This white paper highlights the growth in the use of the technique of Nanoparticle Tracking Analysis as applied to nanoparticles and nanomaterials production and analysis, from the very first reports on nanoscale silver and gold (Lundahl et al. (2008) and Marsh et al. (2008), respectively).
NTA utilizes the properties of both light scattering and Brownian motion in order to obtain the particle size distribution of samples in liquid suspension. A laser beam is passed through the sample chamber, and the particles in suspension in the path of this beam scatter light in such a manner that they can easily be visualized via a 20x magnification microscope onto which is mounted a camera. The camera, which operates at approximately 30 frames per second (fps), captures a video file of the particles moving under Brownian motion within the field of view of approximately 100 μm x 80 μm x 10 μm (Figure 1).
The movement of the particles is captured on a frame-by-frame basis. The proprietary NTA software simultaneously identifies and tracks the center of each of the observed particles, and determines the average distance moved by each particle in the x and y planes. This value allows the particle diffusion coefficient (Dt) to be determined from which, if the sample temperature T and solvent viscosity η are known, the sphere-equivalent hydrodynamic diameter, d, of the particles can be identified using the Stokes-Einstein equation (Equation 1).
where KB is Boltzmann’s constant.
NTA is not an ensemble technique interrogating a very large number of particles, but rather each particle is sized individually, irrespective of the others. An example of the size distribution profile generated by NTA is shown in Figure 2.
In addition, the particles’ movement is measured within a fixed field of view (approximately 100 μm by 80 μm) illuminated by a beam approximately 10 μm in depth. These figures allow a scattering volume of the sample to be estimated; by measuring concentration of the particles within this field of view and extrapolating to a larger volume it is possible to achieve a concentration estimation in terms of particles per mL for any given size class or an overall total.
Nanoparticles and engineered nanoparticles (ENPs) are being increasingly exploited throughout a wide range of industry sectors in order to benefit from the frequently greatly enhanced properties exhibited by materials when they are reduced to the nanoscale. Despite the growing importance of obtaining accurate estimates of size, size distribution and concentration of such nanoscale particles in an increasingly wide range of applications, existing techniques for obtaining such information (e.g. electron microscopy, light scattering) can prove time consuming and complex with results that are difficult to interpret, particularly in samples which are heterogeneous in composition or which contain a range of particle sizes, e.g. are polydisperse.
The effects of ball milling time on the synthesis and consolidation of nanostructured WC–Co composites was investigated by high energy milling in a horizontal ball mill by Hewitt and Kibble (2010) using NTA to determine particle size distribution. This showed that the number of nanosize (<0.2 μm) particles increased with milling time. The onset of the WC–Co eutectic was lowered to 1312°C through an increase in milling time. Hewitt et al. (2009) had previously studied the effect of milling temperature on the synthesis and consolidation of nanocomposite WC–10Co powders using NTA.
Hennart et al. (2012) have also used the NTA principle to characterize the particle size distribution of sub-micron particles suspended in a liquid as well as other techniques such as imaging techniques (SEM, CryoTEM), static light scattering (SLS) and dynamic light scattering (DLS). Their conclusion was that the presence of aggregates frequently severely perturbed results in these other techniques.
Kendall (2011) discussed problems of particle aggregation in ceramics presenting three types of problem to illustrate the theory that small interatomic forces between ceramic particles have a major influence on the aggregates formed during processing, and on the final ceramic product microstructure and strength. The first theoretical problem with ceramic particle aggregation was to define the weak interatomic forces between spheres. The second concerned the better processing that can be applied to dispersed particles to deliver improved ceramic properties by adding polymer to ceramic dispersions to reduce particle attractions which lead to aggregation. The last was the application of polymer extrusion to make improved ceramic fuel cells which can start up in a short time to provide auxiliary power to new applications.
Reduction in the formation of aggregates by the use of surfactants has been investigated using NTA and other techniques. Accordingly, Pollet et al. (2011) used ionic and non-ionic surfactants for the control of platinum nanoparticle aggregation in proton exchange membrane fuel cells. Platinum nanoparticles were prepared in aqueous dispersion using tetradecyltrimethylammonium bromide (C14TAB), cetyltrimethylammonium bromide (C16TAB) and nonylphenolethoxylate (NP9). The aggregation behavior of the nanoparticles was studied using TEM, NTA and DLS. NTA was used specifically to characterize the aggregate’s particle size distribution profile. In further work, the same group used NTA to study the aggregation behavior of these materials to help them conclude that the surfactant molecule selection is vital to obtaining effective fuel cell catalyst (Newton et al. (2011)).
Polymeric systems have also been studied using NTA. Yang et al. (2011) monitored the effects of particle size matched filling of spherical silica on the flowability of epoxy molding compounds for large-scale integrated circuits packaging while Stevens et al. (2012) have investigated nanosponge formation from organocatalytically synthesized poly(carbonate) copolymers. Polleto et al. (2012) have reviewed the use of polymeric nanocapsules in nanocosmetics and nanomedicines comparing a variety of light scattering techniques, including NTA, with EM.
