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Original Article
2025
:18;
1462025
doi:
10.25259/AJC_146_2025

Preparation of photochromic hydrogel toward anticounterfeiting and duplicable data encryption applications

, ,
 Department of Chemistry, College of Science, Princess Nourah bint Abdulrahman University, P.O. Box 84428, Riyadh 11671, Saudi Arabia

*Corresponding author: E-mail address: gmalsnany@pnu.edu.sa (G. Al-Senani)

Received: , Accepted: ,
© 2025 Arabian Journal of Chemistry - Published by Scientific Scholar
Licence
This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

Traditional inks have shown drawbacks, such as low efficiency, low photostability, high cost, and poor durability. Self-healing hydrogels have been more robust and durable than traditional inks. In the current study, a hydrogel ink was prepared from a combination of cellulose microfibrils (CMF; a dispersion agent and microfiller), carboxymethylcellulose (CMC; a hosting agent), and rare-earth strontium aluminate nanoparticles (SAN; a photoluminescent agent). CMF was prepared from sugarcane bagasse, which is a type of agricultural waste. The use of CMF as a dispersion agent and microfiller prevented the agglomeration of SAN. To develop a colorless stamp, SAN must be uniformly dispersed in the nanocomposite hydrogel (CMF@CMC). Different SAN concentrations were combined with CMF and CMC to produce different optical properties. Using transmission electron microscopic (TEM) analysis, CMF has diameters ranging from 15 nm to 50 nm, whereas the SAN morphology demonstrated diameters of 8 nm to 17 nm. The colorimetric coordinates and luminescence spectra showed a color change to green when exposed to ultraviolet radiation. The stamped sheets were excited at 375 nm to produce an emission intensity at 519 nm. The mechanical behavior of stamped papers and the rheological performance of hydrogels were investigated. The morphology of the hydrogel films was explored. The current hydrogel consists of a photostable, reversible, durable, and photochromic nanocomposite, offering a dependable anticounterfeiting solution for a range of commercial merchandise, such as banknotes and commercial products.

Keywords

Carboxymethylcellulose
Data encryption
Hydrogels
Photoluminescence
Rare-earth strontium aluminate nanoparticles

1. Introduction

The forgery of valuable documents, currency, and official certificates has shown detrimental impacts on both national security and the economy [1-3]. Efficient anticounterfeiting measures can be accomplished by using an optically active feature of a material, which is easy to check but challenging to counterfeit [4]. Various optical materials have been recently presented as reliable security encoding agents, such as quantum dots and upconverting nanoparticles [5-7]. Those materials have shown efficient emission and small and consistent particle diameters, which makes them attractive candidates for security authentication. However, they have shown background fluorescence interference on printed surfaces, such as currency and identification cards, owing to their short emission wavelengths [8]. Therefore, the preparation of advanced authentication materials that are highly resistant to emission interference has grown dependent on the usage of new afterglow materials [9-11]. Verifying the validity of important documents, such as identity authentication and banknotes, has grown increasingly important in the last few years. Consequently, numerous methods for verifying authenticity have been reported, such as printing inks. Among the most advanced methods of authentication are barcodes and holography. These systems for authentication are effective; however, their implementation is costly and requires highly advanced techniques [12,13]. Photochromic ink has been recommended to simply develop secure patterns [14-16]. Owing to their weak reflection and high surface area, polymer nanoparticle-dependent photochromic inks give efficient optical properties. However, the polymer nanoparticles typically form aggregates that obstruct the printer nozzles and result in a major conflict [17,18].

