Thin film production from a compressible organic target by laser irradiation: from material destruction at high target pressure to material drop-out at low pressure

Full text: The protein lysozyme is used as a model system for film deposition of complex organic molecules. Its properties are well-known, it has a significant signal for the mass analysis by MALDI (Matrix-Assisted Laser Desorption/Ionization) and an activity test which is not too complicated. We have recently demonstrated that such films can be produced by laser irradiation without an external matrix, but by utilizing the residual water in lysozyme powder as a matrix. Thin film production of an organic material without an external matrix, e.g. toluene or water ice, reduces the background pressure considerably and facilitates the deposition of organic molecules without compounds between the film molecules and the matrix molecules. Uniform thin films of proteins in a dry environment are difficult to produce in the thickness range of 10-100 nm, and typically these films can be used for sensors and for patterning of organic systems, as well as for other biotechnical and pharmaceutical applications. In contrast to most inorganic powders, lysozyme powder can be prepared to a target with a pressure that varies by more than one order of magnitude, i.e. from 10 to 130 bar. A standard deposition at 2 J/cm² with laser light at 355 nm and a target preparation pressure of 60 bar leads to a deposition of about 6 ng/cm² per pulse. The pressed target density at 60 bar is about 0.6 of the crystalline lysozyme density. The target preparation was carried out by pressing the lysozyme powder with a calibrated piston. The increase of target preparation pressure leads to a considerable enhancement of the deposition rate, which varies by a factor of 3 from the lowest pressure at 10 bar up to the highest at 130 bar. However, the number of intact molecules transferred to the film on a substrate falls off by almost two orders of magnitude in the same pressure interval, as determined by quantitative MALDI. The lysozyme molecules are presumably destroyed by the high preparation pressure already in the target or early in the ablation process. It means that the high deposition rate at high target pressure just reflects the high number of lysozyme fragments rather than intact molecules in the film. Films deposited at high target pressure are thus of limited quality, even though they are faster to produce. A surprising observation, which could not be quantified, was that a target prepared with low pressure (< 30 bar) mounted with a perpendicular surface fell out of the holder after few laser shots. The thermal impact of the laser pulses was apparently sufficient to eject material at low internal adhesion. At low pressure a target holder mounted with an inclination of 45^° to the horizontal plane had to be used, such that the expanding lysozyme did not fall out. At such low pressure a substantial part (up to 0.5) of the ablated lysozyme was ejected in chunks, which could be collected on a horizontally placed plate below the target. Most of the films as well as those studied were thus prepared at the optimum target pressure at 60 bar. The influence of the target pressure on these ablation phenomena will be discussed in great detail. (author)