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In order to keep our 3D printed parts safe from being illegally replicated, we often turn to anti-counterfeiting measures, such as embedded identification tags and QR codes, bar codes, and even quantum dots, which are tiny nanocrystals produced from semiconductor materials. Now, a group of researchers from Zhejiang University in China is looking at using upconversion (UC) nanoparticles for anti-counterfeiting.

The UC luminescence process absorbs low-energy photons, eventually converting them to high-energy ones. Nanoparticles that have been doped with lanthanide are being noticed in multiple fields for their use in applications like biological imaging, solar energy storage, point temperature sensing, and anti-counterfeiting. By making raw materials, like resins, functional through the use of components with various optical, magnetic, and electrical properties, we can increase the applications of the finished products.

The process for the synthesis of UC nanoparticles.

The researchers published a paper, titled “3D printing of resin composites doped with upconversion nanoparticles for anti-counterfeiting and temperature detection,” that explains their method for using SLA technology to 3D print organic resin that had been doped with inorganic UC nanoparticles.

The abstract reads, “In our process, the wet-chemistry derived NaYF4: RE (RE: rare earth) nanoparticles with red, green and blue UC emission were incorporated into a resin matrix. We printed out pre-designed 3D structures with high precision and examined the UC emission properties. In a proof-of-concept experiment, we demonstrate that the 3D printed objects have reliable optical anti-counterfeiting based on high concealment in daylight and multi-color UC emission excited by a near-infrared laser at 980 nm. We also show that the 3D part with UC emission can be used for ratiometric temperature sensing from 303.15 K to 463.15 K, making it possible to map the temperature distribution for studying the thermal diffusion process in complex objects.”

(b-d) Typical TEM images for NaYb0.99Er0.01F4, NaY0.8Yb0.19Er0.01F4 and NaY0.8Yb0.155Tm0.045F4 nanoparticles. (e) The XRD patterns for NaY0.8Yb0.19Er0.01F4 and NaY0.8Yb0.155Tm0.045F4 nanoparticles.

SLA 3D printing provides polymer composition with plenty of flexibility and design space for final parts, and adding UC nanoparticles into polymer resin can further increase applications for UC.

“In this work, UC nanoparticles of NaYb0.99Er0.01F4, NaYb0.99Er0.01F4 and NaY0.8Yb0.155Tm0.045Fprepared by wet chemistry were employed as emitters and suitable resin slurry was synthesized to prepare testing samples and pre-designed architectures with inner holes and hollow structures by stereolithography,” the researchers explained. “These organic-inorganic composites show bright UC emission under excitation of a 980 nm laser and high concealment in daylight. Ratiometric temperature sensing based on temperature dependence of intensity ratio of two emission peaks of the Er3+ ions is successfully presented.”

The team synthesized the UC nanoparticles, then performed transmission electron microscopy (TEM) analysis. The nanoparticles were then dispersed into IPDI and mixed with HEMA, before PEG was added; these steps made it possible to add the synthesized nanoparticles homogenously into the slurry, so it would be suitable for SLA 3D printing.

Schematic diagram of the slurry synthesis process, SLA, and luminescence of inorganic-organic composites containing UC nanoparticles of NaYb0.99Er0.01F4, NaY0.8Yb0.19Er0.01F4 and NaY0.8Yb0.155Tm0.045F4.

“The stereolithography was performed with a self-assembled printer whose laser power and scanning speed are adjustable,” the researchers wrote. “100 mm*100 mm*20 mm cubic organic-inorganic composites were printed out for the following tests with a resolution of 0.1 mm. The composite samples are transparent and colorless with sufficient mechanical strength.”

Upon recording the UC emission spectra of the nanocomposites, they determined that the ones with UC nanoparticles had wavelength peaks “in the expected spectral regions.”

Fig. 4: Samples for the illustration of anti-counterfeiting. (a) The hollow ring containing 0.5 wt% UC nanoparticles of NaY0.8Yb0.19Er0.01F4 is colorless and transparent in daylight and shows green emission under the excitation by a 980 nm laser. (b) The hollow ring without UC nanoparticles is indistinguishable with the counterpart containing nanoparticles in daylight and has no green emission under the excitation.

There’s an ever-growing need to develop accessible, inexpensive, and personalized anti-counterfeiting technology. Inorganic and organic luminescent materials based on the photoluminescence process have been used as luminescent inks before, and demonstrate long-wavelength visible emission when exposed to shorter-wavelength excitation, but these are easy to duplicate.

NIR-to-visible luminescent materials based on UC luminescence are more difficult to prepare than UV-to-visible luminescent materials, which are able to be designed with an emission color at a specific excitation power density; by controlling the doping concentration of RE ions, they also have a tunable luminescence lifetime. That’s why researchers, like the group from Zhejiang University, are investigating UC nanoparticles for anti-counterfeiting purposes.

“A striking feature of the lanthanide-doped fluoride nanoparticles we use is their strong UC emission upon excitation by NIR light,” the researchers explained. “As shown in Fig. 4, the hollow ring containing NaY0.8Yb0.19Er0.01F4 nanoparticles presents remarkable green emission under the excitation by a 980 nm laser. Besides, the ring doped with UC nanoparticles and the undoped are not distinguishable under daylight illumination.”

So even if counterfeiters can produce similar-looking fake products, they could potentially be identified quickly with a 980 nm laser.

Complex 3D structures containing UC nanoparticles, showing visible UC emission under excitation of a 980 nm laser.

The researchers also used their UC nanoparticles for temperature detection purposes.

In our experiment, we utilize the fluorescence intensities from two closely spaced energy levels: 2H11/2 and 4S3/2 in NaY0.8Yb0.19Er0.01F4 to measure temperature,” the researchers explained. “The 980 nm laser with an operating power of 3 W was used as the excitation source, and the sample emission spectra were recorded from 303.15 K to 463.15 K.”

The thermal stability of the resin composites was examined, and the team found that from 303.15 K to 463.15 K, the composite’s weight loss was fairly small, which could be because of moist air evaporation and unreacted components; either way, its specific heat capacity didn’t change.

“In summary, resin slurry containing UC nanoparticles suitable for stereolithography was successfully synthesized in our experiment,” the researchers concluded. “Exquisite and sophisticated 3D architectures which exhibit considerable UC emission under NIR light have been fabricated by stereolithography. Their anti-counterfeiting ability was investigated and the results indicated that their concealment in daylight and high luminescent and multi-color UC emission under 980 nm excitation are reliable.”

Co-authors of the paper are Rongping Ni, Bin Qian, Chang Liu, Xiaofeng Liu, and Jianrong Qiu.

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