Luminescent Biolabels Research Articles

Preparation of undecanoic acid-terminated Si particles from rice husks

Luminescent fine Si powder was prepared by an Mg reduction process of SiO2 particles extracted from rice husks. The surface of the powder was terminated by the hydrophilic molecules of undecanoic acid via the hydrosilylation process. After the hydrosilylation of the fine Si powder obtained from rice husks, surface termination by undecanoic acid was confirmed to suppress natural surface oxidation. Transmission electron microscopy observations of the rice husk fine Si powder showed that the Si nanoparticles have a crystalline structure. Room temperature photoluminescence in the wavelength range of 500–1000 nm was observed in the samples taken at each step of the process, and the stability of the luminescence properties was improved via undecanoic acid surface termination. The observation of wavelength dependent microsecond-order photoluminescence indicates that the photoluminescence was due to quantum size effects. On the basis of time-transient light transmittance of rice husk Si colloid, the surface termination of undecanoic acid improved the dispersibility of Si particles in distilled water. The production of luminescent rice husk Si colloid is effective for the preparation of luminescent bio labels and will lead to high value-added recycling of rice husks.

Trap assisted visible light luminescent properties of hydrothermally grown Gd doped ZnO nanostructures

Pristine ZnO and Gd-doped ZnO nanostructures were synthesized via the hydrothermal route to study the influence of Gd doping on the growth and properties of the nanostructures. Synthesized nanostructures were characterized by powder x-ray diffraction technique (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) and UV–visible spectroscopy. Optical study demonstrated that the nanostructures exhibit good visible light transmittance and absorbs ultraviolet light energy. Photoluminescence (PL) study of the nanostructures showed that Gd-doping enhanced the PL activity of the nanostructures by possible introduction of the defect levels. The corresponding augmentation in PL intensity (visible light luminescent intensity) is attributed to comparative increase in crystallinity, complex defects (traps) and electron hole pair recombination rate. This enhanced luminescent property of Gd-doped ZnO nanostructures can be exploited for the luminescence based applications like sensors, UV-Lasers, phosphors, LEDs, luminescent biolabels, etc.

Upconversion rare earth nanoparticles functionalized with folic acid for bioimaging of MCF-7 breast cancer cells

Abstract

Ultra-small Nd(3+)-doped nanoparticles as near-infrared luminescent biolabels of hemin in bacteria.

Near-infrared (NIR) luminescent Nd(3+)-doped nanoparticles (NPs) have attracted considerable attention in bioimaging and biodetection. Here, we demonstrate sub-6 nm NaGdF4:Nd(3+),Fe(3+) NPs as luminescent biolabels of hemin molecules that act as the exogenous electron carriers in microbial communities. Contrary to the severe quenching of the visible luminescence for either upconverting or downconverting NPs, the Nd(3+)-doped NPs show superior properties in avoiding the optical absorption of hemin within the UV and visible spectral regions. A detailed examination showed that the Nd(3+)-doped NPs exhibit no obvious toxic effects on the microbial communities and show scarce influence on the characteristics of labeled hemin molecules in enhancing the reducing power of the fermentation system. More importantly, by monitoring the NIR luminescence of Nd(3+)-doped NPs, the selective accumulation of exogenous electron carriers in bacteria that are lacking reducing power has been revealed for the first time. The application of Nd(3+)-doped NPs as biolabels in bacteria would provide new opportunities for further unravelling the role of exogenous electron carriers in anaerobic digestion.

Lanthanide-doped semiconductor nanocrystals: electronic structures and optical properties

Trivalent lanthanide (Ln3+) ions doped semiconductor nanomaterials have recently attracted considerable attention owing to their distinct optical properties and their important applications in diverse fields such as optoelectronic devices, flat plane displays and luminescent biolabels. This review provides a comprehensive survey of the latest advances in the synthesis, electronic structures and optical spectra of Ln3+ ions in wide band-gap semiconductor nanocrystals (SNCs). In particular, we highlight the general wet-chemical strategies to introduce Ln3+ ions into host lattices, the local environments as well as the sensitization mechanism of Ln3+ in SNCs. The energy levels and crystal-field parameters of Ln3+ in various SNCs determined from energy-level-fitting are summarized, which is of vital importance to understanding the optical properties of Ln3+ ions in SNCs. Finally, some future prospects and challenges in this rapidly growing field are also proposed.

