Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

Summary

We use optical tracking of plasmonic nanoparticles to probe and characterize the frequency movements of aquatic organisms.

Abstract

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticle's position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.

Introduction

Water quality assessment based on chemical and biological indicators is of fundamental importance to gain insight on the state and environmental conditions of an aquatic ecosystem^1-3. Classical methods for chemical water analysis are based on organoleptic properties or the determination of physicochemical parameters. Biological indicators, on the other hand, are animal species whose presence and viability provide insight on the environmental conditions and the effect of pollutants for an ecosystem that they occur in. Typical examples for bioindicators are Copepods, a group of small water crustaceans, which can be found in nearly any water habitat^4,5

Protocol

1. Experimental Setup

1. Use an up-right microscope and a dark field oil condenser with a numerical aperture (NA) = 1.2 for dark field illumination. Use a water immersion objective with 100X magnification and a NA = 1.0 for particle observations and trapping. Use an air objective with 10X magnification and a NA = 0.2 to follow the motion of the Nauplius.
2. Use an optical tweezers setup with a 1,064 nm continuous wave laser coupled into the up-right microscope. Set the laser power of the optical trap to 100 mW (measured with a power meter after the objective).
3. Use a CMOS high-speed camera or a digital single lens reflex (DSLR) camera to detec....

Results

A schematic illustration of the experimental setup is shown in Figure 1A. A dark field configuration is necessary to optically detect the displacement of a 60 nm gold particle in an optical trap¹⁵. The wavelength of 1,064 nm for the trapping laser is chosen to guarantee a stable confinement of the detector gold particle^12,14. A beam splitter in the microscope is used to focus the trapping beam through the objective and a notch filter prevents the trapping laser from entering the det.......

Discussion

Dark field microscopy is a powerful tool for visualizing gold nanoparticles with dimensions below the optical diffraction limit, since the scattering cross section of the metal nanoparticles exceeds their geometric cross section (cp. Figure 2A)¹⁸. In a tweezer setup, this approach even allows to distinguish if only a single or multiple gold nanoparticles are trapped by the laser beam because plasmonic coupling between the particles causes a red-shift of the plasmon resonance frequency¹⁵

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Microscope Zeiss Axio Scope.A1	Carl Zeiss	490035-0012-000	dark field illumination
Water objective Achroplan	Carl Zeiss	440087	100X magnification, NA = 1.0
Air objective Epiplan	Carl Zeiss	442934	10X magnification, NA = 0.2
Dark field oil condenser	Carl Zeiss	445323	NA = 1.2
Cobolt Rumba CW 1,064 nm DPSSL	Cobolt	1064-05-01-2000-500	1,064 nm, CW, λ = 1,064 nm, 2 Watt, TEM00
Beam expander	Edmund Optics		Part no. 1064 2-8X 64414
High Speed Camera Dimax HD	PCO. Germany
Color Camera Canon EOS 500 D	Canon		FAQ-ID: 8201395700
Notch filter StopLine 532/1064	Semrock	A11149-711265	Part no. NF01-532U
Water
Nauplius Artemia salina
Gold colloid	BBInternational	Batch 13741	Diameter 60 nm
MQMie Version 3.2	Dr. Michael Quinten
Mathematica 8.0	Wolfram
Comsol Multiphysics 4.0	COMSOL, Inc.