Synthesis of Fluorescent CdS Quantum Dots of Tunable Light

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Synthesis of Fluorescent CdS Quantum Dots of Tunable Light Emission with a New in Situ Produced Capping Agent Yan-mei Mo, Yu Tang,* Fei Gao, Jun Yang, and Yuan-ming Zhang* Department of Chemistry, College of Life Science and Technology, Jinan University, Guangzhou, Guangdong, 510632, People's Republic of China ABSTRACT: Herein, we report an ambient synthesis of CdS QDs with widely tunable particle size by sulfur source of H2S and a new capping agent R-2-thiotetrahydrothiazole-4-carboxylic acid in situ produced from the reaction of L-cysteine with carbon disulfide in water. XRD, FTIR, UV−vis, PL, and TEM measurements were used to characterize the synthesized CdS QDs. Small particle size in a range of 1−3 nm and five PL colors from dark, deep blue to yellow of CdS QDs were achieved by changing the molar ratio and pH value of preparation. These CdS colloid solutions exhibit high stability even under weak acidity conditions at pH 6.5. The CdS QDs display highest emission intensity at a concentration of 2 mM.

excited at the same wavelength, it is rare to find the particle size of CdS QDs less than 2 nm, and emitting many colors. The particle size was reported with the range 1.37−1.85 nm by estimating from X-ray measurements, and four colors of turbid solution under UV light illumination.27 Rac-penicillamine stabilizing CdS with a size of about 1.4 nm was also reported.22 The small size of CdS is possibly due to the complementary nature of the D-and L-acids allowing for closer packing of the stabilizer on the surface of the Rac-CdS.22 Therefore, it is highly desirable to have CdS nanoparticles widely tunable in size and more fluorescent for various applications. It is well established that the size, shape, and crystal structure of nanoparticles that are prepared by a solution route are strongly influenced by chemical bath conditions, the nature of the capping molecules, and the sources of anion/cation.28,29 Our previous work revealed that the sulfur in thiourea can interact with CdS, and thus form hollow nanowires of CdS/ DBTU nanocomposite by reacting butylamine with CS2 as the sulfur source.30 This result prompted us to synthesize CdS QDs with L-cysteine (L-Cys) and CS2 by taking advantage of two sulfur atoms (Scheme 1) to increase the interaction with CdS

1. INTRODUCTION Over recent decades, many fundamental and applied research works have focused on semiconductor nanoparticles or quantum dots (QDs) for the quantum-size dependence of their electrical and optical properties.1,2 Quantum Dots provide advantages over organic fluorophores, including chemical stability, high photobleaching threshold, and narrow emission spectra.3 Among various semiconductor nanoparticles, CdS QD is a basic and important NP found in a great number of interesting biological analytes, including proteins,4 DNA,5 and metal ions.6 Currently, two main synthetic strategies of CdS QDs with desired optical properties for the biomedical researches and applications have been proposed, i.e., organic-phase and aqueousphase routes. Binding organic molecules to the surface atoms of nanoparticles is important in nanoscience for specific tailoring of physical and chemical properties of nanomaterials.7,8 Thus, trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) were applied as stabilizing agents9,10 in the traditional organicphase route. Some other ligands, such as cationic multivalent polyamine, have also been used recently through ligandexchange methods.11,12 Nevertheless, QDs synthesized in organic solvents suffer from insolubility in the aqueous-phase and incompatibility with the biological system. The aqueous synthesis is reagent-effective, less toxic and more reproducible, which improves the water-stability and biological compatibility. This route is usually concerned with thiol-containing stabilizing reagents, including dihydrolipoic acid (DHLA),13 2- or 3mercaptopropionic acid (MPA),14−16 L-cysteine,17−21 thioglycerol,17 thioglycolic acid (TGA),18 L- and D-penicillamines,22 and glutathione,19which can interact with the surface of QDs as strongly as a covalent bond. Without sulfur atom bonding on CdS QDs, other stabilizing reagents have also been used to prepare CdS QDs, such as citrate,23 polyphosphate,6 polystyrene-b-poly(acrylic acid),7 tyrosine,24 carboxylic- functionalized PVA,25 and bovine serum albumin.26 Although differently sized CdS QDs that are prepared under various conditions, such as temperature, reaction time, capping agent, and ratio of Cd/S, can emit from blue to yellow when © 2012 American Chemical Society

Scheme 1. Illustration of the Preparation Route to Form CdS QDs

and the slow release sulfur source to give small sizes of particles, which is proven to be feasible in this work. Received: Revised: Accepted: Published: 5995