NTA has been used by Kucherov et al. (2012) to analyze the particulate nature of brittle material debris undergoing ballistic impact and proposed that failure waves can be interpreted as the result of the decay of the shock-excited phonon continuum into low frequency peaks in the phonon density of states. Experimental confirmation of this model was reported using fractured particle size analyses and comparing their results with predicted acoustic wavelengths.
Herrington et al. (2012) have studied the effect of the size and size distribution of BaTiO3 nanoparticles on the electro-optic properties of nematic liquid crystals and Jawor-Baczynska et al. (2012) have shown 250 nm glycine-rich nanodroplets are formed on dissolution of glycine crystals but are too small to provide productive nucleation sites. Both studies used NTA, amongst other techniques, for determining nanoparticle size and number.
Homeijer et al. (2010) discussed polymer-induced liquid-precursor (PILP) process in the non-calcium based systems of barium and strontium carbonate and Carja et al. (2010) presented data on nanoparticles of nickel oxide: growth and organization on zinc-substituted anionic clay matrix by a one-pot route at room temperature.
Vogel et al. (2011) have reported a new route for mass production of uniform metal nanoparticles in water by means of laser light induced processes in which NTA showed that pulsed laser ablation from a gold plate in water results in a large amount of nanoparticles with radii in the range of 75 nm with a relatively broad size distribution of sigma = 31%, but that this broad size distribution had been subsequently narrowed in a single irradiation step to sigma = 20% without a significant change of the mean nanoparticle radius utilizing selective laser tailoring. The use of NTA in nanoparticle production studies by laser pyrolysis and laser ablation had both been described earlier (Sentein et al. (2009); Menéndez-Manjón et al. (2009)).
The kinetics of aggregation of initially formed primary particles of alkoxide complexes of rhenium was followed by NTA (Nikonova, 2011). The initial particles with the size below 10 nm aggregated uniformly to spherical particles of several hundred nanometers in size within minutes. The aggregates could be split into initial small particles again by sonication in a standard ultrasound bath in 5 minutes. Reproducible re-aggregation subsequently followed with formation of the same type of spherical aggregates in the same time scale. This was considered important evidence for the formation of the observed oxide particles through a Micelle Templated Self- Assembly of Ligands (MTSAL) mechanism. Nikonova et al. (2011) then demonstrated the role of strongly coordinated inorganic anions on precursor-directed assembly of complex oxide nanobeads using NTA to follow the aggregation process.
Hagmeyer et al. (2011) have demonstrated the self-assembly of calcium phosphate nanoparticles into hollow spheres induced by dissolved amino acids by multiple techniques (AFM, SEM, DLS and NTA) and in more recent work (Hagmeyer et al. (2011)) have gained direct experimental observation of the aggregation of α-amino acids into 100-200 nm clusters in aqueous solution. Their presence was shown by NTA, AFM, and ESI mass spectrometry. Domingos et al. (2010) explored the role of calcium and phosphate in the aggregation of titanium dioxide nanoparticles.
Zhou et al. (2011) have used NTA to determine the hydrodynamic diameters of the nanoparticles suspended in water in their attempts to tune the mechanical properties of liquid crystalline nanoparticles. They reported the synthesis of colloidal nanoparticles with an internal structure forming a gel-like matrix. These nanoparticles were composed of low molecular weight liquid crystal (LC) 4-pentyl-4-cyanobiphenyl (5CB) encapsulated in an LC-based polymer network. Using nanoscopic mechanical analysis, they demonstrated the ability to independently tune the shape anisotropy and stiffness by varying respectively the 5CB concentration and the extent of the polymer cross-linking.
Thermosensitive hydrogels were the subject of another study by de Graaf et al. (2012) in which they developed a micelle-shedding thermosensitive hydrogel based on poly(N-isopropylacrylamide)-poly(ethylene glycol)-poly(N-isopropylacrylamide) as a sustained release formulation for the delivery of the cytostatic agent paclitaxel (PTX). They showed that, at the highest dose, PTX completely inhibited tumor growth for at least 3 weeks with a single hydrogel injection. This promising concept may find application as a depot formulation for sustained, metronomic dosing of chemotherapeutics.
Pinheiro et al. (2012) have reported the preparation and characterization of low dispersity anionic multi-responsive core-shell polymer nanoparticles. The nanoparticles had a glassy poly(methyl methacrylate) (PMMA) core of ca. 40 nm radius and a crosslinked PNIPAM anionic shell with either AA or MA co-monomers, as determined by NTA.
Bewernitz et al. (2012) have studied the same material in a meta-stable liquid precursor phase to investigate its interactions with polyaspartate and this group have proposed that charged polyelectrolytes, like acidic proteins, may be employed by invertebrate organisms to direct crystal growth through an intermediate liquid phase in a process called the polymer-induced liquid-precursor (PILP) process. More recently, Dressick et al. (2012) used NTA for detection and analysis of polyelectrolyte aggregates in their study on divalent–anion salt effects in polyelectrolyte multilayer depositions, while Kim et al. (2012) used DLS to size, and NTA to measure concentration, of the chitosan-lignosulfonates sono-chemically prepared nanoparticles they described.
