Upconversion nanoparticles and their composite nanostructures for biomedical imaging and cancer therapy

Water dispersible upconverting nanoparticles: effects of surface modification on their luminescence and colloidal stability

We present a systematic study on the effect of surface ligands on the luminescence properties and colloidal stability of β-NaYF4:Yb(3+),Er(3+) upconversion nanoparticles (UCNPs), comparing nine different surface coatings to render these UCNPs water-dispersible and bioconjugatable. A prerequisite for this study was a large-scale synthetic method that yields ∼2 g per batch of monodisperse oleate-capped UCNPs providing identical core particles. These ∼23 nm sized UCNPs display an upconversion quantum yield of ∼0.35% when dispersed in cyclohexane and excited with a power density of 150 W cm(-2), underlining their high quality. A comparison of the colloidal stability and luminescence properties of these UCNPs, subsequently surface modified with ligand exchange or encapsulation protocols, revealed that the ratio of the green (545 nm) and red (658 nm) emission bands determined at a constant excitation power density clearly depends on the surface chemistry. Modifications relying on the deposition of additional (amphiphilic) layer coatings, where the initial oleate coating is retained, show reduced non-radiative quenching by water as compared to UCNPs that are rendered water-dispersible via ligand exchange. Moreover, we could demonstrate that the brightness of the upconversion luminescence of the UCNPs is strongly affected by the type of surface modification, i.e., ligand exchange or encapsulation, yet hardly by the chemical nature of the ligand.

Upconversion improvement by the reduction of Na+-vacancies in Mn2+ doped hexagonal NaYbF4:Er3+ nanoparticles

A method of Mn²⁺ doping for the simultaneous control of lattice defects and luminescence output in β-NaYbF₄:Er³⁺ upconversion nanoparticles with a fixed composition of both host and dopants of Ln³⁺ is demonstrated.

Synthesis and characterization of upconversion nanoparticles with shell structure and ligand-free hydrophilic modification

Rare-earth upconversion nanoparticles (UCNPs) with α and β phases were prepared. UCNPs with core–shell structure were prepared and modified to be hydrophilic by ligand-free hydrophilic modification.

Synthesis of a novel bifunctional nanocomposite with tunable upconversion emission and magnetic properties

A method of Co²⁺ ions codoping for significantly enhancing upconversion emission intensity and simultaneous controlling sparamagnetic properties in β-NaYF₄:Yb/Er nanoparticles, with well maintaining their morphology and highly disperse.

Current advances in lanthanide ion (Ln3+)-based upconversion nanomaterials for drug delivery

This review mainly focuses on the recent advances in various chemical syntheses of Ln³⁺-based upconversion nanomaterials, with special emphasis on their application in stimuli-response controlled drug release and subsequent therapy.

Dissolution of upconverting fluoride nanoparticles in aqueous suspensions

The partial dissolution of selected nanoparticles (NaYF₄, LaF₃ and GdF₃) co-doped with Yb³⁺ and Tm³⁺ was detected and compared with respect to their size, chemical composition and structure.

The advances in applying inorganic fluorescent nanomaterials for the detection of hepatocellular carcinoma and other cancers

The advances, drawbacks and application suggestions of QDs, UCNPs and CDs in HCC and other cancer detection fields are discussed.

Near-infrared light-controlled drug release and cancer therapy with polymer-caged upconversion nanoparticles

The core–shell nanocarrier, based on spiropyran-containing copolymer coated upconversion nanocomposites, was successfully prepared via a facile self-assembly process for NIR-triggered drug release and cancer therapy.

Size-dependent upconversion luminescence and temperature sensing behavior of spherical Gd2O3:Yb3+/Er3+ phosphor

In this paper, we found that sensing sensitivity increases with decreasing the particle size, which was confirmed by the Judd–Ofelt theory.

Mesoporous NaYF4:Yb,Er@Au–Pt(iv)-FA nanospheres for dual-modal imaging and synergistic photothermal/chemo-anti-cancer therapy

Mesoporous NaYF₄:Yb,Er@Au–Pt(iv)-FA up-conversion nanoparticles have been designed for dual-modal imaging-guided anti-cancer therapy, and show excellent inhibition toward cancer cells due to the synergistic photothermal/chemo-therapy.

