Charged Gold Nanoparticles Research Articles

Gold Nanoparticle–Polyelectrolyte Complexes with Tunable Structure Probed by Synchrotron Small-Angle X-ray Scattering: Implications for the Production of Colloidal Crystals-Based Nanophotonic Materials

Close-packed colloidal crystals of gold nanoparticles (AuNPs) are of great interest in the field of nanophotonics due to their lattice-pattern-dependent optical properties. One challenge resides in producing such crystalline assemblies by a scalable approach involving polydisperse AuNPs obtained by standard synthesis routes and structuring natural molecules in water at room temperature. Electrostatic complexation between functionalized AuNPs and oppositely charged polyelectrolytes (PELs) is a very simple way to create AuNP self-assemblies of different morphologies and compactness. Our work investigates, using synchrotron small-angle X-ray scattering, their structure as a function of concentration, PEL persistence length and ionic strength. By decreasing the radius, R, of the positively charged gold nanoparticles to a few nm (R ≤ 3 nm) and increasing the polyelectrolyte persistence length, LT, substituting flexible sodium polystyrenesulfonate for natural semiflexible hyaluronan, we tuned the characteristic ratio LT/R up to values of ≥1.85. Such ratios allow the successful formation of a new AuNPs arrangement with a high degree of short-range order at low ionic strength and of crystalline order when interactions are screened by addition of salt. This approach involving commercially available natural water-soluble semiflexible PELs, opens a low-cost and promising way for the one-step production of gold colloidal crystals, which can be of ubiquitous utility in nanophotonics.

Polyethyleneimine/polyethylene glycol–conjugated gold nanoparticles as nanoscale positive/negative controls in nanotoxicology: testing in frog embryo teratogenesis assay–Xenopus and mammalian tissue culture system

Despite the great potential of using positively charged gold nanoparticles (AuNPs) in nanomedicine, no systematic studies have been reported on their synthesis optimization or colloidal stability under physiological conditions until a group at the National Institute of Standards and Technology recently succeeded in producing remarkably stable polyethyleneimine (PEI)-coated AuNPs (Au-PEI). This improved version of Au-PEI (Au-PEI25kB) has increased the demand for toxicity and teratogenicity information for applications in nanomedicine and nanotoxicology. In vitro assays for Au-PEI25kB in various cell lines showed substantial active cytotoxicity. For advanced toxicity research, the frog embryo teratogenesis assay–Xenopus (FETAX) method was employed in this study. We observed that positively-charged Au-PEI25kB exhibited significant toxicity and teratogenicity, whereas polyethylene glycol conjugated AuNPs (Au-PEG) used as comparable negative controls did not. There is a characteristic avidity of Au-PEI25kB for the jelly coat, the chorionic envelope (also known as vitelline membrane) and the cytoplasmic membrane, as well as a barrier effect of the chorionic envelope observed with Au-PEG. To circumvent these characteristics, an injection-mediated FETAX approach was utilized. Like treatment with the FETAX method, the injection of Au-PEI25kB severely impaired embryo development. Notably, the survival/concentration curve that was steep when the standard FETAX approach was employed became gradual in the injection-mediated FETAX. These results suggest that Au-PEI25kB may be a good candidate as a nanoscale positive control material for nanoparticle analysis in toxicology and teratology.

Effect of substrate charge density on the adsorption of intrinsically disordered protein amyloid β40: a molecular dynamics study.

The inhibitory effect of negatively charged gold nanoparticles (AuNPs) on amyloidogenic protein fibrillation has been established from experiments and computer simulations. Here, we investigate the effect of the charge density (σ) of gold (Au) surfaces on the adsorption of the intrinsically disordered amyloid β40 (Aβ40) monomer using molecular dynamics (MD) simulations. On the basis of the binding free energy, some key residues (ARG5, LYS16, LYS28, LEU17-ALA21, ILE31-VAL38) were found to be responsible for preventing the β-sheet formation, which is known to be a precursor for fibrillation. Until a critical charge density (σc) of -0.167 e nm-2, the key residues remained adsorbed on the Au slab. A saturation in the number of condensed counterions (Na+) on Aβ40 was also observed at σc. Beyond σc, the condensation of Na+ occurs only on the Au slab, leading to competition between positively charged key residues and condensed ions. This competition was found to be responsible for the lack of adsorption of the key residues, leading to β-sheet formation for σ > -0.167 e nm-2. This study suggests that if the key residues are not adsorbed, then β-sheet formation is observed, which can then lead to the development of proto-fibrils and subsequently fibrillation. Therefore the surface should have an optimal charge density to be an effective inhibitor of fibrillation.

