Salivary Electrochemical Cortisol Biosensor Based on Tin Disulfide Nanoflakes

Liu et al. Nanoscale Research Letters (2019) 14:189 https://doi.org/10.1186/s11671-019-3012-0 NANO EXPRESS Open Access Salivary Electrochemical Cortisol Biosensor Based on Tin Disulfide Nanoflakes Xinke Liu1*, Sanford P. C. Hsu2,3, Wai-Ching Liu4, Yi-Min Wang4, Xinrui Liu5, Ching-Shu Lo4, Yu-Chien Lin4, Sasza Chyntara Nabilla4, Zhiwen Li1, Yuehua Hong1, Chingpo Lin5, Yunqian Li5, Gang Zhao5 and Ren-Jei Chung4* Abstract Cortisol, a steroid hormone, is secreted by the hypothalamic-pituitary-adrenal system. It is a well-known biomarker of psychological stress and is hence known as the “stress hormone.” If cortisol overexpression is prolonged and repeated, dysfunction in the regulation of cortisol eventually occurs. Therefore, a rapid point-of-care assay to detect cortisol is needed. Salivary cortisol electrochemical analysis is a non-invasive method that is potentially useful in enabling rapid measurement of cortisol levels. In this study, multilayer films containing two-dimensional tin disulfide nanoflakes, cortisol antibody (C-Mab), and bovine serum albumin (BSA) were prepared on glassy carbon electrodes (GCE) as BSA/C-Mab/SnS2/GCE, and characterized using electrochemical impedance spectroscopy and cyclic voltammetry. Electrochemical responses of the biosensor as a function of cortisol concentrations were determined using cyclic voltammetry and differential pulse voltammetry. This cortisol biosensor exhibited a detection range from 100 pM to 100 μM, a detection limit of 100 pM, and a sensitivity of 0.0103 mA/Mcm2 (R2 = 0.9979). Finally, cortisol concentrations in authentic saliva samples obtained using the developed electrochemical system correlated well with results obtained using enzyme-linked immunosorbent assays. This biosensor was successfully prepared and used for the electrochemical detection of salivary cortisol over physiological ranges, based on the specificity of antibody-antigen interactions. Keywords: Cortisol, 2D Tin disulfide nanoflakes, Electrochemical biosensor, Enzyme-linked immunosorbent assay Introduction Cortisol, a steroid hormone, is secreted by the hypothalamic-pituitary-adrenal (HPA) system. It is a well-known biomarker of psychological stress and hence called the “stress hormone” [1, 2]. Cortisol levels follow a circadian rhythm over a 24-h cycle; the highest levels are observed early morning, and the levels progressively reduce by night [3–6]. Excessive levels of cortisol can cause Cushing’s disease, with symptoms of central obesity, purple striae, and proximal muscle weakness. However, reduced levels of cortisol can lead to Addison’s disease, with chronic fatigue, malaise, anorexia, postural hypotension, and hypoglycemia [7–9]. Therefore, maintaining appropriate cortisol balance is essential for human health. * Correspondence: ; 1 College of Materials Science and Engineering, Shenzhen University, No. 3688, Nanhai Ave, Shenzhen 518060, China 4 Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Sec. 3, Zhongxiao E. Rd, Taipei 10608, Taiwan Full list of author information is available at the end of the article A growing interest in the measurement of cortisol as a precursor to medically and psychologically relevant events has developed, among which the most recent affliction is post-traumatic stress disorder (PTSD). The importance of aberrant HPA axis function in PTSD is indisputable; hence, traditional assessment methods are still able to provide abundant evidence and information [10–14]. Recently, many studies have reported the importance of cortisol detection and have identified correlations with different illnesses [15–18]. Various studies have confirmed that cortisol is related to autism spectrum disorder [19], depression [20], suicidal ideation [21], childhood adversity, and externalizing disorders [22]. Although identifying cortisol levels represents an important diagnostic tool, routine laboratory cortisol detection techniques such as chromatography [23, 24], radioimmunoassay [25], electro-chemiluminescent immunoassay [26–28], enzyme-linked immunosorbent assay [28, 29], surface plasmon resonance [1, 30, 31], and quartz crystal microbalance [32] involve extensive © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Liu et al. Nanoscale Research Letters (2019) 14:189 analysis time, are expensive, and cannot be implemented in point-of-care (POC) settings [33]. Therefore, there is currently a need for sensitive, efficient, and real-time determination of cortisol levels. In recent years, electrochemical immunoassay methods, which are established on the specific molecular recognition between antigens and antibodies, have emerged as a promising technology due to salient characteristics, such as involving simple devices, rapid analysis, low cost, label-free POC testing, high sensitivity, and low detection thresholds for cortisol in bio-fluids [34, 35]. Electrical potential changes are ascribed to variations in the concentration of electrochemical redox reactions at the electrode. Secreted cortisol eventually enters the circulatory system and can be found in various bio-fluids such as interstitial fluid [36], blood [37], urine [38], sweat [39], and saliva [40]. The advantages of electrochemical detection of salivary cortisol, which is a non-invasive method, with easy sample collection, handling, and storage, have enhanced its potential for application in POC sensors for real-time measurement [41]. An ideal biosensor should have low detection limits, rapid selectivity, and high sensitivity. In order to fabricate an immunosensor, the immobilizing matrix chosen should possess high surface functionality, high biomolecule loading, and low resistance to electron transport, with a high electron transfer rate [42]. However, metal sulfide nanomaterials have been rarely suggested for the immobilization of proteins for electrochemical biosensing. Therefore, here, tin disulfide was selected as a potential immobilizing matrix for immunosensor development in order to detect cortisol present in saliva. Nano two-dimensional (2D) materials have attracted abundant research interests in the recent decade. There are a variety of kinds of 2D materials ranging from semiconductor to metal and from inorganic to organic [43– 46] and related composite [47–50]. The discovery, manufacturing, and investigation on nano 2D material are prevailing streams in various fields. Nano 2D tin disulfide (SnS2), an n-type semiconductor with a bandgap of 2.18–2.44 eV [51, 52], consists of Sn atoms sandwiched between two layers of hexagonally disposed and cl (...truncated)