FG Anorganische Chemie
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Structural and Magnetic Transitions Caused by Dimer Formation in the CrCl3–MoCl3 Solid Solution
(2024)
In this work, we apply the concept of solid solutions to the two 2D transition metal trihalides CrCl3 and MoCl3. While CrCl3 belongs to the magnetically active CrX3 family, the magnetism in MoCl3 is intrinsically suppressed by the formation of aligned Mo–Mo
dimers, which also distort the regular honeycomb lattice that is typical for the 2D transition metal trihalides. We report suitable synthesis conditions for the gapless solid solution and crystal growth by chemical vapor transport. The CrCl3–MoCl3 solid solution was initially synthesized at 650 °C for 100 h under addition of MoCl5 as mineralizer, and bulk crystals were subsequently grown by vapor transport in a temperature gradient from
600 °C → 550 °C for 60 h. The obtained solid solution exhibits multiple composition-dependent phase transitions at room temperature, as confirmed by powder X-ray diffraction measurements. The possible presence of Mo–Mo dimers in the solid solution
was further investigated by infrared, Raman, electron energy loss, and nuclear quadrupole resonance spectroscopies. The combined results indicate that the Mo–Mo dimers are present over a wide range of compositions. Their orientation changes from parallel alignment for compositions from 70% ≤ c(Mo3+) ≤ 100% to random orientation for samples with c(Mo3+) < 70% content, which significantly affects the course of lattice parameters. Finally, the magnetic properties of the powder samples show a correlation between the Mo3+ content and the transition temperature into the low-temperature phase.
Nanoscale phase separation was induced in the K-doped RuCl3 van der Waals material by annealing, and studied with the goal to find a natural design strategy for the formation of two-dimensional architectures as an alternative to the costly and time-consuming experimental artificial growth methods. Phase conversion was traced by means of thermogravimetric analysis combined with mass spectrometry. The local crystal structure of co-existing K3Ru2Cl9 domains with the sizes of about 100 nm was solved by 3D electron diffraction.
The two-dimensional (2D) chromium trihalides CrCl3, CrBr3, and CrI3 are most famous for their exotic magnetic properties when their crystals get downscaled to nanometer dimensions. One way to tune the properties of such materials and to further increase their potential utility, e.g., in spintronics, is the formation of solid solutions. Here, we present a detailed theoretical and experimental study on the synthesis and nanoplatelet growth of CrBrxCl3–x and CrBrxI3–x solid solutions. Phase pure powder was obtained by tempering the respective parent compounds at 600 and 550 °C, using additional iodine in the case of CrBrxI3–x. The dominant stacking order changes at x = 1.8 for CrBrxCl3–x while remaining similar to CrI3 for the whole CrBrxI3–x series. A consecutive chemical vapor transport from 575 → 525 °C for 2 h (CrBrxCl3–x) or 600 → 550 °C for 3 h with 0.1 mmol additional iodine (CrBrxI3–x) resulted in the deposition of high-quality nanoplatelets on a substrate with only several nanometers in height. The composition can be controlled by the choice of the starting material, since only small shifts in the anion ratio occur during the transport. The deposited CrBrxCl3–x nanoplatelets can then be easily delaminated by ultrasonication in ethanol to reduce the height even further to few-layer dimensions. These nanoplatelets could potentially be used to investigate the property changes (e.g., in terms of magnetic response) for the downscaling of these solid solutions. We further demonstrate the quality of the deposited material by transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy. Raman spectroscopy of the solid solution series reveals a complex evolution of vibrational modes. Photoluminescence measurements on solid solution samples show emission peaks in the near-infrared energy range with the specific energy and intensity being composition and temperature dependent.
