Showing papers by "Younan Xia published in 2010"

Soft lithography for micro- and nanoscale patterning

Abstract: This protocol provides an introduction to soft lithography--a collection of techniques based on printing, molding and embossing with an elastomeric stamp. Soft lithography provides access to three-dimensional and curved structures, tolerates a wide variety of materials, generates well-defined and controllable surface chemistries, and is generally compatible with biological applications. It is also low in cost, experimentally convenient and has emerged as a technology useful for a number of applications that include cell biology, microfluidics, lab-on-a-chip, microelectromechanical systems and flexible electronics/photonics. As examples, here we focus on three of the commonly used soft lithographic techniques: (i) microcontact printing of alkanethiols and proteins on gold-coated and glass substrates; (ii) replica molding for fabrication of microfluidic devices in poly(dimethyl siloxane), and of nanostructures in polyurethane or epoxy; and (iii) solvent-assisted micromolding of nanostructures in poly(methyl methacrylate).

A comparison study of the catalytic properties of Au-based nanocages, nanoboxes, and nanoparticles.

Abstract: We have evaluated the catalytic properties of Au-based nanostructures (including nanocages, nanoboxes, and solid nanoparticles) using a model reaction based on the reduction of p-nitrophenol by NaBH4. From the average reaction rate constants at three different temperatures, we determined the activation energy, the entropy of activation, and the pre-exponential factor for each type of Au nanostructure. The kinetic data indicate that the Au-based nanocages are catalytically more active than both the nanoboxes and nanoparticles probably due to their extremely thin but electrically continuous walls, the high content of Au, and the accessibility of both inner and outer surfaces through the pores in the walls. In addition, a compensation effect was observed in this Au-based catalytic system, which can be primarily interpreted by a model based on kinetic regime switching.

Gold nanocages as photothermal transducers for cancer treatment.

Abstract: Gold nanocages represent a new class of nanomaterials with compact size and tunable optical properties for biomedical applications. They exhibit strong light absorption in the near-infrared region in which light can penetrate deeply into soft tissue. After PEGylation, the Au nanocages can be passively delivered to tumors in animals. Analysis of tissue distribution for the PEGylated Au nanocages showed that the tumor uptake was 5.7 %ID/g at 96 h post injection. The Au nanocages were found not only on the surface, but also in the core of the tumor. By exposing tumors to a near-infrared diode laser (0.7 W/cm2, CW, λ=808 nm) for 10 min, the photothermal effect of the Au nanocages could selectively destroy tumor tissue with minimum damage to the surrounding healthy tissue. Data from functional [18F]fluorodexoyglucose positron emission tomography revealed a decrease in tumor metabolic activity upon the photothermal treatment. Histological examination identified extensive damage to the nuclei of tumor cells and tumor interstitium.

Au@Ag Core−Shell Nanocubes with Finely Tuned and Well-Controlled Sizes, Shell Thicknesses, and Optical Properties

Abstract: NSF [DMR 0804088, ECS 0335765]; NIH [1R01 CA138527]; Ministry of Education Science and Technology [R32 20031]; China Scholarship Council

In vivo molecular photoacoustic tomography of melanomas targeted by bioconjugated gold nanocages.

Abstract: Early diagnosis, accurate staging, and image-guided resection of melanomas remain crucial clinical objectives for improving patient survival and treatment outcomes Conventional techniques cannot meet this demand because of the low sensitivity, low specificity, poor spatial resolution, shallow penetration, and/or ionizing radiation Here we overcome such limitations by combining high-resolution photoacoustic tomography (PAT) with extraordinarily optical absorbing gold nanocages (AuNCs) When bioconjugated with [Nle4,d-Phe7]-α-melanocyte-stimulating hormone, the AuNCs can serve as a novel contrast agent for in vivo molecular PAT of melanomas with both exquisite sensitivity and high specificity The bioconjugated AuNCs enhanced contrast ∼300% more than the control, PEGylated AuNCs The in vivo PAT quantification of the amount of AuNCs accumulated in melanomas was further validated with inductively coupled plasma mass spectrometry (ICP-MS)

Controlling the Shapes of Silver Nanocrystals with Different Capping Agents

Abstract: This paper provides direct evidence to support the role of a capping agent in controlling the evolution of Ag seeds into nanocrystals with different shapes. Starting with single-crystal seeds (spherical or cubic in shape), we could selectively obtain Ag octahedrons enclosed by {111} facets and nanocubes/nanobars enclosed by {100} facets by adding sodium citrate (Na(3)CA) and poly(vinyl pyrrolidone) (PVP), respectively, as a capping agent while all other parameters were kept the same. This research not only offers new insights into the role played by a capping agent in shape-controlled synthesis but also provides, for the first time, Ag octahedrons as small as 40 nm in edge length for optical and spectroscopic studies.

