Optimization of Gold Nanorod Features for the Enhanced Performance of Plasmonic Nanocavity Arrays.

Nanoplasmonic biosensors incorporating noble metal nanocavity arrays are widely used for the detection of various biomarkers. Gold nanorods (GNRs) have unique properties that can enhance spectroscopic detection capabilities of such nanocavity-based biosensors. However, the contribution of the physical properties of multiple GNRs to resonance enhancement of gold nanocavity arrays requires further characterization and elucidation. In this work, we study how GNR aspect ratio (AR) and surface area (SA) modify the plasmonic resonance spectrum of a gold triangular nanocavity array by both simulations and experiments. The finite integration technique (FIT) simulated the extinction spectrum of the gold nanocavity array with 300 nm periodicity onto which the GNRs of different ARs and SAs are placed. Simulations showed that matching of the GNRs longitudinal peak, which is affected by AR, to the nanocavity array's spectrum minima can optimize signal suppression and shifting. Moreover, increasing SA of the matched GNRs increased the spectral variations of the array. Experiments confirmed that GNRs conjugated to a gold triangular nanocavity array of 300 nm periodicity caused spectrum suppression and redshift. Our findings demonstrate that tailoring of the GNR AR and SA parameters to nanoplasmonic arrays has the potential to greatly improve spectral variations for enhanced plasmonic biosensing.

Second harmonic generation hotspot on a centrosymmetric smooth silver surface.

Second harmonic generation (SHG) is forbidden for materials with inversion symmetry, such as bulk metals. Symmetry can be broken by morphological or dielectric discontinuities, yet SHG from a smooth continuous metallic surface is negligible. Using non-linear microscopy, we experimentally demonstrate enhanced SHG within an area of smooth silver film surrounded by nanocavities. Nanocavity-assisted SHG is locally enhanced by more than one order of magnitude compared to a neighboring silver surface area. Linear optical measurements and cathodoluminescence (CL) imaging substantiate these observations. We suggest that plasmonic modes launched from the edges of the nanocavities propagate onto the smooth silver film and annihilate, locally generating SHG. In addition, we show that these hotspots can be dynamically controlled in intensity and location by altering the polarization of the incoming field. Our results show that switchable nonlinear hotspots can be generated on smooth metallic films, with important applications in photocatalysis, single-molecule spectroscopy and non-linear surface imaging.

Hybridization between nanocavities for a polarimetric color sorter at the sub-micron scale.

Metallic hole arrays have been recently used for color generation and filtering due to their reliability and color tunability. However, color generation is still limited to several microns. Understanding the interaction between the individual elements of the whole nanostructure may push the resolution to the sub-micron level. Herein, we study the hybridization between silver nanocavities in order to obtain active color generation at the micron scale. To do so, we use five identical triangular cavities which are separated by hundreds of nanometers from each other. By tuning either the distance between the cavities or the optical polarization state of the incoming field, the transmitted light through the cavities is actively enhanced at specific frequencies. Consequently, a rainbow of colors is observed from a sub-micron scale unit. The reason for this is that the metallic surface plays a vital role in the hybridization between the cavities and contributes to higher frequency modes. Cathodoluminescence measurements have confirmed this assumption and have revealed that these five triangular cavities act as a unified entity surrounded by the propagated surface plasmons. In such plasmonic structures, multi-color tuning can be accomplished and may open the possibility to improve color generation and high-quality pixel fabrication.

New life for an old antibiotic.

Restoring the antibacterial properties of existing antibiotics is of great concern. Herein, we present, for the first time, the formation and deposition of stable antibiotic nanoparticles (NPs) on graphene oxide (GO) sheets by a facile one-step sonochemical technique. Sonochemically synthesized graphene oxide/tetracycline (GO/TET) composite shows enhanced activity against both sensitive and resistant Staphylococcus aureus (S. aureus). The size and deposition of tetracycline (TET) nanoparticles on GO can be controlled by varying the sonication time. The synthesized NPs ranged from 21 to 180 nm. Moreover, ultrasonic irradiation does not cause any structural and chemical changes to the TET molecule as confirmed by Fourier transform infrared spectroscopy (FTIR). The virtue of π-π stacking between GO and TET additionally facilitate the coating of TET NPs upon GO. A time dependent release kinetics of TET NPs from the GO surface is also monitored providing important insights regarding the mechanism of antibacterial activity of GO/TET composites. Our results show that the GO/TET composite is bactericidal in nature, resulting in similar values of minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC). This composite is found to be active against TET resistant S. aureus at a concentration four times lower than the pristine TET. The sensitive S. aureus follows the same trend showing six times lower MIC values compared to pristine TET. GO shows no activity against both sensitive and resistant S. aureus even at a concentration as high as 1 mg/mL but influences the biocidal activity of the GO/TET composite. We propose that the unique structure and composition manifested by GO/TET composites may be further utilized for different formulations of antibiotics with GO. The sonochemical method used in this work can be precisely tailored for the stable deposition of a variety of antibiotics on the GO surface to reduce health risks and increase the spectrum of applications.

A novel sonochemical synthesis of antlerite nanorods.

Antlerite - Cu3(OH)4SO4 was prepared, for the first time, by the sonochemical method from an aqueous solution of CuSO4, without any additives. The source of OH(-) is a result of protonation of SO4(2-) forming HSO4(-) and OH(-). The extreme local conditions inside the cavity that are developed during the bubble collapse (pressure is above 1000atm and the temperature is higher than 5000K) lead to the formation of the crystalline mineral. A suggested mechanism for the mineral formation is proposed. Due to the collapse of the bubbles, the distances between the opposite charge Cu(2+) and SO4(2-) ions is shortened and a crystallization process is initiated. In addition, the reaction is a one-step process with short irradiation time of less than 30min. The chemo-physical analysis of the sonochemically obtained product has revealed the presence of single phase antlerite nanorods.