Fractal Nanostructures Research Articles

In situ self-assembled bismuth oxide fractals enabling highly selective electrosynthesis of formate in flow cells at high current densities

In this work, we present a scalable in-situ growth strategy for self-assembling a carbon felt supported Bi2O3 fractal nanostructure (f-Bi2O3/CF), enabling superior performance towards electrocatalytic CO2-to-formate conversion with an optimal Faradaic efficiency of 95.1% at −0.9 V (vs. RHE) in flow cell. More impressively, the current density reaches an industrially relevant 235.7 mA cm−2 and remains a high FEformate of 87.7%. The enhanced catalytic activity of the f-Bi2O3/CF model is attributed to the increased adsorption energy of OCHO* intermediate by changing the surface charge distribution according to DFT calculations. Using the aforementioned method, a series of other CF supported metal oxide catalysts (i.e., CuO, In2O3, and SnO2) are easily obtained. A brief techno-economic analysis (TEA) model demonstrates the economic potential of electrocatalytic hydrogenation of CO2 into formate in flow cell. Our findings offer a promising pathway for the fabrication of stable and accessible fractal structure, applicable to a wide range of scenarios.

Fractal Design of Hierarchical PtPd with Enhanced Exposed Surface Atoms for Highly Catalytic Activity and Stability.

Hierarchical assembly of arc-like fractal nanostructures not only has its unique self-similarity feature for stability enhancement but also possesses the structural advantages of highly exposed surface-active sites for activity enhancement, remaining a great challenge for high-performance metallic nanocatalyst design. Herein, we report a facile strategy to synthesize a novel arc-like hierarchical fractal structure of PtPd bimetallic nanoparticles (h-PtPd) by using pyridinium-type ionic liquids as the structure-directing agent. Growth mechanisms of the arc-like nanostructured PtPd nanoparticles have been fully studied, and precise control of the particle sizes and pore sizes has been achieved. Due to the structural features, such as size control by self-similarity growth of subunits, structural stability by nanofusion of subunits, and increased numbers of exposed active atoms by the curved homoepitaxial growth, h-PtPd displays outstanding electrocatalytic activity toward oxygen reduction reaction and excellent stability during hydrothermal treatment and catalytic process.

Engineering SERS Properties of Silicon Nanotrees at the Nanoscale

Large specific surface area nanostructures are desirable in a wide range of sensing applications due to their longer light-trapping path and increased absorption. Engineering of the specific nanotree structure which possesses a high branch density turned out to be challenging from the experimental point of view, and certainly not adequately explored. This paper shows how to design substrates with a silicon nanotree structure for surface-enhanced Raman spectroscopy (SERS) applications. Silicon nanotrees were synthesized by a Ag-Au nanocluster-catalyzed low-pressure chemical vapor deposition method (LPCVD). By the presented approaches, it is possible to manipulate branches’ number, length and thickness. The synthesized nanostructures are flexible after immersion in water which improves SERS performance. The amount of sputtered metal played a key role in preserving the flexibility of the nanotree structure. The obtained substrates with highly fractal nanostructure were tested on 4-mercaptophenylboronic acid (MPBA) to match the optimal SERS parameters. The silicon nanotrees fabrication, and particularly obtained SERS substrates plated with Ag and Au nanoparticles, demonstrated good features and a promising approach for further sensor development.

Elucidating Reaction Pathways of the CO2 Electroreduction via Tailorable Tortuosities and Oxidation States of Cu Nanostructures

AbstractCopper‐based 3D fractal nanostructures are integrated on the electrodes using a scalable and ink‐free flame aerosol synthesis technique for electrochemical CO2 reduction. The effects of tortuosity and oxidation state of copper are respectively investigated by isolating each effect from the others. By balancing the intermediate confinement and local availability of CO2, CuO‐derived Cu with optimal tortuosity exhibits a Faradaic efficiency of 65% toward C2+ products at an applied potential of −1.04 V versus reversible hydrogen electrode. A subsequent study of the effects of the oxidation state, which is free from the influence of tortuosity, reveals that Cu2+‐derived Cu demonstrates suppressed hydrogen evolution reaction and a higher C2+/CH4 ratio than metallic Cu. The preference for the formation of both ethanol and n‐propanol versus ethylene, is found to follow the trend from metallic Cu > Cu2+‐derived Cu > Cu+‐derived Cu toward alcohols’ formation. These findings elucidate the underlying causes for the effects of tortuosity of porous Cu electrodes on selectivity and provide insights into the specific effects of the initial oxidation state on various reaction pathways during electrochemical CO2 reduction.

Multispectral Optical Confusion System: Visible to Infrared Coloration with Fractal Nanostructures.

Optical confusion refers to a camouflage technique assimilated with the surroundings through manipulating colors and patterns. With the advances in multispectral imagery detection systems, multispectral camouflage studies on simultaneous deceptions in the visible to infrared ranges remain a key challenge. Thus, creating pixelated patterns is essential for mimicking background signatures by assimilating both colors and patterns. In this study, a multispectral optical confusion system (MOCS) comprising pixelated silicon-based fractal nanostructures (Si-FNSs) is introduced to realize multispectral optical confusion. We analyzed the fractality of the Si-FNSs to understand the relationships between structural characteristics and optical properties with the aggregation phenomenon. The aggregation phenomenon changes the morphological heterogeneity by up to 38.5%, enabling a controllable range of visible reflectivity from 0.01 to 0.12 and infrared emissivity from 0.33 to 0.90. Visible and infrared colors were obtained by controlling the wet-etching time from 10 to 240 min and temperature from 40 to 100 °C. Finally, the MOCS consisting of pixelated Si-FNSs was designed and created by extracting the pattern from the simultaneously captured visible and infrared background images. Using the artificial backgrounds representing these images, we evaluated and compared the multispectral optical confusion performance of the MOCS with conventional camouflage surfaces.

Springback effect and structural features during the drying of silica aerogels tracked by in-situ synchrotron X-ray scattering

The springback effect during ambient pressure drying of aerogels is an interesting structural phenomenon, consisting of a severe shrinkage followed by almost complete re-expansion. The drying of gels causes shrinkage, whereas re-expansion is believed to be linked to repelling forces on the nanoscale. A multi-scale structural characterization of this significant volume change is key in controlling aerogel processing and properties. In this work, hydrophobic, monolithic silica aerogels with high specific surface areas were synthesized by modification with trimethylchlorosilane and ambient pressure drying. Here, we report a multi-method approach focusing on in-situ X-ray scattering to observe alterations of the nanostructured material during the drying of surface-modified and unmodified silica gels. Both show a porous fractal nanostructure, which partially collapses during drying and only recovers in surface-modified samples during the springback effect. Distinct changes of the X-ray scattering data were reproducibly associated with the shrinkage, re-expansion and drying of the gel network. Our findings may contribute to tailor aerogels with specific functionality, as the springback effect has a direct influence on properties (e.g., porosity, pore size distribution), which is directly affected by the degree of re-expansion.

Electronic fractal patterns in building Sierpinski-triangle molecular systems

Fractal features in the electronic properties of Sierpinski triangle flakes and molecular chains are explored simulating experimental synthesized fractal nanostructures.

Achieving geometrical enhancement of fields in chiral nanoplasmonics using fractional calculus

In this paper, an analytical approach has been developed for electromagnetic field enhancement in fractal nanostructure under the quasi-static limit. A metallic fractal embedded in an unbounded, isotropic, and lossless chiral medium has been treated under this approach. In addition, the field enhancement is also noted by imposing surface plasmon resonance condition. To achieve the desired objective, Maxwell divergence equations are considered and written in an appropriate form by mean of fractional calculus.

Reversible capturing and voltammetric determination of circulating tumor cells using two-dimensional nanozyme based on PdMo decorated with gold nanoparticles and aptamer.

Anovel cytosensor was constructed for the ultrasensitive detection and nondestructive release of circulating tumor cells (CTCs) by combining Au nanoparticles-loaded two-dimensional bimetallic PdMo (2D Au@PdMo) nanozymes and electrochemical reductive desorption. The 2D Au@PdMo nanozymes possessed high-efficiency peroxidase-like activity and were assembled with an aptamer composed of a thiol-modified epithelial specific cell adhesion molecule (EpCAM) to strengthen CTCs adhesion. Moreover, the electrode surface was decorated with highly fractal Au nanostructures (HFAuNSs) composites due to the similarity in fractal nanostructure with the CTCs membrane to enhance the CTCs anchoring efficiency and release capability. The captured CTCs could be further efficiently dissociated and nondestructively released from the modified electrodes upon electrochemical reductive desorption. The designed cytosensor showed an excellent analytical performance, with a wide linear range from 2 to 1 × 105 cells mL-1 and low limit of detection (LOD) of 2 cells mL-1 (S/N= 3) at the working potential in the range -0.6 to 0.2V. Asatisfactory CTCs release reaching a range of 93.7-97.4% with acceptable RSD from 3.55 to 6.41% and good cell viability was obtained. Thus, the developed cytosensor might provide a potential alternative to perform CTC-based liquid biopsies, with promising applications in early diagnosis of tumors. Preparation and mechanism of desorption of the cytosensor based on 2D Au@PdMo nanozymes and electrochemical reductive desorption for the detection and release of CTCs. A Preparation procedure of the Apt/Au@PbMo bioconjugates. B Fabrication process of the sandwich-type cytosensor. C Electrochemical signal produced by the Au@PdMo nanozymes. D Mechanism of electrochemical reductive desorption for CTCs release.

Detection and quantification of toxicants in food and water using Ag–Au core-shell fractal SERS nanostructures and multivariate analysis

Here we demonstrate a facile, two step formation of silver core - gold shell (Ag–Au) nanostructures using microelectrodes and assess their performance as surface-enhanced Raman scattering (SERS) substrates to detect and quantify toxicants. Ag nanostructures, serving as the scaffolds for the bimetallic structures, were grown first by using electrochemical deposition on the edges of microelectrodes functionalized with the alkanethiol, 11-mercaptoundecanoic acid. Subsequently, different concentrations of HAuCl4 were used to perform a galvanic reaction on the surfaces of the Ag nanostructures with aqueous droplets being placed on the microelectrodes for 10 min before the substrate was rinsed and dried. Lower HAuCl4 concentrations were found to better preserve the fractal morphology of the formed Ag–Au nanostructures, while higher concentrations resulted in Ag–Au fragments. The SERS enhancement factor for the Ag–Au nanostructures was estimated to have a max value of 6.51 x 105. Combining a data reduction technique with a linear classifier, both identification and quantification were demonstrated with 100% success. The toxicants thiram, thiabendazole, malachite green and biphenyl-4-thiol were all detected and identified at 1 ppm. Lastly, as a proof of concept, the Ag–Au nanostructures were transferred to a PDMS film resulting in a flexible SERS substrate capable of direct detection of thiram on an apple peel without any additional sample pre-treatment.

In Situ Assembly of Gold Nanoparticles in the Presence of Poly-DADMAC Resulting in Hierarchical and Highly Fractal Nanostructures

The presence of the polycationic macromolecule poly(diallyldimethylammonium chloride) (poly-DADMAC) has a strong effect on the shape and size of colloidal gold nanoparticles formed by the reduction of tetrachloroauric acid with ascorbic acid in aqueous solution. It slows down nanoparticle growth and supports the formation of nonspherical, partially highly fractal and hierarchical nanoparticle shapes. Four structural levels have been recognized from the near-spherical gold nanoparticles in the lower nanometer range over compact aggregates in the midnanometer range and flower and star-like particles in the submicron range up to larger filamentous aggregates. High-contrast scanning electron microscope (SEM) images show that single gold nanoparticles and clusters of them are connected by bundles of macromolecules in large aggregates. The investigation showed that a large spectrum of different nanoparticle shapes and sizes can be accessed by tuning the poly-DADMAC concentrations and their ratio to other reactants. The nanoassemblies with a very high specific surface area might be of interest for SERS and heterogeneous catalysis.

Thermodynamic Principles of Precipitation Polymerization and Role of Fractal Nanostructures in the Particle Size Control

The effect of the solvent on the morphology and the particle size of the polymer materials obtained by precipitation polymerization is not yet understood from a physical chemical perspective. Clari...

Nanotopographical Coatings Induce an Early Phenotype-Specific Response of Primary Material-Resident M1 and M2 Macrophages.

Implants elicit an immunological response after implantation that results in the worst case in a complete implant rejection. This biomaterial-induced inflammation is modulated by macrophages and can be influenced by nanotopographical surface structures such as titania nanotubes or fractal titanium nitride (TiN) surfaces. However, their specific impact on a distinct macrophage phenotype has not been identified. By using two different levels of nanostructures and smooth samples as controls, the influence of tubular TiO2 and fractal TiN nanostructures on primary human macrophages with M1 or M2-phenotype was investigated. Therefore, nanotopographical coatings were either, directly generated by physical vapor deposition (PVD) or by electrochemical anodization of titanium PVD coatings. The cellular response of macrophages was quantitatively assessed to demonstrate a difference in biocompatibility of nanotubes in respect to human M1 and M2-macrophages. Depending on the tube diameter of the nanotubular surfaces, low cell numbers and impaired cellular activity, was detected for M2-macrophages, whereas the impact of nanotubes on M1-polarized macrophages was negligible. Importantly, we could confirm this phenotypic response on the fractal TiN surfaces. The results indicate that the investigated topographies specifically impact the macrophage M2-subtype that modulates the formation of the fibrotic capsule and the long-term response to an implant.

Tunable Fractal Nanostructures for Surface-Enhanced Raman Scattering via Templated Electrodeposition of Silver on Low-Energy Surfaces

Here, we demonstrated a facile method for growing nanofeatured silver (Ag) structures on a planar microelectrode platform. The nanostructures were assembled along the electrically insulating substrate, growing from the microelectrode edge with the electrodes acting as the template. This was done by (a) limiting the nucleation to the edges of the electrodes and (b) chemically functionalizing the insulating substrate to promote an interaction between itself and the growing metallic structures. These structures were able to reach lateral lengths greater than 100 μm. Additionally, we performed an extended investigation into how the nanostructure morphology can be changed from spherical aggregates to fractal dendrites by altering the electrical signal and solution composition. We determined citrate to be a necessary additive and showed that it acts as a supporting electrolyte, capping agent, and oxidizing/complexing agent. Next, using optimal deposition conditions, we assembled Ag surface-enhanced Raman scattering (SERS) substrates with an enhancement factor up to 4.55 × 106. Finally, we demonstrated how these SERS nanostructures can also serve as concentration amplification devices, accelerating analyte deposition onto the detection site by means of electrohydrodynamics.

Hierarchical Nanostructuring Array Enhances Mid-Hybridization for Accurate Herbal Identification via ITS2 DNA Barcode.

It remains a technical challenge to accurately identify close species of herbal medicines, especially from adulterants, because of their highly identical phenotypes and chemical compositions. Here, we report a direct, sequencing-free, high-curvature nanostructuring-based electrochemical herb sensor (nanoE-herb sensor) to identify herbal species quickly and accurately using ITS2 barcodes. We engineer a nano-roughened carbon-supported gold nanostructuring array by photolithograph-free, one-step electrodeposition. The 3D fractal nanostructures exhibit a high deflection angle that largely enhances DNA hybridization efficiency, particularly for the midcomplementary hybridization, as compared to the 2D planar surface. More importantly, such a trans-scale array biointerface (including macroscale carbon and nanoscale gold branches) can overcome the detection barrier of slow diffusion of a long genomic sequence and inaccessibility of the sequestered variations in ITS2 secondary structures through the out-protruded 3D functional nanostructures. Our nanoE-herb sensor achieves a detection limit of 0.18 fM for the 64-mer fragment of saffron ITS2 barcode with midhybridization and shows superior specificity against even single-base mismatch. The sensor also precisely differentiates saffron from six other adulterants by directly detecting unpurified asymmetric PCR amplicons (∼500 bp) with ITS2 sequences, suggesting its great potential in the field identification of herbal medicinal species and pathogenic bacteria with specific DNA barcodes.

Nanostructured β‐Bi2O3 Fractals on Carbon Fibers for Highly Selective CO2 Electroreduction to Formate

Abstract3D Bi2O3 fractal nanostructures (f‐Bi2O3) are directly self‐assembled on carbon fiber papers (CFP) using a scalable hot‐aerosol synthesis strategy. This approach provides high versatility in modulating the physiochemical properties of the Bi2O3 catalyst by a tailorable control of its crystalline size, loading, electron density as well as providing exposed stacking of the nanomaterials on the porous CFP substrate. As a result, when tested for electrochemical CO2 reduction reactions (CO2RR), these f‐Bi2O3 electrodes demonstrate superior conversion of CO2 to formate (HCOO−) with low onset overpotential and a high mass‐specific formate partial current density of −52.2 mA mg−1, which is ≈3 times higher than that of the drop‐casted control Bi2O3 catalyst (−15.5 mA mg−1), and a high Faradaic efficiency (FEHCOO−) of 87% at an applied potential of −1.2 V versus reversible hydrogen electrode. The findings reveal that the high exposure of roughened β‐phase Bi2O3/Bi edges and the improved electron density of these fractal structures are key contributors in attainment of high CO2RR activity.

Preparation of silver fractal nanostructures and their application in surface enhanced Raman scattering

Abstract Three kinds of silver fractal nanostructures were prepared by replacement reaction method using copper, tin, aluminum foils and AgNO3 solution. The dentritic, flower bud and shoot bud silver nanostructures had self-similar structural features. Meanwhile, these silver fractal nanostructures had the same structure characters as proper plants in actual biological systems. Furthermore, these silver fractal nanostructures exhibit high surface enhanced Raman scattering (SERS) activity.

Sierpinski gasket-like Pt–Ag octahedral alloy nanocrystals with enhanced electrocatalytic activity and stability

Architectural engineering of noble metal nanocrystals can result in structural diversity and complexity, thereby providing catalysts with multifunctional properties. Herein, a unique Sierpinski gasket-like Pt–Ag octahedral alloy nanostructure with a three-dimensional hyperbranched architecture containing abundant well-defined Pt-rich {111} stable facets and dense low-coordinated active sites is reported. Unlike the well-accepted long-range limiting diffusion govened growth model for fractal structures, our success relies on creating local depletion layer near the crystal surface by surfactant. The as-prepared Sierpinski gasket-like Pt–Ag octahedral nanocrystals allows us to achieve three milestones for electrocatalysts, i.e. high catalytic activity, great durability, and stability towards the methanol oxidation reaction in acidic media. Specifically, the catalyst shows a high specific activity (6.61 mA cm−2, is superior than most of reported Pt-based catalysts), outstanding COads-poisoning tolerance and stability (the morphology and composition of the catalyst are preserved after 2000 cycles). This study points to a new approach to develop highly efficient catalysts in the form of fractal nanostructures with structural diversity and complexity to balance the delicate trade-off between activity and stability of catalysts.

Fabrication of silver dendrite fractal structures for enhanced second harmonic generation and surface-enhanced Raman scattering

Here, we prepared silver (Ag) dendrite fractal nanostructures by a facile electrochemical deposition method. The silver dendrite fractal nanostructures exhibit multiple plasmon resonances, which leads to an enhanced second harmonic generation (SHG). Interestingly, the prepared thin film can be used as plasmonic substrates. As an example application, we selected surface-enhanced Raman scattering (SERS) spectroscopy with tunable sensitivity. Using 1,4-benzenedithiol (1,4-BDT) as a probe molecule, the SERS intensity of dendrite fractal nanostructures is approximately 77 times larger than that of the flower-like nanoplates (one of the structures that appears during the growth), and a low detection limit of 10−14 M 1,4-BDT can be achieved. Our results offer a low-cost and easy strategy in fabricating nonlinear photonic nanodevices and SERS chips.

Advancements in fractal plasmonics: structures, optical properties, and applications.

The structural characteristics of plasmonic nanostructures directly influence their plasmonic properties, and therefore, their potential role in applications ranging from sensing and catalysis to light- and energy-harvesting. For a structure to be compatible with a selected application, it is critical to accurately tune the plasmonic properties over a specific spectral range. Fabricating structures that meet these stringent requirements remains a significant challenge as plasmon resonances are generally narrow with respect to the considered selected spectral range. Adapted from their well-established role in GHz applications, plasmonic fractal structures have emerged as architectures of interest due to their ability to support multiple tunable resonances over broad spectral domains. Here, we review the advancements that have been made in the growing field of fractal plasmonics. Iterative and space-filling geometries that can be prepared by advanced nanofabrication techniques, notably electron-beam lithography, are presented along with the optical properties of such structures and metasurfaces. The distributions of electromagnetic enhancement for some of these fractals is shown, along with how the resonances can be mapped experimentally. This review also explores how fractal structures can be used for applications in solar cell and plasmon-based sensing applications. Finally, the future areas of physical and analytical science that could benefit from fractal plasmonics are discussed.