Colloidal lithography double-nanohole optical trapping of nanoparticles and proteins

Accessible Double Nanohole Raman Tweezer Analysis of Single Nanoparticles

Raman spectroscopy allows for material characterization of nanoparticles; however, probing individual nanoparticles requires an efficient way of isolating and enhancing the signal. Past works have used optical trapping with nanoapertures in metal films to measure the Raman spectra of individual nanoparticles; however, those works required custom laser tweezer systems that provided a transmission signal to verify trapping events as well as costly top-down nanofabrication. Here, we trapped Titania nanoparticles in a commercial Raman system using double nanoholes (DNH) and measured their spectra while trapped. The microscope camera allowed for measuring the trapping event in reflection mode, and a simultaneous Raman spectrum was recorded to allow for material characterization. The Raman signal was comparable to a past work that used particles a million times larger in volume without utilizing double nanoholes, and all other features were similar. The DNHs were created with a colloidal lithography technique and identified in the microscope, as confirmed by electron microscopy registration. Therefore, this approach allows a simple way of characterizing the Raman signal of individual nanoparticles while in solution by using existing commercial Raman systems.

Influence of point mutations on PR65 conformational adaptability: Insights from molecular simulations and nanoaperture optical tweezers

PR65 is the HEAT repeat scaffold subunit of the heterotrimeric protein phosphatase 2A (PP2A) and an archetypal tandem repeat protein. Its conformational mechanics plays a crucial role in PP2A function by opening/closing substrate binding/catalysis interface. Using in silico saturation mutagenesis, we identified PR65 "hinge" residues whose substitutions could alter its conformational adaptability and thereby PP2A function, and selected six mutations that were verified to be expressed and soluble. Molecular simulations and nanoaperture optical tweezers revealed consistent results on the specific effects of the mutations on the structure and dynamics of PR65. Two mutants observed in simulations to stabilize extended/open conformations exhibited higher corner frequencies and lower translational scattering in experiments, indicating a shift toward extended conformations, whereas another displayed the opposite features, confirmed by both simulations and experiments. The study highlights the power of single-molecule nanoaperture-based tweezers integrated with in silico approaches for exploring the effect of mutations on protein structure and dynamics.

Thermometric Analysis of Nanoaperture-Trapped Erbium-Containing Nanocrystals

Temperature changes in plasmonic traps can affect biomolecules and quantum emitters; therefore, several works have sought out the capability of measuring the local temperature. Those works used ionic nanopore currents, fluorescence emission variations, and fluorescence-based diffusion tracking to measure the temperature dependence of shaped nanoapertures in metal films. Here, we make use of a stable erbium-containing NaYF4 nanocrystal that gives local temperature dependence while trapped in the nanoaperture hot spot. Ratiometric analysis of the emission at different wavelengths gives local temperature variation. Since the gold film dominates the thermal characteristic, we find that films of thickness 70, 100, and 130 nm give 0.64, 0.37, and 0.25 K/mW temperature change with laser power. Therefore, using thicker films can be effective in reducing the heating when it is not desired.

Fringe Dielectrophoresis Nanoaperture Optical Trapping with Order of Magnitude Speed-Up for Unmodified Proteins

Single molecule analysis of proteins in an aqueous environment without modification (e.g., labels or tethers) elucidates their biophysics and interactions relevant to drug discovery. By combining fringe-field dielectrophoresis with nanoaperture optical tweezers we demonstrate an order of magnitude faster time-to-trap for proteins when the counter electrode is outside of the solution. When the counter electrode is inside the solution (the more common configuration found in the literature), electrophoresis speeds up the trapping of polystyrene nanospheres, but this was not effective for proteins in general. Since time-to-trap is critical for high-thoughput analysis, these findings are a major advancement to the nanoaperture optical trapping technique for protein analysis.

