Gold Nanoparticle-Biological Molecule Interactions and Catalysis

Increased Range of Catalytic Activities of Immobilized Compared to Colloidal Gold Nanoparticles

Gold nanoparticles (AuNPs) can be described as nanozymes, species that are able to mimic the catalytic activities of several enzymes, such as oxidase/peroxidase, reductase, or catalase. Most studies in the literature focus on the colloidal suspension of AuNPs, and it is obvious that their immobilization could open the doors to new applications thanks to their increased stability in this state. This work aimed to investigate the behavior of surfaces covered by immobilized AuNPs (iAuNPs). Citrate-stabilized AuNPs (AuNPs-cit) were synthesized and immobilized on glass slides using a simple dip coating method. The resulting iAuNPs were characterized (surface plasmon resonance, microscopy, quantification of immobilized AuNPs), and their multi-enzymatic-like activities (oxidase-, peroxidase-, and catalase-like activity) were evaluated. The comparison of their activities versus AuNPs-cit highlighted their added value, especially the preservation of their activity in some reaction media, and their ease of reuse. The huge potential of iAuNPs for heterogeneous catalysis was then applied to the degradation of two model molecules of hospital pollutants: metronidazole and methylene blue.

Temperature-Controlled Reconfigurable Nanoparticle Binary Superlattices

The presence of diffusionless transformations during the assembly of DNA-functionalized particles (DFPs) is highly significant in designing reconfigurable materials whose structure and functional properties are tunable with controllable variables. In this paper, we first use a variety of computational models and techniques (including free energy methods) to address the nature of such transformations between face-centered cubic (FCC) and body-centered cubic (BCC) structures in a three-dimensional binary system of multiflavored DFPs. We find that the structural rearrangements between BCC and FCC structures are thermodynamically reversible and dependent on crystallite size. Smaller nuclei favor nonclose-packed BCC structures, whereas close-packed FCC structures are observed during the growth stage once the crystallite size exceeds a threshold value. Importantly, we show that a similar reversible transformation between BCC/FCC structures can be driven by changing temperature without introducing additional solution components, highlighting the feasibility of creating reconfigurable crystalline materials. Lastly, we validate this thermally responsive switching behavior in a DFP system with explicit DNA (un)hybridization, demonstrating our findings' applicability to experimentally realizable systems.

Recent Advances in Bio-Templated Metallic Nanomaterial Synthesis and Electrocatalytic Applications

Developing metallic nanocatalysts with high reaction activity, selectivity and practical durability is a promising and active subfield in electrocatalysis. In the classical "bottom-up" approach to synthesize stable nanomaterials by chemical reduction, stabilizing additives such as polymers or organic surfactants must be present to cap the nanoparticle to prevent material bulk aggregation. In recent years, biological systems have emerged as green alternatives to support the uncoated inorganic components. One key advantage of biological templates is their inherent ability to produce nanostructures with controllable composition, facet, size and morphology under ecologically friendly synthetic conditions, which are difficult to achieve with traditional inorganic synthesis. In addition, through genetic engineering or bioconjugation, bio-templates can provide numerous possibilities for surface functionalization to incorporate specific binding sites for the target metals. Therefore, in bio-templated systems, the electrocatalytic performance of the formed nanocatalyst can be tuned by precisely controlling the material surface chemistry. With controlled improvements in size, morphology, facet exposure, surface area and electron conductivity, bio-inspired nanomaterials often exhibit enhanced catalytic activity towards electrode reactions. In this Review, recent research developments are presented in bio-approaches for metallic nanomaterial synthesis and their applications in electrocatalysis for sustainable energy storage and conversion systems.

FTDT simulations of local plasmonic fields for theranostic core-shell gold-based nanoparticles

The paper describes the results of finite-difference time-domain (FDTD) mathematical modeling of electromagnetic fields distortion near the surfaces of core-shell gold-based spherical gold nanoparticles (NPs). NPs were consistently functionalized by two shells of different thickness: a water shell, as a model substance for a drug, and an SiO₂ shell, as a capsuling layer. The calculated field values were converted into the electromagnetic field enhancement coefficient and the surface-enhanced Raman scattering (SERS) intensity. Prospects of the theoretical approach for core-shell NPs modeling to evaluate optimal field amplification and light-scattering parameters have been shown. The presented approach could be applied as a basis for performing methods of controlled synthesis for colloidal core-shell theranostic NPs.

