Fluorinated Gold Nanoparticles for Nanostructure Imaging Mass Spectrometry

Abstract

Nanostructure imaging mass spectrometry (NIMS) with fluorinated gold nanoparticles (f-AuNPs) is a nanoparticle assisted laser desorption/ionization approach that requires low laser energy and has demonstrated high sensitivity. Here we describe NIMS with f-AuNPs for the comprehensive analysis of metabolites in biological tissues. F-AuNPs assist in desorption/ionization by laser-induced release of the fluorocarbon chains with minimal background noise. Since the energy barrier required to release the fluorocarbons from the AuNPs is minimal, the energy of the laser is maintained in the low μJ/pulse range, thus limiting metabolite in-source fragmentation. Electron microscopy analysis of tissue samples after f-AuNP NIMS shows a distinct "raising" of the surface as compared to matrix assisted laser desorption ionization ablation, indicative of a gentle desorption mechanism aiding in the generation of intact molecular ions. Moreover, the use of perfluorohexane to distribute the f-AuNPs on the tissue creates a hydrophobic environment minimizing metabolite solubilization and spatial dislocation. The transfer of the energy from the incident laser to the analytes through the release of the fluorocarbon chains similarly enhances the desorption/ionization of metabolites of different chemical nature, resulting in heterogeneous metabolome coverage. We performed the approach in a comparative study of the colon of mice exposed to three different diets. F-AuNP NIMS allows the direct detection of carbohydrates, lipids, bile acids, sulfur metabolites, amino acids, nucleotide precursors as well as other small molecules of varied biological origins. Ultimately, the diversified molecular coverage obtained provides a broad picture of a tissue's metabolic organization.

Keywords: fiber free diet; gut microbiome; mass spectrometry imaging; metabolomics; mice; nanostructure imaging mass spectrometry; perfluorinated gold nanoparticles.

Figure 1.

(A) Schematic of the perfluorinated monolayer on the surface of the f-AuNP. When the laser hits the nanoparticle, the energy is transferred to the surface causing the thermal release of the fluorinated chains; (B) phase separation between the f-AuNPs dissolved in perfluorohexane and the surface of the sample.

Figure 2.

(A) TEM images of f-AuNPs before and after laser irradiation; (B) SEM images obtained after laser irradiation of: mouse brain tissue only, mouse brain tissue with α-cyano-hydroxycinnamic acid (CHCA) matrix, and mouse brain tissue with f-AuNPs.

Figure 3.

Schematic of: (A) the in vivo study; (B) the f-AuNPs NIMS workflow for metabolite imaging in mouse colon samples.

Figure 4.

Number of detected features vs annotated metabolites obtained in the analysis of 5 μm mouse colon slices. Mice were fed with standard diet and metabolome coverage is reported for each chemical class.

Figure 5.

Mass spectrum obtained by f-AuNPs NIMS of a 5 μm mouse colon tissue slice. The insets show the distribution of targeted metabolites in the mice colon. Ion abundances were normalized according to the total signal in each pixel and phosphocholine was used for tissue alignment.

Figure 6.

Overall metabolic organization of the mouse colon under standard diet: (A) lipids, (B) bile acids, (C) carbohydrates, (D) SCFA. Metabolite intensities were normalized according to the total ion signal in each pixel and reported as sum of intensities for each metabolic class. Phosphocholine was used for tissue alignment.

Figure 7.

Metabolic rearrangement of the mouse colon under different diet regimen [standard diet (SD), fiber depleted diet (FDD) or polysaccharides and fiber depleted diet (PFD)]. Metabolite intensities were normalized according to the total ion signal in each pixel and reported as sum of intensities for each metabolic class. Phosphocholine was used for tissue alignment.