The factors that limit the performance of the counter-reference electrode in a commercial screen printed biosensor (SPE) for the quantification of glucose i~ whole blood have been investigated. The electrochemistry of this Ag/AgCI SPE is limited by the resistance of the track (R ~ 125 Q). The open circuit potential at long times (equilibrium potential) depends on the activity of cr in the solution following the Nemst equation, meanwhile at short times (t < 100 s) the potential transient recorded from the moment the electrode is immersed in the solutitn showed some anomalous and irreproducible behaviour, probably associated with some nucleation process occurring at the electrode. The cyclic voltammetry showed also a change of slope at low overpotentials (' = -80 mY) due to the different potentials needed to reduce two different kinds ofAgCI particles; those in contact with Ag and those isolated. We have proposed two different types of counter reference electrodes. One type is based in replacing the Ag from the ink with carbon, meanwhile the other one uses the same Ag/AgCI ink, but adding different amounts of KCl. SPEs employing the first approach show potentials more negative than those of the' commercial SPEs due to the increase of the resistance of the track. Also these electrodes are unable to transfer the required charge predicted for the overall sensor in working conditions, so this formulation was discarded. The SPEs loaded with KCI show shorter equilibration times than the commercial electrodes. The potential of the transient part of the curve is very reproducible. During the transient part of the chronopotentiometry, the potential is established by the gradient ofCr created by the dissolution ofthe cr embedded in the ink. Finally, SPEs treated with light and air show an anomalous behaviour compared with fresh SPEs. This negative effect consisted in a shift towards less negative values of the potential recorded in a cathodic galvanostatic experiment. This anomalous behaviour was similar to that produced by oxidising the Ag in the SPE to A~O, but no mechanism that explains this similarity was found. Also SPEs partially oxidised and reduced in cr solutions showed this shift ofthe potential to less negative values. Since this shift means that it is easier to reduce these oxidised and reduced electrodes we believe that the mechanism involve a facilitated nucleation process at the oxidised or reduced electrodes.

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The calculation of molecular vibrational frequencies has been investigated for a wide range of systems. Anharmonic vibrational frequencies calculated using second-order vibrational perturbation theory have been examined for a set of 88 small molecules (655 normal modes). The performance of different exchange-correlation functionals has been assessed, and anharmonic frequencies calculated using hybrid functionals are found to provide the best agreement with experiment. These functionals are not significantly improved by frequency scaling factors, indicating an absence of significant systematic error, and for the molecules studied the B97-1 and B97-2 functionals give the closest overall agreement with experiment. Experimental anharmonic shifts are found to be closest for the B3LYP and EDF2 functionals, highlighting the importance of an accurate harmonic force field, and investigations using modified hybrid functionals with increased fractions of Hartree-Fock exchange indicate that approximately 20 % is optimal. Partial Hessian harmonic vibrational analysis has been extended in order to cal­culate anharmonic frequencies, and the results show that this technique can be used to calculate accurate frequencies for vibrational modes that are spatially localized at a significantly reduced time cost. Several molecular systems are examined in order to demonstrate the effectiveness of this method, including organic molecules adsorbed on the Si(100)-2xl surface, microsolvated formamide, and the CH stretching region of polycyclic aromatic hydrocarbons.