Stimuli responsive upconversion luminescence nanomaterials and films for various applications

This review highlights recent advances in upconversion luminescence materials in response to various stimuli for a broad spectrum of applications.

Lab on upconversion nanoparticles: optical properties and applications engineering via designed nanostructure

This review aims to summarize recent progress in optical properties and applications engineering of upconversion nanoparticles via the designed nanostructure.

Phonon-assisted energy back transfer-induced multicolor upconversion emission of Gd2O3:Yb3+/Er3+ nanoparticles under near-infrared excitation

By harnessing the phonon-assisted energy back transfer (EBT) from Er³⁺ to nearby Yb³⁺ ions, we obtain continuous multicolor (from green to red) UC fluorescence in the Gd₂O₃:Yb³⁺/Er³⁺ UCNPs.

A dual-mode luminescent probe composed of co-assembled down-conversion CdTe and up-conversion NaYF4:Yb,Tm(Er) nanoparticles

Water-dispersible dual-mode luminescent probes are fabricated by co-assembling down-conversion CdTe and up-conversion NaYF₄:Yb,Tm(Er) nanoparticles via a ligand-exchange strategy.

Smart pH-responsive upconversion nanoparticles for enhanced tumor cellular internalization and near-infrared light-triggered photodynamic therapy

The smart pH-responsive upconversion nanoparticles are promising agents for deep cancer photodynamic therapy applications.

Simultaneous detection of microcysin-LR and okadaic acid using a dual fluorescence resonance energy transfer aptasensor

Algal toxins can cause neurovirulence, hepatotoxicity, and cytotoxicity in humans through the consumption of contaminated water and food. In this work, we presented a novel aptasensor for the simultaneous detection of two algal toxins, microcysin-LR (MC-LR) and okadaic acid (OA). This system employed green and red upconversion nanoparticle (UCNP) luminescence as the donors and two quenchers (BHQ1 and BHQ3) as the corresponding acceptors. The two donor-acceptor couples were fabricated by hybridizing the aptamers with their corresponding complementary DNA. The results indicated that the green and red upconversion luminescence could be quenched by the quencher probes because of their highly overlapping spectrum. In the presence of MC-LR and OA, the aptamers preferred to bind to their corresponding analytes and de-hybridize with the complementary DNA. This effect became sufficiently large to prevent green and red luminescence quenching. Under the optimized experimental conditions, the relative luminescence intensity increased as the algal toxin concentrations increased, allowing for the quantification of MC-LR and OA. The relationships between the luminescence intensity and plotting logarithms of algal toxin concentrations were linear in the range from 0.1 to 50 ng mL(-1) for MC-LR and OA. As a practical application, this type of dual fluorescence resonance energy transfer (FRET) aptasensor was used to monitor the MC-LR and OA levels in naturally contaminated food samples such as fish and shrimps.

The synthesis and mechanism exploration of europium-doped LiYF4 micro-octahedron phosphors with multilevel interiors

Multi-layered hollow LiYF4:Eu(3+) micro-octahedrons, with about 400 nm of single-layer thickness and 300 nm of interlayer space, have been synthesized via a facile hydrothermal route in the presence of surfactant ethylenediamine tetraacetic acid (EDTA). The mechanisms of the morphology evolution of the LiYF4:Eu micro-octahedrons are investigated in detail. Time-dependent experiments indicate that the growth of the micro-octahedrons undergoes four different stages including the aggregation growth of the primary YF3 particle, the transformation of the substance from the orthorhombic-phase YF3 to the tetragonal-phase LiYF4 by the Kirkendall effect with the inward diffusion of quasi-steady state LiF species, adsorption and in situ crystallization, and local Ostwald ripening. The Ostwald ripening process is terminated by the organic adsorption of interlaminar leading to a hollow structure with multilevel interiors. The LiYF4:Eu micro-octahedrons are annealed under the designed temperatures, which leads to the collapse of octahedral structures indicating the role of EDTA on building the octahedron. The spectral measurements show that the calcination approach has a stronger effect on the luminescence tuning of the LiYF4:Eu micro-octahedrons due to the modification of the crystal phase, structure and size. The present study is of great importance in the preparation of rare-earth ion doped LiYF4 hollow materials as well as in applications as building blocks for functional devices.

