Integrative Modulation of Magnetic Resonance Transverse and Longitudinal Relaxivity in a Cell-Viable Bimagnetic Ensemble, γ-Fe2O3@ZnFe2O4.

The potential application of magnetic nanosystems as magnetic resonance imaging (MRI) contrast agents has been thoroughly investigated. This work seeks to attain robust MRI-contrast efficiency by designing an interacting landscape of a bimagnetic ensemble of zinc ferrite nanorods and maghemite nanoparticles, γ-Fe2O3@ZnFe2O4. Because of competing spin clusters and structural anisotropy triggered by isotropic γ-Fe2O3 and anisotropic ZnFe2O4, γ-Fe2O3@ZnFe2O4 undergoes the evolution of cluster spin-glass state as evident from the critical slowing down law. Such interacting γ-Fe2O3@ZnFe2O4 with spin flipping of 1.2 × 10-8 s and energy barrier of 8.2 × 10-14 erg reflects enhanced MRI-contrast signal. Additionally, γ-Fe2O3@ZnFe2O4 is cell-viable to noncancerous HEK 293 cell-line and shows no pro-tumorigenic activity as observed in MDA-MB-231, an extremely aggressive triple-negative breast cancer cell line. As a result, γ-Fe2O3@ZnFe2O4 is a feasible option for an MRI-contrast agent having longitudinal relaxivity, r1, of 0.46 s-1mM-1 and transverse relaxivity, r2, of 15.94 s-1mM-1, together with r2/r1 of 34.65 at 1.41 T up to a modest metal concentration of 0.1 mM. Hence, this study addresses an interacting isotropic/anisotropic framework with faster water proton decay in MR-relaxivity resulting in phantom signal amplification.

Interparticle interaction-dependent jamming in colloids: insights into glass transition and morphology modulation during rapid evaporation-induced assembly.

Understanding the role of interparticle interactions in jamming phenomena is essential for gaining insights into the intriguing glass transition behavior observed in atomic and molecular systems. In this study, we investigate the jamming behavior of colloids with tunable interparticle interactions during evaporation-induced assembly (EIA). By manipulating the interaction among charged colloids using cationic polyethyleneimine (PEI) through electro-sorption and subsequent free polymer induced repulsion, we observe distinct jamming behavior in silica colloids during EIA, depending on the interparticle interactions. Silica colloids with strong repulsive interactions exhibit a repulsive colloidal glass state with a volume fraction of silica colloids in supraparticle ϕ ∼ 0.70. On the other hand, PEI-mediated attractive interactions among silica colloids lead to an attractive colloidal glass phase with a significantly lower ϕ ∼ 0.43. Free polymer induced repulsion of colloids at higher PEI concentration once again results in a repulsive glassy state with ϕ ∼ 0.61. Furthermore, we revealed that interparticle interactions not only influence the jamming behavior but also play a significant role in shaping the morphology of self-assembled structures during EIA, and the assembled structure undergoes a morphological reentrant transition from a doughnut-like shape to a spherical form and again back to a doughnut-like configuration. Jamming-dependent evolution of micropores and dynamics of the confined PEI have been probed using positron annihilation lifetime spectroscopy (PALS) and broadband dielectric spectroscopy (BDS). PALS reveals distinct variations in the micropores of the supraparticles with different PEI loadings, confirming the impact of jamming on the evolution of the micropores within the supraparticles. BDS measurements uncover non-monotonic dynamics of PEI molecules confined in the evolved pore network. It is revealed that the reentrant jamming behavior of colloids, modulated by PEI, holds profound significance for the long-term stability of supraparticles.

Evidence of Size Stratification in Colloidal Glass Microgranules Realized by Rapid Evaporative Assembly.

