Synthesis and Characterization of CdSe/CdS/N-Acetyl-L-Cysteine/Quercetin Nano-Composites and Their Antibacterial Performance

Abstract

We have discovered that quercetin, once coated on the CdSe and CdSe-CdS quantum dots (QDs), becoming highly water soluble. In the present work, we have successfully synthesized CdSe/CdS/N-Acetyl-L-Cysteine(NAC)/Quercetin nano-composites in the aqueous solution. The products were characterized using UV-vis spectroscopy, X-ray powder diffraction, fluorescence spectroscopy, and Fourier transform infrared spectroscopy. The transmission electron microscopy (TEM) tests indicated that our nano-composite products are highly stable with homogeneous particle size and great monodispersity. Quercetin coated nano-composite CdSe/CdS/NAC/Quercetin showed different fluorescence behavior from that of CdSe/CdS/NAC. Most amazingly, the synthesized CdSe/CdS/NAC/Quercetin nano-composite exhibits strong antibacterial activity. The combination of the strong fluorescence and its antibacterial activity makes the quercetin modified quantum dots as a potential candidate for cancer targeted therapy and other cancer treatments.

INTRODUCTION

As one of the important member in flavonoids, quercetin has been reported to possess many useful properties, including anti-flammatory activity, enzyme inhibition, antimicrobial activity, antiallergic activity, antioxidant activity, vascular activity, and cytotoxic antitumor activity.1−3 It is widely distributed in plants and fruits, such as the nuts, tea, onion, grape, and herb.4 Quetcetin, however, limited by its poor solubility in water, has by far not reached its full potential in the medicinal development. Therefore, Gao et al.5 encapsulated quercetin into biodegradable mono-methoxyl poly(ethylene glycol) poly(ε-caprolactone) micelles to treat ovarian cancer rendering the complete dispersible of quercetin in water. Li et al.6 had studied the solution of quercitin and investigated the hydroxyl radical scavenging mechanism of human erythrocytes using quercetin-germanium (IV) complex.

Owing to the unique optical, chemical, and physical properties of quantum dots (QDs), such as high fluorescence, sizedependent broad absorption, high extinction coefficients, and readily size-tenability, which have great potential and excellent advantages in drug delivery and biomedical imaging applications.7−12 Such as Shi et al.13 used three typical CdTe QDs with different maximum emission to detect the interaction between 7-hydroxyflavone/quercetin and human serum albumin in vitro; Jeyadevi R et al.14 used acid-capped cadmium telluride quantum dots as nano-carrier in adjuvant induced arthritic wistar rats and developed nano-carrier for drugs used in the treatment of various degenerative diseases.

However, due to the great toxicity, the use of the singlecore structure of quantum dot has been greatly limited. The core-shell structured QDs has been believed to be a common approach to overcome the confinement in a quantum dots, improve the optical properties, and of course, enhance the luminescence.15−18 Among the variety of components and structures, the CdX (X = S, Se, Te) and core-shell structure such as CdSe(core)-ZnS(shell), CdSe(core)-CdS(shell) and InP(core)-ZnCdSe(shell) have been mostly synthesized. Recently, Hu et al.19 pointed out core-shell CdSe/ZnS QDs could potentially be an excellent fluorescence probe for the flavonoids. Nevertheless, to achieve better bioavailability, it is in great need to develop new functional surface modified core shell QD materials.

In present work, we have combined the synthesis of quercetin with core-shell CdSe/ZnS QDs experience to synthesize CdSe/CdS/NAC/Quercetin quantum dots, to solve the problem of poor solution of quercetin as well as endow the properties of fluorescence, and then the antibacterial effect of the CdSe/CdS/NAC/Quercetin will be tested against four clinical common multidrug resistance pathogens (e.g. Grampositive Staphylococcus aureus and Gram-negative Escherichia coli) in vitro. The present method can be potentially used to develop a new quercetin derivative as a promising anti-cancer Chinese medicine, which can be applied to both drug delivery and cancer therapy.

Figure 1.The preparation procedure of fluorescent quantum dots.

 

EXPERIMENTAL

Materials

Sodium borohydride (NaBH4, 97%), Selenium powder (Se, 99%), 2-propanol, CdCl2·H2O, and Na2S were purchased from Shanghai Zhongqin Chemical Reagent Co. Ltd. N-Acetyl- L-Cysteine (NAC, 99%) and quercetin were obtained from Aladdin Industrial Corporation. Ultrapure water was used throughout the experiments. All other chemicals were of the highest commercially available purity and used as received without further purification.

