Surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy gives the same information as normal Raman spectroscopy but with a significantly enhanced signal. It is also referred to as surface-enhanced Raman scattering (SERS).

It is a spectroscopic method that finds its use in condensed matter chemistry and physics and helps investigate vibrational and rotational modes along with other low-frequency modes within a system.

This article defines the working principle and enhancement mechanisms involved in surface-enhanced Raman spectroscopy.

Also, we have discussed the instrumentation models and the steps required to perform this extremely versatile and modern spectroscopic analysis method.

So for all this interesting information, continue reading!  

What is surface-enhanced Raman spectroscopy? -Definition

SERS is a surface-sensitive technique that enhances light scattering (Raman scattering) by molecules adsorbed on rough metal surfaces or nanostructures. The enhancement factor ranges from 10¹⁰ to 10¹¹ enabling this technique to detect a single molecule at a time.

SERS substrates: Many variants of SERS substrates, for instance, noble metals, rough metal electrodes, semiconductors, nanomaterials and transition metals are reported in literature.

Despite the availability of all these variants, noble metals, for example, Au NPs and Ag NPs, are commonly used due to their straightforward preparation, substantial sensitivity, enhancement, cost-effectiveness, etc.

What is the basic working principle of surface-enhanced Raman spectroscopy?

Background

Raman spectroscopy: It depends on inelastic scattering of photons, resulting in a shift in frequency in comparison to incident light. This process is described as Raman scattering.

Raman scattering: An optical process in which incident excitation light produces scattered light when it interacts with the sample. 

Inelastic scattering denotes that the kinetic energy of photons after interaction with the sample is not conserved. The frequency parameter of monochromatic light also changes once it interacts with the vibrational states of a molecule.

The inelastic scattering happens in two ways:

The kinetic energy either increases (stokes component) or decreases (anti-stokes component).

• Stokes scattering: It is referred to as the scattered light with increased energy relative to incident light.
• Anti-stokes scattering: The scattered light of decreased energy as compared to the energy of the incident radiation.

This energy shift, in addition to vibrational and rotational transitions, gives insight into other low-frequency transitions.

A limitation of Raman scattering: Out of 10⁷ photons, only one photon undergoes Raman scattering. As the Raman signal turns out to be quite weak, thus its use is very limited.

Therefore, after employing conventional Raman spectroscopy for decades, a discovery occurred that metal nanostructures could enhance Raman signals.  This happens when structures are nearby or in contact with the sample; hence, the new advanced version of Raman spectroscopy was named Surface-enhanced Raman spectroscopy.

For instance,

• Silver nanoparticles when incorporated with dye chemicals produced Raman peaks and facilitated single molecule detection at low excitation powers and acquisition times.
• Acquisition time is required for Raman measurement, such as recording a signal.

SERS amplify Raman signals by several orders of magnitude from molecules. This technique is marked by higher scattering efficiencies when molecules adsorb light on rough metal surfaces or colloidal metal nanoparticles.

Even with SERS being a wide and active field of study, there is still ambiguity about the enhancement mechanism.

Enhancement mechanisms

Among many proposed mechanisms, two mechanisms such as electromagnetic (EM) theory and chemical enhancement (CE) theory, are widely accepted to facilitate surface-enhanced Raman spectroscopy.

• Electromagnetic (EM) theory

This mechanism considers the molecule a point dipole that responds to local enhanced fields at or close to the metal surface. These enhanced fields, as a result, couple the incident field with surface plasmons.

• Chemical enhancement (CE) theory

CE theory is based on the chemical interaction between the noble metal and the probe molecules. The noble metal is considered to have a contribution of almost two to three orders of magnitude.

Both of the abovementioned theories work side by side but are still to be explored completely. This is due to the scientists’ facing challenges in investigating the enhancements individually.

Surface-enhanced Raman spectroscopy- Instrumentation and equipment

For the measurement of SER spectra, two approaches regarding instrumentation are observed.

1. Macro-Raman configuration

This configuration facilitates experiments where a high raw SER intensity and low spatial resolution are required.

• SERS substrate is exposed to a focused laser beam at a glancing angle. Laser excites the target species.
• A big collection lens is used to collect the Raman light (stokes).
• The entrance slit of the spectrophotometer is used to focus the light.
• A camera which is a liquid-nitrogen-cooled charged-coupled tool, is used for detection purposes at the final step.

2. Micro-Raman Configuration

This configuration is for experiments where high spatial resolution is required.

• The same aperture objective of high numerical is utilized for both focusing and collecting the laser light.
• After that, a notch filter helps in filtering the Rayleigh scattered light as the scattered light passes through the filter.
• In the final step, the light is focused and directed towards the spectrophotometer and the detector.
• Detector directs the signal to a computer for decoding after recording it.

SERS spectra

• A SERS spectrum is plotted between Raman intensity and Raman shift.

