Adaptive optics approach to surface-enhanced Raman scattering.

Chapter 6 Single molecule sensitivity in surface enhanced Raman scattering using surface plasmon

Surface Enhanced Raman Spectroscopy of 4-Mercaptopyridine Molecules on Pb 3 O 4 Nanoparticles

Developing SERS substrates based on individual gold and silver metals, either with rough surfaces or bare nanoparticles, has certain limitations in practical analysis applications. In order to improve the range of applications of the noble metallic substrates, a comprehensive approach has been proposed for preparing non-traditional SERS nano-substrates by combining tip-enhanced gold nanostars and Raman signal amplification of the silver layer. This preparation process is conducted in two steps, including tuning the sharpness and length of tips by a modified seed growth method followed by coating the silver layer on the formed star-shaped nanoparticles. The obtained AuNS-Ag covered with an average size of around 100 nm exhibited interesting properties as a two-component nano-substrate to amplify the activities in SERS for detecting thiram. The controllable and convenient preparation route of gold nanostars is based on the comproportionation reaction of Au seed particles with Au(iii) ions, achieved by governing the stirring times of the mixture of the Au seed and the growth solution. Thus, the citrate-seed particles decreased in size (below 2 nm) and grew into nanostars with sharp tips. The thickness of Ag covering the Au particles' surface also was appropriately controlled and the tips were still exposed to the outside, which is a benefit for matching with the source excitation wavelength to achieve good SERS performance. The Raman signals of thiram can be instantly and remarkably detected with the enhancement of the substrates. Thiram can be determined without any pretreatment. It was found that the limit of detection for thiram is 0.22 ppm, and the limit of quantification is 0.73 ppm. These experimental results shed some light on developing the SERS method for detecting pesticide residue.

<p>Surface enhanced Raman spectroscopy (SERS) is undoubtedly a powerful tool as it allows one to overcome the major disadvantage of Raman spectroscopy: the weakness of its signal. Enhancement factors (EF) of up to 1010 make it even possible to detect single molecules. However, using it as an analytical tool to make reproducible, quantitative measurements has so far been difficult as the enhancement of the signal is "bought" at the expense of reproducibility: The larger the EF the more the reproducibility of the substrate suffers. This has been dubbed informally the "SERS uncertainty principle" by Natan [1]. While currently a lot of research effort is taking place at the high-EF-side of the spectrum and ever more sophisticated SERS substrates are being explored, in this thesis we would like to make a shift in paradigm and revisit SERS on flat metallic surfaces, which arguably constitute the simplest substrates available. To this end we will show their usefulness in making quantitative measurements and how they are an ideal platform for a new hybrid technique that combines reproducibility and extreme sensitivity with substantial EFs. For making quantitativemeasurements two examples are explored in a systematic way: in the first example (Chapter 2) the determination of an unknown, resonant Raman cross-section is demonstrated on flat metallic films (possibly with some surface roughness) and confirmed with measurements done on more commonly used SERS substrates. Here the quantitative measurement is made possible by introducing a reference molecule as a standard and having statistics as our main ally: even though we do not know the exact EF that the individual molecules experience on the various substrates, we know that on average both, the unknown sample and the known reference, experience the same. In the second example (Chapter 3) we use commercially available flat films for which we verify experimentally that surface roughness is irrelevant. By themselves these substrates yield no enhancement – in fact they even quench the Raman signal. Yet they allow us to calculate and control the electric field on the surface which enables us to determine the orientation of adsorbed molecules by using surface selection rules (SSR). While the first example is mostly empirical, the second one allows us to test our theoretical understanding of plasmonic systems with proper numerical calculations that are in excellent agreement with the observed data. Finally, in Chapter 4, we use those flat films in a special configuration (called the Kretschmann configuration) to excite Surface Plasmon-Polaritons (SPP). This not only allows us to combine the spatial homogeneity of a flat surface with useful EFs easily predicted fromtheory but also to combine the extreme sensitivity of surface plasmon resonance spectroscopy (SPRS) with the analytical power of SERS. It is not our intention to claim that the work presented here is the first attempt to do analytical work with SERS. Rather the newmethods presented in this thesis will add new strategies and tools to the current research effort while the detailed analysis will provide the means to understand them theoretically and in their historical context.</p>

