Flora Schoefbeck

Optical encryption based on nanostructured materials offers an innovative approach to secure data encoding. Here, we report an electrically addressable, colloidal photonic platform using covalent organic framework (COF) particles that enables dynamic, bistable image encryption. Through the electrophoretic self-assembly of size-controlled COF particles within patterned cells, we achieve spatially resolved Bragg reflections visible only under bright-field (BF) microscopy. The encrypted optical states remain hidden to the naked eye due to strong broadband scattering caused by nanoscale surface roughness—a feature tunable via synthesis time. Notably, the system exhibits bistability, maintaining the encoded state even after the removal of the electric field (EF) and enabling full reversibility by field polarity switching. The combination of electrical control, conditional visibility, and algorithmic decoding results in a multi-factor encryption strategy within a single material system. This work positions colloidal COF dispersions as a new class of multifunctional photonic materials for secure displays, anti-counterfeiting, and adaptive optical technologies.

Poly(3,4-ethylenedioxythiophene)-functionalized (PEDOT) MIL-101(Cr) nanoparticles were deposited via drop-casting onto interdigitated electrodes (IDEs). The application of an alternating potential during drop-casting allowed precise spatioselective deposition of the nanoparticles in the interelectrode gaps, generating percolating nanoparticle chains aligned between the finger electrodes. The resulting conductivities of the aligned and chained samples were up to 4 orders of magnitude higher than that of unaligned samples prepared via regular drop-casting. The performance of the unaligned and aligned samples for resistive humidity sensing was investigated, whereby the aligned samples showed a conductivity gain of 230 times at a relative humidity of 90% versus 10%, whereas the unaligned samples showed a lower gain of 40 times at relative humidity 90% versus 10%. Further, the aligned samples could also be utilized as capacitive humidity sensors, showing a change of sensitivity at different frequencies for the aligned sample, whereas unaligned samples show a resistive behavior unsuitable for capacitive sensing. This work demonstrates that simple drop-casting when carried out in conjunction with an applied alternating potential exerts dielectrophoretic control allowing for precise spatioselective deposition, thus leading to higher sample conductivity and enhanced performances for humidity sensing.