Inkjet-Printed Multiwavelength Thermoplasmonic Images for

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Inkjet-printed multi-wavelength thermo-plasmonic images for anti-counterfeiting applications Hongki Kang, Jee Woong Lee, and Yoonkey Nam ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19342 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Applied Materials & Interfaces

Inkjet-printed multi-wavelength thermo-plasmonic images for anti-counterfeiting applications Hongki Kang, Jee Woong Lee, and Yoonkey Nam* Department of Bio and Brain Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea

KEYWORDS Thermo-plasmonics, inkjet printing, photothermal effects, anti-counterfeit, gold nanoparticles

ABSTRACT

Inkjet printing of thermo-plasmonic nanoparticles enables instantaneous, large-area heat pattern generation upon light illumination from distance. By printing multiple metal nanoparticles of different shapes overlaid, we can fabricate multi-wavelength thermo-plasmonic images, which generate different heat patterns from a single printed image depending on the wavelength choice of light. In this work, we propose a novel multi-wavelength thermo-plasmonic image printing that can be used for anti-counterfeit technology. With this technology, ‘printed thermoplasmonic labels’ allow fully secured anti-counterfeit inspection procedure. Input stimulus of near infrared or infrared light illumination and output signal reading of thermal patterns can be

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both completely invisible. Wavelength selective photothermal effect also enables the encryption of the contained information which adds more complexity and thus higher security.

INTRODUCTION. Unique physical characteristics of nanoparticles have opened up numerous novel applications. Among many from science to more practical engineering applications, several anticounterfeiting applications utilizing nanoparticles have been proposed as well.1–3 Key requirements of the adopted technologies for the anti-counterfeiting applications are difficulty in duplication, uniqueness of the verification of contained (or secured) information, and universal applicability of the technology to various substrates or products. As one of the most commonly used technologies, fluorescent materials that emit visible light upon ultraviolet light illumination (also called down-conversion) have been widely printed on various papers such as passport and currency.4–6 In order to make it more difficult to duplicate and more unique, rare up-conversion nanoparticles that emit visible light upon the illumination of longer wavelength light such as near infrared (NIR) or infrared (IR) have been proposed as an alternative.5,7–10 Viewing-angle dependent reflectance change of the nanoparticles with respect to the particle sizes was also suggested for implementing unique structural color patterns.11,12 Unique magnetic response of the nanoparticles was also utilized to implement an anti-counterfeiting application as well.13,14 Modified orientation of the nanoparticles upon strong magnetic field application leads to the change in reflectance, and thus creates color change.13 All of these applications utilized different characteristics of nanoparticles and different input stimulation sources on the nanoparticles, but offered the same method of output signal verification (i.e. visual inspection with naked eyes).


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From the application point of view, the fact that output signal after inspection is publicized can clearly limit feasible applications. In some applications, for example, only the inspector would want to know the results of inspection without sharing with the suspects or subjects. Our eyes cannot see neither the light outside of visible wavelength (400 – 700 nm), nor temperature change. Therefore, if we can develop a method where input and output can be both beyond what we can see, for example, completely secretive anti-counterfeiting inspection can be implemented. In this work, we propose a new concept of nanoparticle based anti-counterfeiting application using thermo-plasmonic effect of metal nanoparticles. Thermo-plasmonic metal nanoparticles can generate localized heat in nanoscale from significantly enhanced light absorption at particular wavelength by localized surface plasmons.15 By inkjet printing overlaid patterns of multiple metal nanoparticle inks that have different peak absorbance wavelength values (e.g. near infrared light, and visible light as described in Figure 1(a)), we can create an image that generates different heat patterns depending on the wavelength of the illuminated light while with naked eyes it is difficult to distinguish the different printed patterns. We chose gold nanorod (GNR) and gold nanosphere (GNS) nanoparticles for this work. GNR can be tuned to absorb NIR light with high thermo-plasmonic efficiency, and GNS can be tuned to absorb visible light while being transparent to NIR light. As illustrated in Figure 1(b), the heat patterns will be only visible through thermal imaging infrared cameras. Utilizing longer wavelength absorbing metal nanoparticles such as gold nanorods, nearly invisible NIR or IR light can be used as the excitation source. With the multi-wavelength thermo-plasmonic image printing method, we can therefore realize a completely secretive information delivery method that can be used for anticounterfeiting applications.


