The Coming of Age of Neodymium: Redefining Its Role in Rare Earth Doped Nanoparticles

Among luminescent nanostructures actively investigated in the last couple of decades, rare earth (RE³⁺) doped nanoparticles (RENPs) are some of the most reported family of materials. The development of RENPs in the biomedical framework is quickly making its transition to the ∼800 nm excitation pathway, beneficial for both in vitro and in vivo applications to eliminate heating and facilitate higher penetration in tissues. Therefore, reports and investigations on RENPs containing the neodymium ion (Nd³⁺) greatly increased in number as the focus on ∼800 nm radiation absorbing Nd³⁺ ion gained traction. In this review, we cover the basics behind the RE³⁺ luminescence, the most successful Nd³⁺-RENP architectures, and highlight application areas. Nd³⁺-RENPs, particularly Nd³⁺-sensitized RENPs, have been scrutinized by considering the division between their upconversion and downshifting emissions. Aside from their distinctive optical properties, significant attention is paid to the diverse applications of Nd³⁺-RENPs, notwithstanding the pitfalls that are still to be addressed. Overall, we aim to provide a comprehensive overview on Nd³⁺-RENPs, discussing their developmental and applicative successes as well as challenges. We also assess future research pathways and foreseeable obstacles ahead, in a field, which we believe will continue witnessing an effervescent progress in the years to come.

Decoupled Rare-Earth Nanoparticles for On-Demand Upconversion Photodynamic Therapy and High-Contrast Near Infrared Imaging in NIR IIb

Rare-earth doped multi-shell nanoparticles slated for theranostic applications produce a variety of emission bands upon near-infrared (NIR) excitation. Their downshifting emission is useful for high-contrast NIR imaging, while the upconversion light can induce photodynamic therapy (PDT). Unfortunately, integration of imaging and therapy is challenging. These modalities are better to be controlled independently so that, with the help of imaging, selective delivery of a theranostic agent at the site of interest could be ensured prior to on-demand PDT initiation. We introduce here multi-shell rare-earth doped nanoparticles (RENPs) arranged in a manner to produce only downshifting emission for NIR imaging when excited at one NIR wavelength and upconversion emission for therapeutic action by using a different excitation wavelength. In this work, multi-shell RENPs with a surface-bound sensitizer have been synthesized for decoupled 1550 nm downshifting emission upon 800 nm excitation and 550 nm upconversion emission caused by 980 nm irradiation. The independently controlled emission bands allow for high-contrast NIR imaging in NIR-IIb of optical transparency that gives high-contrast images due to significantly reduced light scattering. This can be conducted prior to PDT using 980 nm to produce upconverted light at 550 nm that excites the RENP surface-bound photosensitizer, Rose Bengal (RB), to effect photodynamic therapy with high specificity and safer theranostics.

Lanthanide ion (Ln3+ )-based upconversion sensor for quantification of food contaminants: A review

The food safety issue has gradually become the focus of attention in modern society. The presence of food contaminants poses a threat to human health and there are a number of interesting researches on the detection of food contaminants. Upconversion nanoparticles (UCNPs) are superior to other fluorescence materials, considering the benefits of large anti-Stokes shifts, high chemical stability, non-autofluorescence, good light penetration ability, and low toxicity. These properties render UCNPs promising candidates as luminescent labels in biodetection, which provides opportunities as a sensitive, accurate, and rapid detection method. This paper intended to review the research progress of food contaminants detection by UCNPs-based sensors. We have proposed the key criteria for UCNPs in the detection of food contaminants. Additionally, it highlighted the construction process of the UCNPs-based sensors, which includes the synthesis and modification of UCNPs, selection of the recognition elements, and consideration of the detection principle. Moreover, six kinds of food contaminants detected by UCNPs technology in the past 5 years have been summarized and discussed fairly. Last but not least, it is outlined that UCNPs have great potential to be applied in food safety detection and threw new insight into the challenges ahead.

Size-controlled bimodal in vivo nanoprobes as near-infrared phosphors and positive contrast agents for magnetic resonance imaging

