Biomedical Engineering Reference
In-Depth Information
Raman spectroscopy is a noninvasive, nondestructive, and water-insensitive tool that
provides information on the structure and intramolecular and intermolecular interaction
of pathogenic bacteria or toxins (62). The recording of a Raman spectrum often requires
small sample amounts and least sample preparation, and unlike IR spectroscopy, water
can be readily used as a solvent. However, biosystems are prone to auto-fluorescence,
which may deteriorate or completely mask spectra. What's more, the conversion efficiency
of the Raman effect is fairly poor. Only a small amount, 10
6
10
8
, of the laser photon is
converted into Raman photons, limiting the sensitivity of the detection. Thus, surface-
enhanced Raman scattering (SERS) was developed to overcome this problem.
The effect of SERS from molecules adsorbed on an electrochemically roughened surface
with silver colloid was first discovered by Fleischmann et al. (63). This enhancement is due
to electromagnetic and chemical enhancement factors. When the incident light is applied
to a metal surface, molecules adsorbed or in close proximity to the surface create an excep-
tionally large electromagnetic field emitting an SPR. The chemical enhancement is due to
a charge transfer interaction between the metal and adsorbed molecules. Taking advan-
tage of these two factors, Raman signal can be increased by up to six orders of magnitude
or more.
Raman spectroscopy has been employed to identify clinically relevant microcolonies of
S. aureus
,
Staphylococcus epidermidis
,
E. coli
,
Enterococcus faecium
, and
Candida albicans
(64
67). This technique was also employed to detect and differentiate foodborne
pathogens including
E. coli
O157:H7,
B. cereus
,
Saccharomyces cerevisiae
, and
Aspergillus
niger
on the surface of apple (68). Micro-Raman spectroscopy was able to identify bacter-
ial colonies of
Micrococcus luteus
,
Bacillus subtilis
,
Bacillus sphaericus
,
Rhodotorula mucilagi-
nosa
, and
Pseudomonas fluorescence
on agar plates (69,70). The micro-Raman technique
appeared to be suitable for identification of pigment-producing microorganisms, but for
detection of colorless colonies, confocal micro-Raman is ideal (69). Recently, the micro-
Raman technique was employed for possible use in a clean room setting for food process-
ing or pharmaceutical manufacturing for online identification of bacterial cells (70).
Grow et al. (71) described a µSERS technology for fingerprinting of individual bacterial
cells captured selectively on a biochip by antibodies. This technology utilizes a light micro-
scope to localize a single microorganism on a chip, and is hence called µSERS. Antibodies
to target analytes were immobilized on a SERS-active biochip surface and allowed to cap-
ture antigens. Surface-enhanced Raman-spectroscopy fingerprints were collected and
compared with a database for identification. This system was able to differentiate viable
from nonviable cells and could detect
Listeria
species,
Legionella
, and
Cryptosporidium
oocysts at subspecies and strain levels. This system was also able to detect aflatoxin. The
µSERS gave overall weaker signals when antibody-captured cells were analyzed com-
pared to directly deposited cells on SERS surfaces.
Scanning electron microscopy was also used to localize regions in cells with SERS sub-
strate (sliver colloid) for improved vibrational Stokes spectra to discriminate bacterial
strains (72,73). This strategy was used to discriminate cells of
B. subtilis
and
E. coli
. Zeiri et
al. (74) was able to probe the presence of flavin components associated with cell wall
(membrane) of
E. coli
,
Pseudomonas
,
Acinetobacter
, and
Bacillus
using SERS.
18.3.6
Light-Addressable Potentiometric Sensor
Potentiometric sensors work by catalyzing substrates with enzymes, and the resulting
ionic species cause changes in pH, which can be detected by an ion-selective electrode
with sensitivity several order magnitude higher than the standard enzyme immunoassays
(75). In most applications, such as in LAPS, antigen is captured on a solid surface such as
