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Abstract
Rapid prototyping (RP) techniques allow the construction of complex and sophisticated physical models based on personal needs, and the applications of the produced objects can be greatly extended by functionalizing the raw materials (e.g., resins) with components showing electrical, optical and magnetic properties. Here, we demonstrate a simple method for the realization of a three-dimensional architecture through 3D printing of organic resin doped with inorganic upconversion (UC) nanoparticles by using stereolithography technique. 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.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Upconversion (UC) luminescence is an anti-stokes process that absorbs low-energy photons and eventually converts them to high-energy photons. Lanthanide-doped nanoparticles which make use of the rich and long-lived f-f transitions of lanthanide ions to produce multi-color UC emission have sparked great interest in diverse fields, not only for their fundamental scientific importance, but also for their potential applications ranging from compact solid-state lasers, biological imaging and solar energy storage to anti-counterfeiting and point temperature sensing [1–5]. To make photonic devices and three-dimensional bulk materials, UC nanoparticles have been combined with ceramics and glasses [6,7]. To avoid inter-ion interaction in ceramics and glasses to improve efficiency, organic resin has emerged as an important bulk transparent host for UC materials. Polymers (like PDMS, PVP and PMMS) exhibit high impact resistance, light weight, good flexibility, and excellent processability [8–11]. Most polymer resin slurry can be processed by stereolithography which is regarded as the most versatile method with high accuracy and precision among rapid prototyping techniques [12]. In addition, stereolithography offers much flexibility for polymer composition and more design space for final parts, such as a Fresnel lens on the end of optical fiber, micro-scale mounts for single crystal analysis and tissue engineering scaffolds [12–14]. The incorporation of UC nanoparticles into polymer resin for stereolithography could significantly extends UC applications.
In this work, UC nanoparticles of NaYb0.99Er0.01F4, NaYb0.99Er0.01F4 and NaY0.8Yb0.155Tm0.045F4 prepared 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. 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.
2. Experimental demonstration and details
2.1 Materials
Yb(AC)3·4H2O (99.99%), Tm(AC)3·4H2O (99.99%), Er(AC)3·4H2O (99.99%) and Y(Ac)3·4H2O (99.99%) were purchased from Ansheng Inorganic Materials (Ganzhou, China). Oleic acid (OA), 1-octadecene (ODE), NaOH, NH4F, boron trifluoride-methanol (MEOH), 2-hydroxyethyl methacrylate (HEMA), isophorone diisocyanate (mixture of isomers) (IPDI), 2-hydroxyethyl acylate (used as diluent), polyethylene glycol (PEG, average Mn 300), Ditin butyl dilaurate (DBTDL), 2,6-di-tert-butyl-4-methylphenol (BHT) and 2-hydroxy-2-methylpropiophenone (photo-initiator 1173) were purchased from Aladdin Corporation Company (Shanghai, China). Tinuvin 1130 was purchased from BASF Corporation Company (Florham Park, NJ, USA). All of the chemicals were used as-received without further purification.
2.2 Synthesis of UC nanoparticles
The UC nanoparticles were synthesized using a co-precipitate method reported previously with minor modification as shown in Fig. 1(a) [15]. In a typical process for the synthesis of NaY0.8Yb0.155Tm0.045F4, Yb(AC)3·4H2O (0.124 mmol), Tm(AC)3·4H2O (0.036 mmol) and Y(Ac)3·4H2O (0.64 mmol) were added into a 50 mL three-neck flask together with 6 mL OA and 14 mL ODE. The mixture was heated to 423.15 K with constant magnetic stirring and hold for 30 min, and then cooled down to room temperature. Afterwards, 10 mL methanol solution containing 3.2 mmol NH4F and 2 mmol NaOH was added to the flask. The solution was stirred for 30 min at 323.15 K to remove the methanol by evaporation. After removal of the methanol, the solution was heated to 563.15 K in a nitrogen atmosphere and kept for 90 min. After cooling the solution to room temperature, ethanol was added to precipitate the nanoparticles. At last, the nanoparticles were collected by centrifugation. We used the same method to synthesize nanoparticles of NaYb0.99Er0.01F4 and NaY0.8Yb0.19Er0.01F4.
Fig. 1 (a) The process for the synthesis of UC nanoparticles. (b)-(d) Typical TEM images for nanoparticles NaYb0.99Er0.01F4, NaY0.8Yb0.19Er0.01F4 and NaY0.8Yb0.155Tm0.045F4. The corresponding size distributions are shown in the insets in (b)-(d). (e) The XRD patterns for the nanoparticles of NaY0.8Yb0.19Er0.01F4 and NaY0.8Yb0.155Tm0.045F4.
