Aggregation Of Gold Nanoparticles Research Articles

A Sensitive Colorimetric Determination of Metformin Based on Gold Nanoparticles Aggregation in Salt Solution

AbstractThis research attempts to explore a simple and sensitive colorimetric method to detect metformin by using synthesized gold nanoparticles (AuNPs). Spherical AuNPs were synthesized by reducing HAuCl4 with EDTA. Transmission electron microscopy (TEM) and Zeta potential were used to observe AuNPs size and solution stability before and after the addition of metformin. AuNPs became unstable and aggregated after the addition of metformin, and the absorption peak changed from 520 to 654 nm after adding metformin. NaCl concentration is an important factor in AuNPs aggregation, as low as 70 nM metformin can induce the aggregation of AuNPs under 40 mM NaCl. The linear relationship of metformin detecting was gained in the range of 150 nM∼500 nM (R2=0.955). The application of this method to the detection of metformin in actual samples has been verified. Therefore, AuNPs is considered to be a potential multidrug detection tool based on AuNPs surface modifiers.

Block Copolymer Supported Gold Nanoparticles Assemblies with Exposed Gold Surface

Block Copolymer Supported Gold Nanoparticles Assemblies with Exposed Gold Surface

Conformational control of two-dimensional gold nanoparticle arrays in a confined geometry within a vesicular wall

The two-dimensional (2D) assembly of gold nanoparticles (AuNPs) in a confined geometry is a rare phenomenon that has not been experimentally verified for complex systems. In this study, this process was investigated in detail using two types of block copolymers with hydrophobic and hydrophilic blocks and a series of AuNPs of three different sizes protected by hydrophobic ligands. In aqueous solutions, the selected block copolymers self-assembled into vesicular nanostructures with a hydrophobic domain in the wall, which functions as a confined geometrical space for hydrophobic AuNPs (i.e., it exerts a confinement effect and restricts the movement of AuNPs). Small-angle X-ray scattering studies revealed that AuNPs of different sizes assembled differently in the same confined geometry of the vesicular wall. In addition, optimal conditions for the formation of a regular NP array in the hydrophobic domain were determined. The AuNPs successfully self-assembled into a regular 2D lattice structure, forming a shell around the vesicle, when their size matched the thickness of the hydrophobic domain of the vesicular nanostructure. This study provides guidelines for the fabrication of nanoparticle arrays with controlled structures, which could enhance the functionality of materials and their physical properties.

Acid-Responsive Aggregated Gold Nanoparticles for Radiosensitization and Synergistic Chemoradiotherapy in the Treatment of Esophageal Cancer.

Radiotherapy and chemotherapy are limited by insufficient therapeutic efficacy of low-dose radiation and nonspecific drug biodistribution. Herein, an acid-responsive aggregated nanosystem (AuNPs-D-P-DA) loaded with doxorubicin (DOX) is designed for radiosensitization and synergistic chemoradiotherapy. In response to the acid microenvironment of esophageal cancer (EC), small-sized AuNPs-D-P-DA forms large-sized gold nanoparticle (AuNPs) aggregates in tumor tissues to hinder the backflow of AuNPs to the circulation, resulting in enhanced tumor accumulation and retention. Simultaneously, the AuNPs-based radiosensitization is significantly improved because of the high concentration and large size of intratumoral AuNPs, while DOX are delivered and released specifically into tumor cells triggered by the acid microenvironment for chemo-radio synergistic therapy. Acid-responsive AuNPs exacerbate radiation-induced DNA damage, cell apoptosis, cell cycle arrest, and low colony formation ability in vitro and enhance anti-tumor efficacy in vivo compared to un-responsive control. When combined with acid-responsive DOX, the therapeutic efficacy of the formulation is further improved by their synergistic effect. After the treatment of acid-responsive AuNPs plus radiotherapy, fatty acid metabolism is reprogrammed in xenograft models, which provides potential targets for further improvement of radiosensitization. In summary, the acid-responsive AuNPs-D-P-DA nanosystem leverages the radio- and chemotherapeutic synergies of AuNPs-sensitized X-ray irradiation and acid-responsive DOX in the treatment of EC.

