Smartphone‐Assisted Colorimetric Detection of Cr ³⁺ using Vitamin B ₆ Cofactor Functionalized Gold Nanoparticles and Its Applications in Real Sample Analyses

This communication demonstrates a rapid, reproducible and highly selective colorimetric nanosensor for the detection of Cr 3+ ions in aqueous medium using the functionalized gold nanoparticles. The functionalized AuNPs were obtained in three steps: (i) the citrate capped Cit‐AuNPs were prepared by standard procedure, (ii) the thiolated Schiff base CAPLP was generated in‐situ by reacting vitamin B 6 cofactor pyridoxal 5′‐phosphate and cysteamine, and then (iii) the CAPLP was decorated over the surface of Cit‐AuNPs via ligand exchange method yielding spherical and well dispersed nanoparticles of size ∼16±2 nm. The wine red color of CAPLP‐AuNPs solution turned to purplish blue upon addition of Cr 3+ ions along with the SPR band was red‐shifted from 525 nm to 650 nm due to the complexation‐induced aggregation of nanoparticles. With a high selectivity and specificity, this nano‐assembly shows the detection limit down to 2.34 μM and finds applicability over a wide range of pH. The developed nanosensor was applied in environmental water samples for Cr 3+ ions detection and achieved ideal results. More significantly, the visually detectable color change of CAPLP‐AuNPs has been successfully integrated with a smartphone RGBColorValue app to make it useful for on‐site fast measurement and real‐time online analysis of Cr 3+ with the detection limit down to 11 μM.