Polysulfonate Cappings on Upconversion Nanoparticles Prevent Their Disintegration in Water and Provide Superior Stability in a Highly Acidic Medium

Lanthanide-Doped Upconversion Nanoparticles for Single-Particle Imaging

Lanthanide-doped upconversion nanoparticles (UCNPs) have recently demonstrated great promise in single-particle imaging (SPI) due to their exceptional photostability and minimal background fluorescence. However, their limited brightness has posed a significant barrier to wider adoption in SPI applications. This review highlights recent advances in applying UCNPs for SPI, focusing on strategies to enhance their brightness and reduce quenching effects in aqueous environments. Additionally, it summarizes the latest progress in using UCNPs for single-particle tracking and super-resolution imaging, underscoring their potential in biomedical research. Finally, the review outlines current challenges and future directions in this field.

Surface-engineered core-shell upconversion nanoparticles for effective hypericin delivery and multimodal imaging

Early diagnosis and treatment of cancer is rapidly advancing thanks to the development of nanotechnology. Here, upconversion nanoparticles (UCNPs) are particularly promising as they are finding a wide range of applications in drug delivery and tumor imaging. In this report, a novel UCNP-based transport system is proposed for the delivery of the hypericin (Hyp) photosensitizer into malignant tumors. Core-shell NaYF4:Yb3+,Er3+@NaYF4:Nd3+ UCNPs were prepared by thermal decomposition and coated with poly(N,N-dimethylacrylamide-co-2-aminoethyl acrylate)-alendronate [P(DMA-AEA)-Ale], which endowed them with colloidal and chemical stability; finally, Hyp was conjugated. Internalization of CS-UCNP@P(DMA-AEA)-Ale-Hyp nanoparticles by Jurkat cells was successfully validated by multimodal imaging using a microstructural chamber, upconversion luminescence, and Raman microspectroscopy. After irradiation at 590 nm, CS-UCNP@P(DMA-AEA)-Ale-Hyp nanoparticles provided a markedly more effective photodynamic effect than Hyp alone at identical Hyp concentrations due to apoptosis as confirmed by caspase-3 activation. MTT assays showed that Hyp-free nanoparticles were non-cytotoxic, whereas CS-UCNP@P(DMA-AEA)-Ale-Hyp particles significantly reduced cell viability after irradiation. Considering that Hyp release from the nanoparticles was higher in the acidic environment typical of tumors compared to physiological ones, UCNP@P(DMA-AEA)-Ale-Hyp particles are a suitable candidate for future in vivo applications.

Molecular Upconversion Nanoparticles for Live-Cell Imaging

Precise molecular control has become a highly attractive feature to develop the next generation of upconversion materials for autofluorescence-free deep tissue imaging. However, in aqueous environments, upconversion molecules are orders of magnitude dimmer than inorganic upconversion nanoparticles, thereby strongly limiting their applicability to bioimaging. By encapsulating ca. 1,900 upconversion molecules into sub-40 nm polymer nanoparticles, we show that molecular precision and nanomaterial brightness can be combined into a new type of hybrid nanomaterial. The brightness of these molecular upconversion nanoparticles (UCMol-NPs) is almost on par with widely used inorganic upconversion nanoparticles, permitting the experimental demonstration of live-cell imaging with UCMol-NPs, an important step toward advancing molecular upconversion into the application era. Fabrication, characterization, and modeling of UCMol-NPs with various sizes and loadings reveal that significant brightness enhancement is possible. This will be paramount for advancing upconversion beyond the current limits of inorganic nanoparticles and translating them into clinical applications.

Near-infrared DNA biosensors based on polysulfonate coatings for the sensitive detection of microRNAs

