Near-infrared excited luminescence and in vitro imaging of HeLa cells by using Mn2+ enhanced Tb3+ and Yb3+ cooperative upconversion in NaYF4 nanocrystals

Nanomaterials for light-mediated therapeutics in deep tissue

Light-mediated therapeutics, including photodynamic therapy, photothermal therapy and light-triggered drug delivery, have been widely studied due to their high specificity and effective therapy. However, conventional light-mediated therapies usually depend on the activation of light-sensitive molecules with UV or visible light, which have poor penetration in biological tissues. Over the past decade, efforts have been made to engineer nanosystems that can generate luminescence through excitation with near-infrared (NIR) light, ultrasound or X-ray. Certain nanosystems can even carry out light-mediated therapy through chemiluminescence, eliminating the need for external activation. Compared to UV or visible light, these 4 excitation modes penetrate more deeply into biological tissues, triggering light-mediated therapy in deeper tissues. In this review, we systematically report the design and mechanisms of different luminescent nanosystems excited by the 4 excitation sources, methods to enhance the generated luminescence, and recent applications of such nanosystems in deep tissue light-mediated therapeutics.

Understanding the effect of Mn2+ on Yb3+/Er3+ co-doped NaYF4 upconversion and obtaining the optimal combination of these tridoping

In this work, we investigated in detail the upconversion properties of several types of nanoparticles, including NaYF4:5%Yb3+/30%Mn2+, NaYF4:40%Mn2+/x%Yb3+ (x% = 1, 5, 10, 20, 30, and 40), NaYF4:2%Er3+/x%Mn2+ (x% = 20, 30, 40, 50, 60, and 70), NaYF4:40%Mn2+/x%Er3+ (x% = 1, 2, 5, and 10), and NaYF4:40%Mn2+/1%Yb3+/x%Er3+ (x% = 0, 2, 5, and 10). We studied their upconversion emission under 980 nm excitation in both pulsed and continuous wave modes at different synthesis temperatures. The nanoparticles were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and photoluminescence (PL) spectroscopy. The doping of Yb3+ and Mn2+ ions resulted in the nanoparticles assuming cubic and hexagonal crystal structures. The emission intensity increased (106.4 (a.u.*103) to 334.4(a.u.*103)) with increasing synthesis temperature from 120 to 140 °C, while a sharp decrease was observed when the synthesis temperature was increased to 200 °C. The gradual decrease in peak intensity with increasing Mn2+ concentration from 20 to 70% was attributed to energy transfer from Mn2+ to Yb3+. In NaYF4:Mn2+/Yb3+/Er3+ UCNPs, increasing the Er3+ concentration from 0 to 10% led to the disappearance of the blue, orange, and green emission bands. The intense upconversion luminescence pattern with high spatial resolution indicates excellent potential for applications in displays, biological sensors, photodetectors, and solar energy converters.

Up-conversion emission in transition metal and lanthanide co-doped systems: dimer sensitization revisited

Lanthanide (Ln) co-doped transition metal (TM) upconversion (UC) co-doped systems are being intensively investigated for their exciting applications in photonics, bioimaging, and luminescence thermometry. The presence of TM, such as Mo6 + /W6 +, Mn2 +, or Fe3 + determines significant changes in Ln UC emission, such as intensity enhancement, colour modulation, and even the alteration of the photon order. The current mechanism assumes a ground-state absorption/excited-state absorption (ESA/GSA) in TM-Yb dimer followed by direct energy transfer to Er/Tm excited states. We revisit this mechanism by addressing two issues that remain ignored: a dynamical approach to the investigation of the upconversion mechanism and the intrinsic chemical complexity of co-doped TM, Ln systems. To this aim, we employ a pulsed, excitation variable laser across a complete set of UC measurements, such as the emission and excitation spectra and emission decays and analyze multiple grains with transmission electron microscopy (TEM). In the Mo co-doped garnet, the results sustain the co-existence of Mo-free garnet and Mo oxide impurity. In this Mo oxide, the Er upconversion emission properties are fully explained by a relatively efficient sequential Yb to Er upconversion process, with no contribution from Yb-Mo dimer sensitization.

