Luminescent Hybrids Based on Lanthanide Complexes Applied in Information Cryptography

Abstract

Luminescent materials have attracted attention due to their excellent optical performance and luminescence response to physical and chemical stimuli. In this sense, europium (Eu³⁺) complexes supported on Laponite^® clay, responsive to multiple stimuli, were synthesized for anti-counterfeiting and fingerprint development applications. Inks were produced and deposited in different ways on tracing paper and plastic sheets, and it was investigated how information can be revealed and suppressed by coordinating the hybrid with the chelating agent 4,4-trifluoro-1-phenyl-1,3-butanedione (BTFA) and by heating the substrates containing the luminescent inks with a hot air gun. In addition, the produced material was used to develop fingerprints, with the compound being efficient in elucidating papillary lines. This approach presents luminescent composites that are effective in proving document authenticity and elucidating fingerprints, which are gains for the security area.

Keywords:
Laponite^®; luminescence; luminescent inks; fingerprint analysis; security

Introduction

With the advancement of technology, counterfeit documents have been acquiring increasingly realistic characteristics, making them highly like original documents. Counterfeiting of coins, passports, and diplomas, for example, poses an economic, social, and security problem, generating a growing concern among companies and governments increasingly interested in developing mechanisms that make counterfeiting more difficult.¹-³ In this sense, it is important for security markers to be secure, stable, cost-effective, suitable for scale-up production, and unclonable.⁴,⁵ Faced with this issue, various security elements emerge that can be incorporated into documents according to how these elements should be recognized, their cost-effectiveness, and the potential methods of counterfeiting, such as watermarks, holograms, radiofrequency identification, barcodes, and luminescent inks.⁶-⁸

Luminescent inks have become appealing in the fight against counterfeiting due to their excellent information concealment, making it challenging to counterfeit through printing, photography, or any other means.⁸,⁹ Additionally, these luminescent materials are attractive because they possess properties that are difficult to replicate and easy to recognize. Moreover, they are stimuli-responsive,¹⁰ such as light,¹¹ temperature,¹² pH,¹³ and solvent,¹⁴ with the potential for these materials to respond to multiple stimuli.³

Fingerprints also play a role in combating counterfeiting and information storage, as they carry unique information, serving as personal identity recognition, and providing evidence in solving crimes, given their uniqueness and immutability.¹⁵ Fingerprints (FPs) are produced by sebaceous glands and consist of lipids, amino acids, urea, proteins, and salts¹⁶ from the fingertips, forming individual patterns with friction ridges, grooves, and furrows that repeat regularly.¹⁵

Papillary shapes are unique and it is unlikely that these designs will be repeated in different people or even in other skin areas of the same individual. When you touch an object, you can deposit on its surface what are called fingerprints, which are the mirror image of the ridges, which are raised lines, and the furrows, which are lowered lines of the skin, located between the ridges.¹⁷

The deposited latent fingerprints (LFPs) are invisible to the naked eye and require the use of some resource to identify them. LFPs can be classified according to the details of the ridges, where level 1 is not unique to the individual, but the pattern, ridge orientation, core and delta can be identified, level 2 provides information known as fine points, where bifurcations and crossings of the ridges can be observed, and level 3 brings individual characteristics, which include pores, ridge shape, edge contour and scars.¹⁸,¹⁹

In this sense, luminescent materials have been promising in forensic applications, particularly in the anti-counterfeiting of documents²⁰,²¹ and in the decoding of fingerprints.²²,²³ Navami et al.²⁴ developed luminescent nanoparticles of CaZrO₃:Tb³⁺ by combustion, which are efficient both in the visualization of latent fingerprints and in the manufacture of inks for the encryption of information in the fight against forgery.