Microwave-Assisted Synthesis of Biocompatible Europium-Doped Calcium Hydroxyapatite and Fluoroapatite Luminescent Nanospindles Functionalized with Poly(acrylic acid)

Europium-doped calcium hydroxyapatite and fluoroapatite nanophosphors functionalized with poly(acrylic acid) (PAA) have been synthesized through a one-pot microwave-assisted hydrothermal method from aqueous basic solutions containing calcium nitrate, sodium phosphate monobasic, and PAA, as well as sodium fluoride in the case of the fluoroapatite particles. In both cases a spindlelike morphology was obtained, resulting from an aggregation process of smaller subunits which also gave rise to high specific surface area. The size of the nanospindles was 191 (32) × 40 (5) nm for calcium hydroxyapatite and 152 (24) × 38 (6) nm for calcium fluoroapatite. The luminescent nanoparticles showed the typical red luminescence of Eu(3+), which was more efficient for the fluoroapatite particles than for the hydroxyapatite. This is attributed to the presence of OH(-) quenchers in the latter. The nanophosphors showed negligible toxicity for Vero cells. Both PAA-functionalized nanophosphors showed a very high (up to at least 1 week) colloidal stability in 2-(N-morpholino)ethanesulfonic acid (MES) at pH 6.5, which is a commonly used buffer for physiological pH. All these features make both kinds of apatite-based nanoparticles promising tools for biomedical applications, such as luminescent biolabels and tracking devices in drug delivery systems.

Aqueous phase synthesis of upconversion nanocrystals through layer-by-layer epitaxial growth for in vivo X-ray computed tomography

Lanthanide-doped core-shell upconversion nanocrystals (UCNCs) have tremendous potential for applications in many fields, especially in bio-imaging and medical therapy. As core-shell UCNCs are mostly synthesized in organic solvents, tedious organic-aqueous phase transfer processes are usually needed for their use in bio-applications. Herein, we demonstrate the first example of one-step synthesis of highly luminescent core-shell UCNCs in the "aqueous" phase under mild conditions using innocuous reagents. A microwave-assisted approach allowed for layer-by-layer epitaxial growth of a hydrophilic NaGdF4 shell on NaYF4:Yb, Er cores. During this process, surface defects of the nanocrystals could be gradually passivated by the homogeneous shell deposition, resulting in obvious enhancement in the overall upconversion emission efficiency. In addition, the up-down conversion dual-mode luminescent NaYF4:Yb, Er@NaGdF4:Ce, Ln (Eu, Tb, Sm, Dy) nanocrystals were also synthesized to further validate the successful formation of the core-shell structure. More significantly, based on their superior solubility and stability in water solution, high upconversion efficiency and Gd-doped predominant X-ray absorption, the as-prepared NaYF4:Yb, Er@NaGdF4 core-shell UCNCs exhibited high contrast in in vitro cell imaging and in vivo X-ray computed tomography (CT) imaging, demonstrating great potential as multiplexed luminescent biolabels and CT contrast agents.

Time-Gated Orthogonal Scanning Automated Microscopy (OSAM) for High-speed Cell Detection and Analysis

We report a new development of orthogonal scanning automated microscopy (OSAM) incorporating time-gated detection to locate rare-event organisms regardless of autofluorescent background. The necessity of using long-lifetime (hundreds of microseconds) luminescent biolabels for time-gated detection implies long integration (dwell) time, resulting in slow scan speed. However, here we achieve high scan speed using a new 2-step orthogonal scanning strategy to realise on-the-fly time-gated detection and precise location of 1-μm lanthanide-doped microspheres with signal-to-background ratio of 8.9. This enables analysis of a 15 mm × 15 mm slide area in only 3.3 minutes. We demonstrate that detection of only a few hundred photoelectrons within 100 μs is sufficient to distinguish a target event in a prototype system using ultraviolet LED excitation. Cytometric analysis of lanthanide labelled Giardia cysts achieved a signal-to-background ratio of two orders of magnitude. Results suggest that time-gated OSAM represents a new opportunity for high-throughput background-free biosensing applications.

Lanthanide-Doped Inorganic Nanocrystals as Luminescent Biolabels

Trivalent lanthanide ions (Ln(3+)) doped inorganic nanocrystals (NCs) have currently attracted reviving interest and come to the forefront in nanophotonics owing to their potential applications in diverse fields such as luminescent biodetection and bioimaging. As an alternative to conventional biolabels, Ln(3+)-doped NCs show superior features including large stokes shift, multicolor fine-tuning, narrow emission band widths, high photostability, and low toxicity. Particularly, the long-lived luminescence and distinct upconversion (UC) of Ln(3+)-doped NCs are desirable for various bioapplications. The long-lived luminescence of Ln(3+) combined with time-resolved technique can efficiently suppress the interference from short-lived background, resulting in a high signal-to-noise ratio (S/N) and background-free measurements. Near-infrared excited UC emissions of Ln(3+) can bring no autofluorescence and no photodamage to cells or tissues, and thus UC NCs have been regarded as one of the most useful in vivo optical contrast agents. In this review, we outline the most recent development of Ln(3+)-doped NCs as biolabels from the controlled synthesis and surface functionalization of NCs to their bioapplications in heterogeneous and homogeneous biodetection as well as in vitro and in vivo bioimaging.