Various organic colorants have been employed to develop photochromic materials. However, photochromic organic pigments have shown high cost, weak photostability, and poor adsorption [19-21]. Photochromic inorganic agents have shown various advantages, such as photostability, affordability, and durability. Because of their tiny size, inorganic chromic agents can be quickly adsorbed and diffused throughout the colored materials. They have shown endurance and effective quantum yields because their photoluminescence activity is dependent on electron excitation as opposed to the organic agent structural switching that causes steric hindrance [22,23]. Recently, lanthanide-doped photoluminescent pigments, such as rare-earth strontium aluminate nanoparticles (SANs), have been shown to be attractive photochromic compounds [24]. Recently, a few lanthanide-based photoluminescent agents have been reported, such as CaS:Eu2+/Tm3+/Ce3+ (red emission) [25], SrAl2O4:Eu2+/Dy3+ (green emission) [26], and CaAl2O4:Eu2+/Nd3+ (bluish emission) [27]. SAN has been a significant photoluminescent inorganic material that has shown promise in the synthesis of photochromic products due to its photostability, absence of radioactivity, nontoxicity, and efficient optical activity [28]. Thus, the development of photochromic SAN-immobilized nanocomposites can be described as an inventive and effective technique for producing inexpensive and photostable anticounterfeiting materials. However, a significant limitation on the application of photoluminescent materials is their historical use of the particle mold technique [29]. Luminescent particles have been integrated into a wide range of substrates to develop SAN-embedded films, which have resulted in several innovative materials despite their poor mechanical performance. However, many prepared films have demonstrated inadequate durability [30]. Constructing self-healing anticounterfeiting materials has been shown to be an intriguing field of research for various commercial items [31,32]. Self-healing materials have better durability since they can self-repair any damage they sustain [33-42]. Thus, film endurance can be significantly increased with the introduction of self-repairing properties [43,44]. Carboxymethylcellulose (CMC) has been a significant hydrogel due to its availability, adaptability, biodegradability, renewability, mechanical efficiency, and biocompatibility. Additionally, CMC hydrogels can benefit from the inclusion of CMF as a reinforcement agent able to improve the mechanical strength of the CMC-based hydrogels [45-47].

2. Materials and Methods

2.1. Materials

CMC sodium (DS = 0.9; 1500–2500 cP;), sodium hydroxide (NaOH), sodium hypochlorite (NaClO), and sodium sulfite (Na2SO3) were purchased from Sigma-Aldrich (Germany). Boric acid (Merck, Germany), strontium carbonate (Aldrich, Germany), europium oxide (Merck, Germany), aluminum oxide (Sigma-Aldrich, Germany), and dysprosium oxide (Sigma-Aldrich, Germany) were used to synthesize the rare-earth strontium aluminate phosphor [48,49]. Whatman papers with a pore diameter of 11 μm were supplied by Merck (Germany). The research experiments were accomplished at the Natural and Health Sciences Research Center.

2.2. Preparation of CMF

To synthesize cellulose microfibrils (CMFs), a slightly adapted version of a formerly published method was employed [50]. Bagasse was delignified by an aqueous solution of Na2SO3 (1.5%) and NaOH (3.2%) at 100°C for 2.5 h, employing a ratio of 7:1 (solution:bagasse). The blend was washed with distilled water and then treated with an aqueous solution of Na2SO3 (3.0%) and NaOH (2.4%) at 45°C for 1 h, using a ratio of 6:1 (solution:bagasse). The reaction solution was then exposed to NaOH(aq) (25.0%) at 80°C for 1 h, employing a ratio of 6:1, bleached with NaClO (5.0%) at 50°C for 3 h, air-dried, and ground by a Masuko milling system. The preparation of CMF was repeated three times to designate the same microfibril diameters (15-50 nm), confirming reproducibility.

