Nanoparticles (NPs) are useful as matrixes for the analyses of several types of biomolecules (including aminothiols, peptides, and proteins) and for mass spectrometric imaging through surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS), mainly because of their large surface area, strong absorption in the ultraviolet-near-infrared region, and ready functionalization. Metallic NPs, metal oxide NPs, and semiconductor quantum dots, unmodified or functionalized with recognition ligands, have a strong affinity toward analytes; therefore, they allow the enrichment of biomolecules, leading to improved sensitivity with minimal matrix interference in their mass spectra. SALDI-MS using NPs overcomes the two major problems commonly encountered in matrix-assisted laser desorption/ionization mass spectrometry: the presence of "sweet spots" and the high background signals in the low-mass region. In this tutorial review, we discuss the roles played by the nature, size, and concentration of the NPs, the buffer composition, and the laser energy in determining the sensitivity and mass ranges for the analytes. We describe internal standard SALDI-MS methods that allow the concentrations of analytes to be determined with low variation (relative standard deviations: <10%) and we highlight how the simplicity, sensitivity, and reproducibility of SALDI-MS approaches using various NPs allow the analyses of proteins and small analytes and the imaging of cells.
We have developed a highly sensitive and selective fluorescent assay for the detection of acetylcholine (ACh) based on enzyme mimics of Au/Ag nanoparticles (NPs). These NPs were prepared via a one-step solution phase reaction between 13 nm Au NPs and Ag(+) ions in the presence of stabilizing agents such as adenosine triphosphate (ATP) and polyethylene glycol (PEG). Our sensing strategy involves reacting ACh with acetylcholinesterase (AChE) to form choline that is in turn oxidized by choline oxidase (ChOx) to produce betaine and H(2)O(2), which reacts with Amplex UltraRed (AUR) in the presence of bimetallic NPs catalyst to form a fluorescent product. The fluorescence intensity (excitation/emission wavelengths of 540/592 nm) is proportional to the concentration of ACh over a range of 1-100 nM (R(2) = 0.998), with a limit of detection of 0.21 nM (signal/noise = 3). When compared with Au NPs and horseradish peroxidase, the Au/Ag NPs provide 150- and 115-fold higher catalytic activity toward the H(2)O(2)-mediated AUR reaction. The practicality of the assay has been validated by determining the concentrations of ACh in plasma and blood samples, with results of 2.69 ± 0.84 nM (n = 5) and 6.75 ± 1.42 nM (n = 5), respectively. Thus, the present assay holds great potential for the analysis of ACh in biological samples.
We have developed a new internal standard method for the determination of the concentration of captopril (CAP) through surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) using gold nanoparticles (Au NPs). This approach provided linearity for CAP over the concentration range 2.5-25 M (R 2 ϭ 0.987), with a limit of detection (signal-to-noise ratio ϭ 3) of 1.0 M. The spot-to-spot variations in the concentration of CAP through SALDI-MS analyses performed in the absence and presence of the internal standard were 26% and 9%, respectively (15 measurements). This approach provides simplicity, accuracy, precision, and great reproducibility to the determination of the levels of CAP in human urine samples. (J Am Soc Mass Spectrom 2010, 21, 864 -867)
In this study, we combined surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) with HgTe nanostructures as matrix for the detection of several proteins (α1-antitrypsin, trypsin, IgG, protein G) and their complexes. We investigated the effects of several parameters (the concentration and nature of surfactants and metal ions, the pH, and concentration of the analytes in the sample matrixes) on the sensitivity of the detection of these proteins and their complexes. The presence of stabilizing Brij 76 surfactant and Zn(II) ions allowed the detection of weak protein complexes, such as α1-antitrypsin-trypsin and IgG-protein G complexes, at the picomole level. We observed multiply charged states at m/z 72,160 ([α1-antitrypsin + trypsin + H](+)) and 86,585 ([IgG + protein G + 2H](2+)) for the α1-antitrypsin-trypsin and IgG-protein G complexes, respectively. To the best of our knowledge, detection of weak protein complexes and determination of their stoichiometry have not been demonstrated previously when a combination of SALDI-MS and nanostructures were used. This simple and reproducible SALDI-MS approach using HgTe nanostructures holds great potential for the detection of other proteins and their complexes.
Nanomaterials, primarily nanoparticles (NPs), can serve as an alternative matrix for the analysis of various biomolecules through surface-assisted laser desorption/ionization mass spectrometry (SALDI MS). SALDI MS has been developed to overcome poor reproducibility and high background in the lowmass region commonly occurring in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). Various nanomaterials, unmodified or functionalized with recognition ligands, can have a strong affinity toward certain analytes and thus are applicable for their concentration and enrichment from complex biological matrices. In mass spectrometry imaging (MSI), the use of NPs instead of the conventional matrices can improve the spatial resolution up to the cellular level. In this review, the nature of NPs, the methods of sample preparation and approaches for quantitation of biomolecules through SALDI MS are discussed. Practical applications and limitations of SALDI MS employing NPs for separate samples and MSI are mentioned. With regard to the nature of MSI analysis, the use of nanostructured surfaces for MSI is also reflected in this review.
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