As any reader of detective fiction knows, a crucial clue needed to solve a murder is the time of death. The hero detective, typically, is frustrated by the vague responses of the forensics team: “sometime between Tuesday night and Thursday morning” does little to narrow down the range of suspects. Scientists have long tried to help. And forensic science, with DNA analysis at the forefront, now ensures that more real-life criminals can expect a knock on the door, sometimes decades after they thought they had evaded detection. However, it remains effectively impossible to accurately judge the age of a bloodstain. Corpse excluded, bloodstains are typically the most common piece of evidence encountered at a homicidal-crime scene.
Colour could be the key. After blood leaves the body it starts to dry; as it does so, it changes from red to brown. Back in 1907, the Italian researcher Louis Tomellini produced a chart of 12 bloody spots, to illustrate this colour change over a year. As forensic science developed through the twentieth century, so did bloodstain analysis. By the 1960s, researchers were using photospectrometry, recording reflectance spectra and working out how the rate of Tomellini’s colour changes could be affected by different atmospheric conditions. These are useful observations, and forensic analysis of the colour of bloodstains is today a common part of the forensics team’s work. But the results are still too variable for the analysis to stand up in court.
Colour provides more-useful data than many might think. Spectrometry is a valuable technique in many areas, from drug discovery to environmental monitoring. And astronomers use spectrometers to probe the atmospheres of distant exoplanets for conditions that might support life. Spectrometers, in other words, have become indispensable instruments. But they tend to be expensive, complex machines. The most precise can also be bulky, making them difficult to use in the field. On page 67 of this issue, scientists describe a possible step forward. They have built an optical spectrometer that is both small and powerful, and potentially cheap enough to find use in consumer electronics — to detect corked wine perhaps.
Like many modern images, those analysed by the scientists are taken with a smartphone camera. These are selfies from the quantum world: the camera is converted into a spectroscope using suspensions of particles called colloidal quantum dots. Exposed to light, these tiny particles produce vivid colours, with the shade and hue determined by the particle size. With the right mixture of particles, a coating can be applied that can filter and analyse the wavelengths (and so colour) of incoming light.
The research is discussed in a News & Views article on page 39, which describes how it could be used to produce “ubiquitous sensing elements in household devices connected to the Internet”. Beware would-be bloody criminals, your fridge is watching you.
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