Security Printing of Covert Codes using

NIR-to-NIR Upconversion Inks

Prepared by:

Cecilia Douma

Faculty Advisors:

Dr. Stanley May

REU Site Leader, Department of Chemistry

Dr. Alfred Boysen

Professor, Department of Humanities

Program Information:

National Science Foundation

NSF Grant EEC-1263343

Research Experience for Undergraduates

Summer 2014

University of South Dakota

2

TABLE OF CONTENTS

Abstract ..........................................................................................................................................3

Introduction ..................................................................................................................................4

Broader Impact ............................................................................................................................6

Procedure .......................................................................................................................................6

Visualization of NIR Emission ............................................................................................6

Synthesis and Characterization of NaYF4:Yb/Tm ...............................................................8

NIR Emission through Opaque Ink......................................................................................9

Results and Discussion ............................................................................................................10

Visualization of NIR Emission ..........................................................................................10

Synthesis and Characterization of NaYF4:Yb/Tm .............................................................12

NIR Emission through Opaque Ink....................................................................................14

Conclusion ..................................................................................................................................17

References ...................................................................................................................................17

Acknowledgments.....................................................................................................................18

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Abstract

Upconversion nanoparticles (UCNPs) of β-phase NaYF4 doped with Yb and Tm exhibit

‘upconverted’ near-infrared 800 nm emission when excited with 980 nm light. This so-called

NIR-to-NIR luminescence is of interest for security printing because both the NIR excitation and

the upconverted NIR emission can pass through selective protective opaque ink or dye layers and

cannot be detected by the naked eye. The intensity of the emission is affected by the

concentration of Yb and Tm in the nanoparticles. In this report, we optimize dopant

concentrations for the NIR emission. Working with collaborators at SDSM&T, we develop inks

that can be used to print NIR-to-NIR upconversion images. We then demonstrate that these

images can still be read using CCD cameras even when they are covered with visibly opaque

protective coatings.

4

1. Introduction

Upconversion is the process by which low-energy photons are sequentially absorbed,

exciting electrons first from ground state to an excited state and then from that excited state to an

even higher state. When the electron returns to a lower energy state, a high-energy photon is

emitted.1 Upconversion nanoparticles (UCNPs) have been a focus of recent research because of

their wide range of potential applications, including bioimaging, lasers, lighting, and security

printing.2 Of these UCNPs, lanthanide-doped β-phase NaYF4 is recognized for its exceptionally

efficient near-infrared (NIR) to visible upconversion. The concentrations and combinations of

lanthanide dopants influence both the wavelength and the intensity of the emission.3,4,5 For

example, NaYF4 doped with Yb and Tm is known for its visible blue (440-500 nm) emission.

However, the system also emits 800 nm light. This NIR emission is much more intense than the

blue, but it is not visible to the naked eye (Figure 1).

Figure 1: Upconversion emission spectra of NaYF4:Yb/Tm (25/0.3 mol %) excited by a 980 nm laser at a

variety of power densities.

800

600

400

200

0

Inte

ns

ity

(d

N/d

t/d

)

1000900800700600500400

Wavelength (nm)

Power Density

10.1 W/cm2

9.2 W/cm2

7.7 W/cm2

6.1 W/cm2

4.6 W/cm2

3.2 W/cm2

1.9 W/cm2

0.7 W/cm2

980 nm

excitation

800 nm

emission

Blue

emission

5

UCNPs have been explored as a method to improve security printing, since inks made

with UCNPs are invisible unless excited at specific wavelengths and are more challenging to

formulate than traditional security inks. The emission color of the inks can be manipulated, and

the combination of color, luminescence, and image created by UCNP security inks is very

difficult to replicate.6 An additional level of protection could be added if the security ink code

some transmit NIR light. This means that UCNPs could be excited even when they are covered

by black ink. The 800 nm emission of NaYF4:Yb/Tm could also pass through the ink, although

the blue emission would be blocked. The NIR emission would not be visible to the naked eye

but could be visualized using a digital camera or smart phone. Therefore, an UCNP image

printed beneath a layer of ink would be undetectable without appropriate equipment and would

be resistant to tampering efforts by counterfeiters (Figure 2).

