Analysis of metabolites in aqueous solutions by using laser Raman spectroscopy

Biochemical component analysis of human myopic corneal stroma using the Raman spectrum

PURPOSE

This study aimed to measure the Raman spectrum of the human corneal stroma lens obtained from small incision lenticule extraction surgery (SMILE) in Asian myopic eyes using a confocal Raman micro-spectrometer built in the laboratory.

METHODS

Forty-three myopic patients who underwent SMILE with equivalent diopters between - 4.00 and - 6.00 D were selected, and the right eye data were collected. Corneal stroma lenses were obtained during surgery, and the Raman spectra were measured after air drying. The complete Raman spectrum of human myopic corneal stroma lens tissue was obtained within the range of 700-4000 cm-1.

RESULTS

Thirteen characteristic peaks were found, with the stronger peaks appearing at 937 cm-1, corresponding to proline, valine, and the protein skeleton of the human myopic corneal stroma lens; 1243 cm-1, corresponding to collagen protein; 1448 cm-1, corresponding to the collagen protein and phospholipids; and 2940 cm-1, corresponding to the amino acid and lipids, which was the strongest Raman peak.

CONCLUSION

These results demonstrated that Raman spectroscopy has much potential as a fast, cost-effective, and reliable diagnostic tool in the diagnosis and treatment of eye diseases, including myopia, keratoconus, and corneal infection.

Glucose levels between the anterior chamber of the eye and blood are correlated based on blood glucose dynamics

Glycemic control is essential to manage metabolic diseases such as diabetes. Frequent measurements of systemic glucose levels with prompt managements can prevent organ damages. The eye is a glucose highly demanding organ in our body, and the anterior chamber (AC) in the eye has been suggested for a noninvasive blood glucose monitoring site. However, calculating blood glucose levels from measuring glucose levels in AC has been difficult and unclear. In this study, we aimed to examine glucose levels from AC and find a correlation with blood glucose levels. A total of 30 patients with cataracts (men and women, study 1; 7 and 3, study 2; 9 and 11) who visited Keio University Hospital from 2015 to 2018 and agreed to participate in this study were recruited. Glucose levels from AC and the blood were examined by a UV-hexokinase or H2O2-electrode method before/during the cataract surgery. These values were analyzed with regression analyses depending on the groups (blood glucose-ascending and descending groups). In the blood glucose-descending group, glucose levels from AC were strongly correlated with blood glucose levels (a high R2 value, 0.8636). However, the relatively moderate correlation was seen in the blood glucose-ascending group (a low R2 value, 0.5228). Taken together, we showed different correlation ratios on glucose levels between AC and the blood, based on blood glucose dynamics. Stacking data regarding this issue would enable establishing noninvasive blood glucose monitoring from measuring glucose levels in AC more correctly, which will be helpful for proper and prompt managements for glucose-mediated complications.

Very Selective Detection of Low Physiopathological Glucose Levels by Spontaneous Raman Spectroscopy with Univariate Data Analysis

After decades of research on non-invasive glucose monitoring, invasive devices based on finger blood sampling are still the predominant reference for diabetic patients for accurately measuring blood glucose levels. Meanwhile, research continues improving point-of-care technology toward the development of painless and more accurate devices. Raman spectroscopy is well-known as a potentially valuable and painless approach for measuring glucose levels. However, previous Raman studies deal with glucose concentrations that are still order of magnitudes away with respect to human tissues’ physiological concentrations, or they propose enhancement methodologies either invasive or much complex to assure sufficient sensitivity in the physiological range. Instead, this study proposes an alternative non-enhanced Raman spectroscopy approach sensitive to glucose concentrations from 1 to 5 mmol/l, which correspond to the lowest physiopathological glucose level in human blood. Our findings suggest a very selective detection of glucose with respect to other typical metabolites, usually interfering with Raman spectroscopy’s glucose detection. We validate the proposed univariate sensing methodology on glucose solutions mixed with lactate and urea, the two most common molecules found in human serum with concentrations similar to glucose and similar features in the Raman spectra. Our findings clearly illustrate that reliable detection of glucose by Raman spectroscopy is feasible by exploiting the shifted peak at 1125 ± 10 cm–1 within physiopathological ranges.

