Modulating the Microenvironment of Coumarin Dyes within Multihierarchical Supraparticles: En Route toward Versatile Luminescent Dual‐Threshold Temperature Indicators with Ratiometric Readout

Temperature indicators trace (un)desired thermal impacts across their thermal history upon readout at every point of interest via defined irreversible signal changes. Ratiometric luminescent temperature indicators are attractive due to their fast, sensitive, contactless, spatially‐resolved, and self‐referenced readout. However, they have a limited working range as commonly only one specific temperature‐induced physicochemical effect is exploited. Herein, dual‐threshold temperature indicator supraparticles (SPs) are introduced that expand the working range of individual luminescent probes and enhance the extractable information on the thermal history. These SPs derive their functionality from the synergistic interplay of three spectrally distinguishable luminescent species assembled in one multihierarchical, micrometer‐sized particle of hybrid organic‐inorganic nature. This engineered nanostructure enables the stepwise modulation of the physicochemical microenvironment of two incorporated coumarin species at programmable threshold temperatures, resulting in distinct changes in their emission. As these changes are based on dye‐specific mechanisms that are induced in consecutive temperature regimes, the SPs achieve a scarcely reported broad working range (60–200 °C) with temperature‐dependent response times of minutes to seconds. From a broader perspective, the conceptualized SPs pioneer in how multiple individual luminescent probes with different excitation‐emission properties and indicator mechanisms are synergistically united in one advanced indicator system by precise material engineering.


Introduction
[8] Conventional temperature sensors monitor temperature in real-time and thus require permanent signal readout and data storage to provide information on the elapsed thermal history.In contrast, temperature indicators operate autonomously and verify approved conditions or indicate (un)desired temperature events via a defined irreversible signal change that can be read out at every point of interest.
Among different options to realize temperature indicators, [4,8,9,10,11] luminescent systems stand out with their fast, contactless, sensitive, and spatially-resolved readout process. [12][15][16][17][18][19] The spectrally resolved nature of luminescent signals offers diverse readout options for irreversible signal alterations of a probe such as changes in the emission intensity, peak position, band shape, bandwidth, polarization, and lifetime. [20]Among them, the ratiometric detection of emission intensity changes of a probe using a spectrally distinguishable internal reference emission is particularly attractive.It provides the advantage that even small changes in the relative intensity are reliably detectable independent of external factors, such as the probe concentration or the excitation intensity. [12]Moreover, ratiometric temperature indicators typically reveal high contrast emission color changes that are often more easily visible compared to intensity changes (turn on/turn off) of monochromic systems. [13,14]herefore, besides monochromic temperature indicators, [2,21,22] different material concepts for ratiometric luminescent temperature indicators were reported.Most established systems are thin films consisting of blends of organic dyes and thermoplastic polymers.[17][18][19] Their temperature indication mechanism is based on either intramolecular chemical reactions, [13] irreversible thermally-induced excimer formation and dissolution, [15][16][17][18] respectively, or a mixture of both. [14]Each of these material concepts comes with its own merits and drawbacks.However, exploiting physicochemical changes of a single dye species to realize ratiometric temperature indicators poses certain limitations to the spectral separation of the indicator and reference emissions, the options to customize and extend the working range, as well as the observable color changes.
Recently, we introduced another material concept for ratiometric luminescent temperature indicators in the form of supraparticles (SPs) [23] that expands the application options of previously established thin films due to their particulate additive character. [10,24]These hybrid organic-inorganic SPs are built up from different luminescent nanoparticle (NP) types with spectrally distinguishable emissions, which can be simultaneously excited using a single wavelength.The working principle of these SPs is based on irreversible temperature-induced changes in the emission of the organic luminophores and the temperaturestable emission of lanthanide-doped inorganic nanocrystals that act as an internal reference and enable the ratiometric readout.The organic temperature indicators of these SPs were synthesized by incorporating a coumarin dye into thermoplastic polymer NPs.Coumarin dyes are well-known as environmentally sensitive probes and frequently exploited in diverse chemical sensors. [25]Due to a lack of direct thermoresponsive features, to date, coumarin dyes were rarely employed in solid-state temperature indicators.Yet, the sensitivity of coumarin dyes toward their chemical environment offers a great potential to design versatile temperature indicators upon embedding the dye molecules into an appropriate thermoresponsive host material, for instance, a hierarchical SP architecture, as demonstrated in our pioneering work. [23]nspired by a recently presented multiple-threshold temperature indicator based on a single ratiometric organic film, [14] we hypothesize that a coumarin-based ratiometric dual-threshold temperature indicator in the form of a single micrometer-sized particle can be created by exploiting the concept of SPs.To succeed in this endeavor, two different temperature indicator types that are triggered at different threshold temperatures need to be established.
Herein, we present the design, working principle, and adjustability of ratiometric dual-threshold temperature indicator SPs with a two-step eye-readable color change upon exposure to gradually increasing temperatures, which is detectable ex-post at every point of interest.These SPs are assembled from three spectrally distinguishable dye-labeled NP types with emissions in the blue (coumarin 1, C1), green (coumarin 6, C6), and red wavelength range (rhodamine B), respectively.The two coumarinlabeled NPs act as indicators with different susceptibility to temperature events and change their luminescence upon exposure to different temperature ranges irreversibly.The luminescence of the RhB-labeled NPs remains unchanged throughout the investigated temperature range and acts as an internal reference for the ratiometric readout.We demonstrate that temperature-induced alterations in the microenvironment of the coumarin dyes, i.e., polarity changes (C1) and acid formation (C6), within the multihierarchical hybrid organic-inorganic SP architecture are key to achieving the desired functionality.After unraveling the working principle of the SPs, we study their ability to record and classify an exemplary thermal history and investigate their timetemperature behavior.Finally, we exploit the unveiled mechanistic insights and the toolbox-like fabrication process of the SPs via forced assembly using spray-drying as a tool to tune their working range by straightforward synthetic variations.Due to their innovative working principle and their particle-based additive character, the presented ratiometric luminescent dual-threshold temperature indicator SPs expand the horizon of previously established thin film-based ratiometric temperature indicators.

Design of Luminescent Ratiometric Dual-Threshold Temperature Indicator SPs
Aiming to increase the depth of the provided information by a luminescent material, the spectrally resolved nature of luminescent signals provides the chance to combine multiple optical information carriers with spectrally distinguishable emission bands in one system. [24]The assembly of different NP building blocks in defined ratios into SPs with suitable architectures is a versatile method to create such advanced spectral information carriers [26,27,28] or also other functional materials. [29]his concept is herein exploited to prepare ratiometric dualthreshold temperature indicators by combining two differently sensitive types of luminescent temperature indicators with a reference building block in one SP entity.To harness the environmental sensitivity of the emission of coumarin dyes toward irreversible temperature indication, we propose a multihierarchical hybrid organic-inorganic SP design (Figure 1).This material design concept starts with the identification of suitable luminophores (Figure 1a).These luminophores require spectrally distinguishable emissions and the option of turning them into either an irreversible temperature indicator or a temperature-resilient reference emitter.We identified C1 and C6 as luminophores to create temperature indicators.C1 exhibits a dual emission in the blue wavelength range depending on the polarity of its surroundings. [30]In contrast, the emission of C6 in the green wavelength range is sensitive toward acidic environments. [31,32]These two different sensitivities are envisioned to be exploited toward realizing two differently sensitive temperature indicators for the dual-threshold temperature indication functionality of the targeted SPs.As a reference emitter in the red wavelength range, a rhodamine B derivate, i.e., rhodamine B isothiocyanate (RITC), was selected.Rhodamine B is known to show a strong concentration-dependent emission [22] but is less sensitive toward changes in the pH or the polarity of the surrounding medium. [33]o minimize undesired leaching of dye molecules into surrounding media and turn these chosen dyes into temperature indicators and reference building blocks, the dyes are incorporated into suitable thermally sensitive and stable host matrices, respectively (Figure 1b).This processing step introduces the first level of hierarchy to the final SPs.In previous studies, [23,27,34] we exploited a versatile synthetic approach to incorporate hydrophobic coumarin dyes into thermoplastic polymer NPs via an emulsionsolvent evaporation approach utilizing sodium dodecyl sulfate (SDS) as a surfactant (schematic depiction, Figure S1, Supporting Information). [35]This approach offers an almost free choice of dye type, dye concentration, and polymer type.The integration of coumarin dyes into polymer particles is intended for irreversibly modulating their microenvironment upon exposure to temperatures, which exceed the polymer-specific glass transition temperature (T g ).Hence, interchanging the utilized polymer type is envisioned to adjust the working ranges of the two temperature indicator components.Herein, we investigate three different polymer types, i.e., polystyrene (PS), polymethyl methacrylate (PMMA), and polysulfone (PSU), each having a different T g .To realize the temperature-stable reference emitter, RITC shall be integrated into SiO 2 NP via a modified Stöber synthesis (schematic depiction, Figure S2, Supporting Information). [36,37]he covalent linkage of RITC into a thermally stable SiO 2 matrix is assumed to prevent temperature-induced alterations in their environment, particularly intense concentration-dependent emission changes due to excimer formation. [22]he envisaged ratiometric luminescent dual-threshold temperature indicators shall be created by colocalization of the different indicator and reference building blocks in one joint SP entity, which introduces the second level of hierarchy to the material concept (Figure 1c).The forced assembly of different NPs via spray-drying is an attractive method to achieve hierarchically structured functional SPs (schematic depiction, Figure S3, Supporting Information). [38]In this process, a dispersion containing (different) NP types is atomized into a hot chamber.The temperature-induced solvent evaporation forces the NPs within the generated small droplets to cluster together.This process results in the formation of raspberry-like SPs. [39]This method is versatile and almost independent of the surface chemistry of the employed building blocks. [38]Therefore, the composition of the resulting SPs is flexibly altered by adjusting the types and ratios of the utilized NPs. [23,26,27]Additionally, spray-drying is a high throughput method that is established at an industrial scale, which renders the scalability of the fabrication process possible.
The targeted ratiometric dual-threshold temperature indication functionality of the resulting multi-hierarchical SPs is envisioned by us as follows (Figure 1d): the emission spectrum of pristine SPs shows three spectrally distinguishable emission bands in the blue, green, and red wavelength range, respectively.Upon exposure to a first threshold temperature (T 1 ), the blue emission of C1 shall be irreversibly quenched, while the other two emissions remain constant.An elapsed temperature event that exceeds a second, higher threshold temperature (T 2 ) is indicated by an irreversibly altered emission of C6 in the green wavelength range, while the emission of RITC acts as a stable internal reference.

Fabrication and Characterization of Dual-Threshold Temperature Indicator SPs
The first step of the fabrication of the targeted temperature indicator SPs was the synthesis of the three different types of NP building blocks.For a proof-of-concept SP system, PS and PSU were selected as host matrices for the syntheses of C1-and C6-labeled polymer (polymer:dye) NPs, respectively, due to the difference in their T g (differential scanning calorimetry (DSC) measurements, Figure S4, Supporting Information).Scanning electron microscopy (SEM) images reveal that the performed emulsion-solvent evaporation method yields spherical NPs with a polydisperse particle size distribution ranging from roughly 50-300 nm (Figure 2a,b).Dynamic light scattering (DLS) measurements reveal very similar hydrodynamic particle diameters ranging from 100-300 nm for both NP dispersions (Figure S5, Supporting Information).To highlight the versatility of the used method, we performed a systematic synthesis variation, which shows that the particle size distribution is almost independent of the dye type, dye concentration, or polymer type used (Figure S6, Supporting Information).A representative SEM image of the synthesized SiO 2 :RITC NPs shows spherical particles with a very similar diameter of roughly 150 nm (Figure 2c).This size distribution is confirmed by DLS measurements that reveal hydrodynamic particle diameters between 100-240 nm and a low polydispersity index of < 0.1 indicating a monodisperse particle distribution (Figure S5, Supporting Information).
The optical properties of the NPs were investigated via fluorescence spectroscopy.The recorded emission spectra show maxima at 408 nm for PS:C1 NPs (Figure 2d), 503 nm for PSU:C6 NPs (Figure 2e), and 590 nm for SiO 2 :RITC NPs (Figure 2f) matching their blue, green, and red appearance under illumination with UV light (365 nm).Therefore, the different emission peaks achieve the desired spectral separation (>75 nm), making their separate monitoring via ratiometric detection possible.To ultimately achieve ratiometric detection through singlewavelength excitation of the three luminophores, excitation spectra were recorded (Figure S7, Supporting Information).We identified a wavelength of 365 nm as a suitable excitation wavelength as it collectively excites the three dye species and reduces undesired photobleaching. [23]Additionally, this excitation wavelength is commonly used in commercial UV lamps, which can be utilized to follow the detected temperature-induced irreversible color changes of the SPs with the naked eye.To balance the different excitation intensities amongst the three luminophores at the selected excitation wavelength, we adjusted the dye concentrations of the polymer:dye NPs and the mass ratio of the different NP types used for the fabrication of the temperature indicator SPs.For a proof of concept SP system, dye concentrations of 0.01 wt.% C1 and 0.1 wt.% C6 were selected for the PS:C1 and PSU:C1 NPs, respectively, while a mass ratio of PS:C1-PS:C6-SiO 2 :RITC of 5-5-90 was chosen (evaluation standards are reported in the Supporting Information).
A representative SEM overview image of the spray-dried SPs shows spherical particles with diameters ranging from roughly 0.8-10 μm (Figure 2g).Laser diffraction measurements confirm the polydisperse particle size distribution from 0.5-20 μm that peaks around 5 μm (Figure S8, Supporting Information).Such a size distribution is typical for spray-dried particles and results from the polydisperse size of the droplets formed during the atomization of the mixed NP dispersion. [10,23,27,40]Higher magnification SEM images of a single SP (Figure 2h) and its surface (Figure 2i) reveal their raspberry-like architecture consisting of closely assembled NP building blocks.Upon closer inspection of the SP surface, it is possible to distinguish the different NP types based on their different sizes.The uniformly sized inorganic SiO 2 :RITC NPs (≈150 nm) with a share of 90 wt.% form the framework structure of the SP that hosts slightly smaller and larger polymer NPs (50-300 nm), respectively.This assumption is supported by an SEM image with elemental contrast recorded with a backscattered electron detector (Figure S9  polymer NPs that we labeled with iron oxide NPs similar to our previous works. [10,26,27]Iron oxide NPs were employed in order to receive a strong contrast in EDX in the SEM (Figure S10, Supporting Information).These results confirm the hybrid organicinorganic nature of the SPs.
The successful combination of the three different luminescent species is also indicated by a homogenously whitish luminescence emission of the SPs upon excitation with a commercial UV lamp ( exc = 365 nm, Figure 2j).The corresponding emission spectrum exhibits the desired three spectrally distinguishable emission bands at the characteristic wavelengths of C1, C6, and RITC with maxima at 403, 493, and 585 nm.In concert with the literature, the slight shifts of the respective emission maxima between NP dispersions (Figure 2d-f) and SP powders (Figure 2j) are attributed to the different scattering behavior as well as altered interaction between the luminophores, for instance, emission-reabsorption effects. [23,27,34]Despite the multistep fabrication of the SPs, the emission spectra of three independently synthesized SP batches are very similar and indicate the reproducibility of the synthesis (Figure S11, Supporting Information).
To ultimately confirm the coexistence of the three luminophores within one SP entity, individual SPs were characterized via epifluorescence microscopy.It is important to note that these investigations were conducted using a different excitation source compared to the previous fluorescence spectroscopy measurements: instead of light from a Xenon lamp passed through a monochromator, we used a white lamp coupled with a band pass filter for UV excitation, resulting in a spectrally broader and slightly shifted excitation ( exc = 380 ± 10 nm).A representative image shows an individual micrometer-sized SP with a green-white appearance (Figure 2k, additional images are provided in Figure S12, Supporting Information).However, similar to the SP powder, the emission spectrum of an individual SP is dominated by three emission maxima at the characteristic wavelengths of C1, C6, and RITC (Figure 2l).Furthermore, the emission spectra of different individual SPs with varying sizes are similar (Figure S13, Supporting Information).Hence, the fluorescence microscopy analyses convincingly confirm the desired colocalization of the three different NP types into micrometersized, raspberry-like SPs.It is important to note that we attribute the differences in the intensity ratios of the three emission bands between the SP powder and individual SPs to the different excitation sources, spectral response of the complementary metal oxide semiconductor (CMOS) camera-monochromator system, optical filtering of the microscope, altered interactions between the luminophores, and mostly the photobleaching during the microscopy analysis (Figure S14, Supporting Information), particularly seen on the blue emission band.
Before investigating the temperature indication functionality of the prepared SPs, we verified the suitability of the SiO 2 :RITC NPs as temperature-stable reference emitters within the targeted temperature range (50-200 °C, Figure S15, Supporting Information).This finding enables exploiting ratiometric intensity detection for defining signal responses (SRs, Equation ( 1) and Equation ( 2)) to monitor elapsed exposure to threshold temperatures (TT) via the two different temperature indicator emissions of the prepared SPs, i.e., C1 (403 nm) and C6 (493 nm): The temperature indication functionality of the SPs was probed by exposing different powder samples from the same batch to heat treatments from 50-200 °C (10 K steps) for 30 min each.After the powders were cooled down to room temperature (RT), fluorescence spectra were recorded and normalized to the emission of RITC at 585 nm (Figure 3a).Based on this normalization, we determine the course of the two temperature indicator signals of the SPs, SR TT1, and SR TT2 , with increasing experienced maximum temperature compared to their pristine signal (Figure 3b).
The normalized emission spectra reveal significant irreversible changes in the blue and green wavelength range with increasing elapsed maximum temperatures.The emission of RITC exhibits a stable intensity and peak shape but shows a slight bathochromic shift (redshift) similar to our reference experiment (Figure S15, Supporting Information).According to the literature, we attribute this redshift to an increase in distance-dependent dye interactions due to irreversible changes in the SP architecture. [22,27]Based on the recorded changes in the emission signature and color of the SPs (photographs and emission color measurements, Figure S16, Supporting Information), we classify the investigated temperature range into four segments: In thermal history segment I (≤ 60 °C), the emission signature of the SPs remains almost unchanged.In segment II (60-100 °C), the emission of C1 is quenched while the C6 emission shows a slight increase.The quenching of C1 is indicated by a strong decrease of SR TT1 to roughly 20% of the pristine signal in the course of segment II.Importantly, the emission spectrum after exposure to 100 °C reveals a bathochromic shift of the emission maximum from 403 to roughly 430 nm.In segment III (> 100-130 °C), the irreversible bathochromic shift ( max = 435 nm) and quenching of the C1 emission reach a plateau (SR TT1 = 6%-10%) that is maintained throughout the investigated temperature range including segment IV. SR TT2 shows a slight reduction throughout segment III to roughly 95% of its pristine signal.Throughout segment IV (>130-200 °C), the emission of C6 around 500 nm is increasingly quenched, which is indicated by a decrease of SR TT2 to roughly 40%.Additionally, a second bathochromically shifted emission with a maximum at around 550 nm forms.It is important to note that SR TT2 rises again after an exposure to 200 °C, which could cause an uncertainty for the temperature indication and therefore, marks the upper limit of the working range of the presented SPs.
The temperature-induced changes in the fluorescence of the SPs cause an alteration of their emission color (Figure S16, Supporting Information).The initially whitish-blue emission of the SPs turns more whitish-green with increasing experienced temperatures in the thermal history segments I and II.This color change is however hardly detected by the naked eye.After exposure to temperatures in the thermal history segments III and IV, the emission color of the SPs switches from white to yellow and orange, respectively (photographs, Figure 3b).These color changes provide a qualitative assessment of exposure to temperatures in the thermal history segments III and IV with the naked eye.
In sum, the synthesized SPs achieved the desired ratiometric dual-threshold temperature indication functionality based on coumarin dyes by indicating elapsed temperature impact that exceeds a first threshold of 60 °C with a decrease of SR TT1 (C1) while a decrease of SR TT2 (C6) is detected upon exceeding a second threshold of 110 °C.

Mechanistic Investigation of the Temperature Indication Functionality
In the following, we aim to unravel the mechanisms behind the observed emission changes of the coumarin-based temperature indicator species within the multihierarchical SPs.As a starting point, the selected dyes are studied via thermogravimetric analysis (TGA) and DSC.The results from TGA show no significant mass loss for C1 and C6 before reaching 200 °C (mass loss of C1 starts around 200 °C, Figure S17, Supporting Information).Therefore, we assume that a decomposition of the dyes is not the main cause for the observed alterations of the luminescence of the temperature indicator SPs in the studied temperature range (50-200 °C).As reported in our previous study, [23] the DSC analysis of C1 reveals a melting peak with an onset of around 70 °C during the first heating process that upon cooling shows no sign of recrystallization and vanishes in the second heating process (Figure S18a, Supporting Information).The DSC analysis of C6, in contrast, shows no significant endothermic or exothermic process in the investigated temperature range (20-200 °C, Figure S18b, Supporting Information).Next, we investigate irreversible temperature-induced architectural and morphological changes of the multi-hierarchical SPs via SEM imaging analysis.SEM overview images reveal that the spherical, raspberry-like morphology of the SPs, i.e., hierarchy level II, is maintained throughout the entire investigated temperature range (Figure 4a1-c1; Figure S19, Supporting Information).Laser diffraction measurements support this observation but also show the formation of some SP agglomerates after exposure to 200 °C (Figure S20, Supporting Information).A comparison between SEM images of the surface of pristine SPs and SPs after exposure to 120 °C and 200 °C, respectively, exhibits two types of morphological changes in hierarchy level I of the SPs (Figure 4a2-c2).After exposure to 120 °C, some of the incorporated polymer NPs lose their spherical morphology and get deformed (additional SEM images are provided in Figure S21, Supporting Information).This deformation is in concert with a previous study on polymer NPs [22] and results in the formation of soft irregularly shaped clusters that fill up the interstitial pores of the SP (blue dotted circles, Figure 4b2) and leave voids behind (red solid circles, Figure 4b2).According to the T g of PS around 100 °C (DSC, Figure S4, Supporting Information), we assume that these clusters result from softened PS:C1 NPs.After exposure to 200 °C, which exceeds also the T g of PSU around 190 °C (DSC, Figure S4, Supporting Information), more voids and soft clusters are detected (Figure 4c).
The residual SP architecture consists almost exclusively of NPs that reveal a very similar size and are thus likely the temperaturestable SiO 2 :RITC NPs.This assumption is supported by SEM imaging with a backscattered electron detector that provides elemental contrast (Figure S22, Supporting Information).According to our previous studies, [10,23] a softening of the polymer NPs yields an increased mobility of the polymeric chains and the incorporated coumarin dyes.This mobility of the dye molecules makes the desired temperature-induced alteration of their microenvironment possible, for instance, via migration from the bulk to the surface.
In the following, we investigate which effects in the altered microenvironment of the coumarin dyes ultimately cause the observed irreversible changes in their luminescence.First, we focus on the temperature-induced emission changes of C1.Recently, we proposed a mechanism for polymer:C1 temperature indicator NPs in hybrid SPs consisting of mostly rare-earth doped CaF 2 nanocrystals. [23]Herein, we aim to support this mechanism, refine it with additional experiments, and explore if the mechanism is transferable to the introduced dual-threshold temperature indicator SPs.
To do so, we prepared and analyzed different reference systems.The first reference system is a single luminophore SP, in which PSU:C6 and SiO 2 :RITC NPs are replaced by nonlabeled NPs, i.e., PS and SiO 2 .The introduced mass ratio (5-5-90) was maintained.Upon exposure to the same temperature events as the dual-threshold temperature indicator SPs, almost identical irreversible changes of the C1 emission are observed, i.e., the quenching between 60 and 100 °C and the bathochromic peak shift starting at 100 °C (Figure 5a; Figure S23, Supporting Information).These changes are in concert with the changes in the excitation spectra of the SPs (Figure S24, Supporting Information).To investigate the origin of the bathochromic peak shift of C1 in the investigated SPs, we prepared two other reference systems by depositing C1 directly on SiO 2 :RITC NPs and synthesizing SPs from SiO 2 :RITC and PS:C1 NPs, respectively.The emission of C1 on SiO 2 :RITC NPs (SiO 2 :RITC+C1) reveals a significant bathochromic shift to 465 nm compared to SPs in which C1 is incorporated into PS NPs and thus surrounded by non-polar styrene units (Figure 5b).This shift is reflected in the excitation spectrum of this sample by a significant decrease in the absorption band of the emission at 403 nm compared to the SPs carrying PS:C1 NPs (Figure S25, Supporting Information).
Connecting all of these results, we propose the following mechanism for the temperature-induced irreversible emission change of C1 within the presented temperature indicator SPs (Figure 5c).We divide the emission changes into the quenching (60-100 °C, thermal history segment II) and the bathochromic shift of C1 (>100 °C, thermal history segment III-IV), which are assumed to be caused by different processes.
[43][44][45][46] In contrast to the highly emissive planar ICT state, the TICT state of C1 provides an additional non-radiative decay mode by making the rotary motion of the substituent group at position 7 possible, [41,43] which renders the TICT state very weakly to non-emissive. [44,46]Therefore, we as-sume that the observed irreversible quenching of the C1 emission in the thermal history segment II (60-100 °C) is caused by a shift in the dynamic equilibrium toward the weakly to nonemissive TICT state of excited C1 molecules.This shift might be triggered by the detected melting of C1 around 70 °C without recrystallization upon cooling (DSC, Figure S18a, Supporting Information).These assumptions are supported by quantum yield (QY) measurements of the single luminophore SP system that exhibit a reduction of the internal QY from 11% for pristine SPs to 8% and 7% after exposure to 60 °C and 80 °C, respectively (Table S1, Supporting Information).
The bathochromic shift of the C1 emission in the thermal history segments III and IV (>100 °C), is induced by the softening of the PS NPs.This process enables the migration of C1 molecules to an altered microenvironment, for instance, to the surface of the deformed PS clusters.According to the literature (mostly studies on the emission of C1 in different solvents), a bathochromic shift can be caused by an increase in the polarity of its surrounding medium or intermolecular H-bonding (exciplex formation) between the carbonyl oxygen of C1 and, for instance, alcohols. [41,42,46,47]This interaction results in the emission of photons with lower energy. [48]Therefore, we assume that the strong bathochromic shift of the C1 emission upon direct deposition onto SiO 2 :RITC ( max = 465 nm, Figure 5b) is caused by an intermolecular H-bonding of the carbonyl oxygen of C1 and the hydroxyl groups at the surface of the SiO 2 NPs or adsorbed water molecules from the surrounding atmosphere.This assumption is supported by temperature experiments on dried PS:C1 NPs, which do not exhibit a bathochromic shift of their emission after T g -exceeding temperature events (Figure S26, Supporting Information) due to an unaltered, non-polar microenvironment (styrene units).
Compared to the SiO 2 :RITC+C1 reference system (Figure 5b), the bathochromic shift of the C1 emission within the temperature indicator SPs is smaller ( max = 435 nm).We thus assume that just a fraction of the incorporated C1 molecules form intermolecular H-bonds with the hydroxyl groups at the surface of SiO 2 NPs or surface-adsorbed H 2 O molecules.Other C1 molecules show a bathochromic shift of their emission due to the increased polarity of their microenvironment compared to the pristine state within the non-polar PS NP.The bathochromic shift and thus the modulated microenvironment of the C1 molecules remain constant at higher temperatures and thus record T g -exceeding temperature impacts in the history of the SPs.The small intensity fluctuations of the shifted C1 emission in the thermal history segments III and IV are likely caused by changes in the distance-dependent interaction between the C1 molecules (aggregation-induced self-quenching) [49] due to enhanced polymer softening.However, these fluctuations do not significantly impact the signal response of SR TT1 and thus, do not hamper the temperature indication functionality of the SPs.
In sum, the irreversible decrease of the temperature indicator signal SR TT1 (C1) in the thermal history segments II-IV is caused by a combination of conformal changes in the excited state of C1 and an altered microenvironment after exceeding the T g of the C1-hosting polymer.Second, we investigate the emission change of the C6 dye within the temperature indicator SPs.To achieve a better understanding of the temperature-induced formation of the emission around 550 nm, which is partially superimposed by the RITC emission (Figure 3a), we synthesized a single luminophore SP system consisting of unlabeled SiO 2 and PS NPs as well as PSU:C6 NPs.These SPs show similar irreversible temperatureinduced changes in their emission characteristics compared to the temperature indicator SPs when they are subjected to the same temperature events (Figure 5d; Figure S27, Supporting Information).Up to an elapsed temperature impact of 120 °C, the emission intensity of C6 shows a slight rise compared to the pristine state (Figure S27, Supporting Information).We attribute this intensity increase to the evaporation of adsorbed residual water that acts as a quencher from the porous architecture of the SPs.After exposure to higher temperatures (> 120 °C, thermal history segments IV), the emission intensity at roughly 500 nm decreases monotonously, and a second emission maximum at around 550-560 nm forms.The excitation spectra of the SPs after exposure to temperatures in the thermal history segment IV reveal a strong decrease of the absorption band around 460 nm for the emission at 493 nm while the absorption band around 525 nm for the emission at 560 nm shows a strong increase (Figure S28, Supporting Information).These spectral alterations are accompanied by a change in the emission color from green to yellow (Figure 5d, inset) and a decrease of the QY from 14% for pristine PS+PSU:C6+SiO 2 SPs to 2% after exposure to 180 °C for 30 min (Table S2, Supporting Information).According to the literature, C6 shows a large bathochromic shift in its absorption and fluorescence upon protonation due to intramolecular charge transfer interactions from the electron-rich diethylamino moiety to the ring-protonated benzothiazole. [31,50]The reported emission maximum of the monocationic species of C6 ( max ≈ 560 nm) corresponds to the herein observed second emission band after exposure to higher temperatures (> 120 °C).
To corroborate that the emission around 560 nm is caused by the protonation of C6, we prepared a second reference system by adding ≈1 mL of H 2 SO 4 (40%) to the spray-drying dispersion.The resulting SPs reveal a red shift in the fluorescence compared to SPs without added H 2 SO 4 and show an emission band at 560 nm (Figure 5e).This emission matches the second, temperature-induced emission of the PSU:C6 NPs within the SPs in peak structure as well as in the yellow emission color (Figure 5e, inset).These changes are reflected in the excitation spectra by a strong reduction of the absorbance band around 450 nm for the emission at 493 nm and an intense increase of the absorbance band around 525 nm for the emission at 560 nm (Figure S29, Supporting Information).These findings raise the question of which temperature-induced alteration in the microenvironment of the PSU:C6 NPs results in the protonation of C6.Therefore, a third model system was prepared by synthesizing polymer:C6 NPs with another surfactant, i.e., by replacing SDS with cetyltrimethylammonium bromide (CTAB).Interestingly, this SP system does neither show a second emission maximum at around 560 nm upon exposure to 140 °C nor significant quenching (Figure S30, Supporting Information).
Linking all of these results, we propose the following mechanism for the irreversible temperature-induced emission changes of C6 in the temperature indicator SPs (Figure 5f).We assume that the formation of the monocationic species of C6 is attributed to the used surfactant SDS surrounding the PSU:C6 NPs.SDS is known for its temperature-and pH-dependent hydrolysis that results in the formation of 1-dodecanol and sodium hydrogen sulfate. [51,52,53]This reaction creates an acidic environment (likely a mixture of NaHSO 4 and H 2 SO 4 ) [52] that leads to the protonation of C6.This effect becomes dominant with increasing elapsed maximum temperature, which is indicated by the monotonous reduction of the emission of neutral species of C6 at 500 nm and the increase in emission of the monocationic species around 560 nm.Exceeding the glass transition temperature of PSU around 190 °C enhances the fraction of protonated C6 species due to the increased mobility of C6 molecules.These assumptions are supported by temperature experiments on dried PSU:C6 NPs that confirm the temperature-induced protonation of C6 by an intense relative increase in the emission around 560 nm with increasing experienced temperatures (Figure S31, Supporting Information).In sum, the irreversible reduction of the temperature indicator signal SR TT2 (C6) in the thermal history segments III-IV is caused by the formation of the monocationic species of C6 due to temperature-induced hydrolysis of SDS.
A recent study on luminescent SPs showed that the emission characteristics of SP powders can be affected not only by intra-SP but also by inter-SP interactions, e.g., via emission and reabsorption effects. [34]Therefore, we evaluated the contribution of interand intra-SP interactions on the emission characteristics and the emission changes of the temperature indicator SPs based on reference systems with reduced intra-SP interactions and inter-SP interactions, respectively (Figure S32a, Supporting Information).These experiments are thoroughly discussed in the SI, while the main findings are summarized in the following: In the pristine state, systems with reduced inter-and intra-SP interactions reveal only slightly altered emission ratios in comparison to pure temperature indicator SP powders (Figure S32b, Supporting Information).According to this observation, we assume that the signal characteristics of temperature indicator SP powders at the given composition are dominated by the individual emissions of the different colocalized NP species with a minor contribution of inter-and intra-SP interactions.After exposure to 180 °C, however, the different systems reveal changes in their irreversibly altered emission signals (Figure S32c, Supporting Information).While both reference systems show the expected changes in the blue wavelength range, i.e., the quenching and bathochromic shift of the C1 emission, no sign of the formation of a monocationic C6 species is observed for the SP system with reduced intra-SP interactions.The latter SP system comprises a large fraction (50 wt.%) of small-sized SiO 2 NPs (≈ 10 nm, from a NaOHstabilized dispersion, pH ≈ 10) that act as spacers between the luminescent NPs to achieve reduced intra-SP interaction.We assume that the addition of these SiO 2 NP spacers creates an alkaline microenvironment within the porous SPs.Therefore, we attribute the reduced formation of monocationic C6 species within such SPs to a reduced hydrolysis rate of SDS and a buffer effect for the formed acidic species.
Therefore, the experiments demonstrate that inter-SP interactions within the analyzed SP powders have just a minor contribution to the irreversible emission changes.In contrast, unaltered intra-SP interactions within the multi-hierarchical SPs (hierarchy level II) are of great importance to achieve the desired irreversible emission changes of C6.

