Spatially Offset Raman Spectroscopy (SORS) For Surface Investigation

What is the SORS?

Spatially Offset Raman Spectroscopy (SORS) is a type of vibrational spectroscopy that allows chemically specific analysis of materials below their surfaces, enabling non-destructive characterization of subsurface diffusely scattering materials.

Raman spectroscopy uses inelastic scattering of monochromatic light to generate a spectrum unique to a sample. The method typically involves detecting red-shifted photons that result from monochromatic light transferring energy to a molecule's vibrational motion.

The signals of SORS are defined by Raman shift, which is expressed in relative wavenumbers (cm^–1) and is itself an expression of the difference between the absolute wavenumber of the laser wavelength (1/λ_L) and the absolute wavenumber of the Raman emitted photon (1/λ_R). The spectral region of the Raman spectra captured by a spectrograph is called the spectral range and is typically 0–4,000 cm^–1, where most vibrational modes lie, with the signal detected at a wavenumber of 0 cm^–1 representing photons from the incident laser.

What are the benefits of SORS applied in life?

Raman spectroscopy enables precise chemical analysis of objects hidden beneath obscuring surfaces, up to several millimeters. It can be used to analyze bone under the skin, tablets inside plastic bottles, explosives within containers, and counterfeit tablets in blister packs. Additionally, there have been advancements in developing deep non-invasive medical diagnostic techniques using this method.

About SORS

Figure 1 shows different kinds of SORS probes as a) conventional Raman spectroscopy, the Raman signal is collected from the laser illumination zone. b) Point-like spatially offset Raman spectroscopy (SORS) uses near-point illumination and collection areas that are mutually displaced by a spatial offset (Δs). c) Ring-collection SORS uses a point-like illumination geometry with the Raman signal collected through a ring. d) Ring-illumination SORS, or inverse SORS, uses a ringed illumination zone, with the Raman signal collected through a point-like zone at the center of the ring. e) In defocusing SORS, illumination and collection areas remain largely overlapped and their size is controlled by moving the sample relative to the collection optics. f) In transmission Raman spectroscopy (TRS), the illumination and collection zones are on the opposite sides of the sample. In all configurations, illumination and collection beams are labelled L (laser) and R (Raman), respectively.

Figure 1. Variants of SORS.

This affects the choice of laser wavelength when deciding what spectrometer to use. Using a short wavelength can produce a stronger absolute signal, but it's important to balance this with other factors. Photons in the visible and IR ranges are safe, but those with higher energy, like x-rays, can become ionizing radiation and potentially cause cell damage even at low power, particularly in biological materials. Therefore, selecting the highest energy photons isn't always feasible.

Longer wavelengths can improve the signal-to-fluorescence ratio in many biological samples, as many such samples contain compounds that fluoresce under green light. Consequently, wavelengths between 785 – 1024 nm are often used for biological samples. Green lasers at 532 nm are commonly used where fluorescence is less of an issue because they enhance the Raman signal. The 1064 nm excitation wavelength is demonstrated to detect chemicals through barriers like containers, effectively mitigating fluorescence issues from the target chemical. Typically, a Ti:Sapphire laser pumped by 532 nm light from a frequency-doubled Nd:YAG laser provides a tunable laser source in the 690-860 nm range.

Molecular identification can be performed by comparing the measured Raman spectra with those from standard samples stored in a library database.

All depth parameters show an increase with increasing spatial offset, as expected, reflecting the fundamental property of SORS that the increase in spatial offset leads to probing deeper inside the matrix. By comparing the plots for different transport lengths (see Figure 2).

Figure 2. Plots of 10%, 50% (median), and 90% quantile depths versus spatial offset derived from the Monte Carlo simulations for three representative transport lengths.

Specific applications

The research about an approach for subsurface through-skin analysis of salmon using SORS with an excitation wavelength at 830 nm.

In Figure 3, the results of a measurement series through the dark part of the skin is provided. At zero spatial offset, the dominating features of the spectrum are background fluorescence and broad spectral features most likely related to the proteins and pigments of the salmon skin. At 3 and 4 mm spatial offsets, Raman bands related to the fish flesh start to appear in the spectra. The four most prominent peaks (i.e., around 1660 cm^‑1 (cis C=C stretch), 1440 cm^-1 (CH₂ scissoring), 1300 cm^-1 (CH₂ twist), and 1266 cm^‑1 (symmetric = C–H rocking)) are all mainly related to the lipid components of the salmon flesh. At 5 and 6 mm spatial offsets mark optimal choices for Raman measurements through dark salmon skin. Using offsets higher than 6 mm could reduce signal-to-noise.

Figure 3. Conventional Raman and SORS spectra were obtained from measurements performed on intact salmon samples through the dark part of the salmon skin. Spectra are normalized and separated along the intensity axis for clarity.

Raman products

Intins is glad to introduce to customers a comprehensive product set of Raman from Ocean Optic. The product set includes excitation lasers, spectrometers, Raman probes, sample holders for cuvettes, SERS substrates, and safety glasses. Moreover, Intins also provides software for calculating Raman signals and even custom software following customers’ desires. The customers can buy whole the system of Raman as Intins recommended, or chose any product that suit for their setups.

Spectrometer

Currently, there are two spectrometer lines are used for Raman applications such as QE Pro-Raman+ and NIRQuest+1064 Raman. The QE Pro-Raman+  has the Raman shift at 4429 cm¯¹ (532 nm), 2820 cm¯¹ (638 nm), 3002 cm¯¹ (785 nm), the integration time is 8 ms – 3600 s, while the NIRQuest+1064 Raman are used for 2400 cm¯¹ (1064 nm).

Raman Laser

We offer high-power lasers at 532 nm, 638 nm, 785 nm, and 1064 nm Raman excitation wavelengths. These multimode diode lasers produce narrow spectral lines, have an integrated laser driver, and employ thermoelectric cooling for optimal performance.

Raman Probe

Probes for 532 nm, 785 nm, and other wavelengths, are suitable for both lab and industry.

Raman Holder

Ocean Optics offers sample holders for Raman analysis of liquids and solids. The RM-SERS-SHS holder accommodates a standard glass SERS substrate and attaches to a Raman probe. The RM-LQ-SHS holder holds vials and cuvettes. It has a magnetic lid that facilitates sample placement, blocks ambient light, and improves the accuracy of Raman measurements.

SERS substrates

Surface-enhanced Raman spectroscopy (SERS) substrates allow you to make fast, repeatable measurements to identify and quantify SERS active analytes. SERS active chemistry is gold (Au) and silver (Ag) nanoparticles. Its storage lifetime is 1 month or 1.5 months.