Hydrochromic modulator adds visible, infrared and chemical anti-counterfeiting

Researchers in Hong Kong and Singapore developed a flexible hydrochromic optical modulator that uses water and humidity to switch signals across visible, mid-infrared and chemical channels. The multi-mode device could make security labels and encrypted documents harder to copy while staying stable under harsh conditions.

Why it matters: - Counterfeiting and information leakage affect banknotes, medicines, consumer goods, security labels and confidential documents. - Most current anti-counterfeiting tools rely on a single visible signal, which makes them easier to observe and copy. - A device that combines visible, infrared and chemical verification raises the bar for authentication.

What happened: - A team led by Professor Yi Long of The Chinese University of Hong Kong and Professor ZhiLi Dong of Nanyang Technological University developed a dual-band hydrochromic optical modulator. - The work was published in Light: Advanced Manufacturing. - The device uses water and humidity to control optical information in the visible and mid-infrared regions. - The platform also supports chemical authentication through attenuated total reflection Fourier transform infrared spectroscopy. - The source paper is the full report.

The details: - The device combines a porous polymer layer with a low-emissivity layer. - In the dry state, the porous polymer strongly scatters light and appears opaque. - When water fills the pores, scattering drops and the device becomes more transparent. - In the visible channel, that reversible switch can reveal or erase hidden information. - In the mid-infrared channel, the dry state has low emissivity and the wet state has much higher emissivity because of water absorption. - That shift allows hidden thermal patterns to be read with an infrared camera. - In the chemical channel, ATR–FTIR spectroscopy provides expert-level verification. - The researchers used a scalable water-in-oil emulsion process followed by spin coating. - The method can form porous polymer structures on low-emissivity substrates and flexible films. - The optimized device delivered up to 47.2 percent luminance transmittance modulation. - The same device achieved 0.55 mid-infrared emissivity modulation between dry and wet states. - The response was reversible under both direct water contact and changes in relative humidity. - Prototype patterns were readable by the naked eye, infrared imaging or ATR–FTIR spectroscopy, depending on the hidden layer.

Between the lines: - The three-channel design makes copying harder because successful verification requires matching visible, infrared and chemical signatures. - The approach moves hydrochromic security beyond conventional visible-only switching. - The use of a flexible, scalable fabrication process points to easier manufacturing than many lab-only security materials.

What's next: - The team says the platform could be used for security labels, flexible anti-counterfeiting devices, information encryption, humidity-responsive optical systems and smart photonic materials. - The device also retained stable modulation after repeated dry-wet cycling and exposure to high temperature, ultraviolet irradiation and saltwater, which supports further practical testing. - Funding came from the Singapore International Graduate Award, Hong Kong government-backed and university programs, and several research grants.