Raman Sensor Research Articles

Review on All-Fiber Online Raman Sensor with Hollow Core Microstructured Optical Fiber

Raman spectroscopy is widely used for qualitative and quantitative analysis of trace components in scientific fields such as food safety monitoring, drug testing, environmental monitoring, etc. In addition to its demonstrated advantages of fast response, non-destructive, and non-polluting characteristics, fast online Raman detection is drawing growing attention for development. To achieve this desirable capability, hollow core optical fibers are employed as a common transmission channel for light and fluid in the Raman sensor. By enhancing the interaction process between light and matter, the detection sensitivity is improved. At the same time, the Raman spectroscopy signal light collection efficiency is significantly improved. This article summarizes enhancement techniques reported for Raman sensors, followed by a detailed review on fiber-based Raman sensor techniques including theoretical analyses, fabrication, and application based on hollow core photonic crystal fibers and capillary-based hollow core fibers. The prospects of using these fibers for Raman spectroscopy are discussed.

γ-Cyclodextrin capped silver and gold nanoparticles as colorimetric and Raman sensor for detecting traces of pesticide “Chlorpyrifos” in fruits and vegetables

Organophosphorous compounds are extensively used in agriculture as pesticides, which impose great threat to the safety of environment, food and public health. These demands for the development of reliable pesticide detection platform. In this study, noble metal nanoprobes, γ-CD-Ag and γ-CD-Au were developed using a facile one-step green synthetic approach using γ-cyclodextrin (γ-CD) that serves as a platform for colorimetric and Raman sensing of the pesticide, chlorpyrifos (CPP). The noble metal nanoparticles (NPs), owing to their unique optical properties are capable of undergoing colorimetric changes based on variations in their surroundings. The intensity variation of the Raman bands, viz. 417 (1328) and 411 (773) cm-1 as a function of CPP concentration was used to quantify the sensitivity of the γ-CD-Ag and γ-CD-Au nanoprobes. The colorimetric changes as well as enhancement in the Raman bands of CPP resulted due to the interaction of the metal particles with CPP, primarily through the amine and phosphorothioate functional groups. The applicability of the nanoprobe was tested in apple and tomato. The γ-CD-Ag and γ-CD-Au nanoprobes showed excellent colorimetric and Raman response towards the pesticide. The detection limit was found to be 1 ppb and 10 ppb for the nanoprobes, in aqueous solution and apple/tomato, respectively. The methodology developed for pesticide detection is cost-effective and shows relatively fast colorimetric and Raman response. This study thus, provides new insight towards the development of novel nanoprobes for prospective applications in environmental monitoring.

Measurement of Secondary Structure Changes in Poly-L-lysine and Lysozyme during Acoustically Levitated Single Droplet Drying Experiments by In Situ Raman Spectroscopy.

Drying processes such as spray drying, as commonly used in the pharmaceutical industry to convert protein-based drugs into their particulate form, can lead to an irreversible loss of protein activity caused by protein secondary structure changes. Due to the nature of these processes (high droplet number, short drying time), an in situ investigation of the structural changes occurring during a real drying process is hardly possible. Therefore, an approach for the in situ investigation of the expected secondary structural changes during single droplet protein drying in an acoustic levitator by time-resolved Raman spectroscopy was developed and is demonstrated in this paper. For that purpose, a self-developed NIR–Raman sensor generates and detects the Raman signal from the levitated solution droplet. A mathematical spectral reconstruction by multiple Voigt functions is used to quantify the relative secondary structure changes occurring during the drying process. With the developed setup, it was possible to detect and quantify the relative secondary structure changes occurring during single droplet drying experiments for the two chosen model substances: poly-L-lysine, a homopolypeptide widely used as a protein mimic, and lysozyme. Throughout drying, an increase in the β-sheet structure and a decrease in the other two structural elements, α-helix, and random coil, could be identified. In addition, it was observed that the degree of structural changes increased with increasing temperature.