Microfluidic Cultivation and Laser Tweezers Raman Spectroscopy of E. coli under Antibiotic Stress

Analyzing the cells in various body fluids can greatly deepen the understanding of the mechanisms governing the cellular physiology. Due to the variability of physiological and metabolic states, it is important to be able to perform such studies on individual cells. Therefore, we developed an optofluidic system in which we precisely manipulated and monitored individual cells of Escherichia coli. We tested optical micromanipulation in a microfluidic chamber chip by transferring individual bacteria into the chambers. We then subjected the cells in the chambers to antibiotic cefotaxime and we observed the changes by using time-lapse microscopy. Separately, we used laser tweezers Raman spectroscopy (LTRS) in a different micro-chamber chip to manipulate and analyze individual cefotaxime-treated E. coli cells. Additionally, we performed conventional Raman micro-spectroscopic measurements of E. coli cells in a micro-chamber. We found observable changes in the cellular morphology (cell elongation) and in Raman spectra, which were consistent with other recently published observations. The principal component analysis (PCA) of Raman data distinguished between the cefotaxime treated cells and control. We tested the capabilities of the optofluidic system and found it to be a reliable and versatile solution for this class of microbiological experiments.

A technology roadmap of smart biosensors from conventional glucose monitoring systems

The objective of this review article is to focus on technology roadmap of smart biosensors from a conventional glucose monitoring system. The estimation of glucose with commercially available devices involves analysis of blood samples that are obtained by pricking finger or extracting blood from the forearm. Since pain and discomfort are associated with invasive methods, the non-invasive measurement techniques have been investigated. The non-invasive methods show advantages like non-exposure to sharp objects such as needles and syringes, due to which there is an increase in testing frequency, improved control of glucose concentration and absence of pain and biohazard materials. This review study is aimed to describe recent invasive techniques and major noninvasive techniques, viz. biosensors, optical techniques and sensor-embedded contact lenses for glucose estimation.

Raman spectroscopy as a novel tool for monitoring biochemical changes and inter-donor variability in stored red blood cell units

Raman spectroscopy has been used to retrieve biochemical information from the supernatant of stored red blood cells (RBCs), demonstrating that some units of donated RBCs accumulate lactate much more readily than others.

Structural basis for misfolding in myocilin-associated glaucoma

Olfactomedin (OLF) domain-containing proteins play roles in fundamental cellular processes and have been implicated in disorders ranging from glaucoma, cancers and inflammatory bowel disorder, to attention deficit disorder and childhood obesity. We solved crystal structures of the OLF domain of myocilin (myoc-OLF), the best studied such domain to date. Mutations in myoc-OLF are causative in the autosomal dominant inherited form of the prevalent ocular disorder glaucoma. The structures reveal a new addition to the small family of five-bladed β-propellers. Propellers are most well known for their ability to act as hubs for protein-protein interactions, a function that seems most likely for myoc-OLF, but they can also act as enzymes. A calcium ion, sodium ion and glycerol molecule were identified within a central hydrophilic cavity that is accessible via movements of surface loop residues. By mapping familial glaucoma-associated lesions onto the myoc-OLF structure, three regions sensitive to aggregation have been identified, with direct applicability to differentiating between neutral and disease-causing non-synonymous mutations documented in the human population worldwide. Evolutionary analysis mapped onto the myoc-OLF structure reveals conserved and divergent regions for possible overlapping and distinctive functional protein-protein or protein-ligand interactions across the broader OLF domain family. While deciphering the specific normal biological functions, ligands and binding partners for OLF domains will likely continue to be a challenging long-term experimental pursuit, atomic detail structural knowledge of myoc-OLF is a valuable guide for understanding the implications of glaucoma-associated mutations and will help focus future studies of this biomedically important domain family.

Quantitative determination of citric acid in seminal plasma by using Raman spectroscopy

In this study, Raman spectroscopy was first used to study the linear relationship between Raman spectral intensities and citric acid concentrations in aqueous solution. By using the specific Raman band of 942 cm(-1), concentrations of citric acid ranging from 2 to 20 mg/mL were observed linearly (R(2) = 0.993), and the limit of detection was 1.0 mg/mL. Then, citric acid detection in clinical seminal plasma ultrafiltrate samples was performed, and the intensity of the Raman-specific peak demonstrates a good linear correlation (R(2) = 0.946) with citric acid concentrations determined by the enzymatic method. Our results showed that Raman spectroscopy has the potential of being applied to detect concentrations of citric acid in seminal plasma in clinic.

