Plasmonic tweezers are an emerging research topic because of their breakthrough in the conventional diffraction limit and precise manipulation at the nanoscale. Notably, their compatibility with analytical techniques (e.g. fluorescence, surface-enhanced Raman scattering (SERS), and laser desorption/ionization mass spectrometry (LDI MS)) opens up opportunities in optical manipulation and biomedical applications. Herein, we first introduce the structures and trapping forces, followed by a summary of the properties of plasmonic tweezers. The optical trapping of biosamples by plasmonic tweezers are then reviewed, including microorganisms and biomolecules. Finally, we highlight the integration of plasmonic tweezers with analytical techniques towards bioanalytical applications. We conclude with perspectives on the future directions for this topic. We foresee the upcoming era of biological detection by plasmonic tweezing in both academy and industry, which calls for the interest and efforts of scientists from diverse fields.

Plasmonic tweezers for optical manipulation and biomedical applications.

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-Benard 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.

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

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.

https://www.spiedigitallibrary.org/journals/journal-of-biomedical-optics/volume-26/issue-7/070602/Optical-manipulation-advances-for-biophotonics-in-the-21st-century/10.1117/1.JBO.26.7.070602.pdf

Optical manipulation : advances for biophotonics in the 21st century

Microparticle manipulation is mainly needed in several applications like bioanalysis, disease diagnosis, self-cleaning surfaces, as well as drug delivery. The movement of microparticles in a microfluidic situation is actively manipulated by various adjustable mechanisms such as acoustophoresis, electrical forces, thermophoresis, magnetophoresis, as well as optical forces. Using such active manipulation mechanisms, the microparticle movements are obtained required to promote different biological processes. In this paper, we evaluate and deliberate the basic working principles of these microparticle manipulation methods such as the advantages and disadvantages of each method within microfluidic systems focusing on the uses in biomedical devices. These tools are realized and are potentially appropriate for therapeutic, diagnostic, or analytical purposes to present an overview of recent advances in microparticle manipulation for the reader. Finally, we offer viewpoints on the future development of active microparticle manipulation methods.

Active microparticle manipulation: Recent advances

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.

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

Double-nanoholes fabricated by colloidal lithography were used for trapping single colloidal particles and single proteins. A gap separation of 60 nm between the cusps of the double-nanohole was achieved in a gold film of 70 nm thickness sputter coated onglass. The cusp separation was reduced steadily down to 10 nm by plasma etching the colloidal particles prior to sputter coating. Scanning electron microscopy was used to locate a particular double-nanohole and it was registered for later microscopy experiments. 30 nm polystyrene particles, the rubisco protein and bovine serum albumin were trapped using a laser focused through the aperture. Compared to other methods that require top-down nanofabrication, this approach is inexpensive and produces high-quality samples.

Colloidal lithography double-nanohole optical trapping of nanoparticles and proteins.

The single-beam gradient force optical tweezers have transformed various fields of scientific research by enabling manipulation and characterization of single molecules. Conventional optical tweezers pose limitations in trapping particles in the sub-Rayleigh regime. These limitations have been overcome with the help of plasmonic nanoapertures like the double-nanohole aperture. A modified colloidal lithography technique has been used in fabrication of double-nanohole apertures achieving dimensions appropriate for trapping single molecules in this regime. This paper demonstrates optical trapping of a single 10 nm enzyme, rubisco, using double-nanohole apertures fabricated using the modified colloidal lithography technique as well as presents the results from transmission characterization of different double-nanohole apertures carried out using the finite-difference time-domain (FDTD) simulations.

Samuel Mathew

Papers

Colloidal lithography double-nanohole optical trapping of nanoparticles and proteins.

Colloidal lithography for trapping 10 nm enzymes