The instrument problem in the search for life
Astrobiologists have spent decades scanning the solar system and the wider universe for signs that biology exists somewhere other than Earth. The obstacle is not ambition but instrumentation. A biosignature detector has to be exquisitely sensitive, because the traces it hunts may amount to only a handful of molecules in a sample, and it also has to be small and rugged enough to survive a trip on a spacecraft. Building something that satisfies both requirements at once has been, as the researchers frame it, an out-of-this-world engineering challenge. A group at the University of Osaka now describes an electrical technique that may offer a way forward. The findings are set to be published in Nature Communications.
Mirror-image molecules as a signature of biology
Much of the reasoning behind the work rests on amino acids, the molecular building blocks from which proteins are assembled. Many amino acids exist in two forms that mirror one another: identical in chemical formula, but opposite in handedness, or chirality. Life and nonliving chemistry handle those two forms very differently. Organisms build their amino acids almost entirely in the L-form, while their sugars appear overwhelmingly in the D-form. Abiotic chemical and physical processes, by contrast, show no such preference and typically generate both forms in equal proportions. That asymmetry — the ratio of L- to D-forms — is therefore a promising biosignature, and it makes amino acids a natural focus for astrobiology. If a sample from another world came back with a strong handedness bias, it would be a meaningful hint that biology had been at work.
Why bulk measurement falls short
Conventional instruments determine chirality by analyzing large groups of molecules at once. That bulk approach works well in a well-equipped laboratory, but it brings practical difficulties when the goal is a flight-ready instrument. It has been suggested that an electrical detection scheme could sidestep several of these issues at once: such a method would be simpler to implement, less affected by vibration, and free of the chemical reagents that bulk techniques require. Those are precisely the qualities that matter when an instrument must operate far from a laboratory bench.
Single molecules, counted in a nanogap
Advances in nanotechnology let the Osaka team electrically detect individual molecules of biological interest rather than bulk populations. The setup threads molecules through a gap between two gold nanowires. As a molecule traverses that gap, it generates an electrical tunneling current, and the waveform of that current differs depending on which form of the amino acid is passing through. Because those electrical fingerprints are distinguishable, the researchers can count how many molecules of each form move across the gap — a direct, molecule-by-molecule measurement rather than an averaged reading over a crowd. The team also paired the nanogap tunneling technique with artificial intelligence, which they report allowed them to distinguish the two forms.
Samples from a Martian stand-in
The samples analyzed in the study were collected in the hyperarid core of the Atacama Desert, north of Antofagasta in Chile. That region is used as a Mars-analog environment: its extreme dryness and sparse biology make it one of the closest terrestrial approximations of the harsh surfaces found on the red planet. Testing a life-detection technique on material from such a setting is one way to evaluate how the method behaves under conditions that resemble what a planetary mission might encounter. The Atacama image accompanying the work is credited to Christopher E. Carr.
What a compact electrical detector would change
The appeal of the approach lies in what it might enable if it matures into a working instrument. Among the advantages described:
- Individual molecules, rather than large ensembles, are counted, giving a direct read on the L/D ratio that serves as a potential biosignature.
- The measurement is electrical, avoiding the chemical reagents that some established methods depend on.
- Electrical readouts are described as less sensitive to vibration than bulk techniques, a useful property for hardware operating on a lander or rover.
- The approach builds on nanoscale fabrication, in which gold nanowire junctions can be produced on small chips.
Each of those properties speaks to the same bottleneck: the difficulty of shrinking a laboratory-grade analytical method into something that can be flown. A technique that relies on current through a nanogap instead of a reagent bottle and an optical bench is, in principle, easier to miniaturize.
An emerging tool, not a finished mission
The Osaka result is a step in method development rather than a detection of life. Its significance is that it demonstrates a route to distinguishing the two chiral forms of amino acids electrically at single-molecule resolution, and that the approach can be tested against samples from an environment chosen to mimic Mars. Whether the technique eventually becomes part of a future payload depends on further work showing it can operate reliably outside the lab and on the kinds of samples a mission would actually analyze. Still, as astrobiologists continue to weigh where to look and what to look for, an instrument that is small, robust, and reagent-free addresses a recognized gap in the search for biosignatures — both within our solar system and beyond it.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org
