Fractionation and characterization of gold nanoparticles in aqueous solution: asymmetric-flow field flow fractionation with MALS, DLS, and UV–Vis detection

A do-it-yourself benchtop device for highly scalable flow synthesis of protein-based nanoparticles

Synthesis of nanoparticles is typically carried out in batch procedures, which offer limited control of parameters, and a narrow range of possible batch volumes. In contrast, flow synthesis systems, usually having a microfluidic chip as a crucial part, are devoid of these drawbacks. However, large scale devices - millifluidic systems - may offer several advantages over microfluidic systems, such as easier and cheaper production, enhanced throughput, and reduced channel clogging. Here we report a millifluidic system for the generation of protein nanoparticles, using the flow format of the original swift thermal formation technology (STF), which can process batch volume ranging from 100 µl to any practically significant amount. Capabilities of the system are demonstrated with model synthesis of Epirubicin-encapsulated BSA nanoparticles. A better degree of scalability of the synthesis over batch procedure is shown: with a 10-fold working volume increase, hydrodynamic diameter and loading capacity changed by only 10 % and 1 % respectively, compared to 60 % and 30 % for the batch synthesis. Additionally, we provide all engineering drawings, electrical circuits, programming code and nuances of assembly and operation, so that our findings can be easily reproduced. The ease of construction of the device and the superior characteristics of the resulting nanoparticles compared to the batch method indicate application potential in both the biomedical research and industrial spheres.

Centrifugal Field-Flow Fractionation Enables Detection of Slight Aggregation of Nanoparticles That Impacts Their Biomedical Applications

Nanoparticles (NPs) are anticipated to be used for various biomedical applications in which their aggregation has been an important issue. However, concerns regarding slightly aggregated but apparently monodispersed NPs have been difficult to address because of a lack of appropriate evaluation methods. Here, we report centrifugal field-flow fractionation (CF3) as a powerful method for analyzing the slight aggregation of NPs, using antibody-modified gold NPs (Ab-AuNPs) prepared by a conventional protocol with centrifugal purification as a model. While common evaluation methods such as dynamic light scattering cannot detect significant signs of aggregation, CF3 successfully detects distinct peaks of slightly aggregated NPs, including dimers and trimers. Their impact on biological interactions was also demonstrated by a cellular uptake study: slightly aggregated Ab-AuNPs exhibited 1.8 times higher cellular uptake than monodispersed Ab-AuNPs. These results suggest the importance of aggregate evaluation via CF3 as well as the need for careful attention to the bioconjugation procedures for NPs.

Green Synthesis of Bioinspired Nanoparticles Mediated from Plant Extracts of Asteraceae Family for Potential Biological Applications

The Asteraceae family is one of the largest families in the plant kingdom with many of them extensively used for significant traditional and medicinal values. Being a rich source of various phytochemicals, they have found numerous applications in various biological fields and have been extensively used for therapeutic purposes. Owing to its potential phytochemicals present and biological activity, these plants have found their way into pharmaceutical industry as well as in various aspects of nanotechnology such as green synthesis of metal oxide nanoparticles. The nanoparticles developed from the plants of Asteraceae family are highly stable, less expensive, non-toxic, and eco-friendly. Synthesized Asteraceae-mediated nanoparticles have extensive applications in antibacterial, antifungal, antioxidant, anticancer, antidiabetic, and photocatalytic degradation activities. This current review provides an opportunity to understand the recent trend to design and develop strategies for advanced nanoparticles through green synthesis. Here, the review discussed about the plant parts, extraction methods, synthesis, solvents utilized, phytochemicals involved optimization conditions, characterization techniques, and toxicity of nanoparticles using species of Asteraceae and their potential applications for human welfare. Constraints and future prospects for green synthesis of nanoparticles from members of the Asteraceae family are summarized.

