I am automating a small-scale laboratory chemical process. One step is thermal equilibration of DNA in about 0.5 ml of aqueous buffer, which involves raising the liquid to 90°C and then immediately cooling it to 4°C, holding it at each temperature for about 10 minutes. The liquid must not turn to ice, but +/- 3-5 degrees is an acceptable range of temperature control. I'd like the temperature to move from 90C to 4C within about 1 minute.
Ideally, I'd like this to take place in a removable 1.5 ml test tube, or alternatively in a 3 mm OD/1mm ID hose, and be simple and inexpensive to install. It's OK if it requires some manual interventionl, as long as it can be set up in advance. This is going to be a device that is set up during the day and run overnight.
My impression is that a wirewound resistive heater would work best to heat to 90°C. For cooling, I've considered heat pipes and Peltier cooling plates, as well as having the test tube/hose immersed in a small water bath that is heated and cooled from below.
It seems like a Peltier plate would not transfer heat efficiently in and out of the tube, because it would be a flat surface in contact with a round surface, giving minimal contact area. But I haven't seen flexible heat pipes, and I don't know if they'd work to bring the tube to a specific temperature. A water bath seems like a fine solution, but a solution that directly controls the temperature of the tube without introducing a source of contamination would be better. What would you recommend?