Upconversion fluorescence resonance energy transfer in a homogeneous immunoassay for estradiol

Modulation of proteins by rare earth elements as a biotechnological tool

Although rare earth element (REE) complexes are often utilized in bioimaging due to their photo- and redox stability, magnetic and optical characteristics, they are also applied for pharmaceutical applications due to their interaction with macromolecules namely proteins. The possible implications induced by REEs through modification in the function or regulatory activity of the proteins trigger a variety of applications for these elements in biomedicine and biotechnology. Lanthanide complexes have particularly been applied as anti-biofilm agents, cancer inhibitors, potential inflammation inhibitors, metabolic elicitors, and helper agents in the cultivation of unculturable strains, drug delivery, tissue engineering, photodynamic, and radiation therapy. This paper overviews emerging applications of REEs in biotechnology, especially in biomedical imaging, tumor diagnosis, and treatment along with their potential toxic effects. Although significant advances in applying REEs have been made, there is a lack of comprehensive studies to identify the potential of all REEs in biotechnology since only four elements, Eu, Ce, Gd, and La, among 17 REEs have been mostly investigated. However, in depth research on ecotoxicology, environmental behavior, and biological functions of REEs in the health and disease status of living organisms is required to fill the vital gaps in our understanding of REEs applications.

Nanoparticle-Based Bioaffinity Assays: From the Research Laboratory to the Market

Advances in the development of new biorecognition elements, nanoparticle-based labels as well as instrumentation have inspired the design of new bioaffinity assays. This review critically discusses the potential of nanoparticles to replace current enzymatic or molecular labels in immunoassays and other bioaffinity assays. Successful implementations of nanoparticles in commercial assays and the need for rapid tests incorporating nanoparticles in different roles such as capture support, signal generation elements, and signal amplification systems are highlighted. The limited number of nanoparticles applied in current commercial assays can be explained by challenges associated with the analysis of real samples (e.g., blood, urine, or nasal swabs) that are difficult to resolve, particularly if the same performance can be achieved more easily by conventional labels. Lateral flow assays that are based on the visual detection of the red-colored line formed by colloidal gold are a notable exception, exemplified by SARS-CoV-2 rapid antigen tests that have moved from initial laboratory testing to widespread market adaption in less than two years.

Understanding FRET in Upconversion Nanoparticle Nucleic Acid Biosensors

Upconversion nanoparticles (UCNPs) have been frequently applied in Förster resonance energy transfer (FRET) bioanalysis. However, the understanding of how surface coatings, bioconjugation, and dye-surface distance influence FRET biosensing performance has not significantly advanced. Here, we investigated UCNP-to-dye FRET DNA-hybridization assays in H₂O and D₂O using ∼24 nm large NaYF₄:Yb³⁺,Er³⁺ UCNPs coated with thin layers of silica (SiO₂) or poly(acrylic acid) (PAA). FRET resulted in strong distance-dependent PL intensity changes. However, the PL decay times were not significantly altered because of continuous Yb³⁺-to-Er³⁺ energy migration during Er³⁺-to-dye FRET. Direct bioconjugation of DNA to the thin PAA coating combined with the closest possible dye-surface distance resulted in optimal FRET performance with minor influence from competitive quenching by H₂O. The better comprehension of UCNP-to-dye FRET was successfully translated into a microRNA (miR-20a) FRET assay with a limit of detection of 100 fmol in a 80 μL sample volume.

AIEgens: Next Generation Signaling Source for Immunoassays?

