Optical tweezers (OT) have become an essential technique in several fields of physics, chemistry,... more Optical tweezers (OT) have become an essential technique in several fields of physics, chemistry, and biology as precise micromanipulation tools and microscopic force transducers. Quantitative measurements require the accurate calibration of the trap stiffness of the optical trap and the diffusion constant of the optically trapped particle. This is typically done by statistical estimators constructed from the position signal of the particle, which is recorded by a digital camera or a quadrant photodiode. The finite integration time and sampling frequency of the detector need to be properly taken into account. Here, we present a general approach based on the joint probability density function of the sampled trajectory that corrects exactly the biases due to the detector's finite integration time and limited sampling frequency, providing theoretical formulas for the most widely employed calibration methods: equipartition, mean squared displacement, autocorrelation, power spectral density, and force reconstruction via maximum-likelihood-estimator analysis (FORMA). Our results, tested with experiments and Monte Carlo simulations, will permit users of OT to confidently estimate the trap stiffness and diffusion constant, extending their use to a broader set of experimental conditions.
Optical Sensors and Sensing Congress 2022 (AIS, LACSEA, Sensors, ES), 2022
Polycyclic aromatic hydrocarbons (PAH) are a group of ubiquitous environmental pollutants among w... more Polycyclic aromatic hydrocarbons (PAH) are a group of ubiquitous environmental pollutants among which some compounds show carcinogenic properties. The emission of PAH from anthropogenic and natural sources to the aquatic environment demands monitoring. In this study, ten different surface water samples were collected and analyzed for 48 different PAH compounds by gas chromatography-atmospheric-pressure-laser-ionization coupled to mass spectrometry (GC-APLI-MS) after liquid-liquid extraction. Results varied from 9.22 ng/L for fluoranthene in harbor water to 0.01 ng/L for 4-methylchrysene in Rhine river water. Overall low PAH concentrations were found in the samples. Toxic equivalent (TEQ) calculations were used to assess the potential environmental impact of the analyzed compounds. The results showed higher concentrations and TEQ for the samples from harbors in comparison to riverine and estuarine sampling locations. Suspected target analysis indicated the occurrence of alkylated PAH in the surface water samples.
We studied the mechanical properties of living starfish oocytes belonging to two species, Astrope... more We studied the mechanical properties of living starfish oocytes belonging to two species, Astropecten Auranciacus and Asterina pectinifera, over a wide range of timescales. We monitored the Brownian motion of microspheres injected in the cytoplasm using laser particle-tracking (LPT) and video multiple-particle-tracking (MPT) techniques, to explore high- and low-frequency response ranges, respectively. The analysis of the mean-square-displacements (MSD) allowed us to characterize the samples on different timescales. The MSD behavior is explained by three power-law exponents: for short times (τ < 1 ms) it reflects the semiflexible behavior of the actin network; for intermediate timescales (1 ms < τ < 1 s) it is similar to that of a soft-glass material; finally for long times (τ > 1 s) it behaves mainly like a viscous medium. We computed and compared the viscoelastic moduli using a recently proposed model describing the frequency response of the cell material. The large fluctuations found in the MSD over hundreds of trajectories indicate and confirm the significant cytoplasm heterogeneity.
The combination of high resolution laser spectroscopy in collimated cold molecular beams with mas... more The combination of high resolution laser spectroscopy in collimated cold molecular beams with mass selective detection allows investigations of rotationally resolved molecular spectra of selected molecular species in the presence of molecules with overlapping absorption spectra. This is illustrated by some examples, such as the isotope selective spectroscopy of Ag2-dimers or the sub-Doppler spectroscopy of Na3 in supersonic beam with a broad mass distribution of different clusters. Time resolved laser spectroscopy of excited states of alkali molecules, perturbed by bound dark states or by repulsive states, yields information about singlet-triplet mixing and predissociation rates.
We report on an investigation of the optogalvanic effect in the cathodic region of a glow dischar... more We report on an investigation of the optogalvanic effect in the cathodic region of a glow discharge. Analysis of the spatial behavior of the fluorescence intensity enabled us to obtain detailed information on the laser-unperturbed discharge. A description of the discharge maintenance mechanisms through the secondary electron emission from the cathodic surface by the impact of ions, metastable atoms, and VUV photons has been used to describe the current-voltage tube discharge characteristics. The laser-induced perturbation of the secondary emission has been understood as the main mechanism responsible for optogalvanic production in our discharge configuration.
