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 (1.8nm) were prepared from Au NPs (0.47μM) and Man-RSSR-Man (2.5mM) in the presence of sodium borohydride (NaBH ₄; 1.0mM). The highly luminescent properties of Man-Au NDs prepared by the NaBH ₄-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 (5mL), Man-Au NDs were displaced by mannose (100mM) and stabilized by Man-RSSR-Man (5mM). Monitoring the luminescence of Man-Au NDs allowed the detection of E. coli at levels as low as 150CFU/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.