Spectral identification in the attogram regime through laser induced breakdown spectroscopy of single optically-trapped nanoparticles in air

Current trends in nanoengineering are bringing along new structures of diverse chemical compositions that need to be meticulously defined to ensure their correct operation. Few methods can provide the sensitivity required to carry out measurements on individual nanosubjects without tedious sample pre-treatment or data analysis. In the present study, we introduce a pathway for the full elemental identification of single nanoparticles that avoids suspension in liquid media by means of optical trapping and laser-induced plasma spectroscopy. We demonstrate spectroscopic detection and identification of individual Cu nanoparticles of masses down to 73 attograms and report, for the first time, stable optical trapping in air and manipulation of Cu particles from 25 to 70 nm in diameter. 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 7 x exp(-5) Cu atoms in the onset plasma.Universidad de Málaga. Campus de Excelencia Internacional Andalucia Tec

OC-OT-LIBS: A novel approach to the chemical characterization of single particles

Spectral identification of individual micro- and nano-sized particles by the sequential intervention of optical catapulting, optical trapping and laser-induced breakdown spectroscopy is presented [1]. The three techniques are used for different purposes. Optical catapulting (OC) serves to put the particulate material under inspection in aerosol form [2-4]. Optical trapping (OT) permits the isolation and manipulation of individual particles from the aerosol, which are subsequently analyzed by laser-induced breakdown spectroscopy (LIBS). Once catapulted, the dynamics of particle trapping depends on the laser beam characteristics (power and intensity gradient) and on the particle properties (size, mass and shape). Particles are stably trapped in air at atmospheric pressure and can be conveniently manipulated for a precise positioning for LIBS analysis. The spectra acquired from the individually trapped particles permit a straightforward identification of the inspected material. The current work focuses on the development of a procedure for simultaneously acquiring dual information about the particle under study via LIBS and time-resolved plasma images by taking advantage of the aforementioned features of the OC-OT-LIBS instrument to align the multiple lines in a simple yet highly accurate way. The plasma imaging does not only further reinforce the spectral data, but also allows a better comprehension of the chemical and physical processes involved during laser-particle interaction. Also, a thorough determination of the optimal excitation conditions generating the most information out of each laser event was run along the determination of parameters such as the width of the optical trap, its stability as a function of the laser power and the laser wavelength. The extreme sensibility of the presented OC-OT-LIBS technology allows a detection power of attograms for single/individual particle analysis.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Development of a laser-based analytical platform for the LIBS inspection of individual nanoparticles within an atmospheric pressure optical trap

En esta tesis doctoral se presenta un sensor basado en LIBS como técnica de inspección para la caracterización de nanopartículas individuales. El cuerpo de este trabajo se divide en cuatro capítulos en los que se recoge, en primer lugar, el desarrollo del sensor y la parametrización sistemática de los diferentes factores que influyen en el análisis LIBS (densidad de energía, condiciones de adquisición o la posición partícula-láser de análisis) y a la trampa óptica (rigidez de la trampa, manipulación de la partícula atrapada). Posteriormente, se muestran las capacidades analíticas de la tecnología OC-OT-LIBS propuesta haciendo énfasis en los tamaños y masas mínimas detectables haciendo uso de ella y respondiendo a cuestiones fundamentales como los mecanismos de disociación y excitación que tienen lugar durante la interacción láser-materia a través de los espectros obtenidos. Además, dado que el atrapado óptico en aire a presión atmosférica es aún un terreno poco explorado, se recogen resultados novedosos respecto a las posibilidades ofrecidas por la técnica

Polydispersity and fractionation in laser ablation studied by LIBS in an optical trap