Sensitive and convenient detection of miRNA-145 using a gold nanoparticle-HCR coupled system: computational and in vitro validations.

Multiple sclerosis (MS) remains a challenging disease that requires timely diagnosis. Therefore, an ultrasensitive optical biosensor based on hybridization chain reaction (HCR) was developed to detect microRNA-145 (miRNA-145) as an MS biomarker. To construct such a sensor, HCR occurred between specific hairpin probes, as MB1 contains a poly-cytosine nucleotide loop and MB2 has a poly-guanine nucleotide sticky end. By introducing miR-145 as a target sequence, long-range dsDNA polymers are formed. Then, positively charged gold nanoparticles (AuNPs) were incubated with the HCR product, which adsorbed onto the dsDNA polymers due to electrostatic adsorption. This resulted in the precipitation of the AuNPs. By incubating different concentrations of miR-145 with AuNPs, the changes in the UV-vis spectrum of the supernatant were analyzed. The proposed biosensor showed a great ability to detect miR-145 in a wide linear range from 1 pM-1 nM with an excellent detection limit (LOD) of 0.519 nM. Furthermore, the developed biosensor indicated considerable selectivity in discriminating between miR-145 and mismatched sequences. It shows high selectivity in differentiating targets. Interestingly, the proposed method was also able to detect miRNA-145 in the diluted serum samples. In conclusion, this sensing platform exhibits high selectivity and specificity for the detection of circulating microRNAs, which holds great promise for translation to routine clinical applications.

Analyte-triggered in situ "off-on" of Tyndall effect for smartphone-based quantitative nanosensing of Ag+ ions.

This work describes two new colorimetric methods for smartphone-based point-of-care nanosensing of toxic Ag+ ions. They were based on the analyte-triggered in situ "off-on" of Tyndall effect (TE) of non-plasmonic colloid or plasmonic metal nanoprobes. The first TE-inspired assay (TEA) focused on the initial analytical application of precipitation reactions where a non-plasmonic AgCl colloid could be formed once mixing the analyte with a NaCl solution. Such AgCl colloid displayed strong visual TE signals after their irradiation by a laser pointer pen, which unexpectedly achieved a detection limit of ~ 400nM. The second TEA was further designed to reduce the limit down to ~ 78nM using the analyte's oxidizability towards 3,3',5,5'-tetramethylbenzidine molecules. The redox reaction could create positively charged products that could make negatively charged plasmonic gold nanoparticles aggregate through electrostatic interactions to remarkably amplify their TE responses. Both limits were lower than the minimum allowable Ag+ level (~ 460nM) in drinking water issued by the World Health Organization. The satisfactory recovery results for detecting Ag+ ions in river, pond, tap, and drinking water additionally demonstrated good selectivity, accuracy and practicality of the proposed methods for potential point-of-need uses in environmental analysis, public health, water safety, etc.

Raman spectroscopy combined with deep learning for rapid detection of melanoma at the single cell level

Melanoma is an aggressive and metastatic skin cancer caused by genetic mutations in melanocytes, and its incidence is increasing year by year. Understanding the gene mutation information of melanoma cases is very important for its precise treatment. The current diagnostic methods for melanoma include radiological, pharmacological, histological, cytological and molecular techniques, but the gold standard for diagnosis is still pathological biopsy, which is time consuming and destructive. Raman spectroscopy is a rapid, sensitive and nondestructive detection method. In this study, a total of 20,000 Surface-enhanced Raman scattering (SERS) spectra of melanocytes and melanoma cells were collected using a positively charged gold nanoparticles planar solid SERS substrate, and a classification network system based on convolutional neural networks (CNN) was constructed to achieve the classification of melanocytes and melanoma cells, wild-type and mutant melanoma cells and their drug resistance. Among them, the classification accuracy of melanocytes and melanoma cells was over 98%. Raman spectral differences between melanocytes and melanoma cells were analyzed and compared, and the response of cells to antitumor drugs were also evaluated. The results showed that Raman spectroscopy provided a basis for the medication of melanoma, and SERS spectra combined with CNN classification model realized classification of melanoma, which is of great significance for rapid diagnosis and identification of melanoma.