Hamed et al. (2012) described the synthesis, characterization and surface modification of ZnCrFeO4 nanoparticles using the sol gel technique with nanoparticle size controlled through a two-stage annealing process. The resulting nanoparticles were found by EM, AFM and X-ray diffraction studies to have excellent crystal quality while NTA was used to estimate the degree of aggregation present.
During room-temperature synthesis of nanocrystalline and monodisperse titanium dioxide Seisenbaeva et al. (2013) measured the NTA size distribution of the particles redistributed in ethanol by sonication.
While producing amphiphilic copolymers based on polyoxazoline and grape seed vegetable oil derivatives, Travelet et al. (2013) pointed out that DLS results in terms of characteristic size were corroborated using NTA, and also by AFM and TEM imaging, where well-defined spherical and individual nanoparticles exhibited a very good mechanical resistance upon drying
In synthesizing a series of the highly crystalline MFe2O4 ferrite spinel, via a modified Bradley reaction using microwave stimulation, particle size was estimated using theoretical calculations from X-ray data as well as by direct experimental techniques such as TEM, DLS and NTA (Wiglusz et al. (2013)). Polycrystalline MgAl2O4 spinels had also been studied using NTA in earlier work (Goldstein et al., 2009) and Goldstein et al. (2010) had also described the influence of powder type on the densification of transparent MgAl2O4 spinel.
Panda et al. (2013) used NTA to analyze hydroxyapatite (HAp) prepared from fish scale and synthetic body fluid (SBF) solution to confirm that HAp bio-materials from fish scale are physico-chemically and biologically equivalent to the chemically synthesized HAp from SBF.
For the characterization of phase inversion and emulsification properties pre- and post-inversion, Lefsaker (2013) employed several techniques to investigate the properties of his samples including the conductivity, the e-critical cell, the near-infrared spectroscopy (NIR) and a cone and plate rheometer, but also used NTA for determination of the diffusion coefficient of his sample.
The agglomeration of nanoparticle suspensions has been the subject of significant study and NTA has been used to support other techniques (e.g. hydrodynamic chromatography and single particle-inductively coupled plasma mass spectrometry; Rakcheev et al. (2013)) in establishing the size and size distribution of both calibration particles and the agglomerates formed in different nanoparticle systems (Otanicar et al. (2013)).
Using a modified NTA system Jakobi et al. (2011) have determined the stoichiometry of alloy nanoparticles from laser ablation of PtIr in acetone and their electrophoretic deposition on PtIr electrodes. Hartmann et al. (2012) have considered the challenges of testing metal and metal oxide nanoparticles in algal bioassays using titanium dioxide and gold nanoparticles as case studies. Schrittwieser et al. (2012) modelled and developed a biosensor based on optical relaxation measurements of hybrid nanoparticles using NTA to characterize their core asymmetric and magnetic nanoparticles. Using the reprecipitation method, Zhou et al. (2013) synthesized poly(N-vinylcarbazole) nanoparticles (PVK NPs) as a model system. Electrochemical sticking and sensing experiments were then conducted, which involve PVK nanoparticle immobilization on the electrode surface and subsequent oxidative sensing, to enable rapid detection of polymer nanoparticles in aqueous solution. They suggested their technique was better than the other techniques they tried, namely DLS and NTA.
Mesoporous silica is a form of silica and a recent development in nanotechnology. The most common types of mesoporous nanoparticles are MCM-41 and SBA-15. Research continues on these particles, which have applications in catalysis, drug delivery and imaging. Despite their low refractive index and the resultant difficulty in visualizing them when present at small size (e.g. <40 nm) NTA has been used in their detection, analysis and characterization in many applications.
Monodisperse spherical silica particles are potentially available for various applications as building blocks for photonic crystals, chromatography stationary phase and drug support for controlled release. Immobilization of a molecular recognizable unit to the surface of the spherical particles is important in such applications. Okada et al. (2012) used NTA in their study of swellable microsphere of a layered silicate produced by using monodisperse silica particles, showing that silica spheres of submicrometer size were covered by a swellable layered silicate, which plays a role in accommodating cationic species.
Luminescence and imaging studies of 500 nm diameter colloidal silica stained with the transition metal complex [Ru(bpy)3Cl2], [Ru(bpy)3SiNP], have been detailed and suggest that such particles are ideal for particle tracking velocimetry (PTV) or particle imaging velocimetry (PIV) for analysis of fluid flow in microchannels according to Lewis et al. (2012). They used NTA to determine the number distribution of particles in the generated sample of [Ru(bpy)3SiNP] of a certain diameter.
Yang et al. (2011x) obtained relevant particle size distribution to estimate the effects of particle size-matching filling of spherical silica on the flowability of epoxy molding compounds for large-scale integrated circuits packaging.
Yip et al. (2012) have investigated the fluorescence anisotropy metrology of electrostatically and covalently labelled silica nanoparticles by comparing the size of silica nanoparticles using the time-resolved fluorescence anisotropy decay of dye molecules when electrostatically and covalently bound to stable silica nanoparticles. Silica nanoparticles produced using Stöber synthesis of tetraethylorthosilicate (TEOS) are found to be controllable between ~3.1 and 3.8 nm radius by adjusting the relative water:TEOS concentration. While the primary particle size was not detectable by NTA, nanoparticle aggregates in LUDOX® colloids were investigated by tracking analysis of particle diffusion via NTA.
