*2.1. Materials and Reagents*

Nodularin-R and microcystins (MC-LR, -LA, -LY, -LW, -LF, -YR, -WR, -RR) (Enzo, USA) were used as standards. Nodularin-R-ovalbumin (NOD-R-OVA, prepared in our lab) was the coating antigen. Microcystin-LR-keyhole limpet hemocyanin (MC-LR-KLH, prepared in our lab) was used as the immunogen. An anti-MC-LR monoclonal antibody (anti-MC-LR MAb, prepared in our lab) was used for comparison with the nanobodies. pComb3XSS vector and *E. coli* BL21(DE3) available in our lab were used to construct a recombinant vector and a transformed host strain, respectively. Helper phage M13K07 (New England Biolabs, Ipswich, MA, USA) was used for the construction of a phage displayed nanobody library. The anti-VHH-HRP polyclonal antibody from rabbit (Genscript Bio. Co.Ltd, Nanjing, China) was used as a secondary antibody in ELISA. HisPur Ni-NTA resin (GE Healthcare, Beijing, China) was utilized for protein purification. The primers used in this work were synthesized by Invitrogen Biotechnology Co. (Shanghai, China). Other chemical reagents were provided by Sigma (St. Louis, MS, USA) and Thermo Fisher Scientific (Thermo, Waltham, MA, USA).
