Upconversion nanoparticles in biological labeling, imaging, and therapy

Shape-controllable synthesis of hydrophilic NaLuF4:Yb,Er nanocrystals by a surfactant-assistant two-phase system

Water-soluble upconversion nanoparticles (UCNPs) were prepared by a one-pot procedure in a two-phase reacting system. Four kinds of surfactants were tested in the synthesis process as capping agent to tune size and morphology of nanocrystals. Nanoparticles (approximately 70 nm) and rods (400 nm and 2.5 μm) were synthesized, respectively. Then, Fourier transform infrared spectroscopy analysis confirmed the successful linking between UCNP surface and surfactant. Ionic liquids (ILs) and surfactants participated in synthesis process together, competing with each other to cap on UCNPs. ILs still led the competition of capping, while surfactants worked as cooperative assistants to develop functional surface. Further characterizations such as high-resolution transmission electron microscopy and X-ray diffraction indicated the changes in crystallization and phase transformation under the influence of surfactants. In addition, the growth mechanism of nanocrystals and upconversion fluorescence luminance was also investigated in detail. At last, the cytotoxicity of UCNPs was evaluated, which highly suggest that these surface-functionalized UCNPs are promising candidates for biomedical engineering.

Incorporation of computed tomography and magnetic resonance imaging function into NaYF4:Yb/Tm upconversion nanoparticles for in vivo trimodal bioimaging

Rational design and fabrication of multimodal imaging nanoprobes are of great significance for in vivo imaging. Here we report the fabrication of a multishell structured NaYF4:Yb/Tm@NaLuF4@NaYF4@NaGdF4 nanoprobe via a seed-mediated epitaxial growth strategy for upconversion luminescence (UCL), X-ray computed tomography (CT), and magnetic resonance (MR) trimodal imaging. Hexagonal phase NaYF4:Yb/Tm is used as the core to provide UCL, while the shell of NaLuF4 is epitaxially grown on the core not only to provide an optically inert layer for enhancing the UCL but also to serve as a contrast agent for CT. The outermost NaGdF4 shell is fabricated as a thin layer to give the high longitudinal relaxivity (r1) desired for MR imaging. The transition shell layer of NaYF4 not only provides an interface to facilitate the formation of NaGdF4 shell but also inhibits the energy transfer from inner upconversion activator to surface paramagnetic Gd(3+) ions. The fabricated multishell structured nanoprobe shows intense near-infrared UCL, high r1 value of 3.76 mM(-1) s(-1), and in vitro CT contrast effect. The multishell structured nanoprobe offers great potential for in vivo UCL/CT/MR trimodal imaging. Further covalent bonding of folic acid makes the multishell structured nanoprobe promising for in vivo targeted UCL imaging of tumor-bearing mice.

Tuning NaYF₄ Nanoparticles through Alkaline Earth Doping

Phase and size of lanthanide-doped nanoparticles are the most important characteristics that dictate optical properties of these nanoparticles and affect their technological applications. Herein, we present a systematic study to examine the effect of alkaline earth doping on the formation of NaYF₄ upconversion nanoparticles. We show that alkaline earth doping has a dual function of tuning particle size of hexagonal phase NaYF₄ nanoparticles and stabilizing cubic phase NaYF₄ nanoparticles depending on composition and concentration of the dopant ions. The study described here represents a facile and general strategy to tuning the properties of NaYF₄ upconversion nanoparticles.

Multilayer dual-polymer-coated upconversion nanoparticles for multimodal imaging and serum-enhanced gene delivery