Zu et al. (2012) described the preparation of ultrafine polyethylene-silica composite particle with a core-shell structure, using SEM observation and NTA to determine that the composite particles possess a spherical morphology and the mean size is about 160 nm respectively. Bell et al. (2012) have discussed optical methods for the characterization of nanoparticles with the latter study being focused on silica. In exploring the concept of fumed silica nanoparticle-mediated biomimicry for optimal cell–material interactions for artificial organ development, de Mel et al. (2013) used NTA to determine the respective size of the particles under development.
Finally, Jing et al. (2013) have investigated the formation of supported lipid bilayers on silica in relation to lipid phase transition temperature and liposome size. DPPC liposomes ranging from 90 nm to 160 nm in diameter, as measured by NTA, were prepared and used for studies of the formation of supported lipid membranes on silica (SiO2) at temperatures below and above the gel to liquid-crystalline phase transition temperature (Tm). It was found that liposomes smaller than 100 nm spontaneously rupture on the silica surface when deposited at a temperature above Tm and at a critical surface coverage, following a well-established pathway. In contrast, DPPC liposomes larger than 160 nm do not rupture on the surface when adsorbed at 22°C or at 50°C. However, when liposomes of this size are first adsorbed at 22°C and at a high enough surface coverage, after which they are subject to a constant temperature gradient up to 50°C, they rupture and fuse to a bilayer.
Nanoparticles made of silver are increasingly used as additives for materials and coatings with special biological, optical, and electrical properties. Nano-silver absorbs light at a characteristic wavelength (due to metallic surface plasmons), which leads to a yellow color. This was first applied in the coloring of glassware hundreds of years ago. Today, the constant improvement of methods for the production and characterization of nanoparticles allows a better understanding and utilization of nanotechnology. As regards to optical properties, the embedding of nano-silver and nanoparticles from other metals in transparent materials can be tuned to create optical filters that work on the basis of nanoparticle absorption. Another application of nano-silver that is currently established involves conductive nano-inks with high filling degrees that are used to print highly precise continual conductive paths on polymers.
However, the most relevant characteristic of nano-silver is its chemical reactivity. This leads to an antimicrobial effect of silver that is based on strong bonds between silver ions and groups containing carbon monoxide, carbon dioxide, or oxygen and which prevents the spreading of bacteria or fungi. Nano-silver provides a large number of surface atoms for such antibacterial interaction. This has led to many medical applications of nano-silver, such as in catheters or wound dressings. Meanwhile, there are now many consumer products on the market that contain nano-silver, which has partly raised skepticism regarding product safety.
Khaydarov et al. (2012) used NTA to test the aggregation characteristics of silver nanoparticles in the development of a novel method of continuous fabrication of aqueous dispersions of silver nanoparticles using cellulose fibers, showing that the synthesized colloidal dispersions showed a pronounced antibacterial effect, as evidenced by low minimum inhibitory concentration values obtained for Escherichia coli, Staphylococcus aureus and Bacillus subtilis cultures. Hodges (2011) made anti-microbial self-assembling-click monolayers utilizing silver nanoparticles for indwelling medical devices, testing her dispersions with NTA.
Kosmala et al. (2011a) have also reported the development of high concentrated aqueous silver nanofluid and inkjet printing on ceramic substrates in which the effect of substrates, printing temperature and dot spacing on the size and morphology of printed silver features was investigated. NTA was used in the analysis of silver nanoparticles and zeta potential dependence on pH for the nanosilver powders treated with IPA and acetone. The use of high solid loading inks reduces the number of printed layers required for thick, dense and conductive film thus leading to the reduction of the costs, and high efficiency of the printing process (Kosmala et al. (2011b)). Kosmala et al. (2012) also developed a method for the synthesis of silver nanoparticles and fabrication of aqueous Ag inks for inkjet printing using the combination of a triblock copolymer and high intensity focused ultrasound (HIFU) while Yosef and Avnir (2011) entrapped dye molecules within submicron silver particles using NTA to show the existence of silver particle-clusters 150–200 nm in size.
The anti-microbial effect of Murraya koenigii-mediated synthesis of silver nanoparticles against three human pathogenic bacteria was explored by Bonde et al. (2012) using NTA to determine particle size distribution and number concentration during the synthesis. Sable et al. (2012) similarly undertook the phytofabrication of silver nanoparticles by using aquatic plant Hydrilla verticilata with the help of UV-Vis spectroscopy, FTIR, NTA, Electrophoretic Light Scattering (ELS) and SEM.
Chakraborty et al. (2012) investigated the effect of Ag nanoparticle addition and ultrasonic treatment on a stable TiO2 nanofluid to help the separation and recycling of nanoparticles from fluid waste. NTA was used to determine particle size distribution of TiO2 nanoparticles.
