Au NTs Research Articles

Morphological effect on Core@shell AuAg nanoparticles for detecting p‐aminothiophenol dimerization by surface‐enhanced Raman spectroscopy

AbstractA series of non‐spherical metallic Au and bimetallic core@shell AuAg nanoparticles (NPs) have been synthesized for SERS improvements. The bimetallic core@shell AuAg NPs were obtained through a controlled overgrowth of an Ag shell onto the surface of two types of non‐spherical Au NPs, used as seeds, Au nanooctaheda (Au NOc) and Au nanotriangles (Au NTs). This Ag overgrowth was able to produce bimetallic core@shell structures such as nanocubes and nanopyramids, respectively. Transmission electron microscopy (TEM) was used to determine the particle size and particle morphology for metallic and bimetallic NPs, and energy‐dispersive X‐ray (EDX) elemental mapping analysis confirmed the core@shell structure of the bimetallic NPs. The plasmonic absorption bands exhibited for each nanosystem were observed by UV–vis spectroscopy. The concentration of Au and Ag in the bimetallic systems was determined by inductively coupled plasma mass spectrometry (ICP‐MS). After synthesis and characterization, p‐aminothiophenol (PATP) was used as a model analyte to investigate the surface‐enhanced Raman spectroscopy (SERS) capabilities of the synthesized metallic and bimetallic nanosystems. In PATP, a dimerization reaction to 4,4′‐dimercaptoazobenzene (DMAB) is produced when it is adsorbed onto the surface of certain noble metals. This SERS analysis was performed at 10−4 M of PATP and by using two different laser wavelengths (532 and 785 nm) in all cases. In this context, we were able to detect the dimerization reaction of PATP to DMAB only for the bimetallic structures and under the 532 nm laser line. Moreover, we have found that the dimerization capacity also depends on the nanoparticle morphology.

Morphology-Controlled Growth of Crystalline Ag-Pt-Alloyed Shells onto Au Nanotriangles and Their Plasmonic Properties.

The surface plasmon resonance of noble-metal nanoparticles depends on nanoscale size, morphology, and composition, and provides great opportunities for applications in biomedicine, optoelectronics, (photo)catalysis, photovoltaics, and sensing. Here, we present the results of synthesizing ternary metallic or trimetallic nanoparticles, Au nanotriangles (Au NTs) with crystalline Ag-Pt alloyed shells, the morphology of which can be adjusted from a yolk-shell to a core-shell structure by changing the concentration of AgNO3 or the concentration of Au NT seeds, while the shell thickness can be precisely controlled by adjusting the concentration of K2PtCl4. By monitoring the growth process with UV-vis spectra and scanning transmission electron microscopy (STEM), the shells on the Au NT-Ag-Pt yolk-shell nanoparticles were found to grow via a galvanic replacement synergistic route. The plasmonic properties of the as-synthesized nanoparticles were investigated by optical absorbance measurements.

Point-of-Care Biosensing of Urinary Tract Infections Employing Optoplasmonic Surfaces Embedded with Metal Nanotwins.

We report the synthesis of gold nanotwins (Au NTs) on a solid and transparent glass substrate which in turn has been employed for the selective optoplasmonic detection of Escherichia coli (EC) bacteria in human urine for the point-of-care diagnosis of urinary tract infections (UTIs). As compared to the single nanoparticle systems (Au NPs), the Au NTs show an enriched localized surface plasmon resonance (LSPR) due to the enhancement of the electric field under electromagnetic irradiation, e.g., photon, which helps in improving the limits of detection. For this purpose, initially a simple glass surface has been coated with Au NPs, with the help of the linker 3-aminopropyl-triethoxysilane - APTES. The surface has been linked further with another Au NP with the help of the 1,10-alkane-dithiol linker with two thiol ends, which eventually leads to the development of the optoplasmonic surface with Au NTs and an enhanced LSPR response. Subsequently, the EC specific aptamer has been chemically immobilized on the surface of Au NTs with the blocking of free sites via bovine serum albumin (BSA). Remarkably, Raman spectroscopy unfolds a 7-fold increase in the peak intensities with the Au NTs on the glass surface as compared to the surface coated with isolated Au NPs. The enhancement in the LSPR response of glass substrates coated with Au NTs and the EC specific aptamer has been further utilized for the selective and sensitive detection of UTIs. The results have been verified with the help of UV-visible spectroscopy to establish the utility of the proposed sensing methodology. An extensive interference study with other bacterial species unveils the selectivity and specificity of the proposed optoplasmonic sensors toward EC with a detection range of 5 × 103 to 107 CFU/mL. Intuitively, the method is more versatile in a sense that the sensor can be made specific to any other pathogens by simply changing the design of the aptamer. Finally, a low-cost, portable, and point-of-care optoplasmonic transduction setup is designed with a laser light illumination source, a sample holder, and a sensitive photodetector for the detection of UTIs in human urine.

