Optical trapping of silver nanoplatelets

Silver Nanoparticles: Multifunctional Tool in Environmental Water Remediation

Water pollution is a worldwide environmental and health problem that requires the development of sustainable, efficient, and accessible technologies. Nanotechnology is a very attractive alternative in environmental remediation processes due to the multiple properties that are conferred on a material when it is at the nanometric scale. This present review focuses on the understanding of the structure-physicochemical properties-performance relationships of silver nanoparticles, with the objective of guiding the selection of physicochemical properties that promote greater performance and are key factors in their use as antibacterial agents, surface modifiers, colorimetric sensors, signal amplifiers, and plasmonic photocatalysts. Silver nanoparticles with a size of less than 10 nm, morphology with a high percentage of reactive facets {111}, and positive surface charge improve the interaction of the nanoparticles with bacterial cells and induce a greater antibacterial effect. Adsorbent materials functionalized with an optimal concentration of silver nanoparticles increase their contact area and enhance adsorbent capacity. The use of stabilizing agents in silver nanoparticles promotes selective adsorption of contaminants by modifying the surface charge and type of active sites in an adsorbent material, in addition to inducing selective complexation and providing stability in their use as colorimetric sensors. Silver nanoparticles with complex morphologies allow the formation of hot spots or chemical or electromagnetic bonds between substrate and analyte, promoting a greater amplification factor. Controlled doping with nanoparticles in photocatalytic materials produces improvements in their electronic structural properties, promotes changes in charge transfer and bandgap, and improves and expands their photocatalytic properties. Silver nanoparticles have potential use as a tool in water remediation, where by selecting appropriate physicochemical properties for each application, their performance and efficiency are improved.

Optical Trapping, Optical Binding, and Rotational Dynamics of Silicon Nanowires in Counter-Propagating Beams

Silicon nanowires are held and manipulated in controlled optical traps based on counter-propagating beams focused by low numerical aperture lenses. The double-beam configuration compensates light scattering forces enabling an in-depth investigation of the rich dynamics of trapped nanowires that are prone to both optical and hydrodynamic interactions. Several polarization configurations are used, allowing the observation of optical binding with different stable structure as well as the transfer of spin and orbital momentum of light to the trapped silicon nanowires. Accurate modeling based on Brownian dynamics simulations with appropriate optical and hydrodynamic coupling confirms that this rich scenario is crucially dependent on the non-spherical shape of the nanowires. Such an increased level of optical control of multiparticle structure and dynamics open perspectives for nanofluidics and multi-component light-driven nanomachines.

Optical force decoration of 3D microstructures with plasmonic particles

Optical forces are used to push and aggregate gold nanorods onto several substrates creating surface-enhanced Raman scattering (SERS) active hot spots for Raman-based identification of proteins. By monitoring the increase of the protein SERS signal, we observe different aggregation times for different curvatures of the substrates. The slower aggregation dynamics on curved surfaces is justified by a simple geometrical model. In particular, this technique is used to decorate three-dimensional microstructures and to quickly realize hybrid micro/nanosensors for highly sensitive detection of biological material directly in a liquid environment.

Dynamics of the Optically Directed Assembly and Disassembly of Gold Nanoplatelet Arrays

The tremendous progress in nanoscience now allows the creation of static nanostructured materials for a broad range of applications. A further goal is to achieve dynamic and reconfigurable nanostructures. One approach involves nanoparticle-based optical matter, but so far, studies have only considered spherical constituents. A nontrivial issue is that nanoparticles with other shapes are expected to have different local electromagnetic field distributions and interactions with neighbors in optical-matter arrays. Therefore, one would expect their dynamics to be different as well. This paper reports the directed assembly of ordered arrays of gold nanoplatelets in optical line traps, demonstrating the reconfigurability of the array by altering the phase gradient via holographic-beam shaping. The weaker gradient forces and resultant slower motion of the nanoplatelets, as compared with plasmonic (Ag and Au) nanospheres, allow the precise study of their assembly and disassembly dynamics. Both temporal and spatial correlations are detected between particles separated by distances of hundreds of nanometers to several microns. Electrodynamics simulations reveal the presence of multipolar plasmon modes that induce short-range (near-field) and longer-range electrodynamic (e.g., optical binding) interactions. These interactions and the interferences between mutipolar plamon modes cause both the strong correlations and the nonuniform dynamics observed. Our study demonstrates new opportunities for the generation of complex addressable optical matter and the creation of novel active optical technology.

