Multifunctional Optical Sensing with Lanthanide-Doped Upconverting Nanomaterials: Improving Detection Performance of Temperature and Pressure in the Visible and NIR Ranges

Temperature and pressure are fundamental physical parameters in the field of materials science, making their monitoring of utmost significance for scientists and engineers. Here, the NaSrY(MoO4)3:0.02Er3+/0.01Tm3+/0.15Yb3+ nanophosphor is developed as an optical sensor material. Under 975 nm laser excitation, the upconversion characteristics and optical detection performance of the multifunctional sensing platform of temperature and pressure (vacuum) are investigated. We have successfully developed a novel detection platform that enables optical detection of pressure (vacuum) and temperature. This platform utilizes thermally coupled levels (TCLs) and non-TCLs of Er3+ and Tm3+ to achieve ratiometric detection. The multimodal optical temperature and pressure detection based on TCLs and non-TCLs is successfully realized by using different emission bands of double emission centers, which makes it possible for self-referencing optical temperature and pressure measurement modes. These results indicate that the developed nanophosphor is a promising candidate for optical sensors, and our findings suggest potential strategies for modulating the sensor properties of luminescent materials doped with rare-earth ions.

Regulating the upconversion luminescence properties of Tm3+/Yb3+-codoped ZrScW2PO12 microparticles with a negative thermal expansion effect through thermal stimulation for optical thermometry

Tm3+/Yb3+-codoped ZrScW2PO12 microparticles were prepared in order to solve the problems of the severe thermal quenching and unsatisfactory thermometric properties of most luminescent materials. The synthesized materials exhibit a rarely observed negative thermal expansion (NTE) effect, which was verified by in situ X-ray diffraction experiments, performed under high temperature conditions. Upon excitation with a 980 nm laser, bright blue upconversion (UC) emissions originating from Tm3+ were observed. Moreover, owing to the promoted energy transfer, cross-relaxation and non-radiative decay processes at high temperatures triggered by the NTE effect, the observed UC emissions arising from 1G4 and 3F2,3 levels show non-monotonic responses to temperature. By analysing the temperature-dependent luminescence intensity ratio of these UC emissions originating from the non-thermally coupled levels of Tm3+ (1G4 and 3F2,3), the thermometric properties of the prepared microparticles were investigated in detail. Interestingly, the maximum absolute and relative sensitivities of the synthesized compounds are 0.09 and 1.45% K-1, respectively, which are independent of Yb3+ content, but they can be manipulated by employing different sensing modes. Our results manifest that the exploitation of the NTE effect is an efficient way to control the UC luminescence features of rare earth ions and to realize high performance optical thermometry.

Pressure-Induced Remarkable Spectral Red-Shift in Mn2+ -Activated NaY9 (SiO4 )6 O2 Red-Emitting Phosphors for High-Sensitive Optical Manometry

To settle the low sensitivity of luminescent manometers, the Mn2+ -activated NaY9 (SiO4 )6 O2 red-emitting phosphors with splendid pressure sensing performances are developed. Excited by 408 nm, the resulting products emit bright red emission originating from 4 T1 (4 G) → 6 A1 transition of Mn2+ , in which the optimal concentration of the activator ion is ≈1 mol%. Moreover, the admirable thermal stability of the developed phosphors is studied and confirmed by the temperature-dependent emission spectra, based on which the activation energy is derived to be 0.275 eV. By analyzing the pressure-dependent Raman spectra, the structural stability of the synthesized compounds at extreme conditions is verified. Furthermore, the designed phosphors exhibit remarkable spectral red-shift at elevated pressure. Especially, as pressure increases from 0.75 to 7.16 GPa, the emission band centroid shifts from 617.2 to 663.4 nm, resulting in a high sensitivity (dλ/dP) of 7.00 nm GPa-1 , whereas the full width at half maximum (FWHM) increases from 83.0 to 110.6 nm, leading to the ultra-high sensitivity (dFWHM/dP) of 10.13 nm GPa-1 . These achievements manifest that the designed red-emitting phosphors are appropriate for ultrasensitive optical manometry. More importantly, the developed manometer is a current global leader in sensitivity, when operating in the band-width mode, that is, FWHM.

