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Upconverting nanoparticles

Upconverting nanoparticles (UCNPs) are nanoscale materials, typically less than 100 nm in diameter, doped with lanthanide ions such as ytterbium (Yb³⁺), erbium (Er³⁺), and thulium (Tm³⁺), that absorb multiple lower-energy near-infrared (NIR) photons and emit higher-energy visible or ultraviolet light through a nonlinear anti-Stokes optical process known as upconversion.^[1]^[2] These particles are most commonly hosted in inorganic matrices like sodium yttrium fluoride (NaYF₄), which provides a low-phonon-energy environment to minimize non-radiative relaxation and enhance emission efficiency.^[1]^[2] The upconversion mechanism primarily relies on energy transfer upconversion (ETU), where Yb³⁺ ions act as sensitizers to absorb NIR light (typically at 980 nm) and sequentially transfer energy to activator ions like Er³⁺ or Tm³⁺, enabling stepwise excitation to higher energy levels for multicolor emissions such as green, red, or blue.^[1]^[2] Other processes, including excited-state absorption (ESA) and cooperative energy transfer, contribute to the overall efficiency, which can exceed 3-4% in optimized core-shell structures, far surpassing traditional organic dyes or quantum dots in photostability.^[1]^[3] This tunability arises from the ladder-like energy levels of lanthanides, allowing precise control over emission wavelengths by varying dopant concentrations and host compositions.^[2] Synthesis of UCNPs employs methods such as thermal decomposition of rare-earth precursors in high-boiling solvents for monodisperse β-phase particles, co-precipitation for rapid production of α-phase nanocrystals requiring post-annealing, and solvothermal/hydrothermal routes for morphology control using ligands like oleic acid or EDTA.^[1] Key properties include sharp emission bands (full width at half maximum <10 nm), large anti-Stokes shifts (>300 nm), high resistance to photobleaching without degradation over extended periods, and minimal cytotoxicity when surface-modified with biocompatible coatings like silica or polymers, enabling deep-tissue penetration up to several millimeters with NIR excitation.^[1]^[2] These attributes stem from the absence of blinking and low autofluorescence interference in biological environments.^[1] UCNPs have emerged as versatile platforms in biomedical applications, including bioimaging for multiplexed labeling of cancer cells with high signal-to-noise ratios, biosensing via luminescence resonance energy transfer (LRET) for detecting biomarkers like telomerase activity, and phototherapy where NIR-triggered upconversion activates photosensitizers for targeted tumor ablation.^[1]^[2] In tissue engineering, they integrate into 3D-printed scaffolds for real-time monitoring of degradation and bone regeneration, while in drug delivery systems, they enable spatiotemporal release of therapeutics under NIR illumination.^[2] Ongoing advancements as of 2025 focus on enhancing quantum yields through core-shell designs, novel doping strategies, and hybrid nanocomposites to broaden their clinical translation.^[2]^[4]

Fundamentals

Definition and basic properties

Upconverting nanoparticles (UCNPs) are nanoscale particles, typically ranging from 1 to 100 nm in diameter, doped with trivalent lanthanide ions such as ytterbium (Yb³⁺) as a sensitizer and erbium (Er³⁺) or thulium (Tm³⁺) as activators, embedded within a crystalline host lattice like sodium yttrium fluoride (NaYF₄).^[5]^[6] These materials enable the conversion of low-energy near-infrared (NIR) photons into higher-energy visible or ultraviolet (UV) emissions through sequential absorption processes, leveraging the ladder-like energy levels of the lanthanide ions.^[5]^[7] Key properties of UCNPs include high photostability, which resists bleaching under prolonged excitation, making them suitable for extended imaging applications.^[5]^[6] They exhibit narrow emission bands, often less than 10 nm in full width at half maximum (FWHM), due to the shielded 4f-4f electronic transitions of lanthanides.^[5]^[7] Additionally, UCNPs display large anti-Stokes shifts exceeding 300 nm, separating excitation and emission wavelengths to minimize background interference, along with low cytotoxicity in biomedical settings and tunable emission colors achieved by varying dopant concentrations.^[5]^[6]^[7] Unlike downconverting materials, which emit lower-energy photons following Stokes-shifted relaxation, UCNPs produce anti-Stokes emission by accumulating energy from multiple NIR photons.^[5]^[7] The host lattice plays a crucial role by providing a low-phonon-energy environment (e.g., ~350 cm⁻¹ in NaYF₄) that suppresses non-radiative decay pathways, thereby enhancing luminescence efficiency.^[5]^[6] A representative example is the core-shell structure NaYF₄:Yb/Er@NaYF₄, where an inert shell passivates surface defects, reducing quenching and boosting upconversion quantum yield to as high as 4%.^[7]

