Graphene-Based Sensor for the Detection of Cortisol for Stress Level Monitoring and Diagnostics

Biosensors for psychiatric biomarkers in mental health monitoring

Psychiatric disorders are associated with serve disturbances in cognition, emotional control, and/or behavior regulation, yet few routine clinical tools are available for the real-time evaluation and early-stage diagnosis of mental health. Abnormal levels of relevant biomarkers may imply biological, neurological, and developmental dysfunctions of psychiatric patients. Exploring biosensors that can provide rapid, in-situ, and real-time monitoring of psychiatric biomarkers is therefore vital for prevention, diagnosis, treatment, and prognosis of mental disorders. Recently, psychiatric biosensors with high sensitivity, selectivity, and reproducibility have been widely developed, which are mainly based on electrochemical and optical sensing technologies. This review presented psychiatric disorders with high morbidity, disability, and mortality, followed by describing pathophysiology in a biomarker-implying manner. The latest biosensors developed for the detection of representative psychiatric biomarkers (e.g., cortisol, dopamine, and serotonin) were comprehensively summarized and compared in their sensitivities, sensing technologies, applicable biological platforms, and integrative readouts. These well-developed biosensors are promising for facilitating the clinical utility and commercialization of point-of-care diagnostics. It is anticipated that mental healthcare could be gradually improved in multiple perspectives, ranging from innovations in psychiatric biosensors in terms of biometric elements, transducing principles, and flexible readouts, to the construction of 'Big-Data' networks utilized for sharing intractable psychiatric indicators and cases.

Ultrasensitive electrochemical sensor for detection of salivary cortisol in stress conditions

A natural stress response induces elevated cortisol levels in biological fluids, such as saliva. While current sensor technologies can detect cortisol in real time, their sensitivity and reliability for human subjects have not been assured. This is due to relatively low concentrations of salivary cortisol, which fluctuate throughout the day and vary significantly between individuals. To address these challenges, we present an improved electrochemical biosensor leveraging graphene's exceptional conductivity and physicochemical properties. A 1-pyrenebutyric acid N-hydroxysuccinimide ester (PBASE-NHS)-modified commercial graphene foam (GF) electrode is presented to realize an ultra-sensitive biosensor for cortisol detection directly in human saliva. The biosensor fabrication process entails the attachment of anti-cortisol monoclonal antibodies (mAb-cort) onto a PBASE-NHS/GF electrode through noncovalent immobilization on the vertically stratified graphene foam electrode surface. This unique immobilization strategy preserves graphene's structural integrity and electrical conductivity while facilitating antibody immobilization. The binding of cortisol to immobilized mAb-cort is read out via differential pulse voltammetry using ferri/ferro redox reactions. The immunosensor demonstrates an exceptional dynamic range of 1.0 fg mL-1 to 10,000 pg mL-1 (R2 = 0.9914) with a detection limit of 0.24 fg mL-1 (n = 3) for cortisol. Furthermore, we have established the reliability of cortisol sensors in monitoring human saliva. We have also performed multiple modes of validation, one against the established enzyme-linked immunosorbent assay (ELISA) and a second by a third-party service Salimetric on 16 student volunteers exposed to different stress levels, showing excellent correlation (r = 0.9961). These findings suggest the potential for using mAb-cort/PBASE-NHS/GF-based cortisol electrodes for monitoring salivary cortisol in the general population.

Advances in electrochemical biosensor design for the detection of the stress biomarker cortisol

The monitoring of stress levels in humans has become increasingly relevant, given the recent incline of stress-related mental health disorders, lifestyle impacts, and chronic physiological diseases. Long-term exposure to stress can induce anxiety and depression, heart disease, and risky behaviors, such as drug and alcohol abuse. Biomarker molecules can be quantified in biological fluids to study human stress. Cortisol, specifically, is a hormone biomarker produced in the adrenal glands with biofluid concentrations that directly correlate to stress levels in humans. The rapid, real-time detection of cortisol is necessary for stress management and predicting the onset of psychological and physical ailments. Current methods, including mass spectrometry and immunoassays, are effective for sensitive cortisol quantification. However, these techniques provide only single measurements which pose challenges in the continuous monitoring of stress levels. Additionally, these analytical methods often require trained personnel to operate expensive instrumentation. Alternatively, low-cost electrochemical biosensors enable the real-time detection and continuous monitoring of cortisol levels while also providing adequate analytical figures of merit (e.g., sensitivity, selectivity, sensor response times, detection limits, and reproducibility) in a simple design platform. This review discusses the recent developments in electrochemical biosensor design for the detection of cortisol in human biofluids. Special emphasis is given to biosensor recognition elements, including antibodies, molecularly imprinted polymers (MIPs), and aptamers, as critical components of electrochemical biosensors for cortisol detection. Furthermore, the advantages and limiting factors of various electrochemical techniques and sensing in complex biofluid matrices are overviewed. Remarks on the current challenges and future perspectives regarding electrochemical biosensors for stress monitoring are provided, including matrix effects (pH dependence and biological interferences), wearability, and large-scale production.

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.

