Daniel Geißler

Quantum dot biosensors for ultrasensitive multiplexed diagnostics

Time- and color-resolved detection of Foerster resonance energy transfer (FRET) from luminescent terbium complexes to different semiconductor quantum dots results in a fivefold multiplexed bioassay with sub-picomolar detection limits for all five bioanalytes (see picture). The detection of up to five biomarkers occurs with a sensitivity that is 40-240-fold higher than one of the best-established single-analyte reference assays.

Six-Color Time-Resolved Förster Resonance Energy Transfer for Ultrasensitive Multiplexed Biosensing

Simultaneous monitoring of multiple molecular interactions and multiplexed detection of several diagnostic biomarkers at very low concentrations have become important issues in advanced biological and chemical sensing. Here we present an optically multiplexed six-color Förster resonance energy transfer (FRET) biosensor for simultaneous monitoring of five different individual binding events. We combined simultaneous FRET from one Tb complex to five different organic dyes measured in a filter-based time-resolved detection format with a sophisticated spectral crosstalk correction, which results in very efficient background suppression. The advantages and robustness of the multiplexed FRET sensor were exemplified by analyzing a 15-component lung cancer immunoassay involving 10 different antibodies and five different tumor markers in a single 50 μL human serum sample. The multiplexed biosensor offers clinically relevant detection limits in the low picomolar (ng/mL) concentration range for all five markers, thus providing an effective early screening tool for lung cancer with the possibility of distinguishing small-cell from non-small-cell lung carcinoma. This novel technology will open new doors for multiple biomarker diagnostics as well as multiplexed real-time imaging and spectroscopy.

Lanthanides and Quantum Dots as Förster Resonance Energy Transfer Agents for Diagnostics and Cellular Imaging

Luminescent lanthanide labels (LLLs) and semiconductor quantum dots (QDs) are two very special classes of (at least partially) inorganic fluorophores, which provide unique properties for Förster resonance energy transfer (FRET). FRET is an energy-transfer process between an excited donor fluorophore and a ground-state acceptor fluorophore in close proximity (approximately 1-20 nm), and therefore it is extremely well suited for biosensing applications in optical spectroscopy and microscopy. Within this cogent review, we will outline the main photophysical advantages of LLLs and QDs and their special properties for FRET. We will then focus on some recent applications from the FRET biosensing literature using LLLs as donors and QDs as donors and acceptors in combination with several other fluorophores. Recent examples of combining LLLs and QDs for spectral and temporal multiplexing from single-step to multistep FRET demonstrate the versatile and powerful biosensing capabilities of this unique FRET pair. As this review is published in the Forum on Imaging and Sensing, we will also present some new results of our groups concerning LLL-based time-gated cellular imaging with optically trifunctional antibodies and LLL-to-QD FRET-based homogeneous sandwich immunoassays for the detection of carcinoembryonic antigen.

A Rapid, Amplification-Free, and Sensitive Diagnostic Assay for Single-Step Multiplexed Fluorescence Detection of MicroRNA.

The importance of microRNA (miRNA) dysregulation for the development and progression of diseases and the discovery of stable miRNAs in peripheral blood have made these short-sequence nucleic acids next-generation biomarkers. Here we present a fully homogeneous multiplexed miRNA FRET assay that combines careful biophotonic design with various RNA hybridization and ligation steps. The single-step, single-temperature, and amplification-free assay provides a unique combination of performance parameters compared to state-of-the-art miRNA detection technologies. Precise multiplexed quantification of miRNA-20a, -20b, and -21 at concentrations between 0.05 and 0.5 nM in a single 150 μL sample and detection limits between 0.2 and 0.9 nM in 7.5 μL serum samples demonstrate the feasibility of both high-throughput and point-of-care clinical diagnostics.

DMACM-caged adenosine nucleotides: ultrafast phototriggers for ATP, ADP, and AMP activated by long-wavelength irradiation.

The development of new photocleavable adenosine nucleotides based on the photochemistry of [7‐(dimethylamino)coumarin‐4‐yl]methyl (DMACM) esters is described. The phototriggers liberate adenosine triphosphate (ATP), diphosphate, and monophosphate upon UV/Vis irradiation between 334 and 405 nm. The efficiency of photocleavage at long wavelengths is high as a result of a combination of appropriate quantum yields and intensive absorptivities. By using time‐resolved fluorescence spectroscopy, we determined a lower limit of 1.6×109 s−1 for the rate constant of the release of ATP from DMACM‐caged ATP. The favorable properties of DMACM‐caged ATP were confirmed in physiological studies by confocal laser scanning microscopy. We were able to uncage DMACM‐caged ATP in cultures of mouse astrocytes and in brain tissue slices from mice and were also able to measure the effect of photoreleased ATP on the cellular response of astrocytes, namely the ability of the ATP to evoke Ca2+ ion waves.

Critical review of the determination of photoluminescence quantum yields of luminescent reporters.

AbstractA crucial variable for methodical performance evaluation and comparison of luminescent reporters is the photoluminescence quantum yield (Φpl). This quantity, defined as the number of emitted photons per number of absorbed photons, is the direct measure of the efficiency of the conversion of absorbed photons into emitted light for small organic dyes, fluorescent proteins, metal–ligand complexes, metal clusters, polymeric nanoparticles, and semiconductor and up-conversion nanocrystals. Φpl determines the sensitivity for the detection of a specific analyte from the chromophore perspective, together with its molar-absorption coefficient at the excitation wavelength. In this review we discuss different optical and photothermal methods for measuring Φpl of transparent and scattering systems for the most common classes of luminescent reporters, and critically evaluate their potential and limitations. In addition, reporter-specific effects and sources of uncertainty are addressed. The ultimate objective is to provide users of fluorescence techniques with validated tools for the determination of Φpl, including a series of Φpl standards for the ultraviolet, visible, and near-infrared regions, and to enable better judgment of the reliability of literature data. Graphical Abstractᅟ

Recent developments in Förster resonance energy transfer (FRET) diagnostics using quantum dots

AbstractThe exceptional photophysical properties and the nanometric dimensions of colloidal semiconductor quantum dots (QD) have strongly attracted the bioanalytical community over the last approximately 20 y. In particular, the integration of QDs in the analysis of biological components and interactions, and the related diagnostics using Förster resonance energy transfer (FRET), have allowed researchers to significantly improve and diversify fluorescence-based biosensing. In this TRENDS article, we review some recent developments in QD-FRET biosensing that have implemented this technology in electronic consumer products, multiplexed analysis, and detection without light excitation for diagnostic applications. In selected examples of smartphone-based imaging, single- and multistep FRET, steady-state and time-resolved spectroscopy, and bio/chemiluminescence detection of QDs used as both FRET donors and acceptors, we highlight the advantages of QD-based FRET biosensing for multiplexed and sensitive diagnostics. Graphical AbstractQuantum dots (QDs) can be applied as donors and/or acceptors for Förster resonance energy transfer- (FRET-) based biosensing for multiplexed and sensitive diagnostics in various assay formats

Reference materials and representative test materials to develop nanoparticle characterization methods: the NanoChOp project case

This paper describes the production and characteristics of the nanoparticle test materials prepared for common use in the collaborative research project NanoChOp (Chemical and optical characterization of nanomaterials in biological systems), in casu suspensions of silica nanoparticles and CdSe/CdS/ZnS quantum dots (QDs). This paper is the first to illustrate how to assess whether nanoparticle test materials meet the requirements of a “reference material” (ISO Guide 30, 2015) or rather those of the recently defined category of “representative test material (RTM)” (ISO/TS 16195, 2013). The NanoChOp test materials were investigated with small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and centrifugal liquid sedimentation (CLS) to establish whether they complied with the required monomodal particle size distribution. The presence of impurities, aggregates, agglomerates, and viable microorganisms in the suspensions was investigated with DLS, CLS, optical and electron microscopy and via plating on nutrient agar. Suitability of surface functionalization was investigated with attenuated total reflection Fourier transform infrared spectrometry (ATR-FTIR) and via the capacity of the nanoparticles to be fluorescently labeled or to bind antibodies. Between-unit homogeneity and stability were investigated in terms of particle size and zeta potential. This paper shows that only based on the outcome of a detailed characterization process one can raise the status of a test material to RTM or reference material, and how this status depends on its intended use.

Semiconductor Quantum Dots as FRET Acceptors for Multiplexed Diagnostics and Molecular Ruler Application

Applications based on Förster resonance energy transfer (FRET) play an important role for the determination of concentrations and distances within nanometer-scale systems in vitro and in vivo in many fields of biotechnology. Semiconductor nanocrystals (Quantum dots - QDs) possess ideal properties for their application as FRET acceptors when the donors have long excited state lifetimes and when direct excitation of QDs can be efficiently suppressed. Therefore, luminescent terbium complexes (LTCs) with excited state lifetimes of more than 2 ms are ideal FRET donor candidates for QD-acceptors. This chapter will give a short overview of theoretical and practical background of FRET, QDs and LTCs, and present some recent applications of LTC-QD FRET pairs for multiplexed ultra-sensitive in vitro diagnostics and nanometer-resolution molecular distance measurements.

Quantification of Anisotropy-Related Uncertainties in Relative Photoluminescence Quantum Yield Measurements of Nanomaterials – Semiconductor Quantum Dots and Rods

Abstract In order to assess the anisotropy-related uncertainties of relatively determined photoluminescence quantum yields (ΦPL) of molecular emitters and luminescent nanomaterials, we compared ΦPL values measured without and with polarizers using magic angle conditions and studied systematically the dependence of the detected emission intensity on the polarizer settings for samples of varying anisotropy. This includes a dispersion of a spherical quantum dot (QD) with an ideally isotropic emission, a solution of a common small organic dye in a fluid solvent as well as dispersions of elongated quantum dot rods (QDR) with an anisotropic luminescence and a small organic dye in a rigid polymeric matrix, as ideally anisotropic emitter. Our results show that for instruments lacking polarizers, anisotropy-related measurement uncertainties of relative photoluminescence quantum yields can amount to more than 40%, with the size of these systematic errors depending on the difference in emission anisotropy between the sample and the standard.