Ultralow pressure sensing and luminescence thermometry based on the emissions of Er3+/Yb3+ codoped Y2Mo4O15 phosphors

Pressure and temperature are fundamental physical parameters, so their monitoring is crucial for various industrial and scientific purposes. For this reason, we developed a new optical sensor material that allows monitoring of both the physical parameters. The synthesized material exhibits upconversion (UC) emission of Er3+ in the red and green spectral regions under NIR (975 nm) laser irradiation. These UC emissions are strongly temperature-dependent, allowing multimode temperature sensing, either based on the luminescence intensity ratio between thermal-coupled energy levels (TCLs) or non-thermal-coupled energy levels (NTCLs) of Er3+ ions. Meanwhile, the luminescence lifetime of the 4S3/2 state of Er3+ ions was used as the third temperature-dependent spectroscopic parameter, enabling multi-parameter thermal sensing. Moreover, the observed enhancement of laser-induced heating of the sample under vacuum conditions allows for the conversion of the luminescent thermometer into a remote vacuum sensor. The pressure variations in the system are correlated with changes in the band intensity ratio (525/550 nm) of Er3+ TCLs, which are further applied for optical, contactless vacuum sensing. This is because of the light-to-heat conversion effect, which is greatly enhanced under vacuum conditions and manifests as a change in the intensity ratio of Er3+ bands (525/550 nm). The obtained results indicate that an Y2Mo4O15:Er3+/Yb3+ (YMO) phosphor has great application potential for the development of multi-functional and non-invasive optical sensors of pressure and temperature.

The developments of cyan emitting phosphors to fulfill the cyan emission gap of white-LEDs

Future generations of solid-state lighting (SSL) will prioritize the development of innovative luminescent materials with superior characteristics. The phosphors converted into white light-emitting diodes (white LEDs) often have a blue-green cavity. Cyan-emitting phosphor fills the spectral gap and produces "full-visible-spectrum lighting." Full-visible spectrum lighting is beneficial for several purposes, such as light therapy, plant growth, and promoting an active and healthy lifestyle. The design of cyan garnet-type phosphors, like Ca2LuHf2Al3O12 (CLHAO), has recently been the subject of interest. This review study reports a useful cyan-emitting phosphor based on CLHAO composition with a garnet structure to have a cyan-to-green emitting color with good energy transfer. It could be employed as cyan filler in warm-white LED manufacturing. Due to its stability, ability to dope with various ions suitable for their desired qualities, and ease of synthesis, this garnet-like compound is a great host material for rare-earth ions. The development of CLHAO cyan-emitting phosphors has exceptionally high luminescence, resulting in high CRI and warm-white LEDs, making them a viable desire for LED manufacturing. The development of CLHAO cyan-emitting phosphors with diverse synthesis techniques, along with their properties and applications in white LEDs, are extensively covered in this review paper.

Mechanoluminescence and Photoluminescence Heterojunction for Superior Multimode Sensing Platform of Friction, Force, Pressure, and Temperature in Fibers and 3D-Printed Polymers

Endowing a single material with various types of luminescence, that is, exhibiting a simultaneous optical response to different stimuli, is vital in various fields. A photoluminescence (PL)- and mechanoluminescence (ML)-based multifunctional sensing platform is built by combining heterojunctioned ZnS/CaZnOS:Mn2+ mechano-photonic materials using a 3D-printing technique and fiber spinning. ML-active particles are embedded in micrometer-sized cellulose fibers for flexible optical devices capable of emitting light driven by mechanical force. Individually modified 3D-printed hard units that exhibit intense ML in response to mechanical deformation, such as impact and friction, are also fabricated. Importantly, they also allow low-pressure sensing up to ≈100 bar, a range previously inaccessible by any other optical sensing technique. Moreover, the developed optical manometer based on the PL of the materials demonstrates a superior high-pressure sensitivity of ≈6.20 nm GPa-1 . Using this sensing platform, four modes of temperature detection can be achieved: excitation-band spectral shifts, emission-band spectral shifts, bandwidth broadening, and lifetime shortening. This work supports the possibility of mass production of ML-active mechanical and optoelectronic parts integrated with scientific and industrial tools and apparatus.