High-quality water-soluble and surface-functionalized upconversion nanocrystals as luminescent probes for bioimaging

Rare-earth upconversion nanophosphors (UCNPs) have great potential to become excellent biological luminescent labels for fluorescence bioimaging. However, it is still difficult to directly synthesize high-quality water-soluble UCNPs bearing appropriate functional groups using a one-step synthetic strategy. Herein, we report a one-step synthetic strategy for high-quality water-soluble and surface-functionalized UCNPs using a hydrothermal reaction assisted by binary cooperative ligands (HR-BCL). In this system, 6-aminohexanoic acid and oleate were introduced to control nuclear generation and crystal growth of small nanoparticles. The UCNPs synthesized here showed high crystalline and intense upconversion luminescence emission. Fourier-transform infrared and nuclear magnetic resonance spectroscopy indicated that 6-aminohexanoic acid and oleate cooperate as surface ligands to co-control the surface of UCNPs. Thus, the water-solubility of the as-prepared UCNPs can be tuned by changing the molar ratio of 6-aminohexanoic acid to oleate. The AA-modified UCNPs provided a free amine content of (6.0 ± 0.2) × 10−5 mol/g, which renders them dispersible in aqueous solution and allows further conjugation with folic acid (FA) for targeted bioimaging. Furthermore, the amine-functionalized UCNPs show intense near-infrared upconversion luminescence and were successfully applied in the lymphatic capillary bioimaging of small animals with a high signal-to-noise ratio, suggesting that these surface-functionalized UCNPs are promising candidates for luminescent biolabels.

Optical spectroscopy of lanthanides doped in wide band-gap semiconductor nanocrystals

Currently, tripositive lanthanide (Ln3+) ions doped wide band-gap semiconductor nanocrystals (NCs) have been the focus of research interest due to their distinct optical properties and potential applications in optical devices and luminescent biolabels. Because of the low absorptions of parity-forbidden 4f–4f transitions for Ln3+, it is highly anticipated that the luminescence of Ln3+ ions embedded in wide band-gap NC lattices can be sensitized efficiently via exciton recombination in the host. For this purpose, the successful incorporation of Ln3+ into the lattices of semiconductor NCs is of utmost importance, which still remains intractable via conventional wet chemical methods. Here, the most recent progress in the optical spectroscopy of Ln3+ ions doped wide band-gap semiconductor NCs is discussed. Much attention was focused on the optical properties including electronic structures, luminescence dynamics, energy transfer as well as the up-conversion emissions of Ln3+ ions in ZnO, TiO2, SnO2 and In2O3 NCs that were synthesized in our laboratory using wet chemical methods.

Clean and Flexible Modification Strategy for Carboxyl/Aldehyde‐Functionalized Upconversion Nanoparticles and Their Optical Applications

AbstractRare‐earth upconversion nanoparticles (UCNPs) exhibit great potential in luminescent biolabels and other multifunctional probes; however, their applications are limited by their low water solubility and the lack of binding groups. To address these problems, a clean and flexible strategy to modify hydrophobic monodisperse UCNPs into hydrophilic ones that are capped with functional groups is developed. The modification process is implemented by direct oxidation of oleic acid ligands with ozone under specific conditions, where the oleic acid (OA) ligands on the surface of the UCNPs can be converted into azelaic acid ligands (HOOC(CH2)7COOH) or azelaic aldehyde HOOC(CH2)7CHO, as is revealed by Fourier‐transform infrared (FTIR) and nuclear magnetic resonance (NMR) measurements. This oxidation process has no significant side‐effects on the morphology, phase, composition, or luminescent properties of the UCNPs. Free carboxylic acid groups on the surface endow the UCNPs with good water solubility, while aldehyde groups at the surface provide binding sites for amino‐containing molecules via Schiff‐base condensation, such as 2‐(4‐aminophenylethylyl)‐5‐methoxy‐2‐(2‐pyridyl)thiazole (MPTEA) and 2‐aminoethanethiol hydrochloride (NH2CH2CH2SH·HCl, HEMA). A Ce4+ sensor is constructed based on the dual‐emission arising from the different spectral responses of MPTEA and the UCNPs. Facilitated by the covalent linkage between the terminal aldehyde group on the UCNPs and the amino group in HEMA, a hybrid structure of UCNPs and Au NPs is fabricated. The effective coupling between the aldehyde group and the amino group suggests that these functionalized UCNPs have potential in combining other functional units for simultaneous biolabeling, or other optical applications.

