Open Access
Add to BibSonomy
Abstract
High security and convenient operation have been the aim of the anticounterfeiting field. An anticounterfeit system in which multiple information are written on an overlapping area with quantum dot (QD) inks of different fluorescence wavelengths that combines spectroscopy technology for information identification is proposed in this study. Because the emission spectra of QDs can be tuned by simply changing the size, ultraviolet (UV) glue mixed with different sizes of CdSe/ZnS QDs is used as the printing ink with different fluorescence wavelengths. Software-design labels with different information are printed on the overlapping area using inks with different luminescent QDs. The printed information can only be identified by the bandpass filter with the corresponding wavelength under UV light. Under natural light, the information cannot be identified by the naked eye. In the proposed anticounterfeiting system, the excitation light and filter are both indispensables. Our method makes the fabrication of anticounterfeit labels flexible in design, fast in production, and high in information concealment. Meanwhile, the proposed system is quick and convenient, which has huge application potential in the field of display and anticounterfeiting.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Anticounterfeit technology always plays a paramount role in the fields of commercial and national public security, such as brands, banknotes, tickets, and essential documents [1–5]. With the stable and rapid world economic growth, especially the rapid increase of small businesses, sole traders, and personal demands, there is an urgent need to develop low-cost, simple manufacturing, highly secure, and personalized anticounterfeit technology. To meet this emerging requirement, various anticounterfeit systems have been developed in the past decades, e.g., magnetic response and laser holography [6–10]. However, it is complex to manufacture anticounterfeiting products using magnetic response technology [11]. Meanwhile, there is no technical method to prevent counterfeiters from maneuvering laser holography anticounterfeit technology [12].
Besides, simple operation, short manufacturing cycle, and flexible design are advantages of inkjet printing anticounterfeit technology. However, the conventional single-layer has poor identification ability of information density and is much easy to copy. To avoid the abovementioned problems, an ideal inkjet printing anticounterfeit system is proposed in this study. First, anticounterfeit labels containing different information are printed on an overlapping area by different fluorescent inks. Then, the information on different labels can only be seen through the corresponding bandpass filters in ultraviolet (UV) light. By designing the identifier and reasonably controlling the parameters of the inkjet printing process, the miniature multilayer security labels can be well prepared. Moreover, the anticounterfeit labels are undetectable in an ambient environment, making the anticounterfeit highly secure.
2. Experiment
2.1 Luminous characteristics of quantum dot (QD) printing ink
Owing to the straightforward manufacturing method and high quantum yields, we selected the CdSe/ZnS QDs as the fluorescent material [13]. The concentrations of the CdSe/ZnS QDs with different fluorescence wavelengths we selected were all 5 mg/ml. The core–shell structure of QDs coated with a layer of ZnS was to ensure high stability of photoluminescence (PL) properties and improve optical and electronic properties. At the same time, we tested the quantum efficiencies of QDs materials. The QDs with the wavelength of 475 nm is about 85%, and the QDs with the wavelength of 529 nm and 628nm are about 90%. The Jetlab II inkjet printing system (MicroFab Technologies Inc.) was used to fabricate anticounterfeit labels. According to the printing system requirements for the surface tension (20–70 dynes/cm) and viscosity (0–30 cps) of the ink, a suitable UV glue was used as the solvent for CdSe/ZnS QDs. The UV glue is purchased from Polomo Co. In addition, the UV glue could isolate the CdSe/ZnS QDs from air and enhance the long-term stability of PL.
Transmission electron microscopy (TEM) images of the printing ink were obtained using an FEI Tecnai G2 f20 s-twin 200kV [ Figs. 1(a–c)]. Notably, the CdSe/ZnS QDs had uniform particle sizes. They were uniformly distributed in the UV adhesive without aggregation. The average particle sizes in Figs. 1(a–c) are 6, 7.2, and 9 nm, respectively. The corresponding fluorescence wavelength is shown in Fig. 1(d). The excitation wavelength for PL measurement was 300 nm. As the particle size increases, the fluorescence has a redshift. The size depended on the emission peaks, and the fluorescence emission was narrow, which traditional inks (dyes) do not possess. Owing to the above unique optical properties, the QD printing ink has broad application prospects in anticounterfeiting. Because the printed anticounterfeit pattern needs to be cured by UV light, the influence of curing on the optical properties of QD printing inks was also investigated. Figure 2 shows the fluorescence spectrum of the QD printing inks before and after curing. The emission peak does not shift, and the intensity of the emission spectrum change is minute. Therefore, curing does not affect the optical properties of QD printing inks.
