Electrochemical-based biosensors

Electrochemical biosensors offer high sensitivity and excellent selectivity for detecting targets using simple instruments and with fast reaction times (Figure 1). The analytical method using electrochemical biosensors is generally based on electron-transfer processes that occur at the electrode surface and on electroactive materials in the electrolyte [41]. Electrode surfaces coated with bioreceptors can specifically distinguish between biomolecules and targets, and the resulting interactions are converted into distinct electrical signals, enabling both qualitative and quantitative detection of targets [42]. The electrical signal generated from electrochemical measurements will be proportional to the analyte concentration [43]. Signal conversion can take the form of current, potential, impedance, or ion charge. Popular detection methods include cyclic voltammetry (CV), square-wave voltammetry (SWV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) [44,45]. There are three types of electrodes commonly used in electrochemical biosensors. The working electrode serves as the site of redox reactions and functions as a transducer, with its reduction potential depending on the analyte concentration. The reference electrode serves as a comparator to measure the potential at the working electrode, whose reduction potential is independent of the analyte concentration. The counter electrode conducts the entire current required to balance the current at the working electrode [46,47]. Numerous studies have explored the detection of cancer biomarkers using electrochemical-based biosensors, leading to significant advancements in their development. These innovations have enhanced the sensitivity, specificity and practicality of cancer biomarker detection, making electrochemical biosensors a promising tool for molecular diagnostics [40].

Figure 1.: General scheme of an electrochemical and optical biosensor

Cyclic voltammetry (CV) is one of the most influential analytical techniques that analyses the electrochemical properties of an analyte in solution [48]. CV is particularly useful for determining the standard oxidation and reduction potentials of biomarkers, which can indicate the presence of oral cancer markers such as cytokeratin fragment 19 (CYFRA 21-1) or IL-8 in saliva samples [49]. Several recent studies have demonstrated significant progress in the development of electrochemical biosensors for high-sensitivity, non-invasive detection of cancer biomarkers, particularly using CV techniques. For example, Shilpi Verma et al. [50] developed an electrochemical biosensor platform based on a ZnO-rGO nanocomposite for the non-invasive detection of IL-8 as an oral cancer biomarker. This detection system successfully identified IL-8 at low concentrations, ranging from 100 fg mL-1 to 5 ng mL-1, with a sensitivity of 12.46 ± 0.82 μA mL ng-1 and a LOD of 51.53 ± 0.43 pg mL-1. The synthesized nanocomposite exhibits high biocompatibility and excellent electron-transport properties, enabling its use in point-of-care (POC) applications for non-invasive onsite detection.

The label-free EIS approach offers a highly sensitive method for detecting a wide range of analytes, including cancer biomarkers, by monitoring changes in capacitance or charge-transfer resistance on a modified electrode surface [51]. In the study by Joshi et al. [52], the EIS technique was employed to detect two key cancer biomarkers: CEA and CYFRA 21-1. The detection mechanism is based on the interaction between oppositely charged antibodies and antigens, which reduces the overall charge of the complex. This change in charge density is subsequently transferred to the reduced rGO layer, altering the net charge in the p-type rGO layer and decreasing the electrical current passing through the rGO/MEL/antibody device. The sensor demonstrated the ability to quantify biomarker concentrations in saliva samples, with signal variation ranging from 7.14 to 59.1 % for CEA and 6.18 to 64.0 % for CYFRA 21-1, indicating a wide detection range and high sensitivity to changes in biomarker concentration.

In recent years, electrochemical biosensors have undergone extensive modification by integrating nanomaterials, enabling substantial improvements in signal amplification, LODs and assay specificity in cancer biomarker analysis. These enhancements are largely driven by engineered nanostructures that exhibit superior electron-transfer properties [52]. A high-sensitivity bead-based immunoassay with nanofluid preconcentration has been reported for biomarker detection as demonstrated by Fan et al. [53]. This approach employs a bead-based immunosensor integrated with a nanofluidic valve system, enabling real-time antigen quantification through microbiomolecule tracking velocimetry. With the ability to detect prostate-specific antigen (PSA) at an LOD of 50 pg mL-1 in just 20 minutes, this nanoparticle-enhanced platform demonstrates significant improvements in detection speed, sensitivity and the feasibility of multiplexed cancer biomarker analysis. The success of this system highlights how nanomaterials can substantially amplify biosensor performance.

Building upon this concept, Guo et al. [54] introduced a multiplex electrochemical immunoassay capable of simultaneously detecting AFP and CEA in human serum and saliva. Their design employs dual-wavelength quantum dots (QDs at 525 and 625 nm) supported by graphene as a conductive bridge, exploiting the strong signal amplification properties of QDs. The combination of streptavidin-coated CdSe/ZnS QDs and graphene facilitates robust electrochemiluminescent responses, enabling detection within an exceptionally wide dynamic range (0.001 to 0.1 pg mL-1) and achieving an ultralow LOD of 0.4 fg mL-1. This work further reinforces the crucial role of nanostructured materials in enhancing multiplexing capabilities and analytical sensitivity.

In a more advanced development, Kovarova et al. [55] reported a novel magneto-immunosensor that integrates electroactive nanocomposites to simultaneously quantify three ovarian cancer biomarkers: HE4, AFP and cancer antigen 125 (CA-125). This sensor combines multiple nanomaterial labels, including gold nanoparticles (AuNPs), CdTe QDs and PbS QDs, providing distinct electrochemical signatures for each analyte. Moreover, the incorporation of mesoporous silica nanoparticles (SiNPs) significantly boosts electrochemical output by increasing label-loading capacity. Compared to earlier designs, this system demonstrates how strategic nanomaterial engineering can expand multiplexing capability while further lowering LOD.