Noncentrosymmetric Lanthanide-Based MOF Materials Exhibiting Strong SHG Activity and NIR Luminescence of Er3+: Application in Nonlinear Optical Thermometry

Optically active luminescent materials based on lanthanide ions attract significant attention due to their unique spectroscopic properties, nonlinear optical activity, and the possibility of application as contactless sensors. Lanthanide metal-organic frameworks (Ln-MOFs) that exhibit strong second-harmonic generation (SHG) and are optically active in the NIR region are unexpectedly underrepresented. Moreover, such Ln-MOFs require ligands that are chiral and/or need multistep synthetic procedures. Here, we show that the NIR pulsed laser irradiation of the noncentrosymmetric, isostructural Ln-MOF materials (MOF-Er³⁺ (1) and codoped MOF-Yb³⁺/Er³⁺ (2)) that are constructed from simple, achiral organic substrates in a one-step procedure results in strong and tunable SHG activity. The SHG signals could be easily collected, exciting the materials in a broad NIR spectral range, from ≈800 to 1500 nm, resulting in the intense color of emission, observed in the entire visible spectral region. Moreover, upon excitation in the range of ≈900 to 1025 nm, the materials also exhibit the NIR luminescence of Er³⁺ ions, centered at ≈1550 nm. The use of a 975 nm pulse excitation allows simultaneous observations of the conventional NIR emission of Er³⁺ and the SHG signal, altogether tuned by the composition of the Ln-MOF materials. Taking the benefits of different thermal responses of the mentioned effects, we have developed a nonlinear optical thermometer based on lanthanide-MOF materials. In this system, the SHG signal decreases with temperature, whereas the NIR emission band of Er³⁺ slightly broadens, allowing ratiometric (Er³⁺ NIR 1550 nm/SHG 488 nm) temperature monitoring. Our study provides a groundwork for the rational design of readily available and self-monitoring NLO-active Ln-MOFs with the desired optical and electronic properties.

NIR emission of lanthanides for ultrasensitive luminescence manometry-Er3+-activated optical sensor of high pressure

Pressure is an important physical parameter and hence its monitoring is very important for different industrial and scientific applications. Although commonly used luminescent pressure sensors (ruby-Al₂O₃:Cr³⁺ and SrB₄O₇:Sm²⁺) allow optical monitoring of pressure in compressed systems (usually in a diamond anvil cell; DAC), their detection resolution is limited by sensitivity, i.e., pressure response in a form of the detected spectral shift. Here we report, a breakthrough in optical pressure sensing by developing an ultra-sensitive NIR pressure sensor (dλ/dP = 1.766 nm GPa^-1). This luminescent manometer is based on the optically active YVO₄:Yb³⁺-Er³⁺ phosphor material which exhibits the largest spectral shift as a function of pressure compared to other luminescent pressure gauges reported elsewhere. In addition, thanks to the locations of excitation and emission in the NIR range, the developed optical manometer allows high-pressure measurements (without spectral overlapping/interferences) of various luminescent organic and inorganic materials, which are typically excited and can emit in the UV-vis spectral ranges.

Improving performance of luminescent nanothermometers based on non-thermally and thermally coupled levels of lanthanides by modulating laser power

This work sheds light on the pump power impact on the performance of luminescent thermometers, which is often underestimated by researchers. An up-converting, inorganic nanoluminophore, YVO₄:Yb³⁺,Er³⁺ (nanothermometer) was synthesized using the hydrothermal method and a subsequent calcination. This nanomaterial appears as a white powder composed of small nanoparticles (≈20 nm), exhibiting a very intense, green upconverted luminescence (λ_ex = 975 nm), visible to the naked eye. Its emission spectrum consists of four Er³⁺ bands (500-850 nm) and one Yb³⁺ band (>900 nm). The obtained compound exhibits temperature-dependent luminescence properties, hence it is used as an optical nanosensor of temperature. The determined band intensity ratios of the non-thermally coupled levels (non-TCLs) of Yb³⁺/Er³⁺ and thermally coupled levels (TCLs) of Er³⁺ are correlated with temperature, and they are used for ratiometric sensing of temperature. The effects of the pump (NIR laser) power on the luminescence properties of the material, including band intensity ratios, absolute and relative sensitivities and temperature resolution are analysed. It was pointed out that the applied laser power has a huge impact on the values of the aforementioned thermometric parameters, and manipulating the laser power can significantly improve the performance of optical nanothermometers.

Luminescent Nanothermometer Operating at Very High Temperature-Sensing up to 1000 K with Upconverting Nanoparticles (Yb3+/Tm3+)