Integrating enzymes into thermoplastic polymers is challenging due to their lack of robustness with respect to temperature and shear fields during conventional melt processing. In the present study, blown films from low-density polyethylene (LDPE) were prepared containing a technical protease from Bacillus sp. First, LDPE/protease compounds were produced followed by blown film extrusion, both processes at melt mass temperatures of 130 °C or higher. Enzyme activity was proven, both for the LDPE/protease compound and the blown film. The highest enzyme activity in the compound was determined for processing at 132 °C and a screw speed of 75 rpm. The influence of melt temperature and shear fields was studied in detail. Enzyme activities were determined for melt temperatures up to 160 °C and for screw speeds ranging from 75 to 300 rpm during compounding by twin-screw extrusion. The process was also applied for biobased and biodegradable polyesters, where similar protease activity after compounding was verified. Electron microscopy, X-ray diffraction, nuclear magnetic resonance spectroscopy and differential scanning calorimetry served to analyze components and morphology of the enzyme formulation used here. It is proposed that the porous morphology of the protease particles is beneficial for the enzyme to remain active after processing. Additionally, the polymer matrix surrounding the particles protects the protease at elevated temperatures, which can be attributed to thermal insulation. Thus, the right combination of a suited technical enzyme formulation with appropriate mild melt compounding conditions allows enzymes to be incorporated into thermoplastics and retain their activity. This opens the way to use the abundant biological functions of enzymes in thermoplastic applications.
In the search for novel 2D materials with potentially valuable properties, such as a tunable band gap for optoelectronic or catalytic applications, solid solutions hold the potential to significantly expand the inventory of available 2D nanomaterials. In this study, we present for the first time the synthesis of such 2D rhodium trihalide solid solutions: RhBrxCl3−x and RhBrxI3−x. We use thermodynamic simulations and simultaneous thermal analysis to predict conditions for their rational synthesis and to investigate suitable chemical vapor transport (CVT) parameters for these solid solutions. The evolution of the lattice parameters was investigated by powder x-ray diffraction, showing an isostructural relationship of the synthesized compounds and only minor deviation from Vegard's law. The optical band gap of these materials can be tuned in an energy range from 1.5 eV (RhCl3) to 1.2 eV (RhI3) by choosing the composition of the solid solution, while the samples also exhibit photoluminescence in similar energy ranges. Ultimately, the successful deposition of bulk as well as ultrathin 2D nanocrystals of RhBrxCl3−x by CVT from 925 °C to 850 °C is shown, where the composition of the deposited crystals is precisely controlled by the choice of the starting composition and the initial amount of material. The high quality of the obtained nanocrystals is confirmed by atomic force microscopy, high resolution transmission electron microscopy and selected area electron diffraction. For RhBrxI3−x, the CVT from 900 °C to 825 °C is more difficult and has only been practically demonstrated for an exemplary case. According to the observed properties, these novel solid solutions and nanocrystals show a great potential for an application in optoelectronic devices.
DUT-8(Ni) metal-organic framework belongs to the family of flexible pillared layer materials. The desolvated framework can be obtained in the open pore form (op) or in the closed pore form (cp), depending on the crystal size regime. In the present work, we report on the behaviour of desolvated DUT-8(Ni) at elevated temperatures. For both, op and cp variants, heating causes a structural transition, leading to an new, crystalline compound, containing two interpenetrated networks. The state of the framework before transition (op vs. cp) influences the transition temperature: the small particles of the op phase transform at significantly lower temperature in comparison to the macroparticles of the cp phase, transforming close to the decomposition temperature. The new compound, confined closed pore phase (ccp), was characterized by powder X-ray diffraction and spectroscopic techniques, such as IR, EXAFS, and positron annihilation lifetime spectroscopy (PALS). Thermal effects of structural cp to ccp transitions were studied using differential scanning calorimetry (DSC), showing an overall exothermic effect of the process, involving bond breaking and reformation. Theoretical calculations reveal the energetics, driving the observed temperature induced phase transition.
Synthesis of micro- and nanosheets of CrCl3–RuCl3 solid solution by chemical vapour transport
(2022)
Solid solutions of 2D transition metal trihalides are rapidly growing in interest for the search for new 2D materials with novel properties at nanoscale dimensions. In this regard, we present a synthesis method for the Cr1−xRuxCl3 solid solution and describe the behaviour of the unit cell parameters over the whole composition range, which in general follows Vegard's law in the range of a = 5.958(6)CrCl3 … 5.9731(5)RuCl3 Å, b = 10.3328(20)CrCl3 … 10.34606(21)RuCl3 Å, c = 6.110(5)CrCl3 … 6.0385(5)RuCl3 Å and β = 108.522(15)CrCl3 … 108.8314(14)RuCl3 °. The synthesized solid solution powder was subsequently used to deposit micro- and nanosheets directly on a substrate by applying chemical vapour transport in a temperature gradient of 575 °C → 525 °C for 2 h and 650 °C → 600 °C for 0.5 h as a bottom-up approach without the need for an external transport agent. The observed chromium chloride enrichment of the deposited crystals is predicted by thermodynamic simulation. The results allow for a nanostructure synthesis of this solid solution with a predictable composition down to about 30 nm in height and lateral size of several μm. When applying a quick consecutive delamination step, it is possible to obtain few- and monolayer structures, which could be used for further studies of downscaling effects for the CrCl3–RuCl3 solid solution. X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy were used to confirm the purity and quality of the synthesized crystals.
