(Nanowerk Spotlight) Counterfeiting of important documents like passports poses a serious and growing global security threat. Despite the use of various anti-counterfeiting technologies such as specialized inks, holograms and watermarks, the increasing sophistication of counterfeiters means that forgery remains an ongoing risk. A key vulnerability of current methods is that security features like inks are typically printed on document surfaces, making them more accessible for tampering or chemical analysis by fraudsters attempting to decipher their composition.
To combat this, some authorities have begun embedding customized security patterns within polymer films used for document pages, rather than relying solely on surface printing. This makes the chemical structure of security inks much harder to discern and copy. However, achieving high-resolution multicolor fluorescent patterns within the polymer matrix has proven challenging due to the limitations of conventional fluorescent inks.
An ideal photochemical printing solution would allow the fluorescent color at any given position in the film to be precisely controlled by the light dosage applied at that point during the patterning process – analogous to how grayscale values correspond to laser intensity in laser engraving.
Researchers have sought photoresponsive “phototransformer” compounds that could enable such spatially controlled color patterning through structural changes induced by light. To achieve full-color printing, the fluorophores would need to satisfy two key criteria.
First, they must undergo well-defined molecular transformations in response to specified light doses. Secondly, their emission color must shift substantially as the rigidity of the surrounding polymer matrix changes – a property known as rigidochromism.
Conventional fluorophores typically either bleach or constantly emit their inherent color under irradiation, rendering them unsuitable. Some photochromic dyes can generate images via light-induced isomerization or dimerization, but the patterns prove ephemeral due to the reversibility of the photoreactions. Moreover, previously studied compounds show little responsiveness to matrix rigidity. New molecules are needed that combine precise photocontrol with intense rigidochromic shifts.
In a new study published in Advanced Materials (“Multicolor Photochemical Printing Inside Polymer Matrices for Advanced Photonic Anticounterfeiting”), a team led by Prof. Weiguo Huang at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, has developed an innovative photochemical printing system that overcomes the drawbacks of previous approaches. Their work centers on a rationally designed “phototransformer” molecule called diphenanthridinyl fumaronitrile (trans-D5). This substance can undergo two sequential light-induced transformations, first from the initial transform to a cis isomer (cis-D5) and then to a cyclized product (cyclo-D5), accompanied by a dramatic stepwise redshift in emission color from cyan to yellow to red.
Illustration of the photoresponses of a) conventional fluorophores, b) phototransformers. c) The antirigidochromic behavior of the fluorophore, showing a pronounced emission redshifts when the polymer matrix becomes more rigid. d) The light dose-controlled multicolor photochemical printing enables by the synergistic effect of photo transformers and its antirigidochromic behavior. (Image: Reprinted with permission by Wiley-VCH Verlag)
Detailed spectroscopic and crystallographic characterizations revealed that the three isomeric forms of D5 exhibit markedly different molecular packing modes and fluorescence properties, both as pristine solids and when dispersed in polymer matrices. While trans-D5 and cis-D5 show only minor emission changes in polymers, cyclo-D5 exhibited a remarkable 172-nanometer bathochromic shift attributed to strong dipolar interactions between its rigid planar structure and polar groups on the polymer chains. The different emission hues of trans-D5, cis-D5 and cyclo-D5 span a wide gamut across the Munsell color space.
These distinct photophysical characteristics allowed the researchers to demonstrate high-resolution multicolor photopatterning by incorporating trans-D5 into a photocurable polymer precursor containing various acrylate monomers, a crosslinker and a photoinitiator. Irradiating the mixture through a photomask induced a cascade of events – cyclization of trans-D5 to the red-emitting cyclo-D5 form in the exposed areas, matrix hardening due to crosslinking polymerization, and emission color tuning by rigidochromic effects. Regions receiving higher light doses developed more intense red emission due to greater cyclo-D5 production and matrix rigidification, while less-exposed zones remained cyan-emitting.
