Biomedical Engineering Reference
In-Depth Information
Similar to ZnO, nanostructured TiO
2
materials also show high biocompatibility and good reten-
tion of biological activity for protein binding [290,291]. TiO
2
nanotubes, fabricated by low-cost
anodic oxidation of the pure titanium sheet, possess large surface areas and good uniformity and
conformability over large areas, hence they are desirable for electrochemical biosensor design
[291,292]. Liu et al. [293] investigated the coadsorption of protein with thionine on TiO
2
nanotubes
for biosensor design. In the work, they fi rst fabricated TiO
2
nanotubes by anodizing Ti sheets in
a dilute HF solution and then coimmobilized HRP and thionine on the TiO
2
nanotube arrays by
immersing the Ti/TiO
2
electrodes in a mixture of HRP and thionine solution. Electrochemical and
spectroscopic measurements show that the TiO
2
nanotube arrays provide excellent matrices for the
coadsorption of HRP and thionine and that the adsorbed HRP on these TiO
2
nanotube arrays not
only effectively retains its bioactivity but also shows a high affi nity for H
2
O
2
.
14.2.3.2
Nanoparticles in Biosensors
Inorganic NPs, such as metals, semiconductors, and magnetic particles, are clusters of a few hun-
dred to a few thousand atoms that are only several nanometers long. Because of such small sizes,
they have physical, electronic, and chemical properties different from those of bulk metals.
In noble metals, when the size of NPs decreases below the electron mean free path (the distance
the electron travels between scattering collisions with the lattice centers), an intense absorption in
the visible-near-UV that is absent in the spectrum of the bulk material appears, that leads to the sur-
face plasmon band (SPB) observed near 530 nm for NPs. This extinction band arises when the inci-
dent photon frequency is resonant with the collective oscillation of the conduction electrons and is
known as the localized surface plasmon resonance (LSPR) [268,294]. This LSPR gives these metal-
lic NPs brilliant color in colloidal solution that intrigued scientists in the seventeenth century.
The LSPR spectrum depends on the NP itself (i.e., its size, material, and shape) and on the exter-
nal properties such as the dielectric properties of the surrounding environment [295]; the induced
wavelength shifts in the extinction maximum of NPs can be used to detect molecule-induced
changes surrounding the NPs. The selectivity of the sensor is achieved by chemically modifying the
NPs with SA monolayers that can be tailored to incorporate a wide variety of molecular recognition
elements such as enzymes, antibodies, or DNA [296].
In LSPR techniques, a monolayer of noble metal NPs is adsorbed on an optically transparent,
preactivated substrate (e.g., glass functionalized with amine or thiol groups) followed by an activa-
tion step of the particles themselves. Finally, a receptor is immobilized on the particle surface that
binds to the target molecules of the sample under analysis. Spincasting polymers on top of the gold
particles, leading to a refractive index change to which the metal particles respond optically, was
found to increase the performance of this colorimetric biosensor [297,298].
Englebienne et al. fi rst used the refractive index-dependent color change of spherical homoge-
neous 40 nm gold particles to develop a solution-phase immunoassay for monitoring the binding
kinetics of antibody-antigen interactions in real time. After coating with monoclonal antibodies
specifi c for human ferritin, human chorionic gonadotropin (hCG), and human heart fatty acid bind-
ing protein (hFABP), the NPs were incubated in a solution of their respective antigens [299,300].
This incubation caused a redshift in the LSPR extinction as well as an increase in the extinction at
600 nm (bathochromic and hyperchromic effects).
Several studies have been reported on gold NP-based UV-vis technique for the detection of
DNA. This colorimetric detection method is based on the change in absorbance spectra (i.e., color)
as particles are brought together by the hybridization of complementary DNA strands [301-304].
The limits of detection are reported in the range of tens of femtomoles of target oligonucleotide.
These NP aggregation assays represent a 100-fold increase in sensitivity over conventional fl uores-
cence-based assays [301].
Semiconductor quantum dots (QDs) have also been used to develop optical sensors based on
fl uorescence measurements [305,306]. QDs show size-tunable fl uorescence emission and have a