Reflection mode optical trapping using polarization symmetry breaking from tilted double nanoholes

We demonstrate reflection geometry optical trapping using double nanoholes in a metal film. Symmetry breaking of the double nanohole allows for selecting the scattered trapping laser light of orthogonal polarization to the incident beam. This orthogonal polarization light shows a few percent increase when the nanoparticle (e.g., a 20 nm polystyrene particle, or protein bovine serum albumin) is trapped. The reflection geometry simplifies the optical setup and frees up one side of the trap, which has great potential for adding microfluidics to the other side or working with opaque or highly scattering samples.

Coupling Perovskite Quantum Dot Pairs in Solution using a Nanoplasmonic Assembly

Perovskite quantum dots (PQDs) provide a robust solution-based approach to efficient solar cells, bright light emitting devices, and quantum sources of light. Quantifying heterogeneity and understanding coupling between dots is critical for these applications. We use double-nanohole optical trapping to size individual dots and correlate to emission energy shifts from quantum confinement. We were able to assemble a second dot in the trap, which allows us to observe the coupling between dots. We observe a systematic red-shift of 1.1 ± 0.6 meV in the emission wavelength. Theoretical analysis shows that the observed shift is consistent with resonant energy transfer and is unusually large due to moderate-to-large quantum confinement in PQDs. This demonstrates the promise of PQDs for entanglement in quantum information applications. This work enables future in situ control of PQD growth as well as studies of the coupling between small PQD assemblies with quantum information applications in mind.

Accessible high-performance double nanohole tweezers

Nanohole optical tweezers have been used by several groups to trap and analyze proteins. In this work, we demonstrate that it is possible to create high-performance double nanohole (DNH) substrates for trapping proteins without the need for any top-down approaches (such as electron microscopy or focused-ion beam milling). Using polarization analysis, we identify DNHs as well as determine their orientation and then use them for trapping. We are also able to identify other hole configurations, such as single, trimers and other clusters. We explore changing the substrate from glass to polyvinyl chloride to enhance trapping ability, showing 7 times lower minimum trapping power, which we believe is due to reduced surface repulsion. Finally, we present tape exfoliation as a means to expose DNHs without damaging sonication or chemical methods. Overall, these approaches make high quality optical trapping using DNH structures accessible to a broad scientific community.

Nano-Optical Tweezers: Methods and Applications for Trapping Single Molecules and Nanoparticles

Optical tweezers were developed in 1970 by Arthur Ashkin as a tool for the manipulation of micron-sized particles. Ashkin's original design was then adapted for a variety of purposes, such as trapping and manipulation of biological materials^[1] and the laser cooling of atoms.^[2,3] More recent development has led to nano-optical tweezers, for trapping particles on the scale of only a few nanometers, and holographic tweezers, which allow for dynamic control of multiple traps in real-time. These alternatives to conventional optical tweezers have made it possible to trap single molecules and to perform a variety of studies on them. Presented here is a review of recent developments in nano-optical tweezers and their current and future applications.

Optical manipulation: advances for biophotonics in the 21st century

SIGNIFICANCE

Optical trapping is a technique capable of applying minute forces that has been applied to studies spanning single molecules up to microorganisms.

AIM

The goal of this perspective is to highlight some of the main advances in the last decade in this field that are pertinent for a biomedical audience.

APPROACH

First, the direct determination of forces in optical tweezers and the combination of optical and acoustic traps, which allows studies across different length scales, are discussed. Then, a review of the progress made in the direct trapping of both single-molecules, and even single-viruses, and single cells with optical forces is outlined. Lastly, future directions for this methodology in biophotonics are discussed.

RESULTS

In the 21st century, optical manipulation has expanded its unique capabilities, enabling not only a more detailed study of single molecules and single cells but also of more complex living systems, giving us further insights into important biological activities.

CONCLUSIONS

Optical forces have played a large role in the biomedical landscape leading to exceptional new biological breakthroughs. The continuous advances in the world of optical trapping will certainly lead to further exploitation, including exciting in-vivo experiments.

