Printing Upconverting Nanoparticles using a Piezoelectric Inkjet Printer. Prepared by: Shawn Ray McCarther

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Alfred Boysen Professor, Department of Humanities Program Information: National Science Foundation Grant #

1 Printing Upconverting Nanoparticles using a Piezoelectric Inkjet Printer Prepared by: Shawn Ray McCarther Faculty Advisors: Dr. Jon Kellar Dr. Grant Crawford Dr. William Cross Dr. Alfred Boysen Professor, Department of Humanities Program Information: National Science Foundation Grant # EEC Research Experience for Undergraduates Summer 2013 South Dakota School of Mines and Technology 501 E Saint Joseph Street Rapid City, SD 57701

2 TABLE OF CONTENTS Abstract... 3 Introduction... 4 Broader Impact... 5 Procedure... 6 Results Discussion Conclusion Future Work References Acknowledgments

3 Abstract Inkjet printing is developing into more than a way to print digital documents onto paper. It is being used in innovative new ways, depositing precise amounts of various substances onto an ever-increasing number of substrates. There are many types of inkjet printers; this research involves ink formulations for piezoelectric inkjet printing. Piezoelectric inkjet printing offers the potential for exceptional print resolution. Ink formulations in this study were analyzed to meet printer requirements and printed to optimize print characteristics. The solvents need to be able to incorporate a significant weight fraction of polymer additive to print a continuous, thin film. Upconverting nanoparticles (UCNPs) were added to solution and studied to determine the plausibility of using them in security printing. To achieve effective printing, initial research focused on formulating an ink with the necessary rheological properties. This included viscosity and surface tension measurements; contact angle measurements were also taken. Solvents of ethylene glycol (EG) and water, were found to have the appropriate rheology, but did not always remain stable with a significant weight fraction of polymer additive. A solution of EG and 2- buthoxynethanole with 1 weight fraction Polyvinylpyrrolidone (PVP) was found to meet our printer requirements, as well as fully disperse UCNPs; printing is still to be done with this solution. 3

4 1. Introduction Inkjet printing is developing into more than a way to print digital documents onto paper. It is being used in innovative new ways, depositing precise amounts of various substances onto an ever-increasing number of substrates. In the study to follow, ink formulations will be analyzed and printed to optimize print characteristics. Then, upconverting nanoparticles (UCNPs) will be added and studied to determine the plausibility of using them in security printing. The goal of this project is to develop a water-solvent based ink that will properly disperse nanoparticles and covertly for security purposes. The first step was composing an ink compatible with the Trident Piezoelectric Inkjet printer assembly used in this study. Varied ratios of water to ethylene glycol were analyzed with polymer additives to achieve the necessary rheological properties. The properties characterized included viscosity, surface tension, and contact angle. To achieve effective printing, a viscosity in the range of 6 30 cp and surface tension between mn/meter is required. The ink should also remain stable over time. If the inks contain particles, such as the UCNP s, the most common problem is aggregation of the particles and related printing problems such as clogging. When nanoparticles approach each other, small scale molecular interactions can cause the particles to aggregate to reach a lower energy state. This is the reason why capping agents on nano-scale particles are often used. In this case, polyacrylic acid capping agent allows the UCNP s to be evenly dispersed into polar solvents, without later aggregation 4

5 2. Broader Impact Security printing is a broad and comprehensive field very important in the prevention of counterfeiting. While numerous forms of security printing exist, this research focuses on the novel approach of printing upconverting nanoparticles using a piezoelectric inkjet printer. Once successful parameters are created, this process can be scaled up to perform security printing on a high-production line. SDSM&T researchers have previously demonstrated the printing of UCNP s into a quick response (QR) code. When the NIR laser is shone onto the QR code, the upconversion of light waves occurs making the printed code visible. In figure 3, a demonstration using aerosol-jet printing methods is shown. While the up conversion of light was successful with aerosol-jet printing, higher resolution is expected to be achieved with the Trident piezoelectric assembly. Figure 3: demonstration of QR code printed with upconverting nanoparticle ink. 5

3. Procedure The first major milestone to be achieved in this research was to formulate an ink that satisfies the rheological requirements in addition to incorporating a polymer additive.

6 3. Procedure The first major milestone to be achieved in this research was to formulate an ink that satisfies the rheological requirements in addition to incorporating a polymer additive. This was done through formulating inks, characterizing the rheological properties of these inks, and then selecting an ink formulation to further analyze and assess its feasibility for security printing. To begin this research, it was first decided to follow the ink formulation previously designed by an SDSM&T researched for the aerosol jet printer. This ink originally contained a toluene to methyl benzoate ratio of 9:1 with 1% poly(methyl methacrylate) (PMMA) by weight. PMMA was dissolved in this solution by heating to degrees Celsius and stirring for 2-3 hours. Characterization began with the use of a Bohlin Instruments viscometer, shown in figure 4. Figure 4: the Bohlin Instruments Viscometer 6

7 The viscosity was found to be much too low to be functional for the Trident printer assembly, as it requires at least a viscosity of 6 cp. After increasing the weight fraction of PMMA in up to as much as 0.25, adequate viscosity was achieved. However, surface tension measurements could not be accurately performed, and it was predicted that the sticky nature of the solution would not perform well in the printer. Next, we selected to water and ethylene glycol (EG) as a pair of polar solvents. Water was chosen as one of the solvents because we were to analyze UCNPs capped in polyacrylic acid, a capping agent on the nano particles making them dispersible in polar solvents, such as water. EG was chosen as the second polar solvent because it has a viscosity of 16 cp, well within the range needed to print. Varying concentrations of these solvents were formulated, containing various amounts of polyvinyl alcohol (PVA). Viscosity, surface tension, and contact angle measurements were taken on these formulations. Bohlin Instruments viscometer was again used for the viscosity measurements, and the surface tension and contact angle measurements were taken with Drop Pro Imaging Equipment/software. Figure 5 shows a drop being ejected from the Drop Pro for surface tension analysis next to a screenshot of the computer software. 7

steel nozzles. Figure 6 shows contact angle measurements being taken on a piece of steel.