Silver nanocube dimer nanojunctions as plasmon-enhanced Raman sensors

The enhancement factor of the Raman signal in nanojunctions under the plasmon-induced electromagnetic field can be tuned by (1) the conformation of nanojunctions; (2) the conductivity of the ligands/matrices; (3) the irradiation intensity.

Hybrid Sol-Gel Surface-Enhanced Raman Sensor for Xylene Detection in Solution.

This paper reports on the fabrication and characterization of a plasmonic/sol-gel sensor for the detection of aromatic molecules. The sol-gel film was engineered using polysilsesquioxanes groups to capture the analyte, through π-π interaction, and to concentrate it close to the plasmonic surface, where Raman amplification occurs. Xylene was chosen as an analyte to test the sensor. It belongs to the general class of volatile organic compounds and can be found in water or in the atmosphere as pollutants released from a variety of processes; its detection with SERS is typically challenging, due to its low affinity toward metallic surfaces. The identification of xylene was verified in comparison with that of other aromatic molecules, such as benzene and toluene. Investigations were carried out on solutions of xylene in cyclohexane, using concentrations in the range from 0 to 800 mM, to evaluate the limit of detection (LOD) of about 40 mM.

Raman enhancement mechanism and experiments of cavity-enhanced AgNP decorated tapered fiber sensor.

A Raman sensor based on a cavity-enhanced Ag nanoparticle (AgNP) decorated tapered fiber is proposed. Its Raman enhancements are mainly caused by the localized surface plasmon resonance effect of AgNPs decorated on the tapered optical fiber surface and the further reflective laser excitation induced by the capillary-based reflective cavity. We theoretically investigate the backward Stokes power conversion efficiency and cavity enhancement factor of the sensor. The calculated relationship between the cavity enhancement factor ξ and distance L from the tip to reflective rod is also discussed. Subsequently, the proving experiments were carried out for a tapered fiber surface-enhanced Raman scattering (SERS) probe and cavity-enhanced metal decorated tapered fiber Raman sensors. The analytical enhancement factor is 5.51×104 for the tapered fiber SERS probe. Moreover, the predicted curves of the theoretical model are close to the experimental values. This Letter provides a possible way to rigorously quantify the complete coupling efficiency for tapered fiber SERS probes, as well as cavity enhancement factors of cavity-enhanced Raman sensors.

Raman Sensor for the Determination of Gas Solubility

Efficient and environmentally responsible use of resources requires the development and optimization of gas separation processes. A promising approach is the use of liquids that are designed for specific tasks, e.g., the capture of carbon dioxide or other greenhouse gases. This requires an accurate determination of gas solubilities for a broad range of temperatures and pressures. However, state of the art measurement techniques are often very time consuming or exhibit other pitfalls that prevent their use as efficient screening tools. Here, we show that the application of Raman spectroscopy through a compact measuring system can simplify data acquisition for the determination of gas solubilities in liquids. To demonstrate that this approach is expedient, we determined gas solubilities of carbon dioxide in water for three isotherms T = (288.15, 293.15, 298.15) K over a pressure range from p = (0.5–5) MPa and in three imidazolium-based ionic liquids for one isotherm T = 298.15 K at pressures from p = (0.1–5) MPa. When compared to data in the literature, all results are within the reported uncertainties of the measurement techniques involved. The developed analysis method eliminates the need for a lengthy volume or mass calibration of the sample prior to the measurements and, therefore, allows for fast screening of samples, which can help to advance gas separation processes in scientific and industrial applications.