Enhancing Single-Molecule Fluorescence Spectroscopy with Simple and Robust Hybrid Nanoapertures

Plasmonic nanoapertures have found exciting applications in optical sensing, spectroscopy, imaging, and nanomanipulation. The subdiffraction optical field localization, reduced detection volume (~attoliters), and background-free operation make them particularly attractive for single-particle and single-molecule studies. However, in contrast to the high field enhancements by traditional "nanoantenna"-based structures, small field enhancement in conventional nanoapertures results in weak light-matter interactions and thus small enhancement of spectroscopic signals (such as fluorescence and Raman signals) of the analytes interacting with the nanoapertures. In this work, we propose a hybrid nanoaperture design termed "gold-nanoislands-embedded nanoaperture" (AuNIs-e-NA), which provides multiple electromagnetic "hotspots" within the nanoaperture to achieve field enhancements of up to 4000. The AuNIs-e-NA was able to improve the fluorescence signals by more than 2 orders of magnitude with respect to a conventional nanoaperture. With simple design and easy fabrication, along with strong signal enhancements and operability over variable light wavelengths and polarizations, the AuNIs-e-NA will serve as a robust platform for surface-enhanced optical sensing, imaging, and spectroscopy.

Isolating and enhancing single-photon emitters for 1550 nm quantum light sources using double nanohole optical tweezers

Single-photon sources are required for quantum technologies and can be created from individual atoms and atom-like defects. Erbium ions produce single photons at low-loss fiber optic wavelengths, but they have low emission rates, making them challenging to isolate reliably. Here, we tune the size of gold double nanoholes (DNHs) to enhance the emission of single erbium emitters, achieving 50× enhancement over rectangular apertures previously demonstrated. This produces enough enhancement to show emission from single nanocrystals at wavelengths not seen in our previous work, i.e., 400 and 1550 nm. We observe discrete levels of emission for nanocrystals with low numbers of emitters and demonstrate isolating single emitters. We describe how the trapping time is proportional to the enhancement factor for a given DNH structure, giving us an independent way to measure the enhancement. This shows a promising path to achieving single emitter sources at 1550 nm.

Overcoming Diffusion-Limited Trapping in Nanoaperture Tweezers Using Opto-Thermal-Induced Flow

Nanoaperture-based plasmonic tweezers have shown tremendous potential in trapping, sensing, and spectroscopic analysis of nano-objects with single-molecule sensitivity. However, the trapping process is often diffusion-limited and therefore suffers from low-throughput. Here, we present bubble- and convection-assisted trapping techniques, which use opto-thermally generated Marangoni and Rayleigh-Bénard convection flow to rapidly deliver particles from large distances to the nanoaperture instead of relying on normal diffusion, enabling a reduction of 1-2 orders of magnitude in particle-trapping time (i.e., time before a particle is trapped). At a concentration of 2 × 107 particles/mL, average particle-trapping times in bubble- and convection-assisted trapping were 7 and 18 s, respectively, compared with more than 300 s in the diffusion-limited trapping. Trapping of a single particle at an ultralow concentration of 2 × 106 particles/mL was achieved within 2-3 min, which would otherwise take several hours in the diffusion-limited trapping. With their quick delivery and local concentrating of analytes at the functional surfaces, our convection- and bubble-assisted trapping could lead to enhanced sensitivity and throughput of nanoaperture-based plasmonic sensors.

Plasmonic tweezers for optical manipulation and biomedical applications

This comprehensive minireview highlights the recent research on the subtypes, optical manipulation, and biomedical applications of plasmonic tweezers.

Generating and Detecting High-Frequency Liquid-Based Sound Resonances with Nanoplasmonics

We use metal nanostructures (nanoplasmonics) excited with dual frequency lasers to generate and detect high-frequency (>10 GHz) sound wave resonances in water. The difference frequency between the two lasers causes beating in the intensity, which results in a drop in the transmission through the nanostructure when an acoustic resonance is excited. By observing the resonance frequency shifts with changing nanostructure size, the transition from slow to fast sound in water is inferred, which has been measured by inelastic scattering methods in the past. The observed behavior shows remarkable similarities to finite element simulations using a simple Debye model for sound velocity (without fitting parameters).