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Safety is a key parameter for the design and construction of buildings. The most widely used construction material to date is concrete that consists of about 15-20 wt.-% cement, which is responsible for the main concrete properties, i.e., strength and durability. Despite many standards regulating the quality of cement, at present, there exists no viable method to track the cement life cycle “from cradle to grave”. This led to an increasing interest in simple and robust methods for studying the processes and changes occurring during the life cycle of cement. In this context, we explored the applicability of fluorescence measurements which can be performed with relatively inexpensive and miniaturized instrumentation yet require robust optical probes which survive the harsh cement environment. Therefore, we developed a platform of lanthanide-based upconversion nanoparticles (UCNPs), consisting of a NaYF4 matrix doped with Yb3+ and Er3+ with sizes between 20 nm and 55 nm, which show characteristic multi-color emission patterns, composed of narrow bands of varying intensity in the ultraviolet, visible, near-infrared, and short-wave spectral region and examined their potential for cement probing and the non-invasive monitoring of the hydration processes occurring during cement formation.
UCNPs of different size and chemical composition were synthesized via a thermal decomposition approach under inert conditions. The tailor-made design of different emission patterns was achieved by tuning particle size and morphology, material composition, and particle surface chemistry in upscaleable syntheses. For cement probing, different types of UCNPs were added to cement and the evolution of the UCNP emission pattern was used to probe in-situ changes of physico-chemical parameters in the cementitious environment during hydration, utilizing a simple and portable custom-designed optical setup. The observed changes in the UCNP emission patterns are characteristic for a given particle size, surface chemistry, and cement composition. In addition to fluorescence measurements, conventional isothermal heat flow calorimetry was used to study the influence of UCNP addition on cement hydration kinetics. Subsequently, both sets of measurements were correlated. Our results underline the potential of our optical approach ad UCNPs for the non-invasive probing of cementitious systems and cement hydration. This can be also exploited for cutting-edge applications of construction materials such as 3D concrete printing.
The optical properties of these materials strongly depend on the excitation power density, i.e., the number of photons absorbed per time interval. The upconversion quantum efficiencies (ΦUC) of these materials, the excitation power dependent population i.e. the emission characteristics, and the deactivation dynamics are influenced by nanoparticle architecture, doping concentration, and the microenvironment. We will discuss how time resolved measurements can help to understand the fundamental photophysical mechanisms and discuss differences to other nanocrystals like quantum dots.
Lanthanide-based upconversion nanoparticles (UCNPs) like hexagonal 𝛽-NaYF4 UCNPs doped with Yb3+ and Er3+, which efficiently convert 976 nm light to ultraviolet, visible, and near infrared photons, offer new strategies for luminescence-based sensing, barcoding, and Imaging. Their upconversion (UC) luminescence (UCL) features like UCL intensity, quantum yield, relative spectral distribution / UCL luminescence color, and luminescence decay kinetics are, however, strongly influenced by particle size, dopant ion concentration, particle architecture, surface chemistry including presence and thickness of surface passivation and shielding shells, microenvironment/presence of quenchers with high energy vibrations, and excitation power density (P).
We present here a comprehensive study of the influence of excitation power density on the UCL features of different types of UCNPs, focusing on Yb3+ and Er3+ co-doped NaYF4 core-only and core-shell nanostructures with different sizes and doping ion concentration, which underlines the importance of P-dependent optimum dopant concentrations for UCNP performance and the potential of P-tuning of UCL.
The use of inorganic lanthanide-doped upconversion nanoparticles (UCNP) in bioimaging and cellular studies requires biocompatible particles. One possible cause of UCNP toxicity is the release of potentially harmful fluoride and lanthanide ions as revealed by dilution studies in aqueous environments, particularly under high dilution conditions. To address this issue, suitable surface coatings preventing such effects in combination with fast screening methods suited for online monitoring and in situ analyses are desired.
Here we present systematic studies of differently sized β-NaYF4:Yb,Er UCNP stabilized with different surface coatings and hydrophilic ligands varying in binding strength to the particle surface in various aqueous environments at different temperatures and UCNP concentrations. The concentration of the fluoride and lanthanide ions released upon particle dissolution was quantified electrochemically with a fluoride ion-sensitive electrode and inductively coupled plasma optical emission spectrometry (ICP-OES) and monitored fluorometrically, thereby exploiting the sensitivity of the upconversion luminescence to changes in size and surface chemistry. Moreover, changes in surface chemistry were determined with X-Ray photoelectron spectroscopy (XPS). Based upon our results, we could derive optimum screening parameters for UCNP stability studies and determine conditions and coating procedures and ligands for enhancing UCNP stability in aqueous environments.