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2. EXPERIMENTAL METHODS 2.1. Materials and Reagents. With the exception of distilled water, all of the other reagents were of analytic grade and used without further purification. R-2-thiotetrahydrothiazole4-carboxylic acid was synthesized by literature method.31 2.2. Synthesis of R-2-thiotetrahydrothiazole-4-carboxylic Acid (R-TTCA)-Capped CdS Quantum Dots. Under stirring at room temperature, a 10-mL portion of 0.1 M cadmium chloride aqueous solution was dropped into four separated 10 mL of 0.15, 0.2, 0.25, or 0.3 M L-cysteine (L-Cys) aqueous solution (corresponding to the molar ratios of L-Cys/Cd2+ = 3:2, 4:2, 5:2, or 6:2) by adjusting pH value of the mixture to 9.0 with 1.0 M KOH aqueous solution, or into four separated 10 mL (molar ratio of L-Cys/Cd2+ = 4: 2) of 0.2 M L-cysteine aqueous solution by adjusting pH value of the mixture to 7.5, 8.0, 8.5, 9.0, or 9.5 with 1.0 M KOH aqueous solution, respectively. The total volume of reacting solutions under all conditions was controlled to 25 mL by adding water. The solution changed to white turbid after the addition of cadmium chloride solution to L-Cys solution. After the quick addition of 0.2 mL of CS2 with syringe, the reacting mixture turned to yellow immediately and was stirred at room temperature under airtight condition. Clear colorless suspensions of CdS QDs were obtained after 12 h, except a pale yellow suspension of CdS QDs for pH9.5, which could be employed as samples in measurement of UV−vis or PL directly. After precipitation and washing by ethanol, the colloidal quantum dots were obtained by drying under vacuum at 60 °C for 6 h, which was used for XRD or FT-IR measurement. 2.3. Characterization. Using water as solvent, absorption curves were recorded on 900 UV−vis spectrometer with the scan range of 300−700 nm. Fourier transform infrared (FT-IR) spectra were recorded in KBr on a VARIANFTS2000 spectrometer. X-ray powder diffraction (XRD) patterns of samples were carried on a MAC Science MXP-3VA diffractometer equipped with graphite monochromated Cu Ka radiation (λ = 1.5405 Å) in the 2θ range of 10−80°. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observation were conducted on a JEOL JSM-6330F transmission electron microscope by using an accelerating voltage of 300 kV. PL spectra were measured on a VARIAN Cary Eclipse fluorescence spectrophotometer operating with a 350 nm laser beam as a light source, tubevoltage 800 V, and an excitation emission slits of 5 nm.

Figure 1. Absorption curves of different sized CdS QDs obtained at different molar ratios of L-Cys/Cd2+.

Table 1. Average Diameters of CdS QDs Estimated from the Absorbance Spectra and PL Emitting Peaks molar ratio

3:2

4:2

5:2

6:2

absorbance peak (nm) average diameter (nm) PL emitting peaks

382 2.84 564

363 2.36 532

385 2.93 566

387 2.98 575

corresponding to the absorption peaks of 363−387 nm, respectively. D = ( −6.6521 × 10−8)λ 3 + (1.9557 × 10−4)λ 2 − (9.2352)λ + (13.29)

(1)

The molar ratio 4:2 causes the highest blue shift of absorption edge and smallest particle size of CdS QDs. Under lower amounts of L-Cys at molar ratio 3:2, the CdS QDs are not sufficiently covered by R-TTCA, the capping molecules in situ produced from the reaction of L-cysteine with carbon disulfide. Thus, the less stabilization offered by capping molecules results in larger CdS nanocrystals than those obtained at a molar ratio of 4:2. At higher molar ratios 5:2 and 6:2, more L-Cys could produce H2S faster, which is a disadvantage to give small particle sizes because of quick formation of CdS.27 The Fluorescence emission spectra in Figure 2 exhibit a broad band with a highest peak near 532 nm of CdS QDs obtained at a molar ratio 4:2. The other samples obtained at molar ratios 3:2, 5:2, and 6:2 of L-Cys/Cd2+ exhibit peaks near at 564, 566, and 575 nm, respectively. These results are in reasonable agreement with the blue shift of UV−visible absorption spectra (Table 1). In Figure 3, the photos of colloidal solutions taken under UV light display a brightest pale yellow green corresponding to molar ratio 4:2, while the others are yellow. 3.2. The Effects of pH Value on CdS QDs. In order to obtain small particle size of CdS QDs, the molar ratio 4:2 was chosen to discuss the effects of pH value on CdS QDs because of the important role of carboxylic group in stabilization of CdS QDs. The all XRD patterns in Figure 4 display two distinct broad peaks at 2θ = 28°, 48°(JCPDS reference code 41−1049, hexagonal CdS), also a gradual decrease in the full width at halfmaximum (fwhm) of peaks at (2θ = 28°), which indicates that a progressive increase exists in crystallinity and particle size of