Characterization Challenges of Self-Assembled Polymer-SPIONs Nanoparticles: Benefits of Orthogonal Methods

Size and zeta potential are critical physicochemical properties of nanoparticles (NPs), influencing their biological activity and safety profile. These are essential for further industrial upscale and clinical success. However, the characterization of polydisperse, non-spherical NPs is a challenge for traditional characterization techniques (ex., dynamic light scattering (DLS)). In this paper, superparamagnetic iron oxide nanoparticles (SPIONs) were coated with polyvinyl alcohol (PVAL) exhibiting different terminal groups at their surface, either hydroxyl (OH), carboxyl (COOH) or amino (NH₂) end groups. Size, zeta potential and concentration were characterized by orthogonal methods, namely, batch DLS, nanoparticle tracking analysis (NTA), tunable resistive pulse sensing (TRPS), transmission electron microscopy (TEM), asymmetric flow field flow fractionation (AF4) coupled to multi-angle light scattering (MALS), UV-Visible and online DLS. Finally, coated SPIONs were incubated with albumin, and size changes were monitored by AF4-MALS-UV-DLS. NTA showed the biggest mean sizes, even though DLS PVAL-COOH SPION graphs presented aggregates in the micrometer range. TRPS detected more NPs in suspension than NTA. Finally, AF4-MALS-UV-DLS could successfully resolve the different sizes of the coated SPION suspensions. The results highlight the importance of combining techniques with different principles for NPs characterization. The advantages and limitations of each method are discussed here.

Retention of Activity by Antibodies Immobilized on Gold Nanoparticles of Different Sizes: Fluorometric Method of Determination and Comparative Evaluation

Antibody–nanoparticle conjugates are widely used analytical reagents. An informative parameter reflecting the conjugates’ properties is the number of antibodies per nanoparticle that retain their antigen-binding ability. Estimation of this parameter is characterized by a lack of simple, reproducible methods. The proposed method is based on the registration of fluorescence of tryptophan residues contained in proteins and combines sequential measurements of first the immobilized antibody number and then the bound protein antigen number. Requirements for the measurement procedure have been determined to ensure reliable and accurate results. Using the developed technique, preparations of spherical gold nanoparticles obtained by the most common method of citrate reduction of gold salts (the Turkevich–Frens method) and varying in average diameter from 15 to 55 nm have been characterized. It was shown that the number of antibodies (immunoglobulins G) bound by one nanoparticle ranged from 30 to 194 during adsorptive unoriented monolayer immobilization. C-reactive protein was considered as the model antigen. The percentage of antibody valences that retained their antigen-binding properties in the conjugate increased from 17 to 34% with an increase in the diameter of gold nanoparticles. The proposed method and the results of the study provide tools to assess the capabilities of the preparations of gold nanoparticles and their conjugates as well as the expediency of seeking the best techniques for various practical purposes.

Characterization of Gold Nanorods Conjugated with Synthetic Glycopolymers Using an Analytical Approach Based on spICP-SFMS and EAF4-MALS

A new comprehensive analytical approach based on single-particle inductively coupled plasma-sector field mass spectrometry (spICP-SFMS) and electrical asymmetric-flow field-flow-fractionation combined with multi-angle light scattering detection (EAF4-MALS) has been examined for the characterization of galactosamine-terminated poly(N-hydroxyethyl acrylamide)-coated gold nanorods (GNRs) in two different degrees of polymerization (DP) by tuning the feed ratio (short: DP 35; long: DP 60). spICP-SFMS provided information on the particle number concentration, size and size distribution of the GNRs, and was found to be useful as an orthogonal method for fast characterization of GNRs. Glycoconjugated GNRs were separated and characterized via EAF4-MALS in terms of their size and charge and compared to the bare GNRs. In contrast to spICP-SFMS, EAF4-MALS was also able of providing an estimate of the thickness of the glycopolymer coating on the GNRs surface.

