Patent Publication Number: US-11649173-B2

Title: Continuous synthesis of upconverting nanoparticles

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Patent Application No. 63/013,857 entitled “CONTINUOUS SYNTHESIS OF UPCONVERTING NANOPARTICLES” and filed on Apr. 22, 2020, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates to continuous synthesis of upconverting nanoparticles in a fluidly coupled series of microwave reactors. 
     BACKGROUND 
     Upconverting nanoparticles (UCNPs) are nanoparticles that absorb two or more photons of relatively low energy (e.g., 800 nm to 1000 nm) and emit a single photon with higher energy (e.g., 350 nm to 800 nm). In this upconversion process, there is a large anti-Stokes shift between excitation and emission. The emission bands are narrow, and there is no overlap between the excitation and emission bands. UCNPs can be used for multimodal optical-electron microscopy or magnetic resonance imaging. These particles typically do not photobleach, even at high excitation power and duration, and can be excited by relatively low-power, continuous-wave infrared lasers. Since infrared excitation is not photodamaging, does not excite background fluorescence, and has a deeper penetration in tissues than UV or visible excitation, these nanoparticles can be used for live animal and cell imaging, archived tissue imaging, and intraoperative imaging. 
     SUMMARY 
     Methods described in this disclosure allow fabrication of upconverting nanoparticles (UCNPs) in a scalable, continuous flow fashion using a microwave reactor using moderately polar, high boiling point solvent (e.g., plasticizers mixed with oleic acid). These methods demonstrate an increase in production output over current methods by at least a factor of twenty, thereby making UCNPs more suitable for imaging applications (e.g., intraoperative imaging during surgery of tumors). 
     In a first general aspect, synthesizing upconverting nanoparticles includes heating a precursor solution comprising one or more rare earth salts, an alkali metal salt or an alkaline earth salt, and a solvent in a microwave reactor to yield a product mixture; and cooling the product mixture to yield the upconverting nanoparticles. The solvent comprises a plasticizer including one or both of dioctyl terephthalate and bis(2-ethylhexyl) adipate. 
     Implementations of the first general aspect may include one or more of the following features. 
     At least one of the one or more rare earth salts comprises trifluoroacetate. One or more of the rare earth salts comprise one or more of yttrium, ytterbium, and erbium. The solvent further comprises oleic acid. 
     A boiling point of the plasticizer at atmospheric pressure is at least 320° C. A dielectric permittivity of the plasticizer is at least 4 or at least 5. The plasticizer may further include dibutyl phthalate. 
     The first general aspect may include heating the precursor solution to a temperature of at least 280° C. or at least 305° C. Heating the product mixture can include heating for a length of time between 5 minutes and 45 minutes. 
     Some implementations of the first general aspect include flowing the precursor solution through the microwave reactor. 
     In a second general aspect, synthesizing core-shell upconverting nanoparticles includes heating a precursor solution comprising one or more rare earth metal salts, an alkali metal salt or an alkaline earth metal salt, and a solvent in a microwave reactor; combining the upconverting nanoparticles of the first general aspect with the precursor solution to yield a nanoparticle mixture; heating the nanoparticle mixture to yield a product mixture; and cooling the product mixture to yield the core-shell upconverting nanoparticles. The solvent comprises a plasticizer including one or both of dioctyl terephthalate and bis(2-ethylhexyl) adipate. 
     Implementations of the second general aspect may include one or more of the following features. 
     The upconverting nanoparticles can have a cubic crystal structure or a hexagonal crystal structure. A boiling point of the plasticizer at atmospheric pressure is at least 320° C. A dielectric permittivity of the plasticizer is at least 4 or at least 5. The plasticizer may further include dibutyl phthalate. 
     The second general aspect may include heating the precursor solution to a temperature of at least 280° C. or at least 305° C. 
     Some implementations of the second general aspect include flowing the precursor solution through the microwave reactor. 
     The details of one or more embodiments of the subject matter of this disclosure are set forth in the accompanying drawings and the description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS.  1 A and  1 B  are schematic representations of microwave flow reactor configurations for upconverting nanoparticle (UCNP) synthesis.  FIG.  1 A  depicts a batch apparatus, and  FIG.  1 B  depicts a continuous flow apparatus. 
         FIG.  2    shows a typical power and temperature vs. time plot using a commercial microwave reactor for making nanoparticles. 
         FIG.  3    is a transmission electron microscopy (TEM) image of NaYF 4 :ER 0.02 , Yb 0.2  UCNPs synthesized by heating to 300° C. for 5 minutes. 
         FIG.  4    shows typical luminescence for NaYF 4 :Er 0.02 , Yb 0.2  excited with a 980 nm laser. 
         FIG.  5    shows X-ray diffraction patterns of UCNPs as a function of solvent composition. 
         FIG.  6    is a TEM image of UCNPs having a cubic crystal system. 
     