Historical development

The concept of upconversion in lanthanide-doped materials originated in the late 1950s, inspired by advancements in laser physics and the need for efficient infrared photon detection. In 1959, Nicolaas Bloembergen proposed the idea of infrared quantum counters using rare-earth ion-doped solids to convert low-energy photons to higher-energy emissions. This theoretical framework laid the groundwork for practical implementations in the 1960s, when bulk materials exhibited anti-Stokes luminescence. Key early demonstrations included the 1961 observation of two-photon excitation in Eu³⁺-doped CaF₂ by Wolfgang Kaiser and C.G.B. Garrett,^[8] followed by François Auzel's 1966 report of energy transfer upconversion in Yb³⁺-Er³⁺ co-doped phosphate glasses, marking the first efficient infrared-to-visible conversion via sequential photon absorption. Independently, V.V. Ovsyankin and P.P. Feofilov described similar processes in CaF₂:Yb³⁺-Tb³⁺ systems that same year, solidifying rare-earth doping as a cornerstone for upconversion mechanisms.^[9] These bulk material studies, driven by applications in phosphors and lasers, highlighted the photostability and sharp emissions of lanthanide ions but were limited by low efficiency and lack of processability. The transition to nanoscale upconverting materials began in the 1990s, motivated by the nanotechnology boom and interest in size-dependent optical effects, such as enhanced surface-to-volume ratios that could mitigate concentration quenching while enabling colloidal dispersion. Early nanoparticle efforts focused on fluoride hosts for their low phonon energies, with the first report of colloidal upconverting nanoparticles in 2002 by Guangshan Yi and colleagues, who synthesized size-tunable YF₃:Yb³⁺/Er³⁺ nanocrystals via a coprecipitation method, demonstrating visible upconversion under 980 nm excitation. This work shifted focus from bulk to nanomaterials, revealing quantum confinement influences on emission intensity and lifetime. Subsequent milestones included the 2004 synthesis of hexagonal-phase NaYF₄:Yb³⁺/Er³⁺ nanoparticles by Yi's group using EDTA-assisted coprecipitation, which achieved higher efficiency due to the host's favorable crystal structure. In 2006, thermal decomposition methods emerged, with J.W. Stouwdam and F.C.J.M. van Veggel reporting monodisperse NaYF₄:Yb³⁺/Er³⁺ and NaYF₄:Yb³⁺/Tm³⁺ nanocrystals exhibiting multicolor upconversion, paving the way for scalable production. The demonstration of bioimaging potential came in 2008, when D.K. Chatterjee and Y. Zhang applied polyethylenimine-coated NaYF₄:Yb³⁺/Er³⁺ nanoparticles for in vitro cellular labeling, showcasing their low autofluorescence and deep tissue penetration advantages over traditional fluorophores. The 2010s saw rapid evolution through structural innovations, particularly core-shell designs that enhanced brightness by passivating surface defects and reducing quenching. Pioneering work in 2010 by F. Vetrone et al. introduced NaYF₄:Yb³⁺/Er³⁺@NaYF₄ core-shell nanoparticles, boosting upconversion efficiency by over 10-fold via an inert shell that confined lanthanide ions. This architecture, inspired by quantum dot strategies, became widely adopted, with further refinements like active-shell configurations in 2012 by J. Shen et al. incorporating additional dopants in the shell for tunable emissions. These advances were influenced by key figures like Markus Haase, whose group advanced colloidal synthesis, and Paras Prasad, who emphasized biomedical translation. Recent developments up to 2025 have focused on hybrid and multifunctional systems, integrating upconverting nanoparticles with organic components to expand applications. For instance, conjugation with polymers like poly(acrylic acid) or conjugated systems such as polydiacetylene has enabled stimuli-responsive nanocomposites with improved biocompatibility and energy transfer efficiency.^[10] Self-assembly strategies, including electrostatic and hydrophobic interactions, have produced ordered nanocomposites, as reviewed in 2025, where upconverting nanoparticles are assembled into hierarchical structures for enhanced light harvesting and multifunctionality.^[11] These innovations build on the foundational rare-earth doping principles from laser physics, addressing challenges in scalability and integration for emerging fields like theranostics.

