A comparison of edge- and basal-plane pyrolytic graphite electrodes towards the sensitive determination of hydrocortisone

Electrochemical sensors for cortisol detection: Principles, designs, fabrication, and characterisation

Psychological stress is a major factor contributing to health discrepancies among individuals. Sustained exposure to stress triggers signalling pathways in the brain, which leading to the release of stress hormones in the body. Cortisol, a steroid hormone, is a significant biomarker for stress management due to its responsibility in the body's reply to stress. The release of cortisol in bloodstream prepares the body for a "fight or flight" response by increasing heart rate, blood pressure, metabolism, and suppressing the immune system. Detecting cortisol in biological samples is crucial for understanding its role in stress and personalized healthcare. Traditional techniques for cortisol detection have limitations, prompting researchers to explore alternative strategies. Electrochemical sensing has emerged as a reliable method for point-of-care (POC) cortisol detection. This review focuses on the progress made in electrochemical sensors for cortisol detection, covering their design, principle, and electroanalytical methodologies. The analytical performance of these sensors is also analysed and summarized. Despite significant advancements, the development of electrochemical cortisol sensors faces challenges such as biofouling, sample preparation, sensitivity, flexibility, stability, and recognition layer performance. Therefore, the need to develop more sensitive electrodes and materials is emphasized. Finally, we discussed the potential strategies for electrode design and provides examples of sensing approaches. Moreover, the encounters of translating research into real world applications are addressed.

A novel electrochemical immunosensor based on SnS2/NiCo metal-organic frameworks loaded with gold nanoparticles for cortisol detection

In this work, a label-free electrochemical immunosensor using tin sulfide/nickel cobalt metal-organic frameworks (SnS₂/NiCo MOFs) was established for the sensitive etection of cortisol. First, SnS₂/NiCo MOFs were synthesized by doping SnS₂ with NiCo MOF nanocubes by a hydrothermal method. Then, gold nanoparticles (AuNPs) were grown in situ on SnS₂/NiCo MOFs for electrochemical detection. The use of SnS₂/NiCo MOFs promoted the electron transfer rate of AuNPs and enhanced the electrochemical sensing performance of AuNPs@SnS₂/NiCo MOFs-modified electrodes. The large specific surface area of AuNPs@SnS₂/NiCo MOFs provides more active sites for antibody loading. After the prepared immunosensor was incubated with the target analyte, cortisol, the electron transfer impedance increased and the amperometric response decreased, thus establishing a highly sensitive immunosensing method. The sensor had a linear range of 100 fg/mL to 100 ng/mL and a low detection limit of 29 fg/mL. The sensor showed good accuracy and practicability and could be used for the determination of cortisol in saliva.

Cortisol Monitoring Devices toward Implementation for Clinically Relevant Biosensing In Vivo

Cortisol is a steroid hormone that regulates energy metabolism, stress reactions, and immune response. Cortisol is produced in the kidneys' adrenal cortex. Its levels in the circulatory system are regulated by the neuroendocrine system with a negative feedback loop of the hypothalamic-pituitary-adrenal axis (HPA-axis) following circadian rhythm. Conditions associated with HPA-axis disruption cause deteriorative effects on human life quality in numerous ways. Psychiatric, cardiovascular, and metabolic disorders as well as a variety of inflammatory processes accompanying age-related, orphan, and many other conditions are associated with altered cortisol secretion rates and inadequate responses. Laboratory measurements of cortisol are well-developed and based mainly on the enzyme linked immunosorbent assay (ELISA). There is a great demand for a continuous real-time cortisol sensor that is yet to be developed. Recent advances in approaches that will eventually culminate in such sensors have been summarized in several reviews. This review compares different platforms for direct cortisol measurements in biological fluids. The ways to achieve continuous cortisol measurements are discussed. A cortisol monitoring device will be essential for personified pharmacological correction of the HPA-axis toward normal cortisol levels through a 24-h cycle.

Advances on Hormones and Steroids Determination: A Review of Voltammetric Methods since 2000

This article presents advances in the electrochemical determination of hormones and steroids since 2000. A wide spectrum of techniques and working electrodes have been involved in the reported measurements in order to obtain the lowest possible limits of detection. The voltammetric and polarographic techniques, due to their sensitivity and easiness, could be used as alternatives to other, more complicated, analytical assays. Still, growing interest in designing a new construction of the working electrodes enables us to prepare new measurement procedures and obtain lower limits of detection. A brief description of the measured compounds has been presented, along with a comparison of the obtained results.