Ultrasensitive colorimetric detection of amoxicillin based on Tris-HCl-induced aggregation of gold nanoparticles

An ultrasensitive colorimetric aptasensor for the detection of amoxicillin (AMO) based on the Tris-HCl buffer-induced aggregation of gold nanoparticles (AuNPs) was developed. The AuNPs were aggregated by the addition of Tris-HCl buffer. The adsorption of the aptamer on the AuNP surface increased its negative charge density, leading to the enhancement of the electrostatic repulsion between the nanoparticles, thus protecting AuNPs from aggregation in the Tris-HCl buffer. However, the specific binding of the aptamer with AMO induced conformational changes in the aptamer, which reduced the adsorption of the aptamer on the AuNP surface, diminishing the protective effect of the aptamer. This resulted in the aggregation of AuNPs by Tris-HCl buffer, and consequently, color change of the solution containing AuNPs from red to blue. Under optimized conditions, a linear relationship between the absorbance ratio variation (ΔA680/A520) and the AMO concentration was observed in the concentration range of 0.1–125 nM, with a detection limit of 67 pM. The developed biosensor exhibited high selectivity toward AMO. Moreover, this strategy was successfully applied to the detection of AMO in lake water samples. Thus, the present aptasensor is a promising alternative for the simple and ultrasensitive detection of AMO in the environment.

Gold and Cobalt Nanoparticles Dispersed on N‐Doped Carbon Matrix as a Catalyst for 4‐Nitrophenol Reduction

AbstractIn this work, gold, cobalt nanoparticles dispersed on a porous N doped Carbon matrix (Au/Co@N−C), which were prepared by replacement reaction to anchor Au nanoparticles (NPs) on the surface of the carbonized ZIF‐67 (Co@N−C). The Au/Co@N−C composites display excellent catalytic activity for the reduction of 4‐nitrophenol (4‐NP) due to the highly ordered carbon frameworks, which offered porous structure and space to improve the capability of electronic transmission. Moreover, the method of replacement reaction avoided the aggregation of gold nanoparticles effectively, which enhanced the catalytic activity of the Au/Co@N−C composites. The catalytic properties of nanocomposites were assessed through the reduction reaction of 4‐nitrophenol (4‐NP) translate 4‐NP into 4‐aminophenol (4‐AP) with excessive NaBH4, the K app was up to 37.46×10−3 s−1 and the conversion rate of 4‐NP remained at 89 % after five consecutive cycles. Thus, this work has excellent promising preparation of noble metal catalysts and application in the catalysis of phenols.

Kinetic and Mechanistic Insight into the Surfactant-Induced Aggregation of Gold Nanoparticles and Their Catalytic Efficacy: Importance of Surface Restructuring.

Understanding the fundamental interactions between plasmonic metal nanoparticles (MNPs) and small molecules is of utmost importance in various applications such as catalysis, sensing, drug delivery, optoelectronics, and surface-enhanced Raman spectroscopy. Herein, we have investigated the early stage of the aggregation pathway of citrate-stabilized Au NPs with surfactants and explored their catalytic efficacy. Our findings reveal that (17 ± 2)-nm-sized citrate-stabilized Au NPs undergo concentration and time-dependent aggregation with positively charged cetyltrimethylammonium bromide (CTAB). Kinetic analyses revealed the presence of two distinct kinds of aggregates, namely, smaller clusters and a larger branched network of Au nanochains. At longer times and in the presence of higher concentrations of CTAB, these branched networks of Au nanochains transform into dense compact globular aggregates. The catalytic efficacy of Au NPs, branched Au nanochains, and dense compact aggregates has been explored with respect to the reductive hydrogenation of 4-nitophenol in the presence of excess NaBH4. Our study revealed that the catalytic rate decreases in the order of Au NPs > branched Au nanochains > compact aggregates. Interestingly, pre-equilibrating different Au NP samples with excess NaBH4 prior to the onset of the reaction results in similar catalytic activity irrespective of the aggregation state of Au NPs. This observation has been explained by considering efficient surface restructuring via ligand exchange with H- ions and the subsequent disruption of CTAB-induced aggregates of Au NPs. Moreover, the aggregated Au NPs can be recycled over several consecutive cycles for the reductive hydrogenation of 4-NP upon ligand exchange with H- ions. Taken together, our present study highlights the early-stage aggregation kinetics of Au NPs with CTAB surfactants and demonstrates the importance of the surface restructuring of Au NPs on their catalytic efficacy.

