Layer-by-Layer Assembly of Polycations and Polyanions for the Sensitive Detection of Endotoxin

Advances in endotoxin analysis

The outer membrane of gram-negative bacteria is primarily composed of lipopolysaccharide (LPS). In addition to protection, LPS defines the distinct serogroups used to identify bacteria specifically. Furthermore, LPS also act as highly potent stimulators of innate immune cells, a phenomenon essential to understanding pathogen invasion in the body. The complex multi-step process of LPS binding to cells involves several binding partners, including LPS binding protein (LBP), CD14 in both membrane-bound and soluble forms, membrane protein MD-2, and toll-like receptor 4 (TLR4). Once these pathways are activated, pro-inflammatory cytokines are eventually expressed. These binding events are also affected by the presence of monomeric or aggregated LPS. Traditional techniques to detect LPS include the rabbit pyrogen test, the monocyte activation test and Limulus-based tests. Modern approaches are based on protein, antibodies or aptamer binding. Recently, novel techniques including electrochemical methods, HPLC, quartz crystal microbalance (QCM), and molecular imprinting have been developed. These approaches often use nanomaterials such as gold nanoparticles, quantum dots, nanotubes, and magnetic nanoparticles. This chapter reviews current developments in endotoxin detection with a focus on modern novel techniques that use various sensing components, ranging from natural biomolecules to synthetic materials. Highly integrated and miniaturized commercial endotoxin detection devices offer a variety of options as the scientific and technologic revolution proceeds.

Peptide-modified carbon dot aggregates for ultrasensitive detection of lipopolysaccharide through aggregation-induced emission enhancement

This study fabricated yellow-emitting CDs (Y-CDs) by hydrothermal treatment of citric acid and urea and applied them as a fluorescence turn-on platform for sensitive and selective detection of lipopolysaccharide (LPS) based on the non-shifted AIEE of peptide-stabilized CD aggregates. The designed peptide (named K3) consisting of aggregation-active and LPS-recognition units triggered the aggregation of Y-CDs, switching on their fluorescence through the blue-shifted AIEE process. The formed K3-stabilized Y-CD aggregates (K3-YCDAs) specifically interacted with LPS at neutral pH, demonstrating that the sequence of the decorated peptide was highly connected with their selectivity and sensitivity. The K3-YCDAs provided a fast response time (within 5 min) to detect LPS with a quantification range of 0.5-100.0 nM and a limit of detection (LOD, signal-to-noise ratio of 3) of 300.0 pM. By integrating ultrafiltration membranes as a concentration device with K3-YCDAs as a sensing probe, the LOD for LPS was further reduced to 3.0 pM. The determination of picomolar levels of plasma LPS by the K3-YCDAs coupled to the centrifugation ultrafiltration was demonstrated to fall within the specificity range of clinical interest for sepsis patients. Also, the K3-YCDAs served as a fluorescent probe to selectively image and quantify E. coli cells. The distinct advantages of the K3-YCDAs for LPS include fast response time, wide linear range, low detection limit, and excellent selectivity compared to previously reported sensors.

Multiarchitecture-Based Plasmonic-Coupled Emission Employing Gold Nanoparticles: An Efficient Fluorescence Modulation and Biosensing Platform

Surface plasmon-coupled emission (SPCE) is an efficient surface-enhanced fluorescence method based on the near-field coupling process of surface plasmons and fluorophores. Based on this, we developed multiple coupling structures for an SPCE system by introducing gold nanoparticles (AuNPs) with different architectures by adjusting different modification methods and configurations. By assembling AuNPs on a gold substrate through electrostatic adsorption and spin-coating, 40- and 55-fold enhancements were obtained compared to free space (FS) emission, respectively. After theoretical simulations and the optimization of experimental conditions, a novel "hot-spot" plasmonic structure, an intense electromagnetic field within the system, plasmonic properties, and the coupled process were found to be mainly responsible for the diverse enhancement effects observed. For the spin-coating deposition method, new enhancing systems with high efficiency can be easily built without complex modification. Additionally, the subsequent detection system based on the uniform modification of AuNPs through electrostatic adsorption is convenient to establish with high sensitivity and stability, which can broaden the application of SPCE in both fluorescence-based sensing and imaging. This AuNP-enhanced SPCE using an electrostatic adsorption method was designed as an immunosensor to prove feasibility.

Hydrazone ligation assisted DNAzyme walking nanomachine coupled with CRISPR-Cas12a for lipopolysaccharide analysis

In this work, hydrazone ligation assisted DNAzyme walking nanomachine is explored to couple with CRISPR-Cas12a trans-cleavage. Hydrazone ligation with high efficiency can mediate signal input which can be induced by target binding, thereby regulating the performance of DNAzyme walking nanomachine. The product strand from DNAzyme walking nanomachine can further activate the trans-cleavage of Cas12a. So, cascade signal amplification can be achieved to enhance the sensitivity for target detection. Subsequently, hydrazone ligation assisted DNAzyme walking nanomachine coupled with CRISPR-Cas12a has been further developed as a biosensor to analyze lipopolysaccharides. The developed biosensor exhibits a linear range from 0.05 ng/mL to 10⁶ ng/mL and a lowest limit of detection of 7.31 fg/mL. This research provides a new mode for the signal output of DNAzyme walking nanomachine, so as to sensitively analyze different biomolecules.

Effect of Multilayer Termination on Nonspecific Protein Adsorption and Antifouling Activity of Alginate-Based Layer-by-Layer Coatings

Layer-by-layer (LbL) assembly is a versatile platform for applying coatings and studying the properties of promising compounds for antifouling applications. Here, alginate-based LbL coatings were fabricated by alternating the deposition of alginic acid and chitosan or polyethylenimine to form multilayer coatings. Films were prepared with either odd or even bilayer numbers to investigate if the termination of the LbL coatings affects the physicochemical properties, resistance against the nonspecific adsorption (NSA) of proteins, and antifouling efficacy. The hydrophilic films, which were characterized using spectroscopic ellipsometry, water contact angle goniometry, ATR-FTIR spectroscopy, AFM, XPS, and SPR spectroscopy, revealed high swelling in water and strongly reduced the NSA of proteins compared to the hydrophobic reference. While the choice of the polycation was important for the protein resistance of the LbL coatings, the termination mattered less. The attachment of diatoms and settling of barnacle cypris larvae revealed good antifouling properties that were controlled by the termination and the charge density of the LbL films.