SORS Performance of Sublayer Materials with Different Optical Properties under Diffuse Scattering Media

Abstract

Over the past few years, the utilization of spatially offset Raman spectroscopy (SORS) has significantly evolved in its ability to analyze layered turbid materials non-invasively. It is well known that SORS can effectively detect the deeper layer in a high scattering media, and the performance characteristics of SORS have been extensively studied. However, to date, there is a lack of detailed studies of SORS to detect materials with different optical properties. This study aims to fill this gap by constructing a simple bilayer model, in which a target material with different optical properties was covered with a diffuse scattering barrier. By analyzing the Raman intensity from both superficial barriers and underlying target materials, we investigated the SORS performance to probe three typical optical materials with distinct optical properties: strong absorption, high transparency, and strong scattering. It was found that SORS technology can readily detect the samples of different properties under turbid surface coverings, and the typical optical property of the sublayer materials provided a specific SORS feature. Our study demonstrates the great potential of SORS technology for the non-invasive detection of subcutaneous component applications and provides a comprehensive understanding of the SORS characteristic of various materials.

1. Introduction

To achieve non-invasive chemical composition characterization of layered materials, an analytical method with high specificity and the ability to provide chemical analysis at depth is required. Raman spectroscopy is an invaluable technique for chemical analysis owing to its high chemical specificity and sensitivity. However, the conventional Raman spectroscopy method is constrained by a limited detection depth of approximately 100 μm [1,2,3], which is due to the fact that scattered photons from the deeper layer take a longer time to reach the collection point and are prone to diffuse in all directions. As a result, the signal intensity from the sublayer is much weaker than the superficial layer, which leads to a predominance of the collected spectra from the surface. The conventional Raman method is thus not suitable for the detection of layered opaque samples. A technology that can suppress the interference of the superficial material and detect the signal of the sublayer is therefore crucially required.

Spatially offset Raman spectroscopy (SORS) has been proven to be an effective means of sublayer chemical analysis through the opaque surface [4,5]. SORS technology enables deeper detection by varying the distance between the excitation point and the collection point. According to Raman scattering photon migration theory [6], the longer the photons travel through the sample, the more likely they are to propagate laterally. This implies that a certain offset between the excitation point and the collection point brings an overall attenuation of the signal, but the signal from the deeper layer increases significantly. In addition to the inherent advantages of Raman spectroscopy technology, SORS offers two distinct advantages: (1) it can effectively suppress surface fluorescence interference and improve the detection sensitivity of the sublayer [7,8,9], and (2) within a certain range, the Raman signal contribution from the deeper layer can be improved by increasing offset.

The ability of the SORS technique to directly detect deeper substances through surficial barriers has enabled it to be widely used in many fields, such as public health [10], food science [11,12,13,14,15], artwork analysis [16,17], and medical applications [18] in recent years. Nirzari Gupta et al. demonstrated that the SORS method was able to identify and quantify the active ingredients in hand sanitizers through the containers without any prior knowledge [10]. Morey et al. used a handheld spectrometer to distinguish nine different potato varieties according to the content levels of protein and carotenoid [15]. Marco et al. proposed a portable full micro-SORS prototype for in situ analysis of thin, highly turbid layers in paintings and decorative objects [17]. SORS technology also has broad applications in biomedical fields, such as blood quality assessment [19,20,21,22], bone disease diagnosis [23,24,25], cancer diagnosis, etc.

Recently, a study using SORS to detect bones with different mineralization levels has been conducted by Sowoidnich et al. [26]. Polytetrafluoroethylene (PTFE) slice was placed inside the bones with three mineralization levels and at different depths. Their study showed that the SORS signals were quite different under different situations. It brought up a question about how the spectroscopic features of SORS change if the underlying material has a very different optical property, and to what extent SORS can detect underlying materials that are obscured by barriers of varying thickness.

To answer the question above, we investigated the SORS signal’s characteristic of several bilayer models, in which a diffuse tape with varying thicknesses was stuck onto the surfaces of three materials with three distinct optical properties––silicon (Si) with strong absorption, polymethyl methacrylate (PMMA) with high transparency, and PTFE with strong scattering, respectively. The results indicate that the SORS signal features of Si differ greatly from those of PMMA and PTFE due to the former’s strong absorption. Meanwhile, PMMA’s transparency and PTFE’s strong scattering result in their unique SORS signal features. We concluded that the greater the scattering coefficient, the stronger the ability of SORS technology to detect the substances under the thicker obstacle in general. It is well known that human tissue is also a substance with high scattering properties. Hence, this study showcases the potential of the SORS technique for the non-invasive transcutaneous detection of substances of interest.

