Information-Providing Magnetic Supraparticles: Particle Designs to Record Environmental Stimuli with Readout by Magnetic Particle Spectroscopy.

The ability to gather information about materials and products, such as their origin, physicochemical properties or history of experienced environmental stimuli, is valuable for quality control, predictive maintenance, delivery tracking, recycling, and more. Integrating additives capable of recording and storing information into materials offers a flexible approach to create "materials intelligence". Common strategies utilize luminescent markers or DNA sequences that enable object identification and environmental impact monitoring. In contrast to optical methods limited to surface-level analysis, magnetic fields penetrate materials, enabling nondestructive readout even from the inside of opaque or multicomponent objects. While magnetic particle technologies have traditionally been used for biosensing and imaging with highly sensitive instruments like magnetic resonance imaging, these methods are unsuitable for quick, on-site analysis of macroscopic objects. During the past decade, magnetic particle spectroscopy (MPS) has emerged as a faster and more accessible characterization technique. MPS measures the magnetic response of particles in ambient conditions under alternating fields, offering high temporal resolution (∼1-10 s) and more geometric freedom than other magnetometry techniques. Magnetic nanoparticles are a widely studied material class that have been synthesized and optimized, e.g., for various MPS-based application scenarios and to obtain fundamental understanding of magnetic particle systems. Supraparticles (SPs) represent the next structural hierarchy level, as they are composed of one or multiple types of (magnetic) nanoparticles in a defined particulate structure. By ingenious control of structure and composition of such SPs, we have shown that various kinds of information can be obtained from them upon readout with MPS. In this Account, we present SP design concepts facilitating to obtain information about environmental stimuli (e.g., temperature, moisture, UV light, chemical gases) based on irreversible spectral magnetic signal changes upon readout with MPS. Initially, the state of the art on nanoparticles, which provide information by stimulus-induced agglomeration, is summarized. Subsequently, SPs consisting of multiple different nanoparticle types and their capabilities to obtain information on environmental stimuli are considered. Specifically, the advantages of using one or more signal transducing magnetic nanoparticle types used in conjunction with one or more nonmagnetic secondary materials susceptible to the desired environmental stimuli (sensitizer) are discussed. Finally, our latest findings on pronounced large-scale SP structure formation (millimeter-scale) through strongly interacting SPs and their implications on the integration of SPs in macroscopic objects of interest are described. Each of the three structural hierarchy levels, namely nanoparticles, SPs, and the macroscopic object of interest, represents an opportunity on the material level to fine-tune magnetic interactions. However, since the magnetic interactions across these three structural hierarchy levels are interdependent, meaning changes at the nanoparticle level influence the interactions of SPs at the macroscopic level, their control and interpretation in MPS remain challenging and prone to misinterpretation. The application of magnetic SPs as information-providing additives for predictive maintenance, material reuse, recycling, and industrial digitization requires a thorough understanding of all three hierarchical levels. Only then can suitable materials and processes be developed, turning challenges into opportunities for transforming passive matter into perceptual, information-providing systems through the integration of magnetic SPs.