Direct Deposition of (BixSb1–x)2Te3 Nanosheets on Si/SiO2 Substrates by Chemical Vapor Transport
(2022)
The tellurides of bismuth and antimony (Bi2Te3 and Sb2Te3) are prominent members of the V2VI3 material family that exhibit promising topological properties. We provide a method for the rational synthesis of mixed crystals of these materials ((BixSb1–x)2Te3 with x = 0.1, ..., 0.9) by means of a bottom-up chemical vapor transport (CVT) approach. Thermodynamic calculations showed the synthesis to be possible in the temperature range of 390–560 °C without significant enrichment of either component and without adding a transport agent. The starting materials were synthesized and verified by X-ray diffraction (XRD). Optimization experiments showed the ideal conditions for nanosheet synthesis to be T2 = 560 °C, T1 = 390 °C with a reaction time of t = 36 h. Crystals with heights of down to 12 nm (12 quintuple layers) were synthesized and analyzed by means of scanning electron microscopy, energy-dispersive X-ray spectrometry, and atomic force microscopy. High-resolution transmission electron microscopy confirmed the R3̅m crystal structure, high crystallinity, and overall quality of the synthesized (BixSb1–x)2Te3 nanosheets. Magnetotransport measurements revealed that such ternary compounds can have a significantly reduced carrier density compared to the binary parent compounds.
Impacts of TGA furnace parameters for prediction of long-term thermal stability of ionic liquids
(2021)
The concept of maximum operation temperature is established for the prediction of the time dependent thermal stability of ionic liquids based on kinetic evaluation of thermogravimetric analysis. The influence of the furnace control parameters on the maximum operation temperature (MOT) is shown using the example of 1-methyl-3-propylimidazolium iodide ([C3C1im]I) with respect to three different parameter sets of a programmed proportional integral derivative (PID) controller of the TGA. Kinetics of thermal decomposition of [C3C1im]I have been investigated with the implementation of an improved kinetic model. The activation energy obtained using the Kissinger-Akahira-Sunose equation showed variations apparently due to the decomposition degree. The model compound is decomposed by a one-step kinetics, which results from pseudo zero order relationship of the activation energy to the conversion rate. The activation energy, pre-exponential factor, and the activation energy are strongly dependent on the parameters of TGA furnace controller.
New racemic and enantiopure N-phosphorylated thioureas bearing 1-phenylethyl or tetrahydronaphthalenyl fragments were synthesized. According to NMR data assisted by DFT calculations, the preferred conformation is stabilized by an intramolecular hydrogen bond. This form in solution is in equilibrium with dimeric N–H···S hydrogen-bonded associates, the population depending on the concentration. In the crystalline phase the low-energy conformation with an intramolecular H-bond is realized only in the racemic tetrahydronaphthalenyl derivative. In most crystals various types of intermolecular hydrogen bonding are observed, accompanied by the formation of infinite linear chains or helical structures. Due to the conformational lability of compounds and competitive intermolecular H-bonding, multiple polymorphic modifications are formed. Therefore, crystallization of enantiopure 1-phenylethyl derivatives from various solvents results in concomitant polymorphs at room temperature. One of them undergoes reversible two-step phase transitions from the high-symmetry I41 space group (Z′ = 1, no disorder) via the P41 space group (Z′ = 6) to the monoclinic P21 space group (Z′ = 16) accompanied by drastic concerted conformational changes. Notably, the optimization of the crystal packing is observed upon phase transitions with a gradual reduction of the void space in the unit cell from 4.5% to 0.8%. This is a rare case of several high-Z′ polymorphs for one compound, with chirality playing an important role.
