Optical-based biosensors

Optical biosensors are classified into several categories, including colorimetric, fluorometric, luminometric, fibre-optic and surface plasmon resonance (SPR) biosensors. The performance of these optical biosensors continues to improve due to advanced structural designs and innovative biofunctional surfaces that resist nonspecific binding [56]. Among the various types of biosensors, fluorescence-based biosensors have emerged as a particularly prominent technology due to their distinct advantages. These characteristics result in robust biosensors capable of rapidly, accurately and specifically detecting targets in complex samples. The advantages of fluorescence biosensors include their non-invasive nature, ease of use and compatibility with various detection systems. The fundamental principle of fluorescence is that a molecule absorbs light at a specific wavelength (excitation) and then emits light at a different wavelength (emission) [57]. Biosensors utilize this property to generate a signal that correlates with the concentration of the target analyte. By measuring the fluorescence signal intensity, important information about the analyte can be obtained. This principle forms the basis of the specificity and sensitivity of fluorescence biosensors [58]. In recent years, fluorescence biosensors have developed rapidly. For instance, Zhao et al. [59] developed a fluorescence biosensing system based on two-dimensional molybdenum disulfide (MoS₂) for detecting the cancer biomarker CEA. The main sensing mechanism relies on MoS₂ nanosheets' ability to quench fluorescence signals via surface interactions and nonradiative energy transfer. Initially, the fluorescent probe interacts strongly with the MoS₂ surface, resulting in quenching of light emission via FRET or PET. When the target molecule is present, a specific complex forms that shifts the surface interaction equilibrium, causing the fluorescent probe to detach from MoS₂ and thereby restoring the fluorescence signal. The change in fluorescence intensity is proportional to the target concentration, enabling rapid and sensitive quantitative detection. The study successfully detected the CEA biomarker with a LOD value of 34 pg mL-1.

Colorimetric biosensors are practical detection devices that detect the presence and concentration of biomarkers via simple colour changes. The colorimetric method offers advantages for POCT and real-time monitoring due to its flexibility, simple operation, rapid results and versatility across many applications [58]. The basic principle of colorimetric assays is to detect the presence or absence of an analyte and its concentration through a change in colour or its formation. This colour change can be caused by dyes, enzymes, or nanoparticles (NPs) such as AuNPs. Colorimetric assays typically measure changes in light absorption (absorbance) or reflection (reflectance) resulting from chemical or biochemical reactions between the target analyte and a chromogenic probe. The resulting colour change generally arises from changes in optical properties, such as SPR, or from structural shifts. These colour variations can be observed qualitatively (with the naked eye by comparing colours) or quantitatively using reading devices such as scanners, cameras, smartphones, or spectrophotometers [60]. For example, Wang et al. [61] developed a wax-printing-based multilayer μPAD for the colorimetric detection of CEA. This method showed a wide linear range (0.5 to 70 ng mL-1) with a low LOD of 0.015 ng mL-1.

Advancements in optical biosensor modification have created significant opportunities to enhance both sensitivity and multiplexing capabilities for cancer biomarker detection. Various nanomaterial-based modification strategies, such as the incorporation of functionalized layers, the use of labelled nanoparticles, and the integration of microbeads as immobilization platforms, have been shown to markedly improve target-capture efficiency while amplifying the resulting optical signals [61].

For example, Liu et al. [62] developed a multiplex magnetic bead-quantum dot (QD) assay in a microarray format to detect lung cancer biomarkers, including CYFRA 21-1, neuron-specific enolase (NSE) and CEA. By employing magnetic beads and QDs conjugated with specific antibodies, their system successfully detected these biomarkers in serum samples at low concentrations, achieving LOD of 364 pg mL-1 for CYFRA 21-1, 38 pg mL-1 for CEA and 370 pg mL-1 for NSE.

Similarly, Di et al. [63] demonstrated multiple colorimetric strategies for cancer detection. They used modified AuNPs and antibody-conjugated exosomes in a nanozyme-assisted immunosorbent assay (NAISA) to detect exosomal proteins, including CD63, CEA, GPC-3, PD-L1 and HER2, from cell cultures and clinical serum samples. This approach allows differentiation of protein levels without additional labelling steps, resulting in a faster and simpler analytical workflow. Their findings confirm that the NAISA platform enables quantitative, highly sensitive detection of exosomal proteins, with a strong linear response over the concentration range of 13.75-220 μg mL-1.