Evaporation is a ubiquitous phenomenon. Rapid evaporation of the continuous phase from micrometric colloidal droplets can be used to realize nanostructured microgranules, constituting the assembled nanoparticles. One of the important aspects of such nonequilibrium assembly is the nature of the packing of nanoparticles in the microgranules. The present work demonstrates the evidence of size stratification of the nanoparticles in such far-from-equilibrium configurations. Small-angle X-ray scattering, in combination with particle packing simulation, reveals the "large on top"-type stratification in such assembled microgranules, where the larger particles get concentrated at the outer shell of the granules while the smaller particles reside in the core region. It also reveals the presence of local clusters in such a rapid evaporative assembly in aerosolized colloidal droplets.

Amphiphilic interaction-mediated ordering of nanoparticles in Pickering emulsion droplets.

For various industrial processes, the stabilization of an oil phase is crucial and demands a proper balance of complex interactions in an emulsion system. In Pickering emulsions, this is achieved by introducing nanoparticles, which become organized at the oil-water interface. The influence of interparticle interactions towards the formation of a stable emulsion and the ordering of the stabilizing nanoparticles is intriguing and needs attention. In this work, the role of amphiphilic interactions between hydrophilic silica nanoparticles and the Pluronic F127 tri-block co-polymer towards the spontaneous formation of a fairly stable Pickering emulsion has been studied using small-angle X-ray scattering. Unlike the usual random arrangements of the nanoparticles in a conventional Pickering emulsion, we observed highly organized silica nanoparticles at the oil-water interface. The established standard raspberry structural model of the Pickering emulsion fails to explain such strong ordering as observed in the present case. A plausible formation mechanism of the present Pickering emulsion with a high on-surface silica correlation is elucidated on the basis of the combined interactions of the block co-polymer and silica particles. A computer model is developed to elucidate the effects of size and distribution of the surface-decorating nanoparticles and their positional correlation.

Temperature-dependent anomalous viscosity of aqueous solutions of imidazolium-based ionic liquids.

Ionic liquids (ILs) are organic salts with a low melting point compared to inorganic salts. Room temperature ILs are of great importance for their widespread potential industrial applications. The viscosity of aqueous solutions of two imidazolium-based ILs, investigated in the present study, exhibits an anomalous temperature variation. Unlike conventional molecular fluids, the viscosity of 1-methyl-3-octyl imidazolium chloride [OMIM Cl] and 1-methyl-3-decyl imidazolium chloride [DMIM Cl] solutions is found to increase with temperature and then depress. The Small Angle X-ray Scattering (SAXS) data suggest that the lattice parameter of the body-centered cubic lattice formed by the spherical micelles of these ILs, and the morphology of the micelles remain intact over the measured temperature range. The molecular dynamics simulation shows the micelles to be more refined with their integrated structure on increasing the temperature. On further increase of the temperature, the structure is found to be loosened, which is corroborated by the simulation work. The ionic conductivity of these IL solutions shows a trend that is opposite to that of the viscosity. The observed anomalous nature of the viscosity is attributed to the trapped dissociated ions in the network of the micellar aggregates.

Nucleation-Growth Versus Spinodal Decomposition in Fe-Cr Alloys: An Experimental Verification by Atom Probe Tomography and Small Angle Neutron Scattering.

Identifying the operative mode of phase separation [spinodal decomposition (SD) or nucleation-growth (NG)] remains an extremely important area of research. The present work examines this critically in the Fe-Cr system using atom probe tomography (APT) and small angle neutron scattering (SANS), and establishes the framework to distinguish the two different modes of α' phase separation in thermally aged Fe-35 at% Cr and Fe-20 at% Cr alloys. Independent APT analysis determines the mode of phase separation on the basis of (i) the presence/absence of periodic chemical fluctuation through radial distribution function analysis and (ii) interphase interface characteristics (diffuse/sharp). SANS analysis, in contrast, yields virtually indistinguishable correlation peaks for both the modes, which necessitates further investigation of the several different aspects of SANS profiles in the light of APT results. For the first time, key features of SANS profiles have been identified that can unambiguously distinguish SD from NG in the Fe-Cr system: (i) nature of temporal evolution of FWHM of the correlation peak and (ii) appropriate value of γ for fitting with the dynamic scaling model (γ = 6 for SD, Fe-35 at% Cr alloy; γ = 4 for NG, Fe-20 at% Cr alloy).