    
    
     DETAILED DESCRIPTION 
     The ability to govern the nucleation and growth processes in liquid media can be a key to controlling the features of nanoparticles. Solvothermal synthesis of upconverting nanoparticles (UCNPs) involves the decomposition of precursors of fluoride and lanthanide ions at high temperature in the presence of a coordinating ligand and a high-boiling point solvent medium for decomposition reaction to occur. A fine control of the reaction temperature can aid in obtaining UCNPs that are homogeneous both in size and crystalline structure. A high degree of monodispersity can require a high rate of nucleation leading to burst nucleation that precludes further nucleation during nanoparticle growth. The cubic (α) to hexagonal (β) transformation requires prolonged heating for completion. 
     Conventional heating depends on convection currents and the thermal conductivity from the heating device to the medium. A gradual temperature increase can affect nucleation and produce broader nanoparticle size distributions. It can also yield a mixture of particles with cubic and hexagonal crystalline phases, since the reaction time is measured only once the internal temperature is stabilized. On the other hand, using microwave-assisted synthesis, the reaction can be brought up from room temperature to 300° C. in minutes, since heat can be rapidly and uniformly generated within a certain depth of the reaction mixture, depending on the ability of the reaction mixture components to absorb microwave radiation. 
     This disclosure describes synthesis of UCNPs by preparing a precursor solution, heating the precursor solution in one or more heating phases to nucleate and grow the UCNPs, and quenching the reaction. The UCNPs can be prepared in 15-150 minutes reactor time in a one-pot synthesis method. Due at least in part to fast and homogeneous microwave heating, it is possible to obtain a narrow size distribution of 10 nm to 20 nm. The nanoparticles can form stable colloidal dispersions (e.g., in isopropanol) at concentrations as high as 2 mg/mL. 
     Preparing the precursor solution includes combining a solvent and precursor salts in stoichiometric amounts to yield a mixture. The mixture is heated while stirring under alternating inert gas and vacuum to remove residual air and water to yield the precursor solution. The precursor solution is rapidly heated to a temperature above 280° C. in a microwave reactor. Heating the mixture can include two or more intermediate heating phases (e.g., typically at 110° C., 220° C., or up to 280° C. for cubic-phase crystalline particles, or 305° C. for hexagonal phase crystalline particles). Each heating phase can range from about 5 minutes to about 30 minutes in length. After nanoparticle nucleation and growth, the reaction is quenched by cooling the mixture with compressed air to room temperature (e.g., with a minimum average cooling rate of 20° C./min) to yield UCNPs. 
     Suitable precursor salts include acetate and trifluoroacetate salts of alkali metals, alkali rare earth metals, and rare earth elements (Sc, Y, and lanthanides). The precursor salts amounts are calculated stoichiometrically for a final combined rare earth concentration between 1 to 30 millimolar, such that the resulting nanoparticles include a crystalline matrix of a fluoride, for example, NaYF 4 , NaGdF 4 , KGdF 4 , LiYF 4 , YbF 3  or YF 3 , doped with lanthanides (i.e., partially replacing the Y or Gd ion from 0.1 to 100% with one or more lanthanide ions), typically, Er/Yb, Tm/Yb, or Tb/Yb (e.g., NaYF 4 ). 
     The solvent is typically an organic solvent. Suitable solvents include a mixture of oleic acid and a high boiling point (e.g., 320° C. or greater), high dielectric permittivity (e.g., at least 4) plasticizer (e.g., dibutyl phthalate, dioctyl terephthalate, bis(2-ethylhexyl) adipate). Other high boiling point plasticizer esters with a dielectric permittivity larger than 4 are also suitable. The high boiling point, good chemical stability at high temperature, and moderately polar character makes di-n-butyl-phthalate well suited for microwave synthesis. Oleic acid is used as a ligand to control the size and shape of the nanoparticles. Non-coordinating solvent 1-octadecene is a very low-microwave absorber and thus not ideal for this application. Table 1 lists boiling point and dielectric permittivity (ε) of various solvents. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Solvent properties 
               
            
           
           
               
               
               
               
            
               
                   
                 Solvent 
                 Boiling Point (° C.) 
                 ε 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 1-octadecene 
                 315 
                 2.1 
               
