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Photoluminescence Quantum Yields of Luminescent Nanocrystals and Particles in the UV/vis/NIR/SWIR
(2023)
The rational design of functional luminescent materials such as semiconductor quantum dots and lanthanide-based upconversion nanoparticles, all photophysical and mechanistic studies, and the comparison of different emitters require accurate and quantitative photoluminescence measurements. Particularly the reliable determination of the key performance parameter photoluminescence quantum yield (f), the number of emitted per absorbed photons, and the brightness are of special importance for luminescence applications in the life and material sciences and nano(bio)photonics.[1] In this context, examples for absolute measurements of the photoluminescence quantum yields of UV/vis/NIR/SWIR emissive semiconductor quantum dots and rods, made from different materials, and spectrally shifting lanthanide upconversion nanocrystals with different surface chemistries in transparent matrices are presented including excitation wavelength and power density dependent studies utilizing integration sphere spectroscopy.[2,3] In addition, procedures for the absolute determination of the photoluminescence quantum yields of scattering dispersions of larger size quantum rods and differently sized inorganic particles have been developed as well as procedures for the characterization of solid luminescent nanomaterials such as different perovskites and YAG:Cer converter materials.[4] Thereby, challenges and pitfalls of f measurements in different wavelength regions including the SWIR and material-specific effects related to certain emitter classes are addressed, achievable uncertainties are quantified, and relative and absolute measurements of photoluminescence quantum yield measurements are compared to underline limitations of the former approach. Finally, a set of novel UV/vis/NIR quantum yield standards is presented including their certification with a complete uncertainty budget.[5]
Iron Oxide Nanocubes as a New Certified Reference Material for Nanoparticle Size Measurements
(2023)
The rational design and increasing industrial use of nanomaterials require a reliable characterization of their physicochemical key properties like size, size distribution, shape, and surface chemistry. This calls for nanoscale reference materials (nanoRMs) for the validation and standardization of commonly used characterization methods closely matching real-world nonspherical nano-objects. This encouraged us to develop a nonspherical nanoRM of very small size consisting of 8 nm iron oxide nanocubes (BAM-N012) to complement spherical gold, silica, and polymer nanoRMs. In the following, the development and production of this nanoRM are highlighted including the characterization by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) as complementary methods for size and shape parameters, homogeneity and stability studies, and calculation of a complete uncertainty budget of the size features. The determination of the nanocubes’ edge length by TEM and SAXS allows a method comparison. In addition, SAXS measurements can also provide the mean particle number density and the mass concentration. The certified size parameters, area equivalent circular diameter and square edge length, determined by TEM with a relative expanded uncertainty below 9%, are metrologically traceable to a natural constant for length, the very precisely known (111) lattice spacing of silicon. Cubic BAM-N012 qualifies as a certified nanoRM for estimating the precision and trueness, validation, and quality assurance of particle size and shape measurements with electron microscopy and SAXS as well as other sizing methods suitable for nanomaterials. The production of this new iron oxide nanocube RM presents an important achievement for the nanomaterial community, nanomaterial manufacturers, and regulators.
Surface-functionalized polymer beads encoded with molecular luminophores and nanocrystalline emitters such as semiconductor nanocrystals, often referred to as quantum dots (QDs), or magnetic nanoparticles are broadly used in the life sciences as reporters and carrier beads. Many of these applications require a profound knowledge of the chemical nature and total number of their surface functional groups (FGs), that control bead charge, colloidal stability, hydrophobicity, and the interaction with the environment and biological systems. For bioanalytical applications, also the
number of groups accessible for the subsequent functionalization with, e.g., biomolecules or targeting ligands is relevant. In this study, we explore the influence of QD encoding on the amount of carboxylic acid (COOH) surface FGs of 2 μm polystyrene microparticles (PSMPs). This is done for frequently employed oleic acid and oleylamine stabilized, luminescent core/shell CdSe QDs and two commonly used encoding procedures. This included QD addition during bead formation by a thermally induced polymerization reaction and a post synthetic swelling procedure. The accessible number of COOH groups on the surface of QD-encoded and pristine beads was quantified by two colorimetric assays, utilizing differently sized reporters and electrostatic and covalent interactions. The results were compared to the total number of FGs obtained by a conductometric titration and Fourier transform infrared spectroscopy (FTIR). In addition, a comparison of the impact of QD and dye encoding on the bead surface chemistry was performed. Our results demonstrate the influence of QD encoding and the QD-encoding strategy on the number of surface FG that is ascribed to an interaction of the QDs with the carboxylic acid groups on the bead surface. These findings are of considerable relevance for applications of nanoparticle-encoded beads and safe-by-design concepts for nanomaterials.
