Photo-induced Afterglow of Carbon Dots for Dynamic Patterning

Flexible materials with afterglow feature have received considerable attention in the eld of exible electronics. To date, it is still a formidable challenge to develop exible materials with dynamic long afterglow for practical applications. Herein, we report for the rst time photo-induced long afterglow in a exible solid composite lm made of luminescent carbon dots (CDs) and the oxygen-permeable poly pyrrolidone (PVP). The phosphorescent afterglow of the lm can be reversibly activated by photo-irradiation and de-activated by thermal treatment. Impressively, the photo-activation leads to a signicant luminescence lifetime enhancement with a factor of over 3900, and editable afterglow patterns with ne resolution exceeding 1280 dpi are readily achieved by masking and lithography. Furthermore, the retention time of such memorized optical patterns can be tuned from minutes to days by varying temperature, enabling the unique time-temperature indicating function. These ndings not only enrich both the library of the afterglow materials vastly, but also extend the scope of the potiential applications of CDs materials. ,

Since its rst discovery in 2006 25 , carbon dots (CDs), an emerging class of luminescent nanomaterials, have drawn massive research interest for their fascinating PL properties, advanced stability and low-toxicity nature. Based on that, the vast applications of CDs in different elds, including bio-imaging, theranostics, illumination/display, and energy conversion have been explored [26][27][28][29] . Particularly, CDs materials with room temperature long afterglow have recently become a new research hotspot 30-40 , showing superiority in applications compared to the traditional uorescent CDs. For example, timeresolved data encryption and anti-counterfeit patterns have been manufactured, taking advantage of their prolonged emission lifetime [35][36][37][38][39][40] . Despite the ever-growing interest in these novel materials, to the best of our knowledge, researchers have yet to nd a type of CDs-based long afterglow materials with photoinduced optical memory (PIOM) characteristics.
We herein propose, for the rst time, a exible CDs/PVP lm showing photo-induced long afterglow with PIOM effect. In this speci c design, triplet oxygen, a well-known quencher of triplet excitons 41,42 , is intentionally introduced to mediate the phosphorescent afterglow. Upon photo irradiation, CDs embedded in the oxygen-permeable PVP host continuously generate triplet excitons that remove the triplet oxygen through an in situ photodynamic process (Process I and II in Fig. 1a). After that, the irradiated region gradually becomes anoxic and resultantly allows the room temperature phosphorescence (RTP) emission of CDs (Process III in Fig. 1a). The photo-induced phosphorescent long afterglow character remains in the pre-irradiated region for a considerable duration, before environmental oxygen permeates the lm again and causes RTP quenching, erasing the PIOM effect. Notably, the oxygen permeability can be tuned through thermal treatment, resulting in variable retention time of the evoked long afterglow feature (Process IV in Fig. 1a).

Material design and fabrication
To validate our hypothesis, we rstly synthesized a type of CDs containing abundant heteroatoms (N, O) through a solvothermal method (I in Fig. 1b). N, O-containing functional groups can bene t the RTP afterglow of CDs in two major ways. Firstly, N and O atoms contain lone-pair electrons that can facilitate n-π* transitions and enhance the intersystem crossing (ISC) 43,44 , promoting the generation of triplet excitons. Secondly, these functional groups can serve as "anchor spots" for hydrogen bonding xation, which can e ciently reduce non-radiative energy loss. 30 Fig. 4); meanwhile in the powdery state, there was no visualized PL emission whatsoever. However, signi cant long afterglow occurrs in a transparent CDs/PVP composite lm prepared by solvent-casting (II in Fig. 1b). In this speci c composite, PVP acts as a hydrogen-bonded matrix with abundant lactam groups which provide solid xation that suppresses the non-radiative energy loss and bene ts the RTP emission of CDs. Importantly, as a bio-compatible hydrophilic polymer, PVP features excellent oxygen permeability 45 , which fundamentally enables the PIOM design. Furthermore, PVP is reductive and capable of scavenging the generated singlet oxygen 13 , thereby enabling rapid removal of molecular oxygen and a fast responding rate.

