The Application of Phospholipids in Biosensors

Biosensors integrate biological recognition elements with signal transducing devices, offering high specificity and sensitivity for target detection across environmental monitoring, clinical diagnosis, and food safety. Phospholipids, the fundamental components of biological membranes, have emerged as pivotal materials in biosensor design due to their unique amphipathic structure, exceptional biocompatibility, and versatile molecular recognition capabilities. From mimicking native biological microenvironments to serving as active recognition units, phospholipids enhance biosensor performance at multiple levels, driving innovations in detection technology. This article explores the diverse applications of phospholipids in biosensors, highlighting their mechanisms, forms, and practical implications.

Core Properties of Phospholipids and Their Compatibility with Biosensor Functions

Phospholipids consist of hydrophilic head groups and hydrophobic fatty acid tails, a dual-nature structure that enables spontaneous self-assembly into ordered architectures—including phospholipid bilayers, liposomes, and nanodiscs—highly analogous to natural biological membranes. This structural mimicry underpins their compatibility with biosensor requirements.

Firstly, phospholipids’ biocompatibility minimizes toxic effects on biological recognition elements (e.g., enzymes, antibodies, cells), preserving their native conformation and biological activity. For instance, immobilizing enzymes on phospholipid-modified sensor surfaces creates a physiological microenvironment that reduces denaturation, extending the biosensor’s lifespan by 2–3 times compared to unmodified surfaces. Secondly, the reactive head groups of phospholipids (e.g., amino, carboxyl, thiol groups) provide abundant sites for covalent or electrostatic immobilization of recognition molecules. Their hydrophobic domains can also embed membrane proteins, facilitating the construction of high-specificity biosensors based on these integral biomolecules. Additionally, the permeability of phospholipid bilayers can be tuned by adjusting phospholipid types and ratios, enabling selective transport of target analytes and enhancing detection precision.

Typical Application Forms of Phospholipids in Biosensors

The self-assembly versatility of phospholipids gives rise to diverse application forms in biosensors, each tailored to specific detection principles and scenarios.

1. Phospholipid Membrane-Modified Electrodes: Electrochemical Sensing Platforms Mimicking Biomembranes

Phospholipid membrane-modified electrodes are constructed by immobilizing phospholipid monolayers or bilayers on electrode surfaces, primarily functioning as biomimetic platforms for specific recognition and electrochemical signal transduction. Common fabrication methods include the Langmuir-Blodgett (LB) technique and self-assembled monolayer (SAM) method. The LB technique transfers phospholipid monolayers formed at the air-water interface onto electrodes, allowing precise control of membrane thickness and layer count. The SAM method leverages covalent interactions between phospholipid head groups (e.g., thiols with gold electrodes, silanes with glassy carbon electrodes) to form stable self-assembled membranes.

These electrodes excel at detecting biomacromolecules (e.g., proteins, nucleic acids) and small-molecule signaling substances (e.g., neurotransmitters). For example, in acetylcholine biosensors, acetylcholinesterase is immobilized on a phospholipid-modified gold electrode. The phospholipid layer maintains enzymatic activity, and the reaction between acetylcholine and the enzyme generates electroactive products, altering the electrode’s current. This current change is quantified to determine acetylcholine concentration. Additionally, phospholipid membrane-modified electrodes are used to study drug-membrane interactions: monitoring changes in membrane potential and permeability induced by drugs provides insights into their membrane toxicity and mechanisms of action.

2. Liposome-Based Biosensors: Targeted Detection Carriers with Signal Amplification

Liposomes are spherical vesicles enclosed by phospholipid bilayers, with aqueous cores capable of encapsulating signal molecules (e.g., fluorescent dyes, enzymes, quantum dots) and surfaces modifiable with recognition units (e.g., antibodies, aptamers). They act as signal-amplifying carriers for high-sensitivity biosensors, operating on a “recognition-response-signal release” mechanism: binding of target analytes to surface recognition units triggers liposome membrane disruption or increased permeability, releasing encapsulated signal molecules. The signal intensity correlates with target concentration.

