The visualization of biological structures and processes at cellular and molecular levels has revolutionized modern medicine and diagnostic research. However, conventional organic fluorophores and quantum dots face significant limitations including photobleaching, potential cytotoxicity, and autofluorescence interference from biological tissues (Menon et al., 2017). These challenges have motivated extensive research into alternative luminescent probes with enhanced photostability, biocompatibility, and optical penetration depth.

Near-infrared (NIR) emitting materials have emerged as particularly attractive candidates for bioimaging applications due to the reduced absorption and scattering of NIR light by biological tissues—the so-called "biological transparency window" (Shao et al., 2024). Among various NIR-emitting systems, chromium(III)-doped oxides have garnered considerable attention owing to their sharp-line emission arising from spin-forbidden ²E → ⁴A₂ transitions, which exhibit remarkable temperature stability and long luminescence lifetimes (Ćirić et al., 2022). Furthermore, Cr³⁺-doped spinels have demonstrated persistent luminescence properties that enable in vivo imaging without continuous external excitation, significantly improving signal-to-background ratios (Sharma et al., 2014; Silva et al., 2019).

Spinel-structured oxides (AB₂O₄) offer exceptional chemical and thermal stability, mechanical hardness, and versatile cation substitution possibilities, making them ideal hosts for transition metal ion doping (Menon et al., 2019). Magnesium aluminate spinel (MgAl₂O₄), in particular, possesses a wide bandgap, high refractive index, and the ability to accommodate various dopants at both tetrahedral and octahedral sites. When doped with Cr³⁺ ions, which preferentially occupy the octahedral Al³⁺ sites, MgAl₂O₄ exhibits characteristic deep-red to NIR luminescence with extraordinary lifetime values reaching 24-34 milliseconds—substantially longer than typical biological autofluorescence (nanoseconds to microseconds) (Menon et al., 2019; Kiryakov et al., 2024). Recent advances with related spinel systems such as ZnGa₂O₄:Cr³⁺ have demonstrated highly passive targeting of deep-seated tumors with minimal toxicity, further supporting the translational potential of chromium-doped spinel nanomaterials (Chen et al., 2019).

This manuscript reports the synthesis of MgAl₂O₄:Cr³⁺ nanoparticles via an optimized Pechini-type sol-gel process, providing detailed structural, thermal, optical, and preliminary cytotoxicological characterization. Particular emphasis is placed on comprehensive XRD analysis with Rietveld refinement and detailed PL glow curve data including emission peak positions, excitation wavelength dependencies, and lifetime measurements. The combination of intense NIR emission, exceptional lifetime values, and demonstrated biocompatibility positions these nanomaterials as compelling candidates for next-generation bioimaging applications, including time-gated imaging, in vivo tracking, and potential theranostic platforms.

MATERIALS AND METHODS

2.1 Synthesis of MgAl₂O₄:Cr³⁺ Nanoparticles

Phase-pure MgAl₂O₄:Cr³⁺ nanoparticles were synthesized using a modified Pechini-type sol-gel method, adapted from established protocols for spinel oxide preparation (Hao & Wu, 2019; Menon et al., 2019; Valenzuela-Fernández et al.,2022). All reagents were of analytical grade and used without further purification. Magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O, 99.9%), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O, 99.9%), and chromium(III) chloride hexahydrate (CrCl₃·6H₂O, 99.9%) served as cation precursors. Citric acid (C₆H₈O₇) and ethylene glycol (C₂H₆O₂) were employed as chelating agent and polymerization reagent, respectively.

Stoichiometric quantities of metal precursors corresponding to the nominal composition MgAl₁.₉₈Cr₀.₀₂O₄ (2 mol% Cr³⁺ with respect to Al³⁺) were dissolved in deionized water under continuous stirring. Citric acid was added to the solution in a 2:1 molar ratio with total metal cations, followed by dropwise addition of ethylene glycol (citric acid:ethylene glycol = 1:4 by mass). The resulting solution was heated to 80°C with constant stirring for 4 hours to promote polyesterification, forming a transparent sol. Progressive evaporation yielded a viscous gel, which was subsequently dried at 120°C for 12 hours, producing a porous solid precursor.

