The application of emerging luminophores such as near-infrared (NIR) emissive earth-abundant chromium(III) (Cr^III) complexes and triplet- triplet annihilation upconversion (TTA-UC) systems in air as optical reporters for bioimaging or photonic materials for energy conversion requires simple and efficient strategies for their complete protection from luminescence quenching by oxygen. Therefore, we explored the influence of sol-gel synthesis routes on the oxygen protection efficiency of the resulting core and core/shell silica nanoparticles (SiO2 NPs), utilizing the molecular ruby-type luminophores CrPF6 ([Cr(ddpd)2](PF6)3; ddpd = N,N’-dimethyl-N,N’-dipyridin-2-ylpyridin-2,6-diamine) and CrBF4 ([Cr(ddpd)2](BF4)3) with their oxygen-dependent, but polarity-, proticity-, viscosity-, and concentration-independent luminescence as optical probes for oxygen permeability. The sol-gel chemistry routes we assessed include the classical Stöber method and the underexplored L-arginine approach, which relies on the controlled hydrolysis of tetraethoxysilane (TEOS) in a biphasic cyclohexane/water system with the catalyst L-arginine. As demonstrated by luminescence measurements of air- and argon-saturated dispersions of CrPF6- and CrBF4-stained SiO2 NPs of different size and particle architecture, utilizing the luminescence decay kinetics of argon-saturated solutions of CrPF6 and CrBF4 in acetonitrile (ACN) as benchmarks, only SiO2 NPs or shells synthesized by the L-arginine approach provided complete oxygen protection of the Cr^III complexes under ambient conditions. We ascribe the different oxygen shielding efficiencies of the silica networks explored to differences in density and surface chemistry of the resulting nanomaterials and coatings, leading to different oxygen permeabilities. Our L-arginine based silica encapsulation strategy can open the door for the efficient usage of oxygen-sensitive luminophores and TTA-UC systems as optical reporters and spectral shifters in air in the future.

Until now, automation in nanomaterial research has been largely focused on the automated synthesis of engineered nanoparticles (NPs) including the screening of synthesis parameters and the automation of characterization methods such as electron microscopy. Despite the rapidly increasing number of NP samples analyzed due to increasing requirements on NP quality control, increasing safety concerns, and regulatory requirements, automation has not yet been introduced into workflows of analytical methods utilized for screening, monitoring, and quantifying functional groups (FGs) on NPs. To address this gap, we studied the potential of simple automation tools for the quantification of amino surface groups on different types of aminated NPs, varying in size, chemical composition, and optical properties, with the exemplarily chosen sensitive optical fluorescamine (Fluram) assay. This broadly applied, but reportedly error-prone assay, which utilizes a chromogenic reporter, involves multiple pipetting and dilution steps and photometric or fluorometric detection. In this study, we compared the influence of automated and manual pipetting on the results of this assay, which was automatically read out with a microplate reader. Special emphasis was dedicated to parameters like accuracy, consistency, achievable uncertainties, and speed of analysis and to possible interferences from the NPs. Our results highlight the advantages of automated surface FG quantification and the huge potential of automation for nanotechnology. In the future, this will facilitate process and quality control of NP fabrication, surface modification, and stability monitoring and help to produce large data sets for nanomaterial grouping approaches for sustainable and safe-by-design, performance, and risk assessment studies.

