Transmission electron micrograph of nano-gold-SPA probes. The arrowheads pointed to nano-gold-SPA probes. The structure of halo disturbance surrounding the nano-gold was SPA proteins. In keeping the objective of reacting with antibodies, SPA was chosen, for that, SPA could be site-specifically attached to Fc region of antibody, preserving its antigen-combining activity. And nano-gold clustered for silver enhancement.