Simple Synthesis Method and Characterizations of Aggregation-Free Cysteamine Capped PbS Quantum Dot

: Quantum dots have diverse chemical properties with di ﬀ erent ligands attached on the surface. The cysteamine has been used as a ligand for various quantum dots because it has high solubility in water, and it facilitates binding of quantum dot and gold surface. However, the hydrogen bonds in cysteamine cause aggregation of the cysteamine capped quantum dots. In this study, we suggested a simple synthesis method of aggregation-free PbS quantum dot and analyzed the electric and optical properties of the synthesized quantum dot. This study on aggregation-free cysteamine capped quantum dots has the potential to develop advanced quantum dot-based sensor technologies, including biomedical imaging and environmental sensors.


Introduction
Over the past decade, quantum dots have attracted great attention in various research fields and industries because of their distinct electrical and optical properties from bulk materials. Three-dimensional quantum confinements are caused by particle size below Bohr's exciton radius effects on the energy band structure, which is deeply related to the electrical and optical properties of quantum dots [1,2]. Unlike in the bulk semiconductors, the dimension-dependent density of states has the shape of delta function in the quantum dots. This yields the absorption and emission spectra of quantum dots, with narrow full width half maximum (FWHM) centered at particular wavelength. The quantum dots have engineering advantages where the absorption and emission spectra can be adjusted by changing the synthesis conditions. These properties of quantum dots provide high wavelength-selectivity, accuracy, and sensitivity [3][4][5][6]. Therefore, quantum dots are considered one of the most promising materials in various applications, including optoelectronics [7,8], biosensors [9][10][11][12][13][14], energy harvesters [15][16][17], and quantum dot-based lasers [18,19].
In general, the core quantum dot consists of semiconductors, especially the compounds of II-VI or IV-VI materials. The PbS quantum dots, one of IV-VI quantum dots, have been studied in various research and industry fields because it absorbs and emits infrared (IR) radiation in the wavelength range of near and shortwave IR [20][21][22], which corresponds to the absorption energy range between 0.71 to 1.28 eV [23]. The applications of PbS quantum dots with IR have been considered as one of the major subjects in research and industries, including optoelectronics and sensors [24][25][26].
While the core of a quantum dot determines the electrical and optical properties of the quantum dot, the ligand of the quantum dot generally characterizes chemical interactions with other materials including solvents, surfaces, and particles [27]. The development of ligands has made it possible to carry diverse chemical properties to quantum dots, making them more attractive to sensor applications [28,29]. In ligand groups comprising trioctylphosphine oxide (TOPO), trioctylphosphine (TOP), amines, and carboxyl acids, thiols are mainly used to synthesize gold-adherent quantum dots [30,31]. Since gold is one of the most used materials in biosensors [32][33][34][35], PbS quantum dots passivated with thiol-based ligands can have many advantages over other ligands for quantum dots in biosensor applications. Cysteamine, one of the thiols, is distinguished from other thiols because it is soluble in water, the most common and safe solvent. Thus, the various types of cysteamine capped quantum dots have been studied in the fields [36][37][38]. However, the synthesis of cysteamine capped PbS quantum dot has been considered difficult, due to the aggregation caused by hydrogen bonds between cysteamine molecules [39].
In this study, we describe a simple synthesis method of aggregation-free cysteamine capped PbS quantum dots. We employed sodium docusate as a stabilizing agent to maintain lead ions to be monomers, and 2-mercaptoethanesulfonate as an additional ligand to prevent the aggregation. The whole process was conducted in one pot and did not require any extra complex procedures. The size and lattice structure of the quantum dots were characterized by the transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS) confirmed that cysteamine and 2-mercaptoethanesulfonate were well-adhered to the surface of the quantum dot. The absorption characteristics of the quantum dot analyzed by the spectrophotometer shows high absorbance at the wavelength of 1450 nm. The synthesized quantum dots maintain dispersion stability longer than one month.

Synthesis of Aggregation-Free Cysteamine Capped PbS Quantum Dot
Cysteamine capped PbS quantum dot was synthesized in aqueous solution with sodium docusate as a stabilizing agent. A 0.24 mmol (0.105 g) of sodium docusate salt was dissolved in 50 mL of double distilled water with stirring for 30 min on a magnetic stirrer, then a 3 mL of 0.04 M lead chloride aqueous solution was added to the solution. Since the solubility of lead chloride in water (10.8 g/L, 20 • C) is poor, it is recommended to keep the stock solution of lead precursors above 40 • C. After stirring for 10 min, a 0.6 mL of 0.2 M sodium sulfide aqueous solution was added dropwise to the solution and stirred for 10 min. When the sulfur precursor was added, the color of the solution turned dark brown immediately. For ligand exchange, a 1 mL of 0.6 M cysteamine aqueous solution was added into PbS quantum dot colloids and stirred for 10 min. After adding the cysteamine to PbS quantum dots, the texture of suspension was changed. Lastly, a 1 mL of 0.6 M 2-mercaptoethanesulfonate aqueous solution was dropped into cysteamine capped PbS quantum dot and stirred for 10 min, to prevent the aggregation of quantum dots. A brief description of the synthesis method is shown in Figure 1.

Characterizations
The TEM images and EDS measurements of aggregation-free cysteamine capped PbS quantum dot were captured by Titan G2 ChemiSTEM Cs Probe (FEI Company, USA). The synthesized quantum dots were placed on 200 meshes of formvar/copper TEM grid (TED PELLA, Inc., USA) after dipping in colloid for less than 10 s and drying for over 10 min in atmosphere.
The infrared absorption spectrum of the colloidal PbS quantum dot was measured using transmission mode of Cary 5000 UV-Vis-NIR spectrophotometer (Agilent technologies, Inc., USA) in the form of liquids. Because water is not used for infrared spectroscopy due to its high infrared absorption, methanol which has low infrared absorption was used for the solvent. In order to disperse the synthesized quantum dots in methanol, the colloid was centrifuged for 90 min with 7000 of relative centrifugal force (RCF) in a centrifuge. Methanol was added to the precipitated quantum dots, and the colloid was sonicated for 5 min.
The XRD (X-ray diffraction) measurement was conducted in form of powder. The synthesized quantum dot colloid was evaporated in room temperature. The result of XRD was obtained using EMPYREAN (PANanalytical, B.V., Netherlands).

Results and discussions
The colloid of resulting cysteamine capped PbS quantum dot has been well-dissolved in double distilled water over a month. As it contained sodium docusate as a stabilizing agent, lead ion from lead chloride developed in the form of monomers. If the lead precursor is not dissolved as monomers, nuclear growth occurs before nuclei are created, which makes it difficult to control the size of the quantum dots. When the lead precursor is injected to water in which docusate is dissolved, the docusate molecules bind around the lead monomer, and the color of solution turns white. Subsequently, when the lead monomers react with the sulfur precursor, nuclei of PbS quantum dot start to form, and the size of the quantum dot gradually increases according to Ostwald ripening. The size of the quantum dots is usually proportional to the reaction time and temperature. After nuclear growth, cysteamine aqueous solution is added to induce ligand exchange on the surface of the quantum dot. However, since cysteamine has an amine group, there is a possibility that quantum dots may aggregate over time by forming hydrogen bonds in an aqueous solution, and it is necessary to additionally attach another ligand to prevent the aggregation. Since 2-mercaptoethanesulfonate is anionized in an aqueous solution, quantum dots can maintain dispersion stability even if they have cysteamine as a ligand, due to the electrical repulsion generated therefrom. The synthesized aggregation-free PbS quantum dots, as described above, it was confirmed that they did not aggregate and maintained dispersion stability even after one month. Although, some white and yellow

