Biomedical Engineering Reference
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metal NPs. For instance, under atomic bombardment (Ga
+
and In
+
),
the characteristic ions increased with increasing the size of the
AuNPs; however, with cluster projectiles such as C
60
+
and Bi
3
+
, the
deposition of AuNPs and AgNPs onto the sample surface led to a
dramatic decrease in the characteristic ions.
On the other hand, TOF-SIMS can be applied to the determination
of the analytes that have been immobilized on a self-assembled
monolayer (SAM). The secondary ion yield of the analytes can be
eficiently enhanced when the AuNPs were assembled on a SAM.
Kim
et al
. reported that the secondary ion emission of peptides
adsorbed onto the AuNPs/SAM was signiicantly increased when
compared to that adsorbed onto the SAM.
65
In comparison of the
analyte-to-matrix ratio (1: 10
3
−10
6
) in a matrix-enhanced SIMS, the
ratio of analyte to AuNPs/SAM was revealed to be 18−19: 1. Because
of this, it is suggested that the AuNPs would allow a more eficient
secondary ions generation. Moreover, distinct imaging of peptides
and their phosphopeptides adsorbed onto the AuNPs/SAM can be
obtained using TOF-SIMS coupled with microluidic systems. The
same group extended this SIMS-based approach to determine the
kinase activity and to screen inhibitors.
66
Through the formation of Au-S bond, cysteine-terminated
peptides were conjugated to citrate-capped AuNPs that have
been adsorbed onto the 3-aminopropyl triethoxysilane-modiied
glass. After these peptides reacted with kinase, the resulting
phosphopeptides can be easily detected by AuNP-enhanced SIMS
approach, including spectrum and imaging. Quantitative assay of
kinase activity and its inhibition can be determined by comparing
the original unphosphorylated signal with the phosphorylated one
within a single spectrum. A label-free, activity-based assay of matrix
metalloproteinase and its inhibition can be determined using a
similar strategy.
67
Peptide-conjugated AuNPs were electrostatically
attached onto the nonbiofoluling poly(oligo(ethylene glycol)
methacrylate) ilms. After these peptides were digested with matrix
metalloproteinase, the cleaved peptides fragments were analyzed by
AuNP-enhanced TOF-SIMS. The cleavage eficiency of the protease
reaction can be estimated by the ratio of the peak intensity of
uncleaved peptide to that of cleaved peptide. Figure 12.4 illustrates
the entire procedure for the investigation of enzyme activity through
AuNP-enhanced TOF-SIMS.
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