Development of Anti-counterfeiting ink  and  Hydrogel with red luminous Ca²⁺-Doped Y₂O₃:Eu³⁺ (1 mol%) nanoparticles

Luminescent anti-counterfeiting inks have become incredibly important in a variety of international businesses in recent years. These cutting-edge inks are essential for preventing counterfeiting and guaranteeing product authenticity. Polyvinyl alcohol (PVA) is the most suitable matrix among the materials investigated for creating luminous inks. It is a very useful and adaptable media in this cutting-edge field because of its remarkable qualities, which include high optical transparency, low toxicity, water solubility, mechanical strength, biocompatibility, and good film-forming capacity [1].

Luminescent materials, such as lanthanide-doped nanomaterials, quantum dots, organic dyes, and nanoscale metal-organic frameworks, are used in a wide range of anti-counterfeiting ink formulations. Luminescent nanomaterials doped with lanthanides, especially those based on trivalent rare-earth ions (RE³⁺), have become the most promising of these. Their exceptional features, which include large Stokes and anti-Stokes shifts, high quantum efficiency, clearly defined emission bands, and luminescence durations ranging from microseconds to milliseconds, are what make them appealing. Additionally, they have minimal toxicity, exceptional photostability, and excellent chemical and thermal stability. Due to their distinct electronic transitions over a wide spectrum range, RE³⁺ ions are very adaptable for next-generation anti-counterfeiting technologies, enabling customized emission from the ultraviolet (UV) to the infrared (IR) [2, 3].

In this situation, adding divalent calcium ions (Ca²⁺) to the Y₂O₃:Eu³⁺ system is a useful way to change the local crystal field surrounding Eu³⁺ or produce more oxygen vacancies, which may increase the photoluminescence efficiency [4-7].  Furthermore, Ca²⁺ co-doping can improve energy transfer and alter site symmetry, which can improve colour purity and emission intensity [8].  The synergistic effects of Eu³⁺ and Ca²⁺ co-doping in Y₂O₃ still require systematic exploration despite the abundance of research on RE-doped oxides, especially in relation to the structural, morphological, and photoluminescent properties. The need for sophisticated luminous materials that are hard to copy but simple to authenticate has increased because to the growing threat of counterfeit goods in industries including medicine, cash, and branded goods.  In this regard, the crisp emission lines, long-term stability, and excitation flexibility under UV or near-UV light sources of rare-earth (RE) doped phosphors, especially Eu³⁺-activated Y₂O₃, have demonstrated great promise [9,10].  Ca²⁺ ions can be added to the Y₂O₃:Eu³⁺ matrix to further customise emission characteristics and enhance quantum efficiency, which makes these materials ideal for optical tagging and secret security printing [11, 12].The creation of defect states or local symmetry alterations surrounding Eu³⁺ ions, which can alter emission lifetimes and increase red emission intensity, is one of the main benefits of Ca²⁺ co-doping [13]. 

The development of multi-layered or time-gated anti-counterfeiting inks is made possible by these adjustable optical characteristics, where the emission response under various wavelengths or time delays adds another level of complexity to authentication procedures [14].  In order to create safe, printable luminous patterns, Ca²⁺ co-doped Y₂O₃:Eu³⁺ nanoparticles can be distributed in ink formulations or polymeric matrix, including hydrogels. The incorporation of luminous nanophosphors for wearable authentication systems and bio-imaging markers is made possible by hydrogels, which offer a biocompatible, flexible, and transparent medium [15].Furthermore, stimuli-responsive anti-counterfeiting devices can be created by combining photoluminescent properties with the swelling behaviour and responsiveness of hydrogels to external stimuli (such as pH, temperature, moisture) [16,17].

