One material, many possibilities via enrichment of luminescence in La2Zr2O7:Tb3+ nanophosphors for forensic stimuli aided applications

Engineering a single material with multidirectional applications is crucial for improving productivity, low cost, flexibility, least power consumption, etc. To achieve these requirements, novel design structures and high-performance materials are in urgent need. Lanthanide-doped nanophosphors have the greatest strengths and ability in order to tune their applications in various dimensions. However, applications of nanophosphor in latent fingerprints visualization, anti-counterfeiting, and luminescent gels/films are still in their infancy. This study demonstrated a simple strategy to enhance the luminescence of Tb3+ (1–11 mol %) doped La2Zr2O7 nanophosphors by conjugating various fluxes via a simple solution combustion route. The photoluminescence emission spectra reveal intense peaks at ~ 491, 546, 587, and 622 nm, which arises from 5D4 → 7FJ (J = 6, 5, 4, 3) transitions of Tb3+ ions, respectively. The highest emission intensity was achieved in the NH4Cl flux assisted nanophosphor as compared to NaBr and NH4F assisted samples. The colorimetric images of fingerprints visualized using the optimized nanophosphor on forensic related surfaces exhibit level –III ridge details, including sweat pores, the width of the ridges, bifurcation angle, and the successive distance between sweat pores, etc. These results are decisive parameters that clearly support the statement “no two persons have ever been found to have the same fingerprints”. The anti-counterfeiting security ink was formulated using optimized nanophosphor and various patterns were designed by simple screen printing and dip pen technologies. The encoded information was decrypted only under ultraviolet 254 nm light. All the designed patterns are exhibit not just what it looks/feel like and how better it works. As a synergetic contribution of enhanced luminescence of the prepared nanophosphor, the green-emissive films were fabricated, which display excellent flexibility, uniformity, and transparency in the normal and ultraviolet 254 nm light illumination. The aforementioned results revealed that the prepared NH4Cl flux-assisted La2Zr2O7: Tb3+(7 mol %) NPs are considered to be the best candidate for multi-dimensional applications.

www.nature.com/scientificreports/ nitrogen, carbon dioxide, and water vapor followed by the formation of the final product. Similarly, experiments were repeated by the addition of various fluxes, namely NaBr, NH 4 F, and NH 4 Cl (1-5 wt. %) into the precursor solution. Finally, the obtained product was calcined at ~ 800 °C for ~ 3 h and used for further characterizations. The schematic illustration for the synthesis of LZO: Tb 3+ (7 mol %) NPs blended with various fluxes by the solution combustion method was shown in Fig. S1a.
Characterization techniques. The Shimadzu made powder X-ray diffractometer (PXRD) with monochromatic CuKα radiation was used to study the phase purity of the prepared samples. Morphological and particle size analysis was carried out by Hitachi-3000 table top scanning electron microscope (SEM) and Hitachi H-8100 transmission electron microscope (TEM) provided with a LaB 6 filament equipped with EDS (Kevex sigma TM Quasar, USA). Perkin Elmer spectrometer (Spectrum 1000) with KBr pellets was used to perform Fourier IR reflectance (FTIR) of the prepared NPs. The Perkin Elmer spectrophotometer (Lambda -35) was used to study the diffuse reflectance (DR) of the samples. PL studies were performed with Horiba (Jobin Yuvon) spectrofluorimeter maintained at a slit width of 5 nm with xenon lamp as an excitation source. The Nikon D3100/ AF-S digital camera was used to capture developed LFP images and AC labels under normal and UV 254 nm illumination.