2.3. Preparation of SAN

The lanthanide aluminate microparticles were synthesized by the solid-state reaction [22-24,48], whereas the top-down technology was employed to produce SAN [49]. A mixed powder of 1 mol of strontium carbonate, 0.03 mol of dysprosium oxide, 0.2 mol of aluminum oxide, 2 mol of boric acid, and 0.02 mol of europium oxide was suspended in absolute EtOH (450 mL). After 4 h of stirring, the mixture was subjected to 2 h of heating at 90°C, 30 min of ultrasonication at 35 kHz, and 3 h of ball milling. Then, the mixture was sintered in a reductive carbon atmosphere for 4 h at 1350°C. Employing the Triple Roll Mill ES80, the resulting residue was ground and sieved to yield lanthanide aluminate microparticles (11-32 μm). Using the top-down method, 15 g of lanthanide aluminate micropowder was milled in a ball milling tube (stainless steel) that was set on a vibrating disc. For 20 h, the vibrating plate and the vial containing lanthanide aluminate were repeatedly struck by a silicon carbide ball mill (0.1 cm) to produce SAN. The synthesis of SAN was repeated three times to indicate the same nanoparticle diameters (8-17 nm), proving reproducibility.

2.4. Preparation of hydrogels

A solution of CMC (5.0% w/v) in distilled water was mechanically stirred for 3 h and ultrasonicated (Hielscher, Germany) at 20 kHz for 25 min. CMF (10.0% w/w) was added to the above mentioned solution, and then the admixture was stirred for 20 min. Different amounts of SAN were charged into an aqueous dispersion of CMF@CMC, including 0% (SAN0), 0.1% (SAN1), 0.3% (SAN2), 0.5% (SAN3), 0.7% (SAN4), 0.9% (SAN5), 1.1% (SAN6), 1.3% (SAN7), and 1.5% (SAN8) (w/w). Aqueous solutions of SAN/CMF@CMC were obtained from the provided suspensions after homogenization for 30 min. To ensure a homogenous dispersion, the admixtures were stirred for 45 min and ultrasonicated at 20 kHz for 20 min to produce transparent and viscous solutions. Using a wooden stamp, papers were imprinted with the provided hydrogels and then allowed to air-dry for 30 min. In addition, the viscous solutions underwent a casting process in Teflon Petri dishes, and the resultant hydrogel films were left to air-dry for 2 h. The preparation of the photoluminescent hydrogel was repeated three times, indicating the same emission wavelength and confirming reproducibility.

2.5. Analysis methods

2.5.1. Morphological assessment

An energy-dispersive X-ray (EDX) fitted on a scanning electron microscope (SEM; Quanta FEG-250, Czech Republic) was employed to analyze the structural features of the prints. Prints were also examined using X-ray fluorescence (XRF; AXIOS Sequential, Netherlands) to identify their elemental compositions. SAN were ultrasonicated in distilled water for 30 min at 20 kHz to examine their morphology by using a JEM-2100 Plus (Japan). The crystal structure of SAN was examined by a Bruker Advance D8 X-ray diffractometer (XRD; Germany). The morphological results were conducted at three different locations on the sample surface, designating the same morphological features.

2.5.2. Mechanical and rheological studies

Brookfield DVIII was used to determine the rheological properties of the developed hydrogels [51]. Zwick Universal was used to report the mechanical performance of the printed sheets [52]. Both rheological and mechanical studies were measured three times, and the average values were recorded.

2.5.3. Photoluminescence measurements

A JASCO FP-6500 (Japan) with a phosphorescence accessory was employed to measure the emission spectra of the printed papers. In accordance with a previous protocol [53], prints were illuminated with ultraviolet light (375 nm) for 15 s and 120 s to evaluate their photostability. The printed sheet was then left in the dark for 60 min to allow the green emission to return to a white color. The foregoing process was repeated multiple times, gathering the luminescence spectra at the end of each cycle. The photoluminescence spectral profiles were measured at three different locations, giving the same emission wavelength.

2.5.4. Colorimetric studies

Using an Ultrascan Pro (HunterLab, United States), the colorimetric parameters, including CIE Lab coordinates, color strength (K/S), and transparency of prints, were measured. The International Commission on Illumination originally defined the CIE Lab to characterize colors in numerical values. The brightness coordinate is denoted by L*, the yellow-to-blue coordinate is denoted by b*, and the red-to-green coordinate is represented by a* [54]. A Nikon D850 was employed to take photographs of SAN8 under various light conditions. To ensure that the cast film was transparent, a Hitachi U3010 spectrophotometer (Hitachi Co., Ltd., Tokyo, Japan) was used. The color parameters were determined three times, and the average values were recorded. The research experiments were accomplished at the Natural and Health Sciences Research Center.