Effect of Core-Crosslinking on Protein Corona Formation on Polymeric Micelles

Most nanomaterials acquire a protein corona upon contact with biological fluids. The magnitude of this effect is strongly dependent both on surface and structure of the nanoparticle. To define the contribution of the internal nanoparticle structure, protein corona formation of block copolymer micelles with poly(N-2-hydroxypropylmethacrylamide) (pHPMA) as hydrophilic shell, which are crosslinked-or not-in the hydrophobic core is comparatively analyzed. Both types of micelles are incubated with human blood plasma and separated by asymmetrical flow field-flow fractionation (AF4). Their size is determined by dynamic light scattering and proteins within the micellar fraction are characterized by gel electrophoresis and quantified by liquid chromatography-high-resolution mass spectrometry-based label-free quantitative proteomics. The analyses reveal only very low amounts of plasma proteins associated with the micelles. Notably, no significant enrichment of plasma proteins is detectable for core-crosslinked micelles, while noncrosslinked micelles show a significant enrichment of plasma proteins, indicative of protein corona formation. The results indicate that preventing the reorganization of micelles (equilibrium with unimers) by core-crosslinking is crucial to reduce the interaction with plasma proteins.

Aptamer-Target-Gold Nanoparticle Conjugates for the Quantification of Fumonisin B1

Fumonisin B1 (FB1), a mycotoxin classified as group 2B hazard, is of high importance due to its abundance and occurrence in varied crops. Conventional methods for detection are sensitive and selective; however, they also convey disadvantages such as long assay times, expensive equipment and instrumentation, complex procedures, sample pretreatment and unfeasibility for on-site analysis. Therefore, there is a need for quick, simple and affordable quantification methods. On that note, aptamers (ssDNA) are a good alternative for designing specific and sensitive biosensing techniques. In this work, the assessment of the performance of two aptamers (40 and 96 nt) on the colorimetric quantification of FB1 was determined by conducting an aptamer-target incubation step, followed by the addition of gold nanoparticles (AuNPs) and NaCl. Although MgCl₂ and Tris-HCl were, respectively, essential for aptamer 96 and 40 nt, the latter was not specific for FB1. Alternatively, the formation of Aptamer (96 nt)-FB1-AuNP conjugates in MgCl₂ exhibited stabilization to NaCl-induced aggregation at increasing FB1 concentrations. The application of asymmetric flow field-flow fractionation (AF4) allowed their size separation and characterization by a multidetection system (UV-VIS, MALS and DLS online), with a reduction in the limit of detection from 0.002 µg/mL to 56 fg/mL.

Limitations of Nanoparticles Size Characterization by Asymmetric Flow Field‑Fractionation Coupled with Online Dynamic Light Scattering

The development of reliable protocols suitable for the characterisation of the physical properties of nanoparticles in suspension is becoming crucial to assess the potential biological as well as toxicological impact of nanoparticles. Amongst sizing techniques, asymmetric flow field flow fractionation (AF4) coupled to online size detectors represents one of the most robust and flexible options to quantify the particle size distribution in suspension. However, size measurement uncertainties have been reported for on-line dynamic light scattering (DLS) detectors when coupled to AF4 systems. In this work we investigated the influence of the initial concentration of nanoparticles in suspension on the sizing capability of the asymmetric flow field-flow fractionation technique coupled with an on-line dynamic light scattering detector and a UV–Visible spectrophotometer (UV) detector. Experiments were performed with suspensions of gold nanoparticles with a nominal diameter of 40 nm and 60 nm at a range of particle concentrations. The results obtained demonstrate that at low concentration of nanoparticles, the AF4-DLS combined technique fails to evaluate the real size of nanoparticles in suspension, detecting an apparent and progressive size increase as a function of the elution time and of the concentration of nanoparticles in suspension.

