Laser-Induced Transferred Antibacterial Nanoparticles for Mixed-Species Bacteria Biofilm Inactivation.

In the present study, copper and silver nanoparticles with a concentration of 20 µg/cm2 were synthesized using the method of laser-induced forward transfer (LIFT). The antibacterial activity of the nanoparticles was tested against bacterial biofilms that are common in nature, formed by several types of microorganisms (mixed-species bacteria biofilms): Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. The Cu nanoparticles showed complete inhibition of the bacteria biofilms used. In the course of the work, a high level of antibacterial activity was demonstrated by nanoparticles. This activity manifested in the complete suppression of the daily biofilm, with the number of bacteria decreasing by 5-8 orders of magnitude from the initial concentration. To confirm antibacterial activity, and determine reductions in cell viability, the Live/Dead Bacterial Viability Kit was used. FTIR spectroscopy revealed that after Cu NP treatment, there was in a slight shift in the region, which corresponded to fatty acids, indicating a decrease in the relative motional freedom of molecules.

Comparative Analysis of the Silver Nanoparticle's Yield for Pico-Femto-Nanosecond Laser Generation.

Comparative analysis of different laser regimes of silver nanoparticle generation in water was performed for laser pulsewidth in the range of 300 fs-100 ns. Optical spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and method of dynamic light scattering were used for nanoparticle characterization. Different laser regimes of generation were used with varying pulse duration, pulse energy and scanning velocity. The proposed universal quantitative criteria as productivity and ergonomicity of the obtained colloidal solutions of nanoparticles were investigated to compare different laser regimes of production. The efficiency per unit energy for picosecond generation of nanoparticles, free from the influence of nonlinear effects, turns out to be higher by 1-2 orders of magnitude than for nanosecond generation.

FT-IR Analysis of P. aeruginosa Bacteria Inactivation by Femtosecond IR Laser Radiation.

We report the successful inactivation of P. aeruginosa strain by femtosecond infrared (IR) laser radiation at the resonant wavelengths of 3.15 µm and 6.04 µm, chosen due to the presence of characteristic molecular vibrations in the main structural elements of the bacterial cells in these spectral ranges: vibrations of amide groups in proteins (1500-1700 cm-1), and C-H vibrations in membrane proteins and lipids (2800-3000 cm-1). The underlying bactericidal structural molecular changes were revealed by the stationary Fourier-transform IR spectroscopy, with the spectral peaks parameters being obtained by Lorentzian fitting with the hidden peaks revealed by the second derivative calculations, while no visible damage to the cell membranes was identified by scanning and transmission electron microscopy.

Additive Nanosecond Laser-Induced Forward Transfer of High Antibacterial Metal Nanoparticle Dose onto Foodborne Bacterial Biofilms.

Additive laser-induced forward transfer (LIFT) of metal bactericidal nanoparticles from a polymer substrate directly onto food bacterial biofilms has demonstrated its unprecedented efficiency in combating pathogenic microorganisms. Here, a comprehensive study of laser fluence, metal (gold, silver and copper) film thickness, and the transfer distance effects on the antibacterial activity regarding biofilms of Gram-negative and Gram-positive food bacteria (Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Listeria monocytogenes, Salmonella spp.) indicated the optimal operation regimes of the versatile modality. LIFT-induced nanoparticle penetration into a biofilm was studied by energy-dispersion X-ray spectroscopy, which demonstrated that nanoparticles remained predominantly on the surface of the biofilm.

Direct Bactericidal Comparison of Metal Nanoparticles and Their Salts against S. aureus Culture by TEM and FT-IR Spectroscopy.

We report the bactericidal effect of Ag and Cu NPs with different concentrations on methicillin-resistant S. aureus strain in comparison to the effect of AgNO3 and CuCl2 solutions, characterized by microbiological tests, TEM and Fourier-transform infrared spectroscopy. NPs were produced by nanosecond laser ablation in distilled water and characterized by scanning electron microscopy, UV-vis, energy dispersive X-ray, FT-IR spectroscopy, as well as X-ray diffraction, dynamic light scattering size and zeta-potential measurements. Microbiological tests showed antibacterial activity of NPs and metal ion-containing salts. Comparative FT-IR spectroscopy of bacteria, treated with metal NPs and salts, showed the broadening of amide I and II bands, a CH2-related peak and its frequency decrease, indicating the increase of membrane fluidity. The main mechanisms of the antibacterial effect were proposed: Ag and Cu NPs release ions and ROS, which result in lipid peroxidation; AgNO3 forms precipitates on the cell surface, which lead to the mechanical rupture of the membrane and subsequent possible penetration of the precipitates in the emerged damaged spots, complete destruction of the membrane and bacterial death; Cu ions from the CuCl2 solution cause damage to phosphorus- and sulfur-containing biomolecules, which leads to disruption of intracellular biochemical processes. The theories were confirmed by FT-IR spectroscopy and TEM.