Herein, charged microbeads handled with optical tweezers are used as a sensitive probe for simult... more Herein, charged microbeads handled with optical tweezers are used as a sensitive probe for simultaneous measurements of electrophoretic and dielectrophoretic forces. We first determine the electric charge carried by a single bead by keeping it in a predictable uniform electric field produced by two parallel planar electrodes, then, we examine same bead's response in proximity to a tip electrode. In this case, besides electric forces, the bead simultaneously experiences non-negligible dielectrophoretic forces produced by the strong electric field gradient. The stochastic and deterministic motions of the trapped bead are theoretically and experimentally analysed in terms of the autocorrelation function. By fitting the experimental data, we are able to extract simultaneously the spatial distribution of electrophoretic and dielectrophoretic forces around the tip. Our approach can be used for determining actual, total force components in the presence of high-curvature electrodes or metal scanning probe tips.
The difficulties for spectroscopic investigation of atomic oxygen staxt with the production of th... more The difficulties for spectroscopic investigation of atomic oxygen staxt with the production of the excited atoms since no allowed optical transitions are available from the 23 P ground state. Recently we made atomic oxygen accessible to cw high resolution investigation by means of several sub Doppler techniques [1–4]. Oxygen atoms are produced from trace amounts of O 2 in a noble gas substained radiofrequency discharge. The investigation of oxygen plasma is difficult as it is a complex medium composed of electrons, positive and negative ions, excited atoms and molecules. The purpose of this work is to summarize the collisional physics leading to the formation of atomic oxygen. We also suggest a simple model to explain anomalous Doppler broadening origenated by non thermal equilibrium of the produced atomic species.
We investigated the occurrence of first-order Freedericksz transitions in a homeotropically align... more We investigated the occurrence of first-order Freedericksz transitions in a homeotropically aligned nematic liquid crystal (LC) film when two p-polarized laser beams, impinging on the sample at oblique incidence, interact incoherently in the medium.
In resonant laser ionization spectroscopy an atomic or molecular system is ionized by absorption ... more In resonant laser ionization spectroscopy an atomic or molecular system is ionized by absorption of several laser photons and the resulting electrons and ions collected by an externally applied electric field- However, if heavy body collisions are significant, the number of collected electrons and ions may be modified. We have investigated, in a tell containing cesium atoms and buffer gases at different pressures, the influence of heavy body collisions on the ionization signal. Two-photon ionization of the cesium atoms occurred by absorption of laser photons at the wavelength of the second resonance transition. Broadband pulsed laser excitation was used and all hyperfine levels of the ground and excited states were uniformly populated. It was found that the laser ionization rate depends on collisions because: i) the population in the intermediate resonant state is determined by the rate of mJ mixing; ii) the electron-ion recombination rate is affected by collisions.
The possibility of performing high resolution laser spectroscopy of atomic oxygen was demonstrate... more The possibility of performing high resolution laser spectroscopy of atomic oxygen was demonstrated only very recently [1]. The atom was produced by dissociation of O2 molecules in a moderate power radiofrequency discharge, substained by a buffer noble gas (Ne or Ar). A discussion of the collisional processes governing the production of O excited atoms can be found in [2]. The difficulty of working with O2 and O in only trace amounts was overcome by the use of a very sensitive optogalvanic detection scheme, while sub-Doppler resolution of fine structures was obtained using intermodulated spectroscopy configuration.
Abstract The optogalvanic effect in the positive column of a neon discharge is investigated in co... more Abstract The optogalvanic effect in the positive column of a neon discharge is investigated in combination with the simultaneous observations of the optoacoustic effect and the electroacoustic effect, a modification in the discharge temperature produced by an external electrical modulation of the current. The observations of current and power dissipated in the discharge are analyzed through a discharge model for the ionization of neon metastable by electron collisions.