First evidence of nanoparticle uptake through leaves and roots in beech (Fagus sylvatica L.) and pine (Pinus sylvestris L.).

Trees have been used for phytoremediation and as biomonitors of air pollution. However, the mechanisms by which trees mitigate nanoparticle pollution in the environment are still unclear. We investigated whether two important tree species, European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.), are able to take up and transport differently charged gold nanoparticles (Au-NPs) into their stem by comparing leaf-to-root and root-to-leaf pathways. Au-NPs were taken up by roots and leaves, and a small fraction was transported to the stem in both species. Au-NPs were transported from leaves to roots but not vice versa. Leaf Au uptake was higher in beech than in pine, probably because of the higher stomatal density and wood characteristics of beech. Confocal (3D) analysis confirmed the presence of Au-NPs in trichomes and leaf blade, about 20-30μm below the leaf surface in beech. Most Au-NPs likely penetrated into the stomatal openings through diffusion of Au-NPs as suggested by the 3D XRF scanning analysis. However, trichomes were probably involved in the uptake and internal immobilization of NPs, besides their ability to retain them on the leaf surface. The surface charge of Au-NPs may have played a role in their adhesion and uptake, but not in their transport to different tree compartments. Stomatal conductance did not influence the uptake of Au-NPs. This is the first study that shows nanoparticle uptake and transport in beech and pine, contributing to a better understanding of the interactions of NPs with different tree species.

Charged Gold Nanoparticles Promote In Vitro Proliferation in Nardostachys jatamansi by Differentially Regulating Chlorophyll Content, Hormone Concentration, and Antioxidant Activity.

Nardostachys jatamansi is a critically endangered medicinal plant and endemic to the Himalayas, having high commercial demand globally. The accumulation of various secondary metabolites in its shoots and roots with antioxidant potential are well-documented in traditional as well as modern medicine systems. In the present study, we first attempted to investigate the impact of citrate (−ve charge, 11.1 ± 1.9 nm) and CTAB (+ve charge, 19.5 ± 3.2 nm) coated gold nanoparticles (AuNPs) on the in vitro proliferation and antioxidant activities of N. jatamansi. Both the nanoparticles differentially affected the morphological and biochemical parameters, chlorophyll content, internal hormone concentration, and antioxidant activities in a concentration-dependent (10–100 µM) manner. Vigorous shooting was observed in half strength MS medium supplemented with IAA (1 mg/L) with 60 µM citrate-AuNPs (46.4 ± 3.7 mm) and 40 µM CTAB-AuNPs (42.2 ± 3.2 mm). Similarly, the maximum number of roots (5.00 ± 0.67 and 5.33 ± 0.58) and root length (29.9 ± 1.5 mm and 27.3 ± 4.8 mm) was reported in half-strength MS medium with IAA (1 mg/L) supplemented with 60 µM citrate-AuNPs and 40 µM CTAB-AuNPs, respectively. In addition, plants growing on MS medium supplemented with 60 µM citrate-AuNPs and 40 µM CTAB-AuNPs showed significantly enhanced photosynthetic pigments (chlorophyll a and b, carotenoids, and total chlorophyll), internal hormone concentration (GA3, IAA, and ABA), and antioxidant activities (total phenolics, flavonoids, DPPH, and SOD enzyme activity). Moreover, the transcript analysis of ANR1, ARF18, PLY9, SAUR28, GID1A, GRF1, SOD, and CAT further confirmed the role of 60 µM citrate-AuNPs and 40 µM CTAB-AuNPs in the improvement in the growth and antioxidant activities of N. jatamansi. Bearing in mind the urgent requirements of the effective conservation measures of this endangered species, the present findings suggest the elicitation of citrate-AuNPs and CTAB-AuNPs would significantly improve the potential applications of N. jatamansi in the medicinal plant-based industry.

