K+-selective nanospheres: maximising response range and minimising response time

Abstract

Cross-linked K(+) ion-selective copolymer nanospheres have been prepared by free-radical photo-initiated polymerization of n-butyl acrylate (nBA) with hexanedioldiacrylate (HDDA). Nanospheres (<200 nm) containing H(+)-chromoionophore (ETH 5294) and lipophilic salt (KTClPB) for H(+)-sensors, or ETH 5294, a K(+)-selective ionophore (valinomycin) and anionic sites for K(+)-sensors were compared, and the effect of varying the normalised concentrations for beta (R(T)(-)/L(T)) and gamma (C(m)(T)/L(T)) was studied. Experimental data were fitted to theoretical curves for the dynamic response range, based on the effect of changes in the concentration of these lipophilic sensing components incorporated into the spheres, and conditions identified for maximising the response range. A complex valinomycin-K(+) formation constant, log K(IL) = 13.13 +/- 2.22, was obtained in the nBA matrix, and from the calibration curves the apparent acid-dissociation equilibrium constant (pK(a) = 12.92 +/- 0.03) was extracted for the H(+)-sensing system, and the equilibrium exchange constant (pK(exch) = 6.16 +/- 0.03, at pH 7) calculated for the K(+)-sensing nanospheres. A basis for establishing optimum performance was identified, whereby response range and response time were balanced with maximum fluorescence yield. Parameters for achieving nanospheres with a response time <5 minutes, covering 2-3 orders of magnitude change in activity were identified, demanding nanospheres with radius <300 nm and beta(crit) approximately 0.6. An RSD(%) approximately 3% was obtained in a study of the reproducibility of the response of the proposed nanospheres, and selectivity was also evaluated for a K(+)-selective nanosensor using several cations as interfering agents. In most cases, the fluorescent emission spectra showed no response to the cations tested, confirming the selectivity of nanospheres to potassium ion. The nanosensors were satisfactorily applied to the determination of K(+) in samples mimicking physiological conditions.

Evaluating Nanoparticle Sensor Design for Intracellular pH Measurements

“…This equation is in accordance with what has previously been derived for transcribed GFP-based pH indicators. 23 In this field of transcribed pH sensors important considerations on derivation of calibration curves and actual pH measurements have been published. 36,37 The triple-labeled sensor follows an extended version of eq 1:…”

Enzyme Substrates Protective Encapsulation within Polymeric Microspheres

“…This has been proven by numerous research on polyacrylate based ion-selective electrodes [e.g. 17,18] as well as on polymeric microspheres [4,6,16].…”

“…The synthesis of poly(n-butyl acrylate) microspheres was carried out according to the method of Hall, et al [16], with some modifications. A mixture of n-butyl acrylate (nBA) (480 μl), HDDA (220 μl) as cross-linker and DMPP (15 mg) as initiator was prepared using an ultrasound probe for 3 min (Heilsher UP200S, cycle: 0.5; amplitude 50%).…”

“…without microspheres), this solution was added to water based samples. To introduce 4-MUO to the microsphere a simple absorption procedure [16] was applied. Unless otherwise stated, microspheres were prepared directly before use.…”

“…Moreover, due to small size and favorable surface area to size ratio, microspheres are proven to be good candidates for fluorimetric miniature sensors [e.g. 15,16]. Last but not least, lipophilic character of the polymer can result in sufficient protection and/or can be useful to create an enzyme favorable reaction microenvironment.…”