Planar metal–insulator–metal (MIM) optical cavities are attractive for biochemical and environmental sensing applications, as they offer a cost-effective cavity platform with acceptable performances. However, localized detection and scope of expansion of applicable analytes are still challenging. Here, we report a stimuli-responsive color display board that can exhibit local spectral footprints, for locally applied heat and alcohol presence. A thermoresponsive, optically applicable, and patternable copolymer, poly­(N-isopropylacrylamide-r-glycidyl methacrylate), is synthesized and used with a photosensitive cross-linker to produce a responsive insulating layer. This layer is then sandwiched between two nanoporous silver membranes to yield a thermoresponsive MIM cavity. The resonant spectral peak is blue-shifted as the environmental temperature increases, and the dynamic range of the resonant peak is largely affected by the composition and structure of the cross-linker and the copolymer. The localized temperature increase of silk particles with gold nanoparticles by laser heating can be measured by reading the spectral shift. In addition, a free-standing color board can be transferred onto a curved biological tissue sample, allowing us to simultaneously read the temperature of the tissue sample and the concentration of ethanol. The stimuli-responsive MIM provides a new way to optically sense localized environmental temperature and ethanol concentration fluctuations.

Planar and lithography-free metal–insulator–metal (MIM) resonators based on the Fabry–Pérot etalon are attractive for biochemical sensing applications because of their acceptable optical performance and cost-effectiveness. However, injecting analytes into the insulating layer where the optical field is localized (high light–matter interaction) is difficult. Here, planar and lithography-free MIM resonators interacting with their environment are reported. In the MIM, molecules of a liquid can infiltrate the inherent nanopores in the deposited silver nanomembrane and be absorbed into the silk protein hydrogel membrane. The silk layer is swollen when water molecules are absorbed, inducing a large shift in the resonance wavelength. Thus, in this study, the proposed MIM resonator was applied as a highly sensitive water sensor, and a water content as low as 0.008% in organic solvents could be determined by reading the shift in the transmission peak. This limit can be lowered further by using a high-resolution spectrometer and a thicker silk layer. In addition, the area of interaction can be artificially selected by applying an elastomer stamp and a patterned photoresist window.

Inkjet printing is an attractive bottom-up microfabrication technology owing to its simplicity, ease of use, and low cost. This method is particularly suitable for patterning of biomaterials because biofunctionality and bioactivity can be preserved during the patterning process in the absence of harsh conditions such as heat, UV radiation, and plasma. However, it is still challenging to apply this technology to biomaterial-based soft photonics, which requires precise control over morphology and uniformity to confine photons efficiently. This study introduces inkjet printing to create silk protein patterns to emit/guide a single-mode distributed feedback (DFB) laser on a single platform. A thin TiO2 coated grating enables coherent feedback of the generated photons for any shape of the printed silk pattern. The lasing wavelength can be adjusted by adding gold nanoparticles to the silk/dye ink. Photonic components of lasers and waveguides are drawn on a DFB board, and the lasing light can be extracted through adjacent waveguides. The printed components can be reformed by post modification (water-removal and reprinting). Additionally, optically absorptive melanin nanoparticles placed on the waveguide can attenuate the propagating light, thus adding utility for sensing applications. This allows a new method to fabricate cost-effective, easily functionalized, and versatile biomaterial photonic chips for advanced sensing and diagnosis.

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