Unexpected enhancement of pH-stability in Au3+/Ag+ loaded H-bonded layer-by-layer thin films

In this work, a polymeric film was synthesized through a layer-by-layer (LBL) self-assembly technique using polyacrylic acid (PAA) and polyethylene oxide (PEO), resulting in the formation of a hydrogen-bonded LBL film. The formation of these films was evaluated by PMIRRAS and QCM-D. The synergy of these techniques allowed the understanding of the mechanism of formation of the film by showing the H-bonding formation and film growth. Au and Ag metal ions were successfully incorporated into the films, as corroborated by the combination of the information obtained by XRR and PMIRRAS. The films were exposed to increasing pH, showing a pronounced improvement in stability in films loaded with Au ions, extending the stability from pH 4 to 10. This behavior allows the use of this system in a wider range of applications, including the possibility of working in biological conditions. On the other hand, films loaded with Ag disintegrated at pH above 4. At acidic pH (below 3), these films released the Ag ions, which may be useful for the preparation of antibacterial stimuli-responsive nanomaterials. In both cases, the films were adequate to produce metal nanoparticles by metal loading and in situ reduction.

Optical Printing of Single Au Nanostars

Obtaining arrays of single nanoparticles with three-dimensional complex shapes is still an open challenge. Current nanolithography methods do not allow for the preparation of nanoparticles with complex features like nanostars. In this work, we investigate the optical printing of gold nanostars of different sizes as a function of laser wavelength and power. We found that tuning the laser to the main resonances of the nanostars in the near-infrared makes it possible to avoid nanoparticles reshaping due to plasmonic heating, enabling their deposition at the single particle level and in ordered arrays.

Importance of the Structural and Physicochemical Properties of Silica Nanoshells in the Photothermal Effect of Silica-Coated Au Nanoparticles Suspensions

In this work, monodisperse silica-coated gold nanoparticles (NPs) were synthesized and used for obtaining aqueous colloidal dispersions with an optimum relationship between colloidal stability and photothermal activity. The idea behind this design was to produce systems with the advantages of the presence of a silica shell (biocompatibility, potential for surface modification, and protecting effect) with a minimal loss of optical and thermal properties. With this aim, the photothermal properties of NPs with silica shells of different thicknesses were analyzed under conditions of high radiation extinction. By using amorphous, gel-like silica coatings, thicknesses higher than 40 nm could be obtained without an important loss of the light absorption capacity of the colloids and with a significant photothermal response even at low NP concentrations. The effects produced by changes in the solvent and in the NP concentration were also analyzed. The results show that the characteristics of the shell control both, the photothermal effect and the optical properties of the colloidal dispersions. As the presence of a silica shell strongly enhances the possibilities of adding cargo molecules or probes, these colloids can be considered of high interest for biomedical therapies, sensing applications, remote actuation, and other technological applications.

E-waste upcycling for the synthesis of plasmonic responsive gold nanoparticles

One of the current challenges in circular economy is the ability to transform waste into valuable products. In this work, waste of electrical and electronic equipment (WEEE) was used as a gold source to prepare stable gold nanoparticles (AuNP). The proposed methodology involves a series of physical and chemical separation steps, carefully designed according to the complex nature of the selected WEEE and the targeted product. In a first step, pins from microprocessors were separated by mechanical treatments, allowing to concentrate gold in a metallic fraction. A two-step hydrometallurgical method was subsequently performed, to obtain a Au (III) enriched solution. Such solution was used as a secondary raw material to obtain AuNP. For that purpose, a specific synthetic method was developed, adapted to the high acidity and ionic strength of the solution. Thanks to the use of two easily available reducing agents (sodium citrate and ascorbic acid) and a polymeric stabilizer (PVP), it was possible to obtain high purity AuNP presenting a mixture of well-defined spherical and triangular shapes. These AuNP were finally deposited onto glass substrates and present a sensitive response to refractive index changes in the environment, a necessary condition towards application in optical sensors. In summary, this upcycling case study demonstrates that e-waste can successfully replace primary raw materials to obtain highly valuable and useful nanomaterials. These results highlight the potential of urban mining as a sustainable and circular approach to the development of nanotechnologies.

Monitoring Chemical Reactions with SERS-Active Ag-Loaded Mesoporous TiO2 Films

Monitoring chemical reactions that occur in small spaces or confined environments is challenging. Surface-enhanced Raman scattering (SERS) spectroscopy offers the unique possibility to monitor spectral changes with high sensitivity and time resolution. Herein, we report the application of composite mesoporous TiO₂ films loaded with Ag nanoparticles (NPs) to track in situ chemical processes in real time. In particular, the AgNPs@TiO₂ system was employed to monitor two chemical reactions: one occurring on the Ag NPs surface and another taking place in the surrounding solution. In the first case, we monitored the decarboxylation reaction of 4-mercaptobenzoic acid on Ag NPs, which allowed us to identify the conditions that favor it. In the second case, we studied the pH evolution in the nanocavities during a homogeneous alkalization process driven by chloride-assisted glycidol rupture (the Epoxide Route) and compared it with pH measurements by conventional techniques. We therefore demonstrated that the proposed nanodevice provides an excellent performance to monitor dynamic processes occurring either inside the material or in the solution in which it is immersed.

