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Applications of Polymer, Composite, and Coating Materials

Siloxane-Encapsulated Upconversion Nanoparticles Hybrid Composite with Highly Stable Photoluminescence against Heat and Moisture Su Keun Kuk, Junho Jang, Hyeuk Jin Han, Eunsang Lee, Hyeongyeol Oh, Hwea Yoon Kim, Jinhyeong Jang, Kang Taek Lee, Hohjai Lee, Yeon Sik Jung, Chan Beum Park, and Byeong-Soo Bae ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20782 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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ACS Applied Materials & Interfaces

Siloxane-Encapsulated Upconversion Nanoparticles Hybrid Composite with Highly Stable Photoluminescence against Heat and Moisture Su Keun Kuk†‡, Junho Jang†§‡, Hyeuk Jin Han†, Eunsang Lee⊥, Hyeongyeol Oh⊥, Hwea Yoon Kim#, Jinhyeong Jang†, Kang Taek Lee⊥, Hohjai Lee⊥, Yeon Sik Jung†, Chan Beum Park†*, and Byeong-Soo Bae†§*

†Department

of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea §Wearable

Platform Materials Technology Center (WMC), Korea Advanced Institute of Science

and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ⊥Department

of Chemistry, Gwangju Institute of Science and Technology (GIST), 123

Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea #Advanced

Material Research Center, Samsung Advanced Institute of Technology (SAIT),

Samsung Electronics Co KEYWORD: upconversion nanoparticles, siloxane-hybrid composite, sol-gel reaction, photoluminescence stability, micro-barcodes.

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ABSTRACT

Herein, we report a siloxane-encapsulated upconversion nanoparticle hybrid composite (SEUCNP) which exhibits excellent photoluminescence (PL) stability for over 40 days even at elevated temperature, in high humidity, and in harsh chemicals. The SE-UCNP is synthesized through UV-induced free-radical polymerization of sol-gel derived UCNP-containing oligosiloxane resin (UCNP-Oligosiloxane). The siloxane matrix with random network structure by Si-O-Si bonds successfully encapsulates the UCNPs with chemical linkages between the siloxane matrix and organic ligands on UCNPs. This encapsulation results in surface passivation retaining intrinsic fluorescent properties of UCNPs under severe conditions (e.g., 85 °C, 85% relative humidity) and a wide range of pH (from 1 to 14). As an application example, we fabricate a two-color binary micro-barcode based on SE-UCNP via a low-cost transfer printing process. Under near-infrared irradiation, the binary-sequences in our barcode are readable enough to identify objects using a mobile phone camera. The hybridization of UCNPs with a siloxane matrix provides the capacity for highly stable UCNP-based applications in real environments.


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1. Introduction Upconversion nanoparticles (UCNPs) that emit visible light under near-infrared (NIR) irradiation have attracted huge attention for multimodal imaging, cancer therapy, and displays due to their unique properties such as large anti-Stokes shift, sharp emission bands, and low autofluorescence. 1-2

In addition, UCNPs are easily integrated as composites with other various materials for the

synergistic effect of characteristic modification and application.3 The UCNP-based composites improve properties or endow new functionalities of UCNPs and have been widely utilized in various fields.4 In particular, hybridization of UCNP with polymers by simple blending is one of the most popular techniques for the fabrication of hybrid composites.5-7 The integration of UCNPs into the transparent polymer to create an inorganic/organic hybrid material on the macro- or microscale is an effective strategy to attain high-performance UCNP-based applications, which possess the characteristics of both the inorganic UCNPs (multi-photon absorption and anti-Stokes shift) and the organic polymers (flexibility, processibility, transparency, and light-weight). These UCNP/polymer composites are suitable for various applications such as display, sensor, actuator, patterning, encoding, and drug delivery.8-13 However, the emission from UCNPs inside composite is easily weakened by the quenching effect that originates from energy transfer to a non-radiative energy level in surface defects, impurities, ligands, and solvents.14 Notably, the non-radiative relaxation of excited energy level state from surface quenching increases with the rise in temperature, the existence of water molecules, and chemical damage from pH change.15-18 The emission quenching of UCNPs from diverse causes is a huge hurdle to the operation of UCNP-based composites under real environments.


