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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6764−6771
Inkjet-Printed Multiwavelength Thermoplasmonic Images for Anticounterfeiting 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 S Supporting Information *
ABSTRACT: Inkjet printing of thermoplasmonic 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 multiwavelength thermoplasmonic 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 multiwavelength thermoplasmonic image printing process that can be used for anticounterfeit technology. With this technology, “printed thermoplasmonic labels” allow fully secured anticounterfeit inspection procedure. Input stimulus of nearinfrared or infrared light illumination and output signal reading of thermal patterns can be both completely invisible. Wavelength selective photothermal effect also enables the encryption of the contained information, which adds more complexity and thus higher security. KEYWORDS: thermoplasmonics, inkjet printing, photothermal effects, anticounterfeit, gold nanoparticles
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INTRODUCTION
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 either the light outside of visible wavelength (400−700 nm) or 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 anticounterfeiting inspection can be implemented. In this work, we propose a new concept of nanoparticlebased anticounterfeiting application using thermoplasmonic effect of metal nanoparticles. Thermoplasmonic metal nanoparticles can generate localized heat in nanoscale from significantly enhanced light absorption at a 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 1a), we can create an image that generates different heat patterns depending on the wavelength of the illuminated light, whereas 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 thermoplasmonic efficiency, and GNS can be tuned to absorb visible light while being transparent to NIR
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 anticounterfeiting 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 downconversion) have been widely printed on various papers such as passport and currency.4−6 To make it more difficult to duplicate and more unique, rare upconversion 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-angledependent reflectance change of the nanoparticles with respect to 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 anticounterfeiting application.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). © 2018 American Chemical Society
Received: December 20, 2017 Accepted: January 30, 2018 Published: January 30, 2018 6764
DOI: 10.1021/acsami.7b19342 ACS Appl. Mater. Interfaces 2018, 10, 6764−6771
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic illustration of inkjet-printed multiwavelength thermoplasmonic images for anticounterfeiting applications. (a) Overlaid inkjet printing of differently shaped thermoplasmonic nanoparticle inks with different input image patterns for wavelength-selective thermoplasmonic patterns (encryption). (b) Anticounterfeit inspection procedure (decryption). Laser illumination is used as an input stimulus (light excitation) to generate heat patterns. The heat patterns are detected by a thermal imaging IR camera (thermal readout). Upon different wavelengths of the input light, different heat patterns are generated from a single printed image in (a).
To create thermoplasmonic 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 nanoparticle inkjet printing enables high pattern fidelity on any substrate 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 alternating columns (GNS−GNR; placed between the two previous samples for comparison), as shown in Figure 3a. The areal densities of the samples were prepared to be nearly identical. On these samples, we illuminated lights of two different wavelengths (785 and 530 nm) and characterized photothermal efficiency. The printed samples of different nanoparticles showed different colors. Owing 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 dark-field microscopy, both GNS and GNR printed patterns showed gold color, which indicates the scattered light from gold nanoparticles.
light. As illustrated in Figure 1b, the heat patterns will be visible only through thermal imaging infrared cameras. Longer wavelength absorbing metal nanoparticles, such as gold nanorods or nearly invisible NIR or IR light, can be used as the excitation source. With the multiwavelength thermoplasmonic 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 Multiwavelength Thermoplasmonic Nanoparticle Inks. We first synthesized two different shapes of gold nanoparticles for absorbing distinctively different ranges 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 the Experimental Section. The synthesized nanoparticle inks and the shapes of the two nanoparticles are displayed in Figure 2. As shown in the extinction spectra in Figure 2c, the two nanoparticle inks had maximum absorption peaks at 798 and 534 nm. The ratio of extinction values between the two particles was maxed at 579 and 848 nm with 4.85 and 15.82 as ratio, respectively (Figure 2d). This result confirms that the two nanoparticle inks are expected to show distinctive wavelength selectivity of thermoplasmonic effect. Photothermal Contrast Test. We next tested and compared the photothermal efficiency of the nanoparticles. 6765
DOI: 10.1021/acsami.7b19342 ACS Appl. Mater. Interfaces 2018, 10, 6764−6771
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Figure 2. Synthesized thermoplasmonic 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 represent 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; the larger value was always divided by the smaller value) based on the data in (c).
