Hi Vanessa. You can try to clean it using a ultrasonic bath. 

I like to use chromic acid. It should be added to dry cuvettes to avoid localized heating from dilution of the acid. It gets the cuvettes perfectly clean, but it is important to wash the cuvettes very thoroughly afterwards.

Sonicating cuvettes can be problematic as the edges are often bonded with a glue. My PhD supervisor used to go crazy when he saw us sonicating cuvettes.

There is a protocol using nitric acid and ethanol - but it must be done in a fume cupboard with the sash down - personal protective equipment (PPE) etc. And lots of bicarbonate to mop up after.

https://www.youtube.com/watch?v=uFwiZYfEsuY

You may encounter problems convincing the safety people, but we did this for years in our lab. And in view of our supervisors reactions it was generally safer for us to do this than to sonicate. After all you can hide from acid behind PPE and a fumehood sash.

In the youtube clip they use a lot of reactant - you can use much less. The initiation time strongly depends on the conditions and it is often much shorter than in the clip. Nonetheless you get a few seconds to add the acid and cover the cuvette with a beaker and pull down the sash - DO NOT assume you have more time tan that. Gloves lab coat and full face mask of course whenever handling acids and this reaction is a spitter as you can see!

Hellmanex or high pH solutions can eventually cause some etching. Pirahna solution uses larger quantities of acid and the acid is much stronger.

BUT BEFORE DOING the above try this:

This link gives a safer way to clean using using dilute or concentrated nitric acid soaking. It is safer and you can try it first.

http://thelab.photophysics.com/circular-dichroism/cuvette-cleaning/

At the end of any cleaning rinse with water and then acetone and then blow dry or suction it to remove the acetone - then you have a dry cell