Cellulose is a widely used material and a lot of knowledge about its chemistry in bulk, was accumulated through decades [1]. However, making cellulose films is not an easy task. This is due to its insolubility in most volatile solvents. Recently, an experimental protocol was developed, allowing us to prepare ultra-thin films by spin-coating on semiconductor wafers [2]. The method consists on previously silylating the fibres, producing a derivatized cellulose soluble in solvents like tetrahydrofuran, subsequently enabling the spin-coating deposition. Next, the derivatized cellulose film is exposed to hydrochloric acid vapours to be regenerated to cellulose. The surface is then enriched in alcohol groups, which can be chemically activated, namely with isocyanate groups. Actually, rigid molecules containing an isocyanate function in each extremity have the ability of reacting with the cellulose alcohol groups through one of the isocyanate groups. The other one stays free for reacting with other molecules containing amine, alcohol or carboxylic groups.
These ultrathin cellulosic films open new perspectives both from the point of view of fundamental studies and applications. For instance, when deposited on semiconductors, besides protecting their surface from environmental conditions, also allow a further chemical modification or functionalization. These resulting hybrid systems are very useful for sensors [3], as their sensitivity and selectivity both depend on the nature and the molecular arrangement of the active groups grafted on the surface.
In this study, we report the functionalization of cellulose films using 4,4'-methylene bis(phenylisocyanate) to activate the alcohol groups for the inclusion of molecules having biological interest such as porphyrins or DNA single strands as a step towards the fabrication of biosensors. To follow the surface chemistry of cellulose films and to optimize the procedures for their functionalization, a set of techniques of analysis was used: FTIRS in ATR/MIR mode, XPS, HREELS and AFM.
[1] T. Heinze et al, Prog. Polym. Sci. 2001, 26, 1689-1762.
[2] E. Kontturi et al, Chem. Soc. Rev. 2006, 35 (12): 1287-1304.
[3] V. Parra et al, Langmuir, DOI: 10.1021/la063114i
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