Optical glucose sensing in biological fluids: an overview

Lab-created conductive filament based on nickel and graphite particles: An attractive material for the additive manufacture of enhanced electrochemical sensors for non-enzymatic and selective glucose sensing

Developing tailor-made conductive filaments has emerged as a promising niche for producing affordable and high-performance 3D-printed electrochemical sensors. In this context, we propose a novel conductive filament based on graphite, nickel, and polylactic acid (G/Ni/PLA) for the fabrication of non-enzymatic electrochemical sensors aimed at glucose (GLU) determination, a key biomarker in diabetes diagnosis. The materials were thoroughly characterized using morphological, structural, elemental, and electrochemical techniques, which confirmed the effective incorporation of G and Ni into the thermoplastic matrix. Special emphasis was placed on the electrochemical conversion of Ni2⁺ in an alkaline medium (0.1 mol L⁻1 NaOH) into redox-active species (Ni(OH)₂ and NiOOH), which mediate the electrocatalytic oxidation of GLU. Additionally, the influence of varying nickel contents (7.5 %, 10 %, and 12.5 % wt.) on the electrochemical response towards GLU was systematically investigated, with the best performance observed at the highest nickel loading. The innovative 3D-printed G/Ni/PLA sensor was integrated with a batch injection analysis (BIA) system for rapid and sensitive amperometric detection of GLU in artificial biological fluids. The sensor demonstrated a wide linear range (50-1500 μmol L⁻1), a low detection limit (2.6 μmol L⁻1), excellent repeatability (RSD < 9.0 %), and high selectivity, even in the presence of potential interferents such as urea, uric acid, and ascorbic acid. Furthermore, the method was successfully applied to analyze synthetic saliva (a non-invasive sample matrix) and blood plasma under normal and abnormal GLU levels, achieving satisfactory recovery rates ranging from 93 % to 100 %. Therefore, the proposed analytical approach is simple, selective, precise, and accurate, making it highly suitable for non-enzymatic GLU sensing in clinical samples, contributing to the effective diagnosis of diabetes.

Transfer Contrastive Learning for Raman Spectroscopy Skin Cancer Tissue Classification

Using Raman spectroscopy (RS) signals for skin cancer tissue classification has recently drawn significant attention, because of its non-invasive optical technique, which uses molecular structures and conformations within biological tissue for diagnosis. In reality, RS signals are noisy and unstable for training machine learning models. The scarcity of tissue samples also makes it challenging to learn reliable deep-learning networks for clinical usages. In this paper, we advocate a Transfer Contrasting Learning Paradigm (TCLP) to address the scarcity and noisy characteristics of the RS for skin cancer tissue classification. To overcome the challenge of limited samples, TCLP leverages transfer learning to pre-train deep learning models using RS data from similar domains (but collected from different RS equipments for other tasks). To tackle the noisy nature of the RS signals, TCLP uses contrastive learning to augment RS signals to learn reliable feature representation to represent RS signals for final classification. Experiments and comparisons, including statistical tests, demonstrate that TCLP outperforms existing deep learning baselines for RS signal-based skin cancer tissue classification.

ZnO nanorods grown on Cu wire mesh provide a high sensitivity non-enzymatic absorbance glucose sensor

A highly sensitive non-enzymatic absorption-based glucose sensor is introduced that combines ZnO nanorods with the ferrous oxidation-xylenol orange (FOX) assay. ZnO nanorods were successfully synthesized on the surface of a copper wire mesh, exhibiting high crystallinity, purity, and a large surface area. The glucose sensor displays a high sensitivity of 0.394 mM-1 in a wide linear detection range (0 - 6 mM), along with a low limit of detection of 0.25 mM. The proposed assay has an excellent selectivity towards glucose compared with other sugars such as sucrose, maltose, and fructose. The potential use of the non-enzymatic absorbance sensor for diabetes monitoring is demonstrated by measuring the glucose concentration in human blood serum, obtaining values that are consistent with clinical analysis.

Reciprocal polarization imaging of optical activity in reflection

We present reciprocal polarization imaging for the optical activity of chiral media in reflection geometry. The method is based on the reciprocal polar decomposition of backscattering Mueller matrices accounting for the reciprocity of light waves in forward and backward scattering paths. Anisotropic depolarization is introduced to gain sensitivity to optical activity in backscattering. Experiments with glucose solutions show that while the Lu-Chipman decomposition of the backscattering Mueller matrices produces erroneous results, reciprocal polarization imaging correctly retrieves the optical activity of chiral media. The recovered optical rotation agrees with that obtained in the forward geometry and increases linearly with the concentration and thickness of the chiral media. The potential for in vivo glucose monitoring based on optical activity sensing using reciprocal polarization imaging is then discussed.