Upconversion nanoparticles (UCNPs) have been widely explored for various bioapplications because of their unique optical properties, easy surface functionalization, and low cytotoxicity. Herein, we synthesize gadolinium (Gd3+)-doped UCNPs, which are modified first with poly(ethylene glycol) (PEG) and then with two layers of poly(ethylenimine) (PEI) via covalent conjugation and layer-by-layer assembly, respectively. Compared with UCNP-PEG@1×PEI with only one layer of PEI coating, the final complex, UCNP-PEG@2×PEI, with two PEI layers exhibits reduced cytotoxicity and enhanced gene transfection efficiency. It is interesting to find that while free PEI polymer is only effective in gene transfection in a serum-free medium and shows drastically reduced transfection ability if serum is added, UCNP-PEG@2×PEI is able to transfect cells in both serum-free and -containing media and, surprisingly, offers even higher gene transfection efficiency if serum is added. This is likely due to the formation of protein corona on the nanoparticle surface, which triggers the receptor-mediated endocytosis of our UCNP vectors. Considering the upconversion luminescence and magnetic resonance imaging contrasting ability of UCNPs, our novel nanovector could serve as a "trackable" gene-delivery carrier promising for theranostic applications.

Preparation and characterization of decyl-terminated silicon nanoparticles encapsulated in lipid nanocapsules

In this Article, we report on the encapsulation of decyl-modified silicon nanoparticles (decyl-SiNPs) into ∼80 nm lipid nanocapsules (LNCs). The decyl-SiNPs were produced by thermal hydrosilylation of hydride-terminated SiNPs (H-SiNPs) liberated from porous silicon. Various techniques, including Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), UV-vis absorption, dynamic light scattering (DLS), and photoluminescence (PL), were used to characterize their size, shape, colloidal, and optical properties. The results indicate that these nanocapsules feature controllable size, good dispersity, high loading rate of SiNPs, colloidal stability in various media, and bright PL. The PL of decyl-SiNPs loaded LNCs was stable upon heating to 80 °C, but was sensitive to basic solutions due to proton-gated emission of the SiNPs arranged at the LNCs interface between the oil phase and the hydrophilic polyethylene glycol moieties of the surfactant. These luminescent nanocapsules are therefore promising candidates as cellular probes for fluorescence imaging. In addition, it was found that TEM imaging of small-sized decyl-SiNPs could be greatly improved by preliminary negative staining of TEM grids with phosphotungstic acid.

PEGylated upconverting luminescent hollow nanospheres for drug delivery and in vivo imaging

Upconversion luminescent hollow Y2 O3 :Yb(3+) /Er(3+) nanospheres can be synthesized by an etching-free process, which hold promising potential for applications such as drug delivery, angiography, and high-contrast cellular as well as tissue imaging, with no damage from radiation or toxicity.

Synthesis of lanthanide-doped NaYF₄@TiO₂ core-shell composites with highly crystalline and tunable TiO₂ shells under mild conditions and their upconversion-based photocatalysis

NaYF4:Yb,Tm@TiO₂ core-shell composites were synthesized via a facile hydrothermal method. The highly crystalline TiO₂ shell can be uniformly coated onto lanthanide-doped NaYF₄ microrods and nanorods under mild conditions without calcination. The thickness of the TiO₂ shell can be tuned by varying the ratio of fluoride rods and Ti precursors. The microcomposite with a moderate TiO₂ shell shows excellent photocatalytic activity under near-infrared irradiation.

Upconversion-nanophosphor-based functional nanocomposites

Upconversion nanophosphors have the ability to generate visible or near-infrared (NIR) emissions under continuous-wave NIR excitation. Utilizing this special photoluminescent properties, upconversion nanophosphors can be used as key components in complex nanocomposites for a wide range of applications. This review summarizes the basic concept, fabrication strategy, and typical application of upconversion-nanophosphor-based functional nanocomposites. The motivation to design these structures comes from the potential applications in detection, multi-modality bioimaging, and NIR light-induced therapy, as well as the tuning of the upconversion luminescence emissions. This review will give a brief summary of this rapidly developing field, and provide guidance to design and to fabricate new nanocomposites based on upconversion nanophosphors.

Rational design of multifunctional upconversion nanocrystals/polymer nanocomposites for cisplatin (IV) delivery and biomedical imaging

By combining upconversion nanoparticles with the cisplatin (IV) prodrug we have demonstrated that a stable and multifunctional drug delivery system can be designed that will both reduce the drawbacks of cisplatin and give insight in to its in vitro/in vivo imaging. The up/down-conversion fluorescence are detectable and show obvious co-localization, demonstrating that the nanoparticles are rather stable inside cells and retain the UCNPs and block copolymer.