Synthesis of the Quercetin-Coated CdSe/CdS/NAC Nanocomposite

The core-shell quantum dots were prepared according to the published method.20 0.6 mmol of selenium powder and 3.0 mmol of the NaBH4 were dissolved in a three-necked flask contained with 15.0 mL of ultrapure water. The reaction system was degassed and purged with N2 protection for 30 min. The reaction mixture was then heated to 80 °C to obtain the NaHSe solution. The obtained dark red solution was cooled to room temperature under N2 protection. In another three-neck flask, a mixture of 0.6 mmol CdCl2 and 1.0 mmol NAC were prepared in 120 mL ultrapure water. The pH of the mixture was adjusted to 12.0 by adding 1.0 M NaOH solution drop-wise under N2 protection. 3 mL of NaHSe solution prepared earlier was added into the mixture drop-wise at room temperature. The mixture was heated to 106 °C and was kept at this temperature for 50 min. The cold 2-propanol was then added to precipitate the NAC-capped CdSe. The solution of NAC-capped CdSe was slowly brought to room temperature. The CdSe-CdS precursor solution was prepared in a three-necked flask by adding the as-prepared NAC-capped CdSe core to a N2-saturated solution containing 3.0 mmol CdCl2, 0.6 mmol Na2S and 15.0 mmol NAC. The mixture was heated to 106 °C and kept at this temperature for 50 min. The temperature was cooled to the room temperature naturally. After adding the cold 2-propanol, the mixture was centrifuged (10000 r/min for 5 min). The separated NAC-capped CdSe-CdS was added to a mixture of 0.3 mmol quercetin and 120 mL pure water in a beaker under the room temperature. The mixture was stirred for 30 min to precipitate the NAC-capped CdSe/CdS/NAC/Quercetin nanocomposite. The final isolated and deposited products were dried overnight through the drying oven at 40 °C and stored in a refrigerator for the further experiments.

Inhibit Bacteria Growth Experiments

The newly synthesized CdSe/CdS/NAC/Quercetin nanocomposites were tested against the Gram-positive Staphylococcus aurous, Enterococcus faecalis and Gram-negative Escherichia coli, Pseudomonas aeruginosa using optical density method and streak plate method in vitro. 50 mL LB liquid medium was added to 200 μL of bacterial strain liquid. The mixture was incubated for 12 h at 37 °C. The living bacterium number change was tested using the optical density method. The minimum inhibitory concentration (MIC) values were obtained using the streak plate method where the sterile water was used as the control group.

 

RESULTS AND DISCUSSION

HRTEM and SEM Analysis

The images of CdSe/NAC, CdSe/CdS/NAC and CdSe/CdS/NAC/Quercetin nano-composites were characterized using a high resolution transmission electron microscopy (HRTEM). As shown in Fig. 2(a), the HRTEM image of CdSe nanocrystals has a regular shape with an average size of 36 nm. The TEM image for CdSe/CdS in Fig. 2(b) displays spherical particles with excellent monodispersity. The monodispersity can be further observed in the SEM images shown in Fig. 3. The average diameter of the nanocrystals was estimated to be 34−38 nm and 43 nm respectively.

The increase in particle size is due to the over-coating of CdS-shell by nanometer level on the CdSe core. The TEM images of the quercetin modified quantum dots shown in Fig. 2(c) look like the nanocrystal mosaics that are distributed on the surface of organic matter, which were hold together mutually

Figure 2.TEM image of CdSe/NAC(a), CdSe/CdS/NAC(b) and CdSe/CdS/NAC/Quercetin(c) nano-composites.

Figure 3.SEM image of CdSe/NAC(a), CdSe/CdS/NAC(b) and CdSe/CdS/NAC/Quercetin(c) nano-composites.

XRD Study

X-ray diffraction (XRD) patterns for the series of CdSe, CdSe-CdS, Quercetin, CdSe-CdS-Qercetin QDs shown in Fig. 4 were collected using a Rigaku miniflex diffractometer with Cu Kα radiation at 40 kV in the range of 10°≤2θ≤80°. The (111), (220), (311) diffraction peaks of the CdSe (zincblende JCPDS No. 19-0191)21 and CdS (zincblende JCPDS No. 14-1411)22 were observed for CdSe/NAC, CdSe/CdS/NAC. After loading the quercetin, however, the XRD pattern (Fig. 4d) is nearly identical to that of pure quercetin (Fig. 4c). We postulated that quercetin bind with NAC on the surface and it has fully covered the surface of the coreshell CdSe/CdS/NAC quantum dots.

Figure 4.XRD pattern of CdSe/NAC(a), CdSe/CdS/NAC(b), Quercetin(c) and CdSe/CdS/NAC/Quercetin(d).

TG Analysis

The TGA analysis shown in Fig. 5 exhibits the similar weight loss at a rate of 19.8% for both CdSe/NAC and CdSe/CdS/NAC QDs. On the other hand, the CdSe/CdS/NAC/Quercetin nanocomposites display a significant weight loss at 450 °C, which is contributed by the loss of the hydroxyl groups, NAC and quercetin on the QDs surface. All three species show a sharp mass loss at 580 °C, which indicated these compounds are thermally stable. TGA analysis suggests that quercetin was successfully panned together with the N-Acetyl-L-Cysteine coated CdSe@CdS quantum dots.

