1. Biosensing Applications
A biosensor is an analytical system that can detect a specific biological analyte and translate presence and/or concentration information into analytical data, such as electrical, optical, and thermal signals, using a simple, low-cost, and time-effective operation
[1][2][3]. With the advent of nanotechnology, NZ biosensors, including TMDC-based, have witnessed enormous applicability in biomedical domain, particularly diagnostics, due to their intrinsic enzymatic capabilities
[1]. To date, TMDC NZs have been used to detect a variety of biochemical analytes, including tiny biomolecules (such as glucose, cholesterol, glutathione (GSH), and cysteine) as well as macromolecules (e.g., proteins).
TMDC NZ-based biosensing strategies primarily take advantage of their POD-like activity, in which they can oxidize chromogenic substrates (such as TMB, ABTS, and OPD) in the presence of H
2O
2 to produce colored products that can be measured colorimetrically
[4][5][6]. This NZ-based H
2O
2 biosensing is frequently coupled with analyte-specific oxidases such as glucose oxidase (GOx), cholesterol oxidase (ChOx), xanthine oxidase (XOx), and uricase to detect glucose, cholesterol, xanthine, and uric acid, respectively, in biological samples. First, a specific oxidase enzyme metabolizes the bioanalyte in the presence of oxygen to produce a specific acidic product and H
2O
2 as a byproduct. This H
2O
2 is further sensed colorimetrically by NZs as mentioned above. Notably, within the linear detection range, the intensity of color correlates directly with the amount of bioanalyte present in the samples. The GOx/WS
2 biosensor system, for example, was used to detect glucose with a linear range of 5–300 μM and a detection limit of 2.9 μM
[7]. Similarly, cholesterol was successfully detected at concentrations as low as 15 μM using a ChOx/Au nanoparticle-laden MoS
2 nanoribbon system
[8], whereas uricase/MoS
2 nanoflakes sensor could detect uric acid within a range of 0.5–100 μM in human serum samples
[9].
On the contrary, the detection regimes for cysteine and glutathione (GSH) differ substantially. The ability of these materials to prevent oxidation of colorimetric substrates or revert the oxidized colored product (produced via POD-/OD-like activity of NZs) to its pristine unoxidized form is the basis for their sensing
[10]. The color intensity of the reaction mix is inversely proportional to the amount of cysteine or GSH present. Previously, WS
2 nanomaterial with POD-like activity was used to estimate GSH levels as low as 0.061 nM and a linear detection range of 0.1–10 nM. GSH levels in human serum samples could be measured easily and without interference from other substances
[10]. Similarly, cysteine was quantified using Hg
2+ stimulated OD-like activity of MoS
2 QDs-Ag NPs in the 1–100 μM range
[11].
TMDC NZs can also be used to detect biomacromolecules, such as proteins, in a simple and label-free manner. To date, protein biosensing has been approached in a variety of ways. For instance, lipase was found to prevent POD-like activity of MoS
2, allowing its detection at concentrations as low as 5 nM
[12]. Other TMDC NZs-based protein detection strategies utilize nucleic acid aptamer probes due to their target (proteins or other biomolecules) selectivity, chemical stability, and ability to be synthesized in vitro
[13]. ssDNA aptamer probe/MoS
2 nanosheet system was used to detect carcinoembryonic antigen (CEA). In comparison to bare MoS
2 nanosheets, the POD-like activity of aptamer/MoS
2 was ~4.3 times higher, enabling greater oxidation of TMB substrate and consequently higher color intensity. However, when the target analyte, CEA, is present, the attached aptamer probe releases from the MoS
2 nanosheet’s surface and binds with the protein, showing a reduced TMB oxidation. This drop in color intensity can be measured and is inversely proportional to the CEA concentration. Using this method, CEA could be detected in a linear range of 50–1000 ng/mL with the detection limit of 50 ng/mL
[14]. Aptamer-anchored MoS
2/PtCu nanocomposites with strong OD-like activity were used to detect mucin 1 positive cells with high sensitivity and selectivity. Cells such as MCF-7 and A549, which have mucin 1 overexpression, could be detected even in populations as small as 300 cells. The use of NZs with OD-like activity, as in this case, is often advantageous because it surpasses the use of cytotoxic H
2O
2, thus improving the biocompatibility and allowing the biosensor to be used in conjunction with living cells
[15]. Besides, protein-specific antibodies
[16] or antibody/aptamer probes
[17] were also physically/chemically conjugated onto TMDC NZs to detect
Salmonella typhimurium-specific surface proteins and human epididymis-specific protein 4 (HE4) proteins, respectively.
Table 1 summarizes some of the recent TMDC-based NZs that have been used for molecular and macromolecular biosensing so far.
Table 1. TMDC NZs for biosensing applications. TMDC NZs for biosensing applications.
Analyte Detected |
Nanozyme System |
Activity |
Assisting Enzyme |
Detection Type |
Substrate Employed |
Linear Range |
Detection Limit |
Stability |
Biological Samples |
Ref. |
H2O2 |
MoS2 |
POD-like |