Non-enzymatic electrochemical determination of salivary cortisol using ZnO-graphene nanocomposites

Electrochemically deposited ZnO nanoparticles on a pencil graphite electrode (PGE) coated with graphene generate a noteworthy conductive and selective electrochemical sensing electrode for the estimation of cortisol. Electrochemical techniques such as cyclic voltammetry (CV) analysis and electrochemical impedance spectroscopic (EIS) tests were adopted to analyze and understand the nature of the modified sensor. Surface morphological analysis was done using various spectroscopic and microscopic techniques like X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Structural characterization was conducted by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The effect of scan rate, concentration, and cycle numbers was optimized and reported. Differential pulse voltammetric (DPV) analysis reveals that the linear range for the detection of cortisol is 5 × 10⁻¹⁰M − 115 × 10⁻¹⁰ M with a very low-level limit of detection value (0.15 nM). The demonstrated methodology has been excellently functional for the determination of salivary cortisol non-enzymatically at low-level concentration with enhanced selectivity despite the presence of interfering substances.

Electrochemically deposited ZnO nanoparticles on a pencil graphite electrode (PGE) coated with graphene generate a noteworthy conductive and selective electrochemical sensing electrode for the estimation of cortisol.

Electrochemical detection of hydrocortisone using green-synthesized cobalt oxide nanoparticles with nafion-modified glassy carbon electrode

Developing highly sensitive and selective sensors is important for the detection of steroid hormones. Electrochemical sensors are of great interest in this regard. Also utilization of bio-derived substances as an electrode material is environment friendly. In this study, we used green-synthesized cobalt oxide nanoparticles (CoO NPs) along with nafion (Naf) on a glassy carbon electrode to detect hydrocortisone (HC) by voltammetry. Electron microscopy, X-ray diffraction, Raman spectroscopy, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy were used to characterize the CoO NPs prepared using Nigella sativa seeds extract. Cyclic voltammetry and differential pulse voltammetry was utilized for the detection of HC. Only one reduction peak at -0.5 V was observed in the presence of HC in 0.1 M sodium hydroxide, indicating an irreversible electrode process. The Naf-CoO NPs enhanced the active surface area of the glassy carbon electrode (GCE) that resulted in a good response for detecting HC with two linear ranges: 0.001-1 μM and 1-9 μM. In comparison to other published electrochemical sensors, the current sensor displayed a low limit of detection of 0.49 nM, as well as remarkable stability and reproducibility. The sensor exhibited credibility for the sensing of HC in pharmaceutical injections and blood serum samples with recovery percentages ranging from 97.7% to 102.5%. The electrochemical sensor has proved to be valuable for HC detection.

Characterization of graphene/pine wood biochar hybrids: Potential to remove aqueous Cu2

Biochar-based hybrid composites containing added nano-sized phases are emerging adsorbents. Biochar, when functionalized with nanomaterials, can enhance pollutant removal when both the nanophase and the biochar surface act as adsorbents. Three different pine wood wastes (particle size < 0.5 mm, 10 g) were preblended with 1 wt% of three different graphenes in aqueous suspensions, designated as G1, G2, and G3. G1 (S_BET, 8.1 m²/g) was prepared by sonicating graphite made from commercial synthetic graphite powder (particle size 7-11 μm). G2 (312.0 m²/g) and G3 (712.0 m²/g) were purchased commercial graphene nanoplatelets (100 mg in 100 mL deionized water). These three pine wood-graphene mixtures were pyrolyzed at 600 °C for 1 h to generate three graphene-biochar adsorbents, GPBC-1, GPBC-2, and GPBC-3 containing 4.4, 4.9, and 5.0 wt% of G1, G2, and G3 respectively. Aqueous Cu²⁺ adsorption capacities were 10.6 mg/g (GPBC-1), 4.7 mg/g (GPBC-2), and 5.5 mg/g (GPBC-3) versus 7.2 mg/g for raw pine wood biochar (PBC) (0.05 g adsorbent dose, Cu²⁺ 75 mg/L, 25 mL, pH 6, 24 h, 25 ± 0.5 °C). Increased graphene surface areas did not result in adsorption increases. GPBC-1, containing the lowest nanophase surface area with the highest Cu²⁺ capacity, was chosen to evaluate its Cu²⁺ adsorption characteristics further. Results from XPS showed that the surface concentration of oxygenated functional groups on the GPBC-1 is greater than that on the PBC, possibly contributing to its greater Cu²⁺ removal versus PBC. GPBC-1 and PBC uptake of Cu²⁺ followed the pseudo-second-order kinetic model. Langmuir maximum adsorption capacities and BET surface areas were 18.4 mg/g, 484.0 m²/g (GPBC-1) and 9.2 mg/g, 378.0 m²/g (PBC). This corresponds to 3.8 × 10^-2 versus 2.4 × 10^-2 mg/m² of Cu²⁺ removed on GPBC-1 (58% more Cu²⁺ per m²) versus PBC. Cu²⁺ adsorption on both these adsorbents was spontaneous and endothermic.