Membrane Phase Drives the Assembly of Gold Nanoparticles on Biomimetic Lipid Bilayers.

In recent years, many efforts have been devoted to investigating the interaction of nanoparticles (NPs) with lipid biomimetic interfaces, both from a fundamental perspective aimed at understanding relevant phenomena occurring at the nanobio interface and from an application standpoint for the design of novel lipid–nanoparticle hybrid materials. In this area, recent reports have revealed that citrate-capped gold nanoparticles (AuNPs) spontaneously associate with synthetic phospholipid liposomes and, in some cases, self-assemble on the lipid bilayer. However, the mechanistic and kinetic aspects of this phenomenon are not yet completely understood. In this study, we address the kinetics of interaction of citrate-capped AuNP with lipid vesicles of different rigidities (gel-phase rigid membranes on one side and liquid-crystalline-phase soft membranes on the other). The formation of AuNP–lipid vesicle hybrids was monitored over different time and length scales, combining experiments and simulation. The very first AuNP–membrane contact was addressed through molecular dynamics simulations, while the structure, morphology, and physicochemical features of the final colloidal objects were studied through UV–visible spectroscopy, small-angle X-ray scattering, dynamic light scattering, and cryogenic electron microscopy. Our results highlight that the physical state of the membrane triggers a series of events at the colloidal length scale, which regulate the final morphology of the AuNP–lipid vesicle adducts. For lipid vesicles with soft membranes, the hybrids appear as single vesicles decorated by AuNPs, while more rigid membranes lead to flocculation with AuNPs acting as bridges between vesicles. Overall, these results contribute to a mechanistic understanding of the adhesion or self-assembly of AuNPs onto biomimetic membranes, which is relevant for phenomena occurring at the nano–bio interfaces and provide design principles to control the morphology of lipid vesicle–inorganic NP hybrid systems.

Opportunities for Mercuric Ion Spectrophotometric Determination based on Reduction of Gold Nanoparticles Aggregation by N-containing Cyclic Molecules

Opportunities for Mercuric Ion Spectrophotometric Determination based on Reduction of Gold Nanoparticles Aggregation by N-containing Cyclic Molecules

Label-Free Protein Analysis by Pyro-Electrohydrodynamic Jet Printing of Gold Nanoparticles.

The pyro-electrohydrodynamic jet (p-jet) printing technology has been used for the fabrication of confined assemblies of gold nanoparticles with a round shape and a diameter ranging between 100 and 200 μm. The surface-enhanced Raman spectroscopy (SERS) performance of the p-jet substrate was evaluated by using Rhodamine 6G (R6G) as a reference. The results demonstrate that this kind of SERS substrate exhibits strong plasmonic effects and a significant reproducibility of the signal with a coefficient of variation below 15%. We tested the signal behavior also in case of the bovine serum albumin (BSA) as a model analyte, to demonstrate the affinity with biomolecules. Strong SERS activity was measured also for BSA across the whole spot area. The spectral patterns collected in different locations of the sensing area were highly reproducible. This observation was substantiated by multivariate analysis of the imaging datasets and opens the route towards a potential application of this kind of SERS substrate in biosensing.