Wireless Battery-free and Fully Implantable Organ Interfaces

Advances in soft materials, miniaturized electronics, sensors, stimulators, radios, and battery-free power supplies are resulting in a new generation of fully implantable organ interfaces that leverage volumetric reduction and soft mechanics by eliminating electrochemical power storage. This device class offers the ability to provide high-fidelity readouts of physiological processes, enables stimulation, and allows control over organs to realize new therapeutic and diagnostic paradigms. Driven by seamless integration with connected infrastructure, these devices enable personalized digital medicine. Key to advances are carefully designed material, electrophysical, electrochemical, and electromagnetic systems that form implantables with mechanical properties closely matched to the target organ to deliver functionality that supports high-fidelity sensors and stimulators. The elimination of electrochemical power supplies enables control over device operation, anywhere from acute, to lifetimes matching the target subject with physical dimensions that supports imperceptible operation. This review provides a comprehensive overview of the basic building blocks of battery-free organ interfaces and related topics such as implantation, delivery, sterilization, and user acceptance. State of the art examples categorized by organ system and an outlook of interconnection and advanced strategies for computation leveraging the consistent power influx to elevate functionality of this device class over current battery-powered strategies is highlighted.

Non-contact, highly sensitive sugar concentration detection based on Co3Sn2S2 Weyl semimetal thin film sensor by terahertz wave

Blood sugar is an important biomedical parameter of diabetic patients. The current blood sugar testing is based on an invasive method, which is not very friendly for patients who require long-term monitoring, while the non-invasive method is still in the developing stage. In this paper, we design a non-invasive and highly sensitive terahertz wave detector with Co3Sn2S2 semimetal thin film to test sugar concentration. As different concentrations have inconsistent responses to terahertz wave, we can deduce the concentration of the sugar solution to realize real-time highly sensitive detection of blood sugar concentration. This novel method can be further expanded to 6 G edge intelligence for non-invasive and real-time monitoring of blood sugar, and promote the development of 6 G technology.

Sugar Detection in Aqueous Solution Using an SMS Fiber Device

We report on the fabrication and testing of a fiber optics sensor based on multimodal interference effects, which aims at the detection of different types of sweeteners dissolved in water. The device, which has a simple structure, commonly known as the SMS configuration, is built by splicing a segment of commercial-grade, coreless multimode fiber (NC-MMF) between two standard single-mode fibers (SMFs). In this configuration, the evanescent field traveling outside the core of the NC-MMF allows the sensing of the refractive index of the surrounding media, making it possible to detect different levels of sugar concentration. The optical sensor was tested with aqueous solutions of glucose, fructose, and sucrose in the concentration range from 0 wt% to 20 wt% at room temperature. The proposed device exhibits a linear response with a sensitivity of 0.1835 nm/wt% for sucrose, 0.1687 nm/wt% for fructose, and 0.1694 nm/wt% for glucose, respectively, with a sensing resolution of around 0.5 wt%. Finally, we show that, despite having similar concentration behavior, some degree of discrimination between the different sugars can be achieved by assessing their thermo-optical response.

A Sequential Electrospinning of a Coaxial and Blending Process for Creating Double-Layer Hybrid Films to Sense Glucose

This study presents a glucose biosensor based on electrospun core-sheath nanofibers. Two types of film were fabricated using different electrospinning procedures. Film F1 was composed solely of core-sheath nanofibers fabricated using a modified coaxial electrospinning process. Film F2 was a double-layer hybrid film fabricated through a sequential electrospinning and blending process. The bottom layer of F2 comprised core-sheath nanofibers fabricated using a modified process, in which pure polymethacrylate type A (Eudragit L100) was used as the core section and water-soluble lignin (WSL) and phenol were loaded as the sheath section. The top layer of F2 contained glucose oxidase (GOx) and gold nanoparticles, which were distributed throughout the polyvinylpyrrolidone K90 (PVP K90) nanofibers through a single-fluid blending electrospinning process. The study investigated the sequential electrospinning process in detail. The experimental results demonstrated that the F2 hybrid film had a higher degradation efficiency of β-D-glucose than F1, reaching a maximum of over 70% after 12 h within the concentration range of 10-40 mmol/L. The hybrid film F2 is used for colorimetric sensing of β-D-glucose in the range of 1-15 mmol/L. The solution exhibited a color that deepened gradually with an increase in β-D-glucose concentration. Electrospinning is flexible in creating structures for bio-cascade reactions, and the double-layer hybrid film can provide a simple template for developing other sensing nanomaterials.

In situ growth of the CoO nanoneedle array on a 3D nickel foam toward a high-performance glucose sensor

A glucose sensor with high sensitivity and low detection limit is vital for human beings' health. Herein, a CoO nanoneedle array with an unique electronic structure was successfully constructed by a hydrothermal and subsequent high-temperature calcination process. The optimized CoO-400 nanoneedles exhibit a larger electrochemical active surface area, beneficial electronic structure, favorable lattice distortion, and abundant active sites, which effectively promote electrochemical properties toward glucose sensing. The glucose sensor constructed by CoO-400 nanoneedles shows a high sensitivity of 84.23 mA cm^-2 mM^-1 and low detection limit of 4.4 × 10^-7 M, superior to the results from most previous reports. Moreover, outstanding anti-interference ability, superior long-term stability, good repeatability, and satisfactory reproducibility in glucose detection for CoO-400 nanoneedles are also demonstrated.