Figure 2: Diagram of NIR-to-NIR security printing concept.

Paper

UCNPs

Opaque Ink

980 nm

Excitation

800 nm

Emission

Blue Emission

6

2. Broader Impact

Counterfeiting is an area of great concern in the electronics industry, where counterfeit

electronic components are becoming increasingly common. 1,363 counterfeiting incidents were

reported and verified in 2011, a dramatic increase from previous years.7 The most common form

of electronics counterfeiting involves recycling and remarking. Counterfeiters can obtain old,

used, or discarded parts from improperly recycled electronics. They sand off or paint over any

labels or identifying marks, and then they print false labels onto the blank surface and sell the

parts as new. If these improperly labeled parts are used in military, medical, and industrial

technologies, their failure could lead to severe consequences.5 Therefore, it is important for

manufacturers and consumers to be able to verify the authenticity of the electronic components

that they purchase. Currently, most authentication procedures rely on optical inspection of the

part’s characteristics, like its printed labels, indentations, and finish. NIR-to-NIR upconversion

inks have the potential to increase the security of electronic parts because they can be printed

beneath the outer epoxy layer of the part. It would be much more difficult to remove a label

printed below the epoxy layer than a label printed on the surface of the epoxy layer. The

upconversion ink’s NIR emission also could not simply be hidden by an extra layer of paint or

ink on the surface of the part.

3. Procedure

Visualization of NIR Emission

This project relied on a reliable method of visualizing the NIR emission of

NaYF4:Yb/Tm. Since the emission is not visible to the naked eye, a CCD camera was used to

capture images of the excited nanoparticles. To measure the sensitivity of digital cameras to

800 nm light, a Canon Powershot A470 camera was used to record a range of wavelengths from

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500 to 1000 nm as they were projected onto a white sheet of paper by a CARY 5000 UV-Vis-

NIR spectrophotometer. The procedure was repeated using light projected by a FluoroMax 4

spectrometer. For both setups, the camera was unable to adequately detect NIR light. The shell

of the camera was opened, and the factory-installed infrared filter was removed so that the

camera could detect in NIR light.

This modified digital camera was used to visualize the 800 nm emission of the UCNPs.

A piece of white paper with printed UCNPs on it was mounted on a vertical stage, and a 980 nm

diode laser was positioned to excite the printed areas. The modified camera was mounted so that

it record images of the excited UCNPs. A detector was also mounted to record the intensity of

the emissions (Figure 3).

Figure 3: Diagram of spectroscopy setup used to excite printed spots of NaYF4 and to detect and

photograph their emissions.

Modified Digital Camera

980 nm

Diode Laser

Detector

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Synthesis and Characterization of UCNPs

Three separate precursor solutions were formed by reacting yttrium (III) oxide, ytterbium

(III), and thulium (III) oxide with water and acetone and then with oleic acid to form yttrium

oleate, ytterbium oleate, and thulium oleate. Stoichiometrically appropriate quantities of each of

the three precursor solutions, oleic acid (4 mL), and octadecene (16 mL) were added to a three-

neck round-bottom flask containing sodium acetate trihydrate (2 mmol), sodium fluoride (16

mmol), and a magnetic stir bar. The reaction mixture was stirred and heated under vacuum at 80

C for 60 minutes. The temperature was then increased to 120 C, and the reaction mixture was

heated and stirred for an additional 30 minutes. The reaction mixture was heated under argon

gas at 350 C for three hours. Synthesized nanoparticles were washed several times with

acetone, centrifuged, and dried.

The intensity of the upconversion emissions of all newly-synthesized nanoparticles was

determined spectroscopically. After the nanoparticles had been washed and dried, 100 mg of

dried nanoparticles were dispersed in 10 mL of toluene. One solution was made for each unique

combination of dopant concentrations. The upconversion emission spectrum of each

nanoparticle solution was measured using the Blue Wave miniature spectrometer. The

nanoparticles were excited by a 980 nm diode laser at seven different power densities ranging

from 3.23 to 33.31 W/cm2. The absorbance of each solution was measured using a Cary 2000

UV-Vis spectrophotometer. The absorbance spectrum of NaYF4:Yb/Tm contains a peak at 980

nm due to the absorption of the Yb3+ ion. The intensity of this peak corresponds to the relative

number of Yb ions in the solution. The upconversion emission spectra were corrected for

nanoparticle concentration by dividing the original values by the integrated area under the 980

9

nm absorbance peak and multiplying by the molar proportion of Yb in the nanoparticles. The

corrected values are called the “absolute intensity.”

