Use a common chemical technique from the field of forensics to reveal fingerprints in the laboratory.
Detecting fingerprints has been a key aspect of criminal investigations for over 100 years. It is an important forensic tool for two reasons: first, fingerprints are unique (even identical twins have different patterns), and second, fingerprints do not change over time: if you burn or cut your fingertip, the pattern will re-form as it was before. At a crime scene, ‘patent’ fingerprints are easy to detect. These are visible prints left behind by blood-covered fingers, for example. ‘Latent’ fingerprints, however, pose more of a problem. These are invisible prints left by the natural oils and sweat on our skin.
Over time, forensic scientists have developed ways to visualise latent fingerprints. The oldest technique is the powder method, whereby the fingerprint is carefully dusted with dry powder using a fine brush. The powder adheres to the moisture and oil in the fingerprint, making it visible. There are now hundreds of powders with different compositions depending on the surface they are used for. In general, the powders contain pigments to provide contrast, and binders to help them adhere to the fingerprint. The powder method is still used today because it works on almost all non-absorbent surfaces, such as glass.
Another common visualisation technique is cyanoacrylate fuming. This method is used by the police not on site but in the laboratory. Cyanoacrylates are the molecules found in superglue, so the technique is also referred to as the superglue method. Like the powder method, cyanoacrylate fuming is suitable for detecting fingerprints on non-absorbent surfaces. The fuming is carried out in a developing chamber. The cyanoacrylate forms a vapour that adheres to the fingerprint after polymerisation reactions, creating a visible white print.
After development with cyanoacrylate, powders or stains can be applied to the fingerprint to improve the contrast and make it more visible. Fingerprints are often stained with crystal violet solution, which turns them purple.
The activity outlined in this article enables students to detect their own fingerprints using the superglue method. After making their fingerprints visible, students add fluorescent dye to the prints to add contrast. This procedure allows students to experience a safe alternative to staining fingerprints, since crystal violet cannot be used in the classroom due to its toxic and carcinogenic effects. We recommend that students work in small groups of three. Each part of the activity takes approximately 30 minutes.
In this procedure, students detect latent fingerprints using the superglue method to learn about polymerisation reactions. The superglue method involves evaporating superglue in a closed chamber. We used a Tic Tac® box for the chamber and replaced the lid with a reinforced plastic lid to ensure that the vapour could not escape. Alternatively, you could use a Tic Tac® box with a Petri dish for the lid, or exchange the box for a drinking glass, and again use a Petri dish for the lidw1. Note that this equipment should not be used for other purposes after the experiment, so it needs to be disposed of.
Wear a lab coat, gloves and safety glasses. Be careful when handling superglue as it can glue skin – such as fingers and eyelids – together in a few seconds. Cyanoacrylate vapour is a respiratory tract irritant, so the activity must be carried out in a well-ventilated room. If your school laboratory has access to a fume hood, it is advised to carry out step 6 onwards inside the fume hood.
Discuss some of the following questions as a class:
A few minutes after the heating plate is switched on, gaseous superglue rises from the fleece. This superglue vapour adheres to the fingerprint, leaving behind a white trace (see figure 2). This reaction occurs because the cyanoacrylate esters in superglue polymerise on contact with molecules in the fingerprint (e.g. water, fatty acid, amino acids). In this process, a cyanoacrylate ester (most commonly an ethyl cyanoacrylate monomer) becomes a molecular chain (an ethyl cyanoacrylate polymer; see figure 3). At room temperature, an ethyl cyanoacrylate polymer is solid and white, and it is produced in the area where you left your fingerprint. This method works only on dry objects. If the entire piece of aluminium foil were wet, the water would make polymerisation take place on the whole surface of the foil.
In this part of the activity, students stain the fingerprints from part 1 using a handmade dye. To create the dye, students dissolve ink from highlighter pens in ethanol. Students learn about the electromagnetic spectrum and fluorescence, as well as polar and non-polar molecules.
Wear a lab coat, gloves and safety glasses – ideally UV protection glasses. Do not look directly towards the UV lamp or shine the light into someone’s eyes.
Discuss some of the following questions as a class:
When the highlighted filter paper comes into contact with ethanol, the fluorescent dye from the highlighter pen diffuses out from the paper into the solution. To dissolve in ethanol (a polar molecule), the dye must contain some polar components. To stain the superglue fingerprint (comprising non-polar ethyl cyanoacrylate polymers), the dye needs to have non-polar components, too. A highlighter from Herlitz®, for example, would not stain the fingerprints, because the dye (pyranine) is polar (Ducci & Oetken, 2018).
Once the ethanol evaporates off the aluminium foil and the fingerprint is stained with the dye, the fingerprint will fluoresce when irradiated with UV light. This is because the dye contains fluorescent molecules that absorb light of a certain wavelength (such as UV light) and re-emit it at a longer wavelength, such as yellow, orange or pink in the visible spectrum. This isn’t particularly noticeable in daylight, but is more obvious under UV light.