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The commercial availability of stand-alone setups for the determination of absolute photoluminescence quantum yields (φf) in conjunction with the increasing use of integrating sphere accessories for spectrofluorometers is expected to have a considerable influence not only on the characterization of chromophore systems for use in optical and opto-electronic devices, but also on the determination of this key parameter for (bio)analytically relevant dyes and functional luminophores. Despite the huge potential of systems measuring absolute φf values and the renewed interest in dependable data, evaluated protocols for even the most elementary case, the determination of the fluorescence quantum yield of transparent dilute solutions of small organic dyes with integrating sphere methods, are still missing. This encouraged us to evaluate the performance and sources of uncertainty of a simple commercial integrating sphere setup with dilute solutions of two of the best characterized fluorescence quantum yield standards, quinine sulfate dihydrate and rhodamine 101, strongly differing in spectral overlap between absorption and emission. Special attention is dedicated to illustrate common pitfalls of this approach, thereby deriving simple procedures to minimize measurement uncertainties and improve the comparability of data for the broad community of users of fluorescence techniques.
The photoluminescence quantum yield (Φf) that presents a direct measure for the efficiency of the conversion of absorbed photons into emitted photons is one of the spectroscopic key parameters of functional fluorophores. It determines the suitability of such materials for applications in, for example, (bio)analysis, biosensing, and fluorescence imaging as well as as active components in optical devices. The reborn interest in accurate Φf measurements in conjunction with the controversial reliability of reported Φf values of many common organic dyes encouraged us to compare two relative and one absolute fluorometric method for the determination of the fluorescence quantum yields of quinine sulfate dihydrate, coumarin 153, fluorescein, rhodamine 6G, and rhodamine 101. The relative methods include the use of a chain of Φf transfer standards consisting of several 'standard dye' versus 'reference dye' pairs linked to a golden Φf standard that covers the ultraviolet and visible spectral region, and the use of different excitation wavelengths for standard and sample, respectively. Based upon these measurements and the calibration of the instruments employed, complete uncertainty budgets for the resulting Φf values are derived for each method, thereby providing evaluated standard operation procedures for Φf measurements and, simultaneously, a set of assessed Φf standards.
There is an increasing interest in chromophores absorbing and emitting in the near-infrared (NIR) spectral region, e.g., for applications as fluorescent reporters for optical imaging techniques and hence, in reliable methods for the characterization of their signal-relevant properties like the fluorescence quantum yield (Φf) and brightness. The lack of well established Φf standards for the NIR region in conjunction with the need for accurate Φf measurements in transparent and scattering media encouraged us to built up an integrating sphere setup for spectrally resolved measurements of absolute fluorescence traceable to radiometric scales. Here, we present the design of this setup and its characterization and validation including an uncertainty budget for the determination of absolute Φf in the visible and NIR. To provide the basis for better measurements of Φf in the spectral window from ca. 600 to 1000 nm used, e.g., for optical imaging, the absolute Φf of a set of NIR chromophores covering this spectral region are measured and compared to relative values obtained using rhodamine 101 as Φf standard. Additionally, the absolute Φf values of some red dyes that are among the most commonly used labels in the life sciences are presented as well as the absolute quantum yield of an optical probe for tumor imaging.
To establish the methodical basis for the development and certification of fluorescence quantum yield standards, we determined the fluorescence quantum yield Φf of rhodamine 6G (R6G) with two absolute methods with complementary measurement principles, here optical spectroscopy using an integrating sphere setup and pulsed laser photoacoustic spectroscopy (PAS). For the assessment of aggregation- and reabsorption-induced distortions of measured fluorescence quantum yields and procedures for the reliable consideration of such effects, this systematic comparison was performed in ethanol and in water employing different concentrations of R6G. In addition, the relative and absolute fluorescence quantum yields of these solutions were obtained with a calibrated spectrofluorometer and a commercialized integrating sphere setup. Based upon this systematic comparison, experimental advantages and systematic sources of variation were identified for both methods.
Critical review of the determination of photoluminescence quantum yields of luminescent reporters
(2015)
A crucial variable for methodical performance evaluation and comparison of luminescent reporters is the photoluminescence quantum yield (Φ pl). This quantity, defined as the number of emitted photons per number of absorbed photons, is the direct measure of the efficiency of the conversion of absorbed photons into emitted light for small organic dyes, fluorescent proteins, metal–ligand complexes, metal clusters, polymeric nanoparticles, and semiconductor and up-conversion nanocrystals. Φ pl determines the sensitivity for the detection of a specific analyte from the chromophore perspective, together with its molar-absorption coefficient at the excitation wavelength. In this review we discuss different optical and photothermal methods for measuring Φ pl of transparent and scattering systems for the most common classes of luminescent reporters, and critically evaluate their potential and limitations. In addition, reporter-specific effects and sources of uncertainty are addressed. The ultimate objective is to provide users of fluorescence techniques with validated tools for the determination of Φ pl, including a series of Φ pl standards for the ultraviolet, visible, and near-infrared regions, and to enable better judgment of the reliability of literature data.