Ein wichtiges Ziel der Bundesregierung ist der Ausbau des Anteils erneuerbarer Energien an der gesamten Stromerzeugung. Die Entwicklung optimierter Wärme- und Kältespeicher stellt einen wichtigen Baustein bei der Erreichung dieser Zielstellung dar. Elektrisch betriebene Wärmepumpen und Kompressionskältemaschinen sowie dezentrale Klein-BHKW bieten in Verbindung mit entsprechend dimensionierten, thermischen Speichern ein großes Potential zum Lastmanagement in Smart Grids und somit zur Netzintegration von Strom aus erneuerbaren Energien. Die im Projekt entwickelte Methodik zur Suche nach neuen Latentspeichermedien kann für ein breites Anwendungsfeld von Heiz- und Kühlanwendungen in Gebäuden bis hin zur Kraftwerkstechnik eingesetzt werden. Innerhalb der Fördermaßnahme „Anwendungsorientierte Forschung und Entwicklung zur nichtnuklearen Energieforschung“ im 6. Energieforschungsprogramm der Bundesregierung wurde durch die Projektarbeiten das Schwerpunktthema Energiespeicher (3.8) adressiert, wobei der Aspekt der Entwicklung neuer Materialien mit reduzierten Kosten für einen wirtschaftlichen Betrieb im Vordergrund stand. Speziell wurden dabei die Unterpunkte 3.8.4 Thermische Speicher zur Weiterentwicklung, Optimierung und Erprobung von PCM sowie 3.8.5 Übergeordnete Themen mit Modellen und Simulationswerkzeugen für die Auswahl und Bewertung neuer Speichermaterialien adressiert.
Die Projektergebnisse besitzen aber auch Berührungspunkte mit den Schwerpunkten 3.12 Energieeffizienz in Industrie, Gewerbe, Handel und Dienstleistungen sowie 3.4 Solarthermische Kraftwerke, da mit der entwickelten Screening-Methode für diese Anwendungsfälle neue Wärmespeichermaterialien auf der Basis wasserfreier Salze mit höheren Prozesstemperaturen gefunden werden können.
Durch den Einsatz von Methoden zur Modellierung und Simulation von Prozessen erfolgt gleichermaßen die Umsetzung einer energie- und ressourcenschonenden Forschungstätigkeit. Das im Projekt entwickelte Vorgehen hilft dabei, einen äußerst sparsamen Einsatz von Chemikalien, eine niedrige Belastung der Umwelt durch verringerten Entsorgungsaufwand der verbrauchten Stoffe sowie einen deutlich geringeren Aufwand und Energieeinsatz für die Messtechnik zu ermöglichen. Damit wurde zugleich das Schwerpunktthema 3.8.5 Modelle und Simulationswerkzeuge für die Auswahl und Bewertung neuer Speichermaterialien angesprochen.
Temperature-induced change in reactivity of the frequently used ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([C4C1im]BF4) is presented as a prerequisite for the rational screening of reaction courses in material synthesis. [C4C1im]BF4 becomes active with oxidic precursor compounds in reduction reaction at ϑ > 200°C, even without the addition of an external reducing agent. The reaction mechanism of forming red amorphous selenium from SeO2 is investigated as a model system and can be described similarly to the Riley oxidation. The reactive species but-1-ene, which is formed during the decomposition of [C4C1im]BF4, reacts with SeO2 and form but-3-en-2-one, water, and selenium. Elucidation of the mechanism was achieved by thermoanalytical investigations. The mono-tropic phase transition of selenium was analyzed by the differential scanning calorimetry. Beyond, the suitability of the single source oxide precursor Bi2Se3O9 for the synthesis of Bi2Se3 particles was confirmed. Identification, characterization of formed solids succeeded by using light microscopy, XRD, SEM, and EDX.
Black and irregularly shaped crystals of the bismuth‐rich bromide Bi5Br4 were obtained as a by‐product of the reaction of CsBr, Bi, and BiBr3. X‐ray diffraction on a single‐crystal revealed its orthorhombic structure with the space group Pmmn (no. 59) and lattice parameters a = 1800.0(2) pm, b = 1476.1(1) pm, and c = 924.5(2) pm at 296 K. The structure is composed of Bi82+ and Bi95+ polycations and bromidobismuthate(III) anions according to the structured formula Bi5Br4 = Bi20Br16 = Bi82+Bi95+[BiBr5]2–[Bi2Br11]5–. Bi5Br4 is the bismuth‐richest among the bismuth subhalides containing isolated polycations. Extensive differential scanning calorimetry studies indicate that Bi5Br4 decomposes at 262 °C, i.e. one degree below the bismuth‐rich eutectic at 263 °C. All attempts towards a rational synthesis yielded predominantly the neighboring phases BiBr and Bi6Br7.