Broadband Mueller ellipsometer as an all-in-one tool for spectral and temporal analysis of mutarotation kinetics

Spectroscopic Mueller matrix ellipsometry is becoming increasingly routine across physical branches of science, even outside optics. The highly sensitive tracking of the polarization-related physical properties offers a reliable and non-destructive analysis of virtually any sample at hand. If coupled with a physical model, it is impeccable in performance and irreplaceable in versatility. Nonetheless, this method is rarely adopted interdisciplinarily, and when it is, it often plays a supporting role, which does not take benefit of its full potential. To bridge this gap, we present Mueller matrix ellipsometry in the context of chiroptical spectroscopy. In this work, we utilize a commercial broadband Mueller ellipsometer to analyze the optical activity of a saccharides solution. We verify the correctness of the method in the first place by studying the well-known rotatory power of glucose, fructose, and sucrose. By employing a physically meaningful dispersion model, we obtain 2π-unwrapped absolute specific rotations. Besides that, we demonstrate the capability of tracing the glucose mutarotation kinetics from just one set of measurements. Coupling the Mueller matrix ellipsometry with the proposed dispersion model ultimately leads to the precisely determined mutarotation rate constants and spectrally and temporally resolved gyration tensor of individual glucose anomers. In this view, Mueller matrix ellipsometry may stand as an offbeat yet equal technique to those considered classical chiroptical spectroscopy techniques, which may help open new opportunities for broader polarimetric applications in biomedicine and chemistry.

Silicon-photonics-based waveguide Bragg grating sensor for blood glucose monitoring

We demonstrated the design of two different structures, a two-sided structure and a top-surface structure, of glucose waveguide Bragg grating (WBG) sensors in a single-mode silicon-on-insulator (SOI) chip. A two-sided WBG structure was fabricated, and chip preparation was realized by lithography and other processes. A photonic platform for testing the two-sided WBG using glucose was built and completed. When the blood glucose concentration changed by 1 mg/mL, the two-sided WBG had a wavelength offset of 78 pm. The experimental results show that the two structures can achieve the sensing of different blood glucose concentrations. The two-sided WBG had better sensing performance and thus has a wide range of application prospects.

A Dual-Frequency Terahertz Metasurface Capable of Distinguishing the Handedness of Circularly Polarized Light

Circularly polarized light can present more optical properties of chiral materials and is widely used to analyze and detect biomolecules. In this paper, a dual-frequency terahertz circular polarization detection structure, which is based on multilayer metamaterials, is proposed. The proposed structure consists of a dual-frequency quarter-wave plate, a polyimide spacer, and a filter. The simulation results show that the structure can distinguish the handedness of circularly polarized light by filtering. The extinction ratios are 4 dB and 5.26 dB at 0.952 THz and 1.03 THz, respectively, and the maximum transmittance efficiency reaches 40%. Given the advantages of easy integration and dual-frequency operation, our design is bound to facilitate the development of multi-frequency detection in biomedical imaging devices.

Electrochemical Sensing of Glucose Using Glucose Oxidase/PEDOT:4-Sulfocalix [4]arene/MXene Composite Modified Electrode

Glucose is one of the most important monosaccharides found in the food, as a part of more complex structures, which is a primary energy source for the brain and body. Thus, the monitoring of glucose concentration is more important in food and biological samples in order to maintain a healthy lifestyle. Herein, an electrochemical glucose biosensor was fabricated by immobilization of glucose oxidase (GOX) onto poly(3,4-ethylenedioxythiophene):4-sulfocalix [4]arene (PEDOT:SCX)/MXene modified electrode. For this purpose, firstly, PEDOT was synthesized in the presence of SCX (counterion) by the chemical oxidative method. Secondly, MXene (a 2D layered material) was synthesized by using a high-temperature furnace under a nitrogen atmosphere. After that, PEDOT:SCX/MXene (1:1) dispersion was prepared by ultrasonication which was later utilized to prepare PEDOT:SCX/MXene hybrid film. A successful formation of PEDOT:SCX/MXene film was confirmed by HR-SEM, Fourier transform infrared (FT-IR), and Raman spectroscopies. Due to the biocompatibility nature, successful immobilization of GOX was carried out onto chitosan modified PEDOT:SCX/MXene/GCE. Moreover, the electrochemical properties of PEDOT:SCX/MXene/GOX/GCE was studied through cyclic voltammetry and amperometry methods. Interestingly, a stable redox peak of FAD-GOX was observed at a formal potential of –0.435 V on PEDOT:SCX/MXene/GOX/GCE which indicated a direct electron transfer between the enzyme and the electrode surface. PEDOT:SCX/MXene/GOX/GCE also exhibited a linear response against glucose concentrations in the linear range from 0.5 to 8 mM. The effect of pH, sensors reproducibility, and repeatability of the PEDOT:SCX/MXene/GOX/GCE sensor were studied. Finally, this new biosensor was successfully applied to detect glucose in commercial fruit juice sample with satisfactory recovery.

Silicone-containing thermoresponsive membranes to form an optical glucose biosensor

Glucose biosensors that could be subcutaneously injected and interrogated without a physically connected electrode and transmitter affixed to skin would represent a major advancement in reducing the user burden of...