Modification of electrodes with polymer nanocomposites: Application to the simultaneous determination of Zn(II), Cd(II), and Cu(II) in water samples

Currently, there is a need for fast and sensitive analytical methods for monitoring metals in water due to the progressive increase in the presence of metal ions in the environment. These metals reach the environment mainly from industrial activity and heavy metals are non-biodegradable. The present work evaluates different polymeric nanocomposites to carry out the simultaneous electrochemical determination of Cu, Cd, and Zn in water samples. Screen-printed carbon electrodes (SPCE) were modified with the nanocomposites, which were obtained by a mixture of graphene, graphite oxide, and polymers, such as polyethyleneimide, gelatin, and chitosan. These polymers have amino groups in their matrix, giving the nanocomposite the ability to retain divalent cations. However, the availability of these groups plays a fundamental role in the retention of these metals. The modified SPCEs were characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry. The electrode that presented the best performance was selected to determine the concentration of metal ions in water samples by square-wave anodic stripping voltammetry. The obtained detection limits were 0.23 μg L^-1, 0.53 μg L^-1, and 1.52 μg L^-1 for Zn(II), Cd(II), and Cu(II), respectively, with a lineal range of 0.1-50 μg L^-1. The obtained results made it possible to conclude that the method developed using the SPCE modified with the polymeric nanocomposite presented adequate LODs, reasonable sensitivity, selectivity, and reproducibility. Besides, this platform is an excellent tool for developing devices to simultaneously determine heavy metals in environmental samples.

Electrochemical Detection of Cortisol by Silver Nanoparticle-Modified Molecularly Imprinted Polymer-Coated Pencil Graphite Electrodes

The sensitive cortisol detection by an electrochemical sensor based on silver nanoparticle-doped molecularly imprinted polymer was successfully improved. This study describes the method development for cortisol detection in both aqueous solution and biological samples using molecularly imprinted poly(hydroxyethyl methacrylate-N-methacryloyl-(l)-histidine methyl ester)-coated pencil graphite electrodes modified with silver nanoparticles (AgNPs) by differential pulse voltammetry (DPV). The cortisol-imprinted pencil graphite electrode (PGE) has a large surface area because of doped AgNPs with enhanced electroactivity. The prepared molecularly imprinted polymer was characterized by scanning electron microscopy. The DPV response of the synthesized electrode with outstanding electrical conductivity was clarified. Cortisol-imprinted polymer-coated PGEs (MIP), cortisol-imprinted polymer-coated PGEs with AgNPs (MIP@AgNPs), and nonimprinted polymer-coated PGEs with AgNPs (NIP@AgNPs) were evaluated for sensitive and selective detection of cortisol in aqueous solution. Five different cortisol concentrations (0.395, 0.791, 1.32, 2.64, and 3.96 nM) were applied to the MIP@AgNPs, and signal responses were detected by the DPV with a regression coefficient (R2) value of 0.9951. The modified electrode showed good electrocatalytic activity toward cortisol for the linear concentration range from 0.395 to 3.96 nM, and a low limit of detection was recorded as 0.214 nM. The results indicate that the MIP@AgNPs sensor has great potential for sensitive and selective cortisol determination in biological samples.

An Energy-Efficient Flexible Multi-Modal Wireless Sweat Sensing System Based on Laser Induced Graphene

Real-time sweat monitoring is vital for athletes in order to reflect their physical conditions, quantify their exercise loads, and evaluate their training results. Therefore, a multi-modal sweat sensing system with a patch-relay-host topology was developed, which consisted of a wireless sensor patch, a wireless data relay, and a host controller. The wireless sensor patch can monitor the lactate, glucose, K⁺, and Na⁺ concentrations in real-time. The data is forwarded via a wireless data relay through Near Field Communication (NFC) and Bluetooth Low Energy (BLE) technology and it is finally available on the host controller. Meanwhile, existing enzyme sensors in sweat-based wearable sports monitoring systems have limited sensitivities. To improve their sensitivities, this paper proposes a dual enzyme sensing optimization strategy and demonstrates Laser-Induced Graphene (LIG)-based sweat sensors decorated with Single-Walled Carbon Nanotubes (SWCNT). Manufacturing an entire LIG array takes less than one minute and costs about 0.11 yuan in materials, making it suitable for mass production. The in vitro test result showed sensitivities of 0.53 μA/mM and 3.9 μA/mM for lactate and glucose sensing, and 32.5 mV/decade and 33.2 mV/decade for K⁺ and Na⁺ sensing, respectively. To demonstrate the ability to characterize personal physical fitness, an ex vivo sweat analysis test was also performed. Overall, the high-sensitivity lactate enzyme sensor based on SWCNT/LIG can meet the requirements of sweat-based wearable sports monitoring systems.

A Novel Cortisol Immunosensor Based on a Hafnium Oxide/Silicon Structure for Heart Failure Diagnosis

Assessing cortisol levels in human bodies has become essential to diagnose heart failure (HF). In this work, we propose a salivary cortisol detection strategy as part of an easily integrable lab-on-a-chip for detection of HF biomarkers. Our developed capacitive immunosensor based on hafnium oxide (HfO2)/silicon structure showed good linearity between increasing cortisol concentration and the charge-transfer resistance/capacitance. Moreover, the developed biosensor was demonstrated to be highly selective toward cortisol compared to other HF biomarkers such as tumor necrosis factor (TNF-α) and N-terminal pro-brain natriuretic peptide (NT-proBNP). The precision of our developed biosensor was evaluated, and the difference between the determined cortisol concentration in saliva and its expected one is <18%.