WO2018048316A1 - A method for obtaining manganese-doped luminescent nanoparticles of zinc selenide having positive surface charge - Google Patents

A method for obtaining manganese-doped luminescent nanoparticles of zinc selenide having positive surface charge Download PDF

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WO2018048316A1
WO2018048316A1 PCT/PL2017/000079 PL2017000079W WO2018048316A1 WO 2018048316 A1 WO2018048316 A1 WO 2018048316A1 PL 2017000079 W PL2017000079 W PL 2017000079W WO 2018048316 A1 WO2018048316 A1 WO 2018048316A1
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nanoparticles
znse
manganese
selenium
reaction mixture
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WO2018048316A9 (en
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Katarzyna MATRAS-POSTOŁEK
Svitlana SOVINSKA
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Politechnika Krakowska im. Tadeusza Kościuszki
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B19/00—Selenium; Tellurium; Compounds thereof
    • C01B19/007—Tellurides or selenides of metals
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
    • C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
    • C09K11/88—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing selenium, tellurium or unspecified chalcogen elements
    • C09K11/881—Chalcogenides
    • C09K11/883—Chalcogenides with zinc or cadmium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/50—Solid solutions
    • C01P2002/52—Solid solutions containing elements as dopants
    • C01P2002/54—Solid solutions containing elements as dopants one element only
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
    • C01P2002/72—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by d-values or two theta-values, e.g. as X-ray diagram
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/82—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by IR- or Raman-data
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/84—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by UV- or VIS- data
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/04—Particle morphology depicted by an image obtained by TEM, STEM, STM or AFM

Definitions

  • the invention relates to a method for obtaining, in an aqueous medium, manganese-doped luminescent nanoparticles of zinc selenide (ZnSe:Mn) having positive surface charge, in the form of stable aqueous suspensions or nanopowders intended for digestion in water.
  • ZnSe:Mn nanoparticles obtained with the method of the invention can be used in medicine, biology and optoelectronics.
  • Quantum dots Semiconductor nanoparticles having luminescence properties exist in the market as quantum dots (QD).
  • QD quantum dots
  • the term specifies nano structured semiconductors in which the motion of electrons is slowed-down in three directions by potential barriers, forming so- called "box of potential”.
  • the key feature of quantum dots is their ability to emit light waves whose length depends on the size and shape of the nano structures.
  • nanoparticles find application in many areas of life including biology and medicine, especially in the process of diagnosis and treatment of cancer, as well as in the widely understood optoelectronics, in manufacturing diodes, displays and solar panels.
  • the quantum dots market can be now considered a niche market, but due to the wide range of its application a dynamic development of this market is expected.
  • quantum dots are based on the compounds of heavy metals, for instance cadmium (cadmium selenide, cadmium sulfide, cadmium telluride), which are characterized by high toxicity and harmful effect on the environment.
  • cadmium cadmium selenide, cadmium sulfide, cadmium telluride
  • CdSe cadmium selenide
  • the "hot injection” method has become the predominant way for preparing luminescent nanoparticle semiconductors.
  • the method bases on high-temperature pyrolysis of organometallic precursors in a hot organic coordinating-solvent or on mixing the precursors and rapid cooling of the reaction mixture.
  • the reaction media are usually toxic long-chain alkylphosphines, alkylphosphine oxides and alkylamines which boiling points are of about 340°C, such as trioctylphosphine [(C 8 H 17)3 P] (TOP), trioctylphosphine oxide [(C 8 H
  • the method requires the use of an atmosphere of an inert gas (Ar, N 2 ) in order to prevent oxidation of reactants and freshly prepared nanoparticles.
  • the zinc-precursor is most often an organometallic compound, and the selenium-precursor is metallic selenium dissolved in a high-boiling solvent, which at the same time functions as a stabilizer.
  • the advantages of this type of reaction include the ability to obtain nanoparticles of relatively small size and high quantum yield of luminescence. On the other hand, these processes are costly, complicated, energy consuming. In addition, the processes require very high temperatures (280 - 320°C) and the use of toxic substances (organometallic precursors and solvents).
  • colloidal reactions An alternative to this type of known reactions are chemical processes carried out directly in an aquatic environment, broadly understood so-called colloidal reactions, including co-precipitation reactions.
  • the mechanism of colloidal reaction is much simpler and less laborious than the "hot injection" reaction.
  • the main synthesis usually follows the step of selecting or producing selenium, zinc and manganese precursors, and preparing solutions of them.
  • a nucleation phase in consists in that nuclei of nanoparticles form in the initially homogenous solution. In the next stage, growth, the remaining material gradually builds up on the surfaces of the formed stable nuclei.
  • the next phase is the increasing of nanoparticles, after which the process ends with the isolation of the freshly obtained nanoparticles from the reaction mixture (e.g. by precipitation) and optionally further processing the product after the synthesis.
  • the most important parameters of the colloidal reaction which have an influence on the nature of the obtained ZnSe:Mn nanoparticles suspension in aqueous solution, are: the composition and quantity (molar ratio) of precursors introduced into the reaction (usually inorganic salts), the sequence of adding the substrates, pH of the reaction mixture, time and temperature of the reaction, and finally one of the most important parameters which is a stabilizer suitable for a particular reaction mixture, of ZnSe:Mn nanoparticles.
  • these parameters one can control the size, shape, degree of crystalHnity, and surface properties of nanoparticles that directly affect their optical and electrical properties such as luminescence and energy gap, and hence their application properties.
  • Another important aspect is the method of purifying the resulting suspension of ZnSe;Mn nanoparticles and isolating the resulting nanopowder.
  • resulting material can undergo a larger or smaller agglomeration process, which is also essential to its applicability.
  • ZnSe and ZnSe/ZnS nanoparticles show completely different optical and electrical properties than ZnSe nanoparticles doped with manganese atoms.
  • the manganese-doped ZnSe nanoparticles show luminescence in a different visible light range than un-doped nanoparticles.
  • ZnSe:Mn shows luminescence in the range of orange-color visible light, corresponding to wavelength of 580 - 600nm.
  • pure ZnSe shows luminescence in at the wavelength of 450 nm, corresponding to blue light.
  • ZnS shell is intended to enhance the luminescent properties of the obtained nanoparticles and to protect them against oxidation.
  • step 1 selenium-precursor preparation by dissolving in distilled water product of the reaction of powdered selenium and sodium borohydride (NaBH 4 ) in an inert environment (argon or nitrogen);
  • step 2 zinc- precursor (Zn) preparation by dissolving zinc acetate in distilled water in the presence of a thioglycolic acid as a stabilizer and adjusting pH to the value of 8 - 13 due to addition 1M of NaOH;
  • step 3 manganese-precursor preparation by dissolving manganese acetate in distilled water;
  • step 4 main reaction of the obtaining ZnSe:Mn nanoparticles by gradual adding of precursors in different sequence and heating the mixture during the time of 0.5 - 6 h at the temperature of 90 - 120°C;
  • step 5 purifying and precipitating with isopropanol
  • step 1 selenium-precursor preparation by dissolving in distilled water the product of the raction of powdered selenium and sodium borohydride (NaBH 4 ) in an inert environment (argon or nitrogen);
  • step 2 manganese-precursor preparation by dissolving manganese acetate in water containing MPA stabilizer (3-mercaptopropionic acid), and adjusting pH to the value of 7-8 by addition 1 M of NaOH, then heating the mixture at the temperature of 100°C for 0.5 h;
  • step 3 MnSe nanocrystals preparation by injecting with a syringe selenium-precursor into manganese-precursor and heating the mixture in the temperature of 100°C;
  • step 4 zinc-precursor preparation, in the same time, by dissolving zinc acetate in distilled water in the presence of MPA and at pH
  • thiol acids such as 3-mercaptopropionic acid or thioglycolic acid as the stabilizers of ZnSe:Mn nanoparticles entails a change of pH value of the mixture from neutral to highly alkaline, which additionally complicates the whole process and the resulting nanoparticles have a negative charge due to the presence of the carboxyl group.
  • 3-mercaptopropionic acid is a highly toxic substance
  • thioglycolic acid is a highly irritating substance with an unpleasant odor.