3. Results and Discussion

3.1. Photoluminescence analysis

Upon immobilizing SAN within the hydrogel bulk (CMF@CMC), a color-tunable hydrogel was generated. The mechanical strength of the hydrogel was demonstrated to be enhanced by the application of CMF as a dispersing and microfiller agent [55]. In the absence of CMF, no hydrogel was produced during the formulation of the hydrogel. The paper surface was crosslinked with the hydrogel films. In the presence of ultraviolet light, the prints fluoresced green. After the ultraviolet lighting was switched off, reversible phosphorescence was monitored for prints with SAN ratios greater than 0.9%. The reversible fluorescence emission of the prints with SAN contents less than 0.9% was instantaneous after turning off the UV lighting. Figure 1 displays the colorimetric transitions of SAN8 under different lighting conditions. The stamped paper displayed a white shade under daylight, green emission underneath UV rays, and greenish-yellow emission in a dark chamber.

Printed paper (SAN8) showed (a) a white shade under daylight, (b) green emission under ultraviolet radiation, and (c) greenish-yellow emission in the dark.
Figure 1.
Printed paper (SAN8) showed (a) a white shade under daylight, (b) green emission under ultraviolet radiation, and (c) greenish-yellow emission in the dark.

When the SAN ratio was raised, the prints displayed a stronger emission intensity. The prints have shown a reversible photochromism. However, the prints with the SAN contents of 0.9% or less quickly returned to their initial state after the UV illumination source was turned off (i.e., fluorescence emission). After switching off the lighting source, a long-lasting afterglow was detected in prints with the SAN ratios higher than 0.9%. Therefore, SAN5 was shown to have the greenest emission with the best transparency layer. Because the strength of the excitation intensity rose as the SAN content grew, the strength of the emission intensity depends on the SAN content. The fluorescence band at 519 nm makes it clear that Eu(III) was effectively changed into Eu(II). The transition of Eu(II) (4f65d1↔4f7) is responsible for the energy transfer from Dy(III) to Eu(II), which explains the fingerprint emission [28-30]. When exposed to light, Dy(III) causes the relaxation of Eu(II) to the ground state, which leads to the creation of traps that produce photons. It was discovered that the SAN concentration influenced the strength of the excitation band of prints (Figure 2a). Additionally, there was an increase in the emission intensity (519 nm) with extended time exposure to ultraviolet illumination (Figure 2b). The decay time spectra of the paper prints showed a nonlinear declining rate, with a fast early decay followed by a slower decay, as revealed in Figure 3.

(a) Excitation spectra of paper prints at different SAN contents and (b) emission spectra of SAN5 in relation to the duration of UV illumination.
Figure 2.
(a) Excitation spectra of paper prints at different SAN contents and (b) emission spectra of SAN5 in relation to the duration of UV illumination.
Decay time spectra of paper prints.
Figure 3.
Decay time spectra of paper prints.

All prints had an emission wavelength of 519 nm, which is like the powder of SAN [26]. Figure 4(a) shows the excitation and emission spectra for the powder of SAN, displaying an emission wavelength of 519 nm. As shown in Figure 4(b), the paper print (SAN5) was exposed to several cycles of UV radiation for 15 s and 120 s to test its photostability. The UV lighting showed that SAN5 emits a green light. The fluorescence intensity remained constant after being repeatedly cycled between ultraviolet light and darkness, indicating photostability.

(a) Excitation and emission spectra for the powder of SAN and (b) Emission of SAN5 (519 nm) during multiple cycles of exposure to ultraviolet radiation for 15 s and 120 s.
Figure 4.
(a) Excitation and emission spectra for the powder of SAN and (b) Emission of SAN5 (519 nm) during multiple cycles of exposure to ultraviolet radiation for 15 s and 120 s.