Luminogens with aggregation-induced emission (AIEgens) properties have numerous broad applications in fields of chemical and biological analyses due to their exceptional photostability, excellent signal reliability, high quantum yield, and large Stokes' shift. In particular, AIEgens also bring new blood for immunoassay. Since publication of the first 2004 paper, AIEgens-based immunoassays have received significant attention because of their high sensitivity, specificity, accuracy, and reliability. However, until now, there have been no comprehensive literature reviews focused on the evolving field of AIEgens-based immunoassays. Thus, we have extensively reviewed AIEgens-based immunoassays from their basic working principles to specific applications. We focus on several fundamental elements of AIEgens-based immunoassays, including the typical structures of AIEgens, emission mechanism of AIEgens probes, function of AIEgens in immunoassays, and platform of AIEgens-based immunoassays. Then, the representative applications of AIEgens-based immunoassays in food safety, medical diagnostics, and environmental monitoring are explored. Thus, proposals on how to further improve the AIEgens-based immunoassay performance are also discussed, as well as future challenges and perspectives, aiming to provide brief and valid guidelines for choosing suitable AIEgens-based immunoassays according to specific application requirements.

Sensors/nanosensors based on upconversion materials for the determination of pharmaceuticals and biomolecules: An overview

Upconversion materials have been the focus of a large body of research in analytical and clinical fields in the last two decades owing to their ability to convert light between various spectral regions and their particular photophysical features. They emit efficient and sharp ultraviolet (UV) or visible luminescence after excitation with near-infrared (NIR) light. These features overcome some of the disadvantages reported for conventional fluorescent materials and provide opportunities for high sensitivity chemo-and bio-sensing. Here, we review studies that used upconversion materials as sensors for the determination of pharmaceuticals and biomolecules in the last two decades. The articles included in this review were retrieved from the SCOPUS database using the search phrases: "upconversion nanoparticles for determination of pharmaceutical compounds", and "upconversion nanoparticles for determination of biomolecules". Details of each developed upconversion nanoparticles based sensor along with their relevant analytical parameters are reported and carefully explained.

Enhancing FRET biosensing beyond 10 nm with photon avalanche nanoparticles

Förster Resonance Energy Transfer (FRET) between donor (D) and acceptor (A) molecules is a phenomenon commonly exploited to study or visualize biological interactions at the molecular level. However, commonly used organic D and A molecules often suffer from photobleaching and spectral bleed-through, and their spectral properties hinder quantitative analysis. Lanthanide-doped upconverting nanoparticles (UCNPs) as alternative D species offer significant improvements in terms of photostability, spectral purity and background-free luminescence detection, but they bring new challenges related to multiple donor ions existing in a single large size UCNP and the need for nanoparticle biofunctionalization. Considering the relatively short Förster distance (typically below 5-7 nm), it becomes a non-trivial task to assure sufficiently strong D-A interaction, which translates directly to the sensitivity of such bio-sensors. In this work we propose a solution to these issues, which employs the photon avalanche (PA) phenomenon in lanthanide-doped materials. Using theoretical modelling, we predict that these PA systems would be highly susceptible to the presence of A and that the estimated sensitivity range extends to distances 2 to 4 times longer (i.e. 10-25 nm) than those typically found in conventional FRET systems. This promises high sensitivity, low background and spectral or temporal biosensing, and provides the basis for a radically novel approach to combine luminescence imaging and self-normalized bio-molecular interaction sensing.

Development of anti-immunocomplex specific antibodies and non-competitive time-resolved fluorescence immunoassay for the detection of estradiol

Detection of circulatory estradiol has widespread use in various clinical applications. Particularly, the use of estradiol-specific antibodies in immunoassays is routinely used, mainly due to the cost efficiency and simplicity of the sample handling process. However, the circulatory levels of estradiol can be extremely low in some conditions, and beyond the current detection limit of existing competitive immunoassays. We describe the generation of anti-immunocomplex specific antibodies derived from synthetic antibody repertoire and the development of high-performance non-competitive immunoassay for the detection of estradiol. Phage display selections were used to isolate new antibodies from synthetic antibody library with the use of existing estradiol specific Fab fragment. The found antibodies were consecutively used to set up a time-resolved fluorescence-based immunoassay (TRFIA), which can be used to detect estradiol with exceptional sensitivity and specificity. The limit of detection and EC50 were shown to be 3.0 pg mL⁻¹ and 32.4 pg mL⁻¹ respectively.