The Ionic Liquid 1-butyl-3-methylimidazolium tetrafluoroborate [C4C1im]BF4 serves as a commonly solvent in inorganic material synthesis and analytics. Nevertheless, its application is frequently associated with trial and error approaches. Thereupon, detailed knowledge on the thermal behavior is the key information for understanding the reactivity of [C4C1im]BF4. 1-butyl-3-methylimidazolium tetrafluoroborate behaves as a glass in the cold, its glass transition temperature being ϑg = −83 °C. During heating with 10 K·min−1 [C4C1im]BF4 appears to be stable above 350 °C with onset temperatures ϑonset, DSC = 375 °C, ϑonset, DTG = 422 °C, and ϑonset, TG = 437 °C. Thereby, thermal decomposition occurs in a single step reaction forming 1-methyl-1H-imidazole (CH3C3H3N2 or C4H6N2), but-1-ene (C4H8), fluoromethane (CH3F) and boron trifluoride (BF3) as main species, as determined by thermogravimetry coupled with mass spectrometry and FTIR spectroscopy. To be more specific in thermal behavior, the temperature and time dependent stability is evaluated here on the basis of the kinetic model of maximum operation temperature − MOT. Clearly, thermal stability rises with application time, thus being 193 °C for one hour, while reaching only 141 °C for one day, and 114 °C for one week. The incipient decomposition (≤ 1 %) at the calculated time dependent maximum operation temperature finally is verified by optical analysis, infrared (IR), and nuclear magnetic resonance (NMR) spectroscopy.
Three-dimensional architectures constructed via coordination of organic ligands to metal ions (broadly termed metal–organic frameworks, MOFs), are highly interesting for many demanding applications such as gas adsorption, molecular separation, heterogeneous catalysis, molecular sensing, etc. Being constructed from heterogeneous components, such framework solids show characteristic features from both the individual components and framework-specific features. One such interesting physicochemical property is thermal expansion, which arises from thermal vibration from the organic linker and metal ions. Herein, we show a very unique example of thermal responsiveness for the DUT-49 framework, a MOF well-known for its distinctive negative gas adsorption (NGA) properties. In the guest-free form, the framework shows another counter-intuitive phenomenon of negative thermal expansion (NTE), i.e. the lattice size increases with decrease of temperature. However, in the solvated state, it shows both NTE and positive thermal expansion (i.e. lattice size decreases with lowering of temperature, PTE) based on a specific temperature range. When the solvent exists in the liquid form inside the MOF pore, it retains the pristine NTE nature of the bare framework. But freezing of the solvent inside the pores induces the strain, which causes a structural transformation through in-plane bending of the linker and this squeezes the framework by ∼10% of the unit cell volume. This effect has been verified using 3 different solvents where the structural contraction occurs immediately at the freezing point of the individual solvent. Furthermore, studies on a series of DUT-49(M) frameworks with varying metals confirm the general applicability of this mechanism.
Despite chemical vapor transport (CVT) being a widely used method for crystal growth of inorganic substances, detailed mechanistic studies on the course of the crystallization process are rather few. In this study, an elaborated experimental screening run combined with sophisticated modeling of the respective heterogeneous equilibria is presented: Crystal growth of germanium by vapor transport with the addition of iodine has been chosen as a model system for validation of the applied method spectrum. In order to record the course and the interplay of heterogeneous equilibrium and non-equilibrium reactions in the system Ge–I, the experimental setup of high-temperature gas-balance (HTGB) is applied. Additionally, the observed evaporation processes are compared with saturation curves of corresponding volatile substances and, thus, can be assigned to individual species within the system. In this experimental screening, a phase sequence means to examine how the condensed phases undergo iodine depletion and how the gaseous phase undergoes a germanium enrichment when the temperature is increased. This phase screening combined with annealing experiments in the course of the phase sequence helps to analyze stepwise non-equilibrium products and to identify the characteristic species. Subsequently, for the evaluation of the composition of the gaseous phase, and for the deduction of the vapor transport mechanism, thermodynamic modeling by the CalPhaD method is performed. For the reference system, it is confirmed that iodine does not act as the transport agent. Instead, GeI4 is responsible for the volatilization of germanium, forming GeI2. Nevertheless, investigations clearly illustrate how GeI4 forms naturally in the phase sequence in the system Ge–I, which makes direct addition of it unnecessary. The recommended temperature range for vapor transport of germanium spans from 460 to 800 °C. Modeling shows that migration rates for germanium reaches a maximum at a mean temperature between 540 and 550 °C. Finally, vapor transport experiments were performed from 565 to 515 °C and from 690 to 590 °C. By increasing the deposition temperature, a slight decrease of the migration rate was observed, though a positive impact on the crystal’s morphology was also found.
