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Lanthanide-doped upconversion nanoparticles (UCNPs) are of great interest for biomedical applications. Currently, the applicability of UCNP bionanotechnology is hampered by the generally low luminescence intensity of UCNPs and inefficient energy Transfer from UCNPs to surface-bound chromophores used e.g. for photodynamic therapy or analyte sensing. In this work, we address the low-Efficiency issue by developing versatile core-Shell nanostructures, where high-concentration sensitizers and activators are confined in the core and Shell Region of representative hexagonal NaYF2:Yb,Er UCNPs. After Doping concentration optimization, the sensitizer-rich core is able to harvest/accumulate more excitation energy and generate almost one order of Magnitude higher luminescence intesity than conventional homogeneously doped nanostructures. At the same time, the activator Ions located in the Shell enable a ~6 times more efficient resonant energy Transfer from UCNPs to surface-bound acceptor dye molecules due to the short distance between donor-acceptor pairs. Our work provides new insights into the rational design of UCNPs and will greatly encrease the General applicability of upconversion nanotechnologies.
There is an increasing interest in optical reporters like semiconductor and lanthanide-based nanocrystals with emission > 800 nm and recently also > 1000 nm for bioanalysis, medical diagnostics, and safety barcodes. Mandatory for the comparison of different emitter classes and the rational design of the next generation of reporters for the short wavelength infrared (SWIR) region are reliable and quantitative photoluminescence measurements in this challenging wavelength region. This is of special relevance for nanocrystalline emitters like semiconductor quantum dots and rods as well as for upconversion and downconversion nanocrystals, where surface states and the accessibility of emissive states by quenchers largely control accomplishable quantum yields and hence, signal sizes and detection sensitivities from the reporter side. Such measurements are currently hampered by the lack of suitable methods and standards for instrument calibration and validation as well as by the lack of quantum yield standards with emission > 800 nm and especially > 1000 nm.
In this respect, we present the design of integrating sphere setups for absolute and excitation power densitydependent measurements of emission spectra and quantum yields in the wavelength region of 650 to 1650 nm
including calibration strategies and first candidates for potential fluorescence standards. Subsequently, the photoluminescence properties of different types of nanocrystals are presented and discussed including absolute photoluminescence measurements of upconversion and down conversion emission in different solvents.
The direct measure for the conversion efficiency of absorbed photons into emitted light by an emissive species or chromophore is the photoluminescence quantum yield (pl).1 pl is the fluorometric key quantity that controls the suitability of a molecule or material e.g. for application as a label, probe or sensor molecule or as a converter material. A straightforward approach to pl values presents their absolute determination using an integrating sphere set up. This procedure circumvents the use of standards thereby avoiding additional measurements and uncertainties related to their often debated pl values. In this respect, we discuss the experimental requirements on absolute measurements of down and upconversion quantum yields (UC), the later presents currently one of the most challenging spectroscopic measurements due to its multiphotonic character. We explore the challenges for determination of excitation power density (P) dependent pl(P), present the design and characterization of a unique integrating sphere setup for such measurements in the vis to IR spectral region including its calibration, and the influence of the excitation beam profile to perform pl measurements of liquid and solid materials.
As an example for downshifting materials we present doped core-shell CdSe/ZnS semiconductor quantum dots (d-dots) which show a nearly host material and crystal size independent emission with large Stokes shifts and minimum reabsorption.2 Systematic variation of the reaction components, parameters and thickness of the ZnS shell yielded doped nanocrystals with a very high pl, high reproducibility and large quantities. Application of these NCs in the light conversion layers of commercial monocrystalline silicon (mono-Si) solar cells led to a significant enhancement of the external quantum efficiency (EQE) of this devices in the ultraviolet spectral region between 300 and 400 nm. On the other hand, lanthanide-doped up-converting nanoparticles (UCNPs), are promising light converters from the near infrared to the visible region. These NPs show multiple narrow emission bands in the visible (vis) and NIR, excellent photostability, and long luminescence lifetimes. The rational design of brighter UCNP requires an improved understanding of the radiationless deactivation pathways, that are affected by size, surface chemistry, and microenvironment.3 We give an insight into the influences of these parameters on the photophysical key characteristics of the upconversion process such as UC(P), the luminescence decay behavior, the power dependent red-to-green intensity ratio, and intensities of the individual emission bands.
In this review we present new concepts and recent progress in the application of semiconductur quantum dots (QD) as labels in two important areas of biology, bioimaging and biosensing. We analyze the biologically relevant properties of QDs focusing on the following topics: QD surface treatment and stability labeling of cellular structures and receptors with QDs, incorporation of QDs in living cells, cytotoxicity of QDs and influence of the biolocical environment on the biological and optical properties of QDs. Initially, we consider utilization of QDs as agants in high-resolution bioimaging techniques that can provide information at the molecular levels. The deverse range of modern live-cell QD-based imaging techniques with resolution far beyond the diffraction limit of light is examined. In each technique, we discuss the pros and cons of QD use and deliberate how QDs can be further engineered to facilitate their application in the respective imaging techniques and to produce significant improvements in resolution. Then we review QD-based point-of-care bioassays, bioprobes, and biosensors designed in different formats ranging from analytic biochemistry assays and ELISA, to novel point-of-care smartphone integrated QD-based biotests. Here, a wide range of QD-based fluorescence bioassays with optical transduction, electrochemiluminescence and photoelectrochemical assays are discussedc. Finally, this review provides an analysis of the prospects of application of QDs in selected important Areas of biology.
The quantum yield is a critically important parameter in the development of lanthanide-based upconverting nanoparticles (UCNPs) for use as novel contrast agents in biological imaging and optical reporters in assays. The present work focuses on the influence of the beam Profile in measuring the quantum yield (f) of nonscattering dispersions of nonlinear upconverting probes, by establishing a relation between f and excitation light power density from a rate equation analysis. A resulting 60% correction in the measured f due to the beam profile utilized for excitation underlines the significance of the beam profile in such measurements, and its impact when comparing results from different Setups and groups across the world.
There is an increasing interest in molecular and nanoscale with emission > 800 nm and recently also > 1000 nm for bioanalysis, medical diagnostics, bioimaging, and safety barcodes. Mandatory for the comparison of different emitter classes and the rational design of the next generation of reporters for the short wavelength infrared (SWIR) Region are reliable and quantitative photoluminescence measurements in this challenging wavelength region. This is of special relevance for nanocrystalline emitters like semiconductor quantum dots and rods as well as lanthanide-based upconversion and downconversion nanocrystals, where surface states and the accessibility of emissive states by quenchers largely control accomplishable photoluminescence quantum yields and hence, signal sizes and detection sensitivities from the reporter side. Such measurements are currently hampered by the lack of suitable methods and standards for instrument calibration and validation and quantum yield standards with emission > 800 nm and especially > 1000 nm.
In this respect, we present the design of integrating sphere setups for absolute and excitation power density-dependent measurements of emission spectra and photoluminescence quantum yields in the wavelength Region of 650 to 1650 nm including calibration strategies and first candidates for potential fluorescence standards.
Subsequently, the photoluminescence properties of different types of nanocrystals are presented including the upconversion and downconversion emission of differently sized and surface functionalized lanthanide-doped nanoparticles and photoluminescence quenching effects are quantified.
Due to their unique physical properties, particularly their electronic and luminescent properties, graphene quantum dots (GQDs) are expected to be suitable for a wide range of applications in bioimaging, electro-optical and photonic materials or energy harvesting among others.1 Tuning the surface chemistry provides an efficient approach to modulate the fluorescence and distinct electronic properties of GQDs.2 Nevertheless, the role of surface chemistry on the electronic structure of GQDs remains poorly understood. In this presentation, we will compare systematically the electronic and chemical structures of GQDs functionalized with carboxylic and aminated groups to those of non-functionalized GQDs, combining theoretical and experimental approaches, here various photon-based spectroscopies. First, the electronic structure of GQDs was characterized by soft X-ray absorption (XA) and X-ray emission (XE) spectroscopies, probing unoccupied and occupied electronic states, respectively, at the carbon K edge for the first time. The interpretation of the XA/XE spectra was done based on theoretical calculations. Then, the chemical structure of the GQDs was characterized in situ by ATR-FTIR in water, thereby accounting for the importance of the interface between GQDs and water believed to play a central role in the chemical reactivity and the optical properties. We previously demonstrated that monitoring the OH vibrations of water molecules during exposure to humid air was a powerful method to probe H-bonding environment around carbon nanomaterials.3 For GQDs, clear surface-dependent water adsorption profiles are observed and discussed. Finally, UV/Vis absorption and photoluminescence measurements were done to characterize the optical properties of these GQDs. Our results suggest that the surface chemistry of the GQDs affects significantly their electronic structure and optical properties. These findings will contribute to an improved understanding of the structure–activity relationship of GQDs and other carbon nanomaterials with surface modifications.
Quantum dot (QD) based nanomaterials are very promising materials for the fabrication of optoelectronic devices like solar cells, light emitting diodes (LEDs), and photodetectors as well as as reporters for chemo- and biosensing and bioimaging. Many of These applications involve the monitoring of changes in photoluminescence intensity and energy transfer processes which can strongly depend on excitation wavelength or energy. In this work, we analyzed the excitation energy dependence (EED) of the photoluminescence quantum yields (PL QYs) and decay kinetics and the circular dichroism (CD) spectra of CdSe/CdS core/shell QDs with different thicknesses of the surface passivation shell. Our results demonstrate a strong correlation between the spectral position of local maxima observed in the EED of PL QY and the zero-crossing points of the CD profiles. Theoretical analysis of the energy band structure of the QDs with effective mass approximation suggests that these structures could correspond to exciton energy levels. This underlines the potential of CD spectroscopy for the study of electronic energy structure of chiroptically active nanocrystals which reveal quantum confinement effects.