Practical implementation, characterization and applications of a multi-colour time-gated luminescence microscope

Time-gated luminescence microscopy using long-lifetime molecular probes can effectively eliminate autofluorescence to enable high contrast imaging. Here we investigate a new strategy of time-gated imaging for simultaneous visualisation of multiple species of microorganisms stained with long-lived complexes under low-background conditions. This is realized by imaging two pathogenic organisms (Giardia lamblia stained with a red europium probe and Cryptosporidium parvum with a green terbium probe) at UV wavelengths (320-400 nm) through synchronization of a flash lamp with high repetition rate (1 kHz) to a robust time-gating detection unit. This approach provides four times enhancement in signal-to-background ratio over non-time-gated imaging, while the average signal intensity also increases six-fold compared with that under UV LED excitation. The high sensitivity is further confirmed by imaging the single europium-doped Y₂O₂S nanocrystals (150 nm). We report technical details regarding the time-gating detection unit and demonstrate its compatibility with commercial epi-fluorescence microscopes, providing a valuable and convenient addition to standard laboratory equipment.

Rare-Earth doped particles as dual-modality contrast agent for minimally-invasive luminescence and dual-wavelength photoacoustic imaging

Multi-modal imaging is an emerging area that integrates multiple imaging modalities to simultaneously capture visual information over many spatial scales. Complementary contrast agents need to be co-developed in order to achieve high resolution and contrast. In this work, we demonstrated that rare-earth doped particles (REDPs) can be employed as dual-modal imaging agents for both luminescence and photoacoustic (PA) imaging to achieve intrinsic high contrast, temporal and spatial resolution, reaching deeper depth. REDPs synthesized with different surfactants (citric acid, polyacrylic acid, ethylenediaminetetraacetic acid and sodium citrate) exhibit tunable emission properties and PA signal amplitudes. Amongst these samples, sodium citrate-modified REDPs showed the strongest PA signals. Furthermore, since REDPs have multiple absorption peaks, they offer a unique opportunity for multi-wavelength PA imaging (e.g. PA signals were measured using 520 and 975 nm excitations). The in vivo PA images around the cortical superior sagittal sinus (SSS) blood vessel captured with enhanced signal arising from REDPs demonstrated that in addition to be excellent luminescent probes, REDPs can also be used as successful PA contrast agents. Anisotropic polyacrylic acid-modified REDPs were found to be the best candidates for dual-modal luminescence and PA imaging due to their strong luminescence and PA signal intensities.

NIR photoregulated chemo- and photodynamic cancer therapy based on conjugated polyelectrolyte-drug conjugate encapsulated upconversion nanoparticles

The design of nanoplatforms with target recognition and near-infrared (NIR) laser photoregulated chemo- and photodynamic therapy is highly desirable but remains challenging. In this work, we have developed such a system by taking advantage of a conjugated polyelectrolyte (CPE)-drug conjugate and upconversion nanoparticles (UCNPs). The poly(ethylene glycol) (PEG) grafted CPE not only serves as a polymer matrix for UCNP encapsulation, but also as a fluorescent imaging agent, a photosensitizer as well as a carrier for chemotherapeutic drug doxorubicin (DOX) through a UV-cleavable ortho-nitrobenzyl (NB) linker. Upon 980 nm laser irradiation, the UCNPs emit UV and visible light. The up-converted UV light is utilized for controlled drug release through the photocleavage of the ortho-nitrobenzyl linker, while the up-converted visible light is used to initiate the polymer photosensitizer to produce reactive oxygen species (ROS) for photodynamic therapy. The NIR photo-regulated UCNP@CPE-DOX showed high efficiency of ROS generation and controlled drug release in cancer cells upon single laser irradiation. In addition, the combination therapy showed enhanced inhibition of U87-MG cell growth as compared to sole treatments. As two light sources with different wavelengths are always needed for traditional photodynamic therapy and photoregulated drug release, the adoption of UCNPs as an NIR light switch is highly beneficial to combined chemo- and photodynamic therapy with enhanced therapeutic effects.

