Biomedical Engineering Reference
In-Depth Information
Reading the information within a voxel would be carried out by second harmonic
generation using a 1.54-
m laser. This has a wavelength of the fundamental and second
harmonic photon outside the absorption band of the molecule. Figure 1.13 shows the
second harmonic generated from bR in the M and bR states. L1 is the light to switch from
the bR to the M state. L2 is the light to convert from the M back to the bR state. The first
peak intensity corresponds to the second harmonic signal from molecules in the bR state.
When L1 is turned on, molecules in the bR state are photochemically pumped to the M
state. The second harmonic intensity decreases to that of the M state. This change is due
to a large difference in the second-order nonlinearity between the bR and M states. The
contrast ratio between the bR and M states is about 6, which is large enough to reliably
assign each voxel its information state during the reading operation. In addition to the
nondestructive reading operation using second harmonic generation, this method has the
advantage of fast speed and potential for parallel access. Because no molecular transition
is involved in the reading operation, the second harmonic photon is generated instanta-
neously and the reading cycle is likely limited only by the second harmonic photon detec-
tion device. Other methods for two-photon optical memories have been proposed using
organic, semiconductor, and biomolecular materials (35-40). However, some of these
suffer from optical cross talk, or in the case of a bR system based upon its two-photon-
induced photoelectric signal to perform the reading operation, the need for reset opera-
tions to prevent the bR from switching states. Clearly, bR offers interesting optical
properties as well as distinct advantages over other protein systems that make it especially
attractive for integration into smart biosensors.
1.2.1.2 Chemiluminescent Probe-Based Enzyme Biosensors
Chemiluminescent molecules emit visible light, typically accompanying the breakage of an
unstable high-energy bond(s). Because of this property, chemiluminescent molecules
have a wide range of applications, including those in the field of biotechnology. In our stud-
ies, we have utilized a particularly interesting example: chloro 3-(4-methoxy spiro [1,2-
dioxetane-3-2
-tricyclo-[3.3.1.1.]-decan]-4-yl) phenyl phosphate (CSPD). This phenyl
phosphate species is capable of being dephosphorylated by members of a class of phos-
phatase enzymes. We have used alkaline phosphatase in our biosensor studies and it
carries out the initial dephosphorylation step enzymatically, as shown in Figure 1.14. The
dephosphorylated reaction product contains an unstable highly strained four-atom
4.0
FIGURE 1.13
Second harmonic signal from the pho-
tochromic states of bR as a function of
time. The times indicated by L1 and L2
are the times when the light beams are
turned on to switch, respectively, from
the bR to the M state and from the M to
the bR state. Reprinted from Chen, Z.,
Lewis, A., Kumar, J., Tripathy, S.K.,
Marx, K.A., Akkara, J., Kaplan, D.L.
(1994). Second Harmonic Generation of
Bacteriorhodopsin and Its Application
for Three-Dimensional Optical Memory.
In: M. Alper, H. Bayley, D. Kaplan, M.
Navia, eds. Biomolecular Materials by
Design,
Proc. Mat. Res. Soc.
, 330:263-268.
With permission from the Materials
Research Society.
L1 on
L1 on
L1 on
3.0
2.0
1.0
L2 on
L2 on
L2 on
0.0
0
500
1000
1500
Time (s)