Pattern of an Evaporated Colloidal Droplet on a Porous Membrane Dictated by Competitive Processes of Flow and Absorption.

Understanding the deposition pattern formed by an evaporated colloidal drop is of fundamental and technological interest. Such an evaporative process is important in various applications starting from inkjet printing to disease diagnosis. In this work, it is shown that the deposit pattern on a porous membrane can be tuned by varying the colloidal viscosity and membrane pore size. We have used small-angle X-ray scattering (SAXS) in scanning mode for profiling of deposit morphology and also for estimation of the interparticle correlation. It is demonstrated that low viscosity and small pore size favor a centrally dipped pattern owing to the coffee ring effect, which can be modified to a contrasting centrally peaked pattern by increasing the viscosity and pore size. To comprehend the experimental observations, a computer model has been developed using a continuity equation that well corroborates the experimental observations on the final deposited pattern and also provides the time evolution of the pattern. The work provides a way to tune the pattern of colloidal stain on a porous substrate by controlling flow and absorption.

Jamming of Nano-Ellipsoids in a Microsphere: A Quantitative Analysis of Packing Fraction by Small-Angle Scattering.

The packing of particles is ubiquitous, and it is of fundamental importance, particularly in materials science in the nanometric length scale. It becomes more intriguing when constituent particles deviate from spherical symmetry owing to the inherent complexity in quantifying their positional and rotational correlation. For quantitative estimation of packing fraction, it requires a thorough analysis of the positional correlation of jammed particles. This article adopts a novel approach for determination of the packing fraction of strongly correlated nano-ellipsoids in a microsphere using small-angle scattering. The method has been elucidated through a quantitative analysis of structural correlation of nano-hematite ellipsoids in 3D micrometric granules, which are realized using rapid evaporative assembly. Owing to the deviation from spherical symmetry, the conventional analysis of scattering data fails to interpret the actual packing fraction of the anisotropic particles. The structural correlation gets smeared out because of orientation distribution among the packed anisotropic particles, which leads to an anomaly in the estimation of packing fraction using the conventional analysis approach. It is illustrated that consideration of an interparticle distance distribution function of the correlated nano-ellipsoids becomes indispensable in determining their packing fraction.

Structure-Correlated Magnetic Resonance Transverse Relaxivity Enhancement in Superparamagnetic Ensembles with Complex Anisotropy Landscape.

The aim of the work is to explore structure-relaxivity relationship by observing transverse relaxivity enhancement in magnetic resonance imaging (MRI) of differently organized superparamagnetic complex ensembles of zinc ferrite isotropic/anisotropic nanosystems. We observe that superparamagnetic systems show a correlation of MRI-transverse relaxivity, r2/r1, with spatial arrangement of nanoparticles, as well as magnetic easy axes and thermal-energy-dependent anisotropy energy landscape. The presence of highly random/partially aligned easy axes with enhanced anisotropy constant leads to modulation in transverse relaxation. As a result, we achieve highest contrast efficiency in compact ensemble of isotropic nanoparticles and hollow core ensemble. Indeed, core-shell ensemble with combined effect of aligned and randomly oriented easy magnetic axes shows a reduction in MRI contrast efficiency. However, we address a hypothesis for transverse contrast efficiency where we depict the correlation among MRI-transverse contrast efficiency with structural complexity of ensembles, differently arranged primary nanoparticles/magnetic easy axes, anisotropy constant, and collective magnetic behavior. In consequence, we simplify the limitation of quantum mechanical outer-sphere diffusion model of magnetic resonance relaxivity by neglecting the contribution of magnetization and introducing an anisotropy constant contribution with complex structure landscape of easy axes.

Polyethylenimine assisted non-monotonic jamming of colloids during evaporation induced assembly and its implication on CO2 sorption characteristics.