Preparation and characterization of Ag-nanostars@Au-nanowires hierarchical nanostructures for highly sensitive surface enhanced Raman spectroscopy

In this work we study the surface enhanced Raman scattering (SERS) produced by hierarchical nanostructures obtained by coupling different anisotropic nanomaterial of two SERS active metals, namely Ag nanostars (AgNSs) and Au nanowires (AuNWs). Ag nanostars (AgNSs) are prepared, by a two-step one-pot synthesis by reduction of AgNO3 with hydroxylamine, trisodium citrate and NaOH. AuNWs are obtained by electroless templated synthesis in track-etched polycarbonate membranes with following etching of the template. The two precursors are bound together by bridging with the bifunctional cysteamine molecule, obtaining AgNS@AuNW hierarchical structures. Benzenethiol (BT) is adsorbed on the nanostructured material and used as SERS probe to study the amplification of Raman signals. Experimental results indicate significantly larger Raman enhancement when BT is adsorbed onto the AgNS@AuNW in comparison to AuNWs alone or decorated with quasi-spherical silver nanoparticles obtaining AgNP@AuNW. Digital simulations performed by the boundary element method agree with the experimental findings, showing higher number of hot spots and significantly higher SERS enhancements for AgNS@AuNW versus AuNWs or AgNSs or AgNP@AuNW.

Localizing Binding Sites on Bioconjugated Hydrogen-Bonded Organic Semiconductors at the Nanoscale

Hydrogen‐bonded organic semiconductors are extraordinarily stable organic solids forming stable, large crystallites with the ability to preserve favorable electrical properties upon bioconjugation. Lately, tremendous efforts have been made to use these bioconjugated semiconductors as platforms for stable multifunctional bioelectronics devices, yet the detailed characterization of bio‐active binding sites (orientation, density, etc.) at the nanoscale has not been achieved yet. The presented work investigates the bioconjugation of epindolidione and quinacridone, two representative semiconductors, with respect to their exposed amine‐functionalities. Relying on the biotin‐avidin lock‐and‐key system and applying the atomic force microscopy (AFM) derivative topography and recognition (TREC) imaging, we used activated biotin to flag crystal‐faces with exposed amine functional groups. Contrary to previous studies, biotin bonds were found to be stable towards removal by autolysis. The resolution strength and clear recognition capability makes TREC‐AFM a valuable tool in the investigation of bio‐conjugated, hydrogen‐bonded semiconductors.

Protein Interactions with Nanoparticle Surfaces: Highlighting Solution NMR Techniques

In the last decade, nanoparticles (NPs) have become a key tool in medicine and biotechnology as drug delivery systems, biosensors and diagnostic devices. The composition and surface chemistry of NPs vary based on the materials used: typically organic polymers, inorganic materials, or lipids. Nanoparticle classes can be further divided into sub-categories depending on the surface modification and functionalization. These surface properties matter when NPs are introduced into a physiological environment, as they will influence how nucleic acids, lipids, and proteins will interact with the NP surface. While small-molecule interactions are easily probed using NMR spectroscopy, studying protein-NP interactions using NMR introduces several challenges. For example, globular proteins may have a perturbed conformation when attached to a foreign surface, and the size of NP-protein conjugates can lead to excessive line broadening. Many of these challenges have been addressed, and NMR spectroscopy is becoming a mature technique for in situ analysis of NP binding behavior. It is therefore not surprising that NMR has been applied to NP systems and has been used to study biomolecules on NP surfaces. Important considerations include corona composition, protein behavior, and ligand architecture. These features are difficult to resolve using classical surface and material characterization strategies, and NMR provides a complementary avenue of characterization. In this review, we examine how solution NMR can be combined with other analytical techniques to investigate protein behavior on NP surfaces.

