Biomedical Engineering Reference
In-Depth Information
(<500 Da).
6
Therefore, MALDI-MS has not been extensively used for
the characterization of low molecular weight compounds. Moreover,
although the co-crystallization of the analytes and organic matrix is
prerequisite for a successful MALDI-MS analysis, the inhomogeneous
distribution of sample spot requires sweet-spot searching.
7
This
phenomenon leads to poor shot-to-shot and sample-to-sample
reproducibility.
8
To circumvent these problems, matrix-free LDI-MS has
been developed using porous silicon as a sample target for the
sensitive detection of small molecules such as peptides, drugs,
surfactants, and carbohydrates.
9
This well-known technique is
called desorption/ionization on porous silicon.
10
Other approaches
associated with matrix-free LDI-MS include sol-gels,
11
carbon-based
microstructures,
12
silicon nanowires-based silicon wafers,
13
Au-
plated silicon wafers,
14
and germanium nanodots-based chips.
15
For
more information associated with matrix-free LDI MS, the reader is
referred to excellent reviews by Peterson,
9
Huck,
12
and Mrksich.
14
Alternatively, several groups have introduced different types of
matrices for the determination of analytes ranging from small organic
molecules to biopolymers. Early in 1988, Tanaka
et al
. irst reported
that 30 nm cobalt NPs (suspended in glycerol) served as matrices for
the detection of proteins with molecular weights greater than 20000
Da.
16
Sunner
et al
. also utilized micrometer-sized graphite particles
mixed with glycerol for the analysis of peptides and proteins, and
named this method surface-assisted laser desorption/ionization
(SALDI).
17
This method can reduce background in the low mass
region while maintaining the advantages of MALDI. Encouraged by
these results, a series of inorganic micro- and nanomaterials — such
as graphite particles,
18
titanium dioxide NPs,
19
silicon NPs,
20
carbon
nanotubes,
12
gold NPs (AuNPs), silver NPs (AgNPs), and platinum
NPs (PtNPs) — have been extensively studied as potential inorganic
matrices. Compared to organic matrices, the use of inorganic
materials as matrices can provide several advantages, including
high surface areas, better detection reproducibility, easy sample
preparation, lexibility in the sample deposition conditions, and no
dependence on the irradiation wavelength. Moreover, the high molar
matrix-to-analyte ratio (10
7
to 10
9
analytes/particle) is obtained
using nanomaterials as LDI matrices and thus contributing to high
eficient ionization.
21
Table 12.1 lists different kinds of noble metal
nanomaterials that have been used as LDI matrices.
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