QD Products

Upconverting Nanoparticles (UCNPs) and Composite Materials

 

Photoluminescence processing normally involves absorbing light at one wavelength while emitting light at another. There are two kinds of mechanisms in the photoluminescence process: linear down-conversion process and nonlinear up-conversion process. Typical down-conversion photoluminescence normally absorbs one high energy short wavelength photon and emits a low energy long wavelength photon. The up-conversion process is a nonlinear optics process that absorbs low energy light, usually near infrared (NIR) or infrared (IR), converts it to higher energies, visible, or ultraviolet, via multiple absorptions or energy transfer processes. The up-conversion process was observed in core-shell nanoparticles, transition metal, lanthanide and actinide ions doped solid state host. Among them, the highest efficiencies are found in lanthanide doped fluoride materials.


Up-converting nanoparticles (UCNPs) emit exceptional narrow photoluminescence peaks (Figure 1), with typical full-width-at-half-maximum (FWHM) values and full-width-at-10%-maximum (FW10%M) in the ranges less than 20 nm and 30 nm respectively. This is a sharp contrast compared to down-converting organic dyes or quantum dots in the range of 25-60 nm and 60-100 nm respectively. Another unique feature of UCNPs photoluminescence is their NIR or IR excitation, which is especially important for biological applications due to the minimal absorption of tissues in the NIR spectral range. UCNPs are promising materials for low noise deep tissue biolabels, imaging contrast agent, and drug delivery vesicles. UCNPs also has promising applications in NIR detection, NIR laser detection and positioning, and security barcodes.

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Figure 1. Comparison of FL emission of typical organic dye, CdSe/ZnS QDs, and UCNPs downconversion (FWHM: full width at half maximum; FW10%M: full width at 10% maximum).

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Figure 2.High resolution transmission electron microscopy (HRTEM) images of UCNPs in Mesolight.

 

Preparation of high quality (mono-dispersed, high upconversion efficiency, and long term storage stability) UCNPs is very challenging. The most developed solvothermal synthesis has good size control and monodispersion for UCNPs. However, it involves the use of trifluoroacetic acid (TFA) which is a very volatile, corrosive, and extremely hazardous agent. The decomposition of the metal trifluoroacetates also produces various hazard fluorinated chemicals and results in low up-conversion efficiency. Another issue is the multiple emission peaks of UCNPs. For example, NaYF4:Er/Yb and NaYF4:Tm/Yb UCNPs typically exhibit multiple combined emissions generated from different transition states, e.g., NaYF4:Er/Yb UCNPs typically exhibits green (550 nm) and red (655 nm) emissions.


Based on our expertise, Mesolight has developed high quality UCNPs which possess improved luminescence properties. UCNPs products in Mesolight absorb NIR laser excitations (e.g. 980 nm or 1064 nm) and emit visible or NIR emissions. There are four classes of high quality UCNPs products in Mesolight:

 

● Blue-emitting UCNPs (emission peak at 465 nm) (Figure 3)

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Figure 3.Upconverting emission spectrum of blue-emitting high color purity UCNPs (insert: optical picture of the emission) (excitation: 980 nm)

 

● Green-emitting UCNPs (emission peak at 550nm) (Figure 4)

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Figure 4.Upconverting emission spectrum of green-emitting high color purity UCNPs (insert: optical picture of the emission) (excitation: 980 nm)

 

● Red-emitting UCNPs (emission peak at 650 nm) (Figure 5)

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Figure 5.Upconverting emission spectrum of red-emitting high color purity UCNPs (insert: optical picture of the emission) (excitation: 980 nm)

 

NIR-emitting UCNPs (emission peak at 800 nm) (Figure 6)

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Figure 6.Upconverting emission spectrum of NIR-emitting (800 nm) high color purity UCNPs (excitation: 980 nm)

 

These UCNPs products dispersed in organic solvent feature high up-converting efficiency, high light output, high monodispersity, high solution dispersity, tunable size from 6-100 nm, exceptional narrow emission bandwidth, and also exception emission color purity (Figures 2-6).


These high quality UCNPs products can also be transfered from organic solvent into aqueous solution through our proprietary surface ligand modification technology. Figure 7 shows a solution of green-emitting UCNPs water solution, with high dispersity and high upconverting light output. UCNPs products transfer into aqueous media preserve their excellent photoluminescence properties.

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Figure 7. Green-emitting UCNPs in aqueous solution: (left) transparent UCNPs solution and (right) bright green emission when excited with a 980 nm diode laser.


Hybrid composite materials comprising high quality UCNPs embedded in a polymeric matrix have also been made in Mesolight (Figure 8). With our proprietary technology, we have able to fabricate UCNPs/polymer composite materials in a variety of polymer hosts.

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Figure 8. Six UCNPs/polymer composite materials transparent in the visible range (left) and their corresponding green upconverting emissions excited with a 980 nm diode laser (right).

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