3.2. Colorimetric measurements

Both K/S and CIE Lab measurements were employed to assess the transparency and colorimetry of prints. The usage of nanoparticles as fillers has been significant for the preservation of a material’s transparency [56]. As shown in Table 1, the CIE Lab axes were used to examine the color shifts in the paper prints. The fact that every paper print seemed white and resembled the blank paper suggested that the stamped film is colorless. When the SAN ratio is increased, the lack of variations in K/S and CIE Lab can be explained by the low SAN contents. When the SAN content increased, the values of the CIE Lab coordinates remained almost constant. The high value of L* and low values of –a* and +b* indicated a white color of the paper prints, proving the transparency of the printed layer.

Table 1. Coloration screening of paper prints.
Sample K/S L* a* b*
SAN0 0.41 91.49 0.25 +2.19
SAN1 0.97 89.94 –2.88 +2.82
SAN2 1.17 89.70 –2.52 +3.09
SAN3 1.30 89.31 –2.19 +3.37
SAN4 1.42 89.10 –1.95 +3.72
SAN5 1.55 88.94 –1.67 +3.95
SAN6 1.92 88.80 –1.48 +4.30
SAN7 2.24 88.57 –0.29 +4.54
SAN8 2.50 88.22 –1.07 +4.92

3.3. Morphological characterization

A strontium aluminate phosphor (11-32 μm) was synthesized by the solid-state reaction, and the top-down method was used to produce SAN [48,49]. TEM images revealed SAN diameters of 8-17 nm, as depicted in Figures 5(a) and (b). By using TEM analysis, cellulose microfibrils showed diameters of 15-50 nm, as depicted in Figures 5(b-d). XRD analysis of SAN displayed a crystal diameter of 8 nm, as revealed in Figure 6(a). A pure monoclinic phase of SrAl2O4 was verified by the XRD signals. It was also indicated that both Eu2+ and Dy3+ are completely doped in the SrAl2O4 crystal because they were undetectable in the XRD pattern [28-30]. Additionally, the crystal structure of SAN was studied by the selected area electron diffraction (SAED), as illustrated in Figure 6(b). The SAED analysis showed no dislocations or defects, indicating a space of ∼4.45 Å, matching an interplanar space of (0 1 1) for a monoclinic state of SrAl2O4 [11]. An aqueous solution of CMC was charged with an aqueous combination of CMF and SAN. To create various hydrogel solutions, different concentrations of SAN were added. Both SAN-free and SAN-embedded hydrogels had a similar transparency. A wood stamp was used to authenticate a paper surface with the prepared hydrogels. The greenish photochromism in the presence of ultraviolet rays introduces an anticounterfeiting print that is invisible in daytime light.

TEM images of (a-b) SAN and (c-d) CMF .
Figure 5.
TEM images of (a-b) SAN and (c-d) CMF .
(a) XRD pattern and (b) SAED image for SAN.
Figure 6.
(a) XRD pattern and (b) SAED image for SAN.

