Biosensor

1. Definition of biosensors

A biosensor is a device capable of detecting biomolecules, cells, or biological processes.

It enables quantitative or qualitative analysis of specific biomolecules or biological processes by combining biological elements (e.g., enzymes, antibodies, cells, etc.) with sensor technology to convert biological information into measurable signals, such as electrical, optical or chemical signals.

2. Classification of biosensors

A biosensor consists of three main components for target detection in organisms: a recognition element for specific recognition of the target; a converter to convert the detected signal into an outputable signal; and a processor to analyze the output signal.

By utilizing the optical, thermal, and electrochemical properties of different materials, different types of signal responses can be generated, resulting in a variety of biosensors.

Based on the type of output signal, biosensors can be classified as fluorescent sensors, photoelectrochemical sensors, electrochemical sensors, colorimetric sensors, and photothermal sensors.

1) Fluorescent biosensors

Fluorescence is when a substance absorbs a specific wavelength of light and then emits a longer wavelength of light, the difference in wavelength between the two is called the Stokes shift.

Fluorescence spectroscopy is a highly efficient technique for the detection of biomolecules and is widely used in the field of optical biosensors.

The method combines the high sensitivity of fluorescence detection with the high selectivity provided by specific recognition elements (e.g., ligand-binding proteins, antibodies, aptamers).

By using fluorescent nanoparticles (e.g., quantum dots, noble metal nanoparticles) and fluorescent molecules (e.g., fluorescent proteins, small-molecule dyes) as signal output units, a variety of fluorescent biosensors can be constructed.

A variety of fluorescent sensors have been developed based on the principles of fluorescence resonance energy transfer (FRET), internal filtering effect (IFE), and photobiological electron transfer (PET).

2) Photochemical biosensors

A photoelectrochemical (PEC) sensor is a device that utilizes photoelectrochemical principles for detection and analysis.

It generates electrochemical signals related to the concentration of target molecules by exciting chemical reactions in the sample with a light source.

Typically, photoelectrochemical sensors consist of a light source, a working electrode, a reference electrode and a counter electrode.

In this process, a light source excites photosensitive compounds in the sample, such as dye molecules or semiconductor nanoparticles, prompting them to participate in redox reactions or other chemical reactions.

The working electrode is responsible for receiving electrochemical signals generated by chemical reactions, while the reference electrode provides a stable potential as a reference.

Ultimately, a correlation is presented between the signal intensity and the concentration of the target molecule.

Photoelectrochemical biosensors are photoelectrochemical sensors that incorporate biorecognition techniques (e.g. enzyme-substrate, nucleic acid aptamer-target, antigen-antibody, etc.).

Photoelectrochemical biosensors have emerged as a potential alternative for clinical testing due to their ease of operation, rapid response, high selectivity and sensitivity, and instrument miniaturization.

These sensors rely primarily on photoelectric conversion in the presence of light, using photoactive electrodes to produce a measurable current signal.

When the target molecule binds to the biorecognition element, the photoactive substance in the electrode changes, affecting the separation of photogenerated electron-hole pairs, which generates the corresponding photoelectric signal.

Therefore, the selection of materials with high photovoltaic conversion efficiency is crucial for the construction of high-performance photoelectrochemical biosensors.

3) Electrochemical biosensors

Electrochemical biosensors combine electrochemical principles with biorecognition elements for the detection of a wide range of biomolecules, heavy metal ions and contaminants through the specific recognition ability of the recognition element.

These sensors convert the interaction between the biorecognition element and the target molecule into electrical signals (e.g., current, potential, resistance, capacitance, or impedance) through electrochemical conversion techniques and analyze these signals to achieve quantitative or qualitative detection.

Glucometer is one of the most successful electrochemical biosensors widely used for blood glucose monitoring in diabetic patients.

In addition to the miniaturization improvement of blood glucose meters, upgrading their anti-interference capability is also an important direction for future development.

4) Colorimetric Sensor

In practical applications, colorimetric biosensors that enable visual detection are important for the development of portable detection platforms.

Color changes can be triggered by nanoenzymes catalyzing color development reactions or by exploiting the localized surface plasmon resonance effect (LSPR) of noble metals.

By adjusting the properties of the nanomaterials, such as size and morphology, it is possible to modulate the position of the absorption peaks of the materials and thus achieve chromaticity changes.

3. Biosensor applications

With the advancement of sensing technology, biosensors have been widely used in the fields of disease diagnosis, food safety, and environmental monitoring.

Through the principle of DNA base complementary pairing, DNA networks with specific recognition ability can be designed for the detection of disease markers such as miRNAs in the human body, thus enabling disease prevention and diagnosis.

In addition, a series of DNA sequences specific for metal ions (e.g., mercury, silver, copper, and lead) have been identified, and these sequences can be used to develop biosensors for heavy metal ion detection.

By utilizing endogenous metal ions in humans, intracellular fluorescence imaging sensing platforms can also be developed.

With highly specific nucleic acid aptamers, it is possible to create a diagnostic platform that integrates sensing detection and drug release.

When combined with photothermal materials, these platforms can also be used for photothermal therapy, both to recognize cancer cells and to effectively kill them.