An ultrasensitive label-free biosensor based on aptamer functionalized two-dimensional photonic crystal for kanamycin detection in milk

A novel photonic crystal aptamer biosensor SiO2-Au-ssDNA two-dimensional photonic crystal (2D PC), allowing label-free and highly sensitive to kanamycin (KANA), is successfully manufactured. This 2D PC biosensor was prepared via a needle tip flow method, using electrostatic adsorption to introduce negatively charged gold nanoparticles (Au NPs) into the 2D PC, combined with sulfhydryl-modified ssDNA for the rapid measurement. Benefiting from the localized surface plasmon resonance effect of Au NPs and optical response capability of PC, the biosensor has an excellent performance on quantitative analysis of KANA ranging from 5 pg∙mL−1 to 5 μg∙mL−1, with a limit of detection of 1.10 pg∙mL−1. The recovery of KANA is between 97 % and 110 % in the milk samples with relative standard deviation less than 4.8 %, which revealing that the 2D PC biosensor has the excellent performance on the KANA detection in complex conditions.

Four-Level Structural Hierarchy: Microfluidically Supported Synthesis of Polymer Particle Architectures Incorporating Fluorescence-Labeled Components and Metal Nanoparticles.

Hierarchical assemblies of functional polymer particles are promising due to their surface as well as physicochemical properties. However, hierarchical composites are complex and challenging to form due to the many steps necessary for integrating different components into one system. Highly structured four-level composite particles were formed in a four-step process. First of all, gold (Au) nanoparticles, poly(methyl methacrylate) (PMMA) nanoparticles, and poly(tripropylene glycol diacrylate) (poly-TPGDA) microparticles were individually synthesized. By applying microfluidic techniques, polymer nano- and microparticles were formed with tunable size and surface properties. Afterwards, the negatively charged gold nanoparticles and PMMA particles functionalized with a positively charged surface were mixed to form Au/PMMA assemblies. The Au/PMMA composites were mixed and incubated with poly-TPGDA microparticles to form ternary Au/PMMA/poly-TPGDA assemblies. For the formation of composite-containing microparticles, Au/PMMA/poly-TPGDA composites were dispersed in an aqueous acrylamide-methylenebisacrylamide solution. Monomer droplets were formed in a co-flow microfluidic device and photopolymerized by UV light. In this way, hierarchically structured four-level composites consisting of four different size ranges─0.025/0.8/30/1000 μm─were obtained. By functionalizing polymer nano- and microparticles with different fluorescent dyes, it was possible to visualize the same composite particle under two different excitation modes (λex = 395-440 and λex = 510-560 nm). The Au/PMMA/poly-TPGDA composite-embedded polyacrylamide microparticles can be potentially used as a model for the creation of composite particles for sensing, catalysis, multilabeling, and biomedical applications.

CTnI diagnosis in myocardial infarction using G-quadruplex selective Ir(Ⅲ) complex as effective electrochemiluminescence probe

In this work, strong electrochemiluminescence (ECL) emission was achieved by using one type of the G-quadruplex selective iridium (III) complex as an efficient ECL signal probe. Based on the typical sandwich immunoreaction between the cardiac troponin-I antigen (cTnI) and its corresponding antibody, iridium (III) complex was introduced according to its specific interaction with G-quadruplex DNA that modified on the surface of negatively charged gold nanoparticles ((−)AuNPs), inducing an increased ECL signal, which was proportional to cTnI concentration. Based on of this, quantitative detection of cTnI could be realized in the range of 5.0 fg/mL-100 ng/mL, with a detection limit of 1.67 fg/mL. Moreover, the proposed immunosensor was successfully applied for the diagnosis of cTnI in human serums from healthy individuals and acute myocardial infarction (AMI) patients, suggesting a great potential application value in the early diagnosis of AMI.

Field-Effect Capacitors Decorated with Ligand-Stabilized Gold Nanoparticles: Modeling and Experiments

Nanoparticles are recognized as highly attractive tunable materials for designing field-effect biosensors with enhanced performance. In this work, we present a theoretical model for electrolyte-insulator-semiconductor capacitors (EISCAP) decorated with ligand-stabilized charged gold nanoparticles. The charged AuNPs are taken into account as additional, nanometer-sized local gates. The capacitance-voltage (C–V) curves and constant-capacitance (ConCap) signals of the AuNP-decorated EISCAPs have been simulated. The impact of the AuNP coverage on the shift of the C–V curves and the ConCap signals was also studied experimentally on Al–p-Si–SiO2 EISCAPs decorated with positively charged aminooctanethiol-capped AuNPs. In addition, the surface of the EISCAPs, modified with AuNPs, was characterized by scanning electron microscopy for different immobilization times of the nanoparticles.

Enhanced Gold Nanoparticle aggregation with cancer cell DNA for improved radiation therapy and immune response in cancer treatment

In the United States, cancer is the second leading cause of death after heart disease and hundreds of thousands of people pass away each year due to this deadly disease. Many treatments for cancer are nonselective and do not provide long‐lasting immunity in case of recurrence. Gold nanoparticles can provide a more targeted radiation therapy and better effectiveness through heat generation and formation of reactive oxygen species via hydrolysis reaction which can cause DNA damage in the tumor. Unfortunately, gold particles have low photothermal transduction efficiency and must be aggregated to improve performance. To achieve this, positively charged gold nanoparticles were aggregated with MDA MB 231 cancer cell DNA to increase gold nanoparticle aggregation, to elicit higher photothermal efficiency, and to stimulate an immune response. In vitro analysis demonstrated DNA size and concentration dependent gold nanoparticle aggregation, which varied little in the pH range of 5.5 to 8.6. Furthermore, these aggregates elicited an immune response via delivery of DNA for direct transfection of antigen presenting cells. Tumor necrosis factor alpha released from mouse RAW macrophages increased in the presence of the cancer cell DNA aggregated gold particles. This work suggests that the promise of combination therapy using cancer cell DNA not only can improve photothermal therapy of gold nanoparticles, but also can elicit an immune response in cancer treatment.

Transplantation of neuron-inducing grafts embedding positively charged gold nanoparticles for the treatment of spinal cord injury.

In this study, we aimed to investigate the recovery after traumatic spinal cord injury (SCI) by inducing cellular differentiation of transplanted neural stem cells (NSCs) into neurons. We dissociated NSCs from the spinal cords of Fisher 344 rat embryos. An injectable gel crosslinked with glycol chitosan and oxidized hyaluronate was used as a vehicle for NSC transplantation. The gel graft containing the NSC and positively charged gold nanoparticles (pGNP) was implanted into spinal cord lesions in Sprague–Dawley rats (NSC‐pGNP gel group). Cellular differentiation of grafted NSCs into neurons (stained with β‐tubulin III [also called Tuj1]) was significantly increased in the NSC‐pGNP gel group (***p < 0.001) compared to those of two control groups (NSC and NSC gel groups) in the SCI conditions. The NSC‐pGNP gel group showed the lowest differentiation into astrocytes (stained with glial fibrillary acidic protein). Regeneration of damaged axons (stained with biotinylated dextran amines) within the lesion was two‐fold higher in the NSC‐pGNP gel group than that in the NSC gel group. The highest locomotor scores were also found in the NSC‐pGNP gel group. These outcomes suggest that neuron‐inducing pGNP gel graft embedding embryonic spinal cord‐derived NSCs can be a useful type of stem cell therapy after SCI.