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RESULTS. Multi-wavelength thermo-plasmonic nanoparticle inks We first synthesized two different shapes of gold nanoparticles for absorbing distinctively different range of light wavelength. For NIR range, we synthesized gold nanorod (GNR) ink as we have previously utilized for photothermal applications.16,17 For visible range absorption, we synthesized gold nanosphere (GNS) ink as described in Experimental Section. The synthesized nanoparticle inks and the shape of the two nanoparticles are displayed in Figure 2. As shown in the extinction spectra in Figure 2(c), the two nanoparticle inks had maximum absorption peaks at 798 nm and 534 nm, respectively. The ratio of extinction values between the two particles was maxed at 579 nm and 848 nm with 4.85 and 15.82 as ratio, respectively (Figure 2(d)). This result confirms that the two nanoparticle inks are expected to show distinctive wavelength selectivity of thermo-plasmonic effect.

Photothermal contrast test We then tested and compared the photothermal efficiency of the nanoparticles. To create thermo-plasmonic nanoparticle patterns, we utilized a universally applicable inkjet printing process that we recently developed for large-area nanoparticle images.16 As we reported, polyelectrolyte layer-by-layer coating adopted for the nanoparticle inkjet printing enables high pattern fidelity on any substrates by induced contact line pining. With inkjet printing, we can also precisely control the areal density of the nanoparticles to adjust the photothermal intensity. Using this process, we prepared three samples of inkjet-printed nanoparticle microdot arrays: (1) only GNS printed (GNS); (2) only GNR printed (GNR); and (3) 1:1 ratio of GNR and GNS with


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alternating columns (GNS-GNR; placed between the two former samples for comparison) as shown in Figure 3(a). The areal densities of the samples were prepared to be nearly identical. On these samples, we illuminated lights of two different wavelengths (785 nm, 530 nm), and characterized photothermal efficiency, respectively. The printed samples of different nanoparticles showed different colors. Due to the different optical characteristics of the two nanoparticles, the GNS sample showed purple color, and the GNR sample showed achromatic color, with high transparency in visible light (on average 84% and 94% transmittance for GNS and GNR, respectively). Although the areal densities of the two samples were very similar, the GNS sample looked more noticeable as GNS responded to the visible range of light more strongly. The 1:1 GNR-GNS sample showed a mixture of the characteristics of the two. In darkfield microscopy, both GNS and GNR printed patterns showed gold color which indicates the scattered light from gold nanoparticles. The printed substrates preserved their own unique optical characteristics of the nanoparticles. Figure 3(b) shows the extinction spectra of the inkjet-printed GNS and GNR nanoparticle substrates. The extinction characteristics were similar to those in Figure 2(c), which were obtained from suspension. After printing, there were only slight red shift of the peak wavelength (29 nm for both samples) and peak broadening (79% and 116% increases of half width at half maximum for GNS and GNR, respectively) possibly due to plasmonic coupling with nearby nanoparticles. The ratio of extinction spectra, which reasonably represents the photothermal effect contrast, showed that the contrast can be maximized near 566 nm and at longer wavelength (NIR and IR). The mixed GNR and GNS sample showed exactly averaged characteristics of the two individual samples.


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We measured the maximum temperature change of the samples in steady state upon illuminated lights with different light power intensity conditions. From the expected linear relationships as shown in Figure 3(c), we used the slope of regression line as the photothermal efficiency. At the wavelenghth of 785 nm (NIR), GNR sample showed three times larger photothermal efficiency than GNS sample. On the contrary, at the wavelength of 530 nm (green) in Figure 3(d), GNS sample showed more than doubled photothermal effect than GNR sample. With these results, we clearly confirmed that we can achieve significant photothermal effect contrast at both green/yellow light wavelength (530 nm) and near infrared or infrared light (785 nm) ranges.

