Preparation, characterization and cell cytotoxicity of Pd-doped CdTe quantum dots and its application as a sensitive fluorescent nanoprobe
- 99 Downloads
Dopants strongly affect properties and optical behaviors of quantum dots, therefore doped-QDs considered as a novel class of luminescent materials. Following development doping approach and using Pd2+ ions as a dopant, for the first time, in this study, a facile preparation of Pd-doped CdTe QDs was reported with thioglycolic acid as a stabilizer. The essential parameters of the synthesis conditions such as dopant concentration, reaction time and optical properties of the Pd: CdTe QDs was studied. The Pd: CdTe QDs with excellent photostability and biocompatibility, exhibited a greenish emission at 529 nm. Interestingly, these nanoparticles display high fluorescence sensitivity to diazinon (DZN). Under optimal conditions, a proposed sensor was designed based on Pd: CdTe QDs which represented a wide range of linear response (2.3–100 µM), low detection limit [3.3 nM, (S/N = 3)], high stability, and selectivity. Additionally, the newly sensing nanoprobe was used for the determination of DZN in environmental water samples with satisfactory results.
Quantum dots (QDs) have been investigated during in the past few decades because of unique optical properties such as broad and strong absorption, high fluorescence quantum yield, narrow and symmetric emission with tunable colors [1, 2, 3, 4]. These features make them an excellent candidate in diverse applications including solar cells, biosensors, and bioimaging probes [5, 6, 7, 8, 9, 10, 11, 12, 13, 14].
Among the group II–VI semiconductors quantum dots, the Cadmium-based nanoparticles have attracted great attention because of specific features like as absorption coefficient, the narrow bulk band gap, and high photostability [15, 16, 17]. However, the CdTe QDs typically contain the highly toxic heavy-metal atom, Cd, which increases concerns on the safety for various applications [18, 19, 20, 21, 22, 23, 24, 25, 26, 27]. To date, numerous efforts have been dedicated to the design and fabricated of CdTe QDs with lower toxicity, highly efficient fluorescence and tunable emission wavelength [28, 29, 30].
Doping is a widely applied technological progress in nanomaterials science to yield materials with favorable properties and operation . Doped quantum dots with variety of transition-metal ions namely Ag1+, Mn2+, Cu2+, Ni2+, Cr3+, Eu3+ and Yb3+, show that, preserved almost all of the properties of the un-doped QDs and may have added some advantages, for example, reduction of the toxicity, enhanced thermal and environmental stabilities and also mitigated serious problems, for instance, electron–hole pair recombination and leaching [14, 29, 32, 33, 34, 35, 36, 37]. Recent researches indicated that by the addition of noble metals like palladium in the host semiconductor nanoparticle, the optical features of some QDs by changing the band gap were improved [38, 39, 40]. So, considering aspects of favorable environmental effects of palladium, the properties of quantum dots can be improved by doped palladium ions and then, they can be used in various fields such as sensors and catalysts.
Diazinon (DZN) is a popular organophosphate insecticide which widely used on turf, alfalfa, lettuce, almonds, cotton, citrus, and other crops especially rice [41, 42]. Commonly, DZN released into the environment is moderately persistent, so that in the neutral water can stay 6 months. Unfortunately, the residual of the DZN in different parts of the living environment, as soil and water, may have great effects on the aquatic eco-systems and even at low concentrations can be very dangerous for human health [43, 44]. On the other hand, because of high agricultural production demand, pesticide usage is almost inevitable. Thus, the determination of the concentration and trace amounts of DZN in environmental waters with the affordable and highly sensitive analytical method is necessary [45, 46].
2 Materials and methods
Cadmium chloride, palladium(II) acetate, sodium hydroxide, tellurium powder, thioglycolic acid, and DZN were purchased from Sigma Aldrich (St. Louis, MO, USA). Other routine chemicals were purchased from Merck (Darmstadt, Germany) or Sigma Aldrich. Stock standard solutions of DZN with a concentration of 1 × 10−3 mol L−1 was prepared in the ultra-pure water and stored at 4 °C. All reactions were carried out in double deionized water.
Perkin-Elmer LS–55 B fluorescence spectrometer was utilized to record the fluorescence spectra. FT-IR spectra were recorded by using a Perkin-Elmer Spectrum RXI FT-IR spectrometer. The ultraviolet (UV) absorption spectra to study the spectral properties of synthesized quantum dots were collected using an Agilent 8453 (USA) spectrophotometer. Transmission electron microscopy (TEM) images of the samples were obtained on Tecnai T20 microscope operating at 200 kV (FEI). X-ray diffractions (XRD) were performed on Panalytical X’pert PRO diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å) under room temperature. The chemical composition of the nanocrystals was determined by using an SEM with energy dispersive X-ray (EDX) detector INCA Penta FETx3.
