Gold nanoparticle-based colorimetric sensing of dipicolinic acid from complex samples

Dipicolinic acid (DPA) can cause neurotoxicity and is abundant in bacterial spores. Although analytical methods have been reported for DPA detection with high sensitivity, their selectivity toward DPA is declined greatly in the presence of phosphates in the samples. In this study, we developed an approach for DPA detection that is not affected by the presence of phosphates. A colorimetric method based on the use of gold nanoparticles (AuNP) complexed with Ca2+ as sensing agents was explored for DPA detection. Calcium ions and glutathione-capped gold nanoparticles (AuNPs@GSH) can easily form complexes (Ca2+-AuNP@GSH) through GSH-Ca2+ chelation, leading to the aggregation of AuNPs@GSH. The aggregation resulting from the complexes of AuNPs@GSH and Ca2+ can be reversed with the addition of DPA owing to the high formation constant (log Kf = 4.4) between DPA and Ca2+. Furthermore, the color of AuNPs@GSH changes from red to purple when complexed with Ca2+, returning to red upon addition of DPA. The limit of detection of this sensing method toward DPA was estimated to be as low as ~ 2 μM. The feasibility of using the sensing method for quantitative detection of DPA in soil and Bacillus cereus spore samples was also demonstrated.