Investigation on Pollution Level in Surface Sediments of Coastal Area, the Case of Naples and Salerno Gulfs, and in situ Laboratory Raman Researches

The charge transfer-mediated surface enhanced Raman scattering (SERS) of crystal violet (CV) molecules that were chemically conjugated between partially polarized silver nanoparticles and optically smooth gold and silver substrates has been studied under off-resonant conditions. Tyrosine molecules were used as a reducing agent to convert silver ions into silver nanoparticles where oxidised tyrosine caps the silver nanoparticle surface with its semiquinone group. This binding through the quinone group facilitates charge transfer and results in partially oxidised silver. This establishes a chemical link between the silver nanoparticles and the CV molecules, where the positively charged central carbon of CV molecules can bind to the terminal carboxylate anion of the oxidised tyrosine molecules. After drop casting Ag nanoparticles bound with CV molecules it was found that the free terminal amine groups tend to bind with the underlying substrates. Significantly, only those CV molecules that were chemically conjugated between the partially polarised silver nanoparticles and the underlying gold or silver substrates were found to show SERS under off-resonant conditions. The importance of partial charge transfer at the nanoparticle/capping agent interface and the resultant conjugation of CV molecules to off resonant SERS effects was confirmed by using gold nanoparticles prepared in a similar manner. In this case the capping agent binds to the nanoparticle through the amine group which does not facilitate charge transfer from the gold nanoparticle and under these conditions SERS enhancement in the sandwich configuration was not observed.

<p>The work in this thesis focuses on improving the understanding of two key aspects of the interaction between dye molecules and metallic nanoparticles, with particular relevance to Surface Enhanced Raman Spectroscopy (SERS). This is manifested from two main branches of experimental work; the first is concerned with improving the reproducibility of SERS sample preparation using colloidal solutions while the second focuses on directly measuring the absorption spectra of commonly used dye molecules on the surface of colloidal silver nanoparticles. In the first body of work of the thesis, a major step towards improving SERS in colloidal solutions is achieved by highlighting a crucial, but unnoticed possible source of error for such samples; by comparing average enhancement factor measurements on colloidal solutions prepared using different analyte (dye) dilution methods, it is shown that large dye dilution factors can cause extreme variations in nanoparticle coverage across the entire sample. This not only causes analyte-dependent enhancement factors (which is highly undesirable) but can also lead to false identification of single-molecule SERS experiments using the well established bi-analyte method. The errors associated with large dilution factors are interpreted as a competition between dye diffusion and adsorption kinetics. Time dependent fluorescence quenching measurements and finite element modelling (FEM) in COMOSL show that in any system where adsorption competes with diffusion, large dilution factors should be avoided. A simple protocol of half-half dilutions of analytes is proposed as a standard method to be adopted when preparing colloidal solutions for SERS to ensure uniform distribution of analytes is achieved. The second body of work is an experimental investigation of the modification of the energy levels of commonly used dye molecules adsorbed to spherical silver nanoparticles at sub-monolayer concentrations. Through the use of a novel integrating sphere setup, the absorption spectra of Rhodamine 6G, Nile Blue, Rhodamine 700 and Crystal Violet are successfully measured on the surface of silver colloids at ultra-low concentrations where dye-dye interactions are negligible. These results indicate that for most dyes, absorption pectra on the colloid surface are shifted and/or broadened with respect to the free dye in solution. In the most extreme case, a blue shift of almost 90 nm for Crystal Violet suggests a strong chemical interaction with the silver surface. A Mie theory shell model of dye-coated silver spheres is found to accurately reproduce the measured evolution of absorption spectra as the dye concentration on the colloid surface is increased but overestimates the enhancement in absorption, which is interpreted as a result of the adsorption geometry of dyes on the surface, not captured by the shell model. Finally, through careful wavelength dependent SERS measurements, the SERS Raman excitation profile of Crystal Violet is measured and shown to be closely linked to the modified absorbance as obtained in the integrating sphere setup. A standard optical transform model for computing the Raman excitation profile from the modified absorbance is applied and gives good agreement with the measured SERS data. These results represent a direct indication of chemical modifications of resonant molecules used in SERS studies.</p>

