Comparison of methods for reducing the effects of scattering in spectrophotometry

Twists through turbidity: propagation of light carrying orbital angular momentum through a complex scattering medium

We explore the propagation of structured vortex laser beams-shaped light carrying orbital angular momentum (OAM)-through complex multiple scattering medium. These structured vortex beams consist of a spin component, determined by the polarization of electromagnetic fields, and an orbital component, arising from their spatial structure. Although both spin and orbital angular momenta are conserved when shaped light propagates through a homogeneous, low-scattering medium, we investigate the conservation of these angular momenta during the propagation of Laguerre-Gaussian (LG) beams with varying topological charges through a turbid multiple scattering environment. Our findings demonstrate that the OAM of the LG beam is preserved, exhibiting a distinct phase shift indicative of the 'twist of light' through the turbid medium. This preservation of OAM within such environments is confirmed by in-house developed Monte Carlo simulations, showing strong agreement with experimental studies. Our results suggest exciting prospects for leveraging OAM in sensing applications, opening avenues for groundbreaking fundamental research and practical applications in optical communications and remote sensing.

Surfactant-based supramolecular dye assembly: A highly selective and economically viable platform for quantification of heparin antidote

Herein, we have employed a supramolecular assembly of a cationic dye, LDS-698 and a common surfactant sodium dodecyl sulfate (SDS) as a turn-on fluorescent sensor for protamine (Pr) detection. Addition of cationic Pr to the solution of dye-surfactant complex brings negatively charged SDS molecules together through strong electrostatic interaction, assisting aggregation of SDS way before its critical micellar concentration (CMC). These aggregates encapsulate the dye molecules within their hydrophobic region, arresting non-radiative decay channels of the excited dye. Thus, the LDS-698•SDS assembly displays substantial enhancement in fluorescence intensity that follows a nice linear trend with Pr concentration, providing limit of detection (LOD) for Pr as low as 3.84(±0.11) nM in buffer, 124.4(±6.7) nM in 1% human serum and 28.3(±0.5) nM in 100% human urine. Furthermore, high selectivity, low background signal, large stokes shift, and emission in the biologically favorable deep-red region make the studied assembly a promising platform for Pr sensing. As of our knowledge it is the first ever Pr sensory platform, using a very common surfactant (SDS), which is economically affordable and very easily available in the market. This innovative approach can replace the expensive, exotic and specialized chemicals considered for the purpose and thus showcase its potential in practical applications.

Molecular fingerprinting of nanoparticles in complex media with non-contact photoacoustics: beyond the light scattering limit

Optical instruments can probe physical systems even to the level of individual molecules. In particular, every molecule, solution, and structure such as a living cell has a unique absorption spectrum representing a molecular fingerprint. This spectrum can help identify a particular molecule from others or quantify its concentration; however, scattering limits molecular fingerprinting within a complex compound and must be overcome. Here, we present a new, non-contact photoacoustic (PA)-based method that can almost completely remove the influence of background light scattering on absorption measurements in heterogeneous highly scattering solutions and, furthermore, separate the intrinsic absorption of nanoscale objects from their scattering. In particular, we measure pure absorption spectra for solutions of gold nanorods (GNRs) as an example of a plasmonic agent and show that these spectra differ from the extinction measured with conventional UV-VIS spectrophotometry. Finally, we show how the original GNR absorption changes when nanoparticles are internalized by cells.

Simulation Method Linking Dense Microalgal Culture Spectral Properties in the 400-750 nm Range to the Physiology of the Cells

This work describes a method to model the optical properties over the (400-750 nm) spectral range of a dense microalgal culture using the chemical and physical properties of the algal cells. The method was based on a specific program called AlgaSim coupled with the adding-doubling method: at the individual cell scale, AlgaSim simulates the spectral properties of one model, three-layer spherical algal cell from its size and chemical composition. As a second step, the adding-doubling method makes it possible to retrieve the total transmittance of the algal medium from the optical properties of the individual algal cells. The method was tested by comparing the simulated total transmittance spectra for dense marine microalgal cultures of Isochrysis galbana (small flagellates) and Phaeodactylum tricornutum (diatoms) to spectra measured using an experimental spectrophotometric setup. Our study revealed that the total transmittance spectra simulated for the quasi-spherical cells of Isochrysis galbana were in good agreement with the measured spectra over the whole spectral range. For Phaeodactylum tricornutum, large differences between simulated and measured spectra were observed over the blue part of the transmittance spectra, probably due to non-spherical shape of the algal cells. Prediction of the algal cell density, mean size and pigment composition from the total transmittance spectra measured on algal samples was also investigated using the reversal of the method. Mean cell size was successfully predicted for both species. The cell density was also successfully predicted for spherical Isochrysis galbana, with a relative error below 7%, but not for elongated Phaeodactylum tricornutum with a relative error up to 26%. The pigments total quantity and composition, the carotenoids:chlorophyll ratio in particular, were also successfully predicted for Isochrysis galbana with a relative error below 8%. However, the pigment predictions and measurements for Phaeodactylum tricornutum showed large discrepancies, with a relative error up to 88%. These results give strong support for the development of a promising tool providing rapid and accurate estimations of biomass and physiological status of a dense microalgal culture based on only light transmittance properties.

