The Sol-Gel Gateway: Tutorial on cleaning

COATINGS : AN INTRODUCTION TO THE CLEANING PROCEDURES

II.1.4 Procedures

The substrate to be cleaned is placed in the cleaning surfactant solution under ultrasonic agitation for a period ranging from five to fifteen minutes. A beaker of pure water is used to rinse the substrate. This may be combined with ultrasonic agitation to accelerate the process. A rinsing time of five to ten minutes should be adequate. This rinsing is repeated with a second beaker of pure water. Finally, the sample is allowed to soak in a third beaker of pure water for about 2 minutes. This last step is designed to remove small molecules that may have leached into the slightly porous glass surface. Following rinsing, the sample must be dried. This is best achieved by using heated circulated air that has been filtered to remove dust. It is important that the drying be achieved as rapidly as possible, since water has a tendency to adsorb contaminants from ambient air and re-deposit them on the substrate as it dries. In the absence of heated circulated air, compressed nitrogen gas may be used to blow-dry the surfaces. This must be done with care to avoid leaving drying streaks on the surface.
To blow-dry a surface, the procedure outlined below should be used. This is important to avoid drying streaks and surface inhomogeneities that may affect the sol-gel coating deposited on the surface.

The slide is held, using blue polypropylene forceps from Nalgene (reference 6320-0010, Nalge Nunc International, Rochester, NY 14625), as shown in figure 1, below:

Figure 1: Handling of microscope slide during drying with plastic forceps. Slide should be held vertically.

Immediately after removal from the final rinse, the slide is blow dried using clean compressed air (for example, approximately 3 bar pressure, emitted through a nozzle with diameter about 3 mm). The slide is held quasi-vertically and its reverse face (if there is a preferred face) is blow-dried from the top down using a jet of compressed air directed at an angle of about 45° from the vertical. The reason for drying the reverse face first is to maintain a water film on the front face, avoiding water streaks. The front face is dried later, after the possibility of the reverse face generating water streaks has been minimized. The drying of the reverse face is shown schematically in figure 2, below. It is important that the water film be chased down the front of the slide as it dries, leaving no drops or streaks. The drying should begin by chasing the water trapped between the plastic forceps and the slide.

Figure 2: Blow-drying of microscope slide with a flow of compressed nitrogen. The slide should be held vertically and the water chased down the face of the slide. No streaks should be generated.

The water is chased to the bottom of the slide. Any water near the edges is chased from the middle towards the edges. It is important to avoid that the water thus accumulated on the edges of the slide be chased back onto the front face of the slide, thus forming streaks. To achieve this, the jet of compressed air must be halted before it passes the edge of the slide. The angle of the air jet must then be tilted to become more parallel with the glass slide and its position adjusted so that it blows past the edge of the slide. This will blow the water droplets accumulated on the edges off the slide. This is shown in figure 3, below for the lower edge of the slide.

Figure 3: Chasing water that has accumulated on the lower edge of a slide. The stream should be directed past the edge of the slide, at an angle of about 30 to 45° with the edge of the slide. The water should be blown off the slide directly, without spreading across the slide.

A similar concept applies for the vertical edges of the slide, where the compressed air jet is held almost horizontally, and allowed to blow past the edge of the slide. This is done holding the jet at a similar angle with respect to the front surface of the slide and position with respect to the edge of the slide. The water drops being blown off the edge of the slide indicate success of this blowing procedure. These drops do not come back onto the front of the slide, nor do they have any tendency to spread over the back of the slide. They are drawn towards the edge of the slide and are blown off the edge of the slide.

Once the reverse face of the slide has been dried of its water film in this manner, the front face is dried in a similar manner. Maintaining the compressed air blowing past the lower and side edges of the slide while drying the front face will have contributed to minimizing the residual water film on the front side. Water streaks are to be avoided. Care must be taken not to chase water drops on the front or the reverse surface of the slide. If necessary, any water traces remaining on the edges or back of the slide may then be dried off by touching a piece of absorbent paper.

Two parting comments. First of all, tap water, should be avoided. It may form deposits in ultrasonic baths and on the substrates to be cleaned. Secondly, the use of industrial compressed air should be avoided, as it generally contains significant quantities of oil from the compressor.