A Cooperative Effect of Bifunctionalized Nanoparticles on Recognition: Sensing Alkali Ions by Crown and Carboxylate Moieties in Aqueous Media

Abstract

Reported here is a cooperative effect that the sensing efficiency of the active group on gold nanoparticles (GNPs) can be significantly influenced by another proximal functional group. We previously developed a visual sensing scheme for K⁺ by 15-crown-5-CH₂O(CH₂)₁₂SH functionalized GNPs in aqueous matrix. Upon adding K⁺, the GNP solution changes from red to blue. Such a transform is triggered by a 2-to-1 sandwich complexation of crown to K⁺, resulting in the red shift of surface plasmon absorption due to GNP aggregation. Herein, we discover that introducing a second functionality, thioctic acid (TA), onto GNPs significantly affects the sensing efficiency of crown moieties (15-crown-5-CH₂O(CH₂)_nSH and 12-crown-4-CH₂O(CH₂)_nSH, where n = 4, 8, and 12). The rate constant of K⁺ recognition by TA- and 15-crown-5-CH₂O(CH₂)₄S-bifunctionalized GNPs is more than 4 orders of magnitude faster than the others containing longer methylene chains. The same chain-length dependence is also found in the case of Na⁺ sensing by 12-crown-4 functionalized GNPs. The discrepancy in sensing performance is attributed to a cooperative effect that the negatively charged carboxylate of TA may preorganize the crown moiety for K⁺ recognition. This method is applied to measure K⁺ and Na⁺ in human urine by UV−visible spectrometry. By adjusting the concentrations of GNPs, the dynamic ranges tuned for K⁺ and Na⁺ are, respectively, 6.25 μM−1.12 mM and 0.156−4.00 mM, suitable for real samples pretreated simply by 10-fold dilution. The results ([K⁺] = 20.3 mM, [Na⁺] = 45.1 mM) agree with those obtained from ICP-AES ([K⁺] = 19.8 mM, [Na⁺] = 43.8 mM).