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 β (R_T⁻/L_T) and γ (C_mT/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 β_crit ∼ 0.6. An RSD(%) ∼ 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.