Reversible photo-induced long afterglow
The resultant CDs/PVP composite lm initially emitted bright cyan light upon 400 nm excitation, which instantly disappeared as the irradiation switched off. As expected, the rst short irradiation (<0.5 s) did not evoke any observable afterglow in the lm. Impressively, an intense orange phosphorescent afterglow that lasts for several seconds appeared after the lm was continuously irradiated by a 365 nm UV lamp (III in Fig. 1b and Fig. 2a). The photo-activation of afterglow signi cantly prolonged the luminescence lifetime of the material by a factor of 3932 (from 148 μs to 582 ms. Fig. 2b and Supplementary Table 2). It is worthy to note that the afterglow feature aroused by the photo-induced anoxia naturally last for more than 1 h at room temperature, before it gradually disappeared due to oxygen penetration. Within this time, the orange afterglow could be evoked at will by short irradiation. In addition, the photo-induced afterglow feature could be executed by applying thermal treatment shortly. Such an on-off switch could be repeated for multiple cycles without signi cantly losing the original RTP characteristics ( Fig. 2c and Supplementary Fig. 5).
In a further set of experiments, the responding behavior of the photo-induced afterglow was studied.
From Fig. 2d, it was found that the time required to turn-on the afterglow clearly decreased with increasing irradiation power density. Speci cally, the time required to achieve half-maximal RTP intensity (t 1/2 ) was inversely proportional to the irradiation power density ( Supplementary Fig. 6). Estimated from that, with an irradiation power density of 10 mW/cm 2 , the RTP intensity would reach 50% maximum within 4 s and 90% maximum within 20 s under continuous irradiation, allowing the rapid recording of optical information. Meanwhile, it was found that the disappearing speed of the photo-induced afterglow feature was evidently temperature-dependent due to the enhancement of oxygen permeation under higher temperature. For instance, the afterglow feature quickly vanished at 373 K within 15 min, but remained detectable at 253 K even after 48 h (Fig. 2e). Because the disappearing speed of the afterglow feature was fundamentally determined by oxygen permeability, higher temperature could induce faster oxygen permeation, causing the potential long afterglow to perish within a shorter period of time. Additionally, we also found that the retention time could be tuned by further adjusting the molecular weight of the PVP host material, or simply applying surface barrier layers with different thickness (Supplementary Fig. 7).

Veri cation of the oxygen-mediated mechanism
To further illustrate the oxygen-mediated regulation of such a photo-induced long afterglow, a set of control experiments were conducted. A different composite lm was prepared using polyacrylamide (PAM) instead of the PVP polymer. Different from the oxygen-permeable PVP, PAM features minimal oxygen permeability 46 , which leads to a constant anoxia environment in the composite lm. The emission wavelength and decay pro les of the CDs/PAM composite are similar to those of the CDs/PVP composites with photo-induced afterglow (Supplementary Fig. 8 and Table S3). The difference occurred, however, when the delayed emission properties of two different composite lms were examined under intermittent irradiations with a regular "on-off" switching pattern (Fig. 3a). In this case, the CDs/PVP composite lm showed a gradually accumulating RTP intensity with evident memory effect, while the CDs/PAM composite lm showed a constant RTP intensity that almost instantly reached its maximum as the irradiation switched on. A more straight-forward demonstration was shown in Fig. 3b and Supplementary Video 1, where the CDs/PAM composite showed intrinsic long afterglow nature, but CDs/PVP clearly showed memory effect and only emitted long afterglow in the photo-activated region.
The above-mentioned results validated that adequate oxygen permeability of the material was crucial to achieving the unique photo-induced long afterglow. In both composite lms, the host polymers provided hydrogen bonding xations, which suppressed non-radiative transitions and enabled long afterglow. However, only the CDs/PVP composite demonstrated the dynamic long afterglow mediated by a photodynamic oxygen removal process (Fig. 3c). This process was further con rmed by monitoring the characteristic near-infrared (NIR) luminescence of singlet oxygen at 1268 nm 47 . As revealed in Fig. 3d, evident NIR emission was detected from the CDs solution under 400 nm irradiation. The emission intensity, however, dramatically decreased in the presence of PVP, indicating the consumption of the generated singlet oxygen by the PVP macromolecules 13 .
Notably, the photo-induced afterglow was always distinctly localized in the pre-irradiated region, showing promising potential for graphic information processing (IV in Fig. 3b). Based on that, we further applied masking and lithography methods to create designable afterglow patterns on the lm. From Supplementary Fig. 9, we found that the limiting resolution of such patterns was up to 1280 dpi with a standard USAF-1951 target, which equaled to a limiting line resolution of <20 μm. As demonstrated in Fig.  4a, reversible writing-reading-erasing of afterglow patterns could be readily achieved by applying predesigned masks for optical printing (Supplementary Video 2). It's also worth mentioning that the optical printing and afterglow read-out process could be accomplished with a commercial white light LED lamp (Supplementary Fig. 10 and Supplementary Video 3). During this process, the transmittance, morphology, and steady-state PL emission of the lm remained almost unchanged (See Supplementary Figs. 11~12), which altogether made this material naturally suitable for practical applications (See Supplementary Fig.  13).

