Here, we report gold
nanoparticle-coated starch magnetic beads (AuNP@SMBs) that were prepared
by in situ synthesis of AuNPs on the surface of SMBs. Upon functionalization
of the surface with a specific antibody, the immuno-AuNP@SMBs were
found to be effective in separating and concentrating the target pathogenic
bacteria, Escherichia coli O157:H7,
from an aqueous sample as well as providing a hotspot for surface-enhanced
Raman scattering (SERS)-based detection. We employed a bifunctional
linker protein, 4× gold-binding peptide-tagged Streptococcal
protein G (4GS), to immobilize antibodies on AuNP@SMBs and AuNPs in
an oriented form. The linker protein also served as a Raman reporter,
exhibiting a strong and unique fingerprint signal during the SERS
measurement. The amplitude of the SERS signal was shown to have a
good correlation with the concentration of target bacteria ranging
from 100 to 105 CFU/mL. The detection limit
was determined to be as low as a single cell, and the background signals
derived from nontarget bacteria were negligible due to the excellent
specificity and colloidal stability of the immuno-AuNP@SMBs and SERS
tags. The highly sensitive nature of the SERS-based detection system
will provide a promising means to detect the pathogenic microorganisms
in food or clinical specimen.