Optical properties of formation of gold nanoparticle aggregates deposited on quartz glass and application to SPR sensing

The sensing of surrounding medium is a research focus, which is highly dependent on the nanotechnology that exhibits typical features of technical difficulty and high price. In this work, a surrounding medium sensor based on gold nanoparticles (Au NPs) is fabricated. The formation of Au NP aggregates is deposited on quartz glass by alternately immerses the glass sheet in positive charged cationic polymer polyethyleneimine and negative charged citrate coated Au NPs. The formation of Au NP aggregates with different numbers of NPs is fabricated controlling numbers of deposition and deposition time. The formation of aggregates contains less NPs exhibit ability of refractive index sensing by monitoring the surface plasmon resonance (SPR) of Au NPs. The formation of Au NP aggregates can be used as a substitute of Au NPs array for the application of SPR sensing for the merits of easy fabrication and low cost.

Encapsulation of Gold Nanoparticles into Redesigned Ferritin Nanocages for the Assembly of Binary Superlattices Composed of Fluorophores and Gold Nanoparticles.

Nanomaterials with a defined composition and structure can be synthesized by exploiting natural templates or biomolecular matrices. In the present work, we use protein nanocages derived from human ferritin as a nanoscale building block for the assembly of gold nanoparticles and fluorescent molecules in the solid state. As a generalizable strategy, we show that prior to material synthesis, the cargo can be encapsulated into the protein nanocages using a dis- and reassembly approach. Toward this end, a new ligand system for gold nanoparticles enables efficient encapsulation of these particles into the nanocages. The gold nanoparticle-loaded protein nanocages are co-assembled with fluorophore-loaded protein nanocages. Binary superlattices are formed because two oppositely charged ferritin nanocages are used as templates for the assembly. The binary crystals show strong exciton-plasmon coupling between the encapsulated fluorophores and gold nanoparticles, which was spatially resolved with fluorescence lifetime imaging. The strategy outlined here offers a modular approach toward binary nanomaterials with highly ordered building blocks.

A colorimetric sensor for Staphylococcus aureus detection based on controlled click chemical-induced aggregation of gold nanoparticles and immunomagnetic separation.

Staphylococcus aureus (S. aureus) is a pathogen closely associated with foodborne diseases. We prepared a reliable colorimetric sensor to detect S. aureus using click chemical reaction and immunomagnetic separation. Aptamer-functionalized and ALP-labeled Fe3O4 NPs act as separation and signal transduction elements. Under the optimized conditions, the Cu+ generated by signal transduction triggers a click chemistry reaction, which causes the aggregation of azides and alkyne-AuNPs and a color change. The net extinction ratio of Δ(A530/A760) was linearly correlated with the S. aureus concentration from 10 to 106cfumL-1, and the limit detection was 2.4cfumL-1. The recoveries were 91.15 ~ 106.36% for the analysis of spiked food and water samples without pre-enrichment. Therefore, we believe that the detection platform can be easily and accurately used for S. aureus detection, providing a broad prospect for on-site visual detection.

Ultrasensitive Competitive Lateral Flow Immunoassay with Visual Semiquantitative Inspection and Flexible Quantification Capabilities

Antibiotics abuse has caused various problems threatening human health and ecological environment. Monitoring antibiotics residual levels is of great significance, yet still challenging for quantitative point-of-need testing with high-sensitivity and visual capability. Here we developed a competitive lateral flow immunoassay (CLFIA) platform with flexible readout for enrofloxacin (ENR), a regularly added antibiotic. To overcome the limitation of low sensitivity of traditional colloidal gold-based CLFIA, the three-dimensionally assembled gold nanoparticles (AuNPs) within dendritic silica scaffold were fabricated as signal reporters. The assembly structure effectively retained the intrinsic absorption features of hydrophobic AuNPs and greatly enhanced the light extinction ability of a single label for signal amplification. The obtained CLFIA strips can not only achieve qualitative screening of ENR at a very low concentration by naked eye (cutoff value: 0.125 ng/mL), but also enable ultrasensitive quantification of ENR by an optical scanner (limit of detection: 0.00195 ng/mL) or a smartphone (limit of detection: 0.0078 ng/mL). Moreover, to elaborate the visual inspection degree of CLFIA against traditional yes/no interpretation, a novel multirange gradient CLFIA strip was prepared for visually semiquantitative identification of ENR with four concentration ranges. The novel CLFIA platform demonstrated sensitive, specific, and reliable determination of ENR with flexible signal readout and provides a potential and invigorating pathway to point-of-need immunoassay of antibiotics.