In clinical diagnostics, liposome-based biosensors are used to detect tumor markers such as carcinoembryonic antigen (CEA). Liposomes loaded with fluorophores are modified with anti-CEA antibodies, and a quencher is added to the detection system. In the absence of CEA, fluorophores are quenched; CEA binding induces liposome rupture, releasing fluorophores and generating a measurable signal, with a detection limit as low as nanogram levels. In environmental monitoring, these sensors detect heavy metal ions (e.g., Hg²⁺, Pb²⁺): specific binding between metal ions and phospholipid head groups disrupts liposomes, releasing signal molecules for quantitative analysis, offering rapid and straightforward operation.

3. Phospholipid Nanodiscs: Stable Scaffolds for Membrane Protein-Based Sensors

Membrane proteins (e.g., receptors, ion channels) are ideal recognition elements due to their high specificity, but they readily lose activity when removed from native membranes. Phospholipid nanodiscs—disc-shaped nanostructures composed of phospholipid bilayers and amphipathic proteins (e.g., apolipoprotein A-I), typically 5–100 nm in diameter—provide a native-like hydrophobic environment for membrane proteins, stabilizing their conformation and activity. This makes them critical for developing membrane protein-based biosensors.

In drug screening, phospholipid nanodiscs enable the construction of G protein-coupled receptor (GPCR) biosensors. Embedding GPCRs in nanodiscs and immobilizing them on sensor chips allows monitoring of receptor conformational changes upon drug binding via surface plasmon resonance (SPR) technology. SPR detects refractive index shifts on the chip surface, evaluating drug-receptor binding affinity for rapid activity screening. Additionally, nanodiscs facilitate viral detection: leveraging interactions between viral surface proteins and membrane proteins, nanodisc-mediated signal amplification enables high-sensitivity virus detection, with potential applications in rapid infectious disease diagnosis.

Performance Optimization and Challenges of Phospholipid-Based Biosensors

Phospholipid properties directly influence biosensor performance, and tailoring phospholipid composition optimizes key metrics. For example, phospholipids with unsaturated fatty acid chains (e.g., 1-palmitoyl-2-oleoyl phosphatidylcholine) increase membrane fluidity, enhancing the diffusion and binding efficiency of biomolecules. Incorporating cholesterol strengthens membrane stability, reducing interference from environmental factors (e.g., pH, temperature). Modifying head groups (e.g., introducing carboxyl or amino groups) improves the immobilization efficiency of recognition elements, boosting specificity.

Despite their advantages, phospholipid-based biosensors face inherent challenges. Phospholipid membranes exhibit poor long-term stability, degrading via enzymatic hydrolysis or temperature fluctuations, which shortens sensor lifespan. Impurities in complex samples (e.g., proteins, lipids in serum or wastewater) adsorb onto phospholipid surfaces, causing non-specific signal interference and reducing accuracy. Additionally, high production costs of purified phospholipids hinder large-scale industrialization. To address these issues, researchers are developing hybrid membranes (combining phospholipids with polymers) to enhance stability, using surface modification techniques (e.g., polyethylene glycol coating) to reduce non-specific adsorption, and optimizing phospholipid synthesis/purification processes to lower costs.

Conclusion

Phospholipids, with their biomimetic structure, biocompatibility, and structural tunability, are indispensable functional materials in biosensor development. They support biological recognition elements, enable signal amplification, and facilitate targeted detection, spanning the core stages of biosensor construction. As research advances—particularly in hybrid membrane development, intelligent response design, and low-cost synthesis—phospholipid-based biosensors will achieve higher sensitivity, stability, and affordability. Their applications will continue to expand into emerging fields such as point-of-care testing and single-cell analysis, solidifying phospholipids’ role as a cornerstone of modern biosensor technology.