Exposure to GenX and its novel analogs disrupts fatty acid metabolism in male mice

Perfluoroalkyl ether carboxylic acids (PFECAs), including hexafluoropropylene oxide dimer acid (HFPO-DA, GenX), have been widely used as alternatives to perfluorooctanoic acid (PFOA) and subsequently detected in various environmental matrices. Despite this, public information regarding their hepatotoxicity remains limited. Here, to compare the hepatotoxicity of PFECAs and identify better alternatives for GenX, adult male mice were exposed to different concentrations (0.4, 2, and 10 mg/kg/d) of PFOA, GenX, and its analogs (PFMO2HpA and PFMO3NA) for 28 d. Results demonstrated increased hepatomegaly and disturbed fatty acid metabolism with increasing treatment doses. After dimensionality reduction analysis, significant differences were observed in the relative liver weights and liver and serum biochemical parameters among the four clusters. Furthermore, when chemical concentrations in the liver were similar, no differences in the indicators of liver injury associated with fatty acid metabolism were observed among groups in the same clusters. Our results suggest that dimensionality reduction analysis is a useful strategy for analyzing samples exposed to multiple compounds at different doses. Furthermore, PFECAs exhibit similar hepatotoxicities at the same cumulative hepatic concentration in mice with constant body weight, while PFMO2HpA exhibits lower hepatotoxicity compared to GenX at the same dose.

Permittivity-Inspired Microwave Resonator-Based Biosensor Based on Integrated Passive Device Technology for Glucose Identification

In this study, we propose a high-performance resonator-based biosensor for mediator-free glucose identification. The biosensor is characterized by an air-bridge capacitor and fabricated via integrated passive device technology on gallium arsenide (GaAs) substrate. The exterior design of the structure is a spiral inductor with the air-bridge providing a sensitive surface, whereas the internal capacitor improves indicator performance. The sensing relies on repolarization and rearrangement of surface molecules, which are excited by the dropped sample at the microcosmic level, and the resonance performance variation corresponds to the difference in glucose concentration at the macroscopic level. The air-bridge capacitor in the modeled RLC circuit serves as a bio-recognition element to glucose concentration (εglucoseC0), generating resonant frequency shifts at 0.874 GHz and 1.244 GHz for concentrations of 25 mg/dL and 300 mg/dL compared to DI water, respectively. The proposed biosensor exhibits excellent sensitivity at 1.38 MHz per mg/dL with a wide detection range for glucose concentrations of 25-300 mg/dL and a low detection limit of 24.59 mg/dL. Additionally, the frequency shift and concentration are highly linear with a coefficient of determination of 0.98823. The response time is less than 3 s. We performed multiple experiments to verify that the surface morphology reveals no deterioration and chemical binding, thus validating the reusability and reliability of the proposed biosensor.

Metal-Organic-Framework- and MXene-Based Taste Sensors and Glucose Detection

Taste sensors can identify various tastes, including saltiness, bitterness, sweetness, sourness, and umami, and have been useful in the food and beverage industry. Metal-organic frameworks (MOFs) and MXenes have recently received considerable attention for the fabrication of high-performance biosensors owing to their large surface area, high ion transfer ability, adjustable chemical structure. Notably, MOFs with large surface areas, tunable chemical structures, and high stability have been explored in various applications, whereas MXenes with good conductivity, excellent ion-transport characteristics, and ease of modification have exhibited great potential in biochemical sensing. This review first outlines the importance of taste sensors, their operation mechanism, and measuring methods in sensing utilization. Then, recent studies focusing on MOFs and MXenes for the detection of different tastes are discussed. Finally, future directions for biomimetic tongues based on MOFs and MXenes are discussed.

Optical Glucose Sensor Using Pressure Sensitive Paint

A glucose sensor is used as an essential tool for diagnosing and treating diabetic patients and controlling processes during cell culture. Since the development of an electrochemical-based glucose sensor, an optical glucose sensor has been devised to overcome its shortcomings, but this also poses a problem because it requires a complicated manufacturing process. This study aimed to develop an optical glucose sensor film that could be fabricated with a simple process using commercial pressure sensitive paints. The sensor manufacturing technology developed in this work could simplify the complex production process of the existing electrochemical or optical glucose sensors. In addition, a photometric method for glucose concentration analysis was developed using the color image of the sensor. By developing this sensor and analysis technology, the basis for glucose measurement was established that enables two-dimensional, online, and continuous measurement. The proposed sensor showed good linearity at 0–4 mM glucose in an aqueous sample solution, its limit of detection was 0.37 mM, and the response time was 2 min.

Molecular Boronic Acid-Based Saccharide Sensors

Boronic acids can reversibly bind diols, a molecular feature that is ubiquitous within saccharides, leading to their use in the design and implementation of sensors for numerous saccharide species. There is a growing understanding of the importance of saccharides in many biological processes and systems; while saccharide or carbohydrate sensing in medicine is most often associated with detection of glucose in diabetes patients, saccharides have proven to be relevant in a range of disease states. Herein the relevance of carbohydrate sensing for biomedical applications is explored, and this review seeks to outline how the complexity of saccharides presents a challenge for the development of selective sensors and describes efforts that have been made to understand the underpinning fluorescence and binding mechanisms of these systems, before outlining examples of how researchers have used this knowledge to develop ever more selective receptors.