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Here, we report a siloxane-encapsulated UCNP (SE-UCNP) hybrid structure which possesses high fluorescent stability for practical UCNP-based application, as depicted in Figure 1. Compared to hydrocarbon-based polymers, the sol-gel-derived siloxane-based hybrid materials allow low-cost fabrication, adjustable composition, and high thermal stability.19-20 In particular, a silica (SiO2)-like amorphous matrix with random network structure induced by Si-O-Si bonds, provides ease of functionality with diverse organic groups, which can efficiently form composites with various nanomaterials. The Si-O bonds, major bonds of siloxane hybrid materials, have high bonding energy that enhances the physical and chemical stabilities of the nanomaterials encapsulated in the composite.21-24 We found that the siloxane matrix successfully passivates the UCNP’s surface and the SE-UCNP maintained its initial emission property for over 40 days even at a raised temperature, under extreme pH, and in highly humid conditions. As an application example, we fabricated a SE-UCNP-based micro-barcode using a low-cost transfer printing method. The two-color binary encoded micro-barcode was discernible and decodable using mobile phone cameras upon NIR irradiation.

2. Experimental Section 2.1. Synthesis of UCNP dispersed in chloroform. Green emitting NaYF4: Yb (18%), Er (2%) UCNPs were prepared by a coprecipitation method.25-26 YCl3 (0.8 mmol), YbCl3 (0.18 mmol), and ErCl3 (0.02 mmol) were dissolved in 5 mL of methanol and added to a mixture of oleic acid (OA, 6 mL) and 1-octadecene (15 mL). The reaction mixture was heated to 150 °C with vigorous stirring under argon atomsphere until a yellowish transparent solution occurs and then cooling to 50 °C. A methanol solution (10 mL) containing NaOH (2.5 mmol) and NH4F (4 mmol) was slowly


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added into the mixed solution and stirred for 30 min. The solution was then heated to 110 °C for 30 min to remove methanol and water, followed by heating to 305 ℃ for 1.5 h under argon atmosphere. Afterward, the solution was naturally cool to room temperature. The UCNPs were precipitated by the addition of ethanol and washed three times with 1:1 ethanol and water (v/v). The nanoparticles were dissolved in chloroform at the concentration of 20 mg mL-1. For the synthesis of blue emitting NaYF4:Yb (20%), Tm (0.5%) UCNPs, the above process was followed, except for the addition of YCl3 (0.795 mmol), YbCl3 (0.2 mmol), and TmCl3 (0.005 mmol). 2.2. Synthesis of UCNP-Oligosiloxane resin. UCNP-Oligosiloxane resin was synthesized by an in-situ sol-gel condensation27 reaction between diphenylsilanediol (DPSD) and 3methacryloxypropyltrimethoxysilane

(MPTS) in the presence of UCNPs (concentration of

UCNPs: 0.5 wt% of obtained resin). In a two-neck flask, DPSD, MPTS, and the UCNPs were blended by magnetic stirring. Ba(OH)2 · H2O was subsequently added to induce the sol-gel condensation at 80 °C with 6 hours nitrogen purging for solvent evaporation. Following the solgel reaction, photo-initiator (2,2-dimethoxy-2-phenylacetophenone, BDK) was added into the UCNP-Oligosiloxane resin for the UV-induced photopolymerization to obtain the SE-UCNP. 2.3. Synthesis of UCNP/Acrylate resin. UCNP/Acrylate resin was formed by mixing of the UCNPs (concentration of UCNPs: 0.5 wt% of obtained resin) and diacrylate functionalized aliphatic oligomer resin at 80 °C with 6 hours nitrogen purging for solvent evaporation. Afterward, BDK was added to the UCNP/Acrylate resin as a photo-initiator for the UV-induced photopolymerization to obtain the solidified UCNP/Acrylate. 2.4. Preparation of SE-UCNP and UCNP/Acrylate. The UCNP-Oligosiloxane and UCNP/Acrylate resins were dispensed in a stainless steel mold (thickness: 1 mm, radius: 20 mm