NIR photoresponsive crosslinked upconverting nanocarriers toward selective intracellular drug release

An NIR-responsive mesoporous silica coated upconverting nanoparticle (UCNP) conjugate is developed for controllable drug delivery and fluorescence imaging in living cells. In this work, antitumor drug doxorubicin (Dox) molecules are encapsulated within cross-linked photocaged mesoporous silica coated UCNPs. Upon 980 nm light irradiation, Dox could be selectively released through the photocleavage of theo-nitrobenzyl (NB) caged linker by the converted UV emission from UCNPs. This NIR light-responsive nanoparticle conjugate demonstrates high efficiency for the controlled release of the drug in cancer cells. Upon functionalization of the nanocarrier with folic acid (FA), this photocaged FA-conjugated silica-UCNP nanocarrier will also allow targeted intracellular drug delivery and selective fluorescence imaging towards the cell lines with high level expression of folate receptor (FR).

Multicolor tunability and upconversion enhancement of fluoride nanoparticles by oxygen dopant

The ability to manipulate the upconversion luminescence of lanthanide-ion doped fluoride upconversion nanoparticles (UCNPs) is particularly important and highly desired due to their wide applications in color displays, multiplexing bioassays and multicolor imaging. Here, we developed a strategy for simultaneously tuning color output and enhancing upconversion emission of Yb/Er doped fluoride UCNPs, based on adjusting the oxygen doping level. The synthesis of multicolored multifunctional NaGdF4:Yb,Er UCNPs was used as the model host system to demonstrate this protocol. Ammonium nitrate (NH4NO3) was used as the oxygen source and added into the reaction system at the beginning stage of nucleation and growth process of fluoride UCNPs, which facilitates the formation of enough oxygen atoms and the diffusion of these into the fluoride host matrix. The results revealed that multicolour output and upconversion enhancement mainly resulted from the variation of phonon energy and crystal field symmetry of the host lattice, respectively. This strategy can be further expanded to other fluoride host matrices. As an example of an application, multicolored UCNPs were used as a color converter in light emitting diodes, which can effectively convert near-infrared light into visible light. It is expected that these multicolored UCNPs will be promising for applications in multiplexing biodetection, bioimaging (optical and magnetic resonance imaging) and other optical technologies, and the present method for the control of O(2-) doping may also be used in other functional nanomaterials.

Luminescence upconversion in colloidal double quantum dots

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved.

Recent advances in design and fabrication of upconversion nanoparticles and their safe theranostic applications

Lanthanide (Ln) doped upconversion nanoparticles (UCNPs) have attracted enormous attention in the recent years due to their unique upconversion luminescent properties that enable the conversion of low-energy photons (near infrared photons) into high-energy photons (visible to ultraviolet photons) via the multiphoton processes. This feature makes them ideal for bioimaging applications with attractive advantages such as no autofluorescence from biotissues and a large penetration depth. In addition, by incorporating advanced features, such as specific targeting, multimodality imaging and therapeutic delivery, the application of UCNPs has been dramatically expanded. In this review, we first summarize the recent developments in the fabrication strategies of UCNPs with the desired size, enhanced and tunable upconversion luminescence, as well as the combined multifunctionality. We then discuss the chemical methods applied for UCNPs surface functionalization to make these UCNPs biocompatible and water-soluble, and further highlight some representative examples of using UCNPs for in vivo bioimaging, NIR-triggered drug/gene delivery applications and photodynamic therapy. In the perspectives, we discuss the need of systematically nanotoxicology data for rational designs of UCNPs materials, their surface chemistry in safer biomedical applications. The UCNPs can actually provide an ideal multifunctionalized platform for solutions to many key issues in the front of medical sciences such as theranostics, individualized therapeutics, multimodality medicine, etc.