               
                   
                 oleic acid 
                 350 
                 2.46 
               
               
                   
                 di-n-butyl-phthalate 
                 340 
                 6.43 
               
               
                   
               
            
           
         
       
     
     Core-shell nanoparticles can be synthesized by mixing a suspension of cubic or hexagonal core nanoparticles (e.g. NaYF 4  Er, Yb) with a precursor solution (e.g., a precursor made with acetate and trifluoroacetate salts of sodium and ytterbium in stoichiometric ratios to yield NaYbF 4 ) to yield a mixture, and heating the mixture in a microwave reactor with a program similar to that used to obtain core upconversion nanoparticles to yield, for example NaYF 4 :Er, Yb@ NaYbF 4  (core @ shell) nanoparticles. 
       FIGS.  1 A and  1 B  depict microwave flow reactor configurations for UPNP synthesis.  FIG.  1 A  depicts a batch apparatus  100  for UPNP synthesis. Apparatus  100  includes reaction vessel  102  positioned in microwave reactor  104  for receiving the precursor solution. As depicted, reaction vessel  102  is a round bottom flask. However, reaction vessels of other sizes and shapes can be used. Apparatus  100  includes stirrer  106 , temperature sensor  108 , and microwave power control feedback  110 . Opening  112  in reaction vessel  102  provides access for flow of an inert gas (e.g., argon) and evacuation inside the reaction vessel. The flow of gas and evacuation can be alternated. 
       FIG.  1 B  depicts continuous flow apparatus  120  for synthesis of UPNPs by continuously flowing a precursor solution through the apparatus. Reaction vessel  122  is depicted as a spiral borosilicate glass tube positioned in microwave reactor  124 . However, reaction vessels of other sizes and shapes can be used. A precursor solution is provided to inlet  126 , and a nanoparticle suspension leaves reaction vessel  122  through outlet  126 ′. Continuous flow apparatus  120  includes temperature sensor  128  and microwave power control feedback  130 . In one example, a particle yield of 30 mg UCNPs per minute can be obtained in an apparatus having a reaction vessel with a volume per coil of 50 mL, 3 coil turns, a precursor flow rate of 15 mL/minute, a residency time of 10 minutes, and a particle concentration of 2 mg/mL. 
     In many microwave reactors the sample usually is irradiated at one point rather than over the whole sample volume. Because different materials absorb microwaves differently, efficient mixing and fast-responding temperature control is advantageous. Reactor flow, such as that provided by the apparatus depicted in  FIG.  1 B , allows efficient mixing on a macro- or microfluidic scale. 
     The heating rate in a microwave reactor correlates with the complex dielectric permittivity of the reaction components. The microwave absorption cross-section and penetration depth depend on the real part of the dielectric permittivity ε′, while heating due to the dielectric loss through dissipative phenomena is represented by the imaginary part of the dielectric permittivity ε″. 
     As described herein, a mixture of high boiling point polycarboxylic acid esters and oleic acid (OA) is used in the formation UCNPs. Polycarboxylic acid esters (e.g., bis(2-ethylhexyl) adipate (BEHA), di-n-butyl phthalate (DBP), and di-iso-octyl terephthalate (DOTP)) act as solvents with high boiling points and temperature stability. They are soluble in most organic solvents, and have adequate polar and microwave absorbing characteristics. 
     In some implementations, a continuous flow apparatus includes one or more (e.g., three) fluidly coupled microwave reactors configured such that the precursor solution flows sequentially through each reactor to yield UCNPs. 
       FIG.  2    shows a typical power and temperature vs. time plot using a commercial microwave reactor for making UCNPs. For the plot in  FIG.  2   , the precursor solution was heated for 5 minutes to 300° C. 
       FIG.  3    shows a transmission electron microscopy (TEM) image of NaYF 4 :ER 0.02 , Yb 0.2  UCNPs synthesized by heating a precursor solution to 300° C. for 5 minutes. 
       FIG.  4    shows typical luminescence for NaYF 4 :Er 0.02 , Yb 0.2  excited with a 980 nm laser. 
     EXAMPLES 
     Example 1. Trifluoroacetate (TFA) salts of sodium, yttrium, ytterbium, and erbium were dissolved in a mix of oleic acid and di-butyl-phthalate at different ratios by first heating the solution at 120° C. for 10 minutes, followed by heating at 315° C. for 10 to 120 minutes. Trifluoroacetate salts and di-n-butyl phthalate are good microwave absorbers and allow the temperature to rise from 120° C. to 315° C. in approximately 5 minutes. The microwave reactor was a CEM Discover using single mode and continuous power at 2.45 GHz. 
     Colloidal nanoparticle dispersions in 2-propanol at 5 mg/ml were excited with a 500 mW, 980 nm laser. The lifetimes of the green (540 nm) and the red (655 nm) emission were obtained by exciting the samples with 10 ms laser pulses. The emitted light was focused on a Jobin-Yvon HR spectrometer, and detected using a photomultiplier and an averager oscilloscope. The lifetime of these nanoparticles ranges from 200 to 150 μs at 540 nm, and from 320 to 250 μs at 655 nm, and follows a decreasing trend with increasing synthesis time. 
     Example 2. A single mode 2.45 GHz microwave reactor (Discover, CEM, Matthews N.C., USA) equipped with a high temperature PTFE-spill cup and an immersion optical fiber thermometer supplied by its manufacturer was used. All pure solvents were degassed under vacuum and magnetic stirring at room temperature for at least 30 minutes prior to being used. A stock rare-earth precursor solution was prepared in advance and stored up to two months before use. Briefly, a 100 mM total rare earth concentration solution was prepared in advance and stored. 20 mL of oleic acid was degassed under vacuum and stirred for at least 30 minutes in a round bottom flask. Reagents were mixed in stoichiometric proportions to obtain NaY 0.78 F 4 :Yb 0.2 , Er 0.02 . Briefly, 773.05 mg YTFA (TFA=trifluoroacetate), 151.98 mg YbAc (Ac=acetate), 14.99 mg ErAc, 205.2 mg NaTFA, and 125.19 mg NaAc were added to the flask. The mixture was heated to 100° C. in a microwave reactor under argon flow and stirred for 10 minutes. The stock precursor solution was stored at room temperature for a minimum of 45 minutes before using it, and up to two months. 
     An aliquot of the nanoparticle precursor stock solution was combined with an oleic acid (OA): bis(2-ethylhexyl) adipate (BEHA) mixture at different proportions (S1 to S6, see Table 2) into a 125 mL round-bottom flask. The flask was placed in a microwave cavity and kept under a gentle argon flow and stirring using a glass-coated magnet for 10 minutes, and then heated to 300° C. for 5-45 minutes. At the end of the heating program the flask was cooled down to room temperature by a stream of compressed air. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Synthesis parameters 
               