Lanthanide-based upconversion nanoparticles (UCNPs) offer new strategies for luminescence-based sensing and imaging. One of the best studied materials are hexagonal ß-NaYF4 UCNPs doped with 20% Yb3+ and 2% Er3+, which efficiently convert 976 nm light to photons emitted at 540 nm, 655 nm, and 845 nm, respectively, reveal Long luminescence lifetimes (> 100 µs), and are very photostable and chemically inters.[1,2] The properties of their upconversion (UC) luminescence (UCL) are, however, strongly influenced by particle size, concentration and spatial arrangement of dopant Ions, surface chemistry, and microenvironment.[3,4] In addition, the multiphotonic absorption processes responsible for UCL render UCL dependent on excitation power density (P).
The rational design of brighter UCNPs particle architectures encouraged us to assess systematically the influence of these parameters on UCL for differently doped UCNPs relying on the commonly used ß-NaYf4 matrix using steady state and time resolved fluorometry as well as integrating sphere spectroscopy for P varied over almost three orders of magnitude. This includes comprehensive studies of the influence of size and shell, Yb3+ and Er3+ dopand concentrations, and energy Transfer processes from UCNPs to surface-bound organic dyes or vice versa [5]. Our results underline the need for really quantitative luminescence studies for mechanistic insights, the potential of high p to compensate for UCL surface quenching, and the matrix- and P-dependence of the optimum dopand concentration.
Solid state multi-band emitters like lanthanide doped up-conversion nanoparticles (UCNPs) are promising materials for energy and biotechnologies. These materials are excitable in the near infrared (NIR) and show emission bands from UV to SWIR with excellent photostability, and long luminescence lifetimes in the µs range. The efficiency of these materials, i.e. the multiphoton absorption processes, the excitation power dependent population, and deactivation dynamics are strongly influenced by the particle size, structure, doping concentration, surface chemistry, and microenvironment. For applications UCNPs have to be optimized to meet the application relevant optical properties like the upconversion quantum yield (UC) and downshifting quantum yield (PL), luminescence lifetime and emission spectra.
We investigated ß-NaYF4 NPs co-doped with Yb3+ and Er3+ in aqueous and organic media and as powder. For the later, bulk and nanomaterial is compared. For dispersed particles the influence of surface chemistry (ligands), microenvironment (solvent) and size (10 to 43 nm) was studied. Especially for bioapplications the Förster-Resonance-Energy-Transfer (FRET) efficiency from UCNPs to organic dye molecules (rose bengal and sulforhodamine B) was optimized with respect to the UCNP size.
Biophotonics and analytics - Photoluminescence properties of nanocrystals and surface group analysis
(2018)
Correlating the photoluminescence (PL) properties of nanomaterials like semiconductor nanocrystals (QDs) and upconversion nanocrystals (UCNPs) assessed in ensemble studies and at the single particle level and studying their surface chemistry is increasingly relevant for applications of these nanomaterials in the life and material sciences. Here we present a comparison of the spectroscopic properties of ensembles and single nanocrystalline emitters and simple methods for the quantification of functional groups and ligands on particle surfaces. The overall goal of this study was to derive particle architectures and surface chemistries well suited for spectroscopic and microscopic applications.
- UCNPs were succesfully synthesized and characterized
- Various stages of UCNP growth were tracked using different
analytical methods including real time in-situ & time-resolved
luminescence spectroscopy, SAXS and TEM measurements
- Additional size determination will be performed using inductively
coupled plasma - mass spectrometry (ICP-MS)
Common approaches to improve the optical properties of semiconductor quantum dots and lanthanide doped nanophosphors present core/shell structures as radiationless deactivation at the particle surface is usually the main energy loss mechanism. This has led to increasingly sophisticated particle architectures using multishell systems with shells of different chemical composition and thickness and initiated an increasing number of quantitative spectroscopic studies focusing on the key performance parameter photoluminescence quantum yield to identify optimum particle structures. This is particularly challenging in the long wavelength region > 1000 nm and for nonlinear emitters like upconversion nanocrystals. Here, we present suitable absolute methods to quantify the photoluminescence of these different emitters in the vis/NIR/IR and as function of excitation power density and underline the impact of such measurements on a profound mechanistic understanding of the nonradiative deactivation pathways in semiconductor and upconversion nanocrystals of different size and particle architecture in different environments.
The presentation focuses on the current state of the optical spectroscopic studies on plasmon enhanced upconversion luminescent processes of silica-metal core-shell nanocomposites. A general introduction of the upconversion process, the theoretical basis of plasmonic enhancement, the theoretical requirements in regard to NP architecture for plasmon enhanced UC will be outlined. The first proof-of-concept measurements show the difficulty in realization the theoretical measurement parameters in laboratory conditions and underline the need for shifting the experiments to single particle level.“
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.
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.
Upconversion nanoparticles (UCNPs) offer new strategies for luminescence-based sensing and imaging. One of the best studied materials are ..-NaYF4 UCNPs doped with 20 % Yb3+ and 2 % Er3+, which efficiently convert 976 nm light to photons emitted at 540 nm, 655 nm, and 845 nm, respectively, reveal long luminescence lifetimes (> 100 µs), are photostable and chemically inert. Their upconversion (UC) luminescence (UCL) properties are, however, strongly influenced by particle size, surface chemistry, and microenvironment. In addition, the multiphotonic absorption processes responsible for UCL render UCL excitation power density (..) dependent. This makes quantitative UCL measurements as well as the determination of UC quantum yields (.UC) very challenging.
