Adsorption and detection of Escherichia coli using an Au substrate modified with a catecholate-type artificial siderophore–Fe3+ complex

Abstract

A catecholate-type artificial siderophore with a terminal-NH₂ group (1) and its Fe³⁺ complex (2) were prepared. Siderophore 1 was characterized by ¹H NMR, FT-IR, and ESI-TOF MS spectroscopy. The corresponding Fe³⁺ complex 2 was obtained by reaction of 1 with Fe(acac)₃. The absorption band at 500 nm (ε = 4670 M⁻¹ cm⁻¹ at pH 7.0) of the electronic absorption spectrum of 2 is assignable as the LMCT (O_catecholate → Fe³⁺) absorption band. This band indicates the formation of the Fe³⁺ complex of 1. The biological activity of 2 with respect to Escherichia coli was clearly confirmed by observing that it permeates into the cell membrane. The self-assembled monolayer of 2 on an Au substrate, 2/Au, was prepared and its preparation was confirmed by FT-IR reflection–absorption spectroscopy (IR-RAS) and cyclic voltammetry (CV). Furthermore, a quartz crystal microbalance (QCM) chip modified with 2 effectively adsorbed E. coli. M. flavescens, an organism which is incapable of synthesizing siderophores and must therefore use exogenous hydroxamate-type siderophores for growth, did not adsorb on 2/Au. In contrast, E. coli did not adsorb on the hydroxamate-type artificial siderophore–Fe³⁺ complex (3)-modified Au substrate, 3/Au. These results provide preliminary evidence that microbes recognized Fe³⁺ ion-bound siderophores on the surface. The detection limit of 2/Au was ∼10⁴ CFU mL⁻¹.