Potential of Y 2 Sn 2 O 7 :Eu 3+ , Dy 3+ Inorganic Nanophosphors in Latent Fingermark Detection

: In this work, we investigated the potential of Eu 3+ /Dy 3+ -codoped Y 2 Sn 2 O 7 fluorescent nanophosphors to visualize latent fingermarks. We prepared these nanophosphors with various doping concentrations by the conventional coprecipitation reaction. The crystal structure, morphology, luminescence properties, and energy transfer mechanisms were studied. The crystalline phase was characterized by X-ray diffraction and crystal structure refinement using the Rietveld method. XRD measurements showed that the samples crystallized in the pure single pyrochlore phase with few more peaks originated from secondary phases and impurities generated during phosphor production, and that Eu 3+ ions occupied D 3d symmetry sites. The average crystallite size after mechanical grinding was less than 100 nm for all compositions. The optical characterization showed that, when excited under 532 nm, the Eu 3+ /Dy 3+ -codoped Y 2 Sn 2 O 7 samples’ main intense emission peaks were located at 580–707 nm, corresponding to the 5 D 0 → 7 Fj (j = l, 2, 3, and 4) transitions of europium. In fact, the 5 D 0 → 7 F 2 hypersensitive transition is strongly dependent on the local environment and was quite weak in Eu 3+ :Y 2 Sn 2 O 7 at low Eu 3+ doping levels. We found that the presence of Dy 3+ as a codopant permitted enhancing the emission from this transition. The calculated PL CIE coordinates for the synthesized nanophosphors were very close to those of the reddish-orange region and only slightly dependent on the doping level. Various surfaces, including difficult ones (wood and ceramic), were successfully tested for latent fingerprint development with the prepared Eu 3+ /Dy 3+ -codoped Y 2 Sn 2 O 7 fluorescent nanophosphor powder. Thanks to the high contrast obtained, fingerprint ridge patterns at all three levels were highlighted: core (level 1) islands, bifurcation, and enclosure (level 2), and even sweat pores (level 3).


