Apparatus for concentration and detection of polarizable analyte molecules within a fluid medium, the apparatus comprising: a structure (102a, 102b) defining at least one cavity having an inlet, the inlet having a cross-sectional dimension of 200 nm or less; a reservoir for the fluid medium is provided adjacent to, and in fluid communication with, the inlet of the at least one cavity; at least two translocation electrodes (112, 114) positioned relative to the structure (102a, 102b) to enable generation of a direct current, DC, electric field passing through the at least one cavity; at least two trapping electrodes (114, 116a, 116b) positioned relative to the structure (102a, 102b) to enable generation of a time varying electric field proximal to the inlet of the at least one cavity, one of the at least two trapping electrodes being positioned in the reservoir upstream of the inlet of the at least one cavity, and another (116a, 116b) of the at least two trapping electrodes being provided as a continuous layer on the surface of the structure (102a, 102b) adjacent to and surrounding the inlet of the at least one cavity; and a controller configured to: apply a time varying voltage to the at least two trapping electrodes (114, 116a, 116b) to generate a time varying electric field in a proximal region adjacent to the inlet of the at least one cavity to concentrate molecules from the fluid medium within the reservoir in the proximal region by dielectrophoresis; apply a direct current, DC, voltage to the at least two translocation electrodes (112, 114) to generate a translocation electric field passing through the at least one cavity to perform translocation of the concentrated molecules from the proximal region in the reservoir into the at least one cavity. a structure (102a, 102b) defining at least one cavity having an inlet, the inlet having a cross-sectional dimension of 200 nm or less; a reservoir for the fluid medium is provided adjacent to, and in fluid communication with, the inlet of the at least one cavity; at least two translocation electrodes (112, 114) positioned relative to the structure (102a, 102b) to enable generation of a direct current, DC, electric field passing through the at least one cavity; at least two trapping electrodes (114, 116a, 116b) positioned relative to the structure (102a, 102b) to enable generation of a time varying electric field proximal to the inlet of the at least one cavity, one of the at least two trapping electrodes being positioned in the reservoir upstream of the inlet of the at least one cavity, and another (116a, 116b) of the at least two trapping electrodes being provided as a continuous layer on the surface of the structure (102a, 102b) adjacent to and surrounding the inlet of the at least one cavity; and a controller configured to: apply a time varying voltage to the at least two trapping electrodes (114, 116a, 116b) to generate a time varying electric field in a proximal region adjacent to the inlet of the at least one cavity to concentrate molecules from the fluid medium within the reservoir in the proximal region by dielectrophoresis; apply a direct current, DC, voltage to the at least two translocation electrodes (112, 114) to generate a translocation electric field passing through the at least one cavity to perform translocation of the concentrated molecules from the proximal region in the reservoir into the at least one cavity. apply a time varying voltage to the at least two trapping electrodes (114, 116a, 116b) to generate a time varying electric field in a proximal region adjacent to the inlet of the at least one cavity to concentrate molecules from the fluid medium within the reservoir in the proximal region by dielectrophoresis; apply a direct current, DC, voltage to the at least two translocation electrodes (112, 114) to generate a translocation electric field passing through the at least one cavity to perform translocation of the concentrated molecules from the proximal region in the reservoir into the at least one cavity.