Room Temperature Phosphorescence from Natural, Organic Emitters and Their Application in Industrially Compostable Programmable Luminescent Tags

Organic semiconductors provide the potential of biodegradable technologies, but prototypes do only rarely exist. Transparent, ultrathin programmable luminescent tags (PLTs) are presented for minimalistic yet efficient information storage that are fully made from biodegradable or at least industrially compostable, ready‐to‐use materials (bioPLTs). As natural emitters, the quinoline alkaloids show sufficient room temperature phosphorescence when being embedded in polymer matrices with cinchonine exhibiting superior performance. Polylactic acid provides a solution for both the matrix material and the flexible substrate. Room temperature phosphorescence can be locally controlled by the oxygen concentration in the film by using Exceval as additional oxygen blocking layers. These bioPLTs exhibit all function‐defining characteristics also found in their regular nonenvironmentally degradable analogs and, additionally, provide a simplified, high‐contrast readout under continuous‐wave illumination as a consequence of the unique luminescence properties of the natural emitter cinchonine. Limitations for flexible devices arise from limited thermal stability of the polylactic acid foil used as substrate allowing only for one writing cycle and preventing an annealing step during fabrication. Few‐cycle reprogramming is possible when using the architecture of the bioPLTs on regular quartz substrates. This work realizes the versatile platform of PLTs with less harmful materials offering more sustainable use in future.


Introduction
Materials showing persistent luminescence, characterized by extended excited state decay times, have gained much attention recently.3][4][5][6][7][8] The luminescent properties of purely organic materials arise from their unique molecular structures and electronic configurations.In particular, the spin 1 triplet exciton configuration leads to phosphorescence that is as a quantum-mechanically forbidden transition much slower compared to the nanosecond-range allowed fluorescence from spin 0 singlet state. [9]n addition to their optical properties, purely organic luminescent materials offer advantages such as solution processability, mechanical flexibility, and compatibility with various substrates.These characteristics make them suitable for large-scale manufacturing processes and integration into diverse photonic and optoelectronic devices-all which can potentially be made mechanically flexible.
Recently, we reported a novel photonic device architecture based on organic functional materials called programmable luminescent tags (PLTs) that is well suited for various labelling and information exchange applications. [10,11]The high-resolution information inscribed in the PLTs provides high contrast even under continuous illumination.Information can also be erased and rewritten multiple times.PLTs are made from wet processing and can be printed onto any substrate and in large quantities.The design of PLTs is relatively simple allowing for low-cost production since it contains only readily available materials.
Its layered structure is schematically depicted in Figure 1a.The active layer consists of an organic emitter molecule (e.g., NPB, [9] TCATPB, [12] BP-2TA, [10] or BDPB [13] ) showing substantial emission by room-temperature phosphorescence (RTP) when embedded in a polymer host matrix, such as poly(methyl methacrylate) (PMMA) or polystyrene (PS).The matrix rigidifies the emitter leading to the reduction of non-radiative decay channels.The active layer is applied onto a transparent substrate (quartz or PMMA foil) by spin-coating.The RTP intensity correlates with the concentration of molecular oxygen in the film (Figure 1b) because of oxygen-mediated quenching as depicted in Figure 1c.A switching between high O 2 concentration connected to no RTP and low O 2 concentration resulting in significant RTP is possible by UV-light driven consumption of the excited singlet oxygen through a chemical reaction with the local environment (photoconsumption).Therefore, by adding an oxygen-barrier layer (Exceval, an ethylene vinyl alcohol copolymer), the reflux of molecular oxygen can be blocked.In contrast, without the oxygenbarrier layer, RTP is not observed under standard illumination conditions, indicating that the rate of photoconsumption of oxygen must be lower than the rate of oxygen resupply.Illuminating the stack with UV light through a shadow mask, oxygen is consumed only locally and information can be stored within the PLT.Readout is possible through RTP by the emitter molecules in the oxygen-depleted regions.When heated by noncontact (infrared irradiation) or contact (e.g., hot plate) methods, Exceval becomes permeable for oxygen from the surrounding ambient atmosphere leading to the erasure of the written information.As Exceval has a finite oxygen transmission rate, the thickness has direct influence on the image/information retention-the thicker, the longer-of the PLTs.Thus, PLTs can be used for several writing and erasing cycles, as demonstrated and discussed in detail in our earlier work. [10,11]Accordingly, they become flexible, ultrathin, fully transparent, reusable devices for information storage.
Although PLTs represent already a minimalistic yet powerful technology, their application, e.g., in logistics or food labeling, would strongly benefit from a biodegradable, nonhazardous design.Current materials for substrate, host, and emitter can persist in the environment for hundreds of years and cause significant harm to wildlife and ecosystems. [14]17][18][19][20][21][22] Those can be degraded by microorganisms and enzymes, resulting in the formation of carbon dioxide, water, and other nonhazardous natural substances.These materials can be made from a variety of sources, including plant-based materials and biopolymers.There exist several types of polymers that are considered to be biodegradable, among those polylactic acid (PLA), polyhydroxyalkanoates (PHAs), and cellulose derivatives.][34][35] In our earlier PLTs reports, only Exceval already presents a biodegradable solution.
Here, we provide a prototypical design for flexible PLTs completely made from biodegradable or at least industrially compostable materials, so-called bioPLTs.As RTP emitters, we test the quinoline alkaloids quinine, quinidine, cinchonine, and cinchonidine (cf. Figure 3a)), which all perform sufficiently well.PLA can be used as both host and substrate material.Extensive photophysical characterization of the materials and PLTs demonstrate promising performance of the bioPLTs for future ecofriendly information storage, e.g., for food labelling as illustrated in Figure 2 for a banana.