Measuring glucose concentration in a solution based on the indices of polarimetric purity

Polarization imaging is a powerful tool, which can be applied in biomedical diagnosis and many research fields. Here, we propose a new application of the indices of polarimetric purity (IPPs) composed of P1, P2, P3, to describe the glucose concentrations (GC) changes in the scattering system. The results suggest that P1 of the IPPs is a better indicator to GC in the solution than the degree of polarization (DoP) for the forward scattering detection. Meanwhile, the fitting relation among radius of scattering particle, GCs and P1 parameter has also been calculated, in which the error of inversion is no more than 4.73%. In the backscattering detection, the fitted frequency statistical histogram of the IPPs is used to measure the GCs, and their modes can represent changing trend of GCs.

Non-Invasive Blood Glucose Monitoring Technology: A Review

In recent years, with the rise of global diabetes, a growing number of subjects are suffering from pain and infections caused by the invasive nature of mainstream commercial glucose meters. Non-invasive blood glucose monitoring technology has become an international research topic and a new method which could bring relief to a vast number of patients. This paper reviews the research progress and major challenges of non-invasive blood glucose detection technology in recent years, and divides it into three categories: optics, microwave and electrochemistry, based on the detection principle. The technology covers medical, materials, optics, electromagnetic wave, chemistry, biology, computational science and other related fields. The advantages and limitations of non-invasive and invasive technologies as well as electrochemistry and optics in non-invasives are compared horizontally in this paper. In addition, the current research achievements and limitations of non-invasive electrochemical glucose sensing systems in continuous monitoring, point-of-care and clinical settings are highlighted, so as to discuss the development tendency in future research. With the rapid development of wearable technology and transdermal biosensors, non-invasive blood glucose monitoring will become more efficient, affordable, robust, and more competitive on the market.

Review of Non-invasive Glucose Sensing Techniques: Optical, Electrical and Breath Acetone

Annual deaths in the U.S. attributed to diabetes are expected to increase from 280,210 in 2015 to 385,840 in 2030. The increase in the number of people affected by diabetes has made it one of the major public health challenges around the world. Better management of diabetes has the potential to decrease yearly medical costs and deaths associated with the disease. Non-invasive methods are in high demand to take the place of the traditional finger prick method as they can facilitate continuous glucose monitoring. Research groups have been trying for decades to develop functional commercial non-invasive glucose measurement devices. The challenges associated with non-invasive glucose monitoring are the many factors that contribute to inaccurate readings. We identify and address the experimental and physiological challenges and provide recommendations to pave the way for a systematic pathway to a solution. We have reviewed and categorized non-invasive glucose measurement methods based on: (1) the intrinsic properties of glucose, (2) blood/tissue properties and (3) breath acetone analysis. This approach highlights potential critical commonalities among the challenges that act as barriers to future progress. The focus here is on the pertinent physiological aspects, remaining challenges, recent advancements and the sensors that have reached acceptable clinical accuracy.

Modern noninvasive methods for monitoring glucose levels in patients: a review

This paper presents the current state of the art of noninvasive glucose monitoring. In recent years, we can observe constant increase in the incidence of diabetes. About 40% of all performed blood tests apply to the glucose tests. Formerly, this lifestyle disease occurred mainly in rich countries, but now it is becoming more common in poorer countries. It is related to the increase in life expectancy, unhealthy diet, lack of exercise, and other factors. Untreated diabetes may cause many complications or even death. For this reason, daily control of glucose levels in people with this disorder is very important. Measurements with a traditional glucometer are connected with performing finger punctures several times a day, which is painful and uncomfortable for patients. Therefore, researches on other methods are ongoing. A method that would be fast, noninvasive and cheap could also enable testing the state of the entire population, which is necessary because of the number of people currently living with undiagnosed type 2 diabetes. Although the first glucometer was made in 1966, the first studies on glucose level measurement in tear film were documented as early as 1937. This shows how much a noninvasive method of diabetes control is needed. Since then, there have been more and more studies on alternative methods of glucose measurement, not only from tear fluid, but also from saliva, sweat, or transdermally.

Noninvasive glucose detection in exhaled breath condensate

Two-thirds of patients with diabetes avoid regularly monitoring their blood glucose levels because of the painful and invasive nature of current blood glucose detection. As an alternative to blood sample collection, exhaled breath condensate (EBC) has emerged as a promising noninvasive sample from which to monitor glucose levels. However, this dilute sample matrix requires sensors capable of detecting glucose with high resolution at nanomolar and micromolar concentrations. Recent developments in EBC collection methods and highly sensitive glucose biosensors provide a path toward enabling robust and sensitive glucose detection in EBC. This review addresses current and emerging EBC collection and glucose sensing modalities capable of quantifying glucose in EBC samples. We highlight the opportunities and challenges for development and integration of EBC glucose detection systems that will enable clinically robust and accurate EBC glucose measurements for improved glycemic control.