Development and visualization of LFPs using optimized La 2 Zr 2 O 7 :Tb 3+ (7 mol %) (LZOT), NH 4 Cl (4 wt. %) NPs. Fresh fingerprints (FPs) from different healthy donors were collected by washing their hands several times with hand wash and water, subsequently dried in normal air. The thumb finger was rubbed slightly against the forehead and impressed on various substrates with minimal pressure for ~ 3 to 4 s. The developed FPs were invisible to naked eyes and hence called latent FPs (LFPs). To make them visible, the optimized LZOT:NH 4 Cl (4 wt. %) NPs were stained on the LFPs followed by a simple powder dusting technique. The excess powder on the LFPs was removed by smooth to and fro brushing. Finally, the developed FPs were photographed in a digital camera under UV 254 nm light irradiation. The schematic illustration showing LFPs  46 9 CaSn(OH) 6 :Eu 3+ 254 Glass, ceramic tiles, highlighter, aluminum foil, color paper, leaf, currency Type I-III 90 Ghubish et al. 47 10 Fabrication of security ink using LZOT:NH 4 Cl (4 wt. %) NPs. The viscous and luminescent security ink was fabricated using LZOT:NH 4 Cl (4 wt. %) NPs as follows; the stoichiometric amount of the prepared NPs was thoroughly mixed in a ratio of 85:15 v:v ethanol-water solution (1:9 v:v ethanol: water): glycerol for attaining dynamic viscosity. Further, sodium dodecyl sulfonate (3 mg/l) was then added to the above mixture to control the surface tension of the ink. The resulting mixture was ultrasonicated for ~ 20 min to achieve transparent ink. The prepared ink was used to design AC patterns on various surfaces followed by a simple dip pen method. The encoded patterns were in situ photographed under normal as well as UV 254 nm light irradiation.
Screen printing. Screen printing was performed using a mesh with different designs. The prepared inks were poured slowly on the mesh openings and were transferred onto the substrate during the squeezer. Schematic representation of the data encryption and decoding procedure developed by screen printing technique using prepared NPs as a security ink was depicted in Fig. S1c.
Preparation of luminescent hydrogels and flexible films. Firstly, PVA (4 g) was well dissolved in deionized water (~ 30 ml) using a magnetic stirrer for ~ 10 min. Subsequently, formerly prepared luminescent ink (~ 10 ml) was added slowly into the PVA solution and treated ultrasonically by inserting a titanium probe sonicator for ~ 15 min to achieve a uniform solution. Finally, the obtained viscous gel was transferred to specific molds as well as a Petri plate; allowed to dry for ~ 48 h. Later, the obtained patterns and films were photographed using a camera under normal and UV 254 nm light.

Statement of authors and informed consent.
The authors confirmed that all experiments (taking fingerprints of a volunteer/individual) were performed in accordance with relevant guidelines and regulations. An explicit informed consent was obtained from the anonymous volunteer providing the fingerprints. The individual explicitly allowed the authors to use the data in the present publication. And also authors confirmed that all human experimental protocols were approved by a Tumkur University institutional committee. Figure 1a shows the PXRD profiles of pure and LZO:Tb 3+ (1-9 mol %) NPs. Sharp and intense diffraction profiles were indexed to a cubic pyrochlore type structure and well-matched with JCPDS No.:78-1292 61  where Δ r ; acceptable percentage difference, R m ; ionic radii of host ions (R La = 1.16 Å, R Zr = 0.84 Å) and R d ; ionic radii of dopant ions (R Tb = 1.04 Å) in 8-coordinated system. In the present work, the Δ r value between La 3+ and Tb 3+ was found to be ~ 10.34% (< 30%), however Δ r among Zr 4+ and Tb 3+ was obtained to be ~ − 23.80% (< 30%). The obtained Δ r value between La 3+ and Tb 3+ was found to be less than the acceptable value and it signifies effective occupancy of the Tb 3+ ions in the La 3+ site of the LZO lattice rather than the Zr 4+ site. This might be due to dissimilarity in the charge, size, and negative Δ r value between Tb 3+ ions and the Zr 4+ site. In general, fluxes were the most significant role in the fabrication of the NPs, in particular, reducing the firing temperature, improving the crystallinity as well as enhancing the optical and luminescence properties. Hence, to realize the role of various fluxes on the crystallinity of the prepared NPs, we have