Assistant Professor University of Texas at Arlington, United States
Introduction:: There is immense interest in the biomedical community to devise inexpensive, intelligible, and portable sensors that can be used for monitoring health and health risks from environmental hazards. Polydiacetylene (PDA) based materials can be designed using different headgroups to provide a colorimetric response to target molecules or physical stimuli. Diacetylenes undergo a colorless to blue transition upon exposure to UV light and are also mechano-, thermo-, and solvatochromic undergoing a blue to red color change upon exposure to the aforementioned stimuli. PDA-based materials have been widely used in a variety of sensing applications like detection of various solvents like water, explosives, and DNA. These applications have been demonstrated when the PDA-based materials are arranged in lamellar or vesicle form and have not yet been used for spray-on sensing. In this work, we explore the conditions under which PDA-based materials yield a color-changing response, their limitations as well as some examples to be used as spray-on sensors.
Materials and Methods:: Diacetylene amphiphiles were synthesized using protocols previously reported. Diacetylene amphiphiles for spraying were formulated using 100% ethanol as the carrier solvent and the solutions were sonicated to fully suspend the diacetylene monomers. The sample was airbrushed on paper to coat the paper entirely and the formulations were then exposed to 0-100% ethanol for one minute to examine the effect of ethanol on the diacetylene coating. To examine the reversibility, the coating was subjected to over a dozen cycles of heating and cooling at 100°C as well as heating up to 200°C at 25°C intervals. To test the specificity of the sensor for lead, the sensor coated paper was immersed in 0.1mM solutions of different ions and for testing the sensitivity, the paper was submerged in varying concentrations of lead nitrate ranging from 1M to 10-8 M of lead nitrate.
Results, Conclusions, and Discussions:: Our research offers a glimpse at the potential applications these PDA-based spray-on sensors are capable of as well as their limitations. For the initial examination, we used an amphiphilic diacetylene 10,12 pentacosadiynoic acid (PCDA) and derivations of PCDA where the carboxylic acid head group is reacted with 4-aminobenzoic acid to obtain PCDA-Bz which is a reversible, thermally responsive amphiphile, ethylenedioxy-bis-ethylamine to obtain PCDA-EDEA that is responsive to solvents such as methanol, toluene and ethanol, and the PCDA-AB amphiphiles obtained from sequential reaction of PCDA with (aminopropyl)imidazole and bromoacetonitrile which is responds to the presence of water. A dipicolyl amine head group containing amphiphile was also formulated that is lead responsive. The color change observed after exposure of these diacetylene amphiphiles to chemical and thermal stimuli reveal an apparent change in the color of the coating. These results suggest that spray coating of PCDA and its derivatives still retain its sensing abilities. By calculating the percent blue of the reversible PCDA, we were able to show that it can be utilized for temperature sensing applications. However, sequential increase in temperature when heating the reversible sensor showed us that the sensor did not fully return to its original color. Also, at higher temperatures the sensor eventually turns from purple to yellow instead of a blue to red color change that is usually observed. We also demonstrated the use of a spray-on sensor for the purpose of lead detection. We observed that the sensor responded selectively to lead ions in solution with a clear detection limit of up to 0.1mM and a clear color change from blue to red.
In this work, we have shown that diacetylene-containing amphiphiles can be used to spray-coat surfaces that can be turned into sensors for everyday use. Their behavior can be tailored according to the molecule that is being targeted by modifying the head-group of the amphiphile. Our spray-on sensor coating demonstrates that diacetylenes retain the ability to detect UV exposure, temperature, and target molecules such as chemicals and solvents.
