Synthesis and Characterization of Smartphone-Readable Luminescent Lanthanum Borates Doped and Co-Doped with Eu and Dy

Abstract

Despite notable advancements in the development of borate materials, improving their luminescent efficiency remains an important focus in materials research. The synthesis of lanthanum borates (LaBO₃), doped and co-doped with europium (Eu³⁺) and dysprosium (Dy³⁺), by the solid-state method, has demonstrated significant potential to address this challenge due to their unique optical properties. These materials facilitate efficient energy transfer from UV-excited host crystals to trivalent rare-earth activators, resulting in stable and high-intensity luminescence. To better understand their structural and vibrational characteristics, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy were employed to identify functional groups and molecular vibrations in the synthesized materials. Additionally, X-ray diffraction (XRD) analysis was conducted to determine the crystalline structure and phase composition of the samples. All observed transitions of Eu³⁺ and Dy³⁺ in the excitation and emission spectra were systematically analyzed and identified, providing a comprehensive understanding of their behavior. Although smartphone cameras exhibit non-uniform spectral responses, their integration into this study highlights distinct advantages, including contactless interrogation, effective UV excitation suppression, and real-time spectral analysis. These capabilities enable practical and portable fluorescence sensing solutions for applications in healthcare, environmental monitoring, and food safety. By combining advanced photonic materials with accessible smartphone technology, this work demonstrates a novel approach for developing low-cost, scalable, and innovative sensing platforms that address modern technological demands.

1. Introduction

In the coming decades, photonic devices are expected to play an increasingly significant role in human activities. Technologies such as light bulbs, displays, light sensors, scintillation detectors, optical fibers, and lasers are becoming integral to everyday life. Among the materials being explored for advanced photonic applications, lanthanum borates have garnered attention due to their unique optical properties [1,2].

These properties make them particularly suitable for the development of smartphone-compatible fluorescent sensors. Such sensors offer a rapid and convenient method for analyzing various parameters, paving the way for real-time monitoring in fields like healthcare, environmental control, and food safety [3,4]. This integration of photonic materials and smartphone technology represents a promising direction for the future, enabling practical and efficient solutions for everyday challenges [5,6,7].

Energy transfer from the UV-excited borate host crystal to the trivalent rare-earth activators results in host-excited luminescence. A detailed topographic and 3D excitation–emission analysis of their spectra was performed using measurements from both a spectrometer and a mobile-phone camera equipped with a diffraction grating [8].

Smartphones have a different and non-uniform spectral response; however, they offer several advantages, such as contactless interrogation, efficient suppression of UV excitation light, and simultaneous spectral analysis of spatially arranged arrays of fluorescent markers. The ability to leverage smartphone technology for spectral measurements enhances the practicality of these materials for real-world applications [8,9].

The stable fluorescence of these markers at specific wavelengths, compatible with smartphone cameras and mobile applications, enables the development of innovative methodologies for remote analysis and personalized health monitoring [7].

Modern materials chemistry is focused on the development of new inorganic compounds with wide applications in various industrial and technological fields. Like yttrium borates [10], lanthanum has the ability to form compounds with diverse properties that can be used in a number of applications. Lanthanides, due to their unique optical and electrical characteristics, play a key role in the advancement of luminescent materials that are characterized by high color purity and luminous efficiency [11]. The emission characteristics of certain doping ions in phosphors are significantly influenced by their surrounding environments. For instance, Dy³⁺ and Eu³⁺ ions typically display emission bands from the transitions ⁴F_9/2 → ⁶H_15/2, _13/2, _11/2 and ⁵D₀ → ⁷F_1,2,3,4 [12]. Specifically, for the Dy³⁺ ion, the ⁴F_9/2 → ⁶H_13/2 transition is hypersensitive, resulting in a stronger emission band compared to other transitions when Dy³⁺ is situated in a site lacking inversion symmetry. Conversely, if it is in a symmetrical site, the emission band corresponding to the ⁴F_9/2 → ⁶H_15/2 transition becomes the strongest [13,14].

For the Eu³⁺ ion, the ⁵D₀ → ⁷F₂ transition is an electric-dipole transition, and its emission band is strongest when the ion is in a site without inversion symmetry. Conversely, when the ion is in a symmetrical site, the emission from the ⁵D₀ → ⁷F₁ transition becomes more pronounced [15].

The research shows that energy transfers occur from Dy³⁺ to Eu³⁺ in LaBO₃:Dy³⁺/Eu³⁺ phosphors, leading to tunable luminescence. This study highlights the synergy between advanced photonic materials and accessible technology, paving the way for new possibilities in cost-effective and portable sensing solutions. As prospective materials for these technologies, we focused on LaBO₃ doped and co-doped with Eu and Dy. Notably, the use of a smartphone camera for spectral analysis introduces a novel, low-cost methodology for real-time monitoring, which has not been extensively explored in prior research.

2. Materials and Methods

The synthesis of luminescent materials was performed by using the following analytical grade reagents: La₂O₃ (lanthanum (III) oxide), H₃BO₃ (boric acid), Eu₂O₃ (europium (III) oxide) and Dy₂O₃ (dysprosium oxide), purchased from Thermo Fisher Scientific (Waltham, Massachusetts, United States). The samples were synthesized by stoichiometry, in accordance with Equation (1), and boric acid excess was obtained accordingly [11], as presented in

Table 1. A list of sample compositions.

Samples	La2O3 (g)	H3BO3 Excess, %	H3BO3(g)	Eu2O3 mol %/g	Dy2O3 mol %/g	Eu2O3 : Dy2O3 mol %/g
S 1	0.7249	45	0.3987	2/0.07
S 2	0.7249	45	0.3987		2/0.074
S 3	0.7249	45	0.3987			2/1 [0.07/0.037]

.

La₂O₃ + 2H₃BO₃ → 2LaBO₃ + 3H₂O

(1)

The solid-state synthesis was carried out in a muffle furnace, LM 312.07, with a G400 controller and a temperature range of 0–1200 °C and a heating rate of 22.28 °C/min. The prepared mixtures were placed in porcelain crucibles (manufactured by Jizera Porcelain Factory, with a volume of 12 mL, a diameter of 3.5 mm, and a height of 28 mm), and heated up to 1050 °C for 6 h, as shown in Scheme 1. After the set time, the samples remained in the furnace and cooled slowly for 16 h at a cooling rate of 1 °C/min.

3. Research Methods

3.1. XRD Analysis

The crystal structure of the synthesized samples was determined by X-ray diffraction (XRD, Siemens D500 Powder Diffractometer, Berlin, Germany) with CuKα radiation (k = 1.54 A). The crystal structure was modeled using Crystalmaker software 10.6 (CrystalMaker Software Ltd., Oxford, UK; http://www.crystalmaker.com (accessed on 12 December 2024)). The lattice parameters were obtained by using MDI Jade version 9.1 the software (Material Data Inc., Livermore, CA, USA; http://www.materialsdata.com (accessed on 12 December 2024)).

3.2. Infrared Spectroscopy

The FTIR spectra of the samples were recorded in the mid-infrared region (MIR) within the range of 4000–400 cm⁻¹ with a resolution of 2 cm⁻¹ and 25 scans. The instrument used was a Bruker Optics spectrometer Vertex 70 FTIR (Billerica, MA, USA).

The test samples were prepared using the “pelletizing” technique by precisely mixing 150 mg of KBr with 5 mg of the samples in an agate mortar. The mixture was ground and homogenized into a fine powder, which was subsequently pressed using a pellet press to form the final tablets. The sample was measured with the above-described instrument under the selected conditions, using a holder to position the pellet.

3.3. Raman Analysis

For recording the Raman spectra of the synthesized materials, the samples were packed in aluminum discs. The measurements were performed in the range of 4000 to 50 cm⁻¹ using the RAM II (Bruker Optics) equipped with a focused laser beam from an Nd:YAG laser (1064 nm). The spectra were recorded with a resolution of 2 cm⁻¹ and 25 scans.

3.4. Fluorescence Measurement by Spectrometer and Smartphone

The fluorescence spectra were measured using both a standard spectrometer and a mobile-phone-based spectrometer. A laser-driven light source (Energetique) emitting white light in the 190–2500 nm range was connected to a monochromator (Ocean Optics QE 65000) via a 1 mm optical fiber, enabling spectral coverage from 200 to 900 nm.

The experimental setup shown in Figure 1, included a plastic holder with a 1 mm hole, where lead-in and lead-out fibers (600 μm core) were positioned at a 90° angle of direct excitation light to the sample, and the fluorescence emission was measured. The emission was analyzed by the Ocean Optics spectrometer. Additionally, fluorescence was captured using a Samsung Galaxy A51 smartphone (48 MP Main camera F2.0) positioned approximately 15 cm from the sample to ensure proper focus. This setup allowed for comparative analysis of fluorescence spectra using professional and accessible mobile spectrometers. A picture of the spectrum was taken, then it was digitized by Python (Pycharm 2023.2.5), and finally the pixel number was assigned a wavelength after comparison with the spectrum measured by a spectrometer.

4. Results

4.1. XRD Analysis

The results presented in Figure 2 show that the positions of some diffraction peaks are consistent with the standard XRD patterns of LaBO₃ (JCPDS 12-0762). The positions of the main diffraction peaks are around 20.4°, 25.5°, 26.3°, 30.4°, 32.3°, 35.1°, 44.4°, 48.6°, and 51.1°, corresponding to the (0 0 1), (1 1 1), (1 2 0), (2 0 0), (2 1 0), (0 0 2), (1 2 2), (2 3 1), and (3 1 1) planes of LaBO₃ [16]. The intense and sharp diffraction peaks indicate the good crystallinity of the LaBO₃ powder. The space group was Pnam (62), an orthorhombic crystal system, according to the authors in [17]. The analysis results indicate that LaBO₃ can be observed from the pattern indexed as an orthorhombic structure with lattice parameters a = 0.587 nm, b = 0.825 nm, and c = 0.510 nm [18]. The structure of LaBO₃ was visualized using software from card JSPDC00-012-0762.

The crystal structure of orthorhombic lanthanum borate is three-dimensional. Each La³⁺ ion is coordinated by nine O²⁻ ions, forming a nine-coordinate geometry. The La–O bond lengths vary between 2.44 Å and 2.75 Å, indicating slight distortions in the coordination environment. The B³⁺ ions exhibit a trigonal planar geometry, each bonded to three O²⁻ ions with uniform B–O bond lengths of 1.38 Å. There are two distinct O²⁻ sites within the structure. In both sites, each O²⁻ ion is bonded in a distorted single-bond geometry to three equivalent La³⁺ ions and one B³⁺ ion. Despite the similar bonding patterns, these O²⁻ sites are crystallographically inequivalent, contributing to the overall structural complexity [19].

4.2. FTIR Spectroscopy

Figure 3 shows the infrared spectra of LaBO₃ doped with Eu³⁺, Dy³⁺, and co-doped with both rare-earth elements. The spectra provide detailed information on the vibrations in the molecule as they are related to characteristics of the materials. In all three spectra, characteristic peaks associated with the vibrational modes of BO₃ groups are observed, with the main bands located around 1271 cm⁻¹, 941 cm⁻¹, and 710 cm⁻¹. The addition of the rare-earth element Eu³⁺ influences the intensity and position of the peaks, which may be related to specific transitions in the electronic structure of Eu³⁺. The planar trigonal BO₃ groups are characterized by four main vibrational modes [20,21]. They are shown in

Table 2. Vibrational modes of BO₃ and BO₄ groups.

Bands	Vibrational Modes of BO3 Group cm−1	Vibrational Modes of BO4 Group cm−1
Symmetric stretching (ν1)	950	1000
Out–of-planar bending (ν2)	740	950
Asymmetric stretching (ν3)	1260	600
In-planar bending (ν4)	592	<600

.

The trigonal planar borate units ([BO₃]³⁻) possessing the aragonite borate type have stretching (ν₁ and ν₃) and deformation vibrations (ν₂ and ν₄). The doublet absorption bands at 592 and 611 cm⁻¹ are due to in-planar (ν₄) bending of the [BO₃]³⁻ group, and the strong peak at 710 cm⁻¹ is associated with out-of-planar (ν₂) bending of B–O–B. The peak at 941 cm⁻¹ (ν₁) refers to symmetric stretching of three-coordinated bonds [vs(O–B–O)] in the [BO₃]³⁻ groups. The most intense peak is observed at 1271 cm⁻¹, which is characteristic of the asymmetric stretching (ν₃) of the B–O bonds in BO₃ [21,22].

4.3. Raman Spectra

In the Raman spectrum presented in Figure 4, the band at 1379 cm⁻¹ indicates the asymmetric stretching of v_as (B₍₃₎–O). The bending δ(B–O–H) is observed at 1252 cm⁻¹, while the most intense peak at 943 cm⁻¹ corresponds to the symmetric stretching νs(B(₃) –O) [17]. The peak at 632 cm⁻¹ signifies the presence of metaborate groups, characteristic of a ring structure. The band at 606 cm⁻¹ is attributed to vibrations of the bridging bonds in BO₃ groups. The low-intensity split band around 318 cm⁻¹, exhibiting a doublet nature, can be described as La–O bond stretching. The presence of peaks in the range of 124–300 cm⁻¹ can be attributed to La(RE)–O modes [22,23].

Raman spectral analysis identifies the characteristic vibrational states of lanthanum borates, providing information about the ion states and the symmetry of the crystal lattice, as shown in Figure 5.

4.4. Fluorescence Analysis

The photoluminescence spectra of lanthanum borate doped with europium ions have been described in detail in a previous study [7]. Europium ions (Eu³⁺) exhibit two different types of transitions: f−f and f−d. In the fluorescence analysis of lanthanum borate doped with europium ions, the most intense emission peaks were observed at 592 nm (orange) and 615 nm (red) when excited at wavelengths of 284 and 395 nm. The weakest fluorescence was recorded upon excitation at 533 nm. The 615 nm peaks correspond to transitions between the ⁵D₀–⁷F_j multiplets (j = 0, 1, 2, 3, 4), characteristic of Eu³⁺ [24]. Upon excitation at λmax ≈ 290 nm, an energy transfers from the 2p orbitals of O²⁻ to the 4f orbitals of Eu³⁺ was observed. The most intense peak was found in the red region. The peaks in the 350–430 nm range correspond to f−f electron transitions of Eu³⁺ from the ⁷F₀ state to the ⁵D₄, ⁵L₆, and ⁵G_4/5 states [25]. Charge transfer is more intense when the matrix contains Eu³⁺ ions, and the shift of the peaks to longer wavelengths may be attributed to a decrease in the distance between Eu³⁺ and O²⁻ or a reduction in the difference in electronegativity between the ions [26].

The excitation and emission luminescence spectra of LaBO₃ doped with Dy³⁺ ions are given in [6]. The excitation spectrum displays peaks at wavelengths of 330 nm (⁶H₁₅/₂ → ⁴M₁₇/₂), 354 nm (⁶H₁₅/₂ → ⁶P₇/₂), 365 nm (⁶H₁₅/₂ → ⁶P₅/₂), 389 nm (⁶H₁₅/₂ → ⁴F₇/₂), 430 nm (⁶H₁₅/₂ → ⁴G₁₁/₂), and 452 nm (⁶H₁₅/₂ → ⁴I₁₅/₂). These peaks are associated with the intra-configurational f–f transitions of Dy³⁺ ions. The photoluminescence emission spectra exhibit four emission peaks at 477 nm, 569 nm, 660 nm, and 750 nm, with the last two showing very low intensity. The Dy³⁺ emission around 477 nm is attributed to the ⁴F₉/₂ → ⁶H₁₅/₂ transition, which is of a magnetic-dipole nature, whereas the emission at 569 nm (⁴F₉/₂ → ⁶H₁₃/₂) is of an electric-dipole nature.

The magnetic-dipole transition ⁴F₉/₂ → ⁶H₁₅/₂ is more pronounced when Dy³⁺ ions are located in high-symmetry crystalline environments. In contrast, the electric-dipole transition ⁴F₉/₂ → ⁶H₁₃/₂ becomes more intense in environments with lower symmetry of the local crystal structure. The electric-dipole transition (yellow emission) is the most prominent. This indicates that in the synthesized samples, Dy³⁺ ions occupy sites with low symmetry [6].