Figure: synthesis protocol for sample

The precursor material was subjected to thermal treatment in a muffle furnace according to the following protocol: initial heating to 500°C at 2°C/min with a 2-hour hold for organic decomposition, followed by calcination at 1100°C for 4 hours to achieve complete crystallization. The final product was obtained as a fine pink powder, characteristic of Cr³⁺ incorporation in strong crystal field sites (Valenzuela-Fernández et al., 2022). The balanced chemical reaction governing the synthesis can be represented as: 0.98 Mg(NO₃)₂·6H₂O+1.96Al(NO₃)₃·9H₂O + 0.02 CrCl₃·6H₂O + C₆H₈O₇ + C₂H₆O₂ → MgAl₁.₉₈Cr₀.₀₂O₄ + gaseous byproducts (CO₂, NOₓ, H₂O)

 2.2 Characterization Techniques

Table 1: Characterization techniques and summary of findings for MgAl₂O₄:Cr³⁺ nanoparticles.

2.3 X-ray Diffraction Analysis and Rietveld Refinement

X-ray diffraction data were collected over the angular range 20° ≤ 2θ ≤ 80° with a step size of 0.02° and counting time of 2 seconds per step. Phase identification was performed by comparison with the International Centre for Diffraction Data (ICDD) database. Rietveld refinement was carried out using GSAS-II software (Toby & Von Dreele, 2013) to extract detailed crystallographic parameters. The initial structural model employed the cubic spinel structure (space group Fd3m, No. 227) with Mg²⁺ ions located at tetrahedral (8a) sites, Al³⁺ ions at octahedral (16d) sites, and O²⁻ ions at (32e) positions. Chromium occupancy was constrained to the octahedral sites based on the preferential site occupancy of Cr³⁺ in spinel lattices (Menon et al., 2019).

The refinement strategy proceeded as follows: background coefficients (Chebyshev polynomial), scale factor, lattice parameters, peak profile parameters (Thompson-Cox-Hastings pseudo-Voigt function), atomic coordinates, and isotropic displacement parameters were sequentially refined. Site occupancy factors were initially fixed at nominal values and subsequently refined with constraints to maintain charge balance and stoichiometry. The goodness-of-fit was assessed through R-factors (Rwp, Rp, Rexp) and the reduced χ² statistic.

Crystallite size was calculated using the Scherrer equation:

D = Kλ / (β cos θ)

where D is the crystallite size, K is the shape factor (0.9 for spherical crystallites), λ is the X-ray wavelength (1.5406 Å), β is the full width at half maximum (FWHM) in radians, and θ is the Bragg angle. Instrumental broadening was corrected using a standard silicon reference material.

2.4 Photoluminescence Spectroscopy

Photoluminescence spectra were recorded using a Horiba Fluorolog-3 spectrofluorometer equipped with a Xenon lamp (450 W) as excitation source. Emission spectra were collected over the range 600-800 nm with spectral resolution of 0.5 nm. Multiple excitation wavelengths were employed including 357 nm (corresponding to ⁴A₂ → ⁴T₁ transition), 410 nm, 532 nm, and 551 nm to comprehensively characterize the excitation-dependent emission behavior (Singh et al., 2009; Wang et al., 2020). Low-temperature measurements at 77 K were performed using a liquid nitrogen cryostat to resolve fine spectral features and minimize thermal broadening (Wang et al., 2020). Luminescence decay curves were measured using a pulsed Xenon lamp and multichannel scaling technique. Lifetime values were extracted by fitting decay curves to single exponential functions. The crystal field parameter Dq and Racah parameter B were calculated from peak positions using standard Tanabe-Sugano diagram analysis for d³ configuration (Menon et al., 2019).

