Shuai Hou

Gold nanorods core/AgPt alloy nanodots shell: A novel potent antibacterial nanostructure

AbstractIn the light of the current problems of silver nanoparticles (Ag NPs) in terms of antibacterial performance, we have designed a novel trimetallic core/shell nanostructure with AgPt alloy nanodots epitaxially grown on gold nanorods (Au@PtAg NRs) as a potential antibacterial agent. Both Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were studied. The antibacterial activity exhibits an obvious composition-dependence. On increasing the Ag fraction in the alloy shell up to 80%, the antibacterial activity gradually increases, demonstrating a flexible way to tune this activity. At 80% Ag, the antibacterial activity is better than that of a pure Ag shell. The improved antibacterial ability mainly results from the high exposure of silver on the shell surface due to the dot morphology. We thus demonstrate that forming alloys is an effective way to improve antibacterial activity while retaining high chemical stability for Ag-based nanomaterials. Furthermore, due to the tunable localized surface plasmonic response in the near-infrared (NIR) spectral region, additional control over antibacterial activity using light—such as photothermal killing and phototriggered silver ion release—is expected. As a demonstration, highly enhanced antibacterial activity is shown by utilizing the NIR photothermal effect of the nanostructures. Our results indicate that such tailored nanostructures will find a role in the future fight against bacteria, including the challenge of the increasing severity of multidrug resistance.

Au@PtAg core/shell nanorods: tailoring enzyme-like activities via alloying

Pt nanoparticles (NPs) have been reported to demonstrate four kinds of enzyme-like activity including superoxide dismutase (SOD), catalase, oxidase and peroxidase. Some of these activities interfere with each other. For instance, as antioxidant enzyme mimics, their SOD and catalase activities are very helpful in scavenging related reactive oxygen species (ROSs). However, their oxidase-like and/or peroxidase-like activities may simultaneously oxidize some natural antioxidants, thus compromising the final anti-oxidation efficacy. Fine tuning different enzyme-like activities is therefore very important to realize the optimization of their functions. In this paper, our effort is focused in this direction by tailoring the electronic structure of Pt NPs via alloying with Ag. All four enzyme-like activities are found to be weakened by an increased Ag percentage in the alloy, as witnessed by decreased values of Kcat. The variation in the electronic structure also changes the substrate affinity. Introducing silver into Pt weakens the affinity for H2O2, which affects the limit of detection (LOD) for H2O2 and products with H2O2 involved. In contrast to Fe3O4 MNPs, for peroxidase-like activity, hydroxyl radicals are not involved in the oxidation of chromogenic substrates for the alloy nanostructures.

Core–Shell Noble Metal Nanostructures Templated by Gold Nanorods

The main research progress in core-shell noble metal nanostructures templated by gold nanorods (Au NRs) is summarized regarding synthesis, optical, and catalytic properties. Design and fabrication of core-shell hybrid nanostructures are demonstrated to be effective not only for optimizing and expanding intrinsic properties but also for creating novel localized surface plasmon enhanced optical and catalytic functionalities, thus providing great prospects in both fundamental research and potential applications.

Plasmon Enhancement Effect in Au Gold Nanorods@Cu2O Core–Shell Nanostructures and Their Use in Probing Defect States

Au@Cu2O core-shell nanostructures are fabricated to have a plasmon enhancement effect using Au nanorods (Au NRs) as a plasmon-tailorable core. By varying the concentration of Au NRs, we can tune the shell thickness in the range of 10-25 nm. The shell is composed of Cu2O nanocrystallites. Because of the thin shells, the extinction spectra at wavelength >500 nm are dominated by the Au core. However, the large dielectric constant of the shell causes an obvious red shift of the surface plasmon resonance (SPR) band of the Au nanorod. Besides, transverse octupolar SPR appears as a result of the anisotropy of the core and the high dielectric constant of the shell. The anisotropic geometry of the Au NR is found to support the octupolar resonances at smaller sizes than for their spherical counterpart. Theoretical simulations indicate that the transverse SPR bands are divided into two resonances, which are dipolar- and octupolar-dominant, respectively. The Cu2O shell degrades via a defect-mediated oxidative pathway, which is aggravated upon longitudinal SPR excitation. The SPR-mediated local field enhancement and resonance energy transfer are found to enhance the excitation of the defect states in the shell, thus providing a simple yet selective probing strategy for defect states.

