16
strongly localised surface plasmons are formed with incident light in the visible and near-infrared regions, making these materials ideal SERS substrates. A molecule of interest placed near the LSPR field experiences the intensified electromagnetic field leading to enhanced Raman scattering, and thus stronger signals with higher counts in the Raman spectrum [10]. The greatest SERS enhancement occurs within a few nanometres of the plasmonic particle’s surface. This is ideal to study the electrode-electrolyte interface where the electron-transfer occurs at the surface of the metal: a change in the structure of a material or molecule induced by an electron transfer can be captured by SERS as it occurs. To get the most out of the enhanced electromagnetic field, the molecule of interest is sometimes anchored to the SERS substrate in creative ways. This can also be achieved by applying a potential to induce adsorption of the target molecule to the working electrode which is also the SERS substrate [7,10].
Figure 4. Electromagnetic mechanism of the SERS effect. a) Reaction of plasmonic nanoparticles to an electromagnetic field (incident laser); b) Formation of a local “hot spot” between two plasmonic nanoparticles resulting in an enhanced local electromagnetic field and enhanced Raman scattering of a molecule in its vicinity.
With real SERS substrate materials there is not a single nanoparticle or rough edge, but many of them which are in contact or close proximity to each other as shown in Figure 4b. There are positions where the electric fields from various particles or edges overlap, forming an area of intense local electric field. These enhanced locations are known as «hot spots» and they contribute significantly to the electromagnetic SERS enhancement [8]. Due to their diverse and variable properties and their importance to Raman spectroscopy applications, these hot spots have been the focus of research projects that seek to optimise the material, size, and geometry of SERS substrates [11]. Figure 4b illustrates the concept of a hot spot formed by two nanoparticles in contact with each other. In reality, nanoparticles form aggregates and intense hot spots can form in the gaps between the particles [12]. It is important to note that the hot spot is usually very small and localised near the particle surface. The SERS enhancement decreases exponentially with increasing distance from the surface and is essentially negligible at a distance of about 15 nm [8].
Although hot spots are beneficial because they greatly enhance the SERS signal, they also introduce a complication for reproducible experiments. When the concentration of the analyte is low, the interaction of a single molecule in a hot spot can have an appreciable influence on the measured spectrum because the enhancement of the signal at this position is orders of magnitude higher than a typical Raman scattering signal. If the molecule is not anchored in position at the hot spot, it can diffuse away from the ideal position leading to a change in the Raman spectrum. To minimise this issue it is desirable to have numerous hot spots on the SERS substrate. Ideally, there would be full and uniform coverage of the substrate with hot spots; researchers that are developing such materials refer to them as “hot surfaces” [8].
It is important to be aware of how surface enhancement alters the Raman spectrum of the target molecule. The SERS effect does not amplify each band in the Raman spectrum equally.
With SERS, the interaction of the molecule with the substrate is also being probed, and this may give rise to additional peaks in the SERS spectrum. In the case of large molecules, the distance between a given functional group and the LSPR will influence the appearance of the spectra as some vibrational modes will be enhanced [8]. The interpretation of Raman spectra often involves consulting spectral libraries so it is important to keep in mind that experimental SERS spectra may not match the library spectrum.
Shell-isolated nanoparticle enhanced Raman Spectroscopy (SHINERS)
Shell-isolated nanoparticle-enhanced Raman spectroscopy, known as SHINERS, is a type of SERS that was introduced in 2010 [13]. The SHINERS technique is conceptually similar to SERS. With SHINERS, the plasmonic nanoparticle is encapsulated within a thin (less than 10 nm in thickness) silica or alumina shell resulting in what is known as a «SHIN» (shell-isolated nanoparticle). The enhancement effect is the same as with SERS. A region of strong localised electromagnetic field is created as a result of the interaction of the laser light and the plasmonic nanoparticle. The oxide coating is thin and interacts very little with the incident laser light. As with SERS, the formation of hot spots is also possible with SHINERS; it is favoured when the oxide shell is thin enough that the two plasmonic particles within the shells are still sufficiently close [8,9]. Figure 5 illustrates SHINERS and the structure of a spherical gold SHIN. With SHINERS, the Raman scattering enhancement factor is dampened compared to traditional SERS because the molecule of interest is always at a distance from the plasmonic particle. The distance is dictated by the thickness of the oxide shell. Nevertheless, the shell provides several benefits that are mostly related to improved stability and inertness.
Figure 5. Illustrative example of SHINERS featuring a spherical gold nanoparticle in thin silica shell (dielectric material).
Some of these benefits are especially helpful in electrochemical environments.
1. Increased stability concerning the environment, especially in high-temperature applications. Silica and alumina coatings allow silver or gold nanoparticles to be used in SERS applications at temperaturesup to 500 °C [9].
2. Elimination of plasmon-driven side reactions that may occur when the molecule of interest is anchored directly to the SERS substrate. This is often a concern for biological molecules with sensitive structures [8].
3. Possibility to functionalise the oxide coating through a variety of strategies to influence the interaction of the target molecule with the SHIN. For example, to study a chemical reaction taking place with a catalyst. The catalyst can be attached to the SHIN and SHINERS can be used to study the reaction at the catalyst, which would be within the Raman scattering enhancement zone due to its proximity to the plasmonic particle [9].
4. Specifically for EC-Raman studies, there is more flexibility for the working electrode material with SHINERS than with normal SERS. This is because the plasmonic material (i.e., Ag or Au) does not need to be in contact with the electrolyte. Thus, it is possible to use a working electrode material of choice, attach SHINs to the working electrode to create hot spots, and study electrochemical reactions taking place at the working electrode with hyphenated EC-Raman [6,7].
Hyphenated EC-Raman instrumentation and setup
For an EC-Raman experiment, the following components are recommended (see Figure 6):
− A Raman instrument, including the spectrometer and light source − An electrochemical instrument (potentiostat/galvanostat) − An electrochemical cell with an appropriate working electrode
− A means of controlling the position of the sample relative to the Raman optics. In EC- Raman experiments, this is usually the working electrode in the electrochemical cell. This often involves an optical microscope or a positioning stage and probe holder.
The Raman instrument contains everything needed to obtain a Raman spectrum. The excitation source is a laser that provides monochromatic light of a specific wavelength. Specialised optics bring the laser light to the sample, collect the scattered light, and guide it to the spectral analyser. The spectral analyser separates the scattered light into its constituents based on their wavelengths, and the detector converts the optical signal into an electronic signal that is then digitised and displayed as a Raman spectrum.
wavelength (λ) of the excitation source, according to the following relationship:
When designing an EC-Raman setup, consider which laser light source to work with. The most common ones are 532 nm, 785 nm, and 1064 nm. The decision is generally based on the need for strong excitation efficiency balanced with concerns related to the interference of fluorescence. Raman scattering efficiency (Pscattered
) relies strongly on the
INTERNATIONAL LABMATE - FEBRUARY 2023
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80
