Ratiometric response to formaldehyde by 3D silver SERS substrate with polyoxometalate as internal label

Abstract

For surface-enhanced Raman scattering (SERS) analytical technology, the rational design of efficient substrate and effective internal standard is the key issue to implement the reliable detection. In this paper, a novel three-dimensional (3D) silver(I)-polyoxometalate complex of Ag2[Ag(trz)2][H6CrMo6O24]·2H2O (Ag3CrMo6, trz = 1, 2, 4-triazole) has been designed and synthesized, which can be used for the quantitative detection of formaldehyde (FA). Structural analysis reveals that the [H6CrMo6O24]3- polyoxoanions link Ag+ by diverse Ag−O(Mo−O) bonds to form a stable 3D framework. When Ag3CrMo6 is exposed to FA, the Ag+ can be partially reduced to Ag0 species to become the label-free SERS active substrate and and present the enhanced signals ascribed to the Mo−O vibrations. Due to that the reduced Ag0 species is only close to Mo−O groups, the Raman signals for Cr−O bonds are almost constant and can be used as internal standards. Therefore, the IMo−O/ICr−O values can be used as ratiometric SERS signals to fulfill the elusive SERS quantitative analysis for FA with the detection limit of 0.05 ng/L. This work opens up an innovative way to the rational design of label-free SERS substrate based on polyoxoanions and silver ions, by which the quantitative detection can be realized via the ratiometric Raman signals.