Inkjet-printed

multi-wavelength

thermo-plasmonic

images

for

anti-counterfeiting

applications While we achieved significant photothermal effect contrast (three times at 785 nm), the distinctive color difference could be problematic as the patterns can be distinguishable visibly without inspecting thermo-plasmonic effect. In order to make it visibly less distinguishable, therefore, we re-optimized the printing conditions of the GNS ink. We slightly reduced the concentration of the GNS ink droplets by 25% to make it less visible than the previous samples in Figure 3. Furthermore, during the thermo-plasmonic heat generation, we learned that the generated heat diffuses around the printed patterns, and therefore printed patterns with some distance (e.g. edge-to-edge distance up to two times of the size of the patterns) would be allowed to create continuous heat patterns.16 In other words, effective size of the generated heat patterns is two or three times larger than the size of the printed nanoparticles. Therefore, we increased the ratio of dot-to-dot distance to the size of dot patterns further in order to make the printed


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nanoparticle patterns less visible and more transparent while maintaining the thermo-plasmonic image quality. The expected extinction spectra scale of the two nanoparticle conditions would be more evenly balanced (Figure S1). With this effort of modification, we were able to fabricate transparent printed multinanoparticle patterns that were very difficult to distinguish with naked eyes. As shown in Figure 4(a, b) and Figure S2 in Supporting Information, printed patterns composed of two different nanoparticles were quite invisible when they were printed on a transparent film (Figure 4(a)), and it was hard to distinguish which nanoparticle ink was used for which letters. When the two types of nanoparticle patterns were merged, it became even harder to distinguish (Figure 4(b), middle row). Under the dark-field microscopy (Figure 4(c)), the two nanoparticle patterns were slightly distinguishable. However, under bright-field upright microscope, the colors of the two nanoparticle micro patterns were quite similar in terms of color, thus it would be impossible to distinguish which pattern was intended to be encoded as a genuine pattern. We then demonstrated the wavelength selective thermo-plasmonic image printing by illuminating NIR light (808 nm) on the printed overlaid multi-nanoparticle image of Figure 4(b) in the setup described in Figure S3. The top row word, REAL, was only composed of GNRs. The bottom row word, FAKE, was only composed of GNSs. In the middle row, both GNR word and GNS word were printed and overlaid. As shown in Figure 4(d) and Movie S1 in Supporting Information, as soon as the NIR laser turned on, clear heat pattern of word ‘REAL’ was measured through an IR camera from the GNR-only printed pattern. On the other hand, because of the much lower photothermal efficiency of GNS at NIR, GNS-only printed patterns did not generate sufficient heat to be clearly read in the same temperature range. More interestingly, the overlaid words upon the light illumination generated the same heat pattern as the GNR-only


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pattern. Despite the mixture of the two particles at multiple areas over the entire word pattern, we confirmed that the two types of nanoparticles were generating heat independently, and the gold nanorods upon NIR emits surpassingly stronger heat than the gold nanospheres. For anti-counterfeiting technology, it is important that the unique characteristics of the technology should not be duplicated by commonly used tools. For comparison, we tried to duplicate the multi-wavelength thermo-plasmonic image using conventional color laser printer. Despite distinctively different microscopic images in Figure S4, large scale printing image could look quite similar to the multi-wavelength thermo-plasmonic image (Figure 4(e)) especially at straight viewing angles. However, with the typical color laser printer toner materials, it was not possible to observe any photothermal effect as shown in Figure 4(f) and Movie S2 under the same NIR light illumination condition as in Figure 4(d). These results therefore demonstrate both unique characteristic of the wavelength selective photothermal pattern generation using our technology and difficulty in duplication of the characteristic with conventional technology.