MicroRNAs (miRNAs) play crucial roles in the regulation of immune cell differentiation and the immune response during allergic rhinitis (AR). Studies have shown that miRNA-155 is significantly upregulated in AR pathogenesis. Therefore, miRNA-155 can be used as a biomarker for AR diagnosis. Although fluorescent biosensors based on upconversion nanoparticles (UCNPs) have made significant advances in the detection of miRNAs, developing UCNPs with polymer coatings, efficient surface passivation, and DNA functionalization for hybrid sensing in biological media remains challenging. Herein, hairpin DNA1 (H1) is modified into a thin polysulfonic acid layer on UCNPs by sulfonamide bonds, and the fluorescence of the UCNPs is quenched by the fluorescence resonance energy transfer (FRET) process of BHQ3 carried by H1. When the target miRNA-155 is present, the hairpin structure of H1 is opened, allowing BHQ3 to move away from the UCNP surface, and the fluorescence of UCNP is restored. At the same time, hairpin DNA1 (H2) can combine with H1 to replace the miRNA-155 that is bound to H1 with the help of the opening stem ring structure of H1, and the replaced miRNA-155 can continue to react with H1 to amplify the fluorescence signal. Under the optimal experimental conditions, the linear range of miRNA-155 is 0.01-3 nM, with a detection limit of 1.14 pM. Furthermore, the constructed biosensor has been applied to determine miRNA-155 in serum samples, and the spiked recoveries range from 99.8% to 104.8%, which indicates that the developed assay has potential applications in monitoring allergic rhinitis or other miRNA related diseases.

Impact of surface chemistry of upconversion nanoparticles on time-dependent cytotoxicity in non-cancerous epithelial cells

The application of upconversion nanoparticles (UCNPs) for cell and tissue analysis requires a comprehensive understanding of their interactions with biological entities to prevent toxicity or harmful effects. Whereas most studies focus on cancer cells, this work addresses non-cancerous cells with their regular in vitro physiology. Since it is generally accepted that surface chemistry largely determines biocompatibility in general and uptake of nanomaterials in particular, two bilayer surface coatings with different surface shielding properties have been studied: (i) a phospholipid bilayer membrane (PLM) and (ii) an amphiphilic polymer (AP). Both surface modifications are applied to (12-33) nm core-shell UCNPs NaYF4(Yb, Er)@NaYF4, ensuring colloidal stability in biological media. The impact of UCNPs@AP and UCNPs@PLM on non-cancerous epithelial-like kidney cells in vitro was found to differ significantly. UCNPs@PLM did not exhibit any measurable effect on cell physiology, even with prolonged exposure. In contrast, UCNPs@AP caused changes in cell morphology and induced cell-death after approximately 30 h. These variations in toxicity are attributed to the distinct chemical stability of these particles, which likely influences their intracellular disintegration.

Nucleic acid-functionalized upconversion luminescence biosensor based on strand displacement-mediated signal amplification for the detection of trivalent chromium ions

BACKGROUND

Rapid industrial development has generated serious pollution, including the presence of toxic and harmful heavy metal ions. Among them, trivalent chromium ion (Cr3+) is a very important element that poses a threat to life and health in our industrial wastewater pollution. Thus, it is important to develop efficient fluorescence methods for Cr3+ detection. In this study, an upconversion luminescence biosensor for detecting Cr3+ was constructed based on a DNAzyme, strand displacement reaction (SDR), and DNA-functionalized upconversion nanoparticles (UCNPs).

RESULTS

The sulfonate-rich poly (sodium 4-styrene sulfonate) (PSS) was modified onto the surface of UCNPs, forming UCNPs@PSS. Then, NH2-Capture probe DNA (NH2-Cp) was further modified onto the UCNPs@PSS surface through sulfonylation, resulting in UCNPs@PSS@NH2-Cp. The DNAzyme activated by Cr3+ triggered the release of the primer probe (Pp), which initiated the SDR system cycle, thereby releasing a tetramethylrhodamine (TAMRA)-modified signal probe (TAMRA-Sp). Finally, UCNPs@PSS@NH2-Cp bound to TAMRA-Sp through complementary base pairing, causing UCNPs and TAMRA to approach each other. Because of the luminescence resonance energy transfer (LRET) mechanism, the upconversion luminescence (UCL) signal of the UCNPs was quenched by TAMRA, enabling the detection of Cr3+ by the change of I585/I545 ratio. This biosensor has good stability, selectivity, and sensitivity, with a linear range of 0.5-75 nM and a detection limit of 0.135 nM for Cr3+.

SIGNIFICANCE AND NOVELTY

Firstly, based on LRET between UCNPs and TAMRA, the quantitative analysis of Cr3+ is achieved through the changes of ratio fluorescence. Secondly, the specificity of the biosensor is improved by utilizing the specific recognition of DNA enzymes. Thirdly, the signal amplification technology of the SDR cycle greatly improves the sensitivity of biosensor. This biosensor will be useful for future environmental safety monitoring and biopsy of biological fluids.