Using their optimized photochemical printing protocol, Huang’s team generated high-fidelity fluorescent images embedded inside free-standing polymer films by adjusting the light exposure through a patterned mask. Portraits and other intricate designs were faithfully reproduced with resolutions sufficient to define fine details like strands of hair. The printed patterns proved exceptionally resistant to forgery and reverse engineering, combining several layers of security.
First, the use of photomasks enables unique “fingerprint” patterns that would be infeasible to duplicate. Second, the complex light-dose-dependent color mixing achieved through photocyclization and rigidochromic tuning is effectively impossible to reproduce without exact knowledge of the printing process and parameters. Finally, embedding the phototransformer at parts-per-million loading levels renders the fluorescent patterns chemically indecipherable by making any extractable ink traces insufficient for meaningful analysis.
In addition to these anti-counterfeiting advantages, the photo-printed polymer films showed excellent stability and durability. Prolonged exposure to aggressive conditions, including 90% relative humidity, temperatures of 60 °C, and even intense ultraviolet irradiation at 8 mW/cm2 for 5 minutes, caused negligible degradation of the fluorescent images.
The portraits also survived intact when the films were repeatedly bent, twisted and folded, highlighting their exceptional mechanical resilience. This robustness, combined with the flexibility and transparency of the polymer matrix, allowed prototype passport data pages to be created with embedded secure fluorescent portraits. The high-resolution images, which capture intricate details like single strands of hair with remarkable clarity and contrast, are completely invisible under ambient light but vividly revealed under UV excitation.
Notably, neither staining, abrasion, nor delamination compromised the integrity of the printed security features. These results demonstrate the technology’s readiness for real-world document protection applications demanding covert, high-fidelity information storage resistant to accidental damage or deliberate tampering.
The image of portrait after a) exposed to high humidity (90% RH for 1 hour). b) Hexane rinsing. c) Thermal treatment at 60 °C. d) UV-light irradiation (light intensity: 8 mW cm2) for 5 minutes. e–j) The image of the portrait embedded inside polymer film at bending, folding, and twisting states under UV light and day light. k–m) The polymer film laminated on a passport data page with a fluorescent portrait embedded. Scale bars in all images: 1 cm. (Image: Reprinted with permission by Wiley-VCH Verlag)
Beyond its obvious potential for enhancing travel and identity documents, this new photochemical printing technology could transform security across a wide range of industries. Embedding encrypted information within polymer films or coatings could help combat counterfeiting of high-value products like pharmaceuticals, luxury goods, electronics and mechanical parts. Secure patterning of polymeric identity cards, credit cards and bank notes with color-coded or machine-readable fluorescent tags may enable improved anti-fraud and authentication protocols. The long-term stability and chemical inertness of the photo-printed polymer films also suggest applications in archival preservation of records and cultural heritage materials.
More broadly, the ability to generate durable, high-resolution, multicolor images through selective photopatterning of fluorescent dye-polymer composites opens intriguing opportunities in the creative industries. Artists and designers could harness the technology to produce complex, multilayered visual effects through fluorescence patterning within 3D-printed structures, textiles, jewelry and other polymeric media. The option to conceal embedded images that are only revealed under specific lighting conditions lends itself to compelling visual illusions, interactive displays, and smart responsive materials with encoded information content.
The new photochemical printing platform reported by Huang and coworkers sets a new bar for document security by enabling encryption of high-resolution multicolor images inside transparent polymer substrates.
Two key innovations made this possible – the development of phototransformer fluorophores that combine precisely controllable light responses with intense rigidochromic phenomena, and the incorporation of these inks into photocurable matrices that translate light dose information into both color and material properties.
The multiplexed data storage achieved by encoding through light-directed chemical transformations, emission color shifts, and matrix crosslink-density variations makes the resulting security prints effectively unbreakable through known counterfeiting methods.
With a resolution limit near the diffraction limit and a color space spanning the visible spectrum, this new printing technology offers an unprecedented combination of aesthetic quality, anti-forgery protection and creative potential, heralding a step change in how we embed and encode information in polymeric materials.
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