The rational design of next generation molecular and nanoscale reporters and the comparison of different emitter classes require the determination of the fluorometric key performance parameter fluorescence quantum yield (Φf), i.e., the number of emitted photons per number of absorbed photons. Main prerequisites for reliable Φf measurements, which are for transparent luminophore solutions commonly done relative to a reference, i.e., a fluorescence quantum yield standard of known Φf, are reliable and validated instrument calibration procedures to consider wavelength-, polarization-, and time-dependent instrument specific signal contributions, and sufficiently well characterized fluorescence quantum yield standards. As the standard’s Φf value directly contributes to the calculation of the sample’s Φf, its accuracy presents one of the main sources of uncertainty of relative Φf measurements. To close this gap, we developed a first set of 12 fluorescence quantum yield standards, which absorb and emit in the wavelength region of 330−1000 nm and absolutely determined their Φf values with two independently calibrated integrating sphere setups.
Criteria for standard selection and the configuration of these novel fluorescence reference materials are given, and the certification procedure is presented including homogeneity and stability studies and the calculation of complete uncertainty budgets for the certified Φf values. The ultimate goal is to provide the community of fluorescence users with available reference materials as a basis for an improved comparability and reliability of quantum yield data since the measurement of this spectroscopic key property is an essential part of the characterization of any new emitter.
Luminescent Nanoparticles – From Photophysics to the Measurement of Photoluminescence Quantum Yields
(2023)
Inorganic nanocrystals with linear and nonlinear photoluminescence in the ultraviolet, visible, near infrared and short-wave infrared like spectrally shifting lanthanide-based nanoparticles (LnNCs) like NaYF4: Yb, Er and semiconductor quantum dots have meanwhile found applications in the life and material sciences ranging from optical reporters for bioimaging and sensing over security barcodes to solid state lighting and photovoltaics. The identification of optimum particle architectures for photonic applications requires quantitative spectroscopic studies, ideally flanked by single particle studies to assess spectroscopic inhomogeneities on a particle-to-particle level for typical preparation methods. In the following, photoluminescence studies of LnNCs are presented, addressing parameters such as particle size, surface coating, and dopant ion concentration as well as excitation power density mandatory for a profound mechanistic understanding of the nonradiative deactivation pathways in these nanocrystals. In addition, methods for the determination of particle brightness and photoluminescence quantum yield in different spectral windows are presented.
Comparing the performance of molecular and nanoscale luminophores and luminescent microand nanoparticles and estimating achievable signal amplitudes and limits of detection requires a standardizable intensity scale. This initiated the development of the relative MESF (number of molecules of equivalent soluble fluorochromes) and ERF (equivalent reference fluorophores) scales for flow cytometry and fluorescence microscopy. Both intensity scales rely on fluorescence intensity values assigned to fluorescent calibration beads by an intensity comparison to spectrally closely matching fluorophore solutions of known concentration using a spectrofluorometer. Alternatively, the luminophore or bead brightness (B) can be determined that equals the product of the absorption cross section (σa) at the excitation wavelength (σa(λex)) and the photoluminescence quantum yield (Φpl).
