Electrochemical and structural characterization of nanocomposite Agy:TiNx thin films for dry bioelectrodes: the effect of the N/Ti ratio and Ag content

Ag-y:TiNx nanocomposite thin films sputtered with different N/Ti atomic ratios and Ag atomic contents were characterized from the structural and morphological points of view. Their electrochemical behaviour was studied in a synthetic sweat solution, aiming at selecting a suitable material for biolectrode applications. An increase of the N/Ti atomic ratio, which is accompanied by an increase of the Ag atomic content, leads to a substantial increase of the roughness and porosity of the samples, especially for N/Ti ratios >0.2. For N/Ti atomic ratios up to 0.3 (15 at.% Ag) no metallic Ag segregation is visible in the TiNx matrix. Hence, the possible formation of TiAg and Ti2Ag intermetallics or even a Ag/TiAg/Ti2Ag phase mixture, although not demonstrated, should not be disregarded. As for the N/Ti atomic ratio = 0.7 (32 at.% Ag) sample, the Ag phases are predominantly concentrated near the interface with the substrate. The amount of Ag phases at the surface of the films remains somewhat low for all TiN under-stoichiometric films, even for Ag atomic contents up to 32 at.%. When the TiNx matrix reaches the stoichiometric condition (sample with N/Ti atomic ratio = 1 and 20 at.% Ag), Ag segregation occurs and metallic Ag aggregates are visible at the surface of the film, leading to a substantially different electrochemical behaviour. The impedance of the Ag-y: TiNx films in synthetic sweat solution is mainly ruled by the roughness/porosity variation, thus the higher the N/Ti atomic ratio, the lower the impedance. The interfacial film/sweat electrochemical noise and drift were similar for all films and comparable to the results obtained for commercial Ag/AgCl electrodes (except for the N/Ti atomic ratio = 1 and 20 at.% Ag film). In view of the results, it may be concluded that the samples with N/Ti atomic ratios = 0.3 (15 at.% Ag) and 0.7 (32 at.% Ag) are the most appropriate for further bioelectrode development.This research is partially sponsored by FEDER funds through the program COMPETE - Programa Operacional Factores de Competitividade and by national funds through FCT - Fundacao para a Ciencia e a Tecnologia, under the projects PEst-C/EME/UI0285/2011, PTDC/SAU-ENB/116850/2010, PTDC/CTM-NAN/112574/2009 and Programa Pessoa 2012/2013 Cooperacao Portugal/Franca, Project no. 27306UA Porous architectures in GRAded CERamic thin films for biosensors - GRACER. The authors would also like to acknowledge CEMUP for SEM analysis. P. Pedrosa acknowledges FCT for the Ph.D. grant SFRH/BD/70035/2010.