High separation efficiency of gold nanomaterials of different aspect ratio and size using capillary transient isotachophoresis

Two modes of capillary electrophoresis (CE), capillary zone electrophoresis (CZE) and capillary transient isotachophoresis (ctITP), were compared for the detection and separation of spherical gold nanoparticles (AuNPs) and gold nanorods (AuNRs). The development of ctITP using two different leading ions is described. Overall, when compared to traditional capillary zone electrophoresis (CZE), ctITP resulted in improved peak shape and peak efficiency. Specifically, the number of theoretical plates for AuNR samples increased by a factor of 2-2.5 depending on the choice of leading ion. Further, using ctITP two AuNRs differing by aspect ratio were baseline resolved, whereas the same AuNRs could not be separated using CZE or other techniques like single particle inductively coupled plasma mass spectrometry (spICP-MS) and asymmetric flow field-flow fractionation (AF4). The results of this study demonstrate that ctITP is an efficient on-line technique for the improved detection and separation of gold nanomaterials in CE.

Measuring nanoparticles in the size range to 2000 nm

Measurement of light scattered from suspensions of monodisperse nanoparticles in solution ("turbidity") long has been used to derive their size. Following some means of fractionation, the light (monochromatic) scattered by the particles into a set of distinct angles is collected and a non-linear least squares fit was made to an appropriate theory in order to extract their size. For a wide range of particle structures, where this process becomes very complex and of questionable validity, there is a far simpler interpretive means based upon measurements at extremely small, and often inaccessible, scattering angles. A method is described whereby the required small angle values are derived from measurements made over a range of larger, more readily accessible, angles. Although the basis for the analyses developed is the Rayleigh-Gans approximation, the results presented confirm that the method provides meaningful results up to a size of about 2000 nm. The larger sizes are well beyond the RG limits.

Impact of Ionic Strength of Carrier Liquid on Recovery in Flow Field-Flow Fractionation

Asymmetrical flow field-flow fractionation (AF4) and hollow-fiber flow field-flow fractionation (HF5) are techniques widely used in analytical, industrial and biological analyses. The main problem in all AF4 and HF5 analyses is sample loss due to analyte–membrane interactions. In this work the impact of liquid carrier composition on latex nanoparticles (NPs) separation in water and two different concentrations of NH₄NO₃ was studied. In AF4, a constant trend of decreasing the size of 60 and 121.9 nm particles induced by the ionic strength of the carrier liquid has been observed. In contrast, an agglomeration effect of the biggest 356 nm particles was observed when increasing ionic strength, which induced a significant drop of recovery to 35%. H5F provides better resolution and intensified peaks of NPs, but careful optimisation of system parameters is mandatory to obtain good separation.

Facile production of nanocomposites of carbon nanotubes and polycaprolactone with high aspect ratios with potential applications in drug delivery

The geometries and surface properties of nanocarriers greatly influence the interaction between nanomaterials and living cells. In this work we combine multiwalled carbon nanotubes (CNTs) with poly-ε-caprolactone (PCL) to produce non-spherical nanocomposites with high aspect ratios by using a facile emulsion solvent evaporation method. Particles were characterised by dynamic light scattering (DLS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and asymmetric flow field flow fractionation (AF4). Different sizes and morphologies of nanoparticles were produced depending on the concentration of the sodium dodecyl sulphate (SDS), CNTs and PCL. Rod-like PCL-CNT nanostructures with low polydispersity were obtained with 1.5 mg mL-1 of SDS, 0.9 mg mL-1 of CNTs and 10 mg mL-1 PCL. AFM analysis revealed that the PCL and PCL-CNT nanocomposite had comparatively similar moduli of 770 and 560 MPa respectively, indicating that all the CNTs have been coated with at least 2 nm of PCL. Thermogravimetric analysis of the PCL-CNT nanocomposite indicated that they contained 9.6% CNTs by mass. The asymmetric flow field flow fractionation of the samples revealed that the PCL-CNT had larger hydrodynamic diameters than PCL alone. Finally, the drug loading properties of the nanocomposites were assessed using docetaxel as the active substance. The nanocomposites showed comparable entrapment efficiencies of docetaxel (89%) to the CNTs alone (95%) and the PCL nanoparticles alone (81%). This is a facile method for obtaining non-spherical nanocomposites that combines the properties of PCL and CNTs such as the high aspect ratio, modulus. The high drug entrapment efficiency of these nanocomposites may have promising applications in drug delivery.