utilized different amounts of NaBr (1 wt. %), NH 4 F (1 wt. %), and NH 4 Cl (1-5 wt. %) fluxes assisted NPs. PXRD patterns of the LZOT NPs synthesized using all the above fluxes was shown in Fig. 1b. It was evident that all the diffraction profiles were well assigned to standard cubic pyrochlore structure (JCPDS No.: . In addition to this, no obvious peaks belonging to fluxes were revealed. The 1 wt. % of NaBr, NH 4 F, and NH 4 Cl fluxes upsurge the diffraction profile intensities as compared to LZOT NPs. The improvement in the crystallinity after the addition of fluxes was due to several factors, namely solubility, melting point, decomposition property, intermediate compound formation, etc. 63 . Among these fluxes, NH 4 Cl exhibit improved crystallinity. This was mainly attributed to the probable reaction between NH 4 Cl with metal nitrate to form ammonium nitrate. Here, ammonium nitrate plays a dual role; (i) combustible material and (ii) oxidizing agent-assists other materials to burn. Hence, the exothermicity of the redox reaction during synthesis was anticipated to be very high, and also provide the molten medium for mixing of fuel and oxidant as a result of enhancement in the crystallinity 64,65 . However, in the case of NaBr and NH 4 F assisted samples have very low solubility, as well as very high melting point, resulting in no significant changes in the crystallinity when compared to NH 4 Cl. Based on the obtained results, LZOT NPs with different amount (1-5 wt. %) of NH 4 Cl was studied and shown in Fig. 1b. As evident from the figure, the highest crystallinity was achieved for 4 wt. % of NH 4 Cl. The Williamson-Hall (W-H) plots of the prepared NPs were depicted in Fig. 1c (1)  66 . The obtained mean crystallite size and strain were tabulated in Table S1. From the table, the variation in the estimated crystallite size from Scherrer's relation and W-H plots was mainly due to negligence of strain component in the Scherrer's method, however, it is considered in the W-H plots. FT-IR spectra of LZO, LZO:Tb 3+ (1-9 mol %) NPs and LZOT: NaBr, NH 4 F (1 wt. %), and NH 4 Cl (4 wt. %) fluxes recorded in the range 400-4000 cm − 1 was shown in Fig. 1e. The spectra consist of sharp peaks located at ~ 488, 720, and 955 cm -1 , which were attributed to the absorption of La-O, Zr-O, and Zr-O-Zr bonds, respectively 67 . The peak centered at ~ 1388 cm -1 is attributed to NO 3 groups adsorbed on the surface of the LZO:Tb 3+ . The peak centered at 1549 cm -1 was attributed to CO 3 2ionic groups adsorbed on the surface of the LZO:Tb 3+ NPs due to the reaction between the metallic ions and a trace amount of CO 2 from the atmosphere during the synthesis 68 . Figure S2a and b represents the DR spectra of LZO:Tb 3+ (1-9 mol %) NPs and LZOT:NaBr, NH 4 F (1 wt. %) and NH 4 Cl (1 and 4 wt. %) fluxes. The spectra exhibit sharp absorption peaks in the range ~ 200-300 nm, which are ascribed to 4f. → 5d electronic transitions of Tb 3+ ions 69 . The Kubelka-Munk (K-M) function was utilized to determine energy band-gap (E g ) values of the prepared NPs, as described in the previous literature 70 . The E g plots of the LZO:Tb 3+ (1-9 mol %) NPs and fluxes assisted LZOT NPs were depicted in Fig. 1f. As evident from the figure, the E g values were estimated and found to be ~ 3.44-3.51 eV (Table S1). SEM images of pure and LZO:Tb 3+ (1-9 mol %) NPs were shown in Fig www.nature.com/scientificreports/ morphology. The observed porous nature which mainly ascribed to uniform combustion as well as the flame distribution throughout the combustion process. These features were the inherent nature of the combustion synthesis. After the addition of the fluxes in an aqueous medium, the flaky-like structure was clearly retained ( Fig. S3g-i). This was mainly attributed to its excellent solubility, which uniformly distributes and also offers the medium for homogeneous distribution of fuel. Figure S3j and k depicts the TEM, images of the LZOT NPs, and LZOT: NH 4 Cl (4 wt. %) NPs. As observed from the TEM image, particles were agglomerated and their average size was found to be ~ 12 nm. The interplanar spacing was estimated from the HRTEM image ( Fig. S3l) and the value was obtained to be 0.312 nm for the (222) plane. The high crystallinity of the optimized NPs was further confirmed from the selected area of electron diffraction (SAED) pattern (inset of Fig. S3l). Energy-dispersive X-ray (EDAX) spectrum