Recent progress of UCNPs-MoS2 nanocomposites as a platform for biological applications

Composite materials can take advantages of the functional benefits of multiple pure nanomaterials to a greater degree than single nanomaterials alone. The UCNPs-MoS2 composite is a nano-application platform that combines upconversion luminescence and photothermal properties. Upconversion nanoparticles (UCNPs) are inorganic nanomaterials with long-wavelength excitation and short-wavelength tunable emission capabilities, and are able to effectively convert near-infrared (NIR) light into visible light for increased photostability. However, UCNPs have a low capacity for absorbing visible light, whereas MoS2 shows better absorption in the ultraviolet and visible regions. By integrating the benefits of UCNPs and MoS2, UCNPs-MoS2 nanocomposites can convert NIR light with a higher depth of detection into visible light for application with MoS2 through fluorescence resonance energy transfer (FRET), which compensates for the issues of MoS2's low tissue penetration light-absorbing wavelengths and expands its potential biological applications. Therefore, starting from the construction of UCNPs-MoS2 nanoplatforms, herein, we review the research progress in biological applications, including biosensing, phototherapy, bioimaging, and targeted drug delivery. Additionally, the current challenges and future development trends of UCNPs-MoS2 nanocomposites for biological applications are also discussed.

Raspberry-like Nanoheterostructures Comprising Glutathione-Capped Gold Nanoclusters Grown on the Lanthanide Nanoparticle Surface

Bare lanthanide-doped nanoparticles (LnNPs), in particular, NaYF4:Yb3+,Tm3+ NPs (UCTm), have been seeded in situ with gold cations to be used in the subsequent growth of gold nanoclusters (AuNCs) in the presence of glutathione (GSH) to obtain a novel UCTm@AuNC nanoheterostructure (NHS) with a raspberry-like morphology. UCTm@AuNC displays unique optical properties (multiple absorption and emission wavelengths). Specifically, upon 350 nm excitation, it exhibits AuNC photoluminescence (PL) (500-1200 nm, λmax 650 nm) and Yb emission (λmax 980 nm); this is the first example of Yb sensitization in a UCTm@AuNC NHS. Moreover, under 980 nm excitation, it displays (i) upconverting PL of the UCTm (at the blue, red and NIR-I, ca. 800 nm, regions); (ii) two-photon PL of AuNC; and (iii) down-shifting PL of thulium (around 1470 nm). The occurrence of energy transfer from UCTm to AuNCs in the UCTm@AuNC NHS was evidenced by the drastic lengthening of the AuNC PL lifetime (τPL) (from few hundred nanoseconds to more than one hundred microseconds). Initial biological assessment of UCTm@AuNC NHSs in vitro revealed high biocompatibility and bioimaging capabilities upon near-infrared excitation.

Poly(glycerol monomethacrylate)-encapsulated upconverting nanoparticles prepared by miniemulsion polymerization: morphology, chemical stability, antifouling properties and toxicity evaluation

In this report, upconverting NaYF4:Yb3+,Er3+ nanoparticles (UCNPs) were synthesized by high-temperature coprecipitation of lanthanide chlorides and encapsulated in poly(glycerol monomethacrylate) (PGMMA). The UCNP surface was first treated with hydrophobic penta(propylene glycol) methacrylate phosphate (SIPO) to improve colloidal stability and enable encapsulation by reversible addition-fragmentation chain transfer miniemulsion polymerization (RAFT) of glycidyl methacrylate (GMA) in water, followed by its hydrolysis. The resulting UCNP-containing PGMMA particles (UCNP@PGMMA), hundreds of nanometers in diameter, were thoroughly characterized by transmission (TEM) and scanning electron microscopy (SEM), dynamic light scattering (DLS), infrared (FTIR) and fluorescence emission spectroscopy, and thermogravimetric analysis (TGA) in terms of particle morphology, size, polydispersity, luminescence, and composition. The morphology, typically raspberry-like, depended on the GMA/UCNP weight ratio. Coating of the UCNPs with hydrophilic PGMMA provided the UCNPs with antifouling properties while enhancing chemical stability and reducing the cytotoxicity of neat UCNPs to a non-toxic level. In addition, it will allow the binding of molecules such as photosensitizers, thus expanding the possibilities for use in various biomedical applications.