Fig. 1. TEM images of CdSe/ZnS QD printing ink and fluorescence characteristics. (a–c) typical TEM images, with CdSe/ZnS QD particle sizes of 6, 7.2, and 9 nm, respectively. (d) The corresponding fluorescence characteristics of the printing ink with different particle sizes of CdSe/ZnS QDs.
2.2 Precisely controllable miniaturization and high-resolution anticounterfeit labels
The drop-on-demand (DOD) system was employed in the Jetlab II inkjet printing system. A droplet can only be generated when a voltage pulse is applied to the nozzle’s piezoelectric ceramic. Therefore, printing can be accurately controlled by controlling the inkjet printing parameters [14,15]. Figure 3 shows how the inkjet printing system generates and ejects droplets (nozzle diameter = 60µm) at 2 kHz. The printing ink with proper viscosity and surface tension can form stable droplets under the control of printing parameters. The pattern comprises many droplets.
Fig. 3. The generation and ejection process of an 60 µm droplet in the inkjet printing system at 2 kHz.
Moreover, each droplet is a pixel, so the droplet size determines the pattern resolution. We can obtain droplets of different sizes using nozzles of different diameters or by optimizing control parameters. The idea of using this technology makes printing patterns convenient and fast. Figure 4 shows the generated droplets for nozzle diameters of 20 and 80 µm. The droplet size in Fig. 4(a) is 21 µm, in which the nozzle diameter is 20 µm. As shown in Fig. 4(b), the droplet size is 73 µm, in which the nozzle diameter is 80 µm. The inset is the result of the above droplets printed on the substrate. It can be seen that the resolution of the printed pattern can be adjusted by changing the size of the droplets. Nozzle diameter had a significant influence on droplet size. Besides, by only adjusting the printing parameters, droplet diameter can be fine-tuned by about 10% according to the nozzle diameter. Therefore, according to the actual needs, the labels’ micro-degree and resolution can be flexibly and accurately controlled by choosing different nozzle diameters and optimizing the printing parameters.
Fig. 4. Results of droplets produced by different nozzles. (a) Droplet size is 21 µm nozzle when diameter is 20 µm. (b) Droplet size is 73 µm when nozzle diameter is 80 µm. Inset: the result of the droplets printed on the substrate.
2.3 Fabrication of multilayer miniature anticounterfeit labels
A schematic of the fabrication of multilayer miniature anticounterfeit labels is shown in Fig. 5. Using CAD drawing software, the label fabrication becomes flexible and straightforward. The Jetlab II inkjet printing system is shown in Fig. 5(a). According to the label’s resolution and size requirement for application, ideal droplets can be obtained by selecting a suitable nozzle and optimizing printing parameters (Fig. 5(b)). Then, QD inks with different fluorescence peaks are used to print multilayered anticounterfeit labels on an overlapping area, and UV light is used to cure them (Fig. 5(c)). The multi-layer method is that printing layer by layer, and curing after all layers are printed.
Fig. 5. A schematic of the fabrication of multilayer miniature anticounterfeit labels a) Inkjet printing system-based software used for label fabrication. b) Ideal droplets can be obtained by selecting a suitable nozzle and optimizing printing parameters c) Different labels are printed on an overlapping area and cured using UV light. d) The information in each layer is revealed by the corresponding bandpass filter under UV light.
Finally, the anticounterfeit information in each layer is revealed by a corresponding bandpass filter under UV light (Fig. 5(d)). Notably, the pattern information cannot be obtained using only UV light without a corresponding filter. In the absence of UV light, we cannot even find anticounterfeit labels. This technology significantly improves the security of anticounterfeiting.