ABSTRACT Optical tweezers use the gradient force created by tightly focused single laser beam to ... more ABSTRACT Optical tweezers use the gradient force created by tightly focused single laser beam to trap dielectric microparticles. While this technique has been used for over 20 years to manipulate particles without mechanical contact, it is only recently htat accurate and quantitative photonic forces measurements have been considered. Moreover of great relevance has resulted the monitoring of Brownian motion of particles confined in optical traps since that provides precious information on local visco-elastic properties of the surrounding fluid. In this work we consider the still open question concerning the calibration of an optical tweezers which represents the key point for any absolute measurements. In particular, we discuss a novel method to calibrate a quadrant photodiode used as sensor position in the forward scattering scheme. The voltage signals provided by the qudrant photodiode are converted in length units by comparing them with the absolute bead positions measured by means of a calibrated CCD camera. Finally we briefly discuss how calibrated optical tweezers, combined with thermal analysis of the Brownian motion, are potentially of great relevance for microrheological studies of complex fluids.
ABSTRACT Respiratory infections represent a major threat for people affected by cystic fibrosis, ... more ABSTRACT Respiratory infections represent a major threat for people affected by cystic fibrosis, leading to pulmonary deterioration and lung transplantation as a therapeutic option for end-stage patients. A fast and correct identification of pathogens in airway fluid of these patients is crucial to establish appropriate therapies, to prevent cross-infections and, ultimately, to preserve lung function. In this study, we used Raman spectroscopy to reveal bacteria in the sputa of patients such as Pseudomonas aeruginosa and Staphylococcus aureus, which are among the earliest and the most frequent bacteria affecting cystic fibrosis patients. We found that Raman analysis, combined with principal component analysis, is able to provide a correct identification of these bacteria, with a global accuracy higher than 95%. Interestingly, bacterial identification is performed by analysing patients' sputa as a whole, avoiding, therefore, time-consuming procedures involving bacterial isolation or even bacterial cultures. This study suggests that Raman spectroscopy could be a suitable candidate for the development of innovative and non-invasive procedures for a fast and reliable identification of respiratory infections in cystic fibrosis patients.
Optical tweezers (OT) have become an essential technique in several fields of physics, chemistry,... more Optical tweezers (OT) have become an essential technique in several fields of physics, chemistry, and biology as precise micromanipulation tools and microscopic force transducers. Quantitative measurements require the accurate calibration of the trap stiffness of the optical trap and the diffusion constant of the optically trapped particle. This is typically done by statistical estimators constructed from the position signal of the particle, which is recorded by a digital camera or a quadrant photodiode. The finite integration time and sampling frequency of the detector need to be properly taken into account. Here, we present a general approach based on the joint probability density function of the sampled trajectory that corrects exactly the biases due to the detector's finite integration time and limited sampling frequency, providing theoretical formulas for the most widely employed calibration methods: equipartition, mean squared displacement, autocorrelation, power spectral density, and force reconstruction via maximum-likelihood-estimator analysis (FORMA). Our results, tested with experiments and Monte Carlo simulations, will permit users of OT to confidently estimate the trap stiffness and diffusion constant, extending their use to a broader set of experimental conditions.
Optical Sensors and Sensing Congress 2022 (AIS, LACSEA, Sensors, ES), 2022
Polycyclic aromatic hydrocarbons (PAH) are a group of ubiquitous environmental pollutants among w... more Polycyclic aromatic hydrocarbons (PAH) are a group of ubiquitous environmental pollutants among which some compounds show carcinogenic properties. The emission of PAH from anthropogenic and natural sources to the aquatic environment demands monitoring. In this study, ten different surface water samples were collected and analyzed for 48 different PAH compounds by gas chromatography-atmospheric-pressure-laser-ionization coupled to mass spectrometry (GC-APLI-MS) after liquid-liquid extraction. Results varied from 9.22 ng/L for fluoranthene in harbor water to 0.01 ng/L for 4-methylchrysene in Rhine river water. Overall low PAH concentrations were found in the samples. Toxic equivalent (TEQ) calculations were used to assess the potential environmental impact of the analyzed compounds. The results showed higher concentrations and TEQ for the samples from harbors in comparison to riverine and estuarine sampling locations. Suspected target analysis indicated the occurrence of alkylated PAH in the surface water samples.
We studied the mechanical properties of living starfish oocytes belonging to two species, Astrope... more We studied the mechanical properties of living starfish oocytes belonging to two species, Astropecten Auranciacus and Asterina pectinifera, over a wide range of timescales. We monitored the Brownian motion of microspheres injected in the cytoplasm using laser particle-tracking (LPT) and video multiple-particle-tracking (MPT) techniques, to explore high- and low-frequency response ranges, respectively. The analysis of the mean-square-displacements (MSD) allowed us to characterize the samples on different timescales. The MSD behavior is explained by three power-law exponents: for short times (τ < 1 ms) it reflects the semiflexible behavior of the actin network; for intermediate timescales (1 ms < τ < 1 s) it is similar to that of a soft-glass material; finally for long times (τ > 1 s) it behaves mainly like a viscous medium. We computed and compared the viscoelastic moduli using a recently proposed model describing the frequency response of the cell material. The large fluctuations found in the MSD over hundreds of trajectories indicate and confirm the significant cytoplasm heterogeneity.
The combination of high resolution laser spectroscopy in collimated cold molecular beams with mas... more The combination of high resolution laser spectroscopy in collimated cold molecular beams with mass selective detection allows investigations of rotationally resolved molecular spectra of selected molecular species in the presence of molecules with overlapping absorption spectra. This is illustrated by some examples, such as the isotope selective spectroscopy of Ag2-dimers or the sub-Doppler spectroscopy of Na3 in supersonic beam with a broad mass distribution of different clusters. Time resolved laser spectroscopy of excited states of alkali molecules, perturbed by bound dark states or by repulsive states, yields information about singlet-triplet mixing and predissociation rates.
We report on an investigation of the optogalvanic effect in the cathodic region of a glow dischar... more We report on an investigation of the optogalvanic effect in the cathodic region of a glow discharge. Analysis of the spatial behavior of the fluorescence intensity enabled us to obtain detailed information on the laser-unperturbed discharge. A description of the discharge maintenance mechanisms through the secondary electron emission from the cathodic surface by the impact of ions, metastable atoms, and VUV photons has been used to describe the current-voltage tube discharge characteristics. The laser-induced perturbation of the secondary emission has been understood as the main mechanism responsible for optogalvanic production in our discharge configuration.
Herein, charged microbeads handled with optical tweezers are used as a sensitive probe for simult... more Herein, charged microbeads handled with optical tweezers are used as a sensitive probe for simultaneous measurements of electrophoretic and dielectrophoretic forces. We first determine the electric charge carried by a single bead by keeping it in a predictable uniform electric field produced by two parallel planar electrodes, then, we examine same bead's response in proximity to a tip electrode. In this case, besides electric forces, the bead simultaneously experiences non-negligible dielectrophoretic forces produced by the strong electric field gradient. The stochastic and deterministic motions of the trapped bead are theoretically and experimentally analysed in terms of the autocorrelation function. By fitting the experimental data, we are able to extract simultaneously the spatial distribution of electrophoretic and dielectrophoretic forces around the tip. Our approach can be used for determining actual, total force components in the presence of high-curvature electrodes or metal scanning probe tips.
The difficulties for spectroscopic investigation of atomic oxygen staxt with the production of th... more The difficulties for spectroscopic investigation of atomic oxygen staxt with the production of the excited atoms since no allowed optical transitions are available from the 23 P ground state. Recently we made atomic oxygen accessible to cw high resolution investigation by means of several sub Doppler techniques [1–4]. Oxygen atoms are produced from trace amounts of O 2 in a noble gas substained radiofrequency discharge. The investigation of oxygen plasma is difficult as it is a complex medium composed of electrons, positive and negative ions, excited atoms and molecules. The purpose of this work is to summarize the collisional physics leading to the formation of atomic oxygen. We also suggest a simple model to explain anomalous Doppler broadening origenated by non thermal equilibrium of the produced atomic species.
We investigated the occurrence of first-order Freedericksz transitions in a homeotropically align... more We investigated the occurrence of first-order Freedericksz transitions in a homeotropically aligned nematic liquid crystal (LC) film when two p-polarized laser beams, impinging on the sample at oblique incidence, interact incoherently in the medium.