The luminescence spectra of samples co-doped with Eu³⁺ and Dy³⁺ in a molar ratio of 2:1 show four emission bands at 591, 615, 683, and 701 nm for the lanthanum borate matrix, as presented in Figure 6. The most intense luminescent peak for the lanthanum borate matrix is observed upon excitation at a wavelength of 290 nm (Figure 6. Samples containing only Dy³⁺ exhibit insufficient intensity in the red spectral region. Therefore, Eu³⁺ ions, which have a strong red emission, were added alongside Dy³⁺ to achieve highly efficient luminescence.

In the visible spectrum, Dy³⁺ ions produce blue and yellow emissions due to transitions from ⁴F₉/₂ to ⁶H₁₅/₂, ⁶H₁₃/₂, and ⁶H₁₁/₂. The yellow emission is observed when dysprosium ions occupy non-centrosymmetric sites within the crystal lattice. For Eu³⁺ ions, the red emission originating from the ⁵D₀ → ⁷F₂ transition is caused by an electric-dipole transition. The highest intensity is observed when europium ions occupy non-centrosymmetric sites.

The emission spectra of LaBO₃ co-doped with Eu and Dy at excitation wavelengths of 290 nm and 350 nm are presented in Figure 7. When the excitation wavelength was switched to 350 nm, the hypersensitive ⁶P_7/2 level of Dy³⁺ was resonantly excited, followed by rapid nonradiative relaxation to the ⁴F_9/2 level. Consequently, by varying the excitation wavelength from 290 to 350 nm, the emission wavelength, and color output of the same LaBO₃:Eu³⁺, Dy³⁺ particles can be tuned, highlighting the role of selective excitation in controlling their luminescent properties.

The standard procedure is to scan the excitation wavelength λ within a certain range and measure the luminescence spectrum for each excitation wavelength, which allows us to plot excitation–emission spectra, the topographic views of which are shown in Figure 8 and Figure 9. These figures show spectral characteristics of a sample of lanthanum borate doped with europium and dysprosium ions. The spectrum in Figure 8 was obtained using a spectrophotometer, and that in Figure 9 was obtained with a mobile phone. The two graphs represent different levels of accuracy and resolution. The plot from the spectrometer shows clearly defined emission peaks, especially in the red emission of europium ions (~612–620 nm) associated with the electronic transition ⁵D₀–⁷F₂.

Figure 10 a presents the emission spectra of samples S1, S2, and S3, with the most intense emission observed at ~615 nm (red region) for samples S1 and S3, and at 569 nm (yellow region) for sample S2. Sample S1 exhibits the highest fluorescence intensity within the excitation range of 240–400 nm, with the most important peak at 290 nm, attributed to the characteristic transitions of Eu³⁺ ions. The distinct shape of the peaks reflects the high efficiency of the fluorescent transitions of europium ions, contributing to the strong emission in the red region of the spectrum.

Lanthanum borate doped with dysprosium ions (S2) shows a significantly lower intensity compared to S1, with a maximum peak within the narrower excitation range of 300–420 nm and an intensity of approximately 5000 units. The intensity of emission peaks at 591 nm for samples S1 and S3 and at 569 nm for sample S2 when excited by 396 nm and 393 nm, respectively, is dramatically lower, as shown in Figure 10b.

5. Discussion

The ionic radii of Eu³⁺ (0.95 Å) and Dy³⁺ (0.908 Å) are smaller than that of La³⁺ (1.115 Å). Although these differences are relatively small, they can lead to both the substitution of La³⁺ ions and doping within the crystal lattice. Replacing La³⁺ with the smaller Eu³⁺ or Dy³⁺ ions introduces lattice strain, which slightly reduces the lattice parameters (a, b, c), resulting in a minor shrinkage of the unit cell. In this context, both Eu³⁺ and Dy³⁺ act primarily as dopants rather than complete substitutes for La³⁺, partially occupying lattice sites without fully replacing the lanthanum ions. While the orthorhombic structure of LaBO₃ is preserved, slight distortions arise due to the ionic size mismatch, potentially affecting bond angles and La–O bond lengths. These structural modifications are further confirmed by FTIR and Raman spectroscopy, which reveal the occupation of Eu³⁺ and Dy³⁺ ions at non-symmetrical lattice sites, consistent with the observed distortions in the well-defined orthorhombic LaBO₃ framework.