Morphological, Histological and Ultrastructural Changes in Hordeum vulgare (L.) Roots That Have Been Exposed to Negatively Charged Gold Nanoparticles

In recent years, there has been an impressive development of nanotechnology. This has resulted in the increasing release of nanomaterials (NM) into the environment, thereby causing the risk of an uncontrolled impact on living organisms, including plants. More studies indicated the biotoxic effect of NM on plants, including crops. The interaction of nanoparticles (NP) with food crops is extremely important as they are a link to the food chain. The objective of this study was to investigate the effect of negatively charged gold nanoparticles (-) AuNP (at two concentrations; 25 µg/mL or 50 µg/mL) on barley (Hordeum vulgare L.) root development. Morphological, histological and ultrastructural analyses (with the use of stereomicroscope, bright filed microscope and transmission electron microscope) revealed that regardless of the concentration, (-) AuNP did not enter into the plant body. However, the dose of (-) AuNP proved to be important for the plant’s response because different morphological, histological and ultrastructural changes were observed in the treated roots. The NP treatment caused: red root colouration, a local increase in the root diameter and a decreased formation of the root hair cells (on morphological level), damage to the rhizodermal cells, vacuolisation of the cortical cells, a detachment of the cell files between the cortical cells, atypical divisions of the cells, disorder of the meristem organisation (on the histological level), the appearance of periplasmic space, numerous vesicles and multivesicular bodies, electron-dense spots in cytoplasm, alterations in the structure of the mitochondria, breakdown of the tonoplast and the plasmalemma (on the ultrastructural level).

A sensitive surface-enhanced resonance Raman scattering sensor with bifunctional negatively charged gold nanoparticles for the determination of Cr(VI)

Hexavalent chromium (Cr(VI)) pollution in the water system has seriously endangered human health and the environment. Herein, we propose a rapid, simple and sensitive surface-enhanced resonance Raman scattering (SERRS) sensor with the bifunctional negatively charged gold nanoparticles ((−)AuNPs) which employ as not only the oxidoreductase-like nanozyme but also the substrate to determine Cr(VI). (−)AuNPs effectively promoted the conversion of 3,3′,5,5′-tetramethylbenzidine (TMB) into the blue product of 3,3′,5,5′-tetramethylbenzidine diamine (oxTMB) in the presence of Cr(VI) and generated a strong SERRS signal at 1611 cm−1. According to this principle, the Raman intensity difference at 1611 cm−1 exhibited a satisfactory linear relationship with the logarithm of the Cr(VI) concentration from 10−5 to 10−9 M with a low limit of detection (LOD) of 0.4 nM. In addition, the possible SERRS enhancement mechanism, selectivity and reproducibility were also investigated. What's more, the SERRS platform was successfully applied in the complicated water samples, which was anticipated to become a promising analytical method for monitoring of Cr(VI) in the environment.

Amphiphilic Janus Microspheres Prepared by Caged Photoactivatable Alkoxysilane

A simple photolysis route was proposed to prepare Amphiphilic Janus Particles (AJP) based on SiO2 microspheres. The surface of SiO2 microspheres were modified by photoactive alkoxysilane, which was synthesized by dealcoholization condensation of 6-nitroveratroyloxycarbonyl and isocyanatopropyl-triethoxysilane. UV irradiation caused eater-breaking allowed for the precise control of hydrophilic modification of the hemispherical exposed particles surfaces. The component and morphology of the obtained particles were characterized by fourier transform infrared spectroscopy and ultraviolet-visible spectroscopy, and the Janus feature was evaluated by scanning electron microscopy, transmission electron microscopy, and dispersity in the oil–water dual-phases. The following results were obtained. The AJP with 450 nm size processes the hydrophilic amino groups on one side and the hydrophobic 6-nitroveratryloxycarbonyl moieties on the other. Additionally, the AJP were located at the phase boundary between water and n-hexane, and the negative charged gold nanoparticles with 25 nm size were adsorbed only onto the side with the positive charged amino groups. The AJP have interfacial adsorption energies that can be as much as three times larger than that of homogeneous particles and thus exhibit excellent surface activities.

