On the Origin and Underappreciated Effects of Ion Doping in Silica

Measuring Solvent Exchange in Silica Nanoparticles with Rotor-Based Fluorophore

Measuring the diffusivity of molecules is the first step toward understanding their dependence and controlling diffusion, but the challenge increases with the decrease of molecular size, particularly for non-fluorescent and non-reactive molecules such as solvents. Here, the capability to monitor the solvent exchange process within the micropores of silica with millisecond time resolution is demonstrated, by simply embedding a rotor-based fluorophore (thioflavin T) in colloidal silica nanoparticles. Basically, the silica provides an extreme case of viscous microenvironment, which is affected by the polarity of the solvents. The fluorescence intensity traces can be well fitted to the Fickian diffusion model, allowing analytical solution of the diffusion process, and revealing the diffusion coefficients. The validation experiments, involving the water-to-ethanol and ethanol-to-water solvent exchange, the comparison of different drying conditions, and the variation in the degree of cross-linking in silica, confirmed the effectiveness and sensitivity of this method for characterizing diffusion in silica micropores. This work focuses on the method development of measuring diffusivity and the high temporal resolution in tracking solvent exchange dynamics over a short distance (within 165 nm) opens enormous possibilities for further studies.

Unveiling Enhanced Electrostatic Repulsion in Silica Nanosphere Assembly: Formation Dynamics of Body-Centered-Cubic Colloidal Crystals

We demonstrate the effective establishment of long-range electrostatic interactions among colloidal silica nanospheres through acid treatment, enabling their assembly into colloidal crystals at remarkably low concentrations. This novel method overcomes the conventional limitation in colloidal silica assembly by removing entrapped NH4+ ions and enhancing the electrical double layer (EDL) thickness, offering a time-efficient alternative to increase electrostatic interactions compared with methods like dialysis. The increased EDL thickness facilitates the assembly of SiO2 nanospheres into a body-centered-cubic lattice structure at low particle concentrations, allowing for broad spectrum tunability and high tolerance to particle size polydispersity. Further, we uncover a disorder-order transition during colloidal crystallization at low particle concentrations, with the optimal concentration for crystal formation governed by both thermodynamic and kinetic factors. This work not only provides insights into assembly mechanisms but also paves the way for the design and functionalization of colloidal silica-based photonic crystals in diverse applications.

Assembly and Healing: Capacitive and Conductive Plasmonic Interfacing via a Unified and Clean Wet Chemistry Route

Solution-based nanoparticle assembly represents a highly promising way to build functional metastructures based on a wealth of synthetic nanomaterial building blocks with well-controlled morphology and crystallinity. In particular, the involvement of DNA molecular programming in these bottom-up processes gradually helps the ambitious goal of customizable chemical nanofabrication. However, a fundamental challenge is to realize strong interunit coupling in an assembly toward emerging functions and applications. Herein, we present a unified and clean strategy to address this critical issue based on a H2O2-redox-driven "assembly and healing" process. This facile solution route is able to realize both capacitively coupled and conductively bridged colloidal boundaries, simply switchable by the reaction temperature, toward bottom-up nanoplasmonic engineering. In particular, such a "green" process does not cause surface contamination of nanoparticles by exogenous active metal ions or strongly passivating ligands, which, if it occurs, could obscure the intrinsic properties of as-formed structures. Accordingly, previously raised questions regarding the activities of strongly coupled plasmonic structures are clarified. The reported process is adaptable to DNA nanotechnology, offering molecular programmability of interparticle charge conductance. This work represents a new generation of methods to make strongly coupled nanoassemblies, offering great opportunities for functional colloidal technology and even metal self-healing.

Intelligent Colorimetric Indicators for Quality Monitoring and Multilevel Anticounterfeiting with Kinetics-Tunable Fluorescence

The spoilage and forgery of perishable products such as food, drugs, and vaccines cause serious health hazards and economic loss every year. Developing highly efficient and convenient time-temperature indicators (TTIs) to realize quality monitoring and anticounterfeiting simultaneously is urgent but remains a challenge. To this end, a kind of colorimetric fluorescent TTI, based on CsPbBr₃@SiO₂ nanoparticles with tunable quenching kinetics, is developed. The kinetics rate of the CsPbBr₃-based TTIs is easily regulated by adjusting temperature, concentration of the nanoparticles, and addition of salts, stemming from the cation exchange effect, common-ion effect, and structural damage by water. Typically, when combined with europium complexes, the developed TTIs show an irreversible dynamic change in fluorescent colors from green to red upon increasing temperature and time. Furthermore, a locking encryption system with multiple logics is also realized by combining TTIs with different kinetics. The correct information only appears at specific ranges of time and temperature under UV light and is irreversibly self-erased afterward. The simple and low-cost composition and the ingenious design of kinetics-tunable fluorescence in this work stimulate more insights and inspiration toward intelligent TTIs, especially for high-security anticounterfeiting and quality monitoring, which is really conducive to ensuring food and medicine safety.