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or 200 mm) covered on both sides by glass substrates. The surface of glass substrates was treated with trichloro(octadecyl)silane (OTS) to promote easy separation of the cured films from the glass substrates after the UV (λ = 365 nm)-induced free-radical polymerization. 2.5. Fabrication of SE-UCNP micro-barcodes. To fabricate the polydimethylsiloxane (PDMS) mold, pre-polymers and a curing agent (Sylgard 184) were mixed at a weight ratio of 10:1, and the air bubbles in the mixture were removed by degassing in a vacuum. The mixture was then poured onto a silicon wafer and baked at 150 °C for 30 min using a convection oven. By retracting the cured PDMS from the template, surface patterns of the template were securely replicated on the mold. UCNP-oligosiloxane resin was filled into the PDMS mold by a microneedle. UCNP-oligosiloxane resin was then screen printed on an elastomeric PDMS stamp to replicate the surface patterns of the mold. The PDMS pad was brought into contact with various receiver substrates, and the mild pressure was applied uniformly on the back side of the pad with UV light. After 1 min, the pad was retracted for transfer printing, by which patterned SE-UCNP micro-barcodes could be obtained.

3. Results and Discussion The synthesis process of SE-UCNP composite is illustrated in Figure 1b–c. First, we synthesized monodispersed sub-50 nm [NaYF4:Yb (18%), Er (2%)] UCNPs which were passivated by OA (Figure S1a-b).25-26 The hexagonal phase UCNPs emitted major green photoluminescence (PL) at 522 and 542 nm and minor red PL at 656 nm, under 980 nm NIR laser irradiation (Figure S1c-d). We synthesized UCNP dispersed oligosiloxane resin (UCNP-Oligosiloxane resin) via an in-situ sol-gel condensation reaction of silane precursors containing MPTS and DPSD with OA-


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passivated UCNPs.27 When the UCNP-dispersed chloroform solution was mixed with silane precursors, hydrophobic interactions of the OA ligands on UCNP with the methacrylate groups in MPTS and the phenyl groups in DPSD induced the uniform encapsulation of UCNPs through solgel condensation reaction. Afterward, the sol-gel reaction between the hydroxyl groups of DPSD and methoxide groups of MPTS near UCNPs formed the siloxane bonds of the siloxane network.28 The synthesized viscous UCNP-Oligosiloxane resin showed no aggregation of UCNPs in the siloxane matrix (inset in Figure 1b). Lastly, a SE-UCNP was solidified via UV-induced free-

Figure 1. (a) Acquisition of an encoded SE-UCNP hybrid composite by a general mobile phone camera under NIR exposure (980 nm, Power density of 1 W cm-2). (b) Process of in-situ sol-gel condensation reaction of silane precursors (DPSD and MPTS) in the presence of UCNPs. The inset displays photographs of synthesized UCNP-Oligosiloxane resin under NIR irradiation (980 nm, Power density of 1 W cm-2). (c) Free-radical photopolymerization among C=C bonds of methacrylate functional groups in MPTS and oleic acids. The inset displays photographs of a cured SE-UCNP under NIR irradiation (980 nm, Power density of 1 W cm-2).