When it comes to light emission, lanthanide complexes stand out for their excellent luminescence performance, however, these lanthanide complexes can present disadvantages, such as thermal, photochemical, and mechanical instability²⁵ which can be overcome by combining them with inorganic matrices to improve their properties.²⁶ There are several possibilities for integrating these complexes into matrices, such as silica,²⁷ polymers,²⁸ and zeolites.²⁹

Lamellar clays are versatile and promising materials for overcoming the limitations of lanthanide complexes due to their capacity for expansion, as well as being low-cost and biodegradable.²⁵ What makes clay-containing hybrids more attractive is their ability to hinder the coordination of the ion with luminescent properties to hydroxyls, which suppresses luminescence. Therefore, clays are capable of forming hybrids with improved luminescent properties while keeping their structure intact, since the bonds present between the atoms in the layers are covalent and π-π stacking, with the ability to arrange lanthanide ions and complexes without structural compromise.²⁵,³⁰

Considering the excellent properties exhibited by luminescent hybrid materials, here we report the development of a multimodal response (chemical environment and temperature) clay Laponite^®-based luminescent hybrid applied as advanced security ink for printing and stamp. We also show a simple and efficient procedure for decoding latent fingerprints by means of the powder method through the luminescence.

Experimental

Materials and reagents

Laponite^® (BYK Additives, used without purification) (L^®), absolute ethanol (99.3%), dimethylformamide (DMF) (99.8%), methyl benzene (toluene, 99.5%), acetone (99.55%), bidistilled glycerin (99%), sodium hydroxide micropearls (P.A.), tween 20 (P.A), 3-isocyanatopropyl 3-(triethoxysilane) (TESPIC, 95%), 4-hydroxypyridine-2,6-dicarboxylic (chelidamic acid, 97%) (CD), 4,4,4-trifluoro-1-phenyl-1,3-butadione (BTFA, 99%) and europium nitrate (synthesized) (Eu), were used as starting materials for the preparation of the materials. The synthesis methodology is an adaptation of the protocol developed by de Azevedo et al.³¹ The stoichiometric amount of CD and L^® was optimized (Table S1, Supplementary Information (SI) section) until reaching the material with the highest luminescence.

Instrumentation

X-ray diffraction measurements (XRD) were performed on a Rigaku diffractometer, model SmartLab, with Cu Kα radiation. Diffraction patterns were obtained in the angular range 2θ = 5-80°, with an angular step of 0.01°, a speed of 0.6° min^-1, and a counting time of 1 s.

The scanning electron microscopy (SEM) micrography were obtained in a Tescan Mira 3 equipment, operating at 10 kV acceleration. Previously, the materials were fixed on a stub surface with a carbon tape and metalized with gold.

Fourier transform infrared absorption spectrometry (FTIR) analyses were performed in the wavenumber range between 400 and 400 cm^-1 at 40 accumulations using potassium bromide pellet in a PerkinElmer Spectrum 400 spectrophotometer.

The photoluminescent properties (emission and excitation spectra, and luminescence decay curves) were investigated in Horiba Jobin Yvon spectrofluorometer, model Fluorolog-3 equipped with the continuous (450 W) and pulsed (150 W) xenon lamp. The data were collected with the sample holder at a 45° angle to the emission beam.

Thermogravimetric analyses (TGA) were performed in a Shimadzu TGA-60/60H thermogravimetric equipment with air atmosphere, air flow of 100 mL min^-1, and a heating rate of 10 °C min^-1 from environmental temperature up to 900 °C in a platinum sample holder.

Preparation of L^®-CD

The synthetic procedure for functionalization of Laponitel^® (L^®) clay is based in the methodology previously described by Azevedo et al.,³¹ according to Scheme 1. For Laponite^® (L^®) functionalization, 4.85 mmol of chelidamic acid (CD), 2.42 mmol of TESPIC, and 3 mL of DMF were placed in a round-bottom flask, under magnetic stirring, for 24 h at 120 °C under reflux in a nitrogen atmosphere. After this step, 0.52 g of L^® and 1 mL of toluene were added, and the system was maintained under nitrogen reflux for more 24 h at 120 °C. The resulting material was collected by centrifugation (10.000 rpm), washed with DMF and dried at 60 °C in a vacuum oven for 5 h.

Scheme 1
Representation of functionalization of the L^® clay with Eu-based complex.

