The use of quantum dots (QDs) and molecular beacons (MBs) is a recent advance in the field of nanotechnology. These techniques have enabled us to detect a single molecule in a cell, which helps in understanding the dynamics of a cell. The success of these techniques depends on the accurate and efficient analysis of the imaging data these techniques produce. The processing involves--segmentation of the particles, colocalisation and their tracking over multiple frames in 2D and 3D space. In this paper we have used the active contour models: snakes and their variation--GVF (gradient vector field) snakes for segmentation of nano(QD) and cell(MB) images. The results of segmentation have been used to measure the degree of colocalisation for quantum dot images and the gene expression values for molecular beacon images.