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radical addition reaction. The UV curing engendered the formation of carbon single bonds by cross-linking reactions not only between the C=C bonds of MPTS but also between the C=C bond of MPTS and the C=C bond of OA (Figure 1c).29 These two cross-linking reactions imply that the amorphous siloxane matrix network molecularly encapsulates the UCNP and passivates its surface with additional chemical linkages during solidification. The passivation of the UCNP surface by a robust siloxane network is able to diminish the surface defects of the nanoparticles.14, 30 In addition, the chemical linkages between OA and siloxane matrix in SE-UCNPs are possible to decrease the vibrational energy of OA surfactant, which is known to be a cause of non-radiative fluorescence quenching.31 To validate the siloxane bonds by sol-gel condensation reaction and cross-links after UVcuring in UCNP/siloxane hybrid composite, we analyzed the UCNP-Oligosiloxane resin and SEUCNP using Fourier transform infrared (FT-IR) spectroscopy. As shown in Figure 2a, the bands at 1000-1100 cm-1, which correspond to the stretching of the siloxane bond (Si-O-Si), were observed in both UCNP-Oligosiloxane resin and the SE-UCNP. The result indicates that the solgel reaction between the hydroxyl groups of DPSD and the methoxide groups of MPTS for the siloxane bond formation occurred successfully and the UCNPs did not hinder this reaction of silane precursors. Moreover, the bands corresponding to the C=C bond at 1630-1680 cm-1 (C=C stretching) and 950-1000 cm-1 (=C-H bending) of the UCNP-Oligosiloxane resin were observed from the methacrylate group in the oligosiloxane resin and the OA on the surface of UCNPs. However, these C=C bands decreased significantly in the SE-UCNP, which is attributed to the cross-linking among C=C bonds of the methacrylate groups and the OA in the UCNPOligosiloxane resin by the free-radical addition reaction, as shown in Figure 1c. This cross-linked bond formation indicates the surface passivation of UCNPs by siloxane matrix with Si-O-Si bonds.


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Figure 2. (a) FT-IR Spectra of bare UCNP, bare Oligosiloxane resin, UCNP-Oligosiloxane resin and SEUCNP. (b) Photoluminescence spectra of UCNP chloroform solution, UCNP-Oligosiloxane, and SEUCNP (980 nm NIR irradiation, Power density of 1 W cm-2). (c) TOF-SIMS 3D images of the SE-UCNP: Si, Y, Yb, and Er (1000 µm3 of cube).

Figure 2b compares the PL spectra for bare UCNP dispersed chloroform solution, UCNPOligosiloxane resin, and SE-UCNP. The positions of emission (λ = 540 nm) peak remained unchanged during the SE-UCNP synthesis processes, including high-temperature (80 °C) sol-gel condensation and UV-curing steps. To investigate the distribution of UCNPs in the siloxane matrix, we obtained 3D images of visualized element distribution information about the SE-UCNP using time-of-flight secondary ion mass spectrometric analysis (TOF-SIMS). As shown in Figure 2c, the Si (from siloxane matrix), Y, Yb, and Er (All from UCNPs) were uniformly dispersed in


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the cube of 1000 µm3. Additional scanning electron microscope/energy dispersive spectrometer (SEM/EDS) analysis also confirmed the distributed UCNPs inside the SE-UCNP film (Figure S2). These results demonstrate that the UCNPs in the SE-UNCP were homogeneously dispersed inside the siloxane matrix without degradation during the synthesis process. We investigated the PL stability of the SE-UCNP under high-temperature conditions (Figure 3a). For comparison, we tested commercial acryl polymer mixed with UCNPs (UCNP/Acrylate). The acrylate polymer was composed of diacrylate functional groups based on a hydrocarbon backbone but without siloxane bonds (Figure S3). The UCNP/Acrylate was prepared by simple blending of a UCNP solution with acrylate oligomer resin, followed by solidification via UVinduced photopolymerization. The SE-UCNP maintained its initial PL intensity during 40 days aging test at 85 °C, in the air with 5% relative humidity (RH), whereas UCNP/Acrylate showed a drastically decreased PL intensity (c.a. 50% degradation after 40 days). After initial PL drop, the PL of UCNP/Acrylate showed a continuous decrease during aging due to thermal decomposition of acrylate at high temperature in Figure S4. In addition, we fitted the fluorescence lifetime decay curve at 542 nm to explore the non-radiative quenching of SE-UCNP and UCNP/Acrylate by hightemperature aging (Figure 3b). The fluorescence lifetime (0.29 ms) of the SE-UCNP did not change after the aging treatment, but the UCNP/Acrylate exhibited much shorter lifetime (from 0.28 to 0.11 ms) after the aging treatment. The PL quenching and the lifetime decrease of UCNPs at high temperature stem from the increase of the non-radiative quenching rate with elevated temperature.15 The preserved PL and lifetime of SE-UCNP at high temperature demonstrate that the siloxane encapsulation reduced the temperature dependence of UCNP's surface quenching. We ascribe the declined surface quenching of the SE-UCNP at the high temperature to surface passivation of UCNPs by rigid siloxane matrix with chemical cross-links, as we mentioned above.


