Alzheimer’s disease (AD), an age-related neurodegenerative disorder, is the seventh leading cause of death in the United
States. One strong pathological indicator of AD is senile plaques, which are aggregates of fibrils formed from amyloid β
(Aβ) peptides. Thus, detection and inhibition of Aβ aggregation are critical for the prevention and treatment of AD.
Congo red (CR) is one of the most widely used dye molecules for probing as well as inhabiting Aβ aggregation.
However, the nature of interaction between CR and Aβ is not well understood. In this research, we systematically
studied the interaction between CR and Aβ using a combination of optical techniques, including electronic absorption,
fluorescence, Raman scattering, and circular dichroism, to provide detailed information with molecular specificity and
high sensitivity. Compared to CR alone, interaction of the dye with Aβ results in a new absorption peak near 540 nm and
significantly enhanced photoluminescence as well as Raman signal. Our results led us to propose a new model
suggesting that CR exists primarily in a micellar form, resembling H-aggregates, in water and dissociates into monomers
upon interaction with Aβ. This model has significant implications for the development of new strategies to detect and
inhibit brain plaques for treatment of neurological diseases like AD.
Hollow core photonic crystal fiber (HCPCF) employs a guiding mechanism fundamentally different from that in
conventional index guiding fibers. In an HCPCF, periodic air channels in a glass matrix act as reflectors to confine light
in an empty core. As a result, the interaction between light and glass can be very small. Therefore, HCPCF has been used
in applications that require extremely low non-linearity, high breakdown threshold, and zero dispersion. However, their
applications in optical sensing, especially in chemical and biological sensing, have only been extensively explored
recently. Besides their well-recognized optical properties the hollow cores of the fibers can be easily filled with liquid or
gas, providing an ideal sampling mechanism in sensors. Recently, we have demonstrated that by filling up a HCPCF
with gas or liquid samples, it is possible to significantly increase the sensitivity of the sensors in either regular Raman or
surface enhanced Raman scattering (SERS) applications. This is because the confinement of both light and sample inside
the hollow core enables direct interaction between the propagating wave and the analyte. In this paper, we report our
recent work on using HCPCF as a platform for Raman or SERS in the detection of low concentration greenhouse gas
(ambient CO2), biomedically significant molecules (e.g., glucose), and bacteria. We have demonstrated that by filling up
a HCPCF with gas or liquid samples, it is possible to significantly increase the sensitivity of the sensors in either regular
Raman or SERS applications.
This study was designed to examine the possible photosensitization effect of zinc oxide (ZnO) nanowires (NWs) by Au
nanoparticles (AuNPs) by directly monitoring the charge carrier lifetime in AuNP-decorated ZnO NWs. ZnO-Au
nanocomposite structures showed reduced photocurrent compared to pristine ZnO NWs due to the combined effect of
ZnO etching during the AuNPs growth and competitive absorption/scattering effects from AuNPs of incident UV
photons. Ultrafast transient pump-probe spectroscopy was utilized to characterize the charge carrier dynamics. The
bleach recovery of ZnO indicates electron-hole recombination on the 150 ps time scale attributed to shallow donor
recombination. The AuNP-decorated ZnO NWs exhibit a fast decay of 3 ps in addition to the decays observed for ZnO
NWs. This fast decay is similar to the hot electron relaxation lifetime observed for AuNPs in solution. Overall, the
dynamics features for AuNP-decorated ZnO NWs appear as a simple sum of those from AuNPs and ZnO NWs alone.
There is no evidence of photosensitization of the ZnO NWs by AuNPs investigated in this study.
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