In discussing the challenges for physical characterization of silver nanoparticles under pristine and environmentally relevant conditions, MacCuspie et al. (2011) undertook a rigorous physico-chemical characterization by consensus methods and protocols (where available) which enabled an understanding of how the underlying measurement method impacts the reported size measurements, which in turn provided a more complete understanding of the state (size, size distribution, agglomeration, etc.) of the AgNPs with respect to the dispersion conditions. However, the lower sensitivity NTA instruments that was available at the time was found to be incapable of measuring 10, 20 and 40 nm Ag.
To improve the characterization of nanoparticles, including silver, Klein et al. (2011) have, using their expertise in the production and analysis of reference materials, generated a European Commission Joint Research Centre report on a EUR 24693 EN NM-series of representative manufactured nanomaterials in which they assessed ‘300’ silver characterization, stability and homogeneity. Key properties of size and size distribution were studied in an inter-laboratory comparative study using SEM as well as TEM and NTA. As in the work of McCuspie, the sizes of nanosilver tested were found to be outside of the range of NTA instrumentation used.
Ranville et al. (2012) analyzed metal-containing nanoparticles using single particle ICP-MS (Sp ICP-MS) in environmental matrices. Their aim was to develop Sp ICP-MS using spherical monodisperse metal NP “standards” (Au, Ag) and extend this capability to other metal-containing NPs; TiO2, CeO2, ZnO, Ag nanowires, and Carbon Nanotubes (CNTs). Their data comparing Sp ICP-MS to Disc Centrifuge and NTA revealed a broader size distribution when measured by NTA than was detected by the other techniques.
Silver nanoparticles, synthesized using Saccharum officinarum (sugarcane), have been shown to quench and inhibit biofilm formation in Staphylococcus aureus by Masurkar et al. (2012). NTA measurements revealed that the mean size of synthesized silver nanoparticles was found to be 32 nm with a concentration of 17.4×1010 particles/mL. No aggregations or debris were detected on NTA measurements.
The natural synthesis of Ag nanoparticles was further explored by Meshram et al. (2013) who claimed a method that was cost-effective, energy-efficient and easy by using white sugar and sodium hydroxide (NaOH) in the presence of sunlight. They employed visual observation, ultraviolet–visible spectrophotometry, Fourier transform infrared (FTIR), NTA and TEM in their analysis. NTA revealed the polydisperse nature of nanoparticles, 15–30 nm in diameter, while TEM demonstrated the presence of spherical AgNps in the range of 10–25 nm. Similarly, Vezina et al. (2013) have extended this work using white sugar and sodium hydroxide (NaOH) in the presence of sunlight to prepare silver nanoparticles (AgNps) in a simple, eco-friendly and economically sustainable way, making it amenable to large-scale industrial production of AgNps. NTA revealed the polydisperse nature of nanoparticles, 15–30 nm in diameter, while FTIR showed the presence of gluconic acid as capping agent, which increases the stability of AgNps in the colloids. TEM demonstrated the presence of spherical AgNps in the range of 10–25 nm.
Similarly, Dhuldhaj et al. (2012) demonstrated Tagetes erecta mediated phytosynthesis of silver nanoparticles as an eco-friendly approach for nanomaterials synthesis using NTA and TEM to confirm the synthesis of the polydisperse spherical silver nanoparticles of 20-50 nm, with the average size of 30 nm.
Raheman et al. (2011) used NTA and TEM to show that the silver particles synthesized extracellularly by an endophytic fungus were in the range 10-40 nm and exhibited antibacterial activity against human pathogenic bacteria, while Yadav and Rai (2012) described the bioreduction and mechanistic aspects involved in the synthesis of silver nanoparticles using Holarrhena antidysenterica.
Birla et al. (2013) reported the rapid synthesis of silver nanoparticles from Fusarium oxysporum by optimizing physico-cultural conditions and Bonde et al. (2012), from the same group, have similarly described some comparative studies on synthesis of silver nanoparticles by Fusarium oxysporum and Macrophomina phaseolina and its efficacy against bacteria and Malassezia furfur. This group have most recently screened eighteen Phoma sp. for the mycosynthesis of silver nanoparticles (AgNP's). Out of eighteen, seventeen Phoma sp. demonstrated mycosynthesis of AgNP's, which were characterized by UV-Vis spectrophotometry, FTIR, XRD, TEM, SEM, NTA and ELS measurement (Gade et al. (2013a)). Gade has also recently described a ‘green’ extracellular synthesis of silver nanoparticles (SNPs) by Phoma glomerata (MTCC-2210) which showed rapid synthesis in bright sunlight. NTA and EM were used to demonstrate the synthesis of polydispersive and spherical SNPs while FTIR revealed the presence of a protein cap on the silver nanoparticle, which led to increase stability of SNP in the silver colloid (Gade et al. (2013b)).