Nowadays, plasmonic nanoparticles have attracted attention due to the significant improvement in the detection of Raman scattering signals from molecules adsorbed on rough metal surfaces. In the field of surface-enhanced Raman scattering (SERS), it is possible to record spectral signatures of even individual molecules. This phenomenon is partly explained by the significant enhancement of the electromagnetic field at the surface of metal nanoparticles, which actively interacts with the energy levels of adsorbed molecules. The shape, geometry of nanostructures, as well as the size and state of aggregation of the nanomaterial play a key role in the creation of the regions of local enhancement of the electromagnetic field. The localized surface plasmon resonance contributes to a significant enhancement of the electromagnetic field. Due to this, the intensity of the Raman signal of molecules located in these zones increases significantly. There are various approaches to improving the Raman signal of the studied substances. In this work, the extinction cross-sectional spectra of silver nanoparticles under surface plasmon resonance conditions have been calculated using the quasi-dipole approximation, taking into account the influence of the substrate on which the nanoparticles are deposited. The calculations consider the nanoparticle size and the distance between the nanoparticles and the substrate surface. Numerical experiments have demonstrated that the substrate material's permittivity, on which silver nanoparticles are deposited, plays a key role in the design and fabrication of optical structures based on them. It was revealed that a substantial shift of the extinction cross-section peak and an increase in its magnitude at the operating wavelength are primarily driven by surfaces with a high dielectric constant. Based on theoretical research findings, SERS substrates were produced using galvanic replacing and wet chemical methods. Silicon and ITO served as the base materials. Experimental data confirmed that the fabricated SERS substrates are comparable in performance to commercial counterparts. The work was carried out within the SERS project (state registration number 0124U000823). 1. Wang, X., Huang, S. C., Hu, S., Yan, S., & Ren, B. (2020). Fundamental understanding and applications of plasmon-enhanced Raman spectroscopy. Nature Reviews Physics, 2(5), 253-271 2. Schmidt, M. M., Brolo, A. G., & Lindquist, N. C. (2024). Single-molecule surface-enhanced raman spectroscopy: challenges, opportunities, and future directions. ACS nano, 18(38), 25930-25938. 3. William, R. V., Das, G. M., Dantham, V. R., & Laha, R. (2019). Enhancement of single molecule Raman scattering using sprouted potato shaped bimetallic nanoparticles. Scientific Reports, 9(1), 1077.

ABSTRACTLight collection efficiency and specific molecular detection are crucial factors for the performance of bio-chemical molecule sensors. In this paper, the low optical-reflection silicon (Si) substrates which combine reduced optical reflectivity by light trapping effect and high Raman enhancement ability of gold nanoparticles (Au-NPs) coated textured Si substrates are investigated for surface-enhanced Raman scattering (SERS). A fast, single-step and highly controllable nanosecond (ns) laser processing technique is employed to fabricate textured Si substrates under ambient conditions. Parallel arrays of micro-pyramids are fabricated on Si surface by direct laser writing two-dimensional structures. SEM micrographs clearly show well-ordered surface features in the form of micro-pyramid shape with well-defined sharp tips on the laser processed Si substrates. The aggregation of Si micro/nanoparticles on Si surface forms nanocavities and nanogaps and further enhances the surface roughness in order to minimize the optical reflection. The low optical reflection Si substrates exhibit optical reflection below 15% over a broad wavelength range from 300 nm to 1200 nm. The textured Si substrates with high signal reproducibility are successfully applied as SERS substrates to detect a very small concentration of Rhodamine B molecules with an average enhancement factor of the order of ∼107. The low optical reflection and SERS signal amplification are also altered by the variation of laser pulse energy resulting into low optical reflection and high SERS signal intensity over the entire laser-patterned area. The effect of surface roughness on water contact angle was studied after the modification with Polydimethylsiloxane (PDMS), the surfaces show perfect superhydrophobicity with almost no water adhesion. This approach provides a novel high-speed and cost-effective method for fabricating SERS substrate with micro/nano-scale surfaces roughness and low optical reflection for high Raman signal enhancement.

