Synergistic Integration of Spin Crossover and Zinc Oxide in Transparent Films for Active Intelligent Packaging

Abstract

The development of multifunctional smart packaging materials capable of simultaneously monitoring temperature and suppressing microbial contamination is critical for next-generation food and pharmaceutical safety systems. In this study, we report the design and characterization of a polymeric film integrating a spin crossover (SCO)-based thermochromic sensor with zinc oxide (ZnO) nanoparticles serving as an antimicrobial agent. Beyond the individual functionalities, we demonstrate a synergistic effect between SCO and ZnO components. Notably, the SCO transition of the pristine SCO complex is broadened, and the hysteresis width of the transition is decreased (i.e., from 6 K to 1.5 K, 2 K, and 1.5 K for ZnO loading of 0.5%, 1%, and 2%, respectively), in the polysulfone–SCO–ZnO composites. Migration studies reveal that the co-existence of SCO and ZnO does not disrupt the low release profile of active agents, which remains low across ZnO loadings. The polymeric film exhibited dose-dependent antiproliferative activity against MCF-7 breast cancer cells, with a significant reduction in cell viability observed only at the highest tested concentration, indicating cytotoxic potential. This multifunctional platform represents a promising advancement in smart packaging design, enabling real-time thermal indication combined with the integration of ZnO as a literature-established antimicrobial component, within a non-toxic, and visually transparent system. Collectively, the material’s properties offer promising scalability for both food and pharmaceutical packaging applications where visual clarity, antimicrobial integrity, and temperature monitoring are imperative.

1. Introduction

The rapid evolution of packaging technologies in the food and pharmaceutical sectors is increasingly driven by the demand for intelligent, multifunctional systems capable of actively monitoring and enhancing product quality and safety. Traditional passive materials are now being supplanted by advanced packaging designs that integrate sensors, indicators, and antimicrobial agents to interact with both internal and external environmental cues [1,2,3].

Temperature monitoring in packaging has been addressed through various approaches, from irreversible, color-changing ink indicators (time–temperature labels) to electronic sensors. However, many of these systems depend on external readout devices, are relatively costly, or suffer from limited reliability when exposed to fluctuating storage conditions such as variable temperatures or humidity [4,5]. Coordination complexes constitute a versatile class of molecular materials whose tunable optical, magnetic, electronic, and catalytic properties have enabled their integration into diverse technological fields, including sensing, data storage, photonics, and smart packaging, and their performance is often enhanced when embedded within polymeric hosts, conductive polymers, epoxy resins, or hybrid composites that provide structural stability and improved processability [5,6,7,8]. Spin crossover (SCO) complexes, most commonly based on Fe(II), are coordination compounds that undergo a thermally driven, reversible transition between low-spin and high-spin electronic states, a switch that is accompanied by marked changes in optical (thermochromic), magnetic, and structural properties [9,10,11,12,13,14,15,16,17]. This bistability and tunability, achieved by ligand design, counterion choice, and crystal packing, makes SCO materials highly attractive for passive, electricity-free sensing platforms such as temperature and pressure sensors. Importantly, SCO compounds are intrinsically multifunctional materials, in which the thermally driven spin transition is accompanied not only by color changes, but also by pronounced variations in magnetic susceptibility, lattice parameters, birefringence, dielectric response, and even morphology. These coupled responses open additional opportunities for multimodal sensing, where a single SCO-based material could simultaneously provide optical, magnetic, or mechanical readouts of environmental stimuli highlighting their potential for advanced materials that go beyond single-parameter temperature indication [18]. In this frame, beyond their fundamental magnetic behavior, SCO compounds have been increasingly explored as active elements in thermochromic sensing systems, where the reversible optical changes associated with the spin transition can serve as a readout for temperature variation [18]. Indeed, recent demonstrations embed SCO complexes in flexible polymer matrices to build thermochromic sensors that track temperature via visible color change [17,19]. For instance, SCO-based hybrid films have been developed as thermochromic sensors, tracking thermal variations via color changes in polymers using digital imaging and signal processing approaches [20]. Additionally, organic materials incorporating SCO complexes have been reviewed in the context of time–temperature integrator (TTI) devices, where thermally dependent optical signatures enable monitoring of thermal exposure over time [18,21]. On the industrial side, companies exploit leveraging molecular-engineering of smart pigments and coatings to create bespoke thermochromic materials, illustrating that SCO-based systems are transitioning from academic curiosities into commercially viable intelligent packaging solutions [22]. Nonetheless, reliably tuning SCO behavior remains nontrivial. The spin-transition temperature T_1/2, abruptness, and hysteresis width are highly sensitive to crystal packing and intermolecular elastic interactions, because the volume and shape changes between low-spin and high-spin states induce strain that couples spin centers cooperatively. Furthermore, elastic frustration and competing ferro– and antiferro–elastic interactions in the lattice can lead to multi-step or incomplete transitions, complicating the predictability of switching behavior, while long-term stability is also an issue, since repeated thermal cycling may weaken cooperativity and shrink hysteresis due to structural fatigue [23,24]. Our previous study demonstrated that the spin crossover behavior of Fe(II)-based complexes, particularly [Fe(1,10-phenanthroline)₂(NCS)₂], can be effectively modulated through incorporation into chemically distinct polymer matrices such as polylactic acid, polystyrene, and polysulfone [25]. Composites of polysulfone (PSF) with the SCO complex [Fe(1,10-phenanthroline)₂(NCS)₂], revealed a broadened transition in the composite of PSF-SCO indicating a dispersion of local transition temperatures. The study demonstrated that the polymer matrix exerted active influence on the properties of SCO compounds, affecting both their thermodynamic and kinetic behavior. PSF/SCO films exhibited gradual switching, highlighting the matrix’s role in confining the particles and stabilizing spin states. This unorthodox approach seems to be promising for SCO regulation and confinement.

