Writing Self‐Erasing Images using Metastable Nanoparticle “Inks”

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. [1, 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, [3] associated with paper production and recycling. To date, most research on self-erasing media has relied on the use of photochromic molecules [4][5][6][7] -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 [8] the development of photochromic paper that self-erases in 16 to 24 h. While writing with light can be both rapid [9] and accurate, [5,7] photochromic "inks" are not necessarily optimal for transforming light-intensity patterns into color variations, because they have relatively low extinction coefficients, [10] are prone to photobleaching, [11] and usually offer only two colors corresponding to the two states of photoisomerizing molecules. [10] 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 [12] 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. [13] As a result, the NPs aggregate into supraspherical (SS) assemblies, [13][14][15][16] 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.In a wider context, the present system demonstrates the flexibility and promise of non-equilibrium nanostructures to create "smart" materials capable of changing their properties or function on demand in response to external stimuli.Our experiments were based on AuNP (5.6 AE 0.6 nm diameter) or AgNP (5.3 AE 0.3 nm diameter) inks coated with mixed self-assembled monolayers (mSAMs) of dodecylamine (DDA) and photoswitchable azobenzene-terminated thiol (4- Figure 1. Reversible aggregation of photoactive nanoparticles. a) Structural formula of trans-4-(11-mercaptoundecanoxy)azobenzene (trans-MUA). b) UV irradiation of nanoparticles (here, gold) cove...

Metal complexes of azomethine compounds bearing an azo group in the amine fragment: Syntheses, structures, and properties

“…Its ability to undergo photoinduced E/Z isomerization is significant for the development of coordination compounds with the properties of molecular switchers [22]. The E/Z isomerization of the azobenzene frag ment is applied for the formation of metastable nano particles used in the reversible process of image recording/clearing [23].…”

Designing dynamic functional molecular systems

“…Klajn, Grzybowski and co-workers could achieve the dissipative self-assembly of gold nanoparticles in organogel films using light. 26 The gold nanoparticles were decorated by azobenzene functionalized ligands and photoinduced formation of the polar cis azobenzene causes nanoparticle aggregation as a result of favorable dipole-dipole interactions. Here the assembly can be considered dissipative, because thermal relaxation to the thermodynamically favored trans azobenzene isomer would cause disassembly and thus, light energy is required to maintain the assembled structure (Fig.…”

Laser-Induced Formation and Disintegration of Gold Nanopeanuts and Nanowires

“…Lasers also enable spatial control of nanoparticle production and manipulation, which can be utilized in multi-dimensional colorimetric patterning. 24,25 In this article, we report the formation of peanut-shaped gold nanoparticles (Au nanopeanuts) and gold nanowires (AuNWs) by pulsed laser irradiation.…”