Applications for dynamic patterning and time-temperature indication (TTI)
The transportation of many thermal-sensitive cargos like vaccines and medicines relies rmly on the coldchain. Occasionally, cold-chain failures may occur, causing not only nancial loss, but also potential public hygiene hazard 48 . To avoid such issues, time-temperature indicating (TTI) tags like Warmmark (SpotSee TM ) were used to visualize the potential thermal abuse of cargos during storage and transportation. Herein, taking advantage of the editable long afterglow of CDs/PVP composite and its thermal-sensitive retention, a multi-use smart TTI tag carrying renewable logistics data was realized. Following the procedure illustrated in Fig. 4b, editable TTI tags were facilely fabricated. And a conceptual multi-stop cold chain transportation monitored by the CDs/PVP TTI tags was illustrated in Fig. 4c. Herein, a hypothetical 6-stop transportation route was set up, with the logistics datas updated daily. For usage demonstration, two tags were prepared, referring to sample A (well preserved) and sample B (thermally abused), respectively. During the transportation, logistics datas were optically printed upon departure and inspected upon arrival at each stage. The delayed emission photographs of the two tags upon departure and arrival were captured and listed in Fig 4c. In the rst three transport segments (NJ→JZ, JZ→CZ, and CZ→WX), both tags were well-preserved at 253 K. At this stage, all graphic information could be readily recognized upon arrival. At the fourth segment between WX and SZ, while sample A was constantly kept at 253 K, sample B was exposed to room temperature (298 K) for 1 h during this process. As a result, upon arrival at SZ, only the sample A tag retained recognizable barcode pattern. Meanwhile, no information could be read from the sample B tag after multiple attempts ( Supplementary Fig. 14 and Supplementary Video 4), which indicated the potential deterioration of the cargoes. Such a result demonstrated that the CDs/PVP composite could be used as editable TTI tags for niche application.

Discussion
In summary, we presented a rational design of exible composite materials with photo-induced phosphorescent afterglow based on the combination of the CDs phosphor and an oxygen-permeable PVP host. We found that the phosphorescent afterglow can be reversibly switched on-off by light and thermal treatment within seconds. Impressively, the photo-activation aroused a signi cant luminescence lifetime enhancement with a factor of over 3900. With lithography method, editable patterns on this material showed ne resolution (> 1280 dpi), excellent writing-erasing reversibility, and tunable memory retention with thermal sensitivity. Additionally, potential applications of this material were demonstrated in dynamic optical patterning, encryption and time-temperature indication. To the best of our knowledge, this is the rst example of a CDs-based long afterglow material with PIOM effect, which not only opens up a new eld of CDs-based smart optical materials, but also provides novel ideas for the production of exible stimuli-responsive materials with long afterglow.