Methods for Optical Skin Clearing in Molecular Optical Imaging in Dermatology

This short review describes recent progress in using optical clearing (OC) technique in skin studies. Optical clearing is an efficient tool for enhancing the probing depth and data quality in multiphoton microscopy and Raman spectroscopy. Here, we discuss the main mechanisms of OC, its safety, advantages, and limitations. The data on the OC effect on the skin water content are presented. It was demonstrated that 70% glycerol and 100% Omnipaque^TM 300 reduce the water content in the skin. Both OC agents (OCAs) significantly affect the strongly bound and weakly bound water. However, Omnipaque^TM 300 causes considerably less skin dehydration than glycerol. In addition, the results of examination of the OC effect on autofluorescence in two-photon excitation and background fluorescence in Raman scattering at different skin depths are presented. It is shown that Omnipaque^TM 300 is a promising OCA due to its ability to reduce background fluorescence in the upper skin layers. The possibility of multimodal imaging combining optical methods and OC technique is discussed.

Glucose Oxidase-Instructed Fluorescence Amplification Strategy for Intracellular Glucose Detection

The accurate detection of glucose at cellular level remains a big challenge. In this study, a signal amplification strategy mediated by silver nanocube (AgNC), glucose oxidase (GOx), and silver ion fluorescence probe (denoted as AgNC-GOx/Ag⁺-FP) is proposed for amplified intracellular glucose detection. The AgNC is oxidized into Ag⁺ by H₂O₂ generated from GOx-catalyzed glucose oxidation reaction, and Ag⁺ remarkably enhances the red fluorescence of Ag⁺-FP. Our results show that AgNC-GOx/Ag⁺-FP is highly sensitive and specific to glucose and H₂O₂. Afterward, the feasibility of using AgNC-GOx/Ag⁺-FP to detect intracellular glucose is verified in five different cell lines. In summary, a sensitive and specific fluorescence amplification strategy has been developed for intracellular glucose detection.

Highly Responsive and Ultrasensitive Glucose Sensor Based on Au Foam

Glucose concentration is an important physiological index, therefore methods for sensitive detection of glucose are important. In this study, Au foam was prepared by electrodeposition with a dynamic gas template on an Au nanoparticle/Si substrate. The Au foam showed ultrasensitivity, high selectivity, and long-term stability in the quantitative detection of glucose. The foam was used as an electrode, and the amperometric response indicated excellent catalytic activity in glucose oxidation, with a linear response across the concentration range 0.5 μM to 12 mM, and a limit of detection of 0.14 μM. High selectivity for interfering molecules at six times the normal level and long-term stability for 30 days were obtained. The results for electrochemical detection with Au foam of glucose in human serum were consistent with those obtained with a sensor based on surface-enhanced Raman spectroscopy and a commercial sensor. This proves that this method can be used with real samples. These results show that Au foam has great potential for use as a non-enzymatic glucose sensor.

Emerging Applications of Optical Bio-Sensors

In the simplest words, a bio-sensor is an analytic device. In recent years, bio-sensors have shown emerging contribution in medical diagnosis, drug discovery, and treatment process. In this regards, continuous research is ongoing and many more features are being added in the sensing technologies. Optical sensing technology is no more bound in research area but also in the commercial use for the betterment of mankind. There are different types of bio-sensors particularly optical which have already been developed and research is going to expand many more of them. Sensing applications are not limited in glucose, DNA, cancer cell detection, drug discovery, immunological, Hepatitis B virus, and enzyme detection but also many more development is knocking at the door. Therefore, this review paper is focused on the applications and functions of bio-sensors (especially optical) in medical diagnostics and treatment.

Target-responsive DNA hydrogel for non-enzymatic and visual detection of glucose

We have successfully developed a target-responsive aptamer cross-linked hydrogel for the visual detection of glucose, an important biomedical analyte. In this work, the glucose-responsive hydrogel was prepared using the target aptamer and its two short complementary DNA strands grafted onto a linear polyacrylamide chain as cross-linkers. Gold nanoparticles (AuNPs) modified with thiol-PEG were encapsulated in the gel and used as the output signal for visible detection. The complex of glucose and its ligand of boronic acid derivatives (Shinkai's receptor) can bind with the aptamer to disrupt the hydrogel, leading to the release of AuNPs with a distinct red colour in the supernatant. By this method glucose can be detected with the naked eye, and the sensor has a detection limit of 0.44 mM in buffer with the help of UV-Vis spectrophotometry. Furthermore, glucose spiked in 50% urine and 30% serum could also be detected respectively with the naked eye, and glucose was quantitatively detected in 50% urine. The hydrogel system provides a non-enzymatic and visual method for glucose detection, and offers promising applications in biotechnology and biomedicine.