NIR-responsive upconversion nanoparticles stimulate neurite outgrowth in PC12 cells

Nerve regeneration is of diagnostic importance in neuroscience in regards to the treatment of degenerative disease. Owing to the ability to release rare-earth ions and produce ROS during upconversion process, upconversion nanoparticles are first reported for promoting neurite outgrowth. Different charged coating materials which play a critical role in cell attachment, can further lead to different effects on cell differentiation.

The intersection of CMOS microsystems and upconversion nanoparticles for luminescence bioimaging and bioassays

Organic fluorophores and quantum dots are ubiquitous as contrast agents for bio-imaging and as labels in bioassays to enable the detection of biological targets and processes. Upconversion nanoparticles (UCNPs) offer a different set of opportunities as labels in bioassays and for bioimaging. UCNPs are excited at near-infrared (NIR) wavelengths where biological molecules are optically transparent, and their luminesce in the visible and ultraviolet (UV) wavelength range is suitable for detection using complementary metal-oxide-semiconductor (CMOS) technology. These nanoparticles provide multiple sharp emission bands, long lifetimes, tunable emission, high photostability, and low cytotoxicity, which render them particularly useful for bio-imaging applications and multiplexed bioassays. This paper surveys several key concepts surrounding upconversion nanoparticles and the systems that detect and process the corresponding luminescence signals. The principle of photon upconversion, tuning of emission wavelengths, UCNP bioassays, and UCNP time-resolved techniques are described. Electronic readout systems for signal detection and processing suitable for UCNP luminescence using CMOS technology are discussed. This includes recent progress in miniaturized detectors, integrated spectral sensing, and high-precision time-domain circuits. Emphasis is placed on the physical attributes of UCNPs that map strongly to the technical features that CMOS devices excel in delivering, exploring the interoperability between the two technologies.

Spectrally matched upconverting luminescent nanoparticles for monitoring enzymatic reactions

We report on upconverting luminescent nanoparticles (UCLNPs) that are spectrally tuned such that their emission matches the absorption bands of the two most important species associated with enzymatic redox reactions. The core-shell UCLNPs consist of a β-NaYF4 core doped with Yb(3+)/Tm(3+) ions and a shell of pure β-NaYF4. Upon 980 nm excitation, they display emission bands peaking at 360 and 475 nm, which is a perfect match to the absorption bands of the enzyme cosubstrate NADH and the coenzyme FAD, respectively. By exploiting these spectral overlaps, we have designed fluorescent detection schemes for NADH and FAD that are based on the modulation of the emission intensities of UCLNPs by FAD and NADH via an inner filter effect.

Near-infrared light triggered photodynamic therapy in combination with gene therapy using upconversion nanoparticles for effective cancer cell killing

Upconversion nanoparticles (UCNPs) have drawn much attention in cancer imaging and therapy in recent years. Herein, we for the first time report the use of UCNPs with carefully engineered surface chemistry for combined photodynamic therapy (PDT) and gene therapy of cancer. In our system, positively charged NaGdF4:Yb,Er UCNPs with multilayered polymer coatings are synthesized via a layer by layer strategy, and then loaded simultaneously with Chlorin e6 (Ce6), a photosensitizing molecule, and small interfering RNA (siRNA), which targets the Plk1 oncogene. On the one hand, under excitation by a near-infrared (NIR) light at 980 nm, which shows greatly improved tissue penetration compared with visible light, cytotoxic singlet oxygen can be generated via resonance energy transfer from UCNPs to photosensitizer Ce6, while the residual upconversion luminescence is utilized for imaging. On the other hand, the silencing of Plk1 induced by siRNA delivered with UCNPs could induce significant cancer cell apoptosis. As the result of such combined photodynamic and gene therapy, a remarkably enhanced cancer cell killing effect is realized. Our work thus highlights the promise of UCNPs for imaging guided combination therapy of cancer.