<p>The work in this thesis focuses on improving the understanding of two key aspects of the interaction between dye molecules and metallic nanoparticles, with particular relevance to Surface Enhanced Raman Spectroscopy (SERS). This is manifested from two main branches of experimental work; the first is concerned with improving the reproducibility of SERS sample preparation using colloidal solutions while the second focuses on directly measuring the absorption spectra of commonly used dye molecules on the surface of colloidal silver nanoparticles. In the first body of work of the thesis, a major step towards improving SERS in colloidal solutions is achieved by highlighting a crucial, but unnoticed possible source of error for such samples; by comparing average enhancement factor measurements on colloidal solutions prepared using different analyte (dye) dilution methods, it is shown that large dye dilution factors can cause extreme variations in nanoparticle coverage across the entire sample. This not only causes analyte-dependent enhancement factors (which is highly undesirable) but can also lead to false identification of single-molecule SERS experiments using the well established bi-analyte method. The errors associated with large dilution factors are interpreted as a competition between dye diffusion and adsorption kinetics. Time dependent fluorescence quenching measurements and finite element modelling (FEM) in COMOSL show that in any system where adsorption competes with diffusion, large dilution factors should be avoided. A simple protocol of half-half dilutions of analytes is proposed as a standard method to be adopted when preparing colloidal solutions for SERS to ensure uniform distribution of analytes is achieved. The second body of work is an experimental investigation of the modification of the energy levels of commonly used dye molecules adsorbed to spherical silver nanoparticles at sub-monolayer concentrations. Through the use of a novel integrating sphere setup, the absorption spectra of Rhodamine 6G, Nile Blue, Rhodamine 700 and Crystal Violet are successfully measured on the surface of silver colloids at ultra-low concentrations where dye-dye interactions are negligible. These results indicate that for most dyes, absorption pectra on the colloid surface are shifted and/or broadened with respect to the free dye in solution. In the most extreme case, a blue shift of almost 90 nm for Crystal Violet suggests a strong chemical interaction with the silver surface. A Mie theory shell model of dye-coated silver spheres is found to accurately reproduce the measured evolution of absorption spectra as the dye concentration on the colloid surface is increased but overestimates the enhancement in absorption, which is interpreted as a result of the adsorption geometry of dyes on the surface, not captured by the shell model. Finally, through careful wavelength dependent SERS measurements, the SERS Raman excitation profile of Crystal Violet is measured and shown to be closely linked to the modified absorbance as obtained in the integrating sphere setup. A standard optical transform model for computing the Raman excitation profile from the modified absorbance is applied and gives good agreement with the measured SERS data. These results represent a direct indication of chemical modifications of resonant molecules used in SERS studies.</p>

Preface to the special issue dedicated to Professor Richard P. Van Duyne (1945–2019)

The interaction between nanoparticle plasmonic states and molecular electronic states is central to the large enhancements observed in surface‐enhanced Raman scattering (SERS). In this work, we use a model previously used in a fully quantum mechanical description of SERS to explain the enhancement of nontotally symmetric modes for crystal violet (CV) and malachite green (MG) adsorbed on silver nanoparticles. Our explanation is consistent with recent observations of the absorption spectra of dyes on silver nanoparticles that show the absence of charge‐transfer in these systems. Resonance with plasmon and molecular states explains the enhancement of nontotally symmetric modes, with enhancement dominated by the plasmon states. Spectroelectrochemical SERS studies using the same silver nanoparticles that are used to generate SERS in solution show increased SERS intensity for CV with positive potential up to +0.4 V (vs Ag/AgCl), consistent with a red‐shifted plasmon resonance, and decreasing intensity with more positive potentials, consistent with oxidation of the silver nanoparticles. Both crystal violet and malachite green show similar spectroelectrochemical behaviour with 514.5 nm excitation. Our spectroelectrochemical data show that SERS intensities from silver nanoparticles can be optimised with appropriately applied potentials. This work has highlighted the need for more detailed studies of the spectroelectrochemistry of dyes adsorbed on silver nanoparticles to more fully understand how SERS intensities respond to applied potentials. Copyright © 2016 John Wiley & Sons, Ltd.

Hybrid silver nanoparticles with controlled morphology as efficient substrates for surface-enhanced Raman scattering

Surface-Enhanced Raman Scattering (SERS) sensors are versatile biosensors for detecting very low concentrations of molecules. Their molecular and chemical sensitivity crucially depends on the tailoring of their nanostructure. These sensors rely on noble metal or oxide cluster layers, with their crystallinity and nanostructure carefully tuned to enable a maximum of plasmonic hot spots and enabling charge transfer. Typically, such sensors are in thin film geometry. In order to optimize their nanostructure and optical properties of the metal or oxide clusters, cellulose nanofibrils (CNFs) as template for nanoparticle-biopolymer hybrid materials are readily employed due to their high mechanical strength and ability to establish nanoporous, scalable thin films via spray deposition.[1] In order to characterize their nanostructure during synthesis of the hybrid nanoparticle-CNF material, grazing incidence X-ray scattering (GIXS) is ideally suited due to its ability to observe the nanostructural changes during farication and optimization of the sensor materials. The resulting nanostructure is then directly correlation to the sensor capability of the hybrid materials. As a first example, we use the catalytic properties of CNF during thermal decomposition of silver nitrate as precursor for fabrication of size-tuned silver clusters on CNF to tune their optical band-gap. The correlation between hot-spot size and silver nanoparticle size for optimizing the SERS sensitivity is elucidated using GIXS, showing for the first time the optimum hot spot size.[2] In order to facilitate an equal distribution of hot-spots, we make use of the three-dimensional nanoporous structure of the CNF thin films by adding shell-free silver nanoparticles in aqueous solution, again applying spray deposition.[3] The nanoporosity allows for reducing drastically agglomerations of silver nanoparticles, detrimental to hot-spots. At the same time, the spectral position of the plasmon resonance is tuned by the amount of CNF as structural matrix. As a final example we extend these results to novel semiconductor metal oxide nanomaterial (SMON)-based sensors.[4] Here, titanium oxide is deposited via atomic layer deposition (ALD) on the nanoporous CNF network, preserving its three-dimensional morphology. We optimize the nanostructure, crystallinity and rutile-to-anatase ratio in order to obtain superior sensitivity of the hybrid titania/CNF thin films as SMON SERS substrates. The improvement of SERS activity relies on the cooperative modulation of the CNF network and the crystalline states of the deposited titania. To summarize, we present new routes for SERS sensors based on plasmonic and charge transfer properties of hybrid materials by tailoring the hybrid nanostructure.[1] Brett, C. J.; Ohm, W.; Fricke, B.; Alexakis, A. E.; Laarmann, T.; Körstgens, V.; Müller-Buschbaum, P.; Söderberg, L. D.; Roth, S. V. Nanocellulose-Assisted Thermally Induced Growth of Silver Nanoparticles for Optical Applications. ACS Appl. Mater. Interfaces 2021, 13 (23), 27696–27704. https://doi.org/10.1021/acsami.1c07544.[2] Santoro, G.; Yu, S.; Schwartzkopf, M.; Zhang, P.; Koyiloth Vayalil, S.; Risch, J. F. H.; Rübhausen, M. A.; Hernández, M.; Domingo, C.; Roth, S. V. Silver Substrates for Surface Enhanced Raman Scattering: Correlation between Nanostructure and Raman Scattering Enhancement. Appl. Phys. Lett. 2014, 104 (24), 243107. https://doi.org/10.1063/1.4884423.[3] Chen, Q.; Brett, C. J.; Chumakov, A.; Gensch, M.; Schwartzkopf, M.; Körstgens, V.; Söderberg, L. D.; Plech, A.; Zhang, P.; Müller-Buschbaum, P.; Roth, S. V. Layer-by-Layer Spray-Coating of Cellulose Nanofibrils and Silver Nanoparticles for Hydrophilic Interfaces. ACS Appl. Nano Mater. 2021, 4 (1), 503–513. https://doi.org/10.1021/acsanm.0c02819.[4] Chen, Q.; Betker, M.; Harder, C.; Brett, C. J.; Schwartzkopf, M.; Ulrich, N. M.; Toimil‐Molares, M. E.; Trautmann, C.; Söderberg, L. D.; Weindl, C. L.; Körstgens, V.; Müller‐Buschbaum, P.; Ma, M.; Roth, S. V. Biopolymer‐Templated Deposition of Ordered and Polymorph Titanium Dioxide Thin Films for Improved Surface‐Enhanced Raman Scattering Sensitivity. Adv. Funct. Mater. 2022, 32 (6), 2108556. https://doi.org/10.1002/adfm.202108556.

