Development of a Redox-Label-Doped Molecularly Imprinted Polymer on β-Cyclodextrin/Reduced Graphene Oxide for Electrochemical Detection of a Stress Biomarker

Abstract

Cortisol is a major stress biomarker involved in the regulation of metabolic and immune responses. Readily accessible assays with sufficient quantitative and temporal resolution can assist in prevention, early diagnosis, and management of chronic diseases. Whereas conventional assays are costly in terms of time, labor, and capital, an electrochemical approach offers the possibility of miniaturization and detection at the point-of-care. Here, we investigate the biosensor application of molecularly imprinted polypyrrole (PPy) doped with hexacyanoferrate (HCF) and coupled to reduced graphene oxide functionalized with β-cyclodextrin (β-CD). β-CD provides an inclusion site for lipophilic cortisol and was electrochemically grafted simultaneous with reduction of GO. Next, PPy was electrochemically deposited in presence of cortisol template with HCF dopant ions serving as intrinsic redox probe. Thus, the sensor response was evaluated via changes of redox peak current in cyclic voltammetry and demonstrated a broad logarithmic detection range (5 pg/mL to 5000 ng/mL, R 2 = 0.995), with a sensitivity of 8.809 μA log –1 (ng/mL) cm –2 and LOD of 19.3 pM. The sensor was shown to be specific toward cortisol in reference to salivary cortisol concentration in saliva over structural analogues. The sensor was exhibited to determine cortisol in artificial saliva at normal and elevated levels. The good performance and facile electrochemical fabrication of this antibody- and external label-free interface are promising for the development of affordable point-of-care biosensors.

Introducing Nanoscale Electrochemistry in Small-Molecule Detection for Tackling Existing Limitations of Affinity-Based Label-Free Biosensing Applications

“…The LOD for cortisol was calculated to be 3.93 × 10 –13 M (S/N = 3). The LOD is sufficiently low to detect cortisol in various biological fluids, such as sweat and saliva …”

“…The cortisol-selective MICP was prepared by electrochemically copolymerizing β-CD and MB, which resulted in a composite film of β-cyclodextrin polymer (CDP) and polymethylene blue (PMB). The β-CDs in the MICP serve as the recognition element for cortisol in association with cortisol-selective receptor cavities, 38 and PMB serves as signaling redox probes, as shown in Figure 1A. The thickness of the MICP film was adjusted to the nanoscale level (<5 nm) in order to increase the target-to-receptor size ratio for efficient signal transduction in the binding events between small cortisol molecules and the MICP receptors.…”

“…The LOD is sufficiently low to detect cortisol in various biological fluids, such as sweat and saliva. 38 The "PMB wire" embedded in the MICP film can be represented by an RC circuit, and the electron transfer rate (k) could be given by eq 5…”

“…The β-CD, a cyclic oligosaccharide consisting of seven glucose units, contains a hydrophobic inner cavity and a hydrophilic outer side, forming inclusion complexes with guest molecules of appropriate size and shape . Since the size and shape of cortisol are compatible with the inner cavity of β-CD, the cortisol can form a host–guest complex with β-CD via hydrophobic interaction and hydrogen bonding, , which assists in inducing receptor cavities during its electrochemical polymerization. MB also contributes to imprinting cortisol synergistically with β-CD using hydrophobic and hydrogen bonding while acting as a self-reporting probe.…”

Electrochemical Sensor Based on a Composite of Reduced Graphene Oxide and Molecularly Imprinted Copolymer of Polyaniline–Poly(o-phenylenediamine) for Ciprofloxacin Determination: Fabrication, Characterization, and Performance Evaluation

“…GO can be chemically reduced to remove some functional groups and form rGO, a semiconductive material. rGO is a popular transducer and has been demonstrated as a sensitive element for charge-transfer enhancement in electrochemical sensors. − …”