Luminol is an item found in all three Zero Escape games: Zero Escape: Nine Hours, Nine Persons, Nine Doors, Zero Escape: Virtue's Last Reward and Zero Escape: Zero Time Dilemma. Luminol is used to reveal blood (since it reacts to hemoglobin) and other liquids that have been removed from an object. By spraying it on a surface, it will react to traces of the liquid and emit a whitish glow in the dark.
Nine Hours, Nine Persons, Nine Doors
A bottle of luminol is found in the shower room inside of a wooden box that is sitting on a shelf above a toilet. Junpei sprays it on the wall near Nagisa Nijisaki's decimated corpse, where the letters are. When the light is turned off, 3 more letters are revealed: "L, R, L", which is the second half of the instructions on turning the pipe valves.
Virtue's Last Reward
In Virtue's Last Reward, luminol plays a much larger role in the story than just being used in an escape room. It plays a large role in both Luna's and K's endings, as it is used to identify the killer in the game.
The Rec Room has a cabinet with multiple bottles of luminol. While investigating the room, Sigma uses one of the bottles to test for traces on the pool table, revealing the letters A through to F written near the different pockets, one of the clues to the puzzle involving the table and balls.
Later, upon deducing that the old lady was wearing a bracelet when she was killed, which the killer then put on, Sigma works out that whoever killed her would have a reaction from luminol sprayed on their bracelet. He takes both Phi and Dio to the Rec Room before using the luminol of their wrists. Dio's bracelet had a reaction, and he eventually confessed. K later kills Dio as revenge for the old woman's death.
In Luna END, Tenmyouji was met by Sigma and Phi in the Rec Room, and he sprayed luminol at their wrists in an attempt to find the old woman's blood. Upon getting no reaction, he told them both that he now knows neither of them is Zero Sr. After Tenmyouji is killed by the penalty, K takes the luminol from his body and decides to test the substance on Dio, leading to both of their deaths (K wanted to avenge Akane's death). When Sigma and Phi find their bodies, Phi finds the luminol bottle on K's body and turns off the lights, believing K might have used the luminol on Dio before he died. They discover that Dio's bracelet is glowing, but are unsure of why his bracelet would glow. Phi states that Dio's glowing bracelet might make him Zero. Luna later reveals that Dio's bracelet was glowing because he had killed the old woman and took her bracelet, which had her blood on it.
Zero Time Dilemma
Luminol appears in the Infirmary. It is sprayed on the lockers to reveal the codes for them.