In parallel, in materials science, the integration of two distinct functionalities within a single platform presents significant challenges, as the chemical, structural, and interfacial requirements necessary to optimize one function can interfere with or degrade the other, leading to compromised performance or emergent undesired behavior. The coexistence of two active components often introduces complex coupling phenomena, such as strain transfer, electronic perturbation, or local chemical environment modifications, which can alter the intrinsic properties of each constituent. When one of the functionalities is based on SCO, the system becomes particularly intricate, yet offers unique opportunities, because SCO involves a reversible, thermally or optically induced electronic transition (low-spin ↔ high-spin). Cutting-edge studies exemplify that in flexible P(VDF-TrFE) polymer composites containing SCO nanoparticles, the spin transition induces measurable strain (~1%) and significant changes in dielectric permittivity (~40%), demonstrating strong electromechanical coupling that simultaneously affects the SCO behavior [26]. In bilayer heterostructures, silica-coated SCO nanoparticles mechanically coupled to an organic semiconductor (P3HT), showing that spin switching altered electrical conductivity, which is important for memory and sensing devices [27]. Therefore, it is emphasized that dual-functional SCO/polymer systems not only offer the prospect of emergent multifunctionality, but also necessitate careful design and processing to maintain the thermodynamic integrity of both functionalities and exploit their synergistic potential. A particularly promising class of bifunctional materials for applications in food and pharmaceutical packaging, as well as medical materials and surfaces, is those capable of simultaneously sensing temperature changes and exhibiting antimicrobial activity, two functions that are rarely integrated effectively within a single, safe, and scalable platform [28,29,30,31]. Existing platforms such as self-healing AgNP hydrogels or PVA/HPMC–borate hydrogels with in situ incorporation of AgNPs that successfully integrate both functions, are typically quite complex, highlighting the synthetic and design complexity of combining reliable temperature responsiveness, and sustained antimicrobial performance in one scalable platform. Zinc oxide (ZnO) nanoparticles have emerged as one of the most promising candidates for antimicrobial function. ZnO is not only cost-effective and chemically stable, but it also exhibits potent antibacterial activity against both Gram-positive and Gram-negative bacteria through a multifaceted mode of action including the generation of reactive oxygen species (ROS), membrane disruption, and ion release. The antibacterial activity of ZnO nanoparticles originates from a combination of complementary physicochemical mechanisms that act synergistically against a broad spectrum of microorganisms. One of the primary mechanisms involves the generation of reactive oxygen species (ROS), including hydroxyl radicals and superoxide anions, which induce oxidative stress and lead to lipid peroxidation, protein dysfunction, and DNA damage in bacterial cells [32,33,34]. In addition, direct contact between ZnO nanoparticles and bacterial cell membranes can cause structural damage and increased membrane permeability, ultimately resulting in cell lysis [33]. The gradual release of Zn²⁺ ions further contributes to antibacterial activity by interfering with intracellular enzymatic processes and metabolic pathways [34]. The relative contribution of these mechanisms depends strongly on nanoparticle size, dispersion, surface chemistry, and interaction with the surrounding polymer matrix, emphasizing the importance of controlled nanoparticle immobilization in antimicrobial packaging materials to balance efficacy and safety. In the context of food and pharmaceutical packaging, ZnO nanoparticles have been widely investigated as antimicrobial fillers in polymeric matrices due to their broad-spectrum antibacterial activity, chemical stability, and compatibility with industrial processing. ZnO-based nanocomposite films have been successfully developed using both synthetic and bio-based polymers, including polyethylene, polypropylene, polylactic acid, starch, and chitosan, demonstrating enhanced antimicrobial performance while preserving mechanical strength, gas-barrier properties, and optical transparency [32,35,36,37]. Several studies report that ZnO-containing packaging materials can effectively prolong the shelf life of perishable foods by suppressing microbial growth under real storage conditions. Furthermore, ZnO is generally recognized as safe (GRAS) for food contact by the U.S. Food and Drug Administration, facilitating its regulatory acceptance. Despite these advantages, the practical use of ZnO in packaging films faces a well-documented issue which is related to nanoparticle agglomeration. As ZnO loading increases, particle–particle interactions often lead to clustering within the film matrix, resulting in turbidity, uneven distribution, and reduced antimicrobial efficacy [35,38]. Moreover, agglomeration can compromise the aesthetic transparency of the film, a critical feature when integrating visual sensors. Agglomeration also alters the surface area and diffusivity of the nanoparticles, which may lead to uncontrolled high migration of Zn²⁺ ions into food or pharmaceutical products, potentially exceeding permissible limits and raising toxicological concerns [39,40].