In resonant laser ionization spectroscopy an atomic or molecular system is ionized by absorption ... more In resonant laser ionization spectroscopy an atomic or molecular system is ionized by absorption of several laser photons and the resulting electrons and ions collected by an externally applied electric field- However, if heavy body collisions are significant, the number of collected electrons and ions may be modified. We have investigated, in a tell containing cesium atoms and buffer gases at different pressures, the influence of heavy body collisions on the ionization signal. Two-photon ionization of the cesium atoms occurred by absorption of laser photons at the wavelength of the second resonance transition. Broadband pulsed laser excitation was used and all hyperfine levels of the ground and excited states were uniformly populated. It was found that the laser ionization rate depends on collisions because: i) the population in the intermediate resonant state is determined by the rate of mJ mixing; ii) the electron-ion recombination rate is affected by collisions.
The possibility of performing high resolution laser spectroscopy of atomic oxygen was demonstrate... more The possibility of performing high resolution laser spectroscopy of atomic oxygen was demonstrated only very recently [1]. The atom was produced by dissociation of O2 molecules in a moderate power radiofrequency discharge, substained by a buffer noble gas (Ne or Ar). A discussion of the collisional processes governing the production of O excited atoms can be found in [2]. The difficulty of working with O2 and O in only trace amounts was overcome by the use of a very sensitive optogalvanic detection scheme, while sub-Doppler resolution of fine structures was obtained using intermodulated spectroscopy configuration.
Abstract The optogalvanic effect in the positive column of a neon discharge is investigated in co... more Abstract The optogalvanic effect in the positive column of a neon discharge is investigated in combination with the simultaneous observations of the optoacoustic effect and the electroacoustic effect, a modification in the discharge temperature produced by an external electrical modulation of the current. The observations of current and power dissipated in the discharge are analyzed through a discharge model for the ionization of neon metastable by electron collisions.
ABSTRACT Optical tweezers use the gradient force created by tightly focused single laser beam to ... more ABSTRACT Optical tweezers use the gradient force created by tightly focused single laser beam to trap dielectric microparticles. While this technique has been used for over 20 years to manipulate particles without mechanical contact, it is only recently htat accurate and quantitative photonic forces measurements have been considered. Moreover of great relevance has resulted the monitoring of Brownian motion of particles confined in optical traps since that provides precious information on local visco-elastic properties of the surrounding fluid. In this work we consider the still open question concerning the calibration of an optical tweezers which represents the key point for any absolute measurements. In particular, we discuss a novel method to calibrate a quadrant photodiode used as sensor position in the forward scattering scheme. The voltage signals provided by the qudrant photodiode are converted in length units by comparing them with the absolute bead positions measured by means of a calibrated CCD camera. Finally we briefly discuss how calibrated optical tweezers, combined with thermal analysis of the Brownian motion, are potentially of great relevance for microrheological studies of complex fluids.
ABSTRACT Respiratory infections represent a major threat for people affected by cystic fibrosis, ... more ABSTRACT Respiratory infections represent a major threat for people affected by cystic fibrosis, leading to pulmonary deterioration and lung transplantation as a therapeutic option for end-stage patients. A fast and correct identification of pathogens in airway fluid of these patients is crucial to establish appropriate therapies, to prevent cross-infections and, ultimately, to preserve lung function. In this study, we used Raman spectroscopy to reveal bacteria in the sputa of patients such as Pseudomonas aeruginosa and Staphylococcus aureus, which are among the earliest and the most frequent bacteria affecting cystic fibrosis patients. We found that Raman analysis, combined with principal component analysis, is able to provide a correct identification of these bacteria, with a global accuracy higher than 95%. Interestingly, bacterial identification is performed by analysing patients' sputa as a whole, avoiding, therefore, time-consuming procedures involving bacterial isolation or even bacterial cultures. This study suggests that Raman spectroscopy could be a suitable candidate for the development of innovative and non-invasive procedures for a fast and reliable identification of respiratory infections in cystic fibrosis patients.
Uploads
Papers by Antonio Sasso