The effect of phenylalanine ligands on the chiral-selective oxidation of glucose on Au(111)

As typical glucose oxidase nanozymes, gold nanoparticles (Au NPs) have attracted much attention due to their wide-ranging applications. Ligand caps, as the "cure-all solution" for NPs, not only play important roles in the size and shape control of Au NPs but also influence their catalytic activity and selectivity. A deep understanding of the catalytic mechanism and precise description of the important role of ligands can provide possible ways to design functional Au NPs. Here, with the specific example of Au(111) capped with chiral phenylalanine (Phe), the chiral selective oxidation mechanism of glucose and the important role of the ligands were studied via first-principles calculations. All results show that the dehydrogenation of glucose to form glucono delta-lactone (GDL) is favored on clean Au(111), while the subsequent hydrolysis of GDL is the rate-limiting step for glucose oxidation. The flat and nonchiral Au(111) surface shows negligible selectivity in relation to the oxidation of d- and l-glucose, while chiral Phe-Au(111) shows selective adsorption towards d- and l-glucose. l-Phe-capped Au(111) prefers to adsorb d-glucose, while d-Phe-capped Au(111) prefers to adsorb l-glucose. Considering the three steps in the capped ligand catalysis (adsorption, replacement and reaction), we propose that the ligands play key roles in selectively adsorbing reactants before the subsequent exchange and reaction steps.

Development of a Gold Nanoparticle-labeled Sandwich Format Lateral Flow Immunoassay Kit for the Detection of Tropical House Dust Mite Suidasia pontifica

BACKGROUND

The house dust mite Suidasia pontifica (Sp) is an important source of allergens in tropical regions that trigger IgE-mediated allergic reactions such as allergic asthma, atopic dermatitis and allergic rhinitis. Detection of Sp-specific proteins are important in the management and prevention of allergic diseases.

OBJECTIVE

The study aimed to provide a proof of concept for a gold nanoparticle-labeled sandwich format Lateral Flow Immunoassay (LFIA) kit for the detection of Sp-specific proteins.

METHODS

Protein A chromatography-purified rabbit anti-Sp polyclonal antibodies were labeled with gold nanoparticles (AuNP) synthesized from chloroauric acid using the citrate reduction method, then dispensed on a glass fiber pad. Unlabeled antibodies and anti-rabbit IgG were immobilized onto nitrocellulose membrane as test line and control line respectively. Cellulose fiber pad, glass fiber, and the nitrocellulose membrane pad were then assembled as LFIA kit.

RESULTS

Protein-A affinity chromatography purification with pre-concentration yielded 1.45 mg/mL of anti-Sp polyclonal antibodies. Synthesized AuNPs with ~20 nm sizes observed under transmission electron microscope were used for antibody conjugation at an optimal pH of 8.5 (borate buffer) and an optimal ratio of 10 µ L 50µg/mL antibody:100 µ L AuNP. Optimal color intensity and fastest migration time were observed with the treatment of 0.05% Tween20 and 10% sucrose in the conjugate pads; 5% BSA and 0.05% Tween20 in the sample pads, and 1% BSA in the test pads. The limit of detection of the LFIA Sp-specific proteins is 0.076 µg/mL. The sensitivity of the Sp LFIA kit is 83% while the specificity is 100%.

CONCLUSION

This is the first report of a prototype for a cost-effective, rapid, and equipment-free detection of the house dust mite Suidasia pontifica.

Paper-based point-of-care test with xeno nucleic acid probes

Bridging the unmet need of efficient point-of-care testing (POCT) in biomedical engineering research and practice with the emerging development in artificial synthetic xeno nucleic acids (XNAs), this review summarized the recent development in paper-based POCT using XNAs as sensing probes. Alongside the signal transducing mode and immobilization methods of XNA probes, a detailed evaluation of probe performance was disclosed. With these new aspects, both researchers in synthetic chemistry / biomedical engineering and physicians in clinical practice could gain new insights in designing, manufacturing and choosing suitable reagents and techniques for POCT.

In-silico design of nanoparticles for transdermal drug delivery application

Nanoparticles are used in the medical field for various applications like cell imaging, drug delivery, gene and si-RNA delivery, to name a few. Designing nanoparticles for a given application, purely based on the trial and error experimentation, requires a lot of time and effort. In this study we show that computer simulations could help in designing nanoparticles for drug delivery thus reducing the time and cost associated with their design, development and deployment. The permeation of nanoparticles, having various surface chemistries and patterns, through the skin lipid bilayer was studied using constrained and unconstrained molecular dynamics simulations. Interestingly, the permeation mechanism of nanoparticles having the same surface chemistry but different patterns was found to be completely different. Nanoparticles (NPs) were screened based on the free energy of permeation through the skin lipid bilayer. The behavior of the screened NPs was further validated with unconstrained simulations using the skin lipid bilayer. Nanoparticles thus screened through both of the techniques were further used for the co-delivery of a model protein into the skin lipid bilayer. It was observed that the nanoparticles having a 2 : 1 homogeneous ratio of hydrophobic to hydrophilic regions were the most promising in transdermal delivery of proteins. The obtained results are in line with the results of recent permeation experiments on cell and plasma membrane. Our study could help in in-silico design of nanoparticles for delivery of actives through skin. These in-silico experiments thus could help speed up the development process by guiding formulation chemists.

Effect of surface coverage and chemistry on self-assembly of monolayer protected gold nanoparticles: a molecular dynamics simulation study

The PMF between two nanoparticles (of any chemistry and coverage) can be used as a measure to predict their stability.