Transmission electron microscopy (TEM) analysis was performed on a FEG-TEM system (Tecnai G2 F30 S-Twin, Philips-FEI, Netherlands) operated at 300 kV. Figures 1(b)-1(d) show TEM results of the nanoparticles. Typical hexagonal crystals with an average size of around 25 nm were obtained for nanoparticles of NaY0.8Yb0.19Er0.01F4 and NaY0.8Yb0.155Tm0.045F4 as shown in Fig. 1(c) and 1(d). For samples with Yb3+ concentration of 99%, the hexagonal structure is no longer energetically favorable under the similar synthetic conditions [16,17], such that NaYb0.99Er0.01F4 nanoparticles crystallize into cubic with irregular shapes and varying sizes as shown in Fig. 1(a). Powder X-ray diffraction (XRD) patterns were characterized on a RIGAKU D/MAX 2550/PC diffractometer (Japan) with Cu Kα radiation (λ = 0.1542 nm). The XRD patterns for the nanoparticles of NaY0.8Yb0.19Er0.01F4 and NaY0.8Yb0.155Tm0.045F4 in Fig. 1(e) show that all the diffraction peaks can be ascribed to the hexagonal structure of pure crystalline β-NaYF4 (JCPDS no.16-0334). No appearance of other phases in the XRD patterns reveals that Yb3+/Tm3+, Yb3+/Er3+ have been successfully doped into the host lattices of β-NaYF4.
2.3 Synthesis of slurry containing UC nanoparticles and 3D printing of nanocomposites with UC emission
In Fig. 2, we show schematically the slurry synthesis process, stereolithography and UC emission of the test samples. To add the synthesized nanoparticles (NaY0.8Yb0.155Tm0.045F4, NaYb0.99Er0.01F4 and NaY0.8Yb0.19Er0.01F4) homogenously without agglomeration into the slurry, nanoparticles were dispersed into IPDI first, and then mixed with HEMA, forming short chains containing -NCO-. Then PEG was added to form longer chains by polymerization. These reagents were added dropwise to control the synthesis temperature between 333.15 K and 343.15 K in order to get the prepolymer with relatively uniform chain lengths. 2-hydroxyethyl acylate was used to diluent the prepolymer, adjusting viscosity and surface tension to make slurry suitable for stereolithography. Ditin butyl dilaurate (DBTDL), 2,6-di-tert-butyl-4-methylphenol (BHT) and 2-hydroxy-2-methylpropiophenone (photo-initiator 1173) were used as catalyst, polymerization inhibitor and photo-initiator during the synthesis process. Photoresist tinuvin 1130 was added eventually to control extra solidification during stereolithography. The stereolithography was performed with a self-assembled printer whose laser power and scanning speed are adjustable. 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. UC emission spectra of the nanocomposites containing 0.5 wt% NaYb0.99Er0.01F4 (red wavelength), NaY0.8Yb0.19Er0.01F4 (green wavelength) and NaY0.8Yb0.155Tm0.045F4 (blue wavelength) nanoparticles were recorded using an FLS-920 spectrometer. A 980 nm diode laser with an operating power of 3 W was employed as the excitation source. As shown in Fig. 3, the composites incorporated with UC nanoparticles show red, green and blue emission with wavelength peaks in the expected spectral regions. The incorporation into a polymer matrix does not influence the UC emission properties.
Fig. 2 Schematic diagram of the slurry synthesis process, stereolithography and luminescence of inorganic-organic composites containing UC nanoparticles of NaYb0.99Er0.01F4, NaY0.8Yb0.19Er0.01F4 and NaY0.8Yb0.155Tm0.045F4. The nanoparticles were added into IPDI under constant mechanical stirring. The diluents, the photo-initiator 1173 and the photoresist were then added into the prepolymer to make slurry suitable for stereolithography. Testing samples were prepared with a 355 nm laser. Under irradiation of a 980 nm laser, these samples show bright UC emission with red, green and blue wavelengths.
Fig. 3 UC luminescence spectra of resin samples, showing dominant emission peaks in red (NaYb0.99Er0.01F4), green (NaY0.8Yb0.19Er0.01F4) and blue (NaY0.8Yb0.155Tm0.045F4) spectral regions. The excitation wavelength is 980 nm.