Preparation of L^®-(CD)Eu(H₂O)_n

To obtain the L^®-(CD)Eu(H₂O)_n hybrid, an aqueous solution of europium nitrate (Eu) (0.05 mol L^-1, 6 mL) was first prepared and transferred to a borosilicate microwave reactor together with 100 mg of L^®-CD. The mixture was subjected to microwave treatment at 140 °C and 100 W for 20 min. Finally, the material was centrifuged, washed with distilled water, and dried for 5 h at 60 °C in a vacuum oven.³¹

The amount of Eu³⁺ in the L^®-(CD)Eu(H₂O)_n hybrid was quantified by the fluorescence method, in which a calibration curve was obtained from the intensity of the ⁵D₀ → ⁷F₂ transition of the Eu(DPA)₃ complex aqueous coelution, DPA is dipicolinic acid (Figure S12, SI section). The experiment was performed by mixing a solution of Eu(NO₃)₃ and 3 molar equivalents of Na₂DPA solution. After mixing, the mixture was left for 24 h. After this, the luminescence analysis was performed. The same procedure was kept for the supernatant of L^®-(CD)Eu(H₂O)_n synthesis.

Preparation of security paints

The security inks for printing and for pens applications were prepared using water as the dispersion solvent, and the stamp ink was prepared using glycerin as the dispersion solvent. In both cases, the dispersions had a concentration of 5 mg mL^-1 of the L^®-(CD)Eu(H₂O)_n. Initially, L^®-(CD)Eu(H₂O)_n was suspended in the appropriate solvent (water or glycerin) and subjected to an ultrasonic bath for 30 min. Then, 1% (v/v) tween 20 was added, maintaining the ultrasonic bath for more 30 min. Subsequently, the suspensions were then treated with probe ultrasound for 20 min, followed by filtration with a 0.22 μm Millipore filter and resting for one hour.

Deposition of security ink on substrates

For printing security labels, 3 mL of the security ink were inserted into the printer cartridge remanufactured. The coat of arms of the Universidade Federal de Pernambuco, the acronym “UFPE”, circles and stars images, were printed in a single printing cycle on vegetal paper and polyethylene terephthalate (PET) sheet. For writing applications, the ink tube and the 0.5 mm Rollerball pen were washed with water and ethanol. Then, 1 mL of aqueous security ink was introduced into the pen, and the material was deposited on vegetable paper through cursive writing. To stamp the UFPE coat of arms, the L^®-(CD)Eu(H₂O)_n material was suspended in glycerin, and placed in a Petri dish, allowing the stamp to soak in the suspension. The vegetal paper was then stamped with the coated stamp to transfer the design.

Fingerprint preview

Latent fingerprints were collected and visualized using a donor’s fingerprint. Initially, the donor’s fingertips were cleaned with soap and water, were lightly rubbed on the donor’s forehead and pressed onto a glass substrate previously cleaned with isopropyl alcohol. Then, the powder of L^®-(CD)Eu(H₂O)_n was pulverized into the collected fingerprint region using a Fingerprint Brush,³² allowing the latent fingerprint pattern to become visible.

Cell viability assays

The VERO cell line was cultured in 96-well plates at a density of 3 × 10⁵ cells mL^-1, containing RPMI-1640 medium (Sigma Aldrich, St. Louis, MO, USA), supplemented with 10% (m/v) of fetal bovine serum (FBS; Sigma-Aldrich, St. Louis, MO, USA) 1% antibiotic (10,000 international units (IU) mL^-1 (0.6 ug mL^-1) of penicilin g sodium and 10,000 µg mL^-1 of streptomycin, Sigma). The cultures were incubated at 37 °C in a humidified atmosphere containing 5% CO₂ for 24 h for cell adherence. After this period, cell viability was evaluated after incubating the cells with L^®-(CD)Eu(H₂O)_n and L^®-(CD)Eu(BTFA)₃ at concentrations of 100-3 µg mL^-1 for 24 h. Negative control was performed by culturing cells with culture medium. The method used to evaluate cytotoxicity was the MTT (3-(4,5-dimethylthiazol-2-yl bromide)-2,5-diphenyltetrazolium) test. After 24 h of treatment, 10 µL of MTT (5 mg mL^-1 diluted in phosphate buffer, PBS) were added to the wells and incubated for 3 h in the oven at 37 °C. Then, 100 µL of the MTT solubilization solution were added to each well to dissolve the formazan crystals. The plate was kept under stirring for 1 h at room temperature and the optical density was measured at 550 nm in a microplate reader.

Results and Discussion

Structural and morphological analysis

The X-ray diffraction pattern of clay (Figure 1a) shows broad peaks with low intensity, which is common to clays, as they have low crystallinity.³³ The diffraction pattern of L^®-CD (Figure 1a) match the clay pattern with few additional peaks in the region of 15-30° indicating that the functionalization with CD does not significantly change the L^® pristine structure.³⁴ Some additional peaks were observed in the region of 15 and 40°. This change in the profile of the L^®-(CD)Eu(H₂O)_n material (Figure 1a) suggests the formation of a highly crystalline phase. The micrograph of L^® (Figure 1b) shows nanometric structures with non-uniform sizes and a textured structure, due to the stacking of clay platelets. The micrography of L^®-(CD)Eu(H₂O)_n in Figure 1c shows aggregated nanostructures lower than observed for L^®, which we ascribe to the exfoliation and formation of a high crystalline phase.³⁴,³⁵

Figure 1
(a) X-ray diffraction pattern of the materials: L^®, L^®-CD and L^®-(CD)Eu(H₂O)_n, SEM images of (b) L^® and (c) L^®-(CD)Eu(H₂O)_n.

The hybrid formation was monitored by FTIR spectroscopy (Figure 2). The FTIR spectrum for L^® showed a broad absorption band between 3000 and 3900 cm^-1, assigned to the stretching vibration of the O-H bond of the silanol groups and water molecules, and the narrow band between 1567 and 1777 cm^-1 also refers to the bending of the O-H group present in the silanol group.³⁶ In addition, between 815 and 1560 cm^-1 and 400 and 596 cm^-1 there are two characteristic bands of clay, corresponding to the stretching of the Si-O-Si bond, and in the range of 592-812 cm^-1 there is a vibration band related to the stretching of the bonds of the hydroxyl groups linked to magnesium (Mg(OH)₂).³⁷,³⁸

Figure 2
FTIR (KBr) spectra of L^®, CD, L^®-(CD) and L^®-(CD)Eu(H₂O)_n. Characteristic bands of L^® and CD are indicated.

FTIR spectra of L^®-CD confirmed the functionalization of the L^®. The bands at 3606 and 3445 cm^-1, referring to the stretching of the hydroxyl of the carboxylic (COOH) and N-H of the secondary amine groups, respectively, were characteristic of the CD. The signals of CD are shifted to lower wavenumbers when compared to those uncoordinated ligand CD. The characteristic vibrational modes of L^® were also present in the L^®-CD spectrum, between 1777 and 1543 cm^-1, 1222 and 896 cm^-1, 822 and 732 cm^-1 and 528 and 400 cm^-1 were assigned to Si-OH, Si-O-Si, Mg-OH, and Si-O, respectively, confirming the presence of the L^® structure in the hybrid material.

The coordination of the Eu³⁺ ions is evidenced by the appearance of two new peaks at 1568 and 1447 cm^-1, which indicate the asymmetric and symmetric stretching of the carboxylate, respectively.³⁹ The intense band related to the OH stretching remains due to presence of Si-OH of L^® and coordinated H₂O molecule to Eu³⁺.⁴⁰

Luminescent properties

The photoluminescent properties of the materials were obtained in the solid state and at room temperature. As mentioned, the optimized ratio of Eu/L^®-CD was determined based on the intensity and spectral profile of the L^®-(CD)Eu(H₂O)_n (see Table S1, Figures S1 and S2, SI section, for more details). The amount of Eu³⁺ in the L^®-(CD)Eu(H₂O)_n is 0.16 mmol g^-1 (Figure S12, SI section).