Introduction
Since each fingerprint is distinct from others and does not change with age, fingerprint identification technology is well-established in the field of forensic research.Sweat and sebum, which are continuously produced by perspiration, can leave residue on many surfaces when a finger touches them [1][2][3].Fingerprints discovered at crime scenes are called latent fingerprints, which are unable to be seen with the naked eye; therefore, specific methods to develop them must be used.Different physical, chemical, and optical methods can be used to develop these latent fingermarks [1,[4][5][6][7].
Crystals 2024, 14, 300 With state-of-the-art technology, detecting the first two levels of ridge details corresponding to cores (level 1), bifurcations, and terminations (level 2) is quite easy, but the third level (sweat pores) is rather complicated to see, mainly on difficult surfaces such as wood and ceramic.Level 1 details include broad morphological information such as the overall ridge pattern and the fingerprint ridge flow.The second level of details provides information regarding the pattern agreement of individual fingerprint ridges.Level 3 details include sweat pores, curvature, and spots, as well as fingerprint ridge dimensional elements.As a result, the rapid detection of latent fingerprints, which are originally generated by ridges and furrows in the sebum residue left by the finger skin, requires extra trial-and-error procedures.
Numerous research papers have been reported on organic nanophosphors, which may have significant advantages for latent fingerprint development [8] thanks to their interesting properties such as narrow emission lines, large Stokes shifts, long luminescence lifetime, and high-temperature synthesis.Moreover, such powders show their ability to detect latent fingerprints deposited on difficult surfaces such as wood and ceramic, but they usually develop weak fingerprints under ultraviolet (UV) or laser light.
So far, a variety of techniques have been developed for fingerprint visualization, ranging from quantum dots and single-metal deposition methods to superglue fuming and powder-dusting techniques.These traditional methods face several drawbacks, such as poor detection capability and high toxicity.In addition, very few methods are efficient enough to be able to visualize even the third level of fingerprint details, namely sweat pores.Among these techniques, powder dusting has attracted much attention because of its extreme simplicity and effectiveness, provided that the fluorescent powders have excellent absorption and luminescence characteristics.
However, the most common types of powders used at crime scenes have large and non-uniform sizes, so they can hinder the third-level details in the fingerprint pattern due to their airy nature during brush strokes.To overcome these limitations, trivalent rare earth ion-doped pyrochlore nanoparticles, with A 2 B 2 O 7 chemical compositions, are among the most promising powders to improve the visualization of latent fingerprints thanks to their high fluorescence capability [9,10].These nanometer-sized powders are particularly useful on rough surfaces such as wood and multi-colored non-porous objects where normal powders may clutter the surface.In addition, in order to visualize fingerprints well on surfaces of different colors, it is necessary to use fluorescent powders with different color shades to eliminate interference with the background color.Therefore, it is important to study and optimize new materials that have different color shades suitable for this purpose.
Among the various types of pyrochlore, nanopowders based on lanthanide ions (Ln 3+ ) and tin tetracations (Sn 4+ ) are excellent potential hosts for fluorescent ions due to their great chemical stability, high melting point, and small particle size (nm), which depend on the synthesis method and conditions.
In recent years, the red emission of europium ions has been extensively studied both in amorphous and crystalline materials [11,12].The red emission intensity of Eu 3+ ions strongly depends on its local environment [11,13]; therefore, a careful optimization of both the preparation procedure and composition of the compounds is usually crucial for achieving the best performance possible.
On the other hand, Dy 3+ ions have been investigated for their photo-and radioluminescence properties [14][15][16].In fact, they show a white-yellowish emission that appears to be rather stable against changes in the excitation wavelength.Moreover, Dy 3+ ions can be used as codopants to tailor the emission properties of rare-earth ions [15].
Europium ions introduced in Y 2 Sn 2 O 7 have been proven to generate red emission under UV excitation [17,18].In fact, in our previous study [17], we observed that the hypersensitive 5 D 0 → 7 F 2 Eu 3+ transition showed a low emission intensity at doping levels lower than 10% and became very bright at this level.Given its hypersensitive nature, this behavior can be ascribed to the local crystal environment.Therefore, to enhance the Crystals 2024, 14, 300 3 of 12 emission in the red region for better latent fingerprint visualization, we propose using dysprosium ions as codopants in this host material.
In fact, in this work, we successfully synthesized Y 2 Sn 2 O 7 :Eu 3+ , Dy 3+ nanophosphors at various doping concentrations by a coprecipitation method and used them to detect even the third level of detail in fingerprints on wood and ceramic.Therefore, our work will be a step forward for detecting the third level of latent fingerprints in crime scenes.

Synthesis of Phosphor Samples
In this work, all the starting materials were purchased from Sigma-Aldrich (Merck KGaA, Darmstadt, DE, Germany).Stoichiometric amounts of SnCl 2 •2H 2 O, Eu 2 O 3 , Dy 2 O 3 , and Y 2 O 3 were dissolved in 50 mL of 1 M HCl solution in a beaker, and the excess acid was evaporated out repeatedly.Then, ethylene glycol (40 mL) was added to this solution.The solution was slowly heated up to 100 • C followed by adding 2 g of urea, and the temperature was raised to 140 • C. In this step, ethylene glycol was used as the capping agent and urea was used for hydrolysis.At this temperature, the solution became turbid.The temperature was then increased to 150 • C and kept at this value for around 2 h.The precipitate was collected after the reaction by centrifugation and then washed two times with acetone followed by drying under ambient conditions (overnight).The samples thus prepared were finally calcined at 1300 • C in air at a heating rate of 10 • C per minute followed by 30 min of grinding.In a separate experiment, the undoped Y 2 Sn 2 O 7 nanoparticles were also synthesized by a similar method taking only the yttrium and tin precursors.After this, the samples were ground for 60 min at 500 rpm, with a 5 min rest period in a zirconium jar containing zirconium balls.The relevant reaction formulas are shown below:

Characterization
The samples' phase structures were examined utilizing a Panalytical Pro X'Pert MPD (Malvern Panalytical, Malvern, United Kingdom) (40 kV, 30 mA) with CuKa radiation (1.5404A • ) at 30 kV and 15 mA in the range of 10 • -70 • with a step size of 2θ = 0.02.The crystalline phases were identified by comparing the X-ray patterns of the JCPDS database structure.Refining of the crystallographic parameters was performed using the Rietveld fit program.
The hydrodynamic diameter of the powder was measured with Malvern Zetasizer Nano ZS90 (Malvern Panalytical Ltd., Malvern, UK).To separate nanoparticle aggregates, the powder was dispersed in deionized water and sonicated shortly before the test.
The measurement was performed in a glass cuvette with a round aperture at room temperature.The average particle size was calculated by Malvern zeta-sizer software Malvern Zetasizer Nano ZS90 using Dispersion Technology Software 5.1 starting from the autocorrelation function of the light scattered by the nanoparticles.
We observed the morphology of the samples with a high-resolution field emission scanning electron microscope performed with an FEI (Hillsboro, OR, USA) Quanta 450 FEG system operating in a low vacuum.
Photoluminescence (PL) spectra at room temperature were measured using an iHR320 (HORIBA, Ltd., Kyoto, Japan) spectrometer under a fixed excitation wavelength at 532 nm, utilizing a 50 mW laser as an excitation source for emission measurements.The emission monochromator was scanned in the wavelength region between 550 and 730 nm with a resolution of 0.06 nm.

Powder XRD Analysis
The room-temperature powder XRD patterns of the undoped and 2xEu 3+ /2yDy 3+ -codoped Y2Sn2O7 (2x/2y = 0.2/0.01,0.05/0.05,0.05/0.2,0.2/0.05,and 0.1/0.1)samples are shown in Figure 1.The XRD pattern for the parent Y2Sn2O7 is provided for reference.For the (2x/2y = 0.05/0.05,0.2/0.05,and 0.1/0.1)samples, the main crystalline peaks (222), (400), (440), and (622) correspond to the powder diffraction standards, as shown in Figure 1a.These results confirm that the samples annealed at 1300 °C crystallized in a single pyrochlore phase, and the crystal structure belonged to the 3  cubic system space group.Since the ionic radius of the Eu 3+ (r = 0.95 Å) ion [19] and of the Dy 3+ (r = 0.91 Å) ion [20] are similar to that of the Y 3+ ion (r = 0.92 Å), Eu 3+ and Dy 3+ ions can effectively bind to the Y2Sn2O7 host lattice; thus, replacing the Y 3+ ion does not distort the crystal structure.However, for the (2x/2y = 0.2/0.01 and 0.05/0.2) samples with a high doping concentration, a few more peaks are visible in the experimental XRD pattern, possibly from secondary phases and impurities generated during phosphor production [21].Using Rietveld refinements, the element parameters and atomic coordinates obtained by the least squares fitting procedure are shown in Table 1 [22,23].Figure 1b   Using Rietveld refinements, the element parameters and atomic coordinates obtained by the least squares fitting procedure are shown in Table 1 [22,23].Figure 1b   The Rietveld crystal structure refinement results are presented in Table 2.The cubic pyrochlore structure was used to find the lattice parameters, site mixing, and the number of phases.Structural parameters, such as Rp (profile fitting of R-value), Rwp (weighted profile of R-value), and χ 2 (goodness-of-fit factor), obtained from the Rietveld refinement are also presented.The low values of χ 2 and profile parameters (Rp, Rwp) indicate that the derived samples were of better quality and the refinements of the samples were effective.

DLS Analysis
The hydrodynamic diameters acquired from the measurements of the samples Y 1.75 Eu 0.2 Dy 0.05 Sn 2 O 7 and Y 1.8 Eu 0.1 Dy 0.1 Sn 2 O 7 were analyzed.We used deionized water as a solvent to ensure the suspensions had good stability, as reported in previous research works [17,25].We compared the hydrodynamic radius of the samples before and after mechanical grinding.Before grinding, the lowest measured size value was around 291 nm, and the highest diameter was 456 nm.We used high-energy ball milling to crush and grind the materials to maintain the correct fluorescence and to obtain homogeneous and metastable nanocrystalline phases.The smallest-diameter powders were then separated by centrifugation.Usefully, we obtained good results up to 73 nm and 83 corresponding to the Y 1.75 Eu 0.2 Dy 0.05 Sn 2 O 7 and Y 1.8 Eu 0.1 Dy 0.1 Sn 2 O 7 nanopowders, respectively.Thus, the grinder performed an important role in reducing the size of the powders.Figure 2 shows the DLS measurements for the two samples after grinding.