Seed-Mediated Synthesis of Ag Nanocubes with Controllable Edge Lengths in the Range of 30−200 nm and Comparison of Their Optical Properties

Abstract: Silver nanocubes with edge lengths controllable in the range of 30−200 nm were synthesized using an approach based on seeded growth. The keys to the success of this synthesis are the use of single-...

Electrospun nanofibers for neural tissue engineering

Abstract: Biodegradable nanofibers produced by electrospinning represent a new class of promising scaffolds to support nerve regeneration. We begin with a brief discussion on the electrospinning of nanofibers and methods for controlling the structure, porosity, and alignment of the electrospun nanofibers. The methods include control of the nanoscale morphology and microscale alignment of the nanofibers, as well as the fabrication of macroscale, three-dimensional tubular structures. We then highlight recent studies that utilize electrospun nanofibers to manipulate biological processes relevant to nervous tissue regeneration, including stem cell differentiation, guidance of neurite extension, and peripheral nerve injury treatments. The main objective of this feature article is to provide valuable insights into methods for investigating the mechanisms of neurite growth on novel nanofibrous scaffolds and optimization of the nanofiber scaffolds and conduits for repairing peripheral nerve injuries.

Unraveling the Effects of Size, Composition, and Substrate on the Localized Surface Plasmon Resonance Frequencies of Gold and Silver Nanocubes: A Systematic Single-Particle Approach

Abstract: Localized surface plasmon resonances (LSPRs), resulting from the interaction of light with metal nanoparticles, are powerful tools for biological sensors, surface-enhanced spectroscopies, and optical devices. LSPR frequencies are strongly dependent on a nanoparticle’s structure, composition, and local dielectric environment. However, these relationships are prohibitively difficult or impossible to probe from bulk solutions due to the heterogeneity of chemically synthesized products. In this study, systematic single-particle structure−property measurements, coupled with a statistical analysis and FDTD calculations, are performed on silver and gold nanocubes. The dependencies of LSPR frequencies on nanocube size, composition, and substrate dielectric constant are determined. The results obtained represent the most quantitative measurements and analysis to date, yielding predictive rules and fundamental insights into the interactions between nanoparticles and substrates.

The effects of size, shape, and surface functional group of gold nanostructures on their adsorption and internalization by cells.

Abstract: In this study, we examined the effects of size, shape, and surface chemistry of gold nanostructures on their uptake (including both adsorption and internalization) by SK-BR-3 breast cancer cells. We used both spherical and cubic Au nanostructures (nanospheres and nanocages, respectively) of two different sizes, and their surface was modified with poly(ethylene glycol) (PEG), antibody anti-HER2, or poly(allyamine hydrochloride) (PAA). Our results showed that the size of the Au nanostructures influenced their uptake by the cells in a similar way regardless of the surface chemistry, while the shape dependency could vary depending on the surface functional group. In addition, the cells preferred to take up the Au nanostructures covered by different surface groups in the following order: PAA>> anti-HER2> PEG. The fraction of Au nanostructures attached to the cell surface was also dependent on the aforementioned parameters.

Facile Synthesis of Ag Nanocubes of 30 to 70 nm in Edge Length with CF3COOAg as a Precursor

Abstract: This paper describes a new protocol to synthesize Ag nanocubes of 30 to 70 nm in edge length with the use of CF(3)COOAg as a precursor to elemental silver. By adding a trace amount of NaSH and HCl to the polyol synthesis, Ag nanocubes were obtained with good quality, high reproducibility, and on a scale up to 0.19 g per batch for the 70 nm Ag nanocubes. The Ag nanocubes were found to grow in size at a controllable pace over the course of synthesis. The linear relationship between the edge length of the Ag nanocubes and the position of localized surface plasmon resonance (LSPR) peak provides a simple method for finely tuning and controlling the size of the Ag nanocubes by monitoring the UV/Vis spectra of the reaction at different times.

Radially Aligned, Electrospun Nanofibers as Dural Substitutes for Wound Closure and Tissue Regeneration Applications

Abstract: This paper reports the fabrication of scaffolds consisting of radially aligned poly(e-caprolactone) nanofibers by utilizing a collector composed of a central point electrode and a peripheral ring electrode. This novel class of scaffolds was able to present nanoscale topographic cues to cultured cells, directing and enhancing their migration from the periphery to the center. We also established that such scaffolds could induce faster cellular migration and population than nonwoven mats consisting of random nanofibers. Dural fibroblast cells cultured on these two types of scaffolds were found to express type I collagen, the main extracellular matrix component in dural mater. The type I collagen exhibited a high degree of organization on the scaffolds of radially aligned fibers and a haphazard distribution on the scaffolds of random fibers. Taken together, the scaffolds based on radially aligned, electrospun nanofibers show great potential as artificial dural substitutes and may be particularly useful as biomedical patches or grafts to induce wound closure and/or tissue regeneration.

Synthesis of Pd-Au Bimetallic Nanocrystals via Controlled Overgrowth

Abstract: This paper describes the synthesis of Pd−Au bimetallic nanocrystals with controlled morphologies via a one-step seeded-growth method Two different reducing agents, namely, l-ascorbic acid and citric acid, were utilized for the reduction of HAuCl4 in an aqueous solution to control the overgrowth of Au on cubic Pd seeds When l-ascorbic acid was used as the reducing agent, conformal overgrowth of Au on the Pd nanocubes led to the formation of Pd−Au nanocrystals with a core−shell structure On the contrary, localized overgrowth of Au was observed when citric acid was used as the reducing agent, producing Pd−Au bimetallic dimers Through this morphological control, we were able to tune the localized surface plasmon resonance peaks of Pd−Au bimetallic nanostructures in the visible region

Aqueous‐Phase Synthesis of Pt/CeO2 Hybrid Nanostructures and Their Catalytic Properties

Abstract: DOI: 10.1002/adma.201002763 Metal nanocrystals supported on metal oxides often exhibit improved catalytic activity and selectivity as compared to unsupported ones, [ 1–4 ] which is known to arise from several factors, including the shape and size of metal nanocrystals, the metal oxidation state, and the support effect. [ 5–11 ] Early methods for preparing metal nanocrystal on metal oxide hybrid nanostructures mainly involved calcination of metal oxide powders impregnated with metal precursors at high temperatures ( > 400 ° C). [ 12–17 ]

Magnetic-Field-Assisted Electrospinning of Aligned Straight and Wavy Polymeric Nanofibers

Abstract: Electrospinning has emerged as a very attractive approach to the fabrication of nanometer (nm)- and submicron-sized fibers.[1–6] Nanofibers generated using the electrospinning technique have been explored for a wide range of applications such as tissue engineering and bioassay,[5, 7–13] sensing,[14, 15] textile,[16] filtration,[17] electrode,[18] and catalyst supports.[19] In many of these cases, the ability to control the alignment and arrangement of fibers is critical to achieve the designed functions, particularly for tissue engineering that requires the release of proteins and growth factors in a guided fashion.[20–22] Improvements in fiber quality has been achieved in recent year through the optimization of electrospinning conditions and oriented fiber arrays can be produced using either rotating collectors or patterned electrodes.[1, 23–26] Furthermore, when magnetic nanoparticles are mixed with polymers, aligned fibers can also be fabricated.[27]

Gold Nanocages: A Novel Class of Multifunctional Nanomaterials for Theranostic Applications

Abstract: Gold nanocages represent a novel class of nanostructures, well-suited for biomedical applications. They can be readily prepared via the galvanic replacement reaction between silver nanocubes and chloroauric acid. Their optical resonance peaks can be easily and precisely tuned to the near-infrared region from 650–900 nm, the transparent window for blood and soft tissue. Furthermore, their surface can be conveniently conjugated with various ligands for targeting cancer. In this feature article, we highlight recent advances in the large-scale synthesis of gold nanocages and their applications in cancer diagnosis and treatment. Specifically, we have scaled up the production of gold nanocages for in vivo studies and evaluated their tumor targeting capabilities. We have also demonstrated their use as contrast agents for photoacoustic tumor imaging and the mapping of sentinel lymph node, as photothermal transducers for cancer treatment, and as smart carriers for controlled release with a near-infrared laser.