of the optimized LZOT NPs (Fig. S4) signifies the presence of La, Zr, Tb, and O elements, which endorses the effective substitution of the Tb 3+ ions in the host lattice. Figure 2a depicts the PL excitation spectra of LZO:Tb 3+ (1-9 mol %) NPs by monitoring ~ 546 nm emission wavelength at room temperature. The spectrum reveals several well-resolved intense peaks at ~ 317, 328, 339, 351, 377, 396 and 489 nm owing to 7 F 6 → 5 D 0 , 7 F 6 → 5 D 1 , 7 F 6 → 5 L 6 , 7 F 6 → 5 L 9 , 7 F 6 → 5 G 6 , 7 F 6 → 5 D 3 and 7 F 6 → 5 D 4 transitions of Tb 3+ ions, respectively 71 . Among them, the intensity of the excitation peak was maximum at ~ 377 nm, in which efficient energy may transfers from the host to the Tb 3+ ions and it can be being approximately equivalent to traditional NUV LED chips. PL emission spectra of LZO:Tb 3+ (1-9 mol %) NPs excited at ~ 377 nm wavelength were shown in Fig. 2b. The spectra comprised characteristic emission peaks originated from the 5 D 3 and 5 D 4 energy levels to various 7 F J (J = 3, 4, 5, 6) levels. The narrow emission peaks centered at ~ 416, 439, and 466 nm, which ascribed to 5 D 3 → 7 F 5 , 5 D 3 → 7 F 4, and 5 D 3 → 7 F 3 transitions of the Tb 3+ ions, respectively. However, emission peaks at ~ 491, 546, 587 and 622 nm arises from 5 D 4 → 7 F J (J = 6, 5, 4, 3) transitions of Tb 3+ ions, respectively 72 . It was evident from the figure that, emissions originating from the 5 D 4 → 7 F J transitions were more prominent than the 5 D 3 → 7 F J transitions. Among green emissions, the peak at 546 nm ( 5 D 4 → 7 F 5 ) was found to be more intense and had the largest probability for magnetic-dipole transition (ΔJ = ± 1), which was independent of the matrix crystal field and environment of the luminescent center. However, peak centered at 491 nm ( 5 D 4 → 7 F 6 ) related to a forced electric dipole allowed transition, and their intensity is sensitive to the local symmetry around RE ions. The intensity variation might be attributed to cross-relaxation among Tb 3+ ions, which can be expressed as below 73 ;

Results and discussion
Electrons in 5 D 3 state get relaxed at 5 D 4 and the 7 F 6 electrons of Tb 3+ ions are excited to 7 F 0 state. This process declines 5 D 3 → 7 F J transitions, while 5 D 4 → 7 F J transitions become more dominated. As a result, the present NPs show diminished bluish-green emission (400-470 nm) and intense green emission (480-630 nm). The energy level diagram of Tb 3+ ions doped LZO NPs representing probable excitation and emission transitions were depicted in Fig. S5. Normally, the dopant concentration in the phosphors will influence the emission performance. In the present work, the emission intensity increases with the increase of Tb 3+ concentration up to 7 mol % and subsequently, it declines with further increase of dopant concentration was noticed (Fig. 2c). This was mainly attributed to conventional concentration quenching phenomena, which provide clear insight into the non-radiative energy relaxation process between nearby Tb 3+ ions. The critical distance (R c ) between the Tb 3+ ions was estimated using the following relation 74 ; where V; unit cell volume (1254.04 Å 3 ), X c ; critical concentration (0.07), and N; the number of lattice sites in crystallographic unit cell available for dopant ions (8). In the present work, the value of R c was estimated and found to be ~ 8.1 Å. The obtained R c value (> 5 Å) overrules the probability of exchange interaction. Further, no spectral overlap was clearly observed, indicating the occurrence of the radiative re-absorption mechanism. Hence, it was clearly demonstrated that the energy transfer mechanism was directed through multipole-multipole interactions. According to Dexter's theory, the type of multipolar interaction responsible for concentration quenching was elucidated by using the following equation 75 ; here, X; dopant concentration, k 1 and β; constants for each interaction in the same excitation conditions for a given host lattice, and s; series of the electric multipolar interactions (dipole-dipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (q-q) when the values of s are 6, 8 and 10, respectively). The value of s can be calculated from the slope (s/3) of the linear fitted line in Fig. 2d. The value of (-s/3) was found to be − 1.889. Thus, the value of s can be calculated as ~ 6.81 (close to the theoretical value of 6 for the electric d-d