Glyco-conjugate as selective switch-on molecule for Hg²⁺ in the presence of albumin proteins, blood serum milieu and on silica gel solid support

Abstract

The glyco-conjugate, L_S, its Hg²⁺ complex, and the L_O were synthesized and characterized in their solid state by FTIR, diffuse reflectance, and powder XRD. The mercury complex shows dumbbell-like microstructures as a result of the metal ion-induced aggregation of the glyco-conjugate as studied based on scanning electron microscopy. The aggregation was also supported by atomic force microscopy. The Hg²⁺ was detected on L_S-coated silica gel sheets by switch-on fluorescence, wherein, the enhancement in the fluorescence intensity varies linearly with the added Hg²⁺ in the concentration range 5–60 μM. The concentration-dependent variation was also detected on silica gel sheets by viewing their colour under 365 nm light. Hg²⁺ was detected by the L_S-coated silica gel sheets by switch-on fluorescence even in the presence of blood serum. Thus, this method is extendable for the detection and quantification of Hg²⁺ present in blood and or in biological fluids using disposable silica gel sheets. The sensing of Hg²⁺ in HEPES buffer solution is well demonstrated in this paper using various techniques, such as fluorescence, absorption, visual color, ESI MS, and ¹H NMR. In the buffer medium, the LS exhibit selective chromogenic as well as fluorogenic properties towards Hg²⁺ by showing ∼75-fold higher fluorescence emission intensity and the studies were further extended to show that none of the 13 ions studied compete for Hg²⁺. A minimum detection limit of 25 ± 4 ppb was shown by L_S for Hg²⁺ in the buffer. All the experimental studies carried out support the formation of 2 : 1 complex between L_S and Hg²⁺. At higher mole ratios of Hg²⁺, the same yields the oxo-product, viz., L_O. The structure of the 2 : 1 complex was modeled by DFT using B3LYP/LANL2DZ.