In vivo blood glucose quantification using Raman spectroscopy

We here propose a novel Raman spectroscopy method that permits the noninvasive measurement of blood glucose concentration. To reduce the effects of the strong background signals produced by surrounding tissue and to obtain the fingerprint Raman lines formed by blood analytes, a laser was focused on the blood in vessels in the skin. The Raman spectra were collected transcutaneously. Characteristic peaks of glucose (1125 cm^-1) and hemoglobin (1549 cm^-1) were observed. Hemoglobin concentration served as an internal standard, and the ratio of the peaks that appeared at 1125 cm^-1 and 1549 cm^-1 peaks was used to calculate the concentration of blood glucose. We studied three mouse subjects whose blood glucose levels became elevated over a period of 2 hours using a glucose test assay. During the test, 25 Raman spectra were collected transcutaneously and glucose reference values were provided by a blood glucose meter. Results clearly showed the relationship between Raman intensity and concentration. The release curves were approximately linear with a correlation coefficient of 0.91. This noninvasive methodology may be useful for the study of blood glucose in vivo.

Demonstration of remote optical measurement configuration that correlates to glucose concentration in blood

An optical approach allowing the extraction and the separation of remote vibration sources has recently been proposed. The approach has also been applied for medical related applications as blood pressure and heart beats monitoring. In this paper we demonstrate its capability to monitor glucose concentration in blood stream. The technique is based on the tracking of temporal changes of reflected secondary speckle produced in human skin (wrist) when being illuminated by a laser beam. A temporal change in skin's vibration profile generated due to blood pulsation is analyzed for estimating the glucose concentration. Experimental tests that were carried out in order to verify the proposed approach showed good match with the change of the glucose level at the positive slope stage as it was obtained from conventional reference measurement.

Single walled carbon nanotubes as reporters for the optical detection of glucose

This article reviews current efforts to make glucose sensors based on the inherent optical properties of single walled carbon nanotubes. The advantages of single walled carbon nanotubes over traditional organic and nanoparticle fluorophores for in vivo-sensing applications are discussed. Two recent glucose sensors made by our group are described, with the first being an enzyme-based glucose sensor that couples a reaction mediator, which quenches nanotube fluorescence, on the surface of the nanotube with the reaction of the enzyme. The second sensor is based on competitive equilibrium binding between dextran-coated nanotubes and concanavalin A. The biocompatibility of a model sensor is examined using the chicken embryo chorioallantoic membrane as a tissue model. The advantages of measuring glucose concentration directly, like most optical sensors, versus measuring the flux in glucose concentration, like most electrochemical sensors, is discussed.

Experimental and theoretical aspects of glucose measurement using a microcantilever modified by enzyme-containing polyacrylamide

We report a glucose oxidase-containing polyacrylamide hydrogel-coated microcantilever sensor for the measurement of glucose. This enzymatic reaction of glucose results in swelling of the hydrogel due to formation of charged ions (gluconate molecules and protons). The microcantilever undergoes reversible and reproducible bending deflection upon exposure to solutions containing various glucose concentrations due to swelling or shrinking of the hydrogels. The microcantilever deflections increase when the glucose concentrations increase. A theoretical model has been built to correlate volume changes of the gel with microcantilever bending. The calculated data matched with the experimental results very well. Such hydrogel-coated microcantilevers could potentially be used to prepare microcantilever-based chemical and biological sensors when other enzymes are immobilized in the hydrogel.