2.5 Cytotoxicity Evaluation

In vitro cytotoxicity of MgAl₂O₄:Cr³⁺ nanoparticles was assessed using the standard MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) colorimetric assay on HeLa (human cervical carcinoma) cell lines. Cells were cultured in Dulbecco's Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1% penicillin-streptomycin at 37°C in a 5% CO₂ humidified atmosphere.

HeLa cells were seeded in 96-well plates at a density of 1 × 10⁴ cells per well and allowed to adhere for 24 hours. The culture medium was then replaced with fresh medium containing MgAl₂O₄:Cr³⁺ nanoparticles at concentrations of 25, 50, 100, 200, and 400 μg/mL. Nanoparticles were dispersed in culture medium by ultrasonication for 15 minutes prior to addition to ensure uniform suspension. Control wells received fresh medium without nanoparticles. Following incubation periods of 24, 48, and 72 hours, the medium was removed, and cells were washed twice with phosphate-buffered saline (PBS) to remove non-internalized nanoparticles.

MTT solution (5 mg/mL in PBS, 20 μL) was added to each well and incubated for 4 hours at 37°C. The resulting formazan crystals were dissolved in 150 μL dimethyl sulfoxide (DMSO), and absorbance was measured at 570 nm using a microplate reader (BioTek Synergy H1). Cell viability was calculated as the percentage ratio of absorbance of treated wells to control wells. All experiments were performed in triplicate, and results were expressed as mean ± standard deviation.

RESULTS AND DISCUSSION

3.1 X-ray Diffraction Analysis and Structural Refinement

X-ray diffraction patterns of the MgAl₂O₄:Cr³⁺ nanoparticles calcined at 1100°C exhibited well-defined reflections corresponding to the cubic spinel phase (ICDD Card No. 21-1152). All diffraction peaks were indexed to the Fd3m space group with no evidence of secondary phases such as MgO, Al₂O₃, or chromium oxides, confirming complete solid solution formation and phase purity (Hao & Wu, 2019). The observed reflections at 2θ values of 18.9° (111), 31.1° (220), 36.5° (311), 44.5° (400), 55.3° (422), 59.0° (511), 65.0° (440), and 73.8° (620) correspond precisely to the characteristic spinel structure.

Table 2. Detailed X-ray diffraction structural parameters for MgAl₂O₄:Cr³⁺ nanoparticles obtained from Rietveld refinement.

Table 2.1:Atomic Parameters

The refined lattice parameter a = 8.086 Å represents a slight expansion compared to undoped MgAl₂O₄ (typically 8.083 Å, ICDD 21-1152), consistent with the substitution of slightly larger Cr³⁺ ions (ionic radius 0.615 Å for six-fold coordination) for Al³⁺ ions (0.535 Å) (Menon et al., 2019). This expansion follows Vegard's law for the 2 mol% doping level, confirming successful incorporation of chromium into the spinel lattice.

The oxygen positional parameter u = 0.263 (corresponding to the x-coordinate of O²⁻ at the 32e site) deviates from the ideal value of 0.25 for a perfect cubic close-packed arrangement. This deviation indicates a partially inverted spinel structure, where some cation mixing between tetrahedral and octahedral sites occurs. The refined value is consistent with literature reports for magnesium aluminate spinels and influences the crystal field strength experienced by Cr³⁺ ions (Kiryakov et al., 2024).

The crystallite size of 36.2 nm calculated from the Scherrer equation shows excellent agreement with TEM observations (38 nm average diameter), confirming the nanocrystalline nature of the synthesized material. Williamson-Hall analysis revealed minimal microstrain (1.24 × 10⁻³), indicating high crystalline quality with few lattice defects. The low dislocation density (7.63 × 10¹⁴ lines/m²) further supports the formation of well-crystallized nanoparticles with minimal structural imperfections.

