A guanosine-based 2-formylphenylborate ester hydrogel with high selectivity to K+ ions

Abstract

Guanosine-based supramolecular hydrogels are particularly of interest for biomaterial and biomedical purposes, as they are generally biocompatible and stimuli-responsive. We found a strong and long-life transparent hydrogel made by mixing guanosine (G) with 1 equiv. of 2-formylbenzeneboronic acid (2FPB) and KOH. Alkali cations can assist the stacking of individual G-quartet to give extended nanowires, but only K⁺ ion induces the formation of a stable and self-supporting network hydrogel for a couple of months. Data from variable temperature NMR indicated that guanosine 2-formylbenzeneborate ester and G are the key components of the self-assembly. Further, G-2FPB-K⁺ hydrogel solution can induce berberine (BBR) fluorescence, showing high selectivity to K⁺ ion and anti-ion interference capability. A good linear relationship between fluorescent intensity and K⁺ concentration allowed us to directly detect K⁺ levels in human blood serum.

Fig. 1. Self-assembled hydrogel G-2FPB-K⁺ formed by guanosine (1) and 2-formylbenzeneboronic acid (2) in the presence of K⁺ ions.

Fig. 2. (a) PXRD and (b) CD spectroscopy of 50 mM G-2FPB-K⁺ hydrogel (1, 2, and KOH 50 mM each). (c) VT ¹H NMR (left) and VT ¹¹B NMR (right) spectra of a 50 mM G-2FPB hydrogel in D₂O. BF₃·O(C₂H₅)₂ was used as reference to calibrate the chemical shift in VT ¹¹B NMR. 2,2,3,3-(d₄)-3-(Trimethylsilyl)propionic acid sodium salt (0.31 mM) was used to quantify the concentration of different peaks in VT ¹H NMR. (d) The variations of content of 1 and 3 at different temperatures. The result was calculated according to the data from VT ¹H NMR and ¹¹B NMR.

Fig. 3. Images and measurements of a G-2FPB-K⁺ hydrogel. TEM micrograph, (a) 10 mM and (b) 5 mM. (c) AFM topographic image, 5 mM. The inset shows the profiles delimited by the red lines in the main panels. (d) Dynamic frequency sweeps and (e) oscillatory stress sweeps of a 50 mM G-2FPB-K⁺ hydrogel.

Fig. 4. (a) Schematic illustration of fluorescence of G-2FPB-Na⁺–K⁺ system in the presence of BBR. (b) The selectivity of fluorescence enhancement of BBR in G-2FPB-M⁺ solution for K⁺. (c) Selectivity of the 100 mM G-2FPB-Na⁺ solution for the detection of K⁺ against the interference ions in the PB buffer containing 31 μM BBR. Black bar: mono/divalent metal ions. Red bar: K⁺ (0.2 mM, 1 equiv.) and mono/divalent metal ions in the G-2FPB-Na⁺ solution in pH 7.4 PB buffer. Various amounts of different valents cations: (1) 100 equiv.: Li⁺, Na⁺, Cs⁺; (2) 10 equiv.: NH₄⁺, Mg²⁺, (3) 1 equiv.: Rb⁺, Ca²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Fe³⁺. λ_ex = 371 nm, λ_em = 523 nm. (d) The corresponding plot of fluorescence intensity (523 nm) vs. the concentration of KCl.