Optimizing Upconversion Nanoparticles for FRET Biosensing

Upconversion nanoparticles (UCNPs) are some of the most promising nanomaterials for bioanalytical and biomedical applications. One important challenge to be still solved is how UCNPs can be optimally implemented into Förster resonance energy transfer (FRET) biosensing and bioimaging for highly sensitive, wash-free, multiplexed, accurate, and precise quantitative analysis of biomolecules and biomolecular interactions. The many possible UCNP architectures composed of a core and multiple shells doped with different lanthanoid ions at different ratios, the interaction with FRET acceptors at different possible distances and orientations via biomolecular interaction, and the many and long-lasting energy transfer pathways from the initial UCNP excitation to the final FRET process and acceptor emission make the experimental determination of the ideal UCNP-FRET configuration for optimal analytical performance a real challenge. To overcome this issue, we have developed a fully analytical model that requires only a few experimental configurations to determine the ideal UCNP-FRET system within a few minutes. We verified our model via experiments using nine different Nd-, Yb-, and Er-doped core-shell-shell UCNP architectures within a prototypical DNA hybridization assay using Cy3.5 as an acceptor dye. Using the selected experimental input, the model determined the optimal UCNP out of all theoretically possible combinatorial configurations. An extreme economy of time, effort, and material was accompanied by a significant sensitivity increase, which demonstrated the powerful feat of combining a few selected experiments with sophisticated but rapid modeling to accomplish an ideal FRET biosensor.

From Nanothermometry to Bioimaging: Lanthanide-Activated KY3F10 Nanostructures as Biocompatible Multifunctional Tools for Nanomedicine

Lanthanide-activated fluoride-based nanostructures are extremely interesting multifunctional tools for many modern applications in nanomedicine, e.g., bioimaging, sensing, drug delivery, and photodynamic therapy. Importantly, environmental-friendly preparations using a green chemistry approach, as hydrothermal synthesis route, are nowadays highly desirable to obtain colloidal nanoparticles, directly dispersible in hydrophilic media, as physiological solution. The nanomaterials under investigation are new KY₃F₁₀-based citrate-capped core@shell nanostructures activated with several lanthanide ions, namely, Er³⁺, Yb³⁺, Nd³⁺, and Gd³⁺, prepared as colloidal water dispersions. A new facile microwave-assisted synthesis has been exploited for their preparation, with significant reduction of the reaction times and a fine control of the nanoparticle size. These core@shell multifunctional architectures have been investigated for use as biocompatible and efficient contrast agents for optical, magnetic resonance imaging (MRI) and computerized tomography (CT) techniques. These multifunctional nanostructures are also efficient noninvasive optical nanothermometers. In fact, the lanthanide emission intensities have shown a relevant relative variation as a function of the temperature, in the visible and near-infrared optical ranges, efficiently exploiting ratiometric intensity methods for optical thermometry. Importantly, in contrast with other fluoride hosts, chemical dissolution of KY₃F₁₀ citrate-capped nanocrystals in aqueous environment is very limited, of paramount importance for applications in biological fluids. Furthermore, due to the strong paramagnetic properties of lanthanides (e.g., Gd³⁺), and X-ray absorption of both yttrium and lanthanides, the nanostructures under investigation are extremely useful for MRI and CT imaging. Biocompatibility studies of the nanomaterials have revealed very low cytotoxicity in dfferent human cell lines. All these features point to a successful use of these fluoride-based core@shell nanoarchitectures for simultaneous diagnostics and temperature sensing, ensuring an excellent biocompatibility.

Size Control and Improved Aqueous Colloidal Stability of Surface-Functionalized ZnGa2O4:Cr3+ Bright Persistent Luminescent Nanoparticles