Raman shift is the frequency difference between the incident and scattered light beams.

• The interaction between photons and specimen molecules generates Raman spectra and is collected with the aid of optical filters.
• The spectra provide information about chemical species, material conformation and molecular structures.
• Moreover, it gives a “fingerprint” to identify the information above.

Steps to perform surface-enhanced Raman spectroscopy

1. Sample and SERS substrates required in measurements are prepared according to SOP.
2. One calibration set is prepared for each experiment (for building a regression model). Also, one test set (for validating the regression model and comparing results) of the samples is prepared.
3. SERS measurements are performed using the instrument.
4. Raw spectral data is collected to be processed according to the procedure.

Advantages of surface-enhanced Raman spectroscopy

Advantage	Explanation
Low analyte concentration	With the feature of Plasmon-mediated amplification, SERS helps in the analysis of structural fingerprinting for analytes of lower concentration.
Ultra-high sensitivity	The sensitivity of SERS is high enough to detect the extreme trace levels of analyte to single molecules.
Strong resistance to water and air	The final SERS spectrum bears negligible influence of water and/or air. This helps in applying Raman study in the physiological or aqueous environment.
Non-destructive	The analyte structure and composition stay intact during analysis via surface-enhanced Raman spectroscopy.
In-situ (analysis in which the sample is not displaced from its location)	This in-situ technique is especially advantageous to deal with the catalytic oxidation of water at the anode.

Future Martian mission

Considering the importance of the technique, a design of a basic Raman spectrometer with SERS technology is proposed for a future Martian mission. SERS will contribute by adding a metal platform to facilitate sample analysis from the surface of Mars.

How is surface-enhanced Raman spectroscopy different from light-scattering Raman spectroscopy?

Although SERS has the inherent features of Raman spectroscopy, it differs from its parent technique in a following ways.

Light scattering Raman spectroscopy	Surface-enhanced Raman spectroscopy
No signal enhancement is involved. A direct beam of laser targets the sample. – The bonds of the analyte scatter the laser light. – This inelastically scattered light is focused and further processed into the Raman spectrum.	SERS includes a roughened metallic surface in addition to basic Raman characteristics. SERS marked by the enhancement of Raman signal proceeds by following two enhancement mechanisms. 1)  Electromagnetic enhancement mechanism. 2)   Chemical enhancement mechanism.
As only one scattered photon out of the 10 million exhibits Raman scattering so a weak Raman signal is obtained.	A strong signal is obtained.
Light scattering Raman spectroscopy is insensitive.	SERS is comparatively more sensitive due to the enhancement mechanism.
The detection limit is higher for some applications, such as narcotics identification.	The detection limit provided by SERS is suitable for the different applications given in the next section.

Also, read our special article on the simple light scattering Raman spectroscopy.

Why is surface-enhanced Raman spectroscopy important? – Applications

Some of the most useful applications of surface-enhanced Raman spectroscopy in multidisciplinary fields are as follows:

1. Hospitals and diagnostic facilities

Coupling Raman spectrometers with paper-based strips of SERS proves helpful for biological specimen analysis in hospitals and diagnostic centers.

2. Monitoring intracellular environment

SERS can potentially analyze the chemical environment inside the biological cell.

For example, it is used to find the concentration of ions (Mg²⁺, Ca²⁺), redox potential and pH inside the cell.

3. Detection of bacteria

Detection of Escherichia coli bacteria to test the susceptibility of antibiotics is possible using surface-enhanced Raman spectroscopy.

According to a scientific study, nanoroads of rhodamine 6G-modified gold core-silver shell detected bacteria in spiked mouse blood.

Also, within the time span of 3.5 hours, SERS helped differentiate antibiotic resistance.

• Photo-thermal killing of bacteria: In addition to detecting bacteria, with the help of an SER-based approach, scientists reported that the photo-thermal killing of bacteria is possible.
• For instance, in an experiment, SERS not just detected but also annihilated both Gram-positive and Gram-negative bacteria.

4. Environmental analysis

For the past three decades, SERS has facilitated identifying and quantifying pathogens and inorganic and organic contaminants in environmental samples.

Other interesting articles by easytocalculate.com:

• Astronomical spectroscopy
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• Introduction to spectroscopy- Everything you need to know about spectroscopy

References

1. Fornasaro, S., 2020. Surface Enhanced Raman Spectroscopy for Quantitative Analysis: Results of a Large-Scale European Multi-Instrument Interlaboratory Study. Analytical Chemistry, 92, 4053-4064.

2. Samir, K., Prabhat, K., Anamika, D. & Chandra Shakher, P. 2020. Surface-Enhanced Raman Scattering: Introduction and Applications. In: MOJTABA, K. & PARSOUA, A. S. (eds.) Recent Advances in Nanophotonics. Rijeka: Intech open.