Time-Temperature Behavior of Temperature Indication Functionality
Temperature indicators can be categorized into threshold temperature indicators and time-temperature indicators.Threshold temperature indicators show an irreversible signal change upon temperature events that exceed a certain threshold temperature and are suitable to record the experienced maximum temperature of the thermal history. [10,13,14]In contrast, the signal change of time-temperature indicators records the temporal accumulation of experienced temperature events. [4,6,7,11]n the following, we investigate the time-temperature behavior of the presented temperature indicator SPs in light of the gained mechanistic insights.To closely monitor even minor changes in the normalized emission ratio of the three luminophores, we aimed for a second SP system with similar intensities of the three peak maxima.This SP system was achieved by adjusting the concentration of C1 within the PS:C1 NPs (0.005 wt.%) compared to the SPs discussed in Chapter 2.3, which highlights the flexibility of the introduced material concept to tune the resulting SPs.First, the time-temperature behavior of the SPs was studied based on an exemplary thermal history experiment that consisted of multiple consecutive heating events with varying temperatures and a duration of 30 min each (Figure S33, Supporting Information).This experiment was designed to repeatedly expose the SPs to high temperatures, however, a new experienced maximum temperature was not introduced in every temperature event.By measuring the fluorescence characteristics of the SPs after every temperature event at RT, we aim to characterize them either as time-temperature or threshold temperature indicators.
The recorded normalized emission spectra of the SPs reveal distinct irreversible changes corresponding to the previously described mechanism whenever the SPs encountered a new maximum temperature, i.e., after temperature events 1, 2, 4, and 6 (Figure 6a).The other temperature events that do not introduce a new maximum temperature to the thermal history of the SPs (3, 5, 7, 8), yield no significant change in their emission characteristics.This behavior becomes apparent when the changes in the two signal responses of the temperature indicator SPs are displayed as a function of the experienced temperature events (Figure 6b).Temperature events 1 and 2 that expose the SPs to 80 and 120 °C, respectively, introduce new maximum temperatures to the thermal history of the SPs (red dotted line) and are recorded by an increasing change of SR TT1 while SR TT2 remains roughly constant.SR TT2 changes upon exposure to temperature events 4 and 6 that introduced new maximum temperatures of 180 °C and 200 °C, respectively.Both temperature indicator signals remain constant upon exposure to temperature events 3, 5, 7, and 8 that do not alter the experienced maximum temperature of the SPs.
These results demonstrate that the presented temperature indicator SPs act as threshold temperature indicators, which record the experienced maximum temperature in their elapsed history.Moreover, the conducted experiments reveal the dual-threshold indication functionality of the SPs.Temperature events 1-3 up to 120 °C are only recorded by a change of SR TT1 .SR TT2 only changes upon exposure to higher temperatures, herein, 180 and 200 °C.These findings highlight the benefit of the dual-threshold indicator SPs as they expand the working range of SR TT1 (≈60-110 °C) and SR TT2 (≈130-200 °C) upon synergistic combination to 60-200 °C.
After the characterization of the presented SPs as dualthreshold temperature indicators, we investigate the effect of the exposure time (1-120 min) for heat treatments at 100 and 180 °C, respectively, on their emission characteristics.These temperatures were selected as they represent the onset of the glass transition of the utilized polymers, i.e., PS and PSU (DSC, Figure S4, Supporting Information).It is important to note that for each heating time, a different powder sample from the same batch of Normalized intensity / a.u.SPs was analyzed by recording fluorescence spectra at RT after the respective temperature event.

Wavelength / nm
In concert with the previously proposed mechanism, the C1 emission reveals enhanced spectral changes with increasing exposure time upon exposure to 100 °C (Figure 7a).Up to 5 min at 100 °C, the emission intensity of C1 is quenched without a redshift of the emission maximum, which results in a decrease of SR TT1 to 40% of its pristine value (Figure 7c).With increasing heating time (10-120 min) at 100 °C, the emission maximum of C1 shifts bathochromically accompanied by increasing quenching.These effects cause a reduction of SR TT1 to roughly 20% of its pristine value after 30 min at 100 °C before it reaches a plateau value of 10%-15% after 60 min (similar to the thermal history experiment, Figure 6b).
In contrast, the emission of C6 and accordingly SR TT2 exhibit only minor changes upon exposure to 100 °C.We attribute the slight increase of SR TT2 in the first 30 min at 100 °C to the evaporation of residual water from the SP framework.The minor reduction of SR TT2 to roughly 90% of its pristine value upon prolonged exposure to 100 °C is likely caused by a reduction of the spectral crosstalk between C1 and C6, e.g., emissionreabsorption events due to the quenching of C1 and altered fluorophore distances caused by the softening of PS.
For exposure to 180 °C, both temperature indicator components of the SPs show significant changes in their emission with increasing exposure time (Figure 7b).Already after 1 min at 180 °C, the emission of C1 is quenched and bathochromically shifted, which is indicated by an immediate reduction of SR TT1 to the lower plateau value of 15% of its pristine signal.The emission of C6 remains almost unchanged upon short exposure (≤ 2 min) to 180 °C.With increasing exposure time (≥ 5 min) at 180 °C, the emission of the neutral C6 species at around 500 nm gets increasingly quenched while the emission of the monocationic species of C6 at around 560 nm increases.These effects are indicated by a reduction of SR TT2 within the first 30 min at 180 °C before it reaches a plateau value of roughly 40% of its pristine signal between 30-120 min.
Connecting these observations and the results from supporting experiments, conducted with a reproduced SP system at other temperatures (Figures S34 and S35, Supporting Information), with the proposed mechanisms for the two temperature indicator components, we draw the following conclusion: All observed alterations of the emissions of C1 and C6 are temperature-and time-dependent processes that however require a certain threshold temperature to be induced.The quenching of C1 intensifies with increasing temperatures that exceed the first temperature threshold of 60 °C.Similarly, the bathochromic shift of C1 proceeds faster with increasing temperatures but is only triggered after exceeding the T g of PS:C1 at around 100 °C (redshift detected at 100 °C after 10 min, at 105 °C after 5 min, at 115 °C after 2 min, and at 180 °C after 1 min, Figure S34a-d, Supporting Information).Therefore, SR TT1 exhibits a temperaturedependent response time, defined as the time until it reaches the lower plateau value of less than 20% of its pristine signal, for temperature events that exceed 100 °C (Figure S34e, Supporting Information).This response time gets reduced from roughly 60 min at 100 °C to the range of only seconds (60 s) at 180 °C.
In concert with the literature that showed enhanced SDS hydrolysis rates with increasing temperature, [51] we observe an enhanced quenching of the emission of the neutral C6 species at around 500 nm due to their protonation with increasing temperature and exposure time (Figure S35a-d, Supporting Information) above a temperature threshold of roughly 130 °C (Figure 5d).This effect results in an accelerated decrease rate and an enhanced total reduction of SR TT2 with increasing experienced temperature.SR TT2 reaches a plateau value of roughly 50% of its pristine signal for exposure to 170 °C and a plateau value of 40% for exposure to 180 °C after roughly 30 min each (Figure S35e, Supporting Information).For exposure to 190 °C and 200 °C, SR TT2 reaches plateau values of roughly 25% and 20% of its pristine signal after 10 and 5 min, respectively.The lower SR TT2 plateau values and the faster response times at 190 and 200 °C indicate that once the T g of PSU is exceeded, the protonation of C6 is accelerated and enhanced due to polymer softening.Interestingly, the plateau values after exposure to 180 °C and 200 °C match well with the previously performed thermal history experiment, which revealed a signal change of roughly 60% for exposure to 180 °C for 30 min and 80% for exposure to 200 °C for 30 min (Figure 6b).
This finding supports the assumption of temperature-specific plateau values of SR TT2 .It is however important to note that upon prolonged exposure (> 30 min) at 200 °C, a distinct increase of SR TT2 is detected (Figure S35d, Supporting Information), which is assumed to result from enhanced polymer melting processes causing SP aggregation and yielding strongly altered microenvironments of C6 molecules.Therefore, as stated in Chapter 2.2, the working range of the SPs is limited to short exposure times at 200 °C.
In summary, our investigations of the timetemperature behavior of the temperature indicator SPs (PS:C1+PSU:C6+SiO 2 :RITC) demonstrate that the ratiometric detection of changes in their emission signature can differentiate an -prior to the readout -unknown thermal history based on four indicators.These indicators are the relative decrease of SR TT1 and SR TT2 compared to their pristine signal, the bathochromic shift of the emission of C1, and the formation of the monocationic species of C6: • A decrease in SR TT1 , without a reduction of SR TT2 , and without a bathochromic shift of the emission of C1 indicates a temperature event below 100 °C.The extent of the decrease of SR TT1 is time-temperature dependent.Hence, a large decrease results either from short exposure to higher temperatures in the segment, e.g., 90 °C, or long exposure to lower temperatures, e.g., 70 °C.• A decrease of SR TT1 with a residual signal of more than 80% of SR TT2 , a bathochromic shift of the emission of C1 but no sign of an emission by the monocationic species of C6 indicates a temperature event between 100 °C and 130 °C.A large decrease of SR TT1 to the plateau at around 15%-20% of its pris-tine signal is a sign of a longer exposure (>10 min) at 105 °C or exposure to higher temperatures, e.g., 115 °C, for more than 2 min.• A residual signal of SR TT1 of less than 20%, a residual signal of SR TT2 of less than 80%, a bathochromic shift of C1, and evidence of the monocationic emission of C6 indicates a temperature event of more than 130 °C for more than 2 min.In this segment, higher residual signal values of SR TT2 either signalize a short exposure to higher temperatures (190-200 °C) or a long exposure to lower temperatures (140-180 °C).In contrast, lower residual signal values of SR TT2 , e.g., 25%, are only reached upon exposure to higher temperatures (190-200 °C) and thus, specify the experienced maximum temperature.
Such a detailed classification of the thermal history in this broad working range (60-200 °C) is hardly achieved by a single temperature indicator species, which highlights the advantage of the herein-presented luminescent ratiometric dual-threshold temperature indicator SPs.

Adjustability of Dual-Threshold Temperature Indicator SPs
To fully exploit the benefits of the presented material concept and the introduced synthesis approach, it is demonstrated how the variation of the dye concentration and the utilized polymer type tunes the temperature indication functionality of the presented SPs for specific demands.
First, we systematically varied the concentration of C1 in PS:C1 NPs (0.005-0.1 wt%) by using different volumes of a C1 stock solution for their synthesis.The different PS:C1 NPs were individually assembled with PSU:C6 NPs (0.1 wt.% C6) and SiO 2 :RITC NPs in the previously introduced mass ratio (5-5-90) into SPs.With increasing concentration of C1 in PS:C1 NPs, the luminescence color of pristine SPs shifts monotonously from yellow/white to intense blue due to an enhanced emission intensity of C1 (Figure 8a; Figure S36, Supporting Information).Interestingly, normalized fluorescence spectra of pristine SPs with high concentrations of C1 within PS:C1 NPs (0.03-0.1 wt.%) reveal not only an increased intensity of C1 but also an increase in the intensity of C6.We attribute this rise in the intensity of C6 to enhanced crosstalk between C1 and C6 due to the enhanced number of C1 molecules within the confined space of the SPs.
To investigate the effect of the different concentrations of C1 in the PS:C1 NPs on the temperature indication functionality of the SPs, we exposed powder samples of the SPs to different temperature events (similar to Chapter 2.2, for 30 min each) and recorded changes of their fluorescence signature afterward at RT via fluorescence spectroscopy.All SP samples show an irreversible change in the emission of C1 and C6 in concert with the previously proposed mechanism (Figure S37, Supporting Information), which is thereby supported.
Due to the different luminescence colors in their pristine state, these changes result in an altered temperature-induced irreversible change of the luminescence color of the SPs (Figure 8a).By increasing the concentration of C1 in the PS:C1 NPs, the eyereadable color change of the SPs can be adjusted from whiteyellow-orange to blue-green-orange and blue-white-red and thus, customized toward specific use cases.Importantly, the dualthreshold temperature indication functionality is maintained upon increasing the concentration of C1 within the PS:C1 NPs.The temperature-induced change of SR TT1 of the different SP samples is almost independent of the C1 concentration of the employed PS:C1 NPs (Figure S38a, Supporting Information).The temperature-induced reduction of SR TT2 shows a slightly shifted onset toward lower temperatures for higher C1 concentrations (0.03-0.1 wt.%) in the PS:C1 NPs (Figure S38b, Supporting Information).We attribute the shifted onset to the increasing spectral crosstalk between C1 and C6 within the SP samples, which yields a distinct reduction of the emission of C6 due to the quenching C1.However, this effect does not interfere with the dualthreshold temperature indication functionality as the reductions of SR TT1 (60-110 °C) and SR TT2 (100-200 °C) still happen in different temperature ranges.Until this point, all presented temperature indicator SPs were based on PS NPs as the host matrix for C1 and PSU NPs for C6.
Our results indicate that exceeding the T g of the employed polymers has a pronounced effect on the irreversible signal change of C1 and C6 due to their modulated microenvironments.Therefore, as a second option for adjusting the temperature indication functionalities of the SPs, we investigate the effect of the alteration of the T g of the coumarin-hosting polymer NPs.We exchanged the type of polymer used for the synthesis of the polymer:C1 and polymer:C6 NPs for polymers that owe a higher T g for C1, e.g., PMMA and PSU, and a lower T g for C6, e.g., PS and PMMA, respectively (DSC measurements, Figure S4, Supporting Information).This synthetic variation is readily performed by employing different polymer granulates in the chosen emulsionsolvent evaporation method while keeping all other synthesis steps constant.
The temperature indication functionality of the resulting SPs was tested analogously to the variation of the dye concentration and Chapter 2.2.SEM imaging analyses of the SP samples consisting of different polymer:dye NPs demonstrate that the architectural changes of the SPs due to polymer softening, i.e., the formation of irregular-shaped clusters and voids, correspond well to the T g of the respectively utilized polymers (Figures S39 and S40, Supporting Information).Following our proposed mechanisms, the modulation of the microenvironment of the coumarin dyes within these SPs thus occurs after exceeding different temperature thresholds.This assumption is confirmed by the analysis of the luminescence characteristics of the different SP samples, which are discussed in detail in the SI (Figures S41 and S42, Supporting Information) while the main findings are summarized in the following: Increasing the T g of the C1-hosting polymer NPs from PS to PMMA and PSU shifts the decrease of SR TT1 of these SPs to higher experienced temperatures (Figure 8b) while the course of their SR TT2 remains almost unchanged (Figure S41e, Supporting Information).Vice versa, decreasing the T g of the C6-hosting polymer from PSU to PMMA and PS shifts the decrease of SR TT2 of these SPs to lower experienced temperatures (Figure 8c) while the course of their SR TT1 remains constant (Figure S42d, Supporting Information).We assume that the main cause of shifted reductions of SR TT1 and SR TT2 toward higher experienced temperatures is the migration of coumarin dyes from the polymer matrix to an altered microenvironment, which happens only upon T g -exceeding temperature events.A main contributor to the reduction of SR TT1 , particularly for achieving the lower plateau value around 10% of its pristine signal, is the bathochromic shift of the C1 emission that is only induced by T g -exceeding temperature events (Figure S41a-c, Supporting Information).Regarding the shift of SR TT2 , we estimate that the temperature-induced hydrolysis of SDS is likely independent of the chosen C6-hosting polymer NPs as the same amounts of SDS were employed in their synthesis.Therefore, we attribute the observed shift in the course of SR TT2 to the enhancement of the protonation of C6 molecules due to their increased mobility during T g -exceeding temperature events.This assumption is supported by a single luminophore SP system consisting of PS:C6 and SiO 2 NPs that shows a distinct emission of the monocationic species of C6 already after 120 °C (Figure S43, Supporting Information) while SPs with PSU:C6 NPs reveal this effect only after a temperature event of more than 140 °C (Figure 5d).
In sum, the results from the performed synthetic variations support our proposed mechanisms for the coumarin-based temperature indicator species.In addition, the synthetic variations demonstrate the opportunity to customize the working range and the temperature-induced changes in the luminescence color of the SPs toward specific demands and thus, highlight the benefit of the presented multi-hierarchical material design.

Conclusion
In conclusion, we have synthesized and characterized dualthreshold temperature indicator SPs that record the experienced maximum temperature throughout their thermal history over a broad range of temperatures, from 60 up to 200 °C, via irreversible changes in their luminescence.These changes are detected ex-post in a ratiometric manner using a single excitation wavelength or even with the naked eye due to a two-step change in the luminescence color of the SPs.
The indicators derive their functionality from the colocalization of three types of spectrally distinguishable luminescent NPs, i.e., two differently sensitive coumarin-based temperature indicators species and SiO 2 :RITC with a temperature stable emission, into multi-hierarchical, micrometer-sized SPs.We demonstrate that the integration of coumarin dyes into polymer NPs is a versatile, adjustable, and straightforward method to achieve temperature indicators based on the modulation of their microenvironment due to polymer softening upon exposure to T gexceeding temperature events.Based on this design concept, the dual-threshold temperature indication functionality is achieved by the synergistic combination of PS:C1 and PSU:C6 NPs that show irreversible emission changes, which are based on different mechanisms and are thus induced upon exposure to different threshold temperatures.PS:C1 NPs change their emission due to conformation changes of the dye in its excited state as well as changes in the polarity and the interaction potential of the microenvironment of C1.In contrast, the emission changes of PSU:C6 NPs result from the protonation of C6, which is triggered by the temperature-induced hydrolysis of SDS molecules acting as surface-ligands of the NPs.As the irreversible alteration of the C1 and C6 emissions not only consists of changes in the intensity but also in the peak position, the spectral readout of the luminescence of the SPs provides a detailed classification of the maximum temperature within their encountered thermal history.Furthermore, we demonstrate that the chosen multi-hierarchical SP design enables the customization of the temperature-induced change of the luminescence color and the working range of the SPs via readily performed synthetic variations of, e.g., the dye concentration or type of polymer.
The reported maximum temperature indicator SPs stand out with their scalable toolbox-like fabrication process, which uses exclusively inexpensive, abundant, commercially available precursors, and their broad detection range, particularly in the scarcely reported high-temperature range (140-200 °C).With their innovative working principle and particle-based additive character, we firmly believe that the SPs complementary expand the application potential of previously established thin film-based ratiometric temperature indicators.

Experimental Section
Materials and Reagents: Ethanol (EtOH, analytical grade), polystyrene (PS, M w ≈192000), polysulfone (PSU, M w ≈22000), as well as the dyes 7-(diethylamino)−4-methylcoumarin (coumarin Synthesis of Polymer: Coumarin NPs: Polymer:coumarin NPs were synthesized via an emulsion-solvent evaporation method (Figure S1, Supporting Information) similar to the previous work. [23,27,34]Initially, stock solutions of C1 in CH 2 Cl 2 (1 mg mL −1 ), C6 in CHCl 3 (1 mg mL −1 ), and aqueous stock solutions of SDS (0.5 wt.%) as well as CTAB (0.5 wt.%) were prepared.In a typical synthesis of polymer:C1 NPs, containing, e.g., 0.01 wt.% C1, 1 g of the respective polymer (PS, PMMA, or PSU) was dissolved in 0.1 mL of the C1 stock solution and 9.9 mL of DCM under magnetic stirring for several hours.After complete dissolution, 20 mL of the aqueous SDS stock solution was added causing a phase separation.Emulsification of this mixture was performed via sonication (Branson Ultrasonic Sonifier, output: 20, constant duty cycle, 120 s) during which the batch was cooled using an ice bath.From the resulting emulsion, the organic solvent was evaporated under vigorous stirring at RT overnight.The resulting aqueous polymer:C1 NP dispersion was further purified via dialysis against 5 L deionized water (five water exchanges in 48 h).For the synthesis of polymer:C6 NPs, the C6 stock solution, and CHCl 3 were used in the initial synthesis step, keeping all other steps constant.Adjusting the dye concentration of polymer:coumarin NPs was conducted by increasing the volume of the added dye stock solution, maintaining a total volume of the organic solvent of 10 mL.For the synthesis of CTAB stabilized PS:C6 NPs, the SDS stock solution was replaced with the CTAB solution.Non-labeled PS NPs for reference SPs were synthesized following the same procedure but dissolving the polymer in 10 mL of DCM without the addition of a dye solution.Fe 3 O 4 NP-labeled polymer NPs were prepared according to a synthetic procedure of the previous works. [10,26,27]ynthesis of SiO 2 : RITC NPs: The synthesis of dye-labeled SiO 2 NPs was based on the Stöber protocol [37] and adapted from a previously published work with modifications (Figure S2, Supporting Information). [36]or a typical silanization of RITC, 8.90 mg of RITC (0.017 mmol) was dissolved in 2 mL of DMSO in an Ar-flushed plastic tube (Eppendorf) before 13.0 μL of APTES (0.055 mmol) was added.This reaction mixture was stirred in an Ar atmosphere and under the exclusion of light at RT for 24 h.The silanized dye (RITC-APTES) was stored at −18 °C until further use.In a typical synthesis of SiO 2 :RITC NPs, 300 mL of EtOH was mixed with 24 mL of an aqueous ammonia solution (25%) by magnetic stirring.After 10 min, 12 mL TEOS (54.1 mmol) was added.The reaction mixture was stirred for 15 min before 240 μL of the preformed RITC-APTES solution was added.The reaction mixture was stirred under the exclusion of light at RT for 5 h.The resulting pink dispersion was purified via centrifugation steps (Z 326 K, Hermle, 1 × 15 min at 12000 rpm, 2 × 10 min at 10000 rpm).After every step, the separated SiO 2 :RITC NPs were dispersed in water via vortex-mixing (ZX3, VELP Scientifica) and sonication (Sonorex, Badelin).Dye leaching from the particles into the supernatant was observed only during the first washing step, indicative of the incorporation of RITC into SiO 2 NPs.Non-labeled SiO 2 NPs for reference SPs were synthesized following the same procedure but without the addition of dye.
Fabrication of Temperature Indicator SPs: The targeted SPs were synthesized via spray-drying of mixed NP dispersions (Figure S3, Supporting Information) using a lab-scale spray-dryer (Büchi Labortechnik AG, B290 mini connected to a dehumidifier B-296).The NP dispersions employed in the spray-drying process were obtained by mixing the desired types of NP dispersions in the desired mass ratio using magnetic stirring (composition of all presented SP samples, Table S3 and S4, Supporting Information).The particle concentration of the resulting mixed NP dispersions was adjusted to 5 wt.% by the addition of water.The spray-drying parameters were kept constant for every experiment (inlet temperature: 75 °C, pump rate: 0.2 L h −1 , aspirator power: 85%, resulting in a volume flow of ≈ 33 m 3 h -1 (≈ 70 mbar), spraying-gas flow: 470 L h −1 , outlet temperature: ≈ 40 °C).The obtained SP powders were dried at 30 °C and 30 mbar in a vacuum oven (VO29, Memmert) for at least 8 h.
Material Characterization: DSC measurements were performed using a DSC 214 Polyma (Netzsch) on 5-15 mg of sample in the temperature range from 20 to 200 °C (250 °C for PSU) with a heating/cooling rate of 10 K min −1 in an N 2 atmosphere.DLS measurements were carried out with a Litesizer 500 (Anton Paar) on diluted and sonicated NP dispersions.SEM imaging analyses were conducted with a JSM-F100 (JEOL) at a working distance of roughly 6 mm.SPs were spread on carbon pads (Plano) and sputtered with Pt.NP dispersions were applied on Si wafers, dried, and sputtered with Pt.Morphological analyses were performed using a secondary electron detector and applying an acceleration voltage of 2 kV (field emission).Elemental contrast imaging was facilitated using a backscattering electron detector and applying an acceleration voltage of 5 kV.EDX spectroscopy analyses were performed with an acceleration voltage of 15 kV at a working distance of 10 mm in the same SEM device.Laser diffraction measurements were carried out with a Mastersizer 2000 (Malvern Panalytical).SP powders were dispersed in water under mechanical stirring and sonicated using a Hydro 2000S (Malvern Panalytical).TGA was carried out on 2-10 mg of a sample using a TG 209 F1 Libra (Netzsch) in a synthetic air atmosphere (O 2 : 10 mL min -1 , N 2 : 40 mL min −1 ) with a heating rate of 30 K min −1 in the temperature range from 30 to 1000 °C.Fluorescence emission and excitation spectra were recorded with a FP-8500 spectrofluorometer (Jasco) using constant experimental parameters (excitation bandwidth: 5 nm; emission bandwidth: 2.5 nm, response time: 0.2 s, scan speed: 500 nm min −1 , data interval: 0.1 nm).Emission spectra were recorded using an excitation wavelength of 365 nm.Excitation spectra were recorded by monitoring the emission intensity of the respective sample at one specific wavelength (displayed in the figure caption) and varying the excitation wavelength.Powder measurements were conducted with a powder sample cell PH-002 (quartz window size: 0.8 nm, Jasco).All recorded spectra were corrected using a calibrated WI light source ESC-842 (Jasco).Luminescence colors for emission spectra were calculated from spectrally corrected emission spectra using FWLU-879 measurement software (Jasco).Quantum yield measurements were conducted using an integrating sphere (ISF-824, Jasco) with an emission bandwidth of 5 nm on a freshly calibrated spectrometer.All fluorescence measurements were performed at almost constant climate conditions (≈ 25 °C, ≈30% relative humidity).Photographs of NPs and SPs were taken with an iPhone XR (2018) and an iPhone SE (2020), respectively, under UV light excitation ( exc = 365 nm, VL-215-LC UV lamp, Vilber Lourmat).Fluorescence microscopy analyses were performed on an inverted confocal microscope (ECLIPSE Ti, Nikon); the excitation source was a white Mercury lamp coupled with a set of colored bandpass filters to select the excitation band wavelengths.In all the experiments, the excitation was selected to be centered at 380 nm, with a 20 nm width band.The emission was cut by a dichroic mirror and a bandpass filter that has a wavelength cut at 400 nm.The light was focused on individual SPs and collected by an oil-immersion objective 100× with a numerical aperture of 1.4.The circular light spot has a diameter of roughly 30 μm.The sample preparation was done by spin coating a diluted SP suspension on a glass slide to space the individual SPs in the solid film by at least 30 μm from one another.Thereby, only one SP was excited during the measurement, and fluorescence spectra were recorded only from these isolated SPs.To record the fluorescence spectra, the emitted light was focalized on the slit of a Triax monochromator (Horiba Scientific), dispersed by 150 lines grating, and recorded by a cooled CMOS camera.
Temperature Experiments: All temperature experiments on SP samples were performed in a drying chamber (VO29, Memmert) by placing glass vials filled with 10-20 mg of SP powder on the preheated heating plate at the target temperature (50-200 °C, increments of 10-20 K) for the desired duration (e.g., 1-120 min).During the variation of the experienced maximum temperature and the exposure time, respectively, every SP sample was subjected to one temperature event with a fixed temperature or time.In contrast, during the simulation of an exemplary thermal history, the SP samples were repeatedly exposed to different temperature events.Importantly, fluorescence spectra were always recorded after the samples were cooled down to RT.
Statistical Analysis: Whenever mentioned, fluorescence spectra were averaged from three randomly picked powder samples of one SP batch and normalized to the emission of RITC at 585 nm.Temperature indicator signals SR TT1 and SR TT2 were displayed relative to their pristine signal as a function of the experienced maximum temperature or the expe-rienced temperature event.Data points and error bars represent the average and standard deviation of three randomly picked powder samples of one SP batch, respectively.Hydrodynamic particle size distributions obtained from DLS represent the average of three individual measurements from the same batch.The particle size distributions of SPs obtained determined via laser diffraction represent an average of five individual measurements of the same batch.Determination of particle sizes of NPs and SPs from SEM images was performed by measuring at least 50 individual particles.