8 Figure 5: The Drop Pro imaging equipment and computer software. In addition, contact angle measurements were taken on steel, as the Trident Print-head assembly ejects from steel nozzles. Figure 6 shows contact angle measurements being taken on a piece of steel. Contact angle does not need to be in a defined range for initial printing, however, but was investigated to get additional insight into how the rheology of the solutions are changing. 8

9 Figure 6: Contact angle measurements being taken with Drop Pro Imaging Equipment. After formulating table 1, show in the Results, an ink composition approaching the required rheology was selected to begin the printing process. Progress was halted, however, because the Trident Print-head required maintenance for further use; this explained more in depth in the discussion section. The final approach taken to formulate an ink with appropriate rheological properties, a stable weight loading of polymer, and be able to disperse the UCNPs was advised by Dr. Ravi Shankar. Previous research done by Dr. Shankar using the solvent combination of 70 percent ethylene glycol paired with 30 percent 2-buthoxynethanole, in addition to 0.01 weight fraction polyvinylpyrrolidone (PVP). Although this solution did not contain water, the solvents are polar and the UCNPs were expected to disperse. Rheology characterizations were done on this solution in addition to stability analysis and a test to determine if one weight percent UCNPs would 9

10 remain dispersed in this solution over time. This was done by the use of a water sonication device for one minute. 4. Results Table 1 shows the results of the viscosity, Surface tension, and contact angle measurements. The viscosity and surface tension raise and drop, respectively, with the addition of more EG. Table 1: this table shows viscosity and surface tension raising and lowering, respectively, as the ratio of water to ethylene glycol drops Ink containing 1% PVA: varying ratio of water to ethylene glycol Mw(PVA) 31,000 50,000 g/mol, ethylene glycol 99% hydrolyzed Water : EG Viscosity (cps) Surface tension (mn/meter) Contact angle (steel) 90:10 (%) :15 (%) :20 (%) :40 (%) :50 (%) :60 (%) :70 (%) :80 (%) The results of the viscosity and surface tension measurements taken on the ink composed with EG, 2-buthoxynethanole, and PVP yielded a viscosity of 12.2 cp and a surface tension of mn/meter. Additionally, the ink remained stable while being able to disperse the UCNPs with sonication. 10

11 5. Discussion With an ink composition of 20% water, 80% EG, and containing 1 wt% PVA, a rheology near the required values was developed. The surface tension approached the preferred value, although it was slightly higher than desired. The viscosity was in the appropriate range, so preliminarily printing should have been possible. The instability of the ink, however, is expected to have played the key role in hindering performance. To insure a successful print will be possible, a known stable ink, consisting of EG and 2- buthoxynethanole with PVP polymer, was analyzed. This ink, had the following measured rheology: 31.5 mn/meter surface tension, 12.2 cp viscosity. Though UCNP s were able to be successfully dispersed into this ink solution, optimizing its print characteristics for the Trident print-head was not able to be done due to pending maintenance of the assembly. 6. Conclusion Adequate viscosity can be achieved by increasing the ratio of ethylene glycol to water; surface tension neared the required value. However instability of solution occurred at and below 30% H 2 O when using 1.00 weight percent PVA. Because the Trident print-head required maintenance, printing analysis could no longer be performed on the inks. In order to insure the use of a working ink, a known printable solution of EG and 2-buthoxynethanole (instead of water), along with PVP polymer, was composed. With sonication, weight loadings up to 1 weight percent UCNPs were successfully dispersed. 11

12 7. Future Work Future work for this project would include performing a print with the solution of 70% EG, 30% 2-buthoxynethanole, and 1.00 weight percent PVP to analyze and optimize print characteristics. These characteristics, among others, could include print resolution, printhead velocity, drop consistency, and overall visibility of the UCNP s. Additionally, ink solutions composed of water, EG, and PVA can be analyzed with lower weight loadings of PVA to achieve an optimal rheology as well as allowing the inks remaining stable over time. 12

13 References 1. Magdassi, S. (2010). The chemistry of inkjet inks. Singapore: World Scientific Publishing Co. Pte. Ltd. (Magdassi, 2010) 2. Meruga, J., Cross, W., May, P. S., Luu, Q. N., Crawford, A. G., Kellar, J. (2012) Security Printing of Covert Quick Response Codes using Upconverting Nanoparticle Inks. Nanotechnology. 23, (39), Blumenthal, T., Meruga, J., May, P. S., Kellar, J., Cross, W., Ankireddy, K., Vunnam, S., Luu, Q, N. (2012) Patterned Direct Write and Screen Printing of NIR-to-Visible Upconverting Inks for Security Applications. Nanotechnology. 23, (18), Acknowledgements The funding for this research came from the National Science Foundation. Thanks to advisors Dr. Jon Kellar, Dr. William Cross and REU site director Dr. Grant Crawford for their direction and guidance, Professor of English Dr. Alfred Boysen for his critique in writing and speaking, and a special thanks to all of the faculty and staff at SDSM&T and the students working in conjunction with this project. 13

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