3. RESULTS AND DISCUSSION In Scheme 1, H2S could be produced by the reaction of L-Cys with CS2, which is a slow process to release the sulfur source compared with addition of S2− directly.31,32 Then, CdS could be formed from the reaction of H2S with the nearby Cd2+ attracted to carboxylate in L-Cys by electrostatic interaction. The amount of L-Cys and CS2 was excess to make sure all Cd2+ could be converted to CdS. 3.1. Effects of L-Cys/Cd2+ Molar Ratios on CdS QDs. In Figure 1, compared with 512 nm of the bulk CdS absorption edge, the absorptions of CdS QDs obtained at molar ratios 3:2, 4:2, 5:2, and 6:2 of L-Cys/Cd2+ display a largely blue shift to 382, 363, 385, and 387 nm, respectively, indicating the strong quantum confinement of charges in these nanoparticles.33 From the following formula,34 diameters of CdS QDs can be estimated to be in the range of 2.36−2.98 nm (Table 1), 5996

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Figure 2. PL spectra of CdS QDs obtained at different molar ratios of 2+ L-Cys/Cd .

Figure 4. X-ray powder patterns of CdS QDs obtained at different pH values.

quantum dots.27 The rough lines of XRD patterns are possibly caused by existence of organic compounds in CdS QDs35 and the small particle size of CdS nanocrystals. The peaks are fairly broad, even overlapping their neighbors, which indicates the small particle size.36 In Table 2, the size of different nanoparticles obatined at various pH values could be estimated in a range of 1.07−2.70 nm by Debye−Scherrer formula from peaks at 2θ = 28°.37 Figure 5(a) is the FTIR spectrum of prepared R-TTCA, which is consistent with that in the literature.32 The FTIR spectra are simlar for all R-TTCA capped-CdS nanoparticles obtained by precipitating from CdS QDs solution with ethanol, which can be dissolved in water again, thus only the FTIR spectrum of sample prepared at pH 9.0 is shown in Figure 5(b). The absorption peak at 3435 cm−1 indicates the existence of N−H. The bands at 1609 and 1401 cm−1, displaying red shift compared with 1736 and 1483 cm−1 in Figure 5(a), are due to the asymmetric and symmetric carboxylate stretching, which indicates abundant carboxyl groups are tethered on the surface of CdS nanocrystals,38 and thus avoid nanoparticles to coagulate and agglomerate. The absorption of −CS near 1042 cm−1 in CdS QDs decreases largely and displays a small red shift toward lower wavenumbers compared with 1047 cm−1 in the IR spectrum of R-TTCA, which can be attributed to the robust interaction of sulfur atom in R-TTCA with CdS.39 Further support comes from the LC-MS analysis results of M/Z: 163.4 for R-TTCA.

Table 2. Results of Critical Absorption Wavelength, Particle Size Estimated from Absorption Measurements and XRD, PL Peaks and Quantum Yield (QY) in % of CdS QDs Obtained at Different pH Values pH value of preparation

7.5

8.0

8.5

9.0

9.5

critical absorption wavelength (nm) particle size (nm) from UV particle size (nm) from XRD zeta potential (mv) PL peaks (nm) photoluminescence QY in %

314

329

351

361

380

1.55 1.07 −29.6 431 11.4

1.69 1.18 −34.5 454 23.52

1.91 1.40 −42.7 481 16.53

2.28 1.82 −31.7 519 16.58

2.75 2.70 −28.6 547 13.7

In Figure 6, the absorptions of CdS QDs prepared at pH values of 7.5, 8.0, 8.5, 9.0, and 9.5 exhibit largely blue shift to 314, 329, 353, 361, and 380 nm, respectively. All of the nanoparticles lie in the size quantization regime as shown by the blue shift in absorption peaks with respect to the absorption peak of bulk CdS. Thus, the average diameters of the obtained CdS QDS could be estimated in a range of 1.55 to 2.75 nm from absorption measurements34 (Table 2), respectively. It could be found in Table 2 that the particle size becomes larger with increasing the pH value of preparation. In the preparation of CdS QDs, reacting solution could change from colorless to yellow quickly after addition of carbon disulfide, which

Figure 3. Photos of CdS QDs obtained at different molar ratios of L-Cys/Cd2+: (a) under normal light illumination and (b) under UV light illumination (365 nm). 5997

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Figure 5. FTIR spectra of (a) R-TTCA and (b) R-TTCA capped CdS QDs. Figure 7. The θ potentials of CdS QDs obtained at different pH.