In order to assess the anisotropy-related uncertainties of relatively determined photoluminescence quantum yields (ΦPL) of molecular emitters and luminescent nanomaterials, we compared ΦPL values measured without and with polarizers using magic angle conditions and studied systematically the dependence of the detected emission intensity on the polarizer settings for samples of varying anisotropy. This includes a dispersion of a spherical quantum dot (QD) with an ideally isotropic emission, a solution of a common small organic dye in a fluid solvent as well as dispersions of elongated quantum dot rods (QDR) with an anisotropic luminescence and a small organic dye in a rigid polymeric matrix, as ideally anisotropic emitter. Our results show that for instruments lacking polarizers, anisotropy-related measurement uncertainties of relative photoluminescence quantum yields can amount to more than 40%, with the size of these systematic errors depending on the difference in emission anisotropy between the sample and the standard.
The ever-increasing use of fluorescent nanomaterials and micrometer-sized beads in the life and material sciences requires reliable procedures for the measurement of the key performance parameter fluorescence quantum yield (Ff) of scattering particle dispersions and reference systems to evaluate the performance of such measurements. This encouraged us to systematically study, both theoretically and experimentally, the optical determination of photoluminescent quantum yield as a function of the scattering and absorption properties of the sample and the illumination geometry with an integrating sphere method. The latter included measurements with a direct and an indirect illumination. As a representative and easy-to-prepare reference system, we used ethanolic dispersions of 250 nm sized silica particles and the dye rhodamine 101 and systematically varied the concentration of the dye and particles within the typical ranges of spectroscopic and (bio)analytical applications of fluorescent nanomaterials. Based on our measurements, we recommend indirect sample illumination geometry for the accurate measurement of Ff of samples with low or unknown absorption and high scattering coefficients such as dispersions of luminescent particles or fluorescent reporters in biological matrices. This finding is of utmost relevance for all (bio)analytical applications of fluorescent nanomaterials ranging from particle labels and probes over assay platforms to safety barcodes.
Yb,Nd,Er-doped upconversion nanoparticles (UCNPs) have attracted considerable interest as luminescent reporters for bioimaging, sensing, energy conversion/shaping, and anticounterfeiting due to their capability to convert multiple near-infrared (NIR) photons into shorter wavelength ultraviolet, visible or NIR luminescence by successive absorption of two or more NIR photons. This enables optical measurements in complex media with very little background and high penetration depths for bioimaging. The use of Nd3+ as substitute for the commonly employed sensitizer Yb3+ or in combination with Yb3+ shifts the excitation wavelength from about 980 nm, where the absorption of water can weaken upconversion luminescence, to about 800 nm, and laser-induced local overheating effects in cells, tissue, and live animal studies can be minimized. To systematically investigate the potential of Nd3+ doping, we assessed the performance of a set of similarly sized Yb3+,Nd3+,Er3+-doped core- and core–shell UCNPs of different particle architecture in water at broadly varied excitation power densities (P) with steady state and time-resolved fluorometry for excitation at 980 nm and 808 nm. As a measure for UCNPs performance, the P-dependent upconversion quantum yield (Φ) and its saturation behavior were used as well as particle brightness (B). Based upon spectroscopic measurements at both excitation wavelengths in water and in a lipid phantom and B-based calculations of signal size at different penetration depths, conditions under which excitation at 808 nm is advantageous are derived and parameters for the further optimization of triple-doped UCNPs are given.
We present a series of results that demonstrate that the broadband photoluminescence (PL) of aqueous glutathione-capped Ag−In−S (AIS) nanocrystals (NCs) is an inherent property of each NC, rather than a collective characteristic of an NC ensemble. By analyzing parameters affecting the PL features such as the postsynthesis annealing and the deposition of a passivating ZnS shell, we found no correlation between the spectral width of the PL band of AIS (AIS/ZnS) NCs and the density of the lattice defects. Analysis of the PL spectra of a series of size-selected AIS/ZnS NCs revealed that the PL width of fractionated NCs does not depend on the NC size and size distribution. The PL measurements in a broad temperature window from 320 to 10 K demonstrated that the PL width does not decrease with decreasing temperature as expected for an emission arising from thermally activated detrapping processes. Also, we show that the model of the self-trapped exciton can be versatilely applied to reconstruct the PL spectra of different AIS NCs and can account for the effects typically attributed to variations in defect state energy. Measurements of the PL properties of single AIS/ZnS NCs highlighted the broadband nature of the emission of individual NCs. The presented results show that the broadband PL of ternary NCs most probably does not originate from lattice defects but involves the NC lattice as a whole, and, therefore, by tailoring the NC structure, PL efficiencies as high as those reported for binary cadmium or lead chalcogenide NCs can be potentially reached.
Wepresent here a systematic analysis of the influence of Tm3+ Doping concentrations (xTm) on the excitation power (P)-dependent upconversion luminescence and -performance of hexagonal-Phase NaYF4: 20% Yb3+, xTm%Tm3+ upconversion nanoparticles (UCNPs) for xTm of 0.2, 0.5, 0.8, 1.2, and 2.0, respectively. Our results reveal the influence of these differentTm3+ doping concentrations with respect to optimized upconversion quantum yield (ΦUC) values of the variousTm3+ upconversion emission bands, with the highestΦUC values of theTm3+ emission bands above 700 nmresulting for different xTm values as theTm3+ emission bands below 700 nm. This underlines the potential ofTm3+ dopant concentration for colour tuning. Special emphasis was dedicated to the spectroscopic parameters that can be linked to the (de)population pathways of the variousTm3+ energy levels, like the P- and xTm-dependent slope factors and the intensity ratios of selected emission bands. The evaluation of all parameters indicates that not only energy transfer upconversion-, but also crossrelaxation processes between neighbouringTm3+ ions play a vital role in the (de)population of the excited energy levels of Yb3+, Tm3+ codoped nanocrystals.