Mass Spectrometric Bioanalysis Assisted by Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 276

This high detection sensitivity is mainly due to strong absorption
of the UV laser light, lower thermal conductivity, and increased
surface roughness. Because the mass spectra obtained by citrate-
AuNPs contain abundant alkali adducted analyte ions, Russell
et al . modiied citrate-AuNPs with the ionization enhancing ligand,
4-aminothiolphenol. 24 The use of this kind of NPs resulted in a
drastic enhancement in the [M + H] + ion signal, a decrease in the
number of interfering peaks from Au clusters, and an increase
in the useful analyte mass range. The similar phenomena have
been reported in the case of the use of 4-mercaptobenzoic acid-
and α-cyano-4-hydroxy-cinnamic acid (CHCA)-capped AuNPs as
matrices. 25,26 Cheng et al . further discovered that the presence of
glycerol in sample solution can improve the dispersion of CHCA-
capped AuNPs, thereby increasing the sample ionization and shot-
to-shot reproducibility. 26 Moreover, the presence of citric acid in
sample solution can serves as an external proton donor, thereby
increasing the [M + H] + ion signal. In addition to citrate-AuNPs,
bare AuNPs have been used for highly sensitive detection of neutral
carbohydrates without the need of derivatization steps. 27 Under the
optimal concentration of bare AuNPs, the limits of detection (LODs)
at a S / N ratio of 3 are 82, 41, 144, and 151 nM for ribose, glucose,
cellobiose, and maltose. Figure 12.1 compares the use of different
matrices for the analysis of glucose through LDI-MS. The same group
also disclosed that a unique sample preparation, in which samples
are deposited onto the sample plate prior to bare AuNPs, could
offer better detection sensitivity and sample homogeneity when
compared to (a) a deposition of a mixture of matrix and sample
onto the sample plate and (b) a deposition of bare AuNPs onto the
sample plate prior to sample. 28 Except for the cyclic oligosaccharide,
this sample preparation method was also applied to the analyses of
other biological samples, including neutral carbohydrate and steroid,
aminothiols, and peptides as well as proteins. Besides, Nishimura
et al . reported that AuNP-assisted LDI-MS can be conducted for
monitoring glycosyltransferase activity when AuNPs has been
modiied with substrate. 29 Note that glycosyltransferase can catalyze
the transfer of sugars to diverse substrates, such as glycoproteins,
glycolipids, and proteoglycans. Once the substrate-decorated AuNPs
reacted with glycosyltransferase, the product adsorbed onto the
surface of AuNPs can be directly detected by LDI-MS. Interestingly,
when visible laser was used in place of UV laser in LDI-MS, Hori