Understanding the Potential of Fano Resonant Colorimetric Sensors

A South Korean research team led by Associate Professor Gil Ju Lee of Pusan National University's School of Electrical and Electronics Engineering has created a groundbreaking two-dimensional (2D) nanostructured Fano resonant colorimetric sensor (nFRCS). This research was published in Optica.

Colorimetric sensors detect environmental changes by shifting colors visible to the naked eye, requiring no additional equipment and consuming no power. These sensors are crucial for applications like food packaging and artifact preservation, where controlling humidity is vital for maintaining quality.

To function effectively, colorimetric sensors must react quickly, display a linear relationship between color and humidity, cover a broad color spectrum, and remain stable over time. Sensors that generate color through structural changes rather than chemical reactions are generally superior.

Metal-hydrogel-metal (MHM) structures that utilize Fabry-Pérot resonance stand out due to their simplicity and broad range of color generation. These changes occur as the thickness of the hydrogel cavity, which is often filled with swelling materials like chitosan, varies, resulting in different colors. However, conventional designs still face challenges such as slow responsiveness and a limited color range.

To overcome these issues, a research team from South Korea, led by Associate Professor Gil Ju Lee from Pusan National University’s School of Electrical and Electronics Engineering, developed a two-dimensional (2D) nanostructured Fano resonant colorimetric sensor (nFRCS).

Our design introduces nanohole arrays that utilize Fano resonance and plasmonic resonances, significantly enhancing color gamut by controlling the reflectance spectrum from subtractive coloration to additive coloration. Furthermore, these nanohole channels also enhance responsiveness.

Gil Ju Lee, Associate Professor, School of Electrical and Electronics Engineering, Pusan National University

The nFRCS has an MHM structure of silver-chitosan-silver with a thin upper layer and a thick bottom layer. The MHM is also coated with a thin layer of porous germanium (Pr-Ge). This coating is a critical component that converts the MHM from a Fabry-Perot resonator to a Fano resonator, greatly improving color representation.

The sensor also incorporates 2D nanohole arrays (NHAs) into the MHM layer, providing a direct pathway for water vapor to reach and interact with the chitosan layer. Chitosan, a hydrophilic material, absorbs water in high humidity, causing it to swell and changing the sensor's color. In dry conditions, it releases water and shrinks, again causing a color shift based on humidity levels.

These NHAs also improve sensor responsiveness, facilitating interactions like local surface plasmon resonance (LSPR) and surface plasmon polaritons (SPP), further boosting performance. The researchers employed roll-to-plate nano-imprint lithography (NIL) to create the sensor, a cost- and time-efficient method compared to traditional nanofabrication techniques.

Experiments showed that the nFRCS sensor surpassed previous designs in color gamut, achieving 141 % sRGB coverage and 105 % Adobe RGB coverage. It also demonstrated excellent responsiveness, with response and recovery times of 287 and 87 milliseconds, respectively.

Dr. Lee added, “Beyond humidity sensing, the nFRCS can also serve as health monitoring devices, intelligent displays, and interior materials, reacting to external stimuli by generating distinct color shifts. This design could serve as a framework for other types of colorimetric sensors that detect different environmental changes other than humidity.”

This novel sensor represents a major advancement in real-time, zero-power environmental monitoring.

Journal Reference:

Nam, H. J. et. al. (2024) Ultrafast, Fano resonant colorimetric sensor with high chromaticity beyond standard RGB. Optica. doi.org/10.1364/OPTICA.532433