Graphene oxide protected nucleic acid probes for bioanalysis and biomedicine

Recently, the binding ability of DNA on GO and resulting nuclease resistance have attracted increasing attention, leading to new applications both in vivo and in vitro. In vivo, nucleic acids absorbed on GO can be effectively protected from enzymatic degradation and biological interference in complicated samples, making it useful for targeted delivery, gene regulation, intracellular detection and imaging with high uptake efficiencies, high intracellular stability, and very low toxicity. In vitro, the adsorption of ssDNA on GO surface and desorption of dsDNA or well-folded ssDNA from GO surface result in the protection and deprotection of DNA from nucleic digestion, respectively, which has led to target-triggered cyclic enzymatic amplification methods (CEAM) for amplified detection of analytes with sensitivity 2-3 orders of magnitude higher than that of 1:1 binding strategies. This Concept article explores some of the latest developments in this field.

Simultaneous synthesis and amine-functionalization of single-phase BaYF5:Yb/Er nanoprobe for dual-modal in vivo upconversion fluorescence and long-lasting X-ray computed tomography imaging

In this work, we developed a novel and biocompatible dual-modal nanoprobe based on single-phase amine-functionalized BaYF5:Yb/Er nanoparticles (NPs) for upconversion (UC) fluorescence and in vivo computed X-ray tomography (CT) bioimaging for the first time. High-quality water-soluble amine-functionalized BaYF5:Yb/Er NPs with an average size of 24 nm were synthesized by a facile environmentally friendly hydrothermal method for simultaneous synthesis and surface functionalization. Structure investigation based on the Rietveld refinement method revealed that the as-synthesized BaYF5:Yb/Er NPs present a cubic phase structure, which differs from the previously reported tetragonal structure. Under 980 nm excitation, high-contrast green and red UC emissions were observed from HeLa cells incubated with these amine-functionalized NPs. The UC spectra measured from the NPs incubated with HeLa cells presented only green and red UC emissions without any autofluorescence, further revealing that these NPs are ideal candidates for fluorescent bioimaging. In addition, the cell cytotoxicity test showed low cell toxicity of these NPs. These amine-functionalized NPs were also successfully used as CT agents for in vivo CT imaging because of the efficient X-ray absorption efficiency of Ba and doped Yb ions. A prolonged (2 h) signal enhancement of the spleen in a mouse was observed in CT imaging, which can improve the detection of splenic diseases. More importantly, the simultaneous X-ray and UC in vivo bioimaging was demonstrated in a nude mouse for the first time, indicating the as-prepared UCNPs can be successfully used as dual-modal bioprobes. These results demonstrate that BaYF5:Yb/Er NPs are ideal nanoprobes for dual-modal fluorescent/CT bioimaging with low cytotoxicity, non-autofluorescence, and enhanced detection of the spleen.

Biodistribution of sub-10 nm PEG-modified radioactive/upconversion nanoparticles

The biodistribution of lanthanide-based upconversion nanophosphors (UCNPs) has attracted increasing attention, and all of the reported UCNPs display metabolism in the liver and spleen mainly. Herein, ∼8 nm poly(ethylene glycol) (PEG)-coated NaYF4 nanoparticles codoped with Yb(3+), Er(3+), and (or) radioactive (153)Sm(3+) ions were synthesized, through a hydrothermal synthetic system assisted by binary cooperative ligands with oleic acid and PEG dicarboxylic acids. The as-prepared PEG-coating NaYF4:Yb,Er and NaYF4:Yb,Er,(153)Sm are denoted as PEG-UCNPs and PEG-UCNPs((153)Sm), respectively. PEG-UCNPs were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD) analysis, and Fourier-transform infrared (FTIR) spectroscopy. The PEG-UCNPs showed excellent water solubility with a hydrodynamic diameter of ∼10 nm and displayed upconversion luminescence (UCL) under continuous-wave excitation at 980 nm. At the same time, the (153)Sm-doped nanoparticles PEG-UCNPs((153)Sm) displayed radioactivity, and time-dependent biodistribution of PEG-UCNPs((153)Sm) was investigated, through single-photon emission computed tomography (SPECT) imaging and γ-counter analysis. Interestingly, PEG-UCNPs((153)Sm) had a long blood retention time and were partly eliminated through urinary pathways in vivo. Therefore, the concept of fabricating PEG-coated, small nanosize (sub-10 nm) nanoparticles with radioactive property is a useful strategy for providing a potential method to monitor lanthanide nanoparticles renal clearable.