Rare-earth-doped nanoparticles (NPs), such as NaGdF₄ nanocrystals doped with light-emitting rare earth ions, are promising bimodal probes that allow the integration of over 1000 nm near-infrared (OTN-NIR; NIR-II/III) fluorescence imaging and magnetic resonance imaging (MRI) of live bodies. A precise control of the particle size is the key factor for achieving a high signal-to-noise ratio in both NIR fluorescence and MR images and for regulating their function in the body. In this study, size-controlled NaGdF₄:Yb³⁺, Er³⁺ NPs prepared by stepwise crystal growth were used for in vivo bimodal imaging. Hexagonal NaGdF₄:Yb³⁺,Er³⁺ NPs coated with poly(ethylene glycol)-poly(acrylic acid) block copolymer, with hydrodynamic diameters of 15 and 45 nm, were prepared and evaluated as bimodal NPs for OTN-NIR fluorescence imaging and MRI. Their longitudinal (T ₁) and transverse (T ₂) relaxation rates at the static magnetic field strength of 1.0 T, as well as their cytotoxicity towards NIH3T3 cell lines, were evaluated and compared to study the effect of size. Using these particles, blood vessel visualization was achieved by MRI, with the highest relaxometric ratio (r ₁/r ₂) of 0.79 reported to date for NaGdF₄-based nanoprobes (r ₁ = 19.78 mM^-1 s^-1), and by OTN-NIR fluorescence imaging. The results clearly demonstrate the potential of the size-controlled PEG-modified NaGdF₄:Yb³⁺,Er³⁺ NPs as powerful 'positive' T ₁-weight contrast MRI agents and OTN-NIR fluorophores.

Spectral characterization of LiYbF4 upconverting nanoparticles

In light of the recent developments on Yb³⁺-based upconverting rare-earth nanoparticles (RENPs), we have systematically explored the spectral features of LiYbF₄:RE³⁺/LiYF₄ core/shell RENPs doped with various amounts of Tm³⁺, Er³⁺, or Ho³⁺. Tm³⁺-RENPs displayed photoluminescence from the UV to near-infrared (NIR), and the dominant high-photon-order upconversion emission of these RENPs was tunable by Tm³⁺ doping. Similarly, Er³⁺- and Ho³⁺-RENPs with green and red upconversion showed wide color tuning, depending on the doping amount and excitation power density. From steady-state power plot and photoluminescence decay studies we have observed respective changes in upconversion photon order and average lifetime that attest to a number of cross-relaxation processes occurring at higher RE³⁺ doping concentration. Particularly in the case of Tm³⁺-RENPs, cross-relaxation promotes four- and five-photon order upconversion emission in the UV and blue spectral regions. The quantum yield of high-order upconversion emission was on par with classic Yb³⁺/Tm³⁺-doped systems, yet due to the high number of sensitizer ions in the LiYbF₄ host these RENPs are expected to be brighter and thus better suited for applications such as controlled drug delivery or optogenetics. Overall, LiYbF₄:RE³⁺/LiYF₄ RENPs are promising systems to effectively generate high-order upconversion emissions, owing to excitation energy confinement within the Yb³⁺ network and its efficient funneling to the activator dopants.

Contemporary Synthesis of Ultrasmall (sub-10 nm) Upconverting Nanomaterials

Due to their unique photophysical properties, upconverting nanoparticles (UCNPs), i. e. particles capable of converting near-infrared (NIR) photons into tunable emissions in the range of ultraviolet (UV) to NIR, have great potential for use in various biomedical fields such as bioimaging, photodynamic therapy and bioanalytical applications. As far as biomedical applications are concerned, these materials have a number of advantageous properties such as brilliant luminescence and exceptional photostability. Very small "stealth" particles (sub-10 nm), which can circulate in the body largely undetected by the immune system, are particularly important for in vivo use. The fabrication of such particles, which simultaneously have a defined (ultrasmall) size and the required optical properties, is a great challenge and an area that is in its infancy. This minireview provides a concise overview of recent developments on appropriate synthetic methodologies to produce such UCNPs. Particular attention was given to the influence of both surfactants and dopants used to precisely adjust size, crystalline phase and optical properties of UCNPs.

One-pot synthesis of theranostic nanocapsules with lanthanide doped nanoparticles

One-pot synthesis of theranostic nanocapsules with lanthanide doped nanoparticles via interfacial templating condensation for upconversion based photodynamic therapy.

Yb,Nd,Er-doped upconversion nanoparticles: 980 nm versus 808 nm excitation

Yb,Nd,Er-doped upconversion nanoparticles (UCNPs) have attracted considerable interest as luminescent reporters for bioimaging, sensing, energy conversion/shaping, and anticounterfeiting due to their capability to convert multiple near-infrared (NIR) photons into shorter wavelength ultraviolet, visible or NIR luminescence by successive absorption of two or more NIR photons. This enables optical measurements in complex media with very little background and high penetration depths for bioimaging. The use of Nd³⁺ as substitute for the commonly employed sensitizer Yb³⁺ or in combination with Yb³⁺ shifts the excitation wavelength from about 980 nm, where the absorption of water can weaken upconversion luminescence, to about 800 nm, and laser-induced local overheating effects in cells, tissue, and live animal studies can be minimized. To systematically investigate the potential of Nd³⁺ doping, we assessed the performance of a set of similarly sized Yb³⁺,Nd³⁺,Er³⁺-doped core- and core-shell UCNPs of different particle architecture in water at broadly varied excitation power densities (P) with steady state and time-resolved fluorometry for excitation at 980 nm and 808 nm. As a measure for UCNPs performance, the P-dependent upconversion quantum yield (Φ_UC) and its saturation behavior were used as well as particle brightness (B_UC). Based upon spectroscopic measurements at both excitation wavelengths in water and in a lipid phantom and B_UC-based calculations of signal size at different penetration depths, conditions under which excitation at 808 nm is advantageous are derived and parameters for the further optimization of triple-doped UCNPs are given.

