Silica shell/gold core nanoparticles: correlating shell thickness with the plasmonic red shift upon aggregation

Nanoscience and nanotechnology for water remediation: an earnest hope toward sustainability

Water pollution and the global freshwater crisis are the most alarming concerns of the 21st century, as they threaten the sustainability and ecological balance of the environment. The growth of global population, climate change, and expansion of industrial processes are the main causes of these issues. Therefore, effective remediation of polluted water by means of detoxification and purification is of paramount importance. To this end, nanoscience and nanotechnology have emerged as viable options that hold tremendous potential toward the advancement of wastewater treatment methods to enhance treatment efficiency along with augmenting water supply via utilization of unconventional water sources. Materials at the nano level have shown great promise toward water treatment applications owing to their unique physicochemical properties. In this focus article, we highlight the role of new fundamental properties at the nano scale and material properties that are drastically increased due to the nano dimension (e.g. volume-surface ratio) and highlight their impact and potential toward water treatment. We identify and discuss how nano-properties could improve the three main domains of water remediation: the identification of pollutants, their adsorption and catalytic degradation. After discussing all the beneficial aspects we further discuss the key challenges associated with nanomaterials for water treatment. Looking at the current state-of-the-art, the potential as well as the challenges of nanomaterials, we believe that in the future we will see a significant impact of these materials on many water remediation strategies.

Plasmonic and Photothermal Properties of Silica-Capped Gold Nanoparticle Aggregates

Owing to their biocompatibility, gold nanoparticles have many applications in healthcare, notably for targeted drug delivery and the photothermal therapy of tumors. The addition of a silica shell to the nanoparticles can help to minimize the aggregation of the nanoparticles upon exposure to harsh environments and protect any Raman reporters adsorbed onto the metal surface. Here, we report the effects of the addition of a silica shell on the photothermal properties of a series of gold nanostructures, including gold nanoparticle aggregates. The presence of a Raman reporter at the surface of the gold nanoparticles also allows the structures to be evaluated by surface-enhanced Raman scattering (SERS). In this work, we explore the relationship between the degree of aggregation and the position and the extinction of the near-infrared plasmon on the observed SERS intensity and in the increase in bulk temperature upon near-infrared excitation. By tailoring the concentration of the silane and the thickness of the silica shell, it is possible to improve the photothermal heating capabilities of the structures without sacrificing the SERS intensity or changing the optical properties of the gold nanoparticle aggregates.

Dispersion of Hydrophilic "Hedgehog" Microparticles in Liquid CO2 Mixtures

Liquid and supercritical CO₂ are nontoxic and nonflammable reaction media with pressure-variable physical properties. These states of CO₂ also have high solubility limits for gas and liquid hydrocarbons, making them good candidates for "green" hydrophobic solvents in sustainable chemical technologies. However, the dispersion of hydrophilic colloidal nanoscale and microscale particles in CO₂ is challenging due to the tendency of polar particles to aggregate in nonpolar media, limiting their available surface area and catalytic efficiencies. Here we show that native hydrophilic semiconductor particles can be effectively dispersed in a liquid CO₂ mixture with acetonitrile (ACN) without additional chemical or mechanical dispersion techniques. Using surface corrugation as a method to prevent aggregation, we find that geometrically complex particles with a halo of stiff nanoscale spikes disperse and remain suspended longer in liquid CO₂ than those without or with less prominent nanoscale corrugation. For the particles of this size and liquid CO₂ mixtures, individual particle mass remains a prominent factor determining particle sedimentation rate even in the absence of aggregation. Particle dispersion and structural stability are confirmed using a combination of UV-vis spectroscopy, finite-difference time-domain modeling, and electron microscopy. The necessity of the cosolvent (ACN) indicates that particle behavior in liquid CO₂ is vastly different than in traditional liquid-phase solvents and highlights the need for future studies to understand the wetting behavior of hydrophilic particles in high-pressure nonpolar environments.

