Method for preparing water-dispersible quantum dots, colloid and a method for preparing the colloid

Abstract

The method for preparing water-dispersible core-shell quantum dots stabilized with a layer of hydrophilic surface ligands includes making a core of PbS nanocrystals, and obtaining the shell of CdS layer and the surface ligands of dithiocarbamates obtained by reacting amino acids with carbon disulphide, while maintaining the photoluminescence. The emulsion formed of an aqueous solution of amino acid and carbon disulphide includes adding a solution of PbS/CdS in chloroform (CHCl.sub.3). Then, the contents are stirred vigorously for at least 20 hours, then the phases are separated. The upper, aqueous phase, being a solution of PbS/CdS/DTC-amino acid residue, is subjected to purification. The present invention is also colloid and a method for preparing colloid.

Claims

1. A method for preparing water-dispersible core-shell quantum dots stabilized with a layer of hydrophilic surface ligands, the method comprising the steps of: making a core of PbS nanocrystals; and reacting amino acids with carbon disulphide so as to obtain a shell of CdS layer and surface ligands comprised of dithiocarbamates and so as to maintain photoluminescence, wherein the step of reacting amino acids comprises the steps of: adding a solution of PbS/CdS in chloroform to an emulsion of an aqueous solution of amino acid and carbon disulphide so as to form a mixture; stirring said mixture vigorously for at least 20 hours so as to form a stirred mixture; separating said stirred mixture into phases, wherein at least one phase is comprised of an upper aqueous phase, said upper aqueous phase being comprised of a solution of PbS/CdS/DTC-amino acid residue; and purifying said solution of PbS/CdS/DTC-amino acid residue.

2. The method for preparing water-dispersible core-shell quantum dots, according to claim 1, wherein said amino acids are comprised of at least one of lysine, valine, proline, glycine, arginine, alanine, and beta-alanine.

3. The method for preparing water-dispersible core-shell quantum dots, according to claim 1, wherein the step of reacting amino acids further comprises the step of: mixing stoichiometric amounts of amino acids in carbon disulphide, wherein maximum concentration of amino acids is limited by solubility in water.

4. The method for preparing water-dispersible core-shell quantum dots, according to claim 1, wherein at least one phase is comprised of an emulsion phase, said emulsion phase being comprised of dithiocarbamate.

5. The method for preparing water-dispersible core-shell quantum dots, according to claim 1, wherein said solution of PbS/CdS in chloroform has a concentration ranging from 0 to 100 mg/cm3.

6. The method for preparing water-dispersible core-shell quantum dots, according to claim 1, wherein a volume ratio of said solution of PbS/CdS in chloroform to said emulsion has a range of from 1:10 to 10:1.

7. The method for preparing water-dispersible core-shell quantum dots, according to claim 1, wherein the step of purifying said solution of PbS/CdS/DTC-amino acid residue comprises the steps of: centrifuging said solution of PbS/CdS/DTC-amino acid residue; adding acetone so as to form a clouded solution with a first cloudiness; centrifuging said clouded solution so as to form a centrifuged precipitate; dissolving said centrifuged precipitate in a minimum amount of water so as to form a dissolved precipitate solution; and adding acetone to said dissolved precipitate solution so as to form a dispersed colloid solution.

8. The method for preparing water-dispersible core-shell quantum dots, according to claim 7, further comprising the steps of: washing said centrifuged precipitate with acetone, after the step of re-centrifuging said clouded solution, so as to form a washed precipitate, wherein the step of dissolving said centrifuged precipitate is comprised of dispersing said washed precipitate as said centrifuged precipitate in distilled water so as to obtain said dissolved precipitate solution.

9. The method for preparing water-dispersible core-shell quantum dots, according to claim 1, wherein said solution of PbS/CdS/DTC-amino acid residue is comprised of particles sized from 4 to 20 nm.

10. A The method for preparing water-dispersible core-shell quantum dots, according to claim 1, wherein the step of purifying said solution of PbS/CdS/DTC-amino acid residue is comprised of the steps of: adding an excess of a solution of a quaternary ammonium salt to said solution of the PbS/CdS-DTC-amino acid residue so as to form a mixed solution; stirring said mixed solution at room temperature for 24 hours so as to form a stirred solution; centrifuging said stirred solution so as to separate quantum dots from said PbS/CdS-DTC-amino acid residue; washing said quantum dots with distilled water; and dispersing said quantum dots in a phosphate buffer at pH 7.4 so as to obtain a colloidal solution of quantum dots in a PBS buffer.