Physical Principles

Upconversion mechanisms

Upconversion in nanoparticles, particularly those doped with lanthanide ions, relies on several primary mechanisms that enable the absorption of lower-energy photons (typically in the near-infrared) and emission of higher-energy photons (visible or ultraviolet). These mechanisms include excited-state absorption (ESA), energy transfer upconversion (ETU), and photon avalanche (PA). Among them, ETU is the dominant process in most lanthanide-doped upconverting nanoparticles (UCNPs), owing to its efficiency in systems involving sensitizer-activator pairs.^[12]^[9]^[13] Emerging cooperative effects, such as superfluorescence observed in densely doped UCNPs, can further amplify emission efficiency through collective quantum phenomena.^[14] Excited-state absorption (ESA) occurs when a single ion sequentially absorbs multiple photons, first populating an intermediate excited state and then absorbing a second photon to reach a higher emitting state. This process requires precise matching of excitation energy to the intermediate level and is less common in practical UCNPs due to its dependence on high excitation intensities and limited cross-relaxation pathways. In contrast, photon avalanche (PA) involves a nonlinear feedback loop: initial weak ground-state absorption leads to cross-relaxation that populates an intermediate state, enabling resonant excited-state absorption and avalanche-like amplification of the excited population, often with a sharp intensity threshold. PA is rare in nanoparticles but notable for its high nonlinearity in specific dopants like Tm³⁺.^[12]^[13] Energy transfer upconversion (ETU), first elucidated by Auzel in 1966, predominates in lanthanide UCNPs and features ladder-like stepwise energy transfer between a sensitizer ion (typically Yb³⁺) and an activator ion (e.g., Er³⁺ or Tm³⁺). The Yb³⁺ sensitizer, with its simple two-level structure (²F₇/₂ ground state and ²F₅/₂ excited state), efficiently absorbs near-infrared photons around 980 nm due to its large absorption cross-section. It then transfers this energy non-radiatively to the activator via dipole-dipole interactions, progressively populating higher energy levels in a sequential manner. For instance, in Yb³⁺-Er³⁺ systems, the first transfer excites Er³⁺ from its ground state ⁴I₁₅/₂ to the intermediate ⁴I₁₁/₂ level, followed by a second transfer to ⁴F₇/₂ or higher states, enabling green or red emissions upon relaxation. Energy mismatches in these transfers are bridged by phonon interactions with the host lattice, which provides a low-phonon environment to minimize non-radiative losses.^[9]^[12]^[13] The dynamics of these processes are modeled using rate equations that describe the time evolution of population densities N_i in each energy level i. A simplified rate equation for the population of an intermediate excited state (level 2) in an ETU process, incorporating absorption, stimulated emission, spontaneous decay, and energy transfer, is given by:

\frac{dN_2}{dt} = W_{12} N_1 - (W_{21} + A_{21}) N_2 + \beta N_1 N_0

where N_1 and N_0 are the populations of the lower excited and ground states, respectively; W_{ij} are absorption/emission rates between levels i and j; A_{21} is the spontaneous emission rate from level 2 to 1; and \beta is the energy transfer coefficient quantifying the ETU interaction strength. In steady-state conditions (dN_2/dt = 0), these equations reveal power-dependent behaviors, such as quadratic or higher-order scaling of emission intensity with excitation power. Multilevel extensions account for the full lanthanide energy ladder, including cross-relaxation terms.^[15]^[16] Efficiency of upconversion is influenced by dopant ratios and nanoparticle size. Optimal Yb³⁺ concentrations of 20-30 mol% balance sensitization with avoidance of concentration quenching via back energy transfer, maximizing ETU rates in hosts like NaYF₄. Below 50 nm, surface quenching becomes prominent due to increased surface-to-volume ratio, enhancing non-radiative decay through vibrational energy transfer to surrounding ligands or solvent molecules, which reduces overall quantum yield. Core-shell architectures mitigate this by passivating surfaces.^[17]^[18]^[13]