  • the resulting nanoparticles go through a purification process after which they are precipitated from the solution with an alcohol, e.g. isopropanol or ethanol, centrifuged then dried in a vacuum oven.
  • the aim of the present invention is to develop a simple technology for the direct obtaining luminescent ZnSe:Mn nanoparticles in an aqueous solution, having a positive surface charge, and to eliminate highly toxic substances from the process.
  • a method for obtaining manganese-doped luminescent nanoparticles of zinc selenide (ZnSe:Mn) having positive surface charge in which ZnSe:Mn nanoparticles are synthesized in an aqueous reaction-medium containing zinc-precursor in the form of zinc acetate, manganese-precursor in the form of manganese acetate, selenium-precursor, that is the product of the reaction of metallic selenium with sodium borohydride (NaBH 4 ), and a stabilizer from the group of thiol compounds consists in that 2.92 - 5.90 mol of metallic selenium (Se) in a shredded form, preferably granulated, and 6.87 - 14.03 mol of sodium borohydride (NaBH 4 ) are stirred in a reaction vessel with such an amount of demineralized water that weight ratios Se : NaBH 4 : H 2 0 are from 0.38 : 1 : 57.
  • said mixture is heating with stirring at temperature of 75 - 85°C and maintained at this range up to discolouration of the reaction mixture from pink-red color to colorless.
  • the moment of color change indicates that the process of reduction of selenium and formation of selenium salts (HSe-) is completed, and the salt in this case is NaHSe (sodium hydro selenide).
  • the reaction mixture After the introduction into the solution zinc salt, manganese and stabilizer, the reaction mixture is heated to the temperature of 80 - 85°C and maintained at this temperature range for 3.0 - 3.5 hours in order to synthesize ZnSe:Mn.
  • the reaction mixture is cooled to an ambient temperature and the resulting nanoparticles are purified of un-reacted substrates and intermediates formed during the reaction, then the mixture is subjected to membrane-ultrafiltration carried out on an ultra-filtration membrane having a pore size of 50-100 kD, using demineralised water.
  • the ultrafiltration process is carried out up to reduction the electrolytic conductivity of the effluent to the value of 20 - 15 ⁇ S and a pH decrease to the value of 5.0 - 6.2.
  • the aqueous suspension of ZnSe:Mn nanoparticles is stable and can be stored at reduced temperature, preferably at 4 - 8°C, for example for a period of 7 to 14 days or it may be converted to ZnSe:Mn nanopowder.
  • the obtained suspension of ZnSe:Mn nanoparticles after ultrafiltration process is frozen in liquid nitrogen, then lyophilized at temperature from - 30 to -40°C for 3 - 5 hours to obtain nanopowder.
  • the ZnSe:Mn nanopowder obtained after lyophilization has non-compact consistence (powdery consistence), without agglomerates and is characterized by high hydrophilic properties. It can be stored under anhydrous conditions for several months, and it easily dissolves in water or other polar solvents forming stable, high-concentrated dispersions.
  • ZnSe:Mn nanoparticles obtained according to the method of the invention have positive surface charge due to using 2-mercaptoethylamine hydrochloride as a surfactant, that was confirmed by the measurements of zeta potential of nanoparticles, which indicated positive value.
  • 2-mercaptoethylamine hydrochloride Used as a stabilizer 2-mercaptoethylamine hydrochloride is a non-toxic and non- irritant solid substance and does not require changing the pH value of the reaction mixture to high basic values, as opposed to the cases described in the prior art, in which thiol acids were used.
  • Another advantage of the proposed technology is not only the reduction of toxicity, but also the reduction of energy consumption and costs associated with the process, due to elimination expensive and often harmful organometallic precursors and solvents.
  • the process runs at lower temperatures compared to conventional methods.
  • An additional benefit is avoiding long-chain amines as stabilizers, which produce large problems in precipitation and purification of synthesized nanoparticles, as far as known solutions are concerned.
  • the developed technology does not require the use of an inert gas environment, which in consequence makes the whole process considerably simpler, cheaper and easier to apply on an industrial scale.
  • the ZnSe:Mn nanoparticles obtained according to the method of the invention are characterized by relatively small sizes, of the order 20 nm, stable photoluminescence in the range of orange-color visible light corresponding to wavelength of 580 nm, and wavelength of the absorbed light is 340 nm.
  • the undisputed advantage is also that the positively charged amino groups present on the surface of the ZnSe:Mn nanoparticles obtained according to the method of the invention enable further modification of nanoparticle surfaces, as needed.
  • modification the surfaces of ZnSe;Mn nanoparticles with a substance containing free carboxyl groups and a long aliphatic chain e.g. stearic acid
  • stearic acid provides nanoparticles having hydrophobic properties, which in turn allows the solution to be used in the dynamically developing segment of optoelectronics.
  • the modification of nanoparticle surfaces with stearic acid induces a change in their nature from hydrophilic to hydrophobic.
  • ZnSe:Mn nanoparticles very important feature of the obtained ZnSe:Mn nanoparticles is their solubility in organic solvents such as tetrahydrofuran, dimethylformamide, toluene and in polymer solutions, thanks to which the nanoparticles can be used as a components of polymer nanocomposites.
  • polymer nanocomposites can be used as active layers for the production of devices such as light-emitting diodes, displays, solar panels or optical fibers.
  • Fig.l depicts the zeta potential measurement for ZnSe:Mn nanoparticles stabilized with 2-mercaptoethylamine hydrochloride - water dispersion purified in ultrafiltration process;
  • Fig.2 shows three images (labeled a, b, c) of ZnSe:Mn nanoparticles made with a transmission electron microscope (TEM) and the diffraction pattern of nanoparticles
  • TEM transmission electron microscope
  • d made with SAED technique (Selected Area Electron Diffraction);
  • Fig.3 shows the excitation spectrum of ZnSe:Mn nanoparticles
  • Fig.4 shows the emission spectrum of ZnSe:Mn nanoparticles
  • Fig.5 shows the absorption spectrum of ZnSerMn nanoparticles
  • Fig.6 shows the XRD spectrum for ZnSe:Mn nanoparticles
  • Fig.7 shows FT-IR spectrum of ZnSe:Mn nanoparticles
  • Fig.8 shows an exemplary laboratory equipment used in both examples for the synthesis of ZnSe:Mn nanoparticles: 1 - magnetic stirrer, 2 - oil bath, 3 - round-bottomed flask with three necks, 4— reflux condenser, 5 - thermometer, 6 - dropping funnel.
  • step I sodium borohydride (NaBH 4 ) was used, that is a powerful reducing agent which significantly shortens this stage of the reaction.
  • the source of selenium in the described reaction was granulated selenium (Se) reduced with sodium borohydride (NaBH 4 ) in the presence of a protic solvent, in this case distilled water.
  • a protic solvent in this case distilled water.
  • the scheme of laboratory equipment for the synthesis is shown in Fig.8. The equipment consists of: a round-bottomed flask with three necks 3 and having the capacity of 250 cm 3 , a reflux condenser 4 connected to the water source, an oil bath 2 equipped with a temperature controller, a thermometer 5, a dropping funnel 6, and a magnetic stirrer 1.
  • the main reaction step of the preparation of inorganic ZnSe:Mn nanoparticles consists in nucleation of nanocrystals in an aqueous solution by using a solution of zinc- precursors and manganese-precursors having the forms of acetates (zinc acetate dihydrate and manganese acetate tetrahydrate) dissolved in the presence of a stabilizer, which is 2-mercaptoethylamine hydrochloride (HSC 2 H 4 NH 2 -HC1) ; and the solution of reduced selenium prepared in step I.
  • a stabilizer which is 2-mercaptoethylamine hydrochloride (HSC 2 H 4 NH 2 -HC1) ; and the solution of reduced selenium prepared in step I.
  • step I When carrying out step I, in the same time saturated solution of the following substances: zinc acetate dihydrate (Zn(Ac)*2H 2 0), manganese acetate tetrahydrate (Mn(Ac)-4H 2 0) and 2-mercaptoethylamine hydrochloride (HSC 2 H 4 NH 2 -HCl) is prepared in a separate reaction vessel (in a beaker placed on a magnetic stirrer at ambient temperature).
  • Zn(Ac)*2H 2 0 zinc acetate dihydrate
  • Mn(Ac)-4H 2 0 manganese acetate tetrahydrate
  • HSC 2 H 4 NH 2 -HCl 2-mercaptoethylamine hydrochloride
  • the reaction mixture excited with UV-lamp emitting light with a wavelength of 366 nm showed orange light, indicating the successful reaction and formation of ZnSe:Mn nanoparticles in the reaction mixture.
  • the reaction mixture containing ZnSe:Mn nanoparticles was cooled to room temperature and washed with distilled water in membrane-ultrafiltration process.
  • the ultrafiltration process was carried out using 50-100 kDa membrane.
  • the purification process was monitored by measuring the electrolytic conductivity of the effluent mixture.
  • the ultrafiltration was terminated when the electrolytic conductivity of effluent was reduced from 6 - 7 mS to about 20— 15 ⁇ S and pH decreased to the value of 5.0 - 6.2.
  • the purified aqueous suspension of ZnSe:Mn nanoparticles was then frozen in liquid nitrogen and placed in a lyophilizer.
  • the resulting powder of ZnSe:Mn nanoparticles had loose consistency (consistency of loose powder, powder), did not form agglomerates and was characterized by highly hydrophilic properties, easily dissolving in water or other polar solvents forming transparent dispersions, even at high concentration.
  • the scheme of laboratory equipment for the synthesis is shown in Fig.8.
  • the equipment consists of : a round-bottomed flask with three necks (3) having the capacity of 250 cm 3 , a reflux condenser (4) connected to the water source, an oil bath (2) equipped with a temperature controller, a thermometer (5), a dropping funnel (6), and a magnetic stirrer (1).
  • the whole reaction was carried out without the use of an inert gas.
  • step I When carrying out step I, in the same time the solution of the following substances: zinc acetate dihydrate (Zn(Ac)'2H 2 0) and manganese acetate tetrahydrate (Mn(Ac)4H 2 0) and 2-mercaptoethylamine hydrochloride (HSC 2 H 4 NH 2 -HC1) was prepared.
  • the above- mentioned solution was prepared in a separate reaction vessel (in a beaker placed on a magnetic stirrer at ambient temperature).
  • step I i.e. when the solution of selenium was completely discolored
  • a saturated salt solution was gradually dropwise added to the main reaction system containing reduced selenium (Fig.8). The addition time was about 40 min. After completion of the dropwise addition, the reaction mixture was heated to the temperature of 85°C, and maintained at this temperature for 3.5 hours.
  • the reaction mixture excited with UV-lamp emitting light of a wavelength 366 nm showed orange light, indicating the successful reaction and formation of ZnSe:Mn nanoparticles in the reaction mixture.
  • the reaction mixture with ZnSe:Mn nanoparticles was cooled to room temperature and washed with distilled water in the membrane-ultrafiltration process.
  • the reaction mixture was divided in two portions (about 40 cm 3 each), which were placed with stirring in an ultrafiltration column (from Amicon, with a total capacity of 200 cm 3 ) provided with ultrafiltration membrane made of polyethersulfone (Millipore, PFS, Biomax 50 kDa).
  • the purification process was monitored by measuring the electrolytic conductivity of effluent mixture.
  • the ultrafiltration process was terminated when the reduction in the elecrolytic conductivity of the effluent was reduced from 7 mS to about 20— 15 ⁇ S and pH decreased to the value of 6.2.
  • the purified aqueous suspension of ZnSe:Mn nanoparticles was then frozen in liquid nitrogen, placed in a lyophilizer (Gefriertrocknungsanlagen GmbH company Martin Christ model Alpha 2-4 LDPlus) and cooled to the temperature of -40°C for 5 hours.
  • the resulting lyophilized powder of ZnSe:Mn nanoparticles had loose consistency (consistency of loose powder, powder), did not form agglomerates and was characterized by highly hydrophilic properties, easily dissolved in water or other polar solvents forming transparent dispersions, even high-concentrated.
  • the nanoparticles are characterized by a size of the order 20 nm (Fig. 2 a, b, c, d), stable photoluminescence (Fig. 3 and 4) in the range of orange-color visible light corresponding to a wavelength of 580 nm, and light absorbance at 340 nm wavelength (Fig. 5).
  • the quantum yield of luminescence (i.e. the ratio of number of photons emitted to the number of photons of the excitating radiation) as 30% - 35% was defined using results of measurements performed with a spectrofluorimeter.
  • the nanoparticles have a crystal structure of zinc blende, as evidenced by three characteristic peaks of the XRD spectrum in the attached drawings (Fig. 6).
  • Example 1 The laboratory equipment used for the synthesis was as in Example 1. The reaction was carried out without using an inert gas. Appropriate analytical samples: 4.90 mmol of granulated selenium (Se), 1 1.65 mmol of sodium borohydride (NaBH 4 ) and 15 cm 3 of distilled water, were placed in the round- bottomed flask with three necks 3 having the capacity of 250 cm 3 and connected to the reflux condenser 4. The reaction mixture was heated in the oil bath 2 to the temperature of 75°C, so as to reduce metallic selenium. The reduction was carried out for 1 hour, until complete discoloration of the reaction mixture from pink-red color to a completely transparent. The moment of color change indicates completion of the reduction process and forming of selenium salt (HSe-), in this case NaHSe. Stage II
  • step I When carrying out step I, in the same time the solution of the following substances: zinc acetate dihydrate (Zn(Ac)*2H 2 0) and manganese acetate tetrahydrate (Mn(Ac)-4H 2 0) and 2-mercaptoethylamine hydrochloride (HSC 2 H 4 NH 2 -HC1) was prepared.
  • the above- mentioned solution was prepared in a separate reaction vessel (in a beaker placed on a magnetic stirrer at ambient temperature).
  • the reaction mixture excited with the reaction mixture excited with UV-lamp emitting light with a wavelength of 366 nm showed orange light, indicating the successful reaction and formation of ZnSe:Mn nanoparticles in the reaction mixture.
  • the reaction mixture with ZnSe:Mn nanoparticles was cooled to room temperature and washed with distilled water using membrane-ultrafiltration.
  • reaction mixture was divided in two portions (about 30 cm 3 each), which were placed in an ultrafiltration column with stirring (from Amicon, with a total capacity of 200 cm 3 ) provided with ultrafiltration membrane made of polyethersulfone (Millipore, PFS, Biomax 50 kDa).
  • the purification process was monitored by measuring the electrolytic conductivity of effluent mixture.
  • the ultrafiltration process was ended when the reduction in the conductivity of the effluent mixture was reduced from 6 mS to about 20 ⁇ S and pH was decreased to the value of 5.0.
  • the purified aqueous suspension of ZnSe:Mn nanoparticles was then frozen in liquid nitrogen, placed in a lyophilizer (Gefriertrocknungsanlagen GmbH company Martin Christ model Alpha 2-4 LDPlus) and cooled to the temperature of - 30°C for 3 hours.
  • the resulting lyophilized powder of ZnSe:Mn nanoparticles had loose consistency (consistency of loose powder, powder), did not form agglomerates and was characterized by highly hydrophilic properties, easily dissolving in water or other polar solvents forming transparent dispersions, even high-concentrated.
  • luminescent nanoparticles obtained in both embodiments, and in particular their hydrophilic nature and low toxicity enable to use them, without modification, in nano-biomedicine - the field of extremely high potential.
  • the ZnSe:Mn nanoparticles obtained according to the method of the invention have a hydrophilic nature (without surface modification), and due to their characteristics can be used as markers (quantum dots) for picturing pathological tissues, particularly in the diagnosis of tumor cells.
  • quantum dots are also possible to use quantum dots as carriers of anticancer drugs and to improve the treatment.

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Abstract

A method for obtaining manganese-doped luminescent nanoparticles of zinc selenide having positive surface charge, in which nanoparticles are obtained in an aqueous reaction mixture containing a zinc-precursor in the form of zinc acetate (Zn(Ac)-2H2O), a manganese-precursor in the form of manganese acetate (Μn(Αc)·4Η2O), a selenium-precursor which is the product of reaction of metallic selenium and sodium borohydride (Mn(Ac)4H2O), a stabilizer from the group of thiol compounds, characterized in that the stabilizer is 2-mercaptoethylamine hydrochloride (HSC2H4NH2-HCl) in such an amount that the weight ratio Zn(Ac)-2H2O : Mn(Ac)-4H2O : HSC2H4NH2-HCl : H2O is from 8.33 : 1 : 2.76 : 476.19 to 2.56 : 1 :1.35 : 136.67. The synthesis of ZnSe:Mn nanoparticles is performed at a molar ratio of selenium (calculated as metal Se) to Zn(Ac)-2H2O from 0.57 to 1.23.

Description

TITLE:
A method for obtaining manganese-doped luminescent nanoparticles of zinc selenide having positive surface charge
TECHNICAL FIELD:
The invention relates to a method for obtaining, in an aqueous medium, manganese-doped luminescent nanoparticles of zinc selenide (ZnSe:Mn) having positive surface charge, in the form of stable aqueous suspensions or nanopowders intended for digestion in water. The ZnSe:Mn nanoparticles obtained with the method of the invention, both in suspension and powder form, can be used in medicine, biology and optoelectronics.
BACKGROUND ART:
Semiconductor nanoparticles having luminescence properties exist in the market as quantum dots (QD). The term specifies nano structured semiconductors in which the motion of electrons is slowed-down in three directions by potential barriers, forming so- called "box of potential". The key feature of quantum dots is their ability to emit light waves whose length depends on the size and shape of the nano structures. Nowadays, nanoparticles find application in many areas of life including biology and medicine, especially in the process of diagnosis and treatment of cancer, as well as in the widely understood optoelectronics, in manufacturing diodes, displays and solar panels. The quantum dots market can be now considered a niche market, but due to the wide range of its application a dynamic development of this market is expected.
At present, most methods for obtaining quantum dots are based on the compounds of heavy metals, for instance cadmium (cadmium selenide, cadmium sulfide, cadmium telluride), which are characterized by high toxicity and harmful effect on the environment.
One of most used and best-developed quantum dots, having relatively high quantum yield of photoemission brightness is cadmium selenide (CdSe), preparation methods of which are disclosed, for example, in patent and application specifications US2003173541(A1), US 6207229(B1), US2002066401(A1). However, the high toxicity of cadmium selenide and increasingly stringent legal regulations in the field of ecology make it necessary to look for other, safer and more environmentally friendly solutions.
Therefore, a number of studies are currently underway in order to develop new methods for obtaining low-toxic luminescent nanoparticles, characterized by improved properties. The above mentioned studies also concern manganese-doped luminescent nanoparticles of zinc selenide (ZnSe:Mn).
In the scientific and patent literature there are number of reports concerning various methods for obtaining inorganic ZnSe:Mn nanoparticles.
The vast majority of known solutions concerns the methods that use
organometallic precursors of zinc, selenium, manganese and high-boiling solvents - US2002011564(A1), US6780242B2, CN103130201 (WO2014127585 Al); Huaibin Shen, Hongzhe Wang, Xiaomin Li, Jin ZhongNiu, Hua Wang, Xia Chen, Lin Song Li "Phosphine-free synthesis of high quality ZnSe, ZnSe/ZnS, and Cu-, Mn-doped ZnSe nanocrystals" Dalton Transactions, 2009 pp.10534 - 10540; Shinjita Acharya,
Dipankar Das Sarma, Nikhil R. Jana Narayan Pradhan "An Alternate Route to High- Quality ZnSe and Mn-Doped ZnSe Nanocrystals", Journal of Physical Chemistry Letters, 2010 1 (2), pp. 485 - 488; David J. Norris, Nan Yao, Forrest T. Charnock, Thomas A. Kennedy "High-Quality Manganese-Doped ZnSe Nanocrystals", Nano Letters, 2001,1, pp. 3 - 7; Narayan Paradhan, Xiaogang Peng "Efficient and Color- Tunable Mn-Doped ZnSe Nanocrystal Emitters: Control of Optical Performance via Greener Synthetic Chemistry", Journal of the American Chemical Society, 2007, 129, pp.3339 - 3347.
In practice, for obtaining of ZnSe:Mn quantum dots a "hot injection" method is used. The method is based on the work of Christopher Murray, David J. Norris, Moungi G. Bawendi, " Synthesis and characterization of nearly monodisperse CdE (E— sulfur, selenium, tellurium) semiconductor nanocrystallites", Journal of the American
Chemical Society, 1993, 115, pp. 8706 - 8715, which was the first to reveal CdS nanoparticle growth technology. The "hot injection" method has become the predominant way for preparing luminescent nanoparticle semiconductors. The method bases on high-temperature pyrolysis of organometallic precursors in a hot organic coordinating-solvent or on mixing the precursors and rapid cooling of the reaction mixture. The reaction media are usually toxic long-chain alkylphosphines, alkylphosphine oxides and alkylamines which boiling points are of about 340°C, such as trioctylphosphine [(C8H17)3P] (TOP), trioctylphosphine oxide [(C8H|7)3PO] (TOPO) and long-chain amines like
hexadecylamine, octadecylamine, oleylamine. The method requires the use of an atmosphere of an inert gas (Ar, N2) in order to prevent oxidation of reactants and freshly prepared nanoparticles. The zinc-precursor is most often an organometallic compound, and the selenium-precursor is metallic selenium dissolved in a high-boiling solvent, which at the same time functions as a stabilizer. The advantages of this type of reaction include the ability to obtain nanoparticles of relatively small size and high quantum yield of luminescence. On the other hand, these processes are costly, complicated, energy consuming. In addition, the processes require very high temperatures (280 - 320°C) and the use of toxic substances (organometallic precursors and solvents).
Moreover, these processes consist of many steps, even from 5 to 10, which make it difficult to transfer the processes from the laboratory scale to the industrial scale. The necessity of conducting processes in an inert environment (without air), in controlled closed circuits also hinder the process of commercialization. Furthermore, there is the big problem with the purification and precipitation the obtained nanoparticles and intermediates formed during the reaction from organic solvents.
Another method for obtaining ZnSe:Mn nanoparticles having hydrophilic properties is reported in the publication of Narayan Pradhan, David M. Battaglia, Yongcheng Liu, Xiaogang Peng , "Efficient, stable, small and water-soluble doped ZnSe nanocrystal emitters as a non-cadmium biomedical labels", Nano Letters, 2007, 7 (2), pp. 312 - 317. The method consists in the formation of ZnSe:Mn nanoparticles by means of the traditional hot-injection method followed by modification surfaces of nanoparticles with 3-mercaptopropionic acid (MP A), which substitute a long-chain amine. The consequence of the exchange of the ligands on the surfaces of nanoparticles is the change in the nature of nanoparticles from hydrophobic to hydrophilic and weakening of their luminescent properties.
An alternative to this type of known reactions are chemical processes carried out directly in an aquatic environment, broadly understood so-called colloidal reactions, including co-precipitation reactions. The mechanism of colloidal reaction is much simpler and less laborious than the "hot injection" reaction. In colloidal reactions, the main synthesis usually follows the step of selecting or producing selenium, zinc and manganese precursors, and preparing solutions of them. A nucleation phase in consists in that nuclei of nanoparticles form in the initially homogenous solution. In the next stage, growth, the remaining material gradually builds up on the surfaces of the formed stable nuclei. The next phase is the increasing of nanoparticles, after which the process ends with the isolation of the freshly obtained nanoparticles from the reaction mixture (e.g. by precipitation) and optionally further processing the product after the synthesis.
It is well known, that the most important parameters of the colloidal reaction, which have an influence on the nature of the obtained ZnSe:Mn nanoparticles suspension in aqueous solution, are: the composition and quantity (molar ratio) of precursors introduced into the reaction (usually inorganic salts), the sequence of adding the substrates, pH of the reaction mixture, time and temperature of the reaction, and finally one of the most important parameters which is a stabilizer suitable for a particular reaction mixture, of ZnSe:Mn nanoparticles. With these parameters one can control the size, shape, degree of crystalHnity, and surface properties of nanoparticles that directly affect their optical and electrical properties such as luminescence and energy gap, and hence their application properties.
Another important aspect is the method of purifying the resulting suspension of ZnSe;Mn nanoparticles and isolating the resulting nanopowder. Depending on the method used, resulting material can undergo a larger or smaller agglomeration process, which is also essential to its applicability. In the majority of the cases traditional methods are used that consist in precipitating nanoparticles from the aqueous medium with ethanol or other alcohol and then separating them from the rest of the solution by centrifugation and traditional drying (CN 101905862 (A); Chao Wang, Xue Gao, Qiang Ma, Xingguang Su "Aqueous synthesis of mercaptopropionic acid capped Mn2+' doped ZnSe quantum dots", Journal of Materials Chemistry, 2009, 19, pp.7016 -7022). The result of this process is the formation of nanoparticle powder having a high aggregated and agglomerated consistency. Additional operations to break the agglomerates are necessary in order to re-create the suspension. Unfortunately, this is not always possible. The biomedical and optoelectronic industry, in which the ZnSe:Mn nanoparticles are most used, are constantly evolving and increasingly demanding the semiconductor materials used in this field. As far as biomedical applications are concerned, it is required that the nanoparticles are not toxic, have relatively small size and hydrophilic nature due to a suitable stabilizer and water-solubility.
Recently there has been a growing interest in the new methods of direct production of ZnSerMn and ZnSe nanoparticles in aqueous solutions, as it is evidenced by emerging publications and patent descriptions. The majority of the reports concerns the preparation of un-doped nanoparticles ZnSe in aqueous solutions - CN104437559(A), CN105062489(A), WO2014127585(Al), Hongyi Qin, Wenping Jian, Yinan Zhan, Taesung Kim, Zhenhu Jiang, Dong Jian, Dahui Sun " A simple and novel route for the synthesis of water soluble ZnSe quantum dots using the Nano-Se as the reaction intermediate", Materials Letters, 2012, 67, (1), 2012, pp. 28 -÷ 3; Zhengtao Deng, Fee Li Lie, Shengyi Shen, Indraneel Ghosh, Masud Mansuripur, Anthony J. Muscat " Water-based route to ligand-selective synthesis of ZnSe and Cd-doped ZnSe quantum dots with a tunable ultraviolet A to blue photoluminescence", 2009, 25, (1), pp. 434 - 442; Liang Huang, Heyou Han "One-step synthesis of water-soluble ZnSe quantum dots via microwave irradiation", Materials Letters, 2010, 64 (9), pp. 1099 - 1101; Yang Jiao, Dabin Yu, Zirong Wang, Kun Tang, Xiaoquan Sun " Synthesis, nonlinear optical properties and photoluminescence of ZnSe quantum dots in stable solutions", Materials Letters, 2007, 61 (7), pp. 1541 - 1543; Huifeng Qian, Xin Qiu, Liang Li, Jicun Ren "Microwave-Assisted Aqueous Synthesis: A Rapid Approach to Prepare Highly Luminescent ZnSe(S) Alloyed Quantum Dots ", The Journal of Physical Chemistry B, 2006, 110, (18), pp. 9034 - 9040; Feng Jiang, Anthony J. Muscat "Ligand-Controlled Growth of ZnSe Quantum Dots in Water during Ostwald Ripening", Langmuir, 2012, 28, (36), pp. 12931 -12940; J.J.Andrade, A.G. Brasil Jr. P.M.A.Farias, A.Fontes, B.S. Santos " Synthesis and characterization of blue emitting ZnSe quantum dots", Microelectronics Journal,2009, 40 (3), pp. 641 - 643.
Some of these reports describe methods for production core-shell systems, for instance ZnSe/ZnS (CN104877685(A)), or ZnSe:Mn/ZnS (CN 102618289(A), Bich Thi Luong, Eunsu Hyeong; Seokhwan Jia; Nakjoong Kim "Green synthesis of highly UV- orange emitting ZnSe/ZnS:Mn/ZnS core/shell/shell nanocrystals by a three-step single flask method", RSC Advances, 2012, 2 (32), pp.12132 - 12135; Xiaojing Xu, Zhengqing Qi, Zengxia Zhao, Chunlei Wang, Changgui Lu, Xu Shuhong, Yiping Cui, " A two-step method to synthesize water-dispersible Mn.ZnSe/ZnO core/shell quantum dots with pure dopant emission ", New Journal of Chemistry, 2015, 39 (12), pp. 8818 - 8824; Bohua Dong, Lixin Cao, Ge Su, Wei Liu, " Facile Synthesis of Highly Luminescent Water-Soluble ZnSe:Mn/ZnS Core/Shell Doped Nanocrystals with Pure dopant Emission ", The Journal of Physical Chemistry C, 2012, 116 (22 ), Pp. 12258 - 12264; Javelin Ghanbaja, Ghouti Medjahdi, Rapha Schneider "Water-Based Route to Colloidal Mn-Doped ZnSe and Core/Sell ZnSe/ZnS Quantum Dots", Inorganic Chemistry, 2010, 49, pp. 10940 - 10948; Xiao Qi, Xiao Chong "Synthesis and photoluminescence of water-soluble Mn:ZnS/ZnS core/shell quantum dots using nucleation-doping strategy", Optical Materials, 2008, 31 (2), pp. 455 - 460.
It is has been known, that ZnSe and ZnSe/ZnS nanoparticles, show completely different optical and electrical properties than ZnSe nanoparticles doped with manganese atoms. The manganese-doped ZnSe nanoparticles show luminescence in a different visible light range than un-doped nanoparticles. ZnSe:Mn shows luminescence in the range of orange-color visible light, corresponding to wavelength of 580 - 600nm. In turn, pure ZnSe shows luminescence in at the wavelength of 450 nm, corresponding to blue light. In addition, in core/shell systems ZnSe:Mn/ZnS, ZnS shell is intended to enhance the luminescent properties of the obtained nanoparticles and to protect them against oxidation.
Despite the increasing number of scientific and patent publications concerning the aforementioned scope, development of cheap and direct methods for obtaining in aqueous solutions ZnSe:Mn nanoparticles having strictly defined size and morphology and luminescent properties still remains a challenge. For example, the direct synthesis of nanoparticles ZnSe:Mn is disclosed in the literature - Chao Wang, Xue Gao, Qiang Maa, Xingguang Su " Aqueous synthesis of mercaptopropionic acid capped Mn 2 + ' doped ZnSe quantum dots ", Journal of Materials Chemistry, 2009, 19 , pp. 7016 - 7022; Narayan Pradhan, David M. Battaglia, Yongcheng Liu, Xiaogang Peng "Efficient, stable, small and water-soluble doped ZnSe nanocrystal emitters as a non-cadmium biomedical labels "Nano Letters, 2007, 7, (2), pp. 312 - 317 and in two patent applications CN101905862 (A) and CN103320134 (A).
In all the cases the reactions were carried out in several steps in aqueous medium using an inert, argon or nitrogen, environment and as a stabilizer, except for the patent application CN101905862(A), 3 -mercaptopropionic acid was used, which gave the resulting ZnSe:Mn nanoparticles negative surface charge in an aqueous solution.
The patent application CN 101905862 (A) describes the synthesis of ZnSe: Mn nanoparticles which consist of five steps: step 1 - selenium-precursor preparation by dissolving in distilled water product of the reaction of powdered selenium and sodium borohydride (NaBH4) in an inert environment (argon or nitrogen); step 2 - zinc- precursor (Zn) preparation by dissolving zinc acetate in distilled water in the presence of a thioglycolic acid as a stabilizer and adjusting pH to the value of 8 - 13 due to addition 1M of NaOH; step 3 — manganese-precursor preparation by dissolving manganese acetate in distilled water; step 4 - main reaction of the obtaining ZnSe:Mn nanoparticles by gradual adding of precursors in different sequence and heating the mixture during the time of 0.5 - 6 h at the temperature of 90 - 120°C; step 5 - purifying and precipitating with isopropanol, separating centrifuglly and drying. As the consequence of applying thioglycolic acid the surface charge of the nanoparticles was negative due to the presence of the carboxyl group (-COOH) on their surfaces.
In turn, the patent application CN103320134 (A) discloses the synthesis of
ZnSe:Mn nanoparticles, namely MnSe/ZnSe. The synthesis is realized in six steps: step 1— selenium-precursor preparation by dissolving in distilled water the product of the raction of powdered selenium and sodium borohydride (NaBH4) in an inert environment (argon or nitrogen); step 2 - manganese-precursor preparation by dissolving manganese acetate in water containing MPA stabilizer (3-mercaptopropionic acid), and adjusting pH to the value of 7-8 by addition 1 M of NaOH, then heating the mixture at the temperature of 100°C for 0.5 h; step 3 - MnSe nanocrystals preparation by injecting with a syringe selenium-precursor into manganese-precursor and heating the mixture in the temperature of 100°C; step 4 - zinc-precursor preparation, in the same time, by dissolving zinc acetate in distilled water in the presence of MPA and at pH 8, due to addition 1M of NaOH; step 5 - injecting zinc-precursor with a syringe to the main reaction mixture and forming ZnSe:Mn nanoparticles by heating the mixture in an oil bath at the temperature of 100°C. The description of the patent application CN 103320134 (A) does not disclose the step of purification and isolation of the nanoparticles from the reaction mixture.
The above-mentioned patent applications describe multi-step processes
(consisting of at least 5 steps) for obtaining ZnSe:Mn nanoparticles in an aqueous medium. Some of these processes require the use of an inert environment, which further complicates and makes it difficult to transfer the process from the laboratory scale to the industrial scale.
Moreover, the use of thiol acids such as 3-mercaptopropionic acid or thioglycolic acid as the stabilizers of ZnSe:Mn nanoparticles entails a change of pH value of the mixture from neutral to highly alkaline, which additionally complicates the whole process and the resulting nanoparticles have a negative charge due to the presence of the carboxyl group. Moreover, 3-mercaptopropionic acid is a highly toxic substance, and thioglycolic acid is a highly irritating substance with an unpleasant odor. The resulting nanoparticles go through a purification process after which they are precipitated from the solution with an alcohol, e.g. isopropanol or ethanol, centrifuged then dried in a vacuum oven.
The processes according to the known methods give in consequence dry and compact powder.
AIM OF THE INVENTION:
The aim of the present invention is to develop a simple technology for the direct obtaining luminescent ZnSe:Mn nanoparticles in an aqueous solution, having a positive surface charge, and to eliminate highly toxic substances from the process. SUMMARY OF THE INVENTION:
According to the invention, a method for obtaining manganese-doped luminescent nanoparticles of zinc selenide (ZnSe:Mn) having positive surface charge, in which ZnSe:Mn nanoparticles are synthesized in an aqueous reaction-medium containing zinc-precursor in the form of zinc acetate, manganese-precursor in the form of manganese acetate, selenium-precursor, that is the product of the reaction of metallic selenium with sodium borohydride (NaBH4), and a stabilizer from the group of thiol compounds consists in that 2.92 - 5.90 mol of metallic selenium (Se) in a shredded form, preferably granulated, and 6.87 - 14.03 mol of sodium borohydride (NaBH4) are stirred in a reaction vessel with such an amount of demineralized water that weight ratios Se : NaBH4 : H20 are from 0.38 : 1 : 57.91 to 2.31 : 1 : 67.20. In order to reduce metallic selenium, said mixture is heating with stirring at temperature of 75 - 85°C and maintained at this range up to discolouration of the reaction mixture from pink-red color to colorless. The moment of color change indicates that the process of reduction of selenium and formation of selenium salts (HSe-) is completed, and the salt in this case is NaHSe (sodium hydro selenide).
Next, to the reaction mixture is added, by degrees and with stirring, a solution containing 4.80 - 5.13 mole of zinc acetate dihydrate (Zn(Ac>2H20), 0.51 - 1.79 mol of manganese acetate tetrahydrate (Mn(Ac)-4H20), 3.71 - 5.31 mol of a stabilizer in the form of 2-mercaptoethylamine hydrochloride (HSC2H4NH2HC1) and demineralized water in such a volume that the weight ratio of Zn(Ac)-2H20 : Mn(Ac)-4H20 : HSC2H4NH2-HCl : H20 is from 8.33 : 1 : 2.76 : 476 .19 to 2.56 : 1 : 1.35 : 136.67. Thanks to these proportions, the synthesis of nanoparticles is carried out with molar ratio of selenium (based on metallic selenium) to zinc acetate dehydrate from 0.57 to 1.23, which guarantees that the yield of reaction is at least 80%.
After the introduction into the solution zinc salt, manganese and stabilizer, the reaction mixture is heated to the temperature of 80 - 85°C and maintained at this temperature range for 3.0 - 3.5 hours in order to synthesize ZnSe:Mn.
Next, the reaction mixture is cooled to an ambient temperature and the resulting nanoparticles are purified of un-reacted substrates and intermediates formed during the reaction, then the mixture is subjected to membrane-ultrafiltration carried out on an ultra-filtration membrane having a pore size of 50-100 kD, using demineralised water. The ultrafiltration process is carried out up to reduction the electrolytic conductivity of the effluent to the value of 20 - 15 μS and a pH decrease to the value of 5.0 - 6.2.
After the ultrafiltration process the aqueous suspension of ZnSe:Mn nanoparticles is stable and can be stored at reduced temperature, preferably at 4 - 8°C, for example for a period of 7 to 14 days or it may be converted to ZnSe:Mn nanopowder.
Preferably, the obtained suspension of ZnSe:Mn nanoparticles after ultrafiltration process is frozen in liquid nitrogen, then lyophilized at temperature from - 30 to -40°C for 3 - 5 hours to obtain nanopowder.
FAVORABLE EFFECTS OF THE INVENTION:
The ZnSe:Mn nanopowder obtained after lyophilization has non-compact consistence (powdery consistence), without agglomerates and is characterized by high hydrophilic properties. It can be stored under anhydrous conditions for several months, and it easily dissolves in water or other polar solvents forming stable, high-concentrated dispersions.
ZnSe:Mn nanoparticles obtained according to the method of the invention have positive surface charge due to using 2-mercaptoethylamine hydrochloride as a surfactant, that was confirmed by the measurements of zeta potential of nanoparticles, which indicated positive value.
Used as a stabilizer 2-mercaptoethylamine hydrochloride is a non-toxic and non- irritant solid substance and does not require changing the pH value of the reaction mixture to high basic values, as opposed to the cases described in the prior art, in which thiol acids were used.
Another advantage of the proposed technology is not only the reduction of toxicity, but also the reduction of energy consumption and costs associated with the process, due to elimination expensive and often harmful organometallic precursors and solvents. In addition, the process runs at lower temperatures compared to conventional methods. An additional benefit is avoiding long-chain amines as stabilizers, which produce large problems in precipitation and purification of synthesized nanoparticles, as far as known solutions are concerned.
The developed technology does not require the use of an inert gas environment, which in consequence makes the whole process considerably simpler, cheaper and easier to apply on an industrial scale.
In the course of the studies it was stated that the ZnSe:Mn nanoparticles obtained according to the method of the invention are characterized by relatively small sizes, of the order 20 nm, stable photoluminescence in the range of orange-color visible light corresponding to wavelength of 580 nm, and wavelength of the absorbed light is 340 nm.
The worked out, in the development of the invention, possibility of obtaining ZnSe:Mn nanopowder from the reaction solution by means of ultrafiltration and lyophilization, makes it possible to obtain a powder which is characterized by light and loose consistency, without major agglomerates, opposite to the agglomerates obtained with standard techniques of precipitation, centrifuging and drying in a vacuum dryer.
The undisputed advantage is also that the positively charged amino groups present on the surface of the ZnSe:Mn nanoparticles obtained according to the method of the invention enable further modification of nanoparticle surfaces, as needed. This makes it possible to extend the scope of application possibilities of the invention. For example, modification the surfaces of ZnSe;Mn nanoparticles with a substance containing free carboxyl groups and a long aliphatic chain (e.g. stearic acid) provides nanoparticles having hydrophobic properties, which in turn allows the solution to be used in the dynamically developing segment of optoelectronics. The modification of nanoparticle surfaces with stearic acid induces a change in their nature from hydrophilic to hydrophobic. Very important feature of the obtained ZnSe:Mn nanoparticles is their solubility in organic solvents such as tetrahydrofuran, dimethylformamide, toluene and in polymer solutions, thanks to which the nanoparticles can be used as a components of polymer nanocomposites. In turn, polymer nanocomposites can be used as active layers for the production of devices such as light-emitting diodes, displays, solar panels or optical fibers. DESCRIPTION OF THE FIGURES IN THE DRAWINGS:
The subject of the invention in two exemplary embodiments in detail is described below, and the results of the test of the product of Example 1 are shown in the accompanying drawing, in which:
Fig.l depicts the zeta potential measurement for ZnSe:Mn nanoparticles stabilized with 2-mercaptoethylamine hydrochloride - water dispersion purified in ultrafiltration process;
Fig.2 shows three images (labeled a, b, c) of ZnSe:Mn nanoparticles made with a transmission electron microscope (TEM) and the diffraction pattern of nanoparticles
(referred to as d) made with SAED technique (Selected Area Electron Diffraction);
Fig.3 shows the excitation spectrum of ZnSe:Mn nanoparticles;
Fig.4 shows the emission spectrum of ZnSe:Mn nanoparticles;
Fig.5 shows the absorption spectrum of ZnSerMn nanoparticles;
Fig.6 shows the XRD spectrum for ZnSe:Mn nanoparticles;
Fig.7 shows FT-IR spectrum of ZnSe:Mn nanoparticles;
Fig.8 shows an exemplary laboratory equipment used in both examples for the synthesis of ZnSe:Mn nanoparticles: 1 - magnetic stirrer, 2 - oil bath, 3 - round-bottomed flask with three necks, 4— reflux condenser, 5 - thermometer, 6 - dropping funnel.
BEST MODE FOR CARRYING OUT THE INVENTION:
The synthesis of manganese-doped zinc selenide nanoparticles (ZnSe:Mn) in both examples was carried out in aqueous solution, according to the colloid method consisting of three main steps:
I. Reduction of selenium, without using an inert environment, in order to reduce Se form to Se 2- form ;
II. Main reaction of obtaining nanoparticles, conducted in aqueous solution without using an inert medium and without pH changes;
III. Purifying the nanoparticle dispersion with the membrane-ultrafiltration and drying the powder with a lyophilizer. The detailed steps of the examples mentioned above were as follows: Stage I
For the reduction of selenium in step I sodium borohydride (NaBH4) was used, that is a powerful reducing agent which significantly shortens this stage of the reaction. The source of selenium in the described reaction was granulated selenium (Se) reduced with sodium borohydride (NaBH4) in the presence of a protic solvent, in this case distilled water. The scheme of laboratory equipment for the synthesis is shown in Fig.8. The equipment consists of: a round-bottomed flask with three necks 3 and having the capacity of 250 cm3, a reflux condenser 4 connected to the water source, an oil bath 2 equipped with a temperature controller, a thermometer 5, a dropping funnel 6, and a magnetic stirrer 1. Thanks to the selection of the appropriate number of substrates and the order of their addition, it was possible to carry out all the reaction steps, including stage I (reduction) and stage II (main reactions), without the use of inert gas, but in the presence of air. The resulting nanoparticles and intermediates do not oxidize during the reaction.
Stage II
The main reaction step of the preparation of inorganic ZnSe:Mn nanoparticles consists in nucleation of nanocrystals in an aqueous solution by using a solution of zinc- precursors and manganese-precursors having the forms of acetates (zinc acetate dihydrate and manganese acetate tetrahydrate) dissolved in the presence of a stabilizer, which is 2-mercaptoethylamine hydrochloride (HSC2H4NH2-HC1); and the solution of reduced selenium prepared in step I. When carrying out step I, in the same time saturated solution of the following substances: zinc acetate dihydrate (Zn(Ac)*2H20), manganese acetate tetrahydrate (Mn(Ac)-4H20) and 2-mercaptoethylamine hydrochloride (HSC2H4NH2-HCl) is prepared in a separate reaction vessel (in a beaker placed on a magnetic stirrer at ambient temperature).
Stage III
Upon completion of the main reaction, the reaction mixture excited with UV-lamp emitting light with a wavelength of 366 nm showed orange light, indicating the successful reaction and formation of ZnSe:Mn nanoparticles in the reaction mixture. To purify the reaction solution of un-reacted substrates and other intermediates formed during the reaction, the reaction mixture containing ZnSe:Mn nanoparticles was cooled to room temperature and washed with distilled water in membrane-ultrafiltration process.
Preferably, the ultrafiltration process was carried out using 50-100 kDa membrane. The purification process was monitored by measuring the electrolytic conductivity of the effluent mixture. The ultrafiltration was terminated when the electrolytic conductivity of effluent was reduced from 6 - 7 mS to about 20— 15 μS and pH decreased to the value of 5.0 - 6.2.
The purified aqueous suspension of ZnSe:Mn nanoparticles was then frozen in liquid nitrogen and placed in a lyophilizer.
After the lyophilzation the resulting powder of ZnSe:Mn nanoparticles had loose consistency (consistency of loose powder, powder), did not form agglomerates and was characterized by highly hydrophilic properties, easily dissolving in water or other polar solvents forming transparent dispersions, even at high concentration.
EXAMPLES: Example 1
Stage I
The scheme of laboratory equipment for the synthesis is shown in Fig.8. The equipment consists of : a round-bottomed flask with three necks (3) having the capacity of 250 cm3, a reflux condenser (4) connected to the water source, an oil bath (2) equipped with a temperature controller, a thermometer (5), a dropping funnel (6), and a magnetic stirrer (1). The whole reaction was carried out without the use of an inert gas.
Appropriate analytical samples: 2.91 mmol of granulated selenium (Se), 6.87 mrnol of sodium borohydride (NaBH4) and 15 cm3 of distilled water, were placed in the round- bottomed flask with three necks 3 having the capacity of 250 cm3 and connected to the reflux condenser 4. The reaction mixture was heated in the oil bath 2 to the temperature of 85°C so as to reduce metallic selenium. The reduction was carried out for 1.5 hours, until complete discoloration of the reaction mixture from pink-red color to a completely transparent. The moment of color change indicates completion of the reduction process and forming of selenium salt (HSe-), in this case NaHSe.
Stage II
When carrying out step I, in the same time the solution of the following substances: zinc acetate dihydrate (Zn(Ac)'2H20) and manganese acetate tetrahydrate (Mn(Ac)4H20) and 2-mercaptoethylamine hydrochloride (HSC2H4NH2-HC1) was prepared. The above- mentioned solution was prepared in a separate reaction vessel (in a beaker placed on a magnetic stirrer at ambient temperature). For this purpose in 60 cm3 of distilled water the following substrates were mixed: 5.13 mmol of zinc acetate dihydrate, 1.36 mmol of manganese acetate tetrahydrate and 3.71 mmol of 2-mercaptoethylamine hydrochloride. At the end of step I (i.e. when the solution of selenium was completely discolored) a saturated salt solution was gradually dropwise added to the main reaction system containing reduced selenium (Fig.8). The addition time was about 40 min. After completion of the dropwise addition, the reaction mixture was heated to the temperature of 85°C, and maintained at this temperature for 3.5 hours.
Stage III
After the main reaction (stage II), the reaction mixture excited with UV-lamp emitting light of a wavelength 366 nm showed orange light, indicating the successful reaction and formation of ZnSe:Mn nanoparticles in the reaction mixture. To purify the reaction solution of un-reacted substrates and other intermediates formed during the reaction, the reaction mixture with ZnSe:Mn nanoparticles was cooled to room temperature and washed with distilled water in the membrane-ultrafiltration process. For this purpose, the reaction mixture was divided in two portions (about 40 cm3 each), which were placed with stirring in an ultrafiltration column (from Amicon, with a total capacity of 200 cm3 ) provided with ultrafiltration membrane made of polyethersulfone (Millipore, PFS, Biomax 50 kDa). The purification process was monitored by measuring the electrolytic conductivity of effluent mixture. The ultrafiltration process was terminated when the reduction in the elecrolytic conductivity of the effluent was reduced from 7 mS to about 20— 15 μS and pH decreased to the value of 6.2. The purified aqueous suspension of ZnSe:Mn nanoparticles was then frozen in liquid nitrogen, placed in a lyophilizer (Gefriertrocknungsanlagen GmbH company Martin Christ model Alpha 2-4 LDPlus) and cooled to the temperature of -40°C for 5 hours. The resulting lyophilized powder of ZnSe:Mn nanoparticles had loose consistency (consistency of loose powder, powder), did not form agglomerates and was characterized by highly hydrophilic properties, easily dissolved in water or other polar solvents forming transparent dispersions, even high-concentrated.
The study showed that the obtained in this example ZnSe:Mn nanoparticles had positive surface charge which was confirmed by zeta-potential measurements that indicated the average value of the order of + 35 mV (Fig.1 ).
The nanoparticles are characterized by a size of the order 20 nm (Fig. 2 a, b, c, d), stable photoluminescence (Fig. 3 and 4) in the range of orange-color visible light corresponding to a wavelength of 580 nm, and light absorbance at 340 nm wavelength (Fig. 5).
The quantum yield of luminescence (i.e. the ratio of number of photons emitted to the number of photons of the excitating radiation) as 30% - 35% was defined using results of measurements performed with a spectrofluorimeter.
The nanoparticles have a crystal structure of zinc blende, as evidenced by three characteristic peaks of the XRD spectrum in the attached drawings (Fig. 6).
Characteristic spectra derived from an amino group and an aliphatic chain, visible on FT-IR spectrum on attached drawing (Figure 7), evidence the effective stabilization of the surface with the 2-mercaptoethylamine hydrochloride and the hydrophilic nature of nanoparticles which form stable dispersions in solvents having hydrophilic properties.
Example 2
Stage I
The laboratory equipment used for the synthesis was as in Example 1. The reaction was carried out without using an inert gas. Appropriate analytical samples: 4.90 mmol of granulated selenium (Se), 1 1.65 mmol of sodium borohydride (NaBH4) and 15 cm3 of distilled water, were placed in the round- bottomed flask with three necks 3 having the capacity of 250 cm3 and connected to the reflux condenser 4. The reaction mixture was heated in the oil bath 2 to the temperature of 75°C, so as to reduce metallic selenium. The reduction was carried out for 1 hour, until complete discoloration of the reaction mixture from pink-red color to a completely transparent. The moment of color change indicates completion of the reduction process and forming of selenium salt (HSe-), in this case NaHSe. Stage II
When carrying out step I, in the same time the solution of the following substances: zinc acetate dihydrate (Zn(Ac)*2H20) and manganese acetate tetrahydrate (Mn(Ac)-4H20) and 2-mercaptoethylamine hydrochloride (HSC2H4NH2-HC1) was prepared. The above- mentioned solution was prepared in a separate reaction vessel (in a beaker placed on a magnetic stirrer at ambient temperature). For this purpose in 60 cm3 of distilled water the following substrates were mixed: 5.13 mmol of zinc acetate dihydrate, 0.97 mmol of manganese acetate tetrahydrate and 3.71 mmol of 2-mercaptoethylamine hydrochloride. At the end of step I (i.e. when solution of selenium was completely discolored) a saturated salt solution was gradually dropwise added to the main reaction system containing reduced selenium (Fig.8). The addition time was about 20 min. After completion of the dropwise addition, the reaction mixture was heated to the temperature of 80°C, and maintained at that temperature for 3.0 hours. Stage III
After the main reaction, the reaction mixture excited with the reaction mixture excited with UV-lamp emitting light with a wavelength of 366 nm showed orange light, indicating the successful reaction and formation of ZnSe:Mn nanoparticles in the reaction mixture. To purify the reaction solution from un-reacted substrates and other intermediates formed during the reaction, the reaction mixture with ZnSe:Mn nanoparticles was cooled to room temperature and washed with distilled water using membrane-ultrafiltration. For this purpose, the reaction mixture was divided in two portions (about 30 cm3 each), which were placed in an ultrafiltration column with stirring (from Amicon, with a total capacity of 200 cm3 ) provided with ultrafiltration membrane made of polyethersulfone (Millipore, PFS, Biomax 50 kDa). The purification process was monitored by measuring the electrolytic conductivity of effluent mixture. The ultrafiltration process was ended when the reduction in the conductivity of the effluent mixture was reduced from 6 mS to about 20 μS and pH was decreased to the value of 5.0. The purified aqueous suspension of ZnSe:Mn nanoparticles was then frozen in liquid nitrogen, placed in a lyophilizer (Gefriertrocknungsanlagen GmbH company Martin Christ model Alpha 2-4 LDPlus) and cooled to the temperature of - 30°C for 3 hours.
The resulting lyophilized powder of ZnSe:Mn nanoparticles had loose consistency (consistency of loose powder, powder), did not form agglomerates and was characterized by highly hydrophilic properties, easily dissolving in water or other polar solvents forming transparent dispersions, even high-concentrated.
The studies showed that luminescent ZnSe:Mn nanoparticles obtained in this Example are analogous to those of Example 1.
The properties of luminescent nanoparticles obtained in both embodiments, and in particular their hydrophilic nature and low toxicity enable to use them, without modification, in nano-biomedicine - the field of extremely high potential.
The ZnSe:Mn nanoparticles obtained according to the method of the invention have a hydrophilic nature (without surface modification), and due to their characteristics can be used as markers (quantum dots) for picturing pathological tissues, particularly in the diagnosis of tumor cells. In addition, it is also possible to use quantum dots as carriers of anticancer drugs and to improve the treatment.

Claims

PATENT CLAIMS 1. A method for obtaimng manganese-doped luminescent nanoparticles of zinc selenide having positive surface charge, in which ZnSe:Mn nanoparticles are prepared in an aqueous reaction mixture containing a zinc-precursor in the form of zinc acetate, a manganese-precursor in the form of manganese acetate, a selenium-precursor which is the product of reaction of metallic selenium with sodium borohydride, a stabilizer selected from the group of thiol compounds, characterized in that 2.92 - 5.90 mol of shredded metallic selenium and 6.87 - 14.03 mol of sodium borohydride are mixed with such an amount of demineralized water that the weight ratios of Se : NaBH4 : H20 are from 0.38 : 1 : 2.31 to 57.91 : 1 : 67.20 and the system is heated with stirring to the temperature of 75 - 85°C, maintained at this temperature till the reaction mixture discolorates from pink-red color to colorless, then into the reaction mixture is introduced gradually with stirring a solution containing 4.80— 5.13 mole of zinc acetate dihydrate (Zn(Ac)-2H20), 0.51 - 1.79 mol of manganese acetate tetrahydrate (Mn(Ac)-4H20), 3.71 - 5.31 mol of a stabilizer in the form of 2- mercaptoethylamine hydrochloride (HSC2H4NH2-HC1) and demineralized water in such an amount that the weight ratio Zn(Ac)-2H20 : Mn(Ac)-4H20 : HSC2H4NH2-HC1 : H20 is from 8.33 : 1 : 2.76 : 476.19 to 2.56 : 1 :1.35 : 136.67, then the mixture is heated and maintained at the temperature of 80 - 85°C for 3.0— 3.5 hours, and then the reaction mixture is cooled to an ambient temperature and subjected to a membrane- ultrafiltration process, which is carried on an ultrafiltration membrane having a pore size of 50 - 100 kD, using demineralized water, and said ultrafiltration proces is continued until the electrolytic conductivity of the effluent reduces to the value of 20 - 15 mS and pH decreases to the value of 5.0 - 6.2 and after ultrafiltration the obtained aqueous suspension of ZnSe:Mn nanoparticles is stored at reduced temperature, preferably 4 - 8°C, or processed to ZnSe:Mn nanopowder.
2. The method according to claim 1, characterized in that the obtained aqueous suspension of ZnSe:Mn nanoparticles after the ultrafiltration process is frozen in liquid nitrogen, lyophilized at -30 to - 40°C for 3 - 5 hours and resulting ZnSe:Mn nanopowder is stored under anhydrous conditions.
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