The three IR spectra exhibit similar peaks, corresponding to the characteristic vibrational modes of lanthanum borates. The strongest peak intensity is observed for the sample doped with europium ions, which can be attributed to the higher efficiency of these ions in enhancing the characteristic vibrations. For the sample doped with both europium and dysprosium ions, the peak intensity decreases, likely due to competitive interactions between the two types of ions within the structure. The weakest peaks are recorded for the sample doped with dysprosium ions, which may result from the distinct nature of these ions and their influence on the vibrational modes of the matrix [27].

The obtained Raman spectral data show that doping with europium ions leads to significant changes in the symmetric and asymmetric vibrations of the BO₃ groups, accompanied by an increase in the intensity of the characteristic peaks. The addition of dysprosium ions results in weaker intensities but contributes to the appearance of new vibrational modes associated with the La–O and RE–O interactions. The combination of europium and dysprosium ions creates a complex spectral profile, reflecting the synergistic effect between the two rare-earth elements and their influence on the structural properties of lanthanum borate. The results highlight the key role of the dopant composition in controlling the vibrational and structural characteristics of the material.

The high resolution of the spectrometer allows for the detailed analysis of rare-earth ions as well as accurate identification of additional peaks. On the other hand, the graph generated with a mobile phone successfully captures the main emission zones, but with wider lines. Despite the limitations, it is suitable for preliminary assessment, providing a more accessible and rapid method of analysis [28]. PL measurements of samples S1 and S2 by mobile phone can be seen from our previous research [6]. The experiments highlight the advantages and drawbacks of using smartphones for spectral interrogation compared to standard spectrometers. Smartphones provide faster response times of 30–60 fps versus 5–6 fps for spectrometers. They are better suited for color measurements and compatible with IoT technologies. However, they are limited to the visible spectrum and require additional calibration for accurate spectral reproduction due to their non-linearity and saturation effects. Smartphones can also simultaneously interrogate multiple phosphorescent sensors, but certain conditions must be met (isolation from stray light, non-fluorescent substrates, and manageable signal levels). A setup with a modified web camera and integrated silicon LED could expand the usable range to 400–900 nm.

The sample co-doped with europium and dysprosium ions (S3) exhibits intermediate luminescence—higher than S2 but lower than S1. However, the peak intensity in the 240–400 nm range is considerably weaker than that of S1. The spectrum of S3 is dominated by Eu³⁺ transitions, while the contribution of Dy³⁺ is minimal or absent, suggesting that dysprosium ions do not act as sensitizers for europium ions. The energy transfer mechanism occurs when Dy³⁺ ions are excited to high energy levels (⁴F_9/2) in the material by a UV source. Instead of emitting light directly—Dy³⁺ typically emits light around 480 nm (blue) and 570 nm (yellow)—a portion of the energy should transfer non-radiatively to neighboring Eu³⁺ ions through dipole–dipole interactions or quantum mechanical exchange. If this energy reaches the Eu³⁺ ions, they emit their characteristic red light in the range of 610–620 nm, corresponding to the ⁵D₀ to ⁷F₂ transition. In this case, we observe decreasing fluorescent intensity, which leads to the conclusion that Eu and Dy ions are distant from each other and it has obstructed their interactions. Regardless of decreased emission, we established that the effect of co-doping is encoded in different emissions by switching between excitation wavelengths, as shown in Figure 7.

All samples exhibit emission in the orange region of the spectrum (as shown in Figure 10b). However, the intensity is significantly lower compared to the red region. The strongest luminescence in the orange region is again observed for the europium-doped sample (S1). Sample S2 demonstrates a low intensity, confirming that Dy³⁺ is not a suitable activator for generating strong luminescence in this range. Sample S3 shows intermediate intensity, but the presence of dysprosium does not significantly enhance the spectral properties, likely leading to energy competition between the ions.

Factors such as the distance between ions, concentration quenching, symmetry, and defects in the crystal structure influence energy transfer within the lattice. The efficiency of energy transfer is enhanced when Dy³⁺ and Eu³⁺ ions are located close together within the lattice. It is essential to achieve optimal doping levels and ion concentrations; if the concentration of either Eu³⁺ or Dy³⁺ is too high, this can lead to quenching, which suppresses emission. This quenching occurs because the excited states dissipate energy non-radiatively before they can emit light [12,14].

