Gold nanoparticles as assisted matrices for the detection of biomolecules in a high-salt solution through laser desorption/ionization mass spectrometry

Lab-on-a-plate: extending the functionality of MALDI-MS and LDI-MS targets

We review the literature that describes how (matrix-assisted) laser desorption/ionization (MA)LDI target plates can be used not only as sample supports, but beyond that: as functional parts of analytical protocols that incorporate detection by MALDI-MS or matrix-free LDI-MS. Numerous steps of analytical procedures can be performed directly on the (MA)LDI target plates prior to the ionization of analytes in the ion source of a mass spectrometer. These include homogenization, preconcentration, amplification, purification, extraction, digestion, derivatization, synthesis, separation, detection with complementary techniques, data storage, or other steps. Therefore, we consider it helpful to define the "lab-on-a-plate" as a format for carrying out extensive sample treatment as well as bioassays directly on (MA)LDI target plates. This review introduces the lab-on-plate approach and illustrates it with the aid of relevant examples from the scientific and patent literature.

A sample preparation method for gold nanoparticle-assisted laser desorption/ionization time-of-flight mass spectrometry

A sample preparation method to detect small molecules in laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) was developed using bare gold nanoparticles (AuNPs) as matrices. In this sample preparation method, the analyte is deposited first and then followed by the bare AuNPs. Neutral steroids and carbohydrates, which are difficult to ionize, using organic matrices, are cationized efficiently by combining AuNP-assisted LDI-TOF MS with this sample preparation method. As compared to the dried-droplet method (i.e., analyte and bare AuNPs are mixed and dried together), this method offers distinct advantages for improving shot-to-shot reproducibility, increasing the ionization efficiency of the analyte, and reducing sample preparation time.

Colorimetric sensing of silver(I) and mercury(II) ions based on an assembly of Tween 20-stabilized gold nanoparticles

We have developed a rapid and homogeneous method for the highly selective detection of Hg(2+) and Ag(+) using Tween 20-modified gold nanoparticles (AuNPs). Citrate ions were found to still be adsorbed on the Au surface when citrate-capped AuNPs were modified with Tween 20, which stabilizes the citrate-capped AuNPs against conditions of high ionic strength. When citrate ions had reduced Hg(2+) and Ag(+) to form Hg-Au alloys and Ag on the surface of the AuNPs, Tween 20 was removed from the NP surface. As a result, the AuNPs were unstable under a high-ionic-strength solution, resulting in NP aggregation. The formation of Hg-Au alloys or Ag on the surface of the AuNPs was demonstrated by means of inductively coupled plasma mass spectroscopy and energy-dispersive X-ray spectroscopy. Tween 20-AuNPs could selectively detect Hg(2+) and Ag(+) at concentrations as low as 0.1 and 0.1 microM in the presence of NaCl and EDTA, respectively. Moreover, the probe enables the analysis of AgNPs with a minimum detectable concentration that corresponds to 1 pM. This probe was successfully applied to detect Hg(2+) in drinking water and seawater, Ag(+) in drinking water, and AgNPs in drinking water.

Nanomaterial-assisted laser desorption ionization for mass spectrometry-based biomedical analysis

Nanomaterials have been widely used to assist laser desorption ionization of biomolecules for mass spectrometry analysis. Compared with classical matrix-assisted laser desorption ionization, strategies based on nanomaterial-assisted ionization generate a clean background, which is of great benefit for the qualitative and quantitative analysis of small biomolecules, such as therapeutic and diagnostic molecules. As label-free platforms, they have successfully been used for high-throughput enzyme activity/inhibition monitoring and also for tissue imaging to map in situ the distribution of peptides, metabolites and drugs. In addition to widely used porous silicon nanomaterials, gold nanoparticles can be easily chemically modified by thiol-containing compounds, opening novel interesting perspectives. Such functionalized nanoparticles have been used both as probes to extract target molecules and as matrices to assist laser desorption ionization for developing new enzyme immunoassays or for studying DNA hybridization. More recently, semiconductor nanomaterials or quantum dots acting as photosensitive centers to induce in-source redox reactions for proteomics and to investigate biomolecule oxidation for metabolomics have been shown to offer new analytical strategies.

