Surface morphology of thin lysozyme films produced by matrix-assisted pulsed laser evaporation (MAPLE)

Thin films of the protein, lysozyme, have been deposited by the matrix-assisted pulsed laser evaporation (MAPLE) technique. Frozen targets of 0.3-1.0 wt.% lysozyme dissolved in ultrapure water were irradiated by laser light at 355 nm with a fluence of 2 J/cm². The surface quality of the thin lysozyme films of different thickness deposited on 7 mm × 7 mm Si-〈1 0 0〉-wafers was investigated with scanning electron microscopy and atomic force microscopy. Already at comparatively low thickness, ∼20 nm, the substrate is covered by intact lysozyme molecules and fragments. The concentration of lysozyme in the ice matrix apparently does not play any significant role for the morphology of the film. The morphology obtained with MAPLE has been compared with results for direct laser irradiation of a pressed lysozyme sample (i.e. pulsed laser deposition (PLD)).     

Surface morphology of polyethylene glycol films produced by matrix-assisted pulsed laser evaporation (MAPLE): Dependence on substrate temperature

The dependence of the surface morphology on the substrate temperature during film deposition was investigated for polyethylene glycol (PEG) films by matrix-assisted pulsed laser evaporation (MAPLE). The surface structure was studied with a combined technique of optical imaging and AFM measurements. There was a clear difference between the films produced below and above the melting point of PEG. For temperatures above the melting point, the polymer material was distributed non-uniformly over the substrate with growths areas, where cluster-like structures merge into large islands of micrometer size. At these temperatures, the islands in the investigated growth areas cover most of the bottom layer which has a typical height of 50-150 nm.     

Thin films of Cu(II)-o,o′-dihydroxy azobenzene nanoparticle-embedded polyacrylic acid (PAA) for nonlinear optical applications developed by matrix assisted pulsed laser evaporation (MAPLE)

Thin films based on two different metal-organic systems are developed by MAPLE and their nonlinear optical applications are explored. A complex of o,o′-dihydroxy azobenzene with Cu²⁺ cation is found to organize as a non-central symmetric crystallite. A simple protocol is developed for the in situ fabrication of highly monodisperse copper-complex nanoparticles in a polymer film matrix of polyacrylic acid. The thin films were deposited on quartz substrates by MAPLE (matrix assisted pulsed laser evaporation) using a Nd:YAG laser working at 355 nm. Atomic force microscopy (AFM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and optical second harmonic generation (SHG) were performed on the samples. The optical limiting capability of the nanoparticle-embedded polymer film is investigated.     

High fluence deposition of polyethylene glycol films at 1064 nm by matrix assisted pulsed laser evaporation (MAPLE)

Matrix assisted pulsed laser evaporation (MAPLE) has been applied for deposition of thin polyethylene glycol (PEG) films with infrared laser light at 1064 nm. We have irradiated frozen targets (of 1 wt.% PEG dissolved in water) and measured the deposition rate in situ with a quartz crystal microbalance. The laser fluence needed to produce PEG films turned out to be unexpectedly high with a threshold of 9 J/cm², and the deposition rate was much lower than that with laser light at 355 nm. Results from matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis demonstrate that the chemistry, molecular weight and polydispersity of the PEG films were identical to the starting material. Studies of the film surface with scanning electron microscopy (SEM) indicate that the Si-substrate is covered by a relatively homogenous PEG film with few bare spots.     

Growth of thin films of low molecular weight proteins by matrix assisted pulsed laser evaporation (MAPLE)

Thin films of lysozyme and myoglobin grown by matrix assisted pulsed laser evaporation (MAPLE) from a water ice matrix have been investigated. The deposition rate of these two low molecular weight proteins (lysozyme: 14307 amu and myoglobin: 17083 amu) exhibits a maximum of about 1–2 ng/cm² per pulse at a fluence of 1–2 J/cm² and decreases slowly with increasing fluence. This rate is presumably determined by the matrix rather by the proteins. A significant fraction of the proteins are intact in the film as determined by MALDI (Matrix assisted laser desorption ionization) spectrometry. The results for lysozyme demonstrate that the fragmentation rate of the proteins during the MAPLE process is not influenced by the pH of the water solution prior to freezing.     