Lanthanide-based luminescent nanothermometers play a crucial role in optical temperature determination. However, because of the strong thermal quenching of the luminescence, as well as the deterioration of their sensitivity and resolution with temperature elevation, they can operate in a relatively low-temperature range, usually from cryogenic to ≈800 K. In this work, we show how to overcome these limitations and monitor very high-temperature values, with high sensitivity (≈2.1% K^-1) and good thermal resolution (≈1.4 K) at around 1000 K. As an optical probe of temperature, we chose upconverting Yb³⁺-Tm³⁺ codoped YVO₄ nanoparticles. For ratiometric sensing in the low-temperature range, we used the relative intensities of the Tm³⁺ emissions associated with the ³F_2,3 and ³H₄ thermally coupled levels, that is, ³F_2,3 → ³H₆/³H₄ → ³H₆ (700/800 nm) band intensity ratio. In order to improve sensitivity and resolution in the high-temperature range, we used the 940/800 nm band intensity ratio of the nonthermally coupled levels of Yb³⁺ (²F_5/2 → ²F_7/2) and Tm³⁺ (³H₄ → ³H₆). These NIR bands are very intense, even at extreme temperature values, and their intensity ratio changes significantly, allowing accurate temperature sensing with high thermal and spatial resolutions. The results presented in this work may be particularly important for industrial applications, such as metallurgy, catalysis, high-temperature synthesis, materials processing and engineering, and so forth, which require rapid, contactless temperature monitoring at extreme conditions.

Lanthanide Upconverted Luminescence for Simultaneous Contactless Optical Thermometry and Manometry-Sensing under Extreme Conditions of Pressure and Temperature

The growing interest in the miniaturization of various devices and conducting experiments under extreme conditions of pressure and temperature causes the need for the development of small, contactless, precise, and accurate optical sensors without any electrical connections. In this work, YF₃:Yb³⁺-Er³⁺ upconverting microparticles are used as a bifunctional luminescence sensor for simultaneous temperature and pressure measurements. Different changes in the properties of Er³⁺ green and red upconverted luminescence, after excitation of Yb³⁺ ions in the near-infrared at ∼975 nm, are used to calibrate pressure and/or temperature inside the hydrostatic chamber of a diamond anvil cell (DAC). For temperature sensing, changes in the relative intensities of the Er³⁺ green upconverted luminescence of ²H_11/2 and ⁴S_3/2 thermally coupled multiplets to the ⁴I_15/2 ground state, whose relative populations follow a Boltzmann distribution, are calibrated. For pressure sensing, the spectral shift of the Er³⁺ upconverted red emission peak at ∼665 nm, between the Stark sublevels of the ⁴F_9/2 → ⁴I_15/2 transition, is used. Experiments performed under simultaneous extreme conditions of pressure, up to ∼8 GPa, and temperature, up to ∼473 K, confirm the possibility of remote optical pressure and temperature sensing.

Surface Modification of Luminescent LnIII Fluoride Core-Shell Nanoparticles with Acetylsalicylic acid (Aspirin): Synthesis, Spectroscopic and in Vitro Hemocompatibility Studies

Luminescent lanthanide fluoride core-shell (LaF₃ :Tb³⁺ ,Ce³⁺ @SiO₂ -NH₂ ) nanoparticles, with acetylsalicylic acid (aspirin) coated on the surface have been obtained. The synthesized products, which combine the potential located in the silica shell with the luminescent activity of the core, were characterized in detail with the use of luminescence spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) methods. The in vitro effects of the modified luminescent nanoparticles on human red blood cell (RBC) membrane permeability, RBC shape, and sedimentation rate were investigated to assess the hemocompatibility of the obtained compounds. This study demonstrates that LaF₃ : Tb³⁺ 5 %, Ce³⁺ 10 %@SiO₂ -NH₂ nanoparticles with acetylsalicylic acid (aspirin) coated on the surface are very good precursors for multifunctional drug-delivery systems or bio-imaging probes that can be used safely in potential biomedical applications.

3,5-Dihydroxy Benzoic Acid-Capped CaF2:Tb3+ Nanocrystals as Luminescent Probes for the WO4 2- Ion in Aqueous Solution

We report a facile and effective luminescence method for the determination of the WO4 2- ion in aqueous medium at initial pH = 6.3. This is achieved using 3,5-dihydroxybenzoic acid-capped CaF2:Tb3+ (5%) nanocrystals (NCs) as a luminescent probe. This is accomplished based on the energy transfer luminescence from the WO4 2- ion to the Tb3+ ion in small-size CaF2:Tb3+ NCs. Hydroxyl groups on the surface ligand helps in binding the tungstate ion to the surface of the NCs. With the gradual addition of the WO4 2- ion, the intensity of the Tb3+ excitation and emission spectra significantly increased. The linear range of the detection was from 1 to 10 μM for the WO4 2- ion (R 2 = 0.99). The calculated detection limit was 0.4 μM (by applying the 3σ/K criterion).

UV-Vis-NIR absorption spectra of lanthanide oxides and fluorides

Absorption spectra of inorganic lanthanide fluorides and oxides.