The rational design of brighter UCNPs particle architectures and the interest in identifying optimum particle architectures for FRET-based sensing and imaging schemes, which often rely on core-only UCNPs to minimize donor-acceptor distances, encouraged us to assess the influence of particle size and P on UCL. Here, we present the photophysical properties of a series of hexagonal NaYF4 UCNPs with sizes from 10 to 43 nm with different surface ligands dispersed in organic solvents and water studied by steady state and time-resolved fluorometry as well as quantitatively by integrating sphere spectrometry with P varied over about three orders of magnitude. Our results underline the need for really quantitative luminescence studies for mechanistic insights and the potential of high P to compensate for UCL quenching due to high energy phonons and surface effects.
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.
Upconversion nanoparticles (UCNPs) offer new strategies for luminescence-based sensing. The potential of UCNPs to serve as donors in Förster resonance energy transfer (FRET) applications is intensely discussed, owing to their anti-Stokes shifted narrow emission bands, chemical inertness, photostability, and long luminescence lifetimes (> 100 μs). Since FRET is distance dependent, the diameter of the particles is expected to affect the FRET efficiency. In order to identify the ideal particle architecture for FRET-based applications, we performed a systematic spectroscopic study of the influence of the UCNP size on the energy transfer using the organic dyes rose bengal and sulforhodamine B acting as model FRET acceptors for the green upconversion emission. High-quality Yb,Er-doped UCNPs with precisely controlled diameters between 10 and 43 nm were prepared using a high temperature synthesis. The monodisperse, oleate-capped particles were directly modified with the organic dyes by a two-step ligand exchange procedure, resulting in the shortest possible donor-acceptor distance. Successful FRET was demonstrated through the simultaneous drastic reduction of the luminescence intensity and the lifetime of the respective upconversion emission. In contrast to intensity measurements, time-resolved studies on both donor and acceptor luminescence allowed for the elimination of dependencies on excitation power density and particle concentration and for the discrimination between inner filter effects and FRET. The maximum FRET efficiency was observed at a particle diameter around 21 nm, which was attributed to an increasing fraction of the total amount of Er3+ donors inside the UCNPs being within Förster distance. Smaller UCNP diameters did not further improve the FRET efficiency, demonstrating the growing contribution of opposing effects, like the competition of non-radiative surface deactivation, at larger surface-to-volume ratios. This comprehensive understanding of energy transfer processes at the surface of UCNPs is essential for the rational design of upconversion FRET platforms for applications in sensing, imaging, and theranostics with improved sensitivity, reliability and comparability. The energy transfer can also be utilized to shift the luminescence emission by the choice of the organic dye in order to explore applications that require specific emission wavelengths due to interfering substances, while still making use of the advantages of near-infrared excitation.
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.
Upconversion nanoparticles are characterized by their ability to convert low energy near-infrared excitation in high energy emissions in the visible range. This is enabled by sequential absorption of two or more photons via energy transfer from the sensitizer lanthanide ion Yb3+ to an activator lanthanide ion, e.g. Er3+. Yb3+ sensitation has the advantage of 980 nm excitation achieved by commercial available low power cw laser modules.
Nevertheless, for some applications, especially in aqueous media, which is predominantely in bioanalytical applications, there is the drawback of a local Absorption maximum of water at this wavelength. Sample heating can occur, and excitation power density is strongly distance dependent.
To overcome this issue, the idea is to shift the excitation wavelength to 808 nm. This can be achieved by a tandem-excitation using Nd3+ and Yb3+ ions. Nd3+ gets excited by 808 nm irradiation and transfers the energy to Yb3+, which triggers the upconversion emission of the Er3+ ions. Therefore, it is mandatory to separate Nd3+ and Er3+ by core-shell architecture for reducing the energy backtransfer from Er3+ to Nd3+.
To further enhance the upconversion luminescence an additional inert shell formed by the host material NaYF4 reduces the quenching effects caused by surface defects and dangling bonds.
Obtaining high quality upconverting nanocrystals with only little crystal defects and hence, a high luminescence, affords a reliable synthesis route. Only this guarantees the reproducibility of the material and its spectroscopic properties required for future application. The fluorolytic sol-gel synthesis appears to be a convenient attempt, as this is a method with only few steps influencing the material properties, which can be well controlled.
Also creating bright upconverting nanocrystals requires a profound understanding of the interplay of photophysical processes like multiphoton absorption, radiative and non-radiative pathways, and energy transfer in the material.
Based on steady-state and time resolved luminescence measurements at different excitation power densities, the influence of the lanthanide doping ratio and synthesis parameters such as the annealing process on SrF2-nanocrystals obtained via the fluorolytic sol-gel synthesis was systematically studied.