A reversible rhodamine-based sensor (L1) capable of undergoing excimer-fluorescent resonance energy transfer (E m -FRET) was designed and synthesized using a ditopic receptor strategy. Addition of Cu 2+ ions to a solution of L1 induced a ring-open conformation of spirolactam (E m -FRET ON), whilst ring closure was induced upon addition of CH 3 COO À (E m -FRET OFF). Fig. 1 Spectral overlap between naphthyl emission (black) and ringopened rhodamine B absorption (red).

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This research work aims to propose the extraction method using chitosan as the sorbent and gold nanoparticles (AuNPs) as the colorimetric sensor for the development of a simple, costeffective, rapid, sensitive, and selective detection method for histidine. The colorimetric assay is based on the aggregation of AuNPs in the presence of Hg 2+ ions and histidine. The state of AuNPs generally changes from dispersion to aggregation. The change in state is accompanied by a corresponding change in color (from red wine to blue). Therefore, the solid phase extraction (SPE) method using chitosan as the sorbent was used to extract the AuNPs to improve the sensitivity of detection. It was found that the extraction of sensor system using chitosan could increase the detection signal for histidine by ten times. The calibration curve, which is the plot of absorbance ratio  (A 650 /A 528 ) against the concentration of histidine, shows a linear relation in the concentration range of 100 nM-800 nM. The limit of detection (LOD) and limit of quantitation (LOQ) of the method were found to be 99.88 nM and 107.45 nM, respectively. Good recoveries were also obtained (range: 99.75%-104.43%) with relative standard deviations (RSDs) below 5.89% in real water samples. Moreover, this Analytical Sciences