Near-infrared (NIR)-emitting ZnGa₂O₄:Cr³⁺ (ZGO) persistent luminescent nanoparticles (PLNPs) have recently attracted considerable attention for diverse optical applications. The widespread use and promising potential of ZGO material in different applications arise from its prolonged post-excitation emission (several minutes to hours) that eliminates the need for continuous in situ excitation and the possibility of its excitation in different spectral regions (X-rays and UV-vis). However, the lack of precise control over particle size/distribution and its poor water dispersibility and/or limited colloidal stability required for certain biological applications are the major bottlenecks that limit its practical applications. To address these fundamental limitations, herein, we have prepared oleic acid (OA)-stabilized ZGO PLNPs with controlled size (7-12 nm, depending on the type of alcohol used in synthesis) and monodispersity. A further increase in size (8-21 nm), with a concomitant increase in persistent luminescence, could be achieved using a seed-mediated approach, employing the as-prepared ZGO PLNPs from the first synthesis as the seed and growing layers of the same material by adding fresh precursors. To remove their surface oleate groups and make the nanoparticles hydrophilic, two surface modification strategies were evaluated: modification with only poly(acrylic acid) (PAA) as the hydrophilic capping agent and modification with either PAA or cysteamine (Cys) as the hydrophilic capping agent in conjunction with BF₄^- as the intermediate surface modifier. The latter surface modifications involving BF₄^- conferred long-term (60 days and longer) colloidal stability to the nanoparticles in aqueous media, which is related to their favorable ζ potential values. The proposed generalized strategy could be used to prepare different kinds of surface-functionalized PLNPs with control of size, hydrophilicity, and colloidal stability and enhanced/prolonged persistent luminescence for diverse potential applications.

Near-infrared excitation/emission microscopy with lanthanide-based nanoparticles

Near-infrared optical imaging offers some advantages over conventional imaging, such as deeper tissue penetration, low or no autofluorescence, and reduced tissue scattering. Lanthanide-doped nanoparticles (LnNPs) have become a trend in the field of photoactive nanomaterials for optical imaging due to their unique optical features and because they can use NIR light as excitation and/or emission light. This review is focused on NaREF₄ NPs and offers an overview of the state-of-the-art investigation in their use as luminophores in optical microscopy, time-resolved imaging, and super-resolution nanoscopy based on, or applied to, LnNPs. Secondly, whenever LnNPs are combined with other nanomaterial or nanoparticle to afford nanohybrids, the characterization of their physical and chemical properties is of current interest. In this context, the latest trends in optical microscopy and their future perspectives are discussed.

Stability, dissolution, and cytotoxicity of NaYF4-upconversion nanoparticles with different coatings

Upconversion nanoparticles (UCNPs) have attracted considerable attention owing to their unique photophysical properties. Their utilization in biomedical applications depends on the understanding of their transformations under physiological conditions and their potential toxicity. In this study, NaYF4:Yb,Er UCNPs, widely used for luminescence and photophysical studies, were modified with a set of four different coordinatively bound surface ligands, i.e., citrate, alendronate (AA), ethylendiamine tetra(methylene phosphonate) (EDTMP), and poly(maleic anhydride-alt-1-octadecene) (PMAO), as well as silica coatings with two different thicknesses. Subsequently, the aging-induced release of fluoride ions in water and cell culture media and their cytotoxic profile to human keratinocytes were assessed in parallel to the cytotoxic evaluation of the ligands, sodium fluoride and the lanthanide ions. The cytotoxicity studies of UCNPs with different surface modifications demonstrated the good biocompatibility of EDTMP-UCNPs and PMAO-UCNPs, which is in line with the low amount of fluoride ions released from these samples. An efficient prevention of UCNP dissolution and release of cytotoxic ions, as well as low cytotoxicity was also observed for UCNPs with a sufficiently thick silica shell. Overall, our results provide new insights into the understanding of the contribution of surface chemistry to the stability, dissolution behavior, and cytotoxicity of UCNPs. Altogether, the results obtained are highly important for future applications of UCNPs in the life sciences and bioimaging studies.

Upconverting Nanoparticles in Aqueous Media: Not a Dead-End Road. Avoiding Degradation by Using Hydrophobic Polymer Shells

The stunning optical properties of upconverting nanoparticles (UCNPs) have inspired promising biomedical technologies. Nevertheless, their transfer to aqueous media is often accompanied by intense luminescence quenching, partial dissolution by water, and even complete degradation by molecules such as phosphates. Currently, these are major issues hampering the translation of UCNPs to the clinic. In this work, a strategy is developed to coat and protect β-NaYF₄ UCNPs against these effects, by growing a hydrophobic polymer shell (HPS) through miniemulsion polymerization of styrene (St), or St and methyl methacrylate mixtures. This allows one to obtain single core@shell UCNPs@HPS with a final diameter of ≈60-70 nm. Stability studies reveal that these HPSs serve as a very effective barrier, impeding polar molecules to affect UCNPs optical properties. Even more, it allows UCNPs to withstand aggressive conditions such as high dilutions (5 µg mL^-1 ), high phosphate concentrations (100 mm), and high temperatures (70 °C). The physicochemical characterizations prove the potential of HPSs to overcome the current limitations of UCNPs. This strategy, which can be applied to other nanomaterials with similar limitations, paves the way toward more stable and reliable UCNPs with applications in life sciences.

