192 · CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 1: GENERAL APPROACHES 6. OPTIMIZATION OF SENSING MATERIALS USING GRADIENT ARRAYS Sensor material optimization can be performed using gradient sensor materials. Spatial gradients in sensing materials can be generated by varying the nature and concentration of starting components, processing conditions, thickness, and some other parameters. Once a gradient sensor array is formed, it is important to estimate the possibilities to adequately measure the variation of properties along the gradient. These can be intrinsic (thickness, chemical composition, morphology, etc.) or performance (response magnitude, selectivity, stability, immunity to poisoning, etc.) properties. 6.1. VARIABLE CONCENTRATION OF REAGENTS Optimization of concentrations of formulation components can require significant effort because of the nonlinear relationship between additive concentration and sensor response (Papkovsky et al. 1995; Dickinson et al. 1997; Collaudin and Blum 1997; Mills 1998; Levitsky et al. 2001; Eaton 2002; Florescu and Katerkamp 2004; Basu et al. 2005). Concentration-gradient sensor material libraries have been employed for detailed optimization of sensor materials. One-, two-, and three-component composition gradients were made by flow-coating individual liquid formulations onto a flat substrate and allowing