Determination of glucose in human aqueous humor using Raman spectroscopy and designed-solution calibration

Glucose concentrations of in vitro human aqueous humor (HAH) samples from cataract patients were determined using 785 nm Raman spectra and partial least squares (PLS) calibration. PLS models were created from spectra of prepared calibration solutions rather than aqueous humor samples. Spectra were obtained with an excitation energy (100 mW for 150 s), which was higher than can be applied in vivo, to decrease the models' contribution to prediction uncertainty. The solutions contained experimentally designed levels of glucose, bicarbonate, lactate, urea, and ascorbate. Multiplicative signal correction of spectra helped compensate for the +/-20% drift in laser power observed at the sample over six noncontiguous days of data collection. Seventeen HAH samples containing 38-775 mg/dL of glucose exhibited a root-mean-square error (RMSEP) of 22 mg/dL, coefficient of determination (r(2)) of 0.989, and bias of 6 mg/dL when predicted from lower energy (30 s) spectra collected contemporaneously with fifty calibration spectra. Similar results were obtained even when spectral data were gathered separately for human aqueous humor samples and calibration samples: 10 HAH samples, calibrated on 25 solutions measured 3.6 weeks earlier, exhibited an RMSEP of 23 mg/dL, r(2) of 0.992, and bias of 9 mg/dL. The results demonstrate progress toward the determination of glucose levels in patient-derived aqueous humor using laboratory-derived "artificial aqueous humor" calibration solutions.

Glucose determination in human aqueous humor with Raman spectroscopy

It has been suggested that spectroscopic analysis of the aqueous humor of the eye could be used to indirectly predict blood glucose levels in diabetics noninvasively. We have been investigating this potential using Raman spectroscopy in combination with partial least squares (PLS) analysis. We have determined that glucose at clinically relevant concentrations can be accurately predicted in human aqueous humor in vitro using a PLS model based on artificial aqueous humor. We have further determined that with proper instrument design, the light energy necessary to achieve clinically acceptable prediction of glucose does not damage the retinas of rabbits and can be delivered at powers below internationally acceptable safety limits. Herein we summarize our current results and address our strategies to improve instrument design.

Noninvasive and minimally invasive methods for transdermal glucose monitoring

Noninvasive and minimally invasive techniques for monitoring glucose via the skin are reviewed. These approaches rely either on the interaction of electromagnetic radiation with the tissue or on the extraction of fluid across the barrier. The structure and physiology of the skin make the technical realization of transdermal glucose monitoring a difficult challenge. The techniques involving transdermal fluid extraction circumvent and/or compromise the barrier function of skin's outermost and least permeable layer, the stratum corneum, by the application of physical energy. While sonophoresis and microporation methods, for example, are in relatively early-stage development, a device using reverse iontophoresis [the GlucoWatch Biographer (Cygnus, Inc., Redwood City, CA)] is already commercially available. Optical techniques to monitor glucose are truly noninvasive. The tissue is irradiated, the absorbed or scattered radiation is analyzed, and the information is processed, to provide a measure proportional to the concentration of glucose in the dermal tissue. These techniques include near-infrared and Raman spectroscopy, polarimetry, light scattering, and photoacoustic spectroscopy. By contrast, impedance spectroscopy measures changes in the dielectric properties of the tissue induced by blood glucose variation. Large-scale studies in support of efficacy of these methodologies are as yet unavailable. At present, therefore, transdermal fluid extraction technologies are offering greater promise in terms of practical and realizable devices for patient use. The truly noninvasive allure of the optical approach assures continued and intense research activity--for the moment, however, an affordable, efficient, and portable system is not on the immediate horizon.

Noninvasive and minimally-invasive optical monitoring technologies

With recent advancements in micro-fabrication and nano-fabrication techniques as well as advancements in the photonics industry, there is now the potential to develop less invasive portable sensors for monitoring micronutrients and other substances used to assess overall health. There have been many technology innovations in the central laboratory for these substances for overall health status but the primary motivation for the research and development of a portable field instrument has come from a diabetic patient and market-driven desire to minimally invasively or noninvasively monitor glucose concentrations in vivo. Such a sensor system has the potential to significantly improve the quality of life for the estimated 16 million diabetics in this country by making routine glucose measurements less painful and more convenient. In addition, there is a critical need for the development of less invasive portable technologies to assess micronutrient status (iron, vitamin A, iodine and folate), environmental hazards (lead) and for other disease-related substances, such as billirubin for infant jaundice. Currently, over 100 small companies and universities are working to develop improved monitoring devices, primarily for glucose, and optical methods are a big part of these efforts. In this article many of these potentially less invasive and portable optical sensing technologies, which are currently under investigation, will be reviewed including optical absorption spectroscopy, polarimetry, Raman spectroscopy and fluorescence.

