Ni2+-modulated homocysteine-capped CdTe quantum dots as a turn-on photoluminescent sensor for detecting histidine in biological fluids.

The high affinity of histidine to Ni2+ has long been recognized in metal ion affinity chromatography for the separation and purification of histidine-tagged proteins. Besides, such affinity pair has been explored in modern nanotechnology for constructing functional nanoparticle-histidine-tagged protein conjugates. However, the use of Ni2+-histidine affinity pair in conjunction with optically-active nanomaterials for sensor design, to our knowledge, has not been reported yet. Here we report a turn-on photoluminescent sensor for histidine based on Ni2+-modulated homocysteine (Hcy)-capped CdTe quantum dots (QDs) by taking the advantages of this well-known Ni2+-histidine affinity pair and photoluminescent QDs. The photoluminescence of Hcy-capped CdTe QDs can be effectively quenched by Ni2+ due to the binding of Ni2+ to the Hcy on the surface of the QDs and the electron transfer from the photoexcited QDs to Ni2+. The high affinity of histidine to Ni2+ enables Ni2+ to be dissociated from the surface of Hcy-capped CdTe QDs to form stable complex with histidine in solution, thereby recovering the photoluminescence of Hcy-capped CdTe QDs. The Ni2+ induced photoluminescence quenching and subsequent histidine-induced photoluminescence recovery for Hcy-capped CdTe QDs build a solid base for the present QD-based turn-on photoluminescent sensor for detecting histidine. The developed QD-based sensor gives excellent selectivity for histidine over other amino acids with the limit of detection (3 s) of 0.3 μM. The relative standard deviation for 11 replicate detections of 15 μM histidine was 2.7%. The developed sensor was applied to the determination of histidine in human urine samples with recoveries from 94.4% to 106%.