| Summary: | Levofloxacin (LF) is a medically important antibiotic drug that is used to treat a variety of bacterial infections. In this study, three highly sensitive and selective carbon paste electrodes (CPEs) were fabricated for potentiometric determination of the LF drug: (i) CPEs filled with carbon paste (referred to as CPE); (ii) CPE coated (drop-casted) with ion-selective PVC membrane (referred to as C-CPE); (iii) CPE filled with carbon paste modified with a plasticizer (PVC/cyclohexanone) (referenced as P-CPE). The CPE was formulated from graphite (Gr, 44.0%) and reduced graphene oxide (rGO, 3.0%) as the carbon source, tricresyl phosphate (TCP, 47.0%) as the plasticizer; sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (St-TFPMB, 1.0%) as the ion exchanger; and levofloxacinium-tetraphenylborate (LF-TPB, 5.0%) as the lipophilic ion pair. It showed a sub-Nernstian slope of 49.3 mV decade<sup>−1</sup> within the LF concentration range 1.0 × 10<sup>−2</sup> M to 1.0 × 10<sup>−5</sup> M, with a detection limit of 1.0 × 10<sup>−5</sup> M. The PVC coated electrode (C-CPE) showed improved sensitivity (in terms of slope, equal to 50.2 mV decade<sup>−1</sup>) compared to CPEs. After the incorporation of PVC paste on the modified CPE (P-CPE), the sensitivity increased at 53.5 mV decade<sup>−1</sup>, indicating such improvement. The selectivity coefficient (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mrow><mi>log</mi><mo> </mo><mi mathvariant="normal">K</mi></mrow></mrow><mrow><msup><mrow><mi>LF</mi></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup><mo>,</mo><msup><mrow><mi>Fe</mi></mrow><mrow><mo>+</mo><mn>3</mn></mrow></msup></mrow><mrow><mi>pot</mi><mo>.</mo></mrow></msubsup></mrow></semantics></math></inline-formula>) against different interfering species (Na<sup>+</sup>, K<sup>+</sup>, NH<sup>4+</sup>, Ca<sup>2+</sup>, Al<sup>3+</sup>, Fe<sup>3+</sup>, Glycine, Glucose, Maltose, Lactose) were significantly improved by one to three orders of magnitudes in the case of C-CPE and P-CPE, compared to CPEs. The modification with the PVC membrane coating significantly improved the response time and solubility of the LF-TPB within the electrode matrix and increased the lifetime. The constructed sensors were successfully applied for LF determination in pharmaceutical preparation (Levoxin<sup>®</sup> 500 mg), spiked urine, and serum samples with high accuracy and precision.
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