We report a detailed study of hierarchically organized silica-polyethylenimine (PEI) microspheres achieved through evaporation-induced assembly. Due to complex interactions between oppositely-charged silica nanoparticles and PEI, non-monotonic jamming of the colloidal particles is manifested. With an increase in the polymer concentration, the local volume fraction of the silica particles decreases from 0.68 to 0.43 and then increases to 0.55 with further increase. The unusual jamming behaviour of the silica colloids in the presence of PEI provides an avenue for immobilizing PEI without reducing the porosity and specific area in contrast to the conventional impregnation approach. The resultant composite microspheres show good thermal stability and CO2 sorption characteristics. For a 33 wt% PEI loading, the microspheres exhibit a significant CO2 capture capacity of 65 mg g-1 even at room temperature and it is increased to 90 mg g-1 at 75 °C. The variation in the CO2 capture capacity at 0 °C as a function of PEI loading also demonstrated the signature of non-monotonicity owing to the structural modification in the silica-PEI microspheres. The composite microspheres demonstrated fast adsorption kinetics reaching 70% of the total capture capacity in one minute during the CO2 capture. The CO2 cycling adsorption-desorption studies showed good regeneration capability up to 20 cycles.

Hollow TiO2/SiO2 composite microspheres through reactive assembly across immiscible liquid interfaces.

Titania (TiO2) based photocatalysts have shown tremendous potential in tackling important issues related to energy, the environment, and water purification. The tunable morphologies of the TiO2 based multicomponent composites are promising for the improvement of photocatalytic characteristics for practical applications. In this work, we report a one-step facile approach to achieve hollow silica/titania microspheres through the process of reactive assembly at the immiscible interface of micrometer-sized droplets. Scanning electron microscopy and small-angle neutron scattering revealed the hierarchal structure of the microspheres. Elemental mapping of the composite microspheres provided direct evidence of the incorporation of silica nanoparticles into the microspheres. The diffusion of reactant molecules and hydrolysis/condensation reactions across the phase boundary of the interface of two immiscible liquids controls the morphology of the microspheres and the size of TiO2 nanoparticles. The silica/titania composite microspheres show excellent thermal stability against the anatase to rutile phase transition caused by inhibition of the growth of TiO2 nanoparticles due to proximity of the silica nanoparticles. The photoelectrochemical measurements show that TiO2-SiO2 microspheres exhibit superior photocatalytic characteristics compared to the TiO2 microspheres. The kinetics of dye degradation for TiO2-SiO2 microspheres is found to be significantly faster compared to TiO2 microspheres which confirms the superior photocatalytic properties of the composite microspheres.

Unravelling the polyethylenimine mediated non-monotonic stability behaviour of silica colloids: the role of competing electrostatic and entropic interactions.

Polymer-mediated interactions play an important role in the stability of colloids and are therefore paramount for both fundamental as well as scientific interests. The stability of colloids in the presence of neutral polymers depends on several parameters such as the adsorbing/non-adsorbing nature, molecular weight, concentration and temperature, and such systems are well studied. However, the stability behaviour of charged colloids in the presence of charged polyelectrolyte involves complex interaction mechanisms and hence needs attention. The present work reports the study of the stability behaviour of negatively charged silica colloids in the presence of cationic polyethylenimine (PEI) polyelectrolyte using small-angle neutron and X-ray scattering. The intriguing non-monotonic stability behaviour of silica colloids is observed with varying concentrations of PEI. In the low and intermediate PEI concentration regimes, electrosorption of PEI on the silica colloids causes partial screening of charges, leading to aggregation of colloids. The DLVO interaction potential at low and intermediate concentrations of PEI exhibit a reduced repulsion barrier which is responsible for aggregation. In the high concentration regime, the entropic interaction between the free PEI molecules and PEI decorated silica colloids leads to depletion re-stabilization. The combination of DLVO potential and adsorbed PEI mediated enhanced depletion repulsion in the presence of free PEI gives rise to an increased repulsion barrier responsible for the re-stabilization at high PEI concentrations.

