Spectral Identification in the Attogram Regime through Laser‐Induced Emission of Single Optically Trapped Nanoparticles in Air

Abstract

Current trends in nanoengineering are bringing along new structures of diverse chemical compositions that need to be meticulously defined in order to ensure their correct operation. Few methods can provide the sensitivity required to carry out measurements on individual nano‐objects without tedious sample pre‐treatment or data analysis. In the present study, we introduce a pathway for the elemental identification of single nanoparticles (NPs) that avoids suspension in liquid media by means of optical trapping and laser‐induced plasma spectroscopy. We demonstrate spectroscopic detection and identification of individual 25(±3.7) to 70(±10.5) nm in diameter Cu NPs stably trapped in air featuring masses down to 73±35 attograms. We found an increase in the absolute number of photons produced as size of the particles decreased; pointing towards a more efficient excitation of ensembles of only ca. 7×105 Cu atoms in the onset plasma.

Laser-Induced Breakdown Spectroscopy (LIBS): Fast, Effective, and Agile Leading Edge Analytical Technology

“…Additionally, the authors developed a calibration scheme to relate the net signal recorded to the number of photons produced by particles, therefore evaluating the excitation efficiency. Figure 7 sums up the most relevant results reported so far using OC-OT-LIBS, i.e., successful direct detection and characterization of sub-100 nm single Cu particles, 41 with the smallest particle size being 25 nm in diameter, equivalent to a mass of 73 ag. An absolute LOD of approximately 59 ag was estimated for Cu, a limit never been reported before by an optical emission technique.…”

“…The photon yield shows that the excitation efficiency increases as the particle size decreases. [40][41][42] This behavior has been proposed as the reason behind the extreme sensitivity of this approach.…”

“…Typical examples of averaged single particle spectra are provided in the inset. Reprinted from Purohit et al 41 with permission from John Wiley and Sons, Inc. transformed into gas-phase, the size distribution of particles in the ablated mass (also considering the tiny particles generated by nucleation or condensation from the gas phase), the percentage of free atoms and ions per unit of ablated mass, and the percentage of atoms excited to different electronic levels. Despite these unknown answers affecting the quantitative aspects of the technique-a widespread problem in surface analysis techniques-experimental work in LIBS shows that: (a) the atomization efficiency is low, and (b) the particle distribution of the ablated mass is wide and affected by the ablation conditions.…”

“…The photon yield shows that the excitation efficiency increases as the particle size decreases. 40–42 This behavior has been proposed as the reason behind the extreme sensitivity of this approach.

Figure 7.Correlation between averaged single Cu nanoparticle LIBS signal and particle mass (straight line).

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Optical-Trapping Laser Techniques for Characterizing Airborne Aerosol Particles and Its Application in Chemical Aerosol Study

“…The latest generations of OT-LIBS systems (e.g., Figure 11 ), with an increased excitation efficiency for small particles, where air–mass plasma acts as the main source of excitation over direct laser–particle interaction, have pushed the limit of detection down to 58.9 attograms [ 195 ] and later to an extreme sensitivity of 37 attograms [ 196 ] for copper nanoparticles. In fact, in this system, the dissociation and excitation processes are directed by heat transmission from plasma to particle and mass transfer the other way around.…”