DISCUSSION. We proposed a novel image printing technique based on inkjet printing of multiple metal nanoparticles for anti-counterfeiting applications. Compared to most of the previously reported anti-counterfeiting technologies in which often visible lights were used as readout signals, both input stimulus (excitation source) and output signal (readout signal) can be completely invisible, thus allowing fully secretive counterfeit inspection. Compared to other invisible methods such as magnetic response based method, the thermal output patterns can create readable or more complex patterns such as graphics.14 Heat or thermo-sensitive characteristics have also been proposed for anti-counterfeiting applications. However, some still rely on visual pattern changes


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in response to thermal stimuli (e.g. thermochromism).2 There is a method that utilizes heat and temperature change as input and output, respectively.18 However, the method relies on the brief heat flow change during the phase change of materials at different melting points. Therefore, the inspection process is slow because the input stimulus (excitation) requires broad range of temperature change, but the detection time duration is short due to the abrupt phase change procedure. On the other hand, our method in this work can operate instantaneously and for unlimited period of time. This procedure is fully reversible without the need of external input (e.g. erasing data with light in photochromic approach).2 Required expertise in advanced chemical synthesis and inspection instruments that are not commonly available would add extra security and difficulty in duplication. Though clear but rather simple image patterns were used for demonstration in this work, more complex images such as color perception test images or more encrypted images can be printed with ease using versatile inkjet printing system. We observed that the GNR showed higher photothermal efficiency than the GNS. We compared the extinction spectra of the printed samples of the two nanoparticles to anticipate photothermal effect. Based on the extinction value comparison, GNR sample showed approximately two times higher extinction value than GNS sample at 785 nm (Figure 3(b)). However, when the photothermal effect was compared at 785 nm, the GNR sample showed nearly three times higher photothermal effect efficiency than the GNS sample (Figure 3(c)). It has been reported that the nanorod structure is more efficient in generating heat upon light illumination than the spherical shape particles.19,20 It has also been reported that other more branched nanoparticles such as gold nanostar show even higher photothermal efficiency than GNRs.21–23 Therefore, we expect that when in the need of even higher photothermal efficiency contrast, those different types of nanoparticles can be good alternatives.


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For uniform heat generation in micron-scale, it is imperative that we induce uniform assembly of nanoparticles within the printed pattern boundary during drying. However, more significant deposition of nanoparticles at the edge of the nanoparticle ink droplet than in the center, also widely known as coffee-ring or coffee-stain effect, has been the most common failure in uniform nanoparticle assembly. There have been several works reporting conflicting observations about the effect of the shapes of particles on the coffee-ring effect: ellipsoidal particles showing more uniform assembly over spherical particles,24,25 or no difference observed.26 In our inkjet printing process, contact line pinning, a prerequisite condition of coffeering effect, is satisfied with the polyelectrolyte layer-by-layer coating.16 However, no clear difference in terms of coffee-ring effect for both rod-shape and spherical shape nanoparticles was observed. Both showed suppressed coffee-ring effect. Moreover, aggregation of the printed nanoparticles may have affected the thermoplasmonic effect as we saw the red shift and the broadening of the peak absorption spectrum of the printed samples in Figure 3(b). Further improvement on avoiding aggregation and inducing monodisperse states would enhance the wavelength sensitivity and the absorption contrast of the two printed samples. As previously reported, simple silica coating on gold nanoparticles can lead to highly monodisperse nanoparticles without significantly affecting the absorbance spectrum.27,28 The silica coating with optimal thickness also showed significantly enhanced photoacoustic effect.27 We may therefore expect more efficient and stable multi-wavelength thermo-plasmonic image printing with the protective coating layer such as the silica coating on the nanoparticles.

CONCLUSION.


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In this work, we proposed a multi-wavelength thermo-plasmonic image printing technology for anti-counterfeiting applications. Inkjet printing of different thermo-plasmonic nanoparticles with the precise control of optical characteristics enables nearly indistinguishable printed semi-transparent patterns with naked eyes, but clearly distinctive thermal patterns under specific wavelength of light irradiation. With this method, entirely secretive counterfeit inspection is possible by using invisible long wavelength light as stimulus and thermal imaging camera as a decoder. We believe this technology can create unique applications of counterfeit inspection with more advanced security in terms of materials and instruments. Furthermore, the fabrication method of wavelength-selective thermo-plasmonic effects developed in this work can be used for other bioengineering applications such as photothermal therapy, hyperthermia and brain activity modulation where different spatial patterns of thermo-biological effect can be induced by the choice of illuminated light wavelength.15,17