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Figure 3. (a) Change of PL emission intensity of SE-UCNP and UCNP/Acrylate after 40 days aging at 85 °C and 5% RH (980 nm NIR irradiation, Power density of 1 W cm-2, number of experiments = 3). (b) Fluorescence lifetime decay dynamics of SE-UCNP and UCNP/Acrylate before and after aging at 85 °C and 5% RH. The lifetime is calculated by fitting the decay curve with a single exponential function. (c) TGA of bare siloxane and SE-UCNP. The ramping rate is 10 °C/min under N2 gas. (d) 29Si NMR analysis of UCNP-Oligosiloxane resin. Inset scheme is the chemical structure of UCNP-Oilgosiloxane resin demonstrating Si atoms in the resin with bonding states.

We further confirmed the thermal stability of the SE-UCNP under higher temperature (100 °C) to check the effect of encapsulation by the siloxane matrix (Figures S4 and S5). The SE-UCNP exhibited almost unchanged PL intensity for 40 days, whereas the PL of the UCNP/Acrylate decreased to 40% after aging.


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For confirmation of the surface passivation by cross-links between the ligands of UCNPs and siloxane matrix, we conducted thermogravimetric analysis (TGA) of bare siloxane without UCNPs and SE-UCNP. According to the TGA results in Figure 3c, the SE-UCNP exhibited much higher decomposition temperature compared to bare siloxane. The temperature at which 5% weight loss occurred (Td5%) was 388 and 346 °C for SE-UCNP and bare siloxane, respectively. We ascribe the improved thermal stability of SE-UCNP to the formation of additional acrylic bonds from the cross-linking between the OA ligands of UCNPs and the methacrylate groups of the siloxane matrix, previously shown in Figure 2a. Through the cross-links, an SiO2-like encapsulation layer around the UCNPs is able to reduce the surface defects which cause temperature dependency of the surface quenching.14 To investigate the degree of condensation of the siloxane matrix in UCNP/siloxane hybrid structure, we analyzed the UCNP-Oligosiloxane resin using 29Si nuclear magnetic resonance (29Si NMR) (Figure 3d). The inset in Figure 3d shows the chemical formula of UCNP-Oligosiloxane resin, which shows methacrylate (R1) and phenyl (R2) groups in the siloxane matrix. The monomeric, dimeric, and trimeric species of Si atoms in the UCNP-Oligosiloxane resin are expressed as of Tn (MPTS) and Dn (DPSD), where superscript “n” means the number of siloxane bonds of the Si atom. The absence of precursor species such as T0 and D0, and the presence of monomeric species (T1 and D1), dimeric species (T2 and D2), and trimeric species (T3) in 29Si NMR confirmed the formation of siloxane networks with UCNPs in the UCNP-Oligosiloxane resin. The degree of condensation value of the siloxane bonds in UCNP-Oligosiloxane was 86.46% (Table S1), which indicates a highly condensed siloxane network.28 This NMR result validates that siloxane matrix passivating UCNP surface is immensely condensed, which efficiently enhance the thermal stability. Figure S6 shows a transmission electron microscope (TEM) image of a cross-


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sectional SE-UCNP sample prepared by focused ion beam (FIB) and its element mapping result measured using an EDS. The Si and O from siloxane matrix were observed over the entire TEM image, whereas the F, Y, and Yb were partially detected, mostly in the dispersed UCNPs. Taken together, both