The rapid biosynthesis of silver nanoparticles by exploiting the reducing potential of Trapa bispinosa peel extract has been described by Pandey et al. (2013) using NTA to determine the particle size distribution obtained. This produced monodisperse silver nanoparticles (SNPs) within 120 seconds at 30°C, which is the shortest tenure reported for SNP synthesis using plants. Gudadhe et al. (2013) have recently synthesized silver nanoparticles (Ag-NPs) using an extract of Ocimum sanctum leaves that was mixed with agar–agar to prepare an agar-silver nanoparticle (A-AgNp) film. This film was surface-coated over the fruits, Citrus aurantifolium (Thornless lime) and Pyrus malus (Apple), and evaluated for the determination of antimicrobial activity of A-AgNp films using disc diffusion method, weight loss and shelf life of fruits. Their study demonstrated that A-AgNp films possessed antimicrobial activity and also increased the shelf life of fruits used.
Recently, Neumann et al. (2013) have used NTA to investigate the performance of silver nanoparticles in the catalysis of the oxygen reduction reaction in neutral media and generated data showing the nanoparticles produced to be 9 nm in radius. Milczarek et al. (2013) reported a one-step synthesis of softwood lignosulfonate-stabilized silver nanoparticles in an aqueous solution at room temperature. As no particles of diameter greater than 100 nm were detected using NTA, the formation of aggregates that was observed by TEM was considered likely to be an artefact of the TEM sample preparation.
Recently, Luque and his colleagues (Luque et al. (2013)) have evaluated biomass-derived stabilizing agents for colloidal silver nanoparticles via NTA and concluded that NTA has “been proved to be a highly useful, simple and efficient characterization tool to differentiate between capping efficiencies of various biomass-derived stabilizing agents (e.g. starch, alginic acid and a waste-derived hemicellulosic syrup) of aqueous colloidal silver suspensions”.
Electrochemical studies involving the immobilization of nanoparticles from solution at a solid surface followed by anodic stripping voltammetry as a simple technique allowing the analysis of nanoparticle concentrations and identity. Stuart et al. (2013) improved the rate of silver nanoparticle adhesion to ‘sticky electrodes’ in stick and strip experiments at a meso-2,3-dimercaptosuccinic acid (DMSA)-modified gold electrode using NTA to size the adhering particles. The same group also wrote a perspective and guide for experimentalists undertaking electrochemical studies of silver nanoparticles (Tschulik et al. (2013)). This latter report summarized four different electrochemical techniques that have been established and frequently used to characterize various properties of silver nanoparticles. These were based on drop casting, in situ nanoparticle sticking and stripping, transfer sticking and stripping or nanoparticle impacts. NTA was used throughout to confirm nanoparticle size, distribution and concentration.
In applications in medicine and more specifically drug delivery, the dispersion stability of gold nanoparticles plays a significant role on their final performances. With the use of two laser technologies, DLS and NTA, Du et al. (2012) reported a simple method to estimate the stability of nanoparticles dispersed in phosphate buffered saline (PBS). By investigating the effects of sonication treatment and surface modification by five types of surfactants, including nonylphenol ethoxylate (NP9), polyvinyl pyrrolidone (PVP), human serum albumin (HSA), sodium dodecyl sulphate (SDS) and citrate ions on the dispersion stability, the varying self-aggregation and adhesion of gold nanoparticles dispersed in PBS were demonstrated. The results showed that PVP effectively prevented aggregation, while HSA exhibited the best performance in avoiding the adhesion of gold nanoparticle in PBS onto glass and metal. Similarly, Treuel et al. (2012) quantified the influence of polymer coatings on the serum albumin corona formation around silver and gold nanoparticles employing DLS, TEM, SEM, NTA and/or differential centrifugal sedimentation in their study. Aljabali et al. (2011) produced virus-polyelectrolyte-templated gold nanoparticles, his results being supported by NTA data.
Recently, Otsuka et al. (2013) described the self-assembly of maltoheptaose-block-polystyrene (MH1.2k-b-PS4.5k), into micellar nanoparticles and the subsequent encapsulation of gold nanoparticles. The mean hydrodynamic radii (Rh) of the nanoparticles were determined by DLS to be ca. 30 and 80 nm depending on the method of preparation. These results were clearly visualized by TEM, AFM and field emission gun-scanning electron microscope imaging, and complemented by NTA.
Mahl et al. (2011) reported on the possibilities and limitations of different analytical methods for the size determination of a bimodal dispersion of metallic nanoparticles (silver nanoparticles (about 70 nm) and gold nanoparticles (about 15 nm)). Using SEM, TEM, DLS NTA and analytical disc centrifugation, the differences between the methods were highlighted and their ability to distinguish between silver and gold nanoparticles in the mixture demonstrated. The size distribution data from the different methods were clearly different, therefore it was recommended to apply more than one method to characterize the nanoparticle dispersion. In particular, the smaller particles were undetectable by DLS and NTA in the presence of the large particles. For the 1:1 mixture, only electron microscopy and analytical disc centrifugation were able to give quantitative data on the size distribution. On the other hand, it is not possible to make statements about an agglomeration in dispersion with electron microscopy because an agglomeration may also have occurred during the drying process.