Nonlinear Propagation and Filamentation on 100 Meter Air Path of Femtosecond Beam Partitioned by Wire Mesh.

High-intensity (∼1 TW/cm2 and higher) region formed in the propagation of ∼60 GW, 90 fs Ti:Sapphire laser pulse on a ∼100 m path in air spans for several tens of meters and includes a plasma filament and a postfilament light channel. The intensity in this extended region is high enough to generate an infrared supercontinuum wing and to initiate laser-induced discharge in the gap between the electrodes. In the experiment and simulations, we delay the high-intensity region along the propagation direction by inserting metal-wire meshes with square cells at the laser system output. We identify the presence of a high-intensity region from the clean-spatial-mode distributions, appearance of the infrared supercontinuum wing, and occurrence of the laser-induced discharge. In the case of free propagation (without any meshes), the onset of the high-intensity zone is at 40-52 m from the laser system output with ∼30 m extension. Insertion of the mesh with 3 mm cells delays the beginning of the high-intensity region to 49-68 m with the same ∼30 m extension. A decrease in the cell size to 1 mm leads to both delay and shrinking of the high-intensity zone to 71-73 m and 6 m, respectively. Three-dimensional simulations in space confirm the mesh-induced delay of the high-intensity zone as the cell size decreases.

Compact Radio Frequency Discharge-Pumped Slab CO Laser System with a Zinc Germanium Phosphide (ZnGeP2) Sum-Frequency Generator for Remote Sensing of the Atmosphere.

Based on our experimental study of spectral characteristics of an improved compact CO laser system, the prospects of using such a laser system for solving urgent problems of atmospheric sounding were demonstrated. An increase in the number of laser lines due to intracavity generation of the second harmonic and sum-frequencies in a nonlinear zinc germanium phosphide (ZnGeP2) crystal enriches capabilities of the laser. It was shown that such a CO laser system with frequency conversion in ZnGeP2 crystal can be used for detection and measuring concentration of at least 14 minor natural and pollution gases components of the atmosphere. The advantages and disadvantages of various laser operation modes, and the prospects for using its radiation for atmospheric remote sensing in various spectral ranges are discussed.

Terahertz emission pattern from a single-color filament plasma.

We study the angular distribution of different spectral components of the terahertz emission from a single-color laser filament plasma. The opening angle of a terahertz cone is experimentally demonstrated to be proportional to the inverse square root of both plasma channel length and terahertz frequency in the non-linear focusing mode, whereas in the case of linear focusing this dependence breaks down. We also experimentally show that any conclusions of terahertz radiation spectral composition require the angle range from which it is collected to be specified.

Spectrally-selective mid-IR laser-induced inactivation of pathogenic bacteria.

Micrometer-thick layers of Pseudomonas aeruginosa bacteria were prepared on fluorite substrates and scanned by focused mid-IR femtosecond laser radiation that was spectrally tuned to achieve the selective excitation of either the stretching C-H vibrations (3 µm), or stretching C = O, C-N vibrations (6 µm) of the amide groups in the bacteria. The enhanced biocidal efficiency of the latter selective excitation, compared to the more uniform 3-µm laser excitation, was demonstrated by performing viability assays of laser-treated bacterial layers. The bacterial inactivation by the 6-µm ultrashort laser pulses is attributed to dissociative denaturation of lipids and proteins in the cell membranes and intra-cell nucleic acids.

Asymmetric spectral broadening of sub-picosecond laser pulse in BaWO4 crystal: interplay of self-phase modulation, stimulated Raman scattering, and orientational Kerr nonlinearity.

Spectral broadening of 0.3 ps 515 nm laser pulse in a highly Raman-active BaWO4 crystal and fused silica demonstrates significantly different behavior with the incident pulse energy. While the broadening in fused silica is fairly symmetric with respect to the pump laser pulse wavelength, the Stokes wing broadening in the BaWO4 crystal is 2 times wider than that of anti-Stokes wing, the former demonstrating a step-like increase with the pulse energy. To the best of our knowledge, the obtained data are the first clear evidences of the following facts: (i) stimulated Raman scattering with sufficiently high efficiency of conversion to Stokes components slows down spectral broadening induced by self-phase modulation, and (ii) the mechanism of Kerr nonlinearity, which is responsible for self-phase modulation in BaWO4, is of orientational nature. The nonlinear refraction coefficient and its decay time following from our experiments with BaWO4 were estimated as n2≈6.4.10-15cm2/W and τNL≈0.35ps.