2. Materials and Methods

2.1. Bilayer Sample Preparation

In order to investigate the performance of SORS technology on materials with different optical properties, three bilayer models were constructed (Figure 1a). In the bilayer models, we choose sublayer materials as Si with high absorption property, PMMA with high transparency property, and PTFE with strong scattering property, respectively. The turbid scattering 3M tape (Scotch Magic Tape, 3M China Limited, Shanghai, China), which has the same scattering coefficient as human skin [27], was selected as a surface barrier to interfere with the detection of sublayer by SORS. The thickness of the surficial tape was established within a range of 50 to 1250 μm in increments of 50 μm by meticulously applying it onto the sublayer materials, ensuring the absence of any air bubbles between each layer. That is to say, there are a total of 25 samples for each sublayer material, totaling 75 samples. The thickness of the sublayer materials of Silicon, PMMA, and PTFE is 750 μm, 4.4 mm, and 1.8 cm, respectively.

2.2. SORS Instrumentation

Custom-built equipment based on the SORS technique was constructed (Figure 1b) to achieve the Raman measurements, which consists of a laser with a 785 nm excitation wavelength (0785-08-11-M-0500-200, Cobolt AB, Solna, Sweden), an optical system to realize the excitation and collection of Raman photons, a spectrometer (LS 785, Princeton Instruments, NJ, USA), and a charge-coupled device (CCD, 400BRX, Princeton Instruments, NJ, USA) to detect Raman spectra. The laser beam was focused on the sample surface and scattered photons were collected by a fiber bundle. To enable tuning of the spatial offset, a unique round-to-linear fiber bundle was employed, as reported in [28]. This bundle comprises several optical fibers arranged in a circular pattern at the collection end, which are subsequently reconfigured into a linear pattern to align with the entrance slit of the spectrograph. In this way, the signals collected from the fibers during different cycles correspond to different offsets. Through the arrangement of 5 cycles of fibers, the spatial offsets of 0 to 440 μm in increments of 110 μm were attained. This increment was determined by the distance between neighboring fibers. Eventually, the Raman scattering photons are dispersed spectrally in a spectrometer and imaged onto a CCD. All measurements were carried out using the SORS equipment. For each sample, three randomly selected points on the sample surface were probed, and at each point, 9 s spectra (3 s×3 accumulations) were collected, and the average laser power used for the measurements was set at 70 mW.

2.3. Data Analysis

To evaluate the performance of the SORS technique, we calculated the Raman intensity of both outermost and sublayer materials, by integrating the peak area of characteristic peaks of each material. The Raman signal were collected at 521 cm⁻¹ for Si, 811 cm⁻¹ for PMMA (C-O-C stretching), and 734 cm⁻¹ for PTFE (C-F and C-C symmetric stretching) [29,30,31]. A prominent peak at 649 cm⁻¹ of tape was selected as it shows no overlap with the sublayer materials. To facilitate the observation, the exponential fit was performed for the thickness-dependent intensity of underlying PMMA and PTFE. The Lorentzian fit was performed for the attenuation curve of the thickness-dependent intensity of underlying Si. The ratio of signal intensity from sublayer materials to that of surficial tape was calculated to demonstrate the deeper layer detection ability of SORS.

3. Results and Discussion

3.1. SORS Spectra of Different Materials Covered with Diffuse Tape

Figure 2a–c shows the SORS spectra of Si, PMMA, and PTFE covered with diffuse tape at different offsets. The tapes were chosen at a thickness of 600 μm, acting as a turbid barrier to disrupt the detection of target sublayer materials. By comparing the spectra of the pure target materials to those that were covered with tape, our findings demonstrate that SORS can effectively bypass the surficial barrier to detect covered targets. Moreover, high offset yields a stronger signal from sublayer materials, especially for high scattering PTFE (Figure 2d). Upon a thorough examination of the typical Raman band of the tape at 649 cm⁻¹, it is evident that a significant decrease in the tape’s signal at high offsets (Figure 2e). These results demonstrated that SORS have a robust ability to probe hidden or covered materials under diffuse turbid barriers.

Furthermore, the signal to noise (S/N) ratio were calculated to decipher the deep layer probe ability of SORS (Figure 2f). Our results show that PMMA presented a significantly increasing of S/N ratio at higher offset, while PTFE and Si showed a relatively weaker improvement. Those results might be explained as follows: the high transparent PMMA generates much less backscattering laser photons inside tape, thereby leading to a very small noise for SORS detection. Conversely, the high scattering property of PTFE engenders plenty of backscattered laser photons, eventually resulting in a relative low S/N ratio, and the high absorption characteristics of Si result in fewer Raman photons being excited, especially at higher offset.