Origin of the Hierarchical Structure of Dendritic Fibrous Nanosilica: A Small-Angle X-ray Scattering Perspective.

The discovery of dendritic fibrous nanosilica (DFNS) has attracted great attention to the field of catalysis, CO2 capture, drug delivery due to its distinct morphology, and pore size distribution. Despite extensive research, the understanding of the DFNS formation process and its internal structure remains incomplete as microscopy and gas sorption techniques were not able to provide necessary in-depth structural information due to their inherent limitations. In the current work, we present a structural model of DFNS derived using small-angle X-ray scattering (SAXS) supported by 129Xe nuclear magnetic resonance (NMR), which provided intricate details of DFNS and its internal structure. Mechanistic understanding of the DFNS formation and growth process was achieved by performing time-resolved SAXS measurements during the synthesis of DFNS, which unveils the evolution of two levels of a bicontinuous microemulsion structure responsible for intricate DFNS morphology. The validity and the accuracy of the SAXS method and the model were successfully established through a direct correlation among the functionality of the DFNS scattering profile and its pore size distribution, as well as results obtained from the 129Xe NMR studies. It has been established that the DFNS structure originates from direct modulation of the bicontinuous structure controlled by a surfactant, a co-surfactant, and the silicate species formed during hydrolysis and the condensation reaction of the silica precursor.

Probing Kinetics and Mechanism of Formation of Mixed Metallic Nanoparticles in a Polymer Membrane by Galvanic Replacement between Two Immiscible Metals: Case Study of Nickel/Silver Nanoparticle Synthesis.

Galvanic replacement between metals has received notable research interest for the synthesis of heterometallic nanostructures. The growth pattern of the nanostructures depends on several factors such as extent of lattice mismatch, adhesive interaction between the metals, cohesive forces of the individual metals, etc. Due to the difficulties in probing ultrafast kinetics of the galvanic replacement reaction and particle growth in solution, real-time mechanistic investigations are often limited. As a result, the growth mechanism of one metal on the surface of another metal at the nanoscale is poorly understood so far. In the present work, we could successfully probe the galvanic replacement of silver ions with nickel nanoparticles, stabilized in a polymer membrane, using two complementary methods, namely, small-angle X-ray scattering (SAXS) and radiolabeling, and the results are supported by density functional theory (DFT) computations. The silver-nickel system has been chosen for the present investigation because of the high degree of bulk immiscibility caused by the large lattice mismatch (15.9%) and the weak adhesive interaction, which makes it a perfect model system for immiscible metal pairs. Membrane, as a host medium, plays a crucial role in retarding the kinetics of atomic and particle rearrangements (nucleation and growth) due to slower mobility of the atoms (monomers) and particles within the polymer network. This allowed us to examine the real-time concentration of silver monomers during galvanic replacement of silver ions with nickel nanoparticles and evolution of Ni/Ag nanoparticles. From combined experiment and DFT computations, it has been demonstrated, for the first time to the best of our knowledge, that the majority of silver atoms, which are produced on the nickel nanoparticle surface by galvanic reactions, do not form traditional core-shell nanostructures with nickel and undergo a self-governing sequential nucleation and growth of silver nanoparticles via formation of intermediate prenucleation silver clusters, leading to the formation of mixed metallic nanoparticles in the membrane. The surface of NiNPs has a heterogeneous effect on the silver nucleation pathway, which is evident from the reduced critical free energy barrier of nucleation (ΔGcrit). The present work establishes an original mechanistic pathway based on a sequential nucleation model for formation of mixed metallic nanoparticles by the galvanic replacement route, which opens up future possibilities for size-controlled synthesis in mixed systems.

Confinement driven anomalous freezing in nano porous spray dried microspheres.