Thereby, an absolute scale based on fundamental and measurable spectroscopic properties can be realized which is independent of particle size, material, and luminophore staining or labeling density and considers the sensitivity of the optical properties of luminophores to their environment. Aiming for establishing such a brightness scale for light-scattering dispersions of luminescent particles with sizes exceeding a few ten nanometers, we demonstrate how the brightness of quasi-monodisperse 25 nm, 100 nm, and 1 μm sized polystyrene particles (PSP), loaded with two different dyes in varying concentrations, can be obtained with a single custom-designed integrating sphere setup that enables the absolute determination of Φpl and transmittance and diffuse reflectance measurements. The resulting Φpl, σa(λex), imaginary parts of the refractive index, and calculated B values of these samples are given in dependence of the number of incorporated dye molecule per particle. Finally, a unitless luminescence efficiency (LE) is defined allowing for the direct comparison of luminescence efficiencies of particles with different sizes.
Inorganic nanocrystals with linear and nonlinear luminescence in the ultraviolet, visible, near infrared and short-wave infrared like semiconductor quantum dots and spectrally shifting lanthanide-based nanophosphors have meanwhile found many applications in the life and material sciences. This includes optical reporters for bioimaging and sensing, security and authentication barcodes, solid state lighting, converter materials, and photovoltaics. The identification of optimum particle structures requires quantitative spectroscopic studies under application-relevant conditions, focusing on the key performance parameter photoluminescence quantum yield, ideally flanked by single particle studies to assess spectroscopic inhomogeneities on a particle-to-particle level for typical preparation methods. In this context, methods to quantify the photoluminescence of these different nanoscale emitters are shown and utilized as a basis for a profound mechanistic understanding of the nonradiative deactivation pathways in semiconductor and upconversion nanocrystals of different size and particle architecture in different environments. Exemplary for the application potential of such nanomaterials, in addition, the design of optical sensors from different nanomaterials and functional organic dyes is briefly summarized.
Photodynamic therapy (PDT) used for treating cancer relies on the generation of highly reactive oxygen species, for example, singlet oxygen 1O2, by light-induced excitation of a photosensitizer (PS) in the presence of molecular oxygen, inducing DNA damage in close proximity of the PS. Although many precious metal complexes have been explored as PS for PDT and received clinical approval, only recently, the potential of photoactive complexes of nonnoble metals as PS has been discovered. Using the DNA origami technology that can absolutely quantify DNA strand break cross sections, we assessed the potential of the luminescent transition metal complex [Cr(ddpd)2]3+ (ddpd=N,N’-dimethyl-N,N’-dipyridine-2-ylpyridine-2,6-diamine) to damage DNA in an air-saturated aqueous environment upon UV/Vis illumination. The quantum yield for strand breakage, that is, the ratio of DNA strand breaks to the number of absorbed photons, was determined to 1–4%, indicating efficient transformation of photons into DNA strand breaks by [Cr(ddpd)2]3+.
Optical Thermometry is popular among researchers because of its non-contact, high sensitivity, and fast measurement properties. In the present experiment, Er3+/Yb3+/K+ co-doped NaYF4 nanoparticles with different K+ concentrations were synthesized by solvothermal method, and the samples showed bright upconversion green emission under the excitation of a 980 nm laser. The powder X-ray diffractometer and transmission electron microscope were used to characterize the crystal structure and its surface morphology, respectively. The spectral characteristics of nanoparticles with K+ doping concentration from 10% to 30% (Molar ratio) were investigated by fluorescence spectroscopy, and it was observed that the fluorescence intensity reached the maximum at the K+ concentration of 20%, after which the intensity weakened when the K+ content continued to increase. According to the dependence between the luminescence intensity of the sample and the laser power density and fluorescence lifetime, the intrinsic mechanism was carefully investigated. Temperature-dependent spectra of the samples were recorded in the temperature range of 315–495 K, and the maximum values of absolute sensitivity (Sa) and relative sensitivity (Sr) were measured at 0.0041 K−1 (455 K) and 0.9220%K−1 (315 K). The experimental results show that K+/Er3+/Yb3+ triple-doped NaYF4 green fluorescent nanoparticles (GFNs) have good prospects for applications in display devices, temperature sensing, and other fields.
