Fig. 4. (Top) SDS-PAGE gel of UCNP–S-HRP (2), HRP (3) and UCNP–HRP (4) under UV-light (left) and natural light (right); (bottom) enzymatic activity of UCNP–S-HRP via thiol–ene crosslinking, UCNP–S-PEG, and HRP (red dots are superimposed on black squares).