In situ SERS detection of quinolone antibiotic residues in a water environment based on optofluidic in-fiber integrated Ag nanoparticles

In this paper, we present a microstructured optofluidic in-fiber Raman sensor for the detection of quinolone antibiotic residue in a water environment based on Ag surface-enhanced Raman scattering (SERS) substrate grown on the surface of the suspended core of micro-hollow optical fiber (MHF). Here, MHF has a special structure with a suspended core and a microchannel inside, which can become a natural in-fiber optofluidic device. Meanwhile, the self-assembled Ag SERS substrate can be grown on the suspended core's surface through chemical bonds, forming a microstructured optofluidic device with a Raman enhancement effect. Therefore, it can effectively detect the Raman signal of unlabeled trace quinolone antibiotic residue (ciprofloxacin and norfloxacin) inside the optical fiber. The results show that the ciprofloxacin and norfloxacin detection limits (LOD) are 10-10M and 10-11M, respectively. Compared with the maximum residue limit (3.01×10-7mol/L) stipulated by the European Union, the results are much lower, and an ideal quantitative relationship can be obtained within the detection range. Significantly, this study provides an in-fiber microstructured optofluidic Raman sensor for the label-free detection of quinolone antibiotic residue, which will have good development prospects in the field of antibiotic water pollution environmental detection.

On-line SERS detection of bilirubin based on the optofluidic in-fiber integrated GO/Ag NPs for rapid diagnosis of jaundice

In this paper, we propose a self-assembled graphene oxide (GO)/Ag NPs SERS Raman sensor based on a novel type of optofluidic MHF as a point-of-care testing (POCT) device. This device is used to diagnose jaundice and its related diseases through on-line detection of free bilirubin content in human serum. This optofluidic Raman sensor is composed of a microstructured hollow fiber (MHF) with a microstructured channel and a suspended core, which allows the sample solution to flow in the channel while interacting with the strong evanescent field on the suspended core. Here, the suspended core was modified by a GO/Ag NPs SERS substrate. When the sample flows through the channel, and interacts with the strong evanescent field generated by the suspended core, the on-line SERS signal is generated and can be coupled back to the suspended core to be detected. In addition, both the electrostatic interaction and interference between GO/Ag NPs with the target enriched bilirubin. The results show that the detection concentration range of bilirubin aqueous, bilirubin in albumin and bilirubin in human blood are all in the range of 2 μM–100 μM, and all have a good linear response. The limit of detection reaches the order of 10−6 M. This rapid, sensitive and label-free SERS Raman sensor of free bilirubin in blood can detect excessive levels of bilirubin in the actual blood environment of the human body, providing a broad prospect for clinically accurate diagnosis of jaundice and related diseases.

Combined Spectroscopic Analysis of Terrestrial Analogs from a Simulated Astronaut Mission Using the Laser-Induced Breakdown Spectroscopy (LIBS) Raman Sensor: Implications for Mars.

One of the primary objectives of planetary exploration is the search for signs of life (past, present, or future). Formulating an understanding of the geochemical processes on planetary bodies may allow us to define the precursors for biological processes, thus providing insight into the evolution of past life on Earth and other planets, and perhaps a projection into future biological processes. Several techniques have emerged for detecting biomarker signals on an atomic or molecular level, including laser-induced breakdown spectroscopy (LIBS), Raman spectroscopy, laser-induced fluorescence (LIF) spectroscopy, and attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy, each of which addresses complementary aspects of the elemental composition, mineralogy, and organic characterization of a sample. However, given the technical challenges inherent to planetary exploration, having a sound understanding of the data provided from these technologies, and how the inferred insights may be used synergistically is critical for mission success. In this work, we present an in-depth characterization of a set of samples collected during a 28-day Mars analog mission conducted by the Austrian Space Forum in the Dhofar region of Oman. The samples were obtained under high-fidelity spaceflight conditions and by considering the geological context of the test site. The specimens were analyzed using the LIBS-Raman sensor, a prototype instrument for future exploration of Mars. We present the elemental quantification of the samples obtained from LIBS using a previously developed linear mixture model and validated using scanning electron microscopy energy dispersive spectroscopy. Moreover, we provide a full mineral characterization obtained using ultraviolet Raman spectroscopy and LIF, which was verified through ATR FT-IR. Lastly, we present possible discrimination of organics in the samples using LIF and time-resolved LIF. Each of these methods yields accurate results, with low errors in their predictive capabilities of LIBS (median relative error ranging from 4.5% to 16.2%), and degree of richness in subsequent inferences to geochemical and potential biochemical processes of the samples. The existence of such methods of inference and our ability to understand the limitations thereof is crucial for future planetary missions, not only to Mars and Moon but also for future exoplanetary exploration.