3. Results and discussion
According to the above experimental steps, three CdSe/ZnS QDs (fluorescence peak = 475, 529, and 628 nm) were employed as the fluorescent materials for printing anticounterfiet information. The concentrations of the three printing inks were 1, 5, and 10 mg/ml, respectively. Because the nozzle diameter determines the picture size, to obtain pictures using the available camera, a nozzle with a diameter of 60 µm was chosen. The information on miniature patterns made with smaller nozzles could still be identified. Three types of anticounterfeit information were designed in the software, and the different CdSe/ZnS QDs were used to print them on an overlapping area of A4 papers.
Figure 6(a) shows that the prepared pattern of “IOE” by printing QDs of different particle sizes are visible under UV excitation. The “IOE” is a pattern printed in different positions to show the results of inkjet printing of QDs material, and there is no overlay printing, therefore no bandpass filters are used. The size of QDs used to print “I”, “O” and “E” are 6, 7.2 and 9 nm, respectively. Then, we used the QDs to fabricate anti-counterfeiting patterns by multi-layer printing method. The finished multilayer anticounterfeit labels are entirely invisible in daylight via naked eyes, as shown in Fig. 6(b). Moreover, because the three CdSe/ZnS QDs emit light at the same time, we cannot separately distinguish the information carried by labels when only the UV light is used to illuminate the anticounterfeit labels. By contrast, when the corresponding bandpass filter is chosen to identify the information, each layer’s confidential information can be seen under UV light. The center wavelengths of the filters were 630, 530, and 470 nm, respectively. The narrow emission spectra of QDs, as well as the narrow-band filter production technique, are both well-known. Therefore, by selecting a series of QDs with different wavelengths and using the corresponding bandpass filters, a minimal area can load a large amount of information according to this theory, which has excellent application prospects in the anticounterfeiting field.
Fig. 6. Generation of multilayer QD anticounterfeit labels. (a) Multilayer anticounterfeit labels in daylight. (b) The multilayer anticounterfeit labels under UV light. And the multilayer anticounterfeit labels under UV light and identified by the corresponding bandpass filters.
Figure 7 shows that we can make very intricate anticounterfeit patterns in a small area by adjusting printing parameters. Without the corresponding bandpass filter, whether we use UV lamps or not, our electronic devices cannot recognize the quick response code. As shown each layer’s information can be clearly revealed when the corresponding bandpass filter is chosen. The experimental results demonstrate that the proposed anticounterfeit system has many advantages, such as simple pattern making and easy handling, high label accuracy, and high antcounterfeit security.
Fig. 7. Generation of high-resolution anticounterfeiting patterns based on multilayer QDs. And the multilayer anticounterfeit labels under UV light and identified by the corresponding bandpass filters.
In order to ensure the reliability of the preparation pattern of the QDs material, we studied the stability of the QDs material in the natural environment. The luminescent stability of the QDs is measured in 72 h at 25 ◦C. As shown in the Fig. 8, the fluorescence intensity of different color QDs fluctuated within a certain period of time, but the maximum fluctuation is only 7.5%. Therefore, the intensity fluctuation can be ignored. Moreover, the fluorescence wavelength of the QDs does not shift, which provides a strong support for reading information through the bandpass filter.
Fig. 8. The luminescent stability of the QDs, which the fluorescence wavelength at (a) 475nm, (b) 529 nm, (c) 628 nm.
