functionalized CdSe QDs (a), bare AgNPs (b) and MG-modified AgNPs (c); Fluorescence emission spectrum of APBA-functionalized CdSe QDs with λex=400 nm (d).

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3.2 Fluorescence enhancement of CdSe QDs by AgNPs Figure 4A and B shows the fluorescence spectra of CdSe QDs upon the addition of different amounts of AgNPs. It can be seen that the degree of fluorescence enhancement (F/F0: where F and F0 represent the fluorescence intensity of CdSe QDs in the presence and absence of AgNPs) of CdSe QDs increases with the concentration of AgNPs and then tends to a constant value at a concentration of 2.4 nM (Figure 4C). However, when much higher amounts of AgNPs are added, the degree of the fluorescence enhancement decreases. The maximum fluorescence enhancement is about 9-fold when compared with that of CdSe QDs in the absence of AgNPs. At the same time, the emission peak undergoes a significant blue shift from 583 nm to 525 nm as the concentration of AgNPs increases from 0 to 2.4 nM. This observation, which is also observed in the CdS/Au nanocomposite, can be explained by the passivation of trap states on the surface of CdSe QDs.[28]

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cAgNPs / nM Figure 4. Fluorescence spectra of CdSe QDs upon the addition of different concentrations of AgNPs from 0 to 2.4 nM (A) and from 2.4 nM to 19 nM (B) in 0.01 M pH 7.4 PBS. Dependence of fluorescence intensity ratios F/F0 on AgNPs concentration (C).

Recently, it has been demonstrated that thioglycolic acid-functionalized CdTe QDs were self-assembled onto citrate-capped AuNPs by hydrogen binding.[42,

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The

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positively charged CdTe QDs could form fluorescence resonance energy transfer donor-acceptor assemblies with negatively charged AuNPs by electrostatic interactions, which effectively quench the fluorescence intensity of CdTe QDs. 45]

As we know that the pKa of MPA is 4.32,

[21]

[27, 44,

and the carboxylic acid groups in

MPA are almost completely deprotonated in neutral or weakly basic aqueous solution. APBA is a weak acid with pKa about 8.2,[46,

47]

and partially charged at pH 7.4

aqueous solutions. In the present system with a pH of 7.4, both CdSe QDs and AgNPs possess negative charges. Therefore, electrostatic interaction and hydrogen bonds are prevented because of Coulomb repulsion effects between them. As demonstrated in the previous report boronic acids are well-known for their ability to reversibly bind diols.[48] In addition, a control experiment by mixing AgNPs with MPA-modified CdSe QDs was performed under the same experimental conditions. No significant change in the emission spectrum was observed (data not shown). These collective results suggest that the fluorescence change of CdSe QDs is originated from the formation of AgNPs-CdSe QDs complex, in which APBA-functionalized CdSe QDs are covalently bonded with MG-modified AgNPs. The formed AgNPs-CdSe QDs complexes are further confirmed by TEM image. As shown in Figure 2C, ca. 5 nm CdSe QDs are scattering around AgNPs. From the HAADF-STEM image (Figure 2D), we can observe that the CdSe QDs are distributed evenly on the surface of AgNPs. The corresponding EDS-mappings images (Figure 2E, F) show that the Cd element is homogeneously distributed around the Ag atomic mapping region. These findings indicate that APBA-functionalized CdSe QDs are

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successfully assembled onto the surface of MG-functionalized AgNPs. Furthermore, the amount of AgNPs-CdSe QDs complexes initially increases with the concentration of added AgNPs. In this situation, the resonance coupling between AgNPs and CdSe QDs would take place, and the surface plasmons of AgNPs would be excited by the CdSe QDs’ luminescence.[27] This plasmon excitation produces a local enhanced electromagnetic field, which leads to increased excitation rate of CdSe QDs and then results in the enhanced fluorescence. Therefore, the fluorescence emission of CdSe QDs is enhanced with the addition of AgNPs. However, the excess amount of AgNPs probably induces aggregation of as-formed complexes, so the degree of fluorescence enhancement of CdSe QDs decreases. To prove this AgNPs-induced aggregation of complexes, the DLS technique was used to determine the hydrodynamic radius (Rh) of scatterers. The results are shown in Figure S1 (See Supporting Information). An increase in Rh is readily evident with the addition of AgNPs, which indicates the formation of aggregates. To optimize the fluorescence enhancement of AgNPs-CdSe QDs for detection of glucose, the experimental parameters including AgNPs concentration, media pH and incubation time were investigated. As discussed above, in the AgNPs-CdSe QDs system, the maximum fluorescence enhancement was observed as AgNPs concentration is 2.4 nM. Accordingly, the optimal concentration of AgNPs is selected at 2.4 nM. In this study, the boronic acid groups can be ionized by combining with OH− and transform into relatively hydrophilic but unstable borate anions, which can further form more favorable charged and stable borate esters by complexation with

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diol in aqueous medium.[49] Therefore, the media pH greatly affects the formation of

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pH Figure 5. Evolution of relative intensity and the emission position of AgNP-CdSe QDs as a function of the pH.

