Fiber-Optic Microsensor Array Based on Fluorescent Bulk Optode Microspheres for the Trace Analysis of Silver Ions

Abstract

An optical microsensor array is described for the rapid analysis of silver ions at low parts per trillion levels. Because the ionophore o-xylylenebis(N,N-diisobutyldithiocarbamate) (Cu-I) was reevaluated and shown to exhibit excellent selectivity for silver ions, ion-selective electrode (ISE) membranes were optimized and found to exhibit the lowest reported detection limit so far (3 x 10(-10) M). A corresponding Ag+-selective fluorescent optical microsensor array for the rapid sensing of trace level Ag+ was then developed. It was fabricated using plasticized PVC-based micrometer-scale fluorescent microspheres that were produced via a sonic particle casting device. They contained 156 mmol/kg Cu-I, 10 mmol/kg 9-(diethylamino)-5-[4-(15-butyl-1,13-dioxo-2,14-dioxanodecyl) phenylimino]benzo[a]phenoxazine (chromoionophore VII, ETH 5418), 2.3 mmol/kg 1,1' '-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (internal reference dye), and 14 mmol/kg sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and were deposited onto the etched distal end of a 3200-microm-diameter optical fiber bundle. The microarray was characterized by fluorescence spectroscopy in samples containing 10(-12)-10(-8) M AgNO3 at pH 7.4, with selectivity characteristics comparable to the corresponding ISEs. The response time of the microsensor array was found to be less than 15 min for 10(-9) M AgNO3, which is drastically shorter than earlier data on optode films (8 h) and corresponding ISEs (30 min). A detection limit of 4 x 10(-11) M for Ag+ was observed, lower than any previously reported optode or silver-selective ISE. The microsensor array was applied for measurement of free silver levels in buffered pond water samples.

Novel potentiometric and optical silver ion-selective sensors with subnanomolar detection limits

“…Fluorescent microspheres were prepared using a previously described sonic particle-casting apparatus [31]. The casting procedure is based on the coexistence of two streams, a diluted membrane cocktail (core) and purified water (sheath).…”

“…The response slopes for the interfering ions must be recorded since close to theoretical values confirm that selectivity coefficients are not biased [29]. Unfortunately, Ag + -ISEs have only recently been characterized under such precautions [8,15,16,30,31] so that the selectivity behavior of most Ag + -ISEs published earlier is not directly comparable. Similarly, without suppressing ion fluxes in the ISE membranes, the lower detection limit of the sensors is also biased [32,33].…”

“…Similarly, without suppressing ion fluxes in the ISE membranes, the lower detection limit of the sensors is also biased [32,33]. So far, only two papers have reported on Ag + -ISEs with optimized lower detection limits showing values of 10 −9 (100 ppt) [30] and 3 × 10 −10 M Ag + [31].…”

“…Finally, the complex formation constant is directly accessible also from the transmembrane potential of a symmetrically bathed double membrane obtained by joining two preconditioned membranes, one of the usual composition and the other again containing the ion exchanger without the ionophore (sandwich membrane method [39,40]). So far, complex formation constants of only a few Ag + -selective ionophores have been determined by such methods [15,31,39].…”

Synthesis and characterization of monoazathiacrown ethers as ionophores for polymeric membrane silver-selective electrodes

“…Such fluxes can be reduced if primary ions in the inner filling solution are buffered with certain ligands, and thus, their concentration remains constant [27]. A number of different strategies via adjusting the concentration of inner solution have been reported [14,15,28,29]. Herein, we introduce an alternative approach using precipitation method according to the solubility product constant (K sp ) of AgCl to control the concentration of free Ag + in the inner solution.…”