Highly biocompatible zwitterionic phospholipids coated upconversion nanoparticles for efficient bioimaging

The potential of upconversion nanoparticles (UCNPs) in various biomedical applications, including immunoassays, biomedical imaging, and molecular sensing, requires their surface derivatized to be hydrophilic and biocompatible. Here, a new family of compact zwitterionic ligand systems composed with functional phospholipids was designed and used for the surface modification of UCNPs. The zwitterionic UCNPs are hydrophilic, compact, and easily functionalized. It was proved that zwitterionic phospholipids could provide UCNPs with not only extended pH and salt stability but also little nonspecific interactions to positively and negatively charged proteins, low nonspecific adhesion in live-cell imaging process. Most notably, the efficient in vivo tumor imaging performance and long blood circulation half-life suggests the excellent biocompatibility for in vivo imaging of the zwitterionic UCNPs.

Enzyme-responsive cell-penetrating peptide conjugated mesoporous silica quantum dot nanocarriers for controlled release of nucleus-targeted drug molecules and real-time intracellular fluorescence imaging of tumor cells

Here, a set of novel and personalized nanocarriers are presented for controlled nucleus-targeted antitumor drug delivery and real-time imaging of intracellular drug molecule trafficking by integrating an enzyme activatable cell penetrating peptide (CPP) with mesoporous silica coated quantum dots nanoparticles. Upon loading of antitumor drug, doxorubicin (DOX) and further exposure to proteases in tumor cell environment, the enzymatic cleavage of peptide sequence activates oligocationic TAT residues on the QDs@mSiO2 surface and direct the DOX delivery into cellular nucleus. The systematic cell imaging and cytotoxicity studies confirm that the enzyme responsive DOX-loaded CPP-QDs@mSiO2 nanoparticles can selectively release DOX in the tumor cells with high cathepsin B enzyme expression and greatly facilitate DOX accumulation in targeted nucleus, thus exhibiting enhanced antitumor activity in these cells. As contrast, there is limited nuclear-targeted drug accumulation and lower tumor cytotoxicity observed in the cells without enzyme expression. More importantly, significant antitumor DOX accumulation and higher tumor inactivation is also found in the drug resistant tumor cells with targeted enzyme expression. Such simple and specific enzyme responsive mesoporous silica-QDs nanoconjugates provide great promise for rational design of targeted drug delivery into biological system, and may thus greatly facilitate the medical theranostics in the near future.

Biomedical applications of functionalized hollow mesoporous silica nanoparticles: focusing on molecular imaging

Hollow mesoporous silica nanoparticles (HMSNs), with a large cavity inside each original mesoporous silica nanoparticle, have recently gained increasing interest owing to their tremendous potential for cancer imaging and therapy. The last several years have witnessed a rapid development in the engineering of functionalized HMSNs (i.e., f-HMSNs), with various types of inorganic functional nanocrystals integrated into the system for imaging and therapeutic applications. In this article, we summarize the recent progress in the design and biological applications of f-HMSNs, with a special emphasis on molecular imaging. Commonly used synthetic strategies for the generation of high quality HMSNs will be discussed in detail, followed by a systematic review of engineered f-HMSNs for optical, PET, MRI and ultrasound imaging in preclinical studies. Finally, we discuss the challenges and future research directions regarding the use of f-HMSNs for cancer imaging and therapy.

Multihydroxy dendritic upconversion nanoparticles with enhanced water dispersibility and surface functionality for bioimaging

Upconversion nanoparticle (UCNP) as a new class of imaging agent is gaining prominence because of its unique optical properties. An ideal UCNP for bioimaging should simultaneously possess fine water dispersibility and favorable functional groups. In this paper, we present a simple but effective method to the synthesis of a UCNP-based nanohybrid bearing a multihydroxy hyperbranched polyglycerol (HPG) shell by the combination of a "grafting from" strategy with a ring-opening polymerization technique. The structure and morphology of the resulting UCNP-g-HPG nanohybrid were characterized in detail by Fourier transform infrared, (1)H NMR, thermogravimetric analysis, and transmission electron microscopy measurements. The results reveal that the amount of grafted HPG associated with the thickness of the HPG shell can be well tuned. UCNP-g-HPG shows high water dispersibility and strong and stable upconversion luminescence. On the basis of its numerous surface hydroxyl groups, UCNP-g-HPG can be tailored by a representative fluorescent dye rhodamine B to afford a UCNP-g-HPG-RB nanohybrid that simultaneously presents upconversion and downconversion luminescence. Preliminary biological studies demonstrate that UCNP-g-HPG shows low cytotoxicity, high luminescent contrast, and deep light penetration depth, posing promising potential for bioimaging applications.