Colloidal Mechanisms of Gold Nanoparticle Loss in Asymmetric Flow Field-Flow Fractionation

Flow field-flow fractionation is a powerful method for the analysis of nanoparticle size distributions, but its widespread use has been hampered by large analyte losses, especially of metal nanoparticles. Here, we report on the colloidal mechanisms underlying the losses. We systematically studied gold nanoparticles (AuNPs) during asymmetrical flow field-flow fractionation (AF4) by systematic variation of the particle properties and the eluent composition. Recoveries of AuNPs (core diameter 12 nm) stabilized by citrate or polyethylene glycol (PEG) at different ionic strengths were determined. We used online UV-vis detection and off-line elementary analysis to follow particle losses during full analysis runs, runs without cross-flow, and runs with parts of the instrument bypassed. The combination allowed us to calculate relative and absolute analyte losses at different stages of the analytic protocol. We found different loss mechanisms depending on the ligand. Citrate-stabilized particles degraded during analysis and suffered large losses (up to 74%). PEG-stabilized particles had smaller relative losses at moderate ionic strengths (1-20%) that depended on PEG length. Long PEGs at higher ionic strengths (≥5 mM) caused particle loss due to bridging adsorption at the membrane. Bulk agglomeration was not a relevant loss mechanism at low ionic strengths ≤5 mM for any of the studied particles. An unexpectedly large fraction of particles was lost at tubing and other internal surfaces. We propose that the colloidal mechanisms observed here are relevant loss mechanisms in many particle analysis protocols and discuss strategies to avoid them.

Asymmetrical flow field-flow fractionation coupled with multiple detections: A complementary approach in the characterization of egg yolk plasma

The capability of asymmetrical flow field-flow fractionation (AF4) coupled with UV/VIS, multiangle light scattering (MALS) and quasi-elastic light scattering (QELS) (AF4-UV-MALS-QELS) for separation and characterization of egg yolk plasma was evaluated. The accuracy of hydrodynamic radius (Rh) obtained from QELS and AF4 theory (using both simplified and full expression of AF4 retention equations) was discussed. The conformation of low density lipoprotein (LDL) and its aggregates in egg yolk plasma was discussed based on the ratio of radius of gyration (Rg) to Rh together with the results from bio-transmission electron microscopy (Bio-TEM). The results indicate that the full retention equation is more relevant than simplified version for the Rh determination at high cross flow rate. The Rh from online QELS is reliable only at a specific range of sample concentration. The effect of programmed cross flow rate (linear and exponential decay) on the analysis of egg yolk plasma was also investigated. It was found that the use of an exponentially decaying cross flow rate not only reduces the AF4 analysis time of the egg yolk plasma, but also provides better resolution than the use of either a constant or linearly decaying cross flow rate. A combination of an exponentially decaying cross flow AF4-UV-MALS-QELS and the utilization of full retention equation was proved to be a useful method for the separation and characterization of egg yolk plasma.

Multimodal Dispersion of Nanoparticles: A Comprehensive Evaluation of Size Distribution with 9 Size Measurement Methods

PURPOSE

Evaluation of particle size distribution (PSD) of multimodal dispersion of nanoparticles is a difficult task due to inherent limitations of size measurement methods. The present work reports the evaluation of PSD of a dispersion of poly(isobutylcyanoacrylate) nanoparticles decorated with dextran known as multimodal and developed as nanomedecine.

METHODS

The nine methods used were classified as batch particle i.e. Static Light Scattering (SLS) and Dynamic Light Scattering (DLS), single particle i.e. Electron Microscopy (EM), Atomic Force Microscopy (AFM), Tunable Resistive Pulse Sensing (TRPS) and Nanoparticle Tracking Analysis (NTA) and separative particle i.e. Asymmetrical Flow Field-Flow Fractionation coupled with DLS (AsFlFFF) size measurement methods.