Optical properties and characterization

Upconverting nanoparticles (UCNPs) exhibit characteristic emission spectra determined by the dopant ions, typically lanthanides such as Er³⁺ and Tm³⁺, under near-infrared (NIR) excitation. For Er³⁺-doped UCNPs, prominent green emission occurs at approximately 540 nm, corresponding to the ⁴S₃/₂ → ⁴I₁₅/₂ transition, while red emission is observed at around 660 nm from the ⁴F₉/₂ → ⁴I₁₅/₂ transition.^[5]^[19] In Tm³⁺-doped systems, blue emission peaks at about 475 nm, arising from the ¹G₄ → ³H₆ transition.^[5]^[20] These emissions are commonly excited using a 980 nm diode laser, which targets the Yb³⁺ sensitizer to facilitate energy transfer to the activators.^[5] The upconversion luminescence intensity follows a nonlinear power dependence, often with a slope of 2–3 in log-log plots, indicative of two- or three-photon processes dominating under typical excitation conditions.^[5]^[21] Quantum yields for these NIR-to-visible conversions are generally low, ranging from 0.3% to 4% depending on particle size, doping concentration, and host matrix, with optimized core-shell structures achieving up to 3.2% for red emission.^[5]^[22] Luminescence lifetimes in UCNPs, stemming from the forbidden 4f–4f transitions of lanthanide ions, typically span the microsecond regime, such as 65–100 μs for Er³⁺ emissions in passivated nanoparticles.^[18]^[13] These long lifetimes enable time-gated imaging to reduce autofluorescence background. Optical properties are influenced by surface effects, where quenching due to high surface-to-volume ratios in small nanoparticles (<10 nm) reduces efficiency; this is effectively mitigated by inert core-shell designs, such as NaYF₄@NaYF₄, which can enhance emission intensity by factors of 7–30 and extend lifetimes closer to bulk values.^[23]^[5] Additionally, the ratio of green (⁴S₃/₂) to red (⁴F₉/₂) emission intensities in Er³⁺-doped UCNPs varies with temperature, providing a basis for ratiometric sensing with sensitivities up to 1–2% per Kelvin over physiological ranges.^[24]^[25] Characterization of UCNP optical properties relies on steady-state and transient spectroscopy to measure emission spectra, power dependencies, quantum yields, and lifetimes. Steady-state photoluminescence spectroscopy captures emission profiles and intensities under continuous NIR excitation, while time-resolved (transient) spectroscopy resolves decay kinetics in the microsecond domain using techniques like time-correlated single-photon counting.^[5]^[26] Confocal microscopy enables single-particle studies, revealing heterogeneity in brightness and enabling correlation of optical output with particle position or environment.^[27] Complementary structural analyses via transmission electron microscopy (TEM) or scanning electron microscopy (SEM) link size distributions (typically 10–50 nm) to brightness variations, as smaller particles suffer more from surface quenching.^[5]^[26] These methods collectively ensure precise evaluation of UCNP performance for applications requiring stable, tissue-penetrating luminescence.

Chemical Aspects

Synthesis methods

Upconverting nanoparticles (UCNPs), typically lanthanide-doped fluoride hosts such as NaYF₄, are synthesized through various chemical routes that control particle size, phase, and dopant incorporation to optimize luminescence efficiency. Common methods include co-precipitation and thermal decomposition, which enable the production of particles ranging from 5 to 50 nm by adjusting precursor ratios and reaction conditions. These approaches prioritize the incorporation of sensitizers like Yb³⁺ (typically 20 mol%) and activators like Er³⁺ (2 mol%) into low-phonon-energy hosts such as NaYF₄ or NaGdF₄ for dual optical-MRI functionality.^[1] Co-precipitation involves mixing rare-earth salts (e.g., RECl₃) with sodium precursors like NaF in aqueous media, followed by pH adjustment (often to 4-6 with NH₄F) and heating at 50-80°C for 1-2 hours, typically forming α-phase particles that require post-annealing (e.g., at 300°C) to obtain the more efficient β-phase. This method is cost-effective and scalable, yielding particles with good crystallinity but limited monodispersity (typically 20-100 nm), as demonstrated in early syntheses of NaYF₄:Yb,Er UCNPs.^[1] Thermal decomposition, a widely adopted technique for monodisperse UCNPs, entails injecting lanthanide trifluoroacetate precursors into high-boiling solvents like 1-octadecene with oleic acid at 300-320°C under inert atmosphere, allowing precise size control (5-50 nm) via precursor concentration and reaction time (1-2 hours). Pioneered for NaYF₄:Yb,Er nanoparticles, this approach produces highly uniform hexagonal-phase particles but requires organic solvents, posing scalability challenges due to high temperatures and purification needs.^[28] Advanced techniques address limitations in speed, aqueous compatibility, and structural complexity. Hydrothermal and solvothermal syntheses use autoclaves to react precursors (e.g., RE(NO₃)₃ and NaF) in water or organic solvents like ethanol/octadecene at 180-250°C for 12-24 hours, with additives such as oleic acid or CTAB enabling morphology control (e.g., rods or spheres) and phase purity toward the desirable hexagonal β-NaYF₄ over cubic α-phase. Microwave-assisted methods accelerate these processes, achieving NaYF₄:Yb,Er UCNPs in minutes via rapid heating of rare-earth oleates in oleic acid, improving yield and uniformity while reducing energy use compared to conventional heating. Self-assembly strategies facilitate core-shell or alloyed structures, such as successive thermal decomposition to coat NaYF₄:Yb,Er cores with inert NaYF₄ shells (2-5 nm thick) for reduced surface quenching, or template-directed assembly for multifunctional alloys like NaGdF₄ hosts.^[29]^[30]^[28] Key challenges in UCNP synthesis include achieving high yields (>80%) at scale, precise control of the hexagonal phase (which enhances upconversion by 10-100 times over cubic), and uniform dopant distribution to avoid concentration quenching at levels above 4 mol% for Er³⁺. Recent advances up to 2025 emphasize green solvents and hybrid methods to improve biocompatibility and production efficiency for biomedical hosts like NaGdF₄. For example, solvothermal synthesis of lanthanide-doped CaF₂ UCNPs has been reported for alternative hosts with improved stability.^[7]^[28]^[29]^[31]