Nd3+ doped Gd3Sc2Al3O12 nanoparticles: towards efficient nanoprobes for temperature sensing

Development of contactless temperature-probing nanoplatforms based on thermosensitive near-infrared (NIR) light-emitting nanoparticles opens up new horizons for biomedical theranostics at a deep tissue level. Here, we report on the crystallinity and relative thermal sensitivity of NIR emitting Nd³⁺ doped Gd₃Sc₂Al₃O₁₂ (GSAG:Nd³⁺) nanoparticles synthesized by a solvothermal method. The obtained nanoparticles are well-crystallized, with sizes less than 100 nm, and can be dispersed in water without any additional functionalization. Upon excitation at 806 nm, the nanoparticles exhibit emission in the first and second biological optical transparency windows. The temperature sensing properties were evaluated from the luminescence intensity ratio of the thermally coupled emission lines corresponding to the R₁, R₂→Z₅ transitions between the Stark sublevels of the ⁴F_3/2 and ⁴I_9/2 electronic states of Nd³⁺ in the physiological temperature range of 20-50 °C. GSAG:Nd³⁺ nanoparticles exhibit a maximal relative thermal sensitivity of 0.20% °C^-1, higher than that of YAG:Nd³⁺ nanoparticles used as a control, due to the difference in the crystal field of the host matrices. A higher synthesis temperature in the range of 300-400 °C was also provided to improve the crystallinity of the GSAG:Nd³⁺ nanoparticles which results in a higher relative thermal sensitivity. Our results demonstrate the potential of GSAG:Nd³⁺ nanoparticles as luminescence nanothermometers and emphasize the interest of the GSAG matrix itself, which with the presence of Gd, could lead to multimodal diagnostic applications in nanothermometry and magnetic resonance imaging (MRI).

Multifunctional Self-Assembled Supernanoparticles for Deep-Tissue Bimodal Imaging and Amplified Dual-Mode Heating Treatment

Developing multifunctional therapeutic and diagnostic (theranostic) nanoplatforms is critical for addressing challenging issues associated with cancers. Here, self-assembled supernanoparticles consisting of superparamagnetic Fe₃O₄ nanoparticles and photoluminescent PbS/CdS quantum dots whose emission lies within the second biological window (II-BW) are developed. The proposed self-assembled Fe₃O₄ and PbS/CdS (II-BW) supernanoparticles [SASNs (II-BW)] exhibit outstanding photoluminescence detectable through a tissue as thick as 14 mm, by overcoming severe light extinction and concomitant autofluorescence in II-BW, and significantly enhanced T₂ relaxivity (282 mM^-1 s^-1, ca. 4 times higher than free Fe₃O₄ nanoparticles) due to largely enhanced magnetic field inhomogeneity. On the other hand, SASNs (II-BW) possess the dual capacity to act as both magnetothermal and photothermal agents, overcoming the main drawbacks of each type of heating separately. When SASNs (II-BW) are exposed to the dual-mode (magnetothermal and photothermal) heating, the thermal energy transfer efficiency is amplified 7-fold compared with magnetic heating alone. These results, in hand with the excellent photo- and colloidal stability, and negligible cytotoxicity, demonstrate the potential use of SASNs (II-BW) for deep-tissue bimodal (magnetic resonance and photoluminescence) in vivo imaging, while simultaneously providing the possibility of SASNs (II-BW)-mediated amplified dual-mode heating treatment for cancer therapy.

Impact of Quantum Dot Surface on Complex Formation with Chlorin e₆ and Photodynamic Therapy

Nanomaterials have permeated various fields of scientific research, including that of biomedicine, as alternatives for disease diagnosis and therapy. Among different structures, quantum dots (QDs) have distinctive physico-chemical properties sought after in cancer research and eradication. Within the context of cancer therapy, QDs serve the role of transporters and energy donors to photodynamic therapy (PDT) drugs, extending the applicability and efficiency of classic PDT. In contrast to conventional PDT agents, QDs’ surface can be designed to promote cellular targeting and internalization, while their spectral properties enable better light harvesting and deep-tissue use. Here, we investigate the possibility of complex formation between different amphiphilic coating bearing QDs and photosensitizer chlorin e₆ (Ce₆). We show that complex formation dynamics are dependent on the type of coating—phospholipids or amphiphilic polymers—as well as on the surface charge of QDs. Förster’s resonant energy transfer occurred in every complex studied, confirming the possibility of indirect Ce₆ excitation. Nonetheless, in vitro PDT activity was restricted only to negative charge bearing QD-Ce₆ complexes, correlating with better accumulation in cancer cells. Overall, these findings help to better design such and similar complexes, as gained insights can be straightforwardly translated to other types of nanostructures—expanding the palette of possible therapeutic agents for cancer therapy.

