Ｗｒｉｔｉｎｇ Self-Erasing Images using Metastable Nanoparticle “Inks” Angew. Chem. Int. Ed. 2009, 48 Ｗｒｉｔｉｎｇ Self-Erasing Images using Metastable Nanoparticle “Inks” Rafal Klajn, Paul J. Wesson, Kyle J. M. Bishop, and Bartosz A. Grzybowski* 1 Associate Professor of Chemical and Biological Engineering Northwestern University, Evanston/Chicago

Background The development of photosensitive materials has been　a focal subject in polymer science and technology. Materials that store textual or graphical information for a prescribed period of time are desirable for applications in secure communications and also it reducing the costs, both industrial and environmental. First finding of photochromic compounds can be traced back to the middle of the 19th century. To date, most research on self-erasing media has relied on the use of photochromic molecules. Recently Xerox Corporation announced the development of photochromic paper that self-erases in 16 to 24 h, Xerox has filed for patents on the technology, which it calls "erasable paper”. Chem.Rev Volume 100, Number 5 May 2000 Seon-Jeong Lim et al. Angew. Chem. Int. Ed. 2004, 43, 6346 –6350 Fengyu Li et al. Chem. Mater. 2008, 20, 1194–1196 Michael Häckel et al. Adv. Mater. 2007, 19, 227–231 http://www.xerox.com/innovation/news-stories/erasable-paper/enus.html 2

Present Work 　　Here, they 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 nanoparticles embedded in thin, flexible organogel films. As a result of Photoisomerisation of azobenzene group coating to the NPs , NPs aggregate into supraspherical (SS)assemblies whose color depends on duration of UV irradiation*. As we know SS are metastable** , so in the absence of UV irradiation the multi color images written into the films erases spontaneously. *M. Fialkowski et al. J. Phys. Chem. B 2006, 110, 2482 –2496. **R. Klajn et al. Proc. Natl. Acad.Sci. USA 2007, 104, 10305 – 10309. 3

Experimental set-up… Au (5.6±0.6 nm diameter) and Ag NPs (5.3±0.3 nm diameter) inks coated with mixed self-assembled monolayers (mSAMs) of dodecylamine (DDA) and photoswitchable azobenzene-terminated thiol (4-(11 mercaptoundecanoxy) Azobenzene (MUA). 4

NP were dispersed in thin (150 μm) films of syndiotactic poly(methyl methacrylate) (sPMMA) organogel laminated between two flexible poly(vinyl chloride)-coated poly(ethylene terephthalate) sheets (up to 5 ×5 cm). Due to high extinction cross-sections (4.2 ×10-18 m2 for gold and 1.2 ×10-17 m2 for silver), the films were colored brightly: red for AuNPs and yellow for AgNPs. In the absence of UV irradiation, the NPs in the gel had UV/Vis spectra nearly identical to those of free NPs in toluene (λmax,Au =525 nm, λmax,Ag=420 nm) which indicating that the particle were not aggregated. 5

Eg: Gold NPs When films exposed to UV , changed the color depending on duration of irradiation. In the absence of UV irradiation the images erased gradually which controlled by Composition of the mSAMs coating the NPs inks. Number of dipoles induced on the NPs. Accelerated by exposure to visible light or by heating the material. Metastable Aggregate formation.... 6

(10 mWcm-2) for times tirr varying from 0 to 10 s UV/Vis spectra of AuNP (left) and AgNP (right) films exposed to 365 nm UV light (10 mWcm-2) for times tirr varying from 0 to 10 s Red Yellow Pale Blue Violet 7

Why So…?? Dipole moment of Cis≈5 Debye and for Trnas≈1 Debye, which mediated attractive interaction between NPs. Strength depends surface concentration of the MUA tethered to the NPs. For low coverage of MUA <0.23, dipole -dipole force is too weak to cause aggregation. For >0.34, irreversible aggregation occur even in the absence of irradiation. For 0.23<<0.34 ,NPs will aggregate reversibly into disordered, metastable aggregate and will disintegrate in the absence of UV light. 8

Example… 9 Fractional surface coverage of MUA =0.3 Using a light pen (IUV=10 mWcm-2) Irradiation time≈0.8 seconds. The image in the AuNP film self-erases in daylight within 9 h and in 60 s by exposure to intense (0.3 mWcm-2) visible light. Multicolor images could be created with one nanoparticle ink by varying the irradiation dose over different regions of the film. IUV=10 mWcm-2. Purple regions for 0.8 s, Purple-bluish regions for 4 s, Pale-blue regions for 10 s. Self erasing is due to reversion of aggragation to free NPs. Erasing time varies from seconds to several days. Film could be written multiple times. Eg: AuNP film. 9

The factors… tw and te depends on and could be controlled by the intensity of light and by the fractional surface coverage  of the MUA ligands. tw decreases with increasing IUV and vary between 20 s for IUV=0.7mWcm-2 and 0.8 s for IUV= 10 mWcm-2. For a given value of IUV, tw decreased with increasing . te decreased with increasing IVis and vary between te=24 h in IVis= 10 nWcm-2 and te=20 s for IVis=0.8 mWcm-2 halogen lamp) and with decreasing . The progress variable ξ was calculated from the experimental extinction spectra and corresponds to the colormap on the right 10

ADVANTAGES….. Previously we studied ,NPs coated with azobenzene dithiols (ADT) and stabilized in solution by a large excess of didodecyldimethylammonium bromide (DDAB) surfactant*. The absence of the second terminal thiol group enables full reversibility of aggregation. In the absence of dithiol cross-linking the NPs cannot self-assemble into well-ordered crystals, BUT… they aggregate and disaggregate (into similarly sized but orderless structures) much more rapidly. 3. The surfactant-free NPs are stable in gel matrices. This stability extends to high NP concentrations and gives deep colors even to thin gel films. Together, these properties enable dispersion of the NP inks in the gel photopaper and allow for rapid, high-contrast writing using light 11 * R. Klajn et.al. Proc. Natl. Acad. Sci. USA 2007, 104, 10305 – 10309.

Conclusion…. They described a class of self-erasable and rewritable materials in which information is written into metastable nanoparticle “inks”. These materials can be useful for storing sensitive or temporary information like self-expiring bus tickets Although the inks themselves are nontoxic the need to use organogel as the supporting medium is environmentally undesired, and alternative water-based materials using hydrophilic photoswitchable NPs should be considered in future research. 12

Thank You

The erasing is not due to the diffusion of the aggregates, but only to their disassembly, which changes the color by weakening the electrodynamic coupling between proximal NPs. The diffusivities of single NPs and of the supraspheres in the homogeneous sPMMA organogel may be estimated as D= D0 exp(-R0.75),* where D0=kT/6R is the Stokes–Einstein diffusivity, kT is the thermal energy,  is the solvent viscosity, R is the particle or aggregate radius, ≈0.03 is the gel volume fraction, and ≈0.5 1 is a constant specific to a given solute–gel pair. Thus, for a single NP,DNP≈10-6 cm2 s-1; for an aggregate with a radius of approximately 50 nm, the diffusion coefficient drops dramatically to DSS≈ 10-15 cm2 s-1. Therefore, the characteristic times required for the aggregates to diffuse L≈100 m (typical size of the written features) are very long, on the order L2/DSS≈1010 s. Instead, erasure of the images is due entirely to NP disassembly, which is limited by the cis-to-trans isomerization rate and not by diffusion. * B. Amsden, Macromolecules 1998, 31, 8382 – 8395.