Graphene quantum dots (GQDs) have attracted increasing attention due to their favorable optical properties and have been widely used, e.g., in the biomedical field. However, the properties related to the chemical structure of GQDs, resulting in solvent-dependent optical properties, still remain unclear. Herein, we present the synthesis of long-wavelength emitting GQDs with a size of about 3.6 nm via a solvothermal method using oxo-functionalized graphene (oxo-G) and p-phenylenediamine as precursors and their structural and surface chemical analysis by transmission electron and atomic force microscopy (TEM; AFM) as well as Fourier-transform infrared, Raman, and X-ray photoelectron spectroscopy (FTIR; Raman; XPS). Subsequently, the influence of solvent polarity and proticity on the optical properties of the as-prepared GQDs bearing –OH, –NH₂, –COOH and pyridine surface groups was investigated. Based on the results of the absorption and fluorescence (FL) studies, a possible luminescence mechanism is proposed. The observed solvent-induced changes in the spectral position of the FL maximum, FL quantum yield, and FL decay kinetics in protic and aprotic solvents of low and high polarity are ascribed to a combination of polarity effects, intramolecular charge transfer (ICT) processes, and hydrogen bonding. Moreover, the potential of GQDs for the optical sensing of trace amount of water was assessed. The results of our systematic spectroscopic study will promote the rational design of GQDs and shed more light on the FL mechanism of carbon-based fluorescent nanomaterials.

High sensitizer and activator concentrations have been increasingly examined to improve the performance of multi-color emissive upconversion (UC) nanocrystals (UCNC) like NaYF₄:Yb,Er and first strategies were reported to reduce concentration quenching in highly doped UCNC. UC luminescence (UCL) is, however, controlled not only by dopant concentration, yet by an interplay of different parameters including size, crystal and shell quality, and excitation power density (P). Thus, identifying optimum dopant concentrations requires systematic studies of UCNC designed to minimize additional quenching pathways and quantitative spectroscopy. Here, we quantify the dopant concentration dependence of the UCL quantum yield (Φ_UC) of solid NaYF₄:Yb,Er/NaYF₄:Lu upconversion core/shell nanocrystals of varying Yb³⁺ and Er³⁺ concentrations (Yb³⁺ series: 20%‒98% Yb³⁺; 2% Er³⁺; Er³⁺ series: 60% Yb³⁺; 2%‒40% Er³⁺). To circumvent other luminescence quenching processes, an elaborate synthesis yielding OH-free UCNC with record Φ_UC of ~ 9% and ~ 25 nm core particles with a thick surface shell were used. High Yb³⁺ concentrations barely reduce Φ_UC from ~ 9% (20% Yb³⁺) to ~ 7% (98% Yb³⁺) for an Er³⁺ concentration of 2%, thereby allowing to strongly increase the particle absorption cross section and UCNC brightness. Although an increased Er³⁺ concentration reduces Φ_UC from ~ 7 % (2% Er³⁺) to 1% (40%) for 60% Yb³⁺. Nevertheless, at very high P (> 1 MW/cm²) used for microscopic studies, highly Er³⁺-doped UCNC display a high brightness because of reduced saturation. These findings underline the importance of synthesis control and will pave the road to many fundamental studies of UC materials.

Despite considerable advances in synthesizing high-quality core/shell upconversion (UC) nanocrystals (NC; UCNC) and UCNC photophysics, the application of near-infrared (NIR)-excitable lanthanide-doped UCNC in the life and material sciences is still hampered by the relatively low upconversion luminescence (UCL) of UCNC of small size or thin protecting shell. To obtain deeper insights into energy transfer and surface quenching processes involving Yb³⁺ and Er³⁺ ions, we examined energy loss processes in differently sized solid core NaYF₄ nanocrystals doped with either Yb³⁺ (YbNC; 20% Yb³⁺) or Er³⁺ (ErNC; 2% Er³⁺) and co-doped with Yb³⁺ and Er³⁺ (YbErNC; 20% Yb³⁺ and 2% Er³⁺) without a surface protection shell and coated with a thin and a thick NaYF₄ shell in comparison to single and co-doped bulk materials. Luminescence studies at 375 nm excitation demonstrate back-energy transfer (BET) from the ⁴G_11/2 state of Er³⁺ to the ²F_5/2 state of Yb³⁺, through which the red Er^{3+ 4}F_9/2 state is efficiently populated. Excitation power density (P)-dependent steady state and time-resolved photoluminescence measurements at different excitation and emission wavelengths enable to separate surface-related and volume-related effects for two-photonic and three-photonic processes involved in UCL and indicate a different influence of surface passivation on the green and red Er³⁺ emission. The intensity and lifetime of the latter respond particularly to an increase in volume of the active UCNC core. We provide a three-dimensional random walk model to describe these effects that can be used in the future to predict the UCL behavior of UCNC.

We developed a procedure to prepare luminescent LiYF₄:Yb/LiYF₄ and LiYF₄:Yb,Er/LiYF₄ core/shell nanocrystals with a size of approximately 40 nm revealing luminescence decay times of the dopant ions that approach those of high-quality laser crystals of LiYF₄:Yb (Yb:YLF) and LiYF₄:Yb,Er (Yb,Er:YLF) with identical doping concentrations. As the luminescence decay times of Yb³⁺ and Er³⁺ are known to be very sensitive to the presence of quenchers, the long decay times of the core/shell nanocrystals indicate a very low number of defects in the core particles and at the core/shell interfaces. This improvement in the performance was achieved by introducing two important modifications in the commonly used oleic acid based synthesis. First, the shell was prepared via a newly developed method characterized by a very low nucleation rate for particles of pure LiYF₄ shell material. Second, anhydrous acetates were used as precursors and additional drying steps were applied to reduce the incorporation of OH⁻ in the crystal lattice, known to quench the emission of Yb³⁺ ions. Excitation power density (P)-dependent absolute measurements of the upconversion luminescence quantum yield (Φ_UC) of LiYF₄:Yb,Er/LiYF₄ core/shell particles reveal a maximum value of 1.25% at P of 180 W·cm⁻². Although lower than the values reported for NaYF₄:18%Yb,2%Er core/shell nanocrystals with comparable sizes, these Φ_UC values are the highest reported so far for LiYF₄:18%Yb,2%Er/LiYF₄ nanocrystals without additional dopants. Further improvements may nevertheless be possible by optimizing the dopant concentrations in the LiYF₄ nanocrystals.

Ensemble and single particle studies of the excitation power density (P)-dependent upconversion luminescence (UCL) of core and core–shell β-NaYF₄: Yb, Er upconversion nanoparticles (UCNPs) doped with 20% Yb³⁺ and 1% or 3% Er³⁺ performed over a P regime of 6 orders of magnitude reveal an increasing contribution of the emission from high energy Er³⁺ levels at P > 1 kW/cm². This changes the overall emission color from initially green over yellow to white. While initially the green and with increasing P the red emission dominate in ensemble measurements at P < 1 kW/cm², the increasing population of higher Er³⁺ energy levels by multiphotonic processes at higher P in single particle studies results in a multitude of emission bands in the ultraviolet/visible/near infrared (UV/vis/NIR) accompanied by a decreased contribution of the red luminescence. Based upon a thorough analysis of the P-dependence of UCL, the emission bands activated at high P were grouped and assigned to 2–3, 3–4, and 4 photonic processes involving energy transfer (ET), excited-state absorption (ESA), cross-relaxation (CR), back energy transfer (BET), and non-radiative relaxation processes (nRP). This underlines the P-tunability of UCNP brightness and color and highlights the potential of P-dependent measurements for mechanistic studies required to manifest the population pathways of the different Er³⁺ levels.

In the blossoming field of Cd-free semiconductor quantum dots (QDs), ternary I-III-VI QDs have received increasing attention due to the ease of the environmentally friendly synthesis of high-quality materials in water, their high photoluminescence (PL) quantum yields (QYs) in the red and near infrared (NIR) region, and their inherently low toxicity. Moreover, their oxygen-insensitive long PL lifetimes of up to several hundreds of nanoseconds close a gap for applications exploiting the compound-specific parameter PL lifetime. To overcome the lack of reproducible synthetic methodologies and to enable a design-based control of their PL properties, we assessed and modelled the synthesis of high-quality MPA-capped AgInS₂/ZnS (AIS/ZnS) QDs. Systematically refined parameters included reaction time, temperature, Ag:In ratio, S:In ratio, Zn:In ratio, MPA:In ratio, and pH using a design-of-experiment approach. Guidance for the optimization was provided by mathematical models developed for the application-relevant PL parameters, maximum PL wavelength, QY, and PL lifetime as well as the elemental composition in terms of Ag:In:Zn ratio. With these experimental data-based models, MPA:In and Ag:In ratios and pH values were identified as the most important synthesis parameters for PL control and an insight into the connection of these parameters could be gained. Subsequently, the experimental conditions to synthetize QDs with tunable emission and high QY were predicted. The excellent agreement between the predicted and experimentally found PL features confirmed the reliability of our methodology for the rational design of high quality AIS/ZnS QDs with defined PL features. This approach can be straightforwardly extended to other ternary and quaternary QDs and to doped QDs.

We assessed the influence of Yb³⁺ and Er³⁺ dopant concentration on the relative spectral distribution, quantum yield (Ƶ_UC), and decay kinetics of the upconversion luminescence (UCL) and particle brightness (B_UC) for similarly sized (33 nm) oleate-capped β-NaYF₄: Yb³⁺, Er³⁺ upconversion (UC) nanoparticles (UCNPs) in toluene at broadly varied excitation power densities (P). This included an Yb³⁺ series where the Yb³⁺ concentration was varied between 11%Ƀ21% for a constant Er³⁺ concentration of 3%, and an Er³⁺ series, where the Er³⁺ concentration was varied between 1%Ƀ4% for a constant Yb³⁺ concentration of 14%. The results were fitted with a coupled rate equation model utilizing the UCL data and decay kinetics of the green and red Er³⁺ emission and the Yb³⁺ luminescence at 980 nm. An increasing Yb³⁺ concentration favors a pronounced triphotonic population of ⁴F_9/2 at high P by an enhanced back energy transfer (BET) from the ⁴G_11/2 level. Simultaneously, the Yb³⁺-controlled UCNPs absorption cross section overcompensates for the reduction in Ƶ_UC with increasing Yb³⁺ concentration at high P, resulting in an increase in B_UC. Additionally, our results show that an increase in Yb³⁺ and a decrease in Er³⁺ concentration enhance the color tuning range by P. These findings will pave the road to a deeper understanding of the energy transfer processes and their contribution to efficient UCL, as well as still debated trends in green-to-red intensity ratios of UCNPs at different P.

Cd-free I-III-VI group semiconductor quantum dots (QDs) like Ag-In-S and Cu-In-S show unstructured absorption spectra with a pronounced Urbach tail, rendering the determination of their band gap energy (E_g) and the energy structure of the exciton difficult. Additionally, the origin of the broad photoluminescence (PL) band with lifetimes of several hundred nanoseconds is still debated. This encouraged us to study the excitation energy dependence (EED) of the PL maxima, PL spectral band widths, quantum yields (QYs), and decay kinetics of AIS/ZnS QDs of different size, composition, and surface capping ligands. These results were then correlated with the second derivatives of the corresponding absorption spectra. The excellent match between the onset of changes in PL band position and spectral width with the minima found for the second derivatives of the absorption spectra underlines the potential of the EED approach for deriving E_g values of these ternary QDs from PL data. The PL QY is, however, independent of excitation energy in the energy range studied. From the EED of the PL features of the AIS/ZnS QDs we could also derive a mechanism of the formation of the low-energy electronic structure. This was additionally confirmed by a comparison of the EED of PL data of as-synthesized and size-selected QD ensembles and the comparison of these PL data with PL spectra of single QDs. These results indicate a strong contribution of intrinsic inhomogeneous PL broadening to the overall emission features of AIS/ZnS QDs originating from radiative transitions from a set of energy states of defects localized at different positions within the quantum dot volume, in addition to contributions from dimensional and chemical broadening. This mechanism was confirmed by numerically modelling the absorption and PL energies with a simple mass approximation for spherical QDs and a modified donor-acceptor model, thereby utilizing the advantages of previously proposed PL mechanisms of ternary QDs. These findings will pave the road to a deeper understanding of the nature of PL in quantum confined I-III-VI group semiconductor nanomaterials.