Synthesis and characterization of cellulose α-lipoates: A novel material for adsorption onto gold

Novel α-lipoic acid esters of cellulose (cellulose α-lipoates) were synthesized homogeneously in N,N-dimethylacetamide (DMAc)/LiCl using different methods for the in situ activation of the carboxylic acid. Thus, cellulose α-lipoates with degrees of substitution (DS) in the range 0.11 to 1.45 were accessible with N,N'-carbonyldiimidazole and p-toluenesulphonyl chloride as in situ activating agents for the α-lipoic acid. The reactions proceeded totally homogeneous with high yields giving cellulose α-lipoates soluble in DMSO. The α-lipoate moiety containing a S-S function stays intact during the reaction as revealed by FTIR and ¹H NMR spectroscopy. The cellulose α-lipoates showed self-assembly onto gold surface yielding layers with a thickness of 2.9-4.9 nm, which can be confirmed by surface plasmon resonance. The perpropionylated cellulose α-lipoates form films with a comparably low thickness of 0.9 nm.