3. Anti-counterfeiting
3.1 A hollow hand ring containing NaY0.8Yb0.19Er0.01F4
With the rapid expansion of small business, sole traders and personal demands, anti-counterfeiting technology is not only widely used in identity cards, banknotes, tags and important documents. There is an urgent need to develop a low cost, easily accessible, and personalized anti-counterfeiting technology. Both organic and inorganic luminescent materials based on the so-called ‘down-conversion’ process (e.g., photoluminescence) have been used as luminescent ink [18–20], which is featured by long-wavelength visible emission when exposed to shorter-wavelength excitation, mostly ultraviolet (UV) light. However, UV-to-visible down-conversion materials and UV excitation sources have become easily accessible in recent years, making it much easier to duplicate. In contrast, NIR-to-visible luminescent materials based on UC luminescence have complicated preparation process compared with UV-to-visible luminescent materials. Besides, UC materials can be designed with desired emission color at specific excitation power density [21] and tunable luminescence lifetime by controlling the doping concentration of RE ions [22,23]. Combined with the limited access to NIR laser source, the anti-counterfeiting based on UC is difficult to duplicate. Therefore, UC nanoparticles have been investigated in anti-counterfeiting [24–27].
A striking feature of the lanthanide-doped fluoride nanoparticles we use is their strong UC emission upon excitation by NIR light. 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. This ring can be embedded with precious metals, such as gold, silver and platinum or used as a base or pendant for these precious metals to enhance the value of art and substantive values. For counterfeiters, even if they are able to counterfeit products with similar appearance and quality, both the original manufacturer and the store salesmen can easily identify the fakes with a 980 nm laser. In addition, for each batch of commodities, manufacturers can accurately adjust the doping amount to change the luminescence wavelength and intensity ratio. This technique does not require labels that might disappear due to wear and tear. Anti-counterfeiting with UC nanoparticles has high uniqueness, security, easy verification and long preservation life
Fig. 4 Images of samples for the illustration of anti-counterfeiting photographed by a Canon EOS 750D camera with a 100 mm MACRO LENS. (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.
3.2 A hollow lamp shade containing NaYb0.99Er0.01F4 and a piece of rose containing NaY0.8Yb0.155Tm0.045F4
In order to test whether the resin slurry can be used for printing other functional parts during industrial process and test the anti-counterfeiting performance of composites containing NaYb0.99Er0.01F4 and NaY0.8Yb0.155Tm0.045F4, we printed out two other structures: a lampshade and a piece of rose, as shown in Fig. 5. These structures and the hollow ring in Fig. 4 which are hard to produce through traditional casting are easy with stereolithography. The hollow lamp shade containing NaYb0.99Er0.01F4 nanoparticles appears clear and transparent in daylight and gives red emission by excitation with a 980 nm laser. The rose containing NaY0.8Yb0.155Tm0.045F4 nanoparticles gives high luminescence with blue wavelength under excitation. High concealment and multi-color emission can make them behave a similar way as chameleons and provide a strengthened and more reliable anti-counterfeiting effect.
Fig. 5 Complex three-dimensional structures containing UC nanoparticles, showing visible UC emission under excitation of a 980 nm laser. (a) A hollow lamp shade containing NaYb0.99Er0.01F4 nanoparticles appears clear and transparent in daylight and gives red emission by excitation with a 980 nm laser. (b) A piece of rose containing NaY0.8Yb0.155Tm0.045F4 nanoparticles shows blue emission under excitation by a 980 nm laser. The two demonstrated structures are printed with resin slurry containing 0.5 wt% UC nanoparticles using laser scanning speed of 100 mm/s and laser power of 50 mW by stereolithography. The photos were obtained using a Canon EOS 750D camera with a 100 mm MACRO LENS.
4. Temperature-detecting
Temperature is a basic thermodynamic parameter. Optical temperature sensing technology (such as infrared, Raman and glowing) based on temperature-dependent optical response as a non-contact measurement method provides the advantage that the temperature field distribution of the object under test is not damaged and the sensitivity is also guaranteed. However, measuring the absolute fluorescence intensity from a particular level in a temperature-detection system, any change in excitation or in transmission would be interpreted incorrectly as changes in temperature. One way to avoid this problem is to measure the intensity of fluorescence from two different energy levels which have different temperature dependencies. The ratio of intensity provides a measurand that is essentially independent of transmission losses and fluctuation in excitation intensity [28,29]. Due to their closely spaced 4f levels, rare earth ions have been widely used in temperature sensing. 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 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.