Red-emitting upconverting nanoparticles for photodynamic therapy in cancer cells under near-infrared excitation

Upconverting nanoparticles (UCNPs) have attracted considerable attention as potential photosensitizer carriers for photodynamic therapy (PDT) in deep tissues. In this work, a new and efficient NIR photosensitizing nanoplatform for PDT based on red-emitting UCNPs is designed. The red emission band matches well with the efficient absorption bands of the widely used commercially available photosensitizers (Ps), benefiting the fluorescence resonance energy transfer (FRET) from UCNPs to the attached photosensitizers and thus efficiently activating them to generate cytotoxic singlet oxygen. Three commonly used photosensitizers, including chlorine e6 (Ce6), zinc phthalocyanine (ZnPc) and methylene blue (MB), are loaded onto the alpha-cyclodextrin-modified UCNPs to form Ps@UCNPs complexes that efficiently produce singlet oxygen to kill cancer cells under 980 nm near-infrared excitation. Moreover, two different kinds of drugs are co-loaded onto these nanoparticles: chemotherapy drug doxorubicin and PDT agent Ce6. The combinational therapy based on doxorubicin (DOX)-induced chemotherapy and Ce6-triggered PDT exhibits higher therapeutic efficacy relative to the individual means for cancer therapy in vitro.

Sub-10 nm Fe3O4@Cu(2-x)S core-shell nanoparticles for dual-modal imaging and photothermal therapy

Photothermal nanomaterials have recently attracted significant research interest due to their potential applications in biological imaging and therapeutics. However, the development of small-sized photothermal nanomaterials with high thermal stability remains a formidable challenge. Here, we report the rational design and synthesis of ultrasmall (<10 nm) Fe3O4@Cu2-xS core-shell nanoparticles, which offer both high photothermal stability and superparamagnetic properties. Specifically, these core-shell nanoparticles have proven effective as probes for T2-weighted magnetic resonance imaging and infrared thermal imaging because of their strong absorption at the near-infrared region centered around 960 nm. Importantly, the photothermal effect of the nanoparticles can be precisely controlled by varying the Cu content in the core-shell structure. Furthermore, we demonstrate in vitro and in vivo photothermal ablation of cancer cells using these multifunctional nanoparticles. The results should provide improved understanding of synergistic effect resulting from the integration of magnetism with photothermal phenomenon, important for developing multimode nanoparticle probes for biomedical applications.

Targeting polymeric fluorescent nanodiamond-gold/silver multi-functional nanoparticles as a light-transforming hyperthermia reagent for cancer cells

This work demonstrates a simple route for synthesizing multi-functional fluorescent nanodiamond-gold/silver nanoparticles. The fluorescent nanodiamond is formed by the surface passivation of poly(ethylene glycol) bis(3-aminopropyl) terminated. Urchin-like gold/silver nanoparticles can be obtained via one-pot synthesis, and combined with each other via further thiolation of nanodiamond. The morphology of the nanodiamond-gold/silver nanoparticles thus formed was identified herein by high-resolution transmission electron microscopy, and clarified using diffraction patterns. Fourier transform infrared spectroscopy clearly revealed the surface functionalization of the nanoparticles. The fluorescence of the materials with high photo stability was examined by high power laser irradiation and long-term storage at room temperature. To develop the bio-recognition of fluorescent nanodiamond-gold/silver nanoparticles, pre-modified transferrin was conjugated with the gold/silver nanoparticles, and the specificity and activity were confirmed in vitro using human hepatoma cell line (J5). The cellular uptake analysis that was conducted using flow cytometry and inductively coupled plasma mass spectrometry exhibited that twice as many transferrin-modified nanoparticles as bare nanoparticles were engulfed, revealing the targeting and ease of internalization of the human hepatoma cell. Additionally, the in situ monitoring of photothermal therapeutic behavior reveals that the nanodiamond-gold/silver nanoparticles conjugated with transferrin was more therapeutic than the bare nanodiamond-gold/silver materials, even when exposed to a less energetic laser source. Ultimately, this multi-functional material has great potential for application in simple synthesis. It is non-cytotoxic, supports long-term tracing and can be used in highly efficient photothermal therapy against cancer cells.