The plasmonic nanostructures were prepared by an electrical exploding wire technique. The different morphology, surface roughness and hot spots of these nanostructures were examined by field emission scanning electron microscopy (FESEM), scanning transmission electron microscopy (STEM) and atomic force microscope (AFM). Surface enhanced Raman scattering (SERS) performance of the plasmonic nanostructures substrates were detected using Rhodamine 6G (R6G) dye as probe molecules. The surface enhanced Raman scattering SERS enhancement factor of R6G (1x10−6 M) reached (2.3X107) and (2.5X107) at the wave number (1650 cm−1) for the nanostructures AgNW@AgNPs and AgNW@PDA@AgNPs respectively. The plasmonic nanostructures AgNW@AgNPs and AgNW@PDA@AgNPs showed a good linear relationship with a value of the determination coefficient (R2=0.9717 and R2=0.9569) respectively between the intensities of SERS spectra and R6G dye concentration of characteristic spectral peaks (at 1650 cm−1). The nanostructures AgNW@AgNPs and AgNW@PDA@AgNPs have been exhibited a superb reproductions and excellent stability as a SERS substrate to realize the molecular detection. EEW technique has been not involved using of dangerous chemicals and pollutant organic solvents through the fabrication process. These advantages denoted that the substrates of nanostructures AgNW@AgNPs and AgNW@PDA@AgNPs could be considered as a good nominee for organic molecule detection under the very low concentration and used in ultra-sensitive devices and molecular detection through (SERS) applications in biological and chemical fields.

Surface-enhanced Raman scattering (SERS) is a powerful spectroscopic technique that has created exciting opportunities in the field of bioanalytical chemistry, where it combines ultrasensitive detection of biologically relevant molecules with vibrational spectroscopy. Due to the difficulties in preparing reproducible SERS active substrates, SERS has been mainly used as a qualitative tool. In order for SERS to be utilized as a viable tool for quantitative analysis, simple, facile SERS substrates which generate clean, highly reproducible signals must be developed. This dissertation deals with the evaluation of three different methods of SERS that led to the development of a novel substrate for the analysis of biological molecules. In this dissertation, I demonstrated the generation of a SERS signal from pyridine adsorbed to the surface of a roughened silver electrode. The Raman signal was detected utilizing a remote fiber optic probe dispersive Raman instrument. The necessary parameters for generating the optimal SERS signal were established. The SERS spectrum of pyridine was successfully characterized. The synthesis of silver colloid solutions for use as a SERS active substrate was demonstrated. Two different synthetic methods were utilized and the conditions for optimal SERS signal strength were established. SERS signals from solutions of tryptophan were successfully generated and detected using an FT-Raman spectrometer. In this dissertation. I report the successful chemical deposition of a thin silver film on the surface of a quartz/glass fiber filter for use as a SERS active substrate. The successful detection of the SERS signal of solutions of phenylalanine and tryptophan utilizing the silver-coated filters and a FT-Raman instrument was demonstrated. Significant progress was made to develop a quantifiable SERS method utilizing the silver-coated filters. The silver-coated filter SERS method presented here is a novel and promising method for biochemical analysis. The demonstrated ability to detect amino acids and mixtures of amino acids can be further extended to the detection of similar biological molecules such as proteins, tumor markers, and nucleic acids. The use of a FT-Raman instrument combined with this new SERS method allows for a method with both high sensitivity and selectivity.

Photo-induced in-situ synthesized nanocellulose-based SERS substrate integrated with molecularly imprinted polymer for selective detection of pesticide residues.

Experimental and theoretical investigation for a hierarchical SERS activated platform with 3D dense hot spots

Since its discovery in 1974 [1], surface enhanced Raman spectroscopy (SERS) has proven to be a viable technique for increasing the usefulness of traditional Raman-based spectroscopic methods, reporting increases in the intensity of the Raman signal up to 10 orders of magnitude [2]–[5]. The signal amplification provided by SERS substrates is primarily attributed to an electromagnetic effect, relying on the ability to couple incident light to optical modes (surface plasmons) on a roughened metallic surface [3], [6]. Here, we propose laser-induced periodic surface structures (LIPSS) as an effective method of nano-structuring stainless steel substrates for SERS. A nanometric gold layer is subsequently deposited for use as a SERS-active surface. Nano-structuring of steel wafers by femtosecond laser pulses is optimised for use in detecting extracellular vesicles (EVs) from a choriocarcinoma cell line (JEG-3), commonly used as a model for placental syncytiotrophoblasts. A variety of parameters are considered - such as writing laser power, polarization, machining speed, and gold layer thickness - each with implications for the LIPSS nano-structure morphology and, in turn, the SERS enhancement. With our experimental setup we have shown that for stainless steel substrates the Raman signal intensity of EVs increases with the machining laser power used to generate LIPSS; illustrated in Fig. 1.