The lanthanide aluminate powder can be evenly distributed throughout the nanocomposite hydrogel when it is employed in its nanostructure size. The SEM analysis of SAN-integrated cast films is illustrated in Figure 7. The SAN-containing cast films showed porous structures like the SAN-free film. Scanning electron micrographs of prints are shown in Figure 8. The elemental analysis of prints is illustrated in Table 2, which displays the heatmap of the elemental density (EDX; wt%) of different samples at different positions (St₁-St₃). This heatmap organizes the distribution of elements (C, Dy, O, Al, Sr, and Eu) effectively by color intensity and demonstrates that carbon and oxygen are predominant elements in all the samples, owing to the paper cellulose component. The concentrations of other elements, including Sr, Al, Ey, and Dy, are lower owing to the use of pigment traces. SEM images demonstrated that SAN was successfully deposited onto the Whatman paper surface. When the SAN content in the nanocomposite hydrogel was increased, no discernible changes in the stamped sheets’ fiber topologies were detected. There were no discernible differences between the hydrogel-stamped microfibrous paper sheet and blank paper. However, it was found that the quantity of SAN between the paper microfibers was increased by increasing the SAN content in the hydrogel formulations. The homogeneous spreading of SAN on the paper surface can be explained by the coordination binding of the SAN aluminum with the cellulosic hydroxyl of the paper [57]. The SAN particle diameter of 8-17 nm, as revealed by TEM investigation, makes it easy to spread them across the paper surface. EDX testing allowed for the tracking of the elemental contents in the prints. Given their prominence in the paper sheets and the hydrogel bulk (CMF@CMC), carbon, oxygen, and sodium were identified by the EDX analysis as main components. Owing to the application of SAN at low concentrations in the prints, trace quantities of Eu, Dy, Sr, and Al were detectable. The chemical analyses of prints at separate positions were nearly identical, demonstrating that SAN was homogeneously distributed on the paper surface.

(a-d) SEM images of cast films (SAN5) at different positions.
Figure 7.
(a-d) SEM images of cast films (SAN5) at different positions.
SEM analysis of prints; (a-c) SAN1 and (d-f) SAN8.
Figure 8.
SEM analysis of prints; (a-c) SAN1 and (d-f) SAN8.
Table 2. Elemental composition (EDX; wt%) in prints at multiple positions (St1-St3) on paper surface.
Sample C O Na Al Sr Eu Dy
SAN0 58.12 40.20 1.68 – – – –
SAN1 St1 57.85 40.18 1.52 0.22 0.12 0.07 0.04
St2 58.20 40.02 1.37 0.18 0.10 0.08 0.05
St3 57.84 40.22 1.44 0.22 0.15 0.09 0.04
SAN8 St1 56.77 41.00 1.05 0.62 0.35 0.12 0.09
St2 56.36 40.65 1.29 0.89 0.49 0.19 0.13
St3 56.45 40.86 1.17 0.70 0.56 0.16 0.10

Comparing XRF with EDX, it was observed that the former is more precise in determining the elemental compositions of materials [58]. The XRF analysis of SAN1 indicated major contents of SrO (41.74 wt%) and Al2O3 (58.26 wt%), and SAN8 indicated major contents of SrO (42.19 wt%) and Al2O3 (57.71 wt%). However, Dy and Eu were undetected by XRF in the prints due to their low quantities. Both XRF and EDX analyses of the prints showed that the elemental ratios used to produce SAN and luminous films were almost similar.

Infrared spectroscopy revealed the cellulose hydroxyl peak at 3391 cm-1 (stretching), an aliphatic (C-H) peak at 1035 cm-1 (bending), and an aliphatic (C-H) band at 2938 cm-1 (stretching). The carbonyl substituent of the carboxymethylcellulose was reported as the source of the 1731 cm-1 band. An increase in the SAN ratio was observed to cause a decrease in the hydroxyl band intensity on the paper surface. The binding aliphatic (C-H) signal was increasingly apparent at 1035 cm-1 as the SAN ratio increased. The absorbance peaks at 492, 644, and 819 cm-1 were attributed to the lattice peaks of O-Sr, O-Al-O, and O-Al, respectively. The hydrogel’s ability to regenerate itself can strengthen the film’s resistance to deterioration caused by physical contact. Two halves of circles of SAN5 with a relative humidity of 85% were brought into contact to investigate the self-healing efficiency. It was found that the two semicircles had been glued into one circle that can support 100 g of weight. Figure 9 displays SEM images of the hydrogel (SAN5) during the self-healing process. The self-healing activity of the hydrogel is attributed to the hydrogen bond acceptor and the hydrogen bond donor. By motivating the acceptor and donor mobility, moisturizing the affected area helped the border to heal [59].