Luminescent conjugates between dinuclear rhenium complexes and 17α-ethynylestradiol: synthesis, photophysical characterization, and cell imaging

New luminescent conjugates between dinuclear rhenium complexes and an estradiol moiety.

Asynchrony of spectral blue-shifts of quantum dot based digital homogeneous immunoassay

We present a femtomolar digital homogeneous immunoassay for the detection of cancer biomarkers.

Targeted and efficient activation of channelrhodopsins expressed in living cells via specifically-bound upconversion nanoparticles

Optogenetics is an innovative technology now widely adopted by researchers in different fields of biological sciences. However, most light-sensitive proteins adopted in optogenetics are excited by ultraviolet or visible light which has a weak tissue penetration capability. Upconversion nanoparticles (UCNPs), which absorb near-infrared (NIR) light to emit shorter wavelength light, can help address this issue. In this report, we demonstrated the target selectivity by specifically conjugating the UCNPs with channelrhodopsin-2 (ChR2). We tagged the V5 epitope to the extracellular N-terminal of ChR2 (V5-ChR2m) and functionalized the surface of UCNPs with NeutrAvidin (NAv-UCNPs). After the binding of the biotinylated antibody against V5 onto the V5-ChR2m expressed in the plasma membrane of live HEK293T cells, our results showed that the NAv-UCNPs were specifically bound to the membrane of cells expressing V5-ChR2m. Without the V5 epitope or NAv modification, no binding of UCNPs onto the cell membrane was observed. For the cells expressing V5-ChR2m and bound with NAv-UCNPs, both 488 nm illumination and the upconverted blue emission from UCNPs by 980 nm excitation induced an inward current and elevated the intracellular Ca²⁺ concentration. Our design reduces the distance between UCNPs and light-sensitive proteins to the molecular level, which not only minimizes the NIR energy required but also provides a way to guide the specific binding for optogenetics applications.

Bioluminescent Indicator for Highly Sensitive Analysis of Estrogenic Activity in a Cell-Based Format

Estrogens regulate different physiological systems with wide ranges of concentrations. The rapid analysis of estrogens is crucially important for drug discovery and medical diagnosis, but quantitation of nanomolar estrogens in live cells persists as an important challenge. We herein describe a bioluminescent indicator used to detect low concentrations of estrogens quantitatively with a high signal-to-background ratio. The indicator comprises a ligand-binding domain of an estrogen receptor connected with its binding peptide, which is sandwiched between split fragments of a luciferase mutant. Results show that the indicator recovered its bioluminescence upon binding to 17β-estradiol at concentrations higher than 1.0 × 10^-10 M. The indicator was reactive to agonists but did not respond to antagonists. The indicator is expected to be applicable for rapid screening estrogenic compounds and inhibitors, facilitating the discovery of drug candidates in a high-throughput manner.

Fluorescence resonance energy transfer biosensor between upconverting nanoparticles and palladium nanoparticles for ultrasensitive CEA detection

An ultrasensitive biosensor for carcinoembryonic antigen (CEA) was constructed based on fluorescence resonance energy transfer (FRET) between upconverting nanoparticles (UCPs) and palladium nanoparticles (PdNPs). PdNPs was synthesized by the addition of a solution of Na2PdCl4 into a mixture of N2H4·H2O as the reducing agent and 11-mercaptoundecanoic acid (MUDA) as the stabilizer. The CEA aptamer (5'-NH2-ATACCAGCTTATTCAATT-3') was conjugated to hexanedioic acid (HDA) modified UCPs (HDA-UCPs) through an EDC-NHS coupling protocol. The coordination interaction between nitrogen functional groups of the CEA aptamer and PdNPs brought UCPs and PdNPs in close proximity, which resulted in the fluorescence quenching of UCPs to an extent of 85%. And the non-specific fluorescence quenching caused by PdNPs towards HDA-UCPs was negligible. After the introduction of CEA into the UCPs-CEA aptamer-PdNPs fluorescence quenching system, the CEA aptamer preferentially combined with CEA accompanied by the conformational change which weakened the coordination interaction between the CEA aptamer and PdNPs. So fluorescence recovery of UCPs was observed and a linear relationship between the fluorescence recovery of UCPs and the concentration of CEA was obtained in the range from 2pg/mL to 100pg/mL in the aqueous buffer with the detection limit of 0.8pg/mL. The ultrasensitive detection of CEA was also realized in diluted human serum with a linear range from 4pg/mL to 100pg/mL and a detection limit of 1.7pg/mL. This biosensor makes the most of the high quenching ability of PdNPs towards UCPs with negligible non-specific fluorescence quenching and has broad application prospects in biochemistry.

