Fast, sensitive, quantitative, and low-cost analysis of environmental pollutants is highly valuable for environmental monitoring. Due to its single-molecule sensitivity and fingerprint specificity, surface-enhanced Raman spectroscopy (SERS) has been widely employed for heavy metal, organic compound, and pathogen detection. However, SERS quantitation is challenging because 1) analytes do not stay in the strongest enhancing region ("hot spots") and 2) SERS reproducibility is poor. In this dissertation, gold nanoparticle/bacterial cellulose (AuNP/BC) substrates were developed to improve SERS sensitivity by increasing hot spot density within the laser excitation volume. Environmentally relevant organic amines were fixed at "hot spots" by lowering solution pH below the analyte pKa and thus enabling SERS quantitation. In addition, a new SERS internal standard was developed based upon the electromagnetic enhancement mechanism that relates Rayleigh (elastic) and Raman (in-elastic) scattering. Rayleigh scattering arising from the amplified spontaneous emission of the excitation laser was employed as a normalization factor to minimize the inherent SERS signal variation caused by the heterogeneous distribution of "hot spots" across a SERS substrate. This highly novel technique, hot spot-normalized SERS (HSNSERS), was subsequently applied to evaluate the efficiency of SERS substrates, provide in situ monitoring of ligand exchange kinetics on the AuNP surface, and to reveal the relationship between the pKa of aromatic amines and their affinity to citrate-coated AuNPs (cit-AuNPs). Finally, colloidally stable stable pH nanoprobes were synthesized using co-solvent mediated AuNP aggregation and subsequent coating of poly(ethylene) glycol (PEG). These nanoprobes were applied for pH detection in cancer cells and in phosphate buffered aerosol droplets. The latter experiments suggest that stable pH gradients exist in aerosol droplets. / PHD / Traditional analytical methods, such as gas chromatography/mass spectroscopy, liquid chromatography/mass spectroscopy, etc., cannot meet the demand for rapid screening of target environmental pollutants in drinking water. This issue arises due to the requirements for time-consuming sample pre-treatment, well-trained experts, complex instrumental parameter optimization, and scale challenges that limit onsite measurement. Surface-enhanced Raman spectroscopy is a promising approach to overcome these limitations. To improve SERS quantitation, surface-enhanced elastic scattering was developed as a novel internal standard to account for the SERS signal variation caused by substrate heterogeneity (“hot spot” normalization). Compared with traditional SERS internal standards, using scattered light as an internal standard reduces cost, time, interference, and experimental complexity for SERS detection. With this novel approach, the kinetics of adsorption/desorption of guest ligands/citrate onto/from the AuNP surface were quantified in situ and in real time. In addition, the SERS intensities of organic amines acquired at different solution pH values were differentiated using “hot spot” normalization, which revealed the relationship between aromatic amine pK<sub>a</sub> and their affinity to the AuNP surface. Finally, the chemistry in confined aqueous environments, such as aerosol droplets, membrane channels, and cells, is challenging to probe using conventional analytical tools due to their inaccessible small volumes. To address this problem, SERS pH nanoprobes were synthesized and used to detect the pH inside cancer cells and micrometer-sized aerosol droplets.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/93204 |
| Date | 27 February 2018 |
| Creators | Wei, Haoran |
| Contributors | Civil and Environmental Engineering, Vikesland, Peter J., Zhou, Wei, He, Zhen, Marr, Linsey C. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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