- A fibre optic multiple-pass surface plasmon resonance (SPR) sensor coupled with a field-assist capability for detecting extremely low concentrations of charged particles is demonstrated for the first time. The field-assist feature forces charged particles/molecules to the SPR surface, increasing the local concentration for detection and thus the sensitivity by an additional factor of about 100. A 10 pM concentration of 47 nm diameter polystyrene (PS) latex beads and 2 M concentration of salt dissolved in DI water were detected within a few seconds. The equivalent index resolution for atomic size corresponding to ionized chlorine in salt is 2.6 10-8. This technique should offer the potential for the sensitive and fast detection of analytes in a solution. 2007 IOP Publishing Ltd.