Rapid Synthesis of Sub-10 nm Hexagonal NaYF4-Based Upconverting Nanoparticles using Therminol® 66

We report a simple one-pot method for the rapid preparation of sub-10 nm pure hexagonal (β-phase) NaYF4-based upconverting nanoparticles (UCNPs). Using Therminol® 66 as a co-solvent, monodisperse UCNPs could be obtained in unusually short reaction times. By varying the reaction time and reaction temperature, it was possible to control precisely the particle size and crystalline phase of the UCNPs. The upconversion (UC) luminescence properties of the nanocrystals were tuned by varying the concentrations of the dopants (Nd3+ and Yb3+ sensitizer ions and Er3+ activator ions). The size and phase-purity of the as-synthesized core and core-shell nanocrystals were assessed by using complementary transmission electron microscopy, dynamic light scattering, X-ray diffraction, and small-angle X-ray scattering studies. In-depth photophysical evaluation of the UCNPs was pursued by using steady-state and time-resolved luminescence spectroscopy. An enhancement in the UC intensity was observed if the nanocrystals, doped with optimized concentrations of lanthanide sensitizer/activator ions, were further coated with an inert/active shell. This was attributed to the suppression of surface-related luminescence quenching effects.

Upconverting nanocomposites with combined photothermal and photodynamic effects

Lanthanide-doped upconverting nanoparticles (UCNPs) have been studied for diverse biomedical applications due to their inherent ability to convert near-infrared (NIR) excitation light to higher energies (spanning the ultraviolet, visible, and NIR regions). To explore additional functionalities, rational combination with other optically active nanostructures may lead to the development of new multimodal nanoplatforms with theranostic (therapy and diagnostic) capabilities. Here, we develop a nanocomposite consisting of NaGdF₄:Er³⁺, Yb³⁺ UCNPs, mesoporous silica (SiO₂), gold nanorods (GNRs) and a photosensitizer, with integrated functionalities including luminescence imaging, photothermal generation, nanothermometry and photodynamic effects. Under 980 nm irradiation, GNRs and UCNPs are simultaneously excited due to the overlap between the surface plasmon resonance of the GNRs and the absorption of the UCNPs leading to plasmonic enhancement of the upconverted luminescence, while concomitantly creating a temperature gradient. The temperature increase can be determined from the intensity ratio of the upconverted green emission of the UCNPs. Finally, a photosensitizer, zinc phthalocyanine, was loaded into the mesoporous SiO₂. Upon laser irradiation, the upconverted visible light subsequently activates the photosensitizer to release reactive oxygen species. The multifunctional GNR@SiO₂@UCNPs nanocomposites showed strong luminescence signal when incubated in HeLa cervical cancer cells, making them ideal bioprobes for future theranostic applications.

Neto E, Mazali IO, Murugesu M, Sigoli FA. One pot synthesis and systematic study of the photophysical and magnetic properties and thermal sensing of α and β-phase NaLnF4 and β-phase core@shell nanoparticles

Hierarchical nanostructured nanoparticles showing high values of relative thermal sensitivity and magnetization at low temperatures.

BaCaLu2F10:Ln3+ (Ln = Eu, Dy, Tb, Sm, Yb/Er, Yb/Ho) spheres: ionic liquid-based synthesis and luminescence properties

Alkaline earth metal rare earth fluoride BaCaLu₂F₁₀:Ln³⁺ (Ln = Eu, Dy, Tb, Sm, Yb/Er, Yb/Ho) submicrospheres with uniform morphology and size were synthesized via a facile ionic liquid-based hydrothermal route. The down- and up-conversion luminescence has been investigated.

Cr3+/Er3+ co-doped LaAlO3 perovskite phosphor: a near-infrared persistent luminescence probe covering the first and third biological windows

We have developed a novel persistent phosphor of LaAlO₃ perovskite doped with Er³⁺, Cr³⁺ and Sm³⁺ (LAO:Er–Cr–Sm), which exhibits long persistent luminescence (PersL) at 1553 nm due to the Er³⁺:⁴I_13/2 → ⁴I_15/2 transition as well as at 734 nm due to the Cr³⁺:²E(²G) → ⁴A₂(⁴F) transition.