This study focused on the development of novel Ca²⁺ (1mol %) co-doped Y₂O₃:Eu³⁺ (1mol %) nano-phosphors using combustion techniques. The resulting phosphor was characterized using various methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL). Long-term use in biocompatible applications, such as fluorescent hydrogels for biomedical sensing or diagnostics, where both visual and analytical readouts are crucial, is also made possible by the luminous stability of Y₂O₃:Eu³⁺, Ca²⁺ in hydrophilic settings [18]. Therefore, these nanophosphors' multifunctionality makes them promising materials for next-generation smart materials as well as covert ink technologies.

Synthesis of Ca²⁺ (1mol %) Co-doped Y₂O₃:Eu³⁺ (1mol %) Nanophosphors

Using a solution combustion method, Y₂O₃ nanophosphors co-doped with 1 mol % Eu³⁺ and 1 mol % Ca²⁺ were produced. This was achieved by dissolving yttrium nitrate [Y (NO₃)₃·XH₂O], Europium nitrate hexahydrate [Eu (NO₃)₃·6H₂O], Calcium nitrate tetrahydrate, and urea [CO(NH₂)₂] in double-distilled water. The components were mixed in a stoichiometric ratio to form a homogeneous solution. After that, this solution was put in a silica crucible and heated in a preheated muffle furnace to 600°C for 15 minutes. The resulting foamy solid was then easily ground with an agate mortar to produce fine precursor powder, and it was annealed for two hours at 1000°C to improve crystallinity. The combustion process resulted in a frothy sample, which was subsequently ground into a fine powder for further characterization.

Anticounterfiting ink

In order to achieve the target dynamic viscosity, 50 mg of the Y₂O₃:Eu³⁺ (1mol %), Ca²⁺ (1mol %) is carefully dispersed into an ethanol- water solution at a ratio of 50:50. Then this mixture undergoes stirring with a magnetic stirrer at ~ 70°C until it became clear (approximately 10 min). Subsequently, to ensure thorough mixing, the appropriate volume of the PVA solution added. After that, 10 mL of deionized water was used to dissolve 1 gm of polyvinyl alcohol (PVA) with an average molecular weight of 125,000 g/mol for 10% solution and add to that prepared mixture. Also subjected 0.2 gm PEG- 200 (Polyethylene Glycol) in this followed by sonication for 30 min to achieve homogeneity. We obtained AC ink employed for printing AC patterns on various surfaces due to its high viscosity and polymeric composition. These patterns are subsequently observed in real-time while exposed to UV 365 nm light irradiation [19].

Preparation of Ca²⁺ co-doped Y₂O₃: Eu³⁺ hydrogel.

Chitosan with an 85% degree of deacetylation (dd) and 25% glutaraldehyde was used in this process. To proceed with the experiment, firstly, a chitosan solution was prepared by adding 1.7 g of chitosan powder to 1% aqueous acetic acid and left for 8-10 hours at ~50°C with continuous stirring to obtain a 1% (w/v) solution. The viscous and pale yellow chitosan solution was filtered to remove any undissolved particles. Added PEG-200 (2% v/v) and glycerine (1% v/v) to the chitosan solution and stirred until fully mixed (about 20 min). Disperse nanophosphors 60 mg ( ̴ 0.2% w/v) into the chitosan mixture and ultrasonicate (5–10 minutes) to achieve uniform dispersion. Slowly added 0.25 ml glutaraldehyde ( ̴ 0.25% v/v) dropwise while stirring, and the pH was between 4.5 and 6.5. Then wait for 1–2 hours at room temperature to allow Schiff base formation between chitosan amine groups and glutaraldehyde groups. After some time, a slightly yellow color was observed, indicating a reaction to occurred. Then, for desolvation, cold 2-propanol (~4°C) was chosen. Added the chitosan mixture dropwise into 2-propanol under stirring. Then, turbidity was observed immediately, which means phase separation and gel formation occurred, and for a clearer solution, it was separated by centrifugation. Let the gel particles remain undisturbed to form a bulk hydrogel structure. Wash the hydrogel 2–3 times with fresh cold 2-propanol to remove residual acetic acid and unreacted matter. Let the hydrogel place at 4°C for 12–24 hours to stabilize the network and store in distilled water at 4°C. At last, cast the mixture into molds to give any shape, and then we can observe the glowing red colour hydrogel through 365 nm UV light.