NIR Emission through Opaque Ink

The NIR-to-NIR printing concept was tested through a series of proof-of-concept

experiments in which nanoparticle inks or solutions were covered by a layer of opaque black ink.

The ink used in all experiments was a black inkjet ink manufactured by Hobbicolor. The ink

absorbs visible light but transmits both the 800 nm emission of the nanoparticles and the 980 nm

light used to excited the nanoparticles (Figure 4).

Figure 4: Transmission spectra of Hobbicolor Ink-jet ink and Sharpie ink

Upconversion images were printed using a blue UCNP ink of 5 wt% blue OA-UCNPs

(NaYF4: 25%Yb, 0.3%Tm) in an ink base of 1 wt% PMMA dissolved in a solvent of 90:10 v/v

toluene–methyl benzoate.

100

80

60

40

20

0

%T

1000900800700600500400

Wavelength (nm)

Hobbicolor Inkjet Ink Sharpie Ink

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4. Results and Discussion

Visualization of NIR Emission

The photographs in Figure 5 were taken with the unmodified Canon Powershot A470

digital camera. They show 3-pass blue UCNP ink printed on white paper and excited by a 980

nm diode laser. When no filter is used, a bright spot can be seen when both when the laser is

aimed on print (C) and when it is aimed off print (D), suggesting that the camera detected the

intense 980 nm excitation light that reflected off of the paper instead of detecting the 800 nm

emission of interest. To isolate the 800 nm emission, a filter or combination of filters must be

used to block the 980 nm excitation light.

Figure 5: Left: Blue UCNPs printed on white paper and excited by 980 nm diode laser, photographed with

no filter. Right: 980 nm diode laser aimed at blank paper, photographed with no filter

An 850 nm short pass filter, which blocks all wavelengths of light above 850 nm, was an

attractive candidate for isolating the 800 nm emission because the filter transmits the 800 nm

emission of the UCNPs but blocks the 980 nm excitation light from the laser (Figure 6). In some

situations, a 750 nm short pass filter could be used in addition to the 850 nm short pass filter to

block the 800 nm emission so that only the visible blue emission could be detected by the

camera.

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Figure 6: Transmission spectra of 750 nm shortpass filter (red) and 850 nm shortpass filter (black). The

upconversion emission spectrum of NaYF4 is shown for reference in blue.

To test the 850 nm short pass filter, four 3-layer printed circles of blue UCNP ink were

printed on an index card and excited with a 980 nm CW laser (power density of 4 W/cm2). The

modified Canon Powershot, equipped with the 850 nm short pass filter and a 715 long pass filter,

was used to record a video as the card was moved left and right across the stationary laser beam.

Bright spots appeared when the beam passed over the printed UCNPs, and only a dim spot was

visible when the beam passed over unprinted paper, suggesting that 850 nm short pass filter

could effectively block 980 nm excitation light but transmit the 800 nm emission.

100

80

60

40

20

0

Tra

nm

issio

n

900800700600500

Wavelength, nm

750 nm shortpass 850 nm shortpass Blue UCNPs

12

Figure 7 shows a printed UCNP circle excited by a 980 nm laser. The pictures were taken

with the 850 nm short pass filter only, the 850 nm short pass and 715 nm cutoff filters, and the

850 nm short pass and 475 nm cutoff filters, respectively. A bright spot is visible with all three

combinations of filters, suggesting that the 850 nm short pass filter does not block the emissions

of the nanoparticles. The 715 nm cutoff filter blocks visible light, so the bright spot in the center

picture is the 800 nm emission alone. This emission is intense and can be detected by our

camera when the camera is equipped with the 850 short pass filter.

