DMD-based Fabrication of All-Inorganic and Fluorescent Microstructures : Continuous Patterning of Silver Nanoparticles, and Thermoresponsive 3D Holographic Hydrogels

Abstract

Conventional microstructure fabrication methods face significant challenges in achieving both material versatility and dimensional control. Photolithography requires polymer resists that compromise material properties, while techniques such as electron-beam lithography suffer from low throughput and high costs. These limitations have constrained development of functional micro-components that fully exploit properties of diverse material systems. Here, we present a versatile fabrication platform integrating digital micromirror device (DMD)- based optical lithography with two independent material strategies to overcome these challenges. Our first approach enables continuous production of ultrathin all-inorganic microstructures through photoinduced aggregation of noble metal nanoparticles. This method achieves anisotropic microstructure patterning with spatially controlled material distributions. We functionalized Ag and Au nanoparticles with thiol- based photoresponsive ligands and dispersed them in polar solvents with photoacid generators. Under DMD- directed UV exposure in a microfluidic channel, photo-responsive ligands undergo inter-nanoparticles crosslinking and photogenerated and photogenerated acids reduce electrostatic repulsion between particles, causing controlled aggregation into precisely patterned structures. This polymer-free process yields high-aspect- ratio architectures (thickness ~200 nm, aspect ratio >1000:1) while preserving the intrinsic properties of metal nanoparticles. This capability enables fabrication of anisotropic structures, including Janus-type configurations with spatially distinct material domains, supports continuous manufacturing with broad solvent compatibility. Our second approach focuses on the fabrication of thermally activated fluorescent hydrogel microstructures. The structures are prepared by copolymerizing thermo-responsive N-isopropylacrylamide (NIPAM) with 9- (acryloloxy)butyl anthracene-9-carboxylate as a fluorescent component within a PEGDA matrix, followed by digital photopatterning using a dithering mask through DMD lithography system. This creates three-dimensional microstructures with periodically controlled crosslinking density. When the temperature increases above the lower critical solution temperature (LCST), the NIPAM network undergoes a volume phase transition (VPTT) and the microstructure contracts by expelling water. This thermally induced contraction brings anthracene-based fluorophores into closer proximity, and restricts their intramolecular motion. Because non-radiative decay in anthracene derivative mainly occurs through segmental mobility of butyl group in the swollen state, the restriction of molecular motion induced by contraction suppresses these decay pathways and results in a fluorescence enhancement. Furthermore, the bottom of microstructures is mechanically attached to the substrate, creating a vertical gradient in shrinkage. It generates layer-dependent refractive-index modulation, activating three- dimensional volume phase holographic grating (VPHG). This activates holographic patterns that remain invisible below VPTT and appear upon thermal contraction above VPTT, enabling secure, thermally triggered optical information storage. Both strategies leverage the same DMD-based lithography framework but address different material classes and dimensional requirements. The all-inorganic approach targets high-performance plasmonic and catalytic applications where polymer contamination is detrimental, while the hydrogel-based system enables stimuli- responsive optical devices. This versatile platform demonstrates that careful material design combined with precise optical patterning can overcome trade-offs between material functionality, structural complexity, and manufacturing scalability. Our work provides potential toward advanced micro-devices for plasmonics, catalysis, and secure information storage.