Deposition of biopolymer thin films by matrix assisted pulsed laser evaporation

We report on the successful deposition of high quality type I fibrilar collagen thin films by Matrix assisted pulsed laser evaporation (MAPLE). Thin films deposition was performed in a N₂ ambient (20 Pa) using a KrF* laser source (=248 nm,20 ns) operated at a repetition rate of 3 Hz, the incident laser energy at a value within the range (20-35)mJ , and the laser spot area was (3.5-18.5)±0.1 mm². The collagen films were deposited on double face polished 100 single crystalline Si wafers and characterized by Fourier transform infrared spectroscopy, atomic force microscopy and high-resolution transmission electron microscopy. We demonstrate that our thin films are composed of collagen, with no impurities and the roughness can be controlled by the deposition conditions. PACS 52.38.Mf; 82.35.Pq; 83.80.Lz     

Polycaprolactone biopolymer thin films obtained by matrix assisted pulsed laser evaporation

We report the successful deposition of polycaprolactone polymer by MAPLE using a KrF* excimer laser (λ = 248 nm, τ = 7 ns). According to FTIR spectra the deposited films have similar chemical structure to the dropcast material. The fluence plays a key role in optimizing the performances of MAPLE-synthesized polycaprolactone structures. We demonstrated that MAPLE allows for controlling the morphology of films to the level required in targeted drug delivery of pharmacologic agents.     

Nanostructured morphology of polymer films prepared by matrix assisted pulsed laser evaporation

As recently illustrated, nanostructured glassy polymer films with exceptional thermal and kinetic stability can be formed via Matrix Assisted Pulsed Laser Evaporation (MAPLE) (Guo et al. in Nat. Mater. 11:337, 2012). Relative to the standard poly(methyl methacrylate) glass formed on cooling at standard rates, glasses prepared by MAPLE can be 40 % less dense and have a 40 K higher glass transition temperature (T _g ). Furthermore, the kinetic stability in the glassy state can be enhanced by 2 orders-of-magnitude. Here, we examine the stability of the structured morphology. We show that nanostructured glasses may be formed even when the substrate is held at temperatures greater than the polymer T _g during deposition. In addition, we discuss the origin of the enhanced stability and the mechanism of nanostructured film formation within the framework of the Zhigilei model. Finally, we compare the nanostructured morphology to the surface morphology of other MAPLE-deposited films in the literature.     

Laser deposition of cryoglobulin blood proteins thin films by matrix assisted pulsed laser evaporation

We report the first successful deposition of type II cryoglobulin blood protein thin films by matrix assisted pulsed laser evaporation (MAPLE) using a KrF^* excimer laser source (λ = 248 nm, τ_FWHM ≈ 20 ns) operated at a repetition rate of 10 Hz. We demonstrate by AFM and FTIR that MAPLE-deposited thin films consist of starting type II cryoglobulin only, maintaining its chemical structure and biological functionality, being properly collected and processed. The dependence on incident laser fluence of the induced surface morphology is presented. The presence of type II cryoglobulin was revealed as aggregates of globular material in the MAPLE-deposited thin films and confirmed by standard cryoglobulin tests.     

Antibacterial polymeric coatings grown by matrix assisted pulsed laser evaporation

We report on a simple and environmental friendly method to produce composite biocompatible antibacterial coatings consisting of silver nanoparticles (AgNPs, size 40 nm) combined with polymer blends (polyethylene glycol/poly(lactide-co-glycolide), PEG/PLGA blends). The PEG/PLGA&AgNPs coatings were produced by Matrix Assisted Pulsed Laser Evaporation, using a Nd:YAG laser with λ=266 nm. The AgNPs were deposited either on top of a PEG/PLGA layer (i.e., bilayered coating), or simultaneously with the polymers (i.e., blended coating). In both cases, chemical analysis indicated that the polymers preserved their integrity, with no evidence of chemical interaction with the AgNPs. Morphological investigations evidenced homogenous distribution of individual AgNPs on the surface of the coatings, with no signs of aggregation. The size of the AgNPs was ～40 nm, consistent with size of the as-received ones. The presence of AgNPs in the coatings was confirmed by the absorption band at ～420 nm and their stability was checked by monitoring this absorption versus time. After exposure to air, the AgNPs from the bilayered coating showed signs of oxidation. In the blended coating, the oxidation of the AgNPs was prevented by the neighboring polymer molecules. Finally, preliminary investigations confirmed the bacterial killing activity of the coatings against Escherichia coli.     

Uniform thin films of TiO2 nanoparticles deposited by matrix-assisted pulsed laser evaporation

We report morphological and optical properties of a colloidal TiO₂ nanoparticle film, deposited on a quartz substrate by using the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. Atomic Force Microscopy demonstrated that a good uniformity of the deposition can be obtained. The presence of agglomerates with dimensions of about 1 μm in size was noticed. Form UV-vis transmission spectra, recorded in the 200-800 nm range, the optical constants and the energy gap were determined besides the film thickness. The optical constants resulted in agreement with the values reported in literature for TiO₂ nanoparticle thin films.     

