Mass spectrometry imaging (MSI) has made the spatio-chemical characterization of a broad range of small-molecule metabolites within biological tissues possible. However, available matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) suffers from severe background interferences in low-mass ranges and inhomogeneous matrix deposition. Thus, surface-assisted LDI-MS (SALDI-MS) has been an attractive alternative for high-sensitivity detection and imaging of small biomolecules. In this study, we construct a new composite substrate, hydrophobic polydopamine (hPDA)-modified TiO₂ nanotube (TDNT) coated with plasmonic gold nanoparticle (AuNP-hPDA-TDNT), as a dual-polarity SALDI substrate using an easy and cost-effective fabrication approach. Benefitting from the synergistic effects of TDNT semiconductor and plasmonic PDA modification, this SALDI substrate exhibits superior performance for dual-polarity detection of a vast diversity of small molecules. Highly reduced background interferences, lower detection limits, and spot-to-spot repeatability can be achieved using AuNP-hPDA-TDNT substrates. Due to its unique imprinting performance, various metabolites and lipids can be visualized within jatropha integerrima petals, ginkgo leaves, strawberry fruits, and latent fingerprints. More valuably, the universality of this matrix-free substrate is demonstrated for mapping spatial distribution of lipids within mouse brain tissue sections. Considered together, this AuNP-hPDA-TDNT material is expected to be a promising SALDI substrate in various fields, especially in nanomaterial development and life sciences.

Design and discovery of carrier-mediated modified pesticides are vital for reducing pesticide dosage and increasing utilization, yet it remains a great challenge due to limited insights into plant translocation mechanisms. Nanostructure/nanoparticle assisted laser desorption/ionization strategy has established itself as a preferential analytical tool for biological tissue analysis, whereas potential applications in plant sciences are hindered with regard to the inability to slice plant leaves and petals. Herein, we report gold nanoparticle (AuNP)-immersed paper imprinting mass spectrometry imaging (MSI) for the spatiotemporal visualization of pesticide translocation in plant leaves. This approach plays a dual role in preserving spatial information and improving ionization efficiency for pesticides regardless of imaging artifacts due to homogenous AuNP deposition. Using this MSI platform, we proposed the elaborate plant translocation mechanism of agrochemicals for the first time, which is currently poorly understood. The dynamic processes of carrier-mediated pesticides can be clearly visualized, including crossing of plasma membranes by transporters, translocation downward in stems through the phloem, diffusion to the xylem and, conversely, accumulation at margins of the treated leaves. Moreover, this AuNP-assisted paper imprinting method could be highly compatible with laser-based MSI instruments, expediting researches across a broad range of fields, especially in nanomaterial development and life sciences.