Highly selective and sensitive detection of Cr6+ ions using size-specific label-free gold nanoparticles

•Color change is induced by size-specific Au nanoparticles aggregation in the presence of Cr6+ ions.•A newly developed assay method uses 45 nm label-free Au nanoparticles for detection of Cr6+ions.•The sensor exhibits excellent selectivity for Cr6+ions over other metal cations and anions.•The present assay method can detect a Cr6+ionic concentration of ∼ 0.4 nM within 70 min.

Gold nanoparticles (AuNPs) of various sizes were synthesized by modifying the citrate concentration; 45 nm AuNPs were found to respond to Cr6+ ions selectively. Label-free AuNPs, 45 nm, showed localized surface plasmon resonance bands at 530 nm, which decreased linearly upon addition of Cr6+. The addition of Cr6+ also resulted in the appearance of a new band at 750 nm, along with a visible color change in the solution from wine red to violet. The decrease in absorbance and color change were due to AuNP aggregation upon coordination of Cr6+. The detection limit for Cr6+ was 0.4 nM, and excellent selectivity was observed in the presence of other metal ions and anions. The binding site and sensing mechanism for Cr6+ and label-free AuNPs were characterized by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. This method was applied to tap, pond, and waste water samples, and validated using inductively coupled plasma-optical emission spectrometry, illustrating the utility of the sensitive, selective, and simple AuNP sensor in the detection of Cr6+.

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