EXPERIMENTAL SECTION. Plasmonic nanoparticle ink synthesis and characterization Both rod-like and spherical gold nanoparticles were synthesized. Gold nanorod nanoparticle (GNR) was synthesized using a seed mediated method.29 The seed solution was a mixture of 5 ml of 0.5 mM Tetrachloroauric(III) acid (HAuCl4) (520918, Aldrich), 5 ml of 0.2 M cetyltrimethylammonium bromide (CTAB) (H6269, Sigma) and 600 µl of ice-cold 0.01 M sodium borohydride (NaBH4) (71321, Fluka) in deionized water while ultra-sonication for 4 min at 26˚C. The seed solution was kept at room temperature for 2 hours. We mixed 12 µl of the seed


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solution with 5 ml of 0.2 M CTAB, 5 ml of 1 mM HAuCl4, 250 µl of 4 mM silver nitrate (AgNO3) (209139, Sigma-Aldrich), and 70 µl of 78.84 mM L-Ascorbic acid (A5960, Sigma) to grow rod shape seed. The solution was again kept at room temperature until the longitudinal absorption peak was observed around 800 nm. After that, the nanoparticles were washed with deionized water using a centrifuge. Gold nanosphere nanoparticle (GNS) was synthesized based on the following branched nanoparticle recipe.23 150 µl of 20 mM Tetrachloroauric(III) acid (HAuCl4) (520918, Aldrich) was mixed with 2 ml of deionized water and 3 ml of 100 mM HEPES buffer (15630, Gibco, life technologies). pH was adjusted to be 7.4 ± 0.1 at 25°C by adding 1 M NaOH solution. Then the mixture was shaken immediately for 5 sec using a vortex mixer, and the solution was left for 2 hours for reaction. By adjusting the ratio of chemicals, we optimized the GNS to strongly absorb visible light range shown in Figure 2(c). Some GNS particles showed slightly branched structures, but most particles showed sphere shapes. For the surface functionalization of the nanoparticles, both nanoparticle surfaces were coated with methoxyl polyethylene glycol thiol (mPEG-SH) (PG1-TH-5k, Nanocs) for at least 12 hours at room temperature. After the PEG coating, nanoparticle inks were concentrated using centrifuge. Final nanoparticle inks showed maximum absorbance peak around 785 nm for GNR and 530 nm for GNS in water. The average zeta potential values of the nanoparticles were -24.5 mV for GNR and -19.7 mV for GNS (Zetasizer Nano ZS, Malvern). After PEG coating, dialysis of the solution was done within MWCO (Molecular Weight Cut Off) 3500 membrane cassette (ThermoFisher) for two days. The shape of the synthesized nanoparticles was confirmed using a transmission electron microscopy (JEM-3011, JEOL). Extinction spectra of the synthesized nanoparticle inks and printed substrates were measured using a spectrometer covering both


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visible and near-IR range (350 – 1000 nm; USB4000-VIS-NIR-ES with halogen light source HL-2000, Ocean Optics). For stable ink jetting and coffee-ring suppression, the printing ink solvent was finalized as 1:1 ratio of ethylene glycol (324558, Sigma-Aldrich) to deionized water. The printed ink concentration of the nanoparticles were characterized as the maximum peak absorbance value in optical density (OD): 112 OD for GNR, and 137 OD for GNS. Printing process For the inkjet printing of thermo-plasmonic nanoparticles, we used polyelectrolyte layerby-layer (LbL) coating based nanoparticle inkjet printing process we recently developed.16 10 mg/ml of positive polyelectrolyte, poly(allylamine hydrochloride) (PAH) (283215, Aldrich), and negative polyelectrolyte poly(4-styrenesulfonic acid) (PSS) (561258, Aldrich), were dissolved in 10 mM NaCl aqueous solution, respectively. Printing substrates were coated for 5 min in each solution alternatively with deionized water washing in-between. At least 10 bilayers were deposited before the final PAH solution coating. Printing substrates we used for large size images were 20-µm thick transparent and hydrophobic fluorinated ethylene–propylene flexible membrane (FEP membrane) for nanoparticle printing, and 100-µm thick polyethylene terephthalate (PET) film for commercial overhead projector film (OHP film) for color laser printing. For printing in Figure 3, we used microscope glass coverslip (0101030, 22 mm × 22 mm, Marienfeld-Superior) as a printing substrate. Piezoelectric inkjet printing system (UJ200MF, Unijet) with 50-µm nozzle was used in this work. Ink volume was approximately 14 pl with jetting velocity around 2.0 m/s. Nanoparticle GNR inks were filtered (0.45 µm pore size) before loading in order to avoid nozzle clogging. All printing was done in a clean room facility at room temperature. Printed GNR ink areal density