The NaYF4: Yb3+/Tm3+@NaYF4@β-CD upconversion nanoparticles were successfully prepared by the solvothermal method, and the samples were pure hexagonal phase with good crystallinity and homogeneous size, asevidenced by XRD and TEM analysis. The FT-IR analysis shows that β-CD is successfully encapsulated on the surface of NaYF4: Yb3+/Tm3+@NaYF4 nanoparticles. The fluorescence intensity 3and lifetime were significantly increased after coating the inert layer on the surface of core nanoparticles. After further surface modification of β-CD, the fluorescence intensity and fluorescence lifetime were reduced, but the overall fluorescence was stronger. Temperature measurements using the fluorescence intensity ratio technique were found to have relatively low reliability and absolute sensitivity for temperature measurements using thermally coupled energy levels. However, the reliability of temperature measurements using non-thermally coupled energy levels is significantly higher and the absolute sensitivity is much higher than for measurements at thermally coupled levels. Since the maximum absolute sensitivity, maximum relative sensitivity and minimum temperature resolution are determined to be 0.1179 K-1, 2.19 %K 1 and 0.00019 K, respectively, NaYF4: Yb3+/Tm3+@NaYF4@β-CD upconversion nanoparticles are expected to be widely used in the biomedical field due to their feasibility, reliability, non-toxicity and harmlessness.
The core−shell NaYF4:Yb3+/Tm3+@NaYF4:Yb3+ upconversion nanoparticles were successfully prepared by a solvothermal method, and a layer of mesoporous silica (mSiO2) was successfully coated on the periphery of the core−shell nanoparticles to transform their surface from lipophilic to hydrophilic, further expanding their applications in biological tissues. The physical phase, morphology, structure, and fluorescence properties were characterized by X-ray diffraction (XRD), field emission transmission electron microscopy (TEM), Fourier infrared spectroscopy (FT-IR), ζ potential analysis, and fluorescence spectroscopy. It was found that the material has a hexagonal structure with good hydrophilicity and emits intense fluorescence under 980 nm pump laser excitation. The non-contact temperature sensing performance of nanoparticles was evaluated by analyzing the upconversion fluorescence of Tm3+ (1G4 → 3F4 and 3F3 → 3H6) in the temperature range of 284−344 K. The absolute and relative sensitivities were found to be 0.0067 K−1 and 1.08 % K−1, respectively, with high-temperature measurement reliability and good temperature cycling performance. More importantly, its temperature measurement in phosphate-buffered saline (PBS) solution is accurate. In addition, the temperature of the cells can be increased by adjusting the laser power density and laser irradiation time. Therefore, an optical temperature sensing platform was built to realize the application of real-time monitoring of cancer cell temperature and the dual function of photothermal therapy.
Chromium(III) complexes can show phosphorescence from the spin-flip excited doublet states 2E/2T1 in the near-infrared with high photoluminescence quantum yields and extremely long lifetimes in the absence of dioxygen. The prototype molecular ruby, [Cr(ddpd)2]3+ (ddpd = N,N’-dimethyl-N,N’-dipyridine-2-ylpyridine-2,6-diamine), has a photoluminescence quantum yield and a luminescence lifetime of 13.7% and 1.1 ms in deaerated acetonitrile, respectively. However, its luminescence is strongly quenched by 3O2 via an efficient Dexter-type energy transfer process. To enable luminescence applications of molecular rubies in solution under aerobic conditions, we explored the potential of sterically demanding ddpd ligands to shield the chromium(III) center from O2 using steady state and time-resolved photoluminescence spectroscopy. The structures of the novel complexes with sterically demanding ligands were investigated by single crystal X-ray diffraction and quantum chemically by density functional theory calculations. The O2 sensitivity of the photoluminescence was derived from absolutely measured photoluminescence quantum yields and excited state lifetimes under inert and aerobic conditions and by Stern–Volmer analyses of these data. Optimal sterically shielded chromium(III) complexes revealed photoluminescence quantum yields of up to 5.1% and excited state lifetimes of 518 μs in air-saturated acetonitrile, underlining the large potential of this ligand design approach to broaden the applicability of highly emissive chromium(III) complexes.
