Development of a highly selective and sensitive colorimetric detection of manganese(II) ion in environmental water using 3-(4-hydroxy-3-methoxyphenyl)-2,3-dihydropyrazolo [3, 4-b] indole-1(4H)-carbothioamide modified gold nanoparticles: A CIE L*a*b*/

• Development of a sensitive, selective, rapid and efficient novel colorimetric probe for the detection of Mn(II) using TRPIDA_V modified gold nanoparticles. • TRPIDA_V-AuNPs aggregated upon introduction of 2 mg/L of Mn(II). • Developed probe showed high selectivity only towards Mn(II). • The colorimetric probe yielded a detection limit of 0.00691 mg/L with a linear range of 0.5–10 mg/L • The colour difference values increased with increasing Mn(II) standard concentration. This study proposes a sensitive, selective and efficient novel colorimetric probe for the detection of manganese ion (Mn(II)) using 3-(4-hydroxy-3-methoxyphenyl)-2,3-dihydropyrazolo [3, 4-b] indole-1(4H)-carbothioamide (TRPIDA_V) modified gold nanoparticles (TRPIDA_V-AuNPs). TRPIDA_V-AuNPs aggregated upon introduction of 2 mg/L of Mn(II), which led to a change in colour of the dispersed TRPIDA_V-AuNPs suspensions from red to blue and a decrease of the surface plasmon absorption intensity at 520 nm resulting in the formation of a second peak at 655 nm. The colorimetric probe showed high selectivity to Mn(II) and was not selective to other investigated anion and cations as there was no aggregation induced upon addition of 2 mg/L of other anions and cations. Furthermore, the colour change from wine red to blue was only observed in the introduction of 2 mg/L Mn(II) resulting in a second absorption peak at 655 nm. Additionally, the colorimetric probe yielded a detection limit of 0.00233 mg/L with a linear range of 0.5–10 mg/L, showing excellent sensitivity towards Mn(II). The obtained XYZ values displayed similar trends as the RGB values where a rapid decrease in the X values was observed from 1 to 2 mg/L, the Y values increased from 0 to 1 mg/L and decreased significantly at 2 mg/L followed by an increase from 3 to 10 mg/L and the Z values increased rapidly from 0 to 4 mg/L then a decrease at 5 mg/L was observed followed by a rapid increase from 6 to 10 mg/L. It was observed that the colour difference values increased with increasing Mn(II) standard concentration. Hue decreased as the Mn(II) concentration increased and the chroma decrease significantly from 1 to 2 mg/L indicating the TRPIDA_V-AuNPs in the presence of Mn(II) standards solutions changed from vivid to dull as the Mn(II) increases. The obtained results proved the developed colorimetric probe to be highly robust and efficient for the detection of Mn(II) in water samples.