A highly efficient supramolecular photoswitch for singlet oxygen generation in water

A series of water-soluble supramolecular assemblies were constructed from dithienylethene-modified permethyl-β-cyclodextrins and porphyrin derivatives, accompanied by a high FRET efficiency, and could be applied in the control of singlet oxygen generation in a 1% ethanol aqueous solution upon irradiation of different wavelength light. These findings will provide a feasible and convenient way to construct a potential photodynamic therapy material.

The progress of single-band upconversion nanomaterials

An overview of the recent progresses in realizing single-band UC emission through different methods and the related mechanisms are given for the first time. The challenges and future perspectives of these novel NMs are stated.

Surface modification and characterization of photon-upconverting nanoparticles for bioanalytical applications

Photon-upconverting nanoparticles (UCNPs) can be excited by near-infrared light and emit visible light (anti-Stokes emission) which prevents autofluorescence and light scattering of biological samples. The potential for background-free imaging has attracted wide interest in UCNPs in recent years. Small and homogeneous lanthanide-doped UCNPs that display high upconversion efficiency have typically been synthesized in organic solvents. Bioanalytical applications, however, require a subsequent phase transfer to aqueous solutions. Hence, the surface properties of UCNPs must be well designed and characterized to grant both a stable aqueous colloidal dispersion and the ability to conjugate biomolecules and other ligands on the nanoparticle surface. In this review, we introduce various routes for the surface modification of UCNPs and critically discuss their advantages and disadvantages. The last part covers various analytical methods that enable a thorough examination of the progress and success of the surface functionalization.

Chemical sensing and imaging based on photon upconverting nano- and microcrystals: a review

The demand for photostable luminescent reporters that absorb and emit light in the red to near-infrared (NIR) spectral region continues in biomedical research and bioanalysis. In recent years, classical organic fluorophores have increasingly been displaced by luminescent nanoparticles. These consist of either polymer or silica based beads that are loaded with luminescent dyes, conjugated polymers, or inorganic nanomaterials such as semiconductor nanocrystals (quantum dots), colloidal clusters of silver and gold, or carbon dots. Among the inorganic materials, photon upconversion nanocrystals exhibit a high potential for application to bioimaging or biomolecular assays. They offer an exceptionally high photostability, can be excited in the NIR, and their anti-Stokes emission enables luminescence detection free of background and perturbing scatter effects even in complex biological samples. These lanthanide doped inorganic crystals have multiple emission lines that can be tuned by the selection of the dopants.This review article is focused on the applications of functionalized photon upconversion nanoparticles (UCNPs) to chemical sensing. This is a comparatively new field of research activity and mainly directed at the sensing and imaging of ubiquitous chemical analytes in biological samples, particularly in living cells. For this purpose, the particles have to be functionalized with suitable indicator dyes or recognition elements, as they do not show an intrinsic or specific luminescence response to most of these analytes (e.g. pH, oxygen, metal ions). We describe the strategies for the design of such responsive nanocomposites utilizing either luminescence resonance energy transfer or emission-reabsorption (inner filter effect) mechanisms and also highlight examples for their use either immobilized in sensor layers or directly as nanoprobes for intracellular sensing and imaging.