Figure 7: Blue UCNPs printed on white paper and excited by 980 nm diode laser, photographed with 850 nm

short pass filter (left), 850 nm short pass and 715 cutoff filters (center), and 850 nm short pass and 475 nm

cutoff filters (right).

Synthesis and Characterization of UCNPs

Spectroscopic analysis showed that NaYF4 doped with 46.7% Yb and 3.7% Tm produced

the 800 nm emission with the greatest absolute intensity. NaYF4 doped 25% Yb and 0.3% Tm

produced the 800 nm emission with the lowest absolute intensity. When the concentration of Tm

was increased from 0.3% to 2% and then to 3.7%, the visible blue emission was eliminated, but

the absolute intensity of the 800 nm emission increased (Figures 8 and 9).

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Figure 8: Absolute intensity of NaYF4:Yb/Tm emission under 980 nm excitation. An expansion of the visible

region of the spectrum is shown at right.

Figure 9: Log of the absolute intensity of the 800 nm emission vs. log of the excitation power

Optical observations reinforced the spectroscopic findings. For each combination of

dopant concentrations, a spot of nanoparticles in toluene was smeared on white paper. The

nanoparticles were excited with a 980 nm laser and photographed once with the 850 nm short

pass filter and again with the 850 nm short pass and the 750 nm short pass filters (Figure 10).

7.5

7.0

6.5

6.0

Lo

g In

ten

sit

y (

dN

/dt/

d

)

1.41.21.00.80.6

Log Power (W/cm2)

Log (I) =1.38 ± 0.03 * Log (P) + 4.98 ± 0.03Log (I) =1.47 ± 0.1 * Log (P) + 4.88 ± 0.11Log (I) =1.40 ± 0.02 * Log (P) + 5.45 ± 0.02

25%Yb 0.3%Tm 25%Yb 2%Tm 43.6%Yb 3.7%Tm

1.4x106

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Inte

nsit

y, d

N/d

t/d

900800700600500400

Wavelength, nm

25%Yb 0.3%Tm 46.3%Yb 3.7%Tm 25%Yb 2%Tm

14x103

12

10

8

6

4

2

0

Inte

nsit

y, d

N/d

t/d

700650600550500450400

Wavelength, nm

25%Yb 0.3%Tm 46.3%Yb 3.7%Tm 25% Yb 2%Tm

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When only the 850 nm short pass filter is used, the camera can detect both the visible blue

emission and the 800 nm emission of the nanoparticles. When the 750 nm short pass filter is

added, the 800 nm emission is blocked and only the visible blue emission can be detected by the

camera. A bright spot is visible in all three photos with the 850 nm short pass filter only, but in

the photos with both the 850 nm short pass and 750 nm short pass filters, the camera can only

detect a significant emission from the 25% Yb and 0.3% Tm nanoparticles.

Figure 10: UCNPs with three different dopant concentrations printed on white paper and excited by 980 nm

diode laser, photographed with 850 nm short pass filter (top) and 850 nm and 750 nm short pass filters

(bottom)

NIR Emission through Opaque Ink

Spots of UCNP in toluene were smeared on white paper and then covered by layers of

Hobbicolor ink with varying density. The ink-covered spots were photographed once using the

modified digital camera with the 850 nm short pass filter to block the excitation light and again

using the camera with both the 850 nm short pass filter and the 750 short pass filter to block the

excitation light as well as the 800 nm emission (Figure 11). When only the 850 nm short pass

filter was used, a bright 800 nm emission was visible through all ink coatings. When the 750 nm

short pass filter blocked the 800 nm emission, the brightness of the blue emission decreased with

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increasing ink density. No emission was visible when the UCNPs were covered by a 2-pass

layer of Hobbicolor ink.

Figure 11: Blue UCNPs in toluene smeared on white paper and excited by 980 nm diode laser,

photographed with 850 nm short pass filter (right column) and 850 nm and 750 nm short pass filters (left

column). UCNPs were covered in layers of ink of varying density, as described in the diagram at right.

Next, a series of images printed in blue UCNP ink were printed on paper and covered

with a 2-pass layer of Hobbicolor ink (Figure 12). Under 980 nm excitation, the modified

camera could capture well-defined NIR images even when the nanoparticles were completely

covered by opaque ink.