2.3 Preparation of Pd: CdTe quantum dots
Luminescent Pd: CdTe QDs dots were synthesized via one-pot hydrothermal method using thioglycolic acid (TGA) as a stabilizer. Briefly, 300 µL of palladium(II) acetate (1 × 10−3 mol L−1) in acetonitrile was added to 0.15 g of CdCl2 and TGA (80 µL) in the deionized water (50 mL). The pH of the mixture was adjusted to 10 by addition of NaOH solution and stirred for 3 h. Next, for the preparation of sodium hydrogen telluride (NaHTe), the 0.06 g of Te powder with 0.08 g of sodium borohydride added in of deionized water under stirred and N2 purging for 3 h. Then, the prepared sodium hydrogen telluride was immediately injected into the above Cd solution. The resulting solution was transferred into a Teflon-lined stainless steel autoclave and heated in an oven at 100 °C for 3 h. The resultant product was washed with ethanol and kept at 4 °C in the dark for further use.
2.4 Fluorescence study
2.5 Determination of DZN
To confirm the feasibility of DZN detection in environmental water, samples were collected from a suburb of Kermanshah city and Gamasiab river. Before measurement, the 10 mL of the collected water samples were filtrated through 0.22 µm membrane filters to eliminate the suspended solids and then diluted ten times by Tris–HCl buffer (1.0 × 10−3 M, pH 7.1). Subsequently, the FL and absorbance spectra of the solution were recorded containing Pd: CdTe QDs (100 µL) and different amount of DZN after incubation for 1 min.
2.6 Cell viability assays
The cytotoxicity of Pd: CdTe QDs on fibroblast cells were evaluated by using the activity of the lactate dehydrogenase (LDH) method and reported by Linford with some minor changes. In this work, the primary culture of human fibroblast was used as a normal cell that is derived from human skin. The cells were seeded in 25-cm2 tissue culture flasks and maintained in Dulbecco’s MEM supplemented with inactivated fetal bovine serum 10%, penicillin 100 U mL−1 and streptomycin 100 μg mL−1 for 48 h at 37 °C and 5% pCO2. In summary, the cells (in culture medium) were dispensed in 5 × 103 per well in 96-well microplates and allowed to incubate overnight. After 24 h of early cell culture, the fresh medium with nanoparticles at concentrations (1.0, 1.5, 2.0, 2.5, 3.0 µM) was renewed. Again, 100 μL of the media from each well was then transferred to new 96-well plates, and 100 μL of LDH stock was added to each well and cells were incubated at 37 °C for 30 min. Triton 1% was used as a positive control for the extraction test. The LDH release was estimated using a microplate reader at 495 nm according to the manufacturer’s instructions. All measurements were done in triplicate and the mean cell viability was expressed as a percentage of the control.
3 Results and discussion
3.1 The optical properties of Pd: CdTe QDs
3.2 Characterization of Pd: CdTe QD
The surface morphology and particle distribution of Pd: CdTe QDs were confirmed using TEM (Fig. 3b). As can be observed from these pictures, the nanoparticles have average sizes of about 3 nm, and the Pd-doped CdTe samples are well-developed with uniform spherical-shaped morphology with a little agglomeration.
Further evidence on the chemical composition of Pd: CdTe QDs can be obtained from the EDX analysis (Fig. 3c). The EDAX spectra revealed that the nanoparticles comprise an atomic percentage of Te (24.33%), Cd (31.93%) and Pd (4.1%), which show that the amount of palladium ions is not low. More signals like carbon, oxygen, and sulfur are verifying the presence of a capping agent.
To further investigate the existence of thioglycolic acid on the surface of the prepared Pd: CdTe QDs, the FT-IR spectra of free TGA and TGA-capped Pd: CdTe QDs were compared. As shown in Fig. 3d, the S–H stretching vibration bond at about 2569 cm−1 in TGA vanished completely in the spectrum of Pd: CdTe QDs that indicated TGA ligand was attached to Pd: CdTe QDs through covalent bonds between thiol groups and surface Cd and Pd atoms. The characteristic peaks of the asymmetric vibration –COOH at 1712 cm−1 shifted to 1578 cm−1 in Pd: CdTe QDs spectrum, it can be due to binding of the carboxyl group in TGA on to Cd2+ in Pd: CdTe QDs.