Decagram-Scale Synthesis of Multicolor Carbon Nanodots: Self-Tracking Nanoheaters with Inherent and Selective Anticancer Properties

Carbon nanodots (CDs) are a new class of carbon-based nanoparticles endowed with photoluminescence, high specific surface area, and good photothermal conversion, which have spearheaded many breakthroughs in medicine, especially in drug delivery and cancer theranostics. However, the tight control of their structural, optical, and biological properties and the synthesis scale-up have been very difficult so far. Here, we report for the first time an efficient protocol for the one-step synthesis of decagram-scale quantities of N,S-doped CDs with a narrow size distribution, along with a single nanostructure multicolor emission, high near-infrared (NIR) photothermal conversion efficiency, and selective reactive oxygen species (ROS) production in cancer cells. This allows achieving targeted and multimodal cytotoxic effects (i.e., photothermal and oxidative stresses) in cancer cells by applying biocompatible NIR laser sources that can be remotely controlled under the guidance of fluorescence imaging. Hence, our findings open up a range of possibilities for real-world biomedical applications, among which is cancer theranostics. In this work, indocyanine green is used as a bidentate SO_x donor which has the ability to tune surface groups and emission bands of CDs obtained by solvothermal decomposition of citric acid and urea in N,N-dimethylformamide. The co-doping implies various surface states providing transitions in the visible region, thus eliciting a tunable multicolor emission from blue to red and excellent photothermal efficiency in the NIR region useful in bioimaging applications and image-guided anticancer phototherapy. The fluorescence self-tracking capability of SO_x-CDs reveals that they can enter cancer cells more quickly than healthy cell lines and undergo a different intracellular fate after cell internalization. This could explain why sulfur doping entails pro-oxidative activities by triggering more ROS generation in cancer cells when compared to healthy cell lines. We also find that oxidative stress can be locally enhanced under the effects of a NIR laser at moderate power density (2.5 W cm^-2). Overall, these findings suggest that SO_x-CDs are endowed with inherent drug-independent cytotoxic effects toward cancer cells, which would be selectively enhanced by external NIR light irradiation and helpful in precision anticancer approaches. Also, this work opens a debate on the role of CD surface engineering in determining nanotoxicity as a function of cell metabolism, thus allowing a rational design of next-generation nanomaterials with targeted anticancer properties.

NaYF4-based upconverting nanoparticles with optimized phosphonate coatings for chemical stability and viability of human endothelial cells

The increasing interest in upconverting nanoparticles (UCNPs) in biodiagnostics and therapy fuels the development of biocompatible UCNPs platforms. UCNPs are typically nanocrystallites of rare-earth fluorides codoped with Yb³⁺and Er³⁺or Tm³⁺. The most studied UCNPs are based on NaYF₄but are not chemically stable in water. They dissolve significantly in the presence of phosphates. To prevent any adverse effects on the UCNPs induced by cellular phosphates, the surfaces of UCNPs must be made chemically inert and stable by suitable coatings. We studied the effect of various phosphonate coatings on chemical stability andin vitrocytotoxicity of the Yb³⁺,Er³⁺-codoped NaYF₄UCNPs in human endothelial cells obtained from cellular line Ea.hy926. Cell viability of endothelial cells was determined using the resazurin-based assay after the short-term (15 min), and long-term (24 h and 48 h) incubations with UCNPs dispersed in cell-culture medium. The coatings were obtained from tertaphosphonic acid (EDTMP), sodium alendronate and poly(ethylene glycol)-neridronate. Regardless of the coating conditions, 1 - 2 nm-thick amorphous surface layers were observed on the UCNPs with transmission electron microscopy. The upconversion fluorescence was measured in the dispersions of all UCNPs. Surafce quenching in aqueous suspensions of the UCNPs was reduced by the coatings. The dissolution degree of the UCNPs was determined from the concentration of dissolved fluoride measured with ion-selective electrode after the ageing of UCNPs in water, physiological buffer (i.e., phosphate-buffered saline-PBS) and cell-culture medium. The phosphonate coatings prepared at 80 °C significantly suppressed the dissolution of UCNPs in PBS while only minor dissolution of bare and coated UCNPs was measured in water and cell-culture medium. The viability of human endothelial cells was significantly reduced when incubated with UCNPs, but it increased with the improved chemical stability of UCNPs by the phosphonate coatings with negligible cytotoxicity when coated with EDTMP at 80 °C.

