Additive-improved colorimetric nitrite assay with ultrahigh sensitivity based on etching gold nanorods

Gold nanomaterial (GNM)-based colorimetric sensors have gained particular attention in the analytical field because of their instrument-free, low-cost, and easy-to-use features. Herein, a universal strategy for improving the sensitivity of colorimetric assays, which is based on GNM etching, was proposed by using additives to shift the chemical equilibrium of the reactions. As a proof-of-concept demonstration, nitrite was detected with high sensitivity using gold nanorod (AuNR) by monitoring the shift of its longitudinal localized surface plasmonic resonance (Delta lambda) with the use of NH4Br and HCl. These two additives could change chemical equilibriums and promote the etching of more gold atoms at the AuNR ends. The assay is sensitive and 0.001 mu M of nitrite could result in Delta lambda change by 3 nm. A linear relationship between Delta lambda and nitrite concentration was established (Delta lambda =22.8*C-nitrite +9.7) in the range of 0.016-8.0 mu M with a coefficient of 0.999. The reaction time was also accurately controlled by adjusting solution pH with NH3 center dot H2O to terminate the test. The sensitivity of this colorimetric assay toward nitrite was significantly higher than that of the conventional Griess assay, while it was the highest among the reported GNM-based approaches. This method was successfully applied in determining the nitrite content in food samples, including waters, spinach, and rose petals, with less rigorous sample processing. The proposed strategy of using additives to alter chemical equilibriums to improve sensitivity could be extended to other GNM etching sensors.