Material design and fabrication
To validate our hypothesis, we firstly synthesized a type of CDs containing abundant heteroatoms (N, O) through a solvothermal method (I in Fig. 1b). N, O-containing functional groups can benefit 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 fixation, which can efficiently reduce non-radiative energy loss.30,35 As was validated by structural analysis (Supplementary Figs. 1~3, Supplementary Table 1), the as-prepared CDs were highly functionalized with amine, imine, hydroxyl, carboxyl and enamine groups, in agreement with our design principle. At this point, the CDs showed only short-lived fluorescence with a lifetime of 3.3 ns in solution (Supplementary Fig. 4); meanwhile in the powdery state, there was no visualized PL emission whatsoever. However, significant long afterglow occurrs in a transparent CDs/PVP composite film prepared by solvent-casting (II in Fig. 1b). In this specific composite, PVP acts as a hydrogen-bonded matrix with abundant lactam groups which provide solid fixation that suppresses the non-radiative energy loss and benefits the RTP emission of CDs. Importantly, as a bio-compatible hydrophilic polymer, PVP features excellent oxygen permeability45, which fundamentally enables the PIOM design. Furthermore, PVP is reductive and capable of scavenging the generated singlet oxygen13, thereby enabling rapid removal of molecular oxygen and a fast responding rate.
Reversible photo-induced long afterglow
The resultant CDs/PVP composite film initially emitted bright cyan light upon 400 nm excitation, which instantly disappeared as the irradiation switched off. As expected, the first short irradiation (<0.5 s) did not evoke any observable afterglow in the film. Impressively, an intense orange phosphorescent afterglow that lasts for several seconds appeared after the film was continuously irradiated by a 365 nm UV lamp (III in Fig. 1b and Fig. 2a). The photo-activation of afterglow significantly 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 significantly 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. Specifically, the time required to achieve half-maximal RTP intensity (t1/2) was inversely proportional to the irradiation power density (Supplementary Fig. 6). Estimated from that, with an irradiation power density of 10 mW/cm2, 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).
Verification 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 film was prepared using polyacrylamide (PAM) instead of the PVP polymer. Different from the oxygen-permeable PVP, PAM features minimal oxygen permeability46, which leads to a constant anoxia environment in the composite film. The emission wavelength and decay profiles 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 films were examined under intermittent irradiations with a regular “on-off” switching pattern (Fig. 3a). In this case, the CDs/PVP composite film showed a gradually accumulating RTP intensity with evident memory effect, while the CDs/PAM composite film 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 films, the host polymers provided hydrogen bonding fixations, 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 confirmed by monitoring the characteristic near-infrared (NIR) luminescence of singlet oxygen at 1268 nm47. 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 macromolecules13.
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 film. 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 pre-designed 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 film 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 firmly on the cold-chain. Occasionally, cold-chain failures may occur, causing not only financial loss, but also potential public hygiene hazard48. To avoid such issues, time-temperature indicating (TTI) tags like Warmmark (SpotSeeTM) 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 first 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.