Recent trends in carbon nanomaterial-based electrochemical sensors for biomolecules: A review

Abstract

Carbon nanomaterials are advantageous for electrochemical sensors because they increase the electroactive surface area, enhance electron transfer, and promote adsorption of molecules. Carbon nanotubes (CNTs) have been incorporated into electrochemical sensors for biomolecules and strategies have included the traditional dip coating and drop casting methods, direct growth of CNTs on electrodes and the use of CNT fibers and yarns made exclusively of CNTs. Recent research has also focused on utilizing many new types of carbon nanomaterials beyond CNTs. Forms of graphene are now increasingly popular for sensors including reduced graphene oxide, carbon nanohorns, graphene nanofoams, graphene nanorods, and graphene nanoflowers. In this review, we compare different carbon nanomaterial strategies for creating electrochemical sensors for biomolecules. Analytes covered include neurotransmitters and neurochemicals, such as dopamine, ascorbic acid, and serotonin; hydrogen peroxide; proteins, such as biomarkers; and DNA. The review also addresses enzyme-based electrodes that are used to detect non-electroactive species such as glucose, alcohols, and proteins. Finally, we analyze some of the future directions for the field, pointing out gaps in fundamental understanding of electron transfer to carbon nanomaterials and the need for more practical implementation of sensors.

Keywords: Biosensor; Carbon nanotube; Dopamine; Field-effect transistor; Glucose; Graphene.

Figure 1

Comparison of the effect of FSCV scan repetition rate at (A) a carbon nanotube yarn disk microelectrode (CNTYME) and (B) a carbon fiber disk microelectrode (CFME). 1 µM dopamine is detected using a scan rate of 400 V/s. The repetition rate is either 10 Hz (solid black traces) or 100 Hz (dashed red traces). Carbon nanotube yarn electrodes are not dependent on scan repetition frequency. Adapted with permission from reference [47].

Figure 2

FET-based sensor for detection of dopamine. (A)SEM and (B) TEM images of the semiconducting SWCNTs (s-SWCNTs). (C) Typical real time current (ΔI/I₀) changes with dopamine concentration in a PBS solution for FET sensor with s-SWCNT and normal SWCNTS (n-SWCNTs). Adapted with permission from reference [51].

Figure 3

Serotonin fouling at a PEI-CNT fiber microelectrode. (A) Repeated injections of serotonin (1 µM) every 15 s for 25 injections lead to no decrease in current for serotonin at PEI-CNT fiber electrodes (red) as opposed to the 50% decrease for carbon-fiber microelectrodes (CFMEs) (black). (B) Example cyclic voltammograms of 1 µM serotonin for a PEI-CNT fiber microelectrode for the 1st (solid black) and 25th injection (dashed red), approximately 6.25 min apart. The CVs are similar. (C) Example cyclic voltammograms of 1 µM serotonin at CFMEs for the 1st and 25th injection, indicating serotonin fouling does occur at the surface of the CFME. Adapted with permission from reference [68].

Figure 4

CNTs for direct detection of the cancer biomarker OPN. Functionalization scheme for OPN attachment: first, carboxylic acid sites are created on the nanotube sidewall by incubation in a diazonium salt solution. The carboxylic acid group is then activated by EDC and stabilized with NHS. ScFv antibody displaces the NHS and forms an amide bond (surface amine-rich lysine residues responsible for this bond are depicted in red), and OPN binds preferentially to the scFv in the detection step. The OPN epitope is shown in yellow, and the C- and N-termini are in orange and green, respectively. Reprinted (adapted) with permission from reference [160].

Figure 5

Schematic illustrations detailing the fabrication of both a single-HRP and multiple-HRP strategy-based PSA immunosensor. For the single-HRP strategy, the HRP is bound to the antibody. In the multiple HRP strategy, multiple copies of both HRP and the antibody are bound on the MWCNTs, so that a larger quantity of precipitate is formed from the interaction of the labeled antibody and the sensor surface. Adapted with permission from reference [114].

Figure 6

Schematic representation of biosensing of DNA hybridization with EIS. A GCE electrode is modified with FePt–GO. Probe DNA is immobilized by dipping in ssDNA probe solution for 2 h, followed by washing and rinsing steps. Upon hybridization with the target gene, the EIS signal of [Fe(CN)₆]^3−/4− increases (from a to b). Reprinted (adapted) with permission from reference [141]