Measurement of the glucose transport time delay between the blood and aqueous humor of the eye for the eventual development of a noninvasive glucose sensor

In the recent past, several noninvasive optically based methods have been proposed for physiologic glucose sensing. One proposed optical sensing site has been the eye, which, due to its unique optical properties, can be considered as a transparent optical window into the body. In particular, the aqueous humor within the anterior chamber of the eye has been shown to contain glucose levels correlated to those of blood. Concern, however, has been expressed that using the aqueous humor solution as a measure of blood glucose may be problematic due to the potential transport time delay between the blood and the aqueous humor glucose concentrations. This investigation was performed to measure the transport time delay in a rabbit model. The time delay between the blood and aqueous humor glucose concentrations was measured invasively in five New Zealand White rabbits over a series of weeks. An anesthesia protocol containing the drug Xylazine was used to elevate the blood glucose levels to a level commonly seen in diabetic patients. The difference between the glucose peak location times occurring in the blood and aqueous humor glucose response was measured and defined as the transport time delay. The average transport time lag was measured to be under 5 min. This measured time delay indicates that, indeed, the eye could potentially be used as a sensing site for indirect blood glucose measurements and may eventually aid the development of a noninvasive glucose sensor due to its unique optical properties compared to other biological tissues.

Intraocular lens glucose sensor

Ocular spectroscopy, which is the use of the eye to monitor optically the concentration of metabolites in the body, has been successfully applied to monitor aqueous humor glucose concentration. In the United States, 1.7 million intraocular lenses are currently implanted yearly. Because patients with diabetes are more likely to develop cataracts at an earlier age, a relatively high proportion of the patients receiving intraocular lenses have diabetes. Last year, 110,000 patients with diabetes received intraocular lens implants of various materials. We have successfully polymerized a fluorescent complex within a hydrogel intraocular lens that responds well to glucose concentration.

Optical glucose sensing in biological fluids: an overview

Recent technological advancements in the photonics industry have led to a resurgence of interest in optical glucose sensing and to realistic progress toward the development of an optical glucose sensor. Such a sensor has the potential to significantly improve the quality of life for the estimated 16 million diabetics in this country by making routine glucose measurements more convenient. Currently over 100 small companies and universities are working to develop noninvasive or minimally invasive glucose sensing technologies, and optical methods play a large role in these efforts. This article reviews many of the recent advances in optical glucose sensing including optical absorption spectroscopy, polarimetry, Raman spectroscopy, and fluorescent glucose sensing. In addition a review of calibration and data processing methods useful for optical techniques is presented.

Multicomponent blood analysis by near-infrared Raman spectroscopy

We demonstrate the use of Raman spectroscopy to measure the concentration of many important constituents (analytes) in serum and whole blood samples at physiological concentration in vitro across a multipatient data set. A near-infrared (830-nm) diode laser generates Raman spectra that contain superpositions of Raman signals from different analytes. Calibrations for glucose, cholesterol, urea, and other analytes are developed by use of partial least-squares cross validation. We predict six analytes in serum with significant accuracy in a 66-patient data set, using 60-s spectra. The calibrations are shown to be fairly robust against system drift over the span of seven weeks. In whole blood, a preliminary analysis yields accurate predictions of some of the same analytes and also hematocrit. The results hold promise for potential medical applications.

Spectroscopic and Clinical Aspects of Noninvasive Glucose Measurements

Frequent determination of glucose concentrations in diabetic patients is an important tool for diabetes management. This requires repetitive lancing and finger bleeding. Use of noninvasive (NI) detection techniques offers several advantages, such as the absence of pain and exposure to sharp objects and biohazard materials, the potential for increased frequency of testing, and hence, tighter control of the glucose concentrations, and the potential for a closed-loop system including a monitor and an insulin pump. These potential advantages have led to considerable interest in the commercialization of NI glucose monitoring devices. Review of the scientific, patent, and commercial literature indicates that the spectroscopic basis for NI determination of glucose is not yet well established, and attempts at commercialization may be several steps ahead of our understanding the origin and characteristics of an in vivo glucose-specific or glucose-related signal. Several technologies have potential for leading to viable measuring devices, but most of the data are based on in vitro experimentation. Because of the technical complexity of in vivo glucose measurements, this review aims at discussing the gap between the established need and current technology limitations.

