Molybdenum disulfide (MoS2) is a promising two-dimensional (2D) nanomaterial, whose unique adjustable bandgap shows excellent electronic and optical properties in the construction of biosensor interfaces. It not only has the advantages of a high catalytic activity and low manufacturing costs, but it can also further expand the application of hybrid structures through different functionalization, and it is widely used in various biosensors fields.
1. Introduction
Cancer is the world’s leading cause of death and the second most common disease
[1]. At present, more than 200 types of cancers have been found. In general, imaging technologies such as ultrasound, positron emission tomography (PET), magnetic resonance imaging (MRI), and computed tomography (CT) are used for early screening, followed by confirmation through tissue biopsy and histology, so that patients can be treated in a timely manner, which can dramatically reduce cancer mortality
[2][3][4]. However, traditional cancer detection methods are often invasive, expensive, complex, and time consuming. Rapid diagnosis and early prevention are crucial for the clinical treatment and management of cancer
[5]. Cancer biomarkers, as important components of detection, prognosis, and providing an etiological analysis of cancer, are abnormal quantities of biological molecules generated by the body’s response to the disease or directly by the cancer tumor itself, including DNA, RNA, genes, proteins, enzymes, peptides, exosomes, and metabolomics
[6]. So far, the main cancer biomarkers that have been discovered include carcinoembryonic antigen (CEA), carbohydrate antigen 125 (CA125), carbohydrate antigen 15-3 (CA15-3), human epidermal growth factor receptor 2 (HER2), vascular endothelial growth factor 165 (VEGF
165), tissue-specific antigen (TPS), prostate-specific antigen (PSA), alpha-fetoprotein (AFP), squamous cell carcinoma antigen (SCCA), circulating tumour cells (CTCs), microRNAs, and exosomes
[7]. They typically exist in the blood, urine, tears, oral fluids, and other tissues
[8]. Cancer biomarker detection has accelerated the process of cancer diagnosis, and can obtain higher sensitivity and faster cancer screening. Enzyme-linked immunosorbent assay (ELISA)
[9], polymerase chain reaction (PCR)
[10], clustered regularly interspaced short palindromic repeats Cas9 (CRISPR-Cas9)
[11], loop-mediated isothermal amplification (LAMP), time-resolved fluorescence spectroscopy (TR-FS), radioimmunoassay (RIA), and electrophoresis
[12] have been used for the detection of cancer biomarkers
[13]. In addition, emerging technologies such as artificial intelligence, long read sequencing, microarrays, DNA methylation, and liquid biopsy are also committed to the development and high throughput profiling of many biomarkers to strengthen cancer management and improve early screening
[14][15].
In recent years, compared with traditional technologies, biosensors have potential advantages such as a high sensitivity and selectivity, high accuracy, low cost, fast detection, high stability, availability, and ease of operation. They play an important role in diagnosing and quantitatively analysing biomarker concentrations, and are widely used in various fields such as healthcare, food inspection, and environmental testing
[16]. Biosensors use various biomolecules as biometric recognition components, which are fixed on the sensor surface and converted into measurable electronic or optical signals through biological responses with the detection target substance for cancer biomarker detection
[17]. Biosensors can be divided into electrochemical, optical, mass-dependent, and radiation sensitive biosensing platforms based on different transduction principles
[18][19]. Developing efficient and practical biosensors usually requires consideration of the following aspects: (1) synthesis, manufacture, and assembly of suitable sensing materials; (2) selecting appropriate recognition or capture molecules; and the (3) integration of sensor surfaces with biomolecules
[20][21]. With the development of nanotechnology in medicine and biotechnology, more and more researchers are combining different types of nanomaterials with optical, electrical, mechanical, and magnetic sensors to design nanosensors for the detection of cancer biomarkers
[22]. Nanobiosensors are generally composed of nanomaterials and a sensor based on biometric recognition elements
[23]. They can be combined according to the interaction of the affinity bond, covalent bond, cross-linking, capture, and physical adsorption
[24].
Among the various nanomaterials, 2D-layered nanomaterials have attracted widespread research interest due to their quantum confinement, high absorption coefficient, high specific surface area, and tunable bandgap characteristics
[25]. Among them, graphene has excellent physical properties, chemical adjustability, and application potential, and its synthesis, properties, and applications are widely known
[26]. The impressive performance of graphene in various fields has aroused strong interest in the exploration of a wider range of 2D-layered nanomaterials “beyond graphene”
[27]. Transition metal dichalcogenides (TMDs), as a new class of stable inorganic graphene analogues, have been further studied. Among them, MoS
2 is regarded as a representative of TMDs. Its single molecular layer is composed of an atomic layer of transition metal Mo sandwiched between two sulfur elements S
[28]. Mo atoms and S atoms are closely connected by forming a strong covalent bond through coordination, and the interlayer is connected by a weak van der Waals force. This weak connection mode between layers provides conditions for MoS
2 stripping to form a single-layer 2D planar structure
[29], showing unique electronic, optical, mechanical, and chemical properties
[30][31][32]. Most importantly, due to the confinement of electrons/holes in ultra-thin planar structures, MoS
2 is highly sensitive to changes in the microenvironment
[33], thus exhibiting advantages in the construction of biosensing interfaces
[34][35].