SEM Characterization
Figure 3 shows some representative SEM images of all the Y2Sn2O7:2xEu/2yDy samples.Figure 3f depicts a typical SEM image of the ground Y2Sn2O7:20%Eu 3+ /5%Dy 3+ sample.The formation of very tiny nanometer-sized particles with diameters less than 100 nm

SEM Characterization
Figure 3 shows some representative SEM images of all the Y 2 Sn 2 O 7 :2xEu/2yDy samples.Figure 3f depicts a typical SEM image of the ground Y 2 Sn 2 O 7 :20%Eu 3+ /5%Dy 3+ sample.The formation of very tiny nanometer-sized particles with diameters less than 100 nm can be plainly seen.Image analysis of more than 100 particles was performed to find the average diameter of each sample.Table 3 shows the different measured sizes of all samples after ball milling.In all cases, an average diameter of about 100 nm or less was observed.

Photoluminescence
The emission spectra of the Y2Sn2O7:Dy 3+ , Eu 3+ phosphors were recorded with λex = 532 nm. Figure 4 shows the PL spectra of the Y2−2x−2yEuxDySn2O7 (2x/2y = 0.2/0.05,0.1/0.1,0.05/0.2,0.05/0.05,and 0.2/0.01)nanophosphors.All spectra were normalized to the high-    These spectra are in good agreement with the literature [17,18,[26][27][28], but, unlike the Y2Sn2O7:Eu 3+ singly doped compound [17], this doubly doped composition shows a very intense emission in the red region from the hypersensitive Eu 3+ 5 D0→ 7 F2 transition.Several sharp lines are located at 578, 588, 611, 628, and 707 nm [29].These sharp emission peaks are mainly situated in the red spectral region and correspond to transitions from the excited state 5 D0 to lower states 7 FJ (J = 0, 1, 2, 3, 4), respectively [30,31].Due to the screening effect of the outer 5s 2 5p 6 electrons, the crystal field hardly affects the position of the 4f energy levels of Eu 3+ ; on the contrary, it strongly affects the transition probabilities and their selection rules.Eu 3+ ions occupy a site with D3d symmetry in Y2−xEuxSn2O7 [18,32], and this causes electrical and magnetic dipole transitions to occur simultaneously.The 5 D0→ 7 F0 transition observed at 578 nm is forbidden by the Judd-Ofelt theory, but it is usually observed in these crystals due to J-mixing or to the mixing of low-lying charge transfer states into the wavefunctions of the 4f orbitals [17].The 5 D0→ 7 F1 transition has peaks at 588 and 596 nm and it is magnetic dipolar in nature; therefore, it is relatively insensitive to the crystal field.The 5 D0→ 7 F2 transition is centered at 611 and 626 nm, and its intensity is hypersensitive to the crystal environment [13].In a previous article, we observed highly different 5 D0→ 7 F2 transition intensities in Y2−xEuxSn2O7 at different doping levels, and we were not able to obtain an intense emission from this transition at a low Eu doping level.In this case, Dy codoping helps in enhancing this emission, probably due to a stabilization of the Eu 3+ ions' environment.Weaker emissions were also observed from the 5 D0→ 7 F3 (around 650 nm) and 5 D0→ 7 F4 (around 710 nm) transitions.These spectra are in good agreement with the literature [17,18,[26][27][28], but, unlike the Y 2 Sn 2 O 7 :Eu 3+ singly doped compound [17], this doubly doped composition shows a very intense emission in the red region from the hypersensitive Eu 3+ 5 D 0 → 7 F 2 transition.Several sharp lines are located at 578, 588, 611, 628, and 707 nm [29].These sharp emission peaks are mainly situated in the red spectral region and correspond to transitions from the excited state 5 D 0 to lower states 7 F J (J = 0, 1, 2, 3, 4), respectively [30,31].Due to the screening effect of the outer 5s 2 5p 6 electrons, the crystal field hardly affects the position of the 4f energy levels of Eu 3+ ; on the contrary, it strongly affects the transition probabilities and their selection rules.Eu 3+ ions occupy a site with D 3d symmetry in Y 2−x Eu x Sn 2 O 7 [18,32], and this causes electrical and magnetic dipole transitions to occur simultaneously.The 5 D 0 → 7 F 0 transition observed at 578 nm is forbidden by the Judd-Ofelt theory, but it is usually observed in these crystals due to J-mixing or to the mixing of low-lying charge transfer states into the wavefunctions of the 4f orbitals [17].The 5 D 0 → 7 F 1 transition has peaks at 588 and 596 nm and it is magnetic dipolar in nature; therefore, it is relatively insensitive to the crystal field.The 5 D 0 → 7 F 2 transition is centered at 611 and 626 nm, and its intensity is hypersensitive to the crystal environment [13].In a previous article, Crystals 2024, 14, 300 8 of 12 we observed highly different 5 D 0 → 7 F 2 transition intensities in Y 2−x Eu x Sn 2 O 7 at different doping levels, and we were not able to obtain an intense emission from this transition at a low Eu doping level.In this case, Dy codoping helps in enhancing this emission, probably due to a stabilization of the Eu 3+ ions' environment.Weaker emissions were also observed from the 5 D 0 → 7 F 3 (around 650 nm) and 5 D 0 → 7 F 4 (around 710 nm) transitions.
As for Dy:Y 2 Sn 2 O 7 , weak radioluminescence [14] and photoluminescence [15,16] were observed, with the main emission bands located at around 580 nm and 650 nm; therefore, we might expect to observe PL in the same regions.Unfortunately, these are superimposed to the Eu emission bands, so it is not easy to identify specific Dy peaks because their intensity is overwhelmed by the Eu emission bands.However, a careful investigation of the Dy PL features is beyond the scope of this work.
The energy level diagrams of Eu 3+ /Dy 3+ with their main PL emission bands are sketched in Figure 5.  4.  4.

Detection of Latent Fingerprints
The luminescence properties of our powder Y 1.8 Eu 0.1 Dy 0.1 Sn 2 O 7 were demonstrated by using a UV lamp (Figure 7).This allowed us to work under an excitation of 254 nm and to observe with the naked eye the existence of luminescence.When the Y 1.8 Eu 0.1 Dy 0.1 Sn 2 O 7 nanopowder was analyzed under a UV lamp, it was evident that the powder, which was originally white in natural light, turned red-orange under UV light (λ = 254 nm), a characteristic of the existence of luminescence.The color coordinates (x, y) of the Y1.75Eu0.2Dy0.05Sn2O7 phosphors were established be (0.61, 0.38), which are located in the near-red region, as shown in Figure 6.We noti that with the increasing amount of Dy 3+ and decreasing amount of the dopant Eu 3+ , observed a small shift to the reddish-orange zone, such as in the case of Y1.75Eu Dy0.2Sn2O7 (0.56, 0.44) [33].

Detection of Latent Fingerprints
The luminescence properties of our powder Y1.8Eu0.1 Dy0.1Sn2O7 were demonstra by using a UV lamp (Figure 7).This allowed us to work under an excitation of 254 nm a to observe with the naked eye the existence of luminescence.When the Y1.8Eu0.1 Dy0.1Sn nanopowder was analyzed under a UV lamp, it was evident that the powder, which w originally white in natural light, turned red-orange under UV light (λ = 254 nm), a ch acteristic of the existence of luminescence.We tested the nanophosphors as latent fingerprint (LFP) developers; to this end, chose different types of surfaces, both porous and non-porous, like CD, aluminum f wood, and ceramic.First, the donor washed and cleaned their hands; then, they pres their finger on the various surfaces.Then, the Eu 3+ /Dy 3+ -codoped Y2Sn2O7 nanopowd were smoothly stained on the entire surface of the LFP with a feature brush and the exc was gently removed.Figure 8 shows fingerprint images developed under white light aluminum and under 254 nm UV light on different surfaces.A good visualization of LFP was obtained and, in fact, the ridges of the fingerprint were quite visible due to We tested the nanophosphors as latent fingerprint (LFP) developers; to this end, we chose different types of surfaces, both porous and non-porous, like CD, aluminum foil, wood, and ceramic.First, the donor washed and cleaned their hands; then, they pressed their finger on the various surfaces.Then, the Eu 3+ /Dy 3+ -codoped Y 2 Sn 2 O 7 nanopowders were smoothly stained on the entire surface of the LFP with a feature brush and the excess was gently removed.Figure 8 shows fingerprint images developed under white light on aluminum and under 254 nm UV light on different surfaces.A good visualization of the LFP was obtained and, in fact, the ridges of the fingerprint were quite visible due to the small size of the powders.Furthermore, since these nanopowders emit a shimmering reddishorange color, finer fingerprint details, such as pores (level 3), can be successfully developed, especially on wood surfaces and ceramic.Enlarged fingerprint images formed on wood and ceramic under UV light are also shown.The image clearly depicts all three layers of fingerprint ridge patterns: core and whorl (level 1), bifurcation, enclosure, and island (level 2), and sweat pores (Level 3).The pixel values along the red lines show a very good contrast, which clearly distinguishes the brightness of the ridges from the darkness of the furrows.This result confirmed that Y 2 Sn 2 O 7 :Eu 3+ /Dy 3+ nanophosphors showed enhanced luminescence compared with the Y 2 Sn 2 O 7 :Eu 3+ studied in our previous work [17].In fact, thanks to Eu/Dy codoping, we succeeded in obtaining stable luminescence and detecting level 3 of the latent fingerprint on new surfaces such as ceramic, which was not detected in the case of simple doping with europium only [17].
Crystals 2024, 14, 300 11 of 13 In fact, thanks to Eu/Dy codoping, we succeeded in obtaining stable luminescence and detecting level 3 of the latent fingerprint on new surfaces such as ceramic, which was not detected in the case of simple doping with europium only [17].

Conclusions
In this report, 2xEu/2yDy:Y2Sn2O7 nanophosphors were successfully synthesized via the coprecipitation method followed by further calcining treatment, aiming for an efficient visualization of latent fingerprints in forensic science.
shows an example of the result of the fitting procedure.There are two possible sites for oxygen ions, called O and O'.O' ions are in undisturbed locations relative to the fluorite structure (3/8, 3/8, 3/8) and are coordinated by the Y/Eu/Dy cation tetrahedral.Instead, O (3/8, 1/8, 1/8) binds to Y/Eu/Dy and Sn at the next empty 8a site [24].
shows an example of the result of the fitting procedure.There are two possible sites for oxygen ions, called O and O ′ .O ′ ions are in undisturbed locations relative to the fluorite structure (3/8, 3/8, 3/8) and are coordinated by the Y/Eu/Dy cation tetrahedral.Instead, O (3/8, 1/8, 1/8) binds to Y/Eu/Dy and Sn at the next empty 8a site [24].

Figure 6 .
Figure 6.CIE color chromaticity coordinates of Y2Sn2O7:Eu 3+ /Dy 3+ phosphors.Data points for the various samples are represented as stars.

Figure 6 .
Figure 6.CIE color chromaticity coordinates of Y 2 Sn 2 O 7 :Eu 3+ /Dy 3+ phosphors.Data points for the various samples are represented as stars.

Figure 8 .
Figure 8. Developed fingerprint images obtained with Eu/Dy: Y2Sn2O7 (a) under white light on aluminum and under UV light on different surfaces: (b) compact disc, (c) aluminum, (d) wood, and (e) ceramic.Below are enlargements of (d,e) with the three levels of fingerprint ridge patterns visualized and pixel values along the red line.

Figure 8 .
Figure 8. Developed fingerprint images obtained with Eu/Dy: Y 2 Sn 2 O 7 (a) under white light on aluminum and under UV light on different surfaces: (b) compact disc, (c) aluminum, (d) wood, and (e) ceramic.Below are enlargements of (d,e) with the three levels of fingerprint ridge patterns visualized and pixel values along the red line.

Table 2 .
Refined lattice constants, observed phases, and structural fitting parameters of the powder samples with nominal composition Y 2−2x−2y Eu 2x Dy 2y Sn 2 O 7 .

Table 3 .
Measured size of all samples.The emission spectra of the Y 2 Sn 2 O 7 :Dy 3+ , Eu 3+ phosphors were recorded with λ ex = 532 nm.