Edge plane pyrolytic graphite as a sensing surface for the determination of fluvoxamine in urine samples of obsessive-compulsive disorder patients

There is an increasing demand for fast and sensitive determination of antidepressants in human body fluids because of the present scenario of rising depression cases at the global level. A simple and sensitive voltammetric method using edge plane pyrolytic graphite electrode (EPPGE) as a novel sensor is presented for the determination of antidepressant fluvoxamine in urine and blood plasma samples of obsessive-compulsive disorder (OCD) patients. EPPGE is delineated the first time for this determination. EPPGE exhibited strong electrocatalytic activity and enhanced reduction signal towards the sensing of fluvoxamine. Fluvoxamine gave a well-defined reduction peak at ~ - 670 mV using EPPGE. The fluvoxamine reduction peak current was linear to its concentration in the range 5.00 × 10^-9 - 0.1 × 10^-6 mol L^-1 and the limit of detection was found to be 3.5 × 10^-9 mol L^-1. The pre-eminence of EPPGE over mercury electrodes has been proved in terms of sensitivity and imperative analytical parameters. The pH study reveals the involvement of an equal number of electrons and protons in the reduction reaction mechanism. The frequency study indicated the adsorption controlled irreversible reaction mechanism. The stability and reproducibility of the offered sensor were also found most favorable. The interference study confirmed the optimum selectivity of the proposed sensor. The edge plane pyrolytic graphite sensing platform is recommended as a potential contender for the accurate and fast determination of fluvoxamine in depression medications as well as biological specimens of OCD patients.

Voltammetric Pathways for the Analysis of Ophthalmic Drugs

Background:

This review investigates the ophthalmic drugs that have been studied with voltammetry in the web of science database in the last 10 years.

Introduction:

Ophthalmic drugs are used in the diagnosis, evaluation and treatment of various ophthalmological diseases and conditions. A significant literature has emerged in recent years that investigates determination of these active compounds via electroanalytical methods, particularly voltammetry. Low cost, rapid determination, high availability, efficient sensitivity and simple application make voltammetry one of the most used methods for determining various kinds of drugs including ophthalmic ones.

Methods:

In this particular review, we searched the literature via the web of science database for ophthalmic drugs which are investigated with voltammetric techniques using the keywords of voltammetry, electrochemistry, determination and electroanalytical methods.

Results:

We found 33 types of pharmaceuticals in nearly 140 articles. We grouped them clinically into seven major groups as antibiotics, antivirals, non-steroidal anti-inflammatory drugs, anti-glaucomatous drugs, steroidal drugs, local anesthetics and miscellaneous. Voltammetric techniques, electrodes, optimum pHs, peak potentials, limit of detection values, limit of quantification values, linearity ranges, sample type and interference effects were compared.

Conclusion:

Ophthalmic drugs are widely used in the clinic and it is important to determine trace amounts of these species analytically. Voltammetry is a preferred method for its ease of use, high sensitivity, low cost, and high availability for the determination of ophthalmic drugs as well as many other medical drugs. The low limits of detection values indicate that voltammetry is quite sufficient for determining ophthalmic drugs in many media such as human serum, urine and ophthalmic eye drops.

Disposable aptamer-sensor aided by magnetic nanoparticle enrichment for detection of salivary cortisol variations in obstructive sleep apnea patients

We report a disposable point-of-care sensing platform specific to salivary cortisol detection. The sensor is inkjet printed on a paper substrate with a metalloporphyrin based macrocyclic catalyst ink that can electrochemically reduce cortisol, captured by aptamer functionalized magnetic nanoparticles. The sensor consists of a thin magnet disc, aligned at the back of the electrode, in order to populate the magnetic nanoparticle bound cortisol at the sensing electrode area. Proof of concept studies were performed to detect salivary cortisol levels in human subjects with high and low risks for obstructive sleep apnea (OSA). High selectivity was observed to salivary cortisol against a background of closely related steroids.