Figure 3a shows the emission spectrum of the optimized L^®-(CD)Eu(H₂O)_n, which exhibits red emission when excited at λ_Ex = 330 nm and presents a maximum at 614 nm (⁵D₀ → ⁷F₂). The emission spectral profiles contain narrow bands, characteristic of Eu³⁺ materials, from the excited state ⁵D₀ to the ground ⁷F_J levels (J = 0, 1, 2, 3 and 4). In addition, the material exhibits only one symmetric band related to the ⁵D₀ → ⁷F₀ transition, which indicates a low symmetry environment without an inversion center, and the presence of only one site symmetry around Eu³⁺.⁴¹ Furthermore, the excitation spectra (λ_Em = 614 nm) (Figure 3b) exhibit a broad band between 250 and 340 nm, which is assigned to the π* ← π transitions of the CD ligand, indicating that the ligand act as antennas in sensitizing the luminescence of Eu³⁺ ions.³¹ However, the lower relative intensity of the ligand-centered excitation band than that f-f transition (⁵L₆ ← ⁵D_0,1) suggests that this sensitization is not efficient.

Figure 3
(a) Emission (λ_Ex = 330 nm) and (b) excitation (λ_Em = 614 nm) spectra of the L^®-(CD)Eu(H₂O)_n hybrids.

Table S2 (SI section) shows the results of the luminescence lifetime (τ) after excitation at 395 nm and upon monitoring the emission at 614 nm. The single exponential decay profile agrees with Gaussian profile of the ⁵D₀ → ⁷F₀ transition corroborating the presence of a single chemical environment around the Eu³⁺ ion presented hypothesis (Figures S3 and S4, SI section). The lifetime of 0.29 ms is lower than that reported by Azevedo et al.³¹ (0.41 ms), which used mesoporous silica MCM-48 as a support for the (CD)Eu(H₂O)_n complex. This suggests that the support affects the deactivation dynamics of the excited state. Other reports of hybrids containing Eu-complexes display similar values.⁴²-⁴⁴

Encryption and decryption of information

To demonstrate that the L^®-(CD)Eu(H₂O)_n material acts efficiently as an encryption information device, the security ink was inserted in a remanufactured cartridge of a common inkjet printer. The acronym “UFPE”, geometric shapes of stars and circles (Figure S5, SI section) and the Universidade Federal de Pernambuco coat of arms were printed on tracing paper and PET paper using only one printing cycle, as shown in Figures 4a and 4d.

Figure 4
Printed coat of arms of the Universidade Federal de Pernambuco on tracing paper (a, b and c) and PET paper (d, e and f). Images (a) and (d) were taken under natural light. Images (b) and (e) were taken under UV light (365 nm), exposure, demonstrating the luminescence of the printed security ink. Images (c) and (f) show the same printed patterns after spraying with an ethanolic solution of BTFA, and are under UV light (365 nm) exposure.

As can be seen, the printed label is invisible to the naked eye under both visible and UV light. However, after spraying an ethanolic solution of BTFA (5 mM), the label was revealed, displaying a bright red emission. We ascribe it to a change in the maximum wavelength excitation and highly efficient sensitization capability of the Eu-luminescence by the BTFA ligand, as discussed further on. It is important to note that the patterned tracing paper maintains the Universidade Federal de Pernambuco coat of arms label encoded (invisible to the naked eye) under visible and UV (365 nm) light. To decode printed information requiring a combination of the stimulus of UV light (physical stimulus) and the chemical stimulus with BTFA ligand. As already mentioned, this behavior was due to the replacement of the coordinated water molecules by the chelating agent BTFA (Figure 5), which efficiently acts as a sensitizer of the Eu³⁺ luminescence.⁴⁵ Moreover, inkjet printing proved to be efficient in achieving a uniform deposit L^®-(CD)Eu(H₂O)_n on the substrates.⁴⁶ Figure S6 (SI section) shows that the security ink is versatile using as pen ink and stamp ink, maintaining luminescence sensitive to combined chemical (coordination with BTFA ligand) and physical (UV light) stimuli.

