Portable SERS Detection of Dipicolinic Acid with AgAu Nanostructures

Surface-enhanced Raman scattering (SERS) has emerged as a powerful analytical tool, but translating its sensitivities from laboratory Raman microscopes to portable platforms remains a critical challenge for real-world deployment. Here we present a systematic evaluation of Au nanoparticles (Au NPs), Au@Ag core–shells (CS), and Au@AgAu nanorattles (NRs) as SERS substrates for dipicolinic acid (DPA), a key biomarker of Bacillus anthracis spores, using a benchtop portable Raman spectrometer. The nanostructures were synthesized through a stepwise route that yields a coherent series of Au, Ag-rich, and mixed AgAu surface architectures, enabling nanoparticle number-normalized performance comparisons. Benchmarking with 4-aminothiophenol (4-ATP) revealed that while nanorattles provided the strongest per-particle enhancements, core–shell substrates delivered the lowest detection limits, achieving reliable detection of 4-ATP at picomolar levels. Extending to DPA, both CS and NRs achieved detection down to 10–6 M under alkaline conditions, with performance governed by pH-controlled speciation: nanorattles excelled under neutral conditions due to mixed-domain adsorption, while core–shells outperformed under alkaline conditions where the dianion binds strongly to Ag surfaces. DFT calculations of adsorption energies, binding geometries, and theoretical Raman spectra corroborated the experimental findings, showing stronger binding on Ag followed by AgAu surfaces. Importantly, the ∼10–6 M detection limit corresponds to ∼ 106–107 spores mL–1, directly relevant to practical screening scenarios such as enriched swabs or suspicious powders. These findings establish a proof-of-concept baseline for portable SERS detection of spore biomarkers and highlight the interplay between nanostructure architecture, analyte speciation, and excitation wavelength, offering generalizable design principles for the development of field-ready nanosensors.