Few Percent Efficient Polarization-Sensitive Conversion in Nonlinear Plasmonic Interactions Inside Oligomeric Gold Structures.

The backscattering spectra of a 500 nm thick gold film, which was excited near the 525 nm transverse localized plasmon resonance of its constituent, self-organized, vertically-aligned nanorods by normally incident 515 nm, 300 fs laser pulses with linear, radial, azimuthal and circular polarizations, revealed a few-percent conversion into Stokes and anti-Stokes side-band peaks. The investigation of these spectral features based on the nanoscale characterization of the oligomeric structure and numerical simulations of its backscattering response indicated nonlinear Fano-like plasmonic interactions, particularly the partially degenerate four-wave mixing comprised by the visible-range transverse plasmon resonance of the individual nanorods and an IR-range collective mode of the oligomeric structure. Such oligomeric structures in plasmonic films may greatly enhance inner nonlinear electromagnetic interactions and inner near-IR hotspots, paving the way for their engineered IR tunability for broad applications in chemosensing and biosensing.

In Vitro Destruction of Pathogenic Bacterial Biofilms by Bactericidal Metallic Nanoparticles via Laser-Induced Forward Transfer.

A novel, successful method of bactericidal treatment of pathogenic bacterial biofilms in vitro by laser-induced forward transfer of metallic nanoparticles from a polyethylene terephthalate polymeric substrate was suggested. Transferred nanoparticles were characterized by scanning and transmission electron microscopy, energy-dispersive X-ray and Raman spectroscopy. The antibacterial modality of the method was tested on Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas Aeruginosa) bacterial biofilms in vitro, revealing their complete destruction. The proposed simple, cost-effective and potentially mobile biofilm treatment method demonstrated its high and broad bactericidal efficiency.

Similarity of angular distribution for THz radiation emitted by laser filament plasma channels of different lengths.

The influence of plasma channel length on an angular terahertz (THz) radiation distribution is experimentally studied for the channel formed under filamentation of an ultrashort laser pulse. It is shown that the angular distribution of the THz emission depends only on laser intensity in the filament and plasma density of the plasma channel and does not depend on the plasma channel length. A qualitative explanation of the THz emission screening by the filament plasma channel is proposed.

Polarization-Sensitive Surface-Enhanced In Situ Photoluminescence Spectroscopy of S. aureus Bacteria on Gold Nanospikes.

We report the possibility of a time-resolved bacterial live/dead dynamics observation with the use of plasmonic nanospikes. Sharp nanospikes, fabricated on a 500-nm thick gold film by laser ablation with the use of 1030-nm femtosecond pulses, were tested as potential elements for antibacterial surfaces and plasmonic luminescence sensors. Staphylococcus aureus bacteria were stained by a live/dead viability kit, with the dead microorganisms acquiring the red colour, caused by the penetration of the luminescent dye propidium iodide through the damaged cell membrane. Photoluminescence was pumped by 515-nm femtosecond laser pulses with linear (Gaussian beam), circular, azimuthal and radial (Laguerre-Gaussian beam) polarizations, exciting the transverse plasmon resonance of the nanospikes and their apex lightning-rod near-field. According to the numerical electrodynamic modeling, the observed strong increase in the photoluminescence yield for radial polarization, while slightly lower for circular and azimuthal polarizations, compared with the low luminescence intensities for the linear laser polarization, was related to their different laser-nanospike coupling efficiencies.

Surface-Enhanced IR-Absorption Microscopy of Staphylococcus aureus Bacteria on Bactericidal Nanostructured Si Surfaces.

Surface-enhanced IR absorption (SEIRA) microscopy was used to reveal main chemical and physical interactions between Staphylococcus aureus bacteria and different laser-nanostructured bactericidal Si surfaces via simultaneous chemical enhancement of the corresponding IR-absorption in the intact functional chemical groups. A cleaner, less passivated surface of Si nanoripples, laser-patterned in water, exhibits much stronger enhancement of SEIRA signals compared to the bare Si wafer, the surface coating of oxidized Si nanoparticles and oxidized/carbonized Si (nano) ripples, laser-patterned in air and water. Additional very strong bands emerge in the SEIRA spectra on the clean Si nanoripples, indicating the potential chemical modifications in the bacterial membrane and nucleic acids during the bactericidal effect.