11. The method for preparing water-dispersible core-shell quantum dots, according to claim 10, wherein said quaternary ammonium salt is comprised of diallyl-dimethyl-ammonium polyhydroxide.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The present invention is closer presented in the following embodiments and the drawings.

(2) FIG. 1 shows a graph illustration of the zeta potential measured for the sample of PbS/CdS-DTC-Lys in a PbS buffer.

(3) FIG. 2 shows a graph illustration of the EDS spectrum of the PbS/CdS quantum dots.

(4) FIG. 3 shows a graph illustration of the emission and absorption spectra of solutions of the PbS quantum dots (A), the PbS/CdS quantum dots (B) and TEM images and size distributions of PbS (C) and PbS/CdS (D).

(5) FIG. 4 shows a graph illustration of the size distribution of the PbS quantum dots (solution in hexane) obtained by dynamic light scattering, wherein the average particle size is 6.7 nm.

(6) FIG. 5 shows a graph illustration of the size distribution of the PbS/CdS quantum dots (solution in hexane) obtained by dynamic light scattering, wherein the average particle size is 6.8 nm.

(7) FIG. 6 shows a graph illustration of the 1H NMR spectrum recorded for the sample of PbS/CdS-DTC-Lys.

(8) FIG. 7 shows a graph illustration of the 130 NMR spectrum recorded for the sample of PbS/CdS-DTC-Lys.

(9) FIG. 8 shows a TEM image of the sample of PbS/CdS/DTC-Lys.

(10) FIG. 9 shows a graph illustration of the size distribution of the PbS/CdS-DTC-Lys (aqueous solution) quantum dots obtained by dynamic light scattering, wherein the average particle size is 8.6 nm.

(11) FIG. 10 shows a graph illustration of the emission and absorption spectra of solutions of the PbS/CdS/DTC-Lys quantum dots recorded in aqueous solution;

(12) FIG. 11 shows a graph illustration of an emission spectrum before (hexane) and after (H.sub.2O) procedure of the exchange of ligand on the surface of PbS/CdS dots using lysine dithiocarbamate.

DETAILED DESCRIPTION OF THE INVENTION

Example 1: Synthesis of the PbS Quantum Dots According to the Known Method Developed by M. A. Hines et al

(13) 1.0 mmol (223.2 mg) of lead monoxide (PbO), 2.5 mmol (0.79 cm.sup.3) of oleic acid and 12.5 cm.sup.3 of octadec-1-ene are placed in a tri-railed round-bottomed flask equipped with a thermometer, a tap enabling connection to a vacuum line and a silicone septa. The flask is heated on a magnetic stirrer under stirring to 250 C. until complete digestion of PbO. After this time, the flask is cooled to the temperature of 120 C., and the mixture is dehydrated and deoxygenated under reduced pressure, while the flask is periodically filled with nitrogen. After 60 min of degassing, a solution of bis(trimethylsilyl)sulphide (0.5 mmol, 105 l) in 1 cm.sup.3 of octadec-1-ene is quickly injected through the septa. After 5 minutes, the flask is removed from above the source of heat and allows for natural cooling.

(14) Quantum dots are purified by washing the reaction mixture with methanol (2), precipitating the nanoparticles with a mixture of acetone and ethanol (1:1), dispersing in toluene and subsequent precipitating with a mixture of acetone and ethanol (2:1). After centrifugation, the precipitate is dispersed in toluene (8 cm.sup.3) to obtain a colloidal solution.

Example 2: Synthesis of PbS/CdS Core-Shell Quantum Dots with the Use of the Method Developed by Pietryga et al

(15) 0.457 g of CdO, 3 cm.sup.3 of oleic acid and 8 cm.sup.3 of diphenyl ether are placed in a Schlenk flask and heated to 250 C. under a nitrogen atmosphere until complete digestion of CdO. After this time, the flask is cooled to 120 C., and the mixture is dehydrated and deoxygenated under reduced pressure, while the flask is periodically filled with nitrogen. This way, a solution of cadmium oleate is obtained.

(16) 4 cm.sup.3 of a solution of PbS quantum dots in toluene, obtained in the previous step, are placed in tri-railed round-bottomed flask equipped with a thermometer, a capillary enabling a circulation of an inert gas through the solution, and a reflux condenser, and heated to 100 C. while stirring and bubbling nitrogen through it. After one hour, the solution of cadmium oleate is transferred to the flask with PbS and heated at 100 C. for 45 min. After this time, the flask is cooled by immersion in cold water, the solution is washed twice with methanol, and the dots are re-precipitated with a mixture of acetone and ethanol (1:1). The dots are dispersed in chloroform and re-precipitated with a mixture of acetone and ethanol (2:1). The precipitate after centrifugation is dispersed in 10 cm.sup.3 of chloroform to obtain a colloidal solution (Solution B).

Example 3: Preparation of PbS/CdS@DTC-Lys with the Use of the Method According to the Present Invention

(17) 1 cm.sup.3 of a solution of lysine at a concentration of 0.7 M is placed in a vial. Next, 0.7 mmol (34.6 l) of carbon disulphide is injected into the vial, which is then sonicated (10 min). Then, 1 cm.sup.3 of a solution of PbS/CdS (0.1 cm.sup.3 of Solution B+0.9 cm.sup.3 of CHCl.sub.3) is added to the obtained emulsion. After sealing, the vial is placed on a magnetic stirrer and the contents are stirred vigorously for 24 hours.

(18) After this time, stirring is turned off and after separation of phases the upper, aqueous phase, being a solution of PbS/CdS/DTC-Lys, is separated and placed into the centrifuge tube. Next, acetone is added to the solution until the first cloudiness appears, then it is centrifuged and the precipitate is dissolved in a minimum amount of water and then re-precipitated with acetone. After centrifugation, the precipitate is washed once more with acetone and dispersed in 1 cm.sup.3 of distilled water to obtain a colloidal solution.

Example 4: Preparation of PbS/CdS@DTC-Arg

(19) 1 cm.sup.3 of a solution of arginine at a concentration of 0.7 M is placed in a vial. Next, 0.7 mmol (34.6 l) of carbon disulphide is injected into the vial, which is then sonicated (10 min). Then, 1 cm.sup.3 of a solution of PbS/CdS (0.1 cm.sup.3 of Solution B+0.9 cm.sup.3 of CHCl.sub.3) is added to the obtained emulsion. After sealing, the vial is placed on a magnetic stirrer and the contents are stirred vigorously for 24 hours.

(20) After this time, stirring is turned off and after separation of phases the upper, aqueous phase, being a solution of PbS/CdS/DTC-Arg, is separated and placed into the centrifuge tube. Next, acetone is added to the solution until the first cloudiness appears, then it is centrifuged and the precipitate is dissolved in a minimum amount of water and then re-precipitated with acetone. After centrifugation, the precipitate is washed once more with acetone and dispersed in 1 cm.sup.3 of distilled water to obtain a colloidal solution.

Example 5: Preparation of PbS/CdS@DTC-Pro

(21) 1 cm.sup.3 of a solution of proline at a concentration of 0.7 M is placed in a vial. Next, 0.7 mmol (34.6 l) of carbon disulphide is injected into the vial, which is then sonicated (10 min). Then, 1 cm.sup.3 of a solution of PbS/CdS (0.1 cm.sup.3 of Solution B+0.9 cm.sup.3 of CHCl.sub.3) is added to the obtained emulsion. After sealing, the vial is placed on a magnetic stirrer and the contents are stirred vigorously for 24 hours.

(22) After this time, stirring is turned off and after separation of phases the upper, aqueous phase, being a solution of PbS/CdS/DTC-Pro, is separated and placed into the centrifuge tube. Next, acetone is added to the solution until the first cloudiness appears, then it is centrifuged and the precipitate is dissolved in a minimum amount of water and then re-precipitated with acetone. After centrifugation, the precipitate is washed once more with acetone and dispersed in 1 cm.sup.3 of distilled water to obtain a colloidal solution.

Example 6: Preparation of PbS/CdS@DTC-Val

(23) 2 cm.sup.3 of a saturated solution of valine is placed in a vial. Next, 0.43 mmol (21.2 l) of carbon disulphide is injected into the vial, which is then sonicated (10 min). Then, 1 cm.sup.3 of a solution of PbS/CdS (0.1 cm.sup.3 of Solution B+0.9 cm.sup.3 of CHCl.sub.3) is added to the obtained emulsion. After sealing, the vial is placed on a magnetic stirrer and the contents are stirred vigorously for 24 hours.