RESULTS

The multimodal dispersion was identified using AFM, TRPS and NTA and results were consistent with those provided with the method based on a separation step prior to on-line size measurements. None of the light scattering batch methods could reveal the complexity of the PSD of the dispersion.

CONCLUSIONS

Difference between PSD obtained from all size measurement methods tested suggested that study of the PSD of multimodal dispersion required to analyze samples by at least one of the single size particle measurement method or a method that uses a separation step prior PSD measurement.

Size-exclusion chromatography of metal nanoparticles and quantum dots

This review presents an overview of size-exclusion chromatographic separation and characterization of noble metal nanoparticles (NPs) and quantum dots (QDs) over the past 25 years. The properties of NPs and QDs that originate from quantum and surface effects are size dependent; to investigate these properties, a separation technique such as size-exclusion chromatography (SEC) is often needed to obtain narrow distribution NP populations that are also separated from the unreacted starting materials. Information on the size distributions and optical properties of NPs have been obtained by coupling SEC to detection methods such as ultraviolet-visible and/or fluorescence spectroscopy. Problems associated with the sorption of NPs and QDs onto various SEC stationary phases, employing both aqueous and organic eluents, are also discussed here.

Asymmetrical Flow-Field-Flow Fractionation coupled with inductively coupled plasma mass spectrometry for the analysis of gold nanoparticles in the presence of natural nanoparticles

Flow-Field-Flow Fractionation (Flow-FFF), coupled with online detection systems, is one of the most promising tools available for the analysis and characterization of engineered nanoparticles (ENPs) in complex matrices. In order to demonstrate the applicability of Flow-FFF for the detection, quantification, and characterization of engineered gold nanoparticles (AuNPs), model dispersions were prepared containing AuNPs with diameters of 30 or 100nm, natural nanoparticles (NNPs) extracted from a soil sample, and different concentrations of natural organic matter (NOM), which were then used to investigate interactions between the AuNPs and the NNPs. It could be shown that light scattering detection can be used to evaluate the fractionation performance of the pure NNPs, but not the fractionation performance of the mixed samples that also contained AuNPs because of specific interactions between the AuNPs and the laser light. A combination of detectors (i.e. light absorbance and inductively coupled plasma mass spectrometry (ICP-MS)) was found to be useful for differentiating between heteroaggregation and homoaggregation of the nanoparticles (NPs). The addition of NOM to samples containing 30nm AuNPs stabilized the AuNPs without affecting the NP size distribution. However, fractograms for samples with no added NOM showed a change in the size distribution, suggesting interactions between the AuNPs and NNPs. This interpretation was supported by unchanged light absorption wavelengths for the AuNPs. In contrast, results for samples containing 100nm AuNPs were inconclusive with respect to recovery and size distributions because of problems with the separation system that probably related to the size and high density of these nanoparticles, highlighting the need for extensive method optimization strategies, even for nanoparticles of the same material but different sizes.

Characterization and quantification of silver nanoparticles in nutraceuticals and beverages by asymmetric flow field flow fractionation coupled with inductively coupled plasma mass spectrometry