Pettibone and Hudgens (2011) explored gold cluster formation with phosphine ligands: suggesting etching as a size-selective synthetic pathway for small clusters and Yuan et al. (2012a) advocated plasmonic gold nanostars as a potential agent for molecular imaging and cancer therapy reporting also on the spectral characterization and intracellular detection of Surface-Enhanced Raman Scattering (SERS)-encoded plasmonic gold nanostars (Yuan et al. (2012b and 2012c)). The particle hydrodynamic size distribution, concentration and zeta potential were determined by NTA and ELS. Intracellular detection of silica-coated SERS-encoded nanostars was also demonstrated in breast cancer cells. The non-aggregated field enhancement makes the gold nanostar ensemble a promising agent for SERS bioapplications.
Yuan et al. (2012) further described in vivo particle tracking and photothermal ablation using plasmon resonant gold nanostars again using NTA to measure the nanoparticles' hydrodynamic radius, zeta potential, and concentration.
Xie et al. (2012a and 2012b) have used NTA to measure the size and number of hollow gold particles in their study of both SERS investigation of hollow gold nanospheres and synthesis, and NIR optically probed properties of hollow gold nanospheres with localized surface plasmon resonance greater than one micrometer.
The development of gold nanostars was also explored by the group of Vo-Dinh in which a variety of analytical techniques, including NTA, was used to investigate the synthesis of gold nanostars which were tagged with a SERS reporter and linked with an MRI contrast agent Gd3+ (Liu et al. (2013)). In vitro experiments demonstrated the developed nanoprobe to be a potential theranostics agent for future biomedical applications. In an expansion of this work Wang et al. (2013) described a SERS-based detection approach, referred to as “molecular sentinel”plasmonic nanoprobes, to detect an RNA target related to viral infection. This work arose from their earlier work on the production of silica-coated gold nanostars for combined SERS detection and singlet oxygen generation as a potential nanoplatform for theranostics (Fales et al. (2011)).
Given protein-conjugated gold nanoparticles (AuNPs) have been extensively explored for the development of many novel protein assays, James and Driskell (2012) demonstrated that NTA can be used as a rapid and simple analytical tool to monitor bioconjugation and to study protein-protein interactions. Firstly the adsorption of protein A onto gold nanoparticles was analyzed using NTA resulting in a measureable increase in hydrodynamic radius that correlated with protein A concentration. NTA was then used to investigate the binding of mouse IgG to Protein A-conjugated AuNPs and the Ka was measured as 2.00 x 107 M-1. Furthermore, an assay for the detection of mouse IgG was developed using NTA to detect the binding to antibody-AuNP conjugates exhibiting a detection limit of 3.2 ng/mL. However, the formation of aggregates resulting from the use of a polyclonal antibody and multiple binding sites on the antigen prevented the determination of binding affinity for this antibody-antigen system. To measure the binding affinity for this antibody-antigen system the IgG antigen was conjugated to the AuNPs and NTA was used to monitor the binding of the antibody. In this configuration aggregation of conjugates was not detected and a binding affinity constant of 2.80 x 108 M-1 was measured. NTA results obtained in this work were validated by comparison to DLS. This work represented the first evaluation of NTA as an analytical tool for characterizing AuNP bioconjugates, investigating protein-protein binding, and detecting low levels of antigen in a bioassay.
In their work on the generation of representative nanomaterials (e.g. silver) for subsequent use in toxicological studies, the European Commission Joint Research Centre have recently reported their characterization of ‘NM-300’, (a representative manufactured nanomaterial) by a number of sophisticated techniques which resulted in the development and validation of a dedicated method according to ISO17025 principles. They reported that key properties of size and size distribution were studied in an inter-laboratory comparative study using SEM, TEM and NTA (Klein et al. (2011)).
Jiang et al. (2013) used multiple techniques, namely NTA, differential centrifugal sedimentation (DCS), UV-visible spectroscopy (UV), second order spectroscopy (SOS) and TEM to follow the slow agglomeration of gold colloids of approximate diameter 30 nm in the presence of a small concentration of L-cysteine•HCl. This work was described more fully by Jiang (2013).
In a similar vain, Engelbrekt et al. (2013) investigated the complexity associated with the time-dependent physical and chemical properties in aqueous solution during the chemical synthesis of gold nanoparticles (AuNPs) synthesized from gold salt (HAuCl4). Chemical synthesis of AuNPs is a reduction process accompanied by release of ions and protons and formation of solid particles. Dynamic information from redox potential, pH, conductivity, and turbidity of the solution enables distinct observation of reduction and nucleation/growth of AuNPs phases with NTA being used to monitor, in real time, the formation of gold nanoparticles.
The synthesis of iron oxide (magnetic) nanoparticles by a filtrate of Phoma glomerata (a plant pathogen) has been reported by Gudadhe et al. (2012), NTA being used to reveal polydisperse nanoparticles with average size of 56 nm.
Cheng et al. (2012) described the synthesis of carbon-coated, porous and water-dispersive Fe3O4 nanocapsules of about 120 nm (about 50 nm cavity) as measured by NTA and claimed excellent performance for heavy metal removal applications. They showed that when protected by a porous carbon layer, the nanocapsules display excellent acidic resistance and adsorption properties even in an acidic solution (pH = 3).