The detection of glycosphingolipids in brain tissue sections by imaging mass spectrometry using gold nanoparticles

Glycosphingolipids (GSLs) are amphiphilic molecules consisting of a hydrophilic carbohydrate chain and a hydrophobic ceramide moiety. They appear to be involved primarily in biological processes such as cell proliferation, differentiation, and signaling. To investigate the mechanism of brain function in more detail, a more highly sensitive method that would reveal the GSL distribution in the brain is required. In this report, we describe a simple and efficient method for mapping the distribution and localization of GSLs present in mouse brain sections using nanoparticle-assisted laser desorption/ionization imaging mass spectrometry (IMS). We have developed and tested gold nanoparticles (AuNPs) as a new matrix to maximize the detection of GSLs. A matrix of AuNPs modified with alkylamine was used to detect various GSLs, such as minor molecular species of sulfatides and gangliosides, in mouse brain sections; these GSLs were hardly detected using 2,5-dihydroxybenzoic acid (DHB), which is the conventional matrix for GSLs. We achieved approximately 20 times more sensitive detection of GSLs using AuNPs compared to a DHB matrix. We believe that our new approach using AuNPs in IMS could lead to a new strategy for analyzing basic biological mechanisms and several diseases through the distribution of minor GSLs.

Matrix-free LDI mass spectrometry platform using patterned nanostructured gold thin film

A novel matrix-free LDI MS platform using a thin film of patterned nanostructured gold, capped with methyl- and carboxy-terminated self-assembled monolayers (SAMs) is presented. Calibration on the matrix-free LDI surface was performed using a peptide standard mixture available for MALDI analysis. MS analysis for limit of detection was performed using angiotensin I peptide. Peptide fragments from standard protein digests of bovine serum albumin, bovine catalase, and bovine lactoperoxidase were used to carry out peptide mass fingerprinting analysis. Sequence coverage of each protein digest and the number of detected peptide fragments were compared with conventional MALDI MS on a standard MALDI plate. Versatility of the nanostructured gold LDI substrate is illustrated by performing MS analysis on a protein digest using different enzymes and by small molecule MS analysis.

Imaging mass spectrometry with silver nanoparticles reveals the distribution of fatty acids in mouse retinal sections

A new approach to the visualization of fatty acids in mouse liver and retinal samples has been developed using silver nanoparticles (AgNPs) in nanoparticle-assisted laser desorption/ionization imaging mass spectrometry (nano-PALDI-IMS) in negative ion mode. So far, IMS analysis has concentrated on main cell components, such as cell membrane phospholipids and cytoskeletal peptides. AgNPs modified with alkylcarboxylate and alkylamine were used for nano-PALDI-IMS to identify fatty acids, such as stearic, oleic, linoleic, arachidonic, and eicosapentaenoic acids, as well as palmitic acid, in mouse liver sections; these fatty acids are not detected using 2,5-dihydroxybenzoic acid (DHB) as a matrix. The limit of detection for the determination of palmitic acid was 50 pmol using nano-PALDI-IMS. The nano-PALDI-IMS method is successfully applied to the reconstruction of the ion images of fatty acids in mouse liver sections. We verified the detection of fatty acids in liver tissue sections of mice by analyzing standard lipid samples, which showed that fatty acids were from free fatty acids and dissociated fatty acids from lipids when irradiated with a laser. Additionally, we applied the proposed method to the identification of fatty acids in mouse retinal tissue sections, which enabled us to learn the six-zonal distribution of fatty acids in different layers of the retina. We believe that the current approach using AgNPs in nano-PALDI-IMS could lead to a new strategy to analyze basic biological mechanisms and several diseases through the distribution of fatty acids.

Mass spectrometry imaging of pharmacological compounds in tissue sections

The use of MS imaging (MSI) to resolve the spatial and pharmacodynamic distributions of compounds in tissues is emerging as a powerful tool for pharmacological research. Unlike established imaging techniques, only limited a priori knowledge is required and no extensive manipulation (e.g., radiolabeling) of drugs is necessary prior to dosing. MS provides highly multiplexed detection, making it possible to identify compounds, their metabolites and other changes in biomolecular abundances directly off tissue sections in a single pass. This can be employed to obtain near cellular, or potentially subcellular, resolution images. Consideration of technical limitations that affect the process is required, from sample preparation through to analyte ionization and detection. The techniques have only recently been adapted for imaging and novel variations to the established MSI methodologies will further enhance the application of MSI for pharmacological research.