(24) After this time, stirring is turned off and after separation of phases the upper, aqueous phase, being a solution of PbS/CdS/DTC-Val, is separated and placed into the centrifuge tube. Next, acetone is added to the solution until the first cloudiness appears, then it is centrifuged and the precipitate is dissolved in a minimum amount of water and then re-precipitated with acetone. After centrifugation, the precipitate is washed once more with acetone and dispersed in 1 cm.sup.3 of distilled water to obtain a colloidal solution.

Example 7: Preparation of PbS/CdS@DTC-Gly

(25) 1 cm.sup.3 of a solution of glycine at a concentration of 0.7 M is placed in a vial. Next, 0.7 mmol (34.6 l) of carbon disulphide is injected into the vial, which is then sonicated (10 min). Then, 1 cm.sup.3 of a solution of PbS/CdS (0.1 cm.sup.3 of Solution B+0.9 cm.sup.3 of CHCl.sub.3) is added to the obtained emulsion. After sealing, the vial is placed on a magnetic stirrer and the contents are stirred vigorously for 24 hours.

(26) After this time, stirring is turned off and after separation of phases the upper, aqueous phase, being a solution of PbS/CdS/DTC-Gly, is separated and placed into the centrifuge tube. Next, acetone is added to the solution until the first cloudiness appears, then it is centrifuged and the precipitate is dissolved in a minimum amount of water and then re-precipitated with acetone. After centrifugation, the precipitate is washed once more with acetone and dispersed in 1 cm.sup.3 of distilled water to obtain a colloidal solution.

Example 8: Preparation of PbS/CdS@DTC-Ala

(27) 1 cm.sup.3 of a solution of alanine at a concentration of 0.7 M is placed in a vial. Next, 0.7 mmol (34.6 l) of carbon disulphide is injected into the vial, which is then sonicated (10 min). Then, 1 cm.sup.3 of a solution of PbS/CdS (0.1 cm.sup.3 of Solution B+0.9 cm.sup.3 of CHCl.sub.3) is added to the obtained emulsion. After sealing, the vial is placed on a magnetic stirrer and the contents are stirred vigorously for 24 hours.

(28) After this time, stirring is turned off and after separation of phases the upper, aqueous phase, being a solution of PbS/CdS/DTC-Ala, is separated and placed into the centrifuge tube. Next, acetone is added to the solution until the first cloudiness appears, then it is centrifuged and the precipitate is dissolved in a minimum amount of water and then re-precipitated with acetone. After centrifugation, the precipitate is washed once more with acetone and dispersed in 1 cm.sup.3 of distilled water to obtain a colloidal solution.

Example 9. Preparation of PbS/CdS@DTC--Ala

(29) 1 cm.sup.3 of a solution of beta-alanine at a concentration of 0.7 M is placed in a vial. Next, 0.7 mmol (34.6 l) of carbon disulphide is injected into the vial, which is then sonicated (10 min). Then, 1 cm.sup.3 of a solution of PbS/CdS (0.1 cm.sup.3 of Solution B+0.9 cm.sup.3 of CHCl.sub.3) is added to the obtained emulsion. After sealing, the vial is placed on a magnetic stirrer and the contents are stirred vigorously for 24 hours.

(30) After this time, stirring is turned off and after separation of phases the upper, aqueous phase, being a solution of PbS/CdS/DTC--Ala, is separated and placed into the centrifuge tube. Next, acetone is added to the solution until the first cloudiness appears, then it is centrifuged and the precipitate is dissolved in a minimum amount of water and then re-precipitated with acetone. After centrifugation, the precipitate is washed once more with acetone and dispersed in 1 cm.sup.3 of distilled water to obtain a colloidal solution.

Example 10: Preparation of the PbS/CdS Quantum Dots Coated with a Layer of Polyelectrolyte and Stabilised with Lysine Dithiocarbamate

(31) Preparation of the Solution of Polyelectrolyte

(32) In order to coat the water-dispersible quantum dots with a layer of polyelectrolyte, cationic polyelectrolytes can be used. In the exemplary embodiment, diallyl-dimethyl-ammonium polyhydroxide was used. It was obtained in two steps from the commercially available diallyl-dimethyl-ammonium chloride.

