Adjustable nanosensors turn water pressure into fluorescent signals, with stiffness shaping sensitivity

A novel nanovesicle-based platform developed at Institute of Science Tokyo can measure hydrostatic pressure by converting pressure-induced molecular changes into fluorescence signals.

The researchers developed pyrene-modified polyionic complex vesicles and demonstrated that variations in the vesicles’ membrane stiffness regulate their pressure sensitivity. Softer vesicles showed strong sensitivity in the 0.1–50 MPa range, while stiffer vesicles showed pressure-dependent changes in fluorescence lifetime.

These findings enable the investigation of hydrostatic pressure–dependent phenomena in diverse inaccessible environments.

The study is published in ACS Applied Nano Materials.

Pressure sensing at microscopic scales

Hydrostatic pressure is the compressive pressure exerted on an object from all directions by a surrounding fluid at rest. It can influence many physical, chemical and biological processes, making its measurement important in diverse environments, ranging from the deep ocean to living tissues.

Despite its importance, measuring this parameter at microscopic scales remains challenging, particularly in aqueous and biological environments. Existing molecular sensors face limitations such as poor water solubility, insufficient sensitivity in the megapascal range or operational difficulties in biological settings.

Against this backdrop, a research team led by assistant professor Hayato L. Mizuno and associate professor Yasutaka Anraku from Institute of Science Tokyo, Japan, along with professor Gaku Fukuhara from Kyushu University, Japan, developed a nanovesicle platform for hydrostatic pressure sensing. Their study introduces pyrene-modified polyionic complex vesicles, or Pyr-PICsomes, whose membrane stiffness directly regulates their fluorescence response to pressure.

“An important aspect of our platform is that it does not rely on a single sensing mechanism,” says Mizuno. “By changing the membrane stiffness, we can access different fluorescence readouts, providing flexibility in how the pressure is measured.”

Membrane stiffness controls the fluorescence response

PICsomes are polymer-based vesicles that self-assemble in water from oppositely charged polymers. The researchers chemically crosslinked PICsome membranes using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). The EDC concentration was used to adjust the membrane stiffness while maintaining the vesicle size at approximately 100 nm in diameter. They then incorporated pyrene molecules, which act as pressure-sensitive fluorescent probes, into the membrane.

When the pyrene molecules are separated, they fluoresce mainly as individual monomers at around 380 nm. Under hydrostatic pressure, neighboring pyrene molecules are brought closer together, allowing them to form temporary pairs called excimers that emit fluorescent signals at around 500 nm. The change in the excimer-to-monomer fluorescence ratio, therefore, provides a measure of the hydrostatic pressure.

Additionally, the researchers found that the stiffness of the Pyr-PICsome membrane strongly affected how the vesicles responded to pressure. The softest vesicles, with a stiffness of 7.3 pN/nm, showed the largest change in the fluorescence ratio between excimers and monomers across the 0.1–50 MPa range. Their sensitivity was 0.28 MPa⁻¹, compared with 0.02 MPa⁻¹ for the stiffest vesicles, which had a stiffness of 39 pN/nm.

The amount of pyrene also affected the response, with more pyrene molecules leading to greater excimer formation and higher pressure-detection sensitivity.

Stiffer vesicles offer lifetime-based sensing

While investigating the influence of pyrene on the excited-state dynamics of Pyr-PICsomes, the researchers identified another way to detect pressure by measuring fluorescence lifetime, or how long a fluorescent molecule remains in an excited state.

In this case, the stiffer vesicles showed stronger fluorescence lifetime changes as pressure increased. The stiffest vesicles showed a lifetime sensitivity of −0.09 ns/MPa between 0.1 and 50 MPa. These properties make them good candidates for fluorescence lifetime imaging microscopy (FLIM) probes.

These findings suggest that membrane stiffness should be tailored depending on whether the hydrostatic pressure is measured through fluorescence intensity or FLIM.

“This tunability allows the sensing mode to be tailored to different environments and measurement methods, opening possibilities for studying pressure in complex aqueous systems,” notes Mizuno.

Toward pressure measurements in biological environments

Furthermore, the researchers tested the platform’s suitability for complex environments. The covalent crosslinking helps the vesicles maintain their structure in saline conditions, while their hydrophilic outer layer helps protect the membrane environment in biological fluids.

Based on this, the authors propose that Pyr-PICsomes could eventually be used to investigate localized pressure in tissues, cell cultures, ex vivo samples and deep-sea organisms.

This study establishes a materials-design strategy in which membrane mechanics can be used to program optical response behavior. The researchers developed a versatile, water-soluble, biocompatible, robust and tunable platform capable of detecting hydrostatic pressure in complex inaccessible environments. This strategy lays the foundation for smart materials with varied applications across diverse disciplines.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Robert Egan

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