title: Luminescent supraparticles with adjustable ID functionality
Creators: name: Miller, Franziska
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024
Subjects: subject: Supraparticle Luminescence ID Tag Lanthanides anticounterfeiting
subject: DDC Classification::5 Naturwissenschaften::50 Naturwissenschaften::500 Naturwissenschaften und Mathematik

description: The traceability and authentication of products based on object identification are of great value in our fast-moving, interconnected, and digitized world. The authenticated originality of a product's subcomponents makes it possible to transparently trace value chains and ensure the sustainable manufacture and quality of a product. Thus, object identification used for anticounterfeiting measures can contribute to sustainability. At the same time, object identification enables the traceability of product cycles even after the product has reached the end of its life span. In the context of recycling processes, it is possible to separate materials by their type based on their identification (ID) markers. Apart from this, ID tags are also suitable for involvement in digital transformation. More precisely, machine-readable ID tags enable communication and interaction between machines, which is why they could make a decisive contribution to industrial automation and digitization. However, commercial ID systems such as barcodes, serial numbers, or radio-frequency identification (RFID) chips reach their limits when marking the smallest product components, have relatively low counterfeit resistance, and can only be applied superficially to products. Miniaturized, material-based markers as micron-sized additives, can be flexibly incorporated into a wide variety of materials and applied to even the smallest product components. These machine-readable markers are based on intrinsic, physical properties of the marker material and link physical objects with information stored in a database. As an alternative to conventional marking strategies, they, therefore, offer the opportunity to utilize the full potential of object identification. A variety of such markers are based on optical signals, such as luminescence or reflection. There are several ways in which luminescence-based coding can be implemented. Besides graphical and spatially resolved coding, spectral coding is appealing. In this self-referenced, ratiometric coding strategy, relative emission intensity ratios of spectrally distinguishable signals are detected independent of ID tag orientation or concentration and converted into numerical codes. In this work, it is shown that this ratiometric coding principle can also be realized using supraparticles (SPs) with a precisely adjustable ID signature based on luminescence signals. SPs are hierarchically structured, micron-scaled units composed of individual nanoparticles (NPs). These SPs were produced according to a modular principle using forced assembly via spray-drying. In this flexible process, a wide variety of NPs with different physicochemical properties can be combined to form self-contained units. The resulting SPs not only carry combined features of the NPs but also new functionalities that result from NPs interaction within the hierarchically structured entities. The functionalities of SPs can stem from the structure itself, known as emergence, from the colocation of diverse, functional building blocks, or the coupling of individual particles in close proximity. When luminescent NPs with spectrally distinguishable narrow emission bands are combined into SPs, micron-sized ID-carrying additives are created that can be flexibly integrated into objects for their authentication. In Chapter 3.1, SPs are described that carry such a ratiometric luminescent ID signature in detail. For this purpose, four different types of CaF2 NP were synthesized using a co-precipitation reaction, containing one type of Ln3+ ion each (Ln3+ = Tm3+, Tb3+, Dy3+, and Sm3+, respectively). Upon excitation with shortwave UV light, the synthesized CaF2:Ln3+ NPs exhibited characteristic narrow emission bands in the blue, green, yellow, and red spectral regions. A subsequent thermal treatment led to a significant increase in the NPs' emission intensity, thereby enhancing their luminescence properties. Forced assembly of different types of CaF2:Ln3+ NPs in precisely defined quantitative ratios by spray-drying resulted in SPs possessing spectral ID signatures based on relative emission intensity ratios. The ID-bearing additives carry the combined luminescence properties of their NP subunits as self-contained units due to the earlier-mentioned SP colocalization functionality. A thermal treatment step of the SPs after spray-drying activated characteristic optical properties of the Ln3+-containing NPs by crystallization and diffusion processes. By varying the quantity-weighted ratio of each NP type, the relative emission intensity ratios of the resulting SP and thus its ID signature were precisely adjusted. The flexible process of spray-drying makes it possible to equip these ID-bearing SPs with additional nano-sized building blocks, resulting in further ID functionalities of the SPs. In addition to the colocalization effects, coupling effects, i.e., interactions of the NPs near each other, also come into play here. Hence, SP-based ID labels with multi-layered ID systems result, which are described in more detail in Chapter 3.2 By combining the Ln3+-containing CaF2 NPs with organic fluorophores, hybrid ID SPs with more variable optical properties and thus ID coding variety were formed. The divergent response of the organic and inorganic luminescent NPs to high temperatures plays a pivotal role in this context. As described in Section 3.2.1, SPs were established possessing an initially hidden luminescent ID signature, that could be specifically and deliberately revealed. For this purpose, the code-giving Ln3+-containing CaF2 NPs were assembled in defined quantity ratios in the presence of an intensely luminescent fluorophore utilizing spray-drying. The broad emission band of the fluorophore initially dominated the emission spectrum of the hybrid SPs. Deliberate application of heat resulted in the thermal decomposition of the fluorophore with simultaneous crystallization of the Ln3+-containing CaF2 NPs, which is why the characteristic spectral Ln3+-ID signature was revealed in a thermally induced manner. Moreover, by spatially arranging the ID SPs, which exhibit different luminescence hues due to varying luminescence ID signatures, spatially resolved structures were generated. By combining Ln3+-containing CaF2 NPs with polystyrene (PS) NPs containing fluorophore molecules emitting in the blue, green, and red wavelength regions, dual-encoded SPs could be generated. This work is described in detail in Section 3.2.2 Initially, the hybrid SPs exhibit an ID signature, which is derived from the relative emission intensities of the organic fluorophores. By heat treatment, upon thermal decomposition of the organic SP components, the ID signature resulting from the Ln3+ emission signals was activated and the organic ID signature was irreversibly deactivated. Thus, information can be encoded and selectively retrieved or invalidated at multiple levels. By varying the NP quantity ratios, both ID signatures were precisely varied independently of each other. A variety of interactions of the NPs within the SPs have a significant impact on the optical and morphological properties of the SPs. By varying the SP architecture and composition, interactions between NPs (coupling effects) were explored and specifically utilized to optimize the SP ID functionality. For example, the implementation of a core-shell SP structure and the addition of SiO2 NPs had a beneficial effect on the luminescence and morphological properties of the encoded additives. In Section 3.2.3, the high-temperature stable properties of a Ln3+ containing calcium phosphate (CaP) nanomaterial were exploited for the development of refractory post-mortem ID SPs. They were synthesized according to a Pechini sol-gel synthesis with a subsequent annealing step. Through the addition of magnetic core (iron oxide (IO)) - shell (SiO2) NPs and SiO2 NPs into the Eu3+ and Tb3+ ion containing Pechini sol, and subsequently spray-drying the mixture followed by heat treatment, it became feasible to confer magnetic properties upon ratiometrically encoded SPs. Due to the spatial confinement of the magnetic NPs by their SiO2 shell, their transformation to anti-ferromagnetic hematite is delayed when exposed to heat. Accordingly, these thermostabilized SPs retained their magnetic properties after a fire event. As a result, these hybrid SPs could be magnetically separated from fire residues and purified, thereby enabling luminescent signal detection and thus readout of encoded information. In this project, it was again possible to optimize the properties of the post-mortem ID markers concerning their application by adding SiO2 NPs. Uncontrolled sintering of the SPs among each other to form undefined macroscale bulk materials could be prevented by the addition of SiO2 NPs to the SP system. This phenomenon was also used in the work described in Section 3.2.4 to provide NPs with high-temperature properties without sintering. Usually, high-temperature treatment of crystalline NPs is accompanied by undesirable uncontrollable sintering to macroscopic bulk materials. By combining crystalline NPs with SiO2 NPs in excess to form SPs by spray-drying, it was possible to achieve spatial segregation of crystalline NPs from each other within the particle composite. If these hybrid SPs were subjected to thermal treatment, the crystalline NPs could not sinter together. Subsequent treatment of the SPs with caustic soda led to the dissolution of the SiO2 and recovery of a crystalline nano sol with high-temperature properties. Exemplarily, the emission intensity of Eu3+ containing CaF2 NPs as well as the photocatalytic activity of TiO2 NPs were successfully increased using the described method. The work shows that SPs produced by spray-drying carry combined luminescence properties of the assembled luminophores. However, these optical properties are significantly influenced by the interaction of the different NPs placed near each other. Therefore, the NP arrangement and thus the SP architecture, as well as the quantity ratios of individual nanocomponents, represent parameters that could be tuned to optimize the optical properties of the SPs for their ID functionality. In addition, it was possible to generate SP-based, multilayer ID systems in which encoded information can be specifically modified by the effect of temperature. Due to their sophisticated features, these ID systems are particularly suitable for improving anti-counterfeiting. In general, due to their high-temperature stable ID signal, the ID tags developed herein are also interesting for applications where thermal processes are involved.