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was controlled by the number of droplets on the same location (i.e. the number of printing layers) and center-to-center drop spacing. The ink areal density was then characterized using the extinction of the printed ink (optical density), jetted ink volume, the number of printed layers, and drop spacing. For example, when 14 pL droplets of 137 OD ink was printed within 100 µm × 100 µm area for four times (i.e. 4 layers), the areal density was 0.77 OD×pL/µm2. For conventional color laser printing, we used a commercial laser printer (HP Color LaserJet 2700). The printed image was dark red colored (RGB color code: 330000) with 50% opacity.

Printed nanoparticle pattern imaging Printed nanoparticle substrates were characterized in three different microscope setups. On an inverted microscope (IX71, Olympus), dark-field microscopy (U-DCD dark-field condenser, Olympus) and phase-contrast microscopy (IX71, Olympus) were conducted. Bright-field microscopy was conducted using a upright microscope (BXFM, Olympus).

Photothermal effect characterization and multi-wavelength thermo-plasmonic image demonstration NIR light irradiation and subsequent temperature change detection were conducted following the same method that we have recently reported.16,23 We used three light sources. Two are continuous wave laser sources in NIR wavelength: 785 nm CW laser source (BWF1, 450 mW, B&W Tek), and 808 nm CW laser source (4 W, Laserlab Co., Ltd., Anyang, Korea). Smaller power (450 mW) unit was used for up to 5 mm-diameter illumination area, and the other higher power (4 W) unit for larger area illumination (several centimeters). For green light illumination, we used a 530-nm light emitting diode (LED) light source (M530L3-C1, 170 mW,


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Thorlabs). For higher light power density, we took off a front collimator, and placed the green LED close to the nanoparticle printed samples. Using a power detector (XLP12-3S-H2, GentecEO, Québec, Canada), we characterized illuminated power density. Unlike the NIR lasers, the green LED without collimator did not provide uniform light power spatial distribution, which made it difficult to anticipate accurate power density value. Therefore, photothermal effect was characterized based on the overall emitted light power instead of power density. Temperature change by photothermal effect was detected by a handheld infrared camera (E40, FLIR). Extracting raw recording data and subtracting baseline temperature profile right before the illumination for 1 sec, we processed only the profile of temperature change caused by the photothermal effect.


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Figure 1. Schematic illustration of inkjet-printed multi-wavelength thermo-plasmonic images for anti-counterfeiting applications. (a) Overlaid inkjet printing of differently shaped thermoplasmonic nanoparticle inks with different input image patterns for wavelength-selective thermoplasmonic patterns (Encryption). (b) Anti-counterfeit inspection procedure (Decryption). Laser illumination is used as an input stimuli (light excitation) to generate heat patterns. The heat patterns are detected by a thermal imaging IR camera (thermal readout). Upon different wavelength of the input light, different heat patterns are generated from a single printed image in (a).


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Figure 2. Synthesized thermo-plasmonic nanoparticle inks. (a) Both gold nanosphere (GNS) and gold nanorod (GNR) inks of 1 optical density concentration in cuvettes are displayed for comparison in optical characteristics. (b) Transmission electron microscope images of both nanoparticles. Scale bars represents 50 nm. (c) Extinction spectra data of the two nanoparticle inks in (a) showing distinctively different absorption peaks. (d) Extinction contrast spectra (i.e. ratio of the extinction values of the two inks; larger value was always divided by the smaller value) based on the data in (c).


