Optical probes for monitoring, imaging, and sensing of pH are of great interest for the scientific community as pH is a crucial marker for many processes in biotechnology, biology, medical diagnostics, biomedical research, and material corrosion. Thereby, optical pH sensors based on fluorescence have attracted interest in particular as fluorescence offers a high sensitivity down to the single molecule level, can be read out with relatively simple and readily miniaturized instrumentation, and allows online in situ measurements. Also the versatility ranging from molecular and nanosensor formats to planar optodes and fiber-optic sensors, and the non-invasive, non-destructive, and contactless nature of the measurement are application-friendly features. The information content, which is offered by a fluorescence intensity-based sensor, is usually unspecific and limited on the presence or the absence of the chromophore or analyte and can additionally be hampered by fluctuation of the excitation light intensity and changes in fluorophore concentration, e.g., due to photobleaching. Therefore, many fluorescence sensors are utilized in referenced systems, which enable twowavelength ratiometric measurements of the fluorescence intensity by the introduction of an analyte-inert reference with a spectrally distinguishable emission. This work presents the rational design of a versatile, modular, multi-component-based platform for ratiometric optical analyte sensing that can be simply adapted to different formats and measurement geometries. Therefore, readily available analyte-responsive fluorescent boron-dipyrromethene (BODIPY) dyes and near infrared (NIR)-excitable multicolour-Partikelgrößen wurden dafür via Transmissionselektronenmikroskopie (TEM) und Kleinwinkel-Röntgenstreuung (SAXS) bestimmt. Neben der Partikelgröße konnten durch die TEM-Messungen auch Informationen über die Kristallphasen der Nanopartikel erhalten werden. Neben der Erfassung des Partikelwachstums wurde die UCL der UCNPs für die ratiometrische Sensorplattform als Nanolampe und gleichzeitig als Referenzsignal verwendet. Die blaue Upconversion(UC)-Emission der NaYF 4 :Yb 3+ /Tm 3+ UCNPs wurde dabei zur Anregung der pH-sensitiven BODIPY-Farbstoffe verwendet, während die rote UC-Emission als inertes Referenzsignal verwendet wurde. Die Berechnung des Verhältnisses der Emissionsintensitäten der grünen Fluoreszenz des Farbstoffs und der roten UC-Emission des Partikels ermöglicht eine Bestimmung des pH-Werts. Das Potenzial dieser Strategie zur Erfassung des pH-Werts wurde beispielhaft für die Bestimmung der zeitabhängigen Änderungen des pH-Werts einer metabolisierenden Escherichia coli (E. coli)-Suspension gezeigt.

We demonstrate the potential of time-resolved luminescence spectroscopy for the straightforward assessment and in situ monitoring of the stability of upconversion nanocrystals (UCNPs).

An emerging class of inorganic optical reporters are nearinfrared (NIR) excitable lanthanide-based upconversion nanoparticles (UCNPs) with multicolor emission and long luminescence lifetimes in the range of several hundred microseconds. For the design of chemical sensors and optical probes that reveal analyte-specific changes in their spectroscopic properties, these nanomaterials must be combined with sensitive indicator dyes that change their absorption and/or fluorescence properties selectively upon interaction with their target analyte, utilizing either resonance energy transfer (RET) processes or reabsorption-related inner filter effects. The rational development of UCNP-based nanoprobes for chemical sensing and imaging in a biological environment requires reliable methods for the surface functionalization of UCNPs, the analysis and quantification of surface groups, a high colloidal stability of UCNPs in aqueous media as well as the chemically stable attachment of the indicator molecules, and suitable instrumentation for the spectroscopic characterization of the energy-transfer systems and the derived nanosensors. These topics are highlighted in the following feature article, and examples of functionalized core−shell nanoprobes for the sensing of different biologically relevant analytes in aqueous environments will be presented. Special emphasis is placed on the intracellular sensing of pH.

Lanthanide-based, spectrally shifting, and multi-color luminescent upconverting nanoparticles (UCNPs) have received much attention in the last decades because of their applicability as reporter for bioimaging, super-resolution microscopy, and sensing as well as barcoding and anti-counterfeiting tags. A prerequisite for the broad application of UCNPs in areas such as sensing and encoding are simple, robust, and easily upscalable synthesis protocols that yield large quantities of UCNPs with sizes of 20 nm or more with precisely controlled and tunable physicochemical properties from low-cost reagents with a high reproducibility. In this context, we studied the reproducibility, robustness, and upscalability of the synthesis of β-NaYF4:Yb, Er UCNPs via thermal decomposition. Reaction parameters included solvent, precursor chemical compositions, ratio, and concentration. The resulting UCNPs were then examined regarding their application-relevant physicochemical properties such as size, size distribution, morphology, crystal phase, chemical composition, and photoluminescence. Based on these screening studies, we propose a small volume and high-concentration synthesis approach that can provide UCNPs with different, yet controlled size, an excellent phase purity and tunable morphology in batch sizes of up to at least 5 g which are well suited for the fabrication of sensors, printable barcodes or authentication and recycling tags.