Mass-Produced Ionophore-Based Fluorescent Microspheres for Trace Level Determination of Lead Ions

Abstract

The development and characterization of small, uniform, and mass-produced plasticized PVC-based sensing microspheres in view of rapid trace level analysis of lead ions is reported. Micrometer-sized particles obtained via an automated casting process were rendered selective for lead ions by doping them with highly selective components in a manner analogous to traditional optode sensing films. Single particles that contained the lipophilic ionophore N,N,N',N'-tetradodecyl-3-6-dioxaoctane-1-thio-8-oxodiamide (ETH 5493), the chromoionophore ETH 5418 together with a lipophilized indocarbocyanine derivative as internal reference dye (DiIC18), and lipophilic ion-exchanger sites sodium tetrakis[3,5-bistrifluoromethylphenyl]borate, yielded measurable lead responses at the low nanomolar level in pH buffered solutions. The detection limit for single particles was 3 x 10(-9) M at pH 5.7. The microspheres were fabricated via a reproducible formation of polymer droplets within a flowing aqueous phase followed by collection of spherical particles of approximately 13 microm in size. The particles were immobilized and assayed individually in a microflow cell via fluorescence microscopy. Selectivity patterns found were in agreement with those reported earlier for the lead-selective ligand ETH 5493, and all response functions were fully described by theory. In contrast to optode films that necessitated very long equilibration times and large sample volumes in diluted samples of analyte, particles exhibited extremely enhanced equilibrium response times. Thus, for lead sample concentrations at and above 5 x 10(-8) M, response times were approximately 3 min, whereas at the detection limit, complete equilibrium was recorded after just 15 min, with required sample volumes on the order of 1 mL This new class of microspheres appears to be suitable for rapid and sensitive ion detection at trace levels in environmental and biological applications.

Performance evaluation criteria for preparation and measurement of macro- and microfabricated ion-selective electrodes (IUPAC Technical Report)

“…Changes in the concentration of membrane "components" due to their leaching or decomposition are particularly significant when microfabricated electrodes and microsphere optodes are exposed to large volumes of sample or used in long-term in vivo applications, because the total amounts of components are extremely small in these sensing devices [14,28,58,59]. For example, the amount of ionophore in a 5-µm thick, 50-µm diameter disc-shaped membrane cast over a planar sensor surface is only about 0.2 ng.…”

Potentiometric sensors for trace-level analysis

“…In recent years, therefore, research has intensified to develop solid contact electrodes with low LODs. Note that corresponding optical sensors, which do not possess an inner solution, are promising platforms to lower the LODs to trace levels as well [38,46,47].…”

Micro-miniature Autonomous Optical Sensor Array for Monitoring Ions and Metabolites 1: Design, Fabrication, and Data Analysis

“…This can be obtained by adopting optode technology that is based on ion exchange between an aqueous sample and a lipophilic membrane loaded with an ionophore molecule that interacts with a suitable chromoionophore to produce color change of the membrane. [11][12][13][14][15][16][17][18][19][20] The microscopic PVC/BEHS based optode beads used in this work contain a hydrogen ion selective chromoionophore, L(beads), a sodium ionophore, I(beads), and a highly lipophilic anion, TFPB -(beads). When the beads are brought in contact with PBS solution, an ion exchange reaction occurs between the beads and the solution phase whose result is a pH-dependent positive charge shift from the ionophore to the chromoionophore: …”

“…11 The capsule contains immobilized glucose oxidase (GOX) that induces pH to gradually decrease inside the capsule as glucose concentration increases outside, due to the enzymatic production of gluconic acid. The color of the coimmobilized pH-sensitive optode [11][12][13][14][15][16][17][18][19][20] type beads responds to the locally induced pH shifts. Thus, capsule color can be linked to sample glucose concentration using prior calibration.…”