interaction), which signifies that the d-d interaction was the main mechanism for the concentration quenching of Tb 3+ ions in the LZO host. The effect of fluxes on the emission intensity of LZOT NPs was studied and depicted in Fig. 2e. Identical emission profiles were clearly noticed in the without and with flux-assisted NPs. Further, enhancement in the PL emission intensity was achieved for flux (1 wt. %) assisted NPs when compared without flux. This may have been attributed to an increase in crystallinity and phase purity, which will reduce the lattice and surface defects of the NPs. The PL emission was found to be higher (two-fold) in the NH 4 Cl assisted NPs when compared to the NH 4 F and NaBr (1 wt. %). However, the influence of different NH 4 Cl amounts (1-5 wt. %)  www.nature.com/scientificreports/ The CIE color coordinates (x, y) were estimated using the PL emission of the prepared NPs. The estimated color coordinates (x, y) values were denoted by different symbols in the CIE diagram ( Fig. 3a and c). The CIE color coordinates of the NPs located near those of EBU (European Broadcasting Union) for green illumination (x, y = 0.29, 0.60)), showcase the significance of the NH 4 Cl assisted LZOT NPs as a green component in the WLEDs. In addition, correlated color temperature (CCT) was also considered an important parameter to evaluate the color quality of the NPs. In the present work, the CCT of the prepared NPs without and with fluxes were estimated using the following expression 77 ; where, n = (x-x e )/(y-y e ); x, y are the color co-ordinates of sample and x e , y e are chromaticity epicenter (x e = 0.3320, y e = 0.1858). The CCT diagram of the prepared NPs was depicted in Fig. 3b and d. The estimated CCT values were found to be in the range ~ 5200-7000 K, which were fairly equivalent to commercial WLEDs. Hence, the optimized NH 4 Cl assisted LZOT NPs may play a significant role in UV excited cool WLEDs. Further, the color purity of the phosphors was considered an attractive feature which reveals their applicability for plentiful applications. In the present work, the color purity of the prepared NPs was estimated using the following relation 78 ; where (x, y); co-ordinates of a sample point, (x d , y d ); co-ordinates of the dominant wavelength and (x ee , y ee ); coordinates of the illuminated point. The estimated color purity of the optimized NPs was found to be ~ 97%. The estimated photometric properties of the prepared NPs were listed in Table S2. The obtained photometric properties were found to be well accepted as compared to previous literature (Table S3) [79][80][81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97] . The values reveal that the prepared NPs were considered to be an excellent candidate for green color dominance in the UV excited WLEDs. www.nature.com/scientificreports/ The PL decay curves for the 5 D 4 → 7 F 5 (546 nm) transition of Tb 3+ ions in the LZO:Tb 3+ (1-9 mol %) and LZOT:NH 4 Cl (4 wt. %) NPs were shown in Fig. S6 measured under 377 nm excitation. The decay curves were well fitted by using bi-exponential fitting which was expressed as 98 ; where I 1 and I 2 are the intensities at different time intervals and τ 1 and τ 2 are their corresponding lifetimes. Further, the average decay lifetimes can be calculated as; The decay time (τ) values for LZO:Tb 3+ (1-9 mol %) and LZOT:NH 4 Cl (4 wt. %) NPs were estimated and found to be 0.703, 0.574, 0.511, 0.412, 0.384, and 0.798 ms, respectively. The lifetime was found to be higher in the flux-assisted NPs as compared to without flux-prepared samples. Further, the luminescence quantum yield (QY) of LZOT:NH 4 Cl (4 wt. %) NPs was recorded under the excitation of 377 nm. The QY of the optimized NPs was estimated and found 72.53%. It was worth noting that the QY of LZOT:NH 4 Cl (4 wt. %) NPs was greater than some other green-emitting phosphors, such as Sr 3 Gd 1.9 (Si 3 O 9 ) 2 :0.1 Tb 3+ (26.6%) 99 , LiLaSiO 4 :0.08 Tb 3+ ,0.04Sm 3+ (22.34%) 100 . In addition, the temperature-dependent emission spectra of LZOT:NH 4 Cl (4 wt. %) NPs excited at 377 nm as illustrated in Fig. S7. As evident from the figure, the emission intensity gradually decreased with increasing temperature from 300 to 523 K, but still maintained the same profiles. The PL emission intensity at 523 K was 58.82% of that at room temperature, revealing that (LZOT:NH 4 Cl (4 wt. %) NPs had good thermal stability.