Silica Shell Thickness-Dependent Fluorescence Properties of SiO2@Ag@SiO2@QDs Nanocomposites

Silica shell coatings, which constitute important technology for nanoparticle (NP) developments, are utilized in many applications. The silica shell's thickness greatly affects distance-dependent optical properties, such as metal-enhanced fluorescence (MEF) and fluorescence quenching in plasmonic nanocomposites. However, the precise control of silica-shell thicknesses has been mainly conducted on single metal NPs, and rarely on complex nanocomposites. In this study, silica shell-coated Ag nanoparticle-assembled silica nanoparticles (SiO₂@Ag@SiO₂), with finely controlled silica shell thicknesses (4 nm to 38 nm), were prepared, and quantum dots (QDs) were introduced onto SiO₂@Ag@SiO₂. The dominant effect between plasmonic quenching and MEF was defined depending on the thickness of the silica shell between Ag and QDs. When the distance between Ag NPs to QDs was less than ~10 nm, SiO₂@Ag@SiO₂@QDs showed weaker fluorescence intensities than SiO₂@QD (without metal) due to the quenching effect. On the other hand, when the distance between Ag NPs to QDs was from 10 nm to 14 nm, the fluorescence intensity of SiO₂@Ag@SiO₂@QD was stronger than SiO₂@QDs due to MEF. The results provide background knowledge for controlling the thickness of silica shells in metal-containing nanocomposites and facilitate the development of potential applications utilizing the optimal plasmonic phenomenon.

Transformation of L-DOPA and Dopamine on the Surface of Gold Nanoparticles: An NMR and Computational Study

Gold nanoparticles (AuNPs) have found applications in biomedicine as diagnostic tools, but extensive research efforts have been also directed toward their development as more efficient drug delivery agents. The high specific surface area of AuNPs may provide dense loading of molecules like catechols (L-DOPA and dopamine) on nanosurfaces, enabling functionalization strategies for advancing conventional therapy and diagnostic approaches of neurodegenerative diseases. Despite numerous well-described procedures in the literature for preparation of different AuNPs, possible transformation and structural changes of surface functionalization agents have not been considered thoroughly. As a case in point, the catechols L-DOPA and dopamine were selected because of their susceptibility to oxidation, cyclization, and polymerization. To assess the fate of coating and functionalization agents during the preparation of AuNPs or interaction at the nano-bio interface, a combination of spectroscopy, light scattering, and microscopy techniques was used while structural information and reaction mechanism were obtained by NMR in combination with computational tools. The results revealed that the final form of catechol on the AuNP nanosurface depends on the molar ratio of Au used for AuNP preparation. A large molar excess of L-DOPA or dopamine is needed to prepare AuNPs funtionalized with fully reduced catechols. In the case of molar excess of Au, the oxidation of catechols to dopamine quinone and dopaquinone was promoted, and dopaquinone underwent intramolecular cyclization in which additional oxidation products, leukodopachrome, dopachrome, or its tautomer, were formed because of the larger intrinsic acidity of the more nucleophilic amino group in dopaquinone. MD simulations showed that, of the oxidation products, dopachrome had the highest affinity for binding to the AuNPs surface. The results highlight how a more versatile methodological approach, combining experimental and in silico techniques, allows more reliable characterization of binding events at the surface of AuNPs for possible applications in biomedicine.

How to Use Localized Surface Plasmon for Monitoring the Adsorption of Thiol Molecules on Gold Nanoparticles?

The functionalization of spherical gold nanoparticles (AuNPs) in solution with thiol molecules is essential for further developing their applications. AuNPs exhibit a clear localized surface plasmon resonance (LSPR) at 520 nm in water for 20 nm size nanoparticles, which is extremely sensitive to the local surface chemistry. In this study, we revisit the use of UV-visible spectroscopy for monitoring the LSPR peak and investigate the progressive reaction of thiol molecules on 22 nm gold nanoparticles. FTIR spectroscopy and TEM are used for confirming the nature of ligands and the nanoparticle diameter. Two thiols are studied: 11-mercaptoundecanoic acid (MUDA) and 16-mercaptohexadecanoic acid (MHDA). Surface saturation is detected after adding 20 nmol of thiols into 1.3 × 10⁻³ nmol of AuNPs, corresponding approximately to 15,000 molecules per AuNPs (which is equivalent to 10.0 molecules per nm²). Saturation corresponds to an LSPR shift of 2.7 nm and 3.9 nm for MUDA and MHDA, respectively. This LSPR shift is analyzed with an easy-to-use analytical model that accurately predicts the wavelength shift. The case of dodecanehtiol (DDT) where the LSPR shift is 15.6 nm is also quickly commented. An insight into the kinetics of the functionalization is obtained by monitoring the reaction for a low thiol concentration, and the reaction appears to be completed in less than one hour.