Figure 5 shows the effect of pH on the fluorescence intensity and the emission position of AgNPs-CdSe QDs complexes. Clearly, as the pH increases from 7.4 to 11.5, the relative intensity of AgNPs-CdSe QDs complexes gradually decreases accompanied with a red shift of the emission maximum from 525 nm to 570 nm. As mentioned above, the increase of pH value imparts the formation of stable and negatively charged boronate, whereas the Coulomb repulsion between AgNPs and CdSe QDs is also enhanced. In other words, the higher pH is unfavorable for the assembly of CdSe QDs on AgNPs surface. It should be noted that if pH for AgNPs-CdSe QDs system is adjusted to 6.4, the fluorescence is almost completely quenched (data not shown). This may be attributable to the fact that

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APBA-functionalized CdSe QDs in a weak acid environment are relatively unstable and are prone to aggregate. So the physiological pH 7.4 is recommended for use in further experiments. The time dependence of the reaction time between AgNPs and CdSe QDs was investigated, and the result reveals that the fluorescence intensity increases dramatically and then reaches a plateau at 40 min. Therefore, an incubation time of 40 min is selected. 3.3 Detection of glucose based on AgNPs-CdSe QDs complexes As discussed above, AgNPs and CdSe QDs are assembled together through the formation of the reversible boronate ester bonds. It has been found that rigid cis-diols found in many saccharides have stronger affinities than acyclic diols like glycerol with PBA group.[50-52] In the presence of glucose, the free PBA groups will bind with glucose because of the competition driving force, which perturbs the original equilibrium between boronic acid and MG, and results in gradual dissociation of as-formed AgNPs-CdSe QDs complexes. Consequently, the MEF effect will reduce and even disappear (Scheme 1). Figure 6A shows the fluorescence spectra of AgNPs-CdSe QDs complexes upon the addition of different concentrations of glucose. As expected, the fluorescence intensity of AgNPs-CdSe QDs decreases gradually with the increment of glucose concentration. A control experiment was performed in the absence of AgNPs. There is no significant change in the emission spectrum of CdSe QDs. This indicates that fluorescence quenching can be exclusively ascribed to the disaggregation of AgNPs-CdSe QDs complexes induced by glucose. As shown in Figure 6B, a good linear relationship between the fluorescence intensity and the

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glucose concentration is obtained in the range of 2~52 mM, which is over the range of physiological glucose concentration (≈2.5~20 mM). Previous studies show that a linear response across the physiological glucose range is highly desirable for glucose sensor applications.[8,

23]

The calibration curve can be expressed as (F0−F)/F0 =

0.1315+0.00745cglucose (c: mM), where F and F0 represent the fluorescence intensity in the presence of different concentrations of glucose and the absence of glucose, respectively. The corresponding correlation coefficient is 0.991. The detection limit for glucose is 1.86 mM, calculated according to the 3σ/k, where σ represents the standard deviation of nine blank measurements and k is the slope for the range of the linearity used. The relative standard deviation (RSD %) for 11 standard samples each containing 20 mM of glucose is 1.4 %, indicating that the fluorescence response of AgNPs-CdSe QDs complexes toward glucose is highly reproducible.

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cglucose / mM Figure 6. (A) Fluorescence spectra of AgNPs-CdSe QDs complexes upon the addition of different concentrations of glucose in 0.01 M pH 7.4 PBS. (B) Linear plot of fluorescence intensity ratios (F0−F)/F0 versus the concentration of glucose.

In order to examine the selectivity of the as-formed complexes for glucose, the effects of common interfering agents including carbohydrate, amino acids and ions were examined. Figure 7 shows the relative intensity of AgNPs-CdSe QDs complexes with different substances, in which the response for 20 mM glucose without addition of any other substance is defined as 1. It is observed that the investigated agents (except Cu2+) have little effect on the fluorescence intensity of AgNPs-CdSe QDs system. The apparent fluorescence quenching induced by Cu2+ maybe due to the fact that the complexation between Cu2+ and COO− groups on AgNPs surface can induce AgNPs aggregation. The interference of Cu2+ can be previously eliminated by the addition of a masking agent, ethylenediaminetetraacetic acid.[53,

54]

Therefore, this

AgNPs-CdSe QDs system possesses a high selective fluorescence response to glucose

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and can be applied in the determination of glucose.

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Figure 7. Effects of potential interfering substances on the fluorescence intensity of AgNPs-CdSe QDs in the presence of 20 mM glucose. The substance concentrations from left to right are 1×10-2, 1×10-2, 1×10-2, 1×10-2, 1×10-4, 7.2×10-6, 1×10-6, 1×10-5, 1×10-5, 3.6×10-5, 5×10-5, 1×10-5, 2×10-3, 1.6×10-3, 2.9×10-6, 2×10-4, 5×10-5, 5×10-5, 1×10-6, 2×10-6, and 1.5×10-6 M, respectively).

4. CONCLUSIONS In conclusion, we have developed a metal-enhanced QDs fluorescence system by conjugating CdSe QDs with AgNPs through reversible boronate ester bond. The fluorescence enhancement ratio of CdSe QDs can be optimized up to ~ 9. This reversible covalent bonding interactions offer a great opportunity for constructing nanoscale assemblies. Moreover, the as-formed AgNPs-CdSe QDs complexes can be

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used as a fluorescence probe for glucose sensing. The fluorescence of AgNPs-CdSe QDs system is reduced with the addition of glucose through the competitive binding of glucose to the boronic acid ligands. A linear decrease in fluorescence intensity across the physiological glucose range is achieved. The results, shown here, can offer an important reference for studying metal-enhanced QDs fluorescence in solution, which have potential applications in chemical and biological sensors.

ASSOCIATED CONTENT *Supporting Information DLS data as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel: +86-553-3869303. Fax: +86-553-3869303. E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The financial support of the Natural Science Foundation of Anhui Province, China (11040606M61), the Natural Science Foundation of the Anhui Higher Education Institutions of China (KJ2011A137, KJ2012A126) and the Innovation Funds of Anhui Normal University are gratefully acknowledged. REFERENCE

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Fluorescence Enhancement of Cadmium Selenide Quantum Dots Assembled on Silver Nanoparticles and Its Application for Glucose Detection Yecang Tang, Qian Yang, Ting Wu, Li Liu, Yi Ding, Bo Yu

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