NaYF4 nanocrystals with TOPO ligands: synthesis-dependent structural and luminescent properties

A comprehensive characterization of NaYF4 nanocrystals synthesized in trioctylphosphine oxide has been reported in order to present an effective method of monodisperse, small, hexagonal nanocrystal synthesis in a high boiling organic solvent via a co-thermolysis pathway. We observed the influence of temperature, Na/Y precursors ratio and time of the synthesis on the nanocrystals size, shape and crystal structure. For that purpose, we characterized the structure of as-synthesized nanocrystals by X-ray diffraction and transmission electron microscopy. Moreover, all nanocrystals were doped with Eu(3+) ions, which were used as an optical crystal field probe. We applied photoluminescence, PL excitation and absorbance spectra to determine the influence of crystal symmetry, surface to volume ratio and ligands on the optical properties of doped Eu(3+) ions. It was found that trioctylphosphine oxide reduces the free-energy barrier and stimulates the NaYF4 crystallization in the hexagonal phase, even at relatively low temperature. A similar effect was observed when the excess of sodium trifluoroacetate precursors was used. Moreover, the presented nanocrystal evolution within synthesis time confirmed that at suitable conditions NaYF4 crystallized in the hexagonal phase within less than 5 min. Optical spectroscopy investigations confirmed the high quality of small β-NaYF4:Eu(3+) nanocrystals, which are promising candidates for e.g. optical markers in the visible wavelength range.

Quantum dot-based multidonor concentric FRET system and its application to biosensing using an excitation ratio

A plethora of semiconductor quantum dot (QD)-based probes that rely on Förster resonance energy transfer (FRET) have been developed for the optical detection of a wide array of biological targets. To date, the vast majority of these probes have utilized one-step energy transfer between individual donor-acceptor pairs. Here, we report a new multidonor concentric FRET configuration that comprised two fluorescent dyes assembled around a central CdSeS/ZnS QD through peptide linkers. One of these dyes, either Alexa Fluor 555 (A555) or Alexa Fluor 647 (A647), served as an acceptor for both the central QD and the other coassembled dye, Alexa Fluor 488 (A488). The unresolved emission between the A488 and the QD precluded a standard analysis of FRET efficiency from quenching of donor emission intensity or decay time, instead necessitating an analysis of the two energy transfer pathways from deconvolved excitation spectra. When A647 was the terminal acceptor, both the QD-to-A647 and A488-to-A647 energy transfer pathways could be interrogated with blue light, but only the former could be interrogated with violet light. The different degrees of A647 sensitization between these two excitation wavelengths was a predictable function of the above energy transfer efficiencies and dye stoichiometry, and was exploited for quantitative bioanalysis through an excitation ratio, which is in contrast to the conventional use of an emission ratio with FRET-based probes. Detection of the activity of nanomolar concentrations of trypsin, a model protease that hydrolyzed the A488-labeled peptide linker, was demonstrated using both a fluorescence plate reader and a low-cost, compact device that used two low-power light-emitting diodes (LEDs) as excitation sources and a silicon photodiode to detect A647 emission. This multidonor concentric FRET configuration represents a new modality for ratiometric biosensing with QDs and is potentially useful for portable in vitro diagnostics.

Lanthanide upconversion nanoparticles and applications in bioassays and bioimaging: a review

Through the process of photon upconversion, trivalent lanthanide doped nanocrystals convert long-wavelength excitation radiation in the infrared or near infrared region to higher energy emission radiation from ultraviolet to infrared. Such materials offer potential for numerous advantages in analytical applications in comparison to molecular fluorophores and quantum dots. The use of IR radiation as an excitation source reduces autofluorescence and scattering of excitation radiation, which leads to a reduction of background in optical experiments. The upconverting nanocrystals offer excellent photostability and are composed of materials that are not particularly toxic to biological organisms. Excitation at long wavelengths also minimizes damage to biological materials. In this review, the different mechanisms responsible for the upconversion process, and methods that are used to synthesize and decorate upconverting nanoparticles are presented to indicate how absorption and emission can be tuned. Examples of recent applications of upconverting nanoparticles in bioassays for the detection of proteins, nucleic acids, metabolites and metal ions offer indications of analytical advantages in the development of methods of analysis. Examples include multi-color and multi-modal imaging, and the use of upconverting nanoparticles in theranostics.