In the present study, we report a novel bifunctional packaging film that simultaneously incorporates a SCO-based thermochromic indicator and the antimicrobial ZnO nanoparticles into a single polymer matrix. The previously demonstrated strong interfacial interaction of [Fe(1,10-phenanthroline)₂(NCS)₂] with PSF, leading to preserved spin-crossover behavior and exceptionally low migration levels when embedded in PSF, is highly desirable for applications where chemical stability and minimal release of active components are essential. Importantly, this well-characterized SCO–polymer system provides a reliable platform to investigate how the co-presence of ZnO nanoparticles influences SCO behavior, migration, and interfacial stability without introducing additional compatibility constraints. The co-integration of the SCO compound showed that it plays a stabilizing role in ZnO nanoparticle dispersion. Films containing both components exhibit a high degree of optical transparency, unlike those containing ZnO alone, which display characteristic blurriness due to particle aggregation. This unexpected finding suggests a molecular or interfacial interaction between the SCO complex and ZnO that suppresses agglomeration, possibly via electrostatic stabilization, or alteration of the polymer–particle interfacial energy [41,42]. This is important as enhanced dispersion not only improves visual clarity, but also likely contributes to more uniform antimicrobial activity and better-controlled migration behavior. In parallel, in the context of this study the SCO thermo-response was evaluated in the co-presence of ZnO. In addition, the migration profile of both SCO and ZnO was investigated with two-fold scope, to determine whether the active agents leach easily into an alcoholic solution and, secondly, to examine if the co-presence of both materials affects the migration release profile compared to polymer films that contain the active agents individually. ZnO is well-documented for antimicrobial performance in packaging systems (direct antimicrobial assays are beyond the scope of this study and are not reported herein). Nevertheless, the cytotoxic profile of these bifunctional materials was assessed to ensure food safety regulation compliance. In addition, the viability of hormone-dependent breast cancer cells was evaluated to determine the potential antiproliferative activity of these materials. This work represents a step forward in the development of multifunctional films that are not only active (via antimicrobial agents) and intelligent (via thermal indicators), but also environmentally safe, with cytotoxic potential.

