The ability to modulate the physical properties of a supramolecular hydrogel may be beneficial for biomaterial and biomedical applications. We find that guanosine (G 1), when combined with 0.5 equiv of potassium borate, forms a strong, self-supporting hydrogel with elastic moduli >10 kPa. The countercation in the borate salt (MB(OH)(4)) significantly alters the physical properties of the hydrogel. The gelator combination of G 1 and KB(OH)(4) formed the strongest hydrogel, while the weakest system was obtained with LiB(OH)(4), as judged by H-1 NMR and rheology. Data from powder XRD, H-1 double-quantum solid-state magic-angle spinning (MAS) NMR and small-angle neutron scattering (SANS) were consistent with a structural model that involves formation of borate dimers and G4 center dot K+ quartets by G 1 and KB(OH)(4). Stacking of these G4 center dot M+ quartets into G4-nanowires gives a hydrogel. We found that the M+ cation helps stabilize the anionic guanosine-borate (GB) diesters, as well as the G4-quartets. Supplementing the standard gelator mixture of G 1 and 0.5 equiv of KB(OH)(4) with additional KCl or KNO3 increased the strength of the hydrogel. We found that thioflavin T fluoresces in the presence of G4 center dot M+ precursor structures. This fluorescence response for thioflavin T was the greatest for the K+ GB system, presumably due to the enhanced interaction of the dye with the more stable G4 center dot K+ quartets. The fluorescence of thioflavin T increased as a function of gelator concentration with an increase that correlated with the system's gel point, as measured by solution viscosity