Influence of air humidity on 248-nm ultraviolet laser pulse filamentation.

At first glance, the amount of water molecules naturally contained in humid air is negligibly small to affect filamentation of ultrashort laser pulses. However, here we show, both experimentally and numerically, that for ultraviolet laser pulses with 248 nm wavelength this is not true. We demonstrate that with increase of air humidity the plasma channels generated by the ultraviolet laser pulses in air become longer and wider, while the corresponding electron density in humid air can be up to one order of magnitude higher compared to dry air.

Nanosecond-Laser Generation of Nanoparticles in Liquids: From Ablation through Bubble Dynamics to Nanoparticle Yield.

A comprehensive picture of the nanosecond-laser generation of colloidal nanoparticles in liquids is nowadays the demand of their high-throughput industrial fabrication for diverse perspective biomedical, material science, and optoelectronic applications. In this study, using silicon as an example, we present a self-consistent experimental visualization and theoretical description of key transient stages during nanosecond-laser generation of colloidal nanoparticles in liquids: plasma-mediated injection of ablated mass into the liquid and driving the vapor bubble, finalized by the colloid appearance in the liquid. The explored fundamental transient stages envision the basic temporal and spatial scales, as well as laser parameter windows, for the demanded high-throughput nanosecond-laser generation of colloidal nanoparticles in liquids.

Direct laser writing of barriers with controllable permeability in porous glass.

Barriers were produced in porous glass through its local bulk density modification by direct femtosecond writing accompanied by СО2-laser surface thermal densification, to make functional microfluidic elements separated by such physical barriers with different controlled permeability. The separation of multi-component solutions into individual components with different molecule sizes (molecular separation) was performed in this first integrated microfluidic device fabricated in porous glass. Its application in the environmental gas-phase analysis was demonstrated.

On-Fly Femtosecond-Laser Fabrication of Self-Organized Plasmonic Nanotextures for Chemo- and Biosensing Applications.

Surface-enhanced Raman scattering (SERS) and surface-enhanced photoluminescence (SEPL) are emerging as versatile widespread methods for biological, chemical, and physical characterization in close proximity of nanostructured surfaces of plasmonic materials. Meanwhile, single-step, facile, cheap, and green technologies for large-scale fabrication of efficient SERS or SEPL substrates, routinely demonstrating both broad plasmonic response and high enhancement characteristics, are still missing. In this research, single-pulse spallative micron-size craters in a thick Ag film with their internal nanotexture in the form of nanosized tips are for the first time shown to demonstrate strong polarization-dependent enhancement of SEPL and SERS responses from a nanometer-thick covering Rhodamine 6G layer with average enhancement factors of 40 and 2 × 10(6), respectively. Additionally, the first detailed experimental study is reported for physical processes, underlying the formation mechanisms of ablative nanotextures on such "thick" metal films. Such mechanisms demonstrate a complex "hybrid" fluence-dependent ablation character-appearance of spallative craters, typical for bulk material, at low fluences and formation of upright standing nanotips (frozen nanojets), usually associated with thin-film ablation, in the crater centers at higher fluences. Moreover, special emphasis was made on the possibility to reshape the nanotopography of such spallative craters through multipulse laser-induced merging of their small nanotips into larger ones. The presented approach holds promise to be one of the cheapest and easiest-to-implement ways to mass-fabricate various efficient spallation-nanotextured single-element plasmonic substrates for routine chemo- and biosensing, using MHz-repetition-rate femtosecond fiber laser sources with multiplexed laser-beams.

Experimental study of fs-laser induced sub-100-nm periodic surface structures on titanium.

In this work the formation of laser-induced periodic surface structures (LIPSS) on a titanium surface upon irradiation by linearly polarized femtosecond (fs) laser pulses with a repetition rate of 1 kHz in air environment was studied experimentally. In particular, the dependence of high-spatial-frequency-LIPSS (HSFL) characteristics on various laser parameters: fluence, pulse number, wavelength (800 nm and 400 nm), pulse duration (10 fs - 550 fs), and polarization was studied in detail. In comparison with low-spatial-frequency-LIPSS (LSFL), the HSFL emerge at a much lower fluence with orientation perpendicular to the ridges of the LSFL. It was observed that these two types of LIPSS demonstrate different fluence, shot number and wavelength dependencies, which suggest their origin is different. Therefore, the HSFL formation mechanism cannot be described by the widely accepted interference model developed for describing LSFL formation.