2. Materials and Methods

2.1. Materials and Procedures

The SCO complex with the molecular formula [Fe(1,10-phenanthroline)₂(NCS)₂] (1,10-phenanthroline, C₁₂H₈N₂) was synthesized following adapted protocols from the literature [43] and as described in our previous study [25]. For the preparation of the SCO/polymer, ZnO/polymer, and SCO/ZnO composite films, polysulfone (PSF, CAS: 25135-51-7, Sigma-Aldrich, Merck Life Science B.V., Darmstadt, Germany) was used as the polymer matrix and ZnO nanoparticles (NPs) (nanopowder, <100 nm, CAS: 544906, Sigma-Aldrich) as the antimicrobial agent. The films were fabricated using a standard solution casting method with dichloromethane (CH₂Cl₂, CAS: 75-09-2, Scharlau, Barcelona, Spain) as the solvent (Figure 1). Specifically, 0.3 g of each polymer was dissolved in 10 mL of CH₂Cl₂ under magnetic stirring. To prepare the composite materials the following amounts were used: (A) PSF/SCO film (code name: PSF/SCO): 6 mg of the synthesized SCO compound was added to the respective polymer solution after complete dissolution, (B) PSF/ZnO film (code name: PSF/ZnO): 1.5 mg of ZnO NPs was added to the respective polymer solution after complete dissolution, and (C) PSF/SCO/ZnO films: 1.5 mg SCO was added to the respective polymer solution after complete dissolution, while varying amounts of ZnO were added to isolate the three different films with varying loadings of ZnO i.e., 1.5 mg (code name: PSF/SCO/ZnO0.5%), 3 mg (code name: PSF/SCO/ZnO1%), and 6 mg (code name: PSF/SCO/ZnO2%). The resulting mixtures were cast into Pyrex petri dishes (Metrolab, Athens, Greece) of 5 cm diameter. Upon solvent evaporation, in the case of PSF/SCO and PSF/SCO/ZnO, thin films with a light red color and a thickness ranging from 100 to 120 μm were obtained. In the case of PSF/ZnO a non-transparent whitish film was isolated after film casting.

2.2. Experimental Techniques

The morphological features of the polymeric composites before and after the migration release experiments were examined by scanning electron microscopy (SEM). SEM observations were carried out using a Zeiss ZUPRA 35 VP-FEG microscope (Zeiss, Jena, Germany) operating at accelerating voltages between 5 and 20 keV. The instrument was equipped with an energy-dispersive X-ray spectroscopy (EDX) system (Bruker GmbH, Berlin, Germany, Quanta 200) and a backscattered electron (BSE) detector (K E Developments Ltd., Cambridge, UK), enabling elemental composition analysis.

Attenuated total reflectance (ATR) spectra of the SCO complex, pristine polymeric films, and SCO-loaded polymer composites were collected using a Bruker Optics Alpha-P Diamond ATR spectrometer (Bruker Optics GmbH, Ettlingen, Germany).

