Tailoring of the desired selectivity and the turn-on detection range in a self-assembly-based fluorescence sensory system

Abstract

This study demonstrates how to control the selectivity and the turn-on detection range toward the tailoring of an assembly-based fluorescence (FL) sensory system. Assembly-based FL chemosensors composed of oligophenylenevinylene with a varied number of guanidinium receptors (G2, G4 and G6) were newly developed, and their FL response to nucleotides (AMP, ADP and ATP) was investigated. Indeed, G6 exhibited FL emission via self-assembly with ATP. More importantly, the FL response of G6 showed markedly improved selectivity for ATP over ADP and a broadly extended detection range of ATP concentration under adjusted salt conditions. The salt effect on the FL response revealed the competitive binding interactions affecting the subsequent self-assembly process. These studies have unveiled the pivotal binding mechanisms operating in the self-assembly process, which tailor the performance level of the assembly-based sensory system. We believe that this study offers a new design principle of an assembly-based FL chemosensor with high selectivity and the appropriate detection range, being different from the conventional key-and-lock system.

Fig. 1. Chemical structures of G2, G4 and G6.

Fig. 2. (a) Photographs of G2 and G6 (λ _ex = 365 nm) in the presence of ATP (20 μM). (b) Excitation (λ _em = 514 nm) and fluorescence spectra (λ _ex = 388 nm) of G2 and G6 in the presence of ATP (20 μM) in DMSO–water (1 : 9 v/v); green line: G2; red line: G6. Fluorescence titration curve of (c) G2 and (d) G6 upon addition of increasing concentrations of AMP (blue), ADP (green) and ATP (red) in DMSO–water (1 : 9 v/v). Conditions: [G2] = [G6] = 6.0 μM; [HEPES] = 10 mM (pH 7.4); 25 °C; λ _ex = 388 nm; λ _em = 514 nm.

Fig. 3. Fluorescence emission dependence on salt concentration arising from the self-aggregation of G2 (green) and G6 (red) in DMSO–water (1 : 9 v/v). Conditions: [G2] = [G6] = 6.0 μM; [HEPES] = 10 mM (pH 7.4); 25 °C; λ _ex = 388 nm; λ _em = 514 nm.

Fig. 4. Photograph of G6 (λ _ex = 365 nm) in the presence of AMP, ADP and ATP (180 μM for the three) and fluorescence titration curves (λ _ex = 388 nm; λ _em = 514 nm) of G6 upon addition of increasing concentrations of AMP (blue), ADP (green) and ATP (red) under physiological salt conditions. Conditions: [G6] = 6.0 μM; [NaCl] = 125 mM; [KCl] = 5.0 mM; [CaCl₂] = 1.0 mM; [MgCl₂] = 0.5 mM; [HEPES] = 10 mM (pH 7.4); 25 °C.

Fig. 5. Changes in the fluorescence intensity of G6/nucleotide (green: ADP; red: ATP) upon addition of increasing concentrations of (a) NaCl and (b) CaCl₂ in DMSO–water (1 : 9 v/v); conditions: [G6] = 6.0 μM, [nucleotide] = 20 μM, [HEPES] = 10 mM (pH 7.4), 25 °C; λ _ex = 388 nm; λ _em = 514 nm. (c) Fluorescence titration result of G6 upon addition of increasing concentrations of AMP (blue), ADP (green) and ATP (red) with a fixed concentration of CaCl₂ (1.0 mM); conditions: [G6] = 6.0 μM; [HEPES] = 10 mM (pH 7.4); 25 °C; λ _ex = 388 nm; λ _em = 514 nm.