Text

theses

Metal nanoparticles have unique physical and optical properties stemming from their
nanoscale dimensions, inducing strong localized surface plasmon resonance (LSPR), which
lead them to find extensive applications in chemi-sensors and biosensors. By concentrating
the incident electromagnetic (EM) field near the nanostructure, LSPR modes can enhance the
local scattered EM field and affect optical processes such as Raman scattering and
fluorescence, giving rise to the so-called surface-enhanced Raman scattering (SERS) and
plasmon-enhanced fluorescence (PEF). Because they both depend on the distance between the
reporting molecules and the nanoparticles but at different regimes, SERS and PEF become
dominant separately and can be thus leveraged to design orthogonal transduction mechanisms.
In this thesis, the detection sensitivity in solution-based assays enabled by portable SERS and
molecular beacon-based fluorescence are improved by keeping the molecules close to or
properly away from the surface of the nanoparticles.

For the first part, opioid drug molecules are attracted to the surface of silver nanoparticles and
trapped in the “hot spots” to achieve SERS enhancement through aggregation induced by salt
addition. Salting optimization and aggregation dynamics analysis are carried out for
improving drug detection. NaBr solution is chosen as the aggregation inducing salt at theiii
optimal concentration of 1 M with 3 minutes time window for detection. The improved assay
supports a LOD of ~5 ng/mL for fentanyl spiked in urine controls and a LOD of ~0.1 % (10
ng in 10 µg total) mass percent for fentanyl in laced recreational drugs such as heroin or THC,
which surpasses the results achieved in comparable previous reports.

For the second part, fluorophores in molecular beacons (MBs) are pushed away from the
surface of gold nanoparticles via hybridization of the beacons with nucleic-acid strand targets,
but still kept at a well-known distance which depends on the length of the MBs, thus
facilitating PEF and generating enhanced fluorescence signals. A single strand DNA with ten
thymine groups (T10) is also implemented as a spacer to assist the stretching process of MBs
via reduction of steric hindrance. Functionalization of MBs on the surface of gold
nanoparticles is performed along with the optimization of reaction conditions. Based on the
fluorescence enhancement, the proper molar ratio of MBs/ T10-to-gold nanoparticles is 1000/1
for a relatively strong and stable signals enhancement. And the ratio of MBs-to-T10 within the
range of 3~5 has relatively less influence on the result. Calibration curves for the MBs gold
nanoparticles sensor are generated under different concentrations of nucleic-acid strand
targets, indicating a LOD of ~10 nM for room temperature detection and a LOD of ~20 nM at
37.5 ℃

ETD_10798

doi:10.7282/t3-3qmb-v482

M.S.

Includes bibliographical references

ETD graduate

The author owns the copyright to this work.