Migration experiments were conducted by immersing film specimens of defined dimensions in 5 mL of 20% (v/v) ethanol solution within sealed vials. The vials were subsequently maintained in a temperature-controlled incubator. At predetermined time intervals, aliquots of the simulant were withdrawn for analysis to monitor the release of migrated species. Following each measurement, the sampled solution was returned to the respective vial to maintain constant volume. The sampling period extended from 1 to 30 days. The presence of migrated compounds in the simulant was evaluated using UV–Vis spectroscopy (Shimadzu UV-1900, Shimadzu, Kyoto, Japan) over a wavelength range of 200–800 nm with quartz cuvettes, as well as by photoluminescence (PL) spectroscopy employing a Hitachi F2500 spectrometer (Hitachi, Kyoto, Japan) with an excitation wavelength of 325 nm. Quantitative analysis for both methods was performed using calibration curves (Figures S3 and S4).

DC magnetic susceptibility measurements were performed on solid samples packed in gelatin capsules using a Quantum Design 7 T MPMS XL SQUID magnetometer (Quantum Design, San Diego, CA, USA). Magnetization was recorded as a function of temperature over the range of 300 to 100 K under an applied magnetic field of 1000 Oe, in both cooling and heating modes. Diamagnetic corrections were applied to correct for the inherent diamagnetism of the gelatin capsule and the samples using Pascal’s constants [44].

2.3. In Vitro Cell Viability Assessment

• Biochemicals and reagents

Dulbecco’s modified essential medium (DMEM), fetal bovine serum (FBS), L-glutamine, penicillin, streptomycin, amphotericin B, and gentamycin were all obtained from Biosera Ltd. (Cholet, France). All other chemicals used were of the best commercially available grade.

• Cell cultures and conditions

MCF-7 human breast adenocarcinoma cell line was obtained from the American Type Culture Collection (ATCC, HTB-22, Baltimore, MD, USA) and routinely cultured as monolayers at 37 °C in a humidified atmosphere of 5% (v/v) CO₂ and 95% air. Cells were grown in complete DMEM culture medium supplemented with 10% FBS, a cocktail of antimicrobial agents (100 IU/mL penicillin, 100 mg/mL streptomycin, 10 mg/mL gentamicin sulphate, and 2.5 mg/mL amphotericin B) and 2 mM L-glutamine (Biosera, France). Cells were harvested by trypsinization with 0.05% (w/v) trypsin in PBS containing 0.02% (w/v) Na₂EDTA. All experiments were conducted in serum-free conditions (0% FBS), to eliminate the net effects of serum. The tested compounds were dissolved in 100% DCM at a 1 mg/mL concentration of each stock solution and the serial dilutions for final concentrations 1, 10, and 50 μg/mL, according to each experimental design, were performed in DMEM 0% FBS. Solvent controls were also run with each tissue and cell line experiment. Cells were treated with the film and its substitutes for 24 h prior to each experimental procedure. To ensure experimental consistency, control samples containing the same DCM concentrations as the tested compounds were included. Mean cell viability was expressed as percent of residual metabolic activity relative to the solvent controls. The highest DCM concentration used did not exert any significant effect on cell viability.

• WST-1 cell viability assay

MCF-7 cells were seeded in 96-well plates at a density of 7500 cells per well in complete medium. Cells were incubated in complete medium (DMEM, 10% FBS) for 24 h and then the medium was changed to serum-free (DMEM, 0% FBS), and the cells were serum-starved overnight prior to treatment with the tested compounds for 24 h in the concentration range 1–50 μg/mL. To assess cell viability, WST-1 premix reagent was added in each well at 1:10 ratio, mixed gently by pipetting, and incubated for 1 h at 37 °C. The optical absorbance at 450 nm was measured (reference wavelength at 650 nm), using a TECAN Infinite M200 microplate reader (TECAN, Geumcheon District, Seoul, Republic of Korea), according to the manufacturer’s instructions. All experiments were performed in triplicate and repeated independently at least three times.

• Statistical analysis

Data in diagrams are expressed as mean ± standard deviation (SD). Statistical analyses and graphs were made using GraphPad Prism 9 (GraphPad Software, San Diego, CA, USA). Statistically significant differences are indicated by asterisks: * (p < 0.05), compared with untreated (control) cells. Non-statistically significant comparisons (p > 0.05) are not displayed.