A Layer-by-Layer Approach To Retain a Fluorescent Glucose Sensing Assay within the Cavity of a Hydrogel Membrane

A continuous glucose monitoring device that resides fully in the subcutaneous tissue has the potential to greatly improve the management of diabetes. Toward this goal, we have developed a competitive binding glucose sensing assay based on fluorescently labeled PEGylated concanavalin-A (PEGylated-TRITC-ConA) and mannotetraose (APTS-MT). In the present work, we sought to contain this assay within the hollow central cavity of a cylindrical hydrogel membrane, permitting eventual subcutaneous implantation and optical probing through the skin. A "self-cleaning" hydrogel was utilized because of its ability to cyclically deswell/reswell in vivo, which is expected to reduce biofouling and therefore extend the sensor lifetime. Thus, we prepared a hollow, cylindrical hydrogel based on a thermoresponsive electrostatic double network design composed of N-isopropylacrylamide and 2-acrylamido-2-methylpropanesulfonic acid. Next, a layer-by-layer (LbL) coating was applied to the inner wall of the central cavity of the cylindrical membrane. It consisted of 5, 10, 15, 30, or 40 alternating bilayers of positively charged poly(diallyldimethylammonium chloride) and negatively charged poly(sodium 4-styrenesulfonate). With 30 bilayers, the leaching of the smaller-sized component of the assay (APTS-MT) from the membrane cavity was substantially reduced. Moreover, this LbL coating maintained glucose diffusion across the hydrogel membrane. In terms of sensor functionality, the assay housed in the hydrogel membrane cavity tracked changes in glucose concentration (0 to 600 mg/dL) with a mean absolute relative difference of ∼11%.

Green synthesis of copper oxide nanoparticles decorated reduced graphene oxide for high sensitive detection of glucose

A non-enzymatic glucose sensor based on pencil graphite electrode (PGE) modified by copper oxide nanoparticles decorated reduced graphene oxide (CuO(NP)/rGO-PGE) was prepared. XRD patterns showed partially electrochemically reduction of GO and monoclinic structure of CuO on the PGE. The prepared CuO(NP)/rGO exhibited a nanoporous structure by scanning electron microscopy (SEM). Transmittance electron microscopy (TEM) revealed copper oxide nanoparticles were well distributed on rGO and had semispherical shapes with diameter 3-5 nm. Cyclic voltammetry at CuO(NP)/rGO-PGE showed the immobilized CuO(NP)s were highly stable in alkaline solutions and had high electrocatalytic activity toward glucose oxidation. Using amperometry, the detection limit of [0.091 (±0.003) μM] and concentration sensitivity of [4760 (±3.2) μA mM^-1 cm^-2] for glucose was obtained at optimum conditions. The applicability of the sensor was evaluated to determine the glucose concentration in human blood serum samples and the experimental results were comparable with those measured by traditional spectrophotometric methods. The preparation of CuO(NP)/rGO-PGE was reproducible, very simple, fast and inexpensive for practical application.

Gaussian probe beam with high spherical aberration for glucose concentration measurement

We demonstrate that an optical probe beam with high spherical aberration used for glucose concentration measurements gives better sensitivity compared to a probe beam free of aberrations, under similar conditions. We place a singlet focusing lens at a large distance from a laser source with a Gaussian intensity profile to obtain a spherically aberrated probe beam with negligible truncation. The aberrated probe beam propagates through a transparent liquid sample. Intensity profiles of the transmitted beam are recorded by means of a homodyne profiler to perform the glucose concentration measurements accurately.

Recent progress in tissue optical clearing for spectroscopic application

This paper aims to review recent progress in optical clearing of the skin and over naturally turbid biological tissues and blood using this technique in vivo and in vitro with multiphoton microscopy, confocal Raman microscopy, confocal microscopy, NIR spectroscopy, optical coherence tomography, and laser speckle contrast imaging. Basic principles of the technique, its safety, advantages and limitations are discussed. The application of optical clearing agent on a tissue allows for controlling the optical properties of tissue. Optical clearing-induced reduction of tissue scattering significantly facilitates the observation of deep-located tissue regions, at the same time improving the resolution and image contrast for a variety of optical imaging methods suitable for clinical applications, such as diagnostics and laser treatment of skin diseases, mucosal tumor imaging, laser disruption of pathological abnormalities, etc.

Toward the Development of a Glucose Dehydrogenase-Based Saliva Glucose Sensor Without the Need for Sample Preparation

BACKGROUND

Strict glycemic control is known to be a vital key in the management of diabetes mellitus (DM). However, traditional methods face limitations in their efficacy due to the pain and invasiveness of needle pricking which often discourages DM patients from performing the required number of tests per day. Saliva glucose (SG) sensing has long been considered a noninvasive alternative to blood glucose monitoring for diabetes management, however the sample preparation and sensor detection limit have been deemed as challenges to overcome. Herein, we describe a preliminary clinical validation of a disposable SG sensor without any requirement for sample preparation.

METHODS

The sensor utilizes glucose dehydrogenase flavine-adenine dinucleotide (GDH-FAD) in conjunction with disposable screen printed electrodes to measure glucose levels in saliva collected directly from 9 healthy subjects. Cyclic voltammetry and amperometric-time (Amp-it) assays were used to develop calibration curves and test subjects. Sensor calibration was performed using simulated saliva at 6.5 pH and 37ºC.