4.1 Thermal properties after heat treatment
To exclude the potential influence of resin decomposition on the UC emission of the samples, we examined the thermal stability of the resin composites containing NaY0.8Yb0.19Er0.01F4 nanoparticles by TGA and DSC with a ME-51140728, METTLER TOLEDO instrument. The results are shown in Fig. 6. From 303.15 K to 463.15 K, the weight loss of the composite is quite small according to the TGA curve which may be due to evaporation of some moist air and unreacted components and its specific heat capacity does not change as shown from the smooth baseline in DSC. The glass transition temperature of the composite obtained from DSC is 602.15 K which is enough for amorphous thermoplastic in technological applications and sensor in point temperature detecting.
Fig. 6 TDA and DSC curves of the resin composite containing 0.5 wt% UC nanoparticles of NaY0.8Yb0.19Er0.01F4.
4.2 Temperature-dependent UC emission for temperature sensing
As depicted schematically in Fig. 7(a), the excitation of the two 4f levels 2H11/2 and 4S3/2 of erbium (Er3+) through absorption of two NIR photons is facilitated by the presence of ytterbium (Yb3+), which has a lager absorption cross section at 980 nm. As shown in Fig. 7(b), the two peaks centered at 525 nm and 544 nm which can be assigned to transitions from the 2H11/2 and 4S3/2 to the ground state of Er3+ demonstrate a clear evolution with temperature. The noticeable feature of this pair of excited levels is that they are close to each other, thus allowing the 2H11/2 level to be populated from the 4S3/2 by thermal agitation. The temperature T in kelvins can therefore be related with the ratio R of the two fluorescence intensities I525 nm and I544 nm, by 1/T = (k/)[In(C)-In(R)], where R = I525 nm/I544 nm, k is the Boltzmann constant, is the energy gap between the excited levels, and C is a constant depending on the two-level lifetimes, their electronic weight, and the detector sensitivities [30]. In Fig. 7(b), the peak intensities at 525 nm under each temperature are normalized with respect to the emission intensities at 544 nm. In doing so, we can see clearly that with the increasing of the temperature, the intensity at 525 nm decreased compared to that at 544 nm. Figure 7(c) shows a steady increase in the ratio R of fluorescence intensity at 525 nm to 544 nm as the temperature increases. There is a quite good linear relationship between the logarithm of R and the reciprocal of temperature T (1/K), as shown in Fig. 7(d). In order to verify the reusability of the composite doped with nanoparticles as a temperature sensor, we conducted cyclic tests of cooling and reheating of the heated composite. The logarithm of R and the reciprocal of temperature are all perfect linear and the slope remain unchanged. The results suggest that the organic-inorganic composite can be operated with an NIR semiconductor laser of moderate output power as an optical temperature sensor. With the help of a fiber optical spectrometer, it would be possible to map the dynamic temperature distribution of the composite. The main limitation of resin as host material is the reduced temperature detection range, compared with reported higher temperatures using silica hosts [31–33].
Fig. 7 (a) Energy-level diagram of Yb3+/ Er3+ pair and the UC processes under 980 nm excitation. Dotted and twisting arrows indicate nonradiative relaxation, while the solid arrows, labeled I525 nm and I544 nm, represent the radiative transitions used to measure fluorescence intensity ratio. The temperature sensitivity of the NaYF4: Er3+, Yb3+ nanoparticles depends on the population of the closely spaced 2H11/2 and 4S3/2 energy states. (b) Partial UC spectra (500 nm – 600 nm) of resin composite containing 0.5 wt% UC nanoparticles of NaY0.8Yb0.19Er0.01F4 with temperature changing from 303.15 K to 463.15 K. (c) Fluorescent intensity ratio between I525 nm and I544 nm versus temperature of the composite. (d) A plot of logarithm of the intensity ratio between I525 nm and I544 nm versus the inverse of temperature (1/T). The sample was cooled to ambient temperature and heated again to check the reproducibility of temperature sensing. The solid lines are fits to the experimental data.
5. Conclusion
In summary, resin slurry containing UC nanoparticles suitable for stereolithography was successfully synthesized in our experiment. 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. Ratiometric temperature sensing based on transitions between the 2H11/2 and 4S3/2 levels of Er3+ was realized in the range of 303.15 K to 463.15 K with high sensitivity. We also expect that, besides anti-counterfeiting and point temperature sensing, the incorporation of UC nanoparticles into resin slurry and the utilization of stereolithography technique will greatly extend their applications to diverse fields, such as artworks, bioimaging, data storage and solar cells.
Funding
National Key R&D Program of China (Grant No. 2018YFB1107200), the National Natural Science Foundation of China (Grant Nos. 1150432, 61775192, 51472091, 51772270), Open funds of State Key Laboratory of Precision Spectroscopy, East China Normal University and State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences.
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