Upcycling waste expanded polystyrene into UV-excited dual-mode multicolor luminescent electrospun fiber membranes for advanced anti-counterfeiting†

Expanded polystyrene (EPS) is causing severe environmental problems due to its high consumption and non-biodegradability. Upcycling waste EPS into high value-added functional materials is highly advisable in terms of sustainability and environmental concerns. Meanwhile, it is imperative to develop new anti-counterfeiting materials with high security against increasingly high-tech counterfeiting. Developing UV-excited dual-mode luminescent advanced anti-counterfeiting materials that can be excited by commonly used commercial UV light sources (such as 254 nm and 365 nm wavelengths) remains a challenge. Herein, UV-excited dual-mode multicolor luminescent electrospun fiber membranes were fabricated from waste EPS by co-doping with a Eu³⁺ complex and a Tb³⁺ complex via electrospinning. The SEM results prove that the lanthanide complexes are uniformly dispersed in the PS matrix. The luminescence analysis results suggest that all the as-prepared fiber membranes with the different mass ratios of the two complexes can exhibit the characteristic emission of Eu³⁺ ions and Tb³⁺ ions under UV light excitation. The corresponding fiber membrane samples can exhibit intense visible luminescence with different colors under UV lights. Moreover, each membrane sample can display different color luminescence irradiated with UV light at 254 nm and 365 nm, respectively, e.g. show excellent UV-excited dual-mode luminescent properties. This is owing to the different UV absorption properties of the two lanthanide complexes doped in the fiber membrane. Finally, the fiber membranes with different color luminescence from green light to red light were achieved by tuning the mass ratio of the two complexes in the PS matrix and changing UV irradiation wavelengths. The as-prepared fiber membranes with tunable multicolor luminescence are very promising for high-level anti-counterfeiting applications. This work is very meaningful not only to upcycle waste EPS to high value-added functional products but also to develop advanced anti-counterfeiting materials.

A simple and effective method to upcycle waste EPS to UV-excited dual-mode multicolor luminescent membranes for advanced anti-counterfeiting was developed.

A Self-Color-Changing Film with Periodic Nanostructure for Anti-Counterfeit Application

A self-color-changing film aimed at enhanced security and anti-counterfeit packaging is presented. Its function is to change color automatically when flipped under visible light. It is low-cost, takes a few seconds to check by the naked eye, and does not need any special tools to evaluate. The design of the color-changing, anti-counterfeiting film is based on a frequency-selective surface (FSS). The film is designed with aluminum nanocubes. They are laid out as an array in a plane with equal distance from one another. This arrangement allows us to select certain wavelengths of light to pass through by the size of the cubes and the separation distance between them. The performance is evaluated by a finite element analysis (FEA) method. The results show that the intersection of transmittance and the reflectance curves cause the film to change its color automatically when flipped. We also propose a method to predict the color of the film based on the transmittance values. The accuracy of this method is verified by actual colors from experiments with an error of no more than 12.8%, analyzed by the CIE chromaticity diagram.

Unravelling the highly efficient synthesis of individual carbon nanodots from casein micelles and the origin of their competitive constant-blue-red wavelength shift luminescence mechanism for versatile applications

Synthesis of casein-derived carbon nanodots (CND) using a microwave-assisted approach, giving a high product yield (25%), is reported. Casein was used as a sustainable carbon source, and polyvinylpyrrolidone was used as a stabilizer for the nanodots. The size of the prepared amorphous CND corresponds to individual casein coils, which were only partially carbonized. They were obtained due to the disintegration of casein micelles and submicelles within the microwave-assisted solvothermal process. The resulting nanodots had bright photoluminescence, and their electronic structure and optical properties were investigated. A novel competitive model of their luminescence mechanism was introduced to explain a phenomenon beyond the standard models. The synthesized carbon nanodots were used as luminescent ink for anticounterfeit applications. A polymer matrix nanocomposite was prepared by dispersing the nanodots in a flexible and robust poly(styrene-ethylene-butylene-styrene) tri-block copolymer (SEBS) using the solution cast method. For the first time, the effect of CND on the luminescence and mechanical properties of the SEBS/CND self-supporting films was studied. The film was also studied as a phosphor for light-emitting diodes, with a unique experimental setup to avoid self-absorption, which results in low efficiency and eliminates the excess UV transmitted. Because of their high luminescence, photostability, and mechanical properties, these CND could be used as luminescent labels in the packaging and optoelectronics industries.

Synthesis of individual casein based CND with a discrete luminescence mechanism.