Detection of 17β-estradiol in water samples by a novel double-layer molecularly imprinted film-based biosensor

This study reports a novel double-layer molecularly imprinted film (MIF)-based biosensor for rapid, sensitive and highly selective detection of small molecule 17β-estradiol (E2) that is frequently detected in environmental water samples. In this system, the modification of gold surface of SPR chip was performed by 1-dodecanethiol. Then double-layer MIF was generated on the 1-dodecanethiol modified gold surface. The non-modified and imprinted surfaces were characterized by atomic force microscopy (AFM), scanning electron microscope (SEM) and contact angle measurements. Analysis of surface plasmon resonance (SPR) spectroscopy showed that the imprinted sensing film displayed good selectivity for E2 compared to other analog molecules and NIF. A good linear relationship was obtained between the SPR angle and E2 concentrations over a range of 2.50×10(-13)-2.50×10(-)(9)mol/L (R(2)=0.993) with the lowest measurable concentration of 2.50×10(-13)mol/L. The sensor can be regenerated with the mixture of acetic acid and PBS buffer (v/v=1:9) as a desorption agent over tens of times without significant deterioration of the sensor performance. Potential interference of real environmental sample matrix was assessed by spiked samples in several waste seawater effluents. This portable sensor system can be successfully applied for on-site real-time inexpensive and easy-to-use monitoring of E2 or other small molecule pollutants in environmental samples such as effluents or water bodies.

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.

Solid-phase covalent immobilization of upconverting nanoparticles for biosensing by luminescence resonance energy transfer

Monodisperse water-soluble upconverting nanoparticles (UCNPs) were immobilized onto modified glass substrates for development of biosensing surfaces that operated using luminescence resonance energy transfer (LRET). Amine modified UCNPs were prepared from oleic acid capped UCNPs by ligand exchange using o-phosphorylethanolamine (PEA). PEA-UCNPs were covalently immobilized on aldehyde functionalized coverslips. Environmental scanning electron microscopy (ESEM) images indicated a homogeneous distribution of UCNPs on surfaces with a high immobilization density of approximately 1.3 × 10(11) UCNP cm(-2). This is the first account of covalent immobilization of UCNPs for bioassay and biosensor development where the density is on par with the high immobilization densities reported for other types of nanoparticles. The functionality and stability of the immobilized NPs were demonstrated by examining an LRET-based bioassay. The well-known sandwich assay for the detection of thrombin was selected as a model in which UCNPs were used as donors and quantum dots (QDs) as acceptors. The closely packed UCNPs on the glass surface showed a 2.5-fold enhancement in assay sensitivity compared to less-densely packed surfaces. In addition, a 1.5-fold enhancement in energy transfer efficiency was shown for solid-phase compared to solution-phase LRET.

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.

Phospholipid-modified upconversion nanoprobe for ratiometric fluorescence detection and imaging of phospholipase D in cell lysate and in living cells

Phospholipase D (PLD) is a critical component of intracellular signal transduction and has been implicated in many important biological processes. It has been observed that there are abnormalities in PLD expression in many human cancers, and PLD is thus recognized as a potential diagnostic biomarker as well as a target for drug discovery. We report for the first time a phospholipid-modified nanoprobe for ratiometric upconversion fluorescence (UCF) sensing and bioimaging of PLD activity. The nanoprobe can be synthesized by a facile one-step self-assembly of a phospholipid monolayer composed of poly(ethylene glycol) (PEG)ylated phospholipid and rhodamine B-labeled phospholipid on the surface of upconversion nanoparticles (UCNPs) NaYF4: 20%Yb, 2%Er. The fluorescence resonance energy transfer (FRET) process from the UCF emission at 540 nm of the UCNPs to the absorbance of the rhodamine B occurs in the nanoprobe. The PLD-mediated hydrolysis of the phosphodiester bond makes rhodamine B apart from the UCNP surface, leading to the inhibition of FRET. Using the unaffected UCF emission at 655 nm as an internal standard, the nanoprobe can be used for ratiometric UCF detection of PLD activity with high sensitivity and selectivity. The PLD activity in cell lysates is also determined by the nanoprobe, confirming that PLD activity in a breast cancer cell is at least 7-fold higher than in normal cell. Moreover, the nanoprobe has been successfully applied to monitoring PLD activity in living cells by UCF bioimaging. The results reveal that the nanoprobe provides a simple, sensitive, and robust platform for point-of-care diagnostics and drug screening in biomedical applications.

