title: 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)n)SH 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 microM-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).
authors.affiliation: Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30013.
authors.forename: Shu-Yi
authors.initials: SY
authors.lastname: Lin
authors.forename: Chun-hsien
authors.initials: CH
authors.lastname: Chen
authors.forename: Meng-Chieh
authors.initials: MC
authors.lastname: Lin
authors.forename: Hsiu-Fu
authors.initials: HF
authors.lastname: Hsu