Characterizations
The TEM images and EDS measurements of aggregation-free cysteamine capped PbS quantum dot were captured by Titan G2 ChemiSTEM Cs Probe (FEI Company, Hillsboro, OR, USA). The synthesized quantum dots were placed on 200 meshes of formvar/copper TEM grid (TED PELLA, Inc., Redding, CA, USA) after dipping in colloid for less than 10 s and drying for over 10 min in atmosphere.
The infrared absorption spectrum of the colloidal PbS quantum dot was measured using transmission mode of Cary 5000 UV-Vis-NIR spectrophotometer (Agilent technologies, Inc., Santa Clara, CA, USA) in the form of liquids. Because water is not used for infrared spectroscopy due to its high infrared absorption, methanol which has low infrared absorption was used for the solvent. In order to disperse the synthesized quantum dots in methanol, the colloid was centrifuged for 90 min with 7000 of relative centrifugal force (RCF) in a centrifuge. Methanol was added to the precipitated quantum dots, and the colloid was sonicated for 5 min.
The XRD (X-ray diffraction) measurement was conducted in form of powder. The synthesized quantum dot colloid was evaporated in room temperature. The result of XRD was obtained using EMPYREAN (PANanalytical, B.V., Almelo, The Netherlands).

Results and Discussion
The colloid of resulting cysteamine capped PbS quantum dot has been well-dissolved in double distilled water over a month. As it contained sodium docusate as a stabilizing agent, lead ion from lead chloride developed in the form of monomers. If the lead precursor is not dissolved as monomers, nuclear growth occurs before nuclei are created, which makes it difficult to control the size of the quantum dots. When the lead precursor is injected to water in which docusate is dissolved, the docusate molecules bind around the lead monomer, and the color of solution turns white. Subsequently, when the lead monomers react with the sulfur precursor, nuclei of PbS quantum dot start to form, and the size of the quantum dot gradually increases according to Ostwald ripening. The size of the quantum dots is usually proportional to the reaction time and temperature. After nuclear growth, cysteamine aqueous solution is added to induce ligand exchange on the surface of the quantum dot. However, since cysteamine has an amine group, there is a possibility that quantum dots may aggregate over time by forming hydrogen bonds in an aqueous solution, and it is necessary to additionally attach another ligand to prevent the aggregation. Since 2-mercaptoethanesulfonate is anionized in an aqueous solution, quantum dots can maintain dispersion stability even if they have cysteamine as a ligand, due to the electrical repulsion generated therefrom. The synthesized aggregation-free PbS quantum dots, as described above, it was confirmed that they did not aggregate and maintained dispersion stability even after one month. Although, some white and yellow precipitates were formed on the bottom of the solution. We attribute the white and yellow precipitates to lead compounds, including lead sulphate, lead carbonate, and lead oxide, caused by lead atoms not involved in quantum dot formation. Figure 2 shows the images of quantum dots as synthesized and after one month from synthesis.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 9 precipitates were formed on the bottom of the solution. We attribute the white and yellow precipitates to lead compounds, including lead sulphate, lead carbonate, and lead oxide, caused by lead atoms not involved in quantum dot formation. Figure 2 shows the images of quantum dots as synthesized and after one month from synthesis.  Figure 3a shows the TEM images of the synthesized quantum dot. The TEM images shows that the particles with particular diameter are evenly distributed on the grid. Figure 3b shows a TEM image of quantum dots synthesized without adding 2-mercaptoethanesulfonate and severe aggregation of the quantum dots. In Figure 3b, all particles are connected and aggregated with the hydrogen bond from amine functional groups of cysteamine. However, in Figure 3a, the separated particles are laid over the layers. From above, attaching 2-mercaptoethanesulfonate as an additional ligand was an efficient method to prevent the aggregation of cysteamine.  precipitates were formed on the bottom of the solution. We attribute the white and yellow precipitates to lead compounds, including lead sulphate, lead carbonate, and lead oxide, caused by lead atoms not involved in quantum dot formation. Figure 2 shows the images of quantum dots as synthesized and after one month from synthesis.  