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.

An ultrasensitive homogeneous aptasensor for kanamycin based on upconversion fluorescence resonance energy transfer

We developed an ultrasensitive fluorescence resonance energy transfer (FRET) aptasensor for kanamycin detection, using upconversion nanoparticles (UCNPs) as the energy donor and graphene as the energy acceptor. Oleic acid modified upconversion nanoparticles were synthesized through a hydrothermal process followed by a ligand exchange with hexanedioic acid. The kanamycin aptamer (5'-NH2-AGATGGGGGTTGAGGCTAAGCCGA-3') was tagged to UCNPs through an EDC-NHS protocol. The π-π stacking interaction between the aptamer and graphene brought UCNPs and graphene in close proximity and hence initiated the FRET process resulting in quenching of UCNPs fluorescence. The addition of kanamycin to the UCNPs-aptamer-graphene complex caused the fluorescence recovery because of the blocking of the energy transfer, which was induced by the conformation change of aptamer into a hairpin structure. A linear calibration was obtained between the fluorescence intensity and the logarithm of kanamycin concentration in the range from 0.01 nM to 3 nM in aqueous buffer solution, with a detection limit of 9 pM. The aptasensor was also applicable in diluted human serum sample with a linear range from 0.03 nM to 3 nM and a detection limit of 18 pM. The aptasensor showed good specificity towards kanamycin without being disturbed by other antibiotics. The ultrahigh sensitivity and pronounced robustness in complicated sample matrix suggested promising prospect of the aptasensor in practical applications.

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.

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.

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.

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.

Multicolor upconversion nanoparticles for protein conjugation

We describe the preparation of monodisperse, lanthanide-doped hexagonal-phase NaYF₄ upconverting luminescent nanoparticles for protein conjugation. Their core was coated with a silica shell which then was modified with a poly(ethylene glycol) spacer and N-hydroxysuccinimide ester groups. The nanoparticles were characterized by transmission electron microscopy, Raman spectroscopy, X-ray diffraction, and dynamic light scattering. The N-hydroxysuccinimide ester functionalization renders them highly reactive towards amine nucleophiles (e.g., proteins). We show that such particles can be conjugated to proteins. The protein-reactive UCLNPs and their conjugates to streptavidin and bovine serum albumin display multicolor emissions upon 980-nm continuous wave laser excitation. Surface plasmon resonance studies were carried out to prove bioconjugation and to compare the affinity of the particles for proteins immobilized on a thin gold film.

Lanthanide-doped luminescent nano-bioprobes: from fundamentals to biodetection

Trivalent lanthanide (Ln(3+))-doped luminescent inorganic nanoparticles (NPs), characterized by long-lived luminescence, large Stokes and/or anti-Stokes shifts, narrow emission bands and high photochemical stability, are considered to be promising candidates as luminescent bioprobes in biomedicine and biotechnology. In this feature article, we provide a brief overview of the most recent advances in Ln(3+)-doped luminescent inorganic NPs as sensors, which covers from their chemical and physical fundamentals to biodetection, such as controlled synthesis methodology, surface modification chemistry, optical physics, and their promising applications in diverse bioassays, with an emphasis on heterogeneous and homogeneous in vitro biodetection. Finally, some of the most important emerging trends and future efforts toward this active research field are also proposed.