Underwater Time-Gated Standoff Raman Sensor for In Situ Chemical Sensing.

We describe the fabrication of an underwater time-gated standoff Raman sensor, consisting of a custom Raman spectrometer, custom scanner, and commercial diode-pumped pulsed 532 nm laser all located inside a pressure housing. The Raman sensor was tested in the laboratory with samples in air, a tank containing tap water and seawater, and in the coastal Hawaiian harbor. We demonstrate our new system by presenting standoff Raman spectra of some of the chemicals used in homemade explosive devices and improvised explosive devices, including sulfur, nitrates, chlorates, and perchlorates up to a distance of ∼6 m in seawater and tap water. Finally, the Raman spectra of these hazardous chemicals sealed inside plastic containers submersed in the Hawaiian Harbor water are also presented.

Ultra-sensitive slot-waveguide-enhanced Raman spectroscopy for aqueous solutions of non-polar compounds using a functionalized silicon nitride photonic integrated circuit.

We demonstrate an ultra-sensitive waveguide-enhanced Raman sensor for low concentration organic compounds dissolved in water. The spectra are obtained using silicon nitride slot waveguides coated with a thin film of hexamethyldisilazane-modified mesoporous silica. Enriched locally by 600-fold within the coating, a micromolar level of cyclohexanone is probed. The sensor is also capable of simultaneous quantification of multiple analytes, and the adsorbed analytes can be completely released from the coating. These properties make this on-chip Raman sensor promising for diverse applications, especially for the monitoring of non-polar organics and biomolecules in aqueous environments.

MWCNTs/PEDOT: PSS Composite as Guiding Layer on Screen-Printed Carbon Electrode for Linear Range Lactate Detection

The emerging field of nanomaterials could be utilized in biosensors for addressing challenging applications due to its abundant strategic properties. Herein, a composite of multi-walled carbon nanotubes (MWCNTs) and poly (3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT: PSS) was synthesized, and used as guiding layer on screen printed carbon electrode (SPCE) for lactate detection. Lactate plays a substantial role in health care applications. Fundamental characteristics of the composite modified SPCE were studied using FE-SEM, EDS, and Raman spectroscopy and sensor performance was analyzed by electrochemical methods. AFM was used to thoroughly study the screen-printed carbon electrode and as-deposited MWCNT/PEDOT:PSS composite film topography. Various parameters were optimized to achieve the best performance of lactate sensor. The developed sensor provided a wide linear range response (R2 = 0.97) from 1 mM to 10 mM for buffer samples with 35.224 μA mM−1 sensitivity. The proposed sensor was applied for to detect lactate in cancer (MCF-7) cells media. A calculated LOD 4.0 ± 5 μM (S/N) was achieved and the results showed a linear response up to 7 mM. As a result, the advanced approach could be applied for the detection of a range of metabolites using respective enzymes. This approach could open on-line lactate detection in organ-on-a-chip applications.