Figure 5
Representation of the chemical changes of the L^®-(CD)Eu(H₂O)_n hybrid after chemical and physical stimuli.

Up to this point, the luminescent behavior of the printed L^®-(CD)Eu(H₂O)_n and L^®-(CD)Eu(BTFA)₃ hybrids is similar to that reported by Azevedo et al.³¹ for Eu(L)_nDAMIC-SiO₂, where L is the chelidamic acid. It now remains to verify the temperature dependence (second physical stimulus) of the luminescence intensity of the L^®-(CD)Eu(BTFA)₃. Figure 6 shows the temporal evolution of the luminescent behavior of the printed security label on tracing paper when it is heated with a heat gun, with the temperature reaching a maximum of 145 °C, as in a previous report.³¹ It can be seen that in a few seconds, the luminescence disappeared, erasing the coat of arms, as expected, and after heating, the luminescence was progressively restored. The same behavior was observed with a stamp and a pen on tracing paper (Figures S7 and S8, SI section). Depending on the time of exposure to hot air, luminescence may be restored (approximately 2 s); however, this luminescence may be extinguished after prolonged heating due to the removal of BTFA molecules, as represented in Figure 6. Therefore, the luminescent inks obtained with the L^®-(CD)Eu(H₂O)_n hybrid showed great potential to be applied as security devices with multimodal response. The information is decoded by the replacement of a coordinated water molecule by a BTFA ligand and exposure to a common ultraviolet light. The luminescence quenching mediated by heating (physical stimulus) corresponds to the authenticity checks step and restores the system to its initial form. It is important to note that L^®-(CD)Eu(H₂O)_n showed excellent stability and adhesion on both surfaces, tracing paper and the PET paper.

Figure 6
Thermal response of the luminescence of the security label “coat of arms of the Federal University of Pernambuco” printed on tracing paper, under common UV (365 nm) lamp irradiation.

The above-discussed phenomena for the L^®-(CD)Eu(H₂O)_n and L^®-(CD)Eu(BTFA)₃ labels had been accompanied by significant changes in the emission intensity and spectral profile, which then presented a maximum at 611 nm (⁵D₀ → ⁷F₂). Figure 7 shows the emission spectra of the printed L^®-(CD)Eu(H₂O)_n (green line) and L^®-(CD)Eu(BTFA)₃ (red line) on tracing paper. It can be seen that the emission intensity increases by two orders of magnitude, and the spectral profile changes with the replacement of the coordinated water molecule by the BTFA ligand, indicating changes in the coordination polyhedra. The emission spectral profile of L^®-(CD)Eu(BTFA)₃ is the same as that of the Eu(BTFA)₃DAMIC-SiO₂ hybrid, reported by Azevedo et al.³¹ It suggests that the coordination environment of Eu ions in L^®-(CD)Eu(BTFA)₃ is equivalent to that in the Eu(BTFA)₃DAMIC-SiO₂. For this, we suggest that the molecular structure of L^®-(CD)Eu(BTFA)₃ is that shown in Figure S9 (SI section), where the Eu³⁺ ion is eight-coordinated, with one CD (three dentate chelating) and three BTFA molecules with bidentate chelating. The excitation spectra of the L^®-(CD)Eu(BTFA)₃ hybrid show a high intense broad band centered at 345 nm, related to the highly efficient antenna effect of the BTFA ligand. In the spectra, the f-f transitions are not observed (Figure S13, SI section). The emission lifetime increased to 0.36 ms due to the removal of the luminescence suppressor, O-H oscillators, of coordinated water molecules. This behavior is in line with the report by Azevedo et al.,³¹ in which the lifetime was increased from 0.41 to 0.70 ms after replacement of the coordinated water molecules by the BTFA ligands in the hybrids of the silica-grafted Eu-complex (Figure S14, SI section).

Figure 7
Emission spectra (λ_Ex = 365 nm) of the L^®-(CD)Eu(H₂O)_n as deposited on tracing paper (green line), after spraying with BTFA solution (red line), after heating cycles (blue line), and after a second spraying with BTFA solution (orange line).

