Optical microchips based on high-affinity recombinant protein binders—Human serum albumin detection in urine

•Fluorescence biosensors based on albumin-binding domain (ABD) of protein G were developed.•The limit of detection of human serum albumin in urine meets clinically relevant values.•The length of peptide spacer on recombinant binder is key parameter for biosensor performance.•Streptavidin-ABD fusion protein enabled one-step immobilization process and high sensitivity.•Biosensors for other molecular targets recognized by ABD may be fabricated by similar approach.