SNR enhancement of a Raman distributed temperature sensor using partial window-based non local means method

Raman scattering-based distributed sensor is nowadays the most mature and consolidated technology for distributed temperature sensing at long ranges. Even though the various applied methods in recent decades have made important improvements in performance of Raman sensors, they do not thoroughly exploit the high level of correlation and similitude existing in the acquired data. In the present, we exploit a 2D image processing method called non-local means (NLM) algorithm that takes full advantage of, compared to existing methods, highly redundant texture of the 1D data obtained from distributed Raman sensors. One of the main drawbacks of the original NLM filter in distributed sensors area is to establish complete squared similarity windows for denoising the boundary pixels, corresponding to the data placed at the last row of the data matrices, which constrains the best attainable SNR values. To overcome this issue, we introduce an unprecedented approach called partial window (PW) by which we utilize partial similarity windows for denoising the boundary pixels. The PW-based NLM (PW-NLM) technique not only leads to higher levels of SNR compared with the original one, but also has no detrimental effects on the denoised data. The simulation results demonstrate the SNR improvement of 2.8 dB using the PW-based NLM filter over the original NLM filter corresponding to the 17 dB SNR boost compared with the unprocessed data.

In-fiber optofluidic online SERS detection of trace uremia toxin.

In this Letter, we propose a microstructured in-fiber optofluidic surface-enhanced Raman spectroscopy (SERS) sensor for the initial inspection of uremia through the detection of unlabeled urea and creatinine. As a natural microfluidic device, microstructured hollow fiber has a special structure inside. Through chemical bonds, the SERS substrate can be modified and grown on the surface of the suspended core. Here, the silver nanoparticles (Ag NPs) are embedded on the poly diallyl dimethyl ammonium chloride-modified graphene oxide sheet to achieve the self-assembled SERS substrate. The reduced distance between Ag NPs can increase the strong hot spots that generate enhanced Raman signals. Therefore, it can effectively detect the Raman signal of unlabeled trace uremic toxin analytes (urea, creatinine) inside the optical fiber. The results show that under simulated biophysical conditions, the limit detection (LOD) for urea is 10-4M and the linearity is good, especially at the clinical conventional concentration range (2.5-6.5×10-3M). In addition, the online Raman detection of creatinine aqueous solution LOD is 10-6M, which also has good linearity. Significantly, this Letter provides a microstructured optofluidic in-fiber Raman sensor for the preliminary detection of uremia, which will have good development prospects in the field of clinical biomedicine.

Portable Raman leaf-clip sensor for rapid detection of plant stress

Precision agriculture requires new technologies for rapid diagnosis of plant stresses, such as nutrient deficiency and drought, before the onset of visible symptoms and subsequent yield loss. Here, we demonstrate a portable Raman probe that clips around a leaf for rapid, in vivo spectral analysis of plant metabolites including carotenoids and nitrates. We use the leaf-clip Raman sensor for early diagnosis of nitrogen deficiency of the model plant Arabidopsis thaliana as well as two important vegetable crops, Pak Choi (Brassica rapa chinensis) and Choy Sum (Brassica rapa var. parachinensis). In vivo measurements using the portable leaf-clip Raman sensor under full-light growth conditions were consistent with those obtained with a benchtop Raman spectrometer measurements on leaf-sections under laboratory conditions. The portable leaf-clip Raman sensor offers farmers and plant scientists a new precision agriculture tool for early diagnosis and real-time monitoring of plant stresses in field conditions.

Mitigation of photon background in nanoplasmonic all-on-chip Raman sensors.

In the quest for a more compact and cheaper Raman sensor, photonic integration and plasmonic enhancement are central. Nanoplasmonic slot waveguides exhibit the benefits of SERS substrates while being compatible with photonic integration and mass-scale (CMOS) fabrication. A difficulty in pursuing further integration of the Raman sensor with lasers, spectral filters, spectrometers and interconnecting waveguides lies in the presence of a photon background generated by the excitation laser field in any dielectric waveguide constituting those elements. Here, we show this problem can be mitigated by using a multi-mode interferometer and a nanoplasmonic slot waveguide operated in back-reflection to greatly suppress the excitation field behind the sensor while inducing very little photon background.

All-fiber online Raman sensor with enhancement via a Fabry-Perot cavity.