Copper-Ion-Assisted Growth of Gold Nanorods in Seed-Mediated Growth: Significant Narrowing of Size Distribution via Tailoring Reactivity of Seeds

In the well-developed seed-mediated growth of gold nanorods (GNRs), adding the proper amount of Cu(2+) ions in the growth solution leads to significant narrowing in the size distribution of the resultant GNRs, especially for those with shorter aspect ratios (corresponding longitudinal surface plasmon resonance (LSPR) peaks shorter than 750 nm). Cu(2+) ions were found to be able to catalyze the oxidative etching of gold seeds by oxygen, thus mediating subsequent growth kinetics of the GNRs. At proper Cu(2+) concentrations, the size distribution of the original seeds is greatly narrowed via oxidative etching. The etched seeds are highly reactive and grow quickly into desired GNRs with significantly improved size distribution. A similar mechanism can be employed to tune the end cap of the GNRs. Except for copper ions, no observable catalytic effect is observed from other cations presumably due to their lower affinity to oxygen. Considering the widespread use of seed-mediated growth in the morphology-controlled synthesis of noble metal nanostructures, the tailoring in seed reactivity we presented herein could be extended to other systems.

Fabrication of chiral plasmonic oligomers using cysteine-modified gold nanorods as monomers

Generation of circular dichroism (CD) beyond the UV region is of great interest in developing chiral sensors and chiroptical devices. Herein, we demonstrate a simple and versatile method for fabrication of plasmonic oligomers with strong CD response in the visible and near IR spectral range. The oligomers were fabricated by triggering the side-by-side assembly of cysteine-modified gold nanorods. The modified nanorods themselves did not exhibit obvious plasmonic CD signals; however, the oligomers show strong CD bands around the plasmon resonance wavelength. The sign of the CD band was dictated by the chirality of the absorbed cysteine molecules. By adjusting the size of the oligomers, the concentration of chiral molecules, and/or the aspect ratio of the nanorods, the CD intensity and spectral range were readily tunable. Theoretical calculations suggested that CD of the oligomers originated from a slight twist of adjacent nanorods within the oligomer. Therefore, we propose that the adsorbed chiral molecules are able to manipulate the twist angles between the nanorods and thus modulate the CD response of the oligomers.

Peptide-tailored assembling of Au nanorods

By investigating the influence of peptides on the assembling process of Au nanorods induced by 4-mercaptopyridine, two kinds of peptides were identified. The nature of peptides plays an important role in tailoring assembling, which makes potential peptide recognition and detection possible.

Activation of Oxygen-Mediating Pathway Using Copper Ions: Fine-Tuning of Growth Kinetics in Gold Nanorod Overgrowth

Growth kinetics plays an important role in the shape control of nanocrystals (NCs). Herein, we presented a unique way to fine-tune the growth kinetics via oxidative etching activated by copper ions. For the overgrowth of gold nanorods (Au NRs), competitive adsorption of dissolved oxygen on rod surface was found to slow down the overgrowth rate. Copper ions were able to remove the adsorbed oxygen species from the Au surface via oxidative etching, thus exposing more reaction sites for Au deposition. In this way, copper ions facilitated the overgrowth process. Furthermore, Cu(2+) rather than Cu(+) acted as the catalyst for the oxidative etching. Comparative study with Ag(+) indicated that Cu(2+) cannot regulate NC shapes via an underpotential deposition mechanism. In contrast, Ag(+) led to the formation of Au tetrahexahedra (THH) and a slight decrease of the growth rate at similar growth conditions. Combining the distinct roles of the two ions enabled elongated THH to be produced. Copper ions activating the O2 pathway suggested that dissolved oxygen has a strong affinity for the Au surface. Moreover, the results of NC-sensitized singlet oxygen ((1)O2) indicated that the absorbed oxygen species on the surface of Au NCs bounded with low-index facets mainly existed in the form of molecular O2.

Fast characterization of gold nanorods ensemble by correlating its structure with optical extinction spectral features

Owing to unique size- and shape- dependent localized surface plasmon resonance (LSPR) of noble metal nanoparticles (NPs), the optical extinction spectroscopy method (OES) has received much attention to characterize the geometry of metal NPs by fitting experimental UV-vis-NIR spectra. In this work, we aimed to develop a more convenient and accurate OES method to characterize the structural parameters and concentration of the gold nanorods (GNRs) ensemble. The main difference between our approach and previous OES methods is that we solve this inverse spectra problem by establishing the LSPR relation equations of GNRs ensemble so that there is no need of UV-vis-NIR spectra fitting process. The aspect ratio (AR) and AR distribution can be directly retrieved from two of UV-vis-NIR spectral parameters (peak position and full width at half maximum) using the obtained relation equations. Furthermore, the relation equations are modified for applying to the more general GNRs samples by considering the plasmon shift...

L-Cysteine-induced chiroptical activity in assemblies of gold nanorods and its use in ultrasensitive detection of copper ions†

Herein we demonstrated a simple and effective strategy to produce plasmonic optical activity in non-chiral assemblies of gold nanorods (GNRs) via adsorption of L-cysteine (L-Cys). Furthermore, by making use of the catalytic role of Cu2+ in the oxidation of L-Cys by dissolved oxygen, the plasmonic CD intensity can be tuned and used for the detection of copper ions. A dynamic detection range is achieved between 10 pM to 10 nM with a limit of detection (LOD) of 2.6 pM. This strategy offers a simple and ultrasensitive detection of Cu2+ in aqueous solution.