Ultra-Fine Control of Silica Shell Thickness on Silver Nanoparticle-Assembled Structures

To study the distance-dependent electromagnetic field effects related to the enhancement and quenching mechanism of surface-enhanced Raman scattering (SERS) or fluorescence, it is essential to precisely control the distance from the surface of the metal nanoparticle (NP) to the target molecule by using a dielectric layer (e.g., SiO2, TiO2, and Al2O3). However, precisely controlling the thickness of this dielectric layer is challenging. Herein, we present a facile approach to control the thickness of the silica shell on silver nanoparticle-assembled silica nanocomposites, SiO2@Ag NPs, by controlling the number of reacting SiO2@Ag NPs and the silica precursor. Uniform silica shells with thicknesses in the range 5-40 nm were successfully fabricated. The proposed method for creating a homogeneous, precise, and fine silica coating on nanocomposites can potentially contribute to a comprehensive understanding of the distance-dependent electromagnetic field effects and optical properties of metal NPs.

Surface energy transfer in hybrid halide perovskite/plasmonic Au nanoparticle composites

The incorporation of plasmonic metal nanoparticles (NPs) into the multilayered architecture of perovskite solar cells (PSCs) has been a recurrent strategy to enhance the performance of photovoltaic devices from the early development of this technology. However, the specific photophysical interactions between the metal NPs and the hybrid halide perovskites are still not completely understood. Herein, we investigate the influence of Au NPs on the photoluminescence (PL) signal of a thin layer of the CH₃NH₃PbI₃ hybrid perovskite. Core-shell Au@SiO₂ NPs with a tunable thickness of the SiO₂ shell were used to adjust the interaction distance between the plasmonic NPs and the perovskite layer. Complete quenching of the PL signal in the presence of the Au NPs is measured together with the gradual recovery of the PL intensity at a thicker thickness of the SiO₂ shell. A nanometal surface energy transfer (NSET) model is employed to reasonably fit the experimental quenching efficiency. Thus, the energy transfer deactivation is revealed as a detrimental process occurring in the PSCs since it funnels the photon energy into the non-active excited state of the Au NPs. This work indicates that tuning the distance between the plasmonic NPs and the perovskite materials by a silica shell may be a simple and straightforward strategy for further improving the efficiency of PSCs.

A wafer-scale fabrication method for three-dimensional plasmonic hollow nanopillars

Access to nanofabrication strategies for crafting three-dimensional plasmonic structures is limited. In this work, a fabrication strategy to produce 3D plasmonic hollow nanopillars (HNPs) using Talbot lithography and I-line photolithography is introduced. This method is named subtractive hybrid lithography (SHL), and permits intermixed usage of nano-and-macroscale patterns. Sputter-redeposition of gold (Au) on the SHL resist pattern yields large areas of dense periodic Au-HNPs. These Au-HNPs are arranged in a square unit cell with a 250 nm pitch. The carefully controlled fabrication process resulted in Au-HNPs with nanoscale dimensions over the Au-HNP dimensions such as an 80 ± 2 nm thick solid base with a 133 ± 4 nm diameter, and a 170 ± 10 nm high nano-rim with a 14 ± 3 nm sidewall rim-thickness. The plasmonic optical response is assessed with FDTD-modeling and reveals that the highest field enhancement is at the top of the hollow nanopillar rim. The modeled field enhancement factor (EF) is compared to the experimental analytical field enhancement factor, which shows to pair up with ca. 103 < EF < 104 and ca. 103 < EF < 105 for excitation wavelengths of 633 and 785 nm. From a broader perspective, our results can stimulate the use of Au-HNPs in the fields of plasmonic sensors and spectroscopy.

Detection of Silver Nanoparticles in Seawater Using Surface-Enhanced Raman Scattering

Nanomaterials significantly contribute to the development of new solutions to improve consumer products properties. Silver nanoparticles (AgNPs) are one of the most used, and as human exposure to such NPs increases, there is a growing need for analytical methods to identify and quantify nanoparticles present in the environment. Here we designed a detection strategy for AgNPs in seawater using surface-enhanced Raman Scattering (SERS). Three commercial AgNPs coated with polyvinylpyrrolidone (PVP) were used to determine the relative impact of size (PVP-15nmAgNPs and PVP-100nmAgNPs) and aggregation degree (predefined Ag aggregates, PVP-50-80nmAgNPs) on the SERS-based detection method. The study of colloidal stability and dissolution of selected AgNPs into seawater was carried out by dynamic light scattering and UV-vis spectroscopy. We showed that PVP-15nmAgNPs and PVP-100nmAgNPs remained colloidally stable, while PVP-50-80nmAgNPs formed bigger aggregates. We demonstrated that the SERS-based method developed here have the capacity to detect and quantify single and aggregates of AgNPs in seawater. The size had almost no effect on the detection limit (2.15 ± 1.22 mg/L for PVP-15nmAgNPs vs. 1.51 ± 0.71 mg/L for PVP-100nmAgNPs), while aggregation caused an increase of 2.9-fold (6.08 ± 1.21 mg/L). Our results demonstrate the importance of understanding NPs transformation in seawater since this can influence the detection method performance.

Probing Dynamic Features of Phagosome Maturation in Macrophage using Au@MnOx @SiO2 Nanoparticles as pH-Sensitive Plasmonic Nanoprobes

Phagosome maturation in macrophage is essential to the clearance of pathogenic materials in host defence but the dynamic features remain difficult to be measured in real time. Herein, we reported the multilayered Au@MnO_x @SiO₂ nanoparticle as a robust pH-sensitive plasmonic nanosensor for monitoring the dynamic acidification features over the phagosome maturation process in macrophage under darkfield microscopy. For this multilayered nanosensor, the gold nanoparticle core plays a role of signal reporter, the MnO_x shell and the outmost SiO₂ act as the sensing layer and the protecting layer, respectively. After subject to the acidic buffer solution, the MnO_x layer in the multilayered nanoprobe could be decomposed rapidly, resulting in a remarkable spectral shift and color change under darkfield microscopy. We demonstrated this nanosensor for the investigation of single phagosome acidification dynamics by monitoring the color changes of nanoprobes after phagocytosis over time. The nanoprobes after phagocytosized in macrophage displayed a slight color change within the first hour and then cost several minutes to change from red to green in the next stage, indicating the phagosome undergoes a slow first and then fast acidification feature as well as a slow-to-fast acidification translation over the phagosome maturation process. Moreover, we validated that the slow-to-fast acidification translation was dependent on the activation of V-ATPase from the ATP depletion assay. We believed that this nanosensor is promising for studying the dynamic acidification features as well as disorders in phagosome maturation in phagocytic cells, which might provide valuable information for understanding the disease pathogenesis related to phagosome dysfunctions.

Combined NMR and Computational Study of Cysteine Oxidation during Nucleation of Metallic Clusters in Biological Systems

The widespread biomedical applications of silver and gold nanoparticles (AgNPs and AuNPs, respectively) prompt the need for mechanistic evaluation of their interaction with biomolecules. In biological media, metallic NPs are known to transform by various pathways, especially in the presence of thiols. The interplay between metallic NPs and thiols may lead to unpredictable consequences for the health status of an organism. This study explored the potential events occurring during biotransformation, dissolution, and reformation of NPs in the thiol-rich biological media. The study employed a model system evaluating the interaction of cysteine with small-sized AgNPs and AuNPs. The interplay of cysteine on transformation and reformation pathways of these NPs was experimentally investigated by nuclear magnetic resonance (NMR) spectroscopy and supported by light scattering techniques and transmission electron microscopy (TEM). As the main outcome, Ag- or Au-catalyzed oxidation of cysteine to cystine was found to occur through generation of reactive oxygen species (ROS). Computational simulations confirmed this mechanism and the role of ROS in the oxidative dimerization of biothiol during NPs reformation. The obtained results represent valuable mechanistic data about the complex events during the transport of metallic NPs in thiol-rich biological systems that should be considered for the future biomedical applications of metal-based nanomaterials.

Comparative Analysis of Au and Au@SiO2 Nanoparticle-Protein Interactions for Evaluation as Platforms in Theranostic Applications

Gold nanoparticles are utilized in a variety of sensing and detection technologies because of their unique physiochemical properties. Their tunable size, shape, and surface charge enable them to be used in an array of platforms. The purpose of this study is to conduct a thorough spectroscopic characterization of Au and functionalized hybrid Au@SiO₂ nanoparticles under physiological conditions and in the presence of two proteins known to be abundant in serum, bovine serum albumin and human ubiquitin. The information obtained from this study will enable us to develop design principles to synthesize an array of surface-enhanced Raman spectroscopy-based nanoparticles as platforms for theranostic applications. We are particularly interested in tailoring the surface chemistry of the Au@SiO₂ nanoparticles for applications in theranostic technologies. We employ common spectroscopic techniques, with particular emphasis on circular dichroism and heteronuclear single quantum correlation nuclear magnetic resonance (HSQC NMR) spectroscopy, as combinatorial tools to understand protein conformational dynamics, binding site interactions, and protein corona for the design of nanoparticles capable of reaching their intended target in vivo. Our results conclude that protein adsorption onto the nanoparticle surface prevents nanoparticle aggregation. We observed that varying the ionic strength and type of ion influences the aggregation and aggregation rate of each respective nanoparticle. The conformation of proteins and the absorption of proteins on the surface of Au nanoparticles are also influenced by ionic strength. Using two-dimensional [¹⁵N-¹H]-HSQC NMR experiments to compare the interactions of Au and Au@SiO₂ nanoparticles with ¹⁵N-ubiquitin, we observed small chemical shift perturbations in some amino acid peaks and differences in binding site interactions with ubiquitin and respective nanoparticles.

Targeting Oxidative Stress Using Nanoparticles as a Theranostic Strategy for Cardiovascular Diseases

SIGNIFICANCE

Nanomedicine is an application of nanotechnology that provides solutions to unmet medical challenges. The unique features of nanoparticles, such as their small size, modifiable components, and diverse functionality, make them attractive and suitable materials for novel diagnostic, therapeutic, or theranostic applications. Cardiovascular diseases (CVDs) are the major cause of noncommunicable illness in both developing and developed countries. Nanomedicine offers novel theranostic options for the treatment of CVDs. Recent Advances: Many innovative nanoparticles to target reactive oxygen species (ROS) have been developed. In this article, we review the characteristics of nanoparticles that are responsive to ROS, their limitations, and their potential clinical uses. Significant advances made in diagnosis of atherosclerosis and treatment of acute coronary syndrome using nanoparticles are discussed.

CRITICAL ISSUES

Although there is a tremendous potential for the nanoparticle applications in medicine, their safety should be considered while using in humans. We discuss the challenges that may be encountered with some of the innovative nanoparticles used in CVDs.

FUTURE DIRECTIONS

The unique properties of nanoparticles offer novel diagnostic tool and potential therapeutic strategies. However, nanomedicine is still in its infancy, and further in-depth studies are needed before wide clinical application is achieved.

Stereoselective Double Reduction of 3-Methyl-2-cyclohexenone, by Use of Palladium and Platinum Nanoparticles, in Tandem with Alcohol Dehydrogenase

The combination of metal nanoparticles (Pd or Pt NPs) with NAD-dependent thermostable alcohol dehydrogenase (TADH) resulted in the one-flask catalytic double reduction of 3-methyl-2-cyclohexenone to 3-(1S,3S)-methylcyclohexanol. In this article, some assumptions about the interactions between a chemocatalyst and a biocatalyst have been proposed. It was demonstrated that the size of the NPs was the critical parameter for the mutual inhibition: the bigger the NPs, the more harmful for the enzyme they were, even if the NPs themselves were only moderately inactivated. Conversely, the smaller the NPs, the more minimal the TADH denaturation, although they were dramatically inhibited. Resuming, the chemocatalysts were very sensitive to deactivation, which was not related to the amount of enzyme used, while the inhibition of the biocatalyst can be strongly reduced by minimizing the NPs/TADH ratio used to catalyze the reaction. Among some methods to avoid direct binding of NPs with TADH, we found that using large Pd NPs and protecting their surfaces with a silica shell, the overall yield of 3-(1S,3S)-methylcyclohexanol was maximized (36%).

Silica-Coated Plasmonic Metal Nanoparticles in Action

Hybrid colloids consisting of noble metal cores and metal oxide shells have been under intense investigation for over two decades and have driven progress in diverse research lines including sensing, medicine, catalysis, and photovoltaics. Consequently, plasmonic core-shell particles have come to play a vital role in a plethora of applications. Here, an overview is provided of recent developments in the design and utilization of the most successful class of such hybrid materials, silica-coated plasmonic metal nanoparticles. Besides summarizing common simple approaches to silica shell growth, special emphasis is put on advanced synthesis routes that either overcome typical limitations of classical methods, such as stability issues and undefined silica porosity, or grant access to particularly sophisticated nanostructures. Hereby, a description is given, how different types of silica can be used to provide noble metal particles with specific functionalities. Finally, applications of such nanocomposites in ultrasensitive analyte detection, theranostics, catalysts, and thin-film solar cells are reviewed.

Aggregation-Driven Controllable Plasmonic Transition of Silica-Coated Gold Nanoparticles with Temperature-Dependent Polymer-Nanoparticle Interactions for Potential Applications in Optoelectronic Devices

Localized surface plasmon resonance (LSPR) effect relies on the shape, size, and dispersion state of metal nanoparticles and can potentially be employed in many applications such as chemical/biological sensor, optoelectronics, and photocatalyst. While complicated synthetic approaches changing shape and size of nanoparticles can control the intrinsic LSPR effect, here we show that controlling interparticle interactions with silica-coated gold nanoparticles (Au@SiO₂ NPs) is a powerful approach, permitting wide range of optical bandwidth of gold nanoparticles with great stability. The interparticle interactions of Au@SiO₂ NPs are controlled through concentration-, temperature-, and time-dependent polymer-induced interactions. The polymer-induced interactions modulate the state of particle dispersion, resulting an effective plasmonic shift by more than 200 nm. We further explore the microstructure of particle aggregation and explain mechanisms of plasmonic shift based on the results of small-angle X-ray scattering (SAXS) and discrete dipole approximation (DDA) calculation. We show that an effective control of LSPR behavior is now available through trapped aggregation of Au@SiO₂ NPs with temperature variation. We anticipate that the suggested strategy can be employed in many practical applications such as optical bioimaging and optoelectronic devices.

Thermally Stable Metallic Nanoparticles Prepared via Core-Cross-linked Block Copolymer Micellar Nanoreactors

Thermally stable metallic nanoparticles (MNPs) are highly desirable for the melt processing of polymer nanocomposites. However, due to the high surface energy penalty and decreased melting temperature, MNPs are easy to agglomerate and lose their unique properties if there is no protection or confinement layer. In this work, we report a facile and efficient way to synthesize thermally stable MNPs using core-cross-linked polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) reverse micelles as nanoreactors. From infrared results, gold, silver, and palladium ions exhibited distinctive coordination to the 4VP groups with varying chelation strengths. Compared to the non-cross-linked micelles, 1,4-dibromobutane (DBB)-cross-linking of the P4VP cores provided several advantages. First, it prevented severe swelling of the P4VP cores caused by the reducing agents and subsequent merger of swollen micelles. Second, the quaternized P4VP with hydrophilicity enhanced the uptake speed of precursor metal ions into the cores. Third, the cross-linked cores greatly stabilized the MNPs against the high-temperature environment (e.g., 110 °C for 40 h in toluene). In addition, the solubility of the reducing agents also played an important role. Anhydrous hydrazine could swell the P4VP cores and concentric core-shell particle morphology was obtained. On the contrary, triethylsilane could not swell the P4VP cores and thus eccentric core-shell particle morphology was observed. Only the concentric core-shell MNPs exhibited good thermal stability, whereas the eccentric core-shell MNPs did not. This work suggested that these thermally stable MNPs could be good candidates for the melt processing of functional polymer nanocomposites.