Thin films of polyaniline deposited by MAPLE technique

Polyaniline (PAni) has important electro-conductive properties, high absorbance in microwave range and it is also frequently used in gas sensors because of its capability to convert chemical interactions into electrical signals. The methods of obtaining polyaniline in the form of thin films and/or nanostructures are complicated and request special physical and chemical treatments, both on the substrate surface and for the polymer itself.In this paper we applied matrix assisted pulsed lased evaporation (MAPLE) for obtaining thin films and nanostructures of polyaniline. In MAPLE, the target consisting of the material (usually 0.2-3 wt%) dissolved in a solvent is frozen and it is evaporated using a laser. In our case polyaniline-emeraldine salt (PAni-ES) was dissolved in xylene or toluene, frozen in liquid nitrogen and was used as target. The third and the fourth harmonics of a Nd-YAG laser (λ = 355 nm and 266 nm) were used as laser sources. The obtained films have been characterized by atomic force microscopy, dielectric spectroscopy and Fourier transform infrared spectroscopy. The influence of the solvent type and of the laser parameters (wavelengths and fluence) on the polyaniline structures composition and properties has been investigated.     

Thin layers of bovine serum albumin by matrix assisted pulsed laser evaporation

Thin films of bovine serum albumin were prepared by cryogenic matrix assisted pulsed laser evaporation technique under various deposition conditions. Energy density of laser beam changed in the range 0.1-0.5 J cm⁻². Films were deposited in vacuum or in nitrogen ambient. Targets were prepared from bovine serum albumin solution in phosphate buffered physiological saline and with an addition of UV absorbers as dimethylsulphoxide, phthalic acid, or adenine. Polyethylene and silicon (1 1 1) were used as substrates. Film properties were studied with atomic force microscopy and Fourier transform infrared spectroscopy attenuated total reflection. The deposition changed native conformation of albumin, resulting in the formation of water-insoluble aggregates. Addition of laser light absorbers in target solutions did not prevent the damage of albumin structure.     

Thin films of polymer mimics of cross-linking mussel adhesive proteins deposited by matrix assisted pulsed laser evaporation

Mussels secrete specialized adhesives known as mussel adhesive proteins, which allow attachment of the organisms to underwater marine environments. Obtaining large quantities of naturally derived mussel adhesive proteins adhesives has proven to date rather problematic, thus, synthetic analogs of mussel adhesive proteins have recently been developed. We report deposition of 1:100 and 1:1000 poly[(3,4-dihydroxystyrene)-co-styrene)] mussel adhesive protein analogs by matrix assisted pulsed laser evaporation (MAPLE) using an ArF* excimer laser source. The deposited films have been evaluated for their antifouling behavior. The MAPLE-deposited synthetic mussel adhesive protein analog thin films are homogenous and adhesive, making the use of these materials in thin film form a viable option.     

Design challenges for matrix assisted pulsed laser evaporation and infrared resonant laser evaporation equipment

Since the development of the Matrix Assisted Pulsed Laser Evaporation (MAPLE) process by the Naval Research Laboratory (NRL) in the late 1990s, MAPLE has become an active area of research for the deposition of a variety of polymer, biological, and organic thin films. As is often the case with advancements in thin-film deposition techniques new technology sometimes evolves by making minor or major adjustments to existing deposition process equipment and techniques. This is usually the quickest and least expensive way to try out new ideas and to “push the envelope” in order to obtain new and unique scientific results as quickly as possible. This process of “tweaking” current equipment usually works to some degree, but once the new process is further refined overall designs for a new deposition tool based on the critical attributes of the new process typically help capitalize more fully on the all the salient features of the new and improved process. This certainly has been true for the MAPLE process.     

Pulsed laser deposition vs. matrix assisted pulsed laser evaporation for growth of biodegradable polymer thin films

Thin films of poly (lactide-co-glycolide) (PLGA), a biodegradable polymer, were deposited on Si wafers by both conventional pulsed laser deposition (PLD) and matrix assisted pulsed laser evaporation (MAPLE) using chloroform (CHCl₃) as a matrix solvent. This research represents an initial study to investigate the deposition characteristics of each technique at comparable conditions to gain insight into the transport and degradation mechanisms of each approach. The deposited materials were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (¹H NMR), and gel permeation chromatography (GPC) with refractive index (RI) detection. While FTIR and NMR results do not show a measurable departure from the native, in sharp contrast GPC results show a significant change (up to 95%) in molecular weight for both deposition methods. This result makes it clear that it is possible to overlook substantial degradation when incomplete chemical analysis is conducted.Optical transmission measurements of the starting MAPLE targets yielded laser penetration depths on the order of 0.362 cm and 0.209 cm for pure CHCl₃ and 1 wt. % PLGA in CHCl₃, respectively. Straightforward application of the Beer–Lambert law for laser energy deposition predicts a negligible temperature rise of less than 1 K at the target surface, which is in clear contradiction with ablation rates of 1.85 μm/pulse experimentally measured for polymer loaded samples. With an ablation process of this magnitude, the material ejection is likely due to contributions of nonlinear or non-homogeneous laser light absorption rather than evaporation. Severe non-uniformity of the final surface morphologies of the MAPLE films, similar to solvent wicking artifacts found in spin casting supports the spallation scenario in MAPLE. PACS 81.15.Fg; 79.20.Ds; 78.66.Qn; 42.70Jk     

