SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES OF CdS QUANTUM DOTS MODIFIED WITH THIOL GROUP-CONTAINING STABILIZERS

ABSTRACT

The paper describes the synthesis of CdS quantum dots stabilized with thiol-containing stabilizers by a colloidal method. The colloidal solution method does not require high pressure, uses inexpensive and readily available reagents, and has the advantages of controlling the size of CdS quantum dots and the concentration of stabilizers. The basic principles of CdS quantum dots modified with thiol-containing stabilizers as photocatalysts for imaging biomolecules, imaging cancer cells, studying cell structure and organelles, purifying water from organic pollutants, and producing hydrogen gas from water as an alternative renewable energy are described.

АННОТАЦИЯ

В статье описывается синтез квантовых точек CdS, стабилизированных тиолсодержащими стабилизаторами коллоидным методом. Метод коллоидного раствора не требует высокого давления, использует недорогие и легкодоступные реагенты и имеет преимущества контроля размера квантовых точек CdS и концентрации стабилизаторов. Описаны основные принципы работы квантовых точек CdS, модифицированных тиолсодержащими стабилизаторами, в качестве фотокатализаторов для визуализации биомолекул, визуализации раковых клеток, изучения структуры клеток и органелл, очистки воды от органических загрязнителей и получения водорода из воды в качестве альтернативного возобновляемого источника энергии.

Keywords: colloidal chemistry, nanotechnology, biomolecules, photocatalyst, quantum dot, stabilizer, synthesis, spectrum.

Ключевые слова: коллоидная химия, нанотехнологии, биомолекулы, фотокатализатор, квантовая точка, стабилизатор, синтез, спектр.

Introduction

The colloidal solution method is one of the most efficient and simple methods for synthesizing CdS quantum dots stabilized with thiol-containing stabilizers. In this method, nanoparticles are synthesized in a liquid phase and stabilized by stabilizers. The colloidal solution method does not require high pressure during the synthesis process, uses inexpensive and readily available reagents, and has the advantages of controlling the size of CdS quantum dots and the concentration of stabilizers.

Nanostructured semiconductors show great potential for environmental amelioration due to photocatalytic oxidation activated by sunlight or ultraviolet light [1]. CdS quantum dots have unique properties such as stability, self-cleaning, and high photocatalytic degradation. These nanomaterials can be modified with various stabilizers, which improves their photoelectric and photochemical properties [2].

Due to the high light absorption capacity of CdS quantum dots and their low resistance to photocorrosion, CdS quantum dots are currently being used more widely[3].

Experimental part

In the synthesis of CdS nanoparticles, reagents such as cadmium (II) acetate dihydrate (99%, Sigma-Aldrich), sodium sulfide (I) nonahydrate (chemically pure, TatChemProduct), L-cysteine, and sodium hydroxide rhodamine B were used. Cadmium sulfide quantum dots were obtained by the colloidal synthesis method from a supersaturated solution in an aqueous medium by condensation. The synthesis process includes several steps. First, cadmium sulfide nanoparticles were synthesized. For this, 0,25 mmol (66,7 mg) of cadmium acetate dihydrate (Cd(CH₃COO)₂‧2H₂O) was weighed on an electronic balance. The measured 66,7 mg of cadmium acetate dihydrate was placed in a round-bottomed flask and dissolved in 10 ml of distilled water to form a solution. In a beaker, 30 milligrams of sodium sulfide (Na₂S‧9H₂O) (0,125 mmol) was dissolved in 3 ml of distilled water to form a solution. The glycerin bath was placed on a magnetic stirrer, then a stabilizer was added dropwise to the initially prepared cadmium acetate solution until a cadmium complex precipitate was formed. A 2 mol/l sodium hydroxide solution was added to the resulting precipitate until pH=12, and the solution was heated at 85⁰C. In a highly alkaline environment, the pores on the CdS surface react with hydroxide anions to form hydroxyl radicals. A previously prepared sodium sulfide solution was added to the reaction mixture, and the formation of quantum dots was observed. The reaction was carried out for 30 minutes under a nitrogen gas atmosphere. At the end of the reaction, ethanol was added to the system until turbidity was formed. The mixture was placed in test tubes and centrifuged at 6000 rpm for 10 minutes. The precipitate was redissolved in water[4,5].