Gold nanorods as a high-pressure sensor of phase transitions and refractive-index gauge

Gold nanorods (Au NRs), nanospheres and other nanoparticles display numerous superior physicochemical properties, such as resistance to oxidation and aggressive agents, strong enhancement of local electric field and a high absorption coefficient in the visible and near-infrared (NIR) range. The absorption peaks of surface plasmon resonance (SPR) in Au NRs are highly sensitive to their surrounding medium and to its refractive index (RI) changes. However, no applications of NRs for detecting phase transitions have been reported. Here, we show that Au NRs effectively detect phase transitions of compressed compounds, liquid and solid, by measuring their RI. Owing to the direct interaction of the NRs with their surrounding medium, its subtle RI changes can be observed by the use of high-pressure absorption vis-NIR spectroscopy. We have applied a Au NR-based sensor in a diamond anvil cell (DAC) for monitoring the phase transitions of compressed water, its freezing to ice VI and at the subsequent solid-solid phase transition to ice VII, and the monotonic compression and solid-solid phase transitions in urea and thiourea.

Upconverting Lanthanide Fluoride Core@Shell Nanorods for Luminescent Thermometry in the First and Second Biological Windows: β-NaYF4:Yb3+- Er3+@SiO2 Temperature Sensor

Upconverting core@shell type β-NaYF₄:Yb³⁺-Er³⁺@SiO₂ nanorods have been obtained by a two-step synthesis process, which encompasses hydrothermal and microemulsion routes. The synthesized nanomaterial forms stable aqueous colloids and exhibits a bright dual-center emission (λ_ex = 975 nm), i.e., upconversion luminescence of Er³⁺ and down-shifting emission of Yb³⁺, located in the first (I-BW) and the second (II-BW) biological windows of the spectral range, respectively. The intensity ratios of the emission bands of Er³⁺ and Yb³⁺ observed in the vis-near-infrared (NIR) range monotonously change with temperature, i.e., the thermalized Er³⁺ levels (²H_11/2 → ⁴I_15/2/⁴S_3/2 → ⁴I_15/2) and the nonthermally coupled Yb³⁺/Er³⁺ levels (²F_5/2 → ²F_7/2/⁴I_9/2 → ⁴I_15/2 or ⁴F_9/2 → ⁴I_15/2). Hence, their thermal evolutions have been correlated with temperature using the Boltzmann type distribution and second-order polynomial fits for temperature-sensing purposes, i.e., Er³⁺ 525/545 nm (max S_r = 1.31% K^-1) and Yb³⁺/Er³⁺ 1010/810 nm (1.64% K^-1) or 1010/660 nm (0.96% K^-1). Additionally, a fresh chicken breast was used as a tissue imitation in the performed ex vivo experiment, showing the advantage of the use of NIR Yb³⁺/Er³⁺ bands, vs. the typically used Er³⁺ 525/545 nm band ratio, i.e., better penetration of the luminescence signal through the tissue in the I-BW and II-BW. Such nanomaterials can be utilized as accurate and effective, broad-range vis-NIR optical, contactless sensors of temperature.

Optical Pressure Sensor Based on the Emission and Excitation Band Width (fwhm) and Luminescence Shift of Ce3+-Doped Fluorapatite-High-Pressure Sensing

A novel, contactless optical sensor of pressure based on the luminescence red-shift and bandwidth (full width at half-maximum, fwhm) of the Ce³⁺-doped fluorapatite-Y₆Ba₄(SiO₄)₆F₂ powder has been successfully synthesized via a facile solid-state method. The obtained material exhibits a bright blue emission under UV light excitation. It was characterized using powder X-ray diffraction, scanning electron microscopy and luminescence spectroscopy, including high-pressure measurements of excitation and emission spectra, up to above ∼30 GPa. Compression of the material resulted in a significant red-shift of the allowed 4f → 5d and 5d → 4f transitions of Ce³⁺ in the excitation and emission spectra, respectively. The pressure-induced monotonic shift of the emission band, as well as changes in the excitation/emission band widths, have been correlated with pressure for sensing purposes. The material exhibits a high pressure sensitivity (dλ/d P ≈ 0.63 nm/GPa) and outstanding signal intensity at high-pressure conditions (∼90% of the initial intensity at around 20 GPa) with minimal pressure-induced quenching of luminescence.