The surface‐enhanced Raman scattering (SERS) as a novel and efficient analytic technique to probe molecules has attracted tremendous attention. Semiconducting substrates have been widely investigated for their applications into SERS because of their easy integration with electronic devices. In this work, a wafer‐scale semiconducting MoS2 monolayer (2H‐MoS2‐ML) without additional treatment is used as the SERS substrate, which shows the naturally formed MoS2 ML has excellent chemical stability, high uniformity, and high sensitivity. It is found that the detection concentration limit can reach 1 × 10−8 m and the enhancement factor is about 4.5 × 106 for the rhodamine 6G (R6G) under a 532 nm excitation laser, which is the highest SERS performance observed on 2H‐MoS2‐ML up to now. The experimental and computational studies reveal that the photo‐enhanced charge transfer coupled with molecule resonance contribute to remarkable SERS. In addition to R6G, 2H‐MoS2‐ML shows good SERS signals on the detection of amaranth and crystal violet too. The findings not only provide an insightful understanding of the mechanism for the improved SERS performance of semiconducting transition‐metal dichalcogenides (TMDs) MLs, but are helpful for the design of novel SERS substrates. It is expected that the wafer‐scale TMDs may find practical applications in SERS.

Surface enhanced Raman spectroscopy (SERS) has widely been used for material composition analysis because it can provide good selectivity and sensitivity without the labeling process required by fluorescence detection. SERS enhancements on the order of 10 14 have been demonstrated, which can enable the detection of a single molecule. However, further enhancement is necessary to increase the sensitivity of SERS systems, and to make single molecule detection and analysis more practical. In this work, we demonstrate a composite system of silver nanoparticles and an optical microsphere resonator to create an even higher average Raman enhancement than SERS alone. The Raman pump is coupled into the microsphere resonator, where it repeatedly circulates around the surface via total internal reflection in the form of whispering gallery modes (WGMs). Microsphere resonators can have Q-factor values higher than 10 6 , which results in a tremendous local field enhancement. The evanescent field of the WGM interacts with the silver nanoparticles and target analytes, which are adsorbed onto the surface of the microsphere. With this composite system, we demonstrate an increase in Raman enhancement of approximately 300. Engineering improvements to this experimental prototype system may increase the enhancement by an order of magnitude. Further improvements that can leverage the microsphere resonator system to promote stimulated Raman scattering (SRS) may result in a dramatic increase in sensitivity. Ultimately, this composite system will increase the sensitivity of SERS sensor systems, and will bring single molecule detection and analysis systems closer to practical implementation.