The Multipurpose Versatile XRD System (Smart Lab 3kW, Rigaku) was employed for both powder XRD analyses of Ca²⁺ ions co-doped with Y₂O₃:Eu³⁺ phosphor samples, scanning a 2θ range from 10° to 70° to evaluate the structural and phase purity. The surface morphology of Y₂O₃: Eu³⁺ and Ca²⁺ activated phosphor samples was studied using a High-Resolution Field Emission Scanning Electron Microscope with EDS (FE-SEM) (JSM-7610F Plus, JEOL). The luminescence decay profiles for the entire series of  Y₂O₃:Eu³⁺ and Ca²⁺  samples were recorded with a Quanta Master 8450-22 Spectrofluorometer (Horiba), which measures fluorescence across the UV-VIS-NIR range (~200 nm to 3000 nm).

XRD and Structural parameter evaluation

The X-ray diffraction patterns of the calcined Y₂O₃:Eu³⁺ (1mol %), Ca²⁺ (1mol %) doped nanophosphors are shown in Figure 1. The findings demonstrate that there are no discernible impurity peaks in the XRD patterns when compared to the typical JCPDS data. This shows that the trivalent Europium and Calcium ions have effectively integrated into the Y₂O₃ host without altering the crystal structure, and that the synthesized samples are single-phase.

Figure 1: shows the normal JCPDS data of the host Y2O3, as well as the powder X-ray diffractograms of   Y₂O₃:Eu³⁺ (1mol %) , Ca²⁺ (1mol %) samples.

The structural features, such as crystallite size, dislocation density, and microstrain, were ascertained using XRD data.  As shown below, the crystallite size (D) nearly 80 nm was determined using the Hall-Williamsons equation (1) and the Debye-Scherrer's formula (2) [20,21,22]. 

Here, ε stands for the macrostrain in the samples, D is the average crystallite size and β for the full-width at half-maximum (FWHM).  Using the following relation in equation (3) and (4), the values of the microstrain (ε) and dislocation density (δ) were calculated as 6.848 and 1.55108E-07 respectively for Y₂O₃: Eu³⁺ (1mol %), Ca²⁺ (1mol %) nanophosphor.

SEM

A scanning electron microscope was used to analyse the synthesised phosphor powder's surface morphology and crystallite sizes.  At 1000°C, a combustion procedure was used to complete the synthesis. This indicates that the combustion reactions of the mixes proceeded without any problems.  Along with morphological images, Figure 2 (a–b) shows exemplary SEM micrographs of Y₂O₃:Eu³⁺, Ca2⁺ phosphor materials.  The micrograph crystallite diameters vary from 90-120 nm and are of spherical shape.  SEM images show that all of the produced compositions have nearly comparable crystallite sizes, with the average crystallite size falling within the nm.range.

Figure 2: (a–b) displays a typical SEM picture of the nanophosphor sample of Y₂O₃:Eu³⁺   (1mol %), Ca²⁺ (1mol %)

Photo-luminescence (PL) analysis:  

Figures 3 and 4 display the PL excitation and emission spectra of Ca²⁺ (1 mol %) co-doped with Y₂O₃:  Eu³⁺ (1 mol %) nanophosphor, respectively. The PL excitation spectra of 1% Eu³⁺ doped and Ca²⁺   (1 mol %)   co-doped with Y₂O₃ were recorded using the luminescence intensity as a function of the excitation wavelength.  The detection wavelength was chosen at the primary peak of Eu³⁺ at 613 nm, whereas the excitation wavelength was monitored between 320 and 600 nm as shown in Fig. 3.  In this instance, the 230–260 nm range was observed where the charge transfer band was visible.  The excited states of the 4f⁶ configuration and the ⁷F₀ ground state of Eu³⁺ are the source of the transitions between 300 and 500 nm. The 4f-4f transitions of Eu³⁺ ions are represented by the sharp peaks seen at 364 nm (⁷F₀→⁵D₄), 382 nm (⁷F₀→⁵G₂), 394 nm (⁷F₀→⁵L₆), 404 nm (⁷F₀→⁵D₃), 416 nm (⁷F₀→⁵G_j) and 466 nm (⁷F₀→⁵D₂), as well as a peak seen at 533 nm (⁷F₀→⁵D₁) that usually coincides with emission bands.  [25].