Preparation, crystal structure and luminescence properties of a novel single-phase red emitting phosphor CaSr2(PO4)2:Sm3+,Li+

Single-phase CaSr₂(PO₄)₂:Sm³⁺,Li⁺ phosphors were prepared via a high-temperature solid-state method under air. The powder X-ray diffraction patterns, scanning electron microscopy images, photoluminescence spectra, and concentration-dependent emission spectra were measured to characterize the as-prepared phosphors and luminescence decay curves. The results showed that the CaSr₂(PO₄)₂:Sm³⁺,Li⁺ phosphors exhibited red luminescence, and the emission spectra of the phosphors consisted of four sharp peaks at around 565, 601 (the strongest one), 647 and 707 nm. The optimum doping concentration of Sm³⁺ ions was 0.09 (mol concentration), and the mechanism of energy transfer among Sm³⁺ ions was defined to be quadrupole–quadrupole (q–q) interactions using Dexter's theory. The Blasse concentration quenching method was used to determine the critical distance R_c for energy transfer among Sm³⁺ as 10.99 Å. The results indicate that the as-prepared phosphors have good thermal stability with an activation energy of 0.773 eV via temperature-dependent emission spectra. Therefore, CaSr_2−2x(PO₄)₂:xSm³⁺,xLi⁺ materials can be used as red-emitting phosphors for UV-pumped white-light emitting diodes.

Single-phase red emitting CaSr₂(PO₄)₂:Sm³⁺,Li⁺ phosphors with good thermal stability were obtained.

Luminescent-Magnetic Cellulose Fibers, Modified with Lanthanide-Doped Core/Shell Nanostructures

Novel luminescent-magnetic cellulose microfibers were prepared by a dry-wet spinning method with the use of N-methylmorpholine-N-oxide. The synthesized luminescent-magnetic core/shell type nanostructures, based on the lanthanide-doped fluorides and magnetite nanoparticles (NPs)-Fe₃O₄/SiO₂/NH₂/PAA/LnF₃, were used as nanomodifiers of the fibers. Thanks to the successful incorporation of the bifunctional nanomodifiers into the cellulose structure, the functionalized fibers exhibited superior properties, that is, bright multicolor emission under UV light and strong magnetic response. By the use of the as-prepared fibers, the luminescent-magnetic thread was fabricated and used to sew and make a unique pattern in the glove material, as a proof of concept for advanced, multimodal cloths'/materials' protection against counterfeiting. The presence and uniform distribution of the modifier NPs in the polymer matrix were confirmed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis (EDX). The concentration of the modifier NPs in the fibers was determined by inductively coupled plasma mass spectrometry, EDX, and magnetic measurements. The luminescence characteristics of the materials were examined by photoluminescence spectroscopy, and their magnetic field-responsive behavior was investigated by a superconducting quantum interference device.

Multifunctional Optical Sensors for Nanomanometry and Nanothermometry: High-Pressure and High-Temperature Upconversion Luminescence of Lanthanide-Doped Phosphates-LaPO4/YPO4:Yb3+-Tm3

Upconversion luminescence of nano-sized Yb³⁺ and Tm³⁺ codoped rare earth phosphates, that is, LaPO₄ and YPO₄, has been investigated under high-pressure (HP, up to ∼25 GPa) and high-temperature (293-773 K) conditions. The pressure-dependent luminescence properties of the nanocrystals, that is, energy red shift of the band centroids, changes of the band ratios, shortening of upconversion lifetimes, and so forth, make the studied nanomaterials suitable for optical pressure sensing in nanomanometry. Furthermore, thanks to the large energy difference (∼1800 cm^-1), the thermalized states of Tm³⁺ ions are spectrally well-separated, providing high-temperature resolution, required in optical nanothermometry. The temperature of the system containing such active nanomaterials can be determined on the basis of the thermally induced changes of the Tm³⁺ band ratio (³F_2,3 → ³H₆/³H₄ → ³H₆), observed in the emission spectra. The advantage of such upconverting optical sensors is the use of near-infrared light, which is highly penetrable for many materials. The investigated nanomanometers/nanothermometers have been successfully applied, as a proof-of-concept of a novel bimodal optical gauge, for the determination of the temperature of the heated system (473 K), which was simultaneously compressed under HP (1.5 and 5 GPa).

Lifetime nanomanometry - high-pressure luminescence of up-converting lanthanide nanocrystals - SrF2:Yb3+,Er3

Anti-Stokes luminescence of up-converting nanocrystals SrF₂:Yb³⁺,Er³⁺ can be used as a high pressure optical sensor alternative to the ruby fluorescence-scale. In nanocrystalline SrF₂:Yb³⁺,Er³⁺, high pressure reversibly shortens the emission lifetimes nearly linearly up to 5.29 GPa at least. Its advantage is the use of NIR (≈980 nm) radiation, highly penetrable for many materials. The shortening of up-conversion lifetimes has been attributed mainly to the changes in energy transfer rates, caused by decreased interatomic distances and increased overlap integrals between 4f electrons and the valence shells of ligand ions. The origin of high-pressure effects on the luminescence intensity, band ratio and their spectral position has been explained by the increased interactions and distortions of the crystal-field symmetry around the emitting ions in the compressed structure.

Spectroscopic, structural and in vitro cytotoxicity evaluation of luminescent, lanthanide doped core@shell nanomaterials GdVO4:Eu(3+)5%@SiO2@NH2

The luminescent GdVO4:Eu(3+)5%@SiO2@NH2 core@shell nanomaterials were obtained via co-precipitation method, followed by hydrolysis and co-condensation of silane derivatives: tetraethyl orthosilicate and 3-aminopropyltriethoxysilane. Their effect on human erythrocytes sedimentation and on proliferation of human lung microvascular endothelial cells was examined and discussed. The luminescent nanoparticles were synthesized in the presence of polyacrylic acid or glycerin in order to minimalize the agglomeration and excessive growth of nanostructures. Surface coating with amine functionalized silica shell improved their biocompatibility, facilitated further organic conjugation and protected the internal core. Magnetic measurements revealed an enhanced T1-relaxivity for the synthesized GdVO4:Eu(3+)5% nanostructures. Structure, morphology and average grain size of the obtained nanomaterials were determined by X-ray diffraction, transmission electron microscopy and dynamic light scattering analysis. The qualitative elemental composition of the nanomaterials was established using energy-dispersive X-ray spectroscopy. The spectroscopic characteristic of red emitting core@shell nanophosphors was completed by measuring luminescence spectra and decays. The emission spectra revealed characteristic bands of Eu(3+) ions related to the transitions (5)D0-(7)F0,1,2,3,4 and (5)D1-(7)F1. The luminescence lifetimes consisted of two components, associated with the presence of Eu(3+) ions located at the surface of the crystallites and in the bulk.

Synthesis, characterization, and cytotoxicity in human erythrocytes of multifunctional, magnetic, and luminescent nanocrystalline rare earth fluorides

Multifunctional nanoparticles exhibiting red or green luminescence properties and magnetism were synthesized and thoroughly analyzed. The hydrothermal method was used for the synthesis of Eu3+- or Tb3+-doped GdF3-, NaGdF4-, and BaGdF5-based nanocrystalline materials. The X-ray diffraction patterns of the samples confirmed the desired compositions of the materials. Transmission electron microscope images revealed the different morphologies of the products, including the nanocrystal sizes, which varied from 12 nm in the case of BaGdF5-based nanoparticles to larger structures with dimensions exceeding 300 nm. All of the samples presented luminescence under ultraviolet irradiation, as well as when the samples were in the form of water colloids. The highest luminescence was observed for BaGdF5-based materials. The obtained nanoparticles exhibited paramagnetism along with probable evidence of superparamagnetic behavior at low temperatures. The particles' magnetic characteristics were also preserved for samples in the form of a suspension in distilled water. The cytotoxicity studies against the human erythrocytes indicated that the synthesized nanoparticles are non-toxic because they did not cause the red blood cells shape changes nor did they alter their membrane structure and permeabilization.

Synthesis and organic surface modification of luminescent, lanthanide-doped core/shell nanomaterials (LnF3@SiO2@NH2@organic acid) for potential bioapplications: spectroscopic, structural, and in vitro cytotoxicity evaluation

A facile coprecipitation reaction between Ce(3+), Gd(3+), Tb(3+), and F(-) ions, in the presence of glycerine as a capping agent, led to the formation of ultrafine, nanocrystalline CeF3:Tb(3+) 5%, Gd(3+) 5% (LnF3). The as-prepared fluoride nanoparticles were successfully coated with an amine modified silica shell. Subsequently, the obtained LnF3@SiO2@NH2 nanostructures were conjugated with 4-ethoxybenzoic acid in order to prove the possibility of organic modification and obtain a new functional nanomaterial. All of the nanophosphors synthesized exhibited intense green luminescence under UV light irradiation. Based on TEM (transmission electron microscopy) measurements, the diameters of the cores (≈12 nm) and core/shell particles (≈50 nm) were determined. To evaluate the cytotoxic activity of the nanomaterials obtained, their effect on human erythrocytes was investigated. LnF3 nanoparticles were bound to the erythrocyte membrane, without inducing any cytotoxic effects. After coating with silica, the nanoparticles revealed significant cytotoxicity. However, further functionalization of the nanomaterial with -NH2 groups as well as conjugation with 4-ethoxybenzoic acid entailed a decrease in cytotoxicity of the core/shell nanoparticles.