we achieved significant photothermal effect contrast (3 times at 785 nm), the distinctive color difference could be problematic as the patterns can be distinguished visibly without inspecting the thermoplasmonic effect. To make it visibly less distinguishable, therefore, we reoptimized 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 that in the previous samples, as shown in Figure 3. Furthermore, during thermoplasmonic heat generation, we learned that the generated heat diffuses around the printed patterns, and therefore printed patterns with some distance (e.g., edge-toedge distance up to 2 times 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 2 or 3 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 to make the printed nanoparticle patterns less visible and more transparent while maintaining the thermoplasmonic 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 Figures 4a,b and S2 in the Supporting Information, printed patterns composed of two different nanoparticles were quite invisible when they were printed on a transparent film (Figure 4a) 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 4b,
The printed substrates preserved their own unique optical characteristics of the nanoparticles. Figure 3b shows the extinction spectra of the inkjet-printed GNS and GNR nanoparticle substrates. The extinction characteristics were similar to those in Figure 2c, 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 wavelengths (NIR and IR). The mixed GNR and GNS sample showed exactly averaged characteristics of the two individual samples. 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 3c, we used the slope of regression line as the photothermal efficiency. At the wavelength of 785 nm (NIR), GNR sample showed 3 times larger photothermal efficiency than that of GNS sample. On the contrary, at the wavelength of 530 nm (green) in Figure 3d, GNS sample showed more than doubled photothermal effect than that of GNR sample. From these results, we can clearly confirm 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 Multiwavelength Thermoplasmonic Images for Anticounterfeiting Applications. Although 6766
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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, five layers were printed (0.79 OD·pL/μm2). For GNS−GNR sample in the middle, printing conditions are identical to each nanoparticle, 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 represents 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 plotted, 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 under 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).
4d and Movie S1 in the Supporting Information, as soon as the NIR laser was turned on, clear heat pattern of the word REAL was measured through an IR camera from the GNR-only printed pattern. On the contrary, 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 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 emit surpassingly stronger heat than do the gold nanospheres.
middle row). Under dark-field microscopy (Figure 4c), the two nanoparticle patterns became slightly distinguishable. However, under bright-field upright microscope, the colors of the two nanoparticle micropatterns 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 thermoplasmonic image printing by illuminating NIR light (808 nm) on the printed overlaid multinanoparticle image of Figure 4b 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 6767
DOI: 10.1021/acsami.7b19342 ACS Appl. Mater. Interfaces 2018, 10, 6764−6771
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Figure 4. Photothermal effect demonstration of inkjet-printed multiwavelength thermoplasmonic images. (a) Pictures of the inkjet-printed multiwavelength thermoplasmonic images taken with various backgrounds (left, on a hand; right, on a cardboard box). The nanoparticle image is inkjet-printed on an fluorinated ethylene propylene (FEP) membrane (printing resolution: 254 dpi; GNR: five layers, 0.79 OD·pL/μm2; GNS; three 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 thermoplasmonic image in (a) on a white printing paper. (c) Microscope images (dark-field and bright-field) of dotted square areas of each nanoparticle condition 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 noncircular printing patterns. (d) Recorded photothermal image of the sample in (a) upon NIR laser illumination (808 nm, 4.23 mW/mm2) in three-dimensional and two-dimensional plots. Temperature change profile from baseline is processed and plotted. One 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 similarlooking color laser printing in (e) under 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 overhead projector film (OHP) film we used for color laser printing slightly absorbs the NIR light. See Movie S2 for more details. (Please note that when the OHP film is used with the thermoplasmonic image in (d), circular pattern also appears, but clear thermal letters as in (d) still show up distinctively.)
For anticounterfeiting technology, it is important to note that the unique characteristics of the technology should not be duplicated by commonly used tools. For comparison, we tried to duplicate the multiwavelength thermoplasmonic image using a conventional color laser printer. Despite distinctively different microscopic images in Figure S4, large-scale printing image
could look quite similar to the multiwavelength thermoplasmonic image (Figure 4e), 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 4f and Movie S2, under the same NIR light illumination condition, as in Figure 4d. These results therefore 6768
DOI: 10.1021/acsami.7b19342 ACS Appl. Mater. Interfaces 2018, 10, 6764−6771
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particles showing more uniform assembly over spherical particles24,25 or no difference observed.26 In our inkjet printing process, contact line pinning, a prerequisite condition of coffeering effect, occurred because of the polyelectrolyte layer-bylayer coating.16 However, no clear difference in terms of coffeering effect for both rod-shaped and spherical-shaped nanoparticles was observed. Both showed suppressed coffee-ring effect. Moreover, aggregation of the printed nanoparticles may have affected the thermoplasmonic effect, as seen from the red shift and the broadening of the peak absorption spectrum of the printed samples in Figure 3b. 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 multiwavelength thermoplasmonic image printing with the protective coating layer such as the silica coating on nanoparticles.
demonstrate both unique characteristic of the wavelengthselective photothermal pattern generation using our technology and difficulty in duplication of the characteristic with conventional technology.