29Si

NMR and TEM data signify that the condensed siloxane matrix amply

encapsulated the surfaces of the UCNPs. We further monitored the changes of PL intensities at 85 °C and 85% RH, to explore the PL stability of the SE-UCNP in high humidity (Figure 4a). As well known, water molecules on the UCNP surface act as surface oscillators, inhibiting the PL of the nanoparticles through both energy transfer and excitation energy exhaustion.14, 17 Interestingly, our SE-UCNP well maintained its PL intensity for 40 days at 85 °C and 85% RH, whereas the UCNP/Acrylate showed significantly decreased PL intensity (c.a. 50% decrease). We also measured the PL lifetime decays before and after aging at 85 °C and 85% RH (Figure 4b). The PL lifetime of the SE-UCNP after exposure to the high humidity environment was 0.28 ms, quite close to the initial lifetime (0.29 ms). In contrast, the PL lifetime of the UCNP/Acrylate diminished significantly from 0.28 to 0.10 ms. We attribute the long-term stability of SE-UCNP in moist environments to the hindrance of water diffusion by the siloxane network. According to a recent report, strong hydrogen bonding between water molecules and O atoms of siloxane inhibits the diffusion of water molecules into the siloxane network.32 In particular, the dense siloxane network in our SE-UCNP is able to facilitate the formation of hydrogen bonding with water molecules compared to the linear-chain siloxane. Furthermore, the phenyl and methacrylate functional groups in the siloxane matrix should repel water molecules, reducing the water permeability of the SE-UCNP.32 Figure 4c shows the comparison of FT-IR spectra of the SE-UCNP before and after aging at 85 °C and 85% RH. The


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Figure 4. (a) Changes of PL intensity of SE-UCNP and UCNP/Acrylate after 40 days aging at 85 °C and 85% RH (980 nm NIR irradiation, Power density of 1 W cm-2, number of experiments = 3). (b) Fluorescence lifetime decay dynamics of SE-UCNP and UCNP/Acrylate after 40 days aging at 85 °C and 85% RH. (c) Comparison of FT-IR spectra of SE-UCNP before and after 40 days aging at 85 °C and 85% RH. (d) The results of PL changes of SE-UCNP and UCNP/acrylate according to different pH conditions. The samples were soaked in each solution for 2 h (980 nm NIR irradiation, Power density of 1 W cm-2). The PL of the sample at pH 7 after 2 hours was set as a standard for normalization.

bands representing the hydroxyl group (-OH) stretching vibration in the region of 3150-3700 cm1

appeared in the aged SE-UCNP. This result indicates that the siloxane networks trapped water

molecules, hindered their behavior as surface oscillators on UCNPs, and maintained the optical stability of the UCNPs in SE-UCNP in a high moisture environment.


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Moreover, we tested the PL stability of SE-UCNP in various pH conditions (Figures 4d and S7). The acidic conditions cause chemical etching of UCNPs, increasing the number of surface defects that act as surface quenching sites.16 However, we found that the SE-UCNP exhibited unchanged PL intensity over a broad range of pH (from 1 to 14). In contrast, the UCNP/Acrylate showed significantly decreased PL intensity after 2 h storage under acidic conditions (c.a. 70% decrease at pH 1). The result shows that the rigid siloxane network in the SE-UCNP can protect UCNPs from damage in acidic environments. As a demonstration of our SE-UCNP for practical application, we fabricated a SE-UCNPbased micro-barcode by a PDMS-based barcode printing method that is cheap, fast, simple, and reproducible.33 For the fabrication of two-color binary sequences in micro-barcodes, we additionally synthesized a blue light-emitting SE-UCNP [NaYF4:Yb (20%), Tm (0.5%)]. Blue light-emitting UCNPs inside the SE-UCNP did not degrade during the synthesis process and showed superior PL stability under harsh environments (Figures S8-S10). Figure 5a shows a schematic illustration of the entire fabrication process of the micro-barcodes via a PDMS contact transfer printing method using green and blue light-emitting SE-UCNPs. Micro-barcode patterns on a PDMS film were filled by dispensing UCNP-Oligosiloxane resin as encoded and then transferred to the desired substrates. The SE-UCNP-based micro-barcodes were obtained after UV-curing. The width and spacing of the SE-UCNP barcode patterns were tunable through the modulation of etch-mask nanopatterns and dry etching parameters (Figure S11). The line width of patterns of the barcode could be reduced to a size of 200 nm with a large printing area (2×2.5 cm2), as shown in Figure 5b. For practical use, we scaled-up the width of the barcode to microsize, which is detectable under NIR laser excitation using an optical microscope (Figure S12) or a mobile phone camera (Figures 5c and d).