Study of temperature dependence and angular distribution of poly(9,9-dioctylfluorene) polymer films deposited by matrix-assisted pulsed laser evaporation (MAPLE)

Poly(9,9-dioctylfluorene) (PFO) polymer films were deposited by matrix-assisted pulsed laser evaporation (MAPLE) technique. The polymer was diluted (0.5 wt%) in tetrahydrofuran and, once cooled to liquid nitrogen temperature, it was irradiated with a KrF excimer laser. 10,000 laser pulses were used to deposit PFO films on 〈1 0 0〉 Si substrates at different temperatures (−16, 30, 50 and 70 °C). One PFO film was deposited with 16,000 laser pulses at a substrate temperature of 50 °C. The morphology, optical and structural properties of the films were investigated by SEM, AFM, PL and FTIR spectroscopy. SEM inspection showed different characteristic features on the film surface, like deflated balloons, droplets and entangled polymer filaments. The roughness of the films was, at least partially, controlled by substrate heating, which however had the effect to reduce the deposition rate. The increase of the laser pulse number modified the target composition and increased the surface roughness. The angular distribution of the material ejected from the target confirmed the forward ejection of the target material. PFO films presented negligible modification of the chemical structure respect to the bulk material.     

Characterization of zirconium thin films deposited by pulsed laser deposition

Zirconium(Zr) thin films deposited on Si(100) by pulsed laser deposition(PLD) at different pulse repetition rates are investigated. The deposited Zr films exhibit a polycrystalline structure, and the X-ray diffraction(XRD) patterns of the films show the α Zr phase. Due to the morphology variation of the target and the laser–plasma interaction, the deposition rate significantly decreases from 0.0431 /pulse at 2 Hz to 0.0189 /pulse at 20 Hz. The presence of droplets on the surface of the deposited film, which is one of the main disadvantages of the PLD, is observed at various pulse repetition rates. Statistical results show that the dimension and the density of the droplets increase with an increasing pulse repetition rate. We find that the source of droplets is the liquid layer formed under the target surface. The dense nanoparticles covered on the film surface are observed through atomic force microscopy(AFM). The root mean square(RMS) roughness caused by valleys and islands on the film surface initially increases and then decreases with the increasing pulse repetition rate.The results of our investigation will be useful to optimize the synthesis conditions of the Zr films.     

Biocompatible polymeric implants for controlled drug delivery produced by MAPLE

Implants consisting of drug cores coated with polymeric films were developed for delivering drugs in a controlled manner. The polymeric films were produced using matrix assisted pulsed laser evaporation (MAPLE) and consist of poly(lactide-co-glycolide) (PLGA), used individually as well as blended with polyethylene glycol (PEG). Indomethacin (INC) was used as model drug. The implants were tested in vitro (i.e. in conditions similar with those encountered inside the body), for predicting their behavior after implantation at the site of action. To this end, they were immersed in physiological media (i.e. phosphate buffered saline PBS pH 7.4 and blood). At various intervals of PBS immersion (and respectively in blood), the polymeric films coating the drug cores were studied in terms of morphology, chemistry, wettability and blood compatibility. PEG:PLGA film exhibited superior properties as compared to PLGA film, the corresponding implant being thus more suitable for internal use in the human body. In addition, the implant containing PEG:PLGA film provided an efficient and sustained release of the drug. The kinetics of the drug release was consistent with a diffusion mediated mechanism (as revealed by fitting the data with Higuchi's model); the drug was gradually released through the pores formed during PBS immersion. In contrast, the implant containing PLGA film showed poor drug delivery rates and mechanical failure. In this case, fitting the data with Hixson-Crowell model indicated a release mechanism dominated by polymer erosion.     

Multifunctional thin films of lactoferrin for biochemical use deposited by MAPLE technique

Lactoferrin (Lf) is an iron-binding glycoprotein present in almost all mammalian secretions which plays an important role in host defense against microbial and viral infections. The protein has been reported to also have anti-inflammatory activity and antitumoral effects in vitro and in vivo.Thin films of Lf were deposited on silicon, quartz and Thermanox plastic coverslip substrates by Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique, using a Nd:YAG laser working at 266 nm, at different laser fluences (0.1-0.8 J cm⁻²). The deposited layers have been characterized by Fourier Transformed Infra-Red spectroscopy (FTIR), and the morphology of the various substrates was investigated by Atomic Force Microscopy (AFM). The biocompatibility of lactoferrin thin films was evaluated for each substrate, by in vitro biochemical tests.