Gold Nanoparticles Supported on Alumina as a Catalyst for Surface Plasmon-Enhanced Selective Reductions of Nitrobenzene

Au nanoparticles supported on alumina (Au/Al₂O₃) with average particle size of 3.9 ± 0.7 nm and surface plasmon band centerned at 516.5 nm were prepared by deposition-precipitation method, and their photocatalytic activities for the reduction of nitrobenzene using either formic acid in acetonitrile (method A) or KOH in 2-propanol (method B) were investigated. Even at room temperature, the Au/Al₂O₃ was found to be highly active and selective for conversion of nitrobenzene to aniline when used with formic acid in acetonitrile or to azobenzene when performed with KOH in 2-propanol under irradiation with green light-emitting diode (517 nm).

Hybrid Amyloid Membranes for Continuous Flow Catalysis

Amyloid fibrils are promising nanomaterials for technological applications such as biosensors, tissue engineering, drug delivery, and optoelectronics. Here we show that amyloid-metal nanoparticle hybrids can be used both as efficient active materials for wet catalysis and as membranes for continuous flow catalysis applications. Initially, amyloid fibrils generated in vitro from the nontoxic β-lactoglobulin protein act as templates for the synthesis of gold and palladium metal nanoparticles from salt precursors. The resulting hybrids possess catalytic features as demonstrated by evaluating their activity in a model catalytic reaction in water, e.g., the reduction of 4-nitrophenol into 4-aminophenol, with the rate constant of the reduction increasing with the concentration of amyloid-nanoparticle hybrids. Importantly, the same nanoparticles adsorbed onto fibrils surface show improved catalytic efficiency compared to the same unattached particles, pointing at the important role played by the amyloid fibril templates. Then, filter membranes are prepared from the metal nanoparticle-decorated amyloid fibrils by vacuum filtration. The resulting membranes serve as efficient flow catalysis active materials, with a complete catalytic conversion achieved within a single flow passage of a feeding solution through the membrane.

The influence of stabilizers on the production of gold nanoparticles by direct current atmospheric pressure glow microdischarge generated in contact with liquid flowing cathode

Gold nanoparticles (Au NPs) were prepared by direct current atmospheric pressure glow microdischarge (dc-μAPGD) generated between a miniature argon flow microjet and a flowing liquid cathode. The applied discharge system was operated in a continuous flow liquid mode. The influence of various stabilizers added to the solution of the liquid cathode, i.e., gelatin (GEL), polyvinylpyrrolidone (PVP), or polyvinyl alcohol (PVA), as well as the concentration of the Au precursor (chloroauric acid, HAuCl₄) in the solution on the production growth of Au NPs was investigated. Changes in the intensity of the localized surface plasmon resonance (LSPR) band in UV/Vis absorption spectra of solutions treated by dc-μAPGD and their color were observed. The position and the intensity of the LSPR band indicated that relatively small nanoparticles were formed in solutions containing GEL as a capping agent. In these conditions, the maximum of the absorption LSPR band was at 531, 534, and 535 nm, respectively, for 50, 100, and 200 mg L^-1 of Au. Additionally, scanning electron microscopy (SEM) and dynamic light scattering (DLS) were used to analyze the structure and the morphology of obtained Au NPs. The shape of Au NPs was spherical and uniform. Their mean size was ca. 27, 73, and 92 nm, while the polydispersity index was 0.296, 0.348, and 0.456 for Au present in the solution of the flowing liquid cathode at a concentration of 50, 100, and 200 mg L^-1, respectively. The production rate of synthesized Au NPs depended on the precursor concentration with mean values of 2.9, 3.5, and 5.7 mg h^-1, respectively.

Probing structural dynamics of an artificial protein cage using high-speed atomic force microscopy

A cysteine-substituted mutant of the ring-shaped protein TRAP (trp-RNA binding attenuation protein) can be induced to self-assemble into large, monodisperse hollow spherical cages in the presence of 1.4 nm diameter gold nanoparticles. In this study we use high-speed atomic force microscopy (HS-AFM) to probe the dynamics of the structural changes related to TRAP interactions with the gold nanoparticle as well as the disassembly of the cage structure. The dynamic aggregation of TRAP protein in the presence of gold nanoparticles was observed, including oligomeric rearrangements, consistent with a role for gold in mediating intermolecular disulfide bond formation. We were also able to observe that the TRAP-cage is composed of multiple, closely packed TRAP rings in an apparently regular arrangement. A potential role for inter-ring disulfide bonds in forming the TRAP-cage was shown by the fact that ring-ring interactions were reversed upon the addition of reducing agent dithiothreitol. A dramatic disassembly of TRAP-cages was observed using HS-AFM after the addition of dithiothreitol. To the best of our knowledge, this is the first report to show direct high-resolution imaging of the disassembly process of a large protein complex in real time.