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Figure 5. (a) Schematic illustration of the entire process for fabrication of SE-UCNP microbarcodes via the PDMS contact transfer printing method. (b) Scanning electron microscope (SEM) image of patterned SE-UCNP micro-barcodes. Inset image is a photograph of the SE-UCNP micro-barcode on a commercial plastic substrate. (c–d) Green and blue emitting SE-UCNP microbarcodes of various sizes under NIR irradiation (980 nm, Power density of 1 W cm-2). Photographs are acquired by the mobile phone camera. The height of barcode patterns was 10 μm. Photographs of fabricated micro-barcodes on substrates such as (e) currency, (f) a credit card, and (g) human skin. Insets are images of the micro-barcodes acquired by the mobile phone camera (980 nm NIR irradiation, Power density of 1 W cm-2).

We printed SE-UCNP-based micro-barcodes encoding two-color binary sequences onto the various substrates (Figure 5e–g). It was found that the SE-UCNP micro-barcodes were applicable over a range of substrates including paper currency, credit cards, and even human skin. These results demonstrate that our micro-barcodes can be transferred to almost any type of surface, including flexible substrates and biological surfaces. The transferred SE-UCNP barcodes were


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semitransparent on the surface of the substrates and were clearly readable with the mobile phone camera under NIR irradiation. The sequences in the SE-UCNP barcodes were recognizable enough to identify the objects. Taken together, the dual-color-coded SE-UCNP micro-barcodes are promising devices that could encode specific information and allow for covert identification under NIR light. With further study of the expansion of the colors,34 the encoding capacity of our barcodes can be increased, providing a diverse library for practical applications.

4. Conclusion In conclusion, we synthesized a siloxane-encapsulated upconversion nanoparticle hybrid composite exhibiting high PL stability under severe conditions. The UCNPs were uniformly dispersed in the siloxane matrix and retained their initial PL property during in-situ sol-gel condensation and UV-induced free-radical addition reaction. The SE-UCNP showed stable PL performance for long-term aging under harsh conditions, such as high temperature (85 °C / 5% RH and 100 °C / 5% RH), humid environments (85 °C / 85% RH), and acidic conditions. The superior stability of the SE-UCNP was attributed to (1) surface passivation of UCNPs by highly condensed siloxane networks coming from additional cross-linking between the OA on UCNPs and the methacrylate group in the siloxane matrix, and (2) the hindrance of water diffusion by the siloxane matrix. In addition, we demonstrated two-color binary micro-barcodes using green and blue light-emitting SE-UCNP fabricated by a low-cost transfer printing method. The SE-UCNP micro-barcodes were applied to various substrates and recognized by a mobile phone camera under NIR irradiation. Our results demonstrated that siloxane/UCNP hybrid composites possess great PL stability under harsh conditions, suitable for practical imaging and labeling applications that respond to NIR light.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. TEM images. Photographs. Chemical structure. PL spectra. TGA data. Thermal and chemical stability results. Schematic diagram of the setup of fluorescent lifetime decay. Calculation of DOC. (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation (NRF) via the Creative Research Initiative Center (Grant number: NRF-2015 R1A3A2066191) and the Wearable Platform Materials Technology Center (WMC) (Grant number: NRF-2016R1A5A1009926), Re-public of


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Korea. We thank the Korea Basic Science Institute (KBSI) for help with the 29Si NMR spectral measurements.

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Siloxane-Encapsulated Upconversion ... - ACS Publications

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