Self-assembly of microstructured protein coatings with programmable functionality for fluorescent biosensors

Proteins, as genetically programmable functional macromolecules, hold immense potential as biocompatible self-assembling building blocks. Despite their versatility in building coating materials, it has been often hindered from programming their functionality genetically. In this study, we demonstrate a modular self-assembly of protein coatings that are genetically programmable for a biosensor application. We designed recombinant fusion protein building blocks to form microstructured coatings on diverse substrates, such as glass or polymer, through a thermally triggered liquid-liquid phase separation and an orthogonal high-affinity coiled-coil interaction. We incorporated fluorescence proteins into coatings and controlled protein density to enable fluorescence imaging and quantification in a low-resource setting. Then, we created a coating for a calcium biosensor using a genetically engineered calcium indicator protein. This protein coating served as the foundation for our smartphone-based fluorescence biosensor, which successfully measured free calcium concentrations in the millimolar range at which extracellular calcium homeostasis is maintained. Using this fluorescence biosensor, we were able to detect abnormal physiological conditions such as mild or moderate hypercalcemia. We envision that this modular and genetically programmable functional protein coating platform could be extended to the development of highly accessible, low-cost fluorescent biosensors for a variety of targets.