Figure 5.TG Analysis of CdSe/NAC(black line), CdSe/CdS/NAC(red line) and CdSe/CdS/NAC/Quercetin(blue line).

Figure 6.The FTIR spectra of CdSe/NAC(a), CdSe/CdS/NAC(b) and CdSe/CdS/NAC/Quercetin(c).

FTIR Study

Two absorptions at ~1392 cm−1 and 1542 cm−1 for CdSe/ NAC represent the asymmetric −COO− and symmetric −COO− vibrations in NAC, which indicates the deprotonation absorbing of NAC on the surface of QDs. The loading of quercetin leads to a slightly red shift of the peak at 1392 cm−1, and the splitting of the peak at 1542 cm−1. The presence of stretching absorption of O-H can be demonstrated at ~3400 cm−1, and the molecules of NAC on the surface of QDs are associated each other through intermolecular hydrogen bond. The addition of quercetin in the CdSe/CdS/NAC/quercetin nano-composition causes the O-H stretching a slightly blue shift along with a narrowing down in the peak width.23,24 All these observations support the fact that quercetin does attach to the surface of the core-shell CdSe/CdS/NAC quantum dots. The observation of abundant carboxylate anions provides the evidence for the excellent water solubility of the nanocompsoites synthesized in the present work.

UV-Visible Absorption and Photo Luminescence (PL) Study

The UV-visible spectroscopy was performed for CdSe/NAC, CdSe/CdS/NAC and CdSe/CdS/NAC/Quercetin, respectively. The maximum absorption peaks of of CdSe/ NAC, CdSe/CdS/NAC, and CdSe/CdS/NAC/Quercetin occurred at 472 nm, 454 nm, and 372 nm respectively. The attachment of quercetin fundamentally changed the size of core-shell QDs, further changed the electronic structure.

Figure 7.UV absorption spectra and images under daylight(right) and UV light(left, λ=365 nm): CdSe/NAC, CdSe/CdS/NAC and CdSe/CdS/NAC/Quercetin from left to right.

Figure 8.PL emission spectra CdSe/NAC(red line, λem=585 nm, λex=415 nm), CdSe/CdS/NAC(purple line, λem=549 nm, λex=410 nm) and CdSe/CdS/NAC/Quercetin(green line, λem=549, λex=410 nm).

Not surprisingly, the luminescences were observed in the nanocomposites. While the core-shell CdSe/CdS/NAC QDs emits the strongest fluorescence, the flourescence was somewhat quenched with the addition of quercetin. The enhanced fluorescence of CdSe/CdS/NAC in aqueous phase is due to the CdS shell with a larger band gap, which effectively trap the electrons in the core.

Antibacterial Activity

The antibacterial properties of CdSe/CdS/NAC/Quercetin nano-composite had been tested. The results of anti-bacterial efficacy are shown in Fig. 9. The antibacterial activity using optical density method and streak plate method was tested against the Gram-positive Staphylococcus aurous, Enterococcus faecalis and Gram-negative Escherichia coli, Pseudomonas aeruginosa in vitro. Generally, the antibacterial mechanism of flavonoids is considered that phenolic hydroxyl containing molecular structure and the functional groups which combined protein or enzyme through formed hydrogen bonding that may damage or loss of the activity of protein,25 finally, causing cytoplasm shrinked or disintegrated. As a result, the nano-composites synthesized and exhibited antibacterial effect against both Escherichia coli and Staphyococcusaureus in the present work. In addition, a smaller inhibition zones are observed for the Enterococcus faecalis and Pseudomonas aeruginosa at the designed concentrations. The MIC values of the CdSe/CdS/NAC/Quercetin are 3.13 mg/mL for Escherichia coli, 6.25 mg/mL for Pseudomonas aeruginosa, Escherichia coli, and Enterococcus faecalis. Generally, the CdSe/CdS/NAC/Quercetin nano-composites inhibit the growth of both Gram-positive Staphylococcus aureus, Enterococcus faecalisstrain and Gram-negative Escherichia coli, Pseudomonas aeruginosa in the aqueous medium.

Figure 9.Comparison of the number of viable bacteria and the growth curves.

 

CONCLUSION

The CdSe/CdS/NAC/Quercetin nano-composite was synthesized and characterized, which antibacterial effects had been tested. It is indicated that this nanocomposite has been successfully developed to improve the hydrophilicity of quercetin. Furthermore, the results of antibacterial tests indicated that the synthesized CdSe/CdS/NAC/Quercetin nano-composite exhibits strong antibacterial activity. So the combination of the strong fluorescence and its antibacterial activity makes the nanocomposites as a potential candidate for cancer targeted therapy and related fields in future.