In this work a surface-enhanced Raman scattering (SERS) study was carried out, in order to determine the dependence of crystal violet (CV) SERS intensity on the size of gold nanoparticles (GNPs) synthesized by reduction with polyethylene glycol (PEG), with molar masses 200 and 10000. By using UV-Vis spectroscopy and transmission electron microscopy (TEM), the size of the synthesized nanoparticles, in the range of 7–60 nm, showing spherical and polygonal geometries, was determined. Zeta potential measurements were performed in order to assess the stability of the gold colloids. The SERS spectra of different concentrations of CV analyte, using 7 to 60 nm diameter PEG-coated GNPs, were recorded to determine the optimal size of the nanoparticles that provides the highest SERS intensity. This study confi rms that the intensity of the SERS signal increases with increase in the diameter of the GNPs.

Raman Spectroscopy is a time-honored, non-invasive method for analyzing and identifying the molecular composition of materials. However, unenhanced Raman Spectroscopy has extremely low sensitivity which limits its sensing capability. SERS brings rough nano-metallic surfaces in contact with the material molecules to enormously enhance the Raman signals. The sensitivity of SERS can be exploited in probe applications where the spectrometer needs to be brought near the specimen. For example, a long optical fiber coupled to a SERS device can be used to characterize and identify easy-to-reach cancerous tissues in organisms. Unfortunately, background signals in a long fiber can easily mask any signal returning from the end of the probe. A classical solution is to inject nanoparticles and use multiple optical fibers (one to deliver the excitation light and one or more to return the scattered Raman light). However, the coupling between the fibers is poor, reducing the signal strength, and reproducibility between locations, and removal of the injected nanoparticles present difficulties. This work intends to address those challenges by designing and fabricating a low-cost handheld SERS device. The development of this SERS device is broadly split into (a) fabrication of a probe suitable low-cost SERS substrate (b) design and fabrication of the handheld SERS device optics. The method used for the fabrication and characterization of the SERS layer low-cost substrate involved sandpaper imprint patterning of silver nanoparticles. This was accomplished at low cost with inexpensive equipment, readily available materials, and with no chemical or lithographic steps. The handheld SERS optics incorporated a solid-state laser, diffractive optics, the low-cost SERS substrate at one end of a GRIN lens, and a short-tube pathway to the Raman spectrometer. The response of the optical system and imprinted SERS layer was tested to obtain the Raman spectrum from 1nmol to 1mmol Rhodamine 6G suspension. This yielded good Raman scattering results. The developed device was made with a SERS substrate fabrication method which is a low-cost alternative and with no lithography or chemical synthesis. This SERS portable design is also suitable for bio-probes or remote sampling without the disadvantages associated with injected clouds substrates and multiple collection fiber systems.

Synergistic effects of semiconductor substrate and noble metal nano-particles on SERS effect both theoretical and experimental aspects

Surface-enhanced resonance Raman scattering (SERRS) from silver nanoparticles using 514.5-nm excitation has been shown to offer huge potential for applications in highly sensitive multiplexed DNA assays. If the technique is to be applied to real biological samples and integrated with other methods, then the use of gold nanoparticles and longer wavelengths of excitation are desirable. The data presented here demonstrate that dye-labeled oligonucleotide sequences can be directly detected by SERRS using gold nanoparticles in a quantitative manner for the first time. The performance of gold and silver nanoparticles as SERRS substrates was assessed using 514.5-, 632.8-, and 785-nm excitation and a range of 13 commercially available dye-labeled oligonucleotides. The quantitative response allowed the limit of detection to be determined for each case and demonstrates that the technique is highly effective, sensitive, and versatile. The possibility of excitation at multiple wavelengths further enhances the multiplexing potential of the technique. The importance of effectively combining the optical properties of the nanoparticle and the dye label is demonstrated. For example, at 632.8-nm excitation, the dye BODIPY TR-X and gold nanoparticles make a strong SERRS combination with very little background fluorescence. This study allows the choice of nanoparticle and dye label for particular experimental setups, and significantly expands the applicability of enhanced Raman scattering for use in many disciplines.

Comparison between silver and gold nanoparticles stabilized with negatively charged hydrophilic thiols: SR-XPS and SERS as probes for structural differences and similarities

Research on the Raman properties of NiFe/cicada wing composite SERS platform modified by silver nanoparticles