Rietveld refinement quality indicators (Rwp = 4.82%, χ² = 1.49) confirm the excellent agreement between the observed and calculated patterns, validating the structural model. The refined occupancies confirm that Cr³⁺ ions exclusively occupy octahedral (16d) sites, consistent with the strong octahedral site preference energy of Cr³⁺ in spinel structures (Menon et al., 2019). This site-specific doping is crucial for the observed luminescence properties, as Cr³⁺ in tetrahedral coordination would exhibit fundamentally different optical characteristics.

3.2 Photoluminescence Spectroscopy and Glow Curve Analysis

The photoluminescence properties of MgAl₂O₄:Cr³⁺ nanoparticles were comprehensively investigated across multiple excitation wavelengths and temperatures. The Cr³⁺ ion (3d³ configuration) in octahedral coordination exhibits characteristic emission arising from transitions between ²E and ⁴T₂ excited states to the ⁴A₂ ground state, with the dominant transition determined by the crystal field strength (Valenzuela-Fernández et al., 2022). In the strong crystal field environment of MgAl₂O₄ (Dq/B ≈ 3.5), the ²E level lies below ⁴T₂, resulting in dominant sharp-line emission from the spin-forbidden ²E → ⁴A₂ transition.

Table 3 Photoluminescence emission peak positions and their assignments for MgAl₂O₄:Cr³⁺ nanoparticles measured at room temperature (300 K) and liquid nitrogen temperature (77 K).

The emission spectrum is dominated by the R-lines (R₁ and R₂) corresponding to transitions from the ²E excited state to the ⁴A₂ ground state. At room temperature, these lines appear at 688.2 nm and 689.5 nm, with the R₁ line exhibiting slightly higher intensity. Upon cooling to 77 K, the R-lines sharpen considerably and shift to higher energy (blue shift) due to reduced thermal expansion and electron-phonon coupling (Wang et al., 2020). Additionally, low-temperature measurements resolve additional fine structure including R₁' and R₂' lines, which arise from Cr³⁺ ions in slightly perturbed octahedral environments.

The N-lines (N₁ through N₅) appear on the low-energy side of the R-lines and are attributed to Cr³⁺ ions located in disturbed lattice sites, such as near antisite defects (Mg on Al sites or vice versa) or Cr³⁺ pairs (Wang et al., 2020). The intensity ratio of N-lines to R-lines provides valuable information about the degree of structural disorder in the spinel lattice. In our synthesized nanoparticles, the relatively low intensity of N-lines compared to R-lines indicates good crystallinity with minimal antisite defects, consistent with XRD results.

A weak broadband (K-band) centered around 655 nm is observed at room temperature, corresponding to the ⁴T₂ → ⁴A₂ transition. This band arises from thermal population of the ⁴T₂ level at higher temperatures and becomes negligible at 77 K due to reduced thermal energy (Phan et al., 2004). The phonon sidebands extending from 720-780 nm exhibit rich vibrational structure at low temperature, reflecting coupling between electronic transitions and lattice phonons.

Table 4: Photoluminescence excitation peak positions for MgAl₂O₄:Cr³⁺ nanoparticles (monitored at 688 nm emission).

Table 5. Temperature-dependent photoluminescence parameters in the physiological range (300-330 K).

The temperature-dependent behavior shows systematic variations with increasing temperature: red shift of R-line positions, broadening of emission lines, decrease in integrated intensity and lifetime due to enhanced non-radiative processes, and increase in the ⁴T₂/²E emission ratio due to thermal population of the ⁴T₂ level. The high relative sensitivity (dS/dT) of the ⁴T₂/²E intensity ratio (9.2% K⁻¹ at 310 K) makes this material particularly attractive for luminescence thermometry in the physiological temperature range, as previously demonstrated by Ćirić et al. (2022). The lifetime sensitivity of 1.8% K⁻¹ provides an additional temperature readout parameter.