Figure 1 .
Figure 1.Material design of ratiometric luminescent dual-threshold temperature indicator SPs with a multi-hierarchical architecture: a) selection of suitable luminescent dyes; b) hierarchy level I: creation of temperature indicator and reference emitter building blocks by incorporation of luminescent dyes into temperature-sensitive or -stable host matrices; c) hierarchy level II: forced assembly of the different NP types to SPs; d) intended functionality of these SPs, that show a two-step eye-readable color change caused by consecutive quenching of the blue and green emission bands, respectively, upon exposure to gradually increasing temperatures.
, Supporting Information).The formation of an intermixed, raspberry-like structure of the SPs is additionally supported by energy-dispersive Xray (EDX) spectroscopy analyses of model SPs with (partially)

Figure 2 .
Figure 2. a-c) SEM images of PS:C1 NPs (a), PSU:C6 NPs (b), and SiO 2 :RITC NPs (c); d-f) emission spectra normalized to their maximum and photographs ( exc = 365 nm) of an aqueous PS:C1 NP dispersion (d), an alkaline PSU:C6 NP dispersion (e), and a (dried) SiO 2 :RITC NP dispersion (f); g-i) SEM images of spray-dried SPs at different magnifications; j) emission spectrum normalized to the RITC-emission (585 nm) and photograph of pristine SP powder, spectrum represents the average of three randomly picked powder samples of one batch, ( exc = 365 nm); k) fluorescence microscopy image of a single isolated SP in the field of view, and l) fluorescence spectrum of a single isolated SP ( exc = 380±10 nm, normalized to the RITC-emission at 585 nm).

Figure 3 .
Figure 3. a) Normalized fluorescence emission spectra ( exc = 365 nm, normalized to the emission at 585 nm) recorded at RT after heating temperature indicator SPs for 30 min to different temperatures.Spectra represent the average of three randomly picked powder samples of the same SP batch.Red arrows indicate emission changes with increasing experienced temperature.b) Signal responses of the two temperature indicators relative to their pristine state as a function of the experienced temperature.Lines are a guide to the eye.Data points and error bars represent the average and standard deviation of three randomly picked powder samples of one SP batch, respectively.Photographs of pristine SP powder ( exc = 365 nm) as well as SP powders after exposure to 130 °C and 190 °C.

1 Figure 4 .
Figure 4. a-c) SEM images of pristine SPs (a), SPs after exposure to 120 °C for 30 min (b), and SPs after exposure to 200 °C for 30 min (c) at different magnifications: overview (1) and detail (2).Blue dotted circles indicate deformed polymer NPs and red solid circles mark temperature-induced voids in the SP architecture.

2 TFigure 5
Figure 5. a) Fluorescence spectra ( exc = 365 nm) of SPs consisting of PS:C1, PS, and SiO 2 and NPs (mass ratio: 5-5-90) recorded at RT after heating the SPs for 30 min to different temperatures.Red arrows indicate emission changes with increasing experienced temperature.b) Fluorescence spectra ( exc = 365 nm) of reference SPs consisting of PS:C1 and SiO 2 NPs (mass ratio: 10-90) and C1 deposited on SiO 2 :RITC NPs.c) Proposed mechanism of the polymer:C1 temperatures indicator NPs.d) Fluorescence spectra and photographs ( exc = 365 nm) of SP powders consisting of PS, PSU:C6, and SiO 2 NPs (mass ratio: 5-5-90) recorded at RT after heating the SPs for 30 min to different temperatures.e) Fluorescence spectra and photographs ( exc = 365 nm) of SP powders consisting of PS, PSU:C6, and SiO 2 NPs (mass ratio: 5-5-90) with and without the addition of H 2 SO 4 .f) Proposed mechanism of the polymer:C6 temperatures indicator NPs.Spectra in (a) and (d) represent the average of three randomly picked powder samples of one SP batch.The blue and green arrows in (b) and (c) indicate the shift of the coumarin emissions due to their modulated microenvironment.

Figure 6 .
Figure 6.Investigation of the luminescence signal characteristics of temperature indicator SPs across an exemplary thermal history, consisting of repeated exposure to different temperature events (1-8) with a duration of 30 min each: a) fluorescence spectra ( exc = 365 nm, normalized to the RITC-emission at 585 nm) recorded after each temperature event at RT. Spectra represent the average of three randomly picked powder samples of one SP batch.b) Change of the temperature indicator signals SR TT1 and SR TT2 relative to their pristine signal as a function of the experienced temperature event.Lines are a guide to the eye.Data points and error bars represent the average and standard deviation of three randomly picked powder samples of one SP batch, respectively.Red circles indicate the temperature of the respective event.The red dotted line displays the maximum temperature across the elapsed thermal history of the SPs.

Figure 7 .
Figure 7. a-b) Fluorescence spectra ( exc = 365 nm, normalized to the RITC-emission at 585 nm) of temperature indicator SPs as a function of exposure time at 100 °C (a) and 180 °C (b).Spectra were recorded after each temperature event at RT and represent the average of three randomly picked powder samples.c) Temperature indicator signals SR TT1 and SR TT2 relative to their pristine signal as a function of the exposure time at 100 °C and 180 °C.Lines are a guide to the eye.Data points and error bars represent the average and standard deviation of three randomly picked powder samples of one SP batch, respectively.

Figure 8 .
Figure 8. a) Photographs of PS:C1+PSU:C6+SiO 2 :RITC SP powders with varying concentrations of C1 in their pristine state and after exposure to 100 °C, 140 °C, and 200 °C for 30 min each.b) Relative change of SR TT1 of polymer:C1+PSU:C6+SiO 2 :RITC SPs with varying C1-hosting polymer NPs (PS:C1, PMMA:C1, and PSU:C1) compared to their pristine signal as a function of the experienced temperature.c) SR TT2 of PS:C1+polymer:C6+SiO 2 :RITC SPs with varying C6-hosting polymer NPs (PS:C6, PMMA:C6, and PSU:C6) compared to their pristine signal as a function of the experienced temperature.Lines are a guide to the eye.Data points and error bars represent the average and standard deviation of three randomly picked powder samples of one SP batch, respectively.