Application of chromogenic reagents in surface plasmon resonance (SPR)

In this paper, a new simple approach for sensitivity optimization in surface plasmon resonance (SPR) chemosensors based on colorimetric ligands is presented. A new design of SPR sensor with tunable analytical wavelength (λSPR) was constructed for this purpose, to perform studies on the ligand absorbance spectra related sensitivity enhancement. Unlike commercial SPR sensors which operate at one λSPR, the new device can be used for sensitivity analysis at selected λSPR in the range 550–750 nm, offering the possibility to identify the highest sensitivity λSPR in regard to the spectral changes of the selected ligand. Measurements can be easily done in ligand bulk solutions without immobilization steps. Sensitivity enhancement analysis and optimization of λSPR on chromogenic reagents with hypsochromic shift in their absorption spectra are demonstrated in this contribution. Optimal selection of analytical wavelength, set at the absorbance peak of chromogenic reagent Eriochrome Black T (EBT) was observed to result in up to two times increased SPR sensitivity to Cd2+ compared to wavelengths selected in other parts of the ligand absorbance spectra, with a limit of detection (LOD) 0.2 ppm. The sensitivity enhancement at optimal λSPR was observed to be related to increased refractive index (n), drop in extinction coefficient (α) and simultaneous hypsochromic shift of the EBT absorbance spectra causing the λSPR to match the absorbance peak shoulder.