Germanium (Ge) nanowires (NWs) were grown in-plane on nano-structured Si(001)/SiO2
substrates by molecular beam epitaxy using gold (Au) as the solvent. The site-selective NW growth was enabled by a rectangular array of gold droplets on silicon (Si) tips with an Au nuclei density
below 0.25 μm−2 on the surrounding silicon oxide (SiO2). The initial growth of Ge NWs starting from Si–Au droplets with SixGe1−x nucleation from ternary alloy is discussed from a thermodynamic point of view. The in-plane NW elongation occurred within ⟨110⟩ directions on the substrate and NWs were mainly bounded by two 55◦ inclined 111 facets and a less pronounced planar (001) top facet. Fully relaxed crystal lattices of Ge NWs were observed from two-dimensional reciprocal space maps of x-ray diffraction measurements.
Tungsten telluride WTe2 is the sole candidate of a group of two-dimensional layered transition metal dichalcogenides (TMDCs) MX2 with a thermodynamically stable 1T′-structure at room temperature. The binary system W/Te was audited with respect to a rational approach of planning and realization of a bottom-up synthesis of WTe2 nanostructures. Thus, the parameters of the synthesis via chemical vapor transports (CVT) were derived by thermodynamic simulations of the reaction pathway according to the Calphad method. Reflecting on the peritectic melting behavior at 1020 °C, the values of ΔfHm° (298 K) = −26.5 kJ·mol−1 and Sm° (298 K) = 132 J·mol−1 ·K−1 have been obtained.
According to modeling, crystal growth by short time vapor transport is reasonable under the addition of bromine or TeBr4 in the temperature range between 650 and 750 °C. Experimental implementation of crystal growth of WTe2 nanosheets succeeded in a temperature gradient from 725 to 675 °C on yttria-stabilized zirconia (YSZ) (111) substrates, observing the deposition of single crystal sheets of high crystallinity with thicknesses of 15−20 nm (∼20−30 layers). The high crystallinity, pristine morphology, and overall quality of the deposited nanosheets is shown by means of atomic resolution transmission electron microscopy, selected area electron diffraction (SAED), and atomic force microscopy as well as profound double-polarized Raman spectroscopy.
Background: The 3D printing is relevant as a manufacturing technology of functional models for forensic, pharmaceutical and bioanalytical applications such as drug delivery systems, sample preparation and point-of-care tests.
Objective: Melting behavior and autofluorescence of materials are decisive for optimal printing and applicability of the product which are influenced by varying unknown additives.
Methods: We have produced devices for bioanalytical applications from commercially available thermoplastic polymers using a melt-layer process. We characterized them by differential scanning calorimetry, fluorescence spectroscopy and functional assays (DNA capture assay, model for cell adhesion, bacterial adhesion and biofilm formation test).
Results: From 14 tested colored, transparent and black materials we found only deep black acrylonitrile-butadiene-styrene (ABS) and some black polylactic acid (PLA) useable for fluorescence-based assays, with low autofluorescence only in the short-wave range of 300-400 nm. PLA was suitable for standard bioanalytical purposes due to a glass transition temperature of approximately 60°C, resistance to common laboratory chemicals and easy print processing. For temperature-critical methods, such as hybridization reactions up to 90°C, ABS was better suited.
Conclusions: Autofluorescence was not a disadvantage per se but can also be used as a reference signal in assays. The rapid development of individual protocols for sample processing and analysis required the availability of a material with consistent quality over time. For fluorescence-based assays, the use of commercial standard materials did not seem to meet this requirement.