2. Electrochemical Biosensors for Cancer Biomarkers Detection Based on MoS2
Electrochemical sensors are mainly composed of sensitive components, signal transduction components, and nano modified electrode structures. Electrochemical analysis technology is an important detection method in the field of biomedicine. Its basic principle is to analyse the changes in current or impedance signals generated by the interaction between the analyte and the electrode surface. It can monitor the charge movement between reaction interfaces and has significant advantages through its fast response
[36]. In recent years, sensitive electrochemical biosensors have been developed for the detection of cancer biomarkers
[37]. MoS
2 has a hexagonal lattice layered structure, which gives it excellent properties such as a high specific surface area, high electron mobility, thermal stability, catalytic activity, and diamagnetism, which is commonly used in semiconductor materials, catalysts, and lubricating materials, etc.
[38][39][40]. The unique adjustable bandgap characteristic of MoS
2 provides excellent photoluminescence properties, which are widely used in optical devices such as photodetectors. Additionally, MoS
2, as a promising emerging nanomaterial, has low manufacturing costs, rich nanostructures, and is easy to functionalize, making it form hybrid structures with other precious metal nanomaterials, which is widely used in the field of electrochemical sensing
[41]. This chapter divides electrochemical biosensors into potentiometry, amperometry, impedimetry, and photoelectrochemical (PEC) biosensors according to different signal transductions, and introduces the latest application progress of MoS
2 in cancer biomarker detection.
2.1. Potentiometry
Potentiometric sensors obtain information about analytes by measuring the current when potential changes, mainly including chronoamperometry (CA), cyclic voltammetry (CV), differential pulse voltammetry (DPV), and square wave voltammetry (SWV), which are widely used electrochemical analysis methods. These methods fix the biometric elements (such as antibodies, enzymes and aptamers) on the electrode surface, and monitor the current changes triggered when the analyte combines with the biometric element when the potential between the working electrode and the reference electrode remains constant. Within the linear potential range, the monitored peak current value is directly related to the concentration of the target analyte in the solution, so as to realize the detection of the target.
The electronic properties of MoS
2 are highly dependent on its phase structure. The ultra-thin MoS
2 has a good performance, but it is difficult to maintain stability in an independent state and is easy to aggregate
[42]. In order to improve this problem, Ying et al.
[43] used liquid-phase exfoliation and surface modification to synthesize 2H-MoS
2, and used platinum nanowire (Pt NWs) arrays as nanopillars, which were added to the ultra-thin 2D MoS
2 interlayer to form Pt NWs arrays@MoS
2 nano hybrid, which improved the specific surface area and porosity, and could be used as “electronic wires” to catalyze electron transfer at the interface, avoiding folding by creating new dimensions. Thus, stability and current signal enhancement were achieved.
Two main reasons that limit the practical application of MoS
2 in electrochemical sensing are that the strong van der Waals force effect between layers, which leads to aggregation and relatively low conductivity in layers
[44]. In order to overcome these shortcomings, Su et al.
[45] synthesized ionic liquid (IL) functionalized AuNPs/MoS
2/rGO nanocomposites for sensitive detection of cancer-specific target nucleolin. The linear range of the unlabeled electrochemical sensor obtained was 0.5 nM–1.0 μM, and the detection limit was 0.16 nM. Graphene has a large π electronic structure and edge, because of the synergistic effect, and the combination of MoS
2 and graphene can significantly improve the conductivity and large surface area of MoS
2 [46]. In addition, the introduction of AuNPs into nanocomposites can not only fix the thioaptamer through the Au-S bond, which improves the affinity and specificity, but it can also enhance electron transfer and amplify the electrochemical signal.
2.2. Amperometry
Amperometric sensors achieve quantitative detection of analytes by applying a constant voltage to the sensing platform to detect the current generated by the conversion of corresponding electroactive substances. Because of their convenience and high accuracy, they are widely used in the detection of cancer biomarkers. As a result of the excellent catalytic activity of MoS
2 for the reduction in H
2O
2, Ma et al.