Thermodynamics of multilayer protein adsorption on a gold nanoparticle surface

We report the thermodynamics of protein adsorption on negatively charged colloidal gold nanoparticles (GNPs) of 16 nm to 69 nm at pH 7.0. Three biologically important proteins of varying size, namely tetrameric alcohol dehydrogenase (ADH) from Saccharomyces cerevisiae, bovine serum albumin (BSA), and insulin from the porcine pancreas were taken and their adsorption on the GNP surface was investigated by dynamic light scattering (DLS), absorption spectroscopy and zeta potential measurements. The hydrodynamic size of the GNPs was found to increase with protein addition. At very low protein concentrations, the adsorption of these proteins was earlier described by Langmuir-type adsorption isotherms. However, when the protein concentration is raised, the adsorption data are found to fit the BET (Brunauer, Emmett, and Teller) adsorption isotherm well, indicating multilayer formation on the GNP surface. The equilibrium binding constants: KS (monolayer) ∼108-109 M-1, KL (subsequent layers) ∼105-106 M-1, and monolayer thickness are obtained from our measurements. KS is found to increase with the size of the GNPs, while KL does not change significantly with either the size of the GNPs or that of the protein. KL, which originates from protein-protein interaction, compares very well with the protein dimerization constant in solution, indicating that the interaction beyond the first layer is not significantly influenced by the nanoparticles. The first layer forms the soft corona on the GNP surface since this layer, although stable, is removable by centrifugation. But the subsequent layers are weakly bound and we call it an ultra-soft corona in order to distinguish it qualitatively from the strongly bound first layer. The modest magnitudes of Gibbs free energy changes of 43-55 kJ mol-1 for the first layer and 31-35 kJ mol-1 for the subsequent layers indicate that ADH, BSA, and insulin are all physiosorbed on the GNP surface. The temperature dependent DLS data point out that the adsorption of these proteins on GNPs is endothermic but is accompanied by a large increase in entropy. The nature of the BET fits to the adsorption data shows that multilayer adsorption starts even before the monolayer formation is complete, and it does not occur in a layer-by-layer sequence.

Aggregation of Gold Nanoparticles in Presence of the Thermoresponsive Cationic Diblock Copolymer PNIPAAM48-b-PAMPTMA6.

The adsorption of the thermoresponsive positively charged copolymer poly(N-isopropylacrylamide)-block-poly(3-acrylamidopropyl)trimethylammonium chloride, PNIPAAM48-b-PAMPTMA6(+), onto negatively charged gold nanoparticles can provide stability to the nanoparticles and make the emerging structure tunable by temperature. In this work, we characterize the nanocomposite formed by gold nanoparticles and copolymer chains and study the influence of the copolymer on the expected aggregation process that undergoes those nanoparticles at high ionic strength. We also determine the lower critical solution temperature (LCST) of the copolymer (around 42 °C) and evaluate the influence of the temperature on the nanocomposite. For those purposes, we use dynamic light scattering, UV-vis spectroscopy and transmission electron microscopy. At the working PNIPAAM48-b-PAMPTMA6(+) concentration, we observe the existence of copolymer structures that trap the gold nanoparticles and avoid the formation of nanoparticles aggregates. Finally, we discuss how these structures can be useful in catalysis and nanoparticles recovery.

Settling dynamics of nanoparticles in simple and biological media

The biological response of organisms exposed to nanoparticles is often studied in vitro using adherent monolayers of cultured cells. In order to derive accurate concentration–response relationships, it is important to determine the local concentration of nanoparticles to which the cells are actually exposed rather than the nominal concentration of nanoparticles in the cell culture medium. In this study, the sedimentation–diffusion process of different sized and charged gold nanoparticles has been investigated in vitro by evaluating their settling dynamics and by developing a theoretical model to predict the concentration depth profile of nanoparticles in solution over time. Experiments were carried out in water and in cell culture media at a range of controlled temperatures. The optical phenomenon of caustics was exploited to track nanoparticles in real time in a conventional optical microscope without any requirement for fluorescent labelling that potentially affects the dynamics of the nanoparticles. The results obtained demonstrate that size, temperature and the stability of the nanoparticles play a pivotal role in regulating the settling dynamics of nanoparticles. For gold nanoparticles larger than 60 nm in diameter, the initial nominal concentration did not accurately represent the concentration of nanoparticles local to the cells. Finally, the theoretical model proposed accurately described the settling dynamics of the nanoparticles and thus represents a promising tool to support the design of in vitro experiments and the study of concentration–response relationships.