3.2. SORS Features Affected by the Thickness of Surficial Barriers

In the above section, our results demonstrated that SORS is a potent technique to probe hidden or covered materials under diffuse barriers, as well as the different optical properties of sublayer materials resulting in varying SORS behavior. To further evaluate the capability of SORS to detect deeply hidden materials, we investigated the SORS performance in the presence of diffuse tapes with thicknesses that gradually increased. As the thickness of each layer tape is around 50 μm, the bilayer samples with maximum thickness of 1250 μm tapes were prepared by sticking them up to 25 times. The Raman intensity of the sublayer materials was then calculated and plotted versus the tape’s thickness, which are illustrated in Figure 3a–c.

The offset at 0 μm is the paradigm of confocal Raman, in which the excitation site is the same as the collection site. As the tape’s thickness increases, the Raman signal from sublayer materials at 0 μm offset decreases rapidly, indicating that the conventional Raman spectroscopy technique has a limited ability to probe the sublayer targets due to the inference of diffuse barriers. When the detection spatial offset increases, SORS has shown a remarkable ability to probe sublayer materials. Figure 3a–c shows the relation between the sublayer materials’ Raman signal and the tape’s thickness under different spatial offsets. It is interesting to find that high absorption Si of different thicknesses shows an extraordinary result. When the offsets were chosen at 110, 220, 330, and 440 μm, the SORS results showed the optimal detection depth for Si were at 100, 250, 400, and 450 μm, respectively. This extraordinary phenomenon might be attributed to silicon’s high absorption property, in which a specific offset always corresponds to an optimal depth. If the tape’s thickness is smaller than the optimal depth, laser photons will travel a relatively shorter path, and so do the backscattered Raman photons, leading to a relatively weak signal being detected at the specific offset. For thicker tapes, both the excitation and emission photons are more likely to travel laterally, also leading to a weak signal being detected at the given offset.

PMMA and PTFE present very different SORS features from Si, and the signal from these two sublayer materials is always decreased if the tape’s thickness increases (Figure 3b,c). This is due to the thicker diffuse barriers that have impeded the photons’ propagation during both the excitation and emission processes. The high offsets show a relatively slower signal decay, or at a specific thickness, the high offset can probe deeper under the inferencing layers. A detailed comparison between the three materials can be seen in Figure 3d,e. It shows that when the tape thickness was large enough (at 1200 μm), the underlying Si and PMMA were barely sensed, while PTFE still showed a relatively high intensity under 1200 μm thickness tape. This finding indicates that materials with high scattering properties can be probed more easily by the SORS technique than others.

3.3. Ability to Suppress the Interference from Top-Layer

As well as the ability to probe underlying materials, SORS technique have also been proven to suppress the signal interference from the outermost layers. In this section, we compared the Raman intensity ratio between the underlying target and surface materials at different offsets with various tape thicknesses. As shown in Figure 4a–c, it is obvious that under higher offsets, sublayer materials always contribute more signal than surface materials. An unusual result at 330 and 440 μm offset between 50–400 μm thickness for Si from Figure 4a can been noticed. This is the same as the extraordinary phenomenon in Figure 3a, owing to the high absorption property of Si.

Another prominent feature can be observed from the comparison between the intensity ratio of PMMA and PTFE to tape. It shows that the intensity ratio of PMMA to tape is much higher than PTFE to tape when the tape thickness is less than 600 μm. This can be attributed to the difference in the probability of multiple scattering of laser photons inside tapes between two samples. After traversing the tape layer and incidence into the sublayer, much more laser photons experience multiple scattering inside PTFE as its high scattering property. Many laser photons are then backscattered into the tape again, resulting in a relatively higher tape intensity, and a low-intensity ratio of PTFE to tape (Figure 4d,e). On the other hand, for PMMA, there are much fewer backscattered laser photons due to its high transparency, which results in less tape intensity and a high-intensity ratio between PMMA to tape (Figure 4d,e). When the tape’s thickness is larger than 600 μm, the SORS signal is dominated by tape, and the contribution of the backscattered laser also decreases by attenuation of a thicker tape. More sublayer Raman photons can then be backscattered by PTFE, resulting in a higher intensity ratio of PTFE to tape than PMMA (Figure 4f).

4. Conclusions

In summary, we have demonstrated a comprehensive study of SORS technique. By tuning the thickness of surficial covering tape, we investigated the SORS signal features of sublayer materials with distinct optical properties. In general, SORS technique shows its potent ability to probe covered materials, and suppress the interference of surface layer. As for the high absorption property of Si, a specific offset exists that corresponds to an optimal thickness of the tape. The high scattering property of PTFE, when used as a cover, consistently produces stronger signals below tapes under larger offsets. In addition, a considerable number of photons were backscattered into tape by the PTFE under the thinner tapes, leading to a poor intensity ratio to tape than PMMA. This is due to the high transparency property that provides much less backscattered laser photons. Our result provides valuable insights for comprehensively interpreting the SORS characteristics of different optical property materials below turbid barriers with various thicknesses.