Fluorescence spectroscopy is difficult because fluorescence spectra of organic molecules are broad and mostly devoid of features. So a small concentration of fluorescent molecules can create a background in any part of the emission spectrum. So cleaning the glass ware in a fluorescence experiment is not trivial. 

You may think about using a new slide each time. Cleaning glassware for fluorescence experiments is a highly labor intensive process. Waste disposal is also difficult. The laws on waste disposal are very difficult to implement. 

Large glassware is expensive and often unavailable. So it is sometimes worth while to clean and reuse the large glassware. I don't know about microscope slides. 

         If you do decide to reuse the slide, there are important "don'ts".

      Do NOT use soap and detergent. I would recommend not using any detergent because detergents fluoresce brightly. You probably want to clean the slide with a strong acid because the carbonized residue won't fluoresce.

    DO NOT want to  use a strong base to burn away the residue. Bases leave defects on glass surfaces to which organic molecules stick. Fluorescent minerals in the glass may be released by the base. So do not use KOH, NaOH, etc. 

   Human safety and environment protection come first. Pay attention to waste disposal procedures, especially waste disposal laws. Pay attention to safety protocols. Anything that can remove an organic dye can destroy your tissues.  

Use a single rinse with a caustic agent, followed by serial dilution in deionized water. The hard part is disposing of the used caustic agent.

1) Some physical wiping with a brush or chemical wipe may be necessary to get rid of the sticky gobs of waste.

However, DON'T touch the brush or or wipe with soap or detergent. Soap and detergent fluoresce brightly. Pressure on the glass is good. Detergent is not good. 

2) Caustic agent step: Soak for 5 minutes in a caustic agent and pour it out. 

   I used to clean glassware with an aqueous solution of 12% nitric acid. I would fill the glass ware up and leave it there  for 10 minutes.

Nitric acid is a good because it doesn't fluoresce. It is not intrinsically toxic, although it is a dangerous caustic. It carbonizes any organic molecule.

   You need someone to dispose of the nitric acid. Generally, the solution has to be neutralized by titration. Then, the neutral solution can be safely disposed of. Unless you were using something really toxic, of course. 

   The caustic agent should not be left in the container more than 10 minutes. If the surface of the glass is etched, the defects that result can fluoresce. The minerals that leach from the glass will fluoresce.  

3) Serial dilution with deionized or distilled water. 

   The deionized water alone can force adsorbed molecule from the surface of the glass. You don't need a surface agent like soap to clean the glass. You just need lots of clean water.

    There are always impurities adsorbed to the surface of the glass. When the glass is immersed in water, some of the impurities will be dissolved. The glass and water will reach an equilibrium. The pull of the water on the adsorbed molecules will be strongest when there are no molecules in the water. So every time you soak

    Then I would clean the glass ware by serial dilution with deionized water or distilled water. Dilution is necessary after a caustic rinse. The nitric acid would burn the organic substance leaving a carbonized residue. I would fill the container with fresh deionized water, and then spill it out. I repeated about 5 times. 

The last thing to touch the slide has to be deionized water. Anything else that touches the slide can contaminate it.

4) Some glassware can't be cleaned, so has to be replaced. 

Learn to recognize evidence of glass ware contamination. And don't be afraid to repeat questionable results with fresh glassware.

      If you suspect a contaminated glassware, throw the glassware out.  Try to repeat the experiment with new glassware. If you want to check for contamination, soak the uncleaned glassware in water. Check the water for fluorescence.

If the glassware was used on a bright fluorescent molecule (like a laser dye), then it should not be used with weaker fluorescent  molecule (like a salt) later. . 

Of course, there are important rules for the safety and disposal of contaminated glass ware. Glass can cut. Contaminate glassware can itself be organic waste.   

Did I warn you not to use detergent on glassware ?-) Don't !

I forgot to mention one important fact.

Glass ware never arrive clean enough for fluorescence spectroscopy. What you get directly from the factory is always covered with a thin organic residue from the manufacturing environment. Even your slides have to be a bit dirty. 

So using new slides each and every time is not going to work. The glass/silica slides have to be cleaned before each experiment.

So before you do a fluorescence experiment with glass ware, the glass ware has to be cleaned.

I already recommended a procedure. Use one nitric acid rinse, then rinse repeatedly with deionized water. Then it will be ready for a fluorescence experiment.

More is not better. Maybe 12% nitric acid is too much. Maybe you should try soaking it in acid for only five minutes.

However, I still recommend not using soap or detergent. Soap has organic molecules that fluoresce. Many detergents also contain fluorescent dyes to make clothes whiter and brighter !-)  This is not good for glass ware.