Environmentally Safe Mercury(II) Ions Aided Zero-Background and Ultrasensitive SERS Detection of Dipicolinic Acid

Field, reliable, and ultrasensitive detection of dipicolinic acid (DPA), a general biomarker of bacterial spores and especially Bacillus anthracis, is highly desirable but still challenging in current biometric security emergency response system. Herein we report an environmentally safe mercury(II) ions-mediated and competitive coordination interaction based approach for rationally designed surface-enhanced Raman scattering (SERS)-active gold nanoparticles (AuNPs), enabling rapid, ultrasensitive and zero-background detection of DPA without the pretreatment of samples. By means of competitiveness, these papain-capped gold nanoparticles (P-AuNPs) are induced to undergo controllable aggregation upon the addition of Hg²⁺ ions and DPA with a concentration range (1 nM∼8 μM), which correspondingly cause quantitative changes of SERS intensity of cresyl violet acetate (CVa) conjugated AuNPs. The decreased Raman intensity obtained by subtracting two cases of additives that contain only Hg²⁺ and the mixture of Hg²⁺ and DPA is proportional to the concentration of DPA over a range of 1 nM∼8 μM (R² = 0.9824), with by far the lowest limit of detection (LOD) of 67.25 pM (0.01 ppb, S/N = 3:1). Of particular significance, mercury(II) ions actually play two roles in the process of measurements: a mediator for two designed competitive ligands (DPA and papain), and also a scavenger for the possibly blended ligands due to the different interaction time between DPA and the interferent with Hg²⁺ ions, which guarantees the interference-free detection of DPA even under real conditions.