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. 2020 Mar 5;20(5):1422.
doi: 10.3390/s20051422.

Salivary Cortisol Determination on Smartphone-Based Differential Pulse Voltammetry System

Jingjing Liu  1   2   3 , Ning Xu  2 , Hong Men  2 , Shuang Li  1 , Yanli Lu  1 , Sze Shin Low  1 , Xin Li  1 , Lihang Zhu  1 , Chen Cheng  1 , Gang Xu  1 , Qingjun Liu  1
Affiliations

Salivary Cortisol Determination on Smartphone-Based Differential Pulse Voltammetry System

Jingjing Liu et al. Sensors (Basel). .

Abstract

Cortisol is commonly used as a significant biomarker of psychological or physical stress. With the accelerated pace of life, non-invasive cortisol detection at the point of care (POC) is in high demand for personal health monitoring. In this paper, an ultrasensitive immunosensor using gold nanoparticles/molybdenum disulfide/gold nanoparticles (AuNPs/MoS2/AuNPs) as transducer was explored for non-invasive salivary cortisol monitoring at POC with the miniaturized differential pulse voltammetry (DPV) system based on a smartphone. Covalent binding of cortisol antibody (CORT-Ab) onto the AuNPs/MoS2/AuNPs transducer was achieved through the self-assembled monolayer of specially designed polyethylene glycol (PEG, SH-PEG-COOH). Non-specific binding was avoided by passivating the surface with ethanolamine. The miniaturized portable DPV system was utilized for human salivary cortisol detection. A series current response of different cortisol concentrations decreased and exhibited a linear range of 0.5-200 nM, the detection limit of 0.11 nM, and high sensitivity of 30 μA M-1 with a regression coefficient of 0.9947. Cortisol was also distinguished successfully from the other substances in saliva. The recovery ratio of spiked human salivary cortisol and the variation of salivary cortisol level during one day indicated the practicability of the immunosensor based on the portable system. The results demonstrated the excellent performance of the smartphone-based immunosensor system and its great potential application for non-invasive human salivary cortisol detection at POC.

Keywords: differential pulse voltammetry; gold nanoparticles; immunosensor; molybdenum disulfide; salivary cortisol; smartphone.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure A1
Figure A1
Energy-dispersive X-ray spectroscopy (EDS) image of AuNPs/MoS2/AuNPs-modified screen-printed electrode.
Figure A2
Figure A2
Calibration curves for cortisol immunosensor using smartphone-controlled DPV system (red line) and the electrochemical workstation (grey line).
Figure A3
Figure A3
Selective responses of cortisol and substances existed in saliva using the smartphone-controlled DPV system (red box) and the electrochemical workstation (grey box).
Figure 1
Figure 1
The construction and detection procedure of cortisol immunosensor. (a) Fabrication of layer-by-layer self-assembled gold nanoparticles/molybdenum disulfide/gold nanoparticles (AuNPs/MoS2/AuNPs) sandwich-structured screen-printed electrode (SPE), (b) covalent immobilization of cortisol antibody through self-assembled monolayer of polyethylene glycol (PEG).
Figure 2
Figure 2
Structure of the miniaturized differential pulse voltammetry (DPV) system. (a) The diagram of the system, (b) EA/CORT-Ab/SH-PEG-COOH/AuNPs/MoS2/AuNPs biofunctionalized SPE/AuNPs/MoS2/AuNPs biofunctionalized SPE, (c) image of the designed printed circuit board (PCB), (d) the interface of the DPV detection application in the smartphone.
Figure 3
Figure 3
The scanning electron microscope (SEM) image of (a) gold nanoparticles (AuNPs with diameter of approximately 500 nm) modified SPE, (b) molybdenum disulfide/gold nanoparticles (AuNPs/MoS2)-modified SPE, (c) sandwich structured gold nanoparticles/molybdenum disulfide/gold nanoparticles (AuNPs/MoS2/AuNPs) electrode and (d) image of the electrode magnified 50 k times.
Figure 4
Figure 4
Characterization of the prepared immunosensor. (a) cyclic voltammetry (CV) curves of the proposed EA/CORT-Ab/SH-PEG-COOH/AuNPs/MoS2/AuNPs immunosnsor, (b) impedance spectra of AuNPs/MoS2/AuNPs-modified SPE, (c) CV curves of the proposed immunosensor in 5 mM redox couple with different scan rates (from inner to outer: 10 mVs−1–100 mVs−1), (d) the dependence of redox peak currents vs. the square-root of the scan rate..
Figure 5
Figure 5
Electrochemical detection of cortisol using smartphone-controlled DPV system. (a) DPV responses of cortisol from 0.5 nM to 200 nM, (b) calibration curve for cortisol immunosensor.
Figure 6
Figure 6
(a) Selective responses of cortisol and substances existed in saliva using the smartphone-controlled DPV system, inset shows the stability results of the immunosensor, (b) DPV responses of the variation of cortisol level in human saliva during a day based on the smartphone system.
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