Synthesis of CDs:
Brie y, 648 mg (6 mmol) of PBQ was dissolved in 200 mL of ethanol under agitation to form a homogeneous solution. To that solution was added 480 mg (8 mmol) of EDA in one portion to form a hazel dispersion, which was further sealed in an autoclave and maintained at 120 ℃ for 10 h. Afterward, the reaction was naturally cooled down to room temperature. The crude product was collected and concentrated by rotary evaporation under reduced pressure before further puri ed by dialysis (Cutoff Mw: 5000 D. Crude product was dialyzed against DI water for 48 h with water renewed every 12 h) and silica column chromatography (eluent: 90% dichloromethane and 10% methanol). As a result, about 200 mg powdery product was nally obtained.
Preparation of the CDs/PVP composite lm To prepare a CDs/PVP composite lm, 1 g of PVP was fully dissolved in 19 mL of deionized water under agitation at 328 K (55 ℃). Next, 1 mL of ethanol solution containing 5 mg CDs was added to the PVP aqueous solution quickly. The solution was further magnetically stirred for 30 min to ensure a homogeneous dispersion.
Afterward, onto a standard 9 cm x 9 cm square polystyrene petri dish was poured 17.5 g of the CDs/PVP solution. Transferred to a at heater previously adjust to 313 K (40 ℃), the solution was left for solvent evaporation for 10 h. The resultant lm was then heated at 393 K (120 ℃) under reduced pressure (~ 0.1 mbar) for another 2 h for complete drying. The thickness of such as-prepared lm was 80 μm (measured with a micrometer screw gauge). Commercially available PET lamination lms (purchased from Jiwen lamination lm Co. Ltd. Wuxi, China) with different thicknesses (50, 80, 100, 125, 150 μm) were applied to protect the lm from scratch and adjust its oxygen permeability. In this work, typical photophysics data of the material were acquired from a sample coated with 80 μm-thick PET, unless otherwise speci ed. Figure 1 Schematic illustration for the design and fabrication of the CDs/PVP composite lm with reversible photo-induced long afterglow. a Diagram of the processes to achieve photo-induced long afterglow. The CDs/PVP composite lm initially showed no afterglow due to oxygen quenching (I). Upon irradiation, the permeated triplet oxygen was gradually converted to singlet oxygen through a photodynamic process (II), eventually creating an oxygen-free area in the exposed region that showed RTP afterglow (III). Afterward, the regional anoxia would last for a certain duration before oxygen replenishment through diffusion (IV), which could be accelerated by thermal exposure. b The preparation of the CDs/PVP composite lm and demonstration of its PIOM. The CDs synthesized from molecular precursors through solvothermal method (I) were blended with PVP in an aqueous solution and cast into a transparent, exible lm (II). The as-prepared lm initially showed no afterglow after being shortly irradiated (400 nm, 10 mW/cm2, <0.5 s), however, intense afterglow occurred after prolonged irradiation was applied (30 s).   Photo-induced long afterglow patterning and its unique applications thereof. a Illustration of the Ink-free optical printing of dynamic afterglow patterns by masking and lithography. Emblem and Chinese characters (RTP pattern-1 and pattern-2) were created with different masks successively on the same lm after the previous pattern was erased by thermal exposure. Bending the exible lm did not affect the display quality. b Schematic illustration for the use of editable smart tags with thermal sensitive long afterglow memory. Firstly, the logistics data was led and encrypted into a 2D barcode using the Micro-PDF147 format (I). Then, the barcode was printed onto a transparent PET membrane using a commercial static printer to obtain a temporary mask (II). By applying that mask for optical printing (III), a long afterglow pattern containing logistics data was created on the tag (IV). The edited tag was then packaged with the cargo, being regularly inspected, erased, and updated at each stop during the delivery. (c) Flowchart: a hypothetical multi-stop cold-chain transportation route through which thermal-sensitive cargos (sample A and B) were delivered. Photographs: RTP afterglow patterns of tags referring to sample A (top) and sample B (bottom) upon departure and arrival at each stop.