Figure 1: Silver nanoparticles in microwells enhance the Raman light given off when an enzyme (green and red globules) interacts with a compound. The method can distinguish between enzymes with similar structures since different wavelengths are emitted. © 2026 RIKEN Molecular Physiology Laboratory
RIKEN researchers have demonstrated a method that can detect tiny amounts of biomarkers in liquid samples and can distinguish between highly similar biomarkers1. This promises to boost the versatility and usefulness of liquid biopsies.
Liquid biopsies are powerful tools for research and diagnosis since they can detect minute amounts of biomarkers in blood, saliva and urine. In particular, they are often used to detect enzymes that are connected to diseases.
"During the COVID-19 pandemic, liquid biopsies attracted unprecedented attention as a diagnostic method for infectious diseases," notes Rikiya Watanabe of the RIKEN Molecular Physiology Laboratory. "As a result, the effectiveness of liquid biopsies is now being recognized for both testing for infectious diseases but also for a wide range of medical diagnostics."
Liquid biopsies typically detect biomolecules by measuring the fluorescence emitted when a target enzyme reacts with a reporter substrate. This approach enables highly sensitive detection, allowing enzymes to be identified at very low concentrations due to the strong fluorescence signal generated.
However, fluorescence-based detection methods have limited ability to distinguish between closely related enzymes, because fluorescence emission occurs over a broad range of wavelengths.
Raman spectroscopy offers a complementary spectroscopic approach that overcomes this limitation by generating narrow, well-defined spectral peaks that provide distinct molecular fingerprints.
"Compared with fluorescence-based detection, Raman spectroscopy has a key advantage of having an exceptionally high discriminative ability," says Watanabe. "It can distinguish between even subtle differences in molecular structure."
However, Raman spectroscopy suffers from the opposite limitation, as the inherently weak signals hinder rapid detection of enzymes at low concentrations.
Nanoparticles of metals such as gold and silver have the surprising effect of amplifying the Raman signal by roughly a million times. However, this enhancement is difficult to control, which makes it challenging to realize the consistency needed for liquid biopsies.
Now, Watanabe and co-workers have demonstrated a way to enhance the Raman signal in a controllable way. They also showed that their method could distinguish between two very similar enzymes (Fig. 1).
"We developed a novel microchip that significantly improved the reproducibility and quantitative reliability of enhanced Raman signals," says Watanabe. "And we demonstrated its applicability to liquid-biopsy approaches focused on detecting biomarkers."
They used their platform to distinguish between two closely related enzymes, acetylcholinesterase and butyrylcholinesterase, which are very challenging to differentiate using fluorescence-based detection techniques.
Acetylcholinesterase plays a crucial role in regulating neural activity since it hydrolyses the neurotransmitter acetylcholine. The team found that it was present in higher amounts in patients with Alzheimer's compared with those with vascular cognitive disorders.
"This unanticipated variation in acetylcholinesterase levels in patients with dementia suggests that the method may provide valuable contributions to stratifying subtypes of dementia," says Watanabe.