[47] used the hydrothermal method to combine MoS
2 nanoflowers (MoS
2 NFs) with p-type metal semiconductor oxide cuprous oxide (MoS
2@Cu
2O), and, at the same time, the introduction of AuNPs generated MoS
2@Cu
2O-Au complexes by Au-S bonds as nanoprobes for signal amplification. The constructed sandwich immunosensor could detect the cancer marker alpha fetoprotein (AFP) of primary liver cancer in the wide linear range of 0.1 pg/mL to 50 ng/mL, demonstrating good application prospects. Ma et al.
[48] prepared a sandwich-type electrochemical immunosensor for the sensitive detection of CEA by coupling tri-metallic yolk−shell Au@AgPt nanocubes (Au@AgPt YNCs) loaded on amino-functionalized MoS
2 NFs (MoS
2 NFs/Au@AgPt YNCs) with secondary antibodies. As a result of the biphasic synergistic catalysis, the synthesized MoS
2 NFs/Au@AgPt YNCs as a signal label effectively catalyzed the reduction of H
2O
2 to amplify the current signal, and realized the high-precision detection of CEA in the range of 10 fg/mL–100 ng/mL, with an LOD as low as 3.09 fg/mL (S/N = 3). These works provide ideas for the composite modification of MoS
2 with different nano forms and further applications in biosensing platforms.
2.3. Impedimetry
Impedance sensors are an important type of electrochemical sensing that obtains information about analytes by measuring the conductance through interface reactions on the electrode surface. This type of sensor is very sensitive to the change in electrode, and is in an advantageous position in the detection of biomarkers. Therefore, it is also widely introduced into the construction of the electrochemical sensing platform for the detection of cancer biomarkers. Jia et al.
[49] prepared a novel nanohybrid of polyoxometalate-derived MoS
2 nanosheets (pd-MoS
2 NSs) using a hydrothermal method, which exhibited an excellent electrochemical activity and abundant catalytic sites. Furthermore, pd-MoS
2 NSs were vertically grown over β-FeOOH NRs (pd-MoS
2@β-FeOOH), serving as complementary DNA platforms for fixing oncogenes and tumour suppressor miRNA-21, using electrochemical impedance spectroscopy (EIS) to detect miRNA-21, with an LOD as low as 0.11 fM. In addition, microfluidic electrochemical immunochips have been evaluated as a powerful detection platform because of their high sensitivity, low cost, portability, and easy miniaturization. Sri et al.
[50] synthesized MoS
2 NFs using the same method, and electrophoretically deposited them on an indium tin oxide (ITO)-coated glass substrate. Because of the morphology of MoS
2 NFs, antibodies can be effectively fixed on the electrode surface through physical adsorption. The biosensor can sensitively detect tumour necrosis factor-α (TNF-α) between 1–200 pg/mL, with an LOD as low as 0.202 pg/mL. Hu et al.
[51] first prepared MoS
2 by liquid-phase exfoliation and formed a hybrid film with PDDA, designed a three-electrode system in the microfluidic chip, and introduced a MoS
2/PDDA film modified with anti-AFP as the working electrode, Ag/AgCl as the reference electrode, and ITO as the counter electrode. The linear range of AFP detected by EIS was 0.1 ng/mL to 10 ng/mL, with an LOD of 0.033 ng/mL.
2.4. Photoelectrochemistry (PEC)
PEC utilizes photosensitive materials at the electrode interface as signal converters to analyse the electrical signals generated by analytes under light irradiation, combining the advantages of spectral analysis and electrochemical technology. MoS
2 exhibits excellent characteristics of a tunable bandgap in its transition from a blocky structure to a layered structure. The quantum confinement effect led to good visible light absorption and photoelectric conversion efficiency of layered MoS
2 as a direct bandgap semiconductor under visible light excitation, resulting in photocurrent generation. Therefore, it has been introduced into the application of photoelectrochemical sensing platforms. Hu et al.
[52] utilized this mechanism to design a PEC sensing platform based on MoS
2/Au/GaN for the high sensitivity detection of AFP. MoS
2 can suppress the charge transfer of Au/GaN photoelectrodes, leading to a significant decrease in photocurrent. However, the presence of AFP can reduce the inhibitory effect on the photocurrent, thereby utilizing the difference in photocurrent to detect AFP. AFP detection is achieved in a wide linear range of 1.0–150 ng/mL, with an LOD of 0.3 ng/mL. This method has a good sensitivity and high selectivity for AFP detection. Wei et al.
[53] synthesized a light-responsive ZnS/C/MoS
2 nanocomposite to construct a PEC immunosensor for detecting CEA, with a linear range of 2.0 pg/mL–10.0 ng/mL and an LOD of 1.30 pg/mL (S/N = 3), showing good analytical characteristics. In addition to the above sensing methods, other sensing methods based on MoS
2 are listed in
Table 1 to detect various cancer biomarkers.
Table 1. Electrochemical biosensors for cancer biomarkers detection based on MoS2.