(33) A saturated aqueous solution of Ag.sub.2SO.sub.4 is added dropwise to the solution of diallyl-dimethyl-ammonium chloride (20%) until complete precipitation of chlorides. The obtained silver chloride is centrifuged and then a saturated solution of Ba(OH).sub.2 (stoichiometric) is added to the clear solution. The precipitated barium sulphate is centrifuged and the obtained solution of diallyl-dimethyl-ammonium polyhydroxide is concentrated using a rotary evaporator.

(34) Alternatively, diallyl-dimethyl-ammonium polyhydroxide can be obtained by using ion-exchange column chromatography.

(35) Coating Dots with a Layer of Polyelectrolyte

(36) An excess of the solution of diallyl-dimethyl-ammonium polyhydroxide (50 l) is added to an aqueous solution of the PbS/CdS-DTC-Lys quantum dots and after 24 hours of stirring at room temperature the solution is centrifuged (14,000 rpm for 10 min), washed with a small amount of distilled water (0.2 cm.sup.3), and the centrifuged dots are dispersed in a phosphate buffer at pH 7.4, obtaining a colloidal solution of quantum dots in PBS buffer.

(37) The solution of PbS/CdS/DTC-Lys at pH 7.4 (PBS buffer) exhibits a zeta potential equal to 31.4 mV (FIG. 1). After addition of diallyl-dimethyl-ammonium polyhydroxide and washing the centrifuged dots with a distilled water, the solution exhibits a zeta potential equal to +22.0 mV. At the same time, the size is changed from 8.6 to 20.2 nm, which indicates coating the dots with a chain of polyelectrolyte.

(38) In order to determine which of the amino acids allows to obtain stable colloidal solutions, there has been performed a series of experiments (Examples 3-9), in which only the kind of amino acid varied. The greatest stability characterised colloidal solutions prepared using lysine dithiocarbamate. The use of this ligand allows to obtain a colloidal solution stable for at least 3 months. During that time, no loss of dots from the solution as a precipitate or cloudiness was observed. As for other amino acids, i.e. proline, arginine and valine, they make it possible to obtain aqueous solutions that become, however, opalescent after only 24-72 hours.

(39) The EDS (energy dispersion spectroscopy) spectrum recorded using EDAX microprobe installed in a transmission electron microscope (TEM) FEI Tecnai G.sup.2 20 X-TWIN (for the PbS/CdS quantum dots (FIG. 2)) proves the presence of cadmium in the molecules. Moving of the emission and absorption ranges towards shorter wavelengths, visible in FIG. 3, confirms the decrease in the size of PbS particles. FIG. 3 shows the emission and absorption spectra of solutions of the PbS quantum dots (A), the PbS/CdS quantum dots (B) and images taken using a transmission electron microscope (TEM) and size distributions of PbS (C) and PbS/CdS (D).

(40) These facts combined with practically unchanged size of the nanoparticles (FIG. 4 and FIG. 5) clearly confirm obtaining of a PbS/CdS core-shell structure.

(41) For PbS/CdS-DTC-Lys, there was carried out a measurement of .sup.1H NMR and .sup.13C NMR. In the .sup.1H NMR spectrum (FIG. 6) recorded for the sample of PbS/CdS-DTC-Lys peaks characteristic of molecules of lysine are visible.

(42) In the carbon spectrum (FIG. 7), the presence of three signals in the range of 170-190 ppm clearly indicates the presence ofapart from carbon of the carboxyl groupcarbons of the dithiocarbamate group. Two additional signals should be matched with carbons of CS.sub.2.sup. resulting from the reaction of carbon disulphide with one of the two amino groups present in the molecule of lysine. Other signals are characteristic of the molecules of lysine. This confirms the presence of dithiocarbamate ligands on the surface of quantum dots.

(43) The image in FIG. 8, showing a TEM image of the PbS/CdS/DTC-Lys quantum dots, indicates no agglomeration of quantum dots in an aqueous solution. This fact is also confirmed by the measurements carried out by means of dynamic light scattering (DLS), resulting in obtaining a single peak derived from objects with an average size of 8.6 nm (FIG. 9).

(44) FIG. 10 shows the emission and absorption spectra of the PbS/CdS-DTC-Lys quantum dots in an aqueous solution. Positions of the maxima of emission (FIG. 11) and absorption after the exchange of the ligand to DTC-Lys remain unchanged in relation to the starting sample (PbS/CdS).