Mild Homogeneous Synthesis of Gold Nanoparticles through the Epoxide Route: Kinetics, Mechanisms, and Related One-Pot Composites

A new one-pot homogeneous methodology at room temperature to obtain Au nanoparticles (AuNP) on the basis of the epoxide route is presented. The proposed method takes advantage of the homogenous generation of OH^- moieties driven by epoxide ring-opening, mediated by chloride nucleophilic attack. Once reached alkaline conditions, the reducing medium allows the quantitative formation of AuNP under well-defined kinetic control. A stabilizing agent, such as polyvinylpyrrolidone (PVP) or cetyltrimethylammonium chloride (CTAC), is required to maintain the AuNP stable. Meanwhile their presence dramatically affects the reduction kinetics and pathway, as demonstrated by the evolution of the UV/Vis spectra, small-angle X-ray scattering (SAXS) patterns, and pH value along the reaction. In the presence of PVP nanogold spheroids are obtained following a similar reduction mechanism as that observed for control experiments in the absence of PVP. However, if CTAC is employed a stable complex with Au^III is formed, leading to a different reaction pathway and resulting in ellipsoidal-like shaped AuNP. Moreover, the proposed methodology allows stabilize the growing AuNP, by coupling their formation with nonalkoxidic sol-gel reactions, leading to nanocomposite gels with embedded metallic nanoparticles. The epoxide route thus offers a versatile scenario for the one-pot preparation of new metal nanoparticles-inorganic/hybrid matrices nanocomposites with valuable optical properties.

Light-Induced Polymer Response through Thermoplasmonics Transduction in Highly Monodisperse Core-Shell-Brush Nanosystems

Smart nanosystems that transduce external stimuli to physical changes are an inspiring challenge in current materials chemistry. Hybrid organic-inorganic materials attract great attention due to the combination of building blocks responsive to specific external solicitations. In this work, we present a sequential method for obtaining an integrated core-shell-brush nanosystem that transduces light irradiation into a particle size change through a thermoplasmonic effect. We first synthesize hybrid monodisperse systems made up of functionalized silica colloids covered with controllable thermoresponsive poly(N-isopropylacrylamide), PNIPAm, brushes, produced through radical photopolymerization. This methodology was successfully transferred to Au@SiO₂ nanoparticles, leading to a core-shell-brush architecture, in which the Au core acts as a nanosource of heat; the silica layer, in turn, adapts the metal and polymer interfacial chemistries and can also host a fluorescent dye for bioimaging. Upon green LED irradiation, a light-to-heat conversion process leads to the shrinkage of the external polymer layer, as proven by in situ DLS. Our results demonstrate that modular hybrid nanosystems can be designed and produced with photothermo-physical transduction. These remote-controlled nanosystems present prospective applications in smart carriers, responsive bioscaffolds, or soft robotics.

Gold Recycling at Laboratory Scale: From Nanowaste to Nanospheres

The market for products based on nanotechnology, and with it the use of nanomaterials and the generation of nanowaste, increases day by day. Among the vast variety of nanomaterials available, gold nanoparticles (AuNPs) are among the most studied and applied in commercial products. This current situation requires both the development of recovery methods to reduce the amount of nanowaste produced, and new synthetic methods that allow the reuse of recovered gold for new nanomaterial production, keeping in mind both economical and ecological considerations. In this work, a methodology to recover gold from aqueous laboratory nanowaste and transform it into an aqueous HAuCl₄ solution was developed, using extremely simple procedures and readily available chemical reagents (NaCl, HCl, H₂ O₂ ) and allowing the recovery of more than 99 % of the original gold. The experiments were performed by using both simulated and real laboratory nanowastes, and practically the same results were obtained. Moreover, the subsequent use of the obtained aqueous HAuCl₄ solution from the recovered gold to produce spherical AuNPs through a seed-mediated approach was demonstrated. Thus, this work presents for the first time a complete recycling cycle from nanowaste to the reagent and back to the nanomaterial.

TiO2 mesoporous thin film architecture as a tool to control Au nanoparticles growth and sensing capabilities

In this paper, a systematic study regarding the effect of the mesoporous structure over Au nanoparticles (NPs) growth inside and through the pores of mesoporous TiO2 thin films (MTTFs) is presented, and the effect of such characteristics over the composites' sensing capabilities is evaluated. Highly stable MTTFs with different pore diameters (range: 4-8 nm) and pore arrangements (body- and face-centered cubic) were synthesized and characterized. Au NPs were grown inside the pores, and it was demonstrated-through a careful physicochemical characterization-that the amount of incorporated Au and NP size depends on the pore array; being higher for bigger pore diameters and face-centered cubic structures. The same structure allows the growth of more and longer tips over Au NPs deposited at the thin film-substrate interface. Finally, to confirm the effect of the structural characteristics of the composites over their possible applications, the materials were tested as surface-enhanced Raman scattering (SERS)-based substrates. The composites with a higher amount of Au and more ramified NPs were the ones that presented better sensitivity in the detection of a probe molecule (4-nitrothiophenol). Overall, this work demonstrates that the pore size and ordering in MTTFs determine the materials' accessibility and connectivity, and therefore, have a clear impact on their potential applications.

Structural characterization of Au nano bipyramids: reshaping under thermal annealing, the capping agent effect and surface decoration with Pt

Anisotropic gold nanoparticles offer potential applications due to their functionalities and shape-dependent properties. Reshaping noble metal nanoparticles is an interesting field with optical, surface-enhanced Raman spectroscopy, catalytic applications and potential application as a photothermic therapy. This work comprises a structural study on gold nano bipyramids (Au NBPs) and nanodumbbells, and the evolution of Au NBPs capped with cetyltrimethylammonium bromide and dodecanethiol through an in situ and ex situ heating process in high vacuum. Also, we study the reshaping of Au NBPs by the addition of Pt to study the surface modification and the strain generated on a single particle by geometric phase analysis.

Chemical Stability of Mesoporous Oxide Thin Film Electrodes under Electrochemical Cycling: from Dissolution to Stabilization

Mesoporous oxide thin films (MOTF) present very high surface areas and highly controlled monodisperse pores in the nanometer range. These features spurred their possible applications in separation membranes and permselective electrodes. However, their performance in real applications is limited by their reactivity. Here, we perform a basic study of the stability of MOTF toward dissolution in aqueous media using a variety of characterization techniques. In particular, we focus in their stability behavior under the influence of ionic strength, adsorption of electrochemical probes, and applied electrode potential. Mesoporous silica thin films present a limited chemical stability after electrochemical cycling, particularly under high ionic strength, due to their high specific surface area and the interactions between the electrochemical probes and the surface. In contrast, TiO₂ or Si_0.9Zr_0.1O₂ matrices present higher stability; thus, they are an adequate alternative to produce accessible, sensitive, and robust permselective electrodes or membranes that perform under a wide variety of conditions.

One-Step Synthesis of Mesoporous Silica Thin Films Containing Available COOH Groups

Inorganic-organic hybrid mesoporous silica thin films with covalently bonded carboxylic acid groups were synthesized in a one-step procedure, using carboxylic-derivatized alkoxysilanes obtained by photochemical radical thiol-ene addition (PRTEA). The organosilanes were synthesized by clicking mercaptosuccinic or mercaptoacetic thioacids with vinyltrimethoxysilane, using benzophenone as the photoradical initiator. The films were synthesized by evaporation-induced self-assembly of a sol containing a mixture of tetraethoxysilane and different quantities of the organosilanes, without any further treatment after the PRTEA reaction. Two nonionic surfactants were used as templates to produce different pore sizes. Different aging times were also applied. Structural characterization with electron microscopy, porosimetry measurements, and small angle X-ray scattering with two-dimensional detection demonstrated the obtention of mesoporous phases whose degree of ordering depended on the amount of added organosilane. The incorporation of the functional silanes was shown by X-ray photoelectron spectroscopy, and the presence of the COOH groups was confirmed by Fourier transform infrared (FTIR). Finally, the availability of the COOH groups for further chemical modification was demonstrated by FTIR by following the changes in the typical carbonyl IR bands during proton exchange and metal complexation. The proposed simple methodology allows obtaining COOH-modified silica thin films in one step, without the need of hard reaction conditions or deprotection steps. Functionalization with carboxyl groups brings a pH-dependent switch-ability to the pore surface that can be used for multifunctional mesoporous materials design.

Tethering Luminescent Thermometry and Plasmonics: Light Manipulation to Assess Real-Time Thermal Flow in Nanoarchitectures

The past decade has seen significant progresses in the ability to fabricate new mesoporous thin films with highly controlled pore systems and emerging applications in sensing, electrical and thermal isolation, microfluidics, solar cells engineering, energy storage, and catalysis. Heat management at the micro- and nanoscale is a key issue in most of these applications, requiring a complete thermal characterization of the films that is commonly performed using electrical methods. Here, plasmonic-induced heating (through Au NPs) is combined with Tb³⁺/Eu³⁺ luminescence thermometry to measure the thermal conductivity of silica and titania mesoporous nanolayers. This innovative method yields values in accord with those measured by the evasive and destructive conventional 3ω-electrical method, simultaneously overcoming their main limitations, for example, a mandatory deposition of additional isolating and metal layers over the films and the previous knowledge of the thermal contact resistance between the heating and the mesoporous layers.

