P. Serra

Preparation of functional DNA microarrays through laser-induced forward transfer

A functional DNA microarray was prepared through the laser-induced forward transfer (LIFT) technique. In a first experiment, droplets of a buffer solution were spotted onto a substrate at different laser pulse energies. This allowed one to determine that uniform spots with a diameter as small as 40μm could be obtained. In a second experiment, a microarray containing two different human cDNAs and a negative control was spotted through LIFT and submitted to a hybridization assay. The obtained results demonstrated the full functionality of the microarray, which allowed us to prove the viability of LIFT for the production of DNA microarrays.

Laser-induced forward transfer of liquids: Study of the droplet ejection process

Laser-induced forward transfer LIFT is a laser direct-write technique that offers the possibility of printing patterns with a high spatial resolution from a wide range of materials in a solid or liquid state, such as conductors, dielectrics, and biomolecules in solution. This versatility has made LIFT a very promising alternative to lithography-based processes for the rapid prototyping of biomolecule microarrays. Here, we study the transfer process through the LIFT of droplets of a solution suitable for microarray preparation. The laser pulse energy and beam size were systematically varied, and the effect on the transferred droplets was evaluated. Controlled transfers in which the deposited droplets displayed optimal features could be obtained by varying these parameters. In addition, the transferred droplet volume displayed a linear dependence on the laser pulse energy. This dependence allowed determining a threshold energy density value, independent of the laser focusing conditions, which acted as necessary conditions for the transfer to occur. The corresponding sufficient condition was given by a different total energy threshold for each laser beam dimension. The threshold energy density was found to be the dimensional parameter that determined the amount of the transferred liquid per laser pulse, and there was no substantial loss of material due to liquid vaporization during the transfer.

Time-resolved imaging of the laser forward transfer of liquids

Time-resolved imaging is carried out to study the dynamics of the laser-induced forward transfer of an aqueous solution at different laser fluences. The transfer mechanisms are elucidated, and directly correlated with the material deposited at the analyzed irradiation conditions. It is found that there exists a fluence range in which regular and well-defined droplets are deposited. In this case, laser pulse energy absorption results in the formation of a plasma, which expansion originates a cavitation bubble in the liquid. After the further expansion and collapse of the bubble, a long and uniform jet is developed, which advances at a constant velocity until it reaches the receptor substrate. On the other hand, for lower fluences no material is deposited. In this case, although a jet can be also generated, it recoils before reaching the substrate. For higher fluences, splashing is observed on the receptor substrate due to the bursting of the cavitation bubble. Finally, a discussion of the possible mechanisms which lead to such singular dynamics is also provided.

Analysis of the expansion of hydroxyapatite laser ablation plumes

Fast intensified CCD imaging has been used to analyze the expansion of hydroxyapatite laser ablation plumes after KrF excimer laser irradiation. Images as obtained have shown the presence of three different glow regions. A plume-like plasma followed by a smaller and slower plasma cloud freely expand from the first instants of time after the laser pulse. A third glow region, the expansion of which begins over 50 μs after the laser impingement, is formed by hot particles leaving the target. The same experiment has been repeated under the same conditions by changing the laser wavelength to that of ArF. Only the first two plasma clouds have been identified in this case. Finally, in order to relate the detection of hot particles with the presence of droplets in HA pulsed laser deposited films, scanning electron micrographs of the deposited coatings have been obtained showing a much lower density of droplets in the case of ArF irradiation than in that of KrF.

Characterization of hydroxyapatite laser ablation plumes by fast intensified CCD-imaging

ArF excimer laser pulses (193 nm, 20 ns, 150 mJ) have been focused on a hydroxyapatite (HA) target in similar conditions to those normally used for thin film deposition. Fast intensified CCD images of HA laser ablation plumes have been taken in vacuum and under different water vapor pressures ranging from 0.01 mbar to 1 mbar. Images of HA ablation in vacuum have shown a plume freely expanding at a constant velocity of 2.3 X 10 cm/s. HA ablation under a water vapor pressure of 0.01 mbar has revealed an expansion behavior very similar to that of ablation in vacuum. Images taken under a water vapor pressure of 0.1 mbar have shown the formation of a shock structure in the plume. Finally, HA ablation under a water vapor pressure of 1 mbar has revealed the development of some irregularities in the shape of the plume.

