Optical remote sensing of water temperature using Raman spectroscopy

Depth-resolved water temperature measurements using Raman LiDAR

We present the retrieval of depth-resolved temperature measurements in water using Raman LiDAR. Using a 5 m pipe of laboratory water, we recover non-homogeneous temperature profiles with a temperature accuracy ranging between 0.35°C and 0.85°C, and a position resolution of 28 cm.

Monte Carlo modelling for elastic and Raman signals in oceanic LiDAR

We investigate Raman returns in oceanic light detection and ranging (LiDAR), and explore the similarities and differences to conventional elastic returns. We show that Raman returns have much more complex behavior than elastic returns, which likely cannot be well matched by simple models, making Monte Carlo simulations essential. We investigate the correlation between the time of signal arrival and the depth of the Raman event, and find that a linear correlation only exists for judicious choice of system parameters.

Miniaturized 7-in-1 fiber-optic Raman probe

This Letter reports a miniature 7-in-1 fiber-optic Raman probe that eliminates the inelastic background Raman signal from a long-fused silica fiber. Its foremost purpose is to enhance a method for investigating extraordinarily tiny substances and effectively capturing Raman inelastic backscattered signals using optical fibers. We successfully used our home-built fiber taper device to combine seven multimode fibers into a single fiber taper with a probe diameter of approximately 35 µm. By experimentally comparing the traditional bare fiber-based Raman spectroscopy system with the miniaturized tapered fiber-optic Raman sensor using liquid solutions, the novel probe's capability is demonstrated. We observed that the miniaturized probe effectively removed the Raman background signal originating from the optical fiber and confirmed expected outcomes for a series of common Raman spectra.

Temperature-Induced Change of Water Structure in Aqueous Solutions of Some Kosmotropic and Chaotropic Salts

The hydrogen bond structure of water was examined by comparing the temperature dependent OH-stretching bands of water and aqueous NaClO₄, KClO₄, Na₂SO₄, and K₂SO₄ solutions. Results called attention to the role of cations on top of the importance of anions determining the emerging structure of a multi-layered system consisting single water rings or multi-ring water-clusters.

Up-Converting Luminescence and Temperature Sensing of Er3+/Tm3+/Yb3+ Co-Doped NaYF4 Phosphors Operating in Visible and the First Biological Window Range

Accurate and reliable non-contact temperature sensors are imperative for industrial production and scientific research. Here, Er3+/Tm3+/Yb3+ co-doped NaYF4 phosphors were studied as an optical thermometry material. The typical hydrothermal method was used to synthesize hexagonal Er3+/Tm3+/Yb3+ co-doped NaYF4 phosphors and the morphology was approximately rod-like. The up-conversion emissions of the samples were located at 475, 520, 550, 650, 692 and 800 nm. Thermo-responsive emissions from the samples were monitored to evaluate the relative sensing sensitivity. The thermal coupled energy level- and non-thermal coupled energy level-based luminescence intensity ratio thermometry of the sample demonstrated that these two methods can be used to test temperature. Two green emissions (520 and 550 nm), radiated from 2H11/2/4S3/2 levels, were monitored, and the maximum relative sensing sensitivities reached to 0.013 K-1 at 297 K. The emissions located in the first biological window (650, 692 and 800 nm) were monitored and the maximum relative sensing sensitivities reached to 0.027 (R692/650) and 0.028 K-1 (R692/800) at 297 K, respectively. These results indicate that Er3+/Tm3+/Yb3+ co-doped NaYF4 phosphors have potential applications for temperature determination in the visible and the first biological window ranges.

Depth Profiling Investigation of Seawater Using Combined Multi-Optical Spectrometry

Depth profiling investigation plays an important role in studying the dynamic processes of the ocean. In this paper, a newly developed hyphenated underwater system based on multi-optical spectrometry is introduced and used to measure seawater spectra at different depths with the aid of a remotely operated vehicle (ROV). The hyphenated system consists of two independent compact deep-sea spectral instruments, a deep ocean compact autonomous Raman spectrometer and a compact underwater laser-induced breakdown spectroscopy system for sea applications (LIBSea). The former was used to take both Raman scattering and fluorescence of seawater, and the LIBS signal could be recorded with the LIBSea. The first sea trial of the developed system was taken place in the Bismarck Sea, Papua New Guinea, in June 2015. Over 4000 multi-optical spectra had been captured up to the diving depth about 1800 m at maximum. The depth profiles of some ocean parameters were extracted from the captured joint Raman-fluorescence and LIBS spectra with a depth resolution of 1 m. The concentrations of SO42- and the water temperatures were measured using Raman spectra. The fluorescence intensities from both colored dissolved organic matter (CDOM) and chlorophyll were found to be varied in the euphotic zone. With LIBS spectra, the depth profiles of metallic elements were also obtained. The normalized intensity of atomic line Ca(I) extracted from LIBS spectra raised around the depth of 1600 m, similar to the depth profile of CDOM. This phenomenon might be caused by the nonbuoyant hydrothermal plumes. It is worth mentioning that this is the first time Raman and LIBS spectroscopy have been applied simultaneously to the deep-sea in situ investigations.