One-step evaporative jamming of colloidal silica particles in contact-free spray droplets resulted in well-defined powder micro-granules with interstitial nanopores. This paper reports the anomalous freezing behaviour of confined water in the microspheres synthesized using spray drying. It has been revealed that the freezing point of water in these microspheres gets significantly lowered (∼-45 °C) owing to the confinement effect. Thermoporometry results are corroborated with the structural details obtained using complementary techniques of gas adsorption measurements and small-angle x-ray scattering.

Polymer-mediated interaction between nanoparticles during hydration and dehydration: a small-angle X-ray scattering study.

Polymer-mediated interactions such as DNA-protein binding, protein aggregation, and filler reinforcement in polymers play crucial roles in many important biological and industrial processes. In this work, we report a detailed investigation of interactions between nanoparticles in the presence of high volume fractions of an adsorbing polymer. Small-angle X-ray scattering (SAXS) revealed the existence of a stable gel-like structure in the polymer-nanoparticle dispersion, whereby anchored polymer molecules on nanoparticles acted as bridging centres, while basic interactions between nanoparticles remained repulsive. Time-resolved SAXS measurements showed that the local volume fraction of nanoparticles increased during the drying of the dispersion owing to the shrinkage of the gel-like structure. Further, nanoparticle clusters in the dehydrated composite films showed percolated networks of nanoparticles, except for 5% loading that showed a phase-separated morphology as the volume fraction of nanoparticles remained lower than the percolation threshold. A significant restructuring of nanoparticle clusters occurred upon the hydration of nanocomposite films caused by the expansion of polymer networks induced by hydration forces. Temporal evolution of the volume fraction of nanoparticles during dehydration unveiled three distinct stages similar to the logistic growth function and this was attributed to the evaporation of free, intermediate, and bound water in the different stages. A plausible mechanism was elucidated based on the spring action analogy between anchored polymer chains and nanoparticles during hydration and dehydration processes.

A Combinatorial Approach to Reliable Quantitative Analysis of Small Nano-Sized Precipitates: A Case Study with α' Precipitates in Fe-20 at% Cr Alloy.

The quantitative characterization of small nano-sized precipitates poses genuine challenges and is often deficient in accuracy due to the inherent limitations inevitably associated with the individual experimental techniques. A convenient solution is to utilize multiple complementary techniques. The present work demonstrates an effective way to reliably quantify nano-sized precipitates using a combination of complementary techniques of atom probe tomography (APT), small angle neutron scattering (SANS), and transmission electron microscopy (TEM). As a case study, the size (radius, r), number density (NP), volume fraction (ϕ), and chemical composition of Cr-rich α' precipitates are determined in Fe­20 at% Cr alloy, thermally aged at 773 K for 1,000 h. This combinatorial approach utilizes the strength of each technique in such a way that the overall accuracy of quantitative precipitation analysis improves significantly. For example, the superior spatial resolution makes TEM the appropriate technique to estimate the size and size distribution of the precipitates, while APT provides the chemical composition. Similarly, SANS analysis incorporates both the size and the compositional information thus derived independently and provides statiscally averaged quantitative analysis overcoming the field-of-view limitations of both TEM and APT. This combinatorial approach improves the accuracy of quantification and provides the true representation of the microstructure.

Hydrotrope-Driven Self-Assembly in CTAB/ n-Hexanol/Water/Heptane Reverse Micellar System.