Applicable surface enhanced Raman scattering (SERS) active substrates require high enhancement factor (EF), excellent spatial reproducibility, and low-cost fabrication method on a large area. Although several SERS substrates with high EF and relative standard deviation (RSD) of signal less than 5% were reported, reliable fabrication for large area SERS substrates with both high sensitivity and high reproducibility via low-cost routes remains a challenge. Here, we report a facile and cost-effective fabrication process for large-scale SERS substrate with Ag inter-nanoparticle (NP) gaps of 5 nm based on ultrathin alumina mask (UTAM) surface pattern technique. Such closely packed Ag NP arrays with high density of electromagnetic field enhancement ("hot spots") on large area exhibit high SERS activity and excellent reproducibility, simultaneously. Rhodamine 6G molecules with concentration of 1 × 10(-7) M are used to determine the SERS performance, and an EF of ∼10(9) is obtained. It should be noted that we obtain RSDs about 2% from 10 random spots on an area of 1 cm(2), which implies the highly reproducible signals. Finite-difference time-domain simulations further suggest that the enhanced electric field originates from the narrow gap, which agrees well with the experimental results. The low value of RSD and the high EF of SERS signals indicate that the as-prepared substrate may be promising for highly sensitive and uniform SERS detection.

A versatile generation of plasmonic nanoparticle dimers for surface-enhanced Raman scattering (SERS) is presented by combining a DNA origami nanofork and spherical and nonspherical Au or Ag nanoparticles. Combining different nanoparticle species with a DNA origami nanofork to form DNA origami nanoantennas (DONAs), the plasmonic nanoparticle dimers can be optimized for a specific excitation wavelength in SERS. The preparation of such nanoparticle dimers is robust enough to enable the characterization of SERS intensities and SERS enhancement factors of dye-modified DONAs on a single dimer level by measuring in total several thousands of dimers from five different dimer designs, each functionalized with three different Raman reporter molecules and measured at four different excitation wavelengths. Based on these data, SERS enhancement factor (EF) distributions have been determined for each dimer design and excitation wavelengths. The structures and measurement conditions with the highest EFs are suitable for single-molecule SERS (SM-SERS), which is realized by placing single dye molecules into hot spots. We demonstrate that the probability of placing single molecules in a strongly enhancing hot spot for SM-SERS can be increased by using anisotropic nanoparticles with several sharp edges, such as nanoflowers. Combining a Ag nanoparticle with a Au particle in one dimer structure allows for a broadband excitation covering almost the whole visible range. The most versatile plasmonic dimer structure for SERS combines a spherical Ag nanoparticle with a Au nanoflower. Employing the discontinuous Galerkin time domain method, we numerically investigate the bare, symmetric dimers with respect to spectral and near-field properties, showing that, indeed, the nanoflowers induce multiple hot spots located at the edges which surpass the intensity of the spherical dimers, indicating the possibility for SM-SERS. The presented DONA structures and SERS data provide a robust basis for applying such designs as versatile SERS tags and as substrates for SM-SERS measurements.

The surface roughness and nanometer scale structure of Ag films used for surface-enhanced Raman scattering (SERS) are characterized using atomic force microscopy (AFM). Two important types of thin film based SERS-active surface have been examined in this study: (1) Ag island films (AgIF’s) on smooth, insulating substrates and (2) thick Ag films evaporated over both preroughened and smooth substrates. AFM is demonstrated to be capable of quantitatively defining the three-dimensional (3D) structure of these roughened surfaces. The effects of mass thickness, dm, and thermal annealing on the nanostructure of AgIF’s are studied in detail. Particle size histograms are calculated from the AFM images for both ‘‘as-deposited’’ and annealed IF’s with dm=1.8 and 3.5 nm. Quantitative measurements of the SERS enhancement factor (EF) are coupled with the AFM data and interpreted within the framework of the electromagnetic theory of SERS. AFM images for thick evaporated Ag films over a monolayer of polymer nanospheres (AgFON) shows the clear presence of ‘‘random substructure roughness’’ reducing their utility as controlled roughness surfaces. Similar roughness structures are observed for thick evaporated Ag films on smooth, insulating substrates. Nevertheless, AgFON surfaces are demonstrated to be among the most strongly enhancing thin film based surfaces ever studied with EF’s comparable to those found for electrochemically roughened surfaces. Applications of FON surfaces to ultrahigh sensitivity SERS, anti-Stokes detected SERS, and surface-enhanced hyper-Raman spectroscopy (SEHRS) are reported.

Facile fabrication of ultrahigh-sensitivity surface-enhanced Raman scattering sensor using porous silicon photonic crystals prepared on backside silicon wafer