Applications prospect
Visualization of LFPs using LZOT:NH 4 Cl (4 wt. %) NPs. Due to the excellent solid-state PL performance of the prepared NPs, it was used to strengthen its application capability for various fields, especially in forensic science. To explore the practicality of the optimized LZOT:NH 4 Cl (4 wt. %) NPs, we adopted a powder dusting approach for the visualization of LFPs on various substrates. Figure 4a-d shows the visualized FPs using prepared NPs on non-porous surfaces (compact disc, metal scale, glass, and mobile phone screen) under UV 254 nm light illumination. As evident from the figure, the developed FPs with distinguishable ridge details (level I-III) were clearly visible, due to the strong adhesion of the NPs with chemical constituents present in the LFPs. Normally, the chemistry of LFPs residue was more complicated, due to its comprise of several chemical constituents. These components readily form a complex matrix, an emulsion of water, organic and inorganic compounds 96,97 . The chemical residues present in the LFPs were normally very minimal (less than 10 μg) with an average thickness of about 0.1 μm. The LFPs were impressed on the surfaces, nearly 99% of the LFPs contain water 101 . As this water begins to evaporate quickly from the LFPs, subsequently the FPs dry. This process begins to modify certain powders ability to visualize the such FPs. Hence, LFPs dusting powder with specific functional groups which interact with FP residues for improving the visualization ability was highly necessary. In the present work, NH 4 Cl (4 wt. %) flux-assisted NPs can readily interact with water-soluble FPs components typically composed of amino acids (especially serine). Since serine was the most abundant amino acid present in the FPs as compared to other constituents. However, the detection sensitivity of the optimized NPs for LFPs visualization on various porous surfaces, including wood, paper, ticket, and tissue paper (Fig. 4e-h) and semi-porous surfaces, namely glossy paper, plastic card, aluminum foil, and cardboard sheet ( Fig. 4i-l) under UV 254 nm light irradiation were examined. It was clear from the figure that well-defined ridge features enable up to level-I & II details with high sensitivity, low contrast, and without any background hindrance. The grayscale pixel profiles of marked yellow box on the developed FPs (Fig. 4m-o) revealed that prepared NPs were clearly stacked exactly on the ridges rather than furrows due to their nano regime and better adhesive nature. It also supports the above result, in which the green value was visibly high for the ridge regions, however minimal for the furrow regions. Further, 3D interactive plots of the developed FPs also evidenced that the stained NPs were uniformly distributed over the surface of the LFPs (Fig. 4p-r).
Generally, ridges, as well as valleys, are the most significant characteristics of the FPs. These characteristics were normally categorized into three levels 102 . They are, level-I features are the vein feature of the FPs, which comprise a central point, delta, whorl, loop, and arch, which were not enough for personal individualization. Furthermore, level-II features are macroscopic, involving ridge dot, termination, lake, island, bifurcation, the fold of the ridge, and rift valley of the furrow. In addition, level-III features were microscopic characteristics, such as sweat pores, length of the ridge, ridge width, shape of the ridge end, shapes and sizes of the sweat pores, successive distance between pores, scars, ridge bifurcation angle, etc. 103,104 . These features are most significant in forensic investigation but fails to develop and analyse in detail due to the inability of the conventional powders under different circumstances. This makes us motivated to develop efficient NPs, which can enable level-III features in detail. Figure 5A and B represents developed FPs of the two different donors stained with NH 4 Cl (4 wt. %) flux assisted LZOT NPs on glass substrate under UV 254 nm light illumination. It was clearly noticed from the figure that, the NPs adhered well with FPs, showing green emission in the ridgeline, but black in the groove region under UV irradiation. The level-I features, such as whorl, loop, delta, and center dot were clearly revealed. In addition, level-II features, like bifurcation, ridge end, dot, enclosure, bridges, hook, cross