Results and Discussion
To achieve a completely biodegradable PLT, the polymer host, the organic emitter, as well as the substrate need to be exchanged.Previously, we already defined criteria for industrially relevant performance. [10]The polymer host should exhibit an amorphous and homogeneous morphology, be transparent (also with respect to the excitation of the emitter), capable of storing molecular oxygen and sensitive to reaction with singlet oxygen for reduced decomposition of the emitter molecules, as well as show only weak oxygen diffusion.For the emitter, a high RTP quantum yield, ideally low fluorescence, and high sensitivity to oxygen-mediated quenching are required.All materials should have the potential to economically be scaled or-in case of natural materials-sourced and easy to process from solution.In addition to the selection of sufficient materials, it is unclear if the established PLT design depicted in Figure 1a requires modifications.Figure 3 shows the final solution for the bioPLT.The individual material and design choices and its performance are discussed in the following sections.

PLA as Host Material
PLA, a thermoplastic derived from renewable resources such as corn starch or sugarcane, is one of the most widely analyzed ) zoomed in section, where the information is written using a shadow-mask and a UV-LED ( exc = 275 nm).Mind that application of the bioPLT does not cause any defect to the shell or the banana in general, which would affect its durability.c) Activated bioPLT under continuous wave illumination showing high contrast due to the low fluorescence of cinchonine, which was used as natural emitter.d) Persistent luminescence after the UV-LED was switched off.A phosphorescence lifetime of 300 ms allows easy readout.The quality of the writing process could be further improved by using a flexible mask or a focused UV-laser.
types of biodegradable materials.It has been extensively studied for its mechanical, thermal, and oxygen barrier properties, as well as its biodegradability in various environments. [16,20,31,32]espite being claimed to be biodegradable in the literature, its decomposition strongly depends on its morphology and we further consider it to be at least industrially compostable.This means that temperatures beyond its glass-transition temperature (T g ≈ 58 °C) in addition to enzymatic reactions are required to fully decompose it in three months, which are only met in composting plants.A detailed discussion of the decomposition behavior of PLA is provided in the Supporting Information.
There are different PLA types on the market.We chose the Ingeo Biopolymer 4032D because it is transparent in its pure form and can be dissolved in anisole, tetrahydrofuran (THF), and chloroform (CHCl 3 ).When spin coated from these solvents onto quartz substrates, the films appear transparent only in case of THF and CHCl 3 .As described in our previous publication, [36] heating of the film to 180 °C for a few seconds rigidifies the host resulting in increased phosphorescence lifetimes.However, this annealing step strongly limits the substrate choice because only temperature-stable substrates remain applicable, which is discussed further in Section 2.3.We chose THF as solvent because it is known to cause no environmental harm.To test its transparency for UV light, the absorbance of PLA was measured as shown in Figure 4d.PLA exhibits no considerable absorption in the wavelength range that is used to illuminate the active layer ( exc = 275 nm), which makes this polymer highly suitable for fabrication of bioPLTs.It needs to be mentioned that this processing of PLA leads to amorphous film morphologies, which can be decomposed most easily (see Supporting Information for further information).

