Supraparticle designs for micrometer-sized information providers

Concepts such as circular economy, autonomous production of goods (Industry 4.0), as well as tracking and tracing of raw materials across global supply chains aim to improve the resource efficiency, resilience, and sustainability of our economy. The realization of these concepts requires the availability of machine-readable information about the type, origin, fate, and intactness of even small subcomponents or raw materials. Barcode labels and electronic sensors can provide this information for macroscopic products, but obtaining this information about sub-centimeter-sized objects requires the miniaturization of such information providers. In this thesis, it is demonstrated that the forced assembly of nanoparticles and molecules into supraparticles represents a modular approach to create micrometer-sized information providers. The customization of the composition and architecture of supraparticles using spray-drying as a synthesis tool facilitates the fabrication of targeted types of information providers that can be read out by desired spectroscopic methods. In Chapter 3.1, gasochromic supraparticles are presented that undergo a distinct color change when reacting with a specific target gas. First, supraparticles are demonstrated, which record an initial exposure to H2 by an irreversible color change and subsequently monitor the presence of H2 in real time by a reversible color change. This functionality emerges from the interaction of Pd- or Pt-based noble metal nanoparticles, which activate H2, and resazurin molecules, which serve as signal transducers, at the solid-liquid-gas interface of a mesoporous supraparticle based on SiO2 nanoparticles. The modular synthesis of these H2 indicator supraparticles via spray drying facilitates the optimization and customization of their detection performance. Furthermore, the solid-liquid-gas interface within the mesopores of supraparticles can be tailored to create dual gasochromic indicators that visualize both H2 gas and NH3 vapors. In Chapter 3.2, supraparticles that irreversibly record heat treatments lasting from a few seconds to minutes are presented. First, supraparticles are reported that indicate the experienced maximum temperature in the range of 60 to 200 °C through an irreversible eye-readable two step change in their fluorescence, which is quantitatively read out via fluorescence spectroscopy. Second, supraparticles are shown that indicate the encountered maximum temperature from 40 to 170 °C through an irreversible change in their magnetic particle spectroscopy (MPS) signal. MPS facilitates their successful readout independent of the optical constraints, i.e., even from the bulk of opaque objects. The temperature indicator functionalities are derived from the incorporation of fluorescent coumarin dyes or superparamagnetic iron oxide nanoparticles (SPIONs), which serve as signal transducers, into a multi-hierarchical supraparticle architecture containing thermoplastic polymers, which act as sensitizers. In Chapter 3.3, supraparticles with distinguishable and tunable identification (ID) signals are reported, which can link physical objects with associated information from a digital database, such as their type, origin, and fate. First, magnetic supraparticles with adjustable ID signals that are resolved by MPS are presented. These supraparticles can be equipped with additional luminescent ID signals that are resolved by fluorescence spectroscopy, resulting in dual spectrally encoded ID tags. Moreover, hybrid organic-inorganic supraparticles that switch their luminescent ID signal as a result of a specific heat treatment are introduced. The spectral ID signals are obtained by assembling different types of spectrally distinguishable magnetic or/and luminescent signal carriers into supraparticles with a multi-hierarchical architecture. In Chapter 3.4, it is shown that the functionalities of an ID tag and a temperature indicator can be combined in a single luminescent micrometer-sized supraparticle. These so-called communicating supraparticles obtain their functionalities from the assembly of spectrally distinguishable inorganic coding elements and organic temperature indicators in a core satellite architecture. A single fluorescence readout facilitates the detection of both signal responses. From a fundamental perspective, the results of this thesis demonstrate that customizing the interaction of assembled nanoparticles and molecules within tailored supraparticle architectures facilitates the creation of different types of micrometer-sized information providers: Indicator supraparticles can be obtained by combining magnetic or optical signal transducers with stimuli-specific activators or sensitizers. Mesoporous supraparticles can act as micro-reactors with a reaction environment similar to solution chemistry. Multi-hierarchical supraparticle architectures facilitate the control of signal carrier interactions. Multifunctional information providers can be created using core-satellite supraparticle architectures. Furthermore, MPS is identified to resolve changes in the interaction of SPIONs within and between hierarchical sub-structures of supraparticles, which opens up the research field of magnetic stimuli indicators. From the perspective of applications, this work demonstrates the synthesis and functionality of information-providing supraparticles as powders and after their integration into materials. For instance, the integration of H2 indicator supraparticles into coatings or clothing facilitates the detection and localization of H2 leaks within seconds by the naked eye without the need for bulky measurement equipment. The incorporation of magnetic temperature indicator supraparticles into adhesives enables the verification of the desired activation temperature after bonding processes via MPS readout. The application of communicating supraparticles onto objects allows for their identification and indication of the maximum temperature during elapsed heat treatments of only a few seconds with a lateral resolution in the millimeter range. Hence, these supraparticles could play a pivotal role for realizing the initially outlined concepts.