A novel colorimetric method for iron ion(Fe3+) detection was proposed by using asymmetrically modified Janus gold nanoparticles(Janus AuNPs) in this paper.Initially,citrate stabilized AuNPs with relatively large size were prepared through reduction reaction.Then,the large sized citrate stabilized AuNPs were immobilized on the glass slide,through which some binding sites on the surface of AuNPs were concealed.The stabilizing agent of polyethylene glycol(PEG) was further introduced onto the exposed surface of AuNPs to obtain the PEG and citrate asymmetrically modified Janus AuNPs.After the introduction of Fe3+,the aggregation behavior of Janus AuNPs was adjusted in an oriented and controllable manner to form oligomers which could remain stable in aqueous solution for a long time.Such an asymmetrical modification could significantly improve the dynamic detection range(1 μmol/L-10 mmol/L) of AuNPs based colorimetric assay.Meanwhile,a linear calibration curve,i.e.A750/A536=0.129c(μmol/L)+0.317 was also obtained,with a detection limit of 715 nmol/L.Compared with other nanoparticle based colorimetric system for Fe3+detection,this method is simple and sensitive,and surprisingly broadens the linear range for detection.