over, lake, termination, etc. were clearly explored (Fig. S8). Furthermore, the most authenticated level-III features of the FPs of the two different donors, which enclose all dimensional properties of the ridges were revealed and tabulated in Table 2. As evident from the table, level-III dimensionality was varied with donors, which clearly supports the statement "no two persons have ever been found to have the same fingerprints". In addition, SEM images of www.nature.com/scientificreports/ the developed FPs, also reveal the positions of the sweat pores, the distance between successive pores, bifurcation and hook angle, the shape of the ridge end, the width of the ridges, ridge end angle details, etc. (Fig. 5c-j). The chemical residues of the FPs vary over the time after deposition, which depends on various factors, such as atmospheric contamination, humidity, light exposure, temperature, ultraviolet, and other radiations, etc. In the present work, a series of experiments were performed to investigate the influence of external PA on the developed FPs on the glass surface under UV 254 nm illumination (Fig. 6a-f). The photographed images clearly revealed that the FPs were scratched to some degree, however, sufficient ridge features required for personal individualization can be clearly enabled even up to 5 cycles of PA. Pixel profiles and 3D interactive plots of the developed FPs before and after PA, show that the NPs were uniformly distributed and stacked on the ridges rather than furrow region (Fig. 6g-i). Likewise, CA test was also accomplished by soaking the LFPs on the glass surface with acetone and toluene and developed using the optimized NPs (Fig. 6j, j', k and k' . No disruptive interference and clear ridge details can be clearly observed even after chemical treatment. The developed FPs before and after CA were exhibited almost similar emissions without any disruption. The obtained results substantially demonstrated that the present strategy was more efficient in visualizing LFPs with the insignificant effect of powerful external intrusions. The corresponding pixel profiles were clearly demonstrated that the NPs effectively interact www.nature.com/scientificreports/ with amino acids present in the ridge region rather than furrow portions (Fig. 6l). Further, exposure to light on developed FPs can significantly affect FPs compositions. Herein, photo-stability of the developed FPs on the glass upon continuous UV 254 nm (Fig. 6m-r) and 365 nm illumination (Fig. 6s-x) up to ~ 6 h was examined. Well-defined ridge features, which reveal level I-II details without any noticeable luminescence quenching were noticed. This signifies that UV exposure will not have much influence on the visualization ability of the prepared NPs. To evaluate the practicality of the NPs for the visualization of LFPs, we performed the FPs development trials after various FPs aging times (up to 24 days). As displayed in Fig. 7a-e, the gradual decrement in the visualization sensitivity with extended aging was noticed, which ascribed to the slow evaporation of the FPs residue over time. Moreover, LFPs aged for up to 24 days can reveal clear ridges including level I-III features, signifying that the sensitivity of the present NPs was high enough for visualization of aged FPs. Further, the pixel profile value shows greater contrast between fluorescent dark and bright field furrow (Fig. 7f-j). The obtained results were well validated from corresponding 3D interactive plots (Fig. 7k-o). opens up new avenues for practical AC applications. Over the decades, forging/duplicity of important goods or documents, namely certificates, currency, big-name brands, medicines, foods, etc. is a serious threat all over the world that causes a severe negative impact on human health, the world economy, and social development [105][106][107][108] .
To combat this issue, several fluorescent-based materials have been used for AC applications, nevertheless, luminescence quenching, spectral overlap, low quantum efficiency, and toxicity remain a major concern 109 . In this context, we fabricated luminescent-based security inks to authenticate the practicability of the prepared NPs for AC applications. The prepared ink was used in screen printing technology to establish the AC patterns (trees and ice cream) on the paper surface under normal light (Fig. 8a-c and UV 254 nm light illumination (Fig. 8a'-c').