A low-fouling and reusable biosensor for the sensitive electrochemical assay of tuberculosis gene

A low-cost, portable, and reusable biosensor for the rapid and selective quantitation of tuberculosis (TB) gene with low expression levels in early-stage serum specimens is highly desirable but remains a challenge. Herein, a novel biosensor was fabricated using the layer-by-layer assembly of gold nanoparticles (Au NPs), polyethylenimine (PEI) and chondroitin sulfate (CS). The synergistic effect of PEI and CS endows this biosensor antifouling properties towards single proteins, even towards complex biological systems. Before and after the incubation in 10% serum, only 6.4% peak current change of DPV response was obtained. To the best of our knowledge, a few examples have been reported on the successful application of CS in mitigating biofouling. Next, a special gene only representing TB was modified on CS-modified electrode, and this biosensor showed a fast response with good linear concentration dependence on the target DNA in the range 10−14 M to 10−10 M. More importantly, this biosensor based on the reversible bonding of DNA double strand could be reused for at least four times after the regeneration of urea without any passivation, which further reduced the cost of the entire strategy. To some extent, this indicates that the CS-modified biosensor has certain practical application prospect and can be extended to fabricate similar biosensors for the detection of other diseases.

Colorimetric immunoassay via smartphone based on Mn2+-Mediated aggregation of AuNPs for convenient detection of fumonisin B1

Fumonisin B1 (FB1) is one of the most toxic and widely distributed subtypes of the 20 fumonisins, which has attracted much attention in food and feed safety. In this study, a convenient colorimetric immunoassay was proposed for the detection of FB1 based on Mn 2+ -mediated aggregation of gold nanoparticles (AuNPs). After hydrolyzing of ascorbic acid 2-phosphate (AAP) by alkaline phosphatase (ALP), the generated ascorbic acid (AA) decomposes brown MnO 2 shell to Mn 2+ , which growth onto AuNPs and inducing the aggregation of AuNPs. The color change of the solution induced by aggregating of AuNPs can be read by smartphone to avoid subjective differences. The limit of detection (LOD) of this assay was 0.15 ng/mL and the limit of quantification (LOQ) was 0.50 ng/mL. The method has not cross-reaction with other mycotoxins. The average recoveries of FB1 in the maize flour samples ranged from 86.4 % to 110.9 %, which met the food and Drug Administration (FDA) restrictions on FBS in agricultural products. The colorimetric immunoassay can be potentially used for mycotoxin-related food safety monitoring. • A novel immunosensor for FB1 was developed based on Mn 2+ mediated AuNPs aggregation. • With smartphone readings, precise on-site quantitative testing can be carried out without the need of other equipment. • The LOD is lower than maximum residue limit of FB1 set by China, EU, US and FAO.

Understanding the Coupling Mechanism of Gold Nanostructures by Finite-Difference Time-Domain Method

Gold nanoparticle assemblies show a strong plasmonic response due to the combined effects of the individual nanoparticles’ plasmon modes. Increasing the number of nanoparticles in structured assemblies leads to significant shifts in the optical and physical properties. We use finite-difference time-domain (FDTD) simulations to analyze the electromagnetic response of structurally ordered gold nanorods in monomer and dimer configurations. The plasmonic coupling between nanorods in monomers or dimers configurations provides a unique technique for tuning the spectrum intensity, spatial distribution and polarization of local electric fields within and surrounding nanostructures. This study shows an exponential coupling behavior when two gold nanorods are assembled in end-to-end and side-by-side dimer configurations with a small separation distance. The maximum electric field in the gaps between adjacent nanorods in end-to-end dimer configuration describes a more significant enhancement factor than the individual gold nanorod. Our FDTD simulation on dimer in end-to-end assembly for small separation distance up to [Formula: see text][Formula: see text]nm can well explain the observed experimental growth dynamics of gold nanorods.