The excitation wavelength (lexc) dependence of the photoluminescence (PL) quantum yield (FPL) and decay behavior (tPL) of a series of CdSe/CdS quantum dot/Quantum rods (QDQRs), consisting of the same spherical CdSe core and rod-shaped CdS shells, with aspect ratios ranging from 2 to 20 was characterized. lexc between 400–565 nm were chosen to cover the first excitonic absorption band of the CdSe core material, the onset of absorption of the CDs shell, and the region of predominant shell absorption. A strong lexc dependence of relative and absolutely measured FPL and tPL was found particularly for the longer QDQRs with higher aspect ratios. This is attributed to combined contributions from a length-dependent shell-to-core exciton localization efficiency, an increasing number of defect states within the shell for the longest QDQRs, and probably also the presence of absorbing, yet non-emitting shell material. Although the FPL values of the QDQRs decrease at shorter wavelength, the extremely high extinction coefficients introduced by the shell outweigh this effect, leading to significantly higher brightness values at wavelengths below the absorption onset of the CdS Shell compared with direct excitation of the CdSe cores. Moreover, our results present also an interesting example for the comparability of absolutely measured FPL using an integrating sphere setup and FPL values measured relative to common FPL standards, and underline the Need for a correction for particle scattering for QDQRs with high aspect ratios.
Influence of agglomeration and aggregation on the photocatalytic activity of TiO2 nanoparticles
(2017)
Particle aggregation and agglomeration influence the optical properties of materials and therefore theirability to absorb and scatter the incoming radiation, also affecting their photocatalytic activity. Wehave studied the correlation between aggregation and photocatalytic activity for titanium dioxide bymeans of experimental measurements of extinction and photocatalytic activity and calculations of theiroptical properties (extinction, absorption and scattering cross-sections). This approach can be adoptedto quantitatively assess the quantum yields of the heterogeneous photocatalytic systems. The study wasperformed on TiO2PC105 Cristal ACTiVTM, made of aggregated (and agglomerated) primary particles of anatase. The size of the aggregates has been reduced with ultra-sonication. Aqueous suspensions of the obtained materials were characterized by measuring the optical properties (UV–vis extinction), the sizing properties (DLS) and the photocatalytic activity (degradation of phenol under standard con-ditions). The extinction and absorption spectra of the suspensions were derived from the calculatedcoefficients, considering also the size distributions measured with DLS, and revealed that light absorptionis maximized when particle aggregation and agglomeration are avoided, while diffusion of the incomingradiation dominates when large aggregates and agglomerates are present. The present paper represents a valuable approach to the accurate and reproducible measurement of the photocatalytic activity ofTiO2nanoparticles suspensions, thus allowing a more reliable comparison of the properties of different materials.
The implementation of fluorescent methods is of outstanding importance in the field of optical chemical sensor Technology and biosciences. Their bioanalytical applications are manifold including fluorescence microscopy, fluorescence in situ hybridization, DNA sequencing, fluorescence-activated cell sorting, immunoassays, analysis of DNA and Protein microarrays, and quantitative PCR, just to name a few examples. Particularly, fluorescence microscopy is a valuable method in the versatile field of biomedical imaging methods which nowadays utilizes different fluorescence Parameters like emission wavelength/Color and lifetime for the discrimination between different targets. Sectional Images are available with confocal microscopes. Tissue, cells or single cellular compartments can be stained and visualized with fluorescent dyes and biomolecules can be selectively labeled with fluorescent dyes to Monitor biomolecular interactions inside cells or at Membrane bound receptors.
On the other hand , fluorophores can act as indicator (or "molecular probe") to visualize intrinsically colorless and non-fluorescent ionic and neutral analytes such as pH, Oxygen (pO2), metal ions, anions, hydrogen peroxide or bioactive small organic molecules such as Sugars or nucleotides. Thereby, their photoluminescent properties (fluorescence or phoporescence intensity, exitation and/or Emission wavelength, emission lifetime or anisotropy) respond to the presence of these species in their immediate Environment. In general, the use of luminescent probes has the advantage that they can be delivered directly into the sample, and detected in a contactless remote mode. By now, these probes are often encapsulated in different types of nanoparticles (NPs) made from (biodegradable) organic polymers, biopolymers or inorganic materials like silica or bound to their surface.
