(157b) A Low-Cost and Facile Color-Changing Nanofilm for Temperature Monitoring and Recording

To help protect the integrity of bioreagents such as COVID-19 vaccines, a reliable temperature monitoring, and recording device is needed to prevent them wasted due to loss of potency from out-of-range temperature storage in manufacturing or transportation. Current electronic-based temperature monitoring and logging devices are costly, not directly monitoring the temperature of the bioreagents, and facing cyber insecurity with data transmission. A non-electronic-based temperature monitoring and recording sensor based on the inherent properties of smart materials can be a potential solution to overcome the limitation of current devices. Smart nanofilms can be designed to be sensitive to different external parameters such as pH [1, 2], temperature [3, 4], light [5, 6], magnetic field [7, 8], etc. However, incorporating sensitivity to external parameters in the nanofilms mostly requires sophisticated fabrication techniques. In this work, a smart layer-by-layer (LbL) nanofilm has been developed to monitor and record the temperature change of the surrounding environment by changing its inherent structure color.

Previously, a humidity-sensitive nanofilm was fabricated in our lab using the LBL technique [9]. The film showed inherent colors depending on its thickness. Moreover, it could be crosslinked by UV-light and thus designed patterns could be created on it. Under higher relative humidity (even human breath), the film absorbs moisture and could reveal the UV-crosslinked patterned structures. The film was fabricated using chitosan and carboxymethyl cellulose azido derivatives (CMC-N₃), which were deposited on a silicon wafer alternatively as per the LbL technique. With further investigation, it was found that the film could change its color when changing the surrounding temperature under a fixed relative humidity (RH). As the chitosan and CMC-N₃ are polyelectrolyte materials, they are both hydrophilic. So, the film could absorb or desorb moisture from the air and show different thicknesses at different temperatures under a certain RH. When the natural light fell on the film with a certain thickness, it went through the layers of the film to the silicon wafer, reflected, and went through the film again to the outside of it. In different layers, the wavelength of light was absorbed partially, and based on the wavelength of the reflected light a certain color can be seen. As the film showed different thicknesses at different temperatures, the wavelength of the reflected light was different, thus, the color change occurs. This type of thickness-depended color change can also be seen in nature e.g., in Chameleons. They can alter their skin color by changing the distance between the guanine (photonic) crystal layers situated in the upper layer of their skin and thus changing the wavelengths of the reflected light [10].

As shown in the attached Figure, color codes of the film at different temperatures at 40% RH have been achieved and demonstrated the film as a potential temperature sensor. With controlled humidity, this film is expected to change its color irreversibly when the surrounding temperature is above a certain level. If a serial of temperature sensors is incorporated, they can record the history of temperature for the storage samples during a designed time frame. For example, a blue-purple colored film will indicate that the storage condition has been disrupted to over 30⁰C. The film is low-cost and easy to fabricate using chitosan and CMC-N₃, which are both biocompatible and environment-friendly. One unique application of this film is to monitor and record storing, shipping, and handling conditions of temperature-sensitive bioreagents (i.e. COVID-19 vaccines). Furthermore, this film is expected to be potentially used for anti-counterfeiting, smart display devices, and other optical sensors.

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