14 research outputs found
Long-Lifetime Luminescent Europium(III) Complex as an Acceptor in an Upconversion Resonance Energy Transfer Based Homogeneous Assay
Long-lifetime luminescent Eu<sup>III</sup> complexes are widely
used as donors in Förster resonance energy transfer to enable
time-gated detection of sensitized emission from an intrinsically
short-lived acceptor. Here we report a unique energy-transfer system,
where the sensitized acceptor emission has prolonged luminescence
lifetime compared to the donor and the long lifetime is not cut short
upon high energy-transfer efficiency. The infrared-excited, ultraviolet-emitting,
Tm<sup>III</sup>-doped upconverting nanoparticles were used as donors,
and a luminescent Eu<sup>III</sup>-chelate was used as an acceptor.
Upon excitation the sensitized acceptor emission, which is already
spectrally resolved from the donor, can be measured even after the
donor luminescence has decayed. Because of anti-Stokes characteristics,
the time-gated detection is not needed to avoid the autofluorescence.
Thus, the long luminescence lifetime can be further modulated and
utilized, e.g., in background-free molecular sensing, rendering the
system extremely attractive
Homogeneous Detection of Avidin Based on Switchable Lanthanide Luminescence
We have developed switchable lanthanide luminescence-based binary probe technology for homogeneous detection of avidin, which is a tetrameric protein. Two different nonluminescent label moietiesa light-absorbing antenna ligand and a lanthanide ion carrier chelatewere conjugated to separate biotins, which is known as avidin’s natural ligand. The assay was based on binding of the two differently labeled biotins on separate binding sites on the target protein and consequent self-assembly of a luminescent complex from the two label moieties. Specific luminescence signal was observed only at the presence of the target protein. The characteristics of the switchable lanthanide luminescence assay were compared to the reference assay, based on lanthanide resonance energy transfer. Both assays had a limit of detection in the low-picomolar concentration range; however, the lanthanide chelate complementation-based assay had wider dynamic range and its optimization was more straightforward. The switchable lanthanide luminescence technology could be further applied to generic protein detection, using reagents that are analogous to the proximity ligation assay principle
Genetically Encoded Protease Substrate Based on Lanthanide-Binding Peptide for Time-Gated Fluorescence Detection
The study of biomolecular interactions is at the heart
of biomedical research. Fluorescence and Förster resonance
energy transfer (FRET) are potent and versatile tools in studying
these interactions. Fluorescent proteins enable genetic encoding which
facilitates their use in recombinant protein and in vivo applications.
To eliminate the autofluorescence background encountered in applications
based on fluorescent proteins, lanthanide labels can be used as donor
fluorophores. Their long emission lifetime enables the use of time-gating
that significantly improves assay sensitivity. In this work, we have
combined the favorable characteristics of a terbium-ion-containing
lanthanide-binding peptide (Tb<sup>3+</sup>-LBP) and green fluorescent
protein (GFP) in a FRET-based homogeneous protease activity assay.
The used genetically engineered construct had LBP and GFP sequences
at adjacent ends of a linker that encoded the recognition sequence
for caspase-3. Caspase proteases are central mediators in apoptosis
and, consequently, are of great interest in the pharmaceutical industry.
The designed fluorogenic protease substrate was applied for the detection
of caspase-3 activity. We were able to demonstrate, for the first
time, the applicability of a Tb<sup>3+</sup>-LBP–GFP energy-transfer
pair in a protease activity assay. The intrinsically fluorescent and
genetically encodable components enable easy expression of the construct
without the need of cumbersome chemical labeling. By varying the fluorescent
protein and the protease specificity of the internal linker sequence,
the method can be applied for the detection of a wide variety of proteases
Background-Free Referenced Luminescence Sensing and Imaging of pH Using Upconverting Phosphors and Color Camera Read-out
Fluorescence background and problems
with proper signal referencing
severely disrupt the read-out of luminescence sensors and images.
We present a pH sensor film in combination with a simple read-out
technique that overcomes issues of background signals and autofluorescence.
It consists of micrometer-sized upconversion phosphors (UCPs) and
a pH indicator (Neutral Red) that absorbs their green emission. Both
are embedded in a proton permeable hydrogel matrix. The UCPs generate
green and red luminescence upon excitation with IR light of 980 nm
wavelength. Solely the green light of the UCPs is affected by the
pH indicator, while the red emission acts as inert reference signal
for ratiometric measurements. The emission peaks of the UCPs match
the red and green color channels of standard digital cameras. Thereby,
the devised sensor film can be used for referenced ratiometric sensing
and 2D imaging of pH using a color camera read-out. The sensor setup
using common and hand-held devices is cheap and straightforward and
allows for point-of-care measurements. Finally, pH measurements in
human serum samples show the potential of this sensor for imaging
free of interfering background signals
Distance and Temperature Dependency in Nonoverlapping and Conventional Förster Resonance Energy-Transfer
Förster resonance energy-transfer (FRET) is a powerful and widely applied bioanalytical tool. According to the definition of FRET by Förster, for energy-transfer to take place, a substantial spectral overlap between the donor emission and acceptor excitation spectra is required. Recently also a phenomenon termed nonoverlapping FRET (nFRET) has been reported. The nFRET phenomenon is based on energy-transfer between a lanthanide chelate donor and a spectrally nonoverlapping acceptor and thus obviously differs from the conventional FRET, but the mechanism of nFRET and resulting implications to assay design have not been thoroughly examined. In this work, a homogeneous DNA-hybridization assay was constructed to study the distance and temperature dependency of both nFRET and conventional FRET. Capture oligonucleotides were labeled at the 5′-end with a Eu(III)-chelate, and these conjugates hybridized to complementary tracer oligonucleotides labeled with an organic fluorophore at various distances from the 3′-end. The distance dependency was studied with a fluorometer utilizing time-resolution, and the temperature dependency was studied using a frequency-domain (FD) luminometer. Results demonstrated a difference in both the distance and temperature dependency between conventional FRET and nFRET. On the basis of our measurements, we propose that in nFRET thermal excitation occurs from the lowest radiative state of the ion to a higher excited state that is either ionic or associated with a ligand-to-metal charge-transfer state
Environmental Impact on the Excitation Path of the Red Upconversion Emission of Nanocrystalline NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup>
The
mechanism for red upconversion luminescence of Yb–Er
codoped materials is not generally agreed on in the literature. Both
two-photon and three-photon processes have been suggested as the main
path for red upconversion emission. We have studied β-NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> nanoparticles in H<sub>2</sub>O and D<sub>2</sub>O, and we propose that the nanoparticle environment
is a major factor in the selection of the preferred red upconversion
excitation pathway. In H<sub>2</sub>O, efficient multiphonon relaxation
(MPR) promotes the two-photon mechanism through green emitting states,
while, in D<sub>2</sub>O, MPR is less effective and the three-photon
path involving back energy transfer to Yb<sup>3+</sup> is the dominant
mechanism. For the green upconversion emission, our results suggest
the common two-photon path through the <sup>4</sup>F<sub>9/2</sub> energy state in both H<sub>2</sub>O and D<sub>2</sub>O
Effective Shielding of NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> Upconverting Nanoparticles in Aqueous Environments Using Layer-by-Layer Assembly
Aqueous solutions
are the basis for most biomedical assays, but
they quench the upconversion luminescence significantly. Surface modifications
of upconverting nanoparticles are vital for shielding the obtained
luminescence. Modifications also provide new possibilities for further
use by introducing attaching sites for biomolecule conjugation. We
demonstrate the use of a layer-by-layer surface modification method
combining varying lengths of negatively charged polyelectrolytes with
positive neodymium ions in coating the upconverting NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> nanoparticles. We confirmed the
formation of the bilayers and investigated the surface properties
with Fourier transform infrared and reflectance spectroscopy, thermal
analysis, and ζ-potential measurements. The effect of the coating
on the upconversion luminescence properties was characterized, and
the bilayers with the highest improvement in emission intensity were
identified. In addition, studies for the nanoparticle and surface
stability were carried out in aqueous environments. It was observed
that the bilayers were able to shield the materials’ luminescence
from quenching also in the presence of phosphate buffer that is currently
considered the most disruptive environment for the nanoparticles
Photon Upconversion in a Molecular Lanthanide Complex in Anhydrous Solution at Room Temperature
Molecular photon upconversion luminescence
was observed from an
ion-associated complex of an erbium chelate of 2-thenoyltrifluoroacetone
and a near-infrared-emitting cyanine dye in anhydrous solution at
room temperature. In the complex erbium was sensitized by the organic
antenna dye excited at 808 nm. The result was characteristic erbium
emission at 510–565 nm with second-order dependence on the
excitation power, suggesting a dye-sensitized energy transfer upconversion
mechanism. Compared to inorganic upconverting nanoparticles, the organic
molecular dye-sensitized complexes are expected to offer higher molar
absorptivity, smaller well-defined size, and simpler addition of functional
groups
Disintegration of Hexagonal NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> Upconverting Nanoparticles in Aqueous Media: The Role of Fluoride in Solubility Equilibrium
The disintegration
of hexagonal NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> upconverting
nanoparticles (UCNP) was studied by
incubating various nanoparticle concentrations in aqueous suspensions
over time while monitoring the upconversion emission intensity and
measuring the dissolved particle-constituting ion concentrations.
The results revealed that the ions dissolved into water resulting
apparently in anisotropic structural disintegration of the UCNPs as
observed with transmission electron microscopy. The UCNP disintegration
caused partial loss of active ions Yb<sup>3+</sup> and Er<sup>3+</sup> from the host matrix and therefore decrease in the upconversion
luminescence intensity. The decrease, however, was strongly dependent
on the UCNP concentration, and dramatic drop in the intensity was
observed especially at diluted nanoparticle suspensions, where the
nanoparticles disintegrated almost completely until the solubility
equilibrium was achieved. At the concentrated suspensions the equilibrium
was achieved already with minimal disintegration, and the change in
the luminescence intensity was negligible. Further, due to the high
impact of fluoride ions on the solubility equilibrium the disintegration
of the UCNPs could be prevented by adding fluoride to the suspension.
The reported disintegration of NaYF<sub>4</sub>:Yb<sup>3+</sup>,Er<sup>3+</sup> nanoparticles in diluted aqueous suspensions should be taken
into consideration when the UCNPs are used at low concentrations in
analytical applications and in guiding the design of improved shell-stabilized
UCNPs
Quantitative Multianalyte Microarray Immunoassay Utilizing Upconverting Phosphor Technology
A quantitative multianalyte immunoassay utilizing luminescent
upconverting
single-crystal nanoparticles as reporters on an antibody array-in-well
platform was demonstrated. Upconverting nanoparticles are inorganic
rare earth doped materials that have the unique feature of converting
low energy infrared radiation into higher energy visible light. Autofluorescence,
commonly limiting the sensitivity of fluorescence-based assays, can
be completely eliminated with photon upconversion technology because
the phenomenon does not occur in biological materials. Biotinylated
antibodies for three analytes (prostate specific antigen, thyroid
stimulating hormone, and luteinizing hormone) were printed in an array
format onto the bottom of streptavidin-coated microtiter wells. Analyte
dilutions were added to the wells, and the analytes were detected
with antibody-coated upconverting nanoparticles. Binding of the upconverting
nanoparticles was imaged with an anti-Stokes photoluminescence microwell
imager, and the standard curves for each analyte were quantified from
the selected spot areas of the images. Single analyte and reference
assays were also carried out to compare with the results of the multianalyte
assay. Multiplexing did not have an effect on the assay performance.
This study demonstrates the feasibility of upconverting single-crystal
nanoparticles for imaging-based detection of quantitative multianalyte
assays