A noninvasive glucose monitor: preliminary results in rabbits

BACKGROUND

Glucose in the aqueous humor appears to correlate with plasma glucose in humans. It is therefore a potential substrate for noninvasive optical glucose monitoring techniques. We wished to determine the potential for using rabbit aqueous humor as a model to develop a noninvasive method of measuring glucose with Raman spectroscopy.

METHODS

Aqueous humor from 32 rabbit eyes was removed immediately after sacrifice by rapid exsanguination under anesthesia. Raman spectroscopy was performed on the aqueous humor using near infrared (NIR) excitation wavelengths. The Raman spectra from 785 nm and 787.2 nm were subtracted one from the other to eliminate broadband fluorescence. The spectra were then analyzed with linear and nonlinear multivariate analysis and assessed for ability to predict actual aqueous humor glucose concentration. Nine other rabbits were anesthetized with xylazine in order to cause elevation of blood glucose by blocking release of insulin. Blood and aqueous humor glucose were measured at various times after injection of xylazine. Correlation of aqueous humor glucose with simultaneous plasma glucose was assessed.

RESULTS

Partial least squares analysis of raw Raman spectra of aqueous humor showed fair correlation with actual glucose concentration (r2 = 0.76). When the fluorescence spectrum was subtracted prior to linear multivariate analysis correlation was good (r2 = 0.90). When back-propagation with an artificial neural network was added to the analysis, correlation was further improved (r2 = 0.98). Aqueous humor glucose concentration exceeded blood glucose concentration at normoglycemic levels. When blood glucose rose above 200 mg/dL, aqueous humor glucose correlated linearly with plasma glucose.

CONCLUSIONS

Raman spectroscopy can accurately predict glucose concentration in rabbit aqueous humor in vitro. Although rabbit aqueous humor is probably not a good model of human aqueous humor glucose physiology, its reliable correlation with plasma glucose makes it a good model on which to test noninvasive optical sensing techniques.

Introduction to concepts in laser technology for glucose monitoring

Lasers are important tools in many diabetes-related applications, both diagnostic and therapeutic. Despite its wide range of uses, the laser derives all of its advantages over more conventional light sources from a few basic principles. A brief introduction to the fundamental properties of lasers emphasizes these points and suggests ways in which they can be exploited in experiments. Short descriptions of particular laser-based methods for monitoring glucose and related molecules provide introductions to research articles on these subjects.

Feasibility of measuring blood glucose concentration by near-infrared Raman spectroscopy

We report the determinations of glucose concentrations in human whole blood samples made using near-infrared Raman spectroscopy. Raman spectra of blood samples with above-physiological levels of glucose were acquired for 5 min through the wall of a cuvette via fiber optics. Partial least squares analysis was used to predict glucose concentrations in the samples. A root mean squared prediction error of 3.6 mM glucose was achieved with a correlation coefficient of 0.99 between reference and predicted values. This result is the first step in evaluating the potential of near-infrared Raman spectroscopy to perform blood glucose measurement with clinical accuracy. The technique is capable of measuring the concentration of other Raman-active blood constituents; as an example, bicarbonate was also measured. The method could eventually be useful for direct measurement of tissue analytes.

Rapid, noninvasive concentration measurements of aqueous biological analytes by near-infrared Raman spectroscopy

Accurate concentration measurements of glucose, lactic acid, and creatinine in saline solution have beena chieved with near-IR Raman spectroscopy and a partial least-squares analysis. The Raman spectra were acquired remotely through optical fibers. A root-mean-squared prediction error of 1.2 mM for glucose concentration was achieved in 100 s. Concentrations of other analytes were predicted with similar accuracy.

Application of a multivariate technique to Raman spectra for quantification of body chemicals

Raman spectroscopy is a highly specific technique for the identification of molecules by way of the associated characteristic spectra. The aim of this feasibility study is to assess the combination of the multivariate calibration technique of Partial Least-Squares with Raman spectroscopy for the estimation of glucose, lactic acid, and urea concentrations in the presence of each other in a water substrate. The instrument is a CCD-based Raman spectrometer utilizing the 514.5 nm argon laser line. The estimates for the analyte concentrations yielded a standard deviation of concentration residuals of 20.71 mg/dL for glucose, 12.92 mg/dL for lactic acid, and 19.07 mg/dL for urea.