Core/shell-type nanorods of Tb3+-doped LaPO4, modified with amine groups, revealing reduced cytotoxicity

ABSTRACT

A simple co-precipitation reaction between Ln³⁺ cations (Ln = lanthanide) and phosphate ions in the presence of polyethylene glycol (PEG), including post-treatment under hydrothermal conditions, leads to the formation of Tb³⁺-doped LaPO₄ crystalline nanorods. The nanoparticles obtained can be successfully coated with amorphous and porous silica, forming core/shell-type nanorods. Both products reveal intensive green luminescence under UV lamp irradiation. The surface of the core/shell-type product can also be modified with -NH₂ groups via silylation procedure, using 3-aminopropyltriethoxysilane as a modifier. Powder X-ray diffraction, transmission electron microscopy, and scanning electron microscopy confirm the desired structure and needle-like shape of the products synthesized. Fourier transform infrared spectroscopy and specific surface area measurements by Brunauer-Emmett-Teller method reveal a successful surface modification with amine groups of the core/shell-type nanoparticles prepared. The nanomaterials synthesized exhibit green luminescence characteristic of Tb³⁺ ions, as solid powders and aqueous colloids, examined by spectrofluorometry. The in vitro cytotoxicity studies reveal different degree toxicity of the products. LaPO₄:Tb³⁺@SiO₂@NH₂ exhibits the smallest toxicity against B16F0 mouse melanoma cancer cells and human skin microvascular endothelial cell lines, in contrast to the most toxic LaPO₄:Tb³⁺@SiO₂.

Structural, spectroscopic and cytotoxicity studies of TbF3@CeF3 and TbF3@CeF3@SiO2 nanocrystals

ABSTRACT

Terbium fluoride nanocrystals, covered by a shell, composed of cerium fluoride were synthesized by a co-precipitation method. Their complex structure was formed spontaneously during the synthesis. The surface of these core/shell nanocrystals was additionally modified by silica. The properties of TbF₃@CeF₃ and TbF₃@CeF₃@SiO₂ nanocrystals, formed in this way, were investigated. Spectroscopic studies showed that the differences between these two groups of products resulted from the presence of the SiO₂ shell. X-ray diffraction patterns confirmed the trigonal crystal structure of TbF₃@CeF₃ nanocrystals. High resolution transmission electron microscopy in connection with energy-dispersive X-ray spectroscopy showed a complex structure of the formed nanocrystals. Crystallized as small discs, 'the products', with an average diameter around 10 nm, showed an increase in the concentration of Tb³⁺ ions from surface to the core of nanocrystals. In addition to photo-physical analyses, cytotoxicity studies were performed on HSkMEC (Human Skin Microvascular Endothelial Cells) and B16F0 mouse melanoma cancer cells. The cytotoxicity of the nanomaterials was neutral for the investigated cells with no toxic or antiproliferative effect in the cell cultures, either for normal or for cancer cells. This fact makes the obtained nanocrystals good candidates for biological applications and further modifications of the SiO₂ shell.

Magnetic and luminescent hybrid nanomaterial based on Fe(3)O(4) nanocrystals and GdPO(4):Eu(3+) nanoneedles

A bifunctional hybrid nanomaterial, which can show magnetic and luminescent properties, was obtained. A magnetic phase was synthesized as a core/shell type composite. Nanocrystalline magnetite, Fe(3)O(4) was used as the core and was encapsulated in a silica shell. The luminescent phase was GdPO(4) doped with Eu(3+) ions, as the emitter. The investigated materials were synthesized using a coprecipitation method. Encapsulated Fe(3)O(4) was "trapped" in a nano-scaffold composed of GdPO(4) crystalline nanoneedles. When an external magnetic field was applied, this hybrid composite was attracted in one direction. Also, the luminescent phase can move simultaneously with magnetite due to a "trapping" effect. The structure and morphology of the obtained nanocomposites were examined with the use of transmission electron microscopy and X-ray powder diffraction. Spectroscopic properties of the Eu(3+)-doped nanomaterials were studied by measuring their excitation and emission spectra as well as their luminescence decay times. GRAPHICAL ABSTRACT: