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Acoustic levitation is used as a newtool to study concentration-dependent processes influorescence
spectroscopy. With this technique, small amounts of liquid and solid samples can be measured
without the need for sample supports or containers, which often limits signal acquisition and
can even alter sample properties due to interactions with the support material. We demonstrate
that, because of the small sample volume, fluorescence measurements at high concentrations
of an organic dye are possible without the limitation of inner-filter effects, which hamper such
experiments in conventional, cuvette-based measurements. Furthermore, we show that acoustic
levitation of liquid samples provides an experimentally simple way to study distance-dependent
fluorescence modulations in semiconductor nanocrystals. The evaporation of the solvent during
levitation leads to a continuous increase of solute concentration and can easily be monitored by
laser-induced fluorescence.
The optical properties of semiconductor nanocrystals (SC NCs) are largely controlled by their size and surface chemistry, i.e., the chemical composition and thickness of inorganic passivation shells and the chemical nature and number of surface ligands as well as the strength of their bonds to surface atoms.
The latter is particularly important for CdTe NCs, which – together with alloyed CdₓHg₁₋ₓTe – are the only SC NCs that can be prepared in water in high quality without the need for an additional inorganic passivation shell. Aiming at a better understanding of the role of stabilizing ligands for the control of the application-relevant fluorescence features of SC NCs, we assessed the influence of two of the most commonly used monodentate thiol ligands, thioglycolic acid (TGA) and mercaptopropionic acid (MPA),
on the colloidal stability, photoluminescence (PL) quantum yield (QY), and PL decay behavior of a set of CdTe NC colloids. As an indirect measure for the strength of the coordinative bond of the ligands to SC NC surface atoms, the influence of the pH (pD) and the concentration on the PL properties of these colloids was examined in water and D₂O and compared to the results from previous dilution studies with a set of thiol-capped Cd₁₋ₓHgₓTe SC NCs in D₂O. As a prerequisite for these studies, the number of surface ligands was determined photometrically at different steps of purification after SC NC synthesis with Ellman’s test.
Our results demonstrate ligand control of the pH-dependent PL of these SC NCs, with MPA-stabilized CdTe NCs being less prone to luminescence quenching than TGA-capped ones. For both types of CdTe colloids, ligand desorption is more pronounced in H₂O compared to D₂O, underlining also the role of hydrogen bonding and solvent molecules.
Polyamidoamine (PAMAM) dendrimers were used to produce CdSe core/multi-shell fluorescent quantum dots (QDs) which are colloidally stable in aqueous solutions. The size, charge, and optical properties of QDs functionalized with the 4th (G4) and 5th (G5) generation of PAMAM were compared with amphiphilic polymer-covered QDs and used as criteria for the evaluation of the suitability of both water solubilization methods. As revealed by dynamic and electrophoretic light scattering (DLS and ELS), the hydrodynamic sizes of the QDs varied from 30 to 65 nm depending on QD type and dendrimer generation, with all QDs displaying highly positive surface charges, i.e., zeta potentials of around +50 mV in water. PAMAM functionalization yielded stable core/multi-shell QDs with photoluminescence quantum yields (Φ) of up to 45%. These dendrimer-covered QDs showed a smaller decrease in their Φ upon phase transfer compared with QDs made water soluble via encapsulation with amphiphilic brush polymer bearing polyoxyethylene/ polyoxypropylene chains.
An ideal sensor system is a combination of a selective receptor, an effective transducer, and a sensitive detector. To utilize molecularly imprinted polymers (MIPs) as responsive recognition phases in sensors, the employment of fluorescent molecules or nanoparticles (NPs) that show prominent changes in their spectroscopic properties after binding of the target molecule in the MIP’s cavity is particularly attractive. Such fluorescent MIPs (fMIPs) act through target-induced quenching, enhancement, or spectral shifts of the fluorescence. This contribution introduces different strategies of incorporation of fluorescent dyes, probes, and NPs into fMIPs. In addition, various sensing mechanisms are reviewed, and depending on the application of the sensor, the different deployable formats, their advantages, drawbacks, and impact will be presented and discussed.
Ternary semiconductors quantum dots (t-QD) are Cd-free nanocrystals made from I-III-VI group elements like silver or copper, indium and sulfide yielding CIS (CuInS2) or AIS (AgInS2) QDs. They are interesting alternatives to Cd-based QDs for applications as solar concentrators or optically active material for solar cells, light emitting diodes (LED) or reporters for diagnostic assays.
To avoid ligand exchange procedures for high quality QDs, commonly synthesized in high boiling apolar solvents with apolar surface ligands, AIS QDs are synthesized in aqueous solution, required for bioanalytical application, by a microwave-assisted procedure. The surface of these QDs is passivated by a ZnS shell to enhance photoluminescence quantum yield (PL QY) and prevent material decomposition and oxidation. The resulting AIS/ZnS QDs exhibit broad PL spectra in the visible and near infrared, tunable by size and chemical composition.
I will show the simple synthetic procedure and a spectroscopic study of different AIS QDs evaluating their PL properties and stability, PL QY, and PL decay kinetics. The analyzed sample showed long lifetimes, relatively high QY (50%) and good stability. Ligand conjugation is also performed to allow the embedding in polymer matrix, which requires apolar cappng.
The simple aqueous synthesis together with the tunable emission color, the high PL QY, the high absorption coefficients and the long luminescence lifetimes make these t-QDs promising Cd-free materials for many different applications in the material and life sciences.
Optical spectroscopy – Techniques, instrumentation, and typical molecular and nanoscale reporters
(2018)
Different types of optical spectroscopies are introduced with special emphasis on method-inherent limitations and reliable instrument calibration and performance validation. In addition, different classes of molecular and nanocrystalline emitters are presented and the underlying photophysical processes are briefly described.
Ternary semiconductors quantum dots (t-QD) are Cd-free core-only or core-shell semiconductor nanocrystals with a core made from I-III-VI group elements like Cu-In-S (CIS) or Ag-In-S (AIS). To enhance the PL quantum yield (PL QY) and prevent material deterioration and oxidation, these QDs are commonly surface-passivated with ay ZnS shell.
CIS and AIS QDs exhibit broad photoluminescence (PL) bands that cover visible spectrum up to the near infrared (NIR), the spectral position of which being tunable by size, chemical composition of the core (ratio of the components), and surface ligand. They show high absorption coefficients, high PL quantum yields (PL QY of up to 70%), and long luminescence lifetimes in the order of a few hundred nanoseconds that make them promising materials for a broad variety of applications. This makes them interesting alternatives for Cd-based QDs for applications as optically active components in solar concentrators or solar cells, light emitting diodes (LED) or as reporter in the life sciences.
Here we present a systematic spectroscopic study of a set of AIS/ZnS QDs synthesized in water, size selected by fractioned precipitation, and after ligand exchange in an apolar organic solvent as well as embedded in a polymer. The latter was achieved by a photochemically initiated polymerization. The PL properties of these AIS/ZnS including PL QY and the PL decay kinetics assessed in different matrices/environments are shown and discussed as well as possible applications in energy conversion.
Ternary semiconductors Quantum Dots (t-QD) are Cd-free semiconductors nanocrystals made from I-III-VI group elements like CIS or AIS. They are interesting alternatives for Cd-based QDs for applications as optically active components in solar concentrators or solar cells, light emitting diodes (LED) or in the life sciences. To enhance the PL quantum yield (PL QY) and prevent material deterioration and oxidation, these QDs are commonly surface-passivated with by ZnS shell.
AIS QDs exhibit broad photoluminescence (PL) spectra in the visible and near infrared, which are tunable by size, chemical composition (ratio of components or doping), and surface ligand. They show a relatively high absorption coefficient, high PL QY (up to 70%), and long luminescence lifetimes in the order of a few hundred nanoseconds that make them promising materials for a broad variety of applications. Moreover, they can be simply prepared even in water in high quality which avoids further ligand exchange steps for all water-based applications know to decrease QY.
Here we present a systematic spectroscopic study of differently colored AIS/ZnS QDs synthesized in water, size selected by precipitation, transferred to organic solvents via ligand exchange, and embedded in different polymers by a photochemically initiated polymerization. The PL properties including PL QY and the PL decay kinetics measured in different matrices are shown and discussed.
Carbon dots have attracted much attention due to their unique optical, chemical and electronic properties enabling a wide range of applications. The properties of carbon dots can be effectively adjusted through modifying their chemical composition. However, a major challenge remains in understanding the core and surface contributions to optical and electronic transitions. Here, three blue luminescent carbon dots with carboxyl, amino and hydroxyl groups were comprehensively characterized by UV-vis absorption and emission spectroscopy, synchrotron-based X-ray spectroscopy, and infrared spectroscopy. The influence of the surface functionality on their fluorescence was probed by pH-dependent photoluminescence measurements. Moreover, the hydrogen bonding interactions between water and the surface groups of carbon dots were characterized by infrared spectroscopy. Our results show that both core and surface electronic states of blue luminescent carbon dots contribute to electronic acceptor levels while the chemical nature of the surface groups determines the hydrogen bonding behavior of the carbon dots. This comprehensive spectroscopic study demonstrates that the surface chemistry has a profound influence on the electronic configuration and surface–water interaction of carbon dots, thus affecting their photoluminescence properties.
There is an increasing interest in bridging the gap between the photoluminescence (PL) properties of nanomaterials like semiconductor nanocrystals (QDs) commonly assessed in ensemble studies and the PL features of single QDs for life sciences applications such as bioimaging studies or use in microfluidic assays. The fluorescence quantum yield (ΦF) is a key performance parameter for all molecular and nanoscale emitters, increasingly employed in nanoscience, nanotechnology, and medical diagnostics.
ΦF determines not only the signal size together with the reporter´s molar extinction coefficient, yet it is particularly relevant for nanocrystals like QDs with coordinatively bound surface ligands and size- and surface chemistry-dependent PL characteristics.
The importance of ΦF measurements at ultralow concentration encouraged us to explore the potential of fluorescence correlation spectroscopy (FCS) for the relative determination of ΦF of ligand-stabilized CdTe nanocrystals in comparison to molecular dyes with closely matching spectral properties and known ΦF.
We describe a FCS-based method for the relative determination of ΦF of dispersed QDs at ultralow concentrations, and procedures to overcome QD-inherent challenges like complex and power-dependent blinking behavior as well as ligand- and QD-specific aggregation. We could demonstrate the potential of this approach by comparison with steady state ensemble measurements.
Different types of optical spectroscopies are introduced with special emphasis on method-inherent limitations and reliable instrument calibration and performance validation. In addition, procedures for the determination of spectroscopic key parameters like the photoluminescence quantum yield are presented including required instrument calibrations and material-specific effects related to certain emitters.
In Vivo Biotransformations of Indium Phosphide Quantum Dots Revealed by X‑Ray Microspectroscopy
(2019)
Many attempts have been made to synthesize cadmium-free quantum dots
(QDs), using nontoxic materials, while preserving their unique optical properties. Despite impressive advances, gaps in knowledge of their intracellular fate, persistence, and excretion from the targeted cell or organism still exist, precluding clinical applications. In this study, we used a simple model organism (Hydra vulgaris) presenting a tissue grade of organization to determine the biodistribution of indium phosphide (InP)-based QDs by X-ray fluorescence imaging. By complementing elemental imaging with In L-edge X-ray absorption near edge structure, unique information on in situ chemical speciation was obtained. Unexpectedly, spectral profiles indicated the appearance of In−O species within the first hour post-treatment, suggesting a fast degradation of the InP QD core in vivo, induced mainly by carboxylate groups. Moreover, no significant difference in the behavior of bare core QDs and QDs capped with an inorganic Zn(Se,S) gradient shell was observed. The results paralleled those achieved by treating animals with an equivalent dose of indium salts, confirming the preferred bonding type of In3+ ions in Hydra tissues. In conclusion, by focusing on the chemical identity of indium along a 48 h long journey of QDs in Hydra, we describe a fast degradation process, in the absence of evident toxicity. These data pave the way to new paradigms to be considered in the biocompatibility assessment of QD-based biomedical applications, with greater emphasis on the dynamics of in vivo biotransformations, and suggest strategies to drive the design of future applied materials for nanotechnology-based diagnosis and therapeutics.
Semiconductor nanocrystals (quantum dots, QDs) are well known for their superior optoelectronic properties and enabled advancements in several key technologies of the 21st century. II-VI QDs containing the heavy metal element cadmium plaid a central role in this research area. Due to the concerns regarding their toxic potential, the European Union will ban cadmium in 2020 from use in any consumer product.
Due to their comparable optoelectronic properties and a more environmentally friendly character, indium phosphide based QDs (InP-QDs) are a promising alternative. Their large application potential was already shown in their utilization in the newest commercial TV screens. A vital role for highly luminescent QDs for such an application plays the growth of a surface passivating shell around the InP core. The combination of InP/ZnSe/ZnS in so-called core/shell/shell materials exploits the advantages of a small lattice mismatch (3.3%) between InP and ZnSe and the large band gap of ZnS to efficiently confine the excitons in the core and to increase the overall chemical stability. Using a thin ZnSe layer and a thick ZnS shell has shown to improve the optical properties of InP QDs and only recently, the prevention of blinking and mono-exponential decay kinetics could be achieved with a thick ZnSe Shell.
In our contribution, we investigated the influence of the thickness of the intermediate ZnSe layer (1 to 8 monolayers (ML)) on the photophysical properties of InP/ZnSe/ZnS core/shell/shell QDs with a final size of about 10 nm on the ensemble and the single-particle level. By rendering these InP QDs water-soluble, we were able to compare their properties in the organic and aqueous phase and further studied their toxicological potential depending on the chemical composition of their surface shell.
Semiconductor nanocrystals (quantum dots, QDs) are well known for their superior photophysical properties and enabled advancements in several key technologies of the 21st century and numerous technological applications. However, the most studied II-VI semiconductor nanocrystals contain the toxic heavy metal element cadmium, which is limiting their utilization in commercial applications. This has drawn the interest to alternative materials with less toxicity but having similar photophysical features.
The newest generation of TV screens based on QDs have shown that there is a promising environmentally friendly alternative with similar optoelectronic properties, namely indium phosphide (InP) QDs. InP QDs possess a bulk band gap of 1.35 eV with an exciton Bohr radius of ca. 10 nm and thus allow to tune their photoluminescence (PL) from the visible to the near-infrared. Tuning the size and shape of InP QDs and thus tailor their optoelectronic properties can be achieved by different strategies, which range from different types and concentrations of precursors, synthesis temperature or post-synthetic manipulations like etching. The incorporation of other elements like Gallium within the InP core synthesis is another possibility. Using a GaP intermediate layer before growing a ZnS shell has been shown to increase the PL quantum yield, which has been attributed to reduced lattice strain and the removal of phosphor vacancies. Different Ga precursors were investigated but a thorough investigation in terms of their reactivity, localization in the QD and influence on the photophysical properties is lacking to date.
In this contribution we will present the detailed investigation of the presence of two different Ga precursors within the InP core synthesis. Photophysical characterizations (steady-state and PL life-time measurements), transmission electron microscopy, XRD and EDX gave insights into the reactivity of the Ga precursors, the Ga localization in the InP core and influences on the photophysical properties. The variation of the precursor and surfactant concentration and the utilization of different ligands for the Ga precursor allowed tuning the PL emission towards the blue or the red. Depending on the used precursor type we observed the formation of larger-sized InP/GaP core/shell nanocrystals or the formation of InGaP alloy structures enabling to assess the blue range of emission (475 nm).
Gallium – a versatile element for tuning the photoluminescence properties of InP quantum dots
(2019)
With the goal to tune the emission properties of colloidal InP quantum dots, the incorporation of Ga was explored. Unexpectedly, depending on the nature of the gallium precursor, the photoluminescence shifted either to the red (gallium oleate) or to the blue (gallium acetylacetonate). In the first case, larger-sized InP/GaP core/shell nanocrystals were formed, while in the second case the formation of an InGaP alloy structure enabled the blue range of emission (475 nm) to be accessed.
With the goal to improve their photostability, InP-based QDs are passivated with three types of inorganic shells, namely (i) a gradient ZnSexS1−x shell, (ii) an additional ZnS shell on top of the gradient shell with two different thicknesses (core/shell/shell, CSS), (iii) an alumina coating on top of ZnS. All three systems have photoluminescence Quantum yields (PLQY) > 50%and similar PL decay times (64–67 ns). To assess their photostability they are incorporated into a transparent poly (methyl methacrylate) (PMMA) matrix and exposed to continuous irradiation with simulated sunlight in a climate chamber. The alumina coated core/shell system exhibits the highest stability in terms of PLQY Retention as well as the lowest shift of the PL maximum and lowest increase of the PL linewidth, followed by the CSS QDs and finally the gradient shell system. By means of XPS studies we identify the degradation of the ZnS outer layer and concomitant xidation of the emissive InZnP core as the main origins of degradation in the gradient structure. These modifications do not occur in the case of the alumina-capped sample, which exhibits excellent chemical stability. The gradient shell and CSS systems could be transferred to the aqueous phase using surface ligand exchange with penicillamine. Cytotoxicity studies on human primary keratinocytes revealed that exposure for 24 h to 6.25–100 nM of QDs did not affect cell viability. However, a trend toward reduced cell proliferation is observed for higher concentrations of gradient shell and CSS QDs with a thin ZnS shell, while CSS QDs with a thicker ZnS shell do not exhibit any impact.
Oligonucleotide-templated reactions (OTRs) between two reactive hybridization probes allow for the detection of a DNA or RNA of interest by exploiting the target molecule as a catalyst of chemical reactions.
The product of such a reaction commonly exhibits distinct fluorescence properties and can be detected by the means of fluorescence spectroscopy. The vast majority of OTR systems utilize organic dyes as fluorescent reporters. However, the use of brighter emitters, such as semiconductor quantum dots (QDs), has potential to improve the sensitivity of detection by providing brighter signals and permitting the use of probes at very low concentrations. Here we report an RNA-templated reaction between two fluorescently labeled peptide nucleic acid (PNA)-based probes, which proceeds on the surface of a QD. The QD-Bound PNA probe bears a cysteine functionality, while the other PNA is functionalized with an organic dye as a thioester. OTR between these probes proceeds through a transfer of the organic dye to the QD and can be conveniently monitored via fluorescence resonance energy transfer (FRET) from the QD to the Cy5. The reaction was performed in a conventional fluorescence microplate reader and permits the detection of RNA in the picomolar range.
Tempo-spectral multiplexing in flow cytometry with lifetime detection using QD-encoded polymer beads
(2020)
Semiconductor quantum dots (QDs) embedded into polymer microbeads are known to be very attractive emitters for spectral multiplexing and colour encoding. Their luminescence lifetimes or decay kinetics have been, however, rarely exploited as encoding parameter, although they cover time ranges which are not easily accessible with other luminophores. We demonstrate here the potential of QDs made from II/VI semiconductors with luminescence lifetimes of several 10 ns to expand the lifetime range of organic encoding luminophores in multiplexing applications using time-resolved flow cytometry (LT-FCM). For this purpose, two different types of QD-loaded beads were prepared and characterized by photoluminescence measurements on the ensemble level and by single-particle confocal laser scanning microscopy. Subsequently, these lifetime-encoded microbeads were combined with dye-encoded microparticles in systematic studies to demonstrate the potential of these QDs to increase the number of lifetime codes for lifetime multiplexing and combined multiplexing in the time and colour domain (tempo-spectral multiplexing). These studies were done with a recently developed novel luminescence lifetime flow cytometer (LT-FCM setup) operating in the time-domain, that presents an alternative to reports on phase-sensitive lifetime detection in flow cytometry.
Luminescent polymer microbeads are applied in bead-based platforms for biomolecule binding interactions, and the encoding of these beads is important for the realization of optically distinguishable barcodes. While dye- encoded beads often suffer from photobleaching and spectral crosstalk due to the relatively broad dye emission band, quantum dot (QD)-encoded beads absorb in a very broad wavelength range and reveal narrow emission bands, which enables simultaneous excitation of beads stained with differently colored QDs. To address existing limitations of QD encoding, we have explored simple and effective approaches to homogeneously stain polymer particles and identify suitable polymerization reactions and QD surface ligands that tackle these challenges.
Luminescent solar concentrators (LSC) allow to obtain renewable energy from building integrated photovoltaic systems. As promising efficient and long-term stable LSC fluorophores semiconductor nanocrystals like Quantum dots (QDs) with size and composition tunable optoelectronic properties have recently emerged. The most popular II/VI or IV/VI semiconductor QDs contain, however, potentially hazardous cadmium or lead ions, which is a bottleneck for commercial applications. A simple aqueous based, microwaveassisted synthesis for environmentally friendly and highly emissive AgInS2/ ZnS QDs is developed using 3-mercaptopropionic acid (MPA) and glutathione (GSH) and their incorporation into polylaurylmethacrylate (PLMA) polymer slabs integrable in LSC devices (10.4 × 10.4 × 0.2 cm3, G = 12.98).
With this simple approach, optical power efficiencies (OPE) of 3.8% and 3.6% and optical quantum efficiencies (OQE) of 24.1% and 27.4% are obtained, which are among the highest values yet reported.