Quinoline Alkaloids as Emitters
RTP of natural molecules is only scarcely observed so far because most materials' emission is analyzed in solution only, where nonradiative channels govern the triplet state deactivation for purely organic materials.In contrast, several analyses for the quinoline alkaloid quinine have been performed already.An attempt to observe RTP was made already by Levy et al. in 1991 by embedding quinine in sol-gel glasses. [37]However, the resulting fluorescence and phosphorescence peaks were too broad (350-650 nm for fluorescence and 400-700 nm for phosphorescence, respectively) to distinguish them from each other if not measured separately.[40] These characteristics were already used to detect singlet oxygen demonstrating quinine's sensitivity to RTP quenching by molecular oxygen and its potential suitability for application in bioPLTs. [41]Accordingly, we tested quinine and the structurally similar quinoline alkaloids cinchonine, quinidine, and cinchonidine (cf. Figure 1a) for application as RTP emitter.
We embedded all four candidates in PLA leading to smooth thin films of 1800 ± 50 nm thickness.Figure 4a-c shows the photophysical characterization of the films.Under aerated atmosphere, no phosphorescence is visible due to oxygen-mediated quenching.It only appears by measuring in gaseous nitrogen (Figure 4a).Fluorescence as well as phosphorescence peaks are  Table 1.Summary of the emission maxima (fluorescence and phosphorescence; peaks of the vibronic substructure for the latter) of the different quinoline alkaloids in PLA films of 2 wt% along with the phosphorescence lifetimes and quantum yields.well separated.The phosphorescence features are also rediscovered in the delayed spectra at room temperature (Figure 4b) and at 77 K for quinine (Figure S1, Supporting Information).The decay of the RTP intensity can be described using a biexponential fit.The respective data are given in Table 1.All four natural materials show sufficient RTP emission characteristics with a photoluminescence quantum yield (PLQY, Φ Phos ) at 275 nm excitation of at least 4.9% allowing the design of bioPLTs.quinine and quinidine exhibit even stronger fluorescence with Φ Fluo of 7.1% and 6.9%, respectively.Nonetheless, readout under continuouswave illumination appears still reasonable because the fluorescence emission mainly occurs in the UV range (cf. Figure 4a). [10]inchonine and cinchonidine show superior characteristics with Φ Fluo well below Φ Phos .It needs to be noted that PLA further promotes the RTP emission of the quinoline alkaloids presumably by the formation of hydrogen bonds and a suppression of vibrational dissipation. [13]PS:quinine films without the ability to form hydrogen bonds show both weaker RTP emission and shorter phosphorescence lifetime (cf. Figure S2, Supporting Information).
In the following, only quinine and cinchonine are tested for application in PLTs because of the pairwise similar characteristics of quinidine and cinchonidine, respectively.Considering their biotoxicity and a safe application, e.g., in food labeling, the criteria of the REACH regulation, annex XIII for PBT (persistence, bioaccumulation and toxicity), or vPvB (very persistent, very bioaccumulative) are neither met for quinine nor cinchonidine according to safety data sheets and substance databases.This suggests little to no risk for accumulating higher concentrations in the environment and, hence, reaching toxicity levels.It needs to be noted that the amount of luminophores within a standard bioPLT (≈ 4 cm 2 ) is below 0.02 mg (44 mg m −2 ).At such a small dose, the risk is low that acute toxicity levels will be reached (e.g., no observable effect level (NOEL) of quinine: 50-100 mg kg −1 bodyweight/day) [42] especially since oral ingestion is unlikely for the indicated purpose.However, a detailed assessment of the biotoxicity of the bio-PLTs including an evaluation of their decomposition pathways requires analysis in future studies.