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DISCUSSION We proposed a novel image printing technique based on inkjet printing of multiple metal nanoparticles for anticounterfeiting applications. Compared to most of the previously reported anticounterfeiting 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 thermosensitive characteristics have also been proposed for anticounterfeiting applications. However, some still rely on visual pattern changes 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 a 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 an 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. 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. On the basis of the extinction value comparison, GNR sample showed approximately 2 times higher extinction value than GNS sample at 785 nm (Figure 3b). However, when the photothermal effect was compared at 785 nm, the GNR sample showed nearly 3 times higher photothermal effect efficiency than does the GNS sample (Figure 3c). It has been reported that the nanorod structure is more efficient in generating heat upon light illumination than does the spherical-shaped 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. 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
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CONCLUSIONS In this work, we proposed a multiwavelength thermoplasmonic image printing technology for anticounterfeiting applications. Inkjet printing of different thermoplasmonic nanoparticles with the precise control of optical characteristics enables nearly indistinguishable printed semitransparent 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 that 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 thermoplasmonic 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 thermobiological effect can be induced by the choice of illuminated light wavelength.15,17
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EXPERIMENTAL SECTION
Plasmonic Nanoparticle Ink Synthesis and Characterization. Both rodlike 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 ultrasonication was carried out for 4 min at 26 °C. The seed solution was kept at room temperature for 2 h. We mixed 12 μL of the seed 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 seeds. 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 on the basis of the following branched nanoparticle recipe.23 First, 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 N-(2-hydroxyethyl)piperazine-N′ethanesulfonic acid buffer (15630, Gibco, life technologies). pH was 6769
DOI: 10.1021/acsami.7b19342 ACS Appl. Mater. Interfaces 2018, 10, 6764−6771
Research Article
ACS Applied Materials & Interfaces adjusted to be 7.4 ± 0.1 at 25 °C by adding 1 M NaOH solution. Then, the mixture was shaken immediately for 5 s using a vortex mixer, and the solution was left for 2 h for reaction. By adjusting the ratio of chemicals, we optimized the GNS to strongly absorb visible light range shown in Figure 2c. Some GNS particles showed slightly branched structures, but most particles showed sphere shapes. For the surface functionalization of nanoparticles, both nanoparticle surfaces were coated with methoxyl poly(ethylene glycol) thiol (mPEG-SH) (PG1-TH-5k, Nanocs) for at least 12 h at room temperature. After PEG coating, nanoparticle inks were concentrated using centrifuge. Final nanoparticle inks showed the maximum absorbance peak around 785 nm for GNR and 530 nm for GNS in water. The average ζ 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 performed within molecular weight cutoff 3500 membrane cassette (ThermoFisher) for 2 days. The shape of the synthesized nanoparticles was confirmed using transmission electron microscopy (JEM-3011, JEOL). Extinction spectra of the synthesized nanoparticle inks and printed substrates were measured using a spectrometer covering both 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, SigmaAldrich) 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 inkjet printing of thermoplasmonic nanoparticles, we used the polyelectrolyte layer-by-layer coatingbased nanoparticle inkjet printing process we recently developed.16 First, 10 mg/mL positive polyelectrolyte, poly(allylamine hydrochloride) (PAH) (283215, Aldrich) and negative polyelectrolyte poly(4-styrenesulfonic acid) (561258, Aldrich) was dissolved in 10 mM NaCl aqueous solution. 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 ethylenepropylene flexible membranes (FEP membranes) for nanoparticle printing and 100 μm-thick poly(ethylene terephthalate) films 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 to avoid nozzle clogging. All printing was carried out in a clean room facility at room temperature. Printed GNR ink areal density 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 were printed within 100 μm × 100 μm area for 4 times (i.e., four 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 (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 (UDCD dark-field condenser, Olympus) and phase-contrast microscopy (IX71, Olympus) were conducted. Bright-field microscopy was conducted using an upright microscope (BXFM, Olympus). Photothermal Effect Characterization and Multiwavelength Thermoplasmonic 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 (CW) 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 mmdiameter illumination area, and the other higher power (4 W) unit was used 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, 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 (XLP123S-H2, Gentec-EO, 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 the accurate power density value. Therefore, photothermal effect was characterized on the basis of the overall emitted light power instead of power density. Temperature change by photothermal effect was detected by a handheld infrared camera (E40, FLIR). By extracting raw recording data and subtracting baseline temperature profile right before the illumination for 1 s, we processed only the profile of temperature change caused by the photothermal effect.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b19342. Additional Supporting Information figures (PDF) Encrypted thermoplasmonic image heat generation movie for the inkjet-printed thermoplasmonic image sample in Figure 4b (AVI) Similarly printed conventional color laser printed image in Figure 4e, bottom not showing thermoplasmonic patterns (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hongki Kang: 0000-0002-6729-5486 Yoonkey Nam: 0000-0003-4801-9547 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2015R1A2A1A09003605). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 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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REFERENCES
(1) Smith, A. F.; Skrabalak, S. E. Metal Nanomaterials for Optical Anti-Counterfeit Labels. J. Mater. Chem. C 2017, 5, 3207−3215. (2) Yoon, B.; Lee, J.; Park, I. S.; Jeon, S.; Lee, J.; Kim, J.-M. Recent Functional Material Based Approaches to Prevent and Detect Counterfeiting. J. Mater. Chem. C 2013, 1, 2388−2403.
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DOI: 10.1021/acsami.7b19342 ACS Appl. Mater. Interfaces 2018, 10, 6764−6771
Research Article
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DOI: 10.1021/acsami.7b19342 ACS Appl. Mater. Interfaces 2018, 10, 6764−6771