Inorganic-organic coprecipitation: spontaneous formation of enclosed and porous silica compartments with enriched biopolymers

We show that it is possible to spontaneously form all-enclosed compartments with microporous shells and enriched biopolymers via simple coprecipitation of silica and biopolymers. The reaction involves mild conditions and tolerates the random mixing of multiple reagents. Such a synthetic advance points to a new direction for resolving the chicken-egg dilemma of how the early life forms were hosted: without a physical barrier it would be difficult to maintain organized reactions, but without organized reactions, it would be difficult to create a cell membrane. In our synthesis, the divalent cation Ca²⁺ plays a critical role in the co-precipitation and in creating hollow compartments after simple dilution with water. The precursor of silica, poly(silicic acid), is a negatively charged, cross-linked polymer. It could be co-precipitated with negatively charged biopolymers such as DNA and proteins, whereas the remaining silica precursor forms a conformal and microporous shell on the surface of the initial precipitate. After etching, the biopolymers are retained inside the hollow compartments. The fact that multiple favorable conditions are easily brought together in enclosed compartments opens new possibilities in theorizing the host of early life forms.

Diffusion-controlled bridging of the Au Island and Au core in Au@Rh(OH)3 core-shell structure

Hybrid nanostructures have garnered considerable interest because of their fascinating properties owing to the hybridization of materials and their structural varieties. In this study, we report the synthesis of [Au@Rh(OH)₃]-Au island heterostructures using a seed-mediated sequential growth method. Through the thiol ligand-mediated interfacial energy, Au@Rh(OH)₃ core-shell structures with varying shell thicknesses were successfully obtained. On these Au@Rh(OH)₃ core-shell seeds, by modulating the diffusion of HAuCl₄ in the porous Rh(OH)₃ shell, site-specific growth of Au islands on the inner Au core or on the surface of the outer Rh(OH)₃ shell was successfully achieved. Consequently, two types of distinct structures, the Au island-on-[Au@Rh(OH)₃] dimer and Au island-Au bridge-[Au@Rh(OH)₃] dumbbell structures with thin necks were obtained. Further modulations of the growth kinetics led to the formation of Au plate-Au bridge-[Au@Rh(OH)₃] heterostructures with larger structural anisotropy. The flexible structural variations were demonstrated to be an effective means of modulating the plasmonic properties; the Au-Au heterostructures exhibited tunable localized surface plasmon resonance in the visible-near-infrared spectral region and can be used as surface-enhanced Raman scattering (SERS) substrates capable of emitting strong SERS signals. This diffusion-controlled growth of Au bridges in the Rh(OH)₃ shells (penetrating growth) is an interesting new approach for structural control, which enriches the tool box for colloidal nanosynthesis. This advancement in structural control is expected to create new approaches for colloidal synthesis of sophisticated nanomaterials, and eventually enable their extensive applications in various fields.

Study of Nanoparticle-Polymer Interactions via the Mechanical Stretching of Surface-Enhanced Raman Scattering Substrates

It is shown that the aligned electrospun fibers are a convenient platform for studying the mechanical effects on nanomaterials, particularly when using surface-enhanced Raman scattering as a sensitive tool of monitoring. The ligands on the surface of the embedded Au nanoparticles fall off easily with the shear force from the stretching, in contrast to the counterparts protected by polymer/silica shells. Upon stretching, the chains of Au nanoparticles will reversibly break, as revealed by the dramatic changes in the longitudinal plasmon absorption. It is believed that such a platform will open a window for understanding mechanical effects at the nanoscale, and also a new means for synthetic control.

Liquid Nanoparticles: Manipulating the Nucleation and Growth of Nanoscale Droplets

By manipulating the nucleation and growth of solid materials, the synthesis of various sophisticated nanostructures has been achieved. Similar methodology, if applied to liquids, could enable the mass-production and control of ultra-small droplets at the scale of nanoparticles (10^-18  L or below). It would be highly desirable since droplets play a fundamental role in numerous applications. Here we present a general strategy to synthesize and manipulate nanoscale droplets, similar to what has been done to solid nanoparticles in classic solution-synthesis. It was achieved by a solute-induced phase separation which initiates the nucleation of droplets from a homogeneous solution. These liquid nanoparticles have great potentials to be manipulated like their solid counterparts, borrowing from the vast methodologies of nanoparticle synthesis, such as burst nucleation, seeded growth, and co-precipitation. Liquid nanoparticles also serve as a general synthetic platform, to fabricate nanoreactors, drug-loaded carriers, and other hollow nanostructures with a variety of shell materials.

