Arg, Lys and Asp amino acids are known to play a critical role in the adhesion of the RKLPDA engineered peptide on the (101) surface of the titania anatase phase. To understand their contribution to peptide adhesion, we have considered the relevant charge states due to protonation (Arg and Lys) or deprotonation (Asp) occurring in neutral water solution, and studied their adsorption on the (101) anatase TiO₂ surface by ab initio total energy calculations based on density functional theory. The adsorption configurations on the hydrated surface are compared to those on the dry surface considering also the presence of the hydration shell around amino acid side-chains. This study explains how water molecules mediate the adsorption of charged amino acids showing that protonated amino acids are chemically adsorbed much more strongly than de-protonated Asp. Moreover it is shown that the polar screening of the hydration shell reduces the adsorption energy of the protonated amino acids to a small extent, thus evidencing that both Arg and Lys strongly adhere on the (101) anatase TiO₂ surface in neutral water solution and that they play a major role in the adhesion of the RKLPDA peptide.

Natural methylxanthines, caffeine, theophylline and theobromine, are widespread biologically active alkaloids in human nutrition, found mainly in beverages (coffee, tea, cocoa, energy drinks, etc.). Their detection is thus of extreme importance, and many studies are devoted to this topic. During the last decade, graphene oxide (GO) and reduced graphene oxide (RGO) gained popularity as constituents of sensors (chemical, electrochemical and biosensors) for methylxanthines. The main advantages of GO and RGO with respect to graphene are the easiness and cheapness of synthesis, the notable higher solubility in polar solvents (water, among others), and the higher reactivity towards these targets (mainly due to π–π interactions); one of the main disadvantages is the lower electrical conductivity, especially when using them in electrochemical sensors. Nonetheless, their use in sensors is becoming more and more common, with the obtainment of very good results in terms of selectivity and sensitivity (up to 5.4 × 10⁻¹⁰ mol L⁻¹ and 1.8 × 10⁻⁹ mol L⁻¹ for caffeine and theophylline, respectively). Moreover, the ability of GO to protect DNA and RNA from enzymatic digestion renders it one of the best candidates for biosensors based on these nucleic acids. This is an up-to-date review of the use of GO and RGO in sensors.