4. Conclusion
In summary, we proposed a high-security anticounterfeit system. Using the DOD inkjet printing technique, different fluorescence wavelengths of CdSe/ZnS QDs were employed to print anticounterfeit information on an overlapping area. Further, the corresponding bandpass filters were used for security identification. According to the actual requirements, the label’s micro-degree and resolution can be flexibly and accurately controlled by choosing nozzles of different diameters and optimizing printing parameters. Notably, labels’ information is difficult to identify with the naked eyes, whether using UV light. However, when the corresponding bandpass filter is employed in UV light, each layer’s confidential information can be seen. As the QDs with different wavelengths can further carry information, a minimal area can load high-density information using the proposed system. In conclusion, with the advantages of flexible design, simple production, high security, and a large amount of information, the proposed system has great application potential in the anticounterfeiting field.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. H. J. Bae, S. Bae, C. Park, S. Han, J. Kim, L. N. Kim, K. Kim, S. H. Song, W. Park, and S. Kwon, “Biomimetic microfingerprints for anti-counterfeiting strategies,” Adv. Mater. 27(12), 2083–2089 (2015). [CrossRef]
2. Y. Cui, R. S. Hegde, I. Y. Phang, H. K. Lee, and X. Y. Ling, “Encoding molecular information in plasmonic nanostructures for anti-counterfeiting applications,” Nanoscale 6(1), 282–288 (2014). [CrossRef]
3. T. Sun, B. Xu, B. Chen, X. Chen, M. Li, P. Shi, and F. Wang, “Anti-counterfeiting patterns encrypted with multi-mode luminescent nanotaggants,” Nanoscale 9(8), 2701–2705 (2017). [CrossRef]
4. M. You, M. Lin, S. Wang, X. Wang, G. Zhang, Y. Hong, Y. Q. Dong, G. R. Jin, and F. Xu, “Three-dimensional quick response code based on inkjet printing of upconversion fluorescent nanoparticles for drug anti-counterfeiting,” Nanoscale 8(19), 10096–10104 (2016). [CrossRef]
5. R. D. H. Julien Andres, J.-E. Moser, and A.-S. Chauvin, “A New Anti-Counterfeiting Feature Relying on Invisible Luminescent Full Color Images Printed with LanthanideBased Inks,” Adv. Funct. Mater. 24(32), 5029–5036 (2014). [CrossRef]
6. H. Hu, H. Zhong, C. Chen, and Q. Chen, “Magnetically responsive photonic watermarks on banknotes,” J. Mater. Chem. C 2(19), 3695 (2014). [CrossRef]
7. K. Jiang, L. Zhang, J. Lu, C. Xu, C. Cai, and H. Lin, “Triple-Mode Emission of Carbon Dots: Applications for Advanced Anti-Counterfeiting,” Angew. Chem. Int. Ed. 55(25), 7231–7235 (2016). [CrossRef]
8. H. Kim, J. P. Ge, J. Kim, S. Choi, H. Lee, H. Lee, W. Park, Y. Yin, and S. Kwon, “Structural colour printing using a magnetically tunable and lithographically fixable photonic crystal,” Nat. Photonics 3(9), 534–540 (2009). [CrossRef]
9. Y. Liu, K. Ai, and L. Lu, “Designing lanthanide-doped nanocrystals with both up- and down-conversion luminescence for anti-counterfeiting,” Nanoscale 3(11), 4804 (2011). [CrossRef]
10. H. Hu, J. Tang, H. Zhong, Z. Xi, C. Chen, and Q. Chen, “Invisible photonic printing: computer designing graphics, UV printing and shown by a magnetic field,” Sci Rep 3(1), 1484 (2013). [CrossRef]
11. L. Bai, Z. Y. Xie, W. Wang, C. W. Yuan, Y. J. Zhao, Z. D. Mu, Q. F. Zhong, and Z. Z. Gu, “Bio-Inspired Vapor-Responsive Colloidal Photonic Crystal Patterns by Inkjet Printing,” ACS Nano 8(11), 11094–11100 (2014). [CrossRef]
12. D. Vather, I. Naydenova, D. Cody, M. Zawadzka, S. Martin, E. Mihaylova, S. Curran, P. Duffy, J. Portillo, D. Connell, S. McDonnell, and V. Toal, “Serialized holography for brand protection and authentication,” Appl. Opt. 57(22), E131–E137 (2018). [CrossRef]
13. R. F. Wang, X. M. Wei, J. Xie, B. M. Wang, and X. T. He, “One-Step Synthesis of CdSe Quantum Dots by Using Hydrazine Hydrate Reduction of Selenium Dioxide,” Aust. J. Chem. 71(7), 524 (2018). [CrossRef]
14. M. Singh, H. M. Haverinen, P. Dhagat, and G. E. Jabbour, “Inkjet printing-process and its applications,” Adv. Mater. 22(6), 673–685 (2010). [CrossRef]
15. V. Wood, M. J. Panzer, J. L. Chen, M. S. Bradley, J. E. Halpert, M. C. Bawendi, and V. Bulovic, “Inkjet-Printed Quantum Dot-Polymer Composites for Full-Color AC-Driven Displays,” Adv. Mater. 21(21), 2151–2155 (2009). [CrossRef]