3. Results and Discussion

3.1. Characterization of Polymeric Composites

The IR spectrum of the SCO complex is composed of the vibrational modes related to the Fe–N bonds, the 1,10-phenanthroline (phen) ligands, and the thiocyanate (NCS⁻) anions. These vibrational modes are shuffled in three main regions in the low frequency range 90–300 cm⁻¹ where the Fe–N vibrational modes attributed to breathing-type and bending modes of the FeN₆ octahedral core are typically observed: the 400–1600 cm⁻¹ region, associated with C–C and C–N stretching modes within the aromatic rings of the phen, and the stretching vibrations in the region of 2056–2076 cm⁻¹, corresponding to the SCN group [45]. Subsequently, films of the PSF, PSF/SCO, PSF/ZnO, and PSF/SCO/ZnO (0.5, 1, and 2%) films, were characterized by ATR/FTIR spectroscopy (Figure S5). The PSF spectrum displays prominent absorption bands within the 1100–1300 cm⁻¹ range, attributed to both symmetric and asymmetric stretching vibrations of sulfone groups (S=O and S–O). Vibrations associated with aromatic C–C and C–H bonds are detected in the 1450–1600 cm⁻¹ region, while aliphatic CH stretching vibrations occur between 2850 and 3000 cm⁻¹. Additionally, out-of-plane bending of aromatic C–H bonds is evident in the 600–900 cm⁻¹ range [46]. In the PSF/SCO and PSF/SCO/ZnO spectra a weak band near ~2050 cm⁻¹ is noticed, attributed to the SCO compound.

To investigate potential morphological changes and the distribution of the SCO compound within the polymer matrices, cross-sectional SEM and EDX mapping was conducted (Figure 2 and Figure S1). The PSF/ZnO films displayed a uniform dispersion of ZnO nanoparticles throughout the polymer matrix (the fine distribution of SCO in PSF is documented in the previous study [25]). The PSF/SCO/ZnO hybrid revealed both the SCO domains and ZnO particles homogeneously distributed across the film thickness, confirming the effectiveness of the processing method in preventing agglomeration.

3.2. The SCO Behavior in the Polymeric SCO-ZnO Hybrids

To evaluate the thermal response of the composite, we examined the SCO behavior of the PSF/SCO/ZnO system through temperature-dependent magnetic measurements (Figure 3). The χ_MT(T) curve of the PSF/SCO/ZnO composite reveals a transition profile that remains markedly broad and gradual, closely resembling that of PSF/SCO and clearly distinct from the sharper switching of the pristine SCO material; the PSF/ZnO film is purely diamagnetic, as anticipated. The preservation of this flattened transition, indicates that the introduction of ZnO does not restore cooperativity nor narrow the distribution of local spin-transition temperatures imposed by the PSF matrix. Instead, the most evident change introduced by ZnO is the further reduction in the hysteresis width, which decreases from approximately 6 K in PSF/SCO to about 2 K in PSF/SCO/ZnO (this is further slightly decreased to 1.5 K at higher and lower ZnO loading, i.e., 0.5, or 2%, as shown in Figure S2), while the midpoint temperatures remain essentially unchanged. This selective narrowing of the hysteresis, in the absence of any sharpening of the transition, strongly suggests that the ZnO nanoparticles exert an additional mechanical or interfacial decoupling effect.

Rigid inclusions dispersed within PSF can perturb the long-range elastic interactions that normally contribute to the collective bistability of SCO particles, thereby decreasing the energetic separation between the forward and reverse transitions, without altering the heterogeneous microenvironments that determine local T_1/2 values. In this sense, the PSF/SCO/ZnO behavior aligns naturally with the interpretation established for PSF/SCO in the earlier study [25], where the broadening of the transition was attributed to a distribution of transition temperatures arising from matrix-induced variations in confinement, stress, and interfacial contact. The new results show that ZnO does not modify this distribution, but rather fine-tunes the global cooperative balance, yielding a composite in which the SCO transition remains dispersed but the hysteresis loop is mechanically softened.

3.3. Migration Release of the SCO in the Polymeric Composites

To investigate the potential release of functional agents from the developed PSF-based composite films, a systematic migration study was conducted using 20% ethanol which provides a feedback on release safety (20% ethanol is important in packaging materials in food industry and medical applications) [25]. This study encompassed films of pure PSF, PSF containing only SCO compounds, PSF containing ZnO nanoparticles, and PSF/SCO/ZnO hybrids with varied ZnO loadings (0.5%, 1%, and 2%). The dual aim was to (i) assess whether any detectable migration of active components occurred and (ii) determine how the coexistence of SCO and ZnO influences release behavior.

Migration was quantified through both UV–/Vis spectroscopy and PL spectroscopy to ensure reliability. The SCO compound exhibited characteristic absorbance peaks at 266 nm and 509 nm in UV–/Vis, and a PL emission peak at 511 nm, consistent with Fe(II)-based SCO complexes previously reported in literature [47]. Meanwhile, ZnO nanoparticles showed a distinct absorbance peak at 375 nm in UV–/Vis and a PL peak at 388 nm, aligning with known electronic transitions and band-edge emissions of ZnO [48].

The results from both analytical techniques are in agreement (Figure 4, Figure 5, Figures S6 and S7). In the case of PL analysis (Figure 4 and Figure S6), migration from the PSF/SCO system was negligible (release percentage 1.1%), indicating strong interactions between the SCO complex and the PSF matrix which is consistent with earlier findings [25]. In parallel, PSF/ZnO films exhibited a slightly higher migration (release percentage 1.7%), attributable to partial Zn²⁺ leaching, likely caused by ZnO agglomeration, which enhances localized diffusivity. Notably, the ternary systems containing both SCO and ZnO displayed an interesting migration behavior. At 0.5% ZnO loading, a marginal reduction in migration was observed compared to PSF/ZnO alone or PSF/SCO films. In particular, the maximum migration of SCO observed was 0.8%, 0.6%, and 0.9%, for the PSF/SCO/ZnO hybrids with varied ZnO loadings of 0.5%, 1%, and 2%, respectively. In parallel, the corresponding release of ZnO from these three different films was similar and, specifically, 0.7%, 0.6%, and 0.85%, respectively. These values are at very close proximity with UV/Vis analysis (Figure 5 and Figure S7). The lowest migration occurred at 1% ZnO loading, where both UV–/Vis and PL data indicated minimal detectable release of either component, possibly due to improved ZnO dispersion mediated by the SCO compound, which may act as a steric or electrostatic stabilizer. At 2% ZnO, migration increased slightly again, likely due to renewed nanoparticle agglomeration exceeding the stabilizing capacity of the SCO. It is worth noting that this may be corroborated by visual observations since films containing both SCO and ZnO exhibited uniform, transparent red films, while PSF/ZnO film was visibly white and blurry, a sign of nanoparticle aggregation. Nevertheless, it should be noted that in all systems, a very low migration release was achieved and therefore generic conclusions may be insecure. It is also evident though, that the migration trends correlate well with the magnetic findings discussed earlier. The very low release of the SCO component in the PSF/SCO/ZnO hybrid is consistent with the χ_MT(T) results, showing that the SCO particles experience a strongly confining and heterogeneous microenvironment within the PSF matrix, which stabilizes their local positioning and limits their mobility. Likewise, the modest and composition-insensitive ZnO release aligns with the magnetic observation that the addition of ZnO does not disrupt the broad distribution of transition temperatures already imposed by the polymer. Instead, ZnO primarily alters long-range mechanical coupling rather than the local embedding of SCO particles. The slight decrease in migration at 0.5% ZnO loading mirrors the reduced hysteresis width seen magnetically: in both cases, the presence of dispersed ZnO domains appears to reinforce the structural rigidity of the composite without modifying the local environments of the SCO particles. Thus, the migration study confirms the chemical stability of the functional components within the PSF matrix and also reinforces the mechanistic interpretation derived from magnetic measurements, that ZnO acts as a mechanical modifier that influences collective elastic interactions while leaving the microenvironment-driven confinement of the SCO entities largely unchanged. Such mutual interactions between active agents are rarely explored in multifunctional film design but are critical to understanding how co-functionalization affects safety, efficacy, and regulatory compliance [49,50].