Hexagonal NaYF4 doped with 20 % Yb3+ and 2 % Er3+ is an efficient upconversion (UC) phosphor for the conversion of 976 nm excitation light to emission at 845 nm, 800 nm, 655 nm, 540 nm and 410 nm light. The emission behavior of nanoparticles made from this material is strongly influenced by particle size, surface chemistry, and microenvironment. Furthermore their UC emission originates from multiphotonic absorption processes, rendering the resulting luminescence spectra and intensities excitation power density (P) dependent. Therefore the rational design of efficient nm-sized UC particles e.g., for applications in the material and life sciences requires reliable spectroscopic tools for the characterization of the optical properties of these materials like the excitation power density (P)-dependent UC quantum yield (QYUC) in dispersion, which presents a measure for the efficiency of the conversion of absorbed into emitted photons. Up to date the P-dependent absolute measurement of QYUC in aqueous media with an excitation wavelength of 976 nm presents a considerable challenge due to the low absorption coefficients of the UC materials and the absorption of water at this wavelength.
Lanthanide-doped photon-upconversion nanoparticles (UCNPs) have been the Focus of many Research activities in materials and life sciences in the last 15 years because of their potential to convert light between different spectral regions and their unique photophysical properties. To fully exploit the application potential of These facinating nanomaterials, a number of challenges have to be overcome, such as the low brightness, particularly of small UCNPs, and the reliable quantification of the excitation-power-density-dependent upconversion luminescence. In this series of critical Reviews, recent developments in the design, Synthesis, optical-spectroscopic characterization, and application of UCNPs are presented with Special Focus on bioanalysis and the life sciences. Here we guide the reader from the Synthesis of UCNPs to different concepts to enhance their luminescence, including the required optical-spectroscopic assessment to quantify material Performance; surface modification strategies and bioanalytical applications as well as selected examples of the use of UCNPs as reporters in different Assay formats are addressed in part II. Future Trends and challenges in the field of upconversion are discussed with Special emphasis on UCNP Synthesis and material characterization, particularly quantitative luminescence studies.
In Part II of this Review series on lanthanide-doped photon-upconversion nanoparticles (UCNPs), we present and critically discuss the Performance and suitability of UCNPs as background-free luminescent Reporters in bioimaging and bioanalytical applications. The preparation of a biocompatible nanoparticle surface is an integral step for all life - science-related applications. UCNPs have found their way into a large number of diagnostic platforms, homogeneous and heterogeneous assay formats, and sensor applications. Many bioanalytical detection schemes involve Förster resonance energy transfert (FRET), which is still debated for UCNPs and Needs to be much improved. The Need for dedicated and standardized instruments as well as recent studies on the Dissolution and potential toxicity of UCNPs are addressed. Finally we outline future Trends and challenges in the field of upconversion.
At the core of luminescence color and lifetime Tuning of rare earth doped upconverting nanoparticles (UCNPs), is the understanding of the Impact of the particle architecture for commonly used sensitizer (S) and activator (A) Ions. In this respect, a series of core@Shell NaYF4 UCNPs doped with Yb3+ and Ho3+ ions are presented here, where the same dopant concentrations are distributed in different particle architectures following the scheme: YbHo core and YbHo@..., ...@YbHo, Yb@Ho, Ho@Yb, YbHo@Yb, and Yb@YbHo core-Shell NPs. As refealed by quantitative steady-state and time-resolved luminescence studies, the relative spatial Distribution of the A and S ions in the UCNPs and their protection from surface quenching has a critical Impact on ther luminescence characteristics. Although the increased amount of Yb3+ Ions boosts UCNP Performance by amplifying the Absorption, the Yb3+ ions can also efficiently dissipate the energy stored in the material through energy Migration to the surface, thereby reducing the Overall energy Transfer Efficiency to the activator ions. The results provide yet another proof that UC Phosphor chemistry combined with materials Engineering through intentional core@shell structures may help to fine-tune the luminescence Features of UCNPs for their specific future applications in biosensing, bioimaging, photovoltaics, and Display technologies.
Mandatory for the comparison of different emitter classes and the rational design of the next generation of molecular and nanoscale reporters are reliable and quantitative photoluminescence measurements. This is of special relevance for all fluorescence applications in the life and material sciences. In the following, proccedures for the determination of this spectroscopic key parameter are presented including material-specific effects related to certain emitters
We report on a novel and simple approach to surface ligand design of CdSe-based nanocrystals (NCs) with biocompatible, heterobifunctional polyethylene glycol (PEG) molecules. This method provides high transfer yields of the NCs into aqueous media with preservation of the narrow and symmetric emission bands of the initial organic-capped NCs regardless of their interior crystal structure and surface chemistry. The PEG-functionalized NCs show small sizes, high photoluminescence quantum yields of up to 75%, as well as impressive optical and colloidal stability. This universal approach is applied to different fluorescent nanomaterials (CdSe/CdS, CdSe/CdSCdxZn1-xS, and CdSe/CdS/ZnS), extending the great potential of organic-capped NCs for biological applications.