Shell isolated nanoparticles for enhanced Raman spectroscopy studies in lithium–oxygen cells

A critical and detailed assessment of using Shell Isolated Nanoparticles for Enhanced Raman Spectroscopy (SHINERS) on different electrode substrates was carried out, providing relative enhancement factors, as well as an evaluation of the distribution of shell-isolated nanoparticles upon the electrode surfaces. The chemical makeup of surface layers formed upon lithium metal electrodes and the mechanism of the oxygen reduction reaction on carbon substrates relevant to lithium–oxygen cells are studied with the employment of the SHINERS technique. SHINERS enhanced the Raman signal at these surfaces showing a predominant Li₂O based layer on lithium metal in a variety of electrolytes. The formation of LiO₂and Li₂O₂, as well as degradation reactions forming Li₂CO₃, upon planar carbon electrode interfaces and upon composite carbon black electrodes were followed under potential control during the reduction of oxygen in a non-aqueous electrolyte based on dimethyl sulfoxide.

Substrate selection for fundamental studies of electrocatalysts and photoelectrodes: inert potential windows in acidic, neutral, and basic electrolyte

The selection of an appropriate substrate is an important initial step for many studies of electrochemically active materials. In order to help researchers with the substrate selection process, we employ a consistent experimental methodology to evaluate the electrochemical reactivity and stability of seven potential substrate materials for electrocatalyst and photoelectrode evaluation. Using cyclic voltammetry with a progressively increased scan range, we characterize three transparent conducting oxides (indium tin oxide, fluorine-doped tin oxide, and aluminum-doped zinc oxide) and four opaque conductors (gold, stainless steel 304, glassy carbon, and highly oriented pyrolytic graphite) in three different electrolytes (sulfuric acid, sodium acetate, and sodium hydroxide). We determine the inert potential window for each substrate/electrolyte combination and make recommendations about which materials may be most suitable for application under different experimental conditions. Furthermore, the testing methodology provides a framework for other researchers to evaluate and report the baseline activity of other substrates of interest to the broader community.

Automated-immunosensor with centrifugal fluid valves for salivary cortisol measurement

Point-of-care measurement of the stress hormone cortisol will greatly facilitate the timely diagnosis and management of stress-related disorders. We describe an automated salivary cortisol immunosensor, incorporating centrifugal fluid valves and a disposable disc-chip that allows for truncated reporting of cortisol levels (<15 min). The performance characteristics of the immunosensor are optimized through select blocking agents to prevent the non-specific adsorption of proteins; immunoglobulin G (IgG) polymer for the pad and milk protein for the reservoirs and the flow channels. Incorporated centrifugal fluid valves allow for rapid and repeat washings to remove impurities from the saliva samples. An optical reader and laptop computer automate the immunoassay processes and provide easily accessible digital readouts of salivary cortisol measurements. Linear regression analysis of the calibration curve for the cortisol immunosensor showed 0.92 of coefficient of multiple determination, R², and 38.7% of coefficient of variation, CV, for a range of salivary cortisol concentrations between 0.4 and 11.3 ng/mL. The receiver operating characteristic (ROC) curve analysis of human saliva samples indicate potential utility for discriminating stress disorders and underscore potential application of the biosensor in stress disorders. The performance of our salivary cortisol immunosensor approaches laboratory based tests and allows noninvasive, quantitative, and automated analysis of human salivary cortisol levels with reporting times compatible with point-of-care applications.

Immunosensor with fluid control mechanism for salivary cortisol analysis

The purpose of this research is to demonstrate a new design for a cortisol immunosensor for the noninvasive and quantitative analysis of salivary cortisol. We propose a cortisol immunosensor with a fluid control mechanism which has both a vertical flow and a lateral flow. The detected current resulting from a competitive reaction between the sample cortisol and a glucose oxidase (GOD)-labeled cortisol conjugate was found to be inversely related to the concentration of cortisol in the sample solution. A calibration curve using the relative detected current showed a R(2)=0.98 and CV=14% for a range of standard cortisol solutions corresponding to the concentrations of native salivary cortisol (0.1-10 ng/ml). The measurement could be accomplished within 35 min and the cortisol immunosensor could be reused. These results show promise for realizing an on-site and easy-to-use biosensor for cortisol. Used for evaluation of human salivary cortisol levels, the cortisol immunosensor measurement corresponded closely with commercially available ELISA method (R(2)=0.92). Our results indicate the promise of the new cortisol immunosensor for noninvasive, point of care measurement of human salivary cortisol levels.

Direct detection of uric acid in body fluid samples by using boron doped graphite nano particles as the working electrode

Boron doped graphite nano particles were used as functioning elements for creation of electrodes for the detection of uric acid in biological samples. The electrode obtained in this manner was capable of oxidizing ascorbic acid at lower potentials. This provided a desirable solution to the interfering problem encountered in the detection of uric acid in biological samples caused by ascorbic acid. The detectable concentrations for uric acid ranged from 5.0 to 130 µM. The applicability of the electrode was experimentally demonstrated by the successful direct detection of uric acid in real urine samples.