Upconversion nanophosphors Naluf₄:Yb,Tm for lymphatic imaging in vivo by real-time upconversion luminescence imaging under ambient light and high-resolution X-ray CT

Lanthanide upconversion nanophosphor (UCNP) has attracted increasing attention for potential applications in bioimaging due to its excellence in deep and high contrast imaging. To date, most upconversion imaging applications were demonstrated in dark surroundings without ambient light for higher signal-to-noise ratio, which hindered the application of optical imaging guided surgery. Herein, the new established NaLuF₄-based UCNP (NaLuF₄:Yb,Tm, ~17 nm) with bright upconversion emission around 800 nm as imaging signal was used to realize imaging under ambient light to provide more convenient for clinician. Moreover, due to the existance of heavy element lutetium (Lu) in the host lattice, the NaLuF₄:Yb,Tm nanoparticles can also be used as an X-ray CT imaging agent to enhance the imaging depth and in vivo imaging resolution.

In vitro cell imaging using multifunctional small sized KGdF4:Yb3+,Er3+ upconverting nanoparticles synthesized by a one-pot solvothermal process

Multifunctional KGdF4:18%Yb(3+),2%Er(3+) nanoparticles with upconversion fluorescence and paramagnetism are synthesized. The average sizes of the nanoparticles capped with branched polyethyleneimine (PEI) and 6-aminocaproic acid (6AA) are ~14 and ~13 nm, respectively. Our KGdF4 host does not exhibit any phase change with the decrease of particle size, which can prevent the detrimental significant decrease in upconversion luminescence caused by this effect observed in the well-known NaYF4 host. The branched PEI and 6AA capping ligands endow our nanoparticles with water-dispersibility and biocompatibility, which can favor internalization of our nanoparticles into the cytoplasm of HeLa cells and relatively high cell viability. The strong upconversion luminescence detected at the cytoplasm of HeLa cells incubated with the branched PEI-capped nanoparticles is probably attributed to the reported high efficiency of cellular uptake. The magnetic mass susceptibility of our nanoparticle is 8.62 × 10(-5) emu g(-1) Oe(-1). This is the highest value ever reported in trivalent rare-earth ion-doped KGdF4 nanoparticles of small size (≤14 nm), and is very close to that of nanoparticles used as T1 contrast agents in magnetic resonance imaging. These suggest the potential of our KGdF4:Yb(3+),Er(3+) nanoparticles as small-sized multifunctional bioprobes.

Surface-functionalized nanoparticles for biosensing and imaging-guided therapeutics

In this article, the very recent progress of various functional inorganic nanomaterials is reviewed including their unique properties, surface functionalization strategies, and applications in biosensing and imaging-guided therapeutics. The proper surface functionalization renders them with stability, biocompatibility and functionality in physiological environments, and further enables their targeted use in bioapplications after bioconjugation via selective and specific recognition. The surface-functionalized nanoprobes using the most actively studied nanoparticles (i.e., gold nanoparticles, quantum dots, upconversion nanoparticles, and magnetic nanoparticles) make them an excellent platform for a wide range of bioapplications. With more efforts in recent years, they have been widely developed as labeling probes to detect various biological species such as proteins, nucleic acids and ions, and extensively employed as imaging probes to guide therapeutics such as drug/gene delivery and photothermal/photodynamic therapy.

