Methods and devices for the detection and identification in a sample of one or more target molecules which bind to nucleic acid probe molecules are provided. The method includes contacting the sample with a surface that is coated with one or more gradients of nucleic acid or nucleic acid analog probe molecules that bind target molecules in the sample. Gradients are formed by varying a physical, structural or functional property of the probes on the surface; for example, the density of probe molecules bound to the surface. The coating layer or immobilization layer in which the gradient is formed is preferably continuous Determination of the location, speed and/or extent of hybridisation of a nucleic acid on a gradient surface is useful to identify target molecules bound to probes and/or to quantitatively measure the amount of the target in a sample.