RESULTS

The lower limit of detection was determined to be 0.11 mg/dL. A lag time of 15 minutes with a positive correlation between SG and BG levels was found, which agrees with literature results. The detected SG ranges from 2.38 to 3.40 mg/dL over a BG range of 90 to 143 mg/dL.

CONCLUSION

This is the first reported use of measuring SG with GDH-FAD without prior sample preparation. Upon optimization, the sensor has the potential to serve as a supplement to blood glucose monitoring.

High Circular Polarization of Electroluminescence Achieved via Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films

We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor-acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices.

Glucose sensing in oral mucosa simulating phantom using differential absorption based frequency domain low-coherence interferometry

The superluminescent diode based differential absorption frequency domain low-coherence interferometry (FD-DALCI) technique is proposed and demonstrated for sensing physiological concentrations of glucose (0-250 mg/dl) in oral mucosa simulating phantoms (intralipid of concentrations 0.25-0.50%) with wavelengths at 1589 and 1310 nm. The proposed technique allows simultaneous measurements of refractive index based spectral shift and estimation of physiological concentration of glucose in intralipid with scattering characteristics using the differential absorption approach. The sensitivity of the glucose concentration obtained by spectral shift measurement was ≈0.016  nm/(mg/dl), irrespective of the intralipid concentration. The resolution of the glucose level was estimated to be ≈15  mg/dl in 0.25% intralipid and ≈19  mg/dl in 0.5% intralipid using the FD-DALCI technique.

Multispectral Chiral Imaging with a Metalens

The vast majority of biologically active compounds, ranging from amino acids to essential nutrients such as glucose, possess intrinsic handedness. This in turn gives rise to chiral optical properties that provide a basis for detecting and quantifying enantio-specific concentrations of these molecules. However, traditional chiroptical spectroscopy and imaging techniques require cascading of multiple optical components in sophisticated setups. Here, we present a planar lens with an engineered dispersive response, which simultaneously forms two images with opposite helicity of an object within the same field-of-view. In this way, chiroptical properties can be probed across the visible spectrum using only the lens and a camera without the addition of polarizers or dispersive optical devices. We map the circular dichroism of the exoskeleton of a chiral beetle, Chrysina gloriosa, which is known to exhibit high reflectivity of left-circularly polarized light, with high spatial resolution limited by the numerical aperture of the planar lens. Our results demonstrate the potential of metasurfaces in realizing a compact and multifunctional device with unprecedented imaging capabilities.

Behavior of long-period measurements using a small-sized photoacoustic cell for aqueous glucose monitoring

Reliable, noninvasive glucose-monitoring devices are not currently available. From the patient's point of view, it is necessary that glucose-monitoring devices are portable as well as noninvasive. In photoacoustic spectroscopy (PAS), the PA signal induced by the irradiation of the sample with modulated light depends on the optical absorption coefficient of the sample. Unlike the sensitivity of conventional absorption spectroscopy, the sensitivity of PAS scales inversely with the dimensions. An external laser (wavelength of 1550 nm) and a PA cell with a volume of only 4.0 mm(3) were used for monitoring a glucose solution contained in a special sample reservoir. We present PA measurements of glucose in aqueous solutions using a sample reservoir that is suitable for investigations of liquid samples, such as native capillary blood, by performing a long-period measurement.

Non-invasive blood glucose monitor based on spectroscopy using a smartphone

Development of a novel method for non-invasive measurement of blood glucose concentration using smartphone is discussed. Our research work has three major contributions to society and science. First, we modified and extended the Beer-Lambert's law in physics to accommodate for multiple wavelengths. This extension can aid researchers who wish to perform optical spectroscopy. Second, we successfully developed a creative and non-invasive way for diabetic patients to measure glucose levels via a smartphone. Researchers and chemists can now use their smartphones to determine the absorbance and, therefore, concentration of a chemical. Third, we created an inexpensive way to perform optical spectroscopy by using a smartphone. Monitoring blood glucose using a smartphone application that simply uses equipment already available on smartphones will improve the lives of diabetic patients who can continuously check their blood glucose levels while avoiding the current inconvenient, unhygienic, and costly invasive glucose meters.

Vibrational Imaging of Glucose Uptake Activity in Live Cells and Tissues by Stimulated Raman Scattering

Glucose is a ubiquitous energy source for most living organisms. Its uptake activity closely reflects cellular metabolic demand in various physiopathological conditions. Extensive efforts have been made to specifically image glucose uptake, such as with positron emission tomography, magnetic resonance imaging, and fluorescence microscopy, but all have limitations. A new platform to visualize glucose uptake activity in live cells and tissues is presented that involves performing stimulated Raman scattering on a novel glucose analogue labeled with a small alkyne moiety. Cancer cells with differing metabolic activities can be distinguished. Heterogeneous uptake patterns are observed with clear cell-cell variations in tumor xenograft tissues, neuronal culture, and mouse brain tissues. By offering the distinct advantage of optical resolution but without the undesirable influence of fluorophores, this method will facilitate the study of energy demands of living systems with subcellular resolution.