In this Letter, a novel all-fiber online Raman sensor with significant signal enhancement via a Fabry-Perot (FP) cavity is proposed and demonstrated. The FP cavity structure is formed by inserting a long-pass coated fiber and a gold-plated capillary into a silver-lined capillary with a gap. A corroded single-mode fiber is inserted into the gold-plated capillary to guide the excitation light into the FP cavity. The multiple reflections of excitation light in the FP cavity have significantly increased the interaction volume between the light and the sample. Experiment results have demonstrated an enhancement factor of 5 times in the detected Raman signal for ethanol compared to that measured using the silver-lined hollow-core fiber-based Raman cell without FP cavity, or 86 times compared with direct detection using a bare fiber tip. The measurement results are in good agreement with theoretical analyses. This Raman sensor with signal enhancement via the FP cavity has the potential to realize rapid sample replacement and online detection with high sensitivity and high accuracy for biochemical applications.

Elemental estimation of terrestrial analogues from the CanMars rover field campaign using LiRS: Implications for detecting silica-rich deposits on Mars

As space agencies plan for the continuous deployment of rovers and landers to planetary bodies such as the Moon and Mars, an in-depth, quantitative, and qualitative understanding of the observations is essential. One objective of planetary exploration focuses on planetary geochemistry and biochemistry with an emphasis on the search for possible biosignatures and related minerals. To this end, we present the elemental quantification of samples from the CanMars analogue sample return mission conducted in Hanksville, UT, USA. Measurements were carried out in a laboratory at York University, Canada, using the Laser-induced Breakdown Spectroscopy Raman Sensor (LiRS) instrument- a breadboard for a future space concept. A linear Mixture Model (LMM) was used to quantify the abundance of major elements of 10 samples from the resulting Laser-induced Breakdown Spectroscopy LIBS spectra with a calibration set based on the sample mineralogy. We assessed the quantification achieved by LiRS and the LMM by error analysis, which resulted in root mean squared error, absolute error, and percentage relative error of less than 1.299 % ± 0.114% (wt%), 0.959 ± 0.010 (wt%), and 9.613 % ± 1.914% (of wt%), respectively. The results in question suggest that by complementing information obtained from various sources such as Raman spectroscopy, X-ray diffraction, and Reflectance spectroscopy, the quantification of LIBS may be significantly improved, from which subsequent geochemical inferences may be made. Within the scope of the CanMars analogue mission, these results show an advancement over past results with possible implications for ongoing and future sample return missions such as the OSIRIS-REx and the Mars2020 Perseverance Rover.

Phenotypically distinguishing ESBL-producing pathogens using paper-based surface enhanced Raman sensors

Antimicrobial stewardship practices are critical in preventing the further erosion of treatment options for bacterial infections. Yet, at the same time, determination of an infection’s antimicrobial susceptibility requires multiple rounds of culture and expensive lab automation systems. In this work, we report the use of paper-based surface enhanced Raman spectroscopy (SERS) sensors and portable instrumentation to phenotypically discriminate multi-drug resistance with fewer culture steps than conventional clinical microbiology. Specifically, we demonstrate the identification of resistance to varying generations of β-lactam antibiotics by detecting the activity of particular β-lactamase enzymes in a multiplexed assay. The method utilizes molecular reporters that consist of β-lactams with SERS barcodes. Hydrolysis of the β-lactam by β-lactamase enzymes in the sample expels the barcode; the released sulfur-containing barcode is then detected via SERS. Using this approach, we demonstrate the differentiation of E. coli strains with (1) extended spectrum β-lactamase (ESBL), (2) narrow-spectrum β-lactamase, and (3) no resistance, using only a single measurement on a single sample. In addition, we experimentally validate an approach to expand the library of reporters through the simple chemical synthesis of new barcoded β-lactams. Importantly, the reported method determines the susceptibility based on phenotypic β-lactamase activity, which is aligned with current microbiology lab standards. This new method will enable the precise selection of effective β-lactam antibiotics (as opposed to defaulting to drugs of last resort) faster than current methods while using simple steps and low-cost portable instrumentation.