Ultrafast Collective Excited-State Dynamics of a Virus-Supported Fluorophore Antenna

Radiation brightening was recently observed in a multifluorophore-conjugated brome mosaic virus (BMV) particle at room temperature under pulsed excitation. On the basis of its nonlinear dependence on the number of chromophores, the origins of the phenomenon were attributed to a collective relaxation. However, the mechanism remains unknown. We present ultrafast transient absorption and fluorescence spectroscopic studies which shed new light on the collective nature of the relaxation dynamics in such radiation-brightened, multifluorophore particles. Our findings indicate that the emission dynamics is consistent with a superradiance mechanism. The ratio between the rates of competing radiative and nonradiative relaxation pathways depends on the number of chromophores per virus. The findings suggest that small icosahedral virus shells provide a unique biological scaffold for developing nonclassical, deep subwavelength light sources and may open new avenues for the development of photonic probes for medical imaging applications.

Selective Ligand Sensitization of Lanthanide Nanoparticles for Multilevel Information Encryption with Excellent Durability

Durable and multilevel information encryption technology has been of great importance in recent decades. Here, an inkjet printer-adaptable invisible ink was prepared with lanthanide nanoparticles, and optical decoding of information could only be achieved when specific ligand dipicolinic acid was utilized in the presence of UV illumination. In addition, the proposed protocols displayed long shelf life (>one year) and excellent durability even at harsh conditions such as in the presence of strong acids (1 M HCl) and alkalis (1 M NaOH). Meanwhile, such invisible inks could be further employed on a soft matrix via screen-printing, holding great potential for practical applications.

Encapsulation of lead halide perovskite quantum dots in mesoporous NaYF4 matrices with enhanced stability for anti-counterfeiting

All-inorganic cesium lead halide (CsPbX3, X = Cl, Br, and I) perovskite quantum dots (PQDs) have great potential application due to their unique optoelectronic properties. However, poor luminescence stability caused by some inevitable factors such as light, moisture and heat always restricts their practical application. In this work, the stability of CsPbX3 (X = Cl0.5Br0.5, Br, and Br0.5I0.5) PQDs is improved by encapsulating them in stable hollow mesoporous NaYF4:Yb,Tm nanoparticles (HMNPs). Compared to pristine PQDs, HMNP-PQD composites exhibit stable photoluminescence properties that can be maintained for more than 60 days under ambient atmospheric conditions. Thanks to the protection of HMNPs, the composites show much higher long-term stability in highly humid air and enhanced stability against UV light treatment compared to naked CsPbBr3 PQDs. Based on the proposed confinement effects of PQDs coordinated with the hollow mesoporous structure of NaYF4:Yb/Tm, the related structural model of NaYF4:Yb/Tm@PQD composites is discussed. Moreover, dual-mode luminescence can be observed in the NaYF4:Yb/Tm@PQD nanocomposites under 365 nm UV light and 980 nm laser excitation, indicating that the as-designed composites have great potential for dual-mode anti-counterfeiting application. This work provides a new idea for the stabilization and application of CsPbX3 PQDs.

Color-switchable hybrid dots/hydroxyethyl cellulose ink for anti-counterfeiting applications

Many anti-counterfeiting inks have been explored recently, most of them are commonly involved in weak fastness, high cost and long-term toxicity, impeding their real-life applications. Herein, an environment-friendly and inexpensive anti-counterfeiting ink with excellent fastness is reported. The untifake ink is developed by combining hybrid dots (silicon/carbon) with hydroxyethyl cellulose (HEC) binder. Interestingly, the HEC binder can effectively prevent from aggregation-induced quenching of hybrid dots. Subsequently, the customized patterns are successfully transferred onto different surfaces of various substrates including cotton fabric, cellulosic paper, glass, metal, silicon wafer and PET film, using the as-prepared ink by screen-printing technique, exhibiting that the hybrid dots/HEC ink possesses widespread practicability. Notably, fluorescent color of these patterns can be switchable by adjusting environmental pH-value, further imparting the as-prepared ink with excellent covert performance. This new fluorescent hybrid dots/HEC ink will be promising candidates for high-level anti-counterfeiting applications including food packaging, apparel and documents.

Lanthanide-Doped Core@Multishell Nanoarchitectures: Multimodal Excitable Upconverting/Downshifting Luminescence and High-Level Anti-Counterfeiting

The development of luminescent materials with concurrent multimodal emissions is a great challenge to improve security and data storage density. Lanthanide-doped nanocrystals are particularly appropriate for such applications for their abundant intermediate energy states and distinguishable spectroscopic profiles. However, traditional lanthanide luminescent nanoparticles have a limited capacity for information storage or complexity to shield against counterfeiting. Herein, it is demonstrated that the combination of upconverting and downshifting emissions in a particulate designed lanthanide-doped core@multishell nanoarchitecture allows the generation of multicolor dual-modal luminescence over a wide spectral range for complex information storage. Precise control of lanthanide dopants distribution in the core and distinct shells enables simultaneous excitation of 980/808 nm focusing/defocusing laser and 254 nm light and produces complex upconverting emissions from Er, Tm, Eu, and Tb via multiphoton energy transfer processes and downshifting emissions from Eu and Tb via efficient energy transfer from Ce to Eu/Tb in Gd-assisted lattices. It is experimentally proven that multiple visualized anti-counterfeit and information encryption with facile decryption and authentication using screen-printing inks containing the present core@multishell nanocrystals are practically applicable by selecting different excitation modes.

Stimuli-Responsive Naphthalene Diimide as Invisible Ink: A Rewritable Fluorescent Platform for Anti-Counterfeiting

Herein, we report an approach to combat counterfeiting and storage of valuable information based on the solid-state fluorescence switching behavior of isoniazid functionalized naphthalene diimide (ISO_NDI) in response to an external stimuli (i. e., HCl vapor). The unique feature of ISO_NDI is further utilized to develop an invisible ink (ISO_NDI-PVA) with commercial polymer polyvinyl alcohol (PVA). A solid-state fluorescence recovery was observed while loading with HCl vapors. This exclusive property of the material could be applied directly as a security ink for confidential data storage purpose. Based on above strategy, we successfully realized the rewritable application by using ISO_NDI-PVA ink and confirm its practical efficacy on various substrates by creating different patterns. The solid-state fluorescence switching behavior of ISO_NDI-PVA ink exhibited reversible on/off signal for multiple cycles under the influence of HCl/NH₃ vapors. Mechanistic investigation supports a clear participation of intermolecular charge transfer (ICT) phenomenon in the solid-state fluorescence switching property. The ease of fabricating the ink with invisible to visible characteristics in response to HCl vapors provides new opportunities for exploring the application of ISO_NDI-PVA as invisible ink for targeted security applications.

Photoluminescent Oxygeneous-Graphitic Carbon Nitride Nanodot-Incorporated Bioderived Hyperbranched Polyurethane Nanocomposite with Anticounterfeiting Attribute

Anticounterfeiting materials are neo-advanced materials with utility in covert and security strategies. In this context, a photoluminescent, mechanically robust, and thermally stable hyperbranched polyurethane (PU) nanocomposite was fabricated with oxygeneous-graphitic carbon nitride nanodots. The nanocomposite was characterized using infrared, ultraviolet-visible, and photoluminescence spectroscopy, X-ray diffractometry, transmission electron microscopy, and thermogravimetric analysis. The processed nanocomposite demonstrated improved physico-mechanical stability as well as enhanced thermal stability than the pristine PU. The nanocomposite displayed remarkable photoluminescence under long ultraviolet light (365 nm), courtesy of dispersion of oxygeneous-carbon nitride nanodots in the polymer matrix, without any solid-state quenching. The nanocomposite was consequently employed as an ultraviolet light-detectable anticounterfeiting ink material having reinforcing ability.

Blue, green, and red full-color ultralong afterglow in nitrogen-doped carbon dots

Carbon dots (CDs) with tunable emission colors and multiple emission modes are highly desirable in advanced optical anti-counterfeiting. Some pioneering efforts to trigger additional long-lived emission modes, nevertheless, did not perfectly solve the issue of printability and color-tunability in practical applications. Herein, we developed an encapsulating-dissolving-recrystallization route for the synthesis of CD-based anti-counterfeiting inks, and accordingly realized blue, green, and red full-color afterglow emissions from these CD-based inks when printed on paper. The printed inks simultaneously possessed triple emission modes including fluorescence (FL), delayed fluorescence (DF), and room-temperature phosphorescence (RTP), among which the long-lived emissions (DF and RTP) could be selectively activated by using different excitation wavelengths. We believe that the proposed synthetic route in this work may promote the development of multicolor-encoded and multiple-mode-integrated optical anti-counterfeiting systems, and will expand the application of CD-based materials to the fields of sensing, photodynamic therapy and bio-imaging.

A facile and green approach to prepare carbon dots with pH-dependent fluorescence for patterning and bioimaging

Carbon dots prepared with the adoption of ubiquitous natural fruit juices as precursors have good applications in pH sensing, patterning and bioimaging.