LIBS characterization of aerosols produced by laser ablation of bulk samples is strongly influenced by variations of physicohemical properties of the elements integrating the matrix. Among the consequences of these changes are differences on the ablated mass quantities, in the morphology of the aerosolized material or, ultimately, the processes occurring within the plasma such as the so-called fractionation effect[1]. This effect results in spectra that do not represent the bulk composition accurately. Single-particle analysis of these samples constitute a new approach to fundamental studies that are still needed for a better understanding of the involved processes. Herein, the OC-OT-LIBS technology[2] has been employed to monitor the particle-to-particle size dispersion and fractionation effects observed in aerosols generated during laser ablation of copper-based alloys. Under this methodology, particles generated inside of the ablation cell were stably trapped on-line in air at atmospheric pressure in an optical trap. Particles were dislodged using both, high and low fluence regimes, and then conveniently manipulated for precise positioning before LIBS analysis. Size dispersion of the aerosol was confirmed by SEM images. Dendrite-structured agglomerates of nano-particles and micron-sized spherical particles were observed in every case along single spherical particles. Concerning LIBS analysis, reduced fractionation was observed when the aerosol was generated at high laser fluence due to the production of smaller particles and featuring a narrower particle size distribution. In general, the particle size distribution ranged from nanometers to ca. 2 micrometers even under high fluence conditions. In this sense, the possibility offered by OC-OT-LIBS to study the single-particle compositional variation in aerosols with detection power in the sub-femtogram regime opens the door to a new way of understanding the fundamental processes that occur during laser ablation.Universidad de Málaga. Campus de Excelencia Internacional Andalucia Tech

LIBS-Acoustic Mid-Level Fusion Scheme for Mineral Differentiation under Terrestrial and Martian Atmospheric Conditions

The shockwave produced alongside the plasma during a laser-induced breakdown spectroscopy event can be recorded as an acoustic pressure wave to obtain information related to the physical traits of the inspected sample. In the present work, a mid-level fusion approach is developed using simultaneously recorded laser-induced breakdown spectroscopy (LIBS) and acoustic data to enhance the discrimination capabilities of different iron-based and calcium-based mineral phases, which exhibit nearly identical spectral features. To do so, the mid-level data fusion approach is applied concatenating the principal components analysis (PCA)-LIBS score values with the acoustic wave peak-to-peak amplitude and with the intraposition signal change, represented as the slope of the acoustic signal amplitude with respect to the laser shot. The discrimination hit rate of the mineral phases is obtained using linear discriminant analysis. Owing to the increasing interest for in situ applications of LIBS + acoustics information, samples are inspected in a remote experimental configuration and under two different atmospheric traits, Earth and Mars-like conditions, to validate the approach. Particularities conditioning the response of both strategies under each atmosphere are discussed to provide insight to better exploit the complex phenomena resulting in the collected signals. Results reported herein demonstrate for the first time that the characteristic sample input in the laser-produced acoustic wave can be used for the creation of a statistical descriptor to synergistically improve the capabilities of LIBS of differentiation of rocksFundings for this work were provided by the projects UMA18-FEDERJA-272 from the Junta de Andalucía and PID2020-119185GB-I00 from Ministerio de Ciencia e Innovación, Spain. P.P. is grateful to the Junta de Andalucía for his contract under the program Garantía Juvenil. Universidad de Málaga / CBUA"

Hyperspectral LIBS imaging of single-optically trapped particles

The interaction between discrete particles with laser-induced plasma presents some complexities that directly affects the analysis of bulk aerosols by LIBS. Single-particle analysis is not exempt of this kind of phenomena and fundamental studies are still needed for a better understanding of the involved processes [1]. In this work, the OC-OT-LIBS technology [1-4] has been employed to study the distribution of emitting species in the plasma using hyperspectral LIBS imaging. The combination of a typical LIBS imaging system within a bandpass spectral filter offers the possibility for the acquisition of hyperspectral images (Fig. 1). Graphite and copper nanoparticles were studied under this methodology. The bandpass spectral filter selected for graphite and copper were centered at 390 nm and 324.7 nm, respectively. Time-resolved spectroscopy in combination with the acquisition of hyperspectral images allow the separated observation of different species along the plasma lifetime. Hyperspectral photography also provides information concerning on the size and shape of the plasma. In addition, the dual role of air as the atomization and excitation source during the laser-particle interaction is discussed on the basis of spectral evidences. One of the main challenges of the proposed research is to improve the absolute limits of detection of copper previously calculated in ~60 attograms. Emission sensitivity of the hyperspectral images will be compared with those reported in [3] by the same authors, whose calculated the number of absolute photons emitted by the single trapped copper particle by measuring the photon budget of the whole instrument. In this sense, the possibility offered by OC-OT-LIBS to study the single-particle – laser interaction in aerosols in the regime of sub-femtograms opens the door to a new way of knowing the fundamental processes that occur in laser ablation