Optimized Biocompatible Gold Nanotriangles with NIR Absorption for Photothermal Applications

Gold nanotriangles (Au NTs) present singular plasmonic properties that are particularly attractive for photothermal applications. However, most syntheses rely on the use of surfactants which present cytotoxic effects, thus limiting the biomedical possibilities. Moreover, these methods have the disadvantage of producing Au NTs that either present absorption at wavelengths <750 nm or have side lengths >200 nm. In this work, we present the post-synthesis optimization and characterization of biocompatible Au NTs produced in a one-step reduction of Au(III) with thiosulfate ions. We demonstrate that the position of the absorption peaks can be fixed at a desired wavelength by the addition of different SH-containing molecules. In all cases, the stabilized Au NTs can be efficiently separated from spherical gold nanoparticles, a byproduct of most Au NT syntheses, by a method based on depletion forces using sodium dodecyl sulphate (SDS), a low-cost, non-toxic surfactant. In addition, we analyze possible cytotoxic effects of surface sulfide species and residual presence of SDS. First, incubation of the Au NTs with H2S-binding hemoglobin I from Lucina pectinata produced no deleterious effect on the protein. Second, we evaluated cytocompatibility by in vitro cellular tests after incubation with the purified Au NTs. Finally, laser irradiation at 808 nm of the Au NT dispersions yields photothermal conversion efficiencies of around 50%, which remained constant after several heating cycles.

A Three-Dimensional Conductive Scaffold Microchip for Effective Capture and Recovery of Circulating Tumor Cells with High Purity.

Effective acquirement of highly pure circulating tumor cells (CTCs) is very important for CTC-related research. However, it is a great challenge since abundant white blood cells (WBCs) are always co-collected with CTCs because of nonspecific bonding or low depletion rate of WBCs in various CTC isolation platforms. Herein, we designed a three-dimensional (3D) conductive scaffold microchip for highly effective capture and electrochemical release of CTCs with high purity. The conductive 3D scaffold was prepared by dense immobilization of gold nanotubes (Au NTs) on porous polydimethylsiloxane and was functionalized with a CTC-specific biomolecule facilitated by a Au-S bond before embedding into a microfluidic device. The spatially distributed 3D macroporous structure compelled cells to change migration from linear to chaotic and the densely covered Au NTs enhanced the topographic interaction between cells and the substrate, thus synergistically improving the CTC capture efficiency. The Au NT-coated 3D scaffold had good electrical conductivity and the Au-S bond was breakable by voltage exposure so that captured CTCs could be specifically released by electrochemical stimulation while nonspecifically bonded WBCs were not responsive to this process, facilitating recovery of CTCs with high purity. The 3D conductive scaffold microchip was successfully applied to obtain highly pure CTCs from cancer patients' blood, benefiting the downstream analysis of CTCs.

Symmetric and asymmetric epitaxial growth of metals (Ag, Pd, and Pt) onto Au nanotriangles: effects of reductants and plasmonic properties.

The surface plasmon resonance of noble metals can be tuned by morphology and composition, offering interesting opportunities for applications in biomedicine, optoelectronics, photocatalysis, photovoltaics, and sensing. Here, we present the results of the symmetrical and asymmetrical overgrowth of metals (Ag, Pd, and Pt) onto triangular Au nanoplates using l-ascorbic acid (AA) and/or salicylic acid (SA) as reductants. By varying the reaction conditions, various types of Au nanotriangle-metal (Au NT-M) hetero-nanostructures were easily prepared. The plasmonic properties of as-synthesized nanoparticles were investigated by a combination of optical absorbance measurements and Finite-Difference Time-Domain (FDTD) simulations. We show that specific use of these reductants enables controlled growth of different metals on Au NTs, yielding different morphologies and allowing manipulation and tuning of the plasmonic properties of bimetallic Au NT-M (Ag, Pd, and Pt) structures.

Stretchable Electrode Based on Au@Pt Nanotube Networks for Real-Time Monitoring of ROS Signaling in Endothelial Mechanotransduction.

Vascular endothelial cells (ECs) are natively exposed to dynamic cyclic stretch and respond to it by the production of vasoactive molecules. Among them, reactive oxygen species (ROS) are closely implicated to the endothelial function and vascular homeostasis. However, the dynamic monitoring of ROS release during endothelial mechanotransduction remains a steep challenge. Herein, we developed a stretchable electrochemical sensor by decoration of uniform and ultrasmall platinum nanoparticles (Pt NPs) on gold nanotube (Au NT) networks (denoted as Au@Pt NTs). The orchestrated structure exhibited prominent electrocatalytic property toward the oxidation of hydrogen peroxide (H2O2) (as the most stable ROS) while maintaining excellent mechanical compliance of Au NT networks. Moreover, the favorable biocompatibility of Au NTs and Pt NPs promoted the adhesion and proliferation of ECs cultured thereon. These allowed in situ inducing ECs mechanotransduction and synchronously real-time monitoring of H2O2 release. Further investigation revealed that the production of H2O2 was positively correlated with the applied mechanical strains and could be boosted by other coexisting pathogenic factors. This indicates the great prospect of our proposed sensor in exploring ROS-related signaling for the deep understanding of cell mechanotransduction and vascular disorder.

Ni/Au bimetal decorated In2O3 nanotubes for ultra-sensitive ethanol detection

In this work, we report on a high-performance ethanol sensor using Au decorated Ni-doped In2O3 nanotubes (NIO-Au NTs). The morphology, microstructure and surface chemistry of pristine In2O3, NIO and NIO-Au NTs have been discussed by the analysis of SEM, TEM, XRD and XPS spectra. Gas sensing test results reveal that NIO-Au NTs with optimal Au decorated concentration of 2 mol% (NIO-2 mol% Au) exhibits the highest response of 167 towards 50 ppm ethanol at lower working temperature (180 °C), which is about 18 times and 13 times higher than pristine In2O3 NTs and Ni-doped In2O3 NTs, respectively. Moreover, some other important gas sensing properties of NIO-2 mol% Au NTs, such as selectivity and stability, have been proved to be greatly improved. The approach and results proposed in this study are attributed to the catalysis of Ni, the spillover effect of Au and formation of Schottky barrier. The proposed functional hybrid nanomaterial-based sensing strategy offers an effective design for highly sensitive and efficient VOC detection devices, which can operate well at low temperatures also.

Facet-Engineered Surfaces of Two-Dimensional Layered BiOI and Au–BiOI Substrates for Tuning the Surface-Enhanced Raman Scattering and Visible Light Photodetector Response

A simple and facile solution phase synthesis of two-dimensional (2D) layered BiOI single-crystalline square sheets at tunable reaction conditions yielded dominant exposed (001) facets. The bismuth oxyiodide (BiOI) structures analyzed by scanning electron microscopy and atomic force microscopy (AFM) reveal screw-dislocation-driven growth of spiral pyramid-like layer-by-layer (LBL) stacking of ultrathin BiOI sheet of about 2.5 nm. Vertically aligned, oriented Au nanotriangles (Au NTs) allow more analyte molecules to access their sharp tips to form hot-spots thereby enabling development of ultrasensitive plasmonic sensors. Injection of plasmonic hot carriers into the semiconductor at the plasmonic metal–semiconductor interface (Au–BiOI) fabricated under optimal conditions with Au NTs form tunable surface enhanced Raman scattering substrate with maximum enhancement factor on the order of 106 and 27 nanomol as a limit of detection. Electrical transport measurements on BiOI and Au–BiOI LBL 2D square sheet materials under applied pressure using conducting-AFM established the use of these nanosheets as a nanoscale pressure sensor. Photodetector studies show good photoresposnsivity, reproducibility, and fast photoresponse time indicating that the materials is a promising candidate for visible light-photodetector using 454 and 535 nm led lights.

Developing gold nanotubes as photoacoustic contrast agents

Photoacoustic imaging (PAI) combines the spectral selectivity of laser excitation with the high resolution of ultrasound imaging, representing a noninvasive modality that holds remarkable potential for clinical translation. In particular, multispectral optoacoustic tomography has emerged has an innovative PAI technology, which enables high detection specificity by resolving multiple spectral signatures through tissues and accurately decomposing the biodistribution of relevant molecules from non-specific background contributions. Key criteria for developing PAI contrast agents include strong absorbance in the near-infrared (NIR) tissue transparent region and low toxicity. In this study, we have fabricated gold nanotubes (Au NTs) with controlled length and NIR absorption for the application as in vivo MSOT contrast agents. The length control relies on the synthesis of silver nanorod template under certain growth conditions. The Au NTs were coated with a layer of biocompatible polymer, poly(sodium 4-styrenesulfonate) (PSS), to ensure colloidal stability and low cytotoxicity. The PSS-Au NTs have shown excellent MSOT signal enhancement on tissue-mimicking phantom and in vivo, demonstrating the potential for further development towards a theranostic nanoplatform integrating PAI capability and therapeutic functions.

Gold Nanotriangles as Selective Catalysts for Cyclohexanol and Cyclohexanone Production

The search for sustainable catalytic oxidation processes remains a challenge. One process of utmost industrial and economic importance is the selective oxidation of cyclohexane, in the route of nylon-6,6 production, which requires urgent improvement. Herein, Au nanotriangles (Au NTs) were prepared following a three-step (seed preparation, growth and shaping) procedure and applied, for the first time, as catalysts for the selective oxidation of neat cyclohexane to ketone and alcohol (KA) oil (cyclohexanol and cyclohexanone mixture). The Au NTs successfully yield KA oil (up to 14%) under mild conditions (50 °C), using an alternative energy source (microwave irradiation) as reaction promotor.

Flexible Electrochemical Urea Sensor Based on Surface Molecularly Imprinted Nanotubes for Detection of Human Sweat.

Flexible electrochemical (EC) sensors have shown great prospect in epidermal detection for personal healthcare and disease diagnosis. However, no reports have been seen in flexible device for urea analysis in body fluids. Herein, we developed a flexible wearable EC sensor based on surface molecularly imprinted nanotubes for noninvasive urea monitoring with high selectivity in human sweat. The flexible EC sensor was prepared by electropolymerization of 3,4-ethylenedioxythiophene (EDOT) monomer on the hierarchical network of carbon nanotubes (CNTs) and gold nanotubes (Au NTs) to imprint template molecule urea. This sensor exhibited a good linear response toward physiologically relevant urea levels with negligible interferences from common coexisting species. Bending test revealed that this sensor possessed excellent mechanical tolerance and its EC performance was almost not affected by bending deformation. On-body results of human subjects showed that the flexible platform could distinctly respond to the urea levels in volunteer's sweat after aerobic exercise. The new flexible epidermal EC sensor can provide useful insights into noninvasive monitoring of urea levels in various biofluids, which is promising in the clinical diagnosis of diverse biomedical applications.

Analysis of metallic nanoparticles embedded in thin film semiconductors for optoelectronic applications

This paper reports about a study of the local plasmonic resonance (LSPR) produced by metal nanoparticles embedded in a dielectric or semiconductor matrix. It is presented an analysis of the LSPR for different nanoparticle metals, shapes, and embedding media composition. Metals of interest for nanoparticle composition are Aluminum and Gold. Shapes of interest are nanospheres and nanotriangles. We study in this work the optical properties of metal nanoparticles diluted in water or embedded in amorphous silicon, ITO and ZnO as a function of size, aspect-ratio and metal type. Following the analysis based on the exact solution of the Mie theory and DDSCAT numerical simulations, it is presented a comparison with experimental measurements realized with arrays of metal nanospheres. Simulations are also compared with the LSPR produced by gold nanotriangles (Au NTs) that were chemically produced and characterized by microscope and optical measurements.

Optical and biomedical properties of eco-friendly metal nanostructures synthesized using Trigonella foenum-graecum leaf extract

Biogenic, eco-friendly, reliable, and cost-effective metal nanostructures were synthesized using Trigonella foenum-graecum leaf extract, which acted as reducing and stabilizing agent with no additional chemical surfactants. Nanoparticle formation was confirmed from UV–Vis absorption, XRD, and FTIR spectra. Morphology and crystallinity of spherical silver nanoparticles (Ag NPs) and gold nanoparticles (Au NTs) were further confirmed from TEM analysis. Luminescence enhancement and quenching of lanthanide ion complexes were observed and found to be dependent on metal nanostructure concentration. The factors relay on the energy transfers between nanostructures and rare-earth ions (RE). The nonlinear optical studies of Ag NPs and Au NTs were studied using Z-scan and degenerate four wave mixing (DFWM) techniques with picosecond and femtosecond lasers. The surface-enhanced Raman scattering (SERS) of the test molecule, rhodamine 6G, was studied to check the performance of Ag NPs and Au NTs in enhancing the Raman scattering signals. In vitro cytotoxic effect of the Ag NPs and Au NTs has been checked against A549 lung cancer cell lines and the efficiency of toxicity depended on the dosage. These experiments demonstrate that the nanostructures are ideal candidates for applications in optics and biology as PL based devices with appropriate concentrations of RE ions, optical switches, SERS substrates, and therapeutic agents towards cancer cell lines.

(Invited) Stretchable Electrochemical Sensor for Inducing and Monitoring Cell Mechanotransduction in Real-Time

Nearly all kinds of cells within organisms are sensitive to mechanical forces and convert them into specific biochemical responses. This important and sophisticated process is described as mechanotransduction1. Several approaches are classically used to report on the ensuing modifications (gene expression, protein activity, etc.) expressed by cells in response to mechanical stimulations. Nonetheless, acquiring such evidences requires at least tens of minutes preventing any real-time information about the primary biochemical signals underlying mechanotransduction. Monitoring such presumably transient and weak signaling events since very early stages of cell mechanotransduction remains therefore an important challenge. Electrochemical sensing for detecting and quantifying transient release of biochemical molecules by living cells or tissues is widely accepted nowadays. In this regard, flexible and stretchable biocompatible electrochemical sensors should offer ideal platforms for applying mechanical strains and monitoring at the same time electroactive biochemical responses from mechanosensitive cells directly cultured on the sensor. However, up to date, despite several stimulating progresses in stretchable physical sensors and very few emerging successes in wearable electrochemical devices, no breakthrough concept has emerged for inducing cell mechanotransduction and investigating it in real-time by a single electrochemical device. Recently, we reported a high-performance stretchable electrode based on networks of gold nanotubes (Au NTs) deposited on polydimethylsiloxane (PDMS) thin films. Interlacing network of Au NTs endows the sensor with desirable stability against mechanical deformation, and Au nanostructure provides excellent electrochemical performance and biocompatibility. This allowed for the first time, real-time electrochemical monitoring of mechanically sensitive cells on the sensor both in their stretching-free and stretching states as well as sensing of the inner lining of blood vessels.2 However, the sensitivity and stability of the Au NTs/PDMS electrode were not sufficient to detect very weak transient signals triggered from cells by stretching strains only. This limitation was further overcomed by implementing a percolating CNTs network onto the Au NTs backbones through transfer of a free-standing CNTs ultrathin-film onto the Au NTs/PDMS electrode. This hybrid nano-structure provides the sensor with greatly enhanced mechanical and electrochemical performances while granting very good cellular compatibility, which allowed monitoring stretch-induced transient release of vasoactive molecules by endothelial cells cultured on this sensor and submitted to stretching strains. The work represents the first step toward real-time monitoring of cell mechanotransduction by electrochemical techniques, providing a promising way toward many other direct, in real time and in situ measurements of cell mechanotransduction.

Ultrafast synthesis of Au(I)-dodecanethiolate nanotubes for advanced Hg(2+) sensor electrodes.

In this work, an ultrafast and facile method is developed to synthesize Au(I)-dodecanethiolate nanotubes (Au(I)NTs) with the assistance of glycyl-glycyl-glycine (G-G-G). Transmission electron microscopy (TEM) images reveal that the as-prepared Au(I)NTs can be obtained in a 2-h reaction instead of a previous 24-h reaction and are uniform with a hollow structure and smooth surface by virtue of the G-G-G peptide tubular template. According to structural analysis, a possible preparative mechanism is proposed that the G-G-G peptide could help to curl into tube-like morphology in alkaline situation spontaneously to accelerate the formation of Au(I)NTs. Meanwhile, PVDF-stabilized Au(I)NT-modified glassy carbon electrodes present their promising potential for Hg2+ detection.

In vivo ultrasound and photoacoustic monitoring of mesenchymal stem cells labeled with gold nanotracers.

Longitudinal monitoring of cells is required in order to understand the role of delivered stem cells in therapeutic neovascularization. However, there is not an imaging technique that is capable of quantitative, longitudinal assessment of stem cell behaviors with high spatial resolution and sufficient penetration depth. In this study, in vivo and in vitro experiments were performed to demonstrate the efficacy of ultrasound-guided photoacoustic (US/PA) imaging to monitor mesenchymal stem cells (MSCs) labeled with gold nanotracers (Au NTs). The Au NT labeled MSCs, injected intramuscularly in the lower limb of the Lewis rat, were detected and spatially resolved. Furthermore, our quantitative in vitro cell studies indicate that US/PA imaging is capable of high detection sensitivity (1×104 cells/mL) of the Au NT labeled MSCs. Finally, Au NT labeled MSCs captured in the PEGylated fibrin gel system were imaged in vivo, as well as in vitro, over a one week time period, suggesting that longitudinal cell tracking using US/PA imaging is possible. Overall, Au NT labeling of MSCs and US/PA imaging can be an alternative approach in stem cell imaging capable of noninvasive, sensitive, quantitative, longitudinal assessment of stem cell behaviors with high spatial and temporal resolutions at sufficient depths.