pH and oxygen are amongst the most important and frequently measured analytes in the life and material sciences, indicating, e.g., diseases and corrosion processes. This includes the optical monitoring of pH in living cells for studying cellular internalization pathways, such as phagocytosis, endocytosis, and receptor ligand internalization with the aid of molecular and nanoscale fluorescent sensors. Nanoparticle (NP)-based sensors, that are labeled or stained with a multitude of sensor dyes, have several advantages as compare to conventional molecular probes like enhanced brightness, i.e., amplified signals, ease of designing ratiometric systems by combining analyte sensitive and inert reference dyes, and increased photostability. Moreover, this can enable the use of hydrophobic dyes in aqueous environments. Versatile templates and carriers for the fabrication of nanosensors by the staining and/or labelling with different fluorophores and sensor molecules or surface functionalized NP like silica (SiO2-NP) and polystyrene (PS-NP) particles provide. Here we present the design of a versatile platform of color emissive nanosensors and stimuli-responsive microparticles for the measurement of pH, oxygen, and other targets utilizing both types of matrices and sets of spectrally distinguishable sensor and reference dyes and their characterization and demonstrate the applicability of representative sensor particle for cellular studies.
Bioanalytical, diagnostic, and security applications require the fast and sensitive determination of a steadily increasing number of analytes or events in parallel in a broad variety of detection formats and increased sensitivities. This – flanked by recent technical advancements and the availability of simple to use, commercial time-resolved photoluminescence measuring devices at reasonable costs - calls for the exploitation of the species- and environment-specific photoluminescence parameter luminescence lifetime. In this context, time-resolved photoluminescence measurements of different classes of molecular and nanocrystalline emitter and luminescent particles in different time windows are presented and examples for applications such as lifetime multiplexing and barcoding in conjunction with fluorescence lifetime imaging microscopy (FLIM) and flow cytometry are given.
The surface chemistry of nanomaterials controls their interaction with the environment and biological species and their fate and is hence also relevant for their potential toxicity. This has meanwhile led to an increasing interest in validated and preferably standardized methods for the determination and quantification of surface functionalities on nanomaterials and initiated different standardization projects within ISO/TC 229 and IEC/TC 113 as well as interlaboratory comparisons (ILCs) of different analytical methods for the quantification of surface coatings by OECD. Here we present the results of a first ILC on the quantification of the amount of amino functionalities on differently sized inorganic nanoparticles done by division Biophotonics and the National Research Council of Canada (NRC) and the PWI 19257 on the Characterization and Quantification of Surface Functional Groups and Coatings on Nanoobjects approved by ISO/TC 229 (WG2) in fall 2022 that will result in a VAMAS study on this topic organized by division Biophotonics. Key words: nanoparticles, surface analysis, surface functional groups, quantification, optical assay, qNMR, VAMAS, standardization, ICL, quality assurance, reference material.
This paper discusses the feasibility of a novel strategy based on the combination of bioprinting nano-doping technology and laser ablation-inductively coupled plasma time-of-flight mass spectrometry analysis for the preparation and characterization of gelatin- based multi-element calibration standards suitable for quantitative imaging. To achieve this, lanthanide up-conversion nanoparticles were added to a gelatin matrix to produce the bioprinted calibration standards. The features of this bioprinting approach were com- pared with manual cryosectioning standard preparation, in terms of throughput, between batch repeatability and elemental signal homogeneity at 5 μm spatial resolution. By using bioprinting, the between batch variability for three independent standards of the same concentration of 89 Y (range 0–600 mg/kg) was reduced to 5% compared to up to 27% for cryosectioning. On this basis, the relative standard deviation ( RSD ) obtained between three independent calibration slopes measured within 1 day also reduced from 16% (using cryosectioning ) to 5% (using bioprinting), supporting the use of a single standard preparation replicate for each of the concentrations to achieve good calibration performance using bioprinting. This helped reduce the analysis time by approximately 3-fold. With cryosectioning each standard was prepared and sectioned individually, whereas using bio-printing it was possible to have up to six different standards printed simultaneously, reducing the preparation time from approximately 2 h to under 20 min (by approxi- mately 6-fold). The bio-printed calibration standards were found stable for a period of 2 months when stored at ambient temperature and in the dark.