Species resolved analysis of the expansion of hydroxyapatite laser ablation plumes

The plume generated by ablation of hydroxyapatite targets under ArF excimer laser irradiation has been investigated by means of fast intensified charge coupled device (CCD) imaging and optical emission spectroscopy. Results have shown that the plume splits into two plasma clouds as it expands. Time and spatial resolved spectra have revealed that under the experiment conditions emission is mostly due to calcium neutral atoms and calcium oxide molecular radicals. Imaging of the plume with the aid of bandpass filters has demonstrated that the emissive species in the larger and faster plasma cloud are calcium neutral atoms whereas in the smaller and slower one are calcium oxide molecular radicals. Pulsed laser deposition (PLD) is a good technique to deposit thin films of complex compounds, and many authors 1‐6 have used it to obtain hydroxyapatite (HA) coatings for medical prostheses. This motivated us to study the HA laser ablation process through the characterization of the plume by fast intensified CCD imaging. 7,8 The resultant pictures of the plasma generated by ArF excimer laser irradiation of HA targets revealed the presence of two different emissive regions. Thus, in this paper we develop a more comprehensive analysis of the HA laser ablation plume by means of time and space resolved optical emission spectroscopy

Carbon nitride thin films obtained by laser ablation of graphite in a nitrogen plasma

Abstract Carbon nitride thin films were deposited by KrF (248 nm) laser ablation of graphite in a nitrogen atmosphere. A dc or rf nitrogen plasma was also superimposed onto the ablation process. The films were studied using a wide range of characterization techniques such as scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), wavelength-dispersive electron probe X-ray microanalysis (WDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) and Raman spectroscopies. The films were found to have a nitrogen concentration close to 10%. Nitrogen atoms were incorporated substitutionally in the films forming covalent C-N bonds as revealed by XPS, FT-IR and Raman spectroscopies. Spatially resolved optical spectroscopy studies of the ablation plume were also carried out.

Structural characterization of laser-treated Cr 3 C 2 -NiCr coatings

The interconnected porosity of the Cr 3 C 2 –NiCr coatings obtained by high-velocity oxy fuel spraying is detrimental in corrosion and wear resistance applications. Laser treatments allow sealing of their surfaces through melting and resolidification of a thin superficial layer. A Nd:YAG laser beam was used to irradiate Cr 3 C 2 –NiCr coatings either in the continuous wave mode or at different repetition rates in the pulsed one. Results indicated that high peak and low mean laser irradiances are not good, since samples presented deep grooves and an extensive crack network. At low peak and higher mean laser irradiances the surface was molten, and only a few shallow cracks were observed. The interconnected porosity was completely eliminated in a layer up to 80 μm thick, formed by large Cr 7 C 3 grains imbedded in a NiCr matrix.

Evidence of chemical reactions in the hydroxyapatite laser ablation plume with a water atmosphere

The expansion dynamics of the ablation plume generated by KrF laser irradiation of hydroxyapatite targets in a 0.1 mbar water atmosphere has been studied by fast intensified charge coupled device imaging with the aid of optical bandpass filters. The aim of the filters is to isolate the emission of a single species, which allows separate analysis of its expansion. Images obtained without a filter revealed two emissive components in the plume, which expand at different velocities for delay times of up to 1.1 μs. The dynamics of the first component is similar to that of a spherical shock wave, whereas the second component, smaller than the first, expands at constant velocity. Images obtained through a 520 nm filter show that the luminous intensity distribution and evolution of emissive atomic calcium is almost identical to those of the first component of the total emission and that there is no contribution from this species to the emission from the second component of the plume. The analysis through a 780 nm...

Evolution of the plumes produced by laser ablation of a carbon target

Abstract The time evolution of plumes produced by laser ablation of a graphite target has been followed by means of intensified CCD imaging. The images were obtained in the same conditions used for the deposition of microcrystalline diamond. An ArF (193 nm, 23 ns) excimer laser beam was focused onto a pyrolytic graphite target with a fluence around 3 J cm−2. The study was performed in high vacuum and under a hydrogen or helium pressure of 1 mbar. Each image was obtained after a single laser pulse and acquired with aperture times from 5 ns to several microseconds depending on the delay time. Spatially resolved optical spectroscopy studies of the plume were also carried out. In vacuum, the images show a free expansion of the plume with a front velocity of 4 × 106 cm s−1. Spectroscopy results indicate the presence of C2. At tens of microseconds, the trails of some particulates emitted from the target can be observed. With a background pressure of 1 mbar, the plume decelerates and a shock wave structure is developed. In the case of H2, spectroscopy reveals CH formation in the expansion front.