After several heating-cooling cycles, the system restored its initial form (L^®-(CD)Eu(H₂O)_n*), due to BTFA ligand sublimation. The emission spectrum (blue line of Figure 7) showed an increase in the fluorescence related to tracing paper, and the f-f emission bands of Eu³⁺ disappeared. Afterward, a second spraying of the ethanolic solution of BTFA has been made on the printed security label, once again showing the spectral profile of L^®-(CD)Eu(BTFA)₃ (L^®-(CD)Eu(H₂O)_n*)₃* and a bright red emission (see orange line in Figure 7); however, the intensity is half of that presented by L^®-(CD)Eu(BTFA)₃.

The material proved to be stable on the substrate, allowing the luminescence to be activated again even after heating. This multimode information encoding and decoding methodology proved to be interesting and can be used to verify document authenticity, where the information is revealed with chemical stimulus under common UV light, and the luminescence can be extinguished by physical stimuli (heating).

Furthermore, Figure 8b shows the heating-cooling cycle evolution of L^®-(CD)Eu(BTFA)₃ printed label, acquired by continuously monitoring emission at 611 nm (⁵D₀ → ⁷F₂) and under excitation at 365 nm. As qualitatively viewed in Figure 6, the emission intensity drastically decreases with heating and partially restores with cooling. Only after 13 cycles of heating-cooling, a permanent quenching of the luminescence is achieved, which the printed label is invisible to the naked eye. In another experiment, we have continuously heated the decoded security label (L^®-(CD)Eu(BTFA)₃) up to 10 min (Figure 8b) and monitoring emission intensity at 611 nm (⁵D₀ → ⁷F₂). As can be seen, the emission intensity decreases exponentially, however, it does not achieve a threshold.

Figure 8
Temperature dependence of luminescence of decoded security label (L^®-(CD)Eu(BTFA)₃) by monitoring emission at 611 nm (⁵D₀ → ⁷F₂) under continuous heating (a), and by applying heating-cooling cycles (b).

The study of the thermal decomposition of L^®-(CD)Eu(H₂O)_n and L^®-(CD)Eu(BTFA)₃ is very important, because its main application makes use of heating. The study of the heating-cooling cycles shows that L^®-(CD)Eu(BTFA)₃ achieves a threshold after 13 cycles (Figure 8b). Thermogravimetric curves presented in Figure S10 (SI section) showed that the material (paper and L^®-(CD)Eu(BTFA)₃) is stable up to about 180 °C. Heating up to 120 °C only removes physiosorbed water molecules. However, the BTFA ligand sublimates at 135 °C, which is essential to remove the molecules and restore the material to L^®-(CD)Eu(BTFA)₃*. Figure 9 shows the micrographs of the tracing paper and a printed security label before and after the addition of BTFA. Figure 9a shows the image of the tracing paper at two magnifications, which displays only cellulose fibers.⁴⁴ The micrographs in Figures 9b and 9c do not show considerable differences, as expected, since L^®-(CD)Eu(H₂O)_n is a nanometric material and has not been viewed at this magnification. A small change has been verified after spaying BTFA solution at lower magnification; however, at higher magnification, no significant change has been viewed.

Figure 9
SEM images of tracing paper as acquired (a), after printing with L^®-(CD)Eu(H₂O)_n (b) and after spraying with BTFA solution (c).

Viewing fingerprints

The power of the L^®-(CD)Eu(BTFA)₃ system was deposited on the glass blade containing the fingerprint. Figure 10 shows an image of the fingerprint obtained immediately after the material was pulverized, under natural light (Figure 10A) and under ultraviolet light (Figure 10B). Important features capable of identifying the fingerprint of the individual can be observed. Information such as the nucleus (level 1), and island, lake, and end ridge ending (level 2) can be observed. Therefore, the powder under both natural light and ultraviolet light showed the fingerprint with high resolution and validated the hybrid with high potential in decoding fingerprints.⁴⁷ Figure S11 (SI section) shows that the L^®-(CD)Eu(BTFA)₃ material can be used to identify fingerprints on other surfaces.

