Development of Micro-Printing Technology for Fluorescent Encrypted Structure Fabrication

Abstract

Optical encryption and anti-counterfeiting technologies have become increasingly critical across various industries to protect sensitive information and prevent economic losses from counterfeit products. Among various functional nanomaterials, carbon dots (CD) have emerged as promising candidates for security applications due to their exceptional properties including tunable photoluminescence, excellent photostability, biocompatibility, and low toxicity. These unique optical characteristics enable CD to serve as fluorescent markers for information encoding. However, the potential of CD in advanced security applications has been hindered by limitations in available fabrication techniques. Conventional fabrication methods for creating CD-based fluorescent structures have been constrained to either two-dimensional patterns or bulk-scale three-dimensional structures. Lithography based approaches, while capable of high-resolution patterning, are limited to planar geometries and require complex multi-step processes with photomasks. This dimensional constraint presents significant challenges in fabricating high-resolution three-dimensional microstructures that could enable more sophisticated optical information encoding schemes with enhanced security levels. The inability to create microscale 3D fluorescent structures has limited the application of CDs in advanced anti- counterfeiting technologies that require complex structural information encoding. To address these limitations, this study presents a novel fabrication approach for micro-scale three- dimensional fluorescent structures by integrating carbon dots with meniscus-guided micro printing technology. The meniscus-guided printing method operates on the principle of controlled ink deposition through a liquid meniscus formed between a micronozzle and substrate, enabling precise material placement at the microscale. To implement this approach, a comprehensive system encompassing hardware and software components was developed. Precision control software with multi-port serial communication, G-code parsing capabilities, and 9-axis motion control was developed to achieve positioning accuracy of 0.125 µm. This sub-micrometer precision enables the fine control necessary for meniscus formation and ink deposition. A critical technical challenge was establishing a stable process window that balances the rheological properties of CD-incorporated polymer inks with printing parameters. Through systematic optimization examining the relationships between printing speed, nozzle inner diameter, and hydroxypropyl cellulose (HPC) concentration, optimal fabrication conditions were identified. Using the optimal process window, various shapes of three-dimensional microstructures were successfully fabricated. Complex geometries including stacked letter patterns, vertical micro-pillars, and arch-shaped overhang structures were realized with feature sizes at the micrometer scale. All fabricated structures exhibited uniform cyan fluorescence emission under 365 nm UV excitation, confirming that the optical properties of carbon dots were fully preserved throughout the printing processes. Long-term stability testing revealed that the fluorescence intensity remained consistent even after 60 days, demonstrating the long-term stability of the printed structures. To demonstrate the encryption functionality, a 4×20 microarray encoding the text "UNIST" was fabricated at a resolution of 2540 pixels per inch (PPI). The array consisted of alternating pillars with and without CD, creating a binary encoding pattern. This encrypted microarray demonstrated clear dual-state optical response. The pattern remained invisible under white light but was clearly revealed under UV illumination, confirming its suitability for covert information storage and anti-counterfeiting applications. This work presents a platform for fabricating micro-scale three-dimensional fluorescent structures that overcome the dimensional limitations of conventional methods. The meniscus-guided printing approach offers several advantages over lithography-based techniques, including lower equipment costs, simpler process workflows, mask-free direct patterning capability, and compatibility with various substrate materials. Beyond the applications in optical encryption and anti-counterfeiting, this platform technology holds significant potential for extension to other fields requiring fluorescent micropatterns, including biosensors for medical diagnostics, micro-optical components for photonic devices for advanced materials. Furthermore, the methodology is not limited to carbon dots but can be adapted to other functional nanomaterials such as quantum dots, expanding the scope of achievable functionalities. This study establishes a new paradigm for precision additive manufacturing of functional fluorescent nanostructures and is expected to serve as a foundation for the development of next-generation optical security technologies with enhanced complexity and security levels.