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Figure 3. Photothermal effect contrast test using inkjet-printed nanoparticle dot arrays on glass coverslips. (a) Photos of the inkjet-printed nanoparticle array (overall 8 mm wide and 16 mm tall) on 22-mm big glass coverslip. Both dark-field and phase-contrast microscope images are presented. Both are in the same scale, and the scale bar represents 100 µm. Printing resolution is 254 dpi (dot-per-inch) which corresponds to 100 µm center-to-center spacing. For GNS sample, 4 layers were printed (0.77 OD⋅pL/µm2). For GNR, 5 layers were printed (0.79 OD⋅pL/µm2). For GNS-GNR sample in the middle, printing conditions are identical to each nanoparticles, respectively, except that columns are alternatively printed with different inks. (b) Extinction spectra of the three substrates in (a) are displayed. Gray dotted line for the mixed nanoparticle sample was mathematically averaged values of the extinction values of both GNS and GNR samples. Ratio of the extinction spectra of the two GNS and GNR samples is also potted, anticipating photothermal effect contrast. (c) Photothermal effect measurement under NIR wavelength (785 nm) for the three samples in (a) with extracted linear regression slopes compared for photothermal effect efficiency comparison. Average values of the maximum temperature changes in given light illumination conditions were plotted. Standard deviation values of the maximum temperature changes were too small to display as error bars (about 0.1 ° C) (d) The same analysis was performed under green light (530 nm). Please note that different light intensity measure was used in (c; power density) and (d; overall power).


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Figure 4. Photothermal effect demonstration of inkjet-printed multi-wavelength thermoplasmonic images. (a) Pictures of the inkjet-printed multi-wavelength thermo-plasmonic images taken with various backgrounds (left: on a hand; right: on a cardboard box). The nanoparticle image is inkjet-printed on a FEP membrane (printing resolution: 254 dpi; GNR: 5 layers, 0.79 OD⋅pL/µm2; GNS; 3 layers, 0.57 OD⋅pL/µm2). GNS+GNR is merely overlaid printing of GNR word ‘REAL’ and GNS word ‘FAKE’ as described in Figure 1. (b) Another photo of the printed thermo-plasmonic image in (a) on a white printing paper. (c) Microscope images (dark-field and bright-field) of dotted square areas of each nanoparticle conditions in (b). Scale bar represents 200 µm. For GNR printing, between printing layers, misalignment occurred (on the order of a few tens of µm) resulting in non-circular printing patterns. (d) Recorded photothermal image of the sample in (a) upon NIR laser illumination (808 nm, 4.23 mW/mm2) in 3-dimensional and 2-


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dimensional plots. Temperature change profile from baseline is processed and plotted. 1 pixel in the thermal image is approximately 315 µm. See Movie S1 for the entire data. (e) Thermoplasmonic printed images and color laser printed images are shown for comparison. Both photos are taken together in the same exposure condition as shown in Figure S3. (Please note that the two, however, have discernable reflective characteristics depending on the viewing angles.11) (f) Photothermal effect test of the similar looking color laser printing in (e) in the same illumination condition as in (d). No letters are shown. Circular heat pattern which corresponds to the light illumination area is only shown because the OHP film we used for color laser printing slightly absorb the NIR light. See Movie S2 for more details. (Please note that when OHP film is used with the thermo-plasmonic image in (d), circular pattern also appears, but clear thermal letters as in (d) still show up distinctively.)

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional supporting information figures (PDF) Encrypted thermo-plasmonic image heat generation movie for the inkjet-printed thermoplasmonic image sample in Figure 4(b) (AVI) Similarly printed conventional color laser printed image in Figure 4(e, bottom) not showing thermo-plasmonic patterns (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2015R1A2A1A09003605).

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2015R1A2A1A09003605).

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Table Of Contents (TOC)


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Inkjet-Printed Multiwavelength Thermoplasmonic Images for

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