Trace detection and differentiation of uranyl(VI) ion cast films utilizing aligned Ag nanorod SERS substrates

Aligned Ag nanorod arrays prepared by oblique angle vapor deposition (OAD) were utilized to observe the SERS response for cast films of adsorbed uranyl ion complexes. These nanorod arrays consisted of preferentially aligned and tilted cylindrically shaped nanorods (average nanorod length = 858 nm). Routine SERS spectra of cast films of uranyl nitrate resulted in 0.87 pg of material analyzed with a detection limit estimated at 0.87 fg of adsorbed uranyl nitrate. The main spectral feature observed was a strong, broad peak centered at 700 cm−1 assigned to the ν1 symmetric stretch of the uranyl ion, νs(UO2). The complex shape of this band is attributed to multiple uranyl complexes adsorbed onto the Ag surface. Uranyl nitrate cast films formed from ethanol solutions were more uniform and had stronger adsorption to the Ag surface compared to films formed from aqueous solutions. Differentiation of uranyl ion cast films formed from four different uranyl systems (i.e., uranyl nitrate hexahydrate, uranyl acetate dihydrate, uranyl formate monohydrate, and uranyl chloride trihydrate) was accomplished using univariate spectral analysis as well as Partial Least Squares Discriminant Analysis (PLSDA). A simple PLSDA calibration was able to correctly classify 27 SERS spectra of unknown uranyl ion cast films based on the type of anionic ligand present.