In this work, lysozyme-stabilized fluorescent gold nanoclusters (Lyso-AuNCs) and pyridoxal 5′-phosphate (PLP) as a monophosphate ester substrate were used to develop a highly selective and cost-effective bioassay for the detection of alkaline phosphatase (ALP) activity. The vitamin B6 cofactor, PLP, was conjugated with the red-emitting nanoclusters to obtain a probe, PLP_Lyso-AuNCs, via forming a Schiff base linkage between the free amino group of lysozyme and the aldehyde group of PLP. At pH = 10.08, addition of ALP to the yellow-emitting PLP_Lyso-AuNCs solution catalyzed the hydrolysis of PLP and converted it into pyridoxal, which produced a distinct ratiometric fluorescence response and the fluorescent color turned pale white. Using the probe, PLP_Lyso-AuNCs, the ALP activity could be detected down to 0.002 U/L. Further, the changes in the fluorescent color intensity (red, blue, and green) of PLP_Lyso-AuNCs were recorded with the back camera of a smartphone to quantify the ALP activity. Both the fluorimetric and smartphone approaches gave satisfactory recovery percentage, when the practical utility of PLP_Lyso-AuNCs was applied to quantify the ALP activity in environmental and biological samples, such as river and lab tap water, blood plasma, and serum. Finally, a pure white light-emitting nano-assembly was developed by conjugating optimized amounts of both PLP and pyridoxal over the surface of Lyso-AuNCs.

A novel fluorescent receptor L was synthesized by Schiff base condensation of 1-pyrenemethylamine with the vitamin B cofactor pyridoxal. The receptor L is highly selective and sensitive towards Zn ions among other tested metal ions. Upon interaction with Zn, the receptor L showed a distinct fluorescence enhancement at 485 nm due to the excimer formation leading to the fluorescent colour change from blue to bluish-green. Subsequently, when the in situ generated ZnL complex interacted with various anions and amino acids, the addition of HPO and cysteine reinstated the fluorescence of the receptor L due to the demetalation of Zn from the ZnL complex. Accordingly, the receptor L was developed for the highly selective, specific and sensitive detection of three important bioactive analytes, i.e., Zn, HPO and cysteine with a detection limit down to 2.3 × 10 M, 2.18 × 10 M and 1.59 × 10 M, respectively. Additionally, the receptor L was applied to the detection of intracellular Zn ions in live HeLa cells.

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This communication demonstrates a rapid, reproducible and highly selective colorimetric nanosensor for the detection of Cr3+ ions in aqueous medium using the functionalized gold nanoparticles. The functionalized AuNPs were obtained in three steps: (i) the citrate capped Cit‐AuNPs were prepared by standard procedure, (ii) the thiolated Schiff base CAPLP was generated in‐situ by reacting vitamin B6 cofactor pyridoxal 5′‐phosphate and cysteamine, and then (iii) the CAPLP was decorated over the surface of Cit‐AuNPs via ligand exchange method yielding spherical and well dispersed nanoparticles of size ∼16±2 nm. The wine red color of CAPLP‐AuNPs solution turned to purplish blue upon addition of Cr3+ ions along with the SPR band was red‐shifted from 525 nm to 650 nm due to the complexation‐induced aggregation of nanoparticles. With a high selectivity and specificity, this nano‐assembly shows the detection limit down to 2.34 μM and finds applicability over a wide range of pH. The developed nanosensor was applied in environmental water samples for Cr3+ ions detection and achieved ideal results. More significantly, the visually detectable color change of CAPLP‐AuNPs has been successfully integrated with a smartphone RGBColorValue app to make it useful for on‐site fast measurement and real‐time online analysis of Cr3+ with the detection limit down to 11 μM.

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