This paper examines reliable advancements in low-cost DNA- and immuno-chips. Capacitance detection was successfully chosen to develop label-free bio-chips. Probe immobilization was rigorously investigated in order to obtain reliable capacitance measurements. Protein probes immobilized by using usual alkanethiols or thiolated ssDNA probes directly immobilized on gold do not allow sufficient stable capacitance measurements. New alkanethiols improved with ethylene-glycol function are shown in this paper to be more suitable materials for capacitive bio-chip development. Atomic Force Microscopy, Quartz Crystal Microbalance, and Capacitance Measurements were used to demonstrate that ethylene-glycol alkanethiols allow high time stability, smaller errors in detection, and improved ideal behaviour of the sensing surfaces. Measured capacitance is in the range of 8-11 nF/mm(2) for antibody layers and close to 6 nF/mm(2) for DNA probes. It is in the range of 10-12 nF/mm(2) and of 4-6 nF/mm(2) for antigen and DNA detection, respectively. The percentage error in detection is highly improved and it is in the range of 11-37% and of 0,23-0,82% for antigen and DNA, respectively. The reproducibility is also improved and it is close to 0,44% for single spot measurements on ethylene-glycol alkanethiols. A molecular theory attributing these improvements to water molecules strongly coordinated by ethylene-glycol functional groups and to solution ions not entering into probe films is finally proposed. (C) 2008 Elsevier B.V. All rights reserved.