Review of on-line and near real-time spectroscopic monitoring of processes relevant to nuclear material management

Spectroscopic chemometric based on-line monitoring of used nuclear fuel (UNF) reprocessing solutions and characterization of legacy nuclear waste (LNW) stored at Hanford is discussed in this manuscript. Utilizing on-line and near real-time monitoring, as opposed to traditional off-line monitoring, can significantly reduce the cost, risk and improve the efficiency of characterizing UNF and LNW processing streams. Specifically, this manuscript will highlight the benefits of spectroscopy-based monitoring approaches, which generally include the ability to collect data non-destructively. Furthermore, significant literature precedence supports the use of various real-time analysis methods, including chemometric analysis, that enable near-instantaneous conversion of spectroscopic data into information useable by process operators. This approach can accurately quantify and qualify nuclear material in near-real time enabling immediate condition characterization and potential diversion detection within UNF reprocessing streams and LNW. The ability to be applied in a real reprocessing plant and in an actual Hanford waste tank/transfer pipe has been demonstrated by applying this technique to accurately quantify analytes in real UNF streams and LNW samples. The future development of spectroscopy-based on-line monitoring is also discussed in this manuscript.

Thermal monitoring during photothermia: hybrid probes for simultaneous plasmonic heating and near-infrared optical nanothermometry

The control of temperature during photothermal therapy is key to preventing unwanted damage in surrounding tissue or post-treatment inflammatory responses. Lack of accurate thermal control is indeed one of the main limitations that hyperthermia techniques present to allow their translation into therapeutic applications. We developed a nanoprobe that allows controlled local heating, combined with in situ nanothermometry. The design of the probe follows a practical rationale that aims at simplifying experimental requirements and exploits exclusively optical wavelengths matching the first and second biological windows in the near-infrared. Methods: Hybrid nanostructures were chemically synthesized, and combine gold nanostars (photothermal agents) with CaF₂:Nd³⁺,Y³⁺ nanoparticles (luminescent nanothermometers). Both components were simultaneously excited in the near-infrared range, at 808 nm. Following the goal of simplifying the thermal monitoring technique, the luminescent signal was recorded with a portable near-infrared detector. The performance of the probes was tested in 3D tumor spheroids from a human glioblastoma (U87MG) cell line. The location of the beads within the spheroids was determined measuring Nd³⁺ emission in a commercial Lightsheet microscope, modified in-house to be able to select the required near-infrared wavelengths. The temperature achieved inside the tumor spheroids was deduced from the luminescence of Nd³⁺, following a protocol that we developed to provide reliable thermal readings. Results: The choice of materials was shown to work as an optically excited hybrid probe. Depending on the illumination parameters, temperature can be controlled in a range between 37 ºC and 100 ºC. The near-infrared emission of nanothermometers also allows microscopic tracking of the hybrid nanostructures, confirming that the probes can penetrate deeper into the spheroid mass. We observed that, application of optical thermometry in biological environments requires often neglected considerations, since the optical signal changes along the optical path. Accordingly, we developed data analysis protocols that guarantee reliable thermal readings. Conclusions: The prepared hybrid probes are internalized in 3D tumor spheroids and can be used to induce cell death through photothermal effects, while simultaneously measuring the local temperature in situ. We show that luminescent thermometry in biomedical applications requires the development of protocols that guarantee accurate readings. Regarding photothermal treatments, we observe a sharp thermal threshold at around 55 ºC (for 10 min treatments) that separates high survival ratio from complete cell death.

Impact of fluorescence on Raman remote sensing of temperature in natural water samples

A comprehensive investigation into the impact of spectral baseline on temperature prediction in natural marine water samples by Raman spectroscopy is presented. The origin of baseline signals is investigated using principal component analysis and phytoplankton cultures in laboratory experiments. Results indicate that fluorescence from photosynthetic pigments and dissolved organic matter may overlap with the Raman peak for 532 nm excitation and compromise the accuracy of temperature predictions. Two methods of spectral baseline correction in natural waters are evaluated: a traditional tilted baseline correction and a new correction by temperature marker values, with accuracies as high as ± 0.2°C being achieved in both cases.