Initial Biological Assessment of Upconversion Nanohybrids

Nanoparticles for medical use should be non-cytotoxic and free of bacterial contamination. Upconversion nanoparticles (UCNPs) coated with cucurbit[7]uril (CB[7]) made by combining UCNPs free of oleic acid, here termed bare UCNPs (UCn), and CB[7], i.e., UC@CB[7] nanohybrids, could be used as photoactive inorganic-organic hybrid scaffolds for biological applications. UCNPs, in general, are not considered to be highly toxic materials, but the release of fluorides and lanthanides upon their dissolution may cause cytotoxicity. To identify potential adverse effects of the nanoparticles, dehydrogenase activity of endothelial cells, exposed to various concentrations of the UCNPs, was determined. Data were verified by measuring lactate dehydrogenase release as the indicator of loss of plasma membrane integrity, which indicates necrotic cell death. This assay, in combination with calcein AM/Ethidium homodimer-1 staining, identified induction of apoptosis as main mode of cell death for both particles. The data showed that the UCNPs are not cytotoxic to endothelial cells, and the samples did not contain endotoxin contamination. Higher cytotoxicity, however, was seen in HeLa and RAW 264.7 cells. This may be explained by differences in lysosome content and particle uptake rate. Internalization of UCn and UC@CB[7] nanohybrids by cells was demonstrated by NIR laser scanning microscopy.

Formation of phosphonate coatings for improved chemical stability of upconverting nanoparticles under physiological conditions

Upconverting nanoparticles (UCNPs) are being extensively investigated for applications in bioimaging because of their ability to emit ultraviolet, visible, and near-infrared light. NaYF4 is one of the most suitable host matrices for producing high-intensity upconversion fluorescence; however, UCNPs based on NaYF4 are not chemically stable in aqueous media. To prevent dissolution, their surfaces should be modified. We studied the formation of protective phosphonate coatings made of ethylenediamine(tetramethylenephosphonic acid), alendronic acid, and poly(ethylene glycol)-neridronate on cubic NaYF4 nanoparticles and hexagonal Yb3+,Er3+-doped upconverting NaYF4 nanoparticles (β-UCNPs). The effects of synthesis temperature and ultrasonic agitation on the quality of the coatings were studied. The formation of the coatings was investigated by transmission electron microscopy, zeta-potential measurements, and infrared spectroscopy. The quality of the phosphonate coatings was examined with respect to preventing the dissolution of the NPs in phosphate-buffered saline (PBS). The dissolution tests were carried out under physiological conditions (37 °C and pH 7.4) for 3 days and were followed by measurements of the dissolved fluoride with an ion-selective electrode. We found that the protection of the phosphonate coatings can be significantly increased by synthesizing them at 80 °C. At the same time, the coatings obtained at this temperature suppressed the surface quenching of the upconversion fluorescence in β-UCNPs.

Desilylation Induced by Metal Fluoride Nanocrystals Enables Cleavage Chemistry In Vivo

Metal fluoride nanocrystals are widely used in biomedical studies owing to their unique physicochemical properties. The release of metal ions and fluorides from nanocrystals is intrinsic due to the solubility equilibrium. It used to be considered as a drawback because it is related to the decomposition and defunction of metal fluoride nanocrystals. Many strategies have been developed to stabilize the nanocrystals, and the equilibrium concentrations of fluoride are often <1 mM. Here we make good use of this minimum amount of fluoride and unveil that metal fluoride nanocrystals could effectively induce desilylation cleavage chemistry, enabling controlled release of fluorophores and drug molecules in test tubes, living cells, and tumor-bearing mice. Biocompatible PEG (polyethylene glycol)-coated CaF₂ nanocrystals have been prepared to assay the efficiency of desilylation-induced controlled release of functional molecules. We apply the strategy to a prodrug activation of monomethyl auristatin E (MMAE), showing a remarkable anticancer effect, while side effects are almost negligible. In conclusion, this desilylation-induced cleavage chemistry avails the drawback on empowering metal fluoride nanocrystals with a new function of perturbing or activating for further biological applications.