PLA as Substrate Material
Aside from its advantages as a host material, PLA is also available on the market as foil with sufficient mechanical properties rendering it a suitable choice as a flexible, industrially compostable substrate material for bioPLTs.Unfortunately, to date, these foils are not commercially available with thicknesses of at least 120 μm, which would be essential for proper handling dur-ing processing.Nevertheless, according to the manufacturer, Ingeo Biopolymer 4032D is also suitable for film extrusion.For this work, suitable foils for use as substrates were successfully fabricated at IPF Dresden.Differential scanning calorimetry (DSC) measurements revealed also an amorphous morphology of PLA within these foils (cf. Figure S3, Supporting Information).Crystallization would initiate for temperatures beyond 80 °C, which is avoided during processing of the bioPLTs.
Using PLA as substrate as well as host leads to three challenges: (i) the foil starts to soften at around 50 °C and is stable in shape only up to 65 °C which prohibits an additional annealing step during bioPLT production, (ii) it does not behave as an oxygen barrier, and (iii) during spin coating of the active layer on top of it, the foil dissolves by the solvent, which leads to uneven and no longer transparent films.The latter two aspects were overcome by a slightly adjusted yet simple design of the bioPLT as depicted in Figure 3b.An additional Exceval layer between the substrate and the active layer prevents molecular oxygen from diffusing through the substrate into the active layer and protects the foil's surface from dissolving.Since the PLA foils show pronounced hydrophobicity, they are treated with air plasma which renders their surface hydrophilic.Subsequently, the bottom Exceval layer can be applied by spin coating.Here, a layer with a thickness of 2100 ± 50 nm sufficiently separates the active layer from the substrate.