A LIDAR-Compatible, Multichannel Raman Spectrometer for Remote Sensing of Water Temperature

The design and operation of a custom-built LIDAR-compatible, four-channel Raman spectrometer integrated to a 532 nm pulsed laser is presented. The multichannel design allowed for simultaneous collection of Raman photons at two spectral regions identified as highly sensitive to changes in water temperature. For each of these spectral bands, the signals having polarization parallel to (∥) and perpendicular to (⟂), the excitation polarization were collected. Four independent temperature markers were calculated from the Raman signals: two-colour(∥), two-colour(⟂), depolarization(A) and depolarization(B). A total of sixteen datasets were analysed for one ultrapure (Milli-Q) and three samples of natural water. Temperature accuracies of ±0.4 °C-±0.8 °C were achieved using the two-colour(∥) marker. When multiple linear regression models were constructed (linear combination) utilizing all simultaneously acquired temperature markers, improved accuracies of ±0.3 °C-±0.7 °C were achieved.

A New Approach to Measuring the Temperature of Fluids Reaching 300 ℃ and 2 mol/kg NaCl Based on the Raman Shift of Water

The OH stretching band of water is very sensitive to temperature and salinity for the existence of hydrogen bonds between H₂O molecules. In this study, the OH stretching band was deconvoluted into two Gaussian peaks, with peak 1 at approximately 3450 cm^-1 and peak 2 at approximately 3200 cm^-1. The positions of peaks 1 and 2 both shifted to higher wavenumbers with increasing temperature from 50 ℃ to 300 ℃. The effects of salinity in the range of 0-2 mol/kg NaCl on the OH stretching band were also studied. Linearity for the relationship between Raman shift of peak 1 and temperature increased as the salt concentration increased from 0 to 2 mol/kg, while peak 2 displayed an opposing trend. Two temperature calibration models were developed based on the temperature-dependent changes in the Raman frequency shifts of peaks 1 and 2 (precision of 0.9 ℃ and 1.0 ℃, respectively). The calibration models for temperature were successfully applied to determining the temperatures of deep-sea hydrothermal fluids in the Okinawa Trough hydrothermal field. The degree of mixing of hydrothermal fluids and ambient seawater during in situ Raman measurements was estimated by the difference in temperatures determined through these calibration models and those measured through thermocouple sensors.

Double rare-earth nanothermometer in aqueous media: opening the third optical transparency window to temperature sensing

Owing to the alluring possibility of contactless temperature probing with microscopic spatial resolution, photoluminescence nanothermometry at the nanoscale is rapidly advancing towards its successful application in biomedical sciences. The emergence of near-infrared nanothermometers has paved the way for temperature sensing at the deep tissue level. However, water dispersibility, adequate size at the nanoscale, and the capability to efficiently operate in the second and third biological optical transparency windows are the requirements that still have to be fulfilled in a single nanoprobe. In this work, these requirements are addressed by rare-earth doped nanoparticles with core/shell-architecture, dispersed in water, whose excitation and emission wavelengths conveniently fall within the biological optical transparency windows. Under heating-free 800 nm excitation, double nanothermometry is realized either with Ho³⁺-Nd³⁺ (1.18-1.34 μm) or Er³⁺-Nd³⁺ (1.55-1.34 μm) NIR emission band ratios, both displaying equal thermal sensitivities around 1.1% °C^-1. It is further demonstrated that, along with the interionic energy transfer processes, the thermometric properties of these nanoparticles are also governed by the temperature dependent energy transfer to the surrounding solvent (water) molecules. Overall, this work presents a novel water dispersible double ratiometric nanothermometer operating in the second and third biological optical transparency windows. The temperature dependent particle-solvent interaction is also presented, which is critical for e.g. future in vivo applications.

Quantifying Raman OH-band spectra for remote water temperature measurements

Remote water temperature measurements by Raman scattering is a perspective tool for in situ and/or real-time studies for inaccessible areas such as the Arctic region. State-of-the-art laser remote temperature detection techniques are based on temperature-dependent transformation of the Raman OH stretching vibration band. This study compared different approaches quantifying Raman OH-band spectra transformation with temperature: the two-color technique, deconvolution procedure, Raman difference spectroscopy, and centroid technique. Distilled water was probed remotely by compact Raman LIDAR, and the results demonstrated that the Raman OH-band centroid technique achieved the best temperature measurement accuracy (±0.15°C).