The designed patterns were invisible to the naked eye under normal light, while distinctive and sharp luminescence patterns were decoded under UV 254 nm light. However, to make the process simple and cost-effective, we directly designed different patterns with a pen filled with prepared ink. Figure 8d-i displays the AC labels on various surfaces (such as plastic, transparent polyethylene sheet (used for commercial packaging), filter paper, ceramic tile, aluminum foil, and foam) by employing a simple dip pen technique under normal ( Fig. 8d-i) and UV 254 nm light illumination (Fig. 8d'-I'). It was very clear from the figure that, designed AC patterns were invisible under normal light, however corresponding distinctive patterns were decoded upon UV 254 nm light illumination. The obtained results signify that surfaces will not affect the designed patterns. Hence, prepared flux-assisted NPs open wide scope in AC applications, especially signature or personalized security information. Further, the photostability, durability, and mechanical stability of the designed patterns were examined. The AC patterns on the paper surface were continuously illuminated with UV 254 nm for different time periods (1-5 h) (Fig. S9). The obtained results clearly showed that the intensity of the green emission was almost retained even after 5 h prolonged illumination. However, the durability of the patterns on the ceramic tile was examined at varying temperatures from 32, 40, 50, 60, and 70 °C (Fig. S10), which clearly demonstrated that the marginal intensity loss was noticed. The mechanical stability of AC patterns on the aluminum foil was also examined by ultrasonication for 10-50 min at 30 kHz (Fig. S11). The decorated AC patterns on transparent polyethylene sheets retain their luminescence intensity even after sonication in water, which authenticated the stability of the prepared ink. Flexible luminescent hydrogels were highly proficient in converting absorbed energy (like current, electric field, biologic processes, X-ray, chemical reaction, etc.) into electromagnetic radiation 110,111 . They can be extensively used in various applications, such as optoelectronics devices, field-effect transistors, detectors, medical diagnosis, bio-imaging, etc. [112][113][114] . Hence, luminescent gels have been paid much attention due to their outstanding biocompatibility and viscoelastic properties 115 . Herein, luminescent hydrogels with excellent luminescence were fabricated and used for AC applications. The information was encrypted in various scrambled patterns and photographed under normal (Fig. 8j-l) and UV 254 nm light (Fig. 8j'-l'). This encrypted information was decoded by displaying green emissions and hidden information can be realized clearly under UV light as "BUS, SUB, US" and "DIGITAL INDIA". The. Further, flexible luminescent films were most commonly used as labels, packaging, displays, etc., which influenced significant application value in industries as well as life. The prepared films exhibit uniformity and transparency in the visible light Fig. 8m and m'. Further, the luminescent film was highly flexible, and it offers maximum deformation of ~ 200%. Simultaneously, however, the films with green emission under UV 254 nm was also retained their transparent nature (Fig. 8n, n', o, o'). As can be seen   Figures (h and i)  www.nature.com/scientificreports/ from the figure, no variations in the luminescence intensity with an increase in stretching, which might be due to stability in the material density with an increase in surface area.

Conclusion
A low-cost and effective method has been developed for the synthesis of Tb 3+ (1-11 mol %) doped LZO NPs by conjugating the fluxes via a simple solution combustion route. Sharp and intense PXRD profiles were indexed to a cubic pyrochlore-type structure. The improvement in the crystallinity after the addition of NH 4 Cl fluxes exhibits improved crystallinity, which is mainly attributed to the probable reaction between NH 4 Cl with metal nitrate to form ammonium nitrate. The PL emission intensity increases with the increase of Tb 3+ concentration up to 7 mol % and subsequently, it declines due to conventional concentration quenching. PL emission www.nature.com/scientificreports/ was found to be higher (twofold) in the NH 4 Cl assisted NPs when compared to the NH 4 F and NaBr for 1 wt. %. The estimated CIE color coordinates of the NPs located near those of the European Broadcasting Union for green illumination (EBU, (x, y = 0.29, 0.60)), which showcase the significance of the NH 4 Cl assisted LZOT NPs as a green component in the WLEDs. The estimated CCT values were found to be in the range ~ 5000-7000 K, which were fairly equivalent to commercial WLEDs. These obtained colorimetric parameters of the NPs which endorse their usage in high-contrast imaging applications, especially to overcome auto-fluorescent backgrounds. Well-defined ridge features enabling up to level-I-III details with high sensitivity, low contrast, and without any background hindrance were revealed using optimized NPs. The developed films show high photostability against UV irradiation, longer durability, and are highly flexible. The prepared hydrogels were used to encrypt the information and this encrypted information was decoded by displaying green emissions as "BUS, SUB, US" and "DIGITAL INDIA" under UV 254 nm light. To the best of our knowledge, the present work delivers a smart alternative approach to fabricating highly luminescence NPs for various labeling FPs, luminescent security patterning, and flexible films applications.