Poly(N,N-dimethylacrylamide)-coated upconverting NaYF4:Yb,Er@NaYF4:Nd core-shell nanoparticles for fluorescent labeling of carcinoma cells

Upconverting luminescent lanthanide-doped nanoparticles (UCNP) belong to promising new materials that absorb infrared light able to penetrate in the deep tissue level, while emitting photons in the visible or ultraviolet region, which makes them favorable for bioimaging and cell labeling. Here, we have prepared upconverting NaYF4:Yb,Er@NaYF4:Nd core-shell nanoparticles, which were coated with copolymers of N,N-dimethylacrylamide (DMA) and 2-(acryloylamino)-2-methylpropane-1-sulfonic acid (AMPS) or tert-butyl [2-(acryloylamino)ethyl]carbamate (AEC-Boc) with negative or positive charges, respectively. The copolymers were synthesized by a reversible addition-fragmentation chain transfer (RAFT) polymerization, reaching Mn ~ 11 kDa and containing ~ 5 mol% of reactive groups. All copolymers contained bisphosphonate end-groups to be firmly anchored on the surface of NaYF4:Yb,Er@NaYF4:Nd core-shell nanoparticles. To compare properties of polymer coatings, poly(ethylene glycol)-coated and neat UCNP were used as a control. UCNP with various charges were then studied as labels of carcinoma cells, including human hepatocellular carcinoma HepG2, human cervical cancer HeLa, and rat insulinoma INS-1E cells. All the particles proved to be biocompatible (nontoxic); depending on their ξ-potential, the ability to penetrate the cells differed. This ability together with the upconversion luminescence are basic prerequisites for application of particles in photodynamic therapy (PDT) of various tumors, where emission of nanoparticles in visible light range at ~ 650 nm excites photosensitizer.

Photon upconversion through triplet exciton-mediated energy relay

Exploration of upconversion luminescence from lanthanide emitters through energy migration has profound implications for fundamental research and technology development. However, energy migration-mediated upconversion requires stringent experimental conditions, such as high power excitation and special migratory ions in the host lattice, imposing selection constraints on lanthanide emitters. Here we demonstrate photon upconversion of diverse lanthanide emitters by harnessing triplet exciton-mediated energy relay. Compared with gadolinium-based systems, this energy relay is less dependent on excitation power and enhances the emission intensity of Tb3+ by 158-fold. Mechanistic investigations reveal that emission enhancement is attributable to strong coupling between lanthanides and surface molecules, which enables fast triplet generation (<100 ps) and subsequent near-unity triplet transfer efficiency from surface ligands to lanthanides. Moreover, the energy relay approach supports long-distance energy transfer and allows upconversion modulation in microstructures. These findings enhance fundamental understanding of energy transfer at molecule-nanoparticle interfaces and open exciting avenues for developing hybrid, high-performance optical materials.

KLa(0.95-x)GdxF4:Eu3+ hexagonal phase nanoparticles as luminescent probes for in vitro Huh-7 cancer cell imaging

A facile chemical route is reported for synthesizing red-emitting photoluminescent/MRI multi-functional KLa(0.95-x)GdxF4:Eu3+ (x = 0 to 0.4) bio-compatible nanomaterials for targeted in vitro tumor imaging. Hexagonal phase pure nanoparticles show a significant and systematic change in morphology with enhanced photoluminescence due to the substitution of La3+ with Gd3+ ions. Single phase β-KLa(0.95-x)GdxF4:Eu3+ exhibits multifunctional properties, both intense red emission and strong paramagnetism for high-contrast bioimaging applications. These silica capped magnetic/luminescent nanoparticles show long-term colloidal stability, optical transparency in water, strong red emission, and low cytotoxicity. The cellular uptake of coated nanoparticles was investigated in liver cancer cell line Huh-7. Our findings suggest that these nanoparticles can serve as highly luminescent imaging probes for in vitro applications with potential for in vivo and live cell imaging applications.

Nonlinear photoresponse of NaYF4:Yb,Er@NaYF4 nanocrystals under green CW excitation: a comprehensive study

One of the efficient and well-known upconverting nanomaterials is NaYF₄:Yb,Er@NaYF₄, which emits photoluminescence at 545 nm and 660 nm under an excitation of 980 nm. Here, the nonlinearity of β-NaYF₄:Yb,Er@NaYF₄ at 532 nm is investigated using three nonlinear approaches. For the first time, the nonlinear optical conjugation of NaYF₄:Yb,Er@NaYF₄ nanocrystals is observed using the degenerate four-wave mixing method. In the optical bistability study, the optical hysteresis of NaYF₄:Yb,Er@NaYF₄ is measured using the Mach–Zehnder interferometer nonlinear ring cavity, and the results of bistability loops show different behaviors at different power regimes. Finally, the Z-scan technique is used for determining the nonlinear absorption and refraction coefficients, which are calculated in the order of 10⁻⁴ (cm W⁻¹) and 10⁻⁸ (cm² W⁻¹), respectively. The results indicate that by increasing incident powers, optical behaviour changes in both optical bistability and Z-scan. Therefore, the results exhibit that the β-NaYF₄:Yb,Er@NaYF₄ nanocrystals have nonlinear photoresponses at both 980 and 532 nm, which could be promising for photonic devices based on NIR light and visible light.

NaYF₄:Yb,Er@NaYF₄ is an efficient and well-known upconverting nanomaterials at 980 nm, also it has strong optical nonlinearity at 532 nm related to energy states of the Yb/Er system which is determined by a unique approach.