Apparatus for concentration and detection of polarizable analyte molecules within a fluid medium, the apparatus comprising: a structure 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 positioned relative to the structure to enable generation of a direct current, DC, electric field passing through the at least one cavity; at least two trapping electrodes positioned relative to the structure 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 of the at least two trapping electrodes being provided as a continuous layer on the surface of the structure 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 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 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 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 positioned relative to the structure to enable generation of a direct current, DC, electric field passing through the at least one cavity; at least two trapping electrodes positioned relative to the structure 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 of the at least two trapping electrodes being provided as a continuous layer on the surface of the structure 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 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 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 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 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 . <EOS>