Performance of bioPLTs
To fabricate a fully functional bioPLT, the local oxygen concentration within the active layer needs to be sufficiently suppressed to activate RTP emission.We performed tests with only the active emission layer (PLA:quinine) deposited onto glass substrates.Here, even after prolonged UV illumination, no RTP was observed, indicating that the oxygen reflux is stronger compared to the oxygen photoconsumption preventing the ability to write information into the layer.
Accordingly, similar to PLTs made from standard materials, an additional Exceval layer on top of the active layer is required.The layer thickness is 247 ± 2 nm which is sufficiently thick to prevent oxygen from quickly refilling the active layer and quenching the phosphorescence.The final tag is flexible and transparent as can be seen in Figure 5a and consists only of cheap, ready-to-use biodegradable or at least industrially compostable materials.
Writing information in the bioPLT is more challenging in comparison to its nonenvironmentally degradable analog because all four quinoline alkaloids have a lower excitation wavelength compared to the emitter materials NPB and BP-2TA used so far.Regular masks made from PET absorb light in this spectral region rendering them inappropriate.For the analyses within this manuscript, we used a mask made of medium-density fiberboard as shown in Figure 5b.Local photoconsumption of the molecular oxygen and, thus, the activation of RTP was triggered by using a UV-LED with  exc = 275 nm.When the LED was turned off after a sufficient characteristic activation time (see below), a greenish afterglow with high luminescent contrast was observed as shown in Figure 5d,e for cinchonine.Also, with the LED remaining switched on, the phosphorescence could be sufficiently distinguished from the fluorescence (Figure 5c) allowing readout in continuous-wave mode.Analogous photographs for bioPLTs made with quinine are depicted in Figure S4 in the Supporting Information.Although the contrast in continuous-wave mode is worse due to a stronger blue fluorescence, the performance is still satisfying.
Going sustainable comes at a price as Figures 5c-e and especially Figure S4 (Supporting Information) also demonstrate limitations of the bioPLTs.On glass, missing parts of the logo result from a non-ideal activation by using a relatively thick mask.Contactless activation by using a UV-laser or thinner masks from different materials should provide im-proved performance.In case of foils, a distinctive surface roughness leads to slight imperfections in the pattern.Additionally, the missing annealing step might contribute to these imperfections.
After this qualitative evaluation of the bioPLTs, their performance is further analyzed photophysically in terms of activation curves, which are a measure for the photoconsumption of the singlet oxygen under UV illumination and, accordingly, the activation of RTP.The system-specific activation time represents the time at which the phosphorescence intensity reaches 33%.Representative activation curves with respective activation doses for bioPLTs based on quinine and cinchonine on quartz are shown in Figure 6a.They show a well-known sudden increase when the concentration of oxygen is too low to sufficiently quench RTP.Cinchonine shows faster activation with a dose of 141 mJ cm −2 compared to quinine with 201 mJ cm −2 .We associate this behavior to a slightly stronger absorption at  exc = 280 nm and the higher Φ Phos of cinchonine that drive the generation and subsequent photoconsumption of oxygen during activation.After full activation of the bioPLT, the intensity decreases stronger for cinchonine which indicates a pronounced instability under UV light.Surprisingly, bioPLTs on PLA foils (Figure 6b for cinchonine and Figure S5a in the Supporting Information) for quinine) require a significantly smaller activation dose of 19 mJ cm −2 for quinine and 18.5 mJ cm −2 for cinchonine.So far, we can only speculate on the reason for this behavior.We assume, that the missing annealing step in case of foil substrates may affect either the ability of the PLA host to consume molecular oxygen or the intrinsic oxygen concentration in the bioPLT after processing.The annealing might lead to the formation of hydrogen bonds connecting chains and, therefore, blocking potential chain ends for incorporation of or reaction with singlet oxygen.Alternatively, the oxygen might be trapped in the annealed, rigidified film making it harder to remove it from the system and, thus, increasing the activation time.
Regular PLTs have the advantage that they can be rewritten with new information multiple times.Heating the PLT to 95 °C for two minutes makes the Exceval layer permeable for oxygen which refills the active layer and deactivates the RTP.For bio-PLTs this procedure is at least applicable for devices on quartz substrate as the PLA foil does not provide sufficient thermal stability, which is directly connected to its compostability.Four consecutive activation curves, each followed by a heating step for deactivation, are depicted in Figure 6c for a bioPLT using cinchonine and in Figure S5b (Supporting Information) for a bioPLT with quinine.
Again, the cinchonine-based devices show superior performance with smaller deviations between the activation cycles after the first full cycle of activation and deactivation.Three features can be extracted from the measurements for both natural emitters: (i) the RTP contribution to the total intensity decreases, still showing sufficient RTP emission for circa three writing cycles depending on the sensitivity of the reading device, (ii) an increase in the intensity offset caused by additional fluorescent emission of the PLT, which is insensitive to oxygen quenching, and (iii) a decrease in the activation time.Despite the changes in the nature of the emission, the maximum intensity after activation remains relatively unaffected.Unfortunately, spectroscopic analysis of the full bioPLT after each activation cycle is not feasible because of shifts in the measurement spot.This is due to the two different measurement setups (see Section 4) used to evaluate the activation curve and the emission spectrum.However, we performed further spectroscopic analysis of a thin-film sample on quartz substrate before and after illumination by the same UV-LED ( exc = 275 nm) used for activation of the bioPLT.While the illumination was performed for 300 s under nitrogen atmosphere, the spectroscopic analysis occurred under aerated atmosphere to exclude phosphorescence contributions to the fluorescence emission bands.As can be seen in Figure 6d, an additional broad fluorescence signal between 420 and 520 nm appears after illumination resulting in the increase in the intensity offset after each measurement cycle.We assume that the activation leads to a UV-mediated decomposition of emitter molecules accompanied by the formation of a new fluorescent compound.This is further supported by the slight decay in RTP intensity after each activation maximum, when the UV-LED is still illuminating the sample.
Concerning the decrease in the activation time, the strongest difference is observed between the first and the second activation with 7100 and 950 ms, respectively.Already when discussing the strong difference between the activation time of the bioPLT on quartz and PLA, we were suspecting trapped oxygen in the annealed, rigidified film.We assume that these trap states are only partially refilled in the deactivation process leading to a shortening of the activation time.

Conclusion
Programmable luminescent tags already provide a minimalistic and cheap approach for information storage.In this work we presented a fully industrially compostable design made from quinoline alkaloids and polylactic acid that conserves the main promising features of PLTs.The prototypical bioPLTs allow easy, contactfree writing as well as readout even under continuous illumination due to a sufficient contrast between phosphorescence and fluorescence.However, going biodegradable currently comes at a price, especially, considering the flexible bioPLT on PLA.The limited thermal stability inhibits an annealing during production and an erasure of the written information by heating.However, this reduced stability simultaneously supports fast degradation.Possible chemical reaction pathways and products of such bio-PLTs are important to investigate in order to be able to understand and maximize the operational window of these systems.Additionally, a biodegradable emitter with redshifted absorption would also simplify the writing process because standard PETshadow masks could be used.Still, the presented design of bio-PLTs provides a promising template for biodegradable, low-cost information storage.
Hydrophilization of the PLA Foils: For plasma-treatment, an expanded plasma cleaner PDC-002 from Harrick was used.The chamber was evacuated to 0.15 mbar and an air-plasma was applied for one minute.
Thin Film Fabrication: To 6.12 mg of the quinoline alkaloid and 300 mg PLA 6 mL THF was added.The mixture was stirred and gently heated until complete dissolution.150 mg (bottom barrier) and 50 mg (top barrier) Exceval were dissolved in 1 mL water:IPA 9:1 at 120 °C.For spin coating, a speed of 16 rps for 60 s and volumes of 500 μL each were used to produce uniform films.
Emission Measurements: Direct and delayed emission measurements were performed using a CAS 140CTS from Instrument Systems and a triggered 275 nm (Thorlabs, M275L4) LED.For automated data acquisition, the control software SweepMe! was used. [43]All measurements were performed in darkness under nitrogen or ambient conditions.
Phosphorescence Lifetime Measurements: The phosphorescence lifetime was determined using a silicon photodetector PDA100A by Thorlabs.The decays were recorded and fitted using a biexponential fit.The procedure and details can be found in Refs.[44] and [45].
Film Thickness Determination: Film thickness was determined using a profilometer Veeco Dektak 150 from Bruker.A groove has been cut into the film in the middle of the sample using a cannula.A line scan was done at three different positions and the thickness values were averaged.
Activation Curves: In order to determine the dosage required to switch a bioPLT from its dark state (off) to its luminescent state (on), it is imperative to maintain a uniform intensity profile while monitoring the excitation intensity during the exposure process.
The excitation source consists of a 280 nm UV LED (M280L6, Thorlabs) collimated by a lens.To achieve a uniform excitation profile, a topflat diffuser (EDC-20G-1R, RPC Photonics) was positioned after the collimator lens.A focusing lens was then inserted into the beam path to optimize the intensity on the sample.A 6 mm diameter aperture was used to eliminate irregularities at the edges of the excitation profile.The resulting maximum intensity on the sample, denoted I S,max , was quantified to be 19.9 mW cm −2 .
The excitation intensity during activation was recorded using a photodiode (PDA100A2, Thorlabs).To facilitate this, a BaSO 4 -coated reflector was placed adjacent to the aperture to scatter a small portion of the excitation light.This scattered light was then directed through two lenses onto the photodiode chip.A 400 nm short pass filter (XUV0400, Asahi Spectra) was placed in front of the photodiode to prevent stray light from the activated phosphorescence of the PLT from interfering with the measurement.A calibration measurement was performed to determine the calibration factor, denoted as c LED , which correlates the signal recorded by the photodiode (I PD,LED ) to the actual intensity on the sample using the formula Similarly, the light emitted by the sample (I PD,S ) was detected by another photodiode (PDA100A2, Thorlabs).Again, two lenses were used to focus the emitted light onto the photodiode chip.A 450 nm long-pass filter (FELH0450, Thorlabs) was used to capture only the phosphorescent emission, excluding the excitation light and any fluorescence from the emitter.
The electrical signal from the photodiodes was then acquired by a Raspberry Pi microcontroller (RP-PICO) and converted to digital values using its integrated ADC.Before each measurement, a dark measurement was performed to determine the dark current of the photodiodes, which was then subtracted from the measured values.To convert the measurement from time to dose and to account for variations in excitation intensity due to LED heating, the following calculation steps must be performed.First, the accumulated dose (D) is calculated by multiplying the acquisition time of each data point by the current excitation intensity (I S ).Next, the emission of the sample (I PD,S ) is divided by I PD,LED to simulate a constant excitation intensity.
The resulting activation curve, denoted I PD,S,c (D), represents the phosphorescence of the sample as a function of the accumulated dose.This curve is then normalized within a range of 0 to 1, and the activation dose D act is defined as the dose at which the signal (phosphorescence) reaches 0.33.
Fabrication of PLA Substrate Foils: Before the PLA films were produced, the material was dried for 6 h at 80 °C in a vacuum dryer.After drying, the PLA was processed into flat films using the DAVO Visco System E1/30-25D from Reifenhaeuser (Troisdorf, Germany).For this purpose, the melt was discharged at 230 °C from a 250 mm wide slit die and drawn off via open chill-roll 136/350 from COLLIN Lab and pilot solutions (Maitenbeth, Germany) tempered to 70 °C.The screw speed was 50 rpm and the motor speed was 0.58 rpm.The film thickness was about 120 μm.
Photographs: Photographs were taken using a Nokia Nikon D7100 in combination with a Tamron SP 90 mm macro lens.The photographs of the activated tags have been slightly adjusted for better contrast.

Figure 1 .
Figure 1.Schematic representation of a) the general PLT architecture consisting of a substrate, an active layer, and an oxygen barrier and b) the dependence of the phosphorescence intensity on the oxygen concentration in the active layer.c) Simplified Jablonski diagram presenting the oxygen-quenching process.Other nonradiative decay processes and vibronic states are omitted for clarity.

Figure 2 .
Figure 2. Prototypical food labeling based on a bioPLT applied onto a banana.a) Prepared banana and b) zoomed in section, where the information is written using a shadow-mask and a UV-LED ( exc = 275 nm).Mind that application of the bioPLT does not cause any defect to the shell or the banana in general, which would affect its durability.c) Activated bioPLT under continuous wave illumination showing high contrast due to the low fluorescence of cinchonine, which was used as natural emitter.d) Persistent luminescence after the UV-LED was switched off.A phosphorescence lifetime of 300 ms allows easy readout.The quality of the writing process could be further improved by using a flexible mask or a focused UV-laser.

Figure 3 .
Figure 3. a) Chemical structures of materials used in this work for the bioPLT.b) Schematic representation of the biodegradable, minimalistic, and flexible bioPLT stack with two oxygen barrier layers (thicknesses are approximate values).

Figure 4 .
Figure 4. Emission data of thin-films of selected quinoline alkaloids in PLA at room temperature, with  exc = 275 nm, showing a) emission spectra under aerated (dashed line) and nitrogen atmosphere (straight line), b) delayed spectra collected at a delay time of 10 ms showing only the phosphorescence, c) corresponding phosphorescence decay, and d) absorption spectra with additional spectra of pure PLA foil, PLA:quinine thin-film, as well as pure quinine dissolved in THF.

Figure 5 .
Figure 5. Photographs of a) a bioPLT at daylight, b) a medium-density fiberboard shadow-mask made by laser-cutting, c) an activated bioPLT with PLA foil as substrate and cinchonine as natural RTP emitter under continuous wave illumination demonstrating sufficient contrast between fluorescence (deactivated area, widely UV) and phosphorescence (activated area, green), as well as d) an activated tag on quartz glass, and e) an activated tag on foil after the UV-LED is turned off.

Figure 6 .
Figure 6.Activation of the phosphorescence in bioPLTs based on UV-activated photo-consumption of molecular oxygen.a) Activation curves of quinine and cinchonine ( exc = 280 nm) on quartz.The dotted gray line indicates the 33% signal level corresponding to the system-specific activation time.b) Activation curves of PLA:cinchonine bioPLTs on quartz (blue) and PLA foil (orange).c) Activation curves of PLA:cinchonine bioPLTs on quartz for four consecutive measurements (writing cycles), each interrupted by heating the sample on the hot plate at 95 °C for two minutes to allow reflux of molecular oxygen resulting in a deactivation of the bioPLT.d) Thin-film emission data ( exc = 275 nm) of a PLA:cinchonine film on quartz showing the fluorescence response before and after illumination for 300 s under nitrogen atmosphere.The spectra were acquired under aerated atmosphere.