Optical Aggregation of Gold Nanoparticles for SERS Detection of Proteins and Toxins in Liquid Environment: Towards Ultrasensitive and Selective Detection

Optical forces are used to aggregate plasmonic nanoparticles and create SERS-active hot spots in liquid. When biomolecules are added to the nanoparticles, high sensitivity SERS detection can be accomplished. Here, we pursue studies on Bovine Serum Albumin (BSA) detection, investigating the BSA-nanorod aggregations in a range from 100 µM to 50 nM by combining light scattering, plasmon resonance and SERS, and correlating the SERS signal with the concentration. Experimental data are fitted with a simple model describing the optical aggregation process. We show that BSA-nanorod complexes can be optically printed on non-functionalized glass surfaces, designing custom patterns stable with time. Furthermore, we demonstrate that this methodology can be used to detect catalase and hemoglobin, two Raman resonant biomolecules, at concentrations of 10 nM and 1 pM, respectively, i.e., well beyond the limit of detection of BSA. Finally, we show that nanorods functionalized with specific aptamers can be used to capture and detect Ochratoxin A, a fungal toxin found in food commodities and wine. This experiment represents the first step towards the addition of molecular specificity to this novel biosensor strategy.

Optical trapping and optical force positioning of two-dimensional materials

In recent years, considerable effort has been devoted to the synthesis and characterization of two-dimensional materials. Liquid phase exfoliation (LPE) represents a simple, large-scale method to exfoliate layered materials down to mono- and few-layer flakes. In this context, the contactless trapping, characterization, and manipulation of individual nanosheets hold perspectives for increased accuracy in flake metrology and the assembly of novel functional materials. Here, we use optical forces for high-resolution structural characterization and precise mechanical positioning of nanosheets of hexagonal boron nitride, molybdenum disulfide, and tungsten disulfide obtained by LPE. Weakly optically absorbing nanosheets of boron nitride are trapped in optical tweezers. The analysis of the thermal fluctuations allows a direct measurement of optical forces and the mean flake size in a liquid environment. Measured optical trapping constants are compared with T-matrix light scattering calculations to show a quadratic size scaling for small size, as expected for a bidimensional system. In contrast, strongly absorbing nanosheets of molybdenum disulfide and tungsten disulfide are not stably trapped due to the dominance of radiation pressure over the optical trapping force. Thus, optical forces are used to pattern a substrate by selectively depositing nanosheets in short times (minutes) and without any preparation of the surface. This study will be useful for improving ink-jet printing and for a better engineering of optoelectronic devices based on two-dimensional materials.

Laser irradiation in water for the novel, scalable synthesis of black TiO x photocatalyst for environmental remediation

Since 1970, TiO₂ photocatalysis has been considered a possible alternative for sustainable water treatment. This is due to its material stability, abundance, nontoxicity and high activity. Unfortunately, its wide band gap (≈3.2 eV) in the UV portion of the spectrum makes it inefficient under solar illumination. Recently, so-called "black TiO₂" has been proposed as a candidate to overcome this issue. However, typical synthesis routes require high hydrogen pressure and long annealing treatments. In this work, we present an industrially scalable synthesis of TiO₂-based material based on laser irradiation. The resulting black TiO _x shows a high activity and adsorbs visible radiation, overcoming the main concerns related to the use of TiO₂ under solar irradiation. We employed a commercial high repetition rate green laser in order to synthesize a black TiO _x layer and we demonstrate the scalability of the present methodology. The photocatalyst is composed of a nanostructured titanate film (TiO _x ) synthetized on a titanium foil, directly back-contacted to a layer of Pt nanoparticles (PtNps) deposited on the rear side of the same foil. The result is a monolithic photochemical diode with a stacked, layered structure (TiO _x /Ti/PtNps). The resulting high photo-efficiency is ascribed to both the scavenging of electrons by Pt nanoparticles and the presence of trap surface states for holes in an amorphous hydrogenated TiO _x layer.

SERS detection of Biomolecules at Physiological pH via aggregation of Gold Nanorods mediated by Optical Forces and Plasmonic Heating

Strategies for in-liquid molecular detection via Surface Enhanced Raman Scattering (SERS) are currently based on chemically-driven aggregation or optical trapping of metal nanoparticles in presence of the target molecules. Such strategies allow the formation of SERS-active clusters that efficiently embed the molecule at the "hot spots" of the nanoparticles and enhance its Raman scattering by orders of magnitude. Here we report on a novel scheme that exploits the radiation pressure to locally push gold nanorods and induce their aggregation in buffered solutions of biomolecules, achieving biomolecular SERS detection at almost neutral pH. The sensor is applied to detect non-resonant amino acids and proteins, namely Phenylalanine (Phe), Bovine Serum Albumin (BSA) and Lysozyme (Lys), reaching detection limits in the μg/mL range. Being a chemical free and contactless technique, our methodology is easy to implement, fast to operate, needs small sample volumes and has potential for integration in microfluidic circuits for biomarkers detection.

Focus issue introduction: optical cooling and trapping

The year 2015 is an auspicious year for optical science, as it is being celebrated as the International Year of Light and Light-Based Technologies. This Focus Issue of the journals Optics Express and Journal of the Optical Society of America B has been organized by the OSA Technical Group on Optical Cooling and Trapping to mark this occasion, and to highlight the most recent and exciting developments in the topics covered by the group. Together this joint Focus Issue features 32 papers, including both experimental and theoretical works, which span this wide range of activities.

Scaling of optical forces on Au–PEG core–shell nanoparticles

Optical trapping of hybrid core–shell gold–polymer particles is studied.