Figure 3a shows the TEM images of the synthesized quantum dot. The TEM images shows that the particles with particular diameter are evenly distributed on the grid. Figure 3b shows a TEM image of quantum dots synthesized without adding 2-mercaptoethanesulfonate and severe aggregation of the quantum dots. In Figure 3b, all particles are connected and aggregated with the hydrogen bond from amine functional groups of cysteamine. However, in Figure 3a, the separated particles are laid over the layers. From above, attaching 2-mercaptoethanesulfonate as an additional ligand was an efficient method to prevent the aggregation of cysteamine. In Figure 3b, all particles are connected and aggregated with the hydrogen bond from amine functional groups of cysteamine. However, in Figure 3a, the separated particles are laid over the layers. From above, attaching 2-mercaptoethanesulfonate as an additional ligand was an efficient method to prevent the aggregation of cysteamine. Figure 4 shows the TEM image of the synthesized quantum dots in a higher magnitude than in   Figure 5 shows the TEM images of the synthesized quantum dot after one month. As the results in Figure 2, the synthesized quantum dot maintained the dispersion stability after one month. The TEM images of the synthesized quantum dot show that the particles are not aggregated after one month in the microscopic view.   Figure 5 shows the TEM images of the synthesized quantum dot after one month. As the results in Figure 2, the synthesized quantum dot maintained the dispersion stability after one month. The TEM images of the synthesized quantum dot show that the particles are not aggregated after one month in the microscopic view.  Figure 5 shows the TEM images of the synthesized quantum dot after one month. As the results in Figure 2, the synthesized quantum dot maintained the dispersion stability after one month. The TEM images of the synthesized quantum dot show that the particles are not aggregated after one month in the microscopic view.   Figure 6 shows EDS results and it exhibits nitrogen and sodium atoms from the two ligands, indicating that the two ligands were well-attached to the surface of synthesized quantum dot.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 9 Figure 6 shows EDS results and it exhibits nitrogen and sodium atoms from the two ligands, indicating that the two ligands were well-attached to the surface of synthesized quantum dot.     [40]. Figure 8 demonstrates the absorption characteristics of synthesized cysteamine capped PbS quantum dot. The values used in Figure 5 were normalized between 0 to 1. The absorbance of the synthesized quantum dot gradually decreases as the wavelength increases, and the absorbance starts to increase at the wavelength of 1360 nm, reaches a peak at 1450 nm, and then decreases again. The peak of absorbance is determined by the energy between conduction and valence band, which is reversely proportional to the size of the particle. According to Equation (1), the peak of absorbance is calculated to be 1430 nm as the average size of the synthesized quantum dot is 5.2 nm from TEM images.
where E 0 is the absorption energy, and d is the diameter of the quantum dot.     Figure 5 were normalized between 0 to 1. The absorbance of the synthesized quantum dot gradually decreases as the wavelength increases, and the absorbance starts to increase at the wavelength of 1360 nm, reaches a peak at 1450 nm, and then decreases again. The peak of absorbance is determined by the energy between conduction and valence band, which is reversely proportional to the size of the particle. According to Equation (1), the peak of absorbance is where E0 is the absorption energy, and d is the diameter of the quantum dot The measured peak wavelength of absorbance is 1450 nm, which is a good agreement with the theoretical studies.

Conclusion
Among various ligands for quantum dots, cysteamine has attracted attention because of its water-solubility. The water-solubility of ligand offers great benefits in various quantum dot applications. However, the hydrogen bonds between cysteamine molecules prevent the aggregationfree cysteamine capped PbS quantum dots being synthesizing. In this study, we effectively solved The measured peak wavelength of absorbance is 1450 nm, which is a good agreement with the theoretical studies.

Conclusions
Among various ligands for quantum dots, cysteamine has attracted attention because of its water-solubility. The water-solubility of ligand offers great benefits in various quantum dot applications. However, the hydrogen bonds between cysteamine molecules prevent the aggregation-free cysteamine capped PbS quantum dots being synthesizing. In this study, we effectively solved this aggregation problem by additionally attaching 2-mercaptoethanesulfonate to the surface of the quantum dot to repel the attraction force from cysteamine. The structure, size, and IR absorption of the cysteamine capped PbS quantum dots were characterized by TEM, EDS, and spectrophotometer. We believe that this study on aggregation-free cysteamine quantum dots has a promising potential to develop advanced quantum dot-based sensor technologies, including biomedical imaging and environmental sensors.