Long-Term Cryopreservation of Ready-to-Use DNA-Modified Gold Nanoparticle Derivatives: Effect of Preservation Temperature on Their DNA Dissociation and Functional Stability

Abstract The effect of preservation temperature on the DNA dissociation and functional stability of two types of DNA-modified gold nanoparticles (GNPs) was studied. The cryopreservation of both GNPs for 56 days has fully prevented the DNA dissociation and maintained the toehold-mediated DNA displacement reaction capacity without any aggregation of GNPs.

Double-Resonant Nanostructured Gold Surface for Multiplexed Detection

A novel double-resonant plasmonic substrate for fluorescence amplification in a chip-based apta-immunoassay is herein reported. The amplification mechanism relies on plasmon-enhanced fluorescence (PEF) effect. The substrate consists of an assembly of plasmon-coupled and plasmon-uncoupled gold nanoparticles (AuNPs) immobilized onto a glass slide. Plasmon-coupled AuNPs are hexagonally arranged along branch patterns whose resonance lies in the red band (∼675 nm). Plasmon-uncoupled AuNPs are sprinkled onto the substrate, and they exhibit a narrow resonance at 524 nm. Numerical simulations of the plasmonic response of the substrate through the finite-difference time-domain (FDTD) method reveal the presence of electromagnetic hot spots mainly confined in the interparticle junctions. In order to realize a PEF-based device for potential multiplexing applications, the plasmon resonances are coupled with the emission peak of 5-carboxyfluorescein (5-FAM) fluorophore and with the excitation/emission peaks of cyanine 5 (Cy5). The substrate is implemented in a malaria apta-immunoassay to detect Plasmodium falciparum lactate dehydrogenase (PfLDH) in human whole blood. Antibodies against Plasmodium biomarkers constitute the capture layer, whereas fluorescently labeled aptamers recognizing PfLDH are adopted as the top layer. The fluorescence emitted by 5-FAM and Cy5 fluorophores are linearly correlated (logarithm scale) to the PfLDH concentration over five decades. The limits of detection are 50 pM (1.6 ng/mL) with the 5-FAM probe and 260 fM (8.6 pg./mL) with the Cy5 probe. No sample preconcentration and complex pretreatments are required. Average fluorescence amplifications of 160 and 4500 are measured in the 5-FAM and Cy5 channel, respectively. These results are reasonably consistent with those worked out by FDTD simulations. The implementation of the proposed approach in multiwell-plate-based bioassays would lead to either signal redundancy (two dyes for a single analyte) or to a simultaneous detection of two analytes by different dyes, the latter being a key step toward high-throughput analysis.

Aggregation of DNA-Grafted Nanoparticles in Water: The Critical Role of Sequence-Dependent Conformation of DNA Coating.

Aggregation behavior of nanoparticles in water, particularly in the presence of surface coating, is a fundamental behavior to understand as it closely relates to the material function, reactivity, and toxicity. Engineered DNA, an important type of surface coating, is essentially different from conventional polymeric coatings with homogeneous repeating units. The heterogeneous nature of DNA with numerous combinations of the four nucleotides poses a challenge to understand the aggregation of nanoparticles coated with them. In this study, we use both experimental and computational approaches to study how DNA properties influence the aggregation of DNA-conjugated gold nanoparticles (DNA-AuNPs). Aggregation kinetics under different pH values and in three salts (NaCl, CaCl2, and MgCl2) were investigated. Overall, DNA-AuNPs demonstrated similar aggregation behavior of electrosterically stabilized nanoparticles, where the surface coating layer thickness played a determining role. The underlying interactions between DNA-AuNPs were analyzed by an extended DLVO model, and the excluded volume repulsion, which varied as a function of layer thickness, was the dominant stabilizing repulsion. Interestingly, the DNA sequence was found to cause different layer thicknesses and thus profoundly different aggregation behaviors in the presence of cations. The interactions between DNA strands with different sequences and surrounding counterions were investigated via molecular dynamics simulations. The simulation results were consistent with experimental observations and supported the sequence-dependent conformational change of DNA coating. Our study provides new molecular-level insights into aggregation of nanoparticles with heterogeneous surface coating such as DNA.