Biocathode microbial fuel cells (MFCs) have been shown to be useful in reductively changing Cr(VI) into Cr(III)-related precipitates on the surface of electrochemically active bacteria (EAB), but the occurrence of intracellular Cr(VI) reduction in the EAB are still unclear. Based on the four EAB isolated from Cr(VI)-reduced biocathode MFCs and tentatively assigned to Stenotrophomonas sp. YS1, Stenotrophomonas maltophilia YS2, Serratia marcescens YS3 and Achromobacterxylosoxidans YS8, a naphthalimide-rhodamine based Cr(III) fluorescent probe was used for the first time to sense the intracellular Cr(III) ions, inevitably and transiently formed from Cr(VI) reduction in these EAB. More Cr(VI) was reduced in the MFC biocathodes, resulting in intracellular accumulation of total chromium of 45.1 +/- 1.3-60.5 +/- 1.4% with a composite of Cr(III) ions (23.7 +/- 2.6-27.3 +/- 1.1%) and other forms of chromium-complex (18.7 +/- 2.5-32.2 +/- 0.8%), In the open circuit controls, intracellular total chromium only ranged 10.2 +/- 1.5-11.7 +/- 0.8% (Cr(III) ions: 8.2 +/- 0.6-9.5 +/- 0.9%; other forms: 0.2 +/- 0.6-0.3 +/- 0.9%). This study illustrates that cathode directed more intracellular Cr(VI) reduction in the EAB and gives an alternative approach for developing imaging and functionally sensing Cr(III) ions in the MFC biocathodes. (C) 2016 Elsevier B.V. All rights reserved.