3.3 Optimization of analytical conditions
In order to optimal experimental design, the influence of reaction time, acidity and ionic strength were investigated.
3.3.1 Effect of reaction time
3.3.2 Effect of acidity
FL intensity of QDs can be influenced by the pH value of the solution . Therefore, to obtain a highly sensitive DZN detection, this parameter was studied for values between 3 and 9 in Tris–HCl buffer solution (Fig. 4b). In the absence of DZN, protonation of the surface binding thiolates in acidic pH cause decreasing the fluorescence intensity of Pd: CdTe QDs. In higher pH value, the deprotonation of the thiol group in the TGA molecule happens that could strengthen the covalent bond between Cd, Pd atoms, and the TGA molecules, which leads to increased FL intensity of nanoparticles. In the presence of 20 µL of DZN (10 µM) in pH 7.1, the results implied the change of fluorescence intensity reaches to its maximum. Hence, pH 7.1 was chosen to be a suitable pH for DZN detection.
3.3.3 Effect of ionic strength
The influences of ionic strength on the FL intensity of the sensing probe were investigated by NaCl solution (Fig. 4c). The results showed the reaction should be under a lower ionic intensity condition because, with increasing concentration of NaCl, the FL intensity of QDs strongly diminished. Furthermore, the high ionic strength due to the creation of an ion multi-layer barrier around quantum dots and diazinon can affect the formation of the DZN-Pd: CdTe quantum dots complex.
3.4 Determination of DZN with Pd: CdTe QDs
3.5 Mechanism of the “turn-off” fluorescent probe
According to Fig. 6b, the pure DZN show three strong absorption peaks at 219, 251 and 278 nm. During the addition of DZN to the prepared Pd: CdTe QDs solution obvious spectral change occurs in the molar absorption coefficient of quantum dots and DZN, which indicates the quantum dot-DZN system is formed, and quenching type is static quenching. This mechanism was further confirmed by the Stern–Volmer equation. Using fluorescence spectroscopy, we achieved KSV of QDs–DZN complex for various temperatures. The values of Ksv shown in (Fig. 6c). We observed that the temperature relationship with quenching constant (Ksv) is inverse, which confirms that the quenching occurs because of the static process.
3.6 Interference and selectivity studies
3.7 Assay of DZN concentrations in environmental water samples
The determination of diazinon in tap, rain, and river waters
Value found (n = 3)
5.1 ± 0.78
4.88 ± 1.19
4.79 ± 1.43
3.8 Cytotoxicity analysis
It is very important to gain QDs with low cytotoxicity for the purpose of realizing medicine and biological applications [63, 64]. LDH testing is one of the most commonly used techniques for cytotoxicity and cell viability. To measure the activity of Pd: CdTe QDs on the fibroblast cells, LDH assay, was accomplished and the results shown in (Fig. 7c). From the results, it is clear that the Pd: CdTe QDs exhibit dose-dependent cytotoxicity. However, this result reveals the nanoparticles have notable toxicity at 3.0 µM, but, after treatment with the lower concentration of Pd: CdTe QDs (i.e., 1.0, 1.5, 2.0, 2.5 µM) for 48 h, no cytotoxicity effects were observed. Nevertheless, the dosage we use for experiments is much smaller than this concentration which does not have toxic effects.
In this study, a novel Pd: CdTe QDs was designed and synthesized by a facile hydrothermal method and characterized by various physicochemical techniques. Then, it applied as fluorescent nano-sensors for rapid detection of DZN residues in rainwater, tap and river water. Under optimum conditions, the suggested method achieved a good linear and low detection limit. Due to their low cost, simple procedure, low toxicity, rapid detection, and high sensitivity, we suggest that Pd: CdTe QDs as fluorescence sensor have the excellent potential for of low levels the insecticides residue detection in water samples. Furthermore, considering CdTe QDs are known to be toxic due to containing the highly toxic heavy-metal element cadmium, we have explored the toxicity of Pd: CdTe QDs, the results proved Pd: CdTe QDs did not show notable toxicity at the concentrations used in the experiments and, therefore it is proper for biological applications.
The authors would like to thank the Research Council of Kermanshah University of Medical Sciences and Arak University for financial support of this research.
- 53.M.T. Man, H.S. Lee, Sci. Rep. 5, 1 (2015)Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.