Hydrogen Bond-Assisted Colorimetric Picomolar Level Detection of Hg²⁺ Ions in 100% Aqueous Solution

Low-cost naked eye detection methods of toxic metal ions such as Hg²⁺ and Ag⁺ ions in water samples are extremely important owing to their prevalent availability in the environment. Herewith, we have designed a molecular rotor based on julolidine (Jul) as a donor (D) and barbituric acid (BA)/thiobarbituric acid (TBA) as an acceptor (A), which possess higher molar extinction coefficient (ε) values owing to the intramolecular charge transfer interactions. Evaluation of the sensing in neat water suggests the selective colorimetric response only for Hg²⁺ by both the probes, Jul-BA and Jul-TBA, while the latter also responds to the presence of Ag⁺ ions but with a different signal. The detection level is as low as 3.82 pM (8.63 ppt) for mercury and 2.01 pM (3.88 ppt) for silver ions, a much lower value than their permissible limit in drinking water prescribed by the regulatory bodies like US Environmental Protection Agency (EPA). The picomolar level detection in neat water using the colorimetric method by the simple D–A organic chromophore is rare and would have originated from the high ε values of Jul-BA (6.82 × 10⁴ M^–1 cm^–1) and Jul-TBA (16.96 × 10⁴ M^–1 cm^–1) and intermolecular hydrogen-bonded, cyclic, organized hexameric association (rosette) of the probes that provide the cooperativity in binding with the binding constant of Jul-BA with Hg²⁺ (K_a = 1.52 × 10⁵ M^–1) and Jul-TBA with Ag⁺ (K_a = 1.20 × 10⁵ M^–1) and Hg²⁺ (K_a = 1.05 × 10⁵ M^–1). The higher stoichiometries determined from the Job’s plot between the probe to metal 3:1 (Jul-BA-Hg²⁺), 2:1 (Jul-TBA-Ag⁺), and 3:1 (Jul-TBA-Hg²⁺) further confirm that the lowest detection limit is aided by the ordered, organized structure of the probe in water aided by hydrogen bonding, and destruction of intermolecular hydrogen bonding yields feeble response in metal ion detection. The computational studies also substantiated the results. The probe, Jul-TBA, responds selectively to Hg²⁺ ions in the real samples collected from different water sources even with the very high TDS of 2410 ppm.