Targeting CCL21-folic acid-upconversion nanoparticles conjugates to folate receptor-α expressing tumor cells in an endothelial-tumor cell bilayer model

The ability of some malignant cells to evade immunosurveillance has been a major contribution to the inability of the host's immune system to eradicate the neoplastic cells. This has led to the development of various immunological strategies to augment the host immune response as part of cancer treatment. In this study, we developed folic acid (FA)/secondary lymphoid tissue chemokine (CCL21)/upconversion fluorescent nanoparticles (UCNs) conjugates as a targeting and delivery system to attract immune cells to folate receptor (FR) expressing tumor cells. Our data show that FA-conjugated UCNs@mesoporous silica specifically target FR expressing ovarian carcinoma cell line, OVCAR-3, compared to the unconjugated mesoporous silica coated UCNs. Furthermore, the FA-UCNs@mesoporous silica can efficiently cross the endothelial cell monolayer and accumulate in the clusters of OVCAR-3 cells in our endothelial-tumor cell bilayer model. Our migration assay data suggest that the CCL21 loaded into the mesoporous layer is biologically active and can efficiently induce T cells migration in-vitro. No significant cytotoxic effect was observed throughout the study indicating good biocompatibility of the nanoconjugates. As proof-of-concept, we have shown that it is feasible to load biologically active chemokines onto UCNs to modulate T cell migration.

Sensing using rare-earth-doped upconversion nanoparticles

Optical sensing plays an important role in theranostics due to its capability to detect hint biochemical entities or molecular targets as well as to precisely monitor specific fundamental psychological processes. Rare-earth (RE) doped upconversion nanoparticles (UCNPs) are promising for these endeavors due to their unique frequency converting capability; they emit efficient and sharp visible or ultraviolet (UV) luminescence via use of ladder-like energy levels of RE ions when excited at near infrared (NIR) light that are silent to tissues. These features allow not only a high penetration depth in biological tissues but also a high detection sensitivity. Indeed, the energy transfer between UCNPs and biomolecular or chemical indicators provide opportunities for high-sensitive bio- and chemical-sensing. A temperature-sensitive change of the intensity ratio between two close UC bands promises them for use in temperature mapping of a single living cell. In this work, we review recent investigations on using UCNPs for the detection of biomolecules (avidin, ATP, etc.), ions (cyanide, mecury, etc.), small gas molecules (oxygen, carbon dioxide, ammonia, etc.), as well as for in vitro temperature sensing. We also briefly summarize chemical methods in synthesizing UCNPs of high efficiency that are important for the detection limit.

Upconversion nanoparticles for photodynamic therapy and other cancer therapeutics

Photodynamic therapy (PDT) is a non-invasive treatment modality for a variety of diseases including cancer. PDT based on upconversion nanoparticles (UCNPs) has received much attention in recent years. Under near-infrared (NIR) light excitation, UCNPs are able to emit high-energy visible light, which can activate surrounding photosensitizer (PS) molecules to produce singlet oxygen and kill cancer cells. Owing to the high tissue penetration ability of NIR light, NIR-excited UCNPs can be used to activate PS molecules in much deeper tissues compared to traditional PDT induced by visible or ultraviolet (UV) light. In addition to the application of UCNPs as an energy donor in PDT, via similar mechanisms, they could also be used for the NIR light-triggered drug release or activation of 'caged' imaging or therapeutic molecules. In this review, we will summarize the latest progresses regarding the applications of UCNPs for photodynamic therapy, NIR triggered drug and gene delivery, as well as several other UCNP-based cancer therapeutic approaches. The future prospects and challenges in this emerging field will be also discussed.

Toxicity assessments of near-infrared upconversion luminescent LaF3:Yb,Er in early development of zebrafish embryos

This study reports the effects of upconversion nanoparticles (UCNPs) LaF₃:Yb,Er on zebrafish, with the aim of investigating UCNPs toxicity. LaF₃:Yb,Er were prepared by an oleic acid/ionic liquid two-phase system, and characterized by transmission electron microscope and X-ray powder diffraction. 140 zebrafish embryos were divided into six test groups and one control group, and respectively were injected into 5, 25, 50, 100, 200, 400 μg/mL LaF₃:Yb,Er@SiO₂ solution, and respectively were raised for 5 days. Each experiment was repeated ten times. Results showed that water-soluble LaF₃:Yb,Er were successfully prepared, and did not exhibit obvious toxicity to zebrafish embryos under 100 μg/mL, but exhibited chronic toxicities 200 μg/mL in vivo, resulting in malformations and delayed hatching rate and embryonic and larval development. The excretion channels of LaF₃:Yb,Er in adult zebrafish were mainly found in the intestine after being injected evenly for 24 h. In conclusion, the exploration of LaF₃:Yb,Er for in vivo applications in animals and humans must consider UCNPs biocompatibility.