This study evaluated the feasibility of asymmetric flow field flow fractionation coupled with inductively coupled plasma mass spectrometry (AF4-ICP-MS) for separation, characterization and quantification of silver nanoparticles (AgNPs) in complex nutraceutical and beverage samples. For improved determination, different analysis conditions were proposed depending on the NP size, i.e. below 20 nm and in the 20-60 nm range. After optimization of the different experimental parameters affecting the AF4 separation process and the analyte detection, the proposed methods showed a wide dynamic linear range (i.e., in the 10-1000 μg L(-1)) and limits of detection below 28 ng L(-1). A previous probe ultrasonication for 90 s (corresponding to 45 pulses of 2 s) of the tested samples resulted in complete AgNPs disaggregation. As a result, a fast accurate determination was achieved (complete analysis was done in ca. 37 min). The practicality of the proposed methodology for the intended determination was demonstrated by successful determination of the AgNPs present in a variety of nutraceuticals and a beverage at concentration levels in the 0.7-29.5×10(3) μg L(-1) range. A good agreement was observed among these concentration data and those determined by more conventional sample preparation techniques, such as ultracentrifugation and acid digestion. Also, the estimated NP sizes using AF4 compared satisfactorily with those determined by image techniques, i.e. transmission electron microscopy (TEM). All together demonstrated the utility of this novel analytical methodology for the analysis of AgNPs of different size in complex matrices.

Highly stable positively charged dendron-encapsulated gold nanoparticles

We report the development of a novel cationic dendron (TAG1-PCD) and a positively charged gold nanoparticle-dendron conjugate (PCD-AuNP). TAG1-PCD was designed by considering the reactivity, hydrophilicity, and cationic nature that is required to yield a stable gold conjugate in aqueous media. The PCD-AuNPs, nominally 10 nm in size, were synthesized by reduction of chloroauric acid in the presence of TAG1-PCD. The physicochemical properties of PCD-AuNPs were characterized by dynamic light scattering, transmission electron microscopy, UV-vis absorbance, and X-ray photoelectron spectroscopy for investigation of size distribution, shape uniformity, surface plasmon resonance bands, and Au-dendron bonding. Asymmetric-flow field flow fractionation was employed to confirm the in situ size, purity, and surface properties of the PCD-AuNPs. Additionally, the stability of PCD-AuNPs was systematically evaluated with respect to shelf life determination, stability in biological media and a wide range of pH values, chemical resistance against cyanide, redispersibility from lyophilized state, and stability at temperatures relevant to biological systems. Dose dependent cell viability was evaluated in vitro using the human lung epithelial cell line A549 and a monkey kidney Vero cell line. Observations from in vitro studies are discussed. Overall, the investigation confirmed the successful development of stable PCD-AuNPs with excellent stability in biologically relevant test media containing proteins and electrolytes, and with a shelf life exceeding 6 months. The excellent aqueous stability and apparent lack of toxicity for this conjugate enhances its potential use as a test material for investigating interactions between positively charged NPs and biocellular and biomolecular systems, or as a vehicle for drug delivery.

Controlled formation and characterization of dithiothreitol-conjugated gold nanoparticle clusters

We report a systematic study of the controlled formation of discrete-sized gold nanoparticle clusters (GNCs) by interaction with the reducing agent dithiothreitol (DTT). Asymmetric-flow field flow fractionation and electrospray differential mobility analysis were employed complementarily to determine the particle size distributions of DTT-conjugated GNCs (DTT-GNCs). Transmission electron microscopy was used to provide visualization of DTT-GNCs at different states of aggregation. Surface packing density of DTT and the corresponding molecular conformation on the Au surface were characterized by inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy. Results show that DTT increases the aggregation rate of gold nanoparticles (AuNPs) up to ≈100 times. A mixed conformation (i.e., combining vertically aligned, horizontally aligned, and cross-linking modes) exists for DTT on the Au surface for all conditions examined. The primary size of AuNPs, concentration of DTT, and the starting concentration of AuNPs influence the degree of aggregation for DTT-GNCs, indicating that the collision frequency, energy barrier, and surface density of DTT are the key factors that control the aggregation rate. DTT-GNCs exhibit improved structural stability compared to the citrate-stabilized GNCs (i.e., unconjugated) following reaction with thiolated polyethylene glycol (SH-PEG), indicating that cross-linking and surface protection by DTT suppresses disaggregation normally induced by the steric repulsion of SH-PEG. This work describes a prototype methodology to form ligand-conjugated GNCs with high-quality and well-controlled material properties.