Dual Roles of Polymeric Capping Ligands in the Surface-Protected Etching of Colloidal Silica

In this work, we reveal the dual roles of polymeric capping ligands in the hollowing of silica nanospheres during their surface-protected etching. We first show that polymeric capping ligands, if they have a stronger interaction with the surface Si-OH groups than water, can reduce the condensation of the silica network, allowing the diffusion of OH^- ions through the shell to dissolve the inner silica. Also, the polymeric ligands can passivate the surface silica, making it less likely to be dissolved by OH^- ions. The combination of these two roles ensures highly selective etching of the interior of the colloidal silica spheres, making the surface-protected etching a robust process for the synthesis of hollow silica nanoshells. Our insight into the specific roles of the ligands is expected to elucidate the impact of polymeric ligands on the colloidal chemistry of silica, particularly in its condensation and etching behaviors, and offer new opportunities in the design of silica and other oxide-based nanostructures.

A non-surfactant self-templating strategy for mesoporous silica nanospheres: beyond the Stöber method

The well-known Stöber method has been widely used to synthesize nonporous silica nanospheres (NPs), however, in the absence of surfactant templates, the synthesis of mesoporous silica nanospheres (MSNs) has not been achieved. Herein, in the absence of organic surfactant templates, by a simple premixing of three components tetraethoxysilane-water-ethanol (TEOS-H₂O-EtOH) with a precise molar ratio, the parent silica nanoparticles with a low condensation degree and controlled particle size can be readily obtained. Subsequently, via a simple two-step post-treatment, the obtained MSNs exhibited a high surface area (ca. 500 m² g^-1), accessible mesopores (3.0 nm), and a large pore volume (0.87 mL g^-1), similar to those of MCM-41 and SBA-15 silicas. The unique self-templating role of the 'pre-Ouzo' effect of ternary surfactant-free TEOS-H₂O-EtOH systems was proposed to understand the formation of mesoporosity.

The Synthesis of Hollow/Porous Cu2O Nanoparticles by Ion-Pairing Behavior Control

Owing to the properties of low density, large surface areas, excellent loading capacity, high permeability, and interstitial hollow spaces, hollow nanostructures have been widely applied in many important research fields, such as catalysis, drug-controlled release, confined synthesis, optics and electronics, and energy storage. This work provided a simple platform for hollow Cu2O nanostructure synthesis based on the surfactant controlling methodology, which is under the supposed mechanism of ion-pairing behavior at the initial nucleation stage. Thus here, we explore our system in two different directions: (1) we get different types of hollow Cu2O nanoparticles by controlling the surfactant concentration during the synthesis step in colloids, which is critical to the novel structure design and potential application in many different areas and (2) we explore the method to Cu2O hollow particle synthesis to test the hypothesis of the ion-pairing behavior during the initial nucleation by tuning the solvent ratio, cation concentration (such as NH4NO3 addition amount difference in the synthetic step), and selective etching. By tuning the synthetic conditions as well as designing control experiments, we hope to provide a solid understanding of the crystal growth mechanism. Our improved understanding in similar systems (both Cu2O and ZnO systems) will make it easier for interpreting nanostructure formation in new discoveries and, more importantly, in rationally designing various complex nanostructures based on a bottom-up strategy.

Self-Templating Approaches to Hollow Nanostructures

This current research progress on the fabrication of hollow nanostructures by using self-templating methods is reviewed. After a brief introduction to the unique properties and applications of hollow nanostructures and the three general fabrication routes, the discussions are focused on the five main self-templating strategies, including galvanic replacement, the Kirkendall effect, Ostwald ripening, dissolution-regrowth, and the surface-protected hollowing process. Some newly developed synthetic routes are selected and discussed in detail. In conclusion, a summary and the perspectives on the directions that might lead the future development of this exciting field are presented.

Printable Skin-Driven Mechanoluminescence Devices via Nanodoped Matrix Modification

Mechanically driven light generation is an exciting and under-exploited phenomenon with a variety of possible practical applications. However, the current driving mode of mechanoluminescence (ML) devices needs strong stimuli. Here, a flexible sensitive ML device via nanodopant elasticity modulus modification is introduced. Rigid ZnS:M²⁺ (Mn/Cu)@Al₂ O₃ microparticles are dispersed into soft poly(dimethylsiloxane) (PDMS) film and printed out to form flexible devices. For various flexible and sensitive scenes, SiO₂ nanoparticles are adopted to adjust the elasticity modulus of the PDMS matrix. The doped nanoparticles can concentrate stress to ZnS:M²⁺ (Mn/Cu)@Al₂ O₃ microparticles and achieve intense ML under weak stimuli of the moving skin. The printed nano-/microparticle-doped matrix film can achieve skin-driven ML, which can be adopted to present fetching augmented animations expressions. The printable ML film, amenable to large areas, low-cost manufacturing, and mechanical softness will be versatile on stress visualization, luminescent sensors, and open definitely new functional skin with novel augmented animations expressions, the photonic skin.

Confined Nucleation and Growth of PdO Nanocrystals in a Seed-Free Solution inside Hollow Nanoreactor

This paper reports a novel and adaptable hollow nanoreactor system containing a solution of cucurbituril (CB) inside a silica nanoparticle (CB@h-SiO₂) which enables the nucleation and formation of nanocrystals (NCs) to be confined at the seed-free interior solution inside the cavity. The above nanospace confinement strategy restricted the volume of medium available for NC formation to the solution inside the cavity to a few tens of nanometers in size and allowed homogeneous NC nucleation to be examined. Harboring of CB@h-SiO₂ in a Pd²⁺ complex solution confined the nucleation and formation of PdO NCs to the well-isolated nanosized cavity protected by the silica nanoshell, allowing the convoluted formation of clustered PdO NCs to be thoroughly examined. The corresponding temporal investigation indicated that PdO NC clusters evolved via a distinct pathway combining dendritic growth on early nucleated seed NCs and attachment of small intermediate clusters. In addition, the explored strategy was used to fabricate a recyclable nanocatalyst system for selective catalytic oxidation of cinammyl alcohols, featuring a cavity-included Fe₃O₄/PdO nanocomposite.

Unraveling the Growth Mechanism of Silica Particles in the Stöber Method: In Situ Seeded Growth Model

In this work, we investigated the kinetic balance between ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) and subsequent condensation over the growth of silica particles in the Stöber method. Our results reveal that, at the initial stage, the reaction is dictated by TEOS hydrolysis to form silanol monomers, which is denoted as pathway I and is responsible for nucleation and growth of small silica particles via condensation of neighboring silanol monomers and siloxane network clusters derived thereafter. Afterward, the reaction is dictated by condensation of newly formed silanol monomers onto the earlier formed silica particles, which is denoted as pathway II and is responsible for the enlargement in size of silica particles. When TEOS hydrolysis is significantly promoted, either at high ammonia concentration (≥0.95 M) or at low ammonia concentration in the presence of LiOH as secondary catalyst, temporal separation of pathways I and II makes the Stöber method reminiscent of in situ seeded growth. This knowledge advance enables us not only to reconcile the most prevailing aggregation-only and monomer-addition models in literature into one consistent framework to interpret the Stöber process but also to grow monodisperse silica particles with sizes in the range 15-230 nm simply but precisely regulated by the ammonia concentration with the aid of LiOH.

Kinetically-controlled synthesis of ultra-small silica nanoparticles and ultra-thin coatings

The understanding of silica as a polymer-like globule allows us to synthesize ultra-small silica nanoparticles (NPs) via a kinetic controlled process. The synthetic system is quite simple with Tetraethyl orthosilicate (TESO) as the precursor and H2O as the solvent and reactant. The reaction conditions are gentle with a temperature of around 35 to 60 °C with an incubation time of 7-12 hours. The final product of the silica NPs is very uniform and could be as small as 10 nm. The silica NPs can further grow up to 18 nm under the controlled addition of the precursors. Also, these silica NPs can be used as seeds to generate larger silica NPs with sizes ranging from 20 to 100 nm, which can be a useful supplement to the size range made by the traditional Stöber method. Moreover, these ultra-small Au NPs can be used as a depletion reagent or as building blocks for an ultrathin silica coating, which has significant applications in fine-tuning the plasmons of AuNPs and thin spacers for surface enhanced spectroscopies.

Levelling the playing field: screening for synergistic effects in coalesced bimetallic nanoparticles

Depending on the synthetic methods, bimetallic nanoparticles can have either core-shell, phase segregated, alloy, or partially coalesced structures, presenting different degrees of atomic mixing on their surface. Along with the variations of size and morphology, the structural differences make it difficult to compare the catalytic activity of bimetallic nanoparticles. In this article, we developed a facile screening method that can focus on the synergistic effects rather than structural differences. Prefabricated nanoparticles are mixed together to form linear aggregates and coalesced to form bimetallic junctions. Their hollow silica shells allow materials transport but prevent further aggregation. With a level playing field, this screening platform can identify the best bimetallic combination for a catalytic reaction, before optimizing the synthesis. This approach is more advantageous than the conventional approaches where structural difference may have dominant effects on the catalytic performance.

808 nm photocontrolled UCL imaging guided chemo/photothermal synergistic therapy with single UCNPs-CuS@PAA nanocomposite

Recently, incorporating multiple components into one nanostructured matrix to construct a multifunctional nanomedical platform has attracted more and more attention for simultaneous anticancer diagnosis and therapy.