Formation of ordered mesostructured TiO2 thin films: a soft coarse-grained simulation study

Variation of the dispersity index D as water and HCl evaporate distinctly. Right panels show the snapshots of formed mesopores.

Diffusion of single dye molecules in hydrated TiO2 mesoporous films

Fluorescence correlation spectroscopy (FCS) shows how the pore dimensions of thin and hydrated TiO₂ mesoporous calcined films modulate the diffusion of molecules across the pore network.

Understanding the Zr and Si interdispersion in Zr1−xSixO2 mesoporous thin films by using FTIR and XANES spectroscopy

Zr_1−xSi_xO₂ mesoporous oxide thin films were obtained and their interdispersion was demonstrated by careful characterization using spectroscopic techniques.

Selective SERS Sensing Modulated by Functionalized Mesoporous Films

A hybrid material comprising metal nanoparticles embedded in functionalized mesoporous thin films was constructed, and its use as a selective SERS-based sensor was demonstrated. The presence of specific functional groups in the pore network allows control over the surface chemistry of the pores, tuning the selectivity for specific molecules. Amino-functionalized hybrid mesoporous thin films were used in a proof of concept experiment, to discern the presence of methylene blue (MB) in mixtures with acid blue (AB), with no need for any sample pretreatment step. Selective detection of MB was possible through entrapment of AB in the mesoporous matrix, based on its high affinity for amino groups. The sensor selectivity can be tuned by varying the solution pH, rendering a pH responsive surface and thus, selective SERS-based sensing. The developed sensors allow specific detection of molecules in complex matrixes.

Critical aspects in the production of periodically ordered mesoporous titania thin films

Periodically ordered mesoporous titania thin films (MTTF) present a high surface area, controlled porosity in the 2-20 nm pore diameter range and an amorphous or crystalline inorganic framework. These materials are nowadays routinely prepared by combining soft chemistry and supramolecular templating. Photocatalytic transparent coatings and titania-based solar cells are the immediate promising applications. However, a wealth of new prospective uses have emerged on the horizon, such as advanced catalysts, perm-selective membranes, optical materials based on plasmonics and photonics, metamaterials, biomaterials or new magnetic nanocomposites. Current and novel applications rely on the ultimate control of the materials features such as pore size and geometry, surface functionality and wall structure. Even if a certain control of these characteristics has been provided by the methods reported so far, the needs for the next generation of MTTF require a deeper insight in the physical and chemical processes taking place in their preparation and processing. This article presents a critical discussion of these aspects. This discussion is essential to evolve from know-how to sound knowledge, aiming at a rational materials design of these fascinating systems.

Growth and branching of gold nanoparticles through mesoporous silica thin films

Composite materials made of mesoporous oxide thin films containing metallic nanoparticles are of high interest in various fields, including catalysis, biosensing and non-linear optics. We demonstrate in this work the fabrication of such composite materials containing a sub-monolayer of gold nanoparticles (GNPs) of various shapes covered with mesoporous silica thin films. Additionally, the shape of the GNPs (and thus their optical properties) can be modified in situ through seeded growth and branching. Such growth proceeds upon wetting with HAuCl(4) solution, a surfactant (cetyltrimethylammonium bromide, CTAB) and a mild reducing agent (ascorbic acid, AA). The effect of varying several reaction parameters (time and CTAB and AA concentrations) was evaluated, showing that more anisotropic particles are obtained at longer reaction times, lower CTAB concentration and higher AA concentration. The final shape of the GNPs was also found to depend on their initial shape and size, as well as the pore size of the mesoporous film covering them. Because the growth proceeds through the pores of the film, it may lead to shapes that are not easily obtained in solution, such as particles with branches on one side only. Finally, we have confirmed that no damage was induced to the mesoporous silica structure during the growth process and thus the final particles remain well covered by the thin film, which can eventually be used as a filter between the GNPs and the outer medium.

Highly ordered hybrid mesoporous bifunctional thin films

Highly ordered mixed framework mesoporous cubic (Im3m) thin films of (M(1-x)(Si-R)(x))O(2) bearing organic groups (M = Ti or Zr, R = propylamine, propylthiol or phenyl, x < or = 0.2) are obtained by one-pot dip-coating; a second organic function (R' = hexadecyl, phenyl, thiol) can be added by post-grafting with a molecule presenting a group capable to anchor to the M sites, thus leading to bifunctional accessible mesopores.