Surface modification and functionalization

Surface modification of upconverting nanoparticles (UCNPs) is essential to transition them from hydrophobic organic dispersions, typically achieved during synthesis with ligands like oleic acid, to hydrophilic environments suitable for biomedical and technological applications. This post-synthetic process enhances colloidal stability, biocompatibility, and functionality without altering the core upconversion properties. Common strategies include ligand exchange, inorganic coatings, and polymer encapsulation, each tailored to introduce specific functional groups for further conjugation.^[32]^[5] Ligand exchange replaces native hydrophobic oleate ligands with hydrophilic alternatives, such as polyethylene glycol (PEG) derivatives like PEG600 diacid or PEG-thiol (PEG-SH), to impart water solubility. For instance, treatment of oleate-capped NaYF₄:Yb,Er UCNPs with PEG diacid results in stable aqueous dispersions lasting at least two weeks, with preserved upconversion luminescence. Silica coating, often via the Stöber method using tetraethyl orthosilicate (TEOS), forms a biocompatible shell (1-10 nm thick) that shields the UCNP core, minimizes quenching, and provides silanol groups for additional functionalization. Post-synthetic surface doping through ion exchange, such as replacing Na⁺ with RE³⁺ ions, enriches the surface with active lanthanide sites, increasing ligand density from approximately 3.6 to 8.8 molecules/nm² and boosting emission efficiency.^[1]^[32]^[5]^[33] Bilayer formation employs amphiphilic polymers, such as 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-PEG (DSPE-PEG), to encapsulate UCNPs via hydrophobic interactions between alkyl chains and the native ligands, while the PEG corona ensures aqueous stability. This micelle-like structure allows control over ligand density, typically 0.3-7 molecules/nm² depending on polymer chain length and grafting conditions, reducing non-specific protein adsorption. Recent advances include click chemistry, particularly copper-free azide-alkyne cycloaddition, for site-specific conjugation of biomolecules like antibodies directly onto alkyne- or azide-functionalized UCNPs, enabling precise targeting with minimal loss in optical performance. Additionally, self-assembled monolayers incorporating conjugated polymers on UCNP surfaces have been explored to enhance Förster resonance energy transfer (FRET) efficiency to nearby acceptors, as demonstrated in hybrid systems where Tm-doped UCNPs couple with polymer films for super-resolved imaging.^[32]^[34]^[35]^[36] These modifications yield improved outcomes, including zeta potentials of -30 to -50 mV for enhanced electrostatic repulsion and long-term stability in physiological buffers, reduced cytotoxicity through PEG shielding, and facilitated targeted delivery via antibody attachment, as evidenced by increased cellular uptake in vitro without compromising upconversion brightness.^[32]^[1]

Biomedical Applications

Bioimaging and biosensing

Upconverting nanoparticles (UCNPs) have emerged as powerful tools for bioimaging due to their ability to convert near-infrared (NIR) excitation into visible or near-UV emissions, enabling deep-tissue penetration with minimal photodamage. NIR light, typically at 980 nm, scatters and absorbs less in biological tissues compared to visible wavelengths, allowing imaging depths of up to several millimeters to 1 cm in vivo, which surpasses the limitations of traditional fluorescent probes excited by shorter wavelengths.^[37]^[38] This property is particularly advantageous for non-invasive visualization of internal structures, such as tumors, where UCNPs can be conjugated with targeting ligands to accumulate selectively at disease sites.^[39] A key benefit of UCNPs in bioimaging is their sharp emission spectra and long luminescence lifetimes, which eliminate autofluorescence from biological samples and prevent photoblinking, unlike organic dyes and quantum dots that suffer from broad backgrounds and instability under prolonged excitation.^[40]^[5] Multiplexed labeling is facilitated by doping UCNPs with combinations of lanthanide ions, such as Yb³⁺/Er³⁺ for green emission and Yb³⁺/Tm³⁺ for blue, enabling simultaneous detection of multiple targets with a single NIR source and distinguishing signals via spectral separation, akin to RGB coding.^[41]^[42] For instance, in 2020 studies, tumor-targeted UCNPs demonstrated high-contrast in vivo imaging of subcutaneous and orthotopic tumors in mice, revealing tumor margins with resolutions suitable for clinical translation.^[43]^[44] In biosensing, UCNPs enable ratiometric measurements by exploiting temperature-dependent shifts in emission intensity ratios from dual-emissive ions, achieving sensitivities of 1-2% per Kelvin in physiological ranges through analysis of green-to-red ratios in Er³⁺-doped systems.^[45]^[46] For pH and biomarker detection, Förster resonance energy transfer (FRET) mechanisms integrate UCNPs as donors with quenchers or acceptors, where analyte binding modulates energy transfer efficiency; for example, pH-sensitive probes using phenol red conjugates detect intracellular acidity changes with sub-unit pH resolution.^[47]^[48] Biomarker sensing via FRET has been applied in lateral flow assays, where UCNPs enhance sensitivity for rapid diagnostics, such as detecting SARS-CoV-2 antigens at picogram-per-milliliter levels in adapted COVID-19 tests, offering point-of-care advantages over enzyme-linked methods.^[49]^[50]

