Preparation of highly luminescent mannose–gold nanodots for detection and inhibition of growth of Escherichia coli

In this paper, we describe a novel, simple, and convenient method for preparing water-soluble biofunctional gold nanodots (Au NDs) for the sensitive and selective detection of Escherichia coli (E. coli) and the inhibition of its growth. We obtained luminescent mannose-capped Au NDs (Man–Au NDs) from as-prepared 2.9-nm Au nanoparticles (Au NPs) and 29,29′-dithio bis(3′,6′,9′,12′,15′,18′-hexaoxa-nonacosyl α-d-mannopyranoside) (Man–RSSR–Man). To obtain improved quantum yield (>20%), luminescent Man–Au NDs (1.8 nm) were prepared from Au NPs (0.47 μM) and Man–RSSR–Man (2.5 mM) in the presence of sodium borohydride (NaBH4; 1.0 mM). The highly luminescent properties of Man–Au NDs prepared by the NaBH4-assisted method were characterized by UV–vis absorption, photoluminescence, and X-ray photoelectron spectroscopies. The results supported the high-density coverage of the NDs surface by Man–RS ligands. Multivalent interactions between Man–Au NDs and FimH proteins located on the bacterial pili of E. coli resulted in the formation of aggregated cell clusters. After concentrating this agglutinative E. coli from a large-volume cell solution (5 mL), Man–Au NDs were displaced by mannose (100 mM) and stabilized by Man–RSSR–Man (5 mM). Monitoring the luminescence of Man–Au NDs allowed the detection of E. coli at levels as low as 150 CFU/mL. Man–Au NDs were also found to be efficient antibacterial agents, selectively inhibiting the growth of E. coli through Man–Au ND-induced agglutination. Our small-diameter Man–Au NDs, which provided an ultra high ligand density (local concentration) of mannose units for multivalent interactions with E. coli, have great potential for use as an antibacterial agent in other applications.