2-Pass Inkjet Ink

No Inkjet Ink

UCNPs

850 nm short pass filter:

800 nm and blue emission

850 nm and 750 nm short pass filters:

blue emission

No Inkjet Ink

2-Pass Inkjet Ink

Increasing Ink Density

16

Figure 12: Blue UCNP ink images printed on white paper and covered by a 2-Pass layer of black Hobbicolor

inkjet ink excited by 980 nm diode laser.

Blue UCNP ink images were then printed on thin layers of clear epoxy resin and covered

with a layer of epoxy mixed with Hobbicolor ink approximately 1 mm thick (Figure 13). Again,

when the UCNP ink was excited through the opaque epoxy coating, the camera could read well-

defined NIR images. These examples demonstrate the potential for NIR-to-NIR printing in anti-

counterfeiting applications. As we anticipated, the NaYF4:Yb/Tm can be excited through an

opaque coating and emit 800 nm light that can also pass through the opaque coating and be

detected by a digital camera.

17

Figure 13: Blue UCNP ink images printed on epoxy and covered by a 1 mm layer of epoxy mixed with black

Hobbicolor inkjet ink excited by 980 nm diode laser. A: USD logo under ambient room light with no

excitation. B: USD logo with excitation light and ambient room light. C: QR code with excitation and

ambient room light. D. QR code with excited in dark room.

6. Conclusion

Initial experiments prove that the 800 nm emission of NaYF4:Yb/Tm can be detected by a

CCD camera even when the nanoparticles are buried under a layer of opaque ink. By increasing

the concentrations of Yb and Tm in the nanoparticles, the intensity of the 800 nm emission can

be increased, and the blue emission can be eliminated, making the nanoparticles an even more

promising candidate for NIR-to-NIR security printing.

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References

1. Werts, M.H.V. (2000). Luminescent Lanthanide Complexes: Visible Light Sensitised Red

and Near-infrared Luminescence.

2. Suyver, J.F., Grimm, J., van Veen, M.K., Biner, D., Kramer, K.W., & Gudel. (2005).

Upconversion spectroscopy and properties of NaYF4 doped with Er3+, Tm3+ and/or Yb3+.

Journal of Luminescence, 117, 1-12.

3. Meruga, J.M., Baride, A., Cross, W., Kellar, J.J., & May, P.S. (2014) Red-green-blue

printing using luminescence-upconversion inks. Journal of Materials Chemistry C, 2, 2221-

2227.

3. Wang, F., & Liu, X. (2008). Upconversion Multicolor Fine-Tuning: Visible to Near-Infrared

Emission from Lanthanide-Doped NaYF4 Nanoparticles. Journal of the American Chemical

Society, 130, 5642-5643.

4. Yin, A., Zhang, Y., Sun, L., & Yan, C. (2010). Collodial synthesis and blue based multicolor

upconversion emissions of size and composition controlled monodisperse hexagonal

NaYF4:Yb,Tm nanocrystals. Nanoscale, 2, 953-959.

5. Zhao, H., Zhang, X., Wang, X., Gao, H., Zhang, Z., & Cao, W. (2014). Monochromatic

Near-Infrared to Near-Infrared Upconversion Nanoparticles for High-Contrast Fluorescence

Imaging. Journal of Physical Chemistry C., 118, 2820-2825.

6. Cassell, J., & Jaramilla, D. (2012). Reports of Counterfeit Parts Quadruple Since 2009,

Challenging US Defense Industry and National Security. IHS Pressroom.

7. Guin, U., DiMase, D., & Tehranipoor, M. (2013). Counterfeit Integrated Circuits: Detection,

Avoidance, and the Challenges Ahead. Journal of Electronic Testing

Acknowledgments

Funding for this research was provided by the National Science Foundation through a

grant from the Research Experience for Undergraduates program. Special thanks to Dr. Stanley

May, Dr. Alfred Boysen, and Aravind Baride for their guidance, advice, and collaboration.

Thanks also to all faculty, staff, and students at the University of South Dakota and to

collaborators at the South Dakota School of Mines and Technology.