Near-infrared upconversion nanoparticles for bio-applications

Upconversion nanoparticles (UCNs) attract intensive attentions in biomedical applications. They have shown great potential in bioimaging, biomolecule detection, drug delivery, photodynamic therapy and cellular molecules interactions. Due to the anti-Stokes optical property and NIR excitation, UCNs overcome the drawbacks encountered in conventional luminescent biomarkers. High signal to noise ratio, low cytotoxicity and stable high throughput results are obtained using UCNs as luminescent labels or light triggers in biomedical applications. In this review article, the reason for choosing UCNs as biomedical agents, the progress of the UCNs development and case studies of their biomedical applications will be discussed.

Upconversion fluorescence-SERS dual-mode tags for cellular and in vivo imaging

Fluorescent-surface enhanced Raman scattering (F-SERS) dual mode tags showed great potential for bioimaging due to the combined advantages of intuitive, fast imaging of fluorescence and multiplex capability of SERS technique. In previously reported F-SERS tags, organic fluorescent dyes or quantum dots were generally selected to generate fluorescence signal. Herein, we reported the first proof-of-concept upconversion fluorescence (UCF)-SERS dual mode tags based on near infrared (NIR) laser (980 nm) excited upconversion nanoparticles (UCNPs) for live-cell and in vivo imaging. Three components involved in this tag: NaYF4:Yb,Er UCNPs@SiO2 serving as the fluorescent core of the tag; silver nanoparticles in situ grown on the surface of UCNPs@SiO2 for generating characteristic Raman signal; and denatured BSA coating rendering the tag's stability and biocompatibility. The UCF-SERS tags integrated the NIR imaging capability of both fluorescent UCNPs and plasmonic SERS nanoprobe, which facilitated dual mode bioimaging investigation, especially for living animals. Ex vivo experiments revealed that with 980 nm and 785 nm NIR laser irradiations, the UCF and SERS signals of the tags could be detected from 3 and 7 mm deep pork tissues, respectively. Furthermore, the in vivo imaging capabilities of UCF-SERS tags were successfully demonstrated on living mice. The developed dual modality tags held great potential for medical diagnostics and therapy.

MC540 and upconverting nanocrystal coloaded polymeric liposome for near-infrared light-triggered photodynamic therapy and cell fluorescent imaging

In clinic, the application of photodynamic therapy (PDT) in deep tissue is severely constrained by the limited penetration depth of visible light needed for activating the photosensitizer (PS). In this Article, a merocyanine 540 (MC540) and upconverting nanoparticle (UCN) coloaded functional polymeric liposome nanocarrier, (MC540 + UCN)/FPL, was designed and constructed successfully for solving this problem in PDT. Compared with the conventional approaches using UCNs absorbing PSs directly, the combination of UCN and polymeric liposome has unique advantages. The UCN core as a transducer can convert deep-penetrating near-infrared light to visible light for activating MC540. The functional polymeric liposome shell decorated with folate as a nanoshield can keep the UCN and MC540 stable, protect them from being attacked, and help them get into cells. The results show that (MC540 + UCN)/FPL is an individual nanosphere with an average size of 26 nm. MC540 can be activated to produce singlet oxygen successfully by upconverting fluorescence emitted from UCNs. After (MC540 + UCN)/FPL was modified with folate, the cell uptake efficiency increased obviously. More interestingly, in the PDT effect test, the (MC540 + UCN)/FPL nanocarrier further improved the inhibition effect on tumor cells by anchoring targeting folate and transactivating transduction peptide. Our data suggest that the (MC540 + UCN)/FPL nanocarrier may be a useful nanoplatform for future PDT treatment in deep-cancer therapy based on upconversion mechanism.