As illustrated in Fig. 4, The PL emission spectra were recorded at an excitation wavelength of 348 nm and monitored in range 380–700 nm. The notable peaks of Eu³⁺ were discovered at 581 nm with regard to the ⁵D₀→⁷F₀, 588 nm and 594 nm with regard to the ⁵D₀→⁷F_1, 613 nm with regard to the ⁵D₀→⁷F₂   and its weak sub peak at 630 nm with same transition. In this instance, Eu³⁺ ions were substituted in the lattice regions without an inversion centre, as indicated by the major signal for the ⁵D₀→⁷F₂ transition at 613 nm. It is the main cause of the intense red colour [26]. Another peak is seen here at about 416 nm, which is probably caused by Eu³⁺ 4f-4f transitions that originate from excited level ⁵D₁ the lower ground states. After calcium doping, the peak intensities increase by approx. 1.5 times, but the peak positions stay the same as in pure Y₂O₃: Eu³⁺ nanophosphor. Figure 5 illustration depicting the process of energy transfer with Ca²⁺ ions in Y₂O₃:Eu³⁺ nanophosphors.

Figure 3: PL excitation of the Y₂O₃:Eu³⁺ (1mol %), Ca²⁺ (1mol %)   nanophosphor at λem = 613 nm.

Figure 4: The Y₂O₃:Eu³⁺ (1mol %) , Ca²⁺ (1mol %)   nanophosphor PL emission spectra at λex = 348 nm

Figure 5: Illustration depicting the process of energy transfer with Ca²⁺ ions in Y₂O₃: Eu³⁺ nanophosphors.

Evaluation of CIE parameters

For Y₂O₃:Eu³⁺ and Ca²⁺ co-doped Y₂O₃:Eu³⁺ phosphors, the CIE coordinates and correlated colour temperature (CCT) were analysed. The examination of the CIE coordinates (x=0.63, y=0.36) and the outcomes are displayed in Fig. 6.  By calculating the colour correlated temperatures (CCTs), the nature of colour emission was investigated.  The following McCamy empirical equation (3) was used to determine CCT.

CCT(x,y)= 437n³+3601n² - 6861n+5514ϑ31(3) where n is  the ratio of (x−xe) to (y−ye), and xe = 0.3320 and ye = 0.1858.6 [33, 34].  The CCT graphs for the Ca²⁺ (1mol %) co-doped Y₂O₃:Eu³⁺ (1mol %) phosphor are shown in Figure 7. The CCT values is 2130K. Consequently, the manufactured samples' CCT values show a cool white light output.

Figure 6: CIE Colour coordinates of Y₂O₃:Eu³⁺ (1mol %), Ca²⁺ (1mol %).

Figure 7: Y₂O₃:Eu³⁺ (1mol %), Ca²⁺ (1mol %)   u' and v' diagram

Applications of data security

The exceptional photoluminescent characteristics of the Y₂O₃:Eu³⁺, Ca²⁺ nanocomposites created in this work offer promising prospects for real-world data encrypting applications.  Society has always placed a high value on data protection and preservation, but creating sophisticated chemical anti-counterfeiting (AC) systems is fraught with difficulties. In an attempt to stop counterfeiting, a variety of fluorescent materials have been employed; nevertheless, problems with toxicity, spectral overlap, low quantum efficiency, and luminescence quenching remain.  It's difficult to make AC ink that prevents security documents from being forged.  Figure 8a shows AC labels created using Y₂O₃:Eu³⁺, Ca²⁺ composite on various surfaces.  The images created using a basic brush technique on a variety of media (metal, glass, wood, aluminium paper, and cardboard) are shown in Figure 8(a).