Self-organization of nanoparticles into one-dimensional (1D) nanochains leads to new unpredicted physiochemical properties, which are further exploited to develop photonic or electronic devices. Thus, the controlled fabrication of 1D nanochains requires nanotemplate, which acts as building blocks for the self-assembly of nanoparticles. To address this issue, we designed a hydrotrope (sodium salicylate)-based CTAB/ n-hexanol/water/heptane reverse micellar system. Hydrotrope, herein, modulates electrostatic interactions between reverse micellar droplets and paves the way for the formation of self-assembled structures. Small-angle X-ray scattering studies were performed on the CTAB/heptane reverse micellar system by varying hydrotrope concentrations and water-to-surfactant ratios (W x). The aqueous content of the reverse micellar pool is determined from the W x value, where W x = [H2O]/[CTAB] and [CTAB] = 0.05 M. SAXS studies were performed for CTAB/heptane reverse micellar systems at three different W x values, that is, 6, 12, and 16 and represented by W6, W12, and W16, respectively. All SAXS profiles were modeled with a spherical form factor and a Baxter sticky hard sphere structure factor. The interaction between droplets was predicted in terms of stickiness parameter. The effect of W x on the formation of self-assembled structures and forces governing the assembly has been discussed in detail. For the W6 system, the electrostatic repulsion between reverse micellar droplets decreases, resulting in the formation of the 1D chain-like assembly of nanodroplets. In the case of the W12 system, the dual feature of the hydrotrope has been observed, it increases the size of the reverse micellar system and reduces electrostatic repulsion between droplets because of which the formation of chain-like assemblies cannot be determined with accuracy. For the W16 system, the decrease in micellar size with the increase in the hydrotrope concentration has been observed. Thus, our reverse micellar templates may provide a comprehensive method for the fabrication of high aspect ratio 1D nanochains of a variety of materials and harnessing their collective properties for magnetic, catalytic, and opto-electronic applications.

Energetics of ice nucleation in mesoporous titania using positron annihilation spectroscopy.

The low temperature behavior of water and kinetics of ice nucleation in titania mesopores have been probed by positron annihilation lifetime spectroscopy as a function of pore filling. It is revealed that water undergoes complete freezing at around 220 K when more than 50% of the pore volume is filled and such freezing is hindered at lower hydration levels. A model describing progressive trapping of positronium by ice nuclei in liquid water during the phase transition is employed to estimate the energy associated with the nucleation under confinement. It is observed that the energy for ice nucleation in confinement is less than the activation energy for nucleation in bulk water because of the surface assisted nucleation inside the pore. Interestingly, energy for nucleation is seen to decrease with the lowering of hydration level and ascribed to the curtailed hydrogen bonding network of water at lower pore filling.

Evaluating the mechanism of nucleation and growth of silver nanoparticles in a polymer membrane under continuous precursor supply: tuning of multiple to single nucleation pathway.

Size controlled synthesis of nanoparticles in a structured media, such as a membrane, has not yet been achieved successfully in comparison to that in solution due to the lack of mechanistic investigations on the nucleation and growth of nanoparticles in these media. Slower diffusion of precursor and monomer species inside these structured media complicates the nanoparticle formation mechanism. We herein report a novel experimental approach to reveal the mechanism of nucleation and growth during the synthesis of silver nanoparticles in a Nafion-117 membrane using radiolabeling and small angle X-ray scattering (SAXS). The study has been conducted under the conditions of continuous supply of precursor (silver citrate). Repetitive "LaMer type" nucleations have been found to occur in the membrane leading to the formation of polydispersed spherical nanoparticles as evident from time resolved small angle X-ray scattering. These repetitive nucleations have been shown to be responsible for continuous birth of new seeds, which grow to larger particles, mainly by random coagulation introducing non-uniformity in the growth profile of nanoparticles. The additional nucleation events have been successfully ceased by careful tuning of reaction temperature and precursor concentration, thereby eliminating the nanoparticle growth by random coagulation. This has led to the formation of silver nanoparticles with improved morphology and size distributions, which has been manifested in remarkable improvement in the optical quality of the silver nanoparticles. The present study is the first of its kind showing the crucial role of the membrane host in retarding the reaction kinetics which allowed successful probing of temporal variation of monomer concentration during nucleation and growth using a radiotracer. This was hitherto difficult to probe in solution due to its ultrafast kinetics. Additionally, using the experimental monomer concentrations during nucleation, the free energy of activation (ΔGcrit) and the critical radius (rcrit) for nucleation have been estimated and found to be 73 kJ mol-1 and 6.6 Å, respectively. The present work validates the well known theoretical model by La Mer for the synthesis of nanoparticles in a membrane under continuous precursor supply.