(a-c) SEM analysis of hydrogel (SAN5; a thickness of 6 mm) during the self-healing process.
Figure 9.
(a-c) SEM analysis of hydrogel (SAN5; a thickness of 6 mm) during the self-healing process.

3.4. Mechanical screening

The mechanical features of a material help to determine its appropriateness for different applications [52]. The tensile strength (TS) of SAN1 falls with strain percentage relative to SAN0 because Young’s modulus (YM) is dependent on stress/strain. A larger strain percent is achieved by the strong elasticity and stickiness that the prints offer. Figure 10 shows that TS and YM grew from SAN0 to SAN6. This occurs as a result of interfacial contact between the negatively charged cellulose and Al(III) in SAN. Therefore, SAN strengthens the interfacial cross-linking of cellulose paper microfibers. YM and TS were then dropped from SAN6 to SAN8. This could be attributed to the coagulation of SAN, which widens the interstitial spaces between the paper-based cellulosic microfibers. When SAN increased, the distribution of strains hardly changed. The nanocomposite hydrogel (CMF@CMC) matrix maintains SAN on the paper surface by the formation of a coordination bond between aluminum and paper cellulose [60].

Mechanical performance of paper prints.
Figure 10.
Mechanical performance of paper prints.

3.5. Rheology studies

The results of a study on the rheology of SAN5 have been shown in Figure 11. The rheology of hydrogels has a significant impact on their printing efficiency [51]. The current hydrogel showed a viscous fluid because the hydrogel flow rate followed the non-Newtonian rules. The viscosity declined linearly as the shear rate was raised. To preserve equilibrium, the viscosity rapidly decreased as the shearing rate increased.

(a) Viscosity of hydrogels against the SAN content and (b) effect of shear rate on the viscosity of SAN5.
Figure 11.
(a) Viscosity of hydrogels against the SAN content and (b) effect of shear rate on the viscosity of SAN5.

4. Conclusions

To develop a hydrogel ink, SAN (a photochromic agent), CMF (a dispersion agent and microfiller), and CMC (a hosting agent) were combined. The diameters of CMF ranged from 15 nm to 50 nm, while the diameters of SAN were determined to be between 8 nm and 17 nm. The durability, high efficiency, and photostability of the lanthanide-doped SANs made them an ideal luminescent agent for composite hydrogels. The transparency of the prints is due to the homogeneous dispersion of SAN in the hydrogel bulk (CMF@CMC). When exposed to ultraviolet rays at 375 nm, the paper prints fluoresced at 519 nm. The exposure to UV radiation caused the printed layer on a white paper to turn green, as shown by photoluminescence and CIE Lab coordinates. The SAN concentration of 0.9% produced the optimal green emission from the printed paper. The transparency of prints is due to the homogeneous dispersion of SAN in the CMF@CMC hydrogel. The produced hydrogel is inexpensive, strong, and photostable, which effectively provides authenticity for common stamps on a variety of surfaces, such as banknotes. However, it is significant to reduce the amount of the nanophosphor required to develop an optimum hydrogel.

Acknowledgment

This research was funded by the Deanship of Scientific Research and Libraries at Princess Nourah bint Abdulrahman University, through the “Nafea” Program, Grant No. (NP-45-024).

CRediT authorship contribution statement

Ghadah M. Al-Senani: Conceptualization, Supervision, Validation, Data curation, Software, Methodology, Visualization, Investigation, Writing-Original draft preparation, Writing-Reviewing and Editing. Salhah D. Al-Qahtani: Methodology, Data curation, Software, Validation, Writing-Reviewing and Editing. Hesah M. AlMohisen: Conceptualization, Methodology, Visualization, Investigation, Data curation, Validation, Writing-Reviewing and Editing.

Declaration of competing interest

The authors declare that they have no competing interests.

Data availability

All data generated or analyzed during this study are included in this published article.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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