Metal nanoparticles with gain toward single-molecule detection by surface-enhanced Raman scattering.

Abstract: Single-molecule detection via surface-enhanced Raman scattering (SERS) has raised great interest over the past decade. The usual approach toward this goal is to harness the strong surface plasmon resonance of light with complex metallic nanostructures, such as particle aggregates, two-particle gaps, sharp tips, or particles with sharp apexes. Here we propose another route toward the goal by introducing gain medium into single metal nanoparticles with simple geometry. Our calculations show that cubic gold nanobox particles that contain a gain material within the core can create an extremely high enhancement factor of local field intensity larger than 10(8) and a SERS enhancement factor on the order of 10(16)-10(17).

Understanding the SERS Effects of Single Silver Nanoparticles and Their Dimers, One at a Time

Abstract: This Perspective article highlights recent developments in a class of surface-enhanced Raman scattering (SERS) experiments that aim to correlate SERS enhancement factors with the physical parameters of metal nanostructures. In a typical study, the SERS substrate is fabricated by depositing colloidal nanoparticles on a silicon wafer to obtain individual particles isolated from each other or small aggregates such as dimeric units. With the help of registration marks, the same nanoparticle, or dimer of nanoparticles, can be quickly located under a Raman microscope (for SERS spectra) and a scanning electron microscope (for structural characterization). The nanoscale characterization achieved by these studies has resulted in unparalleled investigations into the nature of polarization dependency for SERS, the hot spot nature of single nanoparticles and dimers, and the manipulation of hot spots through shape-controlled synthesis and self-assembly. We discuss the new insights that these studies have offered and t...

Chemical transformations in ultrathin chalcogenide nanowires.

Abstract: We have studied the chemical transformations in ultrathin chalcogenide nanowires with an aim to understand the parameters that control the morphology and crystal structure of the product. Ultrathin Te nanowires were transformed into Ag2Te nanowires with preservation of the single crystallinity. The Ag2Te nanowires were then converted into CdTe, ZnTe, and PbTe using cation-exchange reactions, and the CdTe nanowires were further transformed into PtTe2 nanotubes. On the basis of the solubility products of the ionic solids, the crystal structures of the involved solids, the reaction kinetics, and the reaction conditions for transformations, we were able to reach the following conclusions: (i) The solubility products of ionic solids can be used as a rough criterion to predict if the transformation is thermodynamically favorable or not. (ii) The morphological preservation of reactant nanowires is more sensitive to the change in length rather than the total volume in addition to the lattice matching between the ...

“Aligned-to-random” nanofiber scaffolds for mimicking the structure of the tendon-to-bone insertion site

Abstract: We have demonstrated the fabrication of “aligned-to-random” electrospun nanofiber scaffolds that mimic the structural organization of collagen fibers at the tendon-to-bone insertion site. Tendon fibroblasts cultured on such a scaffold exhibited highly organized and haphazardly oriented morphologies, respectively, on the aligned and random portions.

A Temperature‐Sensitive Drug Release System Based on Phase‐Change Materials

Abstract: Phase-change materials (PCMs), including 1-tetradecanol with a melting point at 38–39 °C and dodecanoic acid with a melting point at 43–46 °C were exploited as thermosensitive materials to demonstrate a new temperature-regulated drug release system. In this approach, colloidal particles containing FITC-dextran were embedded in the PCM matrix and processed as spheres or rods. When temperature was below the melting point of the PCM, there was no release of FITC-dextran due to the hydrophobic nature of the PCM. As the temperature was increased beyond the melting point, the PCM began to melt, the encapsulated particles leached out, and eventually FITC-dextran was released from the colloidal particles. By using colloidal particles made of gelatin, chitosan, and poly(lactic-co-glycolic acid) that have different solubility in water, we could manipulate the release pattern of FITC-dextran. We also demonstrated a dual temperature-regulated drug release system by incorporating two different PCMs into the same device. As an attractive feature, we could easily alter the initiation temperature and release pattern of drugs by judiciously selecting different combinations of PCMs and materials for the colloidal particles.

Engineering the Properties of Metal Nanostructures via Galvanic Replacement Reactions.

Abstract: In this review, we will bring the reader up to date with recent advances in the use of galvanic replacement reactions to engineer highly tunable nanostructures for a variety of applications. We will begin by discussing the variety of templates that have been used for such reactions and how the structural details (e.g., shape, size, and defects, among others) have interesting effects on the ultimate product, beyond serving as a simple site for deposition. This will be followed by a discussion of how we can manipulate the processes of alloying and dealloying to produce novel structures and how the type of precursor affects the final properties. Finally, the interesting optical properties of these materials and some innovative applications in areas of biomedical engineering and catalysis will be discussed, completing our overview of the state of the art in galvanic replacement.

Nucleation and growth mechanisms for Pd-Pt bimetallic nanodendrites and their electrocatalytic properties

Abstract: In a seed-mediated synthesis, nanocrystal growth is often described by assuming the absence of homogeneous nucleation in the solution Here we provide new insights into the nucleation and growth mechanisms underlying the formation of bimetallic nanodendrites that are characterized by a dense array of Pt branches anchored to a Pd nanocrystal core These nanostructures can be easily prepared by a one-step, seeded growth method that involves the reduction of K2PtCl4 by L-ascorbic acid in the presence of 9-nm truncated octahedral Pd seeds in an aqueous solution Transmission electron microscopy (TEM) and high-resolution TEM analyses revealed that both homogeneous and heterogeneous nucleation of Pt occurred at the very early stages of the synthesis and the Pt branches grew through oriented attachment of small Pt particles that had been formed via homogeneous nucleation These new findings contradict the generally accepted mechanism for seeded growth that only involves heterogeneous nucleation and simple growth via atomic addition We have also investigated the electrocatalytic properties of the Pd-Pt nanodendrites for the oxygen reduction and formic acid oxidation reactions by conducting a comparative study with foam-like Pt nanostructures prepared in the absence of Pd seeds under otherwise identical conditions

Targeting gold nanocages to cancer cells for photothermal destruction and drug delivery

Abstract: Importance of the field: Plasmonic nanoparticles provide a new route to treat cancer owing to their ability to convert light into heat effectively for photothermal destruction. Combined with the targeting mechanisms possible with nanoscale materials, this technique has the potential to enable highly targeted therapies to minimize undesirable side effects.Areas covered in this review: This review discusses the use of gold nanocages, a new class of plasmonic nanoparticles, for photothermal applications. Gold nanocages are hollow, porous structures with compact sizes and precisely controlled plasmonic properties and surface chemistry. Also, a recent study of gold nanocages as drug-release carriers by externally controlling the opening and closing of the pores with a smart polymer whose conformation changes at a specific temperature is discussed. Release of the contents can be initiated remotely through near-infrared irradiation. Together, these topics cover the years from 2002 to 2009.What the reader will ga...

Synthesis and characterization of noble-metal nanostructures containing gold nanorods in the center.

Abstract: 2010 WILEY-VCH Verlag Gm Figure 1. A) A schematic image showing the formation of Au@Ag core–shell nanocrystals. B,C) TEM images of (B) the Au nanorods Noble-metal nanocrystals have received increasing interest because they display unique catalytic and optical properties sought for applications such as catalysis, diagnosis, plasmonics, and surface-enhanced Raman spectroscopy (SERS). The catalytic and optical properties of a noble-metal nanocrystal can be tailored by controlling its size, shape, elemental composition, as well as the internal and surface structures. Most recently, special attention has been paid to core–shell, bimetallic nanocrystals because they provide a new system with tunable catalytic and optical properties. In this case, the core–shell structure can be achieved through deposition of a metal on the surface of core made of another metal or via a galvanic replacement reaction between the core and a salt precursor to the second metal. In both cases, the metal nanocrystal serving as the core can have a specific geometric shape and this shape might be able to sustain during the coating or galvanic process. In general, it is a combination of the facets expressed on the core nanocrystal and the degree of lattice mismatch between the two metals that dictates the shape or morphology of the final product. Many attempts have been made to explore Au nanorods as the starting material to construct nanocrystals with a core–shell structure. For example, the ends and the side surface of Au nanorods have been coated with other metals such as Pd, Pt, Ag, Ni, and Au to add new functions to the Au nanorods. Besides coating, other strategies have been reported to generate Au nanocrystals with new geometric shapes by templating against Au nanorods. Specifically, Au nanorods have been transformed into various shapes via overgrowth on the entire surface or the side surface of Au nanorods. However, this strategy has not been widely extended to noble metals other than Au. Only recent studies by Xiang et al. and Becker et al. showed a system, where Ag could grow on specific sides of Au nanorods to generate nanocrystals with a semicircular or triangular morphology. Overall, it remains a challenge to find a simple way to control the overgrowth of other metals on Au nanorods in an effort to generate bimetallic nanocrystals with a core–shell structure and the desired properties. In this Communication, we present a facile method for the synthesis of Ag nanocrystals containing Au nanorods in the center, which will be denoted as Au@Ag for simplicity. Figure 1A shows schematically how the core–shell bimetallic nanocrystals are formed. The final products are mostly octahedrons with a few in other shapes (e.g., decahedrons). Since we used Au nanorods with a uniform size distribution, the core–shell nanocrystals were fairly uniform in terms of both size and shape. In general, the dimensions of the core–shell nanocrystals could be easily tuned by using Au nanorods with different aspect ratios and/or by controlling the amount of AgNO3 added into the reaction system. In a subsequent step, the Au@Ag nanocrystals can also be converted into hollow nanostructures made of Au, Pt, and Pd via galvanic replacement reactions, with Au nanorods encapsulated in the center.

Quantifying the Cellular Uptake of Antibody-Conjugated Au Nanocages by Two-Photon Microscopy and Inductively Coupled Plasma Mass Spectrometry

Abstract: Gold nanocages with localized surface plasmon resonance peaks in the near-infrared region exhibited a broad two-photon photoluminescence band extending from 450 to 650 nm when excited by a Ti:sapphire laser at 800 nm. The bright luminescence makes it possible to explore the use of Au nanocages as a new class of optical imaging agents for two-photon microscopy. In this work, we have demonstrated the use of two-photon microscopy as a convenient tool to directly examine the uptake of antibody-conjugated and PEGylated Au nanocages by U87MGwtEGFR cells. We have also correlated the results from two-photon microscopy with the data obtained by inductively coupled plasma mass spectrometry. Combined together, these results indicate that the antibody-conjugated Au nanocages were attached to the surface of the cells through antibody−antigen binding and then internalized into the cells via receptor-mediated endocytosis. The cellular uptake process was dependent on a number of parameters, including incubation time, inc...

Three-Dimensional Scaffolds for Tissue Engineering: The Importance of Uniformity in Pore Size and Structure

Abstract: To validate the importance of uniformity in pore size and structure of a scaffold for tissue engineering, we fabricated two types of scaffolds with uniform (inverse opal scaffolds) and nonuniform p...

A Sinter-Resistant Catalytic System Based on Platinum Nanoparticles Supported on TiO2 Nanofibers and Covered by Porous Silica

Abstract: Platinum is a key catalyst that is invaluable in many important industrial processes such as CO oxidation in catalytic converters, oxidation and reduction reactions in fuel cells, nitric acid production, and petroleum cracking. Many of these applications utilize Pt nanoparticles supported on oxides or porous carbon. However, in practical applications that involve high temperatures (typically higher than 300 8C), the Pt nanoparticles tend to lose their specific surface area and thus catalytic activity during operation because of sintering. Recent studies have shown that a porous oxide shell can act as a physical barrier to prevent sintering of unsupported metal nanoparticles and, at the same time, provide channels for chemical species to reach the surface of the nanoparticles, thus allowing the catalytic reaction to occur. This concept has been demonstrated in several systems, including Pt@SiO2, [3] Pt@CoO, Pt/CeO2@SiO2, [5] Pd@SiO2, [6] Au@SiO2, [7] Au@SnO2 [8] and Au@ZrO2 [9] core– shell nanostructures. Despite these results, a sinter-resistant system has not been realized in supported Pt nanoparticle catalysts. Improved catalytic or photocatalytic properties are often achieved when metal nanoparticles are supported on oxides such as TiO2 and CeO2 that interact strongly with late transition metals. Herein, we demonstrate a thermally stable catalytic system consisting of Pt nanoparticles that are supported on a TiO2 nanofiber and coated with a porous SiO2 sheath. In this system, the porous SiO2 coating offers an energy barrier to prevent the migration of individual Pt atoms or nanoparticles because of its weak interaction with late transition metals, including Pt. The porous-SiO2/Pt/TiO2 catalytic system was prepared in three steps (Figure 1):