Inorganic nanocrystals with linear and nonlinear luminescence in the ultraviolet, visible, near infrared and short-wave infrared like semiconductor quantum dots and spectrally shifting lanthanide-based nanophosphors have meanwhile found applications in the life and material sciences ranging from optical reporters for bioimaging and sensing over security barcodes to solid state lighting and photovoltaics. These nanomaterials commonly have increasingly sophisticated core/shell particle architectures with shells of different chemical composition and thickness to minimize radiationless deactivation at the particle surface that is usually the main energy loss mechanism [1]. For lanthanide-based spectral shifters, particularly for very small nanoparticles, also surface coatings are needed which protect near-surface lanthanide ions from luminescence quenching by high energy vibrators like O-H groups and prevent the disintegration of these nanoparticles under high dilution conditions. [2,3,4]. The identification of optimum particle structures requires quantitative spectroscopic studies focusing on the key performance parameter photoluminescence quantum yield [5,6], ideally flanked by single particle studies to assess spectroscopic inhomogeneities on a particle-to-particle level for typical preparation methods [7,8], Moreover, in the case of upconversion nanoparticles with a multi-photonic and hence, excitation power density (P)-dependent luminescence, quantitative luminescence studies over a broad P range are required to identify particle architectures that are best suited for applications in fluorescence assays up to fluorescence microscopy. Here, we present methods to quantify the photoluminescence of these different types of emitters in the vis/NIR/SWIR and as function of P and demonstrate the importance of such measurements for a profound mechanistic understanding of the nonradiative deactivation pathways in semiconductor and upconversion nanocrystals of different size and particle architecture in different environments.
A solvothermal method was used to prepare a series of Yb3+/Tm3+/Ca2+ co-doped NaYF4 nanoparticles with different Ca2+ contents. Strong upconversion blue fluorescence could be observed under 980 nm laser excitation of the samples. The effect of different Ca2+ contents on the luminescence intensity was investigated, and it was found that the UV-vis upconversion luminescence increased and then decreased with an increasing Ca2+ concentration during the increase of the Ca2+ content from 0 mol% to 25 mol%, reaching the strongest fluorescence at 15 mol%, which was up to about 28 times stronger than that without Ca2+ doping. Furthermore, the mechanism was investigated, and it was found that the doping of Ca2+ disrupted the symmetry of the crystal field, resulting in a significant enhancement of the overall fluorescence. Applied to fluorescence intensity ratio thermometry, the absolute and relative sensitivities are as high as 0.0418 K−1 and 2.31% K−1, respectively, with a minimum temperature resolution of 0.0129 K.
Ratiometric green–red fluorescent nanosensors for fluorometrically monitoring pH in the acidic range were designed from 80 nm-sized polystyrene (PS) and silica (SiO2) nanoparticles (NPs), red emissive reference dyes, and a green emissive naphthalimide pH probe, analytically and spectroscopically characterized, and compared regarding their sensing performance in aqueous dispersion and in cellular uptake studies. Preparation of these optical probes, which are excitable by 405 nm laser or LED light sources, involved the encapsulation of the pH-inert red-fuorescent dye Nile Red (NR) in the core of self-made carboxylated PSNPs by a simple swelling procedure and the fabrication of rhodamine B (RhB)-stained SiO2-NPs from a silane derivative of pH-insensitive RhB. Subsequently, the custom-made naphthalimide pH probe, that utilizes a protonation-controlled photoinduced electron transfer process, was covalently attached to the carboxylic acid groups at the surface of both types of NPs. Fluorescence microscopy studies with the molecular and nanoscale optical probes and A549 lung cancer cells confirmed the cellular uptake of all probes and their penetration into acidic cell compartments, i.e., the lysosomes, indicated by the switching ON of the green naphthalimide fluorescence. This underlines their suitability for intracellular pH sensing, with the SiO2-based nanosensor revealing the best performance regarding uptake speed and stability.