Selective synthesis of Fe3O4Au x Ag y nanomaterials and their potential applications in catalysis and nanomedicine

In these recent years, magnetite (Fe3O4) has witnessed a growing interest in the scientific community as a potential material in various fields of application namely in catalysis, biosensing, hyperthermia treatments, magnetic resonance imaging (MRI) contrast agents and drug delivery. Their unique properties such as metal-insulator phase transitions, superconductivity, low Curie temperature, and magnetoresistance make magnetite special and need further investigation. On the other hand, nanoparticles especially gold nanoparticles (Au NPs) exhibit striking features that are not observed in the bulk counterparts. For instance, the mentioned ferromagnetism in Au NPs coated with protective agents such as dodecane thiol, in addition to their aptitude to be used in near-infrared (NIR) light sensitivity and their high adsorptive ability in tumor cell, make them useful in nanomedicine application. Besides, silver nanoparticles (Ag NPs) are known as an antimicrobial agent. Put together, the [Formula: see text] nanocomposites with tunable size can therefore display important demanding properties for diverse applications. In this review, we try to examine the new trend of magnetite-based nanomaterial synthesis and their application in catalysis and nanomedicine.

Catalyst Self-Assembly for Scalable Patterning of Sub 10 nm Ultrahigh Aspect Ratio Nanopores in Silicon

Nanoporous silicon (NPSi) has received significant attention for its potential to contribute to a large number of applications, but has not yet been extensively implemented because of the inability of current state-of-the-art nanofabrication techniques to achieve sufficiently small pore size, high aspect ratio, and process scalability. In this work we describe the fabrication of NPSi via a modified metal-assisted chemical etching (MACE) process in which silica-shell gold nanoparticle (SiO2-AuNP) monolayers self-assemble from solution onto a silicon substrate. Exposure to the MACE etchant solution results in the rapid consumption of the SiO2 spacer shell, leaving well-spaced arrays of bare AuNPs on the substrate surface. Particles then begin to catalyze the etching of nanopore arrays without interruption, resulting in the formation of highly anisotropic individual pores. The excellent directionality of pore formation is thought to be promoted by the homogeneous interparticle spacing of the gold core nanocatalysts, which allow for even hole injection and subsequent etching along preferred crystallographic orientations. Electron microscopy and image analysis confirm the ability of the developed technique to produce micrometer-scale arrays of sub 10 nm nanopores with narrow size distributions and aspect ratios of over 100:1. By introducing a scalable process for obtaining high aspect ratio pores in a novel size regime, this work opens the door to implementation of NPSi in numerous devices and applications.

Preparation of fluorescent Au–SiO2 core–shell nanoparticles and nanorods with tunable silica shell thickness and surface modification for immunotargeting

Scalable methods for preparing and modifying Au–SiO₂ core–shell nanoparticles provide a platform for engineering size-dependent multifunctional properties for in vivo biomedical applications.

Nanoparticle colloidal stability in cell culture media and impact on cellular interactions

Nanomaterials are finding increasing use for biomedical applications such as imaging, diagnostics, and drug delivery. While it is well understood that nanoparticle (NP) physico-chemical properties can dictate biological responses and interactions, it has been difficult to outline a unifying framework to directly link NP properties to expected in vitro and in vivo outcomes. When introduced to complex biological media containing electrolytes, proteins, lipids, etc., nanoparticles (NPs) are subjected to a range of forces which determine their behavior in this environment. One aspect of NP behavior in biological systems that is often understated or overlooked is aggregation. NP aggregation will significantly alter in vitro behavior (dosimetry, NP uptake, cytotoxicity), as well as in vivo fate (pharmacokinetics, toxicity, biodistribution). Thus, understanding the factors driving NP colloidal stability and aggregation is paramount. Furthermore, studying biological interactions with NPs at the nanoscale level requires an interdisciplinary effort with a robust understanding of multiple characterization techniques. This review examines the factors that determine NP colloidal stability, the various efforts to stabilize NP in biological media, the methods to characterize NP colloidal stability in situ, and provides a discussion regarding NP interactions with cells.