Figure 8:  (a) AC labels displayed on various surfaces (b) Extended storage period on Fabric

Both natural light and UV 365 nm light are used to observe these AC labels, which were created on various surfaces. Under typical lighting conditions, the patterns exhibit clear, sharp luminescence that is invisible to the human sight.  These outcomes demonstrate how useful the developed ink is for data encrypting applications involving UV source.  When exposed to UV light at 365 nm, the AC patterns on various substrates produce a vivid red hue that is undetectable during the day.  The emission colour stayed the same even after 21 days, demonstrating the longevity of the Y₂O₃:Eu³⁺, Ca²⁺ ink and its suitability for safe applications. This is demonstrated in Fig. 8(b), which evaluates the stability of the AC patterns over time at room temperature (1–21 days). The designs created by AV ink on glass surface were treated with several solvents, including ethanol, acetone, HCl, NH₃, and NaOH, in order to assess the ink's chemical stability.  The digital images displayed in Fig 9 demonstrate the patterns' notable resistance to all the above mentioned solvents.

Figure 9: AC label solvent treatment with different solution in the presence of UV light at 365nm.

Hydrogels with light properties

The capability to create luminescent hydrogels that function under both daylight and UV light conditions provides considerable flexibility in the development of advanced photonic and optoelectronic materials. Fig.10 shows luminescent hydrogels observed under day light and UV light at 365 nm.  In daylight, the hydrogel is transparent and flexible, making it easy to handle and compatible with biological or environmental systems. When exposed to UV light, the embedded Ca²⁺ co-doped Y₂O₃: Eu³⁺ phosphors emit a bright red light, which facilitates real-time visualization, encoding, and optical applications [27, 28]. This dual functionality makes the hydrogel particularly well-suited for various multifunctional uses, including UV-activated biosensors, anti-counterfeiting materials, photo-switchable coatings, and smart displays. Additionally, the luminescence triggered by UV light enables non-contact, remote activation, paving the way for new opportunities in optical communication and responsive systems in soft materials.

Figure 10: (a) Hydrogels observed under Day light (b) UV light at 365 nm

In summary, Ca²⁺ co-doped Y₂O₃: Eu³⁺ nano-phosphors that emit cool white light were synthesized using a urea-assisted solution-combustion method. XRD analysis confirmed phase purity, while results indicated that the crystallites exhibit a semi-spherical shape with dimension 80 nm. SEM images revealed that the morphology of Y₂O₃ co-doped with Europium and Calcium ions was seems to be largely uniform and primarily spherical, demonstrating successful synthesis of the nanophosphors, effective incorporation of Eu³⁺ and Ca²⁺ ions within the Y₂O₃ matrix without significant phase separation. No major morphological changes were observed due to co-doping, suggesting the dopants do not materially alter the overall microstructure. These features confirm that the synthesized material maintains its structural integrity while accommodating the dopants. The optimized Y₂O₃:Eu³⁺ (1 mol%) and Ca²⁺ (1 mol%) phosphor displayed CIE coordinates which are near the NTSC red standards, along with a high correlated color temperature of 2130 K, suggesting it emits a cool white light. These findings underscore its potential for use as a cool-emitting phosphor in near-UV excited red LED applications. Photoluminescence showed that Ca²⁺ co-doping increased charge compensation and decreased non-radiative losses, resulting in enhanced red emission at 613 nm (⁵D₀ → ⁷F₂ transition). Ca²⁺ doped Y₂O₃: Eu³⁺ nanophosphors are hence attractive for red components in solid-state and optoelectronic lighting applications. The optimized NCs are utilized to produce AC labels on different substrates. The resulting AC images exhibit high sensitivity and improved contrast, thanks to their superior fluorescence properties and binding specificity. Overall, the results clearly indicate that the enhanced nano-luminous material hold significant promise for applications in anti-counterfeiting and hydrogel.