Nile Red (or Nile Blue A Oxazone) is a lipophilic dye – that's a fancy term meaning it loves to stick to fats and oils. This is perfect for our purposes, as a major component of latent fingerprint residue is sebaceous secretions, which contain fatty content. Where those secretions are, Nile Red will latch on.
When hit with the right light source (more on that later), Nile Red fluoresces a beautiful orange-red. This gives us a great way to visually locate latent prints that might be hard to see under normal conditions.
Advantages of Nile Red
- Sensitivity: Nile Red is extremely sensitive, so it can pick up faint prints that other techniques might miss.
- Wet Surfaces: One of the major perks of Nile Red is its ability to work even after a porous surface (like paper) has been thoroughly soaked.
- Non-Destructive: It doesn't interfere with possible subsequent DNA analysis.
Equipment
- Fume hood or well-ventilated area: Safety first!
- Glassware: Beakers, stirring rods, graduated cylinder.
- Sprayer or squirt bottle: For applying the solution.
- Forensic light source: Ideally, a laser or an ALS (Alternate Light Source) with excitation wavelengths in the blue-green range (450nm–560nm).
- Orange or red goggles: To view the fluorescence.
- Safety gear: Gloves, lab coat, eye protection.
Ingredients for 1 Liter of Nile Red Working Solution
Here's a common Nile Red formulation recipe. Note that this yields a WORKING solution. We'll have made a concentrated stock previously.
- 500 mL Methanol (HPLC grade for best results)
- 500 mL Deionized Water
- Sodium Hydroxide (a few small pellets - see mixing procedure)
Mixing Procedure
- Make the Stock Solution:
- Dissolve a small amount of Nile Red powder (0.5 g is often plenty) into 100 mL of methanol to create a highly concentrated stock solution. Store this in a dark bottle, it's light-sensitive.
- Mix the Working Solution:
- Slowly add an equal part of your Nile Red stock solution to the 500 mL of methanol while constantly stirring.
- Add a few small pellets of sodium hydroxide and stir. The solution will start to turn a more reddish color. Add NaOH until you get the desired color.
- Add the 500 mL deionized water and stir.
IMPORTANT: Nile Red reagent should be used quickly once prepared (within a few days).
Latent Print Processing with Nile Red
- Pretreatment: Assess your surface. On non-porous items, it can be rinsed, but porous items like paper generally shouldn't get any water treatment before Nile Red.
- Application: Put your item in a fume hood or well-ventilated space. Thoroughly apply the Nile Red solution with a sprayer or squirt bottle. Ensure complete coverage of the area.
- Drying: Let the item air dry in a dark environment.
- Visualization: Don your orange/red goggles, shine your appropriate light source at the item, and boom! You should see any prints light up.
- Photography: Document those developed prints under the light source using an orange filter on your camera.
Storage
- Stock Solution: Store your concentrated Nile Red solution in a dark glass bottle, ideally refrigerated.
- Working Solution: Doesn't store well. Use fresh.
Shelf Life
- Stock Solution: Can last several months if stored correctly.
- Working Solution: A few days, at most.