Drug delivery and phototherapy

Upconverting nanoparticles (UCNPs) facilitate controlled drug delivery by responding to physiological stimuli like pH changes in tumor microenvironments and external near-infrared (NIR) light, often through integration with mesoporous carriers such as silica or metal-organic frameworks. In one representative system, core-shell UCNP@ZIF-8 nanocomposites loaded with the anticancer drug doxorubicin release approximately 37% of the payload at neutral pH (7.4) over 24 hours, but up to 90% under acidic conditions (pH 5.0) combined with 980 nm NIR irradiation at 0.8 W cm⁻² for 5 minutes, enabling selective release in acidic tumor sites.^[51] This NIR responsiveness stems from the upconversion process, where absorbed NIR photons generate heat or energy to trigger structural changes in the carrier, promoting drug diffusion. Targeting specificity is further improved via surface modification, such as folate conjugation, which binds to folate receptor-α overexpressed on cancer cells like ovarian carcinoma lines. Folic acid-functionalized UCNPs coated with mesoporous silica have demonstrated efficient accumulation in folate receptor-positive tumor clusters while crossing endothelial barriers in bilayer models, with no significant cytotoxicity observed.^[52] In photodynamic therapy (PDT), UCNPs serve as energy transducers by converting NIR light (e.g., 980 nm) into visible emissions that activate photosensitizers (PS), such as rose bengal, whose absorption peak aligns with the upconverted green light at approximately 540 nm. This conjugation, often via covalent bonding to NaYF₄:Yb,Er UCNPs, enhances energy transfer efficiency through fluorescence resonance energy transfer, leading to PS excitation and subsequent generation of reactive oxygen species (ROS) like singlet oxygen for targeted cancer cell destruction.^[53] The ROS production mechanism relies on the proximity of PS molecules (e.g., rose bengal loaded into a mesoporous silica shell) to the UCNP core, where upconverted energy directly fuels photochemical reactions, as shown in systems achieving elevated ROS levels under brief NIR exposure (980 nm, 5 minutes) compared to non-coated UCNPs.^[54] Complementing PDT, photothermal therapy (PTT) leverages non-radiative decay in UCNPs, where excited lanthanide ions (e.g., Yb³⁺ and Er³⁺) dissipate energy as heat via multiphonon relaxation and lattice vibrations, raising local temperatures to 50–60°C for inducing apoptosis in tumors without radiative emission.^[55] Recent advancements as of 2025 position rare-earth-doped UCNPs as versatile anti-Stokes nanoprobes for integrated phototherapy, emphasizing improved biocompatibility, deeper tissue penetration, and multimodal combinations like PDT-PTT hybrids to overcome hypoxia and resistance in solid tumors.^[56] Preclinical evaluations from 2023 to 2025 underscore their therapeutic potential; for example, synthetic carbon-based lanthanide UCNPs in photothermal setups administered intravenously (2 mg kg⁻¹) followed by 980 nm laser irradiation (280 mW cm⁻², 10 minutes) resulted in over 90% tumor volume reduction in subcutaneous melanoma mouse models, with complete tumor disappearance in some animals after 21 days.^[57] These outcomes highlight UCNPs' role in minimizing off-target effects while achieving high efficacy in vivo.

Advanced optical techniques

Upconverting nanoparticles (UCNPs) have enabled super-resolution imaging techniques that surpass the diffraction limit of conventional optical microscopy, particularly through stimulated emission depletion (STED) microscopy. In STED, a doughnut-shaped depletion beam suppresses fluorescence emission around the excitation focus, achieving resolutions below 50 nm using NIR wavelengths for both excitation and depletion to minimize tissue damage. For instance, lanthanide-doped NaYF₄ UCNPs, such as those with Yb³⁺ and Tm³⁺ ions, have demonstrated single-particle resolutions of 28 nm (λ/36) and sub-50 nm in deep-tissue imaging up to 93 μm, leveraging 808 nm or 1140 nm depletion lasers with up to 30% efficiency.^[58] Additionally, reversible photoswitching in Tm³⁺-doped NaYF₄ UCNPs allows controlled on/off emission via modulated 810 nm depletion, enabling ~66 nm lateral resolution in STED nanoscopy for dynamic labeling without photobleaching.^[59] UCNPs have also facilitated upconversion lasing, where embedded nanocavities enhance gain for low-threshold visible emission from NIR pumping. Nanocrystal-in-glass microcavities incorporating Yb³⁺/Er³⁺-doped KY₃F₁₀ UCNPs achieve full-color (red, green, blue) lasing with thresholds as low as 83.3 μW under continuous-wave 980 nm excitation, offering slope efficiencies over four times higher than non-cavity systems.^[60] These devices produce pure visible outputs tunable via dopant ratios, with reported lasing in the 2020s highlighting their potential for compact biomedical photonics. In optogenetics, UCNP-opsin hybrids enable noninvasive NIR-triggered neural control by converting 980 nm light to 450 nm blue emission for Channelrhodopsin-2 (ChR2) activation. Yb³⁺/Tm³⁺-doped NaYF₄ UCNPs coated with silica, achieving ~2.5% upconversion efficiency, generate sufficient local blue light (0.34 mW/mm² at 4.5 mm depth) to evoke dopamine release in mouse ventral tegmental area neurons via transcranial NIR pulses.^[61] This approach supports deep-brain applications, such as seizure suppression in the hippocampus, without genetic modification beyond opsin expression.^[61] Thulium doping in UCNPs extends upconversion mechanisms to mid-IR and telecom-relevant wavelengths, enabling detection by converting longer NIR-II excitations to visible or shorter NIR emissions. Tm³⁺-doped NaYF₄ UCNPs excited at 1064–1208 nm produce strong 475 nm (three-photon) or 808 nm emissions, with 1150 nm yielding up to 100-fold brighter blue output than 1064 nm, facilitating high-resolution imaging of cancer cells with low phototoxicity.^[62] This doping strategy supports mid-IR sensing applications by bridging to telecom bands (e.g., ~1550 nm compatibility through NIR-II tailoring), enhancing compatibility with silicon-based detectors.^[63]

Technological Applications

Energy harvesting and photovoltaics

Upconverting nanoparticles (UCNPs) have emerged as a promising material for enhancing photovoltaic (PV) performance by addressing the spectral mismatch between the solar irradiance and the absorption range of conventional silicon solar cells, which typically utilize only about 20-30% of the incident sunlight due to transmission of infrared (IR) photons below the bandgap. By absorbing low-energy IR photons (typically in the 980-1550 nm range) and emitting higher-energy visible or near-infrared light through nonlinear multiphoton processes, UCNPs enable the conversion of otherwise unused sub-bandgap radiation into utilizable wavelengths, potentially increasing overall cell efficiency.^[64] This spectrum modification is particularly effective in rear-side configurations, where UCNPs are integrated as films or layers behind the PV absorber to recapture transmitted IR light without shading the primary absorption area.^[65] A common implementation involves lanthanide-doped UCNPs, such as β-NaYF₄ co-doped with Yb³⁺ as sensitizers and Er³⁺ as activators, which exhibit strong upconversion from 980 nm excitation to green emission around 540 nm, aligning well with silicon's bandgap. These nanoparticles are often embedded in polymer matrices or deposited as thin films on the back reflector of crystalline silicon cells, facilitating energy transfer to the PV material and yielding relative efficiency gains of 2-5% in proof-of-concept tandem designs under simulated solar conditions.^[66] Recent integrations, such as UCNP-sensitized perovskite solar cells, have demonstrated absolute efficiency improvements up to 0.87% for silicon devices and enabled near-infrared harvesting in CsPbI₃ perovskites, pushing overall efficiencies beyond 25% in hybrid systems.^[67]^[68] For instance, core-shell UCNPs with multiband absorption have been layered onto PV back contacts, enhancing short-circuit current density by redirecting IR flux while minimizing thermal losses.^[69] Beyond direct PV enhancement, UCNPs facilitate energy harvesting in wireless sensor networks by leveraging their upconverted luminescence to power low-energy photodetectors or optical transducers under ambient IR illumination, such as from sunlight or waste heat sources. This approach enables self-sustaining operation in remote sensors, where the emitted visible light drives photovoltaic micropanels integrated with the UCNP layer, achieving power densities sufficient for intermittent data transmission without batteries.^[70] Despite these advances, practical deployment faces significant challenges, including low upconversion quantum yields (typically 0.1-4% under non-resonant solar flux), which limit the harvestable energy and require high nanoparticle concentrations that can introduce optical scattering or quenching.^[71] Scalability remains a barrier, as uniform synthesis and integration of UCNPs into large-area PV modules demand cost-effective methods like roll-to-roll processing, while long-term stability under outdoor conditions—against photodegradation and environmental exposure—must be improved for commercial viability.^[72] Ongoing research focuses on core-shell architectures and dye-sensitization to boost quantum efficiency, aiming to bridge the gap toward industrially relevant applications.^[73]

Security and anticounterfeiting

Upconverting nanoparticles (UCNPs) are increasingly utilized in security and anticounterfeiting due to their capacity for near-infrared (NIR)-to-visible light conversion, which enables the creation of invisible features under ambient conditions that become apparent only under specific NIR illumination. These materials, often lanthanide-doped such as with Yb³⁺ and Er³⁺ ions, produce sharp, narrow emission bands in the visible spectrum, making them challenging to duplicate with standard fluorescent dyes or pigments that exhibit broader, Stokes-shifted emissions.^[74]^[74] The primary mechanisms involve anti-Stokes luminescence through processes like excited-state absorption (ESA) and energy transfer upconversion (ETU), where NIR excitation at wavelengths like 980 nm triggers visible emissions, such as green (around 525–555 nm) or red (around 650–680 nm) from Er³⁺-doped UCNPs embedded in inks or polymers. Multi-level authentication is facilitated by modulating emission color via dopant concentrations, intensity through excitation power density, and lifetime for temporal decoding, with reported lifetimes ranging from 111 to 635 μs in core-shell structures that reduce non-radiative decay. This allows for complex, covert patterns, including QR-code-like designs, detectable solely with NIR sources and analyzers, providing layered verification beyond simple visual inspection.^[74]^[75]^[76] Practical examples include Er/Yb-codoped Y₃Al₅O₁₂ nanoparticles incorporated into flexible, water-resistant cellulose papers for document authentication, where 980 nm excitation reveals green luminescence to verify envelopes or labels. In currency and banknotes, UCNP-embedded polymers, such as Er-doped NaHoF₄ variants, form covert green marks integrated into substrates, enhancing anti-forgery measures in high-value items. A 2025 review emphasizes dynamic anticounterfeiting in banknotes using tunable UCNPs with orthogonal excitation-emission profiles for real-time verification.^[75]^[76]^[76] Key advantages arise from the materials' photochemical stability, resistance to photobleaching, and the precision of their narrow emission lines (typically <10 nm full width at half maximum), which resist replication by counterfeiters lacking access to lanthanide synthesis expertise or NIR spectroscopy tools. Integration into everyday substrates like polymers or varnishes further conceals features while maintaining durability under environmental stresses like humidity.^[74]^[74] Recent advances feature hybrid UCNP-quantum dot (QD) systems for enhanced dual-mode security, such as UCNP-perovskite QD nanocomposites that exhibit temperature-dependent emission colors under single NIR excitation, enabling verification via both optical and thermal responses. For instance, combinations of NaYF₄ UCNPs with CsPbX₃ QDs produce multicolor outputs adjustable by ratio, ideal for secure labels with anti-tampering alerts. Core-multi-shell architectures, like NaGdF₄:Yb³⁺@NaHoF₄@NaGdF₄:Yb³⁺, improve upconversion efficiency by over 10-fold compared to core-only designs, supporting tunable, dynamic patterns in inks for advanced anticounterfeiting.^[77]^[78]^[76]

Emerging multifunctional uses

UCNP-conjugated polymers have been developed for flexible displays, where the upconversion process enhances color rendering and energy efficiency in bendable optoelectronic devices. Beyond biomedical realms, UCNPs enable mid-infrared (mid-IR) sensing critical for telecommunications, by upconverting mid-IR signals to visible wavelengths for detection with standard silicon photodetectors. Integration of UCNPs into metasurfaces has achieved high-efficiency IR-to-visible conversion, supporting applications in fiber-optic networks with improved sensitivity.^[79] In environmental monitoring, UCNPs facilitate pollutant detection through upconversion Förster resonance energy transfer (FRET), where analyte binding quenches or modulates emission for selective identification of contaminants like permanganate and dichromate ions in water.^[80] These label-free nanosensors offer rapid, autofluorescence-free analysis with detection limits suitable for real-time water safety assessment.^[80] Looking ahead, emerging opportunities include their potential in quantum technologies, where upconversion mechanisms could enhance quantum sensing and information processing by enabling low-photon-number detection in entangled systems.^[81]

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