WO2016203431A1

WO2016203431A1 - Luminescent bismuth silicates, use and method for producing thereof

Info

Publication number: WO2016203431A1
Application number: PCT/IB2016/053594
Authority: WO
Inventors: Michele BACK; Pietro RIELLO
Original assignee: Universita' Ca' Foscari
Priority date: 2015-06-18
Filing date: 2016-06-17
Publication date: 2016-12-22

Abstract

The present invention relates to luminescent compounds of lanthanide/Yb doped bismuth silicate that present a crystalline structure and are characterized by non-linear optical properties, particularly photon up-conversion following IR or NIR excitation. Furthermore, the present invention describes several synthesis routes for producing said compounds in different form (e.g. particles, core-shell particles, bulk crystal), dimension (e.g. nanoscale or microscale) and crystalline structure, which result in tunable optical properties in view of a specific application. Particularly, there are disclosed Er/ Yb, Ho/Yb, Tm/Yb doped bismuth silicate nanoparticles that emit up-conversion light of the three basic colors (RGB). As said compounds are non-cytotoxic and present attractive optical properties, they find a wide range of applications, both industrial (e.g. lighting, solar energy, anti-counterfeiting) and biomedical (e.g. drug delivery, bio-imaging, nanomedicine).

Description

LUMINESCENT BISMUTH SILICATES, USE AND METHOD FOR PRODUCING THEREOF

Technical Field

The present invention relates generally to luminescent materials i.e. that are able to emit visible light after the excitation with infrared (IR), or ultraviolet (UV), or visible (VIS) radiation and / or also ionizing particles. More specifically, the present invention pertains to luminescent compounds in the form of doped particles (e.g. micro-sized or nano-sized particles) which can exhibit non-linear optical phenomena known in the literature by the terms 'photon up-conversion', 'photon down-conversion', 'photon down-shifting' and also scintillation. Furthermore, the scope of the present invention concerns producing methods of said luminescent compounds.

The phenomenon of up-conversion consists in the sequential absorption of two or more photons which leads to the emission of photons having higher energy than the energy of the excitation photons. Down-conversion consists in the emission of two or more photons with energy lower than the energy of the excitation photons. The phenomenon of photon down-shifting occurs when the absorption of a photon leads to the emission of a single photon with energy lower than the energy of the excitation photons. Finally, scintillation is the emission of photons in the visible range from the recombination of electron-hole pairs generated by ionizing particles and/or radiation.

Background Art

In recent years, materials exhibiting photoactive properties, and particularly non-linear optical properties such as up-conversion, have gained great interest due to the many potential applications in biomedicine, telecommunications, photovoltaic, sensors and lighting.

In the literature there are known various systems exhibiting photon up-conversion, typically from IR or near IR (NIR) to VIS or NIR, and photon down-conversion typically from UV or VIS to VIS or NIR. Generally, these systems consist of a base material called 'matrix' or 'host' doped with one or more ions. Typically such ions are selected from to the series of rare earth: indeed lanthanides present a 'ladder-like' electronic configuration which allows for the nonlinear optical phenomenon of the up-conversion to (and down-conversion) to take place.

From the structural point of view, the matrices already known in the prior art can present an amorphous structure (e.g. a glassy matrix) or a crystalline structure. In turn, the crystal matrices can be prepared both in the form of a 'bulk crystal' (monocrystalline or polycrystalline) and in the form of a nanoparticle or, more generally, of a nanostructure (e.g. a nanotube).

As far as the composition is concerned, the crystalline matrices used so far are primarily fluorides and oxides of metals (Nanoscale, 2013, 5, 23 and references cited therein) and in the scientific and patent literature a large number of compositions are described. Many of these compositions are used in large-scale commercial applications (e.g. Y₂O₃, Y₂O₂S o Gd₂O₃ in lighting), while others are used only in niche applications (e.g. Lu₂O₃) due to the high cost. Fluoride matrices have an high up-conversion efficiency due to the low phonon energy, but they are less stable than the oxides. Furthermore, due to the particular chemistry of the fluorides, fluoride-based particles or nanoparticles cannot bind easily silicon structures without previous encapsulation or functionalization compared to silicon-based systems. This represents an obvious drawback, since most of the research in nanotechnology developed so far, and addressed to applications such as drug delivery, nano-biosensors, bio-marking, photonics, etc., are largely based on the chemistry of silicon and particularly on mesoporous silica nanosystems.

In addition to fluorides and oxides, silicates represent a class of compounds capable of exhibiting up-conversion and other non-linear luminescence phenomena. Most of these relate to compositions of silicate glasses (ad claimed e.g. in CN104099090 or CN102633436), although there are also crystalline matrices based on silicates such as doped yttrium silicates (e.g. CN103059858, CN104212452) or strontium/titanium silicates (e.g. CN101353578).

As far as the patentability of the present invention is concerned, it is important to point out that bismuth compounds have received little attention as a host material for optical applications in spite of the promising features, low toxicity and low cost. Indeed, most of the photoactive properties of bismuth compounds have been exploited for catalyst or electrochemical applications rather than for optical devices.

Furthermore, known up-conversion luminescent systems, including bismuth compounds and targeted to optical applications, refer to glass compositions where the bismuth compound is only a minor constituent, and never represent a major constituent of the host matrix. There are known solutions (e.g. US5230831 and GB1185906A), which include traces of the Bi³⁺ ion as activator of a luminescent material (phosphors) based on rare earth oxides (or more complex compositions). Other known compositions (e.g. CN201210192587A) are complex and are not suitable for large scale applications because of manufacturing limitations (e.g. the synthesis require high temperatures and/or toxic gases).

Only recently, the application WO2015025297A1 describing bismuth oxide compositions with up-conversion properties has been filed. Even if such compositions represents a significant advancement in the field of up-conversion crystalline materials, they cannot emit blue light emissions as the doped bismuth oxide presents a low energy gap. This drawback limits the application in some fields, particularly in lighting where light sources capable of emitting white light are requested.

Greater flexibility in terms of up-conversion emission spectrum, are potentially achievable by using bismuth-doped silicates. In fact, it is known that bismuth silicates have a much higher energy gap than the oxides and also present several crystalline classes and crystalline fields. However, according to the best knowledge of the inventors, no stable bismuth silicate characterized by a non-glassy structure is known which, in combination with appropriate doping ions, exhibits up-conversion (or other non-linear optical phenomena) and it is suitable as host materials in photoactive applications. Known luminescent silicates compositions refer only to glasses containing bismuth compounds.

Furthermore, no synthesis routes are known that, according to the requirements of various applications, allow production of luminescent bismuth silicates in the form of nano-structure, micro-structure and bulk having various crystal structures.

Technical Problem

In view of the above drawbacks of the prior art, in a number of fields, such as lighting, there is still a need for low-cost novel luminescent compounds, which exhibit luminescence as a result of non-linear optical phenomena (e.g. up-conversion). In particular, from the above discussion, it is desirable to have novel luminescent compounds characterized by a non-glassy or crystalline structure, whose absorption/emission spectra can be tailored to meet selective application requirements.

Furthermore, in many other fields, particularly nanomedicine, there is still a need for luminescent compositions, which are compatible with silicon-based microstructure and nanostructure and which therefore can benefit from the expertise already achieved for such systems.

Finally, there is still a need for synthesis routes that allow manufacturing of such luminescent compounds in various shapes or structures and with the desired luminescence characteristics according to the requirements of specific applications.

Object/scope of the invention

Accordingly, the aim of the present invention is to overcome the disadvantages existing in the prior art related to luminescent compounds by disclosing novel luminescent doped bismuth silicate compounds, characterized by a crystalline structure and non-linear optical properties such as photon up-conversion.

Within the scope of the above mentioned aim, a main object of the present invention is to provide stable luminescent compounds consisting of crystalline bismuth silicates advantageously doped with dopant elements, preferably Yb and elements from the lanthanide series, as set forth in the appended independent claim. In particular, an important object of this invention is to synthesize luminescent compounds having peculiar electronic configurations so that they exhibit luminescence as the result of photon up-conversion, down-conversion or down-shifting or as the result of scintillation following the exposure to radiation or ionizing particles.

Still, an important object of the present invention is to provide luminescent compounds consisting of bismuth silicates able to generate red, green and blue (RGB) up-conversion emissions.

Furthermore, another important object of the present invention is to synthesize said luminescent compounds in the form of nanostructures or microstructures or in other forms such as bulk i.e. having dimensions larger than the micron scale. Additionally, said luminescent compounds can be in the form of isolated nanoparticles, or nanoparticles dispersed in a matrix, as well as nanoparticles having a homogeneous or a core-shell structure.

Within the scope of the above mentioned aim, a further object of the present invention is to provide a luminescent material comprising one or more of said luminescent compounds to be used in devices or systems that conveniently exploit the phenomena of up-conversion, down-conversion, down-shifting or scintillation.

Particularly, an important object of the present invention is to provide a luminescent material having a low toxicity, and therefore suitable as a luminescent system in bio-medical applications in combination with drugs or other compounds.

Finally, an important object of the present invention is to provide a luminescent material that present high chemical compatibility with silicon-based micro and nanostructured systems and therefore can take advantage of the technical experiences already known in the art.

In accordance with another aspect of the present invention, it is another object of the present invention to develop a method, as set forth in the appended independent claim, for synthesizing doped bismuth silicates able to exhibit luminescence as the result of photon up-conversion, down-conversion or down-shifting or as the result of scintillation following the exposure to radiation or ionizing particles. Particularly, another main object of the present invention is to provide a method that, by conveniently varying the concentration and type of doping agents, is able to produce luminescent compounds having tailored absorption and emission characteristic according to specific application needs.

Within the scope of the above mentioned aim, it is another object of the present invention to disclose a plurality of methods to allow the skilled in the art to choose the most advantageous synthesis route depending on the desired form or structure of said luminescent bismuth silicates. Still, a further object of the present invention is to provide a plurality of methods for synthesizing said luminescent compounds that offer advantages in terms of costs and production scale-up.

In accordance with another aspect of the present invention, it is another object of the present invention to develop a method, as set forth in the appended independent claim, for converting an electromagnetic radiation based on the luminescent crystalline bismuth silicates advantageously doped with dopant elements, preferably Yb and elements from the lanthanide series.

In accordance with another aspect of the present invention, it is another object of the present invention to disclose the use, as set forth in the appended independent claim, of doped bismuth silicates as a crystalline host in devices or systems that conveniently exploit the phenomena of photon up-conversion, down-conversion, down-shifting or scintillation

Finally, a last object of the present invention is to produce compounds and materials in various forms and structure, based on said luminescence bismuth silicates, with known technologies and at competitive costs with respect to known solutions in the state of the art.

These and still other objects, which will become apparent hereinafter are achieved by luminescent compounds comprising a silicate matrix of bismuth doped with Yb and with one or more elements belonging to the lanthanide series.

Technical Solution

In view of the above disadvantages or drawbacks of the prior art, the present inventors have made a lot of studies on non-linear optical properties of doped bismuth silicates for exploiting the application as host materials e.g. for up-converting application s. These studies were mainly directed to improve the up-converting luminescence intensity and efficiency under different light exposure, the phase and stability of the bismuth silicates , and finally the synthesis process thereof.

After long terms of practice and a lot experiments, the inventors found a new family of luminescent compounds comprising a matrix of silicate bismuth doped with Yb and with one or more elements belonging to the lanthanide series. Said compounds can be expressed by one of the following general formulas:

(Bi_1-(x+y+z)Yb_xM_yLn_z)₂Si₃O₉ (Formula 1)
(Bi_1-(x+y+z)Yb_xM_yLn_z)₄Si₃O₁₂ (Formula 2)
(Bi_1-(x+y+z)Yb_xM_yLn_z)₂SiO₅ (Formula 3)

wherein:

M is selected from the group consisting of: Sc, Y, La, Lu, Li, Na, K, Mg, Ca, Sr, Ba, Mn, Ti, V, Mo, Re, Os, Cr, Fe, Co, Ni, Cu, Zn, In, Al, Ga, Ta, Ge or a combination thereof;
Ln is selected from the group consisting of: Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, or a combination thereof;
x can be any value from about 0.000001 to about 0.4;
y can be any value from 0 to about 0.4;
z can be any value from about 0.000001 to about 0.2;
x, y and z are independent parameters satisfying the relationship: x+y+z < 0.5.

Said luminescent compounds can be produced by using one or more of the synthesis routes described hereinafter, depending on the desired structure, dimensions and luminescence characteristic of the compounds or materials according to the requirements of specific applications in different fields. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

Definitions

For the purpose of understanding the specification and the appended claims, in the following description the chemical elements are defined by means of the respective symbols as reported in a common periodic table of elements. For example, hydrogen is represented by its symbol H; helium is represented by He and so on. Also, it is to be understood that the chemical symbol comprises all isotopes and ions unless stated otherwise. Again, for the sake of brevity, the chemical compounds may be indicated by acronyms widely adopted in the technical field of the present invention.

In the context of the present invention, by the term 'silicate', it is meant a family of compounds having a chemical composition comprising an anionic silicon compound, typically the (ortho-)oxyanion silicate (SiO₄)^4-, but also the units (Si₂O₇)^6- or (SiO₃)². This definition also includes silicates having a more complex composition such as hydroxy-silicates.

Furthermore, in the context of the present invention the term 'particle' will designate an aggregate of atoms, molecules or other fundamental constituents of spherical shape, or also of non-symmetrical shape, having sub-micrometric or super-micrometric dimensions. Particularly, the term 'nanoparticle', 'nanostructure' or 'nanocomposite', singular or plural, will indicate, respectively, particles and structures of dimensions less than about 1 micrometer. With reference to the term 'particle' or 'nanoparticle', the term 'core-shell' will designate a structure having a central, substantially homogeneous material ('core'), not necessarily of spherical symmetry, enclosed by a material having a different composition or structure ('shell') than the core material. For example, a particle with a core-shell structure may have a doped bismuth silicate spherical core enclosed by a silica shell (or vice versa).

Similarly, in the context of the present invention, by the term 'nano-system' it is meant a system, in which one or more chemical entities, such as organic molecules, macromolecules, organometallic or inorganic phases are bonded to one or more compounds, or a material, according to the present invention. The term 'bonded' include not only chemical bonding, but any chemical or physical binding or adsorption between said entities and said compounds or material, as well as chemical entities loading onto said compounds or material.

In addition, with reference to said materials and structures, in the context of the present invention, the terms 'non-glassy' or 'crystalline' will designate a compound that exhibits at least a crystalline phase.

The terms 'luminescent compound' or 'luminescent material' will designate a compound or a material capable of emitting electromagnetic radiation preferably in the visible (hereinafter also referred as VIS) in response to the irradiation by visible light, infrared or near-infrared radiation (hereinafter IR and NIR), ultraviolet radiation (hereinafter UV) and also by ionizing particles (e.g. alpha particles). For the sake of clarity, with reference to Figure 1 cited above, in the following we define the luminescent mechanisms exhibited by the compounds and materials according to the present invention.

The term 'up-conversion' refers to the non-linear optical phenomenon whereby a single photon of energy E₂ is emitted after the absorption of two or more photons of energy E₁ <E₂. Generally IR radiation is absorbed and VIS or NIR radiation is emitted. The term 'down-conversion' refers to the non-linear optical phenomenon whereby two or more photons of energy E₂ are emitted following the absorption of a single photon of energy E₁> E₂. Generally UV or VIS radiation is absorbed and VIS or NIR radiation is emitted. In the context of the present invention, devices or systems that take advantage of these nonlinear mechanisms are defined 'up-converters' and 'down-converters' respectively.

The term 'down-shifting' refers to the phenomenon whereby a photon of energy E₂ is emitted after the absorption of a single photon of energy E₁> E₂. Generally UV or VIS radiation is absorbed and VIS or NIR radiation is emitted. Devices that exploit this mechanism are named 'down-shifter.'

The term 'scintillation' refers to the emission of one or more photons after the absorption of ionizing radiation or particles. The radiation, typically X-rays, gamma rays or alpha particles striking the material generates electron-hole pairs; electrons travel in the material and lose energy by transferring said energy to other electrons; eventually the electron-hole pairs recombine emitting photons, as they return to their ground state. Devices that use this mechanism are defined 'scintillators'.

Finally, the term 'about' as used herein is intended to include values, particularly within 10% of the stated values. The use of 'or' means 'and/or', unless stated otherwise.

It is to be understood that the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.

Process description

The luminescent compounds described by Formulas 1, 2 and 3, can be conveniently manufactured using one or more of the following synthesis routes:

Impregnation of mesoporous silica nanoparticles (MSNs);
Sol-gel synthesis (Pechini's synthesis variant);
Method of polyol;
Hydrothermal synthesis;
Solid state reaction (i.e. milling with heat treatment).

As it will be apparent in the following, the skilled in the art can advantageously choose the best synthesis routes according to the bismuth silicates structure, dimensions or luminescence properties that intends to obtain on the basis of a specific application. For example, the 'method of polyols' is suitable for synthesizing very small nanoparticles, while bulk materials can be produced through the 'solid state reaction' method. The sol-gel method and hydrothermal synthesis are suitable for nano/micro silicate systems with controlled and variable size. Finally, the impregnation process of the mesoporous silica nanoparticles (MSNs) ensure high control over the size of the nano/micro silicate systems containing the luminescent compounds according to the present invention. In the following, we provide a description of each of these synthesis routes.

With reference to the enclosed Figure 4, the luminescent compositions according to Formulas 1, 2 and 3 can present different crystal structures depending on the heat treatment used in the synthesis processes which will be fully described below.

1. Impregnation of mesoporous silica nanoparticles (MSNs)

One of the advantageous route for producing the luminescent compounds described by the Formulas 1, 2 or 3 consists in the impregnation of MSNs with salts of the bismuth precursors, with salts of Yb and with salts of the elements Ln and M. The MSNs impregnation synthesis route, hereinafter described by way of example, but not limitation, is particularly suitable for synthesizing nano/micro spheres of the compounds according to the present invention having a controlled size which can vary in a wide range.

The starting material of the process are MSNs, which are readily available in a wide range of sizes, with diameters ranging from 10 nanometers up to a few microns and a pore volume comprised between 0.7 and 4.5 cm³/g. The impregnation is carried out by dissolving the precursors of Bi, Yb and Ln elements and M in the Formulas 1, 2 or 3 (preferably in the form of nitrates) in a solution of water and nitric acid in a ratio of about 4:1. This solution is then added to a solution of water and nitric acid (ratio of about 4:1) in which the MSNs have been well-dispersed under constant stirring. The mixture thus obtained is stirred at room temperature for between 1 and 24 hours approximately. Finally the solvent is evaporated with the use of a rotavapor. The powders thus obtained are dried at 60-80 °C in an oven for about 12 hours and subsequently heat-treated at a temperature ranging between 400 and 1500 °C and for between 1 and 24 hours, depending on the compound to be synthesized.

Surprisingly, the resulting material consists of a bismuth silicate core surrounded by a silica shell. The inventors believe that this structure is generated through a self-assembly mechanism whereby very small particles nucleated within the pores first tend to aggregate spontaneously and then migrate towards the center of the particles to form the bismuth silicate core. Even if further experimental evidence is needed, this self-assembly mechanism seems to be induced by the concentration of bismuth in the composition according to Formula 1, 2 or 3. To the best knowledge of the inventors, it is not known a synthesis route including such a self-assembly mechanism, particularly for producing luminescent particles or nanoparticles. Therefore, the skilled in the art will appreciate that the method herein disclosed represents a significant improvement of the solutions known in the art.

The wide range in type and size of MSNs available make this simple process ideal for synthesize materials according to the present invention with sizes available between about 10 nanometers and some microns and with varying shapes such as spheres or rods.

2. Sol-gel synthesis (Pechini's synthesis variant)

The starting compounds are bismuth salts that are added to an Yb salt and a salt of one dopant element belonging to Ln or M in the Formulae 1, 2 or 3 according to the desired molar ratio. Said salts, preferably in the form of nitrates, are solubilized by means of a nitric acid solution, to obtain a first homogeneous solution having a molarity in the range between 0.3 and 0.7 molar. Conveniently, the molar ratio between the nitric acid and the precursor salts is preferably equal to 3.

Subsequently, tetraethyl orthosilicate (TEOS) is added to a solution of ethanol, concentrated nitric acid and water (molarity of the solution in a range between 0.3 and 0.7 molar) in order to obtain a second solution. The molar ratio of TEOS and the salts (bismuth and the dopant elements) is preferably equal to that existing between the silicon and the desired bismuth compound, and therefore, with reference to Formulas 1, 2 and 3, a ratio equal to 3:2, 3:4, 1:2. After the first and second solutions are mixed, citric acid is added. Conveniently, it is preferable to maintain a ratio between the moles of citric acid and the mixture of bismuth salts and TEOS preferably equal to 3.

Under continuous stirring, the solution thus obtained is heated, preferably in a sand bath to ensure a very homogeneous temperature distribution. When the solution reaches about 100 °C ethylene glycol is added, preferably in a molar ratio of about 3:2 compared to citric acid. The temperature is then raised to about 120 °C leaving the reaction to progress for a few hours. At the end of the polymerization of ethylene glycol with citric acid, a solid material in the form of a polymer network is obtained. Said polymer network contains the precursors of the compounds whose chemical composition is described by one of the Formulas 1, 2 or 3 according to the present invention. Advantageously, the inventors have found that it is possible to finely control the structure of said polymer network, and particularly the micro-cavities where the bismuth silicate host crystalline grows, by selecting the precursors, the temperature and the duration of said reaction.

In the next step, said solid material is subjected to a heat treatment for burning the polymer network. Said heat treatment is preferably performed in a lab stove at temperature variable between about 400 °C and about 1500 °C for between 30 minutes and 12 hours.

Finally, the compound resulting at the end of the heat treatment is subjected to a rapid quenching treatment, for example, a quenching in air for between about 3 minutes to about 25 minutes. At the end of this step, crystalline nanoparticles having a chemical composition described by Formulas 1, 2 or 3 are obtained. Said nanoparticles surprisingly exhibit up-conversion luminescent properties in the visible range.

By acting on the copolymerization reaction, the luminescent compounds according to the present invention can be synthesized at nanometric scale and also at micrometric scale. Noticeably, the inventors have found that such compounds do not need to have nanometric dimensions for up-conversion (or another non-linear luminescence phenomenon) to occur.

3. Method of polyol

The luminescent compounds described by the Formulas 1, 2 and 3 may be advantageously synthesized by a variant of the so-called 'polyol method', described hereinafter by way of illustration rather than by limitation of the invention set forth in the claims.

The process starts by mixing in the desired molar ratio the following precursors: TEOS; Bi salts; Yb salts; salts of a dopant element Ln or M in the Formulae 1, 2 and 3. Preferably the salts are in the form of nitrate or chloride. Conveniently, the molar ratio of TEOS and all the salts composing the precursors mixture (i.e. Bi salt, Yb salt and Ln and M elements salts) is preferably equal to that existing between the silicon and the desired bismuth compound, and therefore a ratio of 3:2, 3:4, 1:2 with reference to Formulas 1, 2 and 3 respectively.

Said precursors are dissolved in a polyalcohol (e.g. ethylene glycol, diethylene glycol or triethylene glycol) under constant stirring and brought to approximately 120-140 °C for about 5 to about 60 minutes until an homogeneous solution is obtained. Subsequently, the solution is quickly heated to a temperature between 150 °C and 220 °C for between about 5 minutes to about 4 hours, and then cooled to room temperature. The nanoparticles contained in said solution are separated by centrifugation and finally purified, for example by repeated re-dispersion in ethanol followed by centrifugation, for preferably three cycles.