Results and discussion

After the synthesis of CdS quantum dots, L-cysteine ​​is introduced as a coupling agent. L-cysteine is a naturally occurring sulfur-containing amino acid, which can be used with CdS quantum dots to provide storage and secure aggregation. The thiol (-SH) group in L-cysteine interacts with the CdS product to form a passive layer. These surface modifications increase the solubility of quantum dots in various solvents and improve the biocompatibility of the site.

Theoretical calculations have shown that the hexagonal CdS structure leads to the appearance of an internal electric field in the hexagonal structure of CdS, which contributes to the diffusion of charges and their more efficient separation, and in turn enhances its photocatalytic properties.

CdS quantum dots have luminescent properties and have been used in biological imaging. Absorption and luminescence spectra of the synthesized sample (Fig. 1).

Figure 1. Absorption and luminescence spectra of CdS quantum dots stabilized with L-cysteine

They have attracted great interest in bioimaging and fluorescent labeling (in vitro and in vivo). In addition, by adjusting their size and composition, their emission wavelength can be tuned from visible to infrared wavelengths, which may be useful for in vivo imaging, such as sentinel lymph node mapping for image-guided surgery. CdS quantum dots are used as effective markers for imaging cells and tissues due to their high quantum efficiency and ability to emit in different spectral ranges. Once inside cells, CdS quantum dots emit in different colors under ultraviolet (UV) or visible light irradiation. In order to study the effects of heavy metals on the liver of rats, UV imaging of quantum dots in the body of rats fed CdS quantum dots was studied. The purpose of adding CdS quantum dots to the food of the control rats was to monitor the transport of drugs to the target site. The absorption spectra of a sample of their blood plasma were used to determine the presence of CdS quantum dots in the blood (Figures 2 a.b.c.).

a)                                                        b)

c)

Figure 2. Absorption spectra of a sample of rat blood plasma from healthy (a) and control rats (b), absorption spectra of a sample of blood plasma from healthy and control rats (c) in ultraviolet light

The active optical properties of CdS quantum dots make them sensitive to biological molecules, which makes them suitable for use in biosensors. CdS-based biosensors are effective in detecting glucose, DNA, and proteins. They are used in the rapid detection of early-stage diabetes and DNA mutations, in targeted drug delivery, and as biomarkers[6-7].

Since organic substances such as mercaptoethanol, mercaptoacetic acid, mercaptopropionic acid, and L-cysteine were used as stabilizers, the sample was analyzed in the IR spectrum. The results of the analysis can be seen in the IR spectrum in Figure 3.

Figure 3. IR spectrum of CdS quantum dots stabilized with a thiol group stabilizer

CdS quantum dots stabilized with thiol-containing stabilizers increase surface stability and provide a bright luminescence spectrum [8]. The biocompatibility of quantum dots increases the efficiency of drug delivery to the target site. It allows the detection of markers of cancer or viral infections. The advantages of CdS quantum dots stabilized with thiol-containing stabilizers include low toxicity, compatibility with biological systems, good water solubility, and high photoluminescence stability [9].

The photocatalytic properties of CdS quantum dots were studied using methylene blue as an indicator. The photocatalytic reaction rate of CdS quantum dots coated with thiol-containing stabilizers was studied using methylene blue as an indicator. The results can be seen in Figure 4.

Figure 4. Kinetic curves of the decomposition reaction of methylene blue in the presence of CdS quantum dots coated with thiol group stabilizers

Conclusion

In general, CdS quantum dots coated with thiol stabilizers have high stability and biocompatibility, and are considered promising nanomaterials for improving the detection, treatment and therapy processes of diseases in modern medicine.

Photocatalysts developed on the basis of CdS quantum dots have great potential for environmental purification and industrial waste neutralization. Their high photocatalytic activity and optical properties give confidence that they will be widely used in the future.

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