Preparation and Time-Resolved Luminescence Bioassay Application of Multicolor Luminescent Lanthanide Nanoparticles

Because highly luminescent lanthanide compounds are limited to Eu(3+) and Tb(3+) compounds with red (Eu, approximately 615 nm) and green (Tb, approximately 545 nm) emission colors, the development and application of time-resolved luminescence bioassay technique using lanthanide-based multicolor luminescent biolabels have rarely been investigated. In this work, a series of lanthanide complexes covalently bound silica nanoparticles with an excitation maximum wavelength at 335 nm and red, orange, yellow and green emission colors has been prepared by co-binding different molar ratios of luminescent Eu(3+)-Tb(3+) complexes with a ligand N,N,N(1),N(1)-(4'-phenyl-2,2':6',2''-terpyridine-6,6''-diyl)bis(methylenenitrilo) tetrakis (acetic acid) inside the silica nanoparticles. The nanoparticles characterized by transmission electron microscopy and luminescence spectroscopy methods were used for streptavidin labeling, and time-resolved fluoroimmunoassay (TR-FIA) of human prostate-specific antigen (PSA) as well as time-resolved luminescence imaging detection of an environmental pathogen, Giardia lamblia. The results demonstrated the utility of the new multicolor luminescent lanthanide nanoparticles for time-resolved luminescence bioassays.

Visible-light-sensitized highly luminescent europium nanoparticles: preparation and application for time-gated luminescence bioimaging

Time-gated luminescence bioimaging based on microsecond-lifetime luminescent biolabels can provide complete background-free conditions for detecting target cells in an autofluorescence biosample matrix. However, a major drawback of the current lanthanide biolabels is the requirement for UV excitation (<370 nm), which leads to damage to many biological systems and greatly affects the improvement of time-gated luminescence instruments. Herein we describe luminescent europium nanoparticles that have an excitation peak around 406 nm with high quantum yield (∼66%) and fine monodispersity in aqueous solutions. The nanoparticles were prepared by copolymerization of a visible-light-sensitized Eu3+ complex 4,4′-bis(1″,1″,1″,2″,2″,3″,3″-heptafluoro-4″,6″-hexanedion-6″-yl)chlorosulfo-o-terphenyl-Eu3+-2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine conjugated with 3-aminopropyl(triethoxy)silane, free 3-aminopropyl(triethoxy)silane and tetraethyl orthosilicate in a water-in-oil reverse microemulsion. Characterization by transmission electron microscopy and luminescence spectroscopy indicates that the nanoparticles are monodisperse, spherical and uniform in size, <50 nm in diameter, and show strong visible-light-sensitized luminescence with a large quantum yield and a long luminescence lifetime. The new nanoparticles were successfully applied to distinguish an environmental pathogen, Giardia lamblia, within a concentrate of environmental water sample using a time-gated luminescence microscope with pulsed visible light excitation. The method resulted in highly specific and sensitive imaging for Giardia lamblia. These results suggest a broad range of potential bioimaging applications where both long time microscopy observation and high signal-to-background ratio are required for samples containing high concentrations of autofluorescence background.

Lanthanide Bimetallic Helicates forin VitroImaging and Sensing

As the need for targeting luminescent biolabels increases, for mapping selected analytes, imaging of cells and organs, and tracking in cellulo processes, lanthanide bimetallic helicates are emerging as versatile bioprobes. The wrapping of three ligand strands around two metallic centers by self-assembly affords robust molecular edifices with tunable chemical and photophysical properties. In addition, heterometallic helical chelates can be assembled leading to bioprobes with inherent chiral properties. In this paper, we review the literature demonstrating that neutral [Ln(2)(L(CX))(3)] (x=1-3) helicates represent a viable alternative to existing chelating agents for bio-analyses, while featuring specific enhanced properties. These bimetallic chelates self-assemble in water, and at physiological pH the 2:3 (Ln:L(CX)) complex is by far the dominant species, conditional stability constants logbeta(23) being in the range 23-30. The metal ions are 9-coordinate and lie in sites with slightly distorted D(3) symmetry. Efficient protection from water interaction by the tightly wrapped ligand strands results in sizeable photophysical properties, with quantum yields up to 24% for Eu(III) and 11% for Tb(III), while the luminescence of several other visible and/or near-infrared emitting Ln(III) ions is also sensitized. Noncytotoxicity for all the helicates is established for several living cell lines including HeLa, HaCat, MCF-7, 5D10, and Jurkat. We present new data pertaining to the live cell imaging ability of [Eu(2)(L(C1))(3)] and compare the three systems with x=1-3 with respect to thermodynamic stability, photophysics, cell-permeation ability, and targeting capability for sensing in cellulo processes. Prospects of derivatization for characterizing specific biological interactions are discussed.