Figure 6. Absorption curves of different sized CdS QDs obtained at different pH values.

Figure 8. PL spectra of CdS QDs obtained at different pH values.

indicates the start of reaction between L-cysteine and carbon disulfide. Thus, the deeper color of solution at high pH means the faster reaction to produce H2S. However, higher pH would be advantageous to dissolve more H2S. These effects would urge the formation of CdS more quickly, which results in larger particles. It is well established that H2S produced in situ neutralize base, so the pH values of CdS quantum dot solutions reduce to about 6.5, 7.2, 7.8, 8.1, and 8.6, respectively. To our surprise, the CdS quantum dot solutions at pH values of 6.5 and 8.6 could be preserved in a 4 °C refrigerator for two months without observation of any precipitate, while the others could be preserved for five months, which exhibits high stability and high fluorescence. This indicates that there is not much temporal evolution of nanoparticles and capping molecules are effective in preventing the agglomeration and Ostwald ripening of these nanoparticles. The θ potentials indicate the stability order of CdS quantum dot solutions prepared at pH 8.5 > 8.0 > 9.0 > 7.5 > 9.5 (Figure 7 and Table 2), which is consistent with the results of preservation in a 4 °C refrigerator. To the best of our knowledge, the stable CdS quantum dot solution at pH values of 6.5, the weak acidity condition, has not been reported, which will expand the application of CdS QDS. In Figure 8, the CdS QDs at different pH exhibit different fluorescence spectra of visible wavelengths at 431,454, 481,519 and 547 nm, corresponding to the estimated size 1.55, 1.69,

1.91, 2.28, and 2.75 nm (Table 2), respectively. In a photograph (Figure 9) taken under long ultraviolet(365 nm), CdS QDs solutions display five PL colors from dark deep blue to yellow with an increasing pH value of preparation, which also clearly indicates size-dependent emission. It is difficult to find multiple colors of CdS QDs solutions obtained by one method. By using hydrazine-hydrate to reduce sulfur, a new and slow producing source of sulfur, four colors of CdS QDs solutions have been reported.27 But the prepared turbid CdS QDs solutions may limit their application. In order to avoid self-absorbance, the absorbances of the QDs samples and quinine sulfate in 0.1 mol·L−1 of H2SO4 (aq) (with 55% of QY), the standard to determine quantum yields, at excitation wavelength (320 nm) are similar and about 0.1.40 The QY for PL in Table 2 is estimated at room temperature. At the optimum pH value of 8.0, a quantum yield (QY) as high as 23.5% was achieved. In order to apply the CdS QDs at proper concentration, the PL properties of the QDs were examined for samples with five different CdS concentrations, 1, 2, 4, 6, and 10 mM, which were adjusted with water based on the concentration of added Cd2+ under the conditions of excess sulfur source. In emission spectra measured at room temperature (Figure 10), the 2 mM sample exhibits the highest emission intensity, which is much higher than the second one at 1 mM. In literature,15 the highest emission 5998

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Figure 9. Photos of CdS QDs obtained at different pH values: (a) under normal light illumination and (b) under UV light illumination (365 nm).

displays individual particles with sizes of 1−2 nm. In Table 2, the particle size estimated by the Debye−Scherrer formula is smaller than those estimated from absorption and HRTEM measurements. The reason is that the Debye−Scherrer formula estimates the size of the single-crystal domain that has periodic lattices but not the actual particle size (the latter includes lattice domain encapsulated by a thick layer amorphous material).

4. CONCLUSIONS A simple and ambient method for synthesizing fluorescent CdS QDs has been proposed. R-2-thiotetrahydrothiazole-4-carboxylic acid, a new capping agent produced in situ from reaction of L-cysteine and carbon disulfide, was employed to stabilize CdS QDs. Small particle size in a range of 1−3 nm and five PL colors from blue to yellow of CdS QDs were achieved by changing the molar ratio of L-Cys/Cd2+ and pH value of preparation. These CdS colloid solutions preserved in a 4 °C refrigerator exhibit high stability even under weak acidity conditions at pH 6.5. The CdS QDs display their highest emission intensity at a concentration of 2 mM. The quantum yield (QY) of CdS QDs are in the range of 11.43−23.52%.

Figure 10. Concentration effect on PL properties of CdS QDs prepared at pH 9.0.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 0086-20-85220223. Fax: 0086-20-84592606. E-mail: [email protected].

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Natural Science Foundation of Guangdong Province (039213) for financial support.

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Figure 11. TEM images of CdS QDs obtained at pH 8.0.

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