Quantification of the Enhancement Factor in Surface-Enhanced Raman Scattering

<p>This thesis presents a rigorous stepwise methodology towards the accurate measurement and quantification of the SERS enhancement factor (EF), the key parameter in describing the SERS effect. The work represents, we believe, a successful attempt to resolve some of the inconsistencies in the literature and to refocus the field by emphasizing the importance of consistent definitions and rigorous quantification to elucidate matters of fundamental importance in SERS. The success in our approach is that it combines careful experimental measurements upon a sound theoretical framework, and utilizes a 'toolbox' of techniques developed in recent years, such as bi-analyte SERS (BiASERS) techniques for single-molecule (SM) detection, and isotopic editing. In experimental work, we measure the bare Raman cross-sections of five common probes used in SERS as a first step in measuring the analytical enhancement factor (AEF) and single-molecule enhancement factor (SMEF). The methodology in measuring these EFs involved the use of a reference standard of known cross-section along with a careful characterization of the scattering volume through beam profiling experiments. As a guide to validating the reference cross-section we make extensive use of density functional theory (DFT) calculations to obtain estimates for the intrinsic Raman cross-sections of small, non-resonant probes. The results of this work showed that previous upper limits for the EF reported in the literature of 1014 were based on a faulty normalization of the EF. In fact, EFs of 108 were sufficient to see single molecules, which is much lower than previously expected; under optimum conditions, even lower EFs, possibly down to 105 could be sufficient for the SM detection of resonant probes. As a valuable extension of BiASERS, we elaborate on the synthesis of isotopic analogues of a rhodamine dye as ideal partners for SM experiments. The synthesis and definitive characterization of these probes enable their use in an experiment to determine the SM regime in a liquid colloidal sample. Isotopically edited dyes such as these, in combination with the methodologies of EF quantification outlined herein, set the standard for those interested in accurate quantification of the SERS effect. This approach is useful in terms of both basic theoretical questions and applications such as the effective comparison of SERS substrates. Finally, we extend the techniques developed over the thesis to a long-standing and largely unresolved question in SERS: What is the minimum intrinsic Raman cross-section that can be measured as a single molecule in standard SERS conditions. In this work, we explore the SM detection non-resonant probes, which are the molecules of interest for many practical applications such as forensics and biological assays. Specifically, we demonstrate the successful SM detection of isotopically edited adenine probes.</p>

Quantification of the Enhancement Factor in Surface-Enhanced Raman Scattering

<p>This thesis presents a rigorous stepwise methodology towards the accurate measurement and quantification of the SERS enhancement factor (EF), the key parameter in describing the SERS effect. The work represents, we believe, a successful attempt to resolve some of the inconsistencies in the literature and to refocus the field by emphasizing the importance of consistent definitions and rigorous quantification to elucidate matters of fundamental importance in SERS. The success in our approach is that it combines careful experimental measurements upon a sound theoretical framework, and utilizes a 'toolbox' of techniques developed in recent years, such as bi-analyte SERS (BiASERS) techniques for single-molecule (SM) detection, and isotopic editing. In experimental work, we measure the bare Raman cross-sections of five common probes used in SERS as a first step in measuring the analytical enhancement factor (AEF) and single-molecule enhancement factor (SMEF). The methodology in measuring these EFs involved the use of a reference standard of known cross-section along with a careful characterization of the scattering volume through beam profiling experiments. As a guide to validating the reference cross-section we make extensive use of density functional theory (DFT) calculations to obtain estimates for the intrinsic Raman cross-sections of small, non-resonant probes. The results of this work showed that previous upper limits for the EF reported in the literature of 1014 were based on a faulty normalization of the EF. In fact, EFs of 108 were sufficient to see single molecules, which is much lower than previously expected; under optimum conditions, even lower EFs, possibly down to 105 could be sufficient for the SM detection of resonant probes. As a valuable extension of BiASERS, we elaborate on the synthesis of isotopic analogues of a rhodamine dye as ideal partners for SM experiments. The synthesis and definitive characterization of these probes enable their use in an experiment to determine the SM regime in a liquid colloidal sample. Isotopically edited dyes such as these, in combination with the methodologies of EF quantification outlined herein, set the standard for those interested in accurate quantification of the SERS effect. This approach is useful in terms of both basic theoretical questions and applications such as the effective comparison of SERS substrates. Finally, we extend the techniques developed over the thesis to a long-standing and largely unresolved question in SERS: What is the minimum intrinsic Raman cross-section that can be measured as a single molecule in standard SERS conditions. In this work, we explore the SM detection non-resonant probes, which are the molecules of interest for many practical applications such as forensics and biological assays. Specifically, we demonstrate the successful SM detection of isotopically edited adenine probes.</p>

Isotope separation of 176Lu a precursor to 177Lu medical isotope using broadband lasers

A new photoionization scheme accessible by Rhodamine dye lasers is proposed for the isotope separation of 176Lu.$$5d6s^{2}\,{^{2}D_{{3/2}}} (0.0\, {\text{cm}}^{{ - 1}} )\mathop{\longrightarrow}\limits^{{573.8130\, {\text{nm}}}}5d6s6p\,{^{4}F_{{3/2}}^{o}} \left( {17427.28\, {\text{cm}}^{{ - 1}} } \right)\mathop{\longrightarrow}\limits^{{560.3114\, {\text{nm}}}}$$ 5 d 6 s 2 2 D 3 / 2 ( 0.0 cm - 1 ) ⟶ 573.8130 nm 5 d 6 s 6 p 4 F 3 / 2 o 17427.28 cm - 1 ⟶ 560.3114 nm $$6s{6p}^{2}\,{^{4}{P}_{5/2}}\left(35274.5 \,{\text{cm}}^{-1}\right){\to } Autoionization\, State {\to }{Lu}^{+}$$ 6 s 6 p 2 4 P 5 / 2 35274.5 cm - 1 → A u t o i o n i z a t i o n S t a t e → Lu + Optimum conditions for the laser isotope separation have been theoretically computed and compared with the previously reported work. The enrichment of ~ 63% can be obtained with > 22 mg/h production rate even when broadband lasers with bandwidth of 500 MHz are employed for the two step excitation. The simplified system requirements for the photoionization scheme combined with a high production rate of 176Lu than previously reported is expected to reduce the global shortage of 176Lu isotope for medical applications.

Non-Arrhenius Kinetics and Slowed-Diffusion Mechanism of Molecular Aggregation of a Rhodamine Dye on Colloidal Particles

The non-covalent association is important for many fields of science, including processes in living systems. This work elucidates the mechanism of rhodamine 123 molecular aggregation in dispersions of a layered...

MICROPLASTICS TRANSPORT AND MIXING MECHANISMS IN THE NEARSHORE REGION

Microplastics (MP) are emerging pollutants in the marine environment with potential ecotoxicological effects on littoral and coastal ecosystems. A dominate contributing source of microplastic particles is the fragmentation of macroplastics from manufactured goods, alongside laundered synthetic material, abrasion of vehicle tyres and personal care products. The indiscriminate use of plastic and poor management of plastic waste pose serious threat to ecosystem functionality and resilience. Understanding the key underlying transport and mixing mechanisms which influence the behavior of microplastics and their environmental fate are crucial for identify potential microplastic fate-transport pathways from source to sink. This is fundamental for evaluating microplastic interactions and impact on ecosystems. This paper presents laboratory-based tracer measurements for solute and polyethylene (PE) microplastics in the presence of waves. The tests were undertaken in a wave tank equipped with an active absorption paddle-type wave-maker. Fluorescent dye was used to stain the PE particles using a novel staining technique. Rhodamine dye was used as a proxy for the transport of solute pollutants. The temporal and spatial behavior of both microplastics and solute across the nearshore zone was measured using submersible fiber optic fluorometers. Hydrodynamic conditions were designed to create monochromatic waves with a range of wave steepness Sop = 2 - 5 percent. Tracer measurements were conducted at three locations, seaward of the breaker region, breaker region and inner surf zone to provide a comprehensive understanding of mixing across the nearshore. The dispersion coefficients were determined for both solute and PE particles. The results indicate the dominant role of surface and bed generated turbulence in determining mixing and dispersion influenced by wave breaker type and width of the surf zone. The comparison of tracer data suggests that PE particles, with similar density to water, and the solute tracer have a similar transport and mixing behavior under the influence of waves.

Isotope separation of 176Lu a precursor to 177Lu medical isotope using broadband lasers

A new photoionization scheme accessible by Rhodamine dye lasers is proposed for the isotopeseparation of 176Lu. 5d6s2 2D3/2 (0.0 cm-1) - 573.8130 nm -> 5d6s6p 4Fo3/2 (17427.28 cm-1) - 609.6007 nm -> 6s6p2 4P3/2 (33831.46 cm-1) ---> Autoionization State---> Lu+ Optimum conditions for the isotope separation have been derived and compared with thepreviously reported work. The enrichment of ~ 49% can be obtained with > 12 mg / hourproduction rates even when broadband lasers with bandwidth of 500 MHz employed for the twostep excitation. The simplified system requirements for the photoionization scheme with a highproduction rate is expected reduce the global shortage of 176Lu for medical applications.