The synthesis, solution stability and 64Cu2+ labelling of magnetite nanoparticles (NPs) coated with different macrocycles has been reported by Barreto et al. (2011) using NTA to demonstrate that the NPs formed a stable colloidal suspensions in 0.05 M aqueous 2-(N-morpholino)ethanesulfonic acid (MES) buffer, which consist of larger aggregates with a mean hydrodynamic size of about 200 nm.
In a systematic examination of the effect of four common polymers on the size, surface chemistry, colloidal stability, and sedimentation behavior of nanoparticles of zero valent iron (NZVI), Cirtiu et al. (2011) measured the size, surface characteristics and colloidal stability of zero valent iron nanoparticles post and pre-treatment. TEM images and NTA revealed that iron nanoparticles synthesized in the presence of the polymers were larger in diameter, with TEM mean diameters ranging from 84.5 to 189 nm, than the bare-NZVI (59.1 nm), when synthesized with the same initial Fe2+ concentration.
When developing efficient water oxidation catalysts based on readily available iron coordination complexes, Fillol et al. (2011) carried out different analyses to investigate the possible formation of nanoparticles in solution. Experiments performed include DLS and NTA. Catalytic reactions had very low concentration of nanoparticles in solution (< 0.1 ppm), that was below the limit of detection for DLS and accordingly it was not possible to have a reliable size distribution measurement. NTA experiments were shown to be more sensitive in the range of 10 nm to 2000 μm, and measured values of particles/mL were in the same magnitude order 0.76 x 108 particles/mL as the blank experiments. Finally, Kadar et al. (2011) have shown the stabilization of engineered zero-valent nanoiron with Na-acrylic copolymer enhances spermiotoxicity using NTA to detect aggregation behavior.
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ASTM E2834 - 12 (2012) Standard Guide for Measurement of Particle Size Distribution of Nanomaterials in Suspension by Nanoparticle Tracking Analysis (NTA), Active Standard ASTM E2834 Developed by Subcommittee: E56.02|Book of Standards Volume: 14.02, DOI: 10.1520/E2834-12
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Dhuldhaj UP, Deshmukh SD, Gade AK, Yashpal M and Rai MK. (2012) Tagetes erecta mediated phytosynthesis of silver nanoparticles: an eco-friendly approach, Nusantara Bioscience, 4 (3): 109-110 112, November 2012, ISSN: 2087-3948
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Du S, Kendall K, Toloueinia P, Mehrabadi Y, Gupta G and Newton J (2012) Aggregation and adhesion of gold nanoparticles in phosphate buffered saline, Journal of Nanoparticle Research ,Volume 14, Number 3, 758, DOI: 10.1007/s11051-012-0758-z
Engelbrekt C, Jensen PS, Sørensen KH, Ulstrup J and Zhang J (2013) Complexity of Gold Nanoparticle Formation Disclosed by Dynamics Study, J. Phys. Chem. C, 2013, 117 (22), pp 11818–11828, Publication Date (Web): May 9, 2013 (Article), DOI: 10.1021/jp401883h
Fales A, Yuan H and Vo-Dinh T (2011), Silica-Coated Gold Nanostars for Combined SERS Detection and Singlet Oxygen Generation: A Potential Nanoplatform for Theranostics Langmuir, Just Accepted Manuscript, DOI: 10.1021/la202602q
Fillol JL, Codolà Z, Garcia-Bosch I, Gómez L, Pla JJ & Costas M (2011), Efficient water oxidation catalysts based on readily available iron coordination complexes, Nature Chemistry , doi:10.1038/nchem.1140, Published online 04 September 2011
Gade A, Gaikwad S, Duran N, Rai M (2013) Screening of different species of Phoma for Synthesis of Silver nanoparticles, Biotechnology and Applied Biochemistry, DOI: 10.1002/bab.1141
Gade A, Gaikwad S, Duran N, Rai M (2013) Green synthesis of silver nanoparticles by Phoma glomerata, Micron, Available online 24 December , http://dx.doi.org/10.1016/j.micron.2013.12.005
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Gudadhe JA; Bonde SR; Gaikwad SC; Gade AK; Rai MK (2012) Phoma glomerata: A Novel Agent for Fabrication of Iron Oxide Nanoparticles, Journal of Bionanoscience, Volume 5, Number 2, December 2011 , pp. 138-142(5)
Gudadhe JA; Yadav A; Gade A; Marcato PD; Durán N; Rai M (2013) Preparation of an agar-silver nanoparticles (A-AgNp) film for increasing the shelf-life of fruits, Institution of Engineering and Technology, IET Nanobiotechnology, November 2013, 6 pp., DOI: 10.1049/iet-nbt.2013.0010,
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James AE. and Driskell J (2012) Monitoring Gold Nanoparticle Conjugation and Analysis of Biomolecular Binding with Nanoparticle Tracking Analysis (NTA) and Dynamic Light Scattering (DLS), Analyst, 2012, Accepted Manuscript, DOI: 10.1039/C2AN36467K
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Joshi PA; Bonde SR; Gaikwad SC; Gade AK; Abd-Elsalam K; Rai MK (2013) Journal of Bionanoscience, Volume 7, Number 4, August 2013 , pp. 378-385(8)
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