Figure 10
Identification of fingerprint characteristics under natural light (A) and ultraviolet light (B). (a) Core, (b) island, (c) ridge ending (d) lake.

Cell viability

To investigate whether L^®-(CD)Eu(H₂O)_n and L^®-(CD)Eu(BTFA)₃ affect cell viability, VERO cells were cultured in different concentrations (100-3 µg mL^-1) of the materials by MTT assay, as shown in Figure 11.

Figure 11
Cell viability assays of (a) L^®-(CD)Eu(H₂O)_n and (b) L^®-(CD)Eu(BTFA)₃, evaluated by the MTT method using the VERO cell line.

Materials containing clays in their composition become attractive for biological applications. According to Rodrigo et al.,⁴⁸ the biocompatibility and nanoscale characteristics of these materials, such as L^®, make them promising biomaterials.

Both the L^®-(CD)Eu(H₂O)_n and the L^®-(CD)Eu(BTFA)₃ hybrids presented satisfactory results at all concentrations investigated, that is, they showed low toxicity, with few apparent differences. The L^®-(CD)Eu(H₂O)_n hybrid at a concentration of 100 μg mL^-1, for example, showed slightly higher cell viability than the L^®-(CD)Eu(BTFA)₃ hybrid, but both showed satisfactory results, since the results demonstrated insignificant differences when compared to each other and to the control group, with a low cytotoxic effect. These are important results, as these materials can be applied without posing a risk to public health, demonstrating safety for people and the environment and compliance with potential legal regulations.

Conclusions

In this study, a simple, effective and reproducible synthesis method was developed for the functionalization of L^® clay with Eu-complex, resulting in hybrid materials suitable for luminescence-based applications, such as encryption and decryption information. XRD, FTIR and SEM characterizations confirmed the successful functionalization and complexation processes. The luminescent properties showed that both L^®-(CD)Eu(H₂O)_n and L^®-(CD)Eu(BTFA)₃ hybrids exhibit red luminescence, which is expected for materials with europium ions. The L^®-(CD)Eu(H₂O)_n based luminescent ink allowed the printing of security labels on vegetal and PET paper, the use with pens ink and stamp ink, showing responsiveness to chemical stimuli (via BTFA ligand) and physical stimuli (via heating and UV light).

In addition, L^®-(CD)Eu(BTFA)₃ showed good powder dispersion and strong luminescence when applied for latent fingerprint visualization. Preliminary cytotoxicity tests demonstrated low cytotoxicity of the hybrids, supporting their potential safe use in biological and material applications. Overall, these results demonstrate that the synthesized materials are versatile and promising for applications in information security and fingerprint visualization, with further studies needed to explore additional potential uses.

Supplementary Information

The supplementary file (containing synthesis conditions, luminescence study, application of the material in cryptography, material structure and TGA curves) is available free of charge at http://jbcs.sbq.org.br as a PDF file.

Data Availability Statement

All data are available in the text.

Acknowledgments

The authors gratefully acknowledge the financial support provided by National Council for Scientific and Technological Development (CNPq) under grant number 140229/2020-5 and Foundation for Science and Technology Support of the State of Pernambuco (FACEPE) under grant number BFD-0008-1.06/23. R. J. O. acknowledges INCT NanoVida-CNPq (406079/2022-6) for funding. L. L. L. thanks the Chemistry Graduate Program at UFPE, and CNPq, and FACEPE for the fellowships. We would also like to thank Universidade Federal de Pernambuco (UFPE) for providing research infrastructure. Multiuser Center of the Physics Department, UFPE, for X-ray analysis, the Laboratory of Hybrid Compounds, Interfaces and Colloids (CHICO) at the Department of Fundamental Chemistry, UFPE for the SEM analysis.

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Edited by

• Editor handled this article:
  Juliano Alves Bonacin (Associate)

Publication Dates

• Publication in this collection
  27 Oct 2025
• Date of issue
  2025

History

• Received
  27 June 2025
• Accepted
  22 Sept 2025