Use of HSI to measure oxygen saturation in the lower limb and its correlation with healing of foot ulcers in diabetes

AIM

To explore the use of hyperspectral imaging (HSI) to predict healing of diabetic foot ulcers in patients with diabetes.

METHODS

We used a HSI technique that incorporated novel software to account for tissue scattering of light, and was validated using blood samples of varying oxygen saturation assessed by blood gas analysis. HSI was then performed on a population newly presenting with diabetic foot ulcers to a specialist clinic, and associations were sought with healing at 12 and 24 weeks.

RESULTS

The correlation between the results of HSI and blood gas analysis was strong (r = 0.994). A total of 43 patients (mean ± sd age 62.7 ± 12.2 years; 31 men, 12 women; 37 with Type 2 diabetes, six with Type 1 diabetes) with foot ulcers were included in the prospective clinical study and underwent HSI within 16 days of presentation. In all, 26 ulcers healed within 12 weeks and 28 within 24 weeks. There was a negative association between tissue oxygenation assessed by HSI at baseline and healing by 12 weeks (P = 0.009), and this was observed in both infected and non-infected ulcers. There was a significant positive correlation between oxygenation assessed by HSI and time to healing (P = 0.03). No correlations were observed at 24 weeks.

CONCLUSIONS

These findings suggest that HSI may predict healing in routine practice. The fact that the correlation between HSI and healing was negative could be explained by HSI being a measure of oxygenation of haemoglobin and there may be an inverse relationship between this and the oxygenation of extravascular tissue in people with neuropathy and/or microvascular disease.

Improvement of the chemical content prediction of a model powder system by reducing multiple scattering using polarized light spectroscopy

Near-infrared spectroscopy (NIRS) is a powerful non-destructive analytical method used to analyze major compounds in bulk materials and products and requiring no sample preparation. It is widely used in routine analysis and also in line in industries, in vivo with biomedical applications, or in field for agricultural and environmental applications. However, highly scattering samples subvert Beer-Lambert law's linear relationship between spectral absorbance and the concentration. Instead of spectral pre-processing, which is commonly used by NIR spectroscopists to mitigate the scattering effect, we put forward an optical method, i.e., coupling polarized light with NIR spectrometry, to free spectra from scattering effect. This should allow us to retrieve linear and steady conditions for spectral analysis. When tested in visible-NIR (Vis-NIR) range (400-800 nm) on model media, mixtures of scattering and absorbing particles, the setup provided significant improvements in absorber concentration estimation precision as well as in the quality and robustness of the calibration model.

Potential of a spectroscopic measurement method using adding-doubling to retrieve the bulk optical properties of dense microalgal media

In the context of algal mass cultivation, current techniques used for the characterization of algal cells require time-consuming sample preparation and a large amount of costly, standard instrumentation. As the physical and chemical properties of the algal cells strongly affect their optical properties, the optical characterization is seen as a promising method to provide an early diagnosis in the context of mass cultivation monitoring. This article explores the potential of a spectroscopic measurement method coupled with the inversion of the radiative transfer theory for the retrieval of the bulk optical properties of dense algal samples. Total transmittance and total reflectance measurements were performed over the 380-1020 nm range on dense algal samples with a double integrating sphere setup. The bulk absorption and scattering coefficients were thus extracted over the 380-1020 nm range by inverting the radiative transfer theory using inverse-adding-doubling computations. The experimental results are presented and discussed; the configuration of the optical setup remains a critical point. The absorption coefficients obtained for the four samples of this study appear not to be more informative about pigment composition than would be classical methods in analytical spectroscopy; however, there is a real added value in measuring the reduced scattering coefficient, as it appears to be strongly correlated to the size distribution of the algal cells.

Reduction of error in spectrophotometry of scattering media using polarization techniques

Scattering can result in erroneous determination of the concentrations of constituent absorbers in spectrophotometry. This is due to the relationship between attenuation and absorption coefficient becoming nonlinear; hence, the use of the Lambert-Beer law becomes invalid. It has previously been shown that application of polarization techniques can reduce these effects, resulting in a more linear relationship. Here we quantify the impact of this improvement on measurement of the ratio of concentrations for two general absorbing species and show that measurement using polarization-maintaining light is more accurate. This is performed using a generalized version of theory previously dependent on selection of isosbestic wavelengths. For the absorbing species and geometries considered here, the mean error on the estimation of absorber concentration ratio is 18.2% for the case of detection without polarization discrimination. When polarization-maintaining light is extracted, mean errors of 1.2% and 5.1% are achieved for linear and circular polarizations, respectively. The improvement provided by the polarization techniques is observed regardless of the illuminating wavelengths but is achieved at the expense of a reduced signal-to-noise ratio. Taking this into account, for the detection scheme considered with a detector well capacity of 4 x 10(5) electrons the improvement provided by linear polarization-maintaining light is reduced to a factor of 3.6 and for circular polarizations a factor of 2.2.