Exploring and exploiting the synergy of non-covalent interactions on the surface of gold nanoparticles for fluorescent turn-on sensing of bacterial lipopolysaccharide

Simple and Rapid Endotoxin Recognition Using a Dipicolylamine-Modified Fluorescent Probe with Picomolar-Order Sensitivity

Endotoxin is a lipopolysaccharide (LPS) that is found in the outer membrane of the cell wall of Gram-negative bacteria. Due to its high toxicity, the allowable endotoxin limit for water for injection is set at a very low value. Conventional methods for endotoxin detection are time-consuming and expensive and have low reproducibility. A previous study has shown that dipicolylamine (dpa)-modified pyrene-based probes exhibit fluorescence enhancement in response to LPS; however, the application of such probes to the sensing of LPS is not discussed. Against this backdrop, we have developed a simple and rapid endotoxin detection method using a dpa-modified pyrenyl probe having a zinc(II) center (Zn-dpa-C4Py). When LPS was added into Zn-dpa-C4Py solution, excimer emission of the pyrene moiety emerged at 470 nm. This probe can detect picomolar concentrations of LPS (limit of detection = 41 pM). The high sensitivity of the probe is ascribed to the electrostatic and hydrophobic interactions between the probe and LPS, which result in the dimer formation of the pyrene moieties. We also found that Zn-dpa-C4Py has the highest selectivity for LPS compared with other phosphate derivatives, which is probably caused by the co-aggregation of the probe with LPS. We propose that Zn-dpa-C4Py is a promising chemical sensor for the detection of endotoxin in medical and pharmaceutical applications.

Molecular interaction of fibrinogen with zeolite nanoparticles

Fibrinogen is one of the key proteins that participate in the protein corona composition of many types of nanoparticles (NPs), and its conformational changes are crucial for activation of immune systems. Recently, we demonstrated that the fibrinogen highly contributed in the protein corona composition at the surface of zeolite nanoparticles. Therefore, understanding the interaction of fibrinogen with zeolite nanoparticles in more details could shed light of their safe applications in medicine. Thus, we probed the molecular interactions between fibrinogen and zeolite nanoparticles using both experimental and simulation approaches. The results indicated that fibrinogen has a strong and thermodynamically favorable interaction with zeolite nanoparticles in a non-cooperative manner. Additionally, fibrinogen experienced a substantial conformational change in the presence of zeolite nanoparticles through a concentration-dependent manner. Simulation results showed that both E- and D-domain of fibrinogen are bound to the EMT zeolite NPs via strong electrostatic interactions, and undergo structural changes leading to exposing normally buried sequences. D-domain has more contribution in this interaction and the C-terminus of γ chain (γ377-394), located in D-domain, showed the highest level of exposure compared to other sequences/residues.

Citrate-stabilized gold nanoparticles hinder fibrillogenesis of a pathological variant of β2-microglobulin

Nanoparticles have repeatedly been shown to enhance fibril formation when assayed with amyloidogenic proteins. Recently, however, evidence casting some doubt about the generality of this conclusion started to emerge. Therefore, to investigate further the influence of nanoparticles on the fibrillation process, we used a naturally occurring variant of the paradigmatic amyloidogenic protein β₂-microglobulin (β2m), namely D76N β2m where asparagine replaces aspartate at position 76. This variant is responsible for aggressive systemic amyloidosis. After characterizing the interaction of the variant with citrate-stabilized gold nanoparticles (Cit-AuNPs) by NMR and modeling, we analyzed the fibril formation by three different methods: thioflavin T fluorescence, native agarose gel electrophoresis and transmission electron microscopy. The NMR evidence indicated a fast-exchange interaction involving preferentially specific regions of the protein that proved, by subsequent modeling, to be consistent with a dimeric adduct interacting with Cit-AuNPs. The fibril detection assays showed that AuNPs are able to hamper D76N β2m fibrillogenesis through an effective interaction that competes with protofibril formation or recruitment. These findings open promising perspectives for the optimization of the nanoparticle surface to design tunable interactions with proteins.

Interactions of Bacterial Lipopolysaccharides with Gold Nanorod Surfaces Investigated by Refractometric Sensing

The interface between nanoparticles and bacterial surfaces is of great interest for applications in nanomedicine and food safety. Here, we demonstrate that interactions between gold nanorods and bacterial surface molecules are governed by the nanoparticle surface coating. Polymer-coated gold nanorod substrates are exposed to lipopolysaccharides extracted from Pseudomonas aeruginosa, Salmonella enterica and Escherichia coli, and attachment is monitored using localized surface plasmon resonance refractometric sensing. The number of lipopolysaccharide molecules attached per nanorod is calculated from the shift in the plasmon maximum, which results from the change in refractive index after analyte binding. Colloidal gold nanorods in water are also incubated with lipopolysaccharides to demonstrate the effect of lipopolysaccharide concentration on plasmon shift, ζ-potential, and association constant. Both gold nanorod surface charge and surface chemistry affect gold nanorod-lipopolysaccharide interactions. In general, anionic lipopolysaccharides was found to attach more effectively to cationic gold nanorods than to neutral or anionic gold nanorods. Some variation in lipopolysaccharide attachment is also observed between the three strains studied, demonstrating the potential complexity of bacteria-nanoparticle interactions.

Effect of latent heat in boiling water on the synthesis of gold nanoparticles of different sizes by using the Turkevich method

The Turkevich method, involving the reduction of HAuCl4 with citrate in boiling water, allows the facile production of monodisperse, quasispherical gold nanoparticles (AuNPs). Although, it is well-known that the size of the AuNPs obtained with the same recipe vary slightly (as little as approximately 4 nm), but noticeably, from one report to another, it has rarely been studied. The present work demonstrates that this size variation can be reconciled by the small, but noticeable, effect that the latent heat in boiling water has on the size of the AuNPs obtained by using the Turkevich method. The increase in latent heat during water boiling caused an approximately 3 nm reduction in the size of the as-prepared AuNPs; this reduction in size is mainly a result of accelerated nucleation driven by the extra heat. It was further demonstrated that, the heating temperature can be utilized as an additional measure to adjust the growth rate of AuNPs during the reduction of HAuCl4 with citrate in boiling water. Therefore, the latent heat of boiling solvents may provide one way to control nucleation and growth in the synthesis of monodisperse nanoparticles.