Biomedical Engineering Reference
In-Depth Information
surfaces is conveniently situated in between the HOMO and LUMO
of the intramolecular transition. Thus, the new energy states can
contribute resonantly to the Raman scattering cross-section of the
surface complex, increasing its magnitude. A theoretical account
of the chemical enhancement process is summarized in Lombardi
et al. 29
11.1.4
SERS Bioanalytical Applications
There are two different approaches to applying the NP-based SERS
detection techniques in bioanalysis: label-free or labeled. The label-
free methods rely on the species of interest adsorbing directly to
the SERS active surfaces. In the label-free detection scheme, NPs
are introduced into the biological system of interest. Because of the
strong distance-dependent characteristics of the SERS effect, this
approach provides vibrational information of the nano-environment
surrounding the NPs. It has been applied to the study of cellular
imaging 31-34 and microorganism identiications. The label-free
detection has been applied with success in pathogen detection
which requires high sensitivity, rapid response, and beneits from
the elimination of extrinsic labeling steps as well as SERS' ability to
differentiate species and strains of different pathogens. 35-42 This can
be simply achieved by introducing the pathogen cells to a colloidal
metal sol that subsequently adsorbs on the cell surface rendering
each pathogen cell SERS-active as shown in Fig. 11.4. Of note, the cell
Raman spectrum implicitly samples all of the biomolecules of the
entire cell make-up whereas the SERS spectrum only probes the cell
surface composition due to the distance-dependent nature of SERS.
Since the cell surface of different pathogen contains different marker
proteins, the chemically speciic SERS spectra allow one to discern
the different types of bacteria through chemometric analysis.
Although label-free detection is a convenient approach, it also
has a number of issues that one must be aware of. One of the biggest
problems in the label-free approach lies in the non-selective nature
of SERS. Using pathogen detection as an example, Zeiri et al. has
pointed out that SERS spectra obtained from identical strains of
bacteria can produce very different SERS response from different
laboratories. 40-42 Because of the non-selective and distance-
dependence nature of SERS, often, only the molecule that is situated
in the SERS active hot-site will receive optimal enhancement. This
 
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