Writing Self‐Erasing Images using Metastable Nanoparticle “Inks”

TLDR: A conceptually differentSelf-erasing material in which both the “writing” and self-erasure of color images are controlled by the dynamic non-equilibrium aggregation of photoresponsive metal nanoparticles embedded in thin, flexible organogel films.

Abstract: Fans of the “Mission Impossible” movies might recall the selfdestructing messages used to brief the secret agent on the details of his new mission. Even beyond the realm of fictitious espionage, materials that store textual or graphical information for a prescribed period of time are desirable for applications in secure communications. 2] Furthermore, if such materials are rewritable, they can help to limit the use of traditional paper, thereby reducing the costs, both industrial and environmental, associated with paper production and recycling. To date, most research on self-erasing media has relied on the use of photochromic molecules—that is, molecules that isomerize and change color when exposed to light of appropriate wavelength—embedded in or attached to a polymeric or gel matrix. In one widely publicized example, Xerox Corporation recently announced the development of photochromic paper that self-erases in 16 to 24 h. While writing with light can be both rapid and accurate, 7] photochromic “inks” are not necessarily optimal for transforming light-intensity patterns into color variations, because they have relatively low extinction coefficients, are prone to photobleaching, and usually offer only two colors corresponding to the two states of photoisomerizing molecules. Herein, we describe a conceptually different self-erasing material in which both the “writing” and self-erasure of color images are controlled by the dynamic non-equilibrium aggregation of photoresponsive metal (here, gold and silver) nanoparticles (Au and AgNPs “inks”) embedded in thin, flexible organogel films. When exposed to UV light, the trans-azobenzene groups coating the NPs isomerize to cisazobenzene with a large dipole moment. As a result, the NPs aggregate into supraspherical (SS) assemblies, whose apparent color depends on the duration of UV irradiation (Figures 1 and 2). Since the SS are metastable and fall apart spontaneously in the absence of UV irradiation, the two-color and multicolor images written into the films gradually self-erase (Figures 2 and 3). The erasure times can be controlled by the number of dipoles induced on the nanoparticles and can also be accelerated by exposure to visible light or by heating the material. Multiple images can be written into the same film either concurrently or after erasure.