Effect of different silica coatings on the toxicity of upconversion nanoparticles on RAW 264.7 macrophage cells

Upconversion nanoparticles (UCNPs), consisting of NaYF₄ doped with 18% Yb and 2% Er, were coated with microporous silica shells with thickness values of 7 ± 2 and 21 ± 3 nm. Subsequently, the negatively charged particles were functionalized with N-(6-aminohexyl)-3-aminopropyltrimethoxysilane (AHAPS), which provide a positive charge to the nanoparticle surface. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements revealed that, over the course of 24h, particles with thicker shells release fewer lanthanide ions than particles with thinner shells. However, even a 21 ± 3 nm thick silica layer does not entirely block the disintegration process of the UCNPs. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and cell cytometry measurements performed on macrophages (RAW 264.7 cells) indicate that cells treated with amino-functionalized particles with a thicker silica shell have a higher viability than those incubated with UCNPs with a thinner silica shell, even if more particles with a thicker shell are taken up. This effect is less significant for negatively charged particles. Cell cycle analyses with amino-functionalized particles also confirm that thicker silica shells reduce cytotoxicity. Thus, growing silica shells to a sufficient thickness is a simple approach to minimize the cytotoxicity of UCNPs.

Assessing the protective effects of different surface coatings on NaYF4:Yb3+, Er3+ upconverting nanoparticles in buffer and DMEM

We studied the dissolution behavior of β NaYF₄:Yb(20%), Er(2%) UCNP of two different sizes in biologically relevant media i.e., water (neutral pH), phosphate buffered saline (PBS), and Dulbecco’s modified Eagle medium (DMEM) at different temperatures and particle concentrations. Special emphasis was dedicated to assess the influence of different surface functionalizations, particularly the potential of mesoporous and microporous silica shells of different thicknesses for UCNP stabilization and protection. Dissolution was quantified electrochemically using a fluoride ion selective electrode (ISE) and by inductively coupled plasma optical emission spectrometry (ICP OES). In addition, dissolution was monitored fluorometrically. These experiments revealed that a thick microporous silica shell drastically decreased dissolution. Our results also underline the critical influence of the chemical composition of the aqueous environment on UCNP dissolution. In DMEM, we observed the formation of a layer of adsorbed molecules on the UCNP surface that protected the UCNP from dissolution and enhanced their fluorescence. Examination of this layer by X-ray photoelectron spectroscopy (XPS) and mass spectrometry (MS) suggested that mainly phenylalanine, lysine, and glucose are adsorbed from DMEM. These findings should be considered in the future for cellular toxicity studies with UCNP and other nanoparticles and the design of new biocompatible surface coatings.

Time-resolved luminescence spectroscopy for monitoring the stability and dissolution behaviour of upconverting nanocrystals with different surface coatings

We demonstrate the potential of time-resolved luminescence spectroscopy for the straightforward assessment and in situ monitoring of the stability of upconversion nanocrystals (UCNPs). Therefore, we prepared hexagonal NaYF4:Yb3+,Er3+ UCNPs with various coatings with a focus on phosphonate ligands of different valency, using different ligand exchange procedures, and studied their dissolution behaviour in phosphate-buffered saline (PBS) dispersions at 20 °C and 37 °C with various analytical methods. The amount of the released UCNPs constituting fluoride ions was quantified by potentiometry using a fluoride ion-sensitive electrode and particle disintegration was confirmed by transmission electron microscopy studies of the differently aged UCNPs. In parallel, the luminescence features of the UCNPs were measured with special emphasis on the lifetime of the sensitizer emission to demonstrate its suitability as screening parameter for UCNP stability and changes in particle composition. The excellent correlation between the changes in luminescence lifetime and fluoride concentration highlights the potential of our luminescence lifetime method for UCNP stability screening and thereby indirect monitoring of the release of potentially hazardous fluoride ions during uptake and dissolution in biological systems. Additionally, the developed in situ optical method was used to distinguish the dissolution dynamics of differently sized and differently coated UCNPs.