            
           
           
               
               
               
               
            
               
                   
                 Sample 
                 OA (% v/v) 
                 BEHA (% v/v) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 S1 
                 80.0 
                 20.0 
               
               
                   
                 S2 
                 65.0 
                 35.0 
               
               
                   
                 S3 
                 50.0 
                 50.0 
               
               
                   
                 S4 
                 35.0 
                 65.0 
               
               
                   
                 S5 
                 16.7 
                 83.3 
               
               
                   
                 S6 
                 8.4 
                 91.7 
               
               
                   
               
            
           
         
       
     
     Scaled-up continuous flow synthesis: The mixture of the nanoparticle stock precursor solution with oleic acid and a carboxylic acid ester was flowed through microwave reactor coils to heat it to a temperature above 280° C. with a residence time from 5 to 45 minutes. 
       FIG.  5    shows X-ray diffraction patterns of the resulting UCNPs as a function of the OA-BEHA ratio. Sample S1 was consistent with sodium erbium ytterbium yttrium fluoride (reference code 04-022-6704) with an empirical formula of Er 0.01 FY 2 Na 0.5 Y 0.39 Yb 0.1  and having a cubic crystal structure. Sample S6 was seen to be consistent with sodium erbium ytterbium yttrium fluoride (reference code 00-028-1192) with an empirical formula of Er0.04F 4 NaY 0.57 Yb 0.39  (chemical formula Na(Y 0.57 Yb 0.39 Er 0.04 )F 4  and having a hexagonal crystal structure.  FIG.  5    is a transmission electron microscopy image of Sample S5, prepared with a microwave residence time of 15 minutes at 300° C. The nanoparticles have a hexagonal crystalline structure, and diameters ranging from about 40 nm to about 130 nm, with a majority of the nanoparticles having a diameter of about 100 nm. 
     Although this disclosure contains many specific embodiment details, these should not be construed as limitations on the scope of the subject matter or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this disclosure in the context of separate embodiments can also be implemented, in combination, in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. 
     Particular embodiments of the subject matter have been described. Other embodiments, alterations, and permutations of the described embodiments are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results. 
     Accordingly, the previously described example embodiments do not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure.