Al₂O₃/Ag nanostructured substrates prepared by ALD/CVD for matrix-free laser desorption/ionization mass spectrometry

Nanostructure-assisted laser desorption/ionization (NALDI) is a promising matrix-free alternative to matrix-assisted laser desorption/ionization (MALDI) for the analysis of low-molecular-weight compounds. Here, we report the fabrication of NALDI substrates composed of atomic-layer-deposited Al₂O₃ interlayers on AISI 304 steel followed by chemical-vapor-deposited silver nanoparticles (AgNPs). Three independently prepared substrates were used to evaluate the reproducibility of the ALD/CVD approach. Surface morphology, composition, and optical properties were examined by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). SEM and AFM showed granular AgNP layers with mean particle sizes of 31 ± 13, 39 ± 19, and 51 ± 26 nm for plates 1-3, indicating that the initial substrate topography strongly affected the final nanostructure. XPS confirmed the presence of metallic silver, whereas UV-vis diffuse reflectance spectra exhibited a plasmon-related band at approximately 420 nm. The resulting Al₂O₃/Ag substrates were then evaluated in positive-ion NALDI MS using methionine, serine, lactose, and arabinose as model analytes. Amino acids produced protonated ions, metal-ion adducts, and fragment ions, with methionine and serine fragments remaining detectable down to 8 and 20 µg mL^-1, respectively. Saccharides showed extensive fragmentation, and characteristic fragment ions were observed over the full concentration range examined (100-0.05 µg mL^-1). The results demonstrate the feasibility of combining ALD-grown Al₂O₃ with CVD-grown AgNPs to generate matrix-free LDI substrates, while also showing that improved surface homogeneity will be necessary to enhance reproducibility and analytical sensitivity.