Satu Lahtinen

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.

Long-Lifetime Luminescent Europium(III) Complex as an Acceptor in an Upconversion Resonance Energy Transfer Based Homogeneous Assay

Long-lifetime luminescent Eu(III) 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(III)-doped upconverting nanoparticles were used as donors, and a luminescent Eu(III)-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.

Rapid homogeneous immunoassay for cardiac troponin I using switchable lanthanide luminescence.

Homogeneous assays are advantageous because of their simplicity and rapid kinetics but typically their performance is severely compromised compared to heterogeneous assay formats. Here, we report a homogeneous immunoassay utilizing switchable lanthanide luminescence for detection and site-specifically labeled recombinant antibody fragments as binders to improve the assay performance. Switchable lanthanide luminescence enabled elimination of assay background due to division of the luminescent lanthanide chelate into two non-luminescent label moieties. Simultaneous biomolecular recognition of model analyte cardiac troponin I by two antibody fragments brought the label moieties together and resulted in self-assembly of luminescent mixed chelate complex. The assay was very rapid as maximal signal-to-background ratios were achieved already after 6 min of incubation. Additionally, the limit of detection was 0.38 ng/mL (16 pM), which was comparable to the limit of detection for the heterogeneous reference assay based on the same binders (0.26 ng/mL or 11 pM). This is the first study to apply switchable lanthanide luminescence in immunoassays and demonstrates the versatile potential of the technology for rapid and sensitive homogeneous assays.

High gradient magnetic separation of upconverting lanthanide nanophosphors based on their intrinsic paramagnetism

Photon upconverting nanophosphors (UCNPs) have the unique luminescent property of converting low-energy infrared light into visible emission which can be widely utilized in nanoreporter and imaging applications. For the use as reporters in these applications, the UCNPs must undergo a series of surface modification and bioconjugation reactions. Efficient purification methods are required to remove the excess reagents and biomolecules from the nanophosphor solution after each step to yield highly responsive reporters for sensitive bioanalytical assays. However, as the particle size of the UCNPs approaches the size of biomolecules, the handling of these reporters becomes cumbersome with traditional purification methods such as centrifugation. Here we introduce a novel approach for purification of bioconjugated 32-nm NaYF4: Yb3+, Er3+-nanophosphors from excess unbound biomolecules utilizing high gradient magnetic separation (HGMS)-system constructed from permanent super magnets which produce magnetic gradients in a magnetizable steel wool matrix amplifying the magnetic field. The non-magnetic biomolecules flowed straight through the magnetized HGMS-column while the UCNPs were eluted only after the magnetic field was removed. In the UCNPs the luminescent centers, i.e., lanthanide-ion dopants are responsible for the strong upconversion luminescence, but in addition they are also paramagnetic. In this study we have shown that the presence of these weakly paramagnetic luminescent lanthanides actually also enables the use of HGMS to capture the UCNPs without incorporating additional optically inactive magnetic core into them.

Array-in-well serodiagnostic assay utilizing upconverting phosphor label technology.

In this study, a multiplex serological array-in-well assay was constructed for simultaneous detection of serum IgG antibodies against parvovirus B19 and human adenovirus. The array was prepared in streptavidin-coated 96-well microtiter plates by spotting biotinylated parvovirus B19 virus-like-particles, adenovirus type 2 and 5 hexon antigens, negative control of human serum albumin and positive controls of human IgG and anti-human IgG antibodies on the bottom of each well in an array format with a printable area of 2 mm × 2 mm. The array-in-well assay was evaluated with serum samples (n=89) of different antibody status as determined by commercial enzyme immunoassay for parvovirus IgG, and by in-house enzyme immunoassay for adenovirus IgG. The bound serum anti-parvovirus IgG, anti-adenovirus IgG, and total IgG antibodies were detected with anti-human IgG antibody coated photon upconverting nanoparticles and the assay was measured with an anti-Stokes photoluminescence imager. Detection of specific antibodies by the multiplex array-in-well assay was in good agreement (100% for parvovirus B19 and 96% for adenovirus) with the reference results. In conclusion, the array-in-well with upconverting phosphor reporter technology was able to detect antiviral antibodies in human sera, and represents an efficient serodiagnostic concept that is a promising new tool for multiplex serology.

Upconversion Cross-Correlation Spectroscopy of a Sandwich Immunoassay

Abstract Fluorescence correlation and cross‐correlation spectroscopy (FCS/FCCS) have enabled biologists to study processes of transport, binding, and enzymatic reactions in living cells. However, applying FCS and FCCS to samples such as whole blood and plasma is complicated as the fluorescence bursts of diffusing labels can be swamped by strong autofluorescence. Here we present cross‐correlation spectroscopy based on two upconversion nanoparticles emitting at different wavelengths on the anti‐Stokes side of a single excitation laser. This upconversion cross‐correlation spectroscopy (UCCS) approach allows us to completely remove all Stokes shifted autofluorescence background in biological material such as plasma. As a proof of concept, we evaluate the applicability of UCCS to a homogeneous sandwich immunoassay for thyroid stimulating hormone measured in buffer solution and in plasma.

Effective Shielding of NaYF4:Yb3+,Er3+ 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 NaYF4:Yb3+,Er3+ 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.

Photochemical ligation to ultrasensitive DNA detection with upconverting nanoparticles

In this work, we explore a photochemical ligation reaction to covalently modify oligonucleotide-conjugated upconverting nanoparticles (UCNPs) in the presence of a specific target DNA sequence. The target sequence acts as a hybridization template, bringing together a biotinylated photoactivatable oligonucleotide probe and the oligonucleotide probe that is attached to UCNPs. The illumination of the UCNPs by NIR light to generate UV emission internally or illuminating the photoactivatable probe directly by an external UV light promotes the photochemical ligation reaction, yielding covalently biotin functionalized UCNPs that can be selectively captured in streptavidin-coated microwells. Following this strategy, we developed a DNA sensor with a limit of detection of 1 × 10-18 mol per well (20 fM). In addition, we demonstrate the possibility to create UCNP patterns on the surface of solid supports upon NIR illumination that are selectively formed under the presence of the target oligonucleotide.