Stem cell labeling using polyethylenimine conjugated (α-NaYbF4:Tm3+)/CaF2 upconversion nanoparticles

We report on a polyethylenimine (PEI) covalently conjugated (α-NaYbF4:Tm³⁺)/CaF₂ upconversion nanoparticle (PEI-UCNP) and its use for labeling rat mesenchymal stem cells (rMSCs). The PEI-UCNPs absorb and emit near-infrared light, allowing for improved in vivo imaging depth over conventional probes. We found that such covalent surface conjugation by PEI results in a much more stable PEI-UCNP suspension in PBS compared to conventional electrostatic layer by layer (LbL) self-assembling coating approach. We systematically examined the effects of nanoparticle dose and exposure time on rat mesenchymal stem cell (rMSC) cytotoxicity. The exocytosis of PEI-UCNPs from labeled rMSCs and the impact of PEI-UCNP uptake on rMSC differentiation was also investigated. Our data show that incubation of 100-µg/mL PEI-UCNPs with rMSCs for 4 h led to efficient labeling of the MSCs, and such a level of PEI-UCNP exposure imposed little cytotoxicity to rMSCs (95% viability). However, extended incubation of PEI-UCNPs at the 100 µg/mL dose for 24 hour resulted in some cytotoxicity to rMSCs (60% viability). PEI-UCNP labeled rMSCs also exhibited normal early proliferation, and the internalized PEI-UCNPs did not leak out to cause unintended labeling of adjacent cells during a 14-day transwell culture experiment. Finally, PEI-UCNP labeled rMSCs were able to undergo osteogenic and adipogenic differentiation upon in vitro induction, although the osteogenesis of labeled rMSCs appeared to be less potent than that of the unlabeled rMSCs. Taken together, PEI-UCNPs are promising agents for stem cell labeling and tracking.

Quantum dots in bioanalysis: a review of applications across various platforms for fluorescence spectroscopy and imaging

Semiconductor quantum dots (QDs) are brightly luminescent nanoparticles that have found numerous applications in bioanalysis and bioimaging. In this review, we highlight recent developments in these areas in the context of specific methods for fluorescence spectroscopy and imaging. Following a primer on the structure, properties, and biofunctionalization of QDs, we describe select examples of how QDs have been used in combination with steady-state or time-resolved spectroscopic techniques to develop a variety of assays, bioprobes, and biosensors that function via changes in QD photoluminescence intensity, polarization, or lifetime. Some special attention is paid to the use of Förster resonance energy transfer-type methods in bioanalysis, including those based on bioluminescence and chemiluminescence. Direct chemiluminescence, electrochemiluminescence, and charge transfer quenching are similarly discussed. We further describe the combination of QDs and flow cytometry, including traditional cellular analyses and spectrally encoded barcode-based assay technologies, before turning our attention to enhanced fluorescence techniques based on photonic crystals or plasmon coupling. Finally, we survey the use of QDs across different platforms for biological fluorescence imaging, including epifluorescence, confocal, and two-photon excitation microscopy; single particle tracking and fluorescence correlation spectroscopy; super-resolution imaging; near-field scanning optical microscopy; and fluorescence lifetime imaging microscopy. In each of the above-mentioned platforms, QDs provide the brightness needed for highly sensitive detection, the photostability needed for tracking dynamic processes, or the multiplexing capacity needed to elucidate complex systems. There is a clear synergy between advances in QD materials and spectroscopy and imaging techniques, as both must be applied in concert to achieve their full potential.

Multiplexed sensing and imaging with colloidal nano- and microparticles

Sensing and imaging with fluorescent, plasmonic, and magnetic colloidal nano- and microparticles have improved during the past decade. In this review, we describe the concepts and applications of how these techniques can be used in the multiplexed mode, that is, sensing of several analytes in parallel or imaging of several labels in parallel.