Artificial allosteric protein switches (biosensors) hold the promise to deliver disruptive analytical and diagnostic applications. However, their construction is complicated by the limited availability of selective receptor domains. Here, we report the use of mRNA display to rapidly select FN3con-based binding domains to S100A7 protein - a biomarker of heart failure. The crystal structure of the resulting FN3con binding domain in complex with S100A7 dimer revealed that the binding interface of the dimer is formed by similar, but not identical, side-chain interaction networks. Using medium-throughput functional screening, we tested selected binding domains for compatibility with two protein biosensor architectures. The best biosensor demonstrated a dynamic range of 57-fold and a 1 nM limit of detection and was used to establish a rapid homogeneous assay for quantification of S100A7 in clinical saliva samples. The assay was able to distinguish heart failure patient samples from those of healthy donors. Our results demonstrate that mRNA binding domain development and biosensor prototyping pipelines can deliver practically useful biosensors to potentially any target.