Characterizing nanoparticles in complex biological media and physiological fluids with depolarized dynamic light scattering

Light scattering is one of the few techniques available to adequately characterize suspended nanoparticles (NPs) in real time and in situ. However, when it comes to NPs in multicomponent and optically complex aqueous matrices - such as biological media and physiological fluids - light scattering suffers from lack of selectivity, as distinguishing the relevant optical signals from the irrelevant ones is very challenging. We meet this challenge by building on depolarized scattering: Unwanted signals from the matrix are completely suppressed. This approach yields information with an unprecedented signal-to-noise ratio in favour of the NPs and NP-biomolecule corona complexes, which in turn opens the frontier to scattering-based studies addressing the behaviour of NPs in complex physiological/biological fluids.

A one-step short-time synthesis of Ag@SiO2 core-shell nanoparticles

A performance of shell-thickness precise control in silver-silica coating core-shell nanoparticles is presented. 60nm sized citrate-stabilized silver nanoparticles are directly silica coated using a modified Stöber process. Tetraethyl orthosilicate is used as a silica precursor and ammonium hydroxide as catalyst in an alcoholic solvent to promote the seeded silica growth. By simply varying the synthesis reaction time from 4 to 60min, the silica shell thickness is increased from 5.1nm to 76.4nm. This well-controlled synthesis is then transposed to 40, 80 and 100nm sized silver cores in order to show the independence of the silica shell growth on the nanoparticle core size. Optical properties, i.e. localized surface plasmon resonance, of the produced silver-silica core-shell are also investigated.

Superhydrophobic bull's-eye for surface-enhanced Raman scattering

We present a micro-patterned silicon structure that enables the preparation of a substrate for surface-enhanced Raman scattering (SERS) and pre-concentration of the analyte molecules. The structure is designed to produce a hydrophobicity gradient. As a result, a water droplet placed on it will remain centred on the structure as it dries, enabling delivery of materials to its centre. The structure is therefore referred to as a superhydrophobic bull's-eye. A water droplet containing gold colloids placed on it dries to produce a cluster at the bull's-eye centre. A second water droplet placed on it, this time containing analyte molecules, dries such that the molecules are delivered to the gold colloid cluster. We demonstrate the detection of molecules at low concentrations (Rhodamine 6G at 10(-15) M) from small droplets.

Multifunctional gold nanoparticles for diagnosis and therapy of disease

Gold nanoparticles (AuNPs) have a number of physical properties that make them appealing for medical applications. For example, the attenuation of X-rays by gold nanoparticles has led to their use in computed tomography imaging and as adjuvants for radiotherapy. AuNPs have numerous other applications in imaging, therapy and diagnostic systems. The advanced state of synthetic chemistry of gold nanoparticles offers precise control over physicochemical and optical properties. Furthermore gold cores are inert and are considered to be biocompatible and nontoxic. The surface of gold nanoparticles can easily be modified for a specific application, and ligands for targeting, drugs or biocompatible coatings can be introduced. AuNPs can be incorporated into larger structures such as polymeric nanoparticles or liposomes that deliver large payloads for enhanced diagnostic applications, efficiently encapsulate drugs for concurrent therapy or add additional imaging labels. This array of features has led to the aforementioned applications in biomedical fields, but more recently in approaches where multifunctional gold nanoparticles are used for multiple methods, such as concurrent diagnosis and therapy, so-called theranostics. This review covers basic principles and recent findings in gold nanoparticle applications for imaging, therapy and diagnostics, with a focus on reports of multifunctional AuNPs.

Polyvinylpyrrolidone molecular weight controls silica shell thickness on Au nanoparticles with diglycerylsilane as precursor

Several strategies have been described for the preparation of silica-encapsulated gold nanoparticles (SiO(2)-AuNP), which typically suffer from an initial interface between gold and silica that is difficult to control, and layer thicknesses that are very sensitive to minor changes in silane concentration and incubation time. The silica shell thicknesses are normally equal to or larger than the gold particles themselves, which is disadvantageous when the particles are to be used for biodiagnostic applications. We present a facile and reproducible method to produce very thin silica shells (3-5 nm) on gold nanoparticles: the process is highly tolerant to changes in reaction conditions. The method utilized polyvinylpyrrolidone (PVP) of specific molecular weights to form the interface between gold and silica. The method further requires a nontraditional silica precursor, diglycerylsilane, which efficiently undergoes sol-gel processing at neutrality. Under these conditions, higher molecular weight PVP leads to thicker silica shells: PVP acts as the locus for silica growth into an interpenetrating organic-inorganic hybrid structure.