3.4. Post-Migration Structural and Morphological Analysis

To evaluate the structural integrity and morphological stability of the polymeric films following exposure to ethanol-containing media, ATR/FTIR spectroscopy was conducted prior to and after the migration experiments (Figure 6), and SEM analyses were performed (Figure S8). Comparison of the ATR/FTIR spectra of both neat and composite films before and after immersion in 20% ethanol showed no notable alterations. To further assess possible morphological modifications and the dispersion of SCO and ZnO within the polymer matrix, cross-sectional cryo-SEM images were obtained before and after immersion. No discernible changes in morphology or composition were observed, confirming the strong physical entrapment of the guest compounds within the polymer structure.

3.5. Cytotoxicity Assessment

At a next level, we assessed the potential antiproliferative effects of the polymeric film and its individual components, SCO and ZnO, on the growth of the MCF-7 breast cancer cell line. MCF-7 cells were exposed for 24 h to increasing concentrations (1–50 μg/mL, in 10 μg/mL increments; selected concentrations are shown in Figure 7) to evaluate the potential cytotoxicity of the polymeric film and its loadings, as well as to determine any effective antiproliferative concentration. As shown in Figure 7, no significant cytotoxicity was detected in the 1–50 μg/mL range for the ZnO–SCO film; however, at the highest concentration (50 μg/mL), the film induced a statistically significant reduction (~30%) in MCF-7 cell viability. Notably, treatment with ZnO or SCO alone for 24 h did not significantly affect cell viability. Taken together, these findings indicate that the polymeric film exhibits selective cytotoxic activity and potential antiproliferative effects against hormone-dependent MCF-7 breast cancer cells.

4. Conclusions

This study developed multifunctional PSF-based films incorporating a spin crossover (SCO) thermochromic compound and ZnO nanoparticles (at different loadings, i.e., 0.5, 1, and 2%), targeting smart packaging applications that require simultaneous temperature indication and antimicrobial protection. Magnetic measurements revealed that the coexistence of SCO and ZnO preserves the broad, distributed spin transition characteristic of SCO in PSF, while selectively reducing the hysteresis width (i.e., from 6 K to 1.5 K, 2 K, and 1.5 K for ZnO loading 0.5%, 1%, and 2%, respectively), an effect attributed to ZnO-induced modulation of long-range elastic interactions without disturbing the strong microenvironmental confinement of the SCO particles. This structural stabilization is directly reflected in the migration tests in 20% ethanol: both UV/Vis and photoluminescence analyses confirmed very low migration consistent with enhanced particle immobilization shown by the magnetic data. The increase in ZnO loading from 0.5% to 2%, slightly modulated migration behavior, with the 1 wt% ZnO composite exhibiting the most favorable balance. Complementary SEM and EDX analyses verified uniform dispersion of ZnO, and of both ZnO and SCO in the ternary films. Moreover, the cytotoxicity evaluation proposes an alignment with safety requirements, and the observed reduction in MCF-7 cell viability at higher exposure levels highlights an additional, antiproliferative activity that may extend the utility of these materials beyond sensing and established antimicrobial functions. Notably, future studies will focus on the biomaterial-based applications such as the controlled delivery of anti-cancer drugs, enabled by dedicated cell-based experimental systems. Altogether, the combination of reliable thermal signaling, improved dispersion, reduced migration, and bioactive potential positions these PSF/SCO/ZnO composites as promising multifunctional candidates for advanced food and pharmaceutical packaging systems.