One-pot template-free synthesis of NaYF4 upconversion hollow nanospheres for bioimaging and drug delivery

Hollow-structured nanomaterials with fluorescent properties are extremely attractive for image-guided cancer therapy. In this paper, sub-100 nm and hydrophilic NaYF4 upconversion (UC) hollow nanospheres (HNSs) with multicolor UC luminescence and drug-delivery properties were successfully prepared by a facile one-pot template-free hydrothermal route using polyetherimide (PEI) polymer as the stabilizing agent. XRD, SEM, TEM, and N2-adsorption/desorption were used to characterize the as-obtained products. The growth mechanism of the HNSs has been systematically investigated on the basis of the Ostwald ripening. Under 980 nm excitation, UC emissions of HNSs can be tuned by a simple change of the concentration or combination of various upconverters. As a result, the PEI-coated HNSs could be used as efficient probes for in vitro upconversion luminescence (UCL) cell imaging. Furthermore, a doxorubicin storage/release behavior and cancer-cell-killing ability investigation reveal that the product has the potential to be a drug carrier for cancer therapy.

Monodispersed LaF3 nanocrystals: shape-controllable synthesis, excitation-power-dependent multi-color tuning and intense near-infrared upconversion emission

In this study, monodispersed and high-quality hexagonal phase LaF3 nanocrystals with different shapes and sizes were synthesized by a solvothermal method using oleic acid as the stabilizing agent. The as-prepared LaF3 nanocrystals were characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD), and analysis of the upconversion spectra. The TEM results reveal that the samples present high uniformity and monodispersity and are self-assembled into a two-dimensional ordered array. Moreover, the shape, size and structure of the nanocrystals can be readily tuned by adjusting the NaF content. With increasing content of NaF, the shape of the LaF3 nanocrystals changed from particle to rod and the size gradually increased. More importantly, high NaF content favors the formation of one-dimensional nanorods. High Y b(3+) and Er(3+) content is beneficial to synthesizing the hexagonal phase of NaLaF4 nanocrystals. Furthermore, the TEM results show that the shape and size of the LaF3 nanocrystals can also be tuned by doping lanthanide ions, which provides a new route for size and shape control of nanocrystals. In addition, LaF3 nanocrystals co-doped with Y b(3+)/Tm(3+) present efficient near-infrared (NIR)-NIR upconversion luminescence. More importantly, the upconversion luminescent colors can be readily tuned from blue-white to blue by adjusting the excitation power. Therefore, it is expected that these LaF3 nanocrystals with well-controlled shape, size and NIR-NIR upconversion emission have potential applications in biomedical imaging fields.

Recent Advance of Biological Molecular Imaging Based on Lanthanide-Doped Upconversion-Luminescent Nanomaterials

Lanthanide-doped upconversion-luminescent nanoparticles (UCNPs), which can be excited by near-infrared (NIR) laser irradiation to emit multiplex light, have been proven to be very useful for in vitro and in vivo molecular imaging studies. In comparison with the conventionally used down-conversion fluorescence imaging strategies, the NIR light excited luminescence of UCNPs displays high photostability, low cytotoxicity, little background auto-fluorescence, which allows for deep tissue penetration, making them attractive as contrast agents for biomedical imaging applications. In this review, we will mainly focus on the latest development of a new type of lanthanide-doped UCNP material and its main applications for in vitro and in vivo molecular imaging and we will also discuss the challenges and future perspectives.

Advances in noninvasive functional imaging of bone

The demand for functional imaging in clinical medicine is comprehensive. Although the gold standard for the functional imaging of human bones in clinical settings is still radionuclide-based imaging modalities, nonionizing noninvasive imaging technology in small animals has greatly advanced in recent decades, especially the diffuse optical imaging to which Britton Chance made tremendous contributions. The evolution of imaging probes, instruments, and computation has facilitated exploration in the complicated biomedical research field by allowing longitudinal observation of molecular events in live cells and animals. These research-imaging tools are being used for clinical applications in various specialties, such as oncology, neuroscience, and dermatology. The Bone, a deeply located mineralized tissue, presents a challenge for noninvasive functional imaging in humans. Using nanoparticles (NP) with multiple favorable properties as bioimaging probes has provided orthopedics an opportunity to benefit from these noninvasive bone-imaging techniques. This review highlights the historical evolution of radionuclide-based imaging, computed tomography, positron emission tomography, and magnetic resonance imaging, diffuse optics-enabled in vivo technologies, vibrational spectroscopic imaging, and a greater potential for using NPs for biomedical imaging.