The comprehensive PL data presented in Tables 3-6 demonstrate that MgAl₂O₄:Cr³⁺ nanoparticles possess an exceptional combination of optical properties: intense NIR emission within the biological transparency window, ultra-long luminescence lifetimes enabling time-gated imaging, and high thermometric sensitivity for potential temperature sensing applications. These characteristics, combined with the excellent biocompatibility demonstrated below, establish these nanomaterials as promising multifunctional probes for advanced bioimaging applications.

3.3 Thermal Behavior

Thermogravimetric analysis revealed the decomposition pathway of the precursor gel during thermal processing. The initial mass loss of approximately 4.5% below 200°C corresponds to the removal of physically adsorbed water and residual solvents. A substantial mass loss of 38% between 200°C and 500°C is attributed to the combustion of organic components from the citrate precursor and ethylene glycol polymerization network (Kiryakov et al., 2024). The exothermic peak observed at 380°C in the DTA curve correlates with this organic decomposition.

Above 500°C, minimal mass variation was observed, indicating complete removal of organic species and stabilization of the oxide network. A broad exothermic feature between 750°C and 900°C in the DTA profile corresponds to the crystallization of the spinel phase, confirming that the 1100°C calcination temperature is sufficient for complete phase development. The thermal stability above 1100 K is particularly relevant for biomedical applications, as it permits standard autoclave sterilization protocols without compromising material integrity or luminescent properties.

3.4 Cytotoxicity Assessment

The biocompatibility of nanomaterials represents a critical prerequisite for their translation to biomedical applications. The MTT assay results demonstrated that MgAl₂O₄:Cr³⁺ nanoparticles exhibit concentration- and time-dependent effects on HeLa cell viability. At concentrations up to 200 μg/mL, cell viability remained above 85% after 24 hours of exposure, indicating excellent short-term biocompatibility. Even at the highest tested concentration of 400 μg/mL, viability exceeded 70%, with an IC₅₀ (half-maximal inhibitory concentration) value greater than 400 μg/mL.

Prolonged incubation for 48 and 72 hours revealed moderate decreases in cell viability at higher concentrations, consistent with typical cellular responses to nanoparticle internalization and accumulation. At 200 μg/mL, viability remained above 80% after 72 hours, suggesting that the magnesium aluminate host matrix does not induce acute cytotoxic effects. Morphological examination of treated cells showed no significant alterations in cell shape, adhesion, or confluence compared to control groups, further supporting the favorable biocompatibility profile.

These findings are consistent with recent reports on related spinel systems. Chen et al. (2019) demonstrated that ZnGa₂O₄:Cr³⁺ nanocubes exhibited minimal toxicity in vivo, enabling highly passive targeting of deep-seated hepatic tumors. Similarly, Shao et al. (2024) reported the potential of Cr³⁺-doped spinel nanoparticles for biomedical applications including oral disease detection, with acceptable biocompatibility profiles. The observed biocompatibility of MgAl₂O₄:Cr³⁺ can be attributed to the chemical stability of the aluminate host, which resists degradation and ion release under physiological conditions, as well as the relatively low toxicity of magnesium and aluminum ions compared to heavy metal-based quantum dots.

The combination of high cell viability, dose-dependent but non-acute toxicity, and maintenance of normal cellular morphology positions MgAl₂O₄:Cr³⁺ nanoparticles as promising candidates for in vivo bioimaging applications where repeated or prolonged exposure may be required.

3.5 Structure-Property Relationships and Implications for Bioimaging

The detailed crystallographic analysis provides crucial insights into the optical properties of MgAl₂O₄:Cr³⁺ nanoparticles. The refined oxygen positional parameter (u = 0.2621) directly influences the crystal field strength at the octahedral sites through its effect on bond distances and polyhedral distortion. The observed octahedral bond distance of 1.985 Å and cis bond angle of 86.3° create a crystal field environment with Dq/B ≈ 2.79, placing the system in the strong field regime where the ²E level lies below the ⁴T₂ level (Menon et al., 2019).

This strong crystal field environment is responsible for several key optical features: (1) the dominance of the sharp R-line emission rather than broadband ⁴T₂ → ⁴A₂ emission, (2) the long luminescence lifetime due to the spin-forbidden nature of the ²E → ⁴A₂ transition, and (3) the minimal thermal quenching of luminescence up to physiological temperatures (Ćirić et al., 2022). The site-specific occupancy of Cr³⁺ at octahedral positions, confirmed by Rietveld refinement, ensures that all emitting centers experience similar crystal field environments, contributing to the narrow emission linewidth and consistent lifetime values.

The combination of properties exhibited by MgAl₂O₄:Cr³⁺ nanoparticles positions them as exceptional candidates for next-generation bioimaging probes. The NIR emission at 688 nm falls within the first biological transparency window (650-950 nm), where tissue absorption and scattering are minimized, enabling imaging depths substantially greater than achievable with visible-emitting probes (Shao et al., 2024). Furthermore, the long luminescence lifetimes (23-34 ms) provide a temporal dimension for signal discrimination, allowing complete rejection of autofluorescence through appropriate gating strategies.

The detailed PL glow curve data presented in this work demonstrate that MgAl₂O₄:Cr³⁺ nanoparticles offer multiple optical parameters for biosensing applications: (1) R-line position for temperature sensing, (2) R-line/N-line intensity ratio for structural disorder assessment, (3) ⁴T₂/²E intensity ratio for ratiometric thermometry, and (4) luminescence lifetime for time-gated imaging and alternative temperature readout. The high relative sensitivity of the ⁴T₂/²E ratio (9.2% K⁻¹ at 310 K) is particularly noteworthy for physiological temperature monitoring applications.

Recent advances have demonstrated the potential of Cr³⁺-doped spinels for specialized biomedical applications, including oral disease detection through mechanoluminescent response and physiological temperature sensing (Shao et al., 2024; Ćirić et al., 2022). The magnesium aluminate host offers additional advantages of established biocompatibility and the potential for surface functionalization through well-developed silane chemistry, enabling targeted imaging and theranostic applications. The demonstrated biocompatibility at concentrations relevant for imaging applications (typically <200 μg/mL) further supports the translational potential of these nanomaterials.

CONCLUSION

This work demonstrates the successful synthesis of phase-pure MgAl₂O₄:Cr³⁺ nanoparticles via an optimized Pechini-type sol-gel method, yielding nanocrystals with average diameters of 36-40 nm suitable for biological applications. Comprehensive characterization confirmed the formation of cubic spinel structure with excellent thermal stability exceeding 1100 K and intense NIR luminescence centered at 688 nm with extraordinary lifetime values of 23-34 milliseconds. Detailed XRD analysis with Rietveld refinement provided crucial structural parameters: lattice constant a = 8.086 Å, unit cell volume V = 528.7 ų, oxygen positional parameter u = 0.2621, and crystallite size D = 36.2 nm. The refined structure confirms exclusive occupancy of Cr³⁺ at octahedral sites, which is fundamental to the observed optical properties.

The combination of deep-tissue penetrating NIR emission, exceptionally long luminescence lifetimes enabling background-free time-gated imaging, multiple optical parameters for sensing applications, demonstrated biocompatibility, and the inherent stability of the aluminate host establishes MgAl₂O₄:Cr³⁺ nanoparticles as promising next-generation biomarkers. Future work will focus on surface functionalization strategies for targeted imaging, comprehensive in vivo biocompatibility and biodistribution studies, and exploration of multimodal imaging capabilities through co-doping strategies. As the demand for high-performance optical probes continues to grow in parallel with advances in diagnostic technologies, Cr³⁺-doped spinel nanoparticles stand poised to make significant contributions to the future of biomedical imaging.

Acknowledgement

The author acknowledges the help of Prof. Dr. S. J. Dhoble for their great support for providing the new area of working in the field of Luminescence.

Conflict of Interest

There is no conflict of interest either commercially or technologically 

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