The polyoil synthesis concludes with a heat treatment of the solid compounds obtained after the purification step, to increase the degree of crystallinity and to select the desired crystalline phase of the luminescent compound according to the present invention. Such treatment is preferably made in a lab stove at temperature varying in a range between about 300 °C to about 1500 °C for a time between about 30 minutes to about 12 hours.

4. Hydrothermal synthesis

Advantageously, the luminescent compounds described by the Formulas 1, 2 or 3 can also be produced by means of the so-called hydrothermal synthesis.

The precursors mixture contains in the desired molar ratio the following compounds: Bi salts; sodium silicate or TEOS as silicon precursor; Yb salt; a salt of a dopant element Ln or M in the Formulae 1, 2 and 3. Preferably, the Bi salts, the Yb salts and the dopant element salt are in the form of nitrate or chloride.

The precursors of Bi, Yb and of the Ln and M elements are dissolved in a polyol (preferably ethylene glycol or diethylene glycol). The precursor of silicon is instead dissolved in distilled water. The two solutions are then mixed together and the resulting solution is transferred into an autoclave and brought to temperatures comprised in a range between 150 and 200 °C and left for a time varying between 1 and 24 hours.

Finally, after the autoclave is cooled to room temperature, the precipitate is collected, transferred into a crucible and heat-treated to a temperature varying between 400 and 1500 °C for a time between 1 and 24 h. At the end, nanoparticles of a compound according to the present invention are obtained. By means of the hydrothermal synthesis, herein described by way of example but not limitation of the present invention, nanoparticles of different shapes and dimensions can be obtained by properly selecting process parameters.

5. Solid state reaction

Finally, the luminescent compounds according to Formulas 1, 2 and 3 may be advantageously obtained by means of the so-called 'solid state reaction', hereinafter described by way of example but not limitation of the present invention. This synthesis route is particularly suitable for obtaining bulk materials or microcrystalline materials.

The starting compounds are preferably Bi oxides, Yb oxides, Si oxides and oxides of the doping elements belonging to Ln or M in the Formulae 1, 2 or 3. The molar ratio of the compounds is conveniently chosen to obtain the desired luminescent compound according to said formulas.

Such compounds are carefully mixed by means of a number of grinding cycles (e.g. in agate mortar and in a ball mill). Subsequently, the powders thus obtained are heat-treated in a stove at a temperature varying between about 300 to about 1500 °C for between about 1 to about 24 hours depending on the particles structure that is selected for a given application (more details will be provided in the following Examples).

At the end of the heat treatment, a luminescent material of a compound according to the present invention is obtained. The 'solid state' synthesis route concludes with a further grinding cycle to obtain a powder of the desired particle size.

In conclusion, it is apparent to those skilled in the art that herein has been disclosed a plurality of methods for synthesizing the luminescent compounds described by the Formulas 1, 2 and 3, thus achieving one of the main object of the present invention.

Advantageous Effects

Remarkable advantages of the present invention are listed in the following:

The doped bismuth silicate luminescent compositions according the present invention exhibit excellent non-linear optical properties, particularly photon up-conversion from IR/NIR to visible;
The examples of compositions provided below (see Figure 7) , demonstrate that it is possible to generate strong emission of red, green and blue light that properly combined can produce white light, a crucial requirement for lighting applications;
The luminescent compositions according to the present invention are characterized by a crystalline phase (and not a glass phase). The crystal structure can be modified by altering the compositions and/or the process parameters to finely tune the luminescent properties;
Compared to the prior art materials based on fluorides, the novel compounds, being oxides, are more stable. Furthermore, silicates intrinsically present an higher chemical compatibility with silicon-based system than the fluorides. Finally the manufacturing process is more simply and less expensive;
The novel compounds are radiopaque, non-toxic and some compounds show a selective up-converting NIR signal that falls in the therapeutic window. Therefore, they are suitable for biomedical and pharma application (e.g. in bio-imaging or bio-labeling, nanomedicine);
The novel nanoparticles can be easily functionalized or loaded with active molecules (e.g. a drug or a fluorescent dye), and they are easily dispersible in a medium (e.g. a biopolymer), to prepare a biocompatible system, or material;
In terms of manufacturing, the novel compounds are easy to synthesize from low-cost starting materials by means of several reliable synthesis routes. Compounds in forms of nanoparticles, core-shell nanoparticles, microparticles or even bulk crystals can be obtained. These advantages lead to a simplification of the synthesis plant layout, a better process control and cost-saving;
Finally, the production process provide a variety of reliable control means for tailoring and tuning the luminescent properties of the doped bismuth silicate according to specific requirements. A first mean is varying the type (i.e. selecting specific M and Ln in Formula 1, 2, or 3) and concentration of dopants (i.e. selecting specific x, y, and z in Formula 1); a second mean, is combining different compounds according to the present invention; and finally, a last mean is combining different crystalline phases.

Additional objects and advantages of the invention will be set forth in part in the description provided, and in part will be obvious from the description, or may be learned by practice of the invention. It will apparent to those skilled in the art that the advantages of the present invention cannot be achieved by prior art up-converting compounds based on doped bismuth silicates.

Industrial Applicability

The properties of the novel luminescent compounds and materials according to the present invention are highly attractive for a wide range of applications in different markets.

In particular, lighting industry can benefit from this novel technology. In fact, the Examples 1, 2 and 3 provide a demonstration that: firstly, it is possible to finely tune the luminescent properties of the compounds described by the Formulas 1, 2 or 3; secondly, that it is possible to generate strong emission of red, green and blue light (Figure 7) which properly mixed produce white light. These features can be exploited in several application such as: RGB sources; sensitive elements in LED that emits white light by additive mixing; screens or active filters able to increase the luminous efficiency of existing light sources by converting the IR fraction in a visible emission; active screens capable of modulating the spectral characteristics of an existing light source; switchable luminescent piece of furniture.

The peculiar and tunable optical properties of the doped bismuth silicates compounds can advantageously be exploited in anti-counterfeiting systems (e.g. anti-counterfeiting tagging/labelling), but also in the photonics industry where the non-linear optical properties find a wide range of applications (e.g. optical switches, up-converters, down-converters).

Furthermore, the present invention finds application in the solar energy industry e.g. as solar energy harvester or as active coating in standard or multi-junction solar cells to increase photovoltaic efficiency. In fact, by properly selecting luminescent compounds or materials according to the present invention, it is possible to improve the matching of the solar spectrum with the spectral characteristic of the solar cell.

The doped bismuth silicates particles and materials according to the present invention find main applications in biomedicine, nanomedicine and pharma, since they are radiopaque, non-toxic, are available as both particles and nanoparticles, and some compositions (e.g. Yb/Er, Yb/Ho doped bismuth oxide nanocrystal), show a selective up-converting NIR signal that falls in the therapeutic window. In these markets, promising applications cover, just to mention a few: photoactive dye for bio-imaging, bio-labeling, DNA detection, luminescent dye for bioassay; multimodal contrast agent (e.g. combined X-ray and optical tomography), multifunctional contrast agent for combined diagnostic and disease treatment; photoactive carriers for therapeutic compositions in the photodynamic therapy or in the treatment of other human diseases.

In the dental field, the up-conversion mechanism of the doped bismuth silicates compositions can advantageously be exploited to prepare photoactive resins for dental restoration (e.g. a crown or dental filling) which can emit light under irradiation with an IR source. In this way, the dental restorative material can be identified with respect to the natural dental tissue.

Finally, a further potential use of said luminescent doped bismuth silicates compounds is as sensitive element inside a scintillator for detecting ionizing radiation or particles, in light sensor, or wherever it is required the conversion of incident radiation in a more suitable wavelength range.

Description of Drawings

The present invention will be more fully understood by reference to the following drawings which are for illustrative purposes only:

Figure 1 presents schematic diagrams of the non-linear optical processes exhibited by the luminescent compounds according to the present invention. Particularly, (a) shows photon up-conversion; (b) down-conversion, (c) photon down-shifting and (d) the phenomenon of scintillation;
Figure 2 shows, in (a), the SEM micrograph of a luminescent compound according to the best mode of the present invention, while in (b) the high-resolution TEM micrograph of the same compound showing the presence of a core-shell structure;
Figure 3 shows the XRD spectrum of a luminescent compound according to the best mode of the present invention;
Figure 4 shows the XRD spectra, at different temperatures of the heat treatment, of the luminescent compound according to the best mode of the present invention. Said compounds were synthesized by the impregnation process;
Figure 5 shows plot of the Kubelka Munk Function as a function of the excitation light beam. The function is calculated from the reflectance spectrum and it is used to estimate the band-gap of the luminescent compound according to the preferred embodiment of the present invention;
Figure 6 shows the photoluminescence (PL) emission spectrum at a wavelength of 980 nm with reference to the compounds according to best mode of the present invention (co-dopant elements Yb-Ho, Yb-Er and Yb-Tm);
Figure 7 show the position in the color space of the chromaticity coordinates with reference to the luminescent compounds according to the best mode of the present invention (co-dopant elements Yb-Ho, Yb-Er and Yb-Tm);
Figure 8 schematically depicts the use in a device of a luminescent material obtained by combining a plurality of the luminescent compounds according to the present invention. In (a) a series configuration for the luminescent material is shown, while in (b) a parallel configuration is shown.

These figures illustrate and demonstrate various features and embodiments of the present invention, and of the manufacturing method thereof, but are not to be construed as limiting of the invention.

Best Mode

The best mode of the present invention, hereinafter described by way of illustration rather than by limitation of the invention set forth in the claims, refers to the compounds described by the general formula:

(Bi_1-(x+y+z)Yb_xM_yLn_z)₂SiO₅ (Formula 3)

wherein

M is selected from the group consisting of: Sc, Y, La, Lu, Li, Na, K, Mg, Ca, Sr, Ba, Mn, Ti, V, Mo, Re, Os, Cr, Fe, Co, Ni, Cu, Zn, In, Al, Ga, Ta, Ge or a combination thereof,
Ln is Er, Ho or Tm;
x is about 0,10;
y can be any value from 0 to about 0.4;
z can be any value from 0,000001 to about 0,2;
x , y and z are independent parameters satisfying the relationship: x+y+z < 0.5.

Said luminescent compounds can exhibit different crystal structure, by properly choosing the parameters M, Ln, x, y e z, but also the temperature and pressure used during the synthesis route. Particularly, the enclosed Figure 4 (down) clearly demonstrates that it is possible to tune the crystal structure by acting on the temperature of the heat treatment. As a non-limitative example of the present invention, the following table present the most relevant crystal phases found.

No	Spatial Group	Crystal System
36	Cmc21 or	Orthorhombic
62	Pnma or	Orthorhombic
9	C1c1	Monoclinic

Nevertheless, other crystal phases can also be produced. Alternatively, compounds having an amorphous structure can also be synthetized, or also a combination of crystal and amorphous compounds.

With reference to the best mode of the present invention, three luminescent compositions are hereinafter disclosed as non-limitative examples of the present invention. The inventors have advantageously produced such compositions by impregnations of MSNs (synthesis route 'A'), because, surprisingly, it allows to obtain discrete particles characterized by a core-shell structure. Nevertheless, others of the aforementioned synthesis routes can be used to produce discrete particles or nanoparticles, as it will be apparent to the skilled in the art.

EXAMPLE OF LUMINESCENT COMPOUNDS

Example 1: Er-doped luminescent compound

The luminescent composition of the first example is defined by: Ln = Er, y = 0 (M is absent), x = 0.10 and z= 0.02. The mesoporous silica particles used were produced by the inventors following the teachings of Zhen-An Qiao et al. (Chem. Mater. 2009, 21, 3823-3829) and have the following characteristics: mean diameter of about 60 nm; surface area, about 1000 m²/g; total pore volume, about 1.2 m³/g; average pore diameter, approximately 2.4 nm.

The starting compounds are: bismuth nitrate pentahydrate, ytterbium nitrate pentahydrate, erbium nitrate pentahydrate (all purchased from Sigma-Aldrich). The powders obtained by following the impregnation MSNs process described above, were first dried in an oven at 60-80 °C for about 12 hours and subsequently subjected to a heat treatment at about 800-900 °C for about 1.5-2.5 hours.

At the end of the heat treatment, nanoparticles shown in the SEM micrograph of the enclosed Figure 2(a) are obtained. Surprisingly, the nanoparticles present a core-shell structure which is clearly evidenced in the high-resolution TEM micrograph of Figure 2(b). The core of the nanoparticle is composed of bismuth silicate while the shell is composed of silica. As mentioned before, the formation of such a core-shell structure can be explained only by means of a self-assembly mechanism, comprising, firstly, the nucleation of bismuth silicate seed crystals within the pores of the mesoporous silica nanoparticles, and then the spontaneous aggregation and migration towards the center of said seed crystals. This innovative self-assembly process is induced not only by the heat treatment conditions, but also by the concentration of bismuth in the starting composition.

As demonstrated by the XRD spectrum presented in the enclosed Figure 3, the resulting compound is a silicate of composition Bi₂SiO₅ having an orthorhombic structure and a Cmc2₁ space group. The band-gap of the compound according to the present example is equal to 3.68 eV as shown by the Kubelka Munk function obtained from the reflectance spectrum of the enclosed Figure 5.

By exciting the compounds with a laser of wavelength 980 nm, it is possible to obtain the photoluminescence (PL) spectrum that is presented in the enclosed Figure 6 (middle) by way of example but not of limitation of the present invention. The PL shows a bright green emission, which is clearly observable by naked eye, thus demonstrating that the compound of Example 1 is not simply luminescent but it also exhibits luminescence in the form of up-conversion.

Example 2: Ho-doped luminescent compound

The luminescent composition of Example 2 was synthesized using the same process and process parameters as the previous example. It is defined by: Ln = Ho, y = 0 (M is absent), x= 0.10 and z= 0.01. By way of example but not of limitation of the present invention, the enclosed Figure 6 (bottom) shows the PL spectrum of the luminescent composition at an excitation wavelength of 980 nm. Clearly, the sample presents an intense orange light emission that is observable to the naked eye. Therefore, also the compound of Example 2 exhibits up-conversion luminescence.

Example 3: Tm-doped luminescent compound

The last example of luminescent composition according to the best mode of the present is defined by: Ln = Tm, y = 0 (M is absent), x = 0.10 and z = 0.005. Again, the same synthesis route described in Example 1 was used. At an excitation wavelength of 980 nm, the compound exhibits a nonlinear optical phenomenon (i.e. up-conversion) as evidenced by the strong blue light emission, visible to the naked eye, shown in the PL spectrum of Figure 6 (top).

Summing up, Figure 7 presents the chromaticity coordinates in the color space corresponding to the luminescent compounds of the Example 1, 2 and 3 according to the best mode of the present invention. Dopants used are Yb-Ho, Yb-Er and Tb-Tm respectively.

From the examples described above, for illustrative purposes only, it is apparent to those skilled in the art that the main objects of the invention have been fully achieved. In fact, novel compositions and synthesis methods of luminescent doped bismuth silicate able to exhibit up-conversion have been disclosed. Furthermore, it has been shown that by conveniently varying the type and concentration of dopants, it is advantageously possible to synthesize luminescent compounds of bismuth silicate capable of emitting red, green and blue (RGB) up-conversion light emissions.

Mode for Invention

The second embodiment of the present invention, schematically depicted in Figure 8, by way of illustration rather than by limitation of the invention, consists of a luminescent material comprising a plurality of luminescent compounds according to one or more of the Formulas 1, 2 or 3. In fact, by properly selecting the elements Ln or M, or the parameters x, y and z in one or more of the Formulas 1, 2 or 3, it is possible to obtain a plurality of luminescent compounds characterized by different emission spectra (E₁( l ), l₁ )_j and absorption spectra (E₂( l ), l₂ )_j (wherein j = 1, 2 ... n).

This luminescent material may be obtained by combining in series (a) or in parallel (b) said plurality of luminescent compounds as the enclosed Figure 8 schematically shows by way of illustration rather than by limitation of the invention. In the first case, a light source sequentially excites the different luminescent compounds constituting said plurality. For example, in photovoltaic systems this configuration is useful to enhance matching of a sun spectrum and the quantum efficiency of the solar cell and thus to improve PV efficiency. In the second case, the different luminescent compounds are excited independently by means of a single light source. In lighting, this configuration is suitable to obtain light sources with a specific color point in the chromaticity diagram (see Figure 7). For instance, a white light source can be obtained by suitably mixing the individual emissions of the three compositions described in Examples 1, 2 and 3 and by using an excitation source having an emission peak around 980 nm.

From the above, it is evident to the skilled in the art that the configurations of the luminescent compounds in the present embodiment have a purely illustrative character and that other configurations are also possible, for example a combination of the series configuration and parallel configuration.

It is also evident that said luminescent material can be made by means of several known techniques. For example, it is possible to disperse nanoparticles of the individual compounds in a transparent polymer matrix (e.g. an optical grade polycarbonate or methacrylate) in order to develop optically-active flexible polymer films or rigid plates. In addition, by dispersing said nanoparticles in a resin it is possible to prepare a paint which can be applied to glass sheets or optical elements such as filters.

In this way, it has been disclosed a luminescent material having selected optical properties and a method useful for finely tailoring the optical properties of said material and therefore to meet specific application needs.

Finally, in the third embodiment according to the present invention, herein illustrated by way of example but not of limitation, a luminescent nano-system is disclosed. Said nano-system comprises at least one of the luminescent compounds according to one or more of Formulas 1, 2 or 3 and an additional compound, preferably an organic or organometallic compound.

These additional compounds may be enzymes, polypeptides, proteins, antibodies, DNA, RNA, active substances, chemo drugs, immobilizing agents, nanoparticles, inorganic phases of metals, photocatalysts, magnetic oxides, magnetic contrast factors for NMR analysis, contrast factors for optical analysis, or a combination thereof.

Depending on the chemical-physical properties of the nano-system components, said additional compounds are bonded via chemical bonding, physical binding, adsorption. Also, it is also possible to incorporate said additional compounds into said luminescent compounds.

To functionalize the nanoparticles, common-practice techniques may be used. For example, in the case of the luminescent compositions of Examples 1, 2 and 3, well-known functionalization techniques developed for mesoporous silica nanoparticles can be advantageously used.

Thanks to the low cytotoxicity of the bismuth, the compounds according to the present invention are suitable for many biomedical applications. For example, the particles according to the present invention can be combined with specific chemo-drugs such as taxol or doxorubicin to form a bio-system for selective drug delivery in cancer treatment. Furthermore, bio-system comprising said particles are suitable as multimodal contrast agent because the luminescent properties of said particles allows to combine disease treatment and diagnostics.

For illustrative purposes of this embodiment there are provided two examples of nano-systems according to the present invention suitable as photosensitizing agents in photodynamic therapy.

EXAMPLE OF LUMINESCENT MATERIALS OR SYSTEMS

Example 4: Er-doped photosensitive nano-system

In photodynamic therapy (PDT) it is used a photosensitizing agent to produce cytotoxic oxygen that can be used to kill cancer cells. In such therapy it is commonly used a fluorescent dye named Merocyanine 540 (MC540) which is activated with light of 540 nm, i.e. a wavelength outside the therapeutic window that is absorbed only by biological surface tissue. For this reason, the use of MC540 is limited to skin treatment.

The enclosed Figure 6 (middle graph) shows that the compound of Example 1 has an up-conversion emission at around 540 nm (green) following the excitation with IR radiation of wavelength 980 nm. Advantageously, the combination of said compound to Merocianina 540 allows to obtain a nano-photosensitizing system which can be activated with radiations able to penetrate deep biological tissues, for example 980 nm, i.e. within the so-called therapeutic window. In this way, by taking advantage of the up-conversion properties exhibited by the compound of Example 1, it is possible to extend the range of Merocyanine 540 in the PDT therapy.

To functionalize the nanoparticles of the compound of Example 1 various known techniques may be used, for example those described in the patent specification US7563818 in the name of Cancer Research Technology Ltd.

Example 5: Ho-doped photosensitive nano-system

Similarly to the previous example, the compound described in Example 2 can be combined with zinc-phthalocyanine (ZnPc), one of the best photosensitizing agents used in PDT, in order to obtain a nano-system able to activate at a wavelength of 980 nm. In fact, ZnPc is activated with a radiation of 660 nm (close to the limit of the therapeutic window) which is exactly the wavelength emitted from the composition of Example 2 (Figure 6, bottom) as a result of excitation with radiation of wavelength of 980 nm.

In conclusion, It is apparent to those skilled in the art that all the aims and objects of the present invention have been fully achieved by the crystalline lanthanide/Yb doped bismuth silicates compositions and by the manufacturing methods thereof as herein disclosed.

In fact, said novel compositions exhibit nonlinear optical properties since they present luminescence as a result of a photon up-conversion process. Compared to other known up-converting luminescent compounds, such as fluoride-based compounds, the compositions disclosed have excellent optical properties, are non-cytotoxic and present an intrinsic chemical compatibility with silicon-based structures (e.g. mesoporous silica).

Additionally, with the present invention, there have been disclosed several synthesis routes for producing said luminescent compounds that are easy to implement and control.

Said compositions and synthesis routes represent a remarkable achievement with respect to known luminescent compositions as it will be apparent to those skilled in the art by the embodiments and examples provided. Particularly, it has been shown that, by properly selecting the synthesis route, the dopants Ln or M and the parameters x, y and z in the Formulas 1, 2 or 3, it is possible to obtain compounds having the desirable form (e.g. particles, core-shell particles, bulk crystal), dimension (e.g. nanoscale or microscale) and crystalline structure, which result in tunable luminescence properties. Noticeably, the examples provided demonstrate that Er/ Yb, Ho/Yb, Tm/Yb doped bismuth silicate nanoparticles emit a strong up-conversion light of the three basic colors (RGB).

Furthermore, it has been shown that using one of the synthesis route provided, it is surprisingly possible to produce high-quality core-shell lanthanide/Yb doped bismuth silicates nanoparticles by means of a self-assembly process that appears completely new and inventive with respect to the prior art.

Finally, with the present invention, there have been disclosed some uses of said novel luminescent compositions both in industrial sectors (e.g. lighting, solar energy, anti-counterfeiting) and biomedicine (e.g. drug delivery, bio-imaging, nanomedicine).

The invention thus conceived is susceptible of numerous modifications and variations, without departing from the basic concepts as disclosed herein. Although the description and examples above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, any modification of the present invention which comes within the spirit and scope of the following claims is considered part of the present invention.

All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. In practice, all the details such as precursors, solvents, acids used in the synthesis, may be replaced with other technically equivalent elements, as long as they fall within the scope of the present invention. Furthermore, the order of the process steps described above is shown by way of example, but not limitation and can be changed according to convenience.

It will be appreciated that the scope of the present invention fully encompasses other embodiments and examples which may become obvious to those skilled in the art in view of this specification and are therefore within the scope of the claimed invention.

For instance, although the examples above provided refer to a synthesis route based on mesoporous silica nanoparticles impregnation, other processes can be conveniently used to produce the compounds according to the present invention. Similarly, for convenience only up-conversion luminescence spectra were presented with reference to the examples provided. However, the compositions according to the Formulas 1, 2 and 3 also show other nonlinear optical properties, thanks to the asymmetrical crystal structure of many compositions. In practice, by properly selecting the excitation spectrum, those of ordinary skill in the art can test the presence of photon down-conversion, down-shifting and scintillation in an obvious way.

The above description and drawings are only illustrative of preferred embodiments which achieve the aims, objects, features and advantages of the present invention, and they are not intended that the present invention be limited thereto.

In the appended claims, reference to an element in the singular is not intended to mean 'one and only one' unless explicitly so stated, but rather 'one or more.' Where the characteristics and techniques mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example, but not limitation by such reference signs.

Claims

A luminescent compound represented by one or more of the following general formulas:

(Bi_1-(x+y+z)Yb_xM_yLn_z)₂Si₃O₉ (Formula 1)

(Bi_1-(x+y+z)Yb_xM_yLn_z)₄Si₃O₁₂ (Formula 2)

(Bi_1-(x+y+z)Yb_xM_yLn_z)₂SiO₅ (Formula 3)

wherein :

- M is selected from the group consisting of: Sc, Y, La, Lu, Li, Na, K, Mg, Ca, Sr, Ba, Mn, Ti, V, Mo, Re, Os, Cr, Fe, Co, Ni, Cu, Zn, In, Al, Ga, Ta, Ge or a combination thereof;

- Ln is selected from the group consisting of: Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, or a combination thereof;

- x can be any value from about 0.000001 to about 0.4;

- y can be any value from 0 to about 0.4;

- z can be any value from about 0.000001 to about 0.2;

- x, y and z are independent parameters satisfying the relationship: x+y+z < 0.5.
The luminescent compound as recited in claim 1 characterized in that said compound exhibits one or more of the optical phenomena selected from the group consisting of: up-conversion, down-conversion, down-shifting of electromagnetic radiation; scintillation under exposure of ionizing radiations or particles.
The luminescent compound as recited in claim 1 or 2 characterized in that said compound exhibits at least a crystalline phase.
The luminescent compound as recited in one or more of the preceding claims characterized in that the emission or the absorption spectra of said compound are tunable by means of a suitable selection of the parameters Ln, M, x, y, z in said Formulas, said selection being implemented in such a way that variations in the crystalline field or variations in the crystalline phase, or a combination thereof, are induced in the structure of said compound.
The luminescent compound as recited in one or more of the preceding claims characterized in that the structure of said compound is selected from the group consisting of: bulk crystal, nanoparticle, microparticle, nanorod, nanohorm, nanowire, quantum dot, quantum wire, quantum well, nanostructured crystalline material, microstructured crystalline material, or a combination thereof.
The luminescent compound as recited in one or more of the preceding claims characterized in that:

- said compound is represented by said Formula 3;

- M is selected from the group consisting of: Sc, Y, La, Lu, Li, Na, K, Mg, Ca, Sr, Ba, Mn, Ti, V, Mo, Re, Os, Cr, Fe, Co, Ni, Cu, Zn, In, Al, Ga, Ta, Ge, or a combination thereof;

- Ln is selected from the group consisting of: Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, or a combination thereof;

- x can be any value from about 0.000001 to about 0.4;

- y can be any value from 0 to about 0.4;

- z can be any value from about 0.000001 to about 0.2;

- x, y and z are independent parameters satisfying the relationship: x+y+z < 0.5.
The luminescent compound as recited in one or more of the preceding claims characterized in that x can be any value from about 0.0001 to about 0.4.
A luminescent material comprising one or more of the luminescent compounds according to one or more of the preceding claims.
The luminescent material as recited in claim 8 characterized in that said material has a core-shell structure wherein:

- the core comprises said luminescent material ;

- the shell is a material differing from said core for: composition, phase or a combination thereof.
The luminescent material as recited in claim 8 characterized in that said materials has a core-shell structure wherein:

- the shell comprises said luminescent material ;

- the core is a material differing from said shell for: composition, phase or a combination thereof.
The luminescent material as recited in one or more of the claims 8 to 10 characterized in that said materials comprises nanoparticles or particles of said luminescent compositions, said nanoparticles or particles being dispersed in a matrix, preferably a silica matrix or a polymer matrix.
A luminescent material as recited in one or more of the claims 8 to 11 characterized in that said material is porous or mesoporous.
A nano-system comprising:

- the luminescent material as recited in one or more of the claims 8 to 12; and

- at least one compound chemically bound or adsorbed onto said luminescent material, said compound being selected from the group consisting of: enzymes, polypeptides, proteins, antibodies, DNA, RNA, drugs, chemotherapy drugs, chelating agents, nanoparticles, metal oxides inorganic phases, photocatalyzer agents, photosensitizer agents, magnetic oxides, magnetic resonance imaging agents, enhancing agents for optical imaging, or a combination thereof.
The use of the luminescent material according to one or more of the claims 8 to 12 as a radiation converter in an up-converter, down-converter or down-shifter device for converting an exciting radiation of spectrum and peak wavelength (E₁( l ), l₁ ) to an emitted radiation of spectrum and peak wavelength (E₂( l ), l₂ ), wherein said radiation is selected in the group consisting of: infrared electromagnetic radiation, ultraviolet radiation, visible radiation, or a combination thereof.
The use of the luminescent material according to one or more of the claims 8 to 12 in an anti-counterfeiting device or system.
The use of the luminescent material according to one or more of the claims 8 to 12 in a scintillation detector as sensitive element for the detection of ionizing radiation or ionizing particles.
The use of the luminescent material according to one or more of the claims 8 to 12 in a photovoltaic device.
The use of the luminescent material according to one or more of the claims 8 to 12 as a diagnostic means, or for treatment of a condition, disease, or disorder, preferably in the photodynamic therapy.
The use of the luminescent material according to one or more of the claims 8 to 12 in dentistry.
The use of the nano-system according to claim 13 for drug-delivery, or for diagnosis or for treatment of a condition, disease or disorder, preferably in the photodynamic therapy.
The use of the nano-system according to claim 13 for bio-imaging of biological systems, or for targeting biological compositions.
The use of the nano-system according to claim 13 in dentistry.
A method for preparing the luminescent material according to one or more of claims 8 to 12, said method comprising one or more of the following steps:

- defining a concentration for bismuth according to said Formulas 1, 2 or 3;

- selecting an amount of mesoporous silica having mean shape, mean dimension, pores mean dimension and specific surface area suitable for the intended application;

- completely dispersing said amount of mesoporous silica in an aqueous solution of nitric acid, to obtain a dispersion;

- dissolving in a aqueous solution comprising nitric acid a bismuth precursor, an Yb precursor and at least a precursor of a dopant element selected from Ln or M in said Formulas, or a combination thereof, so as to obtain a first solution;

- mixing said dispersion and said first solution to obtain a mixture and then stirring, preferably at room temperature, for a time period of about 1 hour to about 24 hours;

- removing solvents from said mixture, preferably by means of a rotary evaporator, to obtain a particulate;

- drying the particulate obtained at the end of the preceding step for at least 2 hours at a temperature ranging from about 40 to about 120 degrees centigrade to obtain a dried particulate;

- heating said dried particulate, preferably in a stove, at a temperature ranging from about 400 to about 1500 degrees centigrade to for a time period of about 1 to about 24 hours, so as to obtain said luminescent material.
The method as recited in the preceding claim, wherein:

- said material is a core-shell material; or

- said material is a dispersion of particles or nanoparticles in a silica matrix, said particles or nanoparticles consisting of one or more luminescent compounds described by one or more of said Formulas 1, 2 or 3.
The method as recited in claim 24 characterized in that the synthesis of said core-shell material is achieved by means of a self-assembling process of the core, or a self-assembling process of the shell, or a combination thereof.
The method as recited in claim 24 characterized in that the synthesis of said dispersion of particles or nanoparticles in a silica matrix is achieved by means of a self-assembling process of the particles or nanoparticles, or a self-assembling process of the matrix, or a combination thereof.
The method as recited in claims 25 or 26, wherein said self-assembling process comprises the following steps:

- defining a concentration for bismuth according to said Formulas 1, 2 or 3;

- inducing formation of particles or nanoparticles of said luminescent compounds within the pores of said mesopouros silica;

- inducing, preferably by means of a thermal treatment, the preferential binding or accumulation of said particles;

- inducing migration of said particles or nanoparticles, preferably by means of a thermal treatment, to one or more targeted locations of said mesopouros silica, in such a way to form said core;

- inducing migration of said particles or nanoparticles, preferably by means of a thermal treatment, in such a way to form said dispersion of particles or nanoparticles in said silica matrix .
A sol-gel method for preparing the luminescent compound according to any of claims 1 a 7, said method comprising one or more of the following steps:

- adding a silicon precursor to an aqueous solution comprising an alcohol, preferably ethanol, and concentrated nitric acid, to obtain a first solution, having a molar ratio ranging between about 0.1 mol to about 1.0 mol;

- mixing a bismuth precursor, preferably a bismuth nitrate, an Yb precursor and at least a precursor of a dopant element selected from Ln or M, in said Formulas 1, 2 or 3, or a combination thereof, so as to obtain a mixture, wherein the molar ratio between said silicon precursor and all said precursors in said mixture, is preferably the same as the molar ratio between the silicon and the doped bismuth compound according to one of said Formulas;

- completely dissolving said mixture in a aqueous solution comprising nitric acid to obtain a second solution having a molar ratio ranging between about 0.1 mol to about 1.0 mol;

- mixing said first solution to said second solution and then adding an alpha-hydroxycarboxylic, preferably citric acid to obtain a homogeneous solution;

- heating said homogeneous solution at a temperature ranging from about 70 to about 130 degrees centigrade while continuously stirring;

- adding to said homogeneous solution a polyalcohol, preferably ethylene glycol, and then raising the temperature in a range from about 90 to about 150 degrees centigrade to promote a polymerization reaction while continuously stirring said solution for about 1 to about 5 hours until a gel material is obtained;

- heating said material to a temperature ranging from about 400 to about 1500 degrees centigrade for about 0,5 to about 15 hours to burn the polymeric fraction of said gel material and to obtain a powder;

- quenching said powder to room temperature, in less than about 25 minutes by means of a suitable quenching method, to obtain a powder having at least one stable crystalline phase of the luminescent compound described by one or more of the Formulas 1, 2 or 3.
The method as recited in the preceding claim, characterized in that said quenching method is selected from the group consisting of: air quenching, forced air quenching, quenching in a mixture of cooled gases, liquid nitrogen quenching, liquid helium quenching, quenching in cryogenic gas, water quenching, quenching in a liquid medium or a combinations thereof.
A method for preparing the luminescent compound according to any of claims 1 a 7, said method comprising one or more of the following steps:

- mixing a silicon precursor, a bismuth precursor, an Yb precursor and at least a precursor of a dopant element selected from Ln or M, in said Formulas 1, 2 or 3, so as to obtain a mixture, wherein the molar ratio between said silicon precursor and all said precursors in said mixture, is preferably the same as the molar ratio between the silicon and the doped bismuth compound according to one of said Formulas;

- completely dissolving said mixture in a polyalcohol while continuously stirring to obtain a reaction solution;

- heating said reaction solution , at a temperature exceeding about 100 degrees centigrade for a time period of about 5 to about 60 minutes, and then raising the temperature in a range from about 150 to about 220 degrees centigrade;

- maintaining said reaction mixture at a substantial constant temperature for a time period of about 5 minutes to about 4 hours, to obtain a dispersion comprising one or more of the luminescent compounds described by one or more of the Formulas 1, 2 or 3;

- cooling to room temperature said dispersion, and then separating said compound, preferably by centrifugation, so as to obtain a particulate of said compound;

- purifying said particulate, preferably by redispersion in ethanol followed by centrifugation, to obtain a purified particulate;

- heating said purified particulate at a temperature ranging from about 300 to about 1500 degrees centigrade for about 30 minutes to about 12 hours, in such a way to increase the crystalline grade, size or concentration of the desired crystalline phase of said luminescent compounds.
A method for preparing the luminescent compound according to any of claims 1 a 7, said method comprising one or more of the following steps:

- dissolving a silicon precursor in distilled water to obtain a first solution;

- dissolving in a polyoil solution, preferably ethylene glycol or di-ethylene glycol, a bismuth precursor, an Yb precursor and at least a precursor of a dopant element selected from Ln or M, in said Formulas 1, 2 or 3, so as to obtain a second solution, wherein the molar ratio between said silicon precursor and all said precursors in said mixture, is preferably the same as the molar ratio between the silicon and the doped bismuth compound according to one of said Formulas;

- mixing said first solution and said second solution and subject the resulting mixture to an autoclave treatment at a temperature ranging from about 100 to about 400 degrees centigrade for a time period of about 1 to about 24 hours, to obtain a precipitate at the end of the reaction;

- cooling the autoclave to room temperature, collecting said precipitate and performing a heat treatment at a temperature ranging from about 200 to about 1500 degrees centigrade for a time period of about 1 to about 24 hours, to obtain a powder consisting of one or more of the luminescent compounds described by one or more of the Formulas 1, 2 or 3.
A method for preparing the luminescent compound according to any of claims 1 a 7, said method comprising one or more of the following steps:

- mixing a bismuth precursor, a silicon precursor, an Yb precursor and at least a precursor of a dopant element selected from Ln or M, in said Formulas 1, 2 or 3, so as to obtain a mixture, wherein the molar ratio between said silicon precursor and all said precursors in said mixture, is preferably the same as the molar ratio between the silicon and the doped bismuth compound according to one of said Formulas;

- accurately mixing said mixture by means of milling cycles, preferably by means of a ball mill, so as to obtain a finely pulverized homogeneous material;

- heating said pulverized material at a temperature ranging from about 300 to about 1500 degrees centigrade for a time period of about 1 to about 24 hours, so as to obtain at the end of the heating process a material consisting of one or more of the luminescent compounds described by one or more of the Formulas 1, 2 or 3;

- further milling said material to obtain the desired grain size.
A method for preparing the luminescent material according to one or more of claims 8 to 12, comprising one or more method according to one or more of the claims 23 to 32.
The method according to one or more of the claims 28 to 32 characterized in that said silicon precursor is at least one selected from the group consisting of: sodium silicate, TEOS, APTES, MPTS, silane, or a combination thereof.
The method according to one or more of the claims 28 to 32 characterized in that said bismuth precursor is at least one selected from the group consisting of: bismuth oxide, bismuth hydroxide, bismuth acetate, bismuth nitrate, bismuth chloride, bismuth acetylacetonate, bismuth gallate, bismuth citrate, bismuth subsalicylate, bismuth neodecanoate, or a combination thereof.
The method according to one or more of the claims 23 to 32 characterized in that said precursor of the elements Ln or M is at least one selected from the group consisting of: oxide, hydroxide, acetate, nitrate, chloride, citrate, acetylacetonate, or a combination thereof.
The method as recited in any of the claims 23 to 32, further comprising the step of adding at least one additional dopant agent to said bismuth precursor so as to cause the structure of said compound to exhibit: variations in the crystalline field or variations in the crystalline phase, or a combination thereof, wherein said variations are characterized in that at least one optical property of said compound is preserved, said optical property is selected in the group consisting of: up-conversion, down-conversion, down-shifting, scintillation or a combination thereof.
A method of converting an electromagnetic radiation characterized in that said method comprises one or more of the following steps:

- defining a suitable luminescence spectrum according to a specific application;

- defining a color function in a suitable color space, said function defining one or more peak wavelengths for a set of values of the parameters (M, Ln, x, y, z) in one or more of said Formulas 1, 2 or 3;

- matching said luminescence spectrum with one or more set of the parameters (M, Ln, x, y, z);

- selecting one or more compound s as recited in any of the claims 1 to 7 for each element (M, Ln, x, y, z) belonging to said set of values;

- obtaining a luminescent material according to one or more of claims 8 to 12 , said material having a composition corresponding to said one or more set of values (M, Ln, x, y, z) as determined in the preceding step;

- exposing said luminescent material to an exciting electromagnetic radiation of spectrum and peak wavelength (E₁( l ), l₁ ) , wherein said luminescent material emits an emitted radiation of spectrum and peak wavelength (E₂( l ), l₂ ), wherein said radiation is selected in the group consisting of: infrared electromagnetic radiation, ultraviolet radiation or visible radiation or a combination thereof.