Diffusion dynamics and characterization of attogram masses in optically trapped single nanoparticles using laser-induced plasma imaging

Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature . Research is funded by the Spanish Ministerio de Economia y Competitividad (Nos. CTQ2017-82137P and CTQ2014-56058P). P. P. is grateful to the European Union’s NextGenerationEU plan and the Spanish Ministerio de Universidades for his Margarita Salas fellowship under the program “Ayudas para la recualificación del Sistema Universitario español”. Authors are grateful to the Universidad de Málaga/CBUA for fundings for open access charges

Mechanical traits of isolated nuclei inspected via force spectroscopy

The larger stiffness of the nuclei when compared to the rest of the cell imposes a key restriction to cell deformability and their capability to traverse interstices. In contrast, cancerous cells have been reported to exhibit larger and poorly-defined nuclear shapes. Upon probing the mechanical properties of these abnormal nuclei, membrane rigidities were found to be below that of normal nuclei. A plausible explanation is an altered distribution of the nuclear chromatin. This argument is in line with the increased migration capabilities of invasive nuclei and their enhanced adaptability to the abnormal forces these cells experiment. As a response to mechanical stresses, the normal function of the nuclei is altered and can induce changes such as gene expression alteration in the cell. Despite the obvious relevance of the nuclear mechanical traits, few works report data directly acquired on nuclear membranes without any participation from the plasma membrane, which is bound to induce alterations that may disrupt results yielded by high sensitivity tests such as those performed using optical tweezers. In the present work, optical tweezers are used alongside force spectroscopy to test the mechanical traits of isolated nuclear membranes. Membranes’ Young moduli and, therefore, stiffnesses are calculated by performing indentation/retraction cycles inducing gentle deformation on the membranes using an optically trapped microbead. Nuclear membrane responses are studied as a function of the frequency with which cycles were performed to highlight possible dependency on the time lapse over which the perturbation is applied. Additionally, drastic pushing of the trapped bead against the membranes and pulling motions were performed to trigger more dramatic mechanical responses from the nuclei. During those perturbations, maximum indentation depth and maximum tension could be measured from simultaneously acquired confocal microscopy images.Ayudas para la recualificación del Sistema Universitario español modalidad Margarita Salas. NextGenerationEU (UE) & Ministerio de Universidades, Gobierno de España. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Ultrafast laser excitation in atmospheric pressure optical traps for studying attogram mass nanoparticles.

Owing to the exceedingly small mass involved, complete elemental characterization of single nanoparticles demands a highly precise control of signal background and noise sources. LIBS has demonstrated remarkable merits for this task, providing a unique tool for multielemental analysis of particles in the attogram-picogram mass scale [1-3]. Despite this outstanding sensitivity, the air plasma acting as heat source for particle dissociation and excitation is a meddling agent, often limiting the acquisition of an accurate sample signature [4]. Although thermal effects associated with ultrashort laser pulses are known to be reduced when compared to the widely used nanosecond pulse duration regime, attempts to improve nanoinspection performance using ultrafast excitation have remained largely unexplored. Herein, picosecond (ps-) laser pulses are used for the first time as a plasma excitation source for the elemental characterization of single nanoparticles isolated within optical traps in air at atmospheric pressure (Figure 1). Results for ps- excitation of copper particles lead to a mass detection limit of 27 attogram, equivalent to single particles of 18 nm in diameter. Temporal and wavelength-resolved plasma imaging reveals unique traits in the mechanism of atomic excitation in the picosecond regime, leading to a deeper understanding of interaction in single nanoparticle spectroscopy [5].Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech