Dynamic wrinkling pattern exhibiting tunable fluorescence for anticounterfeiting applications

Counterfeiting poses significant economic, health, and security risks. Current anticounterfeiting technologies, while employing various stimuli-responsive molecules and graphically encoded taggants, often lack complexity and are susceptible to replication. Fluorescent patterns offer advantages due to their readily detectable chemical characteristics, and dynamic fluorescence, tunable by external stimuli, enhances security. However, existing fluorescence-based methods remain vulnerable to cloning. Wrinkling patterns, inspired by animal skin's unique fingerprint-like features, provide an additional layer of security due to their randomness and unpredictable formation. Combining responsive fluorescence with dynamic wrinkling patterns into a single tag is challenging but offers significantly enhanced security and information capacity. This research presents a novel approach to achieve this, creating a dynamic dual-function pattern with tunable wrinkles and fluorescence in response to visible light and acid gases.

The authors review existing anticounterfeiting methods, highlighting the limitations of current technologies. They discuss the use of fluorescent patterns and their enhancement through dynamic, stimuli-responsive fluorescence. The inherent challenges in cloning unique surface wrinkles due to their stochastic formation process are also explored. Prior art involving light-triggered and vapor-triggered transitions to enhance security are mentioned, along with the challenges in combining both responsive fluorescence and dynamic wrinkling patterns effectively.

The researchers fabricated a bilayer system using a supramolecular polymer network as the skin layer and poly(dimethylsiloxane) (PDMS) as the soft substrate. The supramolecular network comprised a pyridine-containing copolymer (P4VP-nBA-S) and hydroxyl distyrylpyridine (DSP-OH), which exhibited bright fluorescence. The hydrogen bonding between pyridine and hydroxyl groups provided the rigidity necessary for wrinkle formation. The synthesis and characterization of P4VP-nBA-S and DSP-OH are detailed. The photoisomerization kinetics of DSP-OH under 450 nm light were tracked using UV-vis spectroscopy and ¹H NMR. The effects of acid (CF3COOH) on DSP-OH fluorescence were also investigated using UV-vis and fluorescence emission spectroscopy. A toluene solution of the P4VP-nBA-S/DSP-OH mixture was spin-coated onto the PDMS substrate, followed by thermal treatment to introduce compressive stress and induce wrinkling. The effects of visible light and HCl gas on the wrinkle pattern and fluorescence were studied using atomic force microscopy (AFM), UV-vis spectroscopy, and fluorescence spectroscopy. Selective erasure of wrinkles was demonstrated using photomasks during light irradiation. The reversible nature of both light and acid-induced changes was confirmed through multiple cycles of treatment and recovery.

Irradiation with 450 nm light triggered the photoisomerization of DSP-OH, causing a decrease in the crosslinking density of the supramolecular network and a resulting release of internal stress. This resulted in the simultaneous disappearance of wrinkles and a change in fluorescence from blue to colorless. The characteristic wavelength of the wrinkles remained largely unchanged while the amplitude decreased significantly upon light irradiation. HCl gas treatment weakened the hydrogen bonds, also leading to stress release and wrinkle erasure, along with a fluorescence color change from blue to orange-red. This change was due to the protonation of DSP-OH, which altered its electronic structure. Both the light and acid-induced changes were reversible. The spatial control of stress release using photomasks allowed for selective erasure of wrinkles, creating micropatterns. The high spatial resolution of the light-induced isomerization allowed for the creation of fluorescence micropatterns. The combination of wrinkled topography and fluorescence provided a multilevel anticounterfeiting tag, demonstrated by creating a QR code pattern that could be further modified through HCl treatment and thermal annealing to alter both its morphology and fluorescence. The study demonstrated the potential of this approach for message storage and dynamic displays.

This work successfully addressed the challenge of creating a highly secure anticounterfeiting tag by combining two independent security features: dynamic wrinkles and tunable fluorescence. The orthogonal control of both features through visible light and acid gas offers enhanced security against counterfeiting. The high reversibility and controllability of the system suggest broad applicability in advanced anti-counterfeiting technologies, smart displays, and information storage. The demonstrated ability to create complex and customizable patterns using photomasks increases the complexity and security of the tag significantly. The use of readily available materials and relatively simple fabrication methods adds to its practical potential.

This study successfully demonstrated a robust method for creating a dynamic dual-pattern exhibiting reversible wrinkled topography and tunable fluorescence. The orthogonal control using visible light and acid gas significantly enhances security against counterfeiting. The method's simplicity and versatility suggest broad applicability in various fields, including anticounterfeiting, dynamic displays, and information storage. Future research could explore the integration of this technology with other security measures, such as more complex patterns and the use of different stimuli.

The study primarily focused on the proof-of-concept and demonstration of the technology. Further investigation into the long-term stability and durability of the materials under various environmental conditions is necessary. A comprehensive analysis of the security level against advanced counterfeiting techniques should be undertaken. Expanding the range of stimuli used to control the dual-pattern could enhance its versatility.