Fast-response colorimetric sensor for real-time monitoring has expanded color gamut

Colorimetric sensors detect environmental changes by intuitively changing colors, easily visible to the naked eye without the need for additional equipment. In addition, they operate with zero energy consumption. By visibly changing color, without any additional equipment, these sensors have the potential to play a vital role in applications such as food packaging and preservation of antique objects, where optimal humidity is crucial for quality control.

For accurate humidity detection, colorimetric sensors must cover a wide range of colors, demonstrate a linear correlation between color and humidity, respond quickly, and maintain long-term stability. Sensors that achieve coloring through structural changes are generally more advantageous than sensors based on a chemical reaction.

Among these, metal-hydrogel-metal (MHM) structures using Fabry-Pérot resonance are notable for their simplicity and generation of diversified colors, as changes in the thickness of the hydrogel cavity, often using swelling materials like chitosan lead to different colors. However, conventional designs still suffer from limited color representation and slow responsiveness.

To address these issues, a research team from South Korea, led by Associate Professor Gil Ju Lee of the School of Electrical and Electronic Engineering at Pusan ​​National University, developed a Fano Resonant Colorimetric Sensor (nFRCS) innovative nanostructured in two dimensions (2D).

Dr Lee explains: “Our design introduces nanohole arrays that utilize Fano resonance and plasmon resonances, significantly improving the color gamut by controlling the reflectance spectrum from subtractive to additive coloring. The results are published in the journal Optical.

The nFRCS comprises a silver-chitosan-silver MHM structure with a thin top layer and a thick bottom layer. MHM also has a thin, porous germanium (Pr-Ge) coating. This coating is a key addition that transforms the MHM from a Fabry-Perot resonator to a Fano resonator, significantly improving color representation.

Additionally, nFRCS integrates 2D nanohole arrays (NHA) into the MHM layer that establish a direct pathway for water vapors in the ambient environment to reach and interact with the chitosan layer. Due to the hydrophilic nature of chitosan, in very humid conditions, chitosan absorbs water molecules, causing it to swell, and in dry conditions, releases water molecules, reducing its volume, resulting in a color change depending on the humidity level.

These NHAs also improve sensor responsiveness, and their ordered pattern facilitates additional light-matter interactions such as surface plasmon resonance (SPP) and local surface plasmon resonance (LSPR), further improving performance.

The researchers fabricated the nFRCS sensor using roll-to-plate nanoimprint lithography (NIL), which uses a stamping-like method to transfer the nanoscale patterns onto the MHM layer. Compared to conventional expensive nanostructure fabrication techniques, this method saves time and money.

In experiments, the fabricated nFRCS showed a wide color gamut, exceeding standard RGB (sRGB), showing 141% sRGB coverage and 105% Adobe RGB coverage, surpassing previous studies. Additionally, it demonstrated exceptional responsiveness with response and recovery times of 287 and 87 milliseconds, respectively.

Highlighting the sensor’s wider applications, Dr Lee says: “Beyond humidity detection, nFRCS can also serve as a health monitoring device, smart display and interior materials, responding to stimuli external by generating distinct color changes. This design could serve as a framework for other types of colorimetric sensors that detect different environmental changes other than humidity.

Overall, this innovative sensor marks a significant step forward for real-time, power-free environmental monitoring.