In the orthorhombic structure formed in LaBO₃, Dy³⁺ and Eu³⁺ can occupy asymmetric positions, which is expected to facilitate improved energy transfer between the two ions. However, this material still exhibits a quenching effect, likely due to the formation of defect centers in the structure caused by Dy³⁺ ions [29]. Some authors explain the decrease in the emission intensity of the Dy³⁺ ion as a result of energy transfer via cross-relaxation and resonant energy transfer at doping concentrations above 0.5 mol% [30].

The general observations from photoluminescent analysis are as follows:

• LaBO₃:Eu³⁺ as dominant activators:

The europium-doped sample (S1) exhibits the highest fluorescence intensity under excitation, regardless of the emission wavelength. Eu³⁺ provides strong and distinct luminescence, making it suitable for applications in luminescent materials, particularly for red–orange emission (590–615 nm).

• LaBO₃:Dy³⁺ has limited efficiency: The dysprosium-doped sample (S2) shows significantly lower fluorescence intensity at both 569 nm and 590 nm. Dy³⁺ is not an effective activator, which limits its standalone application in materials requiring high luminescence.
• LaBO₃:Eu³⁺:Dy³⁺ exhibits intermediate intensity, dominated by Eu³⁺ transitions: The presence of Dy³⁺ does not significantly enhance fluorescence and may lead to energy competition between the ions. This suggests a lack of synergistic interaction between Eu³⁺ and Dy³⁺ in this matrix material.
• Eu³⁺ and Dy³⁺ interactions: Comparing the two graphs reveals that co-doping with Eu³⁺ and Dy³⁺ in a 2:1 molar ratio enhances the red emission, characteristic of Eu³⁺. This compensates for the weak red emission observed in Dy³⁺-doped samples.
• Efficient luminescence: Eu³⁺-doped samples demonstrate higher efficiency in the red spectral region, while Dy³⁺-doped samples are more effective in the blue–yellow spectral range.
• Role of symmetry: The non-centrosymmetric distribution of Eu³⁺ and Dy³⁺ in the crystal structure significantly affects the intensities of their respective electric-dipole transitions.

6. Conclusions

Synthesis of lanthanum borates doped with rare-earth elements demonstrates significant potential for the development of smartphone-readable fluorescent sensors. These materials exhibit unique luminescent properties, particularly when doped with rare-earth ions, enabling efficient fluorescence detection. Unlike the standard spectrometer, the smartphone allows for the simultaneous detection of the spectra of arrays of the studied fluorescent markers, of which the spatial distribution will appear different depending on the particular excitation. Further optimization of the synthesis process and the enhancement of sensor sensitivity will be key to advancing this innovative approach. Energy transfer from Dy³⁺ to Eu³⁺ in doped LaBO₃ efficiently enhances the optical properties, including improved red emission and color tunability. This mechanism is essential for the development of high-performance phosphors and optical materials. Figure 7 highlights the unique features of co-doped lanthanum borate, which are not observed in LaBO₃ doped solely with europium or dysprosium. Specifically, we demonstrated that the emission wavelength and color output of LaBO₃:Eu³⁺,Dy³⁺ particles can be tuned by switching the excitation source from 290 to 350 nm. Moreover, we found that the presence of dysprosium ions facilitates the formation of sharper and more fragmented peaks in the orange region, whereas in the spectrum of LaBO₃:Eu³⁺, these appear as broad and diffuse doublets and triplets.

Smartphones allow for the plotting of excitation–emission spectra and accurately identify the individual excitation–emission peaks of the particular samples. However the spectral sensitivity of the smartphone is determined by the transmission spectra of its RGB filters, and with the diminishing smartphone-to-spectrometer intensity ratio, the relative intensities of the different peaks may differ from those measured by a standard spectrometer.

Because the maximum efficiency excitation band λ is practically the same as the LED emission bandwidth, the studied La borates are particularly appropriate for UV LED excitation in the 375–395 nm range to serve as selective fluorescent markers for a number of applications. For example, in healthcare, they can be used for the protection of pharmaceutical products from counterfeiting by integrating them into QR codes. In the food industry, these luminescent materials can be incorporated into product packaging or labels to enhance traceability and quality control. In environmental monitoring, they can be utilized to track pollutants or hazardous substances by embedding fluorescent markers that respond to specific environmental conditions.