We analyzed 1/f noise in silicon nanowire ion-sensitive field-effect transistors (SiNW-ISFETs) having different wire widths ranging from 100 nm to 1 pin and operated under different gating conditions in order to determine the noise source and the sensor accuracy. We find that the gate-referred voltage noise S-VG (power spectral density) is constant over a large range of SiNWs resistances tuned by a DC gate voltage. The measurements of S-VG for SiNWs with two different gate-oxide thicknesses, but otherwise similar device parameters, are only compatible with the so-called trap state noise model in which the source of 1/f noise is due to trap states residing in the gate oxide (most likely in the interface between the semiconductor and the oxide). S-VG is found to be inversely proportional to the wire width for constant wire length. From the noise data we determine a sensor accuracy of 0.017% of a full Nernstian shift of 60 mV/pH for a SiNW wire with a width of 1 pm. No influence of the ions in the buffer solution was found. (C) 2013 Elsevier B.V. All rights reserved.